Thermodynamic properties and phase transitions in the H2O/CO2/CH4 system

Free energy densities as functions of temperature, pressure and the compositions are required for development of a three-component phase field theory able to describe phase transitions involving hydrates. We have evolved the extended adsorption theory due to Kvamme and Tanaka [B. Kvamme, H. Tanaka, J. Phys. Chem. 99 (1995) 7114–7119] by deriving the Gibbs free energy density surface for CO2 and CH4 hydrates. The composite Gibbs free energy surface for the liquid phases has been obtained from an SRK equation of state and gas solubilities measured outside the region of hydrate stability. Our comprehensive thermodynamic model has been shown to predict water–hydrate equilibrium properties that are in a good agreement with experimental data. lar dynamics simulations of hydrates in contact with water at 200 bar and several temperatures have allowed us to estimate hard-to-establish properties needed as input parameters for the practical applications of proposed theories. The 5–95% interval for the interface width of methane hydrate/liquid water system has been estimated to be 8.5 Å. When the interface Gibbs free energy is known as well, the phase field theory contains no adjustable parameters. This work demonstrates how this theory can be applied to the kinetics of hydrate phase transitions. The process of hydrate growth from aqueous solution was found to be rate-limited by mass transport. As prescribed by the combination of the first and the second laws of thermodynamics, solute concentration near the hydrate approaches the equilibrium between hydrate and aqueous solution rather than the solubility limit.