Page 25 - Fluid Catalytic Cracking Handbook

Process Description 9

of refineries are modifying the riser termination devices to minimize
these reactions.
The riser is a vertical pipe. It usually has s 4- to 5-inch (10 to 13
cm) thick refractory lining for insulation and abrasion resistance.
Typical risers are 2 to 6 feet (60 to 180 cm) in diameter and 75 to
120 feet (25 to 30 meters) long. The ideal riser simulates a plug flow
reactor, where the catalyst and the vapor travel the length of the riser
with minimum back mixing.
Efficient contacting of the feed and catalyst is critical for achieving
the desired cracking reactions. Steam is commonly used to atomize
the feed. Smaller oil droplets increase the availability of feed at the
reactive acid sites on the catalyst. With high-activity zeolite catalyst,
virtually all of the cracking reactions take place in three seconds or less.
Risers are normally designed for an outlet vapor velocity of 50 ft/sec
to 75 ft/sec (15.2 to 22.8 m/sec). The average hydrocarbon residence
time is about two seconds (based on outlet conditions). As a consequence
of the cracking reactions, a hydrogen-deficient material called coke is
deposited on the catalyst, reducing catalyst activity.

Catalyst Separation

After exiting the riser, catalyst enters the reactor vessel. In today's
FCC operations, the reactor serves as a housing for the cyclones. In
the early application of FCC, the reactor vessel provided further bed
cracking, as well as being a device used for additional catalyst separation.
Nearly every FCC unit employs some type of inertial separation
device connected on the end of the riser to separate the bulk of the
catalyst from the vapors. A number of units use a deflector device to
turn the catalyst direction downward. On some units, the riser is
directly attached to a set of cyclones. The term "rough cut" cyclones
generally refers to this type of arrangement. These schemes separate
approximately 75% to 99% of the catalyst from product vapors.
Most FCC units employ either single or two-stage cyclones (Figure
1-7) to separate the remaining catalyst particles from the cracked
vapors. The cyclones collect and return the catalyst to the stripper
through the diplegs and flapper/trickle valves (See Figure 1-8). The
product vapors exit the cyclones and flow to the main fractionator
for recovery. The efficiency of a typical two-stage cyclone system
is 99.995+%.