DocX
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Vacuum filtration sucking fluid
So I'm using ny new vacuum pump to vacuum filter some newly synthesised lovely salts.
Problem is, when I run it, the fluid gets sucked into the vacuum hose and into the pump. I hate that.
I use a Buchner connected to a conical flask. Nothing fancy. I don't get why the fluid travels upwards to the hose connector instead of down into the
flask. Anyone had this happen? Is there an easily preventable cause?
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Gammatron
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Flask too small? You could have a 2nd flask as a water trap. Also I have found that because of vacuum lowing the boiling temp of water it will
sometimes condense in the tubing. This was a big problem when I was using an oil filled rotary vane pump but now I use an oilless piston pump and it
handles it a lot better.
[Edited on 10-16-2022 by Gammatron]
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Tsjerk
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Can you post a picture of your setup? This should be an easy fix when we get a view.
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unionised
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Vacuum pumps are designed to suck fluids.
A fluid is something which will flow.
They are not designed to suck liquids or foams.
That trap will work even better to protect your pump if you add a bit of tubing inside the flask. (apologies for the awful drawing of a tube.)
On the other had, doing that makes the trap less good as a "suck back" preventer.
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DocX
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Sure, I'll post one as soon as I get the chance. But it's not a very complicated setup. A Buchner like the one below connected to a 1- liter conical.
And the Buchner funnel connected to a small vacuum pump.
I know a trap would solve the problem, but if possible I would prefer not to complicate the setup.
Although I guess the problem is foam. Liquid being sucked through the porous plate that foams and gets sucked up into the vacuum tube.
[Edited on 20222222/10/16 by DocX]
[Edited on 20222222/10/16 by DocX]
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Gammatron
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Quote: Originally posted by unionised  |
That trap will work even better to protect your pump if you add a bit of tubing inside the flask. (apologies for the awful drawing of a tube.)
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Yeah I noticed that, water could easily get sucked up. I found the pic on google and it worked to illustrate what I was saying, I just use a pipette
through a stopper, not sure how you could attach a tube to the inside of the vacuum port.
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DocX
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But maybe I could attach the hose to the exit tube from the Buchner funnel? And make it so the inflow to the vacuum bottle is further from the vacuum
outlet? That way foam shouldn't travel so easily into the vacuum hose.
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Tsjerk
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Although foam should fall to the bottom without an extra tube, if it doesn't a tube would probably be a fix indeed.
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DocX
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Well, SOMETHING makes fluid enter the vacuum hose. I'll try the extra tube thang (next time I'm doing a filtration) and get back. With pictures.
[Edited on 20222222/10/17 by DocX]
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Texium
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Sounds like you’re using a lot more vacuum than you need for the job. If your only vacuum source is a powerful vacuum pump, I’d recommend
installing a bleed valve so you can adjust the level of vacuum being pulled. Full vacuum for distillations of high boiling stuff and residual solvent
removal, lower vacuum for filtration. Regardless, putting a trap in line to protect your pump (preferably chilled) is ALWAYS a good idea, no matter
what you’re using it for.
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DocX
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Yes, I figured that also. I haven't had time to buy or add a valve though, so just adding a longer outlet from the Büchner funnel is way easier.
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Sulaiman
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A long tube will reduce airflow when there is air to flow
(eg air drying a filter cake)
But when filtering there is negligible airflow,
so full vacuum will be applied.
(boiling/frothing/foaming likely)
CAUTION : Hobby Chemist, not Professional or even Amateur
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DocX
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Sorry, no picture, but the extra bit of tubing attached to the outlet of the Buchner did the trick. The real problem is of course that I'm using
unnecessary strong vacuum, like Texium figured, sucking froth and fluid right into the vacuum pump inlet. But the quick fix did the trick. I
will post a picture, just haven't gotten around to it yet.
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