化學反應工程原理課件 8-1

8MultiphaseReactors8.1Gas-liquidreaction

Themostwidelyusedmodelisthetwo-filmmodel.liquidfilmgasfilmpAGpAi

cAicALzLGcBLcBiLiquidGasinterfaceforgas-liquidandgas-liquid-solidreactionsForagas-liquidreaction

A(g)+BB(l)

RR(l)thereactionstepsare(1)Atransfersfromgasphasetog-linterface;(2)dissolvedAtransfersfromg-linterfacetoliquidandreactswithB.liquidfilmgasfilmpAGpAi

cAicALzLGcBLcBiLiquidGasinterfaceBandRalsotransferinliquid.Atsteadystate,TherateofconversionofAequalstotherateofdiffusionofAthroughtheinterface(massfluxatz=0).liquidfilmgasfilmpAGpAi

cAicALzLGcBLcBiLiquidGasinterface(8.5)DAListhemoleculardiffusivityinliquid.Todeterminethereactionrate,theconcentrationdistributionofAshouldbeobtainedfirst.massfluxthroughtheinterfacekmol/(m2s)(8.5)

A(g)+BB(l)

RR(l)isgenerallyasecondorderreaction.Simplification:Bisinlargeexcessinliquid

firstorderreaction.Themassbalance(diffusion-reactionequation)forAintheliquidfilmis(8.8)Itisbasedonperunitvolume.B.C.:atz=0,

cA=

cAi

atz=L,(8.8)liquidfilmgasfilmpAGpAi

cAicALzLGcBLcBiLiquid

Gasinterfacea

m2/m3

A

transferredthroughtheliquidfilm

Areactedinthebulkliquid

Withthedimensionlessvariables,=cA/cAi,

=z/L,M=kL2/DALthediffusion-reactionequationbecomes(8.10)B.C.:at=0,=1

(8.11)at=1,(8.12)where=1/aL,.volumeofliquidphase

volumeofliquidfilm

M=kL2/DAL

soDAL/L2correspondstotherateofdiffusion.Therefore,M=

rateofreactioninliquidfilm

.

rateofmasstransferinliquidfilm

L2/DAL

isthecharacteristictimeforAtodiffusethroughL

,possiblemaximumvalueLAM=kL2/DAL=

kL(cAi

cAL)Themasstransfercoefficientofliquidphase,kL=DAL/L,isdeterminedexperimentally.=kDAL/kL2Foramoleculardiffusionprocess,themassfluxisLA?Diffusion-reactionequation:B.C.:at=0,=1(8.11)(8.10)at=1,(8.12)where=1/aL,M=kL2/DAL.

ThegeneralsolutionofEq.(8-10)isHattamodulus.Thedimensionlessconcentrationdistributioninliquidphaseis(8.13)From

(RA)=kLcAi

(8.16)where(8.15)isreferredtoastheenhancementfactorofchemicaladsorption.(8-5)rateofphysicalabsorptionDefinetheeffectivenessfactorforgas-liquidreactionasRA:

kmol/(m2s),rA:kmol/(m3s).

isalsoameasureofutilizationofliquidphase.

ratewiththeeffectofmasstransfer

ratewithouttheeffectofmasstransfer=glgl(8.17)i.e.,=1<1

Forapseudo-firstorderirreversiblereaction

rA=kcAi

,(8.18)Discussion:(1)rateofreaction>>rateofmasstransferItisadiffusion-controlled

process,inwhichM>>1,and.theenhancementfactoris(8.15)From

With

(RA)=

kLcAi

(8.16)M=kL2/DAL,and

kL=DAL/L,(8.19)ToincreaseRA:

increasethereactiontemperature,sothatkandDAL

;reducethemasstransferresistanceingasfilm,sothat

cAi;increaseliquidturbulentintensity??withlittleeffect!(8.19)glTheeffectivenessfactorisSince>>1andM>>1,isverysmall,indicatingthatthereactionisnearlycompleteinasmallregionneartheinterface.cAi0LcBLpAGTheholdupofliquidinthereactormaybesmall,suchasinapackedcolumn.Forarapidreaction,iftheamountofBisnotlargeenough,areactioncross-section

mayexists,atwhichtheconcentrationofatleastoneofthereactantsiszero.0LcAireactioncross-sectioncBLsecondorderreaction(2)rateofreaction<<rateofmasstransferItmaybeakineticcontrolledprocess,inwhichM<<1,and,sothattheenhancementfactoris(8.20)and(8.21)FromthedefinitionofandM,Inthiscase,M<<1,but

>>1ingeneral.M>>1<<1??=

rateofreactioninliquidphase

rateofmasstransferinliquidphase

(i)verylarge(i.e.,largeholdupofliquid)

M>>1ispossiblealthoughM<<1,so

1,

1/(M)

Islargeholduphelpful?

Isithelpfultointensifythemasstransferinliquid?cAi0LcBLcALamasstransfercontrolledprocess(ii)isnotverylarge

thenM<<1

M,

1<<1indicatesakineticcontrolledprocess.

Ischemicalreactionhelpfultoabsorption?

0LcBLpAcAatthebeginningcA0LcBLpAatsteadystatettforphysicalabsorptiononly:Absorptionstopsing-lequilibriumofA.withchemicalreaction:Absorptioncontinuesattherateofreaction.Therefore,thereactionishelpful!

1indicatesthatlargeholdupofliquidisalsohelpful!cA0LcBLpAatsteadystateExercise:Chap.8:2,38.2Gas-liquidReactors8.2.1Typesofgas-liquidreactors

(1)packedcolumnsmallholdupofliquidmasstransfercontrolledprocessesrapidreactions

(2)spraytowerformationofsolidGLGL(3)platecolumnlargeholdupofliquidkineticcontrolledprocessessignificantchemicalresistanceandmasstransferresistanceinliquidformationofsolidGL(4)bubblecolumnlargeholdupofliquidsmallinterfacialareaslowreactionbutnotverylargeresistanceofmasstransfer

(5)stirredtankreactorappreciableholdupofliquidandinterfacialareaadaptableGLGLreactora(m2/m3)液含率packedcolumn1200

0.05

2-10spraytower

1200

0.08

40-100platecolumn

1000

0.15

40-100bubblecolumn20

0.984000-104stirredtank200

0.90150-500reactorSelectionofreactors:

(1)rapidreaction(2)significantchemicalresistanceandmasstransferresistanceinliquid(3)kineticcontrol8.2.2Designofbubblecolumnliquid

continuousphasegasdispersephaseDeterminetheheightL.Moleratio

fALQLcB0cBLFgfA0

fALcB

isalsomoleratio.orAssumptions:FgandQLerfacialareaperunitvolumeofgas-liquidmixture(a)isconstant.Assumptions:FgandQLareconstant.Interfacialareaperunitvolumeofgas-liquidmixture(a)isconstant.Thegasflowisaplugflow.

Theconcentrationinbubblesisafunctionofz.Theliquidisamixedflow.

TheconcentrationiscAL

.zLQLcB0cBLFgfA0

fALMaterialbalanceofAin

dz

FgdfA=(RA)aAcdz(8.23)Ac

thecross-sectionalareaRA

basedonunitinterfacialareazdzfAdfAfA(8.24)LQLcB0cBLFgfA0

fAL8.2.3DesignofCSTRAssumption:Bothgasandliquidphasesaremixedflows.

Materialbalances

ofAingasphase

(8.31)ofAinliquidphase(8.32)ofBinliquidphase(8.33)Solvingtheequationssimultaneously.QLFgfA0f

ALcA0cB0cALcBL8.3Gas-liquid-solidReactionsClassification:(1)gas,liquid,andsolidareeitherreactantorproduct.(2)gas-liquidreactionwithsolidcatalyst.(3)oneofthephasesisinertmaterial.Agas-liquidreactionwithsolidcatalystis

A(g)＋BB(l)

RR(l)liquidfilmliquidfilmgasfilmpAGpAiPorouscatalystparticleLiquidGasl-sinterfaceg-linterfacecAicALcASAtransfersdissolvedAtransfersA&

B

diffuse,adsorb&reactRdesorbs&diffusesRates:basedonunitmasscatalystFordilutesolutions,

cAi=

HApAi

.

RA=kcAS

pAGpAiPorouscatalystparticleLiquidGascAicALcAScBLNAG=

kGaL(pAG

pAi

)NAL=

kLaL(cAi

cAL)NAS=

kLSaS(cAL

cAS)+HA