Page 33 - Fluid Catalytic Cracking Handbook

Process Description 1?

enough catalyst to heat the feed and achieve the desired reactor
temperature. In Exxon Model IV and flexicracker designs (see Figure
1-1), the regenerated catalyst flow is mainly controlled by adjusting
the pressure differential between the reactor and regenerator.

Catalyst Separation

As flue gas leaves the dense phase of the regenerator, it entrains
catalyst particles. The amount of entrainment largely depends on the
flue gas superficial velocity. The larger catalyst particles, 50}i-90p, fall
back into the dense bed. The smaller particles, 0|J,-50ji, are suspended
in the dilute phase and carried into the cyclones.
Most FCC unit regenerators employ 4 to 16 parallel sets of primary
and secondary cyclones. The cyclones are designed to recover catalyst
particles greater than 20 microns diameter. The recovered catalyst
particles are returned to the regenerator via the diplegs.
The distance above the catalyst bed at which the flue gas velocity
has stabilized is referred to as the transport disengaging height (TDH).
At this height, the catalyst concentration in the flue gas stays constant;
none will fall back into the bed. The centerline of the first-stage cyclone
inlets should be at TDH or higher; otherwise, excessive catalyst entrain-
ment will cause extreme catalyst losses.

Flue Gas Heat Recovery Schemes

The flue gas exits the cyclones to a plenum chamber in the top of
the regenerator. The hot flue gas holds an appreciable amount of
energy. Various heat recovery schemes are used to recover this energy.
In some units, the flue gas is sent to a CO boiler where both the
sensible and combustible heat are used to generate high-pressure
steam. In other units, the flue gas is exchanged with boiler feed water
to produce steam via the use of a shell/tube or box heat exchanger.
In most units, about two-thirds of the flue gas pressure is let down via
an orifice chamber or across an orifice chamber. The orifice chamber is
a vessel containing a series of perforated plates designed to maintain a
given backpressure upstream of the regenerator pressure control valve.
In some larger units, a turbo expander is used to recover this
pressure energy. To protect the expander blades from being eroded by
catalyst, flue gas is first sent to a third-stage separator to remove the