The process of distillation should be familiar to most readers. The basic concept is that we can separate a mixture of two pure liquids with different boiling points by heating the mixture to a temperature between their respective boiling points. For example, water boils at 100°C and ethanol boils at around 83°C at atmospheric pressure. If we heat the mixture to say 92°C, the ethanol will boil and be transformed into vapour (which is A distillation column is used to make this process more efficient. A schematic diagram of a distillation column is shown below. collected and condensed) while the water will remain as a liquid. This phenomenon is usually quantified by the relative volatility of the two components.

The distillation column itself is made up of a series of stacked plates. A liquid feed containing the mixture of both liquids enters the column at one or more points. The liquid flows over the plates, and vapour bubbles up through the liquid via holes in the plates. As liquid travels down the column, vapour comes in contact with it many times (due to the multiple plates). This is the critical process in distillation columns. The liquid and vapour phases are brought into contact because as one molecule of higher boiling material converts from vapour to liquid phase by energy release, another molecule of the low boiling material utilises the free energy to convert from liquid to vapour phase.