Cracking, in petroleum refining, the process by which heavy hydrocarbon molecules are broken up into lighter molecules by means of heat and usually pressure and sometimes catalysts. Cracking is the most important process for the commercial production of gasoline and diesel fuel.

of heat and usually pressure and sometimes catalysts. Cracking is the most important process for the commercial production of gasoline and diesel fuel.

Schematic diagram of a fluid catalytic cracking unit.

Cracking of petroleum yields light oils (corresponding to gasoline), middle-range oils used in diesel fuel, residual heavy oils, a solid carbonaceous product known as coke, and such gases as methane, ethane, ethylene, propane, propylene, and butylene. Depending on the end product, the oils can go directly into fuel blending, or they can be routed through further cracking reactions or other refining processes until they have produced oils of the desired weight. The gases can be used in the refinery’s fuel system, but they are also important raw materials for petrochemical plants, where they are made into a large number of end products, ranging from synthetic rubber and plastic to agricultural chemicals.

The first thermal cracking process for breaking up large nonvolatile hydrocarbons into gasoline came into use in 1913; it was invented by William Merriam Burton, a chemist who worked for the Standard Oil Company (Indiana), which later became the Amoco Corporation. Various improvements to thermal cracking were introduced into the 1920s. Also in the 1920s, French chemist Eugène Houdry improved the cracking process with catalysts to obtain a higher-octane product. His process was introduced in 1936 by the Socony-Vacuum Oil Company (later Mobil Oil Corporation) and in 1937 by the Sun Oil Company (later Sunoco, Inc.). Catalytic cracking was itself improved in the 1940s with the use of fluidized or moving beds of powdered catalyst. During the 1950s, as demand for automobile and jet fuel increased, hydrocracking was applied to petroleum refining. This process employs hydrogen gas to improve the hydrogen-carbon ratio in the cracked molecules and to arrive at a broader range of end products, such as gasoline, kerosene (used in jet fuel), and diesel fuel. Modern low-temperature hydrocracking was put into commercial production in 1963 by the Standard Oil Company of California (later the Chevron Corporation).