Page 202 - Fluid Catalytic Cracking Handbook

170 Fluid Catalytic Cracking Handbook

• Regenerated catalyst slide (or plug) valve
• Riser
• Reactor-stripper
• Spent catalyst standpipe
• Spent catalyst slide (or plug) valve

Regenerator Catalyst Hopper

In some FCC units, the regenerated catalyst flows through a hopper
prior to entering the standpipe. The hopper is usually internal to the
regenerator and often of an inverted cone design. It provides sufficient
time for the regenerated catalyst to be deaerated before entering the
standpipe. This causes the catalyst entering the standpipe to have
maximum flowing density. The higher the density, the greater the
pressure buildup in the standpipe. In some FCC designs, the regenerated
catalyst hopper is external with fluffing aeration to control the catalyst
density entering the standpipe.

Regenerated Catalyst Standpipe
The standpipe's height provides the driving force for transferring
the catalyst from the regenerator to the reactor. The elevation differ-
ence between the standpipe entrance and the slide valve is the source
of this pressure buildup. For example, if the height difference is 30
3
3
feet (9.2 meters) and the catalyst density is 40 lb/ft (641 kg/m ), the
pressure buildup is:

40 1h 1 ft 2
Pressure Gain = 30 ft x -^ x 2 , = 8.3 psi (57 kp)
3
F
ft 144 in
The key to obtaining maximum pressure gain is to keep the catalyst
fluidized over the length of the standpipe. Longer standpipes will
require external aeration. This compensates for compression of the
entrained gas as it travels down the standpipe. Aeration should be
added evenly along the length of the standpipe. In shorter standpipes
sufficient flue gas is often carried down with the regenerated catalyst
to keep it fluidized and supplemental aeration is unnecessary. Over-
aeration leads to unstable catalyst flow and must be avoided.