Page 218 - Fluid Catalytic Cracking Handbook

186 Fluid Catalytic Cracking Handbook

is particularly useful where the reactor temperature is already
maximized due to a metallurgy constraint.
» Increasing catalyst to oil ratio. The catalyst to oil ratio can be
increased through several knobs including: reducing the FCC feed
preheat temperature, optimizing the stripping and dispersion steam
rate, and using a catalyst that deposits less coke on the catalyst,
Reduction of the catalyst/hydrocarbon time in the riser, coupled with
the elimination of post-riser cracking, reduces the saturation of the
"already produced olefins" and allows the refiner to increase the
reaction severity. The actions enhance the olefin yields and still operate
within the wet gas compressor constraints. Elimination of post-riser
residence time (direct connection of the reactor cyclones to the riser)
or reducing the temperature in the dilute phase virtually eliminates
undesired thermal and nonselective cracking. This reduces dry gas and
diolefin yields.
Adding ZSM-5 catalyst additive is another process available to the
refiner to boost production of light olefins. ZSM-5 at a typical con-
centration of 0.5 to 3.0 wt% is used in a number of FCC units to
increase the gasoline octane and light olefins. As part of the cracking
of low octane components in the gasoline, ZSM-5 also makes C 3, C 4,
and C 5 olefins (see Figure 6-2). Paraffinic feedstocks respond the most
to ZSM-5 catalyst additive.

Gasoline

FCC gasoline has always been the most valuable product of a cat
cracker unit. FCC gasoline accounts for about 35 vol% of the total
U.S. gasoline pool. Historically, the FCC has been run for maximum
gasoline yield with the highest octane.

Gasoline Yield
For a given feedstock, gasoline yield can be increased by:

• Increasing catalyst-to-oil ratio by decreasing the feed preheat
temperature
• Increasing catalyst activity by increasing fresh catalyst addition
or fresh catalyst activity
• Increasing gasoline end point by reducing the main column top
pumparound rate