Cryogenic hydrogen liquefaction by high-expanding supersonic nozzles: A numerical study

Supersonic separation is a promising alternative to traditional cryogenic cycles that use energy-intensive Joule-Thomson valves, as it can increase the liquefaction yield by more than three times in LNG production processes. This study investigates the potential of supersonic nozzles with high area ratios for efficient hydrogen liquefaction through non-equilibrium condensation under realistic operation conditions. A one-dimensional CFD approach is employed to model the condensation process under cryogenic and high-pressure conditions at the nozzle inlet. To accurately characterize cryogenic gas and droplet behavior, the Peng-Robinson equation of state is combined with a revised Gyarmathy growth model, incorporating the Clausius-Clapeyron relation. The results demonstrate that lowering the inlet temperature and increasing the inlet pressure of the nozzle significantly enhance liquefaction yield, achieving a maximum liquid phase production of 22% under hypersonic conditions. Analysis also reveals that hydrogen condensation is very sensitive to the inlet conditions, such that the liquefaction yield is twice as sensitive to inlet temperature as to inlet pressure. Notably, the high specific heat capacity of hydrogen enables the formation of micron-sized hydrogen droplets, which facilitates the separation process.