Hacker News Reader

Comment by avidiax | Hacker News Reader

I suspect that the vapor of the mash is always a mix of the components, and even above the boiling point of methanol, it still produces a mixed vapor. At room temperature, all of the components produce some vapor and will evaporate. This continues as the temperature rises.

It's not clear to me that simple distillation of a methanol/ethanol mixture can produce either pure ethanol or pure methanol at any point, just as it's impossible to distill ethanol and water to pure ethanol (absolute alcohol) if the water is above a small percentage of the mixture.

View on HN

Yup, distillation never produces a pure product. Cask-strength whiskeys contain quite a lot of water, even though nobody is stupid enough to distill at 100C. Even an industrial column still can't go over 96% ABV.

There is always some amount of vapor pressure, even below the boiling point of a substance. Otherwise, neither water nor alcohol would evaporate by themselves at room temperature! The temperature we call the "boiling point" is just the temperature at which the vapor pressure equals the ambient pressure.

You can't distill out pure methanol, as at the boiling point of methanol ethanol also has some vapor pressure, so you distill a mix. However above that boiling point you distilled out all methanol (with a mix of ethanol), and the remaining ethanol should be free from methanol.

This also matches what happens when distilling ethanol from water. You can't distill pure ethanol, but you csn distill ethanol-free water afterwards.

> above that boiling point you distilled out all methanol (with a mix of ethanol)

That's not what studies have shown. Methanol boils off in all phases of distillation, and remains in high concentration at least halfway through.

https://pubs.acs.org/doi/10.1021/acsfoodscitech.3c00627

https://pubs.acs.org/doi/10.1021/acsfoodscitech.1c00025

Thank you for finding these! I remembered they existed but not where

"This also matches what happens when distilling ethanol from water."

Right, normal commercial ethanol production is 95% EtOH, 5% H2O (the constant boiling mixture/azeotrope). That's good enough for most uses but not all. The only problem the average person would ever likely encounter from the residual H2O would be in the application of alcohol-based coatings such as shellac where it can cause whitish discoloration. Painters will occasionally use 99% EtOH which is substantially more expensive (removing that residual H2O requires an altogether different proxess).

Reply on news.ycombinator.com