Design and performance comparison of large-scale hydrogen liquefaction processes based on mixed refrigerant cycle

Liquid hydrogen (LH2) is an efficient carrier for storage and transportation of hydrogen energy. To investigate the effect of mixed refrigerant cycle (MRC) on the performance of large-scale hydrogen liquefiers, three processes with a capacity of 50 TPD LH2 are designed in a relatively achievable way. Process I utilizes a dual-pressure hydrogen Claude cycle with nitrogen precooling as a reference. Process II incorporates a five-component mixed refrigerant cycle in place of the nitrogen cycle. Process III advances further by replacing the hydrogen cycle in Process II with an additional MRC composed of Ne, He and H2. Sensitivity analysis is applied to optimize the parameters of the three processes prior to the comparison of their heat exchangers performance and the energy and exergy analyses. Due to the synergistic effect of smaller temperature difference and higher refrigeration temperature, the employment of MRC for precooling yields a SEC of 1.350 kWh/kg LH2, 57.1% and 66.3% lower than those of nitrogen cycle and liquid nitrogen, respectively. The sensible heat of the Ne/He/H2 mixture shows a better temperature matching than the latent heat of the refrigerant hydrogen when cooling the product hydrogen, reducing energy consumption by 28.4%. The utilization of MRC for precooling generates more exergy loss in the separators, mixers and throttle valves, but the absence of nitrogen expanders and much lower flow rate of refrigerant save more exergy in total. The essential reason for the progressiveness of Process III is that the integrated heat exchanger in the cryostat achieves the logarithmic mean temperature difference (LMTD) of 2.11 K, effectively reducing the exergy loss of the refrigerant compressors, thereby diminishing the thermodynamic irreversibility.