Experimental study of ortho- parahydrogen conversion at 77 k

Hydrogen liquefaction is a viable energy storage and export option on account of its high volumetric energy density; however, it faces several significant challenges. One of these is that hydrogen has two molecular configurations: ortho-hydrogen and para-hydrogen (o-H2 and p-H2). These configurations show different physicochemical properties, which have a significant impact on long-term liquid hydrogen storage. At standard conditions, hydrogen contains 75% o-H2 and 25% p-H2 (so called “normalhydrogen”). Following liquefaction of normal-hydrogen at 22 K, ortho-to-para (OP) conversion can cause serious issues related to heat release caused by it being exothermic. The conversion is ‘forbidden’ and consequently slow (taking of the order of days to weeks) which results in liquid hydrogen evaporation during storage and transport (the heat of conversion exceeds the heat of vaporization). Consequently, production of stable liquid hydrogen requires the use of catalytical conversion of the o-H¬2 content to p-H2 during cooling prior to liquefaction. Current kinetic data for this ortho-para conversion in the literature is however limited, unclear and predominately several decades old; yet it is required for optimized modelling and design of the required combined heat exchanger and conversion unit operations. In this work we will detail a novel experimental apparatus to measure such hydrogen ortho-to-para reaction kinetic data at 77 K, associated with the pre-cooling stage of the hydrogen liquefaction process. Several catalysts including a commercial hydrous ferric oxide (Ionex®) were used for the conversion whilst the resultant o-H2 to p-H2 ratio was quantitatively measured on-line using Raman spectroscopy via an appropriate fiber-optic cable. Conversion data will be presented as a function of flowrate, catalyst mass and pressure and compared against existing literature kinetic data and models. Unexpected conversion in the presence of particulate silica will be highlighted and discussed.