Large-scale hydrogen liquefaction process and its thermodynamic cycle

Objective Hydrogen, as a clean and efficient energy source, has the potential to meet future energy demands. To enhance production efficiency and reduce costs in large-scale hydrogen liquefaction, a new liquefaction process utilizing innovative mixed refrigerants has been developed.

Methods The process features a large-scale hydrogen liquefaction system designed for a liquid hydrogen output of 120 t/d. This system employs liquid nitrogen pre-cooling and a reversed Brayton cycle for deep cooling, using a mixed refrigerant as the working medium, and achieves hydrogen liquefaction through a combination of heat exchanger cooling and expansion cooling. HYSYS was utilized to simulate and analyze the process flow, with key parameters determined through sensitivity analysis and a trial-and-error method. The focus was laid on energy and exergy analysis, with the performance of the liquefaction process illustrated by the composite curve of the heat exchanger.

Results The mixed refrigerant composed of 12％ hydrogen, 7％ neon, and 81％ helium exhibited the highest exergy efficiency. Under this condition, the specific energy consumption was 6.99 kW·h/kg the coefficient of performance was 0.1885, and the exergy efficiency reached 33.96％. The total exergy loss of the system was 2,738.8 kW, primarily attributed to the expander, whose exergy efficiency decreased with lower temperatures. The exergy efficiency of the heat exchanger in the deep cooling section could generally exceed 90％ by limiting the minimum temperature difference through the composite curve of the heat exchanger. Sensitivity analysis indicated that the pressure and temperature of feed hydrogen had minimal impact on the energy consumption of the system.

Conclusion During the deep cooling stage of hydrogen liquefaction, energy consumption can be reduced and exergy efficiency can be improved by adding an appropriate amount of hydrogen and neon to the helium-based mixed refrigerant. The new liquefaction process proposed in this study offers advantages such as low equipment investment, compact structure, and reduced energy consumption, providing a valuable reference for future research on hydrogen liquefaction.