Page 153 - Fluid Catalytic Cracking Handbook

128 Fluid Catalytic Cracking Handbook

R-CH 2-CH 2-CH 2-CH 2-'CH-CH 2-CH 3 -» R-CH 2-CH 2-'CH 2

(4-4)
+ H 2C=CH-CH 2-CH 3
Similar to the methyl radical, the R-*CH 2 radical can also extract a
hydrogen atom from another paraffin to form a secondary free radical
and a smaller paraffin (Equation 4-5).

R,-'CH 2 + R-CH 2-CH 2-CH 2-CH 2-CH 2-CH 2-CH 3 -> R,-CH 3

+ R-CH 2-CH 2-CH 2-CH 2-CH 2-*CH-CH 3

R-*CH ? is more stable than H 3*C. Consequently, the hydrogen extrac-
tion rate of R-*CH 2 is lower than that of the methyl radical.
This sequence of reactions forms a product rich in C } and G,,
and a fair amount of alpha-olefins. Free radicals undergo little branch-
ing (isomerization).
One of the drawbacks of thermal cracking in an FCC is that a high
percentage of the olefins formed during intermediate reactions poly-
merize and condense directly to coke.
The product distribution from thermal cracking is different
from catalytic cracking, as shown in Table 4-2. The shift in product
distribution confirms the fact that these two processes proceed via
different mechanisms,

CATALYTIC CRACKING

Catalytic reactions can be classified into two broad categories:
* Primary cracking of the gas oil molecules
• Secondary rearrangement and re-cracking of cracked products
Before discussing mechanisms of the reactions, it is appropriate to
review FCC catalyst development and examine its cracking properties.
An in-depth discussion of FCC catalyst was presented in Chapter 3.

FCC Catalyst Development

The first commercial fluidized cracking catalyst was acid-treated
natural clay. Later, synthetic silica-alumina materials containing 10 to