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University of Nebraska - LincolnDigitalCommons@University of Nebraska - LincolnPapers in Termal MechanicsChemical and Biomolecular Engineering Researchand Publications12-1-2004Termodynamic Analysis Of Separation SystemsDr.Y. DemirelUniversity of Nebraska Lincoln, [email protected] this and additional works at: htp://digitalcommons.unl.edu/chemengthermalmechPart of the Heat Transfer, Combustion CommonsTis Article is brought to you for free and open access by the Chemical and Biomolecular Engineering Research and Publications atDigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Papers in Termal Mechanics by an authorized administratorof DigitalCommons@University of Nebraska - Lincoln.Demirel, Dr.Y., "Termodynamic Analysis Of Separation Systems" (2004). Papers in Termal Mechanics. Paper 2.htp://digitalcommons.unl.edu/chemengthermalmech/2SEPARATION SCIENCE AND TECHNOLOGY Vol. 39. No.16, pp.3897-3942,2004 REVIEW Thermodynamic Analysis of Separation Systems DepartmentofChemicalEngineering, Virginia PolytechnicInstitute and State University,Blacksburg,Virginia, USA ABSTRACT Separationsystemsmainlyinvolveinterfacialmassandheattransferas wellasmixing.Distillationisamajorseparationsystembymeansof heat supplied from a higher temperaturelevel at the reboiler and rejected inthe condenser ata lowertemperaturelevel.Therefore,it resemblesa heatengineproducingaseparationworkwitharatherlowefficiency. Lostwork(energy) in separation systems is due to irreversibleprocesses ofheat, mass transfer,and mixing,and is directly related to entropy pro- ductionaccordingtotheGouy-Stodolaprinciple.Inmanyseparation systems ofabsorption, desorption,extraction,and membraneseparation, the major irreversibility is the mass transfer process. In the last 30 years or so,thermodynamicanalysishadbecomepopularinevaluatingtheeffi- ciencyofseparationsystems.Thermodynamicanalysisemphasizesthe *Correspondence:YagarDemirel,DepartmentofChemicalEngineering,Virginia PolytechnicInstituteandState University,Blacksburg,VA24061,USA;Fax:540- 231-5022; E-mail:ydemirelOvt.edu. DOI:10.1081/SS-2000411520149-6395 (Print); 1520-5754 (Online) Copyright Q 2004 byMarcel Dekker, Inc.www.dekker.com Request Permissions 1 Order Reprints p 0 we r e d b y RI 5 F,!>?,,-,, #/ 3898Demirel use ofthe second law ofthermodynamicsbeside the first law. and may be applied through (i) the pinch analysis, (ii) the exergy analysis. and (iii) the equipartitionprinciple.The pinchanalysisaimsa betterintegrationofa processwithitsutilities.Itisoneofthemostlyacceptedandutilized methodsinreducingenergycost.Exergyanalysisdescribesthe maximumavailable work whena form ofenergyis convertedreversibly toa reference systeminequilibriumwiththe environmentalconditions; hence,it can relatethe impact ofenergy utilizationto theenvironmental degradation.Ontheotherhand.theequipartitionprinciplestates thata separationoperationwould beoptimumfor aspecified setoffluxes and agiventransfcrareawhenthethermodynamicdrivingforcesareuni- formlydistributedinspaceandtime.Thermodynamicanalysisaimsat identifying.quantifying,and minimizingirreversibilitiesinaseparation system. This study presentsan overviewofthe conventionalapproaches andthethermodynamicanalysistoreduceenergycost,thermodynamic cost.andecologicalcostinseparationsystemswiththemainemphasis ondistillationoperations.Some casestudiesofcostreductionbasedon thethermodynamicanalysisare also included. KeyWords:Distillation;Thermodynamicanalysis;Pinchanalysis; Exergyanalysis: Equipartitionprinciple;Thermoeconomics. INTRODUCTION Distillation is a major separation system inchemical process industries. It usesheatsuppliedathighertemperaturelevels,andrejectsalmostequal amount ofheat in the condenser at lower temperaturelevels yieldinga separ- ationworkofmixtures.Therefore,itisanenergyintensivesystem,and accountsmorethan3%oftheenergydissipationintheUnitedstates.['] In thelast50years,reductionofenergyconsumptionindistillationattracted intensiveresearch.Earlierresearchismainlyconcentratedonoptimum refluxratioandcolumnpressure.Besidethat,theretrofits,suchasheat- integratedcolumns,applicationofheat pumps, changingfeed stage location, andusingfeedsplittinghavealso beenpopular.Later, researchersexplored theuseofprinciplesofthermodynamicsinreducingthecostofseparation systems,particularlyindistillationoperations.Theexcessivecostofsepar- ationsystemsresultspartlybecauseofenergydissipationorlostwork,and combinationofthefirstandsecondlawsofthermodynamicscanidentify andquantifythelostworkdue to irreversibleprocesses.Effortsto minimize theentropyproductionhavebecomepopularsince,accordingtotheGouy- Stodolaprinciple,thelostseparationworkisdirectlyrelatedtoentropy productionresultingfromirreversibleheat,masstransfer,andmixing.This Thermodynamic Analysis ofSeparation Systems3899 innovative approach is called the thermodynamic analysis, and started with the pinch analysis, whichintegrates a process withitsutilitiesin a way to reduce the cost of energy. Later exergy analysis was developed to identify the parts of systems withexcessiveirreversibilitiesand,hence,tocontrolthelostwork. Some researchershave recentlycombined the pinchanalysis and theexergy analysis, andrelatedthermodynamicimperfections withenvironmentalcon- cerns.Basedonthedevelopmentsinnonequilibriumthermodynamics (NET),somerecentresearchhasreportedtheimplicationsoftherateof entropyproductionontheuseofavailableenergyinseparation Equipartitionprincipleisoneresultofsuchresearch,anditstatesthatthe uniformdistributionofthermodynamicforcesinspaceandtimecan improve the thermodynamiceffectiveness of separation systems. The thermo- dynamics approach mayhavewideimplicationsin reducing the energycost, thermodynamiccost(imperfections), andenvironmentaldeterioration.With thisinmind,thisstudypresentsacriticalevaluationofvariousapproaches for reducing the cost of energy in separation systems with the emphasis on dis- tillation.Withinthenextsections, some oftheconventionalapproachesand relativelyrecentinnovativeapproachesofthermodynamicanalysisarepre- sented. Some case studies on reducingthe energy cost byseveral approaches are also presented.Finallythermoeconomics are brieflypresented. SOME CONVENTIONAL APPROACHES Approaches for energy saving in distillationsystems mayvaryaccording to the number of components, nature of mixtures, and utilityconstraints. Most of the conventional approaches involve internal and external modifications and a better integration of columns with the rest of energy exchanging systems. For well-balanced,nearly ideal mixtures, the most useful configuration is to separ- ate purecomponents ineach columninsuccessive order ofdecreasing vola- tility.However,formixtures,suchas thosecontaininglargeproportionsof lessvolatilecomponents,eachcasemustbeconsideredindetailtosave energy,whichmaybes~bstantial. "~Largeconcentrationchangesinmulti- component mixture separations maylead to considerable energy losses, there- fore, the key components should be removed from the feed mixture. As seen in Fig. 1,light-nonkey components can be removed by using an absorber, and the bottomproductsoftheabsorberprovidethefeedtothemaindistillation column. Similarly, heavy-nonkey components are removed by using a prestrip- per, and the over products of the stripper become the feed of the main distilla- tioncolumn.Thesemodificationscanreducetheloadofthecolumnfor debottleneckingandtherequirednumberofstages.["s1Arecentworkcon- siders thefeedcompositionandrelativevolatilitiesforsequencing columns Absorber n 1 Feed Lights Light T Demirel Main column Light bottoms * Heavy Main columnStripperbottoms (a)(b) Figure1.Prefractionationarrangements:(a)removinglightkeyswithabsorber, (b) removing heavykeys withstripper. with respect to their costs of 0~er at i 0n. l ~~Hybrid processesofdistillationand vapor permeation can be alternativesto azeotropic and extractive distillation and lead to improved separationsystems.[7J Thermally coupled multipleeffect distillationcolumns are well known in energysaving;L53S1thefeedissentintotwocolumnsoperatingatdifferent pressures,andtemperaturesofthecondensingvaporsandboilingliquids willbedifferentfromeachotherbyaminimumtemperaturedifference AT,,,.AsFig.2shows,theheatfromthecondensingvaporinthecolumn belowistransferredtothe reboilerwithinthecolumnabove. Feedsplitcan beadjustedtohaveequaldutiesofadjacentboilingandcondensing streams.Therefore,theheatdutiesofboilingandcondensation,Qett,are approximatelyequaltotheheatdutyforasingle effect,Q,dividedbythe number ofeffects(QeR= Q/Nef).This saves energydespite anextra operat- ing and capital cost ofequipment.[']On the other hand. ina diathermal system withheatexchangers allalong thecolumn,or interstage heatexchangers, or internaltraydesignwithheatexchangingcoils,itispossibletoadjustthe flowratioofthephasestovarytheslopes ofoperatinglines.Consequently, theoperatinglinesbecomeclosertoequilibriumcurves,and,hence,the irreversibilityduetomasstransfercanbereduced.Howeverthisrequires Thermodynamic Analysis ofSeparation Systems Figure 2.Two-effect distillation column;[51B: bottom product. D: distillate. tallercolumns,and, hence,an economicalanalysisto evaluate thegainsand losses,[91 althoughthesidereboilersreducetheconsumptionofexpensive high-pressuresteam. Optimumlocations ofheatingand coolingzonescould leadtomaximumexchangeableenergyloads.[101 Suchmodificationshave been mainlycarried out for binarysystems, although some work on heatinte- gration for multicomponent distillationsystemsisalso reported.[","] Column and heat exchanger networkintegrationinrefineryoperations is highlypop~lar.['3-221In the synthesisofa heatexchanger network, the main objective is to determine the maximum energyrecoverybased on the heating and coolingrequirementsofthe processstreams leading to the minimum hot andcoldutilities,whichcanbecalculatedbyalinearprogramming 3902Demirel approach.[51Colunlnretrofitmodelsandheatexchangernetworkretrofit modelsmaybeoptimizedbasedonasuccessivequadraticprogramming solver.['']Lessenergyconsumptionandmorefreecapacityataminimum capitalinvestment maybe possible byinstallingintermediate re boiler^,['^"^' pumps-aroundatcertainlocations.andadjustingthecoolingdutyforeach pump-around,[151preflashingunitsbeforethecrudeoildistillationunit,"61 andreducingtheoperatingpressureandincreasingthepreflashoverhead vapor(Fig.3).L5,181Thereboilerflashingisrelativelythemosteconomical among these configuration^.^'^Forcloseboilingfeeds for whichsmallpres- surechangesarerequired,thecostofcompressionisnottoohigh.Inthe closeboilingsystemofpropylene-propaneseparation,aheatpumplowers theannualcostbyabout3770, andminimizesthefluegasemissionsby about 60%.['~] The optimalitycriterionmaybethe payback periodfor a pre- liminaryeconomicanalysisofheatpump-assisteddistillation~ ~ s t e r n s . ~ ' ~ ]However, designersshould consider the powerconsumption andwaterman- agement aspects ofheat pump operations for an economicalerati ti on.['^-'^] Attainable region analysis is a graphical optimizationmainly used to find a complete set of all possible outcomes from a specified feed set. It is applied to a binarydistillationoperationwithsidecondensers andreboilers,andthe attainable regionofcomposition, cost, andreflux/boilup ratiospaceiscon- structed.['"Cost of the heat transfer units is included in the objective function to be minimized; the optimized solution could reduce column size and energy cost up to1~%. ~". ' ~]Feedconditioningisanotherimportantexternalmodification;itisless expensive compared with theinter heating or inter cooling,and feed precool- ingorpreheatingcanbeusefultounloadthetoporbottomsections ofthe column.Coldfeedmayrequirealargeamountofheatexchange belowthe feedstagetostripthelightcomponents.Usingaprocesssimulator, optimum split ratioand feed locationcan be obtained; feed splittingand pre- heatingwith the bottomproduct can save up to 50% energy (Fig. 4).L'9.'01 Of course,oneshouldevaluatethoroughlytheeconomiccostofinternaland externalmodifications,whichmaybeidentified and evaluated byincorporat- ingtheprinciplesofthermodynamics;forexample,itmaybepossibleto reduce the exergyloss or to distribute the driving forces as evenlyas possible inthemodifiedsystem.Thermodynamicanalysisispresentedinthenext sections. THERMODYNAMIC ANALYSIS Efficiencyinseparation systems is often calculated from thefirstlawof thermodynamics.However,sincethermalenergycannotbeconvertedinto Thermodynamic Analysis of Separation Systems3903 (a) Heat pumping(b) Vapor recompression (c) Reboiler flashing Figure3.Variousdistillation ~onfi~urations;~' ~B:bottomproduct, C:compressor, D:distillate, V: valve. workcompletely,thequalityofthermalenergyshouldbetakeninto Asdistillationsystemsconsumeabout95%ofthetotal thermal energy usedin separationsystems in the United ~t at es , ' ~" a consider- ablepartofthisenergyisdissipatedintotheenvironment. Thermodynamic analysiscanidentifythepartoftotalenergyconvertibletowork,and, hence,thepossiblewaysofreducingthedissipatedenergy. Itcalculates the entropyproductionduetoirreversibilities,whichisdirectlyproportionalto Demirel Figure4.Splitting thefeedinsuchawaythatthenon-heatedfractionkeepsthe refluxratio lowandtheheatedone recovers energyfromthe reboi~er.~"' the dissipationofavailable energy. However, howto usethe thermodynamic analysis to optimize aseparation system hasnot always beenclear. The rateofentropyproductionisalwayspositiveandcalculatedasthe productoffluxes andthermodynamicforces operating withina stern;^"^',^^^ fluxesareexpressed as linear functionsofalltheforces whenasystemis not far from global equilibrium. The keyconcepts for thermodynamic analysis are: (i)availability(exergy),(ii)lostwork(dissipation),and(iii)environmental cost due to thermodynamic imperfections. Ageneral energy balancefor acontrolvolumewithmultiple streams is wherethefirsttermisthechangeininternalenergy,thesecondisthenet changeofenthalpy,kinetic,andpotentialenergiesofflowstreamswithin the control volume, Q,is the heat input rate from the surroundings at tempera- ture To,CQ,is the heatinput rate from a reservoirat temperature T,, and 2% shows the work that includes mechanicalshaft work. expansion or contraction work, and electricalwork. Assuming that the kinetic and potentialenergy over thecontrolvolume are negligible,Eq.(1) becomes Thermodynamic Analysis ofSeparation Systems3905 Entropy balancefor thesame controlvolumeand itssurroundings is Here the first term is the change of entropy of the system, the second is the net rate of entropyflow,- Q,/T,is the rate of decrease in entropy ofsurrounding atTo, and-CQ, /T, isthesumofthe ratesofentropydecrease oftheheat reservoirsatvarioustemperaturesofTi.Theterm@showstherateof entropyproductiondue to irreversibility,whichiszerowhenprocessesand heatflowsbetweenthesystemanditssurroundingarereversible.Equation (3)showsthatentropyis notconserved.EliminationofQ,inEqs.( 2)and (3) yields From the firstand second termswe haveaquantitycalled theavailabilityA: A= H- T,S.Change in A represents the minimum work required to achieve a change.Availabilityisrelatedtothemaximumusefulworkasystemcan deliverwhenitis broughtto equilibriumwiththeenvironmentalconditions inreversiblemode. The last termin Eq. (4) To@ is called therateoflostwork LW or lossofavailability,orexergydestroyed.Thelostworkisaquantitative measureofthethermodynamiccostorimperfectionsinasystem,andis related to availabilitythroughEq.(4) HeatandworktermsinEq.(6)showtransferredavailabilitiesbetweena systemand itsenvironment. Forasteady-stateprocessthelostworkcanbe related to the change inavailability,heat,and workterms.and we obtain 3906Dernirel The lostworkhastwoimportantfeatures:(i)itidentifiesandquantifies the powerlost due to variousirreversibilities,and (ii)itrelates theevolution ofasystem to the environmental conditions.For asteadystate and adiabatic system, Eq.(6) becomes Equation(8)showsthatwhenavailabilitydecreases, and workis trans- ferredfromsurrounding tosystemthenthelostworkwillbepositive:the maximum work that a system can deliver would be the decrease in availability, while the minimumworkwouldcorrespond to the increase inavailability Only, the zero lost work hasno impact on the environment. Reducing the cost in a separation system needs careful elaboration of the concept"cost."Thelostworkcausestheinefficientuseofenergy(lossof exergy),andenvironmentalcostdueto(i)discharginglostexergyintothe environment, and (ii) the depletionofnatural resources becauseof inefficient useoffossilfuels. Hence, the lostworkmayaffect thesustainable develop- mentadversely. Inmostofthecontinuous distillationsystem, thenetavail- abilityincreases becauseoftheheatinputinthe reboiler,and thedifference betweentheavailabilityofproductsandfeedstreamsdeterminesthe minimumworkrequiredfor a reversibleseparation The thermodynamicefficiencyis expressed byusingthe lostwork The thermodynamic efficiency of distillation systems is generally low, and the ther- modynamicanalysis may lead to innovative systems with increased efficiency by decreasing the thermodynamic imperfections and, hence, the lost ~or k. [ ' , ~, ~' ~~" ~' ]Case Study: 1. Distillation of Propylene-PropaneMixture Propylene-propanemixture is a closed boilingmixture. A refluxratioof 15.9 (close tominimum)and200equilibriumstagesarenecessary.Table1 shows the enthalpyand entropies of the saturatedfeed and saturated products Thermodynamic Analysis ofSeparation Systems3907 TableI.Conventionalcolumnoperationfortheseparationofpropyleneand propane. Thermodynamic properties are estimated bySoave-Redlich-Kwong equation ofstate.['] TemperatureFlow rateEnthalpyEntropy StreamKkrnol/hkJ/molkJ/mol K Feed325272.213,338- 4.1683 Distillate3 19.5189.212,243- 13.8068 Bottoms330.911314.687-2.3886 from thesimulationresultswiththe Redlich-Soave equationofstate.15'The reboilerandcondenserdutiesare 8274.72 kW and8280.82 kW, respectively. The reference temperatureis 294 K. The lost work is obtained from Eq. (7) as LW = 1902.58 kW.AvailabilityanalysisyieldsW,,,= C,,,?zA- CinrzA = 140.81 kW, and the thermodynamicefficiency7 is Thelowexergeticefficiencyistypicalfordistillationsystemswithclose boilingmixtures, and whena large amount ofenergyis required in the reboi- ler.Analternativeistousereboiler-liquidflashingasshowninFig.3(c), wherethefeedhasthepressureof108psiabyapower-recoveryturbine. Acompressorisusedtoreturnthereboiledvaportothebottomofthe column.Therequiredreboilerdutyissomewhatlargerthantherequired condenserduty.anauxiliarysteam-heatedreboilerisneeded.So thepower usedinthecompressor is tradedoffwiththelargereductioninthereboiler steam.[51Thealternativedistillationsystemhasproducedthelost work = 501.6 kW, availabilityW,,,,= 38.2 kW.andtheefficiencyof38.21 (38.2 + 501.6) = 7%.Thelostwork,501.6kW, issmallercomparedwith theconventionalcolumnof1902.58 kW.yetthecolumnefficiencyisstill verylow. Case Study: 2. Distillation ofa Five-Component Mixture The second columnhasa feed withfive componentsofethane, propane. n-butane,n-pentane,andn-hexane.Table2showstheconfigurationofthe columnandthesimulationresultsobtainedfromtheAspenPluswiththe Peng-Robinsonequationofstate.Thecolumnhasthecondenserdutyof 3395.336 kWandthereboilerdutyof3432.206 kW.Thecondenserand the 3908Demirel Table2.Columnconfigurationforafive-componentdistillation.Simulation resultsfromAspenPlus(Version-1 1). Thermodynamicpropertiesareestimatedby thePeng-Robinsonequationofstate.Qc = 3395.3367 kW;QR = 3432.2069 kW: N = 14; NF = 7;RR= 8.87. FeedDistillateBottom Flow(kmol/h) Pressure(Atm) Temperature (K) Vaporfraction Enthalpy(kJ/kmol) Entropy (kJ/kmol K) Compositions Ethane n-Propane n-Butane n-Pentane n-Hexane reboilertemperaturesare319.3and400.2K,respectively.Therelerence temperature(dead state temperature),To,is assumed to be300 K.The simu- lation resultsshowthelostwork = 531.37 kW,Wn,i,= 117.49 kW,andthe efficiencyfrom Eq. (12) as18.1%. PINCH ANALYSIS Pinchanalysisoptimizessystemswiththeirutilitiesusingthe principles ofthermodynamics.Thesecondlawdetermines thedirectionofheatflow, and preventscrossovers of the hot and cold stream temperatures.Temperature- enthalpydiagrams calledthecomposite curvesrepresentthe thermalcharac- teristicsofhotandcoldstreams(Fig.5).Hotandcoldstreamscanonly exchangeenergyuptoaminimumallowabletemperaturedifferenceAT,,,. ThetemperaturelevelatwhichAT,,,isobservedinthesystemiscalled thepinchpointorpinchcondition,whichdefinestheminimumdriving force, hence.theminimumentropyproductionallowedinanetwork.Pinch iseasytoidentifybythecompositecurves,andapproaches zeroas thearea forheattransferequipmentapproachesinfinity.Abovethepinch,onlythe hotutilityisrequired,whileonlythecoldutilityisrequiredbelowthe pinch.andnoheatshouldbetranslerredacrossthepinch.Forestimating the minimum hot and cold utilities required, Linnhoff and ~ l o ~ e r ~ ~ ~ ~ ~ ~ ~ devel- opedthetemperature-intervalmethodbasedontheworkof~ o h m a n n . ~ ~ ~ 'Thermodynamic Analysis of Separation Systems jHot utility Figure5.Hotandcold composite curves. Similarly,grandcompositecurvesshowthevariationofheatsupply anddemandinasystem.Thesediagramsenableengineerstoidentifythe suitable utilityand target appropriate loads for variousutilitylevels bymini- mizingtheexpensiveutilitiesandmaximizingtheleastexpensiveutilities, network area, and number of heatexchanger units; theyalso provideinsights foroptimumintegrationofdistillationcolumns.evaporators.condensers, furnaces.and heatpumps to reducetheutilityrequirementsofthecombined system. Anincrease inAT,,,causes higher energycostsand lower capitalcosts (less heatexchanger area). Forexample,anincreaseof5C fromavalueof AT,,,= 10Cdecreasesheatexchangerareaby11%andincreasesthe requiredminimumenergybyabout9%.14']Tofindthevalueofoptimum AT,,,,totalannualcostisplottedagainstAT,,,(Fig.6).Anoptimum AT,,,exists where the totalannual cost ofenergyand capital costs is mini- mized. Once the AT,,,is chosen. minimum hot and cold utilityrequirements can beevaluated fromthe composite curves.Since heatrecoveryand utility system constraints are consideredsystematically,the pinch analysis canesti- mate the reduced annual cost in networks by comparing the cost of fuel and the capital cost ofa network. It is possible to obtainan accurate estimate (within 10%-15%) of overall heat recoverysystem cost without having to designthe system.[423431Pinchanalysishasbeenappliedwidelyinindustryleadingto Demirel Cost totalcost 1capitalcost .-\, - - - _ Figure6.Optimum ATmi, fromenergycostand capitalcost changes. considerablesavings,L413451anditcanidentifyenergytargets,minimum driving forces, and capital cost targets.[461Dhole and ~ i n n h o f f ' ~ ~ ' developed thevaporandliquidcompositecurvesfor representingcombinedheat-and masstransferlossinacolumnsystem.Some ofthepinchtechniquescon- sistofminimizingpressure-dropeffects,waterandwastewater,andplant emission^.[^^-^^^ Pinch analysis can specify the exchanged heat and mass betweenhot/rich andcold/leanstreams basedonthefirstandsecond laws.Accordingtothe second lawofthermodynamics: (heatlmasslost byhotlrichstreams belowthe pinchpoint) -(heat/massgained bycold/leanstreams belowthepinchpoint)5 0 (13) Using the state space approach to processsynthesis, heat and mass exchanger networkrepresentationofdistillationsystemscanbeanalyzedandopti- m i ~ e d . [ ~ ~ ~ ~ ' ] Pinch analysis has also been extended to the integration of chemi- calreactorsystemswithheatoptimizationofindustrial ammonia plant,[531and nitric acidFor example, column grandcom- positecurves;l19-~l.46.4~lcan be usedto modifythe column and heat exchan- ger network; a possible modificationis the useofheatpumpsincolumnsby identifyingtheheatsinksandsources,leadingtoconsiderablesavingand ashorter paybackperiod.[471 Some of theadvantages ofthe pinchanalysis over conventionalones are the ability to set energy cost and capitalcost targetsfor a network, update the Thermodynamic Analysis of Separation Systems Feed Bottoms Reboiler Figure7.Distillationcolumnasaheatenginebetweenreboiler(R)andcon- denser ( c ) . ~ ~ ~ ]processBowsheeting,anddebottleneckingofdistillationcolumns(Fig.7). However,someofthemodificationsimposedbythepinchanalysismay requiresubstantialcapitalinvestmentsandchangesininternalstage design ofdistillationcolumns.Also,pinchanalysismaynotdeliverthedesired result,unlessit isappliedbeforecompletion ofthe processdesignstage and inconsultationwiththeprocessspecialists.Theanalysiswillbesuccessful iftargettemperaturesandutilitiesareset on thebasisofprocessobjectives ratherthanonflowsheeting.For example, aflowsheet maymixtwostreams with different temperatures to prepare a feed for a process.This causes degra- dationofavailable energyor thermodynamicdriving force. To preventthis, the temperaturesof bothstreams should be increasedto the processoperating temperature.Also, heat recovery from specialstreamslike two-phase streams should becompletedinasingle heatexchanger due tophaseseparationand large pressure drops; also, the destinationofprocessstreams should be fully evaluatedtoavoidadverseeffectsofstreamswithhazardouschemicals. However,processintegrationwouldbemorecomplete andmeaningfulifit targetsenvironmentalprotection,emissioncontrol,anddepletionofnatural resourcesbesidethecostofenergy. It isencouraging tonote thatthisissue is addressed in several extensions of the analysis.["7'"1Moreover, the software developed for processintegrationshould be able to interact with the available simulation software toaccess awiderangeofdesign models.[491 Dernirel Case Study: Pinch Analysis: Column Grand Composite Curves: T-H, or Stage-H Forthecolumnconfiguration describedinTable2,thecolumngrand compositecurves(T-H; Stage-flforafive-component mixturehavebeen obtainedusingtheAspenPluscolumn-targeting toolcapability forthermal analysis.Thisanalysisishelpfulinidentifyingthetargetsforappropriate modifications in order to reduce utility and capital costs, improve energy effi- ciency. and facilitate column debottlenecking. Itis basedonthermodynami- callyreversiblecolumnoperationatminimum refluxwithappropriateheat integration. The columngrand composite curves (Figs.8a, 8b) are basedon thepracticalnearminimumthermodynamicconditionapproximationpro- posedbyDholeand~innhoff,' ' ~' andshowthetheoreticalminimum heating andcooling dutieswithin the temperature range. The stage-enthalpy calculations takeinto account the losses or inefficiencies stemming from the actualcolun~ndesign,suchaspressuredrops,multiple-sideproducts,etc. Figure8(a) canbeusefulfor identifying the targets for feed preparation and location.refluxratio,andheatintegrationmodifications.Thecolumn's grand composite curves indicate distortions as significant projections around feedstagelocation(pinch point),ifthecurrentfeedstage isinappropriate. Figure 8b shows a distortion at the pinch point (stages 8 and 9). To compensate inappropriate feedstagelocation, extra localrefluxmaybeneeded. Beside that,afeedstagetoohighuportoolowinthecolumnwilldisplaysharp enthalpychangesonthecondenserandonthereboiler,respectively.The sharp enthalpy changes onthe grandcomposite curvesindicate the needfor adjustment offeedquality.Figure8showsalmostequalenthalpychanges onboththereboilerandcondensersidesofthecurves.Howeverasharp enthalpychangeonthereboilersidesuggeststhatthefeedissubcooled, andapreheatershould beinstalled. Thehorizontalgapbetweenthepinch point and the ordinate inFigure 8a, which is about 200 kW, indicates the pos- sible reduction in heat duties byreducing the reflux ratio withthe expense of increasing number of stages to achieve the specified separation. Obviously, the increaseinthecapitalcostforatallercolumnshouldbetradedoffwith savings in utility costs. Figure 8a also shows that the reboiler side is relatively close to ideal operation while the condenser side is far from ideal operation. The significant area underneath the pinch suggests the need for a side conden- ser at anappropriate temperature level. The need for heat integration through side condensing orside reboiling could bequantified from thearea between the ideal and actual enthalpy profiles after considering the capital cost increase due to the modification. However, external modification offeed conditioning isusuallypreferredtointernalmodificationofheatintegration.Heat integrationbypositioningthesidereboilersandsidecondensersina 3914Demirel column has similarities to useof hot and cold utilities in efficient heat exchan- ger network. Pinchanalysisisawell-establishedtoolindesigninganefficientheat exchangernetwork.Inthecontextofdistillation,thesignificanceofthe pinch is that there should be no side reboiling below the pinch and no side con- densing above the pinch in a heat integrated column. Still, the pinch analysis is constantly being expanded to optimize a whole plant operation containing not onlyheattransfer,butalsoseparationandreactionunitsaswell.Some examples ofsuchexpansionswithcasestudiesareheatintegratedcrudeoil distillationsystems,['01totalprocessenergyintegrationinretrofittingan ammonia plantwith44 hotandcold~treams, "~]heatexchanger networkof a nitricacidplant,L541andcombinationofthechemical reactornetworkwith the heatexchangernetwork.[521 EXERGY ANALYSIS Thequalityofenergyalwaysdegradesinaprocess.Exergyisthe maximumavailablework whensome form ofenergyis convertedreversibly toareferencesystem,whichisinthermodynamicequilibriumwiththe environment, andhasnoabilitytoperformwork.Exergyalsoisameasure ofdistanceofasystem fromglobalequilibrium;astheexergyisconsumed thestatevariablesoftemperature,pressure,andcompositionofsystem approachthoseoftheenvironmentalconditions.['61 Therefore, thereference stateiscalledthedeadstate.["]Thetotalexergyofmulticomponent streamsiscalculatedfromthethreecontributions:exergychangedueto mixing,chemicalexergy, and physicalexergy,['.'5~57-661andis expressed by wherethesubscript o indicates theenthalpy, entropy,and temperatureofthe environment.The exergyofmixingresultsfromtheisothermalandisobaric mixingofstreamsat actualprocessconditions.Thechemicalexergyisthe differenceinchemicalpotentialsbetweentheprocesscomponentsandthe referencecomponentsintheirenvironmentalconcentration,temperature, andpressure.Thephysicalexergyisthemaximumobtainableamountof shaft work (electrical energy) whena stream is brought from process condition (T,P) toequilibriumatambienttemperaturebya reversibleheatexchange. Exergyrelatestheevolutionofaprocesstotheenvironmentalconditions, andconsequentlytotheecologicalimpact.Thisbringsadistinctivefeature to theoptimizationofsystems. Exergy analysis identifiesand quantifies unused parts ofavailable energy anddeterminesthethermodynamicefficiencyofdistillationsystems. Thermodynamic Analysis ofSeparation Systems3915 Traditionally,exergyanalysisisbasedontheoverallthermodynamic efficiencythatistheratioofthelostworktotheidealworkrequiredfor ~ e ~ a r a t i o n . ~ ~ ~ - ~ ~ ] The overallexergyefficiencyfor distillation is the product ofexternalandinternalexergyefficiencies. The externalefficiencydepends on thermal integration among units, coproduction, and recompression of over- head vapor to be used in the reboiler, while the internal exergy depends on the columninternaldesign,feedcompositionandstate,numberofstages,and utilityrequirements.Theexergyefficiencyfordistillationsystemsislow; manyoperatewithabout20%-25%exergeticefficiency,whichcouldbe increasedtoaround60%withcertain modification^.'^]Toseparatea componentwithlowcompositionbydistillationishighlyinefficient,and integrationofseveralfunctionsintosingleequipment,suchasanexchan- ger-dephlegmatororreactivedistillationandabsorptionmayincreasethe efficiency and the investments required. Feed conditions and feed plate location affect irreversibility,and, hence, the efficiency ofseparation systems.[601 Itisacommonapproachtoassumethatthemasstransfer(evaporation orcondensation)is controlled bythevaporphase.Withthisassumption,the exergyanalysis mainlyusesa graphicaltoolcalledthe exergy lossprofiles or the exergy-utilizationdiagrams.[61-641Every process accepts or donates energy inequalamounts:AHd + AH,= 0,whileexergylossorentropychangeis notconserved:ASd + AS,1 0. The exergychangesofenergydonorAXd and acceptor AX,basedon a referencetemperature To are expressed by AX,= AH,- TOAS,= AH,x, (1 5b) where xu and xd are the energylevels, whichshow the ratio ofavailable energy (exergy)tototalenergy,andexpressedbyxd = 1 - ToASd/AHd and xu = 1 - T,AS,/AH,.Then, the exergyloss XI, betweenan energydonor and an energyacceptor is expressedby Therefore, the energy level of the donor process must be greater than or equal to that of the energy level of the acceptor process, and the value of XL is positive as AH,> 0. The abscissa of the exergy-utilization diagram displays the amount of accepted energy, A H,,while the ordinate shows the energy levels of xd and xu; therefore, the area displays the exergy loss.L641 Stage-exergy losses occur due to heat exchanged and mixing between the phasesonstagescausingcooling,heating,condensation,evaporation,and mixing.[641 3916Demirel Forcoolingofthevaporphase,theenergylevelsatstageiare expressed by where Forheatingofthe liquidphase theenergy levels are: where The exergyloss at stagei is expressed byEq. (16): Exergylossesdue to evaporation and condensationareexpressed by where AHQ shows the heat supplied at the energy level rc?Condensation takes place at the liquidphase temperature The vapor flow from the stage i + 1 mixes with the vapor phase on stage i, and the exergy lossis expressed by XLrn.~.i= -RToVi+~X[~i+l,;(lnyi,j- lnyi+~.;)- ( ~1. j - ~i +, . ; ) l (26) Thermodynamic Analysis ofSeparation Systems3917 The liquid flow from stage i-l mixes withthe liquid phase on stage i, and the exergyloss isgivenby Indistillationcolumnsusingsideheatingandcooling,energyutilization diagramscandescribetherelationbetweenexergylossinthecolumnand separation performance. Masstransferinseparationsystems,suchasrectification,absorption, desorption,andmembraneseparation,isoptimumwhentheconditions on the concentrationprofilesprovide the minimalirreversibility.[671Analysis insievetraydistillationcolumnsrevealsthattheirreversibilityonatrayis mostlyduetothebubble-liquidinteraction,andtheexergylosscouldbe reduced considerablyfor the same operating conditions with moderateinvest- m e n t ~ . ~ ~ ~ - ~ ~ ~ Theoptimalconcentrationandtemperatureprofilescanbe derivedbyminimizingtheentropyproductionrateforspecifiedheatand mass fluxes, whichcan be expressed bylinear flux-forcerelations if the trans- portsystem is notfar fromglobal equilibrium.[233'.711 There has beenhesitation and delay for the simulation packages to incor- poratetheexergyanalysisinpropertycalculationsandprocessanalysis. However,forthelast10years,exergyanalysisinsimulationhasbecome popular,L65.66.71-741In1996, Hinderink et a1.[65.661integratedthesubroutines ofexergycalculationswiththeflowsheetingsimulatorofAspenPlusand applied the codes to synthesis gas productionfrom naturalgas. These subrou- tineswere developed byExercom licensed byStork Comprimo.Amsterdam, the Netherlandsforapplyingexergyanalysisasadiagnostictoolinprocess developmentanddesign.17jJExergyanalysiswithinaflowsheetingcan displaytheprocessinefficiencies.Suchananalysis,performedforapartof arefinery,hasrevealedthat70%ofexergylossescanbeprevented.cor- respondingto 40%reductionofprimaryfuelconsumptionfor thecrudeoil distillationcolumn.Inthesamerefinery,splittingthefeedstreamhas reducedthefuelconsumptionby10%.L551For theoptimizationoffeedcon- ditionsandreflux,exergyanalysiscan behelpf~1.[76-7S1Acomplete exergy analysis,however,shouldincludetheexergylossesrelatedtoeconomical costandenvironmentalcost,aswellassuggestionsofmodificationsto reduce theUnlessthat is accomplished,the analysis is mainlyinter- preted as theoretical calculations if system engineers are not trained adequately to implement the results. Consequently, this may undermine the effectiveness ofthe exergy analysis. The computer tools such as Aspen Plus, Hysys. Mathcad, and Pro I1 may be usefulinanalyzingdistillation columnsystems to improve recovery,sepa- ration capacity, and decrease the rate of entropy production. A recent simulation studyL7"suggests that if the positioning of side stream withdrawals and returns is optimized(for example,liquidstreamreturningas vaporentersat aposition where the vapor phase hassimilar composition),heat integrationimproves the recoveryandtheseparationcapacityanddecreasestheexergyloss,butit increases the number ofstages required for a givenseparation. Synthesisstrategiesofsimpleandcomplexdistillationsystemsare basedonheuristicsandalgorithmicanalysis.L61Also,thermodynamic optimumstructureforthesynthesisisoftenseparation trainsmayresultthroughsuccessivemodificationofthermodynamically optimumbuteconomicallyunaffordableflowsheets.Thethermodynamic approachcanbeusedforanalyzingthestructuralstabilityofmulticompo- nentflashanddistillationoperation.1851However,thermodynamicconsider- ationisoftenacomplimentarydesignsupportandmaynotbeafinal selectiontool.[41 Thedesignofasubambientsysteminvolvesdistillation,heatexchanger network, and refrigeration, which are interdependent. The thermodynamic analyti- cal strength of exergy analysis with practical targeting capability of pinch analysis canbecombinedtocalculateexergygrandcompositecurvesforsubambient processes; for example, ethylene and liquefied naturalgas process designs have yieldedanaverageshaftworksavingsof15%overtheresultsobtainedfrom normalpinchanalysis.[451Inananotherindustrialapplication.~861theexergy analysis has beenapplied to cryogenic air distillation plant using Aspen Plus to quantifytheexergylossinvarioussections;compressorsarethesourceofa largeexergylossthatcanbereducedbyhalfbyusingbettercompressors, while thetotalexergyloss canbereducedby2570.[~'~"'Some software tools, suchasSuperTargetofLinnhoffMarchLtd.AspenPinch,andSprintuse pinch analysis;[521obviously, the pinch analysis should be integrated with an econ- omic analyzer for the thermodynamic optimumand the economic optimum. Exergyanalysisforadiabaticanddiabaticdistillationsystemsfor separatingethanolfromwatershowsthatthelargestexergylossoccurs onastagewiththelargestcompositiondifferences,andthetotalexergy lossesare 433.8 kJ/kginanadiabaticdistillationand 248.41 kJ/kginadia- baticoperationcorrespondingto a 42%decrease.1561Foraspecifiednumber ofstages, minimumdistancebetweentheoperatingandequilibriumcurve correspondstooptimumexergyusage.LS71Analysisofaheat-integrateddis- tillationcolumnutilizingtheheatpumpprinciplerevealedthattheexergy lossisconsiderablylowerthanthatofaconventionalc o l ~ r n n . [ ~ ~ - ~ ~ ~ The exergylossprofilesmayleadtosuccessfuldesignmodifications,which areoutlinedfordeethanizercolumnanddistillationofammonia Forexample,throughtraydesignparametersassociatedwith theentropyproduction,anoptimaloperationandenergysavingindistilla- tionsystemscouldbepossible.[9"931 Thermodynamic Analysis ofSeparation Systems Case Study: 1. Exergy Loss Profiles for a Five-Component Distillation Column The Aspen Plus thermal analysis tool estimates the stage exergy loss profiles consistingoftemperature-exergy,stage-exergy,andCarnotfactor(1 - To/ T,,,,)-exergyprofiles.Figure9showsthevaporphasecompositionprofiles andtheexergylossprofilesforthecolumnconfigurationgiveninTable2. Thevaporphasecompositionprofiles(Fig.Ya)candisplaythelevelsof maximumandminimumconcentrationsofthe keycomponents,andthe sharp concentrationchangesaroundthefeedstage.Thestage-exergylossprofiles (Fig. Yb)show the degradationof available work at each stage due to irreversi- bilitysourcesofmomentumloss,thermalloss,andchemicalpotentialloss. Figure Yb clearlyidentifies the excessive loss of exergy on and belowthe feed stage,andsuggests heatintegrationthroughasidecondensershouldbecon- sideredtoreducetheexcessivelossofexergy.Thecombinedexergyand pinchanalysiswouldbearigorousandeffectivetooltooptimizeindividual processor integrated processes. Distillationcolumnsystemoptimizationstartsbyidentifyingtheregions withthelargestexergylossesusingthestage-exergyprofilesofaconverged simulation.Followingthis,columnmodificationsuchasfeedcondition, feed stage location, and possible heat integration based on the more uniform dis- tributionofexergylosswouldbec o n ~i d e r e d . [ ~~- ~' . ~~~Thebestmodifications, whicharefriendlywiththe environmentandcompatiblewiththe restofoper- ationshouldbechosen.Obviously,atthesametime,entropyproductionrate minimizationshouldbes o ~ ~ h t . ~ ~ ~ , ~ ' . ~ ~ ~ Forexample,heatintegrationwiththe combinedadvantagesofdirectvaporrecompressionanddiabaticoperationat halfofthenormalcolumnheightmaybeoneofthebestmodificationsfor closeboilingmixtureseparation.[51Anotherdiabaticoptionistheuseoftwo heatexchangersintegratedinthecolumn replacingthereboilerandcondenser wherelargeexergylossesoccurfrequently.[911Also,changingthefeedstage orsplitting thefeedcanreduceexergylossinacolumnsection;L30~78~981when theexcessiveexergylossduetomixingatthefeedstageisidentified,the design engineer mayuse the prefractionator to reduce the losses. Case Study: 2. Single- and Two-Stage Crude Oil Distillation A1 Muslim et a1.[22Jperformed the exergy analysis ofsingle- and two-stage crude oil distillation. The single-stage system consists ofa crude heating furnace and a 27-tray atmospheric distillation column. The feed is introduced in tray 23. Thetwo-stagesystemconsistsofafurnace,a13-tray atmosphericdistillation Demirel ExergyLoss, (kWj Figure9.(a)Vaporphasecompositionprofilesobtainedfromthesimulations withtheAspenPlusRadfracblockusingthe Peng-Robinsonequationofstate.The columnconfigurationisgiveninTable2.(b)Stageexergylossprofilesobtained fromthe AspenPlus thermalanalysis tool. Thermodynamic Analysis of Separation Systems3921 Table3.Exergyanalysisforsingle-stageandtwo-stagecrudeoildistillation systems.''21 OverallOverallColumn ExergyExergyexergyexergyexergy inputoutputlossefficiencylosses System(Mw)(Mw)(Mw)(Mw) Single-stage498.869.8429.014.0137.2 Two-stage352.0110.9241.131.5121.6 % Difference29.458.843.812511.4 (Reproduced withpermission.) column, another furnace to heat the bottom product of the first unit, and a second distillation column with14 trays. The feed is introduced in tray12. Table 3 com- parestheexergyanalysisofthesystems,andshowsconsiderablereductionin exergylosses. The exergyefficiency ist2'] Case Study: 3. Refinery Operation Optimization by Exergy Analysis ~ i v e r o [ ~ ~ ' reportedexergyanalysisforanexistingrefineryoperation usingthegeneraldefinitionofexergyfromEq.(14).Table4showsthe considerableeconomicalgainsduetothereductioninexergylossesafter theoptimizationstudies. EQUIPARTITION PRINCIPLE The rate of entropy production described by the linear nonequilibrium ther- modynamicsapproachgivesa detailed mathematicalformulation ofthe dissi- patedpower(work) inasystem at local thermodynamicequilibrium.L'.71.943951 Nonequilibriummoleculardynamicssimulations showthattheassumptionof localequilibrium inacolumnsystem is acceptable.1701Forsteadystate linear flux-force relations, constant transport coefficients, and local equilibrium, separ- ation systems with uniform driving forces in space and in time will dissipate less oftheavailableenergy,and, hence.arethermodynamicallyoptimum.[361For example, for agivenflux,acolumnwithuniformdrivingforces issmaller in size,alternatively,itrequireslesscontacttimeforagivensize,andthusa 3922Demirel Table 4.Exergyanalysisand loss reductionin a modifiedrefinery.'561 ExergylossExergyloss beforeafterProposalPaybackNPV'10 optimizationoptimizationinvestmentstimeyearsof Unit( %7 0 ) ($1000)(Months)investments Combined distillation unit Naphtha HDS unit Naphtha reforming unit HDSa unit Catalytic cracking unit Visbreaking unit Utilitiesplant Total "HDS:Hydrodesulphurization. 'NPV:Netpresent value (onlyoperatingcost is takenintoaccount). (Reproducedwithpermission.) higher throughput. One way of achieving uniform driving forces in a distillation system may be the heati nt egrat i ~n. ' ~' ~~' ] The rate of volumetric entropy production due to heat and mass transfer @ for a binary mixture is expressed by[70.71.941 where the J, is the heat flux, Jiis the mass flux for component i, and Xis the thermodynamicforce. Whenthepressure is constant,wehaveVpi,?= VpC, which istheconcentrationdependentpart of thechemicalpotentialgradient. The linear phenomenologicalequations thatfollow from Eq. (29) are Thermodynamic Analysis ofSeparation Systems3923 where Jd (inm3 m-'h-')istherelativemassfluxbetweenheavyandlight componentsacrosstheinterface(Jd = Jl/y,- Jh/y,,), Jh and Jl are themass fluxesofheavyandlightcomponents,respectively,yhandylarethe compositionsofheavyandlightcomponents,respectively,andthecon- stantparameters,Lji,arethelocalphenomenologicaltransportcoefficient, whichcanbedeterminedfromexperiments.Forisothermalconditions,the phenomenologicalcoefficients for masstransferare Usingthechemical force for themass transfer the heatflow becomes On theother hand, Fourier'slaw ofheatconductionwithoutmass transferis (Jq)Jd=o= -kVT(35) Therefore, the thermalconductivity, k,is defined interms ofthe phenomeno- logicalcoefficients Diffusion of the light component is defined by Fick'slaw for the gas phase where D is the diffusion coefficient of the light component and Ac, is the con- centrationdifference oflight component acrossthe distanceAz.The concen- trationdifferencein thegas phase intermsofthetotalpressurePT is where the mole fraction y;is the inlet composition in the liquid. By introducing Eq. (36) into Eq. (34), and with the assumptions of constant driving forces, the average phenomenologicalcoefficient of mass transfer Ll,isobtainedasL7'] 3924Demirel where AHl, and AH,are the heat ofvaporizationsfor the heavy and light com- ponents,respectively.The phenomenological coefficients mayvary consider- ably from enrichingsectionto strippingsection, and thisshould be takeninto accountintheoptimizationcriterion.Inarecentstudy,therateofentropy productionwas calculatedateverystage withthe couplingbetweenthe heat and the masstransport, and verifiedwithexperimentaldata ofethanol-water di ~t i l l a t i on. ' ~~, ~~'The thermal efficiency based on the second law ofthermodynamics may be definedas in Eq.(12) A maximum in the second law efficiencymay be obtained by minimizing theentropyproductionratewithrespecttooneoftheforces.Forexample, assuming thatthe contributiondue tothedifference inchemical potentialis dominant, the change ofthe entropy productionwithrespect to the chemical force can be studied. From Eqs.(29)-(31)the amount ofseparationJd,;and the corresponding rate ofentropygeneration at stage iQiare obtainedas where Xishows the chemicalforceatstagei.Asthelevelofseparationis fixed,theboundaryconditionsfortheforcesarespecified:andanincrease intheforceinonestagemustleadtoareductioninanotherstage.Itis desiredtohaveanincreaseintheflowforagivenentropyproductionrate, and areductionintheentropy productionrate for aspecifiedseparation;the yieldY is defined as the benefit-costratio in an economic sense, and given by WhenthederivativeofYwithrespecttoX,ishigherinonestagethanin another,increasingorreducingthedrivingforceadjusts therateofentropy production.Wecanmaximizetheseparationoutput,byredistributingof forcesbetweenthestages.The distributionisobtainedwiththedifferentia- tiond(l/Xl)/dXI = d(l/X2)/dX2,whichleadstoXI= X2.Theequality offorcesisindependentoftheindividualvaluesofthephenomenological coefficients.The reversibleoperationisalimitcase,andisachievedwhen XIandX2approachzeroandYincreasestowardinfinity.Therefore,the Thermodynamic Analysis ofSeparation Systems3925 practicalimprovement of the second-law efficiency is to apply the relationship between dX1 and dX2. For example, the following relationship atconstant Jd relates the driving forces at two stages. By knowingl la crossthe column, we can determine the possible locations for modifications. A uniform entropy pro- duction rate corresponds to either minimum energycosts for a required sepa- rationandareainvestment,orminimuminvestmentforaspecifiedenergy cost, and leadstothermodynamicallyoptimum design.[701 Stage exergy calculations are used to prepareexergyprofilesthroughout column. Such profilesfirstlyshow the currentlevel ofutilizationofavailable energy, and secondly the effects of operating conditions and design parameters on the efficiencyofoperation. Most ofthe researchers are focusing on how to usetheexergyloss profiles,whichare becomingstraightforwardand partof flowsheeting.Forexample,Ishidaand~ h n o [ ~ l ] and Ishidaand ~ a ~ r a ~ [ ~ ~ , ~ ~ ~preparedtheexergyutilizationdiagramtoshowthetransformedenergy level, andunitheightofthecolumn, and,hence,helptoidentifythe targets for reducingenergyandexergyconsumptions.~971Basedontheexergyloss profiles,modificationsonthefeedstagelocation,feedcondition,andthe useofintermediateexchangers canbe considered.Forexample, exergyloss duetothemixingatthefeedstagecanbeidentifiedandreducedusingan externalmodificationoftheprefractionator.L981Besidethat,theprofiles recentlywereusedtoprovethatauniformdistributionofdrivingforces leadstoahigherthermodynamice f f i c i e n ~~; [ " ~~~' locatingtheheatexchan- gersintheregionswherethelargestdeviationsfromisoforceexist,may lead to the uniform driving forces over the internal stages for a binarydistilla- tionsystem.The resultsofisoforceoperationshouldbe proportionaltothe variationintheprimaryphenomenologicalcoefficient.[991Anisoforceoper- ationofadiabaticcolumnisconsistentwithaminimumexergylossina sectionwhere large refluxratios are avoided.11001 Assumingthatacolumnisareversibleheatengine(Fig.7),work available from thethermal energyis expressed by whereTo is the ambient temperature, and TR and Tc are the temperatures for reboiler and condenser, respectively. The temperature corrections (Carnot efi - ciencies)describe themaximumfractionoftheoreticalwork extractedfrom thermalenergyataparticularambienttemperature.[35"01.'0'10ntheother hand minimum separation work, W,,required for a separation is the net change inavailability Thechangeofavailabilityofseparationisthedifferencebetweenthework supplied bythe heatand the totalworkrequiredfor separationW,, AAs = Wheat- Wts Energy use can be reduced by minimizing the pressuredrop; lost work due to high-pressuredrop(ashighas10 psi)isconsiderableatthecondenserand reboilersystems,andisrelativelylessthroughthetrays(0.1psiorless). Changeofpressureaffectsthedistancefromequilibrium,causesthelarge temperaturedifference, and,hence,utilitycosts betweenthecondenserand reboiler ofdistillationcolumn. Feedtraylocationmayalsobeadjustedto reducethelostwork.Com- monly,thefeedlocationisdeterminedattheminimumutilityloadsand traycountorsimplybytakingintoaccountlight-keyandheavy-keycom- ponentcompositions.Therelativecostoftheheatingandcoolingmedia willalso influence the location of the feed stage. The basic trend of improving thermodynamicefficiencyleadsto tallerand moreslendercolumns. Case Study: Distillation Systems with Isoforce Operation Adiabaticcolumnsare highlyirreversibleand oftenthe irreversibilityis notevenlydistributed.The stage-exergy loss profilesindicate the distribution of stage irreversibility. and hence the distribution of driving forces in a column operation. Figure 9b shows clearly that the operation is far from isoforce oper- ation,especiallyonand belowthefeedstage; and aheatintegration modifi- cationthroughasidecondensershouldbeconsidered.Nonequilibrium thermodynamicapproachmaybeusedtodetenninehowtopositionthe heatintegrationinthecolumn.'991 Thiswillreducetheexcessivelossof exergyand bring the distillationcolumn relatively close to isoforce operation. Distillationcolumnsoperatingwithclosetounifonnthennodynamicforces areanalyzedforseparatingn-pentanefromn-heptane189.99J(Table5) ,and ethanolfrom water(Table 6).L1001Equation(31) showsthatchemicalsepar- ationforceisy,Vp,/T,andshouldbeunifonnthroughoutthecolumn.For the top and bottom parts of the column for ethanol-water separation, a conven- tionalMcCabe-Thielediagramhassmalldistancesbetweentheoperating linesandequilibriumcurve;inthetop,azeotropeexist,andinthebottom partcompositionsareclosetopurecomponents.Inthemiddlepartofthe McCabe-Thiele diagramanoperating line maybe plottedusing T,,,,,, = kx, ,, Thermodynamic Analysis ofSeparation Systems3927 Table5.Reboilerandcondenserdutiesandentropyproductionchangeforthe adiabatic,heatintegratedandoperatingwithisoforceandanear-optimumcolumn for n-pentaneand n-heptane.1991 Reduction in entropy OperationQRM w QcMWproduction(%) Adiabatic2.370.704 Isoforce1.890.732 Nearoptimum1.900.797 (Reproduced with permission.) exp(-C/Rkxi,,,),whereCisachosenconstantdrivingforce,kisHenry's laws'constant, and indices n is the stage number; using this middle operating line,isoforcelinesinethanol-waterseparationareplottedandusedinthe column analysis.[1001This analysis leadsto more thanone isoforceoperating line based on the chosen value ofC and maybe confusing.Table 5 indicates clearly that a thermodynamically optimum distillationcolumn should operate withauniformorclosetouniformdrivingforceinseparation.Thisisin line withtheoperation inwhichexergyloss is distributed evenlywithinthe column. However,inminimizingtheexergylossor the rateofentropypro- duction,oneshouldavoidoperationwithtoosmalldrivingforces(pinchin separation)at anystage. One has to note that the equipartitionprincipleis mainly investigated for binaryseparationsbydistillation,andshouldbeextendedtomulticompo- nentseparationswithnonidealmixturesandbyaccounting for thecoupling betweendrivingforces.However,thegeneralprincipleisnotrestrictedto binarysystemsonly.L361Forexample,Zempetal.[981usedtheexergyloss profiletodeterminethedistributionofdrivingforcesinafive-component distillationcolumn. Still, one has to keep in mind that the treatment ofmulti- component diffusion as opposed to binary diffusion is fundamentallydifferent Table 6.Comparison of the performance ofa diabatic column with a isoforce column operationfor separationofethanol-watermixture.['001 TotalexergyDistillateflowDistillate Operationlosses U/ hratekg/hcomposition (%) Adiabatic44.23 Isoforce14.24 Diabatic15.89 (Reproducedwithpermission.) 3928Demirel and is essentially incompatible with the Fick law, which is not capable of account- ingforcouplingbetweendiffusivefluxesofvariousspecies.'2.3"71*94395.1031 Multicomponentdiffusionismuchmoreappropriatelytreatedbymeans oftheStefan-Maxwellequations,whichinvolvesettingupequationsrela- tingthecorrespondingthermodynamicforcestomassfluxesofallthe components.L95.104.1051 THERMOECONOMICS TheUSDepartmentofEnergyWebsite,"EnergySaversfor Industry PlantManagersand~ngi neer s , " " ~~' offersawidevarietyofenergysaving possibilities.suchas an energymanagement action plan. Energy is conserved inallprocesses.However, theavailable partofenergythatisexergyisnot conserved. The processengineer should minimizetheinput cost ofaprocess byreducingexergylossduetothermodynamicimperfections.Withinthis context,thermodynamicanalysissimultaneouslyconsiderstheinterrelations amongtheuseofenergy,economy,andecology.[561Suchconsiderations mayhave positive impact on sustainable developn~ent.For example, thermo- dynamicanalysisofasolardesalinationunitshowsthatthermoeconomic evaluationofthesystemiscloselyrelatedtoacompleteeconomicanalysis of the possibleimprovements leading to a less irreversible unit.[lo7] Optimizing a plant is complex, since the whole plant should be cost effec- t i ~e. [ " ~- l o]Separation systems should be optimized considering both capital costand operating (energy) cost.[' ' ' I The heuristics ofusinga refluxratioof 1.03-1.3times theminimumrefluxratiois inline with boththe capitalcost and operating costfor binary distillationsystems.ll -'19] The concept of thermodynamics cost relates the thermodynamic limits of separationsystems to finite rate processes1120-1221and considers the environ- mentalimpactthroughthedepletionofnaturalresourceswithintheexergy lossc ~ n c e ~ t . [ ~ ' ~ ~ ] Still, economicanalysisand thermodynamicanalysis per- ceptionsmaynotbeinparallel.Forexample, it isestimated thatadiabatic column has a lower exergyloss (39%)L12'1than does an adiabatic distillation; however, this may not lead to a gain in an economic sense, yet it is certainly a gaininthethermodynamics sense.Thatiswhythethermodynamicanalysis needscarefulinterpretationsandapplicationsofits results.Thermodynamic analysisisalsocapableofquantificationofcouplingintransportpro- cesses.L2.;2.71.94'122] Especiallyindiabaticcolumns,heatandmasstransfer couplingmaybeconsiderableandshouldnotbeneglected.["1.1221The resultsofthermodynamicanalysismaybeinlinewiththoseofeconomic analyseswhenthethermodynamiccostoptimumnotthemaximumthermo- dynamic efficiencyis considered with processspecifications.[1231 Thermodynamic Analysis ofSeparation Systems3929 Although this reviewemphasizes distillation systems, the use ofthermo- dynamic analysis is also becoming popularfor other separationsystems, like supercriticalextraction,L1241desalinationprocesses,L1'51hybridvaporper- meation-distillation,[71 andcroyogenicairseparation.'3"861Forexample, energy requirement analysis of common cycles used in supercritical extraction hasutilizedexergylosses,andanoptimumextractionpressure,whichpro- ducesaminimuminexergylossforspecifiedtemperatureandseparation pressure.['241 Thermodynamicanalysis also has beenused for the economics ofdesalinationtechnologiesbymembranesanddistillation['251 fivemain desalinationsystemsconsideredare:reverseosmosis,electrodialysis,vapor compression, boilingevaporation, and flash evaporation. Exergoeconomicsishighlypopularforanalysisandoptimizationin thermalenergysystems,yetitisfarfromabreakthroughmethodologyfor separationprocessesmainlyduetotheircharacteristicsandcomplexity. The objective ofexergoeconomicsshould be chosenwithcare; for example, theoptimizationshouldtargetbothcapacityexpansionandexergylossin separation systems.[126.1271 Theminimizationofentropy productionis notalwaysan economiccri- terion;sometimes,existingseparationequipmentmaybemodifiedforan evendistributionofforcesorevendistributionofentropyproduction.For example,todetermineaneconomicoptimumforanextractionweassume thattheoperatingcostsarealinearfunctionoftheentropyproduction,and theinvestmentcostsarelinearfunctionofthespaceandtimeofthe process.Then the totalcostCT is expressed as[361 where r is the amortization rate and a,b, and c are the constants related to the costs, V is the volume or size, t isthe time, L is the transport coefficient, X is the driving force, and @ is the rate ofentropy production. Integral in Eq.(48) is subject tothe constraint ofaspecified fluxgiven by The variationaltechnique can be used to minimize the total cost, and the Euler equation for thevariable X is givenby where h isa Lagrangemultiplier.Eq.(50) yields h X= - - = constant 2a (52) Equation(52) showsthatthedistributionofthethermodynamicforce, X,is uniformwhen thetotalcost subject to thespecified flux. J, is minimum. Considera steady-stateoperationinwhich the forces are uniformlydis- tributed;theinvestment cost,c;, ofa transferunitisassumed to belinearly relatedtothesize,V,andtheoperatingcosts,C,,arelinearlyrelatedtothe exergyconsumption where C$is a fixed investment cost and COfisa fixed operating cost, andA and Bare the cost parameters.ExergylossAXc is expressedas Here To is a reference temperature (dead state), and AX,,, is a thermodynamic minimumvalue. The totalflow J = Lm,,.canbewrittenbyusing Eq.(53) where C,is thevariable partofthe investmentcost. Eliminatingthe constant (average) force X,,betweenEq. (55) and the totalentropy production@, , , = JAX,,,weobtain[361 SubstitutingEq. (57) into Eq.(53) and thelatterinto Eq. (54), a relationship betweentheoperatingand investmentcosts isobtained ABT,,J' c, =- LC, +cd+BAXl n Theoptimalsizeisobtainedbyminimizingthetotalcostofoperating andinvestmentscosts,whichislinearlyamortizedwiththeamortization Thermodynamic Analysis of Separation Systems3931 rateT.CT (Ci) = TC; + Co. The minimumofCT isobtained as d c ~ / d C; = 0,and wehave According to Eq. (59), the quantities BT,@,,,whichare related to irreversible dissipation andTV~, ~, should beequal inany transfer unit.Generally, operat- ing costs are linearly related to dissipation, while investment costs are linearly related to the size ofequipment. The optimum size distributionof the transfer units is obtained whenamortization cost is equal to the cost of lost energydue to irreversibility. The cost parameters A and B may be different from one trans- fer unit to another; when A= B, thenQav/Vo,,is a constant, and the optimal sizedistributionreducestoequipartitionofthelocalrateofentropypro- duction.'"]The optimal size ofa transfer unitcan be obtained from Eq.(53) Bydistributingthe entropy productionas evenlyaspossiblealongthespace andtimeline,aneconomicalseparationprocesswouldbedesignedand operated.["]Lateritwasarguedthattheequipartitionofadrivingforce ratherthanequipartitionofentropyproductionratesshouldbeadaptedina binarydistillation.[1001 Dissipationequationsshowthatboththedriving forcesandfluxes playthesame roleinquantifyingtherateofentropypro- d~c t i on. ' ~~' ~] Therefore,equipartitionofentropyproductionprinciplemay pointoutthat theuniformdistributionofdrivingforces is identicalwiththe uniformdistributionoffluxes. Onemajortrendthatappearsisthatofpinchanalysis, exergyanalysis, andequipartitionprinciplesbeingcombinedtoanalyzeprocessandenergy systems.L100.126-1281Thiswillenablethescientiststomodifyexisting systems or design new systems with complete objectives and targets including the environmental concerns and the naturalresources. Thermoeconomics is not a newconcept; however, it has been formulated in a more systematic way, mainlyduring the last 20 years. From an exergetic point of view, cost analysis is performed by using (a) cost accounting methods that use average costs as a basis for a rational price assessment and (b) optimi- zationmethods thatemploymarginal costs inorder to minimize thecosts of the productsofa system or a ~ o m~ o n e n t . ~ ' ~ ' - ~ " ~ To account for the environ- mentalimpactinamoresystematicway,aresource-basedquantifier,called 3932Demirel "extendedexergy,"isemployedtocalculatetheresource-basedvalueof acommodity.['"1Consider aseparationprocesswithoutputs containinghot streams withvariouschemicals havingthe conditions considerablydifferent fromthoseenvironmentaltemperaturesandconcentrations.Toachievea zeroenvironmentalimpact,thesestreamsmustbebroughttoboththermal andchemicalequilibriumwiththesurroundings:thus,thereal(exergetic) costofthezero-impactwouldcorrespondtotheextendedexergyideally requiredtobringtheconditionsofeffluentstoequilibriumconditionswith the s ur r ~undi ngs . [ " ~. ' ~~~Ifanacceptable levelofpollutantor the"tolerable environmental impactlimit"for acertainpollutantwouldbespecified, then theenvironmental costmaybe quantified.Despiteallthesystematic efforts onformulating the thermoeconomics,itsusein designand economicevalu- ations is stilllimited. CONCLUSIONS Energysaving inseparationsystems, particularlyindistillationsystems, is a researchfield that has attracted considerable innovative approaches. A dis- tillationsystemis anessentialseparationprocess yet it is inefficientinusing thermal energy. and mayoperate with adverse environmental impact as it dis- chargesalargeamountofthermalenergyintotheenvironment. Innovative researchincorporatingthe principlesofthermodynamics for energyefficient distillationsystemsisinanadvancedstagethroughpinchanalysis,exergy analysis, and equipartition principle. Thermodynamic analysis simultaneously considers the critical interrelationsamong energycost, thermodynamiccost, andecologicalcost.Thetaskofaprocessengineeristodecidethetarget cost or the costs to be optimized usingthe thermodynamic analysis. The ther- modynamic analysis is still notwidelyused.However, with the currentlevel ofresearchefforts, engineers and scientistsshoulduse the analysis in design. retrofits, economic analysis, and environmental problems. NOMENCLATURE a,b,ccost constant inEq. (48) Aavailability(J mol-'),area(mP2) BBottomproduct(kmol h-') A,Bcost parameters Cconcentration(mol L ' ) ,cost ciffixed investment Ddiffusioncoefficient (m's-'),Distillate.(kmol h-') Thermodynamic Analysis ofSeparation Systems gravitationalacceleration enthalpy heatflux separationflux(mol m-'s-') massflux for componenti (molm-'s-') thermal conductivity(J m ' s-'K) phenomenological coefficient lostwork(J mol-') molar mass(mol) pressure(kPa) heatflux(J mol-' m-') condenser duty(kW) reboilerduty(kW) universalgas constant(J mol-K-I )Refluxratio entropy ( ~mol - ' K-I) time(s) temperature(K) average velocity(m s-l) internalenergy(J) elevation,gas filmthickness (m) liquidmole fraction, distance thermodynamic driving force molfraction volume (m3) yield work(J) GreekSymbols efficiency Lagrance multiplier chemical potential(J mol-' )amortizationrate entropyproductionrate(J K ' sp' ) Subscripts acceptor cooling.condenser cooling water Demirel d evap fs G h 1 L m min max 0 prod R S SYS t donor evaporation flowstream gas heating,heavy light liquid mixing minimization maximization environmental production reboiler separation system total REFERENCES 1.Humphrey,J.L.;Siebert,A.F.Separationtechnologies:Anopportunity for energysavings. Chem.Eng. Prog.1992 (March), 92. 2.Demirel,Y.NonequilibriumTlzer~nodynamics TransportandRate Processesin PI7ysical and BiologicalProcesses: Elsevier:Amsterdam. 2002. 3.Freswater. D.C.; Ziogou, E. Reducing energy requirementsin unit oper- ations. Chem. ThermalEng.1976. 11, 215. 4.Koehler, J.; Aguirre, P.; Blass, E. Evalutionary thermodynamic synthesis ofzeotropicdistillationsequences. GasSep. Purif.1992, 6, 153. 5.Seider,W.D.;Seader,J.D.;Lewin,D.R.Product&ProcessDesigiz Principles, 2ndEdn.; Wiley: New York,2004. 6.Porter, K.E.; Momoh, S.O. Finding the optimum sequence of distillation columns-anequation to replace the rules of thumb'(heuristics). Chem. Eng. 1991, 46, 97. 7.Fahmy, A.;Mewes, D.;Ebert,K. Designmethodologyfor theoptimi- zation ofmembraneseparation for hybrid vapor permeation-distillation processes.Sep. Sci. Technol.2001, 36, 3287. 8.Liu,X.G.;Qian,J.X.Modeling.control,andoptimizationofideal internallycoupleddistillationcolumns.Chem.Eng.Technol.2000, 23, 235. 9.Jones, V.E.; Vais, A.M.; Wilson, J.A. Getting the maximum benefit from a side-reboiler.Chem.Eng.Comm.1999, 171, 195. Thermodynamic Analysis ofSeparation Systems3935 10.Christodoulou,P.A.Energyeconomyoptimizationinseparationpro- cesses-Optimizingtheseparationofsucrose/waterandnon-sugars. Int. Sugar J.1996, 98, 419. 11.Pradubsripetch,D.;Naka.Y.;Fan,Z.Analysisofheatdemandand supplyinmulticomponentdistillationsystems.J.Chem.Eng.Japan 1994, 27, 188. 12.Fraga,E.S.;Zlinkas,A.Evaluationofhybridoptimizationmethodfor theoptimaldesignofheatintegrateddistillationsequences.Adv.Eng. Software2003, 34, 73. 13.Gadalla,M.;Jobson,M.;Smith,R.Increasecapacityanddecrease energyinexistingrefinerydistillationcolumns.Chem.Eng.Process 2003 (April), 44. 14.Ito,A.;Asano,K.Thermaleffectsinnon-adiabaticbinarydistillation effectsofpartialcondensationofmixedvaporsontheratesofheat andmasstransferandpredictionofH.T.U.Chem.Eng.Sci.1982, 37, 1007. 15.Budiman,A.;Ishida, M. Optimalside heatingand cooling ina distilla- tioncolumn. Energy1998, 23, 365. 16.Harbert, W.D.Preflash saves energyin crudeunits. HydrocarbonProc. 1978, 57, 23. 17.Rivero, R.; Anaya, A. Exergy analysis of a distillation tower for crude oil fractionation,andcomputeraidedenergysystemsanalysis,Proc.of Winter AnnualMeeting ofASME1, 25 and 55. Dallas,TX,1990. 18.Frazer, A.C.;Sloley, A.W.Considermodelingtools torevampexisting processunits.HydrocarbonProc.2000,79, 57. 19.Dhole,V.;Buckingham,P.Refinerycolumnintegrationforde- bottleneckingandenergysaving,ESCAPEIVConf,DublinIreland IChemE,Rugby, UK,1994. 20.Gadalla,M.Retrofitdesignofheatintegratedcrudeoildistillation systems. UMIST:Manchester,UK, 2003; Ph.D. thesis. 21.Bagajewicz, M.J.Energysavingshorizonsfortheretrofitofchemical processes.Applicationtocrudefractionationunits. Comp. Chem.Eng. 1998, 23, 1. 22.Al-Muslim,H.;Dincer,I.; Zubair,S.M. Exergyanalysisofsingle-and two-stagecrudeoildistillationunits.J.EnergyResource.Tech.2003, 125, 199. 23.Annakou,0. ;Mizsey, P.Rigorousinvestigationofheatpumpassisted distillation.HeatRecov.Sys. & CHP.1995, 15, 241. 24.Gopichand, S.;Omideyi, T.O.;Kasprzycki,J.;Devotta, S. Theecono- micsofheatpumpassisteddistillationsystems-11.Analysisof ethanol-watermixtures.J.Heat RecoverySys. 1984, 4,271. 3936Demirel 25.Fonyo.Z.;Benko,N.Enhancementofprocessintegrationbyheat pumping.Comp. Chem. Eng.1996,20, S85. 26.Gopichand,S.;Omideyi, T.O.;Kasprzycki,J.; Devotta, S. Theecono- micsofheatpumpassisteddistillationsystems-I.Adesignand economic model.J. HeatRecover.Sys. 1984, 4, 187. 27.Kauchali,S.; McGregor,C.;Hildebrandt,D.Theattainableregionfor simplebinarydistillation.TheannualAIChEmeeting,Dallas,TX, Oct.31-Nov.5, 1999. 28.Kauchali, S.; McGregor, C.; Hildebrandt, D. Binary distillation re-visited using the attainable regiontheory. Comp. Chem. Eng. 2000,24, 23 1. 29.Kohler, J.; Kuen, T.; Blass, E. Minimum energy demand for distillations with distributed componentsand side-productwithdrawals.Chem. Eng. Sci. 1994, 49, 3325. 30.Soave, G.; Feliu, J.A. Saving energy in distillationtowers byfeed split- ting.Appl. ThermalEng. 2002, 22, 889. 31.Cornelissen,R.L.;Hirs,G.G.Exergyanalysisofcryogenicairsepar- ation. EnergyConvers. Mgmt.1998, 1821. 32.Demirel,Y.;Sandler, S.I.h~eversiblethermodynamicsinengineering and science. J. Phy s. Chem. B 2004, 108, 31. 33.Moran,M.J.;Shapiro, H.N.FundamentalsofEngineeringThermody- namics, 4thEdn.; Wiley:NewYork,2000. 34.AssessmentofPotentialEnergySavingsinFluidSeparationTechno- logies:TechnologyReviewandRecommendedResearchAreas;U.S. Dept.ofEnergyOfficeofIndustrialPrograms:Washington,DC. DocumentNo. DOE/ID[24763-11 (Dec.1984). 35.Ognisty, T.P. Analyze distillation columns with thermodynamics.Chem. Eng. Prog.1995 (February), 40. 36.Tondeur, D.;Kvaalen,E. Equipartitionofentropyproduction.Anopti- malitycriterionfortransferandseparationprocesses.Ind. Eng. Chem. Res.1987, 26, 50. 37.Sauar, E.; Rivero, R.; Kjelstrup, S.; Lien, K.M. Diabatic column optimi- zation-comparedtoisoforce columns.EnergyConvers.Mgmt.1997, 38, 1777. 38.Linnhoff, B.; Flower, J.R.Synthesis ofheatexchangernetworks I.Sys- tematic generation of energy optimal networks. AIChE J. 1978a, 24, 633. 39.Linnhoff.B.;Flower,J.R.Synthesisofheatexchangernetworks11. Evolutionarygenerationofnetworks with variouscriteriaofoptimality. AIChEJ.1978b,24, 642. 40.Hohmann, E.C. Optimumnetworksfor heatexchange.Univ.Southern California:Los Angeles,1971; Ph.D. dissertation. 41.Serna, M.; Jimenez,A. Anarea-targetingalgorithmfor thesynthesis of heatexchangernetworks.Chem. Eng. Sci. 2004, 59, 2517. Thermodynamic Analysis of Separation Systems3937 42.Spank,B.TheChemicalEngineer'sResourcepage.http://www. cheresourcess.com/pinchtech2.shtml,2004. 43.TheEnvironmentalTechnology.http://www.envirotechnet.com/ pinchtechnology.com/pinch~home.htm,2004. 44.Al-Kawari,M.A. Pinchtechnology:an efficient tool forchemicalplant energyand capital-costsaving. Appl. Energy2000, 65, 45. 45.Dhole, V.R.; Linnhoff, B. Overall design oflow temperature processes. Comp.Chem. Eng.1994, 13, S105. 46.Dhole,V.R.;Linnhoff,B.Distillationcolumntargets.Comp.Chem. Eng.1993, 17, 549. 47.Ficarella. A.; Laforgia, D. Energy conservation in alcohol distillery with the application of pinch technology. Energy Convers. Mgmt. 1999,40, 1495. 48.Briones,V.;Perez,A.L.;Chavez,L.M.;Mancilla,R.:Garfias,M.; DelRosal,R.;Ramirez,N.Pinchanalysisusedinretrofitdesignof distillationunits.Oil& Gas J.1999, 97, 41. 49.Smith, R.State oftheartinprocessintegration.Appl.ThermalEng. 2000,20, 1337. 50.Bagajewicz,M.J.;Pham.R.;Manousiouthakis, V.Onthestatespace approachtomass/heatexchangernetworkdesign.Chem.Eng.Sci. 1998, 53, 2595. 5 1. Bagajewicz,M.J.;Manousiouthakis,V.Massheat-exchangenetwork representationofdistillationnetworks.AIChEJ.1992, 38,1769. 52.Wang, Y.;Du, J.;Wu, J.; He,G.:Kuang,G.; Fan, X.;Yao, P.;Lu, S.; Li.P.;Tao,J.;Wan,Y.;Kuang.Z.;Tian,Y.Applicationoftotal processenergy-integrationinretrofittinganammoniaplant.Appl. Energy2003, 76,467. 53.Lavric,V.;Bactens,D.;Plesu,V.;DeRuyck,J.Entropygeneration reductionthroughchemicalpinchanalysis.AppliedThermalEng. 2003,23, 1837. 54.Matijasevia,L.;Othmaeia,H.Energyrecoverybypinchtechnology. Appl. Thermal Eng.2002, 22, 477. 55.Doldersum,A.Exergyanalysisprovesviabilityofprocessmodifi- cations.Energy. Convers.Mgmt.1998, 39,1781. 56.Rivero, R.Applicationofthe exergyconceptinthe petroleumrefining and petrochemicalindustry. EnergyConvers.Mgmt. 2002, 43,1199. 57.Petlyuk,F.B.;Platonov,V.M.;Slavinskii,D.M.Thermodynamically optimummethodforseparatingmulticomponentmixtures.Int.Chem. Eng.1965, 12, 555. 58.Fonyo, Z.Thermodynamic analysis ofrectification: I. Reversiblemodel ofrectification.Chem. Eng.1974, 14,18. 3938Dernirel 59.Petlyuk,F.B.Thermodynamicallyreversiblefractionationprocessof multicomponentazeotropicmixtures.Theor.Found.Chem.Eng.1978, 12, 270. 60.Bandyopadhyay,S.Effectoffeedonoptimalthermodynamicperfor- manceofa distillationcolumn.Chem. Eng. J. 2002, 88, 175. 61.Ishida, M.;Ohno, T. Applicationof energy-directionfactor diagram for exergyanalysisofdistillationcolumns.JChem. Eng.Japan1983.16, 281. 62.Ishida,M.;Taprap,R.Applicationofenergy-utilizationdiagram forgraphicexergyanalysisofmulticomponentdistillationcolumn. J. Chem. Eng.Japan1992,25, 396. 63.Ishida, M.; Taprap, R. Introduction of individual energy level for exergy analysisofprocesssystemswithmultiplecomponents.J.Chem.Eng. Japan1993,26. 437. 64.Taprap,R.;Ishida.M.Graphicexergyanalysisofprocessesin distillationcolumnbyenergy-utilizationdiagrams.AIChEJ. 1996,42,1623. 65.Hinderink,A.P.; Kerkhof, F.P.J.M.;Lie, A.B.K.; De Swaan Aron, J.D.; VanderKoo,H.J.Exergyanalysiswithaflowsheetingsimulator-I. Theory:calculatingexergiesofmaterialstreams.Chem.Eng.Sci. 1996, 51,4693. 66.Hinderink,A.P.; Kerkhof, F.P.J.M.;Lie, A.B.K.; De Swaan Aron, J.D.; VanderKoo.H.J.Exergyanalysiswithaflowsheetingsimulator-11. Application;synthesisgasproductionfromnaturalgas.Chem.Eng. Sci. 1996, 51, 4701. 67.Tsirlin.A.M.;Kazakov,V.A.;Berry,S.Finite-timethermodynamics: Limiting performanceofrectificationandminimalentropyproduction inmass transfer.J. Phys. Chem.1998, 98. 3330. 68.Ray,S.; Sengupta, S.P. Irreversibilityanalysis ofa sieve tray in a distil- lation column.Int. J. Heat Mass Transfer 1996, 39,1535. 69.Ray, S.; Panja, A.K.;Sengupta, S.P. Irreversibilityanalysis ofa separa- tionsystemusingsieve traydistillationcolumn. Chem. Eng. Sci. 1994. 49,1472. 70.Ratkje,S.K.;Sauar, E.;Hansen, E.M.;Lien,K.M.Analysisofentropy productionratesfordesignofdistillationcolumns.Ind.Eng.Chem. Res. 1995, 34, 3001. 71.Demirel,Y.;Sandler,S.I.Linearnonequilibriumthermodynamics theoryfor coupledheatandmasstransport.Int.J. HeatMassTransfer 2001.44.2439. 72.Rosen.M.A.;Barry,G.F.TheenhancementoftheSALT-simulation andanalysiscode for first andsecond lawanalysis. InThernzodynamic A17al>,sisand Im1>rovemenr ofEllei-gy Systems(TAIES '89), Proceedings Thermodynamic Analysis ofSeparation Systems3939 bytheInternationalSymposium,Beijing,(China);Ruixian,C., Moran, M.J.,Eds.;Int. AcademicPublishers,PergamonPress:Oxford, 1989,472-480. 73.Rosen, M.A.;Scott, D.S.The enhancementofaprocesssimulatorfor completeenergy-exergyanalysis.InAnah~sis of EnergySystems- Designand Operation; Gaggioli, R.A., Ed.; AES-1, ASME: New York, 1985, 71-80. 74.Yang, Y.; Yang,J.; Zhu, X.: Ling, W.Enhancementand applicationof aflowsheetingsimulatorforsecondlawanalysis,ECOS'92,Inter- nationalSymposiumon Efficiency, Cost, OptimizationandSimulation, Zaragoza, (Spain), ASME: NewYork,1992; 85-91. 75.Lie,A.B.K.;Eigeman,P.M.ExerCom:calculatingexergiesinAspen Plus(PC-bersion)usermanual,StorkComprinoreport61285-0037- 303-01.1994. 76.Maia. M.L.O.; Zemp, R.J. Thermodynamicanalysis of multicomponent distillationcolumn:Identifyingoptimalfeedconditions.BrazilianJ. Chem.Eng. 2000, 17, 751. 77.Yong,P.S.;Moon,H.M.;Yi,S.C.Exergyanalysisofcryogenicair separationprocessforgeneratingnitrogen.J.Ind.Eng.Chem.2002, 8, 499. 78.Fonyo, Z.; Rev, E.;Szitkai, Z.; Emtir, M.; Mizsey, P. Energysavings of integratedand coupleddistillationsystems. Comput. Chem. Eng. 1999. 23, 241. 79.Bjorn, I.N.; Gren. U.; Sbenson, F. Simulation and experimentalstudy of intermediateheatexchangeinasievetraydistillationcolumn.Comp. Chem.Eng., 2002, 26,499. 80.Andersen,T.R.;Siragusa,G.;Andresen,B.;Salamon.P.; Joergensen,S.B. Energyefficient distillationbyoptimaldistributionof heatingandcoolingrequirements.ESCAPE1999, 10. 709. 81.Lynd,L.R.; Grethlein, H.E.Distillationwithintermediateheatpumps and optimalsidestreamreturn. AIChEJ.1986, 32,1347. 82.Mullins, O.C.; Berry, R.S. Minimizationofentropy productionin distil- lation. J. Phys.Chem.1984, 88, 723. 83.Ratkje. S.; Kjelstrup, S.; De Swaan, A.J. Denbigh revisited: reducing lost workinchemicalprocesses.Chem.Eng.Sci.1995, 50, 151. 84.Rivero, R.; Cachot, T.; Ramadane,A.;LeGoff,P.L.Diabaticorquasi reversiblerectification. Int. Chem. Eng. 1994, 43, 240. 85.Hangos,K.M.;Alonso,A.A.;Perkins,J.D.:Ydstie,B.E.Thermody- namicapproachtothestructuralstabilityofprocessplants.AIChEJ. 1999, 45, 802. 86.Yong, P.S.; Moon,H.M.;Yi,S.C. Exergyandcryogenicair separation process for generatingnitrogen.J. Ind. Eng.Chem. 2002, 8. 499. 3940Demirel 87.Rivero, R. Exergysimulation and optimization ofadiabatic and diabatic binarydistillation.Energy2001, 26, 561. 88.Nakaiwa,M.;Huang,C.;Owa,M.;Akiya,T.;Nakam,T.;Sato,M.; Takamatsu,T.;Yashitome,H.Potentialenergysavingsinidealheat- integrateddistillationcolumn. Appl.Thermal Eng. 1998, 18,1077. 89.De Koeijer, G.M.; Kjelstrup, S.; Salamon, P.; Siragusa, G.; Schaller, M.; Hoffmann,K.H.Comparisonofentropyproductionrateminimization methodsfor binarydiabatic distillation.Ind.& Eng. Chem.Res. 2003, 41, 5826. 90.Chang,H.S.;Chuang, S.C.The intrinsicandextrinsic exergylossesof distillationcolumns. J. Chinese Inst. Chem. Eng. 2001, 32, 469. 91.LeGoff,P.;Cachot,T.;Rivero, R.Exergyanalysisofdistillationpro- cesses. Chem.Eng.Technol.1996, 19, 478. 92.Liu, Q.L.; Li, P.; Zhang, Z.B. Energy-savingnonequilibriumthermody- namic analysis for distillation-mathematicalmodel development. Chem. J. Chinese Univ.2001, 22,1209. 93.Kjelstrup,S.; Hafskjold,B. Nonequilibriummoleculardynamicssimu- lationofsteady-stateheatandmasstransportindistillation.Ind.& Eng.Chem. Res.1996,35, 4203. 94.DeGroot,S.R.;Mazur,P.NonequilibriumThermodynamics;North Holland:Amsterdam,1962. 95.Kuiken, G.D.C. Thermodynamics of Irreversible Processes:Applications to Diffusion and Rheology; Wiley: Chichester, 1994. 96.Kjelstrup,S.;deKoeijer,G.M.Transportequationsfordistillation ofethanolandwaterfromtheentropyproductionrate.Chem.Eng. Sci. 2003, 58, 1147. 97.Budiman, A.; Ishida, M. Three-dimensionalgraphical exergy analysis of a distillationcolumn. J.Chem. Eng. Japan1996, 29, 662. 98.Zemp, R.J.;deFaria, S.H.B.; Maria, MDLO. Drivingforcedistribution andexergylossinthethermodynamicanalysisofdistillationcolumn. Comp. Chem. Eng.1997, 21, S523. 99.DeKoeijer,G.M.;Kjelstrup,S.;vanderKooi,H.J.;GroR,B.; Knocke,K.F.;Andersen,T.R.Positioningheatexchangersinbinary traydistillationusingisoforceoperation.EnergyConvers.Mgmt. 2002,43, 157 1. 100.Sauar,E.;Rivero,R.;Kjelstrup,S.;Lien,K.M.Diabaticcolumn optimizationcomparedtoisoforcecolumns.EnergyConvers.Mgmt. 1997, 38,1777. 101.Naka,Y.;Terashita,M.;Hayashiguchi,S.;Takamatsu,T.Aninter- mediateheatingand coolingmethod for a distillationcolumn. J. Chem. Eng. Japan1980,13, 123. Thermodynamic Analysis ofSeparation Systems3941 102.Rivero,R.;Cachot,T.;Ramadane.A.;LeGoff,P.Diabaticorquasi- reversibledistillation:exergyanalysis-industrialapplication.analysis ofthermalandenergysystems.Proc.ofInt.Conf.,Athens,Greece, 1991. 103.Kolev, S.D. Coupled diffusion of multipleionic species in ion-exchange membraneswithfixed ionic groups.Sep. Sci. Technol.2003, 38. 237. 104.Demirel, Y. On the multicomponentdiffusion calculationsby meansof the Maxwell-Stefanequations,Chim. Acta Turc.1986, 13, 114. 105.Krishna,R.;Wesseling,J.A.TheMaxwell-Stefanapproachtomass transfer.Chem.Eng.Sci.,1997, 52, 861. 106.TheU.S.DepartmentofEnergy.EnergySaversforIndustryPlant Managersand Engineers, www.energysavers.gov/industry2003. 107.Arcia-Rodriguez,L.; Gomez-Camacho, C. Exergyanalysis of the SOL- 14 plant.Desalination, 2001. 137, 25 1. 108.Kokossis, A.C.; Floudas, C.A. Synthesis of isothermal reactorseparator- recycle systems. Chem. Eng.Sci.1991, 46,1361. 109.Linhoff,B.;Dundorf,H.;Smith,R.Heatintegrationofdistillation columninto overallprocesses.Chem. Eng.Sci. 1983, 38, 1175. 110.Andrecovich,M.J.;Westerberg,A.W.AMILPforn~ulationforheat- integrateddistillationsequence synthesis. AIChE J.1985, 31, 1461. 111. Jobson. M.; Hildebrandt, D.; Glasser, D. Variables indicating the cost of vapor-liquidequilibriumseparationprocesses.Chem.Eng.Sci.1996, 51, 4749. 112.Koehler, J.; Aguil~e, P.: Biass. E. Minimum refluxcalculationsfor non- idealmixturesusingthereversibledistillationmodel.Chem.Eng.Sci. 1991,46, 3007. 113.Shultz,M.A.;Stewart.D.G.;Hanis,J.M.;Rosenblum,S.P.; Shakur, M.S.;O'Brien.D.C.reducecostwithdividing-wallcolumns. Chem. Eng. Progress2002 (May),64. 114.Rev.E.;Emtir,M.;Szitkai, Z.;Mizsey,P.;Fonyo, Z.Energysavings ofintegratedandcoupleddistillationsystems.Comp.Chem.Eng. 2001, 25, 119. 115.Ricterova,V.Applicationofheatpumpinrectificationdecreases energyconsumptionin thechemicalindustry. EnergyConvers. Mgmt. 1991, 32, 519. 116.Olujic,Z.; Fakhri. F.;de Rijke, A.;de Graauw, J.; Jansen, P.J.Internal heatintegration-thekeytoanenergy-conservingdistillationcolumn. J. Chem. Technol. Biotechnol.2003, 78, 241. 117.Finn.A.J.Rapidassessmentofthermallycoupledsidecolumns.Gas Sep. Purif.1996. 10, 169. 3942Demirel 118.Schuller, M.;Hoffmann.K.H.;Siragusa.G.; Salamon, P.; Andresen,B. Numericallyoptimizedperformanceofdiabaticdistillationcolumns. Comp.Chem. Eng. 2001,25, 1537. 119.Nilsson,K.;Sunden.B.Optimizingarefineryusingthepinch technologyandthemindmethod.HeatRecovery.Sys. & CHP.1994, 14. 211. 120.Kaibel,G.;Biass,E.;Kohler.J.Thermodynamics-Guidelineforthe developmentofdistillationcolumnarrangements.Gas.Sep.Purif. 1990. 4, 109. 121.De Koijer, G.; Rivero, R. Entropy productionand exergy loss in experi- mentaldistillationcolumns.Chem. Eng.Sci. 2003, 58, 1587. 122.Demirel, Y.;Sandler,S.I. Effectsofconcentrationandtemperatureon thecoupledheatandmasstransportinliquidmixtures.Int.J.ofHeat Mass Transfer2002, 45. 75. 123.Sieniutycz,S.; Shiner,J.S.Thermodynamicsofirreversible-processes anditsrelationtochemical-Engineerin-dlawanalysisandfinite- time thermodynamics.J. Non-Equilib.Thermodyn.1994, 19, 303. 124.Smith, R.L.; Inomata, H.;Kanno, M.; Arai, K. Exergy analysis of super- criticalcarbondioxideextractionprocesses.J.Supercrit, Fluids1999, 15, 145. 125.Spiegler, K.S.; El-Sayed, Y.M. The energetics of desalination pi-ocesses. Desalination2001, 134, 109. 126.Zhang,G.:Hua,B.;Chen,Q.Exergoeconomicmethodologyfor analysisandoptimizationofprocesssyslems.Comp.Chem.Eng. 2000. 21. 6 13.127.Chen, Q.L.; Yin. Q.H.; Hua, B. Anexergoeconomicapproach for retrofit offractionating.Energy2002, 27, 65. 128.Sorin,M.;Paris,J.Combinedexergyandpinchapproachtoprocess analysis.Comp.Chem Eng.1997, 21, S23. 129.Bejan, A.;Tsatsaronis,G.;Moran,M.J.Tl~ernzalDesigrzarzdOptimiz- ation: Wiley:NewYork,1996. 130.Sciubba.E.Costanalysisofenergyconservationsystemsviaanovel resource-basedquantifier. Energy2003, 28, 457. 131.Erlach, B.; Serra, L.; Valero, A. Structural theory as standard for thermo- ecnomics.EnergyConvers.Mgmt.1999, 40,1627. 132.Ayres. R.U. Eco-thermodynamics:economics and the second law. Ecol. Economics 1998,26, 180.