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Author: Subject: Vacuum Evaporation
dapper
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[*] posted on 8-11-2006 at 20:18
Vacuum Evaporation


I understand how and why it works.. but could someone explain to me how/with what it is set up? In a home laboratory?

Thanks
Dapp
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enhzflep
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[*] posted on 8-11-2006 at 22:41


If there was a prize awarded for the cheapest. nastiest piece of equipment I'd send you a pic of the award ;).

Before I sourced nitric, I distillled it from KNO3 + H2SO4. With crappy temperature control and zero patience, I was getting heaps of NO2 and wasting my efforts.

I came up with a 4-5 minute job that performed adequetly. Granted, pvc and hot HNO3/NO2 fumes are not very good friends - the tube leading into the condenser had to be replaced every run. However, it did work.

Am not feeling particularly linguistic at the moment, so I've attached a quick hand-draw pic. Hope it helps.

Vacuum.jpg - 43kB
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dapper
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[*] posted on 8-11-2006 at 23:13


sweet gentle jesus. I'm definitely lucky today. So, then .. few questions

'v' bottle - volatile bottle?
oil-filled saucepan? just... a good source of heat?

I wanted to vacuum evap naptha without heat because im afraid of fire/explosion risk.. what do you think? I love the waterfall vacuum. Iced water + bottle recovers solvent..
Sexy, very sexy indeed. did you seal the tubes with silicone, or something else?

Again... very impressed. thanks.
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enhzflep
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[*] posted on 8-11-2006 at 23:29


:D
Nope, the 'V' bottle as in 'V' drink - like red-bull. lol.

Yeah, saucepan is for providing heat. You could use water, but it 'bumps' when boiling, hence the oil.

If everything's sealed ok, naptha shouldn't be a problem. As far as I know, it can only burn with oxygen present so there should be no issue there I believe.

Not that you can't tell anyway, but the water's entirely recycleable. Just collect in a bucket and pour back in when empty.

Never did bother sealing the tubes with anything. Just drilled the holes about 3/4 of a mm smaller than the tube. This was a crappy arrangement as the tube gradually 'shrinks' at the point of constriction. A much better solution would be had by obtaining some inert hose fixtures of some kind - something like what you see on the end of the water taps in the lab. This way, the vacuum will actually make the seal better.

regards.
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[*] posted on 9-11-2006 at 00:09


Quote:
Originally posted by dapper
sweet gentle jesus. I'm definitely lucky tod...
I wanted to vacuum evap naptha without heat because im afraid of fire/explosion risk.. what do you think? I love the waterfall vacuum. Iced water + bottle recovers solvent..
Again... very impressed. thanks.


The aspirator bottle is very old tech, I've seen books from the early 1800s that used it. However it doesn't really pull much of a vacuum, it's better at drawing gases through something. One unit of pressure/vacuum measurement is "inches of water", one inch of water being about 1.9 mm Hg. Even using a large jug you will get less than 100 mmHg difference, your 'vacuum' will be greater than 650 mmHg absolute pressure.

The other problem is that an ice bath isn't going to do much good if you are pulling the solvent off at room temperature. Consider hexane, BP 68.7 C at 760 mmHg. Under 400 mm pressure it boils at 49.5, 100 mm its BP is 15.8; the vapour pressure at 0 C is about 50 mm. So if you boil the solvent at room temperature under about 150 mm Hg, the vapour pressure at your ice-bath condenser is about 1/3 of that.

Use a water bath with a stirrer, boiling stones, or even aquarium pump and bubble stone, to reduce bumping. Water is a lot easier to clean up than vegetable oil, which also gets gummy after long enough exposure to air . If you are worried about igniting fumes, use an immersion heater in the water bath, or a single heat level hotplate (no sparks from a controller) Control the heat level with a lamp dimmer of sufficent amperage rating.

enhzflep's home brew rig would work well for your needs, just as a way to keep solvent fumes from drifting around. You really don't need the aspirator bottle, just a pressure relief bottle (Wulfe bottle).
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Maya
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[*] posted on 10-11-2006 at 05:14


The greater the difference in height between the "coke" bottle with the water exiting and the actual exit the greater the pressure differential. in other words , get the stream of water going down thru as long a pipe as possible before it exits. Also it would help to use glass or something that doesn't deform for these containers as well

[Edited on 10-11-2006 by Maya]
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not_important
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[*] posted on 10-11-2006 at 08:24


That's true, but it takes 33 feet of water to equal 760 mm of mercury. Unless you're working somewhere you can have several stories worth of drain, you're not going to get a good vacuum.
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Baphomet
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[*] posted on 24-11-2006 at 05:43


Niiiice ;) I reckon that picture sums up the whole kitchen chem experience

And not one piece of borosilicate like the big girls blouses use. How good are those camping stoves?
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Heisenberg
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[*] posted on 27-11-2006 at 13:52


I liberated the vacuum pump from a scrap automotive a/c machine at work. Is there anything special I need to do aside from adding a vapor trap( i plan on using an automotive fuel filter) and changing the oil?
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[*] posted on 27-11-2006 at 20:15


A fuel filter isn't a very effective trap, usually cold (solid CO2 or LN2) traps are used, or molecular sieves. Fuel filters are more mechanical filters to remove particulate matter, they don't hurt as they do keep solid bits from getting into the pump but do next to nothing for blocking vapours - even water vapour.

There a several good books on lab vacuum methodology, and other general lab techniques books that have chapters on working with vacuum. Finding or borrowing one of those might be worthwhile.
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enhzflep
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[*] posted on 27-11-2006 at 23:49


Quote:
Originally posted by Baphomet
And not one piece of borosilicate like the big girls blouses use. LOLHow good are those camping stoves?


Those $20 (aussie bucks) stoves are absolutely brilliant! I really cannot say enough good things about them. I bought one over a year ago. The poor thing's been left outside (not under cover) for over a year!

In that time, I have cleaned it not even once. I have covered it in hot sulphuric, caustic, nitric, picric, vege oil and all sorts of nasties. It still lights first click!!:D

The only time it doesn't work is when the gas-can is empty or the burner jets are filled with (rain) water..

Great for a "I couldn't care less about it" heat source - temp control can be difficult, but not unworkable.

[Edited on 28-11-2006 by enhzflep]
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