Solubility prediction of high boiling point impurities in liquid hydrogen by solid-liquid equilibrium methods

Due to the low temperature characteristic of liquid hydrogen (LH₂), most of impurities, such as nitrogen (N₂) and oxygen (O₂), could be dissolved out and converted to solid particles in LH₂ if their amount exceeds their solubility limits. To avoid safety issues such as ice blocking, the impurity solubility and the composition of probable solid air should be accurately predicted. Four different models, including Scatchard-Hildebrand (SH) model, modified SH (MSH) model, PR equation of state (EoS) and SRK EoS, were employed to predict the solubilities of N₂, O₂, water (H₂O), and carbon dioxide (CO₂) in LH₂. For the SH and MSH models, a new approach of dealing with solute-solvent solubility parameters at the solvent's melting temperature was proposed, which could effectively corrected unreasonable prediction by the original models. To improve prediction accuracy, a characteristic parameter of l₁₂ = −0.058 was suggested in the improved MSH model, and the binary interaction coefficient (k₁₂) in form of a bivariate linear formula correlated with temperature and pressure was fitted in the EoS models. The results showed that the solubility levels for N₂, O₂, CO₂, and H₂O were 10⁻⁸, 10⁻¹², 10⁻⁵¹, 10⁻⁸⁹ mol fraction in the normal LH₂ at 20 K. Based on differences in solubility levels of N₂ and O₂, different air existence modes, including complete dissolution of N₂ and O₂ molecules, N₂ dissolution but solid oxygen formation, and solid air formation, could occur successively with the air leakage amount increases.