Page 89 - Synthetic Fuels Handbook

76 CHAPTER THREE

thermally cracked gasoline. Substantial quantities of olefinic gases suitable for poly-
mer gasoline manufacture and smaller quantities of methane, ethane, and ethylene are
produced by catalytic cracking. Sulfur compounds are changed in such a way that the
sulfur content of catalytically cracked gasoline is lower than in thermally cracked gaso-
line. Catalytic cracking produces less heavy residual or tar and more of the useful gas
oils than does thermal cracking. The process has considerable flexibility, permitting the
manufacture of both motor and aviation gasoline and a variation in the gas oil yield to
meet changes in the fuel oil market.
The several processes currently employed in catalytic cracking differ mainly in the
method of catalyst handling, although there is overlap with regard to catalyst type and the
nature of the products.
The catalyst, which may be an activated natural or synthetic material, is employed
in bead, pellet, or microspherical form and can be used as a fixed bed, moving bed, or
fluid bed. The fixed bed process was the first process to be used commercially and uses
a static bed of catalyst in several reactors, which allows a continuous flow of feedstock
to be maintained. Thus, the cycle of operations consists of (a) flow of feedstock through
the catalyst bed, (b) discontinuance of feedstock flow and removal of coke from the
catalyst by burning, and (c) insertion of the reactor on stream. The moving bed process
uses a reaction vessel (in which cracking takes place) and a kiln (in which the spent
catalyst is regenerated) and catalyst movement between the vessels is provided by
various means.
The fluid bed process (Fig. 3.9) differs from the fixed bed and moving bed processes,
insofar as the powdered catalyst is circulated essentially as a fluid with the feedstock. The
several fluid catalytic cracking processes in use differ primarily in mechanical design.
Side-by-side reactor-regenerator construction along with unitary vessel construction (the
reactor either above or below the regenerator) is the two main mechanical variations.

Cold water
Gas to
Waste
recovery
heat
boiler
Water
Fractionator
Flue gas Gasoline
(to final dust Light
collection) Reactor
gas oil
Regenerator
Stripping
steam Heavy
Regen gas oil
Spent
catalyst
catalyst
Fresh feed Slurry
Air
settler
Air blower
Wash oil
Recycle
Slurry
decant oil
FIGURE 3.9 A fluid catalytic cracking (FCC) unit.