In this case, the temperature remains near 300K, well above the constituents' critical temperature (N2
154K), and the attraction between air molecules stays small. The other imperfection of air is the volume of the molecules, which is neglected in the ideal gas theory, but is important at 300bar.
A simple way to check how important this volume is:
- As an ideal gas, a mole at 300K and 400bar would take 62cm3, hence 513kg/m3 for compressed oxygen for instance.
- But liquid oxygen weighs only 1141kg/m3 (at 1atm and 112K), and the liquid's volume resembles the molecules' volume
- So the volume of the gas under these conditions exceeds the one predicted by the ideal gas law.
Under these conditions, met as well in hydraulics pressure accumulators which use a bladder filled with nitrogen, PV hence exceeds nRT, so Z>1.
From Nist, at 300K:200bar Z=1.06