distillation

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Technique used to purify liquids or to separate mixtures of liquids possessing different boiling points. Simple distillation is used in the purification of liquids or the separation of substances in solution from their solvents – for example, in the production of pure water from a salt solution or the recovery of sodium chloride (table salt) from sea water.

The solution is boiled and the vapours of the solvent rise into a separate piece of apparatus (the condenser) where they are cooled and condensed. The liquid produced (the distillate) is the pure solvent; the substances (the solutes), now in solid form, remain in the distillation vessel.

Mixtures of liquids, such as crude oil (unrefined petroleum) or aqueous ethanol, are separated by fractional distillation, or fractionation, in a fractionating column. When the mixture is boiled, the vapours of its most volatile component (the component with the lowest boiling point) rise into the vertical column where they condense to liquid form. As they descend they are reheated to boiling point by the hotter rising vapours of the next component to reach boiling point. This boiling–condensing process occurs repeatedly inside the column. As the column is ascended, progressive enrichment by the lower-boiling-point components occurs; there is thus a temperature gradient inside the column. The vapours of the more volatile components reach the top of the column and enter the condenser for collection before those of the less volatile components. In the fractional distillation of petroleum, groups of compounds possessing similar relative molecular masses and boiling points (the fractions) are tapped off at different points on the column.

The earliest-known reference to the process is to the distillation of wine in the 12th century by Adelard of Bath. The chemical retort used for distillation was invented by Muslims, and was first seen in the West in about 1570.

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