Page 234 - Fluid Catalytic Cracking Handbook

202 Fluid Catalytic Cracking Handbook

« Reactor temperature. An increase in the reactor temperature will
also reduce delta coke by favoring cracking reactions over hydro-
gen transfer reactions. Hydrogen transfer reactions produce more
coke than cracking reactions.
* Catalyst activity. An increase in catalyst activity will increase delta
coke. As catalyst activity increases so does the number of adjacent
sites, which increases the tendency for hydrogen transfer reactions
to occur. Hydrogen transfer reactions are bimolecular and require
adjacent active sites.

FCC ECONOMICS

The cat cracker's operational philosophy is dictated by refinery
economics. Economics of a refinery are divided into internal and
external economics.
Internal economics largely depends on the cost of raw crude and
the FCC unit's yields. The cost of crude can outweigh the benefit from
the cat cracker yields. Refiners who operate their units by a kind of
intuition may drive for more throughput, but this may not be the most
profitable approach.
External economics are factors that are generally forced upon the
refineries. Refiners prefer not to have their operations dictated by
external economics. However, they may have to meet particular
requirements such as those for reformulated gasoline.
To maximize the FCCU's profit, the unit must be operated against
all its mechanical and operating constraints. Generally speaking, the
incremental profit of increasing feed is more than the incremental
profit from increasing conversion. The general target is to maximize
gasoline yield while maintaining the minimum octane that meets
blending requirements.
Because of the high cost of new units and the importance of the
FCC on refinery profitability, improvements should be made to the
existing units to maximize their performance. These performance
indices are:
* Improving product selectivity
* Enhancing operating flexibility
* Increasing unit capacity
* Improving unit reliability