Petroleum is a complex mixture of organic liquids called crude oil and natural gas, which occurs naturally in the ground and was formed millions of years ago. Crude oil varies from oilfield to oilfield in colour and composition, from a pale yellow low viscosity liquid to heavy black ‘treacle’ consistencies. Crude oil and natural gas are extracted from the ground, on land or under the oceans, by sinking an oil well and are then transported by pipeline and/or ship to refineries where their components are processed into refined products. Crude oil and natural gas are of little use in their raw state; their value lies in what is created from them: fuels, lubricating oils, waxes, asphalt, petrochemicals and pipeline quality natural gas.
An oil refinery is an organised and coordinated arrangement of manufacturing processes designed to produce physical and chemical changes in crude oil to convert it into everyday products like petrol, diesel, lubricating oil, fuel oil and bitumen. As crude oil comes from the well it contains a mixture of hydrocarbon compounds and relatively small quantities of other materials such as oxygen, nitrogen, sulphur, salt and water. In the refinery, most of these non-hydrocarbon substances are removed and the oil is broken down into its various components, and blended into useful products. Natural gas from the well, while principally methane, contains quantities of other hydrocarbons – ethane, propane, butane, pentane and also carbon dioxide and water. These components are separated from the methane at a gas fractionation plant.
Petroleum consists of three main hydrocarbon groups:
These consist of straight or branched carbon rings saturated with hydrogen atoms, the simplest of which is methane (CH4) the main ingredient of natural gas. Others in this group include ethane (C2H6), and propane (C3H8).
Hydrocarbons and Naphthenes
With very few carbon atoms (C1 to C4), Hydrocarbons are light in density and are gases under normal atmospheric pressure. Chemically paraffins are very stable compounds. Naphthenes consist of carbon rings, sometimes with side chains, saturated with hydrogen atoms. Naphthenes are chemically stable, they occur naturally in crude oil and have properties similar to paraffins.
Aromatic hydrocarbons are compounds that contain a ring of six carbon atoms with alternating double and single bonds and six attached hydrogen atoms. This type of structure is known as a benzene ring. They occur naturally in crude oil, and can also be created by the refining process. The more carbon atoms a hydrocarbon molecule has, the « heavier » it is (the higher is its molecular weight) and the higher is its the boiling point. Small quantities of a crude oil may be composed of compounds containing oxygen, nitrogen, sulphur and metals. Sulphur content ranges from traces to more than 5 per cent. If a crude oil contains appreciable quantities of sulphur it is called a sour crude; if it contains little or no sulphur it is called a sweet crude.