Crude Oil

Crude oil is a mixture of hydrocarbons. The composition varies depending on its source. North Sea crude is a light oil. This makes it valuable because it needs less processing to make it into the refined products that are most in demand. These are gasoline (petrol), diesel, kerosene (including jet fuel) and fuel oil. It is also low in sulphur which makes it easier to meet new environmental standards which demand a very low sulphur content.

Refinery Processes

The basic refining processes take place in the crude distillation unit (CDU) of the Fuels Zone. Here, crude oil is taken into the atmospheric distillation tower where it is separated into its different fractions. The hydrocarbons in crude oil have different boiling points according to the number of carbon atoms in each molecule and how they are arranged. The oil is heated and the resultant vapours rise up the tower. The vapours cool as they rise and condense onto trays. The lightest compounds condense at the top of the tower and are taken off as LPG (liquefied petroleum gas). The oil then undergoes further processing prior to distribution. The octane rating is increased to improve engine ignition. Sulphur is removed because when products are used the sulphur compounds emitted would smell of rotten eggs and dissolve in rain to form sulphuric acid which would contribute to the problem of acid rain. Other strong smelling compounds are also removed. Heavy residue is taken off at the base of the tower and reprocessed. In the fluid catalytic cracker (FCC) the heavy oil is distilled again, using a chemical catalyst this time, to produce gasoline and diesel. The heaviest sticky residue is redistilled in the vacuum distillation unit then taken to the Lubricants Zone where it is processed to make bitumen, lubricating oils and wax.