工业催化原理-英文课件chapter42

§4.5Diffusioneffect4.5.1DiffusioneffectSolidcatalystmostlyisporouswithlargeinternalsurfacesothatsurfacereactionisusuallycombinedwithdiffusionprocess.Itisnecessarytoeliminatediffuseeffectinthekineticstudies.Reactionatdiffusiveregion:diffusionprocessismuchslowerthantherateofsurfacereactionorwesaydiffusionisaratedeterminingstep.Reactionatkineticregion:reactionatsurfaceisards.§4.5.1DiffusioneffectDuetothediffusionandsurfacereaction,theconcentrationsofreactantaredifferentingasphase,externalanddeepinternalsurfacewhicharetermedasCo,CS,Cxrespectively.Therearefollowinglimitedcases§4.5.1DiffusioneffectgasphaseCoCSexternalsurfaceCxxinternalsurface§4.5.1Diffusioneffectcase1CSCoandCx=0reactionatexternalkineticregion,orreactiononexternalsurfaceiscompleteandrate-determining.case2CSCoandCx=0reactionatexternaldiffusiveregion,reactionatexternalsurfaceisfastandcomplete.§4.5.1Diffusioneffectcase3Co=CS=Cxreactionatinternalkineticregionorreactiononinternalsurfaceisards.case4Co=CSCxreactionatinternaldiffusiveregionorinternaldiffusionisslowandratedetermining.§4.5.2ExternaldiffusionFickdiffusionlaw:therateofreactantdiffusefromgasphasetocatalystsurfacerd=D/L·(Co-CS)hereDisdiffusioncoefficientandListhicknessofinterface.§4.5.2ExternaldiffusionIfchemicalreactionatsurfaceisfirstorderr=kCSatsteadystate(stablecondition)r=rdkCS=D/L·(Co-CS)CS=(D/L)Co/(k+D/L)§4.5.2ExternaldiffusionwhenexternaldiffusionisslowrdsCS=(D/L)Co/(k+D/L)D/LkthenCSCor=kCS=k(D/L)·Co/(k+D/L)rD/L·Cothismeansexternaldiffusioncontrollingreactionsarealwaysfirstorder.§4.5.2Externaldiffusionwhendiffusionisfast,reactionoccuratexternalkineticregionCS=(D/L)Co/(k+D/L)D/LkthenCSCor=kCS=k(D/L)·Co/(k+D/L)r=kCSkCothismeansthediffusioneffectiseliminate§4.5.2ExternaldiffusionHowtoeliminateexternaldiffusion1.Increasethelinearvelocityofflowgas,whichcandecreaseLandenlargeD,tillthereactantconversionremainapproximatelyconstantindicatingthatthediffusioneffectiseliminatedalready.2.Decreasereactiontemperature:sinceDT3/2andkexp(-Ea/RT)Astemperaturegoingdown,kdecreasesmuchfastthanwhatDdoes.ItmakeskD/L§4.5.3InternaldiffusionMainlytherearetwotypesofinternaldiffusionsBulkdiffusion:largeporewhentheporesize(radiusr103nm)isgreaterthanthemeanfreepath,thecollisionsofmoleculesinporetoeachotheraremuchmorethanthecollisionsofmoleculestotheporewall.ItiscalledbulkdiffusionandfollowsthefirstFicklawofdiffusion.Bulkdiffusiondn/dt=-DBS·dC/dxheredn/dtisdiffusionrate,DBisbulkdiffusioncoefficient,Sistheareaofcarryingdiffusion,dC/dxistheconcentrationgradientalongxaxisandthenegativesigndenotesthatmoleculeswoulddiffusefromhigherconcentrationtothelowerdirection.BulkdiffusionaccordingtothetheoryofmolecularmotionDB=1/3vv=(8RT/M)1/2=kT/21/2d2PthenDBT3/2/PthebulkdiffusioncoefficientisproportionaltoT3/2andinverselyproportionaltogaspressurePbutindependentonporesize.KnudsendiffusionKnudsendiffusion(moleculardiffusion):smallporewhentheporesizeisquitesmall(radiusr102nm)comparedwiththemeanfreepath,themolecularcollisionstotheporewallaremuchmorethancollisionstoeachother.ThisisKnudsendiffusion.KnudsendiffusionKnudsendiffusioncoefficientDK=2/3vr=2/3r(8RT/M)1/2DKisproportionaltoT1/2andmeanporeradiusrbutindependentontotalgaspressureP.Itisshowntheporesizeofcatalystwilldirectlyaffectthediffusiontypeandthediffusionrate,henceaffectthereactionratefurther.Forexample:catalyticcrackingofcrudeoil,poresize3.0-10.0nm,r,moleculardiffusionDK=5.3810-7m2s-1§4.5.3InternaldiffusionInternaldiffusionandsurfacereactionwilloccuratthesametimeinthepores.Thefasterdiffusionis,themoreinternalsurfacecanbeutilized.ItisdefinedtheutilizationcoefficientofinternalsurfaceF=r/rowhereristhereactionratewithinternaldiffusioninfluencewhileroistheratewithoutdiffusioninfluence.§4.5.3InternaldiffusionwhenF1,CxCo,itmeansfastdiffusionandhighutilizationofinternalsurfaceorwesaysurfacereactionisrdsandreactionatkineticregion.whenF1,CxCo,itmeansdiffusionisrdsandlowutilizationofinternalsurfaceorreactionatinternaldiffusionregion.§4.5.3InternaldiffusionHowtoeliminatetheinfluenceofinternaldiffusion1.Reducethesizeofcatalystparticleuntilthereactantconversionremainsunchangingindicatingthediffusioneffecteliminated.2.Similartoexternaldiffusioncase,decreasetemperaturealsocaneliminatethediffusioninfluence.Astemperaturegoingdown,kdecreasesmuchfasthencetolowerdownthereactionratewhereasthediffusionratedecreaseslightly.§4.6Influenceofinternaldiffusioneffecttoreactionorderwheninternaldiffusionisimportant,theapparentreactionordern=(no+1)/2noistherealreactionorderwithoutdiffusioninfluence.Itisfoundthatonlyfirstorderreactionisunaffectedbydiffusion.effecttorateconstantwheninternaldiffusionisimportant,theapparentrateconstanttendstoitssquarerootvaluekko1/2§4.6InfluenceofinternaldiffusioneffecttoactivationenergyinternaldiffusionwillmaketheapparentactivationenergytendstothehalfofitsrealvalueEaEao/2whentemperaturegoesup,thereactionrateincreasehencethediffusioninfluencebecomeimportant.Itmakesthereactionfromkineticregionchanges