Page 203 - Fluid Catalytic Cracking Handbook

Unit Monitoring and Control 171

Aside from proper aeration, the flowing catalyst must contain
sufficient 0-40 micron fines to avoid defluidization.

Regenerated Catalyst Slide Valve
The purpose of the regenerated catalyst slide valve is threefold: to
regulate the flow of regenerated catalyst to the riser, to maintain
pressure head in the standpipe, and to protect the regenerator from a
flow reversal. Associated with this control and protection is usually a
I psi to 8 psi (7 K p to 55 K p) pressure drop across the valve.
Riser
The hot-regenerated catalyst is transported up the riser and into the
reactor-stripper. The driving force to carry this mixture of catalyst and
vapors comes from a higher pressure at the base of the riser and the
low density of the catalyst/vapor mix. The large density difference
between the fluidized catalyst on the regenerator side (approximately 40
3
lb/ft ) and the mixture of cracked hydrocarbon vapors and catalyst on the
3
riser side (approximately 1 lb/ft ) drives the system. As for the pressure
balance, this transport of catalyst results in a pressure drop in a range of
5 psi to 9 psi (35 K p to 62 K p). This drop is due to static head and, to a
lesser extent, friction and acceleration of the fluid. In an existing riser,
operating changes, such as higher catalyst circulation or lower vapor
velocity, can affect the density of reaction mixture and increase pressure
drop. This will affect the slide valve differential and percent opening.
Reactor-Stripper

The catalyst bed in the reactor-stripper is important for three reasons:
« to provide enough residence time for proper stripping of the
entrained hydrocarbon vapors prior to entering the regenerator;
• to provide adequate static head for flow of the spent catalyst to
the regenerator; and
• to provide sufficient backpressure to prevent reversal of hot flue
gas into the reactor system.
Assuming a stripper with a 20-ft bed level and a catalyst density
3
of 40 lb/ft , the static pressure is:
3
- n, 40 lbs/ft . . .
20 ft x f— r = 5.5 psi
2
2
144 in /ft