Successfully Specify Three Phase Separators 2

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Successfully pecifyThree-PhaseSeparators

t is often necessaryo separatewoimmiscible liquids, the light andheavy phases,and a vapor.A typi-cal example in petroleum refining

is the separationofwater, and a hydro-carbon liquid and vapor. Little hasbeen published on three-phase(liq-uid/liquid/vapor) separarion, withmost information available only incorporate design files. This articleattemptsto alleviate this situation bycovering he basicsofthree-phasesep-arator design. The authors provide a

Step-by-Step procedure and workedout examples. Further, the examplesoffer guidance n makingassumptionsfor the calculations.

Selectinghree.phare eparatoreAs with two-phase designs, three-

phaseunitscan be eitherverticalor hor-izontal,although hey typically arehori-zontal (seeFigures and2). The verti-cal orientation.Figure 1, s only used fthere is a large amount of vapor to beseparated rom a small amount of the

light and heavy liquid (< l0-207o byweight). Unfortunately, there are nosimple rules for separator selection.Sometimes,both configurationsshouldbe evaluated o decide which is moreeconomical. Further, the available plotspace footprint) may be a factor.

The designof three-phaseseparatorsis similar to their two-phasecounter-parts, except that the liquid sectiondif-fers. For the vertical type, a baffle com-monly keeps the liquid separation

r t -v T -

1,488g,fi(p, pr)( l )

18F

where 1,488converts iscosityof the

sectioncalm to promote the separation.There are different variations of hor-

izontal three-phasevapor-liquid separa-tors. The liquid separationsection isusuallya variationof a device o provideinterface level control, which mayincludea boot or a weir.A boot typical-ly is specifiedwhen he volumeof heavyliquid is not substantial < l5-207o oftotal liquid by weight), while a weir isused when the volume is substantial.Thesehorizontal separators re llustrat-ed in Figure 2. The bucket-and-weirtype design s usedwhen interface evelcontrol may be difficult, such as withheavy oils or when large amountsof anemulsionor a paraffinare present 1).

$tokos'law pplieeSeparatinga vapor from a light liquid

(two-phaseseparation)hasbeencoveredin a previous article (2) and will not bediscussed ere.However,all necessaryinformation for performing this part ofthe calculation s providedhere.The fol-lowing discussion overs he separationof light and heavy liquids.

The flow of rising light droplets inthe heavy liquid phaseor settling heavydroplets n the ight liquid phase s con- ,sidered laminar and is soverned bvStokes'law:

Heresastepwise

procedurefordesigning

liquid/Iiquid/vapor

separutors.

Wayne .Monnery ndWilliamY.Svrcek,

UniversityfCalgary

CHEMTcALNGTNEERTNGRoGREss SEpTEMBERgga . 29



M U L T I P H A S E F L O W

+ D -

1 - n .vent

H"l "rg"I

Baffle

HoldupLisht iq.

+r'-\l H '

lnterfaceheavy iquid

Vapornozzle

Wire mesh

Feednozzle

Maximuml i q u i dlevel

il - *A

Baffle o provide

ca lming one srecommended

SectionA-A

I

Heavyl iquidnozzle

^.[-Lioht r-t iq"uiOnozzle

_THr l

J

I Figure l.Vertical three-phaseseparalors are used with high

vapor loadings

is more diffi-

c u l t ( r e q u i r e s

more time) to set-

tle the droplets

out of the contin-uous phase with

the greater vis-

cosity,since U. is

lower. Practically

speaking, U, is

typically limited

i n c a l c u l a t i o n s

t o 1 0 i n . / m i n

maximum.

For vefiical sep-

arators, he diame-

ter required for

vapor disengage-ment is calculated

as in our previous

article (2). In siz-

ing a separator,he

heightsof the light

and heavy liquids

are assumed,and

the settlingveloci-

ties and settling

t i m e s a r e t h e n

calculated.

The residence imes of the light and

heavy iquids are determinednext.Forthe liquids to separate, he residence

time of the light liquid must be greater

than the time required for the heavy

droplets o settleout of the light liquid

phase; and the residence ime of the

heavy liquid must be greater han the

time required for the light liquid

droplets o rise out of the heavy iquid

phase.fthese conditions renot satis-

fied, then liquid

separation s con-

trolling and the

vessel diametermust be increased.

Holdup time for

liquids must be

added o residence ...

time. The height

of the vertical

three-phase sepa-

rator is calculated

in the same man-

ner as for-the two-

phase ase.

For horizontal separators with a

given diameter, he heights of the light

and heavy liquids are assumedso that

the cross-sectionalareacan be calcu-

lated. With the vapor disengagementarea set by guidelines, the lengths

required by holdup requirementsand

vapor/liquid separationare calculated.

Then, with the assumedheightsof the

light andheavy liquids and calculated

values of settling velocities, the set*

tling times are calculated.

The actual residence imes for the

light and heavy liquids are subse-

quently calculatedandcomparedwith

the required settling times. as in the

vertical case. If the residence times

are not greater than the required set-tling times, then either the diameter

should be increased or. for a given

diameter, the length should be

increased liquid separations con-

trolling). In the subsequentdesign

procedures, the laner approach is

used, along with the procedures dis-

cussed in our previous paper for

vapor/liquid separation 2).

The following desi-rn procedures

and heuristics are a result of a review

of literature sources and accepted

industrial design guidelines.Horizontal designproceduresarepre-

sented for the four separator types

shown in Figure l. The horizontal

design procedures incorporate opti-

mizing the diameter and length by

minimizing the approximate weight

of the shell and heads.To adda degree

of conservatism o the design, he vol-

ume available n the heads s ignored.

oi austi,or causiic

continuousphase rom lb/(ft)(s) to cP.

Simplifying Eq. 1 and convertingthe units of the terminal settling

velocity to in./min from fUs results n:

r r 2 .0615 l x l } - s | f ,QH-pL), r = T

(2)

whereDo is in microns 1 micron=

3.28084 106 eet) andU7, n./min.Eq.2 maybe rewrittenas:

r t kt(Pu - Pr)ur=---T

where

ks=2.0615tl \-sf i .

Values of ft, are given for some sys-

tems n Table 1.

From Eqs. 1-3, it can be seen hat

the settling velocity of a droplet is

inverselyproportional o the viscosi-

ty of the continuousphase.Hence, t

(3 )

Methylethyletonesec-ButyllcoholMethylsobutyletoneNonyl lcohol

FurfuralWaterWaterWaterWater

0is3'0.i630.1630.1630.163

1218989898g8989

30 . SEpTEMBER994 CHEMIcALNGTNEERTNGRocREss



3. Interface ontrolwith weir

Feedin let

l , v

Heavyl iquidoutlet

,* l N , l

' l^n.* * u,n. l

Lightl iquidoutlet

* L N I * -- l

2. InterJaceontrolwith boot 4. Bucket ndweir

Heavy Lightl iquid l iquidoutlet outlet

Heavy iquid

| - t - t - t - l

I L , l L z l L s l L 4 l

Note:N= 112 7,1+6in.

drv=Nozzle ia'

outlet

t - L -

Min.12 n.

Min.12 n.

Vapor

outlet

Light iquidholdu/surge

Vapor

outlet

I Figure 2. Basic designs of horizontal three-phase separators,

Vertical design.procedur€Refer to Figure 1 for dimensions:1. Calculate the verlical terminal

vapor velocity:

U- = ylPt - Pvr t t2 (4)Pv

Calculate he K value, using one ofthe methods n Thble 2 and set Uu =

A.l5Ur for a conservative esign.2. Calculate the vapor volumetric

flow rate:

Q v = 3,600pu(s)

internal. Calculate the vesseldiameter,Dur:

140-,\" 'D""= l# | (6 )

\nu,

If there is a mist eliminator, add 3-6in. Io D, to accommodatea supportring and round up to the next 6-in.

increment to obtain D; if there is nomist eliminator,D = Dur.

4. Calculate the setting velocity ofthe heavy iquid out of the light liquidusingStokes'law(themaximum s 10

in./min):

r t - k t (Pn -P r ) 0 \v H L -

l t r

whereft, is obtained rom Table 1 or is

calculated seeEq. 3) .

5. Similarly, calculate the risingvelocity of the light liquid out of theheavy liquid phaseusing Stokes' aw:

r r - k t ( P r - P t )u u.J

ILH

6. Calculate the light and heavyliquid volumetric flow rutes, Q* and

QnL:

w--o - . - " L L ( 9 )wrL_

60pL

n - W r tunr- 60pa

(8)

WV(10)

7. Assume Hr = I ft (minimum)

and calculate the settling time for the

CHEMTCALENGTNEERTNGpRocREsssEprEMBERrggr31



M U L T I P H A S E

"=,{*t#d)where " s n t.

Note: micron3.2808410{ t

heavy iquid droplets o settle hrough

thisdistance 12 s a conversionactor

for ft to in.):

g. Use Table 3 to determineAo lA.

h. CalculateA= (nl4)D2.

i. CalculateAr.j. Select he argervalueofAr.

k. Calculate the area of the baffle

plate - settling areafor the light liq-

u i d ; A . - A - A o .

10. Calculate the residence ime

of eachphasebasedon the volumes

occupied by the light and heavY

phases:

basedon the requiredholduP

( 1 1 )

8. Assume Hn = | ft (minimum)

and calculatethe settling time for the

light liquid droplets to rise through

this distance:

LzH,(12)

t - ---:,LH -

TIu L H

9. If there is a baffle Plate, calcu-

late the area:

a. Calculate P. - P).b. Assume11*(use9 in. as a mini-

mum) and calculateHt+ H^.

c. Use Figure 3 to obtain G.

d. CalculateAr:

SeeEq. 3) in thebox.

e. Assume Wo = 4 in.

f. CalculateWD/D.

If eLL< tu, or Iur 1 trr, \tctease

the diameterandrepeat he procedure

from Step7 (liquid separation s con-

trolling).Note thatAn= A.

11. Calculate the height of the

light liquid above the outlet (holdup

(1s)

Checkthis r-alueu'ith that assumed

in Step9b to ensure hat the assumed

value is reasonable.ft- surge is not

specified, calculate the surge height

basedon surge ime:

(eu*eo)r,Hs=\---TL

(16)

The minimum is 6 in.

12. Calculate the vessel height

using he guidelines:

He= 6 in. minimum.

H aio h d. + greaterof (2 ft or /1t

+ 0.5 0.

Ho=0.5D or a minimumof :

3 6 i n . + V z d r w i t h o u t m i s t

eliminator),or

24 in.+ t/zd* lwith misteliminator):

H.=+l2H,

t - ----------=,HL_U, ,

s*=w (I4a)

(14b) - HuAuvHL-

Qr t

^ (t.+aeal\/oo,in{0- 0rl\a,=

l-r,

I--T-/\----G-/) Equation13

Ur=0'313

I EquationEl

=2.14ftJs

Q v =415.000lb/h =165.32ft' ls

3.600 /h 0.69731VfflEquation E2

^ 1TVT4STFIs -r-\o=V --irISHs - r0.13tlEquation E3

16.5000lb/hou -

60 minthx 53.95btftt

a EquationE4

= 5.10 t'lmin

l,3o0lb/hvnr=ffi

lEquation E5

= 0.35 t'lmin

53.95 0.6973

32, sEprEMBERlggdCHEMICALENGINEERINGPRoGRESS



10,000

9,000

8,000

2. Calculatehe rapor ' r r lurr - t r i "

flow rate,seeEq. El in thebor.

3. Calculatehe vesselnnerdiam-

eter, eeEq. E3 in thebox.Use1J= 10.5 i.4 . C a l c u l a t eh c : e t t l i n ge l o c i l r r l

theheavy iquid out of the ight iqLridphase.Using Table l , f ts = 0.163Then:

Um = 0.163(62.11 53.95)/0.630= 2.11 n. /min

5. Calculatehe sett l ing elocitl of

the ight iquid out of theheavy iquiclphase:

U ra= 0 .163(62 .1153 .95 ) /0 .761= l.l4 tn.lmin

6. Calculate he l ight and hearIiquid volumetric flow rates.seeEq.E4 andE5 in the box.

7. AssumeHr= 1ft andcalculatethe tirne fbr the heavy iquid to sertleout of the ight iquid phase:

tn , .= 12) 1 .0 )2 . l1 = 5 .7mi n8. AssumeHn = 1 fi andcalculate

the ime lbr thelight liquid to

rise out of theheavy liquidphase:

t r H = ( 1 2 ) ( 1 . 0 )

l l . l J = 6 . 9 m i n

9. Calculatei i l i t i - : , r - r ' : '1 : r i - i1 f - ;1:

r _ r _ i l , ) < r l n t l l l _ i l ) S

lL. t '. . \ : > . r1 . ' t - = I ) i n . .Hr+ Hn= 24

in. L ' . : rg Fr ,rLrre1.G = 9.800 ph/f tz.{ . =

- j r_cal i i t ' r r60 inih) 5.10

- o.-1-iii ' 9..\00gph/iir = 0.25 t2. . \ . . . l l t i l l = J j p . .

\ \ . ,D -1 l r1 l 10 .5) 0 .0317

.an, =Hn+ H, +HR+O^ Ufii

It ' a mist eliminatorpad is used,-,-riional eight s added sshown n

: - - l r e .

Example

Sizea verticalseparator ith a baf'-' : : l l tc andwire-meshmist elimina-: i,.rseparatehe mixture given in

. -,rleJ. Theoperating ressures 165'.., : , rrild t is necessaryo have a

Jr'(rcarboniquid holdup ime of 25:: and a surge ime of only 5 min.1. Calculate he vertical erminal

- .iitr '. Using Table 2, calculateK- ' i t he York Demis te r q r r r t i ons .

. . . Eq .E1 n thebox ., :nJL'1= 0.75 2.14= 2.05 t ls.

co_

oFc

-

-oG

G

r. i

24 in .

/= 4.0184a8

. = _J.9 t64 t

= - i .801705

= l 1.i5348

t. \, to H/D+= HID= \/.\r

= 0.00153756' = 16.787101= 1.299201

: - i.2.923932

. = 1.1.353518'= 11.844824

- = -36.999376

: = 10.529572: = 9.892851

:.luivalentxpressions,uch sH',/D

prpu,lblft3

^=#6ft3ls

60,fF;

1oa,r= r+A,

Q , - r , .T t : 2

- v u :

4u ,

High iquid evelabove nterface

I Figure 3. G is found rom the downconterallowable low.

CHEMICALNGINEERINGROGRESSSEPTEMBER994 33




Experienced 1.0

Trained 1.2

Inexperienced 1.5

Well nstrumented 1. 0

Standardnstrumented 1.2

Poorlvnstrumented 1. 5

F Fuel as nock-outrum

ZOt.ilug n he ncominguelgas ine etween Ltand igh evel hutdown

G.Flare nock-outru m20 o 30min o Hll

Mult ip ly y he ol lowingactorsoptionall:

Personnel Factor Instrumentation Factor

. Using Table 3, AD/A= 0.0095:

1= (nl4)(10.5 t)2= 86.59 tz

A, = (0.0095) 86.59 t2)= g.g21r2

. UseA, = 0.82 ft2.

A, = 86.59 0.82 = 85.11 t210. Calculate he residence ime of

eachphase:

0r. = (1.0 fI) (85.77 112115.10

ft3lmin= 16.8min

lnr = (1.0 f0 (86.59 ftz)10.35

ft3lmin = 241 4 min

11. Calculate he height of the light

liquid above the outlet, based on

holdup:

tlo = (5.10 t3imin)(25 min)/85.77

ftz = 1.5 t

/1s = (5.10 + 0.35)(ft3/min) 5

min)i86.59 t2 = 0.31 tUse I/r = 0.5 ft .

12. Calculate d, according to

Table5:

)"= Qrl(Qr + Q) = (5.10 +

0.35Y(5.10 0.35+ 165.32 60) =

0.0006

Use Eq. E6 (seebox) to calculate

Prpu = pLX+ pll - l.) = (54.55)

(0.0006)+ (0.6913) (l - 0.0006) =

0.730

Qu = 165.32+ (5.10+ 0.35)/60=

165.4I t3/sUseEq. E7 (seebox) to calculated*

dN > 2l in.; use dt = 24 in.

Calculate Ho'.

Ho = 0.5 (10.5)= 5.25 t or

Ho = 24 + 2412= 36 in. = 3.0 ft(minimum)

Use11,= 5.5 ft. From Figure 1, 1t= I ft ands = 0.5 ft. Calculate lur:

Hsw='/,(2.0 f0 + 2 ft = 3 ft

Set11, = 0.5 ft. Final dimensions:

D = 10.5 t, HH= I.0 ft^ HL = 1.0 t,

Hn= 1.5 r, Ho = 0.5 ft. HB.,= 3.0 t,

andHo = 5.5ft. Add 1.5 t for the misteliminator.

Hr= l4'0 ft

H I D = 1 4 . 0 i 1 0 . 5 = 1 . 3

Add 2 ft to H, (Hp = 2.0 ft, Ho =

7.0 f0 so that HlD = 1.52 (HlD

shouldbe n the rangeof 1.5 o 6.0).

Horizontaldesignprocedure:no boot or weir

1. Calculate the vapor volumetric

flow rate,Qu,usingEq. 5.

p,=## x53.es+ffi x62.It 54.55btft3

l EquationE6

)'''= .rr ,

I EquationE7

34 . SEpTEMBER994 CHEMTCALNGTNEERTNGRocREss



2. Calculate the light and heavyliquid volumetric flow rates, Q' and

O711,sing Eqs.9 and 10.

3. Calculate the vertical terminal

velocity,Ur, using Eq. 4. (selecta Kr alue from Table 2) and set Uu =

0.75Ur.

4. Select holdup and surge timest-romTable 6 and calculate he holdupandsurge olumes,V, atd Vr, (unless

surge s otherwisespecified, uchas a>lusvolume):

'= - " {trrQ* tntQ,

e * ' ( h - A v - o * )

Qrr=I A , - A , - A u , l L 1 ? q r\ ' ' . 1 \ . / |

5. Obtain an UD from Table 7 andinitially calculate the diameteraccordingo:

(20)

Calculate the total cross-sectionalilea:

(2r)6. Set the vapor space height, 11,

, rhe larger of 0.2D or 2 ft; I ft if:.-,eres no mist eliminator. Using1 ,/D n Table3, obtainAr/A, andcal---llateAu .

7. Set he heightsof the heavy and

;ht liquids,Hrrand Hr,8. Find (AnL+ AL)lAr, using (I1",

- fl -tlD inTable3,andcalculateA*

9. Calculate he minimum length to

-- -,rmmodatehe iquid holdup/surge:

r _L _

V, +V,(22)

10. Calculate the liquid dropout-:a:

I = H,./L't Q3)

| [. Ca]culate the actual vapor

velocity:

Uw= Qr/Av( ) L \

12. Calculate he minimum lengthrequiredfor vaporiliquid separation:

Lum= UvlQ (2s)

13. If L 1 Lr,,,r, then set L = Lr,*(here, vapor/liquid separationcon-trols). This simply results in someextra holdup and residence ime. If I11 Lr,*, then increaseHu and recal-culate Au, and repeat, starting fromStep 9. If L > L*n, the design isacceptable or vapor/liquid separa-tion. If L )) Lr,,., liquid holdupcon-trols). can onlr,be reducedand .rrr.increased f H,, is reduced.F1, ma1'only be reducedf it is _ureaterhan heminimum specified n Step 6. (With

reduced 11* recalculateAu and repeattheprocedurerom Step9.) Note: Forthis and other calculations, "much

greater than" (>>) and "much lessthan" (<<) meana varianceof greater

than 20Vo.

14. Calculate he settling velocitiesof the heavy iquid out of the light liq-uid phaseand he ight liquid out of theheavy iquid phase,U". and U.o, usingEqs.7 and 8 (find ft, from Thble 1).

15. Calculate he settling imes ofthe heavy iquid out of the ight phase

and the light liquid out of the heavyphase:

tur= 12 (D - Hv- HH)IUHL

tro= 12 HHL/ULH

16. Calculate he residenceof the light and heavy iquids:

0ur= AorLlQu,

(26)

(21)

times

(28)

17.If gHL< tL Hincrease he vessel

arationcontrols):

Q,,

or 0,-, < t", thenlength(liquid sep-

(30)

18. CalculateUD.If UD << 1.5,decrease (unless t is alreadyat its

minimum), and if UD >> 6.0 thenincreaseD; repeat rom Step5.

19. Calculate the thickness of theshell and headsaccording to Table 8.

20. Calculate surface area of theshell and headsaccording o Table8.

21.Calculateheapproximatees-sel weight according o Table8.

22. Increaseor decrease he vesseldiameter by 6-in. increments an drepeat the calculations :until the UDratio ran-eesrom 1.5-6.0.

23. Using the optimum vesselsize

(minimum wei-eht). alculate he nor-mal andhigh liquid levels:

Hau= D - H, (3 )

Anrr= (Am+ Arr) + Vo/L (32)

Obtain 11r.. using Table 3 with thevalue of A*ro/Ar.

Horizontaldesignprocedure:heavy iquid boot

1. Calculate the vapor volumetric

flow rate,Q, usingEq. 5.2. Calcrilate the light and heavy

liquid volumetric flow rates, Q* and

Qnr,perEqs.9 and 10.

3. Calculate the vertical terminalvelocity, Ur, using Eq. 4 (the K value

comes from Table 2) and set Uu =

0.15 Ur.4. Select holdup and surge times

from Table 6 and calculate he holdupand surge volumes, V, and Vr, fromEqs. 18 and 19 (unlesssurge s other-

Vo= To Q,

V s = T s Q r

(18)

( 1 9 )

A _ TED'

4

t r -Au_ (our* ,4 r r )

CHEMTcALNGTNEERTNGRocREss SEpTEMBERgs+ . 35




wisespecified, uchas slug volume).

5. Obtain UD from Table 7 and

initially set he diameteraccording o:

/ ? \

Then calculate the total cross-sec-

tional area,A, usingEq. 21.

6. Set the Yapor spaceheight, F1*

to the larger of 0.2D or 2 ft (l ft if

there is no mist eliminator). Using

HutD in Table 3, obtainAv/Ar andcal-

culateAu.

7. Set he light liquid heights n the

vesseland boot, HrN andHrrt.8. Calculate the cross-sectional

areaof the light liquid above he bot-

tom of the vessel,Ar., using Hrtt/D

in Table 3.

9. Calculate he minimum length to

accommodatehe liquid holdup/surge:

'=(t+

(,q,,- ,)rou=

eu(36)

DishedHeads

Approximate

(34)

10. Calculate the liquid droPout

time, $, usingEq. 23.

1L. Calculate the actual vaPorvelocity,Uuo, singEq.24.

12. Calculate he minimum length

required or liquid/vaporseparation.

Lrr*, usingBq.25.

13. If L 1 Lr,*, then set L = Lr,*

(vapor/liquid separation controls).

This simply results in some extra

holdup and residence time. If I

11Lr,*, then increaseH, and ecalcu-

lateAr, then epeat rom Step9.If L>

Lr,*, the design is accePtable for

vapor/liquid separation. f L >> LMrN,

liquid holdup controls. I can only bereduced atd L*,* increased f I1u is

reduced.Humay only be reduced f it

is greater han the minimum specified

in Step6.

With reduced Hu, recalailate A,

and repeat rom Step 9.

14. Calculate the settling velocity

of the heavy iquid out of the light liq-

uid phase, U"., using Eq. 7 (obtain ks

from Table1).

15. Calculate the settling time of

V., +V"f - n o

" - A r - A u - A r N

the heavy iquid out of the light liquid

phase:

tm= 12 (Hrrn + D - H)IUHL Q5)

16. Calculate he residence ime of

the ight liquid:

19. Calculate the thickness of the

shell and headsaccordingto Table 8.

20. Calculate the surface area ofthe shell and headsaccording o Table

8 .

21. Calculate the aPProximate

weight of the shell and headsaccord-

ing to Table 8.

22.Increaseor decreasehe vessel

diameter by 6-in. increments and

repeat the calculations until 1,/D

ranges rom 1.5-6.0.

23. With the optimum vessel size

(minimum weight), calculatethe nor-

mal andhigh liquid levels:

Hnu= D - Hu (38)

Attrr= Arru + VrlL (39)

DetermineH*rrusingTable 3 from

A*r/Ar.

24. Design the heavy liquid boot:

Set the height of the heavY liquid,

Hur; calculate the rising velocity of

the light liquid out of the heavy iquid

phase,Uru, using Eq. 8 (find ft, from

Note: This volume of light liquid

ignores the light liquid volume in the

boot.

I7. If 1LL< to, then increasethevessel ength (liquid separation on -

trols):

(37)

18. CalculateUD.If UD << 1. 5

then decreaseD (unless t is alreadyat

a minimum) and if UD >> 6.0 then

increaseD; rcpeat rom SteP5.

36 . SEpTEMBER994 cHEMICALNGINEERINGRocREss



Table 1); set U, - 0.15 ULHicalculatethe heavy liquid boot diameter:

Then calculate he settling time of thelight liquid out of the heavy liquidphase:

ttr= l2Hu1/Us1

Calculate he residence ime of theheavy iquid:

the light liquid comparlment usingEq. 44 or read t from Table 9.

(4r) Htu= 0.5D + 1 (44)

whereD is in feet andHrrrin inch-es (roundup to nearestn.). If D < 4.0ft, then Hrm= 9 in. Using HoytD inTable 3, CalculateArrr.

8. Calculate he weir heisht:

tru= l2Hrx/Uro (4e)

14. Calculate minimum I, to facil-itate liquid-liquid separation as thelarser of:

A -ndH*"ut - 4en

( ,,o-,=maxl : f f\ n ur

trrQrr\ (50),A,, I

If eHr < tru, thendiameter.

(42)

the boot

Horizontaldesignprooedure:weir

1. Calculate the vapor volumetricflow rate,Qu,using Eq. 5.

2. Calculate the light and heavl'liquid volumetric flow rates,Qu and

Q11y.aserEqs.9 and 10.

3. Calculate the vertical terminalvaporvelocity, Ur, using Eq. 4 (find Ktiom Table2) and set Uv = 0.l5Ur.

4. Select holdup and surge timestiom Table6, and calculate he holdupand surge volumes, V, and V., fromEqs.18 and 19 (unless urge s other-ir sespecified, uchas a slugvolume).

5. Obtain UD from Table 7 andinitially calculate the diameteraccordingo:

Then calculate he total cross-section-

" larea. . . usingEq.2l .

6. Set he vaporspace eight.Hu .ro the larger of 0.2D or 2 ft (1 ft iflhere is no mist eliminator). UsingH../D in Table3, obtainAu/A, and cal-;ulateAu .

H w = D - H v (4s)

If Hw< 2 ft, increaseD, and repeatthe calculationsrom Step6.

9. Calculate he minimum lengthof the light liquid compartment toaccommodate holdup/surge, 1,. inFigure 2 :

V n + V ,Ar - A r . - AuL

Round to the nearest 7: ft. Theminimum for Lr= d, + 12 in.

10. Set the interface at the heightH,*12, obtaining the heights of theheavy and ight liquids, Hrrand Hrr.

11. For the liquid settling com-partment,calculate he cross-section-al area of the heavy liquid, usingHr/D in Table 3 and calculate thecross-sectionalreaof the ight liquidfrom:

(43) Ar r= Ar - A , - A* (4',7)

12. Calculate the settling velocityof the heavy iquid out of the light liq-uid phase,Uur, and he light liquid outof the heavy liquid phase,U"r, usingEqs 7 and 8 (find ft, from Table 1).

13. Calculate the settling times ofthe heavy iquid out of the light liquidphase and the light liquid out of theheavy iquid phase:

tur= l2Hr1/U*

L1

Round to the nearest% ft.15. Find l,:

L = L t + L z (s1)

16. Calculate the liquid dropouttime, Q,using Eq. 23.

17. Calculate the actual vaporvelocity.U,,.,. singEq.24.

18. Calculate he minimum lengthrequired for vapor/fliquid separation,

1.,rr,.. sing Eq. 25.19. lf L ( Zr'.r- then set L = L*r

(vapor/liquid separation controls).This simply results in some extraholdup and residence time. If L <<Lr,*, then increase Hr, recalatlate A,and repeat he calculations from Step6. If L > L*,r, the design s acceptablefor vapor/liquid separation. If Z >>.Lrr, (liquid separation and holdupcontrol), Z can only be reducedandLr,* increased f Hv is reduced. //umay only be reduced if it is greater

than the minimum specifled n Step 9.With reduced Hr, recalculate A, andrepeat rom Step9.

20. CalculateUD.If UD << 1.5,then decreaseD (unless t is already ata minimum) and repeat from Step 6.If UD >> 6.0, then increase D andrepeat rom Step 5.

21. Calculate he thicknessof theshell and headsaccording to Table 8.

22. Calculate the surfacearea of theshell and headsaccordins to Table 8.

(-+6)

(48). Calculate he low liouid level

CHEMTCALNGTNEERTNGRoGREss SEpTEMBER994 37




23. Calculate he approximate ves-

sel weight according to Table 8.

24. Increaseor decrease he diam-

eter by 6-in. incrementsand repeat he

calculations untiT UD ranges from

1.5-6.0.

25. With the optimum vessel size

(minimum weight), calculatenormal

and high liquid levels:

Hnu=D-H , (52 )

Atrrr= Arrr+ V,1L, (53)

Obtain Hr' using Table 3 with

ANLL|Ar.

Horizontaldesignprocedure:bucket and weir

1. Calculate he vapor volumetric

flow rate,Qu,usingEq. 5.

2. Calculate the light and heavy

liquid volumetric flow rates,Q, and

Qnr,perEqs.9 and 10.

3. Calculate the vertical terminal

vapor velocity, Ur, using Eq. 4 (flnd K

from Table2) and set Uv= 0.75 Ur.

4. Select esidence imes for light

and heavy liquids, 0r, and 0o.. For

sour water stripper feed drums, 0o. =

there is no mist eliminator). Using

Hr/D inTable 3, obtainAu/Arand cal-

culateAu.

7. Calculate ,:

(o,e,,+o-,e-,1, ' . ' ' . ]

60 min for reflnery service,or 10-15

min for chemical-plantservice.For

amine regeneratoreed drums, 0o. =

10-15 min.5. Obtain UD from Table 7 and

initially set he diameteraccording o:

(s4)

0J0n(r1r)

Then calculate he total cross-section-

al area. . usingEq.21

6. Set the vapor spaceheight, 11u,

to the larger of 0.2D or 2 ft. (1 ft if

, = (+(e,,0,,re,,,O,,

f -L l -

1 5 5 )

Ar-4,

8. Calculate the liquid dropout

time,Q,usingEq. 23.

9. Calculate he actualvaporveloc-

Uuo, singEq.24.

10. Calculate the minimum length

required for vapor/liquid separation,

L*,r, using Eq. 25

ll.If Ll 1 Lr,*, then set Lt = Luru

(vapor/liquid separation controls).This simply results in some extra

holdup and residence ime. lf Lt <<

LM,N, h.enncrease lr,. recalculateAu

and repeat he calculations from Step

1. If Lt ) Lr,*, the design s accept-

able or vapor/liquid eparation.

12. Calculate he light liquid layer

thicknessbasedon the heavy liquid

settlinsout:

g,,= 235'ooolblh =343.5'7t3/s3.600 /hx 0.190b/ff

I EquationE8

o,,=45'ooorblh -=18.52ft'lmin

60min/h 40.5 b/ft'I Equation E9

o-, = 7'5oo b/h =2.y2ft'lmin60 min/hx62.0lblfr'

I Equation810

Ur=0'175

I Equation811

, , - ( 4r27r .80 " ' ' -\0.6ru t.7xr/i)

= 11'15t'use1'0 t

I EquationE12

4 = f f i = 1 4 . 5 4 f t- Equation813

tutQrt - 2.0 min x2.02ft3hn1n=0.45 tA,,

I Equation814

s.% t

trrQo - 2.0 min x 18.52 t3lmin =2.41 tALL

I Equation E15

r r Q uure=T

=

I Equation816

rs.02ff

343.51f ls = 4.83 tls7r.08e

38 . SEPTEMBER994 CHEMICALNGINEERINGROGRESS



prop€r-

engli

l ne

w,D,tlni

of Western

degreesn

u 0.00128e'(AsG)Dprr i6 ,V,'

,.hereD" is in microns.13. Calculate the differenc- rl

:rsht between he light and he;:,,.,quidweirs:

i o . r :_ \ H = H r r l l - X l- \

Pu

1-1. esign he ight iquidbucker:': the op of light l iquid u eir '= D

, lssumehe bottonr s at 0.1l5Dl. . r t t ie a holdup/surge t1,prcal l r .' . -i min.); assumeHLL is 6 in., rrl the weir height andLLL is 6 in.

i e the bottom of the bucket.. r,c Table 3 with HrrrlD and

D. calculate Au' and Arr.r.-ulateL,:

:,=

il;::T:1",,-*lllllJli,'

^- =

:#, ?:light

l iquid bor.eesselor -

l:==,1'l

:-i;iliill,1l11;1",,'7\=il,,l,l,1l;;,'.;l:l'"'.'"

D = resseliemeler."r ' l lu'

3' = lJli';,il ,. rmicronsD , = r3prlr 'J isengagementia. . r

t =

; : : ]o.oLoint

eff ic iency. imension-

-' = cfa\ i tar ional onstant, 2.17 r , /sr

6 = baffle iquid toad.gph/fr)

H = heighr. t

H.. = liquid level above baffle, in. or ft

HB,. = liquid height from abor.ebaffle to

leed nozzle. t

H;, = disengagement eight. i

H. = htrldupheisht. i

H- = tei ,sht r . .= l i ; . : : i : : : : :=: : i i l i

: , z z : : . '

0 = volumetr ic iow. l r /sor l t r /min

S - vesselml ler ia l stress alue. s i

ASu = 5psqif gravity differenceberween

Nomenclbture

) l r ' - : -

li. l';L

;JL:

L, L;

LL L

1/

.\ I

I

TC

TH

tut

ttu

1s

TH

TS

Y:,HL

u,.,ou to f heavy iqu id , n . im in

Un = mixture velocity, tVs

Up = boot velocitr'. n.lmin

f = l c f rn i l r J lg l 6 . 1_1 .i . ' . r i n . / r r i n

L = \ i-Lpi \rel . rc i t r . Ll :

,a ,

= - - - . - . - : . : . : '

= : : - : : a i l r denaa t ime. min

- ::'.:::,i:. liquid ifact ion

= l \ : . : : : \ . - P

= : : : . : : : . l b r ' f t ]

= : , : : i r L r l ou i t ime- s

I ighrandheavy iquidr

= '*ftJil],."',0,'o,"0,",,out ol l ight iquid 'min

rprels ul iiii,r;:i::';i'J'

*'=;5.,ff::;l= \u rge rmr .m in .

= al lor lable or izorrnl c loci t l . rl s

= se t t l i nqe loc i t v f he rv r i ou id

dropleis Lrr f l ighr iquid. n.tmin

= ris ingveloci t)of l ighr iquiddroplers

T _ lro+r,)9,,(58)

(Ar,'r_Arr,')

\ssume1., s the argerof D/l2i n .

Design he heavy iquid com-rnt: Set he top of theheavy iq-. - i r '= D - Hv- A11; ssume:r \Lrr_setypically, 5-15 min);: HLL is about6 in. below the:r,ght and LLL is about 6 in..h- bottomof thevessel. sing: v,trh Hrrr/D andHr,,y/D, ca7-i . ..andAr_r...- LLl . i te ., :

i: -.

H,, ;

H , , -

H,. .

H,,,u

HP.

Hsg

n v

H,,,

fts

l

K

L

L*,*

;'':liilr,r-Hj

- - - : - : - - - . . ' -_ - .

= :: : : : : : : - : : : : ,: -= heiSi:t i,'::r iiir:

fle. 11

= i r r r oe he i ohr r i

= totai verticalsepararor eight. i

= vapordisengagement reaheighi. r

= weir height. ft

= heighldrf lerence el i ieen ighi un J

heavy iquid weirs. n.

= Stokes' au. erminal elocirr con-

srant. n. /min)r Pr i t lb/ fr

= terminal eloci tyconsLanl.r , is

= vessel engrh. t

= vapor,4iquideparar ion irrrrnum: .

l

t 6 ,

I - , ; i

: . : : : l ; ; i r i , l ;evei

i : i : i . ; ; i r i t i

: , , . r r q l i i i e re l

r - : . I l i . ' , ' i , l l - . ^ l

' , : a . t I

I _(r, + rr)9,,

(5e)

lAorr- Arrr)

17.Calculate = L, + Lr+ L. + L^.18. Calculate D.I f UD << 1.5.

thendecre seD trnd epeat i'onlStep5 Ii UD >> 6.0. hen ncrease an drepeat i'omStep 5.

19. Calculate he thickness f th eshel1 ndheads ccordingo Table8.

20. Calculate l're surf'ace rea of

cHEMTCALNGTNEERTNGRocnEss sEpTEMBER994 39




shell and headsaccording o Table8.21. Calculatehe approximate es -

sel weight according to Table 8.

22. Increaseor decrease he diam-eter by 6-in. incrementsand repeat he

calculations until UD ranges fromt.5-6.0.

Example2Design a three-phasehorizontal

separatorwith a weir to separate hemixture in Table 10. The operatingpressureand temperature are 25 psig

and 100, espectively, nd t is neces-sary to havea liquid holdup and surgetime of 15 min.

1. SeeEq. E8, box, p. 382. SeeEqs.E9 andE10,box,p. 38

3. K = 0.175 (the Gas Processors

Suppliers'Associationvalue n Table

2 was divided by 2 since here s no

mist eliminator).

SeeEq.E11,box,p. 38

Uv=0 .75x2 .55 = 1 .91 t l s4. Holdup + surgeas specified= 15

min.

Vu+ V, = (15 min) (18.52 t3lmin)= 211.80 t3

Assume 10 min holdup, 5 minsurge.

5. Assume D = l. l .

SeeEq. El2, box,p. 38Ar=n/4 (11.0 | 'z = 95.03 t2

6. Since the mass ate of vaoor is

where : = des ignressurendD= drum ia ,

about 27o f the oading, etF1yo bemuch greater han the minimum.Assume v = 0.70D (0.70)(1.0 0= 7.70 ft. UsingTable3, Av/Ar =

0.148, v= 71.08t27,Hrr r= 0.5) (11.0)7 =I2 .5 in . ,

use 3 n.Hrr/D = 13/(11.0 12)= 0.098UsingTable ,ALLrj/Ar 0.051Aur= (0.051X95.03P)= 4.95112

8. H, = 11.0 1 10= 3.30 t9. SeeEq.El3, box,p. 38Use ,r = 15.0 t.10.HHL- Hrr= 3.3012 1.65 tll. HH//D= 1.65111.00.150FromTable , AH/A, = 0.094Aur= (0.094X95.03t2)= 8.93 t2Au=95.03 71.08 8.93 15.02t l12.FromTable1, ks= 0.333Uur= (0.333)(62.040.5)/0.24

29.83n./minUse10 n./min maximum)Uro= (0.333)(62.040.5)10.682

10.50n./minUse10 n./min maximum)13, tHL= Q2 in./ft)(1.65 t)ilo

in./min= 1.98min, use2.0minstr, = tnr = 2.0 min14. SeeEqs.E14 andE15.box,

p .38Use r = 3.0 t.15 . = 3.0+ 15.0 18.0t16.0 = '7.10

ftll.gI ftls= 4.03s17.See q.El6,box. .38

18. LurN = (4.83 ftls)(4.03 s ) =

19.5 r

19. SinceL 1 Lr,*, set L= 19.5 t(setZ, = 16.0 t, lr = 3.5 t)

20. UD = 19.5/11.0 L78

21. Assume dished heads per

Table11 .

AssumeE = 0.85

Use SA-516 70 carbon steel,design emperature= 650o

S = 17,500psi; from Ref. (3).Corrosionallowance= Vrs n.

P = 2 5 + 3 0 = 5 5 p s i g

SeeEq. E17,box on thispage.

Use r, - 7s n.

SeeEq. E18, box on this page.

Use /" = Vz in; use = Vz n.

22. As = ?I(11.0 0 (19.5 f0 =

673.81 tz

Au= (0.842) 11.0 O'z 101.88 t2

23. SeeEq. E19,box on thispage.

24. In this example, calculationswere performed for only one diame-

ter. However, nearly the minimumUD corcesponded o a diameter of

11.0 fU therefore. he next diameter

should be smaller, resulting in a

laryerUD. Also, calculationsshould

be performed using a diameter of

I 1.5 t .

25. For the light liquid compartment:

Hnrr= Hw= 3'3 ft - 3 ft, 4 in'

AxLr=4.85 185.20116.016.43fP

A*t/Ar= 16.43/95.03 0.713

Using Table 3, HNLrt/D 0.229

Htrrr= (0.229) 11.0)= 2.52 t - 2

ft, 6 in.HLLL=13 n'

Comment: Due to the smal l

amount of heavy liquid and large

amount of vapor, a better design

would have used a boot. A vertical

vessel should be comoared. as

we11.

To eceive freecopyof his article,

send n heReadernquiry ard n his

issue ith henumber 53 ircled,

@

1 _ 55x I32

" - 2 x l 7 M*l|rc=0.307 n.

I Equation 817

r . . = , =0.- -8=81l f 1?? -- *r ,160.495n.H2x 17.5000.850.1x 55

I EquationE18

t =ff' ltff#(on.at

rt22x 0r.88')= 7.s20tbI EquationE19

40 . sEpTEMBER994 CHEMTCALNGTNEERTNGRocnEss

Documents

Successfully Specify Three Phase Separators 2