Liquefying a Gas

  1. Everyone was great about answering my question relating temperature and compression of a gas. Here is ultimately the question I was trying to figure out:

    If a gas is compressed and work is done on the gas to compress it either by piston, centrifuge, or whatever, I understand that the work of compression adds to the kinetic energy of the gas molecules and increases the temperature.

    I may be wrong in this assumption, but I understand that a gas, such as oxygen, can be liquefied by pressure. If compressing a gas raises its temperature (average kinetic energy), how does it liquefy?

    I can imagine two answers: (1) with compression and squeezing of the gas molecules closer together, the intermolecular forces of attraction overcome the kinetic energy, despite the increase in kinetic energy, or (2) my assumption is wrong and that compression must be accompanied by a lowering of temperature or kinetic energy, in which case the forces of attraction still prevail.

    Or a third alternatuive is that I am flat wrong for some other reason.

    Any help?
     
  2. jcsd
  3. ZapperZ

    ZapperZ 30,038
    Staff Emeritus
    Science Advisor
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    You're making the assumption of a purely adiabatic compression, which is never done in liquification of gasses. There's nothing to prevent the process of liquification by a combination of compression and cooling by an external source (heat sink/reservoir). A heat engine has that in one part of its cycle.

    Zz.
     
  4. Yes, it's (2). Keep in mind that the ideal gas law, PV=nRT, is about ideal gases. Ideal means the molecules making up the gas have zero volume and no intermolecular forces (ie- completely elastic collisions) That works well enough as an approximation in most cases, especially when dealing with low densities and high temperatures.

    In reality, things are much more complicated than the ideal condition, as is amply demonstrated by liquification, when the gas goes through changes in state. Liquification takes place because of van der Waals forces between the molecules, and this depends on the specific electron configuration of the molecules.

    As far as I know, in practice liquification is always done by actively cooling the gas as it's compressed. After all, why bother to inhibit cooling? It'd be far too expensive to to reach the necessary pressures for liquification at high temperatures, if it's even possible. Consider the center of the sun, where pressures ar extremely high. Liquification doesn't happen (though the material there isn't even a gas, it's a plasma).

    In many cases, liquification is possible with cooling even without compression. Just go outside in the morning and look and the dew.
     
  5. thanks for the answers.

    so when natural gas is liquified, it is normally liquified by compression and cooling or only cooling?
     
  6. A fairly quick google search indicates that natural gas is liquified mostly through extreme cooling, but it looks like some pressuration is used at least by some manufacturers. It's probably cheaper that way. But it seems the main technology is cryogenic.

    Some gasses can be liquified strictly by pressurization at ambient temperatures, such as butane and propane.

    This isn't my field, so I'm just reporting what I saw on a few websites.
     
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