Catalytic Cracking Process


The fluid catalytic cracking (FCC) process is a process for the conversion of straight-run atmospheric gas oils, vacuum gas oils, certain atmospheric residues, and heavy stocks recovered from other refinery operations into high-octane gasoline, light fuel oils, and olefin-rich light gases. The features of the FCC process are relatively low investment, reliable long-run operations, and an operating versatility that enables the refiner to produce a variety of yield patterns by simply adjusting operating parameters. The product gasoline has an excellent front-end octane number and good overall octane characteristics. Further, FCC gasoline is complemented by the alkylate produced from the gaseous olefinic byproducts because alkylate has superior midrange octane and excellent sensitivity.

In a typical FCC unit, the cracking reactions are carried out in a vertical reactor riser in which a liquid oil stream contacts hot powdered catalyst. The oil vaporizes and cracks to lighter products as it moves up the riser and carries the catalyst powder along with it. The reactions are rapid, and only a few seconds of contact time are necessary for most applications. Simultaneously with the desired reactions, coke, a carbonaceous material having a low ratio of hydrogen to carbon (H/C), deposits on the catalyst and renders it less catalytically active. The spent catalyst and the converted products are then separated; and the catalyst passes to a separate chamber, the regenerator, where the coke is combusted to rejuvenate the catalyst. The rejuvenated catalyst then passes to the bottom of the reactor riser, where the cycle begins again.

Every FCC complex contains the following sections (as above):
● Reactor and regenerator. In the reactor, the feedstock is cracked to an effluent containing hydrocarbons ranging from methane through the highest-boiling material in the feedstock plus hydrogen and hydrogen sulfide. In the regenerator, the circulating spent catalyst is rejuvenated by burning the deposited coke with air at high temperatures.
● Main fractionator. Here the reactor effluent is separated into the various products. The overhead includes gasoline and lighter material. The heavier liquid products, heavier naphtha, and cycle oils are separated as sidecuts, and slurry oil is separated as a bottoms product.
● Gas concentration unit. In this section, usually referred to as the unsaturated gas plant, the unstable gasoline and lighter products from the main fractionator overhead are separated into fuel gas, C3- C4 for alkylation or polymerization, and debutanized gasoline that is essentially ready for use except for possible chemical treating.

Depending on the objectives of the refiner, some unconverted materials in the feedstock boiling range may be recycled to the reactor. In general, conversion, which is typically defined as 100 minus the liquid volume percentage of products heavier than gasoline, is never carried to completion. Some main-column bottoms material, referred to as clarified oil or slurry oil, is a product usually used for fuel oil blending. Light cycle oil, recovered as a sidecut product, is generally used for home heating, although a fraction might be suitable for diesel fuel blending stock.


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