Describe the fractional distillation of crude oil
Crude oil from an oilfield is separated from impurities such as sand and water and is pumped through pipelines to the refinery. At the refinery, fractional distillation is used to separate the crude oil into several fractions, each fraction containing several hydrocarbons which boil within a certain range of temperatures. These different boiling points are roughly related to the number of carbon atoms in the hydrocarbon.
Separation of the hydrocarbons takes place in a fractionating column (fractionating tower). At the start of the refinery process, crude oil is preheated to a temperature of 350–400°C and pumped in at the base of the tower. As it boils, the vapour passes up the tower. It passes through a series of bubble caps and cools as it rises further up the column. The different fractions cool and condense at different temperatures, and therefore at different heights in the column. The fractions condensing at the different levels are collected on trays.
Thus, vapour is rising and liquid falling at each level in the tower. As a result very efficient fractionation occurs. Liquid is taken off at several different levels, the higher the level, the lower the boiling point of the fraction removed. Figure above right shows the process of separation of crude oil into different fractions.
After fractional distillation, impurities are removed. The commonest impurity is sulphur, which is removed and used to manufacture sulphuric acid. If petrol (gasoline) containing sulphur is not purified before it is used in an internal combustion engine, the exhaust fumes will contain oxides of sulphur (SO2 and SO3). These are poisonous gases and will pollute the environment.