Science Questions

How are gases separated for bottling?

Sun, 19th Apr 2009

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Question

Briana asked:

How are gases separated for bottling?

Answer

Dave - So this is how you separate oxygen from nitrogen in the air or helium.  The way that itís done commercially is by cooling air down and all the different gases in air have different points at which they condense so carbon dioxide will come out first, then oxygen, nitrogen and argon.  If you slowly cool it down at different temperatures you take out different gases, different liquid gases.  Helium isnít found in the atmosphere.  You can only find it in the top of oil wells.  Itís caused by radioactive decay.

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Briana asked the Naked Scientists: how do they separate out gases for air tanks, such as oxygen, argon, nitrogen, or helium tanks? Especially since helium is lighter than, say, oxygen, do they have to go higher into the atmosphere to extract it? What do you think? Briana, Mon, 6th Apr 2009

The gases in air can be separated by their different boiling points.
http://encarta.msn.com/media_461543848_761559991_-1_1/boiling_points_of_air_gases.html RD, Mon, 6th Apr 2009

There's not much helium in air- it dissipates into space. However it is formed deep in the earth by radioactive decay and sometimes it gets trapped in much the same way as natural gas.
So Helium is usually extracted from gas wells rather than the air.
Also, while the difference in boiling points is the usual industrial process for separatinfg the gases, it's not the only one. It's possible to make membranes that are more permeable to oxygen than to nitrogen (or the other way round, I'm not certain). You can then use these as sieves to separate the gases.

On a historical note, oxygen used to be obtained by Brin's process which involves the reversible reaction of oxygen with barium oxide. Bored chemist, Tue, 7th Apr 2009

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