milhouse
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Simple distillation question
I am distilling DCM from paint stripper and I have read that DCM boils at 41º Celsius.
I have one thermometer in the paint stripper that reads 46.5º Celsius
I also have a thermometer on top of the three way adapter that reads 36º Celsius
Should I try to get the top thermometer to 41º Celsius? Or is it the temp of the paint stripper that I should try to contain to 41º?
The way that I am doing it now is (( 36 + 46.5 ) / 2) = 41.25 and I have DCM coming over.
If you can help me clear this up it would be greatly appreciated.
[Edited on 9-10-2007 by milhouse]
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Magpie
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I've separated DCM from paint stripper before. There are thread(s) on this. My stripper had two solvents: DCM and methanol. There is also a
non-volatile polymer gunk.
The temperature in the pot is not important.
The thermometer indication in the 3-way adapter is important. You should make your cuts when the this temperature is near (e.g., plus or minus a
degree) the boiling point of solvent you wish to capture. The top of the thermometer bulb should just touch a line extended from the inside bottom
of the 3-way joint condenser tube. If this is not clear refer to any basic organic lab manual. Hope this helps.
[Edited on by Magpie]
2nd edit: Corrected some errors. Have provided a better account in my next post below.
[Edited on by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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milhouse
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Yes, thank you!
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Magpie
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I have to apologize for trying to recount my experience from memory. It appears that I just used a "simple" distillation (no fractionation column).
From my notebook:
Charged 250mL of JASCO "Premium Paint & Epoxy Remover" into a 500 mL RBF. This is a "semi-paste" supposedly 80+% DCM, 15% methanol, and the
remainder white polymer. Set-up for simple distillation with ice-water for condenser coolant & ice-bath for receiving flask. Collected 200 mL at
T= 36-39C.
2nd batch: T=37-40.5C.
Washed product twice with 1.5-2 volumes of distilled water. Dried with Na2SO4 overnight. Decanted off DCM and dried over CaCl2. Filtered this and
redistilled. T=38-39C. Measured sp. gr. at 1.32.
The single most important condition for a successful synthesis is good mixing - Nicodem
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Sauron
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Your two volatile components have boiling points about 20 degrees apart.
To get a decent seperation by simple distillation you will need a delta of 100 degrees.
So what you achieve this way is DCM with less methanol than before. But still impure.
Maybe after 2-3 distillations, the MeOH might be low enough to use molecular sieve to remove it.
But why not just use a column, slow down, and cleanly seperate in one distillation, by doing it as fractionation? A twenty degree delta is plenty
enough for that.
Unless of course there's a pesky binary azeotrope.
Sic gorgeamus a los subjectatus nunc.
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Antwain
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It all comes down to the reflux ratio. For such low boiling solvents I would agree that you would probably not get pure DCM from one distillation, but
if you boil it slowly you can probably do pretty well.
To remove methanol mix with water, keep the bottom fraction, wash again. Then dry with mag sulfate or similar and distill. Pure DCM (if DCM and
methanol was all that was in there).
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Magpie
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Now that I've looked at my notes a little more carefully I see that there is a "pesky azeotrope" of composition 84% DCM/16% methanol, bp = 37.8C,
according to Gmeling's Azeotropic Data. So it looks like the distillation was just a way to separate the solvents from the polymerous gunk.
But you can't argue with my final sp. gr. of 1.32. This speaks of nearly pure DCM. Sp. gr. of methanol is 0.79. The water washes and the subsequent
drying with Na2SO4 and CaCl2 must have been effective in removing the methanol.
The single most important condition for a successful synthesis is good mixing - Nicodem
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Sauron
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True. But you could have done the drying first and achieved the same removal of the methanol, then distilled to remove the polymer gunk.
Either way it was the drying agent that achieved the seperation rather than the distillation, obviously a distillation cannot break an azeotrope.
Anyway well done.
Sic gorgeamus a los subjectatus nunc.
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Antwain
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Electricity aside, I would probably distill the solvent away from the crud first.
polymer + water ---> probably easy enough to clean.
polymer + drying agent ---> sticky mess.
If not before the distillation, then certainly after when you have something which cannot dissolve properly in polar or non polar solvents.
I think I was sleep deprived when I wrote this. Cleaned it up
[Edited on 10-10-2007 by Antwain]
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milhouse
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Thank you all for the help.
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