MadSci Network: Chemistry
Query:

Re: Can you suppply plans for an apparatus that will condense O2 out of the air

Date: Wed May 5 20:36:39 1999
Posted By: John Christie, Faculty, School of Chemistry, La Trobe University, Bundoora, Victoria, Australia
Area of science: Chemistry
ID: 923426023.Ch
Message:

I personally have a real moral dilemma with this sort of question. Liquid 
oxygen is an EXTREMELY dangerous material. It is perilously easy to produce 
using liquid nitrogen and ordinary air. I really would not want to post 
such details on an open web page. I do not have the details at my 
fingertips -- would need to go away to search for them anyway.

On the other hand I believe in the free flow of information and am against 
censorship in general principle, if not always in detail. In instances like 
this, I think that the need to have enough knowledge and resources to find 
the recipe for oneself (it is certainly available in the public domain) 
might be a useful filter in restricting the details to those who might have 
enough intelligence and responsibility to use the knowledge wisely.

Oxygen gas is about 21% of ordinary air. At ordinary temperatures, a mole 
of oxygen gas in air occupies about 120 litres. When condensed to a liquid, 
a mole of oxygen occupies about 60 mL -- that is, it is 2000 times more 
concentrated. Of course at liquid oxygen temperatures, many chemical 
reactions go very slowly. But most oxygen reactions involve free radicals. 
And free radical reactions are often not much affected by temperature. So 
there is the potential for a reaction to get started if any combustible 
material contacts liquid oxygen. And if it does get started, the high 
oxygen concentration will produce dramatic and disastrous effects. There 
will be a huge concentrated supply of oxygen present to sustain any 
reaction that does get started. The heat produced from the reaction will 
cause rapid evaporation of the liquid, accompanied by a 2000-fold 
expansion. The probable result will be an explosion with spot fires at the 
periphery.

It is almost certain that insurance would not cover damage to any premises 
or property where liquid oxygen was being prepared or used.

When I started my University studies in Australia in the early 1960s, 
liquid air (not liquid oxygen, but about 20% liquid oxygen in nitrogen) was 
still being used as a coolant for vacuum lines and the like. Everyone was 
terribly scared when using it, and there were tales of some awful 
accidents. It was universally replaced with liquid nitrogen around 1965, 
and use of liquid air was discontinued.





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