Fractional distillation and petrochemicals

What you'll learn

• What crude oil is, and why it contains many different hydrocarbons.
• How fractional distillation separates crude oil into useful fractions.
• Why evaporation and condensation happen at different heights in the column.
• How fractions are used as fuels and as feedstock for the petrochemical industry.

1. Crude oil: the starting mixture

Crude oil is a dark, thick liquid found underground. It formed over millions of years from the remains of ancient organisms, so it is a fossil fuel: a fuel made from the remains of living things over geological time.

A mixture contains different substances that are not chemically bonded to each other. This matters because crude oil does not need chemical reactions to be separated at first — its hydrocarbons can be separated by their physical properties.

Many of the hydrocarbons in crude oil are alkanes, which are a family of saturated hydrocarbons. For this sub-topic, the key point is not the formula — it is that crude oil contains hydrocarbon molecules of different sizes.

2. Carbon makes many families of compounds

Carbon atoms are unusually useful in chemistry because each carbon atom can form four covalent bonds. This means carbon atoms can join to other carbon atoms to make chains, branches and rings.

That is why there is a huge variety of natural and synthetic carbon compounds. A synthetic compound is one made by humans, often in industry.

A family of compounds is a group of compounds with similar structures and similar chemical properties. In crude oil, hydrocarbons with similar numbers of carbon atoms tend to behave in similar ways.

3. Boiling point: the property used for separation

A boiling point is the temperature at which a liquid changes into a gas throughout the liquid.

Two changes of state are especially important here:

• Evaporation: a liquid changes into a gas.
• Condensation: a gas changes into a liquid when it cools.

Different hydrocarbons in crude oil have different boiling points. Smaller hydrocarbon molecules usually have lower boiling points, so they evaporate more easily. Larger hydrocarbon molecules usually have higher boiling points, so they need higher temperatures to evaporate.

4. What is a fraction?

A fraction is not usually a single pure compound. It is a group of hydrocarbons with similar boiling points.

This is why the process does not separate crude oil into every individual molecule. Instead, it separates crude oil into useful groups.

For example, one fraction might contain relatively small molecules that are useful as petrol, while another contains larger molecules used in diesel oil or heavy fuel oil.

5. Fractional distillation: the full technique

Fractional distillation is the process used to separate crude oil into fractions. It happens in a tall industrial tower called a fractionating column.

The column is very hot at the bottom and cooler at the top. This temperature difference is essential.

Step-by-step: how it works

1. Crude oil is heated in a furnace, which is a high-temperature industrial heater.
2. Many of the hydrocarbons evaporate and become gases.
3. The hot hydrocarbon vapours enter the fractionating column near the bottom.
4. The vapours rise up the column.
5. The column is hottest at the bottom and cooler towards the top.
6. When each hydrocarbon reaches a part of the column that is below its boiling point, it condenses back into a liquid.
7. Liquids are collected from different levels as different fractions.

Small molecules with low boiling points rise higher before they condense. Larger molecules with higher boiling points condense lower down.

6. The main fractions and fuels

Many fuels used in modern life come from crude oil. A fuel is a substance that releases energy when it burns.

Important crude oil fuels include:

• Liquefied petroleum gases, often called LPG, used as fuel in heating and cooking.
• Petrol, used in cars.
• Kerosene, used as aircraft fuel.
• Diesel oil, used in some cars, lorries, buses and trains.
• Heavy fuel oil, used in ships and some power stations.

You do not need to memorise lots of extra fraction names for this specification. Focus on the idea that different fractions contain molecules of different sizes and boiling points.

7. Fractions as feedstock

Crude oil fractions are not only used as fuels. Some are processed to make new substances.

The petrochemical industry is the industry that uses crude oil fractions to make useful chemicals and materials.

Important petrochemical products include:

• Solvents: liquids that dissolve substances, such as in paints and cleaning products.
• Lubricants: substances that reduce friction between moving parts.
• Polymers: very large molecules made from many repeating units, used to make plastics.
• Detergents: cleaning substances that help water remove grease and dirt.

This is why crude oil is described as both a source of fuels and a source of chemical feedstock.

8. Fractional distillation separates — it does not crack

Later, you may learn about cracking, which breaks large hydrocarbon molecules into smaller ones. Fractional distillation is different.

Fractional distillation is a physical separation process. The molecules are not broken apart. They are simply sorted into fractions according to boiling point.

9. How to explain it clearly

A strong GCSE answer should link the process to both changes of state:

• The crude oil is heated so hydrocarbons evaporate.
• The vapours rise up a column with a temperature gradient.
• The column is cooler at the top than at the bottom.
• Hydrocarbons condense when they reach a part of the column below their boiling point.
• Fractions are collected at different levels.