Discontinuous Distillation

The Pot Still (Fig. 103). - In the earliest types of plant the two processes of distillation and rectification were carried out in the same vessel, the latter process being very incompletely effected. A certain volume of the fermented wash was boiled in a large kettle and the vapour so produced led directly to a condenser without passing through any rectifying apparatus. The distillate was then again distilled, and this process had to be repeated several times before strong alcohol was produced. To remove aldehydes and higher alcohols the distillate from the final distillation was collected in three fractions. The first, called " foreshots," contained, besides some ethyl alcohol, most of the aldehydes and esters. The middle fraction contained most of the ethyl alcohol, and the third (feints) contained a large percentage of the higher alcohols.

The plant was simple, but the process was very uneconomical both in time and materials, and the finished alcohol still contained a considerable quantity of aldehydes and higher alcohols. In the manufacture of whisky, brandy, and rum the presence of these by-products is to some extent desirable, as they produce characteristic tastes and odours. For this reason the distillation of these beverages is still sometimes carried out in the above manner in the so-called "Pot Still." Fig. 103 shows a fire-heated pot still. In the earliest form of pot still the neck was not so high as that shown in the figure. The object of lengthening the neck was to prevent the liquid passing over into the condenser, as much frothing takes place in the still, and to produce an increased rectifying action. Many pot stills now in use in Scotland are fitted with " purifiers," which consist of circular vessels cooled by water, placed between the neck and condenser; the condensate produced by them flows back to the still. In some cases the "purifier " consists of a form of multitubular water-cooled dephlegmator. In Ireland the pot stills are much larger, up to 20,000 gallons capacity. The pipe connecting the top of the still with the condenser, called the "Lyne Arm," is from 30 to 40 feet long and is surrounded by a trough through which passes water, the rate of flow of which can be regulated. A pipe from the end of the "lyne arm ' furthest from the still serves to return the liquid condensed in the lyne arm to the still. Similar return pipes are sometimes fitted between the cooling coil and the still. The rate of flow of liquid through these return pipes can be controlled by taps. The point at which the lyne arm enters the condenser is frequently 35 to 40 feet above the top of the still. It is obvious that rectification of the distillate must take place to a considerable extent in such a lyne arm.

Fig. 103.

In Scotland the process of distillation is carried out in two stages. The wash, to which soap is often added, principally to prevent frothing, is distilled in the wash still. One fraction only is collected. The distillation is stopped when the hydrometer shows that the distillate no longer contains alcohol. The residue in the still is run to waste.

The distillate is transferred to a second still, called the "low wines ' still, and redistilled. In this case three fractions are collected, (1) Fore-shots, (2) Whisky, (3) Feints. The collection of the last fraction ceases when the distillate no longer contains alcohol as shown by the hydrometer. The residue in the still is run to waste.

The point at which to begin collecting a new fraction can be judged from the alcoholic strength of the distillate and from the presence or absence of turbidity on adding water to a sample of the distillate. But the decision depends largely on the experience and judgment of the distiller. The first and third fractions are mixed together and added to a subsequent charge in the low wines still, or they may preferably be treated separately. The strength of the middle fraction is about 60 per cent by weight. In Ireland three stills are generally used and the distillate is collected in a larger number of fractions. The strength of the whisky fraction in this case is about 80 per cent by weight.1

The alcoholic strength of the middle or whisky fraction will have a marked effect on its content of impurities. As already stated certain volatile impurities tend to accumulate at a point in the still at which the alcoholic content is from 68 to 73.5 per cent by weight. If the distillate be only raised to a strength of 60 per cent a large part of these substances will pass over into the distillate. But at a strength of slightly over 80 they will be condensed to a considerable extent in the lyne arm, and returned to the still.

1 P. Schidrowitz, J.I.B., 1906, p. 496.

The coefficients of purification of the other impurities vary with the content of ethyl alcohol in the mixture. So that the amount of these in the whisky will also depend to some extent on the alcoholic strength at which the foreshots and whisky fractions are collected.

To prevent amyl alcohol from passing over too freely in the whisky fraction, the alcoholic content of the distillate is raised to a high point before any distillate is collected. When this is the case the separation of the aldehyde and esters will be more difficult. These substances will not be completely eliminated at the end of the fore-shots fraction and will contaminate the whisky fraction to some extent.

The character of the whisky produced will therefore depend upon the amount and nature of the impurities present in the raw spirit and on the methods of controlling the distillation.

Discontinuous Rectifying Stills

The alcohol produced by the stills so far described is not sufficiently pure for many purposes. To produce highly concentrated alcohol containing only traces of volatile impurities a still with a very efficient fractionating column is required. Fig. 104 diagrammatically shows one of these stills. The various types of plates in these columns are described under continuous rectification. The plate with bells is most frequently employed.

Tests of the efficiency of the various forms of plates have been carried out by Barbet and others. It is difficult to make a just comparison between them as varying conditions of working so greatly affect the results obtained. The plates with improved type of bells and directed flow appear to give as good results as any.

Fig. 104.

A. Rectifying column. F. Pipe carrying vapour unconB. Dephlegmator. densed in the dephlegmator

C. Condenser for finished spirit. to the condenser.

D. Vapour pipe. G. Test glass.

E. Reflux pipe from dephlegmator H. Cold water pipe to column. K. Steam pipes.

The number of plates in the column is very considerable. Table 112, taken from Sorel, Rectification de l'alcool, 1895, p. 55, shows the alcoholic content in volume per cent of the liquid on the plates of a discontinuous rectifying column. It will be noticed that there are a very large number of plates on which the alcoholic strength is over 95 per cent by volume.

The object of this, as already explained, is to keep amyl alcohol (and other by-products which behave similarly) from passing over with the distillate until as late a stage of the distillation as possible.

The details of the stills and columns vary greatly in practice, as do also the number of fractions collected and the degree of purity obtained.