Comparative study on thermodynamic performance of hydrogen liquefaction processes with various ortho-para hydrogen conversion methods

Catalytic conversion of ortho-para hydrogen is essential for hydrogen liquefaction. Among the three ortho-para hydrogen conversion methods, i.e., isothermal, adiabatic and continuous conversion, it has already been accepted that the continuous conversion is the most energy-efficient. However, studies on quantitative analyses and the underling mechanisms are still lacking. In this paper, the three conversion methods are evaluated based on a reference 5 t/d-scale hydrogen liquefaction process with a self-developed numerical model. Optimizations are conducted to identify the lowest specific energy consumption (SEC) of the hydrogen liquefaction process with different conversion methods. The three conversion methods are compared in terms of the temperature distribution along the process, the conversion-heat-associated exergy and SEC. The optimization results show that the continuous-conversion based process reaches the lowest SEC, i.e., 11.38 kWh/kgLH2, which is 21.8% and 28.7% lower than the adiabatic and isothermal ones. Exergy analyses indicate that the exergy consumption associated with dissipating the conversion heat is the lowest for the continuous conversion (139.54 kW), followed by the adiabatic conversion and then the isothermal conversion, i.e., 174.94 kW and 273.90 kW, respectively. This work demonstrates the differences quantitively and reveals its mechanism thermodynamically, which would be helpful for understanding, and designing hydrogen liquefaction processes.