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Author: Subject: Purifying Denatured absolute EtOH Superheating problem
angelhair
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[*] posted on 19-4-2008 at 20:44
Purifying Denatured absolute EtOH Superheating problem


To 2L of DAA I added 20g NaOH and refluxed for 2 hours with overhead stiring then switched off heat and changed the setup for distillation.

No matter how fast the stiring, after a while the stuff super heats and explodes out of the flask, on to my shoes.

I did it again and added telfon pieces and a couple of small stir bars, with really fast stirring, same thing.

I used a different set up where I could quickly change to distillation without cooling and it worked out well.

Why this superheating problem?
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MagicJigPipe
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[*] posted on 19-4-2008 at 20:48


How long does it take you to change to distillation? Surely not long enough for the alcohol to cool more than a few degrees.

I have done a similar process with MeOH and did not encounter the same phenomenon.

Why do you add "a couple" of stirbars?




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[*] posted on 19-4-2008 at 20:55


Not long at all, only a few minutes, then it starts distilling OK and after a little while I can see it developing, you know no more little bubbles and it starts bumping, then after about 10 minutes bang. No matter how good the stirring.

I added the small stir bars in the hope that they would act like boiling stones. That;'s what I use instead of stones.
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[*] posted on 19-4-2008 at 21:44


How are you heating the pot?

I think your boilup rate is too high.

Simple distillation or fractional?




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[*] posted on 19-4-2008 at 21:49


Go break a cheap ceramic plate. Thats what I did for boiling stones, though I think someone else broke it for me by accedentally dropping it. I dont think teflon is going to make very good nucleation points, ceramic on the other hand is porous and breaking it makes sharp edges.



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[*] posted on 19-4-2008 at 23:09


The boiling chips we have in lab are teflon bits. Evidently teflon, at least this particular material, is porous and thus serves well.

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[*] posted on 19-4-2008 at 23:18


Teflon can be had in a range of densities, depending on how much it was compressed during sintering. White Teflon thread tape is fairly porous, as are Teflon boiling stones. Teflon coatings on stir bars usually are not porous, because it makes them harder to clean; note that's an issue with the boiling stones if they are recovered and reused.

Second hand ceramics plates are a decent source, as are ceramic tiles.
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[*] posted on 19-4-2008 at 23:21


Teflon boiling chips are commercially available.

Porcelein boiling chips might score the glass and lead to breakage. They need to be small and light.




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[*] posted on 20-4-2008 at 07:36


I'm surprised only 20g of NaOHgives you some much trouble.

What i do to purify EtOH, is leave the EtOH over NaOH/CaO overnight, (20/80g per L) then decant most of itoff the sludge, and distill over a little (5g) NaOH. I found this less troublesome than leave the NaOH/CaO in the flask overnight, and directly distilling. Although this was bumpy, it could achieve a continus, stable boiling rate, by carefully heating up. I stopped when the sludge was gettting very apparent. Don't fill your still more than 1/3 though, and heat slowly, with an oil bath 10-15°C higher than the bp. A mantle is going to cause a lot more superheating.
II would suggest discarding the forerun, as it contains condensation product.
Usually, out of 1L, i'd get 600-750mL of absolute EtOH, and 500mL of extra dry (Mg) EtOH from subsequent distn.

I keep the sludge in a jerrican and use it either in the garden as i would with slacked lime, or to neutralize dilute acids.

[Edited on 20-4-2008 by Klute]
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[*] posted on 21-4-2008 at 14:33


When I tried the same some years ago (posted in this forum) I had no problems. I did the reflux much longer, just to see the colors change. It was also on a much smaller scale.

I think that adding activated carbon and filtering could relieve the problem if it had something to do with the byproducts formed in the condensation reaction. You could also try to do it with the ethanol before the distillation. Apart from that I have no idea.

Klute: Why do you think the forerun contains condensation products? Does it smell fruity? They are inevitably higher boiling than ethanol when formed from acetone or higher ketones (diacetone alcohol, phorone etc, higher condensation products from michael condensation). It is possible that there would be an azeotrope of ethanol. Of course, discarding a funny smelling forerun is not a bad idea.
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[*] posted on 21-4-2008 at 15:07


Yes, it always has a slight smell too it. This could ver well be some ether denaturant present in the denaturated ethanol i use. Also, when mixing the first ~20mL out of a 2L distn with a middle cut, there are some "swirlies" that don't appear when mixing two middle fractions. Obviously, this could not happens when using different grades of ethanol.
Indeed, after 24H over NaOH and 2 hours reflux, most of the carbonyl denaturants are turned to long chained polymers/condensation products and don't cause any problem with the distn.
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[*] posted on 21-4-2008 at 17:07


Klute, I will try it your way.

I have done this many, many times from 250ml up to 5L always in a water bath, around 85 degC, reflux for 2 hours, the colour is orange way before then, change set up, so it has cooled a few degrees then start to distill, comes over fine for a while then problems start regardles how hard the stiring is. Every time!

There are no problems what so ever if I change the set up fast without cooling at all.
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[*] posted on 21-4-2008 at 18:31


I would like to add that teflon seems to make the best "boiling chips". Most of the time, for me, one teflon coated stirbar seems to do the trick. It never seems to "uncharge" like ceramic. Only once have I ever experienced bumping so severe that I had to discontinue the distillation or reaction.

It's probably because I use a hotplate most of the time so my heat-up times are longer. Sometimes I just insulate the top of the flask if I'm having trouble keeping the substance boiling, which is not often.

I honestly don't know what your problem is but, then again, I have never distilled EtOH. Other alcohols never gave me the problem you are having, though.

Good luck.




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[*] posted on 22-4-2008 at 04:17


I only had similar bumping when drying MeOH over CaO/NaOH. I've stopped using this method as it is qualified as "wastefull" in Purification of Laboratory Chemicals. A simple fractionnation yields good enough methanol.

Here are the revelant parts for ethanol:
"Ethanol [64-17-5] M 46.1; b 78.3°; d15 0.79360; d5 0.78506; n 1.36139
[...] Rectified spirit (95% ethanol) is converted to absolute (99.5%) ethanol by refluxing over freshly ignited CaO (250g/L) for 6h, standing overnight, and distilling with precautions to exclude moisture.
[...] Aldehydes can be removed from ethanol by digesting with 8-10g of dissolved KOH and 5-10g of aluminium or zinc per L, followed by distn.
[...] Other purification procedures include pre-treatment with conc. H2SO4 (3mL/L) to eliminate amine impurities, and with KMnO4 to oxidize aldehydes, followed by refluxing over KOH to resinify aldehydes, and distilling to remove traces of H3PO4 and other acidic impurities after passage through silicagel, and drying over CaSO4. Water can be removed by azeotropic distn with DCM (azeotrope boils at 38.1°C and contains 1.8% water) or 2,2,4-trimethylpentane. Ethanol has a pKa15 of 15.5 in water."
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[*] posted on 22-4-2008 at 04:39


Klute: Of course, it must be unreacted denaturant then.
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[*] posted on 22-4-2008 at 05:20


save your broken test tubes, then smash them up, they make great anti-bump chips.



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[*] posted on 1-5-2008 at 19:07


teflon wool is awesome as a boiling chip, i find it works better than the chips themselves and you can make it with a drill bit, teflon block and some practice. i got a bag of it some months back from the engineering shop at a plastic component manufacturing company and now use it broadly for almost everything i used to use glass wool for (except insulating).
I imagine it would be easy to re-use also although i trash mine after use because I have become lazy and wasteful as i get older.




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[*] posted on 20-10-2008 at 17:00


Yes! I made some teflon wool and it is good, but it didn't solve the supperheating problem, only hard overheadstiring and NO drop in temp. If I allow the temp to drop while changing apparatus I get bad problems.

Just on a side note, will distillation from NaOH dehydrate the EtOH to any extent at all?
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[*] posted on 20-10-2008 at 17:25


teflon wool is awesome as a boiling chipand now use it broadly for almost everything i used to use glass wool for .

Suitable replacement for raschig rings in fractionation column?
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[*] posted on 20-10-2008 at 22:44


Quote:
Originally posted by angelhair
Just on a side note, will distillation from NaOH dehydrate the EtOH to any extent at all?

According to Vogel's the answer is no, since dissolving sodium in moist ethanol also can not be used to make anhydrous/absolute ethanol (due to the existence of the lower boiling ethanol/water azeotrope).




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[*] posted on 21-10-2008 at 18:58


Quote:
Originally posted by starman
teflon wool is awesome as a boiling chipand now use it broadly for almost everything i used to use glass wool for .

Suitable replacement for raschig rings in fractionation column?


Yes, i use it in columns often, it also makes a very good splashead by getting an expander from your round bottom filling loosely with wool then a reducer back down to the same side as original, i have found the backpressure on this setup to be negligible c/f a splashhead. Of course you could just use a splashhead, however on a rotary evaporator i find the teflon wool solution superior to using the regular splashheads commonly used, because it nondirectional, unlike a splashead.
Since writing my post here i have used teflon wool for everything and anything and i'm running out, so sad, i'll have to get more however the machine shop i got it from actually had used machining oil on their lathes when making teflon parts, this seems a little dumb to me, especially given i have to wash the shit off. Then again it was free.




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[*] posted on 25-10-2008 at 07:16
As an aside...


Industrially, they use benzene:

http://tinyurl.com/6c69hr

Kinda neat. :)

Ethanol-benzene-water forms an azeotrope that boils at about 64.9C. So they condense that out and do a phase separation after. It would be an interesting process to play with.
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[*] posted on 25-10-2008 at 08:14


I do not understand why you are trying to "purify" absolute alcohol. You can't make it potable and there are few if any applications I can think of where the trace amounts of benzene and acetone remaining from the dehydration process (using a ternary azeotrope to break the binary azeotrope) interfere.

What is your application?




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