Chemgineer
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Reasonable choice of vacuum pump for distillation?
I have a few of those small diaphragm pumps which I use for filtrations, they work quite well but I was wondering if there is a better choice if I
want to run it for much longer for a vacuum distillation?
I've seen some of the liquid ring pumps available with vacuum chambers etc. perhaps one of those would be a good choice? I have some concerns about
damage to pump internals by any gasses or liquids that may carry over though.
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Sulaiman
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I see three general options ;
1) a water-powered aspirator.
Not too expensive, simple setup.
1.1) some members have implemented a closed-loop water recirculation pump system
2) a (relatively) cheap Chinese HVAC rotary vane pump, with frequent oil changes
2.1) take reasonable precautions such as gas-washing and anti-suckback traps,
but overall consider as 'semi-disposable' (a couple of years)
3) 'Western' (expensive) pump, accurate pressure measurement and control, cryogenic traps etc.
PS I have used cheap 12 Vdc pumps continuously for days - more reliable than expected.
[Edited on 12-9-2022 by Sulaiman]
CAUTION : Hobby Chemist, not Professional or even Amateur
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teodor
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Do you need to control the temperature during distillation? The precision of temperature measurement is dependent on the precision of pressure
measurement. If you have no device which is sensitive enough to control the pressure in such a way that the corresponding temperature change will be
in the range of 1-2 degrees you are unable to collect fractions based on the temperature.
The higher vacuum made by electrical pumps compared to a water aspirator requires a more precious manometer (the same absolute change of pressure at a
high vacuum results in higher temperature change compared to a lower vacuum). So, for the purpose of vacuum distillation, I would choose the level of
vacuum pressure created by the pump based on the manometer sensitivity.
[Edited on 12-9-2022 by teodor]
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Rainwater
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Assuming the apparatus is truly sealed and vacuum tight.
Once the vacuum is formed, will sealing the system prevent boiling point changes?
[Edited on 13-9-2022 by Rainwater]
"You can't do that" - challenge accepted
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teodor
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Usually, to break the surface tension of liquid which prevents normal boiling under vacuum (the "bumping" effect), a fine stream of air or inert gas
is introduced through a capillary tube during vacuum distillation for the purpose of creating an additional pressure that breaks the surface tension
allowing vapors to escape the liquid.
This stream is tiny compared to a vacuum pump suction so it doesn't change the vacuum.
Without this stream of gas, the boiling point as determined by a thermometer will be different from a valid boiling point under a vacuum because there
are 2 forces that prevent the vapor to escape in this case: the pressure and the surface tension.
So, there is no way to break surface tension in a "truly sealed" apparatus so the boiling point will depend on surface tension.
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Lionel Spanner
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If it's a low-viscosity liquid with a low amount of suspended solids, vigorous stirring with a magnetic stirrer will break the surface as well.
However this is not generally achievable in a Claisen flask, as the base comes to a point.
Industrial chemist rediscovering the practical pleasures of pure chemistry.
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