Saerynide
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Large scale vacuum distillation
So I have about 10 gallons of mildly dirty DMSO that I need to recover. I basified it to pH 8-9 before vacuum distilling.
On a small scale with 50 ml in the rotavap with an oil bath, colorless DMSO came over at ~112C at 0.02 bar and it was all hunky dory.
The problem arose when I scaled up to 400 ml to test again (didnt want to go all out to 2 liter batches just yet) and couldnt find an oil bath big
enough. So I used an aluminum block on a hotplate with mag stirring instead of rotating the flask.
It was stirring nicely at 500 rpm at 112C, so I slowly pulled the vacuum. At about 0.1 bar, it started to boil nicely and I was thinking "sweet,
maybe this will work after all!" And then the pretty boiling just stopped completely. So I dropped the vacuum. Nothing. So I took it to 0.05 bar,
and I suddenly got a massive bumping storm, and then it just stopped and was still. Then I tried again. Same thing. Everytime I dropped the vacuum,
it would flash, and then it would be still, with no signs of even the teensiest boiling.
I was very irritated at this point, and cared about nothing else except finding the conditions to make a continous controlled boil. I finally did
manage to take it to the vacuum pump's limit of 0.02 bar, with casualties being bumps so violent that the collection flask was half full of dirty DMSO
 However even then, the disillation flask was as still as a stagnant pond 
Any suggestions/comments as to what went wrong and how to fix it?
Should I use a capillary tube instead of mag stirring? Should I pull the vacuum and THEN turn on the heat?
Anyone with vacuum distillation experience on a large scale? Before this, I've never vac distilled more than 100 ml.
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DJF90
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Vacuum ALWAYS before heat. Mag stirring supposedly doesnt work so well under vacuum, so ebulliation ought to be much better. If you have a rotavap I
see no reason why your oil bath won't accomodate a 3L flask or so.
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entropy51
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Quote: Originally posted by DJF90  | | Mag stirring supposedly doesnt work so well under vacuum, so ebulliation ought to be much better. |
I always
use magnetic stirring and I've never had problems with bumping, unless a solid formed or the stirrer stopped. When I worked in industry we always
used mag stirring, even at 5 liter scale.
But the sequence is stirring, then vacuum, then heat.
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DJF90
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Sorry entropy, I got a little confused. Its boiling chips that don't work under vacuum. Mag stirring should be fine. I still dont understand why the
rotavap can't be used though?
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Magpie
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Does the rotovap provide enough turbulence?
[Edited on 28-4-2011 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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Saerynide
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| Quote: | | I still dont understand why the rotavap can't be used though? |
| Quote: | | Does the rotovap provide enough turbulence? |
Life would be much easier if I could use the rotary function. However, I don't have enough oil and neither do the neighboring labs. When I did the
50ml yesterday, I was spinning it in a small dish of oil and I had no bumping problems.
So I guess I was kind of daydreaming yesterday, after the 50 ml went so smoothly. Today, staring at my set up and the 10 gallons, I thought about how
if I were to ever process all of it in a finite amount of time, I would have to do 4L batches, and then start up, shutdown, boundary layers and all
that stuff are actually going to matter...
No longer daydreaming, I decided the logical sequence was: start stirring, slowly pull vacuum then keep it as low as it will go with the regulator
(10m bar seems to be the limit over DMSO). Wait for the DMSO to be degassed and for water vapor to be pulled out, When no more bubbles, start the
condenser fridge pump and get it cold, THEN turn on the heat (with a digital controller).
The DMSO started to come over at 109, but very very slowly. No signs of boiling whatsoever. Slowly took the temp to 120 and it started coming over
faster, but nowhere near fast enough IMO. There was an occasional "pop" but nothing violent this time. However, it still wasnt "boiling"
I managed to get 150 ml of water white DMSO from 400 ml before I started to shut down, because it had already been 3 hrs and I wasnt going to wait
another 3 for the remaining meager 150 ml
I did not want to take the temp up much more because of the danger of decomposition at 150.
I also think the reflux ratio is too high (infact, I have absolutely no need for reflux at all, since the dirty DMSO has only a tiny bit of water and
nonvolatile impurities, and I dont care if water comes over with the DMSO). I see more liquid going back into the distillation flask than coming over
into the receiving one. Maybe too much heat is lost on the way to the condenser, but there is vapor condensing halfway up the coils... Should I turn
up the heat? I was afraid that would just make the whole thing take off uncontrollably because I dont really know how fast/much heat the condenser
can take out.
Our lab also only has pasteur pipettes. Will those hold out under a vacuum if I try to make capillaries out of them? I'm kinda scared theyll implode
and shoot into the flask.
My biggest question: So supposedly, DMSO at 10 mbar should boil at around 80C. Why wont it boil even at 120?
I have a feeling the centripetal force of stirring at 600 rpm is holding the liquid together, with no thanks to its viscosity. I think this is why I
get occasional "pops" because the liquid wants to boil and parts of it is boiling at a molecular level, but the vapor cannot travel to the surface.
Eventually enough vapor accumulates and it bumps.
However, yesterday, when I tried slowing the stirring to see if it would help vapor to escape from the liquid, I would get crazy bumping. The
stirring also cut out at one point yesterday and I didn't notice because I was across the lab. When I noticed and put my hand on the dial, it bumped
so violent I hit the vent button. As a result, I was too afraid today to touch the stirring dial incase touching it caused it to cut out
I suppose the only way to create turbulence is with a capillary tube?
[Edited on 4/29/2011 by Saerynide]
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turd
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Holy jeebus... Stop making rocket science out of a simple vacuum distillation. 
1) Stirring - full blast (and appropriately sized stir bar)
2) Vacuum - full blast
3) Heating - full blast
4) Wait
At the beginning of a new fraction it will bump until the whole apparatus is in equilibrium state. For distillations > 250 ml a column is a good
idea. Not only will the separation be better, but it also servers as a bumping protection. A few months back I distilled a litre of glycerol that way
- no problem, it crystallized in the fridge.
Have you dried your DMSO prior to distillation? I guess it will form an azeotrope, though I didn't check.
BTW: That is beginner material and it's not even chemistry.
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Panache
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seriously you're wasting you're time trying to get off the quick way, you'll end up having many stressful hours (i find bumping a very stressful state
of affairs in the lab).
my suggestion is to go to a metal yard get two 9kg gas cylinders, connect them with a very large u of stainless or mild steel, place 5kg dmso in one,
drill a hole ino it and feed in a capillary and thermocouple, then place it on a heat source (gas burner), connect the u, place the other into a
bucket of cold water, drill a hole, connect your vacuum, start heating, if it begins to bump then open the capillary, however i highly doubt it will
bump, those cylinders are rough inside,
If you're clever you bleed in the remaining dmso through the capillary as the distillation progresses. it may seem too involved, but i guarantee it
will work great.
Otherwise just run a capillary into your current setup, viola no bumping.
ftr. i hate dmso, thoroughly unpleasant and mediocre cf with nmp or dmf. Coud your reaction be done in msm instead it' cheap and basically posesses
the same properties as dmso without the god awful stench.
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Saerynide
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| Quote: | Holy jeebus... Stop making rocket science out of a simple vacuum distillation.
...
Have you dried your DMSO prior to distillation? I guess it will form an azeotrope, though I didn't check.
BTW: That is beginner material and it's not even chemistry. |
I think you misunderstand. I'm not *trying* to make rocket science out of it. If it were that easy and it actually worked the way it normally does,
I wouldn't be asking these questions and my lab wouldnt have already given up - leaving me the only one still going "I must save the Earth!!!". I've
stripped off tons of stuff on a smaller scale both with and without a rotavap, but this stuff seems to have a mind of its own. Perhaps the organic
polymers in the DMSO is wreaking havoc. Mind you, the feedstock is practically black (but it is still at least 95% DMSO, with the other 5% being mostly water, some base and organic crud)
I have not dried the DMSO because we don't care if theres a bit of water in it. In the future, as the DMSO picks up more and more water, I will first
distill at a lower temperature and throw away that fraction.
There are no known azeotropes of DMSO.
And I never said this was chemistry. We are engineers of various sorts and some are bio - no one in our lab is a chemist.
| Quote: Originally posted by Panache | If you're clever you bleed in the remaining dmso through the capillary as the distillation progresses. it may seem too involved, but i guarantee it
will work great.
Otherwise just run a capillary into your current setup, viola no bumping.
ftr. i hate dmso, thoroughly unpleasant and mediocre cf with nmp or dmf. Coud your reaction be done in msm instead it' cheap and basically posesses
the same properties as dmso without the god awful stench. |
My thoughts exactly! My plan for the next step (once I got the bumping issue straight) was to set up a semi-continuous process where DMSO would drip
into the system without breaking the vacuum.
I think on Monday I'm going to try adding something for bump protection (though it will be hard because there isnt much space) and a capillary and
then if that goes well, add a giant sepfunnel to drip in feedstock.
I really hate DMSO too. It reeks, it's oily and sticks to every damn thing it touches and makes everything slippery. However, I did not come up with
the reaction and my team is hell bent on sticking with DMSO
I'm just doing my part to help save the environment here.
Btw, there's no way in hell our institution would let anyone have a hacked-together setup made from scrap If the safety people saw something like that, my butt would be out the door before I could say hocus-pocus.
[Edit]: Maybe I should scrap trying to use the condenser in the rotavap and hack together a separate distillation set up. Then space and orientation
will no longer be an issue and I can let it bump to hells end with a column... However, the vac regulator and the controlled cooling is hooked to the
rotavap and my lab probably wouldnt appreciate it if I removed them
[Edited on 4/30/2011 by Saerynide]
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turd
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| Quote: Originally posted by Saerynide | | [Edit]: Maybe I should scrap trying to use the condenser in the rotavap and hack together a separate distillation set up. |
Definitely. Since you are apparently doing this for your employer, they should give you a distillation bridge that is useful for the
case at hand. With the right gear this should be no problem at all. The flask must be big enough and the heating and stirring strong enough.
It's none of my business and I don't know what your job description is, but it seems very strange that scientific stuff would spend their time
recycling solvents! Usually they only do that when they need ultra-dry/clean solvents and don't trust the lab assistant. What does a drum of fresh
DMSO cost compared to the time it takes you to distill one? The first should be much cheaper.
| Quote: | | However, the vac regulator and the controlled cooling is hooked to the rotavap and my lab probably wouldnt appreciate it if I removed them
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They should provide you with what you need for your work!
| Quote: | | There are no known azeotropes of DMSO. |
That's excellent to know. Next time I need dry DMSO, I will
fractionate it, like I did for glycerol.
| Quote: | | I really hate DMSO too. It reeks, it's oily and sticks to every damn thing it touches and makes everything slippery. |
I see how one can not like DMSO, like all the other polar aprotic solvents. What I dislike most about it: it is not really inert,
like I'd expect it from a solvent. But the smell? Don't you like garlic, the spice of the gods? Very suspicious...
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Magpie
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It might be useful to take a small amount of your feedstock, say 250 mL, and vacuum distill it with a capillary bleed tube. This will reveal the
vacuum and temperature required for the full scale distillation, once you get adequate nucleation/turbulence, of course.
You would need to know the costs of waste disposal if junking the DMSO were an option. But once the group has decided to go green, be politically
correct, etc, that option is off the table.
The single most important condition for a successful synthesis is good mixing - Nicodem
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turd
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If they decided to go green & politically correct, then they also have to buy the necessary stills (typically one per solvent) and personal, or
more realistically contract a certified "green" waste disposal company. They cannot expect their scientific stuff to distill a drum of DMSO through a
roti!
If there's no heavy metals and/or halogenated hydrocarbons in the DMSO, I don't think disposal / recovery should be very expensive?
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smuv
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How big of a stir-bar are you using? Your stirring is probably ineffective. You need a seriously large stir-bar at this scale.
Also, wet solvents bump more, especially when there is a large difference in bp between the water and the base solvent.
And lower pressures promote more bumping (b/c difference between vapor and liquid volume becomes greater).
Finally, you can make an oil bath from a hot plate and a glass or metal container, just b/c it doesn't say BUCCI on the side, doesn't mean you can't
sit it next to your rotovap  .
Edit: also, pipettes to bleed in gas are passe, good stirring will always provide better agitation than this and a rotovap even better. But sure a
standard vwr pipette will take the vacuum, it has such a low surface area the thickness of the glass becomes moot. Also, if it yielded it would
explode, not implode (so you can easily test the pipette on your argon or nitrogen line at 15psi to check its suitability if you are still weary).
[Edited on 4-30-2011 by smuv]
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Saerynide
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So recovering the DMSO was my idea actually. My group normally junks all of it, but we junk so much just to make a gram or two of product and I feel
really really terrible about it. I just can't imagine dumping 10 4L bottles of high quality solvent every other week! 
And though it may be more economical to pay waste disposal for the DMSO than to recover it, it its definately not more economical to buy new DMSO than
to recover it, monetary-wise.
Though my time spent on recovering it may be an issue, which is why I was hoping we could set up a 3-4L batch that could be easily kept at steadystate
distilling away all day in the fume hood and only have someone peek on it once every few hrs, and at the end of the day, throw out the bottoms.
But now that I think about it, maybe junking it is the only viable option... Or
maybe I should suggest we buy a proper solvent still...
None of our labs have lab techs/assistants. The researchers do everything, even autoclaving the disgusting biohazard waste. Gross -_-
smuv:
Ah! That must be it! It must be bumping like crazy because its wet.
We definately sit the oil bath on a hotplate My group would freak if I even suggested filling the rotavap bath with oil
When I ran the 400 ml in the 1 L flask I used a 1nch stir bar because bigger ones tend to not work well in the round bottom and also go out of sync
with the magnet (and that would be really bad in this case).
I'll test the glass pipettes on the nitrogen line. And youre right, they would explode, not implode, because they would be used "inside out"
Magpie: I'll definately try with the small scale first with the capillary.
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Magpie
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This may be useful:
http://www.gaylordchemical.com/bulletins/bulletin107b/Bullet...
The single most important condition for a successful synthesis is good mixing - Nicodem
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smuv
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You need a 2-3 inch stirbar for a 5L flask. You may not be able to get as high stir rpms, but even at lower speeds, stirring will be more efficient
(greater displacement of fluid per revolution).
Also for larger flasks standard flat stirbars work better than you may have experienced with small flasks because the incline is less steep (I hope
you understand what I am saying).
Too bad though about the oil bath, an oil dedicated rotovap bath can be very useful.
[Edited on 5-1-2011 by smuv]
"Titanium tetrachloride…You sly temptress." --Walter Bishop
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Eclectic
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Try dipropylene glycol as your heating bath fluid and a capillary tube or stainless steel bubbling stone to initiate boiling?
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Saerynide
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| Quote: Originally posted by smuv | You need a 2-3 inch stirbar for a 5L flask. You may not be able to get as high stir rpms, but even at lower speeds, stirring will be more efficient
(greater displacement of fluid per revolution).
Also for larger flasks standard flat stirbars work better than you may have experienced with small flasks because the incline is less steep (I hope
you understand what I am saying).
|
I cant even get a 2 inch stirbar to sync properly in a 4L flat bottom vessel of water At 100 rpm, it syncs ok, but 4L of water barely moves. At 200 rpm, I will usually come back to find it rattling
around (water that is, not distillation! lol) I haven't tried in a round vessel, but it doesnt look hopeful lol.
Magpie: I've checked out all their bulletins before starting on this task. Lots of useful info, especially the big manual with all the VLE data and
other things. They seem to make distillation/recovery sound like such an easy task! I'm getting irritated it isnt coming along so easily for us
Eclectic: Whats an bubbling stone? How does it work under vacuum?
[Edited on 5/1/2011 by Saerynide]
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GreenD
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Yeah with DMSO you can't really use large stir bars - the magnetic strength just isn't enough. If you have an over-head stir apparatus that may have
to be your option...
Otherwise, dry the solution first - like they said earlier. If you have condensation wrap the entire thing in aluminum foil up to the condensor
(obviously not on the condensor).
I didn't understand that DMSO bulletin. It basically said do distallation 3 x but used 3 different words for it.
"Evaporate"
"Concentrate"
"Distill"
well no shit.
If you are getting temps & pressures past what you expect it to boil with and no boiling it may be either that you have reached a critical (not
the correct term) concentration - where the bp is maxed out in a solution of water/dmso. OR your gauge may be off or your overall system may be
leaking.
When you set up the distillation make sure you use grease on your glass joints to prohibit vacuum loss - this is crucial in my experience.
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tmb
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Hi. DMSO can be purified by fractional crystallization.
Slowly freeze ca. 80% and discard the liquid (or further crystallize a lower-purity 'second crop').
There is a good article here: www.iupac.org/publications/pac/25/2/0457/pdf/
Water lowers the freezing point towards 0C (for 8% water content), so remove this beforehand with 4A molecular sieves, if it is a large contaminant.
Good luck.
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Saerynide
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GreenD: The bulletin is written for ChemEs, which do things very differently from chem*ists* because we handle everything continuously.
Concentration: Water is removed FROM the feedstock, keeping the bottoms.
1) heat and drive off water, while in the process losing much DMSO to vapor, OR
2) Stripping column, no reflux, to enrich the feedstock in DMSO, OR
3) Fractionally distill off water
All three are different because 1 is just boiling (no plates, no equilibrium stages, nothing) 2 is the bottom half of a distillation column, and 3 (in
a continuous distillation) typically includes rectifying, stripping and reflux
Evaporation: Evaporate out DMSO and dissolved volatiles (bottoms product from concentration step) from nonvolatiles, keeping the tops.
Distillation: Separate water/other volatiles and DMSO from evaporation product
So first you must remove the bulk of the water (if your feedstock is say 10% DMSO only). Then you must remove the nonvolatiles by evaporating the
volatiles (DMSO and water and maybe some other things). Then finally, you must separate the volatiles to get pure DMSO
Continuous distillation is much different from batch distillation.
http://en.wikipedia.org/wiki/Continuous_distillation
[Edited on 5/1/2011 by Saerynide]
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