SUGAR SERIES

Vol. 1. Standard Fabrication Practices for Cane Sugar Mills (Delden) Vol. 2. Manufacture and Refining of Raw Cane Sugar (Baikow) Vol. 3. By-Products of the Cane Sugar Industry (Paturau) Vol. 4. Unit Operations in Cane Sugar Production (Payne) Vol. 5. Noel Deerr: Classic Papers of a Sugar Cane Technologist (Payne,

Compiler) Vol. 6. The Energy Cane Alternative (Alexander) Vol. 7. Handbook of Cane Sugar Engineering (Hugot, 3rd edition) Vol. 8. Management Accounting for the Surgar Cane Industry (Fok Kam) Vol. 9. Chemistry and Processing of Sugarbeet and Sugarcane (Clarke

and GodshalL Editors) Vol. 10. Modern Energy Economy in Beet Sugar Factories (Urbaniec)

sugar series, 10

modern energy economy in beet sugar factories by

K. Urbaniec Division of Thermodynamics, Fluid Mechanics and Heat Transfer, Lund Institute of Technology, Lund, Sweden

Elsevier

Amsterdam — Oxford — New York — Tokyo 1989

ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat 25 P.O. Box 2 1 1 , 1000 AE Amsterdam, The Netherlands

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ISBN 0-444-87294-9 (Vol. 10) ISBN 0-444-41897-0 (Series)

© Elsevier Science Publishers B.V., 1989

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FOREWORD

I t has been v e r y demanding o f t ime and e f f o r t to w r i t e a book on the ene rgy

economy o f beet s u g a r f a c t o r i e s , because the re a re so many a s p e c t s o f t h i s

s u b j e c t t ha t dese rve to be p r e s e n t e d . T r y i n g to e x p l a i n how the energy

requ i rements s h o u l d be adapted to the p r i n c i p l e s o f beet s u g a r manu fac tu re ,

I cou ld not a v o i d d i s c u s s i n g c e r t a i n prob lems o f modern beet s u g a r t e c h n o l o g y .

I hope the s u g a r t e c h n o l o g i s t s can u n d e r s t a n d t h a t f rom t h e i r p o i n t o f v i e w ,

t h i s d i s c u s s i o n may seem incomp le te . I t would be a p p r e c i a t e d i f r e a d e r s would

p o i n t out any o m i s s i o n s or e r r o r s which have been o v e r l o o k e d .

T h i s book c o u l d not have been w r i t t e n w i t h o u t the s t i m u l a t i o n and d i r e c t

a s s i s t a n c e o f P r o f e s s o r Jan Dob rzyck i and D r . W i t o l d L e k a w s k i , to whom s p e c i a l

t hanks are e x p r e s s e d . C e r t a i n i d e a s o r i g i n a t e d f rom d i s c u s s i o n s w i t h the l a t e

P r o f e s s o r S t a n i s l a w Z a g r o d z k i d u r i n g the p e r i o d 1 9 7 9 - 1 9 8 0 .

G ra te fu l a p p r e c i a t i o n i s e x p r e s s e d to the f o l l o w i n g f i r m s and i n s t i t u t i o n s

f o r t h e i r a s s i s t a n c e i n p r o v i d i n g t e c h n i c a l i n f o r m a t i o n , i l l u s t r a t i o n s or

s t a t i s t i c a l d a t a :

- Chemadex, Cukropo l and C u k r o p r o j e k t , Warsaw, P o l a n d ;

- D D S , Copenhagen, Denmark;

- F e i t e n & Gu i l l eaume E n e r g i e t e c h n i k , Nordenham, FRG;

- F r a t e l l i B a b b i n i , Fo r i i , I t a l y ;

- GEA Wiegand , E t t l i n g e n , FRG;

- H o l l y Suga r C o r p o r a t i o n , San F r a n c i s c o , USA;

- K ra f twerk U n i o n , M٧ lhe im , FRG;

- I n s t i t u t f ٧ r L a n d w i r t s c h a f t l i c h e T e c h n o l o g i e und Z u c k e r i n d u s t r i e , TU

B r a u n s c h w e i g , FRG;

- I n s t y t u t P rzemys lu C u k r o w n i c z e g o , Warsaw, P o l a n d ;

- P f e i f e r & Langen , C o l o g n e , FRG;

- R a f f i n e r i e T i r l e m o n t o i s e , B r u s s e l s , B e l g i u m ;

- S o c k e r b o l a g e t , Malmφ, Sweden;

- S p r e c k e l s Suga r D i v i s i o n , Amstar C o r p o r a t i o n , San F r a n c i s c o , USA;

- S t o r d B a r t z , B e r g e n , Norway.

A l a r g e p a r t o f the p r e s e n t book was w r i t t e n d u r i n g my s t a y a t the U n i v e r s i t y

o f Lund , Sweden. I am g r a t e f u l to my f r i e n d P r o f e s s o r Gunnar T y l l e r e d , whase

i n v i t a t i o n made my v i s i t to Sweden p o s s i b l e . Gunnar F e l t b o r g and D r . Jan T jebbes

deserve s p e c i a l t h a n k s f o r t h e i r a s s i s t a n c e i n r e a c h i n g the r e l e v a n t i n f o r m a t i o n

on the Swed ish s u g a r i n d u s t r y . Thanks a re a l s o e x p r e s s e d to the pe rsonne l o f the

l i b r a r y o f S o c k e r b o l a g e t i n A r l φ v where I found a p e r f e c t env i ronment f o r my

V I

l i t e r a t u r e s t u d i e s .

F i n a l l y , I would l i k e to thank t hose A m e r i c a n , B e l g i a n , B r i t i s h , B u l g a r i a n ,

C a n a d i a n , C z e c h , D a n i s h , F r e n c h , German, Greek , H u n g a r i a n , I t a l i a n , P o l i s h ,

S o v i e t , Swed ish and Y u g o s l a v c o l l e a g u e s who he lped me - somet imes unknow ing ly -

th rough i n f o r m a t i o n , a d v i c e , and f r i e n d l y d i s c u s s i o n s .

V I I

PREFACE

The 1970s and 1980s w i l l c e r t a i n l y be remembered as a p e r i o d o f impor tan t

developments i n the energy market and changes i n i n d u s t r y ' s a t t i t u d e towards

energy economy. As e n t i r e n a t i o n s were a f f e c t e d by the economic consequences o f

v i o l e n t f l u c t u a t i o n s i n fue l p r i c e s , much i n t e l l e c t u a l e f f o r t and numerous

p r a c t i c a l a c t i o n s were under taken w i t h the aim o f imp rov ing energy u t i l i z a t i o n .

For example , the ene rgy consumpt ion i n Swed ish i n d u s t r y dec reased by 20%

between 1973 and 1 9 8 3 , w h i l e the t o t a l p r o d u c t i o n volume - e x p r e s s e d i n f i x e d

p r i c e s - remained n e a r l y c o n s t a n t . I n F r a n c e , where the i n d u s t r i a l ou tpu t was

a l s o n e a r l y c o n s t a n t i n the same p e r i o d , the ene rgy consumpt ion i n i n d u s t r y was

reduced by 12%. I n the Federa l R e p u b l i c o f Germany, i n d u s t r i a l ou tpu t r o s e 8%

between 1973 and 1 9 8 4 , but the energy consumpt ion dec reased by 18%. A l t h o u g h

few n a t i o n s have been as s u c c e s s f u l as t h o s e named a b o v e , the t r end towards

s a v i n g energy i s now common. I t has a l s o become v i s i b l e i n s u g a r i n d u s t r i e s

t h roughou t the w o r l d .

The book i s devoted to the problems o f i d e n t i f y i n g the p o t e n t i a l f o r , and

then d e s i g n i n g and imp lemen t i ng , e n e r g y - s a v i n g measures i n beet s u g a r f a c t o r i e s .

As the s u g a r i n d u s t r i e s i n v a r i o u s c o u n t r i e s d i f f e r c o n s i d e r a b l y w i t h r e s p e c t

to the economic c o n d i t i o n s f o r f a c t o r y o p e r a t i o n and the l e v e l o f t e c h n o l o g i c a l

deve lopment , the problem range i s ve ry b r o a d . I t may i n c l u d e the e l i m i n a t i o n o f

f a u l t y o r u n r e l i a b l e a u x i l i a r y equ ipment , o r the i n t r o d u c t i o n o f s i m p l e

improvements i n vapour d i s t r i b u t i o n s c hemes , i n f a c t o r i e s ope ra ted i n c o u n t r i e s

where the need f o r e f f i c i e n t energy u t i l i z a t i o n has not r e a l l y been ve ry u r g e n t

up to now. On the o t h e r h a n d , t he re a re some o t h e r c o u n t r i e s where s u g a r

f a c t o r i e s have l ong s i n c e been ve ry hard p r e s s e d to save ene rgy and where

c o n s i d e r a b l e ach ievements have been no ted i n t h i s f i e l d . F u r t h e r p r o g r e s s may

s t i l l be p o s s i b l e t h e r e , but o n l y i f more advanced e n g i n e e r i n g prob lems a re

s o l v e d .

T a k i n g the D a n i s h s u g a r i n d u s t r y as an examp le , i t can be seen f rom F i g . 1

t h a t the f ue l consumpt ion was d e c r e a s i n g s t e a d i l y d u r i n g the 1950s and 1 9 6 0 s .

A t the b e g i n n i n g o f the 1970s the energy economy reached a c e r t a i n s t a t e o f

m a t u r i t y ; s a v i n g s which c o u l d be ach ieved i n a s i m p l e manner - r o u g h l y s p e a k i n g ,

by a v o i d i n g waste o f energy - had a l r e a d y become a r e a l i t y . F u r t h e r p r o g r e s s

became g r a d u a l l y a q u e s t i o n o f how to mod i fy the f a c t o r i e s i n a l l the a s p e c t s

t ha t cou ld be r e l a t e d to r e d u c t i o n s o f the ene rgy demand, i n c l u d i n g the s u g a r

manu fac tu r i ng p r o c e s s , b y - p r o c e s s e s and a u x i l i a r y p r o c e s s e s , equ ipment , c o n t r o l

V I I I

2^ 5h

cn

•Ľ Ö Ĺ ů

§ 3h CP

^0. ^ 0 ,

X · X X

1955 1960 1965 1970 1975 1980 1985

F i g . 1 . Average consumpt ion o f normal fue l ( h e a t i n g v a l u e 29300 k J / k g ) i n beet s u g a r manufactu'^e i n s e l e c t e d European c o u n t r i e s , w i t h t h e i r 1984 s h a r e i n wor ld p r o d u c t i o n o f beet s u g a r g i v e n i n b r a c k e t s .

s y s t e m s , m o n i t o r i n g methods and p r o c e d u r e s , and even o t h e r a r e a s .

In F i g . 1 , the s t a t i s t i c a l da ta on fue l consumpt ion i n Swed ish and French

s u g a r i n d u s t r i e s a re a l s o shown. I t s h o u l d be po in ted ou t t ha t the da ta a re no t

meant to be compared d i r e c t l y , as the d i f f e r e n c e s must be seen as e x p r e s s i o n s

o f numerous d i s s i m i l a r i t i e s between the i n d u s t r i e s c o n c e r n e d . I n d e e d , t he re a re

even some d i f f e r e n c e s i n the methods used to c a l c u l a t e the ave rage ene rgy

consumpt ion : Swed ish data a re based on p r o d u c t i o n s t a t i s t i c s from 6 wh i te s u g a r

f a c t o r i e s (one raw s u g a r f a c t o r y has been e x c l u d e d ) ; D a n i s h data r e p r e s e n t 5

DDS-owned f a c t o r i e s , f o u r o f them p roduc ing wh i te s u g a r o n l y and one wh i te

s u g a r and r a f f i nade ; F rench da ta have been ave raged from 56 f a c t o r i e s , t h e i r

1984 ou tpu t c o n s i s t i n g o f 80% wh i te s u g a r , 15% s y r u p s and 5% raw s u g a r . I n o t h e r

w o r d s , the d i f f e r e n c e s stem p a r t l y f rom the f a c t t ha t p r o d u c t s w i t h d i f f e r e n t

s p e c i f i c energy demands a re i n v o l v e d . N e v e r t h e l e s s , i t can be conc luded from

a compar ison o f t r ends t h a t both i n Sweden and F r a n c e , the deve lopments towards

s i t u a t i o n s s i m i l a r to t h a t i n Denmark a re f a r advanced .

Under such c i r c u m s t a n c e s , i s one j u s t i f i e d i n w r i t i n g a book devoted s o l e l y

to the energy economy? T a k i n g the g l o b a l p e r s p e c t i v e , t he re i s no doubt t h a t

much work on energy problems i s needed i n the s u g a r i n d u s t r y because the re a re

s t i l l c o u n t r i e s - major beet s u g a r p roduce rs - where the s p e c i f i c energy

consumpt ion i n s u g a r manufac ture i s much h i g h e r than i n the c o u n t r i e s named

above . About 60% o f the w o r l d ' s beet s u g a r comes from f a c t o r i e s c o n s u m i n g , on

a v e r a g e , 2 - 3 t imes more ene rgy per u n i t mass o f bee ts than the D a n i s h s u g a r

f a c t o r i e s .

A s e l e c t i o n o f s t a t i s t i c a l da ta on the ene rgy consumpt ion i n beet s u g a r

manufac ture i n s e v e r a l s u g a r - p r o d u c i n g c o u n t r i e s can be seen i n Tab le 1 . A g a i n ,

IX

TABLE 1

Average consumpt ion o f nonnal f ue l ( h e a t i n g v a l u e 29300 k J / k g ) i n beet s u g a r manufac ture - pu lp d r y i n g e x c l u d e d - i n s e l e c t e d c o u n t r i e s .

Sha re i n wo r l d Normal f ue l Count ry p r o d u c t i o n o f beet Year Bee ts worked consumpt ion

s u g a r 1984 (%) (1000 t o n s ) ( k g / 1 0 0 kg b)

USSR 2 3 . 0 3 1980 64300 FRG 8 . 2 4 1984 20189 3 . 1 ^ USA 6 . 8 9 1980 21320 7 . 6 a Po land 4 . 9 2 1984 15500 7 . 4 C z e c h o s l o v a k i a 2 . 2 2 1984 7540 8 . 5 Greece 0 . 6 2 1981 2560 4 . 6

es t ima ted

the reade r s h o u l d be c a u t i o n e d a g a i n s t d i r e c t compar i sons o f the i n d i c e s g i v e n ,

as w i th t hese c o u n t r i e s coming i n t o the p i c t u r e , one has to be aware o f even

more p ro found d i s s i m i l a r i t i e s than t h o s e between F r a n c e , Sweden and Denmark.

Le t us j u s t ment ion c l i m a t i c c o n d i t i o n s , wh ich a f f e c t the ene rgy demand

c o n s i d e r a b l y : i t i s no t unusua l f o r U S S R , P o l i s h and some US f a c t o r i e s to

p r o c e s s f r o z e n bee ts i n the f i n a l s t a g e o f the s e a s o n , w h i l e t h i s i s unheard o f

i n Greece ( i t i s a l s o ex t remely u n l i k e l y i n Sweden, Denmark and F r a n c e ) . On the

o t h e r h a n d , some o f the c o u n t r i e s l i s t e d had never been known f o r a ve r y

e f f e c t i v e energy u s a g e ; n e i t h e r had they r e a l l y been a f f e c t e d by the

d i s t u r b a n c e s i n the i n t e r n a t i o n a l f ue l market because t h e i r n a t i o n a l economic

p o l i c i e s were d e s i g n e d to c o u n t e r a c t the wor ldw ide t r e n d s . I t s e e m s , however ,

t ha t the p r e s e n t economic deve lopments i n t hese c o u n t r i e s f o l l o w the gene ra l

p a t t e r n , thus s t i m u l a t i n g i n t e r e s t i n a r a t i o n a l i z e d ene rgy economy. I n t h a t

r e s p e c t , the answer to the q u e s t i o n f o rmu la ted above seems to be p o s i t i v e .

How c o u l d the book be shaped i n o r d e r to make i t u s e f u l to the peop le

work ing i n v a r i o u s c o u n t r i e s , where the s u g a r i n d u s t r i e s a re c h a r a c t e r i z e d by

d i f f e r e n t l e v e l s o f s o p h i s t i c a t i o n o f the ene rgy economy? I t seems t h a t when

l o o k i n g a t the energy subsys tem and i t s i n t e r a c t i o n s w i th o t h e r s u b s y s t e m s and

w i th the env i ronment o f the f a c t o r y , an at tempt can be made to s y s t e m a t i z e and

e v a l u a t e the most impor tan t e n e r g y - s a v i n g measures t h a t may come i n t o q u e s t i o n .

Such a s y s t e m a t i c rev iew can prove h e l p f u l to the managers and t e c h n o l o g i s t s i n

s u g a r f a c t o r i e s , where a problem may a r i s e o f c h o o s i n g the most a p p r o p r i a t e s e t

o f measures t ha t b e s t f i t the f a c t o r y ' s un ique n e e d s . I t i s a l s o hoped t ha t the

book can be used i n u n i v e r s i t y - l e v e l c o u r s e s on the ene rgy economy o f s u g a r

f a c t o r i e s , and t ha t i t may be o f i n t e r e s t to d e s i g n e n g i n e e r s as we l l as to

s p e c i a l i s t s engaged i n r e s e a r c h i n t h i s a r e a .

As r e g a r d s the scope o f the book , i t must be conc luded t h a t when a t tempt ing

to cove r the e n t i r e problem f i e l d , i t would be i n a p p r o p r i a t e to r e s t r i c t

a t t e n t i o n to the t r a d i t i o n a l l y r e c o g n i z e d energy economy prob lems o n l y . I n s t e a d ,

i t i s n e c e s s a r y to adopt an i n t e r d i s c i p l i n a r y approach aimed a t d e m o n s t r a t i n g

how the energy demand o f a s u g a r f a c t o r y can be a f f e c t e d by the i n t e r a c t i o n s

between a number o f f a c t o r s , namely :

- l a y o u t and parameters o f the energy c o n v e r s i o n and d i s t r i b u t i o n p r o c e s s e s ;

- l a y o u t and parameters o f the s u g a r manu fac tu r i ng p r o c e s s and b y - p r o c e s s e s ;

- c h a r a c t e r i s t i c s o f the equipment and c o n t r o l s y s t e m s ;

- comple teness and accu racy o f the ene rgy m o n i t o r i n g p r o c e d u r e s .

The book c o n s i s t s e s s e n t i a l l y o f t h ree p a r t s . In Chap te rs 1 to 3 , some

t h e o r e t i c a l background i s g i v e n and e n g i n e e r i n g p r i n c i p l e s f o r c r e a t i n g

e f f i c i e n t energy c o n v e r s i o n and u t i l i z a t i o n s u b s y s t e m s i n s u g a r f a c t o r i e s a re

r ev i ewed . More s p e c i f i c a l l y , Chapter 1 p r o v i d e s an i n t r o d u c t o r y s t r u c t u r i n g

o f the problem f i e l d and a rev iew o f the p e r t i n e n t l i t e r a t u r e . Chap te r 2 i s

devoted to mass and energy b a l a n c e s a s t o o l s f o r a n a l y s i n g ene rgy p r o c e s s e s and

s y s t e m s , and to fundamenta ls o f compu te r -a ided a n a l y s i s and d e s i g n o f ene rgy

s u b s y s t e m s . Chapter 3 p r o v i d e s a rev iew o f e s s e n t i a l e n g i n e e r i n g p r o b l e m s , and

methods f o r t h e i r s o l v i n g , r e l a t e d to improv ing heat economy i n e x i s t i n g

f a c t o r i e s o r d e s i g n i n g new, e f f i c i e n t thermal s u b s y s t e m s .

In the i n i t i a l c h a p t e r s , the impor tance i s i n d i c a t e d o f o t h e r a r e a s t ha t do

not s t r i c t l y be long to energy e n g i n e e r i n g but a re c l e a r l y r e l a t e d to an

e f f i c i e n t energy economy. C o n s e q u e n t l y , i n the second p a r t , i . e . , Chap te rs 4

to 7 , recen t developments i n t hese a r e a s and t h e i r impor tance to ene rgy

c o n v e r s i o n and u t i l i z a t i o n i n s u g a r f a c t o r i e s a re d i s c u s s e d . Chapte r 4 i s

devoted to e n e r g y - e f f i c i e n t p r o c e s s e s , and Chapter 5 to equipment d e s i g n .

Con t ro l sys tems and t h e i r r o l e i n s a v i n g ene rgy a re d i s c u s s e d i n Chapte r 6 . I n

Chapte r 7 , methods and p rocedu res f o r m o n i t o r i n g e n e r g y - r e l a t e d a s p e c t s o f

f a c t o r y o p e r a t i o n a re r ev i ewed . In Chap te rs 6 and 7 , computer a p p l i c a t i o n s i n

the r e s p e c t i v e a r e a s a re a l s o d i s c u s s e d .

I t has been the a u t h o r ' s amb i t i on to i l l u s t r a t e the p r e s e n t a t i o n w i th

s u i t a b l e p r a c t i c a l l y - o r i e n t e d examp les . These a re based m o s t l y on the a u t h o r ' s

expe r i ence f rom 9 y e a r s wo rk ing w i t h an e n g i n e e r i n g company s p e c i a l i z i n g i n the

d e s i g n , e r e c t i o n and m o d e r n i z a t i o n o f s u g a r f a c t o r i e s , as wel l f rom an

a d d i t i o n a l 5 y e a r s o f c o n s u l t i n g and r e s e a r c h f o r the s u g a r i n d u s t r y . S h o r t

examples a re p resen ted i n Chap te rs 1 , 2 , 3 and 7 . I n the t h i r d p a r t o f the b o o k ,

i . e . . Chap te rs 8 and 9 , summaries a re g i v e n o f r e a l - l i f e d e s i g n a n a l y s e s o f

ene rgy subsys tems o f s u g a r f a c t o r i e s , c h a r a c t e r i z e d by d i f f e r e n t l e v e l s o f

s o p h i s t i c a t i o n o f the energy economy.

I t s h o u l d be emphas ized t ha t the p r e s e n t book i s not i n tended to g i v e any

p r e s c r i p t i o n s but r a t h e r to s t i m u l a t e t h i n k i n g and i d e a - g e n e r a t i n g . As i t

X I

d e f i n i t e l y cannot r e p l a c e e n g i n e e r i n g handbooks o r d e s i g n a i d s , an at tempt has

been made to a v o i d the r e p e t i t i o n o f b a s i c t h e o r y and fundamental d a t a , such as

thermodynamic d e f i n i t i o n s and t a b l e s o f p r o p e r t i e s o f water and steam o r

p r o p e r t i e s o f s u g a r s o l u t i o n s ; t h e s e can be found e l s e w h e r e . Fo r r e a d e r s who

may need an i n t r o d u c t i o n to the f u n d a m e n t a l s , ample r e f e r e n c e s a re g i v e n to the

n o w - c l a s s i c a l l i t e r a t u r e , and m o s t l y to A m e r i c a n , E n g l i s h and German s o u r c e s .

Then , i n Append ices 1 and 2 , numer ica l a p p r o x i m a t i o n s o f thermodynamic

p r o p e r t i e s o f wa te r , steam and s u g a r s o l u t i o n s a re d i s c u s s e d . I n Append ix 3 ,

a s h o r t rev iew o f u s e f u l r e l a t i o n s h i p s and da ta r e l a t i n g to c e r t a i n heat

t r a n s f e r phenomena i s g i v e n .

When p r e s e n t i n g p h y s i c o - c h e m i c a l and t e c h n i c a l d a t a , e x a m p l e s , c a l c u l a t i o n s ,

e t c . , S I u n i t s o f measure have been used t h r o u g h o u t the book . Fo r r e a d e r s

accustomed to o t h e r u n i t s . Append ix 4 p r o v i d e s a s e l e c t i o n o f c o n v e r s i o n

f a c t o r s .

X I I

L I S T OF SYMBOLS

a j u i c e d r a f t

b c o n c e n t r a t i o n

Β fue l consump t i on , fue l demand

C s p e c i f i c heat

D steam f low

F s u r f a c e a rea

G mass f l ow

h en tha lpy

k o v e r a l l heat t r a n s f e r c o e f f i c i e n t

Μ mass

Ν e l e c t r i c a l e f f e c t , power

ρ p r e s s u r e

Ρ power demand, power ou tpu t

q , Q heat

heat o f combust ion

h e a t i n g v a l u e

S steam ra te

t temperature

Ô a b s o l u t e temperature

Ä Ô , At temperature d i f f e r e n c e

V s p e c i f i c volume

ô t ime , d u r a t i o n

Mos t f r e q u e n t l y used s u b s c r i p t s or s u p e r s c r i p t s :

b b e e t s , c o s s e t t e s

c condensate

j j u i c e

Ρ pu lp

s steam

V vapour

w water

C h a p t e r 1

THE ENERGY SYSTEM AND I T S ROLE IN A SUGAR FACTORY

1.1 SUBSYSTEMS OF A SUGAR FACTORY

The s u g a r m a n u f a c t u r i n g p r o c e s s , b y - p r o c e s s e s and a u x i l i a r y p r o c e s s e s must

be c o n t i n u o u s l y and r e l i a b l y s u p p l i e d w i t h t h e e n e r g y needed t o p e r f o r m a l l t h e

u n i t o p e r a t i o n s i n v o l v e d . By t h e i r v e r y n a t u r e , t h e p r o c e s s e s a r e c h a r a c t e r i z e d

by p r e d e t e r m i n e d v a l u e s o f c e r t a i n i n p u t , o u t p u t and i n t e r m e d i a t e p a r a m e t e r s ;

o t h e r pa ramete rs may be l i m i t e d by c o n s t r a i n t s r e l a t e d t o p r o c e s s r e q u i r e m e n t s ,

f a c t o r y economy, e n v i r o n m e n t p r o t e c t i o n and o t h e r f a c t o r s . C o n s e q u e n t l y , t h e

f reedom o f c h o i c e o f t h e v a l u e s o f pa rame te rs o f t h e e n e r g y c o n v e r s i o n ,

d i s t r i b u t i o n and u t i l i z a t i o n p r o c e s s e s i s s u b s t a n t i a l l y r e s t r i c t e d . To make

t h i n g s even more c o m p l i c a t e d , u n d e r l o c a l c o n d i t i o n s i n a s p e c i f i c f a c t o r y ,

some c o n s t r a i n t s may be g i v e n o n l y i m p l i c i t l y a n d , q u i t e o f t e n , a r e d i f f i c u l t

t o i d e n t i f y .

I n such c i r c u m s t a n c e s , an i n v e s t i g a t i o n o f t h e e n e r g y economy o f a s p e c i f i c

s u g a r f a c t o r y r e q u i r e s s t u d y i n g v a r i o u s a s p e c t s o f i n t e r a c t i o n s between s u g a r

m a n u f a c t u r i n g , b y - p r o c e s s e s and a u x i l i a r y p r o c e s s e s on t h e one s i d e , and e n e r g y

c o n v e r s i o n , d i s t r i b u t i o n and u t i l i z a t i o n p r o c e s s e s on t h e o t h e r . As t h e

i n t e r a c t i o n s a r e t a k i n g p l a c e i n v a r i o u s s e c t i o n s and components o f t h e f a c t o r y ,

i t c o u l d be c o n c l u d e d t h a t i t i s n e c e s s a r y t o s t u d y t h e e n t i r e f a c t o r y i n g r e a t

d e t a i l . H o w e v e r , i t se ldom makes sense t o i n v e s t i g a t e a l l t h e d e t a i l s a t o n c e ,

as a s t r a i g h t f o r w a r d app roach w o u l d r e s u l t i n a l a r g e vo lume o f i n f o r m a t i o n

w h i c h may be d i f f i c u l t t o h a n d l e and i n t e r p r e t . What i s r e a l l y needed f o r

g r a s p i n g t h e s i t u a t i o n and i d e n t i f y i n g p o t e n t i a l e n e r g y s a v i n g s i s s t r u c t u r e d

i n f o r m a t i o n f rom w h i c h , depend ing on s p e c i f i c n e e d s , r e l e v a n t d a t a on t h e

f a c t o r y r e g i o n s o f i n t e r e s t can be e x t r a c t e d .

The c o n c e p t o f s t r u c t u r e d i n f o r m a t i o n on e n e r g y p r o c e s s e s must be seen i n

c o n n e c t i o n w i t h t h e s t r u c t u r e o f t h e s u g a r f a c t o r y i n q u e s t i o n . T h i s does n o t

n e c e s s a r i l y mean i n v e s t i g a t i n g a l l t h e p h y s i c a l componen ts , l i k e b u i l d i n g s ,

p r o c e s s s t a t i o n s o r equ ipment u n i t s ; t h e s t r u c t u r e may be d e f i n e d i n a manner

s u i t e d t o s p e c i f i c n e e d s . I n t h e l i t e r a t u r e , a v a r i e t y o f app roaches a t

d i f f e r e n t l e v e l s o f c o m p l e x i t y can be f o u n d . A t one end o f t h e c o m p l e x i t y s c a l e ,

t h e app roach employed b y , among o t h e r s , S c h i e b l ( r e f . 1) a n d , more r e c e n t l y ,

K a r r e n ( r e f . 2) can be p l a c e d . H e r e , t h e e n e r g y usage i s i d e n t i f i e d by t h e i n p u t

e n e r g y s t r eam w h i c h can a l s o be c o n s i d e r e d as t h e sum o f power house l o s s e s and

p r o c e s s e n e r g y c o n s u m p t i o n ; t h i s means d i v i d i n g t h e f a c t o r y i n t o power house

and p r o c e s s a r e a s , as i n d i c a t e d i n F i g . 1.1. A b i t c l o s e r t o t h e m i d d l e o f t h e

c o m p l e x i t y s c a l e comes t he s t r u c t u r i n g p r i n c i p l e used by Bal oh ( r e f . 3 ) , t h a t

Γ l o s s e s I

I

1

1 1 s t e a m . ^

2 1 1 e lect r ic 2 1 power 1

1 power 1

fue l

F i g , Ί . Ι . S u g a r f a c t o r y d i v i d e d i n t o power house 1 and p r o c e s s a r e a 2 . Mass and e n e r g y s t reams c r o s s i n g t h e bounda ry shown by t h e dashed l i n e a r e d e t e r m i n e d .

i s , d i v i d i n g t h e f a c t o r y i n t o e i g h t main s e c t i o n s and i d e n t i f y i n g t h r e e

d i f f e r e n t a reas o f e n e r g y p r o c e s s e s . T h i s p r i n c i p l e i s shown s c h e m a t i c a l l y i n

F i g . 1 .2 ; n o t e t h a t t h e s t reams o f e l e c t r i c a l power s u p p l i e d t o t h e f a c t o r y

s e c t i o n s a r e n o t c o n s i d e r e d . Mov ing t o t h e o t h e r end o f t h e c o m p l e x i t y s c a l e ,

t h e approach used by Z a g r o d z k i ( r e f . 4 ) can be p r e s e n t e d . As can be seen i n

F i g . 1 .3 , i t r e l i e s on t h e i d e n t i f i c a t i o n o f e n e r g y s t reams t o and f rom

i n d i v i d u a l equ ipment u n i t s , o r g r o u p s o f u n i t s . Note t h a t o n l y a p a r t o f t h e

f a c t o r y , and o n l y the rma l e n e r g y , a r e c o n s i d e r e d .

Examples o f even more d e t a i l e d app roaches can be f ound i n t h e l i t e r a t u r e , as

i i u e _ g a s _ _ : \ s l u d g e .

L ±i -T^ -{-- -i-T

F i g . 1.2. S u g a r f a c t o r y d i v i d e d i n t o : 1 - l ime k i l n w i t h m i l k - o f - l i m e s t a t i o n , 2 - e x t r a c t i o n s t a t i o n , 3 - j u i c e p u r i f i c a t i o n s t a t i o n , 4 - e v a p o r a t o r , 5 -s u g a r h o u s e , 6 - c o n d e n s a t e t a n k s , 7 - p u l p d r y i n g s t a t i o n , 8 - power h o u s e . Mass and e n e r g y s t reams can be i d e n t i f i e d a t t h r e e d i f f e r e n t b o u n d a r i e s : I -e n t i r e f a c t o r y , I I - h e a t economy a r e a . I I I - p r o c e s s h e a t i n g a r e a .

JP

Ε· exhous i j s team,

η

7 • 5H

EV

η η η

F i g . 1.3. Example o f s t r u c t u r i n g o f s u g a r f a c t o r y equ ipment i n v o l v e d i n t h e e n e r g y p r o c e s s e s . Ε - e x t r a c t i o n s t a t i o n , J P - j u i c e p u r i f i c a t i o n s t a t i o n , SH s u g a r h o u s e , EV - e v a p o r a t o r , V - vacuum p a n s , " - Z - e v a p o r a t o r e f f e c t s , 6 -h e a t e r s , 7 - c o n d e n s e r . V a p o u r s t reams c r o s s i r r t h e bounda ry shown by t h e dashed l i n e and t h e b o u n d a r i e s o f J P , EV and SH can be i d e n t i f i e d .

i l l u s t r a t e d i n F i g . 1.4 ( r e f . 5 ) . T a b l e 1.1 d e m o n s t r a t e s how t h i s s t r u c t u r i n g

p r i n c i p l e makes i t p o s s i b l e t o i d e n t i f y t h e consumpt i on o f h e a t i n g media i n

equ ipment u n i t s o r g roups o f u n i t s i n a model f a c t o r y , t hus p r o v i d i n g a

c o n v e n i e n t b a s i s f o r c a l c u l a t i o n s o f t h e o v e r a l l e n e r g y consumpt i on i n t h e

s u g a r m a n u f a c t u r i n g p r o c e s s o r i n i t s p a r t s .

G e n e r a l l y , a s u g a r f a c t o r y may be a n a l y s e d as a who le o r as a sum o f s m a l l e r

p a r t s ( w h i c h , i n t u r n , may be s u b d i v i d e d t o o ) by emp loy ing t h e n o t i o n o f an

open thermodynamic sys tem ( r e f s . 6 , 7 ) . I t can be d e f i n e d as t h e p a r t o f t h e

p h y s i c a l space w h i c h i s c o n t a i n e d w i t h i n p r e s c r i b e d and i d e n t i f i a b l e b o u n d a r i e s .

The s t a t e o f an open thermodynamic sys tem can be d e f i n e d i n te rms o f q u a n t i t i e s

c h a r a c t e r i z i n g mass and e n e r g y s t reams c r o s s i n g i t s bounda ry ( s e e C h a p t e r 2 ) .

By s p l i t t i n g t h e s y s t e m i n t o s u b s y s t e m s , t h e p o s s i b i l i t y i s c r e a t e d o f

d e t e r m i n i n g mass and e n e r g y s t reams and t h e i r pa rame te rs needed t o d e f i n e t h e

s t a t e s o f t h e s u b s y s t e m s . I t s h o u l d be emphas ized t h a t an i n v e s t i g a t o r i s f r e e

t o d e f i n e t h e b o u n d a r i e s o f t h e s y s t e m and i t s subsys tems i n t h e manner b e s t

s u i t e d t o h i s s p e c i f i c n e e d s . T h i s makes i t p o s s i b l e t o decompose c o m p l i c a t e d

m u l t i v a r i a b l e e n g i n e e r i n g prob lems i n t o a number o f s i m p l e r p rob lems t h a t a r e

e a s i e r t o u n d e r s t a n d and s o l v e . A w e l l p l a n n e d sys tem d e c o m p o s i t i o n a l s o

c r e a t e s p o s s i b i l i t i e s o f i n f o r m a t i o n s t r u c t u r i n g , t h i s i n t u r n g i v i n g a b e t t e r

condenir sa te

J P

6 n: 6

T C ,

e x h p u s t steam

5 H(

π

EV

π

7 :

SH

Β

•Θ D 4 X I ®

F i g . 1.4. A n o t h e r example o f s t r u c t u r i n g o f s u g a r f a c t o r y equ ipment i n v o l v e d i n t h e e n e r g y p r o c e s s e s . JP - j u i c e p u r i f i c a t i o n s t a t i o n , SH - s u g a r h o u s e , A , B, C - vacuum pans A , Β and C , TC - t h e r m o c o m p r e s s o r s , EV - e v a p o r a t o r , 1-4 - e v a p o r a t o r e f f e c t s , 5 - e x t r a c t o r , 6 - h e a t e r s , 7 - c o n d e n s e r , 8 - p u l p p r e s s e s , 9 - t h i c k j u i c e t a n k , 10 m e l t e r , 11 - s y r u p t a n k s , 12 - s u g a r d r y e r . Not shown: condensa te c o n n e c t i o n s . V a p o u r and c o n d e n s a t e s t reams c r o s s i n g t h e bounda ry shown by t h e dashed l i n e and t h e b o u n d a r i e s o f J P , EV and SH a r e i d e n t i f i e d .

i n s i g h t i n t o e n e r g y p r o c e s s e s and p o t e n t i a l e n e r g y s a v i n g s .

The i d e a o f t r e a t i n g a s u g a r f a c t o r y as a thermodynamic sys tem i n w h i c h

subsys tems can be d e f i n e d seems t o be so s i m p l e and o b v i o u s t h a t p e o p l e t e n d t o

s h r u g i t o f f . A c t u a l l y , i t may be t r u e t h a t a common-sense a p p r o a c h t o e n e r g y

a n a l y s e s i s e f f e c t i v e enough i n s o l v i n g s i m p l e p r o b l e m s . When c o n s i d e r i n g

c o m p l i c a t e d and s u b t l e q u e s t i o n s , h o w e v e r , one s h o u l d r e c o g n i z e t h e a d v a n t a g e s

o f a d i s c i p l i n e d , t h e o r e t i c a l l y w e l l f ounded thermodynamic a p p r o a c h . As a m a t t e r

o f f a c t , when t h e e n e r g y economy i n c o n t e m p o r a r y s u g a r f a c t o r i e s becomes so

advanced t h a t t h e was te o f e n e r g y i s l a r g e l y e l i m i n a t e d , v i r t u a l l y no e n e r g y -

s a v i n g measure can be r e g a r d e d as s i m p l e . F o l l o w i n g t h e law o f d i m i n i s h i n g

r e t u r n s , i n v e s t m e n t s i n an advanced e n e r g y economy can b r i n g l i m i t e d g a i n s o n l y ,

so t h e economic a n a l y s e s on w h i c h management d e c i s i o n s a r e based must be

TABLE 1.1

Steam and v a p o u r s t reams (kg /100 kg b ) between equ ipment u n i t s o r g r o u p s i n t h e model f a c t o r y shown s c h e m a t i c a l l y i n F i g . 1.4. Dashed l i n e s s e p a r a t e d a t a r e l a t i n g t o J P , SH and E V .

S o u r c e s

R e c e i v e r s E v a p o r a t o r

1 2

e f f e c t s

3 4

O t h e r s o u r c e s

P r e s s w a t e r h e a t e r E x t r a c t o r Raw j u i c e h e a t e r

0.90 1.97

0.92 Vacuum pan v a p o u r

P r e - l i m e d j u i c e h e a t e r s H e a t e r a f t e r 1 s t c a r b o n a t a t i o n H e a t e r b e f o r e 2nd c a r b o n a t a t i o n 2.50

2.85 3.36 2.62

Condensa te

T h i n j u i c e h e a t e r s 2.24 2.43 1.65

T h i c k j u i c e h e a t e r M e l t e r I n d i r e c t l y h e a t e d tanks D i r e c t l y hea ted tanks Remelt h e a t e r Vacuum pans A

Β C

Sugar d r y e r

0.20 0.45 0.58

12.80 3.40 1.31

0.15

0.18

E x h a u s t steam 0.50

Thermocompressors Condense r

10.00 0.90

L i v e steam 4.00

E v a p o r a t o r t o t a l 12.24 24.57 8.37 6.23

r e l i a b l e i n d e e d . T h i s makes i t n e c e s s a r y t o e n s u r e a h i g h a c c u r a c y o f

e n g i n e e r i n g c a l c u l a t i o n s so as t o p r o v i d e r e l i a b l e i n p u t d a t a f o r economic

a n a l y s e s . F o r examples o f t h e a p p l i c a t i o n o f a d i s c i p l i n e d thermodynamic

app roach i n t h e c a l c u l a t i o n s o f e n e r g y b a l a n c e s , see C h a p t e r s 2 , 3 , 8 and 9.

I t s h o u l d be n o t e d t h a t t h e need f o r d e f i n i n g and s u b s e q u e n t l y decompos ing

a thermodynamic sys tem may a r i s e i n p r a c t i c a l s i t u a t i o n s as d i f f e r e n t a s :

( i ) R a t i o n a l i z i n g , o r p l a n n i n g m o d e r n i z a t i o n o f , t h e e n e r g y economy o f an

e x i s t i n g f a c t o r y , when a t h o r o u g h u n d e r s t a n d i n g o f t h e d e f i c i e n c i e s and

l i m i t a t i o n s o f an e x i s t i n g sys tem i s needed as a f o u n d a t i o n f o r p r o p o s e d

improvements . C h a r a c t e r i s t i c p rob lems a r e d i s c u s s e d i n C h a p t e r s 8 and 9.

( i i ) D e s i g n i n g a new f a c t o r y , i n c l u d i n g i t s e n e r g y s u b s y s t e m . T h i s r e q u i r e s

a n a l y s i n g t h e p r o p e r t i e s o f a sys tem w h i c h does n o t y e t e x i s t w i t h t h e aim o f

s h a p i n g i t o p t i m a l l y , as d i s c u s s e d i n C h a p t e r 9.

( i i i ) M o n i t o r i n g t h e e n e r g y p r o c e s s e s d u r i n g f a c t o r y o p e r a t i o n , when p e r i o d i c

e n e r g y consumpt ion checks o r a t t e m p t e d improvements i n r o u t i n e m o n i t o r i n g

p r o c e d u r e s may r e q u i r e l o c a t i n g new measur ing i n s t r u m e n t s f o r more d e t a i l e d

i n f o r m a t i o n on e n e r g y b a l a n c e s , o r p l a c i n g some s e e m i n g l y r e d u n d a n t

measurements i n o r d e r t o o b t a i n i n f o r m a t i o n needed f o r s y s t e m a t i c c r o s s - c h e c k s

on i m p o r t a n t b a l a n c e d a t a . Problems o f t h i s k i n d a r e p r e s e n t e d i n C h a p t e r 7.

1.2 THE THERMAL SYSTEM AND I T S COMPONENTS

1.2.1 D e f i n i t i o n

T a k i n g advan tage o f t h e f l e x i b i l i t y i n h e r e n t i n t h e n o t i o n o f a s y s t e m

b o u n d a r y , a thermodynamic sys tem can be d e f i n e d w i t h i n a s u g a r f a c t o r y so as t o

i n c o r p o r a t e a l l t h e equ ipment u n i t s i n w h i c h the rma l e n e r g y ( h e a t ) c o n v e r s i o n

and u t i l i z a t i o n p r o c e s s e s a r e c a r r i e d o u t . I n t e r c o n n e c t i n g p i p e s a n d , p o s s i b l y ,

r e l e v a n t a u t o m a t i c c o n t r o l c i r c u i t s can a l s o be taken i n t o c o n s i d e r a t i o n . I t

can f u r t h e r be assumed t h a t t h e e l e c t r i c a l e n e r g y i s d i s r e g a r d e d e x c e p t as

a mean ing fu l o u t p u t i n t h e e n e r g y b a l a n c e o f t h e power h o u s e , o r t h a t i t i s

a l s o a c c o u n t e d f o r as a b a l a n c e i n p u t when equ ipment u n i t s o r g r o u p s a r e

a n a l y s e d . I n t he f o l l o w i n g , such a sys tem w i l l be c a l l e d t h e the rma l sys tem o f

a s u g a r f a c t o r y (an example i s shown s c h e m a t i c a l l y i n F i g . 1 . 5 ) . I t s h o u l d be

no ted t h a t i f a l l t h e e n e r g y p r o c e s s e s , i n c l u d i n g power g e n e r a t i o n and

u t i l i z a t i o n , a r e c o n s i d e r e d , t h e n t h e te rm " e n e r g y s y s t e m " w i l l be u s e d .

O d d l y e n o u g h , t h e n o t i o n o f a t he rma l sys tem i s n o t used i n t h e s u g a r

i n d u s t r y . I n t h e l i t e r a t u r e d e v o t e d t o e n e r g y p r o b l e m s , such terms as " t h e r m a l

scheme o f a s u g a r f a c t o r y " ( r e f . 8 ) , " h e a t c i r c u i t r y " ( r e f . 9 ) , o r " h e a t

economy c i r c u i t " ( r e f . 4 ) a r e u s e d , and most o f t e n , t h e o b j e c t unde r

c o n s i d e r a t i o n i s n o t r i g o r o u s l y d e f i n e d . Some a u t h o r s do n o t use any u n i f y i n g

n o t i o n s f o r t h e o b j e c t o f e n e r g y a n a l y s e s a t a l l . F o r e x a m p l e , Hugo t ( r e f . 10)

t r e a t s e v a p o r a t i o n and h e a t i n g i n d e p e n d e n t l y o f what he c a l l s t h e "s team c y c l e " .

F a i l u r e t o r e c o g n i z e t h e i m p o r t a n c e o f t h e e n t i r e t he rma l s y s t e m t o t h e

e n e r g y econoniy may l e a d t o e n e r g y w a s t a g e . To i l l u s t r a t e t h i s p o i n t , t h e a u t h o r

r e c a l l s a s u g a r f a c t o r y he v i s i t e d f o r a s t u d y on p o s s i b l e e n e r g y s a v i n g s ( t h e

f a c t o r y i s s i t u a t e d i n a c o u n t r y known f o r i t s l o n g - s t a n d i n g i n d u s t r i a l

t r a d i t i o n , and f o r q u i t e e f f i c i e n t e n e r g y u t i l i z a t i o n s t i m u l a t e d by i n a d e q u a t e

domes t i c f u e l r e s o u r c e s ) . I n t he power h o u s e , t h e r e were c l e a n and s h i n i n g

i n s u l a t i o n c o v e r s and f r o n t p a n e l s o f somewhat o u t d a t e d , b u t w e l l m a i n t a i n e d ,

b o i l e r s , c a r e f u l l y m a i n t a i n e d t u r b o - g e n e r a t o r s and p e r f e c t l y f u n c t i o n i n g

a u x i l i a r y equ ipment and measur ing i n s t r u m e n t s . Adequate d a t a r e c o r d s were

a v a i l a b l e f o r t h e e v a l u a t i o n o f e n e r g y b a l a n c e s o f t h e power h o u s e . I n t h e

n e i g h b o u r i n g b u i l d i n g c o n t a i n i n g t h e p r o c e s s e q u i p m e n t , h o w e v e r , e v a p o r a t o r

b o d i e s and vacuum pans were s h i n i n g t o o , b u t numerous p r e s s u r e and t e m p e r a t u r e

i n d i c a t o r s were e i t h e r m i s s i n g o r o u t o f o r d e r . On t h e g round f l o o r ,

m a l f u n c t i o n i n g steam t r a p s and l e a k i n g h o t - c o n d e n s a t e l i n e s were f o u n d . Data

r e c o r d s on e n e r g y u t i l i z a t i o n were i n c o m p l e t e and no h e a t b a l a n c e s o f p r o c e s s

equ ipment c o u l d be e s t i m a t e d . I n s h o r t , t h e r e were s i g n s t h a t t h e f a c t o r y

to ammonia ι water tank |

F i g . 1.5. Example o f a scheme o f a the rma l s y s t e m . I - power h o u s e , I I -e v a p o r a t o r . I I I - b e e t h o u s e , I V - s u g a r h o u s e , V - c o n d e n s i n g and c o o l i n g e q u i p m e n t , 1-4 - e v a p o r a t o r e f f e c t s , 5 - e x t r a c t o r , 6 - h e a t e r s , 7 - c o n d e n s e r , 8 - p u l p p r e s s e s , 9 - t h i c k j u i c e t a n k , 10 - m e l t e r , 11 - s y r u p t a n k s , 12 -s u g a r d r y e r , 13 - c o n d e n s a t e t a n k s , 14 - i n t e r m e d i a t e c o n d e n s a t e t a n k , 15 -main f e e d - w a t e r t a n k , 16 - b o i l e r , 17 - t u r b i n e .

managers t r e a t t he power house and t h e p r o c e s s - h e a t i n g a r e a as i f t h e y were n o t

p a r t s o f t he same sys tem i n w h i c h i n t e r a c t i o n s between e n e r g y p r o c e s s e s

d e t e r m i n e t h e o v e r a l l e n e r g y c o n s u m p t i o n . N e e d l e s s t o s a y , t h e f u e l consump t i on

p e r u n i t mass o f b e e t s p r o c e s s e d was v e r y h i g h i n t h i s f a c t o r y .

Now, even i f l a c k o f awareness o f t h e i m p o r t a n c e o f t h e e n t i r e t he rma l

sys tem i s n o t as d r a s t i c , i t may e a s i l y become a cause o f m i s u n d e r s t a n d i n g , o r

f a i l u r e t o r e c o g n i z e , t h e i n t e r d e p e n d e n c e s g o v e r n i n g t h e e n e r g y economy i n

a s u g a r f a c t o r y . On t h e c o n t r a r y , t h e t o t a l app roach o f w h i c h t h e the rma l

s y s t e m c o n c e p t i s o n l y a p a r t makes i t e a s i e r t o g r a s p t h e e s s e n t i a l f e a t u r e s

o f t h e e n e r g y prob lems a n a l y s e d .

1.2.2 Tasks

I t happens a l l t o o o f t e n t h a t t h e s u g a r t e c h n o l o g i s t s a s s i g n a r a t h e r low

p r i o r i t y t o h e a t c o n v e r s i o n and d i s t r i b u t i o n w i t h i n a f a c t o r y , r e q u i r i n g s i m p l y

t h a t t h e h e a t s u p p l y c o v e r t h e p r o c e s s h e a t demand. L e t us o b s e r v e t h a t such

a r e q u i r e m e n t i s ambiguous even i f a l l t h e p r o c e s s pa rame te rs a r e e x a c t l y

s p e c i f i e d , as a d e f i n i t e p r o c e s s h e a t demand can be s a t i s f i e d by v a r i o u s

the rma l sys tems r e q u i r i n g d i f f e r e n t e n e r g y i n p u t s .

A the rma l sys tem can be a n a l y s e d and e v a l u a t e d i n terms o f c e r t a i n n o t i o n s

w h i c h can a l s o be used t o f o r m u l a t e sys tem t a s k s p r e c i s e l y . I n each equ ipment

u n i t i n t h e p r o c e s s h e a t i n g a r e a o f t h e s y s t e m , a c e r t a i n h e a t s t r e a m q^ s h o u l d

be t r a n s f e r r e d t o some p r o c e s s medium. Heat s t reams can be u n i q u e l y d e f i n e d f o r

i n d i v i d u a l equ ipment u n i t s o r g r o u p s o f u n i t s , as a t f i x e d p r o c e s s p a r a m e t e r s ,

hea t b a l a n c e s o f a l l p r o c e s s p a r t s can be s e t up and n e c e s s a r y h e a t i n p u t s can

be i d e n t i f i e d . We a r e t h u s a b l e t o d e f i n e t h e t o t a l h e a t demand as

Ql = q^. ( 1 . 1 )

where η i s t h e t o t a l number o f equ ipment u n i t s o r g r o u p s i n v o l v e d .

I t i s c h a r a c t e r i s t i c o f the rma l sys tems used i n t he s u g a r i n d u s t r y t h a t

m u l t i p l e use o f h e a t t akes p l a c e . Not o n l y i s t h i s t h e u n d e r l y i n g i d e a o f t h e

m u l t i p l e - e f f e c t e v a p o r a t o r , b u t t h e v a p o u r w i t h d r a w n f rom t h e e v a p o r a t o r i s

a l s o used t o h e a t o t h e r equ ipmen t . On t h e o t h e r h a n d , t h e l o s s e s t o t h e

e n v i r o n m e n t a n n i h i l a t e a p a r t o f t h e h e a t c i r c u l a t i n g i n t h e s y s t e m . T h e r e f o r e ,

t h e r e q u i r e d ( n e t ) h e a t i n p u t t o t h e s y s t e m , i s d i f f e r e n t f r om (and u s u a l l y

much s m a l l e r t h a n ) t h e t o t a l h e a t demand.

I t s h o u l d be p o i n t e d o u t t h a t Q2 i s n o t i d e n t i c a l t o t h e p r i m a r y e n e r g y

i n p u t t o t h e f a c t o r y ( s e e S e c t i o n 1 . 2 . 4 ) . I n F i g . 1 .6 , t h e h e a t s t reams and n e t

h e a t demand a r e i l l u s t r a t e d i n a Sankey d iag ram r e p r e s e n t i n g e n e r g y p r o c e s s e s

i n a the rma l s y s t e m . F o r t h e sake o f s i m p l i c i t y , t h e s u g a r m a n u f a c t u r e i s

d i v i d e d i n t o f i v e s u b p r o c e s s e s ( t h a t i s , t h e r e a r e f i v e equ ipment g r o u p s ) .

ki ln gas,water , cossettes

pulp, heat loss

carbonatatii gas.

heat loss

heat loss ^

k evaporator y l osses

vacuum pan vapour

condensate to boiler

F i g . 1.6. Heat s t reams q . and n e t h e a t demand Qp i n a Sankey d iag ram r e p r e s e n t i n g t h e h e a t f l o w s i n a s u g a r f a c t o r y . 1 - e x t r a c t i o n , 2 - j u i c e p u r i f i c a t i o n , 3 - t h i n j u i c e h e a t i n g , 4 - e v a p o r a t i o n , 5 - c r y s t a l l i z a t i o n .

From t h e p o i n t o f v i e w o f e n e r g y economy, i t i s d e s i r a b l e t h a t t h e r a t i o

Κ = Q T / Q 2 ( 1 . 2 )

i s as l a r g e as p o s s i b l e ; i t t e l l s how many t imes on a v e r a g e t h e h e a t i n p u t i s

c i r c u l a t e d i n o r d e r t o s a t i s f y t h e t o t a l h e a t demand. I n t h e f o l l o w i n g , Κ i s

c a l l e d t h e e f f e c t i v e n e s s r a t i o o f t h e the rma l s y s t e m . The v a l u e o f Κ depends on

t h e sys tem l a y o u t , t h a t i s , t h e number and t y p e s o f equ ipment u n i t s , as w e l l as

v a p o u r and c o n d e n s a t e d i s t r i b u t i o n r o u t e s . F o r a sys tem o f known l a y o u t , t h e

e f f e c t i v e n e s s r a t i o may v a r y , depend ing on t h e the rma l p r o p e r t i e s o f equ ipment

u n i t s and pa rame te rs c h a r a c t e r i z i n g t h e p r o c e s s e s o f e n e r g y d i s t r i b u t i o n and

u t i l i z a t i o n . V a l u e s o f Κ as l a r g e as between 4 and 5 have been r e p o r t e d f o r

s u g a r f a c t o r i e s ( r e f s . 1 1 , 1 2 ) . The t e c h n i q u e s used t o i n c r e a s e Κ a r e d i s c u s s e d

i n C h a p t e r s 3 , 8 and 9. H o w e v e r , d i f f e r e n t l a y o u t s , equ ipment and e n e r g y

p r o c e s s e s l e a d t o d i f f e r e n t i a t e d c o s t s o f i n v e s t m e n t , ma in tenance and o p e r a t i o n .

F o r t h i s r e a s o n , t h e r e i s u s u a l l y a p r a c t i c a l r e q u i r e m e n t imposed on Κ t h a t i t s

10

v a l u e s h o u l d be as l a r g e as a l l o w e d by t h e economic c o n s t r a i n t s and c r i t e r i a

a c c o r d i n g t o w h i c h t h e r e s u l t s o f f a c t o r y o p e r a t i o n a r e e v a l u a t e d . I t may be

n o t e d t h a t w i t h t h e e n e r g y c o s t s v a r y i n g between 3% and 17% o f t h e c o s t o f s u g a r

i n v a r i o u s c o u n t r i e s , t h e most economic v a l u e s o f Κ may be w i d e l y d i f f e r e n t i a t e d .

A l s o , unde r t i m e - v a r y i n g economic c o n d i t i o n s r e s u l t i n g f rom e l e c t r i c i t y t a r i f f s

w h i c h v a r y f rom month t o month d u r i n g t h e autumn and w i n t e r p e r i o d , v a r i a t i o n s

o f t h e e f f e c t i v e n e s s r a t i o d u r i n g p r o d u c t i o n may be j u s t i f i e d ( r e f . 1 3 ) .

I t s h o u l d be n o t e d t h a t o t h e r i n d i c e s have a l s o been p r o p o s e d i n t h e

l i t e r a t u r e f o r e v a l u a t i o n o f t h e e n e r g y p r o c e s s e s t a k i n g p l a c e i n t h e the rma l

s y s t e m . German s o u r c e s ( r e f . 12) employ t h e n o t i o n o f e f f i c i e n c y o f t h e the rma l

sys tem d e f i n e d by t h e f o r m u l a

η = 1 - Q2/Q1 = 1 - 1/K ( 1 . 3 )

1.2.3 B u i l d i n g b l o c k s

The sys tem p a r t s shown i n F i g . 1.5 a r e component g roups o r i n d i v i d u a l

components t h a t c u s t o m a r i l y r e c e i v e s e p a r a t e t r e a t m e n t i n e n e r g y a n a l y s e s .

A l t h o u g h t h e s e p a r t s a r e c e r t a i n l y i m p o r t a n t , such a " d e e p " d e c o m p o s i t i o n does

n o t c o n t r i b u t e much t o u n d e r s t a n d i n g o f t h e e s s e n t i a l f e a t u r e s o f e n e r g y

p r o c e s s e s . A b e t t e r i n s i g h t i s g a i n e d by l o o k i n g a t t h e subsys tems d e f i n e d

a c c o r d i n g t o a n o t h e r d e c o m p o s i t i o n p r i n c i p l e , i n d i c a t e d by t h e dashed l i n e s i n

F i g . 1.5 and a d d i t i o n a l l y i l l u s t r a t e d by t h e scheme shown i n F i g . 1 . 7 ( a ) . The

f u n c t i o n o f t h i s v e r s i o n o f t h e the rma l s y s t e m can be summar ized as f o l l o w s :

- u s i n g p r i m a r y e n e r g y s u p p l i e d i n f u e l , combined g e n e r a t i o n o f h e a t ( c a r r i e d

away by steam e x t r a c t e d f rom t h e t u r b i n e e x h a u s t ) and e l e c t r i c a l power t akes

p l a c e i n t he power h o u s e ,

- hea t s u p p l i e d i n h e a t i n g steam t o t h e e v a p o r a t o r s t a t i o n g e n e r a t e s v a p o u r s

( o f w h i c h a sma l l f r a c t i o n may be d i r e c t e d t o t h e c o n d e n s e r ) and h o t c o n d e n s a t e ;

t he t o t a l q u a n t i t y o f w a t e r e v a p o r a t e d i s p r e d e t e r m i n e d by t h e n e c e s s i t y o f

t r a n s f o r m i n g t h i n j u i c e i n t o t h i c k j u i c e ,

- a p a r t o f t he h o t c o n d e n s a t e i s r e t u r n e d t o t h e power h o u s e ; t h e v a p o u r s and

t h e rema in i ng c o n d e n s a t e a r e used t o t r a n s p o r t h e a t t o t h e j u i c e h e a t e r s , vacuum

pans and o t h e r p r o c e s s equ ipment i n t h e b e e t house and s u g a r h o u s e ,

- a f r a c t i o n o f h e a t s u p p l i e d t o t h e p r o c e s s i s d i s s i p a t e d t o t h e e n v i r o n m e n t

o r c a r r i e d away by o u t f l o w i n g p r o d u c t s o r was te m e d i a , and t h e main p a r t i s

c a r r i e d away by v a p o u r s g e n e r a t e d i n vacuum p a n s ,

- t h e vacuum pan v a p o u r s a r e n o t u t i l i z e d b u t d i r e c t e d t o t h e c o n d e n s e r , where

t h e i r e n e r g y i s a b s o r b e d and removed i n b a r o m e t r i c w a t e r ( i . e . , c o o l i n g w a t e r

mixed w i t h condensed v a p o u r s ) ; t h i s e n e r g y must be f i n a l l y d i s s i p a t e d t o t h e

e n v i r o n m e n t i n a s y s t e m component n o t shown i n t h e scheme ( e . g . c o o l i n g t o w e r s ,

c o o l i n g p o n d , r i v e r o r l a k e ) .

11

-Ό C

t í 3 c χ : -Μ <o

4-> υ χ : to o χ : *f- χ •Μ 3 ω •Γ- C 5 «3 + J

Ε (Ο ^ <ϋ S- ^ — <ο I σ> ω

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• ι - ω to ω ι to S ίΟ -Μ UD

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· · ο -σ · (Λ <υ α i-Ε C ο (υ <υ C ο (Λ ο ι -σ to r— LO Q .

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4 J ^ + J Μ- · ι - <0 S-o 5 σ> <υ σ) to Ε <υ -o <υ Q . s ^ ι 4-> >> ι

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Ο <ο S- ^ CL $-Ο U ro <υ 5 — > Q .

0) ε (Λ Ο ^ ο . «^-Γ-ο «S ω ^ ο > ίΛ

ι— 3 I ^ C ο

<0 ^ 00 0> Q . C i - -·.- ε ω J -

^ 5 ο · — 13 ο V ) •Γ- ο Q . V ) J D > I S-= ^ . ^ i •

ο C S-1^ >> ο

. «ο $ ο . ι— Ν -σ <ο

•I- > • r— O l

σ>·Γ- Γ - I •Γ- 4-> Ζ3

12

The f o l l o w i n g b u i l d i n g b l o c k s can t hus be named w i t h i n t h e s y s t e m : power h o u s e ,

m u l t i p l e - e f f e c t ( m u l t i - s t a g e ) e v a p o r a t o r , p r o c e s s equ ipment i n t h e b e e t house

and i n t h e s u g a r h o u s e , and c o n d e n s e r ( a l s o r e p r e s e n t i n g a s s o c i a t e d c o o l i n g

e q u i p m e n t ) . I n F i g . 1 . 7 ( a ) , t h e w i d t h o f t h e s t r i p s l i n k i n g t h e b l o c k s i n d i c a t e

a p p o x i m a t e l y t h e p r o p o r t i o n a t e e n e r g y s t reams c i r c u l a t e d .

The e s s e n t i a l l a y o u t o f e n e r g y c o n v e r s i o n and d i s t r i b u t i o n p r o c e s s e s

e x p l a i n e d above i s by f a r t h e most commonly a p p l i e d i n c o n t e m p o r a r y s u g a r p l a n t s .

L e t us use i t as a s t a r t i n g p o i n t f o r a s h o r t p r e s e n t a t i o n o f o t h e r p o s s i b l e

l a y o u t s . F o r t he sake o f s i m p l i c i t y , i t w i l l be assumed t h a t t h e pa rame te rs o f

t he s u g a r m a n u f a c t u r i n g p r o c e s s a r e f i x e d , t h a t i s , t h e t o t a l hea t demand

remains c o n s t a n t .

Suppose t h a t i n s t e a d o f d i r e c t i n g t h e e n t i r e e n e r g y s t ream c a r r i e d by vacuum

pan vapou rs t o t h e c o n d e n s e r , a p a r t o f i t i s r e - u s e d i n p r o c e s s h e a t i n g , as

shown s c h e m a t i c a l l y i n F i g . 1 . 7 ( b ) . A p r e r e q u i s i t e ( i . e . an a d d i t i o n a l

c o n s t r a i n t t o be s a t i s f i e d ) f o r t h i s i s t h e e x i s t e n c e o f l o w - t e m p e r a t u r e p r o c e s s

p a r t s where t he t e m p e r a t u r e o f vacuum pan v a p o u r s c o u l d be r e g a r d e d as

s u f f i c i e n t l y h i g h t o a l l o w e f f i c i e n t h e a t i n g . I f p r a c t i c a b l e , t h i s g i v e s t h e

p o s s i b i l i t y o f s a v i n g e n e r g y by r e d u c i n g t h e demand f o r v a p o u r s f rom t h e

e v a p o r a t o r a n d , c o n s e q u e n t l y , d e c r e a s i n g steam demand a t t h e e v a p o r a t o r i n l e t .

H o w e v e r , t h i s p o s s i b i l i t y can be r e a l i z e d o n l y i f t h e e v a p o r a t i o n p r o c e s s i s

m o d i f i e d so as t o s a t i s f y t h e c o n d i t i o n s t h a t t h e t o t a l q u a n t i t y o f w a t e r

e v a p o r a t e d remains unchanged .

Now, suppose t h a t an a t t emp t i s made t o a p p l y one o f t h e above l a y o u t s , b u t

i t t u r n s o u t t h a t t he demand f o r v a p o u r s f rom t h e e v a p o r a t o r i s so sma l l t h a t

t h e r e q u i r e d q u a n t i t y o f w a t e r can be e v a p o r a t e d o n l y i f a l a r g e v a p o u r f l o w t o

t h e c o n d e n s e r i s a l l o w e d . T h i s must be i n t e r p r e t e d as an i n d i c a t i o n t h a t a n o t h e r

e n e r g y - s a v i n g b l o c k s h o u l d be added t o t h e s y s t e m , as shown i n F i g . 1 . 7 ( c ) .

A v a p o u r compresso r r a i s i n g t h e p r e s s u r e o f a p a r t o f t h e v a p o u r s makes i t

p o s s i b l e t o r e c y c l e t h e compressed steam and t o use i t f o r h e a t i n g p u r p o s e s i n

t he e v a p o r a t o r . The r e c y c l e d h e a t d e c r e a s e s t h e h e a t demand a t t h e e v a p o r a t o r

i n l e t , r e s u l t i n g i n a h e a t s a v i n g as shown i n t h e scheme.

As f a r as t h e s u g a r m a n u f a c t u r i n g p r o c e s s i s c o n c e r n e d , m u l t i - s t a g e

e v a p o r a t i o n w i t h v a p o u r w i t h d r a w a l , u t i l i z a t i o n o f vacuum pan v a p o u r s , and

v a p o u r c o m p r e s s i o n a r e t h e main e n e r g y c o n v e r s i o n p r o c e s s e s and t hus a l s o t h e

main f u n c t i o n s o f t h e b u i l d i n g b l o c k s o f t he rma l s y s t e m s . P o s s i b l e v a r i a n t s o f

t h e b u i l d i n g b l o c k s and t h e i r v a r i o u s c o m b i n a t i o n s a r e p r e l i m i n a r i l y p r e s e n t e d

i n t h e r e m a i n i n g S e c t i o n s o f t h i s C h a p t e r . Note t h a t any the rma l s y s t e m has t o

s a t i s f y t h e p r o c e s s - i m p o s e d c o n s t r a i n t s men t ioned a b o v e . O t h e r c o n s t r a i n t s and

t h e i n t e r a c t i o n s between them, seen f rom d i f f e r e n t p o i n t s o f v i e w , a r e d i s c u s s e d

when p r e s e n t i n g t h e d e t a i l s o f t h e b u i l d i n g b l o c k s i n t h e f o l l o w i n g S e c t i o n s and

13

C h a p t e r s .

An i m p o r t a n t b y - p r o c e s s employed i n a m a j o r i t y o f c o n t e m p o r a r y b e e t s u g a r

f a c t o r i e s i s p u l p d r y i n g . I t i s n o t unusua l t h a t i t r e q u i r e s 35% o r even more

o f t h e f a c t o r y ' s o v e r a l l ( i . e . , f o r s u g a r m a n u f a c t u r e and p u l p d r y i n g

c o n s i d e r e d j o i n t l y ) demand f o r p r i m a r y e n e r g y . From t h e p o i n t o f v i e w o f e n e r g y

u t i l i z a t i o n a t y p i c a l d r y i n g p l a n t , c o n s i s t i n g o f a d r y e r and a f u r n a c e where

f u e l i s bu rned i n o r d e r t o g e n e r a t e h o t g a s e s , i s r a t h e r p r i m i t i v e .

C o n s e q u e n t l y , much a t t e n t i o n has r e c e n t l y been d e v o t e d t o r a t i o n a l i z i n g t h e

e n e r g y usage i n p u l p d r y i n g . P o s s i b l e e n e r g y s a v i n g m o d i f i c a t i o n s o f t h e p r o c e s s

l a y o u t and pa ramete rs a r e d i s c u s s e d i n S e c t i o n 1 .2 .8 .

I n a s u g a r f a c t o r y emp loy ing p u l p d r y i n g , an e n e r g y - s a v i n g p o t e n t i a l e x i s t s

a l s o i n t h e the rma l c o u p l i n g between t h i s p r o c e s s and o t h e r f a c t o r y a r e a s . An

e a r l y i d e a , d e v e l o p e d s e v e r a l decades a g o , was t o mix h o t f l u e gas f rom b o i l e r s

w i t h a i r s u p p l i e d t o t h e f u r n a c e , t hus r e d u c i n g t h e f u e l consumpt i on i n t h e

d r y i n g p l a n t . More r e c e n t l y , bo th t h e u t i l i z a t i o n o f d r y e r o u t l e t gas i n t h e

p r o c e s s h e a t i n g i n s u g a r manu fac tu re and t h e u t i l i z a t i o n o f w a s t e h e a t f rom

s u g a r manu fac tu re i n t h e p u l p d r y i n g were i n t r o d u c e d , as shown s c h e m a t i c a l l y i n

F i g . 1 . 7 ( d ) . U s i n g t h e s e t e c h n i q u e s , t h e c o n s t r a i n t s imposed on t h e the rma l

sys tem by t h e s u g a r m a n u f a c t u r i n g p r o c e s s a r e f a v o u r a b l y m o d i f i e d , c r e a t i n g new

p o s s i b i l i t i e s o f e n e r g y s a v i n g s . The u n d e r l y i n g c o n c e p t s a r e d i s c u s s e d i n

S e c t i o n 1 .2 .9 .

1.2.4 Power house

The w o r k i n g p r i n c i p l e o f t h e power houses i n c o n t e m p o r a r y s u g a r f a c t o r i e s

c o n s i s t s o f b u r n i n g f u e l i n a b o i l e r w h i c h s u p p l i e s l i v e steam t o a b a c k

p r e s s u r e t u r b i n e . The t u r b i n e d r i v e s an e l e c t r i c a l g e n e r a t o r w h i c h s u p p l i e s

e l e c t r i c a l power t o t he f a c t o r y ; s i m u l t a n e o u s l y , t h e steam f rom t h e t u r b i n e

e x h a u s t i s d e l i v e r e d t o t h e e v a p o r a t o r s t a t i o n .

T h e r e a r e two i m p o r t a n t c o n s t r a i n t s r e s u l t i n g f rom t h e p r o p e r t i e s o f t h e

power house equ ipment and a f f e c t i n g t h e c h a r a c t e r i s t i c s o f t h e e n t i r e the rma l

s y s t e m :

( i ) The c o n v e r s i o n o f l i v e - s t e a m e n e r g y t a k i n g p l a c e i n t h e t u r b i n e i s

c h a r a c t e r i z e d by a d e f i n i t e r a t i o between e n e r g y c o n v e r t e d i n t o e l e c t r i c i t y and

e x h a u s t - s t e a m e n e r g y w h i c h i s a v a i l a b l e f o r h e a t i n g . From t h e demand

c h a r a c t e r i s t i c s o f t h e e n t i r e f a c t o r y , a d i f f e r e n t p r o p o r t i o n between t h e power

demand and n e t h e a t demand may r e s u l t . O n l y i f t h e e v e n t u a l mismatch p rob lem i s

e f f e c t i v e l y r e s o l v e d by some s p e c i a l measures ( s e e S e c t i o n 1 .5 .3 ) can t h e power

house be r e g a r d e d as a s u i t a b l e e n e r g y s o u r c e f o r a p a r t i c u l a r t he rma l s y s t e m .

( i i ) The c o n v e r s i o n o f p r i m a r y e n e r g y i n t o l i v e - s t e a m e n e r g y t a k i n g p l a c e i n t h e

b o i l e r i s a s s o c i a t e d w i t h e n e r g y l o s s e s o f t h e o r d e r o f 10-20% o r even more .

14

The r e s u l t i n g demand f o r p r i m a r y e n e r g y i s equa l t o t h e sum o f l i v e - s t e a m

e n e r g y and b o i l e r l o s s e s . O n l y i f t h e the rma l c o n n e c t i o n between t h e power

house and p r o c e s s h e a t i n g i s m o d i f i e d can t h e s e l o s s e s be p a r t l y r e c o v e r e d ,

r e s u l t i n g i n a r e d u c t i o n o f t h e t o t a l e n e r g y l o s s f rom t h e the rma l sys tem and

t hus r e d u c i n g t h e demand f o r p r i m a r y e n e r g y .

The prob lems i n d i c a t e d may come i n t o q u e s t i o n i n new o r e x i s t i n g f a c t o r i e s

o p e r a t e d u n d e r d i f f e r e n t l o c a l c o n d i t i o n s and s u b j e c t t o t he i n f l u e n c e o f

d i f f e r e n t economic f a c t o r s . As t h i s d e f i n e s an a r r a y o f w i d e l y d i v e r s i f i e d

s i t u a t i o n s , i t can h a r d l y be imag ined t h a t a steam b o i l e r and a b a c k - p r e s s u r e

t u r b i n e do r e a l l y p r o v i d e a u n i v e r s a l power house s o l u t i o n . A d i s c u s s i o n o f

v a r i o u s s i t u a t i o n s and s o l u t i o n s i s p r e s e n t e d i n S e c t i o n 1.5.

1.2.5 E v a p o r a t o r

The dom ina t i ng e v a p o r a t o r a r rangemen t i s t h a t u s i n g t h e m u l t i p l e - e f f e c t ,

p a r a l l e l - f l o w p r i n c i p l e s c h e m a t i c a l l y shown i n F i g . 1.5. T h r e e t o s i x e f f e c t s

a r e u s e d , f o u r o r f i v e b e i n g t h e commonest s o l u t i o n . The o r i g i n a l i d e a o f u s i n g

t he hea t as many t imes as t h e r e a r e e f f e c t s i n t h e e v a p o r a t o r ( R i l l i e u x

p r i n c i p l e ) has been m o d i f i e d by v a p o u r w i t h d r a w a l f o r h e a t i n g p u r p o s e s . W h i l e

t h i s r e s u l t s i n w o r s e h e a t u t i l i z a t i o n i n t h e e v a p o r a t o r i t s e l f , t h e v a p o u r

w i t h d r a w a l t u r n s o u t t o be d e c i s i v e i n e n s u r i n g a h i g h l y e f f i c i e n t h e a t

u t i l i z a t i o n i n t h e e n t i r e s u g a r f a c t o r y . T h i s p rob lem i s a d d i t i o n a l l y d i s c u s s e d

i n C h a p t e r 3 , and p r a c t i c a l examples a r e g i v e n i n C h a p t e r s 8 and 9.

As a b u i l d i n g b l o c k i n a the rma l s y s t e m , t h e e v a p o r a t o r s h o u l d be adap ted t o

t h e p r o c e s s - i m p o s e d c o n s t r a i n t a l r e a d y m e n t i o n e d , namely t h e p r e d e t e r m i n e d

p e r c e n t a g e o f w a t e r t o be e v a p o r a t e d . A n o t h e r i m p o r t a n t c o n s t r a i n t i s c o n c e r n e d

w i t h t h e j u i c e t e m p e r a t u r e i n t h e h e a t i n g chamber o f t h e f i r s t e v a p o r a t o r

e f f e c t , name ly , i n o r d e r t o p r e v e n t e x c e s s i v e the rma l decay o f s u c r o s e , t h i s

t e m p e r a t u r e must n o t e x c e e d 125-130°C. A number o f p r o p o s a l s on e v a p o r a t o r

a r rangemen t have been made w h i c h aim t o e n s u r e e f f i c i e n t h e a t u t i l i z a t i o n w h i l e

a l s o s a t i s f y i n g t h e c o n s t r a i n t s ; t h e s e p r o p o s a l s have been r e v i e w e d e l s e w h e r e

( r e f . 3 ) . A q u i n t u p l e - e f f e c t e v a p o r a t o r , w i t h p a r a l l e l f l o w o f j u i c e and v a p o u r

i n e f f e c t s 1 t h r o u g h 4 and c o u n t e r - f l o w i n e f f e c t 5, i s shown s c h e m a t i c a l l y i n

F i g . 1.8.

I n modern the rma l sys tems where t h e h e a t i n g needs have been r e d u c e d t o

a minimum, t h e o v e r a l l demand f o r v a p o u r s w i t h d r a w n f rom t h e e v a p o r a t o r may be

l e s s than t h e amount o f w a t e r t o be e v a p o r a t e d , t h u s making t h e w a t e r

p e r c e n t a g e c o n s t r a i n t d i f f i c u l t t o s a t i s f y . An i n c r e a s e d f l o w o f l a s t - e f f e c t

v a p o u r t o t h e c o n d e n s e r can h a r d l y be a c c e p t e d , as t h i s wou ld be a d i r e c t

e n e r g y l o s s r e q u i r i n g a c o r r e s p o n d i n g i n c r e a s e o f t h e h e a t i n g steam s u p p l y t o

t h e f i r s t e f f e c t . H o w e v e r , t h i s s i t u a t i o n can be changed i f u n c o n v e n t i o n a l

e v a p o r a t i o n s t a g e s hea ted w i t h l o w - t e m p e r a t u r e o r was te h e a t a r e a t t a c h e d t o

15

e x h a u s t s team ' \ -

η 128°C

\ th in j u i ce

14 .5%DS.90°C

th ick ju ice 7 2 % D S . 9 4 ° C _

F i g . 1.8. Scheme o f a m u l t i p l e - e f f e c t e v a p o r a t o r f e a t u r i n g c o u n t e r - f l o w o f j u i c e and h e a t i n g v a p o u r i n t he f i f t h e f f e c t . 1-5 - e v a p o r a t o r e f f e c t s , 6 - j u i c e h e a t e r s .

t h e c l a s s i c a l e v a p o r a t o r . An i n t r o d u c t i o n t o p o s s i b l e s o l u t i o n s based on t h i s

p r i n c i p l e i s g i v e n i n S e c t i o n 1 .2 .6 . As an a l t e r n a t i v e , t h e c l a s s i c a l m u l t i

s t a g e e v a p o r a t i o n can be combined w i t h v a p o u r c o m p r e s s i o n , as o u t l i n e d i n

S e c t i o n 1 .2 .7 .

I n each e v a p o r a t o r e f f e c t , w h i l e v a p o u r i s g e n e r a t e d a t a c e r t a i n r a t e ,

condensa te must be d r a i n e d a t an a p p r o x i m a t e l y equa l r a t e f rom t h e h e a t i n g

chamber. The c o n d e n s a t e i s f l a s h e d ( f l a s h - e v a p o r a t e d ) , g e n e r a t i n g a d d i t i o n a l

v a p o u r , t h e e n e r g y o f w h i c h can be u t i l i z e d i n t h e s u b s e q u e n t e f f e c t s . Among

v a r i o u s a r rangements o f t h e c o n d e n s a t e s u b s y s t e m , t h e cascade f l a s h shown

s c h e m a t i c a l l y i n F i g . 1.5 i s t he most e f f e c t i v e s o l u t i o n . F i r s t - e f f e c t and

p o s s i b l y s e c o n d - e f f e c t c o n d e n s a t e a r e t y p i c a l l y used as b o i l e r f e e d - w a t e r , w h i l e

t he condensa tes f rom t h e r e m a i n i n g e f f e c t s can be u t i l i z e d i n t h e p r o c e s s

h e a t i n g and f o r o t h e r p u r p o s e s . V a r i o u s a s p e c t s o f t h e c o n d e n s a t e usage a r e

d i s c u s s e d i n C h a p t e r s 3 , 8 and 9.

The impo r t ance o f t he e v a p o r a t o r t o t h e e n e r g y c o n v e r s i o n and d i s t r i b u t i o n i n

a the rma l sys tem imposes s p e c i a l r e q u i r e m e n t s on t h e e v a p o r a t i o n p r o c e s s , as

w e l l as t he a s s o c i a t e d equ ipment and c o n t r o l s y s t e m s . New deve lopmen ts i n t h e s e

a reas a r e m a i n l y d i s c u s s e d i n C h a p t e r s 5 and 6.

1.2.6 U t i l i z a t i o n o f l o w - t e m p e r a t u r e o r was te h e a t

As t h e vacuum pans a r e s u p p l i e d w i t h t h e h e a t r e q u i r e d f o r s u g a r b o i l i n g , and

the p r o c e s s media a r e c o n t i n u o u s l y hea ted i n o r d e r t o s t a b i l i z e t h e t e m p e r a t u r e s

needed f o r i m p o r t a n t u n i t o p e r a t i o n s , s t reams o f l o w - t e m p e r a t u r e h e a t become

a v a i l a b l e , m a i n l y i n vacuum pan v a p o u r s and s p e n t c a r b o n a t a t i o n g a s . T h e r e a r e

s t i l l many f a c t o r i e s where t h e l o w - t e m p e r a t u r e h e a t i s c o n s i d e r e d u s e l e s s , so

t he vapou rs a r e d i r e c t e d t o t h e c o n d e n s e r and t h e c a r b o n a t a t i o n gas i s

d i s c h a r g e d d i r e c t l y t o t h e a tmosphe re . H o w e v e r , i f t h e s u g a r m a n u f a c t u r i n g

p r o c e s s i s so a r r a n g e d t h a t i n c e r t a i n p a r t s o f t h e w a t e r and j u i c e f l o w s t h e

t empe ra tu re i s low enough ( t h a t i s , l o w e r t han t h e t e m p e r a t u r e o f vacuum pan

16

v a p o u r s o r s p e n t c a r b o n a t a t i o n g a s ) , t h e n t he l o w - t e m p e r a t u r e h e a t can be

u t i l i z e d . T y p i c a l " c o l d " media a r e raw j u i c e a n d , i n some i n s t a n c e s , w a t e r

s u p p l i e d t o t h e e x t r a c t o r , and p r e - l i m e d j u i c e .

One o f t h e o b s t a c l e s t o u t i l i z i n g l o w - t e m p e r a t u r e h e a t i s t h a t t h e n e c e s s a r y

equ ipment may be a b i t t r o u b l e s o m e . I n a j u i c e h e a t e r hea ted by vacuum pan

v a p o u r s , t h e s p e c i f i c volume o f v a p o u r i s so l a r g e t h a t i t n e c e s s i t a t e s a h i g h

f l o w v e l o c i t y . T h i s i n v o l v e s t he r i s k , among o t h e r s , o f t ube v i b r a t i o n a n d , as

t he w a t e r c o n t e n t i n v a p o u r f l o w i n g t h r o u g h t h e h e a t e r i n c r e a s e s , t u b e e r o s i o n .

When r e c o v e r i n g t he c a r b o n a t a t i o n h e a t l o s s i n a s u r f a c e hea t e x c h a n g e r , a low

f i l m c o e f f i c i e n t o f h e a t t r a n s f e r between t h e gas and t h e h e a t i n g s u r f a c e i s

u n a v o i d a b l e , and t hus a r e l a t i v e l y l a r g e h e a t i n g s u r f a c e a r e a may be r e q u i r e d .

I n t he case o f c a r b o n a t a t i o n - g a s r e c i r c u l a t i o n , p o w e r - e x p e n s i v e pumping may a l s o

be n e c e s s a r y .

P r o v i d i n g r e l i a b l e and n o t t o o c o s t l y equ ipment i s a v a i l a b l e , t h e

r e c i r c u l a t e d l o w - t e m p e r a t u r e h e a t can r e p l a c e an e q u i v a l e n t p o r t i o n o f t h e hea t

i n vapou rs w i t h d r a w n f rom t h e e v a p o r a t o r , t hus making i t p o s s i b l e t o r e d u c e t h e

h e a t i n g steam demand a t t he e v a p o r a t o r i n l e t . The h e a t i n g by vacuum pan v a p o u r s

i s a d d i t i o n a l l y d i s c u s s e d i n C h a p t e r 3 , and t h e t e c h n i q u e s used t o r e d u c e t h e

c a r b o n a t a t i o n h e a t l o s s i n C h a p t e r 4 .

C e r t a i n s t reams o f l o w - t e m p e r a t u r e hea t l e a v i n g t h e f a c t o r y a r e so d i f f i c u l t

t o u t i l i z e t h a t t h e y a r e t r a d i t i o n a l l y c a l l e d " w a s t e h e a t " . (Some a u t h o r s use

t h i s te rm f o r a l l t h e hea t s t reams t h a t a r e f i n a l l y d i s c h a r g e d t o t h e

e n v i r o n m e n t , i n c l u d i n g t h e l o w - t e m p e r a t u r e h e a t ) . I t may i n c l u d e , among o t h e r

componen ts , h e a t c a r r i e d by o u t l e t gas f rom t h e p u l p d r y e r , e x c e s s c o n d e n s a t e

f rom t h e e v a p o r a t o r and even b a r o m e t r i c w a t e r p r i o r t o e n t e r i n g t h e c o o l i n g

t o w e r s . I t s h o u l d be emphas ized t h a t t h e u t i l i z a t i o n o f was te h e a t i s now

r e a l i z a b l e , a l t h o u g h n o t a lways e c o n o m i c a l l y f e a s i b l e .

Both l o w - t e m p e r a t u r e and was te hea t can be u t i l i z e d i n j u i c e and s y r u p

e v a p o r a t i o n pe r fo rmed a t a s u f f i c i e n t l y low t e m p e r a t u r e , t h a t i s , u n d e r h i g h

vacuum. S p e c i a l v a p o u r - and g a s - h e a t e d e v a p o r a t o r s a r e b e i n g d e v e l o p e d f o r t h i s

p u r p o s e , as d i s c u s s e d i n C h a p t e r 5. A l t e r n a t i v e methods o f u t i l i z a t i o n o f l o w -

t e m p e r a t u r e hea t employ the rma l c o n n e c t i o n s between s u g a r m a n u f a c t u r e and p u l p

d r y i n g . T h i s i s p a r t i c u l a r l y i n t e r e s t i n g i f t h e s o - c a l l e d l o w - t e m p e r a t u r e d r y i n g

i s a p p l i e d , as o u t l i n e d i n S e c t i o n 1 .2 .9 .

1.2.7 Vapou r compresso rs

As i n d i c a t e d i n S e c t i o n 1.2.3 a b o v e , v a p o u r c o m p r e s s i o n i s a t e c h n i q u e w h i c h

can be used t o r e s o l v e a c o n f l i c t between t h e p o t e n t i a l r e d u c t i o n s o f t h e v a p o u r

demand and t he p r o c e s s c o n s t r a i n t on w a t e r q u a n t i t y t o be e v a p o r a t e d f rom j u i c e .

By i n t r o d u c i n g v a p o u r c o m p r e s s i o n t o t h e the rma l s y s t e m , i t becomes p o s s i b l e t o

e v a p o r a t e the r e q u i r e d w a t e r amount w h i l e r e c i r c u l a t i n g t h e p o r t i o n o f t h e

17

e v a p o r a t i o n h e a t w h i c h exceeds t h e hea t demand o f v a p o u r - h e a t e d equ ipment

o u t s i d e t h e e v a p o r a t o r .

Most o f t e n , t h e v a p o u r t o be compressed i s t aken f rom th-e f i r s t e f f e c t a n d ,

i t s p r e s s u r e r a i s e d , i s s u p p l i e d t o t he h e a t i n g chamber o f t h e same e f f e c t .

A the rma l machine t r a n s f e r r i n g , a t t he expense o f w o r k , h e a t f rom a l o w -

t e m p e r a t u r e body t o a h i g h - t e m p e r a t u r e body i s known as a h e a t pump. I n a

t y p i c a l the rma l sys tem i n a s u g a r f a c t o r y , t h e r e a r e numerous p o t e n t i a l h e a t

pump a p p l i c a t i o n s o t h e r than h e a t r e c i r c u l a t i o n i n t h e f i r s t e v a p o r a t o r e f f e c t .

G e n e r a l l y , t he e n e r g y t r a n s p o r t i s i n i t i a t e d a t t h e h i g h e s t t e m p e r a t u r e / p r e s s u r e

l e v e l c o r r e s p o n d i n g t o t h e pa ramete rs o f l i v e steam and t h e n c o n t i n u e d a t t h e

g r a d u a l l y d e c r e a s i n g t e m p e r a t u r e s and p r e s s u r e s o f media c i r c u l a t e d i n v a r i o u s

p a r t s o f t h e the rma l s y s t e m . The l o w e s t l e v e l , a t w h i c h t h e e n e r g y t r a n s p o r t i s

t e r m i n a t e d , i s d e f i n e d by t h e t e m p e r a t u r e o f b a r o m e t r i c w a t e r and t h e

a t m o s p h e r i c p r e s s u r e . T h e o r e t i c a l l y , t h e h e a t pump p r i n c i p l e can be a p p l i e d

between any two d i f f e r e n t t e m p e r a t u r e / p r e s s u r e l e v e l s . By s u p p l y i n g e n e r g y (as

e l e c t r i c a l power o r l i v e s team) t o t h e h e a t pump, i t becomes p o s s i b l e t o

r e c i r c u l a t e a c e r t a i n amount o f h e a t , t hus c u t t i n g down t h e n e t h e a t demand o f

t he therma l s y s t e m .

I n r e a l i t y , t h e number o f f e a s i b l e h e a t pump a p p l i c a t i o n s i n a t he rma l

sys tem i s l i m i t e d . F o r e x a m p l e , t h e h e a t r e c o v e r y f rom t h e b a r o m e t r i c w a t e r ,

a l t h o u g h r e a l i z a b l e , c a n n o t be pe r fo rmed by v a p o u r c o m p r e s s i o n ; i n s t e a d , i t

r e q u i r e s complex and c o s t l y m a c h i n e r y , making t h e s o l u t i o n uneconomic . F i g u r e

1.9 shows f i v e p o s s i b l e l o c a t i o n s o f v a p o u r compresso rs t o r e c i r c u l a t e h e a t i n

a the rma l sys tem w i t h a q u a d r u p l e - e f f e c t e v a p o r a t o r . I t s h o u l d be p o i n t e d o u t

t h a t t h e s e l o c a t i o n s a r e by no means e q u i v a l e n t t o each o t h e r . I n g e n e r a l , i f

e n e r g y s a v i n g s a r e t o be o b t a i n e d , t hen t h e h e a t r e c i r c u l a t i o n t a k i n g p l a c e i n

a c e r t a i n p a r t o f t h e the rma l s y s t e m must be c o o r d i n a t e d w i t h mass and e n e r g y

Γ "

e x h a u s t ) s t e a m

6

F i g . 1.9. P o s s i b l e l o c a t i o n s o f v a p o u r c o m p r e s s o r s r e l a t i v e t o e s s e n t i a l components o f a the rma l sys tem w i t h a q u a d r u p l e - e f f e c t e v a p o r a t o r s u p p l y i n g s e c o n d - e f f e c t v a p o u r t o vacuum-pan h e a t i n g . 1-4 - e v a p o r a t o r e f f e c t s , 5 - vacuum p a n s , 6 - c o n d e n s e r .

s team

c o m p r e s s e d

v a p o u r

v a p o u r

F i g . 1.10. Work ing p r i n c i p l e o f a j e t - t y p e c o m p r e s s o r .

f l o w s i n o t h e r p a r t s o f t he s y s t e m . F o r e x a m p l e , i f vacuum pan v a p o u r i s

compressed and r e - u s e d t o hea t t h e vacuum p a n s , t h e n t h e demand f o r v a p o u r f rom

the e v a p o r a t o r i s r e d u c e d . As t h e amount o f w a t e r t o be removed f rom j u i c e must

be h e l d c o n s t a n t , t h e d i s t r i b u t i o n o f v a p o u r s w i t h d r a w n f rom t h e e v a p o r a t o r

must be p r o p e r l y a d j u s t e d ; o t h e r w i s e , i t may be n e c e s s a r y t o i n c r e a s e t h e e n e r g y

l o s s r e s u l t i n g f rom t h e f l o w o f l a s t - e f f e c t v a p o u r t o t h e c o n d e n s e r , pe rhaps

making t h e i d e a o f v a p o u r c o m p r e s s i o n m e a n i n g l e s s .

As r e g a r d s t h e e q u i p m e n t , b o t h j e t - t y p e compresso rs ( t h e r m o c o m p r e s s o r s ) and

mechan ica l compresso rs ( t u r b o c o m p r e s s o r s ) can be u s e d . The j e t - t y p e c o m p r e s s o r

( F i g . 1.10) must be c o n t i n u o u s l y s u p p l i e d w i t h l i v e s t eam, b y - p a s s i n g t h e t u r b o

g e n e r a t o r and n o t c o n t r i b u t i n g t o e l e c t r i c i t y g e n e r a t i o n . Depend ing on t h e

i n l e t and o u t l e t p r e s s u r e s o f t h e v a p o u r c o m p r e s s e d , t he c o m p r e s s i o n r a t i o , i . e .

t h e r a t i o o f v a p o u r mass f l o w t o l i v e - s t e a m mass f l o w , v a r i e s as shown i n T a b l e

1.2 ( t h e v a l u e s g i v e n s h o u l d be i n t e r p r e t e d as t h e h i g h e s t a t t a i n a b l e , w h i l e

t h e a c t u a l v a l u e s may a l s o depend on compresso r q u a l i t y ) .

The mechan ica l compresso r ( F i g . 1.11) i s d r i v e n by a m o t o r , t o w h i c h e n e r g y

must be s u p p l i e d c o n t i n u o u s l y . A t p r e s e n t , e l e c t r i c moto rs a r e g e n e r a l l y

r e g a r d e d as most e c o n o m i c , b u t steam t u r b i n e s a r e a l s o used as c o m p r e s s o r

d r i v e s . As a c o n s e q u e n c e , t h e a p p l i c a t i o n o f a mechan ica l c o m p r e s s o r e i t h e r

i n c r e a s e s t h e f a c t o r y ' s power demand, o r r e q u i r e s a c e r t a i n f l o w o f l i v e steam

b y - p a s s i n g t he t u r b o - g e n e r a t o r t o be s u p p l i e d t o t h e c o m p r e s s o r d r i v e .

When c o n s i d e r i n g a l l t he p o s s i b l e s i t u a t i o n s r e l a t e d t o t h e f a c t o r y ' s power

b a l a n c e and hea t b a l a n c e and t h e p o s s i b i l i t y o f c o o p e r a t i o n w i t h an e x t e r n a l

TABLE 1.2

E s t i m a t e d a t t a i n a b l e v a l u e s o f t h e c o m p r e s s i o n r a t i o o f j e t compresso rs o p e r a t e d a t d i f f e r e n t l o c a t i o n s i n t h e the rma l sys tem shown i n F i g . 1.9 a t l i v e - s t e a m paramete rs 38 b a r and 450°C.

L o c a t i o n Compress ion r a t i o

a - 1s t e f f e c t v a p o u r t o e x h a u s t steam 2.4 b - 2nd e f f e c t v a p o u r t o e x h a u s t steam 1.1 c - vacuum pan v a p o u r t o e x h a u s t steam 0.3 d - 3 rd e f f e c t v a p o u r t o 2nd e f f e c t v a p o u r 1.5 e - vacuum pan v a p o u r t o 2nd e f f e c t v a p o u r 0.7

19

F i g . 1.11. T y p i c a l s i n g l e - s t a g e c e n t r i f u g a l c o m p r e s s o r f o r t h e c o m p r e s s i o n o f f i r s t e v a p o r a t o r e f f e c t v a p o u r . A t volume f l o w 50 000 m^/h and r o t a t i o n a l v e l o c i t y 5000 rpm, t h e r o t o r d i a m e t e r i s a b o u t 0.9 m.

power g r i d , i t must be c o n c l u d e d t h a t t h e r e i s no u n i v e r s a l l y o p t i m a l v a p o u r

compress i on t e c h n i q u e . Depending on t h e c o n s t r a i n t s t o be s a t i s f i e d and t h e

economic r e l a t i o n s between t he n e c e s s a r y i n v e s t m e n t s and t h e a t t a i n a b l e e n e r g y

s a v i n g , one t y p e o f equ ipment o r a c o m b i n a t i o n o f bo th may p r e v a i l . Unde r West

European c o n d i t i o n s , i t i s g e n e r a l l y b e l i e v e d t h a t t h e e l e c t r i c a l l y d r i v e n

mechan ica l compresso r r e c i r c u l a t i n g f i r s t - e f f e c t v a p o u r i s most e c o n o m i c .

A l t h o u g h t h i s seems t o be p r o v e d by a number o f r e c e n t i n v e s t m e n t s ( r e f . 1 4 ) ,

t h e e n e r g y - e f f i c i e n t Dan ish f a c t o r i e s employ t u r b i n e - d r i v e n c o m p r e s s o r s

r e c i r c u l a t i n g s e c o n d - e f f e c t v a p o u r ( p o s s i b l y i n c o m b i n a t i o n w i t h j e t - t y p e

c o m p r e s s o r s ) . T h e r e a r e a l s o numerous West European s u g a r f a c t o r i e s r e l y i n g on

j e t - t y p e compresso rs and a t t a i n i n g e x c e l l e n t r e s u l t s ( r e f s . 1 5 , 1 6 ) . I n E a s t e r n

E u r o p e , where t h e economic c o n d i t i o n s a r e d i f f e r e n t , j e t - t y p e c o m p r e s s o r s a r e

u s u a l l y p r e f e r a b l e t o mechan ica l ones ( r e f . 5 ) .

I t may be added t h a t j e t - t y p e compresso rs have r e c e n t l y been s e l e c t e d f o r

v a p o u r c o m p r e s s i o n c i r c u i t s i n Greek and Czech s u g a r f a c t o r i e s ( r e f s . 1 7 , 1 8 ) .

The a p p l i c a t i o n s o f v a p o u r compresso rs a r e d i s c u s s e d i n g r e a t e r d e t a i l i n

C h a p t e r s 3 , 8 and 9.

1.2.8 Pu lp d e h y d r a t i o n

The p u l p d e h y d r a t i o n p r o c e s s i s t y p i c a l l y o p e r a t e d i n p a r a l l e l w i t h t h e

s u g a r m a n u f a c t u r i n g p r o c e s s ( F i g . 1 . 1 2 ( a ) ) . The d e h y d r a t i o n u s u a l l y c o n s i s t s o f

mechan ica l p r e s s i n g o f t h e p u l p t o a d r y - s u b s t a n c e c o n t e n t o f t h e o r d e r o f

20

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pw PP d p i

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PP d p |

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pw

PP d p |

F i g . 1.12. P o s s i b l e schemes f o r t h e p u l p d e h y d r a t i o n p r o c e s s : ( a ) t h e r m a l l y i n d e p e n d e n t , ( b ) t h e r m a l l y i n d e p e n d e n t w i t h gas r e c i r c u l a t i o n , ( c ) p a r t l y h e a t e d w i t h b o i l e r f l u e g a s , ( d ) w i t h t h e u t i l i z a t i o n o f b o i l e r f l u e g a s . pw - p r e s s w a t e r , pp - p r e s s e d p u l p , dp - d r i e d p u l p ; 1 - p r e s s e s , 2 - f u r n a c e , 3 - d r y e r .

20% DS and s u b s e q u e n t the rma l d r y i n g t o abou t 90% DS. W h i l e t h e a v e r a g e e n e r g y

demand i s abou t 1000 kJ p e r kg w a t e r r emoved , t h e c o n t r i b u t i o n s o f t h e two

p r o c e s s e s t o t h i s f i g u r e a r e v e r y d i f f e r e n t i n d e e d . The mechan ica l p r e s s i n g

r e q u i r e s 40-80 k J e l e c t r i c a l e n e r g y p e r kg w a t e r r emoved , w h i l e t h e h e a t demand

i n the rma l d r y i n g i s abou t 3000 kJ p e r kg w a t e r . The d r y i n g and t h e s u b s e q u e n t

p e l l e t i n g o f t h e d r i e d p u l p a r e a l s o a s s o c i a t e d w i t h a power e x p e n d i t u r e o f t h e

o r d e r o f 0.6 kWh p e r 100 kg b e e t p r o c e s s e d .

When compared t o t h e s u g a r m a n u f a c t u r e , t h e u t i l i z a t i o n o f p r i m a r y e n e r g y

s u p p l i e d t o the rma l d r y i n g o f t h e p u l p i s r a t h e r p o o r . A c t u a l l y , i t c a n n o t be

b e t t e r i n a p r o c e s s s t a r t i n g f rom f u e l combus t i on i n a h i g h e x c e s s o f a i r , w h i c h

i s e q u i v a l e n t t o m i x i n g t h e o r e t i c a l l y p o s s i b l e h i g h - t e m p e r a t u r e c o m b u s t i o n gases

w i t h c o l d a i r . T h e r e i s a l s o a l a r g e t e m p e r a t u r e d i f f e r e n c e between t h e gases

and t h e p u l p i n t he d r y e r . F i n a l l y , t h e e n e r g y s t ream i s c a r r i e d away by t h e

o u t l e t gases a f t e r p e r f o r m i n g o n l y one pass t h r o u g h t h e d r y e r .

S u b s t a n t i a l e n e r g y s a v i n g s a r e p o s s i b l e i n p u l p d e h y d r a t i o n i f more w a t e r i s

removed by p r e s s i n g and l e s s by d r y i n g . T h i s can be a c h i e v e d by i n t r o d u c i n g

m ino r p r o c e s s m o d i f i c a t i o n s and a p p l y i n g improved p u l p p r e s s e s , as d i s c u s s e d i n

C h a p t e r 4 . As f a r as t h e c l a s s i c a l the rma l d r y i n g i s c o n c e r n e d , i t can be

c o n c l u d e d t h a t t h e p o s s i b i l i t i e s o f c u t t i n g down t h e e n e r g y e x p e n d i t u r e p e r u n i t

mass o f e v a p o r a t e d w a t e r a r e r a t h e r l i m i t e d . The o n l y e f f e c t i v e e n e r g y - s a v i n g

t e c h n i q u e i s t h e r e c i r c u l a t i o n o f gases f rom the d r y e r o u t l e t , e i t h e r t o t h e

21

d r y e r i n l e t o r t o t h e f u r n a c e i n l e t ( F i g . 1 . 1 2 ( b ) ) . S a v i n g s o f up t o 10-12% o f

t he p r i m a r y e n e r g y i n p u t can be a t t a i n e d ( r e f s . 1 9 , 2 0 ) . H o w e v e r , an e n t i r e l y new

f i e l d o f p o t e n t i a l e n e r g y s a v i n g s can be opened i f t h e p u l p d r y i n g becomes

t h e r m a l l y i n t e g r a t e d w i t h t he s u g a r m a n u f a c t u r e .

1.2.9 Thermal c o u p l i n g between p u l p d r y i n g and s u g a r manu fac tu re

The e n e r g y b a l a n c e o f p u l p d r y i n g can be c o n s i d e r a b l y improved by u t i l i z i n g

t h e was te h e a t f rom t h e b o i l e r f l u e g a s , as shown s c h e m a t i c a l l y i n F i g s . 1 . 1 2 ( c )

and 1 . 1 2 ( d ) . P o s s i b l e s a v i n g s can be e s t i m a t e d a t up t o 12-15% o f t h e d r y e r ' s

h e a t demand.

P a r a l l e l o p e r a t i o n o f a d r y e r hea ted by f l u e gas and a d r y e r w i t h i t s own

f u r n a c e , a l t h o u g h r e a l i z a b l e , has s e r i o u s drawbacks and i s n o t t o be recommended

( r e f . 1 9 ) . I n an a l t e r n a t i v e s y s t e m , t h e b o i l e r f l u e gas r e p l a c e s t h e a i r

admixed w i t h t h e combus t i on gas i n t h e o u t l e t o f t h e d r y i n g - p l a n t f u r n a c e . As

t he f l u e gas t e m p e r a t u r e i s h i g h e r than t h e a i r t e m p e r a t u r e , l e s s combus t i on gas

and thus l e s s f u e l w i l l be consumed f o r a d e f i n i t e t e m p e r a t u r e a t t h e d r y e r

i n l e t . I t i s a l s o p o s s i b l e t o mix t h e b o i l e r f l u e gas w i t h t h e c o m b u s t i o n a i r

p r i o r t o t h e f u r n a c e , r a i s i n g t h e t e m p e r a t u r e i n t h e f u r n a c e i n l e t and making i t

p o s s i b l e t o r e d u c e t h e f u e l demand.

G e n e r a l l y , t h e use o f b o i l e r f l u e gas s e t s s p e c i a l r e q u i r e m e n t s on t h e d r y e r

c a p a c i t y . As shown i n t h e i n v e s t i g a t i o n s o f drum d r y e r s ( r e f . 2 0 ) , t h e most

e f f e c t i v e h e a t u t i l i z a t i o n i s a t t a i n e d a t a c e r t a i n o p t i m a l l o a d o f t h e d r y e r ' s

drum. Load d e v i a t i o n s f rom the o p t i m a l v a l u e cause t h e h e a t consump t i on p e r kg

w a t e r t o i n c r e a s e .

The d i s p r o p o r t i o n i n e n e r g y u t i l i z a t i o n e f f i c i e n c y o f p u l p d r y i n g and s u g a r

manu fac tu re has r e c e n t l y i n s p i r e d r a t i o n a l i z a t i o n measures aimed a t i n t e n s i f y i n g

t he e n e r g y usage i n f a c t o r i e s emp loy i ng p u l p d r y i n g . A t t h e h i g h - t e m p e r a t u r e end

o f t h e p u l p d r y i n g p r o c e s s , e l e c t r i c i t y g e n e r a t i o n ( p r e f e r a b l y i n a g a s - t u r b i n e

s e t ) i s p r o p o s e d t o u t i l i z e t h e the rma l p o t e n t i a l o f t h e h e a t o b t a i n e d t h r o u g h

f u e l c o m b u s t i o n . The changed r e q u i r e m e n t s o f power house o p e r a t i o n a s s o c i a t e d

w i t h g a s - t u r b i n e a p p l i c a t i o n s a r e d i s c u s s e d i n S e c t i o n 1.5.

A t t h e l o w - t e m p e r a t u r e end o f p u l p d r y i n g , a t t e m p t s a r e made t o use w a s t e

h e a t i n t he o u t l e t gases f rom t h e d r y e r f o r h e a t i n g p u r p o s e s i n s u g a r

m a n u f a c t u r e . As t h e o u t l e t gases c o n t a i n d u s t p a r t i c l e s accompanied by s u l p h u r

d i o x i d e , t h i s must be seen i n c o n n e c t i o n w i t h gas c l e a n i n g . Where gas c l e a n i n g

( p o s s i b l y i n c l u d i n g d e s u l p h u r i z a t i o n ) i s r e q u i r e d f o r e n v i r o n m e n t a l r e a s o n s , i t

may be a d v i s a b l e t o comp le te t h e n e c e s s a r y equ ipment w i t h a h e a t - r e c o v e r y

c i r c u i t . The r e c o v e r e d h e a t can be u t i l i z e d i n j u i c e h e a t i n g o r e v a p o r a t i o n , as

has been demons t ra ted i n p r o t o t y p e p l a n t s ( r e f s . 2 1 , 2 2 ) , T h i s c r e a t e s a d d i t i o n a l

p o s s i b i l i t i e s o f s a t i s f y i n g t h e w a t e r remova l c o n s t r a i n t w h i c h i s so i m p o r t a n t

t o t he e n e r g y b a l a n c e o f s u g a r m a n u f a c t u r e .

22

A n o t h e r g roup o f r a t i o n a l i z a t i o n measures o r i g i n a t e s f rom t h e i d e a o f

s p l i t t i n g t he p u l p d r y i n g p r o c e s s i n t o p a r t s pe r fo rmed a t d i f f e r e n t t e m p e r a t u r e

l e v e l s ( r e f s . 9 , 1 9 , 2 3 ) . W h i l e t h e h i g h - t e m p e r a t u r e p r o c e s s i s e s s e n t i a l l y

i d e n t i c a l t o c o n v e n t i o n a l d r y i n g , t h e l o w - t e m p e r a t u r e p r o c e s s c r e a t e s new

p o t e n t i a l f o r t he u t i l i z a t i o n o f was te hea t f rom s u g a r manu fac tu re and

r e d u c t i o n s i n o v e r a l l e n e r g y demand o f t h e f a c t o r y . Waste h e a t can be r e c o v e r e d

f rom vacuum pan v a p o u r s , c o n d e n s a t e , s p e n t c a r b o n a t a t i o n gas and even b a r o m e t r i c

w a t e r , r a i s i n g t h e t e m p e r a t u r e o f a i r s u p p l i e d t o t h e l o w - t e m p e r a t u r e d r y e r t o

55-60°C. By remov ing a s u b s t a n t i a l p a r t o f t h e w a t e r f rom t h e p u l p , t h e f u e l

demand can be r e d u c e d i n t h e f i n a l d r y i n g , where t h e r e q u i r e d d r y s u b s t a n c e

c o n t e n t o f t h e d r i e d p u l p i s a t t a i n e d . The economic p o t e n t i a l s s o c i a t e d w i t h

t h i s s o l u t i o n has a l r e a d y been demons t ra ted i n i n d u s t r i a l - s c a l e p l a n t s

( r e f . 2 4 ) .

V e r y p r o m i s i n g c o n c e p t s o f e n e r g y - s a v i n g the rma l c o u p l i n g between p u l p d r y i n g

and o t h e r s u g a r f a c t o r y subsys tems a r e based on t he a p p l i c a t i o n s o f steam

d r y e r s . The i d e a o f steam d r y i n g i s n o t new t o t h e s u g a r i n d u s t r y , b u t i t i s

o n l y r e c e n t l y t h a t i t has become p o s s i b l e t o i n c l u d e t h e d r y e r s - w h e t h e r

s u p p l i e d w i t h l i v e s t eam, o r w i t h e x h a u s t steam o r v a p o u r f rom t h e e v a p o r a t o r -

i n t o the rma l sys tems u s i n g t h e p r i n c i p l e o f m u l t i p l e h e a t u t i l i z a t i o n w h i c h i s

so c h a r a c t e r i s t i c o f s u g a r m a n u f a c t u r e . I f used i n c o m b i n a t i o n w i t h medium- o r

l o w - t e m p e r a t u r e d r y i n g ( r e f s . 2 5 , 2 6 ) , steam d r y i n g makes i t p o s s i b l e t o

e l i m i n a t e p r i m a r y e n e r g y i n p u t t o t h e p u l p d r y i n g p l a n t and t o a t t a i n

c o n s i d e r a b l e o v e r a l l e n e r g y s a v i n g s .

I t must be a d m i t t e d t h a t t h e the rma l c o u p l i n g between p u l p d r y i n g and s u g a r

manu fac tu re g e n e r a l l y r e q u i r e s t h e a p p l i c a t i o n o f r a t h e r c o s t l y e q u i p m e n t .

T h e r e f o r e , t h e p r a c t i c a b l e e n e r g y s a v i n g s a r e h i g h l y dependen t on economic

f a c t o r s , l i k e t he c o s t s o f f u e l and p o w e r , and t h e c a p i t a l c o s t . W i th t h e

e x c e p t i o n o f t he u t i l i z a t i o n o f b o i l e r f l u e g a s , a l l t h e t e c h n i q u e s men t ioned

above s h o u l d be r e g a r d e d as new deve lopmen ts r a t h e r t han s t a n d a r d i n d u s t r i a l

p r a c t i c e ( f o r a more d e t a i l e d d i s c u s s i o n , see C h a p t e r 4 ) .

1.3 HEAT DEMAND

1.3.1 Scope o f t h e prob lems

Sugar i n d u s t r y p e o p l e t e n d t o t h i n k o f t h e l i n k between h e a t economy and t h e

s u g a r m a n u f a c t u r i n g p r o c e s s i n terms o f how much f u e l s h o u l d be b u r n t i n t h e

b o i l e r s i n o r d e r t o make t h e p r o c e s s r u n . As f u e l p r i c e s go u p , h o w e v e r , t h e

q u e s t i o n o f how t h e p r o c e s s can be a l t e r e d i n o r d e r t o r e d u c e t h e h e a t demand

becomes more and more i m p o r t a n t .

I n S e c t i o n 1 .2 , t h r e e q u a n t i t i e s were i n t r o d u c e d t o e x p r e s s t h e

c h a r a c t e r i s t i c s o f t h e h e a t economy: t o t a l h e a t demand n e t h e a t demand

23

and e f f e c t i v e n e s s r a t i o K. The t r a d i t i o n a l app roach s u g g e s t s t h a t t h e h e a t

economy can be improved by e n s u r i n g t h a t t h e the rma l sys tem i s w e l l d e s i g n e d ,

c a r e f u l l y o p e r a t e d and w e l l m a i n t a i n e d . T h i s means t h a t a t a g i v e n Q- j , a l a r g e Κ

s h o u l d be a t t a i n e d so t h a t Q2 = Q-j /K w i l l become s u f f i c i e n t l y s m a l l .

The p r o c e s s - o r i e n t e d app roach stems f rom t h e o b s e r v a t i o n t h a t t h e n e t h e a t

demand can a l s o be c u t down by r e d u c i n g t h e sum o f h e a t s t reams ( t o t a l h e a t

demand) Q-j. T h i s r e q u i r e s a d j u s t i n g t h e p r o c e s s so as t o make s u g a r m a n u f a c t u r e

l e s s e n e r g y - i n t e n s i v e .

I t s h o u l d be emphas ized t h a t i n i n d u s t r i a l p r a c t i c e , t h e measures t aken t o

i n c r e a s e Κ s h o u l d be t r e a t e d on an equa l b a s i s w i t h t h o s e aimed a t r e d u c i n g Q-j .

I n o t h e r w o r d s , e n e r g y - s a v i n g p r o c e s s a d j u s t m e n t s a r e as i m p o r t a n t as the rma l

sys tem improvements s e r v i n g t h e same p u r p o s e .

I n t h i s S e c t i o n , we s h a l l s t u d y t h e i n f l u e n c e o f p r o c e s s pa rame te rs on t he

sum o f hea t s t reams Q-j . T a k i n g i n t o a c c o u n t t h a t t h e number o f pa rame te rs

c h a r a c t e r i z i n g t h e s u g a r m a n u f a c t u r i n g p r o c e s s and t h e b y - p r o c e s s e s may be q u i t e

l a r g e , we s h a l l r e s t r i c t o u r t r e a t m e n t t o t h e most i m p o r t a n t pa ramete rs t h a t can

a l s o be c o n s i d e r e d a d j u s t a b l e . B e f o r e d i s c u s s i n g t h e d e t a i l s , h o w e v e r , l e t us

ment ion some l i m i t a t i o n s o f t h i s a p p r o a c h .

T h e r e i s much t r u t h i n t h e s a y i n g t h a t s u g a r i s e s s e n t i a l l y p r o d u c e d i n t h e

b e e t f i e l d s and t he f a c t o r y i s o n l y p r o c e s s i n g i t . The i n f l u e n c e o f b e e t g r o w i n g

on t he f a c t o r y ' s h e a t demand i s a t l e a s t t w o f o l d :

( i ) An i n c r e a s e d s u g a r c o n t e n t o f b e e t s i s e q u i v a l e n t t o a r e l a t i v e r e d u c t i o n o f

t h e i n t a k e o f w a t e r and n o n - s u g a r s t o t h e s u g a r m a n u f a c t u r i n g p r o c e s s . As t h e

s u g a r c o n t e n t i s i n c r e a s e d f rom 16 t o 19%, a h e a t s a v i n g ( p e r 1 kg s u g a r

p r o d u c e d ) o f t h e o r d e r o f 6% can be o b t a i n e d .

( i i ) A r e d u c e d c o n t e n t o f n o n - s u g a r s i n b e e t s , t h a t i s , a h i g h e r j u i c e p u r i t y ,

p o s i t i v e l y a f f e c t s t h e mass b a l a n c e s o f t h e p u r i f i c a t i o n and c r y s t a l l i z a t i o n

p r o c e s s e s . As r a w - j u i c e p u r i t y i s i n c r e a s e d f rom 88.5 t o 90%, t h e r e s u l t i n g h e a t

s a v i n g ( p e r 1 kg b e e t ) amounts t o abou t 3%.

U n d e r s t a n d a b l y e n o u g h , d e s p i t e t h i s i n t e r e s t i n g e n e r g y - s a v i n g p o t e n t i a l , i t

i s i m p o s s i b l e t o a d j u s t t he b e e t p r o p e r t i e s q u i c k l y by t a k i n g some t e c h n i c a l l y -

o r i e n t e d measures . T h e r e f o r e , t h e p rob lems o f b e e t g r o w i n g must be c o n s i d e r e d

as be ing beyond t h e scope o f t h e p r e s e n t book .

The e n e r g y demands o f s u g a r manu fac tu re may v a r y c o n s i d e r a b l y , depend ing on

t he t y p e and q u a l i t y o f t he s u g a r . When c o n s i d e r i n g t h e e n t i r e f a c t o r y , o t h e r

f a c t o r s o f impo r tance a r e t h e t y p e and q u a l i t y o f t h e b y - p r o d u c t s , w h e t h e r o r

n o t t h i c k - j u i c e s t o r a g e f o r s u b s e q u e n t p r o c e s s i n g i s e m p l o y e d , w h e t h e r o r n o t

s y r u p s a r e s o l d , e t c . I n t h e c o n t e m p o r a r y s u g a r i n d u s t r y , w h i t e s u g a r

manu fac tu re and t h e c l a s s i c a l o p e r a t i o n a r e d o m i n a n t , b e i n g o f t e n accompanied by

the p r o d u c t i o n o f d r i e d p u l p . Ou r t r e a t m e n t w i l l t h e r e f o r e c o n c e n t r a t e on w h i t e

24

s u g a r f a c t o r i e s , w i t h p u l p d r y i n g t aken i n t o a c c o u n t .

R e t u r n i n g now t o t h e p rob lems t h a t a r e w e l l w i t h i n t h e scope o f t h i s b o o k ,

l e t us quo te t h e f o l l o w i n g a c c o u n t g i v e n by S c h i e b l ( r e f . 1) h a l f a c e n t u r y a g o :

" T h e h e a t i n g steam demand can be d e c r e a s e d i f t h e f o l l o w i n g c o n d i t i o n s a r e

s a t i s f i e d :

- t h e steam demand f o r a u x i l i a r y p u r p o s e s and f o r s u g a r wash i n c e n t r i f u g a l s i s

d e c r e a s e d ,

- t h e f a c t o r y i s o p e r a t e d a t low j u i c e d r a f t ,

- a h i g h c o n c e n t r a t i o n o f t h i c k j u i c e i s m a i n t a i n e d .

A t t h e p r e s e n t s t a t e o f deve lopment o f b e e t s u g a r t e c h n o l o g y , t h e r e a r e no o t h e r

p o s s i b i l i t i e s o f d e c r e a s i n g t h e h e a t demand."

T h i n g s have changed c o n s i d e r a b l y s i n c e t h e s e words were p u b l i s h e d . T h e r e i s

a v a s t l i t e r a t u r e d e v o t e d t o t he methods o f r e d u c i n g t he h e a t demand f o r s u g a r

m a n u f a c t u r e , and t he most i m p o r t a n t s o u r c e s w i l l be r e f e r r e d t o i n t h e

f o l l o w i n g .

I n s t e a d o f naming a few s e l e c t e d e n e r g y - s a v i n g m e a s u r e s , as S c h i e b l d i d ( a n d ,

a c t u a l l y , many o t h e r a u t h o r s t o o ) , l e t us adop t a top -down a p p r o a c h , f i r s t

t a k i n g a b r o a d e r v i e w o f t h e f i e l d o f e x i s t i n g p o s s i b i l i t i e s and t h e n t r y i n g t o

d e f i n e s m a l l e r a reas o f s p e c i a l i n t e r e s t . The n o t i o n o f t h e sum o f h e a t s t reams

can be u t i l i z e d as a s t a r t i n g p o i n t . F o r a s p e c i f i c s u g a r f a c t o r y , i t can be

c o n v e n i e n t l y s p l i t i n t o t h r e e components

where Q ] Q i s t h e sum o f h e a t s t reams t h a t a r e n e c e s s a r y f o r c a r r y i n g o u t u n i t

o p e r a t i o n s t o w h i c h hea t must be d e l i v e r e d , l i k e h e a t i n g , e v a p o r a t i o n ,

c r y s t a l l i z a t i o n and d r y i n g ; Q-j^ i s t h e t o t a l hea t s t ream needed t o b a l a n c e h e a t

d i s s i p a t i o n f rom t h e s e o p e r a t i o n s , and Q-j ^ i s t h e t o t a l h e a t s t ream needed t o

h e a t t h e rooms i n t h e f a c t o r y b u i l d i n g s .

The l a s t q u a n t i t y i s d e l i b e r a t e l y , even i f u n t y p i c a l l y , c o n s i d e r e d t o g e t h e r w i t h

t h e p r o c e s s h e a t demand. As f a r as t h e e n e r g y s u p p l y v i a t h e the rma l sys tem i s

c o n c e r n e d , t h e r e i s no r e a s o n f o r a s e p a r a t e t r e a t m e n t because room h e a t i n g i s

a l s o n e c e s s a r y f o r f a c t o r y o p e r a t i o n .

L e t us o b s e r v e t h a t a t d e f i n i t e p r o c e s s p a r a m e t e r s , components Q-j^ and Q-j^

may depend on f a c t o r s t h a t a r e n o t s p e c i f i c t o t h e p r o c e s s , l i k e t h e q u a l i t y o f

the rma l i n s u l a t i o n o r t h e e f f i c i e n c y o f room h e a t i n g e q u i p m e n t . C o n s e q u e n t l y , i t

may be p o s s i b l e t o c u t down Q-j^ and Q - , ^ , t hus r e d u c i n g t h e t o t a l h e a t s t r e a m ,

w i t h o u t i n t r o d u c i n g any changes t o t h e p r o c e s s . E n e r g y s a v i n g s o f t h i s k i n d a r e

t h e e a s i e s t t o a c h i e v e and w i l l be d i s c u s s e d f i r s t .

The component Q-j^ can be c u t down by a v a r i e t y o f t e c h n i q u e s , t h a t can be

g rouped w i t h r e g a r d t o t h e p r i n c i p l e s a c c o r d i n g t o w h i c h t h e e n e r g y i s s a v e d .

L e t us d e f i n e t h r e e g r o u p s :

25

- r e d u c t i o n s o f t he w a t e r i n t a k e t o t h e s u g a r m a n u f a c t u r i n g p r o c e s s ,

- r e d u c t i o n s o f t he w a t e r t r a n s f e r f rom o t h e r f a c t o r y s e c t i o n s t o t h e s u g a r

h o u s e ,

- i n t r o d u c t i o n o f m o d i f i e d o r new e n e r g y - e f f i c i e n t p r o c e s s e s .

T h i s S e c t i o n i s d e v o t e d t o t h e e n e r g y - s a v i n g measures t h a t can be t a k e n

i n d e p e n d e n t l y o f , o r w i t h i n t he f ramework o f , t h e c o n v e n t i o n a l s u g a r

m a n u f a c t u r i n g p r o c e s s . M o d i f i e d o r new p r o c e s s e s a r e d i s c u s s e d i n C h a p t e r 4 .

1.3.2 H e a t i n g o f f a c t o r y b u i l d i n g s

I n t h o s e p a r t s o f t h e f a c t o r y b u i l d i n g s where a c o m f o r t a b l e t e m p e r a t u r e i s

needed f o r w o r k i n g p e o p l e , h e a t i n g may be n e c e s s a r y d u r i n g t h e o p e r a t i n g and

a p a r t o f t he o f f - s e a s o n p e r i o d . W h i l e t h e f a c t o r y i s i n o p e r a t i o n , t h e h e a t

demand Q ^ ^ i s a sma l l f r a c t i o n o f t h e sum o f t h e h e a t s t reams and t h u s n o t v e r y

i m p o r t a n t t o t he d e s i g n o f t h e the rma l s y s t e m , and c o n t r i b u t e s v e r y l i t t l e t o

the n e t h e a t demand o f t h e f a c t o r y . Depend ing on t h e c l i m a t i c c o n d i t i o n s and t h e

p r o c e s s h e a t demand, h o w e v e r , t he h e a t consumpt i on i n room h e a t i n g d u r i n g t h e

e n t i r e h e a t i n g p e r i o d may be r e s p o n s i b l e f o r 4-6% o f t he f a c t o r y ' s t o t a l annua l

e n e r g y c o n s u m p t i o n .

As c u t t i n g down the hea t demand i n room h e a t i n g i n s t a l l a t i o n s r e q u i r e s

s o l v i n g prob lems t h a t a r e n o t s p e c i f i c t o t h e s u g a r i n d u s t r y , no d e t a i l s w i l l be

d i s c u s s e d h e r e . The main p o i n t s t o be c o n s i d e r e d , p a r t i c u l a r l y i n o l d e r

f a c t o r i e s , a r e :

- improvements o f t he the rma l i n s u l a t i o n o f f a c t o r y b u i l d i n g s ,

- r e d u c t i o n o f t h e h e a t l o s s e s f rom t h e h e a t d i s t r i b u t i o n sys tem ( b y i m p r o v i n g

the rma l i n s u l a t i o n o f t h e h o t - w a t e r p i p e s , i n t r o d u c i n g e f f e c t i v e f l o w c o n t r o l

me thods , e t c . ) ,

- i n t r o d u c t i o n o f a u t o m a t i c r o o m - t e m p e r a t u r e c o n t r o l t o p r e v e n t l o c a l

o v e r h e a t i n g , t hus e l i m i n a t i n g h e a t l o s s e s caused by e x c e s s i v e v e n t i l a t i o n .

When s e l e c t i n g s u i t a b l e h e a t s o u r c e s t o c o v e r t h e hea t demand f o r h e a t i n g

p u r p o s e s , use can be made o f w a s t e - h e a t s u p p l i e s t h a t a r e c h a r a c t e r i s t i c o f t h e

s u g a r i n d u s t r y . As shown i n a s t u d y c a r r i e d o u t f o r a Swed ish s u g a r f a c t o r y

( r e f . 2 7 ) , more than 90% o f t h e hea t demand f o r room h e a t i n g d u r i n g o p e r a t i o n s

can be c o v e r e d by s u p p l y i n g h o t c o n d e n s a t e f rom vacuum p a n s . Unde r M i d d l e -

European c l i m a t i c c o n d i t i o n s , t h i s c o r r e s p o n d s t o 1/4 o f t h e f a c t o r y ' s annua l

hea t consumpt ion f o r h e a t i n g p u r p o s e s , o r an e q u i v a l e n t o f 1-1.5% o f t h e t o t a l

annual e n e r g y c o n s u m p t i o n .

U s i n g more advanced t e c h n i q u e s , t he was te h e a t can be accumu la ted d u r i n g

o p e r a t i o n s and u t i l i z e d f o r h e a t i n g d u r i n g t h e o f f - s e a s o n p e r i o d . F o r e x a m p l e ,

warm w a t e r can be s t o r e d u n d e r g r o u n d and l a t e r s u p p l i e d t o a h e a t pump. The

r e a l i z a b l e s a v i n g s have been e s t i m a t e d a t 60-70% o f t h e f a c t o r y ' s annua l h e a t

consumpt ion f o r h e a t i n g p u r p o s e s , t h a t i s , 2 .4-4 .2% o f t h e t o t a l annua l e n e r g y

26

c o n s u m p t i o n . H o w e v e r , t h e economic p o t e n t i a l o f t h i s s o l u t i o n depends h e a v i l y on

f u e l and power p r i c e s , as w e l l as c a p i t a l c o s t . F o r e x a m p l e , a c o m b i n a t i o n o f

cheap f u e l and r e l a t i v e l y c o s t l y power c o u n t e r a c t s t h e p r o f i t a b i l i t y o f h e a t -

pump a p p l i c a t i o n s , j u s t i f y i n g r a t h e r t h e u t i l i z a t i o n , d u r i n g t h e o f f - s e a s o n

p e r i o d , o f a c o n v e n t i o n a l h e a t i n g sys tem i n w h i c h hea t i s g e n e r a t e d by b u r n i n g

f u e l i n a b o i l e r . F o r such a s o l u t i o n t o be c o m p e t i t i v e , a h i g h b o i l e r

e f f i c i e n c y i s r e q u i r e d .

1.3.3 Heat d i s s i p a t i o n f rom t h e p r o c e s s

Heat l o s s e s f rom t h e s u g a r m a n u f a c t u r i n g p r o c e s s t o t h e e n v i r o n m e n t a r e

caused b y :

- mass and h e a t exchange between t h e a tmosphere and f r e e s u r f a c e s o f h i g h -

t e m p e r a t u r e media i n open t a n k s , s i p h o n s and d i s t r i b u t o r s ,

- hea t exchange between t he a i r and t h e s u r f a c e s o f p r o c e s s equ ipment and

p i p i n g .

I n F i g . 1 .13, t h e e s t i m a t e d h e a t d i s s i p a t i o n f rom t h e w a t e r s u r f a c e i n an

open v e s s e l , a t t y p i c a l ambien t t e m p e r a t u r e , i s shown as a f u n c t i o n o f w a t e r

t e m p e r a t u r e ( r e f . 2 8 ) . A s i m i l a r r a t e o f h e a t d i s s i p a t i o n can be e x p e c t e d a t

20000

10 000

- 5000

o ω χ

2000

1000

500

200

water agi tated

\ \

W ( 3ter at rest

40 50 60 70 80

Water temperature (°C)

90 100

F i g . 1.13. Heat l o s s f rom t h e s u r f a c e o f w a t e r i n an open tank ( a f t e r r e f . 2 8 ) .

27

f r e e j u i c e s u r f a c e s i n t h e j u i c e p u r i f i c a t i o n s t a t i o n , w h i l e t h e h e a t t r a n s f e r

between a i r and m a s s e c u i t e s u r f a c e s i n open v e s s e l s i s l e s s i n t e n s i v e . I f no

p r e c a u t i o n s a r e t a k e n , t h e combined e f f e c t o f h e a t l o s s e s f rom t h e f r e e s u r f a c e s

o f p r o c e s s media i n open v e s s e l s and s i p h o n s may e a s i l y a t t a i n a l e v e l

c o r r e s p o n d i n g t o h e a t i n g - s t e a m consumpt ion o f t h e o r d e r 1-3 kg/100 kg b e e t . I t

i s t hus a d v i s a b l e t o m i n i m i z e t h e a r e a o f f r e e c o n t a c t between a t m o s p h e r i c a i r

and h i g h - t e m p e r a t u r e med ia , by p r o p e r l y s h a p i n g t h e equ ipment u n i t s o r a p p l y i n g

s c r e e n s o r c o v e r s mounted on t h e e q u i p m e n t .

Heat l o s s e s f rom t h e s u r f a c e s o f p r o c e s s equ ipment and p i p i n g depend on t h e

t o t a l s u r f a c e a r e a and t he q u a l i t y o f t he rma l i n s u l a t i o n . When s h a p i n g t h e

equ ipment and p i p i n g i n new o r m o d e r n i z e d f a c t o r i e s , c a r e s h o u l d be t aken o f

t he t r a n s p o r t r o u t e s o f h i g h - t e m p e r a t u r e m e d i a , so t h a t t h e o u t e r s u r f a c e a r e a

o f t h e p i p e s i s m i n i m i z e d . Heat l o s s e s w i l l a l s o be reduced i f t h e number o f

ba re v a l v e s and o t h e r p i p i n g components a l o n g t h e t r a n s p o r t r o u t e s i s m i n i m i z e d .

N o r m a l l y , h e a t l o s s e s f rom a b a r e v a l v e a r e l a r g e r t han f rom 1 m o f t h e p i p e .

The p i p e - l e n g t h e q u i v a l e n t o f a h e a t - d i s s i p a t i n g b a r e f l a n g e i s u s u a l l y abou t

0.5 m. I n F i g . 1.14, t h e e s t i m a t e d h e a t d i s s i p a t i o n f rom b a r e p i p e s , a t t y p i c a l

ambient t e m p e r a t u r e , i s shown as a f u n c t i o n o f t h e t e m p e r a t u r e o f t h e f l u i d

c o n t a i n e d i n t h e p i p e s ( r e f . 2 8 ) .

I t i s d i f f i c u l t t o g i v e q u a n t i t a t i v e i n f o r m a t i o n on t he e n e r g y s a v i n g s w h i c h

4000

2000

_ 1000 4 α Q .

α Φ

100 150

Fluid temperature (°C)

200

F i g . 1.14. Heat l o s s f rom t h e s u r f a c e o f a b a r e p i p e ( a f t e r r e f . 2 8 ) .

28

can be a t t a i n e d by i m p r o v i n g t h e the rma l i n s u l a t i o n , as t h i s depends on t h e

i n i t i a l l o s s l e v e l t o w h i c h t h e s a v i n g s must be compared . I n F i g . 1 .15, t h e

r e l a t i o n s h i p between hea t d i s s i p a t i o n and i n s u l a t i o n t h i c k n e s s a t a d e f i n i t e

p i p e d i a m e t e r i s shown ( r e f . 2 8 ) . Unde r g i v e n economic c o n d i t i o n s , t h e r e i s

a t r a d e - o f f between t h e c o s t o f i n s u l a t i o n and t h e c o s t o f e n e r g y l o s t t o t h e

e n v i r o n m e n t . Depending on l o c a l c o n d i t i o n s , t h e e c o n o m i c a l l y j u s t i f i e d l o s s

l e v e l ( t o be a c c o u n t e d f o r when c o n s i d e r i n g i n s u l a t i o n improvemen ts ) may v a r y

( r e f . 2 9 ) . I t can be e s t i m a t e d t h a t t h e " e n e r g y e q u i v a l e n t " o f t h e d i f f e r e n c e

between p o o r and s a t i s f a c t o r y i n s u l a t i o n o f p r o c e s s equ ipment and p i p i n g i s o f

t he o r d e r o f 4-6% o f t h e sum o f t h e h e a t s t r e a m s .

300

25 50 75 100 Insulat ion thickness (mm)

F i g . 1.15. Heat l o s s f rom t h e s u r f a c e o f an i n s u l a t e d p i p e as a f u n c t i o n o f i n s u l a t i o n t h i c k n e s s ( a f t e r r e f . 2 8 ) .

The hea t d i s s i p a t i o n can a l s o be e f f e c t i v e l y r educed by d e c r e a s i n g t h e

t e m p e r a t u r e l e v e l o f t h e p r o c e s s o r i t s s e c t i o n s . The hea t f l u x r e s u l t i n g f rom

f r e e c o n v e c t i o n f rom a s u r f a c e a t a b s o l u t e t e m p e r a t u r e Τ t o an e n v i r o n m e n t

c h a r a c t e r i z e d by a b s o l u t e t e m p e r a t u r e T ^ i s a f u n c t i o n o f t h e d i f f e r e n c e

ΔΤ = Τ - T g , namely

q ^ = Α ^ ( Δ Τ ) ^ · 2 5 ( ^ 5 )

where i s a c o n s t a n t depend ing on g e o m e t r i c f a c t o r s .

P r o v i d i n g t he t e m p e r a t u r e d i f f e r e n c e ΔΤ i s sma l l i n compar i son w i t h t h e s u r f a c e

t e m p e r a t u r e T , t h e r a d i a t i o n h e a t f l u x can be e x p r e s s e d by t h e f o l l o w i n g

a p p r o x i m a t e r e l a t i o n s h i p

= A ^ T V ( 1 . 6 )

where A ^ i s a c o n s t a n t depend ing on s u r f a c e p r o p e r t i e s and g e o m e t r i c f a c t o r s .

29

I t f o l l o w s f rom t h e above f o r m u l a e t h a t i f t h e s u r f a c e t e m p e r a t u r e Τ i s

d e c r e a s e d a t a c o n s t a n t e n v i r o n m e n t t e m p e r a t u r e T ^ , t h e n t h e r e l a t i v e r e d u c t i o n

i n o v e r a l l h e a t f l u x + q^^ i s l a r g e r t h a n t h e r e l a t i v e r e d u c t i o n i n

t e m p e r a t u r e d i f f e r e n c e Τ - T ^ . T a k i n g i n t o a c c o u n t t h a t t he h e a t l o s s r e d u c t i o n

a p p l i e s t o a t o t a l o f 2-3% o f t h e sum o f t h e h e a t s t r e a m s , o n l y a s u b s t a n t i a l

t e m p e r a t u r e r e d u c t i o n i s r e a l l y i n t e r e s t i n g . O b v i o u s l y , any t e m p e r a t u r e changes

must be t r e a t e d c a u t i o u s l y because o f t he r i s k o f i n t e r f e r i n g w i t h t h e p r o c e s s .

I t has been d e m o n s t r a t e d i n p r a c t i c e t h a t t h e t e m p e r a t u r e i n t h e j u i c e

p u r i f i c a t i o n s t a t i o n can be s u b s t a n t i a l l y r e d u c e d . I n s s p e c i f i c f a c t o r y , t h i s

i s p o s s i b l e o n l y t o the e x t e n t w h i c h can be a c c e p t e d f rom t h e p o i n t o f v i e w o f

p u r i f i c a t i o n r e s u l t s . No g e n e r a l p r e s c r i p t i o n s can be g i v e n h e r e , as t h e

p r a c t i c a b l e t e m p e r a t u r e changes a r e c o n s t r a i n e d by such l o c a l f a c t o r s as b e e t

q u a l i t y and i t s v a r i a t i o n s , r e q u i r e d p u r i f i c a t i o n e f f e c t , equ ipment p r o p e r t i e s

and so o n . Under t h e c o n d i t i o n s c h a r a c t e r i s t i c o f Greek s u g a r f a c t o r i e s ,

p o s s i b l e t e m p e r a t u r e a d j u s t m e n t s have been d i s c u s s e d i n t h e l i t e r a t u r e ( r e f . 3 0 ) .

The p u r i f i c a t i o n method c o n s i d e r e d i s a c l a s s i c a l one c o m p r i s i n g p r o g r e s s i v e

p r e - l i m i n g , main l i m i n g , and d o u b l e - s t a g e c a r b o n a t a t i o n and f i l t r a t i o n . I n F i g .

1.16, t he j u i c e t e m p e r a t u r e i s shown as a f u n c t i o n o f t h e a v e r a g e t ime i t t akes

f o r t he j u i c e t o r e a c h c o n s e c u t i v e p r o c e s s s e c t i o n s . A t a g i v e n p r o c e s s i n g

c a p a b i l i t y , t h i s t ime r e p r e s e n t s t h e volume o f p r o c e s s equ ipment and p i p i n g as

40 60 T i m e ( m i n i

100

F i g . 1.16. J u i c e t e m p e r a t u r e v s . t ime i n j u i c e p u r i f i c a t i o n s t a t i o n s : A - w i t h h o t main l i m i n g o n l y , Β - w i t h c o l d and h o t main l i m i n g . 1 - e x t r a c t i o n , 2 -p r e - l i m i n g , 3 - h e a t i n g o f p r e - l i m e d j u i c e , 4 - main l i m i n g , 5 - 1 s t c a r b o n a t a t i o n , 6 - j u i c e h e a t i n g b e f o r e 1 s t f i l t r a t i o n , 7 - 1 s t f i l t r a t i o n , 8 -j u i c e h e a t i n g b e f o r e 2nd c a r b o n a t a t i o n , 9 - 2nd f i l t r a t i o n , 10 - h e a t i n g o f t h i n j u i c e ( a f t e r r e f . 3 0 ) .

30

w e l l a s , by a n o n l i n e a r t r a n s f o r m a t i o n , t h e a r e a o f o u t e r s u r f a c e s o f equ ipment

and p i p i n g . The d iag ram v i s u a l i z i n g t e m p e r a t u r e as a f u n c t i o n o f t ime t h u s

demons t ra tes t h e h e a t d i s s i p a t i o n p o t e n t i a l o f t he j u i c e p u r i f i c a t i o n s t a t i o n .

As can be s e e n , t h i s p o t e n t i a l ( a p p r o x i m a t e l y r e p r e s e n t e d by t h e s u r f a c e a r e a

under t h e r e s p e c t i v e c u r v e ) i s much s m a l l e r i n case Β than i n case A . T h i s i s

a r e s u l t o f t he a p p l i c a t i o n o f main l i m i n g pe r fo rmed e s s e n t i a l l y a t a low

t e m p e r a t u r e , w i t h a smal l h i g h - t e m p e r a t u r e s e c t i o n o n l y . O t h e r c o n t r i b u t i n g

f a c t o r s a r e : s h o r t r e t e n t i o n t ime o f t h e f i l t r a t i o n o p e r a t i o n s , and a r e l a t i v e l y

low t e m p e r a t u r e o f t h e second c a r b o n a t a t i o n . I t i s r e a l i s t i c t o e x p e c t t h a t i n

case B, t he h e a t consumpt ion w i l l be l e s s t han h a l f o f t h a t i n case A . A l t h o u g h

t he e x a c t f i g u r e wou ld depend on l o c a l c o n d i t i o n s , i t can be e s t i m a t e d t h a t t h e

h e a t demand i n j u i c e p u r i f i c a t i o n w i l l be r e d u c e d t o as low a l e v e l as t h e

e q u i v a l e n t o f abou t 5 kg h e a t i n g steam p e r 100 kg b e e t .

A n o t h e r i n t e r e s t i n g s t u d y on p o s s i b l e t e m p e r a t u r e r e d u c t i o n s i n t h e j u i c e

p u r i f i c a t i o n s t a t i o n has been p u b l i s h e d by S o v i e t a u t h o r s ( r e f . 3 1 ) . I t has been

shown i n a s p e c i f i c s u g a r f a c t o r y t h a t t h e a v e r a g e t e m p e r a t u r e can be d e c r e a s e d

i f p r e - c a r b o n a t a t i o n i s i n t r o d u c e d a t 60°C w i t h t h e CaO r a t e abou t 45% o f t h e

t o t a l . A l t h o u g h t he t e m p e r a t u r e o f t h e f i r s t c a r b o n a t a t i o n remains unchanged a t

85°C, t he hea t l o s s o c c u r r i n g t h e r e i s s u b s t a n t i a l l y r e d u c e d due t o a l o w e r CaO

r a t e (40% o f t he t o t a l , a g a i n s t 80% i n t he o r i g i n a l p r o c e s s ) . The t e m p e r a t u r e o f

t h e second c a r b o n a t a t i o n i s d e c r e a s e d f rom 95^C t o 75^C. The r e s u l t i n g r e d u c t i o n

i n t he hea t d i s s i p a t e d f rom t h e j u i c e p u r i f i c a t i o n p r o c e s s t o t h e e n v i r o n m e n t

can be e s t i m a t e d a t abou t 60%.

P o t e n t i a l t e m p e r a t u r e r e d u c t i o n s i n o t h e r s e c t i o n s o f t h e s u g a r m a n u f a c t u r i n g

p r o c e s s a r e l i m i t e d , b u t s t i l l p o s s i b l e . A n o t h e r s t u d y by S o v i e t a u t h o r s ( r e f .

32) i n d i c a t e s t h a t i n a s p e c i f i c f a c t o r y , t h e b o i l i n g t e m p e r a t u r e s o f A , Β and C

m a s s e c u i t e s can be d e c r e a s e d by abou t 5 K, 10 Κ and 15 K, r e s p e c t i v e l y . A l t h o u g h

t h e new t e m p e r a t u r e reg ime i s aimed a t c u t t i n g down the s u g a r l o s s e s caused by

thermal decay o f s u c r o s e i n vacuum p a n s , t h e hea t d i s s i p a t i o n f rom t h e s u g a r

house can a l s o be r e d u c e d .

1.3.4 Water i n t a k e t o t he p r o c e s s

Most o f t he w a t e r s u p p l i e d t o t h e s u g a r m a n u f a c t u r i n g p r o c e s s i n b e e t s o r

o t h e r mass s t reams ( F i g . 1.17) must u l t i m a t e l y be removed , m a i n l y as v a p o u r o r

as h o t c o n d e n s a t e . The a s s o c i a t e d hea t e x p e n d i t u r e i s a p p r o x i m a t e l y p r o p o r t i o n a l

t o t he amount o f w a t e r s u p p l i e d . I f t h i s amount i s r e d u c e d , and p r o v i d i n g t h e r e

i s no s e r i o u s i n t e r f e r e n c e w i t h t h e p r o c e s s , t hen e n e r g y s a v i n g s can be

o b t a i n e d . The most i m p o r t a n t measures based on t h i s p r i n c i p l e a r e :

- r e d u c t i o n o f t he j u i c e d r a f t ,

- e l i m i n a t i o n o f u n n e c e s s a r y w a t e r a d d i t i o n s t o t h e j u i c e , p a r t i c u l a r l y i n t h e

j u i c e p u r i f i c a t i o n s t a t i o n .

31

water

cosset tes pressed pulpl

..LX sludge

vapour vapour

sugar to d r y e r

molasses

condensate

F i g . 1.17. Main s t reams o f w a t e r and w a t e r - c o n t a i n i n g media e n t e r i n g o r l e a v i n g a s u g a r f a c t o r y . 1 - p r o c e s s a r e a , 2 - e x t r a c t i o n s t a t i o n , 3 - j u i c e p u r i f i c a t i o n s t a t i o n , 4 - e v a p o r a t o r , 5 - s u g a r h o u s e , 6 - c o n d e n s a t e t a n k , 7 - c o n d e n s e r .

- r e d u c t i o n o f t h e w a t e r i n t a k e t o t h e s u g a r h o u s e .

I t i s a l s o p o s s i b l e t o r educe t h e t o t a l w a t e r i n t a k e by i n t r o d u c i n g new

p r o c e s s e s ; e n e r g y - s a v i n g measures o f t h i s k i n d a r e c o n s i d e r e d i n C h a p t e r 4 .

The j u i c e d r a f t , i . e . t h e r a t i o o f r a w - j u i c e f l o w t o c o s s e t t e s f l o w , c a n n o t

be j u d g e d on t he b a s i s o f hea t e x p e n d i t u r e o n l y , as i t i s c e r t a i n l y one o f t h e

most i m p o r t a n t v a r i a b l e s g o v e r n i n g t h e e x t r a c t i o n p r o c e s s , i n f l u e n c i n g a l s o t h e

s u g a r l o s s i n e x h a u s t e d c o s s e t t e s . The s u g a r l o s s depends a l s o on b e e t q u a l i t y ,

c o s s e t t e s q u a l i t y , and on pH and t e m p e r a t u r e d i s t r i b u t i o n s i n t h e e x t r a c t o r , and

t h o s e f a c t o r s may v a r y d u r i n g t he o p e r a t i n g p e r i o d . I t may t h e r e f o r e be

n e c e s s a r y t o v a r y t h e j u i c e d r a f t so as t o keep t h e s u g a r l o s s a t an a c c e p t a b l y

low l e v e l , and t h i s r e q u i r e m e n t may e v e n t u a l l y c l a s h w i t h t h e e n e r g y - b a s e d

r e q u i r e m e n t o f d r a f t m i n i m i z a t i o n .

I f t h e e n e r g y s a v i n g s a r e l a r g e e n o u g h , t h e n t he l o c a l economic c o n d i t i o n s

may s t i m u l a t e d r a f t r e d u c t i o n even a t t h e c o s t o f i n c r e a s e d s u g a r l o s s

( p a r t i c u l a r l y i f s u g a r r e t e n t i o n i n c r e a s e s t h e v a l u e o f e x h a u s t e d c o s s e t t e s s o l d

as f o d d e r ) . T h i s s i t u a t i o n has been a n a l y s e d f o r t r o u g h - and t o w e r - t y p e

e x t r a c t o r s ( r e f . 3 3 ) . Assuming a s u g a r c o n t e n t i n p r e s s e d p u l p o f abou t 2%, i . e .

a s u g a r l o s s abou t 0.6%, j u i c e d r a f t v a l u e s as low as 95.5% f o r a t r o u g h - t y p e

e x t r a c t o r and 98.6% f o r a t o w e r - t y p e e x t r a c t o r have been c o n s i d e r e d . The

r e s u l t i n g e n e r g y s a v i n g s have been e s t i m a t e d a t 5-10% o f t h e f a c t o r y ' s demand

f o r p r i m a r y e n e r g y .

A c t u a l l y , t he p r i c e s o f f u e l s and s u g a r seem t o s t i m u l a t e f a c t o r y o p e r a t i o n

a t an e x t r a c t i o n l o s s s u b s t a n t i a l l y l o w e r t han t h e above v a l u e , so t h e j u i c e

d r a f t s h o u l d be reduced by m o d i f y i n g t h e e x t r a c t i o n p r o c e s s . T h i s depends m a i n l y

on equ ipment a d j u s t m e n t s ( s e e C h a p t e r 5 ) .

32

The w a t e r i n t a k e t o t h e j u i c e p u r i f i c a t i o n s t a t i o n r e s u l t s m a i n l y f rom m i l k -

o f - l i m e a d d i t i o n s t o t he j u i c e and c a r b o n a t a t i o n s l u d g e s w e e t e n i n g - o f f i n t h e

f i l t e r s . A t y p i c a l m i l k - o f - l i m e c o m p o s i t i o n i s 20% CaO and 80% w a t e r , and t h e

CaO r a t e i s o f t h e o r d e r o f 2 kg p e r 1 kg b e e t . The w a t e r i n t a k e i n t h e m i l k - o f -

l ime can be m i n i m i z e d by m a i n t a i n i n g a low CaO r a t e and h i g h CaO c o n c e n t r a t i o n ,

b u t under no c i r c u m s t a n c e s can a r e d u c t i o n o f t h e j u i c e p u r i f i c a t i o n e f f e c t be

a l l o w e d . T h e r e f o r e , a p r e r e q u i s i t e f o r t h i s k i n d o f e n e r g y s a v i n g i s t o a p p l y

p r o p e r l y d e s i g n e d p r o c e s s equ ipment and e f f e c t i v e a u t o m a t i c c o n t r o l o f key

p r o c e s s v a r i a b l e s . U n n e c e s s a r y w a t e r can a l s o be l a r g e l y e l i m i n a t e d i f l i m e

s l a k i n g i s pe r fo rmed u s i n g j u i c e t apped f rom a p r o p e r l y s e l e c t e d p l a c e i n t h e

j u i c e p u r i f i c a t i o n s t a t i o n ( t y p i c a l l y , j u i c e s e p a r a t e d f rom s u b s i d e r s l u d g e i s

u s e d ) .

The s i t u a t i o n w i t h t he s l u d g e s w e e t e n i n g - o f f i s t o some e x t e n t s i m i l a r t o

t h a t w i t h c o s s e t t e s e x h a u s t i o n , namely t h a t t h e r e i s a t r a d e - o f f between t h e

amount o f w a t e r s u p p l i e d and t h e s u g a r l o s s i n s l u d g e . Depend ing on t h e

f i l t r a t i o n scheme and equ ipment u s e d , w a t e r i n t a k e can be l i m i t e d t o 2-4 kg p e r

100 kg b e e t , t h i s r e s u l t i n g i n a f i n a l s u g a r c o n t e n t o f t h e s l u d g e be low

0 .5-0 .7%.

Water i n t a k e t o t h e s u g a r house may be r e q u i r e d f o r :

- d i l u t i o n o f s y r u p s , when t h e p r o c e s s r e q u i r e m e n t s a r e t h a t t h e i r c o n c e n t r a t i o n

has t o be d e c r e a s e d t o a d e f i n i t e v a l u e ,

- s u g a r m e l t i n g ,

- s u g a r wash i n c e n t r i f u g a l s ,

- magma c o n c e n t r a t i o n c o n t r o l d u r i n g s u g a r b o i l i n g i n vacuum p a n s ,

- v i s c o s i t y c o n t r o l d u r i n g C - m a s s e c u i t e c r y s t a l l i z a t i o n ,

- i o n - e x c h a n g e p r o c e s s e s .

V a r i o u s components o f t h e w a t e r i n t a k e , and t h e i r p r o p o r t i o n s , depend on t h e

c r y s t a l l i z a t i o n scheme a p p l i e d . As t h e q u a l i t y o f t h i c k j u i c e and t h e p r o p e r t i e s

o f n o n - s u g a r s v a r y d u r i n g o p e r a t i o n s , i t may be n e c e s s a r y t o a d j u s t t h e s e

components a c c o r d i n g l y . F o r a g i v e n c r y s t a l l i z a t i o n scheme, s p e c i f i c measures

can be taken t o m i n i m i z e t h e t o t a l w a t e r i n t a k e .

The need f o r t h e d i l u t i o n o f s y r u p s u s u a l l y r e s u l t s f rom t h e f i l t r a t i o n

r e q u i r e m e n t s . F o r e x a m p l e , r e m e l t f i l t r a t i o n i s t y p i c a l l y pe r f o rmed a t a d r y

s u b s t a n c e c o n t e n t be low 68%, and i f t h e r e m e l t c o n c e n t r a t i o n exceeds t h i s v a l u e ,

t hen w a t e r must be added . Howeve r , t h e c r y s t a l l i z a t i o n schemes can be so

d e s i g n e d as t o m i n i m i z e o r even e l i m i n a t e t h e need f o r w a t e r a d d i t i o n t o s y r u p s

i n normal o p e r a t i n g c o n d i t i o n s . I t i s a l s o p o s s i b l e t o e l i m i n a t e t h e w a t e r

i n t a k e t o t h e m e l t i n g o p e r a t i o n , u s i n g t h i n j u i c e i n s t e a d .

The w a t e r i n t a k e t o t he vacuum pans can be reduced t o a n e g l i g i b l y sma l l

v a l u e , p r o v i d i n g o t h e r measures a r e t aken t o s e c u r e e f f i c i e n t s u p e r s a t u r a t i o n

33

c o n t r o l d u r i n g t he s u g a r b o l i n g p r o c e s s . To some e x t e n t , t h i s depends on t h e

c r y s t a l l i z a t i o n scheme, b u t vacuum pan au toma t i on ( a u t o m a t i c b o i l i n g c o n t r o l )

seems t o be a d e c i s i v e f a c t o r . T h i s p rob lem i s a d d i t i o n a l l y d i s c u s s e d i n S e c t i o n

1.3.5 and C h a p t e r 6.

The amount o f w a t e r s u p p l i e d t o b a t c h c e n t r i f u g a l s can be o p t i m i z e d w i t h

r e s p e c t t o c r y s t a l l i z a t i o n n e e d s , t h a t i s , f o r maximum y i e l d o f c r y s t a l l i n e

s u g a r o f a d e f i n i t e q u a l i t y ( r e f s . 3 4 , 3 5 ) . A p r e r e q u i s i t e f o r t h e o p t i m i z a t i o n

i s t h a t t h e w a t e r wash o p e r a t i o n s h o u l d be t r e a t e d as a p a r t o f t h e c e n t r i f u g i n g

c y c l e i n w h i c h t i m e , r o t a t i o n a l v e l o c i t y and t h e a p p l i c a t i o n o f s y r u p wash can

a l s o c o n t r i b u t e t o t he f i n a l r e s u l t s . The w a t e r wash o p t i m i z e d i n t h i s manner

can a l s o be c o n s i d e r e d as o p t i m a l w i t h r e s p e c t t o t h e e n e r g y consumpt ion o f t h e

s u g a r h o u s e . E x p e r i e n c e p r o v e s , h o w e v e r , t h a t t h e o p t i m i z a t i o n r e s u l t s may be

v e r y s e n s i t i v e t o l o c a l c o n d i t i o n s . An example o f an o p t i m i z e d c y c l e f o r

A m a s s e c u i t e c e n t r i f u g i n g i n a s p e c i f i c b a t c h c e n t r i f u g a l i s shown i n F i g . 1.18.

Water i n t a k e t o C m a s s e c u i t e c r y s t a l 1 i z e r s t y p i c a l l y s e r v e s t h e p u r p o s e o f

b r i n g i n g down m a s s e c u i t e v i s c o s i t y . As t h e v i s c o s i t y a l s o depends on t h e

t e m p e r a t u r e , i t i s a l s o p o s s i b l e t o o b t a i n a v i s c o s i t y r e d u c t i o n by i n c r e a s i n g

the t e m p e r a t u r e o f t h e m a s s e c u i t e . T h i s app roach has i t s l i m i t a t i o n s , h o w e v e r ,

as t o o h i g h a t e m p e r a t u r e may a d v e r s e l y a f f e c t c r y s t a l l i z a t i o n e f f i c i e n c y .

A n o t h e r v i s c o s i t y - c o n t r o l method w h i c h r e d u c e s t h e w a t e r i n t a k e employs t h e

a d d i t i o n o f mo lasses t o t he m a s s e c u i t e .

I t s h o u l d be p o i n t e d o u t t h a t a n e t w a t e r i n t a k e may a l s o r e s u l t f rom d i r e c t

use o f steam i n t he s u g a r h o u s e . T h i s a p p l i e s t o such o p e r a t i o n s a s :

- vacuum-pan s t e a m i n g ,

- steam wash i n c e n t r i f u g a l s ,

- d i r e c t h e a t i n g o f s y r u p s i n s t o r a g e t a n k s .

1200 Γ

α 1000

800

600

400

200 o

er

^ / ii \ : J I I! 1 114 41-3 ι

I I I I . I I 1 I

\ ^ 4 ^.5

D 60 120 T ime ( s )

180

g r e e n w a s h

F i g . 1.18. O p t i m i z e d c e n t r i f u g i n g c y c l e f o r A m a s s e c u i t e . 1 - c h a r g i n g , 2 -s y r u p w a s h , 3 - w a t e r w a s h , 4 - d i s c h a r g i n g , 5 - c l e a n i n g ( c o u r t e s y D O S ) .

34

T h e r e i s no doub t t h a t t h e s e o p e r a t i o n s s h o u l d be a v o i d e d , o r t h e equ ipment and

i n s t r u m e n t a t i o n s h o u l d make i t p o s s i b l e t o m i n i m i z e t h e steam c o n s u m p t i o n . F o r

e x a m p l e , t h i s can be done w i t h modern a i r - t i g h t s teaming sys tems w h i c h can a l s o

be i n s t a l l e d i n o l d e r vacuum p a n s .

1.3.5 T h i c k j u i c e c o n c e n t r a t i o n and c r y s t a l l i z a t i o n scheme

I t was ment ioned i n S e c t i o n 1.3.1 t h a t t h e hea t demand o f t h e c r y s t a l l i z a t i o n

p r o c e s s can be d e c r e a s e d by r e d u c i n g t h e w a t e r i n t a k e t o t h e s u g a r h o u s e . T h i s

i s a complex q u e s t i o n , o f w h i c h o n l y a p a r t b e l o n g s t o t h e p rob lem f i e l d

d i s c u s s e d i n t h e p r e c e d i n g S e c t i o n . The r e m a i n i n g p a r t s a r e :

- m a i n t a i n i n g a h i g h t h i c k - j u i c e c o n c e n t r a t i o n ,

- o p t i m i z i n g t he scheme and pa ramete rs o f t h e c r y s t a l l i z a t i o n p r o c e s s .

F i g u r e 1.19 shows t h e e s t i m a t e d r e l a t i o n s h i p between t h e hea t demand o f

a s u g a r house emp loy ing t h e c l a s s i c a l t h r e e - b o i l i n g scheme, and t h e

c o n c e n t r a t i o n o f t h i c k j u i c e . When r e d u c i n g t h e w a t e r i n t a k e i n t he t h i c k - j u i c e

s t ream by 50%, t h a t i s , f rom 11.60 t o 5.80 kg p e r 100 kg b e e t , t h e h e a t demand

can be d e c r e a s e d by n e a r l y 1/3 o f i t s i n i t i a l v a l u e (assumed t o c o r r e s p o n d t o

65% D S ) . T h i s i s an i n d i c a t i o n o f a c o n s i d e r a b l e e n e r g y - s a v i n g p o t e n t i a l

a s s o c i a t e d w i t h p o s s i b l e a d j u s t m e n t s o f t h e t h i c k - j u i c e c o n c e n t r a t i o n . H o w e v e r ,

i t s u t i l i z a t i o n depends on w h e t h e r o r n o t c e r t a i n c o n s t r a i n t s can be met .

W a t e r in t h i c k j u i c e ( kg /100 kg b )

11 10 9 8 7

62 64 66 68 70 72 74

C o n c e n t r a t i o n o f t h i c k j u i c e ( % D S )

F i g . 1.19. Heat demand o f t h e t h r e e - b o i l i n g c r y s t a l l i z a t i o n p r o c e s s , as a f u n c t i o n o f t h i c k - j u i c e c o n c e n t r a t i o n .

The d i f f i c u l t i e s appea r a l r e a d y i n t h e e v a p o r a t i o n p r o c e s s , as i n a s p e c i f i c

m u l t i p l e - e f f e c t e v a p o r a t o r , i n c r e a s e d o u t l e t c o n c e n t r a t i o n may r e q u i r e p r o l o n g e d

j u i c e r e t e n t i o n t i m e , and t h e f i n a l e f f e c t s have t o be o p e r a t e d a t i n c r e a s e d d r y

s u b s t a n c e c o n t e n t . T h i s i n d u c e s t h e r i s k o f e x c e s s i v e the rma l decay o f s u c r o s e

and c o l o u r b u i l d - u p , as w e l l as dangerous i n c r u s t a t i o n s w h i c h may be i n i t i a t e d

35

by c o n c e n t r a t i o n f l u c t u a t i o n s i n t h e l a s t e f f e c t . When a t t e m p t i n g t o i n c r e a s e

t h e c o n c e n t r a t i o n o f t h i c k j u i c e , e v a p o r a t o r d e s i g n and t e m p e r a t u r e l e v e l s i n

a l l e f f e c t s , as w e l l as t h e e v a p o r a t o r c o n t r o l s y s t e m , s h o u l d be r e v i e w e d a n d ,

i f n e c e s s a r y , m o d i f i e d f o r s a f e r o p e r a t i o n . E x p e r i e n c e p r o v e s t h a t i t i s n o t

o n l y i n t he i n i t i a l e f f e c t s o p e r a t e d a t h i g h e s t t e m p e r a t u r e s , b u t a l s o i n t h e

f i n a l e f f e c t s , where t h e s i t u a t i o n may become c r i t i c a l w i t h r e s p e c t t o c o l o u r

b u i l d - u p . T h i s may n e c e s s i t a t e r e p l a c i n g t h e e v a p o r a t o r b o d i e s c o n c e r n e d by new

ones e n s u r i n g reduced j u i c e r e t e n t i o n t i m e .

A n o t h e r p rob lem i s a s s o c i a t e d w i t h t h i c k - j u i c e f i l t r a t i o n a t c o n c e n t r a t i o n s

e x c e e d i n g 65-67% DS. C o n v e n t i o n a l f i l t e r s a r e n o t s u i t e d t o h i g h e r

c o n c e n t r a t i o n s and s h o u l d pe rhaps be r e p l a c e d by c e n t r i f u g a l s e p a r a t o r s , wh i ch

have p r o v e d t o work s a t i s f a c t o r i l y i n numerous f a c t o r i e s . T h e r e i s a l s o

a p o s s i b i l i t y o f a p p l y i n g c o n v e n t i o n a l f i l t e r s , n o t n e x t t o t h e e v a p o r a t o r

o u t l e t bu t p r i o r t o t h e l a s t e v a p o r a t o r e f f e c t ; one p o s s i b l e a r rangemen t i s

shown i n F i g . 1.20 ( r e f . 3 6 ) .

e x h a u s t s t e a m

thin j u i ce ϊ [ U % D S . 125°C

r t 3a

Γ 5 5 ^ 6 0 % D S ^

110 °C

3b

] Γ t h i ck ju ice 7 5 % D S . 9 6 %

F i g . 1.20. J u i c e f i l t e r F between e v a p o r a t o r b o d i e s i n t h e t h i r d e f f e c t o f a " f o u r - a n d - a - h a l f - e f f e c t " e v a p o r a t o r ( a f t e r r e f . 3 6 ) .

As r e g a r d s t he o p t i m i z a t i o n o f t h e c r y s t a l l i z a t i o n scheme and p a r a m e t e r s ,

t h i s i s n e c e s s i t a t e d by i n c r e a s e d c o n c e n t r a t i o n s o f t h i c k j u i c e and s y r u p s

because new prob lems a r e c r e a t e d i n t h e b o i l i n g p r o c e s s . Wh i te s u g a r b o i l i n g i n

a b a t c h - t y p e vacuum pan can be r o u g h l y d i v i d e d i n t o t h r e e s t a g e s ( F i g . 1 . 2 1 ) :

( 1 ) E v a p o r a t i o n o f t he i n i t i a l l y drawn u n d e r s a t u r a t e d s o l u t i o n u n t i l a

s u p e r s a t u r a t i o n o f abou t 1.15 i s a t t a i n e d . A h i g h c o n c e n t r a t i o n o f t h i c k j u i c e

i s f a v o u r a b l e w i t h r e s p e c t t o e n e r g y economy, as i t r e s u l t s i n l e s s h e a t

consumed d u r i n g t h i s s t a g e .

( 2 ) Seed ing and c r y s t a l f o r m a t i o n , f o l l o w e d by s l o w c r y s t a l g r o w t h as t h e vacuum

pan i s l o a d e d t o i t s maximum c h a r g e . I n o r d e r t o m a i n t a i n t h e i n t e n s i v e

c i r c u l a t i o n r e q u i r e d f o r u n i f o r m s y r u p s u p e r s a t u r a t i o n and u n i f o r m magma

s t r u c t u r e , t he e v a p o r a t i o n i s c o n t i n u e d w i t h accompany ing i n t a k e s o f

u n d e r s a t u r a t e d s o l u t i o n s . T h e r e i s a r i s k t h a t t h e t h i c k j u i c e c o n c e n t r a t i o n may

36

S t a g e s :

F i g . 1.21. S tages o f t h e w h i t e - s u g a r b o i l i n g p r o c e s s : 1 - e v a p o r a t i o n , 2 -s e e d i n g , c r y s t a l f o r m a t i o n and s l o w c r y s t a l g r o w t h , 3 - a c c e l e r a t e d c r y s t a l g r o w t h .

t u r n o u t t o be j u s t t o o h i g h f o r i n t a k e s meant t o b r i n g t h e s u p e r s a t u r a t i o n down

t o t he r e q u i r e d v a l u e .

( 3 ) A c c e l e r a t e d c r y s t a l g r o w t h as t h e s t r i k e i s t h i c k e n e d u n t i l t h e d e s i r e d

c r y s t a l c o n t e n t has been r e a c h e d . An e v e n t u a l i n t a k e o f h i g h - c o n c e n t r a t i o n t h i c k

j u i c e c o u l d a g a i n be f a v o u r a b l e .

Because o f t h e d i f f i c u l t i e s w h i c h may o c c u r i n s t a g e ( 2 ) , t h e i n t r o d u c t i o n o f

h i g h - c o n c e n t r a t i o n t h i c k j u i c e and s y r u p s c a n n o t be r e g a r d e d as a m ino r

m o d i f i c a t i o n o f t h e c r y s t a l l i z a t i o n p r o c e s s . The s u p e r s a t u r a t i o n can o f c o u r s e

be a d j u s t e d by t a k i n g i n w a t e r , b u t t h i s wou ld c o u n t e r a c t h e a t s a v i n g s . The

p rob lem becomes l e s s c r i t i c a l when s t i r r e d vacuum pans a r e u s e d , making i t

e a s i e r t o m a i n t a i n u n i f o r m s u p e r s a t u r a t i o n i n t h e e n t i r e s t r i k e vo lume .

O p e r a t i o n a l s a f e t y can be f u r t h e r improved by e q u i p p i n g t h e pans w i t h a u t o m a t i c

b o i l i n g c o n t r o l s . The b e s t s o l u t i o n seems t o be t o r e l y on c r y s t a l f o o t i n g ,

w h i c h can h a r d l y be r e g a r d e d as a l i m i t e d m o d i f i c a t i o n b u t r a t h e r as a new

p r o c e s s ; i t i s t h e r e f o r e d i s c u s s e d i n C h a p t e r 4 .

I f t he f o o t i n g p r o c e s s t u r n s o u t t o be t o o advanced w i t h r e g a r d t o t h e s u g a r

house equ ipment a v a i l a b l e , t hen t he c r y s t a l l i z a t i o n scheme w i t h two j u i c e

c o n c e n t r a t i o n s can be a p p l i e d ( r e f . 3 7 ) . I t s e s s e n t i a l i d e a c o n s i s t s o f

t h i c k e n i n g a p a r t o f t h e j u i c e f l o w t o a c o n c e n t r a t i o n e x c e e d i n g 70% DS, w h i l e

t h e r ema in i ng p a r t i s t apped i m m e d i a t e l y a f t e r f i l t r a t i o n , t h a t i s , a t

65-67% DS. H i g h - c o n c e n t r a t i o n j u i c e i s used f o r i n t a k e s d u r i n g t h e f i r s t and

t h i r d s t a g e s o f t h e b o i l i n g p r o c e s s , w h i l e t h e a v a i l a b i l i t y o f l o w - c o n c e n t r a t i o n

j u i c e makes i t e a s i e r t o p e r f o r m t h e c r i t i c a l second s t a g e . I t s h o u l d be made

c l e a r t h a t t h i s method does n o t a l l o w f u l l u t i l i z a t i o n o f t h e h e a t - s a v i n g

p o t e n t i a l o f t he c o n c e n t r a t i o n o f t h i c k j u i c e . I t has been r e p o r t e d t h a t i f

a h a l f o f t he t h i c k - j u i c e s t ream a f t e r f i l t r a t i o n i s t h i c k e n e d t o 74% DS and

t h e o t h e r h a l f remains a t 67% DS, t h e n t h e f a c t o r y ' s steam demand can be r e d u c e d

by abou t 2 kg/100 kg b ( r e f . 3 8 ) . I f t h e e n t i r e t h i c k - j u i c e s t r eam was t h i c k e n e d

37

t o 74% DS, t hen a c c o r d i n g t o F i g . 1.18 a 15% r e d u c t i o n , t h a t i s , by

3.2 kg/100 kg b , w o u l d be p o s s i b l e .

1.4 POWER DEMAND

1.4.1 Scope o f t h e p rob lems

Wi th combined g e n e r a t i o n o f h e a t and e l e c t r i c i t y , t h e l a r g e r t h e steam f l o w

e x t r a c t e d f rom t h e t u r b i n e e x h a u s t , t h e more power can be p roduced i n t h e

e l e c t r i c a l g e n e r a t o r . I n a s u g a r f a c t o r y e q u i p p e d w i t h a p r o p e r l y d i m e n s i o n e d

t u r b o - g e n e r a t o r , and where t h e h e a t consump t i on e x c e e d s t h e e q u i v a l e n t o f

40-45 kg steam p e r 100 kg b e e t , t h e e l e c t r i c a l power s h o u l d be amp le .

The t r e n d towards f u e l s a v i n g s i m p l i e s a r e d u c e d steam f l o w t h r o u g h t h e

t u r b i n e . A t t h e same t i m e , economic f a c t o r s s t i m u l a t e a t e n d e n c y t o mechan ize

and automate a l l t h e u n i t o p e r a t i o n s i n t h e s u g a r i n d u s t r y , t h i s r e s u l t i n g i n

i n c r e a s e d power demand. The i n t r o d u c t i o n o f new e l e c t r i c i t y - c o n s u m i n g equ ipmen t

needed f o r e n v i r o n m e n t a l p r o t e c t i o n , l i k e s l u d g e p r e s s e s , w a s t e - w a t e r t r e a t m e n t

p l a n t s , e t c . , o r t h e a p p l i c a t i o n o f mechan ica l v a p o u r c o m p r e s s o r s , may a l s o

i n c r e a s e t h e power demand. C o n s e q u e n t l y , an i n c r e a s i n g number o f s u g a r f a c t o r i e s

a r e r e a c h i n g t h e p o i n t o f imba lance between t h e steam demand and power demand,

and s p e c i a l measures may be r e q u i r e d t o s e c u r e a r e l i a b l e e n e r g y s u p p l y . I t t hus

becomes i n c r e a s i n g l y i m p o r t a n t t o have s u f f i c i e n t i n f o r m a t i o n on t h e r e l a t i o n

between t h e power demand and t h e f a c t o r y ' s a b i l i t y t o g e n e r a t e i t s own power .

From t h e p o i n t o f v i e w o f e n e r g y b a l a n c e s , a d e s c r i p t i o n o f t h e power demand i n

terms o f t i m e - a v e r a g e d f i g u r e s i s n e e d e d . F o r t h e d i m e n s i o n i n g o f p o w e r -

g e n e r a t i n g and p o w e r - d i s t r i b u t i n g equ ipment and f o r r e l i a b l e c o n t r o l o f t h e

o p e r a t i o n o f t h e e n t i r e e l e c t r i c a l s u b s y s t e m , t h e i n s t a n t a n e o u s demand and i t s

v a r i a t i o n s s h o u l d a l s o be d e f i n e d .

1.4.2 Power ne twork

The e l e c t r i c i t y i s p r o d u c e d as a l t e r n a t i n g c u r r e n t i n a t h r e e - p h a s e s u p p l y .

When compared t o t h e d i r e c t - c u r r e n t i n s t a l l a t i o n s s t i l l i n use i n many

f a c t o r i e s , m o s t l y f o r h i s t o r i c a l r e a s o n s , a l t e r n a t i n g - c u r r e n t sys tems o f f e r t h e

advan tages o f :

- easy t r a n s f o r m a t i o n f rom h i g h t o low v o l t a g e ,

- economic s o l u t i o n f o r power t r a n s m i s s i o n and u t i l i z a t i o n i n l a r g e and medium-

s i z e u n i t s ,

- low c o s t o f t h e m o t o r s .

The s y n c h r o n o u s t h r e e - p h a s e g e n e r a t o r s employ t h r e e g r o u p s o f f i x e d w i n d i n g s

i n w h i c h t h e a l t e r n a t i n g c u r r e n t i s i n d u c e d , and a r o t a t i n g w i n d i n g s u p p l i e d

w i t h d i r e c t c u r r e n t f rom a dynamo ( e x c i t e r ) s i t u a t e d a t t h e end o f t h e r o t o r

s h a f t . The power o u t p u t o f t h e g e n e r a t o r i s c o n t r o l l e d by t h e e x c i t e r c u r r e n t .

S m a l l e r t u r b o - g e n e r a t o r s a r e g e n e r a l l y o f g e a r e d t y p e : t h e t u r b i n e runs a t up t o

38

20 000 rpm a n d , t h r o u g h r e d u c t i o n g e a r s , d r i v e s a g e n e r a t o r r u n n i n g a t 1500 rpm.

A t h i g h e r o u t p u t r a t i n g s , t h e t u r b i n e runs a t 3000 rpm w i t h d i r e c t d r i v e t o

a g e n e r a t o r .

I n o r d e r t o m in im i ze e n e r g y l o s s e s , e l e c t r i c i t y g e n e r a t i o n and t r a n s m i s s i o n

s h o u l d be pe r fo rmed a t low c u r r e n t a n d , c o r r e s p o n d i n g l y , h i g h v o l t a g e . T y p i c a l

v o l t a g e l e v e l s employed a r e s e v e r a l t housands v o l t s . The t r a n s f o r m a t i o n t o t h e

v o l t a g e l e v e l o f motors and o t h e r power r e c e i v e r s ( i . e . power -consuming d e v i c e s )

t akes p l a c e i n t r a n s f o r m e r s w h i c h a r e c o n v e n i e n t l y p l a c e d w i t h i n c l o s e range o f

t h e r e c e i v e r s . The h i g h - v o l t age p a r t o f t he power ne twork i s s e p a r a t e d by

c i r c u i t - b r e a k e r s f rom e n e r g y s o u r c e s ( t h a t i s , g e n e r a t o r s o r s u p p l y l i n e s f rom

t h e e x t e r n a l g r i d ) and t r a n s f o r m e r s . The l o w - v o l t a g e w i n d i n g o f a t r a n s f o r m e r i s

t y p i c a l l y c o n n e c t e d v i a a power c a b l e o r b a r t o a s w i t c h b o a r d , f u r t h e r e n e r g y

d i s t r i b u t i o n t a k i n g p l a c e t o l o c a l s w i t c h b o a r d s o r d i r e c t l y t o l a r g e i n d i v i d u a l

r e c e i v e r s . P o s s i b l e power ne twork a r rangements and t h e i r c o n n e c t i o n s t o t h e

e x t e r n a l power g r i d a r e d i s c u s s e d i n t h e l i t e r a t u r e ( r e f . 3 9 ) .

Among t h e r e c e i v e r s c o n n e c t e d t o t h e power n e t w o r k , a s y n c h r o n o u s e l e c t r i c

motors a r e o f p a r t i c u l a r i m p o r t a n c e . The c o n v e r s i o n o f e l e c t r i c a l i n t o

mechan ica l e n e r g y t a k i n g p l a c e i n t h e s e motors i s accompanied by a l t e r n a t i n g

m a g n e t i z a t i o n o f t h e a c t i v e i r o n . Wi th t h e c h a r a c t e r i s t i c f r e q u e n c y o f t h e

c u r r e n t (50 Hz i n E u r o p e , 60 Hz i n U S ) , t he e l e c t r i c a l e n e r g y i s consumed and

r e c o v e r e d c o r r e s p o n d i n g t o m a g n e t i z a t i o n and c o u n t e r - m a g n e t i z a t i o n . As a r e s u l t ,

i n a d d i t i o n t o t h e f l o w o f e l e c t r i c a l power b e i n g c o n v e r t e d i n t o mechan i ca l work

( c a l l e d e f f e c t i v e power N ^ ) , m a g n e t i z a t i o n e n e r g y i s o s c i l l a t i n g between t h e

s o u r c e and t h e m o t o r s . T h i s i m p l i e s t h a t t he c o n d u c t o r s o f power c a b l e s a r e

c a r r y i n g some a d d i t i o n a l c u r r e n t ; t h e a d d i t i o n a l power i s c a l l e d t h e r e a c t i v e

p o w e r , N ^ . The a c t u a l l o a d o f t h e s o u r c e i s equa l t o t h e g e o m e t r i c sum o f

e f f e c t i v e and r e a c t i v e p o w e r , a l s o c a l l e d t h e a p p a r e n t power N ^ . T h i s phenomenon a

i s u s u a l l y c h a r a c t e r i z e d by t h e s o - c a l l e d power f a c t o r cos φ = Ng/Ng^ ( 1 . 7 )

where φ i s t he phase l a g between t h e e f f e c t i v e power and t h e a p p a r e n t p o w e r .

G e n e r a l l y , each power r e c e i v e r i n a power ne twork can be c h a r a c t e r i z e d , a t

each i n s t a n t , by a s p e c i f i c power f a c t o r . F o r t h r e e - p h a s e a s y n c h r o n o u s m o t o r s ,

w h i c h a r e by f a r t h e most i m p o r t a n t moto rs f o r s u g a r f a c t o r i e s , t h e power f a c t o r

i s a f u n c t i o n o f motor d e s i g n , power r a t i n g and l o a d f a c t o r . I n F i g . 1 .22 ,

a t y p i c a l r e l a t i o n s h i p between t h e power f a c t o r , power r a t i n g and l o a d f a c t o r i s

shown f o r e n c l o s e d - t y p e , 4 - p o l e motors o f J a p a n e s e make w o r k i n g a t 50 Hz ( r e f .

2 8 ) . As can be s e e n , t he l o a d d e v i a t i o n f rom t h e motor r a t i n g i s d e c i s i v e i n

r e d u c i n g cos φ.

The a v e r a g e d power f a c t o r s o f t he r e c e i v e r s d e f i n e t he power f a c t o r o f t h e

g e n e r a t o r . I n a f a c t o r y i n w h i c h no s p e c i a l measures a r e t aken t o improve i t .

39

0.25 0.5 0.75 Load factor

1.0

F i g . 1.22. Power f a c t o r o f e n c l o s e d - t y p e , 4 - p o l e i n d u c t i o n motors o p e r a t e d a t 50 Hz ( a f t e r r e f . 2 8 ) .

cos φ i s u s u a l l y i n t h e range 0 . 6 5 - 0 . 7 0 . L e t us o b s e r v e t h a t t h e maximum

e f f e c t i v e power o f t h e g e n e r a t o r i s d e t e r m i n e d by t h e mechan ica l o u t p u t o f t h e

t u r b i n e . The e l e c t r i c d i m e n s i o n i n g o f t h e g e n e r a t o r m u s t , h o w e v e r , be adap ted

t o t he a p p a r e n t p o w e r , t h a t i s , t h e t u r b i n e o u t p u t d i v i d e d by t h e power f a c t o r .

I n an e x i s t i n g f a c t o r y , t h e r e a c t i v e power o s c i l l a t i n g i n t h e power ne two rk may

cause t h e a p p a r e n t power t o e x c e e d t h e l e v e l f o r w h i c h t h e g e n e r a t i n g and

d i s t r i b u t i n g equ ipment i s d i m e n s i o n e d , even i f t h e t u r b i n e o u t p u t i s

s u f f i c i e n t l y l a r g e t o s u p p l y t h e Sys tem w i t h e f f e c t i v e power . I n o r d e r t o

p r e v e n t t h i s s i t u a t i o n , t h e power f a c t o r o f t h e g e n e r a t o r can be m o d i f i e d by

g e n e r a t i n g r e a c t i v e power i n power c a p a c i t o r s . The c a p a c i t o r s can be c o n n e c t e d

e i t h e r t o t h e main s w i t c h b o a r d o f t h e power h o u s e , o r t o t h e l o c a l s w i t c h b o a r d s

where r e c e i v e r s r e s p o n s i b l e f o r a low power f a c t o r a r e c o n n e c t e d . As i n d i c a t e d

a b o v e , t h e s e r e c e i v e r s a r e a s y n c h r o n o u s motors and p a r t i c u l a r l y ones l o a d e d

be low t h e i r nominal o u t p u t s .

D i s r e g a r d i n g t h e case o f i m p r o p e r l y d i m e n s i o n e d m o t o r s , i t can t h u s be

c o n c l u d e d t h a t when a t t e m p t i n g t o max imize t h e power f a c t o r , c a r e s h o u l d be

taken o f :

- b a t c h c e n t r i f u g a l d r i v e s d u r i n g most o f t h e i r w o r k i n g c y c l e ,

- d r i v e s o f pumps and fans emp loy i ng f l o w c o n t r o l by t h r o t t l i n g d u r i n g t h e

p e r i o d s o f r educed f l o w .

L e t us n o t e t h a t t h e power f a c t o r s o f t h e moto rs men t ioned may v a r y as t h e i r

l oads a r e c h a n g e d , and i t may be n e c e s s a r y t o v a r y t h e r e a c t i v e power g e n e r a t e d

i n t he c a p a c i t o r s a c c o r d i n g l y . H o w e v e r , i f t h e o p e r a t i o n o f a g r o u p o f l a r g e

motors can be c o n t r o l l e d f o r n e a r l y c o n s t a n t t o t a l power c o n s u m p t i o n , i t may

40

a l s o become e a s i e r t o c o n t r o l t h e o v e r a l l r e a c t i v e power o f t h e e n t i r e g r o u p .

T h i s p r i n c i p l e i s a p p l i e d i n t he g r o u p c o n t r o l o f b a t c h c e n t r i f u g a l d r i v e s ( s e e

S e c t i o n 1 . 4 . 3 ) .

1.4.3 Shap ing t he power demand

The s p e c i f i c power demand o f c o n t e m p o r a r y s u g a r f a c t o r i e s i s u s u a l l y o f t h e

o r d e r 2-4 kWh p e r 100 kg b e e t . I t i s i n t e r e s t i n g t o n o t e t h e i n f l u e n c e o f t h e

economy o f s c a l e , namely a l a r g e r p r o c e s s i n g c a p a b i l i t y u s u a l l y i m p l i e s a l o w e r

power demand p e r u n i t mass o f b e e t s p r o c e s s e d . I n F i g . 1 .23, t h e c o n t i n u o u s l i n e

r e p r e s e n t s S o v i e t da ta on t h e s o - c a l l e d b a s i c power demand ( e x c l u d i n g power

consumed i n p u l p d r y i n g and w a s t e - w a t e r t r e a t m e n t ) o f w h i t e - s u g a r m a n u f a c t u r e i n

f a c t o r i e s w i t h p r o c e s s i n g c a p a b i l i t i e s up t o 9000 t o n s p e r day ( r e f . 4 0 ) . The

d i s c r e t e p o i n t s i n t h e d iag ram r e p r e s e n t s t a t i s t i c a l d a t a on o v e r a l l power

c o n s u m p t i o n , i n c l u d i n g p u l p d r y i n g , i n a few West European w h i t e - s u g a r f a c t o r i e s

d u r i n g t he 1985 s e a s o n ; owing t o d i f f e r e n c e s i n p r o c e s s e s and equ ipment

e m p l o y e d , t he i n f l u e n c e o f t h e economy o f s c a l e i s n o t a p p a r e n t f rom t h e s e d a t a .

3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

o o JZ

TD C Ό Ε α; •Ό

χ

3 4 5 6 7

P r o c e s s i n g c a p a b i l i t y ( 1 0 0 0 t / d )

F i g . 1.23. S p e c i f i c power demand as a f u n c t i o n o f t h e p r o c e s s i n g c a p a b i l i t y . The l i n e i n d i c a t e s S o v i e t da ta on w h i t e - s u g a r m a n u f a c t u r e , and t h e p o i n t s -s t a t i s t i c a l da ta on 5 West European f a c t o r i e s .

The c o n t r i b u t i o n s od v a r i o u s f a c t o r y s e c t i o n s t o t h e o v e r a l l power demand may

v a r y , depend ing on l o c a l c o n d i t i o n s . F o r e x a m p l e , t h e e n e r g y consumed by pump

d r i v e s depends on h e i g h t d i f f e r e n c e s between p i e c e s o f equ ipment l o c a t e d a l o n g

t h e t r a n s p o r t r o u t e s o f a l i q u i d . The f o l l o w i n g a p p r o x i m a t e i n d i c e s d e s c r i b e

a t y p i c a l s t r u c t u r e o f t he power demand o f w h i t e - s u g a r m a n u f a c t u r e i n f a c t o r i e s

w i t h p r o c e s s i n g c a p a b i l i t i e s o f 4000-6000 t o n s p e r d a y :

41

- e x t r a c t i o n s t a t i o n 10-12%,

- j u i c e p u r i f i c a t i o n and j u i c e t r a n s p o r t 20-25%,

- s u g a r house 14-20%,

- l ime k i l n , m i l k - o f - l i m e p r e p a r a t i o n and k i l n - g a s pumping 12-16%,

- o t h e r p u r p o s e s 36-43%.

A t t h e p r e s e n t s t a t e o f deve lopmen t o f power ne two rks and t h e i r e q u i p m e n t ,

v a r i o u s t e c h n i q u e s can be used t o m o d i f y t h e power demand t owa rds b e t t e r e n e r g y

economy. F o u r app roaches can be e m p l o y e d .

( i ) I n t r o d u c t i o n o f a l t e r n a t i v e p r o c e s s e s c h a r a c t e r i z e d by a l o w e r power demand.

( i i ) A p p l i c a t i o n o f new equ ipment c h a r a c t e r i z e d by a b e t t e r e f f i c i e n c y o f power

u t i l i z a t i o n .

( i i i ) A p p l i c a t i o n o f a l t e r n a t i v e c o n t r o l methods a t t h e l o c a l (equipm.ent) l e v e l ,

p r e v e n t i n g t h e l o s s e s o f e n e r g y d e l i v e r e d t o e l e c t r i c a l l y d r i v e n e q u i p m e n t ,

p a r t i c u l a r l y unde r v a r i a b l e l o a d .

( i v ) I n t r o d u c t i o n o f new c o n t r o l methods a t t h e subsys tem o r sys tem l e v e l ,

making i t p o s s i b l e t o i n f l u e n c e t h e t ime v a r i a t i o n s o f power demand.

The p o s s i b i l i t i e s o f i n t r o d u c i n g a l t e r n a t i v e p r o c e s s e s v a r y f rom f a c t o r y t o

f a c t o r y , depend ing on t h e o p p o r t u n i t i e s t o i n v e s t , u s u a l l y i n c o n n e c t i o n w i t h

f a c t o r y e x t e n s i o n s . F o r e x a m p l e , h y d r a u l i c b e e t u n l o a d i n g and t r a n s p o r t ,

r e q u i r i n g t h a t abou t 3 m^ w a t e r p e r 1 t b e e t r e c e i v e d i s pumped, i s more e n e r g y -

i n t e n s i v e than d r y u n l o a d i n g and t r a n s p o r t . Power s a v i n g s r e p o r t e d f rom F r e n c h

f a c t o r i e s where t he d r y method has been implemented a r e o f t h e o r d e r o f 0 .3 kWh

p e r 100 kg b e e t ( r e f . 4 1 ) . D i f f e r e n t methods o f d i r t s e p a r a t i o n , h o w e v e r , may

d i f f e r w i t h r e s p e c t t o t h e power c o n s u m p t i o n .

I n t h e p a s t , t o o l i t t l e a t t e n t i o n was p a i d t o t h e power demands o f v a r i o u s

p r o c e s s e s . Now t h i s i s b e g i n n i n g t o c h a n g e , as has been i n d i c a t e d by some r e c e n t

p u b l i c a t i o n s i n w h i c h t h e i n f l u e n c e o f p r o c e s s pa rame te rs on t h e e n e r g y demand

i s s t u d i e d ( r e f s . 4 2 , 4 3 ) . The e n e r g y i n t e n s i t y o f new p r o c e s s e s i s a d d i t i o n a l l y

d i s c u s s e d i n C h a p t e r 4 .

As r e g a r d s t h e e l e c t r i c a l e q u i p m e n t , t h e o p e r a t i o n o f b a t c h c e n t r i f u g a l

d r i v e s w h i c h a r e t h e cause o f l o a d peaks i s p a r t i c u l a r l y i m p o r t a n t . F o r economic

r e a s o n s , a t r e n d has been e s t a b l i s h e d t o w a r d s l a r g e - c a p a c i t y m a c h i n e s , t o d a y

r e a c h i n g 1000-1700 kg p e r c h a r g e . The moments o f i n e r t i a o f r o t o r s i n t h e

l a r g e s t machines a r e o f t h e o r d e r 1000-1400 kg m . I n o r d e r t o a c c e l e r a t e such

r o t o r s t o t y p i c a l r o t a t i o n a l v e l o c i t i e s o f 1000-1500 rpm i n a b o u t 20 w o r k i n g

c y c l e s p e r h o u r , motors r e a c h i n g a power l e v e l o f 250-300 kW a r e r e q u i r e d . E v e r y

a c c e l e r a t i o n s e c t i o n o f t h e c e n t r i f u g i n g c y c l e causes a l a r g e t e m p o r a r y l o a d ,

w h i l e e v e r y d e c e l e r a t i o n s e c t i o n a l l o w s t h e p o s s i b i l i t y o f r e t u r n i n g power t o

t he e l e c t r i c a l subsys tem o f t h e f a c t o r y . The o p e r a t i o n o f b a t c h - c e n t r i f u g a l

d r i v e thus r e s u l t s i n t i m e - v a r y i n g e f f e c t i v e and r e a c t i v e l o a d s . I t i s t h e r e f o r e

42

e s s e n t i a l t h a t d u r i n g t h e o p e r a t i o n o f a g r o u p o f c e n t r i f u g a l s , t h e c y c l e s o f

i n d i v i d u a l machines a r e c o o r d i n a t e d i n o r d e r t o a v o i d s i m u l t a n e o u s a c c e l e r a t i o n

o f s e v e r a l u n i t s .

The c o n t r o l o f b a t c h c e n t r i f u g a l s a c c o r d i n g t o t h i s p r i n c i p l e c o n s t i t u t e s

an example o f equ ipment o p e r a t i o n c o n t r o l a t t h e l o c a l ( equ ipmen t g r o u p , o r

s t a t i o n ) l e v e l . T h i s s o l u t i o n i s p a r t i c u l a r l y u s e f u l f o r l a r g e power r e c e i v e r s

c h a r a c t e r i z e d by s u b s t a n t i a l l o a d v a r i a t i o n s .

I n t h e c o n t e m p o r a r y s u g a r i n d u s t r y , pe rhaps t h e l a r g e s t p o w e r - s a v i n g

p o t e n t i a l i s a s s o c i a t e d w i t h t h e d r i v e s o f pumps and f a n s . On a v e r a g e , t h e s e

machines a r e r e s p o n s i b l e f o r abou t 60% o f t h e e l e c t r i c a l e n e r g y consumed i n

a s u g a r f a c t o r y . D imens ioned f o r t h e l a r g e s t p o s s i b l e f l o w s o f m e d i a , t h e pumps

and fans a r e c o n t r o l l e d u s i n g methods w h i c h i n d u c e e n e r g y l o s s e s , t h a t i s , f l o w

b y - p a s s i n g o r t h r o t t l i n g . T h i s i s a d e c i s i v e f a c t o r i n c a u s i n g i n c r e a s e d

s p e c i f i c power consumpt ion when t h e f a c t o r y ' s p r o c e s s i n g c a p a b i l i t y f a l l s be low

i t s nominal l e v e l ( F i g . 1 . 24 ) . Wi th modern d r i v i n g s y s t e m s , o l d e r c o n t r o l

methods can be r e p l a c e d by t h e v a r i a b l e speed c o n t r o l w h i c h i s d i s c u s s e d i n

C h a p t e r 6. T y p i c a l s a v i n g s a r e o f t h e o r d e r o f 20-40% o f t h e e n e r g y consumed

when u s i n g t h e o r i g i n a l methods .

o o " 1.6 •α

i 1. φ •o ir '• φ o

o- 1.0 50 60 70 80 90 100

A v e r a g e to nomina l d a i l y capab i l i t y ( % )

F i g . 1.24. R a t i o o f a c t u a l t o nominal s p e c i f i c power demand as a f u n c t i o n o f t h e u t i l i z a t i o n o f p r o c e s s i n g c a p a b i l i t y .

A t t h e f a c t o r y l e v e l , a t i m e - v a r y i n g t o t a l power demand may cause

d i f f i c u l t i e s i n s e c u r i n g a r e l i a b l e power s u p p l y . Load peaks e x c e e d i n g t h e

g e n e r a t i n g c a p a b i l i t y o f t h e e l e c t r i c a l subsys tem c a n n o t be met w i t h o u t

s u p p l y i n g power f rom t h e e x t e r n a l g r i d . I n o r d e r t o a v o i d u n n e c e s s a r y e n e r g y

p u r c h a s e s , o r i f no e l e c t r i c i t y s u p p l i e s f rom o u t s i d e t h e f a c t o r y a r e a v a i l a b l e ,

such peaks must be e l i m i n a t e d by t e m p o r a r i l y d i s c o n n e c t i n g c e r t a i n power

r e c e i v e r s . T h i s can be done w i t h o u t r e d u c i n g t h e t o t a l e n e r g y amount r e q u i r e d

f o r normal f a c t o r y o p e r a t i o n , t h a t i s , by c o n t r o l l i n g t h e power demand o n l y when

i t app roaches i t s maximum.

43

The power r e c e i v e r s t o be d i s c o n n e c t e d a r e s e l e c t e d so as n o t t o cause any

damage t o normal f a c t o r y o p e r a t i o n . T y p i c a l l y , t h e s e a r e a e r a t o r s o f w a s t e - w a t e r

t r e a t m e n t p l a n t s , a i r c o n d i t i o n i n g sys tems o f s u g a r and p e l l e t s i l o s , and

e l e c t r i c room h e a t e r s .

E x p e r i e n c e p r o v e s t h a t c o n t r o l o f t h e maximum power demand can be e f f e c t i v e

o n l y i f automated l o a d - m o n i t o r i n g and l o a d - d i s c o n n e c t i n g equ ipment i s u s e d . The

n e c e s s a r y e l e c t r o n i c equ ipment can be r a t h e r i n e x p e n s i v e , as p r o v e d by

a p p l i c a t i o n examples p r e s e n t e d i n t h e l i t e r a t u r e ( r e f s . 4 5 , 4 6 ) . The sys tem i s

measur ing t h e i n s t a n t a n e o u s power demand and compar ing i t w i t h t h e g e n e r a t i n g

c a p a c i t y . I f a p r e d e t e r m i n e d c r i t i c a l d i f f e r e n c e between t h e s e two q u a n t i t i e s

i s a p p r o a c h e d , t h e n a g r o u p o f r e c e i v e r s i s d i s c o n n e c t e d . I n a b r o a d e r l o a d

r a n g e , t h e sys tem o p e r a t i o n i s u s u a l l y s e m i - a u t o m a t i c , a l l o w i n g f o r t h e

i n t e r v e n t i o n o f human o p e r a t o r s , and i t may a l s o i n c o r p o r a t e t r e n d m o n i t o r i n g ,

s i g n a l l i n g o f c r i t i c a l s i t u a t i o n s , p r i n t i n g o f p r o c o t o l s and r e p o r t s , e t c . As

an o p t i o n , t h e m o n i t o r i n g o f t he r e a c t i v e power and t h e a u t o m a t i c c o n t r o l o f

t he power c a p a c i t o r s t o improve t h e power f a c t o r can a l s o be i n c l u d e d .

1.5 INTERACTION BETWEEN POWER GENERATION AND HEAT ECONOMY

1.5.1 E n e r g y p o l i c y c o n s i d e r a t i o n s

The t a s k o f t h e power house can be u n d e r s t o o d as t r a n s f o r m i n g t h e p r i m a r y

e n e r g y s u p p l i e d t o t he f a c t o r y i n t o t h e n e c e s s a r y n e t h e a t i n p u t Q2 and power

i n p u t P ' . The n e t h e a t i n p u t i s o f t e n e x p r e s s e d as t h e mass f l o w o f steam

r e q u i r e d t o c a r r y i t ; t h i s f l o w w i l l be c a l l e d steam demand i n t h e f o l l o w i n g .

Assuming t h e s o - c a l l e d normal steam p a r a m e t e r s , t h a t i s , d r y s a t u r a t i o n s t a t e a t

tg = lOO^C and s p e c i f i c h e a t o f e v a p o r a t i o n r^ = 2256.9 k J / k g , t h e steam demand

can be d e f i n e d as

D' = Q2 / r3 ( 1 . 8 )

The above c o n v e n t i o n i s t o some e x t e n t s i m i l a r t o t h e use o f t h e w i d e l y known

c o n c e p t o f t h e demand f o r normal f u e l ( h e a t i n g v a l u e 29 300 k J / k g ) i n e x p r e s s i n g

t h e demand f o r p r i m a r y e n e r g y . I t s h o u l d be added t h a t i n t h e p r e s e n t b o o k , t h e

te rm "demand" u s u a l l y a p p l i e s t o a p r e d i c t e d o r c a l c u l a t e d q u a n t i t y . I f measured

i n an e x i s t i n g f a c t o r y , t he c o r r e s p o n d i n g q u a n t i t y w i l l r a t h e r be c a l l e d

" c o n s u m p t i o n " .

The most w i d e l y a p p l i e d method o f s u p p l y i n g t h e n e c e s s a r y e n e r g y i n p u t s i s t o

r e l y on combined g e n e r a t i o n o f h e a t and e l e c t r i c i t y i n a steam c y c l e e q u i p p e d

w i t h a b o i l e r and a b a c k - p r e s s u r e t u r b i n e , shown s c h e m a t i c a l l y i n F i g . 1.25

( r e f s . 3 8 , 4 6 , 4 7 ) . N e v e r t h e l e s s , t h e economic c o n d i t i o n s f o r t h i s method b e i n g

w o r k a b l e v a r y c o n s i d e r a b l y between d i f f e r e n t c o u n t r i e s and even between

d i f f e r e n t f a c t o r y l o c a t i o n s .

T a k i n g a g l o b a l p e r s p e c t i v e , t h e r e i s no d o u b t t h a t combined g e n e r a t i o n saves

44

fue l

s t e a m l o s s f 10%

\ f u e l /100%|

c o n d e n s a t e a i r 0 .5%

' l o s s 1.1%

h e a t ^ 76 %

p o w e r 13.4%

F i g . 1.25. E n e r g y sys tem l a y o u t and Sankey d iag ram ( l i v e steam 80 b a r and 520 C , b a c k - p r e s s u r e 3 b a r ) f o r t h e a p p l i c a t i o n o f a b a c k - p r e s s u r e steam t u r b i n e . 1 - b o i l e r , 2 - t u r b i n e , 3 - p r o c e s s .

p r i m a r y e n e r g y and reduces a i r p o l l u t i o n . Where h e a t i n g i s n e c e s s a r y , power can

a d d i t i o n a l l y be o b t a i n e d , u s i n g t h i s me thod , a t t h e expense o f abou t h a l f as

much f u e l as i n a the rma l power p l a n t e q u i p p e d w i t h c o n d e n s i n g t u r b o - g e n e r a t o r s .

T h i s means a l s o t h a t h a l f as much b o i l e r f l u e gas i s d i s c h a r g e d t o t h e

a tmosphere .

A f t e r t h e b i t t e r l e s s o n s o f t h e 1970s, many c o u n t r i e s now r e c o g n i z e t h e

i n d u s t r i a l power houses as b e i n g i m p o r t a n t t o t h e i r e n e r g y p o l i c i e s and l o n g -

term p l a n n i n g i n t h i s f i e l d . Depending on t h e economic s i t u a t i o n and a v a i l a b l e

e n e r g y r e s o u r c e s , governments t r y t o cope w i t h w o r l d market deve lopmen ts by

i n f l u e n c i n g , d i r e c t l y o r i n d i r e c t l y ( e . g . by t a x r e g u l a t i o n s ) , t h e f u e l p r i c e s ,

power p r i c e , and c a p i t a l c o s t . The economic c o n d i t i o n s t hus c r e a t e d f o r power

house o p e r a t i o n may v a r y c o n s i d e r a b l y , as can be demons t ra ted by t h e examples o f

e l e c t r i c i t y p u r c h a s e s i n two i m p o r t a n t b e e t - s u g a r p r o d u c i n g c o u n t r i e s - USSR and

USA. I n t h e USA, a t y p i c a l s i t u a t i o n i s t h a t steam s a v i n g s s h o u l d n o t be

compromised t o keep the e l e c t r i c i t y and steam i n b a l a n c e , as p u r c h a s e d

e l e c t r i c i t y i s c h e a p e r than power g e n e r a t i o n w i t h o u t f u l l u t i l i z a t i o n o f t h e

e x h a u s t s team. C o n t r a r y t o t h a t , i t may be j u s t i f i e d i n t he USSR t o g e n e r a t e

e l e c t r i c i t y w i t h some steam b l o w - o f f even under normal o p e r a t i n g c o n d i t i o n s

( r e f s . 4 8 , 4 9 ) .

F l u c t u a t i n g f u e l p r i c e s s t i m u l a t e an i n t e r e s t i n g deve lopmen t w h i c h has become

v i s i b l e i n f u e l - i m p o r t i n g c o u n t r i e s , namely t h e d i v e r s i f i c a t i o n o f f u e l s t o

e l i m i n a t e t h e dependence on f u e l o i l w h i c h was dominant b e f o r e t h e o i l c r i s e s .

F r a n c e and Sweden can be named as examples o f c o u n t r i e s where t h e s u g a r

i n d u s t r i e s r educed t h e p e r c e n t a g e o f f u e l o i l i n t h e t o t a l e n e r g y consumpt i on

f rom more than 95% t o abou t 50%. I n bo th c o u n t r i e s , t he p e r c e n t a g e o f n a t u r a l

gas was c o n s i d e r a b l y i n c r e a s e d abou t 1985. I n t h e s u g a r i n d u s t r i e s i n o t h e r

c o u n t r i e s , l i k e Denmark, use o f b i t u m i n o u s c o a l as t h e main s u b s t i t u t e f o r f u e l

o i l was p r e f e r r e d .

As r e g a r d s t h e d e c i s i o n s on power house i n v e s t m e n t , w h i c h a r e u s u a l l y

a s s o c i a t e d w i t h l a r g e c a p i t a l e x p e n d i t u r e s , t h e f o l l o w i n g f a c t o r s s h o u l d be

c o n s i d e r e d :

45

- t h e p o s s i b i l i t y o f p u r c h a s i n g power f rom t h e e x t e r n a l g r i d , g r i d r e l i a b i l i t y ,

and t he p o s s i b i l i t y o f s e l l i n g power t o t h e g r i d ,

- f u e l p r i c e ,

- t he r a t i o o f f u e l p r i c e t o power p r i c e ,

- c a p i t a l c o s t ,

- i f a p p l i c a b l e , t h e p r i c e o f power s o l d t o t h e g r i d .

As t h e p a y - b a c k p e r i o d s o f power house i n v e s t m e n t s may be r e l a t i v e l y l o n g , t h e r e

i s t h e r i s k o f f u t u r e f l u c t u a t i o n s o f t h e above f a c t o r s a f f e c t i n g t h e outcome o f

i n v e s t m e n t d e c i s i o n s . As a r e s u l t , t h e r e i s an i n c r e a s i n g i n t e r e s t i n

a l t e r n a t i v e t e c h n o l o g i e s f o r hea t and power g e n e r a t i o n t o r e p l a c e o r supp lement

t h e e x i s t i n g steam c y c l e s .

1.5.2 S o l u t i o n s based on c o n n e c t i o n t o a power g r i d

A s e e m i n g l y e x o t i c a l t e r n a t i v e i s t o a v o i d t h e use o f f u e l s and r e l y on t h e

e x t e r n a l g r i d as t h e o n l y s o u r c e o f e n e r g y . T h i s can be an o p t i m a l s o l u t i o n i f

cheap h y d r o e l e c t r i c power i s a v a i l a b l e , as d e m o n s t r a t e d by a B r a z i l i a n cane

s u g a r f a c t o r y u s i n g e l e c t r i c a l l y hea ted b o i l e r s t o g e n e r a t e steam ( r e f . 5 0 ) .

The a v a i l a b i l i t y o f h y d r o e l e c t r i c power a l s o p l a y e d a d e c i s i v e r o l e i n a d o p t i n g

t h e e x t e r n a l g r i d a l t e r n a t i v e i n t h e A a r b e r g f a c t o r y i n S w i t z e r l a n d n e a r l y f o u r

decades ago ( r e f . 5 1 , 5 2 ) . A s o p h i s t i c a t e d the rma l sys tem e m p l o y i n g v a p o u r

compress i on was a p p l i e d t h e r e i n o r d e r t o a c h i e v e e f f i c i e n t e n e r g y u t i l i z a t i o n

i n s u g a r m a n u f a c t u r e , and s p e c i a l a r rangemen ts were made t o s e l l p r e s s e d p u l p

w i t h o u t d r y i n g . F o l l o w i n g t he changes i n S w i t z e r l a n d ' s e n e r g y b a l a n c e , h o w e v e r ,

i n v e s t m e n t s were made i n t h e e a r l y 1970s p r e p a r i n g f u t u r e power s u p p l i e s f rom

a d e d i c a t e d g e n e r a t i n g u n i t . As h y d r o e l e c t r i c power i s now c o v e r i n g a b o u t 60% o f

t he c o u n t r y ' s e n e r g y demands, a steam c y c l e w i t h a b a c k - p r e s s u r e t u r b i n e has

been p u t i n t o o p e r a t i o n i n A a r b e r g ( r e f . 53) t o s u p p l y n e a r l y h a l f o f t h e power

consumed i n t h e f a c t o r y .

I n s p i t e o f t h e g l o b a l advan tages o f t h e combined g e n e r a t i o n o f h e a t and

e l e c t r i c i t y , l o c a l economic c o n d i t i o n s may j u s t i f y s o l u t i o n s c o n s i s t i n g o f steam

g e n e r a t i o n i n l o w - p r e s s u r e b o i l e r s and power p u r c h a s e s f rom t h e e x t e r n a l g r i d .

I f t h e g r i d r e l i a b i l i t y i s s u f f i c i e n t l y h i g h , t h i s s o l u t i o n i s f u l l y f l e x i b l e

w i t h r e g a r d t o t he r a t i o between h e a t and power consumed. When r e l y i n g on t h i s

a l t e r n a t i v e t o d a y , h o w e v e r , i t i s a d v i s a b l e t o keep open t h e o p t i o n t o a p p l y

combined g e n e r a t i o n i n t h e f u t u r e .

R e t u r n i n g now t o t h e a p p l i c a t i o n o f p o w e r - g e n e r a t i n g u n i t s based on t h e steam

c y c l e , l e t us o b s e r v e t h a t a b a c k - p r e s s u r e t u r b i n e i s j u s t a s p e c i a l case o f

a steam t u r b i n e . I f we d e f i n e A as t h e r a t i o o f steam demand D' ( k g / h ) t o power

demand P' (kW)

A = D ' / P ' ( 1 . 9 )

then f o r A>10, a b a c k - p r e s s u r e t u r b i n e i s u s u a l l y t h e b e s t c h o i c e . F o r A<10,

46

a c o n d e n s i n g / e x t r a c t i o n t u r b i n e ( F i g . 1.26) s h o u l d be r e g a r d e d as a s e r i o u s

a l t e r n a t i v e . I t assumes the use o f e x t r a c t i o n steam i n s t e a d o f e x h a u s t steam

f rom a b a c k - p r e s s u r e u n i t , w h i l e t he r e m a i n i n g steam f l o w i n g t o t h e c o n d e n s e r

g e n e r a t e s some e x t r a power . A l t h o u g h t h i s s o l u t i o n i s e x p e n s i v e , i t can a l s o be

o p e r a t e d d u r i n g t h e o f f - s e a s o n p e r i o d i f power can be s o l d t o t h e e x t e r n a l g r i d .

T h i s seems t o be p a r t i c u l a r l y a t t r a c t i v e t o i n d u s t r i a l combines o f w h i c h t h e

s u g a r f a c t o r y i s o n l y a p a r t , and where h e a t and power a r e needed d u r i n g t h e

o f f - s e a s o n p e r i o d . Examples o f t h i s s o l u t i o n can be f ound i n t h e B u l g a r i a n and

S o v i e t s u g a r i n d u s t r i e s , as w e l l as i n cane s u g a r f a c t o r i e s i n r e g i o n s where

power s u p p l i e s t o t he g r i d a r e r e q u i r e d ( r e f s . 5 4 - 5 6 ) .

s t e a m

f u e l

1 L

1 til -Θ 3

Ί k ' . c o n d e n s a t e

F i g . 1.26. E n e r g y sys tem l a y o u t f o r t h e a p p l i c a t i o n o f a c o n d e n s i n g / e x t r a c t i o n t u r b i n e . 1 - b o i l e r , 2 - t u r b i n e , 3 - p r o c e s s , 4 - c o n d e n s e r .

1.5.3 S o l u t i o n s based on a b a c k - p r e s s u r e t u r b i n e

I n a s u g a r f a c t o r y e q u i p p e d w i t h i t s own power h o u s e , a mismatch p rob lem

between t h e power demand and power p r o d u c t i o n may o c c u r . The e l e c t r i c a l o u t p u t

Ρ o f a b a c k - p r e s s u r e t u r b o - g e n e r a t o r can be e x p r e s s e d as a f u n c t i o n o f steam

f l o w D

Ρ = D/S ( 1 . 1 0 )

where S i s t he steam r a t e , i . e . t he steam consumpt ion p e r u n i t power p r o d u c e d

( k g / k W h ) , o f t he t u r b o - g e n e r a t o r .

D i v i d i n g t h e above e q u a t i o n by t h e d a i l y p r o c e s s i n g c a p a b i l i t y o f t h e f a c t o r y

R, we o b t a i n

P/R = ( D / R ) / S ( 1 . 1 1 )

Assuming a c o n s t a n t steam r a t e , t h i s i s a l i n e a r r e l a t i o n s h i p i l l u s t r a t e d i n

F i g . 1.27. L e t us now e x p r e s s t h e f a c t o r y ' s e n e r g y demand by two i n d i c e s

p ' = P ' / R and d ' = D ' / R r e p r e s e n t i n g power demand and steam demand, r e s p e c t i v e l y ,

p e r u n i t d a i l y c a p a b i l i t y . Two d i f f e r e n t s i t u a t i o n s can be imag ined f o r a b a c k

p r e s s u r e t u r b o - g e n e r a t o r c h a r a c t e r i z e d by steam r a t e S :

( i ) P' < d ' / S

T h i s means t h a t t h e steam f l o w c o v e r i n g t h e f a c t o r y ' s hea t demand i s

47

60 b a r / 5 0 0 ° C / 40 ba r7 420 °C

10 20 30 40 50 60

S team d e m a n d ( k g / I O O k g b )

F i g . 1.27. Power g e n e r a t e d as a f u n c t i o n o f t h e steam f l o w t h r o u g h a b a c k -- p r e s s u r e t u r b i n e .

s u f f i c i e n t l y l a r g e t o g e n e r a t e power needed (d-j i n F i g . 1 . 2 7 ) . The f a c t o r y i s

l i k e l y t o be s e l f - s u f f i c i e n t w i t h r e s p e c t t o power g e n e r a t i o n ,

( i i ) P' > d ' / S

The steam f l o w c o v e r i n g t h e f a c t o r y ' s h e a t demand i s t o o smal l t o s a t i s f y t h e

power demand ( d ^ i n F i g . 1 . 2 7 ) , and s e l f - s u f f i c i e n c y w i t h r e s p e c t t o power

g e n e r a t i o n i s i m p o s s i b l e o r v e r y d i f f i c u l t t o a c h i e v e .

Case ( i ) i s a f a v o u r a b l e s i t u a t i o n . I f no e l e c t r i c i t y s a l e s t o t h e e x t e r n a l

g r i d a r e p o s s i b l e , t hen a p a r t o f t he l i v e steam s h o u l d be d i r e c t e d t o

a t h r o t t l i n g - d e s u p e r h e a t i n g s t a t i o n and s u p p l i e d t o t h e p r o c e s s , t h u s b y - p a s s i n g

t he t u r b i n e . T h i s i s t he most w i d e l y used s o l u t i o n ; i t i s e x t e n s i v e l y d e s c r i b e d

i n t he l i t e r a t u r e ( r e f . 3 9 ) .

Case ( i i ) n e c e s s i t a t e s a c r i t i c a l r e v i e w o f t h e s t r u c t u r e o f t h e power demand,

and a c t i o n s aimed a t s a v i n g power u s i n g t h e t e c h n i q u e s men t ioned i n S e c t i o n

1 .4 .3 . I f no m e a n i n g f u l improvements can be a t t a i n e d , t hen i t becomes n e c e s s a r y

t o m o d i f y t he power house c o n c e p t assumed. T h e r e a r e s e v e r a l p o s s i b l e s o l u t i o n s

i n w h i c h t h e b a c k - p r e s s u r e steam c y c l e remains dom inan t :

- e l e c t r i c i t y p u r c h a s e s f rom t h e e x t e r n a l g r i d t o c o v e r t h e d e f i c i t ;

- u s i n g steam f rom t h e t u r b i n e e x h a u s t t o h e a t combus t i on a i r s u p p l i e d t o t h e

b o i l e r s o r p u l p - d r y e r f u r n a c e , t hus i n c r e a s i n g t h e steam f l o w t h r o u g h t h e

t u r b i n e ;

- i n s t a l l i n g a new b a c k - p r e s s u r e t u r b i n e c h a r a c t e r i z e d by a l o w e r steam r a t e

( i n F i g . 1 .27, t h i s i s i n t e r p r e t e d as moving t o t h e t u r b i n e c h a r a c t e r i s t i c s

shown by t h e dashed l i n e ) ,

- i n s t a l l i n g a new b o i l e r g e n e r a t i n g steam a t a h i g h e r p r e s s u r e and an

a d d i t i o n a l " t o p p i n g " b a c k - p r e s s u r e t u r b i n e ( F i g . 1 . 2 8 ( a ) ) , and s u p p l y i n g steam

48

(α)

fuel

fuel

c o n d e n s a t e

( b )

s t e a m 3 3 fue l

s t e a m

H 3

c o n d e n s a t e

F i g . 1.28. E n e r g y sys tem l a y o u t s f o r t h e m o d e r n i z a t i o n o f c o n v e n t i o n a l b a c k -- p r e s s u r e steam c y c l e s : ( a ) w i t h a t o p p i n g t u r b i n e , ( b ) w i t h a c o n d e n s i n g t u r b i n e . 1 - b o i l e r , 2 - t u r b i n e , 3 - p r o c e s s , 4 - c o n d e n s e r , 5 - t o p p i n g t u r b i n e , 6 - c o n d e n s i n g t u r b i n e .

f rom the e x h a u s t o f t h e " t o p p i n g " t u r b i n e t o t he e x i s t i n g t u r b i n e ;

- i n s t a l l i n g an a d d i t i o n a l c o n d e n s i n g t u r b i n e t o be s u p p l i e d w i t h a p a r t o f t h e

e x h a u s t steam f l o w ( F i g . 1 . 2 8 ( b ) ) ;

- i n cases where a the rma l c o n n e c t i o n between t h e f a c t o r y and an e x t e r n a l h e a t

r e c e i v e r ( e . g . a d i s t r i c t h e a t i n g s y s t e m ) i s p o s s i b l e , a p p l y i n g a b a c k - p r e s s u r e /

/ e x t r a c t i o n t u r b i n e ( r e f . 5 7 ) .

The complement ing power p u r c h a s e s f rom t h e e x t e r n a l g r i d a r e e c o n o m i c a l l y

a t t r a c t i v e , p r o v i d i n g t h e power p r i c e i s s u f f i c i e n t l y low i n r e l a t i o n t o t h e

f u e l p r i c e . T h i s s o l u t i o n i s w i d e l y used nowadays ; v a r i o u s a s p e c t s o f t h e

c o o p e r a t i o n w i t h t h e e x t e r n a l g r i d , i n c l u d i n g t e c h n i c a l d e t a i l s r e l a t e d t o t h e

power n e t w o r k , a r e d i s c u s s e d i n t h e l i t e r a t u r e ( r e f . 3 9 ) .

I n t e r e s t i n g deve lopments can be seen i n F r a n c e , where t h e power t a r i f f s a r e

d i f f e r e n t i a t e d depend ing on t h e t ime o f y e a r , t h e p r i c e s r i s i n g i n O c t o b e r and

November and r e a c h i n g a maximum i n December. T h i s s t i m u l a t e s power p u r c h a s e s i n

t h e i n i t i a l s t a g e o f t h e s e a s o n , w h i l e e l e c t r i c s e l f - s u f f i c i e n c y i s p r e f e r r e d

l a t e r o n . T h e r e f o r e , t i m e - d e p e n d e n t o p e r a t i o n o f t h e e n e r g y sys tems i n t h e s u g a r

i n d u s t r y i s p r o p o s e d so t h a t t h e power demand can v a r y ( r e f . 1 3 ) . D u r i n g t h e

i n i t i a l weeks o f t h e season power can be p u r c h a s e d t o o p e r a t e advanced v a p o u r -

compress i on c i r c u i t s , making i t p o s s i b l e t o m i n i m i z e t h e steam demand and t h u s

a l s o t he f u e l c o n s u m p t i o n . L a t e r o n , no power w i l l be p u r c h a s e d , and t h e f u e l

consumpt ion w i l l be a l l o w e d t o i n c r e a s e .

The second s o l u t i o n f rom t h e above l i s t can be a p p l i e d o n l y i f t h e i n s t a l l e d

b o i l e r and t u r b i n e c a p a c i t i e s a r e l a r g e enough t o a l l o w f o r a steam f l o w

i n c r e a s e . I t r e q u i r e s i n s t a l l i n g s t eam-hea ted a i r p r e h e a t e r s i n w h i c h t h e e n e r g y

o f t h e c o n d e n s i n g e x h a u s t steam can be t r a n s f e r r e d t o t h e combus t i on a i r . The

t e m p e r a t u r e range o v e r w h i c h a i r p r e h e a t i n g must be c o n t a i n e d i s l i m i t e d , t h u s

49

l i m i t i n g t h e a t t a i n a b l e s t e a m - f l o w i n c r e a s e . Where a i r p r e h e a t i n g a p p l i e s t o t h e

b o i l e r s o n l y , t he power g e n e r a t i o n can be i n c r e a s e d by abou t 7-8%. An a n a l y s i s

o f r e l e v a n t e n e r g y b a l a n c e s , i n c l u d i n g t h e case o f p r e h e a t i n g o f a i r d i r e c t e d t o

t h e p u l p - d r y e r f u r n a c e , can be found i n t h e l i t e r a t u r e ( r e f . 5 8 ) .

The t h i r d s o l u t i o n , i . e . a b a c k - p r e s s u r e t u r b i n e w i t h an improved steam r a t e ,

can o n l y be a p p l i e d p r o v i d i n g t h e p r e s s u r e and t e m p e r a t u r e o f t h e l i v e steam a r e

i n c r e a s e d , w h i c h a l s o r e q u i r e s i n s t a l l i n g a new b o i l e r . F i g u r e 1.29 shows

a d iag ram i n d i c a t i n g t he steam r a t e , and t he r a t i o o f t h e e l e c t r i c a l e n e r g y

p roduced t o t he i n p u t o f p r i m a r y e n e r g y i n f u e l d e l i v e r e d , as f u n c t i o n s o f l i v e -

steam p r e s s u r e ( t h e r e may be some s c a t t e r i n t h e d a t a because t h e q u a l i t y o f

t u r b i n e s s u p p l i e d by v a r i o u s m a n u f a c t u r e r s may v a r y ) . L e t us n o t e t h a t t h e steam

r a t e can be r e d u c e d n o t o n l y by i n c r e a s i n g l i v e - s t e a m p r e s s u r e and t e m p e r a t u r e

b u t a l s o by a p p l y i n g t h e r e h e a t c y c l e . F o r a p r o p o s e d u n i t o p e r a t e d a t l i v e -

steam paramete rs 80 b a r and 500^0, w i t h r e h e a t a t 30 b a r and 450^0, an

e s t i m a t e d steam r a t e i s 5.5 kg/kWh ( r e f . 5 9 ) .

The r e m a i n i n g c o n c e p t s named a t t he end o f t h e above l i s t , r e q u i r i n g

r e l a t i v e l y complex power house l a y o u t s , m i g h t be c o n s i d e r e d when s o l v i n g f a c t o r y

e x t e n s i o n o r m o d e r n i z a t i o n p r o b l e m s . I t has been p r o v e d u n d e r s p e c i f i c economic

c o n d i t i o n s t h a t a t o p p i n g t u r b i n e o p e r a t e d a t steam pa rame te rs 100 b a r and 540°C,

h a v i n g t he advan tage o f l a r g e r power p r o d u c t i o n , may be a t l e a s t e c o n o m i c a l l y

e q u i v a l e n t t o an a d d i t i o n a l b a c k - p r e s s u r e t u r b i n e s u p p l i e d w i t h steam a t 40 b a r

and 440°C ( r e f . 6 0 ) . As r e g a r d s an a d d i t i o n a l c o n d e n s i n g t u r b i n e , t h i s i s a

t y p i c a l s o l u t i o n i n cane s u g a r f a c t o r i e s i n w h i c h a l l bagasse i s b u r n e d , and t h e

e l e c t r i c i t y s u r p l u s i s s o l d t o t he e x t e r n a l g r i d ( r e f s . 6 1 , 6 2 ) .

15

SI

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LH

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520 °C

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P r e s s u r e ( b a r )

F i g . 1.29. Steam r a t e , and p e r c e n t a g e o f f u e l e n e r g y c o n v e r t e d t o e l e c t r i c i t y , as f u n c t i o n s o f l i v e steam pa ramete rs a t b a c k - p r e s s u r e 3 b a r .

50

I t s h o u l d be p o i n t e d o u t t h a t a new b o i l e r - t u r b i n e u n i t o p e r a t e d a t i n c r e a s e d

steam p a r a m e t e r s , o r an a d d i t i o n a l b o i l e r - t u r b i n e u n i t , r e q u i r e l a r g e

i n v e s t m e n t s , i n c l u d i n g c o s t l y f o u n d a t i o n s and pe rhaps b u i l d i n g s . I n o r d e r t o

a v o i d t h i s , a l t e r n a t i v e p o w e r - g e n e r a t i o n t e c h n o l o g i e s may be c o n s i d e r e d . The

a l t e r n a t i v e s o l u t i o n s a r e c h a r a c t e r i z e d by l o w e r i n v e s t m e n t c o s t s , and e l i m i n a t e

t h e r i g i d r e l a t i o n s h i p between t h e steam f l o w and t h e power g e n e r a t e d t h a t i s

c h a r a c t e r i s t i c o f t h e steam c y c l e .

1.5.4 C o m b u s t i o n - e n g i n e based s o l u t i o n s

A c o n c e p t w h i c h has r e c e i v e d much a t t e n t i o n i s t o a p p l y g a s - t u r b i n e d r i v e n

g e n e r a t i n g s e t s , w h i c h a r e a v a i l a b l e t o d a y w i t h power r a t i n g s up t o 100 MW. A t

t he p r e s e n t s t a t e o f deve lopmen t o f g a s - t u r b i n e t e c h n o l o g y , h o w e v e r , t h i s l i m i t s

t h e c h o i c e o f f u e l s t o l i q u i d o r gaseous t y p e s .

A g a s - t u r b i n e s e t i n c l u d e s a c o m p r e s s o r r a i s i n g t h e p r e s s u r e o f t h e

a t m o s p h e r i c a i r t o abou t 3-5 b a r and d e l i v e r i n g i t t o a combus t i on chamber where

t h e f u e l i s b u r n e d . Hot combus t i on gases s u b s e q u e n t l y expand i n a t u r b i n e , w h i c h

i s t y p i c a l l y mounted on t h e same s h a f t as t h e c o m p r e s s o r . The r o t a t i o n a l

v e l o c i t y may be as h i g h as 10 000 rpm, so t h e r e i s u s u a l l y a r e d u c i n g g e a r b o x

between t h e t u r b i n e - c o m p r e s s o r s e t and t h e e l e c t r i c a l g e n e r a t o r . The gas

p r e s s u r e i n t h e t u r b i n e e x h a u s t e q u a l s a t m o s p h e r i c p r e s s u r e , and t h e t e m p e r a t u r e

i s t y p i c a l l y abou t 430-550°C. The scheme o f a gas t u r b i n e s e t and a t y p i c a l

Sankey d iag ram a r e shown i n F i g . 1.30. The w e i g h t s and d i m e n s i o n s o f two g a s -

t u r b i n e d r i v e n c o n t i n u o u s - d u t y g e n e r a t i n g s e t s a r e g i v e n i n F i g . 1.31.

fuel

2 h

Qir I

-Θ exhaus t g a s compressor

work 38.3%

h e a t 7 0 . 6 %

IV p o w e r ^ 2 7 5 %

F i g . 1.30. Work ing p r i n c i p l e o f a g a s - t u r b i n e s e t and a c o r r e s p o n d i n g Sankey d i a g r a m . 1 - c o m p r e s s o r , 2 - combus t i on chamber , 3 - t u r b i n e .

An e a r l y i d e a was t o c o n n e c t t h e gas t u r b i n e t o a l o w - p r e s s u r e b o i l e r , i n

w h i c h t h e e n e r g y o f gases l e a v i n g t h e t u r b i n e e x h a u s t can be used t o g e n e r a t e

h e a t i n g s team, as shown s c h e m a t i c a l l y i n F i g . 1 . 32 (a ) ( r e f . 6 3 ) . From t h e p o i n t

o f v i e w o f t h e e n e r g y economy, t h i s s o l u t i o n e n s u r e s t h e l a r g e s t power o u t p u t a t

a g i v e n hea t demand. H o w e v e r , a b o i l e r hea ted by gases a t a r e l a t i v e l y l ow

i n i t i a l t e m p e r a t u r e i s so c o s t l y t h a t t h e economic r e s u l t m i g h t be q u e s t i o n a b l e .

51

(α) (b )

- c 1?

7m 15m

F i g . 1.31. Examples o f g a s - t u r b i n e d r i v e n g e n e r a t i n g s e t s , ( a ) r a t i n g 2.5 MW, w e i g h t 18 t , ( b ) 6.2 MW, 60 t . 1 - a i r i n l e t , 2 - gas o u t l e t , 3 - g e n e r a t o r .

(a) I f u e l I

( b )

2 3 2 3

I f ue l I

0 - 1

g a s g a s

p o w e r 2 7 5 %

heaf 41.6%

p o w e r 2 7 5 %

h e a t 61.7%

p o w e r 20 .0%

"hea t 69 .4%

F i g . 1.32. E n e r g y sys tem l a y o u t s and t y p i c a l Sankey d iagrams f o r t h e a p p l i c a t i o n o f gas t u r b i n e s , ( a ) i n c o n n e c t i o n t o a l o w - p r e s s u r e steam b o i l e r , ( b ) i n c o n n e c t i o n t o a p u l p d r y e r , ( c ) as a t o p p i n g u n i t . 1 - gas t u r b i n e , 2 - s team b o i l e r , 3 - p r o c e s s , 4 - p u l p d r y e r , 5 - steam t u r b i n e .

52

A more a t t r a c t i v e s o l u t i o n c o n s i s t s o f s u p p l y i n g t h e gases f rom t h e t u r b i n e

e x h a u s t d i r e c t l y t o p u l p d r y i n g , as i l l u s t r a t e d i n F i g . 1 . 3 2 ( b ) ( r e f s . 6 4 - 6 7 ) .

A p r o t o t y p e i n s t a l l a t i o n w i t h a gas t u r b i n e r a t e d a t 1200 kW i s o p e r a t e d i n

a F r e n c h f a c t o r y ( r e f . 4 1 ) . Assuming t h a t t h e t o t a l amount o f p u l p p r e s s e d t o

22% DS i s d r i e d t o 90% DS, and t h e e n e r g y b a l a n c e o f t h e t u r b i n e i s e s s e n t i a l l y

i d e n t i c a l t o t h a t shown i n F i g . 1.30, power can be g e n e r a t e d a t t h e r a t e o f

abou t 1.9 kWh p e r 100 kg b e e t .

The most advanced and e c o n o m i c a l l y a t t r a c t i v e c o n c e p t i s based on t h e

a p p l i c a t i o n o f a t o p p i n g g a s - t u r b i n e f rom w h i c h t h e e x h a u s t gases a r e s u p p l i e d

t o a steam b o i l e r e q u i p p e d w i t h i t s own f u r n a c e , as shown s c h e m a t i c a l l y i n

F i g . 1 . 3 2 ( c ) . By b u r n i n g a d d i t i o n a l f u e l i n t h e b o i l e r f u r n a c e , t he t e m p e r a t u r e

o f t h e gases can be r a i s e d , w h i c h a l l o w s t h e h e a t t r a n s f e r s u r f a c e s t o be made

as compact as i n a c o n v e n t i o n a l b o i l e r and t hus no more c o s t l y . The g a s - t u r b i n e

s e t and t he s t e a m - t u r b i n e s e t t o g e t h e r g e n e r a t e more power t han a c o n v e n t i o n a l

steam c y c l e . An i n t e r e s t i n g f e a t u r e o f t h i s s o l u t i o n , demons t ra ted i n a f u l l -

s c a l e i n d u s t r i a l a p p l i c a t i o n i n t h e chemica l i n d u s t r y ( r e f . 6 9 ) , i s t h a t t h e

t o p p i n g g a s - t u r b i n e can be c o n n e c t e d t o a c o n v e n t i o n a l b o i l e r a f t e r m o d e s t l y

e x p e n s i v e b o i l e r m o d i f i c a t i o n s .

The economic j u s t i f i c a t i o n o f t h e t h r e e v a r i a n t s d e s c r i b e d depends on f u e l

and e l e c t r i c i t y p r i c e s , as w e l l as c a p i t a l c o s t . I n s p e c i a l economic c o n d i t i o n s ,

namely h i g h power p r i c e , low D i e s e l - o i l p r i c e and a r e l a t i v e l y l a r g e power

demand d u r i n g t h e o f f - s e a s o n p e r i o d , i t m igh t be p r a c t i c a l t o c o n s i d e r a t o p p i n g

D i e s e l e n g i n e c o n n e c t e d t o a b a c k - p r e s s u r e steam c y c l e , o r t o a p u l p d r y e r

e q u i p p e d w i t h an a u x i l i a r y f u r n a c e ( r e f s . 7 0 , 7 1 ) . D u r i n g t h e o f f - s e a s o n p e r i o d ,

t h e e n g i n e can be o p e r a t e d f o r t h e p u r p o s e o f power g e n e r a t i o n o n l y .

A l t h o u g h r e l y i n g on e x i s t i n g t e c h n o l o g y , t h e f e a s i b i l i t y o f t h e a p p l i c a t i o n

o f D i e s e l e n g i n e and gas t u r b i n e i n t h e s u g a r i n d u s t r y y e t remains t o be

demons t ra ted i n p r a c t i c e . I t can be n o t e d t h a t a f t e r t h e f i r s t wave o f i n t e r e s t

i n c o m b u s t i o n - e n g i n e based s o l u t i o n s i n t h e l a t e 1970s, no l a r g e - s c a l e

i n v e s t m e n t s were u n d e r t a k e n , and some s c e p t i c i s m based on economic

c o n s i d e r a t i o n s was e x p r e s s e d i n t h e l i t e r a t u r e ( r e f . 7 2 ) .

1.5.5 U n c o n v e n t i o n a l t he rma l c o u p l i n g between t h e power house and t h e hea t

economy

A c o n v e n t i o n a l app roach t o t h e l i n k between t h e power house and t h e s u g a r

m a n u f a c t u r i n g p r o c e s s assumes t h a t t h e power house i s f u n c t i o n i n g as an e n e r g y

s o u r c e , and t h e p r o c e s s as an e n e r g y r e c e i v e r . An e n e r g y s t ream f l o w i n g i n t h e

r e v e r s e d i r e c t i o n , namely t h e c o n d e n s a t e r e t u r n e d f rom t h e e v a p o r a t o r t o t h e

b o i l e r s , i s se ldom t r e a t e d as a p a r t o f e n e r g y c o n v e r s i o n and u t i l i z a t i o n

p r o c e s s e s b u t r a t h e r as an a r rangemen t t o s e c u r e p r o p e r q u a l i t y o f t h e b o i l e r

f e e d w a t e r .

53

A c o n s i d e r a b l e e n e r g y - s a v i n g p o t e n t i a l i s a s s o c i a t e d w i t h t h e p o s s i b i l i t y o f

r e c u p e r a t i n g l o w - t e m p e r a t u r e h e a t f rom c e r t a i n s e c t i o n s o f t h e p r o c e s s and

r e t u r n i n g i t f o r r e - u s e i n o t h e r s e c t i o n s . A d i f f i c u l t y i n h e r e n t i n t h i s

app roach i s t o f i n d p r o c e s s media w i t h t e m p e r a t u r e s low enough t o make t h e

a b s o r p t i o n o f t h e r e c u p e r a t e d h e a t p o s s i b l e . ( I t i s p r e c i s e l y f o r t h i s r e a s o n

t h a t l o w - t e m p e r a t u r e p u l p d r y i n g i s so i n t e r e s t i n g as an e n e r g y - s a v i n g m e a s u r e ,

because a i r d i r e c t e d t o a l o w - t e m p e r a t u r e d r y e r i s h e a t e d by w a s t e h e a t w h i c h

wou ld o t h e r w i s e be d i s s i p a t e d t o t h e e n v i r o n m e n t . )

The l o w - t e m p e r a t u r e h e a t can a l s o be r e t u r n e d t o t h e power h o u s e , by h e a t i n g

combus t ion a i r s u p p l i e d t o t h e b o i l e r f u r n a c e s . By making i t p o s s i b l e t o c u t

down t he f u e l consumpt ion i n t h e b o i l e r s , t h i s s o l u t i o n can be e c o n o m i c a l l y

j u s t i f i e d i f t h e f u e l s a v i n g i s l a r g e enough t o pay back t h e i n v e s t m e n t i n t h e

h e a t - r e c u p e r a t i n g and a i r - h e a t i n g e q u i p m e n t .

The d e t a i l s o f t h e the rma l c o u p l i n g between t h e power house and t h e h e a t

economy may v a r y . A s o l u t i o n implemented i n a B e l g i a n f a c t o r y has been d e s c r i b e d

i n t h e l i t e r a t u r e ( r e f . 7 3 ) . The hea t i s r e c o v e r e d f rom s p e n t c a r b o n a t a t i o n gas

i n a d i r e c t - c o n t a c t h e a t e r where t h e t e m p e r a t u r e o f c i r c u l a t i n g w a t e r i s r a i s e d

f rom 50'^C t o a b o u t 80°C ( s e e a l s o S e c t i o n 4 . 2 . 2 ) . Water i s s u b s e q u e n t l y pumped

t o a i r p r e h e a t e r s c o n n e c t e d t o t h e b o i l e r s ( a l t e r n a t i v e l y , i t can be pumped t o

a i r p r e h e a t e r s c o n n e c t e d t o t h e s u g a r d r y e r ) . F u e l s a v i n g s o f t h e o r d e r o f

0.1 kg normal f u e l p e r 100 kg b e e t have been r e p o r t e d f o r t h i s s o l u t i o n .

A i r p r e h e a t e r s can a l s o be s u p p l i e d w i t h h e a t r e c o v e r e d f rom vacuum-pan

v a p o u r s c o n d e n s i n g i n a s p e c i a l c o n d e n s e r ( r e f . 7 4 ) . The t e m p e r a t u r e o f h e a t -

c a r r y i n g w a t e r a t t h e c o n d e n s e r o u t l e t i s a b o u t 58-59°C. F o r t h i s r e a s o n , t h e

economic j u s t i f i c a t i o n o f t h i s s o l u t i o n m igh t be more d i f f i c u l t t o e s t a b l i s h .

F o r a compar i son w i t h o t h e r methods o f u t i l i z a t i o n o f vacuum-pan v a p o u r , see

S e c t i o n 3 . 3 . 1 .

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2 Β. K a r r e n , The p o t e n t i a l f o r e n e r g y s a v i n g i n t h e b e e t s u g a r i n d u s t r y . S u g a r J . , 4 4 ( 1 ) (1981) 8 -13 .

3 T . B a l o h , W ä r m e w i r t s c h a f t , i n : F . S c h n e i d e r ( E d . ) , T e c h n o l o g i e des Z u c k e r s , S c h a p e r V e r l a g , H a n n o v e r , 1968, p p . 705-776.

4 S . Z a g r o d z k i , Gospodarka C i e p l n a C u k r o w n i , WNT, Warszawa , 1979. 5 K. U r b a n i e c , S p r e z a n i e oparow w g o s p o d a r c e c i e p l n e j c u k r o w n i , G a z . C u k r o w . ,

90 (9 ) (1982) 134-136. 6 T . D . E a s t o p and A . McConkey , A p p l i e d Thermodynamics f o r E n g i n e e r i n g

T e c h n o l o g i s t s , 3 rd e d n . , Longman, London and New Y o r k , 1978. 7 G . J . Van Wylen and R . E . S o n n t a g , Fundamenta ls o f C l a s s i c a l T h e r m o d y n a m i c s ,

3 rd e d n . , W i l e y , New Y o r k , 1985. 8 T . B a l o h , Wärmeat las f ü r d i e Z u c k e r i n d u s t r i e , S c h a p e r V e r l a g , H a n n o v e r , 1975. 9 P. V a l e n t i n , E n e r g y c o n s e r v a t i o n s t u d i e s i n t h e b e e t s u g a r i n d u s t r y . I n t .

Sugar J . , 82(982) (1980) 303-309.

54

10 Ε. H u g o t , Handbook o f Cane S u g a r E n g i n e e r i n g , 3 rd e d n . , E l s e v i e r , Amsterdam, 1986.

11 P. V a l e n t i n , U b e r d i e B e e i n f l u s s u n g des P r i m ä r e n e r g i e v e r b r a u c h s i n d e r Z u c k e r i n d u s t r i e , Ζ . Z u c k e r i n d . , 26 (8 ) (1976) 525-534.

12 P. Mosel ( e t a l . ) , O p t i m i e r u n g von E i n d i c k u n g s p r o z e s s e n i n d e r Z u c k e r i n d u s t r i e , Z u c k e r i n d . , 104(12) (1979) 1101-1107.

13 P. G i r a u d , R e d u i r e l e s c o u t s e n e r g e t i q u e s p a r 1 ' u t i l i s a t i o n de l ' e l e c t r i c i t e , I n d . A l i m . A g r i e , 102 (7 -8 ) (1985) 707-710.

14 Κ . Ε . A u s t m e y e r , B rüdenkompress ion i n d e r Z u c k e r i n d u s t r i e , Z u c k e r i n d . , 108(8) (1983) 715-728.

15 R. M i c h e l , P h . T e r n y n c k and P h . B o n n e n f a n t , R e a l i s a t i o n du p o s t e d ' e v a p o r a t i o n dans une u s i n e de 12 000 t / j de b e t t e r a v e s s t o c k a n t 60% du s i r o p p r o d u i t en campagne, I n d . A l i m . A g r i e , 9 4 ( 7 - 8 ) (1977) 701-705.

16 R e p e r t o i r e des S u c r e r i e s e t R a f f i n e r i e s b e i g e s , S u c r . B e i g e , (102) (1984) 21-74.

17 P. C h r i s t o d o u l o u , B e t r i e b s e r f a h r u n g e n m i t dem E i n s a t z e i n e r Wärmepumpe i n V e r d a m p f s t a t i o n e i n e r Z u c k e r f a b r i k , Z u c k e r i n d . , 109(7) (1984) 628-634.

18 P. Ho f fman, O p t i m a l i z a c e e n e r g e t i c k e h o h o s p o d a r s t v i c u k r o v a r u L o v o s i c e , L i s t y C u k r . , 102(7) (1986) 155-161.

19 F. Baunack , T r o c k n u n g , i n : F . S c h n e i d e r ( E d . ) , T e c h n o l o g i e des Z u c k e r s , Schape r V e r l a g , H a n n o v e r , 1968, p p . 845-883.

20 T h . C r o n e w i t z , Wege z u r r a t i o n e l l e n E n e r g i e v e r w e n d u n g b e i d e r S c h n i t z e l -t r o c k n u n g i n d e r Z u c k e r i n d u s t r i e , Z u c k e r i n d . , 105(2) (1980) 129-139.

21 F . Amding, Abwärmenutzung z u r Saf tanwärmung im Zusammenhang m i t d e r S c h n i t z e l t r o c k n u n g , Z u c k e r i n d . , 110(8) (1985) 675-679.

22 P. V e r m e u l e n , Sa f t e i ndamp fung m i t t e l s T r o c k n u n g s a b g a s be i dessen R e i n i g u n g , Z u c k e r i n d . , 110(8) (1985) 681-685.

23 K . E . Aus tmeye r and W. P o e r s c h , N i e d e r t e m p e r a t u r t r o c k n u n g - G r u n d l a g e n und B e t r a c h t u n g e n z u r W i r t s c h a f t l i c h k e i t , Z u c k e r i n d . , 108(9) (1983) 861-868, 108(11) (1983) 1033-1041, 109(5) (1984) 411-419, 110(1) (1985) 28-34.

24 E. S c h r ö t e r , D ie N i e d e r t e m p e r a t u r t r o c k n u n g i n L e h r t e - F u n k t i o n s w e i s e und E r f a h r u n g e n , Z u c k e r i n d . , 111(6) (1986) 545-548.

25 K . E . Aus tmeye r and U . B u n e r t , Abwärmenutzung im Zusammenhang m i t d e r S c h n i t z e l t r o c k n u n g , Z u c k e r i n d . , 110(8) (1985) 659-670.

26 M. Kunz and P. V a l e n t i n , S c h n i t z e l t r o c k n u n g ohne P r i m ä r e n e r g i e e i n s a t z u n t e r a u s s c h l i e s s l i c h e r Nu tzung d e r Abwärme- und E i n d a m p f p o t e n t i a l e d e r Z u c k e r f a b r i k , Z u c k e r i n d . , 111(8) (1986) 741-750.

27 G . F e l t b o r g , Ä t g ä r d e r f ö r a t t minska u p p v ä r m n i n g s k o s t n a d e r n a v i d e t t s o c k e r b r u k , S o c k e r b o l a g e t r e p o r t , 1985.

28 Gu ide Book f o r F a c t o r y E n g i n e e r s on E n e r g y C o n s e r v a t i o n D i a g n o s i s , UNIDO Document I S . 4 4 9 , V i e n n a , 1984.

29 J . S . Hogg ( e t a l . ) . The r o l e o f t h e r m o g r a p h i c s u r v e y i n g i n e n e r g y c o n s e r v a t i o n . I n t . Suga r J . , 85(1011) (1983) 67-71.

30 P. C h r i s t o d o u l o u , D ie O p t i m i e r u n g d e r E n e r g i e w i r t s c h a f t i n d e r Z u c k e r f a b r i k , Ζ . Z u c k e r i n d . , 27 (7 ) (1977) 441-446, 27 (8 ) (1977) 509-515.

31 O . V . M o r o z , A . A . L i p e t s and D.M. K o r i l k e v i c h , P u t i umensheniya p o t e r t e p l a na s t a n t s i i d e f e k o s a t u r a t s i i , Sakh . P r o m . , ( 9 ) (1985) 45-47 .

32 Y u . D . Kot ( e t a l . ) , P r o i z v o d s t v e n n y e i s p y t a n i y a n i z k o t e m p e r a t u r n o g o r e z h i m a , Sakh . P r o m . , ( 2 ) (1985) 20-22 .

33 P . - V . Schmid t and E. Manzke , Zu F r a g e n d e r E n e r g i e w i r t s c h a f t be i d e r T e i l e x t r a k t i o n , L e b e n s m i t t . - I n d . , 2 4 ( 1 ) (1977) 21 -24 , 2 4 ( 2 ) (1977) 77-80.

34 T . P . Ma t v i enko ( e t a l . ) , P r o b e l i v a n i e sakha ra s pr imenen iem v t o r o g o o t t e k a u t f e l y a I k r i s t a l l i z a t s i i , Sakh . P r o m . , ( 8 ) (1984) 31-34.

35 P. M o s e l , H . - R . Kemter and T h . C r o n e w i t z , Z u r Anwendung e i n e r S i r u p d e c k e b e i p e r i o d i s c h a r b e i t e n d e n Z e n t r i f u g e n , Z u c k e r i n d . , 111(3) (1986) 211-216.

36 H. V o g e l e r . E i n Weg z u r hohen D i c k s a f t d i c h t e und d e r e n w i r t s c h a f t l i c h e N u t z u n g , Z u c k e r , 30(12) (1977) 676-683.

37 H . - J . Krombach, M ö g l i c h k e i t e n z u r Senkung des H e i z d a m p f V e r b r a u c h e s im Z u c k e r h a u s , Z u c k e r i n d . , 106(9) (1981) 793-804.

38 W. L e k a w s k i , M o d e r n i z a c j a G o s p o d a r k i C i e p l n e j C u k r o w n i , S T C , Warszawa , 1986.

55

39 W. v . P r o s k o w e t z , K r a f t z e n t r a l e , i n : F . S c h n e i d e r ( E d . ) , T e c h n o l o g i e des Z u c k e r s , S c h a p e r V e r l a g , H a n n o v e r , 1968, p p . 676-704.

40 B . P . E f a n o v and A . G . K u t k o v o y , N o r m i r o v a n i e r askhoda e l e k t r i c h e s k o i e n e r g i i V s a k h a r n o i p r o m y s h l e n n o s t i , S a k h . P r o m . , ( 2 ) (1986) 39-43.

41 C . Longue E p e e , L e c t u r e p r e s e n t e d a t t h e I n t e r n a t i o n a l E x h i b i t i o n SVEKLOVODSTVO, K i e v , May 1986.

42 B. M a y r h o f e r and P. K n e d l i k , D ie R e i n i g u n g von Z u c k e r r ü b e n m i t t e l s L u f t s t r o m , Z u c k e r i n d . , 108(2) (1983) 138-140, 111(2) (1986) 128-132.

43 A . I . Khomenko, O t e p l o v o i e k o n o m i c h n o s t i s i s t e m d i f f u z i y a - d e f e k a t s i y a , S a k h . P r o m . , (11 ) (1983) 42 -47 .

44 V . N . F i l o n e n k o and A . N . Z a g o r u y k o , N e r i t m i c h n o s t r a b o t y s v e k l o s a k h a r n o g o zavoda i u d e l n y e r a s k h o d y e n e r g o r e s u r s o v , S a k h . P r o m . , ( 6 ) (1986) 37-40.

45 U . Zimmer and A . Dambach, S t rombezugsüberwachung im Werk P l a t t l i n g d e r Süddeu tschen Z u c k e r - A G , Z u c k e r i n d . , 108(10) (1983) 940-942.

46 R . A . H . C h i l v e r s , C o n t r o l o f maximum e n e r g y demand u s i n g a m i c r o p r o c e s s o r s y s t e m , P r o c . SASTA, 58 (1984) 111-115.

47 G . H . P i a t t , Steam t u r b i n e deve lopmen t i n t h e b e e t s u g a r i n d u s t r y . I n t . S u g a r J . , 82(982) (1980) 297-302.

48 B . L . K a r r e n , E f f i c i e n c y c o n s i d e r a t i o n s i n t h e use o f p r o c e s s s t e a m . S u g a r J . , 4 7 ( 2 ) (1984) 13-15.

49 V . N . F i l o n e n k o , E f f e k t i v n o s t m e r o p r i y a t i i po s n i z h e n y u r a s k h o d a e n e r g o r e s u r s o v i s b a l a n s i r o v a n n o s t e n e r g o p o t r e b l e n i y a sakha rnogo z a v o d a , Sakh . P r o m . , ( 7 ) (1986) 43-46.

50 J . A . B e z e r r a , The use o f e l e c t r i c b o i l e r s i n s u g a r r e f i n e r i e s as an a l t e r n a t i v e s o u r c e o f s t eam. S u g a r I n d . T e c h n o l . , 44 (1985) 277-297.

51 H . R . B r u n n e r , W. Hoppe and G . v . L e n g y e l - K o n o p i , B e t r a c h t u n g e n z u r E n t w i c k l u n g d e r e n e r g e t i s c h e n K o n z e p t i o n d e r Z u c k e r f a b r i k + R a f f i n e r i e A a r b e r g A G , Z u c k e r i n d . , 106(1) (1981) 42 -47 .

52 H . R . B r u n n e r , D ie Thermokompress ion i n d e r Z u c k e r f a b r i k + R a f f i n e r i e A a r b e r g A G , G e s c h i c h t e - E n t w i c k l u n g - A u s b l i c k , Z u c k e r i n d . , 108(8) (1983) 729-736.

53 H . R . B r u n n e r ( e t a l . ) . D ie V e r d a m p f s t a t i o n d e r Z u c k e r f a b r i k + R a f f i n e r i e A a r b e r g AG und das M u l t i - E n e r g i e - S c h e m a , Z u c k e r i n d . , 110(5) (1985) 393-398.

54 W. L e i b i g , B a s i c e n e r g y and i t s u t i l i z a t i o n i n t h e cane s u g a r i n d u s t r y , Z u c k e r i n d . , 103(5) (1978) 412-416.

55 G . D e r m a l , E n e r g y i n a cane s u g a r c o m p l e x : an o r i g i n a l t e c h n i c a l s o l u t i o n f o r i t s most e f f i c i e n t u s e . Suga r J . , 4 4 ( 6 ) (1981) 5 -8 .

56 T . T o r i s s o n , The p o t e n t i a l f o r g e n e r a t i o n o f p u b l i c e l e c t r i c i t y i n cane s u g a r f a c t o r i e s . S u g a r y A z ú c a r , 7 9 ( 4 ) (1984) 21-31.

57 L. B e r g f o r s , H. H u t t u n e n and J . V i l j a n e n , 20 J a h r e Fernwärmeerzeugung i n d e r Z u c k e r f a b r i k T u r e n k i , Z u c k e r i n d . , 109(7) (1984) 634-637.

58 Κ . Ε . Aus tmeye r and U . B u n e r t , M ö g l i c h k e i t e n z u r V e r b e s s e r u n g d e r E n e r g i e w i r t s c h a f t b e i d e r Z u c k e r g e w i n n u n g , i n : P r o c . 17th C I T S , C o p e n h a g e n , 1983, p p . 333-369.

59 Ε. O t o r o w s k i , R a c j o n a l n e g o s p o d a r o w a n i e p a r a ζ k o t l o w , G a z . C u k r o w . , 9 3 ( 2 ) (1985) 204-206.

60 V . S . Mokhor t and V . N . C h i k i r i s o v , T e k h n i k o - e k o n o m i c h e s k o e s r a v n e n i e r e k o n s t r u k t s i i TEC sakha rnykh z a v o d o v s u s t a n o v k o i n a d s t r o y k i v y s o k o g o d a v l e n i y a , Sakh . P r o m . , ( 7 ) (1983) 40 -42 .

61 R. A n t o i n e , E l e c t r i c i t y e x p o r t f rom cane s u g a r f a c t o r i e s , i n : F . O . L i c h t s Gu ide t o t h e S u g a r F a c t o r y Mach ine I n d u s t r y , F . O . L i c h t GmbH, R a t z e b u r g , 1984, p p . A75-A88.

62 T . E n g b e r g , Steam and power g e n e r a t i o n i n t h e s u g a r i n d u s t r y . I n t . S u g a r J . , 86(1031) (1984) 286-287.

63 N. M a r i g n e t t i and G . M a n t o v a n i , B e t r a c h t u n g e n ü b e r den E i n s a t z d e r G a s t u r b i n e i n d e r Z u c k e r i n d u s t r i e , Z u c k e r , 2 7 ( 9 ) (1974) 470-474.

64 U . H a n t s c h , E i n s a t z von G a s t u r b i n e n i n Z u c k e r f a b r i k e n , Z . Z u c k e r i n d . , 25 (1 ) (1975) 31-32.

65 H. P o h l e r t , D ie Verwendung von G a s t u r b i n e n i n Z u c k e r f a b r i k e n , Z u c k e r , 30(2 ) (1977) 75-76.

56

66 W . J . L e i b i g , Use o f gas t u r b i n e s i n t h e s u g a r i n d u s t r y . S u g a r J . , 40 (12 ) (1978) 13-15.

67 M. B r u h n s , B e i t r a g z u r w i r t s c h a f t l i c h e n B e u r t e i l u n g e i n e r Z u c k e r f a b r i k m i t B rüdenkompress ion und G a s t u r b i n e i n d e r S c h n i t z e l t r o c k n u n g , Z u c k e r i n d . , 107(10) (1982) 945-957.

68 H. L ö f f e l and D. T h i n i u s , G a s t u r b i n e n e i n s a t z im Rahmen d e r K r a f t - W ä r m e -K o p p l u n g , BWK, 37(12) (1985) 482-487.

69 H. L ö f f e l and M. S c h u l z , G a s t u r b i n e f ü r E n e r g i e v e r s o r g u n g s s y s t e m e i n e s I n d u s t r i e b e t r i e b e s , BWK, 36 (6 ) (1984) 243-248.

70 H. Huber and H. L i c h a , E i n Weg z u r Sommerst romerzeugung i n d e r Z u c k e r i n d u s t r i e , Z u c k e r i n d . , 104(1) (1979) 25-29.

71 H . - U . R e i c h e l , Gedanken z u r E n t w i c k l u n g des P r i m ä r e n e r g i e e i n s a t z e s - Dampf und St rom i n d e r Z u c k e r i n d u s t r i e , Z u c k e r i n d . , 107(10) (1982) 936-939.

72 P. V a l e n t i n , D i s k u s s i o n s b e i t r a g z u "Wärmepumpen i n d e r Z u c k e r i n d u s t r i e " , Z u c k e r i n d . , 108(8) (1983) 746-748.

73 Anonymous, R e c u p e r a t i o n t he rm ique s u r buees de seconde c a r b o n a t a t i o n a l a R a f f i n e r i e Notre-Dame a O r e y e , S u c r . B e l g e , 103 (1985) 5-11.

74 T e c h n i c a l i n f o r m a t i o n f rom Wiegand K a r l s r u h e GmbH, E t t l i n g e n , 1986.

57

C h a p t e r 2

ENERGY BALANCES

2.1 PRINCIPLES OF ESTABLISHING ENERGY BALANCES

The e n e r g y b a l a n c e o f a s u g a r f a c t o r y , o r a p a r t o f i t , can be a n a l y s e d on

t h e b a s i s o f t h e f i r s t law o f t h e r m o d y n a m i c s , w h i c h i s e s s e n t i a l l y a s t a t e m e n t

o f t h e p r i n c i p l e o f t he c o n s e r v a t i o n o f e n e r g y . The f i r s t law o f thermodynamics

can be r i g o r o u s l y s t a t e d i n t h e fo rm o f a ma themat i ca l e q u a t i o n , p r o v i d e d t h e

o b j e c t unde r c o n s i d e r a t i o n i s unamb igous l y d e f i n e d as a thermodynamic s y s t e m

( r e f s . 1 , 2 ) . As a l r e a d y ment ioned i n S e c t i o n 1.1, t h e i d e n t i f i c a t i o n o f a

p r e s c r i b e d and i d e n t i f i a b l e bounda ry i s n e c e s s a r y f o r sys tem d e f i n i t i o n ; t h e

boundary s e p a r a t e s t h e sys tem f rom i t s s u r r o u n d i n g s . Once t h e b o u n d a r y has been

d e f i n e d , t h e f i r s t law o f thermodynamics s t a t e s s i m p l y t h a t i n any p r o c e s s , t h e

e n e r g y d e l i v e r e d t o t h e sys tem must equa l t h e sum o f t h e e n e r g y s t o r e d i n i t and

t he e n e r g y removed t o t h e s u r r o u n d i n g s . The mathemat i ca l f o r m u l a t i o n t akes

a c c o u n t o f t h e f a c t t h a t e n e r g y can be a t t r i b u t e d t o m a t t e r c o n t a i n e d i n t h e

sys tem o r , p o s s i b l y , f l o w i n g a c r o s s i t s b o u n d a r y , and t h a t e n e r g y can a l s o be

t r a n s f e r r e d t o and f rom t h e s u r r o u n d i n g s as h e a t and w o r k . I f t h e b o u n d a r y has

been so d e f i n e d t h a t t h e r e i s no mass t r a n s f e r a c r o s s i t , t h e n t h e sys tem i s

s a i d t o be c l o s e d . E n e r g y can e n t e r o r l e a v e a c l o s e d sys tem o n l y as h e a t o r

w o r k .

I n most p rob lems d i s c u s s e d i n t h i s b o o k , h o w e v e r , a t y p i c a l s i t u a t i o n i s t h a t

t he o b j e c t unde r c o n s i d e r a t i o n has t o be i n t e r p r e t e d as an open s y s t e m , t h a t i s ,

i t s boundary has t o be d e f i n e d so as t o a l l o w f o r mass t r a n s f e r t o and f rom t h e

s u r r o u n d i n g s . Such a bounda ry i s sometimes c a l l e d a c o n t r o l s u r f a c e and t h e

sys tem encompassed , a c o n t r o l vo lume .

A p r e r e q u i s i t e f o r d e t e r m i n i n g t h e e n e r g y b a l a n c e o f an open sys tem i s t h a t

t he mass f l o w s c r o s s i n g t h e sys tem bounda ry a r e known. I n c e r t a i n e n g i n e e r i n g

p r o b l e m s , h o w e v e r , i t may i n i t i a l l y be n e c e s s a r y t o c a l c u l a t e some unknown mass

f l o w s . I f t h i s i s t he c a s e , t hen one has t o use a mass b a l a n c e e q u a t i o n w h i c h i s

an e x p r e s s i o n o f t h e law o f mass c o n s e r v a t i o n

M 3 = Δ Μ + ( 2 . 1 )

where i s t h e mass d e l i v e r e d t o t he s y s t e m , Δ Μ i s t h e i n c r e a s e o f t h e s y s t e m

mass, and i s t h e mass removed f rom t h e s y s t e m .

The above e q u a t i o n h o l d s f o r t h e f i n i t e t ime p e r i o d d u r i n g w h i c h t h e

measurements t o d e t e r m i n e M ^ , Δ Μ and have been p e r f o r m e d . I n t h e p rob lems

c o n s i d e r e d h e r e , mass i n f l o w and o u t f l o w u s u a l l y r e s u l t f rom m u l t i p l e mass f l o w s

^ s l ' ^ s 2 » - * ' ^ s p e n t e r i n g t he sys tem and G ^ ^ ^ r 2 " * ' ^ r q " '^s iv ing i t ( F i g . 2 . 1 ) .

58

mass flows entering the system BOUNDARY

niass Uows leaving the systenn

work

F i g . 2 .1 . Mass and e n e r g y b a l a n c e s o f an open thermodynamic s y s t e m .

I t may t h e r e f o r e be c o n v e n i e n t t o e x p r e s s t h e mass b a l a n c e f o r u n i t t ime

% } ' Gsp = ^V^t + ^ ^ + + . . + G , q ( 2 . 2 )

where M^ i s t h e sys tem mass, i . e . t h e mass c o n t a i n e d w i t h i n t h e sys tem b o u n d a r y .

A t y p i c a l case o f an open sys tem i s a s t e a d y - s t a t e sys tem w h i c h i s

c h a r a c t e r i z e d by t i m e - i n v a r i a n t mass and c o n s t a n t mass f l o w s . As t h e t ime

d e r i v a t i v e o f t h e sys tem mass e q u a l s z e r o , t h e b a l a n c e e q u a t i o n becomes

^ s i ^ h z ^1 ^ ^ 2 ^ - - ^ ^ q ( 2 . 3 )

Hav ing de te rm ined t h e mass f l o w s c r o s s i n g t h e sys tem b o u n d a r y , we can r e t u r n

t o t h e main p r o b l e m . A g e n e r a l fo rm o f t h e e n e r g y b a l a n c e e q u a t i o n i s

E3 = ΔΕ + E^ ( 2 . 4 )

where E^ i s t h e e n e r g y d e l i v e r e d t o t h e s y s t e m , ΔΕ i s t h e i n c r e a s e o f t h e

s y s t e m ' s e n e r g y , and E^, i s t h e e n e r g y removed f rom t h e s y s t e m .

U s i n g t h e mass f l o w s and e x p r e s s i n g t h e b a l a n c e f o r u n i t t i m e , we o b t a i n f o r

t h e sys tem shown i n F i g . 2.1

- ^V^t ^ L + G^^h^T . G^^VZ qVq ' ( 2 - 5 )

where h ^ ^ . . , h^p and h ^ - j , . . , h^^ a r e t h e e n t h a l p i e s p e r u n i t mass i n i n f l o w i n g

and o u t f l o w i n g s t r e a m s , E^ i s t h e e n e r g y a s s o c i a t e d w i t h t h e m a t t e r c o n t a i n e d

w i t h i n t h e sys tem b o u n d a r y , L i s t h e mechan ica l power (work p e r u n i t t i m e ) and

Q i s t h e the rma l power ( h e a t p e r u n i t t i m e ) d e l i v e r e d t o t h e s y s t e m . I n s t e a d y -

s t a t e s y s t e m s , t h e t ime d e r i v a t i v e o f E^ i s z e r o , a l l t h e q u a n t i t i e s c o n c e r n e d

a r e c o n s t a n t , and the e n e r g y b a l a n c e e q u a t i o n becomes

• ^ s l ^ s l ' %2\2 h p h , - L * ^ ^ ^ ^ - \ 2 \ 2 ' - ^ V r q " ^ ( ^ . 6 )

I t i s assumed i n e q n s . ( 2 . 5 ) and ( 2 . 6 ) t h a t t h e s i g n o f t h e work i s p o s i t i v e i f

i t i s removed f rom t h e s y s t e m , w h i l e t h e s i g n o f t h e h e a t i s p o s i t i v e i f i t i s

59

d e l i v e r e d t o t h e s y s t e m . The r e s u l t s o f t h e c a l c u l a t i o n s i n w h i c h t h e s e

e q u a t i o n s a r e used a r e c o r r e c t o n l y i f t h i s s i g n c o n v e n t i o n i s o b s e r v e d . One

s h o u l d a l s o be aware o f c e r t a i n i n h e r e n t l i m i t a t i o n s o f t h e b a l a n c e e q u a t i o n s ,

as w e l l as o f t h e c o n d i t i o n s f o r t h e i r c o r r e c t u s e . The d e t a i l s o f t h e prob lems

may v a r y , depend ing on t h e p u r p o s e f o r w h i c h t h e e q u a t i o n s a r e s e t u p , as

e x p l a i n e d b e l o w .

( i ) I n t h e d e s i g n a n a l y s e s , i t i s u s u a l l y assumed t h a t t h e e q u i p m e n t , i n c l u d i n g

a u x i l i a r y d e v i c e s , i s w o r k i n g i n a c c o r d a n c e w i t h t h e t e c h n i c a l s p e c i f i c a t i o n s ,

i . e . t h a t t h e r e a r e no m a l f u n c t i o n i n g steam t r a p s , u n r e l i a b l e v a l v e s , l e a k i n g

p a c k i n g s , e t c . A n o t h e r t y p i c a l assump t i on i s t h a t t h e p r o c e s s i n g c a p a b i l i t y i s

c o n s t a n t (most o f t e n , t h e nominal o r maximum c a p a b i l i t y v a l u e i s assumed) . Even

t h e s e s e e m i n g l y o b v i o u s a s s u m p t i o n s , h o w e v e r , c o n s t i t u t e an i d e a l i z a t i o n o f t h e

e n e r g y p r o c e s s e s a n a l y s e d . The r e a l p r o c e s s e s w i l l c e r t a i n l y be c h a r a c t e r i z e d by

f l u c t u a t i n g p a r a m e t e r s , t h e p r o c e s s i n g c a p a b i l i t y may d e v i a t e f rom t h e v a l u e

assumed, and t h e r e w i l l be some unknown f l o w s o f t h e p r o c e s s media l e a k i n g

between sys tem p a r t s and f rom t h e sys tem t o t h e s u r r o u n d i n g s . Unde r such

c i r c u m s t a n c e s , a l t h o u g h i t i s e s s e n t i a l n o t t o i n t r o d u c e any s i g n i f i c a n t

s y s t e m a t i c e r r o r s i n t o t h e b a l a n c e r e l a t i o n s h i p s , i t i s a l s o m e a n i n g l e s s t o

c o m p l i c a t e t h e b a l a n c e e q u a t i o n s by i n t r o d u c i n g f a c t o r s w h i c h can a c t u a l l y be

n e g l e c t e d w i t h o u t i n c r e a s i n g t h e o v e r a l l u n c e r t a i n t y m a r g i n . T h i s a p p l i e s , i n

t he f i r s t p l a c e , t o t h e e n e r g y s t reams a s s o c i a t e d w i t h t h e h e a t o f

c r y s t a l l i z a t i o n o f s u g a r and t h e mechan ica l work s u p p l i e d t o t h e p r o c e s s

e q u i p m e n t , as t h e i r o r d e r o f magn i tude may be comparab le w i t h t h a t o f t h e

unknown l o s s e s t o t he s u r r o u n d i n g s .

( i i ) I n t h e p r o c e s s m o n i t o r i n g a p p l i c a t i o n s , a number o f d i f f e r e n t s i t u a t i o n s

s h o u l d be r e c o g n i z e d . When i n v e s t i g a t i n g an e x i s t i n g f a c t o r y t o be m o d e r n i z e d ,

t h e c a l c u l a t i o n e r r o r s s h o u l d n o t e x c e e d t h e u n c e r t a i n t y m a r g i n c h a r a c t e r i s t i c

o f t h e s u b s e q u e n t d e s i g n c a l c u l a t i o n s . I n t h e r o u t i n e m o n i t o r i n g t a s k s aimed a t

p r o v i d i n g t h e f a c t o r y managers w i t h i n f o r m a t i o n on how e f f i c i e n t l y t h e e n e r g y i s

u t i l i z e d , t h e s p e c i f i c r e q u i r e m e n t s may v a r y depend ing on t h e e x p e c t e d f a c t o r y

p e r f o r m a n c e . U s u a l l y , t h e d e s i r e d a c c u r a c y i s s i m i l a r t o t h a t t y p i c a l o f t h e

d e s i g n p r o b l e m s . H o w e v e r , t h e r e may be s p e c i a l cases i n w h i c h r a t h e r h i g h

a c c u r a c y i s r e q u i r e d , f o r e x a m p l e , when t h e g u a r a n t e e t e s t s o f equ ipment u n i t s

o r e n t i r e s t a t i o n s have t o be m o n i t o r e d .

( i i i ) As a r u l e , t h e b a l a n c e c a l c u l a t i o n s r e l a t i n g t o s u g a r f a c t o r i e s , o r p a r t s

o f them, a r e pe r f o rmed unde r t h e assump t i on o f s t e a d y - s t a t e c o n d i t i o n s , u s i n g

e q n s . ( 2 . 3 ) and ( 2 . 6 ) . W h i l e t h i s may be p e r f e c t l y c o r r e c t i n most c a s e s , g r e a t

c a r e i s recommended i n i n t e r p r e t i n g t h e r e s u l t s o f e x p e r i m e n t a l i n v e s t i g a t i o n s .

I f t he measurements have been pe r fo rmed w i t h o u t s t r i c t l y m a i n t a i n i n g s t e a d y -

s t a t e c o n d i t i o n s , t h e n n e g l e c t o f t i m e - d e r i v a t i v e s o f and E^ i n e q n s . ( 2 . 2 )

60

and ( 2 . 5 ) , r e s p e c t i v e l y , may become a s o u r c e o f e r r o r s .

2.2 INPUT DATA FOR ENERGY BALANCE CALCULATIONS

2.2.1 N a t u r e o f t he i n p u t d a t a

The c a l c u l a t i o n p r i n c i p l e s p r e s e n t e d i n t h e p r e v i o u s S e c t i o n can be a p p l i e d

t o any thermodynamic sys tem w i t h i n a s u g a r f a c t o r y . F o r a p a r t i c u l a r s y s t e m ,

a s e t o f b a l a n c e e q u a t i o n s can be f o r m u l a t e d , making i t p o s s i b l e t o d e t e r m i n e

t h e v a l u e s o f as many unknown v a r i a b l e s as t h e r e a r e e q u a t i o n s i n t h e s e t . F o r

each e n e r g y b a l a n c e p r o b l e m , an a p p r o p r i a t e s e t o f i n p u t d a t a must be a v a i l a b l e .

I f some r e q u i r e m e n t s have been imposed on t h e s t r u c t u r i n g o f t h e b a l a n c e

r e s u l t s , t h a t i s , i f t h e sys tem i s t o be c o n s i d e r e d as t h e sum o f s p e c i f i c

s u b s y s t e m s , bo th t h e s e t o f e q u a t i o n s and t he s e t o f i n p u t d a t a s h o u l d be

p r e p a r e d i n a manner making i t p o s s i b l e t o s a t i s f y t h e s e r e q u i r e m e n t s . T h i s

i m p l i e s , i n t u r n , t h e n e c e s s i t y n o t o n l y t o a c c o u n t f o r t h e s y s t e m s t r u c t u r e ,

b u t a l s o t o s a t i s f y t h e c o n s t r a i n t s t h a t a r e c h a r a c t e r i s t i c o f t h e s u g a r

m a n u f a c t u r i n g p r o c e s s . F i n a l l y , t h e i n p u t da ta s h o u l d be c o m p l e t e , t h a t i s ,

among t he q u a n t i t i e s a p p e a r i n g i n t h e b a l a n c e e q u a t i o n s , o n l y as many can be

l e f t unknown as t h e r e a r e e q u a t i o n s i n t h e s e t .

The g u i d e l i n e s f o r a d e s c r i p t i o n o f t h e s t r u c t u r e o f a thermodynamic s y s t e m

and i t s subsys tems have been f o r m u l a t e d i n S e c t i o n 1.1, and t y p i c a l s t r u c t u r a l

e lements e n c o u n t e r e d i n the rma l sys tems o f s u g a r f a c t o r i e s have been d i s c u s s e d

i n S e c t i o n 1.2.

C l o s e l y a s s o c i a t e d w i t h t h e i n f o r m a t i o n on t h e sys tem s t r u c t u r e i s t h e

i n f o r m a t i o n on t h e thermodynamic p r o p e r t i e s o f t h e p r o c e s s media and on t h e

equ ipment c h a r a c t e r i s t i c s . I n t h e e q u a t i o n s p r e s e n t e d i n t h e p r e c e d i n g S e c t i o n ,

t h e e n t h a l p i e s o f t h e p r o c e s s media a p p e a r . The e n t h a l p i e s can be d e t e r m i n e d as

f u n c t i o n s o f t e m p e r a t u r e , p r e s s u r e o r o t h e r p a r a m e t e r s . I n p r a c t i c a l

c a l c u l a t i o n s , i n v e r s e f u n c t i o n s may a l s o be n e e d e d , as w e l l as a number o f o t h e r

thermodynamic f u n c t i o n s n o t n e c e s s a r i l y r e l a t e d t o t h e n o t i o n o f e n t h a l p y . The

thermodynamic f u n c t i o n s can be f ound i n d iag ram o r t a b u l a r fo rm i n t h e

l i t e r a t u r e , and t h e i r n u m e r i c a l a p p r o x i m a t i o n s a r e d i s c u s s e d i n A p p e n d i c e s 1

and 2.

As r e g a r d s t h e equ ipment d a t a , two k i n d s o f them a r e o f p a r t i c u l a r

i m p o r t a n c e :

- h e a t l o s s c o e f f i c i e n t s ,

- e q u a t i o n s e x p r e s s i n g t h e o v e r a l l h e a t t r a n s f e r c o e f f i c i e n t s .

A p o s s i b l e app roach t o t h i s p a r t o f t h e e n e r g y - b a l a n c e da ta w i l l be p r e s e n t e d i n

t h e r e m a i n i n g S e c t i o n s o f t h i s C h a p t e r , The v a l u e s o f t h e h e a t l o s s c o e f f i c i e n t s

and t he e q u a t i o n s d e s c r i b i n g t h e h e a t t r a n s f e r c h a r a c t e r i s t i c s o f t h e equ ipment

a r e d i s c u s s e d i n Append i x 3.

As r e g a r d s t h e p r e p a r a t i o n o f da ta on t h e s u g a r m a n u f a c t u r i n g p r o c e s s .

61

t he use o f e q u a t i o n s o f p r o c e s s mass b a l a n c e s h o u l d be named f i r s t . The mass

f l o w s o f t h e p r o c e s s media r e p r e s e n t e d i n t h e e n e r g y b a l a n c e e q u a t i o n s have t o

s a t i s f y t h e e q u a t i o n s o f t h e p r o c e s s mass b a l a n c e . A p r a c t i c a l consequence i s

t h a t t h e m a s s - b a l a n c e i n v e s t i g a t i o n - e x p e r i m e n t a l , t h e o r e t i c a l , o r a

c o m b i n a t i o n o f b o t h - must be comp le ted b e f o r e t h e e n e r g y - b a l a n c e p rob lem i s

a p p r o a c h e d .

S e t t i n g up t h e e q u a t i o n s o f t h e mass b a l a n c e o f a s u g a r m a n u f a c t u r i n g p r o c e s s

can be a d i f f i c u l t p r o b l e m i n i t s e l f . U s u a l l y , i t r e q u i r e s i d e n t i f y i n g t h e

p r o c e s s scheme and c a l c u l a t i n g t h e f l o w s o f mass componen ts : w a t e r , d r y m a t t e r ,

s u c r o s e , and - i f a p p l i c a b l e - s u c r o s e c r y s t a l s . The c a l c u l a t i o n s can be

pe r fo rmed m a n u a l l y ; p o s s i b l y , t w o - o r t h r e e - c o m p o n e n t d iag rams f o r s u g a r

s o l u t i o n s can be used t o s i m p l i f y t h e manual w o r k . Examples o f t h e u p - t o - d a t e

app roach t o t h i s t y p e o f c a l c u l a t i o n method can be f ound i n t h e l i t e r a t u r e

( r e f s . 3 - 5 ) . I n c r e a s i n g l y o f t e n , h o w e v e r , t h e p r o c e s s mass b a l a n c e s a r e

c a l c u l a t e d w i t h t h e a i d o f computer p rog rams . The methods used i n c o m p u t e r i z e d

c a l c u l a t i o n s a r e based e i t h e r on t h e p r o c e s s s i m u l a t i o n app roach ( r e f s . 6 -8 ) o r

on t h e s o l u t i o n o f a sys tem o f l i n e a r e q u a t i o n s ( r e f . 9 ) .

2 .2 .2 Example

The s u g a r m a n u f a c t u r i n g p r o c e s s i s t o be c o n s i d e r e d f o r a f a c t o r y f e a t u r i n g

a t r o u g h - t y p e e x t r a c t o r , a c l a s s i c a l j u i c e p u r i f i c a t i o n s t a t i o n w i t h s u b s e q u e n t

j u i c e d e c a l c i f i c a t i o n and a s t a n d a r d - l i q u o r - b a s e d , t h r e e - b o i l i n g c r y s t a l l i z a t i o n

scheme. The pa rame te rs d e f i n i n g t h e o v e r a l l p r o c e s s c h a r a c t e r i s t i c s a r e g i v e n i n

T a b l e 2 .1 . The scheme o f t h e b e e t house i s shown i n F i g . 2.2 and t h e

c o r r e s p o n d i n g mass b a l a n c e da ta a r e p r e s e n t e d i n T a b l e 2 . 2 . The scheme o f t h e

s u g a r house i s shown i n F i g . 2.3 and t h e mass b a l a n c e d a t a a r e p r e s e n t e d i n

T a b l e 2 . 3 . L e t us a n a l y s e t h e p r o c e s s scheme and t h e mass b a l a n c e d a t a f rom t h e

p o i n t o f v i e w o f t h e i r s u i t a b i l i t y as i n p u t da ta f o r e n e r g y b a l a n c e

c a l c u l a t i o n s .

TABLE 2.1

E s s e n t i a l p r o c e s s da ta f o r t h e Examp le .

Name D imens ion V a l u e

P o l a r i z a t i o n o f c o s s e t t e s % 18.0 J u i c e d r a f t % 110.0 CaO r a t e :

p r e - l i m i n g kg/100 kg b 0.28 main l i m i n g

kg/100 kg b 1.52

2nd c a r b o n a t a t i o n II II 0.22 T h i c k j u i c e c o n c e n t r a t i o n % DS 56.0 Mo lasses p u r i t y % 62.0

62

feed water

ΙΛ

o c !5

Ε

en TD

-D (Λ

Χ )

(Λ

cossettes

EXTRACTOR

HEATER i_

HEATER I

y. L - w e t pulp raw juice

4

press water

PRESSES

pressed pulp

HEATERS to drying

sweet water

PRE-LIMING

HEATERS

MAIN LIMING

1

Φ

Φ

•σ

to lime slaking

i Φ Φ

CARBONATATION I

HEATERS

THICKENERS I

ju ice

VACUUM FILTERS

HEATER

Φ Φ

ω

water -

CARBONATATION I I

THICKENERS Π

s ludge^

thin ju ice to heating and evapora t ion .

SAFETY FILTERS

DECALCI FICATION

1 SULPHITATION

thin juice to sugar house^

ju ice

F i g . 2 . 2 . Scheme o f t he b e e t house c o n s i d e r e d i n t h e Examp le .

63

TABLE 2.2

Mass b a l a n c e o f t h e b e e t house i n t h e Example

No. St ream name T o t a l f l o w

(kg /100 kg b ) C o n c e n t r a t i o n

(% DS) P u r i t y

(%)

1 C o s s e t t e s 100.00 86.90 2 Wet p u l p 84.60 3 P r e s s e d p u l p 27.04 19.00 4 D r i e d p u l p 5.58 92.00 5 P r e s s w a t e r 57.56 0.90 74.50 6 Feed w a t e r 37.04 7 Raw j u i c e 110.00 18.16 88.11 8 J u i c e t o main l i m i n g 135.84 17.88 9 J u i c e t o c a r b o n a t a t i o n I 143.46 17.99

10 J u i c e f rom vacuum f i l t e r s 13.73 12.00 92.38 11 J u i c e t o t h i c k e n e r s I 157.19 17.47 12 J u i c e t o c a r b o n a t a t i o n I I 124.50 16.28 91.48 13 S u b s i d e r s l u d g e I 32.69 20.00 14 Subs i d e r s l u d g e I t o vacuum f i l t e r s 20.69 15 S w e e t e n i n g - o f f w a t e r t o vacuum

f i l t e r s 12.72

16 S l u d g e 8.48 50.00 17 Sweet w a t e r f rom vacuum f i l t e r s 11.02 7.00 89.00 18 J u i c e t o t h i c k e n e r s I I 127.30 92.04 19 S u b s i d e r s l u d g e I I t o p r e - l i m i n g 3.59 18.00 20 J u i c e t o s a f e t y f i l t e r s 125.42 15.76 92.03 21 J u i c e t o s u l p h i t a t i o n 123.71 22 J u i c e t o d e c a l c i f i c a t i o n 118.50 16.68 92.03 23 S w e e t e n i n g - o f f w a t e r t o

d e c a l c i f i c a t i o n 7.00

24 Sweet w a t e r f rom d e c a l c i f i c a t i o n 3.50 1.08 88.00 25 T h i n j u i c e t o e v a p o r a t i o n 122.00 15.51 92.04 26 M i l k - o f - l i m e t o p r e - l i m i n g 1.41 27 " main l i m i n g 7.62 28 " " c a r b o n a t a t i o n I I 1.09 29 K i l n gas t o c a r b o n a t a t i o n I 3.80 30 II II II II J J 0.79

The scheme and t h e da ta d e s c r i b i n g t h e b e e t house p r o v i d e a comp le te

d e f i n i t i o n o f a l l t he mass s t reams t o w h i c h h e a t s h o u l d be d e l i v e r e d . I t i s

p o s s i b l e t o d e t e r m i n e , f o r each s t r e a m , t h e mass f l o w and t h e m a t e r i a l

p r o p e r t i e s a f f e c t i n g t h e e n t h a l p y ( o r t h e s p e c i f i c h e a t ) . No t e m p e r a t u r e s a r e

g i v e n , h o w e v e r , t h i s i m p l y i n g t h a t t h e t e m p e r a t u r e r e q u i r e m e n t s s h o u l d be

s e p a r a t e l y c o n s i d e r e d , p o s s i b l y t o g e t h e r w i t h t h e s t r u c t u r e o f t h e p a r t o f t h e

therma l sys tem w h i c h i s a s s o c i a t e d w i t h t h e b e e t h o u s e .

The scheme o f t h e s u g a r house seems t o be i n c o m p l e t e , as no h e a t i n g

o p e r a t i o n s a r e s p e c i f i e d . The mass b a l a n c e d a t a d e f i n e c o m p l e t e l y a l l t h e t i m e -

a v e r a g e d mass f l o w s o f media w i t h i n t h e s u g a r h o u s e . Once t h e h e a t i n g

o p e r a t i o n s have been s p e c i f i e d ( t y p i c a l l y , h e a t i n g o f t h i c k j u i c e d e l i v e r e d t o

t he s u g a r house and s y r u p s s t o r e d i n i n t e r m e d i a t e t anks i s r e q u i r e d ) , i t w i l l be

p o s s i b l e t o i d e n t i f y a l l t h e mass f l o w s and m a t e r i a l p r o p e r t i e s t h a t a r e

64

thick juice

MELTER

FILTER

standard liquor

VACUUM PANS A 1

1 MIXERS A

1 CENTRIFUGALS A

sugar A

green " syrup A "

1 1 Γ

Q . 13

sz ΙΛ O

VACUUM PANS Β

MIXERS Β 1

1 CENTRIF UGALS Β

sugar Β

1 σ ι >>

3 ί VACUUM PANSC

I

MAGMA MIXER

• white sugar

MIXERS C

z r : CENTRIFUGALS C

L- sugar C —-I

- magma-

molasses

F i g . 2 . 3 . Scheme o f t h e s u g a r house c o n s i d e r e d i n t h e Examp le .

TABLE 2.3

Mass b a l a n c e o f t h e s u g a r house i n t h e Examp le .

No. S t ream name T o t a l f l o w

(kg /100 kg b ) C o n c e n t r a t i o n

{% DS) P u r i t y

{%)

1 T h i c k j u i c e 33.61 56.00 92.02 2 S t a n d a r d l i q u o r 39.88 62.62 93.60 3 A m a s s e c u i t e 29.09 92.50 93.46 4 Green s y r u p A t o vacuum pans C 2.79 82.50 84.60 5 II II II II II g 5.80 82.50 84.60 6 · magma 3.82 82.50 84.60 7 Wash s y r u p A 2.65 75.50 91.96 8 A s u g a r 14.80 99.10 99.85 9 Β m a s s e c u i t e 13.35 92.80 87.18

10 Green s y r u p Β 6.25 83.80 74.20 11 Wash s y r u p Β 1.26 79.50 86.18 12 Β s u g a r 6.27 98.10 98.40 13 C m a s s e c u i t e 7.92 93.50 76.94 14 C s u g a r 3.62 97.10 93.50 15 Magma 7.44 89.60 89.29 16 Mo lasses 4.66 83.50 62.00

65

r e l e v a n t t o t h e e n e r g y b a l a n c e c a l c u l a t i o n s . A g a i n , t h e t e m p e r a t u r e r e q u i r e m e n t s

can c o n v e n i e n t l y be s p e c i f i e d when d e f i n i n g t h e s t r u c t u r e o f t h e a s s o c i a t e d p a r t

o f t he the rma l s y s t e m .

2.3 EXTERNAL ENERGY BALANCE OF A SUGAR FACTORY

The e n e r g y demand i n a s u g a r f a c t o r y i s u l t i m a t e l y d e t e r m i n e d by t h e

i n t e r a c t i o n s between t he s u g a r m a n u f a c t u r i n g p r o c e s s , t h e the rma l sys tem and t h e

power h o u s e . When s o l v i n g c e r t a i n e n g i n e e r i n g p r o b l e m s , i t may be o f i n t e r e s t t o

i n v e s t i g a t e key i n t e r a c t i o n pa ramete rs and t o i d e n t i f y o r e v a l u a t e e s s e n t i a l

p r o p e r t i e s o f t h e t h r e e f a c t o r y subsys tems named, w i t h o u t a n a l y s i n g t h e i r

d e t a i l s . T h i s can be done by s e t t i n g up t h e s o - c a l l e d e x t e r n a l mass and e n e r g y

b a l a n c e o f t h e f a c t o r y . Examples o f a p p l i c a t i o n o f t h i s app roach can be f ound i n

t he l i t e r a t u r e ( r e f s . 1 0 , 1 1 ) .

L e t us c o n s i d e r an open thermodynamic s y s t e m c o m p r i s i n g t h e main p r o c e s s

equ ipment and the rma l equ ipment f rom t h e e x t r a c t i o n s t a t i o n t o t h e s u g a r h o u s e ,

t h a t i s , e x c l u d i n g t h e b e e t wash ing and s l i c i n g s t a t i o n , s u g a r áryer^ l i m e k i l n ,

power h o u s e , b a r o m e t r i c c o n d e n s e r and b a r o m e t r i c - w a t e r c o o l i n g c i r c u i t . A b l o c k

d iagram r e p r e s e n t i n g t h e sys tem i s shown i n F i g . 2 . 4 . I t can be seen t h a t most

mass f l o w s t o and f rom t h e sys tem a r e r o u t i n e l y measured f o r p r o c e s s c o n t r o l

pu rposes o r can be deduced f rom such measurements , so o n l y a few mass f l o w s

s h o u l d be a d d i t i o n a l l y de te rm ined i n o r d e r t o d e s c r i b e t h e mass b a l a n c e f u l l y .

SYSTEM BOUNDARY

Spen t q a s from 1 s f / 2 n d carb.

vapour I 4-

1 cosseHes ^

E S 1

1 E S

1 1 —

o •α α χί 1 δ

J P

^ \ — s i

vapour

EV SH

steann

VQPOur

I sugar

molasse^

F i g . 2 . 4 . B l o c k scheme i l l u s t r a t i n g t h e e x t e r n a l e n e r g y b a l a n c e o f a s u g a r f a c t o r y . ES - e x t r a c t i o n s t a t i o n , JP - j u i c e p u r i f i c a t i o n s t a t i o n , EV -e v a p o r a t o r , SH - s u g a r h o u s e . 1 - b e e t wash ing and s l i c i n g e q u i p m e n t , 2 - l ime k i l n and m i l k - o f - l i m e s t a t i o n , 3 - power h o u s e , 4 - s u g a r d r y i n g and packag ing s t a t i o n , 5 - c o n d e n s e r and c o o l i n g c i r c u i t .

66

The number o f t e m p e r a t u r e ( a n d , p o s s i b l y , p r e s s u r e ) measurements r e q u i r e d t o

i d e n t i f y t h e e n t h a l p i e s o f f l o w i n g media i s a l s o s m a l l .

The i n t e r n a l c o m p l e x i t i e s o f t h e sys tem d e f i n e d above need n o t be known f o r

t h e e x t e r n a l e n e r g y b a l a n c e , w h i c h can be d e s c r i b e d by i n t r o d u c i n g t h e

q u a n t i t i e s c h a r a c t e r i z i n g t h e f l o w i n g media i n t o e q n s . ( 2 . 5 ) o r ( 2 . 6 ) . The n e t

h e a t demand i s r e p r e s e n t e d by t h e h e a t s t ream i n t h e steam d e l i v e r e d f rom t h e

power house and we a r e f r e e t o s t u d y how t o r e d u c e i t by a d j u s t i n g o t h e r

s t r e a m s . O f c o u r s e , i t i s d e s i r a b l e t h a t t h e sum o f o u t f l o w i n g e n e r g y s t reams

s h o u l d be as sma l l as p o s s i b l e , w h i c h can be o b t a i n e d by l i m i t i n g t h e o u t f l o w i n g

mass f l o w s a n d / o r d e c r e a s i n g t h e t e m p e r a t u r e s o f t h e media l e a v i n g t h e s y s t e m .

I n t h e f i r s t p l a c e t h i s a p p l i e s t o v a p o u r s t r e a m s , because t h e v a p o u r e n t h a l p i e s

a r e h i g h e s t . I t i s t h u s e s s e n t i a l t o m i n i m i z e t h e v a p o u r f l o w f rom t h e l a s t

e v a p o r a t o r e f f e c t t o t h e c o n d e n s e r , and t h e e n e r g y o f t h e vacuum pan v a p o u r s

s h o u l d be u t i l i z e d as much as p o s s i b l e b e f o r e t h e v a p o u r s e n t e r t h e c o n d e n s e r .

S i m i l a r recommendat ions a p p l y t o e x c e s s c o n d e n s a t e f rom t h e e v a p o r a t o r , and

s p e n t c a r b o n a t a t i o n g a s .

An i m p o r t a n t c o n c l u s i o n can be drawn f rom t h e e x t e r n a l e n e r g y b a l a n c e , namely

t h a t as l ong as e x c e s s c o n d e n s a t e (ammoniacal w a t e r ) f rom t h e e v a p o r a t o r i s

s u p p l i e d as f r e s h w a t e r t o t h e e x t r a c t i o n s t a t i o n , and t h e c o n d e n s e r l o s s f rom

t h e l a s t e v a p o r a t o r e f f e c t i s h e l d c o n s t a n t , t h e n e t hea t demand i s i n d e p e n d e n t

o f t he amount o f w a t e r e v a p o r a t e d i n t h e e v a p o r a t o r . I f t h e above c o n d i t i o n s a r e

s a t i s f i e d , t hen t h e j u i c e d r a f t can be i n c r e a s e d o r c o n d e n s a t e can be added t o

j u i c e w i t h o u t n e c e s s a r i l y i n c r e a s i n g t h e n e t h e a t demand. On t h e o t h e r h a n d , any

i n t a k e o f e x t r a w a t e r f rom o u t s i d e t h e s y s t e m , f o r example t o vacuum p a n s ,

i n e v i t a b l y i n c r e a s e s t h e n e t hea t demand.

As r e g a r d s t h e c a l c u l a t i o n s o f t h e e n e r g y s t r e a m s , i t s h o u l d be o b s e r v e d t h a t

i f a h i g h a c c u r a c y o f t he e x t e r n a l b a l a n c e i s r e q u i r e d , t h e n i t i s n e c e s s a r y t o

a c c o u n t f o r t h r e e e n t r i e s w h i c h a r e t r a d i t i o n a l l y n e g l e c t e d i n e n g i n e e r i n g

a n a l y s e s .

( i ) Heat o f c a r b o n a t a t i o n r e a c t i o n . Assuming t h a t i t i s g e n e r a t e d a t t h e r a t e o f

70 k J p e r 1 mol o f CaO and t h a t t h e e f f e c t i v e CaO r a t e i s 1.9%, we o b t a i n an

e n e r g y s t ream o f abou t 2400kJ/100 kg b , t h i s b e i n g e q u i v a l e n t t o 1.1 kg steam

p e r 100 kg b.

( i i ) Heat o f c r y s t a l l i z a t i o n o f s u g a r . A t 75°C, t h i s i s 82 k J / k g . M u l t i p l e

c r y s t a l l i z a t i o n - d i s s o l v i n g o p e r a t i o n s i n t h e s u g a r house can be d i s r e g a r d e d , on

t h e assumpt ion t h a t t h e h e a t g e n e r a t e d e q u a l s t h e h e a t a b s o r b e d . H o w e v e r , t h e

s u g a r s t ream l e a v i n g t h e s u g a r house c o r r e s p o n d s t o a d e f i n i t e amount o f h e a t

g e n e r a t e d . A t a t y p i c a l s u g a r y i e l d , t h i s c o r r e s p o n d s t o abou t 0.5 kg steam p e r

100 kg b.

( i i i ) Mechan i ca l work s u p p l i e d t o t he p r o c e s s . Power consumed by t h e motors

67

d r i v i n g t h e pumps, s t i r r e r s and o t h e r equ ipment i s c o n v e r t e d t o mechan i ca l work

and f i n a l l y d i s s i p a t e d - a p a r t t o t h e p r o c e s s m e d i a , and t h e r e s t t o t h e

e n v i r o n m e n t . Assuming a t y p i c a l equ ipment c o n f i g u r a t i o n and a 50:50 e n e r g y

d i s s i p a t i o n , t h e s t ream o f mechan ica l work a b s o r b e d by t h e p r o c e s s media can be

e s t i m a t e d a t abou t 0.8 kg steam p e r 100 kg b.

TABLE 2.4

E x t e r n a l mass and e n e r g y b a l a n c e s o f a s u g a r f a c t o r y c o n s i d e r e d as a thermodynamic sys tem shown s c h e m a t i c a l l y i n F i g . 2 . 4 .

Mass f l o w T e m p e r a t u r e E n t h a l p y E n t h a l p y s t ream No. Name (kg /100 kg b ) ( O Q ) ( k J / k g ) ( M J / l O O kg b )

I n f l o w i n g s t reams

1 C o s s e t t e s 100.0 12 45 4.5 2 M i l k - o f - l i m e 11.9 40 150 1.8 3 K i l n gas 5.3 35 80 0.4 4 Heat o f - - - 2.4

c a r b o n a t a t i o n r e a c t i o n 5 Wash w a t e r t o c e n t r i f u g a l s 1.4 15 63 0.1 6 Wash steam (7 b a r ) t o 1.2 165 2762 3.3

c e n t r i f u g a l s 7 C r y s t a l l i z a t i o n h e a t - - - 1.2 8 Mechan i ca l work t o p r o c e s s - - - 1.8 9 Steam ( 3 . 4 b a r ) t o 52.0 138 2730 142.0

e v a p o r a t o r and vacuum pan s teaming

T o t a l 1-9 171.8 _ _ 157.5

O u t f l o w i n g s t reams

10 P r e s s e d p u l p 14.35% DS 34.3 40 160 5.5 11 C a r b o n a t a t i o n s l u d g e 8.5 60 150 1.3

50% DS 12 Sweet w a t e r t o l ime 6.7 65 267 1.8

s l a k i n g 13 Gas f rom l s t / 2 n d 6.3 82/92 7.3

c a r b o n a t a t i o n 14 Vacuum f i l t e r v a p o u r 0.6 65 2618 1.6 15 V a p o u r f rom t h e l a s t 8.4 90 2660 22.3

e v a p o r a t o r e f f e c t 16 Condensa te t o b o i l e r s 60.2 114 478 28.8 17 Vacuum pan v a p o u r 20.3 62 2613 53.0 18 Suga r f rom c e n t r i f u g a l s 14.2 70 97 1.4 19 H e a t i n g v a p o u r t o s u g a r 0.9 125 2713 2.4

d r y e r 20 Mo lasses 5.0 50 115 0.6 21 C o o l i n g o f C m a s s e c u i t e - - - 0.6 22 Wash steam d i s s i p a t e d f rom 1.0 165 2762 2.8

c e n t r i f u g a l s 23 Steam d i s s i p a t e d f rom 0.5 138 2730 1.4

vacuum pans

T o t a l 10-23 166.9 - - 130.8

24 V a r i o u s l e a k s and h e a t 4 .9 - - 26.7 l o s s e s

T o t a l 10-24 171.8 - - 157.5

68

A q u e s t i o n may be posed abou t t h e pu rpose o f a c c o u n t i n g f o r t h e s e r e l a t i v e l y

smal l i ncoming e n e r g y s t r e a m s , w h i l e s i m u l t a n e o u s l y c a l c u l a t i n g n o t - s o - w e l l

d e f i n e d l o s s e s caused by t h e l e a k s o f media and t h e h e a t t r a n s f e r t o t h e

e n v i r o n m e n t . The answer i s t h a t i t i s o n l y when a l l t h e incoming e n e r g y s t reams

a r e known t h a t i t becomes p o s s i b l e t o e v a l u a t e t h e l o s s e s r e a l i s t i c a l l y . O f

c o u r s e , one can a l s o imag ine e n g i n e e r i n g p rob lems i n w h i c h a p p r o x i m a t e

c a l c u l a t i o n s o f t h e e n e r g y b a l a n c e a r e s u f f i c i e n t t o a r r i v e a t a c o r r e c t

s o l u t i o n , t h i s i m p l y i n g t h a t v e r y sma l l e n e r g y s t reams need n o t be t aken i n t o

a c c o u n t .

I n T a b l e 2 . 4 , an example i s p r e s e n t e d o f t h e e x t e r n a l e n e r g y b a l a n c e w i t h t h e

h e a t o f t h e c a r b o n a t a t i o n r e a c t i o n , h e a t o f c r y s t a l l i z a t i o n o f s u g a r and

mechan ica l work taken i n t o a c c o u n t . As can be s e e n , t h e u n c o n t r o l l a b l e e n e r g y

l o s s e s c o r r e s p o n d t o abou t 19% o f t h e e n e r g y s t ream d e l i v e r e d i n t h e h e a t i n g

s team. Had t h e sma l l i ncoming s t reams been n e g l e c t e d , t h e l o s s e s wou ld have been

c a l c u l a t e d a t abou t 15% o f t h e steam e n e r g y , t h i s b e i n g a c l e a r u n d e r e s t i m a t e .

L e t us o b s e r v e a l s o t h a t t h e d e f i n i t i o n o f t h e thermodynamic sys tem so as t o

e x c l u d e t h e s u g a r d r y e r i s a m a t t e r o f c o n v e n i e n c e . The h e a t demand o f t h e d r y e r

can be c a l c u l a t e d s e p a r a t e l y f rom t h e w a t e r c o n t e n t i n s u g a r l e a v i n g t h e

c e n t r i f u g a l s . Had t h e d r y e r been i n c l u d e d , i t wou ld have been n e c e s s a r y t o

a c c o u n t f o r t h e s t reams o f a i r e n t e r i n g and l e a v i n g t h e s y s t e m , a i r e n t h a l p y

b e i n g a f u n c t i o n o f h u m i d i t y and t e m p e r a t u r e . T h i s wou ld make t h e b a l a n c e

c a l c u l a t i o n s a l i t t l e more l a b o r i o u s .

I t s h o u l d be f i n a l l y no ted t h a t t h e t o t a l hea t demand a n d , c o n s e q u e n t l y ,

t h e e f f e c t i v e n e s s r a t i o o f t h e therma l sys tem c a n n o t be c a l c u l a t e d f rom t h e

e x t e r n a l e n e r g y b a l a n c e . T h i s i s i l l u s t r a t e d by T a b l e 2.4 w h i c h c o n t a i n s t h e

i n f l o w i n g and o u t f l o w i n g e n e r g y s t reams b u t n o t t h e ones c i r c u l a t i n g i n t h e

p r o c e s s .

2.4 MASS AND HEAT BALANCES OF HEAT RECEIVERS

2.4.1 I n t r o d u c t o r y remarks

I t i s o f t e n n e c e s s a r y t o i n v e s t i g a t e t h e mass and h e a t b a l a n c e s o f a s u g a r

f a c t o r y i n o r d e r t o d e t e r m i n e t h e d e t a i l s c o r r e s p o n d i n g t o t h e i n d i v i d u a l

equ ipment u n i t s . Such a s i t u a t i o n may a r i s e when d e s i g n i n g a new the rma l sys tem

f o r g i v e n paramete rs o f t he s u g a r m a n u f a c t u r i n g p r o c e s s , o r when a n a l y s i n g t h e

e n e r g y econorny o f an e x i s t i n g s u g a r f a c t o r y . I n o r d e r t o a r r i v e a t a s o l u t i o n ,

i t i s n e c e s s a r y t o i d e n t i f y a l l t h e h e a t s t reams q^. d e f i n e d i n S e c t i o n 1 . 2 . 2 ;

t h i s makes i t a l s o p o s s i b l e t o c a l c u l a t e t h e t o t a l h e a t demand, t h e n e t h e a t

demand and t he e f f e c t i v e n e s s r a t i o o f t h e the rma l s y s t e m . Examples o f

f o r m u l a t i o n o f t h i s k i n d o f p rob lem can be found i n t h e l i t e r a t u r e ( r e f s .

1 2 - 1 4 ) .

V i r t u a l l y a l l the rma l sys tems i n c o r p o r a t e such equ ipment as j u i c e h e a t e r s .

69

e x t r a c t o r s , hea ted i n t e r m e d i a t e s t o r a g e tanks and vacuum p a n s . G e n e r a l f o r m u l a e

used i n b a l a n c e c a l c u l a t i o n s o f t h e equ ipment u n i t s named a r e p r e s e n t e d i n t h e

f o l l o w i n g . The b a l a n c e s o f o t h e r e q u i p m e n t , n o t c o n s i d e r e d h e r e , can be m o d e l l e d

i n a s i m i l a r manner.

2 .4 .2 J u i c e h e a t e r s

J u i c e h e a t e r s a r e c e r t a i n l y among t h e most i m p o r t a n t components o f t h e

therma l s y s t e m s , as i t i s v i a t h e h e a t e r s t h a t a l a r g e p a r t o f t h e t o t a l h e a t

demand i s t r a n s f e r r e d t o t h e p r o c e s s . The b a l a n c e c a l c u l a t i o n f o r a h e a t e r i s

r a t h e r s i m p l e , b u t t h e p rob lem o f c o o r d i n a t i n g i t w i t h t h e c a l c u l a t i o n s o f o t h e r

p a r t s o f t he the rma l sys tem may be d i f f i c u l t t o s o l v e .

L e t us c o n s i d e r t h e e v a l u a t i o n o f t h e h e a t exchange i n a h e a t e r c h a r a c t e r i z e d

by a g i v e n h e a t i n g s u r f a c e a r e a F and o v e r a l l h e a t t r a n s f e r c o e f f i c i e n t k,

s u p p l i e d w i t h h e a t i n g v a p o u r a t t e m p e r a t u r e t ^ , w h i l e t h e r e q u i r e d f i n a l

t e m p e r a t u r e o f t h e f l u i d hea ted i s t ^ . The q u a n t i t i e s d e t e r m i n i n g t h e mass and

h e a t b a l a n c e s o f t he h e a t e r a r e i n d i c a t e d i n F i g . 2 . 5 ( a ) . ( I t s h o u l d be n o t e d

t h a t t h e c a l c u l a t i o n p r o c e d u r e t o be a p p l i e d i n t h e case o f an unknown h e a t i n g

s u r f a c e a rea i s g i v e n i n r e f . 1 4 . )

(a) (b)

G , t i ,

G v i t v ^ -h-i-H-

iliü i Μ i! ι ι ι ; ι

-h-i-H-

iliü i Μ i! ι ι ι ; ι Ι Μ I I

t2 ,

G c . t c i , t t t t t Miii liiii t t t t t Miii liiii ι ι ι ι ι M i l l

F i g . 2 . 5 . P r i n c i p l e o f mass and h e a t b a l a n c e s o f j u i c e h e a t e r s : ( a ) v a p o u r -h e a t e d , ( b ) c o n d e n s a t e - h e a t e d .

I f t h e f l u i d t e m p e r a t u r e a t t h e h e a t e r i n l e t i s t - j , t h e n t h e r e s u l t i n g f i n a l

t e m p e r a t u r e w i l l be

t2 = - ( t v - t i ) e x p ( - F k / ( ( l + n ) G C ) ) ( 2 . 7 )

where η i s t he h e a t l o s s c o e f f i c i e n t and C i s t h e j u i c e s p e c i f i c h e a t .

N e g l e c t i n g condensa te s u b c o o l i n g , t h e mean l o g a r i t h m i c t e m p e r a t u r e d i f f e r e n c e

can be c a l c u l a t e d as

A t = ( t ^ - t i ) / l n ( ( t ^ - t^)/(t^ - t 2 ) ) ( 2 . 8 )

T a k i n g t he s u b c o o l i n g i n t o a c c o u n t , t h e a p p r o x i m a t e v a l u e o f t h e o u t l e t

t e m p e r a t u r e o f t he c o n d e n s a t e i s

^ c = + 1 ^ ^ 2 ) / δ ( 2 . 9 )

70

D e n o t i n g t h e e n t h a l p y o f d r y s a t u r a t e d steam a t t e m p e r a t u r e t by h " ( t ) , v a p o u r

e n t h a l p y i s t h u s h^ = h " ( t ^ ) , and assuming a l l t h e e n t h a l p i e s a r e e x p r e s s e d i n

k J / k g , condensa te e n t h a l p y i s h^= 4.19 t ^ .

The h e a t t r a n s f e r r e d f rom v a p o u r t o j u i c e i n t h e h e a t e r i s

q = (1 + n ) G C ( t 2 - t ^ ) ( 2 . 1 0 )

and t he h e a t i n g v a p o u r demand

Gv = q / ( h v - \ ) ( 2 . 1 1 )

The f u r t h e r c o u r s e o f t h e c a l c u l a t i o n s depends now on t h e r e l a t i o n between t h e

two f i n a l t e m p e r a t u r e s , t h e r e q u i r e d t ^ and r e s u l t i n g t 2 . Too low a t2 v a l u e i s

an i n d i c a t i o n o f i n s u f f i c i e n t hea t s u p p l y . I n o r d e r t o e n s u r e c o r r e c t h e a t i n g ,

t h e h e a t i n g s u r f a c e a r e a s h o u l d be i n c r e a s e d , o r a n o t h e r h e a t e r s h o u l d be

p l anned n e x t t o t h e one b e i n g e v a l u a t e d ; i n e i t h e r c a s e , i t i s n e c e s s a r y t o

r e p e a t t h e c a l c u l a t i o n a c c o r d i n g t o e q n s . ( 2 . 7 ) - ( 2 . 1 1 ) w i t h a new s e t o f i n p u t

d a t a . I f t ^ t u r n s o u t t o be t o o h i g h , t h e n t h e c o n c l u s i o n c o u l d be drawn t h a t

t o o much hea t w i l l be t r a n s f e r r e d t o t h e j u i c e . I t i s p o s s i b l e t o l e a v e t h e

h e a t i n g s u r f a c e a r e a u n c h a n g e d , h o w e v e r , as i n r e a l i t y , t h e t e m p e r a t u r e c o n t r o l

c i r c u i t w i l l a d j u s t t h e v a p o u r consumpt ion so as t o o b t a i n t h e r e q u i r e d f i n a l

j u i c e t e m p e r a t u r e t ^ . C o n s e q u e n t l y , t h e h e a t t r a n s f e r r e d becomes

q ^ = (1 + n ) G C ( t ^ - t ^ ) ( 2 . 1 2 )

and t h e r e s u l t i n g l o g a r i t h m i c t e m p e r a t u r e d i f f e r e n c e i s

A t ^ = ( t f - t ^ ) / l n ( ( t f - t ^ ) / ( t ^ - t 2 ) ) ( 2 . 1 3 )

The r e s u l t i n g t e m p e r a t u r e and e n t h a l p y o f t h e c o n d e n s a t e a r e

t ^ ^ = ( 6 t ^ + t ^ + t i ) / 8 ( 2 . 1 4 )

and

h ^ f = 4.19 t ^ ^ ( 2 . 1 5 )

and t h e h e a t i n g v a p o u r demand i s

G v f = q f / ( h v - h ^ f ) ( 2 . 1 6 )

I t i s a l s o p o s s i b l e t o d e t e r m i n e t h e e f f e c t i v e ( i . e . , minimum r e q u i r e d ) h e a t i n g

s u r f a c e a r e a as

= q ^ / ( k A t ^ ) ( 2 . 1 7 )

I n t h e case o f c o n d e n s a t e - h e a t e d h e a t e r s , t h e c o u r s e o f t h e c a l c u l a t i o n s and

t h e s u b s e q u e n t d e c i s i o n - m a k i n g p r o c e d u r e may be d i f f e r e n t . T y p i c a l l y , c o n d e n s a t e

mass f l o w G ^ and i n l e t t e m p e r a t u r e t^-j a r e known, a l o n g w i t h h e a t i n g s u r f a c e

a r e a F , o v e r a l l h e a t t r a n s f e r c o e f f i c i e n t k and r e q u i r e d f i n a l t e m p e r a t u r e t ^

o f t h e l i q u i d h e a t e d . The q u a n t i t i e s d e t e r m i n i n g t h e mass and h e a t b a l a n c e s o f

t h e h e a t e r a r e i n d i c a t e d i n F i g . 2 . 5 ( b ) . C o u n t e r - c u r r e n t o p e r a t i o n o f t h e h e a t e r

i s assumed.

71

The d i f f i c u l t y i n i n i t i a t i n g t h e c a l c u l a t i o n s i s t h e u n c e r t a i n t y abou t t h e

f i n a l t e m p e r a t u r e s o f bo th t h e l i q u i d hea ted and t h e c o n d e n s a t e . I t can be

r e s o l v e d by making an i n i t i a l guess t2^- on t h e o u t l e t t e m p e r a t u r e o f t h e l i q u i d

( l a t e r o n , we w i l l be a b l e t o check how good t h e guess i s ) . I t t hus becomes

p o s s i b l e t o e s t i m a t e t h e o u t l e t t e m p e r a t u r e o f t h e c o n d e n s a t e

\ 2 = ^ c l - ^ " ) ^ ^ ( ^ 2 i - t i ) ) / ( 4 . 1 9 ( 2 . 1 8 )

The r e s u l t i n g mean l o g a r i t h m i c t e m p e r a t u r e d i f f e r e n c e i s t h u s

At = ( ( t ^ T - t^^) - ( t ^ 2 - ^ ΐ ) ) / " ' " ( ( ^ ο 1 " ^ 2 i ) / ( ^ c 2 - ^ l ) ) ( 2 · ^ ^ )

and t he h e a t t r a n s f e r r e d can be e s t i m a t e d as

q = kFAt ( 2 . 2 0 )

I t now becomes p o s s i b l e t o c a l c u l a t e an improved a p p r o x i m a t i o n o f t h e f i n a l

t e m p e r a t u r e o f t h e l i q u i d hea ted

t2 = t^ + q / ( ( l + n ) G C ) ( 2 . 2 1 )

I f | t 2 - t2^-| > t ^ , where t ^ i s t h e assumed t e m p e r a t u r e - e r r o r t o l e r a n c e , t h e n

a new guess on t h e v a l u e o f t2^- s h o u l d be made and t h e c a l c u l a t i o n s s h o u l d be

r e p e a t e d , s t a r t i n g f rom e q n . ( 2 . 1 8 ) . Once an a c c e p t a b l e v a l u e o f t 2 has been

f o u n d , t h e c o u r s e o f t h e c a l c u l a t i o n s depends on t h e r e l a t i o n between t2 and

t h e r e q u i r e d f i n a l t e m p e r a t u r e t ^ . I f t 2 i s t o o l o w , t h e n t h e h e a t i n g s u r f a c e

a r e a s h o u l d be i n c r e a s e d , o r a n o t h e r h e a t e r s h o u l d be p l anned i m m e d i a t e l y

f o l l o w i n g t he one b e i n g e v a l u a t e d . I n e i t h e r c a s e , t h e c a l c u l a t i o n s a c c o r d i n g

t o e q n s . ( 2 . 1 8 ) - ( 2 . 2 1 ) must be r e p e a t e d w i t h a new s e t o f i n p u t d a t a . Too h i g h

a t2 v a l u e s h o u l d be i n t e r p r e t e d as an i n d i c a t i o n o f t o o l a r g e a c o n d e n s a t e

f l o w o r t oo l a r g e a h e a t i n g s u r f a c e a r e a . The r e a l i s t i c v a l u e o f t h e h e a t

t r a n s f e r r e d i s

q ^ = (1 + n ) G C ( t ^ - t ^ ) ( 2 . 2 2 )

and t h e f i n a l c o n d e n s a t e t e m p e r a t u r e can be c a l c u l a t e d as

^c2 = ^ c l ^ V ( ^ - ^ ^ ^ c ) ( 2 . 2 3 )

The r e s u l t i n g mean l o g a r i t h m i c t e m p e r a t u r e d i f f e r e n c e i s

Lt^ = ( ( t ^ ^ - t ^ ) - ( t ^ 2 - t i ) ) / l n ( ( t c l - t f ) / ( t c 2 • ^ l ) ) ( 2 . 2 4 )

and t he e f f e c t i v e h e a t i n g s u r f a c e a r e a can be d e t e r m i n e d as

= q ^ / ( k A t f ) ( 2 . 2 5 )

A c o n v e n i e n t f e a t u r e o f t h e f o r m u l a e g i v e n above i s t h a t t h e y a r e e a s y t o

a r r a n g e i n a manner f a c i l i t a t i n g t h e c a l c u l a t i o n s f o r a g r o u p o f h e a t e r s

s u p p l i e d w i t h v a r i o u s h e a t i n g med ia . T y p i c a l l y , a d e f i n i t e t e m p e r a t u r e o f t h e

l i q u i d i s r e q u i r e d a t t h e l a s t h e a t e r o u t l e t and t h e h e a t t r a n s f e r s u r f a c e

a reas a r e g i v e n f o r a l l t h e h e a t e r s . The c a l c u l a t i o n s can be c o m p u t e r i z e d

a c c o r d i n g t o t he f l o w d iag ram shown i n F i g . 2 . 6 . I f t h e i n i t i a l assump t i ons a r e

c o r r e c t , then the temperatures of the l i q u i d between heaters are d e t e r m i n e d and

72

" d e n s a t e W heated L

assign evaporator

effect

t2,ec^n.(2.7)

At^.tcf^^r^vf'^f eans.(2.12)-

(2.17) eons. ( 2 . 8 ) -

(2.11)

set next t^ ecj,ual to

STQP1 ^ 1= ί +1

guess

ecins.(2-ie)-C2.21)

eans. (2 .22) -(2.25)

i CANNOT BE ,ATTAIN ED

I ^ STOP 2 ^

new t 2 i

F i g . 2 . 6 . F low d iag ram o f t h e c a l c u l a t i o n s f o r a g r o u p o f j u i c e h e a t e r s .

73

STOP 1 i s r e a c h e d . I f t h e c a l c u l a t i o n s p r o v e t h a t t h e r e q u i r e d f i n a l t e m p e r a t u r e

o f t he l i q u i d canno t be a t t a i n e d , t h e n a message i s o u t p u t and t h e a c t i o n i s

t e r m i n a t e d a t STOP 2 , t h i s i m p l y i n g t h a t t h e i n p u t da ta must be m o d i f i e d . I t

s h o u l d be o b s e r v e d t h a t a s i n g l e h e a t e r can be c a l c u l a t e d as a s p e c i a l case o f

a h e a t e r g r o u p , t h i s making i t p o s s i b l e t o a p p l y t h e a l g o r i t h m t o most h e a t e r

a r rangements e n c o u n t e r e d i n t he s u g a r i n d u s t r y .

2 . 4 . 3 E x t r a c t o r

A c h a r a c t e r i s t i c f e a t u r e o f t he mass and hea t b a l a n c e s o f t h e e x t r a c t o r i s

t h a t i t may be n e c e s s a r y t o s p l i t t he c a l c u l a t i o n s i n t o p a r t s c o r r e s p o n d i n g t o

v a r i o u s p r o c e s s r e q u i r e m e n t s imposed on e x t r a c t o r p a r t s . The d e t a i l s depend on

e x t r a c t o r d e s i g n , and may v a r y f rom f a c t o r y t o f a c t o r y . C a l c u l a t i o n s o f a t o w e r -

t y p e e x t r a c t o r have been d e s c r i b e d i n t h e l i t e r a t u r e ( r e f . 1 3 ) . I n t h e

f o l l o w i n g , a p o s s i b l e p r o c e d u r e o f t h e a p p r o x i m a t e b a l a n c e c a l c u l a t i o n s o f

a t r o u g h - t y p e (DOS) e x t r a c t o r i s p r e s e n t e d .

The q u a n t i t i e s d e t e r m i n i n g t h e mass and h e a t b a l a n c e s a r e i n d i c a t e d i n

F i g . 2 . 7 . The i d e a o f s p l i t t i n g t h e e q u a t i o n s i n t o two g roups stems f rom t h e

f a c t t h a t c o r r e c t e x t r a c t i o n depends on e f f e c t i v e h e a t i n g o f t h e incoming

c o s s e t t e s i n e x t r a c t o r zone A ; t h i s can be r e f l e c t e d by a r e q u i r e m e n t t h a t t h e

t e m p e r a t u r e s o f t h e j u i c e and c o s s e t t e s between zones A and Β be s u f f i c i e n t l y

h i g h .

cosseites

req,ulrecl temperatures: cossettes t juice t^j

fresh v\ oter Gw.tw pulp

vapour

F i g . 2 . 7 . P r i n c i p l e o f mass and e n e r g y b a l a n c e s o f a t r o u g h - t y p e e x t r a c t o r .

The t o t a l hea t demand o f t h e e x t r a c t o r i s a p p r o x i m a t e l y

Q = (1 + « . ) ( G j h j . GpCptp - 4 . 1 9 ( G ^ t ^ - G p ^ t p J - G ^ C ^ t ^ ) ( 2 . 2 6 )

where m i s t he h e a t l o s s c o e f f i c i e n t , h j i s t h e e n t h a l p y o f j u i c e a t t e m p e r a t u r e

t j , Cp i s t h e s p e c i f i c h e a t o f p u l p , and C^^ i s t h e s p e c i f i c h e a t o f c o s s e t t e s .

74

The e s t i m a t e d t e m p e r a t u r e and e n t h a l p y o f t h e c o n d e n s a t e a t t h e o u t l e t o f zone A

can be c a l c u l a t e d as

t c A = ( " ^ A ' C jtj - Stb) / (S- ' ' (Vj ^ *rb)/2)/8 ( 2 . 2 7 )

= 4.19 t , , ( 2 . 2 8 )

The r e s u l t i n g h e a t demand o f zone A i s

Qa = (1 + m)(G^^Cj^( t^^ - t ^ ) - G.(h^. - h.}) (2.29)

and t he h e a t i n g v a p o u r demand i n zone A i s

S = V ( ^ A - ^ca) (2.30) The t e m p e r a t u r e and e n t h a l p y o f t h e c o n d e n s a t e a t t h e o u t l e t o f zone Β a r e

e s t i m a t e d as

tcB = ("^B ' ( V j ' Vb)/2 - ( V „ - GpwSw ' W^^% ' Sw ' (2-31)

The hea t demand o f zone Β i s t hus

Qb = Q - Qa

and t h e h e a t i n g v a p o u r demand i n zone Β i s

% = v ( ^ B • ^ b )

(2.32)

(2.33)

(2.34)

2.4.4 S u g a r house equ ipment

The e n e r g y p r o c e s s e s i n t h e s u g a r house a r e dominated by t h e h e a t consumpt i on

i n t h e vacuum p a n s , t y p i c a l l y b a t c h p a n s . I n t h e mass and h e a t b a l a n c e

c a l c u l a t i o n s , t h e d i s c r e t e n a t u r e o f vacuum pan h e a t i n g i s u s u a l l y d i s r e g a r d e d

and t he c a l c u l a t i o n s a r e pe r f o rmed f o r t h e e n t i r e m a s s e c u i t e s t reams i n t h e

r e s p e c t i v e s t r i k e s . The q u a n t i t i e s d e t e r m i n i n g t h e mass and h e a t b a l a n c e s o f a

f i c t i v e c o n t i n u o u s pan r e p r e s e n t i n g b a t c h pans a r e i n d i c a t e d i n F i g . 2 . 8 ( a ) .

( Q ) a evaporated water Ge.te

syrups:

(b)

G v > t v ^

Ac

G,b,-t,

massecuite

F i g . 2.8. P r i n c i p l e o f mass and h e a t b a l a n c e s o f s u g a r house e q u i p m e n t : ( a ) ( a ) vacuum p a n , ( b ) s y r u p t a n k .

75

From the p r o c e s s d a t a , t h e sum o f i ncoming mass f l o w s o f s y r u p s and t h e a v e r a g e

v a l u e s o f t h e i r s p e c i f i c h e a t s , i n i t i a l t e m p e r a t u r e s and c o n c e n t r a t i o n s can be

de te rm ined as

G = Σ G . ( 2 . 3 5 )

^a = ( 2 . 3 6 )

t ^ = ( Σ G . C . t . ) / ( G C ^ ) ( 2 . 3 7 )

b^ = ( Σ G . b . ) / G ( 2 . 3 8 )

The mass f l o w o f t h e e v a p o r a t e d w a t e r ( i . e . , vacuum pan v a p o u r ) i s

= G ( l - b ^ / b ^ ) ( 2 . 3 9 )

Hav ing d e t e r m i n e d t h e s p e c i f i c h e a t o f t h e m a s s e c u i t e C ^ , and t h e e n t h a l p y o f

t he vacuum-pan v a p o u r h^ = h " ( t g ) , t h e t i m e - a v e r a g e d h e a t demand i s e x p r e s s e d as

q = (1 + u ) ( G ^ h g + (G - G g ) C ^ t ^ - G C ^ t ^ ) ( 2 . 4 0 )

where u i s t h e h e a t l o s s c o e f f i c i e n t .

L e t us o b s e r v e t h a t even t hough t h e h e a t o f s u g a r c r y s t a l l i z a t i o n i s n e g l e c t e d

h e r e , t h e f o r m u l a can y i e l d c o r r e c t h e a t demand v a l u e s p r o v i d i n g t h e h e a t l o s s

c o e f f i c i e n t i s a p p r o p r i a t e l y d e f i n e d . The e s t i m a t e d v a l u e s o f t h e t e m p e r a t u r e

and e n t h a l p y o f t h e c o n d e n s a t e a r e

^ c = ^ ^ ^ V)/^ ( 2 . 4 1 )

h^ = 4.19 t ^ ( 2 . 4 2 )

and t he t i m e - a v e r a g e d h e a t i n g v a p o u r demand i s

% = q / ( ^ - ^ ) ( 2 - 4 3 )

A s u b s t a n t i a l s h a r e o f t h e h e a t demand o f t h e s u g a r house i s a s s o c i a t e d w i t h t h e

tanks i n w h i c h t h i c k j u i c e , r u n - o f f , r e m e l t a n d , p o s s i b l y , o t h e r media a r e

t e m p o r a r i l y s t o r e d and hea ted t o d e f i n i t e t e m p e r a t u r e s . The q u a n t i t i e s

d e t e r m i n i n g t h e mass and h e a t b a l a n c e s o f an i n d i r e c t l y hea ted tank s u p p l i e d

w i t h h e a t i n g v a p o u r a t t e m p e r a t u r e t ^ a r e i n d i c a t e d i n F i g . 2 . 8 ( b ) . A f t e r

d e t e r m i n i n g t h e i n l e t and o u t l e t e n t h a l p i e s o f t h e s y r u p h-j and h ^ , and assuming

t h e hea t l o s s c o e f f i c i e n t m i s known, t h e h e a t demand can be c a l c u l a t e d as

q = (1 + m)G(h2 - h^ ) ( 2 . 4 4 )

The t e m p e r a t u r e and e n t h a l p y o f t h e c o n d e n s a t e can be e s t i m a t e d as

= (^S ^ 1 ^ ^ 2 ) / ^ (2 - ' ^5 )

h^ = 4.19 t ^ ( 2 . 4 6 )

and t h e h e a t i n g v a p o u r demand i s

^ = q / ( ^ - h^ ) ( 2 . 4 7 )

76

2.5 MASS AND HEAT BALANCES OF A M U L T I P L E - E F F E C T EVAPORATOR

2 . 5 . Ί I n t r o d u c t i o n

The m u l t i p l e - e f f e c t e v a p o r a t o r i s a p r o c e s s s t a t i o n i n w h i c h j u i c e t h i c k e n i n g

takes p l a c e ; i t can a l s o be seen as a b u i l d i n g b l o c k p l a y i n g a d e c i s i v e r o l e i n

t h e m u l t i p l e u t i l i z a t i o n o f e n e r g y i n t h e the rma l s y s t e m . The a c c u r a c y o f t h e

c a l c u l a t i o n s o f e v a p o r a t o r b a l a n c e s d e t e r m i n e s t h e a c c u r a c y w i t h w h i c h t h e mass

and hea t f l o w s w i t h i n t he therma l sys tem can be c a l c u l a t e d . I t s h o u l d t h e r e f o r e

be no ted t h a t i n t h e l i t e r a t u r e , two t e n d e n c i e s c o n c e r n i n g t h e methods o f

e v a p o r a t o r c a l c u l a t i o n s can be i d e n t i f i e d .

( i ) R e l y i n g on s i m p l i f i e d mathemat ica l models w h i c h e n a b l e one t o make

c a l c u l a t i o n s m a n u a l l y , o r w i t h t h e a i d o f a p o c k e t c a l c u l a t o r . A l t h o u g h such

methods a r e c e r t a i n l y u s e f u l i n many a p p l i c a t i o n s , t h e r e i s t h e r i s k t h a t t h e

u s e r may be unaware o f t h e magn i tude o f t h e e r r o r m a r g i n . Because o f t h e

c a l c u l a t i o n e r r o r , i t may be i m p o s s i b l e t o use t h e s e methods i n c e r t a i n d e s i g n

prob lems o r i n t h e o p t i m i z a t i o n o f e v a p o r a t o r s .

( i i ) U s i n g d e t a i l e d mathemat ica l models w h i c h may r e q u i r e t h e c a l c u l a t i o n s t o be

c o m p u t e r i z e d . A number o f models o f t h i s k i n d have been d e s c r i b e d i n t h e

l i t e r a t u r e . H o w e v e r , i t may sometimes be d i f f i c u l t t o use them because t h e

p u b l i c a t i o n s do n o t s p e c i f y model a c c u r a c y and no i n f o r m a t i o n i s g i v e n on the

numer i ca l p rob lems o f model u s e . I n t h e case o f c a l c u l a t i o n methods r e l y i n g on

t h e i t e r a t i v e improvements o f i n i t i a l l y guessed v a l u e s o f unknown v a r i a b l e s ,

an i m p r o p e r l y chosen n u m e r i c a l p r o c e d u r e may c a r r y t h e r i s k o f u n c o n t r o l l a b l e

n u m e r i c a l e r r o r s .

Both app roaches men t ioned w i l l be s t u d i e d i n t h e f o l l o w i n g . A c o n v e n i e n t

s t a r t i n g p o i n t i s t h e s o - c a l l e d g e n e r a l i z e d e v a p o r a t o r s t r u c t u r e . I t can be

u n d e r s t o o d as a s e t o f a l l p o s s i b l e components and t h e i r c o n n e c t i o n s i n

a m u l t i p l e - e f f e c t e v a p o r a t o r . Once a mathemat ica l model o f t h e g e n e r a l i z e d

s t r u c t u r e has been f o r m u l a t e d , one can e a s i l y g e n e r a t e a u n i q u e model o f a

s p e c i f i c e v a p o r a t o r v e r s i o n .

F i g u r e 2.9 shows s c h e m a t i c a l l y a r e p e t i t i v e b l o c k o f t h e s t r u c t u r e o f a c o -

c u r r e n t ( p a r a l l e l f l o w ) m u l t i p l e - e f f e c t e v a p o r a t o r c o m p r i s i n g an a r b i t r a r y

number o f e f f e c t s . The f o l l o w i n g c o n n e c t i o n s between t h e b l o c k and i t s

s u r r o u n d i n g s ( i n c l u d i n g n e i g h b o u r i n g b l o c k s ) can be taken i n t o a c c o u n t :

- v a p o u r w i t h d r a w a l f o r h e a t i n g pu rposes and c o n d e n s a t e r e t u r n f rom h e a t

r e c e i v e r s ,

- s u p p l y o f v a p o u r o b t a i n e d by c o n d e n s a t e f l a s h e v a p o r a t i o n ,

- condensa te w i t h d r a w a l f o r h e a t i n g o r o t h e r p u r p o s e s ( t o t h e r e c e i v e r s o u t s i d e

t h e e v a p o r a t o r ) ,

- condensa te s u p p l y t o t h e c o n d e n s a t e tank i n t h e n e x t e v a p o r a t o r e f f e c t .

I n t h e f o l l o w i n g , t he e f f e c t s o f an N - e f f e c t e v a p o r a t o r w i l l be numbered

77

extracted vapour I

steam I or ;

heating vapour t '

ν ν

i - th effect

to condenser

juice I G Ó . ^ ¿ , b t G'. t.' b.' i I Ά "ί-ΐ ' ί-ΐ' i-1 • I ψ t ' Β

condensate' flash vapouH^

condensatej

returned I condensate

extracted \ condensate!

i - r 1-1

I L

G i , t ¿ • \ EVAPORATOR BOUNDARY

F i g . 2 . 9 . Scheme o f t h e i - t h r e p e t i t i v e b l o c k i n an N - e f f e c t e v a p o r a t o r and t h e symbols used i n t h e mathemat i ca l m o d e l .

1 , 2 , . . , N . C e r t a i n v a r i a b l e s w i l l be i n d e x e d 0 a t i n l e t and N+1 a t o u t l e t .

A t y p i c a l s e t o f i n p u t da ta f o r e v a p o r a t o r c a l c u l a t i o n s c o m p r i s e s t h e

f o l l o w i n g p a r a m e t e r s :

- t h i n j u i c e mass f l o w G ^ ,

- c o n c e n t r a t i o n o f t h i n j u i c e and t h i c k j u i c e Bj^,

- t h i n j u i c e t e m p e r a t u r e t ¿ .

The mass f l o w o f w a t e r t o be e v a p o r a t e d can be c a l c u l a t e d as

G = G¿(1 - b Q / B ^ ) ( 2 . 4 8 )

The f o l l o w i n g p r o c e s s pa ramete rs a r e i n d e p e n d e n t o f t h e mass and e n e r g y b a l a n c e s

and must be s p e c i f i e d b e f o r e b e g i n n i n g t h e c a l c u l a t i o n s :

- t e m p e r a t u r e and p r e s s u r e o f t h e h e a t i n g steam s u p p l i e d t o t h e f i r s t e f f e c t 4-S „ s

P p

- t e m p e r a t u r e d i f f e r e n c e s A t ^ A t 2 , . . , ät^ be tween t h e h e a t i n g steam o r v a p o u r ,

and t he v a p o u r g e n e r a t e d f rom j u i c e ; a l t e r n a t i v e l y , t e m p e r a t u r e d i f f e r e n c e s

between t he h e a t i n g steam o r v a p o u r , and t h e j u i c e , can be s p e c i f i e d .

From the c a l c u l a t i o n s o f o t h e r components o f t h e the rma l s y s t e m , t h e

f o l l o w i n g da ta must a l s o be known:

- mass f l o w s o f v a p o u r s w i t h d r a w n f rom t h e i n d i v i d u a l e f f e c t s f o r h e a t i

pu rposes G ^ , G ^ , . . , G ^ ; ng

78

- mass f l o w s and t e m p e r a t u r e s o f c o n d e n s a t e s r e t u r n e d t o t h e c o n d e n s a t e t a n k s ,

G!|*, G 2 9 . . J GJJ and tp t 2 j . . j tjj;

- c o n d e n s a t e w i t h d r a w a l / s u p p l y c o e f f i c i e n t s r ^ i ^ 2 " * » ' ^Ν '

F o r t h e i - t h e f f e c t , r^ = 0 means t h a t t he e n t i r e c o n d e n s a t e s t r e a m i s w i t h d r a w n

t o t h e s u r r o u n d i n g s , w h i l e r . = 1 means t h a t t he e n t i r e c o n d e n s a t e s t ream i s

s u p p l i e d t o t h e tank i n t h e e f f e c t numbered ( i + 1 ) .

F o r known e v a p o r a t o r d e s i g n s and known c h a r a c t e r i s t i c s o f t h e the rma l

i n s u l a t i o n i n t he i n d i v i d u a l e f f e c t s , t h e f o l l o w i n g the rma l d a t a can be

s p e c i f i e d :

- s u b c o o l i n g ( r e l a t i v e t o t h e s a t u r a t i o n t e m p e r a t u r e ) o f t h e condensa tes l e a v i n g

t he c o n s e c u t i v e h e a t i n g chambers s ^ S 2 » . . , S j ^ ;

- v a p o u r t e m p e r a t u r e d rops due t o t h r o t t l i n g i n t h e p i p e s l i n k i n g n e i g h b o u r i n g

e f f e c t s d p d 2 , . . , d^^;

- hea t l o s s c o e f f i c i e n t s o f t h e e v a p o r a t o r b o d i e s e ^ e 2 , . . , ej^ and c o n d e n s a t e

tanks C p 0 2 » . . > C j^ .

Hav ing s p e c i f i e d a l l t h e i n p u t d a t a , one i s a b l e t o d e t e r m i n e t he v a p o u r

t e m p e r a t u r e s i n t h e e n t i r e e v a p o r a t o r ( f o r e f f e c t s numbered i = 1, 2 , . . , N) as

tV = t ? - A t . ( 2 . 4 9 )

t l , - t ^ - d,. ( 2 . 5 0 )

I t now becomes p o s s i b l e t o c o n s t r u c t a mathemat i ca l d e s c r i p t i o n o f t h e m u l t i

s t a g e e v a p o r a t i o n p r o c e s s , i n t h e fo rm o f a sys tem o f e q u a t i o n s c o n t a i n i n g

unknown mass f l o w s , t e m p e r a t u r e s and j u i c e c o n c e n t r a t i o n s i n o r between t h e

i n d i v i d u a l e v a p o r a t o r e f f e c t s . The f o l l o w i n g thermodynamic f u n c t i o n s must be

known i n o r d e r t o f o r m u l a t e t h e e n e r g y b a l a n c e r e l a t i o n s h i p s :

- h ' ^ ( t , b ) , j u i c e e n t h a l p y as a f u n c t i o n o f t e m p e r a t u r e and c o n c e n t r a t i o n ,

- A T ( t , b ) , b o i l i n g p o i n t e l e v a t i o n as a f u n c t i o n o f t e m p e r a t u r e and

c o n c e n t r a t i o n ,

- h " ( t ) , e n t h a l p y o f d r y s a t u r a t e d steam as a f u n c t i o n o f t e m p e r a t u r e ,

- h ' ( t ) , e n t h a l p y o f s a t u r a t e d w a t e r as a f u n c t i o n o f t e m p e r a t u r e .

2 .5 .2 S i m p l i f i e d model

The mathemat ica l d e s c r i p t i o n o f a s i n g l e e v a p o r a t o r e f f e c t becomes much

e a s i e r t o h a n d l e i f t h e h e a t l o s s e s and c o n d e n s a t e r e t u r n s a r e n e g l e c t e d and t h e

amount o f v a p o u r e v a p o r a t e d f rom j u i c e i s s e t equa l t o t h e amount o f h e a t i n g

v a p o u r o r steam condensed ( t h a t i s , t h e i n f l u e n c e o f t h e t e m p e r a t u r e and

c o n c e n t r a t i o n on t h e j u i c e e n t h a l p y i s d i s r e g a r d e d ) . These assumpt i ons can be

w r i t t e n down, f o r i = 1, 2 , . . , N, as e.¡ = c^. = 0 , οξ* = 0 , and

G^ = G^ - GT + G^^ . , ( 2 . 5 1 )

where G Í i s t h e mass f l o w o f c o n d e n s a t e f l a s h v a p o u r , t o be c a l c u l a t e d f rom t h e

79

r e l a t i o n s h i p

= ( G ' + G ^ _ ^ ) ( h ^ ^ - h ^ ) / ( h [ - h^ ) ( 2 . 5 2 )

where t h e e n t h a l p i e s h ^ ^ , h9 and h t a p p l y t o t h e s u b c o o l e d c o n d e n s a t e l e a v i n g

t h e h e a t i n g chamber , t h e condensa te i n t h e t a n k , and t h e f l a s h v a p o u r ,

r e s p e c t i v e l y . These e n t h a l p i e s a r e d e t e r m i n e d as

h ^ ' = h ' ( t ^ - s . ) ( 2 . 5 3 )

= h ' ( t ^ ^ ^ ) ( 2 . 5 4 )

h f = h " ( t ^ ^ T ) ( 2 . 5 5 )

The mass f l o w o f c o n d e n s a t e t o t h e tank i n t h e n e x t e f f e c t i s

G? = r . ( G ^ + G ^ . ^ - G f ) ( 2 . 5 6 )

The mass b a l a n c e o f t he e v a p o r a t o r can now be d e t e r m i n e d by s o l v i n g an e q u a t i o n

w i t h t he mass f l o w o f t h e l a s t - e f f e c t v a p o u r t o t h e c o n d e n s e r , G ^ ^ ^ , as t h e

unknown v a r i a b l e . L i n k i n g t h e mass b a l a n c e s o f t h e e f f e c t s numbered

N, N - 1 , . . , i , f o r m u l a t e d i n a c c o r d a n c e w i t h e q n . ( 2 . 5 1 ) , we o b t a i n

= ¿ « ^ N - k - i k ) ^ ( 2 . 5 7 )

On t h e o t h e r h a n d , we can c o n c l u d e f rom t h e mass b a l a n c e o f t h e e n t i r e

e v a p o r a t o r t h a t

G ^ ^ l = (G - ^ E ^ i ( G y - G Í ) ) / N ( 2 . 5 8 )

where G i s d e f i n e d by e q n . ( 2 . 4 8 ) .

The m a s s - b a l a n c e e q u a t i o n can now be o b t a i n e d by s u b s t i t u t i n g , f o r

i = Ν , N - 1 , . . , 1, e q n s . ( 2 . 5 3 ) - ( 2 . 5 6 ) i n t o e q n . ( 2 . 1 1 ) , t hus d e t e r m i n i n g g [ as

f u n c t i o n s o f G ^ _ ^ ^ , and s u b s e q u e n t l y s u b s t i t u t i n g t h e s e f u n c t i o n s i n t o e q n .

( 2 . 5 8 ) . F o r a s p e c i f i c number o f e f f e c t s N, t h i s can be done a n a l y t i c a l l y . I t i s

a l s o p o s s i b l e t o d e v i s e a s i m p l e a l g o r i t h m d e l i v e r i n g n u m e r i c a l s o l u t i o n s f o r

a r b i t r a r y N , as shown i n F i g . 2 . 1 0 ( a ) . The a l g o r i t h m i s based on a c h a i n o f

s u b s t i t u t i o n s g e n e r a t i n g an e q u a t i o n o f t h e fo rm

" l l -η<^Μ) ( 2 - 5 9 )

T h i s e q u a t i o n can be s o l v e d by i t e r a t i v e l y i m p r o v i n g a p p r o x i m a t e s o l u t i o n s t o

s a t i s f y t h e c o n d i t i o n

IG^^I - f ( G ^ ^ ^ ) | < g ( 2 . 6 0 )

where g i s a s u f f i c i e n t l y sma l l number.

In o r d e r t o s i m p l i f y t h e f l o w d i a g r a m , t h e c a l c u l a t i o n o f t h e c o n d e n s a t e

mass f l o w a c c o r d i n g t o e q n . ( 2 . 5 6 ) i s n o t shown i n F i g . 2 . 1 0 ( a ) . As t h i s

8 0

(α) eqn.(2A8)

(b )

for i = 1.2. . . . N

eqns.{2.49).(2.58)

{2.53)- (2.55)

Gi = 0

for i=N,N-1,..,1

eqn.(2.57) eqn.(2.52)

- J -

G ^ ^ ^ from(2.58)

eqn.(2A8)

for i=1.2, . . . N

eqns.{2.49).(2.50)

(2.53)-(255).(2.80)

initial guesses

for G ^ , ^

F i g . 2 . 1 0 . F low d iag ram o f t h e a l g o r i t h m s o f e v a p o r a t o r c a l c u l a t i o n s : ( a ) u s i n g t h e s i m p l i f i e d m o d e l , ( b ) u s i n g t h e d e t a i l e d m o d e l .

c a l c u l a t i o n i s pe r f o rmed b e f o r e c h e c k i n g c o n d i t i o n ( 2 . 6 0 ) , t h e mass b a l a n c e o f

t h e e v a p o r a t o r becomes unamb iguous l y d e f i n e d . Known v a p o u r and c o n d e n s a t e f l o w s

make i t p o s s i b l e t o c a l c u l a t e j u i c e f l o w s and c o n c e n t r a t i o n s , f o r

i = 2 , 3 , . . , N - 1 , as

= Q J . ^ - G ? ( 2 . 6 1 )

C o n s e q u e n t l y , b o i l i n g p o i n t e l e v a t i o n s and e f f e c t i v e t e m p e r a t u r e d i f f e r e n c e s i n

t h e e f f e c t s numbered i = 1 , 2 , . . , Ν a r e

Δ Τ . = AT(tr, b . ) ( 2 . 6 3 )

81

A t . = t^ - (tV + Δ Τ . ) ( 2 . 6 4 )

Known t e m p e r a t u r e s and j u i c e c o n c e n t r a t i o n s now make i t p o s s i b l e t o e s t i m a t e

o v e r a l l hea t t r a n s f e r c o e f f i c i e n t s k . i n each e f f e c t , u s i n g g raphs o r f o r m u l a e

c h a r a c t e r i z i n g t h e e v a p o r a t o r d e s i g n a p p l i e d . C o n s e q u e n t l y , h e a t t r a n s f e r

s u r f a c e a reas can be c a l c u l a t e d as

F,. = G^ (h? - h f ) / ( k . A t . ) ( 2 . 6 5 )

2 .5 .3 D e t a i l e d model

I t was o b s e r v e d a l ong t ime ago t h a t t h e a c c u r a c y o f t h e s i m p l i f i e d model may

be i n s u f f i c i e n t when s o l v i n g c e r t a i n e n g i n e e r i n g p rob lems ( r e f . 1 5 ) . The

s y s t e m a t i c e r r o r i n h e r e n t i n t h e s i m p l i f i e d model r e s u l t s m a i n l y f rom n e g l e c t e d

changes i n j u i c e e n t h a l p y between c o n s e c u t i v e e v a p o r a t o r e f f e c t s , n e g l e c t e d h e a t

l o s s e s t o t h e s u r r o u n d i n g s , and i n c o m p l e t e d e s c r i p t i o n o f t h e c o n d e n s a t e f l a s h .

More d e t a i l e d mathemat i ca l models have been c o n s i d e r e d by v a r i o u s a u t h o r s

( r e f s . 1 6 - 1 8 ) . A comp le te s e t o f e q u a t i o n s d e s c r i b i n g a s i n g l e e v a p o r a t o r e f f e c t

i s p r e s e n t e d b e l o w . I t i s assumed t h a t t h e j u i c e c o n c e n t r a t i o n i s known, w h i c h

i m p l i e s t h a t some h i g h e r - o r d e r a l g o r i t h m must s u p p l y c o n c e n t r a t i o n s f o r each

c a l c u l a t i o n s t e p c o r r e s p o n d i n g t o a s i n g l e e f f e c t . T h e r e f o r e , t h e model may

i n c l u d e e q n s . ( 2 . 6 3 ) and ( 2 . 6 4 ) . The e n t h a l p y o f t h e j u i c e i s

h * = h J ( t ^ ' , b . ) ( 2 . 6 6 )

The mass f l o w o f t h e v a p o u r o b t a i n e d i s

G? = G- - G ( + G ^ ^ ^ ( 2 . 6 7 )

The mass f l o w o f t h e h e a t i n g v a p o u r r e q u i r e d t o e v a p o r a t e t h i s amount o f w a t e r

can be c a l c u l a t e d as

G^ = ((1 + e . ) / ( h ^ - h f ) ) ( G ? ( h y - hl^) - G J ( h J _ ^ - h J ) ) ( 2 . 6 8 )

where t h e e n t h a l p y h ? ^ i s d e f i n e d by e q n . ( 2 . 5 3 ) and t h e e n t h a l p y hV o f v a p o u r

g e n e r a t e d i s d e f i n e d as

= h " ( t ^ ) ( 2 . 6 9 )

The t o t a l mass f l o w o f t h e c o n d e n s a t e e n t e r i n g t h e tank i s

G - ^ = G^_. , + G^ + G C ( 2 . 7 0 )

and i t s a v e r a g e t e m p e r a t u r e can be c a l c u l a t e d a p p r o x i m a t e l y as

t f = ( G ? ( t ^ - s . ) + G ^ . ^ t ^ . ^ + G ; ' t ^ ) / G f ( 2 . 7 1 )

The condensa te e n t h a l p y a t t h e a v e r a g e t e m p e r a t u r e i s

h f = h ' ( t f ) ( 2 . 7 2 )

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The mass f l o w o f t h e condensa te f l a s h v a p o u r i s

= G f ( h f - h ^ ) / ( ( h i - h ^ ) ( l + c . ) ) ( 2 . 7 3 )

and t he mass f l o w o f t he c o n d e n s a t e w i t h d r a w n t o t h e s u r r o u n d i n g s i s

G^ = G9^ - - G ^ ( 2 . 7 4 )

I t i s now p o s s i b l e t o d e t e r m i n e t he mass f l o w o f t h e j u i c e a t t h e o u t l e t as

GJ = G J . ^ - G? ( 2 . 7 5 )

and t he j u i c e c o n c e n t r a t i o n a t t h e o u t l e t as

b i = b ^ - i G ^ - j / G ^ ' ( 2 . 7 6 )

As i n t he s i m p l i f i e d m o d e l , t h e above e q u a t i o n s can be f o r m u l a t e d f o r t h e

e f f e c t s numbered 1, 2 , . . , Ν and t h e n combined i n t o one e q u a t i o n ( w i t h one

unknown v a r i a b l e G^^^^) ana logous t o e q n . ( 2 . 5 9 ) . As t h e c h a i n o f s u b s t i t u t i o n s

i s r a t h e r c o m p l i c a t e d , t h e e q u a t i o n can o n l y be s o l v e d n u m e r i c a l l y . C o n t r a r y t o

t h e s i m p l i f i e d m o d e l , d i r e c t i t e r a t i o n s may n o t c o n v e r g e ; i t i s t h e r e f o r e

n e c e s s a r y t o use o t h e r n u m e r i c a l methods . The e q u a t i o n i s c o n v e n i e n t l y r e w r i t t e n

i n t h e fo rm

'^l,-n^l^)-0 ( 2 . 7 7)

I n F i g . 2 . 1 0 ( b ) , t h e f l o w d iag ram o f t h e a l g o r i t h m based on a p p l i c a t i o n o f t h e

s e c a n t method i s shown. A p r e r e q u i s i t e f o r a s u c c e s s f u l i t e r a t i v e c o m p u t a t i o n i s

t o f i n d two i n i t i a l a p p r o x i m a t i o n s o f t h e unknown v a r i a b l e f o r w h i c h e r r o r s o f

e q n . ( 2 . 7 7 ) have o p p o s i t e s i g n s . I t i s u s u a l l y p o s s i b l e t o s e t G ^ ^ ^ = 0 as t h e

f i r s t a p p r o x i m a t i o n , and G^^-j r e s u l t i n g f rom t h e i n i t i a l c h a i n o f s u b s t i t u t i o n s

as t he s e c o n d . When c o m p u t e r i z i n g t h e a l g o r i t h m , i t i s recommended t o check t h e

e r r o r s i g n s f o r bo th a p p r o x i m a t i o n s a n d , i f n e c e s s a r y , t o m o d i f y one o f them

b e f o r e t h e i t e r a t i v e p r o c e s s i s s t a r t e d . Some i n i t i a l a p p r o x i m a t i o n s have a l s o

t o be found f o r s e v e r a l o t h e r v a r i a b l e s whose v a l u e s a r e n o t known as l o n g as

G ^ ^ ^ i s unknown. B e f o r e e n t e r i n g t h e i t e r a t i o n l o o p , i t can be assumed t h a t f o r

i = 2 , 3 , . . , N-1 , t he c o n c e n t r a t i o n s and t h e mass f l o w s o f t h e j u i c e a r e

b. = b._^ + ( B ^ - b Q ) / N ( 2 . 7 8 )

and

G J = G J . ^ b . . i / b . ( 2 . 7 9)

The i n i t i a l guesses f o r t h e mass f l o w s o f t h e f l a s h v a p o u r s f o r i = 1, 2 , . . , N-1

can be

G f = 0.005 G^ ( 2 . 8 0 )

The i t e r a t i o n s i n t h e i n t e r n a l l o o p can be t e r m i n a t e d i f , i n two s u b s e q u e n t

83

i t e r a t i o n s numbered n-1 and n , t h e j u i c e c o n c e n t r a t i o n s a t t h e e v a p o r a t o r o u t l e t

do n o t d i f f e r by more t h a n t h e assumed t o l e r a n c e b^

|bi") - b ( " - 1 ) | < ( 2 . 8 1 )

A f t e r c o m p l e t i n g t h e i t e r a t i o n l o o p c o n t r o l l e d by t h e s e c a n t me thod , a l l t h e

mass f l o w s w i t h i n t h e m u l t i p l e - e f f e c t e v a p o r a t o r a r e known. E q u a t i o n s ( 2 . 6 4 ) and

( 2 . 6 5 ) can t h e n be used t o d e t e r m i n e t h e h e a t exchange a r e a s i n t h e i n d i v i d u a l

e v a p o r a t o r b o d i e s .

2 .5 .4 Example

The d i f f e r e n c e between t h e s i m p l i f i e d model and t h e d e t a i l e d model can be

demons t ra ted by a n u m e r i c a l examp le . The i n p u t d a t a f o r a q u i n t u p l e - e f f e c t

e v a p o r a t o r a r e shown i n T a b l e 2 . 5 , and t h e r e s u l t s o f t h e c a l c u l a t i o n s p e r f o r m e d

u s i n g bo th models a r e shown i n T a b l e 2 . 6 . As can be s e e n , w h i l e t h e r e s u l t s

p roduced by t h e d e t a i l e d model a r e f u l l y a c c e p t a b l e , t h e s i m p l i f i e d model y i e l d s

a n e g a t i v e v a p o u r f l o w f rom t h e l a s t e v a p o r a t o r e f f e c t t o t h e c o n d e n s e r , w h i c h

i s p h y s i c a l l y i m p o s s i b l e .

TABLE 2.5

Main i n p u t da ta f o r t h e mass and h e a t b a l a n c e s o f a q u i n t u p l e - e f f e c t e v a p o r a t o r .

T h i n j u i c e mass f l o w ( t / h ) 219.9 T h i n j u i c e t e m p e r a t u r e ( O C ) 127.0 T h i n j u i c e c o n c e n t r a t i o n {% DS) 14.1 T h i c k j u i c e c o n c e n t r a t i o n (% DS) 52.0 H e a t i n g steam t e m p e r a t u r e ( O C ) 139.0

E f f e c t No. 1 2 3 4 5

Mass f l o w o f v a p o u r w i t h d r a w n ( t / h ) 4.75 49.00 8.10 8.10 6.93 Tempera tu re d i f f e r e n c e between h e a t i n g ( K ) 9.2 8.1 6.8 12.3 13.5 s t e a m / v a p o u r and v a p o u r g e n e r a t e d

( K )

TABLE 2.6

E x c e r p t s f rom t h e c a l c u l a t e d mass and e n e r g y b a l a n c e s o f a q u i n t u p l e - e f f e c t e v a p o r a t o r ( a l l mass f l o w s i n t / h ) .

Q u a n t i t y E f f e c t No. S i m p l i f i e d

model D e t a i l e d

model

Mass f l o w o f v a p o u r g e n e r a t e d 1 67.60 66.85 2 64.01 63.22 3 16.09 15.60 4 9.11 9.66 5 3.10 5.03

Mass f l o w o f c o n d e n s a t e f l a s h 1 1.16 0.98 v a p o u r 2 1.08 0.76

3 1.13 1.00 4 2.08 2.28 5 2.26 2.67

Mass f l o w o f v a p o u r t o t h e c o n d e n s e r - 1 . 5 8 0.76

84

I t s h o u l d be n o t e d t h a t t h e d i f f e r e n c e i n v a p o u r f l o w s t o t h e c o n d e n s e r r e s u l t s

f rom d i f f e r e n t app roaches o f t h e models t o t h e j u i c e f l a s h phenomenon a t t h e

i n l e t t o each s t a g e . As t h i s phenomenon i s d i s r e g a r d e d i n t h e s i m p l i f i e d model

( t h e i n f l u e n c e o f t e m p e r a t u r e and c o n c e n t r a t i o n on j u i c e e n t h a l p y i s n e g l e c t e d ) ,

t h e t o t a l amount o f v a p o u r s g e n e r a t e d i n t h e e v a p o r a t o r i s u n d e r e s t i m a t e d , and

so i s t he v a p o u r f l o w f rom t h e l a s t e f f e c t .

2.6 MASS AND HEAT BALANCES OF A THERMAL SYSTEM

2.6.1 P r i n c i p l e s o f u t i l i z a t i o n o f t h e r e s u l t s o f b a l a n c e c a l c u l a t i o n s

The a l g o r i t h m s used t o c a l c u l a t e t h e m u l t i p l e - e f f e c t e v a p o r a t o r and t h e h e a t

r e c e i v e r s can be combined i n t o one a l g o r i t h m o f a d e t a i l e d mass and h e a t

b a l a n c e o f t h e the rma l s y s t e m . To make a d i s t i n c t i o n f rom t h e e x t e r n a l e n e r g y

b a l a n c e , t h i s p r i n c i p l e i s sometimes c a l l e d e v a p o r a t o r - r e c e i v e r a p p r o a c h . I n

o r d e r t o t u r n t he combined a l g o r i t h m i n t o a t o o l f o r e n g i n e e r i n g a n a l y s e s , i t

s h o u l d be g e n e r a l enough t o make i t p o s s i b l e t o c a l c u l a t e t h e b a l a n c e s o f

v a r i o u s h y p o t h e t i c a l t he rma l s y s t e m s . The a l g o r i t h m s h o u l d a l s o make i t e a s y

f o r t h e u s e r t o h a n d l e p r a c t i c a l s i t u a t i o n s a s s o c i a t e d w i t h s o l v i n g e n g i n e e r i n g

p r o b l e m s . F o r e x a m p l e , when d e s i g n i n g a new the rma l s y s t e m , t h e r e s u l t s o f t h e

c a l c u l a t i o n o f an e v a p o r a t o r m igh t p r o v e t h a t t h e assumed d i s t r i b u t i o n o f

v a p o u r s t o t h e i n d i v i d u a l r e c e i v e r s i s i n c o m p a t i b l e w i t h t h e r e q u i r e d v a l u e o f

t h i c k j u i c e c o n c e n t r a t i o n . A l t e r n a t i v e l y , f o r o t h e r s e t s o f i n p u t d a t a , t h e

r e s u l t s m igh t i n d i c a t e t h a t t he j u i c e t e m p e r a t u r e s assumed i n c e r t a i n p l a c e s

c a n n o t be r e a c h e d . I n b o t h c a s e s , t h e d e s i g n p r o c e d u r e r e q u i r e s t h a t some

changes t o t h e i n p u t da ta be i n t r o d u c e d , and two b a s i c t y p e s o f changes can be

i m a g i n e d : c o r r e c t i o n s o f t h e mass b a l a n c e , o r m o d i f i c a t i o n s o f t h e s t r u c t u r e o f

t he the rma l s y s t e m . Once t h e i n p u t d a t a have been c h a n g e d , t h e c a l c u l a t i o n s must

be r e p e a t e d .

A s l i g h t l y d i f f e r e n t s i t u a t i o n a r i s e s when i n v e s t i g a t i n g t h e mass and h e a t

b a l a n c e s o f an e x i s t i n g s y s t e m . U s u a l l y , t h e b a l a n c e s can be c a l c u l a t e d f rom

i n p u t da ta w h i c h i n c l u d e o n l y a p a r t o f t h e d a t a o b t a i n e d f rom t h e measurements .

The r e s u l t s a r e t hen s e t a g a i n s t t h e r e m a i n i n g measurement d a t a , and an a t t e m p t

i s made t o i n t e r p r e t p o s s i b l e d i s c r e p a n c i e s . Once a h y p o t h e s i s f o r t h e cause o f

t h e d i s c r e p a n c i e s has been f o r m u l a t e d , changes t o t h e i n p u t d a t a a r e i n t r o d u c e d

and t he b a l a n c e c a l c u l a t i o n s a r e r e p e a t e d . ( T h e a p p l i c a t i o n s o f t h i s p r o c e d u r e

a r e men t ioned i n C h a p t e r 3 . )

I t can be c o n c l u d e d f rom t h e above examples t h a t t h e a n a l y s e s o f e n e r g y

b a l a n c e s o f t e n r e q u i r e r e p e t i t i v e c a l c u l a t i o n s . T h i s must be seen i n c o n n e c t i o n

w i t h t he h a n d l i n g o f c o n s i d e r a b l e vo lumes o f d a t a and t h e r e p e t i t i v e use o f

thermodynamic f u n c t i o n s . The p r e s e n t a t i o n o f t h e b a l a n c e r e s u l t s i s a l s o a n o n -

t r i v i a l p r o b l e m , because i t i s n e c e s s a r y t o show a l l t h e d e t a i l s w h i c h may be

needed t o a n a l y s e t h e c o r r e c t n e s s o f t h e b a l a n c e and t o i d e n t i f y t h e n e c e s s a r y

85

changes i n t he i n p u t d a t a .

2 . 6 . 2 C o m p u t e r - a i d e d b a l a n c e c a l c u l a t i o n s

The c h a r a c t e r i s t i c f e a t u r e s o f t h e e n g i n e e r i n g t a s k s d i s c u s s e d i n t h e

p r e c e d i n g S e c t i o n j u s t i f y t h e use o f computers t o automate d a t a h a n d l i n g ,

c a l c u l a t i o n s and p r e s e n t a t i o n o f r e s u l t s . Con tempora ry c o m p u t e r s , i n c l u d i n g t h e

s o - c a l l e d p r o f e s s i o n a l p e r s o n a l c o m p u t e r s , a r e v e r y w e l l s u i t e d t o t h i s k i n d o f

a p p l i c a t i o n . As r e g a r d s t h e d a t a h a n d l i n g , t h e p rob lems can c o n v e n i e n t l y be

s o l v e d u s i n g s t a n d a r d d a t a - b a s e programs t o c r e a t e and upda te t h e d a t a f i l e s .

The c a l c u l a t i o n p r o g r a m , i n c l u d i n g t h e thermodynamic f u n c t i o n s , can be w r i t t e n

i n some h i g h - l e v e l programming l a n g u a g e , l i k e B a s i c , F o r t r a n o r P a s c a l . The

program s h o u l d r e a d most i n p u t d a t a f rom t h e f i l e s , p o s s i b l y a c c e p t i n g t h e

r e m a i n i n g d a t a f rom t h e k e y b o a r d , and t h e r e s u l t s s h o u l d p r e f e r a b l y be s t o r e d i n

t h e f i l e s . I t i s a l s o i m p o r t a n t t o have h i g h l y f l e x i b l e p r e s e n t a t i o n programs

a v a i l a b l e , t o make i t p o s s i b l e t o s e l e c t f rom t h e f i l e s and t o d i s p l a y o r p r i n t

o n l y t h e p a r t o f t h e r e s u l t s w h i c h i s r e a l l y needed a t a p a r t i c u l a r moment.

I t s h o u l d be p o i n t e d o u t t h a t t o c o m p u t e r i z e t h e b a l a n c e c a l c u l a t i o n s i s

pe rhaps more a p rob lem o f o r g a n i z a t i o n t han o f t h e e n g i n e e r i n g a c t i v i t i e s

c o n t e n t . T h e r e f o r e , no s p e c i f i c g u i d e l i n e s on p rogram d e s i g n w i l l be d i s c u s s e d

h e r e , as i t i s n e v e r p o s s i b l e t o e l i m i n a t e t h e need f o r a c a r e f u l a n a l y s i s o f

t h e p r a c t i c a l r e q u i r e m e n t s and c o n d i t i o n s f o r p rog ram u s e . I t may be o f some

i n t e r e s t , h o w e v e r , t o t ake a l o o k a t t h e main f e a t u r e s o f a p r o g r a m , d e v e l o p e d

w i t h t h e p a r t i c i p a t i o n o f t h e p r e s e n t a u t h o r , and l a t e r used i n hund reds o f

a p p l i c a t i o n s ( r e f . 1 9 ) . Most a p p l i c a t i o n cases were r e l a t e d t o d e s i g n p r o b l e m s ,

b u t s e v e r a l dozens o f mass and h e a t b a l a n c e s were a l s o c a l c u l a t e d when

i n v e s t i g a t i n g e n e r g y economy prob lems i n e x i s t i n g s u g a r f a c t o r i e s .

The p rogram was g r a d u a l l y improved d u r i n g t h e p e r i o d 1975-1980, as i t was

d e s i g n e d and coded f o r new v e r s i o n s o f m i c r o c o m p u t e r s . A s i m p l i f i e d f l o w d i a g r a m

v i s u a l i z i n g t h e e s s e n t i a l a c t i o n s and d e c i s i o n s l e f t t o t h e p rogram u s e r and t h e

e s s e n t i a l b l o c k s o f r o u t i n e s p e r f o r m e d by t h e compu te r i s shown i n F i g . 2 .11 . I t

can be seen t h a t t h e d i a l o g u e between t h e u s e r and t h e compute r p l a y s an

i m p o r t a n t r o l e i n t h e f u n c t i o n i n g o f t h e p r o g r a m . I n a d d i t i o n , i t has been t aken

i n t o a c c o u n t t h a t a comp le te a p p l i c a t i o n c y c l e c o m p r i s i n g d a t a i n p u t ,

c a l c u l a t i o n s , p o s s i b l e da ta m o d i f i c a t i o n s and r e - c a l c u l a t i o n s , p r e s e n t a t i o n o f

i n t e r m e d i a t e and f i n a l r e s u l t s , and f i n a l p rob lem a n a l y s i s may r e q u i r e

a c o n s i d e r a b l e t i m e . T h e r e f o r e , i t has been made p o s s i b l e t o i n t e r r u p t , and

l a t e r r e s t a r t , p rog ram a c t i o n a t s e v e r a l p o i n t s between t h e r o u t i n e b l o c k s . The

r e s u l t i n g f l e x i b i l i t y o f p rogram use t u r n e d o u t t o be a c r u c i a l f a c t o r i n i t s

s u c c e s s f u l a p p l i c a t i o n s .

Much a t t e n t i o n has been p a i d t o t h e u s e r ' s c o n v e n i e n c e when u t i l i z i n g t h e

da ta i n p u t r o u t i n e s , as l a r g e d a t a vo lumes a r e a l w a y s a s s o c i a t e d w i t h t h e r i s k

86

key in update

calculations of heat receivers, printouts

:ey in data on thernnal

syslenr) details

corrected At

F i g . 2 .11. S i m p l i f i e d f l o w d iag ram o f a computer program c a l c u l a t i n g t h e mass and hea t b a l a n c e s o f the rma l sys tems o f s u g a r f a c t o r i e s . The boxes marked * deno te d i s k e t t e memory.

o f t y p i n g e r r o r s . I n p u t d a t a a r e l o g i c a l l y d i v i d e d i n t o segments r e l a t e d t o t h e

p r o c e s s mass b a l a n c e , equ ipment p a r a m e t e r s , p r o c e s s h e a t i n g r e q u i r e m e n t s and

the rma l sys tem s t r u c t u r e . W i t h i n each segmen t , a s e r i e s o f e r g o n o m i c a l l y

87

o p t i m i z e d s c r e e n f o rma ts p r o v i d e s g u i d a n c e f o r t h e u s e r and c r o s s - c h e c k s on d a t a

c o r r e c t n e s s and c o n s i s t e n c y , as w e l l as a l l o w i n g c o n v e n i e n t i n t r o d u c t i o n o f a l l

n e c e s s a r y changes and c o r r e c t i o n s t o t h e d a t a a l r e a d y s t o r e d i n t h e compute r

memory.

C o n c e r n i n g t h e o u t p u t s , bo th t o v i d e o s c r e e n and h a r d c o p y , a number o f

o p t i o n s have been i n t r o d u c e d t o e n a b l e t h e u s e r t o choose t h e volume and t h e

fo rm o f t he o u t p u t i n f o r m a t i o n . Among o t h e r s , i t i s p o s s i b l e t o choose f rom f o u r

l anguages f o r o u t p u t d e s c r i p t i o n s . A s e c t i o n o f p r i n t o u t w i t h t h e t e x t i n

E n g l i s h i s shown i n F i g . 2 .12 .

I F A B L Ö ü f E V A P O R A T O R P A R A M E T E R S T A B L E 1

P A F ; : A M E T E R D E S C R I P T I O N

14 Η e 3 1 I. Ι Ί SI S t e a M t E ΐϊι Ρ e r a t Υ E

2 . V a Ρ o u r t e in Ρ E & t u r e

3 . J Υ ;J. C E t E ι ϊ ι ρ e A t ϊ · e

4 • L H I E f U I t θ iTi Ρ E A t . d i f f E r E N c e 5 . E.' V 3 Ρ o a t :I. o Η E F f e C t

¿ . Ε Η t r y S B E Β ι ϊ ι q i.j A i ί t I t \:i 7 *\}aFOIJ r f o r hea t i η η e e d S

8 . J U I C E Q u a n t i t y a f t e r E F F E C T

9. J U i C e C O Ι Ί C E ι ί t r *aF tEr ef f ect 1 0 . U A Ρ O IJ Ρ e S S Υ ν e

1 1 . Τ A Γι S F e r e d H e a t Α Υ a γί t i t y

U N I T

C

C

C

C

% P B

% P B

% P B

% P B

B X

M P A

A T A

KW

E F F E C T N U M B E R

1 ! 2 ! 3 : 4 : 5 136. 0 ! 1 2 8 . 8 Ί 1 2 1 . 5 1110 . 4 ! 9 9 . 8

I I I I I I I I 1 2 9 . 8 ! 1 2 2 . 5 1 1 1 1 . 4 1 1 0 0 . 8 ! 9 0 . 1 I I I I I I I I 1 3 0 . 3 ! 1 2 3 . 2 Ί 1 1 3 . 9 ! 1 0 4 . 4 I 9 4 . 5

5 . 7 ! 5 . 5 ! 7 . 3 ! 5 . 4 ! 4 . 4

J I J J

3 2 . 1 9 ! 2 0 . 3 5 ! 2 5 . 1 5 ! 4 . 3 4 ! 1 . 8 1

! ! ! ! 3 3 . 1 1 Ί 2 5 . 6 5 ! 2 4 . 6 1 ! 3 . 9 4 ! 1 . 4 6

I I I ! 7 . 0 2 ! 2 . 3 6 ! 2 1 . 7 7 ! 4 . 2 3 ! 1 . 8 2

8 2 . 8 3 ! 5 6 . 4 8 ! 3 1 . 3 2 ! 2 6 . 9 9 ! 2 5 . 1 8

1 9 . 7 6 ! 2 8 . 9 8 ! 5 2 . 2 5 ! 6 0 . 6 4 ! 6 5 . 0 1

! ! ! ! 0 . 2 6 8 ! 0 . 2 1 4 ! 0 . 1 5 0 ! 0 . 1 0 4 ! 0 . 0 7 1

2 . 7 3 ! 2 . 1 8 ! 1 . 5 3 ! 1 . 0 6 ! 0 . 7 ; ;

2 0 5 4 7 ! 1 6 0 6 6 ! 1 5 5 6 2 ! 2 5 2 8 ! 9 4 7

F i g . 2 .12 . P a r t o f a p r i n t o u t f rom a compute r p rogram c a l c u l a t i n g t h e mass and hea t b a l a n c e s o f the rma l sys tems o f s u g a r f a c t o r i e s ( c o u r t e s y Chemadex) .

2.7 EXERGY BALANCES

2.7.1 T h e o r e t i c a l backg round

When i n t r o d u c i n g t h e e n e r g y b a l a n c e e q u a t i o n s ( 2 . 5 ) and ( 2 . 6 ) , we have i n

f a c t a c c e p t e d a c o n v e n t i o n a c c o r d i n g t o w h i c h t h e r e p r e s e n t a t i o n o f t h e e n e r g y

f l o w i s based on t he e n t h a l p y o f m a t t e r e n t e r i n g o r l e a v i n g t h e thermodynamic

s y s t e m . The same c o n v e n t i o n i s used when r e p r e s e n t i n g t h e e n e r g y f l o w i n

88

a Sankey d i a g r a m . L e t us o b s e r v e , n o w e v e r , t h a t i f t h e r e f e r e n c e pa rame te rs i n

t h e d e f i n i t i o n o f t h e e n t h a l p y a r e changed ( f o r e x a m p l e , z e r o e n t h a l p y assumed

a t 20°C i n s t e a d o f a t O ^ C ) , t h e n t h e r e p r e s e n t a t i o n o f t h e e n e r g y f l o w i s

changed t o o , even though t h e e s s e n c e o f t h e e n e r g y b a l a n c e does n o t change a t

a l l . I n o r d e r t o a c c e n t u a t e t h e f a c t t h a t t h e e n e r g y b a l a n c e r e p r e s e n t a t i o n i s

so dependent on t h e n o t i o n o f e n t h a l p y , some a u t h o r s have adop ted t h e terms

" e n t h a l p y b a l a n c e " and " e n t h a l p y f l o w d i a g r a m " ( r e f s . 1 4 , 2 0 ) .

The e n t h a l p y - b a s e d e n e r g y b a l a n c e s a r e i n d i s p e n s a b l e t o t h e d e s i g n a n a l y s e s ,

and p a r t i c u l a r l y t o t h e p r e p a r a t i o n o f t h e d a t a f o r equ ipment s e l e c t i o n

d e c i s i o n s . The same a p p l i e s t o t h e m o n i t o r i n g o f e n e r g y p r o c e s s e s , i n c l u d i n g

t h e d e t e r m i n a t i o n o f t he e n e r g y c o n s u m p t i o n . I n a n a l y s e s aimed a t t h e

i d e n t i f i c a t i o n o f t h e e n e r g y - s a v i n g p o t e n t i a l o f p o s s i b l e the rma l sys tem

improvemen ts , h o w e v e r , t h e e n t h a l p y - b a s e d b a l a n c e s can be e x p e c t e d t o d e l i v e r

o n l y a p a r t o f t h e i n f o r m a t i o n r e q u i r e d . The r e a s o n i s t h a t i n r e a l - l i f e

s y s t e m s , t h e e f f i c i e n c y o f e n e r g y u t i l i z a t i o n can be r e d u c e d n o t o n l y by d i r e c t

h e a t l o s s e s t o t h e e n v i r o n m e n t , b u t a l s o by i n d i r e c t l o s s e s known as t h e

the rma l d e g r a d a t i o n o f e n e r g y . F o r e x a m p l e , i f a c e r t a i n amount o f e n e r g y has

been t r a n s f e r r e d f rom a h i g h - t e m p e r a t u r e medium t o a l o w - t e m p e r a t u r e o n e , t h e n

t h e range o f p o s s i b i l i t i e s f o r t h e u t i l i z a t i o n o f t h i s amount o f e n e r g y i s

n a r r o w e d . A s i m i l a r e f f e c t i s o b t a i n e d when t h e f l o w o f an e n e r g y - c a r r y i n g

medium i s t h r o t t l e d down f rom a h i g h e r t o a l o w e r p r e s s u r e . G e n e r a l l y , t h e

p r o c e s s e s r e s u l t i n g i n t h e the rma l d e g r a d a t i o n o f e n e r g y a r e c a l l e d i r r e v e r s i b l e

p r o c e s s e s .

W h i l e t he d i r e c t e n e r g y l o s s e s can be q u a n t i t a t i v e l y d e s c r i b e d on t h e

c o n c e p t u a l b a s i s p r o v i d e d by t h e f i r s t law o f t h e r m o d y n a m i c s , t h e i n d i r e c t

l o s s e s c a n n o t . A q u a n t i t a t i v e d e s c r i p t i o n o f t h e i n d i r e c t e n e r g y l o s s e s r e q u i r e s

use o f t h e n o t i o n s a s s o c i a t e d w i t h t h e second law o f t h e r m o d y n a m i c s , and

p a r t i c u l a r l y t h e n o t i o n o f e n t r o p y . F o r a thermodynamic sys tem w h i c h i s i s o l a t e d

f rom i t s s u r r o u n d i n g s , t h e second law s p e c i f i e s t h a t f o r any i n f i n i t e s i m a l l y

sma l l change o f s t a t e o f t h e s y s t e m , t h e change o f e n t r o p y ( d e n o t e d S ) must be

n o n - n e g a t i v e

dS > 0 ( 2 . 8 1 )

where t h e i n e q u a l i t y s i g n a p p l i e s t o i r r e v e r s i b l e , and t h e e q u a l i t y s i g n t o

r e v e r s i b l e , p r o c e s s e s . I n a f i n i t e p r o c e s s i n i t i a t e d a t s t a t e 1 and t e r m i n a t e d

a t s t a t e 2 , t h e e n t r o p y i n c r e a s e

2 AS = / dS ( 2 . 8 2 )

1

can be u n d e r s t o o d as a measure o f t h e e n e r g y d e g r a d a t i o n caused by t h e p r o c e s s .

As t h e e n t r o p y i s a lways a t t r i b u t e d t o t h e m a t t e r c o n t a i n e d i n t h e s y s t e m , i t

can a l s o be e x p r e s s e d p e r 1 kg mass; i t w i l l t h e n be c a l l e d s p e c i f i c e n t r o p y .

8 9

and deno ted s .

I n an a t t emp t t o c o n s t r u c t a u n i f i e d t h e o r e t i c a l app roach t o b o t h d i r e c t

e n e r g y l o s s e s and e n e r g y d e g r a d a t i o n , a new thermodynamic f u n c t i o n c a l l e d e x e r g y

has been i n t r o d u c e d w i t h t he d e f i n i t i o n

e = h - hQ - T Q ( S - S Q ) ( 2 . 8 3)

where s u b s c r i p t 0 deno tes t h e s t a t e o f thermodynamic e q u i l i b r i u m w i t h t h e

e n v i r o n m e n t . T Q t hus deno tes t h e e n v i r o n m e n t t e m p e r a t u r e and h g , S Q t h e e n t h a l p y

and e n t r o p y , r e s p e c t i v e l y , a t t h e pa rame te rs c o r r e s p o n d i n g t o t h e s t a t e o f

e q u i l i b r i u m ; h and s deno te e n t h a l p y and e n t r o p y , r e s p e c t i v e l y , a t t h e

pa ramete rs f o r w h i c h e x e r g y i s d e f i n e d .

An e x e r g y l o s s t a k i n g p l a c e i n a p r o c e s s can be i n t e r p r e t e d as an i n d i c a t i o n

t h a t , f o l l o w i n g d i r e c t e n e r g y l o s s e s o r t he rma l d e g r a d a t i o n o f e n e r g y ( t h i s

r e s u l t i n g i n e n t h a l p y c h a n g e , e n t r o p y change o r a c o m b i n a t i o n o f b o t h ) , t h e

s t a t e o f t h e sys tem moves c l o s e r t o thermodynamic e q u i l i b r i u m w i t h t h e

e n v i r o n m e n t . O b v i o u s l y , t h i s i s a s s o c i a t e d w i t h a r e d u c t i o n o f t h e range o f

p o s s i b i l i t i e s f o r t h e u t i l i z a t i o n o f sys tem e n e r g y .

U s i n g t h e n o t i o n o f e x e r g y , t h e p r o c e s s e s o f e n e r g y c o n v e r s i o n and

d i s t r i b u t i o n can be d e s c r i b e d by e x e r g y b a l a n c e s . A g r a p h i c a l r e p r e s e n t a t i o n o f

t h e e x e r g y b a l a n c e ( t h a t i s , t h e e x e r g y f l o w d i a g r a m ) i s known as t h e Grassmann

d i a g r a m . I t can g e n e r a l l y be s t a t e d t h a t t h e e x e r g y - b a s e d app roach i s v e r y

g r a p h i c and u s e f u l i n compar i sons o f d i s s i m i l a r s o l u t i o n s o f e n e r g y p r o c e s s e s .

Examples o f s u c c e s s f u l a p p l i c a t i o n s o f e x e r g y a n a l y s e s t o s u g a r t e c h n o l o g y

prob lems can be f ound i n t h e l i t e r a t u r e ( r e f s . 2 2 , 2 3 ) .

As r e g a r d s i n d u s t r i a l p r a c t i c e , i t can be o b s e r v e d t h a t once a few s o l u t i o n

c o n c e p t s have been s e l e c t e d f o r an e n e r g y p r o c e s s , i t becomes n e c e s s a r y t o

d e t e r m i n e t h e p r o c e s s p a r a m e t e r s , s e l e c t t h e equ ipment and e s t i m a t e t h e c o s t s .

O f c o u r s e , c o s t e s t i m a t e s s h o u l d be based on t h e e n t h a l p y b a l a n c e s , p o s s i b l y

p r e p a r e d i n p a r a l l e l w i t h t h e e x e r g y b a l a n c e s . I f t h e f i e l d o f p o s s i b l e

s o l u t i o n s i s l i m i t e d , as i s r a t h e r c h a r a c t e r i s t i c o f t h e s u g a r i n d u s t r y , t h e n

t he e n t h a l p y b a l a n c e a l o n e i s e f f e c t i v e enough as a t o o l f o r s o l v i n g most e n e r g y

e n g i n e e r i n g p r o b l e m s . A f t e r a l l , t h e e x e r g y b a l a n c e i s n o t h i n g more t han a n o t h e r

c o n v e n t i o n f o r d e s c r i b i n g t h e e n e r g y p r o c e s s e s .

2 . 7 . 2 Example

Compare e n t h a l p y - f l o w and e x e r g y - f l o w r e p r e s e n t a t i o n s o f e n e r g y c o n v e r s i o n

and u t i l i z a t i o n i n t h e p u l p d r y i n g p r o c e s s . The amount o f p u l p d e l i v e r e d t o

a d r u m - t y p e d r y e r i s 2 4 kg p e r 1 0 0 kg b e e t , t h e d r y s u b s t a n c e c o n t e n t changes

f rom 2 0 % t o 9 0 % , t h e gas t e m p e r a t u r e i n t h e drum i s 9 0 0 ° C a t i n l e t and l l O ^ C a t

o u t l e t , and t h e e n e r g y consumpt ion i s 2 9 0 0 k J p e r kg w a t e r removed . O t h e r d a t a

a re as f o l l o w s : f u e l used - o i l , f u r n a c e e f f i c i e n c y 0 . 9 3 , h e a t l o s s c o e f f i c i e n t

90

o f t h e d r y e r drum 0 . 0 3 , e n v i r o n m e n t t e m p e r a t u r e 20 C .

E n e r g y c o n v e r s i o n and u t i l i z a t i o n i n t he p u l p d r y i n g p r o c e s s can be d i v i d e d

i n t o t h r e e s t e p s : f u e l c o m b u s t i o n , a i r adm ix i ng t o t h e combus t i on g a s , and

d r y i n g . The Sankey d iag ram ( e n t h a l p y f l o w r e p r e s e n t a t i o n ) i s shown i n

F i g . 2 . 1 3 ( a ) and t h e Grassmann d iag ram ( e x e r g y f l o w r e p r e s e n t a t i o n ) i n

F i g . 2 . 1 3 ( b ) .

( a ) ( b )

fuel 100% fue l 100%

loss Λ ) .

7% VI

loss I

L.

exhaust gas 102.1% exhaus t gas 11.9%

, . pressed Η pulp 0.4%

dr ied pulp 0.1%

F i g . 2 .13 . E n t h a l p y - f l o w ( a ) and e x e r g y - f l o w ( b ) r e p r e s e n t a t i o n s o f t h e p u l p d r y i n g p r o c e s s . 1 - f u r n a c e , 2 - m i x i n g chamber , 3 - d r y e r d rum.

The e n t h a l p y - f l o w app roach seems t o s u g g e s t t h a t s i g n i f i c a n t e n e r g y s a v i n g s

c o u l d o n l y be o b t a i n e d by c u t t i n g down t h e e n t h a l p y f l o w i n t h e gas a t t h e d r y e r

o u t l e t , as t he d i r e c t e n e r g y l o s s e s a r e s m a l l .

I n t h e e x e r g y - f l o w r e p r e s e n t a t i o n , h o w e v e r , c o n s i d e r a b l e e x e r g y l o s s e s a r e

a t t r i b u t e d t o t h e f u r n a c e , t h e m i x i n g chamber and t h e d r y e r d rum. S t a r t i n g f rom

t h e l a s t p r o c e s s s t e p , t h e e x e r g y l o s s i n t h e d r y e r c o u l d be r e d u c e d by

d e c r e a s i n g t h e i n i t i a l gas t e m p e r a t u r e . T h e n , i n t h e m i x i n g chamber , one c o u l d

imag ine t h e e l i m i n a t i o n o f a i r a d m i x i n g , w h i c h i s t h e cause o f t h e e x e r g y l o s s

o c c u r r i n g t h e r e . I n s t e a d , gas c o o l i n g by h e a t exchange w i t h a s t e a m - g e n e r a t i n g

t ube bund le c o u l d be a p p l i e d , and by d e l i v e r i n g steam t o a t u r b o - g e n e r a t o r ,

e l e c t r i c a l e n e r g y c o u l d be p roduced w i t h o u t any e x t r a e n e r g y s u p p l y f rom t h e

e n v i r o n m e n t . F i n a l l y , i n t h e f u r n a c e , t h e e x e r g y l o s s i s u n a v o i d a b l e , as i t i s

a s s o c i a t e d w i t h t h e v e r y n a t u r e o f t h e combus t i on p r o c e s s .

2.8 ANALYSIS OF TRANSIENT ENERGY PROCESSES USING COMPUTER SIMULATION

I n e n g i n e e r i n g prob lems r e l a t e d t o t h e a u t o m a t i c c o n t r o l and m o n i t o r i n g o f

e n e r g y p r o c e s s e s , i t may be n e c e s s a r y t o s t u d y t he dynamic b e h a v i o u r o f t h e

the rma l sys tem unde r c h a n g i n g o p e r a t i n g c o n d i t i o n s . F o r e x a m p l e , i t may be

91

n e c e s s a r y t o e v a l u a t e p o s s i b l e consequences o f t h e a p p l i c a t i o n s o f v a r i o u s

c o n t r o l sys tems w i t h r e s p e c t t o t h e e n e r g y l o s s e s accompany ing t r a n s i e n t s t a t e s

o f t h e e v a p o r a t o r . Prob lems o f t h i s k i n d can be a n a l y s e d w i t h t h e a i d o f

computer s i m u l a t i o n methods .

The e s s e n c e o f computer s i m u l a t i o n i s i l l u s t r a t e d by t h e scheme i n F i g . 2 .14 .

F o r t h e sys tem unde r i n v e s t i g a t i o n ( i . e . , an equ ipment u n i t , a p r o c e s s s t a t i o n

o r a f a c t o r y s e c t i o n ) , an adequate mathemat i ca l model must be f o r m u l a t e d i n

terms o f v a r i a b l e s r e p r e s e n t i n g t h e key p r o c e s s pa rame te rs and t h e e x p r e s s i o n s

( i . e . , e q u a t i o n s , i n e q u a l i t i e s and f u n c t i o n s ) r e p r e s e n t i n g t h e r e l a t i o n s h i p s

between t h e p a r a m e t e r s . Hav ing t r a n s f o r m e d t h e model i n t o a compute r p r o g r a m ,

one i s a b l e t o c a r r y o u t c a l c u l a t i o n s o f t h e b e h a v i o u r o f t h e sys tem i n s t e a d y -

s t a t e and dynamic c o n d i t i o n s .

SYSTEM

1^ II 11

MATHEMATICAL MODEL

input data reflecting operating conditions

_ L _ COMPUTER PROGRAM

' I engineering decisions regarding

sysfenrTproperties.automatic controls, etc.

simulation results predicting system behaviour

F i g . 2 .14 . P r i n c i p l e o f t h e i n v e s t i g a t i o n o f o p e r a t i o n a l c h a r a c t e r i s t i c s o f t e c h n o l o g i c a l sys tems w i t h t h e a i d o f compute r s i m u l a t i o n .

S i m u l a t i o n o f t h e e v a p o r a t o r can be r e g a r d e d as a r e p r e s e n t a t i v e example o f

s i m u l a t i o n prob lems r e l a t e d t o t he e n e r g y economy. The p e r t i n e n t l i t e r a t u r e

r e f l e c t s t h e deve lopmen t o f compute r s i m u l a t i o n t e c h n i q u e s d u r i n g t h e l a s t two

d e c a d e s . I n an e a r l y s t u d y , a l i n e a r i z e d e v a p o r a t o r model s u i t e d t o programming

on an ana log computer was p r o p o s e d ( r e f . 2 4 ) . L a t e r o n , a l e a n i n g t o w a r d s

d i g i t a l computers has been g e n e r a l l y a d o p t e d . S e v e r a l p u b l i c a t i o n s can be named

where e v a p o r a t o r models a r e d e s c r i b e d u s i n g d i f f e r e n t i a l e q u a t i o n s s o l v e d by

such n u m e r i c a l methods as t h e o r t h o g o n a l c o l l o c a t i o n method ( r e f . 2 5 ) , t h e

c o r r e c t o r - p r e d i c t o r method ( r e f . 26) and R u n g e - K u t t a i n t e g r a t i o n ( r e f . 2 7 ) . The

models assume t h e use o f s p e c i a l computer p r o g r a m s , w r i t t e n i n t h e F o r t r a n

l a n g u a g e . To t he knowledge o f t h e p r e s e n t a u t h o r , h o w e v e r , none o f t h e s e

programs has been w i d e l y a p p l i e d .

An a l t e r n a t i v e app roach t o e v a p o r a t o r s i m u l a t i o n assumes t h e a p p l i c a t i o n o f

w i d e l y c i r c u l a t e d g e n e r a l - p u r p o s e s i m u l a t i o n p r o g r a m s . Such programs e n a b l e one

92

t o s o l v e t y p i c a l d i f f e r e n t i a l e q u a t i o n s e n c o u n t e r e d i n t h e s i m u l a t i o n o f

t r a n s i e n t s t a t e s o f v a r i o u s t e c h n o l o g i c a l s y s t e m s . A p r e r e q u i s i t e f o r s u c c e s s f u l

a p p l i c a t i o n o f a s p e c i f i c p rogram i s t o f o r m u l a t e t h e e v a p o r a t o r model i n

a manner c o m p a t i b l e w i t h t h e r e q u i r e m e n t s adop ted by t h e program d e s i g n e r . T h i s

app roach has been implemented i n p r a c t i c e u s i n g a w i d e l y known IBM program

( r e f . 2 8 ) .

Examples o f s i m u l a t i o n r e s u l t s d e s c r i b i n g t h e dynamic b e h a v i o u r o f m u l t i p l e -

e f f e c t e v a p o r a t o r s a r e g i v e n i n F i g . 2 .15 .

( a ) ( b )

iJllUUlr vapour flow

withdrawn from 3rd effect

j r d effect

ju ice temperatures

vapour flow withdrawn from 2nd effect

I

1000 2000

T i m é i s )

3000 4000

3rd effect

vapour consumption

5th effect

5 10

Time (min)

15

F i g . 2 .15 . Examples o f s i m u l a t i o n r e s u l t s d e s c r i b i n g t h e dynamic b e h a v i o u r o f q u i n t u p l e - e f f e c t e v a p o r a t o r s , ( a ) j u i c e t e m p e r a t u r e s a t v a r i a b l e v a p o u r w i t h d r a w a l f rom t h e t h i r d e f f e c t , d i s c h a r g e - c o n t r o l l e d j u i c e l e v e l s ( a f t e r r e f . 2 6 ) , ( b ) consumpt ion o f h e a t i n g v a p o u r s a t v a r i a b l e v a p o u r w i t h d r a w a l f rom t h e second e f f e c t , f e e d - c o n t r o l l e d j u i c e l e v e l s ( a f t e r r e f . 2 8 ) .

REFERENCES

1 T . D . E a s t o p and A . McConkey , A p p l i e d Thermodynamics f o r E n g i n e e r i n g T e c h n o l o g i s t s , 3 rd e d n . , Longman, London and New Y o r k , 1978.

2 G . J . Van Wylen and R . E . S o n n t a g , Fundamenta ls o f C l a s s i c a l The rmodynam ics , 3 rd e d n . , W i l e y , New Y o r k , 1985.

3 J . C u e l , Le b i l a n t he rm ique en s u c r e r i e , S u c r . F r . , 119(21) (1978) 424-434, 119(22) (1978) 455-466.

4 P.W. van d e r Poel ( e t a l . ) , Z u c k e r h a u s s c h e m a t a , e i n B e i s p i e l von I n f o r m a t i o n s v e r b e s s e r u n g m i t H i l f e d e r e l e k t r o n i s c h e n D a t e n v e r a r b e i t u n g , Z u c k e r , 28 (3 ) (1975) 122-131.

5 T . B a l o h , Z u c k e r t e c h n o l o g i s c h e Rechnungen m i t dem Dre ikomponen ten -D iag ramm, Z u c k e r i n d . , 107(6) (1982) 515-525.

6 A . K u b a s i e w i c z , W. Lekawski and K. U r b a n i e c , Automated d e s i g n c a l c u l a t i o n s o f b e e t s u g a r p l a n t s u s i n g m i c r o c o m p u t e r COMPUCORP 425 G , P r o c . 3 rd Symp. Use o f Computers i n Chemica l E n g i n e e r i n g , G l i w i c e , 1974, p p . 213-217.

7 L.W. W e i s s , Computer p rogram t o a i d s u g a r end o p e r a t i o n s . Paper p r e s e n t e d a t 21s t ASSBT M e e t i n g , San D i e g o , 1981.

8. H . R . D e l a n e y , D. G o t t h a r d and J . B . N i c h o l s , Use o f an e n e r g y model i n s u g a r r e f i n i n g . I n t . Suga r J . , 85(1014) (1983) 171-176.

9 R . G . H o e k s t r a , A f l e x i b l e computer p rogram f o r f o u r - c o m p o n e n t m a t e r i a l b a l a n c e s i n s u g a r i n d u s t r y b o i l i n g h o u s e s . I n t . Suga r J . , 85(1016) (1983) 227-232, 85(1017) (1983) 262-265.

93

10 P.M. S i l i n , V o p r o s y T e k h n o l o g i i Sakha rnykh V e s h c h e s t v , P i s h c h e p r o m i z d a t , Moskva , 1950.

11 W. Lekawski and K. U r b a n i e c , M o d e r n i s i e r u n g d e r W 'á rmewi r t scha f t i n Z u c k e r f a b r i k e n , Z u c k e r i n d . , 108(4) (1983) 338-343.

12 Κ. U r b a n i e c and Α . K u b a s i e w i c z , Mode le matematyczne d í a p r o j e k t o w a n i a w i e l o d z i a l o w y c h i n s t a l a c j i w y p a r n y c h , I n z . C h e m . , 7 ( 1 ) (1977) 207-221.

13 T . B a l o h , W ä r m e w i r t s c h a f t , i n : F . S c h n e i d e r ( E d . ) , T e c h n o l o g i e des Z u c k e r s , S c h a p e r V e r l a g , H a n n o v e r , 1968, p p . 705-776.

14 Τ . B a l o h , Wärmeat las f ü r d i e Z u c k e r i n d u s t r i e , S c h a p e r V e r l a g , H a n n o v e r , 1975.

15 A . L . Webre , E v a p o r a t i o n and h e a t i n g , i n : D. S p e n c e r and G . P . Meade ( E d s . ) , Cane Suga r Handbook, W i l e y , New Y o r k , 1948, p p . 134-174.

16 Τ . B a l o h , Wärmetechn ische Berechnung d e r V e r d a m p f s t a t i o n , Z u c k e r - B e i h e f t e , 3 ( 2 ) (1956) 29-74.

17 G . K imenov, E n e r g e t i s c h e U n t e r s u c h u n g e n an e i n e r m e h r s t u f i g e n Ve rdamp f s t a t i o n a l s DampfUmformer, Z u c k e r , 2 5 ( 7 ) (1972) 225-230.

18 V . U r b a n , Matemat i cky model c u k r o v a r n i c k e p r u t o k o v e o d p a r k y p r a c u j i c i se s t o u p a j i c i v r s t v o u , L i s t y C u k r . , 8 9 ( 6 ) (1973) 114-118.

19 G . B a t o r and K. U r b a n i e c , P r o j e k t i e r u n g von Ve rdamp fan lagen i n Z u c k e r f a b r i k e n m i t H i l f e von Compu te rn , Z u c k e r i n d . , 103(12) (1978) 1035-1042.

20 T . B a l o h , E n e r g i e w i r t s c h a f t b e i E i n d a m p f u n g s - und T r o c k n u n g s p r o z e s s e n , Z u c k e r i n d . , 105(1) (1980) 50-61.

21 T . B a l o h , Me thod ik be i e x e r g e t i s c h e n U n t e r s u c h u n g e n i n Z u c k e r f a b r i k e n , Z u c k e r i n d . , 106(1) (1981) 29-40.

22 0. A u e r s w a l d , E x e r g e t i s c h e A n a l y s e e i n e r Z u c k e r f a b r i k m i t B rüdenkompress ion am B e i s p i e l d e r Z u c k e r f a b r i k A a r b e r g , S c h w e i z , Z u c k e r i n d . , 106(9) (1981) 804-815.

23 T . B a l o h , S t u d i e e i n e r Z u c k e r f a b r i k m i t B r ü d e n k o m p r e s s i o n , Z u c k e r i n d . , 109(4) (1984) 285-294.

24 0. W i k l u n d , The c a l c u l a t i o n and c o n t r o l o f m u l t i p l e e f f e c t e v a p o r a t o r s . S o c k e r H a n d l . , 22 (1 ) (1968) 1-22.

25 υ. B o l m s t e d t and Α . J e r n q v i s t , S i m u l a t i o n o f t h e s t e a d y - s t a t e and dynamic b e h a v i o u r o f m u l t i p l e e f f e c t e v a p o r a t i o n p l a n t s . Comp. A i d e d D e s . , 8 ( 3 ) (1976) 142-148, 9 ( 1 ) (1977) 29-40.

26 Μ. M ä k e l ä , Ma temat i sches F o r m u l i e r e n und d i g i t a l e s S i m u l i e r e n e i n e r V e r d a m p f s t a t i o n i n d e r R ü b e n z u c k e r i n d u s t r i e , Z u c k e r i n d . , 106(11) (1981) 989-993.

27 A . L e b e r t ( e t a l . ) , S i m u l a t i o n s u r o r d i n a t e u r d ' u n e v a p o r a t e u r de s u c r e r i e a m u l t i p l e e f f e t s , I n d . A l i m . A g r i e , 9 7 ( 7 - 8 ) (1980) 691-698.

28 Κ. U r b a n i e c and M. S z c z e n i o w s k i , N a c h b i l d u n g e i n e r m e h r s t u f i g e n Ve rdamp f s t a t i o n u n t e r Verwendung des CSMP-Sys tems, Z u c k e r i n d . , 105(7) (1980) 628-631.

94

C h a p t e r 3

SELECTED PROBLEMS OF HEAT ECONOMY

3.1 ANALYSIS AND EVALUATION OF THE HEAT ECONOMY

3.1.1 M e t h o d o l o g i c a l h i n t s

Any a t t emp t t o improve t h e hea t economy o f a s u g a r f a c t o r y b e g i n s w i t h t h e

a c q u i s i t i o n o f i n f o r m a t i o n on t h e e x i s t i n g s t a t e o f t h i n g s , and an i n i t i a l

e f f o r t may be needed t o e s t a b l i s h t h e t y p e and amount o f i n f o r m a t i o n t h a t w i l l

be s u f f i c i e n t . The aim o f an i n i t i a l i n v e s t i g a t i o n i s t o r e a c h a c e r t a i n l e v e l

o f knowledge o f t h e s t r u c t u r e o f t he the rma l s y s t e m , i t s mass and h e a t b a l a n c e s ,

and t he c h a r a c t e r i s t i c s o f t h e equ ipmen t . T h i s s h o u l d e n a b l e one t o p e r f o r m an

o v e r a l l e v a l u a t i o n and t o f o r m u l a t e a d i a g n o s i s o f t he d e f i c i e n c i e s o f t h e h e a t

economy.

L e t us o b s e r v e t h a t i f t h e d i a g n o s i s can be made v e r y a c c u r a t e , t hen i t may

become e a s i e r t o o u t l i n e p o s s i b l e improvements and t h e way t h e y can be

implemented i n t h e f a c t o r y . T h e r e f o r e , i t i s a l w a y s a d v i s a b l e t o have more t han

minimum i n f o r m a t i o n a t h a n d , and a d e t a i l e d knowledge o f t h e scheme o f t h e s u g a r

m a n u f a c t u r i n g p r o c e s s , t h e v a l u e s o f p r o c e s s p a r a m e t e r s , and t h e c h a r a c t e r i s t i c s

o f p r o c e s s equ ipment and c o n t r o l sys tems i s p a r t i c u l a r l y u s e f u l .

I t can t h u s be g e n e r a l l y c o n c l u d e d t h a t t h e more d e t a i l e d a r e t h e a v a i l a b l e

d a t a , t he e a s i e r i t i s t o p e r f o r m an a n a l y s i s o f t h e e x i s t i n g s t a t e o f t h e h e a t

economy. T h e r e i s no p o i n t , h o w e v e r , i n i n v e s t i g a t i n g d e t a i l s w h i c h may be

c o s t l y t o o b t a i n b u t w i l l e v e n t u a l l y t u r n o u t i r r e l e v a n t t o t h e p rob lems o f

i n t e r e s t . T h i s a p p l i e s , i n p a r t i c u l a r , t o t h e mass and h e a t b a l a n c e s , t h e

d e t a i l s o f w h i c h may be d i f f i c u l t t o e s t a b l i s h u n l e s s t ime -consum ing and c o s t l y

measurements a r e p e r f o r m e d . E x p e r i e n c e p r o v e s t h a t a s a t i s f a c t o r y l e v e l o f

knowledge o f t h e b a l a n c e d a t a can o f t e n be o b t a i n e d t h r o u g h a p p r o x i m a t e a n a l y s e s

o f thermodynamic sys tems p r o p e r l y d e f i n e d w i t h i n a f a c t o r y . A p p r o x i m a t e b a l a n c e

c a l c u l a t i o n s can be pe r fo rmed on t h e b a s i s o f e s s e n t i a l d a t a e x t r a c t e d f rom

r o u t i n e f a c t o r y r e c o r d s . I f t he a n a l y s i s i s i n t e n d e d t o c r e a t e a b a s i s f o r

l i m i t e d m o d i f i c a t i o n s o f an e x i s t i n g f a c t o r y , t h i s app roach i s o f t e n , b u t n o t

a l w a y s , e f f e c t i v e enough . A d i s c u s s i o n o f l i m i t e d - s c a l e m o d i f i c a t i o n s o f the rma l

s y s t e m s , i n c l u d i n g p r a c t i c a l e x a m p l e s , i s g i v e n i n C h a p t e r 8.

A d i f f e r e n t s i t u a t i o n a r i s e s when t h e u n d e r l y i n g i n t e n t i o n i s t o m o d e r n i z e

t h e f a c t o r y e x t e n s i v e l y . I n t h i s c a s e , i n - d e p t h s t u d i e s o f new s o l u t i o n s ,

i n c l u d i n g mass and hea t b a l a n c e s c o r r e s p o n d i n g t o bo th t he e x i s t i n g s t a t e and

t he mode rn i zed f a c t o r y , a r e i n d i s p e n s a b l e .

A summary o f i n f o r m a t i o n r e q u i r e m e n t s a s s o c i a t e d w i t h t y p i c a l m o d e r n i z a t i o n s ,

i n c l u d i n g p r a c t i c a l e x a m p l e s , i s g i v e n i n C h a p t e r 9. The need t o a n a l y s e more

95

da ta does n o t n e c e s s a r i l y mean, h o w e v e r , t h a t t h e i n i t i a l s t a t e o f t h e hea t

economy must be known i n f u l l d e t a i l . A g a i n , a p p r o x i m a t e b a l a n c e c a l c u l a t i o n s

t u r n o u t t o be e f f e c t i v e enough i n t y p i c a l m o d e r n i z a t i o n c a s e s .

A p r a c t i c a l c o r r e l a t i o n seems t o e x i s t between t he l e v e l o f s o p h i s t i c a t i o n o f

t h e hea t economy and t he amount o f work needed t o p r e p a r e a d e c i s i o n on t h e most

s u i t a b l e and c o s t - e f f e c t i v e improvemen ts . When t a l k i n g t o t h e managers o f

e n e r g y - e f f i c i e n t f a c t o r i e s , one can u s u a l l y c o n c l u d e t h a t t h e y have a g r a s p o f

t h e s i t u a t i o n and a r e w e l l aware o f t h e a r e a s where improvements a r e n e e d e d , as

w e l l as t he s o l u t i o n s t h a t may come i n t o q u e s t i o n . On t h e o t h e r h a n d , i n a n o t -

s o - e f f i c i e n t f a c t o r y , i t may be t h a t t h e m a n a g e r ' s p e r c e p t i o n o f e n e r g y p rob lems

i s i n c o m p l e t e , no p rob lem h i e r a r c h y e x i s t s and t h e r e i s a l o t o f u n c e r t a i n t y

abou t p o s s i b l e c o u r s e s o f a c t i o n .

F o r an e n e r g y s p e c i a l i s t , t he l a t t e r case c o n s t i t u t e s a r e a l l y c h a l l e n g i n g

s i t u a t i o n . L e t us assume t h a t t he s t a r t i n g p o i n t o f t he i n v e s t i g a t i o n o f a h e a t

economy i s t h e m a n a g e r ' s d i s s a t i s f a c t i o n w i t h t h e e x i s t i n g s t a t e , b u t t h a t t h e r e

i s no c l e a r i d e a o f t h e e x t e n t o f t h e r a t i o n a l i z a t i o n measures needed and t h e

p r i o r i t i e s o f t h e p rob lems t o be s o l v e d . P r i o r t o f o r m u l a t i n g a d i a g n o s i s and

p r o p o s i n g a s e t o f r e m e d i e s , i t i s n e c e s s a r y t o c o l l e c t i n f o r m a t i o n r e l a t e d t o

t he f o l l o w i n g q u e s t i o n s :

( i ) C o n s i d e r i n g t he e x i s t i n g p o s s i b i l i t i e s and l i m i t a t i o n s r e s u l t i n g f rom i t s

l a y o u t and t h e c h a r a c t e r i s t i c s o f t h e e q u i p m e n t , does t h e r e a l pe r f o rmance o f

t h e the rma l sys tem match t h e e x p e c t e d pe r fo rmance ? I f n o t , what a r e t h e r e a s o n s

and how can t h e y be e l i m i n a t e d ?

( i i ) What a r e t h e d e c i s i v e f a c t o r s e n a b l i n g t h e the rma l sys tems t o s a t i s f y t h e

a c t u a l t o t a l hea t demand a t t h e n e t h e a t demand r e c o r d e d , and how can t h e

e f f e c t i v e n e s s r a t i o be i n c r e a s e d ?

( i i i ) What a re t he l i m i t a t i o n s imposed on t h e e n e r g y economy by t he e x i s t i n g

scheme and pa rame te rs o f t h e s u g a r m a n u f a c t u r i n g p r o c e s s and by t h e

c h a r a c t e r i s t i c s o f p r o c e s s equ ipment and c o n t r o l sys tems ? How can t h e s e f a c t o r s

be a d j u s t e d t o r e d u c e t h e t o t a l hea t demand ?

( i v ) How e f f e c t i v e a r e t h e e n e r g y m o n i t o r i n g p r o c e d u r e s based on t h e e x i s t i n g

i n s t r u m e n t a t i o n , and what a c t i o n s can be taken t o improve them ?

I n p r a c t i c e , i t i s n o t p o s s i b l e t o o b t a i n d i r e c t l y p i e c e s o f i n f o r m a t i o n t h a t

f a l l n e a t l y i n t o one o f t h e f o u r c a t e g o r i e s m e n t i o n e d . V e r y o f t e n , one has t o

i d e n t i f y , i n t e r p r e t and s o r t symptoms w h i c h may p o i n t a t some i n f o r m a t i o n

r e l a t e d t o more t han one c a t e g o r y .

T h e r e a r e t h r e e main t e c h n i q u e s o f i d e n t i f y i n g t he symptoms w h i c h

c h a r a c t e r i z e t h e f u n c t i o n i n g o f t he the rma l s y s t e m :

- t o a n a l y s e t h e d a t a i n t h e r o u t i n e f a c t o r y r e c o r d s , t o q u e s t i o n t h e managers

and the t e c h n i c a l p e r s o n n e l , and t o i n s p e c t t he f a c t o r y ( p r e f e r a b l y when i n

96

o p e r a t i o n ) ,

- t o s e t up and t o i n v e s t i g a t e t h e e x t e r n a l mass and e n e r g y b a l a n c e s o f t h e

f a c t o r y ,

- t o a n a l y s e t h e mass and h e a t b a l a n c e s o f t h e the rma l sys tem and i t s

components .

I n more c o m p l i c a t e d c a s e s , i t may be u s e f u l t o s p l i t t h e s y m p t o m - f i n d i n g

p r o c e d u r e i n t o two p a r t s . I n i t i a l l y , b a s i c d a t a a r e c o l l e c t e d t o make t he

b a l a n c e c a l c u l a t i o n s p o s i b l e . Once t h e mass and e n e r g y b a l a n c e s have been

e s t a b l i s h e d , t he f i g u r e s o b t a i n e d and t h e p r e l i m i n a r y d i a g n o s i s a r e compared

w i t h t h e r e a l i t y .

As r e g a r d s t he a n a l y s e s o f t he b a l a n c e f i g u r e s , a v a r i e t y o f t e c h n i q u e s can

be c o n s i d e r e d . I t i s p o p u l a r t o d e f i n e s e v e r a l s e t s o f i n p u t d a t a i n a manner

a l l o w i n g t h e changes o f c e r t a i n i m p o r t a n t f a c t o r s t o be s t u d i e d , and t o

c a l c u l a t e s e v e r a l mass and e n e r g y b a l a n c e s f rom w h i c h i n f o r m a t i o n on t h e

impor tance o f t h e s e f a c t o r s can be e x t r a c t e d . More advanced methods f o r e n e r g y

a n a l y s e s a r e d i s c u s s e d i n C h a p t e r 9.

3 .1 .2 C o l l e c t i n g e s s e n t i a l i n f o r m a t i o n

I n a d d i t i o n t o the r e q u i r e m e n t s men t ioned i n t h e p r e c e d i n g S e c t i o n , i t

f o l l o w s f rom t h e d i s c u s s i o n p r e s e n t e d i n C h a p t e r s 1 and 2 t h a t i n f o r m a t i o n on

t h e e n e r g y economy s h o u l d a l s o be w e l l s t r u c t u r e d . T h i s can be a c h i e v e d u s i n g

t h e top -down a p p r o a c h , t h a t i s , p r e s e n t i n g t h e o v e r a l l p i c t u r e f i r s t , t h e n

a n a l y s i n g t h e e s s e n t i a l b u i l d i n g b l o c k s o f t he therma l s y s t e m , and f i n a l l y

i n v e s t i g a t i n g the c h a r a c t e r i s t i c s o f t h e equ ipment u n i t s .

The f i r s t s t e p s h o u l d be d e v o t e d t o i d e n t i f i c a t i o n o f t h e schemes and

pa ramete rs o f t he s u g a r m a n u f a c t u r i n g p r o c e s s and t he e n e r g y p r o c e s s e s . A l l t h e

d a t a taken f rom t h e e x i s t i n g documents s h o u l d be v e r i f i e d , p r e f e r a b l y d u r i n g

normal f a c t o r y o p e r a t i o n when any changes o r m o d i f i c a t i o n s r e l a t i v e t o t he

documented s t a t e can e a s i l y be o b s e r v e d . When document ing t h e d a t a a c q u i r e d , i t

i s a d v i s a b l e t o p r e s e n t them i n fo rms f a c i l i t a t i n g easy i d e n t i f i c a t i o n o f t he

i n f o r m a t i o n s t r u c t u r e and t he r e l a t i o n s between i m p o r t a n t segments o f

i n f o r m a t i o n . A l t h o u g h t he c o n v e n t i o n a l schemes and t a b l e s l i k e t h o s e used i n t h e

p r e s e n t book a r e s u f f i c i e n t l y e f f e c t i v e i n most p r a c t i c a l a p p l i c a t i o n s , i t may

p r o v e u s e f u l t o combine them w i t h o t h e r fo rms e n a b l i n g one t o l o o k a t t he d a t a

f rom a d i f f e r e n t a n g l e . F i g u r e 3.1 shows an example o f a d iag ram w h i c h makes i t

p o s s i b l e t o match t he a v a i l a b l e h e a t i n g media t o t h e p r o c e s s media t h a t must be

hea ted ( r e f . 1 ) .

Among t h e main f e a t u r e s o f t h e the rma l s y s t e m , t h e s t a b i l i t y o f t h e

o p e r a t i n g pa rame te rs i s o f u tmost i m p o r t a n c e . I n most f a c t o r i e s , pa ramete r

f l u c t u a t i o n s may o c c u r even under p e r f e c t l y normal o p e r a t i n g c o n d i t i o n s , m o s t l y

because t h e r e a r e v a p o u r demand f l u c t u a t i o n s due t o t he b a t c h w i s e o p e r a t i o n

97

c o s s e t t e s

rec i rcu lated ju ice * . p r e s s w a t e r

m a k e - u p water

p re - l imed j u i c e σ J l imed j u i c e ~ c l e a r j u i c e —

t h i n j u i c e TD Φ Ε ω υ o

ju ice in 1 s t e f fec t v a c u u m p a n s A χ

s y r u p s — v a c u u m p a n s Β χ

room h e a t i n g

1s t -e f fec t v a p o u r 2 n d - e f f e c t v a p o u r

c o n d e n s ó t e 3 r d - e f f e c t v a p o u r χ

n o n c o n d e n s a b l e s — v a p o u r f rom vacuum pans A χ

r ec i r cu la ted j u i c e p r e s s w a t e r —

•o Ε en c o

15 20 30 AO 50 60 70 80 90 100 110 120 130

T e m p e r a t u r e { °C )

' i n the e x t r a c t i o n s t a t i o n

F i g . 3 .1 . G r a p h i c a l compendium o f t h e p r o c e s s media t o be h e a t e d , o r hea t r e c e i v e r s , and t he h e a t i n g med ia .

o f vacuum p a n s . Any paramete r change i n t r o d u c i n g a d e v i a t i o n f rom a b a l a n c e d

s t a t e o f t h e the rma l sys tem c r e a t e s t h e r i s k o f i n c r e a s e d e n e r g y l o s s .

T y p i c a l l y , t h e immediate causes o f t h e l o s s a r e i n c r e a s e d v a p o u r f l o w f rom t h e

l a s t e v a p o r a t o r e f f e c t t o t he c o n d e n s e r , o r r e d u c e d t h i c k - j u i c e c o n c e n t r a t i o n

w h i c h r a i s e s t h e h e a t demand o f t h e s u g a r h o u s e . A t v e r y r a p i d pa ramete r

c h a n g e s , t h e r e may be u n f a v o u r a b l e emergency d i s c h a r g e s o f h i g h - t e m p e r a t u r e

media w i t h i n t he therma l sys tem o r even d i r e c t l y t o t h e e n v i r o n m e n t ( e x a m p l e s :

open ing s a f e t y v a l v e s on a steam p i p e l i n e o r on an e v a p o r a t o r b o d y , o v e r f l o w i n g

h o t - j u i c e t a n k s , e t c . ) .

I t f o l l o w s f rom t h e above i n t r o d u c t i o n t h a t a b n o r m a l l y l a r g e pa ramete r

f l u c t u a t i o n s w h i c h a r e d i f f i c u l t t o dampen s h o u l d be t r e a t e d as a symptom o f

dange rous d e f i c i e n c i e s o f t h e the rma l s y s t e m . The u n d e r l y i n g f a c t o r s a r e most

f r e q u e n t l y as f o l l o w s :

- t o o smal l a h e a t i n g s u r f a c e a r e a i n t h e e v a p o r a t o r e f f e c t f rom w h i c h v a p o u r i s

s u p p l i e d t o vacuum p a n s ,

- a f a u l t y c o n t r o l sys tem i n t h e t h r o t t l i n g - d e s u p e r h e a t i n g s t a t i o n , w h i c h i s

d e c i s i v e i n s t a b i l i z i n g t h e p r e s s u r e and t e m p e r a t u r e o f t he e x h a u s t s team,

- a f a u l t y c o n d e n s a t e d r a i n a g e subsys tem c a u s i n g p e r i o d i c a c c u m u l a t i o n o f t he

condensa te i n t h e h e a t i n g chambers o f e v a p o r a t o r b o d i e s .

The e n e r g y consumpt ion o f a s u g a r f a c t o r y may be i n f l u e n c e d n o t o n l y by

98

u n c o n t r o l l a b l e paramete r f l u c t u a t i o n s b u t a l s o by r o u t i n e pa ramete r a d j u s t m e n t s ,

l i k e t h o s e a s s o c i a t e d w i t h a t e m p o r a r i l y i n t r o d u c e d r e d u c t i o n o f t h e p r o c e s s i n g

c a p a b i l i t y . G e n e r a l l y , t h e r e s p o n s e s o f a the rma l sys tem t o changes o f t h i s

k i n d a r e w o r t h s t u d y i n g as an i n d i c a t i o n o f p o s s i b l e sys tem d e f i c i e n c i e s t h a t

need t o be c o r r e c t e d . On t h e o t h e r h a n d , even c o r r e c t l y f u n c t i o n i n g sys tems

w i t h p e r f e c t pa ramete r s t a b i l i z a t i o n a r e a d v e r s e l y a f f e c t e d by c o n t i n u e d

d e v i a t i o n s f rom t h e i r nominal o p e r a t i n g p a r a m e t e r s . T h i s phenomenon s h o u l d be

s t u d i e d w i t h t h e aim o f i d e n t i f y i n g p o s s i b l e i n d i c a t i o n s o f t he e x i s t e n c e o f an

e n e r g y - s a v i n g s t r a t e g y f o r t h e f a c t o r y o p e r a t i o n . Most n o t a b l y , a d a i l y

p r o c e s s i n g c a p a b i l i t y f a l l i n g be low i t s nominal l e v e l u s u a l l y causes t h e e n e r g y

consumpt ion p e r u n i t mass o f b e e t s p r o c e s s e d t o i n c r e a s e . T h e r e a r e r a t h e r few

f a c t o r i e s , h o w e v e r , where t h e managers a r e f u l l y aware o f t h e c o n s e q u e n c e s , i n

te rms o f a d d i t i o n a l f u e l bu rned p e r u n i t mass o f b e e t s p r o c e s s e d , o f a d e c i s i o n

t o reduce t he p r o c e s s i n g c a p a b i l i t y . A n o t h e r i m p o r t a n t f a c t o r i s t he j u i c e

d r a f t , w h i c h a f f e c t s bo th t h e s u g a r l o s s i n e x h a u s t e d c o s s e t t e s and the e n e r g y

demand o f t h e p r o c e s s .

F i g u r e s 3.2 and 3.3 show t h e r e s u l t s o f s t u d i e s o f t h e f u e l consumpt ion under

chang ing o p e r a t i n g c o n d i t i o n s i n two s u g a r f a c t o r i e s w i t h d i f f e r e n t p r o c e s s i n g

c a p a b i l i t i e s . S t a t i s t i c a l d a t a f rom 9 seasons were taken t o c o n s t r u c t t he

d i a g r a m s , and b r a c k e t s deno te seasons c h a r a c t e r i z e d by abnormal c l i m a t i c

c o n d i t i o n s , unusua l b e e t p r o p e r t i e s , e t c .

The a b i l i t y o f t h e therma l s y s t e m ' s p i p i n g t o c r e a t e c o r r e c t f l o w c o n d i t i o n s

f o r a l l t he e n e r g y - c a r r y i n g media i s a n o t h e r i m p o r t a n t p o i n t t o be i n v e s t i g a t e d .

I t i s n o t o n l y t he f l o w c o n d i t i o n s d u r i n g normal f a c t o r y o p e r a t i o n , b u t a l s o

t h o s e i n d u c e d by f a c t o r y s t a r t - u p o r pa ramete r f l u c t u a t i o n s , t h a t may a f f e c t

e n e r g y l o s s e s and equ ipment s a f e t y . The symptoms o f a b n o r m a l i t i e s a re pa ramete r

i n s t a b i l i t i e s , e x c e s s i v e p i p e v i b r a t i o n s , h y d r a u l i c s h o c k s , e t c . G e n e r a l l y ,

t h r e e f a c t o r s a r e o f c r i t i c a l impo r tance t o t he f u n c t i o n i n g o f t h e p i p i n g :

( i ) The d i m e n s i o n s o f t h e p i p e s d e t e r m i n e t he f l o w v e l o c i t i e s under bo th normal

and abnormal o p e r a t i n g c o n d i t i o n s . F low v e l o c i t i e s , i n t u r n , d e t e r m i n e p r e s s u r e

l o s s e s t h a t may e a s i l y become a cause o f d i s t u r b a n c e s i n the rma l sys tem

o p e r a t i o n , such as p r e s s u r e l o s s e s i n t h e v a p o u r p i p e s l i n k i n g t h e e v a p o r a t o r

w i t h t h e vacuum p a n s , o r i n t h e p i p e l i n e s between t h e vacuum pans and t h e

c o n d e n s e r . I t i s recommended t o keep t h e p r e s s u r e l o s s between t h e e v a p o r a t o r

and t he vacuum pans be low 0.1 b a r , w h i c h c o r r e s p o n d s t o a c o n d e n s a t i o n -

t e m p e r a t u r e d rop o f l e s s t han 2 K. ( L e t us o b s e r v e t h a t i n bo th cases m e n t i o n e d ,

t h e p i p e d i m e n s i o n s s h o u l d be based on maximum v a p o u r f l o w s . ) F low v e l o c i t i e s

w h i c h can be recommended f o r d i f f e r e n t p i p e d i m e n s i o n s a r e d i s c u s s e d i n t h e

l i t e r a t u r e ( r e f s . 3 , 4 , 6 ) .

( i i ) The s l o p e s o f h o r i z o n t a l s e c t i o n s o f t he p i p e l i n e s a l l o w g r a v i t a t i o n a l f l o w

99

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4.1

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4.0

3.9

cn ^. 3.8

o Ε Ιο c

3.7

c o α Ε D (Λ C O

o

3.6

3.5

3.4

(71)

3.3

3.2

94 96 98 100 102 104

A v e r a g e to n o m i n a l d a i l y c a p a c i t y { % )

F i g . 3 . 2 . Fue l consumpt ion v s . a v e r a g e d a i l y c a p a c i t y i n two s u g a r f a c t o r i e s w i t h nominal c a p a c i t i e s o f 2100 t / d ( p o i n t s ) and 5800 t / d ( c r o s s e s ) .

o f t he condensa te t o take p l a c e . The recommended s l o p e o f c o n d e n s a t e l i n e s i s a t

l e a s t 2%. The h o r i z o n t a l s e c t i o n s o f steam and v a p o u r l i n e s s h o u l d be i n c l i n e d

t o o , i n o r d e r t o make i t p o s s i b l e t o d r a i n t h e c o n d e n s a t e f o r m i n g t h e r e d u r i n g

t he s t a r t - u p o f t he therma l s y s t e m .

( i i i ) Condensa te d r a i n a g e equ ipment a t t a c h e d t o steam and v a p o u r l i n e s i s

e s s e n t i a l . T h i s i s a p r e r e q u i s i t e f o r a s a f e s t a r t - u p o f t h e the rma l s y s t e m .

An i n s p e c t i o n o f t he p i p i n g aimed a t c h e c k i n g t h e d i m e n s i o n s and s l o p e s , and

t he a v a i l a b i l i t y o f a u x i l i a r y equ ipment ( s e e a l s o S e c t i o n 7 . 7 . 4 ) , can be

combined w i t h a p r e l i m i n a r y i n s p e c t i o n o f t h e the rma l i n s u l a t i o n and can

p r e f e r a b l y be e x t e n d e d t o i n c l u d e an i n s p e c t i o n o f t he i n s u l a t i o n o f t h e main

f a c t o r y equ ipmen t . W h i l e i t i s r e l a t i v e l y e a s y t o g e t a g e n e r a l q u a l i t a t i v e

100

4.3

0 0 ,4 .0

^ § 39

1 ^-^

3.8 [

120 122 124 126 128 130 132 Average juice draft (% )

F i g . 3 . 3 . Fue l consumpt ion v s . a v e r a g e j u i c e d r a f t i n a 2100 t / d f a c t o r y .

p i c t u r e o f t h e s t a t e o f the rma l i n s u l a t i o n , t h e magn i tude o f h e a t d i s s i p a t i o n

l o s s e s remains unknown u n l e s s a s p e c i a l i n v e s t i g a t i o n i s u n d e r t a k e n . A v e r y

e f f e c t i v e t e c h n i q u e w h i c h can be u t i l i z e d f o r t h i s pu rpose i s t h e r m o g r a p h i c

s u r v e y i n g w i t h t he a i d o f i n f r a - r e d - s e n s i t i v e cameras . The u n d e r l y i n g

p r i n c i p l e s , and examples o f t he a p p l i c a t i o n o f t he rmography i n B r i t i s h s u g a r

f a c t o r i e s , a r e p r e s e n t e d i n t he l i t e r a t u r e ( r e f . 2 ) .

A n o t h e r i m p o r t a n t f e a t u r e o f t h e the rma l sys tem i s i t s a b i l i t y t o g u a r a n t e e

s a f e b o i l e r o p e r a t i o n w i t h o u t u n n e c e s s a r y e n e r g y l o s s e s . N o r m a l l y , t h e

condensa tes f rom the t anks i n t h e f i r s t and second e v a p o r a t o r e f f e c t s a r e

s u p p l i e d as f e e d w a t e r t o t he b o i l e r s . I f t he q u a l i t y o f t he c o n d e n s a t e i n a

c e r t a i n tank becomes u n a c c e p t a b l e w i t h r e g a r d t o s a f e b o i l e r o p e r a t i o n , t h e n t h e

e n t i r e amount o f t he h o t condensa te must be removed f rom t h i s p a r t i c u l a r t ank

and f rom the therma l s y s t e m . When r e p l a c i n g i t by make-up w a t e r , abou t 20 kg o f

normal f u e l must be burned i n o r d e r t o hea t 1 m^ w a t e r t o t h e r e q u i r e d

t e m p e r a t u r e .

T h e r e a re s e v e r a l p o s s i b l e causes o f t he d e g r a d a t i o n o f t he q u a l i t y o f

c o n d e n s a t e :

( a ) Too much o f t he gaseous p r o d u c t s o f therma l decay o f s u c r o s e , o r c e r t a i n

nonsuga rs p r e s e n t i n v a p o u r and d i s s o l v e d i n t he c o n d e n s a t e . T h i s phenomenon

endangers the q u a l i t y o f t he condensa te o b t a i n e d f rom f i r s t - e f f e c t v a p o u r .

( b ) J u i c e p e n e t r a t i n g t he h e a t i n g chambers o f t h e e v a p o r a t o r b o d i e s o r h e a t e r s

t h r o u g h l e a k i n g t u b e s . N o r m a l l y , t h e p r e s s u r e d i f f e r e n c e between t he h e a t i n g

chamber and t h e j u i c e space i n an e v a p o r a t o r body wou ld p r e s s t he c o n d e n s a t e

i n t o j u i c e , bu t t he d i r e c t i o n o f l e a k s may be t e m p o r a r i l y r e v e r s e d due t o

p r e s s u r e f l u c t u a t i o n s .

101

( c ) F i r s t - e f f e c t j u i c e c a r r i e d o v e r as foam o r sma l l d r o p l e t s t o t h e h e a t i n g

chamber o f t h e second e f f e c t . T h i s may be caused by e x c e s s i v e f o a m i n g , t o o h i g h

f l o w v e l o c i t y i n t h e v a p o u r chamber o f t h e f i r s t e f f e c t a n d / o r i n e f f i c i e n t

e n t r a i n m e n t s e p a r a t o r s . I t s h o u l d be emphas ized t h a t no s e p a r a t o r can be

c o n s i d e r e d as f u l l y r e l i a b l e a t t o o h i g h v a p o u r - f l o w v e l o c i t i e s ( e x c e e d i n g

1-1.2 m/s i n t h e v a p o u r c h a m b e r ) . I t may t h e r e f o r e happen t h a t a s e p a r a t o r works

p e r f e c t l y w e l l under normal o p e r a t i n g c o n d i t i o n s , b u t j u i c e c a r r y o v e r o c c u r s a t

a b n o r m a l l y low v a p o u r p r e s s u r e s i n d u c e d by pa ramete r f l u c t u a t i o n s i n t h e the rma l

s y s t e m .

The r i s k o f j u i c e c a r r y o v e r i s a l w a y s p r e s e n t i n t h e h e a t i n g chamber o f t h e

second e v a p o r a t o r e f f e c t . I n the rma l sys tems emp loy i ng v a p o u r c o m p r e s s i o n , t h e

f i r s t e f f e c t may a l s o be e n d a n g e r e d . S p e c i a l p r e c a u t i o n s must be t aken t o

m i n i m i z e t h e consequences o f condensa te p o l l u t i o n i n such sys tems ( s e e S e c t i o n

3 . 4 . 4 ) .

A p a r t f rom t h e immediate causes o f t h e p r e s e n c e o f unwanted s u b s t a n c e s i n

t h e c o n d e n s a t e , i t s h o u l d be p o s s i b l e t o d e t e c t t h e danger and t o r e a c t q u i c k l y ,

p r e v e n t i n g b o i l e r damage. To a l a r g e e x t e n t , t h i s depends on t h e i n s t r u m e n t a t i o n

and m o n i t o r i n g p r o c e d u r e s r e l a t i n g t o t h e c o n d e n s a t e c o n t r o l , as w e l l as on t he

p r o c e d u r e s o f r e p l a c i n g t he d i s c a r d e d c o n d e n s a t e by make-up w a t e r .

3 .1 .3 I n t e r p r e t i n g e x t e r n a l b a l a n c e s

By a n a l y s i n g t he e x t e r n a l e n e r g y b a l a n c e and t h e p r o c e s s mass b a l a n c e

t o g e t h e r , i t becomes p o s s i b l e t o d e t e r m i n e c e r t a i n f a c t o r s c a u s i n g e x c e s s i v e

e n e r g y consumpt ion i n a f a c t o r y . These f a c t o r s may be r e l a t e d t o any o f t h e f o u r

q u e s t i o n s l i s t e d i n S e c t i o n 3 . 1 . 1 . As an e x a m p l e , l e t us c o n s i d e r t he e x t e r n a l

e n e r g y b a l a n c e shown i n C h a p t e r 2 , T a b l e 2 . 4 , t o g e t h e r w i t h t he d e s i g n d a t a on

t he p r o c e s s mass b a l a n c e summarized i n T a b l e 3 .1 .

The consumpt ion o f h e a t i n g steam i s c e r t a i n l y v e r y l a r g e ; when c o n v e r t e d t o

normal s team, i t amounts t o 46.7 kg/100 kg b. The main r e a s o n s can be summar ized

as f o l l o w s :

( i ) The t h i c k - j u i c e c o n c e n t r a t i o n o f 65% DS i s r e l a t i v e l y l o w ; t h i s i s c e r t a i n l y

one o f t he p r o c e s s c o n s t r a i n t s w h i c h c o u l d be m o d i f i e d t o d e c r e a s e t h e t o t a l

h e a t demand. Even a t t h i s c o n c e n t r a t i o n , h o w e v e r , t he mass f l o w o f vacuum-pan

v a p o u r s ( T a b l e 2 . 4 , e n t r y 16) i s t o o l a r g e , e x c e e d i n g t he d e s i g n v a l u e ( T a b l e

3 .1 , e n t r y 35) by n e a r l y 13%. T h i s may be caused by t o o l a r g e a w a t e r i n t a k e t o

t he vacuum pans a n d , p r o b a b l y , t o o much wash w a t e r s u p p l i e d t o t h e c e n t r i f u g a l s .

E x c e s s i v e w a t e r i n t a k e s t o t he s u g a r house can a l s o be r e g a r d e d as a p r o c e s s

c o n s t r a i n t t o be m o d i f i e d .

( i i ) The e l e v a t e d t e m p e r a t u r e i n t h e p r e - l i m i n g tank i s m a i n t a i n e d by r e c y c l i n g

a l a r g e f l o w o f h o t j u i c e a f t e r f i r s t c a r b o n a t a t i o n ( T a b l e 3 .1 , e n t r y 9 ) . T h i s

i s a p r o c e s s c o n s t r a i n t p r e v e n t i n g t h e raw j u i c e f rom b e i n g hea ted by l o w -

102

TABLE 3.1

Summary o f t h e d e s i g n da ta on p r o c e s s mass b a l a n c e o f t he f a c t o r y d e s c r i b e d by t h e e x t e r n a l e n e r g y b a l a n c e shown i n T a b l e 2 . 4 .

No. St ream name T o t a l f l o w

(kg /100 kg b) C o n c e n t r a t i o n

(% DS)

1 C o s s e t t e s , p o l . 17.5% 100.0 2 Wet p u l p 90.0 3 P r e s s e d p u l p 34.3 14.35 4 P r e s s w a t e r 55.7 5 Feed w a t e r ( c o n d e n s a t e ) 49.3 6 Raw j u i c e , p u r i t y 88% 115.0 7 J u i c e t o main l i m i n g 176.4 8 J u i c e t o c a r b o n a t a t i o n I 188.3 9 J u i c e f rom c a r b o n a t a t i o n I r e c y c l e d t o

p r e - 1 i m i n g 40.0 10 J u i c e t o d e c a n t e r 148.0 11 S u b s i d e r s l u d g e I t o vacuum f i l t e r s 21.3 12 ^ S u b s i d e r s l u d g e I r e c y c l e d t o p r e - l i m i n g 15.0 13 J u i c e t o s a f e t y f i l t e r s I 130.2 14 J u i c e t o c a r b o n a t a t i o n I I 129.8 15 J u i c e t o t h i c k e n e r s I I 129.0 16 S u b s i d e r s l u d g e I I r e c y c l e d t o p r e - l i m i n g 5.0 17 T h i n j u i c e t o b u f f e r tank 124.0 18 Water ( c o n d e n s a t e ) added f o r c o n t r o l

p u r p o s e s 5.0 19 T h i n j u i c e t o e v a p o r a t o r 129.0 14.3 20 M i l k - o f - l i m e t o p r e - l i m i n g 1.4 21 M i l k - o f - l i m e t o main l i m i n g 9.9 22 M i l k - o f - l i m e t o c a r b o n a t a t i o n I I 0.6 23 T h i c k j u i c e 24.8 65.0 24 Water ( c o n d e n s a t e ) t o r e m e l t Β 3.8 25 Wash w a t e r t o c e n t r i f u g a l s 1.2 26 Condensa te f rom steam wash 0.2 27 Water ( c o n d e n s a t e ) added t o m a s s e c u i t e C 0.3 28 Condensa te f rom vacuum-pan s teaming 0.2 29 Water ( c o n d e n s a t e ) i n t a k e t o vacuum pans 2.5 30 Condensa te f rom d i r e c t h e a t i n g o f s y r u p s 1.1 31 Sugar A t o d r y i n g 14.2 99.1 32 M o l a s s e s 5.0 33 Vapou rs f rom s e l f - e v a p o r a t i o n o f s y r u p s 0.5 34 Water e v a p o r a t e d i n vacuum pans 18.0

t e m p e r a t u r e v a p o u r s f rom t h e l a s t e v a p o r a t o r e f f e c t o r f rom t h e vacuum p a n s . I f

t h i s c o n s t r a i n t can be e l i m i n a t e d , t hen improved u t i l i z a t i o n o f l o w - t e m p e r a t u r e

v a p o u r s becomes p o s s i b l e .

( i i i ) The v a p o u r f l o w f rom t h e l a s t e v a p o r a t o r e f f e c t t o t he c o n d e n s e r ( T a b l e

2 . 4 , e n t r y 14) i s v e r y l a r g e . I t i s ha rd t o b e l i e v e t h a t t h i s can be c o m p a t i b l e

w i t h t h e i n t e n t i o n o f t he d e s i g n e r o f t h e the rma l sys tem o r w i t h t h e w i s h e s o f

t h e o p e r a t i n g p e r s o n n e l . I f an i n s u f f i c i e n t u t i l i z a t i o n o f t h e l a s t - e f f e c t

v a p o u r f o r h e a t i n g p u r p o s e s had i n d e e d been p l a n n e d , i t wou ld have t o be

i n t e r p r e t e d as a thermal sys tem d e f i c i e n c y r e q u i r i n g immediate a c t i o n . More

l i k e l y a r e t he f o l l o w i n g r e a s o n s :

103

- v a p o u r l e a k s t h r o u g h condensa te d r a i n a g e l i n e s i n t he e v a p o r a t o r a r e a , and

pe rhaps t o o much v a p o u r i s w i t h d r a w n a l o n g w i t h t h e n o n c o n d e n s a b l e gases f rom

t h e p r e c e d i n g e v a p o r a t o r e f f e c t , t o g e t h e r c a u s i n g an u n c o n t r o l l a b l e v a p o u r

i n f l o w t o t h e l a s t e f f e c t ,

- f l u c t u a t i o n s i n t h e t h i n - j u i c e f l o w , i n c o m b i n a t i o n w i t h t o o sma l l a volume o f

t he t h i n - j u i c e tank b e f o r e t he e v a p o r a t o r , t h u s f o r c i n g condensa te i n t a k e s t o

t he j u i c e as t h e j u i c e l e v e l i n t h e t ank f a l l s t o o low o r t h e c o n c e n t r a t i o n o f

t h i c k j u i c e becomes t o o h i g h .

The f a c t o r s named above must be seen as i n d i c a t i o n s t h a t t h e therma l sys tem i s

n o t w o r k i n g as o r i g i n a l l y p l a n n e d because o f d e f i c i e n c i e s o f t h e a u x i l i a r y

equ ipmen t .

( i v ) Poor c o n d i t i o n o f t he the rma l i n s u l a t i o n , d i r e c t hea t d i s s i p a t i o n f rom open

t anks c o n t a i n i n g h i g h - t e m p e r a t u r e med ia , and u n c o n t r o l l a b l e l e a k s o f v a p o u r o r

condensa te cause r e l a t i v e l y l a r g e h e a t l o s s e s t o t h e e n v i r o n m e n t ( T a b l e 2 . 4 ,

e n t r y 2 3 ) . A f a c t - f i n d i n g v i s i t t o t he f a c t o r y i n q u e s t i o n w o u l d p r o b a b l y l e a d

t o t he c o n c l u s i o n t h a t bo th t he the rma l sys tem and t h e p r o c e s s equ ipment s h o u l d

be improved i n t h a t r e s p e c t .

3 .1 .4 I n t e r p r e t i n g e v a p o r a t o r - r e c e i v e r b a l a n c e s

I t i s p e r f e c t l y normal t h a t t h e r e a l steam consumpt ion o f an e v a p o r a t o r i s

1-2 kg/100 kg b g r e a t e r than t h e steam demand d e t e r m i n e d f rom t h e m a s s - b a l a n c e

and e n e r g y - b a l a n c e c a l c u l a t i o n s . T h i s i s a r e s u l t o f f l u c t u a t i o n s o f t h e

o p e r a t i n g pa rame te rs t h a t a r e d i s r e g a r d e d i n t h e b a l a n c e e q u a t i o n s based on t h e

s t e a d y - s t a t e a p p r o a c h . A d i f f e r e n c e l a r g e r t han t h a t named a b o v e , h o w e v e r , may

i n d i c a t e t h a t t h e the rma l sys tem does n o t work as d e s i g n e d and p l a n n e d . I t i s

i m p o s s i b l e t o s t u d y t he r e a s o n s f o r t h i s s i t u a t i o n u s i n g t he e x t e r n a l - b a l a n c e

approach o n l y . I n o r d e r t o o b t a i n more i n f o r m a t i o n on t h e e n e r g y p r o c e s s e s , t h e

mass and e n e r g y b a l a n c e s o f t h e e v a p o r a t o r and v a p o u r r e c e i v e r s s h o u l d be

d e t e r m i n e d and a n a l y s e d .

L e t us c o n s i d e r an e x t e n s i o n o f t he example p r e s e n t e d i n t h e p r e c e d i n g

S e c t i o n . I n F i g . 3 . 4 , t h e v a p o u r and c o n d e n s a t e d i s t r i b u t i o n scheme o f t h e same

f a c t o r y i s shown t o g e t h e r w i t h t h e r e s u l t s o f mass- and h e a t - b a l a n c e

c a l c u l a t i o n s pe r f o rmed u s i n g t h e e v a p o r a t o r - r e c e i v e r a p p r o a c h .

As i t t u r n s o u t , t he c a l c u l a t e d steam demand i s 2.0 kg/100 kg b l e s s t han t h e

consumpt ion i n d i c a t e d i n t h e e x t e r n a l b a l a n c e , w h i l e t h e c a l c u l a t e d mass f l o w o f

l a s t - e f f e c t v a p o u r d i r e c t e d t o t h e c o n d e n s e r i s 3.9 kg/100 kg b l e s s t h a n t h e

e x t e r n a l - b a l a n c e v a l u e . T h i s may be an i n d i c a t i o n o f steam and v a p o u r l e a k s

t h r o u g h t h e steam t r a p s i n t h e c o n d e n s a t e d r a i n a g e l i n e s a n d / o r t h r o u g h t h e

v e n t i n g l i n e s , o r t h r o u g h c e r t a i n v a l v e s w h i c h may be c l o s e d b u t a r e n o t f u l l y

t i g h t . ( A d d i t i o n a l i n d i c a t i o n s o f l e a k s o f steam o r h e a t i n g v a p o u r can be

104

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86.7 to COndensate receivers

condensate returns

F i g . 3 . 4 . E x c e r p t s f rom mass and h e a t b a l a n c e s o f t he the rma l sys tem p r e v i o u s l y c o n s i d e r e d i n S e c t i o n 3 .1 .3 ( f l o w s g i v e n i n kg/100 kg b ) .

o b t a i n e d by c h e c k i n g t e m p e r a t u r e s and p r e s s u r e s i n t h e e v a p o r a t o r e f f e c t s .

U s u a l l y , v a p o u r l e a k i n g t o a s p e c i f i c e f f e c t i n d u c e s a t e n d e n c y t o w a r d s

t e m p e r a t u r e and p r e s s u r e i n c r e a s e s w h i c h a r e accompanied by r e d u c e d e v a p o r a t i o n .

I f t h e l e a k s a r e s i g n i f i c a n t , t hen i t may be d i f f i c u l t t o m a i n t a i n a h i g h t h i c k -

j u i c e c o n c e n t r a t i o n a t t h e e v a p o r a t o r o u t l e t . )

F i g u r e 3.5 shows t h e r e s u l t s o f mass- and h e a t - b a l a n c e c a l c u l a t i o n s p e r f o r m e d

f o r t h e same therma l sys tem under t h e f o l l o w i n g assump t i ons c o n c e r n i n g t h e steam

and v a p o u r l e a k s :

( i ) E x h a u s t steam l e a k i n g , a t t h e r a t e o f 1 kg/100 kg b , t h r o u g h a steam t r a p i n

t h e f i r s t e f f e c t and a c o n d e n s a t e f l a s h p i p e t o second e f f e c t v a p o u r .

( i i ) E x h a u s t steam l e a k i n g , a t t h e r a t e o f 1 kg/100 kg b , t h r o u g h t h e same steam

t r a p as above and a n o t h e r steam t r a p a t t h e o u t l e t o f t h e c o n d e n s a t e t ank i n t h e

f i r s t e f f e c t , t o t h e condensa te tank i n t h e f o u r t h e f f e c t .

( i i i ) F i r s t - e f f e c t v a p o u r l e a k i n g , a t t h e r a t e o f 1 kg/100 kg b , t h r o u g h a steam

t r a p i n t he second e f f e c t t o s e c o n d - e f f e c t v a p o u r .

( i v ) S e c o n d - e f f e c t v a p o u r l e a k i n g , a t t h e r a t e o f 5 kg/100 kg b , t h r o u g h a

f a u l t y condensa te d r a i n a g e subsys tem i n t h e vacuum-pan s t a t i o n , t o t h i r d - e f f e c t

v a p o u r .

As can be s e e n , t h e c a l c u l a t e d steam demand i s now 52.9 kg/100 kg b and t h e mass

f l o w o f t h e l a s t - e f f e c t v a p o u r t o t h e c o n d e n s e r i s 7.4 kg/100 kg b ; b o t h v a l u e s

105

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I I 8 5 . 3 to condensate receivers ~ ^

F i g . 3 . 5 . E x c e r p t s f rom mass and h e a t b a l a n c e s o f t h e the rma l sys tem p r e v i o u s l y c o n s i d e r e d i n S e c t i o n 3 . 1 . 3 , w i t h steam and v a p o u r l e a k s taken i n t o a c c o u n t .

a r e p r e t t y c l o s e t o t he r e a l f i g u r e s .

Steam and v a p o u r l e a k s o c c u r r i n g i n t h e c o n d e n s a t e d r a i n a g e subsys tem

c o n s t i t u t e j u s t one p o s s i b l e t y p e o f d e v i a t i o n f rom t h e c o r r e c t o p e r a t i o n o f t h e

therma l s y s t e m . A n o t h e r t y p e o f d e v i a t i o n i s a s s o c i a t e d w i t h l e a k i n g e v a p o r a t o r

t u b e s . As t h e condensa te i s p r e s s e d i n t o j u i c e , t h e r e q u i r e d t h i c k - j u i c e

c o n c e n t r a t i o n may be i m p o s s i b l e t o m a i n t a i n , b u t t h e r e a s o n w i l l be d i f f i c u l t t o

i d e n t i f y . Depend ing on t h e methods o f b a l a n c e c a l c u l a t i o n s a p p l i e d , t h e mass and

e n e r g y b a l a n c e s d e r i v e d f rom t h e v a l u e s o f t h e pa rame te rs measured may i n d i c a t e ,

f o r examp le , t h a t t h e r e i s a c e r t a i n v a p o u r f l o w e n t e r i n g t h e e v a p o r a t o r (when

u s i n g t h e e v a p o r a t o r - b a l a n c e a l g o r i t h m p r e s e n t e d i n C h a p t e r 2 , a r e v e r s e d v a p o u r

f l o w f rom t h e c o n d e n s e r t o t h e l a s t e v a p o r a t o r e f f e c t may be o b t a i n e d ) .

The i n v e s t i g a t i o n s o f e v a p o r a t o r - r e c e i v e r b a l a n c e s can c o n v e n i e n t l y be

e x t e n d e d by d e t e r m i n i n g t h e o v e r a l l hea t t r a n s f e r c o e f f i c i e n t s i n t h e e v a p o r a t o r

b o d i e s , j u i c e h e a t e r s , vacuum pans and o t h e r e q u i p m e n t . ( T h e methods o f

m o n i t o r i n g t h e n e c e s s a r y d a t a and c a l c u l a t i n g t h e c o e f f i c i e n t s a r e d i s c u s s e d i n

C h a p t e r 7 . ) A b n o r m a l l y low h e a t t r a n s f e r i n t e n s i t i e s may be caused by t h e

f o l l o w i n g f a c t o r s e n c o u n t e r e d i n a l l t y p e s o f e v a p o r a t o r s and h e a t e x c h a n g e r s :

- s c a l e b u i l d - u p a f f e c t i n g h e a t i n g s u r f a c e s ;

- t o o h i g h a condensa te l e v e l i n t h e h e a t i n g chamber , due t o i n e f f i c i e n t

condensa te d r a i n a g e ;

106

- d e c r e a s e d v a p o u r c o n d e n s a t i o n t e m p e r a t u r e , due t o t h e p r e s e n c e o f

noncondensab le g a s e s ;

- d e c r e a s e d v a p o u r c o n d e n s a t i o n t e m p e r a t u r e due t o t h e t h r o t t l i n g o f t h e v a p o u r

f l o w .

I n R o b e r t - t y p e e v a p o r a t o r s , t h e o v e r a l l hea t t r a n s f e r c o e f f i c i e n t s a r e

r e d u c e d when t o o h i g h j u i c e l e v e l s a re m a i n t a i n e d . Too low a h e a t t r a n s f e r

c o e f f i c i e n t i n t he f i r s t e v a p o r a t o r e f f e c t may a l s o i n d i c a t e t h a t t h e j u i c e

t e m p e r a t u r e a t t he e v a p o r a t o r i n l e t i s t o o l o w .

I n m u l t i p l e - p a s s t u b u l a r h e a t e r s , t h e hea t t r a n s f e r i n t e n s i t y may be

d e c r e a s e d when t he j u i c e - s i d e s e a l s between t h e passes a r e l e a k i n g . Leaky s e a l s

may cause t h e j u i c e f l o w i n c e r t a i n passes t o d e c r e a s e , w h i c h i s e q u i v a l e n t t o

a r e d u c t i o n o f t he e f f e c t i v e h e a t i n g s u r f a c e a r e a .

3.2 FUNDAMENTALS OF CORRECT OPERATION OF A THERMAL SYSTEM

3.2.1 Condensa te d r a i n a g e

One o f t he e s s e n t i a l r e q u i r e m e n t s o f p r o p e r steam o r v a p o u r h e a t i n g i s a

r e l i a b l e c o n d e n s a t e d r a i n a g e . W h i l e no condensa te s h o u l d accumu la te i n t h e

h e a t i n g chambers o f t he e q u i p m e n t , as t h i s wou ld r educe t h e e f f e c t i v e o v e r a l l

h e a t t r a n s f e r c o e f f i c i e n t , n e i t h e r s h o u l d v a p o u r o r steam be a l l o w e d t o f l o w

t h r o u g h condensa te l i n e s , as t h i s w o u l d be e q u i v a l e n t t o s h o r t - c i r c u i t i n g t h e

therma l s y s t e m . These c o n d i t i o n s can be s a t i s f i e d p r o v i d i n g t h e e n t i r e d r a i n a g e

subsys tem i s p r o p e r l y d e s i g n e d and m a i n t a i n e d . I m p o r t a n t p o i n t s a r e t h e

d imens ions o f t he components , t he l o c a t i o n s o f t h e c o n d e n s a t e - o u t l e t n o z z l e s ,

t h e p o s i t i o n i n g o f t h e condensa te p i p e s and t h e i r v e n t i n g , t h e t y p e , d i m e n s i o n s

and p o s i t i o n i n g o f t h e steam t r a p s and c o n n e c t i o n o f t h e c o n d e n s a t e p i p e s t o

c o r r e c t l y s e l e c t e d r e c e i v e r s . I n t h e e x i s t i n g l i t e r a t u r e , t h e r e a r e r e l a t i v e l y

few s o u r c e s i n w h i c h p r o p e r a t t e n t i o n has been p a i d t o t h e s e p rob lems ( r e f s .

3 , 4 ) .

The d e t a i l s o f a condensa te d r a i n may v a r y , depend ing on t h e p r e s s u r e l e v e l

i n t h e a s s o c i a t e d h e a t i n g chamber. A t h i g h e r p r e s s u r e s c o r r e s p o n d i n g t o e x h a u s t

steam and f i r s t - o r s e c o n d - e f f e c t v a p o u r , t h e c o n d e n s a t e u s u a l l y f l o w s o u t by

g r a v i t y and t h e escape o f v a p o u r i s p r e v e n t e d by a steam t r a p . F u r t h e r d e t a i l s

may depend on t h e t y p e o f steam t r a p s e l e c t e d . Fou r t y p e s a r e most w i d e l y

a p p l i e d i n b e e t - s u g a r f a c t o r i e s :

- f l o a t t y p e ;

- n o z z l e t y p e ;

- N i e s s n e r co lumns ;

- l e v e l c o n t r o l c i r c u i t s a c t i n g as steam t r a p s .

O t h e r d e s i g n s a r e a l s o known ( r e f s . 3 , 4 ) and new deve lopmen ts have r e c e n t l y been

r e p o r t e d ( r e f . 5 ) .

107

A f l o a t - t y p e steam t r a p as shown i n F i g . 3 . 6 ( a ) i s a v e r y p o p u l a r d e v i c e .

A f l o a t opens o r c l o s e s t h e d i s c h a r g e v a l v e , depend ing on t h e c o n d e n s a t e l e v e l

i n t he t r a p . P r o v i d i n g i t i s w e l l m a i n t a i n e d and i t s s i z e has been c o r r e c t l y

chosen w i t h r e g a r d t o t h e a c t u a l o p e r a t i n g c o n d i t i o n s , t h i s d e v i c e e n s u r e s

a r e l i a b l e condensa te d r a i n a g e even a t l a r g e f l o w c h a n g e s . I t s h o u l d be p o i n t e d

o u t , h o w e v e r , t h a t t h e t r a p t h r o u g h p u t depends n o t o n l y on i t s s i z e , b u t a l s o

on t he p r e s s u r e d i f f e r e n c e between t he i n l e t and o u t l e t n o z z l e s .

M a l f u n c t i o n s o f f l o a t - t y p e steam t r a p s a r i s e m o s t l y because o f wear i n t h e

moving p a r t s . A damaged v a l v e s e a t o r v a l v e head may cause v a p o u r l e a k s , and

a damaged l e v e r sys tem may r e s u l t i n v a l v e l o c k - o u t , c a u s i n g i n s u f f i c i e n t

condensa te d r a i n a g e o r steam l e a k s . I t s h o u l d a l s o be remembered t h a t b e f o r e

normal s t e a m - t r a p o p e r a t i o n i s a t t a i n e d d u r i n g a f a c t o r y s t a r t - u p , v e n t i n g o f

t he t r a p chamber i s n e c e s s a r y . C o n s e q u e n t l y , t h e a p p l i c a t i o n s o f f l o a t - t y p e

steam t r a p s a r e a s s o c i a t e d w i t h a r e q u i r e m e n t f o r a c a r e f u l m a i n t e n a n c e . I t i s

t h e r e f o r e n e c e s s a r y t o i n s t a l l t h e d e v i c e s i n e a s i l y a c c e s s i b l e p l a c e s and t o

e q u i p t he d r a i n a g e l i n e s w i t h s h u t - o f f v a l v e s and b y - p a s s s e c t i o n s . I f t h e s e

c o n d i t i o n s a r e n o t s a t i s f i e d , t hen l e a k y steam t r a p s may e a s i l y become a cause

o f r e d u c e d e v a p o r a t o r t h r o u g h p u t o r i n c r e a s e d steam c o n s u m p t i o n .

( a ) ( b )

3J 2 \

F i g . 3 . 6 . Steam t r a p s : ( a ) f l o a t t y p e , ( b ) n o z z l e t y p e . 1 - i n l e t , 2 - o u t l e t , 3 - v e n t , 4 - v a l v e , 5 - f l o a t , 6 - d i a p h r a g m s , 7 - t h r o u g h p u t a d j u s t m e n t s p i n d l e .

A n o t h e r t y p e o f steam t r a p i s t h e n o z z l e d e s i g n shown i n F i g . 3 . 6 ( b ) . I t s

w o r k i n g p r i n c i p l e c o n s i s t s o f t h r o t t l i n g t h e v a p o u r f l o w w h i l e a l l o w i n g f o r a

r e l a t i v e l y f r e e c o n d e n s a t e f l o w . The t h r o t t l i n g e f f e c t i s o b t a i n e d i n a n o z z l e

e q u i p p e d w i t h a sys tem o f d iaphragms w i t h h o l e s o f a r e a s a d j u s t e d t o t h e

o p e r a t i n g c o n d i t i o n s . T h i s d e v i c e i s sma l l and e a s y t o i n s t a l l , and as t h e r e a r e

no c o n s t a n t l y moving p a r t s , a h i g h mechan ica l r e l i a b i l i t y i s e n s u r e d . H o w e v e r ,

t he e f f i c i e n c y o f t he steam t r a p may v a r y w i t h v a r i a b l e o p e r a t i n g c o n d i t i o n s .

108

and f o r condensa te f l o w s w e l l be low t h e i r nominal v a l u e , v a p o u r l e a k s a r e

u n a v o i d a b l e .

I n c e r t a i n d e s i g n s , i t i s p o s s i b l e t o a d j u s t t h e p o s i t i o n o f t he d iaphragms

f o r changed t h r o u g h p u t . T h r o t t l i n g t h e f l o w t o o much, h o w e v e r , may cause t h e

condensa te l e v e l i n t he h e a t i n g chamber t o r i s e e x c e s s i v e l y . N o z z l e - t y p e steam

t r a p s a r e t h e r e f o r e p r e f e r r e d where condensa te d r a i n a g e f rom equ ipment hea ted

w i t h an a lmos t c o n s t a n t steam f l o w , l i k e e v a p o r a t o r s , i s r e q u i r e d .

Vapour l e a k s caused by r a p i d f l o w changes o f s h o r t d u r a t i o n can be e l i m i n a t e d

i f t he n o z z l e - t y p e t r a p i s p r e c e d e d by a w a t e r s e a l i n a U - t u b e , F i g . 3 . 7 ( a ) .

A t r educed f l o w , t he condensa te l e v e l i n t h e i n l e t l e g o f t h e U - t u b e i s l o w e r e d ,

r e d u c i n g t h e p r e s s u r e d i f f e r e n c e a c r o s s t h e steam t r a p and t h u s r e d u c i n g i t s

t h r o u g h p u t . T h i s s o l u t i o n can be recommended f o r t h e c o n n e c t i o n s between t h e

condensa te t anks c o l l e c t i n g t h e c o n d e n s a t e s f rom t h e i n d i v i d u a l e v a p o r a t o r

e f f e c t s . S i m i l a r a r rangemen ts a r e a l s o n e c e s s a r y when a p p l y i n g n o z z l e - t y p e steam

t r a p s i n t h e condensa te d r a i n s a t t a c h e d t o b a t c h vacuum p a n s , where p e r i o d i c

changes o f t he condensa te f l o w o c c u r . T h i s a p p l i c a t i o n case i s s c h e m a t i c a l l y

shown i n F i g . 3 . 7 ( b ) .

( a )

Ε in

I- l . J

(b )

Ε

i - t h e f f e c t v a p o u r

1 t o ( i * l ) t h e f f e c t

1 ' "

L.J

v a p o u r

F i g . 3 . 7 . Recommended a r rangemen ts o f condensa te d r a i n s f e a t u r i n g n o z z l e - t y p e steam t r a p s : ( a ) between two condensa te t a n k s , ( b ) between a vacuum pan and a condensa te m a n i f o l d . 1 - steam t r a p , 2 - n o n - r e t u r n v a l v e , 3 - vacuum p a n , 4 - condensa te m a n i f o l d .

A N i e s s n e r column i s a s i m p l e and s e l f - r e g u l a t i n g d e v i c e r e q u i r i n g o n l y t h a t

enough space i s a v a i l a b l e f o r i t s i n s t a l l a t i o n . The p l a c i n g o f t h e column

r e l a t i v e t o t he h e a t i n g chamber i s shown i n F i g . 3 . 8 . The e f f e c t i v e h e i g h t Η

s h o u l d be l a r g e enough t o g u a r a n t e e t h a t t h e w a t e r s e a l can work p r o p e r l y even

i f t he p r e s s u r e d i f f e r e n c e between t h e h e a t i n g chamber and t h e c o l u m n ' s v a p o u r

chamber i n c r e a s e s due t o pa ramete r f l u c t u a t i o n s o r t o f o u l e d h e a t i n g s u r f a c e s

i n t h e e v a p o r a t o r s t a t i o n . Component s h o u l d be n o t l e s s t han 3 m t o

compensate f o r t h e p r e s s u r e f l u c t u a t i o n s , and component s h o u l d be sma l l

enough t o p r e v e n t t he condensa te f rom a c c u m u l a t i n g i n t he h e a t i n g chamber a t

109

c o n d e n s a t e

F i g . 3 .8 . Scheme o f a condensa te d r a i n a g e l i n e f e a t u r i n g a N i e s s n e r co lumn. 1 - e v a p o r a t o r , 2 - l e v e l g a u g e , 3 - t h r o t t l i n g v a l v e .

a r e d u c e d p r e s s u r e d i f f e r e n c e a c r o s s t h e co lumn . These r e q u i r e m e n t s can be

s t a t e d i n t h e form o f i n e q u a l i t i e s

where Δρ and Δρ . a r e t h e maximum and minimum p r e s s u r e d i f f e r e n c e s , γ i s max mi η c

t h e d e n s i t y o f c o n d e n s a t e i n t h e c e n t r a l p i p e , and g i s t h e a c c e l e r a t i o n o f

g r a v i t y .

I f t h e h e i g h t i s t o o s m a l l , t h e e f f i c i e n c y o f t h e column can be improved by

i n s t a l l i n g a t h r o t t l i n g v a l v e a t t h e c o n d e n s a t e i n l e t . T h i s causes t h e

condensa te l e v e l i n t h e i n l e t p i p e t o r i s e , t h u s c r e a t i n g an a d d i t i o n a l

s a f e g u a r d a g a i n s t v a p o u r e n t e r i n g t he co lumn .

A l l t h e s h o r t c o m i n g s o f t h e c l a s s i c a l steam t r a p s can be a v o i d e d by a p p l y i n g

a c o n t r o l c i r c u i t c o n s i s t i n g o f a l e v e l t r a n s d u c e r , c o n t r o l l e r and c o n t r o l

v a l v e . I t i s a r r a n g e d t o m a i n t a i n a s t a b l e w a t e r s e a l i n t h e d r a i n a g e l i n e . Such

a d e v i c e i s h i g h l y r e l i a b l e , and e a s y t o o p e r a t e and m a i n t a i n . When used i n

a condensa te d r a i n a g e subsys tem c o n n e c t e d t o a m u l t i p l e - e f f e c t e v a p o r a t o r , i t

can e l i m i n a t e t h e steam o r v a p o u r l e a k s between t h e e v a p o r a t o r e f f e c t s

c o m p l e t e l y . A q u a d r u p l e - e f f e c t e v a p o r a t o r w i t h a c o n d e n s a t e d r a i n a g e subsys tem

emp loy ing t h i s i d e a i s shown s c h e m a t i c a l l y i n F i g . 3 . 9 . The c o n d e n s a t e d r a i n a g e

f rom the f i r s t and second e f f e c t s , and t he c o n d e n s a t e f l o w between t h e

condensa te t a n k s , a r e l e v e l - c o n t r o l l e d .

110

to main condensate tank

condensate re turned from heaters and vacuum pans

F i g . 3 . 9 . Scheme o f a condensa te d r a i n a g e subsys tem f e a t u r i n g l e v e l - c o n t r o l l e d h y d r a u l i c s e a l s and condensa te t a n k s .

I n a condensa te d r a i n c o n n e c t e d t o a h e a t i n g chamber o p e r a t e d a t a low

p r e s s u r e , t he escape o f v a p o u r can be e f f e c t i v e l y p r e v e n t e d w i t h o u t u s i n g a

steam t r a p ; i t i s enough t o c o n n e c t t he d r a i n a g e p i p e t o t h e bot tom p a r t o f a

c l o s e d c o n d e n s a t e tank i n w h i c h a c e r t a i n minimum l e v e l o f t he c o n d e n s a t e i s

a l w a y s m a i n t a i n e d . Sometimes t h e d r a i n a g e p i p e can be formed as a U - t u b e , o r

s i p h o n , w i t h t h e two l i q u i d columns i n t h e l e g s o f t he U - t u b e a c t i n g as a

p r e s s u r e - b a l a n c i n g d e v i c e and a h y d r a u l i c s e a l . The s i p h o n can a l s o be used t o

l e t t h e condensa te f l o w f rom one v e s s e l t o a n o t h e r when a d e f i n i t e p r e s s u r e

d i f f e r e n c e between t he v e s s e l s i s t o be m a i n t a i n e d . I t i s i m p o r t a n t t o choose

t h e h e i g h t o f t h e s i p h o n w i t h a s u f f i c i e n t s a f e t y marg in o f a t l e a s t 50%, making

i t p o s s i b l e t o n e u t r a l i z e t h e p r e s s u r e f l u c t u a t i o n s and t h e condensa te f l a s h i n

t h e l o w - p r e s s u r e l e g o f t h e U - t u b e . I n o r d e r t o a v o i d t h e r i s k o f t h e l i q u i d

column b e i n g d e s t r o y e d by t he f l a s h v a p o u r , t he d i a m e t e r o f t h e l o w - p r e s s u r e l e g

s h o u l d be s u f f i c i e n t l y l a r g e t o l i m i t t h e f l o w v e l o c i t y o f t h e c o n d e n s a t e

(assumed t o be f r e e o f v a p o u r b u b b l e s ) t o abou t 0.6 m/s .

I n t he condensa te d r a i n a g e subsys tem shown i n F i g . 3 . 9 , t h e f l o w o f

c o n d e n s a t e s f rom t h e t h i r d and f o u r t h e v a p o r a t o r e f f e c t s and f rom t h e j u i c e

h e a t e r s s u p p l i e d w i t h s e c o n d - , t h i r d - and f o u r t h - e f f e c t v a p o u r s i s

g r a v i t a t i o n a l . As t he condensa te t a n k s a r e l e v e l - c o n t r o l l e d , t h e d r a i n a g e p i p e s

can be c o n n e c t e d t o t h e condensa te t a n k s w i t h o u t u s i n g s i p h o n s .

S p e c i a l condensa te d r a i n a g e p rob lems a r e a s s o c i a t e d w i t h i n t e r m i t t e n t

o p e r a t i o n o f t he b a t c h vacuum p a n s , t h e l a r g e d i s t a n c e between t h e pans and t h e

in

e v a p o r a t o r be ing a c o n t r i b u t i n g f a c t o r . The r e a s o n i s t h a t t h e p r e s s u r e i n t h e

h e a t i n g chambers o f t h e i n d i v i d u a l pans i s s u b j e c t t o l a r g e f l u c t u a t i o n s . D u r i n g

s teaming and o t h e r a u x i l i a r y phases o f t h e b o i l i n g c y c l e , t h e v a p o u r s u p p l y

v a l v e i s c l o s e d w h i l e t he v e n t i n g v a l v e rema ins o p e n , t h i s c a u s i n g t h e p r e s s u r e

i n t h e h e a t i n g chamber t o f a l l . D u r i n g t h e s y r u p - t h i c k e n i n g p h a s e , when t h e h e a t

demand i s l a r g e s t , a l a r g e v a p o u r f l o w r e s u l t s i n a c o n s i d e r a b l e p r e s s u r e l o s s

i n t he s u p p l y l i n e . As t h e v a p o u r f l o w i s much s m a l l e r d u r i n g t h e c r y s t a l - g r o w t h

p h a s e , t he c o r r e s p o n d i n g p r e s s u r e l o s s i s a l s o s m a l l e r . As a c o n s e q u e n c e , even

i f a l l t h e vacuum pans a r e s u p p l i e d w i t h v a p o u r f rom t h e same e v a p o r a t o r e f f e c t ,

t he p r e s s u r e d i f f e r e n c e between t he h e a t i n g chambers o f two vacuum p a n s , one o f

them i n t h e s y r u p - t h i c k e n i n g and t h e o t h e r i n t h e c r y s t a l - g r o w t h p h a s e , may

e a s i l y a t t a i n v a l u e s abou t 0.5 b a r .

I f a condensa te d r a i n c a n n o t a d e q u a t e l y r e s p o n d t o p r e s s u r e f l u c t u a t i o n s ,

t h e r e may be a t e n d e n c y t o w a r d s c o n d e n s a t e a c c u m u l a t i o n i n t h e h e a t i n g chamber

d u r i n g t h e p e r i o d s o f d e c r e a s e d p r e s s u r e . On t h e o t h e r h a n d , when t h e p r e s s u r e

i s r a i s e d , a v a p o u r l e a k may o c c u r i n t h e c o n d e n s a t e l i n e . D r a i n a g e m a l f u n c t i o n s

a re p o s s i b l e even w i t h c o r r e c t l y w o r k i n g steam t r a p s , when t h e d e v i c e s a r e

i n s t a l l e d t o o h i g h ( r e l a t i v e t o t h e h e a t i n g chamber) o r t h e i r o u t l e t s a r e

c o n n e c t e d t o a m a n i f o l d o f t o o smal l a d i a m e t e r .

L e t us a n a l y s e t h e o p e r a t i o n o f a c o n d e n s a t e d r a i n a g e subsys tem e q u i p p e d w i t h

f l o a t - t y p e steam t r a p s , as shown i n F i g . 3 .10 . The c o n d e n s a t e m a n i f o l d i s

c o n n e c t e d t o t h e c o n d e n s a t e tank i n t h e e v a p o r a t o r e f f e c t n e x t t o t h e one

s u p p l y i n g t h e h e a t i n g v a p o u r . Even though t h e c o n d e n s a t e l e a v i n g t h e h e a t i n g

i-th effect vapour

\ Ζ 7 to( i^1) th effect^ vapour

F i g . 3 .10 . P r i n c i p l e o f c o n d e n s a t e d r a i n a g e f rom vacuum pans u s i n g f l o a t - t y p e steam t r a p s and a c o n d e n s a t e m a n i f o l d c o n n e c t e d t o a t a n k .

chambers o f t h e vacuum pans may be s u b c o o l e d , t h a t i s , i t s t e m p e r a t u r e may be

l o w e r t han t h a t o f t he h e a t i n g v a p o u r , i t b e g i n s t o b o i l as soon as i t has

passed t he t r a p and i s e x p o s e d t o t h e p r e s s u r e c o r r e s p o n d i n g t o t h e n e x t

e v a p o r a t o r e f f e c t . The r e s u l t i n g i n c r e a s e o f a v e r a g e s p e c i f i c vo lume i n d u c e s an

112

i n c r e a s e d f l o w v e l o c i t y and a l a r g e r p r e s s u r e l o s s i n t h e m a n i f o l d , t h i s l e a d i n g

t o a r e d u c e d sys tem t h r o u g h p u t and a t e n d e n c y t o w a r d s c o n d e n s a t e a c c u m u l a t i o n i n

t h e i n d i v i d u a l d r a i n s and h e a t i n g chambers .

The s i t u a t i o n d e s c r i b e d can be p r e v e n t e d by i n c r e a s i n g t h e h e i g h t o f t h e

condensa te l e g i n t h e l i n e c o n n e c t i n g t h e h e a t i n g chamber and t h e steam t r a p ,

t h a t i s , by i n s t a l l i n g t h e steam t r a p a t l e a s t 5-6 m be low t h e c o n d e n s a t e o u t l e t

n o z z l e . I n l a r g e - c a p a c i t y s u g a r f a c t o r i e s where t h e vacuum pans must be l o c a t e d

a t a r e l a t i v e l y l ong d i s t a n c e f rom the e v a p o r a t o r , i t i s a d v i s a b l e t o i n s t a l l

a s e p a r a t e condensa te tank c o l l e c t i n g t he c o n d e n s a t e f rom the vacuum p a n s .

Mutua l i n t e r f e r e n c e between t h e pans can be p r e v e n t e d by e l i m i n a t i n g t h e

m a n i f o l d , t h a t i s , c o n n e c t i n g t h e i n d i v i d u a l d r a i n a g e l i n e s d i r e c t l y t o t h e

condensa te t a n k .

Among o t h e r d r a i n a g e v e r s i o n s , a s o l u t i o n based on t h e a p p l i c a t i o n o f a n o n

r e t u r n v a l v e i n s t e a d o f a steam t r a p i s p a r t i c u l a r l y s e n s i t i v e t o t h e r e l a t i o n

between t he l e v e l d i f f e r e n c e and t h e magn i tude o f p r e s s u r e f l u c t u a t i o n s .

R e l i a b l e o p e r a t i o n can be a c h i e v e d u s i n g t h e c o n f i g u r a t i o n shown i n F i g . 3 .11 ,

where t he l e v e l - c o n t r o l p r i n c i p l e i s i n t r o d u c e d t o e l i m i n a t e t h e v a p o u r l e a k s

accompany ing p r e s s u r e peaks i n t h e h e a t i n g chambers o f t he p a n s .

10-12m

F i g . 3 .11. Scheme o f condensa te d r a i n a g e f rom vacuum pans u s i n g n o n - r e t u r n v a l v e s and a l e v e l - c o n t r o l l e d condensa te t a n k .

3 .2 .2 V e n t i n g o f n o n c o n d e n s a b l e s

The e l e v a t e d j u i c e t e m p e r a t u r e i n t h e e v a p o r a t o r causes the rma l decay o f

am ides , b i c a r b o n a t e s , i n v e r t s u g a r and s u c r o s e . As a r e s u l t , ammonia and c a r b o n

d i o x i d e a r e p r o d u c e d . I n a d d i t i o n , p r e s s u r e d r o p s a s s o c i a t e d w i t h j u i c e f l o w

between t he c o n s e c u t i v e e v a p o r a t o r e f f e c t s c o n t r i b u t e t o t h e l i b e r a t i o n o f a i r

d i s s o l v e d i n t h e j u i c e . The p r e s e n c e o f t h e s e gases (known as n o n c o n d e n s a b l e s )

i n h e a t i n g v a p o u r s s h o u l d be r e g a r d e d as an i m p o r t a n t f a c t o r t o be k e p t under

c o n t r o l .

I t i s d i f f i c u l t t o e v a l u a t e t h e amount o f n o n c o n d e n s a b l e s p r o d u c e d i n t h e

e v a p o r a t o r . The amount o f ammonia was e s t i m a t e d a t 0.015 kg/100 kg b by C l a a s s e n

113

( r e f . 7) and 0.005-0.017 kg/100 kg b by D o b r z y c k i ( r e f . 8 ) . The t o t a l amount o f

noncondensab les was e s t i m a t e d a t 0 .024-0.032 kg/100 kg b by Gorokh ( r e f . 9 ) .

An e s t i m a t e g i v e n by Koren ( r e f . 1 0 ) o f t h e c o n c e n t r a t i o n o f n o n c o n d e n s a b l e s i n

t h i r d - e f f e c t v a p o u r f rom a q u a d r u p l e - e f f e c t e v a p o r a t o r was 2 .0-5 .6%. I t seems

t h a t t he a c t u a l f i g u r e s c h a r a c t e r i z i n g t h e p r o c e s s i n g o f b e e t s o f i n f e r i o r

q u a l i t y ( f o l l o w i n g p r o l o n g e d s t o r a g e o r f r o s t damage) may be even h i g h e r .

A t a c o n s t a n t t o t a l p r e s s u r e o f t h e g a s / v a p o u r m i x t u r e i n t h e h e a t i n g chamber

o f an e v a p o r a t o r b o d y , t h e b u i l d - u p o f n o n c o n d e n s a b l e s causes t he p a r t i a l

p r e s s u r e o f v a p o u r t o d e c r e a s e . As a r e s u l t , t h e c o n d e n s a t i o n t e m p e r a t u r e

d e c r e a s e s and t h e e f f e c t i v e t e m p e r a t u r e d i f f e r e n c e between h e a t i n g v a p o u r and

j u i c e i s r e d u c e d . L e t us o b s e r v e t h a t when a t y p i c a l t e m p e r a t u r e d i f f e r e n c e o f

10 Κ i s r e d u c e d by as l i t t l e as 1 .5-2.0 K, t h e r e s u l t i n g r e d u c t i o n o f t h e amount

o f hea t t r a n s f e r r e d i s 15-20%. More d a t a i l l u s t r a t i n g t h e impac t o f

noncondensab les on t h e t e m p e r a t u r e d i f f e r e n c e a f f e c t i n g t he hea t t r a n s f e r a r e

p r e s e n t e d i n T a b l e 3 . 2 .

I t s h o u l d be p o i n t e d o u t t h a t t h e r e a r e a l s o o t h e r d i s a d v a n t a g e o u s

consequences o f t h e p r e s e n c e o f n o n c o n d e n s a b l e s . W h i l e t he f i l m c o e f f i c i e n t o f

h e a t t r a n s f e r a t a h e a t i n g s u r f a c e where t h e c o n d e n s a t i o n o f pu re steam t a k e s

p l a c e i s o f t h e o r d e r o f 10 000 W / ( m ^ K ) , i t does n o t e x c e e d 100 W/(m^K) a t

a s u r f a c e exposed t o a i r . C o n s e q u e n t l y , when t h e s u r f a c e i s exposed t o a m i x t u r e

o f v a p o u r and n o n c o n d e n s a b l e s , t h e f i l m c o e f f i c i e n t o f h e a t t r a n s f e r may be

s u b s t a n t i a l l y r e d u c e d . A t 0 .5-1.0% n o n c o n d e n s a b l e s i n t h e m i x t u r e , t h e r e d u c t i o n

o f t he c o e f f i c i e n t r e l a t i v e t o i t s p u r e - v a p o u r v a l u e i s abou t 50-60%. G e n e r a l l y ,

t h e f i l m c o e f f i c i e n t o f hea t t r a n s f e r i s i n v e r s e l y p r o p o r t i o n a l t o t h e s q u a r e

r o o t o f t h e mass c o n c e n t r a t i o n o f n o n c o n d e n s a b l e s ( r e f . 1 2 ) .

I n o r d e r t o p r e v e n t an e x c e s s i v e c o n c e n t r a t i o n o f n o n c o n d e n s a b l e s , t h e g a s /

v a p o u r m i x t u r e s h o u l d be c o n t i n u o u s l y v e n t e d . W h i l e i t i s p o p u l a r i n p r a c t i c e t o

d i s c h a r g e t h e m i x t u r e t o t h e c o n d e n s e r o r d i r e c t l y t o t h e a t m o s p h e r e , cascade

v e n t i n g between c o n s e c u t i v e e v a p o r a t o r e f f e c t s has been recommended by some

a u t h o r s . The a d v a n t a g e s o f t h i s method a r e h i g h l y d u b i o u s because o f t h e

a c c u m u l a t i o n o f n o n c o n d e n s a b l e s i n f i n a l e v a p o r a t o r e f f e c t s , t h i s making i t

n e c e s s a r y t o v e n t t o t he c o n d e n s e r anyway . A n o t h e r consequence o f cascade

v e n t i n g i s t h a t a t c o n s t a n t h e a t i n g - v a p o u r demand i n t h e the rma l s y s t e m , w a t e r

e v a p o r a t i o n i n t h e e v a p o r a t o r i s r e d u c e d because t h e v a p o u r v e n t e d r e p l a c e s

a p a r t o f t h e v a p o u r w h i c h w o u l d o t h e r w i s e be g e n e r a t e d i n t he a c t u a l e v a p o r a t o r

e f f e c t i t s e l f .

The most e f f e c t i v e method t o v e n t t h e e v a p o r a t o r b o d i e s i s t o l e t t h e e n t i r e

amount o f v a p o u r f rom the p r e c e d i n g e f f e c t f l o w t h r o u g h t h e h e a t i n g chamber i n

t he n e x t e f f e c t . As a p a r t o f t he n o n c o n d e n s a b l e s becomes d i s s o l v e d i n t h e

c o n d e n s a t e , t h e r e s t i s s u p p l i e d i n t h e h e a t i n g v a p o u r t o t h e j u i c e h e a t e r s .

114

TABLE 3.2

R e d u c t i o n o f t h e e f f e c t i v e t e m p e r a t u r e d i f f e r e n c e due t o t h e p r e s e n c e o f a i r i n t h e c o n d e n s i n g v a p o u r , as a f u n c t i o n o f mass c o n c e n t r a t i o n o f a i r , t o t a l p r e s s u r e and t h e o r e t i c a l t e m p e r a t u r e d i f f e r e n c e A t .

( b a r ) ( K ) 2.5 5 7.5 10

0.3 5 11.5 22.9 34.8 48.1 10 5.8 11.5 17.4 24.0 15 3.8 7.6 11.6 16.0 20 2.9 5.7 8.7 12.0

0.5 5 12.6 25.4 39.0 52.1 10 6.3 12.7 19.5 26.0 15 4 .2 8.5 13.0 17.4 20 3.2 6.4 9.8 13.0

0.75 5 13.3 27.2 41.2 55.0 10 6.7 13.6 20.6 27.5 15 4 .4 9.1 13.7 18.3 20 3.3 6.8 10.3 13.8

1.0 5 14.7 28.6 43.9 58.4 10 7.3 14.3 21.9 29.2 15 4 .9 9.5 14.6 19.5 20 3.7 7.1 11.0 14.7

1.5 5 15.2 30.9 46.6 62.4 10 7.6 15.5 23.3 31.2 15 5.1 10.3 15.5 20.8 20 3.8 7.8 11.7 15.6

2.0 5 15.8 32.2 48.8 66.0 10 7.9 16.1 24.4 33.0 15 5.3 10.7 16.3 22.0 20 4 .0 8.1 12.2 16.5

2.5 5 16.6 33.6 51.0 69.0 10 8.3 16.8 25.5 34.5 15 5.5 11.2 17.0 23.0 20 4 .2 8.2 12.8 17.9

3.0 5 17.0 35.0 52.6 71.2 10 8.5 17.5 26.3 35.6 15 5.7 11.7 17.5 23.7 20 4 .3 8.8 13.5 17.8

3.5 5 17.8 35.6 54.0 73.2 10 8.9 17.8 27.0 36.6 15 5.9 11.9 18.0 24.4 20 4 .5 8.9 13.5 18.3

vacuum pans and o t h e r r e c e i v e r s . An i n c r e a s e d c o n c e n t r a t i o n o f n o n c o n d e n s a b l e s

i n t h e h e a t i n g chambers o f h e a t r e c e i v e r s causes l e s s i n c o n v e n i e n c e t h e r e t han

i n t h e e v a p o r a t o r , because t h e a c t u a l t e m p e r a t u r e d i f f e r e n c e s a r e l a r g e r t han

t h o s e c h a r a c t e r i s t i c o f t h e e v a p o r a t o r b o d i e s .

A v e n t i n g sys tem based on v a p o u r w i t h d r a w a l f rom t h e h e a t i n g chambers o f

115

a q u a d r u p l e - e f f e c t e v a p o r a t o r i s shown s c h e m a t i c a l l y i n F i g . 3.12 ( r e f . 6 ) . The

e n t i r e amount o f n o n c o n d e n s a b l e s l i b e r a t e d i n t h e f i r s t e f f e c t and abou t h a l f o f

t he gases f rom t h e second e f f e c t a r e d i r e c t e d t o t h e h e a t e r b e f o r e t h e second

c a r b o n a t a t i o n . T h i r d - e f f e c t v a p o u r and n o n c o n d e n s a b l e s w i t h d r a w n f rom t h e

h e a t i n g chamber o f t h e f o u r t h e f f e c t a r e s u p p l i e d t o t h e h e a t e r b e f o r e h o t main

l i m i n g . I n o r d e r t o m i n i m i z e v a p o u r l o s s e s when v e n t i n g t h e h e a t i n g chambers o f

t h e h e a t e r s named, a u t o m a t i c c o n t r o l can be a p p l i e d by u t i l i z i n g t h e phenomenon

o f c o n d e n s a t i o n t e m p e r a t u r e changes accompany ing t h e changes o f gas

c o n c e n t r a t i o n . The s i g n a l f rom t h e j u i c e - t e m p e r a t u r e c o n t r o l l e r i s a c t i n g , v i a

c o r r e c t i o n e l e m e n t s , on a c o n t r o l v a l v e i n t h e v a p o u r s u p p l y l i n e and a n o t h e r i n

t h e v e n t i n g l i n e . The c o r r e c t i o n e lemen ts a r e t r a n s f o r m i n g t h e s i g n a l i n such

a way t h a t a t t o o h i g h a j u i c e t e m p e r a t u r e , t h e v e n t i n g v a l v e c l o s e s f i r s t and

t h e v a p o u r v a l v e s e c o n d . A t t o o low a j u i c e t e m p e r a t u r e , t h e v a p o u r v a l v e opens

f i r s t and t h e v e n t i n g v a l v e s e c o n d .

Media heated:

α - thin juice

b - clear juice

c - limed juice

steam iL

Pi to the

condenser

F i g . 3 .12 . Scheme o f a v e n t i n g subsys tem f e a t u r i n g a u t o m a t i c c o n t r o l o f t h e d i s c h a r g e o f n o n c o n d e n s a b l e s f rom t h e h e a t i n g chambers o f j u i c e h e a t e r s .

P r a c t i c a l e x p e r i e n c e w i t h t h e above s o l u t i o n i s s a t i s f a c t o r y . N o r m a l l y , t h e

v e n t i n g v a l v e opens o n l y t e m p o r a r i l y , a t t o o low j u i c e t e m p e r a t u r e s i n d u c e d by

j u i c e - f l o w f l u c t u a t i o n s . When t h e v e n t i n g v a l v e c l o s e s , t h e c o n c e n t r a t i o n o f

n o n c o n d e n s a b l e s i n t h e h e a t i n g chamber i s i n c r e a s e d t o a l e v e l e n a b l i n g t h e

gases t o become d i s s o l v e d i n t h e c o n d e n s a t e and d r a i n e d w i t h o u t v a p o u r l o s s .

Howeve r , t h e c o n d e n s a t e s h o u l d be d i r e c t e d t o t h e ammonia-water t ank r a t h e r t han

t o t he condensa te t a n k , because t h e gases l i b e r a t e d by t h e c o n d e n s a t e f l a s h may

o t h e r w i s e r e - e n t e r t h e e v a p o r a t o r .

A n o t h e r c o n t r o l method f o r t h e v e n t i n g o f n o n c o n d e n s a b l e s f rom t h e h e a t i n g

chambers o f e v a p o r a t o r b o d i e s has been t e s t e d i n t h e P o l i s h s u g a r i n d u s t r y

116

( r e f . 1 3 ) . The b u i l d - u p o f n o n c o n d e n s a b l e s t a k i n g p l a c e i n a h e a t i n g chamber can

be d e t e c t e d by compar ing t he t e m p e r a t u r e s o f t h e h e a t i n g v a p o u r f l o w i n g i n t h e

i n l e t n o z z l e and t h a t c o n d e n s i n g a t t h e h e a t i n g s u r f a c e . The s i g n a l f rom a

s p e c i a l t r a n s m i t t e r measur ing t h e t e m p e r a t u r e d i f f e r e n c e may be s u p p l i e d t o an

a u t o m a t i c c o n t r o l l e r open ing t h e v e n t i n g v a l v e .

3 .2 .3 S c a l e p r e v e n t i o n and removal

A c o n s i d e r a b l e q u a n t i t y o f t h e i m p u r i t i e s p r e s e n t i n t h i n j u i c e becomes l e s s

s o l u b l e as t he c o n c e n t r a t i o n o f t h e j u i c e r i s e s , and some o f t h e s e i m p u r i t i e s

may d e p o s i t on t h e h e a t i n g s u r f a c e s o f t he e v a p o r a t o r b o d i e s , f o r m i n g a h a r d

s c a l e . Be ing a poo r c o n d u c t o r o f h e a t , t h e s c a l e d e c r e a s e s t h e h e a t t r a n s f e r

c o e f f i c i e n t s a c r o s s t h e h e a t i n g s u r f a c e s . C o n s e q u e n t l y , t h e e v a p o r a t o r

t h r o u g h p u t i s r e d u c e d and can be a d j u s t e d t o t h e r e q u i r e d v a l u e o n l y i f t h e

t e m p e r a t u r e d i f f e r e n c e s a re i n c r e a s e d , w i t h i n c r e a s e d steam consumpt ion as a

r e s u l t .

The d i s a d v a n t a g e o u s e f f e c t s o f s c a l e f o r m a t i o n depend on t h e the rma l

c o n d u c t i v i t y o f d e p o s i t s and t he s c a l e t h i c k n e s s . The g o v e r n i n g e q u a t i o n i s

k = l / ( l / a ^ + ό / λ + ό ^ / λ ^ + Ι / α ^ ) ( 3 . 4 )

where k i s t h e o v e r a l l h e a t t r a n s f e r c o e f f i c i e n t , a-j and a r e t h e f i l m

c o e f f i c i e n t s o f h e a t t r a n s f e r on t h e j u i c e s i d e and t he v a p o u r s i d e ,

r e s p e c t i v e l y , δ and a re t he t h i c k n e s s e s o f t h e tube w a l l and t h e d e p o s i t

l a y e r , r e s p e c t i v e l y , and λ and a r e t h e the rma l c o n d u c t i v i t i e s o f t h e t u b e

m a t e r i a l and d e p o s i t s , r e s p e c t i v e l y .

The therma l c o n d u c t i v i t y o f d e p o s i t s may v a r y i n t h e range 0 .08-2 .00 W / ( m K ) ,

depend ing on t h e chemica l c o m p o s i t i o n and s t r u c t u r e o f t h e d e p o s i t e d s u b s t a n c e .

T h e r e f o r e , a v e r y t h i n s c a l e may be enough t o a f f e c t t h e h e a t t r a n s f e r

s e r i o u s l y . The l a r g e r t he i n i t i a l o v e r a l l h e a t t r a n s f e r c o e f f i c i e n t

c o r r e s p o n d i n g t o a c l e a n h e a t i n g s u r f a c e , t he more p ronounced i s t h i s e f f e c t , as

shown i n F i g . 3 .13 .

The k i n d o f s c a l e d e p o s i t e d by t he j u i c e d u r i n g e v a p o r a t i o n o b v i o u s l y depends

on many f a c t o r s , and p r i n c i p a l among them i s t h e c o m p o s i t i o n o f m i n e r a l s a l t s i n

t h e raw j u i c e . The j u i c e p u r i f i c a t i o n method a l s o has a marked e f f e c t , as w e l l

as t h e a p p l i c a t i o n o f j u i c e d e c a l c i f i c a t i o n t e c h n i q u e s .

I t i s p o s s i b l e t o p r e v e n t t h e f o r m a t i o n o f s c a l e by u s i n g v a r i o u s a d d i t i o n

p r o d u c t s , a l t h o u g h no p a r t i c u l a r p r o d u c t i s u n i v e r s a l l y e f f i c i e n t i f used w i t h

d i f f e r e n t raw j u i c e p r o p e r t i e s and d i f f e r e n t p u r i f i c a t i o n methods . I n t h e D a n i s h

s u g a r i n d u s t r y , where t he c o n t e n t o f c a l c i u m s a l t s i n b e e t s i s u s u a l l y v e r y l o w ,

s c a l e p r e v e n t i o n i s s i m p l y based on soda a d d i t i o n t o t h e second c a r b o n a t a t i o n .

A number o f m a n u f a c t u r e r s a r e o f f e r i n g p r e p a r a t i o n s based on o r g a n o p h o s p h a t e s

o r p o l y e l e c t r o l y t e s as s c a l e - p r e v e n t i n g a g e n t s , c l a i m i n g a r e d u c t i o n o f s c a l i n g

117

SI

o

1.0

0 .8 φ

0 . 6

ο υ

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C 0.2

Initial heat transfer coefficient:

1 0 0 0 W/(m*K)

2 0 0 0 W/(m^K) 3 0 0 0 W/(m^K)

0 0.2 0 Λ 0.6 O B 1.0

Scale thickness (mm)

F i g . 3 .13 . O v e r a l l h e a t t r a n s f e r c o e f f i c i e n t as a f u n c t i o n o f s c a l e t h i c k n e s s a t s c a l e c o n d u c t i v i t y 0.97 W/ (mK) .

i n t h e e v a p o r a t o r o f up t o 90%. A d i s c u s s i o n o f t h e p r o p e r t i e s o f t h e s e a g e n t s ,

as w e l l as methods o f a p p l i c a t i o n and e x p e r i e n c e s a r i s i n g i n s u g a r f a c t o r i e s ,

can be f o u n d i n t h e l i t e r a t u r e ( r e f s . 1 4 , 1 5 ) .

A n o t h e r s c a l e - p r e v e n t i o n t e c h n i q u e i s based on t h e a c t i v a t i o n o f m o l e c u l e s o f

m i n e r a l s a l t s d i s s o l v e d i n j u i c e i n an e l e c t r i c f i e l d o r i n a m a g n e t i c f i e l d .

The a c t i v a t e d m o l e c u l e s t e n d t o remain i n s u s p e n s i o n r a t h e r t han d e p o s i t i n g on

t he h e a t - e x c h a n g e s u r f a c e s , and some r e p o r t s i n d i c a t e t h a t t h e s c a l e becomes

more e a s i l y removed , sometimes b e i n g changed i n t o m i c r o c r y s t a l 1 i n e mud. The

a c t i v a t i n g d e v i c e c o n s i s t s o f a t u b e , u s u a l l y p l a c e d i n t h e p i p e p r e c e d i n g t h i n -

j u i c e h e a t e r s , and f i e l d - g e n e r a t i n g c i r c u i t r y . Thousands o f u n i t s o f t h i s k i n d

have been s e t i n o p e r a t i o n i n v a r i o u s i n d u s t r i e s d u r i n g t h e l a s t f o u r d e c a d e s .

As r e g a r d s t h e s u g a r i n d u s t r y , i t seems t h a t t h i s method does n o t g i v e v e r y

d e f i n i t e r e s u l t s , and t h e r e have been v a r y i n g r e p o r t s on i t s e f f i c i e n c y ( r e f s .

3 , 1 6 - 1 9 ) . Howeve r , i t c a n n o t be e x c l u d e d t h a t some o f t he u n f a v o u r a b l e r e p o r t s

have r e s u l t e d f rom m i s u n d e r s t a n d i n g s , as t h e e v a l u a t i o n o f t h e e f f i c i e n c y o f

s c a l e p r e v e n t i o n i s a s u r p r i s i n g l y complex p r o b l e m . The a s s o c i a t e d d i f f i c u l t i e s

have been d i s c u s s e d i n a r e c e n t a r t i c l e d e v o t e d t o s t u d i e s o f s c a l e f o r m a t i o n

d u r i n g raw j u i c e e v a p o r a t i o n ( r e f . 2 0 ) .

I t s h o u l d f i n a l l y be p o i n t e d o u t t h a t s c a l e p r e v e n t i o n must a l s o i n c l u d e

p r o p e r p r o c e d u r e s f o r e v a p o r a t o r o p e r a t i o n unde r abnormal c o n d i t i o n s , l i k e t h e

s t a r t - u p w i t h w a t e r r e p l a c i n g j u i c e i n t h e e v a p o r a t o r b o d i e s , o r t h e a d d i t i o n o f

w a t e r t o t h i n j u i c e i n emergency s i t u a t i o n s a r i s i n g where t h e j u i c e f l o w becomes

t o o s m a l l . I n p r i n c i p l e , t h e w a t e r added s h o u l d be o f c o n d e n s a t e o r f e e d - w a t e r

q u a l i t y . The use o f u n t r e a t e d w a t e r must be a v o i d e d even d u r i n g v e r y s h o r t

p e r i o d s , because o f t he r i s k o f f o r m a t i o n o f v e r y t r o u b l e s o m e d e p o s i t s .

V a r i o u s p r o c e d u r e s have been a d v o c a t e d f o r c l e a n i n g e v a p o r a t o r t u b e s and

o t h e r i n c r u s t e d h e a t i n g s u r f a c e s . The commonest p r a c t i c e i s t o b o i l f o r s e v e r a l

h o u r s w i t h c a u s t i c soda s o l u t i o n , t h e n wash w i t h w a t e r and b o i l w i t h d i l u t e

118

h y d r o c h l o r i c a c i d . The d e t a i l s o f t h e p r o c e d u r e must be d e f i n e d so as t o a v o i d

t h e r i s k o f c o r r o s i v e damage t o t h e equ ipmen t , and p a r t i c u l a r l y t o t h e h e a t i n g

t u b e s . I n c o n n e c t i o n w i t h t h i s r e q u i r e m e n t , o t h e r c h e m i c a l s ( i n h i b i t o r s ,

a c t i v a t o r s and p a s s i v a t o r s ) have been d e v e l o p e d t o make i t e a s i e r t o c o n t r o l t h e

c l e a n i n g p r o c e s s . The t y p e and amount o f c h e m i c a l s and t h e b o i l i n g t ime may v a r y

f rom f a c t o r y t o f a c t o r y , depend ing on t h e s c a l e p r o p e r t i e s and tube m a t e r i a l

u s e d . A good a c c o u n t o f t h e p rob lems a s s o c i a t e d w i t h t h e i m p l e m e n t a t i o n o f

chemica l c l e a n i n g methods ( i n t he P o l i s h s u g a r i n d u s t r y ) can be f ound i n t h e

l i t e r a t u r e ( r e f s . 2 1 - 2 4 ) . S p e c i a l i z e d chemica l companies a r e p r e s e n t l y o f f e r i n g

e x t e n d e d i n d i v i d u a l c l e a n i n g and c o n s e r v a t i o n methods , on t he b a s i s o f s c a l e

samp les , s c a l e d - t u b e s e c t i o n s , r e s i n s a m p l e s , e t c .

Where chemica l c l e a n i n g has n o t p r o v e d e f f e c t i v e , mechan ica l o r h y d r a u l i c

t ube c l e a n e r s can be u s e d . E l e c t r i c a l d e s c a l e r s a r e w i d e l y known, c o n s i s t i n g o f

an e l e c t r i c motor and a f l e x i b l e s h a f t t e r m i n a t i n g i n a t o o l w i t h s e r r a t e d

r o l l e r s , w h i c h i s d r i v e n a t a h i g h s p e e d . One o r two passages o f t h e t o o l f o r

each t ube a re u s u a l l y s u f f i c i e n t t o remove t h e s c a l e .

3.3 FUNDAMENTALS OF CORRECT U T I L I Z A T I O N OF CONDENSATES AND VAPOURS

3.3.1 Condensa tes

The u t i l i z a t i o n o f t he e n e r g y o f c o n d e n s a t e s t a k e s p l a c e i n t h e the rma l

c i r c u i t r y a t t a c h e d t o t h e condensa te t a n k s . P r o v i d i n g t h e d r a i n a g e subsys tem i s

w o r k i n g c o r r e c t l y , t he t a s k o f most condensa te t a n k s i s t o a c t as f l o w

s t a b i l i z e r s and t o s e c u r e p r o p e r c o n d i t i o n s f o r t h e c o n d e n s a t e - f l a s h p r o c e s s . As

t h e i n f l o w i n g condensa te i s e x p a n d i n g t o t h e p r e s s u r e m a i n t a i n e d i n t h e t a n k ,

f l a s h v a p o u r , assumed t o be d r y s a t u r a t e d s team, i s g e n e r a t e d ( s e e a l s o S e c t i o n

2 . 5 ) . As a s e c o n d a r y e f f e c t , t h e gases d i s s o l v e d i n t h e condensa te a re

1 i b e r a t e d .

The f l a s h p r o c e s s s h o u l d c o n v e n i e n t l y be pe r f o rmed i n t h i n c o n d e n s a t e l a y e r s

o r s t r e a m s . I n a h o r i z o n t a l t a n k , t h i s can be a c c o m p l i s h e d by s u p p l y i n g t h e

condensa te t o t he upper p a r t and p r e f e r a b l y t o a s p e c i a l n o z z l e p l a c e d above t h e

l i q u i d l e v e l , as shown i n F i g . 3 . 1 4 ( a ) . I t s h o u l d be o b s e r v e d t h a t when

i n t r o d u c i n g t he condensa te be low t h e l i q u i d l e v e l , v a p o u r g e n e r a t i o n may

i n i t i a l l y be i n h i b i t e d by t h e h y d r a u l i c p r e s s u r e , and s u b s e q u e n t i n t e n s i v e

b o i l i n g i n a l a r g e l i q u i d vo lume may i n d u c e tank v i b r a t i o n s .

I n F i g . 3 . 1 4 ( b ) , a scheme f o r a v e r t i c a l t a n k d e s i g n e d t o s a t i s f y t h e

r e q u i r e m e n t s o f e f f i c i e n t c o n d e n s a t e f l a s h i s shown. H i g h - t e m p e r a t u r e c o n d e n s a t e

i s s u p p l i e d f rom be low v i a a n o z z l e t o a s i e v e t r a y p l a c e d above t h e l i q u i d

l e v e l . A p o s s i b l e s u p p l y o f l o w - t e m p e r a t u r e condensa te i s d i r e c t e d t o a n o z z l e

p l a c e d be low t h e o u t l e t n o z z l e . The tank i s u s u a l l y e q u i p p e d w i t h an a u t o m a t i c

l e v e l - c o n t r o l c i r c u i t .

119

(α) ( b )

jT I Χ nozzle nozzle

h i

baffle

sieve tray

Γ

F i g . 3 .14 . D e s i g n p r i n c i p l e s o f c o n d e n s a t e t a n k s : ( a ) h o r i z o n t a l t y p e , ( b ) v e r t i c a l t y p e ( c o u r t e s y Chemadex) . 1 - e x p a n d i n g c o n d e n s a t e , 2 - s u b c o o l e d c o n d e n s a t e , 3 - o u t f l o w i n g c o n d e n s a t e , 4 - f l a s h v a p o u r .

I t s h o u l d be o b s e r v e d t h a t t h e a v a i l a b l e e n e r g y o f t h e c o n d e n s a t e may e i t h e r

be s u p p l i e d t o t h e e v a p o r a t o r i n t h e f l a s h v a p o u r , o r t r a n s f e r r e d t o j u i c e o r

o t h e r media i n c o n d e n s a t e - h e a t e d h e a t e r s . H o w e v e r , i f a p a r t o f t h e h e a t i n g -

v a p o u r demand i s s a t i s f i e d by t h e f l a s h v a p o u r , and t h e c o n d e n s e r l o s s i s

c o n s t a n t , t hen l e s s w a t e r w i l l be e v a p o r a t e d f rom j u i c e i n t h e e v a p o r a t o r . I n

t h e a r rangement shown s c h e m a t i c a l l y i n F i g . 3 . 1 5 ( a ) , a b o u t 1.4 kg f l a s h v a p o u r

i s o b t a i n e d pe r 100 kg b e e t , t h i s r e d u c i n g w a t e r e v a p o r a t i o n i n t h e f i r s t and

second s t a g e s by abou t 2.8 kg/100 kg b. T h i s f i g u r e can be c u t down by a p p l y i n g

d o u b l e - s t a g e condensa te e x p a n s i o n , w h i c h r e q u i r e s u s i n g two c o n d e n s a t e t a n k s as

shown i n F i g . 3 . 1 5 ( b ) . I n t h e f i r s t t a n k , t h e c o n d e n s a t e i s f l a s h e d t o t h e

f i r s t - e f f e c t p r e s s u r e , g e n e r a t i n g abou t 0.7 kg v a p o u r p e r 100 kg b e e t . S e c o n d -

s t a g e f l a s h p r o d u c e s a n o t h e r 0.7 kg v a p o u r p e r 100 kg b e e t . As a r e s u l t , w a t e r

e v a p o r a t i o n w i l l be r e d u c e d by 0.7 + 2 -0 .7 = 2.1 kg/100 kg b , i . e . , 25% l e s s

t han i n t h e p r e v i o u s c a s e .

The f i r s t s t a g e o f t he c o n d e n s a t e f l a s h c h a i n p r e s e n t e d above can be r e p l a c e d

by a h e a t e r t o w h i c h t h i n j u i c e f rom a n o t h e r h e a t e r , hea ted by f i r s t - e f f e c t

v a p o u r , i s s u p p l i e d . T h i s s o l u t i o n , shown s c h e m a t i c a l l y i n F i g . 3 . 1 5 ( c ) , r e d u c e s

w a t e r e v a p o r a t i o n i n t h e e v a p o r a t o r by o n l y 1.4 kg/100 kg b.

A n o t h e r p o s s i b i l i t y o f r e p l a c i n g c o n d e n s a t e f l a s h i n a t a n k by c o n d e n s a t e

c o o l i n g i n a h e a t e r can be f o u n d i n c o n n e c t i o n w i t h t h e l a s t e v a p o r a t o r e f f e c t .

More s p e c i f i c a l l y , t he l a s t - e f f e c t c o n d e n s a t e t ank can be o p e r a t e d a t a p r e s s u r e

equal t o t h a t i n t h e h e a t i n g chamber ( i n s t e a d o f t h a t i n t h e v a p o u r c h a m b e r ) .

120

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34.3 Ί

F i g . 3 .15 . Schemes o f u t i l i z a t i o n o f f i r s t - e f f e c t condensa te i n a q u a d r u p l e -e f f e c t e v a p o r a t o r : ( a ) s i n g l e - s t a g e f l a s h , ( b ) d o u b l e - s t a g e f l a s h , ( c ) t h i n j u i c e h e a t i n g , f o l l o w e d by s i n g l e - s t a g e f l a s h ( f l o w s i n kg/100 kg b ) .

T h i s makes i t p o s s i b l e t o s u p p l y t h e c o n d e n s a t e t o a h e a t e r i n s t a l l e d as t h e

n e x t h e a t i n g s t a g e a f t e r t h e one hea ted by t h e l a s t - e f f e c t v a p o u r . I f a p p l i e d

i n s t e a d o f a f l a s h s t a g e , t h i s s o l u t i o n i n c r e a s e s w a t e r e v a p o r a t i o n i n t h e

e v a p o r a t o r by 1.8-2.5 kg/100 kg b.

I f t he p r i n c i p l e s o f c o r r e c t u t i l i z a t i o n o f c o n d e n s a t e s a r e o b s e r v e d , t h e n

t h e condensa te subsys tem c o n s i s t i n g o f t a n k s , p i p i n g , f i t t i n g s , c o n t r o l s and

measur ing i n s t r u m e n t s l o o k s i m i l a r i n d i f f e r e n t f a c t o r i e s . A l t e r n a t i v e l y , t h e

e s s e n t i a l components can be combined i n t o a s i n g l e p i e c e o f equ ipment known as

t h e compound t a n k . W h i l e t h e compartments o f t h e compound tank c o r r e s p o n d t o t h e

i n d i v i d u a l t a n k s o f t he c o n v e n t i o n a l s o l u t i o n , l e s s p i p i n g and f i t t i n g s can be

u s e d . Depending on l o c a l c o n d i t i o n s , i t may a l s o be e a s i e r t o f i n d a p l a c e f o r

a s i n g l e compound tank r a t h e r t han m u l t i p l e i n d i v i d u a l t a n k s .

121

3 .3 .2 V a p o u r s

The r o l e o f t h e m u l t i p l e - e f f e c t e v a p o r a t o r s t a t i o n i n t h e hea t economy o f

s u g a r f a c t o r i e s has been d i s c u s s e d p r e l i m i n a r i l y i n S e c t i o n 1 .2 .5 . I t s i n f l u e n c e

on t h e n e t h e a t demand o f t he s u g a r m a n u f a c t u r i n g p r o c e s s i s based on t h e

m o d i f i e d R i l l i e u x p r i n c i p l e , t h a t i s , u s i n g t h e h e a t r e p e t i t i v e l y i n t h e

c o n s e c u t i v e e v a p o r a t o r e f f e c t s , and s i m u l t a n e o u s l y d e l i v e r i n g f r a c t i o n s o f t h e

h e a t t o t h e v a p o u r r e c e i v e r s c o n n e c t e d t o each e f f e c t . As a r e s u l t , t h e

e v a p o r a t i o n c o e f f i c i e n t , i . e . t h e r a t i o o f t h e mass o f w a t e r e v a p o r a t e d t o t h e

mass o f h e a t i n g steam consumed i n t he f i r s t e f f e c t , may r e a c h 2 . 3 - 2 . 8 i n a

q u a d r u p l e - e f f e c t and 3 .2 -3 .5 i n a q u i n t u p l e - e f f e c t , e v a p o r a t o r . The l a r g e r t h e

e v a p o r a t i o n c o e f f i c i e n t , t h e l a r g e r can be t h e e f f e c t i v e n e s s r a t i o

c h a r a c t e r i z i n g t h e the rma l s y s t e m .

Numerous t h e o r e t i c a l a n a l y s e s and p r a c t i c a l l y - o r i e n t e d s t u d i e s have been

d e v o t e d t o t h e op t ima l d i s t r i b u t i o n o f v a p o u r s f rom m u l t i p l e - e f f e c t e v a p o r a t o r s

( r e f s . 2 5 - 3 3 ) . T h e i r r e s u l t s can be summar ized i n two r u l e s , t o be o b s e r v e d when

d e s i g n i n g a new therma l sys tem o r m o d e r n i z i n g an e x i s t i n g o n e :

( i ) Each h e a t i n g o p e r a t i o n s h o u l d be a c c o m p l i s h e d u s i n g v a p o u r a t t h e l o w e s t

p o s s i b l e t e m p e r a t u r e .

( i i ) Vapour f l o w f rom the l a s t e v a p o r a t o r e f f e c t t o t h e c o n d e n s e r s h o u l d

approach z e r o .

The t e m p e r a t u r e s o f v a p o u r s f rom the e v a p o r a t o r e f f e c t s a r e d e t e r m i n e d by

t h e number o f e f f e c t s , t h e d e s i g n o f e v a p o r a t o r b o d i e s and t h e h e a t i n g - s u r f a c e

a r e a s i n t h e i n d i v i d u a l e f f e c t s . S i m u l t a n e o u s l y , t h e pa rame te rs o f t h e s u g a r

m a n u f a c t u r i n g p r o c e s s d e t e r m i n e t h e v a p o u r demand and t h e t e m p e r a t u r e r e q u i r e d

f o r t he h e a t i n g o p e r a t i o n s . C o n s e q u e n t l y , i t i s p o s s i b l e t o a s s i g n , t o each

h e a t i n g o p e r a t i o n , a s p e c i f i c e v a p o r a t o r e f f e c t f rom w h i c h h e a t i n g v a p o u r a t

a s u i t a b l e t e m p e r a t u r e can be w i t h d r a w n .

The p o s s i b i l i t i e s f o r u s i n g v a p o u r s f rom d i f f e r e n t e v a p o r a t o r e f f e c t s i n

a g i v e n h e a t i n g o p e r a t i o n a r e l i m i t e d by t h e r e q u i r e d f i n a l t e m p e r a t u r e o f t h e

medium h e a t e d , and by t he minimum t e m p e r a t u r e d i f f e r e n c e ( t e m p e r a t u r e p i n c h )

c h a r a c t e r i s t i c o f t h e h e a t i n g a p p a r a t u s . The r e l a t i o n s between t h e t e m p e r a t u r e s

a r e i l l u s t r a t e d i n F i g . 3 .16 . I t can t h u s be c o n c l u d e d t h a t t h e e f f e c t i v e n e s s o f

t h e u t i l i z a t i o n o f v a p o u r s d e p e n d s , t o a c e r t a i n e x t e n t , on t h e h e a t i n g

equ ipment a v a i l a b l e . A q u a n t i t a t i v e a n a l y s i s o f t h i s r e l a t i o n s h i p can be based

on t he e q u a t i o n e x p r e s s i n g t h e hea t Q t r a n s f e r r e d i n u n i t t ime between t h e

v a p o u r and t h e medium hea ted as

Q = kFAT ( 3 . 5 )

where k i s t h e o v e r a l l h e a t t r a n s f e r c o e f f i c i e n t , F i s t h e h e a t i n g - s u r f a c e a r e a ,

and Δ Τ i s t he mean t e m p e r a t u r e d i f f e r e n c e between h e a t i n g v a p o u r and t h e medium

h e a t e d .

122

\_ D

2 Ο.

Τν vapour

Η-<

pinch

required f inal

temperature

0 100 Heating surface area p a s s e d ( % )

F i g . 3 .16 . Tempera tu re c h a r a c t e r i s t i c s o f j u i c e h e a t i n g . ΔΤ - mean t e m p e r a t u r e d i f f e r e n c e , T ^ - a c t u a l v a p o u r t e m p e r a t u r e , T ^ - l o w e s t p o s s i b l e v a p o u r t e m p e r a t u r e .

F o r a g i v e n amount o f hea t t r a n s f e r r e d , i t may be p o s s i b l e t o r e d u c e t h e mean

t e m p e r a t u r e d i f f e r e n c e (and t h e minimum t e m p e r a t u r e d i f f e r e n c e ) o n l y i f t h e

p r o d u c t kF i s i n c r e a s e d . C o n s e q u e n t l y , t h e p r i n c i p l e o f s e l e c t i o n o f v a p o u r a t

t h e l o w e s t p o s s i b l e t e m p e r a t u r e i m p l i e s t h a t t h e equ ipment c h a r a c t e r i z e d by

a smal l minimum t e m p e r a t u r e d i f f e r e n c e , w h i c h i s e q u i v a l e n t t o a l a r g e k F ,

s h o u l d be p r e f e r r e d . Howeve r , l a r g e kF can o n l y be o b t a i n e d u s i n g h i g h - i n t e n s i t y

h e a t t r a n s f e r , o r l a r g e h e a t i n g - s u r f a c e a r e a s , o r bo th - t h i s i n d u c i n g i n c r e a s e d

h e a t e r c o s t . A t g i v e n u n i t p r i c e s o f e n e r g y and h e a t - e x c h a n g i n g e q u i p m e n t , t h e r e

i s a t r a d e - o f f between t h e e f f e c t i v e n e s s o f t h e e n e r g y u t i l i z a t i o n and t he

i n v e s t m e n t c o s t o f t he h e a t e r s , and an optimum e x i s t s f o r t h e minimum

t e m p e r a t u r e d i f f e r e n c e . ( A c t u a l l y , i t may be p o s s i b l e t o t r e a t t h i s phenomenon

as a b a s i s o f e n e r g y - s y s t e m a n a l y s i s and d e s i g n , as shown i n S e c t i o n 9 . 4 . 3 . )

As r e g a r d s v a p o u r - h e a t e d j u i c e h e a t e r s , i t i s b e l i e v e d a t p r e s e n t t h a t t h e

economic v a l u e s o f t h e minimum t e m p e r a t u r e d i f f e r e n c e s h o u l d n o t e x c e e d 5-10 K,

l o w e r v a l u e s c o r r e s p o n d i n g t o h i g h e r f u e l p r i c e s . T y p i c a l h e a t i n g o p e r a t i o n s

s h o u l d t h u s be a c c o m p l i s h e d u s i n g v a p o u r s a t t e m p e r a t u r e s n o t h i g h e r t han 5-10 Κ

above t h e r e q u i r e d f i n a l j u i c e t e m p e r a t u r e . Even s m a l l e r t e m p e r a t u r e d i f f e r e n c e s

may be adop ted when h e a t i n g w i t h l o w - t e m p e r a t u r e v a p o u r s ( s e e S e c t i o n 3 . 3 . 4 ) .

The l a r g e s t component o f t h e h e a t i n g - v a p o u r demand i s a s s o c i a t e d w i t h t h e

s u g a r b o i l i n g p r o c e s s . S u p p l y i n g t h e vacuum pans w i t h v a p o u r a t t h e l o w e s t

p o s s i b l e t e m p e r a t u r e i s d e c i s i v e i n o p t i m i z i n g t h e d i s t r i b u t i o n o f v a p o u r s f rom

t h e e v a p o r a t o r . F o r t h i s r e a s o n , l e t us t ake a c l o s e r l o o k a t t h e h e a t i n g

r e q u i r e m e n t s o f s u g a r b o i l i n g .

I n t he case o f b a t c h vacuum p a n s , t h e n o t i o n o f t h e minimum t e m p e r a t u r e

d i f f e r e n c e must be adap ted t o t h e d i s c o n t i n u o u s n a t u r e and o t h e r s p e c i a l

f e a t u r e s o f t h e b o i l i n g p r o c e s s . I t i s known t h a t t h e magma t e m p e r a t u r e may be

u n e v e n l y d i s t r i b u t e d i n t h e s t r i k e v o l u m e , t h e d i s t r i b u t i o n b e i n g dependen t on

t h e pan d e s i g n a n d , p o s s i b l y , s t i r r e r e f f i c i e n c y ( r e f s . 3 4 - 3 6 ) . F o r t h e sake o f

123

s i m p l i c i t y , l e t us assume t h a t t h e t e m p e r a t u r e c o n d i t i o n s i n t h e magma can be

a d e q u a t e l y d e s c r i b e d u s i n g t h e mean magma t e m p e r a t u r e . A t c o n s t a n t vacuum,

f o l l o w i n g changes o f t h e b o i l i n g - p o i n t e l e v a t i o n as t h e magma c o n c e n t r a t i o n i s

i n c r e a s e d , t h i s t e m p e r a t u r e v a r i e s d u r i n g t h e b o i l i n g c y c l e as shown

a p p r o x i m a t e l y i n F i g . 3 .17 . F o r t h e same r e a s o n , t h e o v e r a l l h e a t t r a n s f e r

c o e f f i c i e n t a t t he h e a t i n g s u r f a c e v a r i e s as i n d i c a t e d i n t h e same d i a g r a m .

S i m u l t a n e o u s l y , t h e c o n d e n s a t i o n t e m p e r a t u r e i n t h e h e a t i n g chamber may v a r y ( a s

d i s c u s s e d i n S e c t i o n 3 . 1 . 2 ) , becoming t e m p o r a r i l y l o w e r t han t h e t e m p e r a t u r e o f

v a p o u r f rom t h e a s s o c i a t e d e v a p o r a t o r e f f e c t .

oH76

20 UO 60 80 100 Time after seeding (m in )

F i g . 3 .17 . Mean magma t e m p e r a t u r e as a f u n c t i o n o f t ime d u r i n g w h i t e - s u g a r b o i l i n g i n two d i f f e r e n t vacuum pans ( a f t e r r e f . 3 4 ) .

The range o f c h o i c e s o f v a p o u r t e m p e r a t u r e v a l u e s f o r pan h e a t i n g i s l i m i t e d

by t h e b o i l i n g t ime r e q u i r e m e n t w h i c h i s e s s e n t i a l t o bo th t h e s u g a r house

t h r o u g h p u t and p r o d u c t q u a l i t y . A t a g i v e n h e a t i n g - s u r f a c e a r e a , t h e amount o f

h e a t t r a n s f e r r e d s h o u l d be l a r g e enough t o e n s u r e t h a t t he s t r i k e can be

comp le ted w i t h i n t h e r e q u i r e d b o i l i n g t ime τ . I n a d d i t i o n , t h e h e a t t r a n s f e r r e d

p e r u n i t t ime s h o u l d be s u f f i c i e n t l y l a r g e t o make c o r r e c t s t r i k e f i n i s h i n g

p o s s i b l e .

I n o r d e r t o s i m p l i f y t h e f o r m u l a t i o n o f t h e r e q u i r e m e n t s , l e t us assume t h a t

changes o f t h e v a p o u r - c o n d e n s a t i o n t e m p e r a t u r e a r e n e g l i g i b l y s m a l l . T h i s a l l o w s

us t o r e s t r i c t o u r a t t e n t i o n t o t h e f o l l o w i n g pa rame te r s t h a t a p p r o x i m a t e l y

d e s c r i b e t h e t e m p e r a t u r e c y c l e ( F i g . 3 . 1 8 ) :

- t i m e - a v e r a g e d mean magma t e m p e r a t u r e T ^ ;

- h i g h e s t mean magma t e m p e r a t u r e T ^ ;

- vapou r t e m p e r a t u r e T ^ .

124

Time (min)

F i g . 3 .18 . Tempera tu re c y c l e accompany ing t h e s u g a r b o i l i n g p r o c e s s .

The t e m p e r a t u r e c y c l e i s accompanied by t he h e a t t r a n s f e r c y c l e ( F i g . 3 . 1 9 ) ,

w h i c h can be c h a r a c t e r i z e d by t h e f o l l o w i n g pa rame te rs i l l u s t r a t e d i n F i g . 3 .20 :

- t i m e - a v e r a g e d o v e r a l l hea t t r a n s f e r c o e f f i c i e n t k^; a

- l o w e s t o v e r a l l hea t t r a n s f e r c o e f f i c i e n t k^;

- hea t demand p e r one s t r i k e Q^^.

U s i n g t he pa rame te rs l i s t e d a b o v e , i t i s p o s s i b l e t o d e r i v e a s i m p l e

mathemat ica l model o f t he hea t t r a n s f e r r e l a t i o n s h i p s i n a b o i l i n g c y c l e . The

v a p o u r t e m p e r a t u r e s h o u l d be chosen t o s a t i s f y t h e i n e q u a l i t y

w h i c h can be r e w r i t t e n as

Τ < Τ

( 3 . 6 )

V( V) ' a ν ( 3 . 7 )

D e n o t i n g t he maximum a l l o w a b l e h e a t t r a n s f e r r e d p e r u n i t t ime by Q ^ , a n o t h e r

i n e q u a l i t y can be c o n s t r u c t e d t o r e f l e c t t h e c o n d i t i o n s o f s t r i k e f i n i s h i n g as

< V\ - ( 3 . 8 )

o r

( 3 . 9 )

F i g . 3 .19 . O v e r a l l hea t t r a n s f e r c o e f f i c i e n t as a f u n c t i o n o f t ime d u r i n g w h i t e -s u g a r b o i l i n g i n two d i f f e r e n t vacuum pans ( a f t e r r e f . 3 4 ) .

125

Time

F i g . 3 .20 . Heat t r a n s f e r c y c l e accompany ing t h e s u g a r b o i l i n g p r o c e s s .

The above f o r m u l a e a re meant t o p r o v i d e o n l y a q u a l i t a t i v e p i c t u r e o f t h e

therma l a s p e c t s o f s e l e c t i o n o f t h e v a p o u r t e m p e r a t u r e . As i n d i c a t e d by ( 3 . 7 )

and ( 3 . 9 ) , f o r a s p e c i f i c p r o d u c t and a g i v e n vacuum-pan d e s i g n , t h e t e m p e r a t u r e

o f t h e h e a t i n g v a p o u r must be h i g h e r t han a c e r t a i n t h r e s h o l d v a l u e . Most

con tempo ra ry pan d e s i g n s make i t p o s s i b l e t o b o i l w h i t e s u g a r a t a v a p o u r

t e m p e r a t u r e 107-120°C, and l o w - g r a d e p r o d u c t s a t 102-110°C.

From t h e r e a s o n i n g p r e s e n t e d a b o v e , t h e c o n c l u s i o n can be drawn t h a t t h e

v a p o u r t e m p e r a t u r e s e l e c t i o n f o r vacuum-pan h e a t i n g i s v e r y much i n f l u e n c e d by

t h e c o n d i t i o n s o f t h e f i n a l p a r t o f t h e s t r i k e - t h i c k e n i n g p h a s e . A c t u a l l y , t h e

u t i l i z a t i o n o f v a p o u r s f rom t h e e v a p o r a t o r can be improved i f v a p o u r a t a l o w e r

t e m p e r a t u r e i s s u p p l i e d d u r i n g most o f t h e b o i l i n g c y c l e , t h i s b e i n g f o l l o w e d by

h e a t i n g a t a h i g h e r t e m p e r a t u r e when t h e f i n a l p a r t o f t h e s t r i k e - t h i c k e n i n g

phase i s a p p r o a c h e d . A l t h o u g h t h i s i d e a i s c e r t a i n l y r e a l i z a b l e and t h e r e have

been examples o f i t s p r a c t i c a l a p p l i c a t i o n i n b a t c h vacuum pans ( r e f . 3 7 ) , one

has t o r eckon w i t h s i d e - e f f e c t s c o n s i s t i n g o f a d d i t i o n a l f l u c t u a t i o n s o f

e v a p o r a t o r pa rame te rs accompany ing t h e moment o f s w i t c h i n g f rom l o w - t o h i g h -

t e m p e r a t u r e v a p o u r ( t h e f l u c t u a t i o n s r e s u l t i n g , u n a v o i d a b l y , i n an e x t r a e n e r g y

l o s s ) . Howeve r , t h e p o t e n t i a l g a i n s can e a s i l y be a t t a i n e d u s i n g c o n t i n u o u s

vacuum p a n s . A d i s c u s s i o n o f t h e deve lopmen t and a p p l i c a t i o n s o f c o n t i n u o u s pans

i s p r e s e n t e d i n C h a p t e r 5.

R e t u r n i n g now t o t he b a t c h vacuum p a n s , i t can be seen i n F i g s . 3.17 and 3.19

t h a t t h e h i g h e s t mean magma t e m p e r a t u r e t e n d s t o be h i g h e r , and t h e o v e r a l l h e a t

t r a n s f e r c o e f f i c i e n t s i s m a r k e d l y l o w e r , i n t h e n a t u r a l - c i r c u l a t i o n vacuum p a n s .

Both f a c t o r s a c t i n t h e same d i r e c t i o n , making i t n e c e s s a r y t o s e t t h e

d i f f e r e n c e between t he t i m e - a v e r a g e d magma t e m p e r a t u r e and t h e v a p o u r

t e m p e r a t u r e l a r g e r t han i n s t i r r e r - e q u i p p e d p a n s . I t can t h u s be c o n c l u d e d t h a t

t he s t i r r e r - e q u i p p e d vacuum pans make i t p o s s i b l e t o a c c e p t a l o w e r h e a t i n g -

vapou r t e m p e r a t u r e , t h u s s t i m u l a t i n g b e t t e r u t i l i z a t i o n o f v a p o u r s f rom t h e

e v a p o r a t o r .

126

C o n c l u d i n g t h e d i s c u s s i o n o f t h e r u l e o f h e a t i n g w i t h v a p o u r a t t h e l o w e s t

p o s s i b l e t e m p e r a t u r e , l e t us o b s e r v e t h a t i t s a p p l i c a t i o n l e a d s t o s h i f t i n g t h e

e v a p o r a t o r l o a d t owa rds l o w - t e m p e r a t u r e e f f e c t s . As a c o n s e q u e n c e , t h e

e v a p o r a t i o n c o e f f i c i e n t i s i n c r e a s e d and t h e n e t h e a t demand o f t h e f a c t o r y may

be r e d u c e d . I n t he l i t e r a t u r e , examples can be f o u n d o f s u c c e s s f u l a p p l i c a t i o n s

o f t h i s r u l e i n m o d e r n i z a t i o n o f e v a p o r a t o r s t a t i o n s ( r e f s . 3 7 , 3 8 ) .

The second r u l e f o r m u l a t e d a t t h e b e g i n n i n g o f t h i s S e c t i o n , t o m a i n t a i n t h e

v a p o u r f l o w t o t h e condense r c l o s e t o z e r o , means s i m p l y t h a t t h e v a p o u r must

n o t be w a s t e d . On t h e o t h e r h a n d , i t i s n e c e s s a r y t o e v a p o r a t e as much w a t e r i n

t h e e v a p o r a t o r as needed t o a t t a i n a p r e d e t e r m i n e d t h i c k - j u i c e c o n c e n t r a t i o n .

I n a s u g a r f a c t o r y c h a r a c t e r i z e d by a l a r g e hea t demand, so much v a p o u r must be

w i t h d r a w n f rom t h e e v a p o r a t o r f o r h e a t i n g p u r p o s e s t h a t t h i s c o n s t r a i n t i s e a s y

t o s a t i s f y . I f t he hea t demand has been d e c r e a s e d , h o w e v e r , t hen t h e t o t a l

v a p o u r w i t h d r a w a l m igh t be i n s u f f i c i e n t , and i n c r e a s e d v a p o u r f l o w t o t h e

condense r wou ld be t he o n l y p o s s i b i l i t y t o keep t h e t h i c k - j u i c e c o n c e n t r a t i o n

c o n s t a n t . Such a s i t u a t i o n s h o u l d be i n t e r p r e t e d as i n d i c a t i n g t h e n e c e s s i t y t o

m o d i f y t h e a c t u a l e v a p o r a t o r c o n f i g u r a t i o n . G e n e r a l l y , t h r e e s o l u t i o n s can be

c o n s i d e r e d , e i t h e r s e p a r a t e l y o r i n c o m b i n a t i o n :

( 1 ) r e p l a c i n g s e l e c t e d e v a p o r a t o r b o d i e s by o t h e r s w i t h l a r g e r h e a t i n g s u r f a c e s

a n d / o r h i g h e r o v e r a l l hea t t r a n s f e r c o e f f i c i e n t s ;

( 2 ) i n c r e a s i n g t he number o f e v a p o r a t o r e f f e c t s ;

( 3 ) i n t r o d u c i n g a v a p o u r c o m p r e s s i o n c i r c u i t .

The p r i n c i p l e o f t h e f i r s t s o l u t i o n can be seen i n e q n . ( 3 . 5 ) , w h i c h has been

f o r m u l a t e d f o r a h e a t e r b u t a p p l i e s as w e l l t o an e v a p o r a t o r b o d y . F o r a

p r e d e t e r m i n e d amount o f hea t t o be t r a n s f e r r e d f rom h e a t i n g v a p o u r t o j u i c e , an

i n c r e a s e d o v e r a l l hea t t r a n s f e r c o e f f i c i e n t a n d / o r e n l a r g e d h e a t i n g - s u r f a c e a r e a

make i t p o s s i b l e t o r educe t h e mean t e m p e r a t u r e d i f f e r e n c e between v a p o u r and

j u i c e . An a p p l i c a t i o n o f t h i s p r i n c i p l e i s i l l u s t r a t e d i n F i g . 3 .21 , w h i c h shows

v a p o u r t e m p e r a t u r e s i n a q u i n t u p l e - e f f e c t e v a p o r a t o r b e f o r e and a f t e r

m o d e r n i z a t i o n o f t h e 4 t h - e f f e c t b o d y . F o l l o w i n g a r e d u c t i o n o f t h e mean

t e m p e r a t u r e d i f f e r e n c e i n t h e 4 th e f f e c t , t h e t e m p e r a t u r e s o f t h e 4 t h - and 5 t h -

e f f e c t v a p o u r s a r e i n c r e a s e d , making i t e a s i e r t o u t i l i z e t h e s e v a p o u r s f o r

h e a t i n g p u r p o s e s . I t t h u s becomes p o s s i b l e t o s u p p l y 4 t h - and 5 t h - e f f e c t v a p o u r s

t o c e r t a i n r e c e i v e r s t h a t have p r e v i o u s l y been hea ted by 3 r d - and 4 t h - e f f e c t

v a p o u r s , r e s p e c t i v e l y . As a r e s u l t , t h e n e t h e a t demand may be r e d u c e d .

An i n c r e a s e d number o f e v a p o r a t o r e f f e c t s i s an o b v i o u s s o l u t i o n i n s u g a r

f a c t o r i e s where t r i p l e - o r q u a d r u p l e - s t a g e e v a p o r a t o r s a r e u s e d . A t p r e s e n t ,

most e n e r g y e f f i c i e n t f a c t o r i e s r e l y on q u i n t u p l e - e f f e c t e v a p o r a t o r s . T h i s

s h o u l d be seen as an e n t i r e f a m i l y o f e v a p o r a t o r s o l u t i o n s , each c h a r a c t e r i z e d

by a un ique d i s t r i b u t i o n o f h e a t i n g v a p o u r s . Vacuum pans may be hea ted by 2 n d - ,

127

IAO

α 130

t 120 3

ξ 110 α φ 100

90

1 2 3 A 5 Evaporator effect No.

F i g . 3 .21. T e m p e r a t u r e d i s t r i b u t i o n i n a q u i n t u p l e - e f f e c t e v a p o r a t o r . Dashed l i n e s i n d i c a t e r e s u l t s o f t h e m o d e r n i z a t i o n o f f o u r t h - e f f e c t body ( c o u r t e s y C u k r o p r o j e k t ) .

3 r d - , 4 t h - , o r even 5 t h - e f f e c t v a p o u r , o r two d i f f e r e n t v a p o u r s . The l a s t

e f f e c t may work as a c o n c e n t r a t o r , t h a t i s , w i t h v a p o u r w i t h d r a w a l t o t h e

condense r o n l y , o r v a p o u r may a l s o be w i t h d r a w n f o r h e a t i n g p u r p o s e s . Depend ing

on t h e d e s i g n o f t h e e v a p o r a t o r b o d i e s , i t may be n e c e s s a r y t o i n c r e a s e t h e

e x h a u s t - s t e a m p r e s s u r e above t h e l e v e l w h i c h i s s u f f i c i e n t i n q u a d r u p l e - e f f e c t

e v a p o r a t o r s .

Up t o now, s e x t u p l e - e f e c t e v a p o r a t o r s a r e se ldom used i n t y p i c a l w h i t e - s u g a r

f a c t o r i e s . Examples a r e known o f f a c t o r i e s where s e x t u p l e - s t a g e e v a p o r a t o r

s t a t i o n s have been c o n v e r t e d back t o q u i n t u p l e - e f f e c t o n e s , t o make t h e

o p e r a t i o n e a s i e r . S u c c e s s f u l a p p l i c a t i o n s o f s e x t u p l e - e f f e c t e v a p o r a t o r s have

been r e p o r t e d f rom P l a t t l i n g , FRG, and B u c y - l e - L o n g , F r a n c e ( r e f . 3 9 ) . H o w e v e r ,

t h e s e a r e n o t o r d i n a r y f a c t o r i e s , as t h i c k - j u i c e s t o r a g e i s a p p l i e d i n

P l a t t l i n g , and v a p o u r c o m p r e s s i o n i s employed i n t h e the rma l sys tem a t B u c y - l e -

Long .

The i n t r o d u c t i o n o f a v a p o u r - c o m p r e s s i o n c i r c u i t i s men t ioned above as a

t h i r d p o s s i b i l i t y t o m o d i f y an e v a p o r a t o r c o n f i g u r a t i o n . T h i s t e c h n i q u e i s

d i s c u s s e d i n g r e a t e r d e t a i l i n S e c t i o n 3 . 4 .

3 .3 .3 The r o l e o f vacuum sys tems

The a t t a i n a b l e vacuum l e v e l d e f i n e s t h e l o w e s t t e m p e r a t u r e t o w h i c h h e a t

f l o w s can be d i r e c t e d w i t h i n a the rma l s y s t e m . F o r t h i s r e a s o n , t h e f u n c t i o n i n g

o f t h e l a s t e v a p o r a t o r e f f e c t and t h e s u g a r b o i l i n g under vacuum a r e v e r y

i m p o r t a n t t o t h e e f f e c t i v e n e s s o f e n e r g y c o n v e r s i o n p r o c e s s e s t a k i n g p l a c e i n

t he therma l s y s t e m . The d i s c u s s i o n o f t h i s s u b j e c t was i n i t i a t e d i n t h e

p r e c e d i n g S e c t i o n by a n a l y s i n g t h e r e l a t i o n s h i p s c h a r a c t e r i s t i c o f vacuum-pan

128

h e a t i n g . The h i g h e r t he vacuum, t h a t i s , t h e l o w e r t h e p r e s s u r e i n t h e

c o n d e n s e r , t h e l o w e r can be t h e t i m e - a v e r a g e d mean magma t e m p e r a t u r e and t h u s

t h e l o w e r can be t he h e a t i n g - v a p o u r t e m p e r a t u r e , t h i s c o n t r i b u t i n g t o improved

h e a t economy.

As r e g a r d s t he l a s t e v a p o r a t o r e f f e c t , t he vacuum l e v e l d e t e r m i n e s t h e

p r e s s u r e and t h u s t h e j u i c e - b o i l i n g t e m p e r a t u r e . T h i s , i n t u r n , d e f i n e s t h e

t e m p e r a t u r e span a v a i l a b l e f o r t h e m u l t i - s t a g e e v a p o r a t i o n p r o c e s s (be tween t h e

e x h a u s t - s t e a m t e m p e r a t u r e i n t h e f i r s t s t a g e and j u i c e t e m p e r a t u r e i n t h e l a s t ) ,

and i n d i r e c t l y d e f i n e s t h e t e m p e r a t u r e s o f h e a t i n g v a p o u r s and c o n d e n s a t e s f rom

t h e i n d i v i d u a l e v a p o r a t o r e f f e c t s .

I n t he vacuum sys tem o f a s u g a r f a c t o r y , vacuum i s p r o d u c e d i n one o r more

c o n d e n s e r s . The p r e s s u r e i n t h e condense r i s equa l t o t he sum o f t he w a t e r -

s a t u r a t i o n p r e s s u r e c o r r e s p o n d i n g t o t h e t e m p e r a t u r e o f t h e c o n d e n s i n g v a p o u r ,

and t h e p r e s s u r e o f t h e noncondensab le g a s e s . Howeve r , a c o n n e c t i o n between t h e

c o n d e n s e r and an equ ipment u n i t o p e r a t e d under vacuum may i n c l u d e n o z z l e s ,

p i p e s , f i t t i n g s , e n t r a i n m e n t s e p a r a t o r s a n d , p e r h a p s , h e a t i n g chambers o f

h e a t e r s . The f l o w o f v a p o u r t h r o u g h t h e c o n n e c t i o n i s i n e v i t a b l y a s s o c i a t e d w i t h

a p r e s s u r e d r o p , so t he a v a i l a b l e p r e s s u r e i s h i g h e r , i . e . , t h e a v a i l a b l e vacuum

i s l o w e r t han t h a t i n t he c o n d e n s e r ( a c t u a l l y , a p a r t o f t h i s p r e s s u r e d r o p may

o c c u r i n t h e condense r i t s e l f ) . I f , i n a d d i t i o n , t he n o n c o n d e n s a b l e s t e n d t o

accumu la te i n t h e c o n d e n s e r , t h e n t h e i r p r e s s u r e may cause t h e t o t a l p r e s s u r e

i n t h e condense r t o i n c r e a s e , t h i s r e d u c i n g t he a v a i l a b l e vacuum even f u r t h e r .

As a r e s u l t , t h e t e m p e r a t u r e i n t h e equ ipment u n i t may be i n c r e a s e d and a h i g h e r

h e a t i n g - v a p o u r t e m p e r a t u r e may become n e c e s s a r y .

O b v i o u s l y , a t e n d e n c y t o w a r d s i n c r e a s e d t e m p e r a t u r e s o f h e a t i n g v a p o u r s w o u l d

have a d e t r i m e n t a l e f f e c t on t h e h e a t economy. I t can be p r e v e n t e d , h o w e v e r , by

p a y i n g p r o p e r a t t e n t i o n t o t h r e e f a c t o r s c h a r a c t e r i z i n g t he vacuum s y s t e m :

( i ) A v a p o u r - c o n d e n s a t i o n t e m p e r a t u r e c l o s e t o t he i n l e t t e m p e r a t u r e o f c o o l i n g

w a t e r e n t e r i n g t h e c o n d e n s e r ;

( i i ) E f f e c t i v e e v a c u a t i o n o f n o n c o n d e n s a b l e s f rom the c o n d e n s e r ;

( i i i ) S u f f i c i e n t l y smal l p r e s s u r e d r o p s between t h e equ ipment o p e r a t e d under

vacuum and t h e c o n d e n s e r .

M i n i m i z a t i o n o f t h e t e m p e r a t u r e d i f f e r e n c e between t he c o n d e n s i n g v a p o u r and

t h e c o o l i n g w a t e r i s an i m p o r t a n t r e q u i r e m e n t t o be a c c o u n t e d f o r i n c o n d e n s e r

d e s i g n . A d i s c u s s i o n o f t h e a d v a n t a g e s and d i s a d v a n t a g e s o f v a r i o u s d e s i g n s can

be f ound i n t h e l i t e r a t u r e ( r e f s . 3 , 4 0 ) . Many f a c t o r i e s r e l y on s i m p l e , and

q u i t e e f f e c t i v e , c o u n t e r - c u r r e n t s h e l f - t y p e b a r o m e t r i c c o n d e n s e r s . An o u t l i n e o f

a d e s i g n p r e s e n t l y used i n D a n i s h s u g a r f a c t o r i e s can be seen i n F i g . 3 .22 .

A n o t h e r condense r d e s i g n , implemented r e c e n t l y i n t he S o v i e t s u g a r i n d u s t r y ,

c o n s i s t s o f two v e s s e l s c o n n e c t e d i n s e r i e s ( F i g . 3 . 2 3 ) . Vapour e n t e r i n g t h e

129

cooling ρ — ^ water Τ τ - ^ ^

vapour

to vacuum pump

barometric water

F i g . 3 .22 . O u t l i n e o f a c o u n t e r - c u r r e n t s h e l f - t y p e b a r o m e t r i c c o n d e n s e r ( c o u r t e s y D O S ) .

F i g . 3 .23 . Scheme o f a b a r o m e t r i c c o n d e n s e r f e a t u r i n g a c o - c u r r e n t v e s s e l ( a ) and a c o u n t e r - c u r r e n t v e s s e l ( b ) . 1 - v a p o u r i n l e t , 2 - c o o l i n g w a t e r i n l e t , 3 - w a t e r o u t l e t , 4 - vacuum l i n e .

130

f i r s t v e s s e l i s s u b j e c t t o c o - c u r r e n t c o o l i n g as w a t e r f l o w s down t h e s h e l v e s .

The f i r s t p o r t i o n o f w a t e r can be w i t h d r a w n f rom t h e upper p a r t o f t h e v e s s e l .

P r o v i d i n g t h i s w a t e r does n o t mix w i t h t h e main c o o l i n g - w a t e r s t r e a m , t h e upper

p a r t o f t h e f i r s t v e s s e l can be t r e a t e d as an e n t r a i n m e n t s e p a r a t o r . I t i s a l s o

p o s s i b l e t o u t i l i z e t h i s p a r t o f t he c o n d e n s e r as a d i r e c t - c o n t a c t h e a t e r ; f o r

examp le , f r e s h w a t e r s u p p l i e d t o t h e e x t r a c t o r can be hea ted t h e r e . C o - c u r r e n t

c o o l i n g ( u s i n g w a t e r f rom t h e main c o o l i n g c i r c u i t ) c o n t i n u e s i n t h e l o w e r p a r t

o f t he v e s s e l , t h i s be ing f o l l o w e d by c o u n t e r - c u r r e n t c o o l i n g i n t h e second

v e s s e l . I n compar i son t o vacuum sys tems emp loy ing s i m p l e c o u n t e r - c u r r e n t

c o n d e n s e r s , t h i s d e s i g n i s c l a i m e d t o r e d u c e t h e c o o l i n g - w a t e r demand by 25-30%

( r e f . 4 1 ) .

The e v a c u a t i o n o f n o n c o n d e n s a b l e s f rom t h e c o n d e n s e r i s e f f e c t e d u s i n g vacuum

pumps, u s u a l l y o f t h e w a t e r - r i n g t y p e . I n a d d i t i o n t o t h e g e n e r a l c o n d e n s e r

d e s i g n , t h e l o c a t i o n and d i m e n s i o n o f t h e g a s - w i t h d r a w a l n o z z l e i s i m p o r t a n t , as

i t may o r may n o t e n s u r e t h a t t h e gas e v a c u a t i o n i s s u f f i c i e n t and t h e gas

t e m p e r a t u r e i s as low as p o s s i b l e . ( T h e l o w e r t h i s t e m p e r a t u r e , t h e s m a l l e r i s

t he e n e r g y e x p e n d i t u r e i n t h e vacuum pump d r i v e . )

As r e g a r d s t h e p r e s s u r e d r o p s a l o n g t h e v a p o u r p a t h s between equ ipment u n i t s

and t h e c o n d e n s e r , some s p e c i f i c p rob lems may a r i s e depend ing on t h e scheme o f

t h e vacuum i n s t a l l a t i o n . I n t h e case o f i n d i v i d u a l c o n d e n s e r s , i t i s e s s e n t i a l

t o choose channe l d i m e n s i o n s i n a c c o r d a n c e w i t h maximum v a p o u r - f l o w v a l u e s ( f o r

examp le , v a p o u r c h a n n e l s a t t a c h e d t o a vacuum pan s h o u l d be d i m e n s i o n e d f o r t h e

v a p o u r f l o w e x p e c t e d d u r i n g t h e s y r u p - t h i c k e n i n g phase o f t he b o i l i n g c y c l e ) . I f

a c e n t r a l c o n d e n s e r i s e m p l o y e d , t h e n a d e l i c a t e p rob lem a r i s e s o f f l o w

d i s t r i b u t i o n i n p a r a l l e l s e c t i o n s o f t h e vacuum p i p i n g .

L a r g e d i a m e t e r s o f n o z z l e s , v a l v e s and p i p e s i n d u c e a t e n d e n c y t o w a r d s

s e l e c t i n g t o o smal l d i m e n s i o n s , t h i s r e s u l t i n g i n t o o l a r g e p r e s s u r e d r o p s .

T r a d i t i o n a l l y , c a l c u l a t i o n s o f t h e vacuum p i p i n g have been based on t h e

recommended v a l u e s o f f l o w v e l o c i t i e s i n s e r t e d i n t o i n c o m p r e s s i b l e - f l o w f o r m u l a e

e x p r e s s i n g t h e p r e s s u r e d r o p ( r e f . 3 ) . I t can be p r o v e d , h o w e v e r , t h a t n e g l e c t

o f t h e v a p o u r c o m p r e s s i b i l i t y may cause a s y s t e m a t i c e r r o r o f abou t 10% o f t h e

c a l c u l a t e d p r e s s u r e d rop ( r e f . 4 2 ) . An example o f c a l c u l a t i o n s o f t h e vacuum

p i p i n g u s i n g c o m p r e s s i b l e - f l o w f o r m u l a e can be f o u n d i n t h e l i t e r a t u r e

( r e f . 4 3 ) .

I t s h o u l d be added t h a t t h e above d i s c u s s i o n c a n n o t be c o n s i d e r e d as a

comple te p r e s e n t a t i o n o f t h e r e q u i r e m e n t s r e l a t i n g t o e f f i c i e n t vacuum s y s t e m s .

F o r examp le , s t a b i l i t y o f t h e vacuum l e v e l i s v e r y i m p o r t a n t t o bo th t h e h e a t

economy and t he r e l i a b i l i t y o f vacuum-pan o p e r a t i o n . The s t a b i l i t y r e q u i r e m e n t s

t o be a c c o u n t e d f o r i n t h e c o n d e n s e r d e s i g n a r e r e v i e w e d i n t h e l i t e r a t u r e

( r e f . 4 4 ) .

131

3 .3 .4 U t i l i z a t i o n o f l o w - p r e s s u r e v a p o u r s

I n S e c t i o n 1.2, u t i l i z a t i o n o f l o w - t e m p e r a t u r e h e a t and c e r t a i n p rob lems

a s s o c i a t e d w i t h t he equ ipment used f o r t h i s p u r p o s e were p r e l i m i n a r i l y

d i s c u s s e d . I n t h i s a r e a , t h e u t i l i z a t i o n o f vacuum-pan v a p o u r s i s o f p a r t i c u l a r

i m p o r t a n c e . T h i s p rob lem has much i n common w i t h t h e u t i l i z a t i o n o f l a s t - e f f e c t

v a p o u r f rom t h e e v a p o r a t o r .

The p o s s i b i l i t i e s o f h e a t i n g w i t h vacuum-pan v a p o u r s a re l i m i t e d by t h e i r low

t e m p e r a t u r e , 55-65°C. A p o p u l a r s o l u t i o n known f rom l e s s e f f i c i e n t the rma l

sys tems i s t o h e a t f r e s h w a t e r s u p p l i e d t o t h e e x t r a c t o r . T h i s can be d o n e , f o r

examp le , i n t he i n l e t p a r t o f t h e d o u b l e - v e s s e l c o n d e n s e r men t i oned i n t h e

p r e c e d i n g S e c t i o n ; i t has been r e p o r t e d t h a t w a t e r t e m p e r a t u r e s as h i g h as 1-2 Κ

be low the v a p o u r t e m p e r a t u r e can be a t t a i n e d ( r e f . 4 5 ) . I n h i g h l y e f f i c i e n t

thermal s y s t e m s , h o w e v e r , t h e e x t r a c t o r s h o u l d r a t h e r be s u p p l i e d w i t h e x c e s s

c o n d e n s a t e , and o t h e r methods o f u t i l i z a t i o n o f vacuum-pan v a p o u r s s h o u l d be

p r e f e r r e d .

A t y p i c a l e n e r g y - e f f i c i e n t s o l u t i o n i s t o h e a t raw j u i c e i n a h e a t e r o f

a s u i t a b l e d e s i g n . I t seems t h a t h o r i z o n t a l l y - o r v e r t i c a l l y - a r r a n g e d t u b u l a r

h e a t e r s a re most f r e q u e n t l y u s e d , w h i l e a p p l i c a t i o n s o f s p i r a l h e a t e r s a r e a l s o

known. Raw j u i c e can be hea ted t o t h e 50-55°C r e q u i r e d f o r h o t p r e - l i m i n g , t h i s

making i t p o s s i b l e t o u t i l i z e 4-5 kg vacuum-pan v a p o u r p e r 100 kg b e e t . The 2

h e a t i n g s u r f a c e a r e a r e q u i r e d i s t y p i c a l l y o f t h e o r d e r 70-110 m p e r 100 t / d

p r o c e s s i n g c a p a b i l i t y . I f t h e t e m p e r a t u r e i n t h e p r e - l i m e r i s l o w e r , t h e n p r e -

l imed j u i c e can be hea ted i n s t e a d .

The l e a d i n g p r i n c i p l e o f h e a t i n g w i t h vacuum-pan v a p o u r s i s t o aim a t as h i g h

a v a p o u r - c o n d e n s a t i o n t e m p e r a t u r e i n t h e h e a t e r as p o s s i b l e . C o n s e q u e n t l y ,

v a p o u r s f rom pans A a re p r e f e r r e d t o t h o s e f rom Β and C p a n s . P r o p e r p r e c a u t i o n s

s h o u l d a l s o be taken a g a i n s t t h e i n f l u e n c e o f noncondensab le gases c o n s i s t i n g

m a i n l y o f a i r l i b e r a t e d f rom j u i c e and s y r u p s d u r i n g t h e t h i c k e n i n g p h a s e , a i r

e n t e r i n g t h e vacuum pans d u r i n g t h e i n t e r v a l s between t h e b o i l i n g c y c l e s , and

a i r l e a k i n g i n t o t h e vacuum s y s t e m . The volume o f n o n c o n d e n s a b l e s i s u s u a l l y

abou t 1% o f t he v a p o u r vo l ume .

The i n f l u e n c e o f n o n c o n d e n s a b l e s i s e l i m i n a t e d i f t h e h e a t e r i s p r o p e r l y

c o n n e c t e d t o t h e vacuum s y s t e m . I n F i g . 3 .24 , two d i f f e r e n t s o l u t i o n s a r e shown

s c h e m a t i c a l l y . I n case ( a ) , t h e e n t i r e v a p o u r f l o w f rom t h e vacuum pans i s

d i r e c t e d t o t h e h e a t e r and t o t h e c o n d e n s e r , t h i s e f f e c t i v e l y p r e v e n t i n g t h e

a c c u m u l a t i o n o f n o n c o n d e n s a b l e s . The h e a t e r d e s i g n must be adap ted t o a v e r y

l a r g e volume f l o w o f v a p o u r , w h i c h r e q u i r e s a r e l a t i v e l y l a r g e h e a t e r d i a m e t e r

and s p a r s e l y p l a c e d t u b e s , so as n o t t o e x c e e d a v a p o u r p r e s s u r e d rop o f

0 .01-0 .02 b a r . Two s u i t a b l e d e s i g n s i n w h i c h a v a p o u r - f l o w v e l o c i t y o f up t o

40-50 m/s has been assumed a r e shown s c h e m a t i c a l l y i n F i g s . 3.25 (one v a p o u r -

132

(α) (b )

3

- 0

3

Ν /

F i g . 3 .24 . H e a t e r a r rangemen ts s u i t e d t o h e a t i n g w i t h vacuum-pan v a p o u r s : ( a ) h e a t e r s u p p l i e d w i t h t h e e n t i r e v a p o u r f l o w , ( b ) h e a t e r s u p p l i e d w i t h a p a r t o f t he v a p o u r f l o w . 1 - h e a t e r , 2 - foam c a t c h e r , 3 - c o n d e n s e r .

s i d e pass and f o u r j u i c e - s i d e p a s s e s ) and 3.26 ( two and t w e l v e p a s s e s ,

r e s p e c t i v e l y ) . O t h e r d e s i g n s have a l s o been p r e s e n t e d i n t h e l i t e r a t u r e ( r e f .

4 7 ) . G e n e r a l l y , t he h e a t e r d i m e n s i o n s a r e v e r y l a r g e and i t may be d i f f i c u l t t o

f i n d a p l a c e i n an e x i s t i n g f a c t o r y where such a u n i t can be i n s t a l l e d .

I n case ( b ) , t he h e a t e r i s c o n n e c t e d t o a v a p o u r l i n e p a r a l l e l t o t h e main

v a p o u r m a n i f o l d . The v a p o u r f l o w t h r o u g h t h e h e a t e r i s t y p i c a l l y 25-30% o f t h e

t o t a l f l o w , t h i s making i t p o s s i b l e t o r e d u c e t h e d i m e n s i o n s o f t h e u n i t . As t h e

d i a m e t e r s o f t he v a p o u r p i p e s can a l s o be r e d u c e d , i t becomes e a s i e r t o i n s t a l l

t he h e a t e r i n a manner f a c i l i t a t i n g c o n v e n i e n t a c c e s s f o r r e p a i r and

m a i n t e n a n c e . Vapour f rom t h e h e a t e r o u t l e t i s d i r e c t e d t o t h e main v e s s e l o f

a d o u b l e - v e s s e l main c o n d e n s e r (when a c o n d e n s i n g sys tem s i m i l a r t o t h a t

p r e s e n t e d i n S e c t i o n 3 .3 .3 i s a p p l i e d ) , o r t o an i n d i v i d u a l c o n d e n s e r . I n t h i s

c a s e , t h e volume c o n c e n t r a t i o n o f n o n c o n d e n s a b l e s a t t h e v a p o u r o u t l e t may

a t t a i n 4-5%, d e c r e a s i n g t h e c o n d e n s a t i o n t e m p e r a t u r e by abou t 1-1.5 K. I t i s

t h e r e f o r e recommended t o adap t t h e h e a t e r d e s i g n t o t h e f o l l o w i n g r e q u i r e m e n t s

o f e f f i c i e n t v e n t i n g :

uice

8 nn

— C7>

vapour

F i g . 3 .25 . Scheme o f a t u b u l a r h e a t e r hea ted w i t h vacuum-pan v a p o u r , h e a t i n g s u r f a c e a r e a 125 m2. 1 - t u b e s , 2 - i n t e r m e d i a t e t ube s h e e t s , 3 - c o n d e n s a t e -s e p a r a t i n g b a f f l e , 4 - p r o t e c t i v e s c r e e n s ( a f t e r r e f . 4 6 ) .

(α)

133

(b)

baffle

external wall

holes

F i g . 3 .26 . Scheme o f a t u b u l a r h e a t e r h e a t e d w i t h vacuum-pan v a p o u r , h e a t i n g s u r f a c e a r e a 250 m^ ( c o u r t e s y Chemadex) . ( a ) g e n e r a l l a y o u t , ( b ) d e t a i l o f t h e b a f f l e , t o p v i e w .

- v a p o u r - f l o w v e l o c i t y i n t h e f r e e - f l o w ( i . e . d i s r e g a r d i n g t h e p r e s e n c e o f

h e a t i n g t u b e s ) h e a t e r c r o s s - s e c t i o n a r e a c l o s e t o t h e v a p o u r i n l e t s h o u l d be

10-15 m/s , t o p r e v e n t t he d i f f u s i o n o f n o n c o n d e n s a b l e s ;

- c o l d j u i c e s h o u l d e n t e r t h e h e a t i n g t u b e s i n t h e v i c i n i t y o f t h e v a p o u r

o u t l e t , t o s t i m u l a t e i d e n t i c a l f l o w d i r e c t i o n s o f bo th v a p o u r and

n o n c o n d e n s a b l e s .

The above comments and recommendat ions a p p l y a l s o t o t h e h e a t e r s s u p p l i e d

w i t h v a p o u r s f rom t h e l a s t e v a p o r a t o r e f f e c t . A d i s c u s s i o n o f a h e a t e r

a r rangement f o r r a w - j u i c e h e a t i n g u s i n g f i f t h - e f f e c t v a p o u r can be f o u n d i n t h e

l i t e r a t u r e ( r e f . 4 8 ) .

A number o f a l t e r n a t i v e methods o f u t i l i z a t i o n o f l o w - t e m p e r a t u r e v a p o u r s

have a l s o been p r o p o s e d . I n t h e S o v i e t s u g a r i n d u s t r y , d i r e c t - c o n t a c t h e a t e r s

f o r r a w - j u i c e h e a t i n g a r e i n use ( F i g . 3 . 2 7 ) . W h i l e t h i s equ ipment i s s i m p l e and

easy t o imp lement , t h e a d v a n t a g e s o f i t s a p p l i c a t i o n a r e f a r f rom o b v i o u s ,

because t h e j u i c e becomes d i l u t e d w i t h c o n d e n s a t e , t h u s r e q u i r i n g i n c r e a s e d

e v a p o r a t i o n i n t h e e v a p o r a t o r . I n o r d e r t o r e d u c e t h e n e t h e a t demand, i t i s

n e c e s s a r y t o a d j u s t t he d i s t r i b u t i o n o f t h e h e a t i n g v a p o u r s p r i o r t o t h e

imp lemen ta t i on o f a d i r e c t - c o n t a c t h e a t e r ( r e f . 5 0 ) .

A n o t h e r method o f u t i l i z a t i o n o f vacuum-pan v a p o u r s employs an i n t e r m e d i a t e

134

View A

F i g . 3 .27 . Scheme o f a d i r e c t - c o n t a c t j u i c e h e a t e r hea ted w i t h vacuum-pan v a p o u r . 1 - j u i c e i n l e t , 2 - j u i c e o u t l e t , 3 - v a p o u r , 4 - n o n c o n d e n s a b l e s .

w a t e r c i r c u i t f o r hea t t r a n s p o r t t o a i r p r e h e a t e r s i n t h e b o i l e r s o r i n t h e

d r y i n g s t a t i o n s ( s u g a r d r y e r , l o w - t e m p e r a t u r e p u l p d r y e r , e t c . ) . A c o n d e n s e r -

h e a t e r ( " h o t c o n d e n s e r " ) w h i c h has been d e s i g n e d f o r such a p p l i c a t i o n s i s shown

s c h e m a t i c a l l y i n F i g . 3 .28 . A t l e a s t one m a n u f a c t u r e r i s now o f f e r i n g a comp le te

c i r c u i t w h i c h c o n s i s t s o f a c o n d e n s e r - h e a t e r , w a t e r t a n k , pump, and a s p i r a l

hea t e x c h a n g e r f o r r a w - j u i c e h e a t i n g . I t i s c l a i m e d t h a t t h i s s o l u t i o n i s more

economic t han a c o n v e n t i o n a l r a w - j u i c e h e a t e r s u p p l i e d w i t h vacuum-pan v a p o u r .

I n a number o f F r e n c h and West German s u g a r f a c t o r i e s , vacuum-pan v a p o u r s

a re u t i l i z e d i n s p e c i a l e v a p o r a t o r s f o r t h i c k e n i n g j u i c e o r Β s y r u p . As

imp lemen ta t i on o f t h i s method i s v e r y much dependen t on t h e a v a i l a b i l i t y o f

s u i t a b l e equ ipmen t , i t i s d i s c u s s e d i n C h a p t e r 5. I t i s w o r t h n o t i n g t h a t i n t h e

f a c t o r i e s where t h i s method i s a p p l i e d , t h e t o t a l a r e a o f t h e h e a t i n g s u r f a c e s

o f t h e equ ipment hea ted by vacuum-pan v a p o u r s may a t t a i n 380-400 m p e r 1000 t / d

p r o c e s s i n g c a p a b i l i t y .

3.4 VAPOUR COMPRESSION

3.4.1 Compress ion o f v a p o u r s f rom t h e e v a p o r a t o r

An i n t r o d u c t i o n t o t he a p p l i c a t i o n o f v a p o u r c o m p r e s s i o n t e c h n i q u e s was

p r e s e n t e d i n S e c t i o n 1 .2 .7 . T h e r e i s a v a s t l i t e r a t u r e d e v o t e d t o t h e p rob lems

o f comb in ing v a p o u r compress i on c i r c u i t s w i t h v a r i o u s e n e r g y sys tems i n t h e

135

to vacuum pump

1

condensate

water 39 C

cooling water 30"c

cooling ^ water 35 C

φ ) vapour

water 56 C

F i g . 3 .28 . Condenser t o w e r f o r t h e c o n d e n s a t i o n o f vacuum-pan v a p o u r ( c o u r t e s y W i e g a n d ) .

s u g a r i n d u s t r y ( r e f s . 2 , 3 , 5 1 - 5 9 ) . Wor th recommending a l s o i s a b r o c h u r e w h i c h

summar izes t h e p r e s e n t s t a t e - o f - t h e - a r t i n t h e f i e l d o f a p p l i c a t i o n s o f

mechan ica l v a p o u r compresso rs i n v a r i o u s p r o c e s s i n d u s t r i e s ( r e f . 6 0 ) . I n t h i s

S e c t i o n , we s h a l l c o n c e n t r a t e on t h e two s o l u t i o n s most f r e q u e n t l y used i n s u g a r

f a c t o r i e s , namely t h o s e emp loy ing t h e c o m p r e s s i o n o f v a p o u r s f rom t h e f i r s t and

t h e second e v a p o r a t o r e f f e c t s .

I t was i n d i c a t e d i n S e c t i o n 1.2.7 t h a t b e f o r e t h e i n t r o d u c t i o n o f a v a p o u r

compress ion c i r c u i t i n t o a the rma l s y s t e m , i t may be n e c e s s a r y t o r e a r r a n g e t h e

d e t a i l s o f t he d i s t r i b u t i o n o f v a p o u r s f rom t h e e v a p o r a t o r . L e t us c o n s i d e r t h e

example o f a r a t h e r i n e f f i c i e n t the rma l sys tem c h a r a c t e r i z e d by a r e l a t i v e l y

l a r g e steam demand, namely 47.9 kg/100 kg b, c o n s i s t i n g o f :

- h e a t i n g steam a t t h e pa rame te rs c o r r e s p o n d i n g t o t h e t u r b i n e e x h a u s t , s u p p l i e d

t o t h e e v a p o r a t o r and t he s u g a r d r y e r , 46.4 kg/100 kg b;

- l i v e steam t h r o t t l e d t o 7 b a r , s u p p l i e d t o t h e c e n t r i f u g a l s , 1.5 kg/100 kg b.

The f a c t o r y c o n s i d e r e d employs a t r o u g h - t y p e e x t r a c t o r , s u p p l i e d w i t h f r e s h

w a t e r f rom o u t s i d e t h e f a c t o r y , a c l a s s i c a l j u i c e - p u r i f i c a t i o n s t a t i o n , a

q u a d r u p l e - e f f e c t e v a p o r a t o r i n w h i c h t h e j u i c e i s t h i c k e n e d f rom 15.5% DS t o

136

65% DS, and a t h r e e - b o i l i n g c r y s t a l l i z a t i o n scheme. The e s s e n t i a l f e a t u r e s o f

t h e v a p o u r d i s t r i b u t i o n scheme and t h e v a p o u r f l o w s a r e g i v e n i n T a b l e 3 . 3 . L e t

us n o t e t h a t t he f l o w o f l a s t - e f f e c t v a p o u r t o t h e c o n d e n s e r can be r e g a r d e d as

s u f f i c i e n t l y s m a l l , and u n l e s s t h e v a p o u r d i s t r i b u t i o n i s c h a n g e d , no v a p o u r

s t ream i s d i r e c t l y a v a i l a b l e f o r a p o s s i b l e v a p o u r - c o m p r e s s i o n c i r c u i t .

TABLE 3.3

Steam and v a p o u r s t reams (kg /100 kg b) between s o u r c e s and r e c e i v e r s i n a the rma l sys tem consuming 47.9 kg steam p e r 100 kg b e e t .

S o u r c e s

R e c e i v e r s E v a p o r a t o r e f f e c t s O t h e r s

1

P r e s s - w a t e r h e a t e r 1 .95 E x t r a c t o r 2.17 0 .21 R a w - j u i c e h e a t e r condensa te H e a t e r s b e f o r e main l i m i n g 6 .80 H e a t e r a f t e r 1s t c a r b o n a t a t i o n 3.22 H e a t e r b e f o r e 2nd c a r b o n a t a t i o n 2.86 T h i n - j u i c e h e a t e r s 2.52 3.38 T h i c k - j u i c e h e a t e r 0.15 M e l t e r 0.20 I n d i r e c t l y - h e a t e d s y r u p t a n k s 0.45 D i r e c t l y - h e a t e d s y r u p t anks 0.58 Remelt h e a t e r 0.20 Vacuum pans A 12.80

Β 3.40 C 1.31

Vacuum-pan s teaming 1.50 C e n t r i f u g a l s 7 ba r steam 1.50 Sugar d r y e r e x h a u s t steam 0.50 Condense r 0.71

E v a p o r a t o r t o t a l 2.52 32.22 8. .96 0.71

I t can be c o n c l u d e d f rom t h e d a t a p r e s e n t e d i n T a b l e 3.3 t h a t t h e h e a t

economy can be improved by i n t r o d u c i n g t h e u t i l i z a t i o n o f f o u r t h - e f f e c t v a p o u r

and vacuum-pan v a p o u r s . An a d d i t i o n a l improvement can be o b t a i n e d by s u p p l y i n g

t h e e x t r a c t o r w i t h e x c e s s c o n d e n s a t e , i n s t e a d o f f r e s h w a t e r s u p p l i e d a t a l o w e r

i n i t i a l t e m p e r a t u r e . L e t us assume t h a t t h e s e changes a r e i n t r o d u c e d w i t h o u t any

m o d i f i c a t i o n s o f t h e s u g a r m a n u f a c t u r i n g p r o c e s s ; most n o t a b l y , t h e t h i c k - j u i c e

c o n c e n t r a t i o n remains a t 65% DS.

F o l l o w i n g t h e u t i l i z a t i o n o f t h e vacuum-pan v a p o u r s i n r a w - j u i c e h e a t i n g and

o t h e r hea t -economy improvemen ts , t h e demand f o r h e a t i n g v a p o u r s f rom t h e

e v a p o r a t o r i s r e d u c e d . I n o r d e r t o keep t h e t h i c k - j u i c e c o n c e n t r a t i o n c o n s t a n t ,

t h i s can be compensated f o r by a r t i f i c i a l l y w i t h d r a w i n g a s u f f i c i e n t l y l a r g e

v a p o u r s t ream f rom t h e e v a p o r a t o r . L e t us assume t h a t t h i s s t ream can be t a k e n

f rom t h e f i r s t e v a p o r a t o r e f f e c t .

137

The e s s e n t i a l f e a t u r e s o f t h e improved v a p o u r d i s t r i b u t i o n scheme, i n c l u d i n g

t h e f l o w s o f h e a t i n g v a p o u r s f rom t h e e v a p o r a t o r and vacuum p a n s , a r e shown i n

T a b l e 3 . 4 . As can be s e e n , a v a p o u r s t ream o f 10 kg/100 kg b i s w i t h d r a w n f rom

t h e f i r s t e f f e c t . T h i s v a p o u r can be compressed t o t h e e x h a u s t - s t e a m p r e s s u r e

and u t i l i z e d i n t he h e a t i n g chamber o f t h e f i r s t e f f e c t , c u t t i n g down t h e demand

f o r e x h a u s t steam s u p p l i e d t o t h e e v a p o r a t o r .

TABLE 3.4

Steam and v a p o u r s t reams (kg /100 kg b) between s o u r c e s and r e c e i v e r s i n t h e m o d i f i e d the rma l s y s t e m .

S o u r c e s

R e c e i v e r s E v a p o r a t o r e f f e c t s O t h e r s

1

P r e s s - w a t e r h e a t e r 1. ,97 E x t r a c t o r 0. .90 0.92 R a w - j u i c e h e a t e r vacuum-pan v a p o u r H e a t e r s b e f o r e main l i m i n g 2.85 3. ,36 c o n d e n s a t e H e a t e r a f t e r 1 s t c a r b o n a t a t i o n 2.62 H e a t e r b e f o r e 2nd c a r b o n a t a t i o n 2, .50 T h i n - j u i c e h e a t e r s 2. .24 2, .43 1.65 T h i c k - j u i c e h e a t e r 0.15 M e l t e r 0, .20 I n d i r e c t l y - h e a t e d s y r u p t a n k s 0, .45 D i r e c t l y - h e a t e d s y r u p t anks 0, .58 Remel t h e a t e r 0.18 Vacuum pans A 12, .80

Β 3 .40 C 1 .31

Vacuum-pan s teaming 1 .50 C e n t r i f u g a l s 7 ba r steam 1.50 Sugar d r y e r e x h a u s t steam 0.50 Condenser 0, .09 To be w i t h d r a w n 10, .00 (5 .00) *

E v a p o r a t o r t o t a l 12, .24 26 .07 8.37 5, .42

V a p p l i e s t o c o m p r e s s i o n o v e r two s t a g e s

B e f o r e d i s c u s s i n g p o s s i b l e s o l u t i o n s f o r t h e v a p o u r c o m p r e s s i o n c i r c u i t , l e t

us assume t h e f o l l o w i n g v a l u e s o f t h e e n e r g y c o n v e r s i o n and d i s t r i b u t i o n

p r o c e s s e s i n t h e f a c t o r y :

- l i v e steam p r e s s u r e 38 ba r and t e m p e r a t u r e 450°C;

- e x h a u s t steam t e m p e r a t u r e 135°C, f i r s t - e f f e c t v a p o u r t e m p e r a t u r e 126°C ( d r y

s a t u r a t e d steam i n bo th c a s e s , t h a t i s , p r e s s u r e s o f 3.13 ba r and 2.39 b a r ,

r e s p e c t i v e l y ) ;

- power consumpt ion i n t h e f a c t o r y 3 kWh p e r 100 kg b e e t ;

- steam r a t e o f t h e t u r b o - g e n e r a t o r 8 kg /kWh;

- on a v e r a g e , 37.4 % o f t h e h e a t i n g s team, i . e . 17.36 kg/100 kg b , d e l i v e r e d

138

f rom t h e t h r o t t l i n g - d e s u p e r h e a t i n g s t a t i o n .

L e t us a l s o o b s e r v e t h a t 62.6% o f t he h e a t i n g - s t e a m f l o w d e l i v e r e d v i a t h e

t u r b i n e makes i t p o s s i b l e t o g e n e r a t e 3.63 kWh p e r 100 kg b e e t , so t h e f a c t o r y

i s a b l e t o s e l l an e l e c t r i c i t y s u r p l u s o f t h e o r d e r o f 20% o f i t s own power

c o n s u m p t i o n .

3 .4 .2 Mechan i ca l v s . j e t - t y p e compresso rs

To b e g i n w i t h , l e t us c o n s i d e r a compress i on c i r c u i t emp loy ing an

e l e c t r i c a l l y - d r i v e n , s i n g l e - s t a g e mechan ica l c o m p r e s s o r . The v a p o u r c o m p r e s s i o n

p r o c e s s i s shown i n t he M o l l i e r d iag ram i n F i g . 3 .29 . S p e c i f i c power consumpt ion

( p e r 1 kg v a p o u r ) can be c a l c u l a t e d as

P 3 = ( h , 3 - h ^ l ) / { n , n ^ ) ( 3 . 1 0 )

where h^-j i s t h e e n t h a l p y o f f i r s t - e f f e c t v a p o u r , h^^ i s t h e f i n a l v a p o u r

e n t h a l p y i n t he i s e n t r o p i c compress i on p r o c e s s , i s t h e c o m p r e s s i o n e f f i c i e n c y

and i s t he mechan ica l e f f i c i e n c y .

Assuming = 0 .68 , = 0.95 and u s i n g t he pa ramete r v a l u e s l i s t e d i n t h e

p r e c e d i n g S e c t i o n ( e n t h a l p y v a l u e s a c c o r d i n g t o U . G r i g u l l ( E d . ) , P r o p e r t i e s o f

Water and Steam i n S l - U n i t s , 2nd e d n . , S p r i n g e r - V e r l a g , B e r l i n - H e i d e l b e r g - N e w

Y o r k , 1979) , we o b t a i n

Pg = (2764.1 - 2 7 1 4 . 4 ) / ( 0 . 6 8 - 0 . 9 5 ) = 76.9 k J / k g = 0.0214 kWh/kg

2800

Oí

1^2750 o JZ ·•-· c ÜJ

2700

6.95 7.00 7.05 7.10 715

Entropy ( k ^ i R g K ) )

F i g . 3 .29 . Compress ion o f f i r s t - e f f e c t v a p o u r i n a mechan ica l c o m p r e s s o r .

139

The e n t h a l p y o f t he compressed v a p o u r i s

h^^ = h^^ + ( h ^ ^ - h ^ ^ ) / n ^ = 2714.4 + (2764.1 - 2 7 1 4 . 4 ) / 0 . 6 8 = 2787.5 k J / k g

T h i s c o r r e s p o n d s t o a t e m p e r a t u r e o f 163.2°C, t h a t i s , 28.2 Κ above t h e

s a t u r a t i o n t e m p e r a t u r e . The compressed v a p o u r s h o u l d be d e s u p e r h e a t e d by

condensa te i n j e c t i o n and t h e n d i r e c t e d t o t h e h e a t i n g chamber o f t h e f i r s t

e v a p o r a t o r e f f e c t . Assuming t h e c o n d e n s a t e e n t h a l p y c o r r e s p o n d i n g t o t h e l i q u i d

s a t u r a t e d s t a t e a t t h e e x h a u s t - s t e a m p r e s s u r e , t h a t i s , h^ = 567.7 k J / k g , t h e

mass o f condensa te needed t o d e s u p e r h e a t 1 kg compressed v a p o u r can be

c a l c u l a t e d as

= ( h ^ c " • ^ ) ( 2 · ^ ^ '

where h^ i s t h e e n t h a l p y o f e x h a u s t steam (assumed t o be d r y s a t u r a t e d steam a t

3.13 ba r p r e s s u r e ) .

A f t e r i n s e r t i n g t h e e n t h a l p y v a l u e s , we o b t a i n

m^ = (2787.5 - 2 7 2 6 . 6 ) / ( 2 7 2 6 . 6 - 567.7) = 0.028 kg /kg

U s i n g t h e r e s u l t s o f t h e above c a l c u l a t i o n s , we can summarize t h e

consequences o f i n t r o d u c i n g a v a p o u r - c o m p r e s s i o n c i r c u i t based on a mechan ica l

compresso r as f o l l o w s .

( i ) H e a t i n g steam demand i s r e d u c e d by 10 · (1 + 0.028) = 10.28 kg/100 kg b , i . e .

by abou t 21.5% o f t h e i n i t i a l steam demand. T h i s makes i t p o s s i b l e t o c u t down

t h e l i v e steam f l o w t h r o u g h t h e t h r o t t l i n g - d e s u p e r h e a t i n g s t a t i o n t o l e s s t han

41% o f i t s i n i t i a l v a l u e .

( i i ) Power demand i s i n c r e a s e d by 10-0.0214 = 0.214 kWh/100 kg b , i . e . by abou t

7% o f t h e i n i t i a l v a l u e .

L e t us o b s e r v e t h a t t he power o u t p u t remains u n a f f e c t e d , t h i s i m p l y i n g t h a t

t h e r e i s a change i n t h e power b a l a n c e , namely a r e d u c t i o n o f t h e e l e c t r i c i t y

s u r p l u s w h i c h can be s o l d t o t h e e x t e r n a l g r i d .

A d i f f e r e n t r e s u l t i s o b t a i n e d when t h e e l e c t r i c motor d r i v i n g t h e compresso r

i s r e p l a c e d by a steam t u r b i n e . Assuming a steam r a t e o f abou t 12 kg/kWh w h i c h

i s t y p i c a l o f sma l l s i n g l e - s t a g e t u r b i n e s , 2.56 kg l i v e steam p e r 100 kg b e e t i s

needed t o d r i v e t h e c o m p r e s s o r , and t h e expanded steam f rom t h e t u r b i n e e x h a u s t

can be mixed w i t h t h e compressed v a p o u r . I n t h i s c a s e , a r e d u c t i o n o f t h e

h e a t i n g - s t e a m demand and a s i m u l t a n e o u s i n c r e a s e o f t h e l i v e - s t e a m demand

r e s u l t i n a n e t s a v i n g o f abou t 21.5% o f t h e i n i t i a l steam demand, w h i l e t h e

power o u t p u t and t h e power demand remain unchanged . I t i s t h e r e f o r e w e l l w o r t h

c o n s i d e r i n g a t u r b i n e - d r i v e n compresso r as an i n t e r e s t i n g a l t e r n a t i v e t o an

e l e c t r i c a l l y - d r i v e n o n e .

Recen t examples o f t h e a p p l i c a t i o n o f e l e c t r i c a l l y - d r i v e n c o m p r e s s o r s have

been d i s c u s s e d i n t h e l i t e r a t u r e ( r e f s . 6 1 - 6 4 ) . T u r b i n e - d r i v e n c o m p r e s s o r s a r e

known t o be a p p l i e d i n D a n i s h s u g a r f a c t o r i e s (whe re s e c o n d - e f f e c t v a p o u r i s

140

compressed , see S e c t i o n 3 . 4 . 3 ) .

The v a p o u r - c o m p r e s s i o n p r o c e s s can a l s o be pe r fo rmed u s i n g j e t - t y p e

c o m p r e s s o r s . T y p i c a l l y , m u l t i p l e compresso rs a r e i n s t a l l e d i n a s i n g l e

compress i on c i r c u i t f o r r e a s o n s o f f l o w c o n t r o l ( s e e S e c t i o n 3 . 4 . 4 ) . An

i d e a l i z e d compress ion p r o c e s s i n a j e t - t y p e compresso r s u p p l i e d w i t h l i v e steam

i s shown i n t h e M o l l i e r d iag ram i n F i g . 3 .30 . I n o r d e r t o c a l c u l a t e t h e steam

demand, f i n a l v a p o u r e n t h a l p y , e t c . , t h e f o l l o w i n g i n d i c e s c h a r a c t e r i z i n g t h e

i d e a l i z e d p r o c e s s must be known:

- compress i on r a t i o u ( s e e T a b l e 1 . 2 ) ,

3A00

3300

CT3200

>;3100 o £ C

ÜJ 3000

2900

2800

2700

2600 6.95 7.00 7.05 710 7.15

Entropy ( k j / (kg K))

F i g . 3 .30 . Compress ion o f f i r s t - e f f e c t v a p o u r i n a j e t - t y p e c o m p r e s s o r .

141

- e f f i c i e n c y o f t h e l i v e - s t e a m n o z z l e ,

- e f f i c i e n c y o f t h e m ixed -s team ( i . e . , compressed v a p o u r ) n o z z l e η ^ .

The l i v e - s t e a m demand can be d e t e r m i n e d as

= D / u ( 3 . 1 2 )

where i s t h e v a p o u r f l o w .

Assuming u = 2 .40 , we o b t a i n f o r t h e c o m p r e s s i o n c i r c u i t under c o n s i d e r a t i o n

D^ = 10/2.40 = 4.17 kg/100 kg b.

The e n t h a l p y o f t he l i v e s team, a f t e r e x p a n s i o n i n t h e n o z z l e t o t h e p r e s s u r e

o f f i r s t - e f f e c t v a p o u r , can be c a l c u l a t e d as

^ c = ^ " " Ns^ l ( ^ - ^ ^ ^

where h-j i s t he i n i t i a l l i v e - s t e a m e n t h a l p y , and h-j^ i s t h e f i n a l steam e n t h a l p y

i n t h e i s e n t r o p i c e x p a n s i o n p r o c e s s .

I n s e r t i n g e n t h a l p y v a l u e s and assuming n-j = 0 .90 , we o b t a i n

h^^ = 3333.9 - (3333.9 - 2 6 7 3 . 8 ) · 0 . 9 0 = 2739.0 k J / k g .

The e n t h a l p y b a l a n c e o f m i x i n g expanded steam w i t h f i r s t - e f f e c t v a p o u r can be

w r i t t e n as

D l ^ l e ' ^ ^ ^ = C l ^ \K ( 3 . 1 4 )

where h^ i s t h e m ixed -s team e n t h a l p y .

The m ixed -s team e n t h a l p y can t h u s be c a l c u l a t e d as

h^ = (4 .17 -2739 .8 + 1 0 - 2 7 1 4 . 4 ) / ( 4 . 1 7 + 10) = 2721.9 k J / k g .

From a s e p a r a t e e n t r o p y b a l a n c e , we can d e t e r m i n e t h e e n t r o p y o f m ixed steam as

s = 7.0852 k J / ( k g K ) , w h i c h c o r r e s p o n d s t o a s l i g h t l y s u p e r h e a t e d s t a t e . The

e n t h a l p y o f mixed steam a f t e r c o m p r e s s i o n i n t h e n o z z l e t o t h e e x h a u s t - s t e a m

p r e s s u r e can be c a l c u l a t e d as

V = ^ ^ ( ^ s - ( 3 . 1 5 )

where h^^ i s t h e f i n a l steam e n t h a l p y i n t h e i s e n t r o p i c c o m p r e s s i o n p r o c e s s .

I n s e r t i n g e n t h a l p y v a l u e s and assuming = 0 . 8 7 , we o b t a i n

h^^ = 2721.9 + (2772.0 - 2 7 2 1 . 9 ) / 0 . 8 7 = 2779.4 k J / k g .

T h i s c o r r e s p o n d s t o a t e m p e r a t u r e o f 159.4°C, t h a t i s , 24.4 Κ above t h e

s a t u r a t i o n t e m p e r a t u r e . Assuming t h a t m ixed steam i s d e s u p e r h e a t e d u s i n g

condensa te i n j e c t i o n , we can c a l c u l a t e t h e mass o f c o n d e n s a t e p e r 1 kg steam

f rom e q n . ( 3 . 1 1 )

m^ = (2779.4 - 2 7 2 6 . 6 ) / ( 2 7 2 6 . 6 - 567.7) = 0.024 kg /kg

The t o t a l f l o w o f s a t u r a t e d steam s u p p l i e d by t h e v a p o u r - c o m p r e s s i o n c i r c u i t

i s t h u s

s ^ ^ 1 ^ v ' ^ ^ "^c ' ^ ^ ^ ^ ^ ^ ^ ^ - ^ 2 4 ) = 14.52 kg/100 kg b.

We can now summarize t he r e s u l t s o f t h e i n t r o d u c t i o n o f j e t - t y p e c o m p r e s s o r s

142

t o t h e v a p o u r - c o m p r e s s i o n c i r c u i t as f o l l o w s .

( i ) The h e a t i n g - s t e a m demand i s r e d u c e d by 14.52 kg/100 kg b b u t t h e l i v e - s t e a m

demand i s i n c r e a s e d by 4.17 kg/100 kg b. The r e s u l t i n g n e t steam s a v i n g i s

10.35 kg/100 kg b , i . e . , abou t 21.6% o f t h e i n i t i a l steam demand.

( i i ) The l i v e - s t e a m f l o w t h r o u g h t h e t h r o t t l i n g - d e s u p e r h e a t i n g s t a t i o n can be

c u t down t o abou t 16% o f i t s i n i t i a l v a l u e ; h o w e v e r , an even l a r g e r l i v e - s t e a m

f l o w must be s u p p l i e d t o t h e c o m p r e s s o r s .

( i i i ) The power o u t p u t and t he power demand remain u n a f f e c t e d by v a p o u r

c o m p r e s s i o n .

Recen t examples o f t he a p p l i c a t i o n o f j e t - t y p e compresso rs can be found i n

t he l i t e r a t u r e ( r e f s . 6 5 , 6 6 ) . Howeve r , p r a c t i c a l r e s u l t s may d i f f e r

s u b s t a n t i a l l y f rom t h o s e i n d i c a t e d i n t h e above c a l c u l a t i o n , as e x p l a i n e d i n t h e

n e x t S e c t i o n .

3 .4 .3 S e l e c t i n g t he most s u i t a b l e compress i on t e c h n i q u e

A compar i son o f t he v a l u e s o f e s s e n t i a l q u a n t i t i e s c h a r a c t e r i z i n g t he e n e r g y

b a l a n c e s r e s u l t i n g f rom t h e a p p l i c a t i o n s o f t h e t h r e e s o l u t i o n s d e s c r i b e d i n t h e

p r e c e d i n g S e c t i o n i s shown i n T a b l e 3 .5 . As can be s e e n , t h e steam s a v i n g s a r e

a lmos t i d e n t i c a l . The j e t - t y p e c o m p r e s s o r s and t h e t u r b i n e - d r i v e n mechan i ca l

compresso r o f f e r t h e advan tage o f an unchanged power b a l a n c e , w h i l e t h e

e l e c t r i c a l l y - d r i v e n mechan ica l compresso r i n c r e a s e s t h e power demand. I t s h o u l d

be p o i n t e d o u t , h o w e v e r , t h a t t h i s c o n c l u s i o n h o l d s o n l y i f t h e steam s a v i n g

r e l a t i v e t o t h e o r i g i n a l t he rma l sys tem w i t h o u t v a p o u r c o m p r e s s i o n does n o t

exceed t h e l i v e - s t e a m f l o w o r i g i n a l l y d i r e c t e d t o t h e t h r o t t l i n g - d e s u p e r h e a t i n g

s t a t i o n . I n o r d e r t o demons t ra te how d i f f e r e n t r e s u l t s can be o b t a i n e d under

d i f f e r e n t c o n d i t i o n s , a n o t h e r example i s p r e s e n t e d b e l o w .

TABLE 3.5

Compar ison o f e n e r g y b a l a n c e s r e s u l t i n g f rom t h e a p p l i c a t i o n o f v a r i o u s v a p o u r -compress i on t e c h n i q u e s i n a s u g a r f a c t o r y c h a r a c t e r i z e d by an i n i t i a l steam demand o f 47.9 kg/100 kg b.

Q u a n t i t y W i t h o u t

Compressor

D imens ion v a p o u r e l e c t r i c a l l y - • t u r b i n e - j e t -c o m p r e s s i o n d r i v e n d r i v e n t y p e

(kg /100 kg b ) 47.90 37.62 37.62 37.55 (%) 100 78.5 78.5 78.4

(kWh/100 kg b ) 3.000 3.214 3.000 3.000 (%) 100 107 100 100

(kWh/100 kg b ) 3.630 3.630 3.630 3.630 (%) 100 100 100 100

(kWh/100 kg b ) 0.630 0.416 0.630 0.630 (%) 100 66 100 100

143

L e t us assume t h a t t h e i n i t i a l h e a t i n g - s t e a m demand o f t h e f a c t o r y under

c o n s i d e r a t i o n i s 36.4 kg/100 kg b and a l l t h e r e m a i n i n g pa rame te r s o f t h e e n e r g y

c o n v e r s i o n and d i s t r i b u t i o n p r o c e s s e s a r e t h e same as i n t h e f a c t o r y p r e v i o u s l y

c o n s i d e r e d . The a v e r a g e steam f l o w t h r o u g h t h e t h r o t t l i n g - d e s u p e r h e a t i n g s t a t i o n

i s 7.36 kg/100 kg b , i . e . 19.4% o f t h e t o t a l h e a t i n g - s t e a m f l o w . I f we now

c o n s i d e r t h e a p p l i c a t i o n s o f t h r e e v a p o u r - c o m p r e s s i o n c i r c u i t s a n a l o g o u s t o

t h o s e p r e v i o u s l y s p e c i f i e d , t hen t h e r e s u l t i n g r e l a t i v e m o d i f i c a t i o n s o f t h e

e n e r g y b a l a n c e t u r n o u t t o be e n t i r e l y d i f f e r e n t f rom t h o s e f ound i n t h e

p r e v i o u s c a s e . As t he r e d u c t i o n s o f t he h e a t i n g - s t e a m f l o w e x c e e d t h e l i v e - s t e a m

f l o w o r i g i n a l l y s u p p l i e d t o t he t h r o t t l i n g - d e s u p e r h e a t i n g s t a t i o n , t h e steam

f l o w t h r o u g h t h e t u r b i n e and t h e power o u t p u t d e c r e a s e as shown i n T a b l e 3 . 6 .

These e f f e c t s a r e most p ronounced i n t h e case o f j e t - t y p e c o m p r e s s o r s , c a u s i n g

t he s u g a r f a c t o r y t o become h e a v i l y dependen t on e l e c t r i c i t y s u p p l i e s f rom t h e

e x t e r n a l g r i d . When a p p l y i n g a mechan ica l c o m p r e s s o r , a smal l power s u r p l u s can

be e x p e c t e d i n t h e case o f an e l e c t r i c d r i v e , and a smal l power d e f i c i t i n t h e

case o f a t u r b i n e d r i v e .

TABLE 3.6

Compar ison o f e n e r g y b a l a n c e s r e s u l t i n g f rom t h e a p p l i c a t i o n o f v a r i o u s v a p o u r -c o m p r e s s i o n t e c h n i q u e s i n a s u g a r f a c t o r y c h a r a c t e r i z e d by an i n i t i a l steam demand o f 37.9 kg/100 kg b.

Q u a n t i t y W i t h o u t

Compressor

D imens ion v a p o u r e l e c t r i c a l l y - • t u r b i n e - j e t -c o m p r e s s i o n d r i v e n d r i v e n t y p e

(kg /100 kg b) 37.90 27.62 27.62 27.55 (%) 100 72.9 72.9 72.9

(kWh/100 kg b ) 3.000 3.214 3.000 3.000 (%) 100 100 100 100

(kWh/100 kg b ) 3.630 3.270 2.950 2.182 (%) 100 90 81 60

(kWh/100 kg b ) 0.630 0.056 -0 .050 -0 .818 (%) 100 9 -8 -130

I t s h o u l d be emphas ized t h a t T a b l e s 3.5 and 3.6 j u s t summarize t h e r e s u l t s o f

t h e a n a l y s e s c o n c e r n e d w i t h two p a r t i c u l a r s u g a r f a c t o r i e s e q u i p p e d w i t h

p a r t i c u l a r c o m p r e s s i o n c i r c u i t s . These d a t a c a n n o t be c o n s i d e r e d as s u f f i c i e n t

b a s i s f o r g e n e r a l c o n c l u s i o n s . I n r e a l - l i f e e n g i n e e r i n g p rob lems i t i s e s s e n t i a l

t o a c c o u n t f o r t h e i n f l u e n c e o f t h e t y p e o f c o m p r e s s i o n equ ipment on t h e

i n v e s t m e n t c o s t s , as w e l l as o t h e r f a c t o r s o f p r a c t i c a l i n t e r e s t . Depend ing on

t h e i n i t i a l steam-demand l e v e l , t h e r e q u i r e m e n t s imposed on t h e c o m p r e s s i o n

c i r c u i t can be d i f f e r e n t l y f o r m u l a t e d i n d i f f e r e n t f a c t o r i e s . The d e s i g n o f

a v a p o u r c o m p r e s s i o n c i r c u i t can t h u s be c o n s i d e r e d as a m u l t i - v a r i a b l e d e c i s i o n

144

p r o b l e m , i n w h i c h equ ipment t y p e and v a p o u r f l o w s can be o p t i m i z e d f o r t h e b e s t

o v e r a l l economic r e s u l t s .

I n t h i s c o n t e x t , l e t us men t ion t h e p o s s i b i l i t y o f t h e a p p l i c a t i o n o f v a p o u r

c o m p r e s s i o n o v e r two e v a p o r a t i o n s t a g e s . T h e r e a r e i m p o r t a n t p r a c t i c a l

c o n s i d e r a t i o n s w h i c h can j u s t i f y such a s o l u t i o n . I t i s n o t unusua l t h a t t h e

c o n c e n t r a t i o n o f o r g a n i c gaseous s u b s t a n c e s i n t h e f i r s t - e f f e c t v a p o u r i s so

h i g h t h a t when t h e v a p o u r i s compressed and r e c i r c u l a t e d t o f i r s t - e f f e c t

h e a t i n g , t h e q u a l i t y o f f i r s t - e f f e c t condensa te becomes a d v e r s e l y a f f e c t e d . I f

t h e pa rame te rs o f t h e condensa te do n o t s a t i s f y t he r q u i r e m e n t s o f f e e d - w a t e r

q u a l i t y , t h e n a p rob lem i s c r e a t e d w i t h p o s s i b l e a d v e r s e c o n s e q u e n c e s , as

d i s c u s s e d i n S e c t i o n 3 . 1 . 2 . I t i s t h e r e f o r e w o r t h n o t i n g t h a t t he c o n c e n t r a t i o n

o f o r g a n i c gaseous s u b s t a n c e s i n s e c o n d - e f f e c t v a p o u r i s u s u a l l y 3-4 t imes

s m a l l e r t han t h a t i n f i r s t - e f f e c t v a p o u r ( r e f . 6 5 ) .

L e t us r e t u r n f o r a moment t o T a b l e 3 . 4 , i n w h i c h t h e mass f l o w s o f v a p o u r s

f rom t h e e v a p o r a t o r and vacuum pans i n t h e m o d i f i e d the rma l sys tem a r e

p r e s e n t e d . I f we assume t h a t i n s t e a d o f f i r s t - e f f e c t v a p o u r , v a p o u r i s w i t h d r a w n

f rom t h e second e f f e c t t o t h e c o m p r e s s i o n c i r c u i t , t h e n t he r e q u i r e d mass f l o w

o f t h a t v a p o u r can be c a l c u l a t e d f rom t h e c o n s t r a i n t o f c o n s t a n t t h i c k - j u i c e

c o n c e n t r a t i o n . The r e s u l t i n g f l o w v a l u e i s shown i n T a b l e 3.4 i n b r a c k e t s ; i t i s

o n l y h a l f o f t h e e q u i v a l e n t f l o w o f f i r s t - e f f e c t v a p o u r . As a c o n s e q u e n c e , t h e

i n c r e a s e s i n t h e t o t a l v a p o u r f l o w f rom bo th t h e f i r s t and second e v a p o r a t o r

e f f e c t s a r e s m a l l e r t han t h e i n c r e a s e r e s u l t i n g f rom v a p o u r c o m p r e s s i o n o v e r one

e v a p o r a t i o n s t a g e . The o u t l e t c o n c e n t r a t i o n o f j u i c e i n t he f i r s t e f f e c t i s

l o w e r t han i n t he o t h e r s o l u t i o n , t h i s making i t p o s s i b l e t o m a i n t a i n a l a r g e r

v a l u e o f t h e o v e r a l l h e a t t r a n s f e r c o e f f i c i e n t i n t he f i r s t e f f e c t . I t may

t h e r e f o r e be e a s i e r , w i t h r e s p e c t t o t h e r e q u i r e d h e a t i n g - s u r f a c e a r e a i n t h e

f i r s t e v a p o r a t o r e f f e c t , t o implement v a p o u r c o m p r e s s i o n o v e r two s t a g e s . I t can

be added t h a t t h i s s o l u t i o n i s g e n e r a l l y a p p l i e d i n D a n i s h s u g a r f a c t o r i e s and

has a l s o been i n t r o d u c e d t o t h e Greek s u g a r i n d u s t r y .

3 .4 .4 C o n t r o l c o n s i d e r a t i o n s r e l a t i n g t o v a p o u r c o m p r e s s i o n

When s u p p l y i n g t he compressed v a p o u r f rom t h e f i r s t o r second e v a p o r a t o r

e f f e c t t o f i r s t - e f f e c t h e a t i n g , one has t o r eckon w i t h t he r i s k o f j u i c e

c a r r y o v e r c a u s i n g a t e m p o r a r y s u g a r p r e s e n c e i n t h e f i r s t - e f f e c t c o n d e n s a t e . I n

o r d e r t o manage emergency s i t u a t i o n s e f f e c t i v e l y , i t may be a d v i s a b l e t o i n s t a l l

two e v a p o r a t o r b o d i e s and two c o r r e s p o n d i n g c o n d e n s a t e t a n k s i n t h e f i r s t

e f f e c t . I f t h e c o m p r e s s e d - v a p o u r s t ream i s d i r e c t e d t o t h e h e a t i n g chamber o f

one b o d y , and t he o t h e r body i s hea ted w i t h pu re e x h a u s t s team, t h e n t h e r i s k o f

condensa te p o l l u t i o n i s l i m i t e d t o o n l y a p a r t o f t h e condensa te s t r e a m . An

example o f steam and v a p o u r c o n n e c t i o n s i n a v a p o u r c o m p r e s s i o n c i r c u i t

145

u t i l i z i n g t h i s i d e a i s shown i n F i g . 3 . 3 1 ( a ) .

A n o t h e r p o s s i b i l i t y f o r p r e v e n t i n g j u i c e d r o p l e t s f rom b e i n g c a r r i e d o v e r t o

f i r s t - e f f e c t condensa te i s t o w i t h d r a w t h e v a p o u r s t ream t o be c o m p r e s s e d , n o t

d i r e c t l y f rom t h e e f f e c t i n w h i c h v a p o u r i s p r o d u c e d b u t f rom t h e h e a t i n g

chamber o f t h e n e x t e f f e c t . T h i s makes i t p o s s i b l e t o wash t h e v a p o u r t o be

w i t h d r a w n w i t h w a t e r c o n d e n s i n g i n t he h e a t i n g chamber. A c c o r d i n g t o t h i s

p r i n c i p l e , t h e Dan i sh s u g a r f a c t o r i e s employ t h e w i t h d r a w a l o f s e c o n d - e f f e c t

v a p o u r v i a t he h e a t i n g chamber o f t he t h i r d e f f e c t , as shown s c h e m a t i c a l l y i n

F i g . 3 . 3 1 ( b ) .

( b )

F i g . 3 .31. Schemes o f steam and v a p o u r c o n n e c t i o n s p r e v e n t i n g t h e p o l l u t i o n o f f i r s t - e f f e c t c o n d e n s a t e by j u i c e c a r r y o v e r i n t h e compressed v a p o u r : ( a ) c o m p r e s s i o n o v e r one e v a p o r a t i o n s t a g e , ( b ) c o m p r e s s i o n o v e r two e v a p o r a t i o n s t a g e s . 1-3 - e v a p o r a t o r e f f e c t s , 4 - c o m p r e s s o r , 5 - t u r b i n e , 6 - e x h a u s t s team, 7 - l i v e s team, 8 - h e a t i n g v a p o u r t o t h e n e x t e f f e c t , 9 -n o n c o n d e n s a b l e s .

When i n t r o d u c i n g v a p o u r c o m p r e s s i o n t o a the rma l s y s t e m , i t i s d e s i r a b l e t o

e l i m i n a t e , o r a t l e a s t t o r e d u c e , t h e use o f t h e t h r o t t l i n g - d e s u p e r h e a t i n g

s t a t i o n i n c o n t r o l l i n g t h e sys tem t h r o u g h p u t . As r e g a r d s c o m p r e s s i o n c i r c u i t s

emp loy ing mechan ica l c o m p r e s s o r s , t h i s i s s i m p l y a q u e s t i o n o f s e l e c t i n g one o f

t he f l o w - c o n t r o l t e c h n i q u e s a p p l i c a b l e t o gas-pumping equ ipment ( s e e C h a p t e r 7 ) .

The c o n t r o l t e c h n i q u e s e l e c t e d may a f f e c t t h e e n e r g y l o s s e s o c c u r r i n g i n t h e

therma l sys tem under chang ing l o a d s . T h e r e a r e examples o f a p p l i c a t i o n o f t he

most e f f i c i e n t v a r i a b l e - s p e e d c o n t r o l ( r e f . 5 9 ) , and t h e l e s s e f f i c i e n t b y - p a s s

c o n t r o l ( w h i c h happens t o be employed i n t h e scheme shown i n F i g . 3 . 3 1 ( a ) ) .

The case o f j e t - t y p e compresso rs i s d i f f e r e n t , as t h e s e d e v i c e s a r e

e s s e n t i a l l y d e s i g n e d t o work a t d e f i n i t e v a p o u r f l o w s and t h e c o n t r o l ma rg i ns

a re v e r y n a r r o w . I f t he f l o w d e v i a t e s f rom i t s nominal v a l u e , t h e n t h e

e f f i c i e n c y o f a compresso r f a l l s o f f r a p i d l y . I n o r d e r t o make i t p o s s i b l e t o

v a r y t h e c o m p r e s s i o n - c i r c u i t l o a d w i t h o u t s e r i o u s l y r e d u c i n g c o m p r e s s i o n

146

e f f i c i e n c y , i t i s n e c e s s a r y t o a r r a n g e j e t - t y p e compresso rs o f d i f f e r e n t

t h r o u g h p u t s i n b a t t e r i e s , l i k e t h e one shown i n F i g . 3 .32 . A p o p u l a r s o l u t i o n

c o n s i s t s o f u s i n g f o u r c o m p r e s s o r s , t h e i r t h r o u g h p u t s c r e a t i n g a g e o m e t r i c

s e r i e s 1 :2 :4 :8 ( r e f . 6 5 ) . A c o n t r o l sys tem t a k e s c a r e o f t u r n i n g on compresso r

c o m b i n a t i o n s w i t h t o t a l t h r o u g h p u t s g r e a t e r t h a n , b u t c l o s e t o , t h e r e q u i r e d

v a p o u r - f l o w v a l u e s . U s i n g t h i s p r i n c i p l e , t h e a v e r a g e e f f i c i e n c y o f t h e b a t t e r y

a t v a r i a b l e l o a d s may be r e d u c e d o n l y i n s i g n i f i c a n t l y . I t may be added t h a t

t h e shap ing o f t h e compresso r b a t t e r y becomes l e s s i m p o r t a n t when a s t a b l e

e v a p o r a t o r l o a d i s e n s u r e d . I n two Dan i sh s u g a r f a c t o r i e s , v a p o u r c o m p r e s s i o n

c i r c u i t s a r e a p p l i e d w i t h t h e b a t t e r i e s c o m p r i s i n g f o u r and f i v e j e t - t y p e

c o m p r e s s o r s , t he t h r o u g h p u t s o f w h i c h c r e a t e t h e s e r i e s 1 : 1 . 9 4 : 1 . 9 4 : 4 . 5 and

1 : 1 . 0 7 : 2 . 1 4 : 2 . 6 : 4 . 8 4 , r e s p e c t i v e l y .

F i g . 3 .32 . Scheme o f a b a t t e r y o f j e t - t y p e c o m p r e s s o r s . 1 - l i v e s team, 2 - v a p o u r , 3 - compressed v a p o u r .

3 .4 .5 Compress ion o f vacuum-pan v a p o u r s

I t f o l l o w s f rom t h e p r e c e d i n g s e c t i o n s t h a t t h e a p p l i c a t i o n o f a v a p o u r

compress i on c i r c u i t i n an e x i s t i n g the rma l sys tem may n e c e s s i t a t e i n t r o d u c i n g

s u b s t a n t i a l changes i n t he d i s t r i b u t i o n o f v a p o u r s and c o n d e n s a t e s , as w e l l as

i n s t a l l i n g p o s s i b l e r e p l a c e m e n t s f o r some o f t h e e x i s t i n g e v a p o r a t o r b o d i e s .

Because o f l a c k o f space i n t h e e x i s t i n g b u i l d i n g s , o r f o r economic r e a s o n s ,

t h i s may sometimes be d i f f i c u l t t o a c c e p t . I n o r d e r t o w iden t h e c h o i c e o f

p o s s i b l e s o l u t i o n s , t h e c i r c u i t s compress ing vacuum-pan v a p o u r s have been

d e v e l o p e d and implemented i n a few European s u g a r f a c t o r i e s .

A c t u a l l y , t h i s i d e a had a l r e a d y been a p p l i e d f o u r decades ago i n t h e S w i s s

147

f a c t o r y a t A a r b e r g , u s i n g e l e c t r i c a l l y - d r i v e n mechan ica l c o m p r e s s o r s c o n n e c t e d

t o bo th t h e e v a p o r a t i o n and vacuum-pan s t a t i o n s ( r e f . 6 7 ) . The e n t i r e s t ream o f

vacuum-pan v a p o u r s i s compressed f rom 0.25 ba r t o 1.2 ba r p r e s s u r e and

r e c i r c u l a t e d t o pan h e a t i n g . M u l t i - s t a g e c o m p r e s s o r s a r e used f o r t h i s p u r p o s e ;

i n o r d e r t o m i n i m i z e t h e power c o n s u m p t i o n , i n t e r - s t a g e v a p o u r c o o l i n g by

condensa te i n j e c t i o n i s emp loyed . As a r e s u l t , 1 .14-1.16 kg h e a t i n g v a p o u r i s

o b t a i n e d f rom 1 kg vacuum-pan v a p o u r a t a power consumpt ion o f 0 .12 -0 .13 kWh p e r

1 kg vacuum-pan v a p o u r . The power consumpt ion o f t h e s i n g l e - s t a g e compresso rs

i n s t a l l e d i n t h e e v a p o r a t o r a r e a i s 0 .013-0.018 kWh p e r 1 kg v a p o u r c o m p r e s s e d .

As men t ioned i n S e c t i o n 1.5, h o w e v e r , t h e A a r b e r g s o l u t i o n was m o t i v a t e d by

a v e r y s p e c i a l e n e r g y p o l i c y i n w h i c h t h e a v a i l a b i l i t y o f cheap h y d r o e l e c t r i c

power p l a y e d a fundamenta l r o l e . T h e r e f o r e , i t can h a r d l y be r e g a r d e d as a model

e n e r g y sys tem t o be i m i t a t e d on a w i d e r b a s i s . A m o d i f i e d v e r s i o n o f t h e A a r b e r g

sys tem i n c o n n e c t i o n w i t h a f a c t o r y i n w h i c h t he power i s g e n e r a t e d bo th i n

a s t e a m - c y c l e - b a s e d power house and i n a g a s - t u r b i n e g e n e r a t i n g s e t has been

a n a l y s e d by Ba loh ( r e f . 6 8 ) .

I n t h e s o l u t i o n s w h i c h a r e now c o m e r c i a l l y o p e r a t e d i n two F r e n c h f a c t o r i e s

( a w h i t e - s u g a r f a c t o r y and a r e f i n e r y ) , a p a r t o f t h e vacuum-pan v a p o u r s i s

compressed and r e c i r c u l a t e d t o vacuum-pan h e a t i n g ( r e f s . 3 9 , 6 9 ) . L e t us n o t e

t h a t w h i l e t h e A a r b e r g f a c t o r y employs b a t c h vacuum p a n s , t h e F r e n c h

i n s t a l l a t i o n s a r e b u i l t a round c o n t i n u o u s vacuum p a n s . The u n d e r l y i n g i d e a i s t o

reduce t he demand f o r h e a t i n g v a p o u r s f rom t h e e v a p o r a t o r , t h u s making i t

p o s s i b l e t o c u t down t h e h e a t i n g - s t e a m demand. I t s h o u l d be p o i n t e d o u t t h a t

a r e d u c t i o n o f t h e w i t h d r a w a l o f v a p o u r s f rom t h e e v a p o r a t o r has t o be

compensated f o r , so as t o keep t he t h i c k - j u i c e c o n c e n t r a t i o n c o n s t a n t . F o r t h i s

r e a s o n , a r e a r r a n g e m e n t o f t h e v a p o u r - d i s t r i b u t i o n scheme, o r t h e a p p l i c a t i o n o f

a n o t h e r v a p o u r compress i on c i r c u i t i n t h e e v a p o r a t o r a r e a , o r a c o m b i n a t i o n o f

bo th measu res , may be n e e d e d . I f i t i s s u f f i c i e n t t o r e a r r a n g e t h e v a p o u r

d i s t r i b u t i o n o n l y , t h e n t h e n e c e s s a r y i n v e s t m e n t s i n t h e e v a p o r a t o r a r e a may be

l e s s e x t e n s i v e and e a s i e r t o p e r f o r m t han t h o s e n e c e s s i t a t e d by o t h e r v a p o u r

compress ion t e c h n i q u e s .

A d i s a d v a n t a g e o f s o l u t i o n s emp loy ing t h e c o m p r e s s i o n o f vacuum-pan v a p o u r s

i s t h e i r l a r g e power c o n s u m p t i o n . I f an e l e c t r i c a l l y - d r i v e n compresso r i s

a p p l i e d , t hen t h e hea t s a v i n g can be a t t a i n e d a t t h e c o s t o f a p o w e r - c o n s u m p t i o n

i n c r e a s e P^. I n a d d i t i o n , depend ing on t h e i n i t i a l l e v e l o f t h e steam

c o n s u m p t i o n , t h e hea t s a v i n g may be accompanied by a d e c r e a s e P ^ o f t h e power

o u t p u t . As t h e l i v e - s t e a m demand i s r e d u c e d , l e s s a i r w i l l be consumed i n t h e

b o i l e r s and l e s s was te h e a t w i l l be t r a n s p o r t e d i n b a r o m e t r i c w a t e r t o t h e

c o o l i n g t o w e r s , t h i s r e s u l t i n g i n a power-demand d e c r e a s e P ^ . The r e l a t i o n

between t he f u e l s a v i n g and t h e power b a l a n c e change Δ Ρ = P ^ + P ^ - P ^ i s

148

d e t e r m i n e d by t h e pa rame te rs o f t h e compress i on p r o c e s s , as w e l l as by t h e

t u r b i n e steam r a t e and b o i l e r e f f i c i e n c y .

When t h e steam f l o w t h r o u g h t h e t u r b i n e i s r e d u c e d by G ^ , t he r e s u l t i n g

d e c r e a s e o f t h e power o u t p u t i s

P, = G / S ( 3 . 1 6 )

where S i s t h e steam r a t e o f t h e t u r b i n e .

The r e d u c t i o n o f t he hea t demand by causes t he f u e l demand t o d e c r e a s e by

ß r = V(%Vb) ( 3 . 1 7 )

where i s t he h e a t i n g v a l u e o f t h e f u e l , i s t h e s t e a m - p i p i n g e f f i c i e n c y ,

and i s t h e b o i l e r e f f i c i e n c y .

TABLE 3.7

E n e r g y - b a l a n c e m o d i f i c a t i o n s r e s u l t i n g f rom the i m p l e m e n t a t i o n o f vacuum-pan v a p o u r compress ion ( u s i n g mechan ica l c o m p r e s s o r s ) i n two s u g a r f a c t o r i e s c h a r a c t e r i z e d by d i f f e r e n t t u r b i n e steam r a t e s and b o i l e r e f f i c i e n c i e s . E n t r i e s 4-12 a r e g i v e n pe r 1 kg vacuum-pan v a p o u r compressed .

No. Name D imens ion F a c t o r y

1 Steam r a t e , S 2 B o i l e r e f f i c i e n c y , 3 S t e a m - p i p i n g e f f i c i e n c y , η 4 R e d u c t i o n o f t h e steam ^

f l o w t h r o u g h t h e t u r b i n e , G^ 5 R e d u c t i o n o f t h e power o u t p u t , P^ 6 R e d u c t i o n o f t he power demand, P^ 7 Compressor power demand, P^ 8 Change o f t h e power b a l a n c e ,

Pc ^ Pr - Pd 9 Hea t s a v i n g i n t h e e v a p o r a t o r

10 Heat s a v i n g i n t h e t u r b i n e 11 O v e r a l l h e a t s a v i n g , 12 N o r m a l - f u e l s a v i n g , B^

I I I

kg/kWh 8.0 10.5 0.90 0.75 0.98 0.95

kg 1.2 1.2 kWh 0.150 0.114 kWh 0.020 0.020 kWh 0.179 0.179

kWh 0.309 0.273 kJ 2805 2805 kJ 570 502 k J 3375 3307 kg 0.131 0.158

I n T a b l e 3 . 7 , t he c a l c u l a t e d e n e r g y - b a l a n c e changes r e s u l t i n g f rom t h e

i m p l e m e n t a t i o n o f t h e c o m p r e s s i o n o f vacuum-pan v a p o u r s i n two s u g a r f a c t o r i e s

e q u i p p e d w i t h d i f f e r e n t b o i l e r s and t u r b i n e s a r e compared. O b v i o u s l y , an

economic g a i n can be a t t a i n e d o n l y i f t he v a l u e o f t he f u e l saved e x c e e d s t h e

v a l u e o f t h e e l e c t r i c e n e r g y p u r c h a s e d f rom t h e e x t e r n a l g r i d :

B^c^ > APCg ( 3 . 1 8 )

where c^ and c^ a r e t h e p r i c e s o f f u e l and powe r , r e s p e c t i v e l y .

I t s h o u l d be p o i n t e d o u t t h a t t h i s i s o n l y a n e c e s s a r y , b u t n o t a s u f f i c i e n t ,

c o n d i t i o n f o r t h e economic j u s t i f i c a t i o n o f vacuum-pan v a p o u r c o m p r e s s i o n , as

a s e r i o u s economic e v a l u a t i o n r e q u i r e s t h e i n v e s t m e n t s c o s t s t o be t aken i n t o

a c c o u n t .

149

The above i n e q u a l i t y can be t r a n s f o r m e d t o t he f o l l o w i n g fo rm

c^/Cg > Δ Ρ / Β ^ ( 3 . 1 9 )

I t can now be o b s e r v e d t h a t f a c t o r i e s e q u i p p e d w i t h l o w - e f f i c i e n c y b o i l e r s and

t u r b i n e s a r e c h a r a c t e r i z e d by s m a l l e r v a l u e s o f t h e r a t i o Δ Ρ / Β ^ , t h i s i m p l y i n g

t h a t t h e i n e q u a l i t y i s e a s i e r t o s a t i s f y . T h i s does n o t mean, h o w e v e r , t h a t an

o u t d a t e d power house c r e a t e s a b a s i s f o r t h e economic g a i n s f rom vacuum-pan

v a p o u r c o m p r e s s i o n . A c t u a l l y , i t may happen t h a t t h e i n v e s t m e n t s aimed a t

i n c r e a s i n g t h e power house e f f i c i e n c y w i l l , e c o n o m i c a l l y , be more e f f e c t i v e t han

t h o s e r e q u i r e d f o r imp lement ing a v a p o u r - c o m p r e s s i o n c i r c u i t .

I n s t e a d o f an e l e c t r i c a l l y - d r i v e n c o m p r e s s o r , j e t - t y p e compresso rs can be

a p p l i e d . U s i n g l i v e steam a t 38 ba r and 450°C, 2 . 5 - 3 . 0 kg steam a r e needed t o

compress 1 kg vacuum-pan v a p o u r f rom 0.25 t o 1.2 b a r . A f t e r i n j e c t i n g t h e

condensa te t o d e s u p e r h e a t t h e mixed s team, 4 . 1 - 4 . 7 kg s a t u r a t e d steam i s

o b t a i n e d p e r 1 kg vacuum-pan v a p o u r .

A v a p o u r c o m p r e s s i o n c i r c u i t o f t h i s k i n d , o p e r a t e d i n p a r a l l e l w i t h a

c i r c u i t i n w h i c h v a p o u r f rom t h e second e v a p o r a t o r e f f e c t i s c o m p r e s s e d , has

been p r o p o s e d i n t h e l i t e r a t u r e ( r e f . 5 8 ) . C o n s i d e r i n g j o i n t l y t h e f l o w s o f l i v e

steam s u p p l i e d t o bo th c o m p r e s s i o n c i r c u i t s , 5 . 5 - 7 . 0 kg l i v e steam p e r 1 kg

vacuum-pan v a p o u r s h o u l d be s u p p l i e d t o j e t - t y p e c o m p r e s s o r s . The r e d u c t i o n o f

t he n e t hea t demand i s e q u i v a l e n t t o abou t 1.65 kg steam p e r 1 kg vacuum-pan

v a p o u r compressed .

Due t o a r e l a t i v e l y l a r g e l i v e - s t e a m demand, t h e f i e l d o f p o t e n t i a l

a p p l i c a t i o n s o f t h i s t e c h n i q u e seems t o be l i m i t e d t o t h e m o d e r n i z a t i o n o f

f a c t o r i e s i n w h i c h a s u b s t a n t i a l p a r t o f t h e h e a t i n g - s t e a m f l o w must be s u p p l i e d

f rom t h e t h r o t t l i n g - d e s u p e r h e a t i n g s t a t i o n . Under such c o n d i t i o n , - l i v e steam can

be d i r e c t e d t o t h e compresso rs w i t h o u t a f f e c t i n g t h e power o u t p u t o f t h e power

h o u s e . On t h e o t h e r h a n d , i f t h e e n t i r e steam f l o w has o r i g i n a l l y been s u p p l i e d

v i a t he t u r b i n e , t h e n u s i n g l i v e steam i n a v a p o u r - c o m p r e s s i o n c i r c u i t r e s u l t s

i n a r e d u c t i o n o f t h e power o u t p u t o f abou t 0 . 4 - 0 . 5 kWh p e r 1 kg vacuum-pan

v a p o u r compressed . T h i s seems t o be a c c e p t a b l e o n l y i f s u f f i c i e n t l y cheap power

i s a v a i l a b l e f rom an e x t e r n a l e l e c t r i c g r i d .

A n o t h e r d i s a d v a n t a g e o f j e t - t y p e c o m p r e s s o r s i s t h e i r l i m i t e d f l e x i b i l i t y

under v a r i a b l e - l o a d c o n d i t i o n s . When used i n c o n n e c t i o n w i t h b a t c h vacuum p a n s ,

t h e compress i on c i r c u i t i s e x p e c t e d t o compensate f o r q u i c k changes o f h e a t i n g -

v a p o u r demand. T h i s r e q u i r e m e n t i s much e a s i e r t o s a t i s f y when a p p l y i n g a

mechan ica l c o m p r e s s o r . T h e r e a r e b e t t e r chances f o r a c o m p e t i t i v e p o s i t i o n o f

j e t - t y p e compresso rs i n f a c t o r i e s emp loy ing c o n t i n u o u s vacuum p a n s , e s p e c i a l l y

i f t he i n d i c e s g i v e n above c o u l d be improved by i n c r e a s i n g t h e c o m p r e s s i o n

r a t i o . T h i s m igh t be p o s s i b l e when r e p l a c i n g c o n v e n t i o n a l s i n g l e - n o z z l e d e v i c e s

by t he m u l t i p l e - n o z z l e , v a r i a b l e t h r o a t - a r e a d e s i g n ( " s t a t o - c o m p r e s s o r s " )

150

a c c o r d i n g t o a F r e n c h p a t e n t . A d i s c u s s i o n o f t h e a p p l i c a t i o n o f m u l t i p l e - n o z z l e

compresso rs i n a v a p o u r c o m p r e s s i o n c i r c u i t r e c i r c u l a t i n g v a p o u r f rom a

c o n t i n u o u s vacuum pan can be f ound i n t h e l i t e r a t u r e ( r e f . 7 0 ) . F o r a d e v i c e

u t i l i z i n g l i v e steam a t 24 ba r and 320°C and r a i s i n g t h e v a p o u r p r e s s u r e f rom

0.3 ba r t o 1.2 b a r , a c o m p r e s s i o n r a t i o o f 0.70 has been r e p o r t e d ( r e f . 7 1 ) .

T h i s v a l u e i s t w i c e t h a t a t t a i n a b l e i n a s i n g l e - n o z z l e c o m p r e s s o r .

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56 C . H . I v e r s o n , Mechan i ca l v a p o r - r e c o m p r e s s i o n - f a l l i n g f i l m e v a p o r a t i o n . Sugar J . , 44 (1 ) (1981) 15-20.

57 Κ. U r b a n i e c , S p r e z a n i e oparow w g o s p o d a r c e c i e p l n e j c u k r o w n i , G a z . C u k r o w . , 90 (9 ) (1982) 134-136.

58 K . E . A u s t m e y e r , B rüdenkompress ion i n d e r Z u c k e r i n d u s t r i e , Z u c k e r i n d . , 108(8) (1983) 715-728.

59 J . B o z e c , E v o l u t i o n de l a consommation t he rm ique dans Γ i n d u s t r i e s u c r i e r e , I n d . A l i m . A g r i e , 100(7 -8 ) (1983) 477-480.

60 Mechan ische B r ü d e n k o m p r e s s i o n , V D I - G e s e l 1 s c h a f t E n e r g i e t e c h n i k , D ü s s e l d o r f , 1987.

61 T . L u b i e n s k i , E r s t e S c h r i t t e i n d e r B r ü d e n k o m p r e s s i o n , Z u c k e r i n d . , 105(11) (1980) 1087-1088.

62 H. W e i d n e r , D i e B rüdenkompress ion i n e i n e r R o h z u c k e r f a b r i k , Z u c k e r i n d . , 108(8) (1983) 736-742.

63 U . J a c o b s e n , Der e i n s t u f i g e R a d i a l k o m p r e s s o r , Z u c k e r i n d . , 108(8) (1983) 742-746.

64 M. B u r t i n and J . - C . G i o r g i , Recompress ion de l a v a p e u r : l a s o l u t i o n o r i g i n a l e de l a s u c r e r i e de G u i g n i c o u r t , S u c r . F r . , 125(82) (1984) 117-121.

65 P. C h r i s t o d o u l o u , B e t r i e b s e r f a h r u n g e n m i t dem E i n s a t z e i n e r Wärmepumpe i n d e r V e r d a m p f S t a t i o n e i n e r Z u c k e r f a b r i k , Z u c k e r i n d . , 109(7) (1984) 628-634.

66 P .Ho f fman , O p t i m a l i z a c e e n e r g e t i c k e h o h o s p o d a r s t v i c u k r o v a r u L o v o s i c e , L i s t y C u k r . , 102(7) (1986) 155-161.

67 H . R . B r u n n e r , D i e Thermokompress ion i n der Z u c k e r f a b r i k + R a f f i n e r i e A a r b e r g A G , G e s c h i c h t e - E n t w i c k l u n g - A u s b l i c k , Z u c k e r i n d . , 108(8) (1983) 729-736.

68 T . B a l o h , S t u d i e e i n e r Z u c k e r f a b r i k m i t B r ü d e n k o m p r e s s i o n , Z u c k e r i n d . , 109(4) (1984) 285-294.

69 J . - C . G i o r g i , La r e c o m p r e s s i o n de v a p e u r de c u i t e , i n : P r o c . 17th C I T S , Copenhagen , 1983, p p . 279-290.

70 J . C u e l , Economies d ' e n e r g i e en r a f f i n e r i e p a r u t i l i z a t i o n e t / o u r e c o m p r e s s i o n de v a p e u r s i s s u e s d'une c u i t e c o n t i n u e a s s o c i e e a de m a l a x e u r s - c r i s t a l 1 i s e u r s Continus sous v i d e p r o f o n d , I n d . A l i m . A g r i e , 103(7-8 ) (1986) 669-675.

71 C . Longue E p e e , L e c t u r e p r e s e n t e d a t the I n t e r n a t i o n a l E x h i b i t i o n SVEKLOVODSTVO, K i e v , May 1986.

153

C h a p t e r 4

ENERGY SAVINGS BY PROCESS MODIFICATIONS

4.1 INTRODUCTION

I n s y s t e m a t i c a l l y e s t a b l i s h i n g ways t o r e d u c e t h e n e t h e a t demand, we

s e p a r a t e d measures aimed a t r e d u c i n g t h e h e a t demand o f t he i n d i v i d u a l p r o c e s s e s

f rom t h o s e f a c i l i t a t i n g an i n c r e a s e o f t h e e f f e c t i v e n e s s r a t i o o f t h e the rma l

s y s t e m . When b e g i n n i n g a d i s c u s s i o n o f e n e r g y - s a v i n g p r o c e s s m o d i f i c a t i o n s , one

m igh t pe rhaps e x p e c t t h a t o n l y measures b e l o n g i n g t o t h e f o r m e r g roup w o u l d be

c o n s i d e r e d . A c t u a l l y , t h e p rob lem i s more i n v o l v e d as t h e r e a r e examples o f

p r o c e s s e s t h a t have been i n t r o d u c e d f o r t h e s o l e p u r p o s e o f making the rma l

sys tem improvements p o s s i b l e . M o r e o v e r , a p r o c e s s may a f f e c t t h e e n e r g y demand

d i r e c t l y ( ow ing t o a d i r e c t r e l a t i o n s h i p between e n e r g y demand and p r o c e s s

p a r a m e t e r s ) o r i n d i r e c t l y ( ow ing t o t h e i n f l u e n c e on t h e pa rame te rs o f o t h e r

p r o c e s s e s ) . F i n a l l y , t he power demand o f a p r o c e s s may a l s o become an i m p o r t a n t

i s s u e .

I t i s sometimes d i f f i c u l t t o t e l l w h e t h e r a c e r t a i n e n e r g y s a v i n g can be

a t t r i b u t e d t o a p r o c e s s improvement o r t o deve lopmen ts i n equ ipment o r c o n t r o l

s y s t e m s . T h i s C h a p t e r p r e s e n t s a r e v i e w o f p rob lems i n w h i c h r e - t h i n k i n g o f

p r o c e s s r e q u i r e m e n t s p l a y s a p a r t i c u l a r l y i m p o r t a n t r o l e . O f c o u r s e , t h i s i s

j u s t a c o n v e n t i o n ; t h e n e x t two c h a p t e r s a r e d e v o t e d t o complementary p rob lems

where equ ipment and c o n t r o l sys tems come i n t o t h e f o r e g r o u n d .

The p r e s e n t a u t h o r i s aware o f t h e f a c t t h a t i t i s n o t e a s y t o c o v e r a b r o a d

p rob lem f i e l d i n w h i c h new deve lopmen ts a r e s t e a d i l y t a k i n g p l a c e . I t i s h o p e d ,

h o w e v e r , t h a t i d e n t i f i c a t i o n o f t h e main d i r e c t i o n s o f e f f o r t can have a l a s t i n g

v a l u e . I n t h i s c o n t e x t , c e r t a i n p u b l i c a t i o n s o r p u b l i c a t i o n s e r i e s d e s e r v e t o be

n o t e d , as t h e y p r o v i d e d i n v a l u a b l e h e l p i n t h e s t u d i e s o f w o r l d w i d e t r e n d s

( r e f s . 1 -3 ) . I n t h e f o l l o w i n g , t h e s u b j e c t i s s p l i t i n t o f o u r p a r t s :

- j u i c e p u r i f i c a t i o n ;

- s u g a r c r y s t a l l i z a t i o n ;

- u n c o n v e n t i o n a l p r o c e s s e s ;

- p u l p d e h y d r a t i o n .

I t m igh t be a rgued t h a t t h e c o n c e p t o f p r e s e n t i n g p r o c e s s e s as means t o

reduce e n e r g y demand i s t a k i n g t h i n g s t o o f a r ; a f t e r a l l , t h e s u g a r i n d u s t r y i s

n o t a f i e l d o f e n e r g y - s a v i n g c o n t e s t s . I t s h o u l d t h e r e f o r e be p o i n t e d o u t t h a t

t he p r i o r i t i e s o f d i f f e r e n t a s p e c t s o f f a c t o r y o p e r a t i o n have been d i s c u s s e d i n

C h a p t e r 1. Even i f t h e p r e s e n t C h a p t e r may be f o u n d p r o v o c a t i v e , i t i s hoped

t h a t i t can s t i m u l a t e u s e f u l i d e a s w h i c h w i l l e v e n t u a l l y f i n d t h e i r way i n t o

p r a c t i c e .

154

4 .2 J U I C E P U R I F I C A T I O N

4.2.1 I n f l u e n c e on t h e e n e r g y demand

Bee t s u g a r f a c t o r i e s a l l o v e r t h e w o r l d employ t h e method o f p u r i f i c a t i o n o f

raw j u i c e based on l ime and ca rbon d i o x i d e a d d i t i o n s . The aim o f j u i c e

p u r i f i c a t i o n i s t o remove nonsuga rs as f a r as p o s s i b l e , i n o r d e r t o p r o d u c e

c l e a r t h i n j u i c e w i t h h i g h p u r i t y and s t a b i l i t y p r e v e n t i n g q u a l i t y changes

d u r i n g e v a p o r a t i o n . Due t o v a r y i n g b e e t c o m p o s i t i o n and d i f f e r e n t methods o f

h a r v e s t i n g , t r a n s p o r t , s t o r a g e and e x t r a c t i o n , many v e r s i o n s o f t h e j u i c e

p u r i f i c a t i o n method a r e i n u s e . The v e r s i o n s may d i f f e r w i t h r e s p e c t t o t h e

d i s t r i b u t i o n o f l ime between i n d i v i d u a l p r o c e s s s t a g e s , t h e amount o f r e c y c l e d

j u i c e o r s l u d g e , t h e t e m p e r a t u r e s and r e s i d e n c e t imes c h a r a c t e r i z i n g v a r i o u s

p r o c e s s s t a g e s .

I n a r e f e r e n c e v e r s i o n o f t he c l a s s i c a l j u i c e p u r i f i c a t i o n method , t he t o t a l

r e q u i r e m e n t o f CaO amounts t o 80-95% o f t he c o n t e n t o f nonsuga rs i n raw j u i c e .

10-12% goes t o the p r e - 1 i m e r , 5-7% to t he j u i c e p r i o r t o second c a r b o n a t a t i o n

and t he rema inder i s added t o t he main l i m e r . The t e m p e r a t u r e s a r e 55-65°C i n

t h e p r e - l i m i n g , 85°C i n t h e main l i m i n g and f i r s t c a r b o n a t a t i o n , and 94°C i n

t h e second c a r b o n a t a t i o n ( r e f . 4 ) .

The pa rame te rs named above d e f i n e t h e therma l c h a r a c t e r i s t i c s o f t h e j u i c e

p u r i f i c a t i o n p r o c e s s . I t s i n f l u e n c e on t he t o t a l e n e r g y demand i n s u g a r

manu fac tu re i s more c o m p l i c a t e d t han j u s t d i r e c t l y c o n t r i b u t i n g t o t h e h e a t

demand. I n t h e f i r s t p l a c e , t h i n j u i c e q u a l i t y d e t e r m i n e s the r e q u i r e m e n t s on

t h e s u g a r c r y s t a l l i z a t i o n p r o c e s s , t h u s a f f e c t i n g t h e e n e r g y demand i n t h e s u g a r

house ( s e e S e c t i o n 4 . 3 ) . T h e n , t he t e m p e r a t u r e i n p r e - l i m i n g i s d e c i s i v e i n

w h e t h e r o r o r n o t i t i s p o s s i b l e t o u t i l i z e l o w - t e m p e r a t u r e hea t (vacuum-pan

v a p o u r s , l a s t - e f f e c t v a p o u r , c o n d e n s a t e ) i n raw j u i c e h e a t i n g . A c t u a l l y , t h i s

p o s s i b i l i t y depends a l s o on t h e t e m p e r a t u r e o f raw j u i c e and o t h e r f a c t o r s , so

one s h o u l d pe rhaps i n v e s t i g a t e i t by a n a l y s i n g a subsys tem c o m p r i s i n g e x t r a c t i o n

and j u i c e p u r i f i c a t i o n ( r e f . 5 ) . F o r examp le , i f l a r g e f l o w s o f h o t j u i c e o r

s u b s i d e r s l u d g e a r e r e c y c l e d t o t h e p r e - 1 i m e r f rom f i r s t o r second

c a r b o n a t a t i o n , t h e n l e s s h e a t i n g o f raw j u i c e i s n e e d e d . T h i s must be

compensated f o r by more hea t d e l i v e r e d t o o t h e r s t a g e s o f j u i c e h e a t i n g , where

t h e t e m p e r a t u r e s a r e t o o h i g h t o a l l o w t h e u t i l i z a t i o n o f l o w - t e m p e r a t u r e h e a t .

I t s h o u l d a l s o be o b s e r v e d t h a t l a r g e r e c y c l e s may r e q u i r e c o n s i d e r a b l e power

consumpt ion i n j u i c e pumping.

C o n c e r n i n g d i r e c t e n e r g y l o s s e s a s s o c i a t e d w i t h j u i c e p u r i f i c a t i o n ,

a q u a l i t a t i v e d i s c u s s i o n o f t h e i r r e d u c t i o n by p r o c e s s t e m p e r a t u r e changes has

been g i v e n i n S e c t i o n 1 .3 .3 . The main p a r t o f t h e s e l o s s e s , amount ing under

c e r t a i n c i r c u m s t a n c e s t o as much as 1/10 o f t h e n e t hea t demand o f t h e

f a c t o r y , o c c u r s i n t h e c a r b o n a t a t i o n s . I n t he f o l l o w i n g , t h e e n e r g y - s a v i n g

155

p o t e n t i a l a s s o c i a t e d w i t h t he c a r b o n a t a t i o n p r o c e s s i s d i s c u s s e d .

4 . 2 . 2 Heat b a l a n c e o f c a r b o n a t a t i o n

A t y p i c a l a r rangemen t o f t h e equ ipment a s s o c i a t e d w i t h t h e c a r b o n a t a t i o n

p r o c e s s i s shown i n F i g . 4 . 1 . Gas f rom t h e l ime k i l n , c o n t a i n i n g C O ^ , i s d e -

d u s t e d , washed and d e l i v e r e d t o a w a t e r - r i n g c o m p r e s s o r . I t s p r e s s u r e r a i s e d t o

1 .6-1 .8 b a r and a t a t e m p e r a t u r e abou t 35°C, t h e gas f l o w s t o two s e p a r a t e ,

a t m o s p h e r i c - p r e s s u r e c a r b o n a t a t i o n t a n k s , where i t i s b r o u g h t i n t o c o n t a c t w i t h :

- i n f i r s t c a r b o n a t a t i o n - j u i c e a f t e r main l i m i n g , d e l i v e r e d a t a b o u t 85°C,

- i n second c a r b o n a t a t i o n - j u i c e a f t e r f i r s t f i l t r a t i o n , d e l i v e r e d a t a b o u t

94°C.

I n bo th c a r b o n a t a t i o n t a n k s , mass and h e a t exchange t a k e s p l a c e between t h e

j u i c e and t h e c a r b o n a t a t i o n g a s . As t h e gas f l o w s f rom t h e b u b b l e r t o t h e j u i c e

s u r f a c e , i t s p r e s s u r e f a l l s t o a t m o s p h e r i c p r e s s u r e and t h e t e m p e r a t u r e

i n c r e a s e s , f i n a l l y a p p r o a c h i n g t h e j u i c e t e m p e r a t u r e . The gas a l s o becomes

s a t u r a t e d w i t h v a p o u r e v a p o r a t e d f rom j u i c e . I t s e n t h a l p y r a i s e d c o n s i d e r a b l y

above t h e i n l e t v a l u e , t h e s p e n t gas i s d i s c h a r g e d t o t h e a t m o s p h e r e , t h i s

c a u s i n g an e n e r g y l o s s .

LIME KILN J D E - D U S T E R

I water I

WASHER

5^

limed juice_ CARBONATATION I CARBONATATION I I

COMPRESSOR

clear juice_

F i g . 4 . 1 . Scheme o f t h e equ ipment a r rangemen t a s s o c i a t e d w i t h t h e c a r b o n a t a t i o n p r o c e s s .

The l o s s can be s t u d i e d u s i n g e q n . ( 2 . 6 ) . F o r t h e thermodynamic sys tem

c o m p r i s i n g a s i n g l e t ank shown i n F i g . 4 . 2 , t h e e n e r g y b a l a n c e i s

• ^ j i ^ j i ^ ^ ^ ^ ^ = Vj2^ V V ' E ( ' · ^ '

where G j ^ and G^-j a r e t h e mass f l o w s o f j u i c e and c a r b o n a t a t i o n gas ( u n d e r s t o o d

as d r y g a s ) , r e s p e c t i v e l y , a t i n l e t ; G^^ and G ^ ^ a r e t h e mass f l o w s o f t h e same

media a t o u t l e t ; h^^ and h^^ a r e t h e e n t h a l p i e s o f j u i c e a t i n l e t and o u t l e t ;

hg^ and h^^ a r e t h e e n t h a l p i e s o f c a r b o n a t a t i o n gas ( p e r 1 kg d r y g a s ) a t i n l e t

and o u t l e t ; Qp i s t he h e a t o f c a r b o n a t a t i o n r e a c t i o n , and i s t h e h e a t

d i s s i p a t e d t o t h e e n v i r o n m e n t by r a d i a t i v e and c o n v e c t i v e h e a t exchange ( b o t h

Qp and a r e e x p r e s s e d p e r u n i t t i m e ) .

The b a l a n c e e q u a t i o n can be r e w r i t t e n t o r e f l e c t t h e f a c t t h a t t h e e n e r g y l o s t

156

Gas Gg2 Gas Gg i

Juice out

Gj2 '^\2

J u i c e in

Gji.hj i

F i g . 4.2. C a r b o n a t a t i o n tank as a thermodynamic s y s t e m . F o r e x p l a n a t i o n o f t h e s y m b o l s , see t e x t .

by d i s c h a r g i n g s p e n t gas and by h e a t exchange w i t h t h e e n v i r o n m e n t has a c t u a l l y

been removed f rom t h e j u i c e

( 4 . 2 )

( 4 . 3 )

( 4 . 4 )

V g 2 - ^ g l ^ l ^ = ^ • i ^ ' j i - «j2^2 ^ \ The h e a t l o s s can t h u s be e x p r e s s e d e i t h e r as

\ - %2\2 - Λ^ " o r as

Q L = G j ^ h - T - G j 2 h j 2 ^ Q R

Now, l e t us o b s e r v e t h a t t h e h e a t o f r e a c t i o n c a n n o t be n e g l e c t e d i n e n e r g y

b a l a n c e s . I f we assume t h a t i t i s g e n e r a t e d a t 70 kJ /kmo l ( r e f . 6) and t h a t t h e

amount o f CaO i n v o l v e d i n b o t h c a r b o n a t a t i o n s i s 2 kg/100 kg b , t h e n we a r r i v e

a t t h e v a l u e = 2500 kJ /100 kg b , t h i s b e i n g e q u i v a l e n t t o a steam amount

abou t 1.1 kg/100 kg b.

The above e x p r e s s i o n s e n a b l e us t o e v a l u a t e t h e magn i tude o f t h e combined

h e a t l o s s f rom t h e c a r b o n a t a t i o n s . I n t h e r e f e r e n c e v e r s i o n o f t h e p r o c e s s

men t ioned i n t h e p r e c e d i n g S e c t i o n , a t 40% CO^ c o n t e n t , one needs a p p r o x i m a t e l y

3.4 k g / l O O kg b o f k i l n gas i n f i r s t c a r b o n a t a t i o n and 0.7 k g / l O O kg b i n second

c a r b o n a t a t i o n . Even a t t h e most advan tageous p r o c e s s p a r a m e t e r s , t h e combined

l o s s c a n n o t be e x p e c t e d t o be l o w e r t han 5000 kJ /100 kg b , o r a steam e q u i v a l e n t

157

o f abou t 2.2 kg/100 kg b. G e n e r a l l y , assuming t h a t as much gas i s d e l i v e r e d t o

t h e p r o c e s s as needed t o n e u t r a l i z e t h e a c t i v e CaO i n t h e j u i c e , t h e e n e r g y l o s s

depends on t h e f o l l o w i n g f a c t o r s :

- CaO r a t e ;

- j u i c e t e m p e r a t u r e ;

- CO^ c o n t e n t i n incoming k i l n g a s ;

- CO2 u t i l i z a t i o n .

A t low i n i t i a l CO2 c o n t e n t , s a y 30% and a t l i m i t e d CO2 u t i l i z a t i o n , t h e combined

c a r b o n a t a t i o n h e a t l o s s can r e a c h 10 000-12 000 kJ /100 kg b , o r a steam

e q u i v a l e n t o f 4 . 4 - 5 . 3 kg/100 kg b. I n modern b e e t s u g a r f a c t o r i e s c h a r a c t e r i z e d

by steam demand o f t h e o r d e r o f 30 kg steam p e r 100 kg b e e t , t h e c a r b o n a t a t i o n

h e a t l o s s t h u s becomes one o f t h e l a r g e s t i d e n t i f i a b l e components o f t h e n e t

hea t demand.

A number o f s o l u t i o n s have been p r o p o s e d t o r e d u c e t h e c a r b o n a t a t i o n h e a t

l o s s by c u t t i n g down gas f l o w and o u t l e t e n t h a l p y . W i t t e and Sch iweck ( r e f . 6)

d e s c r i b e a sys tem based on r e c y c l i n g s p e n t gas f rom second t o f i r s t

c a r b o n a t a t i o n . The f l o w o f k i l n gas t o f i r s t c a r b o n a t a t i o n can be r e d u c e d by

10%; t he r e s u l t i n g steam s a v i n g has been e s t i m a t e d a t 0.5 kg/100 kg b , t h a t i s ,

up t o 1/5 o f t h e c a r b o n a t a t i o n l o s s . The same r e f e r e n c e r e p o r t s r e s u l t s o f

e x p e r i m e n t s w i t h a r a w - j u i c e h e a t e r h e a t e d by s p e n t c a r b o n a t a t i o n g a s . No

e s t i m a t e o f t h e a t t a i n a b l e e n e r g y s a v i n g i s g i v e n ; t h e s a v i n g w o u l d p r o b a b l y be

d e t e r m i n e d by an economic t r a d e - o f f between t h e v a l u e o f e n e r g y s a v e d and t h e

h e a t e r c o s t .

A n o t h e r s o l u t i o n based on a s i m i l a r app roach has been r e p o r t e d by a B e l g i a n

team ( r e f . 7 ) . Spen t gas f rom second c a r b o n a t a t i o n i s b r o u g h t i n t o d i r e c t

c o n t a c t w i t h w a t e r ; t he r e c u p e r a t e d h e a t a b s o r b e d by t h e w a t e r i s t h e n used t o

p r e h e a t a i r d e l i v e r e d t o t h e b o i l e r s o r t o t h e s u g a r d r y e r . The i m p l e m e n t a t i o n

i n a s u g a r f a c t o r y r e s u l t e d i n an e s t i m a t e d e n e r g y s a v i n g o f t h e o r d e r o f 1 kg

steam pe r 100 kg b e e t . S i m i l a r r e s u l t s have been r e p o r t e d f rom F r e n c h s u g a r

f a c t o r i e s where w a t e r i s hea ted by s p e n t gases f rom bo th c a r b o n a t a t i o n s i n a

s p e c i a l c o n d e n s e r , t o w h i c h an e x h a u s t f a n f o r gas pumping i s c o n n e c t e d ( r e f . 8 ) .

I n F i g . 4 . 3 , t h e e s s e n c e o f t h e s o l u t i o n s r e v i e w e d above i s p r e s e n t e d i n

a s i m p l i f i e d Sankey d i a g r a m . T h e y can be c l a s s i f i e d as h e a t r e c u p e r a t i o n

t e c h n i q u e s n o t a f f e c t i n g t h e p r i n c i p l e o f t h e c a r b o n a t a t i o n p r o c e s s .

4 . 2 . 3 M o d i f i c a t i o n s o f c a r b o n a t a t i o n

C a r b o n a t a t i o n h e a t l o s s can a l s o be r e d u c e d by c h a n g i n g p r o c e s s p a r a m e t e r s

i n a way f a c i l i t a t i n g a r e d u c t i o n o f t h e d i f f e r e n c e Mg2hg2 " ' ^g l ' ^g l

( 4 . 3 ) . More s p e c i f i c a l l y , i t i s p o s s i b l e t o r e d u c e t h e gas e n t h a l p y i n c r e a s e , as

w e l l as t o improve CO^ u t i l i z a t i o n , t h u s c u t t i n g down t h e gas f l o w . Two

158

2

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to gas ) o h <

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1 energy recycled to the process

\ carbonatat ion loss

F i g . 4 . 3 . E n e r g y - f l o w p r i n c i p l e o f sys tems r e d u c i n g t h e c a r b o n a t a t i o n hea t l o s s by r e c u p e r a t i o n .

t e c h n i q u e s can be used f o r t h i s pu rpose ( r e f . 9 ) :

- h e a t i n g and h u m i d i f i c a t i o n o f t h e k i l n gas p r i o r t o c a r b o n a t a t i o n ;

- c a r b o n a t a t i o n a t i n c r e a s e d p r e s s u r e .

The p r i n c i p l e on w h i c h bo th methods a r e based can be e x p l a i n e d i n a g r a p h

showing c a r b o n a t a t i o n gas e n t h a l p y ( p e r 1 kg o f d r y g a s ) as a f u n c t i o n o f

t e m p e r a t u r e and p r e s s u r e ( F i g . 4 . 4 ) . The g r a p h has been c o n s t r u c t e d u s i n g t h e

r e l a t i o n s h i p between e n t h a l p y p e r 1 kg d r y gas and m o i s t u r e c o n t e n t an

t e m p e r a t u r e , and t h e r e l a t i o n s h i p between m o i s t u r e c o n t e n t , t o t a l p r e s s u r e and

1 2 Pressure ( b a r )

F i g . 4 . 4 . E n t h a l p y o f c a r b o n a t a t i o n gas v s . p r e s s u r e and t e m p e r a t u r e .

159

( 4 . 5 )

p a r t i a l p r e s s u r e o f steam

X = ( m g / m g ) P g / ( p - Pg)

where X i s t h e m o i s t u r e c o n t e n t i n kg /kg d r y g a s , m^ i s t h e mo la r w e i g h t o f

s team, m^ i s t h e a v e r a g e mo la r w e i g h t o f g a s , p^ i s t h e p a r t i a l p r e s s u r e o f

s team, and ρ i s t h e t o t a l p r e s s u r e .

I n t h e g r a p h , examples a r e shown o f t h e e n t h a l p y i n c r e a s e o f c a r b o n a t a t i o n

gas i n t h e c o n v e n t i o n a l p r o c e s s ( c a s e A) and o f gas w h i c h has been p r e l i m i n a r i l y

hea ted t o 70°C w i t h accompany ing h u m i d i f i c a t i o n , a t 1.3 ba r o u t l e t p r e s s u r e

( c a s e B ) . The e n t h a l p y i n c r e a s e i n case A i s 1830 k J / k g d r y g a s , and i n case Β

960 k J / k g d r y g a s . T a k i n g i n t o a c c o u n t t h a t case Β w o u l d a l s o i n v o l v e a

r e d u c t i o n o f t h e gas f l o w due t o b e t t e r CO^ u t i l i z a t i o n , t h e combined h e a t l o s s

can i n t h i s e x e m p l a r y case be abou t 50% o f t h a t i n t h e c o n v e n t i o n a l p r o c e s s .

The f i r s t i d e a i s gas h e a t i n g and h u m i d i f i c a t i o n a t t h e expense o f was te h e a t

f rom o t h e r s e c t i o n s o f t h e s u g a r m a n u f a c t u r e . Waste h e a t can be a v a i l a b l e i n h o t

condensa te e x t r a c t e d d i r e c t l y f rom t h e e v a p o r a t i o n s t a t i o n ( a t 95-100°C) o r i n

t he condensa te w h i c h has a l r e a d y passed j u i c e h e a t e r s ( a b o u t 75°C) . Mass and

hea t exchange t a k i n g p l a c e i n t h e c a r b o n a t a t i o n t ank between t h e h e a t e d ,

h u m i d i f i e d k i l n gas and t h e j u i c e t h e n r e s u l t s i n l e s s e v a p o r a t i o n f rom t h e

j u i c e and s m a l l e r j u i c e t e m p e r a t u r e d r o p . C o n s e q u e n t l y , even t hough t h e

t e m p e r a t u r e and h u m i d i t y o f t h e s p e n t gas a r e i d e n t i c a l t o t h o s e i n t h e

c o n v e n t i o n a l p r o c e s s , t h e hea t l o s s i s r e d u c e d by abou t 1 /3, i . e . a t l e a s t

0.8 kg steam p e r 100 kg b e e t . An improvement o f CO^ u t i l i z a t i o n o f t h e o r d e r

10-15% can a l s o be e x p e c t e d . T h i s method can be c l a s s i f i e d as a p r o c e s s

m o d i f i c a t i o n , and i t s w o r k i n g p r i n c i p l e i s shown i n a Sankey d iag ram i n

F i g . 4 . 5 ( a ) .

As r e g a r d s t he c o n f i g u r a t i o n o f t h e e q u i p m e n t , t h i s method r e q u i r e s add ing t o

(B)

CD ^ cr <

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COMPRESSOR

Ι Λ additional

i pumping power Ι

F i g . 4 . 5 . E n e r g y - f l o w p r i n c i p l e s o f t he m o d i f i e d c a r b o n a t a t i o n s : ( a ) w i t h gas h e a t i n g and h u m i d i f i c a t i o n , ( b ) a t i n c r e a s e d p r e s s u r e . 1 - e n e r g y o f k i l n g a s , 2 - hea t t r a n s f e r r e d f rom j u i c e t o g a s , 3 - e n e r g y o f s p e n t g a s .

160

t h e c a r b o n a t a t i o n tank a s c r u b b e r f i l l e d w i t h R a s c h i g r i n g s o r some o t h e r t y p e

o f p a c k i n g . T h e r e , gas f l o w i n g i n an upward d i r e c t i o n i s hea ted and h u m i d i f i e d

by c o n d e n s a t e d i s p e r s e d on t h e p a c k i n g s u r f a c e .

The second method i m p l i e s t h a t t h e p r o c e s s c a n n o t be pe r f o rmed i n a

c o n v e n t i o n a l a t m o s p h e r i c - p r e s s u r e c a r b o n a t a t i o n t a n k ; i n s t e a d , a p r e s s u r e v e s s e l

s h o u l d be a p p l i e d and t he gas pump s h o u l d be o p e r a t e d a t i n c r e a s e d p r e s s u r e . The

thermodynamic consequence o f i n c r e a s e d p r e s s u r e i s t h a t t h e p a r t i a l p r e s s u r e o f

s a t u r a t e d steam i n t h e s p e n t gas remains c o n s t a n t ( i t depends on j u i c e

t e m p e r a t u r e o n l y ) , so t h e f i n a l m o i s t u r e c o n t e n t p e r 1 kg d r y gas i s r e d u c e d .

T h i s r e s u l t s i n r e d u c e d f i n a l e n t h a l p y p e r 1 kg d r y g a s . C o n s e q u e n t l y , j u i c e

e v a p o r a t i o n and j u i c e t e m p e r a t u r e d r o p caused by mass and h e a t exchange between

t h e j u i c e and t h e gas a r e s m a l l e r t han i n t h e a t m o s p h e r i c - p r e s s u r e p r o c e s s .

The w o r k i n g p r i n c i p l e o f c a r b o n a t a t i o n a t i n c r e a s e d p r e s s u r e i s i l l u s t r a t e d

i n F i g . 4 . 5 ( b ) . A t a gas p r e s s u r e above t he j u i c e s u r f a c e i n t h e tank abou t

1.7 b a r , t h e o v e r a l l h e a t l o s s can be r e d u c e d by n e a r l y 1 /2 , i . e . a t l e a s t

1.2 kg steam p e r 100 kg b e e t . T h i s e f f e c t can be m a g n i f i e d i f t h e gas i s

p r e h e a t e d and h u m i d i f i e d b e f o r e i t e n t e r s t he c a r b o n a t a t i o n t a n k s .

Advan tageous the rma l e f f e c t s o f i n c r e a s e d c a r b o n a t a t i o n p r e s s u r e have been

v e r i f i e d i n l a b o r a t o r y - s c a l e e x p e r i m e n t s ( r e f . 1 0 ) . C o n c e r n i n g t h e combined

e f f e c t o f i n c r e a s e d p r e s s u r e and gas h e a t i n g and h u m i d i f i c a t i o n , some i n i t i a l

r e s u l t s i n d i c a t e t h a t i t may be r e a l i s t i c t o e x p e c t an improvement i n CO^

u t i l i z a t i o n by a f a c t o r o f 1 .2 , i . e . i n f i r s t c a r b o n a t a t i o n , f rom a b o u t 70% t o

84%. T h i s w o u l d c o n t r i b u t e t o a r e d u c t i o n o f t h e c a r b o n a t a t i o n h e a t l o s s by 2 / 3 ,

i . e . a t l e a s t 1.6 kg steam p e r 100 kg b e e t .

A l t h o u g h t h e the rma l e f f e c t s can be r e g a r d e d as e x p e r i m e n t a l l y v e r i f i e d , t h e

a p p l i c a t i o n p o t e n t i a l o f c a r b o n a t a t i o n a t i n c r e a s e d p r e s s u r e i s n o t o b v i o u s , as

i t depends on economic f a c t o r s . I n p a r a l l e l t o t h e g a i n r e s u l t i n g f rom f u e l

s a v i n g s , one has t o c o n s i d e r on t h e c o s t s i d e :

- i n c r e a s e d i n v e s t m e n t c o s t s o f gas c o m p r e s s o r s and c a r b o n a t a t i o n t a n k s ;

- i n c r e a s e d power consumpt ion i n gas c o m p r e s s o r s .

C o n s e q u e n t l y , t h e o v e r a l l economic r e s u l t i s v e r y much dependen t on season

l e n g t h , f u e l p r i c e , power c o s t and c a p i t a l c o s t ( r e f . 1 1 ) .

4 .3 SUGAR CRYSTALL IZAT ION

4.3.1 Scope o f t h e p rob lems

The e n e r g y demand o f t h e s u g a r house u s u a l l y c o r r e s p o n d s t o 50-70% o f t h e n e t

h e a t demand and 14-20% o f t h e t o t a l power demand o f a w h i t e - s u g a r f a c t o r y . I n

a f a c t o r y c h a r a c t e r i z e d by d e f i n i t e l e v e l s o f equ ipment q u a l i t y and p r o c e s s

a u t o m a t i o n , t h e e x a c t f i g u r e s depend on t h e l a y o u t and pa rame te r s o f t h e s u g a r

c r y s t a l l i z a t i o n p r o c e s s . ( L e t us r e c a l l t h a t i n t h e p r e s e n t book , we a r e m a i n l y

161

i n t e r e s t e d i n t h r e e - b o i l i n g c r y s t a l l i z a t i o n . )

T a k i n g i n t o a c c o u n t t h a t a l a r g e p a r t o f t h e f a c t o r y ' s h e a t demand i s

c o n c e r n e d , a p rob lem m igh t be posed o f a r r a n g i n g t h e c r y s t a l l i z a t i o n p r o c e s s so

as t o m i n i m i z e t h e hea t demand o f t h e s u g a r h o u s e . O n l y s i m p l i f i e d p rob lem

f o r m u l a t i o n s can be a t temp ted and o n l y a p p r o x i m a t e s o l u t i o n s s o u g h t , h o w e v e r ,

because t h e r e i s a m u l t i t u d e o f p r o c e s s c o n s t r a i n t s a s s o c i a t e d w i t h s u g a r o u t p u t

and s u g a r q u a l i t y . A c t u a l l y , one m igh t even n o t e t h a t t h e c r y s t a l l i z a t i o n

p r o c e s s i s v e r y much dependen t on t h e q u a l i t y o f j u i c e s e n t e r i n g t h e s u g a r

h o u s e . T h i s w o u l d i m p l y t h a t i n o r d e r t o a r r a n g e t h e c r y s t a l l i z a t i o n p r o c e s s

o p t i m a l l y , t he e n t i r e f a c t o r y s h o u l d be c o n s i d e r e d . N e e d l e s s t o s a y , such an

approach wou ld n o t be v e r y p r a c t i c a l i f one had t o c o n c e n t r a t e on t h e e n e r g y

economy.

L e t us o b s e r v e t h a t t h e p rob lem becomes even more complex i f we a l l o w f o r

p o s s i b l e m o d i f i c a t i o n s o f t h e equ ipmen t , m a i n l y vacuum p a n s , as w e l l as

m o d i f i c a t i o n s o f t h e a u t o m a t i c c o n t r o l s y s t e m s . F o r t h e sake o f s i m p l i c i t y o f

p r e s e n t a t i o n , t h e s e q u e s t i o n s a r e c o n s i d e r e d s e p a r a t e l y i n C h a p t e r s 5 and 6. I t

s h o u l d n e v e r t h e l e s s be a d m i t t e d t h a t s a v i n g e n e r g y by r a t i o n a l i z i n g t h e s u g a r

c r y s t a l l i z a t i o n p r o c e s s i m p l i e s t h e n e c e s s i t y o f t o u c h i n g t h e most i n t r i c a t e

p rob lems o f s u g a r t e c h n o l o g y .

T h r e e i m p o r t a n t e n e r g y - s a v i n g c o n c e p t s a r e d i s c u s s e d i n t h e p r e s e n t S e c t i o n :

- t h e Dan i sh c r y s t a l l i z a t i o n scheme;

- c o o l i n g c r y s t a l l i z a t i o n ;

- c r y s t a l f o o t i n g t e c h n i q u e s .

4 . 3 . 2 Dan i sh c r y s t a l l i z a t i o n scheme

The e s s e n t i a l i d e a o f t h e Dan i sh c r y s t a l l i z a t i o n scheme i s t o e n s u r e t h a t

t he q u a l i t y o f Β p r o d u c t i s h i g h enough t o mix i t w i t h A p r o d u c t , and t o t r e a t

t h e m i x t u r e as w h i t e s u g a r . T h i s r e d u c e s t h e combined m a s s e c u i t e c i r c u l a t i o n

c o n s i d e r a b l y , b r i n g i n g abou t a r e d u c t i o n o f t h e t o t a l h e a t e x p e n d i t u r e i n s u g a r

b o i l i n g . As a f i r s t a p p r o x i m a t i o n , t h e h e a t s a v i n g can be e s t i m a t e d as t h e h e a t

r e q u i r e d t o e v a p o r a t e t h e amount o f w a t e r t h e o r e t i c a l l y needed t o d i s s o l v e

Β s u g a r , p l u s t he amount o f wash w a t e r needed when c e n t r i f u g i n g t h i s s u g a r as

A s u g a r . When compared t o a c o n v e n t i o n a l t h r e e - b o i l i n g scheme i n w h i c h t h i c k

j u i c e a t 72% DS i s used t o d i s s o l v e Β s u g a r , t h e steam s a v i n g i s o f t h e o r d e r o f

9-13% o f t h e w h i t e s u g a r o u t p u t , o r - u s i n g v a l u e s c h a r a c t e r i s t i c o f D a n i s h

f a c t o r i e s - abou t 1 .2-1 .7 kg/100 kg b. I f t h e c o n v e n t i o n a l scheme employs

d i s s o l v i n g Β s u g a r i n w a t e r , t h e n t h e s a v i n g amounts t o 12-14% o f t h e w h i t e

s u g a r o u t p u t , o r abou t 1 .6-1 .9 kg steam p e r 100 kg b e e t .

The s u g a r house scheme a p p l i e d i n DOS s u g a r f a c t o r i e s and t h e scheme o f s u g a r

f l o w t h r o u g h c r y s t a l l i z a t i o n s t a g e s a r e shown i n F i g . 4 . 6 . The main q u a l i t y

162

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r e q u i r e m e n t c o n c e r n e d w i t h Β s u g a r i s i t s low c o l o u r . I t i s known t h a t i n a

number o f c o u n t r i e s , t h e a t t e m p t s t o implement t h e D a n i s h scheme f a i l e d because

t h i s r e q u i r e m e n t c o u l d n o t be s a t i s f i e d r e l i a b l y . T h i s i s u n d e r s t a n d a b l e , as t h e

c o n d i t i o n s f o r a r e l i a b l e o p e r a t i o n o f t h e D a n i s h scheme - i n t e n d e d m a i n l y t o

p r e v e n t c o l o u r b u i l d - u p - have been s p e c i f i e d as f o l l o w s ( r e f . 1 2 ) .

( i ) I n t h e j u i c e p u r i f i c a t i o n , c o l o u r i n g m a t t e r e x h i b i t i n g s p e c i a l a f f i n i t y t o

s u g a r s h o u l d be e l i m i n a t e d . T h i s r e q u i r e s t h e p o l y m e r i z a t i o n o f p h e n o l i c

compounds so t h a t t h e y can be removed w i t h t h e s l u d g e , as w e l l as d e s t r o y i n g

i n v e r t s u g a r . I n p r a c t i c a l t e r m s , an a d e q u a t e l y l o n g j u i c e r e t e n t i o n t ime s h o u l d

be e n s u r e d a t an a l k a l i n i t y l e v e l e x c e e d i n g 0 . 4 - 0 . 5 g C a O / l O O ml ( w h i c h

c o r r e s p o n d s t o main l i m i n g ) .

( i i ) F i r s t f i l t r a t i o n must be e f f e c t i v e enough t o keep t h e s l u d g e c o n t e n t i n

t he f i l t r a t e be low 20 ppm.

( i i i ) S u l p h i t a t i o n o f t h i n j u i c e s h o u l d e n s u r e a pH v a l u e o f abou t 8.7 a t 20°C.

( i v ) I n t h e s u g a r h o u s e , pH v a l u e s s h o u l d be k e p t l o w e r t han 8 . 5 - 9 . 0 .

( v ) No u n n e c e s s a r y r e c y c l e s o f n o n s u g a r s and c o l o u r i n g m a t t e r s h o u l d be

t o l e r a t e d i n t h e s u g a r h o u s e .

( v i ) H igh c r y s t a l q u a l i t y s h o u l d be e n s u r e d i n b o i l i n g o f C s u g a r .

( v i i ) H i g h - p r e c i s i o n c e n t r i f u g i n g o f m a s s e c u i t e s i s r e q u i r e d .

P r a c t i c a l e x p e r i e n c e p r o v e s t h a t c o n d i t i o n s ( i v ) - ( v i i ) can be r e l i a b l y met

o n l y i f t h e s u g a r house o p e r a t i o n i s v e r y w e l l s t a b i l i z e d w i t h r e s p e c t t o t h e

q u a l i t y o f b o i l i n g and t he t e m p e r a t u r e s o f m a s s e c u i t e s b e f o r e c e n t r i f u g i n g . I n

s i m p l e t e r m s , i t can be c o n c l u d e d t h a t t h e D a n i s h c r y s t a l l i z a t i o n scheme i s

p r a c t i c a b l e o n l y i n w e l l e q u i p p e d , h i g h l y au toma ted , c a r e f u l l y m a i n t a i n e d and

s k i l f u l l y o p e r a t e d s u g a r f a c t o r i e s , and a s u f f i c i e n t l y h i g h b e e t q u a l i t y seems

t o be a p r e r e q u i s i t e . The q u a l i t y r e q u i r e m e n t s s h o u l d be u n d e r s t o o d t o a p p l y t o

b e e t s a t t h e e n t r a n c e t o t h e s l i c i n g s t a t i o n , t h a t i s , w i t h t h e consequences o f

b e e t d e t e r i o r a t i o n d u r i n g t r a n s p o r t and s t o r a g e t a k e n i n t o a c c o u n t . Among t h e

q u a l i t y pa rame te rs c o n c e r n e d , t h e amino-N c o n t e n t seems t o be o f c o n s i d e r a b l e

i m p o r t a n c e . I n Denmark, i t i s kep t be low 100 mg Ν p e r 100 g s u g a r as p r a c t i c a l

e x p e r i e n c e has shown t h a t h i g h e r amino-N c o n t e n t s make i t d i f f i c u l t t o o b t a i n

a s u f f i c i e n t l y h i g h p u r i t y o f t h i c k j u i c e ( r e f . 1 3 ) .

4 . 3 . 3 C o o l i n g c r y s t a l l i z a t i o n

The c o o l i n g c r y s t a l l i z a t i o n i s n o t a new i d e a , as i t i s g e n e r a l l y a p p l i e d i n

C m a s s e c u i t e c r y s t a l l i z a t i o n and i t has a l s o been p r a c t i s e d i n t he

c r y s t a l l i z a t i o n o f h i g h - p u r i t y m a s s e c u i t e s i n t h e cane s u g a r i n d u s t r y . W h i l e t h e

e v a p o r a t i n g c r y s t a l l i z a t i o n employs t h e e v a p o r a t i o n o f w a t e r f o r t h e

s u p e r s a t u r a t i o n c o n t r o l , t h e c o o l i n g c r y s t a l l i z a t i o n r e l i e s on a r e d u c t i o n o f

t he s o l u b i l i t y o f s u c r o s e i n w a t e r w i t h d e c r e a s i n g t e m p e r a t u r e . T h i s phenomenon

164

i s i l l u s t r a t e d i n a d iag ram i n F i g . 4 .7 ( a f t e r r e f . 1 4 ) . I t has been e s t i m a t e d

t h a t f o r h i g h - p u r i t y m a s s e c u i t e s , a t e m p e r a t u r e d e c r e a s e o f 2 Κ makes i t

p o s s i b l e t o i n c r e a s e t h e c r y s t a l y i e l d by 1% ( r e f . 1 5 ) .

4.0 Γ

3.5μ

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3 .ομ

2.5k

2 . 0 L - ^ AO 50 60 70 80

Temperature (**C)

F i g . 4 . 7 . S o l u b i l i t y o f s u c r o s e i n w a t e r as a f u n c t i o n o f t e m p e r a t u r e ( a f t e r r e f . 1 4 ) .

The m a s s e c u i t e can be c o o l e d i n an a t m o s p h e r i c - p r e s s u r e w a t e r - c o o l e d

c r y s t a l 1 i z e r . An equ ipment c o n f i g u r a t i o n making i t p o s s i b l e t o combine

e v a p o r a t i n g c r y s t a l l i z a t i o n and " p u r e - c o o l i n g " c r y s t a l l i z a t i o n i n t o one

c r y s t a l l i z a t i o n s t a g e i s s c h e m a t i c a l l y shown i n F i g . 4 .8 ( a f t e r r e f . 1 6 ) . The

m a s s e c u i t e i s d i s c h a r g e d f rom vacuum pans t o c o n v e n t i o n a l m i x e r s , f rom w h i c h i t

i s d i r e c t e d t o c o o l i n g c r y s t a l 1 i z e r s where an adequate r e t e n t i o n t ime - up t o

4-5 h f o r A , and 6-7 h f o r Β m a s s e c u i t e - must be e n s u r e d . As t h e m a s s e c u i t e i s

cooling water

VACUUM PANS

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COOLING CRYSTALLIZERS

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F i g . 4 . 8 . Scheme o f a c r y s t a l l i z a t i o n s t a g e emp loy ing e v a p o r a t i n g c r y s t a l l i z a t i o n and " p u r e - c o o l i n g " c r y s t a l l i z a t i o n .

165

c o o l e d down f rom t h e i n i t i a l t e m p e r a t u r e o f 70°C t o a b o u t 40-50°C, t h e c r y s t a l

c o n t e n t can be s u b s t a n i t a l l y i n c r e a s e d , i t s a t t a i n a b l e f i n a l v a l u e depend ing on

t h e DS c o n t e n t o f t h e r e c i r c u l a t e d s y r u p . F o r a c r y s t a l 1 i z e r d e s i g n known as

a c o o l i n g - c r y s t a l l i z a t i o n t o w e r t e s t e d i n t h e s u g a r i n d u s t r y i n FRG, t h e

a t t a i n a b l e r e l a t i v e i n c r e a s e o f t h e c r y s t a l c o n t e n t i s shown as a f u n c t i o n o f

f i n a l m a s s e c u i t e t e m p e r a t u r e and s y r u p DS c o n t e n t i n F i g . 4 . 9 . H o w e v e r , t h e

p o t e n t i a l f o r improvement o f c r y s t a l y i e l d i n d i c a t e d i n t h i s d iag ram t u r n s o u t

t o be d i f f i c u l t t o a c h i e v e owing t o t h e p rob lems o f m a i n t a i n i n g a s u f f i c i e n t l y

i n t e n s i v e hea t t r a n s f e r between t h e m a s s e c u i t e and t h e c o o l i n g s u r f a c e s .

0.50 Γ

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0 - L 70 60 50 40 30 F ind massecuite temperature {°C)

F i g . 4 . 9 . R e l a t i v e i n c r e a s e o f t h e c r y s t a l c o n t e n t i n t h e c o o l i n g -c r y s t a l l i z a t i o n t o w e r v s . f i n a l m a s s e c u i t e t e m p e r a t u r e and DS c o n t e n t o f r e c i r c u l a t e d s y r u p ( a f t e r r e f . 1 7 ) .

A n o t h e r method o f c o o l i n g c r y s t a l l i z a t i o n , i n v e n t e d i n F r a n c e , employs a

vacuum c r y s t a l 1 i z e r i n w h i c h m a s s e c u i t e i s b o i l i n g as i t expands t o a p r e s s u r e

as low as abou t 0.09 b a r . The c r y s t a l 1 i z e r i s fo rmed as a h o r i z o n t a l c y l i n d r i c a l

v e s s e l e q u i p p e d w i t h a r i b b o n s t i r r e r , and t h e r e q u i r e d r e t e n t i o n t ime i s a b o u t

1 h . A p o s s i b l e equ ipment c o n f i g u r a t i o n i s shown s c h e m a t i c a l l y i n F i g . 4 . 1 0 . The

f l o w o f r e c i r c u l a t e d s y r u p (80-82% DS) i s a b o u t h a l f o f t h e t o t a l s y r u p f l o w .

P r a c t i c a l v a l u e s o f t h e r e l a t i v e c r y s t a l - y i e l d i n c r e a s e a r e abou t 0 .25 -0 .30

( i . e . , t he c r y s t a l c o n t e n t o f t h e m a s s e c u i t e i s i n c r e a s e d by a f a c t o r o f 1.25-

1 .30 ) .

As t h e c o o l i n g c r y s t a l l i z a t i o n makes i t p o s s i b l e t o i n c r e a s e t h e c r y s t a l

y i e l d i n a c r y s t a l l i z a t i o n s t a g e w i t h o u t a d d i t i o n a l h e a t e x p e n d i t u r e , i t can be

used t o r e d u c e t h e hea t demand o f t h e s u g a r h o u s e . The a t t a i n a b l e s a v i n g s depend

on t h e c o o l i n g method a p p l i e d and t h e l a y o u t o f t h e c r y s t a l l i z a t i o n scheme.

T a k i n g a c o n v e n t i o n a l t h r e e - b o i l i n g scheme w i t h t h i c k - j u i c e c o n c e n t r a t i o n o f

75% DS as a b a s i s f o r c o m p a r i s o n s , i t can be c o n c l u d e d t h a t t h e same w h i t e - s u g a r

166

80°C CONTINUOUS VACUUM PAN

massecuite

O.IAbar 68-70°C

0.09 bar 55-60°C

VACUUM CRYSTALLIZER1

VACUUM CRYSTALLIZER 2

CENTRIFUGALS

to the next crystal l izat ion

stage

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F i g . 4 . 1 0 . Scheme o f a c r y s t a l l i z a t i o n s t a g e emp loy ing e v a p o r a t i n g c r y s t a l l i z a t i o n and c o o l i n g c r y s t a l l i z a t i o n under vacuum ( a f t e r r e f . 1 8 ) .

o u t p u t can be o b t a i n e d u s i n g t w o - b o i l i n g schemes f e a t u r i n g combined e v a p o r a t i o n -

and c o o l i n g - c r y s t a l l i z a t i o n o f t h e w h i t e s u g a r . I n t h e case o f " p u r e - c o o l i n g "

c r y s t a l l i z a t i o n , assuming a c r y s t a l - y i e l d i n c r e a s e abou t 0 .32 , t h e t h e o r e t i c a l

hea t demand ( c a l c u l a t e d f rom t h e r e q u i r e d e v a p o r a t i o n ) o f t h e s u g a r house i s

r e d u c e d by 48%. I f vacuum c r y s t a l l i z a t i o n i s employed and t h e c r y s t a l y i e l d i n

w h i t e - s u g a r c r y s t a l l i z a t i o n i n c r e a s e s by 0 .33 , t h e r e s u l t i n g r e d u c t i o n o f t h e

t h e o r e t i c a l hea t demand i s n e a r l y 53% ( r e f . 1 6 ) .

The e n e r g y - s a v i n g p o t e n t i a l o f c o o l i n g c r y s t a l l i z a t i o n has y e t t o be

i n v e s t i g a t e d . Up t o now, t h e deve lopmen t i n t h i s a r e a has been s t i m u l a t e d m a i n l y

by t h e i n d u s t r y ' s i n t e r e s t i n i m p r o v i n g s u g a r q u a l i t y . The u n d e r l y i n g i d e a i s t o

t ake advan tage o f bo th t h e n e g l i g i b l e c o l o u r i n c r e a s e c h a r a c t e r i s t i c o f t h e

c o o l i n g c r y s t a l l i z a t i o n , and t h e r e d u c t i o n o f t h e amount o f s y r u p s accompany ing

t he c r y s t a l - y i e l d i n c r e a s e . The l a t t e r f a c t o r makes i t p o s s i b l e t o r e d u c e s y r u p

r e c i r c u l a t i o n i n t he c r y s t a l l i z a t i o n scheme, t h i s r e s u l t i n g i n r e d u c e d c o l o u r

b u i l d - u p . I n a d d i t i o n , new p o s s i b i l i t i e s t o c o n t r o l t h e c r y s t a l g r o w t h a r e

c r e a t e d , t h i s making i t p o s s i b l e t o c o n t r o l f i n a l c r y s t a l s i z e and g r a n u l o m e t r i c

d i s t r i b u t i o n . Howeve r , when a t t e m p t i n g t o d e s i g n a c r y s t a l l i z a t i o n p r o c e s s

a iming t o r educe t he hea t demand o f t h e s u g a r h o u s e , one w o u l d have t o c o n s i d e r

c r y s t a l - q u a l i t y c o n s t r a i n t s w h i c h have n o t been f u l l y i n v e s t i g a t e d so f a r

( r e f . 1 9 ) .

4 . 3 . 4 C r y s t a l f o o t i n g t e c h n i q u e s

The c o n c e p t o f c r y s t a l f o o t i n g o p e r a t i o n was i n t r o d u c e d w i t h t h e e s s e n t i a l

aim o f i m p r o v i n g c r y s t a l q u a l i t y and e s p e c i a l l y t o make t h e g r a n u l o m e t r i c

d i s t r i b u t i o n more u n i f o r m . The u n d e r l y i n g i d e a i s t o r a t i o n a l i z e t h e i n i t i a l

167

phase o f t h e s u g a r b o i l i n g p r o c e s s as d i s c u s s e d i n S e c t i o n 1 .3 .5 . I n s t e a d o f

p e r f o r m i n g i t i n e v e r y s t r i k e i n e v e r y vacuum p a n , t h e i n i t i a l s y r u p t h i c k e n i n g

and c r y s t a l f o r m a t i o n i s c o n c e n t r a t e d i n a s p e c i a l i z e d u n i t . The seed magma

o b t a i n e d t h e r e i s s u b s e q u e n t l y d e l i v e r e d t o vacuum pans where t h e b o i l i n g can be

s t a r t e d d i r e c t l y f rom the c r y s t a l g r o w t h p h a s e .

A c r y s t a l f o o t i n g t e c h n i q u e can be i n t r o d u c e d i n t o c r y s t a l l i z a t i o n schemes i n

a v a r i e t y o f w a y s , depend ing on t h e k i n d o f f o o t i n g u n i t and t h e scheme i n

q u e s t i o n . A s i m p l e method c o n s i s t s o f p r e p a r i n g t h e seed magma e v e r y second

s t r i k e i n a s e l e c t e d b a t c h vacuum pan t o a b o u t 2 /3 o f i t s v o l u m e , pumping a h a l f

o f t h e magma t o a n o t h e r pan and s u b s e q u e n t l y b o i l i n g s u g a r i n bo th pans i n

p a r a l l e l . P r a c t i c a l e x p e r i e n c e p r o v e s t h a t i t may r e d u c e t h e a v e r a g e b o i l i n g

t ime by abou t 20%. Fewer s y r u p - t h i c k e n i n g phases pe r f o rmed i n t h e e n t i r e vacuum-

pan s t a t i o n r e d u c e t h e s t a t i o n ' s h e a t demand by a few p e r c e n t .

A n o t h e r method c o n s i s t s o f p r e p a r i n g t h e seed magma as a m i x t u r e o f s y r u p s

and c r y s t a l l i n e Β and C s u g a r s i n a s p e c i a l m i x e r . When d e l i v e r i n g t h e magma t o

vacuum pans A , t he b o i l i n g o f A s u g a r can be s t a r t e d f rom the c r y s t a l g r o w t h

p h a s e . E x p e r i m e n t s have d e m o n s t r a t e d t h e p o s s i b i l i t y o f r e d u c i n g t h e a v e r a g e

b o i l i n g t ime by abou t 25% and c u t t i n g down t h e h e a t demand by 5% ( r e f . 2 0 ) .

A method w h i c h r e c e i v e d much a t t e n t i o n i n t h e l a t e 1970s i s t o mix g r e e n

s y r u p A and n o n - a f f i n e d C s u g a r t o seed magma s u b s e q u e n t l y used i n vacuum pans

B. I t has been p r o v e d n o t t o b r i n g any s i g n i f i c a n t e n e r g y s a v i n g s . The h e a t

demand can be e f f e c t i v e l y r e d u c e d , h o w e v e r , i f t h i s p r o c e d u r e i s a p p l i e d t o

a p a r t o f t h e C - s u g a r s t ream o n l y a n d , i n a d d i t i o n , p a r t o f t h e Β s u g a r i s m ixed

w i t h wash s y r u p A t o seed magma used i n vacuum pans A ( F i g . 4 . 1 1 ) . Such a

" d o u b l e - f o o t i n g " t e c h n i q u e shows an e n e r g y - s a v i n g p o t e n t i a l o f t h e o r d e r o f up

t o 3 kg steam p e r 100 kg b e e t ( r e f s . 2 1 , 2 2 ) .

The b e s t c o n t r o l o f c r y s t a l q u a l i t y can be a t t a i n e d when a p p l y i n g a s p e c i a l

f o o t i n g u n i t i n w h i c h bo th c o o l i n g - and e v a p o r a t i n g - c r y s t a l 1 i z a t i o n , used a t

d i f f e r e n t s t a g e s o f t h e w o r k i n g c y c l e , a r e employed t o p r o d u c e t h e seed magma.

The equ ipment c o n f i g u r a t i o n can be e i t h e r a c o m b i n a t i o n o f a s e p a r a t e

c r y s t a l l i z e r and a vacuum p a n , o r a s i n g l e p i e c e s c h e m a t i c a l l y shown i n F i g .

4.12 ( r e f s . 2 3 , 2 4 ) . I f comp le ted w i t h t h i c k - j u i c e c o n d i t i o n i n g making i t

p o s s i b l e t o c o n t r o l t h i c k - j u i c e p a r a m e t e r s a c c u r a t e l y a t t h e e n t r a n c e t o t h e

s u g a r h o u s e , t h e c r y s t a l f o o t i n g t e c h n i q u e p r o v i d e s a v e r y e f f e c t i v e t o o l f o r

m a s t e r i n g s u g a r c r y s t a l l i z a t i o n a t h i g h t h i c k - j u i c e c o n c e n t r a t i o n s .

A p r e r e q u i s i t e f o r i t s s u c c e s s f u l a p p l i c a t i o n i s t h a t t h e s u g a r house i s

e q u i p p e d w i t h s t i r r e d vacuum pans o f s u i t a b l e d e s i g n and w i t h e f f e c t i v e

a u t o m a t i c b o i l i n g c o n t r o l s ; t h e s e q u e s t i o n s a r e d i s c u s s e d m a i n l y i n C h a p t e r s 5

and 6. I n c o n c l u s i o n , t h e impo r tance o f t h e c r y s t a l f o o t i n g t e c h n i q u e s t o t h e

e n e r g y economy does n o t l i e i n some d i r e c t e n e r g y - s a v i n g e f f e c t s b u t r a t h e r i n

168

thin juice thick juice

4 Β remelt Β seed magma

U L L I STAGE A

MELTER Β FOOTING UNIT Β

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i STAGE Β

C seed magma C remelt

FOOTING UNIT C MELTER C

STAGE C

white sugar

W<hrixi- • C suga r •

molasses

F i g . 4 ,11 . S i m p l i f i e d scheme o f t h r e e - b o i l i n g w h i t e - s u g a r c r y s t a l l i z a t i o n u s i n g " d o u b l e f o o t i n g " .

i t s p o t e n t i a l t o u t i l i z e e f f e c t i v e l y t h e i n f l u e n c e o f i n c r e a s e d t h i c k - j u i c e

c o n c e n t r a t i o n on t h e hea t demand i n t h e s u g a r house ( s e e S e c t i o n 1 . 3 . 5 ) .

( a ) slurry

cooling water

® condenser

Γ-CXl·

ISOLUTION * * TANK

I MIXER

HXl -thick juice

remelt

syrup

steam

to vacuum pans ^

y supersaturation about 1.05

* y 7/.-75V0DS

r—t><l—' "5<

CONDITIONED IREMELT* TANK

MIXER

to vacuum

5Γ m

pans

F i g . 4 . 1 2 . F o o t i n g u n i t s emp loy ing bo th e v a p o r a t i n g - and c o o l i n g - c r y s t a l l i z a t i o n t o p roduce seed magma: ( a ) w i t h a vacuum pan and a w a t e r - c o o l e d s t i r r e d v e s s e l ( a f t e r r e f . 2 3 ) , ( b ) w i t h a s p e c i a l l y d e s i g n e d vacuum p a n / c r y s t a l 1 i z e r and v a c u u m - c o n t r o l l e d c o o l i n g ( a f t e r r e f . 2 4 ) .

169

4.4 UNCONVENTIONAL ENERGY-SAVING PROCESSES IN SUGAR MANUFACTURE

4.4.1 U n d e r l y i n g c o n c e p t s

The s u g a r m a n u f a c t u r i n g p r o c e s s emp loy i ng e x t r a c t i o n , j u i c e p u r i f i c a t i o n w i t h

l ime and c a r b o n d i o x i d e , j u i c e t h i c k e n i n g by e v a p o r a t i o n and f i n a l l y ,

c r y s t a l l i z a t i o n , i s a p p a r e n t l y a s s o c i a t e d w i t h a c e r t a i n minimum e n e r g y

e x p e n d i t u r e w h i c h c a n n o t be f u r t h e r r e d u c e d . Assuming an o p t i m a l c o n f i g u r a t i o n

o f t h e the rma l s y s t e m , an e s t i m a t e o f t h e minimum i n p u t o f p r i m a r y e n e r g y abou t

2.3 kg normal f u e l p e r 100 kg b e e t has been g i v e n i n D a n i s h s o u r c e s ( r e f . 2 5 ) .

Even though t h i s v a l u e can s t i l l be d i s c u s s e d , p r a c t i c a l e x p e r i e n c e p r o v e s t h a t

e x t r e m e l y low hea t demand i n c o n v e n t i o n a l s u g a r m a n u f a c t u r e can o n l y be a t t a i n e d

a t t h e expense o f i n c o n v e n i e n t p r o c e s s m o d i f i c a t i o n s , t h e s e i n t u r n r e q u i r i n g

m o d i f i e d equ ipment and c o n t r o l s y s t e m s , as w e l l as i n c r e a s e d c o m p l e x i t y o f t h e

the rma l s y s t e m . I t seems t h a t t h e r e i s no o t h e r way i n w h i c h t h e c o n s t r a i n t s

i n h e r e n t i n t h e c o n v e n t i o n a l s u g a r m a n u f a c t u r i n g p r o c e s s can be s a t i s f i e d .

A number o f a l t e r n a t i v e p r o c e s s e s m i g h t p o s s i b l y be employed i n s u g a r

manu fac tu re t o remove o r change t h e c o n s t r a i n t s imposed on e n e r g y c o n v e r s i o n and

u t i l i z a t i o n t e c h n i q u e s . T h i s m igh t open e n t i r e l y new p o s s i b i l i t i e s f o r e n e r g y

demand r e d u c t i o n s .

To b e g i n w i t h , t h e i n i t i a l phase o f s u g a r m a n u f a c t u r i n g c o n s i s t s i n f a c t o f

j u i c e s e p a r a t i o n ; t h i s can be done by methods o t h e r t han e x t r a c t i o n . N e x t , j u i c e

p u r i f i c a t i o n can be p e r f o r m e d , a t l e a s t p a r t l y , w i t h o u t l ime and C O ^ .

E v a p o r a t i o n i s n o t t h e o n l y method s u i t e d t o j u i c e t h i c k e n i n g ; s i m i l a r l y ,

e v a p o r a t i n g - c r y s t a l 1 i z a t i o n can be a i d e d o r even r e p l a c e d by o t h e r methods o f

s e p a r a t i o n o f c r y s t a l l i n e s u g a r . C o n s e q u e n t l y , one can imag ine a s u g a r f a c t o r y

w i t h o u t e x t r a c t o r s , l i m e r s , c a r b o n a t a t i o n t a n k s , e v a p o r a t o r s and vacuum p a n s .

F o r t h e t ime b e i n g , such a v i s i o n b o r d e r s on s c i e n c e f i c t i o n , so no a t t e m p t w i l l

be made t o a n a l y s e i t as a w h o l e . I f one l o o k s a t t h e componen ts , h o w e v e r , t h e n

t he a p p l i c a t i o n p r o s p e c t s t u r n o u t t o be more r e a l i s t i c . I n t h e f o l l o w i n g , s h o r t

r e v i e w s o f t h e most p r o m i s i n g c o n c e p t s a r e g i v e n .

4 . 4 . 2 J u i c e s e p a r a t i o n

As an a l t e r n a t i v e t o e x t r a c t i o n , j u i c e s e p a r a t i o n f rom r a s p e d o r s l i c e d b e e t

b r e i can be c o n s i d e r e d . T h i s method was w i d e l y used i n t h e 19th c e n t u r y and

u l t i m a t e l y abandoned because t h e r e c o v e r y o f s u c r o s e was n o t s u f f i c i e n t l y

c o m p l e t e . I t i s now be ing s t u d i e d , h o w e v e r , w i t h some s u b s t a n t i a l m o d i f i c a t i o n s .

One p o s s i b l e v e r s i o n i s t o a p p l y l o w - t e m p e r a t u r e , c o u n t e r - c u r r e n t wash ing o f

b e e t b r e i ( r e f . 2 6 ) . The p r i n c i p l e o f t h e p r o c e s s and i t s e s s e n t i a l p a r a m e t e r s

a re g i v e n i n F i g . 4 . 1 3 ( a ) and t h e r e t e n t i o n t ime o f t h e b r e i can be e s t i m a t e d a t

10 m i n u t e s . The key p rob lem i s t o d e s t r o y c e l l membranes e f f e c t i v e l y so t h a t

s u g a r can be washed o u t a t low t e m p e r a t u r e . T h i s may r e q u i r e d o u b l e - s t a g e

170

(α) disintegrated beet

tissue 23VoDS acid f i l t rat ion

aid p reserva t ive water

raw juice 16.4% D S *

( b ) powdered lime 0.6

disintegrated ^

beet t issue 100

press juice

^ ^ A S H - "

-STAGE U

- - W A S H - '

. ^ S T A G E 2 .

" - W A S H - - '

.^STAGE 3 . PRESS

^ ^ A S H - "

-STAGE U

" - W A S H - - '

.^STAGE 3 .

b re i3 Ί

PRESS

pressed brei 35% DS

TANK 80°C

water 12.5 powdered lime

1 Γ PRESS 1 PRESS 2 35 bar 70 bar

raw juice

• pressed brei AOVoDS

F i g . 4 . 1 3 . Schemes o f j u i c e s e p a r a t i o n f rom b e e t b r e i : ( a ) t r i p l e - s t a g e c o u n t e r -c u r r e n t wash ing and p r e s s i n g o f b r e i , ( b ) d o u b l e - s t a g e p r e s s i n g . Mass f l o w s g i v e n i n kg/100 kg b. * / i n c l u d i n g 2% f i b r e s u b s t a n c e .

d i s i n t e g r a t i o n o f t h e b e e t t i s s u e : f i r s t i n a d i s k m i l l , t h e n i n a homogen ize r

o r a b e a t e r m i l l . S u c r o s e r e c o v e r y can be e s t i m a t e d as 0.97 m u l t i p l i e d by t h e

e f f i c i e n c y o f d i s i n t e g r a t i o n o f c e l l membranes. An e f f i c i e n c y o f 0.95 can e a s i l y

be o b t a i n e d , r e s u l t i n g t h u s i n s u c r o s e r e c o v e r y abou t 0 .92 . A t 16% s u g a r i n

b e e t s , 13.8% s u g a r i n raw j u i c e can be o b t a i n e d .

The a d v a n t a g e s o f t h e p r o c e s s a r e :

- h i g h p u r i t y o f s e p a r a t e d j u i c e ;

- no hea t e x p e n d i t u r e ;

- h i g h v a l u e o f c o n c e n t r a t e d b r e i as animal f e e d .

The most s e r i o u s d i s a d v a n t a g e i s t h e r e l a t i v e l y h i g h power demand o f t h e

d i s i n t e g r a t i o n equ ipmen t : 0.44 kWh/100 kg b has been r e p o r t e d f rom a p r o t o t y p e

i n s t a l l a t i o n . N e v e r t h e l e s s , economic c o m p e t i t i v e n e s s a g a i n s t c o n v e n t i o n a l

e x t r a c t i o n has a l r e a d y been c l a i m e d a t sma l l p r o c e s s i n g c a p a b i l i t i e s up t o

1200 t / d .

A n o t h e r s o l u t i o n employs d o u b l e - s t a g e p r e s s i n g o f b e e t b r e i a t e l e v a t e d

t e m p e r a t u r e ( r e f . 2 7 ) . The p r o c e s s i s s c h e m a t i c a l l y shown i n F i g . 4 . 1 3 ( b ) ; t h e

r e t e n t i o n t ime o f t h e b r e i can be e s t i m a t e d a t 20 m i n u t e s . D o u b l e - s t a g e p r e s s i n g

o f f e r s t h e a d v a n t a g e s o f v e r y h i g h s u c r o s e r e c o v e r y and v e r y h i g h c o n t e n t o f

d r y s u b s t a n c e i n t h e p r e s s e d b r e i . A t 16% s u g a r i n b e e t s , 15.5% s u g a r i n raw

j u i c e seems t o be a r e a l i s t i c f i g u r e . As t h i s p r o c e s s has been t e s t e d on

171

a l a b o r a t o r y s c a l e o n l y , f u r t h e r r e s e a r c h i s needed b e f o r e any e v a l u a t i o n can be

g i v e n o f i t s t e c h n i c a l and economic f e a s i b i l i t y .

4 . 4 . 3 J u i c e p u r i f i c a t i o n and t h i c k e n i n g

J u i c e p u r i f i c a t i o n i s a p r o c e s s e s s e n t i a l l y aimed a t remov ing n o n s u g a r s , and

j u i c e t h i c k e n i n g one aimed a t remov ing e x c e s s w a t e r f rom s u g a r s o l u t i o n s . These

f u n c t i o n s can be pe r f o rmed emp loy ing membrane f i l t r a t i o n p r o c e s s e s ( r e f s . 28-30)

known as u l t r a f i l t r a t i o n and h y p e r f i 1 t r a t i o n ; t h e l a t t e r t e c h n i q u e i s a l s o

c a l l e d r e v e r s e o s m o s i s . The c h a r a c t e r i s t i c s o f b o t h p r o c e s s e s a r e g i v e n i n

T a b l e 4 . 1 . U l t r a f i l t r a t i o n c o n c e n t r a t e s h i g h - m o l e c u l a r s u b s t a n c e c o l l o i d s and

suspended p a r t i c l e s , w h i l e h y p e r f i 1 t r a t i o n c o n c e n t r a t e s l o w - m o l e c u l a r s u b s t a n c e s

and s e p a r a t e s t h e s o l v e n t . The a p p r o x i m a t e l i m i t between t h e two p r o c e s s e s i s

a m o l e c u l a r w e i g h t o f 500-1000.

TABLE 4.1

Main f e a t u r e s o f u l t r a f i l t r a t i o n and h y p e r f i 1 t r a t i o n p r o c e s s e s .

U l t r a f i l t r a t i o n H y p e r f i 1 t r a t i o n

Minimum s i z e o f p a r t i c l e s s e p a r a t e d ( m i c r o n s ) 10-200 1-10 Examples o f s u b s t a n c e s n o t s e p a r a t e d w a t e r , e t h a n o l , w a t e r , e t h a n o l , Examples o f s u b s t a n c e s n o t s e p a r a t e d

l a c t i c a c i d . l a c t i c a c i d s u g a r s , s a l t s . l o w e r o r g a n i c compounds

P r e s s u r e range ( b a r ) 1-10 20-100 A p p l i c a t i o n s o u t s i d e s u g a r i n d u s t r y s e p a r a t i o n o f w a t e r

p r o t e i n s d e s a l i n a t i o n

The membranes, u s u a l l y 100-400 m i c r o n s t h i c k , a r e m a n u f a c t u r e d f rom p o l y m e r i c

m a t e r i a l s c a s t on a p o l y e s t e r o r p o l y p r o p y l e n e s u p p o r t . The d i f f e r e n c e between

u l t r a f i l t r a t i o n and h y p e r f i 1 t r a t i o n membranes l i e s i n t h e i r s t r u c t u r e s . The s i z e

o f t he membrane i s l i m i t e d by i t s s t r e n g t h . The membranes a r e mounted i n modu les

p r o v i d i n g a l s o n e c e s s a r y f l o w c h a n n e l s ; t u b u l a r , s p i r a l - w o u n d , p l a t e - a n d - f r a m e

and h o l l o w - f i b r e d e s i g n s a r e u s e d . The modules can be c o n n e c t e d t o g e t h e r and

e q u i p p e d w i t h pumps, v a l v e s , t a n k s , a u t o m a t i c c o n t r o l s , e t c . , t o c r e a t e a

membrane f i l t r a t i o n sys tem as shown s c h e m a t i c a l l y i n F i g . 4.14 ( r e f . 3 0 ) .

An u l t r a f i l t r a t i o n u n i t can be i n c o r p o r a t e d i n s u g a r m a n u f a c t u r e as j u i c e

p u r i f i c a t i o n equ ipmen t , a c c o r d i n g t o a scheme shown i n F i g . 4.15 ( r e f . 3 0 ) .

W h i l e t he p u r i t y o f u l t r a f i l t e r e d j u i c e can be as h i g h as t h a t o f t h i n j u i c e

l e a v i n g c o n v e n t i o n a l p u r i f i c a t i o n s t a t i o n s , i n v e r t s u g a r i s however n o t

e l i m i n a t e d , t h i s g i v i n g r i s e t o e x c e s s i v e c o l o u r f o r m a t i o n . T h e r e f o r e , f u r t h e r

t r e a t m e n t w i t h 0.05 kg l ime p e r 100 kg b e e t , o r by i o n e x c h a n g e , may be

n e c e s s a r y .

The a d v a n t a g e s o f u l t r a f i l t r a t i o n a r e l ime s a v i n g and e l i m i n a t i o n o f

172

sugar solution

water

permeate

concentrate

F i g . 4 .14 . Scheme o f a membrane f i l t r a t i o n sys tem ( a f t e r r e f . 3 0 ) .

water

raw juice ] SCREENING — ^ P R E - T R E A T M E N T —

ULTRAFILTRATION 80°C

thin juice SULPHITATION

CLARIFICATION OR FILTRATION

sludge

permeate

LIMING

concentrate

F i g . 4 .15 . J u i c e p u r i f i c a t i o n scheme i n c l u d i n g an u l t r a f i l t r a t i o n u n i t ( a f t e r r e f . 3 0 ) .

c a r b o n a t a t i o n hea t l o s s e s . I f a c h i e v e d d u r i n g a f a c t o r y e x t e n s i o n , t h i s can make

i n v e s t m e n t i n t h e l ime k i l n u n n e c e s s a r y . C o s t e s t i m a t e s based on p i l o t - s c a l e

t r i a l s can be f ound i n the l i t e r a t u r e ( r e f . 3 1 ) .

A h y p e r f i l t r a t i o n u n i t can be used t o remove w a t e r f rom j u i c e , t h u s r e d u c i n g

t h e e v a p o r a t o r l o a d . Two p o s s i b l e l o c a t i o n s i n a s u g a r m a n u f a c t u r i n g l i n e a r e

shown s c h e m a t i c a l l y i n F i g . 4.16 ( r e f . 3 2 ) . A t t h e p r e s e n t s t a t e o f deve lopmen t

o f h y p e r f i 1 t r a t i o n membranes, 30-35% DS seems t o be t h e upper l i m i t o f

p r a c t i c a b l e j u i c e c o n c e n t r a t i o n s . T h i s c o r r e s p o n d s t o an a t t a i n a b l e steam s a v i n g

o f t h e o r d e r o f 2.7 kg/100 kg b. Power consumpt ion i n j u i c e pumping, h o w e v e r ,

may be as h i g h as 0.8 kWh/100 kg b. C o s t e s t i m a t e s o b t a i n e d by e x t r a p o l a t i n g t h e

f i g u r e s f rom p i l o t - s c a l e t e s t s a r e g i v e n i n t h e l i t e r a t u r e ( r e f . 3 1 ) .

The p rob lem w i t h t h e membrane f i l t r a t i o n sys tems p r e s e n t l y a v a i l a b l e i s t h a t

module s i z e s a r e r e l a t i v e l y s m a l l , t h i s i n c r e a s i n g t h e i n v e s t m e n t c o s t s o f

l a r g e - c a p a c i t y u n i t s . The c o s t o f membranes i s a l s o h i g h . N e v e r t h e l e s s , i f t h e

f u e l c o s t i s h i g h , i t may be j u s t i f i e d t o c o n s i d e r e n e r g y - s a v i n g membrane

f i l t r a t i o n sys tems as s e r i o u s a l t e r n a t i v e s t o e x t e n s i o n s o f t h e c o n v e n t i o n a l

equ ipmen t . F u r t h e r deve lopmen ts i n membrane t e c h n o l o g y can be e x p e c t e d t o

173

improve t h e c o m p e t i t i v e n e s s o f such s o l u t i o n s ; i n t h e f i r s t p l a c e , t h i s seems t o

a p p l y t o h y p e r f i 1 t r a t i o n ( r e v e r s e o s m o s i s ) u n i t s .

cossettes EXTRACTION

pulp

row juice HYPERFILTRATION VERSION 1

concentrate

permeate

LIMING

CARBONATATION

thick juice EVAPORATION

Ε 1

permeate

t FILTRATION

"1 concentrate HYPERFILTRATION SULPHITATION

VERSION 2 SULPHITATION

s ludge

F i g . 4 . 16 . P o s s i b l e l o c a t i o n s o f h y p e r f i 1 t r a t i o n u n i t s i n a scheme o f a s u g a r m a n u f a c t u r i n g p r o c e s s ( a f t e r r e f . 3 2 ) .

4 . 4 . 4 Sugar c r y s t a l l i z a t i o n

The p o s s i b i l i t i e s f o r a r a d i c a l change i n s u g a r c r y s t a l l i z a t i o n methods

depend v e r y much on t he r e s u l t s t h a t can be o b t a i n e d i n t h e p r e c e d i n g s e c t i o n s

o f t h e s u g a r m a n u f a c t u r i n g l i n e . I f h i g h enough t h i c k - j u i c e p u r i t y c o u l d be

assumed, t h e n t h e c o m p l i c a t e d m u l t i - s t a g e c r y s t a l l i z a t i o n c o u l d be r e p l a c e d , f o r

examp le , by s p r a y d r y i n g . I m p l i c a t i o n s on t h e e n e r g y s i d e , as w e l l as t h e

consequences f o r i n v e s t m e n t c o s t s , wou ld be enormous.

S t a y i n g w i t h i n t h e f rames d e f i n e d by t h e p r e s e n t s t a t e o f deve lopmen t o f

j u i c e s e p a r a t i o n and p u r i f i c a t i o n , m u l t i - s t a g e c r y s t a l l i z a t i o n seems t o be t h e

o n l y f e a s i b l e s o l u t i o n . C o n s i d e r a b l e e n e r g y s a v i n g s can be o b t a i n e d , h o w e v e r , i f

t h e e v a p o r a t i n g c r y s t a l l i z a t i o n i s r e p l a c e d by some l e s s e n e r g y - i n t e n s i v e

method. A d o p t i n g t he i d e a u s e d , f o r examp le , i n c e r t a i n European p a t e n t s

c o n c e r n e d w i t h mo lasses d e s u g a r i z a t i o n , t h e a p p l i c a t i o n o f f r e e z e

c r y s t a l l i z a t i o n has been p r o p o s e d ( r e f s . 3 3 , 3 4 ) . I t s p r i n c i p l e r e q u i r e s a

r e f r i g e r a n t s u b s t a n c e t o be added t o t h e s u c r o s e s o l u t i o n . As t h e r e f r i g e r a n t

a b s o r b s h e a t f rom the s o l u t i o n , t h e w a t e r c r y s t a l l i z e s . E v e n t u a l l y , t h i s b r i n g s

abou t t he s u p e r s a t u r a t i o n o f t h e s o l u t i o n and t h e f o r m a t i o n o f s u g a r c r y s t a l s .

I n t h e n e x t s t e p , s u g a r i s s e p a r a t e d f rom i c e c r y s t a l s t h a t a r e s u b s e q u e n t l y

washed w i t h w a t e r . Sugar c r y s t a l s a r e washed w i t h s y r u p , d r a i n e d , f i l t e r e d and

c e n t r i f u g e d .

T h i s method i s so new t o t h e s u g a r i n d u s t r y t h a t a l o t o f work i s needed t o

c l a r i f y i t s a p p l i c a t i o n p o t e n t i a l . On t h e e n e r g y s i d e , t h e c h o i c e o f t h e

r e f r i g e r a n t and t h e c o n c e p t o f t h e r e f r i g e r a t i o n c i r c u i t seem t o be d e c i s i v e i n

d e t e r m i n i n g t h e a t t a i n a b l e s a v i n g s .

174

4.5 PULP DEHYDRATION

4.5.1 L i n e s o f deve lopment

I t i s c h a r a c t e r i s t i c o f h i s t o r i c a l deve lopmen ts i n t he s u g a r i n d u s t r y t h a t

e n e r g y usage i n t he s u g a r m a n u f a c t u r i n g p r o c e s s has been t r e a t e d more s e r i o u s l y

than t h a t i n d r y i n g t h e p u l p . F o l l o w i n g t h e e n e r g y c r i s e s o f t h e 1970s, i t was

r e a l i z e d t h a t w h i l e t h e e n e r g y sys tems o f s u g a r manu fac tu re a r e r a t h e r e l a b o r a t e

and s t e a d i l y i m p r o v i n g , r e l a t i v e l y p r i m i t i v e p u l p - d r y i n g sys tems can be

r e s p o n s i b l e f o r as much as 1/3 o f t h e p r i m a r y - e n e r g y i n p u t in a s u g a r f a c t o r y .

T h i s s t i m u l a t e d much r e s e a r c h and numerous p r a c t i c a l a c t i o n s w h i c h g r a d u a l l y

began t o g i v e p r a c t i c a l r e s u l t s . I n c e r t a i n c o u n t r i e s , t h e e n e r g y s a v i n g s i n

p u l p d r y i n g have been q u i t e s p e c t a c u l a r , as can be seen i n F i g . 4 . 1 7 . The

p r o g r e s s was a c h i e v e d owing t o combined deve lopmen ts i n p r o c e s s e s , equ ipment

and c o n t r o l s y s t e m s . I n t he p r e s e n t a u t h o r ' s o p i n i o n , h o w e v e r , r e - t h i n k i n g o f

p r o c e s s r e q u i r e m e n t s p l a y e d a p a r t i c u l a r l y i m p o r t a n t r o l e .

1978 1980 1982 1984

F i g . 4 . 1 7 . S t a t i s t i c a l d a t a on e n e r g y consumpt ion i n p u l p d r y i n g i n FRG and Sweden, 1977-1985. The v a l u e s g i v e n f o r bo th c o u n t r i e s a r e n o t d i r e c t l y comparab le because o f t he d i f f e r e n c e s i n mo lasses d o s a g e .

I t s h o u l d n o t be f o r g o t t e n t h a t t h e e s s e n t i a l p rob lem w i t h t h e we t p u l p i s

how t o u t i l i z e i t . The most w i d e l y a c c e p t e d s o l u t i o n c o n s i s t s o f p r e s s i n g ,

d r y i n g and p e l l e t i n g the p u l p so t h a t i t can be e a s i l y s t o r e d , t r a n s p o r t e d and

s o l d as animal f e e d a d d i t i v e . T h e r e a r e numerous o t h e r p r o p o s a l s , h o w e v e r , t h a t

a l s o d e s e r v e s e r i o u s c o n s i d e r a t i o n . Depend ing on economic and v a r i o u s l o c a l

( e . g . , e n v i r o n m e n t a l ) c o n d i t i o n s i n a p a r t i c u l a r f a c t o r y , t h e b e s t c h o i c e may

v a r y .

An i n t e r e s t i n g p o s s i b i l i t y i s t o a v o i d t r e a t i n g t h e p u l p as a b y - p r o d u c t and

t o u t i l i z e i t w i t h t h e aim o f i m p r o v i n g t h e f a c t o r y ' s e n e r g y b a l a n c e . T h i s can

be done by c o n v e r t i n g t h e p r e s s e d p u l p t o b i o g a s i n an a n a e r o b i c f e r m e n t a t i o n

p r o c e s s . I t has been demons t ra ted i n l a b o r a t o r y - s c a l e e x p e r i m e n t s , and p a r t l y

c o n f i r m e d i n a p i l o t p l a n t , t h a t 90% o f t h e o r g a n i c m a t t e r p r e s e n t i n p u l p can

175

be c o n v e r t e d t o methane, t h e r e s t b e i n g a was te w h i c h needs t o be d i s p o s e d o f

( r e f . 2 , 3 5 ) . B i o g a s g e n e r a t e d f rom t h e e n t i r e amount o f p u l p can be s u p p l i e d t o

t he b o i l e r s . A l t e r n a t i v e l y , a p a r t o f t h e p u l p may be c o n v e r t e d t o methane,

wh i ch i s s u b s e q u e n t l y bu rned i n a d r y e r f u r n a c e , making i t p o s s i b l e t o d r y t h e

rema in ing p a r t . The economic p o t e n t i a l o f b i o g a s p r o d u c t i o n f rom t h e p u l p has

y e t t o be demons t ra ted i n a f u l l - s c a l e i n d u s t r i a l a p p l i c a t i o n .

A s o l u t i o n w i d e l y p r a c t i s e d i s t o s e l l t h e p r e s s e d p u l p d i r e c t l y , as f o d d e r .

I f t r a n s p o r t and s t o r a g e a r e p r o v i d e d by t h e c u s t o m e r s , t h e a d v a n t a g e s a r e

g r e a t . I t can be seen f rom t h e e x p e r i e n c e s o f numerous c o u n t r i e s , h o w e v e r , t h a t

t he o v e r a l l c o s t s o f t h e e n t i r e d i s t r i b u t i o n and s t o r a g e sys tem - s e r v i n g a

p r o d u c t w h i c h c o n t a i n s abou t 80% w a t e r - may be h i g h e r t han t h e v a l u e o f s a v i n g s

o b t a i n e d i n t h e f a c t o r y . T h e r e f o r e , t h i s s o l u t i o n may be d i f f i c u l t t o

s u b s t a n t i a t e i n w e l l - b a l a n c e d economies . N e e d l e s s t o s a y , i t a l s o r e q u i r e s l o n g -

te rm m a r k e t i n g .

A n o t h e r p o s s i b i l i t y c o n s i s t s o f s t o r i n g t h e p r e s s e d p u l p i n t h e f a c t o r y a r e a .

T h i s r e q u i r e s t h e a p p l i c a t i o n o f a s u i t a b l e p r e s e r v a t i o n p r o c e d u r e e n s u r i n g p u l p

f e r m e n t a t i o n aimed a t l a c t i c a c i d f o r m a t i o n ( r e f s . 3 7 , 3 7 ) . The s i m p l e s t method

i s t o e n s i l e t h e p u l p i m m e d i a t e l y a f t e r p r e s s i n g , t h a t i s , a t 45-50°C. A c o r r e c t

f e r m e n t a t i o n i s a t t a i n e d i f t h e e n s i l e d p u l p i s c o o l e d a t a d a i l y r a t e o f

0 . 5 - 1 . 0 K. No chemica l a d d i t i v e s a r e r e q u i r e d , b u t t h e a d d i t i o n o f m o l a s s e s has

been shown t o i n c r e a s e l a c t i c a c i d f o r m a t i o n . A l t e r n a t i v e l y , c o o l e d p u l p can be

e n s i l e d , p o s s i b l y w i t h chemica l p r e s e r v a t i v e s o r i n a 85:15 m i x t u r e w i t h b e e t

f r a g m e n t s . S u c c e s s f u l i n d u s t r i a l a p p l i c a t i o n s o f p u l p e n s i l a g e a r e known.

R e t u r n i n g now t o t h e p u l p d e h y d r a t i o n me thod , r e f e r e n c e can be made t o

S e c t i o n s 1.2.8 and 1.2.9 where t h e i m p o r t a n c e o f e n e r g y - s a v i n g p r o c e s s

m o d i f i c a t i o n s was s t r e s s e d . I n F i g . 4 . 1 8 ( a ) , t h e i n f l u e n c e o f t h e f i n a l DS

c o n t e n t on t h e s p e c i f i c e n e r g y demand i n mechan i ca l and the rma l d e h y d r a t i o n i s

shown. The e n e r g y demand p e r u n i t mass o f w a t e r removed by mechan ica l p r e s s i n g

i s v e r y low a t low DS c o n t e n t , b u t i t i n c r e a s e s r a p i d l y a t DS c o n t e n t s above

a c e r t a i n l i m i t . C o n c e r n i n g the rma l d r y i n g , i t s s p e c i f i c e n e r g y demand i s

r e l a t i v e l y c o n s t a n t o v e r a w ide range o f DS c o n t e n t s . The i n t e r s e c t i o n p o i n t

between t h e a p p l i c a t i o n r a n g e s o f bo th me thods , h o w e v e r , i s d e t e r m i n e d by

o v e r a l l economic r e s u l t s r a t h e r t han by e n e r g y i s s u e s o n l y . I t t u r n s o u t t h a t

w i t h i n c r e a s i n g DS c o n t e n t , t h e i n v e s t m e n t c o s t s o f p r e s s e s i n c r e a s e more

r a p i d l y t han t h e s p e c i f i c e n e r g y demand, s h i f t i n g t h e i n t e r s e c t i o n p o i n t t o w a r d s

l o w e r DS v a l u e s .

Numerous s t u d i e s o f a p p l i c a t i o n r a n g e s o f bo th d e h y d r a t i o n methods have been

p u b l i s h e d ( r e f . 3 8 - 4 3 ) . A g raph d e p i c t i n g t h e r e l a t i o n s h i p between p r e s s i n g and

thermal d r y i n g i s shown i n F i g . 4 . 1 8 ( b ) . As can be s e e n , a t 8% DS i n e x h a u s t e d

c o s s e t t e s , i n c r e a s i n g t h e DS c o n t e n t o f p r e s s e d p u l p f rom 22% t o 30% r e s u l t s i n

176

(α)

Ο Φ ϊ I

^1

III·"" g I

I l l l l l l l l l l l l á l l l " " " ' t h e r m a l

mechanical

J 0 20 40 60 80 100

Final DS content in pu lp(%)

20 40 60 80

DS content in pulp ( % )

F i g . 4 .18 . C h a r a c t e r i s t i c s o f mechan ica l and therma l p u l p d e h y d r a t i o n : ( a ) s p e c i f i c e n e r g y demand v s . f i n a l DS c o n t e n t , ( b ) w a t e r amount i n p u l p v s . DS c o n t e n t ( a f t e r r e f . 5 1 ) .

an i n c r e a s e o f w a t e r amount removed by therma l d e h y d r a t i o n o f abou t 1/7.

S i m u l t a n e o u s l y , t h e w a t e r amount removed by the rma l d e h y d r a t i o n i s r e d u c e d by

abou t 1/3.

I n t h e f o l l o w i n g , t h r e e e n e r g y - s a v i n g t e c h n i q u e s r e l a t e d t o p u l p d e h y d r a t i o n

t e c h n o l o g y a r e d i s c u s s e d :

- p r e s s i n g a t i n c r e a s e d f i n a l DS c o n t e n t o f t h e p u l p ;

- l o w - t e m p e r a t u r e d r y i n g ;

- steam d r y i n g .

4 . 5 . 2 P r e s s i n g t o h i g h DS c o n t e n t

A mechan ica l p r e s s o f c o n t e m p o r a r y d e s i g n u t i l i z e s t h e combined e f f e c t o f

p r e s s u r e and r e t e n t i o n t ime on t h e f i n a l DS c o n t e n t o f t h e p u l p . T h i s phenomenon

has been e x t e n s i v e l y s t u d i e d f o r d i f f e r e n t p r e s s d e s i g n s ; sample r e s u l t s a r e

shown i n F i g . 4.19 ( r e f . 4 4 ) . The nominal r e t e n t i o n t i m e , c o r r e s p o n d i n g t o t h e

nominal c a p a c i t y o f t h e p r e s s , d e t e r m i n e s i t s d i m e n s i o n s and t h u s t h e i n v e s t m e n t

c o s t . When t h e r o t a t i o n a l v e l o c i t y o f t he r o t o r i s r e d u c e d , l o n g e r r e t e n t i o n

t ime i s e n s u r e d and a h i g h e r DS c o n t e n t can be a t t a i n e d ; t h i s i m p l i e s , h o w e v e r ,

t h a t t h e c a p a c i t y u t i l i z a t i o n d e c r e a s e s . T h e r e f o r e , r e a l p r o g r e s s i s a c h i e v e d

o n l y i f t h e p r e s s d e s i g n i s improved t o g i v e a h i g h DS c o n t e n t i n t h e most

economica l o p e r a t i n g c o n d i t i o n s .

Up t o now, t he e s t a b l i s h e d p r e s s m a n u f a c t u r e r s i n t r o d u c e d o n l y l i m i t e d

changes t o t h e i r p r o d u c t s ( r e f s . 4 4 , 4 5 ) . Among t h e new d e s i g n s , a F r e n c h

s o l u t i o n a t t a i n i n g 50% DS was s u c c e s s f u l l y t e s t e d , bo th on p i l o t and i n d u s t r i a l

s c a l e s ( r e f . 4 2 ) . The c o n c e p t o f p u l p c e n t r i f u g i n g a l s o d e s e r v e s t o be n o t e d .

177

10 20 30 40 Retention time (min)

50 60

F i g . 4 . 19 . A t t a i n a b l e DS c o n t e n t o f p r e s s e d p u l p v s . r e t e n t i o n t ime and p r e s s u r e .

I t i s a w e l l known phenomenon t h a t t h e r e s u l t s o f p r e s s i n g a r e t o some e x t e n t

dependen t on t he p r o p e r t i e s o f t h e we t p u l p , most n o t a b l y on t h e c o n d i t i o n o f

p e c t i n s . I f t he p e c t i n s a r e decomposed d u r i n g e x t r a c t i o n , p r e s s i n g becomes

d i f f i c u l t . F o r t h i s r e a s o n , t o o h i g h t e m p e r a t u r e s and t o o l ong p u l p r e t e n t i o n

t imes i n t he e x t r a c t o r s h o u l d be a v o i d e d , as w e l l as p u l p r e c y c l e s . M o r e o v e r ,

advan tageous e f f e c t s can be a c h i e v e d by c o n t r o l l e d i n f e c t i o n by l a c t i c a c i d

b a c t e r i a , g i v i n g low pH o f t h e p u l p and good p r e s s i n g . As t h i s a l s o causes t he

s u g a r l o s s e s i n t h e e x t r a c t i o n t o i n c r e a s e , t h e f e a s i b i l i t y o f t h e method i s n o t

o b v i o u s . A r e f e r e n c e can be made t o f u l l - s c a l e e x p e r i m e n t s i n two A u s t r i a n

f a c t o r i e s , where m i c r o b i a l i n f e c t i o n i n t o w e r e x t r a c t o r s was c o n t r o l l e d t h r o u g h

c o n t i n u o u s f o r m a l i n d o s i n g v i a h i g h - p r e c i s i o n m e t e r i n g sys tems ( r e f . 4 6 ) . I n one

c a s e , t h e r e s u l t s were c l e a r l y p o s i t i v e ; i n t h e o t h e r f a c t o r y , t h e v a l u e o f

s u g a r l o s t a n n i h i l a t e d t h e e f f e c t o f e n e r g y s a v i n g . I t can a l s o be men t ioned

t h a t t h e r e have been examples o f i m p r o v i n g we t p u l p p r o p e r t i e s by d o s i n g

s u l p h u r i c a c i d t o p r e s s w a t e r (pH v a l u e abou t 4 ) .

I n r e c e n t y e a r s , m a i n l y on an e m p i r i c a l b a s i s , t h e a d d i t i o n o f p r e s s i n g a i d s

has become w i d e s p r e a d ( r e f s . 4 4 , 4 7 ) . C a l c i u m s a l t s - C a ( H S 0 2 ) 2 , C a C l 2 , CaSO^ -

a re t he most p o p u l a r because o f t h e i r low c o s t . The a i d s a r e added i n s o l u t i o n

o r s l u r r y t o t h e e x t r a c t i o n f e e d w a t e r o r t o t h e p u l p a t t h e e x t r a c t o r o u t l e t .

T e s t s o f l i m i n g o f f r e s h c o s s e t t e s have a l s o been p e r f o r m e d ( r e f . 4 8 ) . D i f f e r e n t

r a t i o s o f a i d / b e e t s a r e u s e d , bu t 500 g p e r 1 t b e e t i s a b o u t t h e upper l i m i t .

More s y s t e m a t i c s t u d i e s o f t h e e f f e c t o f p r e s s i n g a i d s , p u b l i s h e d r e c e n t l y ,

de te rm ine an upper l i m i t o f t h e l o a d i n g o f c a l c i u m s a l t s a t 4 m i l l i g r a m

e q u i v a l e n t s p e r 100 g b e e t ( r e f . 4 9 ) . The i n c r e a s e i n the d r y s u b s t a n c e c o n t e n t

o f t he p r e s s e d p u l p i s 3-4%. I t has a l s o been e s t a b l i s h e d t h a t t h e a i d s

c o n t a i n i n g t r i v a l e n t i o n s , e . g . k^^{SO^)^, may r a i s e t h e p u l p DS c o n t e n t even

f u r t h e r .

178

The amounts o f s a l t s used as p r e s s i n g a i d s a r e so sma l l t h a t t h e y seem t o

d i s a p p e a r i n t h e j u i c e p u r i f i c a t i o n p r o c e s s , a l t h o u g h s l i g h t l y i n c r e a s e d s u g a r

l o s s i n mo lasses can be e x p e c t e d e s p e c i a l l y when u s i n g C a C l ^ ( r e f . 5 0 ) . The

c o n d i t i o n s change when mo lasses i s added t o t h e p u l p . I t i s known t h a t i t s

o s m o t i c e f f e c t on p u l p p a r t i c l e s r a i s e s t h e amount o f w a t e r t h a t can be removed

f rom t h e p u l p . A f t e r add ing as much m o l a s s e s as 3-4 kg/100 kg b , h o w e v e r , t h e

s u g a r c o n t e n t i n t h e p r e s s f i l t r a t e may become so h i g h t h a t i t c a n n o t be

r e c y c l e d t o t he e x t r a c t o r ( p a r t i c u l a r l y i f t h e mo lasses i s added i n t h e second

p r e s s i n g , see b e l o w ) . One p o s s i b l e s o l u t i o n i s t o t h i c k e n t h e f i l t r a t e i n

a s p e c i a l e v a p o r a t o r and t o r e c y c l e i t t o t h e p r e s s e s ( r e f . 3 8 ) .

The improvements i n p r e s s d e s i g n , p u l p c o n d i t i o n i n g i n t h e e x t r a c t o r and

a p p l i c a t i o n o f p r e s s i n g a i d s can be combined w i t h d o u b l e - s t a g e p r e s s i n g . I t was

i n i t i a l l y t e s t e d w i t h o u t p r e s s i n g a i d s , g i v i n g a DS i n c r e a s e o f up t o 10% above

t h a t a t t a i n e d i n t h e f i r s t s t a g e . U s i n g p r e s s i n g a i d s , 35-40% DS i n t h e p u l p can

be a t t a i n e d , b u t an e c o n o m i c a l l y j u s t i f i e d l e v e l seems t o be somewhat l o w e r .

4 . 5 . 3 L o w - t e m p e r a t u r e d r y i n g

I t was men t ioned i n S e c t i o n 1.2.8 t h a t owing t o t h e p r o c e s s l a y o u t and

p a r a m e t e r s , t h e e n e r g y u t i l i z a t i o n i n c o n v e n t i o n a l the rma l d r y i n g i s p o o r .

Assuming t h a t h i g h - t e m p e r a t u r e gases s h o u l d be f e d t o t h e d r y e r , b u r n i n g o f f u e l

c a n n o t be a v o i d e d and o n l y a p a r t o f t h e e n e r g y demand can be s a t i s f i e d

u t i l i z i n g b o i l e r f l u e g a s e s . T h e r e f o r e , a p r o p o s a l has been made t o i n t r o d u c e

a d r y i n g p r o c e s s w i t h t h e i n i t i a l gas t e m p e r a t u r e low enough t o u t i l i z e was te

hea t f rom t h e s u g a r m a n u f a c t u r i n g p r o c e s s . C a l l e d l o w - t e m p e r a t u r e d r y i n g , t h i s

p r o c e s s has p r o v e d t e c h n o l o g i c a l l y f e a s i b l e i n a few a p p l i c a t i o n s . As t h e c o s t

o f t h e n e c e s s a r y equ ipment i s v e r y h i g h , h o w e v e r , i t c a n n o t be seen as t h e

u l t i m a t e e n e r g y - s a v i n g s o l u t i o n b u t r a t h e r as a n o t h e r new sys tem component t o be

u t i l i z e d i n e n e r g y - e f f i c i e n t f a c t o r i e s .

The s t reams o f was te h e a t t h a t can be c o n s i d e r e d f o r u t i l i z a t i o n a r e

a v a i l a b l e i n t h e f o l l o w i n g med ia :

- b a r o m e t r i c w a t e r ;

- vacuum pan v a p o u r ;

- condensa te (ammonia w a t e r ) ;

- s p e n t c a r b o n a t a t i o n g a s ;

- v a p o u r s f rom l ime s l a k i n g ;

- f l u e gas f rom b o i l e r s ;

- p o s s i b l y , s p e n t gas f rom h i g h - t e m p e r a t u r e p u l p d r y i n g .

H e a t i n g o f a i r has been e x t e n s i v e l y s t u d i e d i n a number o f p u b l i c a t i o n s ( r e f s .

5 1 - 5 4 ) . The a t t a i n a b l e t e m p e r a t u r e i s o f t h e o r d e r o f 50-70°C. The e x a c t v a l u e

s e l e c t e d , as w e l l as o t h e r p r o c e s s pa rame te rs - i n c l u d i n g a i r h u m i d i t y a t t h e

179

d r y e r o u t l e t - d e t e r m i n e t h e e n e r g y demand p e r 1 kg w a t e r removed f rom t h e p u l p ,

as a p p r o x i m a t e l y shown i n t h e d iag ram o f F i g . 4 . 2 0 . When compared t o h i g h -

t e m p e r a t u r e d r y i n g , s a y a t 500°C a i r t e m p e r a t u r e a t t h e d r y e r i n l e t , 40-80% more

e n e r g y p e r 1 kg w a t e r i s needed i n l o w - t e m p e r a t u r e d r y i n g ( a t a i r h u m i d i t y

chang ing f rom 50% t o 90%). I n o r d e r t o m i n i m i z e t h e s p e c i f i c e n e r g y demand, t h e

o u t l e t t e m p e r a t u r e s h o u l d be low and o u t l e t h u m i d i t y h i g h , t h i s i m p l y i n g t h a t i t

i s v e r y d i f f i c u l t t o d r y t h e p u l p t o a h i g h DS c o n t e n t . C o n s e q u e n t l y , l o w -

t e m p e r a t u r e d r y i n g i s most s u i t a b l e as a f i r s t s t a g e p r e c e d i n g h i g h - t e m p e r a t u r e

d r y i n g , where t h e f i n a l DS c o n t e n t can be a t t a i n e d . I t has been p r o v e d t h a t such

a s o l u t i o n can save more e n e r g y t han a p a r a l l e l c o m b i n a t i o n o f bo th t y p e s o f

d r y i n g ( r e f . 5 1 ) .

— — air temperature at d rye r inlet — a i r humidity at d rye r outlet

5500 Γ

20 AO 60 80

Air temperature at d ryer outlet {°C)

100 120

F i g . 4 . 2 0 . S p e c i f i c e n e r g y demand i n l o w - and med ium- tempera tu re d r y i n g as a f u n c t i o n o f p r o c e s s pa rame te rs ( a f t e r K . K r ö l l , T r o c k n u n g s t e c h n i k , 2nd e d n . , S p r i n g e r - V e r l a g , B e r l i n , 1978) .

A p o s s i b l e d e s i g n f o r a l o w - t e m p e r a t u r e d r y e r o f t h e t r a v e l l i n g - s c r e e n t y p e

i s shown s c h e m a t i c a l l y i n F i g . 4.21 ( t h e d r y e r c o n s i s t s o f m u l t i p l e c e l l s l i k e

t h e one shown i n c r o s s - s e c t i o n ) . I t i s c h a r a c t e r i z e d by a s p e c i f i c power demand

abou t 50 kWh p e r 1 t w a t e r removed , t h e a i r f a n s b e i n g r e s p o n s i b l e f o r most o f

i t and t h e c o n v e y o r and s c r e e n d r i v e s f o r t h e r e s t . The pa rame te rs o f a l o w -

t e m p e r a t u r e d r y e r o p e r a t e d i n a s u g a r f a c t o r y i n FRG a r e l i s t e d i n T a b l e 4 . 2 .

O t h e r d e s i g n s have been d e s c r i b e d i n t h e l i t e r a t u r e ( r e f s . 5 5 - 5 7 ) .

As t he i n t r o d u c t i o n o f l o w - t e m p e r a t u r e d r y i n g has a d i s a d v a n t a g e o u s e f f e c t on

180

A pulp in

4- A-A air out

i L

7 - - - r - - z - z - . - ^

\ / _ o u i pulp out

A i F i g . 4 .21 . Scheme o f a l o w - t e m p e r a t u r e d r y e r ( a f t e r r e f . 5 3 ) . 1 - a i r h e a t e r , 2 - a i r f a n , 3 - a i r d i s t r i b u t o r and s c r e e n .

TABLE 4.2

Paramete rs o f a l o w - t e m p e r a t u r e d r y e r ( a f t e r r e f . 5 1 ) .

VäTüe pe r 1 t w a t e r removed a t

e v a p o r a t i n g c a p a c i t y ( t / h ) Q u a n t i t y T o t a l v a l u e

25 30

Thermal c a p a c i t y (kW) ρ 34600 1384 1153 H e a t i n g s u r f a c e a r e a i n a i r h e a t e r s (m ) 43370 1735 1446 A i r f l o w ( t / h ) 2830 113 94 Power demand, i n c l u d i n g w a s t e - h e a t r e c o v e r y equ ipment (kW) ^ 1500 60 50 A v e r a g e a i r t e m p e r a t u r e a t h e a t e r o u t l e t ( C) 50 A v e r a g e p u l p r e t e n t i o n t ime (m in ) 25 D imens ions (m) ^ 36X16X19 T o t a l s c r e e n a r e a (m ) 600 T o t a l w e i g h t ( t ) 1600

t h e power b a l a n c e o f t h e f a c t o r y , a p r o p o s a l was made t o c r e a t e an a d d i t i o n a l

therma l c o n n e c t i o n between p u l p d r y i n g and s u g a r m a n u f a c t u r e , w i t h t h e aim o f

i n c r e a s i n g the steam demand o f t h e e v a p o r a t i o n s t a t i o n , w i t h o u t i n c r e a s i n g t h e

p r i m a r y - e n e r g y i n p u t t o t h e f a c t o r y . I n t h i s way , t h e steam f l o w t h r o u g h t h e

t u r b i n e can be i n c r e a s e d and a d d i t i o n a l power g e n e r a t e d . Assuming t h a t t h e

a d d i t i o n a l c o n n e c t i o n c o n s i s t s o f s u p p l y i n g t h i r d - e f f e c t v a p o u r t o an a i r

h e a t e r , an a i r t e m p e r a t u r e abou t 90°C can be a t t a i n e d ; t h i s s o l u t i o n i s known as

med ium- tempera tu re d r y i n g . As can be seen i n F i g . 4 . 2 0 , i t s s p e c i f i c e n e r g y

demand does n o t d i f f e r s i g n i f i c a n t l y f rom t h a t o f l o w - t e m p e r a t u r e d r y i n g . The

a i r f l o w and s c r e e n s u r f a c e i n t h e d r y e r can be c o s i d e r a b l y r e d u c e d , h o w e v e r ,

t h i s r e s u l t i n g i n reduced power demand and i n v e s t m e n t c o s t s . U s i n g medium-

181

t e m p e r a t u r e d r y i n g f o l l o w e d by c o n v e n t i o n a l d r y i n g w i t h b o i l e r f l u e g a s , t h e

t o t a l p u l p amount can be d r i e d u s i n g o n l y was te and l o w - t e m p e r a t u r e h e a t f rom

o t h e r f a c t o r y s e c t i o n s . A s u i t a b l e l a y o u t o f t h e e n e r g y sys tem i s shown

s c h e m a t i c a l l y i n F i g . 4.22 ( a f t e r r e f . 5 8 ) .

steam

fuel

9 0 Ϊ

3 r d - e f f e c t ^ I vapour

pressed pulp

flue gas 205°C dried pulp

F i g . 4 . 2 2 . E n e r g y sys tem l a y o u t f o r d o u b l e - s t a g e p u l p d r y i n g u s i n g was te and l o w - t e m p e r a t u r e h e a t f rom b o i l e r s and s u g a r m a n u f a c t u r e ( a f t e r r e f . 5 8 ) . 1 - b o i l e r , 2 - t u r b i n e , 3 - s u g a r m a n u f a c t u r i n g p r o c e s s , 4 - med ium- tempera tu re d r y e r , 5 - c o n v e n t i o n a l d r y e r .

4 . 5 . 4 Steam d r y i n g

I n t h e s e a r c h f o r a l t e r n a t i v e s t o c o n v e n t i o n a l t he rma l d e h y d r a t i o n , steam

d r y i n g has r e c e i v e d much a t t e n t i o n i n r e c e n t y e a r s . I n a d d i t i o n t o t h e o b v i o u s

p r o p e r t y o f making i t easy t o i n t e g r a t e t h e h e a t economy i n s u g a r m a n u f a c t u r e

and p u l p d r y i n g , t h i s method can a l s o improve t h e q u a l i t y o f t h e p u l p , as i t i s

d r i e d i n t h e absence o f a i r and t h u s w i t h o u t t h e r i s k o f o x i d a t i o n . Among f o u r

i n d u s t r i a l - s c a l e steam d r y e r s p r e s e n t l y i n o p e r a t i o n ( r e f s . 1 3 , 5 9 - 6 1 ) , t h e

Swed ish u n i t p r o d u c e s h i g h - q u a l i t y d r i e d p u l p w h i c h i s s u b s e q u e n t l y t r a n s f o r m e d

t o a f o o d a d d i t i v e ( r e f . 62) and t h e r e m a i n i n g ones seem t o be used m a i n l y f o r

t he pu rpose o f i m p r o v i n g e n e r g y economy.

The thermodynamic mechanism o f steam d r y i n g i s somewhat d i f f e r e n t f rom t h a t

o f c o n v e n t i o n a l d r y i n g . The d r i v i n g f o r c e o f t h e mass exchange between a p u l p

p a r t i c l e and t h e h e a t i n g a g e n t i s t h e d i f f e r e n c e between t h e steam t e m p e r a t u r e

and t he s a t u r a t i o n t e m p e r a t u r e a t t h e p r e s s u r e m a i n t a i n e d i n t h e d r y e r . As t h e

t e m p e r a t u r e o f t he p a r t i c l e e n t e r i n g t h e d r y e r i s l o w e r t han t h e steam

t e m p e r a t u r e , t he p a r t i c l e may i n i t i a l l y a b s o r b w a t e r by c o n d e n s a t i o n .

E v a p o r a t i o n b e g i n s a f t e r t h e s a t u r a t i o n t e m p e r a t u r e has been a t t a i n e d a t t h e

p a r t i c l e s u r f a c e , and t he s a t u r a t i o n zone i s g r a d u a l l y e x t e n d e d t o t h e c e n t r e o f

t he p a r t i c l e . O n l y i n t he l a y e r s f rom w h i c h w a t e r has been removed can t he

p a r t i c l e t e m p e r a t u r e d i f f e r s i g n i f i c a n t l y f rom t h e s a t u r a t i o n v a l u e , w h i c h i s

u n l i k e l y t o happen i n a w e l l d e s i g n e d d r y e r . I n t h i s way o v e r h e a t i n g o f t h e p u l p

182

0.5 1.0 1.5 2.0 Water content ( k g / k g DS)

2.5

F i g . 4 . 2 3 . Changes o f a v e r a g e p u l p t e m p e r a t u r e and w a t e r c o n t e n t d u r i n g steam

d r y i n g ( a f t e r r e f . 3 8 ) .

The steam t e m p e r a t u r e s r e p o r t e d a r e i n t h e range 132-189^0. The w o r k i n g

p r i n c i p l e s o f two d r y e r d e s i g n s a r e shown s c h e m a t i c a l l y i n F i g . 4 . 2 4 . I t seems

t h a t t h e s o l u t i o n s have y e t t o be o p t i m i z e d w i t h r e s p e c t t o t h e i r economic

f e a s i b i l i t y .

The d r y e r a p p l i e d i n a s u g a r f a c t o r y i n FRG i s o f t h e t r a v e l l i n g - s c r e e n t y p e

( F i g . 4 . 2 4 ( a ) , a f t e r r e f . 6 1 ) . The d i m e n s i o n s g i v e n a p p l y t o a u n i t r a t e d 20 t / h

e v a p o r a t e d w a t e r . The d r y e r c o n s i s t s o f n i n e c e l l s l i k e t h e one shown i n c r o s s -

s e c t i o n . The s a t u r a t i o n t e m p e r a t u r e o f t h e r e c i r c u l a t e d v a p o u r i s 132°C. Due t o

t h e p r e s e n c e o f c i r c u l a t i o n f a n s , i t s s p e c i f i c power demand i s s i m i l a r t o t h a t

o f a l o w - t e m p e r a t u r e d r y e r , t h a t i s , abou t 50 kWh/1 t w a t e r . As t h e d r y e r i s

(a) vapour

pulp in

40 m

pulp out

condensate vapour

F i g . 4 . 24 . w o r k i n g p r i n c i p l e s o f steam d r y e r s : ^7!^|5",^^;Π;α. ( b ) f l u i d i z e d b e d . 1 - h e a t e r , 2 - s c r e e n s , 3 - f a n , 4 t l u i a i z e o

can be a v o i d e d , w i t h a p o s i t i v e e f f e c t on t h e q u a l i t y o f t h e f i n a l p r o d u c t .

A d iag ram o f p u l p t e m p e r a t u r e changes d u r i n g steam d r y i n g i s shown i n F i g . 4 . 2 3 .

183

hea ted by e x h a u s t s team, i t s p r e s e n c e i n an e n e r g y sys tem f a c i l i t a t e s i n c r e a s e d

power g e n e r a t i o n . I f combined w i t h a l o w - t e m p e r a t u r e d r y e r used i n t h e i n i t i a l

p r o c e s s s t a g e , t h i s d e s i g n makes i t p o s s i b l e t o d r y t h e e n t i r e p u l p amount

w i t h o u t d i r e c t e x p e n d i t u r e o f p r i m a r y e n e r g y . A s u i t a b l e e n e r g y - s y s t e m l a y o u t i s

shown s c h e m a t i c a l l y i n F i g . 4 . 2 5 ( a ) .

( a ) Qir I

pressed pulp

flue gas

steam

fuel

(b)

waste heat

vapour

steam

fuel - Θ

vapour

steam ^ I ^ 1

pressed pulp dried pulp

U partly dried pulp

dried pulp

F i g . 4 .25 . E n e r g y sys tem l a y o u t s f o r s t e a m - d r y i n g o f t h e p u l p : ( a ) d o u b l e - s t a g e d r y i n g , steam d r y e r hea ted w i t h e x h a u s t steam ( a f t e r r e f . 3 8 ) , ( b ) steam d r y e r hea ted w i t h h i g h - p r e s s u r e steam ( a f t e r r e f s . 1 3 , 6 3 ) . 1 - b o i l e r , 2 - t u r b i n e , 3 - s u g a r m a n u f a c t u r i n g p r o c e s s , 4 - l o w - t e m p e r a t u r e d r y e r , 5 - steam d r y e r , 6 - w a s t e - h e a t r e c o v e r y s u b s y s t e m , 7 - v a p o u r w a s h e r .

A D a n i s h d e s i g n emp loy ing t h e c e l l u l a r f l u i d i z e d - b e d t e c h n i q u e i s shown i n

F i g . 4 . 2 4 ( b ) ( a f t e r r e f s . 1 3 , 6 3 ) . The f l u i d i z e d bed c o n s i s t s o f m u l t i p l e c e l l s

a r r a n g e d i n a c i r c l e s u r r o u n d i n g t h e t u b u l a r h e a t e r s i t u a t e d i n t h e c e n t r e o f

a v e r t i c a l v e s s e l . I n a p r o t o t y p e a p p l i c a t i o n , t h e d r y e r i s r a t e d 6 t / h

e v a p o r a t e d w a t e r . I t i s hea ted by steam a t 12 ba r and 220°C f rom a b o i l e r w h i c h

happens t o be a v a i l a b l e i n t h e f a c t o r y i n q u e s t i o n ; t h i s steam does n o t

184

c o n t r i b u t e t o power g e n e r a t i o n . The v a p o u r p r e s s u r e i s m a i n t a i n e d a t 3.5 ba r and

t he t e m p e r a t u r e i s 162°C above t he f l u i d i z e d bed and 189°C a t t h e h e a t e r o u t l e t .

The r e c i r c u l a t e d v a p o u r f l o w i s 35-40 t imes l a r g e r t han t h e w a t e r e v a p o r a t i o n .

As v a p o u r e n e r g y i s d i r e c t e d t o t h e e v a p o r a t o r s t a t i o n , t he hea t consumpt i on i n

s u g a r manu fac tu re can be r e d u c e d . A p o s s i b l e e n e r g y - s y s t e m l a y o u t i s shown

s c h e m a t i c a l l y i n F i g . 4 . 2 5 ( b ) .

C o n s i d e r a b l e hea t s a v i n g s can be imag ined i n s u g a r f a c t o r i e s emp loy ing s team-

d r y i n g o f t h e p u l p and v a p o u r c o m p r e s s i o n . F o r examp le , i t i s p r o p o s e d t o

s u p e r h e a t v a p o u r w i t h d r a w n f rom the second e v a p o r a t o r e f f e c t and s u b s e q u e n t l y

compressed , and t o d e l i v e r i t t o a steam d r y e r ( r e f . 6 4 ) . T h i s w o u l d make i t

p o s s i b l e t o i n t e g r a t e e n e r g y economy i n s u g a r manu fac tu re and p u l p d r y i n g i n

v a r i o u s t y p e s o f e n e r g y s y s t e m s , i n c l u d i n g t h o s e emp loy ing gas t u r b i n e s .

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1 L. R o s e n b e r g , T e c h n o l o g i c a l changes i n some o f E u r o p e ' s s u g a r p r o d u c i n g c o u n t r i e s . S u g a r . J . , 4 6 ( 5 ) (1983) 7-11.

2 E. R e i n e f e l d , Uber d i e Kampagne 1985, Z u c k e r i n d . , 111(4) (1986) 303-313. 3 E.W. K r a u s e , New equ ipment and p r o c e s s e s i n t h e s u g a r i n d u s t r y , i n : F . O .

L i c h t Yearbook and D i r e c t o r y , R a t z e b u r g , 1985, p p . E5 -E36 . 4 H. S c h i w e c k , T h . C r o n e w i t z and G . W i t t e , Some t h o u g h t s on t he c l a s s i c a l

method o f j u i c e p u r i f i c a t i o n . Sugar J . , 47(11) (1985) 18-22. 5 A . I . Khomenko, 0 t e p l o v o i e k o n o m i c h n o s t i s i s t e m d i f f u z i y a - d e f e k a t s i y a ,

Sakh . P r o m . , (11 ) (1983) 42-47 . 6 G . W i t t e and H. S c h i w e c k , D i e Ausnu t zung des Wärme inha l t es von C a r b o n a t a -

t i o n s b r ü d e n , Z u c k e r i n d . , 109(8) (1984) 706-710. 7 Anonymous, R e c u p e r a t i o n t h e r m i q u e s u r buees de seconde c a r b o n a t a t i o n a l a

R a f f i n e r i e Notre-Dame a O r e y e , S u c r . B e i g e , 103 (1985) 5-11. 8 T e c h n i c a l i n f o r m a t i o n f rom F i v e s - C a i l Babcock , L i l l e , 1986. 9 W. Lekawski and K. U r b a n i e c , E n e r g y s a v i n g t h r o u g h m o d i f i c a t i o n o f t h e

c a r b o n a t a t i o n p r o c e s s , Z u c k e r i n d . , 110(9) (1985) 810-813. 10 Τ . B o g u m i l , E x p e r i m e n t a l i n v e s t i g a t i o n s o f t h e c a r b o n a t a t i o n p r o c e s s a t

i n c r e a s e d p r e s s u r e , Z u c k e r i n d . , 111(6) (1986) 565-568. 11 Κ. U r b a n i e c , Heat economy improvements a s s o c i a t e d w i t h t h e c a r b o n a t a t i o n

p r o c e s s i n b e e t s u g a r p l a n t s . Paper p r e s e n t e d a t t h e I n t e r n a t i o n a l C o n f e r e n c e " Improvement o f t h e Bee t Sugar P r o d u c t i o n " , Warszawa, May 1987.

12 R . F . Madsen, V e r s c h i e d e n e Z u c k e r h a u s k o n z e p t e und i h r E i n f l u s s a u f den E n e r g i e v e r b r a u c h , Z u c k e r i n d . , 111(12) (1986) 1121-1126.

13 R . F . Madsen, P r o g r e s s i n Dan i sh s u g a r p r o d u c t i o n w i t h i n t h e p a s t d e c a d e , Paper p r e s e n t e d a t t h e I n t e r n a t i o n a l C o n f e r e n c e " Improvement o f t h e B e e t Sugar P r o d u c t i o n " , Warszawa, May 1987.

14 K. Wagne rowsk i , D. Dabrowska and C . D a b r o w s k i , Prob leme d e r M e l a s s e r s c h ö p f u n g , Ζ . Z u c k e r i n d . , 12(9) (1962) 664-671.

15 Η. S c h i w e c k , M ö g l i c h k e i t e n z u r Senkung des E n e r g i e b e d a r f s im Z u c k e r h a u s , Z u c k e r , 30(10) (1977) 525-534.

16 K . E . Aus tmeyer and R. Marwede, E n t w u r f und B i l a n z i e r u n g w e i t e r f ü h r e n d e r Z u c k e r h a u s k o n z e p t e , Z u c k e r i n d . , 112(3) (1987) 193-201.

17 S. M a t u s c h , P r a k t i s c h e E r f a h r u n g e n m i t den K ü h l u n g s k r i s t a l 1 i s a t o r KKT, Z u c k e r i n d . , 112(4) (1987) 274-276.

18 H. E i c h h o r n , A r b e i t s w e i s e m i t k o n t i n u i e r l i c h e n Vakuum-Ma ischen , System B e g h i n - S a y , i n d e r R a f f i n e r i e E l s d o r f , Z u c k e r i n d . , 112(2) 114-117.

19 D. S c h l i e p h a k e , K . E . Aus tmeyer and R. Hempelmann, K ü h l u n g s k r i s t a l l i s a t i o n von Magmen h ö h e r e r R e i n h e i t , Z u c k e r i n d . , 110(4) (1987) 269-273.

20 A . R . S a p r o n o v , V . l . T u z h i l k i n and A . P . S h c h e r e n k o , Sovremennye n a p r a v l e n i y a ν u l u c h s h e n i i k r i s t a l l i z a t s i i s a k h a r a , Sakh . P r o m . , ( 5 ) (1985) 42-44 .

185

21 Ε. R e i n e f e l d , Über d i e Kampagne 1979, Z u c k e r i n d . , 105(4) (1980) 329-340. 22 E. R e i n e f e l d , Über d i e Kampagne 1981, Z u c k e r i n d . , 107(5) (1982) 369-380. 23 K . E . A u s t m e y e r , A n a l y s i s o f s u g a r b o i l i n g and i t s t e c h n i c a l c o n s e q u e n c e s .

I n t . Sugar J . , 88 ( 1 9 8 6 ) , P a r t I (1045) 3 - 7 , P a r t I I (1046) 23-29 , P a r t I I I (1047) 50-55.

24 H. Sch iweck and M. M u n i r , Das H e r s t e l l e n e i n e s gemeinsames K r i s t a l l f u s s magmas f ü r W e i s s z u c k e r - 1 und -2 nach dem V e r f a h r e n d e r Süddeu tschen Z u c k e r - A G , Paper p r e s e n t e d a t t h e I n t e r n a t i o n a l C o n f e r e n c e " Improvement o f t he Bee t Sugar P r o d u c t i o n " , Warszawa, May 1987.

25 R . F . Madsen and W. Ko fod N i e l s e n , D ie Kampagne 1977 i n d e r " A / S De Danske S u k k e r f a b r i k k e r " , Z u c k e r i n d . , 103(10) (1978) 831-839.

26 Anonymous, E rzeugung von R ü b e n r o h s a f t d u r c h m e h r s t u f i g e G e g e n s t r o m -Auswaschung von z e r k l e i n e r t e m Rübenmate r i a l be i n i e d r i g e n T e m p e r a t u r e n , Z u c k e r i n d . , 110(8) (1985) 709-710.

27 J . M . R a n d a l l , R . H . Edwards and E. Z a r a g o s a , E x p r e s s i o n o f j u i c e f rom s u g a r bee t t i s s u e . Paper p r e s e n t e d a t 23rd ASSBT M e e t i n g , San D i e g o , F e b r u a r y 1985.

28 T . B a l o h , Reversosmose i n d e r T e c h n o l o g i e des Z u c k e r s , Ζ . Z u c k e r i n d . , 25 (8 ) (1975) 452-456.

29 S . E . B i c h s e i and A . M . S a n d r e , A p p l i c a t i o n o f membrane t e c h n o l o g y t o j u i c e c o n c e n t r a t i o n . I n t . Sugar J . , 84(1005) (1982) 266-268.

30 W. Ko fod N i e l s e n , S . K r i s t e n s e n and R . F . Madsen, P r o s p e c t s and p o s s i b i l i t i e s i n a p p l i c a t i o n o f membrane f i l t r a t i o n sys tems w i t h i n t h e b e e t and cane s u g a r i n d u s t r y . Sugar T e c h . R e v . , 9 ( 1 ) (1982) 59-117.

31 T . R . Hanssens ( e t a l . ) . U l t r a f i l t r a t i o n as an a l t e r n a t i v e f o r raw j u i c e p u r i f i c a t i o n i n t h e b e e t s u g a r i n d u s t r y , Z u c k e r i n d . , 109(2) (1084) 152-156.

32 W. Cape l i n . Bee t j u i c e c o n c e n t r a t i o n by r e v e r s e o s m o s i s . I n t . Sugar J . , 84(1007) (1982) 323-324.

33 P . J . Wrobel and J . A . H e i s t , Sugar c r y s t a l l i z a t i o n f rom b e e t j u i c e s and mo lasses u s i n g t h e h y d r a t e f r e e z i n g p r o c e s s . I n t . Sugar J . , 89(1062) (1987) 111-117.

34 S . E . B i c h s e l , M. C l e a r y and T . S . B a r r o n , Steam consumpt ion r e d u c t i o n by e u t e c t i c f r e e z e c r y s t a l l i z a t i o n o f s u c r o s e . Paper p r e s e n t e d a t 23rd ASSBT M e e t i n g , San D i e g o , F e b r u a r y 1985.

35 K. B u c h h o l z ( e t a l . ) , U n t e r s u c h u n g e n z u r B i l d u n g von B i o g a s aus R ü b e n p r e s s -s c h n i t z e l n , Z u c k e r i n d . , 11(9) (1986) 837-845.

36 E. T h i e r , K o n s e r v i e r u n g s t e c h n i s c h e und w i r t s c h a f t l i c h e A s p e k t e des P r e s s s c h n i t z e l a b s a t z e s , Z u c k e r i n d . , 106(1) (1981) 60-65.

37 J . B e c k h o f f and C . H e l l e r , P r e s s s c h n i t z e l - e i n e A l t e r n a t i v e z u r S c h n i t z e l t r o c k n u n g , Z u c k e r i n d . , 108(3) (1983) 213-217.

38 M. Kunz and P. V a l e n t i n , S c h n i t z e l t r o c k n u n g ohne P r i m ä r e n e r g i e e i n s a t z u n t e r a u s s c h l i e s s l i c h e r Nu tzung de r Abwärme- und E i n d a m p f P o t e n t i a l e d e r Z u c k e r f a b r i k , Z u c k e r i n d . , 111(8) (1986) 741-750.

39 T h . C r o n e w i t z ( e t a l . ) . Über den E i n f l u s s v e r s c h i e d e n e r G r ö s s e n a u f den E n e r g i e b e d a r f und S t a u b e m i s s i o n von S c h n i t z e l t r o c k n u n g s a n l a g e n u n t e r B e r ü c k s i c h t i g u n g des V e r w e i l Z e i t v e r h a l t e n s d e r S c h n i t z e l während d e r T r o c k n u n g , Z u c k e r , 28 (8 ) (1975) 401-410.

40 H. H u b e r , Bestimmung des o p t i m a l e n E n e r g i e v e r b r a u c h s f ü r d i e E n t w ä s s e r u n g d e r e x t r a h i e r t e n S c h n i t z e l , Z u c k e r , 30 (9 ) (1977) 485-489.

41 T h . C r o n e w i t z , Wege z u r r a t i o n e l l e n E n e r g i e v e r w e n d u n g be i d e r S c h n i t z e l -t r o c k n u n g i n d e r Z u c k e r i n d u s t r i e , Z u c k e r i n d . , 105(2) (1980) 129-139.

42 M. Demaux, P r e s s a g e e t sechage des p u l p e s de b e t t e r a v e s . F a c t e u r s de r e d u c t i o n des c o u t s . I n d . A l i m . A g r i e , 102 (7 -8 ) (1985) 723-730.

43 Μ. Demaux, C o u t de l ' e n e r g i e e t p r e s s a g e des p u l p e s de b e t t e r a v e s , I n d . A l i m . A g r i e , 103(7 -8 ) (1986) 661-667.

44 O p t i m i e r u n g d e r mechan ischen S c h n i t z e l a b p r e s s u n g , Z u c k e r i n d . , 106(11) (1981) 965-981.

45 Symposium: " S c h n i t z e l a b p r e s s u n g - S tand 1987" , Z u c k e r i n d . , 112(7) (1987) 571-579.

46 F. Hol l a u s and G . P o l l a c h , V e r b e s s e r u n g d e r S c h n i t z e l a b p r e s s u n g d u r c h g e s t e u e r t e I n f e k t i o n , Z u c k e r i n d . , 111(11) (1986) 1025-1030.

186

47 P. C a u l k i n s , G . Holman and L . Norman, Gypsum - c o s t - e f f e c t i v e p r e s s i n g a i d . Sugar J . , 47 (12) (1985) 21-23.

48 J . M . R a n d a l l , W. Camirand and E . M . Z a r a g o s a , E n e r g y r e d u c t i o n by c o s s e t t e l i m i n g , Z u c k e r i n d . , 107(1) (1982) 38-46.

49 Κ. B u c h h o l z , R. T a r r a c h and K . - M . B l i e s e n e r , Chemische A s p e k t e d e r mechan ischen S c h n i t z e l e n t w ä s s e r u n g , Z u c k e r i n d . , 111(1) (1986) 23-27 .

50 E . R e i n e f e l d , Über d i e Kampagne 1980, Z u c k e r i n d . , 106(5) (1981) 397-406. 51 D. S c h r ö d e r , E i n i g e Gedanken zum E i n s a t z e i n e r N i e d e r t e m p e r a t u r t r o c k n u n g

i n n e r h a l b d e r S c h n i t z e l t r o c k n u n g , Z u c k e r i n d . , 108(2) (1983) 126-135. 52 P. V a l e n t i n , E r h ö h t e Abwärmenutzung d e r Z u c k e r f a b r i k i n d e r N i e d e r

t e m p e r a t u r t r o c k n u n g , Z u c k e r i n d . , 108(11) (1983) 1025-1033. 53 K . E . Aus tmeyer and W. P o e r s c h , N i e d e r t e m p e r a t u r t r o c k n u n g - G r u n d l a g e n und

B e t r a c h t u n g e n z u r W i r t s c h a f t l i c h k e i t , Z u c k e r i n d . , 108(9) (1983) 861-868, 108(11) (1983) 1033-1041, 109(5) (1984) 411-419, 110(1) (1985) 28-34.

54 H. K l e b e r , Anwärmung von L u f t f ü r d i e N i e d e r t e m p e r a t u r - T r o c k n u n g , Z u c k e r i n d . , 110(8) (1985) 686-688.

55 W. K u n z , D ie N i e d e r t e m p e r a t u r t r o c k n u n g i n V e r b i n d u n g m i t d e r t r a d i t i o n e l l e n S c h n i t z e l t r o c k n u n g , Z u c k e r i n d . , 108(9) (1983) 868-870.

56 Anonymous, N i e d e r t e m p e r a t u r - S c h n i t z e l t r o c k n u n g i n d e r Z u c k e r f a b r i k A r t e n a y , Z u c k e r i n d . , 108(2) (1983) 135.

57 E . S c h r ö t e r , D i e N i e d e r t e m p e r a t u r t r o c k n u n g i n L e h r t e - F u n k t i o n s w e i s e und B e t r i e b s e r f a h r u n g e n , Z u c k e r i n d . , 111(6) (1986) 545-549.

58 K . E . Aus tmeyer and U . B u n e r t , Abwärmenutzung im Zusammenhang m i t d e r S c h n i t z e l t r o c k n u n g , 110(8) (1985) 659-670.

59 Anonymous, P i l o t a n l a g e f ü r S c h n i t z e l t r o c k n u n g m i t t e l s Dampf, Z u c k e r i n d . , 110(1) (1985) 54.

60 Anonymous, Dampf t rockne r f ü r S c h n i t z e l , Z u c k e r i n d . , 110(8) (1985) 707-708. 61 Anonymous, S team-hea ted p u l p d r y e r , i n : F . O . L i c h t Yearbook and D i r e c t o r y ,

R a t z e b u r g , 1986, p p . G5-G9 . 62 C . Gudmundson, p e r s o n a l commun ica t i on . 63 A . S l o t h J e n s e n ( e t a l . ) . Bee t p u l p d r y i n g i n s u p e r h e a t e d steam under

p r e s s u r e , Z u c k e r i n d . , 112(10) (1987) 886-891. 64 Ε . O t o r o w s k i , P u l p d r y i n g . Paper p r e s e n t e d a t t h e I n t e r n a t i o n a l C o n f e r e n c e

" Improvement o f t he Bee t Sugar P r o d u c t i o n " , Warszawa, May 1987.

187

C h a p t e r 5

EQUIPMENT DESIGN FOR E F F I C I E N T ENERGY U T I L I Z A T I O N

5.1 SCOPE OF THE PROBLEMS

The deve lopments o f e n e r g y - s a v i n g p r o c e s s e s and equ ipment a r e m u t u a l l y

d e p e n d e n t . W h i l e t h e p r o c e s s r e q u i r e m e n t s i n f l u e n c e equ ipment d e s i g n , i t i s t h e

a t t a i n a b l e equ ipment c h a r a c t e r i s t i c s w h i c h a r e o f t e n d e c i s i v e i n c o n s t r a i n i n g

t h e pa rame te rs o f a p r o c e s s . T h i s a p p l i e s t o l a r g e s e c t i o n s o f t h e s u g a r

m a n u f a c t u r i n g p r o c e s s i n w h i c h i n t e r a c t i o n s between numerous i n t e r c o n n e c t e d

equ ipment u n i t s a r e t a k i n g p l a c e ( j u i c e p u r i f i c a t i o n , s u g a r c r y s t a l l i z a t i o n ,

e t c . ) , as w e l l as t o u n i t o p e r a t i o n s p e r f o r m e d i n s p e c i a l i z e d equ ipment

( e x t r a c t i o n , h e a t i n g , e t c . ) .

By i n t r o d u c i n g s e l e c t i v e d e s i g n changes o r a p p l y i n g e n t i r e l y new equ ipment

d e s i g n s , i t becomes p o s s i b l e t o c u t down t h e power demand and t h e t o t a l h e a t

demand o f a s u g a r f a c t o r y . I n a d d i t i o n , improved c h a r a c t e r i s t i c s o f t h e

equ ipment u n i t s o f w h i c h a the rma l sys tem i s composed may c o n t r i b u t e t o an

improved e f f e c t i v e n e s s r a t i o and t h u s r e d u c e d n e t h e a t demand. C o n s i d e r a b l e

p r o g r e s s has been a c h i e v e d i n t h e s e a r e a s i n r e c e n t y e a r s . I n t h e p r e s e n t

C h a p t e r , d e s i g n t r e n d s a r e r e v i e w e d r e l a t i n g t o t h e f o l l o w i n g e q u i p m e n t :

- e x t r a c t o r s ,

- e v a p o r a t o r s ,

- h e a t e x c h a n g e r s ,

- vacuum p a n s ,

- c e n t r i f u g a l s .

5.2 EXTRACTORS

The e s s e n t i a l f u n c t i o n o f an e x t r a c t o r can be d e s c r i b e d as c o u n t e r - c u r r e n t

l e a c h i n g o f c o s s e t t e s . I n i t i a l the rma l breakdown o r d e n a t u r a t i o n o f c e l l

membranes i s r e q u i r e d t o make s u c r o s e e x t r a c t i o n p o s s i b l e . D e n a t u r a t i o n b e g i n s

a t 50-60°C and becomes a lmos t i n s t a n t a n e o u s a t t e m p e r a t u r e s above 90°C. F o r t h i s

r e a s o n , i t i s n e c e s s a r y t o s u p p l y hea t t o t h e e x t r a c t o r , w h i c h i s t h u s a l s o

a component o f t h e the rma l s y s t e m .

As t h e d r i v i n g f o r c e o f s u c r o s e e x t r a c t i o n i s t h e d i f f e r e n c e o f c o n c e n t r a t i o n

between t h e c o s s e t t e s and t h e e x t r a c t i n g j u i c e , t h e c o n c e n t r a t i o n o f t h e j u i c e

can n e v e r exceed t h a t i n t h e e x t r a c t e d m a t e r i a l . T a k i n g i n t o a c c o u n t t h a t t h e

c o s s e t t e s c o n t a i n abou t 95% c e l l j u i c e , t h e j u i c e d r a f t can n e v e r be l o w e r t han

95%; t o a t t a i n t h i s t h e o r e t i c a l l i m i t , an i d e a l e x t r a c t o r o f i n f i n i t e l e n g t h

wou ld be r e q u i r e d . I n r e a l i t y , i n o r d e r t o i n c r e a s e t h e d r i v i n g f o r c e and

reduce equ ipment s i z e , e x c e s s w a t e r i s f e d t o t h e e x t r a c t o r , t h i s c a u s i n g

188

i n c r e a s e d j u i c e d r a f t . The p r a c t i c a l r ange o f j u i c e d r a f t s i s 105-140%.

From t h e t r e n d t owa rd e n e r g y s a v i n g , two b a s i c r e q u i r e m e n t s t o w h i c h

e x t r a c t o r d e s i g n e r s must r e s p o n d f o l l o w :

- p r o d u c t i o n o f c o l d raw j u i c e , w h i c h a l l o w s t h e r e c o v e r y o f l o w - t e m p e r a t u r e

hea t w h i c h wou ld o t h e r w i s e be w a s t e d ;

- t h e p o s s i b i l i t y o f o p e r a t i n g t h e e x t r a c t o r a t low j u i c e d r a f t , w h i c h r e s u l t s

i n a r e d u c t i o n o f t he amount o f w a t e r t o be e v a p o r a t e d .

U s i n g a c l a s s i f i c a t i o n w i t h r e s p e c t t o t h e way t h e c o s s e t t e s a re t r a n s p o r t e d

w i t h i n t h e e x t r a c t o r , f o u r d e s i g n s a r e t h e most w i d e l y used t o d a y : t o w e r t y p e ,

s c r o l l ( o r t r o u g h ) t y p e , moving bed ( o r b e l t ) t y p e and drum t y p e . T h e i r w o r k i n g

p r i n c i p l e s a r e shown s c h e m a t i c a l l y i n F i g . 5 .1 . As can be s e e n , t he p r o d u c t i o n

o f c o l d raw j u i c e has been c h a r a c t e r i s t i c o f t he t r o u g h - t y p e (DOS) e x t r a c t o r

o n l y . I n t h i s d e s i g n , hea t i s t r a n s f e r r e d t o t h e e x t r a c t i o n m i x t u r e f rom v a p o u r

c o n d e n s i n g i n h e a t i n g j a c k e t s a t t a c h e d t o t he t r o u g h . I t seems t h a t a t l a r g e

c a p a c i t i e s , h o w e v e r , t he h e a t i n g j a c k e t s a r e n o t e f f e c t i v e enough t o e n s u r e

a c o r r e c t t e m p e r a t u r e d i s t r i b u t i o n i n t h e e x t r a c t i o n m i x t u r e , so 3000 t / d i s t h e

p r a c t i c a l l i m i t o f t he c a p a c i t y o f t r o u g h - t y p e e x t r a c t o r s t o d a y .

T h e r e i s no c o n s t r a i n t o f t h i s k i n d r e l a t i n g t o t h e o t h e r e x t r a c t o r t y p e s .

(a)

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h 5 (b) Μ

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7 ^ Γ 6

F i g . 5 .1 . Work ing p r i n c i p l e s o f e x t r a c t o r s : ( a ) t o w e r , ( b ) moving b e d , ( c ) t r o u g h , ( d ) drum. Μ - h e a t e x c h a n g e r c o s s e t t e s - j u i c e ( m i x e r ) , Η - j u i c e h e a t e r . 1 - c o s s e t t e s , 2 - f r e s h w a t e r , 3 - p r e s s w a t e r , 4 - raw j u i c e , 5 -e x h a u s t e d c o s s e t t e s , 6 - s team, 7 - r e c i r c u l a t e d j u i c e , 8 - e x t r a c t i o n m i x t u r e .

189

because h e a t i s s u p p l i e d v i a s e p a r a t e hea t e x c h a n g e r s t o t h e j u i c e r e c i r c u l a t e d

a t t he r a t e o f up t o 300%. O r i g i n a l l y i n v e n t e d t o d e l i v e r h o t raw j u i c e , t h e s e

d e s i g n s must be m o d i f i e d by add ing a h e a t e x c h a n g e r ( m i x e r ) i n w h i c h j u i c e i s

c o o l e d , i n c o u n t e r - f l o w , by incoming c o s s e t t e s . The m i x e r can be i n s t a l l e d as a

s e p a r a t e u n i t , b u t i n new d r u m - t y p e d e s i g n s , i t can a l s o be s t r u c t u r a l l y

i n t e g r a t e d w i t h t h e e x t r a c t o r p r o p e r . The a d d i t i o n o f a m i x e r r e s u l t s i n

i n c r e a s e d i n v e s t m e n t c o s t s , w h i c h can a p p a r e n t l y be o f f - s e t by c o s t r e d u c t i o n s

a s s o c i a t e d w i t h e n e r g y s a v i n g s .

The a p p l i c a t i o n o f m i x e r s i n new e x t r a c t i o n s t a t i o n s can be c o n s i d e r e d as

s t a n d a r d p r a c t i c e nowadays , and m i x e r s a r e a l s o added t o t h e e x i s t i n g e x t r a c t o r s

( r e f . 1 ) . One case has been r e p o r t e d o f a t r o u g h - t y p e a p p a r a t u s ( f o r m e r l y used

as an e x t r a c t o r ) a p p l i e d as a m i x e r l i n k e d t o a d r u m - t y p e e x t r a c t o r ( r e f . 2 ) .

The a t t a i n a b l e j u i c e t e m p e r a t u r e i s u s u a l l y 11-15 Κ above t h e c o s s e t t e s

t e m p e r a t u r e . Depend ing on t h e e x t r a c t o r t y p e and l o c a l c o n d i t i o n s , steam s a v i n g s

o f t he o r d e r o f 0.5 kg/100 kg b have been r e p o r t e d ( r e f . 3 ) ; t h i s f i g u r e s h o u l d

be t r e a t e d c a u t i o u s l y , as i t depends on t h e accompany ing c o r r e c t i o n s o f t h e

therma l sys tem as w e l l . As p o i n t e d o u t by G e n i e ( r e f . 4 ) , h o w e v e r , i t makes

l i t t l e sense t o i n v e s t t o o much i n a m i x e r subsys tem because t h e the rma l r e t u r n s

d i m i n i s h i f a c e r t a i n m i x e r s i z e i s exceeded a n d , i n a d d i t i o n , s u c r o s e l o s s e s

i n c r e a s e as a r e s u l t o f i n c r e a s e d b a c t e r i a l a c t i v i t y on a c c o u n t o f low

t e m p e r a t u r e s and l ong r e t e n t i o n t i m e s .

Lower i ng o f t h e j u i c e d r a f t has a l r e a d y been d i s c u s s e d i n S e c t i o n 1 .3 .4 .

Be ing a l w a y s a p remise f o r r e d u c e d e n e r g y c o n s u m p t i o n , i t becomes a lmos t a

n e c e s s i t y i n modern s u g a r f a c t o r i e s i n w h i c h hea t consumpt ion has been d e c r e a s e d

t o a c e r t a i n l e v e l . The a s s o c i a t e d d e c r e a s e i n t h e demand f o r h e a t i n g v a p o u r s

may l e a d t o t he i m p o s s i b i l i t y o f f u l l u t i l i z a t i o n o f v a p o u r s f rom t h e

e v a p o r a t i o n p r o c e s s , u n l e s s v a p o u r c o m p r e s s i o n i s a p p l i e d . L o w e r i n g o f t h e j u i c e

d r a f t wou ld be much more c o n v e n i e n t , b u t i t r a i s e s e x t r a c t o r d e s i g n p rob lems

wh i ch s t i l l remain t o be s o l v e d . A b e t t e r u n d e r s t a n d i n g o f t h e d e s i g n

r e q u i r e m e n t s has been a c h i e v e d i n r e c e n t y e a r s , owing t o advances i n t h e t h e o r y

o f t h e e x t r a c t i o n p r o c e s s ( r e f s . 5 - 8 ) .

Each e x t r a c t o r can be c h a r a c t e r i z e d by a number o f mass t r a n s f e r u n i t s , w h i c h

can be d e f i n e d as t h e number o f s t e p s i n a p e r f e c t b a t c h e x t r a c t o r t h a t w o u l d be

r e q u i r e d t o p r o d u c e t he same j u i c e f rom t h e same c o s s e t t e s w i t h t h e same l o s s o f

s u g a r i n e x h a u s t e d c o s s e t t e s ( f o r ma themat i ca l f o r m u l a e , see r e f . 6 ) . P r a c t i c a l

v a l u e s range f rom 8 t o 16; t h e h i g h e r t h e f i g u r e t h e b e t t e r i s t h e e x t r a c t o r ,

and t h e l o w e r t h e j u i c e d r a f t t h a t can be m a i n t a i n e d a t a g i v e n s u g a r l o s s . F o r

t he d r u m - t y p e d e s i g n , t he r e q u i r e m e n t s have been summar ized by G e n i e ( r e f . 4 ) i n

a g raph ( F i g . 5 . 2 ) . As can be s e e n , a r e d u c t i o n i n j u i c e d r a f t r e q u i r e s a

s t r u c t u r a l change i n e x t r a c t o r d e s i g n ; i . e . , an i n c r e a s e d number o f t r a n s f e r

190

101 evaporated water less per 100 kg beets

I L 10 15

No. of t ransfer units

20

F i g . 5 . 2 . R e d u c t i o n o f j u i c e d r a f t v s . number o f t r a n s f e r u n i t s o f e x t r a c t o r f o r 0.2% s u c r o s e l o s s on b e e t s and 18% DS i n p r e s s e d p u l p ( a f t e r r e f . 4 ) .

u n i t s . S i m i l a r r e l a t i o n s h i p s e x i s t f o r o t h e r e x t r a c t o r t y p e s , t h i s s t i m u l a t i n g

a l s o numerous m o d e r n i z a t i o n s o f e x i s t i n g u n i t s . When i n c r e a s i n g t h e h e i g h t o f

a t o w e r o r t h e l e n g t h o f a t r o u g h , o t h e r improvements a r e i n t r o d u c e d as w e l l ,

p a r t i c u l a r l y e a s i l y c o n t r o l l e d d r i v e s , improved m i x i n g / t r a n s p o r t i n g e l e m e n t s ,

more e f f e c t i v e h e a t - s u p p l y s u b s y s t e m s , e t c . ( r e f s . 1 , 9 - 1 1 ) .

5.3 EVAPORATORS

5.3.1 C o n v e n t i o n a l a p p l i c a t i o n s

I t was p o i n t e d o u t i n S e c t i o n 3 .3 .2 t h a t a h i g h h e a t t r a n s f e r i n t e n s i t y

s h o u l d be r e g a r d e d as an i m p o r t a n t f e a t u r e o f t h e e v a p o r a t o r s . A t low e n e r g y

c o s t , i n t e n s i v e hea t t r a n s f e r has been t r e a t e d as a means t o r e d u c e h e a t i n g

s u r f a c e a r e a s and t h u s i n v e s t m e n t c o s t s . The t r e n d t o w a r d s e n e r g y s a v i n g s

i n i t i a t e d changes i n t h e a t t i t u d e o f d e s i g n e r s . I n a m u l t i p l e - e f f e c t e v a p o r a t o r

s t a t i o n , a h i g h p r i o r i t y i s g i v e n nowadays t o m a i n t a i n i n g v a p o u r t e m p e r a t u r e s

f a c i l i t a t i n g u t i l i z a t i o n o f l o w - g r a d e h e a t f rom t h e f i n a l e f f e c t s . As a

c o n s e q u e n c e , t e m p e r a t u r e d i f f e r e n c e s between v a p o u r s i n c o n s e c u t i v e e f f e c t s may

be s m a l l e r t han t h o s e recommended i n t h e p a s t , even a t t h e expense o f l a r g e r

h e a t i n g s u r f a c e a r e a s .

The t r a d e - o f f between h e a t i n g s u r f a c e a r e a and t e m p e r a t u r e d i f f e r e n c e has

been s t u d i e d by numerous a u t h o r s ( r e f s . 1 2 - 1 5 ) . A l t h o u g h no u n i v e r s a l d e s i g n

p r e s c r i p t i o n s have been f o r m u l a t e d , t h i s work n e v e r t h e l e s s c o n t r i b u t e s t o b e t t e r

u n d e r s t a n d i n g o f t h e r e l a t i o n s h i p s i n v o l v e d . L e t us c o n s i d e r an a p p r o x i m a t e

f o r m u l a g i v e n by Ba loh ( r e f . 15) f o r t h e op t ima l t e m p e r a t u r e d i f f e r e n c e Δ Τ ^ ^ ^ i n

an e v a p o r a t o r e f f e c t i n w h i c h t h e j u i c e t e m p e r a t u r e ( a b s o l u t e v a l u e ) i s Τ

( 5 . 1 )

where c , i s t h e annual c o s t o f 1 m h e a t i n g s u r f a c e a r e a , c i s t h e e n e r g y c o s t , a β

T ^ i s t he e n v i r o n m e n t t e m p e r a t u r e , k i s t h e o v e r a l l h e a t t r a n s f e r c o e f f i c i e n t ,

and τ i s t h e d u r a t i o n o f t he o p e r a t i n g s e a s o n .

191

As can be s e e n , t h e t e m p e r a t u r e d i f f e r e n c e i n an e v a p o r a t o r body s h o u l d be

r e d u c e d a t h i g h e n e r g y c o s t , h i g h l y i n t e n s i v e h e a t t r a n s f e r and p r o l o n g e d

s e a s o n s . I t s h o u l d be i n c r e a s e d , h o w e v e r , when t h e e v a p o r a t o r c o s t i n c r e a s e s

r e l a t i v e t o t h e e n e r g y c o s t , o r when t h e c a p i t a l c o s t i s i n c r e a s e d .

A n o t h e r i m p o r t a n t f e a t u r e o f t h e e v a p o r a t o r s i s t h e j u i c e r e t e n t i o n t ime and

i t s d i s t r i b u t i o n . A t j u i c e t e m p e r a t u r e s above 105°C, t h e r e t e n t i o n t ime

d e t e r m i n e s t h e amount o f h y d r o l i z e d s u c r o s e and decomposed i n v e r t s u g a r , t h i s

a f f e c t i n g c o l o u r f o r m a t i o n i n t h e j u i c e . As p o i n t e d o u t i n S e c t i o n 4 . 3 , a h i g h

j u i c e c o l o u r , n e c e s s i t a t i n g a l a r g e m a s s e c u i t e c i r c u l a t i o n , may i n d i r e c t l y

i n c r e a s e t h e e n e r g y demand o f t he s u g a r h o u s e . The p rob lem o f c o l o u r f o r m a t i o n

i n t h e e v a p o r a t o r s t a t i o n has been s t u d i e d i n r e c e n t y e a r s ( r e f . 16) and t h e

a s s o c i a t e d r e q u i r e m e n t on e v a p o r a t o r d e s i g n i s f a i r l y c l e a r : t h e mean j u i c e

r e t e n t i o n t ime s h o u l d be as s h o r t as p o s s i b l e , and t h e r e t e n t i o n t ime spec t rum

s h o u l d be c o n c e n t r a t e d c l o s e t o t h e mean v a l u e .

From the v a r i e t y o f r e q u i r e m e n t s men t i oned a b o v e , i t i s n e c e s s a r y t o e v a l u a t e

t he e x i s t i n g e v a p o r a t o r d e s i g n s c r i t i c a l l y . A t l e a s t t h r e e d i s a d v a n t a g e o u s

p r o p e r t i e s o f t h e p o p u l a r R o b e r t - t y p e e v a p o r a t o r can be i d e n t i f i e d :

- a p a r t o f t h e a v a i l a b l e t e m p e r a t u r e d i f f e r e n c e becomes l o s t because o f t h e

h y d r o s t a t i c h e a d ;

- t h e o v e r a l l h e a t t r a n s f e r c o e f f i c i e n t d e c r e a s e s r a p i d l y w i t h i n c r e a s i n g j u i c e

c o n c e n t r a t i o n ; i n t h e l a s t e v a p o r a t o r e f f e c t , i t s v a l u e may be 5-6 t imes s m a l l e r

t han t h a t i n t h e f i r s t e f f e c t ;

- mean j u i c e r e t e n t i o n t ime i s o f t h e o r d e r o f s e v e r a l m i n u t e s ; i n a d d i t i o n ,

owing t o n a t u r a l c i r c u l a t i o n i n a r e l a t i v e l y l a r g e l i q u i d v o l u m e , t h e s p e c t r u m

o f r e t e n t i o n t i m e s i s r a t h e r w i d e .

F o r t h i s r e a s o n o t h e r e v a p o r a t o r t y p e s , p a r t i c u l a r l y t h i n - f i l m t y p e s , have

r e c e i v e d a t t e n t i o n i n r e c e n t y e a r s . Among numerous d e s i g n s t h a t a r e r e v i e w e d

e l s e w h e r e ( r e f . 1 7 ) , t h e f a l l i n g - f i l m e v a p o r a t o r s seem t o be b e s t s u i t e d t o t h e

r e q u i r e m e n t s . T h r e e v e r s i o n s o f t h e t u b u l a r f a l l i n g - f i l m d e s i g n a r e shown

s c h e m a t i c a l l y i n F i g . 5.3 (where a s k e t c h o f a R o b e r t - t y p e u n i t i s a l s o g i v e n

f o r c o m p a r i s o n ) . T h i n j u i c e i s s p r e a d on t h e v e r t i c a l t u b e b u n d l e by means o f

a d i s t r i b u t o r d e v i c e , and f l o w s as a f i l m on t h e i n n e r t u b e w a l l f rom t o p t o

bo t t om. The t u b e s used i n t h e s u g a r i n d u s t r y a r e 6-12 m l o n g . I n o r d e r t o

p r e v e n t v i b r a t i o n s , t h e t u b e s a r e l e d t h r o u g h b a f f l e s p l a c e d abou t 2 m a p a r t . As

a r e s u l t o f h e a t i n g t h e c h e s t w i t h e x h a u s t steam o r v a p o u r , v a p o u r i s g e n e r a t e d

f rom the j u i c e . H a v i n g l e f t t h e t ube b u n d l e , t h e c o n c e n t r a t e d j u i c e f a l l s i n t o

t h e e v a p o r a t o r base and t h e v a p o u r f l o w s t h r o u g h a s e p a r a t o r t o t h e o u t l e t

n o z z l e .

The f u n c t i o n i n g o f t h e j u i c e d i s t r i b u t i o n d e v i c e i s o f c r i t i c a l impo r tance t o

t h e e f f i c i e n c y and r e l i a b i l i t y o f t he f a l l i n g - f i l m e v a p o r a t o r . I f w e t t i n g o f

192

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a t ube w a l l becomes i n s u f f i c i e n t , t h e r e i s a r i s k t h a t t h e l i q u i d f i l m w i l l t e a r

a p a r t , t h i s r e s u l t i n g i n s c a l e f o r m a t i o n on t h e t ube w a l l . I t i s t h e r e f o r e

e s s e n t i a l t h a t t h i s d e v i c e works w i t h o u t c l o g g i n g , e n s u r i n g a l s o u n i f o r m j u i c e

d i s t r i b u t i o n on t he t u b e - s h e e t s u r f a c e . The w o r k i n g p r i n c i p l e s o f f o u r p a t e n t e d

d i s t r i b u t o r d e s i g n s a r e shown i n F i g . 5 .4 .

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F i g . 5 .4 . J u i c e d i s t r i b u t i o n d e v i c e s employed i n f a l l i n g - f i l m e v a p o r a t o r s : ( a ) and ( b ) c i r c u l a r a r r a n g e m e n t s , ( c ) b a r s and tube i n s e r t s under m u l t i p l e n o z z l e s , ( d ) b a f f l e s under a s p r i n k l e r .

I n o r d e r t o a v o i d i n s u f f i c i e n t t ube w e t t i n g a t r e d u c e d l o a d , a r e c i r c u l a t i o n

pump can be a t t a c h e d t o t h e e v a p o r a t o r . U s i n g r e c i r c u l a t i o n , s a f e o p e r a t i o n can

be a c h i e v e d a t l o a d s as low as abou t 40% o f t he nominal v a l u e .

An e s t i m a t e o f t he a t t a i n a b l e o v e r a l l hea t t r a n s f e r c o e f f i c i e n t o f t he

f a l l i n g - f i l m e v a p o r a t o r as a f u n c t i o n o f j u i c e c o n c e n t r a t i o n i s shown i n

F i g . 5.5 ( a f t e r r e f . 1 8 ) , w i t h an a n a l o g o u s r e l a t i o n s h i p c h a r a c t e r i s t i c o f t he

R o b e r t - t y p e d e s i g n . As can be s e e n , w h i l e t h e r e i s no n o t i c e a b l e d i f f e r e n c e i n

i n t he f i r s t e v a p o r a t o r e f f e c t , t he v a l u e o f k a t j u i c e c o n c e n t r a t i o n s 35-70% DS

i n a f a l l i n g - f i l m u n i t can be up t o 100% above t h a t i n a R o b e r t - t y p e e v a p o r a t o r .

F o r t h i s r e a s o n , r e p l a c e m e n t s o f R o b e r t - t y p e u n i t s by f a l l i n g - f i l m ones i n f i n a l

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20 30 AO 50 60 Mean juice concentration ( % DS)

70

F i g . 5 .5 . O v e r a l l hea t t r a n s f e r c o e f f i c i e n t s o f R o b e r t and f a l l i n g - f i l m e v a p o r a t o r s ( a f t e r r e f . 1 8 ) .

e v a p o r a t o r e f f e c t s have been u n d e r t a k e n i n numerous f a c t o r i e s .

The mean j u i c e r e t e n t i o n t ime i n a f a l l i n g - f i l m u n i t i s t y p i c a l l y abou t

1.5 m i n . T a k i n g s p e c i a l measu res , i t can be c u t down t o l e s s than 30 s ( r e f . 1 9 ) ,

t h i s r e q u i r i n g p r o p e r p r e c a u t i o n s i n t h e f i e l d o f a u t o m a t i c c o n t r o l and s a f e t y

d e v i c e s . A s , i n a d d i t i o n , t he spec t rum o f r e t e n t i o n t imes i s q u i t e n a r r o w ,

f a l l i n g - f i l m u n i t s match v e r y w e l l t h e r e q u i r e m e n t s o f a p p l i c a t i o n i n t h e

i n i t i a l e v a p o r a t o r e f f e c t s . I t has a l s o been r e p o r t e d t h a t a t low j u i c e

c o n c e n t r a t i o n s , o n l y n e g l i g i b l e s c a l i n g may o c c u r i n a f a l l i n g - f i l m e v a p o r a t o r

even though s c a l e f o r m a t i o n was a s e r i o u s p rob lem i n a R o b e r t - t y p e u n i t

p r e v i o u s l y u s e d .

I n t h e f i r s t e v a p o r a t o r e f f e c t , t h e p rob lem o f e n t r a i n m e n t s e p a r a t i o n becomes

c r i t i c a l because o f t he r i s k t h a t j u i c e c a r r y o v e r w i l l l e a d t o t h e p r e s e n c e o f

s u g a r i n t h e s e c o n d - e f f e c t c o n d e n s a t e . I t seems t h a t t h e deve lopmen t o f

d i f f e r e n t e n t r a i n m e n t s e p a r a t o r s has advanced so f a r t h a t t h e y can be s a f e l y

a p p l i e d w i t h i n t h e i r r e s p e c t i v e r a n g e s o f o p e r a t i o n ( r e f s . 2 0 - 2 3 ) . C e n t r i f u g a l

s e p a r a t o r s a r e c h a r a c t e r i z e d by a r e l a t i v e l y l a r g e v a p o u r v e l o c i t y , 10-40 m /s ,

and a p r e s s u r e d rop o f 10-100 mm H^O. The p o p u l a r " z i g z a g " and c u r v i l i n e a r

b a f f l e s a re most e f f e c t i v e a t v a p o u r v e l o c i t i e s 3-12 m/s , w i t h r e s u l t i n g

p r e s s u r e d r o p s 12-25 mm H^O. F i n a l l y , mesh pads can be used a t 1.5-10 m/s and

12-50 mm H2O. W h i l e t h e c e n t r i f u g a l s e p a r a t o r s c a n n o t s t o p v e r y f i n e j u i c e

d r o p l e t s s m a l l e r t han 10 m i c r o n s , t he mesh pads a r e e f f e c t i v e down t o a d r o p l e t

d i a m e t e r o f 5 m i c r o n s . The l a t t e r d e s i g n i s however p rone t o p a r t i a l c l o g g i n g

by d r y s u b s t a n c e s o f t h e j u i c e , t h i s r e s u l t i n g i n t h e pe r fo rmance d e t e r i o r a t i n g

w i t h t i m e . I n o r d e r t o p r e v e n t d e p o s i t b u i l d - u p , p e r i o d i c a l wash ing w i t h w a t e r

may be r e q u i r e d . A d e t a i l o f a f a l l i n g - f i l m e v a p o r a t o r w i t h e n t r a i n m e n t

s e p a r a t i o n augmented by a mesh pad i s shown s c h e m a t i c a l l y i n F i g . 5 .6 .

195

^condensate

MESH PAD

F i g . 5 .6 . I n t e g r a t e d e n t r a i n m e n t s e p a r a t o r e q u i p p e d w i t h a mesh pad a t t h e base o f a f a l l i n g - f i l m e v a p o r a t o r .

5 .3 ,2 U n c o n v e n t i o n a l a p p l i c a t i o n s

I t was men t ioned i n S e c t i o n 1.2.5 t h a t s u b s t a n t i a l r e d u c t i o n s o f t h e t o t a l

h e a t demand i n s u g a r manu fac tu re may cause t h e o v e r a l l v a p o u r demand i n t h e

e v a p o r a t o r t o become s m a l l e r t han t h e amount o f w a t e r t o be e v a p o r a t e d f rom

j u i c e . A p o s s i b l e s o l u t i o n t o t h i s p rob lem i s t o combine c o n v e n t i o n a l m u l t i

s t a g e e v a p o r a t i o n w i t h one o r two e v a p o r a t i o n s t a g e s hea ted by was te h e a t ; l o w -

t e m p e r a t u r e v a p o u r s o b t a i n e d i n t h e a d d i t i o n a l e v a p o r a t o r s can be d i r e c t e d t o

t h e c o n d e n s e r . I t s h o u l d be o b s e r v e d t h a t t h i s i s an a l t e r n a t i v e t o a v a p o u r

c o m p r e s s i o n c i r c u i t i n w h i c h an e q u i v a l e n t amount o f f i r s t - o r s e c o n d - e f f e c t

v a p o u r i s r e c i r c u l a t e d . Compared w i t h v a p o u r c o m p r e s s i o n , i t has t h e a d v a n t a g e

o f n o t a f f e c t i n g t h e power b a l a n c e o f t h e f a c t o r y .

Two d i f f e r e n t c o n c e p t s o f u n c o n v e n t i o n a l e v a p o r a t i o n have been implemented i n

p r a c t i c e r e c e n t l y .

( i ) T h i c k e n i n g o f j u i c e between c o n v e n t i o n a l e v a p o r a t i o n s t a g e s o r a f t e r t h e

l a s t e v a p o r a t o r e f f e c t , u s i n g vacuum-pan v a p o u r s ( r e f s . 2 4 - 2 6 ) .

( i i ) P r e - e v a p o r a t i o n o f t h i n j u i c e p r i o r t o t h e c o n v e n t i o n a l m u l t i - s t a g e

e v a p o r a t i o n p r o c e s s , u s i n g o u t l e t gases f rom p u l p d r y i n g ( r e f . 2 7 ) .

A n o t h e r p r o p o s a l , s t i l l i n t h e d e s i g n s t a g e , c o n s i s t s o f p r e - e v a p o r a t i n g a p a r t

o f t h e t h i n j u i c e f l o w i n a d o u b l e - e f f e c t e v a p o r a t o r hea ted by vacuum-pan

v a p o u r s ( r e f . 2 8 ) .

I t i s c h a r a c t e r i s t i c o f a l l t h e s e c o n c e p t s t h a t i n o r d e r t o u t i l i z e t h e l o w -

g rade h e a t , t h e j u i c e t e m p e r a t u r e i n t h e e v a p o r a t o r s h o u l d be s u f f i c i e n t l y l o w .

The j u i c e i s t h e r e f o r e c o o l e d down p r i o r t o e n t e r i n g t h e s p e c i a l e v a p o r a t o r and

warmed up a f t e r l e a v i n g i t .

196

I n case ( i ) , a t 0.12 bar e v a p o r a t i o n p r e s s u r e and 54 C nominal j u i c e

t e m p e r a t u r e , t h e r e i s a p rob lem t h a t t h e j u i c e may become s u p e r s a t u r a t e d a t

u n d e s i r a b l e paramete r f l u c t u a t i o n s . F o r t h i s r e a s o n , i f t h e assumed t h i c k - j u i c e

c o n c e n t r a t i o n exceeds 72-73% DS, t hen t h e s p e c i a l e v a p o r a t o r i s more s a f e l y

u t i l i z e d p r i o r t o t he l a s t e v a p o r a t o r e f f e c t , as shown i n F i g . 5 . 7 ( a ) . A t l o w e r

c o n c e n t r a t i o n s , i t can be a t t a c h e d t o t h e e v a p o r a t o r o u t l e t . F i g . 5 . 7 ( b ) . The

p rob lem w i t h t he s p e c i a l e v a p o r a t o r i s t h a t t he p r e s s u r e s o f t h e h e a t i n g v a p o u r

and t he v a p o u r g e n e r a t e d f rom j u i c e a r e v e r y l o w . As a c o n s e q u e n c e , l a r g e c r o s s -

s e c t i o n s o f t he f l o w c h a n n e l s a re r e q u i r e d and h i g h v a p o u r v e l o c i t i e s a re

d i f f i c u l t t o a v o i d i n c e r t a i n e v a p o r a t o r p a r t s .

F i g . 5 . 7 . J u i c e e v a p o r a t i o n u s i n g vacuum-pan v a p o u r s i n c o n n e c t i o n w i t h c o n v e n t i o n a l f o u r - s t a g e e v a p o r a t i o n : ( a ) between s t a g e s 3 and 4 , ( b ) a f t e r s t a g e 4 . 1 - s p e c i a l e v a p o r a t o r , 2 - hea t e x c h a n g e r , 3 - s team, 4 - vacuum-pan v a p o u r , 5 - t h i n j u i c e , 6 - t h i c k j u i c e .

To t he knowledge o f t h e p r e s e n t a u t h o r , a l l t he e v a p o r a t o r s hea ted by vacuum-

pan v a p o u r s a re o f t he f a l l i n g - f i l m t y p e . A t l e a s t one m a n u f a c t u r e r i s known t o

o f f e r a d e s i g n n o t v e r y d i f f e r e n t f rom t h o s e shown i n F i g . 5 . 3 ; i t s h o u l d be

c o n n e c t e d t o a s e p a r a t e condense r ( r e f . 2 6 ) . A compet ing s o l u t i o n c o n s i s t s o f

an e v a p o r a t o r , condense r and j u i c e tank i n t e g r a t e d i n a t o w e r - l i k e u n i t shown

s c h e m a t i c a l l y i n F i g . 5.8 ( a f t e r r e f . 2 4 ) . The mass and hea t b a l a n c e d a t a g i v e n

i n t he f i g u r e c o r r e s p o n d t o a s u g a r f a c t o r y w i t h a p r o c e s s i n g c a p a b i l i t y o f

7200 t / d . As can be s e e n , t he s p e c i a l e v a p o r a t o r makes i t p o s s i b l e t o e v a p o r a t e

8.3 kg w a t e r pe r 100 kg b e e t u s i n g o n l y s e c o n d a r y hea t w i t h o u t c o n t r i b u t i n g t o

p r o c e s s h e a t i n g .

197

EVAPORATOR

vacuum pan vapour 60°C. 7.7 kg/100 kg b_

ju ice 53%DS

^ ^cool ing water 28°C

1 β barometric water 38^C^

THICK JUICE TANK

F i g . 5 .8 . F a l l i n g - f i l m e v a p o r a t o r hea ted by vacuum-pan v a p o u r and f e a t u r i n g an i n t e g r a t e d condense r and t h i c k - j u i c e tank ( a f t e r r e f . 2 4 ) .

The s o l u t i o n under ( i i ) i s known f rom a p r o t o t y p e a p p l i c a t i o n i n a 6000 t / d

s u g a r f a c t o r y ( r e f . 2 7 ) . A c t u a l l y , i t was s t i m u l a t e d by t h e n e c e s s i t y o f

c l e a n i n g t h e o u t l e t gases f rom the p u l p d r y e r . Due t o v e r y s t r i n g e n t

e n v i r o n m e n t a l r e q u i r e m e n t s , d o u b l e - s t a g e c l e a n i n g ( c y c l o n e s f o l l o w e d by a

w a s h e r ) was a d o p t e d . The a v a i l a b i l i t y o f c l e a n gas a t 68°C made i t p o s s i b l e t o

i n c l u d e a g a s - h e a t e d e v a p o r a t o r i n t h e new i n s t a l l a t i o n . As t h e v a p o u r p r e s e n t

i n t he gas condenses i n t he h e a t i n g chamber , t h e u n i t can be e x p e c t e d t o

f u n c t i o n as an a d d i t i o n a l g a s - c l e a n i n g s t a g e i n w h i c h t h e f i n e s t d u s t p a r t i c l e s

a r e s e p a r a t e d w h i l e SO^ and NO^ d i s s o l v e i n w a t e r .

The f l o w o f t h i n j u i c e d e l i v e r e d t o p r e - e v a p o r a t i o n i s 110 t / h , i . e . , a b o u t

1/3 o f t h e t o t a l t h i n j u i c e f l o w . The i n f l o w i n g j u i c e a t 15.6% DS i s c o o l e d down

t o 60°C and expanded t o 44°C i n t he e v a p o r a t o r s ; t h e o u t f l o w i n g j u i c e a t

21.5% DS i s warmed up t o 90°C. Two f a l l i n g - f i l m e v a p o r a t o r s w i t h o u t j u i c e

c i r c u l a t i o n were i n s t a l l e d f o r t h i s a p p l i c a t i o n . Each u n i t has a h e a t i n g s u r f a c e 2

o f 1750 m c o n s i s t i n g o f t u b e s 51 mm o u t e r d i a m e t e r and 8 m l o n g . The t u b e s a r e

p e r i o d i c a l l y washed on t h e gas s i d e , u s i n g h o t w a t e r i n t r o d u c e d a t t h e upper

t ube s h e e t and f l o w i n g i n a f i l m on t he o u t e r t u b e w a l l s .

198

To comple te t h i s r e v i e w , a n o t h e r unusua l e v a p o r a t o r a p p l i c a t i o n can be

m e n t i o n e d . The medium t h i c k e n e d i s g r e e n s y r u p Β l e a v i n g a Q u e n t i n u n i t . I n

o r d e r t o make t h e i o n exchange p r o c e s s p o s s i b l e , t h e s y r u p i s d i l u t e d t o 64% DS

b e f o r e e n t e r i n g t h i s u n i t , w h i c h r e q u i r e s add ing up t o 2.9 kg w a t e r p e r 100 kg

b e e t . As t h e d i l u t e d s y r u p i s r e t u r n e d t o t he c r y s t a l l i z a t i o n s u b s y s t e m , t h i s

w a t e r must be e v a p o r a t e d . I f i t i s done i n C vacuum p a n s , t h e n a c o r r e s p o n d i n g

i n c r e a s e o f t h e hea t demand o f t h e s u g a r house can be e x p e c t e d . A l t e r n a t i v e l y ,

e x c e s s w a t e r can be e v a p o r a t e d u s i n g was te h e a t .

W i th i t s t e m p e r a t u r e r e d u c e d t o 50°C, s i m i l a r l y t o t h e case d i s c u s s e d a b o v e ,

t h e s y r u p i s d e l i v e r e d t o an e v a p o r a t o r hea ted by vacuum-pan v a p o u r s . A f a l l i n g -

f i l m u n i t ( w i t h o u t s y r u p r e c i r c u l a t i o n ) e q u i p p e d w i t h i t s own c o n d e n s e r has been

s e l e c t e d f o r t h i s a p p l i c a t i o n . The main p o i n t s o f t h e d e s i g n a n a l y s i s and a

summary o f o p e r a t i o n a l r e s u l t s can be f ound i n t h e l i t e r a t u r e ( r e f . 2 8 ) . T h i s

s o l u t i o n has much i n common w i t h t h e e v a p o r a t o r shown i n F i g . 5 .8 .

5.4 HEAT EXCHANGERS

T h e r e a re no r e v o l u t i o n a r y new d e s i g n s i n t h e j u i c e h e a t e r f i e l d , b u t t h e

r e q u i r e m e n t s d i s c u s s e d i n S e c t i o n 3 .3 .2 n e c e s s i t a t e a more c a r e f u l app roach t o

t he e x i s t i n g v a r i e t y o f c h o i c e s . From t h e p o i n t o f v i e w o f e n e r g y u t i l i z a t i o n ,

t h r e e f a c t o r s s h o u l d be taken i n t o a c c o u n t :

- t h e the rma l r e s i s t a n c e o f t h e h e a t i n g s u r f a c e , p a r t i c u l a r l y under r e a l

o p e r a t i n g c o n d i t i o n s , a s s o c i a t e d w i t h t h e r i s k o f s c a l e b u i l d - u p ;

- t he p o s s i b i l i t y o f m a i n t a i n i n g an e c o n o m i c a l l y j u s t i f i e d r e l a t i o n s h i p between

t h e hea t t r a n s f e r i n t e n s i t y and t h e p r e s s u r e d r o p i n t h e l i q u i d h e a t e d ;

- t he p o s s i b i l i t y o f o b t a i n i n g pu re c o u n t e r - f l o w , w h i c h i s e s p e c i a l l y i m p o r t a n t

i n t he case o f r e c u p e r a t i o n o f l o w - t e m p e r a t u r e h e a t .

T h r e e w i d e l y used j u i c e h e a t e r d e s i g n s a r e shown s c h e m a t i c a l l y i n F i g . 5 . 9 .

The f a c t o r s men t ioned above a r e d e c i s i v e i n q u a l i f y i n g t h e p l a t e h e a t e x c h a n g e r

as t he d e s i g n w h i c h can be adap ted most e a s i l y t o d i f f e r e n t o p e r a t i n g

c o n d i t i o n s . I n a d d i t i o n t o v e r y h i g h o v e r a l l h e a t t r a n s f e r c o e f f i c i e n t s , p l a t e

hea t e x c h a n g e r s g i v e t he u s e r a r e a l chance o f easy m a i n t e n a n c e , as t h e y a r e

o f f e r e d t o d a y w i t h :

- v a r i o u s channe l g e o m e t r i e s p r o v i d i n g f o r s e l f - c l e a n i n g e f f e c t s , and t h u s

s u i t e d t o p a r t i c l e - c o n t a i n i n g l i q u i d s ;

- p l a t e m a t e r i a l s s u i t e d t o v a r i o u s c o r r o s i o n r e q u i r e m e n t s ;

- p a c k i n g m a t e r i a l s w h i c h make i t p o s s i b l e t o o p e r a t e p l a t e h e a t e x c h a n g e r s

s a f e l y a t t e m p e r a t u r e s up t o 260°C.

I n e a r l y a p p l i c a t i o n s i n t h e s u g a r i n d u s t r y , t h e c l a s s i c a l v e r s i o n o f t h e

p l a t e hea t e x c h a n g e r was dominan t . As i t i s c h a r a c t e r i z e d by v e r y smal l p l a t e

s p a c i n g (somet imes l e s s than 1 mm), s c a l e b u i l d - u p n o t o n l y causes a r e d u c t i o n

199

(α) (b)

Ε (c )

1 ^

F i g . 5 .9 . Schemes o f j u i c e h e a t e r s : ( a ) t u b u l a r , ( b ) p l a t e , ( c ) s p i r a l . 1 - i n f l o w i n g j u i c e , 2 - o u t f l o w i n g j u i c e , 3 - h e a t i n g v a p o u r , 4 - c o n d e n s a t e , 5 - s p e n t v a p o u r .

o f t h e hea t t r a n s f e r i n t e n s i t y , b u t a l s o a r a p i d i n c r e a s e o f t h e p r e s s u r e d r o p

on t h e j u i c e s i d e . T h i s e x c l u d e s t h e use o f c l a s s i c a l p l a t e h e a t e r s i f t h e r e i s

a r i s k o f heavy s c a l i n g , as on raw j u i c e and p a r t i c u l a r l y i n t h e t e m p e r a t u r e

range o f p r o t e i n d e p o s i t ( b e l o w 70°C) .

I n r e c e n t y e a r s , a new g e n e r a t i o n o f p l a t e h e a t e r s has been i n t r o d u c e d t o t h e

marke t . The m o d i f i e d d e s i g n i s l e s s s e n s i t i v e t o s c a l e b u i l d - u p . A s i d e - e f f e c t

o f t h e changed f l o w - c h a n n e l g e o m e t r y , h o w e v e r , i s t h a t t h e o v e r a l l h e a t t r a n s f e r

c o e f f i c i e n t i s s m a l l e r t han i n t he c l a s s i c a l v e r s i o n . A summary o f o p e r a t i o n a l

r e s u l t s o b t a i n e d w i t h t he new p l a t e h e a t e r s used on raw j u i c e can be f o u n d i n

t h e l i t e r a t u r e ( r e f . 3 0 ) .

T h e r e a r e s t i l l c e r t a i n a p p l i c a t i o n s i n a s u g a r f a c t o r y where o t h e r h e a t

e x c h a n g e r d e s i g n s can p r o f i t a b l y be u s e d . Examples can be c i t e d o f s u g a r

f a c t o r i e s e q u i p p e d w i t h v a r i o u s t y p e s o f j u i c e h e a t e r s o p t i m a l l y s e l e c t e d f o r

t h e r e q u i r e m e n t s c h a r a c t e r i s t i c o f e v e r y h e a t i n g s t a g e . T a b l e 5.1 g i v e s a

summary o f t h e da ta on t u b u l a r , s p i r a l and p l a t e h e a t e r s i n s t a l l e d i n a 6600 t / d

West European f a c t o r y . I t i s i n t e r e s t i n g t o n o t e , h o w e v e r , t h a t i n t h e D a n i s h

s u g a r i n d u s t r y , o n l y t u b u l a r h e a t e r s a r e used and t h e p l a t e u n i t s have been

200

TABLE 5.1

J u i c e h e a t e r s o f d i f f e r e n t d e s i g n s i n a 6600 t / d s u g a r f a c t o r y .

L o c a t i o n H e a t i n g medium H e a t e r t y p e H e a t i n g s u r f a c e

a r e a (m^)

Raw j u i c e 5th v a p o u r t u b u l a r 250X2 II II condensa te s p i r a l 38X2

P r e - l i m e d j u i c e vacuum-pan v a p o u r II 150X2 II II condensa te II 69X2 II II 4 th v a p o u r t u b u l a r 150X2

C l e a r j u i c e 3 rd v a p o u r p l a t e 83* T h i n j u i c e II II II 345*

II II 2nd v a p o u r II 210* II II 1s t v a p o u r II 83+116 II II e x h a u s t steam II 64+89

s i n g l e u n i t ( o t h e r h e a t e r s a r e i n s t a l l e d two i n p a r a l l e l )

f ound t o o d i f f i c u l t t o v e n t ( r e f . 3 1 ) .

A n o t h e r j u i c e h e a t e r d e s i g n w h i c h d e s e r v e s t o be men t ioned i s t h e segmented

t u b u l a r h e a t e r i n t r o d u c e d i n t h e S o v i e t s u g a r i n d u s t r y ( r e f s . 3 2 - 3 4 ) . I t i s

c h a r a c t e r i z e d by j u i c e v e l o c i t y abou t 3 m/s o r even h i g h e r , t h i s r e d u c i n g s c a l e

b u i l d - u p . As can be seen i n F i g . 5 .10 , t h e j u i c e f l o w pa th i s s t r e a m l i n e d t o

e n s u r e a low p r e s s u r e l o s s . A p p a r e n t l y , t h e c o n d i t i o n s f o r t he f l o w o f t he

h e a t i n g v a p o u r a r e f a r f rom opt imum, and t h e use o f v a p o u r e j e c t o r s f o r

improvement o f t he hea t t r a n s f e r has been r e p o r t e d ( r e f . 3 3 ) .

3 condensate

heating vapour

Top view J u i c e out

F i g . 5 .10 . Scheme o f a segmented t u b u l a r h e a t e r .

201

I n hea t e x c h a n g e r a p p l i c a t i o n s o t h e r t han j u i c e h e a t i n g between e x t r a c t i o n

and e v a p o r a t i o n , t he t r e n d s a r e s i m i l a r . A l t h o u g h t h e t u b u l a r d e s i g n i s s t i l

v e r y much i n u s e , t h e demand f o r e x c h a n g e r s e n s u r i n g a h i g h h e a t t r a n s f e r

i n t e n s i t y i s i n c r e a s i n g . F o r examp le , i n t h e u n c o n v e n t i o n a l e v a p o r a t i o n c i r c u i t s

d i s c u s s e d i n t he p r e c e d i n g S e c t i o n , i t i s e s s e n t i a l t h a t t h e t e m p e r a t u r e o f

j u i c e r e t u r n e d t o t h e p r o c e s s i n g l i n e i s as c l o s e as p o s s i b l e t o t h e i n i t i a l

t e m p e r a t u r e . As a r u l e , p l a t e hea t e x c h a n g e r s a r e s e l e c t e d f o r t h i s a p p l i c a t i o n .

S t i l l a n o t h e r u s e f u l f e a t u r e o f t he p l a t e h e a t e x c h a n g e r s i s t h e i r compac tness ,

w h i c h makes them easy t o i n s t a l l i n a l i m i t e d s p a c e , t h i s b e i n g a c h a r a c t e r i s t i c

r e q u i r e m e n t o f f a c t o r y m o d e r n i z a t i o n s o r e x t e n s i o n s .

5.5 VACUUM PANS

5.5.1 B a t c h - t y p e u n i t s

The therma l a s p e c t s o f vacuum-pan d e s i g n were p r e l i m i n a r i l y d i s c u s s e d i n

S e c t i o n 3 . 3 . 2 . I t can be no ted t h a t a l a r g e o v e r a l l hea t t r a n s f e r c o e f f i c i e n t

and a l a r g e h e a t i n g s u r f a c e a r e a a re c e r t a i n l y d e s i r a b l e f e a t u r e s o f t h e b a t c h -

t y p e u n i t s . T a k i n g i n t o a c c o u n t t he e s s e n t i a l r o l e o f t h e b o i l i n g p r o c e s s i n

s u g a r m a n u f a c t u r e , h o w e v e r , o t h e r r e q u i r e m e n t s s h o u l d pe rhaps be g i v e n even

h i g h e r p r i o r i t y . Most o f a l l , t he vacuum pan s h o u l d e n s u r e a r e a s o n a b l y s h o r t

b o i l i n g t i m e , h i g h c r y s t a l y i e l d and h i g h c r y s t a l q u a l i t y . A c t u a l l y , t h e s e

f a c t o r s can a l s o be l i n k e d t o t h e e n e r g y demand:

- i n c r e a s e d c r y s t a l y i e l d r e s u l t s i n r e d u c e d m a s s e c u i t e c i r c u l a t i o n ;

- improved c r y s t a l q u a l i t y c o n t r i b u t e s t o b e t t e r c o n d i t i o n s f o r c e n t r i f u g i n g ;

i . e . , l o w e r w a t e r consumpt ion i n c e n t r i f u g a l s and t h u s a r e d u c e d amount o f

s y r u p s .

On t he b a s i s o f advances o f t h e t h e o r y o f c r y s t a l l i z a t i o n and accumu la ted

e x p e r i e n c e , t h e i n a d e q u a c i e s o f b a t c h vacuum pans a r e now r e l a t i v e l y w e l l

u n d e r s t o o d . The h e a t i n g s u r f a c e a r e a i s t y p i c a l l y t o o l a r g e d u r i n g t h e f i r s t

s t a g e o f t h e b o i l i n g c y c l e ( c f . S e c t i o n 1 . 3 . 5 ) . A h i g h w a t e r e v a p o r a t i o n r a t e

r e s u l t i n g f rom bubb le b o i l i n g causes t h e o c c u r r e n c e o f z o n e s o f t o o h i g h

s u p e r s a t u r a t i o n , where c r y s t a l c o n g l o m e r a t e s a r e c r e a t e d o r s e c o n d a r y n u c l e a t i o n

t a k e s p l a c e . I n t he r e g i o n s c l o s e t o t h e h e a t i n g s u r f a c e , where t h e t e m p e r a t u r e

i s h i g h e r , t he s o l u t i o n may be u n d e r s a t u r a t e d , c a u s i n g t h e c r y s t a l s t o d i s s o l v e .

As t he l o c a l p r o c e s s e s a r e i m p o s s i b l e t o c o n t r o l , t h e g r a n u l o m e t r i c d i s t r i b u t i o n

o f c r y s t a l s i s a d v e r s e l y a f f e c t e d .

D u r i n g t h e l a s t s t a g e o f t h e b o i l i n g c y c l e , t h e h e a t i n g s u r f a c e a r e a i s

u s u a l l y t o o s m a l l . The e v a p o r a t i o n r a t e i s i n s u f f i c i e n t t o m a i n t a i n t h e d e s i r e d

c o n c e n t r a t i o n g r a d i e n t i n t h e s o l u t i o n a t t h e s u r f a c e s o f c r y s t a l s .

The s i t u a t i o n can be improved i f an e f f i c i e n t s t i r r e r i s i n s t a l l e d i n t h e

vacuum p a n . D u r i n g s o l u t i o n t h i c k e n i n g and c r y s t a l f o r m a t i o n , i n t e n s i v e m i x i n g

202

r e d u c e s t h e d a n g e r s o f l o c a l o v e r - and u n d e r - s a t u r a t i o n . I n t h e h i g h l y v i s c o u s

m a s s e c u i t e o b t a i n e d d u r i n g the l a s t s t a g e o f t h e b o i l i n g , t h e s t i r r e r h e l p s t o

i n c r e a s e t he c i r c u l a t i o n , t h i s i n c r e a s i n g t h e o v e r a l l h e a t t r a n s f e r c o e f f i c i e n t

and i n t e n s i f y i n g t h e e v a p o r a t i o n . P o s i t i v e e f f e c t s can a l s o be o b t a i n e d by

chang ing t h e way t h e vacuum pan i s u t i l i z e d i n t h e s u g a r b o i l i n g p r o c e s s , t h a t

i s , by emp loy ing t h e c r y s t a l f o o t i n g t e c h n i q u e , i n t r o d u c i n g c o n t r o l l e d vacuum

c h a n g e s , e t c . ( r e f s . 3 5 , 3 6 ) ; t h e s e methods have been r e v i e w e d i n S e c t i o n 4 . 3 .

(a) (b) ( c )

F i g . 5 .11. Examples o f s t i r r e r a r rangemen ts i n b a t c h vacuum p a n s : ( a ) s t i r r e r w i t h i n c a l a n d r i a , i n w a r d c i r c u l a t i o n , ( b ) s t i r r e r above c a l a n d r i a , ou twa rd c i r c u l a t i o n , ( c ) s t i r r e r be low c a l a n d r i a , ou twa rd c i r c u l a t i o n ( a f t e r r e f . 3 6 ) .

I t s h o u l d be added t h a t a t p r e s e n t , t h e r e q u i r e m e n t s o f t h e c r y s t a l g r o w t h

s t a g e o f t h e b o i l i n g c y c l e a r e n o t f u l l y known. W h i l e r e s e a r c h i n t h i s a r e a

c o n t i n u e s , t he p r e s e n t s t a t e o f knowledge on t h e d e s i g n o f modern vacuum pans

can be summarized as f o l l o w s .

( i ) Among v a r i o u s t y p e s o f pans shown i n F i g . 5 .11 , t h e c a l a n d r i a d e s i g n s w i t h

ample downtakes and f l a t o r s t r e a m l i n e d bot toms a r e p r e f e r r e d t o o t h e r s o l u t i o n s

( r e f s . 3 6 , 3 7 ) .

( i i ) The hyd rodynam ics o f m a s s e c u i t e c i r c u l a t i o n seem t o be w e l l u n d e r s t o o d .

The o p t i m i z a t i o n o f l e n g t h and d i a m e t e r o f h e a t i n g t u b e s and t h e d i a m e t e r o f

t h e downtake has been s t u d i e d ( r e f s . 3 8 , 3 9 ) .

( i i i ) I t has t aken a l ong t ime t o a r r i v e a t a s a t i s f a c t o r y d e s i g n o f s t i r r e r

p r o p e l l e r s and a p r o p e r a r rangement o f n o z z l e s r e l a t i v e t o t h e s t i r r e r and t h e

h e a t i n g s u r f a c e ( F i g . 5 . 1 2 ) . The speed o f p r o p e l l e r r o t a t i o n i s c a r e f u l l y

s e l e c t e d , t o e n s u r e i n c r e a s e d hea t t r a n s f e r d u r i n g t h e l a s t s t a g e o f t h e b o i l i n g

c y c l e , w h i l e a l s o keep ing t h e power demand a t as low a l e v e l as p o s s i b l e ( r e f s .

3 6 , 4 0 ) .

The a d v a n t a g e s o f f o r c e d c i r c u l a t i o n a r e so e v i d e n t t h a t i t has become

p o p u l a r t o i n s t a l l s p e c i a l l y d e s i g n e d s t i r r e r s i n o l d , n a t u r a l - c i r c u l a t i o n

vacuum p a n s . I t has been p r o v e d by measurements made i n a vacuum pan i n s t a l l e d

i n Β s t r i k e t h a t such a s t i r r e r can i n c r e a s e t h e h e a t t r a n s f e r c o e f f i c i e n t

d u r i n g t h e f i n a l s t a g e o f b o i l i n g ( a t magma c o n c e n t r a t i o n 90-93% DS) by

100-400%, and s h o r t e n t h e b o i l i n g t ime by 1/3 ( r e f . 1 ) .

203

F i g . 5 .12 . Deve lopment o f f o r c e d - c i r c u l a t i o n vacuum p a n s : ( a ) d e s i g n p r o p o s a l f rom 1896, ( b ) d e s i g n f rom 1949, ( c ) scheme o f a vacuum pan f o r a f t e r p r o d u c t b o i l i n g ( a f t e r r e f . 4 0 ) . 1 - j u i c e i n l e t , 2 - v a p o u r o u t l e t , 3 - m a s s e c u i t e o u t l e t , 4 - h e a t i n g chamber , ( 5 ) s t i r r e r .

As p r a c t i c a l e x p e r i e n c e p r o v e s , i f a s o p h i s t i c a t e d mechan ica l d e s i g n o f a

b a t c h vacuum pan i s combined w i t h e f f e c t i v e a u t o m a t i c b o i l i n g c o n t r o l s , t h e n

t h e r e s u l t s can be r e a l l y s a t i s f a c t o r y . N e v e r t h e l e s s , t h e s u g a r t e c h n o l o g i s t s

a re now aware o f t h e f a c t t h a t i n b a t c h p a n s , n o t h i n g more t han a t r a d e - o f f

between c o n f l i c t i n g r e q u i r e m e n t s o f d i f f e r e n t s t a g e s o f b o i l i n g can be a t t a i n e d .

S u b s t a n t i a l l y improved r e s p o n s e t o t he p r o c e s s r e q u i r e m e n t s can o n l y be e n s u r e d

i n c o n t i n u o u s vacuum p a n s .

5 .5 .2 C o n t i n u o u s u n i t s

I t has l ong been known t h a t t h e r e i s an e n e r g y - s a v i n g p o t e n t i a l i n c o n t i n u o u s

vacuum p a n s . One o f t h e r e a s o n s i s t h e i r i n h e r e n t a b i l i t y t o e l i m i n a t e t h e

f l u c t u a t i o n s o f t h e h e a t i n g v a p o u r demand. The r e s u l t i n g s t a b l e l o a d on t h e

e v a p o r a t o r s t a t i o n makes i t p o s s i b l e t o s t a b i l i z e t h i c k - j u i c e c o n c e n t r a t i o n and

t o a v o i d c o n d e n s e r l o s s e s e f f e c t i v e l y , w i t h r e d u c e d n e t h e a t demand as a r e s u l t .

A d i r e c t e n e r g y s a v i n g i s a l s o o b t a i n e d because s teaming a f t e r e v e r y

d i s c o n t i n u o u s b o i l i n g c y c l e i s no l o n g e r n e c e s s a r y . The advances i n b a t c h

vacuum-pan d e s i g n and h i g h l y e f f i c i e n t c o n t r o l s y s t e m s , h o w e v e r , made i t more

d i f f i c u l t f o r c o n t i n u o u s vacuum pans t o compete . D u r i n g t h e 1970s, v a r i o u s t y p e s

o f c o n t i n u o u s pans were t r i e d : g r o u p s o f i n t e r c o n n e c t e d b a t c h p a n s , h o r i z o n t a l

s i n g l e - o r m u l t i p l e - c o m p a r t m e n t u n i t s , c r y s t a l l i z a t i o n t o w e r , e t c . ( r e f s . 38,

4 1 - 4 4 ) . T h e r e were r e p o r t s t h a t s a t i s f a c t o r y o p e r a t i o n o f c e r t a i n d e s i g n s had

been o b t a i n e d , b u t m a i n l y on l o w - g r a d e s t r i k e s . Even t hough t h e number o f

c o n t i n u o u s pans was s l o w l y i n c r e a s i n g , t h e r e were a l s o r e p o r t s t h a t some o f them

204

were taken o u t o f o p e r a t i o n because o f u n s a t i s f a c t o r y r e s u l t s ( r e f . 4 5 ) . Two

p rob lems p r o v e d t o be most d i f f i c u l t t o s o l v e :

- t h e p e r i o d f o r w h i c h c o n t i n u o u s o p e r a t i o n c o u l d be m a i n t a i n e d was r a t h e r

s h o r t , because o f i n c r u s t a t i o n s w i t h i n t h e u n i t , e s p e c i a l l y on h i g h - p u r i t y

m a s s e c u i t e s ;

- t he g r a n u l o m e t r i c d i s t r i b u t i o n o f c r y s t a l s was w i d e r than t h a t a t t a i n a b l e i n

w e l l o p e r a t e d , modern ba t ch p a n s .

As p o i n t e d o u t by Aus tmeyer and F r a n k e n f e l d ( r e f . 4 6 ) , i n p r a c t i c e no

c o n t i n u o u s vacuum pan c o u l d be s u c c e s s f u l l y implemented w i t h o u t c r y s t a l f o o t i n g .

I t seems now, h o w e v e r , t h a t accumu la ted e x p e r i e n c e i s b e g i n n i n g t o b r i n g

p o s i t i v e r e s u l t s . S e v e r a l u n i t s o f a w e l l known h o r i z o n t a l m u l t i p l e - c o m p a r t m e n t

d e s i g n (80 pans d e l i v e r e d o r o r d e r e d up t o 1986) a r e now o p e r a t e d w i t h c r y s t a l

f o o t i n g ( r e f . 2 5 ) . A u n i t r a t e d 31 t / h Β m a s s e c u i t e , w i t h h e a t i n g s u r f a c e a r e a o f

540 m^, i s s u p p l i e d w i t h 12.5 t / h seed magma. S i m i l a r a p p a r a t u s r a t e d a t

17.5 t / h C m a s s e c u i t e , w i t h a h e a t i n g s u r f a c e a r e a o f 754 m , i s s u p p l i e d w i t h

6.1 t / h seed magma. Bo th u n i t s a r e hea ted w i t h v a p o u r a t 100°C, o f w h i c h a p a r t

i s i n j e c t e d d i r e c t l y i n t o t h e magma w i t h t h e aim o f i m p r o v i n g c i r c u l a t i o n .

A summary o f o p e r a t i o n a l r e s u l t s can be f ound i n t h e l i t e r a t u r e ( r e f . 4 7 ) . As

t h e f l o w o f seed magma l a r g e r t han 30% o f t he vacuum pan o u t p u t i s a c l e a r

d i s a d v a n t a g e , r e s e a r c h i s under way t o r educe t h i s f i g u r e . T h e r e a r e r e p o r t s

t h a t by m o d i f y i n g pan d e s i g n and c o n t r o l p r i n c i p l e s , o p e r a t i o n w i t h l e s s than

5% seed magma w i l l be p o s s i b l e ( r e f . 4 8 ) .

From t h e d e s i g n p r i n c i p l e men t i oned a b o v e , a new v e r s i o n o f a h o r i z o n t a l

c o n t i n u o u s pan e v o l v e d i n Sou th A f r i c a ( r e f . 4 9 ) . On t he b a s i s o f p o s i t i v e

Φ 6.75m

F i g . 5 .13 , Scheme o f a c o n t i n u o u s vacuum p a n , o f L a n g r e n e y t y p e ( a f t e r r e f . 5 2 ) . 1 - s t a n d a r d l i q u o r , 2 - seed magma, 3 - m a s s e c u i t e , 4 - h e a t i n g s u r f a c e s , 5 - s t i r r e r .

205

r e s u l t s o b t a i n e d i n the cane s u g a r i n d u s t r y , i t has r e c e n t l y been i n t r o d u c e d t o

a European b e e t s u g a r f a c t o r y ( r e f . 5 0 ) .

A n o t h e r h o r i z o n t a l s i n g l e - c o m p a r t m e n t d e s i g n was m o d i f i e d by a d o p t i n g a

c i r c u l a r shape o f v e s s e l w i t h a s t i r r e d o u t l e t s e c t i o n , as shown i n F i g . 5.13

( r e f s . 5 1 , 5 2 ) . The d i m e n s i o n s g i v e n i n t h e f i g u r e a p p l y t o a u n i t r a t e d a t

17 t / h C m a s s e c u i t e , w i t h h e a t i n g s u r f a c e a r e a o f 620 m^. I t i s n o r m a l l y

F i g . 5 .14. Scheme o f a c o n t i n u o u s vacuum p a n , o f BMA t y p e ( a f t e r r e f . 54) 1 - f e e d l i q u o r , 2 - seed magma, 3 - m a s s e c u i t e , 4 - v a p o u r , 5 - h e a t i n g s u r f a c e s , 6 - s t i r r e r s , 7 - s team.

206

o p e r a t e d w i t h c r y s t a l f o o t i n g . Numerous a p p l i c a t i o n s o f t h i s u n i t a r e known i n

t he cane s u g a r i n d u s t r y ; i t i s a l s o used i n C s t r i k e i n a F r e n c h b e e t s u g a r

f a c t o r y ( r e f . 5 1 ) .

The t o w e r d e s i g n has r e c e n t l y been adop ted by a n o t h e r m a n u f a c t u r e r , and t h e

r e s u l t s seem t o be b e t t e r t h i s t i m e . The u n i t c o n s i s t s o f a cascade o f s e v e r a l

c r y s t a l l i z a t i o n chambers , where t h e m a s s e c u i t e f l o w s f rom t h e t o p t o w a r d s t h e

bot tom under g r a v i t y , as shown i n F i g . 5.14 ( r e f s . 3 6 , 5 3 , 5 4 ) . The d i m e n s i o n s

g i v e n i n t h e f i g u r e a p p l y t o a u n i t r a t e d a t 45 t / h A m a s s e c u i t e , w i t h a h e a t i n g

s u r f a c e a r e a o f 1590 m^. The seed and t h e f e e d s o l u t i o n a r e f e d c o n t i n u o u s l y

i n t o t h e f i r s t chamber ; p a r t o f t he f e e d s o l u t i o n i s a l s o f e d t o chambers 2

t o 4 . Each c r y s t a l l i z a t i o n chamber i s adap ted t o t h e o p e r a t i o n a l r e q u i r e m e n t s

o f t h e r e l e v a n t p a r t o f t he b o i l i n g c y c l e i n te rms o f v o l u m e , h e a t i n g s u r f a c e ,

s t i r r e r c h a r a c t e r i s t i c s , e t c . F o r examp le , t he s t i r r e r s used i n t h e upper

chambers a r e h i g h - s p e e d a g i t a t o r s , w h i l e t h o s e i n t h e l o w e r chambers a r e l o w -

speed t u r b i n e s t i r r e r s augment ing m a s s e c u i t e c i r c u l a t i o n . I n c r u s t a t i o n s f o r m i n g

i n c e r t a i n chambers can be removed i n d i v i d u a l l y w i t h o u t f u l l y i n t e r r u p t i n g

o p e r a t i o n o f t h e u n i t . The t o w e r i s hea ted w i t h v a p o u r a t 90°C and a h i g h h e a t

t r a n s f e r i n t e n s i t y i s m a i n t a i n e d by c o n t r o l l i n g t h e m a s s e c u i t e l e v e l s i n t h e

i n d i v i d u a l chambers .

The c r y s t a l l i z a t i o n t o w e r s a r e o p e r a t e d s u c c e s s f u l l y i n A s t r i k e i n a few

f a c t o r i e s i n FRG. A r e d u c t i o n o f steam demand by abou t 5% has been r e p o r t e d f rom

one f a c t o r y . A l t h o u g h c o r r e c t i o n s and improvements a r e s t i l l b e i n g i n t r o d u c e d

i n t o t h i s d e s i g n , t h e o p e r a t i o n a l r e s u l t s a r e s a t i s f a c t o r y ( r e f s . 5 5 , 5 6 ) . The

f l o w o f seed magma i s k e p t be low 20% o f t h e pan o u t p u t .

5.6 CENTRIFUGALS

5.6.1 I n t r o d u c t i o n

The d i r e c t i n f l u e n c e o f c e n t r i f u g a l s on t h e e n e r g y consumpt ion i n a f a c t o r y

i s a s s o c i a t e d w i t h e l e c t r i c i t y - c o n s u m i n g d r i v i n g m o t o r s . Even more i m p o r t a n t ,

h o w e v e r , i s t h e i n d i r e c t i n f l u e n c e a s s o c i a t e d w i t h t h e q u a l i t y o f c e n t r i f u g e d

s u g a r and w i t h t h e e f f i c i e n c y o f s y r u p s e p a r a t i o n , because t h e s e f a c t o r s a f f e c t

t he mass f l o w s and t h u s t h e o v e r a l l e n e r g y consumpt ion i n t h e s u g a r h o u s e . I n

t h e c o n t e m p o r a r y s u g a r i n d u s t r y , b o t h b a t c h and c o n t i n u o u s c e n t r i f u g a l s a r e i n

u s e . Modern d e s i g n s o f bo th mach ines a r e shown s c h e m a t i c a l l y i n F i g . 5 .15 . Each

o f them has i t s a d v a n t a g e s and d i s a d v a n t a g e s ; t h i s a p p l i e s t o e n e r g y p rob lems

as w e l l as t o o t h e r a s p e c t s o f s u g a r c e n t r i f u g i n g ( r e f . 5 7 ) .

5 .6 .2 B a t c h mach ines

The deve lopmen t o f b a t c h c e n t r i f u g a l s has f o r many y e a r s been c h a r a c t e r i z e d

by a t r e n d t o w a r d s b i g g e r b a s k e t s , i . e . i n c r e a s e d c a p a c i t y . T h i s i s t h e cause

o f t h e p rob lem w i t h l a r g e - c a p a c i t y i r r e g u l a r l y r u n n i n g e l e c t r i c a l d r i v e s

207

F i g . 5 .15. Schemes o f c e n t r i f u g a l s : ( a ) b a t c h m a c h i n e , ( b ) c o n t i n u o u s mach ine . 1 - d r i v i n g m o t o r , 2 - b a s k e t , 3 - m a s s e c u i t e i n l e t , 4 - s u g a r o u t l e t , 5 -s y r u p - c o l l e c t i n g c a s i n g , 6 - s u g a r - c o l l e c t i n g c a s i n g , 7 - wash n o z z l e s .

ment ioned i n S e c t i o n 1 .4 .3 . D u r i n g t h e 1970s and 1980s, much e f f o r t was s p e n t on

t he deve lopment o f d r i v e s t h a t can work e f f i c i e n t l y a t t h e speed changes

c h a r a c t e r i s t i c o f t h e c e n t r i f u g i n g c y c l e .

Many fo rms o f e l e c t r i c a l d r i v e have been a p p l i e d t o t h e b a t c h m a c h i n e s . I n

t he 1970s, p o l e - c h a n g e a s y n c h r o n o u s a l t e r n a t i n g c u r r e n t mo to rs became v e r y

p o p u l a r . The p r i n c i p l e o f speed c o n t r o l can be seen i n t h e e q u a t i o n e x p r e s s i n g

t h e number o f r e v o l u t i o n s o f an a s y n c h r o n o u s motor ( i n rpm)

η = ( 6 0 f / p ) ( l - s ) ( 5 . 2 )

where f i s t h e s t a t o r f r e q u e n c y , ρ i s t h e number o f p o l e s , and s i s t h e motor

s l i p .

The s l i p can be d e f i n e d as

s = ( n ^ - n ) / n ^ ( 5 . 3 )

where n^ i s t h e s y n c h r o n o u s number o f r e v o l u t i o n s .

208

S t a n d a r d s i n g l e - s p e e d a s y n c h r o n o u s moto rs c o n n e c t e d t o a f i x e d - f r e q u e n c y s o u r c e

have a l a r g e s l i p d u r i n g r u n n i n g - u p , t h i s b e i n g t h e cause o f e n e r g y l o s s e s . I n

p o l e - c h a n g e m o t o r s , m u l t i p l e w i n d i n g s w i t h d i f f e r e n t numbers o f p o l e s a r e

i n s t a l l e d . By s w i t c h i n g f rom one w i n d i n g t o a n o t h e r , s t e p - c h a n g e s o f t h e number

o f r e v o l u t i o n s can be o b t a i n e d . I n t h i s w a y , a v e r a g e s l i p and a s s o c i a t e d e n e r g y

l o s s e s d u r i n g r u n n i n g - u p can be r e d u c e d . By r e d u c i n g f o u r t o f i v e d i f f e r e n t

speeds r a t h e r than a s i n g l e - s p e e d m o t o r , l o s s e s can be r e d u c e d by 75-80%. I n

a d d i t i o n , e l e c t r i c a l r e g e n e r a t i v e b r a k i n g can be u t i l i z e d f o r p a r t i a l r e c o v e r y

o f t he k i n e t i c e n e r g y o f t h e mass b e i n g c e n t r i f u g e d .

Advances i n s e m i c o n d u c t o r t e c h n o l o g y made i t p o s s i b l e f o r d i r e c t c u r r e n t

d r i v e s t o become h i g h l y c o m p e t i t i v e i n r e c e n t y e a r s . The c o m p l i c a t e d and c o s t l y

Ward -Leona rd sys tems have been r e p l a c e d by s i m p l e and r o b u s t t h y r i s t o r -

c o n t r o l l e d d r i v e s ( r e f . 5 8 ) . As t h e d . c . motor i s n o t dependen t on c u r r e n t

f r e q u e n c y , t h e r e a r e no s l i p l o s s e s d u r i n g r u n n i n g - u p and b r a k i n g . More

e f f i c i e n t e n e r g y r e c o v e r y t e c h n i q u e s can a l s o be a p p l i e d i n d . c . d r i v e s . F o r

t h i s r e a s o n , t he d . c . motor uses l e s s e n e r g y t han a . c . mo to rs w i t h p o l e - c h a n g e .

A n o t h e r s o l u t i o n based on t h e a p p l i c a t i o n o f s e m i c o n d u c t o r d e v i c e s i s t h e

f r e q u e n c y c o n v e r t e r a . c . d r i v e ( r e f s . 5 9 , 6 0 ) . The speed c o n t r o l p r i n c i p l e

c o n s i s t s o f v a r y i n g t h e s t a t o r f r e q u e n c y ; i t i s a l s o n e c e s s a r y t o v a r y t h e

v o l t a g e p r o p o r t i o n a t e l y t o t h e f r e q u e n c y . The f r e q u e n c y c o n v e r t e r c o n v e r t s t h e

c o n s t a n t a l t e r n a t i n g q u a n t i t i e s o f t h e mains i n t o v a r i a b l e v a l u e s s u i t a b l e f o r

t h e speed s e t t i n g s o f t h e m o t o r . T h i s e n a b l e s t h e speed o f t h e motor t o be

i n f i n i t e l y v a r i a b l e . I f a c o n v e r t e r w i t h a d i r e c t c u r r e n t i n t e r m e d i a t e c i r c u i t

i s u s e d , t h e n e n e r g y r e c o v e r y when b r a k i n g i s p o s s i b l e w i t h o u t any a d d i t i o n a l

e f f o r t : t he motor i s s i m p l y o p e r a t i n g as a g e n e r a t o r and f e e d i n g e n e r g y back

i n t o t h e ma ins . The mains s i d e o f t h e c o n v e r t e r can a l s o be e q u i p p e d w i t h an

o s c i l l a t i o n c i r c u i t w h i c h compensates t h e b a s i c amount o f t h e c o n t r o l r e a c t i v e

powe r , t h u s e n s u r i n g a h i g h power f a c t o r .

The a v e r a g e e f f i c i e n c y o f t h e f r e q u e n c y c o n v e r t e r a . c . d r i v e i s comparab le

w i t h t h a t o f t h e d . c . d r i v e . The power d iag rams o f bo th d r i v e s i n a c e n t r i f u g i n g

c y c l e a r e compared i n F i g . 5.16 ( a f t e r r e f s . 5 8 - 6 0 ) .

Modern c e n t r i f u g a l d r i v e s a r e s u p p l i e d w i t h comp le te c o n t r o l s f o r t h e

t e c h n o l o g i c a l f l o w o f o p e r a t i o n s . E l e c t r o n i c c i r c u i t s t e n d t o d o m i n a t e , and

m i c r o p r o c e s s o r a p p l i c a t i o n s a r e i n c r e a s i n g l y f r e q u e n t . F o r t h e c y c l i c s e q u e n c e ,

a memory programmable c o n t r o l i s u t i l i z e d . I n many c a s e s , n o t o n l y i s t h e who le

c y c l e a u t o m a t i c b u t g r o u p s o f mach ines a r e a l s o a u t o m a t i c a l l y l i n k e d , f o r smooth

b a t t e r y o p e r a t i o n .

The b a t c h s u g a r c e n t r i f u g a l i s n e a r l y 150 y e a r s o l d . I t can now be c o n s i d e r e d

a v e r y t h o r o u g h l y d e v e l o p e d m a c h i n e , and f u r t h e r p r o g r e s s c o n s i s t s o f t h e

r e f i n e m e n t o f d e t a i l s . Among t he d e t a i l s w h i c h a re r e l a t e d t o e n e r g y p r o b l e m s .

209

filling speeding up centrifuging braking discharging

F i g . 5 .16. Power - t ime d iagrams o f b a t c h c e n t r i f u g a l d r i v e s : ( a ) f r e q u e n c y c o n v e r t e r a . c . d r i v e , ( b ) d . c . d r i v e ( a f t e r r e f s . 5 8 - 6 0 ) . Shaded a r e a s i n d i c a t e e n e r g y r e c o v e r e d d u r i n g b r a k i n g .

wash sys tems s h o u l d be named. As a l r e a d y men t i oned i n S e c t i o n 1 .3 .4 , t h e

o p t i m i z a t i o n o f w a t e r wash i s i m p o r t a n t t o t h e e n e r g y consumpt ion i n t h e s u g a r

h o u s e . Modern wash sys tems s h o u l d t h e r e f o r e be f l e x i b l e enough t o make i t

p o s s i b l e t o o p t i m i z e t h e s p r a y g e o m e t r y , as w e l l as t h e t i m i n g and d u r a t i o n o f

t he wash . Examples o f c o n s i d e r a b l e improvements i n m a s s e c u i t e c i r c u l a t i o n

o b t a i n e d by wash o p t i m i z a t i o n can be f o u n d i n t h e l i t e r a t u r e ( r e f . 6 1 ) .

5 .6 .3 C o n t i n u o u s mach ines

Wi th r e s p e c t t o power demand, c o n t i n u o u s c e n t r i f u g a l s a r e c l e a r l y p r e f e r a b l e

t o b a t c h t y p e s . The d r i v e i s u s u a l l y f rom a s t a n d a r d a . c . m o t o r . The motor i s

smal l compared w i t h t h a t o f a b a t c h mach ine , s i n c e t h e h i g h a c c e l e r a t i n g /

d e c e l e r a t i n g l o a d s a r e no l o n g e r p r e s e n t .

The dom ina t i ng c o n t i n u o u s - a c t i o n d e s i g n p r i n c i p l e i s t h a t o f a v e r t i c a l - a x i s ,

c o n e - b a s k e t mach ine . The pe r f o rmance o f t h e c o n t e m p o r a r y c o n t i n u o u s c e n t r i f u g a l

i n l o w - g r a d e s t a t i o n s i s u s u a l l y e q u i v a l e n t t o t h a t o f t h e b a t c h mach ine . I n

t he c e n t r i f u g i n g o f h i g h - p u r i t y m a s s e c u i t e s , h o w e v e r , t h e c o n t i n u o u s c e n t r i f u g a l

i s g e n e r a l l y i n f e r i o r t o t h e b a t c h one ( r e f . 6 2 ) . I n c e r t a i n a p p l i c a t i o n s ,

c o n t i n u o u s c e n t r i f u g i n g i s f o l l o w e d by a f i n i s h i n g c r y s t a l t r e a t m e n t i n b a t c h

mach ines .

The main d i s a d v a n t a g e o f c o n t i n u o u s c e n t r i f u g i n g i s t h e c r y s t a l damage

r e s u l t i n g f rom c o l l i s i o n s o f c r y s t a l s , e j e c t e d a t a h i g h s p e e d , w i t h t h e s u g a r -

210

c o l l e c t i n g c a s i n g . T h i s has a d i r e c t d e t r i m e n t a l e f f e c t on t h e g r a n u l o m e t r i c

d i s t r i b u t i o n o f c r y s t a l s ; i n d i r e c t l y , i n c r e a s e d m a s s e c u i t e c i r c u l a t i o n i n t h e

s u g a r house can be a r e s u l t . F o l l o w i n g y e a r s o f r e s e a r c h work on t h i s p rob lem

( r e f s . 2 5 , 6 3 ) , a t l e a s t one m a n u f a c t u r e r i s now o f f e r i n g a d e v i c e w h i c h can be

i n s t a l l e d i n c o n t i n u o u s mach ines t o r educe c r y s t a l damage. Formed as a wheel

p l a c e d between t h e baske t and t h e c a s i n g , i t i s r o t a t e d a t a speed l o w e r t han

t h a t o f t h e b a s k e t ( r e f . 6 4 ) .

A n o t h e r p rob lem i s t h a t t h e wash a c t s d i f f e r e n t l y i n a c o n t i n u o u s m a c h i n e ,

so t h e wash c o n t r o l canno t be q u i t e so f i n e as w i t h a w e l l d e s i g n e d b a t c h

mach ine . The s i t u a t i o n improves as work on t h e r e f i n e m e n t o f wash sys tems and

o t h e r d e t a i l s c o n t i n u e s . T a k i n g i n t o a c c o u n t t h a t t h e c o n t i n u o u s machine i s 50

y e a r s y o u n g e r t han t he b a t c h o n e , i t seems t h a t i t s e n e r g y - s a v i n g p o t e n t i a l has

n o t y e t been f u l l y u t i l i z e d .

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sposobom, Sakh . P r o m . , ( 9 ) (1980) 43-46. 11 K. Vukov and I . S i p o s , V e r s u c h e z u r e n e r g i e s p a r e n d e r E rhöhung d e r g e w i n n

ba ren Zuckermenge be i d e r R ü b e n e x t r a k t i o n , Paper p r e s e n t e d a t t h e I n t e r n a t i o n a l C o n f e r e n c e " Improvement o f t h e Bee t Suga r P r o d u c t i o n " , Warszawa, May 1987.

12 P. V a l e n t i n , D i e A b h ä n g i g k e i t des H e i z w ä r m e b e d a r f s von H e i z f 1 ä c h e n g r ö s s e und R o h s a f t a b z u g , Z u c k e r i n d . , 104(8) (1979) 695-701.

13 P. Mosel ( e t a l . ) , O p t i m i e r u n g von E i n d i c k u n g s p r o z e s s e n i n d e r Z u c k e r i n d u s t r i e , Z u c k e r i n d . , 104(12) (1979) 1101-1106.

14 A . A . Knyazev and V . N . G o r o k h , V l i y a n i e u d e l n o i p l o s h c h a d i p o v e r k h n o s t i n a g r e v a v y p a r n o i u s t a n o v k i na r a s k h o d t e p l o v o i e n e r g i i d l y a t e k h n o l o g i c h e s k i k h nuzhd s v e k l o s a k h a r n o g o z a v o d a , Sakh . P r o m . , ( 3 ) (1976) 41-43 .

15 T . B a l o h , O p t i m i e r u n g von Z u c k e r f a b r i k s a n l a g e n u n t e r B e r ü c k s i c h t i g u n g des E n e r g i e h a u s h a l t s , Z u c k e r , 29(10) (1976) 541-548.

16 K. V u k o v , I . Körmendy and H.M. L o k o , A u f e n t h a l t s z e i t und S a f t v e r f ä r b u n g i n e i n e r V e r d a m p f s t a t i o n , Z u c k e r i n d . , 108(12) (1983) 1144-1149.

17 S . Z a g r o d z k i and A . K u b a s i e w i c z , Hea t economy i n b e e t s u g a r f a c t o r y e v a p o r a t i o n . Sugar T e c h . R e v . , 5 ( 1 / 2 ) (1977/78) 1-154.

18 P. T o b e , F a l l i n g - f i l m e v a p o r a t o r s f o r t h e cane s u g a r i n d u s t r y , W i e g a n d , E t t l i n g e n , 1986.

211

19 U . C u r d t s , L e c t u r e p r e s e n t e d a t t h e I n t e r n a t i o n a l C o n f e r e n c e " Improvement o f t h e B e e t Sugar P r o d u c t i o n " , Warszawa, May 1987.

20 A . K u b a s i e w i c z , W y p a r k i . K o n s t r u k c j a i O b l i c z a n i e , WNT, Warszawa, 1977. 21 Anonymous, S e p a r a t e u r s de g o u t t e s EUROFORM pou r i n s t a l l a t i o n s d ' e v a p o r a t i o n

dans 1 ' i n d u s t r i e s u c r i e r e , S u c r . F r . , 118(June 1977) 262-265. 22 J . Kwasn iak , P o l i s h P a t e n t 79 895. 23 D.M. Humm, E n t r a i n m e n t s e p a r a t o r s f o r vacuum pans and e v a p o r a t o r s . Sugar J . ,

44(12) (1982) 8-14. 24 E. R e i n e f e l d , Über d i e Kampagne 1984, Z u c k e r i n d . , 110(5) (1985) 367-377. 25 Ε . R e i n e f e l d , Über d i e Kampagne 1985, Z u c k e r i n d . , 111(4) (1986) 303-313. 26 T e c h n i c a l i n f o r m a t i o n f rom Wiegand , i n : F . O . L i c h t s Yearbook and D i r e c t o r y ,

R a t z e b u r g , 1983, p p . H71-H74. 27 P. V e r m e u l e n , Sa f t e i ndamp fung m i t t e l s T r o c k n u n g s a b g a s be i dessen R e i n i g u n g ,

Z u c k e r i n d . , 110(8) (1985) 681-685. 28 W. L e k a w s k i , p e r s o n a l commun ica t i on . 29 Ε . Hess and H . v . Mal l a n d , E i n d i c k u n g s a n l a g e f ü r Rohzucke r 2 - A b l a u f ,

Z u c k e r i n d . , 109(4) (1985) 295-300. 30 N . R . T w a i t e , H . J . D a v e n p o r t and E . K . M a c d o n a l d , E n e r g y r e d u c t i o n and p r o c e s s

i n t e g r a t i o n . I n t . Suga r J . , 88(1055) (1986) 217-219, 88(1056) (1986) 230-236.

31 Ν. L o f t , p e r s o n a l commun i ca t i on . 32 A . A . Pochechun ( e t a l . ) , P r o i z v o d s t v e n n y e i s p y t a n i y a s e k t s i o n n o g o p o d o g r e v a

t e l y a soka p e r e d I I s a t u r a t s i e i , Sakh . P r o m . , ( 2 ) (1983) 43-45 . 33 Y u . S . R a z l a d i n ( e t a l . ) , Nagrev d i f f u z i o n n o g o soka ν s e k t s i o n n y k h a p p a r a t a k h

d l y a z a v o d a moshchnos tyu 6 t y s . t , Sakh . P r o m . , ( 1 ) (1984) 35-38. 34 V . G . B e l i k ( e t a l . ) , S e k t s i o n n y e p o d o g r e v a t e l i , Sakh . P r o m . , ( 6 ) (1986)

35-37. 35 H. S c h i w e c k , M ö g l i c h k e i t e n z u r Senkung des E n e r g i e b e d a r f s im Z u c k e r h a u s ,

Z u c k e r , 30(10) (1977) 525-535. 36 K . E . A u s t m e y e r , A n a l y s i s o f s u g a r b o i l i n g and i t s t e c h n i c a l c o n s e q u e n c e s .

I n t . Sugar J . , 88 ( 1 9 8 6 ) , P a r t I (1045) 3 - 7 , P a r t I I (1046) 23-29 , P a r t I I I (1047) 50-55.

37 C h . M o l l e r , Sugar b o i l i n g t h e o r y and p r a c t i c e . I n t . Sugar J . , 85(1045) (1983) 163-165.

38 E. H u g o t , Handbook o f Cane Sugar E n g i n e e r i n g , 3 r d e d n . , E l s e v i e r , Amsterdam, 1986.

39 H . N . Gupta and S . J . Pande, Optimum d e s i g n o f a vacuum p a n . I n t . Sugar J . , 88(1048) (1986) 66-68.

40 E r f a h r u n g e n m i t R ü h r w e r k s - A p p a r a t e n , Z u c k e r i n d . , 105(3) (1980) 227-244. 41 Μ. D m i t r o v s k i and A . Η . Kokke, US P a t e n t 3 981 739. 42 A . G e n a r t , I n s t a l l a t i o n d ' u n c r i s t a l 1 i s e u r c o n t i n u a l a R a f f i n e r i e N o t r e -

Dame a O r e y e , S u c r . B e i g e , 98(11) (1979) 337-345. 43 F. L a n g r e n e y , E r s t e E r g e b n i s s e des k o n t i n u i e r l i c h a r b e i t e n d e n E i n d a m p f -

k r i s t a l l i s a t o r s , B a u a r t " L a n g r e n e y " , Z . Z u c k e r i n d . , 26(12) (1976) 772-776. 44 W. W o z n i a k i e w i c z and M. W i e k l u k , P o l i s h P a t e n t 54 086. 45 K. T e s c h , E r f a h r u n g e n m i t R ü h r w e r k s - K o c h a p p a r a t e n i n d e r Z u c k e r f a b r i k

O c h s e n f u r t , Z u c k e r i n d . , 105(3) (1980) 240-242. 46 K . E . Aus tmeyer and T h . F r a n k e n f e l d , Der Weg z u r k o n t i n u i e r l i c h e n K r i s t a l l

f u s s g e w i n n u n g , Z u c k e r i n d . , 112(1) (1987) 36-45 . 47 H. H e r o l d , K o n s t r u k t i o n und A r b e i t s w e i s e d e r k o n t i n u i e r l i c h e n Kochappa ra te

von F i v e s - C a i l Babcock i n E l s d o r f , Z u c k e r i n d . , 112(2) (1987) 118-122. 48 Anonymous, Seed ing a c o n t i n u o u s vacuum pan w i t h l e s s t h a n 10% magma,

Z u c k e r i n d . , 112(3) (1987) 211. 49 P.W. R e i n , E x p e r i e n c e w i t h c o n t i n u o u s vacuum pans i n T o n g a a t - H u l e t t S u g a r ,

I n t . Sugar J . , 89(1058) (1987) 28-34. 50 H . - E . U e c k e r , K o n t i n u i e r l i c h e r K o c h a p p a r a t B a u a r t T o n g a a t - H u l e t t ,

Z u c k e r i n d . , 112(3) (1987) 202-203. 51 F . L a n g r e n e y , Le p o i n t s u r l e c r i s t a l 1 i s e u r c o n t i n u non c o m p a r t i m e n t e ,

I n d . A l i m . A g r i e , 102(7 -8 ) (1985) 673-678.

212

52 Anonymous, Le c r i s t a l 1 i s e u r c o n t i n u ( L i c . L a n g r e n e y ) a l a S u c r e r i e de M a r i e , I n d . A l i m . A g r i e , 102(7-8 ) (1985) 713-717.

53 Ε . R e i n e f e l d , Über d i e Kampagne 1983, Z u c k e r i n d . , 109(5) (1984) 399-411. 54 E . D . B o s s e , A new e v a p o - c r y s t a l 1 i z a t i o n t o w e r f o r w h i t e s u g a r and low raw

p r o d u c t s . Sugar y A z ú c a r , 8 1 ( 5 ) (1986) 33-56. 55 Κ. Her rman , Neue Systeme z u r k o n t i n u i e r l i c h e n K o c h a r b e i t - BMA-Verdampfungs-

K r i s t a l l i s a t i o n s t u r m , Z u c k e r i n d . , 112(4) (1987) 277-280. 56 J . Be low , D ie V K T - A n l a g e i n L e h r t e , Z u c k e r i n d . , 112(4) (1987) 280-284. 57 J . O . S m i t h , Recen t p r o g r e s s i n s u g a r c e n t r i f u g i n g . Sugar T e c h . R e v . ,

4 ( 1 ) (1976/77) 49-87 . 58 Η. G r a s s , S p e z i a l a n t r i e b e f ü r d i e Z u c k e r i n d u s t r i e , Z u c k e r i n d . , 107(9) (1982)

863-868. 59 H. G r a s s , D i e E n t w i c k l u n g e l e k t r i s c h e r Z u c k e r z e n t r i f u g e n a n t r i e b e ,

Z u c k e r i n d . , 110(2) (1985) 132-136. 60 T e c h n i c a l i n f o r m a t i o n f rom Fe i t en&Gu i1 leaume E n e r g i e t e c h n i k , Nordenham,

1985. 61 E. R e i n e f e l d , Über d i e Kampagne 1982, Z u c k e r i n d . , 108(4) (1983) 307-319. 62 P. C r e d o z , J . Ledoux and G . J o u r n e t , The deve lopmen t o f c o n t i n u o u s

c e n t r i f u g i n g i n t h e f i e l d o f h i g h - p u r i t y s u g a r s , Sugar y A z ú c a r , 75 (2 ) (1980) 34-42 .

63 Ε . R e i n e f e l d , Über d i e Kampagne 1980, Z u c k e r i n d . , 106(5) (1981) 397-407. 64 Anonymous, K r i s t a l l r a d z u r V e r r i n g e r u n g von K r i s t a l 1bruch i n k o n t i n u i e r l i

chen Z e n t r i f u g e n , Z u c k e r i n d . , 112(1) (1987) 34.

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C h a p t e r 6

AUTOMATIC CONTROL FOR E F F I C I E N T ENERGY U T I L I Z A T I O N

6.1 SCOPE OF THE PROBLEMS

A u t o m a t i c p r o c e s s c o n t r o l i s i n d i s p e n s a b l e t o t h e c o n t e m p o r a r y s u g a r

i n d u s t r y . When imp lement ing o p t i m i z e d p r o c e s s e s o r p u t t i n g improved equ ipment

i n t o o p e r a t i o n , t h e a u t o m a t i c c o n t r o l o f t e n p l a y s a l e a d i n g r o l e i n e n s u r i n g

t h a t t h e i n t e n t i o n s o f a t e c h n o l o g i s t o r an equ ipment d e s i g n e r become a r e a l i t y .

To a l a r g e e x t e n t , t h i s i s a l s o t r u e i n t h e case o f improvements i n t r o d u c e d i n t o

t h e e n e r g y economy.

T h i s C h a p t e r p r e s e n t s a r e v i e w o f t h e a p p l i c a t i o n s o f a u t o m a t i c c o n t r o l t h a t

a r e p a r t i c u l a r l y u s e f u l i n o p t i m i z i n g e n e r g y c o n v e r s i o n and u t i l i z a t i o n i n t h e

s u g a r f a c t o r y . A c t u a l l y , t h e f u n c t i o n i n g o f a u t o m a t i c c o n t r o l s can be r e l a t e d t o

t h e e n e r g y economy i n a v a r i e t y o f w a y s . To b e g i n w i t h , examples can be named o f

a u t o m a t i c c i r c u i t s c o n t r o l l i n g e n e r g y p r o c e s s e s d i r e c t l y , such as combus t i on

c o n t r o l i n a b o i l e r o r i n a p u l p d r y i n g f u r n a c e ( r e f . 1 ) . T h i s t y p e o f c o n t r o l

a p p l i c a t i o n w i l l n o t be d i s c u s s e d h e r e , h o w e v e r , as i t i s n o t s p e c i f i c t o t h e

s u g a r i n d u s t r y .

A n o t h e r g roup o f a u t o m a t i c c o n t r o l c i r c u i t s s e r v e s t h e p u r p o s e o f s t a b i l i z i n g

p r o c e s s p a r a m e t e r s , w h i c h has some i n f l u e n c e on t h e e n e r g y demand o f t h e

p r o c e s s e s a f f e c t e d . T h i s can be e x e m p l i f i e d by t h e a u t o m a t i c c o n t r o l o f m i l k - o f -

l ime d e n s i t y and m i l k - o f - l i m e f l o w t o t h e j u i c e p u r i f i c a t i o n s t a t i o n . H e r e , t he

s e t t i n g s a r e a l w a y s a r r a n g e d t o e n s u r e t h e CaO doses r e q u i r e d by t h e j u i c e

p u r i f i c a t i o n p r o c e s s . C o n s e q u e n t l y , t he amount o f e x c e s s w a t e r i n t r o d u c e d i n

m i l k - o f - l i m e t o t h e p r o c e s s v a r i e s depend ing on t h e q u a l i t y o f t h e a u t o m a t i c

c o n t r o l , w i t h v a r y i n g hea t demand i n t h e e v a p o r a t i o n p r o c e s s as a r e s u l t . I n

p r i n c i p l e , a u t o m a t i c c o n t r o l s o f t h i s k i n d need n o t be d i s c u s s e d i n g r e a t e r

d e t a i l , as t h e e n e r g y - r e l a t e d r e q u i r e m e n t s on t h e i r o p e r a t i o n a r e r a t h e r

o b v i o u s . One p o s s i b l e e x c e p t i o n i s t h e f l o w c o n t r o l , e s p e c i a l l y i n a p p l i c a t i o n s

c o n c e r n e d w i t h l a r g e f l u i d s t r e a m s . I n t h i s c a s e , t h e w o r k i n g p r i n c i p l e o f t h e

c o n t r o l module may d i r e c t l y i n f l u e n c e t h e power demand o f f l u i d pumping under

chang ing f l o w s .

C e r t a i n c o n t r o l sys tems can be r e l a t e d t o t h e e n e r g y economy because t h e y

a f f e c t p o s s i b l e f l u c t u a t i o n s o f steam and v a p o u r f l o w s i n t h e the rma l s y s t e m .

Such f l u c t u a t i o n s can i n d u c e e x t r a hea t l o s s e s c h a r a c t e r i s t i c o f t r a n s i e n t

s t a t e s o f t h e e v a p o r a t o r , w i t h a d e t r i m e n t a l e f f e c t on t h e t i m e - a v e r a g e d

e f f e c t i v e n e s s r a t i o o f t h e the rma l s y s t e m . L e t us n o t e t h a t t h e r e a r e examples

o f a u t o m a t i c c o n t r o l s m a i n l y s e r v i n g t h e p u r p o s e o f s t a b i l i z a t i o n o f f l o w s o f

p r o c e s s med ia , l i k e t h e sys tem o f c o o r d i n a t i o n o f f l o w s i n t h e j u i c e

214

p u r i f i c a t i o n s t a t i o n , o r t h e sys tem o f c o o r d i n a t i o n o f b a t c h equ ipment o p e r a t i o n

i n t he s u g a r h o u s e . A p p l i c a t i o n s o f such sys tems r e s u l t i n s t a b l e hea t demand i n

r e s p e c t i v e s e c t i o n s o f t h e s u g a r m a n u f a c t u r i n g p r o c e s s , l e a d i n g t o r e g u l a r i z e d

e v a p o r a t o r o p e r a t i o n .

P a r t i c u l a r l y i m p o r t a n t a p p l i c a t i o n s o f a u t o m a t i c c o n t r o l a re c o n c e r n e d w i t h

p r o c e s s e s i n w h i c h t h e r e q u i r e m e n t s o f s u g a r m a n u f a c t u r e a r e l i n k e d t o g e t h e r

w i t h t h o s e o f e n e r g y economy. I t was p o i n t e d o u t i n t h e p r e c e d i n g c h a p t e r s t h a t

t h e r e q u i r e m e n t s b e l o n g i n g t o t h e s e two g r o u p s o f t e n c o n f l i c t . The way such

c o n f l i c t s a r e r e s o l v e d may d e p e n d , t o a l a r g e e x t e n t , on t h e c o n t r o l s ys tem

a p p l i e d . The a s s o c i a t e d m u l t i v a r i a b l e , m u l t i - o b j e c t i v e c o n t r o l p rob lems a r e

o f t e n v e r y d i f f i c u l t t o s o l v e , h o w e v e r . T h i s s t i m u l a t e s t h e use o f computer

t e c h n o l o g y , w h i c h i s p a r t i c u l a r l y w e l l s u i t e d t o complex c o n t r o l t a s k s .

I t can be c o n c l u d e d f rom t h e above i n t r o d u c t i o n t h a t v a r i o u s a s p e c t s o f

a u t o m a t i c c o n t r o l and v a r i o u s c o n t r o l a p p l i c a t i o n s may a f f e c t t h e e f f i c i e n c y o f

e n e r g y u t i l i z a t i o n i n a s u g a r f a c t o r y . I n t h e s u b s e q u e n t S e c t i o n s , s i x d i f f e r e n t

p rob lem a r e a s a r e d i s c u s s e d :

- deve lopment o f compu te r -based c o n t r o l s y s t e m s ;

- f l o w c o n t r o l u s i n g v a r i a b l e speed d r i v e s ;

- e x t r a c t i o n c o n t r o l ;

- e v a p o r a t i o n c o n t r o l ;

- s u g a r c r y s t a l l i z a t i o n c o n t r o l ;

- p u l p d r y i n g c o n t r o l .

6 .2 COMPUTER-BASED CONTROL SYSTEMS

I t i s c h a r a c t e r i s t i c o f c o n v e n t i o n a l c o n t r o l t e c h n o l o g y t h a t t h e app roach t o

a p p l i c a t i o n s c o n s i s t s o f i d e n t i f y i n g n e c e s s a r y c o n t r o l f u n c t i o n s and s e l e c t i n g

s u i t a b l e c o n t r o l modules w h i c h add up t o a s y s t e m . I n t h e case o f a complex

c o n t r o l t a s k , a m u l t i t u d e o f c o n t r o l modules o r a s p e c i a l i z e d c o n t r o l l e r must be

a p p l i e d . F o r t h i s r e a s o n , such t a s k s may be v e r y c o s t l y t o au tomate .

The a d v e n t o f d i g i t a l computers made i t p o s s i b l e t o change t h e c o n v e n t i o n a l

a p p r o a c h . I n s t e a d o f p e r f e c t i n g t h e sys tem by a d d i n g , each t i m e , modules f o r

a d d i t i o n a l f u n c t i o n s , a s i n g l e module - t h a t i s , t h e c e n t r a l p r o c e s s o r o f t h e

computer - can be u t i l i z e d f o r i n f o r m a t i o n p r o c e s s i n g . A l l t h e n e c e s s a r y

o p e r a t i o n s , i n c l u d i n g c o m p u t a t i o n s , a r e c a r r i e d o u t s e q u e n t i a l l y a c c o r d i n g t o

a p r e - e s t a b l i s h e d p rog ram. The speed o f t h e computer i s u s u a l l y h i g h enough t o

f o l l o w t h e e v o l u t i o n o f s i g n a l s coming f rom t h e i n s t a l l a t i o n b e i n g c o n t r o l l e d .

T h e r e a re numerous examples o f c o m p u t e r - o r m i c r o p r o c e s s o r - b a s e d sys tems

e f f e c t i v e l y r e p l a c i n g c o n v e n t i o n a l l o o p s c o n t r o l l i n g combus t i on i n b o i l e r s and

p u l p d r y i n g f u r n a c e s , d e l i v e r y o f m a t e r i a l s t o l ime k i l n s , m i l k - o f - l i m e f l o w t o

j u i c e p u r i f i c a t i o n , o p e r a t i o n o f f i l t e r s t a t i o n s , c o o r d i n a t i o n o f f l o w s i n t h e

215

m a n u f a c t u r i n g l i n e between e x t r a c t i o n and e v a p o r a t i o n , e t c . { r e f s . 2 - 9 ) .

S i g n i f i c a n t e n e r g y s a v i n g s r e s u l t i n g f rom improved c o n t r o l have been r e p o r t e d i n

some c a s e s ( r e f s . 2 , 3 , 8 ) .

Owing t o t h e deve lopment o f ha rdware and s o f t w a r e , t h e computer i s no l o n g e r

s o l e l y a d a t a p r o c e s s i n g machine b u t a l s o an e n g i n e e r i n g t o o l c a p a b l e o f

c o n t r o l l i n g complex i n s t a l l a t i o n s , as w e l l as c o l l e c t i n g d a t a f o r d i r e c t

management o f t h e p r o c e s s o r f o r t r a n s m i s s i o n t o o t h e r c o m p u t e r s . T h i s

s t i m u l a t e s n o t o n l y t he r e p l a c e m e n t o f c o n v e n t i o n a l c o n t r o l l o o p s by compu te r -

c o n t r o l l e d equ ipmen t , b u t a l s o t h e deve lopmen t o f d a t a a c q u i s i t i o n , t r a n s m i s s i o n

and p r o c e s s i n g sys tems t h a t c o u l d n o t e x i s t w i t h o u t computer t e c h n o l o g y . Two

p o s s i b l e p r i n c i p l e s o f a r c h i t e c t u r e o f complex c o m p u t e r - b a s e d c o n t r o l sys tems

a r e shown i n F i g . 6 .1 . The f u n c t i o n s o f such sys tems can be t a i l o r e d t o t h e

needs o f i n d i v i d u a l s t a t i o n s , p r o c e s s s e c t i o n s o r even an e n t i r e f a c t o r y and can

i n c l u d e a u t o m a t i c p r o c e s s c o n t r o l as w e l l as d a t a m o n i t o r i n g , p r o c e s s i n g and

r e c o r d i n g , h a n d l i n g o f a l a r m s , e t c .

(Q)

PROCESS

(b)

PROCESS

F i g . 6 .1 . Schemes o f complex c o m p u t e r - b a s e d c o n t r o l s y s t e m s : ( a ) h i e r a r c h i c a l s y s t e m , ( b ) d i s t r i b u t e d s y s t e m . 1 - s u p e r v i s o r y c o m p u t e r , 2 - dua l p r o c e s s c o n t r o l compu te r , 3 - d i s k s t o r a g e , 4 - c o n t r o l l o o p s , 5 - l o c a l c o n t r o l c o m p u t e r s , 6 - l o c a l a r e a n e t w o r k s i n c l u d i n g m u l t i p l e c o n t r o l l o o p s , 7 - i n p u t / o u t p u t i n t e r f a c e , 8 - d a t a b u s , 9 - d a t a commun ica t ion l i n k .

The impo r tance o f computer t e c h n o l o g y t o t h e d e s i g n and o p e r a t i o n o f s u g a r

f a c t o r i e s has been d i s c u s s e d e l s e w h e r e ( r e f s . 1 0 - 1 9 ) . C o n c e r n i n g e n e r g y economy,

new p o s s i b i l i t i e s o f improved e n e r g y u t i l i z a t i o n a r e d e r i v e d f rom t h e f o l l o w i n g

f e a t u r e s o f c o m p u t e r - b a s e d c o n t r o l s y s t e m s :

- i n t e g r a t i o n o f c o n t r o l o f i n d i v i d u a l s t a t i o n s , w i t h i n t e r a c t i o n s between them

taken i n t o a c c o u n t ;

216

- i n t e g r a t i o n o f management and u t i l i z a t i o n o f d a t a o r i g i n a t i n g f rom v a r i o u s

s o u r c e s , such as t h e l a b o r a t o r y , a u t o m a t i c measur ing i n s t r u m e n t s , c o u n t e r s ,

f a c t o r y r e c o r d s , e t c .

I t i s f e l t t h a t t h e s e new p o s s i b i l i t i e s have n o t y e t been f u l l y r e a l i z e d .

Examples o f t h e i r u t i l i z a t i o n a r e men t ioned i n t h e s u b s e q u e n t S e c t i o n s and i n

C h a p t e r 7.

6.3 FLOW CONTROL USING VARIABLE SPEED DRIVES

6.3.1 I n t r o d u c t i o n

I t has a l r e a d y been men t ioned i n S e c t i o n 1.4.3 t h a t t h e d r i v e s o f pumps, f a n s

and compresso rs a r e t y p i c a l l y r e s p o n s i b l e f o r abou t 60% o f t h e e l e c t r i c a l e n e r g y

consumed i n a s u g a r f a c t o r y . Among the l a r g e s t power consumers , t h e f o l l o w i n g

mach ines can be named:

- b o i l e r f e e d pumps;

- b e e t pumps;

- main j u i c e pumps i n t h e j u i c e p u r i f i c a t i o n s t a t i o n ;

- w a s t e - w a t e r pumps;

- f o r c i n g f a n s and e x h a u s t f a n s i n t h e b o i l e r h o u s e ;

- mechan ica l v a p o u r c o m p r e s s o r s ;

- a i r f a n s i n l o w - t e m p e r a t u r e p u l p d r y e r s .

As t h e p r o c e s s i n g c a p a b i l i t y o f t h e f a c t o r y o r t h e t h r o u g h p u t s o f t h e i n d i v i d u a l

s t a t i o n s v a r y , t h e f l o w s o f media d e l i v e r e d by pumps and f a n s a r e v a r i e d t o o .

T h i s i s e n s u r e d by a u t o m a t i c modules w h i c h a d j u s t t h e f l o w s t o t he r e q u i r e d

v a l u e s . W i th t h e f l o w c o n t r o l methods w i d e l y a p p l i e d , t he most economica l

o p e r a t i o n o f t h e sys tem c o m p r i s i n g t h e mach ine , i t s d r i v e and t h e c o n t r o l module

i s a c h i e v e d a t t h e maximum f l o w . A t r e d u c e d f l o w , e n e r g y d i s s i p a t i o n t a k e s p l a c e

i n t h e sys tem and t h e d r i v e consumes more power t han r e a l l y n e c e s s a r y .

U s i n g v a r i a b l e speed c o n t r o l , t h e power consumed by t he d r i v e o f a pump o r

a f a n can be m i n i m i z e d f o r any f l o w v a l u e . The a t t a i n a b l e s a v i n g depends on t h e

c o n t r o l method r e p l a c e d ; t y p i c a l l y , i t i s o f t h e o r d e r o f 20-40% o f t h e

e l e c t r i c a l e n e r g y consumed by t h e d r i v e d u r i n g t h e e n t i r e o p e r a t i o n .

6 .3 .2 Pump d r i v e s

The t a s k o f t h e pump a t a g i v e n l i q u i d f l o w i s t o g e n e r a t e a p r e s s u r e h i g h

enough t o overcome the p r e s u r e l o s s i n t h e h y d r a u l i c sys tem t o w h i c h t h e l i q u i d

i s d e l i v e r e d . The p r e s s u r e l o s s i s an a p p r o x i m a t e l y q u a d r a t i c f u n c t i o n o f t h e

f l o w , w h i l e t h e p r e s s u r e g e n e r a t e d by a r o t o d y n a m i c pump a t a c o n s t a n t r a t e o f

r e v o l u t i o n depends on t h e f l o w , a c c o r d i n g t o t h e pump c h a r a c t e r i s t i c s .

C o n s e q u e n t l y , t h e p r e s s u r e e q u i l i b r i u m between an u n c o n t r o l l e d pump and t h e

h y d r a u l i c sys tem i s o b t a i n e d a t a d e f i n i t e f l o w v a l u e , as shown i n F i g . 6 . 2 .

When the r e q u i r e d f l o w d e v i a t e s f rom t h e e q u i l i b r i u m v a l u e , i t i s n e c e s s a r y t o

217

Flow ( k g / s )

F i g . 6 . 2 . Pump c h a r a c t e r i s t i c s ( 1 ) and p r e s s u r e l o s s i n a h y d r a u l i c sys tem ( 2 ) . Hs - s t a t i c h e a d , - dynamic h e a d , Η - e f f e c t i v e h e a d , G - e q u i l i b r i u m f l o w . Dashed l i n e s i n d i c a t e pump c h a r a c t e r i s t i c s a t d i f f e r e n t r a t e s o f r e v o l u t i o n .

overcome the mismatch between t h e c h a r a c t e r i s t i c s o f t h e pump and t h a t o f t h e

h y d r a u l i c s y s t e m .

U s i n g a b y - p a s s c o n t r o l , a d i f f e r e n c e i s c r e a t e d between t h e f l o w o f l i q u i d

p a s s i n g t h r o u g h t h e pump and t h e f l o w o f t h e l i q u i d d e l i v e r e d t o t h e s y s t e m .

The pump i s o p e r a t e d a t a f l o w l a r g e r than r e a l l y n e e d e d , and t h e e x c e s s l i q u i d

i s r e c i r c u l a t e d t o t h e s u c t i o n n o z z l e . T h i s i s accompanied by e n e r g y d i s s i p a t i o n

i n t he b y - p a s s v a l v e . The power consumed by t h e pump d r i v e can be e x p r e s s e d as

= ( G ^ + G ^ ) g H / n b = N^,^ + ( 6 . 1 )

where G^ i s t h e mass f l o w o f l i q u i d d e l i v e r e d t o t h e h y d r a u l i c s y s t e m , G^ i s t h e

mass f l o w o f r e c i r c u l a t e d l i q u i d , g i s t h e a c c e l e r a t i o n o f g r a v i t y , Η i s t h e

e f f e c t i v e pump h e a d , x]^ i s t h e pump e f f i c i e n c y , i s t h e power consumed t o

d e l i v e r t h e f l o w o f l i q u i d r e a l l y n e e d e d , and i s t h e power l o s t by l i q u i d

r e c i r c u l a t i o n .

A method w h i c h i s by f a r t h e most p o p u l a r i n t h e s u g a r i n d u s t r y c o n s i s t s o f

f l o w c o n t r o l by t h r o t t l i n g . U s i n g a t h r o t t l i n g v a l v e i n s t a l l e d between t h e

d i s c h a r g e n o z z l e o f t he pump and t h e h y d r a u l i c s y s t e m , t h e dynamic head can be

changed and t h u s a d i f f e r e n c e can be c r e a t e d between t h e p r e s s u r e g e n e r a t e d by

t he pump and t he p r e s s u r e a t t h e sys tem i n l e t . The pump i s o p e r a t e d a t an

e f f e c t i v e head l a r g e r t han r e a l l y n e e d e d , w h i c h i s accompanied by e n e r g y

d i s s i p a t i o n i n t h e t h r o t t l i n g v a l v e . The power consumed by t h e pump d r i v e can be

e x p r e s s e d as

\ = G^g (H + H ^ / n ^ = N^^ + ( 6 . 2 )

where i s t h e e f f e c t i v e head l o s t by t h r o t t l i n g , i s t h e pump e f f i c i e n c y ,

N^^ i s t he power consumed t o d e l i v e r mass f l o w G ^ a t e f f e c t i v e head H , and N-j i s

t he power l o s t by t h r o t t l i n g . I t s h o u l d be n o t e d t h a t i s d i f f e r e n t f rom τ]^

because t h e pump i s o p e r a t e d a t a n o t h e r r e g i o n o f i t s c h a r a c t e r i s t i c s .

T y p i c a l l y , t h e e f f i c i e n c y o f a r o t o d y n a m i c pump o p e r a t e d a t c o n s t a n t speed

218

d e c r e a s e s when moving away f rom t h e nominal o p e r a t i n g c o n d i t i o n s , t h i s

c o n t r i b u t i n g t o t he e n e r g y d i s s i p a t i o n i n d u c e d by t h e t h r o t t l i n g c o n t r o l .

The e n e r g y d i s s i p a t i o n e f f e c t s c h a r a c t e r i s t i c o f t h e f l o w c o n t r o l methods

d i s c u s s e d above can be e l i m i n a t e d by u s i n g a v a r i a b l e speed c o n t r o l . I t s

p r i n c i p l e can be seen i n F i g . 6 . 2 , where pump c h a r a c t e r i s t i c s c o r r e s p o n d i n g t o

d i f f e r e n t speeds a r e i n d i c a t e d by dashed l i n e s . By v a r y i n g t h e r e v o l u t i o n r a t e

o f t h e pump i m p e l l e r , t he e f f e c t i v e pump head can be n e a r l y i d e a l l y a d j u s t e d t o

t h e c h a r a c t e r i s t i c s o f t he h y d r a u l i c s y s t e m . C o n s e q u e n t l y , t h e pump d r i v e

consumes o n l y as much power as r e a l l y needed t o d e l i v e r t h e r e q u i r e d f l o w o f t h e

l i q u i d . A n o t h e r advan tage o f a v a r i a b l e r e v o l u t i o n speed i s t h a t t h e e f f i c i e n c y

o f t h e pump d e v i a t e s o n l y n e g l i g i b l y f rom i t s maximum v a l u e . A compar i son o f

power consumpt ion f o r v a r i a b l e speed c o n t r o l and t h r o t t l i n g c o n t r o l i s shown i n

F i g . 6 . 3 .

100

- 80 c

o I 60 3 § AO

20

0 20 AO 60 80 100 Flow ( 7 · )

F i g . 6 . 3 . Power consumed by a pump d r i v e a t v a r i a b l e f l o w . 1 - c o n t r o l by t h r o t t l i n g , 2 - v a r i a b l e speed c o n t r o l .

I n a s p e c i f i c a p p l i c a t i o n , t he e n e r g y s a v i n g w h i c h can be a t t a i n e d by u s i n g

v a r i a b l e speed c o n t r o l depends on t h e l o a d c h a r a c t e r i s t i c s o f t h e pump. The

f e a s i b i l i t y o f t he method i s a m a t t e r o f e c o n o m i c s , as t h e v a l u e o f e n e r g y saved

s h o u l d be we ighed a g a i n s t t h e i n c r e a s e d i n v e s t m e n t c o s t o f t h e d r i v e ( r e f . 2 0 ) .

6 .3 .3 Fan d r i v e s

The f l o w o f gas d e l i v e r e d by a f a n can a l s o be v a r i e d by u s i n g a b y - p a s s o r

t h r o t t l i n g c o n t r o l a c c o r d i n g t o t h e p r i n c i p l e s d i s c u s s e d i n t h e p r e c e d i n g

S e c t i o n ; one p o s s i b l e d i f f e r e n c e i s t h a t t h e t h r o t t l i n g v a l v e can be i n s t a l l e d

i n t h e s u c t i o n l i n e o f t h e f a n . A n o t h e r c o n t r o l method used i n c o n n e c t i o n w i t h

l a r g e - c a p a c i t y f a n s o p e r a t e d a t c o n s t a n t speed employs t h e p o s i t i o n i n g o f i n l e t

219

g u i d e vanes f o r a v a r i a b l e p e r i p h e r a l component o f t h e gas v e l o c i t y a t i n l e t .

A c c o r d i n g t o E u l e r ' s pump and t u r b i n e e q u a t i o n , t h e t h e o r e t i c a l head can be

e x p r e s s e d as

"2 "2u l ^ l u ^ ( 6 . 3 )

where u i s t h e v e l o c i t y o f t r a n s p o r t a t i o n a t t h e mean r o t o r d i a m e t e r , c^ i s

t h e p e r i p h e r a l component o f t h e gas v e l o c i t y , and t h e s u b s c r i p t s deno te 1 -

i n l e t and 2 - o u t l e t ; g i s t h e a c c e l e r a t i o n o f g r a v i t y .

As can be s e e n , t he t h e o r e t i c a l head - and t h u s t h e e f f e c t i v e head t o o - v a r i e s

as c ^^ i s v a r i e d . When t h e a n g l e o f i n c l i n a t i o n o f t h e i n l e t g u i d e vanes i s

changed , t he gas f l o w e n t e r i n g t h e f a n r o t o r i s a l s o c h a n g e d . T h i s c o n t r o l

method i s more economica l t han c o n t r o l by t h r o t t l i n g , p a r t i c u l a r l y a t f l o w s

s m a l l e r t han 70-75% o f t h e maximum v a l u e .

S i m i l a r l y t o t h e case o f t h e pump, h o w e v e r , i t i s v a r i a b l e speed c o n t r o l

w h i c h e n s u r e s t h e most e n e r g y - e f f i c i e n t f a n o p e r a t i o n . A compar i son o f power

consumpt ion c u r v e s r e p r e s e n t i n g t h r e e d i f f e r e n t c o n t r o l methods i s shown i n

F i g . 6 . 4 . O b v i o u s l y , t h e f e a s i b i l i t y o f v a r i a b l e speed c o n t r o l i n a s p e c i f i c f a n

a p p l i c a t i o n depends on economic f a c t o r s ( r e f . 2 0 ) .

20 40 60 Flow ( " /«)

80 100

F i g . 6 . 4 . Power consumed by a f a n d r i v e a t v a r i a b l e f l o w . 1 - c o n t r o l by t h r o t t l i n g , 2 - p o s i t i o n i n g o f i n l e t g u i d e v a n e s , 3 - v a r i a b l e s p e e d .

6.4 EXTRACTION CONTROL

The dynamic r e s p o n s e o f t h e e x t r a c t i o n p r o c e s s p e r f o r m e d i n c o n t e m p o r a r y

c o n t i n u o u s e x t r a c t o r s i s v e r y s l o w . U s i n g c o n v e n t i o n a l c o n t r o l c i r c u i t s , t h e

t ime span between s e t - p o i n t a d j u s t m e n t and a t t a i n m e n t o f a d e s i r e d r e s u l t i s o f

t h e o r d e r o f s e v e r a l h o u r s . M a i n l y f o r t h i s r e a s o n , a human o p e r a t o r i s o f t e n

unab le t o c o n t r o l t h e e x t r a c t i o n p r o c e s s v e r y e f f e c t i v e l y , a l t h o u g h good r e s u l t s

can c e r t a i n l y be a c h i e v e d by e x p e r i e n c e d p e r s o n n e l . I f a d i s t u r b a n c e , such as

t o o l a r g e a j u i c e d r a f t , i s d e t e c t e d , t h e n s e v e r a l h o u r s may be needed t o b r i n g

t he s i t u a t i o n back t o n o r m a l . I n t h e meant ime, t h e f a c t o r y must be o p e r a t e d

220

under abnormal c o n d i t i o n s i n d u c i n g an e x c e s s i v e h e a t c o n s u m p t i o n .

E f f e c t i v e e x t r a c t i o n c o n t r o l can be e n s u r e d i f t h e c o n v e n t i o n a l c o n t r o l l e r s

a r e r e p l a c e d by a compu te r -based c o n t r o l s y s t e m . T h i s makes i t p o s s i b l e t o

m o n i t o r t h e t r e n d s o f pa rame te rs measured and t o employ c o n t r o l a l g o r i t h m s

p r e v e n t i n g u n d e s i r a b l e s i t u a t i o n s . The s t a b i l i z a t i o n o f e x t r a c t i o n pa rame te r s

f a c i l i t a t e s e f f i c i e n t e n e r g y usage i n t h e e n t i r e s u g a r m a n u f a c t u r i n g p r o c e s s .

I f a d i s t u r b a n c e o c c u r s , t h e computer i s a b l e t o r e s t o r e t h e d e s i r e d paramete r

reg ime more q u i c k l y than a human o p e r a t o r .

V a r i o u s e x t r a c t o r t y p e s men t ioned i n S e c t i o n 5.2 r e q u i r e d i f f e r e n t app roaches

t o t h e i r c o n t r o l . The e s s e n t i a l f e a t u r e s o f c o n t r o l sys tems used i n c o n n e c t i o n

w i t h t o w e r , drum and m o v i n g - b e d e x t r a c t o r s have been p r e s e n t e d i n t h e l i t e r a t u r e

( r e f s . 1 4 , 2 1 , 2 2 ) . C o n t r o l o f t h e t r o u g h e x t r a c t o r i s p a r t i c u l a r l y s i m p l e , and

can be summarized he re f o r i l l u s t r a t i o n p u r p o s e s . An o u t l i n e o f a t r o u g h

e x t r a c t o r , w i t h i n d i c a t i o n s o f t h e measured and c o n t r o l l e d p a r a m e t e r s , i s g i v e n

i n F i g . 6 .5 . The i n p u t s a r e as f o l l o w s ( r e f . 6 ) :

- l e v e l s i n b e e t s i l o ;

- b e l t w e i g h e r on b e l t f o r c o s s e t t e s ;

- c o s s e t t e s l e v e l a t j u i c e e n d ;

- 4 b u b b l e - t u b e l e v e l s a t i n t e r m e d i a t e p o i n t s i n t h e t r o u g h ;

- f r e s h - w a t e r f l o w ;

- p r e s s - w a t e r f l o w ;

- l e v e l i n p r e s s - w a t e r t a n k .

The e s s e n t i a l t a s k o f t he c o n t r o l sys tem i s t o keep t h e l e v e l o f t h e c o s s e t t e s

Speed control of slicing machines

Speed control of h e l i c e s ^

^ Θ— Juice flow control

Press water and fresh water flow control

Temperature control

F i g . 6 . 5 . Work ing p r i n c i p l e o f t h e a u t o m a t i c c o n t r o l o f a t r o u g h e x t r a c t o r ( a f t e r r e f . 6 ) . P o i n t s o f measurements : C - r a t e o f d e l i v e r y o f c o s s e t t e s , L0 -L6 - l e v e l s , T 0 - T 5 - t e m p e r a t u r e s , F l and F2 - f l o w s .

221

i n t he e x t r a c t o r s a t i s f a c t o r y under a l l c o n d i t i o n s . I n a d d i t i o n , t h e computer

t a k e s c a r e o f t h e t e m p e r a t u r e c o n t r o l , as w e l l as o f t h e s t a r t - u p and s h u t - d o w n

p r o c e d u r e s , i n c l u d i n g s t a r t i n g and s t o p p i n g o f a l l m o t o r s , open ing and c l o s i n g

o f main v a p o u r v a l v e , e t c .

The computer can a l s o be c h a r g e d w i t h d r a f t o p t i m i z a t i o n . T h i s p rob lem

o r i g i n a t e s f rom t h e f a c t t h a t t h e d r a f t and t he s u g a r l o s s i n e x h a u s t e d

c o s s e t t e s a r e i n t e r r e l a t e d . The i n f l u e n c e o f t h e d r a f t on e n e r g y demand can be

c a l c u l a t e d f rom t h e e v a p o r a t o r hea t b a l a n c e . The s u g a r c o n c e n t r a t i o n i n

e x h a u s t e d c o s s e t t e s can be e x p r e s s e d u s i n g S i l i n ' s f o r m u l a

b^ = ( ( a - l ) b ^ / ( a e x p ( ( ( a - 1 ) / a ) ( A T L x / y ) ) - 1) ( 6 . 4 )

where a i s t h e j u i c e d r a f t , b^ i s t h e s u g a r c o n c e n t r a t i o n i n t h e incoming

c o s s e t t e s , A i s a c h a r a c t e r i s t i c c o n s t a n t , Τ i s t h e a b s o l u t e t e m p e r a t u r e , L i s

t he l e n g t h o f 100 g c o s s e t t e s , τ i s t h e e x t r a c t i o n t i m e , and μ i s t h e j u i c e

v i s c o s i t y .

F o r g i v e n p r i c e s o f s u g a r and f u e l , i t t h u s becomes p o s s i b l e t o e x p r e s s t h e

v a l u e o f s u g a r l o s t i n e x h a u s t e d c o s s e t t e s , and t h e v a l u e o f t h e n e c e s s a r y

e n e r g y i n p u t , as f u n c t i o n s o f t he j u i c e d r a f t . T h i s makes i t p o s s i b l e t o

d e t e r m i n e t h e op t ima l d r a f t v a l u e .

A more r e l i a b l e d e t e r m i n a t i o n o f t h e s u g a r l o s s can be a c h i e v e d u s i n g o n - l i n e

measurements o f t h e s u g a r c o n t e n t o f t h e p r e s s w a t e r . Examples a r e a l s o known o f

t h e a p p l i c a t i o n s o f i n d u s t r i a l r e f r a c t o m e t e r s w i t h a u t o m a t i c j u i c e samp l i ng f o r

o n - l i n e measurements o f t h e j u i c e c o n c e n t r a t i o n . F u r t h e r m o r e , t h e o p t i m i z a t i o n

model can be e x t e n d e d t o i n c o r p o r a t e t h e consequences o f t h e e x t r a c t i o n o f

n o n s u g a r s , by p r e d i c t i n g t h e d i s t r i b u t i o n o f t h e e x t r a c t e d s u g a r between w h i t e

s u g a r and m o l a s s e s . I n t h i s c a s e , t h e op t ima l d r a f t depends a l s o on t h e p r i c e o f

m o l a s s e s .

Summaries o f o p e r a t i o n a l r e s u l t s o b t a i n e d w i t h c o m p u t e r - b a s e d e x t r a c t i o n

c o n t r o l can be f o u n d i n t h e l i t e r a t u r e ( r e f s . 2 1 , 2 3 ) . The r e p o r t e d r e d u c t i o n s o f

t h e j u i c e d r a f t a r e o f t h e o r d e r o f 1-2%.

6.5 EVAPORATION CONTROL

I t f o l l o w s f rom t h e c h a r a c t e r i s t i c s o f t h e the rma l sys tem t h a t two a s p e c t s o f

a u t o m a t i c e v a p o r a t i o n c o n t r o l a r e p a r t i c u l a r l y i m p o r t a n t t o t h e e n e r g y economy,

namely e f f e c t i v e s t a b i l i z a t i o n o f t h e c o n c e n t r a t i o n o f t h i c k j u i c e , and

e f f e c t i v e s t a b i l i z a t i o n o f v a p o u r p r e s s u r e s i n t h e i n d i v i d u a l e v a p o r a t o r

e f f e c t s . As p o i n t e d o u t by Z a g r o d z k i t h r e e decades ago ( r e f . 2 4 ) , p r e s s u r e

s t a b i l i z a t i o n i n t h e e f f e c t f rom w h i c h v a p o u r i s w i t h d r a w n f o r vacuum-pan

h e a t i n g i s a d e c i s i v e f a c t o r i n e n s u r i n g s t a b l e e v a p o r a t o r o p e r a t i o n .

O t h e r c o n t r o l r e q u i r e m e n t s a r e l i n k e d t o t h e c h a r a c t e r i s t i c f e a t u r e s o f t h e

m u l t i p l e - e f f e c t e v a p o r a t o r s t a t i o n . I t i s e s s e n t i a l t o e n s u r e economica l

222

Operation o f t he ind iv idua l e f f e c t s , t h a t i s , t o maintain optimal c o n d i t i o n s f o r

hea t t r a n s f e r and t o m in im i ze s u c r o s e d e c a y . C o n c e r n i n g t h e p r o c e s s d y n a m i c s ,

f l e x i b l e a d a p t a t i o n o f t h e e v a p o r a t i n g c a p a c i t y o f t h e e n t i r e s t a t i o n t o

changes i n t h e f l o w o f t h i n j u i c e e n t e r i n g t h e e v a p o r a t o r i s p r i m a r i l y r e q u i r e d .

When p e r f o r m i n g t he n e c e s s a r y f u n c t i o n s , t h e a u t o m a t i c c o n t r o l sys tem s h o u l d

be a b l e t o e l i m i n a t e d i s t u r b a n c e s r e s u l t i n g f rom f l u c t u a t i o n s o f t h e f o l l o w i n g

q u a n t i t i e s :

- t h i n j u i c e f l o w and c o n c e n t r a t i o n ;

- j u i c e l e v e l s i n t h e e v a p o r a t o r b o d i e s ;

- h e a t i n g v a p o u r demand ( e s p e c i a l l y v a p o u r w i t h d r a w n f o r vacuum-pan h e a t i n g ) .

To t h e g e n e r a l r e q u i r e m e n t s l i s t e d a b o v e , v a r i o u s e x t e n s i o n s can be added

depend ing on t h e t y p e o f e v a p o r a t o r emp loyed . F o r examp le , t h e R o b e r t t y p e and

f a l l i n g - f i l m e v a p o r a t o r s a r e p r e f e r a b l y f e e d - c o n t r o l l e d , w h i l e t h e c l i m b i n g - f i l m

e v a p o r a t o r must be p r o v i d e d w i t h a h y d r a u l i c s e a l a t t h e o u t l e t , t h i s i m p l y i n g

d i s c h a r g e c o n t r o l . M o r e o v e r , owing t o t h e d i f f e r e n c e s i n j u i c e vo lumes

c o n t a i n e d , t h e dynamic r e s p o n s e o f t h e t h i n - f i l m e v a p o r a t o r s i s s e v e r a l t imes

f a s t e r t han t h a t o f t h e R o b e r t e v a p o r a t o r s .

The m u l t i t u d e o f r e q u i r e m e n t s c h a r a c t e r i z i n g v a r i o u s e v a p o r a t o r s t a t i o n s can

be met o n l y by a p p l y i n g v a r i o u s c o n t r o l s y s t e m s . T y p i c a l s o l u t i o n s a r e r e v i e w e d

e l s e w h e r e ( r e f s . 2 5 - 2 7 ) . The p r o c e s s e s t o be c o n t r o l l e d a r e v e r y comp lex ,

h o w e v e r , as t h e y c o n s i s t o f i n t e r r e l a t e d r a p i d phenomena o f h e a t t r a n s f e r and

r e l a t i v e l y s l o w phenomena o f mass t r a n s f e r . F o r t h i s r e a s o n , i t may be d i f f i c u l t

t o a t t a i n s a t i s f a c t o r y r e s u l t s w i t h t y p i c a l c o n t r o l s y s t e m s , and t h e v a r i a t i o n s

f ound i n p r a c t i c e seem t o be i n f i n i t e . T h i s can be e x e m p l i f i e d by a u t o m a t i c

c o n t r o l o f t h e c o n c e n t r a t i o n o f t h i c k j u i c e . Fou r d i f f e r e n t s o l u t i o n s a r e shown

s c h e m a t i c a l l y i n F i g . 6 . 6 . The sys tems under ( a ) , ( b ) and ( c ) a r e t o o p r i m i t i v e

t o p e r f o r m s a t i s f a c t o r i l y under a l l c o n d i t i o n s . B e t t e r r e s u l t s can be o b t a i n e d

w i t h t he f o u r t h s y s t e m , i n w h i c h t h e f l o w o f v a p o u r f rom t h e l a s t e v a p o r a t o r

e f f e c t i s measured and compared w i t h t h e r e q u i r e d v a l u e d e t e r m i n e d on t h e b a s i s

o f measurements o f j u i c e fow and c o n c e n t r a t i o n b e f o r e t h e l a s t e f f e c t . T h i s

p r i n c i p l e can be a l s o m o d i f i e d by i n t r o d u c i n g a c o r r e c t i o n o f t h e r e q u i r e d

v a p o u r f l o w depend ing on t h i c k - j u i c e c o n c e n t r a t i o n measured .

I t may be added t h a t even more complex sys tems o f s t a b i l i z a t i o n o f t h i c k -

j u i c e c o n c e n t r a t i o n a r e e n c o u n t e r e d i n p r a c t i c e . A t r e q u i r e d c o n c e n t r a t i o n

v a l u e s a p p r o a c h i n g 75% DS, due w e i g h t s h o u l d be g i v e n t o t h e s a f e t y r e q u i r e m e n t

c o n c e r n e d w i t h t h e r i s k o f t h i c k j u i c e becoming s u p e r s a t u r a t e d . As t h e s t a t e o f

s a t u r a t i o n depends on j u i c e t e m p e r a t u r e , a d d i t i o n a l measurements become

n e c e s s a r y and e x t e n d e d d e c i s i o n - m a k i n g must be i n c l u d e d i n t h e c o n t r o l

a l g o r i t h m .

A l t h o u g h good r e s u l t s can be o b t a i n e d u s i n g c o n v e n t i o n a l e v a p o r a t o r c o n t r o l .

223

(α)

to condenser

η -βίο) to condenser

F i g . 6 .6 . P o s s i b l e s o l u t i o n s o f t h e a u t o m a t i c c o n t r o l o f t h i c k - j u i c e c o n c e n t r a t i o n : ( a ) t h i c k - j u i c e r e c i r c u l a t i o n , ( b ) t h i n - j u i c e b y - p a s s , ( c ) v a r i a b l e p r e s s u r e i n t h e l a s t e v a p o r a t o r e f f e c t , ( d ) v a r i a b l e v a p o u r f l o w f rom n e x t t o t h e l a s t e f f e c t t o t h e c o n d e n s e r . D, F - measurements o f d e n s i t y and f l o w , r e s p e c t i v e l y ; R, C - r e g i s t r a t i o n and c o n t r o l , r e s p e c t i v e l y ; U F , FFRC - e lements r e s p o n s i b l e f o r t h e c o m p u t a t i o n o f r e q u i r e d f l o w v a l u e and t he compar i son between a c t u a l and r e q u i r e d f l o w v a l u e s , r e s p e c t i v e l y .

224

t h e above example can be seen as an i n d i c a t i o n o f t he f a c t t h a t e f f e c t i v e

c o n t r o l a l g o r i t h m s must be q u i t e c o m p l i c a t e d , and t h e i r i m p l e m e n t a t i o n u s i n g

c o n v e n t i o n a l c o n t r o l t e c h n o l o g y may be r a t h e r c o s t l y . F o r t h i s r e a s o n , c o m p u t e r -

based e v a p o r a t o r c o n t r o l i s now g a i n i n g p o p u l a r i t y ( r e f s . 1 , 3 , 1 4 , 2 2 ) . A s o l u t i o n

r e c e n t l y implemented i n a s e x t u p l e - e f f e c t e v a p o r a t o r can be c i t e d as an example

( r e f . 2 8 ) . The f u n c t i o n s pe r f o rmed by t h e computer p rogram a r e l o g i c a l l y d i v i d e d

i n t o t h r e e modu les .

( i ) P r e s s u r e c o n t r o l .

( i i ) S u p p r e s s i o n o f f l u c t u a t i o n s o f j u i c e c o n c e n t r a t i o n .

( i i i ) S t a b i l i z a t i o n o f t h i c k - j u i c e c o n c e n t r a t i o n a t a p r e d e t e r m i n e d l e v e l .

The l o c a t i o n s o f t h e c o n t r o l v a l v e s i n t h e e v a p o r a t o r scheme and t h e p o i n t s o f

measurement o f t h e e s s e n t i a l v a r i a b l e s a r e shown s c h e m a t i c a l l y i n F i g . 6 . 7 .

Module ( i ) i s r e s p o n s i b l e f o r a d j u s t m e n t s o f t h e p o s i t i o n o f t h e b y - p a s s v a l v e

between f i r s t - and f o u r t h - e f f e c t v a p o u r , as w e l l as o f t h e p o s i t i o n s o f t h e

v a l v e s on v a p o u r l i n e s c o n n e c t e d t o t h e c o n d e n s e r . Module ( i i ) a d j u s t s t h e

p o s i t i o n o f t he v a l v e c o n t r o l l i n g t h i n - j u i c e f l o w t o t h e i n l e t o f t h e f o u r t h

e f f e c t . The t h i r d module c o n t r o l s t h e b y - p a s s f l o w o f f o u r t h - t o s i x t h - e f f e c t

v a p o u r and t he f l o w o f s i x t h - e f f e c t v a p o u r t o t h e c o n d e n s e r . I f t h e

c o n c e n t r a t i o n o f t h i c k j u i c e t e n d s t o be t o o l o w , t h e b y - p a s s v a l v e c l o s e s and

t h e v a p o u r f l o w t o t h e c o n d e n s e r i s i n c r e a s e d . I n t h e o p p o s i t e c a s e , t h e b y - p a s s

f l o w i s i n c r e a s e d , t h i s be ing e q u i v a l e n t t o " n e g a t i v e c o n d e n s a t i o n " o f s i x t h -

e f f e c t v a p o u r .

I n t o t a l , t h e sys tem u t i l i z e s 28 ana log i n p u t s ( f l o w s , p r e s s u r e s and

c o n c e n t r a t i o n s ) , 6 b i n a r y i n p u t s ( s t a t e o f a c t i v a t i o n o f c o n t r o l f u n c t i o n s ) and

6 ana log o u t p u t s ( v a r i a b l e s c o n t r o l l e d ) .

thin juice

1 2 3 U 5 6

lb

to ^^condenser

thick juice

F i g . 6 . 7 . Work ing p r i n c i p l e o f t h e a u t o m a t i c c o n t r o l o f a s e x t u p l e - e f f e c t e v a p o r a t o r ( a f t e r r e f . 2 8 ) . P o i n t s o f measurement : a -d - f l o w s , e and f -c o n c e n t r a t i o n s , g - j - p r e s s u r e s .

225

6.6 SUGAR CRYSTALL IZAT ION CONTROL

6.6.1 I n t r o d u c t i o n

The use o f a u t o m a t i c c o n t r o l i n t he s u g a r house was i n i t i a l l y s t i m u l a t e d by

t he r e q u i r e m e n t s o f s u g a r q u a l i t y , r e q u i r i n g r e p r o d u c i b l e r e s u l t s o f t h e

c r y s t a l l i z a t i o n p r o c e s s . R i s i n g e n e r g y c o s t s e x p o s e d t h e s i g n i f i c a n c e o f c o n t r o l

methods f o r i n c r e a s i n g t he e f f i c i e n c y o f e n e r g y u t i l i z a t i o n . The f o l l o w i n g

f a c t o r s a s s o c i a t e d w i t h t h e q u a l i t y o f p r o c e s s c o n t r o l a r e p a r t i c u l a r l y

i m p o r t a n t :

- e f f e c t i v e s t a b i l i z a t i o n o f p r o c e s s p a r a m e t e r s ;

- e x a c t d i s t r i b u t i o n o f mass f l o w s i n t h e c r y s t a l l i z a t i o n scheme;

- smooth w i t h d r a w a l o f h e a t i n g v a p o u r s f rom t h e e v a p o r a t o r s t a t i o n ;

- e n e r g y - e f f i c i e n t s u g a r b o i l i n g i n vacuum p a n s .

P r a c t i c a l e x p e r i e n c e p r o v e d t h a t good r e s u l t s can be a t t a i n e d i f a t w o - l e v e l

approach t o t h e c o n t r o l o f t h e s u g a r house i s a d o p t e d . The l o w e r l e v e l i n c l u d e s

t h e c o n t r o l l e r s o f t h e i n d i v i d u a l equ ipment u n i t s , w h i l e t h e upper l e v e l i s

c r e a t e d t o e n s u r e t h e c o o r d i n a t i o n o f i n t e r r e l a t e d p r o c e s s e s . A n o t h e r p r a c t i c a l

c o n c l u s i o n i s t h a t t h e c o m p l e x i t y o f c o n t r o l a l g o r i t h m s j u s t i f i e s t h e use o f

compu te r -based c o n t r o l s y s t e m s .

A p r e r e q u i s i t e f o r t h e e f f e c t i v e n e s s o f s u g a r house c o n t r o l i s t h e

s t a b i l i z a t i o n o f p r o c e s s i n p u t s . I n c o n t e m p o r a r y s u g a r f a c t o r i e s o p e r a t e d a t

t h i c k - j u i c e c o n c e n t r a t i o n s a p p r o a c h i n g 75% DS, t h e s t a b i l i z a t i o n o f pa rame te rs

o f t h e t h i c k j u i c e and s y r u p s becomes c r i t i c a l . I n o r d e r t o e n s u r e r e p r o d u c i b l e

r e s u l t s o f f e e d i n t a k e s t o t h e vacuum pans and t o r e d u c e t h e amount o f w a t e r

drawn f o r c o n t r o l p u r p o s e s , t h e r i s k o f u n d e s i r a b l e c r y s t a l f o r m a t i o n s h o u l d be

e l i m i n a t e d . As t h e s t a t e o f s a t u r a t i o n depends a l s o on s o l u t i o n t e m p e r a t u r e ,

t h i s i s no l o n g e r o n l y a q u e s t i o n o f c o n t r o l l i n g t h e c o n c e n t r a t i o n .

The t h i c k - j u i c e pa rame te rs can be s t a b i l i z e d i n a c o n d i t i o n e r shown

s c h e m a t i c a l l y i n F i g . 6 .8 . The j u i c e l e a v i n g t h e e v a p o r a t o r i s expanded i n a

v e s s e l i n s t a l l e d b e f o r e t h e t h i c k - j u i c e t a n k . The r e q u i r e d j u i c e t e m p e r a t u r e i s

m a i n t a i n e d by c o n t r o l l i n g t he p r e s s u r e . The v e s s e l i s a l s o l e v e l - c o n t r o l l e d . I f

t he s e l f - e v a p o r a t i o n o f t h i c k j u i c e i n c r e a s e s t h e c o n c e n t r a t i o n above t h e

r e q u i r e d v a l u e , t h e n t h e o u t l e t s t ream i s d i l u t e d w i t h t h i n j u i c e .

6 .6 .2 Sugar b o i l i n g

The e s s e n t i a l p a r t o f t h e hea t consumed i n t h e b a t c h vacuum pan i s needed t o

e v a p o r a t e w a t e r f rom s u g a r s o l u t i o n . I n t h e f i r s t phase o f t h e b o i l i n g c y c l e ,

t h e i n i t i a l l y drawn s o l u t i o n i s t h i c k e n e d i n o r d e r t o a t t a i n t h e r e q u i r e d

s u p e r s a t u r a t i o n . I n t h e r e m a i n i n g p h a s e s , w a t e r i n s u g a r s o l u t i o n s o r , i n some

i n s t a n c e s , pu re w a t e r i s drawn t o t h e vacuum pan f o r c o n t r o l p u r p o s e s . Water

i n t a k e s immed ia te l y a f t e r s e e d i n g a r e r e q u i r e d t o s t a b i l i z e t h e s u p e r s a t u r a t i o n .

226

to condenser

to sugar house

F i g . 6 . 8 . Scheme o f a u t o m a t i c t h i c k - j u i c e c o n d i t i o n i n g . 1 - c o n d i t i o n i n g v e s s e l , 2 - t h i c k - j u i c e t a n k . P, L , D - p r e s s u r e , l e v e l and d e n s i t y , r e s p e c t i v e l y ; I , C - i n d i c a t i o n and c o n t r o l , r e s p e c t i v e l y .

L a t e r o n , c r y s t a l g r o w t h i s a s s o c i a t e d w i t h t h e need t o draw w a t e r m a i n t a i n i n g

t he b a l a n c e between e v a p o r a t i o n and c r y s t a l l i z a t i o n . F i n a l l y , w a t e r may be drawn

i n o r d e r t o keep t h e s t r i k e b e f o r e i t i s d i s c h a r g e d .

The impor tance o f an a u t o m a t i c b o i l i n g c o n t r o l t o e f f i c i e n t e n e r g y

u t i l i z a t i o n l i e s i n e l i m i n a t i n g t h e i n f l u e n c e o f i n d e t e r m i n i s t i c f a c t o r s -

a s s o c i a t e d w i t h t he i n t e r v e n t i o n s o f a human o p e r a t o r - on t h e amount o f w a t e r

drawn and t h u s on e n e r g y s p e n t d u r i n g t h e b o i l i n g c y c l e . The e s s e n t i a l f u n c t i o n s

o f c o n t e m p o r a r y a u t o m a t i c c o n t r o l s can be summarized as f o l l o w s ( r e f s . 2 9 - 3 5 ) .

( i ) C o n t i n u o u s measur ing o f t h e s y r u p and m a s s e c u i t e l e v e l s f o r a u t o m a t i c

c h a r g i n g , a u t o m a t i c t h i c k e n i n g and a u t o m a t i c change o f f e e d s u p p l y , vacuum and

h e a t i n g v a p o u r s u p p l y .

( i i ) C o n t i n u o u s c o n t r o l o f s u p e r s a t u r a t i o n , u s u a l l y by measur ing t h e d i e l e c t r i c

v a l u e o r c o n d u c t i v i t y o f t h e m a s s e c u i t e .

( i i i ) C o n t i n u o u s c o n t r o l o f t he vacuum l e v e l i n t h e p a n .

( i v ) C o n t i n u o u s c o n t r o l o f t h e h e a t i n g v a p o u r s u p p l y d u r i n g t h e who le c y c l e .

( v ) C o n t i n u o u s c o n t r o l o f t h e m a s s e c u i t e t e m p e r a t u r e .

( v i ) C o n t i n u o u s measur ing o f t h e power consumpt ion o f t he s t i r r e r , t h i s a l l o w i n g

f i n a l t h i c k e n i n g t o t h e optimum m a s s e c u i t e c o n c e n t r a t i o n b e f o r e d i s c h a r g e .

I n a compu te r -based c o n t r o l s y s t e m , t h e computer a l s o t a k e s c a r e o f a l l v a l v e

o p e r a t i o n s d u r i n g s t a r t i n g and s t o p p i n g o f t he vacuum-pan c y c l e .

The f u n c t i o n s under ( i i i ) and ( i v ) a r e p a r t i c u l a r l y u s e f u l i n m i n i m i z i n g h e a t

c o n s u m p t i o n . A t t h e b e g i n n i n g o f t h e c r y s t a l g r o w t h p h a s e , i t i s d e s i r a b l e t o

i n c r e a s e t h e t e m p e r a t u r e o f t h e s u g a r s o l u t i o n ; t h i s can be done by i n c r e a s i n g

t h e p r e s s u r e i n t h e p a n . I n t h i s w a y , t he c r y s t a l l i z a t i o n can be i n c r e a s e d

w i t h o u t s p e n d i n g a d d i t i o n a l e n e r g y t o i n t e n s i f y e v a p o r a t i o n . L a t e r o n , when t h e

c r y s t a l g r o w t h becomes l i m i t e d by t h e e v a p o r a t i o n , t h e p r e s s u r e s h o u l d be

227

r e d u c e d , t h i s r e s u l t i n g i n r e d u c e d s o l u t i o n t e m p e r a t u r e and t h u s i n c r e a s e d h e a t

f l u x a t t h e h e a t i n g s u r f a c e . An a d d i t i o n a l e n e r g y - s a v i n g e f f e c t i s a l s o

o b t a i n e d , namely e x t r a c r y s t a l l i z a t i o n due t o r e d u c e d s o l u b i l i t y o f s u c r o s e

( i . e . , w i t h o u t h e a t e x p e n d i t u r e ) . The p r i n c i p l e o f p r o g r a m - c o n t r o l l e d

t e m p e r a t u r e changes d u r i n g t h e b o i l i n g c y c l e i s shown i n F i g . 6 . 9 ( a ) . When

p e r f o r m i n g t h e t e m p e r a t u r e r e d u c t i o n , t h e c o n t r o l a l g o r i t h m s h o u l d a v o i d

e x c e s s i v e t e m p e r a t u r e g r a d i e n t s , w i t h t h e a s s o c i a t e d r i s k o f f a l s e g r a i n

f o r m a t i o n .

(a) (b )

lower limit

Time Time

F i g . 6 . 9 . P r i n c i p l e s o f p r o g r a m - c o n t r o l l e d changes o f i m p o r t a n t v a r i a b l e s d u r i n g t h e a u t o m a t i c b o i l i n g c y c l e : ( a ) t e m p e r a t u r e , ( b ) h e a t i n g - v a p o u r f l o w . 1 - e v a p o r a t i o n o f t he i n i t i a l l y drawn s o l u t i o n , 2 - s e e d i n g and c r y s t a l f o r m a t i o n , 3 - c r y s t a l g r o w t h , 4 - t ime i n t e r v a l o f f l o w c o n t r o l u s i n g o p t i m i z a t i o n a l g o r i t h m .

The c o n t r o l o f h e a t i n g v a p o u r s u p p l y makes i t p o s s i b l e t o a v o i d u n n e c e s s a r y

w a t e r i n t a k e s accompanying t h e advanced c r y s t a l g r o w t h p h a s e . D u r i n g t h i s p a r t

o f t h e b o i l i n g c y c l e , t h e c o n t r o l a l g o r i t h m a d j u s t s t h e s e t - p o i n t v a l u e o f t h e

v a p o u r f l o w depend ing on t h e d r y s u b s t a n c e c o n t e n t o f t h e s o l u t i o n drawn and t h e

r a t e a t w h i c h t he m a s s e c u i t e l e v e l i s r a i s e d . The p r i n c i p l e o f p r o g r a m -

c o n t r o l l e d changes o f t h e v a p o u r f l o w i s shown i n F i g . 6 . 9 ( b ) .

The f u n c t i o n s under ( i ) and ( i i i ) can be u t i l i z e d t o v a r y t h e d u r a t i o n o f t h e

b o i l i n g c y c l e , a c c o r d i n g t o t h e r e q u i r e m e n t s d e f i n e d by t he u p p e r - l e v e l c o n t r o l .

More s p e c i f i c a l l y , i t i s p o s s i b l e t o r a i s e t h e m a s s e c u i t e l e v e l a t a

p r e d e t e r m i n e d r a t e , t h u s a f f e c t i n g t h e t ime needed t o r e a c h t h e maximum. I n

a d d i t i o n , t he s e t - p o i n t v a l u e o f t h e vacuum l e v e l and t h e moment when i t s change

i s i n i t i a t e d can be a d j u s t e d i n o r d e r t o i n f l u e n c e t h e r a t e o f c r y s t a l g r o w t h .

The c o m p l e x i t y o f c o m p u t e r - b a s e d b o i l i n g c o n t r o l s can be i l l u s t r a t e d by t h e

pa rame te rs o f one o f t h e sys tems a v a i l a b l e on t h e marke t ( r e f . 3 6 ) . I t uses 8

ana log and 32 d i g i t a l i n p u t s , as w e l l as 4 a n a l o g and 16 d i g i t a l o u t p u t s .

228

Communicat ion w i t h t h e u s e r i s p o s s i b l e v i a d i s p l a y , p r i n t e r , k e y b o a r d and u s e r -

d e f i n e d p u s h - b u t t o n s . The sys tem i s a l s o e q u i p p e d w i t h a da ta communica t ion

c h a n n e l , w h i c h p e r m i t s da ta t r a n s m i s s i o n t o and f rom t h e upper c o n t r o l l e v e l .

6 .6 .3 C o o r d i n a t i o n o f s u g a r house o p e r a t i o n s

The r e p r o d u c i b l e r e s u l t s o f automated s u g a r b o i l i n g make i t p o s s i b l e t o

u n d e r t a k e t h e t a s k o f c o o r d i n a t i n g t h e p r o c e s s e s o f m u l t i - s t a g e c r y s t a l l i z a t i o n

t o a c h i e v e t he b e s t o v e r a l l r e s u l t s . The c o n c e p t o f t h e q u a l i t y o f r e s u l t s ,

h o w e v e r , i s f a r f rom o b v i o u s . L e t us f o r m u l a t e two h y p o t h e t i c a l o b j e c t i v e s o f

c o o r d i n a t i o n :

- m a x i m i z i n g w h i t e s u g a r o u t p u t p e r 100 kg b e e t p r o c e s s e d ;

- m a x i m i z i n g s u g a r house t h r o u g h p u t , e x p r e s s e d i n amount o f t h i c k j u i c e

p r o c e s s e d i n u n i t t i m e .

On t h e b a s i s o f t h e s e o b j e c t i v e s , two d i f f e r e n t c o n t r o l s t r a t e g i e s can be

d e f i n e d ; i t i s a l s o p o s s i b l e t o combine them i n t o one compromise s t r a t e g y ( r e f .

3 7 ) . A n a l o g o u s l y , one can imag ine t he m i n i m i z a t i o n o f t h e e n e r g y demand b e i n g

a c c e p t e d as one o f t h e c o n t r o l o b j e c t i v e s , and s e a r c h i n g f o r e n e r g y s a v i n g s

be ing i n c l u d e d i n t he c o n t r o l s t r a t e g y .

I n any c o n t r o l s t r a t e g y , a c c o u n t s h o u l d be t aken o f numerous c o n s t r a i n t s

r e l a t e d t o t h e i n t e r a c t i o n o f c o n t i n u o u s and b a t c h e q u i p m e n t , s t o r a g e vo lumes

a v a i l a b l e , l i m i t a t i o n s o f h e a t i n g v a p o u r s u p p l y , e t c . P r a c t i c a l e x p e r i e n c e

p r o v e s t h a t v i o l a t i o n s o f t h e s e c o n s t r a i n t s a r e t h e p r i m a r y cause o f t h e

d i s t u r b a n c e s o c c u r r i n g i n m a n u a l l y o p e r a t e d c r y s t a l l i z a t i o n s u b s y s t e m s .

A t t h e p r e s e n t s t a t e o f deve lopmen t o f c o o r d i n a t i o n c o n t r o l , a t t e n t i o n i s

c o n c e n t r a t e d on a v o i d i n g d i s t u r b a n c e s i n mass and e n e r g y f l o w s w i t h i n t h e s u g a r

h o u s e . A c o n t r i b u t i n g f a c t o r i s t h a t t h e e x i s t i n g c r y s t a l l i z a t i o n subsys tems and

t h e i r e n v i r o n m e n t a r e o f t e n n o t p a r t i c u l a r l y s u i t e d t o a u t o m a t i c s u g a r house

o p e r a t i o n . T h i s r e s u l t s i n a m u l t i t u d e o f s p e c i a l c o n s t r a i n t s t o be a c c o u n t e d

f o r i n t h e c o n t r o l a l g o r i t h m s . T y p i c a l examples a r e : l i m i t e d c a p a c i t y o f t h e

vacuum s y s t e m , t o o smal l s t o r a g e t a n k s , l i m i t e d range o f o p e r a t i o n o f t h e

e v a p o r a t o r c o n t r o l , e t c . F o r t h i s r e a s o n , t h e g e n e r a l o b j e c t i v e s o f c o o r d i n a t i o n

i n t h e e s t a b l i s h e d sys tems a r e l e s s i m p o r t a n t t han t h e c o n t r o l f u n c t i o n s aimed

a t s a t i s f y i n g t h e c o n s t r a i n t s , l i k e s c h e d u l i n g o f o p e r a t i o n o f b a t c h vacuum pans

and c e n t r i f u g a l s , p r e v e n t i n g tank o v e r f l o w , e t c . ( r e f s . 3 8 - 4 1 ) . These f u n c t i o n s

c o n s i s t m a i n l y o f s u p e r v i s i n g t h e l e v e l s i n s y r u p t a n k s and m a s s e c u i t e m i x e r s

and a d j u s t i n g t h e w o r k i n g c y c l e s o f t h e b a t c h equ ipmen t . I n t h i s manner ,

a r e g u l a r i z a t i o n o f s u g a r house o p e r a t i o n can be o b t a i n e d .

The r e s u l t s o b t a i n e d w i t h t h e c o o r d i n a t i o n sys tems a r e c l e a r l y p o s i t i v e .

The a t t a i n a b l e e n e r g y s a v i n g s can be i l l u s t r a t e d by r e d u c t i o n o f t h e amount o f

w a t e r drawn t o b a t c h vacuum pans i n a s u g a r f a c t o r y i n FRG ( r e f . 4 2 ) . A f t e r

imp lement ing t h e c o o r d i n a t i o n s y s t e m , w a t e r i n t a k e s t o vacuum pans C were

229

reduced f rom t h e i n i t i a l l e v e l o f a b o u t 30 kg p e r 1 t magma t o z e r o . I n A

s t r i k e , w a t e r i n t a k e s were n e a r l y h a l v e d f rom abou t 12 t o 7 kg p e r 1 t magma.

I t i s f e l t t h a t f u r t h e r improvements a r e needed i n t h e methods o f

c o o r d i n a t i o n o f s u g a r house o p e r a t i o n . The e x i s t i n g c o o r d i n a t i o n sys tems do n o t

f u l l y u t i l i z e t h e t e c h n o l o g i c a l p o t e n t i a l o f i n t e g r a t e d c o n t r o l o f i n d i v i d u a l

s t a t i o n s and i n t e g r a t e d management o f d a t a on t h e d e t a i l s o f t h e s u g a r

c r y s t a l l i z a t i o n p r o c e s s .

6.7 PULP DRYING CONTROL

The goa l o f t h e p u l p d r y i n g p r o c e s s i s t o d r y t h e incoming p r e s s e d p u l p t o

a d e f i n i t e m o i s t u r e c o n t e n t . T y p i c a l l y , t h e f i n a l m o i s t u r e c o n t e n t o f t h e d r i e d

p u l p s h o u l d n o t be l o w e r t han 5-6%; i n t h e s u b s e q u e n t p e l l e t i n g , i t i s

e v e n t u a l l y i n c r e a s e d t o 9-11%. These v a l u e s a r e d e s i r a b l e w i t h r e s p e c t bo th t o

e n e r g y economy and t h e keep ing q u a l i t y o f t h e d r i e d p u l p .

The t a s k o f t h e a u t o m a t i c c o n t r o l s ys tem i s t o keep t h e f i n a l m o i s t u r e

c o n t e n t o f t h e d r i e d p u l p a t a p r e d e t e r m i n e d l e v e l . I t s h o u l d be p o s s i b l e t o

e l i m i n a t e d i s t u r b a n c e s r e s u l t i n g m a i n l y f rom f l u c t u a t i o n s o f t h e r a t e o f

d e l i v e r y o f p r e s s e d p u l p , t h e m o i s t u r e c o n t e n t o f p r e s s e d p u l p , and t h e q u a l i t y

o f p r e s s e d p u l p . The r e v i e w o f c o n t r o l p rob lems p r e s e n t e d be low i s l i m i t e d t o

t h e p rob lems c h a r a c t e r i s t i c o f d r u m - t y p e d r y e r s hea ted by combus t i on g a s e s .

C o n t r o l o f t h e combus t i on p r o c e s s i s e x c l u d e d f rom t h e r e v i e w as i t i s n o t

s p e c i f i c t o t h e s u g a r i n d u s t r y .

T h e r e a r e two main r e a s o n s f o r t h e d i f f i c u l t i e s e n c o u n t e r e d i n p u l p d r y i n g

c o n t r o l :

- a c c u r a t e measurements o f t h e m o i s t u r e c o n t e n t o f t h e p u l p a r e d i f f i c u l t t o

make;

- t h e p r o c e s s i s c h a r a c t e r i z e d by a l a r g e r a t i o o f gas f l o w t o p r e s s e d p u l p

f l o w ; i t s v a l u e depends on t h e d e t a i l s o f t h e p r o c e s s , and i s o f t h e o r d e r 2-4

i n h i g h - t e m p e r a t u r e d r y e r s ( l o w e r v a l u e s b e i n g c h a r a c t e r i s t i c o f d r y e r s

f e a t u r i n g gas r e c i r c u l a t i o n o r u t i l i z a t i o n o f b o i l e r f l u e g a s ) .

Because o f t h e f i r s t p r o b l e m , most a u t o m a t i c c o n t r o l sys tems p r e s e n t l y i n use

r e l y on measurement o f t h e e x i t gas t e m p e r a t u r e , i n d i r e c t l y r e p r e s e n t i n g t h e

f i n a l m o i s t u r e c o n t e n t o f t h e d r i e d p u l p . Depend ing on t h e a c t u a l v a l u e

measured , t h e sys tem a d j u s t s t h e e n e r g y i n p u t t o t h e d r y e r f u r n a c e . P o s s i b l e

c o r r e c t i o n s o f s e t t i n g s a r e i n t r o d u c e d a c c o r d i n g t o t h e r e s u l t s o f t h e

l a b o r a t o r y a n a l y s e s o f d r i e d p u l p s a m p l e s . The w o r k i n g p r i n c i p l e o f t h e d r y e r

c o n t r o l a f f e c t s t h e way t he d r y e r o p e r a t o r s a c t . I n o r d e r t o a v o i d t h e r i s k o f

t o o h i g h f i n a l m o i s t u r e c o n t e n t , t h e y t e n d t o o v e r d r y t he p u l p , w i t h t o o l a r g e

an e n e r g y consumpt ion as a r e s u l t .

The g a s / p u l p r a t i o , a l o n g w i t h a l a r g e s p e c i f i c volume o f t h e g a s , r e s u l t s i n

a c o n s i d e r a b l e d i f f e r e n c e between t h e r e t e n t i o n t ime o f t h e gas and t h a t o f

230

t he p u l p i n t h e d r y e r drum. W h i l e t h e t ime needed f o r t he p u l p t o r e a c h t h e

d r y e r o u t l e t i s o f t he o r d e r o f one h o u r , t he r e t e n t i o n t ime o f t h e gas may be

s e v e r a l s e c o n d s . F o r t h i s r e a s o n , i f a d i s t u r b a n c e o c c u r s and t he h e a t b a l a n c e

o f t he d r y e r c h a n g e s , t h e e x i t gas t e m p e r a t u r e i s changed w i t h i n s e c o n d s , b u t

15-20 m inu tes a r e needed b e f o r e t h e f i n a l m o i s t u r e c o n t e n t o f t h e d r i e d p u l p

i s c h a n g e d . T h i s t ime l ag i s t h e u n d e r l y i n g cause o f t h e d i f f i c u l t i e s a s s o c i a t e d

w i t h c o n t r o l l i n g t r a n s i e n t p r o c e s s e s i n t h e d r y e r . F o r examp le , i n p u l p d r y i n g

s t a t i o n s e q u i p p e d w i t h two o r more d r y e r s w o r k i n g i n p a r a l l e l , one o f them i s

d e s t i n e d t o a b s o r b t h e s w i n g s i n t h e p r o c e s s . When e x p e c t i n g f l u c t u a t i o n s o f t h e

p a r a m e t e r s , t h e o p e r a t o r u s u a l l y t r i e s t o c a t c h t h e w o r s t c o n d i t i o n . A t y p i c a l

r e s u l t i s t h a t t h e e x i t gas t e m p e r a t u r e t e n d s t o be t o o h i g h , and t h e e n e r g y

consumpt ion l a r g e r than r e a l l y n e c e s s a r y .

V a r i o u s sys tems o f a u t o m a t i c p u l p d r y i n g c o n t r o l have been p r o p o s e d t o

overcome t h e t y p i c a l d i f f i c u l t i e s men t ioned above ( r e f s . 4 3 , 4 4 ) . C o n s i d e r a b l e

improvements have been a t t a i n e d by u s i n g new methods o f measurement o f t h e

m o i s t u r e c o n t e n t o f t h e p u l p ( r e f s . 2 2 , 4 5 ) . As t h e c o n t r o l f u n c t i o n s a r e

e x t e n d e d and c o n t r o l a l g o r i t h m s become i n c r e a s i n g l y c o m p l e x , h o w e v e r , t h e

c o n v e n t i o n a l sys tems become c o m p l i c a t e d and c o s t l y . A c o n t r i b u t i n g f a c t o r i s

t h a t t h e e n e r g y - s a v i n g e x t e n s i o n s o f t h e p u l p d r y i n g p r o c e s s , l i k e u t i l i z a t i o n

o f b o i l e r f l u e gas o r gas r e c i r c u l a t i o n , r e q u i r e t h e a d d i t i o n o f e x t r a c o n t r o l

l o o p s t o t he b a s i c s y s t e m .

A l t h o u g h an e x p e r i e n c e d d r y e r o p e r a t o r can a t t a i n v e r y good r e s u l t s u s i n g

a c o n v e n t i o n a l c o n t r o l sys tem and new deve lopmen ts a r e s t i l l p o s s i b l e i n t h i s

f i e l d ( r e f . 4 6 ) , t he f u t u r e i s c e r t a i n l y i n c o m p u t e r - b a s e d s y s t e m s . The

advan tages o f computer a p p l i c a t i o n have been c l e a r l y d e m o n s t r a t e d i n a s u g a r

f a c t o r y i n FRG, where computer t e c h n o l o g y was i n t r o d u c e d as a l o g i c a l s t e p i n

dr ied pulp

F i g . 6 .10 . P o i n t s o f measurement o f main v a r i a b l e s i n t h e a u t o m a t i c c o n t r o l o f a p u l p d r y e r : 1 - combus t ion gas t e m p e r a t u r e , 2 - t e m p e r a t u r e i n t h e drum, 3 - o u t l e t t e m p e r a t u r e , 4 - f i n a l m o i s t u r e c o n t e n t , 5 - f u e l f l o w , 6 - a i r f l o w , 7 - f e e d e r c a p a c i t y , 8 - i n i t i a l m o i s t u r e c o n t e n t . F - f u r n a c e , D - d r u m , A - a f t e r d r y e r .

231

i m p r o v i n g t h e e x i s t i n g p u l p d r y i n g c o n t r o l s ( r e f . 4 7 ) . The mathemat i ca l model o f

t he p r o c e s s dynamics was i d e n t i f i e d on t h e b a s i s o f measurements r e c o r d e d i n t h e

a c t u a l p u l p d r y i n g s t a t i o n , u s i n g a method c o m p r i s i n g c o r r e l a t i o n a n a l y s i s and

e s t i m a t i o n o f model p a r a m e t e r s . A s k e t c h o f a d r y e r , w i t h i n d i c a t i o n o f i n p u t

and o u t p u t v a r i a b l e s o f t h e m o d e l , i s shown i n F i g . 6 .10 .

The computer c o n t r o l s t h r e e d r y e r s w o r k i n g i n p a r a l l e l . I t was e s t i m a t e d t h a t

t he imp lemen ta t i on o f t h i s sys tem r e s u l t e d i n a f u e l s a v i n g o f abou t 2.5%. I t i s

n o t e w o r t h y t h a t t h i s s a v i n g was o b t a i n e d r e l a t i v e t o d r y e r o p e r a t i o n s u p e r v i s e d

by e x p e r i e n c e d o p e r a t o r s u s i n g w e l l - f u n c t i o n i n g c o n v e n t i o n a l c o n t r o l s . S i m i l a r

r e s u l t s a r e r e p o r t e d f rom o t h e r cases o f a p p l i c a t i o n o f c o m p u t e r - b a s e d d r y e r

c o n t r o l s .

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12 Η. P a s c h o l d , E i n s a t z von P r o z e s s r e c h n e r n i n b e l g i s c h e n und n i e d e r l ä n d i s c h e n Z u c k e r f a b r i k e n , Z u c k e r i n d . , 105(4) (1980) 343-344.

13 L . W e n z e l , B e i s p i e l e von A u t o m a t i s i e r u n g s k o n z e p t e n i n Z u c k e r f a b r i k e n , Z u c k e r i n d . , 107(10) (1982) 934-936.

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15 H . S . B i r k e t t , Computer a p p l i c a t i o n s . Sugar J . , 46(10) (1984) 10-12. 16 Anonymous, A u t o m a t i s a t i o n e t i n f o r m a t i q u e i n d u s t r i e l l e , S u c r . F r . ,

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D C I - S y s t e m im Werk P l a t t l i n g d e r S ü d d e u t s c h e n Z u c k e r - A G , Z u c k e r i n d . , 111(4) (1986) 321-328.

18 T h . C r o n e w i t z , W e c h s e l w i r k u n g e n be i d e r E n t w i c k l u n g von D a t e n v e r a r b e i t u n g und V e r f a h r e n s t e c h n i k - M ö g l i c h k e i t e n d e r P r o z e s s f ü h r u n g i n d e r Z u k u n f t , Z u c k e r i n d . , 112(2) (1987) 103-107.

19 P. P e t e r s , A u t o m a t i s i e r u n g s - und P r o z e s s d a t e n e r f a s s u n g s a n i agen i n s ü d d e u t s c h e n Z u c k e r f a b r i k e n - e i n e S t a n d o r t b e s t i m m u n g , Z u c k e r i n d . , 112(2) (1987) 107-114.

232

20 J . M e r k l , E n e r g i e e i n s p a r u n g m i t d r e h z a h l r e g e ! b a r e n D r e h s t r o m a n t r i e b e n i n d e r Z u c k e r i n d u s t r i e , Z u c k e r i n d . , 110(2) (1985) 1082-1089.

21 P. M o s e l , E r f a h r u n g e n m i t e inem P r o z e s s r e c h n e r f ü r v o l l a u t o m a t i s c h e n B e t r i e b d e r E x t r a k t i o n , Z u c k e r , 27(10) (1974) 528-541.

22 P.W. van d e r P o e l , N . H . M . de V i s s e r and C . C . B l e y e n b e r g , Deve lopments i n computer and o n - l i n e c o n t r o l i n t h e b e e t s u g a r f a c t o r y . Sugar T e c h . R e v . , 9 ( 1 ) (1982) 1-58.

23 G . W i n d a l , B. P o r t a l e s and D. Maes, Le p o i n t s u r Γ a u t o m a t i s a t i o n des d i f f u s e u r s Continus p a r l e p rocede I R I S , S u c r . F r . , 126(93) (1985) 145-149.

24 S . Z a g r o d z k i , 0 zasadach samoczynne j r e g u l a c j i s t a c j i w y p a r n e j , G a z . C u k r o w . , 60 (4 ) (1958) 105-108.

25 W. Grasmann and 0. P a r i s e k , Messen und Rege ln des D u r c h f l u s s e s und d e r D i c h t e von D i c k s a f t und D ü n n s a f t , Z u c k e r , (1969) 432-438.

26 D ie Rege lung d e r V e r d a m p f s t a t i o n i n d e r Z u c k e r f a b r i k , T e c h n i c a l i n f o r m a t i o n f rom S iemens , F ü r t h , 1975.

27 T . W . B a k e r , E v a p o r a t i o n and h e a t i n g , i n : G . T . Meade and J . C . Chen ( E d s . ) , Cane Sugar Handbook, W i l e y , New Y o r k , 1977, p p . 185-235.

28 J . C . G i o r g i , P. G i r a u d and A . D e l e u r e n c e , G e s t i o n au tomat ique de l ' a t e l i e r d ' e v a p o r a t i o n , S u c r . F r . , 126(93) (1985) 123-128.

29 H . - F . K o r n , D ie P r a x i s des k o n t r o l l i e r t e n und a u t o m a t i s i e r t e n K o c h e n s , Z u c k e r , 19(13) (1966) 337-349.

30 R . J . Bass and J . Donovan , M i c r o p r o c e s s o r c o n t r o l o f s u g a r b o i l i n g , S u c r . B e i g e , 95 (8 ) (1976) 421-433.

31 G . Windal and A . D e l e u r e n c e , R e s u l t a t s i n d u s t r i é i s de l a c o n d u i t e des c u i t e s pa r m i c r o - o r d i n a t e u r , S u c r . F r . , 9 6 ( 3 ) (1979) 121-122.

32 P.W. van d e r Poel ( e t a l . ) , A u t o m a t i s i e r u n g d e r K o c h s t a t i o n d e r C S M - Z u c k e r -f a b r i k B r e d a , Z u c k e r i n d . , 107(2) (1982) 113-117.

33 P.W. van d e r Poel ( e t a l . ) , E n e r g i e e i n s p a r u n g e n be i d e r V e r d a m p f u n g s k r i s t a l l i s a t i o n , Z u c k e r i n d . , 108(10) (1983) 934-939.

34 D. Hoks , A u t o m a t i c a l l y c o n t r o l l e d s u g a r b o i l i n g s y s t e m . Sugar J . , 46 (7 ) (1983) 8 -9 .

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37 D. P i o t r o w s i c i and K. U r b a n i e c , Op t ima l c o n t r o l o f b a t c h - c o n t i n u o u s c r y s t a l ! f z a t i on o f s u g a r . Paper p r e s e n t e d a t 7 th I n t e r n a t i o n a l C o n g r e s s CHISAr^B l , P r a g u e , September 1981.

38 G . W i n d a l , A u t o m a t i s a t i o n pa r m i c r o - o r d i n a t e u r des c u i t e s 2eme e t 3eme j e t de l a S u c r e r i e de T o u r y , S u c r . F r . , 9 4 ( 3 ) (1977) 129-135.

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40 G . Windal and A . D e l e u r e n c e , A p p l i c a t i o n a l a c r i s t a l 1 i s a t i o n du p r o c e d e Cheops de g e s t i ó n c o o r d o n n e e , I n d . A l i m . A g r i e , 9 8 ( 7 - 8 ) (1981) 581-588.

41 Β. H a r r i s o n and J . R u z i c k a , S u p e r v i s o r y c o n t r o l and d a t a a c q u i s i t i o n f o r p r o c e s s . Sugar J . , 48 (1 ) (1985) 5 -9 .

42 W. Assenmacher , Η. Merensky and K. W ö h r l e , A u t o m a t i s c h e S t e u e r u n g e i n e r K o c h s t a t i o n m i t d i s k o n t i n u i e r l i c h e n K o c h a p p a r a t e n , Z u c k e r i n d . , 111(6) (1986) 549-554.

43 F. Baunack , T r o c k n u n g , i n : F . S c h n e i d e r ( E d . ) , T e c h n o l o g i e des Z u c k e r s , Schaper V e r l a g , H a n n o v e r , 1968, p p . 845-883.

44 H .A . P a s c h o l d , D i e Rege lung von a d i a b a t i s c h e n und n i c h t - a d i a b a t i s c h e n T r o c k n e r n am B e i s p i e l von Trommel - und d a m p f b e h e i z t e n R ö h r e n b ü n d e l t r o c k n e r n , Z u c k e r i n d . , 103(12) (1978) 1036-1030.

45 H. Kemter , Rege lung von S c h n i t z e l t r o m m e l t r o c k n e r n , Z u c k e r , 30(1 ) (1977) 7-10 . 46 H . P . G i l d e r s l e e v e , Bee t p u l p d r y i n g c o n t r o l . Sugar J . , 44 (5 ) (1981) 15-18. 47 P. Mosel ( e t a l . ) , Führung e i n e r T r o m m e l t r o c k n e r a n l a g e f ü r P r e s s s c h n i t z e l

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233

C h a p t e r 7

MONITORING OF THE HEAT CONSUMPTION

7.1 INTRODUCTORY REMARKS

7.1.1 Scope o f t h e p rob lems

The r e q u i r e m e n t s o f hea t economy i n a s u g a r f a c t o r y a r e , on t h e one hand , t o

e n s u r e a c o n t i n u o u s hea t s u p p l y t o a l l s e c t i o n s o f t h e p r o c e s s and t o m a i n t a i n

p r o p e r o p e r a t i n g c o n d i t i o n s f o r t h e f a c t o r y e q u i p m e n t , as t h e s e a r e t h e

p r e r e q u i s i t e s f o r smooth f a c t o r y o p e r a t i o n , and on t h e o t h e r h a n d , t o m i n i m i z e

hea t consumpt ion i n t he p r o c e s s . A l l t h e s e t a s k s c a n n o t be f u l f i l l e d w i t h o u t

m o n i t o r i n g h e a t consumpt ion i n i n d i v i d u a l equ ipment u n i t s and p r o c e s s s t a t i o n s ,

o v e r s e e i n g equ ipment o p e r a t i o n , i d e n t i f y i n g causes f o r hea t l o s s e s and

d e t e r m i n i n g t h e p o s s i b i l i t i e s f o r improvements i n h e a t economy.

I t i s t r a d i t i o n a l l y u n d e r s t o o d t h a t t he p rob lem a r e a under c o n s i d e r a t i o n

i n c l u d e s :

- f u e l and f u e l s t o r a g e ;

- f u e l combus t ion and steam g e n e r a t i o n i n b o i l e r s ;

- e l e c t r i c i t y g e n e r a t i o n i n t h e power h o u s e ;

- steam and v a p o u r u t i l i z a t i o n f o r p r o c e s s p u r p o s e s .

A long w i t h s a t i s f y i n g t h e s p e c i f i c needs o f t h e s u g a r i n d u s t r y , i t i s

n e c e s s a r y i n most c o u n t r i e s t o m o n i t o r f a c t o r y o p e r a t i o n a c c o r d i n g t o g e n e r a l

r e g u l a t i o n s c o n c e r n i n g t h e o p e r a t i o n o f e n e r g y - s u p p l y equ ipmen t . The r e g u l a t i o n s

u s u a l l y r e q u i r e keep ing o p e r a t i n g r e c o r d s and p e r i o d i c a l l y e v a l u a t i n g them f rom

t h e p o i n t o f v i e w o f e n e r g y u t i l i z a t i o n . T h i s a p p l i e s i n p a r t i c u l a r t o such

equ ipment a s :

- steam and w a t e r b o i l e r s ;

- t u r b i n e s ;

- l a r g e hea t r e c e i v e r s ( h e a t e x c h a n g e r s , d r y e r s , e t c . ) ;

- c o n t r o l l i n g and measur ing equ ipmen t .

I t can be s t a t e d t h a t , u n l i k e d e s i g n p r o b l e m s , m o n i t o r i n g p rob lems a s s o c i a t e d

w i t h t he hea t economy i n s u g a r f a c t o r i e s a r e n o t v e r y w e l l c o v e r e d i n t h e

l i t e r a t u r e . I n t h i s C h a p t e r , t h e b a s i c f a c t o r s needed t o m o n i t o r and e v a l u a t e

hea t economy c o r r e c t l y and e f f e c t i v e l y under a c t u a l o p e r a t i n g c o n d i t i o n s a r e

b r o u g h t t o g e t h e r .

As o n l y t h e most i m p o r t a n t t o p i c s can be d i s c u s s e d h e r e , t h e h i g h e s t p r i o r i t y

i s a t t r i b u t e d t o p rob lems d i r e c t l y a s s o c i a t e d w i t h e n e r g y s a v i n g s . The p rob lems

o f c o n t r o l l i n g s p e c i f i c a s p e c t s o f t h e o p e r a t i o n o f i n d i v i d u a l mach ines and

equ ipment u n i t s a r e r e g a r d e d as be ing beyond t h e scope o f t h i s C h a p t e r .

I t has been assumed t h r o u g h o u t t h i s C h a p t e r t h a t t h e s u g a r f a c t o r y under

234

c o n s i d e r a t i o n i s c h a r a c t e r i z e d by an a v e r a g e l e v e l o f p r o c e s s i n s t r u m e n t a t i o n .

The advanced measur ing and a u t o m a t i c m o n i t o r i n g sys tems used i n some modern

p l a n t s a r e l a r g e l y i g n o r e d h e r e .

7 .1 .2 T h e o r e t i c a l background

I n t h e p u b l i c a t i o n s d e v o t e d s o l e l y t o t h e e n e r g y economy ( r e f s . 1 - 4 ) , and

a l s o i n t he s p e c i a l c h a p t e r s i n w e l l known books on b e e t s u g a r t e c h n o l o g y ( r e f s .

5 - 9 ) , m o n i t o r i n g o f t he hea t consumpt ion i s t r e a t e d o n l y m a r g i n a l l y . The

i n f o r m a t i o n g a t h e r e d i n t h e s e s o u r c e s may p r o v e i n s u f f i c i e n t as a b a s i s f o r

p r a c t i c a l e n g i n e e r i n g a n a l y s e s a s s o c i a t e d w i t h t h i s p r o b l e m . I t may t h e n become

n e c e s s a r y t o r e l y on g e n e r a l t h e o r e t i c a l r e l a t i o n s h i p s , p a r t i c u l a r l y i n such

f i e l d s as t h o s e l i s t e d b e l o w .

( i ) The f i r s t law o f t he rmodynamics , as a b a s i s f o r d e t e r m i n i n g t h e e n e r g y

b a l a n c e s o f therma l p r o c e s s e s and f l o w s i n s t a t i o n s , equ ipment o r equ ipment

p a r t s . T h i s i m p o r t a n t m a t t e r has been d i s c u s s e d i n g r e a t e r d e t a i l i n S e c t i o n

2 .1 . F o r an e x t e n s i v e t h e o r e t i c a l t r e a t m e n t r e f s . 10 and 11, o r o t h e r l i t e r a t u r e

on a p p l i e d t he rmodynamics , may be c o n s u l t e d .

( i i ) The thermodynamics o f w a t e r and s team, w h i c h a r e t h e b a s i c media i n t h e

thermal p r o c e s s e s . The books men t ioned above a l s o i n c l u d e c h a p t e r s d e v o t e d t o

t h i s t o p i c . T a b l e s o f thermodynamic f u n c t i o n s a r e g i v e n i n r e f s . 12, 13 and

o t h e r s o u r c e s , w h i l e f o r n u m e r i c a l a p p r o x i m a t i o n s f o r c a l c u l a t o r - o r c o m p u t e r -

a i d e d c a l c u l a t i o n s . A p p e n d i x 1 may be c o n s u l t e d .

( i i i ) C o m b u s t i o n . T h i s p r o c e s s i s p a r t i c u l a r l y w o r t h a t t e n t i o n , because i t may

g i v e r i s e t o c o n s i d e r a b l e e n e r g y l o s s e s ; l i t e r a t u r e as a b o v e .

( i v ) Heat t r a n s f e r , as a g roup o f phenomena c o n t r i b u t i n g t o t h e mechanism o f

most hea t l o s s e s . I n a d d i t i o n t o t h e above l i t e r a t u r e , r e f s . 14 and 15 can be

recommended.

( v ) The f o u n d a t i o n s o f i n d u s t r i a l measurement , as a b a s i s f o r t h e d a t a g a t h e r i n g

and da ta i n t e r p r e t a t i o n on w h i c h hea t -economy m o n i t o r i n g must r e l y . F o r

t e c h n i c a l d e t a i l s and f o r i n f o r m a t i o n on e r r o r a n a l y s i s , r e f s . 15-18 may be

c o n s u l t e d .

I t s h o u l d be added t h a t p r o p e r a n a l y s i s and e f f e c t i v e s o l v i n g o f t h e p rob lems

a s s o c i a t e d w i t h t h e u t i l i z a t i o n o f steam and v a p o u r f o r p r o c e s s p u r p o s e s o f t e n

r e q u i r e deep u n d e r s t a n d i n g o f t h e s u g a r m a n u f a c t u r i n g p r o c e s s .

An e s s e n t i a l p r o c e s s i n hea t -economy m o n i t o r i n g i s t o i d e n t i f y t h e h e a t

b a l a n c e s o f t h e o b j e c t s under c o n s i d e r a t i o n , i . e . equ ipment u n i t s , p r o c e s s

s t a t i o n s , o r even t h e e n t i r e f a c t o r y . The i d e n t i f i c a t i o n o f a h e a t b a l a n c e

c o n s i s t s o f e s t a b l i s h i n g t h e bounda ry o f a thermodynamic s y s t e m , d e t e r m i n i n g t h e

mass and e n e r g y s t reams c r o s s i n g t h e b o u n d a r y , and r e c o g n i z i n g t h e magn i tude o f

each s t r e a m . I n p r a c t i c e , i t r e q u i r e s t h a t measurements o f t h e a p p r o p r i a t e

235

pa ramete rs a re t a k e n . Not a l l pa rame te rs need t o be known, h o w e v e r , as t h e

t h e o r e t i c a l b a l a n c e d e s c r i p t i o n e n a b l e s one t o c o n s t r u c t a sys tem o f e q u a t i o n s .

The measurements s h o u l d s u p p l y as many pa ramete r v a l u e s as r e q u i r e d f o r

d e t e r m i n i n g t he r e m a i n i n g ones f rom t h e e q u a t i o n s .

When p e r f o r m i n g t h e measurements , i t i s n e c e s s a r y t o o b s e r v e t h e c o n f o r m i t y

o f t h e s t a t e o f t he thermodynamic sys tem c o n s i d e r e d w i t h t h e c o n d i t i o n s f o r

w h i c h t h e t h e o r e t i c a l b a l a n c e d e s c r i p t i o n has been f o r m u l a t e d . I n p a r t i c u l a r ,

when u s i n g t h e f o r m u l a e o r i g i n a t i n g f rom e q n s . ( 2 . 3 ) o r ( 2 . 6 ) , s t e a d y - s t a t e

c o n d i t i o n s must be m a i n t a i n e d t h r o u g h o u t t h e t e s t p e r i o d . I f f l u c t u a t i o n s o f t h e

pa rame te rs c a n n o t be a v o i d e d , t h e t e s t p e r i o d s h o u l d be l ong enough t o e n s u r e

t h e v a l i d i t y o f mean v a l u e s ( t h e recommendat ions c o n c e r n i n g t e s t d u r a t i o n s f o r

i n d i v i d u a l s t a t i o n s a r e g i v e n b e l o w ) .

I n an e x i s t i n g f a c t o r y , t h e p r e r e q u i s i t e f o r measurements l e a d i n g t o a

c o r r e c t h e a t b a l a n c e i s t h a t t h e sys tem be e q u i p p e d w i t h good measu r i ng

equ ipmen t . C l e a r l y , e r r o n e o u s measurements may l e a d t o a f a l s e b a l a n c e and

f i n a l l y t o wrong c o n c l u s i o n s . I t i s t h u s recommended t h a t more measu r ing

i n s t r u m e n t s be i n s t a l l e d t han a r e t h e o r e t i c a l l y needed f o r s o l u t i o n o f t h e

sys tem o f b a l a n c e e q u a t i o n s . Measurement d a t a can t h e n be checked by compar ing

b a l a n c e r e s u l t s o b t a i n e d i n d i f f e r e n t w a y s .

7 .1 .3 Example

When i n v e s t i g a t i n g t he e n e r g y b a l a n c e o f t h e s u g a r h o u s e , i t i s n e c e s s a r y t o

overcome d i f f i c u l t i e s due t o t h e p e r i o d i c o p e r a t i o n o f t h e vacuum pans and

c e n t r i f u g a l s . F o r examp le , t he mass and e n e r g y b a l a n c e s o f a b a t c h vacuum pan

noncondensable gases

A U X I L I A R Y B O U N D A R I E S | "

cooling , A - b 1 water ' / r , ^ - " '

S U G A R H O U S E E Q U I P M E N T

~ " S Y S T E M " B O U N D A R Y ~ "

leaks

I ' heat losses

' sugar

4 -molasses

condensate

J

F i g . 7 .1 . E n e r g y b a l a n c e o f t h e s u g a r h o u s e .

236

s h o u l d be a n a l y s e d f o r t he e n t i r e b o i l i n g c y c l e , w i t h t he a i d o f b a l a n c e

r e l a t i o n s h i p s based on e q n s . ( 2 . 1 ) and ( 2 . 4 ) . F o r s t a b i l i z e d s u g a r house o u t p u t ,

h o w e v e r , t h e b a l a n c e can r e l y on t h e r e l a t i o n s h i p s c o n c e r n i n g s t e a d y - s t a t e

c o n d i t i o n s , i . e . e q n s . ( 2 . 3 ) and ( 2 . 6 ) . The sys tem bounda ry i s p l a c e d as shown

i n F i g . 7 .1 , where an a u x i l i a r y bounda ry f o r t h e c o n d e n s e r b a l a n c e i s a l s o

i n d i c a t e d . The measurements n e c e s s a r y f o r e s t a b l i s h i n g the b a l a n c e s s h o u l d be

pe r f o rmed o v e r an adequate t i m e , so t h a t r e l i a b l e mean v a l u e s can be c a l c u l a t e d

f o r steam and condensa te f l o w s , b a r o m e t r i c w a t e r t e m p e r a t u r e , e t c . I n an o v e r a l l

e n e r g y b a l a n c e f o r t he s u g a r h o u s e , one can n e g l e c t r e l a t i v e l y smal l e n e r g y

s t reams such as c r y s t a l l i z a t i o n hea t and work pe r f o rmed by d r i v i n g moto rs

( e s p e c i a l l y i n m i x e r s , c e n t r i f u g a l s and pumps) . A n o t h e r c o n v e n i e n t assump t i on i s

t h a t t h e mass l o s s e s caused by u n c o n t r o l l a b l e l e a k s ( l e a k i n g s e a l s , e v a p o r a t i o n

f rom open t a n k s , e t c . ) a r e t r e a t e d j o i n t l y as mass s t ream G ^ ^ . The e n e r g y l o s s e s

i n l e a k s , h o w e v e r , may c o n v e n t i o n a l l y be added t o t h o s e r e s u l t i n g f rom hea t

t r a n s f e r t o t he e n v i r o n m e n t , and t r e a t e d j o i n t l y as hea t Q . Assuming t h a t a l l

t he rema in ing q u a n t i t i e s shown i n F i g . 7.1 have been d e f i n e d f rom t h e

measurements , t he e q u a t i o n s o f mass and e n e r g y b a l a n c e s can be w r i t t e n as

• sl " %2 " %3 " ^ 4 " <^s5 = ^1 ^ 2 * ^ 3 + ^ 4 + ^ 5

%A^ ^ S2^2 ^ ^ s 3 ^ 3 ^ «^54^4 ^ ^ 5 ^ 5 =

= ^^^^ * \z\z' ^ 3 ^ 3 ^ ^ 4 ^ 4 - ^

Hence t he t o t a l l eak s t ream and t o t a l u n c o n t r o l l a b l e hea t l o s s a r e

^ 5 = » sl ^ hz * " '^s4 ^ - ( ^ 1 ^ ^2 * ^ 3 * ^ 4 ^

Q - G^^h^T + G 2 2 ^ ^ 3 ^ 3 " ^ 4 ^ 4 -

- (Sl^l * S2^2 ' ^ S 3 h s 3 ^ 4 ^ 4 + Ss^S^

T y p i c a l b a l a n c e d a t a and r e s u l t s a r e g i v e n i n T a b l e 7 .1 .

TABLE 7.1

Mass and e n e r g y b a l a n c e o f t h e s u g a r h o u s e , c a l c u l a t e d f o r 100 kg b e e t .

St ream name G

(kg /100 kg b ) t

( °C) h

( k J / k g ) G-h

( k J / 1 0 0 kg b )

k I n l e t

1 T h i c k j u i c e 28.6 103.3 306 8752 2 T h i n j u i c e 2.4 95 371 890 3 Water 9.4 80 335 3149 4 V a p o u r s 26.4 2754 72706 5 C o o l i n g w a t e r 360.0 20 84 30240

j O u t l e t

1 Sugar 14.0 20 23 322 2 M o l a s s e s 3.6 20 105 378 3 Condensa te 25.2 95 398 10030 4 B a r o m e t r i c w a t e r 382.0 55 230 87860 5 Leaks 2.0 Hea t l o s s e s 17147

237

7 .1 .4 M o n i t o r i n g and r e d u c t i o n o f t h e h e a t consumpt ion

The hea t economy i n a s u g a r f a c t o r y can be t r e a t e d as a sys tem kep t i n

o p e r a t i o n by a f u e l s u p p l y . As t h i s sys tem i s r e l a t i v e l y c o m p l i c a t e d , i t s

f u n c t i o n i n g can be e v a l u a t e d o n l y on t h e b a s i s o f r e c o r d e d v a l u e s o f c e r t a i n

pa ramete rs c h a r a c t e r i z i n g s e l e c t e d the rma l p r o c e s s e s . I t has been s u g g e s t e d

above t h a t t h e d a t a on sys tem o p e r a t i o n can be a p p l i e d t o i d e n t i f y h e a t l o s s e s

and t h e i r p o s s i b l e r e d u c t i o n . M o n i t o r i n g r e s u l t s t h u s e n a b l e one t o p o i n t o u t

w h e r e , and i n what manner , t h e e x i s t i n g i n s t a l l a t i o n can be improved and what

e f f e c t s can be e x p e c t e d .

E x p e r i e n c e p r o v e s t h a t i f no s y s t e m a t i c m o n i t o r i n g o f t h e hea t economy has

been c a r r i e d o u t p r e v i o u s l y , i n i t i a t i n g such a c t i o n may i m m e d i a t e l y y i e l d

advan tageous r e s u l t s . An i n v e n t o r y and d o c u m e n t a t i o n o f t h e e x i s t i n g the rma l

s y s t e m , even w i t h o u t hea t b a l a n c e s , e n a b l e s one t o d i s c o v e r t h e a b n o r m a l i t i e s

w h i c h a r e n o t v i s i b l e i n t h e f u n c t i o n i n g o f i n d i v i d u a l equ ipment u n i t s and

s t a t i o n s , l i k e e r r o r s i n c o n d e n s a t e l i n e s c a u s i n g i n c o m p l e t e u t i l i z a t i o n o f

condensa te e n e r g y , and e r r o r s i n v a p o u r l i n e s c a u s i n g sma l l u n c o n t r o l l a b l e

v a p o u r f l o w s between e v a p o r a t o r e f f e c t s . The most i m p o r t a n t c o n c l u s i o n on ways

t o r e d u c e h e a t consumpt ion can o f c o u r s e be drawn f rom t h e hea t b a l a n c e s o f

i n d i v i d u a l s t a t i o n s and o f t h e e n t i r e f a c t o r y . I t i s a n o t h e r m a t t e r , h o w e v e r ,

i n what manner t h e c o n c l u s i o n s a r e implemented i n p r a c t i c e . I n p r i n c i p l e , t h e

improvements a r e a l w a y s aimed a t r e d u c i n g h e a t consumpt i on p e r u n i t amount o f

raw m a t e r i a l , o r u n i t amount o f s u g a r . The e s s e n t i a l a i m , h o w e v e r , i s t o r e d u c e

m a n u f a c t u r i n g c o s t s . The economic f a c t o r s s h o u l d f i n a l l y d e c i d e w h e t h e r t h e

p o t e n t i a l r e d u c t i o n o f hea t consumpt ion i s u t i l i z e d o r n o t .

I t s h o u l d a l s o be remembered t h a t o n l y i n e x c e p t i o n a l c a s e s can t h e measures

r e d u c i n g hea t consumpt ion be r e g a r d e d as " p u r e " h e a t economy c o r r e c t i o n s . As

a r u l e , c o r r e c t i v e a c t i o n s a r e c o n c e r n e d w i t h t h e f a c t o r s a f f e c t i n g bo th p r o c e s s

and hea t economy. As an examp le , c o n s i d e r t h e c o n c e n t r a t i o n o f t h i c k j u i c e ,

a pa ramete r whose impo r tance t o t h e h e a t consumpt ion i n s u g a r m a n u f a c t u r e has

been d i s c u s s e d i n S e c t i o n 1 .3 .5 . Any c o r r e c t i o n o f t h i s p a r a m e t e r , h o w e v e r , i s

u n t h i n k a b l e w i t h o u t p r i o r a n a l y s i s o f p o s s i b l e consequences i n s u g a r y i e l d and

s u g a r q u a l i t y .

7.2 QUALITY OF WATER AND STEAM

7.2.1 Scope o f t h e q u a l i t y i n s p e c t i o n

I n o r d e r t o m a i n t a i n p r o p e r o p e r a t i n g c o n d i t i o n s f o r b o i l e r s and t u r b i n e s ,

i t i s n e c e s s a r y t o i n s p e c t t he q u a l i t y o f w a t e r and steam i n t h e the rma l sys tems

o f s u g a r f a c t o r i e s s y s t e m a t i c a l l y a n d , i f r e q u i r e d , t o t ake c o r r e c t i v e a c t i o n .

The aim o f t h e i n s p e c t i o n i s t o e s t a b l i s h , by measurement o r l a b o r a t o r y

a n a l y s i s , t h e amounts o f i m p u r i t i e s i n :

- f e e d w a t e r s u p p l i e d t o t h e b o i l e r ;

238

- b o i l e r w a t e r and s team;

- c o n d e n s a t e , p a r t i c u l a r l y f rom t h e e v a p o r a t i o n s t a t i o n .

The pa rame te rs o f make-up w a t e r a re a l s o i m p o r t a n t , and t h u s t h e q u a l i t y

i n s p e c t i o n i n c l u d e s a n a l y s e s o f raw w a t e r and o f w a t e r a t t h e i n d i v i d u a l s t a g e s

o f t h e p u r i f i c a t i o n p r o c e s s .

Wi th r e s p e c t t o t h e i r p h y s i c a l p r o p e r t i e s , t he i m p u r i t i e s can be c l a s s i f i e d

a s :

- mechan ica l i m p u r i t i e s , w i t h p a r t i c l e s i z e s above 0.1 m i c r o n , a p p e a r i n g as

suspended m a t t e r o r s e d i m e n t ;

- c o l l o i d a l i m p u r i t i e s , w i t h p a r t i c l e s between 1 nm and 0.1 m i c r o n , c r e a t i n g

s u s p e n s o i d s ;

- m o l e c u l a r i m p u r i t i e s , w i t h p a r t i c l e s be low 1 nm a p p e a r i n g i n s o l u t i o n s .

W i th r e s p e c t t o t he chemica l p r o p e r t i e s , t h e c u m u l a t i v e c o n c e n t r a t i o n s o f

c e r t a i n g r o u p s o f compounds a re i m p o r t a n t ( e x a m p l e s : h a r d n e s s , a l k a l i n i t y ) , b u t

so a l s o t he i n d i v i d u a l c o n c e n t r a t i o n s o f some s p e c i f i c e lemen ts and compounds

( e x a m p l e s : o x y g e n , S i O ^ ) .

Owing t o t h e e x t r e m e l y h i g h q u a l i t y r e q u i r e m e n t s o f w a t e r and steam t h a t

r e s u l t f rom t h e advanced p o w e r - g e n e r a t i o n t e c h n o l o g i e s used i n l a r g e - s c a l e

e l e c t r i c i t y g e n e r a t i o n , t h e p rob lem o f q u a l i t y i n s p e c t i o n and c o n t r o l has

e v o l v e d i n t o an e n t i r e b ranch o f e n g i n e e r i n g s c i e n c e ( r e f . 1 9 ) . O n l y a sma l l

p a r t o f i t i s r e a l l y a p p l i c a b l e t o the rma l sys tems a s s o c i a t e d w i t h s u g a r

t e c h n o l o g y .

The d e t a i l s and p r i n c i p l e s o f q u a l i t y i n s p e c t i o n o f w a t e r and steam may v a r y

i n d i f f e r e n t s u g a r f a c t o r i e s , i n a c c o r d a n c e w i t h i n s t a l l a t i o n o r equ ipment

t y p e s , l o c a l c o n d i t i o n s o r l o c a l r e g u l a t i o n s . As a r u l e , t h e c h i e f t e c h n o l o g i s t

and c h i e f power e n g i n e e r j o i n t l y bea r t h e r e s p o n s i b i l i t y f o r i n s p e c t i o n

p r i n c i p l e s ; somet imes, t h e agreement o f t h e equ ipment m a n u f a c t u r e r i s r e q u i r e d .

7 .2 .2 Condensa te

The aim o f t h e i n s p e c t i o n o f c o n d e n s a t e f rom t h e e v a p o r a t o r s t a t i o n i s t o

e s t a b l i s h w h e t h e r o r n o t i t i s s u i t a b l e as a f e e d - w a t e r component t h a t can be

r e t u r n e d t o t h e b o i l e r h o u s e . ( T h e q u a l i t y o f d i r e c t f e e d w a t e r f o r t h e b o i l e r s

i s i n s p e c t e d s e p a r a t e l y . ) A t t e n t i o n s h o u l d be f o c u s e d on m o l e c u l a r i m p u r i t i e s

and p a r t i c u l a r l y on s u g a r c o n c e n t r a t i o n , w h i c h may i n d i c a t e l e a k i n g t u b e s i n

t h e e v a p o r a t o r o r t h e p r e s e n c e o f j u i c e d r o p l e t s i n v a p o u r s f rom t h e f i r s t

e f f e c t .

The p r e s e n c e o f s u g a r makes i t p r a c t i c a l l y i m p o s s i b l e t o d i r e c t c o n d e n s a t e

t o t h e b o i l e r , as t h i s m igh t cause f o u l i n g o f t h e h e a t i n g s u r f a c e and o t h e r

dange rous e f f e c t s . As h i g h c o n d e n s a t e t e m p e r a t u r e may i n d u c e t he d e c o m p o s i t i o n

o f s u c r o s e w h i c h t h e n becomes u n d e t e c t a b l e , o p e r a t o r s a r e recommended t o r e l y on

239

t h e d e t e r m i n a t i o n o f oxygen c o n s u m p t i o n , w h i c h i s a measure o f t he c o n c e n t r a t i o n

o f o r g a n i c compounds. A c c o r d i n g l y , a n a l y s e s i n c o n d e n s a t e q u a l i t y i n s p e c t i o n

t y p i c a l l y dea l w i t h p H , s u g a r p r e s e n c e and o x y g e n c o n s u m p t i o n .

As f a r as t h e c o n d e n s a t e b y - p a s s i n g t h e b o i l e r s i s c o n c e r n e d , t h e i n s p e c t i o n

i a aimed a t d e t e c t i o n o f s u g a r as an i n d i c a t i o n o f imp rope r o p e r a t i o n o f

equ ipment ( j u i c e d r o p l e t s o r j u i c e foam i n v a p o u r s ) , o r l e a k i n g t u b e s . R o u t i n e

i n s p e c t i o n may r e l y on samples t aken f rom c o n d e n s a t e t a n k s . Once t h e p r e s e n c e

o f s u g a r i n a t ank has been d e t e c t e d , h o w e v e r , i t may become n e c e s s a r y t o o b t a i n

condensa te samples f rom the o u t l e t p i p e s c o n n e c t e d t o t h e i n d i v i d u a l equ ipment

u n i t s .

An i m p o r t a n t f a c t o r t o be a c c o u n t e d f o r i n e n s u r i n g t h e e f f i c i e n c y o f

condensa te q u a l i t y i n s p e c t i o n i s t h e f r e q u e n c y o f a n a l y s e s . D e t e c t i o n o f s u g a r

i n condensa te f rom t h e f i r s t and second e v a p o r a t o r e f f e c t s s h o u l d be p e r f o r m e d

c o n t i n u o u s l y . I f t h e r e a r e no a u t o m a t i c a n a l y s e r s , t h i s means t h a t t h e t e s t s a r e

r e p e a t e d o v e r a c y c l e o f s e v e r a l m i n u t e s a t a p o s i t i o n i n t he v i c i n i t y o f t h e

e v a p o r a t o r . A c c u r a t e l a b o r a t o r y t e s t s f o r s u g a r , t o g e t h e r w i t h o x y g e n

consumpt ion measurements , a r e r e q u i r e d a t l e a s t e v e r y 2 h o u r s . O t h e r c o n d e n s a t e

a n a l y s e s may be pe r f o rmed w i t h t h e same f r e q u e n c y , as i n t h e case o f f e e d - w a t e r

i n s p e c t i o n . An example o f a comp le te s e t o f c o n d e n s a t e a n a l y s e s , a c c e p t e d by t h e

b o i l e r m a n u f a c t u r e r f o r a s p e c i f i c s u g a r f a c t o r y , i s g i v e n i n T a b l e 7 . 2 .

TABLE 7.2

Example o f a s e t o f condensa te a n a l y s e s .

Q u a n t i t y D imens ion T e s t s ^ p e r ^ 8 - h o u r

pH 4 T o t a l h a r d n e s s m v a l / l i t r e 1 Oxygen consumpt ion mg KMnO¿^ / l i t r e 4 A l k a l i n i t y m v a l / l i t r e 2 Sugar p r e s e n c e 4*

* / number o f l a b o r a t o r y t e s t s aimed a t v e r i f i c a t i o n o f r o u t i n e t e s t s

7 .2 .3 Feed w a t e r and make-up w a t e r

Feed w a t e r s u p p l i e d t o t h e b o i l e r s c o n s i s t s o f e v a p o r a t o r c o n d e n s a t e t o w h i c h

make-up w a t e r i s added . The demand f o r make-up w a t e r i s u s u a l l y o n l y a few

p e r c e n t on b e e t s . G e n e r a l l y , q u a l i t y r e q u i r e m e n t s c o n c e r n i n g f e e d w a t e r depend

on t h e b o i l e r t y p e and steam p r e s s u r e . I n some c o u n t r i e s , s t a n d a r d s o r o t h e r

r e g u l a t i o n s have been i n t r o d u c e d i n t h i s f i e l d . Q u a l i t y recommendat ions i s s u e d

i n Be lg ium can be f ound i n t h e l i t e r a t u r e ( r e f . 2 0 ) . B e l o w , t h e r e q u i r e m e n t s

f o r m u l a t e d i n P o l i s h r e g u l a t i o n s ( r e f . 21) a r e r e v i e w e d .

The f e e d w a t e r must be c o l o u r l e s s and f r e e f rom mechan ica l i m p u r i t i e s . A pH

240

above 7 i s r e q u i r e d and t he oxygen consumpt ion s h o u l d be as low as p o s s i b l e ,

p r e f e r a b l y be low 5-10 mg K M n O ^ / l i t r e . The w a t e r h a r d n e s s s h o u l d be as low as

p o s s i b l e , b u t i t s a l l o w a b l e upper l i m i t depends on t h e b o i l e r t y p e , steam

p r e s s u r e and t he hea t f l u x a t t h e h e a t i n g s u r f a c e ( f o r examp le , i n t h e case o f

w a t e r - t u b e r a d i a n t b o i l e r s a t 40 ba r o p e r a t i n g p r e s s u r e , f e e d w a t e r w i t h a

h a r d n e s s o f up t o 0.01 m v a l / l i t r e can be a c c e p t e d ) . The o x y g e n c o n t e n t s h o u l d be

as smal l as p o s s i b l e , n o t e x c e e d i n g 0 .02 -0 .03 m g / l i t r e f o r modern b o i l e r s . The

CO^ c o n t e n t - w i t h w h i c h t h e danger o f c o r r o s i v e a c t i o n i s a s s o c i a t e d - must n o t

exceed 20 m g / l i t r e .

Depending on t he b o i l e r p r e s s u r e , d i f f e r e n t l e v e l s o f a l k a l i n i t y and o f t h e

c o n t e n t o f s i l i c o n d i o x i d e , phospha tes and i r o n compounds can be a c c e p t e d . The

c o n c e n t r a t i o n o f o i l - t y p e i m p u r i t i e s c a n n o t e x c e e d a l i m i t w h i c h has been

d e f i n e d a t 10 m g / l i t r e f o r o l d e r b o i l e r s w i t h l a r g e w a t e r vo lume and a t 0 . 5 - 1 . 0

m g / l i t r e f o r modern b o i l e r s o p e r a t e d a t 40 ba r steam p r e s s u r e .

The same r e g u l a t i o n d e f i n e s t h e s e t o f o b l i g a t o r y f e e d - w a t e r a n a l y s e s . F o r

t he most w i d e l y used b o i l e r t y p e s , i t i s n e c e s s a r y t o d e t e r m i n e :

- p H ;

- oxygen c o n s u m p t i o n ;

- h a r d n e s s ;

- d i s s o l v e d oxygen c o n t e n t ;

- mechan ica l i m p u r i t i e s ;

- a l k a l i n i t y ;

- S i O ^ c o n c e n t r a t i o n ;

- CO^ c o n c e n t r a t i o n ;

- o i l - t y p e i m p u r i t i e s ;

- w a t e r a p p e a r a n c e .

F o r o l d e r b o i l e r s o p e r a t e d a t a p r e s s u r e be low 16 b a r , o n l y t h e f i r s t 5

a n a l y s e s on t h e l i s t a n d , a d d i t i o n a l l y , t he d e t e r m i n a t i o n o f t e m p o r a r y h a r d n e s s ,

a r e o b l i g a t o r y . An i m p o r t a n t s e c t i o n o f t h e r e g u l a t i o n s s t i p u l a t e s t h a t f o r

modern b o i l e r s n o t men t ioned i n t h e g e n e r a l s e c t i o n , t h e r e q u i r e m e n t s f o r m u l a t e d

by t h e b o i l e r ' s m a n u f a c t u r e r a r e d e c i s i v e .

W a t e r - q u a l i t y i n s p e c t i o n i n t h e w a t e r - t r e a t m e n t i n s t a l l a t i o n s may be aimed a t

c o n t r o l l i n g t h e f i n a l r e s u l t o f t h e t r e a t m e n t , t o g e t h e r w i t h t h e r e s u l t s o f u n i t

o p e r a t i o n s such as c o a g u l a t i o n , f i l t r a t i o n , d e c a r b o n i z a t i o n , and s o f t e n i n g .

The d e t a i l e d i n s p e c t i o n scheme s h o u l d be adap ted t o l o c a l c o n d i t i o n s , w h i c h

means t o t he p r o p e r t i e s o f t h e raw w a t e r , t he t r e a t m e n t p r o c e s s s t r u c t u r e and

r e q u i r e m e n t s imposed on f e e d - w a t e r p a r a m e t e r s . I n t h e case o f i o n exchange

t r e a t m e n t , c o n t r o l o f t h e i o n - e x c h a n g e r a c t i v i t y must be i n c l u d e d .

C o n c e r n i n g t h e f r e q u e n c y o f a n a l y s e s , t h e r e q u i r e m e n t s f o r m u l a t e d by t h e

b o i l e r m a n u f a c t u r e r a re b i n d i n g on o p e r a t o r s . U s u a l l y , a comp le te s e t o f

241

a n a l y s e s s h o u l d be pe r f o rmed t h r e e t imes a d a y , i . e . once p e r 8 - h o u r w o r k i n g

s h i f t . Loca l c o n d i t i o n s may make i t n e c e s s a r y t o p e r f o r m c e r t a i n a n a l y s e s more

f r e q u e n t l y , w h i l e o t h e r s may even be o m i t t e d . I n t h e case o f new b o i l e r s ,

h o w e v e r , such changes must be a c c e p t e d by t h e m a n u f a c t u r e r .

An example o f a comple te s e t o f w a t e r a n a l y s e s i n t he d i f f e r e n t s t a g e s o f

a t r e a t m e n t p r o c e s s , and f e e d - w a t e r a n a l y s e s a c c e p t e d by t h e b o i l e r m a n u f a c t u r e r ,

i s shown i n T a b l e 7 . 3 .

7 .2 .4 B o i l e r w a t e r and steam

Steam g e n e r a t i o n i n b o i l e r s i s i n h e r e n t l y a s s o c i a t e d w i t h t h e i n c r e a s i n g

c o n c e n t r a t i o n o f i m p u r i t i e s i n t he b o i l e r w a t e r . T h i s , i n t u r n , c r e a t e s t he

danger o f d i s t u r b a n c e s i n b o i l e r o p e r a t i o n .

Mechan i ca l i m p u r i t i e s o f o r g a n i c o r i g i n t e n d t o i n c r e a s e b o i l e r f o a m i n g ; i f

combined w i t h m i n e r a l i m p u r i t i e s , t h e y cause t h e f o r m a t i o n o f a dange rous k i n d

o f b o i l e r s c a l e . Mechan i ca l i m p u r i t i e s o f m i n e r a l o r i g i n m igh t become

t r a n s f o r m e d i n t o s l u d g e o r h a r d b o i l e r s c a l e . C o l l o i d a l i m p u r i t i e s a l s o cause

foaming and s l u d g e f o r m a t i o n ; c o l l o i d a l s i l i c o n d i o x i d e i s p a r t i c u l a r l y

d a n g e r o u s , as i t becomes t r a n s f o r m e d i n t o b o i l e r s c a l e w h i c h i s v e r y d i f f i c u l t

t o remove. O i l o r l u b r i c a n t p r e s e n t i n b o i l e r w a t e r becomes s i n t e r e d on t h e

h e a t i n g s u r f a c e s , making h e a t t r a n s f e r more d i f f i c u l t ; e v e n t u a l l y , t h i s may l e a d

t o b u r n i n g o f t h e b o i l e r t u b e s . S i m i l a r e f f e c t s a r e i n d u c e d by t h e p r e s e n c e o f

o r g a n i c m o l e c u l a r i m p u r i t i e s ( e . g . s u c r o s e ) , w h i l e m i n e r a l m o l e c u l a r i m p u r i t i e s

cause s c a l e f o r m a t i o n o r c o r r o s i o n .

Q u a l i t y i n s p e c t i o n o f b o i l e r w a t e r i s aimed a t d e t e r m i n i n g w h e t h e r o r n o t t h e

c o n c e n t r a t i o n o f i m p u r i t i e s e x c e e d s t h e a l l o w a b l e l i m i t . The i n s p e c t i o n

r e q u i r e m e n t s can be f o r m u l a t e d by t h e b o i l e r m a n u f a c t u r e r o r by b o i l e r

i n s p e c t i o n a u t h o r i t i e s . The g e n e r a l p r i n c i p l e i s t h a t t h e c o n c e n t r a t i o n o f

i m p u r i t i e s must be smal l enough t o e n s u r e :

- e l i m i n a t i o n o f c o r r o s i o n p r o c e s s e s ;

- l i m i t a t i o n o f b o i l e r s c a l e and s l u d g e f o r m a t i o n ;

- m a i n t a i n i n g t h e p r o p e r steam p u r i t y , w h i l e h e a t l o s s e s r e s u l t i n g f rom t h e

b o i l e r blowdown a r e kep t r e a s o n a b l y s m a l l .

A c c o r d i n g t o P o l i s h r e g u l a t i o n s , q u a l i t y i n s p e c t i o n o f t h e b o i l e r w a t e r

s h o u l d c o m p r i s e , f o r most b o i l e r s , p a r t i c u l a t e and d i s s o l v e d m a t t e r , a l k a l i n i t y ,

S i O ^ c o n t e n t and P^O^ c o n t e n t . F o r o l d e r b o i l e r s , i t i s enough t o d e t e r m i n e

p a r t i c u l a t e and d i s s o l v e d m a t t e r , and a l k a l i n i t y .

I n o r d e r t o o b t a i n a t e m p o r a r y pH i n c r e a s e and t o c o u n t e r a c t s l u d g e

p r e c i p i t a t i o n , phospha te may be added t o b o i l e r w a t e r . H o w e v e r , e f f e c t i v e

c o n t r o l o f t h e c o n c e n t r a t i o n o f i m p u r i t i e s r e l i e s on b o i l e r b lowdown. W i th t h e

r e s u l t s o f a n a l y s e s o f b o i l e r w a t e r and make-up w a t e r , i t i s p o s s i b l e t o

p r e d e t e r m i n e t h e n e c e s s a r y blowdown r a t e i n o r d e r t o m a i n t a i n a c o n s t a n t d e s i r e d

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c o n c e n t r a t i o n i n t h e b o i l e r w a t e r ( f o r d e t a i l s , see r e f . 2 2 ) . I t s h o u l d be

emphas ized t h a t e x c e s s i v e blowdown means a l o s s o f e n e r g y and s h o u l d t h e r e f o r e

be a v o i d e d .

Steam l e a v i n g t h e b o i l e r c o n t a i n s i m p u r i t i e s o r i g i n a t i n g f rom t h e b o i l e r

w a t e r , p a r t i c u l a r l y i f foaming o c c u r s . Such i m p u r i t i e s - e s p e c i a l l y s i l i c o n

d i o x i d e and i r o n compounds - a r e d a n g e r o u s , as t h e y f a l l i n t o t h e t u r b i n e and

t h e r e fo rm d e p o s i t s w h i c h a f f e c t t h e d u r a b i l i t y o f t h e t u r b i n e e l emen ts and

d i s t u r b t h e i r f u n c t i o n ( e . g . h i n d e r t h e movement o f v a l v e p a r t s ) . C o n t r o l o f

steam p u r i t y s h o u l d be by c o n t r o l o f t h e i m p u r i t y c o n t e n t o f t h e b o i l e r w a t e r .

The l i m i t o f i m p u r i t y c o n c e n t r a t i o n i n s team, as w e l l as t h e method o f i t s

i n s p e c t i o n , s h o u l d be d e t e r m i n e d by c o n s u l t a t i o n w i t h t h e b o i l e r and t u r b i n e

m a n u f a c t u r e r s , and f i n a l l y v e r i f i e d i n p r a c t i c a l o p e r a t i o n . T y p i c a l l y , s team

q u a l i t y m o n i t o r i n g s h o u l d i n c l u d e :

- S i O ^ c o n c e n t r a t i o n ;

- o v e r a l l Fe c o n c e n t r a t i o n ;

- e l e c t r i c a l c o n d u c t i v i t y o f t h e c o n d e n s a t e a t 20°C.

The f r e q u e n c y o f a n a l y s e s o f b o i l e r w a t e r and steam s h o u l d f o l l o w t h e

r e q u i r e m e n t s o f t h e b o i l e r m a n u f a c t u r e r : t y p i c a l l y , t h e r e q u i r e d f r e q u e n c y i s

t w i c e d a i l y . P r a c t i c a l e x p e r i e n c e p r o v e s , h o w e v e r , t h a t f o r e f f i c i e n t q u a l i t y

c o n t r o l , t h e b a s i c pa rame te rs o f b o i l e r w a t e r (pH and e l e c t r i c a l c o n d u c t i v i t y )

must be d e t e r m i n e d more f r e q u e n t l y .

An example o f a s e t o f b o i l e r w a t e r and steam a n a l y s e s , a c c e p t e d by t h e

b o i l e r m a n u f a c t u r e r f o r a s p e c i f i c s u g a r f a c t o r y , i s g i v e n i n T a b l e 7 .4 .

TABLE 7.4

Example o f a s e t o f b o i l e r w a t e r and steam a n a l y s e s .

Q u a n t i t y D imens ion A l l o w a b l e

v a l u e s T e s t s p e r

8 - h o u r s h i f t

B o i l e r w a t e r

pH E l e c t r i c a l c o n d u c t i v i t y A l k a l i n i t y " p " P2O5 c o n t e n t SÍO2 c o n t e n t

yS /cm m v a l / 1 i t r e m g / 1 i t r e m g / 1 i t r e

7 50-5000 0 . 1 - 6 . 0

3-10 ca 25

4 1-2

1 2

i r r e g u l a r

Steam Fe c o n t e n t SÍO2 c o n t e n t

m g / 1 i t r e m g / 1 i t r e

max. 0.02 max. 0.02

1 1

7 .2 .5 Sampl ing o f w a t e r and steam

Water samples a r e c o l l e c t e d and p r e s e r v e d i n g l a s s b o t t l e s w h i c h a r e washed

i n advance w i t h soap o r soda s o l u t i o n , t h e n t a p w a t e r and f i n a l l y w i t h d i s t i l l e d

w a t e r . S i m i l a r l y washed g l a s s p l u g s , o r c o r k s b o i l e d i n d i s t i l l e d w a t e r , a r e

u s e d . The method o f samp l i ng w a t e r s h o u l d be a d a p t e d t o t h e t y p e o f w a t e r s o u r c e

and t o t h e a n a l y s i s r e q u i r e d ( r e f . 2 3 ) . F o r e x a m p l e , raw w a t e r f rom a pump o r

244

a p i p e l i n e s h o u l d f l o w f o r abou t 10 m inu tes b e f o r e a sample i s c o l l e c t e d ; i f

t h e a n a l y s i s i s c o n c e r n e d w i t h t h e c o n t e n t o f d i s s o l v e d g a s , sample a e r a t i o n

must be a v o i d e d .

Condensa te and b o i l e r w a t e r a r e sampled a c c o r d i n g t o s t a n d a r d methods ( r e f .

24) and samp l ing equ ipment can a l s o be s t a n d a r d i z e d ( r e f . 2 5 ) . Condensa te samples

can be o b t a i n e d t h r o u g h s p e c i a l gauge cocks i n s t a l l e d i n r e l e v a n t p i p e l i n e s , o r

t h r o u g h d r a i n cocks t h a t a r e p a r t s o f w a t e r - l e v e l i n d i c a t o r s . The l a t t e r

p o s s i b i l i t y i s p a r t i c u l a r l y i m p o r t a n t i f t h e condensa te p r e s s u r e i s l o w e r t han

t he a t m o s p h e r i c p r e s s u r e ; o t h e r w i s e , d r a i n cocks o r a i r - e s c a p e cocks a t t h e

steam t r a p s can be u s e d .

F e e d - w a t e r and b o i l e r - w a t e r samples must be c o o l e d . U s u a l l y , s i m p l e c o i l -

t y p e , w a t e r - c o o l e d hea t e x c h a n g e r s a r e i n s t a l l e d i n c o n n e c t i o n w i t h t he r e l e v a n t

t e s t c o c k s . I t s h o u l d be p o i n t e d o u t t h a t such c o o l e r s may be s u b j e c t t o

h y d r a u l i c t e s t s , t o g e t h e r w i t h t h e e n t i r e b o i l e r .

Sample c o o l i n g i s a l s o n e c e s s a r y i n t h e case o f steam s a m p l i n g ; steam i s

c o l l e c t e d i n condensa te form ( r e f . 2 6 ) . L i k e f e e d - w a t e r and b o i l e r - w a t e r

s a m p l i n g , condensa te f l o w must be a l l o w e d p r i o r t o sample c o l l e c t i o n , so t h a t

any i m p u r i t i e s i n i t i a l l y p r e s e n t i n t he p i p e and i n t he c o o l e r a r e washed away.

7 .2 .6 D e t e r m i n a t i o n o f p h y s i c a l p r o p e r t i e s

From t h e p o i n t o f v i e w o f s u i t a b i l i t y f o r h e a t economy s y s t e m s , o n l y

p h y s i c a l p r o p e r t i e s o f w a t e r r e l a t e d t o t h e c o n c e n t r a t i o n o f i m p u r i t i e s a r e

i m p o r t a n t .

The appearance o f w a t e r i s e v a l u a t e d by t r a n s p a r e n c y and t u r b i d i t y t e s t s .

Such t e s t s a r e g e n e r a l l y s t a n d a r d i z e d ( r e f . 2 7 ) .

The measure o f c o n c e n t r a t i o n o f n o n - v o l a t i l e i m p u r i t i e s i n w a t e r i s t h e d r y

m a t t e r c o n t e n t , i . e . t he amount o f d r y m a t t e r r e m a i n i n g a f t e r 1 l i t r e o f w a t e r

has been e v a p o r a t e d ; i t s w e i g h t i s d e t e r m i n e d a f t e r d r y i n g a t 105°C. T e s t

p r o c e d u r e s a r e a l s o s t a n d a r d i z e d ( r e f . 2 8 ) .

The t o t a l s a l t c o n t e n t i n w a t e r can be e x p r e s s e d i n d i r e c t l y by t h e w a t e r

d e n s i t y . F o r r a p i d measurements o f d e n s i t y , ae rome te r s c a l i b r a t e d i n k g / l i t r e

o r d e g r e e s Baume (°Bé) can be u s e d .

The c o n t e n t o f s a l t s , a c i d s and bases can be e x p r e s s e d by t h e s p e c i f i c

e l e c t r i c a l c o n d u c t i v i t y o f t h e w a t e r . As t h i s depends on t e m p e r a t u r e , i t i s

measured a t 20°C. I n t he case o f measurements made a t d i f f e r e n t t e m p e r a t u r e s ,

c o n d u c t i v i t y v a l u e s s h o u l d be c o r r e c t e d a c c o r d i n g t o s t a n d a r d i z e d f o r m u l a e . As

c o n d u c t i v i t y measurements can e a s i l y be r e c o r d e d a u t o m a t i c a l l y , t h e y a r e w i d e l y

a p p l i e d i n w a t e r and steam q u a l i t y i n s p e c t i o n . M e a s u r i n g p r o c e d u r e s a r e

s t a n d a r d i z e d ( r e f . 2 9 ) .

245

7 .2 .7 T o t a l h a r d n e s s and t e m p o r a r y h a r d n e s s

Hardness i s a measure o f t h e t e n d e n c y o f w a t e r t o p r o d u c e d e p o s i t s and t o

fo rm b o i l e r s c a l e . Ha rdness i s caused by c a l c i u m and magnesium s a l t s t h a t appear

as b i c a r b o n a t e s , Ca(HC02)2 ^Qi^^O^'^Z' s a l t s o f i n o r g a n i c a c i d s ; t h a t

i s , s u l p h a t e s , c h l o r i d e s and n i t r a t e s ( C a S O ^ , M g C l ^ , Cdi(HÖ^)^) and o t h e r s . Water

h a r d n e s s can be e x p r e s s e d i n d e g r e e s , o r g r a m - e q u i v a l e n t s o r m i l l i g r a m -

e q u i v a l e n t s p e r u n i t vo lume . I n s e v e r a l European c o u n t r i e s , t h e s o - c a l l e d German

deg ree o f h a r d n e s s has been a p p l i e d ; i t c o r r e s p o n d s t o 10 g CaO i n 1 m* o f

w a t e r , o r 10 mg CaO i n 1 l i t r e .

The t o t a l h a r d n e s s o f w a t e r can be d e t e r m i n e d u s i n g t h e v e r s e n a t e method ,

t h a t i s , by t i t r a t i n g t he w a t e r sample w i t h sodium v e r s e n a t e ( r e f . 3 0 ) .

T o t a l h a r d n e s s can be r e g a r d e d as t h e sum o f t e m p o r a r y h a r d n e s s , caused by

c a l c i u m and magnesium b i c a r b o n a t e s , and permanent h a r d n e s s , caused by o t h e r

c a l c i u m and magnesium s a l t s . Tempora ry h a r d n e s s can be d e t e r m i n e d by t i t r a t i n g

w a t e r w i t h h y d r o c h l o r i c a c i d .

7 .2 .8 Oxygen consumpt ion and s u g a r c o n t e n t

Oxygen consumpt ion and a r e l a t e d q u a n t i t y , t h e chemica l o x y g e n demand, a r e

c o n v e n t i o n a l measures o f t h e c o n c e n t r a t i o n o f o r g a n i c i m p u r i t i e s and some o t h e r

e a s i l y o x i d i z a b l e i n o r g a n i c compounds.

The oxygen consumpt ion can be d e t e r m i n e d by t i t r a t i n g w a t e r w i t h p o t a s s i u m

permanganate s o l u t i o n . I t i s e x p r e s s e d i n t h e m i l l i g r a m s o f KMnO^ used t o

o x i d i z e t he i m p u r i t i e s i n 1 l i t r e o f w a t e r . By c o n v e r t i n g t h i s q u a n t i t y i n t o

t h e oxygen amount, we o b t a i n t h e chemica l o x y g e n demand.

T e s t s o f t he p r e s e n c e o f s u c r o s e i n w a t e r (2 m g / 1 i t r e o r more) can be

pe r f o rmed u s i n g t h e w e l l known a l p h a - n a p h t o l method . W i t h l a r g e r s u g a r

c o n c e n t r a t i o n s , c l a s s i c a l a n a l y s i s t e c h n i q u e s known i n s u g a r t e c h n o l o g y can be

a p p l i e d ( r e f . 3 1 ) . C o n d u c t o m e t r i c and o t h e r methods f o r c o n t i n u o u s measurement ,

p a r t i c u l a r l y s u i t a b l e f o r c o n d e n s a t e q u a l i t y m o n i t o r i n g , a r e a l s o i n u s e .

7 .2 .9 Hydrogen i o n c o n t e n t , a l k a l i n i t y and o t h e r p r o p e r t i e s

The v a l u e o f pH i n aqueous s o l u t i o n s can be d e t e r m i n e d by c o l o r i m e t r i c

a n a l y s i s o r by e l e c t r i c a l measurement . The l a t t e r method i s t h e most w i d e l y

u s e d ; i t c o n s i s t s o f measur ing t h e e l e c t r o m o t i v e f o r c e o f t h e s o - c a l l e d

p H - m e t r i c c e l l , i . e . two e l e c t r o d e s immersed i n t h e s o l u t i o n . I t s h o u l d be

p o i n t e d o u t , h o w e v e r , t h a t t h e measu r i ng e r r o r i s h i g h l y dependen t on t h e

e l e c t r o d e c o n d i t i o n . T h e r e f o r e , pH me te rs s h o u l d be t e s t e d o f t e n by measu r ing

t h e pH v a l u e s o f two r e f e r e n c e s o l u t i o n s . F o r r e l i a b l e measurement , i t i s

n e c e s s a r y t o e l i m i n a t e t h e p o s s i b i l i t y o f a c c i d e n t a l l y p o l l u t i n g w a t e r samples

w i t h any i m p u r i t i e s . The t e m p e r a t u r e o f t h e w a t e r t e s t e d s h o u l d be 20°C.

A l k a l i n i t y o f w a t e r i s caused by t h e p r e s e n c e o f h y d r o x i d e s and s a l t s o f

246

e lements b e l o n g i n g t o t h e p o t a s s i u m and c a l c i u m g r o u p s . I t can be d e t e r m i n e d by

t i t r a t i n g w a t e r w i t h h y d r o c h l o r i c a c i d .

O t h e r chemica l a n a l y s e s i m p o r t a n t f o r w a t e r q u a l i t y i n s p e c t i o n a r e :

- oxygen ( u n d e r s t o o d as d i s s o l v e d f r e e o x y g e n ) c o n t e n t ;

- c a r b o n d i o x i d e c o n t e n t ;

- phospha te c o n t e n t ;

- s i l i c o n d i o x i d e c o n t e n t ;

- o i l ( u n d e r s t o o d as o i l - t y p e i m p u r i t i e s ) c o n t e n t ;

- i r o n ( u n d e r s t o o d as t o t a l i r o n i n v a r i o u s compounds) c o n t e n t .

7.3 FUEL ANALYSIS

7.3.1 Sampl ing o f f u e l s

F u e l s a r e e v a l u a t e d u s i n g t h e r e s u l t s o f l a b o r a t o r y t e s t s p e r f o r m e d on an

a v e r a g e d l a b o r a t o r y samp le . Samples s h o u l d be t aken f rom t h e f u e l t r a n s p o r t s

d u r i n g u n l o a d i n g , so t h a t n e c e s s a r y c l a i m s t o t h e s u p p l i e r can be made, and a l s o

f rom the f u e l s t ream i n t r o d u c e d t o t h e b o i l e r f u r n a c e , so t h a t t h e a c t u a l

h e a t i n g v a l u e can be a c c o u n t e d f o r i n t h e e n e r g y b a l a n c e . The most i m p o r t a n t

p r o p e r t i e s o f coa l and o t h e r s o l i d f u e l s a r e t h e p e r c e n t a g e s o f c o m b u s t i b l e

m a t t e r , m o i s t u r e and a s h . The c h a r a c t e r i s t i c s o f i m p o r t a n t c o a l t y p e s a r e

s t a n d a r d i z e d i n many c o u n t r i e s ( r e f . 3 2 ) . I n t h e case o f l i q u i d f u e l s , such

p r o p e r t i e s as v i s c o s i t y , t h e p e r c e n t a g e s o f s u l p h u r and vanadium o x i d e , and

f i n a l l y t h e p e r c e n t a g e o f w a t e r , a r e p a r t i c u l a r l y i m p o r t a n t . F o r d e t a i l s ,

n a t i o n a l s t a n d a r d s s h o u l d be c o n s u l t e d ( r e f . 3 3 ) .

C o r r e c t samp l ing o f f u e l i s a p r e r e q u i s i t e f o r r e l i a b l e d e t e r m i n a t i o n o f i t s

p r o p e r t i e s . The f o l l o w i n g c o n c e p t s a r e a p p l i e d i n d e f i n i n g samp l i ng p r o c e d u r e s :

- a p a r e n t l o t i s t h e amount o f a d e f i n i t e t y p e o f f u e l , n o t g r e a t e r t han

1000 t o n s ;

- a p r i m a r y sample i s a sample t aken f rom one p l a c e i n t h e p a r e n t l o t , o r a

s i n g l e sample taken f rom a f l o w i n g - f u e l s t r e a m ;

- a g r o s s sample i s a sample c o n t a i n i n g a l l p r i m a r y s a m p l e s ;

- an a v e r a g e d sample i s a p a r t o f t h e g r o s s sample d e s t i n e d f o r l a b o r a t o r y

a n a l y s e s .

I n t h e case o f c o a l o r o t h e r s o l i d f u e l s , i t i s d e s i r e d t h a t each p a r t i c l e i n

t h e p a r e n t l o t be g i v e n t he same p r o b a b i l i t y o f b e i n g chosen f o r t h e sample as

e v e r y o t h e r p a r t i c l e , d e s p i t e d i f f e r e n c e s i n p a r t i c l e s i z e o r s h a p e . P r i m a r y

samples s h o u l d p r e f e r a b l y be c o l l e c t e d f rom a f u e l s t ream on a c o n v e y o r , w i t h

t h e f r e q u e n c y a d j u s t e d t o t h e s i z e o f t h e p a r e n t l o t and t o t h e r e q u i r e d number

o f samp les . D i r e c t samp l ing f rom r a i l w a y c a r s , t r u c k s o r f u e l p i l e s i s a l l o w e d

o n l y i f t h e r e i s no p o s s i b i l i t y o f samp l i ng f rom c o n v e y o r s . The g r o s s sample i s

fo rmed by m i x i n g a l l p r i m a r y s a m p l e s ; c a r e s h o u l d be t aken t h a t t h e samples a r e

247

hand led so as t o m i n i m i z e changes i n f u e l p r o p e r t i e s . F o r d e t a i l s , see t h e

a p p r o p r i a t e s t a n d a r d s ( r e f . 3 4 ) .

I n t h e case o f l i q u i d f u e l s , i t i s i m p o r t a n t t o a p p l y t h e same method o f

samp l ing when c o l l e c t i n g t h e p r i m a r y samples f rom w h i c h t h e a v e r a g e d sample w i l l

be o b t a i n e d . Sampl ing o f f u e l s i n s t o r a g e t a n k s s h o u l d be pe r f o rmed i n such

a way t h a t no p a r t i c u l a r l a y e r o f l i q u i d i s p r e f e r r e d ; i f a b a t c h o f l i q u i d i s

run i n t o a t a n k , t hen t h e f i r s t samp l ing s h o u l d o c c u r n o t e a r l i e r t han two h o u r s

l a t e r . Sampl ing f rom p i p e l i n e s may e i t h e r be done c o n t i n u o u s l y , p r o p o r t i o n a l l y

t o t h e f u e l f l o w , o r p e r i o d i c a l l y a t c o n s t a n t f r e q u e n c y ; p e r i o d s w i t h c o n s t a n t

f l o w c o n d i t i o n s a r e p r e f e r r e d .

The p r i m a r y sample vo lume i s d e f i n e d by t h e c a p a c i t y o f t h e samp l i ng d e v i c e ,

and s h o u l d be f i x e d f o r a g i v e n f u e l l o t . The g r o s s sample vo lume s h o u l d be

l a r g e enough f o r t h e p r e p a r a t i o n o f an a v e r a g e d sample o f a t l e a s t 3 kg . I n t h e

case o f a r b i t r a t i o n a n a l y s e s pe r f o rmed i n two o r t h r e e d i f f e r e n t l a b o r a t o r i e s ,

a v e r a g e d samples o f up t o 9 kg may be r e q u i r e d . F o r d e t a i l s on samp l ing o f

l i q u i d f u e l s , see r e f . 35.

7 .3 .2 D e t e r m i n a t i o n o f coa l p r o p e r t i e s

The m o i s t u r e c o n t e n t i s c e r t a i n l y one o f t h e most i m p o r t a n t p r o p e r t i e s o f

s o l i d f u e l s , as i t can a d v e r s e l y i n f l u e n c e b o i l e r c a p a c i t y and e f f i c i e n c y .

A h i g h m o i s t u r e c o n t e n t makes f u e l i g n i t i o n more d i f f i c u l t , w h i l e t h e h e a t i n g

v a l u e d e c r e a s e s .

A t low t e m p e r a t u r e s o f f l u e gas a p p r o a c h i n g t h e b o i l e r o u t l e t , c o n d e n s a t i o n

o f v a p o u r s o r i g i n a t i n g f rom f u e l m o i s t u r e i n t h e p r e s e n c e o f SO^ and SO^ causes

c o r r o s i o n o f b o i l e r p a r t s . A t f r e e z i n g ambien t t e m p e r a t u r e s , f u e l m o i s t u r e may

cause s e r i o u s p rob lems i n t r a n s p o r t a t i o n and u n l o a d i n g .

I n chemica l a n a l y s e s , t h e c o n c e p t s o f f r e e , i n h e r e n t and t o t a l m o i s t u r e a r e

a p p l i e d . F r e e m o i s t u r e i s t he p a r t o f t h e w a t e r i n s o l i d f u e l t h a t e v a p o r a t e s

w h i l e a t t a i n i n g e q u i l i b r i u m w i t h t h e e n v i r o n m e n t ( w a t e r removed by d r y i n g a t

ambien t t e m p e r a t u r e ) . I n h e r e n t m o i s t u r e i s t h e p a r t o f t h e w a t e r t h a t rema ins i n

t h e f u e l a f t e r a t m o s p h e r i c d r y i n g ; i t can be d e t e r m i n e d by f i n d i n g t h e mass

decrement o f f u e l w i t h o u t f r e e m o i s t u r e , a d d i t i o n a l l y d r i e d a t 1 0 5 - l l Ü ° C .

A s h , i . e . m i n e r a l m a t t e r , d e c r e a s e s t h e h e a t i n g v a l u e o f f u e l . A h i g h ash

c o n t e n t makes t he combus t i on p r o c e s s more d i f f i c u l t , and r e d u c e s b o i l e r c a p a c i t y

and e f f i c i e n c y . F l y ash i n f l u e gas may be r e s p o n s i b l e f o r t h e f o u l i n g o f

h e a t i n g s u r f a c e s and e r o s i o n wear o f b o i l e r p a r t s . I n o r d e r t o d e t e r m i n e t h e ash

c o n t e n t , a c o a l sample o f 1-2 g mass i s bu rned i n a l a b o r a t o r y oven a t 815^C,

and t he r e s i d u e i s a d d i t i o n a l l y r o a s t e d .

C o m b u s t i b l e m a t t e r i n c o a l i n c l u d e s t h e e l emen ts t h a t t a k e p a r t i n t h e

combus t ion p r o c e s s , i . e . c a r b o n , h y d r o g e n and s u l p h u r . Knowledge o f t h e c o n t e n t

o f c o m b u s t i b l e e lemen ts may be e s p e c i a l l y i m p o r t a n t i f t h e r e i s no p o s s i b i l i t y

248

o f d i r e c t l y d e t e r m i n i n g t he combus t i on h e a t o r h e a t i n g v a l u e o f t h e f u e l , as

t h e s e q u a n t i t i e s can be a l t e r n a t i v e l y c a l c u l a t e d by an i n d i r e c t method ( s e e

S e c t i o n 7 . 3 . 4 ) . Sometimes i t may be i m p o r t a n t t o d e t e r m i n e t h e c o n t e n t o f

c o m b u s t i b l e m a t t e r i n b o i l e r a s h , as t h i s w o u l d a l l o w c a l c u l a t i o n o f t h e e n e r g y

l o s s r e s u l t i n g f rom i n c o m p l e t e c o m b u s t i o n .

F o r d e t a i l s o f coa l a n a l y s i s and s t a n d a r d s c o n c e r n i n g t h e l a b o r a t o r y

p r o c e d u r e s , t he l i t e r a t u r e s h o u l d be c o n s u l t e d ( r e f . 3 6 , 3 7 ) .

7 .3 .3 P r o p e r t i e s o f f u e l o i l

Perhaps t h e most i m p o r t a n t p r o p e r t y o f f u e l o i l i s t he v i s c o s i t y . L i g h t f u e l

o i l s o f v i s c o s i t y up t o 2°E ( d e g r e e s E n g l e r ) a t 20^C may be s u p p l i e d t o t h e

b u r n e r s w i t h o u t p r i o r h e a t i n g . Heavy f u e l o i l s o f h i g h v i s c o s i t y must be hea ted

e i t h e r o n c e , i . e . b e f o r e pumping, o r t w i c e , i . e . b e f o r e pumping and b e f o r e

d i s p e r s i n g i n b u r n e r s . The n e c e s s a r y t e m p e r a t u r e (up t o 150°C) depends on t h e

o i l t y p e , and can be f ound i n d iag rams o r nomographs ( r e f . 3 3 ) .

O t h e r e s s e n t i a l pa rame te rs a re t h e s u l p h u r c o n t e n t and vanadium o x i d e

c o n t e n t . S u l p h u r i s i m p o r t a n t n o t o n l y f o r e n v i r o n m e n t a l r e a s o n s ; i t may be

r e s p o n s i b l e f o r l o w - t e m p e r a t u r e c o r r o s i o n i n t h e o u t l e t p a r t s o f t h e b o i l e r .

Vanadium o x i d e , V20^ , may cause h i g h - t e m p e r a t u r e c o r r o s i o n o f b o i l e r p a r t s .

7 .3 .4 Heat o f combus t i on and h e a t i n g v a l u e

The h e a t o f c o m b u s t i o n , Q ^ , i s t h e amount o f e n e r g y r e l e a s e d pe r u n i t

q u a n t i t y o f f u e l i n compressed o x y g e n a t c o n s t a n t v o l u m e ; i t i s a d d i t i o n a l l y

assumed t h a t t h e f u e l t e m p e r a t u r e b e f o r e combus t i on and t h e t e m p e r a t u r e o f t h e

combus t ion p r o d u c t s i s 20°C, and t h e w a t e r i n i t i a l l y p r e s e n t i n t h e f u e l , o r

c r e a t e d by b u r n i n g o f h y d r o g e n , i s f i n a l l y c o n d e n s e d .

The d e t e r m i n a t i o n o f h e a t o f combus t i on i s one o f t he most i m p o r t a n t

l a b o r a t o r y measurements needed f o r hea t -economy m o n i t o r i n g , and t h u s p a r t i c u l a r

c a r e and a c c u r a c y i s r e q u i r e d . The s t a n d a r d method a p p l i c a b l e t o s o l i d and

l i q u i d f u e l s i s t he b o m b - c a l o r i m e t e r method . The a p p a r a t u s i s r e q u i r e d t o meet

a s t a n d a r d s p e c i f i c a t i o n , and t h e p r o c e d u r e used i s a l s o s t a n d a r d i z e d ( r e f . 3 7 ) .

The bomb i s a smal l s t a i n l e s s - s t e e l v e s s e l i n w h i c h a smal l mass o f t h e f u e l

( a b o u t 1 g ) i s h e l d i n a c r u c i b l e ( F i g . 7 . 2 ) . I f t h e f u e l i s s o l i d , i t i s

u s u a l l y c r u s h e d and t hen p r e s s e d i n t o t h e fo rm o f a p e l l e t i n a s p e c i a l p r e s s .

The p e l l e t i s i g n i t e d by f u s i n g a p i e c e o f w i r e w i t h w h i c h i t i s i n c o n t a c t ; t h e

p e l l e t can a l s o be made w i t h t h e f u s e w i r e p a s s i n g t h r o u g h i t .

The w i r e fo rms p a r t o f an e l e c t r i c a l c i r c u i t w h i c h can be comp le ted by a

f i r i n g b u t t o n w h i c h i s s i t u a t e d i n a p o s i t i o n remote f rom t h e bomb. I f a l i q u i d

f u e l i s be i ng t e s t e d , i t i s c o n t a i n e d i n a g e l a t i n e c a p s u l e and t h e f i r i n g may

be a s s i s t e d by i n c l u d i n g i n t h e c r u c i b l e a l i t t l e p a r a f f i n o f known h e a t o f

c o m b u s t i o n . The c r u c i b l e c a r r y i n g t h e f u e l i s l o c a t e d i n t h e bomb, and t h e t o p

249

o f t he bomb i s sc rewed down. Oxygen i s t h e n a d m i t t e d s l o w l y u n t i l t h e p r e s s u r e

i s 20-35 b a r , depend ing on t he f u e l t y p e . The bomb i s l o c a t e d i n t h e c a l o r i m e t e r

and a measured q u a n t i t y o f w a t e r i s pou red i n t o t h e c a l o r i m e t e r .

F i g . 7 . 2 . Scheme o f t h e bomb c a l o r i m e t e r . 1 - i n s u l a t i n g c o v e r , 2 - w a t e r , 3 - bomb, 4 - i n s u l a t i n g f e e t , 5 - f u e l sample i n a c r u c i b l e .

The c a l o r i m e t e r i s c l o s e d , t h e e x t e r n a l c o n n e c t i o n s t o t he c i r c u i t a r e made,

and an a c c u r a t e thermometer o f t h e f i x e d - r a n g e o r t h e Beckman t y p e i s immersed

t o a p r o p e r dep th i n t h e w a t e r . The w a t e r i s s t i r r e d by a m o t o r - d r i v e n s t i r r e r

and t e m p e r a t u r e o b s e r v a t i o n s a r e t aken e v e r y m i n u t e . A t t h e end o f t h e f i f t h

m i n u t e , t he c h a r g e i s f i r e d and t e m p e r a t u r e r e a d i n g s a r e t aken e v e r y 10 seconds

u n t i l t h e r e a d i n g s b e g i n t o f a l l ; t h e n , t he f r e q u e n c y o f r e a d i n g s can a g a i n be

r e d u c e d t o e v e r y m i n u t e . The measured t e m p e r a t u r e r i s e , w h i c h s h o u l d n o t e x c e e d

2-3 K, i s c o r r e c t e d f o r v a r i o u s l o s s e s a c c o r d i n g t o t h e f o r m u l a e g i v e n i n t h e

r e s p e c t i v e s t a n d a r d s . The hea t o f combus t i on o f t h e f u e l t e s t e d i s f i n a l l y

c a l c u l a t e d f rom the h e a t b a l a n c e e q u a t i o n , a l s o g i v e n i n t h e s t a n d a r d s , and t h e

measur ing e r r o r can be e s t i m a t e d .

I f , f o r any r e a s o n , t he bomb c a l o r i m e t e r method c a n n o t be u s e d , b u t t h e

chemica l c o n s t i t u t i o n o f t h e f u e l i s known, t h e n i t i s p o s s i b l e t o c a l c u l a t e

t he hea t o f combus t ion u s i n g a p p r o p r i a t e f o r m u l a e . F o r e x a m p l e , i n t h e case o f

European coa l t y p e s , t h e s o - c a l l e d D u l o n g ' s f o r m u l a may be a p p l i e d

= 341C + 1444{H - (0 + Ν - l ) / 8 ) + 93S ( k J / k g ) ( 7 . 1 )

where C , H , 0 , N , and S a re t h e p e r c e n t a g e s o f c a r b o n , h y d r o g e n , o x y g e n ,

n i t r o g e n and s u l p h u r .

The a c c u r a c y o f t h e s e s o - c a l l e d i n d i r e c t methods f o r d e t e r m i n a t i o n o f t h e h e a t

o f combus t i on i s l i m i t e d . A t a c a r b o n c o n t e n t o f a b o u t 86%, and an o x y g e n

c o n t e n t be low 7.5%, t h e a c c u r a c y o f t h e D u l o n g ' s f o r m u l a i s a b o u t ±2%.

250

The h e a t i n g v a l u e o f t h e f u e l i s equa l t o t h e h e a t o f combus t i on minus t h e

l a t e n t h e a t o f t h e w a t e r coming f rom t h e f u e l d u r i n g c o m b u s t i o n . T h i s

c o r r e s p o n d s t o t h e f a c t t h a t w a t e r v a p o u r formed i n f u r n a c e s i s e m i t t e d i n t h e

f l u e gas t o t h e a tmosphe re , and t h e l a t e n t h e a t i s l o s t ( c o n t r a r y t o a bomb

c a l o r i m e t e r measurement ) .

The h e a t i n g v a l u e o f t h e f u e l , Q ^ , can be c a l c u l a t e d as a f u n c t i o n o f t h e

combus t ion h e a t , Q ^ , t h e h y d r o g e n p e r c e n t a g e Η and t h e m o i s t u r e p e r c e n t a g e W.

A g a i n , d e t a i l e d f o r m u l a e can be f o u n d i n t h e r e l e v a n t s t a n d a r d s . F o r e x a m p l e ,

P o l i s h S t a n d a r d s s t a t e t h a t f o r l i q u i d f u e l s

= - 25 .19(9H + W) ( k J / k g ) ( 7 . 2 )

and f o r s o l i d f u e l s

= - 24 .55 (8 .9H + W) ( k J / k g ) ( 7 . 3 )

I t i s n o t e w o r t h y t h a t i n t h e case o f l i q u i d f u e l s , e m p i r i c a l f o r m u l a e a r e

a v a i l a b l e f o r c a l c u l a t i o n o f t h e h y d r o g e n c o n t e n t as a f u n c t i o n o f t h e h e a t o f

c o m b u s t i o n , w h i l e t h e h y d r o g e n c o n t e n t i n s o l i d f u e l s must be e x p e r i m e n t a l l y

d e t e r m i n e d .

I n d i r e c t c a l c u l a t i o n o f t h e h e a t i n g v a l u e u s i n g t h e f o r m u l a e g i v e n i n t h e

l i t e r a t u r e i s a l s o p o s s i b l e . F o r examp le , t h e f o l l o w i n g f o r m u l a may be a p p l i e d

t o European coa l t y p e s :

= 339(C - 0 . 7 5 ( 0 / 2 ) ) + 1193(H - 0 . 1 2 5 ( 0 / 2 ) ) + 105S - 25W. ( k J / k g ) ( 7 . 4 )

where i s t he p e r c e n t a g e o f i n h e r e n t m o i s t u r e ; o t h e r symbo ls as a b o v e .

7 .3 .5 Fue l s t o r a g e i n s p e c t i o n

Coa l p i l e s s h o u l d be v i s u a l l y i n s p e c t e d d a i l y d u r i n g t h e f i r s t 3 months a f t e r

t h e i r d e l i v e r y and t w i c e a week t h e r e a f t e r . A f i r e h a z a r d i s i n d i c a t e d b y :

- s p o t - w i s e d r y i n g o f t he p i l e s u r f a c e a f t e r r a i n f a l l ;

- e s c a p i n g steam c l o u d s ;

- d e p o s i t s o f ash and s u l p h u r a p p e a r i n g on t h e p i l e s u r f a c e ;

- e s c a p i n g smoke o f c h a r a c t e r i s t i c o d o u r .

A more p r e c i s e i n s p e c t i o n p r o c e d u r e r e q u i r e s t h a t t h e t e m p e r a t u r e o f t he c o a l

i n t h e p i l e s be measured . The s t a n d a r d measurement f r e q u e n c y i s t w i c e a week

d u r i n g t he f i r s t 2 months and once e v e r y two weeks t h e r e a f t e r . A c o a l

t e m p e r a t u r e above 45°C s h o u l d be i n t e r p r e t e d as an i n d i c a t i o n o f f i r e h a z a r d . I f

t h e t e m p e r a t u r e r i s e s above 60°C, t h e r e l e v a n t f u e l l o t must i m m e d i a t e l y be

d i r e c t e d t o t h e f u r n a c e .

A c c o r d i n g t o P o l i s h r e g u l a t i o n s , c o a l - s t o r a g e y a r d s w i t h c a p a c i t i e s l a r g e r

t han 500 t o n s must be e q u i p p e d w i t h permanent t e m p e r a t u r e - m e a s u r i n g s y s t e m s ,

c o n s i s t i n g o f i n s t r u m e n t s spaced l e s s t han 5 m a p a r t . The s i m p l e s t i n s t r u m e n t i s

a m e r c u r y - i n - g l a s s thermometer p l a c e d i n s i d e a s t e e l t ube t h a t can be i n s e r t e d

251

i n t h e f u e l l a y e r . S p e c i a l s i g n a l l i n g the rmometers o r remote measu r i ng sys tems

can a l s o be a p p l i e d .

Fue l o i l i s h e l d i n s t o r a g e t a n k s o u t s i d e t h e b o i l e r h o u s e , and an

i n t e r m e d i a t e tank w i t h a c a p a c i t y o f a b o u t 8 - h o u r s ' b o i l e r s u p p l y i s u s u a l l y

i n s t a l l e d i n , o r c l o s e t o , t h e b o i l e r h o u s e . As h e a v y o i l r e q u i r e s h e a t i n g

b e f o r e i t can be pumped t o t h e b u r n e r s , t e m p e r a t u r e c o n t r o l i n t h e t a n k s and

p i p e l i n e s i s e s s e n t i a l . A s t a n d a r d s o l u t i o n t o t h i s p rob lem i s an a u t o m a t i c a l l y

c o n t r o l l e d h e a t i n g sys tem comp le te w i t h r e m o t e - t e m p e r a t u r e measurements and

d e v i c e s w a r n i n g o f e x c e s s t e m p e r a t u r e .

7.4 BOILERS

7.4.1 Methods o f measurement

The b o i l e r s can be r e g a r d e d as a s p e c i a l p a r t o f t h e the rma l s y s t e m , where

c o n s i d e r a b l e e n e r g y l o s s e s can o c c u r o r c o n s i d e r a b l e e n e r g y s a v i n g s can be

a t t a i n e d . E n e r g y d e l i v e r e d i n f u e l can be e f f i c i e n t l y u t i l i z e d i f b o i l e r s a r e

m a i n t a i n e d i n p r o p e r c o n d i t i o n . B o i l e r e f f i c i e n c y i s one o f t h e i m p o r t a n t

i n d i c e s t o be s y s t e m a t i c a l l y checked and c a r e f u l l y w a t c h e d , because even sma l l

d e v i a t i o n s f rom i t s op t ima l v a l u e , i f m a i n t a i n e d f o r a l o n g p e r i o d , may cause

c o n s i d e r a b l e e n e r g y l o s s e s .

I n some c o u n t r i e s , g e n e r a l s a f e t y r e g u l a t i o n s may a p p l y t o t h e b o i l e r s

i n s t a l l e d i n s u g a r f a c t o r i e s . T h i s can be e x e m p l i f i e d by t h e s o - c a l l e d

"Measurement l i s t f o r b o i l e r o p e r a t i o n " . T a b l e 7 . 5 , w h i c h i s o b l i g a t o r y i n

Po land ( r e f . 2 1 ) . I t s t a t e s wha t pa rame te r s s h o u l d be r e c o r d e d a t l e a s t e v e r y

h o u r , f o r r o u t i n e c h e c k i n g o f b o i l e r o p e r a t i o n .

A b o i l e r scheme w i t h an i n d i c a t i o n o f pa rame te r s needed f o r t h e b o i l e r e n e r g y

b a l a n c e i s shown i n F i g . 7 . 3 . The e s s e n t i a l p rob lem o f b o i l e r c h e c k i n g i s t o

t g . p g . D Tsuperheated steam

ι Η JTT Κ

\ \ \ \ \ \ \ \ ^ \ \ \ \ \ f e e d - w a t e r ^ ^ | j \ ^ p ^ . t ^ 21

F i g . 7 . 3 . Scheme o f a b o i l e r . M e a s u r i n g p o i n t s a r e i n d i c a t e d a c c o r d i n g t o t h e l i s t o f measurements . T a b l e 7 .5 .

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TABLE 7.5

Measurement l i s t f o r b o i l e r o p e r a t i o n , a c c o r d i n g t o P o l i s h r e g u l a t i o n s ( r e f . 2 1 ) .

No. Parameter B o i l e r c a p a c i t y 3 ^ ^ ^ ^ ^

1-5 t / h above 5 t / h

1 F e e d - w a t e r p r e s s u r e χ χ p^ 2 Steam p r e s s u r e a t b o i l e r o u t l e t r χ p^ 3 Steam p r e s s u r e i n b o i l e r drum χ χ 4 F e e d - w a t e r t e m p e r a t u r e χ χ t ^ 5 Tempera tu re a f t e r f e e d - w a t e r h e a t e r χ χ 6 Steam t e m p e r a t u r e a t b o i l e r o u t l e t χ χ 7 B l a s t a i r p r e s s u r e χ χ 8 A i r p r e s s u r e b e f o r e a i r h e a t e r χ r 9 A i r p r e s s u r e a f t e r a i r h e a t e r - r

10 F l u e gas t e m p e r a t u r e b e f o r e f e e d - w a t e r h e a t e r r r

11 F l u e gas t e m p e r a t u r e b e f o r e a i r h e a t e r - r 12 F l u e gas t e m p e r a t u r e b e f o r e ch imney χ χ 13 A i r t e m p e r a t u r e b e f o r e a i r h e a t e r - r 14 A i r t e m p e r a t u r e a f t e r a i r h e a t e r - r 15 D r a u g h t i n f u r n a c e above s t o k e r r χ 16 D r a u g h t b e f o r e f e e d - w a t e r h e a t e r r r 17 D r a u g h t a f t e r s u p e r h e a t e r r r 18 D r a u g h t b e f o r e a i r h e a t e r - r 19 D r a u g h t b e f o r e ch imney χ χ 20 CO2 c o n t e n t i n f l u e gas b e f o r e ch imney x * χ 21 F e e d - w a t e r f l o w χ χ 22 Steam f l o w x * * χ D 23 C o m b u s t i b l e m a t t e r i n ash r χ 24 Fue l f l o w χ χ Β

* 7 * * 7 X = r e q u i r e d ; r = recommended; e x c e p t h a n d - f i r e d f u r n a c e s ; o n l y i n b o i l e r s e q u i p p e d w i t h s u p e r h e a t e r s .

de te rm ine how much e n e r g y i s consumed i n t h e g e n e r a t i o n o f a d e f i n i t e amount o f

steam a t t h e r e q u i r e d p r e s s u r e and t e m p e r a t u r e . Two i n d i c e s can be u s e d , t h e

b o i l e r e f f i c i e n c y η and t he e n e r g y consumpt ion p e r 1 kg steam q .

η = D ( h ^ - h^ ) / (BQ^) -100% ( 7 . 5 )

where D i s t h e mass o f steam g e n e r a t e d i n t he b o i l e r , h^ i s t h e e n t h a l p y o f

steam a t t h e b o i l e r o u t l e t , h i s t h e e n t h a l p y o f f e e d w a t e r , Β i s t h e mass o f w

f u e l consumed i n t h e b o i l e r f u r n a c e , and i s t h e h e a t i n g v a l u e o f f u e l .

The e n e r g y consumpt ion p e r 1 kg steam can be c a l c u l a t e d as

q = BQ^/D ( 7 . 6 )

I n o r d e r t o d e t e r m i n e t he above i n d i c e s , i n a d d i t i o n t o t h e h e a t i n g v a l u e o f

f u e l , t he f o l l o w i n g q u a n t i t i e s must be f ound f rom measurements :

- t h e mass o f f u e l consumed;

- t h e mass o f steam g e n e r a t e d ;

- t he steam p r e s s u r e and t e m p e r a t u r e ( f o r e n t h a l p y d e t e r m i n a t i o n ) ;

- t he mass o f f e e d w a t e r d e l i v e r e d t o t h e b o i l e r ( c h e c k on s t e a m - f l o w

253

measurement ) ;

- t h e f e e d - w a t e r t e m p e r a t u r e ( f o r e n t h a l p y d e t e r m i n a t i o n ) .

The amount o f s o l i d f u e l i s w e i g h e d , p r e f e r a b l y by an a u t o m a t i c s c a l e w h i c h

i s c a l i b r a t e d d a i l y . I n t h e absence o f s c a l e s , a v o l u m e t r i c method can be u s e d .

F u e l - o i l consumpt ion can be measured by f l o w me te rs i n s t a l l e d i n t he s u p p l y

p i p e l i n e . F o r a c c u r a t e hea t b a l a n c e s , i t i s recommended t h a t f l o w r e a d i n g s be

taken e v e r y f i v e m i n u t e s .

The amount o f steam i s measured by f l o w me te rs and e v e n t u a l l y checked a g a i n s t

t he i n d i c a t i o n s o f a w a t e r meter i n s t a l l e d on t he f e e d - w a t e r s u p p l y l i n e . I f a

h i g h - a c c u r a c y b a l a n c e i s r e q u i r e d , i t i s recommended t h a t t he s o o t b l o w e r s and

t he b o i l e r blowdown a re t e m p o r a r i l y c u t o f f ; i f t h e blowdown c a n n o t be a v o i d e d ,

t hen a p p r o p r i a t e c o r r e c t i o n s must be i n t r o d u c e d i n t h e f e e d - w a t e r meter

i n d i c a t i o n s .

Mean v a l u e s o f t h e w a t e r and steam p a r a m e t e r s can be e s t i m a t e d w i t h t h e a i d

o f t e m p e r a t u r e and p r e s s u r e r e c o r d s f rom t h e r e c o r d i n g i n s t r u m e n t s . F o r a c c u r a t e

e n e r g y b a l a n c e s , t h e pa rame te rs can be c a l c u l a t e d as mean v a l u e s o f i n s t r u m e n t

r e a d i n g s taken e v e r y 15 m i n u t e s .

A r o u t i n e b o i l e r check c o n s i s t s o f o b s e r v i n g t h e i n d i c a t i o n s o f t he i n s t a l l e d

measur ing i n s t r u m e n t s and r e c o r d i n g t h e i m p o r t a n t r e a d i n g s a t l e a s t e v e r y h o u r .

I n f o r m a t i o n g a t h e r e d i n t h i s way g e n e r a l l y e n a b l e s us t o e v a l u a t e b o i l e r

o p e r a t i o n , e s p e c i a l l y f u e l c o n s u m p t i o n , and t o e s t a b l i s h how i t i s i n f l u e n c e d by

l o a d f l u c t u a t i o n s r e s u l t i n g f rom o p e r a t i o n o f t h e s u g a r f a c t o r y .

P e r i o d i c checks s h o u l d be aimed a t a n a l y s i n g t h e e n e r g y b a l a n c e s o f t he

b o i l e r u n i t s a t l e a s t e v e r y 10 d a y s . E n e r g y b a l a n c e c a l c u l a t i o n s s h o u l d be based

on t h e r e s u l t s o f measurements t aken e v e r y 15 m inu tes d u r i n g a 6 - h o u r t e s t

p e r i o d ( r e f . 3 9 ) . The p r e r e q u i s i t e f o r a r e l i a b l e e n e r g y b a l a n c e i s s t a b i l i z e d

b o i l e r o p e r a t i o n d u r i n g t h a t t i m e .

7 .4 .2 Combus t ion e v a l u a t i o n

The e s s e n t i a l r e q u i r e m e n t s t o be a c c o u n t e d f o r i n combus t i on e v a l u a t i o n a r e

r e l a t e d t o t h e phenomena o c c u r r i n g i n t h e b o i l e r f u r n a c e . The combus t i on p r o c e s s

i n c l u d e s f u e l d e c o m p o s i t i o n and o x i d a t i o n o f c o m b u s t i b l e componen ts , i . e .

c a r b o n , s u l p h u r and h y d r o g e n , i n t o ca rbon d i o x i d e , s u l p h u r d i o x i d e and w a t e r ,

r e s p e c t i v e l y . Two b a s i c r e q u i r e m e n t s f o r e f f e c t i v e combus t i on a r e t h u s a

s u f f i c i e n t l y h i g h t e m p e r a t u r e and an adequa te o x y g e n s u p p l y .

E v a l u a t i o n o f t he combus t ion p r o c e s s i s based on f l u e - g a s a n a l y s i s , aimed a t

d e t e r m i n i n g t he c o n t e n t o f c a r b o n d i o x i d e , c a r b o n monox ide and o x y g e n . The

measured CO^ c o n t e n t e n a b l e s us t o e v a l u a t e t h e combus t i on p r o c e s s by compar i son

w i t h t he t h e o r e t i c a l CO^ c o n t e n t c a l c u l a t e d f rom s t o i c h o m e t r i c r e l a t i o n s h i p s . I f

t h e r e a l CO^ c o n t e n t i s l o w e r t han t h e t h e o r e t i c a l v a l u e , and t he 0^ c o n t e n t i s

h i g h , t h i s i n d i c a t e s an e x c e s s i v e a i r s u p p l y o r a l e a k y b o i l e r e x h a u s t c h a n n e l .

254

From s i m u l t a n e o u s f u r n a c e t e m p e r a t u r e and d r a u g h t measurements , i t i s p o s s i b l e

t o d e t e r m i n e t h e r e a s o n s f o r t h e s i t u a t i o n .

The CO c o n t e n t i n d i c a t e s i n c o m p l e t e c o m b u s t i o n , r e s u l t i n g e i t h e r f rom

i n a d e q u a t e a i r f e e d o r , i n t h e case o f s o l i d f u e l s , f rom t o o t h i c k a f u e l l a y e r

on t h e s t o k e r .

The u l t i m a t e goa l o f combus t i on e v a l u a t i o n i s t o d e t e r m i n e t h e e n e r g y l o s s e s

r e s u l t i n g f rom t h e combus t ion p r o c e s s and t o t ake a p p r o p r i a t e c o r r e c t i v e

measu res . The ch imney l o s s depends on t h e CO^ c o n t e n t and t h e f l u e - g a s

t e m p e r a t u r e . T h i s i s t h e l a r g e s t o f t h e e n e r g y l o s s e s , n o r m a l l y amount ing t o

8-12% o f t h e e n e r g y d e l i v e r e d i n t h e f u e l . I n o l d e r b o i l e r t y p e s , i t can e x c e e d

20%. The ch imney l o s s can be c a l c u l a t e d f rom t h e S i e g e r t f o r m u l a

= a ( t ^ - t ^ (%) ( 7 . 7 )

0.10

0.09

0.08

0.07

0.05

0.04

1

16 18 CO2 content in f lue gas (%)

20 22

F i g . 7 .4 . C o e f f i c i e n t α i n t h e S i e g e r t f o r m u l a ( a f t e r r e f . 4 0 ) . S o l i d l i n e s -c o a l and l i g n i t e , dashed l i n e s - wood .

255

where α i s a c o e f f i c i e n t depend ing on t h e c o n t e n t , f u e l t y p e and t h e f u e l

m o i s t u r e c o n t e n t ( t h e v a l u e o f α can be f o u n d i n F i g . 7 . 4 ) , t ^ i s t h e f l u e - g a s

t e m p e r a t u r e a t t h e b o i l e r o u t l e t i n and t ^ i s t h e ambien t t e m p e r a t u r e i n °C.

F o r g a s - and o i l - f i r e d b o i l e r s , f o r m u l a e e x p r e s s i n g t h e ch imney l o s s as a

f u n c t i o n o f CO^ and 0^ c o n t e n t i n t h e f l u e gas and t e m p e r a t u r e d i f f e r e n c e

t - t can be f ound i n t h e l i t e r a t u r e , and even c a l c u l a t o r p rograms t o g 3

automate t h e c a l c u l a t i o n s o f t h e ch imney l o s s have been p u b l i s h e d ( r e f . 4 1 ) .

The i n c o m p l e t e combus t ion l o s s , S j , depends i n p r i n c i p l e on t h e c o n t e n t o f

c o m b u s t i b l e gases ( C O , h y d r o c a r b o n s ) i n t h e f l u e g a s . I t amounts t o 0 .5-1.5%

i n b o i l e r s e q u i p p e d w i t h modern f u r n a c e s , and up t o 5% i n o l d e r b o i l e r s w i t h

h a n d - f i r e d f u r n a c e s . Due t o measurement d i f f i c u l t i e s , d e t e r m i n a t i o n o f t h e

i n c o m p l e t e combus t ion l o s s u s u a l l y r e l i e s on t h e CO c o n t e n t o f t h e f l u e g a s .

F i g . 7 .5 . We see t h a t 1% CO i n f l u e gas c o r r e s p o n d s t o an e n e r g y l o s s o f

a p p r o x i m a t e l y 4-6%.

The ash l o s s , S ^ , depends on t h e c o m b u s t i b l e m a t t e r c o n t e n t i n t h e a s h . T h i s

depends m o s t l y on t h e f u e l t y p e and t h e f u r n a c e t y p e . The ash l o s s i n o i l - f i r e d

CO content in f lue gas (%)

0.3 O.A 0.6 0.8 1.0 1.5

10 8 6 5 4 3 2 1.5 1.0 0.8 0.6 Incomplete combustion loss (%)

F i g . 7 .5 . I n c o m p l e t e combus t i on l o s s S j ( a f t e r r e f . 4 0 ) .

2 5 6

Mass of ash as α percentage of fuel mass (%)

3 A 5 6 7 8 9 1 0 1 1 1 2 1 3 Κ 1 5 1 6

A s h l o s s ( % )

F i g . 7 . 6 . Ash l o s s ( a f t e r r e f . 3 7 ) .

b o i l e r s does n o t exceed 1 . 5 % , w h i l e i n t h e case o f c o a l - f i r e d b o i l e r s , i t can be

0 . 5 - 4 . 0 % i n pu l v e r i z e d - c o a l f u r n a c e s , 5 - 1 4 % i n s t o k e r - f i r e d b o i l e r s , and 6 - 1 8 %

i n h a n d - f i r e d f u r n a c e s . The ash l o s s can be e s t i m a t e d as a f u n c t i o n o f t he ash

mass and t h e c o m b u s t i b l e m a t t e r c o n t e n t C i n t h e ash samples ( F i g . 7 . 6 ) .

The r a d i a t i o n hea t l o s s , S ^ , i s caused by therma l r a d i a t i o n f rom b o i l e r w a l l s .

I t depends on b o i l e r c a p a c i t y ( F i g . 7 . 7 ) . T y p i c a l v a l u e s a r e 0 . 5 % i n l a r g e

b o i l e r s and up t o 1 0 % i n smal l b o i l e r s w i t h i n s u f f i c i e n t the rma l i n s u l a t i o n .

3.2

θ 2 Λ

o .1.6

^0.8 α:

\ \

8 10 ' lA '18 20 AO 60 80 100 12 16

Boiler capacity ( t / h )

F i g . 7 . 7 . R a d i a t i o n l o s s S R ( a f t e r r e f . 4 0 ) . 1 - b o i l e r s e q u i p p e d w i t h a i r h e a t e r s and f e e d - w a t e r h e a t e r s , 2 - b o i l e r s w i t h o u t h e a t e r s .

257

7 . 4 . 3 Steam g e n e r a t i o n

A steam g e n e r a t i o n check s h o u l d be d i r e c t e d t o t h r e e e s s e n t i a l a r e a s :

o p e r a t i o n a l s a f e t y , t h e b a s i c o p e r a t i n g pa rame te r s and t h e c o r r e c t n e s s o f t h e

steam g e n e r a t i o n p r o c e s s . S a f e t y r e q u i r e m e n t s c o n c e r n i n g measur ing and

s i g n a l l i n g equ ipmen t , as w e l l as o v e r - p r e s s u r e p r o t e c t i o n , a re g o v e r n e d by

s a f e t y r e g u l a t i o n s i n most c o u n t r i e s and a r e o m i t t e d h e r e .

As t o b a s i c o p e r a t i n g p a r a m e t e r s , t h e d e v i a t i o n s f rom t h e i r nominal v a l u e s

canno t exceed t he l i m i t s t h a t a r e r e q u i r e d by o t h e r p o w e r - h o u s e e q u i p m e n t ,

m a i n l y t h e t u r b i n e s and t h e steam t h r o t t l i n g - d e s u p e r h e a t i n g s t a t i o n . The steam

p r e s s u r e and t e m p e r a t u r e a r e a u t o m a t i c a l l y s t a b i l i z e d i n modern b o i l e r s , and

t h u s t he t a s k o f t he o p e r a t i n g p e r s o n n e l i s l i m i t e d t o c h e c k i n g t he c o r r e c t n e s s

o f t he a u t o m a t i c c o n t r o l .

As t o t h e steam g e n e r a t i o n p r o c e s s , i t s h o u l d be remembered t h a t i t c o n s i s t s

o f t he p r o c e s s e s i n t h r e e b o i l e r p a r t s : f e e d - w a t e r h e a t e r , b o i l e r p r o p e r and

s u p e r h e a t e r . These p r o c e s s e s can be e v a l u a t e d on t h e b a s i s o f t h e r e s u l t s o f

measurements s p e c i f i e d i n S e c t i o n 7 . 4 . 1 . R e c o r d i n g i n s t r u m e n t s s h o u l d p r e f e r a b l y

be u s e d ; o t h e r w i s e , r e a d i n g s t aken a t l e a s t e v e r y hou r s h o u l d be i n c l u d e d i n

o p e r a t i o n r e p o r t s .

7 . 4 . 4 E n e r g y b a l a n c e o f t he b o i l e r

I t i s recommended t h a t e n e r g y b a l a n c e s be s e t up a t l e a s t e v e r y t e n days

( t e s t c o n d i t i o n s a r e e x p l a i n e d i n S e c t i o n 7 . 4 . 1 ) . I n a d d i t i o n t o pa ramete r

v a l u e s , t h e h e a t i n g v a l u e o f t he f u e l must a l s o be known. The mean v a l u e s o f t h e

pa rame te rs a re a p p l i e d i n t he c a l c u l a t i o n s .

The e n e r g y o f t he f u e l s u p p l i e d t o t h e b o i l e r , Q p , i s t r a n s f o r m e d i n t o steam

e n e r g y , Q ^ , and e n e r g y l o s s e s , Qj^.

Qp = Q 3 + Q L ( 7 . 8 )

The e n e r g y s u p p l i e d can be c a l c u l a t e d as

Qp = B Q H ( 7 . 9 )

where Β i s t he mass o f f u e l s u p p l i e d t o t h e b o i l e r , and Q^ i s t h e h e a t i n g v a l u e

o f t he f u e l .

The e n e r g y consumed i n steam g e n e r a t i o n can be c a l c u l a t e d f rom t h e f o r m u l a

Qs = D ( h ^ - h^) ( 7 . 1 0 )

where D i s t he mass o f steam g e n e r a t e d , a c c o r d i n g t o f l o w measurement a t t h e

b o i l e r o u t l e t , h^ i s t he e n t h a l p y o f steam ( f o u n d i n steam t a b l e s o r d i a g r a m s ,

f o r t he measured p r e s s u r e p^ and t e m p e r a t u r e t ^ ) , and h i s t h e e n t h a l p y o f t he

s s w

f e e d w a t e r ( f o u n d i n t a b l e s f o r t he measured t e m p e r a t u r e t ^ ) .

I f Qp and Q^ have been c a l c u l a t e d , t h e b o i l e r e f f i c i e n c y can a l s o be

c a l c u l a t e d a c c o r d i n g t o e q n . ( 7 . 5 ) . The a c c u r a c y o f t h e e f f i c i e n c y v a l u e depends

258

m a i n l y on measur ing e r r o r s i n t h e measurements o f steam f l o w and f u e l mass. I n

some i n s t a n c e s , b o i l e r e f f i c i e n c y can be c a l c u l a t e d more a c c u r a t e l y i f t h e h e a t

l o s s e s have been d e t e r m i n e d ( s e e S e c t i o n 7 . 4 . 2 ) . The r e l e v a n t f o r m u l a i s

η = 100 - ( S ^ + S j + S^ + S^) (%) ( 7 . 1 1 )

7 .4 .5 Example

A c o a l - f i r e d b o i l e r has been t e s t e d and t h e f o l l o w i n g d a t a summarize t h e t e s t

r e s u l t s :

- coa l c o n s u m p t i o n , Β = 5550 k g / h ;

- coa l h e a t i n g v a l u e , = 21440 k J / k g ;

- steam f l o w , D = 32900 k g / h ;

- steam e n t h a l p y , h^ = 3172 k J / k g ;

- f e e d - w a t e r e n t h a l p y , h , = 488 k J / k g . w

The e s s e n t i a l e n e r g y b a l a n c e components a r e t h u s :

- e n e r g y s u p p l i e d t o t he b o i l e r

Qp = 5550-21440 = 1.19-10^ k J / h ;

- e n e r g y consumed i n steam g e n e r a t i o n

Q 3 = 32900· (3172 - 488) = 0 .88-10^ k J / h ;

- e n e r g y l o s s

Q L = Qp - Q 3 = 0 .31-10^ k J / h .

I t i s now p o s s i b l e t o c a l c u l a t e t he b o i l e r e f f i c i e n c y

η = ( 0 . 8 8 - 1 0 ^ / 1 . 1 9 · 1 0 ^ ) · 1 0 0 = 73.9%

and t h e e n e r g y consumpt ion p e r 1 kg steam

q = 5550-21440/32900 = 3617 k J / k g .

7.5 TURBO-GENERATORS

7.5.1 Method o f measurement

The e s s e n t i a l p rob lem o f t u r b o - g e n e r a t o r c h e c k i n g i s t o d e t e r m i n e t h e h e a t

consumpt ion i n e l e c t r i c i t y g e n e r a t i o n . I t i s c o n v e n t i o n a l l y e x p r e s s e d as t h e

steam r a t e , S , t h a t i s , t he mass o f steam consumed i n t h e t u r b o - g e n e r a t o r p e r

1 kWh o f e l e c t r i c a l e n e r g y p r o d u c e d :

S = M/E (kg /kWh) ( 7 . 1 2 )

where Μ i s t h e mass o f steam s u p p l i e d t o t h e t u r b i n e w i t h i n a c e r t a i n t ime

p e r i o d i n kg , and Ε i s t h e e l e c t r i c a l e n e r g y p r o d u c e d a t t h e same t ime i n kWh.

The same i n d e x can a l s o be c a l c u l a t e d as

S = D/N (kg /kWh) ( 7 . 1 3 )

where D i s t h e mass f l o w o f steam i n k g / h , and Ν i s t h e e l e c t r i c a l e f f e c t o f

t h e t u r b o - g e n e r a t o r i n kW.

259

The e s s e n t i a l i n f o r m a t i o n c o n t a i n e d i n t h e S v a l u e can a l t e r n a t i v e l y be

e x p r e s s e d u s i n g t h e c o n c e p t o f t h e o v e r a l l e f f i c i e n c y o f t h e t u r b o - g e n e r a t o r

( s e e S e c t i o n 7 . 5 . 5 ) . I n o r d e r t o c a l c u l a t e bo th i n d i c e s , t h e f o l l o w i n g

measurements a r e n e c e s s a r y :

- steam amount, Μ ( k g ) o r steam mass f l o w , D ( k g / h ) ;

- amount o f e l e c t r i c a l e n e r g y , Ε (kWh) o r e l e c t r i c a l e f f e c t , Ν ( k W ) ;

- i n l e t p r e s s u r e o f s team, p^ ( b a r ) ;

- i n l e t t e m p e r a t u r e o f s team, t ^ ( ° C ) ;

- o u t l e t t e m p e r a t u r e o f s team, t^ ( °C ) ( r e q u i r e d o n l y f o r t h e c a l c u l a t i o n o f

o v e r a l l e f f i c i e n c y ) .

The way i n w h i c h t h e t e s t i s p e r f o r m e d and t h e f r e q u e n c y o f t h e measurements

d e t e r m i n e how t h e t e s t r e s u l t s can be i n t e r p r e t e d . A r o u t i n e check i s based on

measurements r e p e a t e d e v e r y 30 m i n u t e s ; t u r b i n e o p e r a t i o n i s d e t e r m i n e d by steam

g e n e r a t i o n i n t h e b o i l e r and by t h e accompany ing e l e c t r i c a l and the rma l l o a d s .

The r e s u l t i n g S v a l u e r e f l e c t s n o t o n l y t h e t u r b o - g e n e r a t o r q u a l i t y b u t a l s o

t he method o f i t s u t i l i z a t i o n under a c t u a l l o a d c o n d i t i o n s .

As t o t h e p e r i o d i c c h e c k i n g , t h i s i s aimed a t t e s t i n g t u r b i n e o u t p u t and

e f f i c i e n c y under s t a b i l i z e d o p e r a t i n g c o n d i t i o n s . The t u r b i n e s h o u l d be b r o u g h t

i n t o the rma l e q u i l i b r i u m i n a d v a n c e , b e f o r e t h e t e s t i s i n i t i a t e d ; t h e l o a d

s h o u l d be s t a b i l i z e d and t h e n h e l d c o n s t a n t d u r i n g t h e e n t i r e t e s t . I t i s

recommended t h a t t h e f r e q u e n c y o f t h e measurements be r e l a t e d t o t h e d u r a t i o n o f

t h e t e s t . F o r examp le , t he recommendat ion o f r e f . 42 i s t h a t t h e t e s t d u r a t i o n

can be 15 o r 60 m i n u t e s , and t h e recommended measu r i ng f r e q u e n c y i s shown i n

T a b l e 7 .6 .

TABLE 7.6

Recommended f r e q u e n c y o f i n s t r u m e n t r e a d i n g s i n t u r b o - g e n e r a t o r t e s t s .

Measurement ^ ^ " ^ ^ ^ P " 15 m i n u t e s 60 m inu tes

E l e c t r i c a l e f f e c t 1 min 1 min Steam f l o w 1 min 1 min Tempera tu re o f incoming steam 1 min 5 min P r e s s u r e o f incoming steam 1 min 5 min Tempera tu re o f e x h a u s t steam 1 min 5 min P r e s s u r e o f e x h a u s t steam 1 min 5 min E l e c t r i c i t y meter i n d i c a t i o n s b e g i n n i n g and Steam c o u n t e r i n d i c a t i o n s end o f t e s t

7 .5 .2 Steam consumpt ion

The t r u e steam c o n s u m p t i o n , a q u a n t i t y on w h i c h t h e b a l a n c e and t h e r o u t i n e

c h e c k i n g o f t h e t u r b o - g e n e r a t o r a r e b a s e d , can be d e t e r m i n e d f rom s t e a m - c o u n t e r

i n d i c a t i o n s a t t h e b e g i n n i n g and end o f t h e t e s t . I t i s n e c e s s a r y t o m o d i f y t h e

260

c o u n t e r i n d i c a t i o n s i f t h e steam p r e s s u r e and t e m p e r a t u r e d e v i a t e f rom t h e

nominal f l o w - m e t e r v a l u e s a t t h e t ime o f t h e t e s t . I n t h e case o f p e r i o d i c

checks aimed a t t h o r o u g h t u r b i n e i n s p e c t i o n , t h e steam consumpt ion v a l u e s h o u l d

be a d d i t i o n a l l y c o r r e c t e d f o r p r e s s u r e and t e m p e r a t u r e d e v i a t i o n s , a c c o r d i n g t o

t h e f o r m u l a

D = - M ^ ) f ^ / T ( k g / h ) ( 7 . 1 4 )

where τ i s t he t e s t d u r a t i o n i n h , M-j and a r e t h e m o d i f i e d c o u n t e r

i n d i c a t i o n s a t t h e b e g i n n i n g and a t t h e end o f t h e t e s t i n kg , and fj^ i s a

c o r r e c t i o n f a c t o r .

I f t h e measurements a re taken f rom a steam f l o w meter and η m o d i f i e d f l o w

v a l u e s D p D2, . . , D ^ have been r e c o r d e d , t hen t he f o l l o w i n g f o r m u l a i s u s e d :

D = O ^ f , ( 7 . 1 5 )

where D ^ i s t h e mean v a l u e o f t he steam f l o w . I f some measurements f a l l beyond

t h e ±2.5% i n t e r v a l a round t h e mean v a l u e , t h i s i s c a l c u l a t e d as

D = ( d / n ) Σ / D T ) 2 ( 7 . 1 6 ) ^ i = l ^

O t h e r w i s e , t h e mean a r i t h m e t i c v a l u e s h o u l d be t a k e n .

The c o r r e c t i o n f a c t o r , f j^ , i s c a l c u l a t e d f o r a b a c k - p r e s s u r e t u r b i n e as t h e

p r o d u c t o f t h r e e c o e f f i c i e n t s r e f l e c t i n g t h e d e v i a t i o n s o f i n l e t p r e s s u r e , i n l e t

t e m p e r a t u r e and o u t l e t p r e s s u r e f rom t h e i r nominal v a l u e s . The d a t a on t h e s e

c o e f f i c i e n t s s h o u l d be s u p p l i e d i n d iag ram o r t a b l e fo rm by t h e t u r b i n e

m a n u f a c t u r e r .

7 .5 .3 T u r b o - g e n e r a t o r power o u t p u t

I n power o u t p u t measurements , t he same g e n e r a l p r i n c i p l e i s a p p l i e d as i n t h e

case o f steam c o n s u m p t i o n . The b a l a n c e and r o u t i n e c o n t r o l o f t h e t u r b o

g e n e r a t o r s h o u l d be based on a c t u a l o u t p u t v a l u e s , w h i l e i n p e r i o d i c c o n t r o l ,

c o r r e c t i o n s f o r t h e d e v i a t i o n s o f o p e r a t i n g pa rame te r s a r e n e c e s s a r y .

I n t he case o f measurements t aken f rom an e l e c t r i c m e t e r , t h e t u r b o - g e n e r a t o r

power o u t p u t i s c a l c u l a t e d as

Ν = (E2 - E ^ ) A f p / T (kW) ( 7 . 1 7 )

where τ i s t h e t e s t d u r a t i o n i n h , E^ and E2 a r e t h e e l e c t r i c meter i n d i c a t i o n s

a t t h e b e g i n n i n g and a t t h e end o f t e s t i n kWh, A i s t h e e l e c t r i c meter c o n s t a n t

and f p i s t h e c o r r e c t i o n f a c t o r .

I f t he r e a d i n g s a re taken f rom a power o u t p u t m e t e r , t h e n t h e f o l l o w i n g

f o r m u l a i s u s e d :

Ν = N^fp (kW) ( 7 . 1 8 )

where i s t h e mean a r i t h m e t i c v a l u e o f t h e power o u t p u t , i n kW.

The c o r r e c t i o n f a c t o r , f ^ , i s a g a i n c a l c u l a t e d on t h e b a s i s o f t u r b i n e

261

c h a r a c t e r i s t i c s , t a k i n g i n t o a c c o u n t t h e pa ramete r d e v i a t i o n s f rom t h e i r nominal

v a l u e s .

7 .5 .4 T u r b o - g e n e r a t o r e n e r g y b a l a n c e

I n t h e e n e r g y b a l a n c e o f a t u r b o - g e n e r a t o r , a l l i ncoming and o u t g o i n g mass

and e n e r g y f l o w s s h o u l d be a c c o u n t e d f o r :

- incoming s team;

- e x h a u s t s team;

- e l e c t r i c a l e n e r g y p r o d u c e d ;

- hea t d i s s i p a t e d t o t he e n v i r o n m e n t .

The scheme o f a t u r b o - g e n e r a t o r w i t h an i n d i c a t i o n o f t h e sys tem bounda ry and

e n e r g y s t reams i s shown i n F i g . 7 . 8 ( a ) .

(a) SYSTEM BOUNDARY

live steam TURBINE

r heat loss

L

electrical effect

>

exhaust steam

F i g . 7 .8 . E n e r g y c o n v e r s i o n p r i n c i p l e o f a t u r b o - g e n e r a t o r w i t h a b a c k - p r e s s u r e t u r b i n e , ( a ) e n e r g y b a l a n c e scheme, ( b ) steam e x p a n s i o n p r o c e s s shown i n t h e M o l l i e r d i a g r a m .

Assuming t h a t t h e f l o w s o f incoming steam and e x h a u s t steam a r e e q u a l , i t

f o l l o w s f rom the f i r s t law o f thermodynamics t h a t

Dh^ = Ν + Dk^ - Q ( 7 . 1 9 )

where Q i s t h e hea t l o s s , and h^ and h-j a r e t h e e n t h a l p i e s o f incoming steam and

e x h a u s t s team, r e s p e c t i v e l y .

7 .5 .5 T u r b i n e and t u r b o - g e n e r a t o r e f f i c i e n c y

The p r o c e s s t h a t i s t h e o r e t i c a l l y p o s s i b l e i n a steam t u r b i n e c o n s i s t s o f

t r a n s f o r m i n g i n t o mechan ica l work a p a r t o f t he steam e n e r g y , equa l t o t h e

262

i s e n t r o p i c e n t h a l p y d rop w i t h t h e e x p a n s i o n f rom i n i t i a l t o f i n a l steam

p r e s s u r e . T h i s i s shown i n t h e M o l l i e r d i a g r a m , F i g . 7 . 8 ( b ) . As t h e r e a r e a l w a y s

f r i c t i o n l o s s e s and i n t e r n a l l e a k s a s s o c i a t e d w i t h steam e x p a n s i o n i n t h e

t u r b i n e , t h e r e a l p r o c e s s f o l l o w s t h e p a t t e r n shown i n t h e d i a g r a m , w h i l e o n l y

a p a r t o f t he i s e n t r o p i c e n t h a l p y d rop i s t r a n s f o r m e d i n t o mechan ica l w o r k .

The u t i l i z a t i o n o f t h e i s e n t r o p i c e n t h a l p y d rop i n a t u r b i n e can be e x p r e s s e d

by t h e s o - c a l l e d i n t e r n a l e f f i c i e n c y

^ i = ( ^ - - ^2^ ( ^ - 2 0 )

where h^ i s t he t h e o r e t i c a l e n t h a l p y v a l u e a f t e r i s e n t r o p i c e x p a n s i o n t o t h e

f i n a l p r e s s u r e , and t he r e m a i n i n g symbo ls a r e as a b o v e .

The v a l u e o f t h e i n t e r n a l e f f i c i e n c y i s o f p r i m a r y impo r t ance i f t h e combined

g e n e r a t i o n o f hea t and e l e c t r i c i t y i s c o n s i d e r e d . The h i g h e r t h e e f f i c i e n c y

v a l u e , t h e l a r g e r t he amount o f e l e c t r i c a l e n e r g y t h a t can be g e n e r a t e d f rom t h e

same amount o f s team. I f t h e p e r i o d i c t u r b i n e i n s p e c t i o n s r e v e a l t h a t t h e

i n t e r n a l e f f i c i e n c y i s d e c r e a s i n g , t h e n one may e x p e c t t h a t t h e c o n t r o l v a l v e s ,

i n t e r n a l s e a l s o r b l a d e sys tem r e q u i r e r e p a i r .

A c t u a l l y , o n l y a p a r t o f t h e mechan ica l work o b t a i n e d i n t h e t u r b i n e can be

t r a n s f o r m e d i n t o e l e c t r i c a l e n e r g y . E n e r g y l o s s e s i n t h e t u r b o - g e n e r a t o r a r e

caused by f r i c t i o n i n t he b e a r i n g s and p o s s i b l y i n t h e t r a n s m i s s i o n gea r between

t h e t u r b i n e and g e n e r a t o r , e n e r g y consumpt ion i n a u x i l i a r y mechanisms, and h e a t

d i s s i p a t i o n i n t he g e n e r a t o r w i n d i n g . These l o s s e s a r e c o n v e n t i o n a l l y a lowed

f o r , t o g e t h e r w i t h l o s s e s caused by steam l e a k s t h r o u g h t h e t u r b i n e s e a l s and by

hea t d i s s i p a t i o n f rom the s u r f a c e o f t h e t u r b i n e c y l i n d e r .

The o v e r a l l u t i l i z a t i o n o f t h e i s e n t r o p i c e n t h a l p y d rop i n a t u r b o - g e n e r a t o r

can be e x p r e s s e d by t h e o v e r a l l e f f i c i e n c y

= n . . 3 6 0 0 / ( S ( h ^ - h ^ ) ) ( 7 . 2 1 )

I t f o l l o w s f rom e q n s . ( 7 . 2 0 ) and ( 7 . 2 1 ) t h a t once t he q u a n t i t i e s d i s c u s s e d i n

S e c t i o n s 7 . 5 . 1 - 7 . 5 . 4 have been d e t e r m i n e d , i t becomes p o s s i b l e t o c a l c u l a t e t h e

i n t e r n a l e f f i c i e n c y o f t h e t u r b i n e and t h e o v e r a l l e f f i c i e n c y o f t h e t u r b o

g e n e r a t o r . No s i m p l e method e x i s t s , h o w e v e r , f o r d e t e r m i n a t i o n o f t h e t u r b i n e

mechan ica l e f f i c i e n c y , η ^ , t h e t r a n s m i s s i o n gea r e f f i c i e n c y , η ^ ^ , and t h e

g e n e r a t o r e f f i c i e n c y , η ^ . I t can t h u s be u s e f u l t o remember t h a t

V t g ' ^ g = ( ^ - 2 2 )

7 .5 .6 Example

A t u r b o - g e n e r a t o r d r i v e n by a b a c k - p r e s s u r e t u r b i n e has been t e s t e d and t h e

f o l l o w i n g d a t a summarize t h e t e s t r e s u l t s (mean v a l u e s ) :

- e l e c t r i c a l e f f e c t , N^ = 6593 kW; m

- steam f l o w , = 58830 k g / h ;

263

- i n l e t p r e s s u r e and t e m p e r a t u r e o f s team, p^ = 34.33 b a r , t ^ = 433.2°C

( e n t h a l p y h^ = 3300 k J / k g ) ;

- o u t l e t p r e s s u r e and t e m p e r a t u r e o f s team, p-j = 3.52 b a r , t^ = 193.3°C

( e n t h a l p y h^ = 2851 k J / k g ) .

C o r r e c t i o n f a c t o r s , f rom d iagrams s u p p l i e d by t h e t u r b i n e m a n u f a c t u r e r , a r e

f ^ = 1-0.997-0.99 = 0.987 = 1 / f^ .

The c o r r e c t e d v a l u e s o f t h e e l e c t r i c a l e f f e c t and steam f l o w a r e t h u s

Ν = 6593/0.987 = 6679 kW,

D = 58830-0.987 = 58064 k g / h .

The steam r a t e i s

S = 58064/6679 = 8 . 6 9 kg/kWh.

The t h e o r e t i c a l e n t h a l p y v a l u e a f t e r i s e n t r o p i c e x p a n s i o n t o t h e o u t l e t

p r e s s u r e , f rom steam t a b l e s , i s h^ = 2732 k J / k g .

The i n t e r n a l t u r b i n e e f f i c i e n c y i s

η. = (3300 - 2851) / (3300 - 2732) = 0 .790.

The o v e r a l l t u r b o - g e n e r a t o r e f f i c i e n c y i s

= 0 .790-3600 / (8 .69 (3300 - 2851) ) = 0 .729.

7.6 PROCESS HEATING EQUIPMENT

7.6.1 E v a p o r a t o r

J u i c e t h i c k e n i n g i n a m u l t i p l e - e f f e c t e v a p o r a t o r depends on h e a t t r a n s m i s s i o n

f rom t h e h e a t i n g steam t o t h e v a p o u r s . C a s c a d e - t y p e h e a t i n g o f c o n s e c u t i v e

e v a p o r a t o r e f f e c t s e n s u r e s m u l t i p l e u t i l i z a t i o n o f hea t i n t h e c o n s e c u t i v e

e v a p o r a t i o n s t a g e s . Heat i s f u r t h e r u t i l i z e d i n s u g a r b o i l i n g , j u i c e h e a t i n g and

o t h e r p r o c e s s s e c t i o n s .

P r o p e r f u n c t i o n i n g o f t h e e v a p o r a t o r s t a t i o n i s v e r y much dependen t on t h e

f u n c t i o n i n g o f t h e i n d i v i d u a l e v a p o r a t o r e f f e c t s and t h e i r a u x i l i a r y e q u i p m e n t .

The c a s c a d e - h e a t i n g p r i n c i p l e works w e l l o n l y i f i n t e n s i v e hea t t r a n s f e r i s

e n s u r e d i n each e f f e c t .

I n t he case o f R o b e r t - t y p e e v a p o r a t o r s , t h e j u i c e l e v e l i n t h e h e a t i n g t u b e s

i s o f p r i m a r y i m p o r t a n c e . A c c o r d i n g t o some s o u r c e s ( r e f . 6 ) , t h e o p t i m a l j u i c e

l e v e l i s 20-30% o f t ube h e i g h t . The v a l u e s g i v e n i n o t h e r s o u r c e s a r e 10-15%

h i g h e r ( r e f . 8 ) . Even i f a d i f f e r e n c e o f , s a y , 10% tube h e i g h t has a l i m i t e d

i n f l u e n c e on t h e h e a t t r a n s f e r i n t e n s i t y , i t s h o u l d be remembered t h a t marked

d e v i a t i o n s f rom t h e s e v a l u e s wou ld have an u n f a v o u r a b l e e f f e c t on j u i c e

t h i c k e n i n g .

F o r t h e c o r r e c t o p e r a t i o n o f f a l l i n g - f i l m e v a p o r a t o r s , s u f f i c i e n t l i q u i d

w e t t i n g o f t h e tube w a l l i s n e c e s s a r y . Too sma l l a j u i c e f l o w i s a s s o c i a t e d

w i t h t h e r i s k t h a t t h e f i l m w i l l t e a r a p a r t , t h i s r e s u l t i n g i n l o c a l

s u p e r s a t u r a t i o n and i n c r u s t a t i o n s f o r m i n g on t h e t ube w a l l . P r a c t i c a l e x p e r i e n c e

p r o v e s t h a t l i q u i d w e t t i n g can be e v a l u a t e d u s i n g volume f l o w p e r u n i t l e n g t h o f

264

tube c i r c u m f e r e n c e a t t he o u t l e t . As a f i r s t a p p r o x i m a t i o n o f minimum w e t t i n g

f o r t he e v a p o r a t i o n o f s u g a r s o l u t i o n s , t h e v a l u e o f 15 1 i t r e s / ( h - c m ) can be

u s e d .

Heat t r a n s f e r can a l s o be h i n d e r e d by i r r e g u l a r i t i e s i n steam c o n d e n s a t i o n on

t he o u t e r s u r f a c e s o f t he h e a t i n g t u b e s . T h i s can be a v o i d e d i f n o n c o n d e n s a b l e s

a r e v e n t e d e f f e c t i v e l y , and i f t h e condensa te d r a i n a g e p r e v e n t s t h e c o n d e n s a t e

l e v e l f rom r i s i n g above t h e o u t l e t n o z z l e s . Ex t reme c a r e i s recommended h e r e , as

bo th l e a k i n g steam t r a p s and e x c e s s i v e v e n t i n g may cause u n n e c e s s a r y v a p o u r

l o s s e s . F o r d e t a i l s , see condensa te d r a i n a g e under S e c t i o n 3.2.1 and v e n t i n g

under S e c t i o n 3 . 2 . 2 .

I n many s u g a r f a c t o r i e s , s c a l e b u i l d - u p on t he i n t e r n a l s u r f a c e s o f h e a t i n g

t u b e s and t h e accompanying r e d u c t i o n o f t h e hea t t r a n s f e r i n t e n s i t y may l e a d t o

a s i t u a t i o n where t h e t o t a l t e m p e r a t u r e d e c r e a s e i n t h e e v a p o r a t o r ( f r o m

h e a t i n g - s t e a m t e m p e r a t u r e t o t h i c k - j u i c e t e m p e r a t u r e ) becomes i n s u f f i c i e n t f o r

t he r e q u i r e d j u i c e t h i c k e n i n g . I f t he p r o c e s s i n g c a p a b i l i t y i s h e l d c o n s t a n t ,

a l o w e r c o n c e n t r a t i o n o f t h i c k j u i c e must be a l l o w e d w h i c h , i n t u r n , causes

i n c r e a s e d hea t consumpt ion i n t h e s u g a r house and f i n a l l y i n t h e e n t i r e s u g a r

f a c t o r y . The s i t u a t i o n can be b r o u g h t back t o normal i f t h e s c a l e i s removed .

A c t u a l l y , e a r l i e r s c a l e removal i s recommended, as i t a l l o w s e l i m i n a t i o n o f n o n -

op t ima l hea t u t i l i z a t i o n ; a l s o , i t i s e a s i e r t o remove t h e t h i n n e r s c a l e l a y e r s .

The c h o i c e o f t he moment a t w h i c h s c a l e removal s h o u l d be u n d e r t a k e n can be

based on o b s e r v a t i o n o f t he o v e r a l l h e a t t r a n s f e r c o e f f i c i e n t s i n t h e e v a p o r a t o r

e f f e c t s . T h i s r e q u i r e s measur ing t h e q u a n t i t i e s needed t o c a l c u l a t e t he h e a t

t r a n s f e r c o e f f i c i e n t s .

As p r e s e n t e d a b o v e , t h e p u r p o s e o f t h e e v a p o r a t o r check i s t o d e t e r m i n e t h e

c o u r s e o f t h e m u l t i - s t a g e e v a p o r a t i o n p r o c e s s , and p o s s i b l y t o i n t r o d u c e some

p r o c e s s c o r r e c t i o n s aimed a t r e d u c t i o n o f t h e steam c o n s u m p t i o n . I n a w e l l

d e s i g n e d and p r o p e r l y m a i n t a i n e d e v a p o r a t o r , i t w o u l d be enough t o check t h e

t e m p e r a t u r e d i f f e r e n c e s i n t h e i n d i v i d u a l e f f e c t s and t h e o v e r a l l c o n c e n t r a t i o n

i n c r e a s e . I f , h o w e v e r , t e c h n i c a l c h a r a c t e r i s t i c s and c o r r e c t n e s s o f o p e r a t i o n

have t o be i n v e s t i g a t e d , t h e n t h e f o l l o w i n g measurements a r e n e c e s s a r y :

- steam f l o w t o t h e f i r s t e f f e c t , ( kg /100 kg b ) ;

- t e m p e r a t u r e and p r e s s u r e i n h e a t i n g chambers o f i n d i v i d u a l e f f e c t s , t p t ^ ,

. . , ( ° C ) ; p^ , p^ , . . , ( b a r ) ;

- t e m p e r a t u r e and p r e s s u r e i n v a p o u r chambers o f i n d i v i d u a l e f f e c t s , t p t^, . . ,

( ° C ) ; p^ , p^ , . . , ( b a r ) ;

- t h i n - j u i c e f l o w , G¿ (kg /100 kg b ) ;

- t h i n - j u i c e t e m p e r a t u r e , t ^ , and j u i c e t e m p e r a t u r e i n t he i n d i v i d u a l e f f e c t s ,

t l t ¡ , . . , ( ° C ) ;

- t h i n - j u i c e c o n c e n t r a t i o n , b^ , and j u i c e c o n c e n t r a t i o n s a t c o n s e c u t i v e o u t l e t s ,

265

b p b^ , . . , (% D S ) .

I t i s i m p o r t a n t t o adap t t he measur ing t e c h n i q u e s t o t he p e c u l i a r i t i e s o f

m u l t i - s t a g e e v a p o r a t i o n , p a r t i c u l a r l y i n t e m p e r a t u r e d e t e r m i n a t i o n . I n t h e

f o l l o w i n g , i t i s assumed t h a t t h e the rmometers i n t h e v a p o u r chambers a r e p l a c e d

c l o s e t o t he t o p o f t h e h e a t i n g t u b e s , t h u s i n d i c a t i n g t h e t e m p e r a t u r e s o f t h e

s u p e r h e a t e d v a p o u r s ( s a t u r a t i o n t e m p e r a t u r e + b o i l i n g p o i n t e l e v a t i o n ) .

I t s h o u l d be p o i n t e d o u t t h a t r e l i a b i l i t y o f t he t e s t r e s u l t s can be a c h i e v e d

o n l y i f t he measurements a re pe r f o rmed d u r i n g s t e a d y - s t a t e e v a p o r a t o r o p e r a t i o n ,

when paramete r and j u i c e - l e v e l o s c i l l a t i o n s a r e l i m i t e d . Ex t reme c a r e i s

recommended when c o l l e c t i n g j u i c e samples f o r c o n c e n t r a t i o n measurements . I n

p r i n c i p l e , t e s t cocks f o r j u i c e samp l ing s h o u l d be p r o v i d e d w i t h w a t e r c o o l e r s ,

so t h a t t h e j u i c e can be c o o l e d t o abou t 20°C. D u r i n g t h e t e s t , t h e cocks a r e

opened so t h a t a s t e a d y j u i c e f l o w i s e n s u r e d , and t h e samples a r e c o l l e c t e d i n

v e s s e l s l a r g e enough t o e n s u r e c o n c e n t r a t i o n a v e r a g i n g .

S u f f i c i e n t samp l ing a c c u r a c y can be o b t a i n e d even i f t he j u i c e samples have

n o t been c o o l e d . T h e n , h o w e v e r , t he c o n c e n t r a t i o n v a l u e s d e t e r m i n e d i n t h e

l a b o r a t o r y s h o u l d be c o r r e c t e d f o r s e l f - e v a p o r a t i o n e f f e c t s ( t h e s e e f f e c t s must

be t aken i n t o a c c o u n t i n t h e samples o f t h i n j u i c e and j u i c e f rom t h e f i r s t and

second e v a p o r a t o r e f f e c t s ) . I f t he l a b o r a t o r y - e s t i m a t e d c o n c e n t r a t i o n v a l u e

i s b * , t h e n t h e t r u e c o n c e n t r a t i o n i s

b = b * / ( l + Am) (% DS) ( 7 . 2 3 )

where Am i s t he mass o f w a t e r e v a p o r a t e d f rom 1 kg o f j u i c e

Am = (h - h J / ( 2 6 7 6 - h ) ( k g ) ( 7 . 2 4 ) a

where h i s t h e j u i c e e n t h a l p y i n t h e t e s t cock o u t l e t , w h i c h can be c a l c u l a t e d

as a p p r o x i m a t e l y

h = 4.19(1 - 0 . 5 2 b / 1 0 0 ) t ( k J / k g ) ( 7 . 2 5 )

and h i s t h e e n t h a l p y o f t h e b o i l i n g j u i c e a t a t m o s p h e r i c p r e s s u r e . F o r t y p i c a l a

j u i c e c o n c e n t r a t i o n s , i t can be c a l c u l a t e d as

t h i n j u i c e h^ = 421 - 2.19b ( k J / k g )

j u i c e a f t e r f i r s t e f f e c t h^ = 423 - 2.20b ( k J / k g ) a

j u i c e a f t e r second e f f e c t h = 430 - 2.23b ( k J / k g ) . a

Once t h e above q u a n t i t i e s have been measu red , i t becomes p o s s i b l e t o

i d e n t i f y t h e c o u r s e o f t he m u l t i - s t a g e e v a p o r a t i o n p r o c e s s . From t h e d r y m a t t e r

b a l a n c e e q u a t i o n f o r t h e i - t h e v a p o r a t o r e f f e c t

G¿bo = GJb . ( 7 . 2 6 )

we can c a l c u l a t e t h e j u i c e f l o w

= G ¿ b Q / b . ( kg /100 kg b ) ( 7 . 2 7 )

N e x t , t he mass o f e v a p o r a t e d w a t e r can be c a l c u l a t e d

266

W. = G^_^ - ( kg /100 kg b ) ( 7 . 2 8 )

The e f f e c t i v e t e m p e r a t u r e d i f f e r e n c e i n t h e i - t h e f f e c t i s

A t . = - ( 7 . 2 9 )

( T h i s f o r m u l a i s v a l i d o n l y i f tV i s t h e v a p o u r t e m p e r a t u r e measured i n t h e

v a p o u r chamber c l o s e t o t h e j u i c e s u r f a c e . I f t h e t e m p e r a t u r e i s measured a t t h e

v a p o u r chamber o u t l e t , t hen t h e e f f e c t i v e d i f f e r e n c e must be r e d u c e d by t h e

v a l u e o f b o l i n g p o i n t e l e v a t i o n . )

U s i n g t h e q u a n t i t i e s d e t e r m i n e d a c c o r d i n g t o t h e above f o r m u l a e , t h e o v e r a l l

h e a t t r a n s f e r c o e f f i c i e n t can be d e t e r m i n e d f rom a g r a p h g i v e n i n r e f . 2 . I t i s

a l s o p o s s i b l e t o a p p l y a s i m p l e i t e r a t i v e a l g o r i t h m w h i c h can c o n v e n i e n t l y be

c o m p u t e r i z e d . The a l g o r i t h m uses t h e thermodynamic f u n c t i o n s h " ( t ) and h ' ( t ) ,

i . e . e n t h a l p i e s o f d r y s a t u r a t e d steam and s a t u r a t e d w a t e r as f u n c t i o n s o f

t e m p e r a t u r e , and h ' ^ ( t , b ) , i . e . j u i c e e n t h a l p y as a f u n c t i o n o f t e m p e r a t u r e and

c o n c e n t r a t i o n . The amount o f h e a t t r a n s f e r r e d i n u n i t t ime i s

= (1 + e . ) ( W . ( h " ( t ^ ) - h ' ( t p ) + G ^ ( h J ( t ^ . p b . . ^ ) - h J ( t J , b . ) ) ) ( 7 . 3 0 )

where e / i s t h e h e a t l o s s c o e f f i c i e n t o f t h e i - t h e v a p o r a t o r e f f e c t .

The o v e r a l l hea t t r a n s f e r c o e f f i c i e n t can be c a l c u l a t e d f rom t h e f o r m u l a

k. = Q . / ( F . A t . ) ( 7 . 3 1 )

I n c o n c l u s i o n , l e t us s p e c i f y t h e recommendat ions f o r e v a p o r a t o r c h e c k i n g .

A r o u t i n e check s h o u l d c o n s i s t s o f r e c o r d i n g t h e j u i c e and v a p o u r t e m p e r a t u r e s

i n t h e i n d i v i d u a l e f f e c t s and t h e c o n c e n t r a t i o n s o f t h i n and t h i c k j u i c e . The

r e c o r d i n g f r e q u e n c y s h o u l d be a t l e a s t e v e r y 0 . 5 - 1 . 0 h o u r .

I n p e r i o d i c c h e c k s , a more d e t a i l e d i n v e s t i g a t i o n o f t h e e v a p o r a t o r ,

i n c l u d i n g t h e d e t e r m i n a t i o n o f hea t t r a n s f e r c o e f f i c i e n t s , i s n e c e s s a r y . I n

o r d e r t o e n s u r e t he r e l i a b i l i t y o f r e s u l t s , t e s t d u r a t i o n s o f a t l e a s t 4-6 h o u r s

s h o u l d be a d o p t e d . I n t h e case o f h e a t t r a n s f e r o b s e r v a t i o n s , t h e t e s t s s h o u l d

be pe r f o rmed e v e r y 10 d a y s , and even more f r e q u e n t l y when marked d r o p s i n h e a t

t r a n s f e r c o e f f i c i e n t s have been o b s e r v e d .

7 .6 .2 Example

I n a s u g a r f a c t o r y o p e r a t e d a t a c a p a c i t y o f 4000 t / d , a 4 - h o u r t e s t o f t h e

e v a p o r a t o r s t a t i o n has been p e r f o r m e d . The h e a t t r a n s f e r a r e a s i n f o u r 2

c o n s e c u t i v e e v a p o r a t o r e f f e c t s a r e : 2100, 2400, 2100 and 1050 m . The mean

v a l u e s o f t h i n - j u i c e f l o w and t h i n - j u i c e c o n c e n t r a t i o n a r e Gjj = 124.9 kg/100 kg

b e e t , bg = 13.9% DS. O t h e r t e s t d a t a a r e g i v e n i n t h e upper p a r t o f T a b l e 7 . 7 .

U s i n g e q n s . ( 7 . 2 7 ) , ( 7 . 2 8 ) and ( 7 . 2 9 ) , j u i c e f l o w s , amounts o f w a t e r

e v a p o r a t e d and e f f e c t i v e t e m p e r a t u r e d i f f e r e n c e s can be c a l c u l a t e d , as shown i n

t h e c e n t r e o f T a b l e 7 . 7 .

267

F i n a l l y , t h e g raph g i v e n i n r e f . 2 can be u s e d . The r e s u l t i n g v a l u e s o f

o v e r a l l h e a t t r a n s f e r c o e f f i c i e n t s a r e shown i n t h e l o w e r p a r t o f T a b l e 7 . 7 .

TABLE 7.7

Example o f e v a p o r a t o r t e s t r e s u l t s .

E f f e c t No . 1 2 3 4

J u i c e c o n c e n t r a t i o n a t o u t l e t (% DS) 21 35 52 60 Tempera tu re i n h e a t i n g chamber ( °C) 134.0 127.4 119.7 110.0 Tempera tu re i n v a p o u r chamber ( °C) 128.5 121.0 112.9 103.7

J u i c e f l o w (kg /100 kg b ) 82.0 49.3 33.2 28.7 Water e v a p o r a t i o n (kg /100 kg b ) 43.0 32.7 16.1 4.5 E f f e c t i v e t e m p e r a t u r e d i f f e r e n c e ( K ) 5.5 6.4 6.8 6.3

Heat t r a n s f e r c o e f f i c i e n t ( W i m h ) ) 3.84 2.20 1.14 0.70

7 .6 .3 E x t r a c t o r

The e s s e n t i a l e x t r a c t o r f u n c t i o n s , i . e . p r o p e r e x h a u s t i o n o f c o s s e t t e s and

o b t a i n i n g h i g h - p u r i t y raw j u i c e , a r e dependen t on r a p i d h e a t i n g o f t h e i n f l o w i n g

c o s s e t t e s f o r d e n a t u r a t i o n o f l i v i n g c e l l s , f o l l o w e d by e x t r a c t i o n a t a

t e m p e r a t u r e l e v e l o f 70-74°C. The p a r t o f t h e h e a t economy r e l a t e d t o e x t r a c t i o n

i s aimed p r e c i s e l y a t t h a t o p e r a t i o n . The f o l l o w i n g d i s c u s s i o n a p p l i e s m o s t l y t o

t he t r o u g h - t y p e (DOS) e x t r a c t o r s , b u t t h e g e n e r a l recommendat ions and comments

on m o n i t o r i n g p r i n c i p l e s and methods a r e a l s o v a l i d f o r o t h e r e x t r a c t o r t y p e s .

T h e r e a r e f o u r h e a t i n g j a c k e t s i n t r o u g h - t y p e e x t r a c t o r s ; two o f them a r e

u s u a l l y hea ted by s e c o n d - e f f e c t v a p o u r a t a t e m p e r a t u r e o f 112-115°C, w h i l e t h e

o t h e r two a r e hea ted by t h i r d - e f f e c t v a p o u r a t 103-105°C. E x t r a c t o r m o n i t o r i n g

i s aimed m a i n l y a t c h e c k i n g w h e t h e r t h e r e q u i r e m e n t s o f e x t r a c t i o n t e m p e r a t u r e

and c o r r e c t n e s s o f h e a t s u p p l y a r e f u l f i l l e d . The p r e r e q u i s i t e f o r c o r r e c t

e x t r a c t o r o p e r a t i o n i s e f f i c i e n t c o n d e n s a t e d r a i n a g e and e f f i c i e n t v e n t i n g o f

h e a t i n g j a c k e t s ; t h e c o n t r o l p rob lems i n t h i s a r e a were d i s c u s s e d i n S e c t i o n s

3.2.1 and 3 . 2 . 2 . O t h e r the rma l p rob lems c a n n o t be s e p a r a t e d f rom t h e e x t r a c t i o n

p r o c e s s i t s e l f . F o r c o r r e c t i n t e r p r e t a t i o n o f t h e check r e s u l t s , i t i s

recommended t h a t c o n s i d e r a t i o n be g i v e n t o h e a t t r a n s f e r i n two p a r t s o f t h e

e x t r a c t o r : A - c o s s e t t e s h e a t i n g z o n e ; Β - e x t r a c t i o n p r o c e s s zone (whe re t h e

t e m p e r a t u r e s o f j u i c e and c o s s e t t e s a r e e q u a l ) , as shown i n F i g . 7 . 9 . G d e n o t e s

t he mass f l o w i n k g / l O O kg b , t i s t h e t e m p e r a t u r e i n °C, and C i s t h e s p e c i f i c

hea t i n k J / ( k g K ) . The s u b s c r i p t s d e n o t e : b , c o s s e t t e s ; pw, p r e s s w a t e r ; w , f r e s h

w a t e r ; j , raw j u i c e ; p , p u l p ( e x h a u s t e d c o s s e t t e s ) ; m, j u i c e - c o s s e t t e s m i x t u r e

between zones A and B.

U s i n g t h e d e f i n i t i o n o f t h e e f f i c i e n c y o f h e a t exchange between j u i c e and

c o s s e t t e s a c c o r d i n g t o r e f . 43

Ε = 1 0 0 . G . C . ( t ^ - t j ) / ( G ( ^ C ^ , ( t ^ - t ( , ) ) (%) ( 7 . 3 2 )

268

G j C j t j Β

Τ"

Gw CvA/t V

GpCptp

F i g . 7 .9 . Heat b a l a n c e scheme o f t h e t r o u g h e x t r a c t o r . A - f r o n t p a r t , Β - r e a r p a r t . Fo r t h e e x p l a n a t i o n o f s y m b o l s , see t e x t .

we can e x p r e s s t he hea t demand f o r c o s s e t t e s h e a t i n g as

= ( (100 - E ) / E ) G j ^ C ^ ( t ^ - t ^ ) ( k J / 1 0 0 kg b) ( 7 . 3 3 )

T a k i n g i n t o a c c o u n t t h a t = 1 0 0 - G / a , where a i s t h e raw j u i c e d r a f t i n %, t he

r e l a t i o n s h i p s between e x t r a c t i o n pa rame te rs can be shown d i a g r a m m a t i c a l l y

( F i g . 7 . 1 0 ) . I t can e a s i l y be seen t h a t t he the rma l phenomena a re h i g h l y

dependen t on such p r o c e s s f a c t o r s a s :

- c o s s e t t e s q u a l i t y , as l o w e r q u a l i t y causes a d e c r e a s e d Ε v a l u e ;

80 h

S 60

c D O Ε O ^ A O O Οι

Χ

20 h

50

A O

L_ Ζ)

Ε ο. Ε Φ

Φ ο

30

20

10

5 10 Cossettes temperature (°C)

15

α = 115 α=120

α=110

^ Ε = 8 0 /

E = go

Ε = 70

Ε =80

Ε =90

Ε = 100

F i g . 7 .10 . R e l a t i o n s h i p between e x t r a c t i o n pa rame te rs i n t h e f r o n t p a r t o f t h e e x t r a c t o r ( a f t e r r e f . 4 3 ) . a - j u i c e d r a f t ( % ) , Ε - hea t exchange e f f i c i e n c y {%

269

- d e - a e r a t i o n o f t he j u i c e - c o s s e t t e s m i x t u r e , as i n s u f f i c i e n t d e - a e r a t i o n causes

Ε t o d e c r e a s e ;

- j u i c e d r a f t , as i t s i n c r e a s e i m m e d i a t e l y causes i n c r e a s e d h e a t consumpt ion i n

t he e x t r a c t o r ;

- s t a b i l i z a t i o n o f t he j u i c e l e v e l a t t he o u t l e t s c r e e n , as an i n c o r r e c t j u i c e

l e v e l may a l s o reduce t he v a l u e o f E .

The d iag ram a l s o d e m o n s t r a t e s t h a t c o r r e c t e x t r a c t o r o p e r a t i o n n e c e s s a r i l y

i n v o l v e s v a r i a b l e j u i c e t e m p e r a t u r e as a f u n c t i o n o f t h e c o s s e t t e s t e m p e r a t u r e .

I f t h e c o s s e t t e s t e m p e r a t u r e i s l o w , a l o w e r j u i c e t e m p e r a t u r e s h o u l d be

a c c e p t e d , w i t h s u b s e q u e n t j u i c e h e a t i n g i n t h e h e a t e r s b e f o r e t h e p u r i f i c a t i o n

s t a t i o n . The op t ima l t e m p e r a t u r e d i f f e r e n c e between j u i c e and c o s s e t t e s i s

10-15 K.

I n o r d e r t o a c h i e v e a comp le te e v a l u a t i o n o f t h e the rma l c o n d i t i o n s , t he

e x t r a c t o r check s h o u l d i f p o s s i b l e c o n s i s t o f measu r i ng a l l t he q u a n t i t i e s

i n c l u d e d i n t h e h e a t b a l a n c e ( s e e F i g . 7.9 and t h e e x p l a n a t i o n o f s y m b o l s ) . F o r

c o r r e c t i n t e r p r e t a t i o n o f t he t e s t r e s u l t s , t h e f o l l o w i n g d a t a a r e a l s o

n e c e s s a r y :

- t he c o s s e t t e s l e n g t h ;

- t h e j u i c e d r a f t ;

- comments on c o s s e t t e s q u a l i t y , d e - a e r a t i o n o f t h e j u i c e - c o s s e t t e s m i x t u r e and

t h e j u i c e l e v e l a t t h e o u t l e t s c r e e n .

Known v a l u e s o f t he i n w a r d and o u t w a r d mass f l o w s make i t p o s s i b l e t o

c a l c u l a t e t h e t h e o r e t i c a l hea t demand ( w i t h o u t l o s s e s t o t h e e n v i r o n m e n t ) f rom

t h e e n e r g y b a l a n c e e q u a t i o n

F o r hea t -economy m o n i t o r i n g p u r p o s e s , t h e r e a l h e a t consumpt ion w o u l d be

i n t e r e s t i n g , bu t t he measur ing equ ipment c o n v e n t i o n a l l y i n s t a l l e d a t t h e

e x t r a c t o r s does n o t a l l o w f o r such a measurement . I f a d d i t i o n a l equ ipment f o r

t h e d e t e r m i n a t i o n o f c o n d e n s a t e f l o w i s i n s t a l l e d , t h e n c o n d e n s a t e f l o w s ( e q u a l

t o v a p o u r f l o w s ) and Gg , i n kg/100 kg b , can be d e t e r m i n e d . P r o v i d i n g t h a t

t he v a p o u r t e m p e r a t u r e s t ^ and t g , and c o n d e n s a t e t e m p e r a t u r e s t ^ ^ and t ^ g , a r e

a l s o measured , i t becomes p o s s i b l e t o c a l c u l a t e t h e h e a t consumpt ion i n bo th

p a r t s o f t h e e x t r a c t o r

QA = S ^ ^ A " " ^ - ^ ^ ^ C A ) ^9 b) ( 7 . 3 5 )

Qß = Gg(hg - 4 . 1 9 t ^ g ) ( k J / 1 0 0 kg b) ( 7 . 3 6 )

where h^ and hg a r e t h e e n t h a l p i e s o f v a p o u r s i n k J / k g , a t t e m p e r a t u r e s t ^ and

t g , r e s p e c t i v e l y .

Depend ing on t h e c o s s e t t e s t e m p e r a t u r e , t h e v a p o u r consumpt ion i n P a r t A i s

1 .5-2.0 kg/100 kg b. I n P a r t B, a v a p o u r f l o w o f up t o 1 kg/100 kg b i s r e q u i r e d

270

w i t h o u t p r e s s - w a t e r h e a t i n g , o r somewhat l e s s i f t h e p r e s s w a t e r i s h e a t e d .

Tempera tu re m o n i t o r i n g a t t h e e x t r a c t o r must be r o u t i n e l y pe r f o rmed d u r i n g

t he e n t i r e o p e r a t i o n s . A h e a t b a l a n c e d e t e r m i n a t i o n , p o s s i b l y i n c l u d i n g

c o n d e n s a t e - f l o w measurements , may become n e c e s s a r y i f unusua l p rob lems o c c u r i n

e x t r a c t o r o p e r a t i o n .

7 .6 .4 J u i c e h e a t e r s

The hea t s u p p l y t o t he j u i c e o r o t h e r media i n t h e h e a t e r s i s aimed a t

a t t a i n i n g t he t e m p e r a t u r e needed f o r such u n i t o p e r a t i o n s as l i m i n g ,

c a r b o n a t a t i o n , f i l t r a t i o n , e t c . A t t e n t i o n s h o u l d be d i r e c t e d , h o w e v e r , n o t o n l y

t o t he f u n c t i o n i n g o f each p a r t i c u l a r h e a t e r , b u t a l s o t o t h e o p e r a t i o n o f a l l

h e a t e r s v i e w e d as an i m p o r t a n t p a r t o f t h e hea t economy. From t h a t p o i n t o f

v i e w , hea t economy m o n i t o r i n g s h o u l d compr i se c h e c k i n g t h e t e m p e r a t u r e s o f t h e

media f o r t h e i r c o r r e s p o n d e n c e w i t h p r o c e s s r e q u i r e m e n t s , as w e l l as i n s p e c t i n g

t he s u i t a b i l i t y o f t h e d i s t r i b u t i o n o f h e a t i n g media ( c o n d e n s a t e and v a p o u r s

f rom the e v a p o r a t o r and vacuum p a n s ) . The e s s e n t i a l p r i n c i p l e o f h e a t - s a v i n g

media d i s t r i b u t i o n i s t o s u p p l y t h e h e a t e r w i t h a medium a t a t e m p e r a t u r e h i g h

enough t o hea t t h e j u i c e , b u t n o t t o o h i g h . Depend ing on h e a t e r d e s i g n , hea t

t r a n s f e r a r e a and j u i c e v e l o c i t y , t h e v a p o u r t e m p e r a t u r e s h o u l d be n o t h i g h e r

than 5-10 Κ above t h e f i n a l j u i c e t e m p e r a t u r e ( s e e a l s o S e c t i o n 3 .3 .2 on

u t i l i z a t i o n o f v a p o u r s ) .

R e g a r d i n g i n d i v i d u a l h e a t e r s , t h e y s h o u l d be t h o r o u g h l y i n s p e c t e d f o r t h e i r

f u n c t i o n i n g as hea t r e c e i v e r s . I n t h e case o f v a p o u r h e a t i n g , t h e c o n d e n s a t e

l e v e l i n t h e h e a t i n g chamber i s p a r t i c u l a r l y i m p o r t a n t ( s e e a l s o S e c t i o n 3.2.1

on condensa te d r a i n a g e ) . The c o n d e n s a t e l e v e l s h o u l d n o t be a l l o w e d t o r i s e

above t h e o u t l e t n o z z l e s , as t h i s w o u l d r e d u c e t h e e f f e c t i v e o v e r a l l hea t

t r a n s f e r c o e f f i c i e n t , making i t more d i f f i c u l t t o a t t a i n t h e p r o p e r j u i c e

t e m p e r a t u r e . S i m i l a r p rob lems o c c u r i f h e a t t r a n s f e r i s hampered by t h e

a c c u m u l a t i o n o f n o n c o n d e n s a b l e s ; t h e h e a t e r check s h o u l d t h u s i n c l u d e i n s p e c t i o n

o f t h e v e n t s ( s e e S e c t i o n 3 . 2 . 2 ) . The h e a t t r a n s f e r i n t e n s i t y can a l s o be

reduced by s c a l e b u i l d - u p i n t h e h e a t i n g t u b e s ; t h i s i s a n o t h e r p rob lem t o be

a c c o u n t e d f o r i n t he i n s p e c t i o n .

I n o r d e r t o make c a l c u l a t i o n s o f h e a t b a l a n c e p o s s i b l e , t h e f o l l o w i n g

measurements a r e r e q u i r e d :

- j u i c e mass f l o w , G (kg /100 kg b ) ;

- j u i c e i n l e t t e m p e r a t u r e , t^ ( ^ C ) ;

- j u i c e o u t l e t t e m p e r a t u r e , t ^ ( ° C ) ;

- j u i c e c o n c e n t r a t i o n , b (% D S ) ;

- v a p o u r o r condensa te ( s e e b e l o w ) mass f l o w , G^ (kg /100 kg b ) ;

- v a p o u r t e m p e r a t u r e , t ^ , and c o n d e n s a t e t e m p e r a t u r e , t ^ ( °C) ( o r , i n t h e case

271

o f condensa te h e a t i n g , i n i t i a l and f i n a l t e m p e r a t u r e s , t ^ - j , t ^ ^ ( ° ^ ) »

condensa te l e v e l i n t he h e a t i n g chamber s h o u l d be wa tched a t t h e same t i m e ) .

The measurements s h o u l d be pe r f o rmed d u r i n g s t e a d y - s t a t e h e a t e r o p e r a t i o n ,

p o s s i b l y a t t he j u i c e f l o w c o r r e s p o n d i n g t o t he nominal p r o c e s s i n g c a p a b i l i t y o f

t he f a c t o r y . The recommended t e s t d u r a t i o n i s a t l e a s t 1-2 h o u r s , w i t h an

i n s t r u m e n t - r e a d i n g f r e q u e n c y o f 5-10 m i n u t e s . A f t e r mean pa ramete r v a l u e s have

been c a l c u l a t e d , t he hea t t r a n s f e r i n t e n s i t y s h o u l d be e v a l u a t e d f i r s t . I f t he

c o r r e c t v a l u e o f t h e v a p o u r ( o r i n l e t c o n d e n s a t e ) t e m p e r a t u r e i s accompanied by

an i n s u f f i c i e n t j u i c e t e m p e r a t u r e i n c r e a s e ( w i t h p r o p e r condensa te d r a i n a g e and

v e n t i n g ) , t hen s c a l i n g on t he h e a t i n g s u r f a c e s may be s u s p e c t e d .

A t known j u i c e - m a s s f l o w , G , t he hea t consumed can be c a l c u l a t e d as

Q = G C ( t 2 - t ^ ) ( l + n ) ( k J / 1 0 0 kg b ) ( 7 . 3 7 )

where the s p e c i f i c hea t o f t he j u i c e , C ( k J / ( k g K ) ) , can be f ound i n t a b l e s o r

c a l c u l a t e d as a f u n c t i o n o f t h e j u i c e c o n c e n t r a t i o n , b, and η i s t h e hea t l o s s

c o e f f i c i e n t .

On v a p o u r h e a t i n g , hea t consumpt ion on t h e v a p o u r s i d e can be d e t e r m i n e d o n l y

i f p e r f e c t o p e r a t i o n o f t h e steam t r a p s i s e n s u r e d ( t h a t i s , o n l y i f steam

leakage i n t h e condensa te s t ream i s e l i m i n a t e d ) . The condensa te f l o w can be

measured by a v o l u m e t r i c method , by c a t c h i n g t he c o n d e n s a t e i n a s p e c i a l

c o n t a i n e r . The h e a t consumpt ion can t hen be c a l c u l a t e d as

Q = G ^ ( h " - 4 . 1 9 t ^ ) ( k J / 1 0 0 kg b) ( 7 . 3 8 )

where h" d e n o t e s v a p o u r e n t h a l p y i n k J / k g . I n t h e case o f condensa te h e a t i n g , t he c o r r e s p o n d i n g f o r m u l a i s

Q = G ^ - 4 . 1 9 ( t ^ ^ - t ^ 2 ) kg b) ( 7 . 3 9 )

As t o t h e f r e q u e n c y o f h e a t e r check measurements , an i n s p e c t i o n o f

t e m p e r a t u r e s b e f o r e most i m p o r t a n t u n i t o p e r a t i o n s s h o u l d be p e r f o r m e d r o u t i n e l y

d u r i n g t he e n t i r e s e a s o n . Any i r r e g u l a r i t i e s n o t i c e d i n t he d i s t r i b u t i o n o f

h e a t i n g media t o t h e i n d i v i d u a l h e a t e r s must i m m e d i a t e l y be c o r r e c t e d . D e t a i l e d

i n v e s t i g a t i o n s o f h e a t e r o p e r a t i o n may be r e q u i r e d i f d i f f i c u l t i e s o c c u r i n t h e

h e a t i n g o f p r o c e s s med ia .

7 .6 .5 Ba tch vacuum pans

Sugar b o i l i n g i s a d e c i s i v e o p e r a t i o n f o r s u g a r y i e l d and s u g a r q u a l i t y . The

b o i l i n g t i m e , combined w i t h i n s t a l l e d pan v a p a c i t y , can be a l i m i t a t i o n t o s u g a r

f a c t o r y p r o c e s s i n g c a p a b i l i t y . I n a d d i t i o n , t h e h e a t consumpt ion f o r s u g a r

b o i l i n g may i n v o l v e 40-50% o f t he t o t a l p r o c e s s - h e a t c o n s u m p t i o n . As a

c o n s e q u e n c e , i n v e s t i g a t i o n s o f hea t consumpt ion i n vacuum pans must be ser: as

one o f t he most s e r i o u s t a s k s i n hea t -economy m o n i t o r i n g .

I n p r a c t i c e , a hea t economy check i n t h e s u g a r house canno t be s e p a r a t e d f rom

272

a t e c h n o l o g i c a l check . The r e a s o n i s t h a t t he h e a t consumpt ion f o r s u g a r b o i l i n g

depends on t h e amounts and c o n c e n t r a t i o n s o f t h i c k j u i c e and r e m e l t , as w e l l as

on t he amount o f w a t e r i n t r o d u c e d i n t o t h e s u g a r house v i a such o p e r a t i o n s as

d i l u t i o n o f s y r u p s , wash ing and i n t a k e s t o vacuum p a n s .

The t o t a l consumpt ion o f v a p o u r s f o r s u g a r b o i l i n g i n a vacuum pan ( o r i n

a g roup o f vacuum pans ) can be e x p r e s s e d as t h e sum

= + G ^ (kg /100 kg b ) ( 7 . 4 0 )

where G^ i s t he v a p o u r consumpt ion f o r b o i l i n g s u g a r s o l u t i o n s , and G ^ i s t h e

v a p o u r consumpt ion f o r t h e e v a p o r a t i o n o f a d d i t i o n a l w a t e r f rom i n t a k e s . Both

q u a n t i t i e s depend on t e c h n o l o g i c a l pa rame te rs a c c o r d i n g t o t he f o r m u l a e ( r e f . 8)

G ^ = u M ^ Í C g í t ^ - t ^ ) + (1 - b ^ / b ^ ) ( h ^ - h ) ) / ( h ^ - h^) ( kg /100 kg b ) ( 7 . 4 1 )

G ^ = W(h^ - h ) / ( h ^ - h^) (kg /100 kg b) ( 7 . 4 2 )

where u i s t h e l o s s c o e f f i c i e n t ( 1 . 1 - 1 . 2 ) , i s t h e t o t a l amount o f s u g a r

s o l u t i o n s s u p p l i e d t o t he vacuum pan i n kg/100 kg b , bj^ i s t h e f i n a l

c o n c e n t r a t i o n o f m a s s e c u i t e i n % DS, i s t he mean s p e c i f i c h e a t o f t h e

s o l u t i o n s i n k J / ( k g K ) , t|^ i s t h e b o i l i n g t e m p e r a t u r e i n °C, t ^ i s t h e mean

i n i t i a l t e m p e r a t u r e o f t h e s o l u t i o n s i n °C, h , i s t h e e n t h a l p y o f t h e vacuum-pan w

v a p o u r , h i s t h e e n t h a l p y o f w a t e r a t t e m p e r a t u r e t ^ , h^ i s t h e e n t h a l p y o f t h e

h e a t i n g v a p o u r , h^ i s t he condensa te e n t h a l p y ( a l l e n t h a l p i e s i n k J / k g ) , and W

i s t he amount o f w a t e r f rom i n t a k e s , i n kg/100 kg b.

The above r e l a t i o n s h i p s p r o v e t h a t t h e most i m p o r t a n t f a c t o r i n o b t a i n i n g

hea t s a v i n g s i n s u g a r b o i l i n g i s adhe rence t o t h e t e c h n o l o g i c a l g u i d e l i n e s . I t

i s p a r t i c u l a r l y i m p o r t a n t t o m a i n t a i n h i g h v a l u e s i n t h e c o n c e n t r a t i o n s o f t h i c k

j u i c e , s y r u p s and r e m e l t . S h o u l d any o f t h e c o n c e n t r a t i o n s d e c r e a s e , t h e

r e s u l t i n g d e c r e a s e i n b^ v a l u e wou ld r e s u l t i n an i n c r e a s e d v a l u e o f v a p o u r

c o n s u m p t i o n , G ^ . Water i n t a k e s t o vacuum pans s h o u l d be r e d u c e d t o a minimum; o t h e r w i s e , t h e f a c t o r G wou ld cause an u n n e c e s s a r y i n c r e a s e i n v a p o u r

w

c o n s u m p t i o n . On t h e o t h e r h a n d , t h e vacuum i n t he pans i s a l s o i m p o r t a n t , as

t h i s i s t h e e s s e n t i a l f a c t o r i n e n s u r i n g i n t e n s i v e h e a t t r a n s f e r , w h i c h i s

n e c e s s a r y t o m a i n t a i n s h o r t b o i l i n g t imes w i t h o u t i n c r e a s i n g e n e r g y e x p e n d i t u r e .

As t o t he i n d i v i d u a l vacuum p a n s , t h e i r o p e r a t i o n as hea t r e c e i v e r s s h o u l d

be t h o r o u g h l y i n v e s t i g a t e d . The most i m p o r t a n t p o i n t s a r e :

- condensa te d r a i n a g e f rom t h e h e a t i n g chamber ;

- v e n t i n g o f t he h e a t i n g chamber ;

- e l i m i n a t i o n o f i n c r u s t a t i o n s f rom the hea t t r a n s f e r s u r f a c e .

The measurements needed f o r m o n i t o r i n g p u r p o s e s a r e :

- b o i l i n g t e m p e r a t u r e , t j ^ ;

- v a p o u r p r e s s u r e , Pj^;

- h e a t i n g - v a p o u r t e m p e r a t u r e , t ^ ;

273

- condensa te t e m p e r a t u r e , t ^ .

V a l u e s o f t he above pa rame te rs e n a b l e u s , e v e n t u a l l y , t o d e t e r m i n e t h e e n t h a l p y

v a l u e s a p p e a r i n g i n e q n s . ( 7 . 3 9 ) and ( 7 . 4 0 ) . ( I t s h o u l d be p o i n t e d o u t t h a t

p e r f o r m i n g t he measurements e n t a i l s ove rcoming s p e c i f i c o b s t a c l e s r e l a t e d t o

suga r b o i l i n g ( r e f . 1 6 ) . ) A t t h e end o f t he s t r i k e , t h e amount o f m a s s e c u i t e M^,

and t he f i n a l c o n c e n t r a t i o n b|^, become known.

The measurements s h o u l d compr i se a t l e a s t two s t r i k e s . The f r e q u e n c y o f

i n s t r u m e n t r e a d i n g s s h o u l d be adap ted t o t h e s t r i k e d u r a t i o n ( a t l e a s t 10-15

r e a d i n g s ) .

U s i n g t h e e s t i m a t e d mean v a l u e o f t h e c o n c e n t r a t i o n o f s u g a r s o l u t i o n s

s u p p l i e d t o t h e vacuum p a n , t h e amount o f w a t e r e v a p o r a t e d f rom t h e s o l u t i o n s

can be c a l c u l a t e d as

= M ^ ( b . / b ^ - 1) ( kg /100 kg b) ( 7 . 4 3 )

D i r e c t measurement o f t he t o t a l v a p o u r consumpt ion i s o f t e n i m p o s s i b l e f o r

l a c k o f f l o w me te rs a t i n d i v i d u a l vacuum p a n s . P r o v i d i n g t h a t c o n d e n s a t e

d r a i n a g e and v e n t i n g do n o t cause v a p o u r l e a k s , v o l u m e t r i c d e t e r m i n a t i o n o f t h e

condensa te amount, equa l t o G ^ , can p o s s i b l y be u s e d . Condensa te f l o w can a l s o

be measured w i t h t h e a i d o f an i n d u c t i v e f l o w meter i n s t a l l e d as shown

s c h e m a t i c a l l y i n F i g . 7. 11. A c o n d e n s a t e tank e q u i p p e d w i t h a w a t e r - l e v e l

i n d i c a t o r and a s e p a r a t e s i g h t g l a s s make i t p o s s i b l e t o check t h e i n t e g r a l o f

t he f l o w meter r e c o r d s , and t o check t h e c o n d i t i o n s f o r c o r r e c t measurement

( l a r g e movements o f t he condensa te column must be a v o i d e d ) .

Wi th p r o p e r vacuum-pan o p e r a t i o n , t h e v a p o u r consumpt ion d u r i n g one s t r i k e

F i g . 7 .11. Measurement o f t he c o n d e n s a t e f l o w f rom t h e h e a t i n g chamber o f a vacuum pan ( a f t e r r e f . 4 4 ) . 1 - vacuum p a n , 2 - c o n d e n s a t e tank w i t h a w a t e r -l e v e l i n d i c a t o r , 3 - f l o w m e t e r , 4 - s i g h t g l a s s , 5 - p r e s s u r e - b a l a n c i n g p i p e .

274

s h o u l d n o t exceed t he v a l u e

G. = ( 1 . 0 2 t o 1.15)W^ ( k g / l O O kg b ) ( 7 . 4 4 ) L e

I n r e f . 6, t h e upper l i m i t o f 1.20Wg i s m e n t i o n e d , b u t t h i s seems t o be

i n a p p r o p r i a t e i n t h e s e days o f e n e r g y s h o r t a g e s .

A v e r y u s e f u l a d d i t i o n t o t h e m o n i t o r i n g t e c h n i q u e s d e s c r i b e d above i s t h e

measurement o f t o t a l v a p o u r consumpt ion i n vacuum p a n s . T h i s r e q u i r e s

i n s t a l l a t i o n o f a f l o w meter on t h e main p i p e l i n e s u p p l y i n g v a p o u r t o t h e p a n s .

Even though t h i s method i s n o t e n t i r e l y a c c u r a t e when a p p l i e d t o s a t u r a t e d

s team, i t g i v e s a much-needed p r a c t i c a l o r i e n t a t i o n . F o r r o u t i n e vacuum-pan

c h e c k s , a c t u a l f l o w v a l u e s and f l o w o s c i l l a t i o n s can be e s t i m a t e d , w h i l e i n

p e r i o d i c c h e c k s , t h e o v e r a l l l e v e l o f v a p o u r consumpt ion and abnormal

consumpt ion jumps can be e v a l u a t e d and a n a l y s e d . I n p r i n c i p l e , i f t h e

c o n f i g u r a t i o n o f t he s u p p l y p i p e s a l l o w s , v a p o u r - f l o w measurement equ ipment

c o u l d be i n s t a l l e d a t each i n d i v i d u a l p a n .

7 .6 .6 O t h e r hea t r e c e i v e r s

I n a d d i t i o n t o t he main h e a t r e c e i v e r s d i s c u s s e d a b o v e , t h e r e a r e numerous

smal l h e a t r e c e i v e r s i n a s u g a r p l a n t : t h e s u g a r d r y e r , m e l t e r , hea ted s t o r a g e

t a n k s ( c o n t a i n i n g t h i c k j u i c e , r e m e l t , s y r u p s , m o l a s s e s , e t c . ) , n o z z l e sys tems

f o r wash ing and s t e a m i n g , and mo lasses pumping s y s t e m . I f u s e d , cube s u g a r

p r o d u c t i o n and p u l p p e l l e t i n g a l s o c o n t r i b u t e t o hea t c o n s u m p t i o n .

As each o f t h e i n d i v i d u a l hea t r e c e i v e r s men t ioned above i s r e s p o n s i b l e f o r

a v e r y sma l l p o r t i o n o f t h e t o t a l h e a t c o n s u m p t i o n , t h e y a r e u s u a l l y p o o r l y

s u p e r v i s e d o r even t r e a t e d m a r g i n a l l y . N e v e r t h e l e s s , t h e y d e s e r v e more a t t e n t i o n ,

as t h e i r combined hea t consumpt ion may c o n s t i t u t e a c o n s i d e r a b l e p o r t i o n o f t h e

t o t a l .

The e s s e n t i a l p rob lem i n t he m o n i t o r i n g o f smal l hea t r e c e i v e r s i s t h e

d e t e c t i o n o f e x c e s s i v e - t h a t i s , o u t o f a l l p r o p o r t i o n t o p r o c e s s

r e q u i r e m e n t s - consumpt ion o f steam o r v a p o u r s . Even i f t h e equ ipment i s

p r o p e r l y m a i n t a i n e d , t h i s may happen because o f o p e r a t o r e r r o r s ; t y p i c a l c a s e s

a r e s u g a r wash ing and t he s t e a m i n g - o u t o f vacuum p a n s .

App rox ima te v a l u e s o f the rma l pa rame te rs and t he consumpt ion o f h e a t i n g

media i n v a r i o u s u n i t s a r e g i v e n i n T a b l e 7 .8 . The o p e r a t i o n o f t h e s u g a r d r y e r

r e q u i r e s t h a t some p r o c e s s pa rame te rs a re a d d i t i o n a l l y measu red . I t i s

recommended t h a t t h e m o i s t u r e c o n t e n t i n s u g a r f rom c e n t r i f u g a l s be m a i n t a i n e d

a t 0 .5-1 .5%, t h e a i r f l o w a t 5-15 kg/100 kg b , and t h e a i r t e m p e r a t u r e a t

110-120°C.

Smal l hea t r e c e i v e r s s h o u l d be examined a t t h e b e g i n n i n g o f t h e s e a s o n , and

a l s o f o l l o w i n g t e m p o r a r y s h u t - d o w n s o r equ ipment r e p a i r s . I n t h e case o f

equ ipment where hea t consumpt ion depends on o p e r a t o r q u a l i f i c a t i o n s , a d d i t i o n a l

275

TABLE 7.8

H e a t i n g o f v a r i o u s smal l h e a t r e c e i v e r s .

Equipment H e a t i n g medium Consumpt ion ( kg /100 kg b )

Sugar d r y e r e x h a u s t s team, I s t - e f f e c t v a p o u r be low 1.0

S t o r a g e t a n k s ( comb ined) 2 n d - e f f e c t v a p o u r be low 1.5 M e l t e r 2 n d - e f f e c t v a p o u r 0 . 2 - 0 . 3 Sugar wash ing steam 5 b a r , 200°C 1 .5-2 .0 S t e a m i n g - o u t o f vacuum pans e x h a u s t s team,

1 s t - o r 2 n d - e f f e c t v a p o u r be low 1.5 Pu lp p e l l e t i n g steam 5 ba r be low 0.2

s p o t checks a r e recommended ( t h i s a p p l i e s t o s u g a r wash ing i n c e n t r i f u g a l s and

t o t h e s t e a m i n g - o u t o f vacuum pans and p i p e l i n e s ) .

7.7 AUXIL IARY EQUIPMENT

7.7.1 T h r o t t l i n g - d e s u p e r h e a t i n g s t a t i o n

The t h r o t t l i n g - d e s u p e r h e a t i n g s t a t i o n s e r v e s t o r educe t h e steam p r e s s u r e and

d e c r e a s e i t s t e m p e r a t u r e so t h a t c e r t a i n d e f i n i t e v a l u e s o f bo th p a r a m e t e r s a r e

a t t a i n e d . I n the rma l s y s t e m s , such s t a t i o n s a r e a p p l i e d t o t r a n s f o r m l i v e steam

i n t o l o w - p r e s s u r e s team, t o supp lemen t t h e t u r b i n e - e x h a u s t s team, o r t h e

i n t e r m e d i a t e - p r e s s u r e steam ( u s u a l l y abou t 5 b a r ) t o be used i n c e n t r i f u g a l s .

The t h r o t t l i n g - d e s u p e r h e a t i n g p r o c e s s i s shown i n t h e M o l l i e r d i ag ram i n

F i g . 7 .12 . The p r e s s u r e r e d u c t i o n c o n s i s t s o f t h r o t t l i n g t h e steam f l o w i n a

r e d u c i n g v a l v e a t c o n s t a n t e n t h a l p y (segment A B ) . The t e m p e r a t u r e i s d e c r e a s e d

by i n j e c t i n g w a t e r a t s t a t e D i n such an amount ( a u t o m a t i c a l l y c o n t r o l l e d ) t h a t

a f t e r m i x i n g i t w i t h steam a t s t a t e B, t h e f i n a l s t a t e o f t h e m i x t u r e C i s c l o s e

t o s a t u r a t i o n . I n p r a c t i c e , steam a t t h e o u t l e t o f t h e t h r o t t l i n g - d e s u p e r h e a t i n g

s t a t i o n may be s l i g h t l y s u p e r h e a t e d , t h a t i s , i t s t e m p e r a t u r e may be 30-40 Κ

above t h e s a t u r a t i o n t e m p e r a t u r e .

F o r t h e e n t i r e the rma l s y s t e m , t h e f u n c t i o n i n g o f t h e t h r o t t l i n g -

d e s u p e r h e a t i n g s t a t i o n t h a t s u p p l i e s make-up steam t o t h e e x h a u s t steam i s o f

p a r t i c u l a r i m p o r t a n c e . The demand f o r e x h a u s t steam f l u c t u a t e s , f o l l o w i n g t h e

changes i n e v a p o r a t o r l o a d s . The t h r o t t l i n g - d e s u p e r h e a t i n g s t a t i o n s h o u l d

compensate f o r t h e s e f l u c t u a t i o n s , so t h a t c o n s t a n t p r e s s u r e i s m a i n t a i n e d a t

t h e i n l e t t o t h e h e a t i n g chamber o f t h e f i r s t e v a p o r a t o r e f f e c t .

The check o f t h e t h r o t t l i n g - d e s u p e r h e a t i n g s t a t i o n s h o u l d be aimed a t

v e r i f i c a t i o n o f t h e p r o c e s s p a r a m e t e r s . I n a d d i t i o n t o t h e mean pa ramete r

v a l u e s , f l u c t u a t i o n s o f a c t u a l v a l u e s a r e a l s o i m p o r t a n t . D u r i n g a t e s t o f a t

l e a s t 1-2 h o u r s , t h e f o l l o w i n g i n s t r u m e n t r e a d i n g s a r e r e q u i r e d e v e r y 5 m i n u t e s :

- l i v e steam p r e s s u r e and t e m p e r a t u r e ;

276

Entropy

F i g . 7 .12 . Work ing p r i n c i p l e o f t he t h r o t t l i n g - d e s u p e r h e a t i n g s t a t i o n . SC - s a t u r a t i o n c u r v e , p^ - p r e s s u r e o f l i v e s team, pg - p r e s s u r e o f make-up s team; A - s t a t e o f steam a t i n l e t , Β - s t a t e o f steam a f t e r r e d u c i n g v a l v e , C - s t a t e o f steam a t o u t l e t , D - s t a t e o f w a t e r b e f o r e d e s u p e r h e a t e r .

- steam p r e s s u r e a f t e r t he r e d u c i n g v a l v e ;

- steam p r e s s u r e and t e m p e r a t u r e a f t e r t h e d e s u p e r h e a t e r ;

- p r e s s u r e and t e m p e r a t u r e o f w a t e r s u p p l i e d t o t h e d e s u p e r h e a t e r .

F l u c t u a t i o n s o f steam p r e s s u r e can be e s t i m a t e d s i m p l y by i n c r e a s i n g t h e

f r e q u e n c y o f t he i n s t r u m e n t r e a d i n g s , f o r example t o e v e r y m i n u t e . T e m p e r a t u r e

f l u c t u a t i o n s can be e v a l u a t e d o n l y r o u g h l y , as t h e l ag o f i n d u s t r i a l

thermometers i s t o o l a r g e . The r e s u l t s s h o u l d be e v a l u a t e d a g a i n s t t he f o l l o w i n g

g u i d e l i n e s :

- a t t he mean i n l e t - p r e s s u r e v a l u e , c o n s t r a i n e d w i t h i n t he r e g i o n ±5% a round t h e

nominal v a l u e , t h e t o l e r a n c e o f t h e mean v a l u e o f t he r e d u c e d p r e s s u r e i s a l s o

- v e r y good r e d u c e d - p r e s s u r e s t a b i l i z a t i o n means t h a t p r e s s u r e f l u c t u a t i o n s do

n o t exceed ±2% o f i t s mean v a l u e . Howeve r , l a r g e r f l u c t u a t i o n s can be a l l o w e d ,

p r o v i d e d t h a t d i s t u r b a n c e s i n t h e sys tem o p e r a t i o n ( e s p e c i a l l y t he t u r b i n e ) a r e

a v o i d e d ;

- t he steam t e m p e r a t u r e a f t e r t he d e s u p e r h e a t i n g s h o u l d be h i g h e r than t h e

s a t u r a t i o n t e m p e r a t u r e , because t h i s i s t he o n l y way t o e l i m i n a t e t h e

u n c o n t r o l l a b l e p r e s e n c e o f e x c e s s w a t e r . Howeve r , s u p e r h e a t i n g s h o u l d n o t

exceed 40 K.

I n o r d e r t o i n t e r p r e t t e s t r e s u l t s c o r r e c t l y , a t t e n t i o n s h o u l d be g i v e n t o

t he o p e r a t i n g c o n d i t i o n s . T h e r e a r e a t l e a s t t h r e e r e q u i r e m e n t s f o r normal

277

o p e r a t i o n o f t h e t h r o t t l i n g - d e s u p e r h e a t i n g s t a t i o n ; name ly , l o a d f l u c t u a t i o n s

s h o u l d be no more i n t e n s i v e than under a v e r a g e o p e r a t i n g c o n d i t i o n s , t h e b o i l e r

l o a d s h o u l d be r e a s o n a b l y l o w e r t han i t s maximum c a p a c i t y , and t h e p r e s s u r e o f

w a t e r s u p p l i e d t o t h e d e s u p e r h e a t e r s h o u l d be a t l e a s t 4 b a r h i g h e r t han t h e

reduced steam p r e s s u r e .

I t i s recommended t h a t t h e t h r o t t l i n g - d e s u p e r h e a t i n g s t a t i o n i s checked a t

t he b e g i n n i n g o f t h e season and t h e n p e r i o d i c a l l y e v e r y t e n days o r two weeks .

7 .7 .2 B a r o m e t r i c c o n d e n s e r s

The v a p o u r - c o n d e n s i n g s t a t i o n p l a y s an i m p o r t a n t r o l e , as i t i n f l u e n c e s such

i m p o r t a n t p r o c e s s e s as s u g a r b o i l i n g , w h i l e a l s o i n t e r a c t i n g w i t h t h e the rma l

sys tem because i t a b s o r b s l a r g e amounts o f hea t t o be d i s s i p a t e d t o t h e

e n v i r o n m e n t . An i m p o r t a n t p r o c e s s r e q u i r e m e n t i s t h a t t he c o n d e n s e r s s h o u l d

e n s u r e t he c o n s t a n t low p r e s s u r e ( h i g h vacuum) t h a t i s n e c e s s a r y t o o b t a i n low

b o i l i n g t e m p e r a t u r e s and r a p i d t h i c k e n i n g o f s u g a r s o l u t i o n s i n t he b a t c h vacuum

p a n s . A therma l r e q u i r e m e n t i s t h a t t he t e m p e r a t u r e o f b a r o m e t r i c w a t e r s h o u l d

be as low as p o s s i b l e , bu t h i g h enough t o e n s u r e r a p i d hea t d i s s i p a t i o n i n t h e

c o o l i n g t o w e r s . I n d o u b l e - s t a g e c o n d e n s i n g s y s t e m s , i t may be i m p o r t a n t t o g e t

w a t e r f rom t h e f i r s t s t a g e w i t h a t e m p e r a t u r e s u f f i c i e n t l y h i g h t o s e r v e as f e e d

w a t e r f o r t h e e x t r a c t i o n p r o c e s s . S i m u l t a n e o u s l y , t h e o p e r a t i o n o f t h e v a p o u r -

condens ing s t a t i o n i s dependen t on e f f i c i e n t e v a c u a t i o n o f n o n c o n d e n s a b l e s by

vacuum pumps and a l s o on hea t d i s s i p a t i o n f rom t h e c o o l i n g t o w e r s .

The e s s e n t i a l p rob lem i n c o n d e n s e r i n s p e c t i o n i s t o d e t e r m i n e t h e c o o l i n g -

w a t e r consumpt ion w h i l e c o l l e c t i n g i n f o r m a t i o n on t h e c o n d e n s a t i o n p r o c e s s . T h i s

makes i t p o s s i b l e t o check w h e t h e r p r o c e s s r e q u i r e m e n t s a r e s a t i s f i e d , and

whe the r t he w a t e r consumpt ion can be d e c r e a s e d . The recommended c o n d e n s e r - t e s t

d u r a t i o n i s 4-6 h o u r s . The f o l l o w i n g measurements s h o u l d be r e c o r d e d e v e r y 20-30

m i n u t e s :

- t h e vacuum i n t h e condense r a t t h e v a p o u r i n l e t n o z z l e l e v e l ;

- t h e vacuum i n t h e upper p a r t o f t h e c o n d e n s e r above t h e c o o l i n g w a t e r i n l e t ;

- t h e t e m p e r a t u r e o f t he v a p o u r s s u p p l i e d t o t h e c o n d e n s e r ;

- t he mass f l o w o f c o o l i n g w a t e r ;

- t he t e m p e r a t u r e o f t he c o o l i n g w a t e r b e f o r e t h e c o n d e n s e r ;

- t he t e m p e r a t u r e o f t h e b a r o m e t r i c w a t e r a t t he c o n d e n s e r o u t l e t ( a t bo th

o u t l e t s i n a d o u b l e - s t a g e c o n d e n s i n g s t a t i o n ) .

T e s t r e s u l t s s h o u l d be i n t e r p r e t e d a g a i n s t t h e f o l l o w i n g g u i d e l i n e s :

- t h e d e s i r a b l e vacuum i n t h e c o n d e n s e r c e n t r e i s abou t 0.8 b a r (600 mm H g ) , as

a g a i n s t 0.88 ba r (660 mm Hg) i n i t s upper p a r t ;

- t h e d e s i r a b l e t e m p e r a t u r e o f t h e b a r o m e t r i c w a t e r i n a s i n g l e - s t a g e c o n d e n s e r

i s 45-50°C;

278

- i n a d o u b l e - s t a g e c o n d e n s e r , t h e t e m p e r a t u r e o f t he b a r o m e t r i c w a t e r s h o u l d

be 60-65°C i n t he f i r s t s t a g e and 40-45°C i n t h e second s t a g e ;

- t he d i f f e r e n c e between t h e s a t u r a t i o n t e m p e r a t u r e a t a c t u a l c o n d e n s e r p r e s s u r e

and t he b a r o m e t r i c w a t e r t e m p e r a t u r e s h o u l d n o t e x c e e d 5 K.

O p e r a t i n g c o n d i t i o n s a t t he t ime o f t he t e s t s h o u l d be a c c o u n t e d f o r i n t h e

e v a l u a t i o n o f t e s t r e s u l t s . The e f f i c i e n c y o f t he vacuum pumps and t h e

t e m p e r a t u r e o f t he c o o l i n g w a t e r a r e e s p e c i a l l y i m p o r t a n t , as w e l l as v a p o u r -

f l o w f l u c t u a t i o n s , because a t e m p o r a r y f l o w i n c r e a s e may i n d u c e l o w e r e d vacuum.

Under normal f a c t o r y - o p e r a t i n g c o n d i t i o n s , p o s s i b l e d i f f i c u l t i e s i n

m a i n t a i n i n g a p p r o p r i a t e vacuum, as w e l l as t o o l a r g e a d i f f e r e n c e between

s a t u r a t i o n and b a r o m e t r i c - w a t e r t e m p e r a t u r e s , i n d i c a t e t h a t t he condense r i s

o v e r l o a d e d by e x c e s s i v e v a p o u r f l o w . I n t h i s c a s e , t he f l o w v e l o c i t y o f t h e

v a p o u r s i n t h e f r e e - f l o w c r o s s - s e c t i o n a r e a o f t h e c o n d e n s e r s h o u l d be v e r i f i e d .

The a p p r o x i m a t e v a l u e ( n e g l e c t i n g h e a t l o s s e s t o t h e e n v i r o n m e n t ) o f t h e v a p o u r

mass f l o w , G^, can be c a l c u l a t e d f rom the e q u a t i o n s o f c o n d e n s e r mass and

e n e r g y b a l a n c e s

+ = G , (7.45)

where G. i s t he unknown mass f l o w o f b a r o m e t r i c w a t e r i n k g / h , G, i s t h e known

D W mass f l o w o f c o o l i n g w a t e r i n k g / h , and h ^ , h^ and hj^ a re e n t h a l p i e s i n k J / k g ,

w h i c h can be d e t e r m i n e d as f u n c t i o n s o f known t e m p e r a t u r e s t ^ , t ^ and t ^ . Upon

d e t e r m i n a t i o n o f t h e v a p o u r mass f l o w

^ = • - \ ^ ( k g / h ) (7.47)

t he f l o w v e l o c i t y can be c a l c u l a t e d as

w = G^vy((TTd2/4)-3600) ( m / s ) (7.48) where v ^ i s t he s p e c i f i c v o l u m e , i n m'^/kg, o f d r y s a t u r a t e d steam a t t e m p e r a t u r e

t ^ , and d i s t h e i n n e r d i ame te r o f t h e condense r b o d y , i n m. The f l o w v e l o c i t y

s h o u l d n o t exceed 50-60 m/s .

I t i s recommended t h a t t h e c o n d e n s e r s a r e checked a t t he b e g i n n i n g o f t h e

s e a s o n ; a f t e r t h a t , i n s p e c t i o n s h o u l d be u n d e r t a k e n whenever d e v i a t i o n s f rom

normal c o n d e n s e r o p e r a t i o n o c c u r .

7.7.3 Steam t r a p s

As emphas ized i n S e c t i o n 3.2.1, one o f t h e e s s e n t i a l r e q u i r e m e n t s o f p r o p e r

steam o r v a p o u r h e a t i n g i s r e l i a b l e condensa te d r a i n a g e . T h i s r e q u i r e m e n t can be

s a t i s f i e d , p r o v i d i n g t h a t t h e e n t i r e sys tem i s p r o p e r l y d e s i g n e d and m a i n t a i n e d .

I n p r a c t i c e , i t i s n o t unusua l t h a t d e s i g n e r r o r s o r m a k e s h i f t m o d i f i c a t i o n s

cause steam consumpt ion i n i n d i v i d u a l equ ipment u n i t s t o i n c r e a s e by 25-50% above t he a c t u a l n e e d . I t s h o u l d be u n d e r s t o o d he re t h a t t he p r e r e q u i s i t e f o r

279

r o u t i n e c h e c k i n g o f steam t r a p s i s t o e l i m i n a t e such e r r o r s .

Check ing o f steam t r a p s i s aimed a t d e t e c t i n g and e l i m i n a t i n g m a l f u n c t i o n i n g

t r a p s . Condensa te d r a i n a g e can be o b s e r v e d a t w a t e r - l e v e l gauges t h a t i n d i c a t e

t h e condensa te l e v e l i n t h e h e a t i n g chambers o f t h e r e l e v a n t a p p a r a t u s . N o z z l e -

t y p e steam t r a p s a r e o f t e n e q u i p p e d w i t h s i g h t - g l a s s e s t h a t make i t p o s s i b l e t o

o b s e r v e steam l e a k s . When i r r e g u l a r i t i e s a r e d e t e c t e d , i t may be n e c e s s a r y t o

r e f e r t o t h e measur ing i n s t r u m e n t s i n t h e the rma l s y s t e m ; h o w e v e r , i f t h e

p r e s s u r e d rop a c r o s s t he t r a p does n o t d i f f e r much f rom i t s nominal v a l u e , t r a p

damage can be s u s p e c t e d . A damaged t r a p must be r e p l a c e d w i t h o u t d e l a y .

Steam t r a p o p e r a t i o n can a l s o be i n v e s t i g a t e d u s i n g an u l t r a s o n i c sound

d e t e c t o r ( r e f . 4 5 ) . A l e s s r e l i a b l e method employs a s t e t h o s c o p e , o r even a

metal r o d t o u c h i n g t he t r a p s u r f a c e (an i n i t i a l t r i a l s h o u l d be made t o e n s u r e

t h a t t h i s method i s e f f i c i e n t ) .

7 .7 .4 Steam and v a p o u r p i p e l i n e s

The equ ipment and machines i n a the rma l sys tem a re i n t e r c o n n e c t e d by

p i p e l i n e s f o r t r a n s p o r t o f s team, v a p o u r o r w a t e r . Steam and v a p o u r p i p e l i n e s

a re p a r t i c u l a r l y i m p o r t a n t because o f t h e i r i n f l u e n c e on t h e e n e r g y u t i l i z a t i o n .

I t s h o u l d be p o i n t e d o u t t h a t c o r r e c t o p e r a t i o n o f a steam p i p e l i n e d e p e n d s ,

i n t he f i r s t p l a c e , on s a t i s f y i n g b a s i c c o n s t r u c t i o n r u l e s such as p i p e -

p o s i t i o n i n g w i t h t he s l o p e needed f o r c o n d e n s a t e f l o w - o f f , a p p l y i n g n e c e s s a r y

d r a i n a g e and v e n t i n g , p r o v i d i n g a p p r o p r i a t e the rma l i n s u l a t i o n , i n s t a l l i n g b y

pass l i n e s , e t c . These r u l e s a r e p a r t i c u l a r l y i m p o r t a n t f o r s a f e s t a r t - u p and

s h u t - d o w n , as w e l l as f o r t h e smooth r e p a i r and ma in tenance o f i n d i v i d u a l

equ ipment u n i t s w i t h o u t a f f e c t i n g t he o p e r a t i o n o f t h e e n t i r e s y s t e m . I t i s n o t

unusua l i n p r a c t i c e , h o w e v e r , t o f i n d t h a t t h e s e r u l e s a r e n o t c o m p l e t e l y

s a t i s f i e d due t o d e s i g n e r r o r s o r m a k e s h i f t m o d i f i c a t i o n s . I t s h o u l d be

u n d e r s t o o d he re t h a t t h e p r e r e q u i s i t e f o r normal p i p e l i n e c h e c k i n g i s t o

e l i m i n a t e e r r o r s o f t h a t k i n d .

Check ing o f steam p i p e l i n e s i s aimed a t :

- s t eam- leakage d e t e c t i o n and e l i m i n a t i o n ;

- t h e r m a l - i n s u l a t i o n i n s p e c t i o n and r e p a i r .

S team- leakage d e t e c t i o n c o n s i s t s o f f i n d i n g l e a k a g e s t o t h e e n v i r o n m e n t and

u n c o n t r o l l e d f l o w s t h r o u g h f a u l t y d r a i n s o r v e n t s , as w e l l as f l o w s i n d u c e d by

l e a k i n g v a l v e s i n b y - p a s s o r r e s e r v e l i n e s . T h i s may be a d i f f i c u l t t a s k i n

complex p i p i n g s y s t e m s ; i t can be made e a s i e r i f t he rmometers and manometers a r e

i n s t a l l e d i n a l l t h e p i p i n g s e c t i o n s . I t i s s u f f i c i e n t t o p e r f o r m s t e a m - l e a k a g e

checks a t t h e b e g i n n i n g o f t h e season and a f t e r t e m p o r a r y s h u t - d o w n s o r

equ ipment r e p a i r s .

The aim o f t h e checks o f the rma l i n s u l a t i o n i s t o i d e n t i f y and e l i m i n a t e

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u n n e c e s s a r y h e a t l o s s e s f rom the p i p e l i n e s u r f a c e . Wet o r damaged i n s u l a t i o n

s e c t i o n s s h o u l d immed ia te l y be r e p a i r e d . I n cases o f we t i n s u l a t i o n , i t i s

enough t o e l i m i n a t e t h e w a t e r i n f l o w , as hea t f l u x f rom the p i p e s u r f a c e w i l l

cause s u b s e q u e n t d r y i n g . I n s u l a t i o n c h e c k i n g s h o u l d be pe r f o rmed a t t h e f a c t o r y

s t a r t - u p , and a f t e r equ ipment o r p i p i n g r e p a i r s . A t t e n t i o n s h o u l d be d i r e c t e d

t o p i p e s e c t i o n s c l o s e t o f i t t i n g s , hange rs and compensa t ion p i e c e s .

7 .7 .5 V e n t s

The h e a t i n g chambers o f equ ipment hea ted by v a p o u r s , such as e v a p o r a t o r s ,

j u i c e h e a t e r s and p r i m a r i l y vacuum p a n s , s h o u l d be c o n t i n u o u s l y v e n t e d .

Noncondensab le gases p r e s e n t i n c o n d e n s i n g v a p o u r s t e n d t o accumu la te i n t h e

l o w e r r e g i o n s o f h e a t i n g chambers . I t i s u s u a l l y a t t h e s e p o i n t s , and

p a r t i c u l a r l y where t h e v a p o u r p a t h s e n d , t h a t v e n t i n g n o z z l e s s h o u l d be

i n s t a l l e d ( r e f . 4 6 ) ; t he n o z z l e s s h o u l d be c o n n e c t e d t o p i p e s e q u i p p e d w i t h

v a l v e s t h a t make i t p o s s i b l e t o c o n t r o l t he f l o w .

I f v e n t i n g does n o t p r e v e n t t h e a c c u m u l a t i o n o f noncondensab le g a s e s , t hen

t h e i r i n c r e a s e d p a r t i a l p r e s s u r e causes t he p a r t i a l p r e s s u r e o f t he steam t o

d e c r e a s e , w h i c h i n t u r n d e c r e a s e s t he c o n d e n s a t i o n t e m p e r a t u r e . I n s u f f i c i e n t

v e n t i n g t h u s r e s u l t s i n a d e c r e a s e d e f f e c t i v e t e m p e r a t u r e d i f f e r e n c e and l e s s

i n t e n s i v e hea t t r a n s f e r w h i c h means:

- d e c r e a s e d j u i c e e v a p o r a t i o n i n t h e e v a p o r a t o r s ;

- d e c r e a s e d f i n a l j u i c e t e m p e r a t u r e i n t h e h e a t e r s ;

- l o n g e r b o i l i n g t ime i n t h e ba t ch vacuum p a n s .

V e n t i n g a l w a y s causes a c e r t a i n amount o f steam t o e s c a p e , t o g e t h e r w i t h t he

noncondensab le g a s e s . I t i s t h u s i m p o r t a n t t o r educe t h a t amount t o a minimum

a n d , i f p o s s i b l e , t o r e c o v e r h e a t f rom t h e e s c a p i n g m i x t u r e . Most o f t e n , v e n t i n g

p i p e s a re c o n n e c t e d a t s e l e c t e d p l a c e s i n t h e therma l s y s t e m , as e x p l a i n e d i n

S e c t i o n 3 . 2 . 2 .

When c o n t r o l l i n g t h e f l o w o f a g a s - v a p o u r m i x t u r e , i t i s recommended t h a t t h e

v a l v e s p i n d l e be t u r n e d n o t more t han 1/4 t o 1/3 r e v o l u t i o n a t a t i m e . I t s h o u l d

be o b s e r v e d t h a t t h i s k i n d o f f l o w c o n t r o l i s r e a l i z a b l e o n l y i n t h e case o f

v e n t s opened t o the a t m o s p h e r e , as e x c e s s i v e v e n t i n g i s i n d i c a t e d t h e r e by

v i s i b l e steam o u t f l o w .

The a c c u r a t e c o n t r o l o f v e n t i n g t h a t i s p a r t i c u l a r l y needed i n t he

e v a p o r a t o r s wou ld r e q u i r e i n s t a l l a t i o n o f p r e c i s i o n thermometers a t t he steam

n o z z l e b e f o r e the h e a t i n g chamber i n l e t and i n t he h e a t i n g chamber c l o s e t o

t he v e n t i n g n o z z l e . Open ing o f t h e c o n t r o l v a l v e s h o u l d r e s u l t i n a t e m p e r a t u r e

d i f f e r e n c e o f abou t 1 K. As i t i s p r a c t i c a l l y i m p o s s i b l e t o measure such a smal l

t e m p e r a t u r e d i f f e r e n c e w i t h adequate a c c u r a c y i t i s recommended t h a t , i n s t e a d o f

two t he rmome te r s , a s p e c i a l measur ing sys tem be used i n c l u d i n g f o u r r e s i s t a n c e

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thermometers and an e l e c t r i c a l b r i d g e e q u i p p e d w i t h f o u r r e s i s t o r s ( r e f . 4 7 ) .

T h i s i s p a r t i c u l a r l y u s e f u l i n t he second and t h i r d e v a p o r a t o r e f f e c t s where t h e

c o n c e n t r a t i o n o f n o n c o n d e n s a b l e s i s h i g h e s t .

Check ing o f v e n t i n g sys tems s h o u l d p r i m a r i l y be c a r r i e d o u t a t t h e b e g i n n i n g

o f t he s e a s o n , a f t e r t e m p o r a r y s h u t - d o w n s and a f t e r equ ipment r e p a i r s . A l s o i n

cases o f i r r e g u l a r i t i e s , i n d i c a t e d by t o o low f i n a l j u i c e t e m p e r a t u r e s i n t h e

h e a t e r s o r t o o s l o w s u g a r b o i l i n g i n t h e vacuum p a n s , a v e n t i n g check s h o u l d be

a s t a n d a r d r o u t i n e .

7.8 PULP DRYER

7.8.1 Methods o f measurement

I t i s assumed t h r o u g h o u t t h i s S e c t i o n t h a t t h e d r y e r under c o n s i d e r a t i o n i s

a c l a s s i c a l d r u m - t y p e d r y e r hea ted by combus t i on g a s e s . Compared t o a the rma l

sys tem i n s u g a r m a n u f a c t u r e , a p u l p d r y e r e q u i p p e d w i t h i t s own f u r n a c e i s

e x t r e m e l y p r i m i t i v e , as t he hea t i s u t i l i z e d o n l y once ( t h e same a p p l i e s t o

d r y e r s u t i l i z i n g f l u e gases f rom b o i l e r s ) . Any h e a t l o s s i n p u l p d r y i n g i s t h u s

i r r e c o v e r a b l e , and t h a t i s why t he d r y i n g p r o c e s s s h o u l d be c a r e f u l l y m o n i t o r e d .

A d r u m - t y p e p u l p d r y e r i s shown s c h e m a t i c a l l y i n F i g . 7 .13 , t o g e t h e r w i t h

a l i s t o f q u a n t i t i e s t h a t appear i n t h e mass and h e a t b a l a n c e s .

pressed pulp

B . Q H

exhaust gas t 2 . C 0 2

F i g . 7 .13 . P o i n t s o f measurement i n p u l p d r y e r c h e c k . 1 - f u r n a c e , 2 - f e e d e r , 3 - a f t e r d r y e r . F o r t he e x p l a n a t i o n o f s y m b o l s , see t e x t .

The e s s e n t i a l p rob lem o f m o n i t o r i n g e n e r g y usage i n a p u l p d r y e r i s t o

de te rm ine t h e hea t consumpt ion w h i l e d r y i n g a d e f i n i t e amount o f p u l p . The h e a t

consumpt ion can c o n v e n t i o n a l l y be e x p r e s s e d by two i n d i c e s : t he d r y e r

e f f i c i e n c y , η, and the hea t consumpt ion f o r t he e v a p o r a t i o n o f 1 kg o f w a t e r , q .

The d r y e r e f f i c i e n c y can be d e f i n e d as

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where i s t h e t h e o r e t i c a l h e a t demand f o r e v a p o r a t i n g w a t e r f rom t h e p u l p i n

k J , Β i s t h e f u e l consumpt ion i n kg , and i s t h e h e a t i n g v a l u e o f f u e l i n

k J / k g .

The hea t consumpt ion f o r e v a p o r a t i o n o f 1 kg w a t e r can be c a l c u l a t e d f rom t h e

f o r m u l a

q = BQ^/W ( k J / k g ) ( 7 . 5 0 )

where W i s t h e mass o f w a t e r e v a p o r a t e d i n t h e d r y e r i n kg .

I n o r d e r t o de te rm ine t h e above i n d i c e s , i t i s n e c e s s a r y t o measure t he

f o l l o w i n g q u a n t i t i e s :

- mass o f f u e l consumed, Β ( k g ) ;

- h e a t i n g v a l u e o f f u e l , ( k J / k g ) ;

- mass o f p r e s s e d p u l p , ( k g ) , o r mass o f d r i e d p u l p , G^ ( k g ) ;

- mass o f mo lasses added b e f o r e t h e d r y e r , G ^ ( k g ) ;

- d r y m a t t e r c o n t e n t i n p r e s s e d p u l p , s^ ( % ) , i n d r i e d p u l p , S2 ( % ) , and i n

m o l a s s e s , s ^ ( % ) ;

- p r e s s e d p u l p t e m p e r a t u r e b e f o r e t h e d r y e r , t^^ ( ° C ) ;

- gas t e m p e r a t u r e b e f o r e t h e d r y e r drum, t^ ( ° C ) , and a t t h e d r y e r o u t l e t ,

( °C ) .

I n o r d e r t o a t t a i n s a t i s f a c t o r y measur ing a c c u r a c y , c a r e i s r e q u i r e d i n t a k i n g

r e p r e s e n t a t i v e p u l p samples f o r t h e d e t e r m i n a t i o n o f d r y m a t t e r c o n t e n t .

Tempera tu re measurements b e f o r e t h e d r y e r o u t l e t a r e a l s o v e r y i m p o r t a n t . The

f l u e gas t e m p e r a t u r e b e f o r e t he drum must n o t be measured a t t h e f u r n a c e o u t l e t ,

bu t i n s t e a d where t he f l u e gas has a l r e a d y been mixed w i t h s e c o n d a r y a i r ( f e d

f o r t e m p e r a t u r e a d j u s t m e n t p u r p o s e s ) and a l s o w i t h a i r l e a k i n g i n t h r o u g h gaps

between t h e f u r n a c e and t he drum, as w e l l as a round t he p u l p i n t a k e . T e m p e r a t u r e

d e t e c t o r s s h o u l d t h u s be i n s t a l l e d a t t he drum beyond t he p u l p i n t a k e . As t o t h e

o u t l e t t e m p e r a t u r e measurements , s y s t e m a t i c e r r o r s due t o i r r e g u l a r i t i e s o f t he

t e m p e r a t u r e d i s t r i b u t i o n i n t h e drum o u t l e t must be a v o i d e d . The r i g h t p l a c e f o r

t e m p e r a t u r e measurement i s t h e o u t l e t n o z z l e o f t he e x h a u s t f a n .

A v e r y u s e f u l a d d i t i o n t o t he above measurements i s d e t e r m i n a t i o n o f t h e CO2

c o n t e n t i n t he gas a t t he d r y e r o u t l e t . Gas samples s h o u l d be taken f rom t h e

o u t l e t n o z z l e o f t h e e x h a u s t f a n .

I n t h e case o f a p e r i o d i c d r y e r c h e c k , i t i s recommended t h a t t h e sys tem o f

measur ing used makes i t p o s s i b l e t o d e t e r m i n e t h e hea t consumpt ion i n d i c e s and

a l s o t o i d e n t i f y t h e r e a s o n s f o r any i r r e g u l a r i t i e s d e t e c t e d . P a r t i c u l a r l y

u s e f u l a re t h e measurements o f CO2 c o n t e n t i n t h e gas b e f o r e t h e d r y e r drum and

a t t he d r y e r o u t l e t , as t he d i f f e r e n c e i n CO2 c o n t e n t shows w h e t h e r t h e drum i s

p r o p e r l y s e a l e d . I n a d d i t i o n , i f t he o u t l e t CO2 c o n t e n t and t e m p e r a t u r e a r e

known, t h e n app rox ima te v a l u e s o f hea t consumpt ion and gas h u m i d i t y can be f o u n d

i n t h e d iagrams g i v e n i n r e f . 8 o r o t h e r s o u r c e s .

283

R o u t i n e d r y e r m o n i t o r i n g s h o u l d be based on i n s t r u m e n t r e a d i n g s t aken e v e r y

h o u r . Then t he mean v a l u e s o f pa rame te rs ( c a l c u l a t e d o v e r t h e e n t i r e t e s t

p e r i o d ) r e f l e c t n o t o n l y t h e q u a l i t y o f t h e d r y e r - f u r n a c e s y s t e m , b u t a l s o t h e

i n f l u e n c e o f d i s t u r b a n c e s i n t h e o p e r a t i n g c o n d i t i o n s , such as c h a n g i n g m o i s t u r e

c o n t e n t o r mass f l o w o f t h e p r e s s e d p u l p .

I n p e r i o d i c c h e c k s , t he i n v e s t i g a t i o n s a r e aimed a t t he d e t e r m i n a t i o n o f

d r y e r e f f i c i e n c y . R e l i a b l e t e s t r e s u l t s can o n l y be o b t a i n e d i f t h e d r y e r l o a d

i s s t a b i l i z e d and no s i g n i f i c a n t d i s t u r b a n c e s o c c u r . O t h e r w i s e , t h e the rma l

c a p a c i t y o f t h e d r y e r may i n t r o d u c e a c o n s i d e r a b l e h e a t - b a l a n c e e r r o r . The t e s t

d u r a t i o n s h o u l d be 8-12 h o u r s , w i t h pa ramete r r e a d i n g s taken e v e r y 30 m inu tes

and p u l p samples c o l l e c t e d e v e r y h o u r .

7 .8 .2 Mass and hea t b a l a n c e s o f a p u l p - d r y i n g p l a n t

The mass b a l a n c e can be a n a l y s e d on t h e b a s i s o f t h e e q u a t i o n d e s c r i b i n g t h e

d r y m a t t e r s t ream

G^s^ = G2S2 ( 7 . 5 1 )

P r o v i d e d t h a t t h e p u l p samples a r e r e a l l y r e p r e s e n t a t i v e , v e r y a c c u r a t e v a l u e s

o f s^ and S2 can be o b t a i n e d f rom l a b o r a t o r y a n a l y s e s . T h u s , e q n . ( 7 . 5 1 ) makes

i t p o s s i b l e t o v e r i f y t h e i n d i c a t i o n s o f t h e p r e s s e d - p u l p s c a l e s . The mass o f

d r i e d p u l p l e a v i n g t h e d r y e r , G ^ , can be d e t e r m i n e d by d i r e c t i n g t h e d r i e d p u l p

i n t o a c o n t a i n e r . Once t h e b a t c h has been w e i g h e d , one can c a l c u l a t e G^ f rom

e q n . ( 7 . 5 1 ) .

The mass o f w a t e r e v a p o r a t e d i n t h e d r y e r can be c a l c u l a t e d as

W = G^(S2 - S ^ ) / S 2 + G^(S2 - s ^ ) / s 2 ( k g ) ( 7 . 5 2 )

The t h e o r e t i c a l hea t demand f o r w a t e r e v a p o r a t i o n can be c a l c u l a t e d as

= W(h^ - h^) ( k J ) ( 7 . 5 3 )

where h^ i s t he e n t h a l p y o f t h e v a p o u r s i n t h e gas a t t h e d r y e r o u t l e t i n k J / k g ,

and h^ i s t h e e n t h a l p y o f w a t e r i n t h e p r e s s e d p u l p i n k J / k g . The e n t h a l p y h^

can be d e t e r m i n e d f rom steam t a b l e s o r d i a g r a m s . A c t u a l l y , v a p o u r s i n t h e o u t l e t

gases a r e s u p e r h e a t e d ; t h e t e m p e r a t u r e i s t ^ and t h e p r e s s u r e s h o u l d be

u n d e r s t o o d as t h e p a r t i a l p r e s s u r e o f H^O i n g a s . H o w e v e r , a s a t i s f a c t o r y

a p p r o x i m a t i o n i s o b t a i n e d by assuming t h a t t h e p r e s s u r e i s 1 ba r ( t h e r e s u l t i n g

e r r o r does n o t e x c e e d 1%). The o t h e r e n t h a l p y v a l u e i s h^ = 4 .19 tp^ ( k J / k g ) .

7 .8 .3 Economica l p u l p d r y e r o p e r a t i o n

I n a r e a s o n a b l y w e l l o p e r a t e d p u l p d r y e r w i t h an i n d i v i d u a l f u r n a c e , a

t y p i c a l e f f i c i e n c y v a l u e i s 0 . 7 5 - 0 . 8 5 , w h i l e h e a t consumpt ion p e r 1 kg

e v a p o r a t e d w a t e r does n o t e x c e e d 3140-3560 k J / k g .

I n a d r y e r hea ted by b o i l e r f l u e g a s , t h e e f f i c i e n c y v a l u e c o n v e n t i o n a l l y

c a l c u l a t e d p e r t o t a l amount o f f u e l consumed i n t h e b o i l e r i s 0 . 1 0 - 0 . 1 5 , and

284

t he hea t consumpt ion ( c a l c u l a t e d i n t h e same way) amounts t o 21500-28000 k J / k g .

Keep ing the thermal i n d i c e s o f a d r y e r a t an a p p r o p r i a t e l e v e l r e q u i r e s good

u n d e r s t a n d i n g o f t he r e l a t i o n s h i p s t h a t a f f e c t i t s hea t b a l a n c e , s c h e m a t i c a l l y

shown i n a s i m p l i f i e d Sankey d iag ram i n F i g . 7 .14 .

( i ) The s m a l l e s t p o s s i b l e h e a t consumpt ion i s r e l a t e d t o t he t h e o r e t i c a l h e a t

demand, c a l c u l a t e d a c c o r d i n g t o e q n . ( 7 . 5 3 ) .

( i i ) Pu lp d r y i n g r e q u i r e s t h a t t he e n e r g y i n t r o d u c e d i n t o the d r y i n g gas must

exceed t he t h e o r e t i c a l demand by t h e combined e n e r g y l o s s i n t h e o u t l e t g a s ,

p l u s t he hea t l o s s f rom the d r y e r drum.

( i i i ) I n o r d e r t o g e n e r a t e t h e r i g h t amount o f gas a t t h e p r o p e r t e m p r e r a t u r e ,

i t i s n e c e s s a r y t o burn an amount o f f u e l t h a t c o r r e s p o n d s t o t he above amount

o f e n e r g y , p l u s combus t ion l o s s e s and hea t l o s s f rom the f u r n a c e t o t he

e n v i ronment .

ω l_ φ >

•σ ω

o QJ Χ

χ Cí ώ

Losses from furnace

Cf m

Heat theoretically needed

Exhaust and radiation

losses

F i g . 7 .14. Heat b a l a n c e o f t h e p u l p d r y i n g p l a n t .

I t f o l l o w s f rom ( i ) t h a t p r o c e s s t o l e r a n c e s s h o u l d be o b s e r v e d , and t h e f i n a l

d r y m a t t e r c o n t e n t i n d r i e d p u l p s h o u l d n o t exceed t h e r e q u i r e d v a l u e .

The need t o m i n i m i z e t he e x h a u s t l o s s i s a p p a r e n t i n ( i i ) ; t h e o u t l e t gas

t e m p e r a t u r e s h o u l d n o t exceed 90-115°C, and t h e gas f l o w s h o u l d be m i n i m a l . An

e q u i v a l e n t f o r m u l a t i o n o f t h e l a t t e r r e q u i r e m e n t i s t o keep t h e gas t e m p e r a t u r e

b e f o r e t he d r y e r drum r a t h e r h i g h , a t 800-900°C.

F i n a l l y , ( i i i ) i m p l i e s t h a t gas g e n e r a t i o n s h o u l d p r o c e e d w i t h e x c e s s a i r

be low 100-150%; o t h e r w i s e , t he gas t e m p e r a t u r e b e f o r e t h e d r y e r drum w o u l d be

t o o l o w . I n a c t u a l combus t ion c o n d i t i o n s , t h e amount o f e x c e s s a i r i s l o w e r .

The rema in ing a i r ( s e c o n d a r y a i r ) i s i n t r o d u c e d s e p a r a t e l y b e f o r e t he drum i n l e t

and u t i l i z e d f o r g a s - t e m p e r a t u r e a d j u s t m e n t . U n c o n t r o l l a b l e a i r l e a k s t h r o u g h

t h e gaps between t h e drum and t he f u r n a c e , and a t t he p u l p i n t a k e , a r e c l e a r l y

d i s a d v a n t a g e o u s , l e a d i n g sometimes t o an u n a c c e p t a b l y l a r g e t e m p e r a t u r e d r o p .

The l e a k s may a l s o cause t e m p e r a t u r e d i s t r i b u t i o n i r r e g u l a r i t i e s o v e r t he drum

285

c r o s s - s e c t i o n , hamper ing t h e p u l p - d r y i n g p r o c e s s .

I n keep ing t h e e n t i r e a i r f e e d under c o n t r o l , CO^ measurement i n t h e gas a t

t he d r y e r o u t l e t i s e s p e c i a l l y u s e f u l . P r o v i d i n g t h e chemica l c o m p o s i t i o n o f t h e

f u e l and t he r e q u i r e d e x c e s s a i r a r e known, i t i s p o s s i b l e t o d e t e r m i n e t h e

d e s i r a b l e v o l u m e t r i c c o m p o s i t i o n o f t he f l u e gas b e f o r e t h e d r y e r drum. T a k i n g

i n t o a c c o u n t t h e f u e l m o i s t u r e and t h e w a t e r e v a p o r a t e d f rom t h e p u l p , t h e

recommended c o n t e n t a t t h e d r y e r o u t l e t can a l s o be d e t e r m i n e d ( a n d ,

p r e f e r a b l y , e x p e r i m e n t a l l y v e r i f i e d ) . The d r y e r o p e r a t o r s h o u l d keep t h e a c t u a l

c o n t e n t c l o s e t o t he recommended v a l u e , a v o i d i n g l o w e r v a l u e s t h a t i n d i c a t e

t o o much e x c e s s a i r .

As a c o n c l u d i n g comment t o ( i i i ) , i t s h o u l d be s t a t e d t h a t t h e f u r n a c e must

be m a i n t a i n e d i n p r o p e r c o n d i t i o n i n o r d e r t o keep combus t i on l o s s e s and h e a t

d i s s i p a t i o n a t a minimum.

As a m a t t e r o f f a c t , p u l p - d r y i n g o p e r a t i o n c o n s i s t s o f ma tch ing t h e

r e q u i r e m e n t s on t h e e f f e c t o f t h e d r y i n g p r o c e s s and t h e e n e r g y c o n s u m p t i o n .

The pa ramete r v a l u e s g i v e n above make i t p o s s i b l e t o a c h i e v e a compromise i n

d r y e r c a p a c i t y , c l o s e t o i t s nominal l e v e l . C a p a c i t y changes r e q u i r e m o d i f y i n g

paramete r v a l u e s , wh i ch c a u s e s , as a r u l e , i n c r e a s e o f h e a t c o n s u m p t i o n . F o r

t h i s r e a s o n , t h e f i r s t p r i n c i p l e o f p u l p - d r y e r o p e r a t i o n i s t o keep t h e c a p a c i t y

c o n s t a n t ; i f p o s s i b l e , c l o s e t o i t s nominal v a l u e .

7 .8 .4 Example

An o i l - f i r e d , d r u m - t y p e p u l p d r y e r has been t e s t e d i n a 12-hour t e s t and t h e

f o l l o w i n g d a t a summarize t h e t e s t r e s u l t s :

- mass o f f u e l o i l consumed, Β = 14077 kg ;

- o i l h e a t i n g v a l u e , = 40400 k J / k g ;

- mass o f p r e s s e d p u l p , G-j = 223510 kg ;

- mass o f mo lasses added b e f o r e d r y e r , G ^ = 8975 kg ;

- d r y m a t t e r c o n t e n t i n p r e s s e d p u l p , s-j = 17%, i n d r i e d p u l p , S2 = 90.8%, and

i n m o l a s s e s , s = 76.3%;

- p r e s s e d p u l p t e m p e r a t u r e b e f o r e d r y e r , t^-j = 20 C ;

- gas t e m p e r a t u r e b e f o r e d r y e r drum, t-j = 803^0, and a t d r y e r o u t l e t , t2 = 102°C.

The mass o f w a t e r e v a p o r a t e d i n t h e d r y e r

W = 223510(90.7 - 1 7 ) / 9 0 . 7 + 8975(90.7 - 7 6 . 3 ) / 9 0 . 7 = 183042 kg

The v a p o u r e n t h a l p y a t t he d r y e r o u t l e t , f rom steam t a b l e s

h^ = 2682 k J / k g

The t h e o r e t i c a l hea t demand

= 183042(2682 - 4 .19 -20) = 4 .756-10^ kJ

The d r y e r e f f i c i e n c y

η = 4 .756-10^ / (14077-40400) = 0.836

286

The hea t consumpt ion f o r e v a p o r a t i o n o f 1 kg w a t e r

q = (14077·40400) /183042 = 3106 k J / k g

7.9 COMPUTER-AIDED MONITORING

I t can be c o n c l u d e d f rom p r e c e d i n g S e c t i o n s o f t h i s C h a p t e r t h a t once methods

o f i d e n t i f i c a t i o n o f t h e h e a t b a l a n c e s needed t o s u p e r v i s e t h e h e a t economy

i n a s u g a r f a c t o r y have been e s t a b l i s h e d , t h e m o n i t o r i n g p r o c e d u r e s can be

r o u t i n e l y a p p l i e d on a r e p e t i t i v e b a s i s . The p r a c t i c a l m o n i t o r i n g t h e n c o n s i s t s

m a i n l y o f da ta a c q u i s i t i o n , da ta p r o c e s s i n g and r e p o r t g e n e r a t i n g , and can be

automated w i t h t he a i d o f a d i g i t a l compu te r . T h i s a p p l i e s , i n t he f i r s t p l a c e ,

t o t he r o u t i n e m o n i t o r i n g t a s k s ( r o u t i n e c h e c k s ) w h i c h a r e pe r f o rmed most o f t e n ,

so t h a t t he i n i t i a l e f f o r t needed t o p r e p a r e c o m p u t e r i z e d t o o l s can be p a i d back

most q u i c k l y .

The impor tance o f c o m p u t e r - a i d e d m o n i t o r i n g t o t h e e n e r g y economy l i e s i n t h e

f a c t t h a t , by r e d u c i n g t h e amount o f r e p e t i t i v e and t e d i o u s work a s s o c i a t e d w i t h

the p r e p e r a t i o n o f t he hea t b a l a n c e s , i t c r e a t e s t he p o s s i b i l i t y o f

s y s t e m a t i c , d e t a i l e d e v a l u a t i o n o f t he e n e r g y p r o c e s s e s . T h i s a p p l i e s n o t o n l y

t o t h e i n d i v i d u a l s t a t i o n s d i s c u s s e d i n S e c t i o n s 7 . 2 - 7 . 8 , b u t a l s o t o t he e n t i r e

therma l s y s t e m . On t h i s b a s i s i t becomes p o s s i b l e t o c o n t r o l and o p t i m i z e , i n

r e s p e c t o f t he e n e r g y economy o f t h e e n t i r e f a c t o r y , t h e o p e r a t i n g c o n d i t i o n s a t

t h e most d e c i s i v e p r o c e s s s t a t i o n s .

A g e n e r a l scheme o f t h e d a t a f l o w on w h i c h c o m p u t e r - a i d e d m o n i t o r i n g o f t h e

f a c t o r y o p e r a t i o n i s based can be seen i n F i g . 7 .15 . The p r a c t i c a l a p p l i c a t i o n s

o f t he g e n e r a l i d e a a r e d i f f e r e n t i a t e d w i t h r e s p e c t t o d a t a a c q u i s i t i o n

t e c h n i q u e s , d a t a - b a s e s t r u c t u r e and v o l u m e , t he d a t a - b a s e u p d a t i n g and s e a r c h

t e c h n i q u e s u s e d , u s e r programs a v a i l a b l e and d a t a p r e s e n t a t i o n methods emp loyed .

By a d o p t i n g t h i s k i n d o f s t r u c t u r i n g o f da ta p r o c e s s i n g f u n c t i o n s , h o w e v e r , one

i s a b l e t o a p p l y w i d e l y c i r c u l a t e d , w e l l p r o v e n s o f t w a r e components f a c i l i t a t i n g

MEASURING INSTRUMENTS

DATA INPUT USER TERMINALS TERMINALS

Software system

Updating programs

1 Data acquisi t ion

1 programs

Data

base

User programs T T

Search

programs

PRINTER

PLOTTER

F i g . 7 .15 . Scheme o f da ta f l o w i n c o m p u t e r - a i d e d m o n i t o r i n g o f t h e f a c t o r y o p e r a t i o n .

287

smooth i m p l e m e n t a t i o n and h i g h r e l i a b i l i t y o f t h e m o n i t o r i n g s y s t e m .

M o n i t o r i n g o f t h e e n e r g y economy can be t r e a t e d as one o f t h e f u n c t i o n s o f

an e x t e n s i v e m o n i t o r i n g sys tem c o v e r i n g v a r i o u s a s p e c t s o f f a c t o r y o p e r a t i o n , o r

i t can be pe r f o rmed by a s p e c i a l i z e d s y s t e m . The d a t a management f o r m o n i t o r i n g

p u r p o s e s can be i n t e g r a t e d w i t h t h a t r e q u i r e d f o r c o m p u t e r - b a s e d a u t o m a t i c

c o n t r o l , o r can be t r e a t e d i n d e p e n d e n t l y o f t h e a u t o m a t i c c o n t r o l f u n c t i o n s .

Among t h e m o n i t o r i n g p rob lems w h i c h must be s o l v e d i n o r d e r t o r e c o g n i z e t h e

c o n d i t i o n o f t h e e n e r g y p r o c e s s e s , e v a p o r a t o r m o n i t o r i n g i s o f c r i t i c a l

i m p o r t a n c e . I t d e t e r m i n e s t he q u a l i t y o f i n f o r m a t i o n on t he most i m p o r t a n t

pa ramete rs o f t he e n e r g y p r o c e s s e s , and i s d e c i s i v e i n c r e a t i n g p o s s i b i l i t i e s

o f c o r r e c t l y d i a g n o s i n g t h e i n a d e q u a c i e s o f t h e e n e r g y economy.

The s i m p l e s t app roach t o e v a p o r a t o r m o n i t o r i n g i s t o use a p a r t o f t h e d a t a

on f a c t o r y o p e r a t i o n , s t o r e d o f f - l i n e i n t h e computer memory f o r s t a t i s t i c a l

p u r p o s e s , f o r d e t e r m i n a t i o n o f t he t r e n d s i n pa ramete r v a l u e s and f o r p e r i o d i c

mass and h e a t b a l a n c e c a l c u l a t i o n s . The e v a p o r a t i o n p r o c e s s can be i d e n t i f i e d i n

terms o f a v e r a g e v a l u e s o f t h e p a r a m e t e r s , t y p i c a l l y c a l c u l a t e d once p e r day o r

once pe r s h i f t . An example o f a p p l i c a t i o n o f t h i s app roach can be f o u n d i n

r e f . 45.

A more advanced e v a p o r a t o r m o n i t o r i n g p r o c e d u r e i s based on o n - l i n e r e c o r d i n g

o f t h e e s s e n t i a l p a r a m e t e r s , measured c o n t i n u o u s l y i n t he i n d i v i d u a l e f f e c t s ,

and o f f - l i n e s t o r i n g o f t he da ta on j u i c e c o n c e n t r a t i o n s p e r i o d i c a l l y d e t e r m i n e d

by l a b o r a t o r y a n a l y s e s . The c a l c u l a t i o n s o f mass and h e a t b a l a n c e s can o n l y be

pe r f o rmed u s i n g t i m e - a v e r a g e d v a l u e s o f t h e pa rame te r s c o r r e s p o n d i n g t o t h e

p e r i o d s between l a b o r a t o r y a n a l y s e s . H o w e v e r , t h e r e c o r d e d v a l u e s o f p r e s s u r e s ,

t e m p e r a t u r e s and f l o w s can be d i s p l a y e d o r p r i n t e d , t h u s making i t p o s s i b l e t o

p e r f o r m d e t a i l e d a n a l y s e s o f pa ramete r changes d u r i n g f a c t o r y o p e r a t i o n .

Examples o f a p p l i c a t i o n o f t h i s t y p e o f m o n i t o r i n g p r o c e d u r e s have been

ment ioned i n t h e l i t e r a t u r e ( r e f . 4 9 ) .

A m o n i t o r i n g p r o c e d u r e o f f e r i n g a lmos t c o n t i n u o u s a v a i l a b i l i t y o f comp le te

da ta on t he e v a p o r a t i o n p r o c e s s i s t o measure c o n t i n u o u s l y , and t o r e c o r d o n

l i n e , a l l t h e pa rame te rs needed t o c a l c u l a t e t h e mass and h e a t b a l a n c e s . T h i s

does n o t n e c e s s a r i l y mean t h a t a l l t h e j u i c e c o n c e n t r a t i o n v a l u e s c h a r a c t e r i z i n g

the s t a t e o f t h e e v a p o r a t o r have t o be a u t o m a t i c a l l y measu red . I t i s p o s s i b l e t o

i d e n t i f y c e r t a i n d e t a i l s o f t he mass and h e a t b a l a n c e s o f t h e e v a p o r a t o r u s i n g

i n d i r e c t measurements , as d e m o n s t r a t e d by an example r e p o r t e d i n r e f . 50. The

amount o f hea t t r a n s f e r r e d i n t h e e v a p o r a t o r b o d i e s can be d e t e r m i n e d u s i n g t h e

measurements o f condensa te f l o w , and t h e c o n d e n s e r l o s s f rom t h e l a s t e f f e c t can

be c a l c u l a t e d by measur ing t h e v a p o u r f l o w t o t h e c o n d e n s e r . I t i s i n t e r e s t i n g

t o no te t h a t t he s p e c i a l hea t -economy m o n i t o r i n g sys tem d e s c r i b e d i n r e f . 50

employs a d a t a a c q u i s i t i o n u n i t h a n d l i n g 64 i n p u t s i g n a l s , and t h e da ta

288

( i n c l u d i n g t he c a l c u l a t i o n s o f t h e mass and hea t b a l a n c e s ) a re p r o c e s s e d by two

h a n d - h e l d programmable c a l c u l a t o r s .

I t can be e x p e c t e d t h a t t h e deve lopmen t o f computer t e c h n o l o g y and t h e r i s i n g

impo r tance o f e n e r g y economy w i l l r e s u l t i n w i d e s p r e a d use o f c o m p u t e r - a i d e d

m o n i t o r i n g o f e n e r g y p r o c e s s e s . Numerous m o n i t o r i n g sys tems o f t h i s k i n d a r e

p r e s e n t l y be i ng implemented i n v a r i o u s c o u n t r i e s .

REFERENCES

Symbols o f N a t i o n a l S t a n d a r d s used b e l o w : ASTM - USA, BS - G r e a t B r i t a i n , NF -F r a n c e , DIN - FRG, COST - USSR, PN - P o l a n d .

1 K. S c h i e b l , W ä r m e w i r t s c h a f t i n d e r Z u c k e r i n d u s t r i e , Τ . S t e i n k o p f f V e r l a g , D r e s d e n / L e i p z i g , 1939.

2 Τ . B a l o h , Wärmeat las f ü r d i e Z u c k e r i n d u s t r i e , Schaper V e r l a g , H a n n o v e r , 1975.

3 S . Z a g r o d z k i and A . K u b a s i e w i c z , Heat economy i n b e e t s u g a r f a c t o r y e v a p o r a t i o n , Sugar T e c h . R e v . , 5 ( 1 / 2 ) (1977/78) 1-154.

4 S . Z a g r o d z k i , Gospodarka C i e p l n a C u k r o w n i , WNT, Warszawa, 1979. 5 P. H o n i g , P r i n c i p l e s o f Sugar T e c h n o l o g y , E l s e v i e r , Amsterdam, 1963. 6 F. S c h n e i d e r ( E d . ) , T e c h n o l o g i e des Z u c k e r s , Schaper V e r l a g , H a n n o v e r , 1968. 7 R .A . M c G i n n i s ( E d . ) , Bee t Sugar T e c h n o l o g y , Beet Sugar Dev . F o u n d . ,

F o r t C o l l i n s , 1971. 8 J . D o b r z y c k i ( E d . ) , P o r a d n i k I n z y n i e r a - C u k r o w n i c t w o , WNT, Warszawa, 1973. 9 D. Urban ( e t a l . ) , Z u c k e r h e r s t e l l u n g , F a c h b u c h v e r l a g , L e i p z i g , 1980.

10 F. B o s n j a k o v i c , T e c h n i s c h e Thermodynamik , T . S t e i n k o p f f V e r l a g , D r e s d e n , 1965.

11 T . D . Eas top and A . McConkey, A p p l i e d Thermodynamics f o r E n g i n e e r i n g T e c h n o l o g i s t s , 3 rd e d n . , Longmans, London and New Y o r k , 1978.

12 U . G r i g u l l ( E d . ) , P r o p e r t i e s o f Water and Steam i n S l - U n i t s , 2nd e d n . , S p r i n g e r - V e r l a g , B e r l i n - H e i d e l b e r g - N e w Y o r k , 1979.

13 M.P. V u k a l o v i c h , T e p l o f i z i c h e s k i e S v o i s t v a Vody i Vodyanogo P a r a , M a s h i n o s t r o e n i e , Moskva , 1967.

14 J . P . Holman, Heat T r a n s f e r , 5 th e d n . , M c G r a w - H i l l , H a m b u r g - L o n d o n - P a r i s , 1981.

15 J . C h u d z i n s k i ( e t a l . ) , P o r a d n i k T e r m o e n e r g e t y k a , 2nd e d n . , WNT, Warszawa, 1974.

16 J . D o b r z y c k i , A u t o m a t y z a c j a w P r z e m y s l e C u k r o w n i c z y m , WNT, Warszawa, 1974. 17 J . S tanek ( E d . ) , Handbuch d e r M e s s t e c h n i k i n d e r B e t r i e b s k o n t r o l l e ,

A k a d e m i e v e r l a g , L e i p z i g , 1979. 18 R .P . B e n e d i c t , Fundamenta ls o f T e m p e r a t u r e , P r e s s u r e , and F low Measurements ,

W i l e y , New Y o r k , 1969. 19 R. F r e i e r , K e s s e l s p e i s e w a s s e r , K ü h l w a s s e r - T e c h n o l o g i e , B e t r i e b s a n a l y s e ,

W a l t e r de G r u y t e r , B e r l i n , 1963. 20 F. C o g e t and M. W i n k e l , Le t r a i t e m e n t des eaux de c h a u f f e r i e s dans l e s

s u c r e r i e s , S u c r . B e i g e , 102 (1984) 5-11. 21 R e g u l a t i o n s o f t h e P o l i s h M i n i s t r y o f M i n i n g and E n e r g y ( i n P o l i s h ) ,

M o n i t o r P o l s k i , (51) ( 1 9 6 7 ) . 22 R . H . L . Howe, B o i l e r - w a t e r c o n t r o l f o r e f f i c i e n t steam p r o d u c t i o n , i n :

R. Greene ( E d . ) , P r o c e s s E n e r g y C o n s e r v a t i o n , M c G r a w - H i l l , New Y o r k , 1982, p p . 185-188.

23 Sampl ing o f i n d u s t r i a l w a t e r , ASTM D 510-68, BS 1328:1968; PN-74 /C-04620 . 24 Sampl ing o f w a t e r f rom b o i l e r s , ASTM 860-54, BS 1328:1968; PN-74 /C-04620 . 25 Equipment f o r samp l ing i n d u s t r i a l w a t e r and s team, ASTM 1192-70;

PN-74/C-04620. 26 Sampl ing o f s team, ASTM 1066-69; BS 3285; PN-74/C-04621. 27 Appearance o f w a t e r , ASTM D 1889-71; BS 2690: P a r t 9 ; NF Τ 90-002 /50 ;

PN-79 /C-04583.

289

28 P a r t i c u l a t e and d i s s o l v e d m a t t e r i n w a t e r , ASTM D 1888-67; NF Τ 90-029 /70 ; COST 18164-72; PN-78/C-04541.

29 E l e c t r i c a l c o n d u c t i v i t y o f w a t e r , ASTM D 1125-61; BS 2690: P a r t 9 ; NF Τ 90-031 /73 ; PN-77 /C-04542.

30 Water h a r d n e s s , ASTM D 1126-67; BS 1427:1962; NF Τ 90-003 /58 ; PN-71 /C-04554. 31 F. S c h n e i d e r ( E d . ) , Sugar A n a l y s i s - ICUMSA M e t h o d s , ICUMSA, P e t e r b o r o u g h ,

1979. 32 Coal and o t h e r s o l i d f u e l s , NF Μ 10-002; GOST 19292-73; PN-82/G-97001. 33 Fue l o i l s , ASTM D 396-73; BS 2869; NF Μ 15-010 and 011/68; DIN 51603-66;

GOST 10585-63; PN-75/C-96024. 34 Sampl ing o f c o a l , ASTM D 2234-72; BS 1017:1977; NF Μ 01-001; DIN 51701;

GOST 16479-70; PN-80/G-04502. 35 Sampl ing o f l i q u i d f u e l s , ASTM D 270; BS 3195:1959; NF Μ 07-001 /60 ;

DIN 51570; GOST 2517-60; PN-66 /C-04000 . 36 A . A . A v d e e v a , B . S . B e l o s e l s k i i and M.N. K r a s n o v , K o n t r o l T o p l i v a ν E l e k t r o -

s t a n t s i y a k h , E n e r g i y a , Moskva , 1973. 37 H. K a r o l c z u k , R a c j o n a l n a Gospodarka Weglem E n e r g e t y c z n y m , WNT, Warszawa,

1978. 38 Heat o f combus t i on by bomb c a l o r i m e t e r . S o l i d f u e l s , BS 1016: P a r t 5;

DIN 51900; GOST 147-74; PN-81/G-04513. L i q u i d f u e l s , ASTM D 240-64; NF Μ 07-030/65; GOST 6712-53; PN-71 /C-04062 .

39 Requ i remen ts and a c c e p t a n c e t e s t s , steam b o i l e r s , I S O / T C - 6 4 P u b l i c a t i o n s No. 40-50 ( 1 9 5 7 - 5 9 ) ; DIN 1942; PN-72/M-3128.

40 P. O r l o w s k i , K o t l y Parowe w E n e r g e t y c e P r z e m y s l o w e j , WNT, Warszawa, 1976. 41 T . A . S t o a , C a l c u l a t i n g b o i l e r e f f i c i e n c y and e c o n o m i c s , i n : R. Greene ( E d . ) ,

P r o c e s s E n e r g y C o n s e r v a t i o n , M c G r a w - H i l l , New Y o r k , 1982, p p . 245-250. 42 Requ i rements and a c c e p t a n c e t e s t s , steam t u r b i n e s , l E C No. 45 /1970;

PN-71/M-35520. 43 A . K u b a s i e w i c z and W. L e k a w s k i , P r z e b i e g wymiany c i e p l a w e k s t r a k t o r z e

ko ry towym, G a z . C u k r o w . , 83 (3 ) ( 1 9 7 5 ) . 44 K . E . A u s t m e y e r , A n a l y s i s o f s u g a r b o i l i n g and i t s t e c h n i c a l c o n s e q u e n c e s .

I n t . Sugar J . , 88 ( 1 9 8 6 ) , P a r t I (1045) 3 - 7 , P a r t I I (1046) 23-29 , P a r t I I I (1047) 50-55.

45 S . J . V a l l e r y , A re y o u r steam t r a p s w a s t i n g e n e r g y ? , i n : R. Greene ( E d . ) , P r o c e s s E n e r g y C o n s e r v a t i o n , M c G r a w - H i l l , New Y o r k , 1982, p p . 170-184.

46 D. V o i t and A . H u t s i n p i 1 l e r , A p r a c t i c a l app roach t o t h e v e n t i n g o f n o n c o n d e n s a b l e s . Paper p r e s e n t e d a t 23rd ASSBT M e e t i n g , San D i e g o , F e b r u a r y 1985.

47 S . Z a g r o d z k i and J . D o b r z y c k i , Removal o f i n c o n d e n s a b l e gases f rom c a l a n d r i a s . I n t . Sugar J . , 71 (1969) 235-237.

48 B . L . K a r r e n and M.K. F a v i e l l , A computer app roach t o t e c h n i c a l r e c o r d s i n t he b e e t s u g a r f a c t o r y l a b o r a t o r y , S u c r . B e i g e , 99 (2 ) (1980) 63-80.

49 D. P i o t r o w s k i and K. U r b a n i e c , Anwenderprogramme f ü r den P r o z e s s r e c h n e r e i n s a t z i n Z u c k e r f a b r i k e n , Z u c k e r i n d . , 106(2) (1981) 135-138.

50 C h . M o l l e r and H. J a n s d o r f , Heat economy and s u p e r v i s o r y computer c o n t r o l . I n t . Sugar J . , 87(1034) (1985) 26-31.

290

C h a p t e r 8

S T E P - B Y - S T E P IMPROVEMENTS OF E X I S T I N G ENERGY SYSTEMS

8.1 THE S T E P - B Y - S T E P APPROACH

8.1.1 I n t r o d u c t i o n

From t h e t e c h n i c a l s t a n d p o i n t , i t i s d i f f i c u l t t o make a c l e a r d i s t i n c t i o n

between s t e p - b y - s t e p improvements and an e x t e n s i v e m o d e r n i z a t i o n o f a s u g a r

f a c t o r y . Managers o f f a c t o r i e s o p e r a t e d under d i f f e r e n t c o n d i t i o n s wou ld

p r o b a b l y i n t e r p r e t t h e s e two terms d i f f e r e n t l y , depend ing on t he economic

r e s o u r c e s w h i c h a re a v a i l a b l e t o them. I t seems, h o w e v e r , t h a t t he most d i s t i n c t

d i f f e r e n c e s between s t e p - b y - s t e p improvements and a m o d e r n i z a t i o n can be f o u n d

i n the me thodo logy o f i m p l e m e n t a t i o n o f changes d e c i d e d upon .

W h i l e t h e m o d e r n i z a t i o n o f a f a c t o r y c o n s i s t s o f a package o f e x t e n s i v e

changes t o be i n t r o d u c e d a t o n c e , s t e p - b y - s t e p improvements may i n v o l v e numerous

s m a l l e r u n d e r t a k i n g s s p r e a d o v e r a l o n g e r t ime p e r i o d . On t h e b a s i s o f r e p e a t e d

r e v i e w s o f t he f a c t o r y ' s n e e d s , t h e o b j e c t i v e s a r e p e r i o d i c a l l y upda ted and

r a t i o n a l i z a t i o n measures a r e s e l e c t e d f rom a l i m i t e d f i e l d o f p o s s i b l e

s o l u t i o n s , t he l i m i t a t i o n s be ing d e f i n e d by t h e a v a i l a b l e economic r e s o u r c e s .

C o n s e q u e n t l y , improvements i n t h e ene rgy -economy a r e a can be d e c i d e d upon and

implemented o n l y i f t h e y r e a l l y a r e more u r g e n t than o t h e r a c t i o n s a l s o

c o n s i d e r e d d e s i r a b l e . H i g h p r i o r i t y i s u s u a l l y a s s i g n e d , h o w e v e r , t o measures

wh i ch improve s u g a r y i e l d o r p r o d u c t q u a l i t y , o p e r a t i o n a l s a f e t y and equ ipment

r e l i a b i l i t y , w h i l e a l s o b e n e f i t i n g e n e r g y economy.

W i t h i n t he g e n e r a l f rame t h u s o u t l i n e d , v a r i o u s c o u r s e s o f a c t i o n may be

adop ted i n a s p e c i f i c f a c t o r y i n a c c o r d a n c e w i t h t h e l o c a l c o n d i t i o n s . P o s s i b l e

r a t i o n a l i z a t i o n measures i n t he a r e a o f e n e r g y economy i n s u g a r manu fac tu re can

be s y s t e m a t i z e d by d i s t i n g u i s h i n g between t h r e e ways t o reduce t h e e n e r g y demand,

( i ) B r i n g i n g t h e e n e r g y - s y s t e m o p e r a t i o n i n t o p a r i t y w i t h i t s nominal

c a p a b i l i t i e s , by e l i m i n a t i n g u n n e c e s s a r y d e v i a t i o n s f rom t h e r e q u i r e d c o u r s e o f

e n e r g y p r o c e s s e s and i m p r o v i n g m a l f u n c t i o n i n g subsys tems and components o f t h e

e n e r g y s y s t e m . T y p i c a l measures a r e as f o l l o w s :

- e l i m i n a t i n g steam and v a p o u r l e a k s i n t he condensa te l i n e s ;

- e l i m i n a t i n g the causes o f abnormal pa ramete r f l u c t u a t i o n s ;

- i m p r o v i n g condensa te d r a i n a g e f rom s team- and v a p o u r - h e a t e d e q u i p m e n t ;

- i m p r o v i n g t he w i t h d r a w a l o f n o n c o n d e n s a b l e s f rom t h e h e a t i n g chambers o f

e v a p o r a t o r s and h e a t e r s ;

- p r e v e n t i n g t h e f o r m a t i o n o f s c a l e ;

- s e c u r i n g p r o p e r q u a l i t y o f t h e condensa te r e t u r n e d f rom t h e e v a p o r a t o r t o t h e

b o i l e r s ;

291

- s e c u r i n g p r o p e r f u n c t i o n i n g o f t h e measu r ing i n s t r u m e n t s w h i c h a r e e s s e n t i a l

i n m o n i t o r i n g e n e r g y c o n v e r s i o n , d i s t r i b u t i o n and u t i l i z a t i o n p r o c e s s e s .

( i i ) Reduc ing t h e t o t a l e n e r g y demand o f t h e s u g a r m a n u f a c t u r i n g p r o c e s s . Among

o t h e r s , t h e f o l l o w i n g measures can be u n d e r t a k e n :

- r e d u c i n g t h e hea t d i s s i p a t i o n f rom t h e p r o c e s s equ ipment and p i p i n g t o t h e

e n v i r o n m e n t ;

- r e d u c i n g t h e e n e r g y consumpt ion f o r a u x i l i a r y p u r p o s e s i n t h e p r o c e s s p l a n t ;

- r e d u c i n g t h e w a t e r i n t a k e t o t h e p r o c e s s ;

- r e d u c i n g t h e t o t a l w a t e r i n t a k e t o t h e s u g a r h o u s e ;

- r e p l a c i n g o u t d a t e d p r o c e s s - e q u i p m e n t u n i t s by new ones f a c i l i t a t i n g a b e t t e r

e n e r g y economy;

- i m p r o v i n g t h e a u t o m a t i c c o n t r o l s f o r b e t t e r e n e r g y u t i l i z a t i o n i n t h e p r o c e s s .

( i i i ) I m p r o v i n g t h e c a p a b i l i t i e s o f t he e n e r g y s y s t e m , i n c l u d i n g t h e

e f f e c t i v e n e s s r a t i o o f t h e therma l s y s t e m . T y p i c a l measures a r e as f o l l o w s :

- r e d u c i n g t h e e n e r g y l o s s e s and t h e e n e r g y consumpt ion f o r a u x i l i a r y p u r p o s e s

i n t he power h o u s e ;

- i m p r o v i n g t h e power f a c t o r o f t h e e l e c t r i c a l s u b s y s t e m ;

- r e d u c i n g t h e hea t d i s s i p a t i o n f rom t h e components o f t h e e n e r g y sys tem t o t h e

e n v i r o n m e n t ;

- i m p r o v i n g t h e u t i l i z a t i o n o f c o n d e n s a t e s ;

- i m p r o v i n g t h e u t i l i z a t i o n o f l o w - p r e s s u r e v a p o u r s ;

- o p t i m i z i n g t h e u t i l i z a t i o n o f v a p o u r s f rom t h e e v a p o r a t o r ;

- r e p l a c i n g o u t d a t e d e n e r g y - s y s t e m equ ipment by more modern and e f f i c i e n t

m a c h i n e r y ;

- i m p r o v i n g a u t o m a t i c c o n t r o l s t o a c h i e v e b e t t e r e f f i c i e n c y o f e n e r g y c o n v e r s i o n

and d i s t r i b u t i o n p r o c e s s e s .

8 .1 .2 Rev iew o f examples

T h e r e i s a v a s t l i t e r a t u r e d e v o t e d t o s t e p - b y - s t e p improvements i n t h e e n e r g y

economy o f s u g a r f a c t o r i e s , a l t h o u g h i t i s f e l t t h a t p rob lems o f r a t i o n a l i z a t i o n

o f power b a l a n c e s a r e n o t a d e q u a t e l y c o v e r e d . Some p u b l i c a t i o n s s i m p l y p r e s e n t

p a r t i c u l a r measures unde r t aken i n s p e c i f i c f a c t o r i e s ( r e f s . 1 , 2 ) . V e r y v a l u a b l e

i n f o r m a t i o n can be f ound i n t h e a r t i c l e s a t t e m p t i n g t o draw g e n e r a l i z e d

c o n c l u s i o n s f rom t h e e x p e r i e n c e s o f c o n s u l t a n t s , e n g i n e e r i n g companies o r s u g a r

i n d u s t r y managers i n v o l v e d i n e n e r g y - r a t i o n a l i z a t i o n programmes implemented i n

a number o f s u g a r f a c t o r i e s ( r e f s . 3 - 1 2 ) .

The most i n t e r e s t i n g g roup o f p u b l i c a t i o n s i s t h a t d e v o t e d t o t h e e x p e r i e n c e s

accumu la ted i n s p e c i f i c f a c t o r i e s d u r i n g l o n g e r p e r i o d s o f s t e p - b y - s t e p

improvements ( r e f s . 1 3 - 1 6 ) . As p r e s e n t a t i o n s o f t h i s k i n d a r e r a t h e r s c a r c e , l e t

us a d d i t i o n a l l y c o n s i d e r two examples o f s t e p - b y - s t e p improvement programmes

292

wh i ch have been e f f e c t e d d u r i n g a p e r i o d o f 15 y e a r s i n Swed ish s u g a r f a c t o r i e s .

The da ta p r e s e n t e d be low have been e x t r a c t e d by t h e p r e s e n t a u t h o r f rom t h e

p u b l i s h e d o p e r a t i o n s r e p o r t s .

The p r o c e s s i n g c a p a b i l i t i e s o f t he f a c t o r i e s c o n s i d e r e d a r e 2100 and 5800

t o n s p e r d a y . As bo th f a c t o r i e s be long t o t he same company, i t can be assumed

t h a t t h e y have been s u b j e c t t o t he p r e s s u r e o f i d e n t i c a l economic s t i m u l a t o r s

and t h a t t h e same l e v e l o f t e c h n o l o g i c a l e x p e r t i s e has been a v a i l a b l e t o them.

T h e i r s t a r t i n g p o s i t i o n s i n 1970 can be summarized as f o l l o w s :

- bo th f a c t o r i e s were equ ipped w i t h t o w e r - t y p e e x t r a c t o r s , c l a s s i c a l j u i c e

p u r i f i c a t i o n s t a t i o n s , q u i n t u p l e - e f f e c t e v a p o r a t o r s and t h r e e - b o i l i n g

c r y s t a l l i z a t i o n schemes w i t h t h e a f f i n a t i o n o f C s u g a r ;

- bo th f a c t o r i e s were e q u i p p e d w i t h o i l - f i r e d b o i l e r s o p e r a t e d a t l i v e - s t e a m

pa rame te rs 40 ba r and 430°C;

- i n bo th c a s e s , abou t 10% o f t he power demand was c o v e r e d by power p u r c h a s e s

f rom the e x t e r n a l g r i d ;

- as t h e f l u e gases f rom b o i l e r s i n t h e l a r g e r o f t he two f a c t o r i e s were

u t i l i z e d i n t he p u l p - d r y e r f u r n a c e , no e c o n o m i z e r s were i n s t a l l e d t h e r e , t h i s

r e s u l t i n g i n a b o i l e r e f f i c i e n c y 8% l o w e r t h a n t h a t i n t h e s m a l l e r f a c t o r y ;

- f o l l o w i n g i n v e s t m e n t s made d u r i n g 1950s and 1960s, t h e l a r g e r f a c t o r y was

g e n e r a l l y e q u i p p e d w i t h more modern m a c h i n e r y and a u t o m a t i c c o n t r o l c i r c u i t s .

T a b l e s 8.1 and 8.2 l i s t t he r a t i o n a l i z a t i o n measures t h a t were implemented i n

t he e n e r g y economy and o t h e r r e l a t e d a r e a s i n bo th f a c t o r i e s d u r i n g t h e p e r i o d

1970-1985. The r e s u l t s were c a r e f u l l y c o n t r o l l e d a s , s t a r t i n g f rom the f i r s t o i l

c r i s i s i n 1974, t he f a c t o r i e s adop ted e n e r g y - m o n i t o r i n g p r o c e d u r e s based on

f r e q u e n t 24 -hou r e n e r g y - c o n s u m p t i o n t e s t s . I n a d d i t i o n , d e t a i l e d i n v e s t i g a t i o n s

o f t he e n e r g y economy emp loy ing one-week t e s t p e r i o d s were pe r f o rmed e v e r y

second o r t h i r d y e a r .

I n bo th f a c t o r i e s , t he m o d i f i c a t i o n s o f j u i c e h e a t i n g and t h e improvements

i n t r o d u c e d i n t he s u g a r houses seem t o have p l a y e d a d e c i s i v e r o l e i n r e d u c i n g

t he e n e r g y c o n s u m p t i o n . The t o t a l w a t e r i n t a k e and t he m a s s e c u i t e c i r c u l a t i o n

were r e d u c e d i n t he s u g a r h o u s e s , and t he pa ramete r f l u c t u a t i o n s o r i g i n a t i n g

f rom b a t c h w i s e o p e r a t i o n o f t he s u g a r house equ ipment were s u b s t a n t i a l l y

1 i m i t e d .

I n t he l a r g e r f a c t o r y , most o f t he i n v e s t m e n t s p r o p o s e d t o implement t h e

ene rgy -economy improvements were pe r f o rmed d u r i n g t h e a c t u a l p e r i o d . On t h e

c o n t r a r y , t he managers o f t he s m a l l e r f a c t o r y were n e i t h e r a b l e t o m o d e r n i z e t h e

t owe r e x t r a c t o r f o r l o w e r j u i c e d r a f t , n o r a b l e t o m o d i f y t h e j u i c e p u r i f i c a t i o n

s t a t i o n f o r l o w e r CaO r a t e ( t h r o u g h o u t t he p e r i o d o f i n t e r e s t i n c l u d i n g t h e 1985

s e a s o n , t h e CaO r a t e was 20-25% h i g h e r than i n t he l a r g e r f a c t o r y ) .

N e v e r t h e l e s s , t h e r e s u l t s o f t h e 1 5 - y e a r deve lopmen ts a r e q u i t e i m p r e s s i v e i n

293

TAB

LE

8.Ί

En

erg

y-s

avin

g

an

d

oth

er

rela

ted

m

ea

sure

s in

tro

du

ce

d

19

70

-19

85

, a

nd sta

tisti

ca

l d

ata

o

n

no

rma

l-fu

el

(he

ati

ng

va

lue

2

93

00

kJ/k

g)

co

nsu

mp

tio

n

in

sug

ar

ma

nu

factu

re

in

a

58

00

t/d

facto

ry.

\/,^

^w,

c^r

^^

'-c

A^

^^

-A^

^

No

rma

l fu

el

cons

umed

Y

ea

r S

pe

cif

ica

tio

n

^^^^^^^

19

66

-19

69

3.7

0-4

.17

1970

A

uto

ma

tic

bo

ilin

g

co

ntr

ols

in

sta

lle

d

on

va

cuu

m

pa

ns

C.

3.9

3

1971

T

wo

n

ew

li

me

d-j

uic

e

he

ate

rs

wit

h

incre

ase

d

he

ati

ng

su

rfa

ce

a

rea

s in

sta

lle

d.

He

ati

ng

3

.79

su

rfa

ce

a

rea

in

fo

urt

h

eva

po

rato

r e

ffe

ct

inc

rea

se

d.

Te

mp

era

ture

co

ntr

oll

ers

in

sta

lle

d

on

h

ea

ters

b

efo

re fi

rst

an

d

seco

nd

ca

rbo

na

tati

on

. 19

72

Le

ve

l c

on

tro

lle

rs

ins

tall

ed

o

n

wa

ter

se

als

(a

cti

ng

a

s st

ea

m

tra

ps

) in

co

nd

en

sate

3

.58

d

rain

ag

e

lin

es

fr

om

2

nd

, 3

rd

an

d

4th

e

va

po

rato

r e

ffe

cts

. 19

74

Th

erm

al

insu

lati

on

o

f fu

el

tan

ks

imp

rove

d.

Ne

w

bo

ile

r fo

r o

ff-s

ea

so

n

he

ati

ng

a

nd

ro

om

3.5

6

tem

pe

ratu

re

co

ntr

oll

ers

in

sta

lle

d.

On

e

ne

w

vacu

um

pan

A

e

qu

ipp

ed w

ith

a sti

rre

r a

nd

an

a

ir-t

igh

t,

low

-pre

ss

ure

ste

am

ing

syste

m i

nsta

lle

d.

1975

T

hre

e

co

nti

nu

ou

s a

ffin

ati

on

c

en

trif

ug

als

in

sta

lle

d.

On

e

co

nti

nu

ou

s ce

ntr

ifu

ga

l 3

.23

in

sta

lle

d

in

Β

str

ike

fo

r a

cce

pta

nce te

sts

. 19

76

Fo

ur

co

nti

nu

ou

s c

en

trif

ug

als

in

sta

lle

d

in

Β

str

ike

. O

ne

co

nti

nu

ou

s ce

ntr

ifu

ga

l 3

.42

in

sta

lle

d

in

C

str

ike

fo

r a

cce

pta

nce te

sts

. 19

77

Ne

w

ste

am

ing

-ou

t sy

ste

m

usin

g

se

co

nd

-eff

ect

va

po

ur

ins

tall

ed

in

a

ll va

cuum

p

an

s A

. 3

.21

19

78

Op

tim

iza

tio

n

of

C-m

asse

cu

ite

sta

tio

n

co

mp

lete

d.

3.3

6

1979

E

con

om

ize

r in

sta

lle

d

in

on

e

bo

ile

r.

Ve

nti

ng

o

f n

on

con

de

nsa

ble

s im

pro

ve

d.

Wa

ter

inta

ke

3

.38

to

ca

rbo

na

tati

on

slu

dg

e

sw

ee

ten

ing

-off

re

du

ce

d.

1980

F

low

m

ete

r in

sta

lle

d

on

w

ate

r su

pp

ly

to

the

su

ga

r h

ou

se.

3.2

8

1982

S

tirr

er

ins

tall

ed

in

o

ne

va

cuu

m

pa

n

A.

3.2

0

1983

T

ow

er

extr

acto

r m

od

ern

ize

d

for

low

er

juic

e

dra

ft.

Au

tom

ati

c b

oil

ing

c

on

tro

ls

on

va

cuu

m

3.1

8

pa

ns

A

mo

de

rniz

ed

. C

on

de

nsa

te-h

ea

ted

pla

te

he

at

exc

ha

ng

er

ins

tall

ed

a

s th

ick

-ju

ice

he

ate

r in

ste

ad

o

f a

tub

ula

r h

ea

t e

xch

an

ge

r h

ea

ted

by

thir

d-e

ffe

ct

va

po

ur.

19

84

Syr

up

w

ash

teste

d

in

on

e

ce

ntr

ifu

ga

l in

A str

ike

. 3

.15

19

85

Imp

rove

d

oil

bu

rne

rs

ins

tall

ed

in

o

ne

bo

ile

r.

Mic

roc

om

pu

ter-

ba

se

d

bo

ile

r co

ntr

ol

3.1

3

syst

em

im

ple

me

nte

d.

294

TAB

LE

8.2

En

erg

y-s

avin

g

an

d

oth

er

rela

ted

m

ea

sure

s in

tro

du

ce

d

19

70

-19

85

, a

nd sta

tisti

ca

l d

ata

on

no

rma

l-fu

el

(he

ati

ng

v

alu

e

29

30

0 kJ/k

g)

co

nsu

mp

tio

n

in

sug

ar

ma

nu

factu

re

in

a

21

00

t/d

fac

tory

.

^ e

.jr-

4.·

N

orm

al

fue

l co

nsum

ed

Ye

ar

Sp

ec

ific

ati

on

^

^^

/^^

^

19

67

-19

70

3.8

8-4

.02

1971

N

ew

m

ea

su

rin

g

instr

um

en

ts

insta

lle

d

on

the

eva

po

rato

r sta

tio

n.

Eq

uip

me

nt

op

era

tors

4

.09

instr

ucte

d

on

the

pri

nc

iple

s

of

smo

oth

o

pe

rati

on

o

f A

-ma

sse

cu

ite sta

tio

n.

1972

E

ne

rgy

econ

omy

an

aly

se

d

an

d

eq

uip

me

nt

mo

de

rniz

ati

on

p

rog

ram

me

pro

po

se

d.

3.9

8

1973

T

he

rma

l in

su

lati

on

re

pla

ce

d

on

thre

e

eva

po

rato

r b

od

ies

an

d

on

e

co

nd

en

sa

te

tan

k.

4.1

7

1974

S

tirr

er

insta

lle

d

in

on

e

vacu

um

pan

A.

New

b

oile

r fo

r o

ff-s

ea

so

n

he

ati

ng

insta

lle

d.

3.9

6

1975

N

ew

m

ea

su

rin

g

instr

um

en

ts

insta

lle

d

on

the

extr

acto

r a

s w

ell

as

on

va

cuum

p

an

s Β

a

nd

C.

3.4

9

1977

P

erf

orm

an

ce

of

C-m

asse

cu

ite

sta

tio

n

an

aly

se

d

an

d

mo

de

rniz

ati

on

p

rop

ose

d.

3.6

6

1978

C

on

tin

uo

us

ce

ntr

ifu

ga

ls

insta

lle

d

for

incre

ase

d

thro

ug

hp

ut

of

C-m

asse

cu

ite

sta

tio

n.

3.7

7

1979

A

uto

ma

tic

tem

pe

ratu

re

co

ntr

ol

cir

cu

it

insta

lle

d

on

exh

au

st

ste

am

su

pp

ly

to

the

3.6

5

eva

po

rato

r sta

tio

n.

Le

ve

l co

ntr

olle

rs

insta

lle

d

on

wa

ter

se

als

(a

cti

ng

a

s st

ea

m t

rap

s)

in

co

nd

en

sa

te

dra

ina

ge

lin

es

b

etw

ee

n

eva

po

rato

r e

ffe

cts

3

an

d

4

as

we

ll

as

4

an

d

5.

1980

S

yru

p

was

h im

ple

me

nte

d

in

ce

ntr

ifu

ga

ls

A

an

d

B.

En

erg

y ec

onom

y a

na

lyse

d.

3.7

3

1981

C

he

mic

al

sc

ale

p

reve

nti

on

im

ple

me

nte

d.

3.4

5

1982

S

tea

m-t

urb

ine

dri

ve

n

kiln

-ga

s

pum

ps

rep

lace

d

by

ne

w e

lec

tric

all

y-d

riv

en

o

ne

s.

Ne

w

3.4

0

he

ate

r fo

r th

in ju

ice

insta

lle

d.

Va

po

ur

dis

trib

uti

on

sc

hem

e a

dju

ste

d

for

be

tte

r u

tiliza

tio

n

of

low

-te

mp

era

ture

va

po

urs

. E

ne

rgy

econ

omy

an

aly

se

d.

1983

A

uto

ma

tic

leve

l-co

ntr

ol

cir

cu

its

in

th

e

eva

po

rato

r sta

tio

n

mo

de

rniz

ed

. S

pir

al

he

at

3.4

6

exc

ha

ng

ers

h

ea

ted

by

co

nd

en

sa

te

an

d

fou

rth

-eff

ect

vap

ou

r im

ple

me

nte

d

as

raw

-ju

ice

he

ate

rs.

Inte

gra

ted

a

uto

ma

tic

co

ntr

ol

of

the

be

et

ho

use

im

ple

me

nte

d.

1984

S

tirr

ers

in

sta

lle

d

in

all

vacu

um

pa

ns

A.

Co

nti

nu

ou

s ce

ntr

ifu

ga

ls

insta

lle

d

in

Β str

ike

. 3

.14

Pro

gra

mm

ab

le

co

ntr

ol

of

A

ce

ntr

ifu

ga

ls

imp

lem

en

ted

. C

he

mic

al

sc

ale

p

reve

nti

on

im

pro

ve

d.

1985

3

.23

295

bo th c a s e s . The f u e l consumpt ion e x p r e s s e d i n kg normal f u e l p e r 100 kg b e e t has

been reduced by abou t 20% i n t he l a r g e r f a c t o r y and by abou t 30% i n t he s m a l l e r .

I n o r d e r t o i n i t i a t e ene rgy -economy improvements u s i n g t h e s t e p - b y - s t e p

a p p r o a c h , s u f f i c i e n t i n f o r m a t i o n must be a v a i l a b l e on t h e e x i s t i n g s t a t e o f

t h i n g s and p o s s i b l e c o u r s e s o f a c t i o n . I n t h e f o l l o w i n g , two examples a re

p r e s e n t e d o f t h e r e s u l t s o f s t u d i e s i n t e n d e d t o c r e a t e c o n v e n i e n t s t a r t i n g

p o i n t s f o r s t e p - b y - s t e p improvements .

The f i r s t example ( S e c t i o n 8 .2 ) i s r e p r e s e n t a t i v e o f r a t h e r s m a l l , n o t - s o -

modern f a c t o r i e s c h a r a c t e r i z e d by r a t h e r p o o r e n e r g y u t i l i z a t i o n and v e r y

l i m i t e d i n i t i a l knowledge o f measures t h a t can be taken t o improve i t . The

i n i t i a l f u e l consumpt ion i n s u g a r manu fac tu re i s abou t 6.7 kg normal f u e l p e r

100 kg b e e t . P o s s i b l e improvements a r e s t u d i e d on t he b a s i s o f mass and hea t

b a l a n c e s w h i c h a r e c a l c u l a t e d u s i n g i n p u t d a t a e x t r a c t e d f rom r o u t i n e f a c t o r y

r e c o r d s . The h e a t s a v i n g w h i c h can be o b t a i n e d by t a k i n g t h e most u r g e n t

r a t i o n a l i z a t i o n measures (a imed m a i n l y a t r e d u c i n g t h e e n e r g y w a s t e ) i s

e s t i m a t e d a t abou t 20% o f t he i n i t i a l hea t c o n s u m p t i o n . I n o r d e r t o p r e p a r e

d e c i s i o n s on f u r t h e r improvemen ts , a d e t a i l e d d e s i g n s t u d y i s recommended.

The second example ( S e c t i o n 8 .3 ) i s c o n c e r n e d w i t h a m e d i u m - c a p a c i t y s u g a r

f a c t o r y c h a r a c t e r i z e d by a q u i t e e f f e c t i v e e n e r g y economy. The i n i t i a l f u e l

consumpt ion i n s u g a r manu fac tu re i s 3.5 kg normal f u e l pe r 100 kg b e e t . An

a n a l y s i s o f ways t o improve t he hea t economy i s pe r f o rmed u s i n g t h e r e s u l t s o f

measurements o f t he pa rame te rs o f e n e r g y c o n v e r s i o n and u t i l i z a t i o n p r o c e s s e s .

I n t h i s manner , a r e l i a b l e b a s i s i s c r e a t e d f o r e v a l u a t i o n o f t he consequences

o f smal l improvements o f t h e v a p o u r d i s t r i b u t i o n . The e s t i m a t e d hea t s a v i n g i s

o f t he o r d e r o f 2% o f t he i n i t i a l hea t c o n s u m p t i o n .

The e x p e r i e n c e p r o v e s t h a t even h i g h l y e f f i c i e n t modern e n e r g y sys tems can be

improved u s i n g t he s t e p - b y - s t e p a p p r o a c h . To i l l u s t r a t e t h i s p o i n t . S e c t i o n 8.4

p r e s e n t s a summary o f e n e r g y - s a v i n g measures t aken d u r i n g a 1 0 - y e a r p e r i o d i n

a l a r g e r a w - s u g a r f a c t o r y . A t p r e s e n t , t h e f a c t o r y i s consuming l e s s t han 2 kg

normal f u e l p e r 100 kg b e e t . T h i s example i n d i c a t e s a l s o t h e impo r tance o f

c o o r d i n a t i n g t he hea t and power b a l a n c e s a t a v e r y low hea t demand. I n a d d i t i o n ,

t he d i f f i c u l t y i s demons t ra ted o f d i s t i n g u i s h i n g between s t e p - b y - s t e p

improvements and a m o d e r n i z a t i o n . A l t h o u g h t h e g e n e r a l app roach t o t h e

improvements can be c o n s i d e r e d as e v o l u t i o n a r y , some o f t h e s t e p s t aken i n v o l v e

e x t e n s i v e t e c h n o l o g i c a l changes and r a t h e r c o s t l y i n v e s t m e n t s .

8.2 FACTORY CHARACTERIZED BY POOR I N I T I A L ENERGY U T I L I Z A T I O N

8.2.1 I n t r o d u c t o r y remarks

The example p r e s e n t e d i n t h i s S e c t i o n i s based on a r e a l case i n v e s t i g a t e d by

t he p r e s e n t a u t h o r a few y e a r s a g o . The c o n s u l t a n t was i n v i t e d t o t h e f a c t o r y i n

296

q u e s t i o n by a manager whose i n t e r e s t i n e n e r g y economy was o b v i o u s l y s t i m u l a t e d

by t h e r a p i d l y r i s i n g o i l p r i c e s a t t h a t t i m e . A n o t h e r c o n t r i b u t i n g f a c t o r was

t h e p l a g u e o f f r e q u e n t b o i l e r f a i l u r e s t h a t s e r i o u s l y a f f e c t e d t h e o p e r a t i o n a l

r e s u l t s . T h e r e was an e x p e c t a t i o n t h a t some q u i c k a c t i o n s c o u l d be u n d e r t a k e n

d u r i n g t h e s u b s e q u e n t o f f - s e a s o n p e r i o d , w i t h t he aim o f a c h i e v i n g s u b s t a n t i a l

e n e r g y s a v i n g s a l r e a d y i n t he n e x t s e a s o n . The manager was r e a d y t o c o n t i n u e

t he r a t i o n a l i z a t i o n o f t he e n e r g y economy i n coming y e a r s , b u t he made i t c l e a r

t h a t i n t h e f o r e s e e a b l e f u t u r e , no i n v e s t m e n t f u n d s w o u l d be a v a i l a b l e f o r an

e x t e n s i v e m o d e r n i z a t i o n o f t h e f a c t o r y .

The i n v i t a t i o n came as t he season app roached i t s e n d . No a c t u a l d a t a were

a v a i l a b l e on t h e d e t a i l s o f t he f a c t o r y ' s e n e r g y b a l a n c e . As t h e r e was no t ime

l e f t f o r p r e p a r i n g a d d i t i o n a l measurements , i t became c l e a r t h a t e v a l u a t i o n o f

t he mass and e n e r g y b a l a n c e s s h o u l d be pe r f o rmed on t h e b a s i s o f t h e d a t a

e x t r a c t e d f rom t h e f a c t o r y r e c o r d s o r measured by t h e e x i s t i n g i n s t r u m e n t a t i o n .

8 .2 .2 B a s i c f a c t o r y d a t a and scheme o f t h e s u g a r m a n u f a c t u r i n g p r o c e s s

P r o c e s s i n g c a p a b i l i t y : 2100 t / d .

P o l a r i z a t i o n o f c o s s e t t e s : 16.18%.

E x t r a c t o r : t r o u g h - t y p e .

J u i c e d r a f t : 120%.

R a w - j u i c e c o n c e n t r a t i o n and p u r i t y : 15.80% DS and 86.82%.

Pu lp p r e s s e d t o : 17.3% DS.

P o l a r i z a t i o n o f p r e s s e d p u l p : 1%.

K i l n g a s : 30-32% CO^ v o l .

J u i c e p u r i f i c a t i o n a c c o r d i n g t o t he c l a s s i c a l scheme, c o m p r i s i n g :

- h o t p r e - l i m i n g a t 45°C, CaO r a t e 0.30 kg/100 kg b;

- main l i m i n g a t 85°C, CaO r a t e 2.01 kg/100 kg b ;

- c a r b o n a t a t i o n I a t 80-85°C;

- d o u b l e - s t a g e f i l t r a t i o n I ;

- c a r b o n a t a t i o n I I a t 93-97°C;

- s i n g l e - s t a g e f i l t r a t i o n I I .

P u r i f i c a t i o n e f f e c t : 36%.

T h i n - j u i c e c o n c e n t r a t i o n and p u r i t y : 15.63% DS and 91.19%.

E v a p o r a t o r s t a t i o n : q u a d r u p l e - e f f e c t , R o b e r t - t y p e b o d i e s .

T h i c k - j u i c e c o n c e n t r a t i o n : 61.4% DS.

Sugar h o u s e :

- r a w - s u g a r a d d i t i o n 3.21 kg/100 kg b;

- t h r e e - b o i l i n g scheme w i t h t h e a f f i n a t i o n o f C s u g a r and raw s u g a r ;

- Β s u g a r me l t ed i n w a t e r ;

- m i x t u r e o f a f f i n e d C s u g a r and raw s u g a r m e l t e d i n t h i n j u i c e .

297

Sugar o u t p u t ( i n c l u d i n g s u g a r i n t r o d u c e d as raw s u g a r ) : 15.73 kg/100 kg b.

Power h o u s e :

- o i l - f i r e d b o i l e r s , s i x u n i t s r a t e d 12.5 t / h and one 6.5 t / h , a v e r a g e

e f f i c i e n c y abou t 80%;

- l i v e steam pa ramete rs 25 ba r and 425°C;

- two b a c k - p r e s s u r e t u r b i n e s r a t e d 3 MW e a c h ;

- b a c k - p r e s s u r e 3.3 b a r .

Steam s u p p l y t o t he s u g a r m a n u f a c t u r i n g p r o c e s s :

- l i v e steam t h r o t t l e d t o 4 .9 ba r t o t h e c e n t r i f u g a l s ;

- h e a t i n g steam 3.3 bar ( e x h a u s t steam and t h r o t t l e d l i v e steam) t o t he

rema in ing r e c e i v e r s .

H e a t i n g steam c o n s u m p t i o n : 56-57 kg/100 kg b.

Normal f u e l c o n s u m p t i o n : abou t 6.5 kg/100 kg b.

The schemes o f t h e bee t house and s u g a r house a r e shown i n F i g s . 8.1 and 8 . 2 ,

r e s p e c t i v e l y .

8 .2 .3 Scheme o f t he therma l sys tem

The scheme i s shown i n F i g . 8 . 3 . T h e r e a r e s e v e r a l q u e s t i o n a b l e d e t a i l s t o

a c c o u n t f o r i n t h e e v a l u a t i o n o f t he hea t economy:

- t h e h e a t i n g sys tems o f t h e f a c t o r y b u i l d i n g s and s u g a r s i l o s a r e s u p p l i e d w i t h

e x h a u s t s team;

- m e l t e r , r e m e l t h e a t e r , t h i n - j u i c e h e a t e r and s y r u p t a n k s a r e h e a t e d by e x h a u s t

s team;

- s t e a m i n g - o u t o f vacuum pans i s p e r f o r m e d u s i n g e x h a u s t s team;

- most condensa te f rom s team- and v a p o u r - h e a t e d e q u i p m e n t , and t h e c o n d e n s a t e

f rom t h e h e a t i n g s y s t e m s , i s w a s t e d ;

- t he f o u r t h e v a p o r a t o r e f f e c t i s o p e r a t e d as a c o n c e n t r a t o r , i m p l y i n g t h a t t h e

e v a p o r a t o r i s e s s e n t i a l l y o p e r a t e d as a t r i p l e - e f f e c t ;

- t he v a p o u r d i s t r i b u t i o n scheme i s r a t h e r p r i m i t i v e and t h e t e m p e r a t u r e s o f t h e

v a p o u r s s u p p l i e d t o t h e i n d i v i d u a l h e a t e r s do n o t s a t i s f y t he r e q u i r e m e n t o f

minimum t e m p e r a t u r e d i f f e r e n c e s ;

- t h e scheme o f t he c o n d e n s a t e subsys tem i n t he e v a p o r a t o r a r e a i s a l s o

p r i m i t i v e , r e s u l t i n g i n poo r u t i l i z a t i o n o f t h e c o n d e n s a t e e n e r g y ;

- t he h e a t i n g s u r f a c e a r e a s o f t he e v a p o r a t o r b o d i e s and most j u i c e h e a t e r s a r e

v e r y l a r g e .

8 .2 .4 A d d i t i o n a l i n f o r m a t i o n a c q u i r e d i n t h e f a c t o r y

I n t h e p r o c e s s h e a t i n g a r e a , a number o f m a l f u n c t i o n i n g subsys tems and

components were i d e n t i f i e d :

- a l l t h e steam t r a p s were o f t he f l o a t t y p e and some o f them were l e a k i n g

v a p o u r t o t he condensa te l i n e s ;

298

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to boi ler — — house — 1 L to 6^

F i g . 8 . 3 . Scheme o f t h e the rma l sys tem i n the p r o c e s s h e a t i n g a r e a . Condensa te d r a i n a g e l i n e s n o t shown i n t he p i c t u r e a r e c o n n e c t e d t o t h e sewer s y s t e m . The meaning o f numbers d e n o t i n g equ ipment u n i t s i s i d e n t i c a l t o t h a t i n F i g . 1.5.

- s t a r t - u p v e n t s i n s e v e r a l steam t r a p s were kep t p e r m a n e n t l y o p e n , l e a k i n g

v a p o u r t o t he e n v i r o n m e n t ;

- v e n t i n g o f noncondensab les was c l e a r l y i n a c h a o t i c c o n d i t i o n , h e a t e r v e n t s

be ing kep t c l o s e d and vacuum-pan v e n t s l e a k i n g t o o much v a p o u r t o t h e

e n v i r o n m e n t ;

- a number o f measur ing i n s t r u m e n t s were m i s s i n g o r o u t o f o r d e r .

I n t he power house a r e a , t he equ ipment seemed t o be r a t h e r w e l l m a i n t a i n e d .

Among t h e measur ing i n s t r u m e n t s , f l u e gas a n a l y s e r s were o u t o f o r d e r and steam

f l o w meters were c l e a r l y n o t w o r k i n g r e l i a b l y . The most s e r i o u s p rob lem seemed

t o be t he poo r q u a l i t y o f make-up w a t e r , w i t h r e s u l t i n g c o r r o s i o n damage

f r e q u e n t l y o c c u r r i n g i n t he b o i l e r t u b e s .

C o n c e r n i n g t he hea t d i s t r i b u t i o n and u t i l i z a t i o n p r o c e s s e s , a number o f

301

d e f i c i e n c i e s i n t he thermal sys tem and i n t h e s u g a r m a n u f a c t u r i n g p r o c e s s can be

immed ia te l y i d e n t i f i e d :

- e x c e s s i v e hea t l o s s e s due t o l a r g e s e c t i o n s o f the rma l i n s u l a t i o n b e i n g i n

poo r c o n d i t i o n o r m i s s i n g ( e . g . a few h e a t e r s , v a p o u r p i p e l i n e s f rom t h e t h i r d

e v a p o r a t o r e f f e c t and a number o f c o n d e n s a t e p i p e s ) ;

- e x c e s s i v e hea t consumpt ion i n t he r o o m - h e a t i n g s y s t e m , due t o l a c k o f room

t e m p e r a t u r e c o n t r o l ;

- e x c e s s i v e hea t l o s s e s due t o u n c o v e r e d t a n k s i n t h e s u g a r house (most o f t h e

e x i s t i n g c o v e r s kep t p e r m a n e n t l y o p e n ) ;

- n o n - o p t i m a l o p e r a t i o n o f t he e v a p o r a t o r and e x c e s s i v e pa ramete r f l u c t u a t i o n s

due t o i n a d e q u a t e a u t o m a t i c c o n t r o l s .

I t g r a d u a l l y became c l e a r d u r i n g t he v i s i t t o t h e f a c t o r y t h a t i t s e n e r g y -

economy p rob lems were caused m a i n l y by a was te o f hea t i n t h e p r o c e s s h e a t i n g

a r e a . T h i s was accompanied by a was te o f c o n d e n s a t e s , and t h u s e x c e s s i v e

consumpt ion o f make-up w a t e r w h i c h must be added t o t h e b o i l e r f e e d . When

o v e r l o a d i n g t h e w a t e r - t r e a t m e n t s t a t i o n by a l a r g e f l o w o f p o l l u t e d r i v e r w a t e r ,

t he q u a l i t y o f make-up w a t e r became u n s a t i s f a c t o r y . T h i s c o n t r i b u t e d t o

a c c e l e r a t e d c o r r o s i o n , w h i c h was t h e immediate cause o f b o i l e r - t u b e f a i l u r e s .

T a k i n g i n t o a c c o u n t t h a t no p o w e r - b a l a n c e p rob lems were d e t e c t e d and t h a t t he

power house seemed t o be w e l l m a i n t a i n e d and o p e r a t e d , t h e above c o n c l u s i o n s

p o i n t a t p r o c e s s h e a t i n g as t h e e s s e n t i a l p rob lem t o w h i c h a t t e n t i o n s h o u l d be

d i r e c t e d when p e r f o r m i n g s u b s e q u e n t s t a g e s o f t h e a n a l y s i s .

8 .2 .5 Mass b a l a n c e o f t h e s u g a r m a n u f a c t u r i n g p r o c e s s

The c a l c u l a t e d mass b a l a n c e o f t h e b e e t house i s shown i n T a b l e 8 . 3 . The

pa rame te rs o f t he e x t r a c t i o n and j u i c e p u r i f i c a t i o n p r o c e s s e s can be r e g a r d e d as

e s s e n t i a l l y c o r r e c t .

The c a l c u l a t e d mass b a l a n c e o f t he s u g a r house i s shown i n T a b l e 8 . 4 . The

pa rame te rs o f g r e e n s y r u p s A and B, i . e . h i g h p u r i t i e s and r e l a t i v e l y low

c o n c e n t r a t i o n s , i n d i c a t e t h a t t o o much wash w a t e r i s s u p p l i e d t o A and Β

c e n t r i f u g a l s . As a r e s u l t , c r y s t a l s d i s s o l v e i n e x c e s s w a t e r , c a u s i n g a r e d u c e d

c r y s t a l y i e l d and i n c r e a s e d c i r c u l a t i o n o f m a s s e c u i t e s i n t h e s u g a r h o u s e . T h i s

c o n t r i b u t e s t o t h e e x c e s s i v e h e a t demand i n t h e s u g a r h o u s e .

8 .2 .6 Heat b a l a n c e o f t he the rma l sys tem and h e a t economy e v a l u a t i o n

The hea t b a l a n c e c a l c u l a t i o n s were p e r f o r m e d a c c o r d i n g t o t h e e v a p o r a t o r -

r e c e i v e r a p p r o a c h . The r e s u l t s , e x p r e s s e d i n steam and v a p o u r f l o w s , a r e shown

i n T a b l e 8 . 5 . The b a l a n c e da ta seem t o c o n f i r m t h e i m p r e s s i o n g a i n e d f rom t h e

r e v i e w o f t he the rma l sys tem scheme t h a t t h e v a p o u r and condensa te d i s t r i b u t i o n

i s r a t h e r p r i m i t i v e . I t can a l s o be seen t h a t t h e c a l c u l a t e d steam demand i s

3 .5 -4 .5 kg/100 kg b l e s s t han t h e steam consumpt i on t aken f rom t h e f a c t o r y

302

TABLE 8.3

Mass b a l a n c e o f t h e b e e t h o u s e .

No. St ream name T o t a l f l o w C o n c e n t r a t i o n P u r i t y

No. St ream name (kg /100 kg b) (% DS) (%)

1 C o s s e t t e s 100.00 85.70

2 Wet p u l p 85.00 3 P r e s s e d p u l p 30.70 17.30 4 P r e s s w a t e r 54.30 1.50 5 F r e s h w a t e r 50.70 6 Raw j u i c e 120.00 15.17 86.82 7 J u i c e t o p r e - l i m i n g 123.76 8 J u i c e t o main l i m i n g 125.28 9 J u i c e t o c a r b o n a t a t i o n I 135.35

10 J u i c e f rom vacuum f i l t e r s 17.37 11 J u i c e t o t h i c k e n e r s I 152.45 12 J u i c e t o c a r b o n a t a t i o n I I 130.20 13 S u b s i d e r s l u d g e I 22.25 14 S w e e t e n i n g - o f f w a t e r t o vacuum f i l t e r s ; 14.33 15 S ludge 9.55 16 Sweet w a t e r f rom vacuum f i l t e r s 9.55 17 J u i c e t o t h i c k e n e r s I I 130.16 18 S u b s i d e r s l u d g e I I t o p r e - l i m i n g 3.76 19 J u i c e t o f i n e f i l t e r s 126.40 20 T h i n j u i c e 126.40 13.56 91.19 21 T h i n j u i c e t o e v a p o r a t i o n 119.57 22 M i l k - o f - l i m e t o p r e - l i m i n g 1.52 23 M i l k - o f - l i m e t o main l i m i n g 10.07 24 K i l n gas t o c a r b o n a t a t i o n I 4.49 25 K i l n gas t o c a r b o n a t a t i o n I I 0.40

r e c o r d s . Most p r o b a b l y , t h i s i s an i n d i c a t i o n o f steam a n d / o r v a p o u r l e a k s

o c c u r r i n g i n t h e condensa te d r a i n a g e s u b s y s t e m .

On t he b a s i s o f t he da ta t h u s a c q u i r e d , t h e f o l l o w i n g g e n e r a l c o n c l u s i o n s

were d r a w n .

( i ) The the rma l sys tem i s c l e a r l y w o r k i n g l e s s e f f e c t i v e l y t han c o u l d be

e x p e c t e d on t h e b a s i s o f i t s d e s i g n and equ ipment c h a r a c t e r i s t i c s . The most

i m p o r t a n t r e a s o n s a r e :

- steam and v a p o u r l e a k s w i t h i n t h e sys tem o r d i r e c t l y t o t he e n v i r o n m e n t ;

- u n r e l i a b l e v e n t i n g o f n o n c o n d e n s a b l e s ;

- e x c e s s i v e hea t d i s s i p a t i o n t o t h e e n v i r o n m e n t .

( i i ) The d e s i g n and pa rame te rs o f t he s u g a r m a n u f a c t u r i n g p r o c e s s i n t h e s u g a r

house a r e a a r e c o n t r i b u t i n g t o t h e e x c e s s i v e t o t a l h e a t demand. The e s s e n t i a l

d e f i c i e n c i e s a r e :

- u n n e c e s s a r y w a t e r i n t a k e t o Β r e m e l t ;

- t o o l a r g e w a s h - w a t e r consumpt ion i n A and Β c e n t r i f u g a l s .

( i i i ) The e f f e c t i v e n e s s o f t h e h e a t d i s t r i b u t i o n scheme i n t he p r o c e s s h e a t i n g

a r e a i s t o o l o w . The u n d e r l y i n g r e a s o n s a r e :

- was te o f c o n d e n s a t e s ;

303

TABLE 8.4

Mass b a l a n c e o f t he s u g a r h o u s e .

7Γ ¡ri77~T7Z T o t a l f l o w C o n c e n t r a t i o n P u r i t y No. St ream name ^^^^^^^ ./ o/ ^

1 T h i c k j u i c e 26.24 61.40 91 .14 2 T h i n j u i c e 6.83 13.56 91 .19 3 Raw s u g a r 3.21 98.73 98.70 4 A m a s s e c u i t e 41.33 91.90 93.74 5 Green s y r u p A 24.15 79.70 89.00 6 Wash s y r u p A 3.84 74.19 91.94 7 Whi te s u g a r 15.73 99.95 99.80 8 Β m a s s e c u i t e 18.51 92.00 88.39 9 Green s y r u p Β 8.15 81.80 79.20

10 Wash s y r u p Β 3.79 74.18 85.69 11 Β s u g a r 7.59 99.50 97.50 12 C m a s s e c u i t e 13.32 94.60 82.93 13 C s u g a r 8.56 98.50 92.50 14 M o l a s s e s 4.76 87.60 63.60 15 A f f i n a t i o n m a s s e c u i t e 21.26 90.00 91.93 16 A f f i n a t i o n s y r u p 11.04 78.81 87.42 17 A f f i n e d C s u g a r 10.65 98.00 95.69 18 Β r e m e l t 11.86 65.00 97.55 19 C - a f f . r e m e l t 17.48 65.00 95.32 20 Water t o c e n t r i f u g a l s A 1.53 21 Steam t o c e n t r i f u g a l s A 1.03 22 Water t o c e n t r i f u g a l s Β 1.02 23 Water t o Β r e m e l t 4.11

TABLE 8.5

Steam and v a p o u r f l o w s (kg /100 kg b) between s o u r c e s and r e c e i v e r s i n t he p r o c e s s - h e a t i n g a rea o f t h e the rma l s y s t e m .

S o u r c e s No. R e c e i v e r s E x h a u s t E v a p o r a t o r e f f e c t s n4-höv-c

steam 1 2 3 4 ^^"^^^^

1 E x t r a c t o r 1.18 1.85 2 R a w - j u i c e h e a t e r s 1.23 c o n d e n s a t e 3 L i m e d - j u i c e h e a t e r s 0.44 6.30 4 H e a t e r i n c a r b o n a t a t i o n I 1.30 5 C l e a r - j u i c e h e a t e r 3.67 6 T h i n - j u i c e h e a t e r s 5.85 7 T h i c k - j u i c e h e a t e r 0.45 8 M e l t e r 0.40 9 I n d i r e c t l y - h e a t e d s y r u p t a n k s 0.38

10 D i r e c t l y - h e a t e d s y r u p t a n k s 0.55 11 Remel t h e a t e r 0.36 12 Vacuum pans A 18.61 13 Vacuum pans Β 3.46 14 Vacuum pans C 2.67 15 Vacuum-pan s teaming 1.80 16 O t h e r smal l r e c e i v e r s 0.50 17 Sugar d r y e r 0.50 18 Condense r 0.58 19 E v a p o r a t o r t o t a l 5.85 30.03 10.67 0.58 20 E x h a u s t - s t e a m consumpt ion 51.58

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- no u t i l i z a t i o n o f f o u r t h - e f f e c t v a p o u r ;

- i n a d e q u a t e u t i l i z a t i o n o f t h i r d - e f f e c t v a p o u r ;

- u n n e c e s s a r y h e a t i n g w i t h e x h a u s t s team;

- u n n e c e s s a r y u t i l i z a t i o n o f e x h a u s t steam f o r a u x i l i a r y p u r p o s e s .

8 .2 .7 P o s s i b l e improvements and i m p l e m e n t a t i o n s t r a t e g y

E v a l u a t i o n o f t h e hea t economy i n d i c a t e s t h a t s u b s t a n t i a l improvements can be

o b t a i n e d by t a k i n g v a r i o u s r a t i o n a l i z a t i o n measu res . As t he pa rame te rs o f t he

equ ipment a r e s u f f i c i e n t l y w e l l s u i t e d t o t h e f a c t o r y ' s n e e d s , t he s t e p - b y - s t e p

approach a v o i d i n g l a r g e i n v e s t m e n t s seems t o be s u f f i c i e n t t o c u t down t h e hea t

consumpt ion c o n s i d e r a b l y .

The imp lemen ta t i on s t r a t e g y e v o l v e s n a t u r a l l y f rom t h e f o l l o w i n g l o g i c a l

sequence o f c o r r e c t i v e a c t i o n s .

( i ) F i r s t , t h e was te o f e n e r g y s h o u l d be i m m e d i a t e l y e l i m i n a t e d by r e p a i r i n g o r

r e p l a c i n g a l l t h e m a l f u n c t i o n i n g a u x i l i a r y components o f t h e therma l s y s t e m ,

t h a t i s :

- l e a k i n g steam t r a p s ;

- u n r e l i a b l e v e n t s ;

- i n o p e r a t i v e measur ing i n s t r u m e n t s .

( i i ) S e c o n d , t h e t o t a l hea t demand o f t he s u g a r m a n u f a c t u r i n g p r o c e s s s h o u l d be

reduced b y :

- r e p a i r i n g o r i n s t a l l i n g damaged o r m i s s i n g s e c t i o n s o f t h e the rma l i n s u l a t i o n

i n t h e p r o c e s s h e a t i n g a r e a ;

- e l i m i n a t i n g t he use o f e x h a u s t steam f o r a u x i l i a r y p u r p o s e s and r e p l a c i n g i t

by f i r s t - o r s e c o n d - e f f e c t v a p o u r ;

- m o d i f y i n g t he c r y s t a l l i z a t i o n scheme t o t h e fo rm shown i n F i g . 8 . 4 , t h u s

c u t t i n g down the t o t a l w a t e r i n t a k e t o t he s u g a r house (B r e m e l t i s p r e p a r e d

u s i n g t h i n j u i c e i n s t e a d o f w a t e r ) ;

- r e d u c i n g t h e consumpt ion o f wash w a t e r i n A and Β c e n t r i f u g a l s ;

- r e d u c i n g t h e t o t a l w a t e r i n t a k e t o t h e s u g a r house even f u r t h e r by i n c r e a s i n g

t he t h i c k - j u i c e c o n c e n t r a t i o n .

The e s t i m a t e d mass b a l a n c e o f t he s u g a r house a f t e r w a t e r i n t a k e r e d u c t i o n s i s

shown i n T a b l e 8 . 6 .

( i i i ) The e f f e c t i v e n e s s r a t i o o f t he the rma l sys tem s h o u l d be i m p r o v e d . F i r s t o f

a l l , i t i s n e c e s s a r y t o e l i m i n a t e t h e was te o f c o n d e n s a t e s by i n s t a l l i n g

condensa te d r a i n a g e l i n e s l i n k i n g a l l t h e h e a t e r s w i t h p r o p e r l y s e l e c t e d

r e s p e c t i v e condensa te t a n k s . W h i l e i m p r o v i n g t h e e n e r g y r e c o v e r y , t h i s w i l l

r educe t he d e f i c i t o f make-up w a t e r and c o n t r i b u t e t o improved b o i l e r

r e l i a b i l i t y . F u r t h e r improvement i n t h i s r e s p e c t can be o b t a i n e d by r e p l a c i n g

t he o l d - f a s h i o n e d steam h e a t i n g o f f a c t o r y b u i l d i n g s by w a t e r h e a t i n g . To

comple te t h e c o r r e c t i v e a c t i o n s r e l a t e d t o t h e c o n d e n s a t e subsys tem

306

T o t a l f l o w C o n c e n t r a t i o n P u r i t y No. St ream name ^ ^ ^ / ^ ^ ^ (o/^j

1 T h i c k j u i c e 24.57 65.00 91.14 2 T h i n j u i c e 7.91 13.56 91.19 3 Raw s u g a r 3.21 98.73 98.70 4 A m a s s e c u i t e 31.27 92.00 94.11 5 Green s y r u p A 12.43 81.00 85.00 6 Wash s y r u p A 3.25 76.93 92.91 7 Wh i te s u g a r 16.05 99.96 99.95 8 Β m a s s e c u i t e 11.80 93.00 86.29 9 Green s y r u p Β 5.24 83.00 72.00

10 Wash s y r u p Β 1.26 79.27 85.29 11 Β s u g a r 5.66 99.50 97.50 12 C m a s s e c u i t e 7.86 93.50 76.87 13 C s u g a r 3.55 98.50 95.50 14 Mo lasses 4.62 83.50 60.00 15 A f f i n a t i o n m a s s e c u i t e 11.99 90.50 92.33 16 A f f i n a t i o n s y r u p 6.17 78.64 84.69 17 A f f i n e d C s u g a r 6.06 99.00 98.52 18 Β r e m e l t 9.73 65.00 97.03 19 C a f f . r e m e l t 10.06 65.00 97.91 20 Water t o c e n t r i f u g a l s A 0.47 21 Steam t o c e n t r i f u g a l s A 0.16 22 Water t o c e n t r i f u g a l s Β 0.35 23 Water t o c r y s t a l 1 i z e r s C 0.31

i t i s n e c e s s a r y t o a d d , t o t h e e x i s t i n g equ ipment and p i p i n g , a c o n d e n s a t e tank

and a few f l a s h - v a p o u r c o n n e c t i o n s making i t p o s s i b l e t o u t i l i z e f u l l y t h e

a v a i l a b l e condensa te e n e r g y i n t h e e v a p o r a t o r .

The n e x t s t e p t o improve t h e e f f e c t i v e n e s s r a t i o c o n s i s t s o f e l i m i n a t i n g

p r o c e s s - e q u i p m e n t h e a t i n g w i t h e x h a u s t s team, i m p r o v i n g u t i l i z a t i o n o f t h i r d -

e f f e c t v a p o u r and i n t r o d u c i n g t h e u t i l i z a t i o n o f f o u r t h - e f f e c t v a p o u r . T h i s

amounts t o a r e c o n s t r u c t i o n o f t he v a p o u r d i s t r i b u t i o n scheme and must be

c o o r d i n a t e d w i t h t h e i n s t a l l a t i o n o f t h e condensa te d r a i n a g e l i n e s men t i oned

b e f o r e .

The m o d i f i e d v a p o u r d i s t r i b u t i o n scheme i s shown i n F i g . 8 . 5 . I n o r d e r t o

take f u l l advan tage o f t h e r e c o n s t r u c t i o n o f t h e e v a p o r a t i o n s u b s y s t e m , t h e

e v a p o r a t o r s h o u l d a l s o be e q u i p p e d w i t h a u t o m a t i c l e v e l c o n t r o l s .

Equipment r e p a i r and changes n e c e s s a r y f o r i n t r o d u c i n g t h e m o d i f i c a t i o n s

men t ioned under ( i ) and ( i i ) can e a s i l y be comp le ted d u r i n g one o f f - s e a s o n

p e r i o d . I t can be e s t i m a t e d t h a t t h e r e s u l t i n g r e d u c t i o n o f t h e steam

consumpt ion w i l l be a t l e a s t 4 . 5 - 5 . 0 kg/100 kg b. As r e g a r d s r e c o n s t r u c t i o n o f

t h e v a p o u r d i s t r i b u t i o n scheme, i t must be p r e c e d e d by a d e s i g n s t u d y on t h e

n e c e s s a r y p i p i n g m o d i f i c a t i o n s . A p r e l i m i n a r y h e a t b a l a n c e ( e x p r e s s e d i n steam

and v a p o u r f l o w s ) o f t h e m o d i f i e d the rma l sys tem i s p r e s e n t e d i n T a b l e 8 . 7 . As

TABLE 8.6

M o d i f i e d mass b a l a n c e o f t h e s u g a r h o u s e .

307

to ammonia water

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F i g . 8 . 5 . M o d i f i e d scheme o f t h e the rma l sys tem i n t h e p r o c e s s h e a t i n g a r e a . Condensa te d r a i n a g e l i n e s n o t shown i n t he p i c t u r e a r e c o n n e c t e d t o t h e ammonia w a t e r t a n k . The meaning o f numbers d e n o t i n g equ ipment u n i t s i s i d e n t i c a l t o t h a t i n F i g . 1.5.

can be s e e n , t h e steam consumpt ion can be r e d u c e d t o abou t 41.8 kg/100 kg b ,

w h i c h i s n e a r l y 10 kg/100 kg b l e s s t han i n d i c a t e d i n t he h e a t b a l a n c e o f t h e

e x i s t i n g the rma l sys tem ( T a b l e 8 . 5 ) .

I t s h o u l d be p o i n t e d o u t t h a t even a f t e r t h e c o m p l e t i o n o f t h e

r a t i o n a l i z a t i o n programme p r o p o s e d a b o v e , t h e h e a t economy w i l l remain r a t h e r

p r i m i t i v e , as t h e e f f e c t i v e n e s s r a t i o o f a the rma l sys tem w i t h o u t f u l l

u t i l i z a t i o n o f t h e l o w - t e m p e r a t u r e h e a t c a n n o t be v e r y h i g h . U n l e s s a d e t a i l e d

d e s i g n s t u d y i s p e r f o r m e d , i t i s i m p o s s i b l e t o d e t e r m i n e w h e t h e r o r n o t t h e

a p p l i c a t i o n o f more advanced s o l u t i o n s c o u l d be e c o n o m i c a l l y v i a b l e . I t i s

t h e r e f o r e recommended t o e v a l u a t e t h e e n e r g y economy a g a i n a f t e r two o r t h r e e

y e a r s o f t h e s t e p - b y - s t e p improvement programme, and t o r e p e a t t h e a n a l y s i s o f

p o s s i b l e e n e r g y - s a v i n g measu res . The i n d i v i d u a l measures can be e v a l u a t e d

308

TABLE 8.7

Steam and v a p o u r f l o w s (kg /100 kg b) between s o u r c e s and r e c e i v e r s i n t he p r o c e s s - h e a t i n g a r e a o f t h e m o d i f i e d the rma l s y s t e m .

No. R e c e i v e r s S o u r c e s E x h a u s t E v a p o r a t o r e f f e c t s

steam Ί 2 3 4 "

1 E x t r a c t o r 1 .18 1 .87 2 R a w - j u i c e h e a t e r s 0, .30 condensa te 3 L i m e d - j u i c e h e a t e r s 2.44 4, .33 4 H e a t e r i n c a r b o n a t a t i o n I 1.32 5 C l e a r - j u i c e h e a t e r s 1, .14 2.54 6 T h i n - j u i c e h e a t e r s 2.21 2 .99 7 T h i c k - j u i c e h e a t e r 0, .35 8 9

M e l t e r I n d i r e c t l y - h e a t e d s y r u p t a n k s

0, 0,

.40

.30 10 D i r e c t l y - h e a t e d s y r u p t anks 0, .24 11 Remelt h e a t e r 0, .26 12 Vacuum pans A 13, .04 13 Vacuum pans Β 2, .87 14 Vacuum pans C 1, .42 15 Vacuum-pan s t e a m i n g - o u t 1, .80 16 O t h e r smal l r e c e i v e r s 0 .50 17 Sugar d r y e r 0 .50 18 Condenser 1. .02 19 E v a p o r a t o r t o t a l 2.21 25. .99 8.16 4. .63 20 E x h a u s t - s t e a m consumpt ion 41 , .77

e c o n o m i c a l l y , c r e a t i n g a b a s i s f o r d e c i s i o n s on t h e f u t u r e c o u r s e o f a c t i o n . I f

t he l a c k o f i n v e s t m e n t f u n d s c o n t i n u e s , t h e n e x t s t a g e o f t he s t e p - b y - s t e p

improvement programme can be p l a n n e d ; o t h e r w i s e , a m o d e r n i z a t i o n o f t he f a c t o r y

can be c o n s i d e r e d .

8.3 FACTORY CHARACTERIZED BY GOOD I N I T I A L ENERGY U T I L I Z A T I O N

8.3.1 I n t r o d u c t o r y remarks

The example p r e s e n t e d i n t h i s S e c t i o n i s based on a r e a l case i n v e s t i g a t e d

by a s p e c i a l i z e d team a few y e a r s ago i n Sweden ( r e f . 1 7 ) . The i n v e s t i g a t i o n was

p l anned i n advance by the f a c t o r y manager ( as a r u l e , e v e r y Swed ish s u g a r

f a c t o r y has i t s e n e r g y economy i n v e s t i g a t e d e v e r y t h i r d o r f o u r t h y e a r ) .

The f a c t o r y was known f o r i t s r a t h e r low steam c o n s u m p t i o n , abou t 30-32 kg

pe r 100 kg b , r e s u l t i n g f rom i n v e s t m e n t s pe r f o rmed m a i n l y i n t h e 1950s and 1960s

and s t e p - b y - s t e p improvements i n t r o d u c e d i n t h e 1970s. F o r economic r e a s o n s , t h e

s t e p - b y - s t e p app roach i s a l s o l i k e l y t o remain t h e o n l y p r a c t i c a l method o f

i n t r o d u c i n g improvements i n t he f o r e s e e a b l e f u t u r e .

I t was p l anned t o a c q u i r e t h e n e c e s s a r y da ta d u r i n g 5 measurements s e s s i o n s

o r g a n i z e d a p p r o x i m a t e l y e v e r y two weeks f rom t h e b e g i n n i n g t o t he end o f t h e

s e a s o n . T h i s a r rangement was f o r s t u d y i n g t h e pa rame te rs o f t h e hea t u t i l i z a t i o n

p r o c e s s e s as f u n c t i o n s o f t i m e , t h u s making i t p o s s i b l e t o i d e n t i f y t he

309

i n f l u e n c e o f t h e s c a l e b u i l d - u p . The u l t i m a t e goa l was t o p r e p a r e a comp le te

s u r v e y o f t h e hea t u t i l i z a t i o n i n t h e s u g a r m a n u f a c t u r i n g p r o c e s s and f o r

a u x i l i a r y p u r p o s e s , and p a r t i c u l a r l y i n t h e e v a p o r a t o r s t a t i o n , s u g a r house and

a number o f smal l steam r e c e i v e r s . On t h e b a s i s o f t h e s u r v e y r e s u l t s , t h e most

e f f e c t i v e measures t o reduce t h e hea t demand c o u l d be d e t e r m i n e d .

D u r i n g t h e o f f - s e a s o n p e r i o d p r e c e d i n g t h e a c t u a l s e a s o n ' s o p e r a t i o n s , a

s p e c i a l measur ing sys tem i n d e p e n d e n t o f t h e f a c t o r y ' s measu r ing i n s t r u m e n t s was

i n s t a l l e d w i t h t h e aim o f p r o v i d i n g v e r y a c c u r a t e d a t a on t h e most i m p o r t a n t

p a r a m e t e r s , namely p r e s s u r e s and t e m p e r a t u r e s i n t h e e v a p o r a t i o n s u b s y s t e m ,

i n c l u d i n g c o n d e n s a t e t a n k s . O r i f i c e s were i n s t a l l e d i n t h e main v e n t i n g l i n e s

i n o r d e r t o make i t p o s s i b l e t o e v a l u a t e t h e f l o w s o f n o n c o n d e n s a b l e s .

A r rangements were a l s o made w i t h t h e l a b o r a t o r y t o p e r f o r m a number o f e x t r a

a n a l y s e s , m o s t l y o f t he d r y s u b s t a n c e c o n t e n t and p o l a r i z a t i o n ( a s w e l l as

a l k a l i n i t y and c a l c i u m s a l t s i n t h e case o f samples taken f rom t h e j u i c e

p u r i f i c a t i o n s t a t i o n ) o f j u i c e and s y r u p s a m p l e s . The p o i n t s o f samp l i ng were

p l a n n e d t o a c h i e v e a h i g h a c c u r a c y o f i d e n t i f i c a t i o n o f t he p r o c e s s mass b a l a n c e

and o f t h e the rma l p r o p e r t i e s o f a l l t h e i m p o r t a n t mass s t r e a m s . I n t h e

e v a p o r a t o r a r e a , j u i c e samples were t o be c o l l e c t e d i n a manner a l l o w i n g

d e t e r m i n a t i o n o f i n l e t and o u t l e t j u i c e c o n c e n t r a t i o n s i n a l l t he e v a p o r a t o r

b o d i e s .

I t was p l a n n e d t o a c q u i r e t h e r e m a i n i n g d a t a f rom t h e o r d i n a r y f a c t o r y

i n s t r u m e n t a t i o n , and t o take c e r t a i n i n s t r u m e n t r e a d i n g s s o l e l y f o r t h e pu rpose

o f c r o s s - c h e c k i n g t h e i n d i c a t i o n s o f t h e s p e c i a l measu r ing s y s t e m . A f t e r

c a l c u l a t i n g t he t ime a v e r a g e d v a l u e s o f t h e key p a r a m e t e r s , e v e r y measurement

p e r i o d ( l a s t i n g abou t one w o r k i n g s h i f t ) was e x p e c t e d t o p r o d u c e abou t 300 d a t a

t o be used i n t h e hea t b a l a n c e c a l c u l a t i o n s .

8 . 3 .2 B a s i c f a c t o r y d a t a and scheme o f t h e s u g a r m a n u f a c t u r i n g p r o c e s s

P r o c e s s i n g c a p a b i l i t y : 3700 t / d .

P o l a r i z a t i o n o f c o s s e t t e s : 15.7%.

E x t r a c t o r : t r o u g h - t y p e .

J u i c e d r a f t : 120%.

R a w - j u i c e c o n c e n t r a t i o n : 15.0%.

P u l p p r e s s e d t o : 21.4% DS.

K i l n g a s : 36-38% CO^ v o l .

J u i c e p u r i f i c a t i o n a c c o r d i n g t o t he c l a s s i c a l scheme, c o m p r i s i n g :

- h o t p r e - l i m i n g a t 60-65°C;

- main l i m i n g a t abou t 85°C;

- c a r b o n a t a t i o n I a t 80-85°C;

- d o u b l e - s t a g e f i l t r a t i o n I ;

- c a r b o n a t a t i o n I I a t 92-95°C;

310

- d o u b l e - s t a g e f i l t r a t i o n I I .

T h i n - j u i c e c o n c e n t r a t i o n : 14.2% DS.

E v a p o r a t o r : q u i n t u p l e - e f f e c t , f a l l i n g - f i l m b o d i e s i n t h e f i r s t and f i f t h

e f f e c t s , R o b e r t - t y p e b o d i e s i n t h e r e m a i n i n g e f f e c t s .

T h i c k - j u i c e c o n c e n t r a t i o n : 64.8% DS.

Sugar h o u s e :

- t h r e e - b o i l i n g scheme w i t h t h e a f f i n a t i o n o f C s u g a r ;

- a f f i n e d C s u g a r me l t ed i n t h i n j u i c e ;

- s t a n d a r d l i q u o r p r e p a r e d f rom t h i c k j u i c e , Β s u g a r and C - a f f . r e m e l t .

Power h o u s e :

- o i l - f i r e d b o i l e r r a t e d 50 t / h , e f f i c i e n c y 91%;

- l i v e - s t e a m pa rame te rs 60 ba r and 470°C;

- b a c k - p r e s s u r e t u r b i n e r a t e d 6 MW;

- b a c k - p r e s s u r e 3.5 b a r .

Steam s u p p l y t o t h e s u g a r m a n u f a c t u r i n g p r o c e s s : steam 3.5 bar ( i n p r i n c i p l e ,

e x h a u s t steam o n l y , and t h r o t t l e d l i v e steam i f n e c e s s a r y ) .

H e a t i n g steam c o n s u m p t i o n : 30-32 kg/100 kg b.

Normal f u e l c o n s u m p t i o n : abou t 3.6 kg/100 kg b.

The schemes o f t h e b e e t house and s u g a r house a r e shown i n F i g s . 8.6 and 8 . 7 ,

r e s p e c t i v e l y . I t can immed ia te l y be seen t h a t t h e s u g a r m a n u f a c t u r i n g p r o c e s s

has been c a r e f u l l y p l a n n e d t o m i n i m i z e w a t e r i n t a k e s and t o e l i m i n a t e

u n n e c e s s a r y r e c y c l i n g o f t h e p r o c e s s med ia .

8 .3 .3 Scheme o f t h e therma l sys tem

The scheme i s shown i n F i g . 8 . 8 . A number o f d e t a i l s d e s e r v e t o be n o t e d , as

t h e y i n d i c a t e t h a t a h i g h d e g r e e o f r a t i o n a l i z a t i o n o f t h e e n e r g y economy has

a l r e a d y been a t t a i n e d .

I n t he p r o c e s s h e a t i n g a r e a , t h r e e f a c t o r s make i t p o s s i b l e t o a t t a i n a h i g h

e f f e c t i v e n e s s r a t i o :

- t h e a p p l i c a t i o n o f a q u i n t u p l e - e f f e c t e v a p o r a t o r s t a t i o n and t h e u t i l i z a t i o n

o f f i f t h - e f f e c t v a p o u r i n r a w - j u i c e h e a t i n g ;

- t he u t i l i z a t i o n o f t he vacuum-pan v a p o u r s i n r a w - j u i c e h e a t i n g ;

- t h e u t i l i z a t i o n o f t h e c o n d e n s a t e i n h e a t i n g p r e - l i m e d j u i c e .

The e v a p o r a t o r s t a t i o n c o n s i s t s o f n i n e b o d i e s , two o f them o f t h e f a l l i n g -

f i l m t y p e and t he rema inde r o f t h e R o b e r t t y p e . A scheme o f j u i c e and v a p o u r

c o n n e c t i o n s , i n c l u d i n g v e n t i n g l i n e s , i s shown i n F i g . 8 . 9 . ( H e a t i n g s u r f a c e

a r e a s a r e g i v e n i n T a b l e 8 . 9 . )

A f a l l i n g - f i l m body i n t h e f i r s t e v a p o r a t o r e f f e c t i s c h a r a c t e r i z e d by a

l a r g e o v e r a l l hea t t r a n s f e r c o e f f i c i e n t w h i c h rema ins n e a r l y c o n s t a n t t h r o u g h o u t

t h e s e a s o n . T h i s i s a d e c i s i v e f a c t o r keep ing t h e e v a p o r a t i o n c a p a c i t y a t a h i g h

l e v e l , even i f s c a l e b u i l d - u p t a k e s p l a c e i n t h e second and t h i r d e f f e c t s .

311

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to sugar house

F i g . 8 . 6 . Scheme o f e x t r a c t i o n and j u i c e p u r i f i c a t i o n . 1 - h e a t e x c h a n g e r i n wh i ch condensa te ( f r e s h w a t e r ) warms up p r e s s w a t e r .

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F i g . 8 . 8 . Scheme o f t he the rma l s y s t e m . 14 - e x t r a c t o r , 18 - p r e s s - w a t e r h e a t e r ; t h e r ema in i ng numbers have t h e same meaning as i n F i g . 1.5. D u r i n g t h e measurements , h e a t e r 6"*" was o u t o f o p e r a t i o n .

314

to the 2nd thin juice heater

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F i g . 8 . 9 . Scheme o f j u i c e and v a p o u r c o n n e c t i o n s i n t h e e v a p o r a t o r s t a t i o n . D o t t e d l i n e s i n d i c a t e v e n t i n g c o n n e c t i o n s .

A n o t h e r f a l l i n g - f i l m body i n t h e f i f t h e v a p o r a t o r e f f e c t a l s o p l a y s an i m p o r t a n t

r o l e . I t s l a r g e hea t t r a n s f e r c o e f f i c i e n t makes i t p o s s i b l e t o keep t h e

t e m p e r a t u r e o f t h e f i f t h - e f f e c t v a p o u r s u f f i c i e n t l y h i g h t o u t i l i z e t h e l o w -

t e m p e r a t u r e h e a t , even d u r i n g t he f i n a l weeks o f t h e s e a s o n . T h i s w o u l d be v e r y

d i f f i c u l t t o a c h i e v e w i t h a R o b e r t - t y p e b o d y .

V e n t i n g l i n e s f rom c e r t a i n e v a p o r a t o r b o d i e s and v e n t i n g l i n e s f rom s e l e c t e d

h e a t e r s a r e c o n n e c t e d t o v a p o u r s u p p l y p i p e s o f o t h e r h e a t e r s . T h i s makes i t

p o s s i b l e t o u t i l i z e t he e n e r g y o f t h e m i x t u r e o f v a p o u r and n o n c o n d e n s a b l e s

p r i o r t o d i s c h a r g i n g i t t o t he e n v i r o n m e n t o r t o t h e c o n d e n s e r .

P l a t e hea t e x c h a n g e r s a r e a p p l i e d as c l e a r - j u i c e h e a t e r s ( b e f o r e

c a r b o n a t a t i o n I ) and as j u i c e h e a t e r s a f t e r c a r b o n a t a t i o n I I . Owing t o t h e i r

l a r g e hea t t r a n s f e r c o e f f i c i e n t s , t h e s e h e a t e r s can be s u p p l i e d w i t h v a p o u r s o f

r e l a t i v e l y low t e m p e r a t u r e s .

I n t h e power house a r e a , b o i l e r blowdown i s p e r f o r m e d v i a a f l a s h tank

c o n n e c t e d t o t h e e x h a u s t - s t e a m p i p e l i n e . F l a s h v a p o u r o b t a i n e d f rom t h e b o i l e r

w a t e r ( s u b s e q u e n t l y d i s c h a r g e d t o t h e sewer sys tem) i s t h u s mixed w i t h e x h a u s t

s team. A n a l o g o u s l y , t he v e n t i n g l i n e f rom t h e main f e e d - w a t e r t ank i s c o n n e c t e d

t o t h e p i p e s u p p l y i n g t h i r d - e f f e c t v a p o u r t o t h e h e a t i n g chamber o f t h e f o u r t h

e v a p o r a t o r e f f e c t . I n t h i s a r r a n g e m e n t , t h e f e e d w a t e r i s f l a s h e d f rom t h e

p r e s s u r e ^ o f s e c o n d - e f f e c t v a p o u r t o t h a t o f t h i r d - e f f e c t v a p o u r .

8 .3 .4 I n f o r m a t i o n o b t a i n e d f rom t h e measurements

U s i n g t h e r e c o r d e d v a l u e s o f j u i c e c o n c e n t r a t i o n s , as w e l l as v a p o u r and

condensa te t e m p e r a t u r e s measured i n t h e e v a p o r a t o r a r e a , mass and hea t b a l a n c e s

o f t h e e v a p o r a t o r were c a l c u l a t e d f o r each measurement s e s s i o n . An example o f

a comple te s e t o f i n p u t d a t a and c a l c u l a t i o n r e s u l t s ( c o r r e s p o n d i n g t o t h e

315

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second s e s s i o n , i . e . t h e t h i r d week o f t h e s e a s o n ) i s g i v e n i n T a b l e 8 . 8 . As can

be s e e n , t he r e s u l t s i n c l u d e hea t f l u x e s and o v e r a l l hea t t r a n s f e r c o e f f i c i e n t s

i n t h e i n d i v i d u a l e v a p o r a t o r b o d i e s . T h i s makes i t p o s s i b l e t o e v a l u a t e t h e

i n f l u e n c e o f s c a l e b u i l d - u p on t h e hea t t r a n s f e r i n t e n s i t y i n t h e e v a p o r a t o r

s t a t i o n .

The o v e r a l l hea t t r a n s f e r c o e f f i c i e n t s i n t h e i n d i v i d u a l e v a p o r a t o r e f f e c t s

a r e shown as f u n c t i o n s o f t ime i n F i g . 8 .10 . As can be s e e n , t h e t e n d e n c i e s

c h a r a c t e r i s t i c o f R o b e r t - t y p e b o d i e s have been r e g i s t e r e d i n e f f e c t s 2 -4 . The

r e d u c t i o n o f t h e hea t t r a n s f e r i n t e n s i t y i n t he f o u r t h e f f e c t was so d r a s t i c

a f t e r f i v e weeks t h a t a s t a n d - b y body (48) was c o n n e c t e d t o t h i s e f f e c t . As

r e g a r d s the f a l l i n g - f i l m b o d i e s , no s c a l e b u i l d - u p i s v i s i b l e i n t h e f i r s t

e f f e c t , w h i l e t h e hea t t r a n s f e r i n t e n s i t y i n t he f i f t h e f f e c t d e c r e a s e s l i k e

t h a t i n R o b e r t - t y p e b o d i e s .

U s i n g t he r e c o r d e d v a l u e s o f t he t e m p e r a t u r e s o f h e a t i n g v a p o u r s , as w e l l as

j u i c e t e m p e r a t u r e s and j u i c e f l o w s , t he hea t b a l a n c e s o f j u i c e h e a t e r s were

2500

- 2000

ε

§ 1500

ΙΛ C

I 1000

α χ :

6 500h

1st effect

4 6

Time (weeks)

10

F i g . 8 .10 . Changes o f t he a v e r a g e d o v e r a l l hea t t r a n s f e r c o e f f i c i e n t s i n t h e e v a p o r a t o r s t a t i o n ( s t a n d - b y body c o n n e c t e d t o t h e f o u r t h e f f e c t a f t e r 5 w e e k s ) .

317

c a l c u l a t e d f o r each measurement s e s s i o n . An example o f a comp le te s e t o f i n p u t

da ta and c a l c u l a t i o n r e s u l t s ( c o r r e s p o n d i n g t o t h e second s e s s i o n ) i s g i v e n i n

T a b l e 8 . 9 . As t h e r e s u l t s i n c l u d e t h e o v e r a l l hea t t r a n s f e r c o e f f i c i e n t s o f t he

i n d i v i d u a l h e a t e r s , t h e e f f e c t s o f s c a l e b u i l d - u p i n t h e h e a t e r s can be s t u d i e d .

TABLE 8.9

Heat b a l a n c e s o f t h e j u i c e h e a t e r s .

Medium hea ted Raw j u i c e P r e -

1 i med j u i c e

C l e a r j u i c e J u i ce a f t e r 2nd c a r b . ' *

T h i n j u i c e Medium hea ted

1 2

P r e -1 i med j u i c e

1* 2*

J u i ce a f t e r 2nd c a r b . ' * 1 2 3 4

I n p u t d a t a : H e a t i n g s u r f a c e a r e a (m^) 125 125 259 70.2 70.8 100 150 150 150 150 H e a t i n g v a p o u r . e x e f f e c t No. ^ 5 5 4 4 4 3 3 2 1 h a u s t t e m p e r a t u r e ( C) 77.7 77.7 92.5 92.5 92.5 108.1 108.1 117.3 128.8 137.4 J u i c e t e m p e r a t u r e ( C)

i n l e t 42.3 42.3 75.1 80.7 88.7 85.0 95.5 105.5 116.0 124.4 o u t l e t 60.1 58.8 84.3 88.7 91.5 98.5 105.5 116.0 124.4 133.0

J u i c e f l o w (kg /100 kg b) 56.6 106.4 147.2 138.2 138.2 116.8 113.7 113.7 113.7 113.7

R e s u l t s : Vapour demand (kg /100 kg b ) 3.09 2.86 1.86 1.86 0.65 2.70 1 .96 2.08 1 .69 1.76 O v e r a l l hea t t r a n s f e r c o e f f i c i e n t (W/ (m2K) ) 950 850 720 3660 4280 1680 1970 2790 1340 1360

V p l a t e hea t e x c h a n g e r s

The o v e r a l l h e a t t r a n s f e r c o e f f i c i e n t s i n t h e i n d i v i d u a l j u i c e h e a t e r s a r e

shown as f u n c t i o n s o f t ime i n F i g . 8 .11 . These c o e f f i c i e n t s a re g e n e r a l l y v e r y

h i g h , w h i c h i n d i c a t e s t h a t t h e h e a t e r s a r e u t i l i z e d e f f e c t i v e l y . The e f f e c t s o f

t h e s c a l e b u i l d - u p can be seen e x a c t l y where t h e y m igh t be e x p e c t e d , namely i n

r a w - j u i c e , p r e - 1 i m e d - j u i c e and c l e a r - j u i c e h e a t e r s .

I t can be c o n c l u d e d t h a t t he j u i c e - t e m p e r a t u r e p r o f i l e s d e t e r m i n e d d u r i n g t h e

measurement s e s s i o n s a r e e s s e n t i a l l y c o r r e c t . A compar i son o f t e m p e r a t u r e

p r o f i l e s c o r r e s p o n d i n g t o t h e b e g i n n i n g and end o f t h e season i s shown i n

F i g . 8 .12 . N o t e w o r t h y a re t h e v e r y smal l v a l u e s - somet imes as low as 2 Κ - o f

t he minimum t e m p e r a t u r e d i f f e r e n c e i n t h e t h i n - j u i c e h e a t e r s , w h i c h a r e o f

t u b u l a r d e s i g n .

The e x h a u s t steam consumpt ion was d e t e r m i n e d a t 30 .5 -34 .0 kg/100 kg b i n

measurement s e s s i o n s 1-4. I t i n c r e a s e d t o 43.1 kg/100 kg b i n measurement

s e s s i o n 5, w h i c h was r e p r e s e n t a t i v e o f t h e f i n a l s t a g e o f t h e s e a s o n . C o n c e r n i n g

318

4000 h

3000 h

2000 h

clear juice (PHE)

c ω 'ο

I o

(Λ C O

1000 h

3000 h α o»

2

O

2000 h

1000 h

A 6 Time (weeks)

F i g . 8 .11. O v e r a l l hea t t r a n s f e r c o e f f i c i e n t s i n t he j u i c e h e a t e r s as f u n c t i o n s o f t i m e . PHE - p l a t e hea t e x c h a n g e r s . * d e n o t e s two h e a t e r s c o n n e c t e d i n p a r a l l e i .

319

F i g . 8 .12 . T e m p e r a t u r e s o f h e a t i n g v a p o u r s ( dashed l i n e s ; numbers deno te e v a p o r a t o r e f f e c t s ) and j u i c e ( s o l i d l i n e s ) . T h i c k l i n e s - f i r s t week o f t h e s e a s o n , t h i n l i n e s - t e n t h week o f t h e s e a s o n .

t he hea t consumpt ion components , p a r t i c u l a r a t t e n t i o n was p a i d t o t h e s u g a r

h o u s e . The h e a t i n g - v a p o u r demand o f t h e vacuum pans was c a l c u l a t e d on t h e b a s i s

o f t h e mass b a l a n c e o f t h e s u g a r c r y s t a l l i z a t i o n p r o c e s s . The demand f i g u r e s

c o r r e s p o n d i n g t o t h e f i v e measurement s e s s i o n s were c o n t a i n e d i n t h e range

14 .1-16 .8 kg/100 kg b , w i t h a t e n d e n c y t o i n c r e a s e as t h e season p r o g r e s s e d . As

t he v a p o u r used i n vacuum-pan s t e a m i n g - o u t was s u p p l i e d v i a a s e p a r a t e s u p p l y

l i n e , i t was p o s s i b l e t o measure i t s consumpt ion d i r e c t l y , and i n measurement

s e s s i o n s 1-4 t h e r e s u l t s were n e a r l y i d e n t i c a l , a t 0 . 9 - 1 . 0 kg/100 kg b. The

v a l u e d e t e r m i n e d i n measurement s e s s i o n 5 was 1.4 kg/100 kg b.

I n a d d i t i o n t o t he e x h a u s t s team, s a t u r a t e d b o i l e r steam ( w i t h d r a w n f rom t h e

b o i l e r drum) t h r o t t l e d t o 12 ba r i s s u p p l i e d t o t h e f a c t o r y f o r such p u r p o s e s as

h e a t i n g f u e l o i l b e f o r e t he b u r n e r s , as w e l l as o i l a t o m i z a t i o n i n t h e b u r n e r s

i n s t a l l e d i n t h e b o i l e r f u r n a c e and i n t h e l ime k i l n . I n t h e f i v e measurement

s e s s i o n s , t he consumpt ion o f 12 ba r steam remained n e a r l y c o n s t a n t a t

0 .3 -0 .4 kg/100 kg b.

320

8 .3 .5 Heat b a l a n c e o f t h e therma l sys tem

The hea t b a l a n c e c a l c u l a t i o n s were pe r f o rmed i n t h e f o l l o w i n g manner :

- u s i n g t h e r e s u l t s o f f l o w measurements ( o r t he c a l c u l a t e d mass b a l a n c e d a t a )

and t he t e m p e r a t u r e v a l u e s measu red , t he v a p o u r o r steam consumpt ion was

d e t e r m i n e d f o r a l l hea t r e c e i v e r s , and t h e t o t a l consumpt ion o f v a p o u r s f rom t h e

i n d i v i d u a l e v a p o r a t o r e f f e c t s was c a l c u l a t e d ;

- u s i n g t he v a l u e s o f t h e e v a p o r a t o r pa rame te rs measu red , t h e mass and hea t

b a l a n c e s o f t h e e v a p o r a t o r , i n c l u d i n g v a p o u r f l o w s ( a v a i l a b l e f o r p r o c e s s

h e a t i n g ) f rom t h e i n d i v i d u a l e f f e c t s , were c a l c u l a t e d .

Owing t o measurement e r r o r s , t h e t o t a l v a p o u r consumpt ion may d i f f e r f rom

t h e c a l c u l a t e d a v a i l a b l e v a p o u r f l o w . I f t h e d i f f e r e n c e i s l e s s t han

TABLE 8.10

Steam and v a p o u r f l o w s (kg /100 kg b ) between s o u r c e s and r e c e i v e r s i n t he thermal s y s t e m .

No. R e c e i v e r s E x h a u s t steam 1

S o u r c e s E v a p o r a t o r e f f e c t s O t h e r s

10

11

12 13

14 15 16 17 18 19

20 21 22 23

E x t r a c t o r R a w - j u i c e h e a t e r s P r e - 1 i m e d - j u i c e h e a t e r s C l e a r - j u i c e h e a t e r s H e a t e r a f t e r c a r b o n a t a t i o n I I 2.70 T h i n - j u i c e h e a t e r s 1.76 1.69 2.08 1.96 O i l t a n k s 0.25 Pu lp d r y i n g and p e l l e t i n g 0.02 H e a t i n g o f p r o d u c t s t o r e s 0.50 H e a t i n g o f f a c t o r y b u i l d i n g s 0.19 H e a t i n g o f o f f i c e b u i l d i n g s 0.16 Main f e e d - w a t e r tank 0.13 M o l a s s e s and a f f . - s y r u p t a n k s 0.12 M e l t e r C 0.02 Sy rup A t anks 0.37 Sy rup Β t anks 0.12 M e l t e r Β 0.33 Vacuum pans A , B, C 14.13 Vacuum-pan s t e a m i n g - o u t 0.86 C e n t r i f u g a l s 0.22 Sugar d r y e r 0.48 E v a p o r a t o r t o t a l 1.8 4.4 E x h a u s t - s t e a m consumpt ion 30.6 O i l b u r n e r s i n b o i l e r and l ime k i l n

0.83

1.86 2.51

5.95 vacuum-pan v a p o u r

condensa te

18.8 5.2 6.0

12 ba r steam 24

321

1 kg/100 kg b , t hen t he a c c u r a c y o f t h e measurements can be r e g a r d e d as

s a t i s f a c t o r y . I t can be added t h a t t h i s v a l u e i s e q u i v a l e n t t o d e t e r m i n i n g j u i c e

c o n c e n t r a t i o n s i n t he f i r s t and second e v a p o r a t o r e f f e c t s w i t h an a c c u r a c y o f

0.1% DS.

The hea t b a l a n c e d e t e r m i n e d u s i n g t h e r e s u l t s o f t h e second measurement

s e s s i o n ( t h i r d week o f t he s e a s o n ) i s p r e s e n t e d i n T a b l e 8 .10 .

8 .3 .6 E v a l u a t i o n o f t he hea t economy and i d e n t i f i c a t i o n o f p o s s i b l e improvements

The r e s u l t s o f t h e measurements and t h e s u b s e q u e n t hea t b a l a n c e c a l c u l a t i o n s

c o n f i r m e d t h a t t h e hea t economy i s q u i t e e f f e c t i v e . No s u b s t a n t i a l d i s c r e p a n c i e s

were o b s e r v e d between t he p o t e n t i a l e f f e c t i v e n e s s o f t h e e n e r g y c o n v e r s i o n and

d i s t r i b u t i o n p r o c e s s e s and t he r e a l b e h a v i o u r o f t h e the rma l s y s t e m . From

a d e t a i l e d r e v i e w o f t he hea t r e c e i v e r s , h o w e v e r , a number o f p o s s i b l e smal l

improvements o f hea t u t i l i z a t i o n can be i d e n t i f i e d .

As r e g a r d s t he s u g a r h o u s e , i t was o b s e r v e d d u r i n g s e s s i o n 5 t h a t t h e v a p o u r

consumpt ion i n vacuum-pan s t e a m i n g - o u t changed w i t h o u t a p p a r e n t r e a s o n ; t h i s

i n d i c a t e s t h a t i f more a t t e n t i o n i s p a i d t o t he f u n c t i o n i n g o f t h e s teaming

equ ipmen t , t hen v a p o u r can be s a v e d . As some smal l s y r u p t a n k s a r e hea ted by

f i r s t - e f f e c t v a p o u r , a hea t s a v i n g can be o b t a i n e d u s i n g s e c o n d - e f f e c t v a p o u r

i n s t e a d . F u r t h e r m o r e , some o t h e r s y r u p t a n k s a r e d i r e c t l y hea ted by s e c o n d -

e f f e c t v a p o u r w i t h o u t any a r rangemen ts t o s e c u r e a u n i f o r m t e m p e r a t u r e

d i s t r i b u t i o n i n t h e s y r u p vo lume . T h i s i s a s s o c i a t e d w i t h a r i s k o f l o c a l l y

o v e r h e a t e d zones c a u s i n g e x c e s s i v e hea t c o n s u m p t i o n . T h i s s i t u a t i o n can be

improved e i t h e r by imp lement ing i n d i r e c t h e a t i n g , o r by i n s t a l l i n g m i x i n g

d e v i c e s i n d i r e c t l y hea ted t a n k s .

As r e g a r d s smal l e x h a u s t - s t e a m r e c e i v e r s , some o f them can a c t u a l l y be

s u p p l i e d w i t h f i r s t - o r s e c o n d - e f f e c t v a p o u r . I n t h e f i r s t p l a c e , t h i s a p p l i e s

t o o i l - t a n k h e a t i n g , as w e l l as t o h e a t i n g o f f a c t o r y b u i l d i n g s . The h e a t i n g

sys tem i n t h e o f f i c e b u i l d i n g can a l s o be s u p p l i e d w i t h s e c o n d - e f f e c t v a p o u r .

F i n a l l y , i t was o b s e r v e d t h a t t h e f l o w o f t h e m i x t u r e o f v a p o u r and

noncondensab les w i t h d r a w n f rom t h e e v a p o r a t o r by v e n t i n g c o u l d be r e d u c e d

w i t h o u t any a d v e r s e e f f e c t s i n t h e hea t t r a n s f e r i n t e n s i t y . The a s s o c i a t e d

e n e r g y - s a v i n g p o t e n t i a l can be u t i l i z e d , p r o v i d i n g t h e o p e r a t i n g p e r s o n n e l

o p e r a t e t he v e n t i n g sys tem more c a r e f u l l y .

The e s t i m a t e d e f f e c t s o f t h e improvements l i s t e d above were c a l c u l a t e d u s i n g

a computer p rogram f o r e v a p o r a t o r b a l a n c e c a l c u l a t i o n s . The r e s u l t s a r e l i s t e d

i n T a b l e 8 .11. O n l y i n t he case o f r e p l a c i n g e x h a u s t steam by v a p o u r s i n t h e

h e a t i n g o f c e r t a i n r e c e i v e r s , h o w e v e r , can t h e e s t i m a t e s be t r e a t e d as t h e

v a l u e s o f e x p e c t e d s a v i n g s . O t h e r e s t i m a t e s , o b t a i n e d on t h e " i f - t h e n " b a s i s ,

i n d i c a t e t h e o r d e r o f magn i tude b u t l e a v e a marg in o f u n c e r t a i n t y abou t t h e

322

TABLE 8.11

E s t i m a t e d h e a t i n g - s t e a m s a v i n g s r e s u l t i n g f rom v a r i o u s improvements o f t h e h e a t economy.

~ I 77. ~ E s t i m a t e d steam No. S p e c i f i c a t i o n 3 ^ ^ . ^ g

1 Reduc ing t he v a p o u r consumpt ion i n vacuum-pan s t e a m i n g - o u t by 15% 0, .06

2 R e p l a c i n g f i r s t - e f f e c t v a p o u r by s e c o n d - e f f e c t v a p o u r i n t h e h e a t i n g o f smal l s y r u p t a n k s 0 .02

3 Reduc ing t h e consumpt ion o f s e c o n d - e f f e c t v a p o u r i n t he d i r e c t h e a t i n g o f s y r u p t a n k s 0 .20

4 R e p l a c i n g e x h a u s t steam by f i r s t - and s e c o n d - e f f e c t v a p o u r i n t he h e a t i n g o f o i l t anks and f a c t o r y b u i l d i n g s 0 .28

5 R e p l a c i n g e x h a u s t steam by s e c o n d - e f f e c t v a p o u r i n t he h e a t i n g o f t h e o f f i c e b u i l d i n g 0, .06

6 Reduc ing t he f l o w o f v a p o u r - n o n c o n d e n s a b l e s m i x t u r e w i t h d r a w n f rom t h e e v a p o r a t o r by 25% 0, .10

a t t a i n a b l e v a l u e s o f t h e s a v i n g s . (Mos t n o t a b l y , t h i s a p p l i e s t o t h e vacuum-pan

s t e a m i n g - o u t and t o t he v e n t i n g o f n o n c o n d e n s a b l e s . I f r e d u c t i o n s o f t h e v a p o u r

f l o w s by 15% and 25%, r e s p e c t i v e l y , can be a t t a i n e d , t hen steam s a v i n g s o f

0.06 kg/100 kg b and 0.10 kg/100 kg b , r e s p e c t i v e l y , seem t o be w i t h i n r e a c h .

The assump t i ons on v a p o u r - f l o w r e d u c t i o n , h o w e v e r , can o n l y be p r o v e d by

p r a c t i c a l r e s u l t s . ) T h e r e f o r e , T a b l e 8.11 can o n l y be r e g a r d e d as a r e v i e w o f

p o s s i b l e r a t i o n a l i z a t i o n measures and t h e i r r e l a t i v e i m p o r t a n c e , bu t n o t t h e i r

a b s o l u t e e f f e c t s .

8.4 FACTORY FEATURING AN ADVANCED ENERGY SYSTEM

8.4.1 I n t r o d u c t o r y remarks

A l t h o u g h t h e p r e s e n t book i s assumed t o be c o n c e r n e d m a i n l y w i t h w h i t e - s u g a r

f a c t o r i e s , i t seems r e a s o n a b l e t o g i v e , i n t h i s S e c t i o n , a summary o f s t e p - b y -

s t e p improvements i n t r o d u c e d on an advanced e n e r g y sys tem i n a r a w - s u g a r

f a c t o r y . P u t t i n g a s i d e t h e p r o c e s s - s p e c i f i c a s p e c t s o f t h i s p a r t i c u l a r c a s e , i t

may be i n t e r e s t i n g t o see how much e n e r g y can be s a v e d , and how i m p o r t a n t t h e

power b a l a n c e becomes i n a modern f a c t o r y u t i l i z i n g e n e r g y v e r y e f f i c i e n t l y .

The i n f o r m a t i o n p r e s e n t e d be low i s e x t r a c t e d p a r t l y f rom p u b l i c a t i o n s ( r e f s .

18,19) and p a r t l y f rom the m a t e r i a l o b t a i n e d d i r e c t l y f rom P f e i f e r & L a n g e n

Company, C o l o g n e , FRG ( r e f . 1 9 , 2 0 ) .

The A p p e l d o r n f a c t o r y was e r e c t e d i n t he p e r i o d 1975-77. I t s main p r o d u c t i s

raw s u g a r d e s t i n e d f o r f u r t h e r p r o c e s s i n g i n a r e f i n e r y owned by t h e same

company.

The i n i t i a l p r o c e s s i n g c a p a b i l i t y was 4500 t / d . Howeve r , t h e equ ipment was

d imens ioned t o make i t p o s s i b l e t o i n c r e a s e t he p r o c e s s i n g c a p a b i l i t y w i t h o u t

f u r t h e r heavy i n v e s t m e n t . The l a y o u t o f t he p r o c e s s s t a t i o n s and t h e main

323

f a c t o r y b u i l d i n g were a l s o d e s i g n e d t o f a c i l i t a t e f u t u r e e x t e n s i o n s . F o l l o w i n g

t h e e v o l u t i o n o f f u e l and power p r i c e s i n FRG, advan tage was taken o f t h e s e

s p e c i a l f e a t u r e s o f t he A p p e l d o r n f a c t o r y , and a number o f improvements were

i n t r o d u c e d a im ing t o improve t h e o v e r a l l economic r e s u l t s a n d , i n p a r t i c u l a r , t o

r e d u c e t he e n e r g y c o s t s .

A t t h e i n i t i a l steam consumpt ion l e v e l o f abou t 27 kg/100 kg b, t h e f a c t o r y

c o u l d i n p r i n c i p l e be c o n s i d e r e d as r a t h e r e n e r g y - e f f i c i e n t . W i th c a r e f u l l y

d e s i g n e d p r o c e s s and therma l sys tem schemes as w e l l as modern equ ipment and

a u t o m a t i c c o n t r o l s , i t was p o s s i b l e t o p r e v e n t u n n e c e s s a r y e n e r g y was te and t o

keep t h e e n e r g y c o n v e r s i o n , d i s t r i b u t i o n and u t i l i z a t i o n p r o c e s s e s w e l l under

c o n t r o l . T h e r e f o r e , when d e s i g n i n g and imp lemen t ing t h e improvemen ts , a t t e n t i o n

was t u r n e d t o two g r o u p s o f r a t i o n a l i z a t i o n m e a s u r e s :

- r e d u c i n g t h e t o t a l e n e r g y demand o f t h e s u g a r m a n u f a c t u r i n g p r o c e s s ;

- i m p r o v i n g t he c a p a b i l i t i e s o f t he e n e r g y s y s t e m .

8 .4 .2 E v o l u t i o n o f t h e s u g a r m a n u f a c t u r i n g p r o c e s s

The f a c t o r y i s e q u i p p e d w i t h a t o w e r e x t r a c t o r , c l a s s i c a l j u i c e p u r i f i c a t i o n

s t a t i o n , m u l t i p l e - s t a g e e v a p o r a t o r s t a t i o n and b a t c h - t y p e c r y s t a l l i z a t i o n

equ ipmen t . I n i t i a l l y , a j u i c e d r a f t abou t 125 kg/100 kg b was m a i n t a i n e d . The

e v a p o r a t o r s - f o u r R o b e r t - t y p e and two f a l l i n g - f i l m u n i t s , w i t h h e a t i n g s u r f a c e 2

a r e a s o f 2000 m each - were a r r a n g e d i n a q u i n t u p l e - e f f e c t s t a t i o n . A t a t h i n -

j u i c e c o n c e n t r a t i o n abou t 14% DS, a t h i c k - j u i c e c o n c e n t r a t i o n o f 66% DS was

m a i n t a i n e d .

The i n i t i a l v e r s i o n o f t h e s u g a r house was based on a s i n g l e - b o i l i n g

c r y s t a l l i z a t i o n p r o c e s s , w i t h raw s u g a r and s y r u p as t h e f i n a l p r o d u c t s . I n

o r d e r t o p roduce mo lasses needed i n t h e p u l p - d r y i n g p l a n t , a l o w - g r a d e p r o d u c t

s t a g e was added t o t h e c r y s t a l l i z a t i o n scheme. I n 1978, t h e c a p a c i t y o f t he l o w -

g rade s t a t i o n was s u f f i c i e n t t o p r o c e s s abou t h a l f o f t h e s y r u p f l o w . A t 66% DS

t h i c k - j u i c e c o n c e n t r a t i o n , t h e t h e o r e t i c a l w a t e r e v a p o r a t i o n i n t h e s u g a r house

was 8.7 kg/100 kg b.

F o l l o w i n g a d j u s t m e n t s o f t h e f a c t o r y e q u i p m e n t , t h e p r o c e s s i n g c a p a b i l i t y

a t t a i n e d 5000 t / d a l r e a d y i n 1979. I n o r d e r t o r e d u c e t he e n e r g y demand, t h e

w a t e r i n t a k e t o t h e p r o c e s s was r e d u c e d by g r a d u a l l y d e c r e a s i n g t h e j u i c e d r a f t

t o 117%. I n 1981, t h e e v a p o r a t i o n p r o c e s s was m o d i f i e d by a t t a c h i n g a v a p o u r

compress i on c i r c u i t t o t h e e v a p o r a t o r s t a t i o n and r e a r r a n g i n g v a p o u r and

condensa te d i s t r i b u t i o n t o t h e hea t r e c e i v e r s . As t h i s made i t p o s s i b l e t o

i n c r e a s e t h i c k - j u i c e c o n c e n t r a t i o n t o 68% DS, t h e hea t demand o f t h e s u g a r house

was r e d u c e d . Howeve r , a Q u e n t i n s t a t i o n was i n s t a l l e d i n t h e s u g a r house and t h e

c a p a c i t y o f t he l o w - g r a d e s t a t i o n was e x t e n d e d t o p r o c e s s t h e e n t i r e s y r u p f l o w ,

t h i s i n c r e a s i n g t h e hea t demand.

324

I n 1986, t he p r o c e s s i n g c a p a b i l i t y was r a i s e d t o 6200 t / d . Owing t o e x t e n s i o n

o f t h e e x t r a c t o r ( t h e a v e r a g e e x t r a c t i o n t ime was changed f rom 87 t o 120 m i n ) ,

t he j u i c e d r a f t a t t a i n e d a v e r y low v a l u e o f 109%. A new f a l l i n g - f i l m u n i t w i t h 2

a h e a t i n g s u r f a c e a r e a o f 2500 m was i n s t a l l e d as t h e f i f t h e v a p o r a t o r e f f e c t ,

and t h e t h i c k - j u i c e c o n c e n t r a t i o n was i n c r e a s e d t o 72-73% DS. Even t hough t h e

c r y s t a l l i z a t i o n scheme was t r a n s f o r m e d t o i n c o r p o r a t e " t w o - a n d - a - h a l f - b o i l i n g "

( r aw s u g a r s I and I I , and a l o w - g r a d e p r o d u c t ) , h i g h t h i c k - j u i c e c o n c e n t r a t i o n

and t he i n t r o d u c t i o n o f c r y s t a l f o o t i n g r e s u l t e d i n t h e t h e o r e t i c a l w a t e r

e v a p o r a t i o n i n t h e s u g a r house be ing r e d u c e d t o 8.2 kg/100 kg b.

8 .4 .3 E v o l u t i o n o f t h e therma l sys tem

The 1978 v e r s i o n o f t h e therma l sys tem i s shown s c h e m a t i c a l l y i n F i g . 8 .13 ,

t o g e t h e r w i t h t he da ta on mass and hea t b a l a n c e s . The e v a p o r a t o r consumed

26.8 kg steam pe r 100 kg b. Owing t o a r a t h e r low hea t demand o f t he s u g a r

h o u s e , i t was i m p o s s i b l e t o u t i l i z e t he e n t i r e amount o f v a p o u r s , and a

r e l a t i v e l y l a r g e f l o w o f l a s t - e f f e c t v a p o u r t o the c o n d e n s e r had t o be a c c e p t e d .

The condensa te e n e r g y was n o t f u l l y u t i l i z e d . I n t h e power h o u s e , one t u r b o

g e n e r a t o r d r i v e n by a b a c k - p r e s s u r e t u r b i n e s u p p l i e d w i t h l i v e steam a t 58 bar

and 500°C was enough t o c o v e r t he power demand o f t h e f a c t o r y . Howeve r , a p a r t

o f t h e h e a t i n g - s t e a m f l o w had t o be s u p p l i e d v i a t h e t h r o t t l i n g - d e s u p e r h e a t i n g

s t a t i o n .

F o l l o w i n g t he e x t e n s i o n s o f t he f a c t o r y and t he r e s u l t i n g i n c r e a s e i n power

demand, a second t u r b o - g e n e r a t o r was i n s t a l l e d i n t he power h o u s e . A t h e o r e t i c a l

p o s s i b i l i t y a r o s e t o s e l l t h e power s u r p l u s t o t h e e x t e r n a l g r i d ; h o w e v e r , t h e

f a c t o r y was o f f e r e d a power p r i c e w h i c h t u r n e d o u t t o be t o o low t o c o v e r t h e

c o s t . T h i s s t i m u l a t e d a t h o r o u g h a n a l y s i s and r e - o r i e n t a t i o n o f t h e e n e r g y

p o l i c y t owa rds f a r - r e a c h i n g power and hea t s a v i n g s .

I t was d e c i d e d t o m o d i f y t h e the rma l sys tem by i n t r o d u c i n g t he c o m p r e s s i o n o f

f i r s t - e f f e c t v a p o u r i n an e l e c t r i c a l l y - d r i v e n mechan ica l c o m p r e s s o r . I n o r d e r t o

change t h e mass and h e a t b a l a n c e s o f t h e e v a p o r a t o r , r a w - j u i c e h e a t i n g w i t h

vacuum-pan v a p o u r s and p r e - 1 i m e d - j u i c e h e a t i n g w i t h condensa te were imp lemen ted .

T h i s made i t p o s s i b l e t o r e c i r c u l a t e 27.3 kg f i r s t - e f f e c t v a p o u r p e r 100 kg b e e t

i n t he v a p o u r - c o m p r e s s i o n c i r c u i t , and t o d e c r e a s e t h e h e a t i n g - s t e a m consumpt i on

i n t h e e v a p o r a t o r t o 23.6 kg/100 kg b. The 1981 v e r s i o n o f t h e the rma l sys tem i s

shown s c h e m a t i c a l l y i n F i g . 8 .14 .

Among t h e m o d i f i c a t i o n s i n t r o d u c e d a f t e r 1981, e x t e n s i o n o f t he e v a p o r a t o r

s t a t i o n p l a y e d t he main r o l e . F o l l o w i n g t he i n s t a l l a t i o n o f a f a l l i n g - f i l m 2

e v a p o r a t o r w i t h a h e a t i n g s u r f a c e a r e a o f 2500 m i n t h e l a s t e f f e c t , t h e t o t a l 2

h e a t i n g s u r f a c e a r e a o f t h e e v a p o r a t o r r e a c h e d 14500 m . T h i s made i t p o s s i b l e

t o m a i n t a i n 73% DS t h i c k - j u i c e c o n c e n t r a t i o n a t r e d u c e d t e m p e r a t u r e d i f f e r e n c e s

i n t h e i n d i v i d u a l e f f e c t s . I t t h u s became p o s s i b l e t o d e c r e a s e t he t e m p e r a t u r e

325

Fig.

8

.14

. S

imp

lifi

ed

sche

me

of

the

ther

mal

sy

stem

of

the

App

eldo

rn

fact

ory

, 19

81

vers

ion

(aft

er

ref.

18

).

Fig.

8

.13

. S

imp

lifi

ed

sche

me

of

the

ther

mal

sy

stem

of

the

App

eldo

rn

fact

ory

, 19

78 v

ersi

on

(aft

er

ref.

18

).

Stea

m a

nd

vapo

ur

flow

s ar

e gi

ven

in

kg/lO

O

kg b

.

326

Fig.

8.

16.

Sim

pli

fied

sch

eme

of t

he

ther

mal

sy

stem

of

the

App

eldo

rn

fact

ory

, 19

86 v

ersi

on

(aft

er

ref.

18).

Fig. 8.15.

Scheme o

f the

util

izat

ion

of c

ondensate

in t

he A

ppel

dorn f

actory (after

ref. 18).

327

(and t h e c o r r e s p o n d i n g s a t u r a t i o n p r e s s u r e ) o f t h e h e a t i n g s team, t h i s r e s u l t i n g

i n an i n c r e a s e d i s e n t r o p i c e n t h a l p y d r o p i n t h e t u r b i n e and an i n c r e a s e o f t h e

power g e n e r a t e d . Owing t o l o w e r j u i c e t e m p e r a t u r e s i n t h e i n d i v i d u a l e v a p o r a t o r

e f f e c t s , s u c r o s e decay was a l s o r e d u c e d , w i t h a c o n s i d e r a b l e r e d u c t i o n o f t h i c k -

j u i c e c o l o u r as a r e s u l t ( r e f . 1 7 , 1 8 ) .

V e r y i n t e n s i v e u t i l i z a t i o n o f t h e c o n d e n s a t e e n e r g y was a l s o implemented

( F i g . 8 . 1 5 ) . I n a s e r i e s o f hea t e x c h a n g e r s c o n n e c t e d t o t h e room h e a t i n g

s y s t e m , 1 i m e d - j u i c e and r a w - j u i c e h e a t i n g , f u e l - o i l h e a t i n g and w a s t e - w a t e r

t r e a t m e n t s y s t e m , t he t e m p e r a t u r e o f t he c o n d e n s a t e f a l l s f rom i t s i n i t i a l

v a l u e o f 92°C t o as low as 37°C. I n a d d i t i o n , depend ing on t h e t e m p e r a t u r e

l e v e l , p a r t s o f t he condensa te s t ream a re u t i l i z e d a s :

- wash w a t e r i n t h e c e n t r i f u g a l s ;

- s w e e t e n i n g - o f f w a t e r i n t he s l u d g e p r e s s e s ;

- f r e s h w a t e r i n t h e e x t r a c t o r .

The scheme o f t h e thermal s y s t e m , w i t h t h e da ta on mass and hea t b a l a n c e s

c h a r a c t e r i s t i c o f t he 1986 s e a s o n , a r e shown i n F i g . 8 .16 . As can be s e e n , t h e

h e a t i n g - s t e a m consumpt ion i n t h e e v a p o r a t o r was r e d u c e d t o 18.5 kg/100 kg b.

8 .4 .4 E v o l u t i o n o f t h e power b a l a n c e

When d e s i g n i n g t h e A p p e l d o r n s u g a r f a c t o r y , much e f f o r t was s p e n t on e n s u r i n g

a low power demand. F o r examp le , j u i c e pumps i n t h e e x t r a c t i o n and j u i c e

p u r i f i c a t i o n s t a t i o n s were e q u i p p e d w i t h t h y r i s t o r - c o n t r o l l e d d . c . d r i v e s ,

making i t p o s s i b l e t o a p p l y v a r i a b l e speed c o n t r o l . As a r e s u l t , power

consumpt ion as low as 2.54 kWh/100 kg b was a c h i e v e d i n t h e f i r s t s e a s o n .

The i n s t a l l a t i o n o f an e l e c t r i c a l l y - d r i v e n v a p o u r compresso r caused t h e power

demand o f t h e f a c t o r y t o i n c r e a s e by 13%. I n o r d e r t o r e s t o r e t h e r e l a t i o n

between power g e n e r a t e d and power consumed, a number o f r a t i o n a l i z a t i o n measures

were t a k e n :

- an e x t e n s i o n o f t he p r o c e s s i n g c a p a b i l i t y o f t h e f a c t o r y t ook p l a c e w i t h o u t

any e x t e n s i o n o f t h e b e e t s t o r a g e y a r d s ;

- t he e x i s t i n g w a s t e - w a t e r t r e a t m e n t p l a n t was r e p l a c e d by an a n a e r o b i c p l a n t

c h a r a c t e r i z e d by a l o w e r e n e r g y demand;

- o n l y a p a r t o f t he p r e s s e d p u l p was d i r e c t e d t o t he d r y i n g p l a n t (50% i n 1986) .

A r e v i e w was a l s o u n d e r t a k e n o f t h e e l e c t r i c d r i v e s and e l e c t r i c a l l y - d r i v e n

machines i n t h e e n t i r e f a c t o r y . F o r examp le , a l l t he t r o u g h c o n v e y o r s were

i n v e s t i g a t e d t o i d e n t i f y t h o s e t h a t c o u l d be r e p l a c e d by more e n e r g y - e f f i c i e n t

b e l t c o n v e y o r s . Howeve r , p a r t i c u l a r a t t e n t i o n was p a i d t o t h e f l o w m a c h i n e r y

and t h e p o s s i b i l i t i e s o f r e p l a c i n g t h r o t t l i n g c o n t r o l by v a r i a b l e speed c o n t r o l ,

o r i n t r o d u c i n g i n t e r m i t t e n t i n s t e a d o f c o n t i n u o u s o p e r a t i o n . T a k i n g advan tage o f

f a l l i n g p r i c e s o f f r e q u e n c y - c o n t r o l l e d a . c . d r i v e s , v a r i a b l e - s p e e d d r i v e s were

implemented i n b e e t pumps and k i l n - g a s c o m p r e s s o r s , as w e l l as i n p u l p p r e s s e s .

328

TABL

E 8.

12

Powe

r co

nsum

ed a

nd p

ower

su

pplie

d to

th

e A

ppel

dorn

su

gar

fact

ory

, in

kW

h/10

0 kg

b

(aft

er

ref.

18).

1977

19

78

1979

19

80

1981

19

82

1983

19

84

1985

19

86

Bee

t st

orag

e ya

rds

0.27

2 0.

261

0.27

6 0.

294

0.26

4 0.

290

0.23

3 0.

164

0.14

0 0.

160

Bee

t ho

use

0.87

0 0.

860

0.85

6 0.

843

1.01

8 0.

980

0.96

3 0.

945

0.92

2 0.

830

Suga

r ho

use

0.61

4 0.

610

0.61

2 0.

633

0.60

9 0.

584

0.55

4 0.

642

0.63

0 0.

670

Pul

p-dr

ying

p

lan

t 0.

338

0.33

4 0.

350

0.34

3 0.

330

0.33

4 0.

271

0.26

0 0.

210

0.28

0 Po

wer

hous

e 0.

155

0.15

7 0.

160

0.15

4 0.

142

0.15

4 0.

159

0.15

8 0.

160

0.17

3 W

ater

ci

rcu

its

and

com

pres

sed-

air

supp

ly

0.15

5 0.

157

0.17

6 0.

156

0.16

2 0.

149

0.16

0 0.

157

0.13

0 0.

140

Was

te-w

ater

tre

atm

ent

0.13

3 0.

201

0.22

8 0.

260

0.25

1 0.

263

0.17

8 0.

200

0.11

0 0.

110

Vapo

ur

com

pres

sor

0.39

0 0.

410

0.38

0 0.

290

0.24

0 0.

330

Powe

r ge

nera

ted

2.47

3 2.

506

2.61

2 2.

642

3.02

2 3.

012

2.75

2 2.

640

2.41

4 2.

534

Powe

r pu

rcha

sed

0.06

4 0.

074

0.04

6 0.

041

0.14

4 0.

152

0.14

6 0.

176

0.12

8 0.

159

Tota

l po

wer

su

pplie

d 2.

537

2.58

0 2.

658

2.68

3 3.

166

3.16

4 2.

898

2.81

6 2.

542

2.69

3

TABL

E 8.

13

Powe

r co

nsum

ed a

nd p

ower

su

pplie

d to

th

e A

ppel

dorn

su

gar

fact

ory

, in

kW

h/t

suga

r (a

fter

re

f. 18

).

1977

19

78

1979

19

80

1981

19

82

1983

19

84

1985

19

86"

Bee

t st

orag

e ya

rds

20.8

9 18

.63

19.2

8 21

.73

18.8

0 20

.94

16.5

4 11

.63

9.49

10

.70

Bee

t ho

use

66.7

9 61

.49

59.8

4 62

.23

73.3

4 71

.94

69.8

8 67

.76

65.4

8 53

.30

Suga

r ho

use

47.1

4 43

.62

42.8

0 46

.71

43.3

0 42

.16

39.4

7 45

.57

43.7

0 42

.90

Pulp

-dry

ing

pla

nt

25.9

4 23

.85

24.4

7 25

.34

23.4

8 24

.08

19.3

1 18

.30

14.8

1 18

.20

Powe

r ho

use

11.9

0 11

.19

11.2

0 11

.41

10.0

7 11

.10

11.3

0 11

.24

10.9

4 11

.20

Wat

er

circ

uit

s an

d co

mpr

esse

d-ai

r su

pply

11

.90

11.1

9 12

.07

11.5

2 11

.49

10.7

6 11

.40

11.1

5 9.

27

8.80

W

aste

-wat

er

trea

tmen

t 10

.18

14.1

7 15

.95

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329

As a r e s u l t , t h e t o t a l i n s t a l l e d power o f t h e d . c . d r i v e s r e a c h e d a l e v e l o f

3200 kW, and t h a t o f t he f r e q u e n c y - c o n t r o l l e d a . c . d r i v e s 1900 kW, w i t h t h e

v a r i a b l e speed d r i v e s d o m i n a t i n g i n t h e b e e t house and i n t h e s u g a r h o u s e . As

r e g a r d s t h e equ ipment i n s t a l l e d i n t he power house and i n t h e p u l p - d r y i n g

p l a n t , i t t u r n e d o u t t h a t t h e p o t e n t i a l e n e r g y s a v i n g s a r e t o o smal l t o j u s t i f y

t he i n v e s t m e n t c o s t s o f v a r i a b l e - s p e e d d r i v e s t h e r e .

The e v o l u t i o n o f t h e power consumpt ion i n s e v e n s e c t i o n s o f t h e f a c t o r y (and

t he v a p o u r c o m p r e s s o r ) i s i l l u s t r a t e d i n T a b l e s 8.12 and 8 .13 . As can be s e e n ,

t h e power consumpt ion o f t he v a p o u r compresso r has been o f f - s e t by t h e power

s a v i n g s a t t a i n e d i n t he b e e t s t o r a g e y a r d s , b e e t h o u s e , p u l p - d r y i n g p l a n t , w a t e r

s u p p l y sys tem and w a s t e - w a t e r t r e a t m e n t p l a n t . I t i s a l s o i n t e r e s t i n g t o see

t h a t i n t he s u g a r h o u s e , w h i l e t he power consumpt ion p e r 100 kg b e e t has

i n c r e a s e d due t o t he e x t e n s i o n s o f t he c r y s t a l l i z a t i o n scheme, a r e d u c t i o n o f

t he power consumpt ion p e r 1 t s u g a r was a c h i e v e d .

I n T a b l e s 8.12 and 8 . 1 3 , d a t a a r e a l s o g i v e n on t h e power s u p p l i e d t o t h e

f a c t o r y . I t can be c o n c l u d e d f rom t h e s t e e p i n c r e a s e s o f power g e n e r a t e d and

p u r c h a s e d w h i c h took p l a c e i n 1981 t h a t t h e hea t s a v i n g men t ioned i n t he

p r e c e d i n g S e c t i o n was a t t a i n e d a t t h e expense o f i n c r e a s e d power c o n s u m p t i o n .

I t can a l s o be seen t h a t t he measures t aken t o a d j u s t t h e power g e n e r a t i o n t o

chang ing power demand were q u i t e e f f e c t i v e , as a t p r e s e n t , t h e power p u r c h a s e d

does n o t e x c e e d 5-6% o f t he t o t a l power s u p p l i e d .

REFERENCES

1 L. S z y d l o , W. Lekawski and K. U r b a n i e c , M o d e r n i z a c j a g o s p o d a r k i c i e p l n e j Cukrown i K l e c i n a , G a z . C u k r o w . , 9 3 ( 7 - 8 ) (1985) 134.

2 N .K . P o l i s h c h u k , I s p o l z o v a n i e e n e r g o r e s u r s o v na E rken -Shakharskom sakharnom z a v o d e , Sakh . P r o m . , ( 6 ) (1986) 39-40.

3 K. U r b a n i e c , Ocena p r a k t y c z n y c h m o z l i w o s c i o s z c z e d z a n i a p a l i w a w g o s p o d a r c e e n e r g e t y c z n e j c u k r o w n i , G a z . C u k r o w . , 8 9 ( 4 ) (1981) 80-81.

4 Y u . D . G o l o v n y a k and L . G . B e l o s t o t s k i i , S h i r o k o v n e d r y a t nauchnye r a z r a b o t k i d l y a s n i z h e n i y a raskhoda t o p l i v a , Sakh . P r o m . , ( 8 ) (1981) 21-24.

5 K. U r b a n i e c , R a c j o n a l i z a c j a g o s p o d a r k i c i e p l n e j w c u k r o w n i a c h , G a z . C u k r o w . , 92 (2 ) (1984) 27-28.

6 C . H . I v e r s o n , W i t h e r g o e s t t h o u , oh BTU ? , Sugar J . , 45 (11) (1983) 17-22. 7 J . B o z e c , E v o l u t i o n de l a consommation t h e r m i q u e dans 1 ' i n d u s t r i e s u c r i e r e ,

I n d . A l i m . A g r i e , 100(7 -8 ) (1983) 477-480. 8 Anonymous, E n e r g y p rogram a t I m p e r i a l S u g a r , Sugar J . , 4 7 ( 1 ) (1984) 20. 9 B. K a r r e n , E x p e r i e n c e o f e n e r g y s a v i n g i n t he Canad ian s u g a r i n d u s t r y , i n :

F . O . L i c h t s Gu ide t o t h e Sugar F a c t o r y Mach ine I n d u s t r y , F . O . L i c h t GmbH, R a t z e b u r g , 1984, p p . A75-A88.

10 L . L . N e v i l l e , H o l l y Sugar C o r p o r a t i o n ' s c a p i t a l improvement p r o g r a m , Sugar y A z ú c a r , 80 (2 ) (1985) 49 ,52 .

11 G . K o w a l s k a , Po rzadkowan ie g o s p o d a r k i c i e p l n e j na p r z y k l a d z i e cuk rown i w i e l k o p o l s k i c h , G a z . C u k r o w . , 94 (4 ) (1986) 52-53.

12 W. L e k a w s k i , M o d e r n i z a c j a G o s p o d a r k i C i e p l n e j C u k r o w n i , S T C , Warszawa, 1986. 13 E . V . M l o d z y a n o w s k i i , V . S . B e r e z y u k and K . N . S a v c h u k , Ekonomnoe i s p o l z o v a n y e

e n e r g o r e s u r s o v , Sakh . P r o m . , ( 7 ) (1981) 22-27 . 14 E. Krupka and J . S z a d k o w s k i , Gospodarka c i e p l n a w Cukrown i G o s l a w i c e ,

G a z . C u k r o w . , 89 (1 ) (1981) 2 -5 .

330

15 A . I . Khomenko, Ekonomya t o p i i v n o - e n e r g e t i c h e s k i k h r e s u r s o v - i t o g i i z a d a c h i , Sakh . P r o m . , (2) (1983) 35-39.

16 L . P . I g n a t e v ( e t a l . ) , O p y t r a b o t y po s n i z h e n i y u raskhoda t o p i i v n o -e n e r g e t i c h e s k i k h r e s u r s o v na A l e k s a n d r i i s k o m sakharnom z a v o d e , Sakh . P r o m . , (10) (1985) 32-34.

17 G. F e l t b o r g , p e r s o n a l commun ica t i on . 18 H. W e i d n e r , D i e B rüdenkompress ion i n e i n e r R o h z u c k e r f a b r i k , Z u c k e r i n d . ,

108(8) (1983) 736-742. 19 υ . C u r d t s , E i n Weg z u r V e r b e s s e r u n g d e r W ä r m e w i r t s c h a f t e i n e r R o h z u c k e r

f a b r i k - am B e i s p i e l d e r Z u c k e r f a b r i k A p p e l d o r n , Paper p r e s e n t e d a t t h e I n t e r n a t i o n a l C o n f e r e n c e " Improvement o f Bee t Sugar P r o d u c t i o n " , Warszawa, May 1987.

20 A . Co l sman , p e r s o n a l commun ica t i on .

331

C h a p t e r 9

DESIGN OF MODERNIZED OR NEW ENERGY SYSTEMS

9.1 THE MODERNIZATION OF ENERGY SYSTEMS

C o n t r a r y t o the s t e p - b y - s t e p a p p r o a c h c o n s i d e r e d i n t h e p r e c e d i n g C h a p t e r ,

m o d e r n i z a t i o n o f a s u g a r f a c t o r y i n v o l v e s e x t e n s i v e changes t h a t a r e i n t r o d u c e d

e i t h e r a l l a t once o r i n a few s t e p s pe r f o rmed d u r i n g c o n s e c u t i v e o f f - s e a s o n

p e r i o d s . T y p i c a l l y , t h i s i s a s e r i o u s and c o s t l y u n d e r t a k i n g , v e r y se ldom aimed

s o l e l y a t e n e r g y s a v i n g s . Most o f t e n t h e r e a r e s e v e r a l t e c h n o l o g i c a l o b j e c t i v e s ,

among w h i c h an e x t e n s i o n o f t h e p r o c e s s i n g c a p a b i l i t y has a l e a d i n g r o l e and

the o t h e r s s e r v e the pu rpose o f c u t t i n g down m a n u f a c t u r i n g c o s t s . As t h e

u n d e r t a k i n g has t o be comp le ted w i t h i n a d e f i n i t e c o s t / t i m e f r amework , i t may be

d i f f i c u l t t o a r r i v e a t an i n i t i a l p rob lem f o r m u l a t i o n , and even more so t o f i n d

t he s o u g h t - a f t e r s o l u t i o n . T h e r e f o r e , m o d e r n i z a t i o n u s u a l l y r e q u i r e s t he

e x p e r t i s e o f s p e c i a l i z e d e n g i n e e r i n g compan ies .

P u b l i c a t i o n s r e l a t e d t o t h e m e t h o d o l o g i c a l p rob lems o f m o d e r n i z a t i o n o f s u g a r

f a c t o r i e s a r e r a t h e r s c a r c e , as t h i s s u b j e c t i s r e g a r d e d as a p a r t o f t h e know-

how o f t h e companies i n v o l v e d . A book and a few a r t i c l e s can be f ound r e v i e w i n g

t h e g e n e r a l d e s i g n p r o c e d u r e s , as w e l l as t h e s p e c i a l i z e d q u e s t i o n s o f e n e r g y

economy improvements ( r e f s . 1 - 3 ) .

The i n v e s t i g a t i o n o f m o d e r n i z a t i o n r e q u i r e m e n t s b e g i n s w i t h d r a w i n g up an

i n v e n t o r y o f e x i s t i n g f a c t o r y s u b s y s t e m s . The number o f subsys tems t y p i c a l l y

d e f i n e d i s 25-30. F o r e v e r y s u b s y s t e m , c r u c i a l d a t a on p r o c e s s a p p a r a t u s and

machines a r e r e g i s t e r e d , as w e l l as on a s s o c i a t e d e l e c t r i c a l e q u i p m e n t , c o n t r o l

c i r c u i t r y and measur ing i n s t r u m e n t s . D raw ings a r e s i m u l t a n e o u s l y p r e p a r e d o f

e s s e n t i a l f a c t o r y b u i l d i n g s , t o g e t h e r w i t h a s i t e p l a n . The i n f o r m a t i o n t h u s

a c q u i r e d makes i t p o s s i b l e t o e v a l u a t e t h e f a c t o r y ' s t e c h n o l o g i c a l base and t h e

s t a t e o f t h e b u i l d i n g s .

The n e x t s t e p u s u a l l y c o n s i s t s o f d i s c u s s i n g t h e da ta w h i c h a r e p a r t i c u l a r l y

i m p o r t a n t t o d e t e r m i n a t i o n o f t h e o b j e c t i v e s and i d e n t i f i c a t i o n o f t h e c o s t - and

t i m e - r e l a t e d c o n s t r a i n t s o f t h e m o d e r n i z a t i o n . The da ta o f c r u c i a l i m p o r t a n c e

a r e as f o l l o w s :

- b e e t p o l a r i z a t i o n ;

- s u g a r l o s s e s i n t h e m a n u f a c t u r i n g p r o c e s s , i n c l u d i n g t h e i r d i s t r i b u t i o n

between e x t r a c t i o n , c a r b o n a t a t i o n s l u d g e , mo lasses and u n d e f i n e d l o s s ;

- p u r i t i e s o f raw j u i c e , t h i c k j u i c e and m o l a s s e s ;

- t he main f e a t u r e s o f t h e j u i c e p u r i f i c a t i o n scheme, i n c l u d i n g t e m p e r a t u r e s o f

e s s e n t i a l p r o c e s s s t e p s ;

- CaO r a t e s and main f e a t u r e s o f t h e l i m e - s l a k i n g p r o c e s s ;

332

- t he main f e a t u r e s o f t he p u l p - p r e s s i n g and p u l p - d r y i n g p r o c e s s e s ;

- t he main f e a t u r e s o f t he s u g a r c r y s t a l l i z a t i o n scheme and s u g a r b o i l i n g

p r o c e s s ;

- au toma t i on r e q u i r e m e n t s ;

- e n v i r o n m e n t a l p r o t e c t i o n r e q u i r e m e n t s ;

- the f u e l t y p e and the e x p e c t e d l e v e l o f f u e l demand i n t h e power house and i n

the p u l p - d r y i n g p l a n t ;

- power consumpt ion r e q u i r e m e n t s .

Hav ing a c q u i r e d n e c e s s a r y knowledge o f t h e c o n d i t i o n o f t h e f a c t o r y and t he

p r i o r i t i e s o f t he m o d e r n i z a t i o n o b j e c t i v e s , t he d e s i g n e n g i n e e r s s h o u l d be a b l e

t o c a l c u l a t e and p r e p a r e s k e t c h e s o f r e c o n s t r u c t i o n p r o p o s a l s f o r t h e most

i m p o r t a n t f a c t o r y s e c t i o n s . Depending on s p e c i f i c c o n d i t i o n s , t he f o l l o w i n g

m a t e r i a l may be r e q u i r e d :

- scheme o f b e e t r e c e p t i o n , u n l o a d i n g , t r a n s p o r t and s t o r a g e ;

- mass b a l a n c e s o f e x t r a c t i o n , j u i c e p u r i f i c a t i o n , e v a p o r a t i o n and s u g a r

c r y s t a l l i z a t i o n ;

- h e a t b a l a n c e o f t he therma l s y s t e m ;

- h e a t b a l a n c e o f t he p u l p - d r y i n g p l a n t ;

- v e r i f i c a t i o n o f r a t i n g o f t h e e x i s t i n g equ ipment and recommendat ions on t he

s e l e c t i o n o f new equ ipment u n i t s ;

- e n e r g y ( h e a t and power ) b a l a n c e o f t h e power h o u s e .

C o m p l e t i o n o f t h i s s t e p makes i t p o s s i b l e t o d e t e r m i n e t h e scope o f

equ ipment p u r c h a s e s and c o n s t r u c t i o n w o r k , as w e l l as t o p l a n d i s m a n t l i n g o f

t he e x i s t i n g e q u i p m e n t , c o n s t r u c t i o n w o r k , a s s e m b l i n g o f new equ ipment and

p i p i n g , i n s t a l l a t i o n o f e l e c t r i c a l equ ipment and c o n t r o l c i r c u i t r y , e t c .

The s o l u t i o n s a r e u s u a l l y p r e p a r e d i n a number o f v e r s i o n s and d e s c r i b e d i n a

r e p o r t on p o s s i b l e and recommended c o u r s e s o f a c t i o n , c o s t p r o j e c t i o n s and

p r e l i m i n a r y t i m e - s c h e d u l e s . T h i s c o n s t i t u t e s a b a s i s f o r d e c i s i o n - m a k i n g on

d e t a i l e d m o d e r n i z a t i o n d e s i g n s .

The d e c i s i o n s t e p d e s e r v e s most a t t e n t i o n as i t d e t e r m i n e s , t o a g r e a t

e x t e n t , the economic r e s u l t s o f t h e e n t i r e u n d e r t a k i n g . The c o s t s t r u c t u r e o f

a t y p i c a l m o d e r n i z a t i o n case i s c h a r a c t e r i z e d by a p p r o x i m a t e l y equa l

c o n t r i b u t i o n s o f t he f o l l o w i n g c o s t componen ts :

- t he c o s t o f t he c o n s t r u c t i o n w o r k ;

- t he c o s t o f p u r c h a s i n g and a s s e m b l i n g t he main e q u i p m e n t ;

- t he c o s t o f p i p i n g , e l e c t r i c a l e q u i p m e n t , c o n t r o l and i n s t r u m e n t a t i o n

c i r c u i t r y , e n g i n e e r i n g s e r v i c e s , e t c .

P r i o r t o making t h e d e c i s i o n s , one can a l s o c o n s i d e r t h e m o d e r n i z a t i o n c o s t s as

a sum o f two componen ts :

( i ) t he c o s t o f t h e i n c r e a s e o f p r o c e s s i n g c a p a b i l i t y ;

333

( i i ) the c o s t o f t he o p t i m i z a t i o n o f d e t a i l e d s o l u t i o n s .

I t can be e s t i m a t e d t h a t component ( i i ) i s t y p i c a l l y o f t he o r d e r o f 15-20% o f

t he t o t a l c o s t . By o p t i m i z i n g t he s o l u t i o n s , h o w e v e r , one may i n f l u e n c e t h e

economic r e s u l t s o f t he m o d e r n i z a t i o n t o a d e g r e e comparab le w i t h t h a t

a s s o c i a t e d w i t h t he p r o c e s s i n g - c a p a b i l i t y i n c r e a s e . A c t u a l l y , t h i s i s where t h e

impo r t ance o f improvements o f t h e e n e r g y economy l i e s . I t i s n o t unusua l t h a t

t h e s e improvements a r e d e c i s i v e i n o p t i m i z i n g t h e m o d e r n i z a t i o n s o l u t i o n s .

O p t i m i z a t i o n o f t h e e n e r g y economy o f a m o d e r n i z e d s u g a r f a c t o r y i s a

d e c i s i o n p rob lem unde r many c o n s t r a i n t s . W i t h i n t h e f i e l d o f f e a s i b l e s o l u t i o n s ,

each e n e r g y - s a v i n g t e c h n i q u e o f t h o s e c o n s i d e r e d i n C h a p t e r s 3 t o 7 and

s y s t e m a t i z e d i n S e c t i o n 8.1 can be c o n s i d e r e d f o r a p p l i c a t i o n . The s e t o f

t e c h n i q u e s s a t i s f y i n g t he c o n s t r a i n t s and p r o m i s i n g t h e b e s t economic r e s u l t s

c o n s t i t u t e s t h e d e s i r e d s o l u t i o n ( s e e a l s o S e c t i o n 9 . 4 ) .

I t can be c o n c l u d e d f rom t h e above i n t r o d u c t i o n t h a t i t i s d i f f i c u l t t o

r e p o r t a m o d e r n i z a t i o n example i n g r e a t d e t a i l . As a r u l e , t h e p r e s e n t a t i o n s

p u b l i s h e d a r e v e r y c o n c i s e and f e w , i f a n y , t e c h n i c a l d e t a i l s a r e g i v e n

( r e f s . 4 - 7 ) . T r y i n g t o change t h i s s i t u a t i o n a l i t t l e w i t h o u t e x c e e d i n g t h e

space a v a i l a b l e i n t h e p r e s e n t b o o k , summaries a r e p r e s e n t e d i n S e c t i o n s 9.2

and 9.3 o f two r e a l - l i f e c a s e s : one f a c t o r y c h a r a c t e r i z e d b y r a t h e r p o o r

i n i t i a l e n e r g y u t i l i z a t i o n and a n o t h e r i n w h i c h t h e e n e r g y economy was q u i t e

d e c e n t . Bo th m o d e r n i z a t i o n s were u l t i m a t e l y aimed a t e x t e n s i o n s o f t h e

p r o c e s s i n g c a p a b i l i t y , t h i s b e i n g v e r y much dependen t on improvements o f t h e

e n e r g y economy. Pu lp d r y i n g was n o t t aken i n t o c o n s i d e r a t i o n .

The p r e s e n t a t i o n s c o n c e n t r a t e on t h e most r e l e v a n t p a r t s o f t h e m o d e r n i z a t i o n

c o n c e p t s , namely t h o s e c o n c e r n e d w i t h p o s s i b l e v e r s i o n s o f t h e r e c o n s t r u c t e d

thermal sys tems and t h e i r h e a t b a l a n c e s . I t i s n o t e w o r t h y , h o w e v e r , t h a t t h e

impo r tance o f t h e power b a l a n c e s has a l s o come i n t o f o c u s . I n t h e f i r s t e x a m p l e ,

p r e s e n t e d i n S e c t i o n 9 . 2 , owing t o a r e l a t i v e l y l a r g e n e t h e a t demand, t h e

f a c t o r y i s g e n e r a l l y a b l e t o s e l l a power s u r p l u s t o t h e e x t e r n a l g r i d . I t i s

o n l y t he most e n e r g y - e f f i c i e n t v e r s i o n o f t h e the rma l sys tem w h i c h r e d u c e s t h e

steam f l o w t h r o u g h t h e t u r b i n e t o a v a l u e l e s s t h a n t h a t r e q u i r e d f o r e l e c t r i c a l

s e l f - s u f f i c i e n c y . The second example p r e s e n t e d i n S e c t i o n 9.3 i s c o n c e r n e d w i t h

an e n e r g y sys tem i n w h i c h p o s s i b l e s i g n i f i c a n t h e a t s a v i n g s a r e accompanied by

a w i d e n i n g power d e f i c i t . I n t h i s s i t u a t i o n , measures t o r e d u c e t h e power

demand become an i m p o r t a n t p a r t o f t h e m o d e r n i z a t i o n , and t h e magn i t ude o f

complementary power p u r c h a s e s f rom t h e e x t e r n a l g r i d t akes a p l a c e i n t h e

e v a l u a t i o n o f m o d e r n i z a t i o n v e r s i o n s .

C o n c l u d i n g t h i s C h a p t e r and t h e e n t i r e b o o k , a summary i s g i v e n i n S e c t i o n

9.4 o f p r i n c i p l e s o f e n e r g y - s y s t e m d e s i g n u s i n g o p t i m i z a t i o n me thods .

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9.2 FACTORY CHARACTERIZED BY POOR I N I T I A L ENERGY U T I L I Z A T I O N

9.2.1 I n t r o d u c t o r y remarks

The f a c t o r y was e r e c t e d a t t h e b e g i n n i n g o f t h i s c e n t u r y . I n t h e 1930s and

1940s s a t e l l i t e p r o d u c t i o n f a c i l i t i e s were b u i l t , i n c l u d i n g an a l c o h o l

d i s t i l l e r y , a c a r b o n d i o x i d e p l a n t and a m a c h i n e - s h o p . A f t e r numerous

e x t e n s i o n s and m o d e r n i z a t i o n s o f t h e s u g a r f a c t o r y , a p r o c e s s i n g c a p a b i l i t y o f

3000 t / d was a t t a i n e d i n the 1960s. By comb in ing t h e s t e p - b y - s t e p app roach w i t h

two l i m i t e d - s c a l e m o d e r n i z a t i o n s d u r i n g a p e r i o d o f s e v e r a l y e a r s e n d i n g l a t e

i n t he 1970s, t he b u l k o f t he o l d p r o c e s s equ ipment was r e p l a c e d by new

m a c h i n e r y . The p r o c e s s i n g c a p a b i l i t y was t hen i n c r e a s e d t o 3900 t o n s p e r d a y ,

w i t h t h e maximum t h r o u g h p u t o f t he e x t r a c t i o n s t a t i o n e s t i m a t e d a t a b o u t

5000 t / d .

As t h e r e were o n l y min imal i n v e s t m e n t s i n t he the rma l sys tem d u r i n g t he most

r e c e n t p e r i o d o f f a c t o r y improvemen ts , symptoms o f i n a d e q u a t e i n s t a l l e d b o i l e r

c a p a c i t y became v i s i b l e a t t h i s p r o c e s s i n g c a p a b i l i t y . The managing s t a f f was

aware o f t he o u t d a t e d h e a t economy and t h e f a c t t h a t no f u r t h e r f a c t o r y

e x t e n s i o n s can be p l anned u n l e s s t h e the rma l sys tem i s t h o r o u g h l y m o d e r n i z e d .

As a m a t t e r o f f a c t , t h i s s i t u a t i o n was n o t u n e x p e c t e d . I t was d e l i b e r a t e l y

a l l o w e d t o c o i n c i d e w i t h t h e n e c e s s a r y i n s t a l l a t i o n o f r e p l a c e m e n t s f o r two o l d

e v a p o r a t o r b o d i e s i n the 2nd e f f e c t , t h e i r c e r t i f i c a t e s o f p r e s s u r e - v e s s e l

o p e r a t i o n above 2 b a r b e i n g due t o e x p i r e . C o n s e q u e n t l y , an e n g i n e e r i n g company

was h i r e d t o d e s i g n the n e c e s s a r y r e c o n s t r u c t i o n o f t h e therma l s y s t e m .

A number o f c o n s t r a i n t s and assump t i ons were f o r m u l a t e d i n advance by t h e

managing s t a f f o f t he f a c t o r y :

- the e n e r g y s a v i n g s s h o u l d make i t p o s s i b l e t o i n c r e a s e t h e p r o c e s s i n g

c a p a b i l i t y t o 5000 t / d , b u t t he n e s e s s a r y e x t e n s i o n s o f t h e p r o c e s s equ ipment

w i l l be c o n s i d e r e d a t a l a t e r d a t e ;

- no i n v e s t m e n t f unds a r e a v a i l a b l e f o r m o d e r n i z a t i o n o f t h e power house

e q u i p m e n t ;

- t he f a c t o r y has t o be s e l f - s u f f i c i e n t i n power , as t h e e x t e r n a l g r i d i s n o t

r e l i a b l e enough d u r i n g t h e w i n t e r p e r i o d o f peak l o a d s ;

- steam consumpt ion i n t he p r o d u c t i o n f a c i l i t i e s o u t s i d e t h e s u g a r f a c t o r y i s

w e l l under c o n t r o l and need n o t be a n a l y s e d ;

- i n t he s u g a r f a c t o r y , t he j u i c e d r a f t s h o u l d be i n c r e a s e d and t he c r y s t a l l i z

a t i o n scheme s h o u l d be a d j u s t e d t o i n c r e a s e t h e s u g a r o u t p u t ;

- due t o the c h a r a c t e r i s t i c s o f t h e e x i s t i n g t h i c k - j u i c e f i l t r a t i o n e q u i p m e n t ,

t h i c k - j u i c e c o n c e n t r a t i o n s h o u l d n o t exceed 65% DS.

Under such c i r c u m s t a n c e s , t h e t a s k o f t h e d e s i g n e r s was l e s s c o m p l i c a t e d

than i n a t y p i c a l case o f f a c t o r y m o d e r n i z a t i o n . I t was p o s s i b l e t o l i m i t t he

335

e x t e n t o f t he i n v e n t o r y o f t he e x i s t i n g f a c t o r y subsys tems t o t he c o l l e c t i o n o f

e s s e n t i a l d a t a r e q u i r e d f o r c a l c u l a t i o n s o f t h e mass and h e a t b a l a n c e s o f t h e

therma l s y s t e m . O n l y f o r t he case o f t h e e v a p o r a t o r s t a t i o n and h e a t e r s

( t o g e t h e r w i t h t h e a s s o c i a t e d p i p i n g and a u x i l i a r y equ ipmen t ) were d e t a i l e d

schemes and l a y o u t d r a w i n g s p r e p a r e d . An i n v e n t o r y o f t h e r e l e v a n t s u p p o r t i n g

s t r u c t u r e s was a l s o drawn u p , and a s e p a r a t e e x a m i n a t i o n o f t he measur ing

i n s t r u m e n t s and c o n t r o l c i r c u i t s was u n d e r t a k e n .

9 .2 .2 B a s i c f a c t o r y d a t a

P r o c e s s i n g c a p a b i l i t y : 3900 t / d .

P o l a r i z a t i o n o f c o s s e t t e s : 14.0-14.5%.

E x t r a c t i o n s t a t i o n : two t r o u g h - t y p e e x t r a c t o r s .

J u i c e d r a f t : 108%.

R a w - j u i c e c o n c e n t r a t i o n and p u r i t y : 15.29% DS, 88.5%.

Pu lp p r e s s e d t o : 10.5% DS and 15.0% DS ( 4 / 5 and 1/5 o f t h e w e t - p u l p f l o w ,

r e s p e c t i v e l y ; t he f o r m e r p a r t i s s o l d t o f a r m e r s and t h e l a t t e r d i r e c t e d t o

a d r y e r ) .

K i l n g a s : 26.8% CO2 v o l .

J u i c e p u r i f i c a t i o n a c c o r d i n g t o t h e c l a s s i c a l scheme, c o m p r i s i n g :

- h o t p r e - l i m i n g a t 50°C, CaO r a t e 0.22 kg/100 kg b ;

- main l i m i n g a t 8 6 ^ C , CaO r a t e 1.30 kg/100 kg b ;

- 1 s t c a r b o n a t a t i o n a t 82°C;

- d e c a n t i n g and f i l t r a t i o n ;

- 2nd c a r b o n a t a t i o n a t 94°C, CaO r a t e 0.10 kg/100 kg b ;

- d o u b l e - s t a g e 2nd f i l t r a t i o n .

T h i n - j u i c e c o n c e n t r a t i o n and p u r i t y : 13.42% D S , 92.40%.

E v a p o r a t o r : q u a d r u p l e - e f f e c t , R o b e r t - t y p e b o d i e s .

T h i c k - j u i c e c o n c e n t r a t i o n : 60-62.5% DS.

Sugar h o u s e :

- s t a n d a r d - l i q u o r based t h r e e - b o i l i n g scheme w i t h t h e a f f i n a t i o n o f C s u g a r ;

- Β s u g a r and a f f i n e d C s u g a r m e l t e d i n t h i n j u i c e ;

- s t a n d a r d l i q u o r p r e p a r e d f rom t h i c k j u i c e and r e m e l t .

Sugar o u t p u t : 10.89 kg/100 kg b.

Power h o u s e :

- c o a l - f i r e d b o i l e r s , two u n i t s r a t e d 30 t / h and t h r e e u n i t s r a t e d 20 t / h ,

a v e r a g e e f f i c i e n c y abou t 60%;

- l i v e steam paramete rs 22 b a r , 320°C;

- two b a c k - p r e s s u r e t u r b i n e s r a t e d 3.3 MW e a c h , steam r a t e s a b o u t 11 kg /kWh;

- b a c k - p r e s s u r e 3.2 b a r .

The f a c t o r y s e l l s a power s u r p l u s t o t he e x t e r n a l g r i d .

336

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Steam s u p p l y t o t he s u g a r m a n u f a c t u r i n g p r o c e s s :

- l i v e steam t h r o t t l e d t o 6 b a r t o t he c e n t r i f u g a l s and vacuum-pan s t e a m i n g ;

- h e a t i n g steam 3.2 b a r ( e x h a u s t steam and t h r o t t l e d l i v e s team) t o t h e

r e m a i n i n g r e c e i v e r s .

Steam consumpt ion a c c o r d i n g t o measurements : 44 .5 -48 .6 kg/100 kg b .

Power c o n s u m p t i o n : e s t i m a t e d a t abou t 2.5 kWh/100 kg b.

Normal f u e l c o n s u m p t i o n : 6 .20-7 .00 kg/100 kg b.

I t s h o u l d be n o t e d t h a t i n p a r a l l e l t o s u p p l y i n g e n e r g y t o t h e s u g a r f a c t o r y ,

t he power house s u p p l i e s power and steam t o t h e f o l l o w i n g p r o d u c t i o n f a c i l i t i e s :

- a l c o h o l d i s t i l l e r y ;

- c a r b o n - d i o x i d e p l a n t ;

- m a c h i n e - s h o p .

The combined power demand o f t h e s e f a c i l i t i e s does n o t e x c e e d 0.8 MW, and t h e

h e a t i n g steam demand i s o f t h e o r d e r o f 6 t / h . The demand on 6 b a r steam i s

n e g l i g i b l y s m a l l .

The scheme o f v a p o u r and c o n d e n s a t e d i s t r i b u t i o n , and t h e main r e s u l t s o f t h e

mass and h e a t b a l a n c e c a l c u l a t i o n s o f t h e the rma l sys tem o f t h e s u g a r f a c t o r y

b e f o r e m o d e r n i z a t i o n , a r e shown i n F i g . 9 .1 . I t can i m m e d i a t e l y be r e c o g n i z e d

t h a t t h e r e a r e a few q u e s t i o n a b l e d e t a i l s t h a t s h o u l d be e l i m i n a t e d :

- the q u a d r u p l e - e f f e c t e v a p o r a t o r i s e s s e n t i a l l y o p e r a t e d as a t r i p l e - e f f e c t one

because t h e r e i s no h e a t i n g w i t h l a s t - s t a g e v a p o u r ;

- the 1 s t - and 2 n d - e f f e c t c o n d e n s a t e s a r e f l a s h e d t o t h e a t m o s p h e r e ;

- a s i g n i f i c a n t p a r t o f t h e c o n d e n s a t e s i s w a s t e d ;

- the f l o w o f c o n d e n s a t e r e t u r n e d t o t h e power house i s t o o sma l l t o e n s u r e an

adequate f e e d - w a t e r s u p p l y .

A c t u a l l y , vacuum-pan s teaming w i t h 6 b a r steam - as men t ioned e a r l i e r - a l s o

b e l o n g s t o t h i s l i s t .

A n o t h e r o b s e r v a t i o n i s t h a t t h e c a l c u l a t e d demand on h e a t i n g steam i s l o w e r

than the consumpt ion found i n t he measurements . Two main r e a s o n s f o r t h i s

d i s c r e p a n c y can be i d e n t i f i e d :

- v a p o u r l e a k s t h r o u g h f l o a t - t y p e steam t r a p s i n t he c o n d e n s a t e d r a i n a g e l i n e s

i n the e v a p o r a t o r a r e a ;

- f r e q u e n t pa ramete r i n s t a b i l i t i e s , caused by i n a d e q u a t e t h r o u g h p u t and u n s t a b l e

c o n t r o l s o f t he t h r o t t l i n g - d e s u p e r h e a t i n g s t a t i o n d e l i v e r i n g 3.2 b a r s team.

I t s h o u l d be added t h a t t he f o l l o w i n g q u e s t i o n a b l e d e t a i l s , n o t shown i n

F i g . 9 . 1 , were a l s o f ound i n the v a p o u r d i s t r i b u t i o n scheme:

- e x t r a c t o r h e a t i n g by I s t - e f f e c t v a p o u r o n l y ; t h i s was m o t i v a t e d by t h e f a c t

t h a t i t h e l p e d t o p r o v i d e a h i g h t e m p e r a t u r e o f t h e e x t r a c t i o n m i x t u r e , t hus

making i t p o s s i b l e t o m a i n t a i n a v e r y low j u i c e d r a f t ;

- m u l t i p l e - s t a g e h e a t i n g o f p r e - l i m e d j u i c e and t h i n j u i c e where v a p o u r s a t

338

the t e m p e r a t u r e s r e q u i r e d i n t he f i n a l s t a g e s were a l s o used i n t he p r e c e d i n g

s t a g e s ; t h i s c o n t r i b u t e d t o i n a d e q u a t e u t i l i z a t i o n o f l o w - t e m p e r a t u r e v a p o u r s .

9 .2 .3 F i e l d o f s o l u t i o n s

The e n g i n e e r i n g team p r o p o s e d t h e f o l l o w i n g s t r a t e g y o f m o d e r n i z a t i o n , t o be

implemented i n two s t e p s .

( i ) W h i l e i n s t a l l i n g the n e c e s s a r y r e p l a c e m e n t s o f t h e e v a p o r a t o r b o d i e s i n t h e

2nd e f f e c t , the rma l sys tem c o r r e c t i o n s can be i n t r o d u c e d t o r e d u c e t h e n e t h e a t

demand and make i t p o s s i b l e t o o p e r a t e t h e f a c t o r y a t a h i g h e r j u i c e d r a f t and

i n c r e a s e d s u g a r o u t p u t .

( i i ) The e v a p o r a t o r and t h e v a p o u r d i s t r i b u t i o n scheme can be r e c o n s t r u c t e d t o

improve the e f f e c t i v e n e s s r a t i o o f t h e the rma l sys tem and t hus r e d u c e t h e n e t

h e a t demand even f u r t h e r .

When a n a l y s i n g t he o p e r a t i o n o f t h e e x t r a c t i o n s t a t i o n a t a h i g h e r j u i c e

d r a f t , i t was c o n c l u d e d t h a t bo th t h e f l o w o f e x t r a c t i o n f e e d - w a t e r and t he f l o w

o f p r e s s w a t e r can be i n c r e a s e d . The d r y s u b s t a n c e c o n t e n t o f t h e p r e s s e d p u l p

can be i n c r e a s e d t o 11.2% DS and 15.6% DS ( 4 / 5 and 1/5 o f t h e w e t - p u l p f l o w ,

r e s p e c t i v e l y ) , t h i s b e i n g w e l l w i t h i n the o p e r a t i n g range o f t h e e x i s t i n g p u l p

p r e s s e s . I n t h i s w a y , more p r e s s w a t e r i s o b t a i n e d and a d d i t i o n a l e n e r g y

s a v i n g s i n p u l p d r y i n g become p o s s i b l e . C o n c e r n i n g t h e c r y s t a l l i z a t i o n scheme,

i t t u r n e d o u t t h a t an i n c r e a s e d s u g a r o u t p u t r e q u i r e s a l a r g e r m a s s e c u i t e

c i r c u l a t i o n , t h i s making an i n c r e a s e d h e a t demand o f t h e s u g a r b o i l i n g p r o c e s s

u n a v o i d a b l e even a t t h i c k - j u i c e c o n c e n t r a t i o n o f 65% DS (as compared t o 60% DS

p r i o r t o t he m o d e r n i z a t i o n ) .

Two therma l sys tem v e r s i o n s were p r o p o s e d f o r s t e p ( i ) and t h r e e v e r s i o n s f o r

s t e p ( i i ) . I n t h e f o l l o w i n g , t h e s e v e r s i o n s a r e d e n o t e d A l , A2 and 81, 82 , 83.

Each o f them assumes t h a t t he f o l l o w i n g changes a r e i n t r o d u c e d t o t he the rma l

s y s t e m :

- vacuum-pan s teaming i s pe r f o rmed u s i n g 2 n d - e f f e c t v a p o u r ;

- c o n d e n s a t e s f rom a l l i m p o r t a n t v a p o u r r e c e i v e r s a r e r e t u r n e d t o the c o n d e n s a t e

tanks ( t h i s r e q u i r e s i n s t a l l i n g new t a n k s , as t h e vo lumes o f t h e e x i s t i n g ones

a r e t o o sma l l t o accommodate i n c r e a s e d c o n d e n s a t e f l o w s ) ;

- cascade f l a s h i n g o f c o n d e n s a t e s i s a p p l i e d ;

- e x t r a c t o r s a r e hea ted by 1 s t - and 3 r d - e f f e c t v a p o u r ;

- m u l t i p l e - s t a g e j u i c e h e a t i n g i s pe r fo rmed u s i n g v a p o u r s o f d i f f e r e n t

t e m p e r a t u r e s , s t a r t i n g f rom t h e l o w e s t p o s s i b l e t e m p e r a t u r e ( i n some v e r s i o n s ,

t h i s may r e q u i r e i n s t a l l i n g new h e a t e r s , as t h e h e a t i n g s u r f a c e a r e a s o f t h e

e x i s t i n g ones may be t o o sma l l when u t i l i z e d a t r e d u c e d t e m p e r a t u r e

d i f f e r e n c e s ) ;

- l e v e l - c o n t r o l l e d h y d r a u l i c s e a l s a r e a p p l i e d i n t h e c o n d e n s a t e d r a i n a g e l i n e s

c o n n e c t e d t o e v a p o r a t o r b o d i e s l a and l b ;

339

No. E x i s t i n g A l A2 Bl B2 B3

l a 1500 1500 1500 1500 1500 1500 l b 1500 1500^ 1500^ 1500^ 1500^ 1500^ 2a 1460 1800^ 1800? 1800^ 1800f 1800^ 2b 1460 1800^ 1800^ 1800^ 1800^ 1800^ 3a 1320 1320 1460 1320 1460 1460 3b - 1460 1460 1460 1460 1460 4 900 900 1320 900 1320 1320 5 - - - - 900 900

' new b o d i e s

A2. Q u a d r u p l e - e f f e c t e v a p o r a t o r w i t h i n c r e a s e d h e a t i n g s u r f a c e a r e a s i n the 2 n d ,

3 rd and 4 th e f f e c t s ; and the f o l l o w i n g a d j u s t m e n t s o f t h e the rma l s y s t e m :

- 4 t h - e f f e c t v a p o u r i s u t i l i z e d i n r a w - j u i c e and 1 i m e d - j u i c e h e a t i n g ;

- o t h e r d e t a i l s a r e e s s e n t i a l l y i d e n t i c a l t o t h o s e o f v e r s i o n A l .

F o r more i n f o r m a t i o n , see F i g . 9 . 3 . I t i s n e c e s s a r y t o i n s t a l l f o u r new

c o n d e n s a t e tanks i n t h i s v e r s i o n .

- a new t h r o t t l i n g - d e s u p e r h e a t i n g s t a t i o n t o s u p p l y 3.2 b a r steam i s i n s t a l l e d

and e q u i p p e d w i t h an a u t o m a t i c c o n t r o l c i r c u i t s t a b i l i z i n g e x h a u s t - s t e a m

p r e s s u r e .

The c o n f i g u r a t i o n s o f t he e v a p o r a t o r s t a t i o n employed i n t h e d i f f e r e n t

v e r s i o n s a re p r e s e n t e d i n T a b l e 9 .1 . The main f e a t u r e s o f t h e i n d i v i d u a l

v e r s i o n s a r e r e v i e w e d b e l o w .

A l . Q u a d r u p l e - e f f e c t e v a p o r a t o r w i t h i n c r e a s e d h e a t i n g s u r f a c e a r e a s i n t he 2nd

and 3 rd e f f e c t s , and t he f o l l o w i n g a d j u s t m e n t s o f t h e therma l s y s t e m :

- 4 t h - e f f e c t v a p o u r i s u t i l i z e d i n r a w - j u i c e h e a t i n g and i n d i r e c t h e a t i n g o f

e x t r a c t i o n f e e d - w a t e r i n a p r e c o n d e n s e r ;

- an a d d i t i o n a l c o n d e n s a t e tank i s i n s t a l l e d t o c o l l e c t t h e c o n d e n s a t e d r a i n e d

f rom the 3 rd e v a p o r a t o r e f f e c t ;

- a u t o m a t i c l e v e l c o n t r o l l e r s a r e i n s t a l l e d i n 2nd - and 3 r d - e f f e c t c o n d e n s a t e

tanks t o e n s u r e e f f e c t i v e h y d r a u l i c s e a l s i n r e s p e c t i v e c o n d e n s a t e d r a i n a g e

l i n e s .

The d i s t r i b u t i o n o f v a p o u r s and c o n d e n s a t e s , and t h e r e s u l t s o f mass and h e a t

b a l a n c e c a l c u l a t i o n s , a r e shown i n F i g . 9 . 2 . I n a d d i t i o n t o two new e v a p o r a t o r

b o d i e s ( w h i c h a r e a l s o i n c l u d e d i n o t h e r m o d e r n i z a t i o n v e r s i o n s ) , t h i s v e r s i o n

r e q u i r e s i n s t a l l i n g f o u r new c o n d e n s a t e t a n k s .

TABLE 9.1

E v a p o r a t o r c o n f i g u r a t i o n s i n d i f f e r e n t m o d e r n i z a t i o n v e r s i o n s .

2 Body H e a t i n g s u r f a c e a r e a (m )

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B l . Q u a d r u p l e - e f f e c t e v a p o r a t o r w i th vapour c o m p r e s s i o n , a n d :

- a h e a t i n g scheme s i m i l a r to t ha t o f v e r s i o n A l ;

- vacuum-pan vapour i s u t i l i z e d in r a w - j u i c e h e a t i n g ;

- i n a d d i t i o n to the b a s i c condensa te subsys tem i d e n t i c a l to t ha t o f v e r s i o n A l ,

an au tomat ic l e v e l c o n t r o l l e r i s i n s t a l l e d i n the 4 t h - e f f e c t condensa te tank and

a condensa te tank f l a s h e d to 4 t h - e f f e c t vapour i s i n s t a l l e d as a f i n a l l i n k i n

the condensa te tank c h a i n ;

- compress ion o f I s t - e f f e c t vapour i s performed u s i n g j e t - t y p e c o m p r e s s o r s ;

- compressed vapour i s d i r e c t e d to the h e a t i n g chamber o f body l a ;

- exhaus t steam i s d i r e c t e d to the h e a t i n g chamber o f body l b ;

- i n o rde r to reduce compress ion work , a temperature d i f f e r e n c e o f 6 Κ i s

ma in ta ined a c r o s s the h e a t i n g s u r f a c e i n body l a , w h i l e the c o r r e s p o n d i n g

f i g u r e in body lb i s 10 K;

- s e p a r a t e condensa te tanks a re i n s t a l l e d to c o l l e c t condensa tes from b o d i e s l a

and l b .

The d i s t r i b u t i o n scheme f o r vapou rs and condensa tes and e x c e r p t s from the mass

and heat ba lance data a re shown i n F i g . 9 . 4 . T h i s v e r s i o n r e q u i r e s i n s t a l l i n g

s i x new condensa te t anks and one new h e a t e r .

B 2 . Q u i n t u p l e - e f f e c t e v a p o r a t o r , a n d :

- condensa te tank c o l l e c t i n g 5 t h - e f f e c t condensa te i s no t connected to the

condensa te tank c h a i n ;

- condensa te i s u t i l i z e d in h e a t i n g p r e - l i m e d j u i c e , p r e h e a t i n g a i r be fo re the

s u g a r d r y e r , k i l n - g a s h e a t i n g and h u m i d i f i c a t i o n be fo re the c a r b o n a t a t i o n t a n k s ,

and room h e a t i n g ;

- vacuum-pan v a p o u r s a re u t i l i z e d in r a w - j u i c e h e a t i n g ;

- vacuum pans A are heated by 2 n d - e f f e c t v a p o u r , but vacuum pans Β and C by 3 r d -

e f f e c t vapour ( the h e a t i n g s u r f a c e a r e a s i n vacuum pans A tu rned out to be too

s m a l l , p r e c l u d i n g the use o f vapour a t a lower t e m p e r a t u r e ) ;

- t h i n - j u i c e h e a t i n g i n the f i n a l s t a g e i s per formed u s i n g exhaus t s team.

For more i n f o r m a t i o n , see F i g . 9 . 5 . I t i s n e c e s s a r y to i n s t a l l f i v e new

condensa te tanks and th ree new h e a t e r s i n t h i s v e r s i o n .

B 3 . Q u i n t u p l e - e f f e c t e v a p o r a t o r w i th vapour c o m p r e s s i o n , a n d :

- a h e a t i n g scheme e s s e n t i a l l y the same as i n v e r s i o n B 2 ;

- a c o n t i n u o u s c h a i n o f condensa te tanks a p p l i e d between the 2nd and 5 th

e v a p o r a t o r e f f e c t s ;

- t h i c k j u i c e a f t e r the 4 th e f f e c t i s d i r e c t e d to the s u g a r house where

s t a n d a r d l i q u o r i s p r e p a r e d ;

- s t a n d a r d l i q u o r i s re tu rned to the 5 th e f f e c t and t h i ckened to 11% D S ;

- compress ion o f I s t - e f f e c t vapour i s per formed u s i n g an e l e c t r i c a l l y - d r i v e n

mechanica l c o m p r e s s o r ;

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- compressed vapour i s d i r e c t e d to the h e a t i n g chamber o f body l a ;

- exhaus t steam i s d i r e c t e d to the h e a t i n g chamber o f body l b ;

- i n o r d e r to reduce compressor power demand, a temperature d i f f e r e n c e o f 6 Κ

i s ma in ta ined between the h e a t i n g chamber and vapour chamber i n body l a , w h i l e

the c o r r e s p o n d i n g f i g u r e i n body l b i s 10 K;

- s e p a r a t e condensa te tanks are i n s t a l l e d to c o l l e c t condensa tes from b o d i e s l a

and l b .

The d i s t r i b u t i o n scheme f o r vapou rs and c o n d e n s a t e s , and s e l e c t e d r e s u l t s o f

mass and heat ba lance c a l c u l a t i o n s , a re shown i n F i g . 9 . 6 . T h i s v e r s i o n r e q u i r e s

i n s t a l l i n g s i x new condensa te t anks and f i v e new h e a t e r s .

9 . 2 . 4 Compar ison o f s o l u t i o n s

When a n a l y s i n g the m o d e r n i z a t i o n s t r a t e g y o u t l i n e d i n the p reced ing S e c t i o n s ,

each v e r s i o n o f the modern ized f a c t o r y can be c h a r a c t e r i z e d by a v e c t o r o f

a t t r i b u t e s c o m p r i s i n g inves tment c o s t , fue l s a v i n g and power demand i n c r e a s e .

U s i n g the ac tua l p r i c e s o f fue l and power, the consequences o f m o d e r n i z a t i o n

can thus be e x p r e s s e d i n economic t e rms . The economic data can a l s o be combined

i n t o some s y n t h e t i c i n d e x , l i k e the p e r i o d o f r e t u r n on i nves tmen t . T h i s makes

i t p o s s i b l e to compare the s o l u t i o n s and to s e l e c t the most f e a s i b l e one o f

v e r s i o n s B l , B2 and B 3 .

The Inves tmen t c o s t was c a l c u l a t e d on the b a s i s o f p r i c e s quoted by the

s u p p l i e r s o f the main equipment ( e v a p o r a t o r b o d i e s , j u i c e h e a t e r s , condensa te

t a n k s , pumps and vapour c o m p r e s s o r s ) . To the equipment p r i c e s , the f o l l o w i n g

es t ima ted c o s t components were added :

- p i p i n g and a u x i l i a r y equ ipment ;

- measur ing d e v i c e s and con t r o l c i r c u i t s ;

- thermal i n s u l a t i o n ;

- d e s i g n documen ta t i on ;

- c o n s t r u c t i o n work ;

- a s s e m b l i n g o f equ ipment , p i p i n g and i n s t r u m e n t a t i o n .

Es t ima ted v a l u e s o f the a t t r i b u t e s o f the m o d e r n i z a t i o n v e r s i o n s were taken

from r e s u l t s o f the d e s i g n a n a l y s i s summarized i n the p r e v i o u s S e c t i o n .

I t was agreed w i th the f a c t o r y managers t h a t no d e t a i l e d economic a n a l y s i s i s

r e q u i r e d , because the impor tance o f the r e c o n s t r u c t i o n o f the thermal sys tem

l i e s ma in l y i n making i t p o s s i b l e to ex tend the p r o c e s s i n g c a p a b i l i t y ; however ,

t h i s w i l l be d e s i g n e d and a n a l y s e d a t a l a t e r d a t e . In o rde r to reduce the

comp lex i t y o f the compar ison o f the v a r i o u s v e r s i o n s , approx imate v a l u e s

( n e g l e c t i n g the i n f l u e n c e o f c a p i t a l c o s t ) o f the p e r i o d o f r e t u r n on

i n v e s t m e n t , i n y e a r s , were c a l c u l a t e d a c c o r d i n g to the fo rmu la

τ = I / A ( 9 . 1 )

where I i s the inves tment c o s t and A i s the annual s a v i n g .

347

The r e s u l t s o f the compara t i ve a n a l y s i s a re summarized i n Tab le 9 . 2 . Due to

f l u c t u a t i o n s o f p r i c e s and c u r r e n c y exchange r a t e s i n the i n t e r n a t i o n a l marke t ,

i t would make l i t t l e sense to quote the a b s o l u t e l e v e l o f the economic e s t i m a t e s

i n the o r i g i n a l c u r r e n c y . A t the moment o f p u b l i c a t i o n o f the p r e s e n t b o o k ,

a coup le o f y e a r s a f t e r the a n a l y s i s was comp le ted , these data would be o f

h i s t o r i c a l v a l u e o n l y . T h e r e f o r e , the c o s t s and s a v i n g s a re g i v e n r e l a t i v e to

the inves tmen t c o s t a s s o c i a t e d w i th the m o d e r n i z a t i o n v e r s i o n Al ( t h i s c o s t i s

taken as ^00%),

TABLE 9 . 2

Compar ison o f main t e c h n i c a l and economic parameters o f d i f f e r e n t m o d e r n i z a t i o n v e r s i o n s .

Thermal sys tem v e r s i o n E x i s t i n g Al A2 Bl B2 B3

H e a t i n g - s t e a m demand ( k g / 1 0 0 kg b) 4 2 . 4 4 1 . 2 4 0 . 3 3 7 . 7 3 6 . 8 3 1 . 9 6 bar steam demand ( k g / 1 0 0 kg b) 2 . 5 1.0 1.0 1.0 1.0 1.0 Norma l - fue l demand ( k g / 1 0 0 kg b) 6 . 0 0 5 . 6 2 5 . 5 0 5 . 1 2 4 . 6 0 3 . 0 7 L i v e - s t e a m demand i n vapour compress ion ( k g / 1 0 0 kg b) - - - 6 . 1 0 - -Power demand i n vapour compress ion (kWh/100 kg b) - - - - - 0 . 3 2 Condensate f low to the b o i l e r house

( k g / 1 0 0 kg b) 4 0 . 5 4 6 . 5 4 5 . 2 4 7 . 0 4 1 . 5 3 5 . 7 Tota l h e a t i n g s u r f a c e a rea i n the^ e v a p o r a t o r (m ) 8140 10280 10420 10280 11740 11740 R e l a t i v e inves tmen t c o s t (%) - 100 111 116 148 234 Va lue o f coa l saved per s e a s o n {7o) - 58 70 107 157 205 Approx imate p e r i o d o f r e t u r n on inves tmen t ( y e a r s ) - 1 .73 1 .59 1 .08 0 . 9 4 1 .14

As can be seen i n Tab le 9 . 2 , v e r s i o n s B l , B2 and B3 are e c o n o m i c a l l y more

a t t r a c t i v e than Al and A 2 . T h i s i n d i c a t e s t h a t a f t e r the f i r s t m o d e r n i z a t i o n

s t e p has been comp le ted , the second s t e p s h o u l d be taken as soon as p o s s i b l e .

Among the Β v e r s i o n s , i t i s Bl t ha t i s c h a r a c t e r i z e d by the l owes t i nves tmen t

c o s t ; B3 o f f e r s the l a r g e s t fue l s a v i n g , and B2 seems to p r o v i d e a t r a d e - o f f

between inves tmen t c o s t and fue l s a v i n g .

From the data g i v e n i n S e c t i o n 9 . 2 . 2 , the t o t a l power demand (vapour

compress ion exc luded ) o f the s u g a r f a c t o r y a t the p r o c e s s i n g c a p a b i l i t y o f

5000 t / d , p l u s o the r p r o d u c t i o n f a c i l i t i e s , can be es t ima ted a t 6 . 0 MW. Add ing

a 10% s a f e t y m a r g i n , the power demand w i l l equal the r a t i n g o f the t u r b i n e s .

To genera te 6 . 6 MW in the t u r b o - g e n e r a t o r s , a steam f low o f about 73 t / h i s

r e q u i r e d . S u b t r a c t i n g 6 t / h consumed o u t s i d e the s u g a r f a c t o r y , a minimum

h e a t i n g - s t e a m demand o f 67 t / h , o r 3 2 . 2 k g / 1 0 0 kg b , i s o b t a i n e d . Look ing now a t

the c h a r a c t e r i s t i c s o f v e r s i o n B 3 , i t can be seen t ha t the t o t a l power demand,

vapour compress ion i n c l u d e d , amounts to 6 . 7 MW and the h e a t i n g - s t e a m demand i s

348

a l i t t l e l e s s than the minimum v a l u e . T h i s e x c l u d e s the p o s s i b i l i t y o f

implement ing v e r s i o n B3 w i thou t p u r c h a s i n g power from the ex te rna l g r i d , o r

mode rn i z i ng the power h o u s e . T h e r e f o r e , v e r s i o n B3 does no t s a t i s f y the

c o n s t r a i n t s l i s t e d i n S e c t i o n 9 . 2 . 1 .

9 . 3 FACTORY CHARACTERIZED BY GOOD I N I T I A L ENERGY U T I L I Z A T I O N

9 . 3 . 1 I n t r o d u c t o r y remarks

The f a c t o r y was b u i l t in the e a r l y 1970s w i th an i n i t i a l p r o c e s s i n g

c a p a b i l i t y o f 4000 t ons per day . The i n i t i a l f ue l consumpt ion was about 5 . 3 kg

normal fue l per 100 kg bee t . Du r i ng a p e r i o d o f about ten y e a r s , r e l y i n g m o s t l y

on the s t e p - b y - s t e p a p p r o a c h , the p r o c e s s i n g c a p a b i l i t y was i n c r e a s e d to

5900 t / d . T h i s was accompanied by the o p t i m i z a t i o n o f the s u g a r manu fac tu r i ng

p r o c e s s w i th r e s p e c t to the fue l demand, and numerous improvements o f the

thermal s y s t e m . Among o t h e r s , the u t i l i z a t i o n o f vacuum-pan vapou rs i n two

t u b u l a r r a w - j u i c e h e a t e r s was i n t r o d u c e d , a l ong w i th improvements o f the

u t i l i z a t i o n o f low- tempera tu re p r imary vapours and c o n d e n s a t e . As a r e s u l t ,

n o r m a l - f u e l consumpt ion dec reased to 3 . 5 - 3 . 7 k g / 1 0 0 kg b.

When a p r o c e s s i n g c a p a b i l i t y o f 5900 t / d was a t t a i n e d , d i f f i c u l t i e s a r o s e i n

m a i n t a i n i n g p roper v a l u e s o f c r u c i a l p r o c e s s pa rame te rs . The tempera tu res o f

e x t r a c t i o n and main l i m i n g tended to be too l ow , and the c o n c e n t r a t i o n o f t h i c k

j u i c e d e c r e a s e d . T h i s was accompanied by vacuum- leve l i n s t a b i l i t i e s d i s t u r b i n g

the s u g a r b o i l i n g p r o c e s s . An e v a l u a t i o n o f the mass and heat b a l a n c e s o f the

e v a p o r a t o r i n d i c a t e d a l s o a l a r g e f low o f l a s t - e f f e c t vapour t o the c o n d e n s e r .

A f t e r the r e s u l t s o f the s e a s o n had been r e v i e w e d , i t was conc luded t h a t the

o p e r a t i o n a l d i f f i c u l t i e s caused too h i g h s u g a r l o s s e s . I t a l s o became c l e a r t ha t

no f u r t h e r r e d u c t i o n o f the energy consumpt ion i s p o s s i b l e u n l e s s the e x i s t i n g

p r o c e s s equipment and thermal sys tem a re mode rn i zed . C o n s e q u e n t l y , an

e n g i n e e r i n g team was c a l l e d i n to a n a l y s e the s i t u a t i o n and to d e s i g n the

n e c e s s a r y m o d i f i c a t i o n s .

A d e t a i l e d i n v e n t o r y o f 12 f a c t o r y subsys tems i n the s u g a r m a n u f a c t u r i n g l i n e

was p r e p a r e d , s t a r t i n g from the beet wash ing s t a t i o n and end ing a t the C

m a s s e c u i t e s t a t i o n . A rev iew o f impor tan t parameters o f 15 o t h e r s u b s y s t e m s was

a l s o c a r r i e d o u t . Wh i le most o f the data needed to i n i t i a t e a m o d e r n i z a t i o n

s t u d y i n the heat economy area were o b t a i n e d , i t was found t h a t the data on the

f a c t o r y ' s power ba lance were no t s u f f i c i e n t l y d e t a i l e d . Tak i ng i n t o accoun t t ha t

the modern ized f a c t o r y cannot be s e l f - s u f f i c i e n t i n power, i t became c l e a r t h a t

ways to reduce the power demand i n a l l r e l e v a n t f a c t o r y s u b s y s t e m s s h o u l d be

s t u d i e d and p roper measures s h o u l d be t a k e n . I t was t h e r e f o r e recommended t h a t :

- measurements be made o f the power consumpt ion i n major power r e c e i v e r s d u r i n g

the nex t s e a s o n , to a n a l y s e the r a t i n g o f motors and t r a n s f o r m e r s ;

- a d e t a i l e d s t udy be under taken o f the w a t e r - s u p p l y and was te -wa te r t rea tment

349

Subsystems, with the aim o f reducing the power demand;

- a d e t a i l e d s t udy be under taken o f f low c o n t r o l r equ i remen ts i n the s u g a r

manu fac tu r i ng p r o c e s s , w i th the aim o f m o d e r n i z i n g the c o n t r o l sys tems f o r

be t t e r ene rgy u t i l i z a t i o n .

9 . 3 . 2 B a s i c f a c t o r y data and heat ba lance

P r o c e s s i n g c a p a b i l i t y : 5900 t / d .

P o l a r i z a t i o n o f c o s s e t t e s : 15 .7%.

E x t r a c t i o n s t a t i o n : two t h r o u g h - t y p e e x t r a c t o r s .

J u i c e d r a f t : 115%.

Raw- ju i ce c o n c e n t r a t i o n and p u r i t y : 15.0% DS and 8 8 . 4 % .

Pu lp p r e s s e d t o : 27 .6% D S .

K i l n g a s : 35% CO^ v o l .

J u i c e p u r i f i c a t i o n a c c o r d i n g to the c l a s s i c a l scheme, c o m p r i s i n g :

- ho t p r e - l i m i n g a t 4 5 - 5 0 ° C ;

- main l i m i n g a t 79 -80°C ( r e q u i r e d tempera tu re : 8 2 - 8 5 ° C ) ;

- 1 s t c a r b o n a t a t i o n a t 77°C ( r e q u i r e d tempera tu re : 8 0 - 8 2 ° C ) ;

- d o u b l e - s t a g e 1 s t f i l t r a t i o n ;

- 2nd c a r b o n a t a t i o n a t 9 6 ° C ;

- d o u b l e - s t a g e 2nd f i l t r a t i o n .

D e c a l c i f i c a t i o n o f t h i n j u i c e by ion exchange .

T h i n - j u i c e c o n c e n t r a t i o n and p u r i t y : 15.4% DS and 9 1 . 8 % .

E v a p o r a t o r : q u a d r u p l e - e f f e c t , R o b e r t - t y p e b o d i e s ; N i e s s n e r columns a p p l i e d i n

the condensa te d r a i n a g e s u b s y s t e m .

T h i c k - j u i c e c o n c e n t r a t i o n : 61 .9% DS ( r e q u i r e d v a l u e : 65% D S ) .

S u g a r h o u s e :

- t h r e e - b o i l i n g scheme w i th the a f f i n a t i o n o f C s u g a r ;

- 60% o f Β s y r u p p r o c e s s e d i n a Quent in u n i t .

Power h o u s e :

- o i l - f i r e d b o i l e r s , ave rage e f f i c i e n c y 92%;

- l i v e steam parameters 40 b a r , 4 3 0 ^ 0 ;

- b a c k - p r e s s u r e 2 . 9 b a r ;

- f eed -wa te r pump d r i v e n by a steam t u r b i n e .

Steam s u p p l y to the s u g a r manu fac tu r i ng p r o c e s s :

- l i v e steam t h r o t t l e d to 8 bar to the c e n t r i f u g a l s ;

- h e a t i n g steam 2 . 9 bar ( exhaus t steam and t h r o t t l e d l i v e steam) to the

rema in ing r e c e i v e r s .

Hea t ing steam c o n s u m p t i o n : 3 5 . 5 k g / 1 0 0 kg b.

Power c o n s u m p t i o n : 2 . 8 5 kWh/100 kg b.

Normal fue l c o n s u m p t i o n : 3 . 6 8 k g / 1 0 0 kg b.

The s i m p l i f i e d scheme o f vapour and condensa te d i s t r i b u t i o n , and the r e s u l t s

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o f mass and heat ba lance c a l c u l a t i o n s o f the thermal sys tem be fo re m o d e r n i z a t i o n

a re shown in F i g . 9 . 7 . No d e t a i l s o f the vapour and condensa te c o n n e c t i o n s i n

the p r o c e s s h e a t i n g a rea a re g i v e n , as t h e s e were found e n t i r e l y c o r r e c t .

9 . 3 . 3 F i e l d o f s o l u t i o n s

The e n g i n e e r i n g team proposed the f o l l o w i n g s t r a t e g y o f m o d e r n i z a t i o n , to be

implemented in f o u r s t e p s .

( i ) I n t r oduce equipment m o d i f i c a t i o n s and minor thermal sys tem c o r r e c t i o n s ,

aimed a t s e c u r i n g p roper p r o c e s s parameters and e l i m i n a t i n g u n n e c e s s a r y energy

l o s s e s .

( i i ) Conver t the e x i s t i n g e v a p o r a t o r i n t o a q u i n t u p l e - e f f e c t o n e , i n o r d e r to

a t t a i n a h i g h c o n c e n t r a t i o n o f t h i c k j u i c e and to i n c r e a s e the e f f e c t i v e n e s s

r a t i o o f the thermal s y s t e m .

( i i i ) I n c r e a s e the h e a t i n g s u r f a c e a rea i n the f i r s t e f f e c t (two a l t e r n a t i v e 2

s o l u t i o n s can be c o n s i d e r e d : the e x i s t i n g R o b e r t - t y p e body w i t h 1800 m

h e a t i n g - s u r f a c e a rea can be e i t h e r removed from the f a c t o r y , o r o n l y t e m p o r a r i l y

d i s c o n n e c t e d ) .

( i v ) I n t r o d u c e a vapour compress ion c i r c u i t and a d d i t i o n a l l y i n c r e a s e the

e f f e c t i v e n e s s r a t i o o f the thermal sys tem th rough improved u t i l i z a t i o n o f low-

temperature vapours (each s o l u t i o n c o n s i d e r e d i n the p reced ing s t e p g e n e r a t e s

two p o s s i b l e v e r s i o n s ) .

The p roposa l can be c o n v e n i e n t l y rev iewed by summar iz ing the main f e a t u r e s

o f two i n te rmed ia te s o l u t i o n s t ha t may r e s u l t from the comp le t ion o f s t e p s ( i )

and ( i i ) , as wel l a s f o u r p o s s i b l e v e r s i o n s among which a c h o i c e must be made

when e x e c u t i n g s t e p s ( i i i ) and ( i v ) . Tab le 9 . 3 shows c o n f i g u r a t i o n s o f the

e v a p o r a t o r s t a t i o n f o r a l l v e r s i o n s , t oge the r w i th data on the j u i c e

c o n c e n t r a t i o n s .

A l . An i n te rmed ia te s o l u t i o n r e s u l t i n g from s t e p ( i ) :

- steam j a c k e t s o f the e x t r a c t o r s a re heated by 2 n d - and 3 r d - e f f e c t v a p o u r s , and

2 n d - e f f e c t vapour i s a d d i t i o n a l l y i n j e c t e d i n t o the e x t r a c t i o n m i x t u r e , but no te

tha t vapour i n j e c t i o n may a d v e r s e l y a f f e c t the e f f e c t i v e n e s s r a t i o , be ing

p r i m a r i l y aimed a t s e c u r i n g a c o r r e c t temperature d i s t r i b u t i o n i n the e x t r a c t i o n

p r o c e s s ;

- the b u f f e r tank between p r e - l i m i n g and hot main l i m i n g i s conve r ted to a l ime r

i n which c o l d main l i m i n g can be pe r fo rmed ;

- one o f the h e a t e r s used h i t h e r t o f o r r a w - j u i c e h e a t i n g w i th vacuum-pan vapour

i s conve r ted to l i m e d - j u i c e h e a t i n g ( p r i o r to hot main l i m i n g ) w i th l a s t - e f f e c t

v a p o u r , a change which does not a f f e c t the e f f e c t i v e n e s s r a t i o ( the f low o f

l a s t - e f f e c t vapour to the condenser was anyway too l a r g e ) but s e c u r e s a c o r r e c t

temperature i n the hot main l i m i n g ;

- k i l n - g a s hea t i ng and h u m i d i f i c a t i o n a p p a r a t u s i s i n s t a l l e d be fo re the 1 s t

353

c a r b o n a t a t i o n ;

- a new b u f f e r tank i s i n s t a l l e d a f t e r the 2nd c a r b o n a t a t i o n to s e c u r e an

adequate j u i c e r e t e n t i o n t ime needed to s t a b i l i z e CaCO^ c r y s t a l s ;

- improved steam t r a p s are i n s t a l l e d i n condensa te d r a i n a g e l i n e s o f vacuum pans

A , and au tomat ic l e v e l con t r o l i s a t t ached to the condensa te tank c o l l e c t i n g

condensa tes from the h e a t i n g chambers o f the vacuum p a n s ;

- improved v e n t i n g o f the h e a t i n g chambers o f the vacuum pans i s implemented;

- improved v e n t i n g o f the h e a t i n g chambers o f the 2nd e v a p o r a t o r e f f e c t and o f

the j u i c e hea te r s heated w i th 1 s t - and 2 n d - e f f e c t vapours i s implemented;

- p i p e s o f i n c r e a s e d d iameters are i n s t a l l e d i n the c o n n e c t i o n s between the

vacuum pans and the c o n d e n s e r , as we l l as between the l a s t e v a p o r a t o r e f f e c t and

the c o n d e n s e r ;

- the C m a s s e c u i t e s t a t i o n i s extended by i n s t a l l i n g two v e r t i c a l - t y p e

c r y s t a l 1 i z e r s w i th a c a p a c i t y o f 150 m^ e a c h .

In s p i t e o f a number o f improvements i n t r o d u c e d to the thermal s y s t e m , the

measures l i s t e d above cannot be expec ted to reduce the fue l c o n s u m p t i o n .

A c t u a l l y , when b r i n g i n g p r o c e s s h e a t i n g back to n o r m a l , the t o t a l heat demand i s

i n c r e a s e d . Heat s a v i n g s can o n l y be o b t a i n e d by t a k i n g the next m o d e r n i z a t i o n

s t e p .

Conce rn ing the power demand, two minor improvements were p r o p o s e d :

- a t h y r i s t o r - c o n t r o l l e d d . c . d r i v e i n s t a l l e d i n the o u t l e t s e c t i o n o f the beet

washer ( to make bee t - f l ow con t r o l p o s s i b l e , and to save p o w e r ) ;

- a t h y r i s t o r - c o n t r o l l e d d . c . d r i v e a p p l i e d i n the j u i c e pump a f t e r 1 s t

c a r b o n a t a t i o n ( to min imize the i n f l u e n c e o f pumping on the s t r u c t u r e o f d e p o s i t s

to be f i l t e r e d , and to save power ) . I t s h o u l d be o b s e r v e d , however , t h a t an

i n c r e a s e o f the t o ta l power demand can be expec ted f o l l o w i n g the i n s t a l l a t i o n o f

a s t i r r e d c o l d main l imer and two C m a s s e c u i t e c r y s t a l 1 i z e r s .

A 2 . Another i n te rmed ia te s o l u t i o n , r e s u l t i n g f rom s t e p ( i i ) :

- the e v a p o r a t o r s t a t i o n i s extended by i n s t a l l i n g two f a l l i n g - f i l m b o d i e s w i t h

h e a t i n g s u r f a c e a r e a s o f 1400 m and 1000 m , to be used as the 4 th and 5th

e f f e c t s , r e s p e c t i v e l y ;

- no changes a re i n t r o d u c e d to the 1 s t and 2nd e v a p o r a t o r e f f e c t s , but the 3 rd

e f f e c t i s extended by add ing a R o b e r t - t y p e body p r e v i o u s l y used i n the 4 th

e f f e c t ;

- the condensa te d r a i n a g e subsys tem i s ex tended by i n s t a l l i n g two condensa te

t anks a t t ached to new e v a p o r a t o r b o d i e s ;

- a t h i c k - j u i c e c o n d i t i o n e r o f the vacuum type i s i n s t a l l e d a t the e v a p o r a t o r

o u t l e t to s t a b i l i z e the f i n a l c o n c e n t r a t i o n o f the t h i c k j u i c e , by means o f

s e l f - e v a p o r a t i o n o r t h i n - j u i c e i n t a k e , a t a l e v e l o f 70% D S .

The d i s t r i b u t i o n o f vapours and condensa tes and the r e s u l t s o f mass and heat

354

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ba lance c a l c u l a t i o n s o f the modern ized thermal sys tem a re shown i n F i g . 9 . 8 .

I n o r d e r to s t a b i l i z e the e x t r a c t i o n parameters and t h i c k - j u i c e c o n c e n t r a t i o n ,

as we l l as to m in im ize heat l o s s e s caused by the v e n t i n g o f n o n c o n d e n s a b l e s , i t

was a l s o p roposed to i n s t a l l some a d d i t i o n a l c o n t r o l equ ipment :

- au tomat i c c o n t r o l o f the temperature d i f f e r e n c e between raw j u i c e and incoming

c o s s e t t e s , by means o f a v a r i a b l e f l ow o f vapour i n j e c t e d i n t o the e x t r a c t i o n

m i x t u r e ;

- au tomat i c c o n t r o l o f the t h i c k - j u i c e c o n c e n t r a t i o n a t the o u t l e t o f the t h i c k

j u i c e c o n d i t i o n e r ;

- au tomat i c c o n t r o l o f the v e n t i n g o f the vacuum p a n s ;

- au tomat i c c o n t r o l o f the v e n t i n g o f the c l e a r - j u i c e hea te r (heated by 2 n d -

e f f e c t v a p o u r ) .

B l . Compress ion o f 2 n d - e f f e c t v a p o u r .

I n a d d i t i o n to the changes d e s c r i b e d a b o v e , a f a l l i n g - f i l m body w i t h a 2

h e a t i n g s u r f a c e a rea o f 2400 m i s i n s t a l l e d as the 1 s t e v a p o r a t o r e f f e c t

( r e p l a c i n g the o l d R o b e r t - t y p e b o d y ) . I n the vapour c o m p r e s s i o n c i r c u i t , 2 n d -

e f f e c t vapour i s r e c y c l e d to the h e a t i n g chamber o f the I s t - e f f e c t u s i n g an

e l e c t r i c a l l y - d r i v e n mechanica l c o m p r e s s o r . Other changes a re as f o l l o w s :

- t h i c k - j u i c e c o n c e n t r a t i o n 74% D S ;

- r a w - j u i c e h e a t i n g i n a s p i r a l hea te r u s i n g ho t water f rom a "ho t c o n d e n s e r " i n

which vacuum-pan vapours a re c o n d e n s e d ;

- p r e - l i m e d j u i c e h e a t i n g w i t h l a s t - e f f e c t vapour i n two t u b u l a r heat e x c h a n g e r s

p r e v i o u s l y used as raw j u i c e h e a t e r s ;

- t h i n - j u i c e h e a t i n g i n f o u r s t a g e s ;

- the s t e a m - t u r b i n e d r i v e n feed -wa te r pump r e p l a c e d by an e l e c t r i c a l l y - d r i v e n

o n e .

The d i s t r i b u t i o n o f vapou rs and condensa tes and the r e s u l t s o f mass and heat

ba lance c a l c u l a t i o n s o f t h i s v e r s i o n a re shown i n F i g . 9 . 9 . I t can be f u r t h e r

es t ima ted t ha t the combined power demand o f the vapour compresso r and the f e e d -

water pump i s about 920 kW. Owing to reduced heat demand, the power demand o f

the c o m b u s t i o n - a i r f a n s and the b a r o m e t r i c - w a t e r pumps can s i m u l t a n e o u s l y be

reduced by about 300 kW.

B 2 . Compress ion o f I s t - e f f e c t vapour to a f a l l i n g - f i l m body .

Two f a l l i n g - f i l m b o d i e s , 1500 m^ ( l a ) and 2400 m^ ( l b ) , a re i n s t a l l e d i n the

1 s t e v a p o r a t o r e f f e c t and I s t - e f f e c t vapour i s r e c y c l e d to the h e a t i n g chamber

o f body l a u s i n g an e l e c t r i c a l l y - d r i v e n mechan ica l c o m p r e s s o r . A new tank i s

i n s t a l l e d to c o l l e c t the condensa te d r a i n e d f rom body l a . Other d e t a i l s remain

the same as i n the p reced ing v e r s i o n , excep t t h a t the e x h a u s t - s t e a m tempera ture

i s 3 Κ l owe r , r e s u l t i n g i n a lower b a c k - p r e s s u r e and thus more power genera ted

i n the t u r b o - g e n e r a t o r . Fo r data on mass and heat b a l a n c e s , see F i g . 9 . 1 0 .

358

The combined power demand o f the vapour compressor and the f eed -wa te r pump i s

about 660 kW. The power demand r e d u c t i o n r e s u l t i n g f rom reduced heat demand i s

the same as i n v e r s i o n B l .

C I . Compress ion o f I s t - e f f e c t vapour to a R o b e r t - t y p e body.

A f a l l i n g - f i l m body , 2400 m^, i s added to the e x i s t i n g R o b e r t - t y p e body i n

the 1 s t e v a p o r a t o r e f f e c t . The vapour compress ion c i r c u i t remains i d e n t i c a l to

t ha t o f the p reced ing v e r s i o n , but the compressed vapour i s r e c y c l e d to the

h e a t i n g chamber o f body l b . As the h e a t i n g s u r f a c e a rea o f the R o b e r t - t y p e body

i s l a r g e r than t ha t o f the f a l l i n g - f i l m body , the temperature d i f f e r e n c e between

h e a t i n g - s t e a m and vapour can be r e d u c e d , r e s u l t i n g i n a reduced power demand by

the compresso r . The mass and heat b a l a n c e s a re n e a r l y the same as i n the

p reced ing v e r s i o n . The combined power demand o f the compressor and the f e e d -

water pump i s about 600 kW. The power-demand r e d u c t i o n i n the rema in ing

equipment i s i d e n t i c a l to t h a t i n v e r s i o n s Bl and B 2 .

C 2 . Thermocompress ion o f I s t - e f f e c t v a p o u r .

The e v a p o r a t o r s t a t i o n i s i d e n t i c a l to t ha t o f the p r e c e d i n g v e r s i o n but the

I s t - e f f e c t vapour i s compressed u s i n g j e t - t y p e c o m p r e s s o r s . Wh i le the mass and

heat b a l a n c e s o f the thermal sys tem i n the p r o c e s s - h e a t i n g a rea remain i d e n t i c a l

to t hose o f v e r s i o n s B2 and C I , the mass and energy b a l a n c e s o f the power house

and the 1 s t e v a p o r a t o r e f f e c t a re changed as shown i n F i g . 9 . 1 1 . The power

demand o f the feed -wa te r pump i s app rox ima te l y equal to the power-demand

r e d u c t i o n r e s u l t i n g from reduced heat demand, so the t o t a l power demand i s equal

to t ha t o f v e r s i o n s A l , A2 and B l .

l osses 0.5

F i g . 9 . 1 1 . E x c e r p t s from mass and heat b a l a n c e s o f the modern ized thermal s y s t e m , v e r s i o n C 2 .

359

9 . 3 . 4 Compar ison o f s o l u t i o n s

An approx imate economic a n a l y s i s o f the m o d e r n i z a t i o n p r o p o s a l was p r e p a r e d ,

to compare the s o l u t i o n s ( B l , B 2 , CI and C2) and to s e l e c t the most f e a s i b l e

v e r s i o n . The gene ra l approach adopted was s i m i l a r to t h a t p r e s e n t e d i n S e c t i o n

9 . 2 . 4 , w i th the f o l l o w i n g e x t e n s i o n s :

- the v e c t o r o f a t t r i b u t e s i n c l u d e s a d d i t i o n a l s u g a r p r o d u c t i o n ;

- when a n a l y s i n g the inves tmen t c o s t , i t s h o u l d be taken i n t o accoun t t ha t i f a

c e r t a i n equipment u n i t i s removed from the f a c t o r y i n q u e s t i o n , i t can be

c o n s i d e r e d f o r a p p l i c a t i o n i n o t h e r s u g a r f a c t o r i e s o f the same company;

- the p o s s i b i l i t y o f f u t u r e changes i n fue l and power p r i c e s s h o u l d be a l l owed

f o r ;

- when c a l c u l a t i n g the p e r i o d o f r e t u r n on i n v e s t m e n t , c a p i t a l c o s t and

i n c r e a s e d main tenance c o s t s h o u l d be accoun ted f o r .

In v e r s i o n s Bl and B 2 , removal o f the e x i s t i n g R o b e r t - t y p e body from the 1 s t

e v a p o r a t o r e f f e c t was assumed. As i t can be a p p l i e d i n ano the r s u g a r f a c t o r y ,

the va lue o f t h i s equipment u n i t was deducted f rom the i nves tmen t c o s t s o f t h e s e

v e r s i o n s .

The es t ima ted r e s u l t s o f the m o d e r n i z a t i o n , t h a t i s , the fue l s a v i n g , power

demand i n c r e a s e and a d d i t i o n a l s u g a r p r o d u c t i o n , were taken f rom the d e s i g n

a n a l y s i s p resen ted i n the p r e v i o u s S e c t i o n . Two economic e s t i m a t e s were

determined f o r fue l s a v i n g s and power demand i n c r e a s e s :

1 . u s i n g the ac tua l p r i c e s o f fue l o i l and power ;

2 . u s i n g the f o r e c a s t ave rage p r i c e s f o r the i n i t i a l s e a s o n s w i t h the modern ized

f a c t o r y ; e . g . f o r a p e r i o d o f t h ree y e a r s , a f ue l o i l p r i c e i n c r e a s e d by 50% and

power p r i c e by 35%.

The c a p i t a l c o s t and i n c r e a s e d main tenance c o s t were j o i n t l y e s t i m a t e d , u s i n g

an e q u i v a l e n t i n t e r e s t r a te o f 0 . 1 3 . As a f i r s t a p p r o x i m a t i o n , the p e r i o d o f

r e t u r n on i n v e s t m e n t , i n y e a r s , was c a l c u l a t e d u s i n g the fo rmu la

τ = I / ( A - r l ) ( 9 . 2 )

where I i s the i nves tmen t c o s t , A i s the annual s a v i n g , and r i s the e q u i v a l e n t

i n t e r e s t r a t e .

The r e s u l t s o f the compara t i ve a n a l y s i s a re shown i n Tab le 9 . 4 . As i n Tab le

9 . 2 i n S e c t i o n 9 . 2 . 4 , the c o s t s a re g i v e n r e l a t i v e to the i nves tmen t c o s t o f

a s e l e c t e d v e r s i o n . I t has been assumed t ha t the i nves tmen t c o s t a s s o c i a t e d w i th

mode r n i za t i on s t e p ( i ) i s 100%.

The con ten t s o f Tab le 9 . 4 can be summarized as f o l l o w s :

- the i n te rmed ia te s o l u t i o n s a re e c o n o m i c a l l y h i g h l y a t t r a c t i v e ;

- the economic r e s u l t s o f f u r t h e r i nves tmen ts i n the ene rgy economy a re

dependent on developments i n the fue l ma rke t ;

360

TAB

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361

- among the v e r s i o n s c o n s i d e r e d , CI and C2 a re c h a r a c t e r i z e d by the s h o r t e s t

p e r i o d s o f r e t u r n on i n v e s t m e n t .

9 . 4 OPTIMIZATION OF ENERGY SYSTEMS

9 . 4 . 1 P r a c t i c a l meaning o f d e s i g n o p t i m i z a t i o n

With the t r a d i t i o n a l d e s i g n me thods , improvements a re i n t r o d u c e d to ene rgy

sys tems u s i n g the l e a r n i n g - b y - e x p e r i e n c e a p p r o a c h . When e v a l u a t i n g the r e s u l t s

o b t a i n e d from s u g a r f a c t o r y o p a r a t i o n , the o p e r a t o r s and d e s i g n e r s l e a r n f rom

t h e i r m i s t a k e s . As a new f a c t o r y i s e r e c t e d o r an e x i s t i n g one i s m o d e r n i z e d ,

a t tempts a re made to improve the energy economy i n r e l a t i o n to e a r l i e r

s o l u t i o n s . The r e s u l t s a re ve ry much dependent on the e n g i n e e r ' s i n t u i t i o n and

e x p e r i e n c e , and i t may be i m p o s s i b l e to determine j u s t how c l o s e a d e s i g n i s to

the rea l minimum energy demand. On the o t h e r h a n d , i t i s i n c r e a s i n g l y o f t e n

r e q u i r e d t ha t e n e r g y - c o s t s a v i n g s s h o u l d be ba lanced a g a i n s t c a p i t a l i n ves tmen ts

and economic and o p e r a t i n g c o n s t r a i n t s to i d e n t i f y the most c o s t - e f f e c t i v e

d e s i g n i n any g i v e n s i t u a t i o n . I n o r d e r to make i t p o s s i b l e f o r e n g i n e e r s to use

t h i s a p p r o a c h , new compute r -a ided methods have been p roposed f o r e n e r g y - s y s t e m

d e s i g n .

In the f o l l o w i n g , s h o r t rev iews a re g i v e n o f the u n d e r l y i n g p r i n c i p l e s o f

sys tem s y n t h e s i s by mathemat ical programming and the p r o c e s s i n t e g r a t i o n

t e c h n i q u e . Both methods o r i g i n a t e d f rom the needs o f gene ra l p r o c e s s e n g i n e e r i n g ,

and p a r t i c u l a r l y from the n e c e s s i t y to shape ene rgy sys tems o f complex and o f t e n

e n t i r e l y new chemical p r o c e s s e s r a t i o n a l l y , where i t may be i m p o s s i b l e to use

the e v o l u t i o n a r y a p p r o a c h . I n the s u g a r i n d u s t r y , the s i t u a t i o n i s d i f f e r e n t

because the p r o c e s s has changed r e l a t i v e l y l i t t l e o v e r many d e c a d e s . When

a p p l i e d to an e x i s t i n g s u g a r f a c t o r y , the new methods migh t j u s t i n d i c a t e t h a t

the p r o c e s s i s o p e r a t i n g c l o s e to minimum ene rgy demand and any improvement can

be ach ieved o n l y by i n t r o d u c i n g new u n i t o p e r a t i o n s and equ ipment . When new

s o l u t i o n s are c o n s i d e r e d , however , the new methods may prove u s e f u l i n s t u d y i n g

t h e i r e n e r g y - s a v i n g p o t e n t i a l and max im iz ing p o s s i b l e p r o f i t s .

9 . 4 . 2 E n e r g y - s y s t e m s y n t h e s i s u s i n g mathemat ica l programming methods

"Mathemat ica l programming" i s the common name o f s e v e r a l mathemat ica l

t echn iques t ha t at tempt to s o l v e prob lems by m i n i m i z i n g o r max im iz i ng a f u n c t i o n

( c a l l e d the o b j e c t i v e f u n c t i o n ) o f s e v e r a l independent v a r i a b l e s . T y p i c a l

i n d u s t r i a l a p p l i c a t i o n s i n c l u d e de te rm in i ng the optimum a l l o c a t i o n o f r e s o u r c e s

( i . e . , c a p i t a l , raw m a t e r i a l s , manpower, e t c . ) to o b t a i n maximum p r o f i t o r

minimum c o s t f o r the p r o j e c t , c h o o s i n g the optimum v a l u e s o f d e s i g n v a r i a b l e s to

o b t a i n minimum c o s t o r maximum th roughpu t o f the equipment u n i t , e t c . Opt imal

a l l o c a t i o n o f r e s o u r c e s o r opt imal v a l u e s o f d e s i g n v a r i a b l e s must be determined

under c o n d i t i o n s where the re a re a l t e r n a t i v e u s e s o f r e s o u r c e s o r a l t e r n a t i v e

362

d e s i g n s , and where p h y s i c a l , economic and o t h e r c o n s t r a i n t s must be met. The

c o n s t r a i n t s take the form o f e q u a t i o n s o r i n e q u a l i t i e s c o n t a i n i n g the same

problem v a r i a b l e s as appear i n the o b j e c t i v e f u n c t i o n .

R e s t r i c t i n g our a t t e n t i o n to the energy economy o f s u g a r f a c t o r i e s , we can

s t a t e tha t f o r a g i v e n scheme and known parameters o f the s u g a r m a n u f a c t u r i n g

p r o c e s s , a l t e r n a t i v e e n e r g y - s y s t e m d e s i g n s can be c o n s i d e r e d . Each d e s i g n i s

d e f i n e d by :

- a sys tem s t r u c t u r e ( i . e . a s e t o f components and t h e i r c o n n e c t i o n s ) ;

- parameters o f the energy c o n v e r s i o n , d i s t r i b u t i o n and u t i l i z a t i o n p r o c e s s e s

( f l ows o f e n e r g y - c a r r y i n g med ia , t e m p e r a t u r e s , e t c . ) .

Le t us assume t h a t the s e t o f p o s s i b l e sys tem s t r u c t u r e s i s l i m i t e d to a few

v e r s i o n s and the problem c o n s i s t s o f de te rm in ing the v a l u e s o f η unknown

v a r i a b l e s ×2, . . , x^ c h a r a c t e r i z i n g the components and the ene rgy p r o c e s s e s

o f each v e r s i o n . The c o n s t r a i n t s e t t h a t d e s c r i b e s a t y p i c a l ene rgy sys tem

c o n s i s t s l a r g e l y o f the f o l l o w i n g r e l a t i o n s h i p s .

( i ) E q u a t i o n s f o r the mass and energy b a l a n c e s f o r p r o c e s s u n i t s and equipment

i tems c o n s i d e r e d , i n c l u d i n g m u l t i p l e - e f f e c t e v a p o r a t o r , j u i c e h e a t e r s , e x t r a c t o r ,

t u r b i n e , e t c .

( i i ) E q u a t i o n s f o r heat and power demand.

( i i i ) Upper and lower bounds f o r the independent v a r i a b l e s .

( i v ) E q u a t i o n s and i n e q u a l i t i e s t ha t a re f a c t o r y - d e p e n d e n t .

U s i n g the n o t a t i o n χ = (X ] ,X2> . . fXp )5 we may w r i t e down the gene ra l form o f the

c o n s t r a i n t s e t as

fT(x) = 0 i = 1 , 2 , . . , ρ ( 9 . 3 )

^ j ( x ) < 0 j = 1 , 2 , . . , q ( 9 . 3 )

The o b j e c t i v e f u n c t i o n f o r an ene rgy sys tem can range from ve ry s i m p l e to

q u i t e complex. The s i m p l e s t c o n s i s t o f a s i n g l e v a r i a b l e r e p r e s e n t i n g , f o r

example , the l i v e steam demand, o r the t o t a l fue l demand. I n e i t h e r c a s e , the

o b j e c t i v e f u n c t i o n i s m i n i m i z e d .

A comprehens ive o b j e c t i v e can be d e f i n e d as the sum o f o p e r a t i n g expenses

( i n c l u d i n g f u e l , e l e c t r i c power, f eed -wa te r make-up f o r the b o i l e r , e t c . ) and

the c o s t ^ f c a p i t a l r e c o v e r y , p l u s a r e t u r n on inves tmen t f o r major equ ipment .

F o r a new energy sys tem ( i n a modern ized o r an e n t i r e l y new f a c t o r y ) a t the

d e s i g n s t a g e , t h i s o b j e c t i v e f u n c t i o n r e p r e s e n t s the t o t a l v a r i a b l e c o s t o f the

sys tem and i s a l s o m i n i m i z e d .

Between the two t ypes o f o b j e c t i v e f u n c t i o n ment ioned a b o v e , f u n c t i o n s o f

i n te rmed ia te comp lex i t y can be i m a g i n e d . S e l e c t i o n o f a p a r t i c u l a r o b j e c t i v e

f u n c t i o n , to r e f l e c t the w i s h e s and e x p e c t a t i o n s o f the d e c i s i o n - m a k e r s , i s

o f t en t r e a t e d as a pa r t o f the d e s i g n s t u d y . I n a p r e l i m i n a r y d e s i g n , i t may be

s u f f i c i e n t to m in im ize the t o t a l steam o r fue l demand. I n a d e t a i l e d d e s i g n , the

363

o b j e c t i v e f u n c t i o n s h o u l d i n c l u d e a l l the e s s e n t i a l f a c t o r s t h a t a f f e c t the

economic r e s u l t s o f f a c t o r y o p e r a t i o n .

Hav ing s p e c i f i e d the o b j e c t i v e f u n c t i o n F(x^) , we can fo rmu la te the

mathemat ica l programming problem which i s an a b s t r a c t r e p r e s e n t a t i o n o f the

problem o f opt imal s y n t h e s i s o f the ene rgy s y s t e m . Among a l l the p o s s i b l e x ' s we

a re s e e k i n g such an xP ( i . e . , x°,X2>.. »2< ) t h a t the o b j e c t i v e f u n c t i o n a t t a i n s

i t s minimum

F ( x ° ) = mjn F ( x ) ( 9 . 5 )

Of c o u r s e , x ° can be accepted o n l y i f i t s a t i s f i e s the c o n s t r a i n t s ( 9 . 3 ) and

( 9 . 4 ) .

From the mathemat ical p o i n t o f v iew the p rob lem, c o n s i s t i n g o f c o n d i t i o n s

( 9 . 3 ) - ( 9 . 5 ) , can be e i t h e r l i n e a r o r n o n l i n e a r . I n the former c a s e , the

f u n c t i o n s F , f , and f . must be l i n e a r , t h a t i s , i t s h o u l d be p o s s i b l e to e x p r e s s ' J η

each o f them i n the form ^E^aj^Xj^, where a p a2» . . j a re known c o n s t a n t s . I f

a t l e a s t one o f the f u n c t i o n s i n v o l v e d i s n o n l i n e a r , then the e n t i r e problem i s

s a i d to be n o n l i n e a r . Depending on the prob lem t y p e , d i f f e r e n t mathemat ica l

p rocedures must be a p p l i e d to f i n d a s o l u t i o n .

I t i s an i n h e r e n t p r o p e r t y o f the problems o f e n e r g y - s y s t e m o p t i m i z a t i o n t ha t

some o f the r e l a t i o n s h i p s ment ioned under ( i ) , ( i i ) and ( i v ) a re n o n l i n e a r . I n

p r i n c i p l e , i t may be p o s s i b l e to t r a n s f o r m such r e l a t i o n s h i p s i n t o l i n e a r ones

and to app l y wel l p r o v e n , r e l i a b l e l i n e a r programming methods to f i n d a s o l u t i o n

( r e f . 8 ) . I t has a l s o been demons t ra ted , however , t h a t n o n l i n e a r prob lems can be

e f f e c t i v e l y s o l v e d u s i n g a p p r o p r i a t e numer ica l methods ( r e f s . 9 , 1 0 ) . Fo r

example , opt imal s y n t h e s i s o f a thermal sys tem f e a t u r i n g a q u a d r u p l e - e f f e c t

e v a p o r a t o r has been fo rmu la ted and s o l v e d as a n o n l i n e a r programming prob lem

w i th 2 4 - 2 6 v a r i a b l e s and 19 -24 c o n s t r a i n t s , the e x a c t number o f v a r i a b l e s and

c o n s t r a i n t s depending on the sys tem s t r u c t u r e c o n s i d e r e d ( r e f . 1 1 ) .

I t i s worth n o t i n g t ha t the f i r s t s u c c e s s f u l a t tempts to i n t r o d u c e the

methods o f opt imal sys tem s y n t h e s i s to the s u g a r i n d u s t r y took p l ace a t the

b e g i n n i n g o f the 1 9 7 0 s , when t h i s approach was r e l a t i v e l y new. I t can be seen i n

the l i t e r a t u r e , however , t ha t a w ide r i n t e r e s t i n the a p p l i c a t i o n o f

o p t i m i z a t i o n methods to the food i n d u s t r i e s began some ten y e a r s l a t e r ( r e f s .

1 2 , 1 3 ) . T a k i n g advantage o f the development o f mathemat ica l t e c h n i q u e s , i t i s

now p o s s i b l e to o p t i m i z e the sys tem s t r u c t u r e a l o n g w i t h the parameters o f the

components and p r o c e s s e s .

9 . 4 . 3 E n e r g y - s y s t e m d e s i g n u s i n g the techn ique o f p r o c e s s i n t e g r a t i o n

A d i s a d v a n t a g e o f the opt imal s y n t h e s i s approach d i s c u s s e d i n the p r e c e d i n g

S e c t i o n i s t h a t the t r a n s l a t i o n o f r e a l - l i f e d e s i g n problems to a b s t r a c t

mathemat ical fo rmulae i s both d i f f i c u l t and t i m e - c o n s u m i n g . Even w i th computer

364

programs tha t automate the min imum-seek ing c o m p u t a t i o n s , a l o t o f e f f o r t must be

spen t on the i d e n t i f i c a t i o n o f c o n s t r a i n t s and t h e i r mathemat ica l f o r m u l a t i o n ,

p r e p a r a t i o n o f i n p u t data i n accordance w i th the mathemat ica l c o n v e n t i o n a s s u m e d ,

e t c . Once the o p t i m i z a t i o n r e s u l t s have been o b t a i n e d , however , the d e s i g n e r s

tend to t r e a t the f i g u r e s w i th some s u s p i c i o n because they a re u s u a l l y unab le to

con t r o l i n t u i t i v e l y the p r o c e s s o f a r r i v i n g a t a s o l u t i o n . A b e t t e r i n s i g h t can

o n l y be g a i n e d i n an i n d i r e c t manner , by r e p e a t i n g the o p t i m i z a t i o n compu ta t i ons

f o r m u l t i p l e s e t s o f i n p u t data and a n a l y s i n g the s o l u t i o n ' s s e n s i t i v i t y to

changes o f impor tan t i n p u t parameters ( l i k e the p r i c e s o f f ue l and power , c a p i t a l

c o s t r a t e , e t c . ) . Owing to the a s s o c i a t e d work load and the p s y c h o l o g i c a l b a r r i e r ,

t h i s approach may be d i f f i c u l t to adopt as a p a r t o f the e n g i n e e r i n g a c t i v i t i e s .

P r o c e s s i n t e g r a t i o n i s a t echn ique to f a c i l i t a t e s y s t e m a t i c thermodynamic

a n a l y s i s o f comp l i ca ted energy s y s t e m s . O r i g i n a t i n g from the work on mathemat ica l

t o o l s to s y n t h e s i z e e n e r g y - o p t i m a l heat exchanger networks ( r e f s . 1 4 , 1 5 ) , i t can

he lp the u s e r to unde rs tand how and where a v a i l a b l e energy can b e s t be s u p p l i e d

and r e - u s e d w i t h i n the p r o c e s s , and a t what temperature i t s h o u l d be r e j e c t e d

from the p r o c e s s ( r e f s . 1 6 , 1 7 ) .

An i n t r o d u c t i o n to the r e a s o n i n g c h a r a c t e r i s t i c o f p r o c e s s i n t e g r a t i o n can be

g i v e n by u s i n g the s o - c a l l e d compos i te c u r v e s i n a g raph showing cumu la t i ve heat

l o a d s as f u n c t i o n s o f tempera tu re . The g raph can be c o n s t r u c t e d f rom mass and

heat ba lance data c o n s i s t i n g o f the mass f l o w , en tha lpy o r s p e c i f i c h e a t , s u p p l y

temperature and r e q u i r e d ( t a r g e t ) temperature f o r each p r o c e s s s t r e a m .

The hot compos i te r e p r e s e n t s the amount o f heat a v a i l a b l e a t v a r i o u s

tempera tures o f the hot p r o c e s s media (exhaus t s t e a m , h e a t i n g v a p o u r s , condensa te

c o n d e n s a t e , e t c . ) . T h i s heat must be removed to dec rease the e n t h a l p i e s o f the

ho t med ia , i n accordance w i t h the assumed methods o f t h e i r u t i l i z a t i o n . The c o l d

compos i te r e p r e s e n t s the amount o f heat r e q u i r e d a t v a r i o u s tempera tu res o f the

c o l d p r o c e s s media ( c o s s e t t e s , p r e s s w a t e r , j u i c e i n v a r i o u s s t a g e s o f the

p r o c e s s , s y r u p s , e t c . ) . T h i s heat must be s u p p l i e d to i n c r e a s e the tempera tu res

o f the c o l d media to t h e i r r e q u i r e d v a l u e s , as d e f i n e d by the p r o c e s s n e e d s .

Assuming a h y p o t h e t i c a l s i t u a t i o n t ha t the re i s no heat r e c o v e r y i n the ene rgy

s y s t e m , i t would be n e c e s s a r y to s u p p l y the e n t i r e heat amount r e p r e s e n t e d by

the c o l d compos i te i n the b o i l e r f u e l . S i m u l t a n e o u s l y , the heat amount a v a i l a b l e

i n the hot media would need to be removed from the p r o c e s s u s i n g c o o l i n g wa te r .

U s i n g heat r e c o v e r y , t ha t i s , a l l o w i n g f o r some o f the heat a v a i l a b l e i n the ho t

s t reams to cove r the heat demand o f the c o l d s t r e a m s , i t becomes p o s s i b l e to

reduce the fue l demand. T h i s can be done i n a v a r i e t y o f w a y s , and e x p e r i e n c e

p roves t ha t some o f the h e a t - r e c o v e r y s o l u t i o n s may be p r e f e r a b l e to o t h e r s .

The p o t e n t i a l f o r heat r e c o v e r y by heat exchange between hot and c o l d p r o c e s s

media can be i n v e s t i g a t e d by f i x i n g the r e l a t i v e p o s i t i o n s o f the hot and c o l d

365

100 150 200

Heat load (MW)

150

Heat load (MW)

F i g . 9 . 1 2 . Examples o f cumu la t i ve heat l o a d s as f u n c t i o n s o f t empera tu re . 1 -hot c o m p o s i t e , 2 - c o l d c o m p o s i t e , 3 - p i n c h p o i n t , 4 - minimum heat s u p p l y , 5 - minimum heat r e j e c t i o n .

compos i te c u r v e s , as shown i n F i g . 9 . 1 2 . The d i s t a n c e between them i n the

d i r e c t i o n o f the temperature a x i s must be g r e a t e r t h a n , o r equal t o , the minimum

accep tab le temperature d i f f e r e n c e c h a r a c t e r i s t i c o f the hea t -exchange equipment

a v a i l a b l e ( i n a way, t h i s temperature d i f f e r e n c e r e f l e c t s the a t t a i n a b l e o v e r a l l

heat t r a n s f e r c o e f f i c i e n t , see S e c t i o n 3 . 3 . 2 ) . Once the minimum temperature

d i f f e r e n c e has been d e f i n e d , the r e l a t i v e p o s i t i o n s o f both c u r v e s become f i x e d

and the amounts o f heat to be s u p p l i e d , exchanged and r e j e c t e d can be de te rm ined .

I t a l s o becomes p o s s i b l e to i d e n t i f y the p i n c h p o i n t , t ha t i s , the p o i n t on the

graph where the compos i te c u r v e s a re s e p a r a t e d by the minimum temperature

d i f f e r e n c e .

The p inch p o i n t s e p a r a t e s two d i s t i n c t r e g i o n s o f the p r o c e s s . A t

tempera tures above the p i n c h - p o i n t t empe ra tu re , a l l the heat a v a i l a b l e i n the

hot media can be t r a n s f e r r e d to the c o l d m e d i a , and the heat d e f i c i t must be

ba lanced by s u p p l y i n g f u e l . Below the p i n c h - p o i n t t empe ra tu re , a l l the h e a t i n g

needs o f the c o l d media can be s a t i s f i e d u s i n g the heat a v a i l a b l e i n the hot

med ia , and the s u r p l u s heat must f i n a l l y be r e j e c t e d . I t can thus be conc luded

t ha t the re s h o u l d be no heat t r a n s f e r a c r o s s the p i n c h , as any heat amount

t r a n s f e r r e d w i l l i n c r e a s e the d e f i c i t i n the upper r e g i o n ; t h i s w i l l l ead to

i n c r e a s e d fue l demand, and more i n s t a l l e d h e a t i n g s u r f a c e a rea than r e a l l y

needed. Heat t r a n s f e r a c r o s s the p i nch s h o u l d t h e r e f o r e be a v o i d e d i n a new

d e s i g n . When i n v e s t i g a t i n g p o s s i b l e improvements i n an e x i s t i n g f a c t o r y , the

366

p i n c h - p o i n t temperature s h o u l d be determined and the c a s e s o f i n c o r r e c t h e a t i n g

s h o u l d be i d e n t i f i e d .

I t s h o u l d be p o i n t e d ou t t h a t the above c o n c l u s i o n s r e l a t i n g to the e n e r g y

t a r g e t s and heat t r a n s f e r a r rangements can be drawn be fo re i n i t i a t i n g d e s i g n

work , and the i n f o r m a t i o n thus a c q u i r e d can be t r ea ted as a d e s i g n g u i d e l i n e .

A c t u a l l y , i f the p i n c h - p o i n t temperature i s known, then a d d i t i o n a l i n f o r m a t i o n

can be ob ta i ned r e g a r d i n g o t h e r energy p r o c e s s e s t o o . Fo r examp le , i t s h o u l d be

c l e a r t ha t a vapour compress ion c i r c u i t can save energy o n l y i f i t t akes the

heat from below the p inch and s u p p l i e s i t to a temperature l e v e l above the p i nch

p o i n t where t he re i s a heat d e f i c i t . T h i s i s an unambiguous c r i t e r i o n making i t

p o s s i b l e to i d e n t i f y economic a p p l i c a t i o n s o f vapour c o m p r e s s i o n .

As can be seen i n F i g . 9 . 1 2 , w h i l e the minimum accep tab le temperature

d i f f e r e n c e a f f e c t s the r e l a t i v e p o s i t i o n s o f the compos i te c u r v e s , i t a l s o

de termines the w id th o f the r e g i o n o f o v e r l a p r e p r e s e n t i n g p o s s i b l e heat

e x c h a n g e , and the w id th o f the r e g i o n r e p r e s e n t i n g n e c e s s a r y heat s u p p l y . T a k i n g

i n t o accoun t the c o s t s o f h e a t - e x c h a n g e r s u r f a c e s and energy and a p p l y i n g

compute r -a ided o p t i m i z a t i o n methods , i t becomes p o s s i b l e to f i n d the most

economic v a l u e o f the minimum temperature d i f f e r e n c e . The r e s u l t i n g d e s i g n

g u i d e l i n e s can then be t r e a t e d as opt imal w i th r e s p e c t to o v e r a l l f a c t o r y

economy. U s i n g t hese g u i d e l i n e s , the most economic e n e r g y - s y s t e m o p t i o n s to be

c o n s i d e r e d i n the d e t a i l e d d e s i g n a re e a s i l y i d e n t i f i e d .

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1 N . P . Romensk i i ( E d . ) , R e k o n s t r u k t s i y a i Tekhn i cheskoe P e r e v o o r u z h e n i e Sakharnykh Zavodov , T e k h n i k a , K i e v , 1 9 8 5 .

2 H. Wunsch , E r k e n t n i s s e und E r f a h r u n g e n bei der P lanung von K a p a z i t δ t s e rwe i te rungen i n Z u c k e r f a b r i k e n , Z u c k e r i n d . , 1 0 7 ( 1 0 ) ( 1982 ) 9 3 2 - 9 3 4 .

3 W. Lekawski and K. U r b a n i e c , M o d e r n i s i e r u n g der Wδrmewi r tscha f t i n Z u c k e r f a b r i k e n , Z u c k e r i n d . , 1 0 8 ( 4 ) (1983) 3 3 8 - 3 4 3 .

4 R. M i c h e l , P h . Ternynck and P h . B o n n e n f a n t , R e a l i s a t i o n du pos te d ' e v a p o r a t i o n dans une u s i n e de 12000 t / j de b e t t e r a v e s s t o c k a n t 60% du s i r o p p r o d u i t en campagne, I n d . A l i m . A g r i e , 9 4 ( 7 - 8 ) (1977 ) 7 0 1 - 7 0 5 . Η. C y r k l a f f (e t a l . ) , M o d e r n i z a c j a g o s p o d a r k i c i e p l n e j cukrowni Che lmza , Gaz . C u k r o w . , 9 2 ( 7 - 8 H . R . B runner (e t a l .

(1984) 1 5 6 - 1 5 7 . , D ie Ve rdamp fs ta t i on der Z u c k e r f a b r i k + R a f f i n e r i e

Aa rbe rg AG und das M u l t i - E n e r g i e - S c h e m a , Z u c k e r i n d . , 1 1 0 ( 5 ) (1985 ) 3 9 3 - 3 9 8 . / P . Hof fman, Optimal i z a c e e n e r g e t i c k e h o h o s p o d a r s t v i c u k r o v a r u L o v o s i c e ,

L i s t y C u k r . , 1 0 2 ( 7 ) (1986 ) 1 5 5 - 1 6 1 . 8 J . K . C l a r k and N . E . He im i ck , How to op t im i ze the d e s i g n o f s team s y s t e m s ,

i n : R. Greene ( E d . ) , P r o c e s s Energy C o n s e r v a t i o n , M c G r a w - H i l l , New Y o r k , 1 9 8 2 , pp . 1 5 3 - 1 6 4 .

9 A . Kubas iew icz (e t a l . ) , Op tyma l i zac ja g o s p o d a r k i c i e p l n e j cukrowni za pomoca maszyny matematyczne j , Gaz . C u k r o w . , 8 3 ( 7 ) (1975 ) 1 6 5 - 1 6 7 .

10 A . Kubas iew icz (e t a l . ) . Optimum d e s i g n o f thermal sys tems o f s u g a r p l a n t s . Paper p resen ted a t V I I I I n t e r n a t i o n a l Confe rence on I n d u s t r i a l E n e r g e t i c s , Gdansk , September 1 9 7 5 .

11 A . Kubas iew icz (e t a l . ) . Some a s p e c t s o f computer ized d e s i g n o f thermal sys tems o f beet s u g a r p l a n t s , i n : P r o c . Symp. Computers i n the D e s i g n and E r e c t i o n o f Chemical P l a n t s , K a r l o v y V a r y , September 1 9 7 5 , pp . 5 9 9 - 6 0 7 .

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12 I . S a g u y , O p t i m i z a t i o n t h e o r y , t e c h n i q u e s , and t h e i r imp lementa t ion i n the food i n d u s t r y : i n t r o d u c t i o n . Food T e c h n . , ( 1982 ) ( 7 ) 8 7 .

13 D. Depeyre and P h . L u c a s , S y n t h e s e de p rocedes e t a m e l i o r a t i o n e n e r g e t i q u e du procede s u c r i e r , I n d . A l i m . A g r i e , 1 0 2 ( 7 - 8 ) ( 1985 ) 7 4 3 - 7 4 8 .

14 Β . L i n n h o f f and J . R . F l o w e r , S y n t h e s i s o f heat exchanger n e t w o r k s , A I C h E J . , 2 4 ( 4 ) (1978 ) 6 3 3 - 6 5 4 .

15 D. Bo land and B. L i n n h o f f , The p r e l i m i n a r y d e s i g n o f ne tworks f o r heat exchange by s y s t e m a t i c methods , Chem. E n g i n e e r , (1979 ) (4 ) 2 2 2 - 2 2 8 .

16 B. Goublomme, Comment abo rde r le probleme de l a r e d u c t i o n des c o u t s e n e r g e t i q u e s dans l e s s u c r e r i e s , S u c r . B e i g e , 103 (1985 ) 2 7 - 3 0 .

17 N . R . T w a i t e , H . J . Davenpor t and E . K . M a c d o n a l d , Energy r e d u c t i o n and p r o c e s s i n t e g r a t i o n . I n t . S u g a r J . , 88 ( 1 9 8 6 ) , P a r t I : ( 1055 ) 2 1 7 - 2 1 9 , P a r t I I : ( 1056 ) 2 3 0 - 2 3 6 .

368

Appendix 1

NUMERICAL APPROXIMATIONS OF THERMODYNAMIC PROPERTIES OF WATER AND STEAM

Data on the thermodynamic p r o p e r t i e s o f water and steam are i n d i s p e n s a b l e

to e n g i n e e r i n g c a l c u l a t i o n s r e l a t e d to the energy economy o f s u g a r f a c t o r i e s .

Most o f ten used are data on the p r o p e r t i e s o f s a t u r a t e d water and d ry s a t u r a t e d

s team, as wel l as superhea ted s team. They can be found i n genera l t a b l e s o f

the p r o p e r t i e s o f water and steam ( r e f . 1 ) , o r i n s p e c i a l i z e d t a b l e s , d iag rams

and nomographs where the ranges o f the parameters are adapted to the needs o f

the s u g a r i n d u s t r y ( r e f . 2 ) .

In computer ized c a l c u l a t i o n s , o r when u s i n g h a n d - h e l d programmable

c a l c u l a t o r s to automate p a r t s o f the c a l c u l a t i o n p r o c e d u r e s , t a b l e s o r d iag rams

o f thermodynamic p r o p e r t i e s s h o u l d p r e f e r a b l y be r ep l aced by s u i t a b l e f u n c t i o n a l

r e l a t i o n s h i p s . T h i s requ i rement i s nowadays r e c o g n i z e d by the p u b l i s h e r s o f

i n t e r n a t i o n a l l y known t a b l e s o f thermodynamic p r o p e r t i e s o f water and s t e a m ,

where mathemat ical formulae are a l s o g i v e n f o r most thermodynamic f u n c t i o n s

( r e f . 1 ) . These formulae are i n t e n d e d , however , to combine thermodynamic

c o n s i s t e n c y w i th h i g h accu racy over broad ranges o f parameter v a l u e s . To s a t i s f y

t h i s c o n d i t i o n , the mathemat ical e x p r e s s i o n s c o n s i s t o f many terms and the

c o e f f i c i e n t s are g i v e n w i th s e v e r a l - d i g i t a c c u r a c y . Such fo rmulae may be

i n c o n v e n i e n t to u s e , e s p e c i a l l y when r e l y i n g on smal l -memory comput ing hardware .

However, i f the parameter ranges are narrow and the accu racy c o n d i t i o n s a re no t

ve ry s t r i n g e n t , then the l eng thy e x p r e s s i o n s can be rep laced by more c o n c i s e

o n e s . Numerous s imp le fo rmulae d e s i g n e d f o r use w i t h i n d e f i n i t e i n t e r v a l s o f

parameter v a l u e s can be found in the l i t e r a t u r e , and some o f them have been

e l abo r a ted to s a t i s f y the needs o f the s u g a r i n d u s t r y .

The most impor tan t app rox ima t i on fo rmulae a re g i v e n i n Tab le A l . U n l e s s

o the rw i se s t a t e d , t h e i r ranges o f v a l i d i t y s h o u l d be unde rs tood to c o i n c i d e w i th

the ranges o f parameters no rma l l y encounte red i n the s u g a r i n d u s t r y . The maximum

r e l a t i v e d i f f e r e n c e between the t a b u l a t e d data and the a p p r o x i m a t i o n s does no t

exceed 0 .3% and the ave rage e r r o r i s t y p i c a l l y l e s s than 0 . 1 % .

REFERENCES

1 U. G r i g u l l ( E d . ) , P r o p e r t i e s o f Water and Steam i n S l - u n i t s , 2nd e d n . , S p r i n g e r - V e r l a g , B e r l i n - H e i d e l b e r g - N e w Y o r k , 1979 .

2 T. B a l o h , Wδrmeat las f ٧ r d ie Z u c k e r i n d u s t r i e , Schaper V e r l a g , Hannove r , 1 9 7 5 . 3 A . I l l y e s , Anwendung von N δ h e r u n g s g l e i c h u n g e n i n der Wδrmetechn ik ,

Ζ . Z u c k e r i n d . , 2 6 ( 1 2 ) (1976) 7 6 3 - 7 6 5 . 4 . G, Ba to r and Κ. U r b a n i e c , P r o j e k t i e r u n g von Verdampfan lagen i n Z u c k e r f a b r i k e n

mi t H i l f e von Computern, Z u c k e r i n d . , 103 (12 ) (1978) 1 0 3 5 - 1 0 4 2 . 5 W. Reed , The smal l programmable c a l c u l a t o r i n a s u g a r r e f i n e r y . S u g a r J . ,

P a r t I : January 1 9 7 9 , 1 3 - 2 0 , P a r t I I : Feb rua ry 1 9 7 9 , 1 3 - 2 0 .

369 TA

BLE

Al

Ap

pro

xim

ati

on

form

ula

e fo

r th

erm

od

yna

mic

fu

ncti

on

s o

f w

ate

r an

d st

ea

m

(te

mp

era

ture

in

p

ressu

re

in

ba

r,

en

tha

lpy

in

kJ

/kg

, s

pe

cif

ic

volu

me

in

m^

/kg

).

Sp

ec

ific

ati

on

^^"^

^^^

R

efe

ren

ce

va

lid

ity

Sa

tura

tio

n

tem

pe

ratu

re a

s

0.2

5-6

.0 b

ar

t^^^ =

1

59

.52

ρ^·

""^^

^ -

59

.96

3

a fu

ncti

on

of

pre

ssu

re

t^^^

=

(58

5.4

3 +

2

02

.2

log

p

)/(4

.98

7

- lo

g

p)

- 1

7.7

8 5

^

Sa

tura

tio

n

pre

ssu

re a

s

65

-16

0°C

p

^^^

=

((t

+

59

.96

)/1

59

.52

)^'^

^^

3

a fu

ncti

on o

f te

mp

era

ture

p,,.

=

ex

p(2

.30

26

(11

.06

6 t

- 4

34

)/(1

.8

t +

3

96

) -

2.6

74

4)

5^

sa

t

afS

1S

L?io

roT

Sera

f"e

2

0-1

60

°C

h'

=

1.5

43

. 4

.12

20

t .

0.5

44

94

(t2

/10

00

) 4

En

tha

lpy

of

dry

sa

tura

ted

st

ea

m

20

-16

0°C

h

" =

2

50

0 +

1

.81

3 t

+

(0.4

71

t^

- 0

.01

10

4

t^)/

10

00

4

as a

fu

ncti

on o

f te

mp

era

ture

6

5-1

60

°C

h"

=

13

09

.1

+

54

6.3

5(t

+

5

9.9

6)°

·^^°^

3

Sp

ecif

ic

volu

me

of

sa

tura

ted

w

ate

r ^.^

^ o

^

^,

^

y^

^^

^^

_

^^

^^

^

_

0

00

35

3 ^2

^ 3

as

a

fun

cti

on

of

tem

pe

ratu

re

^

^

Sp

ec

ific

vo

lum

e o

f dry

sa

tura

ted

0

.25

-1.2

b

ar

v"

=

1/(

0.1

05

+

0.6

10

5 ρ

-

0.0

30

3 p

^)

3

ste

am

a

s

a

fun

cti

on

of

pre

ssu

re

1.2

-6.0

b

ar

v"

=

1/(

0.0

53 +

0

.54

62

ρ -

0.0

04

55

3 p

^)

3

En

tha

lpy

of

su

pe

rhe

ate

d

ste

am

a

s

h

=

10

77

.81

+

0.7

74

58

t +

0

.00

01

37

t^

- ^a

Srlssuie

"""

^^"^

P^^

^^^^

^ -

13

0.5

3(p

+

0

.68

9)(

log

ρ +

1

.16

15

)/(1

.8

t -

21

8)

' B

riti

sh

u

nit

s

use

d in

th

is

refe

ren

ce

370

Appendix 2

NUMERICAL APPROXIMATIONS OF THERMODYNAMIC PROPERTIES OF SUGAR SOLUTIONS

Data on the thermodynamic p r o p e r t i e s o f s u g a r s o l u t i o n s can be found i n

the t a b l e s and d iag rams g i v e n i n r e f . 1 and o t h e r s o u r c e s . As i n the p r o p e r t i e s

o f water and s team, when c a l c u l a t i n g the energy b a l a n c e s , these da ta s h o u l d

p r e f e r a b l y be a v a i l a b l e i n the form o f f u n c t i o n a l r e l a t i o n s h i p s . Mos t p u b l i s h e d

r e l a t i o n s h i p s are based on data measured f o r pure s u c r o s e s o l u t i o n s and thus

y i e l d o n l y approx imate v a l u e s when used f o r t e c h n i c a l s u g a r s o l u t i o n s . However,

i n t y p i c a l e n g i n e e r i n g c a l c u l a t i o n s , t h e i r a c c u r a c y can be rega rded as

s u f f i c i e n t l y h i g h .

A few app rox ima t i on fo rmulae a re g i v e n i n Tab le A 2 . U n l e s s o the rw i se s t a t e d ,

the ranges o f v a l i d i t y s h o u l d be unde rs tood to c o i n c i d e w i th the ranges o f

parameters n o r m a l l y encounte red in the s u g a r i n d u s t r y . The maximum r e l a t i v e

d i f f e r e n c e between the t a b u l a t e d data and the a p p r o x i m a t i o n s does no t exceed

0.5% except f o r the second d e n s i t y f o r m u l a , the maximum e r r o r o f wh ich i s

about 2%.

REFERENCES

1 T. B a l o h , Wδrmeat las f ٧ r d i e Z u c k e r i n d u s t r i e , Schaper V e r l a g , Hannover , 1975 . 2 A . I l l y e s , Anwendung von N δ h e r u n g s g l e i c h u n g e n i n der Wδrmetechn ik ,

Ζ . Z u c k e r i n d . , 2 6 ( 1 2 ) (1976) 7 6 3 - 7 6 5 . 3 G. Ba to r and Κ. U r b a n i e c , P r o j e k t i e r u n g von Verdampfan lagen i n Z u c k e r f a b r i k e n

mi t H i l f e von Computern , Z u c k e r i n d . , 103 (12 ) (1978) 1 0 3 5 - 1 0 4 2 . 4 W. R e e d , The smal l programmable c a l c u l a t o r i n a s u g a r r e f i n e r y . S u g a r J . ,

Pa r t I : January 1 9 7 9 , 1 3 - 2 0 , P a r t I I : Feb rua ry 1 9 7 9 , 1 3 - 2 0 .

371

TAB

LE A

2

Ap

pro

xim

ati

on

form

ula

e fo

r th

erm

od

yna

mic

fu

ncti

on

s

of

su

ga

r so

luti

on

s

(te

mp

era

ture

in

°C

, b

.p.e

. in

K

, co

nce

ntr

ati

on

in

%

DS

, p

uri

ty in

%

, e

nth

alp

y in

k

J/k

g,

sp

ec

ific

h

ea

t in

k

J/(

kg

K),

d

en

sit

y in

k

g/m

^).

Sp

ec

ific

ati

on

Sid

itν

R

efe

ren

ce

Bo

ilin

g-p

oin

t e

lev

ati

on

, a

s

20

-14

0°C

Δ

Τ =

f,

+

(t

/10

0)f

p +

(t

/10

0)^

f-.

3

a fu

ncti

on o

f co

nce

ntr

ati

on

10

-90%

D

S

^

and

wa

ter

sa

tura

tio

n

tem

pe

ratu

re

f-j

=

ex

p(-

1.5

25

4 +

0

.02

29

62

b +

0

.00

02

16

3 b

)

- 0

.2

f2

=

ex

p(-

3.2

02

1

+

0.0

06

67

43

b

- 0

.00

01

16

1 b

^)

- 0

.15

f3

=

ex

p(-

1.4

27

8 -

0.0

24

38

2 b

+

0

.00

06

04

7 b

^)

Sp

ec

ific

h

ea

t a

s

a

fun

cti

on

of

C

=

4.1

94 +

t(

t -

72

)/1

08

15

5 -

tem

pe

ratu

re,

co

nce

ntr

ati

on

an

d

_

(b/1

00

)(2

.72

2

- 0

.00

75

t .

0.0

04

6(1

00

- r)

) ^

Sp

ec

ific

h

ea

t a

s

a

fun

cti

on o

f

tem

pe

ratu

re

an

d

co

nce

ntr

ati

on

C =

4

.18

68 -

0.0

25

58

14

b +

0

.00

00

75

36

bt

(pu

rity

a

bo

ut

90%

) E

nth

alp

y a

s

a

fun

cti

on o

f ^n

, T/

inO

r o

te

mp

era

ture

a

nd

co

nce

ntr

ati

on

in

ano/

nc h

=

1

.5 +

(4

.12

2 -

0.0

25

12

b)t

+

(5

.5 +

0

.37

5 b

)(t/

10

0)'

^ 3

(p

uri

ty

ab

ou

t 9

0%

) '"

'^"/

^

Spi

ννtrr

/aνr

^coν

^nνν

atio

n ρ

=

^^

^-^

"

0-0

^38

3 t

- 0

.00

38

4

. (b

t)/(

0.2

46 t

- 0

.26

8)

2

De

nsity

as

a

fun

cti

on o

f co

nce

ntr

ati

on

(te

mp

era

ture

10

-70%

D

S ń

=

1

02

2.5

3 b

^'

^^-^

'^^^

exp

(0.0

05

55

3 b

) 70

-130

OC

)

372

Appendix 3

CALCULATIONS OF HEAT TRANSFER PHENOMENA

I t was assumed th roughou t Chapter 2 and o the r r e l e v a n t p a r t s o f the p r e s e n t

book t h a t data on the thermal p r o p e r t i e s o f equipment a re a v a i l a b l e whenever

needed i n the c a l c u l a t i o n s o f ene rgy b a l a n c e s o f f a c t o r y s u b s y s t e m s . In p r a c t i c e ,

such data a re o f ten unknown f o r s p e c i f i c equipment u n i t s , and i n o rde r to make

the ba lance c a l c u l a t i o n s p o s s i b l e , c e r t a i n a s s u m p t i o n s may be r e q u i r e d . The aim

o f t h i s Appendix i s to s y n t h e s i z e i n f o r m a t i o n which can prove h e l p f u l i n making

r e a l i s t i c a s s u m p t i o n s about the c h a r a c t e r i s t i c s o f thermal equ ipment .

In the energy ba lance o f an equipment u n i t , heat l o s s e s to the env i ronment

are accounted f o r by m u l t i p l y i n g the heat e f f e c t i v e l y t r a n s f e r r e d w i t h i n the

u n i t by the heat l o s s c o e f f i c i e n t . I t s h o u l d be emphasized t ha t d i f f e r e n t

d e f i n i t i o n s o f such c o e f f i c i e n t s can be found i n the l i t e r a t u r e , and t h e i r

ac tua l v a l u e s may depend on equipment d e s i g n , the e f f i c i e n c y o f thermal

i n s u l a t i o n and l o c a l c o n d i t i o n s ( f o r example , ou tdoor l o c a t i o n o f a p a r t i c u l a r

u n i t ) . I n the fo rmulae g i v e n i n Chapter 2 , the f o l l o w i n g v a l u e s o f heat l o s s

c o e f f i c i e n t s can be u s e d :

- j u i c e h e a t e r s 0 . 0 3 - 0 . 0 5 ;

- e x t r a c t o r s 0 . 0 5 - 0 . 1 0 ;

- vacuum pans 0 . 0 3 - 0 . 1 2 ;

- s y r u p t anks 0 . 0 3 - 0 . 0 5 .

As r e g a r d s the e v a p o r a t o r s , exper imenta l work has been repo r ted on the

de te rm ina t i on o f heat l o s s c o e f f i c i e n t s . Zag rodzk i and S o k o l o w s k i ( r e f . 1) found

tha t the heat l o s s c o e f f i c i e n t o f a R o b e r t - t y p e e v a p o r a t o r body i n the second

e f f e c t o f a q u a d r u p l e - e f f e c t e v a p o r a t o r was 0 . 0 0 1 5 ; w i th an a d d i t i o n a l

i n s u l a t i o n l a y e r , the c o e f f i c i e n t was reduced to 0 . 0 0 0 9 . These r e s u l t s are in

good agreement w i th the va lue o f 0 . 0 0 1 1 measured by Hogg e t a l . ( r e f . 2 ) . Tak ing

i n t o accoun t heat d i s s i p a t i o n from j u i c e , vapour and condensa te p i p e l i n e s ,

a c o e f f i c i e n t va lue o f 0 . 0 0 2 5 was recommended by the l a t t e r a u t h o r s , w h i l e i n

o l d e r s o u r c e s , v a l u e s o f up to 0 . 0 3 can be found ( r e f . 3 ) .

I t seems t ha t f o r most e v a p o r a t o r s , a heat l o s s c o e f f i c i e n t between 0 . 0 0 2 5

and 0 . 0 1 can be assumed. When u s i n g the c a l c u l a t i o n a l g o r i t h m p resen ted i n

Chapter 2 , i d e n t i c a l v a l u e s can u s u a l l y be assumed f o r both the e v a p o r a t o r body

and the condensate tank .

The v a l u e s o f o v e r a l l heat t r a n s f e r c o e f f i c i e n t s used i n the d e s i g n

c a l c u l a t i o n s o f thermal sys tems are d e c i s i v e i n e n s u r i n g a p roper c h o i c e o f

the a r e a s o f h e a t i n g s u r f a c e s in equipment u n i t s . As the p r a c t i c a l v a l u e s o f

heat t r a n s f e r c o e f f i c i e n t s may v a r y w i th t i m e , depending on s c a l e b u i l d - u p .

373

i t i s d i f f i c u l t to c o r r e l a t e d e s i g n data and r e a l i t y . A l t h o u g h the f i l m

c o e f f i c i e n t s o f heat t r a n s f e r c h a r a c t e r i z i n g c l e a n h e a t i n g s u r f a c e s can be

c a l c u l a t e d u s i n g d i m e n s i o n l e s s r e l a t i o n s h i p s known from the t heo ry o f heat

t r a n s f e r and w ide l y c i t e d in the l i t e r a t u r e , i t i s common to r e l y i n s t e a d on

p r a c t i c a l l y v e r i f i e d f i g u r e s . Care s h o u l d be t a k e n , however , o f the

c o m p a t i b i l i t y o f a l l da ta used i n the e q u a t i o n g o v e r n i n g heat t r a n s f e r a c r o s s

the h e a t i n g s u r f a c e

Q = k F At

where Q i s the heat t r a n s f e r r e d i n u n i t t i m e , k i s the o v e r a l l heat t r a n s f e r

c o e f f i c i e n t , F i s the h e a t i n g s u r f a c e a r e a , and At i s the mean temperature

d i f f e r e n c e .

In the j u i c e h e a t e r s , i t i s e s s e n t i a l t ha t the va lue o f k be determined u s i n g

the same d e f i n i t i o n o f the h e a t i n g s u r f a c e a rea as assumed i n the ac tua l

c a l c u l a t i o n . For t u b u l a r h e a t e r s , the i n n e r s u r f a c e a rea o f the tubes i s

t y p i c a l l y u s e d . Depending on the tube d iameter and wa l l t h i c k n e s s , i t may d i f f e r

by up to 12-15% from the ou te r s u r f a c e a rea o f the t u b e s .

For vapou r -hea ted t u b u l a r hea te r s ope ra ted a t c o r r e c t v a l u e s o f the j u i c e

f low v e l o c i t y , the o v e r a l l heat t r a n s f e r c o e f f i c i e n t s d e f i n e d a t the i n n e r

s u r f a c e area o f the tubes can u s u a l l y be assumed as f o l l o w s :

- raw j u i c e 6 0 0 - 8 0 0 W / ( m ^ K ) ;

- c l e a r j u i c e 700 -1000 W / ( m ^ K ) ;

- t h i n j u i c e 9 0 0 - 1 2 0 0 W / ( m ^ K ) ;

- t h i c k j u i c e and s y r u p s 4 0 0 - 6 0 0 W / ( m ^ K ) .

These v a l u e s can be t r ea ted as rough e s t i m a t e s o n l y ( c f . exper imenta l v a l u e s

c i t e d in Chapter 8 ) . As r e g a r d s p l a t e and s p i r a l h e a t e r s , the s c a t t e r o f

p r a c t i c a l v a l u e s o f heat t r a n s f e r c o e f f i c i e n t s i s so l a r g e t h a t one can o n l y

r e l y on the data s u p p l i e d by the equipment m a n u f a c t u r e r s .

In the case o f e v a p o r a t o r s , d e f i n i t i o n s o f both the h e a t i n g s u r f a c e a rea and

the e f f e c t i v e temperature d i f f e r e n c e are e s s e n t i a l . The temperature d i f f e r e n c e ,

a c c o r d i n g to the d e f i n i t i o n g i v e n i n Chapter 2 and most f r e q u e n t l y used i n the

l i t e r a t u r e , i s

At = t^ - ( t^ + ΔΤ)

where t^ i s the temperature o f the h e a t i n g steam (vapou r ) c o n d e n s i n g i n the

h e a t i n g chamber, t^ i s the temperature o f s a t u r a t e d vapour i n the vapour chamber

and AT i s the b o i l i n g p o i n t e l e v a t i o n . However, some a u t h o r s take the e f f e c t i v e

temperature d i f f e r e n c e as

At = t^ ^ t^

Adop t i ng t h i s d e f i n i t i o n , the v a l u e s o f the o v e r a l l heat t r a n s f e r c o e f f i c i e n t

c a l c u l a t e d f o r e v a p o r a t o r e f f e c t s opera ted a t h i g h e r j u i c e c o n c e n t r a t i o n s

(50-70% DS) may be up to 50-60% g r e a t e r than those c o r r e s p o n d i n g to the

374

d e f i n i t i o n o f At adopted i n the p r e s e n t book .

For a p r o p e r l y d imens ioned and c o r r e c t l y ope ra ted R o b e r t - t y p e e v a p o r a t o r ,

the o v e r a l l heat t r a n s f e r c o e f f i c i e n t can be c a l c u l a t e d from B a l o h ' s fo rmu la

( r e f . 3)

k = 5 . 2 3 - 1 0 ^ ( b 2 ^ + b^^^ + 800) (W{mh)) where b^.^ and b^^^ are j u i c e c o n c e n t r a t i o n s ( i n % DS) a t i n l e t and o u t l e t ,

r e s p e c t i v e l y . T h i s fo rmu la i s known to g i v e somewhat too h i g h v a l u e s o f the heat

t r a n s f e r c o e f f i c i e n t a t the h i g h e s t j u i c e c o n c e n t r a t i o n s .

Approx imate v a l u e s o f the o v e r a l l heat t r a n s f e r c o e f f i c i e n t i n both R o b e r t -

type and f a l l i n g - f i l m e v a p o r a t o r s a re a l s o g i v e n , as f u n c t i o n s o f mean j u i c e

c o n c e n t r a t i o n , i n F i g . 5 . 5 .

REFERENCES

1 S . Zag rodzk i and A . S o k o l o w s k i , Pomiary s t r a t c i e p l n y c h w apa ra tach wypa rnych , Gaz. C u k r o w . , 8 1 ( 4 ) (1973 ) 8 1 - 8 5 .

2 J . S . Hogg (e t a l . ) . The r o l e o f the rmograph ic s u r v e y i n g in energy c o n s e r v a t i o n . I n t . S u g a r J . , 8 5 ( 1 0 1 1 ) (1983) 6 7 - 7 1 .

3 T. B a l o h , Wδrmeat las f ٧ r d ie Z u c k e r i n d u s t r i e , Schaper V e r l a g , Hannover , 1 9 7 5 .

375

Appendix 4

and from g i v e n tj^ i n Κ

tp = 1.8(t,^ - 2 7 3 . 1 5 ) + 32 .

UNITS OF MEASUREMENT

The S I sys tem o f u n i t s used th roughou t t h i s book i s based on the f o l l o w i n g

u n i t s :

- mass i n k i l o g r a m s ( k g ) ;

- l eng th i n metres (m) ;

- t ime i n seconds ( s ) .

The temperature i s measured i n deg rees C e l s i u s ( ° C ) , and the a b s o l u t e

temperature i n K e l v i n s ( K ) . The temperature d i f f e r e n c e i s e x p r e s s e d i n K.

The met r i c sys tem o f u n i t s ( a l s o known as the t e c h n i c a l sys tem) i s based on

the f o l l o w i n g u n i t s :

- f o r ce in k i l o g r a m s f o r c e , o r k i l o p o n d s ( k g f o r k p ) ;

- l e n g t h i n m;

- t ime in s .

The mass i s measured i n k g . The temperature i s measured i n ^ C , and the a b s o l u t e

temperature i n deg rees K e l v i n ( ° K ) . The temperature d i f f e r e n c e can be e x p r e s s e d

i n °C o r \ .

The B r i t i s h sys tem o f u n i t s ( a l s o known as p o u n d - s e c o n d - f o o t sys tem) i s

based on the f o l l o w i n g u n i t s :

- mass i n pounds ( l b . ) ;

- l eng th i n f e e t ( f t . ) ;

- t ime in seconds (denoted s e c ) .

The f o r ce i s e x p r e s s e d i n pounds f o r c e ( I b f . ) . The temperature i s measured i n

degrees F a h r e n h e i t ( ^ F ) . The temperature d i f f e r e n c e i s a l s o e x p r e s s e d i n ^ F .

The c o n v e r s i o n from S I to me t r i c o r B r i t i s h u n i t s can be performed u s i n g

the c o n v e r s i o n f a c t o r s g i v e n in Tab le A 4 .

The c o n v e r s i o n fo rmu la f o r c a l c u l a t i o n o f the temperature i n °F from a g i v e n

temperature t^ i n °C i s

tp = 1.8 t(. + 32

376

TAB

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377

INDEX

A i r p r e h e a t i n g , 1 5 7 , 342 A l k a l i n i t y o f wa te r , 2 3 8 , 2 4 3 , 245 Apparen t power, 38 Appearance o f wa te r , 2 4 0 , 244 Ash con ten t o f c o a l , 247

Ba romet r i c c o n d e n s e r , see condenser Ba romet r i c wa te r , 2 , 1 0 , 1 6 , 2 2 , 6 5 , 2 7 7 , 357 Batch c e n t r i f u g a l , 3 3 , 2 0 6 , 228 Batch vacuum p a n , 7 4 , 1 1 0 , 1 2 2 , 1 2 5 , 1 4 7 , 1 6 7 , 2 0 1 , 2 2 5 , 2 2 8 , 2 3 5 , 2 7 1 , 2 7 7 , 280 B i o g a s p r o d u c t i o n , 175 B o i l e r blowdown, 2 4 1 , 2 4 3 , 2 5 5 , 313 B o i l e r check , 253 B o i l e r e f f i c i e n c y , 2 6 , 1 4 8 , 2 5 1 , 2 5 8 , 2 9 2 , 3 1 0 , 3 3 5 , 349 B o i l e r l o s s :

a s h , 255 ch imney, 254 incomplete c o m b u s t i o n , 255 r a d i a t i o n , 256

B o i l e r water q u a l i t y , 241 B o i l i n g p o i n t e l e v a t i o n , 7 8 , 1 2 3 , 2 6 5 , 3 7 0 , 373 Bomb c a l o r i m e t e r , 248 Boundary o f thermodynamic s y s t e m , 3 , 6 , 5 7 , 234 B . p . e . , see b o i l i n g p o i n t e l e v a t i o n

C a r b o n a t a t i o n , 5 , 2 9 , 6 1 , 6 3 , 6 6 , 154 , 1 5 9 , 2 9 6 , 2 9 8 , 3 0 2 , 3 0 9 , 3 1 1 , 318 C a r b o n a t a t i o n g a s , 2 , 9 , 1 5 , 2 2 , 5 3 , 6 5 , 1 5 5 , 1 5 7 , 178 C e n t r i f u g a l d r i v e , 3 9 , 4 1 , 2 0 7 , 209 Chemical c l e a n i n g o f t u b e s , 117 C l o s e d thermodynamic s y s t e m , 57 Co lou r b u i l d - u p , 3 4 , 1 6 3 , 166 Combined g e n e r a t i o n o f heat and e l e c t r i c i t y , 1 0 , 4 3 , 262 Combus t ib le ma t te r :

i n a s h , 2 5 2 , 255 in c o a l , 247

Composi te c u r v e , 364 Compress ion r a t i o , 1 8 , 140 , 150 Condensa te :

d r a i n a g e , 1 5 , 9 7 , 9 9 , 1 0 3 , 1 0 5 , 1 0 9 , 1 1 1 , 1 1 8 , 2 6 4 , 2 7 0 , 2 7 2 , 2 7 8 , 2 9 0 , 2 9 3 , 3 0 2 , 3 3 2 , 338

f l a s h i n g ( f l a s h - e v a p o r a t i o n ) , 1 5 , 7 6 , 7 9 , 8 2 , 119 p o l l u t i o n , 1 0 1 , 144 q u a l i t y , 100 , 117 , 2 3 9 , 290

Condensate t a n k , 2 , 7 , 3 1 , 7 6 , 7 9 , 8 1 , 1 0 0 , 1 0 8 , 1 1 0 , 1 1 2 , 1 1 8 , 1 2 0 , 1 4 4 , 2 3 9 , 2 7 3 , 2 9 4 , 3 0 5 , 3 3 8 , 3 4 2 , 3 4 6 , 353

Condense r , 3 , 7 , 1 0 , 1 4 , 1 7 , 3 1 , 4 6 , 4 8 , 6 5 , 7 7 , 7 9 , 8 3 , 9 7 , 1 0 2 , 1 0 4 , 1 1 3 , 1 2 1 , 1 2 6 , 1 2 8 , 1 3 0 , 134 , 2 2 3 , 2 7 7 , 2 8 7 , 3 4 8 , 352

C o n t i n u o u s c e n t r i f u g a l , 2 0 6 , 2 0 9 , 2 9 3 , 294 C o n t i n u o u s vacuum p a n , 1 2 5 , 1 4 7 , 1 4 9 , 1 6 6 , 203 Con t ro l s u r f a c e , 57 Con t ro l vo lume, 57 C o o l i n g - c r y s t a l l i z a t i o n tower , 165 C o s s e t t e s m i x e r , 189 C r y s t a l f o o t i n g , 3 6 , 166 , 2 0 4 , 324

378

C r y s t a l 1 i z a t i o n : c o o l i n g , 1 6 3 , 167 e v a p o r a t i n g , 1 6 3 , 167 , 173 f r e e z e , 173 under vacuum, 165

C r y s t a l l i z a t i o n scheme: D a n i s h , 161 s i n g l e b o i l i n g , 323 t h r e e - b o i l i n g , 3 4 , 6 1 , 1 6 1 , 1 6 5 , 2 9 2 , 2 9 6 , 3 1 0 , 3 3 5 , 349 t w o - a n d - a - h a l f - b o i l i n g , 324 t w o - b o i l i n g , 166 w i th c r y s t a l f o o t i n g , 1 6 6 , 168 w i th two j u i c e c o n c e n t r a t i o n s , 36

C r y s t a l l i z a t i o n tower , 2 0 3 , 206

Decompos i t i on o f a s y s t e m , 3 , 10 D u l o n g ' s f o r m u l a , 249

E f f e c t i v e power, 38 E f f e c t i v e n e s s r a t i o , 9 , 2 3 , 9 5 , 1 2 1 , 1 8 7 , 2 1 3 , 2 9 1 , 3 1 0 , 3 3 8 , 352 E l e c t r i c a l c o n d u c t i v i t y o f wa te r , 243 E l e c t r i c a l d e s c a l e r , 118 Energy b a l a n c e , 5 , 8 , 5 7 , 6 0 , 6 5 , 8 4 , 8 7 , 9 6 , 1 0 3 , 1 5 5 , 2 3 4 , 2 5 3 , 2 5 7 , 2 6 1 , 2 6 9 ,

2 7 8 , 2 9 6 , 382 Energy s y s t e m , 5 , 1 7 4 , 1 8 1 , 1 8 3 , 2 9 0 , 2 9 5 , 3 2 2 , 3 3 3 , 3 6 2 , 364 E n t h a l p y b a l a n c e , 88 En t ra inment s e p a r a t o r , 1 0 1 , 1 3 0 , 194 E u l e r ' s e q u a t i o n , 219 E v a p o r a t i o n , 6 , 1 2 , 2 4 , 3 5 , 1 4 4 , 1 4 7 , 2 1 3 , 2 1 5 , 2 2 1 , 2 2 6 , 2 6 4 , 2 8 7 , 307 E v a p o r a t i o n c o e f f i c i e n t , 121 E v a p o r a t o r :

c l i m b i n g - f i l m , 222 d o u b l e - e f f e c t , 195 f a l l i n g - f i l m , 1 9 1 , 2 2 2 , 2 6 3 , 3 1 0 , 3 1 4 , 3 1 6 , 3 2 3 , 3 5 3 , 3 5 7 , 374 m u l t i p l e - e f f e c t , 8 , 1 2 , 1 5 , 3 4 , 7 6 , 8 3 , 1 2 1 , 1 9 0 , 2 2 1 , 2 6 3 , 3 2 3 , 349 q u a d r u p l e - e f f e c t , 17 , 1 2 0 , 1 2 6 , 2 9 6 , 3 3 5 , 3 3 9 , 342 q u i n t u p l e - e f f e c t , 1 4 , 8 3 , 1 2 6 , 1 3 5 , 2 9 2 , 3 1 0 , 3 2 3 , 342 R o b e r t - t y p e , 1 0 6 , 1 9 1 , 2 2 2 , 2 6 3 , 2 9 6 , 3 1 0 , 3 1 4 , 3 1 6 , 3 2 3 , 3 3 5 , 3 4 9 , 3 5 2 , 3 5 7 ,

3 5 9 , 3 7 2 , 374 s e x t u p l e - e f f e c t , 127 , 224 t r i p l e - e f f e c t , 1 2 6 , 2 9 7 , 337 t h i n - f i l m , 1 9 1 , 222

Evapo ra to r check , 2 6 4 , 266 E v a p o r a t o r - r e c e i v e r a p p r o a c h , 8 4 , 1 0 3 , 301 E x e r g y , 8 9 , 90 Ex te rna l energy b a l a n c e , 6 5 , 8 4 , 1 0 1 , 103 E x t r a c t o r :

b e l t t y p e , see mov ing-bed type drum t y p e , 1 8 8 , 2 2 0 , mov ing-bed t y p e , 1 8 8 , 220 s c r o l l t y p e , see t r ough type tower t y p e , 3 1 , 7 3 , 1 8 8 , 2 2 0 , 2 9 2 , 323 t r ough t y p e , 3 1 , 6 1 , 7 3 , 1 3 5 , 1 8 8 , 2 2 0 , 2 6 7 , 2 9 6 , 3 0 9 , 3 3 5 , 349

E x t r a c t o r check , 269

Feed-water q u a l i t y , 238 F i l m c o e f f i c i e n t o f heat t r a n s f e r , 1 6 , 1 1 3 , 1 1 6 , 373 F l o a t - t y p e steam t r a p , 1 0 6 , 1 1 1 , 2 9 7 , 337

379

Flow c o n t r o l : by p o s i t i o n i n g o f i n l e t gu ide v a n e s , 219 by t h r o t t l i n g , 4 2 , 2 1 7 , 219 b y - p a s s , 4 2 , 1 5 5 , 2 1 7 , 219 v a r i a b l e s p e e d , 4 2 , 1 4 5 , 2 1 6 , 2 1 8 , 327

F r e q u e n c y - c o n v e r t e r ( - c o n t r o l l e d ) a . c . d r i v e , 2 0 8 , 3 2 7 , 329 Fuel s t o r a g e , 2 3 3 , 250

Gas t u r b i n e , 2 1 , 5 0 , 184 Grassmann d i a g r a m , 89

Hardness o f wa te r , 2 3 8 , 2 4 0 , 2 4 2 , 245 Heat b a l a n c e , 6 , 8 , 1 8 , 6 8 , 7 3 , 7 6 , 8 4 , 9 6 , 1 0 4 , 2 6 8 , 2 7 0 , 2 8 1 , 2 8 3 , 2 8 7 , 3 0 1 ,

3 0 6 , 3 0 9 , 3 1 4 , 3 2 0 , 3 2 4 , 3 2 7 , 3 3 2 , 3 3 6 , 3 3 9 , 3 4 2 , 3 4 6 , 3 4 9 , 352 Heat l o s s :

by d i s s i p a t i o n to the env i ronmen t , 2 4 , 2 6 , 1 0 0 , 1 0 3 , 1 5 5 , 2 8 4 , 2 9 1 , 301 by f r e e c o n v e c t i o n , 28 by r a d i a t i o n , 28

Heat l o s s c o e f f i c i e n t , 6 0 , 6 9 , 7 3 , 7 5 , 7 8 , 8 9 , 2 6 6 , 2 7 1 , 372 Heat o f c a r b o n a t a t i o n r e a c t i o n , 6 6 , 155 Heat o f combus t i on , 248 Heat o f c r y s t a l l i z a t i o n o f s u g a r , 5 9 , 6 6 , 7 5 , 236 Heat pump, 1 7 , 25 Hea te r , see j u i c e heater Hea t ing and hum id i f i c a t i o n o f k i l n g a s , 1 5 8 , 3 4 2 , 352 Hea t ing v a l u e o f f u e l , 1 4 8 , 2 4 6 , 2 5 2 , 257 Hot c o n d e n s e r , 134 , 357 Hydrogen i on c o n t e n t , see pH H y p e r f i l t r a t i o n , 171

J u i c e c a r r y o v e r , 1 0 1 , 1 4 4 , 194 J u i c e d r a f t , 2 4 , 3 0 , 6 1 , 6 6 , 9 8 , 1 8 7 , 2 1 9 , 2 2 1 , 2 6 8 , 2 9 2 , 2 9 6 , 3 0 9 , 3 2 3 , 3 3 4 ,

3 3 8 , 349 J u i c e h e a t e r :

c o n d e n s a t e - h e a t e d , 6 9 , 1 1 9 , 2 7 1 , 293 d i r e c t - c o n t a c t , 130 , 133 p l a t e , 1 9 8 , 2 9 3 , 3 1 4 , 3 1 7 , 373 segmented, 200 s p i r a l , 1 3 1 , 1 9 9 , 2 9 4 , 3 5 7 , 373 t u b u l a r , 1 0 6 , 1 3 1 , 1 9 9 , 2 9 3 , 373 v a p o u r - h e a t e d , 6 9 , 1 2 2 , 271

J u i c e heater check , 271 J u i c e p u r i f i c a t i o n , 2 , 2 3 , 2 9 , 4 1 , 6 1 , 1 1 6 , 1 3 5 , 1 5 3 , 1 6 3 , 1 6 9 , 1 7 1 , 2 1 3 , 2 1 6 ,

2 9 2 , 2 9 6 , 3 0 9 , 3 1 1 , 3 2 3 , 3 3 1 , 3 3 5 , 349 J u i c e s e p a r a t i o n , 169

K i l n g a s , 9 , 6 2 , 6 7 , 1 5 5 , 1 5 7 , 1 5 9 , 2 9 6 , 3 0 9 , 3 3 5 , 349

Law o f mass c o n s e r v a t i o n , 57 Law o f thermodynamics :

f i r s t , 5 7 , 8 8 , 2 3 4 , 261 s e c o n d , 88

L e v e l - c o n t r o l l e d h y d r a u l i c (water ) s e a l , 1 0 6 , 1 0 9 , 1 1 2 , 2 9 3 , 2 9 4 , 3 3 8 , 342 L i n e a r programming, 363

Mass b a l a n c e , 5 7 , 6 1 , 6 3 , 6 7 , 7 4 , 7 9 , 8 3 , 9 4 , 1 0 1 , 2 3 5 , 2 8 1 , 2 8 3 , 2 8 7 , 2 9 6 , 3 0 1 , 3 0 5 , 3 1 4 , 3 3 2 , 3 3 5 , 3 3 7 , 3 3 9 , 352

Mean l o g a r i t h m i c temperature d i f f e r e n c e , 69 Membrane f i l t r a t i o n , 171 Minimum temperature d i f f e r e n c e , 1 2 1 , 365 M o i s t u r e con ten t o f c o a l , 247 Motor s l i p , 207

380

Net heat demand, 8 , 2 2 , 2 5 , 6 6 , 9 5 , 1 2 1 , 1 4 9 , 1 5 3 , 1 6 0 , 187 , 2 0 3 , 3 3 3 , 338 N i e s s n e r co lumn, 1 0 6 , 1 0 8 , 349 N o n c o n d e n s a b l e s , 9 7 , 1 1 2 , 1 1 5 , 1 2 8 , 130 , 2 6 4 , 2 7 0 , 2 8 0 , 2 9 0 , 3 0 0 , 3 0 2 , 3 0 9 , 314 N o n l i n e a r programming, 363 Normal f u e l , 4 3 , 53 Normal steam pa rame te r s , 43 N o z z l e - t y p e steam t r a p , 1 0 6 , 279 Number o f mass t r a n s f e r u n i t s , 189

O b j e c t i v e f u n c t i o n , 362 Open thermodynamic s y s t e m , 3 , 5 7 , 65 Optimal s y n t h e s i s , 363 O v e r a l l heat t r a n s f e r c o e f f i c i e n t , 6 0 , 6 9 , 8 1 , 1 0 6 , 1 1 6 , 1 2 1 , 1 2 4 , 1 4 4 , 1 9 3 ,

1 9 8 , 2 0 1 , 2 6 4 , 2 6 6 , 2 7 0 , 3 1 5 , 3 6 5 , 372 O v e r a l l t u r b o - g e n e r a t o r e f f i c i e n c y , 2 5 9 , 262 Oxygen consumpt ion o f wa te r , 2 3 9 , 2 4 2 , 245 Oxygen con ten t o f wa te r , 2 4 0 , 2 4 2 , 246

pH, 2 3 9 , 2 4 2 , 245 P e r i o d o f r e t u r n on i nves tmen t , 3 4 6 , 359 P inch p o i n t , 365 P o l e - c h a n g e a . c . motor , 207 Power c a p a c i t o r , 3 9 , 43 Power demand, 1 3 , 1 8 , 3 7 , 4 0 , 4 5 , 1 5 3 , 1 6 0 , 170 , 1 7 9 , 1 8 2 , 187 , 2 0 9 , 2 1 3 , 3 2 7 ,

3 3 2 , 3 3 4 , 3 3 7 , 3 4 6 , 3 5 3 , 357 Power f a c t o r , 3 8 , 4 3 , 2 0 8 , 290 Power h o u s e , 1 , 6 , 1 0 , 3 9 , 4 3 , 4 5 , 4 9 , 5 2 , 1 4 9 , 2 3 3 , 2 9 0 , 2 9 7 , 3 0 0 , 3 1 0 , 3 1 4 ,

3 2 4 , 3 3 2 , 3 3 5 , 3 3 7 , 348 Power ne twork , 37 P r e s s i n g a i d s , 177 Pu lp d e h y d r a t i o n , 1 9 , 1 5 3 , 174 Pu lp d r y e r :

drum t y p e , 2 1 , 8 9 , 2 2 9 , 2 8 1 , 285 f l u i d i z e d - b e d t y p e , 183 low- tempera tu re , 2 2 , 5 3 , 179 s team, 2 2 , 182 t r a v e l l i n g - s c r e e n t y p e , 1 7 9 , 182

Pu lp d rye r check , 282 Pu lp d rye r e f f i c i e n c y , 2 8 1 , 2 8 3 , 285 Pu lp d r y i n g :

h i g h - t e m p e r a t u r e , 2 2 , 179 low- tempera tu re , 1 6 , 2 2 , 1 7 6 , 178 medium-temperature, 2 2 , 178 s team, 2 2 , 181

Pu lp e n s i l a g e , 175 Pu lp f e r m e n t a t i o n , 175 Pu lp p r e s s i n g , 1 9 , 176 , 178

Quent in u n i t , 1 9 8 , 3 2 3 , 349

R e a c t i v e power, 3 8 , 43 Reverse o s m o s i s , see h y p e r f i l t r a t i o n

S a m p l i n g : f u e l , 246 j u i c e , 265 s team, 244 wa te r , 243

Sankey d i a g r a m , 8 , 4 4 , 5 0 , 8 8 , 9 0 , 1 5 8 , 283 S c a l e , 1 0 5 , 1 1 6 , 1 9 3 , 198 , 2 4 1 , 2 4 5 , 2 6 4 , 2 7 0 , 3 0 9 , 3 1 6 , 372 Secan t method, 8 0 , 82 Seed magma, 167 , 204 S i e g e r t ' s f o r m u l a , 254

381

S i l i n ' s f o r m u l a , 221 S i m u l a t i o n , 6 1 , 91 S ludge s w e e t e n i n g - o f f , 3 2 , 302 S o l u b i l i t y o f s u c r o s e , 1 6 3 , 1 6 6 , 227 S t e a d y - s t a t e c o n d i t i o n s , 5 8 , 9 1 , 235 Steam demand, 1 6 , 4 3 , 4 6 , 1 3 5 , 1 3 9 , 1 4 2 , 1 4 9 , 3 0 1 , 3 3 7 , 3 4 7 , 362 Steam q u a l i t y , 2 3 7 , 243 Steam r a t e , 4 6 , 4 9 , 137 , 1 3 9 , 1 4 8 , 2 5 8 , 2 6 3 , 335 Steam t r a p , 6 , 5 9 , 1 0 3 , 1 0 6 , 1 1 1 , 2 7 8 , 2 9 3 , 3 0 0 , 353 Steam t u r b i n e :

b a c k - p r e s s u r e , 1 3 , 4 4 , 4 7 , 2 6 0 , 2 9 7 , 3 1 0 , 324 b a c k - p r e s s u r e / e x t r a c t i o n , 48 c o n d e n s i n g , 48 c o n d e n s i n g / e x t r a c t i o n , 46 t o p p i n g , 46

Sugar b o i l i n g : c o n t r o l , 3 3 , 2 0 3 , 2 2 6 , 293 p r o c e s s , 1 5 , 3 2 , 3 5 , 1 1 1 , 1 2 2 , 1 2 5 , 1 2 7 , 1 6 7 , 2 0 1 , 2 0 3 , 2 2 5 , 2 2 7 , 2 7 1 , 332

Sugar l o s s : i n exhaus ted c o s s e t t e s , 3 1 , 1 8 9 , 221 i n s l u d g e , 32

S u r r o u n d i n g s o f thermodynamic s y s t e m , 5 7 , 88

Temperature p i n c h , see minimum temperature d i f f e r e n c e Thermal decay o f s u c r o s e , 14 , 3 0 , 3 4 , 100 Thermal d e g r a d a t i o n o f e n e r g y , 88 Thermal i n s u l a t i o n , 2 5 , 2 7 , 7 8 , 9 9 , 1 0 3 , 2 7 9 , 2 9 3 , 3 0 1 , 3 0 5 , 3 4 4 , 372 Thermal s y s t e m , 6 , 1 0 , 1 6 , 2 3 , 6 0 , 6 5 , 6 8 , 8 4 , 8 8 , 9 0 , 9 4 , 1 0 0 , 1 0 3 , 1 2 1 , 1 2 7 ,

1 3 1 , 1 3 6 , 1 4 5 , 1 5 3 , 187 , 2 1 3 , 2 2 1 , 2 5 1 , 2 7 5 , 2 7 7 , 2 7 9 , 2 8 7 , 2 9 7 , 3 0 0 , 3 0 5 , 3 1 0 , 3 2 0 , 3 2 3 , 3 3 2 , 3 3 4 , 3 4 0 , 3 4 3 , 3 4 8 , 3 5 4 , 373

Thermodynamic f u n c t i o n s , 6 0 , 7 8 , 8 4 , 8 9 , 2 3 4 , 3 6 8 , 370 Thermodynamic s y s t e m , 3 , 6 , 5 7 , 6 0 , 6 7 , 8 8 , 2 3 4 , 235 Thermograph ic s u r v e y i n g , 100 T h r o t t l i n g - d e s u p e r h e a t i n g s t a t i o n , 4 7 , 9 7 , 1 3 8 , 1 4 2 , 1 4 5 , 1 4 9 , 2 5 7 , 2 7 5 , 324 T h y r i s t o r - c o n t r o l l e d d . c . d r i v e , 2 0 8 , 3 2 7 , 353 Tota l heat demand, 8 , 1 2 , 2 2 , 6 8 , 9 5 , 1 8 7 , 1 9 5 , 3 0 2 , 3 0 5 , 3 2 3 , 353 Tu rb ine e f f i c i e n c y :

i n t e r n a l , 262 m e c h a n i c a l , 262

T u r b o - g e n e r a t o r check , 258

T u r b o - g e n e r a t o r e f f i c i e n c y , see o v e r a l l t . - g . e .

U l t r a f i l t r a t i o n , 171

Vacuum-pan check , 272 Vacuum-pan s t e a m i n g - o u t , 3 3 , 2 0 3 , 2 7 5 , 3 0 8 , 3 1 9 , 337 Vacuum-pan s t i r r e r , 1 2 2 , 1 2 5 , 2 0 2 , 2 0 4 , 206 Vapour c o m p r e s s i o n , 1 1 , 1 6 , 1 2 6 , 1 3 4 , 1 3 7 , 1 4 0 , 1 4 2 , 1 4 4 , 1 4 7 , 1 4 9 , 1 8 4 , 1 9 5 ,

3 2 4 , 3 4 2 , 3 4 7 , 3 5 7 , 3 6 0 , 366 Vapour compresso r :

e l e c t r i c a l l y d r i v e n , 1 3 8 , 1 4 2 , 1 4 5 , 1 4 7 , 1 4 9 , 3 2 4 , 3 4 2 , 357 j e t t y p e , 1 8 , 1 4 0 , 1 4 2 , 1 4 5 , 1 4 9 , 3 2 4 , 3 4 2 , 358 m e c h a n i c a l , 1 8 , 1 3 8 , 1 4 2 , 1 4 5 , 147 , 1 4 9 , 3 2 7 , 3 4 2 , 357 t u r b i n e d r i v e n , 1 3 9 , 142

Ven t i ng (o f n o n c o n d e n s a b l e s ) , 1 1 2 , 1 1 5 , 2 6 4 , 2 7 0 , 2 7 2 , 2 8 0 , 2 9 0 , 3 0 0 , 3 0 3 , 3 1 4 , 3 5 3 , 357

Waste h e a t , 1 4 , 2 1 , 2 5 , 5 3 , 147 , 1 5 9 , 1 7 8 , 1 8 0 , 185

382

Water i n t a k e : to j u i c e p u r i f i c a t i o n s t a t i o n , 32 to C m a s s e c u i t e c r y s t a l 1 i z e r s , 33 to p r o c e s s , 3 0 , 2 9 1 , 3 1 0 , 323 to s u g a r h o u s e , 3 2 , 3 4 , 2 9 1 , 3 0 2 , 305 to vacuum p a n s , 3 2 , 6 6 , 1 0 1 , 2 2 5 , 2 2 8 , 271