A column still also called a continuous still, patent still, or coffey still is a variety of still that consists of two columns invented by the Irish man Aeneas Coffey. The first, called the analyser, has steam rising and wash descending through several levels. The second column called the rectifier carries the alcohol from the wash where it circulates until it can condense at the required strength. Column stills behave like a series of single pot stills, except in a long vertical tube. The tube is filled with either porous packing, or bubble plates. The rising vapor, which is low in alcohol, starts to condense in the cooler, higher level of the column. The temperature of each successively higher stage is slightly lower than the previous stage, and so the vapor in equilibrium with the liquid at each stage is progressively more enriched in alcohol. Whereas a single pot still charged with wine might yield a vapor enriched to 40-50% alcohol, a column still can achieve a vapor alcohol content of 96%; an azeotropic mixture of alcohol and water. Further enrichment is only possible by absorbing the remaining water using other means, such as hygroscopic chemicals.
This is an example of a fractional distillation, in that it yields a narrow fraction of the distillable components. This technique is frequently employed in chemical synthesis, in this case the component of the still responsible for the separation is a fractionating column.
A continuous still can, as its name suggests, sustain a constant process of distilliation. This along with the higher alcohol concentration of the final distillate is its main advantage over a pot still which can only work in batches. Continuous stills are charged with pre-heated feed liquor at some point in the column. Heat (usually in the form of steam) is supplied to the base of the column. Stripped (alcohol free) liquid is drawn off at the base while almost pure alcohol is condensed after migrating to the top of the column. The disadvantage of a continuous still, at least for the production of alcohol, is that the alcohol produced is not purified from higher boiling point contaminants such as methanol and acetaldehyde.
Column stills are frequently used in the production of grain whisky.