smuv - 23-9-2010 at 20:35
Does anyone have any data about the ethanol water azeotrope at pressures above one atmosphere. Essentially what I am looking for is @ pressure X the
ethanol water azeotrope has a mole fraction Y of water and X of ethanol.
I am working on a little project that requires a large amount fairly anhydrous ethanol (basically, I want to make lots of ethylene) and am considering
running a pressure swing distillation to achieve this. Of course I could run a vacuum distillation...but if a pressure swing distillation is possible
there are some very appealing aspects to running it under super atmospheric-pressure.
P.S. I did a fair amount of scifinder searching as well as google scholar-ing. I am having trouble putting what I want into a concise keyword though.
Thanks.
[Edited on 9-24-2010 by smuv]
kmno4 - 26-9-2010 at 12:13
15 minutes of googling gave this:
http://www.hyper-tvt.ethz.ch/distillation-azeotrope-pressure...
smuv - 26-9-2010 at 14:43
Thanks for your time.
Does not look very promising however, the azeotrope seems pretty inelastic to increasing pressure.
chief - 27-9-2010 at 11:50
As far as I remember Ethanole can be dried using Na2SO4 ... ; the latter can easily be recycled by heating ...
not_important - 27-9-2010 at 12:26
Yeah, EtOH-H2O above ~500 mmHg is pretty uninteresting and generally not too helpful for getting to anhydrous; reduced pressure is the way to go.
smuv - 28-9-2010 at 13:39
@Chief, there are many desiccants that will remove water from ethanol, but few do it with selectivity. Therefore most desiccants form both hydrates
and alcoholates (or whatever you want to call that) when drying ethanol. When the alcohol is present in such an excess with respect to water, in
order to achieve reasonable dryness a huge excess of desiccant must be used and large losses of alcohol are experienced due to (in)selectivity of
descant. The best descants for rough drying of ethanol are CaO (not realistically re-usable) and Zeolites (re-usable).
@not_important: of course I have considered vac distillation, its just at present I am not properly equipped to condense the ethanol/protect the pump.
I guess now I am leaning toward zeolites.
not_important - 28-9-2010 at 15:08
Just takes an water aspirator pump, run double condensers with the first using whatever running water you have, the second - a vertical coil type
works well, uses water that's run through crushed ice (if your tap water isn't cold enough). Outside temperatures should soon be dropping, making
reduced pressure distillation more attractive.
My point was that elevated pressures are not the way to go, too much futzing around to be worth it. Reduced pressure is the simplest and cheapest, mol
sieves and the like next. best
[Edited on 28-9-2010 by not_important]
smuv - 28-9-2010 at 19:14
Well, now that I have actually looked at the vapor pressure of ethanol at various temperatures (I certainly overestimated them when considering all
this), I agree not_important vac distillation seems viable for me (in winter my water is VERY cold).
P.S. A nice vapor pressure calculator for various solvents: http://www.ddbst.com/en/online/Online_Calc_vap_Form.php
h2o2guru - 29-9-2010 at 10:59
have you thought about doing a liquid CO2 separation. alcohol will preferentially dissolve in liquid CO2 , venting the water then CO2 will give
anhydrous ethanol.
Patents for this would be ~ 20-30 years old.
peach - 9-10-2010 at 06:03
We need a stickied thread with no posts and just the title "Distill alcohol under vacuum" 
It needs to be under 80-70mBar to break.
Running it through the vapour pressure calculator (which is handy, I've not seen that one), it's getting close to the minimum you're going to be able
to easily condense.
You'll want ice in the condensers water and ice around the receiver, or a lot of it will end up getting sucked through and out to the aspirator /
pump.
Save up lots of plastic drinks bottles, fill them with water and chuck them in the freezer. Then dump loads of them into the bucket the condenser is
running from. I'd go for filling it with bottles. You only need enough water in there to cover the bottles and pump. Or just use bags of ice if you're
in the US. I use bottles because only one supermarket does premade bags around here (Tescos), and they're not always open at 3am when something I'm
doing begins to cook, or I want some beer 
. I have ice cube trays and those
bags to make them, but the bags are annoying and I don't like throwing out plastic after one use.
Putting salt or anti-freeze into the water prior to freezing it doesn't result in a bath / loop that stays at that below zero temperature as it goes
through the latent heat change (solid to liquid); which is where all the cooling power is. It will come out the freezer as solid / slush, then rise in
a fairly normal way to liquid.
An aspirator is a good idea because the pressure is low enough to do it, but not far off. So it'll just break it, and not drag the temperature down
stupidly low.
On the subject of vacuum pumps, I've been reading John's version of Playboy, some of the vacuum pump manuals. One of them, from Leybold, mentioned how
flammables usually become none combustible once they get into the tens of mBar range and down.
I wonder if that's to do with the oxygen being sucked out and replaced with the flammable (so it's saturated and there's no oxidizer present), or if
it also applies when the correct mix is still there, if it's something to do with the densities. I wouldn't expect Leybold to make a mistake on that,
so I'm guessing it does have something to do with the densities. For instance, it may not be enough to support a continuous combustion.
Another example, gas has a fairly odd way of responding to vacuums in terms of conduction. It gets easier to ionize the gas as it goes down, then goes
back up.
As with lots of other things, like designing sparkless hotplates, fumes, burns, the problem isn't so much with the experiment running, it's when the
human comes in and tinkers with it.
E.g. Sodium still are fine if they're left alone, refluxing. It's when the student / member of staff turns up to get some solvent, or quench the
still, that it has a fit.
[Edited on 9-10-2010 by peach]