Blackbeltjawa
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Vacuum for distillations
Hi, I have tried an aspirator with little luck. My water pressure is exceedingly high so you would think it would pull a good vacuum but it's barely
sufficient for filtering and doesnt suffice for distillations. Should I buy another (perhaps more expensive) aspirator, buy a laboratory vacuum pump
or is there something that I might be missing in using the one I have? If the suggestion is a vacuum pump could anyone suggest a model that's both
affordable and produces a reliable vacuum? My distillations as of now have a pretty wide range in boiling temps but I could need greater accuracy on
the vacuum pressure later down the line but my budget is kind of narrow right now. What features are important when shopping for a vacuum pump?
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macckone
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If you aren't getting sufficient vacuum for filtering then I would definitely replace the aspirator. They aren't expensive.
I have switched to a modified aquarium pump to provide vacuum.
It doesn't get as low as my aspirator set up did but is generally sufficient.
For really low vacuum you have to have a real vacuum pump.
If you need less than 50mm hg, that is really the only way to get it.
Calcium chloride 'enhanced' water with a premium aspirator can get lower but it requires a lot of flow and the solution has to be really cold.
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weilawei
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Depends on what you want to do with it. Pump solvents through it? Buy a cheap single stage rotary vane. Pump nasty acidic stuff? Buy a chemical-rated
membrane pump (more expensive). For analysis or if you're going to be nice (always using a cold trap and gas ballast), a 2-stage rotary vane is nice.
The HVAC pumps on Amazon or eBay should all work fine for filtering and stripping stuff. I like to get 10mm hose barb connectors and heavy wall Tygon
tubing. Get a cheap analog manometer. You'll need vacuum oil, a dewar, cold trap, and a coolant. This could be dry jce in isopropanol, but I find
isopropanol cooled in the freezer in the dewar first to be just fine for short runs.
I recommend spending some time familiarizing yourself with how they work, startup and shutdown procedures.
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Swinfi2
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My aspirator was kinda sucky (not in a good way) as the check valve kept bleeding, so its weak vacuum wouldn't even hold. My solution to the problem
was to saw the pump out of an old freezer and hot-wire it. Sure exposed live mains is kinda hazardous but... (https://www.youtube.com/watch?v=BfAC05eYGHs) For a switch I use a brick.
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beerwiz
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I bought a 9CFM 10 micron pump, it's strong yes but I put a needle valve between the pump and the distillation setup and it works like a charm. I can
turn the needle valve to just enough vacuum that it boils but it doesn't bump, you eyeball this. Works perfect and it's cheap. I use a rotavap with a
-120C cold trap and I change the oil in the oil pump with mineral oil, don't buy that vacuum pump oil crap. Mineral oil from the pharmacy is the best
vacuum pump oil, and it doesn't produce oil mist, I get deeper vacuum with mineral oil compared to any other vacuum pump oil I've tried.
I used to hate doing distillations and tried to avoid them at any cost but now they are a pleasure to do and super fast.
[Edited on 10-10-2018 by beerwiz]
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S.C. Wack
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Many can get by with a diaphragm pump.
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macckone
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modified aquarium pump is actually a diaphragm pump and you can get them for $15.
it seems feasible you can buy a two stage and stack the stages.
I didn't do that with mine. I use one side for pressure and the other for vacuum.
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zed
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Ummm. Some aspirators are advertised to require a modest length of tubing (perhaps with the exit end submerged in water) attached to the exhaust
outlet.
In days of yore, my units didn't require it, but designs vary. Some folks have reported poor vacuum, without that auxiliary tubing.
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macckone
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there are two types of flowing liquid pumps:
Bernoulli - also commonly called aspirator pumps
Sprengle - usually uses mercury but can also be water
a sprengle pump requires the terminal end to be submerged.
a bernoulli pump does not
HOWEVER one thing a bernoulli pump does require is a very high flow rate where a sprengle pump only requires a really long tube (30+ inches with
mercury and up to 30 ft with water).
Some people say a bernoulli pump requires a high head pressure but the flow rate is really the key factor since it is the flow that creates the
vacuum. There is a resistance in the aspirator and piping so there needs to be a minimum pressure to achieve a given flow. Generally residential
piping is about 45psi but can be between 40 and 80.
A cheap fountain pump can do 8 ft of lift (4 psi) at 400 gph.
An aspiarator with a 1/2 in (1.27cm) main diameter and a 3/16 in (.5 cm) inner diameter can achieve minimum vacuum. Of course your mileage may vary
if there is turbulence or the back pressure prevents that flow rate.
From an engineering standpoint the 3rd derivative of the slope of the tube wall leading to the construction should be smooth to achieve laminar flow
and least resistance.
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