Investigating Mixed Refrigerant Solidification in Hydrogen Liquefaction Processes

Within hydrogen liquefaction cycles, refrigerant mixtures reach highly critical conditions, with temperatures approaching the solidification temperatures of the pure components. To ensure the feasibility of the hydrogen liquefaction process, it is necessary to verify that such mixtures do not solidify under specified temperature and pressure conditions. This verification is carried out through the minimization of Gibbs free energy by simulating a RGibbs reactor in Aspen Plus® V11. For a given refrigerant mixture, comprising light hydrocarbons, hydrogen and nitrogen, the solidification temperature is determined as a function of pressure. The solidification curve is plotted for four mixed refrigerants that have been utilized in the literature for the precooling section of the hydrogen liquefaction process. The composition of the mixed refrigerant greatly affects the minimum temperature at which a refrigerant can be safely used without incurring solid precipitation, which varies between 60 K and 100 K. This work provides a guide for selecting the appropriate mixed refrigerant composition and operating conditions, with the aim to enhance the efficiency, reliability, and safety of hydrogen liquefaction technologies.