Or boiling booze to catch a contact high? ![[barf]](http://www.armedpolitesociety.com/Smileys/default/barf_smiley.gif "Barf!")
Dont knock it, till you try it.
It is really multitasking, when you think about it. How many times have you had to say "Hey hold my beer and watch this?"
Why the magic "95%" (190 proof) number?
Because a lot of interesting things happen when you hit that concentration of alcohol in water.
For one thing, as I mentioned, 100% alcohol wants to absorb water until it gets to about 95%. So when you try to distill it past that point it is very difficult to separate the water out in the mixture of 5% steam and 95% alcohol vapor. So when it is condensed and dribbles into the receiving vessel, you've still got that 5% water in it.
Special techniques are required to "kick it over" that 95% point, which is why 100% alcohol is so expensive.
So you can't just use your grandmother's little copper still to do it.
Terry, 230RN
(Image credit in properties.)
A water/ethanol binary mixture forms an
Azeotrope at atmospheric pressure and a concentration ~95%*. ~95%* concentration is the equilibrium point for ethanol and water, and since water vapor is in the air, it will absorbe into the ethanol solution until the solution is ~95%* ethanol. An azeotrope forms from the solution activity of two species deviating from an ideal condition. In the case of ethanol and water, it due to hydrogen interaction.
Here is a hand drawn water etoh azeotrope I pulled off the internet:
This is clearly distorted to demonstrate the properties of the azeotrope. I show this to demonstrate that the liquid boiling temperature is equal the vapor dewpoint temperature. Since the mass transfer agent in distillation is energy, the two components cannot be seperated any further. Now boiling points are directly relate to atmospheric pressure, so the azeotrope will not be a problem at a different pressure.
Here is an interesting
McCabe Thiele diagram:
This is the method for determining the minimum number of stages a distillation column will need, and also the approximate composition at each stage. Note: true equilibrium will not be obtained. This would required an infinite amount of stages, as in reality each stage does not go to equilibrium. This is more of a starting point for designing a distillation column, and it shows what is going on.
The bottom line is the operating line, the top line is the solution equilibrium. You will notice that the equilibrium line approaches the operating line as the composition approaches the azeotrope. I offer this diagram as quick and easy way of understanding the distillation process. The process shows that there is a negative separation above the azeotrope (where the operating line crosses the equilibrium line) this is impossible, so the drawing shows where distillation will no longer separate water and ethanol.
Two methods that are used to distill 100% ethanol:
The addition of benzene
benzene will cause a two liquid phase tertiary mixture, that "breaks" the azeotrope. It does this by changing the molecular interaction of the mixture. The benzene can then be recovered from the remaining water, because the they arent soluble.
Also pressurizing the distillation column will avoid the azeotrope. Azeotropes are dependent on the ambient pressure. However, this raises the amount of energy required to distill the solution. This holds true for a negative and positive pressurization.
*the azeotrope is 95.6% etoh water by weight. Like i said, equilibrium will never be obtained so 95% is what the final product is. I assume that 95.5% would required a column significantly larger than the one for 95%, and the consumer is not willing to pay extra for 0.5% more alcohol (think cost benefit analysis).
**I left most of my text books at work, as usual, so I couldnt really do a thorough review for accuracy. I basically wrote this from memory. If I made any technical errors, please point them out.
