Steam Cycle Simulation Aspen Plus v8 - Inside...

Rev0.0 ‐1‐ January1,2015

SteamCycleSimulation–AspenPlusv8.6TheattachedgivesstepstosetupasimulationinAspenPlusv8.6tomodelasimpleRankinesteamcycleforelectricityproduction.Thesystemconsistingof:

Fuelsidewithnaturalgasfeed,airblower,combustionchamber,&fuelsideofthesteamboiler.

Steamsidewithsteamturbine,steamcondenser,condensatepump,&steamsideoftheboiler.

Thesimulationwillbesetupassumingisentropicstepsfortherotatingequipment.WhenthesimulationissetuptheoverallPFDshouldlooklikethefollowingfigure.

Createnewsimulationfile

StarttheprogramfromStart,AllPrograms,AspenTech,ProcessModelingV8.6,AspenPlus,AspenPlusV8.6.WhentheprogramopenschoosetheNewbutton.ChoosetheGasProcessingthentheGasProcessingwithMetricUnitstemplate.ClicktheCreatebutton.


DefinetheComponents&thePropertyModels

Specifycomponents,fluidpropertypackages,&crudeoilassays

Thefirststepistodefineasetofpurechemicalspeciestorepresent:

Steamasmodeledbypurewater&usingpropertycorrelationsconsistentwiththeASMESteamTables.

Thenaturalgasfuel,air,&combustionexhaustaspurelightcomponentsmodeledbythePeng‐Robinsonequationofstate(EOS).

Nowlet’saddcomponentstomodelthefuelsideofthesystem.GobacktotheComponentListsitem&clickontheAddbuttontocreateComponentList‐2.Weneedcomponentsforthefollowing:

Steam.Fornowwe’llmodelaspurewater. Naturalgas.Fornowlet’smodelthisasapossiblemixtureofmethane,ethane,&propane. Air.Fornowwe’llmodelthisasamixtureofoxygen&nitrogen. Combustiongases.Attheminimumwe’llalsoneedcarbondioxideandwater(whichwe

alsoneedformodelingthesteam).However,we’llalsowanttotakeintoaccountincompletecombustion(formingcarbonmonoxide)aswellasNOxformation(fornowjustasNO,NO2,&N2O).

ClicktheFindbuttontobringupthedatabanksearchform.Youcanentereithertheentireformula,partofaname,orseveralotherpossiblesearchitemstofindallofthedesiredchemicalspecies.Whenthepropercompoundisfound,selectitinthelist&clickAddselectedcompounds.ThefollowingfigureshowsasearchforH2O.Asyouareaddingcompoundsyoumaybeaskedwhethertoaddorreplacethecompoundalreadyinthelist;choosetheAddoption.


BelowisanexampleofcomponentsretrievedfromtheAspendatabanks.Therearetwoissueswithdefaultmannerinwhichthislistispresented.One,theComponentIDsarenotverydescriptiveofthecompound(especiallyascomparedtotheAliasvalues).Two,theorderdoesnotgroupthecompoundsinaconvenientmanner.Wecanaddressbothoftheseissuesbeforeproceedingmuchfurther.

Let’schangetheComponentIDvaluestomostlymatchtheAliasvalues.SelecttheComponentIDvalueeitherbydouble‐clickingonitorbyclicking&thenpressingtheF2key.Onceselected,typeinthenewID&presstheEnterkey.AspenPluswillaskwhatyoureallywanttodobymakingthischange;clicktheRenamebutton.ChangeallIDs.


NowpresstheReorderbutton.Aformpopsupthatwillallowyoutomoveselectedcompoundsupordownsothattheyinaconvenientorder.PressClosewhendone.


Thenextstepistoassurethatanappropriatefluidpropertypackagehasbeenchosenforthesecompounds.ClickonMethodsintheAllItemslistontheleft.FromhereweseethatthePeng‐RobinsonEOShasbeenspecifiedasthebasemethod(perthechoiceoftemplateoriginallychose).AlsotheASMEsteamtableoptionhasbeenspecifiedforcaseswhenonlywaterispresentinthestream.Thesearethedesiredoptionssowecancontinueon.

Nowisagoodtimetosavethefilebeforewestartsettinguptheprocesssimulation.ClicktheFiletab&thentheSaveAsitem.ChoosetheAspenPlusBackupoption.Setup&SolvetheFlowsheet

WorkingUnits

ActivatetheSimulationoption.Notethatyou’llseeablankflowsheet.WewouldliketoshowthecalculationswithamodifiedsetofSIunits,inparticular:

Temperatureas°C. Pressureasbar(absolute). Massflowaskg/sec. Molarflowaskg.mol/sec. HeatdutyaskJ/sec. PoweraskW.


UndertheHometabclicktheUnitSetsbutton.InthelistofunitsetsclickontherowforSI‐CBAR&pressEdit.Proceedingthroughthevarioustabsallowsyoutodeterminewhatwillbeusedforthedisplayoftheresultsaswellasthedefaultunitsfortheinput.Mostoftheunitsarewhatwedesire,butnotall.Forexample,youcanseethatMassFlowwillbereportedinkg/hr,notquitewhatwewant.

Let’spulldownthelistsassociatedfortheFlowrelatedvalues&pickoptionsthatareintermsofseconds,nothours.GointotheHeattab.ChangeHeatrelatedvaluesfromJtokJandpowerrelatedvaluesfromWtokW.

SteamCycle

WewillwanttocreateasimpleRankinecyclewiththefollowingprocessconditions: Saturatedsteamproductionat125bar. Finalcondensationto20°C. Steamturbineoperatingatidealreversibleconditions. Condensatepumpoperatingatidealreversibleconditions. Noextrapressuredropthroughheatexchangersorpiping.


Let’splacethefollowingunitsfromtheModelPalettetotheflowsheet1:2Heaters,aturbine(asPressureChangers,Compr,ICON3),&Pump.Ultimatelyitwillbedepictedasfollows(withrotationofthepumpicon).

Connecttheunitswiththefollowingstreams:

IntheModelPaletteclickontheMaterialstreambutton.Drawasfollows:o DrawastreamintotheredarrowofPUMP;callitCONDNSAT.o DrawastreamfromtheredarrowoutofPUMP&intotheredarrowofBOILER;callit

HP‐STEAM.o DrawastreamfromtheredarrowoutofBOILER&intotheredarrowofSTMTURBN;

callitAIR‐2.o DrawastreamfromtheREDarrowoutofSTMTURBN&intotheredarrowof

CONDNSR;callitEXHAUST.o DrawastreamfromtheredarrowoutofCONDNSR;callitCOND‐2.

IntheModelPaletteclickontheHeatstreambutton.Drawasfollows:o DrawastreamoutofthebluearrowofBOILER;callitQ‐BOILER.o DrawastreamoutofthebluearrowofCONDNSR;callitQ‐CONDSR.

IntheModelPaletteclickontheWorkstreambutton.Drawasfollows:o DrawastreamoutofthebluearrowofPUMP;callitW‐PUMP.o DrawastreamoutofthebluearrowofSTMTURBN;callitW‐TURBN.

Let’sstarttoinitializethewatercirculatingthroughthesteamloop.

1IftheModelPaletteisnotvisiblechoosetheViewtab&clickontheModelPalettebuttonorpresstheF10key.


Double‐clickontheCONDNSATstream.SelectTemperature&VaporFractionfortheFlashType.Enter20CfortheTemperature&0fortheVaporFraction(i.e.,asaturatedliquid).SpecifytheCompositionaspurewater;entera1forH2OinthelistwiththeMole‐Fracoption.Let’suseaflowbasisof1kg/s.

Nowlet’ssettheoperatingparametersforthevariousunits.DoubleclickonthePUMPicontoopentheinputsheet.MakesurethePumpoptionisspecifiedunderModel.SpecifytheDischargePressureas125bar.Finally,todefinethisasanidealpumpspecifya1forboththePump&DriverEfficiencies.

Nowlet’sdefinetheoperatingconditionsforthesteamsideoftheboiler.DoubleclickontheBOILERicontoopenitsinputform.Wewanttospecifytheoutletsteamassaturatedvapor.Wecouldspecifythevaporfraction.Insteadwe’lldefine0°Cofsuperheat.Wealsowanttospecifyazeropressuredrop.Wecandothisbyspecifyingazerovalueforthepressure.


Nowlet’sdefineoperatingconditionsfortheturbine.DoubleclickontheSTMTRBNicontoopentheinputsheet.MakesuretheTurbineoptionisspecifiedunderModel.Wewanttodefinethisasanidealturbinesospecifya1forboththeIsentropic&MechanicalEfficiencies.WeneedtospecifysomethingabouttheDischargePressureeventhoughwedon’treallyknowwhatitis,onlythatiscorrespondstothewatervaporpressureat20°C.Fornowspecifyavalueof0.1bar;we’llfixitlater.Weknowthatthisisacondensingsteamturbine.ThedefaultfortheTurbinemodelisthatonlyvaporwillexit,sothiswillhavetobechanged.ClickontheConvergencetab.PulldownthelistforValidphases&changetoVapor‐Liquid‐FreeWater.

Nowlet’sdefinetheoperatingconditionsforthesteamcondenser.Doubleclickontheexchangericontoopenitsinputform.Wewanttospecifytheoutletassaturatedliquid.WewanttomakesurethatoneoftheFlashTypeoptionsisVaporfraction&settheappropriatevalueas0(i.e.,saturatedliquid).Wealsowanttospecifyazeropressuredrop(i.e.,letthedischargepressuresettingfromtheturbinecontrolthis).MakesurethatoneoftheFlashTypeoptionsisPressure&settheappropriatevalueas0(i.e.,zeropressuredrop).Wenowhaveenoughsettingstobeabletorunthesimulation.Openthecontrolpanel(itemundertheHometab)&pressRun.Somewarningsmaycomeupbuttheywillbeaddressedlater.WecansummarizetheresultsontheflowsheetbymodifyingtheStreamResultssettings.SelecttheMainFlowsheet.SelecttheModifytab&selecttheTemperature,Pressure,&VaporFractionitems.Clickinthelowerleft‐handcorneroftheStreamResultssection.Onthepop‐upformselectthe


Heat/Workitem.Also,changetheformatforthePressurevaluetoshowthreedecimalplaces(i.e.,as“%.3f”).DothesamefortheVaporfractionformat.ClickOK.

Wecannowseeasummaryoftheresultsontheflowsheetbelow.Onethingtonoteisthatourguessfortheturbine’sdischargepressurewasinerror.Thepressureshouldactuallybe0.019bartocorrespondwithacondenseroutlettemperatureof20°C.Wecouldgobackandchangethevaluemanually.However,we’lluseoneofAspenPlus’soperationstoautomaticallysetittomatchthecondenseroutlet.

WewillusetheCALCULATORoperationto“feedforward”theCondenser’spressuretotheturbine’sdischargepressure.Eventhoughwecoulduseacalculatorwithoutseeinganyindicationontheflowsheetwe’llinsteadputaniconontheflowsheettogiveanindicationthatitisthere.


FromtheModelPalette,chooseManipulators,Calculator,&selectICON1.Placeitontheflowsheetneartheexhauststream.(Fortheflowsheetshownitisrotatedverticallysoitcan“hang”belowtheline.)RenameitSET‐P.Double‐clickontheicontoopenupitsinputform.DefinethevariablePCNDSRasthepressurecalculatedforstreamCONDNSAT(i.e.,thesaturationpressureat20°C).SpecifythisasanImportvariable(i.e.,thevaluehastobecalculatedbyAspenPlus&willbe“read”bythecalculatoroperation).

NowdefinethevariablePTURBNasthesteamturbine’sdischargepressure.SpecifythisasanExportvariable(i.e.,thevaluebe“written”asaparameterinadownstreamoperation).


Finally,definetherelationshipintheCalculatetab.UsingtheFortranmethodenterthestatement“PTURBN=PCNDSR”(startingincolumn6).

Nowwecanrerunthesimulationandgettheresultssummarizedbelow.WecanseethattheCALCULATORhasdoneitsjob;theoutletpressurefromthesteamturbineisnowthesameastheinlettothecondenser.However,thetemperaturesaredifferent.What’swrong?

TheproblemisthatmostofthecalculationsaredonewiththedefaultPENG‐ROBproperties,notthedesiredSTEAM‐TAproperties.TheexceptionistheoutletoftheturbinewhichrecognizestheliquidformedasFreeWater&usestheSTEAM‐TAoptionforitsproperties.Hence,theinconsistency.WecanforcetheunitstousetheSTEAM‐TAoptiontodothecalculationsbymakingmodificationstoeachunit’sBlockOptionssettings.IntheSimulationtreestructureforeachoperationselectBlock


Options.Underthepull‐downlistforPropertymethodselectSTEAM‐TA.TheformforSTMTRBNisshownasanexample.

Nowwhenwererunthesimulationwegetconsistentresults.Noticethattherearesubtlechangestotheheat&workstreams.Forexample,theboilerheatisnow2,581kJ/sec;itwas2,808kJ/secwhencalculatedbythePENG‐ROBmethod(adifferenceof9%).


InpreparationforadditionalchangesweneedtomodifythesettingsforSTMTRBN.OnitsinputformclickontheConvergencetab.ChangetheValidphasestoVapor‐Liquid‐Liquid.Whenwererunwegetaminorwarningthattheoutletisbelowitsdewpoint(whichwealreadyknowsincethisisacondensingturbine).

Whendealingwiththepositive&negativevaluesfortheheat&workstreamsrememberthetwoconventionsusedbyAspenPlus:

IftheheatorpowerstreamisanoutletofaunitthenAspenPlushascalculatedthevaluetomakeotheroperatingspecifications(suchastheoutlettemperatureinanexchanger).IfitisaninlettoaunitthenAspenPlususesthevaluetodeterminetheoutletconditions.

Heatrepresentsenergytoorfromtheunitoperation;itisinthedirectionofthearrowiftheheatispositiveorintheoppositedirectionifitisnegative.Work,ontheotherhand,representsenergytoorfromtheuniverse;theenergyflowisintheoppositedirectionasthatforheat.

SinceQ‐BOILERisnegativeforaheatstreampointingawayfromtheBOILER,thentheenergyflowsintotheboiler’sfluid.SinceW‐TURBNisnegativeforaworkstreampointingawayfromtheSTMTURBN,thentheenergyflowsoutoftheturbine’sfluid.Fromtheresultsshownwecancalculatethethermalefficiencyofthissteamcycle.Weshouldalwaysmakeuseoftheabsolutevaluesfortheheat&workstreams.Forthissteamcycle:

netth

boiler

W‐TURBN W‐PUMPW 1078 130.4126

Q Q‐BOILER 2581

.


Fuel&CombustionSystem

Wewillwanttocreateasimplenaturalgasburner/boilerwiththefollowingprocessconditions: Naturalgasisavailableatindustrialdeliverypressure,20bar‐g&15°C.Wewill

characterizethenaturalgasas100%methane. Airisavailableat25°C.Wewillcharacterizetheairasa21/79O2/N2molarmixtureand

bonedry(i.e.,nowater).Wewanttoaddenoughairsothatthereis20%excessoxygenbasedoncompletecombustionofthenaturalgas.

Thecombustionprocessoccursnearatmosphericconditionssothenaturalgasmustbeletdowninpressure.However,ablowerisneededtopushtheairintothecombustionchamber.

Thepressuredropthroughtheburner/boiler/fluecombinationis0.3bar. Thefluegasisemittedat120°Ctopreventanyliquiddropout&subsequentcorrosion

problems.Let’splacethefollowingunitsfromtheModelPalettetotheflowsheet:Valve,Compressor2,RGibbsReactor,&Heater3.Ultimatelyitwillbedepictedasfollows.(We’lldiscusstheSETcalculatorsaswego.)

Connecttheunitswiththefollowingstreams:

IntheModelPaletteclickontheMaterialstreambutton.Drawasfollows:o DrawastreamintothebluearrowoftheLETDOWNvalve;callitFUELGAS.o DrawastreamfromthebluearrowoutoftheLETDOWNvalve&intothebluearrowof

theCOMBSTNreactor;callitLP‐GAS.o DrawastreamintothebluearrowoftheAIRBLWRcompressor;callitAIR.o DrawastreamfromthebluearrowoutoftheAIRBLWRcompressor&intotheblue

arrowoftheCOMBSTNreactor;callitAIR‐2.o DrawastreamfromthebluearrowoutoftheCOMBSTNreactor&intothebluearrow

oftheHRSGexchanger;callitCOMBGAS.o DrawastreamfromtheredarrowoutoftheHRSGexchanger;callitFLUEGAS.

IntheModelPaletteclickontheHeatstreambutton.Drawasfollows:

2Notethatthecompressorhasbeenrotatedverticallytogettheinletstreambelowthecompressor&theoutletstreamabove.3Notethattheheatexchangerhasbeenrotatedverticallytogettheheatstreambelowtheexchanger.


o DrawastreamoutofthebluearrowoftheHRSGexchanger;callitQ‐HRSG. IntheModelPaletteclickontheWorkstreambutton.Drawasfollows:

o DrawastreamoutofthebluearrowoftheAIRBLWRcompressor;callitW‐BLOWER.Let’sstartsettingparametersfortheinletstreams.Let’sinitializethenaturalgasstreamfirst.Double‐clickontheFUELGASstream.SelectTemperature&PressurefortheFlashType.Enter15CfortheTemperature&20bargforthePressure.Enter1fortheC1valueasMole‐Frac.Let’suseaflowbasisof1kg.mol/sec.Nowlet’sinitializetheAIRstream.Double‐clickontheAIRstream.SelectTemperature&PressurefortheFlashType.Enter25CfortheTemperature&0bargforthePressure.Enter0.21fortheO2&0.79fortheN2valuesasMole‐Frac.Asastartingpointlet’sdefinetheflowrateas12kg.mol/hr.Let’sspecifytheoutletpressureof0.3bar‐gafterthelet‐downvalve.Double‐clickonLETDOWN.Specify0.3bargastheOutletpressure.


Wewanttomaketheairbloweranidealreversiblecompressor.Double‐clickonAIRBLWR.SelecttheCompressorasModel.PulldowntheTypelist&chooseIsentropic.Specify1fortheIsentropic&MechanicalEfficiencies.

Nowit’stimetomodelthecombustionportionofthefuelgasburner.Therearevariousoptionsfordoingthis.Oneofthesimplest(andwouldnormallybedoneforhandcalculations)wouldbetodefineallcombustionreactions&specifytheextentofconversionforeach.Instead,we’regoingtotakeadvantageofthefullthermodynamiccapabilitiesofAspenPlus&useareactorthatwillminimizetheGibb’sfreeenergy.Allwehavetodoislisttheexpectedproducts&AspenPluswillcalculatetheresultingproductdistributionthathonorsthematerial&energybalancesaswellasanychemicalequilibriumlimitations.DoubleclickontheRGibbsReactoricon.SetPressureto0(torepresentazeropressuredrop)&specify0fortheHeatDuty(tosignifyadiabaticoperation).That’sprettymuchit.Thedefaultistoincludeallspeciesinthecomponentlistaspotentialproducts.

Nowlet’sseehowmuchheatcanbetransferredoutofthecombustiongasesbyspecifyingthecombustiongassideoftheboiler.Doubleclickontheheatericon.Settheconditionstotheoutletconditionsoutthestack:120C&0barg.


Wehaven’taddressedthecalculatoroperationsyetbutwecanstillrunthesimulation.TheresultsaresummarizedontheFlowsheetshowthatthecombustiontemperaturewillbe1734°C.Wecandouble‐clickontheCOMBGASstream&seethattherewillbesomeCO&NOxformedattheseconditions.

Therearestillacoupleitemstobedoneto“cleanup”thesimulation&formatoftheresults.Thefirstisforamatterofconvenience–howshouldwespecifythepressureoftheAIR‐2streamoutoftheairblower?RightnowthepressureintotheCOMBSTNoperationissetseparatelyforthetwoinletstreams(LP‐GAS&AIR‐2).Ifastudywastobeperformed&thepressureweretochangethenhavingthespecificationsintwoseparatelocationscouldleadtothembeingchangeddifferently.Itsurewouldbenicetosetitonlyinonelocation&thenhavetheotherlocationupdateautomatically.WecandothiswithaCALCULATORoperation.FromtheModelPalette,chooseManipulators,Calculator,&selectICON1.PlaceitontheflowsheetneartheAIR‐2stream.(Fortheflowsheetshownitisrotatedtotheleftsoitcan“hang”offtheline.)RenameitSET‐AP.


Double‐clickontheicontoopenupitsinputform.DefinethevariablePFUELasthepressureforstreamLP‐GAS(i.e.,thepressureoutofthelet‐downvalve).SpecifythisasanImportvariable(i.e.,thevaluehastobecalculatedbyAspenPlus&willbe“read”bythecalculatoroperation).

NowdefinethevariablePAIRastheairblower’sdischargepressure.SpecifythisasanExportvariable(i.e.,thevaluebe“written”asaparameterinadownstreamoperation).

Finally,definetherelationshipintheCalculatetab.UsingtheFortranmethodenterthestatement“PAIR=PFUEL”(startingincolumn6).

Thesecondchangeinvolvesaconvenientwaytomakesurethatthecorrectamountofairisaddedtomatchthe“excessoxygen”spec.Theamountofstoichiometricoxygenisdeterminedfromthecombustionreactions.Formethane,ethane,&propanethereactionsare,respectively:


CH4+2O2CO2+2H2O C2H6+3.5O22CO2+3H2O C3H8+5O23CO2+4H2OThisshowsthatweneedtoknowthecompositionofthefuelgas(inmolaramounts)todeterminethestoichiometricamountofoxygenneeded.The“excess”partisadditionaloxygen(asamultiplier)thatisadded.ThefinalconsiderationisthatthespecificationinAspenPlusisnotjustfortherateofoxygenbutratheroftheair;sowehavetotakeintoaccountthecompositionoftheairaccountforthelargeamountofnitrogenalsobeintroducedintotheCOMBSTNoperation.Sincewehavesetthecompositionofthefuelgastobepuremethane&thebasisflowrateto1kg.mol/secthenthestoichiometricoxygenflowrateistwicethis,2kg.mol/sec.Wealsoneedtoincreasethisby20%toincludethedesiredexcess.Andweneedtotakeintoaccounttheoxygencontentintheairtodeterminetheairrate.Sooverall:

2

2

O

air

O

1 2 1 0.211.43kg.mol/sec

0.21

excessstoichn f

ny

.

WecoulddothesecalculationspriortorunningAspenPlusandentertheairrate.OrwecoulddothecalculationswithinAspenPlus.FromtheModelPalette,chooseManipulators,Calculator,&selectICON1.PlaceitontheflowsheetneartheAIRstream.(Fortheflowsheetshownitisrotatedtotherightsoitcan“hang”offtheline.)RenameitSET‐AFLO.Double‐clickontheicontoopenupitsinputform.DefinethevariableAIRFLOasthemolarflowofthestreamAIR(i.e.,thepressureoutofthelet‐downvalve).SpecifythisasanExportvariable.


Nowlet’sstartdefiningImportvariables.FirstdefinethevariableYO2astheO2molefractionintheair.SpecifythisasanImportvariable.

Nextlet’sdefineImportvariablesforthecombustibleportionsofthefuelgas(eventhoughwe’veonlyusedmethanewehaveincludedthepossibilityforethane&propane,too).DefinethevariablesC1FLO,C2FLO,&C3FLO.MakesurethesearespecifiedasImportvariables.

Finally,definetherelationshipintheCalculatetab.UsingtheFortranmethodenterthestatement:AIRFLO=2.*C1FLO+3.5*C2FLO+5.*C3FLOAIRFLO=AIRFLO*(1.+0.2)AIRFLO=AIRFLO/YO2(startingincolumn6).

Thesimulationcanbererungivingtheresultssummarizedbelow.Notethatcorrectairflowhasbeencalculated,11.43kg.mol/sec.


Onemoremodification,thattodirectlyshowthemolefractionsofallofthestreams(sincerightnowtheresultsareonlyshownasmolarflows).ExpandtheSetupoptionsintheleft‐handSimulationtreestructure.SelectReportOptions.NotethatMoleFlowbasisoptionisspecifiedbutnoneoftheFractionbasisoptions.SelecttheMoleoption.


Rerunthesimulation.Nowwhenyoudouble‐clickontheCOMBGASstreamyouwillnotonlyseethecompositionoutofCOMBSTNinmolarflowsbutalsoasmolefractions.

TyingtheTwoSystemsTogether

Eventhoughthesteamcycle&fuelgassystemsareinthesameAspenPlusflowsheettheyarereallymodeledseparately.Thesteamcyclehasconvergedwithabasisof1kg/secwatercirculationrate&thefuelsystemhasconvergedwithabasisof1kg.mol/secfuelgas.Wewilltiethesystemstogetherby“pushing”thedutyfromthefuelsideoftheboilertothesteamside&adjustingthewatercirculationrateinthesteamcycletoensurethisistheonlyheatneededforthesteamcycle.Nowlet’sconnectthetwosystems.

RenamethestreamQ‐BOILERtoQ‐RESID(for“residual”). RenamethestreamQ‐HRSGtoQ‐BOILER. Right‐clickonQ‐BOILER,selectReconnect,ReconnectDestination,&attachtoblueinlet

arrowontheBOILERexchanger.


Runthesimulation.Noticethatforthecombinationoffuelgasrate&watercirculationratethereistoomuchgeneratedfromthecombustionsideoftheboilertobeabsorbedbythesteam.Wecanseethisbecausethe“residual”heatfromthesteamside,Q‐RESID,is763,426kJ/sec.Since766,007kJ/secwasgeneratedfromthecombustionsidethenonly2,581kJ/secwasneededinthesteamside.

Theresultsshowthatwereallyneed296.8kg/secwatercirculatinginthesteamcycletoabsorballoftheheatfromthecombustionside.Wecouldenterthisvaluemanuallybutthenwewouldhavetodothehandcalculationoveragainifanyconditionsweretochange.InsteadwewillletAspenPluscalculatetheproperflowrate.FromtheModelPaletteselectaDesignSpec(theoneshowninthePFDistheDesignoption&rotatedtotheright).ChangethenametoADJ‐WFLO.Double‐clickonADJ‐WFLOtogetitsinputforms.CreateavariableRESIDUALtorepresenttheresidualheataroundtheboiler(i.e.,thedifferencebetweentheheatgeneratedonthecombustionside&theheatneededonthesteamside).


SelecttheSpectab.DesignatetheRESIDUALvariableastheSpecvariable,setitsTargetvalueto0(I.E.,tomatchupthecombustionside&steamsiderequirements).SetitsconvergenceToleranceto0.5.SelecttheVarytab.ToadjustthemassflowoftheCONDENSATstreamfirstchooseStream‐VarastheType4.Let’sassumethattheUpperlimitislessthan500kg/sec;we’llsettheLowerlimitas0.1(aslightlypositivenumber).SettheStepsizeas0.01&theMaximumstepsizeas0.1.

Wecanlookattheresults&seethattheanticipatedwatercirculationratehasbeenfound,296.8kg/sec.

4DonotchooseMass‐FlowastheType;thiswillpointtotheflowofanindividualcomponent,nottheentirestream.


AdditionalStream&UnitAnalyses

Thereareadditionalanalysesthatwemaywanttoperformforthissimulation.Sincethegoaloftheprocessistocreatepowerweshouldbeveryinterestedtodeterminethevariousthermalefficienciesofthesystems.Tocalculatetheefficiencyoftheboilerweneedtodeterminetheheatingvalueofthefuelgasused.Todothiswewillmakeuseofthebuilt‐innet&grossheatingvalues(lower&higher,respectively).ExpandtheSetupitemintheleft‐handtreestructureoftheSimulationitems.UnderPropertySetscreateaNewsetcalledHEATVALS.Editthatpropertyset&addthepropertiesQVALNET&QVALGRS.

Nextwewanttoaddthesepropertiestothesimulationreport.UnderSetupintheleft‐handtreestructurechooseReportOptions.GototheStreamtab&clickonPropertySets.


SelectHEATVALSintheAvailablepropertysetslist&press>.ThiswillmoveHEATVALStotheSelectedpropertysetslist.ClickonClose.

Nowwecanrerunthesimulation.NowwhenwelookattheResultsforastreamwewillseethenet&grossheatingvaluesatthebottomofthelist.

Wecannowstarttocalculatevariousefficienciesforthecombinedfuel/steamsystem.

Boilerefficiency.Thiswillbetheamountofheatthatistransferredoutofthecombustionsectionofthesystemintothesteamsystem.Thiscanbebasedeitheronthelower(net)heatingvaluebutmorenormallyonthehigher(gross)heatingvalue:

boilerHHV

766,007kJ/sec0.8601

HHV 55515.1kJ/kg 16.043kg/sec

Qm

.

Steamcyclethermalefficiency.Thishasalreadybeencalculatedastheratioofthenetwork

producedbythesteamcycletotheboilerheatin:

turbine pump

th

boiler

319,838kW 3,716kW0.4127

766,007kJ/sec

W W

Q

.

Overallefficiency.Thisisnormallycalculatedastheproductofthecombustionside’s

efficiency&thesteamcycle’sefficiency:

HHV th 0.8601 0.4127 0.3550 .


However,thisdoesnottakeintoaccounttheenergyneededtoruntheairblower.Instead,weshouldusetheratioofthenetworkproducedtotheenteringheatingvalue(again,intermsofHHV):

turbine pump

total,HHV

319,838kW 3,716kW 7622kW0.3464

HHV 55515.1kJ/kg 16.043kg/sec

blowerW W W

m

.

Let’ssetupanExcelspreadsheettodothesecalculations.Youcanstartwithaspreadsheetwithlabelsthatlooklikebelow.NotethatvaluesthatwillbedeterminedfromtheAspenPlussimulation(eitherasaninputoracalculatedvalue)areinabluefont&willhavealightgreenbackground.

Theinformationwewanttoputintothistable&useforcalculationswillcomefromstreamresults(Material,Heat,&Work)aswellasequipmentinformation(i.e.,modelresults).Wecouldcopy&pasteindividualdatavaluesbetweentheAspenPlussimulationandthespreadsheet;itismoreflexibletocopyentiretablesofresultstothespreadsheet&thenpickoutthevaluesdesired.Performthefollowingsteps:

Inyourspreadsheetcreatethreenewstabs&callthemMaterialTable,HeatTable,&WorkTable.

InyourAspenPlussimulationselecttheStreamsoptionunderResultsSummaryintheleft‐handtreestructure.ThedefaultshowstheMaterialtabselected.ClicktheCopyAllbutton.GototheMaterialTabletabinyourspreadsheet&selectcellA1.Right‐click&selectPaste.Youmaywanttoadjustcolumnwidthssoyoucanmorereadilyreadallofthevalues.


InyourAspenPlussimulationselecttheHeattab.Selectthesquareintheupperleftpartofthetable&click(youshouldseetheentiretablehighlighted).Right‐clickthisupperleftsquareofthetable&selectCopy.GototheHeatTabletabinyourspreadsheet&selectcellA1.Right‐click&selectPaste.Youmaywanttoadjustcolumnwidthssoyoucanmorereadilyreadallofthevalues.


InyourAspenPlussimulationselecttheWorktab.Selectthesquareintheupperleftpartofthetable&click(youshouldseetheentiretablehighlighted).Right‐clickthisupperleftsquareofthetable&selectCopy.GototheHeatTabletabinyourspreadsheet&selectcellA1.Right‐click&selectPaste.Youmaywanttoadjustcolumnwidthssoyoucanmorereadilyreadallofthevalues.


InyourAspenPlussimulationselecttheModelsoptionunderResultsSummaryintheleft‐handtreestructure.ThedefaultshowsasummaryreportwiththeHeatertabselected.ClicktheSendtoExcelbutton.UsethedefaultformofOnetableperExcelworksheet.SelecttheoptiontoAddtablestoexistingworkbook;clicktheBrowsebutton&findthespreadsheetthatyou’vecreated.ClickontheExporttablestoExcelbutton.WhendoneclickOKforOpenExcelFile.Youshouldseetabsforthevarioustypesofequipmentinyoursimulation.


Nowthatwehavetheresultsinthespreadsheetlet’sstarttoconnectingthecellvaluesintheSummarypage.Manyofthevaluescanbereferencedtoasinglecell,e.g.,themassflowrateofthefuelgasas“='MaterialTable'!I6”,thesteamturbinepoweras“='WorkTable'!D2”,orthesteamturbinemechanicalefficiencyas“=Compr!E17”.Thetotalmolarflowrateofthefuelgasisalittlemorecomplicatedsincethetotalvalueisnotreportedinthematerialtable;itcanbedeterminedasthesumofallthemolarflowratesoftheindividualcomponents,“=SUM('MaterialTable'!I11:I21)”.NotethateventhoughtheunitsonthevaluescouldbeextractedfromtherowdescriptionincolumnAofthesheetsitiseasiertoenterthemastextvalues.

Someadditionalcleanup:

Itisconvenienttoformatthenumberslargerthan1,000toanumberwithnodecimalplaces&commaseparators.

Thesignsontheheat&worktermsaredependentonwhetherthevaluesaretransferringinoroutofaparticularunit.Onlytheabsolutevaluesshouldbereportedhere(importanthereonlyforthepowertermassociatedwiththesteamturbine).


Nowwewanttoaddformulastocalculatetheefficiencyvalues:

CellE2,“=B3*B4” CellE3,“=B3*B5” CellH2,“=E4/E2” CellH3,“=E4/E3” CellH5,“=(E8‐E7)/E4” CellH7,“=(E8‐E7‐E6)/E2” CellH8,“=(E8‐E7‐E6)/E3”

WenowhaveaspreadsheetcreatedwithafairlyflexibleformatthatallowsustocalculatenewefficienciesformodificationstotheAspenPlussimulation.Allwewouldhavetodoiscopyinthenewstreamtables&modelresults.Forexample,wecangetderivenewefficiencyvaluesforthefollowingchangesinoperatingparameters:

Pressuredropthroughthefuelgassystemis0.2bar(not0.3bar). Theisentropicefficienciesofallrotatingequipmentis85%(not100%)&themechanical

efficienciesare95%(not100%). 150°Cofsuperheatsuppliedtothesteam.


We’llskipthedetailsofallofthechangestotheAspenPlussimulation.Howeverthespreadsheetshownbelowshowsallofthestepsoftheefficiencycalculations.

Steam Cycle Simulation Aspen Plus v8 - Inside...

Documents

Transcript of Steam Cycle Simulation Aspen Plus v8 - Inside...