Can ethanol be purified by reverse osmosis?

13 March 2011

Question

Hi guys great show.  
Question for you all regarding reverse osmosis water purifiers.
 
I am making my own home brew spirits with dextrose and yeast, then it got me wondering if I could get rid of the water by putting it through the RO plant.  
Before I boil off the fusels, ethanol and methanol.
 
So when using a RO water plant, if you where to use a 10% alcohol solution would you get any alcohol (ethanol&methanol) through the membrane when processed.  
Look forward to your answer.
 
Dave Glossop

Answer

Dave - I don't want to comment on the legality of this so let's stick to the science!

What is reverse osmosis in the first place? This is used quite often for purifying water, to get salt out of it. Essentially, you have a membrane, which will let water through but not salt, for example.

So, if you apply pressure to the side with the salt water, the water molecules are forced through the membrane but the salt can't get through. So you end up with more salty water on one side, and fresh water on the other side.

Essentially, what the questionner is attempting to do is get a membrane which will let water through but not alcohol; then apply pressure to it, and that pumps the water out leaving a stronger solution of the alcohol behind (and all the other stuff in there).

I imagine it would end up tasting very, very strong, because very few of the taste molecules are going to get through so it's going to end up with a very, very flavoursome brew.

I also think it depends on the membrane. I've looked up that some membranes are better at this than others. You'd have to pick one which is particularly good at noticing a difference between the alcohol and the water. And if you get one which works, if you pick the right membrane, there's no reason why it wouldn't work...

Comments

10% ethanol solution is equivalent to 50 bar osmotic pressure. So you would need a very high pressure to concentrate 10% ethanol and overcome its osmotic pressure. Typical RO systems work/are calibrated for up to 50 bar pressure.
Furthermore, after concentration of about 3% even the tightest RO passes half of the ethanol through, so the losses will be high.

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