A new approach to the modeling of deactivation in the conversion of methanol on zeolite catalysts.docx

A new approach to the modeling of deactivation in the conversion of methanol on zeolite catalysts.docx

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Anewapproachtothemodelingofdeactivationintheconversionofmethanolonzeolitecatalysts

Copyright©2009ElsevierLtdAllrightsreserved.

Anewapproachtothemodelingofdeactivationintheconversionofmethanolonzeolitecatalysts

TonV.W.Janssens

a,

aHaldorTopsøeA/S,Nymøllevej55,DK-2800Lyngby,Denmark

Received8January2009; 

revised18February2009; 

accepted8March2009. 

Availableonline1May2009.

Abstract

Thedeactivationofazeolitecatalystintheconversionofmethanoltohydrocarbonsisdescribedasareductionoftheeffectiveamountofcatalystwithtimeonstream.Withtheassumptionsthattheconversionofmethanolisafirst-orderreaction,andthatthelossofactivecatalystisproportionaltotheconversion,anexpressionfortheconversionwithtimeonstreamisobtained,whichdescribestheexperimentaldatawell.Thisexpressioncontainstherateconstant,thatcharacterizestheactivity,andadeactivationcoefficientthatdescribesthedeactivationbehaviorasparameters.Itisshownthatactivecatalystsshowamoresuddendecreaseinconversion,andthatthedeactivationratedeterminesthetimeatwhichthedecreaseinconversionisobserved.Iftheinitialconversioniscloseto100%,thelifetimeto50%conversiondoesnotdependontheactivity,andthedeactivationcoefficientisdirectlyderivedfromtheexperimentaldata,bydividingthemeasuredlifetimeto50%conversionbytheappliedcontacttime.Thelifetimetoallotherconversionlevelsisdependentonbothdeactivationandactivity,whichimpliesthatacatalystlifetimetobreakthroughofmethanoldoesnotscalewiththedeactivationrate.Likewise,itisshownthattheconversioncapacityisagoodcharacterizationofthedeactivation,andthiscanbereadilycalculatedastheproductofthespacevelocityofmethanol（WHSV）andthelifetimeto50%conversion.Theamountofconvertedmethanolatotherconversionlevelsdependsonthedeactivation,theactivity,andappliedcontacttime（spacevelocity）,andisthereforelessappropriatetouseasacharacterizationofthedeactivationbehavior.

Graphicalabstract

Thelifetimeto50%conversionisentirelydeterminedbythedeactivationproperties;differencesintheslopeofthedecayreflectdifferentcatalyticactivity.

Full-sizeimage（17K）

High-qualityimage（120K）

Keywords:

Conversionofmethanoltohydrocarbons;Zeolite;Methanoltogasoline;ZSM-5;Deactivation;Kineticmodel;Catalystlifetime;Conversioncapacity

ArticleOutline

1.Introduction

2.Experimental

3.Thedeactivationmodel

4.Discussion

4.1.Catalystlifetime

4.2.Methanolconversioncapacity

4.3.Finalremarks

5.Conclusions

References

1.Introduction

Themethanoltogasoline（MTG）reactionistheconversionofmethanoloverazeolite-basedcatalysttolightolefins（C2–C4）andliquidproductsintheboilingpointrangeofgasoline,whichtypicallyoccursinthetemperaturerangeof300–400 °C.IntheTIGAS（TopsøeIntegratedGASoline）process,theMTGreactioniscombinedwiththemethanolsynthesis,andinthiswayanefficientprocessconvertingsynthesisgastogasolineisobtained[1]and[2].

TheMTGreactioncanberegardedasasequentialreaction,consistingofthefollowingsteps[3]:

Methanol

DME→lightolefins→gasolineproducts

andthisisknowntooccuronavarietyofzeolites[4].TheZSM-5zeolite,however,isthemostcommonlyappliedone,sinceitappearstobesuperiorforthisreaction.TheMTGreactionoveraZSM-5-basedcatalystis,likemanyotherhydrocarbonreactionsoverzeolitecatalysts,alwaysaccompaniedbycokeformation,whichleadstodeactivationofthecatalyst.Thedeactivationbycokeformationoccursthroughblockingtheaccesstotheactiveacidsites,eitherbydepositionofcarbonaceouscompoundsdirectlyontheacidsitesitselfandinthemicroporechannelsofthezeolite（internalcoke）,orbyblockingtheentrancetothemicropores,therebypreventingthediffusionofmethanolmoleculesintothezeolitestructure（externalcoke）[5],[6],[7],[8],[9],[10]and[11].Lossofactivityduetothecokeformationis,inprinciple,reversible,andthecatalyticactivitycanberestoredbyaregenerationthatremovesthedepositedcoke.Acommonprocedureistoburnoffthecokewithoxygenat500–600 °C[7],[8],[12]and[13].Regeneratedcatalystsoftenshowasomewhatloweractivitythanfreshones,possiblyduetodealuminationofthezeolite,whichtypicallyoccursatelevatedtemperatures（>500 °C）inthepresenceofwater.

Asaconsequenceofthedeactivation,thetotalamountofmethanolthatcanbeconvertedoveraZSM-5catalystislimited,anditisthereforeimportanttoknowthedeactivationbehaviorofthecatalyst.Tocharacterizecatalystdeactivation,itisrequiredtodescribehowthecatalyticactivitydecreasesduringthecatalystlifetime.Differentapproachesforsuchadescriptionhavebeendeveloped.Theapproachoftenencounteredintheliteratureisbasedonadescriptionofcarbonformationasafunctionofcontacttime,e.g.bytheempiricalVoorhiesequation[11],[14]and[15].Thecatalystactivityisthenrelatedtothecarboncontentinthecatalyst,using（semi-）empiricalrelations,resultinginthedescriptionoftheactivityasafunctionoftime[15],[16],[17],[18]and[19].Thepercolationmodeldescribesthecokeformationinthezeolitechannelsandtheaccessibilityoftheactivesitesinmoredetail,fromwhichthentheactivityisderived[20],[21],[22]and[23].Clearly,intheseapproaches,catalystdeactivationisdirectlyrelatedtotheformationofcoke,anddeactivationbydealuminationisnotincludedassuch.Theadvantageofthesemodelsisthatthecatalystdeactivationisrelatedtothechemistryofcarbonformationinthezeolitechannels,whichishelpfulinthedesignofzeolitestructureswithastableactivityforMTG.

Anotherapproachistodescribethechangeincatalyticactivitywithtime,andincorporatethisinakineticmodel.Thisyieldsaphenomenologicaldescription,andisindependentofthecauseofdeactivation.Usually,theactivationisdefinedbytheratiooftherateexpressionafteragiventimeonstreamandthatforthefreshcatalyst,whichresultsessentiallyinatime-dependentrateconstant[24].Thedeactivationoftendependsonparameters,suchasconversion,currentactivity,orcertainproductconcentrations,andistypicallydescribedbysimilarexpressionsasareactionrate,basedonrateconstantandanactivationenergyfordeactivation.Thedeactivationisthencharacterizedbyoneormorerateconstantsandactivationenergies[25],[26],[27],[28]and[29].

Inthisarticle,itwillbeshownthatcatalystdeactivationinMTGalsocanbedescribedasalossoftheeffectiveamountofactivecatalystinthereactorduringcatalystoperation,insteadofadecreaseintherateconstant,alsoresultinginaphenomenologicaldescriptionofthedeactivation.Therateatwhichtheeffectiveamountofactivecatalystdecreasesisthenadirectmeasureforthecatalystdeactivation.Thisapproachissimilartotheconceptofaneffectivespacevelocity,whichwasproposedpreviouslybyDahlandKolboe[30]and[31]andSapre[32],asareducedamountofactivecatalystresultsinaneffectivelylowercontacttimeorhigherspacevelocity.

Tomeasurethecatalystdeactivationexperimentally,theconversionwithtimeonstreamisfollowedunderconstantprocessconditions.Thedeactivationrateisthenderivedfromamodelthatdescribestheobserveddecreaseinconversionintermsofalossofactivecatalyst.Inthisarticle,experimentaldataforafewselectedZSM-5zeolites,showingdifferentdeactivationbehavior,arepresented.Thedeactivationratesdeterminedinthiswaycan,inprinciple,beusedforfurtherstudyoftheinfluenceofcatalystproperties,e.g.acidity,particlesize,orofprocessconditions,e.g.temperature,pressure,feedcomposition,onthecatalystdeactivation.Thescopeofthepresentarticle,however,isconfinedtothederivationofadeactivationmodel,andtheproceduresthatcanbeusedtoextractinformationonthedeactivationbehaviorfromtheexperimentaldata,withtheaimtodevelopamethodforthecharacterizationofcatalystdeactivation.

Themodelthatisderivedhereisbasedontheassumptionsthattheconversionofmethanolisafirst-orderreaction,andthatthedeactivationrateisproportionaltothetotalconversionofmethanolandDME.Therateconstantandthedeactivationrateappearastheparameterstobedetermined.Sincethedeactivationisregardedasareductionoftheamountofactivecatalyst,orofthenumberactivesites,ratherthanareductionoftheintrinsicactivityofanactivesite,therateconstantdoesnotchangewithtimeinthemodel.Althoughthisseemstobeaseveresimplification,itturnsoutthatitdescribestheobservedconversionofmethanolwithtimeonstreamwell,and,inaddition,asurprisinglyeasymethodtoextractthedeactivationratefromtheexperimentaldataisfound.Theexperimentallydetermineddeactivationratesandrateconstantsareusedtocalculatethecatalystlifetimeandtheconversioncapacity.Themodelgivesaclearinsightonhowthecatalyticactivity,thespacevelocity,andthedeactivationbehavioraffectthemeasuredconversions,thecatalystlifetime,andtheconversioncapacityofthecatalyst.

2.Experimental

Tomeasurethedeactivation,thetotalconversionofmethanoltohydrocarbons,i.e.thehydrocarbonyieldcalculatedonaC-atombasis,ismeasuredasafunctionoftime.ThoughitdoesnotincludetheconversionofmethanoltoDME,thisisreferredtoas“conversion”throughoutthispaper;methanolandDMEareassumedtobeinequilibriumandtreat