化学反应工程原理课件 7-1

7DesignandAnalysis

ofHeterogeneousCatalyticReactors

Thereactorsdiscussedinthischapter:

packedbedcatalyticreactor

fluidizedbedreactor

Objectives:desiredconversion

reactionvolume(amountofcatalyst)7.1TransportPhenomenainaPackedBed7.1.1Flowoffluidinafixedbed

walleffect

transitionfromlaminartoturbulentflow

pressuredropThepressuredropthroughapackedbedis

(fromPrinciplesofChemicalEngineering)whereu0isthesuperficialvelocity,andaisthespecific(external)surfacearea.Withtheequivalentdiameterds=6/a,(7.1)Re<10,laminarflow;Re>1000,fullydevelopedturbulence.(7.1)(7.2)andwherefisthefrictionalcoefficient,Exercise:Chap.7:37.1.2AxialmassandheattransferItistheratioofconvectiontodiffusion,whereDaistheaxialdiffusioncoefficient

includingmoleculardiffusion,vortexdiffusionandsoon.=ReSc

Definethe

Pecletnumberforaxialmasstransferas

DefinePecletnumberforaxialheattransferasItistheratioofconvectiontothermaldiffusion,whereeaistheaxialeffectiveconductioncoefficientincludingconduction,convectionandradiation.=RePrPecletnumberWhenPe>>1,thebackmixingcanbeneglectedwithrespecttotheconvection,i.e.PFR.Chap.5:

rateofconvection

rateofdispersionTheparticleReynoldsnumberis

ExperimentsshowthatwhenRep>10,(Pea)m=2forgas,(Pea)m=0.31forliquid.NotetherelationPe=(Pea)m

L/dp

andL/dp>>50formostoffixedbedreactors.Pe>>100PFRNon-isothermalcondition:

homogeneousreactors:

(Pea)m

(Pea)h

fixedbedreactors:

heattransfer:influidandsolidphasesmasstransfer:influidphaseonly(Pea)h

(Pea)m/3ForL/dp>150

PFR7.1.3RadialmassandheattransferThemassandheattransferinradialdirectionalwaysexists.heatcarrier7.2MathematicalModel

forFixedBedReactorspseudo-homogeneousmodel

theeffectsofmassandheattransferarenegligible.heterogeneousmodelsimultaneousmomentum,heatandmasstransfer.Heterogeneousmodel:Simplifications:largeL/dp(negligiblebackmixing)largeG(negligibleresistanceforheatandmasstransferinboundarylayers)averageT

andcincrosssection

PFR

(forfluidoutsideparticlesandparticles)+

(effectofinternaldiffusion)or0Massbalance:Energybalance:(7.9)(perunitvolume)Forcoolingorheatingmedium:(perunitmasscatalyst)(7.13)(7.10)Momentumbalance:

Theeffectofpressuredifferenceonreactionsmaybeneglectedexceptunderhighpressureandlargepressuredifference.(7.11)Boundaryconditions:

atZ=0,XA=0,

T=T0,Tc=Tc0,

p=p0

(7.12)Calculations:

XAL(heightofthefixedbed)

XA,Loperationcondition

IfL/dpisnotsolargethattheeffectofbackmixingshouldbeconsidered.Forisothermalsinglereactionswithconstant-volume,thedispersionmodelis(7.18)Boundarycondition:(closed-closed)(7.20c)atZ=L,(7.20a)atZ=0,Dataforreactordesign:(1)dataonreactionkinetics

orderofreaction,rateconstant,…(2)dataonthermodynamics

heatofreaction,heatcapacity,equilibriumconstant,…(3)dataontransportphenomena

viscosity,diffusivity,conductivity,…

poresizedistribution,p,b,specificsurfacearea,…7.3AdiabaticFixedBedReactors7.3.1TypesofadiabaticfixedbedreactorssinglepackedbedreactorstagedpackedbedreactorsRadialheattransfer?No!gasincoolant

withintercooling

withcoldshotcoolingfeedoroneofreactants7.3.2Calculation

foradiabaticfixedbedreactorsForasingleadiabaticfixedbedreactor,thecalculationissimilartothatinChap.4.Fromtheenergybalance,wehave

T

T0=XA(7.23)whereFromthemassbalance,weobtain

thelength(orheight)ofthereactor(7.24)

or

thevolumeofcatalyst(7.25)7.3.3StagedadiabaticfixedbedreactorsHereweonlyconsiderthosewithintercooling.gasinheatcarriercA1,T1cA1,T20cA0,T0cA2,T2anadiabaticfixedbedreactorheatexchangerFigure7-4.Reversibleexothermicreaction:equilibriumoptimumtemp.XATABCDEFgasinheatingcA1,

T1cA1,T20cA0,T0cA2,T2cA2,T30cA3,T3ABDCFEGoperatinglineOptimization:

minimumcostforproduct

maximumthroughput

minimumamountofcatalyst:Forcertainnumberofstages,rateandcompositionoffeed,andfinalconversion,thevariablesforoptimizationareXAiandTi0,total2N1variables.Exercises:Chap.7:6,157.4PackedBedReactors

withHeatExchangedp<D/8toavoidchanneling,butiftheparticleistoosmall,thepressuredifferenceincreases.heatcarrierfeed

product

Shellandtubereactor:Comparisonwithadiabaticpackedbedreactors:

moreuniformtemperaturedistribution

lessbackmixing

easiertoscale-upRelationoftemperatureandconversion:adiabatic:theslopeofoperatinglineisconstant;withheatexchange:theslopechanges.

FromEqs.(7-9)and(7-10),(7.32)rateofheattransferrateofheatreleaseFigure7.6:reversibleexothermicreactionadiabatic(AK)XATequilibriumoptimumtemp.AK(7.32)rateofheatreleaserateofheattransferbetter

withheatexchange:

hotspot

mayexist.Theposition:dT/dz

=0.Figure4C:reversibleendothermicreaction

adiabatic

XATZ

withheatexchange:

coldpoint

mayexistFigure7.7:axialtemperaturedistributionTemperaturerunaway:thetemperatureatthehotspotincreasesrapidlyasthefeedtemperatureincreases.TZ7.5PackedBedReactorswithSelf-heating

Thereactionisexothermicandthefeedisheatedbytheheatofreaction.

countercurrent

cocurrent

T0LT0LThepackedbedreactorswithself-heatingisaspecialcaseofthosewithheatexchange.Bothmaybeoperatedcountercurrentlyor

cocurrently,andhavethephenomenaofhotspot,temperaturerunaway,….

7.6EffectsofParameters

Weusuallyconcernwiththeeffectsofparametersonthetemperatureathotspot,temperatureT0andconcentrationwA0

ofthefeedtemperatureofcoolantTc

operatingpressurePrateofheattransferU/d…Itneedsextensivelycomputation.Exercises:Chap.7:12,17

7.7FluidizedBedReactorsMainparts:shellgasdistributorheatexchangedeviceinternalsgas-solidseparatorparticlechargeanddischargedevice

lgulgpumffixedbedMNfluidizedbedApplications:

gas-solidliquid-solidgas-liquid-solidComparisonoffixedandfluidizedbedreactors:(1)backmixing(2)particlesizeandfluidvelocity(3)lossofcatalystandreplacement(4)controloftemperature(5)design7.8CatalyticReactorsinLaboratoryexperimentalmethodsforfindingreactionrateasafunctionoftemperature,pressure,composition,…7.8.1Basicrequirements

Todeterminetheintrinsickinetics,effectsofheatandmasstransfershouldbeeliminated.

Thegradientsofconc.andtemp.resultinginmassandheattransfer:

insideandoutsideparticle

radialandaxialdirectionofreactorIftheeightgradientsareeliminated,thereactorisamixedflowreactor

(reactorwithoutgradients).Atleast,theflowisaplugflow.Basicrequirements:

isothermalandidealflow.7.8.2Typesofexperimentaldevices(1)Integral(plugflow)reactorwithsmalldiameter,e.g.,D

1cm(PFR),mergedinanisothermalb