大三下all反应工程基础chap3

Chapter2-Summary1.TheconversionXisthemolesofAreactedpermoleofAfed.Forbatchsystems:Forflowsystems:ForreactorsinserieswithnosidestreamsBatchCSTRPFRPBR2.MolebalanceequationsinfourcommonreactorsIntegralDifferentialIntegralDifferentialChapter1Chapter23.If–rA=f(X)isgiven,thefollowinggraphicaltechniquescanbeusedtosizeaCSTRandaplug-flowreactorPFRCSTRAdditionaldiscussionsonCSTR:1.Idealreactor,immediatemixingtouniformstateinsideCSTRQuestion:Howtoimplementfastmixingofliquidsorgases?2.VolumeofCSTRvs.Conversion(X)XFA0/-rAV1X2X1V2X1X2-rA~f(T)exothermic,gas-phasereactionT1:lowtemperatureT2:hightemperature-rA,1-rA,2CSTR:0-dimensionV1V2SpacetimeChapter3RateLawsandStoichiometrySuccessismeasurednotsomuchbythepositiononehasreachedinlife,asbytheobstaclesonehasewhiletryingtosucceed.BookerT.WashingtonOverviewHowtoobtaintherateofreactionasafunctionofconversionStep1:(Part1)definetheratelaw,relatingtherateofreactiontotheconcentrationsofthereactingspeciesandtotemperatureStep2:(Part2)defineconcentrationsforflowandbatchsystemsanddevelopastoichiometrictablesothatonecanwriteconcentrationsasafunctionofconversionPart1RateLaws3.1BasicdefinitionsTypesofreactions

Molecularityofareactionisthenumberofatoms,ions,ormoleculesinvolved(colliding)inareactionstep

unimolecular,bimolecular,termolecularHomogeneousHeterogeneousIrreversibleReversibleExamples:UnimolecularreactionBimolecularreactionTermolecularreaction3.1.1RelativeratesofreactionRateofformationofC=(RateofdisappearanceofA)Similarly:ReactionstoichiometryExample:3.2ThereactionorderandtheratelawRatelaworkineticexpression:algebraicequationthatrelates–rAtothespeciesconcentrationskA:rateconstant,orspecificrateofreaction3.2.1PowerlawmodelsandelementaryratelawsReactionorder:referstothepowers,e.g.,

,

Overallorderofthereaction,nABAProductsReactionorder:nZero-order(n=0)First-order(n=1)Second-order(n=2)Third-order(n=3)Unit:Elementaryreaction:evolvesasinglestepThestoichiometriccoefficients:identicaltothepowersintheratelawFollowsanelementaryratelaw,BUT,nonelementaryreactionRatelaws,determinedbyexperimentalobservationCollisiontheoryPathways3.2.2NonelementaryratelawsHomogeneousreactionsApparentreactionorders...Apparent

firstorder(N2O),overallfirstorderorderof-1(O2),firstorder(N2O),overall0orderHeterogeneousreactionsRatelawintermsofpracticalpressuresratherthanconcentrations(T)(B)Langmuir-HinshelwoodkineticsAdsorptionconstants,kPa-1IdealgaslawComputationalfluiddynamics(CFD)withdetailedchemistry计算流体力学耦合基元反应动力学详细考察壁面导热、孔径、活性比表面积影响，指导催化剂设计、反应器设计、优化原料气体组成，等等。CO2+2H2OCH4+2O2CH4+½O2CO+2H2CFDwithdetailedchemistryCFD表面机理3.2.3ReversiblereactionsAllratelawsforreversiblereactionsmustreducetothethermodynamicrelationshipRelatingthereactingspeciesconcentrationsatequilibrium.Atequilibrium:Forwardandreversespecificreactionrateconstants,kBandk-BConcentrationequilibriumconstantwhere,Concentrationequilibriumconstantequilibriumconstant:

decreaseswithincreasingtemperatureforexothermicreactions;

increaseswithincreasingtemperatureforendothermicreactions.k:toparticularspeciesAtequilibrium:Nochangeinthetotalnumberofmolesandtheheatcapacityterm,CP=0TKCEndothermicTKCExothermic3.3Thereactionrateconstantk:specificreactionrateortherateconstantalwaysstronglydependentontemperature!catalysttotalpressureingas-phasereactionsionicstrength,solvent,etc.,inliquid-phaseArrheniusequationA:pre-exponentialfactororfrequencyfactorE:activationenergy,J/molorcal/molR:gasconstant,8.314J/mol·K=1.987cal/mol·KT:absolutetemperatureWhyisthereanactivationenergyThemoleculesneedenergytodistortorstretchtheirbondssothattheybreakthemandthusformnewbondsThestericandelectronrepulsionforcesmustbeeasthereactingmoleculescomeclosetogetherActivationenergy:abarriertoenergytransfer(fromkineticenergytopotentialenergy)betweenreactingmoleculesthatmustbee.PotentialenergyreactantsproductsA+BCA-B-CAB+CEAB+EICEIA+EBCHRxEBEnergybarrierEA-B-CReactioncoordinateDeterminationoftheactivationenergyDataset:(T1,kA1),(T2,kA2),(T3,kA3),...(Tn,kAn)lnk1/TSlope=-(E/R)LowEHighEHowtoexperimentallydeterminetheactivationenergy,EEvsk:Temperaturesensitivity

FromLevenspiel’sbookBatchCSTRPFRPBRDifferentialformAlgebraicformIntegralformDesignequationsBatchCSTRPFRPBRMolebalanceequationsinfourcommonreactorsIntegralDifferentialIntegralDifferentialChapter1Chapter2PresentstatusofourapproachtoreactorsizinganddesignRatelaw+BeabletodesignisothermalreactorsPart2StoichiometryGeneralreaction“PermoleofA”3.5BatchsystemsSpeciesInitially(mol)Change(mol)Remaining(mol)ABCDI(inerts)TotalsThechangeinthetotalnumberofmolespermoleofAreactedTodeterminethereactionrateasafunctionofconversionX,weneedtoknowtheconcentrationsofthereactingspeciesasafunctionofconversion.3.5.1EquationsforbatchconcentrationsBatchconcentrationDefinetheparameterWeneedV(X)toobtainCj=hj(X)3.5.2Constant-volumebatchreactionsystemsNochangeinvolumeasthereactionprogressesConstant-volumeConstant-densityWater-gasshiftreactionGas-phasereactions:Liquid-phasereactions:Essentiallyaconstant-volumereactionprocessException:polymerizationprocessesTosummarizeforliquid-phasereactions:Constant-volumebatchreactionsystemsExamples:p71,limitingreactant3.6FlowsystemsEnteringFA0FB0FC0FD0FI0LeavingFAFBFCFDFIStoichiometrictableforaflowsystemSpeciesFlowratetoreactor(mol/time)Changewithinreactor(mol/time)Effluentratefromreactor(mol/time)ABCDI(inerts)Totals3.6.1EquationsforconcentrationinflowsystemsDefinitionofconcentrationforaflowsystem3.6.2Liquid-phaseconcentrationsForliquids,volumechangewithreactionisnegligiblewhennophasechangesaretakingplace.3.6.3ChangeinthetotalnumberofmoleswithreactioninthegasphaseSynthesisofammoniaExamples:

...Batchreactorswithvariablevolumet=0:SeldomencounteredEquationofstateZ:compressibilityfactorValidforbothbatchandflowsystemsCompleteconversionX=1,NT=NTfVariable-volumebatchFlowreactorswithvariablevolumetricflowrateAtentrance:Gaslaw:ConcentrationintermsofconversionforgasflowsystemsandGas-phaseconcentrationasafunctionofconversionConcentrationsinavariable-volumegasflowsystemNotconstant!Examples,P77-83Closure.WritetheratelawintermsofconcentrationandtheArrheniustemperaturedependenceUsethestoichiometrictabletowritetheconcentrationsintermsofconversiontofinallyarriveatarelationshipbetweentherateofreactionandconversion.StoichiometryRateLawMoleBalanceChapter3Chapter3Chapter1Chapter4:combineandevaluateChapter8:energybalance(isothermalreactordesign)

Stoichiometry3.7MeasuresotherthanconversionIntermsofNA,NBorFA,FBTobeincludedinChapter4.3.8ReactionwithcondensationPhase-changeGas-phasereactionCVDprocess:thinfilmdepositiononsolidsurfaces.Ingeneral:Example:P86-87Summary–PartI1.RelativeratesofreactionforthegenericreactionTherelativeratesofreactioncanbewritteneitheras2.Reactionorderisdeterminedfromexperimentalobservation:3.ThetemperaturedependenceofaspecificreactionrateisgivenbytheArrheniusequationA:frequencyfactorE:activationenergyEquilibriumconstantSummary–Part24.ThestoichiometrictableforthereactionSpeciesFlowratetoreactor(mol/time)Changewithinreactor(mol/time)Effluentratefromreactor(mol/time)ABCDI(inerts)Totals5.Inthecaseofidealgases,Batchconstantvolume:Flowsystems:GasLiquid6.Forgas-phasereactions,7.ForpressibleliquidsHoldforgas-phasereactionscarriedoutatconstantvolumeinbatchsystems8.Intermsofgas-phasemolarflowrates,theconcentrationofspeciesiisForsilicontobeusedinthemanufactureofmicroelectronicdevices,itmustcontainlessthan150partspertrillionofimpurities.Thisultrapuresiliconcanbeproducedbyreactingmetallurgicalsilicon(98%pure)andHCltoformtrichlorosilaneandotherproducts.Thismixtureisdistilledandthenthetrichlorosilaneisreactedwithhydrogenat1100Ctoformpolycrystallinesilicon,HCl,andothersilaneproductsinthefollowingreaction:

Setupastoichiometrictableforthefollowingcase:x=1y=2z=2Thetotalpressuremaybetakentobe2atmandthefeedisstoichiometric.Sketchtheconcentrationsofeachspeciesasafunctionofconversion.Thefirststepinsettingupourstoichiometrictableistodeterminethestoichiometriccoefficients(a,b,c,d,&e).

Aftermakingsuretheelementsarebalancedonbothsidesoftheaboveequation,thecompleteequationlookslike: