Contrary to pot still distillation, which is a batch-oriented process whereby the stills need to cool down and be cleaned after each round of distilling before being heated up again, column still distillation is a continuous process. One of the first column stills was designed by Aeneas Coffey, an Irish distiller, and was patented in the first half of the 19th century. The structure usually consists of two interconnected columns, an analyser and a rectifier. Both are divided into a series of compartments, separated by perforated plates.
Wash, the low-alcohol fluid retained after fermentation, is fed into the column through a copper pipe attached to the top of the analyser, where the wash is poured over the first perforated plate. The liquid starts to trickle through the compartments and makes its way to the bottom of the column while steam simultaneously rises from the bottom and ascends through the plates, making contact with the liquid and stripping it of alcohol.
The alcoholic fumes travel from the analyser through a pipe to the bottom of the rectifier and start ascending again. Since the various alcohols fractionate at different temperatures, the spirit divides into various parts. The heavier alcohols condense back into liquid form and lie on the plates. Only the lighter alcoholic vapors make it to the top of the column and are collected in a condenser. The eventual spirit contains approximately 90 to 94.8% abv.
This is an edited feature provided by SMWS ambassador Hans Offringa from his comprehensive A Field Guide to Whisky: An expert compendium to take your passion and knowledge to the next level which you can purchase by clicking on the link.