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Science

Making crude oil useful

Fractional distillation

Hydrocarbons have different boiling points. They can be solid, liquid or gas at room temperature,

  • small hydrocarbons with only a few carbon atoms have low boiling points and are gases
  • hydrocarbons with between five and 12 carbon atoms are usually liquids
  • large hydrocarbons with many carbon atoms have high boiling points and are solids.

Because they have different boiling points, the substances in crude oil can be separated using fractional distillation.

The fractionating column

Fractional distillation is different from distillation in that it separates a mixture into a number of different parts, called fractions. A tall column is fitted above the mixture, with several condensers coming off at different heights. The column is hot at the bottom and cool at the top. Substances with high boiling points condense at the bottom and substances with lower boiling points condense on the way to the top.

The crude oil is evaporated and its vapours condense at different temperatures in the fractionating column. Each fraction contains hydrocarbon molecules with a similar number of carbon atoms.

Fractioning column

Oil fractions and their uses

Back to Carbon chemistry index

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