Page 49 - Modeling of Chemical Kinetics and Reactor Design

Reaction Mechanisms and Rate Expressions 19

The following mechanism has been postulated for the gas phase
decomposition of acetaldehyde:

CH CHO  → CH + CO
Thermal decomposition
4
3
Acetaldehyde Methane Carbon monoxide
Initiation


CH CHO → CH + CHO * (1-77)
*
k 1
3
3
This thermal initiation generates two free radicals by breaking a
covalent bond. The aldehyde radical is long-lived and does not markedly
influence the subsequent mechanism. The methane radical is highly
reactive and generates most reactions.
Propagation


CH + CH CHO  → CH + CH CO * (1-78)
*
k 2
4
3
3
3
r

CH CO → CO + CH * (1-79)
*
k 3
3 3
r r
The propagation reactions use a methyl radical and generate another.
There is no net consumption, and a single initiation reaction can result
in an indefinite number of propagation reactions.
Termination

CH + CH  → C H 6 (1-80)
*
*
k 4
3
3
2
The following mechanism has been postulated for the decomposition
of ethane into ethylene and hydrogen. The overall rate expression is
first order in ethane.
Initiation

C H → CH + CH * 3 (1-81)
*
k 1
2
3
6