Solubility and freeze-out kinetics of solid nitrogen in liquid hydrogen

The freeze-out of trace impurities poses a critical risk to the reliable and energy-efficient operation of hydrogen liquefaction plants. Nitrogen is one of the most common impurities, yet its solubility in liquid hydrogen remains uncertain. Here, we report the first characterisation of both the solubility and freeze-out kinetics of solid nitrogen in liquid hydrogen using an optical microscopy apparatus. The new equilibrium data were used to refine the solubility model for this binary in the freely available software tool ThermoFAST, yielding a predicted nitrogen solubility of about 0.3 ppm at (0.1 MPa, 20 K). The kinetic measurements show that once this limit is exceeded, solid nitrogen nucleates rapidly at subcoolings below 2 K. Together, these results provide the first direct and quantitative insight into the equilibrium and kinetic conditions governing nitrogen freeze-out in liquid hydrogen, equipping engineers with essential data and models to better assess formation risk and improve the safety and cost-effectiveness of hydrogen liquefaction. • First simultaneous measurements of solubility and freeze-out kinetics of N 2 in LH 2 • Improved solubility model achieves 43 % lower RMS error across all SFE data. • Kinetic analysis shows solid N 2 nucleates rapidly at subcoolings <2 K. • Low interfacial free energy explains faster nucleation relative to other cryogenic solids. • Outcomes provide engineers with essential data/tools to mitigate N 2 freeze-out risk.