Separation of ethyl acetate–isooctane mixture by heteroazeotropic batch distillation

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Identification number: DOI : 10.1016/j.cherd.2013.10.010

Official URL:

http://dx.doi.org/10.1016/j.cherd.2013.10.010

This is an author-deposited version published in: http://oatao.univ-toulouse.fr/

Eprints ID: 11119

To cite this version:

Ooms, Tom and Vreysen, Steven and Van Baelen, Guy and Gerbaud, Vincent

and Rodriguez-Donis, Ivonne

Separation of ethyl acetate–isooctane mixture by

heteroazeotropic batch distillation. (2014) Chemical Engineering Research and

Design, vol. 92 (n° 6). pp. 995-1004. ISSN 0263-8762

O

pen

A

rchive

T

oulouse

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Separation

of

ethyl

acetate–isooctane

mixture

by

heteroazeotropic

batch

distillation

Tom

Ooms

_,

_Steven

_Vreysen

_,

_Guy

_Van

_Baelen

_,

Vincent

Gerbaud

_,

_Ivonne

_{Rodriguez-Donis}

a_{ThomasMoreKempen,Kleinhoefstraat4,B-2440Geel,Belgium}

b_{UniversitédeToulouse,INP,UPS,LGC(LaboratoiredeGénieChimique),4alléeEmileMonso,F-31432Toulouse}

Cedex04,France

c_{CNRS,LGC(LaboratoiredeGénieChimique),F-31432ToulouseCedex04,France}

d_{InstitutoSuperiordeTecnologíasyCienciasAplicadas(InSTEC),AveSalvadorAllendeyLuaces,LaHabana}

10400,Cuba

a

b

s

t

r

a

c

t

Thispaperstudiestheseparationofanethylacetate–isooctanemixturebyheterogeneousazeotropicdistillation inabatchrectifyingcolumn.Aninitiallistof60candidateswasstudiedbutonlymethanolandacetonitrilewere obtainedaspotentialheterogeneousentrainers.Theseentrainersformalowboilingheterogeneousazeotropewith isooctane.Experimentalverificationofthemiscibilitygapwithisooctanewasperformedat25◦_{Cforeachentrainer}

givingasmallerregionformethanolthanforacetonitrile.Feasibilityoftheheterogeneousazeotropicbatch distilla-tionwascarriedoutexperimentallyinalaboratorybatchdistillationcolumnhaving44theoreticalequilibriumstages andusingahighrefluxratio.Severaldistillatefractionsweretakenasafunctionofthetemperatureatthetopof thecolumn.Forbothmethanolandacetonitrile,themainfractionwasdefinedbythecondensedvaporprovidinga liquid–liquidsplitoftheisooctane/entrainerheteroazeotropeintothedecanter.Ethylacetateimpuritywasdetected inbothdecantedphases,butinmuchloweramountwhenusingacetonitrileasentrainer.Theprocesswith acetoni-trilealsoresultedinashorteroperatingtimeandhigherpurityandrecoveryyieldofisooctaneasthemaindistillate product.Pureethylacetateremainedintotheboilerattheendofeachprocess.

Keywords: Heteroazeotropic distillation; Solvent recovery; Batch distillation; Ethyl acetate–isooctane mixture; Entrainerselection;Methanol;Acetonitrile

1.

Introduction

Solventrecoveryisbecomingamajorissueinthe pharmaceu-ticalandspecialtychemicalindustries.Aswasteregulations arebecomingstricter,theeconomicbenefitsofhaving recov-ery systems for the reuse of solvents in processes are significant.Recoveryoforganicsolventsisgenerallypracticed becauseofincreasedsolventcostandpotentialsolvent short-ages.Pharmaceuticalandchemicalindustriesareusinglarge amountsofsolventsforproductionand cleaningoperation thatmayendupaswastestreams(WhimandJohnson,1996;

∗_{Correspondingauthorat:UniversitédeToulouse,INP,UPS,LGC(LaboratoiredeGénieChimique),4alléeEmileMonso,F-31432Toulouse}

Cedex04,France.Tel.:+33534323651.

E-mailaddresses:steven.vreysen@khk.be(S.Vreysen),vincent.gerbaud@ensiacet.fr(V.Gerbaud).

Stoye,2000;Seyleretal.,2006).Batchprocesseshavebeenthe preferredoperatingmodeinthemanufacturingofhigh-value addedchemicals,sincethedemandforhigh-valuechemicals areseasonalandlowinvolume(HalimandSrinivasan,2007). Amongallseparationtechniques,distillationisbyfarthe mostusedtechniquefortheseparationofwastestreamsin theirindividual components.Unfortunatelytheuseof con-ventional batchdistillationisoftenlimitedbythefrequent presenceofazeotropes.Therefore,nowadayswastestreams are often incinerated when azeotropes are involved (Van Baelenetal.,2010).Reducingincinerationandincreasingthe

recoveryofthesesolventsbydistillationwilldecrease emis-sionsofgreenhousegassesintheair,lowerthecostofwaste incinerationandwillleadtoamoreefficientandsustainable useofrawmaterials(Seyleretal.,2006).

Advanced distillation techniques, like heteroazeotropic batchdistillationonwhichwefocushere,havetobeapplied torecoversolventsfromwastestreamscontainingazeotropes. Althoughdistillationrequiresalsoalotofenergy,thereisstill abenefitexpectedfromanecologicand economicpointof view,especiallyin mixturescontainingexpensivesolvents, likeisooctane.

Mostprocessesfortheseparationofazeotropicorlow rel-ative volatility zeotropic mixtures AB involve the addition ofan entrainer E, leading to ahomogeneous ternary mix-tureABEorexhibitingabinaryheterogeneousmiscibilitygap (Doherty and Malone, 2001). Simulation and experimental resultshave shown keydifferences between homogeneous andheterogeneousbatchdistillationandledtothe publica-tionofgeneralfeasibilityrules(Rodriguez-Donisetal.,2001a,b; Skourasetal.,2005).Indeed,someadvantages of heteroge-neous batchprocessescomparedtohomogeneoussystems are:(i)moresuitablecandidateentrainersandhence,more designalternativesfortheseparationofnon-idealmixtures, (ii)simplifieddistillationsequencesthankstotheliquid–liquid phasesplitoccurringinsomepartsofthecolumnandinthe decanter,(iii)theadditionofasmalleramountofentrainerto theoriginalmixture,(iv)theuseofmoreflexiblerefluxpolicies throughanycombinationoftheentrainer-richphaseandthe distillate-richphase,(v) dependingontherefluxpolicy,the typicalunstableseparatricesofresiduecurvemaparenota limitanymorefortheseparationofcomponentslocatedin dif-ferentbasicdistillationregions(LangandModla,2006;Hegely etal.,2013).

Inthisstudyweareconcernedwiththeseparationofan organic waste composed of ethylacetate (bp=77.2◦_{C) and}

isooctane(bp=99.1◦_{C),commonlyusedinthe}

pharmaceuti-calindustry.Thisbinarymixtureexhibitsaminimumboiling temperatureazeotropeat76.3◦_{Cunderatmosphericpressure}

with0.844ofethylacetatemolarcomposition(Gmehlingetal., 2004).Accordingtothefeasibilityrulesforbatchazeotropic distillation(Rodriguez-Donisetal.,2001b;Skourasetal.,2005), non-conventional distillation processes such as heteroge-neous azeotropic batch distillation (HABD) can be feasible to separate bothcomponents into high purity fractions. It requirestheadditionofanentrainerwhichhastoforma het-erogeneousbinaryazeotropewithoneofthekeycomponents and havingthe lowestboilingtemperature oftheresulting ternarymixture.Theformationofaternaryazeotropeshould be avoided as it pollutes the main product distillate with unwanted compounds. However, good performance of the heteroazeotropicbatchdistillationisnotlimitedbythe pres-enceofextrahomogeneousbinaryazeotropes.Hence,feasible ternarydiagramsforminimumboilingazeotropesarethe 2.0-2b,2.0-2cand3.0-2classdiagramsaccordingtoSerafimov’s classificationhavingaphysicaloccurrenceof21.1%,0.9%and 8.4%,respectively(Hilmenetal.,2002;Kivaetal.,2003).

In this paper, the choice of the entrainer is first dis-cussed in order to find the best entrainer option for heterogeneous azeotropic batch distillation. Second, the topological and thermodynamics properties of the ethyl acetate–isooctane–entrainermixtureresidue curvemap are described.Feasibilityoftheheterogeneousazeotropicbatch distillation(HABD) processisevaluatedtakinginto account the extend of the liquid–liquid vapour equilibrium region

into the residue curve map, the liquid–liquid splitratio at thebinaryheteroazeotropeandthepurityofbothdecanted phases. Finally,experimentsareruninapackedlaboratory batchdistillation columnunderhighrefluxratiotovalidate the feasibility of the separation ofethyl acetate–isooctane mixturebyheterogeneousbatchazeotropicdistillation.The puritiesofisooctaneasdistillateproductandethylacetateas boilerproductarethencomparedtothepreliminaryresults givenbytheresiduecurvemapanalysis.

2.

Entrainer

selection

for

the

separation

of

ethyl

acetate–isooctane

mixture

The overall performance of non-ideal mixtures separation stronglyreliesonthechoiceofasuitableentrainerE.Assessing feasibilityrequirestheevaluationoftheabilityofEtoform binary andternary azeotropeswithAor B.Azeotropic ten-dency can be approximately estimated via the study of chemicalinteractions(homologousseries,polarity,hydrogen bonding aptitude) together with heuristicson boiling tem-perature differences (Berg, 1969;Perry et al., 1997;Doherty andKnapp,1993;Gerbaudetal.,2006).Accuratepredictionof azeotropiccompositionandtemperatureundertheoperating pressurerequireseitherexperimentaldata(Gmehlingetal., 2004;GmehlingandOnken,1982)orcalculationusing ther-modynamicmodelsofvapor–liquidandvapor–liquid–liquid phaseequilibria,likeactivitycoefficientmodelsorequation ofstatesorgroupcontributionmethods(Bossenetal.,1993; GmehlingandMöllmann,1998;Theryetal.,2004).

First,theselectionofpotentialfeasibleentrainers(E)forthe separation oftheminimum temperatureboiling azeotropic mixtureethylacetate(A)–isooctane(B)isperformedusingthe RegsolExpert®_{wizardtoolwhichcombineschemicalinsight}

and thermodynamiccalculationstofindsuitableentrainers (Gerbaudetal.,2006).Asetof60componentswasselectedas candidateentrainers belongingtoseveralchemicalfamilies andhavingboilingtemperaturelowerthanethylacetate(light E),higherthanisooctane(heavyE)andintermediatebetween bothoriginalcomponents(intermediateE).Themodified UNI-FACDortmundversion1993(Gmehlingetal.,1993)wasused for estimating the existence of binary azeotropic mixtures (A)–(E)and(B)–(E)and,also,theeventualoccurrenceofternary azeotrope ABE.Predicted azeotropeoccurrencewasfurther verifiedagainstexperimentalevidence(Gmehlingetal.,2004, thiswork).

Analysing all possible generic mixtures A–B separation withanyentrainerE,Rodriguez-Donisetal.deriveda com-pleted set ofnecessary entrainer selection rules for batch azeotropicdistillationwereelucidated(Rodriguez-Donisetal., 2001a,b). Thoserules were implemented into the decision makingsoftwareRegSolExpert®_{forRegeneratingSolventsby}

batchdistillation,alongwithfeasibilityrulesforclassicaland pressure swing batchdistillation (ProsimS.A.,2012).In the batchrectification(resp.stripping)process,thatusuallystarts with an infinite reflux (resp. reboil) operation followed by distillate (resp. bottomproduct) removal,the column over-head(resp.bottom)composition underinfinitereflux(resp. reboil)istheleast(resp.most)volatilecomponent;anunstable (resp.stable)node;ofthedistillationregionwheretheglobal mixturecompositionlies,enablingtorecoveritasdistillate (resp.bottomproduct)(Bernotetal.,1990,1991).Separationof minimumboilingazeotropicmixtureswithhomogeneousor heterogeneousentrainerisonlyfeasibleif:

Table1–Candidateentrainer(E)forseparatingethylacetate(A)–isooctane(B)bybatchazeotropicdistillation.Azeotropic

datapredictedwiththemodifiedDortmundUNIFACmodel.

Entrainer Type AzeoAE AzeoBE AzeoABE Sequence

Diethylether Light None None None SSS

Acetone Light None HomPmin[un] None SSS

Ethylformate Light None HomPmin[un] None SSS

Methanol Light HetPmin[un] HomPmin[s] None R

Ethanol Light HomPmin[s] HomPmin[s] HomPmin[un] SSS

1-Butanal Light None HomPmin[un] None SSS

1-Pentyne Light None None None SSS

n-Hexane Light HomPmin[un] None None SSS

Acetonitrile Intermediate HetPmin[un] HomPmin[s] None R

n-Butylamine Intermediate HomPmin[un] None None SSS

3-Methylpentane Intermediate HomPmin[un] None None SSS

Acrylonitrile Intermediate HomPmin[s] HomPmin[s] HomPmin[un] SSS

Water Heavy HetPmin[s] HetPmin[s] HetPmin[un] RRR

Nitromethane Heavy None HetPmin[s] None Extractive

(1) By introducing light and intermediate entrainers and requiringtheuseoftwoconsecutivebatchstripping col-umnconfiguration (S–S).(1.a) thesimplestcaseisgiven byanintermediateentrainerformingnoazeotropewith either (A) or (B). (1.b) Theremaining casesimpose the formationofaternaryazeotropeoramaximumboiling azeotrope,moredifficulttofindinpractice.

(2) Aheavyhomogeneousentrainercanbealsousedifthe boundariesintheternaryresiduecurvemapexhibitsome curvatureand,inthiscase,theseparationrequiresthree consecutivebatchdistillationtasks,allinvolvingstripping columnconfiguration(SSS).

(3) Heterogeneousentrainers(light,intermediateandheavy) increasetheseparationalternativesandcanbeusedwith abatchrectificationcolumnwithatopdecanter.(3.a)The simplestalternative isgivenbytheformationofa het-erogeneousbinaryazeotrope(A–E)or(B–E)beingthesole unstablenodeoftheresiduecurveasitwaspointedout byRodriguez-Donisetal.(2001b).Aftercondensationthe heteroazeotropicgivesrisetoaliquid–liquidsplitinthe decanterfromwhichtheproduct-richphaseiswithdrawn asdistillate.Inthiscase,separationof(A)and(B)canbe reachedusingonlyonebatchseparationtaskina rectify-ingcolumnconfiguration(R).(3.b)Asequenceofseveral batchdistillationtasksisusedinthepresenceofaternary heterazeotrope.

Thecriterion(3.a)governsthesearchofafeasibleentrainer for the separation ethyl acetate–isooctane mixture. As a result ofthe entrainer screening, 13 of the 60 preselected entrainerswerefoundsuitablefortheseparationoftheethyl acetate–isooctanemixtureandarereportedinTable1along withthesuitablebatchdistillationprocesssequence,with rec-tifier(R)orstripper(S)columnconfiguration.Nohomogeneous entrainercouldmatchthesimplestalternative(1.a).Mostof themmakefeasibletheseparationusingasequenceofthree batchstrippercolumnwheretherecoveryyieldofthe compo-nentsdependsstronglyonthecurvaturedegreeoftheresidue curvemapboundaries.

Regarding heavy entrainers, water forms three hetero-geneousazeotropicmixtures: twobinariesand oneternary givingaverycomplexscenariooftheresultingternaryresidue curvemap.NitromethaneisalsomentionedinTable1asit lead tothe formation ofa heteroazeotrope withisooctane but,beingasaddlepointazeotrope,thefeasibleprocessisnot

heteroazeotropic distillation, but heterogeneous extractive distillation(Rodríguez-Donisetal.,2003a,b).

Twoheterogeneousentrainerswerefoundmatchingwith thecriterion(3.a),alightcomponent(methanol)andan inter-mediatecomponent(acetonitrile).Methanolandacetonitrile werethenselectedformoredetaileddesignofthe heteroge-neousazeotropicbatchdistillationprocessinabatchrectifier withadecanter.

3.

Design

of

heterogeneous

batch

distillation

using

residue

curve

map

analysis

model (Fredenslund et al., 1975). The modified Dortmund UNIFACmodelusedinRegSolExpert®_{(ProSimS.A.)uses}

tem-peraturedependentparametersandfittedatomicradiusand volumewhereastheoriginalUNIFACmodelhasno tempera-turedependencyandusesBondi’smethodfortheradiusand volume(Bondi,1964;Gmehlingetal.,1993).Althoughtheyalso havedifferentparametervaluesforthegroupcontributions, thepredicted valuearecloseinthesystemwestudiedand consistentwiththeexperimentaldataavailable.

Bothmethanolandacetonitrileentrainersformone homo-geneousazeotropewithethylacetateandoneheterogeneous azeotrope with isooctane. As the ternary mixtureinvolves threebinary azeotropicmixturesand noternaryazeotrope, theresultingresiduecurvemapcorrespondstothe3.0-2class according to Serafimov’s classification, with an estimated statisticoccurrenceof8.4%amongreportedternaryazeotropic mixtures(Kivaetal.,2003).Theresiduecurvemapexhibitstwo unstable boundariesconnectingthebinary heteroazeotrope to each homogeneous binary azeotrope and dividing the compositionspaceinthreebasicdistillationregions.These boundariesusuallyrestricttheseparationofthecomponents byhomogeneousazeotropicdistillation,butnotanymorein thecaseofheterogeneousentrainers(Rodríguez-Donisetal., 2002;LangandModla,2006).Theefficiencyofheterogeneous azeotropicdistillationislargelydeterminedbythe tempera-tureandcompositionofthebinaryheteroazeotropeandbythe compositionofeachliquidphaseafterdecantation.Table2

displaystheboilingtemperatureandthecompositionofeach azeotropicmixturealongwiththepurityofeachliquidphase afterdecantationat25◦_{Ccomputed bytheoriginalUNIFAC}

0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100

homogeneous mixture

LLE experimental envelope VLE UNIFAC envelope LLE UNIFAC envelope

Binodal plot

(isocotane, methanol) = (24.8% , 75.2%) at 58.5°C (ethyl acetate, methanol) = (28.7% , 71.2%) at 60.8°C (isooctane , ethyl acetate) = (19.0% , 81.0%) at 75.5°C

E th yl_a ce ta te (m o_le % ) Me tha nol ( mo le% )

Isooctane (mole%)

Binary azeotrope

77.2°C [sn]

75.5°C [s]

60.8°C [s]

58.5°C [un] 99.1°C [sn]

64.7°C [sn]

Fig.1– Methanol–ethylacetate–isooctane3.0-2classternarydiagramandliquid–liquidandliquid–liquid–vapour

equilibriumenvelopesestimatedbytheoriginalUNIFACmodel.

0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 E th yl_a ce ta te (mo le % ) homogeneous mixture

(isocotane, acetonitrile) = (36.1% , 63.9%) at 68.3°C

(ethyl acetate, acetonitrile) = (70.6% , 29.4%) at 74.8°C

(isooctane , ethyl acetate) = (19.0% , 81.0%) at 75.5°C

LLE experimental envelope

VLE UNIFAC envelope

LLE UNIFAC envelope

Binodal plot Binary azeotrope Ace ton itrile (mo le%)

Isooctane (mole%)

77.2°C [sn]

75.5°C [s] 74.8°C [s]

68.3°C [un] 99.1°C [sn]

81.5°C [sn]

Fig.2– Acetonitrile–ethylacetate–isooctane3.0-2classternarydiagramandliquid–liquidandliquid–liquid–vapour

equilibriumenvelopesestimatedbytheoriginalUNIFACmodel.

Table2–Predictedtemperatureandcompositionofbinaryheterogeneousazeotropicmixtureandliquid–liquid

equilibriumdataat25◦_C.

Entrainer(E) Heteroazeotrope

isooctane(B)–entrainer(E)

Entrainer heavyphase Isooctane lightphase (Heavy/light) moleratio Methanol(64.7◦_{C) x} E=0.7523 T=58.5◦C xE=0.9476 xB=0.8924 3.30 Acetonitrile(81.5◦_{C) x} E=0.6382 T=68.3◦C xE=0.9788 xB=0.9682 1.78

model and represented inFigs. 1and 2. Similar data (not shown)wereobtainedwiththemodifiedDortmundUNIFAC modelusedinRegsolExpert®_{(ProSimS.A.).}

Theheterogeneousbinaryazeotropehasthelowestboiling pointinthemixtureforbothentrainers.ForaHABDprocess, mixingagivenamountoftheentrainerintotheinitialcharge allowsthewithdrawalofabinaryheteroazeotropeasvapour topproductusingtherequirednumberofequilibriumplates intothecolumn.Aftercondensationofthisvapour,twoliquid phasesaredecanted.Theentrainer-richphaseistheheavy liquidphaseanditcanbedirectlysentbackatthetopofthe columnasliquidreflux.Theisooctane-richlightphaseiskept asdistillateproduct.

Comparisonofmethanolandacetonitrilecanbeperformed fromanalysisofFigs.1and2alongwithTable2.Some impor-tantremarkscanbedoneasfollowing:

(a) Thesizeoftheimmiscibilitygaptogetherwiththepurity ofisooctane-richlightphasedeterminestheeasinessof the HABDprocess and the distillate purity. Comparing

Figs.1and2,acetonitrileprovidesthelargest immiscibil-ityregionandthebestpurity,0.9682comparedto0.8924 givenbymethanol(seeTable2).However,afurther purifi-cationofisooctaneisexpectedtobedonebyanadditional batchdistillationstepifapurerisooctaneissought.

(b) Whenthe compositionofthebinary heteroazeotropeis closetotheentrainerapex,theleverruleunderlyingthe phasesplitintothedecanterhintsthatlessamountofthe distillatephasecanbedrawn,providingaprohibited oper-atingtime.ThisisdisplayedinTable2bythe(heavy/light) molephaseratio,representingtheliquid–liquidsplitratio intothedecanter.Avalueoftheliquid–liquidsplitratio approachingtounityisrecommendedinordertoprovide atthesametimeenoughentrainer-richphaseasaliquid refluxtowardsthecolumntopandanadequatedistillate flowrateoftheisooctane-richphase.Inthissense ace-tonitrileshowsthemostpromisingvalue.

(c) Alowerboilingtemperatureofthebinaryheteroazeotrope may provide a separation with less energy demand. Methanolasalightentrainerdisplaysalower tempera-turevalueofthebinaryheteroazeotropewithisooctane. However, it is very close to the boiling temperature ofthe homogeneous azeotrope methanol–ethyl acetate (see Fig. 1). On the other hand,acetonitrile leads to a largerdifferencebetweentheboilingtemperatureofits heteroazeotropicmixturewithisooctaneandthe homo-geneous azeotropewith ethylacetate (seeFig. 2). As a compromise,acetonitrileispreferable.

(d) Ethylacetate remains into the boiler atthe end ofthe batchdistillation process for bothentrainers. Itcan be obtained with a very high molar purity, depending on theinitialchargeoftheentrainerandtherigorous con-trolofrefluxpolicy(Rodríguez-Donisetal.,2002;Skouras etal.,2005).However,ifthestillcontentisfinallypolluted withtheentrainer,therecoveryyieldofethylacetatewill bebetteriftheoverallcompositionofthehomogeneous binaryazeotropeisclosertoentrainer apex.Ithappens formethanol(compareFigs.1and2).

(e) The overall composition of the homogeneous binary azeotropeentrainer–ethylacetate maypreventthe con-centration of ethyl acetate into the still during batch operation.Indeed,onceisooctaneiscompletelydistilled out, it may happen that the still composition reaches the binary sideentrainer–ethyl acetate onthe segment limitedbytheentrainerapexandthehomogeneousbinary azeotrope. Inthis segment the ethylacetate is notthe stablenodeanymore,butitistheentrainer.Hence,the entrainer is then concentrated into the still whereas ethylacetate isfullytaken out ofthe columnas distil-late given by the minimum boiling azeotropic mixture entrainer–ethylacetate.Inthatcase,methanoloffersthe bestoptionlikein(d).

As a whole, those arguments point out acetonitrile as a better entrainer than methanol for separating ethyl acetate–isooctanebyheterogeneousazeotropic batch distil-lation.

4.

Experimental

validation

of

the

separation

of

ethyl

acetate–isooctane

by

heterogeneous

azeotropic

batch

distillation

Theheterogeneousazeotropicdistillationisfeasiblewhenthe binaryheteroazeotrope,beingthesoleunstablenodeofthe residue curvemap, isobtainedatthe columntopand fur-therundergoesaliquid–liquidsplittingintothedecanterafter condensation.

Regardingthechoiceofmethanolandacetonitrileas can-didate entrainers,the lack ofthe liquid–liquid–vapour and

liquid–liquid equilibriumexperimentaldata forthe studied mixturespromptedustomeasuretheimmiscibilitygapbefore running heteroazeotropic batchdistillation process experi-ments.

4.1. Materialsandsampleanalysis

99.0mass%gradeethylacetate(FisherScientific)and isooc-tane(2,2,4-trimethylpentane)(Sigma–Aldrich)and99.9mass% acetonitrile, methanol and 1-propanol (VWR Prolabo) were used. All sampleswere analysed intriplicate bygas chro-matography (Chromopack 9002) equipped with a flame ionisationdetector(FID)usingaCP-Sil5CBcolumnof25m length,0.32mminternaldiameterand 5␮mfilmthickness. Thetemperatureoftheinjectoranddetectorweresetat200◦_C.

The injectionvolume was0.1␮l. For the samplesobtained bythedistillationexperimentwithacetonitrileasentrainer thetemperature oftheGCovenwassetat80◦_C,raisedfor

4minatarateof10◦_{C/minandheldfor2minat120}◦_C.The

internalstandardwas1-propanolandsamplesweredilutedin methanol.Forthesamplesobtainedbythedistillation exper-imentwithmethanolasentrainer,thetemperatureoftheGC ovenwassetat80◦_{Cfor4min,andraisedfor2minatarateof}

20◦_{C/min.Theinternalstandardwasacetonitrileandsamples}

weredilutedin1-propanol.

Thestandarddeviationbetweenthethreereplicatesofthe samesamplewasalwayslowerthan2.5%.

Foreachsamplethesumofvolume%ofentrainer, isooc-tane and ethyl acetate was 100%±2.5%. All results were normalisedto100.0%.

4.2. Experimentalverificationofheterogeneous azeotropicdistillation

4.2.1. Determinationoftheimmiscibilitygapforeach ternarymixture

Experimentaldeterminationoftheliquid–liquidphase enve-lopeofeachternarymixturewasdoneandcomparedtothe calculatedliquid–liquidphaseenvelopeat25◦_Cdisplayedin

Figs.1and2.Severalternarymixtureswithdifferenttotal con-centrationsweremadebymixingthepureproducts.Therefore each time a fixed amount of ethyl acetate, isooctane and entrainer(methanoloracetonitrile)wasagitatedwitha mag-neticstirrerinaclosedthermostatedvesselfor4h.Afterthe agitation,themixturewasleftfordecantationfor12h. Sam-plesweretakenfromeachliquidphaseandanalysedfollowing the methodology described inSection 4.1. Finally, toverify theextentoftheliquid–liquidphasesplitting,wealsoreport onehomogenous sample.Heretheliquid–liquid separation justdisappeared.Sothissamplerepresentsanupperlimitfor ethylacetatecontentbeforephaseseparationdisappears.The experimentaldataarereportedinTables3and4andplotted inFig.2.

WecomparedthosemeasuredLLEvaluesat25◦_Cwith

pre-dicted onesbythe same UNIFACmodelthanthe oneused duringtheentrainerselectionstep.

In the caseofmethanol, Fig. 2shows some differences betweenthemeasurementsandtheliquid–liquidequilibrium computed using the UNIFAC model. Thetwo phase region spanissmallerthantheonepredictedandthemaximum con-centrationofethylacetateissmallerintheexperiments.Also, theisooctaneconcentrationoftheisooctane-richphase(light) andthemethanolconcentrationofthemethanol-rich(heavy)

Table3–Experimentalcompositionoftheheavyand

lightphaseofthesamplesusedforliquid–liquid

equilibriumdataat25◦_Cfortheethyl

acetate–isooctane–methanolsystem(mole%).

Sample Ethylacetate Isooctane Methanol

Sample1 xA=0.0000 xB=0.2690 xE=0.7310 Heavyphase1 xA=0.0000 xB=0.1131 xE=0.8869 Lightphase1 xA=0.0000 xB=0.8026 xE=0.1974 Sample2 xA=0.0083 xB=0.2360 xE=0.7556 Heavyphase2 xA=0.0082 xB=0.1124 xE=0.8794 Lightphase2 xA=0.0090 xB=0.8005 xE=0.1905 Sample3 xA=0.0196 xB=0.2370 xE=0.7434 Heavyphase3 xA=0.0186 xB=0.1178 xE=0.8636 Lightphase3 xA=0.0240 xB=0.7468 xE=0.2293 Sample4 xA=0.0299 xB=0.2395 xE=0.7307 Heavyphase4 xA=0.0296 xB=0.1980 xE=0.7724 Lightphase4 xA=0.0324 xB=0.6648 xE=0.3028 Homogenous xA=0.0613 xB=0.4212 xE=0.5175

phaseweresmallerthanthosecalculatedusingtheUNIFAC model.

In the case of acetonitrile, the calculated liquid–liquid envelopeliesundertheexperimentaloneandextendsfarther towardstheisooctanevertexcomparedtoourmeasurements. Nevertheless,theexperimentalisooctaneconcentrationofthe isooctane-richphaseislargerwhenusingacetonitrile com-pared to the oneusing methanol. However,the maximum molarcompositionofisooctaneinthelightphaseisnearto 90%.Anadditionaldistillationstepwillberequiredformore pureproduct.

Theseexperimentalresultsconfirmtheselectionof ace-tonitrile as an adequate heterogeneous entrainer as was elucidatedabovefromTable2predictions.

4.2.2. Experimentalvalidationofheterogeneousazeotropic distillationinalaboratorybatchdistillationcolumn

Fig.3showsthebatchdistillationcolumnusedforthe sep-aration of the mixture acetonitrile–ethyl acetate–isooctane andthemixturemethanol–ethylacetate–isooctane.This dis-tillation columnisdesignedtohavea maximalamount of theoretical plates within the available lab space. It has a lengthof1.50mand an inner diameterof0.03m. To min-imiseheat lossesthe columnhasaradiationscreen and a vacuummantle.Thecolumnisfilledwith3cm×3cmraschig ringsmeaningabout44theoreticalplates.TheHETP(height equivalenttoatheoreticalplate)ofthepackedcolumnandthe

Table4–Experimentalcompositionoftheheavyand

lightphaseofthesamplesusedforliquid–liquid

equilibriumdataat25◦_Cfortheethyl

acetate–isooctane–acetonitrilesystem(mole%).

Sample Ethylacetate Isooctane Acetonitrile

Sample1 xA=0.0000 xB=0.2330 xE=0.6770 Heavyphase1 xA=0.0000 xB=0.0348 xE=0.9652 Lightphase1 xA=0.0000 xB=0.8991 xE=0.1009 Sample2 xA=0.1010 xB=0.4338 xE=0.4652 Heavyphase2 xA=0.1221 xB=0.0642 xE=0.8137 Lightphase2 xA=0.0833 xB=0.7443 xE=0.1724 Sample3 xA=0.1945 xB=0.3793 xE=0.4262 Heavyphase3 xA=0.2310 xB=0.1186 xE=0.6503 Lightphase3 xA=0.1618 xB=0.6124 xE=0.2258 Sample4 xA=0.2274 xB=0.3676 xE=0.4050 Heavyphase4 xA=0.2573 xB=0.1935 xE=0.5492 Lightphase4 xA=0.2186 xB=0.4803 xE=0.3010 Homogenous xA=0.2588 xB=0.3542 xE=0.3870

Fig.3–Batchdistillationcolumnconfigurationatlaboratory

scale.

associatedplatenumberwasexperimentallydefinedby run-ningtheseparationofaheptane–methylcyclohexanemixture (Heringtonetal.,1979).Theboilerwasheatedwithan elec-tricalheatingjacketof0.6kWatabout40%ofitsmaximum heating capacity.Thecondenserwascooledwithtapwater ofabout10–15◦_{Cuntilthecondensatewasatroom}

temper-ature(25◦_{C)andtheliquidrefluxwascontrolledthroughthe}

open/closetimeofasolenoidvalve.Temperaturewas mea-sured with a Pt-100 temperature data loggerat the top of

0 54 56 58 60 62 64 66 68 70 72 74 76 78 T o ptemp e ra tu re (°C) 100 50 Fractio Fractio Fractio Fractio Fractio 200 150 on 1 on 2 on 3 on 4 on 5 3 250 00 350 Tim e (m inutes 450 400 ) 550 500 600 650 700

Fig.4–EvolutionofthetoptemperaturewithoperationtimeduringtheHABDprocesswithmethanolasentrainer.

thecolumn.Inallexperimentstotalrefluxwasapplieduntil thecolumnreachesthesteadystatedeterminedbyunvarying temperatureatthetopofthecolumn.Thenthedistillatewas drawnunderafixedrefluxratio.Severalfractionswere col-lectedaccordingtothetemperaturevariationatthetopand furtheranalysedbyGC-FID.Thedistillationwasstoppedwhen thetoptemperaturereachedtheboilingtemperatureofethyl acetate.

4.2.2.1. Experimental validation of the separation of ethyl

acetate–isooctane with methanol as entrainer. The total

amount of initial ternary mixture fed having a total composition of xF={0.550/0.283/0.167} (methanol/ethyl

acetate/isooctane)intotheboilerwas0.841L(10.811mol).The amountofentrainerwascalculatedbytaking5%ofentrainer volumemorethanthetheoreticalcompositionofthebinary heteroazeotrope with isooctane computed by the original UNIFACmodel(Table2)inordertomakesurethatisooctane can bedistilled by this heteroazeotrope,taking in account the total liquid holdup of the column. The heterogeneous azeotropicdistillationprocesswascarriedoutundera con-stantrefluxratioof20,toensurethattheheteroazeotropeis obtainedinthevapouroverhead,asthetheoreticalfeasibility criterion 3.arequests. Smallerreflux ratio values were not testedbutwouldbeneededinindustrialoperation.

Fig.4showsthevariationofthetemperatureatthetopof thecolumnduringthebatchoperationandthefivedistillate fractionsthatweretaken.Thetemperatureatthetopofthe columnwasverystableduringthedistillationexperimentfor nearly8hnearto58◦_{C,closetothetargetedboiling}

tempera-tureofthebinaryheteroazeotropepredictedbybothUNIFAC models(58.5◦_C).

Table5showsthefivedistillatefractionsthatweretaken depending on the top temperature. Thefirst fraction(only 0.012L) was homogeneous, because ofthe high content of ethylacetate.Fraction2correspondedtoaheterogeneoustop vapourcompositionandconstitutedthemaindistillate frac-tion having0.435L. Thenextfractions3, 4and 5werethe transitioncutstowardspureethylacetateintotheboiler.The wholeprocesstook11hand36minuntilpureethylacetate wasobtainedintheresidue(Fig.5).

Table6showstheoverheadvapourcompositioncomputed from the composition ofheavyand lightphase offraction 2. This fraction 2 was decanted at 25◦_{C. The}

methanol-rich phase contains almost 15mole% ofisooctane and the isooctane-richphase exhibitsonly67.5mole% ofisooctane, significantlylower than thepredicted valueforbinary het-eroazeotrope (see Table 2). This is mainly due to the fact thatthecondensedvapourdrawnatthetopofthecolumn alsocontained4.6mole%ofethylacetate(Table6),indicating thatabetterseparationrequiresahighernumberof equilib-riumstages.Sobothliquidphaseshavealowerpuritythan expectedaswellandthemoleratioofbothphases(4.96)is higherthanpredicted(3.3).Alltheseexperimentalresults con-firmthatmethanolisnotafavourableentraineraspredicted fromtheanalysisofFig.1andTable2.

Table7showsthemolarmassbalanceofthedistillation experiment. The residue contains 100mole% ofpure ethyl acetateattheendofthedistillationprocess.Exceptfraction2, allotherfractionswerehomogenousandweretakentogether inthemassbalance.Thedistillationexperimentwasdonein duplicateandthesolventlossesinthemassbalanceinboth testswerenearlyconstantandthereforenotonlydueto exper-imentalerrors. Thesolventlosses inthemass balanceare

Table5–Volume,timeandtoptemperatureforeachdistillatefraction.

Fraction Volume(L) Time(hours:minutes) Toptemperature(◦_C)

Begin End Begin End

Fraction1 0.012 0:00 0:12 57.8 57.9 Heavyfraction2 0.305 0:12 8:20 57.9 58.1 Lightfraction2 0.130 Fraction3 0.050 8:20 9:30 58.1 58.4 Fraction4 0.050 9:30 10:37 58.4 75.7 Fraction5 0.050 10:37 11:36 75.7 76.5 Residue 0.178 11:36 – 76.5 –

500 450 400 350 300 250 200 150 100 50 0 64 66 68 70 72 74 76 78 T op tem pe rature (° C)

Time (minutes)

Fraction 1 Fraction 2 Fraction 3 Fraction 4 Fraction 5

Fig.5–EvolutionofthetoptemperaturewithoperationtimeduringtheHABDprocesswithacetonitrileasentrainer.

Table6–Experimentalliquid–liquidsplittingforfraction2at25◦_C.

Component Overheadvapor(mole%) Heavyphase(mole%) Lightphase(mole%) heavy/lightphase(moleratio)

Methanol 71.9 81.3 25.3 4.96

Ethylacetate 4.6 4.0 7.2

Isooctane 23.5 14.7 67.5

mainlyduetotheliquidretentioninthecolumnaswellas toevaporationlossestotheatmosphereatthesolenoidvalve andsamplepointsofthecolumn,althoughallpossibleactions wereundertakentominimisethesesolventlosses.The recov-eryyieldofmethanolduringthemainfraction2was72.3%and 78.0%forisooctane.AccordingtoTable7,therecoveryyieldof ethylacetatewas59.3%.Itcouldbeimprovedbyavoidingthe withdrawaloffraction5,asthetoptemperaturevstimecurve displayedinFig.4showsaboilingtemperaturenearpurethe ethylacetatevalueforthisfraction.

4.2.2.2. Experimental validation of the separation of ethyl acetate–isooctanewithacetonitrileasentrainer. Forthe sepa-ration ofethylacetate–isooctane mixturewithacetonitrile, 0.772L(8.154mol)ofinitialternarymixturewerefedintothe boiler havingacompositionofxF={0.404/0.375/0.221}

(ace-tonitrile/ethyl acetate/isooctane). The amount of entrainer wasagain calculatedbytaking5%ofthe entrainer volume inexcessaswell.Fig.4showsthetemperatureevolutionat the top ofthe columnasa functionofthe operating time andthefivedistillatefractionsthatweretaken.Table8shows thevolumeofthesefivedistillatefractionsandtheirrelated temperatureandoperatingtime.

Table 9shows the overheadvapour composition during fraction 2and the compositionof the relatedphases after decantationat25◦_C.

Unlike tothe methanol case,liquid–liquid splittinginto thedecanterappearsfromthebeginningofthebatch oper-ation,which isnotsurprisingasFig. 2showsthat theLLE regionismuchlarger,comparedtotheonewithmethanol(see

Fig.1).Besides,thecompositionofethylacetateinthe over-headvapour(1.5%inTable9)islowerthantheexperimental valuereportedinTable6formethanol.Hence,thesame dis-tillationcolumnwiththesamerefluxratioperformsabetter separationwhenusingacetonitrile.

Fraction2representsthemaindistillateproduct.The tem-perature of fraction 2 is nearby 68.0◦_{C, very close to the}

predicted value of the binary heteroazeotrope by UNIFAC (Table2).Noexperimentaldataofthebinaryheterazeotrope acetonitrile–isooctane were found in the usual literature (Gmehlingetal.,2004)butitcanbeassumedthatthereal tem-peraturevalueliesslightlybelowthefraction2temperature becausetheoverheadvapourcompositionhasasmallcontent inethylacetate(1.5mole%inTable9).Thenextfractions3–5 containthetransitiontopureethylacetate.Thewholeprocess took8hand20minuntilpureethylacetatewasobtainedin theresidueintotheboiler.

Fraction2isconsideredasthemainproduct.Theoverall compositionoffraction2and thecorrespondingheavyand light phasecomposition along withthe mole ratio ofboth phasesareclosetothetheoreticalcompositionpredictedby theoriginalUNIFACmodel(Table2).

As seenin Table9,the heavy andthe lightphase have a high content of acetonitrile and isooctane, respectively. Both phases also contain less ethyl acetate than when using methanol as entrainer. Those experimental results confirmthatacetonitrileisthepreferredentrainerforthe het-eroazeotropicbatchdistillationprocess.

Table10showsthemolarmassbalanceofthedistillation experiment.Fractions1and2wereheterogeneousandwere

Table7–MolarmassbalanceofHABDwithmethanolasentrainer.

Component Input Outputstreams Balance

Heterogeneous fraction

Homogeneous fraction

Residue Total-IN Total-OUT Deviation(%)

Methanol 5.950 4.305 0.960 0.000 5.950 5.265 −11.5

Ethylacetate 3.054 0.272 0.894 1.812 3.054 2.979 −2.5

Isooctane 1.807 1.409 0.213 0.000 1.807 1.621 −10.3

Table8–Volume,timeandtoptemperaturerangeofthedistillatefractions.

Fraction Volume(L) Time(hours:minutes) Toptemperature(◦_C)

Begin End Begin End

Heavyfraction1 0.004 0:00 0:10 67.2 68.0 Lightfraction1 0.006 Heavyfraction2 0.161 0:10 6:20 68.0 69.2 Lightfraction2 0.247 Fraction3 0.050 6:20 7:15 69.2 74.1 Fraction4 0.030 7:15 7:52 74.1 75.6 Fraction5 0.020 7:52 8:20 75.6 76.1 Residue 0.202 8:20 – 76.1 –

Table9–Experimentalliquid–liquidsplittingforfraction2at25◦_C.

Component Overheadvapor(mole%) Heavyphase(mole%) Lightphase(mole%) Heavy/lightphase(moleratio)

Acetonitrile 63.6 94.6 8.9 1.77

Ethylacetate 1.5 1.8 0.9

Isooctane 34.9 3.6 90.2

Table10–MolarmassbalanceofHABDwithacetonitrile.

Component Input Outputstreams Balance

Heterogeneousfraction Homogeneousfraction Residue Total-IN Total-OUT Deviation(%)

Acetonitrile 3.293 2.874 0.236 0.000 3.293 3.110 −5.6

Ethylacetate 3.054 0.067 0.620 2.056 3.054 2.744 −10.1

Isooctane 1.807 1.574 0.161 0.000 1.807 1.735 −4.0

Total 8.154 4.515 1.020 2.056 8.154 7.589 −6.9

combinedtogetherinthemassbalance.Alsothehomogenous fractions3,4and5weretakentogetherinthemassbalance. Thenegativesigninthecolumndeviationindicatessolvent losses.Thereasonsforthesesolventlosseswere explained abovefortheexperimentswithmethanol.Againthe distil-lation wasdone induplicate, withnearly constant solvent losses.Therecoveryyieldofacetonitrilefromthe heteroge-neousfractions1and2was87.3%and87.1%forisooctane, within experimental uncertainty.The recoveryyieldofthe ethylacetateremainedintothestillwas67.3%.Allrecovery yieldsarehigher10%thanthoseobtainedwithmethanol.

Inconclusion,acetonitrileperformstheseparationofethyl acetate–isooctanemixtureinashorteroperatingtime(8.2h) thanmethanol(11.36h)andwithbetterpuritiesandrecovery yieldsforbothoriginalcomponentsand theheterogeneous entrainer.Thiswasanticipatedfromtheentrainerchoicestep andlaterconfirmedbytheexperiments.

5.

Conclusions

Theseparation of anethylacetate–isooctane mixture can-notbeachievedbyconventionaldistillationprocess,because aminimumboilinghomogenousazeotropeexists.Therefore the use ofHABDfor the separationof this binary mixture wasinvestigated.Acetonitrileandmethanolwereselectedas thebest entrainers,usingthe RegSolExpert® _softwarefrom

an initial listcontaining 60 candidates belongingto differ-ent chemical families. Later, the feasibility of HABD was assessed, based on the computation of the residue curve mapforbothentrainerswhereUNIFACwasusedasa ther-modynamicmodel.Bothheterogeneousentrainersprovidea ternarydiagrammatchingtothe3.0-2classdiagram accord-ingtoSerafimov’sclassification.Analysisoftheresiduecurve map and the liquid–liquid envelope pointed out that ace-tonitrile was a more promising candidate than methanol

becauseoftheentrainercompositionintheheteroazeotrope isooctane–entrainer,awiderimmiscibilitygapwithahigher compositionofisooctaneinthelightphaseandthehigh tem-perature difference respectingto other involved azeotropic mixtures.Experimentalassessmentoftheternary immisci-bilitygapunder25◦_{Cwascarriedoutinagreementwiththe}

predictionsbyUNIFAC.ExperimentalverificationoftheHABD process showed that acetonitrile isa betterheterogeneous entrainerbecauseitperformstheseparationoftheazeotropic componentsinashorteroperatingtimeandprovideshigher purityand recoveryyieldforall components. Further opti-misationoftheheterogeneousazeotropicdistillationisnow compulsoryinordertoimprovethecolumndesignsoasto reducethesolventlossesandtoestablishtheoptimalvalues fortheessentialoperatingparameterssuchastherefluxratio, initial amountoftheentrainer,heatdutytotheboiler,etc. Accordingtoourknowledge,thisisfirstcontributionofthe separationofethylacetate–isooctanemixtureusing hetero-geneousazeotropicdistillationprocess.

Acknowledgement

ThisworkwasfundedbyTheEnvironmental&Energy Tech-nology Innovation Platform (MIP), founded by the Flemish Government.

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