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Comment by leni536 | Hacker News Reader

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.

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> 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).

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