Page 31 - Fluid Catalytic Cracking Handbook

Process Description 15

Air provides oxygen for the combustion of coke and is supplied by
one or more air blowers. The air blower provides sufficient air velocity
and pressure to maintain the catalyst bed in a fluid state. The air enters
the regenerator through an air distributor (Figure 1-11) located near
the bottom of the vessel. The design of an air distributor is important
in achieving efficient and reliable catalyst regeneration. Air distributors
are typically designed for a 1.0 psi to 2.0 psi (7 to 15 Kpa) pressure
drop to ensure positive air flow through all nozzles.
There are two regions in the regenerator: the dense phase and the
dilute phase. At velocities common in the regenerator, 2 ft/sec to 4 ft/sec
(0.6 to 1.2 m/sec), the bulk of catalyst particles are in the dense bed
immediately above the air distributor. The dilute phase is the region
above the dense phase up to the cyclone inlet, and has a substantially
lower catalyst concentration.

Standpipe/Slide Valve

During regeneration, the coke level on the catalyst is typically
reduced to 0.05%. From the regenerator, the catalyst flows down a
transfer line commonly referred to as a standpipe. The standpipe
provides the necessary pressure to circulate the catalyst around the
unit. Some standpipes extend into the regenerator, and the top section
is often called a catalyst hopper. The hopper, internal to the regener-
ator, is usually an inverted cone design. In units with "long" catalyst
standpipes, external withdrawal hoppers are often used to feed the
standpipes. The hopper provides sufficient time for the regenerated
catalyst to be "de-bubbled" before entering the standpipe.
Standpipes are typically sized for a flux rate in the range of 100 to
2 2
300 lb/sec/ft (500 to 1,500 kg/sec/m ) of circulating catalyst. In most
cases, sufficient flue gas is carried down with the regenerated catalyst
to keep it fluidized. However, longer standpipes may require external
aeration to ensure that the catalyst remains fluidized. A gas medium,
such as air, steam, nitrogen, or fuel gas, is injected along the length
of the standpipe. The catalyst density in a well-designed standpipe is
3
3
in the range of 35 to 45 lb/ft (560 to 720 kg/m ).
The flow rate of the regenerated catalyst to the riser is commonly
regulated by either a slide or plug valve. The operation of a slide valve
is similar to that of a variable orifice. Slide valve operation is often
controlled by the reactor temperature. Its main function is to supply