How Do Varying THF Concentrations Affect the CH₄ Cage Occupancy in CH₄+THF sII Hydrates? A Thermodynamic Approach

Hydrate-based gas storage holds a huge potential to become a promising technology in the future, provided that the storage conditions are moderate enough. Tetrahydrofuran (THF), a widely known promoter, is used in this study to understand its effect on hydrate phase equilibrium conditions and cage occupancies thereafter. Here, we present a fugacity-based thermodynamic model to study the three-phase liquid-hydrate-vapor (L-H-V) equilibria of CH4 hydrate in pure water (sI hydrate) and in 0.48–5.56 mol % THF solutions (sII hydrate). The model uses the vdW-P theory for the hydrate phase, Peng–Robinson–Stryjek–Vera (PRSV) EOS for the vapor phase, and a modified UNIFAC method for liquid phase calculations. We have introduced a guest–guest interaction parameter for the Langmuir constant estimation. The model also considers the occupancy of CH4 in large cages of the CH4+THF sII hydrate. The AADs of the model predictions from the experimental values for CH4 hydrate in pure water and THF solutions are 1.52 and 2.75%, respectively. Although THF reduced CH4 hydrate phase equilibrium pressures drastically in the beginning at 0.48 mol %, no significant changes are observed beyond 3.0 mol % THF concentration. With the increase in THF concentration, the CH4 occupancy in small and large cages decreased, and the THF occupancy in large cages increased. CH4 occupancy in sII hydrate large cages is minimal but increases slowly with the temperature. Based on the findings, CH4 hydrates can be stored at moderate conditions of 298.15 K and 4.71 MPa using an optimal 3.0 mol % THF concentration. The thermodynamic approach developed in this study also provides a significant basis for future hydrate-based gas storage technology.