化学反应工程英文课件Chapter-8

化学反应工程Chapter8PotpourriofMultipleReactionsChapter7consideredreactionsinparallel.Thesearereactionswheretheproductdoesnotreactfurther.Thischapterconsidersallsortsofreactionswheretheproductformedmayreactfurther.Herearesomeexamples:ARSTABRRBSSBTSeriesARSReversibleandirreversibleSeriesparallel,orconsecutive-competitiveARSTUDenbighsystem化学反应工程ARSReversibleARSReversiblenetworkWedeveloporpresenttheperformanceequationsofsomeofthesimplersystemsandpointouttheirspecialfeaturessuchasmaximaofintermediates.化学反应工程proceedonlyinthepresenceoflight,thattheystoptheinstantthelightisturnedoff,andthatforagivenintensityofradiation,therateequationsare1AArkC12RARrkCkC2SRrkC(2)(3)(4)8.1IRREVERSIBLEFIRST-ORDERREACTIONSINSERIESForeasyvisuali’zationconsiderthatthereactions12kkARS(1)化学反应工程QualitativeDiscussionAboutProductDistribution.ConsiderthefollowingtwowaysoftreatingabeakercontainingA:First,thecontentsareuniformlyirradiated;Second,asmallstreamiscontinuouslywithdrawnfromthebeaker,ir’radiated(照射),andreturnedtothebeaker;therateofabsorptionof‘radiantenergyisthesameinthetwocases.Thetwoschemes(计划,方案)areshowninFigs.8.1and8.2.化学反应工程Figure8.1Concentration-timecurvesifthecontentsofthebeakerareirradiateduniformly.化学反应工程Figure8.2Concentration-timecurvesforthecontentsofthebeakerifonlyasmallportionofthefluidisirradiatedatanyinstant.化学反应工程Inthefirstbeaker(showninFig.8.1),whenthecontentsarebeingirradiatedallatthesametime,thefirstbitoflightwillattackAalonebecauseonlyAispresentatthestart.TheresultisthatRisformed.WiththenextbitoflightbothAandRwillcom’pete;however,Aisinverylargeexcesssoitwillprefe’rentially(优先地)absorbthe‘radiantenergytodecomposeandformmoreR.DuringthisprocessAdisappearsandproductsareformed.IstheproductdistributionofRandSdifferentinthetwobeakers?Letusseewhetherwecananswerthisquestionqualitatively(定性地)forallvaluesoftherateconstants.化学反应工程Thus,theconcentrationofRwillrisewhiletheconcentrationofAwillfall.ThisprocesswillcontinueuntilRispresentinhighenoughconcentrationsothatitcancompetefavorablywithAforthe‘radiantenergy.Whenthishappens,amaximumRconcentrationisreached.AfterthisthedecompositionofRbecomesmorerapidthanitsrateofformationanditsconcentrationdrops.AtypicalconcentrationtimecurveisshowninFig.8.1.化学反应工程Intheal’ternative(二中择一的)way(showninFig.8.2)oftreatingA,asmallfractionofthebeaker’scontentsiscontinuouslyremoved,ir‘radiated,andreturnedtothebeaker.Althoughthetotalabsorptionrateisthesameinthetwocases,theintensityofradiationreceivedbytheremovedfluidisgreater,anditcouldwellbe,iftheflowrateisnottoohigh,thatthefluidbeingirradiatedreactse’ssentiallytocompletion.Inthiscase,then,AisremovedandSisreturnedtothebeaker.So,astimepassestheconcentrationofAslowlydecreasesinthebeaker,Srises,whileRisabsent(缺席的).ThisprogressivechangeisshowninFig.8.2.化学反应工程Thesetwomethodsofreactingthecontentsofthebeakeryielddifferentproductdistributionsandrepresentthetwoextremesinpossibleoperations,onewithamaximumpossibleformationofRandtheotherwithaminimum,ornoformation,ofR.Howcanwebestcharacterizethisbehavior?Wenoteinthefirstmethodthatthecontentsofthebeakerremainhomogeneousthroughout,allchangingslowlywithtime,whereasinthesecondastreamofhighlyreactedfluidiscontinuallybeingmixedwithfreshfluid.Inotherwords,wearemixingtwostreamsofdifferentcompositions—calledbackmix.Nobackmix！Hasbackmix!化学反应工程Forirreversiblereactionsinseriesthemixingoffluidofdifferentcompositionisthekeytotheformationofintermediate.Themaximumpossibleamountofanyandallintermediatesisobtainediffluidsofdifferentcompositionsandatdifferentstagesofconversionarenotallowedtomix.Theabovediscussionsuggeststhefollowingrulegoverningproductdistributionforreactionsinseries.Nobackmix!化学反应工程Astheintermediateisfrequentlythedesiredreactionproduct,thisruleallowsustoevaluatethee’ffectivenessofvariousreactorsystems.Forexample,plugflowandbatchoperationsshouldbothgiveamaximumRyieldbecauseherethereisnomixingoffluidstreamsofdifferentcompositions.Ontheotherhand,themixedreactorshouldnotgiveashighayieldofRaspossiblebecauseafreshstreamofpureAisbeingmixedcontinuallywithanalreadyreactedfluidinthereactor.化学反应工程Thefollowingexamples‘illustratethepointjustmade.Wethengiveaquantitative(定量的)treatmentwhichwillverifythesequalitative(定性的)findings.EXAMPLE8.1FAVORABLECONTACTINGPATTERNSFORANYSETOFIRREVERSIBLEREACTIONSINSERIES,NOTJUSTARSWhichcontactingpatternofFigs.E8.1,whenproperlyoperated,cangiveahigherconcentrationofanyintermediate,thecontactingpatternontheleftortheoneontheright?化学反应工程FigureE8.1a,b,c,d化学反应工程SOLUIONFocusingonthemixingruleforreactionsinseries,thattheextentofmixingofstreamsofdifferentcompositionshouldbeminimized,wereason(v.推论)forpart(a):Theleftpatternisbetter;infactitisthebestpossibleflowscheme.forpart(b):LookingatFigs.6.5,6.6,and6.16,6.17ofChapter6weseethattheleftisclosertoplugflowforbothfirst-andsecond-orderreactions.Sowegeneralizethistoanypositiveorderreaction.化学反应工程forpart(c):Therightpatternisbetterbecauseitisclosertoplugflow.forpart(d):‘Turbulentflowhaslessintermixingoffluidsofdifferentages,lessbypassing;hence,therights