If you are trying to get to the azeotrope starting with 10% abv beer, the minimum reflux ratio that you need is 2.4:1. This means that for every 1 liter of product that you take off, you need to send 2.4 liters (or about 70% of the liquified vapor) back down your column. Theoretically, to achieve this minimum you need an infinite number of plates; in practice, you can get close enough to azeotrope that you can't measure the difference with around 25 theoretical plates. To a limited degree, you can reduce the number of plates and still get the same proof by increasing the reflux ratio. At 100% reflux, you can get aeotrope with about 7 plates from a 10% beer (but of course 100% reflux gives you no product and is only useful as a science fair project). Nixon and McCaw advocate short columns and around 90% reflux (10 liters down the column for even one that you take off).
This is all dictated by the laws of physics. The only ways around this are:
- increase the proof of your beer (decreases reflux for azeotropic product)
- reduce your product purity requirement (reduce the required proof of the product)
You can also use these same figures and increase your product production rate by increasing the boiler power, but only if you correspondingly increase your condenser heat removal capacity (to maintain the reflux ratio needed for your HETP), and making sure that your column diameter is sufficient to maintain vapor flow up the column at a resasonable rate (12 - 20 in/sec; calculated as I described in an earlier posting).
Note that when you start your batch still off with 10% abv, it won't stay this way. As product comes off, the %abv of the beer must reduce and therefore you need to monitor still head temperature (as a proxy for proof) throughout your run and as you see the temperature increase, you must increase your reflux rate to get back to azeptrope (or whatever proof you desire). At some point, your reflux ratio will be so high that you decide that it isn't worth it anymore and you will stop the run and use the low wines on a subsequent run.
I cannot emphasize enough that the "correct" way to get your still set up and going the way that you want it is to start with 100% reflux and get the column up into equilibirium. This could easilt take a couple of hours. If you can't achieve equlibrium with 100% reflux, you aren't removing enough heat or don't have enough HETP or have some other issue such as fugitive heat loss from the column, reflux running down the column walls, or plugged packing, etc. If your beer was only ethanol and water, then at equilibrium the still heat temperature would be around 172.5 degrees F which is the boiling point of the azeotrope. You would then SLOWLY start to take off product, carefully watching the thermometer, until you saw it rise to a new, higher temperature at equilibrium, and then you would slightly increase the reflux ratio back to azeotrope and keep watching and adjusting, for reasons stated earlier. Of course you have other stuff in your beer than ethanol and water and some of it has a lower BP than the azeotrope, so your initial equilibrium temperature will be a little lower depending upon how much methanol and other stuff is in your beer. As a spirit distiller, you probably want to very slowly bleed off this stuff (foreshots) and discard it, then start collecting the heads and the main run and stop at the tails. I'm more interested in fuel ethanol and can afford to take this other stuff off as product, as long as I see that it is small (my engine won't get a headache :-).
Get you column into equilibrium and keep it in equlibrium at all times!!! This is what reflux distillation is all about. Patience is the watchword here. If you don't have the patience to do this, get yourself a high quality continuous still and then once you get it up into equilibirium and establish the required minimum reflux ratio, you can just leave it there without further adjustment (which doesn't mean unattended -- you are making a highly flammable fuel, regardless of what you actually intend to do with it later :-).
From: Gavin Flett <gavin_flett@...>
To: new_distillers <firstname.lastname@example.org>
Sent: Mon, May 7, 2012 10:48 pm
Subject: RE: [new_distillers] Re: My reflux column