This is the most important step in the whole process of producing pure alcohol from sugar. And an essential step. Any description of alcoholic beverage production which does not include it is describing the production of an impure product, a type of whiskey or moonshine. It may be palatable if carefully prepared but it certainly will not be pure alcohol.
Because of its importance it will be described in some detail, a detail which unfortunately may be intimidating to some and boring to others. To those in the first category we say this: Once you have assembled the equipment and made a few runs it will all become incredibly routine. It's like riding a bicycle a lengthy description of how to do it would probably decide you to take up walking instead, but once you've set off down the road there's no looking back. It's easy!
In fractional distillation the vapours emerging from the boiling mixture are passed 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 interface on the packing material there is an exchange between liquid and vapour, the vapour giving up its latent heat to the descending liquid. Thus, the liquid is turned into vapour while the vapour is condensed back to liquid. The newly-formed vapour rises and the same exchange takes place on the next surface it meets up the column. Similarly, the descending liquid meets rising vapour at an interface in the packing further down the column.
The more volatile components of the mixture which have entered the vapour phase rise to the top of the column while the less volatile components which have gone into the liquid phase flow 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. It is not too dissimilar to what is happening in a heat exchanger where we also have counter-current flow — the cold water is being warmed by the hot vapour while the hot vapour is being progressively cooled to lower temperatures by the flow of water in the opposite direction.
In a commercial operation, which runs continuously, the different components of the mixture are concentrated at various heights within the column, and can be drawn off, and this continues indefinitely. Methanol, for example, would be continuously withdrawn from the top of the column while ethanol would be continuously removed from a point a little further down. The chemicals in each draw are not completely pure, but are much purer than they were before.
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 condensed vapour from the top of the column. First come the acetone and then the methanol and any other low boiling point compounds. They are discarded. Then the ethanol starts to appear, and when it does a por tion 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.
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