ASpen Plus HEAT EXCHANGER NETWORKS - pinch casestu

HEATEXCHANGERNETWORKSCHE396SENIORDESIGNSubmittedbyMichelleVillasinKeithObenzaMaryLanuzaToanNguyenOrbitCHE396SeniorDesignHeatExchangerNetworksTableofContentsMinimumheatingandcoolingrequirements…………………………………………..1MinimumEnergyHeatExchangerNetwork………………………………………….11LoopsandPaths………………………………………………………………………14ReducingNumberofExchangers……………………………………………………..15DistillationColumns…………………………………………………………………..22PinchMethod…………………………………………………………………………27StreamSplitting……………………………………………………………………….34References…………………………………………………………………………….36CHE396SeniorDesignHeatExchangerNetworks1HeatExchangerNetworksEnergyconservationisimportantinprocessdesign.Inindustrialexperience,thecalculationoftheminimumheatingandcoolingrequirementsrevealsignificantenergysavings.Specifically,ImperialChemicalIndustriesintheUnitedKingdomandUnionCarbideintheUnitedStateshavebothstatedtheresultsofnumerouscasestudiesthatindicate30%to50%energysavingscomparedtotraditionalpractice.Therefore,energyintegrationdesignprocedureisaverybeneficialtoolandisanimportantphaseindeterminingthecostofpreliminarydesign.Thefirststepintheenergyintegrationanalysisisthecalculationoftheminimumheatingandcoolingrequirementsforaheat-exchangernetwork.Inanyprocessflowsheet,thereareseveralstreamsthatneedtobeheatedandtherearesomethatneedtobecooled.Intheaceticanhydrideproduction,forexample,thereactionstreaminthesecondreactormustbecooled,whiletheliquidproductcomingoutofthesamereactormustbeheatedfordistillation.Forthatreason,coolingwaterisneededtolowerthetemperatureofthereactorstream,andsteamisneededforheatinginthedistillationcolumn.Therearetwolawsforheatintegrationanalysis.Thefirstlawstatesthatthedifferencebetweentheheatavailableinthehotstreamsandtheheatrequiredforthecoldstreamsisthenetamountofheatthatmustberemovedorsupplied.Considerthisexample.Supposethereare6streamsgiven,threethatneedtobeheatedandtheotherthreeneedtobecooled.Theheatassociatedwitheachstreamcanbecalculatedbyusingthefollowingequation:Qi=FiCpi)Ti(1)Forourcasestudy,sixrepresentativestreams,threestreamstobecooledandthreetobeheatedup,werechosen.FigureAandTable1showsthedescriptionsofthechosenstreams.CHE396SeniorDesignHeatExchangerNetworks2FigureASchematicDiagramofCaseStudyAceticAnhydridePlantTable1DescriptionsofStreamsStreamDescription1Freshacetonegoinginthesystem.2Freshaceticacidgoinginthesystem.3Distillationcolumn1reboiler.4Recycleaceticacidgoingtoreactor2.5Flash/condenser6RecycleaceticacidgoingtoabsorberAbsorberCondenserR2R1DistillationColumn1DistillationColumn2AceticAnhydrideAceticAcidAcetone123456CHE396SeniorDesignHeatExchangerNetworks3Forstream1,Q1=(1000Btu/hroF)(250-120)=130x103Btu/hrTable2showstheresultsforeachstream.Table2FirstLawCalculationStreamNo.ConditionFCp(Btu/hroF)Tin(oF)Tout(oF)Qavailable105Btu/hr1Cold.489377133-2.742Cold217377129-1.133Cold5.0x105156196-2054Hot1.23x1042447721.05Hot2.75x1051761281326Hot10462441291.2Total=-50.25Asshowninthetable,50.25x105Btu/hrmustbesuppliedfromutilitiesifnorestrictionsontemperature-drivingforcesarepresent.However,thecalculationforthefirstlawdoesnotconsiderthefactthatheatcanonlybetransferredfromahotstreamtoacoldstreamifthetemperatureofthehotstreamsurpassesthatofthecoldstream.Therefore,asecondlawstatesthatapositivetemperaturedrivingforcemustexistbetweenthehotandthecoldstreams.Foranyheat-exchangernetworks,thesecondlawmustbesatisfiedaswellasthefirstlaw.AsimplewaytoencompassthesecondlawwaspresentedbyHohmann,Umedaetal.,andLinhoffandFlower.Adescriptionoftheiranalysisisshowninaccordingly.Ifaminimumdrivingforceof10oFbetweenthehotandthecoldstreamsischosen,agraphcanbeestablishedshowingtwotemperaturescalesthatareshiftedby10oF,oneforthehotstreamsandtheotherforthecoldsteams.Then,streamdataisplottedonthisgraph(Figure1).Nextaseriesoftemperatureintervalsaregeneratedcorrespondingtotheheadsandthetailsofthearrowsonthegraph(Figure2).CHE396SeniorDesignHeatExchangerNetworks4Figure1ShiftedTemperatureScaleFigure2TemperatureIntervalsIneachinterval,heatfromanyhotstreamsinthehigh-temperatureintervalscanbetransferredtoanyofthecoldstreamsatlower-temperatureintervals.Forastartingpoint,heattransferineachintervalwouldbeconsideredseparately.Thenecessaryequationisshownbelow.Qi=[Σ(FCp)hot,i-Σ(FCp)cold,i]ΔTi(2)Forexample:Q1=[1046+1.12x4](244-206)=5.07x105Thus,forthefirstinterval,avalueof5.07x105isobtained.ThevaluesforotherintervalsareshowninFigure3.Noticethatthesummationoftheheatavailableinalltheintervalsisthesameasthenetdifferencebetweentheheatavailableinthehotstreamsandthatinthecoldstreamsobtainedusingthefirstlaw.2502402302202102001901801701601501401301201101009080702402302202102001901801701601501401301201101009080706045612324423020619617616616615614313313912912911912811887777767CHE396SeniorDesignHeatExchangerNetworks5Figure3NetEnergyRequiredatEachInterval-21.211.3Takingalltheheatavailableatthehighestinterval(206to244oF),transferittothenextlower-temperatureinterval(176to206oF)andrepeatforallintervals.Sincetheheatistransferredtoalower-temperatureinterval,thesecondlawissatisfied.FromFigure4,itcanbeseenthattheavailabl