The Column

A still on every floor

Vapour from the boiler rises through holes in the first plate and bubbles through the thin layer of liquid lying on it. Some of the water in the vapour condenses there, some of the alcohol in the liquid boils off, and what leaves the plate is richer than what arrived: one step up the staircase. The same happens on the next plate, and the next. The liquid, which grows weaker as it gives up its alcohol, runs down through a pipe at the side of each plate, the downcomer, to the plate below.

Real plates are not perfect: each achieves something like half to four fifths of an ideal equilibrium step, which the model here takes as 75%. The caps over the holes, bubble caps, make sure the vapour is forced through the liquid instead of just slipping past it.

Reflux: the column's fuel

A plate only works if there is liquid on it, and the liquid at the top has to come from somewhere. So the vapour leaving the column is partly condensed in a dephlegmator and sent back down as reflux. The reflux ratio is how much goes back for each part drawn off. Turn it down and the plates run dry and the spirit weakens; turn it up and the spirit gets stronger, but you draw off only a trickle and spend much more steam.

Push both plates and reflux and the strength stops near 96%, just short of the azeotrope from chapter 2: no column can pass it.

The continuous still

A pot still is a batch: fill, run, empty, clean. In 1830 Aeneas Coffey, a former Irish excise officer, patented a still that never stops. Its first column, the analyser, takes hot wash in at the top and steam in at the bottom; the steam strips the alcohol out as the wash falls from plate to plate, and spent wash drains out of the base. The alcoholic vapour then climbs the second column, the rectifier, where it is concentrated plate by plate. Grain whisky, most rum and almost all neutral spirit are made in columns descended from his.

Strength is a choice

A column can make a spirit too pure to taste of anything, and sometimes that is the goal: neutral spirit for vodka and gin must reach at least 96% in the EU. When flavour matters, the law stops the still short. Scotch must come off below 94.8%; bourbon at no more than 80%, so that the grain still speaks. Many American distilleries follow their column with a simple pot-like doubler, and many small distilleries today use a hybrid: a pot with a short column on top, plates that can be switched on or off.