Comprehensive Characterization of Interfacial Behavior for the Mixture CO2 + H2O + CH4: Comparison between Atomistic and Coarse Grained Molecular Simulation Models and Density Gradient Theory

The accurate description of the phase equilibria and interfacial behavior of the ternary mixture H2O + CO2 + CH4 is of fundamental importance in processes related with enhanced natural gas recovery, CO2 storage, and gas-oil miscibility analysis. For this reason, the physical understanding and theoretical modeling of this remarkably complex mixture, in a wide range of thermodynamic conditions, constitutes a challenging task both for scientists and engineers. This work focuses on the description of the interfacial behavior of this mixture, with special emphasis on several regions that yield different scenarios (vaporliquid, liquidliquid, and vaporliquidliquid equilibria) and in pressure and temperature ranges related with the practical applications previously mentioned. A comparison between three alternative approaches has been performed: atomistic Monte Carlo simulations (MC), coarse grained molecular dynamics (CG-MD) simulations, and density gradient theory (DGT) have been used to characterize the interfacial region, describing in detail complex phenomena, including preferential adsorption and wetting phenomena even in the ternary triphasic region. Agreement between the results obtained from different methods indicate that the three alternative approaches are fully equivalent to analyze the interfacial behavior. It has been also found that the preferential adsorption of CO2 over H2O interface is greater if compared to CH4 in all conditions characterized. In fact, we have also demonstrated that CH4 under triphasic conditions has very limited influence on the complete wetting of the binary system H2O + CO2.