CHEMCAD中磁阻柱的收敛方法

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MethodstoConvergeReluctantColumnsinCHEMCAD

ColumnModelsinCHEMCAD

TherearethreesteadystatedistillationmodelsinCHEMCAD:TOWR(Inside-outmethod),SCDS(simultaneouscorrectiondistillation),andTPLS(TOWRunitwithadditionalin/outconnections).Theliquid-liquidextractor(EXTR)isamodifiedformoftheSCDScolumn,asthemathematicsaresimilar.Ingeneral,theSCDS/EXTRcolumnisbettersuitedtoactivitycoefficientKvaluesandtheTOWRisbettersuitedtoequationsofstate.

Difficultieswithcolumns

Inappropiatethermodynamicselection

Inappropiatecolumnspecification

Rigidcontstraintsforconvergence

TroubleshootingDifficultColumns

Whiletryingtoconvergeacolumnitisinterestingtoseewhythemodelisfailing.Toseetheresultsoftheiterationsthatfailed,followthesesteps.

InCHEMCAD,gotothRUNmenu.

ChooseConvergence.

Intheconvergencedialog,checkmarkDisplayTraceWindowandGenerateRunHistory.

ThefirstsettingactivatesadisplaywindowwhichcontainsinformationonthecurrentstatusofcalculationswhenCHEMCADruns.Thelattercheckmarkpreventsthiswindowfrombeingrefreshedwitheachunit/iteration.Foracolumn,thetracewindowwillshowthepredictedcompositionprofilefromeachiteration,andtheerror(relativetopreviousiteration)term.Oftenthisinformationhelpsidentifyaproblem.Forexample,ifastageisdryingup,eitheryouhavetoomuchheatortoohighavaporflowinthecolumn.

Thermodynamics

Separationunitsarehighlydependentontheselectionofvaporliquidequilibrium(VLE)Kvaluemodel.TheuserwillrememberthatKvaluemodelsareusedtocalculatethecompositionoftheliquidandvaporphasesofamixtureatagiventemperatureandpressure.ThereareseveraldifferentKmodelswhicharedesignedfordifferentchemicalsystems.

Example:Figures1and2arexvsyplotsofthesystemwater–ethanolusingNRTLandSRK,respectively.Thereisasignificantdifferencebetweentheseparationsyouwillobtaininamodelusingthesetwomodels.Forthissystem,NRTLwouldbeareasonablechoicewhileSRKwouldbeunrealistic.Quiteoftenyouwillnotbeabletorunacolumnmodelifyouhaveanunrealisticthermodynamicselection.

Figure

SEQFigure\*ARABIC

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Figure

SEQFigure\*ARABIC

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ColumnSpecification

StagenumberingconventioninCHEMCADistoptobottom,1toN.Astageisconsideredthespaceaboveaplate.Ifacondenserispresentitisstage1;ifareboilerispresentitisstageN.Tomodelacolumnwhichhastenstagespluscondenserandreboileronemustspecify12stagesinCHEMCAD(10+condenser+reboiler=12).

Ifacondenserispresent,thefeedmustnotenterstage1,asthatisthereboiler.Topstagefeedsshouldenterstage2,thetopstage(plate),ifacondenserispresent.Likewise,ifareboilerispresentabottomplatefeedisconnectedtostage(N-1),notstageN.

ConvergingtoRigidConstraints

Typicallytheuserhasaspecificationtowhichtheywishtodesireacolumn.Quiteoftenthisisarestrictivecomposition,suchasmassfractionofakeycomponentineitherthebottomsortops.

ConvergingacolumnmodelinCHEMCADissimilartoconvergingacolumnintherealworld;itisdifficulttogodirectlytohighpurityseparation.Itisbesttostartwithaneasytarget,suchasrefluxratioandbottomsflowrate.Oncethecolumnisconvergedtothissimplespecification,we‘tighten’thespecificationstowardthegoal.

ProcedureforConvergingfromloosespecifications:(seeexample,below)

Setupthecolumn

VerifythatthetargetisthermodynamicallyfeasiblewithyourselectedVLEKmodel.

OntheSPECIFICATIONSpage,set‘loose’specificationssuchas‘RefluxRatio’and‘bottomsmass/moleflowrate

Runthecolumnandconverge.Changethespecificationsifnecessary.

GototheCONVERGENCEpageofthecolumndialog.Settheinitialflagto0ReloadColumnProfile.ThissettinginstructsCHEMCADtousethecurrentconvergedprofileasit’sstartingpoint(initialconditions)initerativecalculations.

OntheSPECIFICATIONSpagechangetomoretightspecifications.Runthecolumn.

Ifthecolumnconverges,tightenthespecificationsandrunagain.Ifthecolumnfailstoconverge,donotsavetheprofileofthefailedattempt.Relaxthespecificationsandrunthecolumnagain.

Repeatstep6untilyoureachthetarget.

ProcedureforInitialconvergenceofaDifficultcolumn

Oftenitisdifficulttoobtainthefirstconvergenceonacolumn.Ifthecolumnisrunwithnocondenserorreboiler,onedoesnothavetheoptionof‘loose’specifications.Ifthecolumnhasacondenserorreboiler,relaxingspecificationsdoesnotalwayshelp.

Ontheconvergencepageofthecolumndialog,specifyestimatesifyoucanmakereasonableestimates.Notethatabadguesswillmakethecolumnmoredifficulttoconvergethannoestimate.

Removenonkeycomponentsfromthefeed(s)toobtainthefirstconvergence.

Example:ifthefeedisprimarilyethanol,water,andnitrogenbutalsohasoxygen,ethane,andcarbondioxidepresentremovethelattercomponentsfromthefeedstream.Convergethecolumnwithafeedofethanol,water,andnitrogen.Settheintialflagto0ReloadColumnProfile,returntheothercomponents,andruntheunitagain.

Specifyalargernumberofiterationsontheconvergencepageofthecolumndialog.Thedefaultis50,butoften52iterationswillfindtheanswer,asperMurphy’sLaw.

Tryanalternatecolumnmodel.IfyouarecurrentlyusingtheSCDStrythesameseparationwithaTOWRorviceversa.Thetwomodelsuseextremelydifferentmathematicalmodels;oftenonewillfindananswerin10iterationswhiletheotherisdifficulttoconverge.Itisnotpossibletoobtaindifferentanswerswiththecolumns;themodelsarenumericalmethodstofindastablecompositionprofile.

Considerapartialcondenser.Ifyouhaveacondenserpresentbuthaveasignificantamountoflightends,youmayhavedifficultyconvergingthecolumn.Thedefaultcondensertype,total,requiresthatnovaporleavesstage1.Iflightendsarepresent,thismaynotbepossiblewithoutcryogenictemperatures.Changecondensermodetopartialandthelightendsgaseswillslippastthecondenser.

Example:Recoveryofhighpurityethanol

AnSCDScolumnwith8stages,acondenser,andreboilerisusedtoseparateamixturesof1000lb/hrethanol,1000lb/hrwater.Temperatureis70°Fandpressureis15psia.Thegoalistoobtainhighpurityethanolinthedistillate.

TheKmodelissettoNRTL(Thermodynamicsmenu>KValue),whichiscapableofaccuratelymodelingthenonidealsystemwater-alcohol.Itisimportanttonotethatthereisanazeotropeinthissystemathighlevelsofalcohol.

Specify10stagesforthecolumn,withfeedonstage3.

Nowset‘loose’specificationsforthecondenserandreboiler.Forcolumnswithtotalcondensers,arefluxratioof1.2isagoodspecificationforthecondenser.Agoodinitialspecificationforreboilersismassflowrate.Thereare2000lb/hrenteringthecolumn;setthebottomstohave1500lb/hr.

Runtheflowsheet.Thecolumnwillconverge.Itisnowtimetotightenthespecificationstowardthetarget.Visualinspectionofthetopsstreamshowsthatmassfractionofethanolis0.86.Changethecondenserspecificationtodistillatecomponentmassfraction,setethanolasthecomponent,andsetthespecas0.91.Visualinspectionofthebottomsshowsthat~950lb/hrofwaterishere.Changethereboilerspecificatontobottomcomponentmassflowrate,setthecomponenttoWater,andspecify980lb/hr.

Thenextstepiscrucial:gotothespecificationspage.Ontherighthandsideisthefieldinitialflag.Setthistomode0:reloadcolumnprofile.Thissettinginstructsthemodeltousethecurrenttemperature,pressure,andcompositionprofileofthecolumnasthestartingvaluesforthenextcalculation.Bystartingfromthecurrentconvergedprofile,thenumericalmethodpackagescanfindatightersolution.Notethatthereareothermodeswhichallowyoutospecifyaprofileyourself.Theseareparticularlyusefulinstartingamodelofanexistingunit;youalreadyhavearoughprofileacrossthestages.

Runthecolumn.Itwillconverge.Nowchangethesettingonthecondenserto0.95andrunthecolumn.Thefollowingmessagewillappear.

Thecolumnhasnotconverged.CHEMCADisaskingifyouwanttosavethecurrentprofile,whichisnotaconvergedresult.Ifyousavetheprofileyoumayrunmoreiterationsofthecolumnmethods;sometimesthisleadstoasolution.Ifmoreiterationsdoesnotconvergethecolumn,youwillhavetostartthiscolumnoverfromthebeginning.Thiscanbetedious;itisofteneasiertomakesmallchangesratherthanjumps.Press[NO]todiscardthecurrentprofile,CHEMCADwillreverttotheconvergedprofilefrombefore(topspecification0.91,bottomspecification980).

Thejumpfrom0.91to0.95wastoolargetobesolvedin40iterations(defaultnumberofiterationsforSCDS).Wewilltryasmallerchange.Setthetopspecificaitonto0.93andthebottomspecificationto990.Runthecolumn.

WereturntoThermodynamicstocheckthefeasibilityofdistillingforhigherpurity.ZoominginonaTPXYplot(Plotmenu>TPXY)ofthesystemweseethatwearequiteclosetotheazeotrope.Itwillbedifficulttoobtainhigherpurity;mores