Coffey still - Patent Still - Column Still

a continuous distillation

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 first used at the Cameron Bridge Grain Distillery. The design was enhanced and patented in 1831 by an Irishman, Aeneas Coffey.

This new still was called the 'Continuous Still' (also 'Column', or 'Patent', or 'Coffey Still'). In simple terms, consists of two columns, one of which has steam rising and wash descending through successive storeys inside (referred to as the 'Rectifier'). The steam stripped out the alcohol from the wash and carried over to the second column (referred to as the 'Analyzer') where it circulates until it can condense at the required strength.


The column on the right is the column still, however it is attached to a potstill.
 

The first column (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, formed 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, sr

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 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 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 (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 rendered free of lower-boiling-point contaminants (such as methanol and acetaldehyde).

The "column" still is a series of vertical sections that contain trays with bubble caps as shown in the next picture. As the boiling whisky vapors travel up through the trays, it cools a bit and droplets of whiskey and water drop out of the vapor and trickled down through the trays to the bottom of the still. The highly concentrated whiskey is taken off the top of the still where it is cooled.

The whiskey vapor from the boiling mash travels up through the bubble caps. The floor of the bubble tray has condensed liquid which the vapor bubbles through. As the vapor bubbles through the liquid some of the vapor cools and condenses. As the liquid builds up in the tray it will overflow the drain pipe and flow down the column to be reboiled.

The benefits of the continuous still is a cheaper and purer spirit.

  • Has two main parts, a 'Rectifier' and an 'Analyzer', which both resemble tall, wide tubes.
  • They are both filled with steam.
  • The liquid being distilled enters a pipe travelling down the rectifier, and is heated almost to boiling point.
  • The alcohol from the primary liquid vaporizes and is channelled along with the steam back to the base of the Rectifier.
  • Here it mixes with more steam around the pipes, bring with it more liquid to be distilled, hence a 'Continuous Still'.
  • Roughly two-thirds up the Analyser, the vapour hits a cold plate condensing it into a liquid. This is channelled out as a distilled product.

Today, alternatives open to distillers are to use the labour intensive pot still which carries the fragrances and flavours of the raw material, or the faster, cheaper continuous still with its potential for high strength, pure but tasteless spirit.

(click to enlarge)

How does it work:

Steam is fed into the base of the analyser and hot wash into the top. As the two meet on the surface of the perforated plates, the wash boils and a mixture of alcohol vapours and uncondensed steam rises to the top of the column. The spent wash runs down and is led off from the base.

The hot vapours enter the rectifier at the base and as they rise through the chambers they partially condense on the sections of a long coil through which wash is flowing. The spirit vapour condenses at the top of the rectifier and is run off through a water-cooled condenser to the spirit safe and on to the spirit receiver. Once the spirit begins to be collected it runs continuously until the end of distillation.

Legend:

A. Analyzer
B. Rectifier
1. Wash
2. Steam
3. Liquid out
4. Alcohol vapor
5. Recycled less volatile components
6. Most volatile components
7. Condenser


Because of the rectifying element present in this process, the distillate is generally lighter in aroma than most Malt Whiskies. It consequently has a milder character and requires less time to mature.

"
(click to enlarge)