Boiling point of ethyl alcohol

[perfection]

Easily googled i have found that the boiling point of ethyl alcohol is 78.37 degrees Centigrade.

I would believe that this is the boiling point of PURE ethyl alcohol at 1 atm pressure

Distillation however occurs on a (fermented) wash or a low wine (the distillate after the first distillation of a pot still process) which means the quatum of alcohol would be in the region of 7-12% and 28-32% respectively.  

My question:  Although, the ethanol will vaporize at various speeds at various temperatures, what would be the boil point of the alcohol when in amixture where its strength is 15% or 30% - will it still achieve a boil to vapour at 78.37 (other things being equal)?  How does one  approach thinking about this?

[meerkat]

In general, mixtures of two liquids will boil at temperatures between the boiling points of the two pure liquids and the boiling point will vary with the concentration.  The ethanol-water mixture is a bit different in that it forms an azeotrope. From the first sentence above we would expect the boiling point (at atmospheric pressure) of a mixture of ethanol and water to be between 100°C (boiling point of pure water) and 78.37°C (boiling point of pure ethanol). In most cases this is true.  However, a mixture containing 95.58 mass % ethanol will boil at 78.15°C, which is lower than the boiling point of pure ethanol. This is called the azeotrope. To really split hairs, it is called a minimum boiling azeotrope because you can also get maximum boiling azeotropes where the boiling point of the mixture is higher than either of the pure boiling points.

The existence of the azeotrope is why we cannot achieve 100% ethanol by normal distillation.  The lowest temperature occurs at the top of the column and for ethanol-water this would be the azeotrope temperature of 78.15°C and no matter how much taller you made the column you could never go beyond the 95.58 mass % concentration.

I have attached a table of boiling point data. In addition to showing the boiling point at various liquid concentrations it also shows the composition of the vapour that is generated. Between 100°C and the azeotrope ethanol is more volatile than water and there will be a higher concentration of ethanol in the vapour than was in the boiling liquid. If this were not so, distillation columns would not work.  The "VLE" in the title on the attachment stands for Vapour Liquid Equilibrium - sorry for the jargon.

 

Carey and Lewis MF Mass and ABV.pdf

[perfection]

Thank you for taking the time out to help me Meerkat - 

Would you please in a few lines, explain its consequence to distillation?   Distillation is a controlled process and the lower the temperatures the slower (and more thorough?) the distillation.  So do distillers stay clear (below) this temperature (boiling point of the liquid at various concentrations) for a better separation  or is there some other significance for pot and column still distillation?

Thanks

[meerkat]

It is important to remember that the temperature is not an independent variable that we can set arbitrarily. The temperatures in the table I posted earlier are boiling point temperatures and are fixed by the composition of the boiling liquid. The only way to change the boiling point for a given concentration is to change the pressure, but I am assuming here that everything is being done at normal atmospheric pressure.

Let me take the data from the 4th row in the table as an example and assume you have material from a previous stripping run at 32.26 %ABV.  If you put this in a pot still and start heating it, the temperature will rise but it will not boil until the temperature reaches 85.3°C. Spirit at this composition can only be distilled at 85.3°C and at any lower temperature there will be no boiling and therefore no distillation. If you continue heating the pot and boiling the spirit the boiling point will slowly rise as the concentration of the ethanol in the pot decreases - because more ethanol than water has been removed by distillation.  The new temperature is just an indicator of the new composition in the pot, and cannot be increased (or decreased) arbitrarily while maintaining boiling.

Even when we use reflux on a column to "control the temperature" we are not truly controlling the temperature as an independent variable. Changing the reflux rate changes the composition in the column and the measured temperature is just an indication of that changed composition. The measured temperature can be used to interpolate the data in my table to work out what the actual composition is because we know it is at its boiling point.  We are all guilty of talking of controlling the temperature, but strictly that is not true.  The temperature is just a proxy for the composition.

We could take this analogy a step further and say we do not really even measure the temperature. Just as we have no direct way to measure the composition inside the column, we actually have no way to directly measure its temperature.  We measure the length of a column of mercury in a pencil thermometer (or the resistance of an RTD probe) and from this length (or resistance) we infer a temperature.  And in turn from this inferred temperature we infer a composition.

I have heard of distillers preferring "slow distillation" but have no direct experience of this myself. True distillation is not impacted by the rate at which it occurs (as long as the column is still operating properly) so if the taste/smell of the product changes with the rate of distillation there is some other phenomenon occurring.  We know that in alcohol distillation there are some chemical reactions going on - particularly between any sulfur compounds and the copper - so I can accept that the rate at which a product is distilled can affect its quality but it is not the distillation itself that is having that effect.

[Silk City Distillers]

4 hours ago, perfection said:

Distillation is a controlled process and the lower the temperatures the slower (and more thorough?) the distillation.  So do distillers stay clear (below) this temperature (boiling point of the liquid at various concentrations) for a better separation  or is there some other significance for pot and column still distillation?

Can I be so bold as to summarize?  This is a common thought, but it's totally wrong.  You can not control the temperature of boiling.

The temperature the pot will boil at depends on how much alcohol there is.  More alcohol, boils lower.  You can only control heat input, this is, the SPEED of boiling.  More power in, more vapor generated, but the temp doesn't go up.  Less power may may the temp fall, but this is only after it's stopped boiling and producing vapor.  Over the course of the run, the temp in the pot will rise, but this is a function of the alcohol being removed, and the boiling point increasing as a result.

 

 

[HedgeBird]

http://www.kelleybarts.com/PhotoXfer/ReadMeFirst/UnderstandingPredictingPotstillRun.html

The chart on this page is basically the same info as Meerkat's table but I think makes it easier to visualize.  It also contains info on the alcohol concentration you get/produce at the various boiling points.   If I was teaching a class on distillation it would probably start with this chart.