(218f) The Thermodynamic Modeling of Methane-Carbon Dioxide-Liquid Water–Hydrates

Extensively existing methane hydrates (MH) in nature could be a next-generation clean energy resource. The development of methane from MH by carbon dioxide (CO₂) injection has attracted more attention because the substitution of methane by carbon dioxide in the hydrate phase helps to maintain the local safety. This process involves in the dissociation of MH and the formation of mixed hydrates of carbon dioxide-methane in the place of CH₄-hydrate. Knowledge of Vapor (V)-Liquid Water (Lw)-Hydrate (H) equilibrium conditions of mixed CO₂-CH₄ hydrate is required for the process. While classic cubic equations have been applied to investigate the phase equilibria, the statistical association fluid theory (SAFT) equation of state along with van der Waals-Platteeuw theory is used in this work to account for the effect of association interactions in the mixture

In this work, we analyzed the thermodynamic properties of CO₂-water, CH₄-water, and CO₂-CH₄-water system based on the latest version of SAFT equation of state, i.e. SAFT2-RPM, which is applied to calculate the fugacities in vapor and liquid phases. Residual Helmholtz free energy is computed as the sum of hard sphere repulsion, hard chain formation, dispersion and association terms. Van der Waals-Platteeuw model is employed to calculate the fugacity of water in the liquid and hydrate phases. The interaction between water and CO₂/CH₄ in hydrate cavities is calculated by Langmuir constant. The Kihara cell potential with spherical cell assumption is applied to estimate the cavity potential function.

The three-phase V-Lw-H equilibrium conditions for CO₂-CH₄-water system are modeled at various temperatures ranging from 273.15 to 286.15 K and various CO₂ vapor concentrations in the mixture. The calculated equilibrium conditions are in a good agreement with experimental data with the average deviation less than 1%.