Petroleum refining is the process of transforming crude oil into useful products such as gasoline, diesel fuel, asphalt base, fuel oils, heating oil, kerosene, and liquefied petroleum gas. The refining process involves breaking down crude oil into its various components, which are then selectively reconfigured into new products. The process of refining crude oil involves three basic steps: separation, conversion, and blending.

The separation process involves piping crude oil through hot furnaces, and the resulting liquids and vapors are discharged into distillation units. Inside the distillation units, the liquids and vapors separate into petroleum components, called fractions, according to their boiling points. Heavy fractions are on the bottom and light fractions are on the top.

The conversion process involves rearranging molecules to add value. One example of this is cracking, which splits molecules to create gasoline molecules. Other refinery processes rearrange molecules rather than splitting them to add value. For example, alkylation makes gasoline components by combining some of the gaseous byproducts of cracking.

The blending process involves mixing various streams into finished petroleum products. The various grades of motor fuels are blends of different streams or “fractions” such as reformate, alkylate, catalytically cracked gasoline, etc. Refineries blend compounds obtained either from their internal refining process operations or externally to make gasoline that meets specifications for acceptable motor vehicle performance.

In addition to crude oil, refineries and blending facilities add other oils and liquids during processing to produce the finished products that are sold to consumers. These other oils and liquids include liquids that condense in natural gas wells, natural gas plant liquids from natural gas processing, and liquefied gases from the refinery.

The petroleum refining process is complex and requires precise temperature and pressure parameters. The equipment and monitoring required to ensure the proper progression of these processes is complex and has evolved through the advancement of the scientific field of petroleum engineering.