So-called Third Stage Separator (TSS) Packages were orginally designed for long-term reliable protection of Power Recovery Units in FCC plants. Nowadays a modern TSS is also capable of reducing the catalyst in the flue gas to meet environmental legislations. For optimal efficiencies of the cyclonic TSS, and for removal of catalyst fines from the underflow stream, a Fourth Stage Separator (FSS) is installed. FSS systems are typically designed to handle 3 up to 5 % of the total flue gas flow, being extracted at the bottom of the TSS system.

Where in the past often a cyclonic FSS was sufficient to separate the cat fines from the TSS underflow, nowadays much more efficient filter type FSS units are selected to assist the TSS in reducing the total stack emission level to meet the stringent emission regulations.

When installing the proven Dahlman Fourth Stage Separator technology, particulate matter in the refinery stack can be reduced to << 40 mg/Nm3.