Avoiding the formation of solid oxygen in hydrogen liquefaction plants is essential for safe and economic plant operation. As a pure substance, the freezing point of oxygen is about 35 K above the temperature at which hydrogen is liquefied, meaning that solid oxygen can freeze-out of liquid hydrogen even when its concentration has been reduced to trace levels. The formation of solid oxygen poses a number of threats to hydrogen liquefaction plants including the potential to block cryogenic heat exchange equipment; subsequent accumulation of the solid phase can also create a potentially explosive mixture of oxygen and hydrogen. To prevent these dangerous freeze-out events from occurring, the concentration of oxygen in the feed to a hydrogen liquefier must be decreased below its solubility limit in the fluid phase. However, for oxygen, this solubility limit is not known; previous research suggests that it is below 1 part per million (ppm) but the exact solubility was not able to be resolved within the resolution of the technique.
This uncertainty in the solubility limit means that absorbers used to reduce the concentration of impurities in hydrogen must be over-sized (increasing costs) to ensure safe operation of the plant. In this project, high-resolution optical measurements of the solid-fluid equilibrium temperature for the oxygen-hydrogen binary will be used to tune thermodynamic models of the solubility of oxygen in liquid hydrogen to close this knowledge gap. This will allow Air Products to better assess the risk of a solid oxygen freeze-out even occurring during the production of liquid hydrogen.
Partners: The University of Western Australia, Air Products
Project Leader: Prof. Eric May
Duration: 1 year