Page 159 - Fluid Catalytic Cracking Handbook

134 Fluid Catalytic Cracking Handbook

* Higher octane in the gasoline fraction. Isoparaffins in the gasoline
boiling range have higher octane than normal paraffins.
* Higher-value chemical and oxygenate feedstocks in the C 3/C 4
fraction. Isobutylene and isoamylene are used for the production
of methyl tertiary butyl ether (MTBE) and tertiary amyl methyl
ether (TAME). MTBE and TAME can be blended into the gasoline
to reduce auto emissions.
* Lower cloud point in the diesel fuel. Isoparaffins in the light cycle
oil boiling range improve the cloud point.

Hydrogen Transfer Reactions

Hydrogen transfer is more correctly called hydride transfer. It is a
bimolecular reaction in which one reactant is an olefin. Two examples
are the reaction of two olefins and the reaction of an olefin and
a naphthene.
In the reaction of two olefins, both olefins must be adsorbed on
active sites that are close together. One of these olefins becomes a
paraffin and the other becomes a cyclo-olefin as hydrogen is moved
from one to the other. Cyclo-olefin is now hydrogen transferred with
another olefin to yield a paraffin and a cyclodi-olefin. Cyclodi-olefin
will then rearrange to form an aromatic. The chain ends because
aromatics are extremely stable. Hydrogen transfer of olefins converts
them to paraffins and aromatics (Equation 4-11).

4 C rlH 2n -» 3 C n H 2n+2 + C nH 2n^

olefins —> paraffins + aromatic (4-11)

In the reaction of naphthenes with olefins, naphthenic compounds
are hydrogen donors. They can react with olefins to produce paraffins
and aromatics (Equation 4-12).

3 C nH 2n + C mH 2m -» 3 C n H 2n+2 + C m H 2m^ 6
olefins + naphthene —> paraffins + aromatic (4-12)

A rare-earth-exchanged zeolite increases hydrogen transfer reactions.
In simple terms, rare earth forms bridges between two to three acid
sites in the catalyst framework. In doing so, the rare earth protects