Page 160 - Fluid Catalytic Cracking Handbook

Chemistry of FCC Reactions 135

those acid sites. Because hydrogen transfer needs adjacent acid sites,
bridging these sites with rare earth promotes hydrogen transfer reactions.
Hydrogen transfer reactions usually increase gasoline yield and
stability. The reactivity of the gasoline is reduced because hydrogen
transfer produces fewer olefins.
Olefins are the reactive species in gasoline for secondary reactions.
1
Therefore, hydrogen transfer reactions indirectly reduce "overcracking'
of the gasoline.
Some of the drawbacks of hydrogen transfer reactions are:
• Lower gasoline octane
* Lower light olefin in the LPG
• Higher aromatics in the gasoline and LCO
* Lower olefin in the front end of gasoline

Other Reactions
Cracking, isomerization, and hydrogen transfer reactions account for
the majority of cat cracking reactions. Other reactions play an impor-
tant role in unit operation. Two prominent reactions are dehydro-
genation and coking.

Dehydrogenation. Under ideal conditions (i.e., a "clean" feedstock
and a catalyst with no metals), cat cracking does not yield any appre-
ciable amount of molecular hydrogen. Therefore, dehydrogenation
reactions will proceed only if the catalyst is contaminated with metals
such as nickel and vanadium.

Coking. Cat cracking yields a residue called coke. The chemistry
of coke formation is complex and not very well understood. Similar
to hydrogen transfer reactions, catalytic coke is a "bimolecular"
reaction. It proceeds via carbenium ions or free radicals. In theory,
coke yield should increase as the hydrogen transfer rate is increased.
It is postulated [4] that reactions producing unsaturates and multi-ring
aromatics are the principal coke-forming compounds. Unsaturates such
as olefins, diolefins, and multi-ring polycyclic olefins are very reactive
and can polymerize to form coke.
For a given catalyst and feedstock, catalytic coke yield is a direct
function of conversion. However, an optimum riser temperature will
minimize coke yield. For a typical cat cracker, this temperature is