Page 210 - Fluid Catalytic Cracking Handbook

178 Fluid Catalytic Cracking Handbook

The reactor pressure is not directly controlled; instead, it floats on
the main column overhead receiver. A pressure controller on the
overhead receiver controls the wet gas compressor and indirectly
controls the reactor pressure. The regenerator pressure is often con-
trolled directly by regulating the flue gas slide or butterfly valve. In
some cases, the flue gas slide or butterfly valve is used to control the
differential pressure between the regenerator and reactor.
The reactor or stripper catalyst level controller is controlled with
a level controller that regulates the movement of the spent catalyst
slide valve. The regenerator level is manually controlled to maintain
catalyst inventory.

Regenerated and Spent Catalyst Slide Valve
Low Differential Pressure Override
Normally, the reactor temperature and the stripper level controllers
regulate the movement of the regenerated and spent catalyst slide
valves. The algorithm of these controllers can drive the valves either
fully open or fully closed if the controller set-point is unobtainable.
It is extremely important that a positive and stable pressure differential
be maintained across both the regenerated and spent catalyst slide
valves. For safety, a low differential pressure controller overrides the
temperature/level controllers should these valves open too much. The
shutdown is usually set at 2 psi (14 K p).
The direction of catalyst flow must always be from the regenerator
to the reactor and from the reactor back to the regenerator. A negative
differential pressure across the regenerated catalyst slide valve can
allow hydrocarbons to back-flow into the regenerator. This is called
a flow reversal and can result in an uncontrolled afterburn and
possible equipment damage. A negative pressure differential across
the spent catalyst slide valve can allow air to back-flow from the
regenerator into the reactor with equally disastrous consequences.
To protect the reactor and the regenerator against a flow reversal,
pressure differential controllers are used to monitor and control the
differential pressures across the slide valves. If the differential pressure
falls below a minimum set-point, the pressure differential controller
(PDIC) overrides the process controller and closes the valve. Only
after the PDIC is satisfied will the control of the slide valve return to
the process.