Will high-pressure compression of air cause a Hydrogen EXPLOSION? 2

[DougSmith]

Hi all,

I am an MSChE working as an environmental consultant. I have a client who has designed a 1600-gallon vacuum enclosure in which he can open 30-gallon drums of waste and collect any tritium (isotope of hydrogen) which may be present in the drums. Tritium has many of the same chemical properties as hydrogen, so I'll refer to both hydrogen and tritium as "hydrogen" below.

The drums contain an oily sludge and are estimated to be almost or completely full of the sludge. Thus the mass of tritium in any drum is expected to be very small. There is almost no physical data and no laboratory data on the drum contents because of the radioactivity. It's not something you can just pour into a beaker and play with!

My job is to contact vendors and specify a vacuum pump and a compressor. The client wishes to collect any tritium with the vacuum pump and then compress it to about 2200 psi and store in in welding cylinders for later disposal.

The compressor vendor is concerned about the possibility of explosion. The lower and upper explosive limits of hydrogen in air are 4% and 75% by volume, respectively. I do not expect the concentration of hydrogen to be within the 4% to 75% explosive limits at anytime during the collection and compression process.

The compressor vendor's concern is that at pressure this high (up to 2200 psi), the oxygen and hydrogen are far more prone to react (e.g., ignite or explode) than at atmospheric pressure. Certainly this much pressure will heat up the air and will decrease the volume so that the O2/H2 diatomic molecules will collide more frequently.

Does anyone have experience in compressing air to 2200 psi, especially air that is slightly richer in hydrogen than atmospheric air? Is the vendor's concern justified? Do the normal upper/lower explosive limits apply at high pressure?

Thank you for any guidance you can provide,

Doug Smith

 

[rbcoulter]

I would be concerned also. I don't know the hydrogen explosion envelope but it should be checked out. Also, what else is in those drums? A very low vacuum, many things will be volatized. Many of these can be VOCs which are just as big of a concern as the hydrogen (maybe more so).

4% to 75% is a wide range. Let's assume that the vacuum enclosure is initially air/oxygen free. When a drum is opened, then could some dissolved oxygen be released from its contents? How can you be sure that the oxygen to hydrogen ratio (or oxygen to VOC ratio) is safe?

I wouldn't be compressing anything unless I was absolutely sure of the composition of the gas to be compressed.

 

[butelja]

Why not do an atmospheric pressure purge of air from the vacuum chamber with nitrogen and eliminate the explosion potential entirely?

 

[JKEngineer]

I am going to be vague in this response because I am relating work that is 15-20 years old. The Bureau of Mines had some old publications on explosive mixtures and their behavior. I think they related some of the behavior to conditions like pressure. I also think that pressure would be a concern, because I think that there were effects to be encountered. I suggest you try to find these old reports. (There were several and they were sizable.) Or find other sources that relate the flammability of a mixture to the mixture conditions and components. I do think that you have cause for concern. Jack M. Kleinfeld, P.E. Kleinfeld Technical Services, Inc.
Infrared Thermography, Finite Element Analysis, Process Engineering

http://www.KleinfeldTechnical.com

 

[Flareman]

Bureau of Mines Publication 503 does have some data (as suggested by JKEngineer. I can make a pdf copy of a couple of pages if you're interested. Let me know at flareman_xs@Netzero.net.
There's a little bit on Deuterium too, which is along the lines of your interest.
The bottom line, by my interpretation, is that the potential temperature increase due to compression is more likely to be a problem than the pressure itself but you need to read this stuff to get a feel for it.

The BOM publications are not widely available anymore. I was checking this very issue recently and the GPO told me to go and find my Local Federal Deposit Library and ask them. The copied sections which I have came from the ASME library in NY (at probably 25 cents a page) or perhaps the Inst Mech E library in the UK, I don't remember now.

NASA handle hydrogen under pressure all the time. Perhaps you can get to someone in Cape Canaveral and ask them for information. Start with a switchboard call to "United Space Alliance" and slog your way to someone who knows about this.

 

[sciguyjim]

Besides the concerns others have voiced, I'd like to add this. If there is oxygen in the gas mixture with the hydrogen, or any organic vapors, it may withstand being compressed but sudden changes in pressure, either sudden compression or sudden decompression, can cause the oxygen to spontaneously react with the other gases. I've heard of serious accidents from both types of pressure changes when oxygen was present.

 

[DougSmith]

I posted the original message. Since then, the client has changed the process so that the captured air/tritium stream only needs to be compressed to 200 psi rather than 2,200 psi. Quite a difference!

My thanks to JKEngineer for the Bureau of Mines suggestion and to Flareman for sending me a copy. The BOM document showed that the lower explosive limit for hydrogen was actually higher at 2,200 psi (around 8%) than at atmospheric (4%). That was good news if the 2,200 psi requirement was still in effect.

All the required equipment has been specified and quoted. Thank you to all who replied.

Doug Smith

 

[JKEngineer]

Glad to help. Jack M. Kleinfeld, P.E. Kleinfeld Technical Services, Inc.
Infrared Thermography, Finite Element Analysis, Process Engineering

http://www.KleinfeldTechnical.com