UnsatisfactoryYield
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Registered: 5-1-2023
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Vacuum distillation/sublimation/filtration setup
Hi,
I'm just getting my lab set up and could use some advice. I have a few syntheses planned that will need a vacuum of 10-15mmHg to distill organic
product (if I want to replicate published procedure at least).
Watching NurdRage's video on aspirators, it seems like you can get 16.5mmHg with a $20 pump, some ice water, and an aspirator - pretty impressive.
He also mentions potentially achieving better results using a fluid with lower vapor pressure, like ethylene glycol - but I'd rather not introduce
those vapors as a contaminant. What if I were to just use an aqueous CaCl2 solution at -10 or -20C?
I was going to bite the bullet and spend $2k on a Welch DryFast, so if this method gives decent results, it would save me a lot of headache.
What kind of vacuum setup would you recommend for a beginner?
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Fery
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Registered: 27-8-2019
Location: Czechoslovakia
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Cheap water aspirator pump = necessary.
If you have extra money buy cheap Chinese 2-stage oil rotary vane vacuum pump on ebay (circa 200 US$).
I use the water aspirator for vacuum filtrations, for vacuum distillations of compounds with high boiling point upto 200 C, to remove volatile low
boiling point solvents (not to mess oil in oil pump), also when there are corrosive vapors.
I use the 2-stage oil rotary vane vacuum pump for distillations of everything boiling above 200 C. My pump (cheap one from ebay, cca 200 US$) reduced
boiling points of o-nitrotoluene from 222 to 70 C, benzyl acetate from 214 to 58,5 C, 1-acetylnaphthalene from 300 to 135-142 C, benzylideneacetone
from 260-262 to 86-95 C.
When circulating a liquid through water aspirator pump, the vacuum strength is always slightly worse than the liquid vapor pressure at the liquid
temperature. You also need powerful circulating pump which generates pressure of about 3-5 atm and. Using weak circulating pump able to pressure
liquid only upto 1 atm = to transport e.g. water only upto 10 m height is not enough for the water aspirator pump vacuum distillations. The pressure
of tap water is approximately 5 atm (in my country, from municipality pipes). The pressure of water pumped from own wells into pipes inside houses is
usually set with automated pressure control devices to be between 3-5 atm (it oscillates, which is not good for stable vacuum). I tried also weak
circulating pump submerged in 10 L bucket of water which produced pressure of only 1 atm and it was suitable only for vacuum filtrations, because
there was only weak vacuum and air was continually entering through filter paper, this continuous air flow prevented water suckbacks, when connecting
to vacuum distillation apparatus such weak vacuum was unstable and caused water suckbacks (no air flow entering the apparatus = the pump was flooded
with a little of water at which it immediately lost the vacuum strength + stability and quickly flooded completely - observed on glass pumps by eyes
and invisible on plastic pumps but the behavior the same = flooding and losing vacuum). When you connect a water aspirator pump to municipality pipes
and you runs it for hours it could be quite expensive, the price for water will very likely increase in future and in some countries the water is very
rare and precious.
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Dr.Bob
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There are plenty of cheap vacuum pumps out there, if you aren't using it a lot, that makes more sense than a $2K pump. If you want a pump that can
run for 5 years continuously, however, a Welch is it. I like the acid resistant ones a lot.
But an aspirator for short bursts is OK, or the water pump is fine, as long as it does at least 5 gal a minute or better.
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Rainwater
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If you have a 3d printer, you can get one that way. I can provide a scad file if there is interest
Or you can make one with everyday items.
A water aspirator is a very very simple device..
All you need is a tubing T, tubing and some superglue.
[Edited on 10-1-2023 by Rainwater]
"You can't do that" - challenge accepted
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Rainwater
National Hazard
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found it.
Attachment: vac valve.stl (1.9MB)
This file has been downloaded 195 times
Attachment: vac valve.scad (5kB)
This file has been downloaded 200 times
"You can't do that" - challenge accepted
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