Effect of Air Flow Rate on Carrot Drying

8/13/2019 Effect of Air Flow Rate on Carrot Drying

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EFFECT OF AIR FLOW RATE ON CARROT DRYINGA. Mulet

a, A. Berna

a, M. Borr

a& F. Pinaga

b

aChemical Engineering Department, University of Illes Balears, 07071, Palma de Mallorca

SPAINbInstituto de Agroqumica y Tecnologa de Alimentos, Valencia, SPAIN

Available online: 02 Apr 2007

To cite this article:A. Mulet, A. Berna, M. Borr & F. Pinaga (1987): EFFECT OF AIR FLOW RATE ON CARROT DRYING, Drying

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D R Y I N G T E C H N O L O G Y 5 2 ) , 2 4 5- 25 8 1 9 8 7 )

EFFECT OF AIR FLOW RATE ON CARROT DRYING

A. Mulet, A. Berna, Mil Borras and F. FinagarChemical Engineering Department

University of Illes Balrars07071 Palma de Ma1 lorca (SPAIN)

*Institute de Asroqulmica y Tecnologla de AlimentosValencia (SPAIN)

Key words and phrases:mass transfer, phase resistance, drying kinetics,vegetables drying

ABSTRACTThe influence of air flow rate on the kinetics of

drying 10 ~10 x1 0 carrot cubes is presented. For thisgeometry kinetic equations are available, for the firstfalling rate drying period.

Drying air flows of 1000, 2000 2500, 3000, 4000,5000, 6000, 8000 and 9000 kg/m h were employed. It wasfound that for flow rates above 6000 kg/mL h the valueof D/I : remains almost constant, thus indicating thatwhen the air flow rate is higher it has no influence onthe dl-ying rate. The influence of air flow rate oncarrot drying has been determined, hence allowingoptimal flow rate calculation under economicconstrictions.

INTRODUCT

Since 1973, when oil price increased sharply,people have become aware of the importance of energy

Copyright 987 y Marcel Dekker Inc.


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46 MULET ET AL

c o s t s o n e c on o m ic s o f p r o c e s s i n g . F ood i n d u s t r y a ndd r y i n g i n p a r t i c u l a r h a v e n o t e s c a p e d t h i s d y na m ic s .U nder t h e s e c i r c u m s t a n c e s , t h e r e s e a r c h i n t e r e s t o nd r y i n g p r o c e s s e s h a s b e en f o c u s e d o n p r o u u c t q u a l i t yi mp ro ve m en t a n d e n e r g y s a v i n g s . In b o t h c a s e s i t isn e c e s s a r y t o know t h e me ch an is m an d k i n e t i c s o f d r y i n gf o r e a c h p a r t i c u l a r m a t e r i a l . E ne rg y s a v i n g s c a n b ea c h i e v e d b y d r y e r / d r y i n g o p t i m i z a t i o n o r / a n d u s e of newe n e r g y s o u r c e s 1 k e s o l a r e n e r g y .

O ne o f t h e m o s t com mon w ay s o f d r y i n g is u s i n g h o ta i r a s a d r y i n g a g e n t . H e at a n d m ss t r a n s f e r b e tw ee nt h e a i r a n d t h e s o l i d t a k e p l a c e i n o p p o s i t ed i r e c t i o n s . One way o f s a v i n g e n e r g y i n t h i s d r y i n gp r o c e s s is t o d e c r e a s e t h e a i r f l o w i n t h e d r i e r . T h i sis b e n e f i c i a l i n tw o d i f f e r e n t w a y s ; t h e f i r s t is t h es a v i n g i n e n e r g y ( a nd i n v e s t m e n t ) n ee de d t o c i r c u l a t et h e a i r a nd t h e s ec o n d is a d e c r e a s e i ~ i h e a i r h e a t i n gr e q u i r e m e n t s . T h e s e b e n e f i t s c o u l d b e o u t w e ig h e d b y ar e d u c t i o n i n t h e d r y i n g r a t e , when e x t e r n a l h e a t a n dm as s t r a n s f e r r e s i s t a n c e s becom e i m p o r t a n t . T he ser e s i s t a n c e s i n c r e a s e w i t h d e c r e a s i n g a i r f l o w . Too p t i m i z e t h e d r y e r o p e r a t i o n i t i s i m p o r ta n t t oq u a n t i f y t h e s e e f f e c t s .

T h e s o l i d p h a s e r e s i s t a n c e d e pe n d s o n t h ec h a r a c t e r i s t i c s o f t h e s o l i d b e i n g d r i e d , t h e f l u i dt h a t is t r a n s f e r e d a n d t e m p e r a t u r e . When t h i sr e s i s t a n c e c o n t r o l s t h e d r y i n g p r o c e s s , a n i n c r e a s e o nt h e a i r v e l o c i t y w i l l n o t a f f e c t t h e d r y i n g r a t e o f t h es o l i d . A s t h e t r a n s f e r w i t l ~ i n h e s o l i d is a ss um e d t obe by d i f f u s i o n , t h e f l u x is d e s c r i b e d b y F i c k s l aw ,

The e x t e r n a l r e s i s t a n c e ( g a s p h a s e ) t o m as st r - a n s f e r - is d i r e c t l y r e l a t e d t o a i r v e l o c i t y t h ro u g ht h e m as s t r a n s f e r c o e f f i c i e n t b et we en a i r a n d s o l i dk > I n c r e a s i n g t h e a i r v e l o c i t y wi i n c r e a s e k. a n d


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I R FLOW R TE ON C RROT DRYING 247

t h e r e s i s t a n c e w i l l d i m i n i s h (S be rw oo d e t a 1 . , 1 9 7 5 ) . I ft h e c o n t r o l l i n g s t e p is t h e f l u i d p h as e t r a n s f e r , t h e nt h e f l u x w i l l b e

The mass t r a n s f e r c o e f f i c i e n t ( k . , ) c a n be o b ta i n e df r o m s e v e r a l c o r r e l a t i o n s , s om e o f t he m wer-e r e p o r t e db y S k e l l a n d ( 1 9 7 4 ) . I n g e n e r a l , c o r r e l a t i o n s a r e b as edo n t h e h e a t a nd m s t r a n s f e r a n a lo g y r e l a t e d t o ap a r t i c l e s h a p e . F a s t e r n a k a n d G a u v i n s c o r r e l a t i o n1960 ) is c o n s i d e r e d i n t h i s p a p e r . T h e se a u t h o r s

o b t a i n e d e x p e r im e n t a l d a t a f o r h e a t a n d m as s t ~ ~ a n s f e r ,u lld er 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 , b e tw e en a l u i da n d s o l i d s o f d i f f e r e l i t s h a p e s a nd o r i e n t a t i o n s . T h i se q u a t i o n is

w h e r e

a nd t h e v a l i d i t y r a n g e 5 0 0 R e 5 0 0 0

Th e c h a r a c t e r i s t i c d im e i l s i on , d , u se d f n r t h ec a l . c u l a t i o r 1 o f t h e S h a nd Re n u m h e ~ - s , is d e f i n e d a s t h et o t a l s u r f a c e a l - ea of t h e b od y d i v i d e d b y t h e p e r i m e t e ro f t h e maximum p r o j e c t e d a r e a p e r p e n d i c u l a r t o f l o w .From t h i s c o r r e l a t i o n i t is f ou nd t h a t t h e r e l a t i o nb e t w e e n k a nd a i r v e l o c i t y i .s

s a C o n s e q u e n ce , a i r v e l o c i t y w i l l g r e a t l yi n f l u e n c e m a s s t r a n s f e r r e s i s t a n c e . I f t h e c o n t r o l i n gr e s i s t a n t c e is w i t h i n t h e s o l i d p h a s e t h e d r y i n g r a t ew i l l b e c o n s t a n t ; t . h i s r a t e w i l l i n c r e a s e w i t h a i rv e l o c i t y i f t h e c o n t r o l l i n g p h a s e is i n t h e g a s . Theo v e r a l l r e s i s t a n c e is t h e sum o f e x t e r n a l a n d i n t e r n a l


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48 M U L E T ET AL

resistances. Mass flux is inversely proportional to theresistance and will be affected differently by airvelocity, depending on the controlling resistance.

From these considerations a logical conclusion isthat for a given temperature, for each nwterial, sizeand shape, there is a threshold air velocity value overwhich no increase in drying rate is observed. This willbe the maximum air velocity to be used. OtherConsiderations taken into aciount could suggest a lowervelucity but by no means it will be economic to use ahigher one. From these considerations, it seemsinteresting to stablish a methodology to determlne thethreshold value of the air velocity to be used, thiswas one of the objectives of the study. Results on theinfluence of hat air velocity on the drying rate ofcarrots are presented and the threshold value of airvelocity is determined. The agreement betweenexperimental and computed threshold value is good.

PROCEDURES

Figure 1 shows a schematic. diagram of thelaboratory dryer used for experimental work. Threepart.=, an be distinguished: air flow rate control,heating control system and drying chamber.

A 0 5 HP SIEMENS fan was used and air flow ratewas measured with a rotametel- aud manually controlled.The rotameter allowed measurements up to 40 m5 /h withan accuracy of 0.5 m /h.

The heating system consisted of an electkic 7 Wheater plac2d inside the chamber. Heating control wasachieved by a PI controller, adjusting the dryingchamber temperature. The temperature was measured witha thermocouple and digitally displayed on the controlpanel.


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25 MULET ET AL

product was obtained according to the AOAC method(1970).

RESULTS

Experiments were carried out at different airvelocities, but at a constant air temperature andinitial bed heiyht. The influence of bed height and airtemperature on the drying rate bas been reported byothers (Madarro et al. 1901, Rossell6, 1983;. Loadingsof less than 10 kg/m:? bed height had no influence onthe drying rate. When air temperatures were higherthan 70,. , the dried product partially lost it.s freshproduct characteristics. Due to the interest on dryingat near room temperatures (solar dryers), a temperatureof 30'' C was chosen to carry out the experiments, thelow drying rate facilitates the observance of thekinetics.

The alr flow range was selected accordine to theresults oi Mitchell and Potts (1958) who concluded thatfor air flow rates greater than 4200 kg/m h noinfluence of this variable was observed on the dryingrate

Based on this information, we decided to carry outexperiments under the following operating conditions:

Air temperature: 30Loading 10 kg/m:Air flow : 1000, 2000, 2500, 3000, 4000,

5000, 6000, 8000, 9000 kg/m-: hThe mean air humidity was: 50-40 relative

humidi ty.

No case hardenins was observed under theseoperating conditions.

Drying curves for some of these experiments areshown, in ,figure 2 in the form of dimensionless


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A I R FLOW RATE ON CARROT DRYING 2 5

FIGURE 2.- Drying curves for carrot cubes at severalair flow rates.

moisture, = W W, )/ W, - W. versus drying time.Equilibrium data were obtained from literatureCarbonell et al. 1 9 8 4 ) .

It can be observed figure 3 that almost all thedrying takes place during the falling rate period. Asmall constant. drying rate period was observed for thelower air flow rates. The drying rate increases withair flow.

The objective of this study was limited to thefirst difussional period falling rate). Equations forthis period are known and can be found in theliterature Mulet et al ., 1983 . Dimensionless moistureas a time function, for sufficiently high values ofthis variable, is described by equation 6)

where D is an effective diffusivity


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252 MULET ET AL.

I

FIGUR 3.- Drying rate curves for carrot cubes at twoair flow rates.

Data points representing the falling rate periodwere obtained fl-om the plot of In versus time. Oneof these plots, for three different air flows, is shownin -figure 4 The points that were on a strajght line,after those with dimenaionless moisture greater than0 6 were eliminated, were considered to be within thefirst diffussional drying perlod. The value 0 6 isconsidered to be a reasonable starting point for thediffusional period Perry and Green, 1984 . Valuesof D/r2 obtained from the slope of the straieht lines- 3 n . D/4r.) are shown in Table 1. in this table

values of mass flux density t N 1 obtained by usingFick s law are also shown. Calculated values


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I R FLOW R TE ON C R ROT DRYING

FIGURE 4. Graphical selection of th first diffusional drying period.


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254 MULET T AL.

TABLE 1Experimental Diffusivities and Experimental

Calculated Mass Flux for Different Air Flow Rates.

Air flow rate D/r2: 10~; I? . Ne.:(kg/m;: h h ' ) (kg water /h m-:)

N from experimental dataN A - from Pasternak and Gauvin correlation

Experimental mass flux values versus air flow ar-eplotted in figure 5 straight line can be fitted tothe left zone of the plot. The slope of the line is0.528, which agrees well with the expected value of0.514 from the Pasternak and Gauvin s correlation. Theright zone of the plot is almost an horizontal line,indicating that the air flow had no influence on thedrying rate, thus the internal resistance wascontrolling. The slope changing zone (figure 5indicates a change of the controlling phase, in ourexperiments this zone was about 6000 kg air/h m: . Thisvalue is higher than the one reported by Mitchell andPotts < 1 9 5 8 , although the influence of air flow ratesgreater than 4200 kg air/h m .;is small.

The shape of a log--log lot of D/r ::? ersus linearair velocity would be similar to that of N,. From the


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AIR FLOW RATE ON CA RROT DRYING

F I G U R E 5. M ass f l u x v a r i a t i o n v e r s u s a i r f l o w .

r e s u l t i n g l e a s t s q u a r e e q u a t i o n s f i t t o t w o s t r a i g h tl i n e s t s p o s s i b l e t o c om p ut e t h e m as n it u d o f D / r i 'f o r a g i v e n l i n e a r a i r v e l o c i t y . The k no w le d ge o f D / r 'a l l o w s m o i st u l- e c a l c u 1 a t i o n : s u s i n g e q u a t i o n 6

F i g u r e s ho w s t h e v a r i a t i o n o f D/r::': w i t h a i r f l o wr a t e . I t c a n b e o b s e rv e d t h a t fr o m a b o u t 6 kg/m:.:ha i r f l o w r a t e D/r : . : ' r e m a in s p r a c t i c a l l y c o n s t a n t . se x p e c t e d t h e a p p a r e n t d i f f u s i v i t y < D / I - ' : ) o b t a i n e d f r ome x p e r i m e n t a l d a t a v a r i e s w i t h a i r v e l o c i t y . T h e re s a na i r f l o w v a l u e a b o v e w hic h t h e a i r v e l o c i t y h a s n oi n f l u e n c e o n t h e d r y i n g r a t e m ea n i n g t h a t t h e d r y i n gp r o c e s s s c o n t . r o l l e d by t h e i n t e r n a l r e s i s t a n c e . T h i sa i r f l o w r a t e v a l u e s d i f f i c u l t t o d e t e r m in e m a i n l yd ue t o t h e e x p e r i m e n t a l e r r o r s . E x pe r i m en t a l p o i n t s


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MULET ET AL

FIGURE 6 .- Air flow influence on apparent diffusivity.Results from Rossell6 (1983,.

dispersion could be explained by the actualnon-uniformity of raw material and drying air (ambient)throughout the year.

The relationship between D/rY; and air flow rate(G) could be well described by equations 7 ) and 8 ) .

D/r :: exp (-6.602 0.528 In G) 7 )

for G 6 kg/m2h, and

for G > 6 kg/m'h


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AIR FLOW RATE ON CARROT DRYING 257

where the numerical parameters have been obtained fromexperimental points by the use of a least squaretechnique. The mean error is 4. and the standarddeviation 1.51 lo-

NOMENCLATURE

c gas phase concentration of waterc Interphase water concentration

ConcentrationApparent dif fusivityCharacteristic distanceAir flow ratej factor for mass transferMass transfer coaf cientMass fluxA half of thicknessReynolds numberSchmidt numberSherwood numberTimeAir velocityMoistureCritical moistureEquilibrium moistureLength on X axisDimensionless moisture

REFERENCESAOAC 1970. Official methods of analysis (1lw *d. , p.

876 45. 31.Carbonell. J . V . Madarro A. Pifiaga F. and Pefia. J L

1984. DeshidrataciBn de frutas y hortalizas conaire ambiente. CinCtica de adsorci5n y desorcibnde agua en zanahoi-ias. Rev. Agroquim. Tecnol.Aliment. 24 (1): 94-104.


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258 MUL T ET AL.

Madarro, A., PiRaga, F., arbonell, J . V . and Pefia, . L .1981. DeshidrataciBn de frutas y hortalizas conalre ambiente. I Ensayos exploratorios conzanahorias. Rev. Agroqul m. Tecnol. Alim. 21 4),525-533.

Mitchell, T. . and Potts,C.S. 1958. Through-circulationdrying of vegetables. 111 . Carrots. J . Sci. FoodAgric. 9 93-98.

Mulet, A., Rossell6, C . , isaga, F., Carbonell, J . V .and Berna, A. 1993. Mecanismo y cinetica delsecado de zanahorias con aire caliente. Rev.Agroquim. Tecnol. Aliment. 23 3 1 , 369-377.

Pasternak, I.S. and Gauvin, W H 1960. Turbulent heatand mass transfer from stationary particles. Can.J. Chem. Eng. , 39, 35-42.

Perry, R. . and Green, D. 1984. Chemical EngineersHandbook, 6 , Ed. McGraw-Hill Book Co. N Y

Rossell6, C. 1983. Contribuci6n a1 estudio del secadode hortalizas en Mallorca. Secado de zanahoriascon aire caliente en un secadero tipo SARGENT.Tesis dr Licenciatura. Facultad de Ciencias.Universitat de les Illes Balears.

Sherwood, T. Pigford, R. . and Wilke, C. . 1975.Mass transfer. McGraw-Hill Book Co., N Y

Skelland, A. . . 1974. Difussional mass transfer. JohnWiley Sons Inc.

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Effect of Air Flow Rate on Carrot Drying