化学反应工程英文课件Chapter-4

化学反应工程Chapter4IntroductiontoReactorDesign4.1GENERALDISCUSSIONSofarwehaveconsideredthemathematicalexpressioncalledtherateequationwhichdescribestheprogressofahomogeneousreaction.Therateequationforareactingcomponentiisanintensivemeasure,andittellshowrapidlycomponentiformsordisappearsinagivenenvironmentasafunctionoftheconditionsthere,orV)volumeofregionewithinthconditions(V1reactionbyfdtdNriiThisisadifferentialexpression.化学反应工程Inreactordesignwewanttoknowwhatsizeandtypeofreactorandmethodofoperationarebestforagivenjob.Becausethismayrequirethattheconditionsinthereactorvarywithpositionaswellastime,thisquestioncanonlybeansweredbyaproperintegrationoftherateequationfortheoperation.Thismayposedifficultiesbecausethetemperatureandcompositionofthereactingfluidmayvaryfrompointtopointwithinthereactor,dependingontheendothermicorexothermiccharacterofthereaction,therateofheatadditionorremovalfromthesystem,andtheflowpatternoffluidthroughthevessel.Ineffect,then,manyfactorsmustbeaccountedforinpredictingtheperformanceofareactor.Howbesttotreatthesefactorsisthemainproblemofreactordesign.化学反应工程Equipmentinwhichhomogeneousreactionsareeffectedcanbeoneofthreegeneraltypes:thebatch,thesteady-stateflow,andtheunsteady-statefloworsemibatchreactor.Thelastclassificationincludesallreactorsthatdonotfallintothefirsttwocategories.ThesetypesareshowninFig.4.1.化学反应工程Figure4.1Broadclassificationofreactortypes.(a)Thebatchreactor.(b)Thesteady-stateflowreactor.(c),(d),and(e)Variousformsofthesemibatchreactor.化学反应工程Thesteady-stateflowreactorisidealforindustrialpurposeswhenlargequantitiesofmaterialaretobeprocessedandwhentherateofreactionisfairlyhightoextremelyhigh.Supportingequipmentneedsaregreat;however,extremelygoodproductqualitycontrolcanbeobtained.Asmaybeexpected,thisisthereactorthatiswidelyusedintheoilindustry.Thebatchreactorissimple,needslittlesupportingequipment,andisthereforeidealforsmall-scaleexperimentalstudiesonreactionkinetics.Industriallyitisusedwhenrelativelysmallamountsofmaterialaretobetreated.化学反应工程Thesemibatchreactorisaflexiblesystembutismoredifficulttoanalyzethantheotherreactortypes.Itoffersgoodcontrolofreactionspeedbecausethereactionproceedsasreactantsareadded.Suchreactorsareusedinavarietyofapplicationsfromthecalo’rimetrictitrationsinthelaboratorytothelargeopenhearth(炉膛)furnacesforsteelproduction.Thestartingpointforalldesignisthematerialbalanceexpressedforanyreactant(orproduct).Thus,asillustratedinFig.4.2,wehave化学反应工程Figure4.2Materialbalanceforanelementofvolumeofthereactor.化学反应工程volumeofelementinreactantofonaccumulatiofratevolumeofelementwithinthereactionchemicaltoduelossreactantofratevolumeofelementoutflowreactantofratevolumeofelementintoflowreactantofrate(1)Wherethecompositionwithinthereactorisuniform(independentofposition),theaccountingmaybemadeoverthewholereactor.化学反应工程Wherethecompositionisnotuniform,itmustbemadeoveradifferentialelementofvolumeandthenintegratedacrossthewholereactorfortheappropriateflowandconcentrationconditions.Forthevariousreactortypesthisequationsimplifiesonewayoranother,andtheresultantexpressionwhenintegratedgivesthebasicperformanceequationforthattypeofunit.Thus,inthebatchreactorthefirsttwotermsarezero;inthesteady-stateflowreactorthefourthtermdisappears;forthesemibatchreactorallfourtermsmayhavetobeconsidered.化学反应工程Innonisothermaloperationsenergybalancesmustbeusedinconjunctionwithmaterialbalances.Thus,asillustratedinFigure4.3,wehaveevolumofelementnheatwithiofonaccumulatiofratemevoluofelementithinreactionwbyheatofncedisappearaofratevolumeofelementoutflowheatofratevolumeofelementintoflowheatofrate(2)化学反应工程Figure4.3Energybalanceforanelementofvolumeofthereactor.化学反应工程ThematerialbalanceofEq.1andtheenergybalanceofEq.2aretiedtogetherbytheirthirdtermsbecausetheheateffectisproducedbythereactionitself.Whenwecanpredicttheresponseofthereactingsystemtochangesinoperatingconditions(howratesandequilibriumconversionchangewithtemperatureandpressure),whenweareabletocompareyieldsforalternativedesigns(adiabaticversusisothermaloperations,singleversusmultiplereactorunits,flowversusbatchsystem),andwhenwecanestimatetheeconomicsofthesevariousalternatives,thenandonlythenwillwefeelsurethatwecanarriveatthedesignwellfittedforthepurposeathand.化学反应工程SymbolsandRelationshipbetweenCAandXAForthereactionaA+bB→rR,withinertsiI,Figs.4.4and4.5showsthesymbolscommonlyusedtotellwhatishappeninginthebatchandflowreactors.Thesefiguresshowthattherearetworelatedmeasuresoftheextentofreaction,theconcentrationCAandtheconversionXA.However,therelationshipbetweenCAandXAisoftennotobviousbutdependsonanumberoffactors.Thisleadstothreespecialcases,asfollows.化学反应工程Figure4.4Symbolusedforbatchreactors.化学反应工程0VVVforand1and10X0X1XAAA0AAA0AA0AAA0AAAAAdXCdCXCCCdCdXCCX(3)Figure4.5Symbolsusedforflowreactors.TorelatethechangesinBandRtoAwehave化学反应工程bXCaXCrCCbCCaCCBB0AA0R0RBB0AA0or(4)SpecialCase2.BatchandFlowSystemsofGases