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Controlling stills by temperature

PostPosted: Mon Apr 22, 2013 10:32 am
by blond.chap
There have been a few questions recently about controlling stills based either on maintaining the temperature of a reflux condenser or the temperature of a boiler. Just a quick lesson in physical chemistry:

Simply put, you cannot use control of boiler or reflux condenser temperature to get perfect distillation

The problem with this idea is that evaporation and condensation of multi-component mixtures isn't as simple as: component X boils at 79degC, component Y boils at 80degC. There are 2 problems with this idea:

1. Liquids begin to evaporate well before their boiling points, as they approach the boiling point they evaporate more and more rapidly, until at the boiling point the temperature cannot increase any more, and all energy is spent evaporating the liquid. A simple example is your newly made spirit, if you leave the seal off it at ambient temperature, you will slowly lose volume, ambient temperature is well below ethanol's boiling point. This means that if you try to hold a condenser/boiler at 77degC in an attempt to get all undesirable components without any ethanol (hearts) (boil at 78degC), there will still be a lot of ethanol coming through.

2. Each component in a mixture interacts with each other component to change the combined boiling point. E.g. ethanol boils at 78.37degC, water boils at 100degC. But a mixture of 67% ethanol, 33% water boils at 80degC. This is the same with methanol, acetone, butanol, propanol etc. When there is a mixture, you can't think of them having their own individual boiling point, you need to think of it as: "At this liquid %methanol, the vapour % methanol will be..."

The reason that multi-stage distillation works is by exploiting the difference between the % of ethanol when in the vapour state and the % ethanol when in the liquid state at a certain temperature. As an example: For a wash of 10% ethanol at it's boiling point of 90degC, when the vapour evaporates it will be 61% ethanol. If we then condense this vapour we'll result in a liquid of 61% ethanol. If you then re-evaporate this liquid at its boiling point of 80.4degC, the vapour that will be 82% ethanol. You can continue in this way to get up to the azeotrope of 95.5% ethanol. (temperature, %vapour, and %liquid are from the graph below).

The paragraph above describes what happens if you do multiple pot still runs, or alternatively run through a plated column (re-evaporation occurs at the plates. It works the same way in a packed column, the re-evaporation happens all over the packing surface.

In summary, you can't get pure ethanol by only controlling boiler or reflux condenser temperature, neither can you separate all methanol, acetone etc by the same method. The components enjoy being mixed too much for it to be that simple.


Also please note that where distillers use a recommended temperature range for the reflux condenser, this is to allow better control. Very cold coolant will be difficult to control because small changes in coolant flow will change the coolant load by a large amount.