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University of Mississippi University of Mississippi
eGrove eGrove
Honors Theses Honors College (Sally McDonnell Barksdale Honors College)
2016
Optimization of Fluidized Bed Isothermal Reactor in a Styrene Optimization of Fluidized Bed Isothermal Reactor in a Styrene
Production Process Production Process
Matthew Hamilton Peaster University of Mississippi. Sally McDonnell Barksdale Honors College
Follow this and additional works at: https://egrove.olemiss.edu/hon_thesis
Part of the Chemical Engineering Commons
Recommended Citation Recommended Citation Peaster, Matthew Hamilton, "Optimization of Fluidized Bed Isothermal Reactor in a Styrene Production Process" (2016). Honors Theses. 675. https://egrove.olemiss.edu/hon_thesis/675
This Undergraduate Thesis is brought to you for free and open access by the Honors College (Sally McDonnell Barksdale Honors College) at eGrove. It has been accepted for inclusion in Honors Theses by an authorized administrator of eGrove. For more information, please contact [email protected].
1
OptimizationofaFluidizedBedIsothermalReactorina
StyreneProductionProcess
MattPeasterCommittee:Dr.AdamSmith___________________________________________________
Date______________________________________________
Dr.JohnO’Haver__________________________________________________
Date______________________________________________
Mr.DavidCarroll__________________________________________________
Date______________________________________________
2
Acknowledgements
Iwanttoacknowledgeafewpeoplewhowereinstrumentalinthecompletionofthisthesis.First,IwanttothankDr.AdamSmithwhoplayedamajorroleintrainingmetobecomeachemicalengineer.WithoutAdamIwouldnotbean
engineerandwouldnothavecompletedthisthesis.Heprovidedinvaluableadviceandguidanceduringthisentireprocessandwasalwaysavailableformyquestionsthroughoutmyentirecollegecareer.Secondly,Iwanttothankbothofmyparentsforsupportingandprayingformeformyentirelifeinallmyacademicendeavors.Myfather,especially,deservesacknowledgementforinstillinginmealoveand
desireforlearningforitsownsakeandforGod’sglory.Finally,IwanttopraiseGodforhisgraceandmercytomethroughoutthisthesiswritingprocess,andIwantto
acknowledgethatHedeservesallthegloryforitscompletion.
3
Abstract
Thefocusofthisthesisistoexplaintheoptimizationofafluidizedbed
isothermalreactorinastyreneproductionprocess.Thefirstsectionofthethesis
givesasummaryofchemicalprocessoptimizationingeneral.Thenextportionof
thethesisgivesanintroductiontochemicalprocesssimulationsoftware,andit
explainshowsimulationsoftwareaidsinthedesignandoptimizationofchemical
processes.Thethirdsectionofthethesisgivesabriefoverviewofanoptimization
projectofastyreneproductionprocessthatwascompletedintheprevious
semesterwithagroupofthree.Thefinalsectionexplainstheoptimizationofa
fluidizedbedreactorinthestyreneproductionprocessdiscussedintheprevious
sectionofthethesis.Theresultsofthereactoroptimizationproducedareactor
systemthathasatotalfluidizedcatalystbedvolumeof75.4m3with15reactorsin
parallel.Theoptimizedreactoroperatesatatemperatureof715°Candapressureof
75kPa,anditproducesatotalflowrateofstyreneof193kmol/hrandyieldof
ethylbenzenetostyreneof68%.
4
Contents:
I. IntroductiontoEngineeringOptimizationandDesign…………………………..3
II. IntroductiontoChemicalProcessSimulationSoftware………………..……….7
III. SummaryoftheOptimizationoftheUnit500StyreneProduction
Process……………………………………………………………………………………………...11
IV. FluidizedBedIsothermalReactorOptimization…………………………………..15
V. References…………………………………………………………………………………………24
VI. Appendix………………………………………………………………………………………...…25
5
IntroductiontoEngineeringProcessOptimizationandDesign Thissectionoverviewskeyengineeringprocessdesignandoptimization
conceptsandpurpose.Theultimategoaalofprocessdesignandoptimizationisto
improvetheprocess.RichardTurton’sbooknamedAnalysis,Synthesis,andDesign
ofChemicalProcessesdefinesoptimizationas“theprocessofimprovinganexisting
situation,device,orsystemsuchasachemicalprocess1.”Theactivityof
optimizationinvolvesusingcreativeapproachestoexaminemultipleoptionsfor
processchangesthatfocusonoptimizingachosenobjectivefunction.Theobjective
functionofaprocessisamathematicalfunctionthatthepersonoptimizingattempts
tominimizeormaximizebyfindingthebestvaluesforthedecisionvariables.The
decisionvariables,ordesignvariables,foraprocessarethosevariablesthatthe
engineerhasadegreeofcontrolover.Thesevariablesmaybeoftwodifferent
types,continuousordiscrete.Continuousvariablesaresuchthingsastemperature
andpressure,whilediscretevariablesareintegervaluessuchasthenumberof
stagesinanabsorptioncolumn.Alldecisionvariableshavecertainvaluelimitations
calledconstraints.Aconstraintforanoptimizationcaninvolvemultipledecision
variables.Therefore,thetruegoalofoptimizationistominimizeormaximizeone
ormoreobjectivefunctionswhileremainingwithintheconstraintsofthedecision
variables.Foralloptimizationproblemsaglobaloptimumexists.Theoptimumis
thepointwheretheobjectivefunctionreachesthebestpossiblevaluewithall
decisionvariableswithintheirconstraints.Theglobaloptimumisthebestpossible
solutiontoanoptimizationproblem.Thisvaluewillneverbefoundinany
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optimizationordesignproblem,buttheobjectiveistogetascloseaspossibletothe
globaloptimumvalue.
Alloptimizationsbeginwithaninitialbasecase.Therefore,adefined
processmustexistthattheoptimizationprocesscanimproveupon.Thebasecase
processdesignmaybeanactualoperatingplantorjustaconceptualprocess
flowsheet,butitmustbeadefinedprocess.Tostarttheoptimizationofaprocess,
selectingthebestbasecasedesignavailableforthestartingpointisideal.Analysis
ofthebasecasedesignmustbeabletogiveacalculationoftheobjectivefunctionof
theoptimization.Therefore,thebasecasedesignneedstocontainatleastenough
detailtoproducethecalculationsnecessaryforfindingtheobjectivefunctionofthe
optimization.Itisalsoimperativethattheanalysisofthebasecasealsoincludes
enoughdetailtoshowtheresultofchangingkeydecisionvariablesontheobjective
function.Findingthevaluesofkeydecisionvariablesthatmaximizeorminimizethe
objectivefunctionisthegoalofaprocessoptimization.Therefore,acalculationof
theeffectofthedecisionvariablesontheobjectivefunctionmustbepossibleinthe
basecase.
Animportantstepinbeginningtheoptimizationofaprocessistochoose
thescopeofthebasecasetooptimize.Thescopemaybeasinglepieceof
equipment,multiplepiecesofequipment,oranentireplant.Afterchoosingthe
scopeoftheoptimizationofthebasecase,thenextstepintheoptimizationprocess
istochoosetheobjectivefunction.Asstatedearlier,selectionoftheobjective
functionmusthaveanextrememaximumorminimumvalueasitsgoal.Choosing
theobjectivefunctionwiselyisveryimportanttothesuccessoftheoptimization.If
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avagueorbadlychosenobjectivefunctionisthegoalofanoptimization,thenthe
resultsoftheoptimizationwillnotbeuseful.Inmostprocessoptimizations,the
objectivefunctionchosenisonewithunitsofdollars.Commonlyusedobjective
functionsarethenetpresentvalueofaprocess(NPV)ortheequivalentannual
operatingcost(EAOC).Dependingonthescopeoftheprocesschosentooptimize,
theobjectivefunctionmaynotalwaysbedirectlycenteredoneconomics.
Therefore,asmallerscopemayhaveasitsobjectivefunctionthemaximumyieldof
areactorortheminimizationoftheconcentrationofsomecontaminantfroma
wastestream.Themostimportantpartofchoosingtheobjectivefunctionisto
confirmthatarationalbasisforitsselectionastheobjectivefunctionexistswhether
itismonetaryornonmonetary.
Afterchoosingthescopeanddefiningtheobjectivefunctionofthe
optimizationprocess,anevaluationofthebasecaseprocessneedstotakeplacein
ordertodecidethetargetsofanoptimizedprocess.Theinitialanalysisofthebase
caseproducesagoalfortheoptimization,anditalsochartsoutapathbywhichto
movetowardsthesolution.Thisanalysisusuallyleadstotheidentificationofthe
mostimportantdecisionvariables.Thekeydecisionvariablesaretheonesthat
affecttheobjectivefunctioninthelargestway.Somedecisionvariablesaffectthe
processandtheobjectivefunctionmorethanothers.Therefore,variousdecision
variablesprovetobemoreimportantorlessimportantbasedonthebasecase
analysis.Identificationandprioritizationofthedecisionvariablesisthelaststep
beforetrulybeginningtooptimizetheprocess.Theoptimizationprocesstakes
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placebyvaryingthedecisionvariabletofindthevaluesthatgivetheoptimum
objectivefunction.
Themethodsoffindingtheseoptimumdecisionvariablesaretopologicaland
parametricoptimization.Typically,topologicaloptimizationisthefirstmethodof
optimizationemployed.Topologicalconsiderationsusuallycomefirstin
optimization,becauseitismucheasiertooptimizeparametricallyafterthe
designationoftheflowsheettopology.Someprocessesrequiretheuseoftopological
andparametricoptimizationproceduressimultaneously,butconsiderationofany
largechangesinprocesstopologyusuallycomesfirstintheoptimizationprocess.
Themainfocusesoftopologicaloptimizationincludefindingtheoptimummethod
forthefollowingissues:eliminationofunwantedby-products,rearrangementor
eliminationofequipment,alternativeseparationmethodsorreactorconfigurations,
andimprovedheatintegration.Addressingthesequestionsaccordingtotheorder
inwhichtheyarelistedisbeneficialinfindingtheoptimizedtopologyforaprocess.
Aftersettingthetopologyoftheprocessflowsheet,thenextstepoftheoptimization
istousethemethodofparametricoptimizationtofindtheoptimumparametersfor
theprocess.Examplesofsomeimportantissuestoaddressinparametric
optimizationarethefollowing:reactoroperatingconditions,single-passconversion
inthereactor,recoveryoftheunreactedmaterials,refluxratios,operatingpressure
ofseparators,andpurityofproducts.Muchofthetimethetoolusedforbothtypes
ofoptimizationissimulationsoftwarethatcanvarymultipledecisionvariablesat
thesametimewithintheirconstraintsinordertomaximizeorminimizeagiven
objectivefunction
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IntroductiontoChemicalProcessSimulationSoftware
Chemicalprocesssimulationsoftwareisaveryusefulandeffectivetooltoaid
intheoptimizationofachemicalprocess.Simulatorscancarryoutbothtopological
andparametricoptimizations.Aprocesssimulatorisverypowerfultoolthat
engineersusetoaidinoptimization,design,andtroubleshootingofchemical
processes.Allprocesssimulatorshavesixmaincomponents.Theseelementsare
thefollowing:componentdatabase,thermodynamicmodelsolver,flowsheet
builder,unitoperationblocksolver,dataoutputgenerator,andaflowsheetsolver.
Theengineerusingthesimulatormustbeveryfamiliarwiththesoftwaresystem
andabletousealltheseelementseffectivelyinordertosetupaprocessaccurately.
Eachpartofthesimulatorhasadifferentfunction.Thecomponentdatabasestores
alltheconstantsneededtocalculatephysicalpropertiesfromthethermodynamic
models.Thethermodynamicmodelsolverusesachosenthermodynamicsystemto
calculateandestimateproperties.Theflowsheetbuilderdisplaysgraphicallythe
flowofthestreamsandequipment.Theunitoperationblocksolverperforms
numerouscalculationsonvariouspiecesofequipmentintheprocess.Output
reportsanddatagenerationcomefromthedataoutputgenerator.Thiselementofa
simulatorcancustomizesimulationresultsandconsolidatetheminareportor
graphicalform.Theflowsheetsolvergovernsthesequenceoftheflowsheet
calculations,anditcontrolstheoverallconvergenceofaprocesssimulation.
Inordertosetupaprocesssimulationauserneedstofollowafewgeneral
steps.Thefirststepinsettingupaprocessistheselectionofallthechemical
componentspresentintheprocessfromthecomponentdatabase.Afterselecting
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thecorrectchemicalsfromthedatabase,thenextstepistoselectathermodynamic
packagetomakethecalculationsinthesimulator.Selectionofthethermodynamic
modelisaveryimportantpartofthesimulationsetup,becausechoosingan
incorrectmodelforthesimulatorproducesinaccurateresultsthatarenotusefulto
theuser.Sometimesthethermodynamicmodelisdifferentforeachpieceof
equipment.Someoptionsforthesemodelsincludepackagesthatcalculateforone
liquidphaseortwoliquidphases.Theusermustbesuretoknowthephasesand
conditionsineachpieceofequipmentinordertoaccuratelysetupthe
thermodynamicmodel.Havingselectedthecorrectthermodynamicmodelforeach
pieceofequipment,thenextstepistoinputtheparticularflowsheettopology.
Creationoftheflowsheettopologyinvolvesdesignatingandspecifyingtheinputand
outputstreamsforeachpieceofprocessequipmentinthesimulation.Definitionof
thefeedstreampropertiescomesnextinthesetup.Theusermustspecifyallofthe
propertiesofthestreamsfeedingintotheprocessincludingthetemperature,
pressure,flowrate,vaporfraction,andcompositionofthestreamsinorderto
accuratelysimulatetheprocess.Afterspecifyingthefeedstreamproperties,the
parametersoftheprocessequipmentneedspecification.Theseparameterswillbe
someofthevariablesthatchangeinordertooptimizefortheobjectivefunction.
Thefinalstepinthesimulationsetupistheselectionofhowtodisplaytheresults
andthemethodofconvergence.Afterselectingtheconvergencemethodandthe
desireddisplayoftheresults,theusercanrunthesimulationandobtainasolution.
Inordertooptimizeaprocessusingasimulator,thebasecaseprocessmust
besetupcorrectlyinthesimulatoraccordingtothestepsdiscussedinthelast
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paragraph.Aftersettingupthebasecaseprocessinthesimulator,theusercanuse
themethodsoftopologicalandparametricoptimizationtoimprovetheprocess.To
optimizeaprocessinasimulator,anobjectivefunctionneedstobeselectedforthe
processorthepieceofequipmenttobeoptimized.Makingsurethattheprocess
operatesinsideofitsconstraintsisacrucialpartofoptimizationwithasimulator.If
theengineerusingthesimulatorisnotcareful,convergenceontheobjective
functioncanoccuratconditionsoutsidetheprocessconstraints.Iftheprocessdoes
notremaininsidetheconstraints,thenanysolutionconvergeduponisuseless.The
decisionvariablesfortheoptimizationmaybetopologicalorparametricinnature.
Mostprocesssimulatorshavesomesortofoptimizerelement,orthecapabilityto
runcasestudiesonaprocess.Optimizersandcasestudiesarebothusefulmethods
ofoptimizingwithaprocesssimulator.Manytimestheuseremploysbothtoolsto
optimizeaprocess.Thecasestudyfeatureofasimulatorwithtakeaninputofa
certainparameter,anditwillgraphicallydisplaytheeffectsofvaryingthe
parameteroveraspecifiedrangeofvaluesusingadesignatedstepsizeonachosen
objectivefunction.Thistoolisveryusefulinoptimization,becausetheusercan
obtainagraphicalrepresentationofhowcertainparametersaffecttheprocessand
theobjectivefunction.Fromthisinformationthechoiceofthebestparametersto
maximizeorminimizetheobjectivefunctionismuchmoreobvious.
Anoptimizerisalsoavaluabletooltouseinoptimization.Theoptimizer
elementofaprocesssimulatormakescalculatingthebestparameterstoachievean
objectivefunctionextremelyefficient.Thefirststepinusinganoptimizeristo
designateanobjectivefunctionandchoosetomaximizeorminimizeit.Afterthis
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theuserselectstheconstraintsfortheoptimizertooperateunder.Thefinalstepin
settinguptheoptimizerisselectingthekeydecisionvariables.Thedecision
variablesneedaspecificationfortherangeandstepsizeofvaluesinwhichto
optimize.Theoptimizerhasthecapabilitytocalculatetheobjectivefunctionusing
asmanydecisionvariablesastheuserwishestoinput.Aftersettingupthe
optimizercorrectly,theusercanruntheoptimizer,anditwillconvergeonthe
objectivefunctionbychangingthechosenparameterswithinthespecifiedranges
undertheconstraintsgiven.Attheclickofabuttonthesimulationsoftwareallows
anengineertofindthebestpossiblevaluesforthedecisionvariablesneededto
reachadesiredobjectivefunction.Iftheoptimizerdoesnotfindasolutionthefirst
timethatitisrun,thenoneormorerangesofvaluesforparametersmayneed
changingorexpansioninorderfortheoptimizertoconvergeonasolution.
Sometimestheoptimizerdoesnotafindasolution,becausenosolutionexistsfor
theparametersgivenwiththeequipmentspecificationsdefinedbytheuser.The
optimizermayrequireredefinitionoftheequipmentorprocessspecificationsin
ordertofindasolution.Runningcasestudiesonaprocessorspecificpieceof
equipmentbeforesettinguptheoptimizerisusuallybeneficial.Thegraphical
resultsfromthecasestudygiveagoodideaofhowchangesincertainparameters
affectanobjectivefunction.Withthisknowledgetheusercaninitiallysetupthe
rangesofvaluesforvariablesintheoptimizermoreaccurately.Optimizerandcase
studyfunctionsareverybeneficialfeaturesofprocesssimulatorsthatallow
engineerstoimproveprocessesmoreefficiently.
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SummaryoftheOptimizationofUnit500StyreneProductionProcess
Asamemberofathree-personteamlastsemester,Itookpartinthe
optimizationofastyreneproductionplant.AppendixAtothisthesiscontainsthe
completedetailsoftheoptimization.Thefollowingisabriefsummaryofthe
optimizationprojectinordertohelpclarifythepurposeofthefluidizedbed
isothermalreactoroptimizationthatisthemainsubjectofthisthesis.Thegoalof
thestyreneplantistoconvertethylbenzene,viaacatalyticreaction,tostyrene.
Styreneisamonomerthatpolymerizestocreatepolystyrenebetterknownas
Styrofoam.Theproductionrequirementfortheprocessis100,000tonnes/yrof
styreneof99.5wt%purity.Ourobjectiveasateamwastooptimizetheprocessin
ordertomaximizethenetpresentvalue,orNPV,whilesatisfyingasetofgiven
constraints.
Thefirststepintheoptimizationprocesswastodoapreliminaryanalysisof
thebasecaseinordertoidentifypotentialrevenueandcalculatetheeconomic
potentialoftheplant,Theeconomicpotentialisthepotentialmaximumprofit
possiblefortheprocess.Thiscalculationassumesthatallproductsseparate
perfectlyandthatallproductscanbesoldatthepureproductprices.Calculationsof
economicpotentialshowedthatitwaspossiblefortheprocesstobeprofitable.
Sincetheprocesshadthepotentialtobeprofitable,webeganamoredetailed
analysisofthebasecaseprocess.Thefirststepinimprovingtheprocessisto
determinethecurrentbasecasevaluefortheobjectivefunctionasastandardforto
improveupon.Therefore,thefirstpartofourprojectinvolvedsettingupthebase
caseplantinaprocesssimulatorandcalculatingthenetpresentvalueofthecurrent
14
process.Aftercalculatingthebasecasenetpresentvalue,webeganconductinga
sensitivityanalysisofthebasecaseprocessinordertofindoutwhichvariables
affecttheobjectivefunctionthemost.Asensitivityanalysishelpstopinpointwhich
areasoftheprocessaremostimportanttothemaximizationorminimizationofa
desiredobjectivefunction.Thesensitivityanalysisindicatedthatchangesinraw
materials,revenue,utilities,andthefixedcapitalinvestmenthavethelargestimpact
onthenetpresentvalueoftheprocess..Sincethesefactorsprovedtobekey
variablesinmaximizingthenetpresentvalue,wedecidedtooptimizebyaddressing
theseissuesfirst.
Thenextstepintheoptimizationprocessbeganbydesigninganewreactor
section.ThereactorsectionplaysthebiggestroleinmaximizingtheNPV,becauseit
convertsrawmaterialtoproduct.Therefore,sinceoursensitivityanalysisshowed
thatrawmaterialsandrevenuehadthegreatesteffectontheNPV,itwasobvious
thatweshouldfocusonthereactorsfirst.Mostofthetimeoptimizationofthe
reactorsectioncomesfirstinachemicalprocessoptimization.Itmakessenseto
optimizethereactorsectionatthebeginningoftheoptimizationprocess,because
thereactorinletandexitstreamspecificationsdeterminetherequirementsforthe
feedsectionandseparationsectionoftheplant.Optimizationofthereactor
involvedanalyzingvarioustemperature,pressure,molarcompositionandvolume
conditionswithinthegivenconstraintsoftheproject.AppendixApresentsthe
constraintsindetail.Originallythebasecaseoperatedwithtwoadiabaticplugflow
reactorsinseries.Weoptimizedthereactorsectionwiththeyieldofethylbenzene
tostyreneasourobjectivefunction.Sincestyreneisbyfarthemostprofitable
15
productofthereactions,andethylbenzeneisaveryexpensiverawmaterial;yield
seemedtobethemostappropriateobjectivefunction.Yieldinvolvesmaximizing
theconversionoftherawmaterialtodesiredproduct..Weimprovedthereactor
performancebyredefiningsomeoftheinletparametersandusingfiveparallel
adiabaticreactorsinsteadoftheoriginaldesign.Themainsubjectofthisthesisisa
furtheroptimizationofthereactorsectionofthisplant.Iwillexplainthedetailsof
thisreactoroptimizationlaterinthethesis.Atthispoint,Iwanttocontinueto
summarizetheoptimizationoftheentireplant.
Afteroptimizingthereactorsectionoftheplant,wedecidedtooptimizethe
feedsectiontotheplantinordertofitthereactorinletrequirements.The
sensitivityanalysisexhibitedthatutilitycostwasveryimpactfulontheobjective
functionofmaximizingtheNPVoftheprocess.Inordertoaddressthisissue,we
foundwaystointegrateheatinthefeedsectiontoachievetherequiredreactorinlet
conditions.Feedsectionoptimizationinvolvedafewtopologicalchanges.We
rearrangedtheorderinwhichtheprocessstreamflowedthroughvariouspiecesof
equipment.Heatintegrationinthefeedsectiondecreasedtheutilitycostofthe
plant,anditincreasedtheNPVoftheprocess.
Thenextportionoftheplantthatwefocusedonwastheseparationsection.
Theseparationsectioninaprocessisextremelyimportant.Improvementofthe
separationsectionallowsmoreoftheproductthatismadeinthereactortobesold
toincreaserevenuefortheplant.Theseparationsectionisalsocrucialin
separatingoutunreactedrawmaterialssothattheycanberecycledandreusedin
thereactor.Iftheseparationsectionisnotefficient,thenrawmaterialandproduct
16
willgotowaste.InordertomaximizetheNPVoftheprocess,optimizationofthe
separationsectionofaplantisvital.Weoptimizedtheseparationsectionby
changingsomeofthespecificationsofthestreamleadingintothe
liquid/liquid/vaporseparator.Thisprocessvesselisthefirstpieceofequipment
thatseparateswastefromproductintheprocess.Increasingtheefficiencyofthis
vesselhelpedsavestyreneproductandunusedethylbenzenefrombeingwasted.
Wealsoredefinedsomeparametersofthebothdistillationcolumnsinorderto
obtainbetterseparation.Thechangesmadetothedistillationcolumnsincreased
therevenuebyproducingasellabledistillatestreamfromthefirstdistillation
columnandbyseparatingethylbenzenefromstyrenemoreefficientlyinthesecond
column.Parametricandtopologicalchangestotheseparationsectionincreasedthe
NPVoftheprocess.AppendixAgivesmoredetailsonthechangesmadetothe
separationsectionofUnit500andtheresultingincreaseoftheNPV.
Thefinalstepinouroptimizationprocesswastoaddressthefixedcapital
investmentcostoftheprocess.Thesensitivityanalysisshowedthatthechangesin
thefixedcapitalinvestmentaffectedtheNPVoftheprocessgreatly.Inorderto
makesurethatwemaximizedtheNPV,weresearchedwaystodecreasethefixed
capitalinvestmentfortheplantinordertomaximizetheobjectivefunction.Fixed
capitalinvestmentforaplantincludesthecostofthephysicalprocessequipment,
andcertainconstructionmaterialsforequipmentaremuchmoreexpensivethan
others.Wemadeafewchangestomaterialsofconstructionofafewofthepiecesof
equipmentintheplantthatdecreasedthefixedcapitalinvestmentfortheplant.
Thebiggestchangeinfixedcapitalinvestmentcamefromchangingthematerialof
17
constructionforbothofthedistillationcolumnsfromtitaniumtocarbonsteel.
Titaniumismuchmoreexpensivethancarbonsteel,andcertaintemperature,
pressure,andchemicalsrequireitsuse.Afterresearchingthepropertiesofthe
chemicalspresentintheprocessandanalyzingthetemperatureandpressure
operatingconditionsintheplant,wedecidedthatcarbonsteelwasanappropriate
materialformostoftheprocessequipment.Theseconstructionmaterialchanges
decreasedthefixedcapitalinvestmentfortheprocessandincreasedtheNPVforthe
process.Thesechangesconcludedouroptimizationprocess.Overall,ourprocess
optimizationincreasedtheNPVdrastically,butwerecommendedfurther
optimizationtotheprocessbeforemovingforwardwiththenewplantdesign.
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FluidizedBedIsothermalReactorOptimization
Myindividualprojectforthisthesiswastooptimizeafluidizedbedisothermal
reactorfortheproductionofstyrene.Anisothermalreactormaintainsthesame
inletandexitstreamtemperature,andafluidizedbedisaconfigurationwherethe
catalystparticlesarefullysuspendedinafluid.Whenabedreachesfluidizationthe
pressuredropacrossthereactorremainsconstantwithincreasingsuperficial
velocity,butthebedheightcontinuestoincreasewithincreasingfluidflow.
Minimumfluidizationvelocityisthesuperficialvelocityofthefluidinafluidized
bedatwhichthedragforcebytheupwardmovingfluidisequaltotheweightofthe
solidparticles.Acrucialoperatingconstraintgivenintheprojectstatementforthis
fluidizedbedreactoristhatthesuperficialgasvelocityinthereactorremainswithin
therangeof3to10timestheminimumfluidizingvelocity.Figure1belowdisplays
theeffectofsuperficialvelocityonpressuredropacrossafluidizedbedreactor.
Figure1:Effectofsuperficialvelocityonpressuredropandbedheightofafluidized
bedreactor
19
Therequirementsforthisparticularreactorwerethatthereactorbe
simulatedinthesimulationsoftwareSimSciPro/IIusinganisothermalplugflow
reactor.Aninternalheatexchangerexistsinsidethereactorthatprovidesthe
isothermalcapabilities.Thereactoroperatingtemperatureisconstrainedbythe
operatingrangeofthecatalyst.ThesamedesignconstraintsfortheUnit500
reactorappliedtothefluidizedbedreactor.TheUnit500reactortemperature
constraintswereamaximumoperatingtemperatureof1000Kwithamaximumof
50Kvariationintemperatureoverthelengthofthereactor.Thepressure
constraintforthereactorwasanoperatingpressureintherangeof0.75to2.5bar.
Anotherconstraintofthereactordesignwasthattheinletmolarcompositionfor
thefluidizedbedreactorbethesameastheinletmolarcompositionofthe
optimizedUnit500reactor.Thisincludesasteamtoethylbenzeneratioof15.6to1.
Beforesettingupthereactorsimulation,Ineededtocalculatetheminimum
fluidizationvelocityforthissystem.Tofindtheminimumfluidizationvelocity,umf,I
usedtheWenandYucorrelationgivenasfollows:
𝑅𝑒!,!" =!!"!!!!
!!= [1135.69+ 𝐴𝑟]!.! − 33.7(1)
WhereRep,mfistheReynoldsnumber;AristheArchimedesnumber,
𝐴𝑟 = !!!(!!!!!)!!!!!!
; dpistheparticlediameter;ρgisthedensityofthegas,μgisthe
gasviscosity;ρsisthecatalystdensity;andgistheaccelerationduetogravity.The
projectstatementstatedthatthecatalystparticlediameteris300μm,andthe
densityofthecatalystis2000kg/m3.TheoptimizedPro/IIflowsheetprovidedthe
valuesforthegasdensityandviscosityatoperatingconditionsof685°Cand190
20
kPa.Usingtheseequationsandvalues,Ifoundtheminimumfluidizationvelocityto
beumf=0.032m/s.Therangeforthesuperficialgasvelocityforvaluesof3to10
timestheminimumfluidizationvelocityis0.096to0.320m/s.Icalculatedthese
velocityvaluesassumingthatthegasandcatalystdensityandviscositychangeonly
negligiblywithintheoperatingrangefortemperatureandpressure.Therefore,the
mainconstraintontheoptimizationwasthatthesuperficialgasvelocitystaysinside
therangestatedabove.Aftercalculatingthevelocityforthereactor,Iverifiedthat
thepressuredropacrossthelengthofthefluidizedreactorwasequaltozerousing
thefollowingequation:
∆𝑃 = 𝑔 1− 𝜀 𝜌! − 𝜌! 𝐿(2)
WhereΔPisthepressuredropacrossthereactor,gisaccelerationduetogravity,ε
isthevoidfractionofthefluidizedbed,ρsistheparticledensity,ρgisthegasdensity,
andListhelengthofthereactororinthisparticularcasetheheightofthefluidized
bed.Atthemaxcalculatedfluidvelocityofug=0.32andthegivenparticlediameter
ofdp=300μm,thevoidfractionofthebedisequaltonearly1.Withthisvoid
fraction,thepressuredropalongafluidizedbedofanylengthisnearly0.This
pressuredropagreeswiththeinformationdisplayedinFigure1above.After
determiningthevelocityconstraintandthepressuredropforthereactor,Ibegan
settingupthereactorsimulationinordertooptimize.
ThefirststepinsettingupthenewreactorinPro/IIwastoinputthe
chemicalsintheprocessandsetupthereactionkineticsforthereactor.The
reactionproceedsaccordingtothefollowingsetofreactions,andthechemicals
21
listedundertheequationsalongwithsteamaretheonlyonespresentinthe
process:
C6H5C2H5↔C6H5C2H3+H2(1)
EthylbenzeneStyreneHydrogen
C6H5C2H5→C6H6+C2H4(2)
EthylbenzeneBenzeneMethane
C6H5C2H5+H2→C6H5CH3+CH4(3)
EthylbenzeneHydrogenTolueneMethane
Afterenteringthechemicalsintotheprocesssimulatorandsettingupthe
reactionkinetics,Ichoseathermodynamicpackagetocarryoutthecalculationsfor
thesimulator.Thethermodynamicmodelselectedtocalculatesolutionsina
processsimulatorisextremelyimportant,andchoosingthewrongmodelgives
resultsthatarenotaccurateoruseful.IchosetheSRKSimScipackageasthemodel
forthisreactor.ThisthermodynamicmodelusestheSoave-Redlich-Kwong
equationofstatetomakethermodynamiccalculations.Ifoundasuitable
thermodynamicpackagebyanalyzingtheprocessusingFigure1.
22
Figure2:GuidelinesforSelectionofThermodynamicPackage
Inthestyreneproductionprocess,nopolarorhydrogenbondingispresent;
hydrocarbonswithgreaterthanfivecarbonsexistintheprocess;molecular
hydrogenispresentintheprocess,andthetemperatureoftheprocessisgreater
than250K.FollowingFigure1asaguidelinetheSRKSimScipackageprovedtobea
correctchoice.
Inordertobegintheoptimizationanddesignofthereactor,Isetupthe
processflowsheetforthereactor.Asmentionedearlier,thereactorchosenforthis
projectwasafluidizedbedisothermalplugflowreactor.Fluidizedbedreactors
haveabubblingnature.Inordertocompensateforbubblinginthereactor,
simulationofthereactorrequiresthatsomeofthefeedgasbypassthecatalystin
thereactor.Thereactorsimulationrequiredafeedgasbypassof10%.Witha10%
feedgasbypassthesingle-passconversioninthereactorcanonlyreachamaximum
23
conversionof90%.Thebypassstreammusthaveaheatexchangerandavalve
addedtoitinordertobeabletomatchthetemperatureandpressureinthereactor
outletstreamwhenitrecombines.
Duringtheflowsheetsetup,Ispecifiedtheinletstreampropertiesandthe
initialspecificationsforthereactor,heatexchanger,andvalve.Themolar
compositionofthefeedstreamtothereactorremainedthesameasthemolar
compositionoftheoptimizedreactorfromtheoriginaloptimizationproject.After
settinguptheflowsheetforthereactor,Ibegantheactualoptimizationprocess.
Thefirststepinoptimizingthereactorwastochooseanobjectivefunction
fortheoptimization.Iselectedthemaximumoutputofstyreneinthestreamexiting
thereactorasmyobjectivefunction.Sinceproductionofstyrenefromethylbenzene
istheobjectiveoftheoriginalprocess,Idecidedthatmaximizingthestyreneoutof
thereactorwasthebestwaytooptimize.Thenextstepintheoptimizationwasto
selectcertainparameterstooptimizeandtodefinetherangesoverwhichtovary
them.Thechosenparameterswerethereactorinletstreamtemperatureand
pressure,thetotalvolumeofcatalyst,andthenumberofreactorsinparallel.The
onlyconstraintenteredintotheoptimizerwasthatthesuperficialgasvelocityinthe
reactorremainsintherange0.096to0.320m/s.Theactualspecificationsforthe
reactorwerethatthetemperatureremainsconstantacrossthereactorandthatno
pressuredropoccursacrossthereactor.Anotherspecificationforthereactorwas
thelengthofthereactororthefluidizedbedheight.Inordertofindtheoptimal
innerdiameterforthereactor,thedimensionofreactorlengthneedstobespecified
inthereactor.Pro/IIoptimizesforreactorcatalystvolume.Therefore,itis
24
appropriatetochoosediameterorlengthasaspecification.Thevaluerangesfor
theoptimizationparameterswereasfollows:inlettemperaturefrom450to700°C,
inletpressurefrom75kPato250kPa,innerdiameterof0to8m,andthenumberof
reactorsfrom1to15reactorsinparallel.Aconstraintofthereactoristhatitcannot
operateatemperaturehigherthan1000Kor727°C.Thepressureconstraintforthe
reactoristhatoperatingpressureneedstostaybetween75kPaand250kPa.The
innerdiameterrangeandthenumberofreactorsrangearenotasintuitive.Since
thisreactoroptimizationprojecthasnoeconomicinvestmentconstraints,itis
feasibletodesignaninfinitenumberofinfinitelylargereactors.Inordertostay
withinreason,Iassumedamaxreactordiameterof8mandamaxnumberof
parallelreactorsof15inordertokeepthefixedcapitalinvestmenttoareasonable
amount.
Togetabetterunderstandingofhoweachvariableofthereactoraffectedthe
objectivefunction,Iperformedcasestudiesthatincluded:theinnerdiameter,
catalystbedheight,numberofreactors,temperature,andpressureversusthe
productionofstyrene.Thecasestudiesshowedthatifeverythingelseremains
constantthatanincreaseintheinnerdiameterforthereactordecreasesthegas
velocityinthereactor,anditwillalsodecreasetheyield.Alargerfluidizedbed
heightseemstolowertheyieldofthereactor,butitdoesnotaffectthegasvelocity
ofthereactorifeveryotherparameterremainsconstant.Increasingthenumberof
reactorsreducesthevelocity,butitalsoreducestheyieldofstyrene.Casestudies
ontheeffectoftemperatureandpressureontheproductionofstyreneshowedthat
decreasingthepressureincreasedtheyieldandthevelocityinthereactorand
25
decreasingthetemperaturedecreasedtheyieldandvelocityofthereactor.The
figuresbelowdisplaytheeffectsofpressure,temperatureandcatalystbeddiameter
onstyreneproduction.
Figure2:Displayoftheeffectofchangingreactorinletpressureonstyrene
production
0
50
100
150
200
250
0 50 100 150 200 250 300StyreneProduction(kmol/hr)
Pressure(kPa)
StyreneProductionvs.Pressure
Pressure
26
Figure3:Displayoftheeffectofvaryingreactorinlettemperatureonstyrene
production
Figure4:Displayoftheeffectofvaryingcatalystbeddiameteronstyrene
production
Theprocesssimulatorcontainsacalculatorfeaturethathasthecapabilityto
outputtheresultsofparameteroptimizationsandtocalculatesolutionstouser-
definedformulas.Inordertooutputthesolutionsfortheoptimization,Isetupa
0
20
40
60
80
100
120
140
160
0 200 400 600 800
StyreneProduction(kmol/hr)
Temperature(˚C)
StyreneProductionvs.Temperature
Temperature
151152153154155156157158159160161162
0 2000 4000 6000 8000 10000 12000
StyreneProduction(kmol/hr)
DiameterofCatalystBed(mm)
CatalystBedDiametervs.StyreneProduction
Diameter
27
calculatortodisplaytheresultsoftheoptimizercalculations.Theresultsdisplayed
fromthecalculatorweretheconversionofethylbenzene,theselectivityof
ethylbenzenetostyrene,theyieldofstyrenetoethylbenzene,andthemaximum
velocityofthegasinsidethereactor.Asecondcalculatordisplayedtheheightof
thefluidizedbed,thediameter,thetotalfluidizedcatalystvolume,thetemperature,
andthepressureofthereactor.Theresultsoftheoptimizerconfirmedthetrends
exhibitedinthecasestudies.Theoptimizedreactorsystemhasatotalfluidized
catalystbedvolumeof75.4m3.Forthisvolumeofcatalysts,theoptimizedsystem
requirestheemploymentof15reactorsinordertokeepthesuperficialgasvelocity
withintherequiredrange.Theoptimizedreactoroperatesatatemperatureof
715°Candapressureof75kPa.Theoptimizedreactorparametersproducedatotal
flowrateofstyreneoutofthereactorsystemof193kmol/hrandyieldof
ethylbenzenetostyreneof68%.Theoriginaloptimizedreactorproduced123
kmol/hrstyrene.Therefore,thefluidizedbedisothermalreactorincreasedthe
styreneproductionby57%.
28
References:
1Turton,Richard.Analysis,Synthesis,andDesignofChemicalProcesses.Upper
SaddleRiver,NJ:PrenticeHallPTR,2012.Print.
(1) (2)MacCabe,WarrenL.,andPeterHarriot.UnitOperationsofChemical
Engineering.5thed.NewYork:McGraw-Hill,1994.Print.
29
Appendix:
I.AppendixA……………………………………………………………………………………………………28
30
AppendixA
OptimizationofStyreneProductionProcess
ChE451:ProcessDesignNishalBhikha
WilsonLook
MattPeaster
December12th,2015
31
ExecutiveSummary:
Byperformingasensitivityanalysis,wedeterminedthatchangesintheraw
materials,revenue,fixedcapitalinvestment(FCI),andutilitieshavethemostimpact
inincreasingthenetpresentvalue(NPV)oftheproposedprocess.Anoversightin
thebasecasereactordesignforcedustomakemajormodificationstoourprocessin
thereactorsectionresultinginanincreaseinrawmaterialcostfortheoptimized
plant.Thebasecasepressuredropacrossthereactorsresultsinanincrediblyhigh
velocitythatwasnotaccuratelyaccountedforintheoriginalbasecasedesign.We
determinedthatitwasnecessarytohavefiveadiabaticreactorsinparallelto
producethetargetof100,000tonnes/yrofstyrene.
Additionally,heatintegrationgreatlyimprovedtheutilitycost.Essentially
one“hot”stream,theeffluentfromthereactor,needstobecooledinpreparationfor
theseparationsection.Wedecidedtousethisstreamtopreheatthelow-pressure
steaminertbeforeitentersthefiredheater.Wealsousedthereactoreffluent
streamtovaporizethecombinedethylbenzenefeedbeforeusingcoldutilitiesto
reachthedesiredseparationfeedtemperature.Theheatintegrationforthe
optimizedprocessreducedtheutilitycostby$12million/yrfromthebasecase.
Furthermore,modificationstothefirstdistillationcolumnallowedustosell
apurifiedbenzene/toluenestreamtoincreaserevenue.Wealsoloweredraw
materialcostsbyoptimizingtheliquid/liquid/vaporseparatortoreduce
ethylbenzeneandstyrenelostinthefuelgas.Optimizationstotheseparation
sectionallowedustorecyclemoreethylbenzeneandtakemoreofthestyrene
producedtoactualproductwithoutlosingittofuelgas.Wearealsoabletosell
32
morefuelgasduetothesechanges.Optimizationstotheseparationsectionresulted
inanincreaseofrevenueof$15million/yr.Wealsoreducedthefixedcapital
investmentbychangingthematerialsofconstructionforvariouspiecesof
equipment.Wereplacedthetitaniumdistillationcolumnswithcarbonsteel
distillationcolumns,andwealsoreplacedstainlesssteelwithcarbonsteelforafew
heatexchangers.Changesinconstructionmaterialsforprocessequipmentreduced
ourfixedcapitalinvestmentby$117million.
ImplementationofthechangesmentionedaboveresultedinNPVof-$412
millionthattranslatestoanequivalentannualoperatingcost(EAOC)of$72.9
millionfortheoptimizedcase.ThisEAOCismuchlowerthantheprojectedcostof
purchasing100,000tonnes/yrfor$160million/yr.Table1givesasummaryofthe
bottomlineresultsofoptimizationoftheUnit500styreneproductionprocess.
Table1:BottomLineResultsofUnit500Optimization
OptimizedCase($M/yr) BaseCase($M/yr)
Revenue 185 170
RawMaterials -137 -118
Utilities -57 -69
TotalFCI -136 -253
CostofManufacturing -260 -278
NetPresentValue -412 -558
EAOC 73 98
33
Table1showsthatchangesmadetotheseparationsectionincreasedthetotal
revenueoftheprocessby$15million/yr.Therawmaterialcostfortheprocess
actuallyincreasedintheoptimizedcaseby$19million/yr.Thisincreaseinraw
materialcostwasduetothefactthatthebasecasereactordesignwasnotpossible
duetothepressuredropacrossthereactor.Wereducedtheutilitycostforthe
optimizedcaseby$12million/yrbyintegratingheatmoreefficiently,andchanges
madetothematerialsofconstructionforcertainpiecesofprocessequipment
loweredtheFCIcostby$117million/yr.Overall,Table1illustratesthatthebottom
lineforouroptimizedstyreneprocessoverthelifeoftheprojectis-$412million/yr
andanEAOCof$72.9million/yr.ThisoptimizedNPVis$146milliongreaterthan
thebasecaseprocessdesignNPV.EventhoughtheNPVforthebasecaseplantis
muchlowerthantheNPVfortheoptimizedstyreneplant,wesuggestthattheplant
needsfurtheroptimizationandamoredetailedestimate.Werecommend
optimizingthereactorsfurthertofindareactorsystemwithahigheryieldof
ethylbenzenetostyrene.Theseparationsectionalsoneedsfurtheroptimization.
Theliquid/liquid/vaporseparatorstilllosessomeethylbenzeneandstyrenetofuel
gas,andfurtheroptimizationmayprovideasolutiontothisproblem.
34
Contents:
I. Nomenclature5
II. Introduction7
III. Results10
IV. ProcessDescription24
V. Discussions28
VI. ConclusionsandRecommendations32
VII. SafetyandEnvironmentalConcerns:33
VIII. References35
IX. Appendix36
35
Nomenclature:
FT-temperaturecorrectionfactor
Fp-pressurefactor
Fm-materialfactor
Kn-ConstantsfromTurton(TableA.1)
Cn-ConstantfromTurton(TableA.2)
Bn-ConstantfromTurton(TableA.4)
Q-Duty,kW
h-Localheattransfercoefficient,W/m2K
L-lengthofreactor,m
bfw-boilerfeedwater
cw-coolingwater
lps-lowpressuresteam
hps-highpressuresteam
A-area,m2
V-volume,m3
D-columndiameter,m
H-columnheight,m
W-work,kW
Nol-numberofoperatinglabor
P–pressure,kPa
ρ-density,kg/m3
m-massflowrate,kg/hr
36
mw-molecularweight
COM-costofmanufacturing,$
FCI-fixedcapitalinvestment,$
CTM-totalmodulecost,$
CGR-grassrootscost,$
Col-costofoperatinglabor,$
Δp-pressurechangeacrossreactor,kPa
Vo-superficialvelocity,m/s
ε-voidfraction
Φs-sphericity
Dp-diameterofsphericalparticle
μ-viscosity,cP
t-Holduptimeforsizingvessels,m
η-efficiency
ΔTLM-logmeantemperaturedifference,C°
U-overallheattransfercoefficient,W/m2K
RM–RawMaterials,$
Ut-Utilities,$
WT-WasteTreatment,$
37
Introduction:
Thepurposeofthisreportistodescribetheoptimizationofastyrene
productionprocess.Styrenepolymerizestoproducepolystyrene,whichisa
lightweightsubstancewithavarietyofindustrialusessuchaspackaging,foam
insulation,andfoodcontainers(1).Productionofstyreneoccursfromthe
dehydrogenationofethylbenzeneasseeninEquations1through4.Thestyrene
productionprocess,Unit500,discussedinthisreportisonlyaportionofalarger
plantthatmanufacturesbenzene,ethylbenzene,andpolystyrene.Theprocess
conceptdiagraminFigure1illustratesasimplifiedversionofthestyreneprocessin
Unit500.
Figure1:ProcessConceptDiagramoftheProductionofStyrene
AsillustratedinFigure1,ethylbenzenereactsinareversiblereactiontoproduce
styreneandhydrogen.Twoundesiredsidereactionstakeplaceintheprocess.In
thefirstundesiredreactionethylbenzenereactstoproducebenzeneandethylene,
andinthesecondundesiredreactionethylbenzenereactswithhydrogentoproduce
tolueneandmethane.TheonlyrawmaterialneededfortheUnit500styrene
productionprocessisethylbenzene.Figure1showsthatthereactedethylbenzene
38
producesthreeseparatesellableproductsintheprocess.Theseproductsarea
benzene/toluenemixture,fuelgas,andthedesiredproductstyrene.Since
productionofstyreneisthegoaloftheprocessandthemostprofitableproductof
theethylbenzenereactions,themainobjectiveoftheoptimizationoftheprocessis
tomaximizetheyieldofethylbenzenetostyreneandtooptimizetheseparationof
theproductcomponentsfromunreactedethylbenzene.
Initially,wecalculatedaneconomicpotential,asseeninTable2,forthe
processinordertogetanideaofthepotentialrevenueoftheprocess.The
economicpotentialcalculationassumesthatallcomponentsseparateperfectlyand
thatwecansellallproducts.Table2illustratesthattheprocessbuys136kmol/hr
ofethylbenzene,anditproducesandsells120kmol/hrofstyrene,113kmol/hrof
hydrogen,8kmol/hrofbenzeneandtoluene,and7kmol/hrofMethaneand
ethylene.FromthiseconomicpotentialcalculationinTable2,weseethatthis
processhasthepotentialtoproduce$8,830dollars/hrwithperfectseparationand
theabilitytosellallproducts.Sincetheprocessprovedtobeprofitable,wedecided
toproceedwithoptimization.
39
Table2:EconomicPotentialofStyreneProductionProcess
Components Flow Rate Value Density Molar Mass
Total Cost/Revenue
Pure (kmol/hr) ($/kg) (BTU/lbmol) (kg/m3) (kg/kmol) ($/hr) Ethylbenzene 136 0.900 - 866.0 106 -12,950
Styrene 120 1.598 - 909.0 104 19,975 Hydrogen 113 - 51,600 0.099 2 305 Benzene 8 0.919 - 876.5 78 576 Toluene 8 1.033 - 866.5 92 764 Methane 7 - 21,400 - 16 59 Ethylene 7 - 20,500 - 28 99
Economic Potential ($/hr) 8,830
ThemainobjectiveofoptimizationwastomaximizetheNPVoftheprocess
whilesatisfyingasetofgivenconstraintsthatmainlyincludetheproduction
requirementsforstyreneandthereactorandseparationsectionoperating
temperatures.TheproductionrequirementforUnit500is100,000tonnes/yrof
styreneof99.5wt%purity.Thereactordesignedtoproducethestyrenemustnot
havetemperaturethatexceeds1000K,andthetemperaturedropacrossthereactor
cannotbegreaterthan50K.Afterthereactor,thestyreneproducedhassome
specificconstraintsintheseparationsection.Inordertopreventthepolymerization
ofstyreneintheseparationsection,thetemperaturemustremainbelow125°C.
Table3summarizestheeconomicconstraintsfortheproject.
40
Table3:EconomicParametersfortheStyreneProject
Parameters Value
OperatingLaborCost $59,580peroperatorperyear
CorporateTaxRate 35%
DepreciationMethod 7yearMACRS
MARR 12%
OperatingHoursPerYear 8000
Whendesigningtheoptimizedstyrenecase,weassumedourentireprocess
operatedatsteadystate.Tosimplifyourheatexchangercalculations,weassumed
thatthetemperaturecorrectionfactoris0.9withnophasechangeand1forphase
change.
Beforebeginningoptimization,weperformedasensitivityanalysisonthe
process.Theresultsofthesensitivityanalysisindicatedthatchangesinraw
materials,revenue,utilities,andFCImosteffectivelymaximizetheNPV.
Westartedtheoptimizationbyfocusingthereactortoincreasetheyieldof
rawmaterialtoproduct.Inordertomoreefficientlyuserawmaterialsandincrease
revenue,weoptimizedtheseparationsectiontomoreeffectivelyseparate
ethylbenzenefromourproducts.Afteroptimizingthereactorandseparation
sections,weaddressedtheutilitycostsbyintegratingheatinordertofindthemost
economicaluseofenergyintheprocess.Finally,weresearchedtheconstruction
materialsoftheprocessequipmentandmadetheappropriatechangestoreducethe
FCI
41
Results:
Figure 2: Process Flow Diagram for the Optimized Plant
42
Table4:StreamTablesforUnit500StyreneOptimizedPlant1
StreamNo. 1 2 3 4 5 9Temperature(ºC)
136 107 350 160 830 685
Pressure(kPa) 210 200 180 600 550 190VaporMoleFraction
0 0 1 1 1 1
TotalFlow(kg/hr)
18,400 56,300 56,300 148,000 148,000 204,000
TotalFlow(kmol/hr)
174 531 531 8,210 8,210 8,741
CompFlow(kmol/hr)
Water 8,210 8,210 8,210Ethylbenzene 170 526 526 526Styrene 1.21 1.21 1.21Hydrogen Benzene 1.74 1.74 1.74 1.74Toluene 1.74 1.85 1.85 1.85Ethylene Methane
StreamNo. 10 11 12 13 14 15Temperature(ºC)
653 465 361 270 170 51
Pressure(kPa) 160 145 125 110 80 120VaporMoleFraction
1 1 1 1 1 0.02
TotalFlow(kg/hr)
204,000 204,000 204,000 204,000 204,000 204,000
TotalFlow(kmol/hr)
8,880 8,880 8,880 8,880 8,880 8,880
CompFlow(kmol/hr)
Water 8,210 8,210 8,210 8,210 8,210 8,210Ethylbenzene 364 364 364 364 364 364Styrene 123 123 123 123 123 123Hydrogen 100 100 100 100 100 100Benzene 20.4 20.4 20.4 20.4 20.4 20.4Toluene 23.9 23.9 23.9 23.9 23.9 23.9Ethylene 18.7 18.7 18.7 18.7 18.7 18.7Methane 22 22 22 22 22 22
1Thesetablescontainroundedvaluestoincreasereadability.Ifthecomponentmolarflowrateis0,thentraceamountsofthecomponentactuallyexist.
43
Table4:StreamTablesforUnit500StyreneOptimizedPlantCont.
StreamNo. 16 17 18 20 21 22Temperature(ºC) 50.8 50.8 50.8 50.8 63.4 116Pressure(kPa) 105 105 105 65 35 55VaporMoleFraction
1 0 0 0.0004 0 0
TotalFlow(kg/hr) 2,420 54,300 147,500 54,300 1,170 50,500TotalFlow(kmol/hr)
170 527 8,185 527 13 478
CompFlow(kmol/hr)
Water 21 4.95 8,185 4.95 0.03 Ethylbenzene 5.50 358 0 358 1.04 356Styrene 1.58 121 0 121 0.15 121Hydrogen 99.6 0.12 0 0.12 Benzene 1.99 18.4 0 18.4 3.28 Toluene 0.94 22.9 0 22.9 8.49 0.11Ethylene 18.0 0.66 0.66 0 Methane 21.7 0.28 0 0.28 0
StreamNo. 23 24 26 27 28 29Temperature(ºC) 90.8 124 63.4 124 50.8 92.6Pressure(kPa) 25 55 200 200 200 210VaporMoleFraction
0 0 0 0 0 0
TotalFlow(kg/hr)
37,900 12,600 1,170 12,600 37,900 148,000
TotalFlow(kmol/hr)
357 121 13 121 8,185 357
CompFlow(kmol/hr)
Water 0.03 8,184 Ethylbenzene 356 0.59 1.04 0.59 0.10 356Styrene 1.21 120 0.15 120 0 1.21Hydrogen 0 Benzene 3.28 0.01 Toluene 0.11 8.49 0.05 0.11Ethylene 0 Methane 0 0.03
44
Table4:StreamTablesforUnit500StyreneOptimizedPlantCont.
StreamNo. 30 31 32 33 34Temperature(ºC) 455 829 352 194 216Pressure(kPa) 585 229 200 95 135VaporMoleFraction
1 1 1 1 1
TotalFlow(kg/hr) 148,000 148,000 56,300 204,000 204,000TotalFlow(kmol/hr)
8,210 8,210 531 8,880 8,880
CompFlow(kmol/hr)
Water 8,210 8,210 8,210 8,210Ethylbenzene 526 364 364Styrene 1.21 123 123Hydrogen 100 100Benzene 1.74 20.4 20.4Toluene 1.85 23.9 23.9Ethylene 18.7 18.7Methane 22 22
StreamNo. 35 36 37 38 39Temperature(ºC) 63.4 54.8 120 55 172Pressure(kPa) 35 35 105 90 240VaporMoleFraction
1 1 1 1 1
TotalFlow(kg/hr) 2,700 5,120 5,120 5,120 5,120TotalFlow(kmol/hr)
36.2 207 207 207 207
CompFlow(kmol/hr)
Water 4.92 25.9 25.9 25.9 25.9Ethylbenzene 0.75 6.24 6.24 6.24 6.24Styrene 0.09 1.68 1.68 1.68 1.68Hydrogen 0.12 100 100 100 100Benzene 15.1 17.1 17.1 17.1 17.1Toluene 14.3 15.2 15.2 15.2 15.2Ethylene 0.66 18.7 18.7 18.7 18.7Methane 0.28 22 22 22 22
45
Table5:PartialEquipmentSummaryUnit500StyreneOptimizedPlantHeatExchangersE-511A=585m2
1-2exchanger,fixedtubesheet,316SSQ=47.567GJ/hrShellsidepressure–200kPaTubesidepressure–145kPaPrice:$1,600,000
E-512A=832m21-2exchanger,floatinghead,316SSQ=90.4035GJ/hrShellsidepressure–600kPaTubesidepressure–160kPaPrice:$3,400,000
E-513A=889m21-2exchanger,fixedtubesheet,CarbonSteelQ=38.6456GJ/hrShellsidepressure–4200kPaTubesidepressure–125kPaPrice:$1,220,000
E-514A=682m21-2exchanger,fixedtubesheet,CarbonSteelQ=36.5574GJ/hrShellsidepressure–1100kPaTubesidepressure–110kPaPrice:$1,540,000
E-515 A=603m21-2exchanger,fixedtubesheet,CarbonSteelQ=424.653GJ/hrShellsidepressure–600kPaTubesidepressure–95kPaPrice:$900,000
E-516A=880m21-2exchanger,fixedtubesheet,CarbonSteelShellsidepressure–200kPaTubesidepressure–135kPaPrice:$2,850,000
E-517A=306m21-2exchanger,floatinghead,CarbonSteelShellsidepressure–200kPaTubesidepressure–35kPaPrice:$308,000
E-518A=356m21-2exchanger,fixedtubesheet,CarbonSteelShellsidepressure–600kPaTubesidepressure–55kPaPrice:$440,000
E-519A=786m21-2exchanger,floatinghead,CarbonSteelShellsidepressure–200kPaTubesidepressure–25kPaPrice:$530,000
E-520A=756m21-2exchanger,floatinghead,CarbonSteelShellsidepressure–600kPaTubesidepressure–55kPaPrice:$850,000
E-521A=117m21-2exchanger,floatinghead,CarbonSteelShellsidepressure–200kPaTubesidepressure–105kPaPrice:$220,000
ReactorsR-511316stainlesssteelpackedbedVoidfraction=0.5Volume=126m3Price:$20,300,000
46
Table5:PartialEquipmentSummaryUnit500StyreneOptimizedPlantCont.FiredHeaterH-511Fireheater–refractorylined,stainlesssteeltubesrequiredheatload=124.25GJ/hr80%thermalefficiencymaximumpressureratingof600kPaPrice:$13,000,000
VesselsV-511CarbonSteelMaximumoperatingpressure=200kPaVerticalHeight=7.44mDiameter=2.48mVolume=36m3Price:$310,000
V-512CarbonSteelHorizontalL/D=3V=13.2m3
Price:$92,000
V-513CarbonSteelHorizontalL/D=3V=51.8m3
Price:$218,000
TowersT-511CarbonSteel38SieveTrays65%efficienttraysFeedontray60.5metertrayspacingcolumnheight=22mdiameter=4.53mmaximumpressureratingof100kPaPrice:$4,100,000
T-512CarbonSteel122SieveTrays65%efficienttraysFeedontray310.4metertrayspacingcolumnheight=53mdiameter=8.37mmaximumpressureratingof100kPaPrice:$68,000,000
47
Table5:PartialEquipmentSummaryUnit500StyreneOptimizedPlantCont.CompressorsandDrivesC-511CarbonSteelActualW=104kW76%adiabaticefficiencyPrice:$280,000
C-512A-CCarbonSteelActualW=2,182kW76%adiabaticefficiencyPrice:$9,850,000
C-513CarbonSteelActualW=248kW76%adiabaticefficiencyPrice:$614,000
C-514CarbonSteelActualW=210kW76%adiabaticefficiencyPrice:$530,000
D-511A/BElectric/ExplosionProofActualW=116kW90%efficiencyPrice:$310,000
D-512A-C/D-FElectric/ExplosionProofActualW=2,424kW90%efficiencyPrice:$3,530,000
D-513A/BElectric/ExplosionProofActualW=276kW90%efficiencyPrice:$525,000
D-514A/BElectric/ExplosionProofActualW=233kW90%efficiencyPrice:$621,000
PumpsP-511A/BCarbonsteel–centrifugalActualPower=5.74kWEfficiency70%ElectricDrivePrice:$70,500
P-512A/BCarbonsteel–centrifugalActualPower=1kWEfficiency70%ElectricDrivePrice:$47,000
P-513A/BCarbonsteel–centrifugalActualPower=1kWEfficiency70%ElectricDrivePrice:$47,000
P-514A/BCarbonsteel–centrifugalActualPower=1kWEfficiency70%ElectricDrivePrice:$47,000
P-515A/BCarbonsteel–centrifugalActualPower=1kWEfficiency70%ElectricDrivePrice:$47,000
P-516A/BCarbonsteel–centrifugalActualPower=5.34kWEfficiency70%ElectricDrivePrice:$257,000
48
Table6:UtilitySummaryforUnit500E-512 E-513 E-514 E-515 E-516lps bfwàhps bfwàmps bfwàlps cw
148,000kg/hr 16,000kg/hr 13,000kg/hr 4,030kg/hr 10,400,000kg/hr
E-517 E-518 E-519 E-520 E-521lpsàbfw cw lpsàbfw cw cw
16,600kg/hr 645,000kg/hr 42,700kg/hr 2,170,000kg/hr 20,600kg/hr
IllustratedaboveinFigure2isaprocessflowdiagramoftheoptimized
processfollowedbystreamtablesinTable4,apartialequipmentsummaryinTable
5,andutilitysummaryinTable6.Asstatedintheintroduction,theoptimizationof
thestyreneproductionprocessbeganbyperforminganeconomicsensitivity
analysisonthebasecaseasillustratedbelowinFigure3.
Figure3:SensitivityAnalysisofUnit500StyreneProcess
-$900
-$800
-$700
-$600
-$500
-$400
-$300
-30% -20% -10% 0% 10% 20% 30%
NetPresentValue
(millionsofdollars)
Changeofcomponent
SensitivityAnalysis
RawMaterialsUtilities
OperatingLaborFCI
Revenue
Note:UtilitiesandFCIoverlap
49
ThedatashowninFigure3indicateshowchangesinthelistedareasaffecttheNPV.
Optimizationbeganbyaddressingthemostsensitiveareasfirst.Sincetheplanthas
arequiredyearlyproductionof100,000tonnes/yrofstyrene,therevenuefrom
styrenecannotchange.AsillustratedinFigure3,theareamostsensitivetochanges
istherawmaterialcostfollowedbytheutilitycostandthefixedcapitalinvestment
cost.Inordertoaddressthesensitivityoftheprocesstochangesinrawmaterials,
webeganoptimizationbyfirstfocusingonthereactorandoptimizingforincreased
yieldofethylbenzenetostyrene.Followingthereactoroptimization,weoptimized
theutilitiesfortheprocess.Utilitycostwasthesecondmostsensitivetochangeas
seeninFigure3.Wereducedtheutilitycostbyusingthehotprocessstreamfrom
thereactortoheatthelow-pressuresteambeforeitenteredthefiredheater.
Finally,weanalyzedthepossibilityofchangingsomeoftheprocessequipment’s
materialofconstructioninordertotryandreducetheFCI.Theseparationsection
wasallstainlesssteelinthebasecase,andresearchonconstructionmaterials
showedthatcarbonsteelisanappropriateandlessexpensivealternativeto
stainlesssteelforthisapplication.ThismaterialchangereducedtheFCI.
WeusedtheSimSciPRO/IIsimulationsoftwaretomodelourprocesstoaid
optimization.ThefirststepinsettingupaPRO/IIsimulationisselectingan
appropriatethermodynamicmodel.WeusedFigure4toanalyzeourprocessand
arriveatasuitablemodel.
50
Figure4:GuidelinesforSelectionofThermodynamicPackageinPro/II
Inthestyreneproductionprocessnopolarorhydrogenbondingispresent;
hydrocarbonswithgreaterthanfivecarbonsexistintheprocess;hydrogenis
presentintheprocess;andthetemperaturefortheprocessisgreaterthan250K.
TheseconditionsledtothechoiceoftheSRKSIMSCIthermodynamicpackage.The
SRKpackageusestheSoave-Redlich-Kwongequationofstatetomake
thermodynamiccalculations.Allofprocessonthefrontendfeedsectionusesthe
SRKpackageforoneliquidphase.TheheatexchangerE-516andtheflashvesselV-
511usethetwoliquidphaseSRKpackage.AfterbeingcompressedinC-512,Stream
34hasaliquidorganicphaseandaliquidwaterphase.Therefore,thetwoliquid
phaseSRKpackageismostappropriateforE-516andV-511.Itisimportanttonote
thattheseparationofethylbenzeneandstyreneissomewhatdifficulttomodeldue
51
tothesimilaritiesbetweenthecomponents.Assuch,weappliedtheIdealpackage
toourseconddistillationcolumnandstyrenepump.
Theobjectiveofthereactoroptimizationwastodesignareactorthatgives
thegreatestyieldofethylbenzenetostyrene.Weinvestigatedtheoptimizationof
anadiabaticandisothermalplugflowreactor.Wedeterminedthattheprocess
requiredatleastfiveparallelreactorstokeepthevelocityofthesysteminapossible
rangewithouthavingchokedflow.Forthisreasontherawmaterialcostfor
ethylbenzeneincreasedfromthebasecasetotheoptimizedcase.Thebasecase
failedtotakethevelocityintoaccountthusdeliveringanimpracticalscenario.An
economicanalysisonboththeisothermalandadiabaticplugflowoptimized
reactors,illustratedinTable7,indicatedthattheadiabaticsethadthepotentialto
providegreaterprofit.Thisresultpromptedourteamtoproceedoptimizingthe
processusingtheadiabaticplugflowreactors.
Table7:EconomicAnalysisComparingIsothermalandAdiabaticReactors
Isothermal AdiabaticFeedintoReactor(kmol/hr) 7,345 8,986.5EBFeedintoReactor(kmol/hr)
442.5 541.2
StyreneProduced(kmol/hr) 120.5 120.5RecycleEthylbenzene(kmol/hr)
222.8 356.3
RecycleStyrene(kmol/hr) 1.05 1.484RecycleToluene(kmol/hr) 0.2555 0.197ExtraFiredHeater(GJ/hr) 17.5 EconomicAnalysis: StyreneProduced($M/yr) 160.5 160.5EthylbenzeneBuy($M/yr) -168 -141.3ExtraFireHeaterCost($M/yr) -1.55 NetProfit($M/yr) -7.55 19.2
52
Theobjectiveofthefeedsectiondesignwastosatisfytheoptimizedreactor’s
requiredinletconditions.OptimizationofthereactorshowedthatStream9must
haveasteamtoethylbenzeneratioof15.6,atemperatureof685°C,andapressure
of190kPa.
Intheseparationsection,weanalyzedmultipledistillationtowersand
liquid/liquid/vaporseparatorspecifications.Wedeterminedthatincreasingthe
pressureto120kPaandloweringthetemperatureto51°CinStream15reducedthe
amountofethylbenzeneandstyrenelosttothefuelgasstream.Thisallowedthe
processtorecyclemoreethylbenzeneandtotakemoreofthestyreneproducedin
thereactortoactualproduct.Furthermore,modificationstoT-511allowedusto
producea90mol%benzene/toluenestreamtobesoldtoincreaserevenue.These
modificationstoT-511includedreducingthetoptraypressureandtemperatureto
35kPaand63.4°Crespectively.Thenumberofactualtrayswasreducedto38with
atrayefficiencyof65%.Thechangesmadetotheseparationsectionincreasedthe
totalrevenuefortheprocessby$15million/yr.
Afterconcludingtheoptimizationoftheseparationsection,weintegrated
heatinordertoreduceutilitycosts.Theprocessneedsheattodrivethereaction.
Afterthereaction,theprocessstreamneedscoolinginordertoseparateand
preventthepolymerizationofstyrene.Withthisinmind,wechosetousethe
reactoreffluentinsteadhotutilitiestopreheatthelow-pressuresteamfeedandthe
combinedethylbenzenefeed.Thereactoreffluent,Stream10,heatsthelow-
pressuresteaminE-512toatemperatureof455°Cbeforeitentersthefiredheater.
ThisreducestherequireddutyandfuelgascostfortheH-511bymorethan50%.
53
Afterpreheatingthelow-pressuresteam,theprocessstreamleavingE-512,Stream
11,heatsthecombinedethylbenzenefeedstreaminE-501to350°C.Aseriesof
heatexchangersthencoolstheprocessstreamto51°Cleadingtotheseparation
section.Thesechangesinheatintegrationreducedtheutilitycostby$12
million/yr.
Wealsoinvestigatedthematerialsofconstruction.Researchonthenatureof
hydrogenembrittlementandcorrosivematerialsinmetalsshowedthatcarbonsteel
isasuitablematerialforourprocess(2)(3)(4).Hydrogenembrittlementoccurs
whenmonoatomichydrogenispresent.Thismonoatomichydrogencanseepinto
themetaloftheprocessequipmentandcreateasmallpressurepocket.Overtimeas
moreandmorehydrogensettlesinthispocket,cracksoccurwhichchallengethe
integrityoftheequipment.Stainlesssteelisresistanttohydrogenembrittlement.
MonoatomichydrogenisonlypresentinR-511forthisprocess.Therefore,R-511
materialisstainlesssteel(2)(3)(4).Figure5showsthetemperatureconstraints
forstainlesssteel,andFigure6showsthetemperaturelimitationsofcarbonsteel.In
Figure5andFigure6themaximumallowablestressisthemaximumworking
pressureofthematerial,andthispressureisafunctionoftheoperating
temperatureoftheprocessequipment.Theoperatingtemperaturesincarbonsteel
vesselsneedstoremainbelow400°Cinordertomaintainintegrityasseenin
Figure6.E-511andE-512,whichoperateattemperaturesof455°Cand656°C
respectively,needstainlesssteelconstructionwhichmaintainsstressintegritywith
operatingtemperaturesuptoalmost700°CasseeninFigure5.SinceT-511andT-
512havenomonoatomichydrogenpresentandtheoperatingtemperaturesare
54
relativelylow,carbonsteelisagoodchoiceofconstructionmaterialinsteadof
titanium.Wechosethesematerialsinsteadoftitaniumbecausetheincreased
corrosionresistanceisunnecessaryinthiscase.Thechangesinmaterialsof
constructionreducedtheFCIby$117million.
Figure5:MaximumAllowableStressforStainlessSteel(3)
55
Figure6:MaximumAllowableStressforCarbonSteel(3)
Thecalculationforthetotalcostofmanufacturing(COM)without
depreciationis:
𝐶𝑂𝑀 = 0.18𝐹𝐶𝐼 + 2.73𝐶!" + 1.23(𝑈𝑡 + 𝑅𝑀 +𝑊𝑇) (5)
Table8showsthecomponentsincludedintheCOMcalculation.Table9showsa
summaryofcomponentsfortheFCI,andTable10showstheutilitycostbytypefor
ourplant.
Table8:CostofManufacturingSummaryforOptimizedUnit500
Component Cost($M)RawMaterials 132.5WasteWater 0.07Utilities 56.5
FixedCapitalInvestment 135.5OperatingLabor 0.89
CostofManufacturing 259
56
Table7givesadescriptionoftotalcostofmanufacturingfortheoptimizedstyrene
process.Thecostofmanufacturingtakesintoaccountthefixedcapitalinvestment
fortheprocessalongwithanyrecurringcostsfortheprocess.Althoughthefixed
capitalinvestmentfortheprocessisnotarecurringcost,therawmaterials,
wastewatertreatment,utilities,andoperatinglaborareallrecurringyearlycosts.
Thewastewatertreatmentandoperatinglaborcostdidnotchangefromthebase
casetotheoptimizedcasefortheplant.Therawmaterialcostincreasedwhilethe
utilitycostandthefixedcapitalinvestmentdecreased.Theoverallcostof
manufacturing,asseeninTable8,is$259million.ThisCOMisan$18million
decreasefromthebasecase.
Table9:SummaryofFixedCapitalInvestmentforOptimizedUnit500
Unit Price($K)HeatExchangers $13,800
Pumps 312Reactors 20,300Towers 71,600Vessels 617
Compressors 11,500Drives 4,990
FiredHeater 12,700Total $135,500
FromTable9itisapparentthatthedistillationtowersandreactormakeupmake
upalargeportion,67%,ofthefixedcapitalinvestment.Heatexchangers,
compressors,andthefiredheatercontribute28%tothetotalFCI.Pumps,vessels,
anddrivesforthecompressorscontributetotherestofFCI.Optimizationofthe
towersandchangesintheconstructionmaterialsofsomeoftheplantequipment
57
gaveanFCIof$135.5asseeninTable9.Thisisan$117milliondecreasefromthe
basecase.
Table10:UtilityCostbyTypeforOptimizedUnit500Utility Electric
Power(kW)
HighPressureSteam(kg/hr)
MediumPressureSteam(kg/hr)
LowPressureSteam(kg/hr)
CoolingWater(kg/hr)
FuelGas(GJ/hr)
BoilerFeedWater(kg/hr)
Totals 7,910 -16,040 -13,075 203,000 13,200,000 124.25 -26,200TotalYearlyCost
($K/yr)
311 (3,850) (3,095) 47,600 1,560 11,000 (514)
Total $M56.5Table10givesasummaryofthetotalutilitycostforUnit500.Drasticreductionof
thefuelgascostduetoheatintegrationdecreasedtheutilitycostfortheplant.
Optimizationofthereactoralongwithchangesinthefeedsectionsetupallowedus
toreducethetotalamountoflow-pressuresteamneededfortheprocess.This
helpedtoreducetheutilitycostaswell.Thetotaloptimizedplantutilitycostof
$56.5million/yrisa$12million/yrdecreasefromthebasecase.
58
ProcessDescription:
Fresh98mol%ethylbenzenewith1mol%Benzeneand1mol%toluene,
Stream1,combineswithrecycledethylbenzene,inStream29,asStream2.Aheat
exchanger,E-511,heatsStream2from107°Cand200kPato350°Cand180kPa
usingthereactoreffluentfromE-512.Theheatedstream,Stream3,iscompressed
viaacompressorC-511to352°Cand200kPa.Low-pressuresteamisfedtothe
processasStream4andheatedfrom160°Cand600kPato455°Cand585kPaina
heatexchanger,E-512,bythehotreactoreffluent,Stream10.Thesteamleaving
E-512,Stream30,isfurtherheatedinafiredheater,H-511,to830°Cand550
kPa.SuperheatedsteamexitingH-511,Stream5,isfedtoavalve.Stream31exits
thevalveat829°Cand229kPaandcombineswithStream32,thestreamleavingC-
511.Theresultingvapormixture,Stream9,isfedtofiveparalleladiabaticplugflow
reactors,R-511A-E,at685°Cand190kPa.Theethylbenzenefedtothereactor
reactscatalyticallyaccordingtothefollowingreactions:
C6H5C2H5↔C6H5C2H3+H2(1)
EthylbenzeneStyreneHydrogen
C6H5C2H5→C6H6+C2H4(2)
EthylbenzeneBenzeneMethane
C6H5C2H5+H2→C6H5CH3+CH4(3)
EthylbenzeneHydrogenTolueneMethane
Thereactoreffluent,Stream10,exitsat653°Cand160kPaandisusedto
heatthelow-pressuresteaminE-512.TheprocessstreamexitingE-512,Stream11,
59
isfedtoE-511at465°Cand145kPa.Afterbeingcooled,Stream12exitsE-511at
361°Cand125kPaandissentthroughaseriesofheatexchangers.Thefirstheat
exchanger,E-513,coolsStream12to270°Cand110kPabyvaporizingboilerfeed
watertoproducehigh-pressuresteam.ThecooledstreamexitingE-513,Stream13,
entersasecondheatexchanger,E-514.BoilerfeedwaterinE-514coolsStream13
to194°Cand95kPaandcreatesmediumpressuresteam.ThestreamexitingE-514,
Stream33,isfedtoathirdheatexchanger,E-515,wheretheproductstreamis
cooledto170°Cand80kPausingboilerfeedwatertocreatelowpressure
steam.Theresultingstream,Stream14,iscompressedtoapressureof135kPaand
atemperature216°Cinacompressor,C-512.ThestreamexitingC-512,Stream34,
entersanotherheatexchanger,E-516,whichcoolsthestreamto51°Cand120kPa
usingcoolingwater.ThestreamexitingE-516,Stream15,isfedtoa
liquid/liquid/vaporseparator,V-511.ThewaterrichstreamleavingV-511is
pumped,viaP-511,toapressureof200kPa,andissentoutoftheprocessto
treatmentaswaste.
TheorganicliquidstreamleavingV-511entersavalveatatemperatureof
51andapressureof105kPa.Thestreamexitingthevalve,Stream20,isfedtoa
distillationcolumn,T-511,atapressureof65kPaandatemperatureof51°C.T-511
contains38actualsievetraysandoperateswithatoptraypressureof35kPaanda
bottomtraypressureof55kPa.Theoverheadnoncondensablevaporstreamfrom
thecolumnmixeswithStream16,thevaporstreamfromV-511.Theresulting
stream,Stream36,iscompressedfrom35kPaand55°Cto105kPaand120°Cina
compressor,C-513.ThestreamexitingC-513,Stream37,issenttoaheat
60
exchanger,E-521,whereitisheatedto55°Cand90kPa.Theresultingstream,
Stream38,issenttoasecondcompressor,C-514,whereitiscompressedto240kPa
and169°Candissoldasfuelgas.TheoverheadvaporstreamfromT-511is
condensedusingcoolingwaterin,E-518,andthecondensateiscollectedinthe
refluxdrum,V-512.TheliquidstreamleavingT-511isfedtoarefluxpump,P-512,
whereitissplitintotwoseparatestreams.Oneportion,Stream21,isfedtothe
pump,P-514,andissoldasa90mol%purebenzene/toluenemixture.Thesecond
portionisreturnedtothecolumntoprovidereflux.
Stream22,thebottomsproductfromT-511,contains99.5%ofthe
ethylbenzenefedtothecolumnandissenttoadistillationcolumn,T-512,at116°C
and55kPa.T-512contains122realsievetraysandoperateswithatoptray
pressure25kPaandabottomtraypressureof55kPa.Theoverheadvaporstream
fromthecolumn,whichcontains99%oftheethylbenzenefedtothecolumn,is
condensedusingcoolingwaterinE-520.Thecondensateiscollectedinareflux
drum,V-513.ThestreamleavingV-513issplitintotwoseparatestreams.Oneofthe
streams,Stream23isfedtoapump,P-516.ThestreamexitingP-516,Stream29,is
senttothefeedsectionasarecyclestreamat93°Cand210kPaandismixedwith
theethylbenzeneinStream1.Thesecondstreamisreturnedtothecolumn,T-512,
toprovidereflux.Stream24,thebottomsproductfromT-512,containsessentially
allofthestyrenethatwasfedtothecolumn,anditispumpedtoapressureof200
kPa,viapumpP-515.Thestreamexitingthepump,Stream27,exitstheprocessas
the99.5wt%purestyreneproduct.
61
Discussion:
Theresultsstatedthatthefiveadiabaticplugflowreactorsaretheoptimized
caseforthestyreneprocess.Thisisquitedifferentfromthebasecasewhichuses
twoplugflowreactorsinseries.ByusingtheEquation6,wefoundthatthevelocity
throughthepackedbedwasunrealisticallyhighwhenpairedwithareasonable
pressuredrop(5).
∆!!= !"#!!!
!!!!!!
(!!!)!
!!+ !.!"!!!!
!!!!
!!!!! (6)
Weobtainedtwodifferentscenariosforreactordesignsfromclass.Inorder
tooptimizeboththeisothermalreactorandtheadiabaticreactor,weranmultiple
casestudiesoncertainreactorparameterstogetanideaofwhichconditionsgave
theoptimizedcase.Withbothsetsofoptimizedreactorsinhand,weperformedan
economicanalysisofbothsystems.Ourcalculationsshowthattheadiabatic
reactorshadthepotentialtoproducemoreprofitthantheisothermalplugflow
reactors.Mostofthedifferenceinprofitcamefromthedecreasedrawmaterialcost
intheadiabaticreactorduetoincreasedrecycledethylbenzene
Afterchoosingtheadiabaticreactorsforourprocess,wedesignedasimple
feedsectionalmostidenticaltothebasecaseinordertosatisfytheoptimizedinlet
conditions(temperature,pressure,andsteamtoethylbenzeneratio).Welater
modifiedthefeedsectionviaheatintegration.ThisreducedthedutyrequiredforH-
511tosuperheatthesteam,therebyreducingutilityandFCIcosts.Afterheating
Stream4,thereactoreffluent,Stream11,heatstheethylbenzenestream,Stream2,
inE-511,removingtheneedforhighpressuresteam.Overall,theheatintegration
62
ontheprocessreducedtheutilitycostbyapproximately$12million/yrcompared
tothebasecase.
Lookingintotheseparationsection,wenoticedtheoriginalfuelgas
compressorhadacompressionratiogreaterthan3.Inordertoabidebythe
heuristics,wereplaceditwithtwocompressorswithanintercooler.Wealso
noticedthatweloseanappreciableamountofethylbenzeneandstyreneinV-511.
Weattemptedtodecreasethislossbyadjustingtheflashparameters.Casestudies
showedthatreducingthetemperatureandincreasingthepressureofStream15
decreasedtheethylbenzeneandstyrenelostinStream16.Weachievedthe
modifiedconditionsbyaddingC-512andincreasingthedutyofE-516.After
lookingattheflashconditionsweinvestigatedpurifyingthebenzene/toluene
streaminordertoincreaserevenue.Aneconomicanalysisshowedthatwecould
potentiallysellthisstreamforroughly$9million/yr.Thisledtothechangein
specificationsforT-511.WereducedthetoptraypressureandtemperatureinT-
511to35kPaand63.4°C.Thesechangeshelpedtogetabetterseparationofthe
componentsinthetower.Theincreaseinseparationproducedmorebenzeneand
tolueneinthedistillatestream.Thisallowsustosellthestreamfor50%ofthepure
benzeneandtolueneprices,anoptionthatwasnotviableinthebasecase.
Alternativesthatweexploredduringoptimizationincludedfurtherpurifying
thebenzene/toluenestream,thelocationofC-512,andheatintegration.Themain
alternativetothe90mol%benzene/toluenestreamisa99.5mol%benzenestream
thatwecansellatfullprice.Toachievethis,weneedtoimplementathird
distillationcolumnwiththeassociatedheatexchangersandvessel.Ouranalysis
63
showedthatwewouldgainroughly$900thousand/yrbyimplementingthisthird
distillationcolumn.The$900thousand/yrisamuchlowerprofitthansellingthe
90mol%benzene/toluenestreamfor$9million/yr.Therefore,wedecidedtonot
useathirddistillationcolumn.C-512isplacedbetweenE-515andE-516because
thisisthelastandcoolestpointwheretheprocessstreamisavapor.This
minimizestheworkdonebythecompressor.Weinvestigatedmultipleplacements
forC-512.Whenplacedearlierintheprocess,thedutyandutilitycostincreasefor
C-512.PreheatingStreams1andStream29isanalternativetopreheatingStream2
withthereactoreffluent.Weconcludedthatseparatingtheeffluentintotwo
separatestreamsinordertopreheatinthisfashionisnotaseconomicallyprofitable
askeepingthestreamtogether.
Ouroptimizedprocessdoeshavedesignconcerns.RefertoTable11tosee
theseconcernsandtheirrespectivejustifications.
Table11:ProcessConditionsMatrix
Equipment ReactorsandSeparators OtherEquipment
HighTemp.LowPres. Exchangers Valve
E-511
XE-512
X
R-511 X T-511
X
T-512
X V-511
X
V-512
X V-513
X
Valve1
X
64
Table11:ProcessConditionsMatrix(cont.)Unit Causefor
ConcernJustification
E-511 ΔTLM>100 LowerutilitycostthanhavingalowerΔTLME-512 ΔTLM>100 LowerutilitycostthanhavingalowerΔTLMR-511 HighTemp. Favorableequilibriumconversionforendothermicreaction.
T-511LowPressure Styrenecan'tbeabove125ºCsowemustoperateatlowTemps
andPressure.
T-512LowPressure Styrenecan'tbeabove125ºCsowemustoperateatlowTemps
andPressure.
V-511LowPressure Styrenecan'tbeabove125ºCsowemustoperateatlowTemps
andPressure.
V-512LowPressure Styrenecan'tbeabove125ºCsowemustoperateatlowTemps
andPressure.
V-513LowPressure Styrenecan'tbeabove125ºCsowemustoperateatlowTemps
andPressure.Valve1 LargeΔP Expanderdoesn'tworkduetothehighlossofthermoenergy.MixingStreams31&32
GreatlyDifferingTemperatures
Steamisneededtoprovideadrivingforceformasstransfer.
T-512 ColumnHeight HeuristicforColumnHeightof53mmax,somodifydimensions
65
Conclusion&Recommendations:
Thebasecaseaspresentedisnotpossible.Thereactorspresentedinthe
basecasewillproduceachokedflowduetotheextremelyhighpressuredrop.We
recommendfiveadiabaticreactorsinparalleltoachievetherequiredproduction
rate.Theinletstreamtothereactorneedstohavetemperatureandpressure
conditionsof685°Cand190kPawithasteamtoethylbenzeneratioof15.6.
Furthermore,usingthereactoreffluent,Stream10,topreheatthelow-pressure
steaminE-512beforeitentersH-511lowersthefuelgascostbymorethan50%.
EmployingthestreamexitingE-512,Stream11,tovaporizetheethylbenzenefeed
streaminE-511alsoreducesutilitycost.Thisheatintegrationreducesthetotal
utilitycostfromthebasecaseby$12million/yr.TheadditionofC-512andE-516
allowedustoreducethetemperatureandincreasethepressureofStream15
leadingtotheliquid/liquid/vaporseparator.Bychangingtheseconditions,weare
abletoincreasethestyreneintheproductstreamandtheethylbenzeneinthe
recyclestream.Thisreducestherawmaterialcostandtakesmoreofthestyrene
producedinthereactortoactualproduct.ModificationstoT-511ofloweringthe
toptraytemperatureandpressuregavetheprocesstheabilitytosella90mol%
benzene/toluenemixture.Theseparationsectionoptimizationincreasedthe
revenuefortheprocessby$15million/yr.Changingtheconstructionmaterialsfor
T-511andT-512fromtitaniumtocarbonsteelgreatlyreducestheFCI.Thetotal
decreasefromthebasecaseinFCIafterthesechangesis$117million.ThenewNPV
oftheoptimizedprocessis-$412million.ThisNPVgivesanEAOCof$72.9million
whichiswellbelowtheprojected$160million/yrtobuystyrene.
66
Withtheseconsiderationsinmind,werecommendfurtheroptimizationon
theprocessandamoredetailedestimateoftheNPV.Specificareasforfurther
optimizationincludetheflashconditionsofV-511,reactordesign,andcalculating
thepressuredropsacrossthedistillationcolumns.Althoughouroptimizations
savedalargeamountofstyreneandethylbenzenefrombeinglosttofuelgasinV-
511,asignificantamountofethylbenzeneisstillbeinglost.Werecommendlooking
intoV-511forabetteroptimization.Furtheroptimizationofthereactortoincrease
theyieldofethylbenzenetostyreneisalsoastrongrecommendation.
67
SafetyandEnvironmentalConcerns:
Thefirstandforemostgoalofanoptimizationprojectistodesignaprocess
thatissafeforothers,yourself,andtheenvironment.Ouroptimizedstyrene
processpresentsplantoperatorswithafewpotentiallyhazardoussituations.High
temperaturesandpressuresexistinmanyareasoftheprocessespeciallyintheheat
exchangers,reactors,andpiping.Inordertosafelyoperatethesepiecesof
equipment,correctplacementofappropriateinsulationisanecessity.Vesselsand
pipeswithhigh-pressurefluidsmustemploysafetyvalveswhereneeded.Careful
andregularmaintenanceoftheprocesscontrolsystemsisarequirementforany
safeprocessoperation.Also,thoroughtrainingofoperatorsinthesystemcontrols
andemergencyprotocolsisveryimportanttothehealthandsafetyoftheplantand
thepeopleinit.Operators,maintenancecrews,andcontractlaborneedtowearthe
appropriatepersonalprotectiveequipmentatalltimeswheninsidetheplant.
FollowingtheguidelinespresentedbyOSHA,theOccupationalSafetyandHealth
Administration,isagoodsafetypractice.
Afewenvironmentalconcernsarealsopresentinourprocess.The
wastewaterexitingtheplantcontainstracesoforganics.Beforewastewaterenters
theenvironment,treatmentandremovaloftheorganicsneedstotakeplace.The
fuelgasstreamthatisbeingsoldalsocontainssomenoncondensablegasesthat
couldbeharmfultohumansortheenvironmentwhenburned.Propercontainment
ofthesegasesandthefuelgasburnedinthefiredheaterisasignificant
environmentalsafetyconcernforthisprocess.CarefulobservingEPAregulations
forwastewaterandfuelgasisessentialtopreservingtheenvironment.Safety
68
considerationsneedcontinuedre-evaluationasfurtherdesignoptimizationstake
place
69
References:
(1)http://www.wisegeek.com/what-are-the-different-uses-of-polystyrene.html
(2)http://www.slac.stanford.edu/cgi-wrap/getdoc/slac-tn-64-047.pdf
(3)Turton,Richard.Analysis,Synthesis,andDesignofChemicalProcesses.Upper
SaddleRiver,NJ:PrenticeHallPTR,2012.Print.
(4)http://www.engineeringtoolbox.com/metal-corrosion-resistance-d_491.html
(5)MacCabe,WarrenL.,andPeterHarriot.UnitOperationsofChemicalEngineering.
5thed.NewYork:McGraw-Hill,1994.Print.
70
Appendix:
TableofContentsLocalHeatTransferCoefficients37SampleCalculations38 Conversion38 Selectivity38 Yield38 SizingVessel39 SizingHeatExchanger39 SizingDistillationColumn40 SizingPumps40 SizingCompressors41 SizingDrives41 SizingFiredHeater41CashFlowStatement42
71
LocalHeatTransferCoefficientsHeatTransferTo h(W/m2K)
LiquidOrganic 600CondensingSteam 6,000BoilingOrganic 5,000VaporOrganic 100DesuperheatingSteam 200BoilingWater 8,000CoolingWater 1,000PartiallyCondensingOrganic 3,000CondensingOrganic 1,500