Continuous Rectification

The raw spirit to be rectified is supplied to the still with alcoholic content of 334 per cent to 38 per cent by weight. In this case it is not found necessary to neutralise the liquid before rectification, as the acids are largely removed in the preliminary purification, and therefore do not remain for long periods in contact with the concentrated alcohol.

The raw spirit is supplied at a point near the top of a column similar in principle to the wash column of a continuous still, steam being admitted near the base.

The head products tend to pass away from the upper part of the column mixed with some ethyl alcohol. To free them as far as possible from the latter, a few plates are placed above the feed plate.

The distillate leaving the top of the column is rich in head products for which K' >1. The raw alcohol flows down the column, being progressively freed from head products, and finally leaves the base of the column with a greatly reduced content of head products, but with its content of ethyl alcohol and higher alcohols almost unchanged.

This preliminary purification is in general only efficient for the removal of substances for which K' is considerably over 1 at the greatest alcoholic strength which exists in the purifying column.

The less volatile by-products are best removed in the final rectifying column.

The partially purified alcohol is then supplied to a point near the base of a rectifying column, in which it is caused to boil by steam introduced below the supply plate. The distribution of the various constituents of the mixtures takes place as in the column of a discontinuous rectifying still.

But in this case the composition of vapour and liquid at any point in the column remains constant throughout the rectification. This is very important, as the various constituents can be run off continuously from the points in the column at which they accumulate. In the discontinuous still this is not generally possible, or is very much more difficult to control, as these points are continually changing as the rectification proceeds.

The remaining head products (mixed with a certain proportion of ethyl alcohol) pass continuously uncondensed through the condenser at the top of the column. The ethyl alcohol is run off from a plate somewhat lower down.

Impurities such as iso-amyl acetate, iso-butyl alcohol, and propyl alcohol are continuously run off from the point in the column where the alcoholic strength is about 68 to 73.5 per cent by weight. Amyl alcohol is run off from a point at which the alcoholic strength is 334 per cent by weight.

The withdrawal of these two classes of by-products is not begun until they have had time to accumulate to a considerable extent. This is generally not the case until the still has been running for about two days. Bub it is not advisable to allow them to accumulate beyond a certain point for fear of contaminating the finished spirit. For this reason the mixtures withdrawn always contain much ethyl alcohol and have to be fractionated separately, or, if the impurity is only slightly soluble in water, they may be purified by washing with water and decantation.

A comparison of the two methods of rectification just described shows that the fixed conditions of the continuous process combined with the preliminary purification of the dilute spirit renders the removal of the volatile by-products much easier and more efficient - the head products being largely removed under the most favourable conditions, and the higher alcohols and other less volatile impurities being continuously prevented from mounting to the point in the column from which the finished spirit is withdrawn.