Much of what needs to be said about the principles of distillation was covered in the chapter on beverages. There, the distinction was made between the comparatively simple pot stills used in the manufacture of whisky and the more elaborate still with fractionating column used to remove all the impurities and leave a pure alcohol, as in the manufacture of gin and vodka. The present chapter will explain just what is involved in carrying out a fractional distillation and how you go about it, but first a few words about principles. These will let you know just why a certain procedure is being followed. There is nothing more irritating in an instruction manual than to be told arbitrarily to do something without an explanation as to why it is necessary.
Firstly, as mentioned earlier, distillation does not make anything. It is a purification process in the same sense that a water-softener removes hardness from water. In the case of alcohol, distillation does not and cannot produce a drop of alcohol, and there will be the same amount of it at the end of the purification (or in practice a little less) as there was at the beginning.
Distillation separates the various chemical compounds produced during fermentation, using the difference in boiling points to effect the separation. The boiling points at standard atmospheric pressure of some of the more important chemicals found in the beer produced by fermenting sugar with yeast are shown in the table below. The same chemicals are found to a greater or lesser extent in commercial wines, beers and whiskies, but in these beverages they are, of course, quite happily consumed. Sometimes with unfortunate consequences the next morning!
* Footnote. In Appendix III will be found a detailed description of the mechanisms involved in distillation, a subject which should be of interest to all those who wish to know exactly why something happens in addition to knowing how.
Boiling Point, oC.
Ethyl acetate Ethyl alcohol Propyl alcohol Water
When a mixture of these compounds is boiled the most volatile, i.e. the ones with the lowest boiling points such as acetone and methanol in the above table, will vaporize first. When they distil over they are referred to as the "heads". There is no clear-cut separation of the various compounds so the heads are still coming over when the ethanol starts to appear. Similarly, before all the ethanol has distilled over, the "tails" will begin to appear in the distillate. These tails are the compounds at the lower end of the above table, i.e. those with the highest boiling points such as propyl, butyl and amyl alcohols. These alcohols are known collectively as "fusel oils" and, like methanol and some of the other compounds, are quite poisonous.
In such a system there is probably a small fraction in the middle which is pure ethyl alcohol but most of it will be contaminated with either heads or tails. One could discard the first heads and the last tails and redistil the middle fraction, repeating this process over and over again until the last of the impurities had been wrung out of the ethanol. Unfortunately, apart from being very time consuming, the loss of ethanol on repeated redistillation would be such that the final yield of pure alcohol would be virtually zero.
Fortunately, it is possible to overcome this problem by a very elegant procedure called fractional distillation, a process which has already been described to some extent in the equipment section. It will be useful to refer back to the diagrams in that section as you read on.
In fractional distillation the vapours emerging from the boiling mixture pass up a column packed with small pieces of glass, ceramic, stainless steel, or other inert material. Each of these pieces can hold a small amount of liquid, either internally (if they have internal crevices) or in the interstices between adjacent particles. At the top of the column the emerging vapour is condensed into a liquid by means of cold water running through a heat exchanger. The condensed liquid runs back down the column until it reaches the boiler where it is reheated, converted into vapour once more, and once again moves up the column.
At equilibrium, which may take several hours to achieve, the system consists of vapour rising up the column meeting a flow of liquid running down the column. At each vapour-liquid interface on the packing material within the column a partial separation occurs wherein the more volatile components of the mixture go into the vapour phase and rise to the top while the less volatile components go into the liquid phase and are carried down into the boiler. At equilibrium, the many components in the mixture become stacked up in the column in the order of their boiling points, the most volatile at the top and the least volatile at the bottom.
In a commercial operation, which runs continuously, the different components of the mixture are drawn off at various heights within the column, and this continues indefinitely. Methanol, for example, would be continuously drawn off from the top of the column while ethanol would be continuously removed from a point a little further down.
Very small operations such as we are concerned with here do not employ a continuous system. Rather, fractional distillation is carried out batch-wise. After column equilibrium is established, with acetone and methanol at the top and fusel oils at the bottom, we start to progressively draw off liquid from the top of the column. First come the acetone and then the methanol and any other low boiling point compounds. Then the ethanol starts to appear, and when it does a portion of it is drawn off and bottled for use. The remainder is allowed to run back down the column to continue the counter-current flow and the purification process. Eventually the ethanol will be exhausted and the higher alcohols, the so-called fusel oils, will start to emerge. At this point (or in practice somewhat before) the boiler is switched off.
Water is an important constituent of the fermentation broth and with a boiling point of 100 oC. lies intermediate between the least and the most volatile components of the mixture. It has one important difference from the other components, however, in that it forms an azeotrope with ethanol. An azeotrope is a mixture of two liquids with a boiling point lower than either constituent. In the case of ethanol and water the azeotrope occurs at a mixture of about 95% ethanol and 5% water. As far as the system is concerned it "thinks" that this mixture is a single liquid with the lower boiling point and proceeds to separate it on that basis. The ethanol which is purified by a fractionating column is not, therefore, pure 100% ethanol but pure 95%, the "impurity" being pure water. No amount of re-distillation under the conditions we are using will influence this percentage.
If it is absolutely essential to remove all the water, for example if it is to be mixed with gasoline to produce gasohol, then it is necessary to break the azeotrope by adding a third component such as benzene. For our purposes, however, where we are going to dilute the alcohol with water to 40% anyway, the presence of 5% water is of no consequence.
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