Originally By Tony Ackland
Ethanol/Water Distillation Equalibrium Demonstration
This graph is of the "ethanol-water equilibrium" eg a liquid of 15% alcohol will be in equilibrium with a vapour at 65% alcohol. If this 65% vapour is then cooled to form a liquid (it will remain at 65%), the new liquid would then be at equilibrium with a 84% vapour, and so on. If you have a pot still, just set the plates to one.
You can see that due to the shape of the curve, most of the gains are early on; to get to the really high % purity, you need to take lots of steps later on. There is no way around this. If you want high purity, you have to work hard for it. Also note (particularly for inefficient columns with the equivalent of only 1-2 plates) that the starting % can also affect the final % achieved - hence a good idea to use the better yeasts.
Each of these "steps" represents an "ideal plate" where enough mingling of liquid & vapour allows them to come to equilibrium. If you don't allow enough mingling (equilibrium), then you won't achieve a full step, but end up a little shy of the target. You get the first step free - its the boiler/pot.
1 = 53%
2 = 80%
3 = 87%
4 = 90%
5 = 92%
6 = 92.6%
7 = 93.3%
8 = 93.8%
9 = 94.2%
10 = 94.4%
One way of doing these steps is to do many single distillations, collect the vapour that comes off, condense it, clean out the still, and run it through the still again. This why pot stillers do double & triple distillations to get into the 80+ % range. But a Reflux column allows this to happen continuously; if given enough surface area to equilibrate on, the vapour can have gone through multiple distillations by the time it gets to the top of the column.
For each plate to work, it has to be at a particular temperature,
slightly cooler than the one below, and warmer than the one above. Only
then will it achieve its equilibrium and an increase in the alcohol purity.
The differences are really fine too its all happening only between 78.1 C
and 82.2 C quite a tight band to walk between.
1) when a vapor condenses then the resulting liquid has the same composition as the vapor, and the temperature at which this occurs is the same as the boiling point of that mixture. The boiling point lowers as the proportion of volatiles increases, so the temperature as you go up a column naturally decreases. One sticking point is that many think that a vapor only condenses when it encounters a surface that is cooler than the boiling point, but this is not so. Condensation occurs when there is a path for the latent heat of vaporization/condensation to be removed from the vapor, and the resulting liquid will remain at its boiling point if no further heat is removed.
2) when this liquid re-evaporates then the resulting vapor is richer in the most volatile components.
The packing is there simply to hold intermediate distillate in place so it can be bathed in hot, rising vapor and allow this second process to occur. As volatiles are further extracted from the intermediate distillate, the boiling point of what remains increases and the depleted liquid builds up, eventually dripping down the packing to a hotter level where it can again be stripped of more volatiles.
A cooling tube placed near the bottom of a column simply interrupts this natural progression and serves no useful purpose in the separation process. In contrast, the top cooling tube IS useful as it helps to return some of the vapor arriving at the top of the column to the packing, where it has a further chance of being stripped more thoroughly. This is what a condenser placed on top of a compound column does, but with more efficiency.