Measurement of the vapour-liquid equilibrium of binary and ternary mixtures of CO₂, N₂ and H₂, systems which are of relevance to CCS technology

The p-T phase envelopes of four binary and two ternary mixtures of carbon dioxide (CO₂), nitrogen (N₂) and hydrogen (H₂) have been reported at temperatures between 252 and 304K. The compositions of the binary mixtures are x_N2=0.0201 and 0.0399 for CO₂+N₂ and x_H2=0.0300 and 0.0499 for CO₂+H₂, respectively. For the two ternary mixtures of CO₂+N₂+H₂, the compositions are x_N2=0.020, x_H2=0.030 and x_N2=0.040, x_H2=0.030. The experimental data show that the bubble-point pressures of the CO₂-permanent gas mixtures increase substantially when adding only several percent (mole fraction) of N₂, H₂ or both in CO₂. H₂ has the largest effect on the bubble-point pressure. For a mixture with 5% H₂ in CO₂, the bubble-point pressure reaches 10.1MPa at 288.15K, corresponding to an increase of 99% from the vapour pressure of pure CO₂. These vapour-liquid-equilibrium data have also been compared to those predicted using the GERG-2004 and Peng-Robinson (PR) equations of state. Between the two models, the PR equation of state gives the smaller deviations (2.5-4.2%) for the bubble-point lines of CO₂+H₂ when using temperature-dependent binary interaction parameters, while both models show excellent agreement (0.4-2.8%) between the experimental and predicted data for CO₂+N₂. We also successfully model the CO₂+H₂ data using molecular simulation.