Page 158 - Fluid Catalytic Cracking Handbook

Chemistry of FCC Reactions 133

the cracking reactions decreases with decreasing chain length. With
short chains, it is not possible to form stable carbenium ions.
The initial products of beta-scission are an olefin and a new carbenium
ion (Equation 4-9). The newly-formed carbenium ion will then continue
a series of chain reactions. Small ions (four-carbon or five-carbon) can
transfer the positive charge to a big molecule, and the big molecule
can crack. Cracking does not eliminate the positive charge; it stays
until two ions collide. The smaller ions are more stable and will not
crack, They survive until they transfer their charge to a big molecule,

i+
R _ r "H CH PH — PH CH
IV. " " V,' 1 1 V*' 1. !••-> V--' 1 !••) V--J. .I') " \.~^ I to
+
-* CH 3 — CH = CH 2 + C H 2 — CH 2 — CH 2R (4-9)
Because beta-scission is mono-molecular and cracking is endo-
thermic, the cracking rate is favored by high temperatures and is not
equilibrium-limited.

Isomerization Reactions

Isomerization reactions occur frequently in catalytic cracking, and
infrequently in thermal cracking. In both, breaking of a bond is via
beta-scission. However, in catalytic cracking, carbocations tend to
rearrange to form tertiary ions. Tertiary ions are more stable than
secondary and primary ions; they shift around and crack to produce
branched molecules (Equation 4-10). (In thermal cracking, free radicals
yield normal or straight chain compounds.)

CH 3 — CH, -— C+H — CH, — CH 2R -» CH 3 — C+ — CH — CH 2R

H CR

or

CH — CH 2 — CH 2R

(4-10)

Some of the advantages of isomerization are: