Distillation How it Works

The mechanism by which alcohol can be purified by distillation is a subject shrouded in mystery for most amateurs. There are those who know that the process involves the boiling of a dilute, impure alcohol and separating the various constituents by means of the difference in their boiling points, but just how or why this separation takes place is known only vaguely.

Take a mixture of methanol, ethanol and water for example. The first has a boiling point of 64.7 oC while the second boils at 78.4 oC. So it is thought that by heating the mixture to 64.7 oC. and holding it there the methanol will boil off. Raise the temperature to about 78 oC. and the ethanol will boil off, leaving the water behind.. This is completely wrong and has led to many disappointments.

In this book we have attempted to shed a little light on the subject, but it is apparent from readers' comments that there is an unsatisfied thirst for additional information. In this discussion, therefore, we shall go into the mechanism of distillation in somewhat more depth. Let's start by talking about vapour pressure.

Vapour pressure.

All liquids (and solids too for that matter) have a vapour pressure. That's why we can smell them — molecules escape from the surface and penetrate our nostrils. Every substance is a collection of molecules held together by mutual attraction and vibrating about their mean position. The higher the temperature the faster they vibrate.

At the surface of a liquid vibration enables some of the molecules to escape the clutches of their neighbours in the body of the liquid and enter the vapour phase, and the higher the temperature and the more the molecules vibrate the greater the number which are able to escape. The vapour pressure of a substance is the contribution these freed molecules make to the pressure of the surrounding atmosphere. This may be illustrated by a simple experiment. Take a glass tube about a metre long, closed at one end, and fill it with mercury. Upend it in a beaker of mercury and the mercury in the tube will fall and leave a vacuum above it. The column of mercury is being held up by the pressure of the surrounding atmosphere and the height of the column is a measure of the atmospheric pressure.

Now introduce a few drops of water into the bottom of the tube. The water floats to the top of the mercury and will be seen to boil rapidly. Continue adding water until there is some liquid water floating on the mercury and you will notice that the mercury column has been lowered by about an inch. This is the vapour pressure of water at that temperature. If the temperature is raised the V.P. will increase also, and when ca. 100 oC. is reached the mercury level in the column will be the same as in the beaker and the column will be full of water vapour. Repeat the experiment with methanol instead of water and you will find that the tube will finally be empty of mercury at 64.7 oC, the boiling point of methanol. The vapour pressure of a liquid at its boiling point equals atmospheric pressure.

Latent heat of vaporization.

It takes a certain amount of energy to raise the temperature of a liquid from, say, room temperature to its boiling point, but it takes very much more energy to convert the boiling liquid into vapour, even though the temperature stays the same. This energy is called the latent heat of vaporization and is large because it has to work against the mutual attraction of the molecules in the liquid and provide them with enough kinetic energy to remain apart. So you cannot raise the temperature of boiling water by pouring more energy into it — it will stay at 100 oC.

Mixtures.

Take pure methanol, B.P. 64.7 oC. and start adding water. The boiling point of the mixture will rise the more water you add, indicating that the molecules — both water and methanol —are finding it more and more difficult to escape from the mixture to form vapour. The water molecules exert a higher attraction than the methanol molecules and this attraction extends to all the molecules in the mixture. This is the crux of the distillation process — that a mixture boils at some temperature depending on the relative concentrations of its constituents and produces a vapour which is a mixture of the two. The constituent with the higher vapour pressure will contribute more molecules to the vapour than will the constituent with the lower vapour pressure. In the case of a methanol/water mix, whatever mixture you started out with you end up with a vapour which is richer in methanol than water. Condense this vapour and then re-boil it and the result will be a vapour with yet more methanol than before. This process is illustrated in the diagrams below.

Take a mixture of methanol and water with X% methanol by volume. The top left dot charts the boiling point of the liquid mixture as being Tx C.

It must be emphasised again that the boiling point of a mixture is not the boiling point of either of the constituents, but lies somewhere in between (if in any doubt about this, please read again the paragraphs above.)

The vapour from this mixture contains more methanol than the mix it came from, let's say Y% methanol (shown by the second dot at Tx oC) and this new mixture condenses at Ty oC. Note that this new mixture with more methanol condenses at a lower temperature than it took to boil the mixture it came from.

We now take this condensed liquid and heat it again until it boils. Once again, the vapour contains more methanol than it did before in mixture that's boiling, and that this vapour will therefore condense at an even lower temperature.

Subsequent vaporizations and condens-ations are plotted in this chart. As the concentration of the condensed liquid approaches 100% methanol the boiling point, as might be expected, decreases at each stage and eventually approaches the boiling point of pure methanol 64.7 oC.

Joining up these dots gives us two curves. The upper one may be called the vapour line. Anything above it is vapour above the boiling point for that mixture, and anything below the lower liquid line is liquid below the boiling point for that mixture.

It is sometimes said that any point lying in between the lines represents a transition phase between liquid and vapour, but a little reflection will show the fallacy of this view. We chose to start at a certain concentration of methanol, but another concentration would have resulted in a similar set of points offset either to the left or right of those shown.

An area where vapour condenses, hangs around and then vaporizes again is termed a 'plate'. It may be likened to an actual plate or tray fitted in a column.

The diagrams above relate to a methanol-water mixture, and are quite simple. In the case of an ethanol-water mixture we would find that there is a kink in the bottom of the curves. This results from the fact that ethanol and water form an azeotropic mixture when the concentration of ethanol is around 95%. Subsequent vaporization of liquid at this concentration will not yield vapour with a higher concentration of ethanol but one of the same concentration as the liquid. If we started with a mixture that had more than 95% ethanol, then the concentration of the vapour would be less and, once again, the system would tend to settle at the azeotropic point. Distillation alone cannot give a concentration of ethanol higher than 95%

So that's what happens when we heat a mixture of two volatile liquids. The constituent with the highest vapour pressure will appear in greater and greater quantity in the vapour as we boil the mixture, then condense it, then repeat the boil-condense cycle over and over again.

So the question is, how do you get enough cycles of boiling-condensing-boiling into a device which is suitable for use by amateurs. The answer lies in using a column packed with surfaces where the vapours rising from the boiler meet the liquid falling from the condenser in the still-head. At each surface the hot vapour gives up its latent heat to the descending liquid and re-vaporizes it. So one gets a whole series, probably many hundreds, of mini-distillations down (or up) the length of the column. As noted in the book, it is possible to provide the very large surface required by packing the column with stainless steel filaments.

Reflux and Balance.

A column packed as described will enable the boiling-condensing cycle to be repeated many times and the constituents of the original mix will start to separate out, the most volatile at the top. Condensed liquid that runs back down the column is termed the reflux. It is richer in the most volatile constituents than the vapour rising to meet it, and you will recall that its boiling point is lower than the vapour further down in the column. It therefore boils as it passes down the packing and the resulting vapour is even richer in the volatile constituents.

This process may be 'hurried along' by condensing out all the vapour that reaches the top of the column and returning it as reflux. By this means, the most volatile constituent of a

mix is concentrated in the top section of the column, the less volatile constituents being confined to the lower section. A high degree of purity is achieved in this manner.

The process of separation takes time as many cycles of boiling-condensation have to occur before the lightest constituent is fully isolated in the top section of the column. When no further variation in concentration of the various constituents occurs along the length of the column, the column is said to be in balance. As the boiling point varies according to the relative mix of the constituents, it follows that the temperature of the column will be high at the bottom and will decrease the higher you go. When the column is balanced then the temperatures along the length of the column are stable and exhibit no variation with time. The top section of the column will be at the boiling point of the most volatile constituent.

With the column balanced, a start may be made on withdrawing the lightest constituent condensed at the top. However, only a small amount of the total condensed may be withdrawn if balance is to be maintained. The quantity withdrawn compared to that which is supplied is termed 'Reflux Ratio'. As noted in the book, experience has shown that a reflux ratio of 1:10 in a column about 1 metre long and between 25 and 35 mm diameter gives consistently good results.

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Responses

  • fastolph
    How is fractional distillation powered and how it works?
    3 months ago

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