Quantitative cryogenic orthohydrogen conversion kinetics

Liquid hydrogen is a widely considered option for storing and transporting hydrogen. Hydrogen however exists as two nuclear spin isomers: ortho- and parahydrogen. Conversion from orthohydrogen to parahydrogen (OPC) is an exothermic reaction that should, ideally, be completed during liquefaction. Reaction kinetic data for this catalyzed conversion is however limited. To help address this, a cryogenic hydrogen facility was designed and constructed, which allows for systematic determination of OPC kinetic data as a function of space velocity, catalyst loading, temperature and pressure. An in-line Raman spectrometer is used to quantify effluent hydrogen isomer composition. Determination of OPC kinetic data is demonstrated using the commercial catalyst, IONEX, for temperatures of (30 to 70) K, pressures of (2 MPa and 4 MPa) and hydrogen flow rates of (10 to 500) SCCM. This measurement range, which encompasses hydrogen in both a supercritical and a liquid phase, covers the typical operating conditions encountered during hydrogen liquefaction. A first-order reaction kinetic model for the conversion reaction, as is widely used in the design of hydrogen liquefaction processes, was fitted to the resultant 516 data points across this measurement range, which allowed for re-tuning of the required kinetic parameters. A combined plate-fin heat exchanger and reactor was then simulated using these revised parameters, which led to an increase of 33 % in the required reactor volume hosting the catalyst.