jwarr
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Vacuump Pump Operation
I just recently obtained a 2.5 CFM AC-type vacuum pump (which works wonderfully by the way). I'm curious on a couple points though and hoping I can
get some clarification from people with similar experience.
Currently I have my vacuum pump attached to a bleeder valve which then just connects to the vacuum nipple on my flask, take off adapter, etc.
1. When performing a distillation under vacuum should I establish a vacuum, close the bleeder valve, and then simply remove the vacuum pump? Will
the effect of the vapor pressure produced by whatever it is I'm distilling be negligible in compared to the vacuum? Or do I need to keep the vacuum
pump on for the entire duration of the distillation?
2. I'll be working with MeOH, DCM, and H2O as my solvents. Should I worry about setting up a vacuum trap or vacuum reservoir? I'm not trying to
make this pump last for centuries and money is a big factor.
Thanks in advance for the help guys.
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aonomus
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Think about it, as you start to boil your crude product, the pressure inside the distillation rig will rise until the vapour pressure of what ever
fraction is equal with that in the gaseous phase. You need to keep the vacuum running during the length of the distillation otherwise it will just
stop boiling.
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entropy51
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| Quote: | | I'll be working with MeOH, DCM, and H2O as my solvents. Should I worry about setting up a vacuum trap or vacuum reservoir? |
It's standard practice to place a dry ice cooled trap between the distillation and the pump. You might squeak by with using the gas
ballast and frequent oil changes, but it is Abuse of Pump.
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jwarr
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Entropy, could you explain what you mean by a gas ballast?
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Arrhenius
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A gas ballast is a device built into certain higher end vacuum pumps. If you have an A/C service pump, I doubt it has one. A gas ballast helps to
de-gas the pump oil and pull oil back in from the mist trap (in Edwards pumps). Basically the point is that if you don't have a cold trap, you'll
tend to contaminate your pump oil quickly and the pump won't hold a good vacuum when it's hot. You don't really need a high-vac pump for manipulating
DCM or methanol, but it'll pump off water pretty well. If you're vacuum distilling reaction products, I'd say you want a dry ice/acetone trap.
Obviously if your vacuum changes during a distillation, so does the distillation temperature, which makes it hard to tell what you're fractionating.
[Edited on 6-11-2009 by Arrhenius]
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