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The Fluid Catalytic Cracker (FCC) plant converts heavy hydrocarbons into lighter ones by means of reacting the feed with a fluidized cataylst under high temperature (900F). The FCC unit provides a refinery with considerable flexibility by allowing the operator to shift the performance of the unit to increase the yield of a particular product, for example, to make more diesel fuel or more gasoline.
Vacuum gas oil (VGO) is used as the primary feed to the FCC. The unit can also process residuals (for example, from either atmospheric distillation, vacuum distillation, and/or visbreaking), thus increasing the yield of product as well as contributing to the useful disposal of the heavy residues.
As shown in the diagram below, the three main components in an FCC unit are the Reactor, the Regenerator, and the Fractionator. The Reactor consists of a riser pipe and a reaction chamber, although with most modern FCC units the reaction occurs in the riser pipe and the reaction chamber acts more like a cyclone to separate spent catalyst from the products. Products from the reaction process include primarily cat cracked gasoline and light hydrocarbons (C4 and lighter), but also include both light and heavy gas oil, cycle oil, and coke. Coke is carbon that ends up as a deposit on the cataylst causing it to become spent, or inactive. All products except the spent catalyst are sent to the Fractionator column, where they are separated according to their weight and then sent elsewhere in the refinery for further processing. The heaviest product, cycle oil, is recycled back to the Reactor where it is further cracked in a process called recycling to extinction.
The Regenerator receives the spent catalyst from the reactor, where the coke is removed from the catalyst by reacting it with oxygen from heated air (1,100F) to form carbon dioxide. The regenerated catalyst is sent back to the Reactor, ready to convert more feed in a continuous steady-state process. |
