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An oil refinery is an industrial process plant where crude oil is processed and refined into more useful petroleum products, such as gasolinediesel fuel, asphalt base, heating oil, kerosene, and liquefied petroleum gas. Oil refineries are typically large  industrial complexes with extensive piping running throughout, carrying streams of fluids between large chemical processing units.

Raw or unprocessed crude oil is not generally useful in its raw or unprocessed form, as it comes out of the ground. Although "light, sweet" (low viscosity, low sulfur) crude oil has been used directly as a burner fuel for steam vessel propulsion, the lighter elements form explosive vapors in the fuel tanks and so it was quite dangerous, especially in warships. Instead, the hundreds of different hydrocarbon molecules in crude oil are separated in a refinery into components that can be used as fuels, lubricants, and as feedstock in petrochemical processes that manufacture such products as plastics, detergents, solvents, elastomers and fibers such as nylon and polyesters. Petroleum fossil fuels are burned in internal combustion engines in order to provide power to operate ships, automobiles, aircraft engines, lawn-mowers, chainsaws, and other pieces of power equipment. These different hydrocarbons have different boiling points, which means they can be separated by distillation. Since the lighter liquid products are in great demand for use in internal combustion engines, a modern refinery will convert heavy hydrocarbons and lighter gaseous elements into these higher value products.

Oil can be used in so many ways because it contains hydrocarbons of varying molecular masses, forms and lengths such as paraffins, aromatics, naphthenes, alkenes, dienes, and alkynes. While the molecules in crude oil include many different atoms such as sulfer and nitrogen, the most plentiful molecules are the hydrocarbons, which are molecules of varying length and complexity made of hydrogen and carbon atoms, and a small number of oxygen atoms. The differences in the structure of these molecules is what confers upon them their varying physical and chemical properties, and it is this variety that makes crude oil so useful in such a broad range of applications.

Once separated and purified of any contaminants and impurities, the fuel or lubricant can be sold without any further processing. Smaller molecules such as isobutane and propylene or butylenes can be recombined to meet specific octane requirements of fuels by processes such as alkylation or less commonly, dimerization. Octane grade of gasoline can also be improved by catalytic reforming, which strips hydrogen out of hydrocarbons to produce aromatics, which have much higher octane ratings. Intermediate products such as gasoils 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 are large scale plants, processing from about a hundred thousand to several hundred thousand barrels of crude oil per day. Because of the high capacity, many of the units are operated continuously (as opposed to processing in batches) at steady state or approximately steady state for long periods of time (months to years). This high capacity also makes process optimization and advanced process control very desirable.



The world's first oil refineries were set up by Ignacy Łukasiewicz near Jasło, Austrian Empire (now in Poland) in the years 1854-56 but they were initially small as there was no real demand for refined fuel. As Łukasiewicz's kerosene lamp gained popularity the refining industry grew in the area.

The first large oil refinery opened at Ploieşti, Romania in 1856. Several other refineries were built at that location with investment from United States companies before being taken over by Nazi Germany during World War II. Most of these refineries were heavily bombarded by US Army Air Forces in Operation Tidal Wave, August 1, 1943. Since then they have been rebuilt, and currently pose somewhat of an environmental concern.

Another early example is Oljeön, Sweden, now preserved as a museum at the UNESCO world heritage site Engelsberg. It started operation in 1875 and is part of the Ecomuseum Bergslagen.

At one time, the world's largest oil refinery was claimed to be Ras Tanura, Saudi Arabia, owned by Saudi Aramco. For most of the 20th century, the largest refinery of the world was the Abadan refinery in Iran. This refinery suffered extensive damage during the Iran-Iraq war. The world's largest refinery complex is the "Centro de Refinación de Paraguaná" (CRP) operated by PDVSA in Venezuela with a production capacity of 956,000 barrels per day (152,000 m³/d) (Amuay 635,000 bbl/d (101,000 m³/d), Cardón 305,000 bbl/d (48,500 m³/d) and Bajo Grande 16,000 bpd). SK Energy's Ulsan refinery in South Korea with a capacity of 840,000 bbl/d (134,000 m³/d) and Reliance Petroleum's refinery in Jamnagar, India with 660,000 bbl/d (105,000 m³/d) are the second and third largest, respectively.

Oil Refining in the United States


Early US refineries processed crude oil to recover the kerosene. Other products were considered wastes and were often dumped directly into the nearest river. The invention of the automobile shifted the demand to gasoline and diesel, which remain the primary refined products today. Refineries pre-dating the US Environmental Protection Agency were not subject to any environmental protection regulations. Today, national and state legislation requires refineries to meet stringent air and water cleanliness standards. In fact, obtaining a permit to build a modern refinery is perceived by many American oil companies to be so difficult and costly that no new refineries have been built (though many have been expanded) in the United States since 1976. As a result, some believe that this may be the reason that the US is becoming more and more dependent on the imports of finished gasoline, as opposed to incremental crude oil. On the other hand, studies have revealed that accelerating merger activity in the refining and production sector has reduced capacity further, resulting in tighter markets in the United States in particular.