反应工程基础(程易)chapter9-rtd

Informationneededtopredictwhatareactorcando Reactor Input Output PerformanceequationRelatesinputtooutput Contactingpatternorhowmaterialsflowthroughandcontacteachotherinthereactor Kineticsorhowfastthingshappen Ifveryfast thenequilibriumtellswhatwillleavethereactor Ifnotsofast thentherateofchemicalreaction andmaybeheatandmasstransfertoo willdeterminewhatwillhappen FluidizedBedReactor Case1 Case2 Case3 RTDsofgasandsolids GasRTDs SolidsRTDs Bi modalRTD Mixingindiscimpellersystems Tiltedconfiguration Structureinaneccentricstirredtank Concentricorbitsina3 discsystem http sol rutgers edu shinbrot Group Index html 高粘体系的液体混合现象 Chapter9DistributionsofResidenceTimesforChemicalReactors Overview NonidealreactorsPart 1 characterize non idealreactorsResidenceTimeDistribution RTD E t Meanresidencetime tmVariance 2Cumulativedistributionfunction F t Part 2 predictconversionandexitconcentrationsbasedonRTDRTDnotunique models Part1CharacterizationandDiagnostics 9 1Generalcharacteristics TwomajorusesoftheRTDtocharacterizenonidealreactors1 Todiagnoseproblemsofreactorsinoperation2 Topredictconversionoreffluentconcentrationinexisting availablereactorswhenanewreactionisusedinthereactor Examples Channeling Tankreactor Deadzone Bypassing Thethreeconcepts RTDMixingModel Todescribethedeviationsfromthemixingpatternsassumedinidealreactors Tocharacterizethemixinginnonidealreactors 9 1 1RTDfunction Residencetime thetimetheatomsspentinthereactorPlug flowreactor batchreactorAlltheatomsinthereactorshavethesameresidencetimeCSTRFeedsmixedimmediately butwithdrawncontinuously RTD somemoleculesleavequickly othersoverstaytheirwelcome RTD acharacteristicofthemixingthatoccursinachemicalreactor 9 2MeasurementoftheRTD RTDisdeterminedexperimentallybyinjectinganinertchemical molecule oratom calledatracer intothereactoratsometimet 0andthenmeasuringthetracerconcentration C intheeffluentstreamasafunctionoftimeTracer nonreactive easilydetectable similarphysicalpropertiestothefluid noadsorptiononthewallsorsurfaces etc PulseinputandStepinput 阶跃注入 脉冲注入 9 2 1Pulseinputexperiment Reactor Feed Injection Detection Effluent C C t C C t t t Pulseinjection Stepinjection Stepresponse Pulseresponse C C t t Pulseinjection Pulseresponse Onlyflowcarriestracer Nodiffusion E t residenttimedistributionfunctionhowmuchtimedifferentfluidelementshavespentinthereactor C t t Pulseresponse E t t Fractionofmaterialleavingthereactorthathasresidedinthereactorfortimesbetweent1andt2 t1 t2 ProblemsusingPulseinput Pulse canbehardtoobtainareasonablepulseattheinjectionpointLongtailsofthemeasuredC t curve Convolutionintegral 卷积 PulseImperfectpulseStep Ageneraldescription Outputconcentration Inputconcentration Input Equivalentform 9 2 2Steptracerexperiment C C t t Stepinjection Stepresponse Stepinjection AdvantageofF t easierexperimentsDrawbacks differentiation errorlargeamountoftracer 9 3CharacteristicsoftheRTD E t exit agedistributionfunction agedistributionoftheeffluentstreami e thelengthsoftimevariousatomsspendatreactionconditions 9 3 1Integralrelationships ThecumulativeRTDfunctionF t 9 3 2Meanresidencetime Thefirstmomentgivestheaveragetimetheeffluentmoleculesspentinthereactor Spacetimeoraverageresidencetime V Intheabsencetodispersion forconstantvolumetricflow 0 tm 9 3 3OthermomentsoftheRTD Thesecondmomentaboutthemeanisthevariance Thethirdmoment skewness ThetwoparametersmostcommonlyusedtocharacterizetheRTDare and 2 9 3 4NormalizedRTDfunction E representsthenumberofreactorvolumesoffluidbasedonentranceconditionsthathaveflowedthroughthereactorintimet WhyweuseanormalizedRTD Theflowperformanceinsidereactorsofdifferentsizescanbecompareddirectly Example allperfectlymixedCSTR 9 3 5Internal agedistribution I representstheageofamoleculeinsidethereactor I thefractionofmaterialinsidethereactorthathasbeeninsidethereactorforaperiodtimebetween and CSTR P633推导过程 9 4RTDinidealreactors 9 4 1RTDsinbatchandplug flowreactors Plugflowreactor PropertiesofDiracdeltafunction Forplugflow E t t Out F t t 1 0 9 4 2Single CSTRRTD In Out Accumulation Fromtracerexperiment E F 1 0 1 0 9 4 3Laminarflowreactor U Theminimumtimethefluidmayspendinthereactor 0 5 E 0 5 F 1 PFR CSTR LFR NormalizedRTDfunctionforalaminarflowreactor 9 5Diagnosticsandtroubleshooting 9 5 1Generalcomments 9 5 2SimplediagnosticsandtroubleshootingusingtheRTDforidealreactors A TheCSTR Perfectoperation P b Bypassing BP c Deadvolume DV Summary B Tubularreactor a PerfectoperationofPFR P b PFRwithchanneling Bypassing BP c PFRwithdeadvolume DV Summary 9 5 3PFR CSTRseriesRTD CSTR PFR PFR CSTR RTDisnotuniquetoaparticularreactorsequence CSTR PFR PFR CSTR E t t PFR 1 CSTR Example comparingsecond orderreactionsystems CSTR PFR PFR CSTR CSTR PFR PFR CSTR 1 2 Part2PredictingConversionandExitConcentration 9 6ReactormodelingusingtheRTD RTD Model Kineticdata ExitconversionandExitconcentration ModelsforpredictingconversionfromRTDdata Zeroadjustableparametersa Segregationmodelb Maximummixednessmodel 2 Oneadjustableparametera Tanks in seriesmodelb Dispersionmodel 3 TwoadjustableparametersRealreactorsmodeledascombinationsofidealreactors RTD tellshowlongthevariousfluidelementshavebeeninthereactor butdoesnottellanythingabouttheexchangeofmatterbetweenthefluidelements i e themixing Mixingofreactingspecies oneofthemajorfactorscontrollingthebehaviorofchemicalreactors Forfirst orderreactions Conversionisindependentofconcentration OncetheRTDisdetermined theconversioncanbepredicted Forreactionsotherthanfirstorder RTDisnotsufficient Model toaccountforthemixingofmoleculesinsidethereactor Macromixing Producesadistributionofresidencetimeswithout however specifyinghowmoleculesofdifferentagesencounteroneanotherinthereactor Micromixing Describeshowmoleculesofdifferentagesencounteroneanotherinthereactor Twoextremes Completesegregation Allmoleculesofthesameagegroupremaintogetherastheytravelthroughthereactorandarenotmixedwithanyotherageuntiltheyexitthereactor 2 Completemicromixing Moleculesofdifferentagegroupsarecompletelymixedatthemolecularlevelassoonastheyenterthereactor 9 7Zero parametermodels 9 7 1Segregationmodel Mixingoftheglobulesofdifferentagesoccurshere Mixingoccursatthelatestpossiblemoment Eachlittlebatchreactor globule exitingtherealreactoratdifferenttimeswillhaveadifferentconversion X1 X2 X3 RTD Model Kineticdata ExitconversionandExitconcentration Meanconversionofthoseglobulesspendingbetweentimetandt dtinthereactor Conversionachievedinaglobuleafterspendingatimetinthereactor X Fractionofglobulesthatspendbetweentandt dtinthereactor Segregationmodel Summary ifwehavetheRTD thereactionrateexpression thenforasegregatedflowsituation i e model wehavesufficientinformationtocalculatetheconversion Considerafirst orderreaction Forabatchreactor ForconstantvolumeandwithNA NA0 1 X solution Meanconversionforafirst orderreaction Example ApplicationsofthesegregationmodelforanidealPFR aCSTR andalaminarflowreactor first orderreaction 1 PFR Chapter4 2 CSTR Chapter4 3 Laminarflowreactor Hilder M H Trans IchemE59p143 1979 9 7 2Maximummixednessmodel Segregationmodel mixingoccursatthelatestpossiblepoint Maximummixednessmodel mixingoccursattheearliestpossiblepoint Segregationmodel Maximummixednessmodel Thevolumeoffluidwithalifeexpectancybetween and TherateofgenerationofthesubstanceAinthisvolume Maximummixednessgivesthelowerboundonconversion X whenn 1 Molebalance 9 7 3Segregationvs maximummixednesspredictions If then O Levenspiel P358 a b c