Liquefying air, or oxygen enriched air.

[billybobjoe]

Well I have searched the forums and haven’t found anything, and searched Google, so I am posting a new topic.

I happen to have an old refrigerant compressor that will pump to 500 psi before it starts to sound strained. Now I am sure that it could do more, but I don’t want to hurt it. So my question, could I compress air to 500 psi, and then cool it. Just store the air in some form of tank (say a propane tank). Once the tank has come to a normal temp again (assuming it would get hot from compression) immerse it in dry ice/ acetone slurry and cool out only the oxygen, argon and co2, but leave the majority of the nitrogen as a gas.

I have racked my brain, but I haven’t thought of any reasonable ways of storing my oxygen enriched liquefied air, perhaps store it as a gas, and cool it as needed. As an aside, I was wondering if there would be any ways to separate out the oxygen, or remove say the argon and any liquefied nitrogen but not cause significant losses of my oxygen.

Edit: sp.

[nbk2000]

Oxygen doesn't liquify at the temperature of Dry Ice/Acetone slurry.

Now, if you compress air, cool it, and release the pressure through a needle valve, recycling the cooled air through the circuit, then eventually it'll get cold enough to condense out the oxygen/nitrogen into your dewar flask collector.

You then let the 'liquid air' evaporate, which causes it to lose the lower boiling-point components like argon/xenon/nitrogen, leaving behind liquid oxygen. :)

The Amateur Scientist Collection CD has a relevant article that you could use.

[billybobjoe]

I will have to look into The Amateur Scientist Collection on CD.

Would I get enough cooling prior to releasing through a needle valve by cooling in dry ice/ acetone slurry that it would only have to be released once?

Maybe it was a hair brained idea, but I was thinking that if I compressed it, its boiling point would be closer to zero and then I could simply compress more air into the cold cylinder, cool that. Then once it was fully chilled i could "siphon" my liquid air off. :)

[Jacks Complete]

Yes, that would certainly help. The first step is to ensure your compressor is in the shade. Then fill it fully, and wait for it to cool down to ambient. Then you can top it up, and repeat. Now you can start to cool it with a water jacket or something cheap, to bring it down to about 5 degrees, and then you can get clever with your dry ice.

Commercial systems re-circulate the air, as NBK mentioned, and they achieve very impressive efficencies, turning some 50% of the compressed air into liquid which stays cool enough to remain liquid.

As regards fractional distillation, you will be bloody lucky to get anything to work. The idea of freezing it just enough to get only the O2 is also impossible. You are talking multimillion pound gear for doing that at cryo temps. Even a simple centrifuge and extraction may well be beyond your safe capabilities.

I'd go for the centrifuge, anyway. Look up the densities of LN and LOX, and Argon, etc. and you should be able to use a regular centrifuge design that has been modified a bit without killing yourself. Just remember that wood and steel aren't very strong at these temps, and most plastics are brittle as ice too.

Think about selling the liquid noble gases if you start doing a lot of this, since they are very expensive, and even just LN enriched with others will probably be worth money. Won't make you back the electric costs, mind.

[nbk2000]

There's no need for distillation for enrichment. The other gases boil off before oxygen does, leaving it behind.

And, yes, you'll need to recirculate the air that you're trying to cool, otherwise you'll never get it cold enough to condense.

UTFSE (Google) for more details.

[Jacks Complete]

Boiling off the LN seems a bit wasteful, though, and the fraction of air that liquifies which is LN is likely to be more than the 73% in air, as it goes liquid at a higher temperature, so you would need some exotic thermometer that can cope with the difference between LOX and LN temperatures, to get a 10% or so yeild.

Forgot to mention the obvious, LOX sets things burning really easily. Which is probably the goal, but take care anyway.