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Petroleum refining is the processes used to produce fuels, chemicals, and gas by treatment of petroleum. Petroleum has been known and used for thousands of years, but systematic separation of its components has only been carried out for just over a century. Initially, petroleum was refined almost entirely to produce fuels. Since the Second World War the use of refinery products as a source of petrochemicals has become more important, but over 90 percent of crude petroleum is still used for fuel. The key to petroleum refining is the initial separation of hydrocarbons into various groups of similar compounds. The groups are distinguished by their boiling-points, and they are separated by fractional distillation. A group of hydrocarbons with similar boiling-points is called a fraction. Each fraction has a distinct treatment within the refinery. Fractions for which there is little demand may be converted to other fractions by later refinery processes. Refinery gas is the petroleum fraction with the lowest boiling-point, and does not condense in a fractional distillation column.

Propane and butane may be extracted from refinery gas to make liquefied petroleum gas. The residual gas, containing mainly hydrogen, methane, and ethane, is used as a fuel to operate the refinery. The most economically important product of petroleum refining is the range of fractions called petrol, which boils at 30-140 degrees Celsius. Light petrol condenses at boiling-points of 30-80 degrees Celsius, right at the top of the fractionation column. It is used to make fuel for motor cars and other petrol-engined vehicles. Next down the column, at boiling- points of 80-190 degrees Celsius, naphthas are drawn off. They may be used in blending fuels. Individual naphthas are separated and used to make solvents, and as a raw material in producing many organic chemicals. Much of the naphtha fraction is reformed for use in petrol. The fraction next below the naphthas in the fractionation column condenses at boiling-points of 190-250 degrees Celsius. This fraction contains the kerosenes, which include paraffin, traditionally burnt with a wick for heating and lighting. This fraction is now more important for making aviation fuel for jet aircraft. The final group of fractions condensing in the column is diesel oil, or gas oil, with boiling-points in the range 250-350 degrees Celsius. Their main use is in diesel engines.

Heavier oil which does not evaporate in the initial fractional distillation passes through the bottom of the column. In some refineries these residues pass on to another stage of vacuum distillation. Products separated this way include lubricating oils and petroleum jelly (used as a grease, or as a base for making ointments). Separating individual compounds from the various fractions and residues is done by several methods. Solvent extraction, for example, is another way of extracting lubricating oil from residues. Further solvent treatment can eliminate undesirable contaminants from lubricating oil or kerosenes. Some substances are removed or separated by crystallization, in which the heavier fractions are cooled until waxes crystallize, and other semi-solids solidify. The solid particles are then filtered out. Preparing fractions or products for final use involves many complicated processes. Impurities, of which the most important are sulphur compounds, are generally removed by hydro treatment.

In blending, different fractions are mixed to achieve specific properties. For example, fuel-oils for domestic and industrial heating are a blend of heavy residue oils with lighter fractions which reduce their viscosity. Oils to be burnt in engines generally need fuel additives blended in to improve their performance and safety. Chemical treatment of fractions to change them into other fractions or into feedstocks for petrochemicals is a large and growing part of refinery work. These processes include cracking, in which heavier hydrocarbons in residues are broken down into lighter fractions, particularly petrol. In hydro treatment, unsaturated hydrocarbons may be saturated with hydrogen. To make slightly heavier hydrocarbons, or to turn straight-chain molecules into ring molecules, a reforming process is used. This produces more petrol, and many aromatic hydrocarbons for use in the chemical industry to make explosives, synthetic rubbers, food preservatives, and many other specialist chemicals.

Other building-up processes include polymerisation, in which identical molecules combine, and alkylation, in which hydrocarbon groups or chains are added to molecules. Storage facilities are a vital part of the work of a refinery, which may have hundreds of tanks, generally above ground and about 30 metres in diameter. A huge network of pipes connects the tanks with various processes, so that a tank may be used for storing intermediate fractions, separate compounds, or finished product awaiting transport to users or chemical factories. Large tanks hold the crude oil delivered to the refinery, with each tank used for oil from a particular source. Switching between crude tanks enables selection of the crude to give the properties best suited to the refinery's current workload. Transport of crude to the refinery is by pipeline or by oil tanker (most refineries are near the sea). Transport of finished products is generally by pipeline, road, or rail.
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