4.7-4.10 - Crude oil fractions and fractional distillation

Crude oil as a mixture

Crude oil is a mixture. That means it contains many different substances that are physically mixed together, rather than chemically joined into one compound.

The substances in crude oil are hydrocarbons. A hydrocarbon is a compound that contains hydrogen and carbon only.

Because crude oil is a mixture, its hydrocarbons can be separated by a physical method. The hydrocarbons do not need to be reacted or changed into new substances for this separation.

A fraction is not usually one pure hydrocarbon. It is a group of hydrocarbons with similar boiling points. Fractions are collected over a boiling point range, so each fraction still contains a mixture of hydrocarbons.

The important exam contrast is:

How fractional distillation works

Fractional distillation separates crude oil because the hydrocarbons in crude oil have different boiling points. In an oil refinery, crude oil is heated before it enters the fractionating column. Much of it vaporises and the hot vapours enter near the bottom of the column.

[DIAGRAM: asset_name: Crude oil fractions and fractional distillation - diagram 01; asset_slug: c50_crude_oil_fractions_and_fractional_distillation__diagram_01; recommended_method: image_gen; description: A monochrome industrial fractional distillation column for crude oil, showing heated crude oil vapours entering the lower column, vapours rising through a hot-bottom/cool-top temperature gradient, and refinery gases, gasoline, kerosene, diesel, fuel oil and bitumen removed from top to bottom.]

The column has a temperature gradient. It is hotter at the bottom and cooler at the top. As vapours rise up the column, they cool.

When a hydrocarbon vapour reaches a region of the column below its boiling point, it condenses to a liquid. Fractions are then tapped off or removed at different heights. Low-boiling fractions are collected higher up the column. High-boiling fractions condense lower down or remain near the bottom.

For a Pearson-style description, build the chain in this order:

Avoid saying that fractional distillation breaks hydrocarbon molecules into smaller molecules. It separates the mixture into fractions; chemical breaking of molecules belongs to a later topic.

Main fractions and uses

You need to know the main fractions from crude oil in the order they are collected. The order below runs from the top of the fractionating column to the bottom.

The names matter. Pearson questions may give a diagram with unnamed outlets and ask for the top and bottom fractions. The top fraction is refinery gases, not just "gas". The bottom fraction is bitumen.

Some names vary in everyday use. Gasoline is often called petrol. Kerosene may be used as aircraft fuel. In exam answers, use the specification names unless the question clearly accepts an everyday alternative.