I read that one of the common methods to make untaxed “denatured” ethanol undrinkable is to contaminate it with methanol, because the boiling points are similar.
Suppose that someone wanted to separate the 2, just for kicks, leaving the ethanol in a pure enough form to be safe to drink. How could it be done?
You could distill the methanol away from the ethanol, since it has a lower boiling point. Then distill the ethanol out of what’s left, to separate it from its impurities.
Then do it again. Lather, rinse, repeat – to further separate out any remaining methanol and other stuff.
There might be other tricks to bind up unwanted stuff, or even the methanol.
certainly something like this was done. as I mentioned in the other thread (the one where this all came up in the first place), people in my hometown were arrested during Prohibition for doing exactly this, and selling the distilled-out ethanol in New York City. I’m just saying I’d be very leery of the alcohol thus obtained.
Along time ago, in an organic chemistry class, I was required to separate a mixture of methanol and ethanol by distillation as part of a practical exam. So, yes it can be done. I have done it. I, however, was working in a lab with precision equipment and with only a few ounces of the mixture. I would not have cared to drink my own product, even so.
The boiling points aren’t what I would even consider close, being some 14 degrees C apart, but at the lower boiling point of methanol, 64.7 deg C, ethanol will have an appreciable vapor pressure, and you’ll boil off a lot of ethanol at that temperature. Ethanol boils at 78.4 deg C, and likely by that temperature you’ll have mostly ethanol left, but not much of the original volume. Plus, a lot of denatured alcohol uses benzene, which is almost impossible to distill away, because it forms an azeotrope with both ethanol and water.
Or like British tramps throughout the 20th century you could just drink the meths straight.
“I think you’ll be amused by this Red Biddy. Insouciant and playful, but a powerful aftertaste.”
As has been mentioned above, denatured ethanol can be denatured with other things. But yes, distillation, under the best conditions, will give you some purified ethanol, even though you will sacrifice a bunch of the ethanol in the “heads” and “tails” of the distillation. And you spend fuel, and time, and the money associated with the initial purchase and other costs thereof. But in the end, you win, you get ethanol.
Its still bootlegging, and its still a crime. To use, a still, in that way. 
The purpose of denaturing ethanol is just to keep honest people honest. If you insist on evading the law, they’ll get you anyway, is the assumption, I think, law enforcement makes.
On another forum, I just smack down posts like this one. “Its illegal, and we won’t teach you how to do it, or we’d be liable” YMMV here on the SDMB.
I don’t see any rational reason to do it. My local liquor store’s shelves are groaning under the weight of thousands of bottles of perfectly safe to drink alcohol that probably also tastes better. I simply wondered if it is chemically possible to separate it at all. I should have asked about Benzene - how would you get the benzene out?
Whatever the liquor tax is, it is in no way worth the cost of distilling it, much less whatever crazy thing you would have to do to remove Benzene. I’m just wondering if it is even doable in a way that leaves a safe product.
I would say it is doable. It wouldn’t be economical, and you’d have to worry about whether the original ethanol was contaminated with something other than methanol, because the manufacturer, knowing that it was not intended for human consumption, might have been a little casual with the production procedure. Distilled alcohol can contain significant amounts of fusel oil which is definitely undesirable.
It would be like combining hydrochloric acid and sodium hydroxide in the proper stoichiometry to make salt water that you could safely drink. It could be done, but I’d start with pharmaceutical grade reagents to be safe.