Parametric Analysis of Ortho-to-Para Conversion in Hydrogen Liquefaction

Hydrogen is an energy carrier and is produced just like electricity. Hydrogen is liquefied for storage and transportation purposes to overcome the shortcomings of its low molecular weight and energy density per unit volume. The liquefaction of hydrogen is different from that of other substances as it involves a reactive transformation of its isomers: ortho-hydrogen and para-hydrogen. As the temperature decreases, the equilibrium concentration shifts toward a higher para- content from the normal concentration of 25 % at 25 °C. Para-hydrogen is preferred because of its lower boil-off rate, which is a major challenge at cryogenic temperatures. Ortho-para conversion, heat leak, sloshing, and flashing are considered as the reasons for such losses. The self-conversion rate of hydrogen in a non-equilibrium state is extremely slow; however, at cryogenic temperatures, o-p conversion is an exothermic affair. From the liquefaction point of view, this exothermic heat of conversion is an added work, increasing he liquefaction energy requirement by about 15 %. Catalysts are used to achieve the equilibrium concentration of p-H₂ at a finite rate. Little work has been done from the process systems point of view regarding o-p H₂ conversion. Therefore, parametric analysis of this vital conversion reaction, the spatial distribution of intermediate heat exchangers, and impact on the energy efficiency of the liquefaction process have been studied and partially presented here.