Mother's Home-Scale Vacuum Distillery

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VACUUM ANYONE?

As we mentioned earlier, this still can be built as either a normal pressure or a vacuum-aided apparatus (if the latter feature is eliminated, however, approximately 6-1/2 times more surface area must be added to the reboiler heat exchanger). In its vacuum mode, as Goosen chose to build it, the distillery uses a watersealed and—cooled Kinney "liquid ring" air pump to draw off the device's internal atmosphere, and this air is removed from a spot close to the alcohol discharge point, through the Liebig condenser's center tube.

In the course of our experimentation, we've found a vacuum pressure of 18 to 22 inches of mercury to be about the ideal range in which to work. At the high end of this scale, the reboiler temperature needs to be only 125°F, that at the top of the stripper tower 115°F, and that in the uppermost section of the rectifier 95°F, in order for distillation to take place. A greater vacuum increase would, of course, lower the boiling point still further, but would also have the undesirable effect of simultaneously decreasing the volume of vapors that could be carried through the columns, which in effect would reduce the capacity of a givendiameter tube by half, or more, in comparison to an equivalent pipe in a normal still ... and thus lower the distillery's production capability.

Under a high vacuum load, any negatively aspirated still will have a tendency to "vapor lock", since there isn't enough atmosphere present to lift the heavy vapors and keep them flowing. To cure this problem, we've introduced an adjustable "artificial leak" into the system, which promotes a limited vapor flow and also provides a means of controlling the depth of the vacuum draw. Furthermore, negative-atmosphere stills tend to heat up evenly throughout, thereby discouraging proper fractionation . . . but this quirk has been checked by the introduction of cool mash into the top of the stripper, as well as by the use of a heat exchanger in the reflux section of the rectifier column.

As a result of our compensating for the idiosyncrasies inherent in the vacuum system, we're able to feed the still at a rate of 48 gallons per hour, and collect about six gallons of fuel-grade alcohol over the same length of time.

A BOON TO SMALL-SCALE FUEL PRODUCERS

On the surface, it might appear that the application of vacuum principles to the distillation of alcohol would enable a fuel producer to reduce his or her still's energy requirements by an extraordinary measure. Unfortunately, the only true "economy of BTU" present in a vacuum system occurs in the saving of sensible heat, or that which is required to raise the temperature of the mash from ambient to the brink of boiling. But the additional energy that's then needed to convert the heated liquid into vapor (this is known as latent heat) is still very much a factor in both normal atmosphere and vacuum setups ... so the increase in thrift of operation in a negative-pressure setup isn't as great as you might think.

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