Technical guidance: Stages of production: Continuous Distillation
(Coffey Patent Still Distilling Process – diagram at SPIR4290)
Grain, neutral and some fruit spirits are manufactured by continuous distillation. This method of distillation which uses ‘patent’ stills is more efficient than the traditional pot still. The distillation apparatus consists of two columns, the ‘analyser’ and ‘rectifier’. These columns are made up of a number of compartments separated by diaphragms or plates. The plates are perforated with small holesto permit the upward passage of steam in the analyser and spirit vapour in the rectifier. Broadly speaking, the analyser performs the function of the wash still and the rectifier the low wines still.
Wash is introduced near the top of the still and percolates slowly down through the plates. Steam enters from the bottom of the column and rises up through the plates. As thewash proceeds down the column it is heated by the rising steam. The alcohol and vapour,known as low wines are boiled off. By the time the wash reaches the bottom of the ‘analyser’ all that remains are solids and water (spent wash).
The low wines produced in the ‘analyser’ are fed into the ‘rectifier’ where they condense and separate out into various factions. The ethanol faction (spirit) is drawn off and collected in the spirits receiver, the remaining liquid is either returned to the ‘analyser’ for re-distillation or extracted as ‘fuseloil’.
‘Fusel oil’, a by-product of the distillation process, consists mainly of a mixture of ‘higher’ alcohols (amyl, butyl, propanol and so on), water and ethanol. Disposal of fusel oil is discussed in SPIR4160.
The ethanol produced by continuous distillation is of a higher strength and purer quality than the spirits obtained from pot stills. There are, however, limits to how pure the spirit produced can be if they are to be used in the manufacture of certain types of drinks. Under Council Regulation (EEC) No 1576/89 which lays down general rules on thedefinition, description and presentation of spirit drinks:
- whisky must be distilled at a strength less than 94.8% alcohol by volume; and
- fruit spirit must be distilled at a strength less than 86% alcohol by volume,
to retain the aroma and taste derived from the raw materials used.
There are no limitations placed on the upper strength of neutral spirits.