Fractional distillation of crude oil

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Crude oil despite being one of the world's most valuable natural resources is a smelly 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. Some of these hydrocarbon molecules are small molecules such as the gases methane, ethane and propane which are dissolved in the crude oil. There are also many medium sized hydrocarbon molecules such as those found in petrol or diesel as well as large hydrocarbon molecules that are found in bitumen, tar and greases. Now recall that hydrocarbons are compounds containing only the elements hydrogen and carbon.

At an oil refinery

There are thousands of valuable hydrocarbon molecules all mixed together in crude oil and it is the job of an oil refinery to separate out these valuable hydrocarbons from the crude oil. At an oil refinery this mixture of hydrocarbons present in the crude oil is separated into different fractions which contain similar sized molecules. This is easily done since similar sized molecules have similar boiling points.

The fractionating column

As we mentioned above crude oil is not very useful when it first comes out of the ground because it is a mixture of many different hydrocarbon molecules. Now at an oil refinery the crude oil is separated into different useful parts called fractions. This separation takes place in a tall structure called a fractionating column and the process used to separate the crude oil into its fractions is called fractional distillation.

The image below shows a fractionating column some of the valuable hydrocarbon fractions that are obtained from crude oil and the uses these fractions are put to.

Fractional distillation - how it works

The mixture of hydrocarbons present in crude oil is separated into various fractions at an oil refinery. These fractions are themselves mixtures of many thousands of similar sized hydrocarbon molecules. Since these molecules are similar in size they have boiling points that are relatively close to each other. The crude oil is separated in a fractionating column. 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 bottom of the tall fractionating column. This is a tall narrow cylinder with a temperature gradient. The column is hot near the bottom close to the furnace and becomes cooler the higher you go up the column.


• The hot vapours rise up inside the fractionating column. When these hot vapours meet a surface cooler than their boiling point they condense and form a liquid. This is a similar process that happens when steam from a kettle meets a cool window and condensation forms on the glass.
  
  The steam from a kettle may be around 100⁰C. When this hot steam meets the window which is cooler than the boiling point of water the steam molecules lose energy and the steam condenses to form droplets of water. The same process happens inside the fractionating column. Hot hydrocarbon vapours rise up the column until they meet a bubble cap or metal tray that is cooler than their boiling point. The hot hydrocarbon vapours then condense and the liquid hydrocarbon formed is run off and 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 consists mainly of bitumen and greases.

Self-check

Match up the terms with their correct definitions. Simply click the term and then its correct definition. Correct responses will turn green.

Crude oil and its fractions

The larger the hydrocarbon molecule the higher will be its boiling point, this is largely due to more and stronger intermolecular bonding between the hydrocarbon molecules. This stronger intermolecular bonding between the hydrocarbon molecules means it takes more energy to separate the molecules which means they will be less volatile and less flammable. Click here for more info on how intermolecular bonding affects the boiling points of hydrocarbons.

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 and similar sizes.

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

• The fuels petrol, diesel and kerosene


• Oil to lubricate engines and machinery


• Feedstock molecules in the naphtha fraction which may end up being used in medicines, cosmetics, solvents, detergents, plastics as well as many other everyday products are all separated out from the crude oil.

Self-check

Try the quick activity below to review your understanding of fractional distillation:

Self-check: Quick Quiz

Try the quick quiz below to review your understanding of fractional distillation:

Self-check summary

Complete the paragraphs below by filling in the blanks to complete the sentences summarising the main points mentioned above. The words to complete the sentences are shown in the yellow boxes below. Simply select the correct word from the drop-down menus to complete the two paragraphs.

Summary table

Why not use the summary table below to make set of revision flashcards or revison notes on the fractional distillation of crude oil?

Key Points

• Crude oil is a mixture of many different sized hydrocarbon molecules.


• At an oil refinery 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.


• 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.


• Large hydrocarbon molecules condense near the bottom of the fractionating column while smaller hydrocarbon molecules condense higher up.

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