Page 29 - Fluid Catalytic Cracking Handbook

Process Description 13

the falling catalyst tends to entrain steam, thus reducing the effective–
ness of stripping steam.
It is important to minimize the amount of hydrocarbon vapors
carried through to the regenerator, but not all the hydrocarbon vapors
can be displaced from the catalyst pores in the stripper. A fraction of
them are carried with the spent catalyst into the regenerator. These
hydrocarbon vapors/liquid have a higher hydrogen-to-carbon ratio than
the coke on the catalyst. The drawbacks of allowing these hydrogen-
rich hydrocarbons to enter the regenerator are as follows:

* Loss of liquid product. Instead of the hydrocarbons burning in the
regenerator, they could be recovered as liquid products.
« Loss of throughput. The combustion of hydrogen to water pro-
duces 3.7 times more heat than the combustion of carbon to
carbon dioxide. The increase in the regenerator temperature caused
by excess hydrocarbons could exceed the temperature limit of the
regenerator internals and force the unit to a reduced feed rate
mode of operation.
* Loss of catalyst activity. The higher regenerator temperature
combined with the formation of steam in the regenerator reduces
catalyst activity by destroying the catalyst's crystalline structure.
The flow of spent catalyst to the regenerator is typically controlled
by a valve that slides back and forth. This slide valve is controlled
by the catalyst level in the stripper. The catalyst height in the stripper
provides the pressure head, which allows the catalyst to flow into the
regenerator. The exposed surface of the slide valve is usually lined
with refractory to withstand erosion. In a number of earlier FCC
designs, lift air is used to transport the spent catalyst into the regener-
ator (Figure 1-10).

REGENERATOR–HEAT/CATALYST RECOVERY

The regenerator has two main functions: it restores catalyst activity
and supplies heat to crack the feed. The spent catalyst entering the
regenerator contains between 0.4 wt% and 2.5 wt% coke, depending
on the quality of the feedstock. Components of coke are carbon,
hydrogen, and trace amounts of sulfur and nitrogen. These burn
according to the following reactions.