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Replacing gas in a fixed volume without losing pressure?

  1. Oct 26, 2013 #1
    Hi everyone! Sorry if I am posting this in the wrong place.

    Im doing my Msc in chemistry, and I have a little problem with my current experimental setup. This setup have been used before by Phd students, but not for the same experiments I want to do. Any advice would be greatly appriciated!

    The experimental setup consists of a stainless steel cell with a fixed volume placed inside an incubator (large fridge). There are three valves connected to the incubator, (1) introduction of water (by applying a vacuum to the cell beforehand) and evacuating the cell of water, (2) application of pressure, which is connected to a chosen gas flask, (3) a sampling valve for gas sampling and evacuating the cell of gas.

    Im want to form methane hydrates, which form at high pressures and low temperatures. I fill the cell with an amount of water, and apply pressure determined through the pressure of the gas flask. The container is then cooled to form hydrates, which gives two phases: hydrate and excess methane gas. What I would like to do is apply carbon dioxide gas, which is thought to replace the methane molecules in the hydrates. However, if I evacuate all the methane gas from the container, the hydrates will melt. I need to evacuate the cell of excess methane gas, while adding CO2 gas without a pressure drop.

    I was thinking of gradually adding CO2 gas, while evacuating the excess methane gas from another valve. But I think they will form a mixture through diffusion, which makes it hard to completely evacuate the methane gas, no matter how many times i try.

    Any ideas?
  2. jcsd
  3. Oct 26, 2013 #2


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    If your temperature remains low enough, the bulk of the methane hydrate will remain frozen.
    There will be some residual methane in any event as you introduce the CO2, from slow breakup of methane hydrate. So you must expect some mixing.
    What reactions are you working to determine? CO2 mixes very well with water and gives rise to carbonic acid.
    So there will be both chemical as well as physical effects to monitor in your experiment.
  4. Oct 26, 2013 #3


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    You will need to devise a separation process - for example, if one of the gasses is lighter than the other, remove the lighter gas from the top of the cell while introducing the heavier gas at the bottom. Or vice versa.

    If you can get any kind of separation you can then continue to bleed off the "unwanted" gas plus an admixture of the CO2 by injecting CO2 at an over-pressure, and continue to flow the gasses in and out for a longer time. Then the CO2 will slowly dilute the remaining methane, while maintaining pressure. When you are done you can reduce the CO2 pressure.
  5. Oct 27, 2013 #4
    First, thank you for your answer.

    As one of the valves is located at the bottom of the cell (introduction of water), it might be possible to attach this one to the CO2 source after the hydrates have been formed? Considering that methane is the lighter gas, this can then be removed through the sampling valve at the top of the cell, like you explained. However as the hydrates which form in the cell are solid will the CO2 be able to "pass" this solid phase and displace the methane?

    What about replacing the sampling valve with a "safety valve" that automatically releases gas above a certain set pressure. By applying CO2 gas at the bottom, this valve will "bleed of" gas higher than this set pressure, eg at pressures above 80bar. I guess this method will consume much gas before the cell is diluted of methane?

    I know there is also a gas booster available on my faculty. By incorporating this, the pressures are not limited by the pressure in the gas flasks.
  6. Oct 27, 2013 #5
    I would like to examine the kinetics of a CO2-CH4 exhange in the hydrates. Pressure and temperature will be logged on a computer, while gas sampling will be analyzed and quantified on a GC. Im aware of the high solubility of CO2, which is why I would probably measure the pH after each experiment.
  7. Oct 27, 2013 #6


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    Seems to me the solution might be to flush the system with an inert liquid such as propane before introducing the CO2.
    That way you do not have unknown methane residues to confuse your measurement and you avoid the need to depressurize.
  8. Nov 1, 2013 #7


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    Maybe use lighter weight helium from above to expel the heavier reaction gases from the bottom of the container.
    Maybe consider a gas centrifuge for separation of the expelled gas.
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