Page 65 - Modeling of Chemical Kinetics and Reactor Design

Reaction Mechanisms and Rate Expressions 35

4. If component i takes place in more than one elementary step, then

± ( ) = ∑ ± ( ) r i (1-147)
r
i
All elementary steps
For example, at 480°C the gaseous decomposition of acetaldehyde
has an order with respect to concentration of 3/2. The main
reaction is CH CHO = CH + CO
4
3
A possible reaction mechanism is:

Initiation

1. CH CHO → CH + CHO *
*
k 1
3
3
Propagation

*
k 2
2. r CH + CH CHO  → CH + CH CO *
3
3
3
4
∗
∗

k 3
3. CH CO → CH + CO
3 r 3
Termination

4. CH + CH  → C H 6
*
*
k 4
3
3
2
It is assumed that for each initiation there are many propagation
cycles before termination. The main reaction is therefore given by the
addition of the propagation steps alone, which gives the correct
stoichiometric equation. A small amount of ethane, C H , is expected
6
2
due to the termination reaction.
The rate expressions are:
( +r CH CO * ) net = dC CH CO * = kC CH 3 * C CH CHO − k C CH CO * (1-148)
3
2
3
3
dt
3
3
( +r ) = dC CH 3 * = kC + k C
3
3
CH 3 * dt 1 CH CHO 3 CH CO *
net
(1-149)
− kC * C CH CHO − k C 2 *
2
4
2
CH 3 3 CH 3