crude oil distillation

Higher and foundation tiers

Fractional distillation of crude oil

Crude oil despite being one of the world's most valuable natural resources is a useless, smelly and thick black "liquid" when it comes out of the ground. The main reason for this is that it is a mixture of thousands of different hydrocarbon molecules; now recall that hydrocarbons are compounds containing only the elements hydrogen and carbon. There are thousands of different hydrocarbons all mixed together in crude oil. Some of the hydrocarbon molecules are small, some are medium sized and some are large. At an oil refinery this mixture of hydrocarbons present in the crude oil is separated into different fractions of hydrocarbons which contain similar sized molecules; this is easily done since similar sized molecules have similar boiling points.

The image below shows some of the valuable hydrocarbon fractions that are obtained from crude oil.

Explanation of what happens to crude oil at an oil refinery, how it is separated out into useful fractions.  Fractional distillation of crude oil

Fractional distillation

The mixture of hydrocarbons present in the crude oil are separated out into various fractions at the oil refinery, now these fractions are themselves mixtures of many thousands of similar sized hydrocarbon molecules and since these molecules are similar in size they will have boiling points which are relative close to each other. The crude oil is separated in a fractionating column at the oil refinery similar to the one shown in the image above. To separate the crude oil into its various fractions the crude oil is: Chemistry student working in the school science lab

The larger or longer the hydrocarbon molecule the higher will be its boiling point. The fractionating column will basically separate out molecules according to their boiling points into separate fractions. Each fraction will contain many different hydrocarbon molecules each with similar boiling points. As you can see lots of very valuable substance such as fuels, petrol, diesel, kerosene for modern transport, oil to lubricate engines and machinery as well as feedstock molecules in the naphtha fraction which may end being used in medicines, cosmetics, solvents, detergents, plastic as well as many other everyday products. Each fraction collected will contain a mixture of many different molecules. These molecules will be of a similar size with similar boiling points.

Key Points

Practice questions

Check your understanding - Questions on Fractional distillation of oil.

Check your understanding - Quick Quiz on fractional distillation of oil.

Check your understanding - Additional questions on Fractional distillation of oil.