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 smaller molecules such as isobutane and propylene or butylenes can be recombined to meet specific octane requirements by processes such as alkylation or less commonly, dimerization. Octane can also be improved by catalytic reforming , which strips hydrogen out of hydrocarbons to produce aromatics, which have much higher octane ratings. Intermediate products can even be reprocessed to break a heavy, long-chained oil into a lighter short-chained one, by various forms of cracking such as Fluid Catalytic Cracking, Thermal Cracking, and Hydrocracking. The final step in gasoline production is the blending of fuels with different octane ratings, vapor pressures and other properties to meet product specifications.

Oil refineries can become very large and sprawling complexes with vast numbers of pipes running all throughout the facility. The refining process can cause many different chemicals to be released into the atmosphere, so a (sometimes very intense) odor sometimes 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 (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.

The largest oil refinery in the world is in Ras Tanura, Saudi Arabia, owned by Saudi Aramco. In fact, that city has prospered because of the "mega refinery" and the adjacent oil seaport.

External links

• Discover Petroleum, an educational site from the UK
• A billion barrels ago..., stories in the Ras Tanura refinery