fractional distillation

Process used to split complex mixtures (such as petroleum) into their components, usually by repeated heating, boiling, and condensation; see distillation. In the laboratory it is carried out using a fractionating column.

Fractional distillation is used to separate mixtures of miscible liquids, such as ethanol and water. The process depends on the components of the mixture having different boiling points. The liquid is heated so that it turns into a gas. The vapours pass up a fractionating column where they are gradually cooled. As each of the components of the mixture cools to its boiling point, it turns back into a liquid. The different components of the mixture condense at different levels in the fractionating column and thus may be separated.

In industry, fractional distillation is used to separate the compounds in crude oil (unrefined petroleum) into useful fractions, each fraction containing compounds with similar boiling points. Air is also separated by fractional distillation. This is done by cooling air until it condenses and then allowing the temperature of the liquid air to rise. Each gas will distill off at its own boiling point.