反应工程基础(程易)chapter9-rtd-283507575-课件

InformationneededtopredictwhatareactorcandoReactorInputOutputPerformanceequationRelatesinputtooutputContactingpattern

orhowmaterialsflowthroughandcontacteachotherinthereactor

Kineticsorhowfastthingshappen.Ifveryfast,thenequilibriumtellswhatwillleavethereactor.Ifnotsofast,thentherateofchemicalreaction,andmaybeheatandmasstransfertoo,willdeterminewhatwillhappen.FluidizedBedReactorCase1Case2Case3RTDsofgasandsolidsGasRTDsSolidsRTDsBi-modalRTDMixingindiscimpellersystemsTiltedconfigurationStructureinaneccentricstirredtank

Concentricorbitsina3-discsystem/~shinbrot/Group_Index.html高粘体系的液体混合现象Chapter9

DistributionsofResidenceTimesforChemicalReactorsOverviewNonidealreactorsPart-1:characterize(non)idealreactors

ResidenceTimeDistribution(RTD),E(t)

Meanresidencetime,tm

Variance,

2

Cumulativedistributionfunction,F(t)

Part-2:predictconversionandexitconcentrationsbasedonRTD

RTD

notuniquemodelsPart1

CharacterizationandDiagnostics9.1GeneralcharacteristicsTwomajorusesoftheRTDtocharacterizenonidealreactors

1.Todiagnoseproblemsofreactorsinoperation

2.Topredictconversionoreffluentconcentrationinexisting/availablereactorswhenanewreactionisusedinthereactorExamples:ChannelingTankreactorDeadzoneBypassingThethreeconceptsRTDMixingModel-Todescribethedeviationsfromthemixingpatternsassumedinidealreactors-Tocharacterizethemixinginnonidealreactors9.1.1RTDfunctionResidencetime:thetimetheatomsspentinthereactorPlug-flowreactor,batchreactor

AlltheatomsinthereactorshavethesameresidencetimeCSTR

Feedsmixedimmediately,butwithdrawncontinuously“RTD”:somemoleculesleavequickly,othersoverstaytheirwelcome.RTD:acharacteristicofthemixingthatoccursinachemicalreactor9.2MeasurementoftheRTDRTDisdeterminedexperimentallybyinjectinganinertchemical,molecule,oratom,calleda

tracer,intothereactoratsometimet=0andthenmeasuringthetracerconcentration,C,intheeffluentstreamasa

functionoftimeTracer:

nonreactive,easilydetectable,similarphysicalpropertiestothefluid,noadsorptiononthewallsorsurfaces,etc.PulseinputandStepinput阶跃注入脉冲注入9.2.1PulseinputexperimentReactorFeedInjectionDetectionEffluentCCtCCtttPulseinjectionStepinjectionStepresponsePulseresponseCCttPulseinjectionPulseresponseOnlyflowcarriestracer(Nodiffusion)E(t):residenttimedistributionfunction

howmuchtimedifferentfluidelementshavespentinthereactorC(t)tPulseresponseE(t)tFractionofmaterialleavingthereactorthathasresidedinthereactorfortimesbetweent1andt2t1t2ProblemsusingPulseinput:“Pulse”:canbehardtoobtainareasonablepulseattheinjectionpointLongtailsofthemeasuredC(t)curveConvolutionintegral（卷积）PulseImperfectpulseStepAgeneraldescription:Outputconcentration~InputconcentrationInputEquivalentform9.2.2SteptracerexperimentCCttStepinjectionStepresponseStepinjectionAdvantageofF(t):

easierexperimentsDrawbacks:differentiationerrorlargeamountoftracer9.3CharacteristicsoftheRTDE(t):exit-agedistributionfunction,

agedistributionoftheeffluentstreami.e.,thelengthsoftimevariousatomsspendatreactionconditions9.3.1IntegralrelationshipsThecumulativeRTDfunctionF(t)9.3.2MeanresidencetimeThefirstmomentgivestheaveragetimetheeffluentmoleculesspentinthereactor.Spacetimeoraverageresidencetime,=V/Intheabsencetodispersion,forconstantvolumetricflow,=0=tm9.3.3OthermomentsoftheRTDThesecondmomentaboutthemeanisthevarianceThethirdmoment,skewnessThetwoparametersmostcommonlyusedtocharacterizetheRTDareand2.9.3.4NormalizedRTDfunction,E():representsthenumberofreactorvolumesoffluidbasedonentranceconditionsthathaveflowedthroughthereactorintimet.WhyweuseanormalizedRTD?Theflowperformanceinsidereactorsofdifferentsizescanbecompareddirectly.Example:allperfectlymixedCSTR:9.3.5Internal-agedistribution,I():representstheageofamoleculeinsidethereactorI():thefractionofmaterialinsidethereactorthathasbeeninsidethereactorforaperiodtimebetweenand+CSTR:P633推导过程9.4RTDinidealreactors9.4.1RTDsinbatchandplug-flowreactorsPlugflowreactor:PropertiesofDiracdeltafunctionForplugflowE(t)tOutF(t)t1.09.4.2Single-CSTRRTDIn–Out=AccumulationFromtracerexperiment:E()F()1.01.09.4.3LaminarflowreactorUTheminimumtimethefluidmayspendinthereactor:0.5E()0.5F()1PFRCSTRLFRNormalizedRTDfunctionforalaminarflowreactor9.5Diagnosticsandtroubleshooting9.5.1Generalcomments9.5.2SimplediagnosticsandtroubleshootingusingtheRTDforidealreactorsA.TheCSTRPerfectoperation(P)(b)Bypassing(BP)(c)Deadvolume(DV)SummaryB.Tubularreactor(a)PerfectoperationofPFR(P)(b)PFRwithchanneling(Bypassing,BP)(c)PFRwithdeadvolume(DV)Summary9.5.3PFR/CSTRseriesRTDCSTR+PFRPFR+CSTRRTDisnotuniquetoaparticularreactorsequence.CSTRPFRPFRCSTRE(t)tPFR1/CSTRExample:comparingsecond-orderreactionsystemsCSTR+PFRPFR+CSTRCSTRPFRPFRCSTR(1)(2)Part2

PredictingConversionandExitConcentration9.6ReactormodelingusingtheRTDRTD+Model+KineticdataExitconversionandExitconcentrationModelsforpredictingconversionfromRTDdataZeroadjustableparameters

a.Segregationmodel

b.Maximummixednessmodel2.Oneadjustableparameter

a.Tanks-in-seriesmodel

b.Dispersionmodel3.Twoadjustableparameters

RealreactorsmodeledascombinationsofidealreactorsRTD:tellshowlongthevariousfluidelementshavebeeninthereactor,butdoesnottellanythingabouttheexchangeofmatterbetweenthefluidelements(i.e.,themixing)Mixingofreactingspecies:oneofthemajorfactorscontrollingthebehaviorofchemicalreactors.

Forfirst-orderreactions,ConversionisindependentofconcentrationOncetheRTDisdetermined,theconversioncanbepredicted.Forreactionsotherthanfirstorder,RTDisnotsufficient.Model:toaccountforthemixingofmoleculesinsidethereactorMacromixing:

Producesadistributionofresidencetimeswithout,however,specifyinghowmoleculesofdifferentagesencounteroneanotherinthereactor.Micromixing:

Describeshowmoleculesofdifferentagesencounteroneanotherinthereactor.Twoextremes:Completesegregation:Allmoleculesofthesameagegroupremaintogetherastheytravelthroughthereactorandarenotmixedwithanyotherageuntiltheyexitthereactor(2)Completemicromixing:

Moleculesofdifferentagegroupsarecompletelymixedatthemolecularlevelassoonastheyenterthereactor.9.7Zero-parametermodels9.7.1SegregationmodelMixingoftheglobulesofdifferentagesoccurshere.Mixingoccursatthelatestpossiblemoment.Eachlittlebatchreactor(globule)exitingtherealreactoratdifferenttimeswillhaveadifferentconversion.(X1,X2,X3...)RTD+Model+KineticdataExitconversionandExitconcentrationMeanconversionofthoseglobulesspendingbetweentimetandt+dtinthereactor=ConversionachievedinaglobuleafterspendingatimetinthereactorXFractionofglobulesthatspendbetweentandt+dtinthereactorSegregationmodelSummary:ifwehavetheRTD,thereactionrateexpression,thenforasegregatedflowsituation(i.e.,model),wehavesufficientinformationtocalculatetheconversion.Considerafirst-orderreaction:Forabatchreactor:ForconstantvolumeandwithNA=NA0(1-X)solutionMeanconversionforafirst-orderreactionExample:ApplicationsofthesegregationmodelforanidealPFR,aCSTR,andalaminarflowreactor(first-orderreaction)(1)PFR:Chapter4(2)CSTR:Chapter4(3)LaminarflowreactorHilder,M.H.Trans.IchemE59p143(1979)9.7.2MaximummixednessmodelSegregationmodel:mixingoccursatthelatestpossiblepoint.Maximummixednessmodel:mixingoccursattheearliestpossiblepoint.SegregationmodelMaximummixednessmodelThevolumeoffluidwithalifeexpectancybetweenand+TherateofgenerationofthesubstanceAinthisvolume:Maximummixednessgivesthelowerboundonconversion(X)whenn>1.Molebalance9.7.3Segregationvs.maximummixednesspredictionsIfthenO.Levenspiel,P358(a)(b)(c,d,e)9.8Usingsoftwarepackage