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# Does a Mixture of Gases Segregate by MW in a Vessel?3

## Does a Mixture of Gases Segregate by MW in a Vessel?

(OP)
Hi guys. I should know the answer to this but I don't. Does a mixture of gases in a container segregate/stratify by MW/density?

Suppose I have a mixture of CO2, H2S, CH4, and H20 in a tank vapor space, or in a closed vessel. Over time, does the mixture segregate out and stratify by MW?

### RE: Does a Mixture of Gases Segregate by MW in a Vessel?

I'd say no because thermal agitation of gas molecules at normal temperatures are more powerful than gravimetric separation thus it never separates.

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### RE: Does a Mixture of Gases Segregate by MW in a Vessel?

It really is the other way around. If I open a container of dry ice in an open field and let it sublimate then for the first several hours the atmosphere around the container will be approximately 100% CO2 (which is heavier than air). Once all the dry ice is gone, the CO2 concentration will immediately begin declining until the atmosphere around the container is just air. Say that the container was an ice chest (open top and closed side and bottom). Once the dry ice is gone, the atmosphere within the ice chest will be 100% CO2 for a time, but mixing at the mouth of the ice chest will draw the CO2 away and within a few hours the chest will be full of air.

So the idea of gravity separation of gases can happen for very short duration at the beginning of the contact, but will not survive very long, and certainly will not re-separate once mixing happens.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

### RE: Does a Mixture of Gases Segregate by MW in a Vessel?

(OP)
Thanks guys.

David - I appreciate your example but I was inquiring about a closed container. Your example of the CO2 in an open field is valid, but If I open the solid CO2 inside a closed vessel, will the CO2 form a layer at the bottom of 100% (or nearly 100%) CO2?

If there is no segregation, then why do we say that H2S in air will collect in enclosed low areas (e.g. sumps, well cellars, pits)?

And why are water heaters placed up on pedestals inside a garage, in the possibility of gasoline vapors being in the air and settling near the floor?

I'm trying to talk myself into the following: For a typical process plant tank or vessel, I think the gases do segregate, but I don't think completely. For the time frames we are interested in, for example, a quiescent tank vapor space, for practical calculations I think it's safe to say the gas nearest the liquid level in a tank is probably a high mole fraction of the heaviest component. In the same manner, the composition of the gas mixture in a well cellar is probably mostly H2S at the bottom of the cellar.

### RE: Does a Mixture of Gases Segregate by MW in a Vessel?

Differing gas densities can delay mixing of two gases, but once mixed, normal gravity will never cause gases or fluids to separate. Brownian motion and molecular diffusion are much more powerful forces at a molecular level than gravity.

Gas centrifuges operate at up to one million g.

### RE: Does a Mixture of Gases Segregate by MW in a Vessel?

The dry ice example was just trying to illuminate the discussion. Works the same way in a closed vessel. H2S kills people in low lying areas because of the delay for mixing. If you stop an H2S leak the air around it will be breathable in a little while.

Think about taking gas samples. We just grab one point because you get the same answer if you grab several. If I squirt blue molecules into a red gas in an enclosed vessel, in a short time I will have purple gas regardless of the differences in molecular weight.

If you suck a hard vacuum on the head space in a tank (assuming it is not an API tank and is designed for vacuum) and then introduce air, the air will enter the tank rapidly and will not mix with the vapors that came off the tank. Once the velocity difference between the two gases is used up, mixing begins. In a little while you will reach 100% saturation of the volatile vapors and everything will reach an equilibrium that has the gas very much non-stratified.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

### RE: Does a Mixture of Gases Segregate by MW in a Vessel?

I think some people confuse gases with liquids. Gases do not coexist in separate phases, nor do they segregate again into separate phases after mixing based on density in normal gravity. Density differences do delay mixing, to an amount determined in part by the diffusivity of the gas. Hydrogen for instance tends to diffuse rapidly and hence doesn't tend to stratify as much as people might otherwise assume.

You can use very high gravity (i.e. gas centrifuges) to separate gas components from one another, but they do not form separate phases.

### RE: Does a Mixture of Gases Segregate by MW in a Vessel?

If temps are low enough that the normal energy transfer/mixing from Brownian motion is largely suppressed then there will be stratification, but we are talking about a few degrees above the boiling point.
Calculate the mean-free-path, distance between gas molecules colliding. You will see that it is very short.
At room temp gases are moving 400-1800 m/sec (depends on gas) and colliding with each other about every 10 microns. This is what keeps gases mixed.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

### RE: Does a Mixture of Gases Segregate by MW in a Vessel?

(OP)
Guys thanks for the helpful replies and the great discussion. Much appreciated! Pete

### RE: Does a Mixture of Gases Segregate by MW in a Vessel?

You _can_ separate mixtures of gasses - after all thats whats goes on in a GC column and i think its also used in some nuclear processes where uran hexaflourid gas is used to separate the U235 from the U238 in high speed gas centrifuges. But for any "normal" circumstance you can assume that a mixed gas phase will n_not_ settle out layered by its MW or any other property.

### RE: Does a Mixture of Gases Segregate by MW in a Vessel?

Anecdote: "we" used to use the fact that gas mixing takes time to our advantage when filling the hydrogen-filled casings of rotating electrical equipment of various types following internal maintenance; the air within was first displaced upward with CO2 until an Orsat analyzer determined the top exiting gas was > 99% CO2. The next step was to displace the CO2 downward with H2 until Orsat analysis proved the exiting bottom gas was > 99% H2.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

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