Known since antiquity, the concentration of alcohol by the application of heat to a fermented liquid mixture is perhaps the oldest form of distillation. However, the technique is now widely used for a variety of liquids in chemistry and in the production of petroleum products, among other fields.
The device used to effect a distillation is referred to as a still and consists, at minimum, of a pot in which the source material is heated, a condenser in which the heated vapor is cooled back to the liquid state, and a receiver in which the concentrated or purified liquid is collected.
The equipment may effect separation by one of two main methods. Firstly the vapours given off by the heated mixture may consist of two liquids with significantly different boiling points. Thus, the vapour that is given off is in the vast majority of one or the other liquid, which after condensation and collection effects the separation.
The second method (fractional distillation) is more effective at separating liquids with similar boiling points. This method relies upon a gradient of temperatures existing in the condenser stage of the equipment. Often in this technique, a vertical condenser , or column, is used. By extracting products that are liquid at different heights up the column, it is possible to extract liquids that have different boiling points. The greater the distance over which the temperature gradient in the condenser is applied leads to easier and greater separation.
Under ideal conditions, distillation can be used to effect a near-complete separation of one type of liquid from another. The fractions can further purified by a second distillation.
It is also possible to separate fractions by cooling, using differences in their freezing points.