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.

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:

• Passed through a hot pipe in a furnace as shown in the image above, this vaporises most of the hydrocarbon molecules present in the crude oil.
• The hot vapours then enter the tall fractionating column. This is a tall narrow cylinder with a temperature gradient; which gets cooler the higher you go up the column.
• The hot vapours rise up inside the fractionating column. When the hot vapours meets a surface cooler than its boiling point then it will condense and form a liquid. This is why for example if you boil a kettle too close to a cool window condensation collects on the window. The steam from the kettle which maybe around 130⁰C meets the window which is cooler than the boiling point of the water; so the steam will condense. The same process will happen inside the fractionating column. Hot vapours will rise until they meet a bubble cap cooler than their boiling point, the hot vapours will then condense and the liquids formed will be run off and collected.
• Any fraction or part of the oil that was not vaporised falls to the bottom of the fractionating column and is collected; this fraction will consist of bitumen (tar) and greases.

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

• Crude oil is a mixture of many different sized hydrocarbon molecules.
• Crude oil is separated out into different fractions at an oil refinery. Here the crude oil is heated until most of it vaporises. The hot vapours then enter a tall fractionating column where they condense at different heights inside the column.
• The fractionating column has a temperature gradient; it is hot at the bottom and cooler at the top.
• Small hydrocarbon molecules have low boiling points; large hydrocarbon molecules have high boiling points. Inside the fractionating column large hydrocarbon molecules condense at the bottom and as you rise up the column the smaller molecules condense higher up.

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