comparison of extractive distillation and pressure-swing

December 30, 2020

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China Petroleum Processing and Petrochemical Technology 2020,Vol.22,No.4,pp137-146Simulation and Optimization

Comparison of Extractive Distillation and Pressure-Swing Distillation for Methanol and Acetonitrile Separation

Han Dongmin; Chen Yanhong(Department of Chemical Engineering, Shengli College, China University of Petroleum, Dongying, 257061)

Abstract: In thepresentwork,acomparativestudyof theextractivedistillationandpressureswingdistillation formethanol-acetonitrileseparationisperformedforthefirsttime.Differentseparationalternatives,includingtheconventionalextractivedistillation, theextractivedistillationwithvaporor liquidside-stream,thepressure-swingdistillationwithorwithoutfullheatintegration,andtheheat-pumpassistedpressure-swingdistillationarerigorouslysimulatedandoptimizedbasedontheminimumtotalannualcost(TAC)viathesequential iterativestrategy.TheresultsshowthatTACandCO2 emissionof thenewextractivedistillationwithvaporside-stream(Vapor-SED)aresimilar to thoseof theextractivedistillationwithliquidside-stream(Liquid-SED).Furthermore,theVapor-SEDandLiquid-SEDcanachieve30.01%and30.56%reductioninTACand23.32%and23.49%reductioninCO2emission,respectively,overthemostcompetitivefullyheat-integratedPSDconfiguration.Hence,theextractivedistillationwithvapororliquidside-streamappearstobeabetteroptioneconomicallyandenvironmentallyfortheseparationofmethanolandacetonitrile.Key words:azeotrope;extractivedistillation;pressureswingdistillation;TAC;methanol/acetonitrile

1 Introduction

Methanol and acetonitrile,which are important rawmaterials inchemical industry,havebeenwidelyusedasextractionsolvents, syntheticorganicmaterials,etc.Sincemethanolandacetonitrileformaminimumboilinghomogeneous azeotrope at atmospheric pressure and 63.5°Cwith themixturecontaining81%ofmethanol,it is impossible to separate methanol and acetonitrile mixtureby conventional distillationmethod.Hence,somespecialdistillationtechnologiessuchaspressure-swingdistillation(PSD),orextractivedistillation(ED)areneededfor thisseparation.Perhapsthereareseveralmethodscapableof separatinganazeotropicmixture.However, for different azeotropic system, themostappropriate separation method must achieve a best economicsandenvironmentaleffectofthesystem.Many researchers havemade comparisons betweenextractive distillation process and pressure-swingdistillation process for various azeotropic systems.Forexamples,Ghuge,etal.[1] studied theseparationofTHF-watersystemusingextractiveandpressure-swingdistillationmethods.HefoundthatextractivedistillationwithDMSOas the entrainer appeared tobe abetter

optionforthissystem.Luo,etal.[2] studied the separation ofisopropylalcoholanddiisopropylethermixtureusingthese twomethods. Itwasrevealed that thefullyheat-integratedpressure-swingdistillationprocesswasmoreattractive in termsof steady-stateeconomics.Similarstudieswerecarriedout forseparationofacetoneandchloroform[3],methanolandchloroform[4],di-n-propylether and n-propylalcohol[5],acetonitrileandn-propanol[6] byextractivedistillationandpressure-swingdistillation.Itcanbeconcludedthatfordifferentazeotropicsystem,theperformanceofpressure-swingdistillationandextractivedistillationvariesfromsystemtosystem.There is a problemof high energy consumption inthe distillation-based processes. Consequently, itis important to reduce the energy consumption and improve theeconomicperformance[7].Lotsofenergy-saving technologies have been proposed and applied to thedistillationprocesses,suchas theheat-integrateddistillation[8-10], theheat-pumpassisteddistillation[11-12],the dividing-wall column[13-14], the reduced-pressure

Received date:2020-03-26;Accepted date:2020-05-18.Corresponding Author: Ms.HanDongmin,Telephone:+86-13615460529;E-mail:dongminzi@126.com.

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distillation[15-19], etc.As forextractivedistillation, theextractivedividingwallcolumn(EDWC)[14]hasdrawnmuchattentioninrecentyears.However, thedesignandcontrolof theEDWCarecomplicated.Tututi-Avila[20] proposedanovelliquidside-streamextractivedistillationsystembasedonthethermallycoupleddistillationsystem.Andthisconfigurationismoreenergy-efficient thantheEDWCsystem.Wang[21-22]andMa[23] furtherprovedtheefficiencyandcontrollabilityof thisconfiguration.Butthere isno reporton thesimulationof thevaporside-streamextractivedistillationsystem.Asregardspressure-swingdistillation(PSD),thepartialorfullheatintegrationtechnology[24] and theheat-pump technology[25] can be appliedtoreducetheenergycost.But these integrationtechnologiesmayalsoincreasethecapitalcost.Thus, itisnecessarytoinvestigatethePSDprocessesmodifiedbydifferentheatintegrationtechnologiesinmoredetailfordifferentazeotropicmixtures.To thebestofourknowledge, thedirect comparisonbetween extractive distillation and pressure-swingdistillationformethanolandacetonitrileseparationisnotreportedintheopenliterature.Thepurposeofthisworkis tocompare these twomethods for theseparationofmethanolandacetonitrilemixture.Asfor theextractivedistillation system, theoptimumentrainerwas firstlychosenbasedontheVLEcurvesandtheresiduecurve.Twoenergy-efficientextractivedistillationprocesses,including the liquidside-streamextractivedistillationsystem (Liquid-SED) and the vapor side-streamextractivedistillationsystem(Vapor-SED),aredevelopedbasedon the conventional process. In regard to thepressure-swingdistillation,configurationswithfullheatintegration (HIPSD)andheat-pumpassistedpressure-swingdistillation(HPAPSD)processesareanalyzed.Theparametersofall theprocessesareoptimizedbasedontheminimumTAC.Furthermore,all theprocesseshavebeen compared based on the environmental and economic performance.Finally, themost economicandenergyefficientprocess is identifiedamongvariousprocessschemes.

2 Design Basis

In thiswork, a feed flow rateof3000kg/hwith thefeedstock containing 50%ofmethanol and 50%of

acetonitrilewas takenas thebasis for the simulation.The product purity specification of acetonitrile andmethanolwasspecifiedat99.5%.AspenPlus7.2wasusedtosimulatealltheprocesses.TheWilsonmodelforthermodynamicpropertiesstudywasusedtodescribethenon-idealityofliquidandidealvaporphasebehavior[21-22].

2.1 Basis of economic analysis

Theeconomicanalysesareevaluated in termsof totalannualcost(TAC),whichisthesumoftheoperatingcostandtotalcapitalcostdividedby3years(paybackperiod).Themorecalculationdetailscanbefoundinreferences[14]and[26].

2.2 CO2 emissions

CO2 emissions can reflect the environmental impactofdifferentprocesses. Itcanbecalculatedforagivenamountof fuelburnt.CO2emissions(kg/h)arerelatedas[27],

= × ×

where thefuelnetheatingvalue(NHV)is39771kJ/kgandthemasspercentagecarboninfuel(C)is86.5%whenheavyoil isusedas thefuel;α is theratioofCO2 and carbonmolarmasses(3.67).QFuel represents the heat duty fromfuelburnt (kJ/h),which iscalculated through thefollowingexpression.

whereQProc represents theprocessheatduty(kW),λProc (kJ/kg) is the latentheatofutilizedsteam;hProc (kJ/kg)is themassenthalpyofutilizedsteam;TFTB (°C) is theflametemperatureoftheboilerfluegas;Tstack(°C)isthestacktemperature;andT0(°C)istheambienttemperature(25°C).Asfor thesteamboiler, theflametemperature(TFTB) and stack temperature (Tstack) are adopted as1800°Cand160°C,respectively.

3　 Methanol-Acetonitrile Separation Using Extractive Distillation

3.1 Selection of entrainer

It is important toselectanappropriateentrainerfor theextractivedistillationprocess.Organicsolventssuchasaniline, ethyleneglycol,dimethyl formamide (DMF)and chlorobenzene are generally used as entrainers in the

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extractivedistillationprocess.Figure1(a)showstheeffectofdifferententrainersonVLEofmethanol/acetonitrilewithanentrainer/feedmolarratioof1.Itcanbeseenthataniline and chlorobenzene can both greatly enhance the relativevolatilitybetweenmethanolandacetonitrile. Inordertocomparethesetwoentrainersinmoredetail,the

Figure 1 Effect of different entrainers for the separation: (a) effect of differernt entrainers on VLE of methanol/

acetonitrile, (b) residue curve maps for methanol/acetonitrile/chlorobenzene system and methanol/acetonitrile/

aniline system at 1 atm

residuecurvemapsforbothmethanol-acetonitrile-anilineandmethanol-acetonitrile-chlorobenzenesystemsat1atmareanalyzed(Figure1(b)).ItcanbeseenfromFigure1(b)thatpuremethanolandacetonitrilearesaddlepointswhileanilineandchlorobenzenearestablenodes.Thereisnodistillationregion in the residualcurves forboththe twosystems.Thecurves illustrate thefeasibilityofanilineandchlorobenzeneasentrainersformethanolandacetonitrileseparation.Inaddition,whenusinganilineasthe entrainer，the intersectionpointof the isovolatilitycurveandthemethanol-entraineredgeof thetriangleiscloser to themethanolcorner,whichmeansthatanilineisamoreeffectiveentrainerthanchlorobenzeneforthissystem to some extent[26,28].Therefore,anilineischosenastheentrainerinthesimulation.

3.2　 Flowsheet of the conventional extractive distilla-tion system (CED)

The conventional extractive distillation process consists of an extractive distillation column (EDC) and anentrainerrecoverycolumn(ERC)(Figure2).Highpuritymethanol and acetonitrile products are obtained as the distillatesoftheEDCandERC,respectively.ForCEDsystem,thevariousdesignvariablesincludingthe operating pressure of EDC (P1), the entrainerflowrate (S), the totalnumberof stagesofEDC(NT1)andERC(NT2), the feedstage locations (NF1,NF2 and NFS),andthemolarrefluxratioofEDC(RR1)andERC(RR2)needtobedeterminedforachievinganoptimumperformance.Theoptimizationworkwasdonethroughasequential iterativestrategyreported inourpreviouswork[28].Figure2presents theflowsheetof theoptimalsystem.

3.3　 Flowsheet of the vapor side-stream extractive distillation system (Vapor-SED)

Thevapor side-streamextractivedistillation system(Vapor-SED)formethanolandacetonitrileseparationwassimulatedandoptimizedinthissection.Figure3showsthe flowsheetof theVapor-SEDprocess.Methanol iswithdrawnatthetopofthefirstcolumn(T1).AvaporsidestreamiswithdrawnnearthebottomofT1andisfedtothesecondcolumn(T2).AcetonitrileisremovedfromthetopofT2.TheentrainerobtainedfromthebottomofT1 and T2 is recycled to T1.

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In theVapor-SEDprocess, theoptimizationvariablesinclude theentrainer flowrate (S), the totalnumberofstagesofT1(NT1)andT2 (NT2), thefeedstage locations(NF1,NF2 and NFS),themolarrefluxratioofT1(RR1)andT2(RR2), thesidevaporlocation(NVS),andtheflowrateofthesidevaporstream(VS).Theoptimizationworkwasdonethroughasequentialiterativestrategy(Figure4)tofindtheoptimaldesigns.Figure 5 shows the optimization procedure of the

Vapor-SED system. It is observed that the optimalflowrate of the entrainer is 4 800kg/h, the optimaltotal number of stages is 48 forT1 and 21 forT2,respectively.As for T1, the best feed position ofentrainer isat the5th stage, thebest feedpositionofazeotrope is at the31th stageand thebest side-drawvapor location is at the42th stage.Theoptimal feedlocationofT2isatthe14thstage.Figure3presentstheflowsheetoftheoptimalsystem.

Figure 2 Flowsheet of the optimal conventional extractive distillation process (CED)

Figure 3 Flowsheet of the optimal Vapor-SED process

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3.4　 Flowsheet of the liquid side-stream extractive distillation system (Liquid-SED)

The flowsheet of the liquid side-stream extractivedistillationsystem(Liquid-SED)issimilartothatoftheVapor-SEDsystem,except that thevaporsidestreamisreplacedbya liquidsidestream.Figure6presents theflowsheetoftheoptimalsystem.

4　 Methanol-Acetonitrile Separation Using Pressure-Swing Distillation

The PSD process includes a low pressure column(LPC) and a high pressure column (HPC) (Figure7). The rawmaterial and the recycled distil latestreamfromtheLPCarefedtotheHPC.Highpurityacetonitrile isobtained from thebottomofHPCandthedistillatestreamisfedtotheLPC.Then,thehighpuritymethanolisdrawnfromthebottomoftheLPC

andthedistillatestream, thecompositionofwhich isclosetotheazeotrope,isrecycledtothehighpressurecolumn.

Figure 4 Optimization procedures for Vapor-SED process

Figure 5 Optimization data of Vapor-SED

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4.1 Selection of pressure

Figure 8 exhibits the influence of pressure on theazeotropiccompositionandazeotropic temperatureformethanolandacetonitrilebinarysystem.Itcanbeseenthat themole fractionofmethanol in the azeotropessignificantly increases from0.62 to 0.95when thepressurechangesfrom0.1atmto5atm. Itmeans thatthepressure-swingdistillation(PSD)isfeasiblefor theseparationofmethanolandacetonitrile.WhenthehigherpressureisperformedinHPC,theless

reboilerdutiesof thecolumnsareneeded.Buthigherpressurewould lead tohigh temperature requirementfor thereboiler.Inorder tousethelowpressurestream(433K)andensure the temperaturedifferencebetweenthereboilerandthestreamshouldbegreaterthan20K,whilethepressureofHPCissetat5atm.Thepressureof theLPCisoptimizedbyminimizingTAC.Figure9displays theeffectofpressureof theLPConTAC.Asshown inFigure9, theTAC first decreases and thenincreaseswithadecreasingpressureoftheLPC.Thiscan

Figure 6 Flowsheet of the optimal Liquid-SED process

Figure 7 The optimal flow sheet of the conventional PSD process

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occurwhenthepressureoftheLPCisbelow0.6atm,theexpensivechilledutilityfor thecondenseroperation isneeded.Thus,thepressureoftheLPCissetat0.6atm.

Figure 9 Effect of pressure of the LPC on TAC

4.2 Process optimization

4.2.1 PSD without heat integration

ThePSDprocessisoptimizedbysequentialiterativestrategyreportedbyWang[6],andtheflowrateofbottomproductsin the twocolumns isadjusted tomaintain thepurityspecificationformethanolandacetonitrile.Figure7exhibitsthedetailedinformationoftheoptimizedPSDprocess.

4.2.1 PSD with full heat integration

It canbe seen fromFigure7 that thecondenserdutyofHPC is 2.146MWand the reboiler duty ofLPCis1.777MW.Meanwhile, the temperaturedifferencebetweenthecondenserofHPC(385.1K)andthereboilerofLPC(325.2K)is large.It indicatesthat thefullheatintegrationcanbeused to reduce theTAC.Figure10shows thedetailed informationof theoptimizedPSDprocesswithfullheat integration(HIPSD).TheTACof

theHIPSDprocessis0.913×106$/a,whichisby45.88%lowerthanthatofthePSDprocess.

Figure 10 The optimal flowsheet of the PSD process with full heat integration

4.2.2 PSD with heat pump technology

Given that the temperature difference between thecondenserofLPC(313.9K)and the reboilerofLPC(325.2K) is small, thePSDprocesscoupledwith theheatpumptechnology(HPAPSD)is investigatedinthiswork. In theHPAPSDprocess, thevaporstreamfromtheLPCiscompressed toheat the liquidstreamof thereboiler.Figure11showstheflowsheetandtheoptimizedparameters in theHPAPSDprocess.Theoperatingcostof theHPAPSDprocess is 0.676×106 $/awhich cansave0.658×106$/aascomparedwith theconventionalPSD process.TheTAC of theHPAPSD process is 1.067×106$/a,whichisby36.75%lowerthanthatofthePSDprocess.Theresultsdemonstratethattheuseofheatpumptechnologyshowsabettereconomicperformance.

5　 Comparison of PSD and Extractive Distillation for Methanol-Acetonitrile Separation

ThekeyeconomicperformanceandCO2emissionofalloftheprocessesaresummarizedinTable1.Theresultsshowthatthetotalannualcostoftheextractiveprocess(0.68×106 $/a)issubstantiallysmallerthanthatofthepressure-swingdistillationprocess(1.687×106$/a).Heatintegrationandheat pump technology can be applied to thepressure-swing

Figure 8 Effect of pressure on azeotropic composition and temperature

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processtoreducetheoperatingcost.AsshowninTable1,incomparisonwiththeconventionalPSDprocess,theHIPSDprocessandtheHPAPSDprocesscansaveTOCby55.62%and49.33%,canreduceTACby45.88%and36.75%,andcandecreaseCO2emissionby45.06%and42.23%,respectively.ThePSDconfigurationwithfullheat integration is more economical as compared to the heatpumptechnology,becausethecapitalcostincreasessignificantlyincaseofHPAPSDsystem.Incomparisonwiththeconventionalextractivedistillation,theVapor-SEDsystemandtheLiquid-SEDsystemcansaveTACby6.03%and6.76%,andcanreduceCO2emissionby13.95%and14.13%,respectively.Thiscanoccurbecausetheuseofthesidestreamcolumncanreducetheremixingdegreeofthecomponentsinthecolumnsothattheoperatingcostissaved.TheLiquid-SEDsystemrequires30.56%lessTACand23.49%lessCO2emissionthanthoseoftheHIPSDsystem.Inaddition,inordertoevaluatetheeconomicsofrelevantprocessesmorecomprehensively,wefurtherinvestigatetheeffectofpaybackperiodonTAC(Figure12).Itcanbeseenthatwhenthepaybackperiodincreasesfrom1yearto15

years,theTACdropssignificantlyinthefirst4yearsandlaterflattens.TheTACoftheconventionalPSDprocessisthehighestateachpointonthepaybackperiod,whiletheTACoftheLiquid-SEDandVapor-SEDprocessesistheleast.TheseresultsfurtherindicatethattheVapor-SEDsystemandtheLiquid-SEDsystemaremuchmoreattractiveformethanol/acetonitrilemixtureseparationincomparisonwithconventionalextractivedistillationandPSDprocesses.

Figure 12 The effect of payback period on the TAC ■—CED;●—Vapor-SED;▲—Liquid-SED;▼—PSD;◆—HIPSD;

◄—HPAPSD

Figure 11 The optimal flowsheet of the PSD process with heat pump technology

Table 1 Economic analysis results of different processes Item Reboilerduty,MW Condenserduty,MW TOC,106$/a TCC,106$ TAC,106$/a CO2emission,kg/h

CED 1.867 -1.267 0.474 0.618 0.680 717.94

Vapor-SED 1.372 -1.821 0.427 0.633 0.639 617.82

Liquid-SED 1.369 -1.817 0.427 0.621 0.634 616.47

PSD 4.208 -4.133 1.334 1.059 1.687 1466.49

HIPSD 4.347 -2.239 0.592 0.963 0.913 805.73

HPAPSD 4.191 -2.451 0.676 1.173 1.067 847.21

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6 Conclusions

Inthispaper,differentprocessesforseparatingmethanolandacetonitrilemixturehavebeendevelopedandoptimized.Also, thecomparisonhasbeencarriedoutbetweentheextractivedistillationwithvapororliquidside-streamandthepressureswingdistillationwithheatintegrationorheatpumpbasedontheenvironmentalandeconomicperformance.Theresultsshowthat theTACandCO2emissionofthePSDprocesswithoutintegrationarethelargestamongalltheprocesses.Eventhoughtheheatintegrationtechnologyandheatpump technology requirebya45.88%anda36.75%lessTAC,respectively,ascomparedtothoseoftheconventionalPSDprocess,theenergycostandTACarestillmuchlargerthanthoseoftheextractivedistillationprocesses.Furthermore,theVapor-SEDandLiquid-SEDgivea30.01%anda30.56%reductioninTAC,a23.32%anda23.49%reduction in CO2emission,respectively,ascomparedwiththemostcompetitivefullyheat-integratedPSDprocess.Therefore, upon considering the apparent economicand environmental benefits, the proposed extractivedistillationwithliquidorvaporside-streamisanattractive choicefortheseparationofmethanolandacetonitrile.

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Novel Cyclohexanol Dehydrogenation Catalyst for Manufacture of Cyclohexanone Passed Appraisal

OnJuly29,2020theproject“Commercialapplicationofnovelcyclohexanoldehydrogenationcatalystformanufactureofcyclohexanone”undertakenbytheSINOPECNanjingChemicalResearchInstitute(NCRI)haspassedtheappraisalofresearchachievementsorganizedbytheSINOPECScienceandTechnologyDivision.Theexpertsattendingtheappraisalmeetinghaveadmitted that theperformance indicatorsof thedehydrogenationcatalystNDH6have reached theadvancedlevelofsimilarinternationalcatalysts.Currently the SINOPEC Nanjing Chemical Industry Company has applied this catalyst tomanufacturecyclohexanone fromcyclohexanolwith the capacityofprocessunit reaching160kt/a.Theheat-conductingoil system is adopted in both benzene hydrogenation andcyclohexanoldehydrogenationprocesses, inwhichtheoutlet temperatureofheat-conductingoil isabout240°Cforpreheatingbenzene,while the temperatureofheat-conductingoil forpreheating thecyclohexanoldehydrogenationsystem is230℃at the initial stage

andthenincreasesto260℃atthefinalstage.NCRIhasindependentlydeveloped theNDH6typecyclohexanoldehydrogenationcatalyst,whichwith theadditionofcocatalyst can further improve the low-temperaturecatalyticactivityandextendthecyclelengthofcatalystduringthelowerpreheatingtemperaturestage(<210℃).The surplusheat frombenzenehydrogenationcanbeused to maintain the heat needed by the dehydrogenation system, resulting in reducedoperatingcostandenergyconsumptionfordehydrogenationoftherecyclestream.Theresultofcommercialapplicationof theNDH6typecatalystinthe60kt/acyclohexanoldehydrogenationunithas revealed that under standard conditions the catalyst operated smoothly,with thecyclohexanol conversionreachingmorethan55.0%andthecyclohexanoneselectivityexceeding99.0%.Comparedwith the traditionalCu-Zndehydrogenationcatalyst, theNDH6 typecatalystcanincreasethecyclohexanolconversionby4%⸻5%alongwithareductionofsteamconsumptionequatingto1.34t/h.