Page 209 - Fluid Catalytic Cracking Handbook

Unit Monitoring and Control 177

aeration of the standpipes. In a well-fluidized standpipe, the ex-
3 3
pected catalyst density is in the range of 35 - 45 lb/ft (561 kg/m to
3
721 kg/m ).
If the catalyst density in the spent catalyst standpipe was 40 lb/ft'*
3 3 3
(640 kg/m ) instead of 20 lb/ft (320 kg/m ), the pressure buildup
would have been 4.0 psi instead of 2.0 psi. The extra 2 psi (13.8 K p)
can be used to circulate more catalyst or to lower the reactor pressure.
3 3
In the regenerated catalyst standpipe, a 40 lb/ft (640 kg/m ) catalyst
^ ' 3
density versus a 25.4 lb/ft (407 kg/m") density produces 3 psi (20,7
K p) more pressure head, again allowing an increase in circulation or
a reduction in the regenerator pressure (gaining more combustion air).

Process control instrumentation controls the FCC unit in a safe,
monitored mode with limited operator intervention. Two levels of
process control are used:

• Basic supervisory control
• Advanced process control (APC)

Basic Supervisory Control

The primary controls in the reactor-regenerator section are flow,
temperature, pressure, and catalyst level.
The flow controllers are often used to set desired flows for the fresh
feed, stripping steam, and dispersion steam. Each flow controller
usually has three modes of control: manual, auto, and cascade. In
manual mode, the operator manually opens or closes a valve to the
desired percent opening. In auto mode, the operator enters the desired
flow rate as a set-point. In cascade mode, the controller set-point is
an input from another controller.
The reactor temperature is controlled by a temperature controller that
regulates the regenerated catalyst slide valve. The regenerator tempera-
ture is not automatically controlled but depends on its mode of
operation. In partial combustion, the regenerator temperature is con-
trolled by adjusting the flow of combustion air to the regenerator. In
full burn, the regenerator temperature is a function of operating
conditions such as reactor temperature and slurry recycle.