Crude petroleum (mineral oil) is the term usually applied to the gases occurring naturally in the oilfields, the liquid from the wells, and the solids which are dissolved in, or have been separated from, the liquid. The composition of crude petroleum varies with the locality of occurrence. But all contain alkanes (straight- and branched-chain from about C1 to C40), cycloalkanes or naphthenes, and aromatic hydrocarbons. The low-boiling fractions of almost all petroleums are composed of alkanes; it is the composition of the higher-boiling fractions which differs according to the source of the petroleum. In addition to hydrocarbons, there are also present compounds containing oxygen, nitrogen, sulphur and metallic constituents.

Composition of crude petroleum

If the residue of petroleum, after removal of volatile compounds, contains a large amount of alkanes or wax, the petroleum is classified as paraffinic or paraffin base oil. If naphthenes predominate, the petroleum is classified as asphaltic or asphalt base oil. The crudes from the well in Pennsylvania, Irak and Rumania are paraffinic; those from Baku and Venezuela are asphaltic; and those from Oklahoma, Taxas and Mexico are intermediate in composition, and may be classified as paraffinic and asphaltic.

Distillation of petroleum

The crude oil is nearly always associated with water and sand; hence the crude petroleum discharged from the top of the well contains water and sand in suspension. The mixture is passed, under pressure, into cylindrical tanks, and the gas, oil and solids are drawn off separately.

Except for the low-boiling hydrocarbons, no attempt is made to separate the individual hydrocarbons. The crude oil is fractionated by continuous distillation into four main fractions : Petrol (gasoline), Kerosene (Kerosine, paraffin oil), gas oil (heavy oil) and lubricating oil. The residue may be fractionated by means of vacuum-distillation to give light, medium and heavy lubricating oils, paraffin wax, and asphaltic bitumen. Each of the four main fractions may be further split up by batch distillation into fractions of narrow boiling range. Recently, it has been possible to isolate individuals by ‘superfractionation’. The final number of fractions taken depends on the purpose in view. Below table shows one set of fractions that may be obtained.