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There is a little bit of seperation at the extreme heights of the atmosphere, but the pressures are essentially non-exsistant.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
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Errorist, maybe this will help. Gas molecules have weight, and that holds them down. The earth's atmosphere weights 14.7 pounds per square inch at sea level. That's ten tonnes per square meter. Air pressure at any particular altitude simply measures the amount (mass) of air above it.
If you could stand up a 200 mile high tube, it would reach to the "top" of the atmosphere. In a sense the atmosphere doesn't have a "top" because it keeps getting thinner and thinner, but 100 miles is effectively the top. At that altitude, the air molecules are so far apart that a space vehicle can stay in orbit for days before drag brings it down. But even 1000 miles up, there is some atmospheric drag; just very, very little.
The thing that causes molecules to escape the Earth is temperature; hot molecules move faster than cold ones. Once you get up above 80 or 90 miles, the molecules are so far apart that if a molecule gets hit by an energetic photon and it gains a lot of kinetic energy, it can fly in a straight line a very long way without hitting another molecule. If it doesn't hit anything and it's moving fast enough, the molecule will fly away from the earth. It needs the same speed that a rocket needs to escape, though. Otherwise it will do the same thing that a slower rocket would do; its path would describe an arc. It would rise, bend over, then fly straight back down.
A hydrogen atom has a mass of 1; a hydrogen molecule, 2; an oxygen atom, 16; an oxygen molecule, 32; a carbon dioxide molecule, 44. So a particular energetic photon--say, an x-ray photon--absorbed by a hydrogen atom will impart a LOT more velocity to it than to a carbon dioxide molecule. That's why hydrogen escapes from the earth and oxygen or CO2 don't. It's not because hydrogen has a form of antigravity; it's because it can reach a higher velocity with the same photon input.
A pipe 1 meter in diameter, open to the atmosphere at the bottom and to space at the top, will have ten tonnes of gas inside it, whether that gas is hydrogen or something else, or a mixture. The gas column will behave basically the same regardless of the composition; the pressure will decrease as you go up. Maybe a hydrogen column will be 101 or 110 or 90 miles high instead of 100; in other words, the change in molecular weight will change the exact way the density changes with altitude, but not a lot. If anything, the walls of the pipe might shield the hydrogens inside from ultraviolet and other energetic photons, depriving the atoms at the very top of the column--where they are very far apart and can travel long distances without collision--of the energy source they need to fly upward and out. You may actually get LESS hydrogen escaping from the tube than could escape from the open atmosphere as a result. Another factor that will decrease the leakage is the fact that only atoms flying straight up will escape; any flying diagonally will run into the walls of the tube and slow down. This is also not true in the open atmosphere, where diagonally moving atoms can escape as well.
-- RobS
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You are essentially correct Rob... *nod nod* Though the temperature/velocity thing governs a great deal of the amount of gas which escapes the Earth's pull, and alot of hydrogen burned off is from protons via solar wind and not so much photons by EM radiation.
Further, since gasses compress and gravity affects all gas particles, there is no way to push a contiguous column of gas up to the required altitude, you have to push it with somthing that the gas cannot pass through, like the walls of a gas tank.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
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I agree!! You can push it up with the existing atmosphere by letting it back in through the bottom and letting the column equalize with the outside pressure.Of course, you could put a barrier between the two gasses.Since, the H2 is already on the top you my not need that barrier.
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Ummmmm nooo, I don't think you got it... if you did that, then the weight of the hydrogen, which will want to assume a similar pressure as the atmosphere outside the tube, would actually force the air out DOWN the tube.
If there were no hard barrier between the gasses, then just as stated above, the hydrogen will be pulled back down to assume roughly the same volume and roughly double the pressure of the atmosphere admitted.
You have to come up with a microscale rationale to get the gas molecules up the tube! There is no possible macroscale trick you can use! Its against the laws of physics.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
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Nope the outside air pressure is about 15 psig at sea level, and the H2 pressure inside the tube is 7 psig. So the outside air would rush in the tube.
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No, the air admitted into the bottom won't reach the altitude it would outside the tube, because of the hydrogen pushing down on it. In fact it would level off aproximatly where the sum of the lengths of the two gas columns (air and hydrogen) are equal to the height of the atmosphere outside, give or take a little.
Again, you must stop trying to address the problem with pressure differentials and flows, because it is fundimentally impossible to move much gas this way. You HAVE to aproach the problem from the molecular level up!
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Would the H2 still be on the top of the column once the pressure was equalized?
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If you give it enough time to come to equilibrium then no, most of the gas will have mixed together pretty evenly. The very low pressure top regions of the column would be slightly hydrogen rich, the pressure is too low to do anything with.
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What elements are left for it to mix with? What if it was He instead of H2?
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If you admit air, then its mostly nitrogen/oxygen/argon etc... helium would also fraction slightly rich at the top, though not as high as hydrogen would. The altitude helium can achieve at equilibrium is much lower than hydrogen.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
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I would think the atmosphere we are letting in has already mixed with H2, and the H2 that was introduced into the sytsem would not be be able to mix any more.
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You aren't thinking microscale... you can pack any number of gas molecules of any type into a given space, all the way up to the point where it begins to liquify or solidify... gasses are infinitly soluble in eachother, Charles' Law of partial pressures I think.
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If they don't combine in time they will seperate. I have seen it happen many times purging generators out.
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No, they won't. In fact, when you are purging a system you are generally not pushing all the offending gas out with only one volume of inert gas, but rather pumping large amounts of inert gas through it in the hopes that it will drag/push some of the bad gas with it.
Again, a gas does not behave like a liquid, miscible or not, and under zero gravity field a mixture of gasses will be perfectly mixed because the motion of molecules in a gas is entirely random, reguardless of the identity of the molcules. But since we live under a pretty strong gravity well, then as RobS mentions above the faster moving hydrogen molecules will reach higher altitudes than heavier ones, hence the air is richer in the lighter gasses the higher you go. But, since humans don't ordinarily deal with large height differences, this is less a problem in every day life... however since the tube is so long the extremes apply.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
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Ok, why purge the hydrogen out the top of the generator casing while admiting the carbon dioxide in the bottom?
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If the system is still hot, then you want to get rid of the hydrogen without having to push it through a hot machine, risking ignition.
Although all gasses will diffuse to their natural gravity-fractioned concentrations over time, the effect is not instantainious: you can put a little puff of Argon (heavy) over somthing in a open container to keep some of the air off of it, but it won't last very long since the gas will diffuse anyway to an essentially 100% mixed condition.
NOTE that this will not in any way affect the equilibrium concentration induced by gravity, which is a nearly homogeneous system, and will in any event not affect the altitude which the molecules can reach.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
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But,it shows how the gasses do seperate even under pressure. I will imagine they will do the same in the tube extending into space. Also, once the hydrogen is purged from the generator casing out the top by the CO2,the CO2 is then purged back out the bottom with compressed atmospheric air admitted in top.So, it works both ways.
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Oui no no no...
They don't naturally seperate except over very large height differences induced by gravity. If you have two clouds of gas at equal height adjacent to eachother, they will mix for intents and purposes perfectly if you give them a few minutes on our small human scale.
You still aren't thinking small, why would any a seperation cause the velocity of the gas molecules to increase without getting too hot? Even if the gasses were truely imiscible then the weight of the hydrogen would still simply push whatever gas you inject out the bottom. If you sealed the tube, then the weight of the hydrogen (gasses do have weight) would just compress the lift gas under it, because it would be pushed down. The gas under the column of hydrogen is also influenced by gravity, and when forces down by the weight of hydrogen, would compress until both the air and hydrogen are the same height as the outside atmosphere, getting you nowhere.
When you are purging a generator, its happening fairly quickly, which doesn't give the gas enough time to throughly mix... but this does not change the underlying physics problem, that hydrogen molecules are (mostly) too slow to reach 400km.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
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There would be no need for it, and the concept would be readily proved quite preposterous, as soon as all concerned understand the molecular, microscale nature of gasses.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
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Ok,why does the H2 purity meter show otherwise at the top vent? As the H2 is pushed out the top by the C02 being admitted through bottom,the percentage of H2 goes down on the meter. As the CO2 is added to the bottom the H2 escapes the top vent. It takes about two volume exchanges to see any change at all.
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The fact that it takes multiple volumes actually proves me correct:
1: The CO2 probably does not reach all the nooks and cranies of the mechanism before it reaches the outlet, hence does not come into contact with all the hydrogen readily.
2: The fact that the hydrogen "guage" doesn't suddenly drop of to near-zero means that the gasses are indeed mixing, and not like oil/water which would be displaced almost immediatly.
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I agree they are still mixing. I have seen that on the purity meter before. Many times I have seen the purity go down during the purging process. However, if I stop during the process and come back later the purity of the H2 rises back up again. This proves the elements seperate again. The CO2 settles to the bottom again over time.
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There will be a LITTLE bit of seperation a equilibrium, but it will be pretty well mixed. I doubt the H2 overall will return to pre-purge concentrations, and then there is experimental error to consider induced by temperature change... Again, how will this get the gas 400km straight up?
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In the same manner in which the generator was purged out.In the bottom with atmosphere and out the top the H2 exits to space.Well if you can get it to the extosphere inside the tube, from there you could beam it to a higher altitude. BTW if you just let the hydrogen in the bottom of the tube how high will it rise up the tube?
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