Page 35 - Modeling of Chemical Kinetics and Reactor Design

Reaction Mechanisms and Rate Expressions 5

TYPICAL REACTION MECHANISMS

CRACKING OF ALKANES (Paraffins C H 2n+2 )
n
Pyrolysis of alkanes is referred to as cracking. Alkanes from the
paraffins (kerosene) fraction in the vapor state are passed through a
metal chamber heated to 400–700°C. Metallic oxides are used as a
catalyst. The starting alkanes are broken down into a mixture of
smaller alkanes, alkenes, and some hydrogen.

o
−
Alkanes  → Smaller alkanes + alkenes + H 2
400 700
C
For example:
o
400 700 C−
2CH CH CH  → CH + CH CH = CH + CH = CH + H (1-1)
3 2 3 4 3 2 2 2 2
Propene Ethene
Mechanism

The reaction proceeds via a free radical mechanism with homolytic
fission occuring between the carbon-carbon atoms. The mechanism of
reactions for the cracking of propane is:

Chain initiation:
1. CH CH − CH → CH CH ∗ + CH ∗ (1-2)
3 2 3 3 2 3
Chain propagation:

2. CH CH → CH = CH + H * (1-3)
*
3 2 2 2
3. CH CH CH + H → H + CH CHCH 3 (1-4)
˙
*
3
2
3
2
3
˙
4. CH CH CH + CH → CH + CH CHCH (1-5)
*
3 2 3 3 4 3 3
Chain termination:
∗
5. 2CH CH → CH = CH + CH CH (1-6)
3 2 2 2 3 3
˙
6. 2CH CHCH → CH CH = CH + CH CH CH 3 (1-7)
2
3
3
3
3
2