The influence of the thermodynamic model of equilibrium-hydrogen on the simulation of its liquefaction

In view of a possible hydrogen infrastructure a number of studies on large-scale hydrogen liquefiers state that conventional plant efficiencies may be substantially increased. Generally those studies consider different (i) working conditions, (ii) operational unit performances and (iii) thermodynamic models, making it difficult to compare their results. The present work focuses specifically on the third issue by assessing the influence of the thermodynamic modeling of the fluid on the simulation outcomes. Numerical approaches to compute the heat capacities as well as the equations of state of hydrogen forms (orthohydrogen and parahydrogen) and their mixtures (equilibrium-hydrogen and normal-hydrogen) are described here. The attention is on equilibrium-hydrogen because the ortho-to-para conversion, which is naturally slow and largely exothermic, is to be catalytically promoted during liquefaction in order to reduce the liquefaction work compared to batch conversion. Because to the knowledge of the authors no commercial code comprises equilibrium-hydrogen, a fluid file for REFPROP is developed and used as a reference case for comparison against four other modeling alternatives that can be readily executed with commercial codes. The comparison is based on the analysis of the cooling curves, which are temperature profiles as functions of specific enthalpies, employed to calculate the ideal liquefaction work and estimate the real one. The results indicate that the choice of the heat capacity model is crucial for the accurate simulation of the overall process, whereas the choice of the equation of state is of minor importance for the overall process, but turns crucial for analyzing local processes and for the dimensioning of the operational units, like turbomachines, that are influenced by volume flows.