A unit operation used in petrochemical refineries to convert the heavy components in crude oil into useful products such as fuels for motorized vehicles. They consist of a catalytic reactor front end section in which cracking takes place, reducing long-chain hydrocarbon molecules into smaller short-chain molecules. In the light or back end section, separation takes place where the cracked (p. 154) molecules are fractionated through a series of columns to produce fuel gas, fuel oil, LPG, diesel, and gas oil.