Tuning effect of DIOX on the thermodynamics and cage occupancy of CH4/CO2 + DIOX mixed hydrates

Abstract

Hydrate technology could provide a novel approach to deliver convenient CO2-rich natural gas. The identification of a suitable and environmental-friendly promoter for enhancing CO2/CH4 hydrate kinetics is critical. In this study, we evaluated the thermodynamics, kinetics, and morphology of CH4/CO2 + DIOX mixed hydrates in the presence of a thermodynamic promoter 1,3-dioxolane (DIOX) at and below its stoichiometric concentration (SC, CDIOX = 5.56 mol%). Due to the thermodynamic preference of DIOX enclathration in mixed hydrates formation, a decrease in CDIOX in the unconverted solution was observed post nucleation below SC and the hydrates formed exhibit significant heterogeneity in cage occupancy. As CDIOX in the unconverted solution decreases, the DIOX concentration in hydrate (RDIOX) and the thermodynamic stability of the newly formed hydrates decreases. Based on the step-wise heating design, the thermodynamic stability of the hydrates formed below SC with high-RDIOX is close to that of hydrates formed at SC, while hydrates with low-RDIOX undergo dissociation at their P-T trajectories above the measured phase equilibria. Raman measurement indicates that the lower-RDIOX hydrates possess a higher CO2 composition. This study provides new insights into the tuning effect of DIOX on mixed hydrates formation and the basis for using low thermodynamic promoter concentration in hydrate-based technologies.