A petroleum refinery is an industrial process plant where crude oil is transformed and refined into useful products such as gasoline, diesel fuel, asphalt base, fuel oils, heating oil, kerosene, and liquefied petroleum gas. The refining process breaks crude oil down into its various components, which are then selectively reconfigured into new products. Petroleum refineries are complex and expensive industrial facilities that involve many different processing units and auxiliary facilities such as utility units and storage tanks. The refining process involves three basic steps: separation, conversion, and treatment.

• Separation: Crude oil is piped 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.

• Conversion: Cracking is a process that rearranges molecules in crude oil to add value. Other refinery processes rearrange molecules rather than splitting molecules to add value. Fluid catalytic cracking is an example of a conversion process.

• Treatment: The finishing touches occur during the final treatment. To make gasoline, refinery technicians carefully combine a variety of streams from the processing units. Octane level, vapor pressure ratings, and other special considerations determine the final formulation of gasoline.

Refineries can produce high-value products such as gasoline, diesel fuel, and jet fuel from light crude oil with simple distillation. However, the physical characteristics of crude oil determine how refineries process it. Crude oils are classified by density (API gravity) and sulfur content. Less dense (lighter) crude oils (with higher API gravity) generally have more light hydrocarbons. 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, liquefied gases from the refinery, and unfinished oils that are produced by partially refining crude oil.