Journal 01 Sr ir-nuf.c &. Industrial ResearchV0J. 68. October 2()()9. pp. R71-n:,

Entrainer for batch distillation of acetic acid -water system

SV Gadek ar '. R v N~ik~and S N Kaulh

'Department 01 Chemical Engineering. Bharati Vidyapceth University College ur EnglJ1eenng. Punr-. India'Nalional Chemical Laboratory. Pune.Tndia

'National Environmental Engineering Research Institute. lag rUT. India

Received 19 Februarv 2008: revised N June 2009: accepted 21 July 21/0Y

Acetic acid-water sysre m (AW) poses a challc nge in rcactiv e disri llation. acetic acid purity and loss of accnc acid in aqueoussolutions. Feasible separation or AW has been studied usin~ emrainer s but acetic acid loss in lean phase cannot he avoided. Thisstudy presents feasible rq:ion 10 operate column with toluene and benzene as entruincr for AW. Liquid -Iiqurd equilibrium data andnature of lie line for eihv 1 acetate are given pnoriry.

Introduction

Under batch distillation. to reduce further rccovervof pure components. products are highly dependent oninitial mixture compositions. Use of batch rectifier andstripper for hcieroazcotropic systems has gainedpopularity'. Hillman: studied many process alternativesto achieve successful batch distillation operations.Distillation of heterogeneous mixture. which requiredecanter for handling liquid -liquid phase split, differsfrom ideal systems. Vapor condenses and condensatecollects in decanter as entrainer rich phase and entrainerlean phase. Options are reported"? for sending entrainerrich phase to column. Application of entrainer hassmoothened process of complex heterogeneous systems.Efficiency of azeotropic batch distillation depends largcl~upon choice of suitable entrainer for the system.

Acetic acid-water (AW) system was studied withcertain entrainer on lab scale in batch mode of operationand showed: a) decrease in loss of acetic acid fromaqueous layer of distillate: and b) reduced reaction timeto complete equilibrium controlled reaction with hi,!,'hconversion for a particular configuration or banhdistillation assembly. Parameters (role of reflux.modeof operation. ernraincr inventory. and feed position I [or

• Author for correspondenceTel: 91 -020-30273670: Fax: Y 1-020-254427-70Evrnail: principal_miICtlc@.milpune.com

feasible separation have been reported". Simulations havebeen done to find out suitable cnrrainer for AVril.

This study presents toluene. benzene and ethyl acetatea~ an entrainer for A \\1 to reduce amount of acetic acidloss in aqueous phase of distillate using Residue CurveMap (ReM).

Experimental

Residue Curve Map (RCM)

RCM'] is 3 geometrical tool to find feasible separation.column sequencing and optimum parameters for distillationoperations. RCM are ternary diagrams displaying tracesof liquid compositions remaining in a single batch still a" aresult of vaporization over a period of time. These liquidtraces are known as residue curves. According to RCMtechnique". any ternary diagram is divided into nodes(stable and unstable). saddles and distillation regions. Astable node is highest boiling vertex in a distillation region.All residue curves in a distillation region point towards astable node. An unstable node is generally lowest boilingvertex in any distillation region. where all residue curveswill originate from an unstable vertex. A saddle point. anintermediate point in a distillation region. has neither residuecurves coming or g0ing through it. Residue curve runsfrom unstable node 10 stable node via a saddle point.Distillation boundary is a hazy line joining a stable node toazeotrope present in system ()f formed by joining two

872 .I SCl lND RES \'01. 611 OCTOBER :n09

------ --~----.-Table 1- LL[ daia Ior AWT system" (experime rual ne

lines In rnoleor 31 <lwe)

SN("I Left phase Righi phase-.- --------

Water Arctic acid Water Acetic acid

95.371 ".~74 0.507 J.369

: 90.923 8.959 0.753 3.46:-

3 88.869 10.982 0997 ·LS6'.. R5.R59 I~ 959 1.232 7687

5 R2.310 17.468 1.469 8.816I> 79.24:'- ::041>6 1.687 12.72::7 72.854 26.519 :.129 16.675

8 62.796 35.572 2.775 21091

9 51U20 39517 4.066 24.126

azeoiropes in the system. It divides ternary plot intodistillation regions.

System was studied properly by vapor liquidequilibrium data for AW using toluene and benzene asentrainer. Tie line was compared for A\V with toluene.benzene and ethyl acetate.

Results and DiscussionJ. Acetic Acid -Water -Toluene (AWT) System

Liquid-liquid equilibrium (LLE) data (Table I) forAWT was referred from DECHEMAI4

. In AWT. aceticacid is highest boiling vertex (BP I Igoe) and becomesstable node (Fig. I). There is presence of minimumboiling azeotropes between water and toluene at 84.4°C(Azeo I) and acetic acid and toluene at I04.SoC(Azeo 2). Due to presence of azeotropes. entire ternarydiagram is divided into three distillation regions (A, Band C). One of the distillation boundaries is formedbetween acetic acid and Azeo I and other between AzeoI and Azeo 2 (Table 2). In Region A, acetic acid isstable node, and water-saddle point and Azeo I isunstable node. In Region B. acetic acid is stable node.and Azeo 2-saddle point and Azeo I is unstable node. InRegion C. toluene is stable node, and Azeo 2-saddle pointand Azeo I is unstable node.

Under vapor-liquid equilibrium (VLE) data, there isno much difference in region A between liquid and vaporcomposition. suggesting that for feed in region A, timerequired will be more (Fig. 2). In region C, stable nodeis toluene. so nature of distillate will be richer in toluene.which is undesired (Fig. I). Distillation in region 13wasfound to be more superior to be operated. There weresignificam jumps between liquid and vapor cornposuior»for points in region B. indicating that with minimum timeand stages one can get desired output.Intensity of jump

WATER TOlUEf'iE(l JOOr)

l\1oJe 0/.

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Fig. 1-1":lIure of residue curve !\'I acetic acid-water-toluene

~yqt"ln

• Azeou ope

A Liquid Compos it ion

• Vapor Cornposit ioo". Disfilbtion bounchry

WATER (100°c) 50'i'.

Auo J(!,14,4Cc)

TO I. \![J"E( "0°')

:::1\101(' 'X,

\\'~tt'r

Fig. 2- Triangular diagram she-w 109 selected VLE along withbimodal curve

ACET1C <\Cll) i,1U;:<'<l

--E- -----1\10h °0":It<<'1

Fl~ ~- Fl~3~lhlerea-on 10I'p"" .:It,'\,.o lumn (uccuc ~H'-!J- \\ .ucr-toluene •.\ ,ll'lll}

GADEKAR ('I al: ENTRAINER I·OR I3ATel1 DlSTI LLATION OF ACETIC .ACID - WATER SYSTEM 8/3

Point number

Table 2- Expcrirnenral VLE darn for A\\ -I ternary ,~qrm

Liquid cornposuion

Mole"cVapor composuu-n

MQle'!cWater Accuc 3C id Water Ar eric acid

R('XlOlI CI J(I I 5 ~o 3535 7RS :;5.~~2 101 2'1 ~5.97 :?)) I~)h:::.' 100 9.11 :::; 40 '1-l 34.2-l 10~.: 2 157 39.2 9 88 :s -175 102 5.3 41.5 9.75 30.:?7(-, 103 7.14 :<2.55 5911 1:'.077 104 0214 J:l.3 I 7 30508 ID5 5.91 I 1:\.67 6904 28.848Region B9 98 5.5<) 53.-17 2.3 17.3610 90.S 24.5 54.7 I 55.8:- lUII ns 11.39 50.23 6S 30.2::'Rr!iiol1ll

12 10:'.5 2-1.05 n.20 51.5 -131-' 101.5 -10.,0 48.42 64.27 -'4.3114 10::> -I2.RO 47.628 62 36.915 92 -11.27 52.45 62.19 1) 7'216 94 }-I5 6::'.29 53.28 :'40917 100 63.02 36.77 76.S 22.3518 101 46.6 50.70 83.51 15.9019 IOU 62:.4 '7.36 7::>.19 :>7.39

- - -_.------ - - --- ---

for some points is such that it shows only O-IOLiiJ io_~ ofacetic acid In aqueous phase.

Points 9 and J 0 (Fig. :1) shows significant jumps withdistillate composition below R9( tic line. Region B wouldbe most favorable region for column operation sincedistillate collected in this region has shown somemagnificent jumps. indicating efficient separation withoutmuch loss of acetic acid in aqueous solution. Accordi ngto nature of VLE data in this region and conceptualmethodology, feed position in shaded portion of region Bwould lead to negligible loss in acetic acid (0-2%) andmay lead to less time in product separation.

2. Acetic Acid -Water -Benzcne (A'VB) System

LLE data for AWB system is given in Table 3. VLEdata for some points are given in Table 4. In AWBsystem, acetic acid is highest boiling vertex and forms astable node. There is presence of azeotrope betweenbenzene and water at 70.5cC (Azeo I) and acetic acidand benzene at 80cC r Azco ~). Due to presence ofazeotropcs. entire ternary diagram is divided into threedistillation regillns (A, Band O. One of the distillationboundaries is formed bctw een acetic acid and Azco Iand other between Azeo I and Azco 2 (Fig.« L In RegionA. acetic acid is stable node. water - saddle point andAzeo I is unstable node. ln Region B. acetic acid is

Table 3- Experimentat LLE data for :\v.:B systemrexperirncntal tie line in mok'H-!

SNo. Left phase Righr phase

Water Acetic acid \\'3ICT Acetic acid

I 95.000 4.500 1.025 2.9432 76.400 22.800 ~.933 10.7803 57.270 39.353 6192 23.482<1 41.500 47.500 10.000 30.0005 30.500 49.000 16 ()(){) 40.000

stable node, Azeo 2- saddle point and Azeo J is unstablenode. In Region C. benzene is stable node. Azeo 2- saddlepoint and Azeo I is unstable node. One of the distillationboundaries is present between acetic acid and Azeo Iand other distillation boundary is between Azeo I andAzeo 2. It is quite similar to AWT system, Nature ofresidue curves in region A is showing still path to beincreasing and become richer in acetic acid (stable node).It is similar nature of residue curves in region B. Entireresidue curves are approaching stable node acetic acid.

In Region C. space available is very small asazeotrope between acetic acid and benzene (9S'7c) isnear to benzene pure node. Distillate for feed points inthis region may exhibit azeotropic composit ion, Presenceof azeotrope in acetic acid and benzene (Azeo 2) makes

.I SClI~D RES VOL f>R OCTOBER 2(lO')

A(L11C" ACln (l ,g<'c)

Table 4: Experimental VLE data for /\\\'B ~\·'tl'm

Point Temp. Liquid composition Vapor compositionnumber -c Mole% Mole'!c

----------Water Acetic acid Wain Acetic acid

79.50 3850 11.760 0.970 11.990'.? ,9.50 6.370 33 ""{lO 6.9 Jlt 32.590., 79.R() 13.3OC 42.780 -1.91() 17.520. 7'1 SO 14.54:; -16.362 ') :;,In 19.740~:; XO.OO 1.397 -19.330 II ~9S 27.6<106 R200 3.450 :'-1.400 < 150 1:'>.3207 97.00 69.120 2S.R20 6~.870 29.380S 101.00 26.080 6S.500 82800 16.360

Table 5- LLE data for acetic ac id- water-erhyl acetate system"(experimental tie lines in moleo/r at 30re)

S No. Left phase Ri!1ht phase

Water Acetic acid Water Acetic acid

I 18.547 3.091 97.329 0.8992 22391 5.892 96.236 1.7623 26.777 8.368 94.975 2.666,--...,4 31.5) 3 10.504 93.791 3.-190!

5 34A57 12.332 92.804 4.294

separation tedious. Ultimate limit of residue curves inthis region is entrainer via an azeotrope. For feed in regionC points I and 2 show vapor and still composition verynear to each other. which is time consuming and notsuitable for distillation operation. Even by having benzeneas an entrainer. fced portion in region B i~ showi ng goodjumps. Distillate collected was having vcr y less loss ofacetic acid in aqueous phase. Point- fl. ~ and -I areshowing higher jumps. which were h a v ing feed

AC ETIC AODOJSCc)

I,Ii

,,101.% ~ /,

ACf',lc Acid ;/; /

1i // ./~O"/o ,/"-I . .

/ /j >-.I /',,-+

I" i

!LL__~

I

I / .-----c.=====:::::=====::::::::=;;~~~~=~'£L..,2(8o-c)

~'" T£R (J OOOc) so-/.AZ«) J(iOS'c) BENZE..l"[.(SO.)C:c)

~---

ri~ ."- RCM and Ica-rble reason 10 operate column (aceuc acid-W;!ICr-OCJlICJ1C' ,~,""em)

composition in region B. whereas when feed compositionare falling in Region A. there is very small differencebetween vapor and liquid composition (point 7), indicatingthat large number of stages are required to bring desiredseparation. Nature of residue curves for this system andshaded portion. which may give best separation. areshown in Fig. S. For feed lying in Region B and according10 feed tendency. 0 wide gap is shown between distillateand bottom composition. It may be quite possible thatwhen feed is in shaded region, distillate composition willex hi hit only I ·20/r loss of acetic acid at gain of minimumnumber of stages in the column.

AWT as an ernrainer is same to results showing withAW B as an entrainer. In both systems, there is sure lossof acetic acid in aqueous phase. This loss of componentB (acetic acid) in Evlean phase and falling liquid level insti II may hamper amount of heat transferred and therebyvapor flow rate. Nature of tie line causes a loss ofcomponent B. In current work, selection of entrainerbased on nature of tie line and decanter composition isgiven preference.

If an entrainer has reverse nature of tie line such asit is more towards water node and away from entrainernode, separation might be quite simpler. Such is the caseof entrainer like acetic acid -water-ethyl acetate(AWE)" system (Table 5). In this system, there is onlyone minimum boiling azeotrope present between waterand ethyl acetate at 70.4cC. Ternary plot is thereforedivided into tw o distillation regions. As number ofazcoiropcs increases. complexities in column increases.thcr cby reducing chances of feasible separations. Fromn.uurc of residue curve for thc system. it is found that inboth distillation regions. residue curve approaches stable

~,'>' ••• Jts

.:~ ~r .:

C.'\DEKAR tf "I. ENTRAINER FOR 13/\TCH D1S11LLI\TION OF ACETJC .-\CJD -WATER SYSTEM 875

ACr.TI( AUD (l18"c)

r"\101f' Cj'e

-\UOllf Ar-id/.

I/~/ '

, \ - Dhtilbllon bound2r). \\t" \~. \.

\

'\,~-;" .

\\ ..-;\.. ..,\ / 1 \ ,.'.~\ Ii,> :'\1olf' "/0

I ; \ F..lh~1AH'f21f/ \,

.\/_---

ETHYL ACETATEI77"C)

f\1ol~ 'Y".\V2tt'r

FI~ (> Nature 01 tIC line tor acetic acid-water-cthvl acetate ~y"cm

node acetic acid only. At heterogeneous region in AWEsystem. it covers approx . only 20%. Therefore. thereare fewer chances of complexities in column. If oneconsiders same feed positions of feasible region B fortoluene and benzene as an entrainer in same position forethyl acetate as an erurainer and compare with natureof tic line. one would find that in most of the cases therewill he 0ge loss of acetic acid in aqueous phase. This i~purely due to opposite nature of tie line. which is moretowards water node and away from ethyl acetate node(Fig. 6).

ConclusionsAW system has mystery in much industrial

processes. Proper experimental VLE study of the systemto study nature of the bottom and distillation products isprerequisite, like for toluene and benzene. Region B wasfound to be favorable, yet acid loss was not completelyeliminated. Working with these entrainers, threedistillation regions, which add up to complexities incolumn, were found. Nature of tie line is importantparameter to consider acid free disposal of water.Entrainer like ethyl acetate having tie line towards purewater node is considered a good option and may result ingood separation and negligible loss of acetic acid inaqueous phase. This entrainer with acetic acid· watersystem leads to only two distillation regions. Entrainer

tendency to form azeotrope with acetic acid is alsoimportant factor; entraincr that does not form azeotropewith acetic acid should be preferred. In other words,less the number of distillation regions less is the amountof complexities in column and more is the tendency toachieve pure product.

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