11
$\begingroup$

I am preparing to do a vacuum distillation and am unsure exactly how to manage the vacuum. The vacuum pump I have is a two-stage rotary vane pump which is capable of 250 microns or lower. It has a triple inlet, one 1/4", 3/8" and 1/2".

My glassware is typical distillation glassware including a Graham condenser and a vacuum takeoff joint that connects between the condenser and the receiver.

My concern is that if I apply too much vacuum that it will cause a glass implosion. Is this not a concern and I can just operate the vacuum pump freely, or do I need to take special steps to limit the degree of vacuum?

I watched a YouTube video of a guy doing a vacuum distillation and I noticed that he did not use a capillary bubbler. Under what circumstances can the bubbler be dispensed with?

Another concern is the need for a manometer. I have read that I need to use a closed-end manometer (very expensive) to measure the pressure in a vacuum distillation. Why can't I use a normal vacuum gauge? Also, if I am doing an azeotropic separation such as a water-ethanol separation, is it critical I monitor the pressure? I am assuming it is.

$\endgroup$
  • $\begingroup$ @Jan I do not have enough reputation to create new tags. There is no distillation tag. There is no vacuum tag even. Pretty lame that a forum supposedly about chemistry has no tag for distillation. $\endgroup$ – Shaka Boom Jun 14 '15 at 22:16
  • 2
    $\begingroup$ There is an experimental-chemistry which would have fitted your question a lot better. $\endgroup$ – Jan Jun 14 '15 at 22:25
  • $\begingroup$ There is a safety tag, which fits too. $\endgroup$ – Geoff Hutchison Jun 14 '15 at 23:43
  • $\begingroup$ Currently, the tag distillation currently only refers to this question. We just graduated not so long ago, so it is not that unusual that we do not yet have every tag that we might need as a community. There are 44 questions to which this tag could probably be applied, so it looks like a tag we might need. However, we do not need duplicate tags. If all questions tagged distillation also share another tag, then we may not need the new tag. $\endgroup$ – Ben Norris Jun 15 '15 at 0:58
  • $\begingroup$ @Ben Norris: I suppose your reasoning is technically correct, but I do think distillation is an essential chemistry tag, and I was surprised to hear we did not have this tag, which is why I added it. $\endgroup$ – user467 Jun 15 '15 at 1:27
8
$\begingroup$

This pump is way overkill for a vacuum distillation. The problem is not glass implosion necessarily (round bottom jointed glassware is designed to be put under vacuum, less clear on the condenser), but rather that you may not be able to condense your product given the low vacuum. Vacuum distillation is typically done with an aspirator that gets down to 25 Torr (25,000 microns) or so. At too low a pressure, you may need a very cold receiving vessel and condenser, and depending upon the solvent dry ice slush may not even be enough. You need to determine the boiling point of your solvent at low pressures to figure this out. Your pump is for Schlenk lines and similar applications.

I don't see a reason not to use a vacuum gauge (and you are not supposed to use mercury anymore in any application), though again, depending upon the setup, it is possible to harm the gauge. You should try to monitor pressure as part of the experimental process.

Not using the capillary bubbler seems foolish, as bumping is a major problem with vacuum distillations. A capillary bubbler is a very thin tube (like when you draw glass tubing after melting it) that goes into one joint of the stillpot (drawn end in the reaction mixture). When vacuum is applied, air is forced through the tube, but because the drawn end is so narrow it only allows small bubbles into the mixture. These take the place of boiling chips which will not form bubbles as all gases are extracted from the chips via the vacuum.

$\endgroup$
  • $\begingroup$ Right, I have been figuring that out. I either have to get a "dry" vacuum pump with a regulator, or else use a vacuum regulator with my existing pump. The problem there is that a 3 CFM vacuum regulator is pretty expensive. $\endgroup$ – Shaka Boom Jun 15 '15 at 0:02
  • $\begingroup$ How about a water aspirator? Cheap! $\endgroup$ – user467 Jun 15 '15 at 0:04
  • $\begingroup$ Just looked up the 3 CFM, and unfortunately this is probably what you need for your overkill pump. You need a weaker pump! I don't know where you are working, but if you are in an educational or professional lab, do you have access to house vacuum? That tends to be low enough for vacuum distillation. $\endgroup$ – user467 Jun 15 '15 at 0:17
  • $\begingroup$ @ShakaBoom: If this answer is sufficient, could I ask you to accept it? Thanks in advance! $\endgroup$ – user467 Jun 22 '15 at 0:15
  • $\begingroup$ @trb456: Could you clarify what a capillary bubbler is and how it should be configured in a vacuum distillation apparatus? (Or address the same question here?) I can't find any component by that name, though I know some bump countermeasure is required for vacuum distillation! $\endgroup$ – feetwet Sep 13 '15 at 3:04
3
$\begingroup$

If you can, use a digital vacuum guage. I never use a bubbler as it can draw air in and introduce oxygen into your material as a result. Not ideal. You could try and use nitrogen through the capillary, but trying to manage the flow balanced with the vacuum drawing it in is a pain. Plant vessels never use bubblers and they distil under vacuum just fine.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.