Does a Mixture of Gases Segregate by MW in a Vessel?
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?
Thanks in advance!!
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?
Thanks in advance!!





RE: Does a Mixture of Gases Segregate by MW in a Vessel?
http://www.eng-tips.com/threadminder.cfm?pid=1529
Use translation assistance for Engineers forum
Note the rules include No Student posting
RE: Does a Mixture of Gases Segregate by MW in a Vessel?
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?
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?
Gas centrifuges operate at up to one million g.
RE: Does a Mixture of Gases Segregate by MW in a Vessel?
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?
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?
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?
RE: Does a Mixture of Gases Segregate by MW in a Vessel?
RE: Does a Mixture of Gases Segregate by MW in a Vessel?
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]