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Gases compression at critical point. LNG (Methane) storage.

  1. Jul 2, 2015 #1
    There are claims or at least assumptions that: "upon approaching "critical point" pure gases become super compressible. You could compress them many times without pressure increase and store like this if you maintain exact temperature and pressure needed. It opens possibility to superdense gas storage. Impurities tend to destroy this state by triggering convertion to a gas phase. If critical point temperature is exceeded this state is destroyed."

    Methane critical point is -82 C at 45 atm. Does it open a new possibilities to store methane or natural gas at liquid form? Natural gas isn't pure gas, does it have a critical point? Could some additives extend temperature brackets of a critical point?
  2. jcsd
  3. Jul 3, 2015 #2
    No. Say you maintain -62oC (i.e., comfortably above the critical temperature). You would need to compress it to 1000 atm to reach a density of about 450 kg/m3 which is the density of liquid methane at 1 atm and -182oC. Storing it underground at 13oC and 1000 atm gets you to a density of about 350 kg/m3. If you mean pressures far beyond 1000 atm, the mechanical and cost issues quickly overwhelm any possible advantage.
  4. Jul 3, 2015 #3
    I thougt that pressurising gas near critical point and pressure relieves you from need to maintain pressure after gas been liquefied. So, for example you compressed methane at -82 and 45 atm. to 1000 atm. and reduced pressure again to 45 atm. You do not need to maintain pressure of 1000 atm. after liquefaction.
    Am I wrong? They seem store CO2 and methane like this already. What is a point of it?
  5. Jul 3, 2015 #4
    No. Reducing pressure will just allow the liquid to revaporize.
  6. Jul 3, 2015 #5
    Liquid CO2 is usually stored at ambient temperature and about 56 atm. One example is a CO2 fire extinguisher. Pull the handle and the liquid immediately vaporizes out the nozzle. Methane stored as a liquid is at cryogenic temperatures and modest pressure. Liquid storage is used to minimize the volume.
  7. Jul 3, 2015 #6
    To store methane at ambient temperature and pressure in liquid form we need to increase its boiling point up 200 C. If you pressurise water to 100 atm. it will rise its boiling point more than 200 C. Why the same thing doesn't work with methane? Is there some chemical additives which could dramatically increase methane boiling point? For example, there exist additives which they add to diesel fuel to reduce its boiling point and prevent from freezing. How do they work? And how much could they change it?
  8. Jul 3, 2015 #7
    Because water at 300 C is below its critical temperature and methane at ambient temperature is above its critical temperature.
  9. Jul 3, 2015 #8
    Well, I mean, if we compress water at 20 C to 100 atm it will rise its boiling point to 309 C. 280 C more up. But if we compress liquid methane at -164 C to 100 atm it will not rise its boiling point to +20 C. Not even to -80C (below its critical temperature). Because if it would we would be able to store liquid methane at ambient temperature and not to high pressure. But probably I'm grasping what you mean... What if we mix liquid methane with some long chained hydrocarbons? For example polyethylene? How much it should rise its boiling point? And what proportion could work?
    Last edited: Jul 3, 2015
  10. Jul 4, 2015 #9
    Congratulations, you have discovered that water behaves very differently from a non-polar substance like methane.
    Methane has no useful solubility in polyethylene.
    For another possibility, search "adsorption of methane on activated carbon."
  11. Jul 4, 2015 #10
    I meant some kind of liquid long chaned hydrocarbons similar to polyethilene by molecular mass and carbon chain length.
    Adsorbents do not provide better energy density than CNG. And adsorbent degrade pretty fast. I meant storing methane in solved rather than adsorbed form. For example if you want to increase boiling point of gasoline you could mix it with heavier hydrocarbons. What if you mix liquid methane with heavier hydrocarbons and later would separate it with help of some membrane before burning? Also, what about solving methane in some supercritical liquids, for example in heavy hydrocarbons?
  12. Jul 4, 2015 #11
    The technical term for what you're looking for is "magic dust."
  13. Jul 4, 2015 #12
    :smile: It could be, but nowdays they find and synthesise more and more substances with really unusual properties...
  14. Jul 10, 2015 #13
    What is known about methane solubility in supercritical fluids (SF)? It is claimed SF are excellent as a solvents. Is there some exact data on how many volumes of methane you could solve in a record SF?
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