cabal
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Which pump when working with flammable gas?
I'm looking for some safety guidance on upcoming hydrogenation experiments.
I'll be evacuating the reaction vessel with my membrane vacuum pump and then flood the vessel with H2 at 1 atm. The paper I'm using for reference
suggests to then evacuate the vessel a second time before flooding it again with H2.
As H2 and oxygen can create a flammable and potentially explosive mixture, I'd like to know whether it's safe to use a membrane pump to evacuate the
hydrogen-flooded vessel.
I'm using only a 300 ml vessel so I'm not concerned about my personal safety as much as I'm concerned about damaging my pump.
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DJF90
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Water aspirator
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cabal
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Will a water aspirator be just generally be safer or is it an absolute must when working with flammable gases?
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Elemental Phosphorus
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It will just generally be safer, but a lot of people tend to like aspirators for their relative simplicity, and when using a glass aspirator there is
little risk of damaging your pump with anything corrosive. You would likely be fine using a membrane pump, but a glass water aspirator is a good
investment since they are not too expensive and find good use when working with corrosive gases.
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DJF90
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Don't use a diaphragm/membrane pump as part of the pumping cycle involves compression, which is best avoided with flammable gas/air(or oxygen)
mixtures
[Edited on 1-9-2017 by DJF90]
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cabal
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Fantastic. Thank you both for your input. I was hesitant to use the water aspirator as my tap is on the other side of the lab from where I have
installed my H2 cylinder and I'd prefer not repositioning it.
Silly as this may sound, I could just hook a water aspirator onto a pump that draws water from a bucket to get the best of both worlds (and do the
environment some good in the process), right?
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Dr.Bob
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We use water aspirators to evacuate hydrogenations, much safer. If you use a recirculating pump, it needs to be outside, as the hydrogen will then
just be released inside if not. Hydrogen has a very wide explosive range, only a few percent in air will explode if exposed to a spark, like in a
pump motor. So just waste some water for the minute it takes to evacuate the flask.
If you cycle the flask through 2 or 3 cycles of weak vacuum and hydrogen, that is enough, and afterwards, then cycle it through nitrogen and vacuum 3
times to deactivate the catalyst and solvent so it does not catch fire when exposed to air.
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DJF90
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Dr Bob's experience mirrors my own. Just a quick note to say that if you're doing this in academia or industry, there is likely to be a COP or SOP for
this type of reaction, and if so then you"ll need to read it and sign off on it before undertaking the experiment. It'll also inform you of whats
advocated by your department with regards to equipment and procedure.
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