Fluid catalytic cracking

Claim 1

In a fluidized catalytic cracking process comprising a riser zone, a reaction vessel, a stripping zone vertically mounted below said reaction zone, and a regeneration zone wherein regenerated catalyst from said regeneration zone is contacted at cracking conditions with hydrocarbon oil charge in said riser zone, wherein catalyst and hydrocarbon vapors are disengaged in said reaction vessel, wherein a fluidized bed of catalyst is maintained in said reaction vessel, wherein catalyst from said fluidized bed is withdrawn into the upper portion of said stripping zone, wherein stripping vapors are introduced into the lower portion of said stripping zone, wherein stripping vapors and stripped hdyrocarbons are vented from the top of the stripping zone into the upper portion of said reaction vessel above the surface of the fluidized catalyst bed, and wherein stripped catalyst from the bottom of the stripping zone is withdrawn into the regeneration zone; the improvement which comprises: a. restricting flow of stripping vapors and stripped hydrocarbon from said stripping zone to said reactor to maintain a selected differential pressure between the top of stripping zone and the upper portion of the reaction vessel; b. accumulating a bed of fluidized catalyst in the reaction vessel sufficient to provide a weight of fluidized caTalyst equivalent to the pressure differential between the top of the stripping zone and the upper portion of the reaction vessel; and c. flowing catalyst from the bottom of said fluidized bed into upper portion of said stripping zone at a rate sufficient to maintain the volume of fluidized bed of step (b).