My friend Peter Schmidt-Nielsen revitalized an old conversation of ours on twitter, and it led to me having a much better sense of what’s up with azeotropic concentrations and why it is that you “can’t” concentrate ethanol above its azeotropic point of 95% via distillation and boiling.
The level 1 argument goes:
When you boil off vapors from a 50% ethanol mixture, the ethanol boils off more readily, and so you can condense the vapor back into a 60% ethanol mixture (and redistill your now-lower-concentration liquid leftovers, and so on). If you repeat this process, the ethanol concentration will keep climbing, but it asymptotes—at 94%, you’ll only get to 94.5%, and 94.5% will only get you to 94.7%, and so on. If all you have is a collection of mixtures at concentrations less than 95%, everything you do to them will only give you more of the same. So you can’t make pure ethanol via distillation.
[N.B.: This chart is for propanol rather than ethanol, but it’s more nicely labeled than the ethanol charts I could find, and also the relevant regions end up a little more visually distinct here. But it’s the same qualitative story as with ethanol, just with more of the action in the [0.95,1] regime. See here for a real ethanol graph.]
The level 2 rejoinder goes:
Hang on a moment, though. What if I get up to 95%, and then just give myself a tiny little push by adding in a bit of pure ethanol? Now my vapor will be lower concentration than my starting liquid, but that’s just fine—it means that the leftover liquid has improved! So I can do the same kind of distillation pipeline, but using my liquid as the high-value enriched stages, and carry myself all the way up to 100% purity. Once I pass over the azeotropic point even a little, I’m back in a regime where I can keep increasing my concentrations, just via the opposite method.
But the level 3 counterargument goes:
Although this technically works, your yields are crap—in particular, the yield of pure ethanol you can get out this way is only ever as much as you added initially to get over the hump of the azeotropic point, so you don’t get any gains. Suppose you start with 1 unit of azeotropic solution and you add x units of pure ethanol to bring yourself up to a 0.95+0.05x1+x concentration solution. Now any distillation and remixing steps you do from here will keep concentrations above 0.95, for the same reasons they kept it below 0.95 in the other regime. But if the concentrations can’t drop below 0.95, then conservation of water and ethanol mass means that you can only ever get x units of pure ethanol back out of this pool—if you had any more, the leftovers would have to average a concentration below 0.95, which they can’t cross. So your yield from this process is at most as good as what you put in to kickstart it.
I’m not sure what level 4 is yet, but I think it might be either of:
Use variation in vapor-liquid curves with atmospheric pressure to get more wiggle room and extract way better yields by dynamically moving the pressure of your distillery up and down.
Force higher concentrations via osmosis with the right filter around the azeotropic point to get the boost, and anti-distill to get pure ethanol out of that, which might turn out to be more energy-efficient than using osmosis to go that high directly.
(People who actually know physics and chemistry, feel free to correct me on any of this!)
My friend Peter Schmidt-Nielsen revitalized an old conversation of ours on twitter, and it led to me having a much better sense of what’s up with azeotropic concentrations and why it is that you “can’t” concentrate ethanol above its azeotropic point of 95% via distillation and boiling.
The level 1 argument goes:
The level 2 rejoinder goes:
But the level 3 counterargument goes:
I’m not sure what level 4 is yet, but I think it might be either of:
Use variation in vapor-liquid curves with atmospheric pressure to get more wiggle room and extract way better yields by dynamically moving the pressure of your distillery up and down.
Force higher concentrations via osmosis with the right filter around the azeotropic point to get the boost, and anti-distill to get pure ethanol out of that, which might turn out to be more energy-efficient than using osmosis to go that high directly.
(People who actually know physics and chemistry, feel free to correct me on any of this!)