Petroleum is an oily flammable bituminous liquid that may vary from almost colorless to black, occurs in many places in the upper strata of the earth, is a complex mixture of hydrocarbons of varying volatility, together with small quantities of substances which contain oxygen, nitrogen, sulphur and ash derived from the vegetable and animal organisms from which the petroleum was formed, and is prepared for use as gasoline, naphtha, or other products by various refining processes.

Fractional distillation is useful for separating a mixture of substances with narrow differences in boiling points, and is the most important step in the refining process. The crude oil is conveyed to the refinery by pipeline, oil tanker or tank wagon first has water and solid contaminants removed from it by sedimentation and are then split up by fractional distillation. The crude oil is pumped through tube stills in which it is heated to 280°-300°C. It is then admitted to a large fractionating column in which the gases, the readily volatile petrol constituents, and the kerosene (paraffin oil) are distilled off.

The remaining distillation residue, which is already of a viscous consistency, is pumped through a second tube still, in which it is reheated, and is then passed to a second fractionating column. In this column, which operates under vacuum, various grades of oil are distilled off (gas oil, diesel oil, cylinder oil, machine oil, etc.), while asphalt, mineral pitch, coke like residues and inorganic matter remain behind.

Except gas, nearly all the petroleum fractions require further processing whereby their content of deleterious impurities ash, sulphur and nitrogen compounds, gumming and polymerizing substances is reduced or these impurities are removed altogether, either by chemical conversion or by physical adsorption with such substances as active charcoal, silica gel, kieselguhr, fuller’s earth, etc.

Many petroleum fractions have to be treated with additives in order to acquire the desired properties. For e.g., petrol must undergo further chemical processing to give it good anti-knock and ignition properties, reduce its odour, and make it resistant to ageing. Similarly machine oils have to be non-resinous, pale-coloured, odourless and oxidation-resistant; additives which further improve the properties of the oil are also employed.

Despite the separation of the multiple mixture of which petroleum consists into a number of fractions, each of which contains fewer constituents than the initial crude petroleum, each fraction still comprises many different constituents for e.g. petrol contains upwards of a hundred.

The correct fractionation of petroleum is therefore a difficult art which, in addition to the necessary knowledge involves the use of much complex measuring equipment and costly apparatus. For instance, petroleum contains corrosive substances. Because of this the giant modern petroleum fractionating columns are made of special high-grade steel. Their operation is well nigh fully automatic.

Very few of the components come out of the fractional distillation column ready for market. Many of them must be chemically processed to make other fractions. For example, only 40% of distilled crude oil is gasoline; however, gasoline is one of the major products made by oil companies.