Bubble points and densities of H2 (up to ∼ 5%) in CO2-rich binary systems

In this study, experimental measurements of the bubble points of binary mixtures containing varying concentrations of CO₂ (99.5 %, 99 %, 98.5 %,98 %, and ∼ 95 %) with hydrogen (H₂) were made. These measurements were carried out from low temperatures (240.20 K) up to 294.84 K (with uncertainties of 0.14 K) using the constant composition expansion method. The experimental data were used to validate two thermodynamic models - the Peng-Robinson and Multi-Fluid Helmholtz Energy Approximation Equation of state (PR-EoS and MFHEA-EoS). From the results, the presence of H₂ in CO₂ at concentrations between 0.5 and 5 % caused a significant (∼19–980 %) positive deviation from the phase behaviour of CO₂ stream compared to that of pure CO₂. This effect intensified with higher concentrations of H₂ and decreased with rising temperatures. Both models demonstrated good agreement with the experimental bubble point data, exhibiting <4 % average deviation for the system. Notably, the PR-EoS outperformed the MFHEA-EoS, showing <3 % average deviation. Densities of CO₂ (99.5 %) with H₂ were measured at 278.14, 298.34, 323.55, and 348.40 K and pressures up to 35 MPa. While the densities of CO₂ (94.99 %) with H₂ were measured at 278.06, 288.13, 298.26, and 323.53 K and pressures up to 35 MPa using a vibrating tube densimeter which was calibrated using water and hydrogen. For the 99.5 % CO₂ binary mixture, the average absolute relative deviations (AARD) of the model predictions were 0.09 % and 0.26 % against MFHEA and PR EoS respectively. The AARD of the model predictions for the 94.99 % CO₂ were 0.33 % and 1.49 % with MFHEA and PR EoS respectively. Furthermore, even at low concentrations (0.5 %), the presence of H₂ led to a substantial reduction (>35 %) in the density of the mixture compared to that of pure CO₂ at lower pressure conditions with this effect becoming more pronounced at higher temperatures and concentrations of H₂. Both models predicted the densities of the system well (with <2 % deviations from the experimental data), though MFHEA-Eos was more accurate with <0.4 % maximum relative deviation (MaxRD) for all the data points.