I agree an open dewar is not likely to liquefy air as the N2 vapors are constantly blanketing the dewar opening preventing air from entering. A somewhat similar safety issue with LIN dewars is as follows
I led the investigation for two incidents involving liquid Nitrogen traps. Many gas suppliers collect and purify pure metal hydride gases such as arsine, phosphine or germane in aluminum cylinders immersed in dewars filled with LIN. In July of 2001 I flew out to Tulsa to investigate an incident in which an aluminum cylinder containing 65 lbs of arsine suddenly ruptured releasing its contents into the air. Over a 100 people thought they had been exposed and rushed to the hospital.
I was baffled as to what may have happened. Based on the interviews and the incident timeline as well as testing we were able to determine that the operator had opened a new system valve allowing air to be sucked into an evacuated cylinder. He was still able to pull a vacuum on the cylinder and didn't think anything was wrong. He then proceeded to fill the cylinder with 65 lbs of arsine that he manufactured. This was then placed in the outside dock area to warm up the frozen arsine. During this time the liquid air and arsine vaporized and partially mixed. As4 which is a decomposition product of arsine is pyrophoric. This ignited the arsine/air mixture. Since it was not well mixed there was a deflagrating reaction that was triggered at the liquid level of the arsine. This propagated upward compressing and heating the mixture above it (pressure piling) causing a detonation with enough force to rip the brass cylinder valve out of the cylinder neck stripping away the threads. The hot reaction of the arsine/air mixture continued flowing out of the cylinder opening and it was hot enough to melt the aluminum interior surface of the cylinder neck.
In testing we conducted after this incident we immersed a aluminum cylinder into a LIN dewar with a vacuum on it, within minutes after opening the valve we liquefied 5 lbs of air in the cylinder. Pulling a vacuum enriched the oxygen concentration because of the difference in boiling points.
In 2011 another gas supplier had a germane cylinder that they immersed in a LIN dewar. Air liquefied in the aluminum cylinder when the operator opened the wrong valve. After the cylinder warmed up the operator quickly (1 min) added hydrogen to the germane cylinder to a pressure of 800 psig. The heat of compression ignited the air/germane mixture, testing has shown that increasing the pressure dramatically lowers the autoignition temperature of a germane/air mixture. At 500 psig it is 20 C! The pressure suddenly jumped to over 1,400 psig and then decayed rapidly to 935 psig. This caused the cylinder to jump and imbed itself in the hood ceiling. Since the cylinder was intact we were able to analyze the gaseous and solid byproducts. Based on a material balance we determined that 81 liters of air liquefied in the 47 liter cylinder based on the N2 we found. There was no O2 since it all reacted with the germane. The air/germane reaction caused the remaining germane to initiate a self sustaining decomposition reaction as evidenced by the solid germanium and the H2 generated.
Eugene Ngai
Chemically Speaking LLC
www.chemicallyspeakingllc.com
