Distillation principles

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AmErICAn DIstIllInG InstItutE | 1

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Distillation Principles

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Most artisan stills are of the reflux or pot column design because of the inherent flexibility that they offer.

HOw DISTILLATION wORkS
Distillation is a physical process where compounds are separated by virtue of their different boiling points and vaporpressures. The separation in distillation occurs when a mixture of compounds in the still is brought to boil. As a simplification, assume that the still contains only ethanol and water. Ignoring the azeotrope discussed below, for every mix ratio of ethanol to water there is one and only one new boiling point that lies between the boiling point of either. Conversely, for each boiling point, there isone and only one ratio of ethanol in the kettle, and an enriched ratio in the vapor and the distillate. If you know the temperature in the kettle, you can look up the exact ethanol ratio both in the kettle and in the vapor/distillate, in a simple table [figure 1/page 5]. Assume a mixture of 90% water and 10% ethanol (by volume) is to be separated by distillation. Water has a boiling point of 212ºFand ethanol has a boiling point of 173ºF, but this 10% ethanol mixture will boil at 197ºF; it will not boil at 173ºF. The vapor above the liquid will be 61% ethanol, as will the distillate. In a simple kettle, the ethanol percentage will drop during the boil because more ethanol than water is being removed, and neither are being replenished. This alone accounts for the increase in boiling pointfrom start to finish— the ratio changes, so the BP changes. Note that we started with 10% ethanol in the kettle, and now have a distillate at 61% ethanol, a 6-fold increase in strength. Referring to the table and graph below, if we now distill this condensate again, the new distillate will be 86%, and if we distill that, we will have a 91%, and again 92%, and again 93% and after six distillations,we may get 94%. As the concentration of the impurity (water) decreases, it becomes more difficult to remove. This

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notion is very important for other products of fermentation in our wash. No matter what the concentration and boiling point of a given impurity, some of it will escape the kettle and find its way to the distillate throughout the distilling run. Thismeans that head cuts can never be precise because these lighter impurities do not all vaporize before the hearts begin. Likewise, some tail impurities manage to vaporize well before they are expected. Compounds with boiling points between water and ethanol, such as diacetyl at 190ºF may be impossible to remove by distillation. Therefore, distilling a bad wash never makes a good whiskey— and a goodwhiskey always starts with a good wash. High-separation vodka stills employ a reflux column with many plates where vapors can condense, then like small kettles, re-vaporize the new enriched liquid, further enriching the vapor. Multiple cycles of condensation and re-boiling, one cycle per plate, occur in a single pass as vapors rise through the column before distillate is drawn from the still nearthe top. even these stills can not enrich beyond 96.5% ABV because ethanol and water form an azeotrope where some mix ratios have a boiling point not between the boiling points of the constituents. This prevents complete distillation. Nonetheless, reflux columns attached to pot still can sharpen separation, making head and tail cuts easier, but most believe that this leads to a lesser whiskeybecause the cleaner separation strips away character. Artisan distillers want to preserve the character of their whiskey, so if a column is employed, the plates are opened to reduce reflux and more closely match the results from the neck and arm of a traditional whiskey still. If your kettle temperature is 198.5ºF then your kettle contains 9% ethanol and the vapor contains 59% ethanol. As the run...
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