Fluid catalytic cracking* (FCC) is the most important conversion process used in petroleum refineries. It is widely used to convert the high-boiling hydrocarbonfractions of petroleum crude oils into more valuable 
gasoline, olefinic gases and other products.
. Cracking of petroleum hydrocarbons for conversion of
heavy fractions into lighter fractions was originally done by thermal cracking which has been almost completely replaced by catalytic cracking because it produces more gasoline with a higher octane rating. It also produces byproduct gases that are more olefinic, and hence more valuable, than those produced by thermal cracking.
The feedstock to an FCC is usually that portion of the crude oil that has an initialboiling point of 340 ° or C
higher at atmospheric pressure and an average molecular weight ranging from about 200 to 600 or higher. The FCC process vaporizes and breaks the long-chain molecules of the high-boiling hydrocarbon liquids into much shorter molecules by contacting the feedstock, at high temperature and moderate pressure, with a fluidized powdered catalyst.
In effect, refineries use fluid catalytic cracking to correct the imbalance between the market demand for gasoline and the excess of heavy, high boiling range products resulting from the distillation of crude oil. As of 2006, FCC units were in operation at 400 petroleum refineries worldwide and about one-third of the
crude oil refined in those refineries is processed in an FCC to produce high-octane gasoline and fuel 
During 2007, the FCC units in the United States processed a total of 834,300,000 litres 
(5,300,000 barrels) per day of feedstock and FCC units worldwide processed about twice that amount.
Flow diagram and process description:
The modern FCC units are all continuous processes which operate 24 hours a day for as much as 2 to 3 years between shutdowns for routine maintenance.
There are a number of different proprietary process designs that have been developed for modern FCC units. Each design is available under a license that must be purchased from the design developer by any petroleum refining company desiring to construct and operate an FCC of a given design. Basically, there are two different configurations for an FCC unit: the "stacked" type where the reactor and the catalyst regenerator are contained in a single vessel with the reactor above the catalyst regenerator and the "side-by-side" type where the reactor and catalyst regenerator are in two separate vessels. These 
are the major FCC designers and licensors:
ExxonMobil Research and Engineering (EMRE)
Shell Global Solutions International
Stone & Webster Engineering Corporation (SWECO) / Institut Francais du Petrole (IFP) Universal Oil Products (UOP)
Kellogg Brown & Root (KBR)
Each of the proprietary design licensors claims to have unique features and advantages. A complete discussion of the relative advantages of each of the processes is well beyond the scope of this article. Suffice it to say that all of the licensors have designed and constructed FCC units that have operated quite satisfactorily.
The process flow diagram of a typical FCC unit, just below, is based on one of the above side-by-side configurations:
Reactor and Regenerator:
As depicted in the above diagram, the preheated high-boiling petroleum feedstock (at about 315 to 430 ° consisting of long-chain hydrocarbon molecules is combined with recycle slurry oil from the bottom of C)
the distillation column and injected into thecatalyst riser where it is vaporized and cracked into smaller molecules of vapor by contact and mixing with the very hot powdered catalyst from the regenerator. All of the cracking reactions take place in the catalyst riser. The hydrocarbon vapors "fluidize" the powdered catalyst and the mixture of...