malford
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Distillation Question
Let's say I have an Erlenmeyer flask to receive a distilled compound. I will distill under vacuum of -18.6 in. Hg. To apply the vacuum, I have a
vacuum adapter fitted to the top of the aforementioned flask. This has a 10mm serrated barb going to the vacuum system and it has a long drip tube
going down into the flask.
My logic is this: to ensure that all of the compound is condensed and none is sucked out through the vacuum system, rather than pulling the vacuum
right after the condenser, I'll allow the vapor to travel down the vacuum adapter's long, narrow tube into the flask then expand into the large,
ice-cold flask to ensure condensation.
My question is: what will happen once enough distilled product fills the flask to the bottom of the drip tube? Would it be problematic for the tube to
be below the level of the distilled liquid that has been collected? Theoretically, no vapors should be left at that point, but any that are, should be
pulled through the liquid which will be very cold and can assist condensation.
Vacuum adapter: http://www.chemglass.com/product_view.asp?pnr=CG-1049-E
[Edited on 18-8-2013 by malford]
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blogfast25
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What precisely do you mean by that? 18.6" Hg is 472 mm Hg [edited value]. Why the minus sign? Do you mean actual pressure in the system = 760 mm
(atmospheric pressure) - 472 mm = 288 mm Hg? 288/25.4 = 11.3" Hg
Or do you mean actual pressure in the system = 47.2 mm Hg ((1 - 47.2/760) * 100 = 94 % vacuum)?
[Edited on 18-8-2013 by blogfast25]
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BromicAcid
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That is essentially the principle behind the vacuum trap, vapors are directed to the bottom of the trap immersed in dry ice or something similar and
then by giving them a long path up the side of the flask they condense. The issue as you say however is once you get your liquid level to the drip
tip. Distillation will continue as the vacuum will continue to draw on the system but if you release the vacuum from the direction of the vacuum pump
then your liquid will be forced up the tube and back to your distillation pot. Releasing from the distillation pot side will eliminate the
possibility of suck-back. Be aware that most Erlenmeyer flasks are not designed to be used under vacuum however I have yet to see one fail (knock on
wood).
When we do a bulk strip of solvent at work we will collect solvent in a similar manner, usually once the liquid level is above the tube the
distillation will speed up as the cold liquid is better able to condense the vapors and thus less solvent vapor makes its way through which would
otherwise work against the vacuum.
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BromicAcid
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I thought the OP meant that atmospheric pressure is 29.92 inches mercury so that he would be distilling at 11.32 inches Hg (18.6 inches less than
atmospheric) or about 288 mm Hg. Though I must confess to not caring much about whatever pressure he was at only the situation he presented for the
distillation.
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blogfast25
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@Bromoc:
Yep, I got my inches wrong (1" = 25.4 mm, NOT 2.54 mm).
11.32" absolute pressure is what he means.
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malford
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Quote: Originally posted by BromicAcid  | | usually once the liquid level is above the tube the distillation will speed up as the cold liquid is better able to condense the vapors and thus less
solvent vapor makes its way through which would otherwise work against the vacuum. |
Bingo. So, as I suspected, taking this to the extreme of having the drip tube reach the bottom of the receiving flask would be ideal because the
ice-cold liquid distillation product (as such because the flask is submerged in an ice bath) aids condensation.
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