Joule-Thomson effect

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In physics, the Joule-Thomson effect, or Joule-Kelvin effect, is a process in which the temperature of a gas is decreased by letting the gas expand adiabatically.

It is named for James Prescott Joule and William Thomson, 1st Baron Kelvin who established the effect in 1852 following earlier work by Joule on Joule expansion in which a gas expands at constant internal energy.

1 Description
2 Negative Joule-Thomson effect
3 Applications
4 Bibliography
5 See also
6 External links

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Description

The relationship between temperature, pressure and volume of a gas is simply described by the gas laws. When volume is increased, the gas laws do not uniquely determine what happens to the pressure and temperature of the gas. In general, when a gas expands adiabatically, the temperature may either decrease or increase, depending on the initial temperature and pressure. For a fixed pressure, a gas has a Joule-Thomson (Kelvin) inversion temperature, above which expansion causes the temperature to rise, and below which expansion causes cooling. For most gases, at atmospheric pressure this temperature is fairly high (above room temperature), and so gases can be cooled by expansion.

The change of temperature with respect to change of pressure in a Joule-Thomson process (adiabatic condition) is the Joule-Thomson (Kelvin) coefficient

<math>\mu_{JT} = \left( {\partial T \over \partial P} \right)_H</math>

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Negative Joule-Thomson effect

One notable exception on the rule that temperature decreases with expansion of the gas is helium, whose Joule-Thomson inversion temperature at one atmosphere is about 40 K (−233 °C). The only other gas that warms upon expansion at standard conditions is hydrogen. The physical reason behind this is as follows. During collisions, kinetic energy is temporarily converted into potential energy. This means that a drop in density leads to a drop in the number of collisions per time unit, hence causing a decrease in potential energy, which due to the conservation of energy in turn leads to an increase in kinetic energy, and hence temperature. In the case of hydrogen and helium, the conversion of potential energy into kinetic energy more than outweighs the increase in potential energy due to electromagnetic forces.

The Joule-Thomson inversion temperature is the temperature where the coefficient changes sign.

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Applications

The effect is applied in the Linde technique as a standard process in the petrochemical industry for example, where the cooling effect is used to liquefy gases, in many cryogenic applications (e.g. for the production of liquid oxygen, nitrogen and helium) for liquifying gases. Only when the Joule-Thomson coefficent for the given gas at the given temperature is greater than zero, the gas can be liquefied at that temperature. In other words, a gas may only be liquefied below its critical temperature.

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