IndustrialCatalyticProcesses forFineandSpecialtyChemicals
Editedby
SunilS.Joshi
CSIR-NationalChemicalLaboratory,Pune,India
VivekV.Ranade
CSIR-NationalChemicalLaboratory,Pune,India
KannanSrinivasan CSIR-CentralSaltandMarineChemicalsResearchInstitute,Councilof ScientificandIndustrialResearch,Bhavnagar,India
A.Venugopal IndianInstituteofChemicalTechnology,Hyderabad,India
P.Unnikrishnan CatalysisDivision,CSIR-NationalChemicalLaboratory,Pune,India MayukhG.Warawdekar FineResearchandDevelopmentCentrePvt.Ltd.,Mumbai,India
J.Yadav IndianInstituteofChemicalTechnology,Hyderabad,India
Notations
Abbreviations
BCRbubblecolumnreactor
CFDcomputationalfluiddynamics
COFscovalentorganicframeworks
CSTRcontinuousstirredtankreactor
CVDchemicalvapordeposition
DPdepositionprecipitation
GLgas-liquid
GLSgas-liquid-solid
HTshydrotalcites
LSliquid-solid
MOFsmetalorganicframeworks
O/Woilinwater
PAMAMpolyamidoamine
PVDphysicalvapordeposition
SMSIstrongmetalsupportinteraction
STRstirredtankreactor
TOFturnoverfrequency
TONturnovernumber
TS-1titaniumsilicalite-1
W/Owaterinoil
ZIFzeoliticimidazoleframework
Notations
a distance
a phasefraction
aB interfacialarea,m2/m3
ap surfaceareaoftheparticle,m2/m3 A reactant A oritsconcentration/pre-exponentialfactor/Arrheniusconstant
AE externalsurfaceareaofacatalyst
Nu Nusseltnumber
NA Avogadronumber
ne numberofeddiesperunitvolume
ni numberofbubblesperunitvolume
p reactionorder/pressure,Pa
P product P oritsconcentration/probabilityfactororstericfactor/power
Pc probabilityofcollision
PH2 partialpressureshydrogen
PO saturationvaporpressure/powernumber
Ps effectivesolidpressure
PrPrandtlnumber
Q product Q oritsconcentration/flowrate,m3/s/molarconcentrationsofthe speciesinsolution/impellerdischargeflow
q* relativediffusivityfactor
Qa0 internalheatofadsorption
qie collisionrateofbubbleswithturbulenteddies
r rateofreaction,subscriptindicatingspecificreaction/product/radiusofthe particle,m
R radius(m)/rateofreaction/universalgasconstant
RA reactionrateconstantforformation/consumptionofA
Re Reynoldsnumber
Reb Reynoldsnumberforbubble
Rep particleReynoldsnumber
s factorforimpeller/surfacetension
S sourceterms/degreeofsupersaturation
S0 initialstickingprobability
Sct turbulentSchmidtnumber
Sie collisioncross-sectionalarea
Sh Sherwoodnumber
Sc Schmidtnumber
Sq stickingcoefficient
St totalsurfacearea
T localgranulartemperaturetankdiameter,mandtemperature, K/temperature
td timefordiffusion
tk turbulenttimescale
tO inductionperiod
U, U, uG, uL superficialvelocity,m/s
upt particleterminalvelocity(m/s)
V, V0 bubblevolume,m3
Preface
Fineandspecialtychemicalsareessentialforeverythingwedoinourdailylives.These chemicalscatertoseveralkeyapplicationsrequiredformaintainingandenhancingourquality oflife,andwillbecomeincreasinglyimportant.Thefineandspecialtychemicalssectoris facingmanychallengestodayforvarietyofreasons,suchasfragmentedcapacity,relatively lowcapitalandtechnologyintensity,fastererosionofmarginsduetocommoditizationof products,therisingcostsofrawmaterialsandenergy,andstricterenvironmentalregulations. Thesechallengesalsooffernewopportunitiestoinnovateandcreateacompetitiveedge. Catalysisandcatalyticprocessesarethekeysfordevelopinggloballycompetitiveand environmentallybenignmethodsofconvertingnaturalresourcesintofineandspeciality chemicals.Replacementofthestoichiometricreactionsbythecatalyticreactions,development andimplementationofnewcatalystsystemsandtechnologiestomaketheprocesses environmentallyfriendly,energyefficiencyandbeinggloballycompetitivearetheneedsof thehour.
Withthisbackground,wehavestartedalargeandambitiousprogramentitledIndusMagic (anacronymforinnovate,developandup-scalemodular,agile,intensifiedandcontinuous processes;see www.indusmagic.org formoreinformation).CSIR-NationalChemical Laboratory(NCL),whichisapremierresearchlaboratoryintheareaofchemicalandallied sciencesinIndia,isthenodallaboratoryforexecutingtheIndusMagicprogram.CSIR-NCL interactscloselywiththechemicalindustryinIndiaandabroadanddevelopsknowledgebases andintellectualpropertytoaddressrelevantproblemsofthisindustry.AspartoftheIndus Magicprogram,weworkcloselywiththefineandspecialtychemicalssectortoidentify industryneeds.Theindustrialcatalysisandcatalyticprocesseswasidentifiedasoneofthekey needsandwasincorporatedasoneofthemajorsub-programsofIndusMagic.We organizedaworkshoponindustrialcatalysisandcatalyticprocessesaspartofthiswork (see http://induscap.ncl.res.in formoreinformation).Theworkshopbroughttogetherseveral expertsonindustrialcatalysisfromresearchinstitutes,academia,andindustry.Thisbook essentiallyoriginatedfromtheIndusCapworkshop.
Catalysts(homogeneousorheterogeneous)reducetheactivationenergybarrierfor transformationsandfacilitatebettercontrolonselectivity.Therefore,thedevelopmentand
CatalysisandCatalyticProcesses
V.V.Ranade,S.S.Joshi CSIR-NationalChemicalLaboratory,Pune,India
1.1Introduction
Chemicalandalliedindustriesmanufactureproductsthatareessentialforcreatingand sustainingmodernsocieties.Thechemical(andbiological)transformationsnecessaryto maketheseessentialproductsofteninvolvetheuseofcatalysts.Thecatalyst(whichcanbe eitherhomogeneousorheterogeneous)providesareducedactivationenergybarrierto transformationsandfacilitatesbettercontrolonselectivity.Thedevelopmentandselection oftherightcatalyst,therefore,canmakeasubstantialimpactonprocessviabilityand economics.Besidestherightcatalyst,itisessentialtodeveloptherightreactortypeandprocess intensificationstrategiesforeffectivetranslationofthelaboratoryprocesstopractice.
Withstrictenvironmentalregulations,risingrawmaterialprices,depletingfeedstocks,andacall forgreenchemistryasdrivingforces,thechemicalindustryfacesalargerchallengewithboth opportunitiesandrisks.Catalysisisofparamountimportanceinthechemicalindustryduetoits directinvolvementintheproductionof80%ofindustriallyimportantchemicals.Catalystsare involvedinmorethan $10trillioningoodsandservicesoftheglobalgrossdomesticproduct (GDP)annually.Itisestimatedthattheglobaldemandoncatalystsismorethan $30billion,anda veryrobustgrowthisprojectedinthefuture.Thereisanurgentneedtodevelopcost-effective andenvironmentallybenignmethodsofconvertingnaturalresourcesintofineandspecialty chemicalsusinghighlyefficientcatalystsandemployingcleanermethodologies.The advancementsincatalysisandapplicationstothechemicalindustryareverysignificantandare responsibleforcleanerprocesses.Replacementofthestoichiometricreactionsbycatalytic reactionsandapplicationofnewcatalystsystemsandtechnologiestomaketheprocesses environmentallyfriendly,energyefficient,andgloballycompetitivearecurrentneeds.
Acatalystisasubstancethatprovidesanalternativerouteofreactionwheretheactivation energyislowered.Catalystsdon’taffectthechemicalequilibriumassociatedwithareaction; theymerelychangetheratesofreactions.Catalystsareclassifiedinavarietyofdifferent ways.Thecommonlyusedclassificationbyreactionengineersisbasedonnumberof phases,suchas
http://dx.doi.org/10.1016/B978-0-12-801457-8.00001-X
• homogenouscatalysis(catalystandsubstrateinsamephase)or
• heterogeneouscatalysis(solidcatalystandsubstrateisagasand/orliquid)
Basicconceptsofcatalysisarebrieflyintroducedinthefollowingsection. Itisimportanttocombinetheunderstandingofcatalysiswithkeyreactionengineeringexpertise totranslatethepotentialofacatalystintheformofapracticallyimplementedcatalyticprocessor plant.Anycatalyticreactorhastocarryoutseveralfunctionslikebringingreactantsintointimate contactwiththeactivesitesonacatalyst(toallowchemicalreactionstooccur),providingan appropriateenvironment(temperatureandconcentrationfields)foradequatetime,andallowing forremovalofproducts.Areactorengineerhastoensurethattheevolvedreactorhardwareand operatingprotocolsatisfyvariousprocessdemandswithoutcompromisingsafety,the environment,andeconomics.Naturally,successfulreactorengineeringrequiresbringingtogether betterchemistry[thermodynamics,catalysis(replacereagent-basedprocesses),improved solvents(supercriticalmedia,ionicliquids),improvedatomefficiency,wasteprevention—leave nowastetotreat]andbetterengineering(fluiddynamics,mixingandheatandmasstransfer,new waysofprocessintensification,computationalmodels,andreal-timeprocessmonitoringand control).Someoftheseaspectsarebrieflydiscussedin Section1.3.Organizationofthisbookis outlinedinthelastsectionofthechapter.
1.2CatalystsandCatalyticReactions
Theword catalyst wasfirstusedin1835.Overtheyears,ithasbeenestablishedthata catalystinfluenceskineticsofaprocesswithoutundergoinganychangeitself.Acatalyst doesnotalterthethermodynamicsofareaction.Insimplewords,acatalystalterstheroute withoutalteringthedestination(see Fig.1.1).Heretherouteisthemetaphorforactivation energy—minimumenergyinputforachemicalsystemtoundergoachemicalreactionand thetransitionstateofachemicalreaction.
Fig.1.1
Acatalystallowsreactiontoproceedthroughanalternativepath.
Thetermsthatareoftenusedinthecontextofcatalyticactivityareturnovernumber(TON),to definetheproductivityofacatalyst,andturnoverfrequency(TOF),todefinethecatalyst activityorTONperunittime.
TheTOFisdefinedintermsofactivecatalyticcenters,suchas
TOF ¼ volumetricrateofreaction numberofcenters=volume ¼ moles volume time volume moles ¼ time 1
TOFmaybeinarangeof10 2 to102 forindustrialapplications.
TheTONisdefinedasameasureofcapacityofthecatalystforacceleratingthereactionsuchas
TON ¼ TOF Lifetimeofacatalyst
TypicallyTONisintherangeof106 to107 forindustrialapplications.
Theroleofacatalystbecomesevenmoreimportantwhenmultiplereactionsare thermodynamicallyfeasible.Insuchcases,anappropriatecatalystmanipulatesthereaction ratesinsuchawaythatselectivitytowardadesiredproductincreases.Severalfactorsand parametersinfluencetheoverallperformanceofacatalyst.Theselectionofacatalystfor anindustrialprocessthereforedependsontheroleitissupposedtoplay.Theeffectofacatalyston kineticsofthereactionneedstobeunderstoodindetailtogetaninsightaboutthesurface chemistryinvolvedthatwouldhelpinthedesignofaspecificcatalysis.Itisthereforeimportantto understandtheelementarystepsthroughwhichacatalystinfluencesoverallperformance.
Homogeneouscatalyststypicallyformacomplexwithoneofthereactants,whicheventually transformsitintotheproductafterinteractingwithotherreactants.Theprocessisessentially similartohomogeneousreactionsintheabsenceofacatalystandisoftencontrolledby mixingreactantsandacatalystspeciesonamolecularlevel.Incontrasttothis,ina heterogeneouscatalyst,severaladditionalstepsareinvolvedalongwithreactionoccurring onthecatalystsurface,suchas
• externaldiffusiontowardacatalystpellet
• internaldiffusiontowardacatalystsurface
• molecularadsorptiononacatalystsurface
• surfacereaction
• desorptionfromacatalystsurface
• internaldiffusionawayfromthecatalystsurface
• externaldiffusionawayfromthecatalystpellet
Thesestepsneedtobeunderstoodtoselectanappropriatereactorandoperatingstrategy.This procedurewillbediscussedlaterinthisbook.
Heterogeneouscatalysisallowseasyseparationandreuseofacatalyst.Anexampleof heterogeneouscatalysisisHaber’sprocess,whereironpowderisusedasacatalysttoenablethe
Homogeneouscatalystsusedinindustrialchemistryaregenerallyfromorganometallic compounds(compoundswithametal-carbonbond).Thecentralmetalatomisboundtoorganic andinorganicligands.Thecatalystenvironmentcanbeeasilymodifiedtoalterthecatalytic propertiesbymanipulatingligands.Transitionmetalsplayamajorroleinthedevelopment oftheseorganometalliccomplexes.Thisisbecauseoftheavailabilityof d-orbitalsoftransition metals,whichallowligandstobondinsuchawaythattheyareavailableforfurtherreaction. Rhodiumphosphine-basedmetalcomplexessuchas[RhCl(PPh3)3]havebeenfoundtobe aneffectivecatalystforthehydrogenationofolefins.Onaccountofthestabilityoftransition metalcomplexes,theprocesstemperaturesaregenerallylimitedto200°C,andthisbecomes alimitationofhomogeneouscatalysis.Becausethecatalystiscompletelydispersedinthe reactionmedia,thesesystemsfacedifficultiesinseparationorrecoveryofcatalysts.
Significanteffortshavebeenandarebeingspentondecipheringmechanismsofhomogeneous catalysistofacilitatefurtherdevelopmentofnewcatalystsystems.Tolman[2]proposeda mechanismwithwhichareactioniscatalyzedbyhomogenousorganometalliccomplexes, whichwasreferredtoasthe16or18electronrule(see Fig.1.2).Itpostulatestheroleofthe oxidationstateandcoordinationnumberofthemetalcenterofthetransitionmetalcomplex. TheorganometalliccomplexesreferredtoarethetransitionmetalcomplexeswithCO,N2, CN ,RNC,PR3, π-aryl, π-allyl, –SiR3,and π-acylligands,whichhavehighligandfield strengthandcovalentbonding.Thetwomajorpostulatesoftheruleareasfollows[2]:
• Diamagneticorganometalliccomplexesoftransitionmetalsexistinanymeasurable quantityonlyifthevalenceshellofcentralmetalcontains16or18electrons.
• Theintermediatesthatareformedduringthecourseofthereactionshouldalsocontain16 or18valenceshellelectrons.
Saturated 18e complex
Unsaturated 16e complex
Unsaturated 16e complexes Saturated 18e complex
Fig.1.2
π complex, 18e
Cycleexplainingthe16/18electronrule.
Tounderstandthecatalyticcycleinhomogeneouscatalysis,astoichiometricreactionwith well-definedtransitionmetalcomplexescanbeusedtoelucidatethestepsinvolved.Labeled compoundscanalsobeusedtovalidatethepostulatedreactionmechanismbyemploying spectroscopicidentificationtechniques.Variousinsituspectroscopytechniquessuchasinfraredspectroscopy(IR),nuclearmagneticresonance(NMR),electronspinresonance(ESR),and Ramanareveryhelpfulindevelopingabetterunderstandingofhomogeneouscatalysis.Ithas beenobservedthatInfraredspectroscopyhasbeenveryusefulinstudyingcarbonylcomplexes.
1.2.3HeterogeneousCatalysts
Theuseofheterogeneouscatalystsinthechemicalindustrybeganintheearly1800swith Faradaybeingamongthepioneersofheterogeneouscatalysisanddiscoveringtheuseof platinumforoxidation.ThesesystemswereinuseduringtheSecondWorldWarfor reactionssuchasdehydrogenationofmethylcyclohexanetoformtolueneinthepresence ofPt-Al2O3 orinalkaneisomerizationusingCr2O3-Al2O3.Afterthewar,withdiversification inchemicalssynthesizedandadvancementoftechnology,heterogeneouscatalystswere usedforthehydrocrackingofhigh-boilingpetroleumusingNi-aluminosilicatetoformfuels. Thisrevolutionizedtheautomobileindustry.Anotherapplicationofsolidcatalystswasin thesynthesisofpolyethylenefromethylenebypolymerizationinthepresenceofZiegler-Natta (TiCl4-Al(C2H5)3)catalysts.Heterogeneouscatalystsareusedforinnumerablereactions suchasoxidation,nitration,coupling,condensation,andhydrogenation.
Heterogeneouscatalysisfacilitatesalargenumberofchemicalreactions.Theuseof heterogeneouscatalystsinfinechemicalsisgainingimportancebecauseofthefollowing reasons[3]:
• Becausethecatalystisnotinthesamephaseasthereactingmolecules,itallowsforahigher possibilityofcatalystrecoveryandrecyclability.Chemicalbondsareformedwiththe catalysteitherthroughphysisorptionorchemisorptionduringthereactionandbroken thereaftertoregeneratethecatalyst,albeitwithlossofactivityinsomecases[4].
• Solidacidcatalystsareeasiertohandleincomparisonwithconventionalmineralacids suchasH2SO4 andhydrofluoricacid(HF).Theyreducecapitalcostandalsoensure materialsafetybecausetheyhavelesscorrosivity.
• Heterogeneouscatalystsforbulkchemicalshavebeenusedsincethebeginningof chemicalindustries,hencetheprocessesandtheirrolesinthemechanismoftheorganic synthesisarewellunderstoodinmostcases.Thereforetheycanbedownscaledfor theirapplicationsinfinechemicalstosomeextent.
• Myriadcatalystswithacidicorbasicpropertiesexistorhavebeendesignedtosynthesize particularspecies,whichensuresproductmaximization.Mixedmetaloxide,clays, zeolites,silica,alumina,zirconia,andheteropolyacidsareafewclassesofcatalysts
usedpredominantly.Theycanbemodifiedtoalargeextentthroughimpregnationof homogeneouscatalystsormetalsandstructuralchanges.
• Microporousandmesoporousstructuresorsievesandhoneycomb-likestructuresallow heterogeneouscatalyststobehighlyshapeandstereoselective.Thesedesignsgive enzyme-likeefficiencytothecatalyst.
Anewstageofdevelopmentinheterogeneouscatalystscamewiththeobjectiveofusing renewablefeedstocksandenvironmentallybenignprocessesandtechniquesfordownstream wastereduction.Catalyststhathavehighefficiencyandbettersurfacepropertiesarebeing developedforprocessintensification[5].
Acatalystfacilitatesreactionthroughtheformationofcomplexeswithreactingspecies.The productformeddoesn’thavethetendencytobondwiththecatalyst,whichimpliesthecatalyst surfaceisregenerated.However,thisisonlypartiallycorrect.Thesurfaceandstructureofthe catalystaremodifiedwitheachreaction.Forinstance,inthecaseofapuremetalcatalyst, surfaceroughnessandcrystallinitychange,whereasinthecaseofmetaloxidesthereisachange intheratioofmetalandoxygen.Commercialcatalystsaregenerallyavailableinvarious physicalformssuchaspowder,pellets,granules,andextrudates.Poresizeplaysamajorrole instructureandthereforeincatalyticperformance(conversion,selectivity,yield,TOF,and TON).Porouscatalystsofferalargesurfacearea,theabilitytosupportvariedchemical functionalities,andtheabilitytoformdifferentnetworksaccordingtotheapplications.Broadly catalystsareclassifiedintothreekindsofporousmaterials:
• Microporous:Porediameterislessthan2nm.Atypicalexampleofamicroporouscatalyst iszeolite.Ithasacrystallineandwell-definedstructure.Ithasasilicon,aluminum,and oxygenframework,andwateroranothercationmaybepresentinthepores.Activated carbonisalsomicroporousadsorbentandhasvaryingorigins,thermalresistance,and porosity,dependingonthemethodofsynthesis.
• Mesoporous:Porediameterisbetween2and50nm.Mesoporoussolidsaresynthesized throughatemplatingapproach,whereinsurfactantsareusedfordirectingthestructure. Subsequently,thesurfactantisremoved,andamesoporoussystemisobtainedthat replicatesthesurfactantassembly[6].
• Macroporous:Porediameterisgreaterthan50nm.Macroporousmaterialcanbe synthesizedbyasol-gelmethodsuchasporoussilica,alumina,andzirconiagels.Inthe caseofzirconiagels,ametalsaltprecursorisusedfortheepoxidemediatedsol-gelmethod followedbyphaseseparation.Morphologyofthecatalystwouldbegovernedby temperatureandamountofsolventsorreactantsused[7].
Activityistherateatwhichareactionproceedsinpresenceofacatalyst.Theactivityofthe heterogeneouscatalystdependsonthereactionconditionsoftemperature,pressure,and catalystloadingwithrespecttoreactantsandonreactorconditionssuchasflowrateand surfaceareaofreactor.Anothercharacteristicofacatalystisselectivity,whichistheextent
Thefirststepinanyreactionengineeringanalysisisformulatingamathematicalframeworkto describetherate(andmechanism)bywhichonechemicalspeciesisconvertedintoanotherin theabsenceofanytransportlimitations(chemicalkinetics).Therateisthemass,inmolesofa species,transformedperunittime,whereasthemechanismisthesequenceofindividual chemicalevents,whoseoverallresultproducestheobservedtransformation.Althoughthe knowledgeofmechanismisnotnecessaryforreactionengineering,itisofgreatvaluein generalizingandsystematizingthereactionkinetics.Theknowledgeofrateoftransformation, however,isessentialforanyreactionengineeringactivity.Therateoftransformingone chemicalspeciesintoanothercannotbepredictedwithaccuracy.Itisaspecificquantitythat mustbedeterminedfromexperimentalmeasurements.
Measuringtherateofchemicalreactionsinthelaboratoryisitselfaspecializedbranchof scienceandengineering.Therateisformallydefinedasthechangeinmolesofacomponent perunittimeandperunitvolumeofreactionmixture.Itisimportantthatthisratebean intrinsicpropertyofagivenchemicalsystemandnotafunctionofanyphysicalprocesssuchas mixingorheatandmasstransfer.Thus,theratemustbealocalorpointvaluereferringtoa differentialvolumeofreactionmixturearoundthatpoint.Itis,therefore,essentialtoseparate theeffectsofphysicalprocessesfromthemeasuredexperimentaldatatoextractthe informationabouttheintrinsicreactionkinetics.Itisadifficulttask.Moreinformationabout chemicalkineticsandlaboratoryreactorsusedforobtainingintrinsickineticscanbefound intextbookslikeSmith[12],Levenspiel[13],andDoraiswamyandSharma[14].Assuming thatsuchintrinsicratedataisavailable,chemicalkineticistshavedevelopedanumberof valuablegeneralizationsforformulatingrateexpressions,includingthoseforcatalytic reactions.Varioustextbookscoveraspectsofchemicalkineticsindetail[12,13,15].
Oncetheintrinsickineticsisavailable,theproductionrateandcompositionoftheproductscan berelated,inprinciple,tothereactorvolume,reactorconfiguration,andmodeofoperation. Thisisthecentraltaskofareactionandreactorengineeringactivity.Thefirststepofreactor engineeringistoselectasuitablereactortype.Incatalyticreactors,multiplephasesare almostalwaysinvolved(seeexamplescitedinRefs.[14,16–19]).Severaltypesofreactorsare usedforsuchcatalyticandmultiphaseapplications.Broadly,thesereactorsmaybeclassified basedonpresenceofphases,suchas
• gas-liquidreactors:stirredreactors,bubblecolumnreactors,packedcolumns,andloop reactors;
• gas-liquid-solidsreactors:stirredslurryreactors,three-phasefluidizedbedreactors(bubble columnslurryreactors),packedbubblecolumnreactors,tricklebedreactors,andloop reactors;or
• gas-solidreactors:fluidizedbedreactors,fixedbedreactors,andmovingbedreactors.
Existenceofmultiplephasesopensupavarietyofchoicesinbringingthesephasestogetherto react.KrishnaandSie[20]havediscussedathree-levelapproachforreactordesignand selection:
