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separating liquid oxygen and nitrogen

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IamSmooth

Thu Sept 13 2012, 03:58AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

I've seen fractional distallation and membrane filters. Any other ways?

Could I submerge a magnet and "pull" the oxygen out if I'm talking about a small quantity.

The reason I ask is this: membrane filters seem to be very expensive. Fractional distallation requires that I store the evaporated N2 and reliquify it. I don't have a means to capture the evaporated N2. If I want to cool it, this requires another cooling apparatus. It will be a project enough just to get the air liquified.

AndrewM

Thu Sept 13 2012, 04:13AM

Registered Member #49 Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362

If you have LN2 or can get it, the liquid air that condenses on the side of the reservoir will be LOX rich.

Doesn't get any easier than that.

Ash Small

Thu Sept 13 2012, 07:14AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Have you considered separating them before cooling?

You could use zeolytes (molecular sieves) or just burn off the oxygen, then remove the CO2,etc.

IamSmooth

Thu Sept 13 2012, 12:59PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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When liquifying air, Andrew, there will be a mixture. I'm not trying to get O2; I'm trying to get N2, so the condensed air won't help.

Ash, I've thought about burning the air, but I feel this will get complicated. I don't want to have any combustion going on near my setup. Further, the combustion would have to be complete, or else I will have other gases to deal with in the end.

I like the idea of the membranes, but they are expensive. Does anyone think my magnet idea is reasonable? Since O2 is paramagnetic, I could dip a large neodymium magnet into the mixture and just pull the O2 out. I just want to get the O2 content low enough that it is not a hazard for the short time I have the liquid.

Bored Chemist

Thu Sept 13 2012, 08:15PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

A magnet won't work.
Adding ammonia to air then passing it through a hot copper oxide catalyst will give you wet nitrogen (with a little argon etc). a cold trap would take out most of the water. A desiccant trap would do even better.

In practice this doesn't work as well as it looks because you get oxides of nitrogen. Also the reaction is rather exothermic so you have to be careful to keep it under control

IamSmooth

Thu Sept 13 2012, 09:32PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

why won't a magnet work?

Ash Small

Thu Sept 13 2012, 11:16PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Google 'zeolites'. I'm experiencing some browser problems at the moment, but would have provided some links earlier otherwise.

I'm not talking 'membranes', I'm talking 'molecular sieves'.

(Sorry about the 'browser problems'.)

IamSmooth

Fri Sept 14 2012, 02:40AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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I think the sieves are what I meant. You are talking about the micron tubules that allow the faster gases to get through, and reject the larger molecules.

The question is where can I get one without paying a lot of money?

Ash Small

Fri Sept 14 2012, 07:08AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

IamSmooth wrote ...

I think the sieves are what I meant. You are talking about the micron tubules that allow the faster gases to get through, and reject the larger molecules.

No. there are numerous zeolites. Cat litter is one. They use another in oxygen concentrators to remove the nitrogen. Others are used with a backing pump in place of a high vacuum pump for vacuum experiments.

It would need some more research, but if you can find one that absorbs O2.......

It may be easier to react the O2 with something else though, and remove it that way, depending on what's available to you.

EDIT:

EDIT: Found this, not sure hw relevant t is:

"Portable" depends entirely on what purity of N2 you need. If 98-99% is OK, then a PSA system can do that. The adsorbent used is a carbon molecular sieve. They're made by firing carbonaceous material first to create a porous body and then to with a controlled amount of oxidizer (H2O or CO2) to burn out the pores. Sometimes there's another firing in inert gas to desorb CO2 and CO species; I suspect this is useful for N2 separation, but I don't have anything definitive. Bituminous coal is one typical feedstock of the adsorbent.

This is a kinetic separation; the pores have to be (on average) small enough so that the difference in diffusion between O2 and N2 is large enough. The O2 diffuses faster through the pore; presumably this is some not-quite-bonding effect between O2 and carbon that causes a lowering of surface tension and subsequently lower boundary-slip friction. So a lot depends on the adsorbent processing.

If you need higher purity N2, then what's typically done is a deoxygenation step, which is in three parts. The first is to introduce H2 into the mostly-N2 gas stream. The second is catalytic oxidation of the H2; this is just a catalytic converter run "backwards", since we're removing O2 and not a combustible from the output gas. The third is a dehydration step."

EDIT: Also found this:http://aussiezeolite.com.au/yahoo_site_admin/ assets/docs/DK2772_ch22.166185814.pdf

IntraWinding

Fri Sept 14 2012, 01:26PM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

On a small scale you could slowly pass air over heated powdered metal.

An old chemists way to strip out oxygen from a air is to bubble it through an alkaline solution of pyrogallol (a photographic developer).

You can buy little packets that look like those silica gel bags you see everywhere but which absorb oxygen instead of moisture. Unfortunately you have to order a shed load. The ones I found were based on absorbing oxygen by the rusting of finely powdered iron and went on to discover that a type of cheap disposable hand warmer sold in a local shop was iron based too, so it's clearly quite an energetic/rapid reaction. Here's the list of ingredients for the hand warmers I found online in the MSDS (Material Safety Data Sheet) .

Iron
water
cellulose
vermiculite
activated carbon
sodium chloride

Another cheaper possibility if you want large quantities might be to use a tank of some single celled organism that can strip all the oxygen out of bubbled through air without suffocating itself. You could try growing a bunch of bugs from soil samples, say an inch underground where they have to fight to get oxygen but it's not totally absent. The nice thing is they would evolve to use oxygen in the way you supply it over time! You might develop your own product!
This is informative:

You can remove CO2 by passing through Sodium Hydroxide solution.

I looked into oxygen removal previously because a friend was trying to grow some anaerobic microbes (oxygen hating) for an art project and was struggling to eliminate oxygen for them.

What do you want to do this for?

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