Significance of ortho-para hydrogen conversion in the performance of hydrogen liquefaction process

Abstract

Hydrogen is an energy vector and is produced just like electricity. In order to overcome the shortcomings associated with its low molecular weight and energy density per unit volume, hydrogen is liquefied for storage and transportation purposes. The liquefaction of hydrogen differs from that of other substances as it involves the reactive transformation of its isomeric states. At 25 °C, molecular hydrogen consists of 75% orthohydrogen and 25% of parahydrogen. As the normal boiling point, hydrogen essentially exists in the para-state, which is preferred because of its lower boil-off gas rate. However, the conversion of ortho-to-para hydrogen is an exothermic reaction, and this enthalpy of conversion enhances the total reversible work by about 15%. Little work has been done regarding ortho-to-para hydrogen conversion from the process systems point of view. Therefore, parametric analysis of this vital conversion reaction was studied with potential impact on the performance of cryogenic heat exchangers, reactors configuration and mode of operation, and probable impact on the energy efficiency of the liquefaction process. An alternate approach to simulate the reaction is also proposed. The results show that the current approaches to process design need to be changed. The study opens avenues for more in-depth analysis and optimization approaches to present a holistic framework for future integrated energy systems.