Composition Of Gas

Estimations have been made of the composition of the gas evolved during fermentation. Table 92 shows the results of one of these experiments.

The fermenting vessel was about two-thirds full of mash, the space above the latter being occupied by air before inoculation.

Table 92

The percentage of carbon dioxide did not vary from 9.30 a.m. 29. vi. '16 to the end of the fermentation, which lasted 36 hours. Total gas evolved, 42,694 cubic feet. The high percentage of hydrogen in the gas evolved during the early hours is partly due to the greater solubility of the carbon dioxide.

The production of acetone and n-butyl alcohol at various stages in the fermentation is shown in Table 93.

Table 93

Acidity is expressed in c.c. of N/10 alkali required to neutralise 10 c.c. of mash.

Wort, similar to that used in alcoholic fermentation for the production of ethyl alcohol, can also be used. The wash at the end of the fermentation of a starchy mash usually contains about 0.6 per cent of acetone, and. 1.4 per cent of n-butyl alcohol, and a much smaller proportion of ethyl alcohol. When wort is used the concentration may be much higher than this.

The Distillation Process

The distillation of the fermented wash is carried out either in a Coffey still slightly modified for the purpose, or in one of Messrs. Blair, Campbell & M'Lean's acetone stills. Both these stills will produce almost pure acetone in one operation. But when the acetone is required for the manufacture of cordite, redistillation is necessary in order to remove carbon dioxide and traces of substances which affect the stability of cordite. This final distillation is carried out in an intermittent still with a very efficient fractionating column.

Preliminary Distillation In Coffey's Still

See Fig. 121, p. 312.

The fermented wash is supplied to the still in the usual manner, but its temperature is from 35° to 40° C. As it passes down the rectifying column in the wash pipe its temperature must be slightly under 56° C. at the level of the spirit plate, where acetone is drawn off from the still. The difference between 35° C. and 56° C. is so small that it is difficult to keep the temperature low enough at the spirit plate. To overcome this difficulty either the wash pipe may be brought into the rectifying column lower down, and its upper coils replaced by an independent cold water coil, or the temperature of the wash may be reduced by mixing it with cold water.

After leaving the rectifying column, the wash enters the top of the analyser down which it flows, leaving the base of the column as spent wash, free from acetone and n-butyl alcohol.

The condensate leaving the base of the rectifier generally consists of a dilute solution of n-butyl alcohol in water. This is pumped back to the top of the analyser. Provision is made for running off the n-butyl alcohol from one of the lower plates of the rectifying column. The method of draw-'ing off the liquid from the plate is shown in Fig. 89. When the still is working smoothly, an azeotropic mixture of n-butyl alcohol and water can be run off steadily. It is then cooled in a condenser, and passes to a separator, in which two layers are formed; the upper layer consists of 85 per cent n-butyl alcohol, and 15 per cent water; the lower, of 12 per cent n-butyl alcohol and 88 per cent water. The lower layer is pumped back into the top of the analyser. When very concentrated wash is distilled the mixture of n-butyl alcohol and water is run off from the base of the rectifying column. In either case the n-butyl alcohol obtained contains some ethyl alcohol and acetone. It is most important to keep up a steady flow of n-butyl alcohol from the rectifying column, so as to prevent its gradually passing up the column and contaminating the acetone distillate. Acetone is drawn off from the spirit plate. The gases dissolved in the wash leave the still by a pipe passing into the air from the top of the rectifier, carrying with them a very small amount of acetone, which can be removed by passing through a scrubber, down which water trickles. The acetone from the spirit plate flows into a cooler, where its temperature is reduced to 15° C, the specific gravity being about 0.800. Having only been in contact with the gases at a temperature of about 56° ft, the amount of carbon dioxide dissolved in the acetone is much smaller than is the case when the acetone is cooled in presence of excess of the gases. This point is of some importance when the acetone is required for the manufacture of cordite.

Fig. 89.

Acetone of over 99 per cent purity can be produced with Coffey's still, and the n-butyl alcohol which is simultaneously run off contains less than 1 per cent of acetone.

Messrs. Blair, Campbell & M'Lean's Continuous Acetone Still,

Fig. 90. - The fermented wash is distilled in a boiling column, having plates of the bell type, described under alcohol distillation (p. 303). The wash flows from the reservoir a through the regulator b, and preheater c, in which it is heated by a heat exchange with the spent wash. It then enters the wash column d just above the top-most plate, and flows away as spent wash, practically free from acetone and n-butyl alcohol, by a trapped pipe at the base of the column.

Steam is introduced below the lowest plate at z, the supply being automatically controlled by the regulator g. The rising vapourbubbles through a depth of about 1 inch of liquid on each plate. The vapour leaving the top of the boiling column by pipe h enters the base of the rectifying column, which is of the same type as the boiling column, and is also provided with plates of the bell type.

Fig. 90.

The vapour leaving the top of the rectifying column enters a water-cooled tubular dephlegmator K, which provides the necessary condensate for the rectifying column. The vapour passing uncondensed through this dephlegmator enters at the top of a water-cooled condenser L, and is there completely condensed. Part of the condensate so produced flows to the test glass o as finished acetone, and part is returned by pipe n to the top of the rectifying column with the liquid condensed in the dephlegmator k. The gas originally dissolved in the wash is led from the base of the condenser l to a gas scrubber m. This consists of a cylindrical tower filled with pieces of coke down which water is made to flow. By this means the gases are freed from acetone and the dilute aqueous solution of acetone so produced flows back through a trapped pipe v to the boiling column.