The Basics of Distillation

Well, this has been much delayed, but I wanted to write down what makes distillation possible. It’s really not a complicated idea. The first part is what I already wrote about: You create a solution (somehow) that has a mixture of liquids with the idea that you want to separate them. Not all liquids are mutually soluble, but since water is known as the “universal solvent,” most things (solids and liquids) can be dissolved in water. Therefore a common task that needs to be done is to separate water from the stuff that is mixed with it.

Evaporation is the key to distillation. Moreover, another important point is that not all liquids have the same boiling point. Oh, and the fact that boiling is a reversible process. Let’s step back: You don’t have to boil a liquid for it to evaporate. A pot of water, left alone, will turn into an empty pot (if you are patient enough). If you enclosed that water in a vessel of some kind and applied heat to it, you’d be encouraging it to evaporate at a faster rate, up to and including boiling it. But remember, boiling is reversible. When the water vapor gets far enough from the heat, it will turn back into a liquid (i.e., it will condense).

If our imaginary vessel is big enough such that the water vapor can get away from the heat, it will condense on the sides and slide back down to the [still boiling] water. This brings up an important point: Such a vessel can’t be closed, can it? For most liquids, the space consumed by a vapor is MUCH larger than that consumed by the equivalent liquid. Thus a closed vessel would be under enormous pressure…probably dangerous levels of pressure. So the vessel will need to be open. Now, can we make the fact that a vapor wants to condense back to a liquid as it cools off work for us?

If the imaginary vessel has a level somewhat above the boiling water that is intentionally kept at a temperature less of 100 °C, the water will condense at that level. The act of changing phase back to a liquid will give off energy that will warm up the level, so it will take work to keep that level cool enough. I hated Thermodynamics in college, so I won’t even try to figure out how much energy this would take. Intuitively, it will take energy to keep that level at or below 100 °C.

When water is distilled (to purify it), the idea is that water vapor can be collected and condensed into “purer” water, leaving behind the impurities that might have been dissolved solids, and perhaps other liquids. Any liquids that boil at a lower point than water will be in the mix as well, unless great care is take to make them condense at a lower level, so the vapors that reach the water collection level are mostly water.

Separating liquids is easier when they have significantly different boiling points. For instance, Ethanol (78.4 °C) and water (100 °C). It’s not a perfect matter of just “boil the solution and catch everything that condenses at the desired boiling/condensing point temperature” because the vapor is a mixture of gases and the condensate won’t be a perfectly pure, isolated liquid.

Think about it: Even without applying heat, some of the water is vaporizing (evaporating) anyway. A glass of Scotch will have Ethanol wafting away even when you are not trying to boil it (don’t boil Scotch in my presence, please). Trust me on this, even though you can’t smell it, water is wafting away as well. And those other smells? The volatile compounds that make up the pleasing aromas and flavors? You can smell them because they are leaving the glass. When a Chemist says “volatile” they are referring to a tendency to evaporate. Common usage seems to equate volatility with inflammability, but that’s a different adjective entirely: Inflammable.

The trick with distillation, since it’s hard to get what you want after only one pass through the still, is to take the result and distill it further. After one pass, the distillate is not perfect, it’s perfected: It’s closer to where you want to be than whatever you started with. That’s what is done in the production of whisk(e)y.

Scotch is typically distilled twice, but some are triple distilled (makes the initial new make spirit higher ABV, compared to a double-distilled product, but the initial ABV and the bottle strength have no relationship with each other due to aging, mixing with water, etc.). As an aside, most Irish whisky (I’m particularly fond of Bushmills 16) is triple-distilled.

Bruichladdich has created a quadruple-distilled monster called “X4” that is nicknamed “Perilous Whisky.” When it went into the casks it was over 90% ABV. As a publicity stunt, they used X4 as fuel in a racing car. Seriously. Aside: I object to the plan to mix it down and sell it at 50% ABV. It’s closer to single-malt vodka. Come on…Glenfarclas makes a 60% ABV cask-strength expression. At least go for 66% ABV.

Now that some of the more mundane aspects of distilling are behind us, I’ll take a look at how a pot still actually works.

1 Comment»

[…] to the coming attractions here: I will be writing a sequel to my distilling article either tonight or tomorrow night that will show how that distilling information is applied to […]

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