Sciencemadness Discussion Board

Separating DCM and methanol

Runningbear - 11-10-2009 at 02:29

If you have a mixture of DCM and methanol and you plan to separate them using fractional distillation, could you add anything to the mixture to help make the separation cleaner? (assuming you didn't plan to save the methanol). Or is the temp difference enough to make the fractions very clean.

thanks in advance,

Runningbear

JohnWW - 11-10-2009 at 02:50

Have you looked on Google for any data suggesting whether DCM and methanol form an azeotrope?

P.S.: They DO, unfortunately, form an azeotrope! See http://www.solvent--recycling.com/azeotrope_1.html , which says:
Component A: Dichloromethane
Component B: Methanol
Boiling Point A: 40 °C
Boiling Point B: 64.5 °C
Azeotrope Boiling Point: 37.8 °C
Azeotrope Wt.% A: 92.7 %

This means that it is a constant-minimum-boiling-point azeotrope, like water-ethanol, preventing further purification by only distillation beyond 92.7% dichloromethane.

[Edited on 11-10-09 by JohnWW]

entropy51 - 11-10-2009 at 09:47

Quote: Originally posted by Runningbear  
If you have a mixture of DCM and methanol and you plan to separate them using fractional distillation, could you add anything to the mixture to help make the separation cleaner? (assuming you didn't plan to save the methanol). Or is the temp difference enough to make the fractions very clean.

thanks in advance,

Runningbear


Since you don't want the methanol, why not just wash the methanol out with water? Doesn't get much cleaner than that. You can then dry the DCM and distill it.

There's at least one thread on this, in one of which Magpie described his method. Best to look at it.

Runningbear - 11-10-2009 at 15:29

thanks guys,

I have tried the wash with water Entrophy, but there is some type of emulsifier present that is creating three layers, the middle layer being this messy soap like mixture. It's reducing the yield on the DCM from 87% to about 20%.

I thought there might be another option but I may have to go back and experiment with different wash techniques.

thanks,

Runningbear

entropy51 - 11-10-2009 at 15:51

Runningbear, that's the reason I suggested consulting that other thread. You said CH3OH and CH2Cl2, no mention of other junk being present. I thought about mentioning it, but didn't. Sorry, I should have known better, but took you literally.

Do a quick distillation so that both CH3OH and CH2Cl2 distill over. Don't distill too close to dryness, or the other crap will mess up your flask. (You could even try putting a little mineral oil in the pot to keep the junk from sticking to the distillation flask, just an idea, I haven't tried it.)

Wash the distillate, now free of emulsifier with water to remove the CH3OH.

mr.crow - 11-10-2009 at 17:42

How much methanol should you put back in when its distilled? To prevent it from turning into phosgene

So its

1) Crude distill
2) wash with water to get rid of ~10% methanol
3) dry with CaCl2
4) fractional distill

entropy51 - 11-10-2009 at 18:08

Quote: Originally posted by mr.crow  
How much methanol should you put back in when its distilled? To prevent it from turning into phosgene

So its

1) Crude distill
2) wash with water to get rid of ~10% methanol
3) dry with CaCl2
4) fractional distill


CH2Cl2 won't turn into phosgene when distilled, with or without CH3OH. Just don't distill to dryness (not ever, not anything).

Yes, that's a procedure that works for some of the commercial mixtures, but it might depend on what else is in the mix.

mr.crow - 11-10-2009 at 18:37

Oh I mean putting methanol in for storage, sry

entropy51 - 11-10-2009 at 18:49

Quote: Originally posted by mr.crow  
Oh I mean putting methanol in for storage, sry


Sorry, I misunderstood. I've put ethanol in CHCl3 to prevent COCl2 formation, but I've never heard of needing in in CH2Cl2. Maybe I'm missing something. I think even CHCl3 may not really need it if kept tightly closed, cool and dark, but I do it anyways. I guess it couldn't hurt, but I would use very little, certainly less than 1%.

Runningbear - 11-10-2009 at 19:19

thanks entrophy. The answer is kind of obvious when you spell it out like that;)

Runningbear