Composition And Amount Of The Condensate Returned To The Still

If a very large proportion of the vapour evolved in the still be condensed, the composition of the condensate will approximate to that of the vapour evolved. If a very small proportion of the vapour be condensed the composition of the condensate will approximate to that of the original liquid in the still. The residual uncondensed vapour will contain a higher percentage of alcohol in the first than in the second case. If a large proportion of the vapour evolved in the still be condensed, the alcoholic strength of the vapour as it rises from plate to plate in the column will increase rapidly, and a comparatively small number of plates will be required to produce alcohol of high concentration. But the large amount of condensate returned to the still entails a correspondingly high heat consumption in the still to produce a definite weight of finished product.

If, on the other hand, only a small proportion of the vapour be condensed the alcoholic strength of the residual uncondensed vapour will only rise slowly as it passes from plate to plate up the column, and a large number of plates will be required to produce alcohol of the same strength as before. But the amount of condensate returned to the still being smaller than in the first case the heat consumption in the still will be less for the same output of finished product.

In practice the plates should be as numerous as is reasonably possible so as to reduce the fuel expenses.

Condenser or Dephlegmator (see Fig. 116). - The function of the condenser is (1) to produce the amount of condensate required for the efficient working of the plates of the column. (2) It may also to some extent increase the alcoholic strength of the vapour passing on uncon-densed to the cooler. This effect is only produced to any marked extent when the alcoholic strength of the vapour entering the condenser is low. To attain this object as far as possible the vapour should pass as slowly as possible through the condenser. The temperature difference between cooling liquid and vapour should be as small as possible. The vapour should rise and the cooling liquid fall in the condenser. A large proportion of the vapour must be condensed. The condensate should be kept in as small drops as possible, so as to ensure as perfect contact as possible between vapour and condensed liquid.

1 Of. L. Gay; Chim. et Ind., 1920, 4, 178-188.

Some of these requirements are fulfilled in the best German dephleg-mators, which can really be better described as rectifiers and condensers combined. They enable the column to be built with fewer plates, but their construction is complicated, and it is questionable whether a rather higher rectifying column with ordinary tubular condenser is not preferable on account of simplicity of construction.

The condenser also serves to preheat the wash to be distilled, which replaces the cooling water in many continuous stills.

Use Of Wash To Replace Water As Cooling Liquid In The Condenser

If the alcoholic strength of the wash is not above 9 per cent the wash alone will suffice to provide the necessary condensate for the rectifying column when the alcoholic content of the finished product does not exceed 90 to 92 per cent by volume (85.7 to 88.3 per cent by weight). But if it is required to produce spirit containing 94.6 per cent alcohol by weight, the wash will only be sufficient to provide the necessary condensate provided its alcoholic content is not above 3 per cent.

Position Of The Condenser

If the condenser be placed above the top of the rectifying column, the volume of the condensate on the upper plates is large and gradually decreases as it flows downwards. But if several condensers be arranged at intervals down the column the upper plates receive little condensate, though the total flow returned to the still be the same in both cases.

This latter arrangement is not so efficient as the former and necessitates a larger number of plates in the column than would be required to produce the same result with the condenser above the top of the rectifying column.

Should the Rectifying Column be lagged ? - If the column be made of metal a considerable loss of heat may take place by radiation and air currents. When the wash is preheated in the condenser this means a complete loss of a considerable amount of heat. In any case the loss of heat which will take place from all points on the outside of the column will lead to a certain amount of condensation of the vapour. As previously stated, it is preferable to supply the whole volume of condensate to the top plate of the column. Condensation produced by cooling the surface of the column produces a similar effect to several condensers arranged at intervals down the column, and should therefore be avoided.

The loss of heat from a square meter of surface per hour for each degree (centigrade) difference of temperature between the surface and surrounding air is approximately: Copper . . . . . . 5 to 6.5 calories.

Wrought iron . . . . . 8 ,, 9 „

Cast iron . . . . . . 9 ,, 10 „

The following is an example of what a high percentage of the heat supplied to a still may be lost in this way :-

Total heat required to produce 100 litres of spirit amounts to from 9000 to 12,000 calories.

The loss is much lower in the larger still, but even here it amounts to at least 5 per cent of the total heat supplied, when the column is not lagged. With the columns lagged the loss is reduced to 1 per cent of the total heat supplied.