Petroleum refinery process in which heavy oil is passed through metal chambers called catalytic cracking or cat cracking under pressure and high temperature in the presence of catalysts such as alumina, silica, or zeolites. Alkylation is the transfer of an alkyl group from one molecule to another. When many molecules of a simple compound join together, the product is called a polymer and the process is termed polymerization.
Fluid catalytic cracking (FCC), a type of secondary unit operation, is primarily used in producing additional gasoline in the refining process. Isomerization is the process in which hydrocarbon molecules are rearranged into a more useful isomer. Reforming is another process in which hydrocarbon molecules are rearranged into other molecules, usually with the loss of a small molecule such as hydrogen.