Thermodynamic modeling and process-integration-based optimization of hydrogen liquefaction systems incorporating ortho-para hydrogen conversion

Ortho-para hydrogen conversion plays a crucial role in hydrogen liquefaction; however, its thermodynamic effects on hydrogen liquefaction systems have not yet been systematically evaluated. In this context, an integrated analytical model for ortho-para hydrogen conversion has been developed. This model facilitates the identification of optimal parameters for various types of converters and enables an exploration of how different conversion methods affect energy consumption within hydrogen liquefaction systems. With the energy balance of adiabatic, isothermal, and continuous ortho-para hydrogen conversion considered alongside a single-pass refrigerant cycle, relationships among power consumption, hydrogen, and refrigerant parameters are established. The analysis also delves into how adjustments in para hydrogen concentration affect refrigerant parameters and overall energy efficiency. The effect of ortho-para hydrogen conversion on the hot composite curve is examined, and a correlation is proposed between the inlet and outlet parameters of ortho-para hydrogen conversion, as well as the degree of deviation from continuous conversion, represented by an offset triangle. Based on the assessment of the distinctions between adiabatic, isothermal, and continuous conversions, a key variable optimization strategy is developed using the offset triangle graphical approach. When the continuous conversion, four-stage adiabatic conversion, and four-stage isothermal conversion are utilized in a hydrogen liquefaction system with the binary refrigerant precooling cycle, the specific energy consumptions (SEC) are 6.8172 kWh/kg LH2 , 8.0433 kWh/kg LH2 , and 13.2669 kWh/kg LH2 , respectively. These values correspond to SEC increases of 23.00 %, 45.12 %, and 139.38 %, respectively, compared with the no-conversion case. Following optimization, the SEC of the system with four-stage adiabatic conversion is reduced by 7.45 %, while the coefficient of performance (COP) improves by 8.03 %, compared with the corresponding unoptimized four-stage adiabatic conversion case.