1. Properties of the hydrogen molecule

1.1 Molar mass

The molar mass of hydrogen is low (2.016 g/mol), fourteen times lower than that of nitrogen (28.013 g/mol).

This low molar mass has several consequences for hydrogen properties and the liquefaction process.

Under given thermodynamic conditions (pressure and temperature), the kinetic energy of the molecules is fourteen times lower for hydrogen than for nitrogen. This characteristic rules out the use of centrifugal compressors for hydrogen. Indeed, the operation of these compressors is based on the transformation of kinetic energy into compression energy. For a given hydrogen compression ratio, too many impellers would be required to keep within an acceptable speed range (below 30,000 rpm), and this solution would be uneconomical....