Nitrox gas separation

[Damselfish]

A few times on this board it's been said that partial pressure fills can take a little time to mix completely and that is the reason for rolling the tank in those cases. I don't know if that is true but it sounds like someone heard that and extrapolated to believing the air separates. Silly.

Too often places that don't get the mix right will blame anything but themselves or their analyzer.

 

[Andy077]

liger,

I guess I will accept your opinion (you being a ChemE) over the "experts" who posted above. But I am curious as to whether or not, in your opinion, 3000 psi has any impact on the possibility of the gas mix separating somewhat ? I got a nitrox fill the other day and personally witnessed the different analyses before and after rolling the tank around on the floor for a few minutes. Is it not theoretically possible for a "mixture" of gasses having different densities to separate ?

boat

That's two issues, when pure O2 is filled on top of air already in the tank, especially if it is done gently, and there is a big temperature difference between the two, there may be incomplete mixing due to one layering on top of the other, but once mixed, they are in complete solution with each other and do not come out.

Example, take a cup of very salty water with red food coloring. and a cup of tap water with blue.
Gently add the blue to the red, if you are careful, you will get layers, but once you stir them together, they stay mixed forever.

(analytical chemistry by day)

 

[egp]

There is a logical basis for the mistake, which depends on how the nitrox was produced.

If the nitrox was produced by putting pure O2 in the tank, followed by topping it off with air, it is possible that the two gases might not yet be mixed. In this case, rolling the tank would cause the two to mix.

If the nitrox was mixed in a continuous blend method, then only mixed nitrox goes in the tank, and the gases would never be separated.

Therefore, although everyone is correct that the gases never separate, the tank could very well have had unmixed gases and needed the rolling.

 

[Blackwood]

However, you really shouldn't be storing your tanks below -297 degrees F, it'll make the metal very brittle.

True, but steel is about 3% stronger at -300 than it is a room temperature, so it may even itself out

 

[Andy077]

True, but steel is about 3% stronger at -300 than it is a room temperature, so it may even itself out

Well, according to my fire textbooks, it's 15% stronger at 500 degrees too!

Fun fact, if you leave a container of liquid nitrogen exposed to the air for too long, eventually it will condense the oxygen out of the air near its surface, and the liquid O2 will sing to the bottom, eventually ending up with a light blue puddle of LOX under your liquid N2.

Of course, once the N2 boils off, you're likely to blow yourself up.

 

[ligersandtions]

liger,

I guess I will accept your opinion (you being a ChemE) over the "experts" who posted above. But I am curious as to whether or not, in your opinion, 3000 psi has any impact on the possibility of the gas mix separating somewhat ? I got a nitrox fill the other day and personally witnessed the different analyses before and after rolling the tank around on the floor for a few minutes. Is it not theoretically possible for a "mixture" of gasses having different densities to separate ?

boat

I'd have to bust out my old p-chem book to really answer this correctly....but I'm at work and that's not a book I have here. So I'll go ahead and answer as best I can. Brownian motion causes random movement of particles. The travel of small particles such as nitrogen and oxygen is governed by Brownian motion....effects of gravity (which is what would cause the particles to separate) are so small in comparison to the effects of Brownian motion. If I recall correctly, the effects of Brownian motion to the effects of gravity is something on the order of 10^3 to 10^4. Based on this fact, particles will continue to move randomly, rather than be organized into respective layers based on their (slightly) different molecular weights. There is, of course, the possibility that the particles will randomly move to a place where all the oxygen is at the bottom and all the nitrogen is at the top, but that would be a lot of calculations required to tell you what the actual probability is (so it would be best to just assume that it will never happen).

Out of curiosity, how different were your oxygen readings before and after rolling the tank on the ground? Was it within the calibration limits? Is it possible that your analyzer needs to be re-calibrated?

 

[boat sju]

liger and egp,

When I witnessed the "rolling the tank" experiment my tank had been topped off with nitrox. There had been about 700 psi of air in it initially. It was the shops analyzer. I don't remember the before and after readings but it wasn't just decimals. Seems it was at least a full unit (ie. 29 to 30 or whatever). Don't get me wrong : I wouldn't bet a month's salary that it wasn't just user error or the analyzer hadn't truly settled yet or whatever, but I think it's a fun discussion.

 

[Andy077]

I'd have to bust out my old p-chem book to really answer this correctly....

That way lies madness.

 

[Netsvan]

Have I heard of people saying that? *sigh*... yep, I've heard of it. Is it absurd? Absolutely.

Let's look at it from a basic scientific perspective. The following are the (rounded) molecular weights (atomic weight, for Argon) of the gases that make up "air" and their rounded fractions of air they make up.

Gas|g/mol|%
Argon|40|1
Oxygen|32|21
Nitrogen|28|78

Consider a disused basement (the scary kind that get kids' adrenaline really running hot). Let's say it has nice 10-foot ceilings (the better to have tall, dusty shelves, eh? :biggrin. If the gases separated, you'd end up with an inch and a quarter of argon on the floor, with just over two feet of pure oxygen floating on top of it, with almost eight feet of nitrogen filling up the rest of the room. If you walked into the basement without stirring it, the nitrogen would asphyxiate you, although when you pass out and fall into the oxygen zone, you may well survive. (You could crawl in the oxygen layer over to the fan... of course, if the fan, socket, or switch were in the oxygen layer, you just might die in a flash fire when you turn it on.)

(Imagine if the atmosphere separated like that. Hehe, you'd be able to get oxygen fills for dirt cheap, but buying nitrogen to dilute it would be expensive.)

Now, admittedly, it *is* possible to do fun things like pouring CO₂ into an empty aquarium to put out a flame, as CO₂'s molecular weight of 44 g/mol (and the usual cold temperature of freshly sublimated or decanted CO₂) yields a higher density than air, but it doesn't just stay there permanently. Even the slightest air movement will stir it up and away, and in quiescent surroundings, you'd still have diffusion to disperse it.

what he said post 3

 

[Blackwood]

Well, according to my fire textbooks, it's 15% stronger at 500 degrees too!

Really? That's surprising. I would have thought the lower density at higher temperatures would weaken it.

Fun fact, if you leave a container of liquid nitrogen exposed to the air for too long, eventually it will condense the oxygen out of the air near its surface, and the liquid O2 will sing to the bottom, eventually ending up with a light blue puddle of LOX under your liquid N2.

Of course, once the N2 boils off, you're likely to blow yourself up.

Sounds like a challenge