Page 58 - Modeling of Chemical Kinetics and Reactor Design

28 Modeling of Chemical Kinetics and Reactor Design

since little or no product is formed. The rate then increases to a
maximum as the products are formed and then decreases to a low value
as the reactants are consumed. Consider the following mechanism for an
autocatalytic reaction:

1 k
2A → B + C (1-116)

A + B  → AB (1-117)

k 2
k

2AB → 3B + C (1-118)
3
where AB is an intermediate complex.
The observed rate of reaction is the rate of formation of species:

+ ( r ) = k C 2 + k C 2 (1-119)
C 1 A 3 AB
The concentration of the reaction intermediate AB may be deter-
mined by using the steady state approximation for intermediates,

2
+ ( r AB ) = k C C B − k C 2 AB ≈ 0 (1-120)
3
2
A
Therefore,
 k  05 .
=
C AB  2 • CC B (1-121)
A
 2 k 3 
Substituting Equation 1-121 into Equation 1-119 gives

+ ( r C ) = k C 2 A + k 2 CC B (1-122)
A
1
2
Therefore, the mechanism results in a rate expression in which species
B is responsible for the autocatalytic behavior.

GAS-SOLID CATALYTIC REACTIONS

Consider a gaseous reactant flowing through a bed of solid catalyst
pellets. The physical steps involved are the transfer of the component