Page 204 - Fluid Catalytic Cracking Handbook

172 Fluid Catalytic Cracking Handbook

Spent Catalyst Standpipe
From the bottom of the stripper, the spent catalyst flows into the
spent catalyst standpipe. Sometimes the catalyst is partially defluidized
in the stripper cone. To counter this, "dry" steam is usually added
(through a distributor) to fluidize the catalyst prior to its entering the
standpipe. The loss of fluidization in the stripper cone can cause a
buildup of dense phase catalyst along the cone walls. This buildup can
restrict catalyst flow into the standpipe, causing erratic flow and
reducing pressure buildup in the standpipe.
Like the regenerated catalyst standpipe, the spent catalyst standpipe
may require supplemental aeration to obtain optimum flow char-
acteristics. "Dry" stearn is the usual aeration medium.

Spent Catalyst Slide or Plug Valve
The spent catalyst slide valve is located at the base of the standpipe.
It controls the stripper bed level and regulates the flow of spent
catalyst into the regenerator. As with the regenerated catalyst slide
valve, the catalyst level in the stripper generates pressure as long as
it is fluidized. The pressure differential across the slide valve will be
at the expense of consuming a pressure differential in the range of
3 psi to 6 psi (20 kp to 40 kp).
In earlier Model II and Model III FCC units, spent catalyst was
transported into the regenerator using 50% to 100% of combustion
air. This spent cat riser was designed for a minimum air velocity of
30 ft/sec (9.1 m/sec).

Case Study

A survey of the reactor-regenerator circuit of a 50,000 bpd (331
3
m /hr) cat cracker produced these results:

Reactor dilute phase (dome) pressure = 19.0 psig/131 K p
Reactor catalyst dilute phase bed level = 25.0 ft/7.6 m
Reactor-stripper catalyst bed level = 18.0 ft/5.5 m
3
Reactor-stripper catalyst density = 40 Ib/ft /640 kg/m 3
Spent, catalyst standpipe elevation = 14.4 ft/4.4 m
Pressure above the spent catalyst slide valve = 26.1 psig/180 K p
Spent catalyst slide valve AP (@ 55% opening) = 4.0 psi/27.6 K p