Temperature, absolute Joule-Thomson inversion

This rapid adiabatic expansion is sufficient to cool the nitrogen to below its boiling point of 77°K, so this is a way to make liquid nitrogen. There is a temperature for each gas called the Joule-Thomson inversion temperature and cooling occurs if the initial temperature is below that temperature but the gas heats upon expansion if the initial temperature is above the inversion temperature. At room temperature He is above its inversion temperature and will actually heat up upon expansion. Although there is also a pressure effect, there are absolute temperatures for this effect. For He the temperature is 51°K, for H2 202°K, for N2 621°K, and for O2 it is 764°K (see discussion at http //en.citizendium.org/wiki/Joule-Thomson effect). Thus, air (N2 + O2) can be liquefied by adiabatic expansion starting from room temperature and 1 atm, but He and H2 must be precooled to below their Joule-Thomson inversion temperatures. 

Older proeesses used Joule-Thomson eooling entirely. The Joule-Thomson effeet is defined as the eooling that oeeurs when a highly eompressed gas is allowed to expand in sueh a way that no external work is done. This eooling is inversely proportional to the square of the absolute temperature. The system worked satisfaetorily, but it required mueh higher pressures to remove the same amount of energy. 

---