Plates With Bells

In another form of boiling column the liquid flows from plate to plate by means of reflux tubes similar to those in Coffey's still. A slightly greater depth of liquid is maintained on each plate, but not in this case by the pressure of the ascending vapours. The perforations in the plates are replaced by a comparatively small number of larger holes, each of which is covered with the arrangement shown diagrammatically in Fig. 107. The vapour passes through the central pipe a (Fig. 107) (the upper rim of which is above the surface of liquid on the plate) and is forced to bubble through the liquid on the plate by the bell b. The depth of liquid is determined by the height to which the top of the drop or overflow pipe c rises above the plate. The depth of liquid through which the vapour bubbles is determined by the difference of level between the rim of the bell b and the top of the overflow pipe c. Fig. 108 shows the arrangement of bells on the plate. Fig. 109 represents a form of bell designed by

Fig. 107.

Fig. 108.

Fig. 109.

Barbet. The slits on the side tend to break up the vapour as it enters the liquid. Fig. 110 shows a bell designed for use with thick wash; the conical form is intended to prevent the deposit of solid matter from the wash. The efficiency of this form of plate depends on the proper arrangement of these holes and bells over the surface of the plates. They must be arranged so as to cause the wash to flow evenly all over the plate.

When properly designed this type of plate does not get blocked even by thick wash; and has the advantage over the perforated plates that the bells can be replaced one by one without dismantling the column when the slit becomes worn out. The liquid remains on the plate if by chance the pressure falls too low.

Fig. no.

Sloping Columns

Fig. 1ll shows the Guillaume Inclined Column.

Fig. 111.

This apparatus is constructed of an upper and lower part which can be easily separated, for cleaning purposes. The lower half is fitted with upstanding baffle plates d, which alternately stop short of either side.

The wash which enters at the top by pipe 1 follows a zigzag course down the column as shown by the arrow. The rate of flow is regulated by a valve at a, and the spent wash leaves the column by b. ,The upper half is provided with depending baffle plates (3) extending across the whole width of the column.

Steam enters the base of the column by pipe 2, On its way up the column the steam must pass under each of the depending baffle plates (3) attached to the upper half of the column. In so doing it bubbles through the down-flowing wash, carrying with it all the volatile products of the fermentation.

This column is said to work very well with thick wash, and a more even flow of the liquid is obtained than in the ordinary plate column. This is important, as it means more complete removal of the volatile products. Blocking of any part of the passage is not likely to occur ; it has also the advantage of being more compact and less lofty than some of the other forms of boiling columns. The top of this column being . so low enables the condensate from the base of the rectifying column to flow back into it by gravity. With a high boiling column, this can only be attained by having an unduly high building.

2. The Full Column. - The "full column " is completely filled with wash. The wash which enters at the top and flows away at the base is made to circulate by a large number of baffle plates arranged in the column as shown in Fig. 127, d. Steam enters at the base. It is not so efficient as the plate type of column, as it is impossible to effect so thorough a mixture of the vapour and liquid, but it is not so liable to blockage when thick wash is being used.

Preheaters

Before entering the boiling column the fermented wash is subjected to a preliminary heating in some form of heat exchanger. The wash may either be used as a substitute for cold water in the condenser, dephlegmator, or rectifying column, as in Coffey's still, or a special vessel may be constructed in which a heat exchange takes place between the hot spent wash and the cold wash to be distilled. The former method has the advantage of compactness and economy of design. No extra apparatus is required, but it may not be practicable when very thick wash is used. It should be noted here that the arrangement adopted in Coffey's still for preheating the wash is not the ideal one from the point of view of economy of rectification.

Theoretically the best arrangement is to have one dephlegmator at the top of the rectifying column so that every plate of the rectifier has the full benefit of the maximum volume of the condensate. If part of the condensate is produced at points lower down the column the efficiency of the rectifier is diminished (see p. 273). However, the most vigorous condensation must take place at the top where the wash is cold. When the wash has become fairly hot the amount of condensation of head products and ethyl alcohol must be greatly diminished.

For thick wash the preheater must be specially designed to prevent x the deposit of solid matter from the wash and to facilitate cleaning and the removal of obstructions should any be formed. Whatever system of distillation be adopted, a large amount of heat is always wasted unless it be utilised for some purpose external to the distillation.

Rectification

In the earliest types of still the concentration of the alcohol was effected by repeated fractionation. In a discontinuous apparatus this was a slow and wasteful process. The distillation and rectification are now in the majority of cases carried out by continuous processes. But the final rectification is frequently carried out in a discontinuous still. In some systems the continuous distillation is independent of the continuous rectification ; in others the two processes are combined and carried out simultaneously. In the former the vapour produced by distillation is condensed and the liquid so produced is fed to an independent rectifying plant. In the latter the alcoholic vapour produced by distillation is led directly into a rectifying apparatus. The latter is evidently more economical of heat. The process of rectification is essentially the same in both cases. The number of plates required in the rectifying column to obtain spirit of a certain strength depends on (1) the alcoholic content of the wash, (2) the strength of the alcoholic vapour supplied by the boiling column, (3) the amount of liquid refluxed by the dephlegmator. Ceteris paribus, the larger the number of plates the less the amount of dephlegmation required and the smaller the consumption of steam by the still.