Page 80 - The engineering of chemical reactions

64 Reaction Rates, the Batch Reactor, and the Real World
bed system that replaces the need to shut down to remove coke from the catalyst. This
process is calledjuidized catalytic cracking, or FCC.
The FCC reactor is really two reactors with solid catalyst pellets cycled between them.
The vaporized gas oil is fed along with fresh catalyst to the first, called the reactor, and the
spent catalyst is separated from the products in a cyclone and sent to the regenerator, where
air (now sometimes 02) is added to oxidize the carbon. The flows of reactants, products,
air, and catalyst are indicated in Figure 2-13. The reactor cracks the hydrocarbon and forms
coke on the catalyst. Then in the regenerator the coke is burned off and the catalyst is sent
back into the reactor.
The “magic” of FCC is that the reactor-regenerator combination solves both the
heat management and coking problems simultaneously. Burning off the coke is strongly
exothermic, and this reaction in the regenerator supplies the heat (carried with the hot
regenerated catalyst particles) for the endothermic cracking reactions in the reactor.
Typical operating conditions of these components of the FCC reactor are indicated
in Table 2-5. The residence time in the regenerator is longer than in the reactor, and it is
therefore considerably larger.
Many reactor-regenerator configurations have been developed with quite different
shapes as this technology has evolved and FCC units have become larger. A modem FCC
reactor resembles the Space Shuttle (but one hopes it stays on the ground) in size, tank
configurations, and engineering complexity. The FCC reactor is without question the most
complex and important equipment in chemical engineering.

Heavy oil processing

The “bottom of the barrel” contains heavy, smelly compounds that have polyaromatic rings
and that contain up to several percent of S and N in aromatic rings and in side chains
sulfides and amines. This fraction will not boil below temperatures where the molecules
begin to crack, and it is called “residual” oil or “vacuum resid” if it boils at reduced pressure.
This fraction also contains perhaps 0.1% of heavy metals tied up as porphyrin rings in the
polyaromatics. All these species are severe poisons to either FCC or catalytic reforming

Fkure 2-13 Sketch of a fluidized cat- c o , z - I products
I
I
al& cracking reactor (FCC) for cracking I spent I
heavy petrole& fractions into the boiling
range needed for gasoline. catalyst
egenerator reactor

7 - T

air