Crude oil fractions as fuels and feedstocks

Crude oil is separated into fractions: mixtures of hydrocarbons with similar boiling ranges. Once separated, the fractions can be used directly or processed further. Some are used as fuels; others provide starting materials for making chemicals.

Crude oil fractions as fuels

A fuel releases energy when it burns. Crude oil fractions provide fuels for transport, heating and industry:

• Liquefied petroleum gases (LPG) can be used for heating and cooking.
• Petrol is used in many cars.
• Kerosene is used as an aircraft fuel.
• Diesel oil is used in vehicles such as cars and lorries.
• Heavy fuel oil is used in ships and for industrial heating.

Crude oil fractions as feedstocks

A feedstock is a starting material used to make other chemicals. Fractions used as petrochemical feedstocks can be processed into a wide range of useful carbon-based substances, including:

• Solvents, which dissolve other substances.
• Lubricants, which reduce friction between moving surfaces.
• Polymers, which are used to make materials such as plastics.
• Detergents, which help remove dirt and grease.

Carbon atoms can bond to one another to form chains and other arrangements. Different carbon compounds can have similar structures and form families of compounds, while differences in their structures give them different properties. This helps explain why there are so many natural and synthetic carbon compounds with a wide range of uses.

Fractional distillation of crude oil

Crude oil is a mixture of many different hydrocarbons, rather than a single substance. Hydrocarbons are compounds made from hydrogen and carbon. Because the hydrocarbons in crude oil have different boiling points, they can be separated into groups by fractional distillation.

Separation in the fractionating column

Crude oil is heated so that many of its components evaporate. The vapours enter a fractionating column, which is hotter at the bottom and cooler at the top. As the vapours rise, they cool. When a hydrocarbon vapour reaches a region cool enough, it condenses into a liquid.

Hydrocarbons with higher boiling points condense lower down the column, where it is still relatively hot. Those with lower boiling points remain as vapours for longer and condense higher up, where the column is cooler. Liquids are collected at different levels of the column.

Crude oil fractions

Each liquid collected is called a fraction. A fraction is not normally a single hydrocarbon: it contains hydrocarbons with a similar range of boiling points and usually a similar number of carbon atoms. The hydrocarbons in the same fraction therefore have similar properties.