simple dew point control - aspenplus...
TRANSCRIPT
Rev2.1 ‐1‐ January9,2018
SimpleDewPointControl–AspenPlusv10StepsarepresentedtosetupasimulationinAspenPlusv8.8tomodelasimpledewpointcontrolsystemconsistingof:
Gaschiller Flashseparator Liquidstabilizerwithgas
recycle&compression Productgascompression Simplepropane
refrigerationloopWhenthesimulationissetuptheoverallPFDshouldlooklikethefiguretotheright.BasisAgasplantisprocessing100MMscfd(drybasis)toproduceaspecpipelinegasaswellasapipelinerawmixliquidproduct(YGrade).Thefollowingareknownconditionsforthefeedstockandspecificationfortheproducts:
Thecompositionofthefeedgasisshowninthefollowingtable.
Thegasenterstheplantat400psia&120°F. Thegasisnearlysaturatedwithwaterattheinlet
conditions,48lbwaterperMMscfdrygas. Theproducedpipelinegasshouldhaveagrossheating
valuebetween905to1050Btu/scf1&ahydrocarbondewpointnohigherthan15°F.
Theproducedpipelinegasshouldbedeliveredtothepipelineat1000psiaandnohigherthan120°F.
Theproducedliquidsshallbeexportedviapipeline&stabilizedtohaveaTVP(truevaporpressure)@100°Fnogreaterthan103psia.
Component Mol%N2 0.357CO2 0.194C1 80.980C2 13.238C3 3.438i‐C4 0.431n‐C4 0.742i‐C5 0.199n‐C5 0.156n‐C6 0.163n‐C7 0.065n‐C8 0.026n‐C9 0.010
Apropanerefrigerationloopwillbeusedtoprovidethechillingduty.Thecondenserwill
operateat120°F.Theminimumapproachtemperaturewithinthechillerwillbe10°F. Aircoolerswillbeusedtocoolgases&liquidsto120°F.
CreatenewsimulationfileWhenrunningunderWindows10youcanstarttheprogramfromStart,theallprogramslist,AspenPlus,AspenPlusV10.Whentheprogramopenschoosethenewbutton.thereareseveraltemplates
1Ifthegrossheatingvaluespeccannotbeachievedsetthechilledseparatortothelowestreasonabletemperaturewhenusingasimplepropanechillingloop,‐30°F.
Rev2.1 ‐2‐ January9,2018
thatcanbechosen.SelecttheGasProcessingoptionintheleft‐handcolumn&chosetheGasProcessingwithEnglishUnitstemplate.PressCreate.
SaveasyougoOneofthethingsyou’llwanttodoistosaveyourfilesasyougo.ThefirsttimeyougototheSaveAsoptionyou’llhaveseveralformatsfromwhichtochoose.ThereareadvantagestosaveastheAspenPlusBackup(BKP)format–thefilestendtobesmaller&lesslikelytobecomecorrupted.Forthisproblemlet’susethename“SimpleDewPointControlV10.”
DefinetheComponents&thePropertyModelsSpecifycomponents,fluidpropertypackages,&crudeoilassaysThefirststepistoaddasetofpurechemicalspeciestorepresentthegas&waterphases.WhenyouopenanewfilethedefaultscreenshouldbetheComponent‐Specificationsform.(Ifnot,press
Rev2.1 ‐3‐ January9,2018
theSpecificationsitemunderComponentsintheleft‐handcolumn.)Wwillwanttoaddthefollowingpurecomponents:water,nitrogen,carbondioxide,methane,ethane,propane,i‐butane,n‐butane,i‐pentane,n‐pentane,n‐hexane,nheptane,n‐octane,&n‐nonane.OneofthedirectwaystodothisistopressFind&usethesearchformtofindthedesiredcomponents.ThefollowingformsshowasearchforH2O;keyphrasescanbeusedwiththeEqualsorContainsoptionstofindallcomponents.Foreachsucceedingcompoundyouwillbeaskedtoreplaceoneofthecompoundsoraddtothelist;chooseaddtothelist.
Rev2.1 ‐4‐ January9,2018
Whenyoustartaddingtheothercomponentsyouhaveanextraquestiontoanswer,whethertoaddorreplacethecurrentcomponent.YouwillprobablywanttochooseAdduntilyou’veaddedallofyourcomponents.Therearevarioustricksforfindinggroupsofcompounds.Forexample,bysearchingforn‐Alkanesthatcontain“ethane”youcangetthelighthydrocarbons.Youcanselect&addallasagroup.
Rev2.1 ‐5‐ January9,2018
Afterfindingallofthecomponentsyoushouldhavealistthatlookssimilartothefollowingform.
Rev2.1 ‐6‐ January9,2018
AspenPluswillretrieveinformationabouteachcomponent&alsocreateaComponentIDforthissimulation.YouarefreetochangetheseIDstomatchyourpersonaldesires.Forexample,youcanchangetheIDforMETHA‐01toC1bydoublingclickingonthattextitem;afterchangingthetextvalue&pressingenterAspenPluswillverifythatyouwanttoRenamethecomponent&replacethatcomponentwithsomethingelse.Thiscanbedoneforallofthecomponentstocreate(IMHO)morereasonableIDs.
Rev2.1 ‐7‐ January9,2018
Anotherissueistheorderthatthecomponentsmayhavebeenextracted.Ihaveapersonalpreferenceforalistintheorderofwater,lightgascomponents,&thenthehydrocarbonsinincreasingcarbonnumberorder.Thisisnotthecurrentorder.YoucanchangetheorderbypressingtheReorderbutton&thenusingtheup&downarrowstoputcomponentsinyourpreferredorder.
Thenextstepwouldnormallybetopickafluidpropertypackage.However,whenwechosetheGasProcessingoptionwhenwecreatedthesimulationthePeng‐Robinsonequationofstatemethodwaschosenasthedefault.WecanseethisbyselectingMethods&Specificationsintheleft‐handcolumn.NoticethattheBaseMethodisPENG‐ROB.Wewillkeepthisdefaultselection.
Rev2.1 ‐8‐ January9,2018
TheremaystillbeitemstobeaddressedbeforewecanentertheSimulationsection(youcantellthisifthereisa symbolintheleft‐handcolumn).Tofindoutwhatneedstobedoneclickthe button.YoumaygetaformthatallowsustomodifyvaluesforthePeng‐Robinsonbinaryinteractioncoefficients.Ifyougetthis,donotchangeanyofthevalues.
Rev2.1 ‐9‐ January9,2018
Nowwhenyoupressthe (Next)buttontheprogramshouldshowyouthatyoucangoontothenextstep.SelectGototheSimulationenvironment&pressOK.Setup&SolvetheFlowsheetGasChilling&SeparationWhenyouactivatethesimulationenvironment&you’llseeablankflowsheet.Wewillwanttocreateafeedstream&attachittoaHeater.Theoutletwillbeattachedtoathree‐phaseflashseparator.
EnsurethatthemodelPaletteisvisible.Ifitisnot,presstheViewtab&clickModelPalette.AshortcutkeyistopressF10.
Placethefollowingunitsontheflowsheet:
AHeater,COMBINE.(Youmaywanttochooseoneofthesquaresforitsiconinsteadofaheatexchanger).
AnHeater,CHILLER AFlash3separator,COLDSEP.
AsshownintheBFDabove,connecttheunitswithmassstreamsDRYFEED,FEEDWATR,WETFEED,CHILLED,COLDVAP,COLDLIQ,&COLDWATRaswellastheHeatstreamQ‐CHILLR.(Rememberthatstreamnamescanonlybe8charactersinlength&willalwaysbecapitalized.)
Rev2.1 ‐10‐ January9,2018
Double‐clickontheDRYFEEDstreamtoopenuptheentryformstospecifycomposition&conditions.EnsurethattheFlashTypeisTemperature&Pressure.EntertheflowratewithaMolebasis&usetheMMscfdunits.UseMole‐fracforthecomposition(drybasis,i.e.,nowater);youdonotneedtomakesurethenumbersaddto1,theprogramwillnormalizeasappropriate.WewanttodothesamethingforthewaterportionofthefeedrepresentedbythestreamFEEDWATR.Double‐clickontheFEEDWATRstreamtoopenuptheentryformsforthisstream.Enter4,800lb/dayusingtheMassbasis(torepresentthe48lb/MMscfwatercontent).Enterthepressure&thetemperature.SincethisispurewateryoucanspecifythecompositioneitherusingMass‐FracorMole‐Frac.
Rev2.1 ‐11‐ January9,2018
WemixtogethertheseparateDRYFEED&FEEDWATRstreamstodefinethewetfeedtothegasplant.NormallywewoulddothiswithaMixeroperation,butinsteadwe’regoingtodoitwithaHeatertobeabletospecifytheactualtemperatureofthewetfeed.Double‐clickonCOMBINEtoopentheinputform.Specifythe120°FoutletTemperature.SpecifythePressureas400psia(wecouldhavespecifiedazerotodenoteazeropressuredropbutspecifyingtheactualpressuregivesusasinglepointofcontrolfortheinletpressure).PulldowntheValidphaseslist&chooseVapor‐Liquid‐FreeWater.Wenowwanttomodelthegassideofthechiller.Wewillultimatelyuseadifferentoperationtomodelboththeprocess&coolantsidesoftheexchanger,butherewe’lljustmodeltheprocessfeedsidewithaHeater.Double‐clickonCHILLER.FornowspecifythePressureas0psia(tosignifyazeropressuredrop).PulldowntheValidphaseslist&chooseVapor‐Liquid‐FreeWater.Fornowlet’sspecifytheoutletTemperatureas15°F(thespecvalueforthedewpointoftheproducedgasinthepipeline).Finally,let’sspecifytheoperationforthecoldseparator.Double‐clickonCOLDSEP.SettheFlashTypeasDuty&Pressure.SpecifythePressureas0psia(tosignifyazeropressuredrop)&theDutyas0MMBtu/hr(tosignifyadiabaticoperation).PressingNextshowsthatalloftherequiredspecificationshavebeenmade.PressOKtorunthesimulation.AtabfortheControlPanelshouldopenup&indicatethatthesimulationhasrunsuccessfully.(NoticethatthereisawarningconcerningthenumberofphasesintheCOMBINEblock;sincenofreewatershouldformfromthisoperationthiscanbeignored.)
Rev2.1 ‐12‐ January9,2018
Whataresomeoftheresults?Wecangetanoverviewbypostingsummaryconditionsontheflowsheet.ClickonStreamResultsintheModifytaboftheribbon.SelectTemperature,Pressure,Massflowrate,&Heat/Duty.PressOK.Nowthesenumbersarepostedontheflowsheet.
Rev2.1 ‐13‐ January9,2018
NoticethatallvaluesarecalculatedforthestreamsoutofCOLDSEPareat15°F.Thismeansthatthevaporoutoftheseparator,COLDVAP,isatitsdewpointat15°F.Thismakesthepipeline’sdewpointspec,right?No,notreally.Buthowwouldweknowthis?WecanlookatthephaseenvelopeforCOLDVAPtodetermineifthevaporwillhaveaminimumdewpointtemperatureatallpressuresitislikelytoexperienceinthepipeline.ClickonstreamCOLDVAP;intheribbonundertheHometabselectthepulldownlistStreamAnalysis&selectPTEnvelope.MakesuretheStreamIDisCOLDVAP.PressRunAnalysis.Youwillseeaphasediagramshowingthebubblepoint&dewpointcurves;fromthediagramyoucanseethatthecricondenthermisabout20°F.SelecttheResultsforPTENV‐1intheleft‐handcolumn;fromthetableofvaluesyoucanseethatthehighesttemperature(essentiallythecricondentherm)is19.8°F.Thisoccursat647psia.
Rev2.1 ‐14‐ January9,2018
Rev2.1 ‐15‐ January9,2018
Thepressureatwhichthecricondenthermoccursisverymuchinthepossiblerangeofpipelineoperatingpressures.Sincethegasinthepipelinewillexperiencepressureslowerthantheinlet’s1000psia,itisappropriatetousethecricondenthermasthecontrollingvalueforthisspec.Andsincethetemperatureis20°F,thisgasdoesnotmakethisspec.Fornowwe’llusetrial‐and‐errortodetermineanappropriatetemperatureforthecoldseparator.NotethatifwespecifythetemperatureoutofCHILLERas9°FwegetacricondenthermofCOLDVAPofjustunder15°F.Havewemettheheatingvaluespec?Wecandeterminethisbymakinguseofthebuilt‐innet&grossheatingvalues&addingtothestreamreport.ExpandtheSetupitemintheleft‐handtreestructureoftheSimulationitems.UnderPropertySetscreateaNewsetcalledHEATVALS.Editthatpropertyset&addthepropertiesQVALGRS&QVALNET.(Forreasonstobenotedlater,makesurethatQVALGRSisthefirstinthelist.)
Rev2.1 ‐16‐ January9,2018
Nowwecanrerunthesimulation&lookfortheResultsforCOLDVAP.Gotothebottomofthestreamreport&clickon<addproperties>.TowardthebottomofthelistclickonbothHEATVALSproperties&clickOK.AtthebottomoftheEditStreamPropertyTemplateformpressOK.NowwhenyoulookatthestreamreportforCOLDVAPyou’llseethegross&netheatingvalues.
Rev2.1 ‐17‐ January9,2018
Thebadnewsisthateventhoughthenet&grossheatingvaluesarecalculated&reportedatthebottomofthelistthesevaluesareinmassunits,notthescfmolarunitsthatwereallywant.We’llhavetodosomeunitconversionsusingtheMassflow&Moleflowvalues:
6
hrBtu lb 2423048.9 211179daylb hr
HHVscfMMscf 1098.809
MMscfday
Btu1182.3
scf
Thisvalueistoohigh&willrequiremoreheavyhydrocarbonsberemoved.Butbeforewefocusonthislet’saddadditionalprocessingtostabilizetheliquidformed(sincethiswillinvolverecyclingbacktheevolvedgas).LiquidStabilizationDeterminationofliquid’sTVPvalueThenextstepistodetermineiftheproducedliquidwillmaketheTVPspecof103psia.Let’saddaHeaterTVPCALContoCOLDLIQ&useittodeterminethebubblepointpressureat100°F.SettheVaporfractionto0(tocorrespondtoaliquidfractionofexactly1).
Rev2.1 ‐18‐ January9,2018
Runthesimulation.Therearevariouswaystocheckforthecalculatedvaporpressure;let’slookatthestreamResultsforLIQUID.ThisshowsthattheTVPis654psia,muchhigherthanthedesired103psia.Wecanlookatthecompositiontoseetheproblem–theamountofmethaneisroughlythesameastheethane&propane.ThisismuchtoohighforaY‐gradeNGLliquidmix.Howcanweseethemolefractions?Thesearepartofthestreamreport,butyoumayhavetopressthe+signontheMoleFractionsoptiontoexapandthelist.
Rev2.1 ‐19‐ January9,2018
SetuptheStabilizercolumn
Wecanprocessthehigh‐pressureliquidinastrippingcolumntoremovetheselightends.Let’saddtwounitsinbetweenthecoldseparator&theTVPcalculation:
AValve,VLV‐001 APetroFracSTRIPcolumn,STAB.
ConnectwithmaterialstreamsFLSHDLIQ,STABGAS,&STABLIQasshownabove.Double‐clickonVLV‐100.SpecifytheOutletPressureas200psia.Nowlet’sdefinethestabilizingcolumnasa11‐stagecolumnwithakettlereboiler.(Rememberthatthereboilerwillactasthe11thstage,sothereareonly10stagesinthecolumnitself.)Double‐clickonSTAB.Setthenumberofidealstagesto11.SpecifyNone‐TopFeedfortheCondenserbutaKettleastheReboilertype.Weneedgiveanestimatefortherateeitheroutthetoporbottomofthecolumn.Roughly6900lb/hrisbeingfedtothecolumn;if¾ofthisisstrippedoffasvolatilegasthenthebottomsflowshouldbeabout1,500lb/hr.
Rev2.1 ‐20‐ January9,2018
Let’slookattheStreamstab.Theinformationshouldalreadyhavebeenspecifiedfortheproductstreams.Ensurethatthefeedtothecolumngoestothetopstage,1.ByspecifiyingAbove‐Stagethenanyvaporthatmightbecreatedbyflashingthroughtheinletnozzleofthetowerwillnotcontacttheotherfluidsonthetopstagebutratheronlymixwiththevaporfromthetopstage.Let’slookatrunningthetowerat200psiawithanegligiblepressuredrop.Specify200forboththetop&bottomstagepressure.Wecannowrunthesimulation.Lookingattheprocessflowdiagramwecanseethatthecolumnoperateswithatemperatureof416°F.But,theliquidproducedhasaTVPat100°Fofonly3psia(asseenfromtheresultsforTVPCALC).Thisismuchlowerthanitneedstobe.
Rev2.1 ‐21‐ January9,2018
OneofthereasonsforusingaPetroFraccolumnisthatitisrelativelyeasytospecifyoperatingconditionsonthecolumn.Let’sspecific200°Fasthereboilertemperature.SelectDesignSpecificationsintheleft‐handcolumn&presstheNewbutton.SelectStagetemperatureastheType&assignittothereboiler(thelaststage,11).Setthevalueto200(thedefaultunitsbeingusedare°F).OntheVarytabselecttheBottomsFlowRateastheadjustedvariable.Nowwhenwererunthecolumnweseethatthereboilertemperatureis,indeed,200°F,buttheTVPat100°Fis68psia(stilllowerthannecessary).
Wecouldadjustthetower’sdesignspecbytrail‐and‐error,butthatwouldbeinconvenientaswemakeotherchangesthataffectthecolumnoperation.However,wecanaddaflowsheet‐leveldesignspectovarythereboilertemperaturetomakethisspec.SelectDesignSpecsunderFlowsheetingSpecsintheleft‐handcolumn.Createanewspec,DS‐TVP.First,we’lldefinethetargetvariableundertheDefinetab.CreateanewvariableTVP&associateitwiththepressureoftheLIQUIDstream(i.e.,thecalculatedbubblepointpressureat100°F).Next,specifythevalueontheSpectab.
Rev2.1 ‐22‐ January9,2018
Finally,weneedtospecifythereboiler’stemperatureasthevariabletovary.ThisisnotstraightforwardtodefinesinceitisitselfadesignspecforthePetroFracblock.FortheBlockSTABspecifyVALUEastheVariable;thiskeywordsignifiesthatwearemodifyingsomethingthathasbeendefinedasaDesignSpec.SpecifythatitisthefirstofSTAB’sdesignspecs(andithappenstobetheonlyone,too)byspecifying1forthevalueofID1.Nowthetrickypart,definingupper&lowerlimitsfortheiterations.Ifwewereoperatingthecolumnat103psiathenthereboilerwouldbeat100°F–thiswouldmakeareasonablelowerlimit.Atelevatedpressuresthenthereboilertemperaturewouldbehigher.Wealreadyknowthat200°Fistoohigh,butthiswouldmakeareasonableupperlimit.Nottootricky.Thetrickyparthastodowiththeunits–eventhoughweareworkingwithtemperatureunitsof°FwemustspecifytheLower&UppervaluesinAspenPlus’sintrinsicunits,Kelvin.Thevaluesof100°F&200°Fareapproximately311K&366K,
Rev2.1 ‐23‐ January9,2018
respectively;thesearethevaluesspecifiedfortheselimits.Finally,specifyreasonablefractionalvaluesfortheperturbation&maximumstepsizes,0.1&0.5,respectively.Nowwecanrerunthesimulation.Thesummaryintheflowsheetshowsthatoperatingthereboilerat166°Fwillgivealiquidthathasa103psiaTVPat100°F.
Whatdoesthestabilizedliquidlooklike?Double‐clickonSTABLIQ&selectResultsintheleft‐handcolumn.(Remembertoexpandanylistofvaluesthatyouwanttoexamine.)Notethatthereisessentiallynomethane&verylittleethane–allofthismaterialhasbeenstrippedoutintotheoverheadvaporstream.
Rev2.1 ‐24‐ January9,2018
Let’slookathowmuchgashasbeenstrippedout.Double‐clickonSTABGAS.LookundertheResultsarea(expandingthelistsasnecessary&addingtheheatingvalues).Noticethatthisgashasveryhighconcentrationsofmethaneðane(about90mol%).Butcouldthisbedirectlyproducedaspipelinegas?SelectProperties.NotethattheHHVistoohigh,1466Btu/scf(ascalculatedfromthegrossheatingvalue,massflowrate,&molarflowrate).Morethanlikelyitwon’tmakethedewpointspeceither.There’snotalotofitcomparedtotheCOLDVAPbutitwon’tmakethepropertiesofthetotalproducedgasanybetter.RecycleofRecoveredGasOnemightaskwhywedidn’tincludeacondenseronthestabilizercolumn.Acondenserwouldallowustowashthepropane&heavier(C3+)backdownthecolumn&outwiththeStabilizedLiquid.WecaneffectivelygetthiseffectbyreconfiguringtheprocesstorecycletherecoveredgasfromthestabilizingcolumnupstreamofCHILLER.However,sincetherecoveredgasisproducedatalowerpressure,itmustbecompressedtoahigherpressureconsistentwiththeoriginalfeedgas.
Let’saddtwounits:
Rev2.1 ‐25‐ January9,2018
ACompr,RECCOMP AMixer,RECMIX.
ConnectwithmaterialstreamsRECGAS&TOTALandaddtheworkstreamW‐RECCMP.NotethattheiconforRECCOMPhasbeenflippedonthePFDshownabove.Thisdonebyright‐clickingonRECCOMP,selectingRotateIcon,&thenFlipHorizontal.Double‐clickonRECCOMP.SettheTypetoIsentropic.SelectDischargePressure&setitsvalueto400psia.SettheIsentropicefficiencyto0.75(areasonabledefaultadiabaticvalue).
Runthesimulation.NotethatthereisarecyclestreambutAspenPlussetsitupautomaticallywithoutanythingspecialbeingdone.IfonewastochecktheControlPanelyou’dseethatittook5iterationstoconvergethisrecycle.
Rev2.1 ‐26‐ January9,2018
Howhasaddingtherecyclegasaffectedthefinalresults?ThereisnotagreatdealofRecycledGasbeingmixedwiththefreshfeed(1,063lb/hrvs.218,257lb/hr)sothecompositionofCOLDVAPdoesnotchangebymuch.Wecanrerun&checkPTENV‐1toseethatthecricondenthermisstillabout14.4°F,makingspec.Butwewouldexpecttheproducedgastoalsohaveasimilarhigherheatingvalue&itwillbeabovethespec.Let’slookatthenewResultsforCOLDVAP&calculatethegrossheatingvaluesonanscfmolarunitbasis:
6
hrBtu lb 2423046.6 212180daylb hr
HHVscfMMscf 1099.1861
MMscfday
Btu1183.2
scf
Rev2.1 ‐27‐ January9,2018
ThisHHVishigherthanthespecvalue.WecantrytodecreasetheHHVbyreducingthetemperatureofCHILLER.Let’slowerthistemperaturetothelowestlimitreasonableforasimplepropanechillingloop,‐30°F.Reducingthistemperaturedoesshiftmoreoftheheavyendsoutoftheproducedgas&theHHVislower.However,theHHVofCHILLEDisstilltoohigh,1152Btu/scf(ascalculatedusingthevaluesbelow&theequationabove).Unfortunately,thisisprettymuchthebestwecandowhenusingachilledsingle‐stageflashseparationunit.CalculatorBlockforHHVinMolarUnitsNoticethatwehavehadtodosidecalculationsfortheheatingvalueinunitsofBtu/scfsincethevalueisreportedinmassunits.Wecanaddacalculatorblocktodothiscalculationforus.SincethisvalueisonlyforreportingpurposeswewillwriteitsvaluetotheControlPanel.Intheleft‐handtreestructureexpandtheFlowsheetingOptionscategory&selectCalculator.PresstheNewbutton.GiveitthenameGASHHV.Nowwe’llpullinthevalueforBtu/lbgrossheatingvalueasGROSS,lb/hrmassflowrateasMASS,&MMscf/daymolarflowrateasMOLES.Creatingtheflowratevariablesisstraightforward–theReferenceTypeisStream‐Var,selectedtheMIXEDsubstream,&
Rev2.1 ‐28‐ January9,2018
chooseMASS‐FLOWANDMOLE‐FLOWasappropriate.Thegrossheatingvalueisalittlemorecomplicated.HeretheReferenceTypeisStream‐PropyouhavetoselectadefinedPropSet.We’vedefinedHEATVALSasboththegrossandnetheatingvalues.AspenPluswillusethefirstvalueforitscalculations(butwillgivewarningsthatitisdoingso).Ifyouwanttoeliminatethewarningthencreateanewpropertysetwithjustthegrossheatingvalue,QVALGRS.Let’senterthestatementforthecalculationasacoupleFortranstatements.ThevariableHHVwillbecalculatedwiththefirststatement&writtentotheContolPanelwiththesecond.(Don’tmaketheWRITEstatementmorecomplicatedthanthisunlessyouhaveaFortrancompileronyourcomputer;thisissimpleenoughthatAspenPluswillinterpretthecodewithitsowncapabilities.)Finally,wewillspecifywhentocalculatethevaluebyspecifyingeachoftheinternalvariableasInputvariables.So,thiscalculatorblockwillberecalculatedeverytimeoneormoreofthesevariableshaschanged.NowwhenyourunthesimulationyoucanchecktheControlPanel&findtheHHVintheproperunits.
Rev2.1 ‐29‐ January9,2018
PreventionofFreezinginDPCSeparatorTheinletfeedgasisnearlywatersaturatedattheentrytotheprocess.WhenthewaterdropsoutofthegasphasewhenitiscooledthereisapotentialforfreezingintheCHILLER&COLDSEP.Atypicaltechniquetopreventiceorhydrateformationistoinjectethyleneglycol(EG)upstreamoftheCHILLER.AnaqueoussolutionofEGhastheabilitytosuppresstheformationofice.Init’spurestateEGhasafreezingpointof8°F,butaqueoussolutionshavefreezingpointsthatarelower.Noticefromthechartontheright1onemaygetfreezingprotectionto‐30°ForlowerbymaintainingaEGconcentrationinwaterof85wt%to50wt%.Whataretheappropriateconcentrationstoconsiderforourprocess?
Wewouldliketomakesurethatthereisfreezingprotectionfortheentireconcentrationrangebefore&afterthewaterisabsorbed.
Wewantprotectionnotonlyattheprocesstemperaturebutalsothecoldesttemperatureatthetubewall.Thismeanswehavetoprotectbelowthe‐30°Fprocesstemperaturebuttothecoolanttemperatureof‐40°Forlower.
BasedontheseconsiderationswewillwantaconcentratedEGsolutionof83wt%(protectionto‐40°F,thecoldesttubetemperatureexpectedinCHILLER).Thisshouldbeinjectedatasufficientratesothatitwillbedilutedtonolowerthan80wt%(protectionto‐50°F)2.TobeabletoaddanEGsolutionwemustaddethyleneglycoltothecomponentlist.ReturntothePropertiessection.SelectComponentstoviewthecomponentlist.PressFind,enter“glycol”intheContainsbox,&pressFindNow.ThecomponentETHYLENE‐GLYCOLshouldbeinthemiddleofthelist;select&pressAddselectedcompounds.PresstheAddbutton.Double‐clicktheComponentIDtochangeETHYL‐01toEG.ReordertoputEGbetweenH2O&N2.
1EngineeringandOperatingGuideforDOWTHERMSR‐1andDOWTHERM4000InhibitedEthyleneGlycol‐basedHeatTransferFluids,DowChemicaltechnicalpublication,http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_010e/0901b8038010e413.pdf?filepath=/heattrans/pdfs/noreg/180‐01190.pdf&fromPage=GetDoc2Notethateventhoughwecouldtrytooperateintheregionoflowerglycolconcentrations(60wt%dilutedto55wt%)thenormalpracticeistooperateinthehigherconcentrationrange;ifexcesswatercomesinwiththegasthenthehigherconcentrationsactuallygetbetterfreezeprotection,notworse.
Rev2.1 ‐30‐ January9,2018
ReturntotheSimulationsection.Let’saddastreamfortheethyleneglycol,EG,intotheRECMIX.Double‐clickonthestreamEG.Specifythecompostionas83wt%EG&17wt%H2O.UndertheStatevariablessetthepressureto400psia&itstemperatureto60°F(typicalforundergroundstorage;we’llfindoutamorereasonabletemperaturelater).Fornowsetthemassflowrateto5,333lb/hr(thisshouldmaketheColdWaterstreamabout80wt%glycol).
Rev2.1 ‐31‐ January9,2018
Runthesimulation.DoubleclickonCOLDSEP&selectStreamResultssowecanlookattheeffectofaddingtheglycol.Lookingatthemassfractionsthesplitoftheglycolbetweentheoil,gas,&waterphaseslookveryreasonable.Butlookatthetemperatures–thetemperatureofthestreamsoutofCOLDSEPareabout1°differentthanthatcomingin(eventhoughtheyshouldbethesame).ThetemperatureissuecanberesolvedbygoingbacktotheCHILLERspecifications&changetheValidphasestoVapor‐Liquid_DirtyWater.Nowrerunthesimulation.Thetemperaturediscrepancyhasdisappeared.PropaneRefrigerationLoopThenextdetailwecanisarefrigerationlooptobeabletocoolthefeed&recyclegasestoCHILLER.Wewillnotactuallyadda“loop”butratherasequentialsetofoperationsthatare“brokenopen”afterthecondenser.Addthefollowingequipmenttotheflowsheet:
AHeater,CHILL‐C ACompr,REFCOMP AHeater,REFCOND AValve,VLV‐REF.
Rev2.1 ‐32‐ January9,2018
Inaddition,adda“dummy”Heater,NET‐LOOP,tocalculatethelow&highpressuresforthesaturatedvapor&liquidconditions.Thiswillbeusefulfordetermining&feeding‐forwardvariousprocessconditions.TherearetwoissueswithmodelingthisrefrigerationloopinAspenPlusthatwillrequiresomeadvancedsetup.
Wedon’tknowtheflowrateofthepropanerefrigerantsinceitwillincreaseordecreasetobalancetheheatloadinthechiller.
Wedon’tknowthepressureoutofthecompressor,onlythattheeffluentfromthecondenserwillbeatasettemperature.
Let’screatethestreamREFVAPthatrepresentsthepropanerefrigerantattheoutletofthechiller.Setitscompositionas100%C3.SetitsFlashTypetoTemperature&VaporFraction,theTemperatureto‐40°F,&theVaporfractionto1(torepresentasaturatedvapor).Wedon’tknowtheflowratefortherefrigerant.Fornow,setitsTotalflowrateto100lb/hr.Let’sreallystarttheloopcalculationsforthesaturatedliquidoutoftherefrigerant’scondenser,streamHPLIQ.We’llspecifytheconditionsoutofthe“dummy”HeaterNET‐LOOPassaturatedvaporat120°F.
Rev2.1 ‐33‐ January9,2018
NowwewanttospecifythepressuredropacrossVLV‐REF.Wedon’treallyknowwhatthispressureisthough–well,wekindado,sincewe’vecalculatedthepressurewhenwedefinedstreamREFVAP,butwedon’thavethatinformationavailabletousyet.Sofornow,let’sjustassumethatthepressureis10psia;we’lladjustthistothecorrectvaluelater.Next,let’ssettheconditionsforthecoldsideofthechiller,CHILL‐C.Wewanttherefrigeranttoleaveassaturatedvaporat‐40°F.Butwealsoknowthedutyrequiredbytheprocessside,theheatstreamQ‐CHILLR.Inactualoperationwewouldvarytherefrigerantflowratetobeabletoprovidethisamountofcoolingfromthevaporizationacrosstherefrigerant’ssideofthischiller.Butwecan’tsetthatviaCHILL‐C.Thewaywe’regoingtomodelthisisto:
specifytheoutletconditionsforstreamREFVAP‐2(temperature&phasecondition),
letAspenPluscalculatetheenthalpychangeassociatedwiththegivenflowrate,and
figureoutaresidualamountofheatneeded(representedbyheatstreamQ‐RESID)above&beyondthatcalculatedinQ‐CHILLR.
So,settheoperatingconditionsforCHILL‐CasaFlashTypeofVaporFraction&TemperaturewiththeTemperatureas‐40°F(tomatchREFVAP)&theVaporfractionof1(todenoteasaturatedvapor).Nowlet’scompressthisrefrigerant.WeknowthepressureviathecalculationforstreamHPLIQbutwecan’tdirectlyaccessit.So,we’lltemporarilysetadischargepressure.SpecifytheTypeofCompressortoIsentropic.Setareasonabledefaultefficiencyto0.75.Fornow,settheDischargePressureto150psia.
Rev2.1 ‐34‐ January9,2018
Let’ssettheconditionsfortherefrigerant’scondenser,REFCOND.AswesetthecalculationsforHPLIQwewillbecoolingtherefrigeranttoasaturatedliquidstateat120°F.SowespecifyaFlashTypeofVaporFraction&Temperaturewiththeappropriatevalues.SpecifyingVaporfractionof0denotesasaturatedliquid.Nowwecanrunthesimulation&examinetheresults.Weseethatthereareacouplediscrepancieswiththespecificationsontheoperationofthisrefrigerationloop:
TheoutletpressureofVLV‐REFisslightlylow&doesnotmatchupwiththepressurerequiredtogiveasaturatedvaporinREFVAP‐2.Itshouldbeabout16psia.
TheoutletpressureonREFCOMPisnothighenoughtomatchupwiththerequiredpressuretogiveasaturatedliquidlikethatinHPLIQ.Itshouldbeabout244psia.
TheflowrateismuchtoolowsincethevaporizationoftherefrigerantinCHILL‐C“absorbs”aninsignificantamountoftheheatfromQ‐CHILLR.AlmostallQ‐CHILLR’sheatpassesthroughtoQ‐RESID.
Let’sfirsttakecareofthetemperature&pressurediscrepancies.Wecouldrunthesimulation,examinetheresults,andmanuallyupdatetheoutletconditionsforREFCOMP&VLV‐REF.Instead,let’slookathowwecantransferthisinformationprogrammaticallyusingTransferblocks.First,let’stalkaboutadirectmethodthatweultimatelywillnotuse:
WecantransferthepressureofREFVAPtotheoutletofVLV‐REFinafeedforwardmannerusingaTransferblock.ExpandFlowsheetingoptionsintheleft‐handtreestructure&selectTransfer;presstheNewbutton&specifythenameTRN‐P.IntheFromtabspecifythepressurefromREFVAPastheinformationtothetransferred.IntheTotabright‐clickonVariableNumber&selectNew;thenspecifythisasthereferencepressureoutofVLV‐REF.
Rev2.1 ‐35‐ January9,2018
Thisnormallyisaneasy&directmethodtocopyvaluesfromoneunittoanotherinafeed‐forwardmanner.However,wewanttotransfermultiplevariables,eachofwhichwouldrequiretheirownTransferstatement.WecanalsodothiswithasingleCalculatorblock.SelecttheCalculatoritemunderFlowsheetingOptionsintheleft‐handtreestructure.PresstheNewbutton&namethecalculatorblockREFLOOP.Let’sdefinevariables:
TLOW&PLOWforthetemperature&pressureconsistentwiththerefrigerantatthechilleroutlet,streamREFVAP.
THIGH&PHIGHforthetemperature&pressureconsistentwiththerefrigerantatthecondenseroutlet,streamHPLIQ.
PVLVforthepressureoutofvalveVLV‐REF(variableP‐Out)&intothechiller. TCHILLforthetemperatureoutoftheexchangerCHILL‐C(variableTEMP). PCMPforthedischargepressurefromthecompressorREFCOMP(variablePRES). TCNDforthetemperatureoutoftheexchangerREFCOND(variableTEMP).
Rev2.1 ‐36‐ January9,2018
NowwecanuseasetofFortranstatementstoequatethepressures&temperaturesfromREFVAP&HPLIQtotheblocksdownstreamoftheseinitialstreamcalculations.
Finally,let’ssetthestreamvariablesasimportedvariables&theblockvariablesasexportedvariables.(Theorderisnotimportant.)
Oneadvantagetousingacalculatorblocktosetthesevaluesisthatwecanincorporateoffsetstotheblockvariables.Forexample,ifthereisanon‐zeropressuredropinthecondenserREFCONDthenwecouldincludethatinsettingtheREFCOMP’sdischargepressure(assomethinglikePCMP=PHIGH+DELTAP).Nowwhenwere‐runthesimulationwecanseethatthepressuresarematchedup.
WealsoneedtoadjusttheflowrateintherefrigerationlooptobalancetheheatrepresentedinQ‐CHILLR;wecandothisusingaDesignSpectomaketheresidualheatstreamQ‐RESIDtobezero.
Rev2.1 ‐37‐ January9,2018
ExpandFlowsheetingOptionsintheleft‐handtreestructure&createanewDesignSpecDS‐FLOW.DefineaVariableRESIDUALastheheatstreamQ‐RESID.GototheSpectab&setTHETargetvalueforRESIDUALas0withaToleranceof0.1.OntheVarytabspecifytheadjustablevariableasthemassflowrateinREFVAP.TokeepthisgeneralallowaLowerlimitof0;fornowlet’sassumetheUpperlimittobe500,000lb/hr.
Rev2.1 ‐38‐ January9,2018
Nowwhenwere‐runthesimulationweseethattheactualrefrigerantflowrateis276,668lb/hrofpropane&requires79,963hpforthecompressor.
ProductCompressionThefinalstepinthissimplesimulationistoaddcompressionforthefinalproductgas.Addtotheflowsheettheunit:
ACompr,PRODCOMPConnectusingmaterialstreamPRODGAS&workstreamW‐PRDCMP.
Double‐clickonPRODCOMPtosetupitsparameters.SpecifytheTypeofCompressortoIsentropic.Setareasonabledefaultefficiencyto0.75.FornowsettheDischargePressureto1000psia.Runthesimulation.
Rev2.1 ‐39‐ January9,2018
Notethatoutlettemperatureislessthan120°F,soafinalcoolerisnotneededtobeabletointroducethisgasintothepipeline.
AdditionaldetailtotheFlowsheetTheremanydetailsthatcanbeaddedtothisflowsheet.InparticularwewilladddetailforregeneratingtheEG.EthyleneGlycolRegenerationTheinitialflowsheetassumesthat83wt%ethyleneglycol(EG)canbemadeavailabletotheprocess.ThisEGisnotafreshfeed,butratheritisrecirculatedafterthewaterpickedupinCOLDSEPisstrippedout.WewillbeaddingthefollowingmajoroperationstoregeneratetheEGare:
astrippingcolumnwithareboiler&partialcondenser.UsetheRADFRACFRACT1unit.
across‐exchangertorecoverheatfromthestrippedEG.UseanMHeatXunit.
apumptobringtheleanEGuptotheinjectionpressure.UsethePUMPunit.
Connectstreamsasshowninthefigureabove.UsetheexistingstreamCOLDWATRtoconnecttoEGHX.Fornow,let’snotcloseofftheEGrecyclebutrathercreateanewstreamforthepumpoutlet,EG‐RETRN.
Rev2.1 ‐40‐ January9,2018
Let’sdefinetheEGstripper,EGSTRIP.Doubleclickonthismodule;onthefirsttabset: KeeptheEquilibriumCalculationtype. Setthenumberofstagesto4.Thiswillcreateonestageforthereboiler,oneforthe
condenser,&2stageswithinthecolumnitself. Setthecondensertype
toPartial‐Vapor. SettheValidPhasesto
Vapor‐Liquid. EstimatetheReflux
Ratioas0.15(bymole)andtheBottomsRateas5333lb/hr(theratespecifiedfortheEGrecyclestream).Thesewillonlybeusedasestimates&willultimatelybereplacedbyotherdesignspecsonthecolumn.
ClickontheStreamstab.SettheRICHEGfeedstreamtostage3(thebottomstagerepresentingatray)&specifyAbove‐Stage.
EGstrippersoperatenearatmosphericconditiontokeepthereboilertemperaturesaslowaspossible.We’llfirstassumeazeropressuredropacrossthecolumn.SettheStage1/CondenserPressureto1atm&allpressuredropstozero.
Rev2.1 ‐41‐ January9,2018
Eventhoughwe’vegivenanapproximatespecificationonthebottomofthisstripper(i.e.,thebottomsrate)whatwereallywanttospecifyglycolconcentration(83wt%).Let’sexpandtheSpecificationsitemintheleft‐handtreestructure,selectDesignSpecifications,&addaNewspec.SettheMasspurityas0.83.BasethispurityasfractionEGoutofanH2O&EGmixture(Componentstab).Finally,thisspecwillbeappliedtotheLEANEGstream.
Rev2.1 ‐42‐ January9,2018
Toachievethisspecwemustadjustthereboileroperation.UnderSpecificationsclickVary&createaNewitem.SelectBottomsrateastheType.Putinfairlytightboundsonthisflowrate–uselower&upperboundsof5000to5500lb/hr.
Let’sdefinethecrossexchangerthatwillpreheatthecoldwater/EGfeedandrecoverheatfromtheleanglycolashotstripperbottoms.Bythewayyou’veattachedthestreamsyoushouldhaveCOLDWATRastheCOLDsideinletstream&LEANEGastheHOT.Let’signorepressuredropsfornow,sokeepthePressurevaluesas0.We’dliketostartthecalculationswithoutcreatingaheat‐basedrecycleloop,so,let’sspecifytheoutlettemperaturefortheCOLDsideas200°F.(ThisshouldallowthedutyrequiredtobepassedontotheHOTsideinafeed‐forwardmanner.)
WemustfinishspecifyingthepumpfortheEGreturnbeforewecanrunthesimulation.SpecifytheDischargepressureas400psia(tomatchuptheotherinletpressures)&thepumpefficiencyas0.75.
Rev2.1 ‐43‐ January9,2018
Let’srunthesimulation&lookattheoverallresultsfortheEGstrippingsection.OnethingwecanseeisthatthevaporoffofEGSTRIPisalittlehigherthanexpected,215°F.Wewouldexpectittobecloserto212°Fifitwasnearlypurewater.
Let’slookatthecompositionsofthetop&bottomstreamsfromEGSTRIP.Double‐clickonEGSTRIP&selecttheStreamResultsoptionintheleft‐handtreestructure.ExpandtheMassFractionsitem.NotethatthebottomstreamLEANEGisasexpected,83wt%EGwithminimalamountsofhydrocarboncomponents.ButtheoverheadWATERVAPhasabout2wt%EGinit;thisrepresentsalossthat(1)needstobemadeupintheprocess&(2)needstobeaccountedforwhendischargingtotheenvironment.
FurthertuningoftheEGStripperoperationcouldbeperformedtoreducetheamountofEGlosttoWATERVAP.Let’salsolookatthereturntemperatureoftheleanEG,EG‐RETRN.Noticethatthepumpoutletis‐1°F.Thisisnotablefortworeasons:
Thisislowerthantheinitialspecthattheethyleneglycolwouldbeenteringat60°F.EGHXactuallyallowsustogettoocoldbyrecoveringtoomuchrefrigerationintheCOLDWATRstream.
Infact,thistemperaturemayactuallybetoolow.Typicalreturntemperaturesshouldbe40to55°F.ThishighertemperaturecouldbedirectlyspecifiedinEGHX;BUTassoonasyouchangethespecfromoneontheoutletofthehotsidetooneonthecoldsideyousetupa
Rev2.1 ‐44‐ January9,2018
recycleloop.ButyoucanmanuallyreducethetemperatureofTHERICHEGstreamuntilthetemperatureofEG‐RETRNrisesabove40°F.Reducingthespecfrom200°Fto160°Fwilldothis.
OptimizingProcessThebasicprocesshasnowbeensetup.Notethattherearethreemajorpowerusers:
ProductGasCompressor–4,024hp RecycleGasCompressor–112hp RefrigerationCompressor–7,963hp
Inadditiontherearetwomajorheatusers:
Stabilizer’sreboiler–3.3MMBtu/hr EGstripper’sreboiler–0.5MMBtu/hr.
Aquestionforoptimization–cananyofthesestreamsbereducedtoreducetheoperatingexpensefortheprocess?Somethoughts:
MostofthesevaluesaredependentontheoperatingconditionsofCOLDSEP.Thissetstheamountofgasthatneedstoberecompressed,theamountoflightendstotheSTABthatneedtobestrippedoff,compressed,&recycledback,andtheamountofwaterabsorbed®eneratedinEGSTRIP.
Thebigoperatingcostandonethatcanbeaddressedwithfurtherdesignisthepowerneededfortherefrigerationloop.Therearetwowaysthatthiscouldbedone:
o WecouldtrytorecovertherefrigerationfromthecoldstreamsfromCOLDSEP.Bydoingsotherewouldbelessrefrigerationdutyneeded,reducingthepowerrequirementforREFCOMP.Also,bywarmingCOLDLIQbeforegoingtoSTABtheamountofreboilerdutywillalsobereduced.However,notethatbyincreasingthetemperatureofthegasbeforePRODCOMPtherequiredpowerinthiscompressorwillincrease,possiblynegatingthemajorityofthepowersavings.
o Wecouldincreasethenumberofrefrigerationstagesofcompressionwithassociaterecycleoftheintermediategasesfromtheintermediatestageeconomizers.Itistypicalthatatwo‐stagesystemcansaveabout20%ofthepowerrequiredbytherefrigerationsystem.