A refining process involving decomposition and molecular recombination of organic compounds, especially hydrocarbons obtained by means of heat, to form molecules suitable for motor fuels, monomers, petrochemicals, etc. A series of condensation reactions takes place, accompanied by transfer of hydrogen atoms between molecules which brings about fundamental changes in their structure. Thermal cracking, the older method, exposes the distillate to temperatures of approximately 540–650 C (1000–1200F) for varying periods of time; it is no longer used for gasoline, but is still of value in producing hydrocarbon gases for plastics monomers. The development of premium fuels for airplanes and automobiles resulted from the use of catalysts in cracking. In this process, hydrocarbon vapors are passed at approximately 400 C (750F) over a metallic catalyst (e.g., silica-alumina or platinum); the complex recombinations (alkylation, polymerization, isomerization, etc.) occur within seconds to yield high-octane gasoline. Among the chemical changes induced are conversion of alicyclic compounds (cyclohexane) to aromatic compounds, and of straight-chain to branched-chain structures (isomerization). Cracking reactions are exothermic. Free radicals, carbonium ions, and other chain-initiating agents are involved in these rearrangements.Catalytic cracking is carried out by either the moving-bed or the fluid-bed technique. In the former the catalyst is pelleted, while in the latter it is finely divided. Instances in which cracking does not involve production of gasoline are the steam cracking of methane or naphtha to form synthesis gas, thermal cracking of naphtha to ethylene, and thermal decomposition of methane to carbon black and hydrogen.See Catalysis; Synthesis Gas; Fluidization; Pyrolysis.
