Example

From 112.8 kilos of wash containing 10 per cent alcohol (by weight), temperature 20° C, to produce spirit containing 85.76 per cent of alcohol by weight, the wash being preheated to 70° C. in the condenser,

Table 110

Alcoholic strength (weight per cent) of liquid and vapour on the top plate of a wash column when the alcoholic strength and temperature of the wash varies.

In these circumstances the strength of the liquid and vapour on the top plate of the wash column will be 12.3 and 54.2 per cent by weight respectively (see Table 110).

From Table 109 it is seen that to produce 100 kilos water practically free from alcohol as outflow from the wash column when the strength of liquid on the top plate is 12.7 per cent the minimum heat consumption is 12,000 calories, and the number of plates required in the wash column is 15.

Table 106 shows that with a minimum abstraction of 5000 calories in the condenser per 10 kilos of finished product of the required strength (85.76 per cent by weight) the column will require 8 plates.

Let us now attempt to assess the heat requirements of the still in the given case by another method.

Let us assume as the basis of calculation that 100 kilos of spent wash practically free from alcohol leave the boiling column. This quantity of liquid (assuming that the amount of steam used for heating purposes, and condensed in the wash, is left out of account) would be produced from 112.8 kilos of the original wash, which would also yield 13.16 kilos of finished spirit (containing 85.76 per cent of alcohol by weight).

Let the specific heat of the wash be 1.01, the latent heat of vaporisation of water and alcohol being 544 and 205 respectively.

Then the total heat requirements of the still will be made up, as already stated, of the following items (see page 277): (1) Heat of vaporisation of finished product leaving the top of the still. Total weight of finished product 13.16 kilos containing 11.28 kilos of alcohol and 1.88 kilos of water.

This heat therefore amounts to (11.28 x205) +(1.88 x544) =3335.1 calories.

(2) Heat required to raise the wash from its temperature when preheated to its boiling point on the top plate of the wash column. From Table 102 the boiling point of the liquid on the top plate of the wash column is 90° C. The wash is preheated to 70° C. The amount of heat required here is: 112.8 x 1.01 (90 - 70) =2278.5 calories.

(3) The amount of heat taken up in the wash in the condenser: 112.8 x 1.01 (70 - 20) =5696.4 calories.

To these must be added the amount of heat required for the three small additional requirements referred to on p. 278. This amounts to 884 calories. For the calculation of this see Hausbrand, loc. cit. p. 92.

These four items together give a total of 12,194 calories as the total heat requirements in the still.

If these figures be compared with those given on Tables 106 and 109, it will be seen that the calculated total heat requirement of the still is nearly equal to that given in the table.

In comparing the heat abstracted in the condenser as given in Table 106 with that found by calculation, it must be remembered that the former is for an output of 10 kilos of spirit and that the amount obtained is 13.16 kilos.

By reference to Table 105 it is seen that for a production of 1 kilo of alcohol the greatest abstraction of heat in the condenser for the interval between spirit of 12.3 per cent and 85.76 per cent by weight is 442 calories. To produce 13.16 kilos of such spirit the amount required here will therefore be at least 5816 calories, which is only very slightly greater than the figure obtained in the calculation, i.e. 5696 calories.

The wash here has been alone sufficient practically to supply the necessary condensate to the rectifying column. Had it been required to produce a finished product containing 94 per cent of alcohol from the same wash a considerable amount of cold water in addition to the wash would have been required in the condenser.

It must be remembered that the figures in the tables referred to represent the minimum theoretical heat requirements. In practice it would therefore be necessary to use a certain amount of cooling water in addition to the wash (of. p. 273).