Page 157 - Fluid Catalytic Cracking Handbook

132 Fluid Catalytic Cracking Handbook

R — CH 2 -— CH 2 — CH 2 — CH 3 (removal of H~ @ Lewis site)

+
_» R _ c H — CH 2 — CH 2 — CH 3 (4-8}
Both the Bronsted and Lewis acid sites on the catalyst generate
carbenium ions. The Bronsted site donates a proton to an olefin
molecule and the Lewis site removes electrons from a paraffin mole-
cule. In commercial units, olefins come in with the feed or are pro-
duced through thermal cracking reactions.
The stability of carbocations depends on the nature of alkyl groups
attached to the positive charge. The relative stability of carbenium ions
is as follows [2] with tertiary ions being the most stable:

Tertiary > Secondary > Primary > Ethyl > Methyl
C ~ C + P P P+ P R P P + P P + P*
R . '"•"""" V-- "" V.- V_^ \*s V~" V-"' JLX. V-' V_-- V-- *—• V,,'
c

One of the benefits of catalytic cracking is that the primary and
secondary ions tend to rearrange to form a tertiary ion (a carbon with
three other carbon bonds attached). As will be discussed later, the
increased stability of tertiary ions accounts for the high degree of
branching associated with cat cracking.
Once formed, carbenium ions can form a number of different
reactions. The nature and strength of the catalyst acid sites influence
the extent to which each of these reactions occur. The three dominant
reactions of carbenium ions are:

* The cracking of a carbon-carbon bond
* Isomerization
* Hydrogen transfer

Cracking Reactions

Cracking, or beta-scission, is a key feature of ionic cracking. Beta-
scission is the splitting of the C-C bond two carbons away from the
positive-charge carbon atom. Beta-scission is preferred because the
energy required to break this bond is lower than that needed to break
the adjacent C-C bond, the alpha bond. In addition, short-chain hydro-
carbons are less reactive than long-chain hydrocarbons. The rate of