As the liquid cannot pass through the holes in the partitions it can only pass downward through the drop-pipe tubes h. By this means the mash is spread in a thin stratum over each partition to the depth of the seal g and is fully exposed to the steam forcing its way up through the holes, the alcohol it contains being thus volatilized at every step.
In its course downwards the wash is met by the steam passing up through the perforations, and the whole of the spirit which it contains is thus converted into vapor. As soon as the chamber B is nearly full of the spent wash, its contents are run off into the lower compartment by opening a valve in the pipe V. By means of the cock E, they are finally discharged from the apparatus. This process is continued until all the wash has been pumped through.
The course taken by the steam will be readily understood by a glance at the figure. When it has passed through each of the chambers of the analyzer, the mixed vapors of water and spirit pass through the pipe i into the rectifying column. Ascending again, they heat the coiled pipe m, and are partially deprived of aqueous vapors by condensation. Being thus gradually concentrated, by the time they reach the opening at w they consist of nearly pure spirit, and are then condensed by the cool liquid in the pipe, fall upon the partition and are carried away by the pipe y to a refrigerator W. Any uncondensed gases pass out by the pipe R to the same refrigerator, where they are deprived of any alcohol they may contain. The weak liquor condensed in the different compartments of the rectifier descends in the same manner as the wash descends in the other column; as it always contains a little spirit, it is conveyed by means of the pipe S to the vessel L in order to be pumped once more through the apparatus.
The condensed spirit gathered over the plates v passes out through the pipe y to the condensing worm T. If any vapors escape the condensing plates they pass into R and are condensed in the worm T also. From worm T the spirit flows into a suitable receiver Z.
Before the process of distillation commences, it is usual, especially when the common Scotch stills are employed, to add about one lb. of soap to the contents of the still for every 100 gallons of wash. This is done in order to prevent the liquid from boiling over, which object is effected in the following way: The fermented wash always contains small quantities of acetic acid; this acts upon the soap, liberating an oily compound which floats upon the surface. The bubbles of gas as they rise from the body of the liquid are broken by this layer of oil, and hence the violence of the ebullition is considerably checked. Butter is sometimes employed for the same purpose.
Fig. 19.—Rotary Current Still.
Figs. [19] and [20] show a diagrammatic section and a plan of a still used for thick mashes which are liable to burn. This comprises a circular chamber B supported over suitable heating means, having on its bottom a series of concentric partitions b which divide the bottom of the chamber into shallow channels for the mash. Running diametrically through the chamber is a partition.