Page 201 - Fluid Catalytic Cracking Handbook

Unit Monitoring and Control 169

« Maximize catalyst circulation
* Ensure steady circulation
* Maximize the available pressure drop at the slide valves
* Minimize the loads on the blower and the wet gas compressor
A clear understanding of the pressure balance is extremely important
in "squeezing" the most out of a unit. Incremental capacity can come
from increased catalyst circulation or from altering the differential
pressure between the reactor-regenerator to "free up" the wet gas
compressor or air blower loads. One must know how to manipulate
the pressure balance to identify the "true" constraints of the unit.
Using the drawing(s) of the reactor-regenerator, the unit engineer
must be able to go through the pressure balance and determine whether
it makes sense. He or she needs to calculate and estimate pressures,
densities, pressure buildup in the standpipes, etc. The potential for
improvements can be substantial.

Basic Fluidization Principals

A fluidized catalyst behaves like a liquid. Catalyst flow occurs in
the direction of a lower pressure. The difference in pressure between
any two points in a bed is equal to the static head of the bed between
these points, multiplied by the fluidized catalyst density, but only if
the catalyst is fluidized.
FCC catalyst can be made to flow like a liquid, but only if the
pressure force is transmitted through the catalyst particles and not the
vessel wall. The catalyst must remain in a fluidized state as it makes
a loop through the circuit.
To illustrate the application of the above principals, the role of each
major component of the circuit is discussed in the following sections,
followed by an actual case study. As a reference, Appendix 8 contains
fluidization terms and definitions commonly used in the FCC.

Major Components of the Reactor-Regenerator Circuit

The major components of the reactor-regenerator circuit that either
produce or consume pressure are as follows:
* Regenerator catalyst hopper
* Regenerated catalyst standpipe