化学反应工程原理课件 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.

根据反应机理推导速率方程时，假定：(1)反应由一系列基元反应依次进行，并应用质量作用定律确定基元反应的速率；(2)若某基元反应速率<<其它基元反应速率，则称为速率控制步骤，其反应速率即为整个反应的速率，其余基元反应处于

“拟平衡态”；(3)各基元反应速率基本相同时，各反应均处于“拟定态”。Note:Themechanismisassumed.Thesamerateequationmaybeobtainedfromdifferentmechanism.Themechanismshouldbetestedexperimentally.Empiricalrateequation:basedonexperiments.Generalform:

powerfunction

(2.19)

hyperbolicfunctionTheconcentrationofproductmayaffecttherate.Forareversiblereaction,

AA+BBRRitmaybeassumedthatFromthechemicalequilibriumfor

idealgas,(2.24)

isreferredtoasthestoichiometricnumber.Chemicalequilibriumconstant:化学计量数的物理意义为反应机理中速率控制步骤出现的次数。(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正常动力学反常动力学

ForreactionsobeyingtheArrheniuslaw:

irreversiblesinglereactions:T,r;

multiplereactions:??Forareversiblereaction,(2.32)endothermic

reversiblereaction（可逆吸热）exothermicreversiblereaction（可逆放热）Top

rXATr4

r3

r2

r1

r=0r4

>r3

>r2

>r1