Simple Dew Point Control - AspenPlus v10inside.mines.edu/~jjechura/GasProcessing/Simple... · The gas is nearly saturated with water at the inlet conditions, 48 lb water per MMscf

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.


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


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.


Whenyoustartaddingtheothercomponentsyouhaveanextraquestiontoanswer,whethertoaddorreplacethecurrentcomponent.YouwillprobablywanttochooseAdduntilyou’veaddedallofyourcomponents.Therearevarioustricksforfindinggroupsofcompounds.Forexample,bysearchingforn‐Alkanesthatcontain“ethane”youcangetthelighthydrocarbons.Youcanselect&addallasagroup.


Afterfindingallofthecomponentsyoushouldhavealistthatlookssimilartothefollowingform.


AspenPluswillretrieveinformationabouteachcomponent&alsocreateaComponentIDforthissimulation.YouarefreetochangetheseIDstomatchyourpersonaldesires.Forexample,youcanchangetheIDforMETHA‐01toC1bydoublingclickingonthattextitem;afterchangingthetextvalue&pressingenterAspenPluswillverifythatyouwanttoRenamethecomponent&replacethatcomponentwithsomethingelse.Thiscanbedoneforallofthecomponentstocreate(IMHO)morereasonableIDs.


Anotherissueistheorderthatthecomponentsmayhavebeenextracted.Ihaveapersonalpreferenceforalistintheorderofwater,lightgascomponents,&thenthehydrocarbonsinincreasingcarbonnumberorder.Thisisnotthecurrentorder.YoucanchangetheorderbypressingtheReorderbutton&thenusingtheup&downarrowstoputcomponentsinyourpreferredorder.

Thenextstepwouldnormallybetopickafluidpropertypackage.However,whenwechosetheGasProcessingoptionwhenwecreatedthesimulationthePeng‐Robinsonequationofstatemethodwaschosenasthedefault.WecanseethisbyselectingMethods&Specificationsintheleft‐handcolumn.NoticethattheBaseMethodisPENG‐ROB.Wewillkeepthisdefaultselection.


TheremaystillbeitemstobeaddressedbeforewecanentertheSimulationsection(youcantellthisifthereisa symbolintheleft‐handcolumn).Tofindoutwhatneedstobedoneclickthe button.YoumaygetaformthatallowsustomodifyvaluesforthePeng‐Robinsonbinaryinteractioncoefficients.Ifyougetthis,donotchangeanyofthevalues.


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.)


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.


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.)


Whataresomeoftheresults?Wecangetanoverviewbypostingsummaryconditionsontheflowsheet.ClickonStreamResultsintheModifytaboftheribbon.SelectTemperature,Pressure,Massflowrate,&Heat/Duty.PressOK.Nowthesenumbersarepostedontheflowsheet.


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.



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.)


Nowwecanrerunthesimulation&lookfortheResultsforCOLDVAP.Gotothebottomofthestreamreport&clickon<addproperties>.TowardthebottomofthelistclickonbothHEATVALSproperties&clickOK.AtthebottomoftheEditStreamPropertyTemplateformpressOK.NowwhenyoulookatthestreamreportforCOLDVAPyou’llseethegross&netheatingvalues.


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).


Runthesimulation.Therearevariouswaystocheckforthecalculatedvaporpressure;let’slookatthestreamResultsforLIQUID.ThisshowsthattheTVPis654psia,muchhigherthanthedesired103psia.Wecanlookatthecompositiontoseetheproblem–theamountofmethaneisroughlythesameastheethane&propane.ThisismuchtoohighforaY‐gradeNGLliquidmix.Howcanweseethemolefractions?Thesearepartofthestreamreport,butyoumayhavetopressthe+signontheMoleFractionsoptiontoexapandthelist.


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.


Let’slookattheStreamstab.Theinformationshouldalreadyhavebeenspecifiedfortheproductstreams.Ensurethatthefeedtothecolumngoestothetopstage,1.ByspecifiyingAbove‐Stagethenanyvaporthatmightbecreatedbyflashingthroughtheinletnozzleofthetowerwillnotcontacttheotherfluidsonthetopstagebutratheronlymixwiththevaporfromthetopstage.Let’slookatrunningthetowerat200psiawithanegligiblepressuredrop.Specify200forboththetop&bottomstagepressure.Wecannowrunthesimulation.Lookingattheprocessflowdiagramwecanseethatthecolumnoperateswithatemperatureof416°F.But,theliquidproducedhasaTVPat100°Fofonly3psia(asseenfromtheresultsforTVPCALC).Thisismuchlowerthanitneedstobe.


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.


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,


respectively;thesearethevaluesspecifiedfortheselimits.Finally,specifyreasonablefractionalvaluesfortheperturbation&maximumstepsizes,0.1&0.5,respectively.Nowwecanrerunthesimulation.Thesummaryintheflowsheetshowsthatoperatingthereboilerat166°Fwillgivealiquidthathasa103psiaTVPat100°F.

Whatdoesthestabilizedliquidlooklike?Double‐clickonSTABLIQ&selectResultsintheleft‐handcolumn.(Remembertoexpandanylistofvaluesthatyouwanttoexamine.)Notethatthereisessentiallynomethane&verylittleethane–allofthismaterialhasbeenstrippedoutintotheoverheadvaporstream.


Let’slookathowmuchgashasbeenstrippedout.Double‐clickonSTABGAS.LookundertheResultsarea(expandingthelistsasnecessary&addingtheheatingvalues).Noticethatthisgashasveryhighconcentrationsofmethane&ethane(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:


ACompr,RECCOMP AMixer,RECMIX.

ConnectwithmaterialstreamsRECGAS&TOTALandaddtheworkstreamW‐RECCMP.NotethattheiconforRECCOMPhasbeenflippedonthePFDshownabove.Thisdonebyright‐clickingonRECCOMP,selectingRotateIcon,&thenFlipHorizontal.Double‐clickonRECCOMP.SettheTypetoIsentropic.SelectDischargePressure&setitsvalueto400psia.SettheIsentropicefficiencyto0.75(areasonabledefaultadiabaticvalue).

Runthesimulation.NotethatthereisarecyclestreambutAspenPlussetsitupautomaticallywithoutanythingspecialbeingdone.IfonewastochecktheControlPanelyou’dseethatittook5iterationstoconvergethisrecycle.


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


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,&


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.


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.


ReturntotheSimulationsection.Let’saddastreamfortheethyleneglycol,EG,intotheRECMIX.Double‐clickonthestreamEG.Specifythecompostionas83wt%EG&17wt%H2O.UndertheStatevariablessetthepressureto400psia&itstemperatureto60°F(typicalforundergroundstorage;we’llfindoutamorereasonabletemperaturelater).Fornowsetthemassflowrateto5,333lb/hr(thisshouldmaketheColdWaterstreamabout80wt%glycol).


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.


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.


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.


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.


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).


NowwecanuseasetofFortranstatementstoequatethepressures&temperaturesfromREFVAP&HPLIQtotheblocksdownstreamoftheseinitialstreamcalculations.

Finally,let’ssetthestreamvariablesasimportedvariables&theblockvariablesasexportedvariables.(Theorderisnotimportant.)

Oneadvantagetousingacalculatorblocktosetthesevaluesisthatwecanincorporateoffsetstotheblockvariables.Forexample,ifthereisanon‐zeropressuredropinthecondenserREFCONDthenwecouldincludethatinsettingtheREFCOMP’sdischargepressure(assomethinglikePCMP=PHIGH+DELTAP).Nowwhenwere‐runthesimulationwecanseethatthepressuresarematchedup.

WealsoneedtoadjusttheflowrateintherefrigerationlooptobalancetheheatrepresentedinQ‐CHILLR;wecandothisusingaDesignSpectomaketheresidualheatstreamQ‐RESIDtobezero.


ExpandFlowsheetingOptionsintheleft‐handtreestructure&createanewDesignSpecDS‐FLOW.DefineaVariableRESIDUALastheheatstreamQ‐RESID.GototheSpectab&setTHETargetvalueforRESIDUALas0withaToleranceof0.1.OntheVarytabspecifytheadjustablevariableasthemassflowrateinREFVAP.TokeepthisgeneralallowaLowerlimitof0;fornowlet’sassumetheUpperlimittobe500,000lb/hr.


Nowwhenwere‐runthesimulationweseethattheactualrefrigerantflowrateis276,668lb/hrofpropane&requires79,963hpforthecompressor.

ProductCompressionThefinalstepinthissimplesimulationistoaddcompressionforthefinalproductgas.Addtotheflowsheettheunit:

ACompr,PRODCOMPConnectusingmaterialstreamPRODGAS&workstreamW‐PRDCMP.

Double‐clickonPRODCOMPtosetupitsparameters.SpecifytheTypeofCompressortoIsentropic.Setareasonabledefaultefficiencyto0.75.FornowsettheDischargePressureto1000psia.Runthesimulation.


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.


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.


Eventhoughwe’vegivenanapproximatespecificationonthebottomofthisstripper(i.e.,thebottomsrate)whatwereallywanttospecifyglycolconcentration(83wt%).Let’sexpandtheSpecificationsitemintheleft‐handtreestructure,selectDesignSpecifications,&addaNewspec.SettheMasspurityas0.83.BasethispurityasfractionEGoutofanH2O&EGmixture(Componentstab).Finally,thisspecwillbeappliedtotheLEANEGstream.


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.


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


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&regeneratedinEGSTRIP.

Thebigoperatingcostandonethatcanbeaddressedwithfurtherdesignisthepowerneededfortherefrigerationloop.Therearetwowaysthatthiscouldbedone:

o WecouldtrytorecovertherefrigerationfromthecoldstreamsfromCOLDSEP.Bydoingsotherewouldbelessrefrigerationdutyneeded,reducingthepowerrequirementforREFCOMP.Also,bywarmingCOLDLIQbeforegoingtoSTABtheamountofreboilerdutywillalsobereduced.However,notethatbyincreasingthetemperatureofthegasbeforePRODCOMPtherequiredpowerinthiscompressorwillincrease,possiblynegatingthemajorityofthepowersavings.

o Wecouldincreasethenumberofrefrigerationstagesofcompressionwithassociaterecycleoftheintermediategasesfromtheintermediatestageeconomizers.Itistypicalthatatwo‐stagesystemcansaveabout20%ofthepowerrequiredbytherefrigerationsystem.

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Simple Dew Point Control - AspenPlus v10inside.mines.edu/~jjechura/GasProcessing/Simple... · The gas is nearly saturated with water at the inlet conditions, 48 lb water per MMscf