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Author: Subject: changing flasks during vacuum distillation (no cow, etc)?
EmmisonJ
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[*] posted on 15-11-2009 at 11:30
changing flasks during vacuum distillation (no cow, etc)?


i'm no stranger to distillations however i'm about to attempt my first vacuum distillation so this is a whole new ballgame to me. i have a 2-stage vacuum pump rated at 15 microns

i will need to change flasks halfway through distillation and don't have a cow/pig. i was planning on just leaving the distillation going as-is (keep vacuum on, keep heat on, etc) and just disconnect the vacuum hose from the vacuum pump, quickly change out flasks (to avoid superheating), and reconnect the vacuum tubing to the vacuum pump. my only worry about this is that i've read there is a risk of implosion due to drastic changes in pressure (the apparatus going from vacuum to atmospheric too quickly?).

my vacuum pump has a gas ballast valve and i thought of opening that up fully before disconnecting to at least reduce vacuum a little bit. a friend of mine basically does his vac distillations this way and has never had a problem but uses a cheaper vac pump that is probably more equivalent to an aspirator, should i be worried? in the long-run i will be getting a 3-way adapter that connects to the vacuum pump so that i can bleed in pressure slowly, etc, but no local hardware stores have the parts i need in the threading size that i need so that will have to be put on the back-burner for the moment.

[Edited on 15-11-2009 by EmmisonJ]
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Arrhenius
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[*] posted on 15-11-2009 at 12:29


I've never seen an apparatus implode. Atmospheric pressure (outside flask) just isn't enough to break a flask under any vacuum.

There are a few ways people work around the issues you've described. 1.) use a cow 2.) use a distillation apparatus with stopcocks which isolate the receiver.

I would recommend you briefly stop heating the boiling flask while you swap the receiver. As soon as you break the vacuum, the boiling flask begins to heat up, and when you pull a vacuum again it will almost certainly bump. Same reasoning why you pull a vacuum first, then start heating at the very beginning. Another decent idea is to install some sort of valve that will allow you to graduate re-apply the vacuum (still, would remove heating on boiling flask, even if it takes you 10 seconds to change receiver).

Many 'gas ballast' devices don't really break the vacuum all that much, so I wouldn't recommend that. While you're building the 3-way valve, you should build a cold finger too.
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aonomus
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[*] posted on 15-11-2009 at 13:23


Big discussion regarding this topic here: http://www.sciencemadness.org/talk/viewthread.php?tid=4284
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EmmisonJ
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[*] posted on 15-11-2009 at 17:13


ah there's the post i was looking for. i tried searching and didn't manage to find it.

thanks Arrhenius for the advice, i'll just give it a go as-is for now and try to carefully disconnect the vacuum tubing while in operation. you were really helpful, thanks!
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anotheronebitesthedust
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[*] posted on 15-11-2009 at 22:43


In the past I've simply pulled the hose off of the adapter, while the pump is still running and the distilling flask was hot. I really had no idea there was a correct procedure. In the future I am going to try to get ahold of the cow adapter, and until then I will always at least turn off the heat source as well as the actual pump and let it sit for 15 minutes before pulling off the hose.
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