View of the Shell/Valero Martinez oil refinery

An oil refinery is an industrial process plant where crude oil is processed and refined into useful petroleum products.

Raw or unprocessed ("crude") oil is not very useful in the form it comes in out of the ground. It needs to be broken down into parts and refined before use in a solid material such as plastics and foams, or as petroleum fossil fuels as in the case of automobile and airplane engines.

Oil can be used in so many various ways because it contains hydrocarbons of varying lengths such as paraffins, aromatics, napthenes (or cycloalkanes), alkenes, dienes, and alkynes.

Crude oil is separated into fractions by fractional distillation. The heaver fractions, that emerge from the bottom of the fractionating column are often broken up (cracked) to make more useful products.

Hydrocarbons are molecules of varying length and complexity made of hydrogen and carbon. Their various structures give them their differing properties and thereby uses. The trick in the oil refinement process is separating and purifying these. All these different hydrocarbons have different boiling points, which means they can be separated by distillation.

Once separated and any contaminants and impurities have been removed, the oil can be either sold without any further processing, or recombined to meet specific requirements (as in the case of fuels with different octane ratings) or even be reprocessed to break a heavy, long-chained oil into a lighter short-chained one.

Oil refineries can become very large and sprawling complexes with vast numbers of pipes running all throughout the facility. The refining process causes many different chemicals to be released into the atmosphere, so a (sometimes very intense) odor accompanies the presence of a refinery. Environmental groups have lobbied many governments to increase restrictions on how much material refineries can release, and many refineries have installed equipment and changed practices to lessen the environmental impact. In the United States, there is strong pressure to prevent the development of new refineries, and none have been built in the country for more than three decades. However, many already-existing refineries have expanded during that time. Both environmental and safety concerns mean that oil refineries are usually located a safe distance away from major urban areas. Nevertheless, there are potentially dangerous exceptions to this rule, a particularly notable one being the Santa Cruz refinery 1 (http://www.atan.org/en/refineria/vertidos/elcolmo.htm) (Tenerife, Spain), which is sited in a densely-populated city center and next to the only two major evacuation routes in and out of the city.