Steam Cycle Simulation Aspen Plus V8

SteamCycleSimulation–AspenPlusv8.6
TheattachedgivesstepstosetupasimulationinAspenPlusv8.6tomodelasimpleRankinesteam
cycleforelectricityproduction.Thesystemconsistingof:
 Fuelsidewithnaturalgasfeed,airblower,combustionchamber,&fuelsideofthesteam
boiler.
 Steamsidewithsteamturbine,steamcondenser,condensatepump,&steamsideofthe
boiler.
Thesimulationwillbesetupassumingisentropicstepsfortherotatingequipment.
WhenthesimulationissetuptheoverallPFDshouldlooklikethefollowingfigure.
Createnewsimulationfile
StarttheprogramfromStart,AllPrograms,AspenTech,ProcessModelingV8.6,AspenPlus,Aspen
PlusV8.6.WhentheprogramopenschoosetheNewbutton.ChoosetheGasProcessingthentheGas
ProcessingwithMetricUnitstemplate.ClicktheCreatebutton.
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DefinetheComponents&thePropertyModels
Specifycomponents,fluidpropertypackages,&crudeoilassays
Thefirststepistodefineasetofpurechemicalspeciestorepresent:
 Steamasmodeledbypurewater&usingpropertycorrelationsconsistentwiththeASME
SteamTables.
 Thenaturalgasfuel,air,&combustionexhaustaspurelightcomponentsmodeledbythe
Peng‐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’llalsowanttotakeintoaccountincomplete
combustion(formingcarbonmonoxide)aswellasNOxformation(fornowjustasNO,NO2,
&N2O).
ClicktheFindbuttontobringupthedatabanksearchform.Youcanentereithertheentireformula,
partofaname,orseveralotherpossiblesearchitemstofindallofthedesiredchemicalspecies.
Whenthepropercompoundisfound,selectitinthelist&clickAddselectedcompounds.The
followingfigureshowsasearchforH2O.Asyouareaddingcompoundsyoumaybeaskedwhether
toaddorreplacethecompoundalreadyinthelist;choosetheAddoption.
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BelowisanexampleofcomponentsretrievedfromtheAspendatabanks.Therearetwoissueswith
defaultmannerinwhichthislistispresented.One,theComponentIDsarenotverydescriptiveof
thecompound(especiallyascomparedtotheAliasvalues).Two,theorderdoesnotgroupthe
compoundsinaconvenientmanner.Wecanaddressbothoftheseissuesbeforeproceedingmuch
further.
Let’schangetheComponentIDvaluestomostlymatchtheAliasvalues.SelecttheComponentID
valueeitherbydouble‐clickingonitorbyclicking&thenpressingtheF2key.Onceselected,typein
thenewID&presstheEnterkey.AspenPluswillaskwhatyoureallywanttodobymakingthis
change;clicktheRenamebutton.ChangeallIDs.
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NowpresstheReorderbutton.Aformpops
upthatwillallowyoutomoveselected
compoundsupordownsothattheyina
convenientorder.PressClosewhendone.
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Thenextstepistoassurethatanappropriatefluidpropertypackagehasbeenchosenforthese
compounds.ClickonMethodsintheAllItemslistontheleft.FromhereweseethatthePeng‐
RobinsonEOShasbeenspecifiedasthebasemethod(perthechoiceoftemplateoriginallychose).
AlsotheASMEsteamtableoptionhasbeenspecifiedforcaseswhenonlywaterispresentinthe
stream.Thesearethedesiredoptionssowecancontinueon.
Nowisagoodtimetosavethefilebeforewestartsettinguptheprocesssimulation.ClicktheFile
tab&thentheSaveAsitem.ChoosetheAspenPlusBackupoption.
Setup&SolvetheFlowsheet
WorkingUnits
ActivatetheSimulationoption.Notethatyou’llseeablankflowsheet.
WewouldliketoshowthecalculationswithamodifiedsetofSIunits,inparticular:
 Temperatureas°C.
 Pressureasbar(absolute).
 Massflowaskg/sec.
 Molarflowaskg.mol/sec.
 HeatdutyaskJ/sec.
 PoweraskW.
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UndertheHometabclick
theUnitSetsbutton.In
thelistofunitsetsclick
ontherowforSI‐CBAR&
pressEdit.Proceeding
throughthevarioustabs
allowsyoutodetermine
whatwillbeusedforthe
displayoftheresultsas
wellasthedefaultunits
fortheinput.Mostofthe
unitsarewhatwedesire,
butnotall.Forexample,
youcanseethatMass
Flowwillbereportedin
kg/hr,notquitewhatwe
want.
Let’spulldownthelists
associatedfortheFlow
relatedvalues&pick
optionsthatareinterms
ofseconds,nothours.
GointotheHeattab.
ChangeHeatrelated
valuesfromJtokJand
powerrelatedvalues
fromWtokW.
SteamCycle
WewillwanttocreateasimpleRankinecyclewiththefollowingprocessconditions:
 Saturatedsteamproductionat125bar.
 Finalcondensationto20°C.
 Steamturbineoperatingatidealreversibleconditions.
 Condensatepumpoperatingatidealreversibleconditions.
 Noextrapressuredropthroughheatexchangersorpiping.
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Let’splacethefollowingunitsfromtheModelPalettetotheflowsheet1:2Heaters,aturbine(as
PressureChangers,Compr,ICON3),&Pump.Ultimatelyitwillbedepictedasfollows(withrotation
ofthepumpicon).
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&clickontheModelPalettebuttonorpresstheF10
key.
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Double‐clickontheCONDNSAT
stream.
SelectTemperature&Vapor
FractionfortheFlashType.Enter
20CfortheTemperature&0for
theVaporFraction(i.e.,a
saturatedliquid).
SpecifytheCompositionaspure
water;entera1forH2Ointhe
listwiththeMole‐Fracoption.
Let’suseaflowbasisof1kg/s.
Nowlet’ssettheoperatingparametersforthevariousunits.
DoubleclickonthePUMPicontoopentheinput
sheet.MakesurethePumpoptionisspecified
underModel.
SpecifytheDischargePressureas125bar.
Finally,todefinethisasanidealpumpspecifya
1forboththePump&DriverEfficiencies.
Nowlet’sdefinetheoperatingconditionsforthe
steamsideoftheboiler.Doubleclickonthe
BOILERicontoopenitsinputform.
Wewanttospecifytheoutletsteamassaturated
vapor.Wecouldspecifythevaporfraction.
Insteadwe’lldefine0°Cofsuperheat.
Wealsowanttospecifyazeropressuredrop.We
candothisbyspecifyingazerovalueforthe
pressure.
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Nowlet’sdefineoperating
conditionsfortheturbine.Double
clickontheSTMTRBNicontoopen
theinputsheet.Makesurethe
Turbineoptionisspecifiedunder
Model.
Wewanttodefinethisasanideal
turbinesospecifya1forboththe
Isentropic&MechanicalEfficiencies.
Weneedtospecifysomething
abouttheDischargePressureeven
thoughwedon’treallyknowwhat
itis,onlythatiscorrespondstothe
watervaporpressureat20°C.For
nowspecifyavalueof0.1bar;we’ll
fixitlater.
Weknowthatthisisacondensing
steamturbine.Thedefaultforthe
Turbinemodelisthatonlyvapor
willexit,sothiswillhavetobe
changed.ClickontheConvergence
tab.PulldownthelistforValid
phases&changetoVapor‐Liquid‐
FreeWater.
Nowlet’sdefinetheoperatingconditionsforthe
steamcondenser.Doubleclickontheexchanger
icontoopenitsinputform.
Wewanttospecifytheoutletassaturatedliquid.
WewanttomakesurethatoneoftheFlashType
optionsisVaporfraction&settheappropriate
valueas0(i.e.,saturatedliquid).
Wealsowanttospecifyazeropressuredrop(i.e.,
letthedischargepressuresettingfromthe
turbinecontrolthis).Makesurethatoneofthe
FlashTypeoptionsisPressure&setthe
appropriatevalueas0(i.e.,zeropressuredrop).
Wenowhaveenoughsettingstobeabletorunthesimulation.Openthecontrolpanel(itemunder
theHometab)&pressRun.Somewarningsmaycomeupbuttheywillbeaddressedlater.
WecansummarizetheresultsontheflowsheetbymodifyingtheStreamResultssettings.Selectthe
MainFlowsheet.SelecttheModifytab&selecttheTemperature,Pressure,&VaporFractionitems.
Clickinthelowerleft‐handcorneroftheStreamResultssection.Onthepop‐upformselectthe
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Heat/Workitem.Also,changetheformatforthePressurevaluetoshowthreedecimalplaces(i.e.,as
“%.3f”).DothesamefortheVaporfractionformat.ClickOK.
Wecannowseeasummaryoftheresultsontheflowsheetbelow.Onethingtonoteisthatour
guessfortheturbine’sdischargepressurewasinerror.Thepressureshouldactuallybe0.019bar
tocorrespondwithacondenseroutlettemperatureof20°C.Wecouldgobackandchangethevalue
manually.However,we’lluseoneofAspenPlus’soperationstoautomaticallysetittomatchthe
condenseroutlet.
WewillusetheCALCULATORoperationto“feedforward”theCondenser’spressuretotheturbine’s
dischargepressure.Eventhoughwecoulduseacalculatorwithoutseeinganyindicationonthe
flowsheetwe’llinsteadputaniconontheflowsheettogiveanindicationthatitisthere.
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FromtheModelPalette,chooseManipulators,Calculator,&selectICON1.Placeitontheflowsheet
neartheexhauststream.(Fortheflowsheetshownitisrotatedverticallysoitcan“hang”belowthe
line.)RenameitSET‐P.
Double‐clickontheicontoopen
upitsinputform.Definethe
variablePCNDSRasthepressure
calculatedforstreamCONDNSAT
(i.e.,thesaturationpressureat
20°C).SpecifythisasanImport
variable(i.e.,thevaluehastobe
calculatedbyAspenPlus&willbe
“read”bythecalculator
operation).
NowdefinethevariablePTURBN
asthesteamturbine’sdischarge
pressure.SpecifythisasanExport
variable(i.e.,thevaluebe
“written”asaparameterina
downstreamoperation).
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Finally,definetherelationshipintheCalculate
tab.UsingtheFortranmethodenterthe
statement“PTURBN=PCNDSR”(startingin
column6).
Nowwecanrerunthesimulationandgettheresultssummarizedbelow.Wecanseethatthe
CALCULATORhasdoneitsjob;theoutletpressurefromthesteamturbineisnowthesameasthe
inlettothecondenser.However,thetemperaturesaredifferent.What’swrong?
TheproblemisthatmostofthecalculationsaredonewiththedefaultPENG‐ROBproperties,notthe
desiredSTEAM‐TAproperties.Theexceptionistheoutletoftheturbinewhichrecognizestheliquid
formedasFreeWater&usestheSTEAM‐TAoptionforitsproperties.Hence,theinconsistency.
WecanforcetheunitstousetheSTEAM‐TAoptiontodothecalculationsbymakingmodifications
toeachunit’sBlockOptionssettings.IntheSimulationtreestructureforeachoperationselectBlock
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Options.Underthepull‐downlistforPropertymethodselectSTEAM‐TA.TheformforSTMTRBNis
shownasanexample.
Nowwhenwererunthesimulationwegetconsistentresults.Noticethattherearesubtlechanges
totheheat&workstreams.Forexample,theboilerheatisnow2,581kJ/sec;itwas2,808kJ/sec
whencalculatedbythePENG‐ROBmethod(adifferenceof9%).
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Inpreparationforadditionalchangeswe
needtomodifythesettingsfor
STMTRBN.Onitsinputformclickonthe
Convergencetab.ChangetheValid
phasestoVapor‐Liquid‐Liquid.Whenwe
rerunwegetaminorwarningthatthe
outletisbelowitsdewpoint(whichwe
alreadyknowsincethisisacondensing
turbine).
Whendealingwiththepositive&negativevaluesfortheheat&workstreamsrememberthetwo
conventionsusedbyAspenPlus:
 IftheheatorpowerstreamisanoutletofaunitthenAspenPlushascalculatedthevalueto
makeotheroperatingspecifications(suchastheoutlettemperatureinanexchanger).Ifitis
aninlettoaunitthenAspenPlususesthevaluetodeterminetheoutletconditions.
 Heatrepresentsenergytoorfromtheunitoperation;itisinthedirectionofthearrowifthe
heatispositiveorintheoppositedirectionifitisnegative.Work,ontheotherhand,
representsenergytoorfromtheuniverse;theenergyflowisintheoppositedirectionas
thatforheat.
SinceQ‐BOILERisnegativeforaheatstreampointingawayfromtheBOILER,thentheenergyflows
intotheboiler’sfluid.SinceW‐TURBNisnegativeforaworkstreampointingawayfromthe
STMTURBN,thentheenergyflowsoutoftheturbine’sfluid.
Fromtheresultsshownwecancalculatethethermalefficiencyofthissteamcycle.Weshould
alwaysmakeuseoftheabsolutevaluesfortheheat&workstreams.Forthissteamcycle:
W‐TURBN  W‐PUMP 1078  13
W
th  net 

 0.4126 .
Q boiler
Q‐BOILER
2581
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Fuel&CombustionSystem
Wewillwanttocreateasimplenaturalgasburner/boilerwiththefollowingprocessconditions:
 Naturalgasisavailableatindustrialdeliverypressure,20bar‐g&15°C.Wewill
characterizethenaturalgasas100%methane.
 Airisavailableat25°C.Wewillcharacterizetheairasa21/79O2/N2molarmixtureand
bonedry(i.e.,nowater).Wewanttoaddenoughairsothatthereis20%excessoxygen
basedoncompletecombustionofthenaturalgas.
 Thecombustionprocessoccursnearatmosphericconditionssothenaturalgasmustbelet
downinpressure.However,ablowerisneededtopushtheairintothecombustion
chamber.
 Thepressuredropthroughtheburner/boiler/fluecombinationis0.3bar.
 Thefluegasisemittedat120°Ctopreventanyliquiddropout&subsequentcorrosion
problems.
Let’splacethefollowingunitsfromtheModelPalettetotheflowsheet:Valve,Compressor2,RGibbs
Reactor,&Heater3.Ultimatelyitwillbedepictedasfollows.(We’lldiscusstheSETcalculatorsaswe
go.)
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&the
outletstreamabove.
3Notethattheheatexchangerhasbeenrotatedverticallytogettheheatstreambelowtheexchanger.
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
o DrawastreamoutofthebluearrowoftheHRSGexchanger;callitQ‐HRSG.
IntheModelPaletteclickontheWorkstreambutton.Drawasfollows:
o DrawastreamoutofthebluearrowoftheAIRBLWRcompressor;callitW‐BLOWER.
Let’sstartsettingparametersfortheinletstreams.
Let’sinitializethenaturalgas
streamfirst.
Double‐clickontheFUELGAS
stream.
SelectTemperature&Pressure
fortheFlashType.Enter15Cfor
theTemperature&20bargfor
thePressure.
Enter1fortheC1valueasMole‐
Frac.
Let’suseaflowbasisof1
kg.mol/sec.
Nowlet’sinitializetheAIR
stream.
Double‐clickontheAIRstream.
SelectTemperature&Pressure
fortheFlashType.Enter25C
fortheTemperature&0barg
forthePressure.
Enter0.21fortheO2&0.79for
theN2valuesasMole‐Frac.
Asastartingpointlet’sdefine
theflowrateas12kg.mol/hr.
Let’sspecifytheoutletpressureof0.3
bar‐gafterthelet‐downvalve.
Double‐clickonLETDOWN.Specify
0.3bargastheOutletpressure.
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Wewanttomaketheairbloweran
idealreversiblecompressor.
Double‐clickonAIRBLWR.Select
theCompressorasModel.Pulldown
theTypelist&chooseIsentropic.
Specify1fortheIsentropic&
MechanicalEfficiencies.
Nowit’stimetomodelthecombustionportionofthefuelgasburner.Therearevariousoptionsfor
doingthis.Oneofthesimplest(andwouldnormallybedoneforhandcalculations)wouldbeto
defineallcombustionreactions&specifytheextentofconversionforeach.Instead,we’regoingto
takeadvantageofthefullthermodynamiccapabilitiesofAspenPlus&useareactorthatwill
minimizetheGibb’sfreeenergy.Allwehavetodoislisttheexpectedproducts&AspenPluswill
calculatetheresultingproductdistributionthathonorsthematerial&energybalancesaswellas
anychemicalequilibriumlimitations.
DoubleclickontheRGibbsReactoricon.
SetPressureto0(torepresentazero
pressuredrop)&specify0fortheHeat
Duty(tosignifyadiabaticoperation).
That’sprettymuchit.Thedefaultisto
includeallspeciesinthecomponentlist
aspotentialproducts.
Nowlet’sseehowmuchheatcanbetransferredoutofthecombustiongasesbyspecifyingthe
combustiongassideoftheboiler.
Doubleclickontheheatericon.Settheconditionsto
theoutletconditionsoutthestack:120C&0barg.
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Wehaven’taddressedthecalculatoroperationsyet
butwecanstillrunthesimulation.Theresultsare
summarizedontheFlowsheetshowthatthe
combustiontemperaturewillbe1734°C.Wecan
double‐clickontheCOMBGASstream&seethat
therewillbesomeCO&NOxformedatthese
conditions.
Therearestillacoupleitemstobedoneto“cleanup”thesimulation&formatoftheresults.The
firstisforamatterofconvenience–howshouldwespecifythepressureoftheAIR‐2streamoutof
theairblower?RightnowthepressureintotheCOMBSTNoperationissetseparatelyforthetwo
inletstreams(LP‐GAS&AIR‐2).Ifastudywastobeperformed&thepressureweretochangethen
havingthespecificationsintwoseparatelocationscouldleadtothembeingchangeddifferently.It
surewouldbenicetosetitonlyinonelocation&thenhavetheotherlocationupdate
automatically.WecandothiswithaCALCULATORoperation.
FromtheModelPalette,chooseManipulators,Calculator,&selectICON1.Placeitontheflowsheet
neartheAIR‐2stream.(Fortheflowsheetshownitisrotatedtotheleftsoitcan“hang”offtheline.)
RenameitSET‐AP.
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Double‐clickontheicontoopen
upitsinputform.Definethe
variablePFUELasthepressurefor
streamLP‐GAS(i.e.,thepressure
outofthelet‐downvalve).Specify
thisasanImportvariable(i.e.,the
valuehastobecalculatedby
AspenPlus&willbe“read”bythe
calculatoroperation).
NowdefinethevariablePAIRas
theairblower’sdischarge
pressure.SpecifythisasanExport
variable(i.e.,thevaluebe
“written”asaparameterina
downstreamoperation).
Finally,definetherelationshipintheCalculate
tab.UsingtheFortranmethodenterthe
statement“PAIR=PFUEL”(startingincolumn6).
Thesecondchangeinvolvesaconvenientwaytomakesurethatthecorrectamountofairisadded
tomatchthe“excessoxygen”spec.Theamountofstoichiometricoxygenisdeterminedfromthe
combustionreactions.Formethane,ethane,&propanethereactionsare,respectively:
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CH4+2O2CO2+2H2O
C2H6+3.5O22CO2+3H2O
C3H8+5O23CO2+4H2O
Thisshowsthatweneedtoknowthecompositionofthefuelgas(inmolaramounts)todetermine
thestoichiometricamountofoxygenneeded.The“excess”partisadditionaloxygen(asa
multiplier)thatisadded.ThefinalconsiderationisthatthespecificationinAspenPlusisnotjust
fortherateofoxygenbutratheroftheair;sowehavetotakeintoaccountthecompositionofthe
airaccountforthelargeamountofnitrogenalsobeintroducedintotheCOMBSTNoperation.
Sincewehavesetthecompositionofthefuelgastobepuremethane&thebasisflowrateto1
kg.mol/secthenthestoichiometricoxygenflowrateistwicethis,2kg.mol/sec.Wealsoneedto
increasethisby20%toincludethedesiredexcess.Andweneedtotakeintoaccounttheoxygen
contentintheairtodeterminetheairrate.Sooverall:
nO2
1  fexcess  2 1  0.2
stoich

 11.43kg.mol/sec .
nair 
0.21
yO2
 
WecoulddothesecalculationspriortorunningAspenPlusandentertheairrate.Orwecoulddo
thecalculationswithinAspenPlus.
FromtheModelPalette,chooseManipulators,Calculator,&selectICON1.Placeitontheflowsheet
neartheAIRstream.(Fortheflowsheetshownitisrotatedtotherightsoitcan“hang”offtheline.)
RenameitSET‐AFLO.
Double‐clickontheicontoopenup
itsinputform.Definethevariable
AIRFLOasthemolarflowofthe
streamAIR(i.e.,thepressureoutof
thelet‐downvalve).Specifythisas
anExportvariable.
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Nowlet’sstartdefiningImport
variables.Firstdefinethevariable
YO2astheO2molefractioninthe
air.SpecifythisasanImport
variable.
Nextlet’sdefineImportvariablesfor
thecombustibleportionsofthefuel
gas(eventhoughwe’veonlyused
methanewehaveincludedthe
possibilityforethane&propane,
too).DefinethevariablesC1FLO,
C2FLO,&C3FLO.Makesurethese
arespecifiedasImportvariables.
Finally,definetherelationshipintheCalculatetab.
UsingtheFortranmethodenterthestatement:
AIRFLO=2.*C1FLO+3.5*C2FLO+5.*C3FLO
AIRFLO=AIRFLO*(1.+0.2)
AIRFLO=AIRFLO/YO2
(startingincolumn6).
Thesimulationcanbererungivingtheresultssummarizedbelow.Notethatcorrectairflowhas
beencalculated,11.43kg.mol/sec.
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Onemoremodification,thattodirectlyshowthemolefractionsofallofthestreams(sinceright
nowtheresultsareonlyshownasmolarflows).
ExpandtheSetupoptionsinthe
left‐handSimulationtree
structure.SelectReportOptions.
NotethatMoleFlowbasisoption
isspecifiedbutnoneofthe
Fractionbasisoptions.Selectthe
Moleoption.
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Rerunthesimulation.Nowwhenyoudouble‐click
ontheCOMBGASstreamyouwillnotonlyseethe
compositionoutofCOMBSTNinmolarflowsbut
alsoasmolefractions.
TyingtheTwoSystemsTogether
Eventhoughthesteamcycle&fuelgassystemsareinthesameAspenPlusflowsheettheyare
reallymodeledseparately.Thesteamcyclehasconvergedwithabasisof1kg/secwatercirculation
rate&thefuelsystemhasconvergedwithabasisof1kg.mol/secfuelgas.Wewilltiethesystems
togetherby“pushing”thedutyfromthefuelsideoftheboilertothesteamside&adjustingthe
watercirculationrateinthesteamcycletoensurethisistheonlyheatneededforthesteamcycle.
Nowlet’sconnectthetwosystems.
 RenamethestreamQ‐BOILERtoQ‐RESID(for“residual”).
 RenamethestreamQ‐HRSGtoQ‐BOILER.
 Right‐clickonQ‐BOILER,selectReconnect,ReconnectDestination,&attachtoblueinlet
arrowontheBOILERexchanger.
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Runthesimulation.Noticethatforthe
combinationoffuelgasrate&watercirculation
ratethereistoomuchgeneratedfromthe
combustionsideoftheboilertobeabsorbedby
thesteam.Wecanseethisbecausethe“residual”
heatfromthesteamside,Q‐RESID,is763,426
kJ/sec.Since766,007kJ/secwasgeneratedfrom
thecombustionsidethenonly2,581kJ/secwas
neededinthesteamside.
Theresultsshowthatwereallyneed296.8kg/secwatercirculatinginthesteamcycletoabsorball
oftheheatfromthecombustionside.Wecouldenterthisvaluemanuallybutthenwewouldhave
todothehandcalculationoveragainifanyconditionsweretochange.InsteadwewillletAspen
Pluscalculatetheproperflowrate.
FromtheModelPaletteselectaDesignSpec(theoneshowninthePFDistheDesignoption&
rotatedtotheright).ChangethenametoADJ‐WFLO.
Double‐clickonADJ‐WFLOtoget
itsinputforms.Createavariable
RESIDUALtorepresentthe
residualheataroundtheboiler
(i.e.,thedifferencebetweenthe
heatgeneratedonthe
combustionside&theheat
neededonthesteamside).
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SelecttheSpectab.Designatethe
RESIDUALvariableastheSpecvariable,
setitsTargetvalueto0(I.E.,tomatchup
thecombustionside&steamside
requirements).Setitsconvergence
Toleranceto0.5.
SelecttheVarytab.Toadjustthemass
flowoftheCONDENSATstreamfirst
chooseStream‐VarastheType4.
Let’sassumethattheUpperlimitisless
than500kg/sec;we’llsettheLower
limitas0.1(aslightlypositivenumber).
SettheStepsizeas0.01&theMaximum
stepsizeas0.1.
Wecanlookattheresults&seethattheanticipatedwatercirculationratehasbeenfound,296.8
kg/sec.
4DonotchooseMass‐FlowastheType;thiswillpointtotheflowofanindividualcomponent,nottheentire
stream.
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AdditionalStream&UnitAnalyses
Thereareadditionalanalysesthatwemaywanttoperformforthissimulation.Sincethegoalofthe
processistocreatepowerweshouldbeveryinterestedtodeterminethevariousthermal
efficienciesofthesystems.
Tocalculatetheefficiencyoftheboilerweneedtodeterminetheheatingvalueofthefuelgasused.
Todothiswewillmakeuseofthebuilt‐innet&grossheatingvalues(lower&higher,respectively).
ExpandtheSetup
itemintheleft‐hand
treestructureofthe
Simulationitems.
UnderPropertySets
createaNewset
calledHEATVALS.
Editthatproperty
set&addthe
propertiesQVALNET
&QVALGRS.
Nextwewant
toaddthese
propertiesto
thesimulation
report.Under
Setupinthe
left‐handtree
structure
chooseReport
Options.Goto
theStreamtab
&clickon
PropertySets.
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SelectHEATVALSintheAvailablepropertysetslist&
press>.ThiswillmoveHEATVALStotheSelected
propertysetslist.ClickonClose.
Nowwecanrerun
thesimulation.
Nowwhenwe
lookattheResults
forastreamwe
willseethenet&
grossheating
valuesatthe
bottomofthelist.
Wecannowstarttocalculatevariousefficienciesforthecombinedfuel/steamsystem.
 Boilerefficiency.Thiswillbetheamountofheatthatistransferredoutofthecombustion
sectionofthesystemintothesteamsystem.Thiscanbebasedeitheronthelower(net)
heatingvaluebutmorenormallyonthehigher(gross)heatingvalue:
766,007kJ/sec 
Qboiler
HHV 

 0.8601 .
 HHV  m 55515.1kJ/kg 16.043kg/sec 


Steamcyclethermalefficiency.Thishasalreadybeencalculatedastheratioofthenetwork
producedbythesteamcycletotheboilerheatin:
Wturbine  Wpump 319,838kW   3,716kW 
th 

 0.4127 .
Qboiler
766,007kJ/sec 
Overallefficiency.Thisisnormallycalculatedastheproductofthecombustionside’s
efficiency&thesteamcycle’sefficiency:
HHV th   0.8601 0.4127   0.3550 .
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However,thisdoesnottakeintoaccounttheenergyneededtoruntheairblower.Instead,
weshouldusetheratioofthenetworkproducedtotheenteringheatingvalue(again,in
termsofHHV):
Wturbine  Wpump  Wblower 319,838kW   3,716kW   7622kW 
total,HHV 

 0.3464 .
 HHV  m
55515.1kJ/kg 16.043kg/sec 
Let’ssetupanExcelspreadsheettodothesecalculations.Youcanstartwithaspreadsheetwith
labelsthatlooklikebelow.NotethatvaluesthatwillbedeterminedfromtheAspenPlussimulation
(eitherasaninputoracalculatedvalue)areinabluefont&willhavealightgreenbackground.
Theinformationwewanttoputintothistable&useforcalculationswillcomefromstreamresults
(Material,Heat,&Work)aswellasequipmentinformation(i.e.,modelresults).Wecouldcopy&
pasteindividualdatavaluesbetweentheAspenPlussimulationandthespreadsheet;itismore
flexibletocopyentiretablesofresultstothespreadsheet&thenpickoutthevaluesdesired.
Performthefollowingsteps:
 Inyourspreadsheetcreatethreenewstabs&callthemMaterialTable,HeatTable,&Work
Table.
 InyourAspenPlussimulationselecttheStreamsoptionunderResultsSummaryintheleft‐
handtreestructure.ThedefaultshowstheMaterialtabselected.ClicktheCopyAllbutton.
GototheMaterialTabletabinyourspreadsheet&selectcellA1.Right‐click&selectPaste.
Youmaywanttoadjustcolumnwidthssoyoucanmorereadilyreadallofthevalues.
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
InyourAspenPlussimulationselecttheHeattab.Selectthesquareintheupperleftpartof
thetable&click(youshouldseetheentiretablehighlighted).Right‐clickthisupperleft
squareofthetable&selectCopy.GototheHeatTabletabinyourspreadsheet&selectcell
A1.Right‐click&selectPaste.Youmaywanttoadjustcolumnwidthssoyoucanmore
readilyreadallofthevalues.
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
InyourAspenPlussimulationselecttheWorktab.Selectthesquareintheupperleftpartof
thetable&click(youshouldseetheentiretablehighlighted).Right‐clickthisupperleft
squareofthetable&selectCopy.GototheHeatTabletabinyourspreadsheet&selectcell
A1.Right‐click&selectPaste.Youmaywanttoadjustcolumnwidthssoyoucanmore
readilyreadallofthevalues.
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
InyourAspenPlussimulationselecttheModelsoptionunderResultsSummaryintheleft‐
handtreestructure.ThedefaultshowsasummaryreportwiththeHeatertabselected.Click
theSendtoExcelbutton.UsethedefaultformofOnetableperExcelworksheet.Selectthe
optiontoAddtablestoexistingworkbook;clicktheBrowsebutton&findthespreadsheet
thatyou’vecreated.ClickontheExporttablestoExcelbutton.WhendoneclickOKforOpen
ExcelFile.Youshouldseetabsforthevarioustypesofequipmentinyoursimulation.
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Nowthatwehavetheresultsinthespreadsheetlet’sstarttoconnectingthecellvaluesinthe
Summarypage.Manyofthevaluescanbereferencedtoasinglecell,e.g.,themassflowrateofthe
fuelgasas“='MaterialTable'!I6”,thesteamturbinepoweras“='WorkTable'!D2”,orthesteam
turbinemechanicalefficiencyas“=Compr!E17”.Thetotalmolarflowrateofthefuelgasisalittle
morecomplicatedsincethetotalvalueisnotreportedinthematerialtable;itcanbedeterminedas
thesumofallthemolarflowratesoftheindividualcomponents,“=SUM('MaterialTable'!I11:I21)”.
Notethateventhoughtheunitsonthevaluescouldbeextractedfromtherowdescriptionin
columnAofthesheetsitiseasiertoenterthemastextvalues.
Someadditionalcleanup:
 Itisconvenienttoformatthenumberslargerthan1,000toanumberwithnodecimal
places&commaseparators.
 Thesignsontheheat&worktermsaredependentonwhetherthevaluesaretransferring
inoroutofaparticularunit.Onlytheabsolutevaluesshouldbereportedhere(important
hereonlyforthepowertermassociatedwiththesteamturbine).
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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”
Wenowhaveaspreadsheetcreatedwithafairlyflexibleformatthatallowsustocalculatenew
efficienciesformodificationstotheAspenPlussimulation.Allwewouldhavetodoiscopyinthe
newstreamtables&modelresults.Forexample,wecangetderivenewefficiencyvaluesforthe
followingchangesinoperatingparameters:
 Pressuredropthroughthefuelgassystemis0.2bar(not0.3bar).
 Theisentropicefficienciesofallrotatingequipmentis85%(not100%)&themechanical
efficienciesare95%(not100%).
 150°Cofsuperheatsuppliedtothesteam.
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We’llskipthedetailsofallofthechangestotheAspenPlussimulation.Howeverthespreadsheet
shownbelowshowsallofthestepsoftheefficiencycalculations.
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