Atmospheric fractional distillation, the first and core step in the refining process, uses heat to separate the numerous different hydrocarbon components present in oil into several fractions, depending on their boiling point. The first volatile products of the crude oil are gaseous hydrocarbons of one to four carbon atoms that were dissolved in the oil. Liquid petroleum gases (LPG) can be used as fuel or converted to useful chemicals. With increasingly higher boiling range and number of contained carbon atoms, the next fractions are gasoline, naphtha, jet fuel, kerosene, gas oil, and heating and fuel oils. Finally, the heaviest products with boiling temperatures above 600 °C, known as residual oil, can be separated into such individual constituents as coke, asphalt, tars, and waxes. Alternatively, it can be further processed by cracking to produce lighter fractions and maximize the output of the most desirable products such as gasoline and diesel fuel.
After atmospheric distillation, the relative amounts of usable fractions obtained from crude oil do not coincide with the commercial needs, and therefore additional "downstream" processing is needed. In general, these processes are designed to increase the yield of lighter, higher-value products such as gasoline. Downstream operations include vacuum distillation, cracking, reforming, alkyla-tion, isomerization, and oligomerization.
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