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nimgoldman
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Yes but drying all the way to anhydrous requires more sieves (over 20 grams of sieves per gram of water) and this leads to significant loss of ethanol
which adheres to the sieves' surface.
Maybe distilling the ethanol off the sieves afterwards might save some alcohol but it is extra work. Using only minimum amount of sieves (high quality
zeolites are expensive and regenerating them in a home lab setup is also a lengthy process).
From Wikipedia:
| Quote: | | Rectified spirit is typically distilled in continuous multi-column stills at 96–96.5% ABV and diluted as necessary. |
I guess they simply ensure the ethanol is distilled to the point of being practically azeotropic and then the amount of water added is always the
same.
I don't know if there is any filtration or chemical purification in the pharmaceutical-grade ethanol production process (besides distillation), but
I've the common impurities (esters and fusels) can be removed by activated charcoal filtration below 50% ABV and by treatment with mild alkali (e.g.
baking soda) to hydrolyze esters, namely ethyl acetate into ethanol and acetic acid, which can be more easily separated by fractional distillation. I
found table salt is also effective in aiding the separation process.
[Edited on 13-2-2019 by nimgoldman]
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DavidJR
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Quote: Originally posted by nimgoldman  | Yes but drying all the way to anhydrous requires more sieves (over 20 grams of sieves per gram of water) and leads to significant loss of ethanol
which adheres to the sieves.
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I don't think that's correct - I've seen figures of around 20% by weight water capacity for molecular sieves 3A, which would correspond to 5 grams of
sieves per gram of water.
You do lose some amount of product but I don't think it's a whole lot.
Alternatively you might want to start with another drying agent to get it to >= 99% then add sieves to get the last little bit.
[Edited on 13-2-2019 by DavidJR]
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nimgoldman
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I have a different experience in my practice.
When I start with say 1 000 mL of 92% ABV ethanol and add just 5 g of 3A sieves per gram of water (Carl-Roth brand), that is 400 g of sieves, the
final concentration of ethanol after 2 days is about 97-98%. I have to add huge excess to get near anhydrous in reasonable time, sacrificing
significant portion of ethanol by adherence to the sieves surface. The drying process is statistical and I think the theoretical absorption capacity
can be approached only asymptotically.
I was thinking about recovering some ethanol by distilling it off the sieves.
My 3A sieves are tested for water absorbtion capacity of at least 15.5% (weight of water per weight of sieves) - that roughly corresponds to the above
figures.
[Edited on 13-2-2019 by nimgoldman]
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DavidJR
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According to my calculations, 1 litre of 92% v/v ethanol contains 99g of water, so you'd actually need 500g. Also I'm not sure how long one has to
wait as I have a tendency to add sieves and forget about it for a week.
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S.C. Wack
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If Alcotec turbo yeast and sugar is so clean, why did they immediately get a reputation as unclean, why do they sell carbon, yeast mixed with carbon,
and special vodka yeast they say actually gives a clean product...they seem eager to give the impression that they either sell a good product or it
can be cleaned up.
Could your confidence in yeast, carbon, the number of theoretical plates possible in your setup and the number actually attained in practice, and
sieves (particularly in grossly wet alcohol) be over-rated? Ethanol becomes exponentially more difficult to purify by distillation as the water
content decreases; I suspect most members would not be using a column with too many plates and too high a reflux ratio.
There are tables that say how much water to add to x amount if your ethanol tests y and you want z.
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nimgoldman
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| Quote: Originally posted by DavidJR | | According to my calculations, 1 litre of 92% v/v ethanol contains 99g of water, so you'd actually need 500g. Also I'm not sure how long one has to
wait as I have a tendency to add sieves and forget about it for a week. |
Yes I made a mistake in my calculation. I just subtracted the volume of alcohol, not realizing there is more water due to packing of molecules. Now it
makes sense.
Yes I will look up the tables. I was more interested in the theory behind the mixing ratios or whether they are just measured empirically.
As for the purity, I don't know what other significant impurities could the yeast produce.
I used Lalvin EC 1118 yeast earlier (usually sold as wine or champagne yeast), but these were more expensive, plus I had to purchase the extra
nutrient - the nutrient is not necessary and indeed adds impurities, but it makes the yesat multiply and thus speed up the process consierably.
Note that I need ethanol pure just enough for simple syntheses and washing glassware.
I think I will filter the alcohol with AC when it is diluted below 50% v/v as the carbon is said to be much more efficient when water will present.
Too strong alcohol is said to be able to actually pull the adsorbed impurities back.
I will also check pH and neutralize any acids and try to hydrolyze esters (namely ethyl acetate). I am thinking about treatment with KOH and sulfuric
acid to remove possible aldehyde impurities.
The still does not have much plates nor the reflux ratio can be controlled. I can control the pot temperature and also thinking about adding a
makeshift dephlegmator to improve the reflux.
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Tsjerk
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First try to measure properly diluted ethanol. And stop using molecular sieves to dry very wet ethanol. Start with some anhydrous salt, and only dry
the last percent with sieves. Then properly calculate the amount of water you need. I'm almost sure that will fix your problem (two problems; losing
much ethanol to the sieves and the density/percentage thing).
Don't try to measure pH of anything not being at least 95% water, or at least don't try to deduct anything out of it. It won't work as any equipment
that will give you a value you can do any calculations with won't be designed or won't be calibrated to do so. Anything you can measure pH with will
give you a hint of it being acidic or not, not how acidic it is. Or maybe it will tell you it is acidic while it is not. Any pH meter swings between
pH 4 and 11 while being in perfectly neutral and pure 3 molar KCl storing buffer for example.
| Quote: Originally posted by Tsjerk | | Quote: Originally posted by nimgoldman | | Then I look up its w/w concentration from table and calculate how much water to add to reach azeotropic concentration, which I believe is 95.5% w/w
according to Wiki, though sources vary. |
Can you tell us how you do this calculation? |
I asked you didn't I? A lovely demonstration experiment is to load 50 ml 99.9% ethanol on top of 50 ml of water in a measuring cylinder and make 4 ml
"disappear" buy stirring it.
[Edited on 14-2-2019 by Tsjerk]
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nimgoldman
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As for the drying, I browsed many threads to realize no common drying agent works properly with ethanol. For example, magnesium sulfate is said to not
working at all, calcium chloride complexes with ethanol etc.
Most sources talk about rigorous drying, though, not about removing just some water from dilute alcohol.
I found some people use cornmeal or cellulose. I will investigate this further.
So far I will distill up to 92%, then pre-dry with - say - MgSO4 and then finally dry with sieves.
I calculate density using volumetric flask and a calibrated scale.
Then I look up the wt% concentration from density using table found on the internet.
The target w/w concentration of an azeotrope is 95.5% so the target concentration can be expressed as
| Code: | 0.955 = w_EtOH / (w_total + w_H2O) |
where w_EtOH is the weight of pure ethanol in the solution and w_total is the weight of the solution.
Solving for w_H2O gives us the weight of water to add to reach the desired concentration.
[Edited on 19-2-2019 by nimgoldman]
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