A column still, also called a continuous still, patent still or Coffey still, is a variety of still consisting of two columns invented in 1826 by Robert Stein, a Clackmannanshire distiller, and it was first used at the Cameron Bridge Grain Distillery in Fife, Scotland. The design was enhanced and patented in 1831 by an Irishman, Aeneas Coffey. The first column (called the analyzer) 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, formed in a long vertical tube. The tube is filled with either porous packing or bubble plates. The rising vapour, 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, so the vapour in equilibrium with the liquid at each stage is progressively more enriched with alcohol. Whereas a single pot still charged with wine might yield a vapour enriched to 40-50% alcohol, a column still can achieve a vapour 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 hydrophilic chemicals or azeotropic distillation.
A column still 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 distillation. This, along with the ability to produce a higher concentration of alcohol in 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 (approximately alcohol-free) liquid is drawn off at the base, while alcoholic spirits are condensed after migrating to the top of the column.
Column stills are frequently used in the production of grain whisky. Distillation by column still is the traditional method for production of Armagnac although distillation by pot still is allowed. The use of column stills for the distillation of Cognac is forbidden. Distillation by column stills are permitted for Calvados A.O.C. and Calvados Domfrontais. Calvados Pays d'Auge A.O.C. is required to be distilled by pot still.
