drw@culdev1.UUCP (Dale Worley) (10/07/87)
doug@ndcheg.UUCP (Doug Price) writes: > Since xenon > is 4.56 more dense than air at 21 C, I wonder how you can keep a homogenous > mixture of xenon/O2 without going to zero gravity? Using the Maxwell-Boltzman distribution to examine how the density of xenon and, say, oxygen vary with altitude: density is proportional to exp[ - m g h / k T ] where m = mass of atom/molecule, g = earth's accelleration, h = altitude, k = Boltzman's constant, T = absolute temperature you can figure out that the density of xenon varies with altitude only a little more than the density of oxygen does, and the difference in densities varies about as fast. Remember, nonzero temperature tends to keep things stirred up! (but it's significant only if they have a *real* small mass) > Also, 300 liters of > 800 psig xenon will run you approximately $3600 (U.S.). It seems to me you're going to need around 1000 atm, which is 15000 psi, which must cost around US$72,000 for 300 liters, and that's only the volume of a human body. And building the storage tank would be hard. But it would *still* be *fun*!!! Dale -- Dale Worley Cullinet Software ARPA: culdev1!drw@eddie.mit.edu UUCP: ...!seismo!harvard!mit-eddie!culdev1!drw Give me money or kill me!
doug@ndcheg.UUCP (Doug Price) (10/08/87)
In article <1617@culdev1.UUCP>, drw@culdev1.UUCP (Dale Worley) writes: > doug@ndcheg.UUCP (Doug Price) writes: > > Since xenon > > is 4.56 more dense than air at 21 C, I wonder how you can keep a homogenous > > mixture of xenon/O2 without going to zero gravity? > > Using the Maxwell-Boltzman distribution to examine how the density of > xenon and, say, oxygen vary with altitude: > > density is proportional to exp[ - m g h / k T ] > > where m = mass of atom/molecule, g = earth's accelleration, h = > altitude, k = Boltzman's constant, T = absolute temperature > > you can figure out that the density of xenon varies with altitude only > a little more than the density of oxygen does, and the difference in > densities varies about as fast. Remember, nonzero temperature tends > to keep things stirred up! (but it's significant only if they have a > *real* small mass) What I meant was that since xenon is more dense than oxygen, it would settle towards the bottom of the storage vessel when gravity is present. It is known that mixtures of gases will separate if their densities vary by a large amount and if there is no strong intermolecular forces to keep the mixture homogeneous. Linde (producer of laboratory gases) recommends that cylinders containing such mixtures be rolled around prior to use so the gases are well mixed. > Doug Price U. of Notre Dame ..!iuvax!ndmath!ndcheg
drw@culdev1.UUCP (Dale Worley) (10/09/87)
doug@ndcheg.UUCP (Doug Price) writes: > What I meant was that since xenon is more dense than oxygen, it would > settle towards the bottom of the storage vessel when gravity is present. > It is known that mixtures of gases will separate if their densities vary > by a large amount and if there is no strong intermolecular forces to keep > the mixture homogeneous. Linde (producer of laboratory gases) recommends > that cylinders containing such mixtures be rolled around prior to use so > the gases are well mixed. > Yes. And what I was pointing out was that (assuming you know the temperature) the *equilibrium* separation is *determined* by the fact that it all equilibrates to the Maxwell-Boltzman distribution, and that on laboratory scales at room temperature, the separation is very small. The "mixtures" that separate in a laboratory setting are most likely not mixed on a molecular scale, but rather were made by injecting certain amounts of two gasses into a bottle, leaving a gas which is a mosaic of macroscopic-scale regions of the two components. Compare to pouring one liquid into another. The two naturally form two layers, which diffusion will slowly mix completely. One can apply this method to the atmosphere. The M-B distribution gives you the fall-off in air density with altitude. But note, the e-folding height differs for each molecular species (being smaller for heavier species), so once you get up real high, hydrogen and helium predominate. Dale -- Dale Worley Cullinet Software ARPA: culdev1!drw@eddie.mit.edu UUCP: ...!seismo!harvard!mit-eddie!culdev1!drw Give me money or kill me! Exercise your childishness -- remember, we are all 10 in some base.