Thermophysical properties of CO₂ + N₂ and CO₂ + H₂ mixtures: Molecular simulation and thermodynamic model validation

Molecular simulations were performed to evaluate thermophysical properties namely density, viscosity, and vapor-liquid equilibrium (VLE) of pure CO₂ and its binary mixtures with N₂ and H₂ as impurities. These simulations were conducted under pressures and temperatures relevant to carbon capture, utilization, and storage (CCUS) operations. Molecular dynamics (MD) simulations were employed to estimate density and viscosity, while VLE data were obtained using Grand Equilibrium (GE) Monte Carlo (MC) simulations. The results were compared with predictions from several models. These included the Span and Wagner and the Multi-Fluid Helmholtz Energy Approximation (MFHEA) equations of state, which were used for density and VLE predictions. For viscosity of pure CO₂, the Laesecke et al. correlation was applied. Mixture viscosities were evaluated using the Lennard-Jones (LJ), Residual Entropy (ES-NIST), and SUPERTRAPP models. The Pxy data for CO₂ + N₂ and CO₂ + H₂ binary mixtures showed excellent agreement with both MFHEA model predictions and experimental data. The GE method proved effective in predicting VLE behavior; however, further analysis is needed, especially near-critical conditions. In terms of density, excellent agreement was achieved, with average uncertainties below 1 kg/m³. However, larger deviations were generally observed in the supercritical region, as well as at higher impurity concentrations. Viscosity data exhibited higher uncertainties compared to experimental values, particularly at lower temperatures and higher densities. Nevertheless, most results remained within the simulation uncertainty bounds, for which average values of 4.51% and 4.81% were observed for pure CO₂ and the binary mixtures, respectively. This highlights the reliability of our modeling approach in predicting thermophysical properties.