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 valuable hydrocarbons all mixed together in crude oil and it is the job of an oil refinery to separate out all these valuable hydrocarbons from the 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 fractionating column

Fractional distillation- how it works

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. 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 and turns them into a hot gas.


• The hot vapours then enter the tall fractionating column. This is simply a tall narrow cylinder with a temperature gradient; this tall column is hot close to the bottom and the furnace but 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 a kettle maybe around 130⁰C; now when this hot steam meets the window; which is cooler than the boiling point of the water ; then the steam molecules will lose energy and the steam will condense to form droplets of water on the window. Well the same process will happen inside the fractionating column hot hydrocarbon vapours will rise up the fractionating column until they meet a bubble cap (metal tray) cooler than their boiling point, the hot vapours will then condense and the liquid hydrocarbon formed will be run off and be collected.


• Any fraction or part of the crude 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.

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Crude oil and its fractions

The larger the hydrocarbon molecule the higher will be its boiling point. The fractionating column will basically separate out hydrocarbon molecules according to their boiling points into separate parts or fractions. Each fraction will contain many different hydrocarbon molecules each with similar boiling points; these fractions will also contain hydrocarbon molecules of a similar size.

As you can see from the image above lots of very valuable substance such as:

• The fuels petrol, diesel and kerosene


• Oil to lubricate engines and machinery


• Feedstock molecules in the naphtha fraction which may end being used in medicines, cosmetics, solvents, detergents, plastic as well as many other everyday products.

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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. Where they condense depends on their boiling point.


• 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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