The still shown in the accompanying illustration (fig. 4) is a type designed by Messrs. Robert Willison of Alloa for Scotch grain whisky distilleries. This Coffey still is a double still con sisting of two adjacent columns, termed respectively rectifier and analyser. Both columns are subdivided into a number of chambers by perforated copper plates. The main structure is of wood firmly braced with iron. Each compartment communicates with the next by means of a drop pipe standing slightly above the level of the plate and passing downwards into a cup, which forms a water seal or joint. Each compartment is also fitted with a safety valve in case of the plates choking or of the pressure ris ing unduly. At the beginning of the operation both columns are filled with steam at a pressure of about 5 pounds. The steam at the base of the analyser passes upwards through it, thence to the bottom of the rectifier by means of the pipe B (termed the low wines vapour pipe), and then up through the rectifier. When both columns are filled with steam the wash is pumped up from the wash charger through the copper pipe A nearly to the top of the rectifier, which it enters at the point A. The pipe A runs from the top to the bottom of the rectifier, forming a double bend in each compartment, and the wash (contained in the pipe) travels down in a zig-zag course until it reaches the base of the rectifier at the point C. From here (still remaining in pipe A) it is pumped to the top of the analyser, where it emerges from the pipe and covers the plate of the top compartment. As there is an upward pressure of steam the wash is not able to pass through the per forations of the copper plate forming the base of the compart ment, but collects until its level reaches the top of the first drop pipe. Through this is passes into the cup on the plate below and so out on to the next plate. The drop pipes being trapped by the cups, the steam cannot pass upwards through the former. In this way the wash passes through compartment to compartment on the analyser until it reaches the bottom, and then passes out by means of the spent wash siphon. The steam, on its passage up through the analyser, carries with it the alcoholic vapours and other volatile matters contained in the wash. The alcoholic vapours pass from the top of the analyser to the bottom of the rectifier, and then upwards through the latter from compart ment to compartment. In so doing they are gradually cooled by the wash flowing down through the pipe A. This gradual cool ing causes the less volatile constituents to condense and so to flow downwards through the column until they reach the base of the rectifier. At a certain point in the upper part of the rectifier (marked S in the illustration) the bottom of the corn partment in question is formed not of a perforated plate, but of a stout copper sheet, pierced by a fairly wide pipe, which stands up about 2 in. above the level of the former. This is termed the spirit plate. It is so placed that the alcoholic vapours condense either on or just above it. The alcohol passes out from the spirit plate chamber from one of the two pipes shown in the illustration (either to the spirits or to the feints receiver as the case may be), and is then further cooled, to complete the condensation, by being run through coils immersed in flowing cold water.
In order to render the condensation still more perfect the upper chambers of the rectifier are fitted with coils through which cold water is passed. The vapours condensed by this fall upon the spirit plate. The vapours which have an appreciably lower boiling point than ethylic alcohol, such as the aldehydes, together with a large volume of carbonic acid gas derived from the wash, pass out of the top of the rectifier by means of the "incondensi ble gas" pipe E, and thence to a separate condensing coil. The
spirit retained is of high strength, generally about 64 over proof. The less volatile constituents of the wash, generally termed "fusel oil," which pass out of the base of the rectifier, are cooled and then passed to the oil vessel. After the apparatus has been worked for some time the fusel oil which floats in a layer on the top of the contents of the oil vessel is skimmed off. The watery layer from the oil vessel, which still contains a little alcohol, is again passed through the apparatus to remove all of the latter.
The distillation is controlled by an operator standing on the platform P. The operator is able by means of the sampling ap paratus X to determine the quality and strength of the spirit and of the wash. He is able, by regulating the quantity of steam admitted to the apparatus, by modifying the rate of pumping, and by running the spirit either to the spirit or to the feints receiver, as the case may be, to control the strength and quality of the product in much the same manner as does the pot still distiller.
By-products of Fermentation and Distillation.—Although the principal ingredient of all spirits is, of course, ethyl alcohol diluted with different proportions of water, there are present in varying quantities, other substances which impart to each type its individual characteristics and afford a means of determining its origin. These by-products are formed mainly during fermen tation, but are also to a certain extent pre-existent in the raw ma terials or may be generated during the operations preceding and succeeding fermentation. Their nature is complex and varies sensibly according to the raw materials and the methods of malt ing, mashing, fermentation and distillation. The by-products or secondary ingredients may be classified as follows: (a) higher alcohols, (b) esters, (c) fatty acids, (d) fatty aldehydes and acetals, (e) furfural, (f) terpene, terpene hydrate and ethereal oils, (g) volatile bases.
The higher alcohols—commonly known as fusel oil—consist of mixtures of fatty alcohols containing three or more atoms of carbon. The fusel oil British pot still spirits is chiefly composed of amyl, butyl and propyl alcohols, their relative proportion being in that order, but in the preparation of British spirits in the patent still the amyl and butyl ingredients are to a great extent removed and the propyl is the predominating higher alcohol in the finished product. The butyl in the grain spirits, as also in brandy, is chiefly the normal, whilst that con tained in spirit produced from potatoes is chiefly iso-butyl. The esters, formed by the interaction of alcohols and acids, chiefly during the fermenting and distilling operations, consist almost entirely of fatty acid radicles in combination with ethyl and to a minor extent amyl alcohol. Ethyl acetate (acetic ester) is the chief of these, others being ethyl valerate, butyrate and pro pionate. Oenanthic ether (ethyl pelargonate) although only representing a small proportion—about one per cent—of the total secondary ingredients present in brandy, is one of that spirit's most pronounced characteristics. The acid present is chiefly acetic—formed by the oxidation of the ethyl alcohol—but small quantities of other acids are also found. Furfural, which is prob ably formed chiefly from the pentoses contained in the wort, occurs to a greater or less extent in most spirits, but it is par ticularly characteristic of pot still products.