Table 108. Wash Column

Theoretical minimum amount of heat (in great calories) to be supplied to wash column to produce 100 kilos of outflow water free from alcohol, from wash the alcoholic content of which varies from 15 to 0.5 per cent by weight.

Table 109 shows the results obtained on each plate of a wash column when the alcoholic strength of the wash and heat consumption vary, but outflow of water from the base of the column remains constant.

For instance, if the original wash contains 845 per cent of alcohol, seven plates will be required if the heat consumption is 20,000 calories, but about seventeen plates are necessary if the heat consumption is only 10,000 calories.

Table 109

Wash Column.

Alcohol content (weight per cent) of liquid and vapour on each plate of a wash column when 10,000, 12,000, 20,000, 50,000, and 125,000 great calories of heat are supplied at the base of the column, per 100 kilos of water leaving the column.

10,000 calories. 12,000 calories 20,000 calories. 50,000 calories. 125,000 calories

The amount of heat given in Table 105 represents not only that required to free the wash from alcohol but also to effect the concentration of the spirit in the rectifying column.

This total is made up in the following way: (1) Latent heat of vaporisation carried off to the cooler by the finished product.

(2) If the wash is preheated in the condenser it will be supplied to the top plate of the wash column at a temperature above that at which it entered the condenser, but below its boiling point. It cannot be heated to its boiling point in the condenser, for the temperature of the strong alcoholic vapour in the condenser is only about 78° C. to 85° C, and it is impossible to heat the wash to a temperature of more than 8 to 10 degrees below that of the vapour in the condenser. Heat must therefore be supplied in the wash column to raise the wash to its boiling point on the top plate of the wash column.

(3) To produce the necessary condensate in the rectifying column heat is abstracted in the condenser. This amount of heat must have originally been supplied in the wash column, and is represented by the amount of heat taken up by the wash as it is preheated in the condenser.

These three items together account for the greater part of the heat requirements of the still, and together represent the total given in Table 105.

But there are three other small amounts of heat which must be added to make up the total heat requirements of the still.

(a) The heat required to raise the temperature of the condensate from that at which it leaves the condenser to that existing on the top plate of the wash column.

(b) The heat required to raise the temperature of the wash from its boiling point on the top plate of the wash column to its temperature as it leaves the base of the wash column (about 102° C). In calculating the amount of steam to supply the total requirements of the still, the temperature of the spent wash as it leaves the wash column must be taken into account. This temperature is about 102° C. From this it follows that 1 kilo of saturated steam will yield to the liquid in the column 637 - 102 = 535 calories.

(c) Replacement of heat unavoidably lost by radiation, etc., from the still.

To determine the number of plates in the rectifying and wash columns and capacity of the condenser required to produce spirit of a . certain concentration, the following procedure may be adopted. It is first necessary to determine the alcoholic strength of the liquid and vapour on the top plate of the wash column. This will depend upon the alcoholic strength of the wash and on the temperature to which the wash has been preheated in the condenser.

Table 110 gives the necessary information for wash containing 1, 5, 7, and 10 per cent of alcohol by weight. It will be noted that the higher the temperature of the wash the lower the alcoholic strength of the liquid and vapour on the top plate of the wash column.

Having found the alcoholic strength on the top plate of the wash column, the number of plates in the rectifying column and capacity of the condenser required to raise the alcoholic strength to the required extent can be found from Table 106 or 107.

The number of plates required in the wash column is found in Table 107, which also gives the heat requirements of the still as already explained.

A concrete example will make clear how the information respecting wash and rectifying columns given in the tables can be combined so as to show the most economical working arrangement theoretically possible.