Optimization of Fluidized Bed Isothermal Reactor in a ...

University of Mississippi University of Mississippi

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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

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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

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OptimizationofaFluidizedBedIsothermalReactorina

StyreneProductionProcess

MattPeasterCommittee:Dr.AdamSmith___________________________________________________

Date______________________________________________

Dr.JohnO’Haver__________________________________________________

Date______________________________________________

Mr.DavidCarroll__________________________________________________

Date______________________________________________

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Acknowledgements

Iwanttoacknowledgeafewpeoplewhowereinstrumentalinthecompletionofthisthesis.First,IwanttothankDr.AdamSmithwhoplayedamajorroleintrainingmetobecomeachemicalengineer.WithoutAdamIwouldnotbean

engineerandwouldnothavecompletedthisthesis.Heprovidedinvaluableadviceandguidanceduringthisentireprocessandwasalwaysavailableformyquestionsthroughoutmyentirecollegecareer.Secondly,Iwanttothankbothofmyparentsforsupportingandprayingformeformyentirelifeinallmyacademicendeavors.Myfather,especially,deservesacknowledgementforinstillinginmealoveand

desireforlearningforitsownsakeandforGod’sglory.Finally,IwanttopraiseGodforhisgraceandmercytomethroughoutthisthesiswritingprocess,andIwantto

acknowledgethatHedeservesallthegloryforitscompletion.

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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%.

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Contents:

I. IntroductiontoEngineeringOptimizationandDesign…………………………..3

II. IntroductiontoChemicalProcessSimulationSoftware………………..……….7

III. SummaryoftheOptimizationoftheUnit500StyreneProduction

Process……………………………………………………………………………………………...11

IV. FluidizedBedIsothermalReactorOptimization…………………………………..15

V. References…………………………………………………………………………………………24

VI. Appendix………………………………………………………………………………………...…25

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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

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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

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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

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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

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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

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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

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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

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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




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



PumpsP-511A/BCarbonsteel–centrifugalActualPower=5.74kWEfficiency70%ElectricDrivePrice:$70,500

P-512A/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.


andPressure.


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

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