Variations in cat cracking include Selective Component Cracking (SCC) for polypropylene production, a two-vessel and external-reactor design for processing heavy residue feeds, and a UOP process for converting gas oils and resid feedstocks. All crackers employ a steam stripper to remove hydrocarbons entrained in the spent catalyst.
Stripper level control allows sufficient residence time for stripping steam to displace hydrocarbons for recovery. It also maintains sufficient pressure to keep air in the regenerator from reverse flow into the reaction system, thereby causing a hazard. A waste heat recovery steam drum would also require monitoring.