Page 264 - Modeling of Chemical Kinetics and Reactor Design

234 Modeling of Chemical Kinetics and Reactor Design

FLUID CATALYTIC CRACKING (FCC) UNIT

The fluid catalytic cracking (FCC) process, as shown in Figure 4-
10, converts straight-run atmospheric gas oil, vacuum gas oils, and
heavy stocks recovered from other operations into high-octane gaso-
line, light fuel oils, slurry oil, and olefin-rich light gases. These
products undergo further processing and separation in the FCC unit
main fractionator and other vessels downstream of the FCC reactor.
The gasoline produced has good overall octane characteristics and an
excellent octane number. The catalysts used are mixtures of crystalline
alumina silicates (known as zeolites), active alumina, silica-alumina,
clay, and rare earth oxides.
In the FCC, an oil feed composed of heavy hydrocarbon molecules
is mixed with catalyst and enters a fluidized bed reactor. The long
molecules react on the surface of the catalyst and are cracked into
lighter product molecules (e.g., gasoline), which leave the reactor from
the top. During the cracking process, carbon and other heavy uncracked
organic materials are deposited on the surface of the catalyst resulting
in its deactivation. The catalyst is then taken into a regenerator where
the deposited surface material is burned with air. The regenerated
catalyst returns to the reactor after it has been mixed with fresh feed.
The activity of the newer catalysts is so intense that much of the
cracking takes place in the line returning the regenerated catalyst to
the reactor. This process is referred to as the transfer line cracking.
A salient feature of the fluidized bed reactor is that it operates at
nearly constant temperature and is, therefore, easy to control. Also,
there is no opportunity for hot spots (a condition where a small
increase in the wall temperature causes the temperature in a certain
region of the reactor to increase rapidly, resulting in uncontrollable
reactions) to develop as in the case of the fixed bed reactor. However,
the fluidized bed is not as flexible as the fixed bed in adding or
removing heat. The loss of catalyst due to carryover with the gas
stream from the reactor and regenerator may cause problems. In this
case, particle attrition reduces their size to such an extent where they
are no longer fluidized, but instead flow with the gas stream. If this
occurs, cyclone separators placed in the effluent lines from the reactor
and the regenerator can recover the fine particles. These cyclones
remove the majority of the entrained equilibrium size catalyst particles
and smaller fines. The catalyst fines are attrition products caused by