The main fractionator takes hot reactor effluent straight from the FCC riser, carrying catalyst fines and coke precursors. It is one of the dirtiest distillation services in a refinery, and run length is usually limited by fouling rather than by capacity.

The bottom section takes the worst of it. Grid packing and shed decks are used in preference to trays where coking is severe, because they tolerate deposits without blocking outright.