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.

Quote: Originally posted by DJF90

Mag stirring supposedly doesnt work so well under vacuum, so ebulliation ought to be much better.

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]

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.

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]

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.

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...

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]

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.

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]