Fluid Catalytic Cracker (FCC)
The FCC is considered by many as the heart of a modern petroleum refinery. FCC is the tool refiners use to correct the imbalance between the market demand for lighter petroleum products and crude oil distillation that produces an excess of heavy, high boiling range products. The FCC unit converts heavy gas oil into gasoline and diesel.
The FCC process cracks heavy gas oils by breaking the carbon bonds in large molecules into multiple smaller molecules that boil in a much lower temperature range. The FCC can achieve conversions of 70-80% of heavy gas oil into products boiling in the heavy gasoline range. The reduction in density across the FCC also has the benefit of producing a volume gain (i.e., combined product volumes are greater than the feed volume). Since most petroleum products are sold on a volume basis, this gain has a significant effect on refinery profitability.
FCC reactions are promoted at high temperatures 950-1020˚F (510-550˚C) but relatively low pressures of 10-30 psi (1-2 bar). At these temperatures, coke formation deactivates the catalyst by blocking reaction sites on the solid catalyst. The FCC unit utilizes a very fine powdery catalyst know as a zeolite catalyst that is able to flow like a liquid in a fluidized bed - hence the name "Fluid Cat Cracker". Catalyst is continually circulated from the reactor to a regenerator where coke is burned off in controlled combustion with air creating carbon monoxide, carbon dioxide, sulfur oxides (SOX) and nitrous oxides (NOX) as well as some other combustion products.
Feedstock gas oil is preheated and mixed with hot catalyst coming from the regenerator at 1200-1350˚F (650-735˚C). The hot catalyst vaporizes the feedstock and heats it to reaction temperature. To avoid overcracking, which reduces yield at the expense of gasoline, reaction time is minimized. The primary reaction occurs in the transfer line (or riser) going to the reactor. The primary purpose of the reactor is to separate catalyst from reaction products.
FCC products are more highly unsaturated than distillation products. Naphtha in the gasoline range has good octane. Distillate range products have low pour points but poorer combustion qualities. Light end products are highly olefinic (unsaturated) and are used as feedstock for further upgrading processes like alkylation. With sulfur concentration of gasoline reducing, FCC products (gasoline and distillates) may require desulfurization through a HDS Unit prior to blending.
Air emissions are a growing concern for FCC units. Emissions include catalyst fines, SOX and NOX components. Electrostatic precipitators and scrubbers are used to reduce air emissions. As air quality concerns grow, more equipment to reduce SOX and NOX are expected.
