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In [[physics]], the '''Joule-Thomson effect''' is a process in which the temperature of a real gas, as differentiated from an [[ideal gas]],  is either decreased or increased by letting the gas expand freely at constant [[enthalpy]] (which means that no heat is transferred to or from the gas, and no external work is extracted).<ref name=Perry>{{cite book | author=Perry, R.H. and Green, D.W. | title=Perry's Chemical Engineers' Handbook | publisher=McGraw-Hill Book Co. | year=1984 | id=ISBN 0-07-049479-7}}</ref><ref name=Roy>{{cite book|author=Bimalendu Narayan Roy|title=Fundamentals of Classical and Statistical Thermodynamics|edition=|publisher=Wiley|year=2002|id=ISBN 0-470-84313-6}}</ref><ref name=Edmister>{{cite book|author=Wayne C. Edmister and Byunk Ik Lee|title=Applied Hydrocarbon Thermodynamics|edition= 2nd edition (Volume 1)|publisher=Gulf Publishing|year=1984|id=ISBN 0-87201-855-5}}</ref>. Ideal gases neither heat nor cool upon being expanded at constant enthalpy.


It's 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]].
The Joule-Thomson effect is sometimes referred to as the ''Joule-Kelvin effect''. Engineers often refer to it as simply the ''J-T effect''.
==Description==
When a real gas expands freely at constant enthalpy , the temperature may either decrease or increase, depending on the initial temperature and pressure.  For any given pressure, a real gas has a Joule-Thomson '''[[inversion temperature]]''',<ref name=Roy/> above which expansion at constant enthalpy causes the gas temperature to rise, and below which expansion at constant enthalpy causes the gas to cool. For most gases at [[atmospheric pressure]], the inversion temperature is fairly high (above room temperature), and so most gases at those temperature and pressure conditions are cooled by [[isenthalpic]] expansion.
By contrast, an [[isentropic]] expansion of a gas, in which the gas does positive [[mechanical work]] while expanding, always causes the gas temperature to decrease.
In other words, a real gas expansion through a well-insulated [[valve]] (in which the gas has done no mechanical work and no [[heat transfer]] has occurred) may undergo either a decrease  or increase in temperature. However, a gas expansion through a [[gas turbine]] driving a [[compressor]] or other mechanical equipment always undergoes a drop in temperature because the gas has done work.
==The Joule-Thomson coefficient==
The change of temperature with a decrease of pressure in a Joule-Thomson process is the '''Joule-Thomson coefficient''':<ref name=Perry/><ref name=Edmister/><ref>[http://www.chem.arizona.edu/~salzmanr/480a/480ants/jadjte/jadjte.html Joule Expansion] (by W.R. Salzman, Department of Chemistry, [[University of Arizona]])</ref>
:<math>\mu_{JT} = \left( {\partial T \over \partial P} \right)_H</math>
The value of <math>\mu_{JT}</math> is typically expressed in [[Kelvin|K]]/[[Pascal (unit)|Pa]] or [[Celsius|°C]]/[[bar (unit)|bar]] and depends on the specific gas, as well as the temperature and pressure of the gas before expansion.
For all real gases, it will equal zero at some point called the '''inversion point''' and, as explained above, the Joule-Thomson '''inversion temperature''' is the temperature where the coefficient changes sign (i.e., where the coefficient equals zero).  The Joule-Thomson inversion temperature depends on the pressure of the gas before expansion.
In any gas expansion, the gas pressure decreases and thus the sign of <math>\partial P</math> is always negative. With that in mind, the following table explains when the Joule-Thomson effect cools or heats a real gas:
{| class="wikitable"
!If the gas temperature is!!then <math>\mu_{JT}</math> is!!since <math>\partial P</math> is!!thus <math>\partial T</math> must be!!so the gas
|-
|align=center|below the inversion temperature||align=center|positive||always negative||align=center|negative||align=center|cools
|-
|align=center|above the inversion temperature||align=center|negative||always negative||align=center|positive||align=center|heats
|}
[[Helium]] and [[hydrogen]] are two gases whose Joule-Thomson inversion temperatures at one [[atmosphere (unit)|atmosphere]] are very low (e.g., about −222 °C for helium).  Thus, helium and hydrogen will warm when expanded at constant enthalpy at typical room temperatures.  On the other hand [[nitrogen]] has an inversion temperature of 621 K (348 °C) and [[oxygen]] has an inversion temperature of 764 K (491 °C): the two most abundant gases in air can be cooled by the Joule-Thomson effect at typical room temperatures.<ref name=Perry/>
It should be noted that <math>\mu_{JT}</math> is always equal to zero for ideal gases (i.e., they will neither heat nor cool upon being expanded at constant enthalpy).
==Physical mechanism==
As a gas expands, the average distance between [[molecule]]s grows.  Because of intermolecular attractive forces, expansion causes an increase in the [[potential energy]] of the gas.  If no external work is extracted in the process (''free expansion'') and no heat is transferred, the total energy of the gas remains the same because of the [[conservation of energy]].  The increase in potential energy thus means a decrease in [[kinetic energy]] and therefore in temperature. 
A second mechanism has the opposite effect. During gas molecule collisions, kinetic energy is temporarily converted into potential energy. As the average intermolecular distance increases, there is a drop in the number of collisions per time unit, which causes a decrease in average potential energy. Again, total energy is conserved, so this leads to an increase in kinetic energy (temperature).  Below the Joule-Thompson inversion temperature, the former effect (work done internally against intermolecular attractive forces) dominates, and free expansion causes a decrease in temperature.  Above the inversion temperature, the latter effect (reduced collisions causing a decrease in the average potential energy) dominates, and free expansion causes a temperature increase.
==Applications==
As for how the Joule-Thomson effect is achieved in practice:
* The real gas is allowed to expand through a throttling device (usually a [[valve]]) which must be very well insulated to prevent any heat transfer to or from the gas.
* There must be no external work extracted from the gas during the expansion (the gas must not be expanded through a [[turbine]], for example).
The effect is applied in the [[Linde cycle]], a process used in the [[petrochemical industry]] for example, where the cooling effect is used to liquefy gases, and also in many [[cryogenic]] applications (e.g., for the production of liquid [[oxygen]], [[nitrogen]] and [[argon]]). Only when the Joule-Thomson coefficient for the given gas at the given temperature is greater than zero can the gas be liquefied at that temperature by the Linde cycle. In other words, a gas must be below its inversion temperature to be liquified by the Linde cycle. For this reason, simple Linde cycle liquifiers cannot normally be used to liquify [[helium]], [[hydrogen]] and [[neon]].
== References ==
{{reflist}}

Revision as of 20:57, 17 February 2008

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