化學(xué)反應(yīng)工程原理課件 2-1

2FundamentalsofReactionKinetics

Chemicalkineticsisthebasisforobtaining(1)reactionmechanism;(2)equationofreactionrate.2.1RateofChemicalReaction

1.Definitionofreactionrate

Forareaction

AA+BB=RR(2.1)basedonunitvolumeofreactingfluid,thereactionrateisNote:

rispositive.Therelationbetweenmolenumbernandmolarconcentrationcis

nA

=

VcA

sothatthereactionrateisexpressedasForaconstant-volumereactor,

Forgases,thereactionratemaybeexpressedas

SincecA=cA0(1XA)Theyarevalidforconstant-volumereactors.

2.Therelationofreactionratesforcomponents

BecausednA

:

dnB:

dnR=A:B:R

,wehave(constant)

(2.3)Thegeneralexpressionisor(2.4)

3.ReactionrateondifferentbasisForacontinuousreactor,reactantAentersthereactorinmolarflowrate

FA0,basedonunitvolume,thereactionrateisdVFAFA0FAdFA

(2.7)Formultiphasereactions,itmaybebasedonunitinterfacialarea

Forfluid-solidsystems,itmaybebasedonunitmassofsolid,whereaisinterfacialarea,andWisusuallythemassofcatalyst.or(2.8)or(2.9)Therelationiswherebisthestackingdensity,andaVistheexternalspecificarea.(2.10)

4.Determinationofreactionrate

byexperimentNotethedifferencein

intrinsickinetics

apparentkineticsExercise:Chap.2:1

pAt2.2ReactionRateEquationswhereistheconcentrationvector.Foranelementaryreaction

AA+BB

PP+SStherateequationisForacertainreactingsystemundercertainpressure,therateequationis(2.11)(2.12)Foranonelementaryreaction,weassumeasequenceofelementaryreactions:reactionmechanism(reactionpath)overallreactionrate,verifiedexperimentally.

根據(jù)反應(yīng)機(jī)理推導(dǎo)速率方程時(shí)，假定：(1)反應(yīng)由一系列基元反應(yīng)依次進(jìn)行，并應(yīng)用質(zhì)量作用定律確定基元反應(yīng)的速率；(2)若某基元反應(yīng)速率<<其它基元反應(yīng)速率，則稱為速率控制步驟，其反應(yīng)速率即為整個(gè)反應(yīng)的速率，其余基元反應(yīng)處于

“擬平衡態(tài)”；(3)各基元反應(yīng)速率基本相同時(shí)，各反應(yīng)均處于“擬定態(tài)”。Note:Themechanismisassumed.Thesamerateequationmaybeobtainedfromdifferentmechanism.Themechanismshouldbetestedexperimentally.Empiricalrateequation:basedonexperiments.Generalform:

powerfunction

(2.19)

hyperbolicfunctionTheconcentrationofproductmayaffecttherate.Forareversiblereaction,

AA+BBRRitmaybeassumedthatFromthechemicalequilibriumfor

idealgas,(2.24)

isreferredtoasthestoichiometricnumber.Chemicalequilibriumconstant:化學(xué)計(jì)量數(shù)的物理意義為反應(yīng)機(jī)理中速率控制步驟出現(xiàn)的次數(shù)。(2.25)?2.3EffectofTemperatureonReactionRate

AtagivenT,therateconstantkisaconstant.Chemicaltheorypredictsthatk

Tnexp(E/RT)WeusuallyuseanArrhenius-typerelation

k=

Aexp(E/RT)

(2.26)

AandEaredeterminedwith

lnk=lnA

E/RT(2.26a)Note:

Theunitofkdependsontheexpressionofreactionrate,rateequation,andconcentration.Forareactioningasphase,conc.pcyk

kp

kc

ky

Foridealgasandapower-lawrateequation,

kc=(RT)kp=(RT/p)ky

(2.27)

p-totalpressure,-overallreactionorder.

Forareversiblereaction,wemayobtainfromEq.(2-26a)FromEq.(2-25),wehaveor(2.28)(2.29)chemicalequilibriumconstantTheeffectofTonKPmaybeexpressedbythevan’tHoffequation,Hrtheenthalpydifferenceofreaction:Hr>0,endothermicreaction;Hr<0,exothermicreaction.Therefore,(2.30)(2.31)Inmostofreactions,thereactionrateincreaseswithtemperature.However,somecomplexconditionsmayoccur,e.g.rTrTrT正常動(dòng)力學(xué)反常動(dòng)力學(xué)

ForreactionsobeyingtheArrheniuslaw:

irreversiblesinglereactions:T,r;

multiplereactions:??Forareversiblereaction,(2.32)endothermic

reversiblereaction（可逆吸熱）exothermicreversiblereaction（可逆放熱）Top

rXATr4

r3

r2

r1

r=0r4

>r3

>r2

>r1