Hydrogen ortho-para conversion: process sensitivities and optimisation

Abstract

Liquid hydrogen has potential as a storage vector for large-scale hydrogen transportation. Hydrogen liquefaction is however a highly energy intensive process, particularly due to the necessary exothermic spin-isomer conversion from ortho- to parahydrogen. We explore the operation of the main cryogenic plate-fin heat-exchangers (PFHXs) used in hydrogen liquefaction; simultaneous dynamic processes occurring within the PFHX include the heterogeneously catalysed spin-isomer conversion, heat transfer and pressure loss. In this work, these coupled processes are simulated for a 100 tonne/day hydrogen reactant cooled by a helium refrigerant in a PFHX. For the process configuration utilised in this work, a reactor volume of 24.0 m3 and a coolant flow rate of 600 tonne/day were necessary to achieve and outlet parahydrogen fraction of 0.99. Pressure drop through the PFHX packed bed was consistently negligible. We also explored the sensitivity towards model parameters. The outlet parahydrogen fraction is determined to be considerably (∼10 times) more sensitive to reaction rate kinetics relative to heat transfer: therefore, the reactor geometry will be primarily determined by conversion kinetics. The current quantitative understanding of these rate kinetics is however limited. Future experimental research thus needs to focus on quantifying ortho- to parahydrogen kinetics. Destination: Chemical Engineering and Processing: Process Intensification