Structural and dynamic analyses of CH4-C2H6-CO2 hydrates using thermodynamic modeling and molecular dynamic simulation

Adding CO2 into the CH4-C2H6 (C1-C2) hydrate system introduces much complexity to the stability of C1-C2-CO2 hydrates due to the occupancy competition of CO2 for the small and large hydrate cages with CH4 and C2H6 molecules. Also, the effect of weak hydrogen bonding between CO2 and cage H2O molecules has not been elucidated. In this work, the thermodynamic modeling and molecular dynamic (MD) simulation methods are applied to explore the impact of gas composition on the motion of H2O and gas molecules in the C1-C2-CO2 hydrates. According to the calculated results of the thermodynamic model, the sII C1-C2 hydrates are transformed to stable sI hydrates as CO2 is added. The results of the MD simulation indicate that the stability of C1-C2-CO2 hydrates is reduced as the CO2 composition increases. The weak hydrogen bonds between CO2 and cage H2O molecules facilitate the reorientation of H2O. The gas composition will influence the average translational motion as well as the amplitude and the frequency of oscillations in the translational and rotational curves of gas molecules. The variation in the average MSD curves of CH4 molecules is due to different cage occupancy induced by the competition of CO2 for the large hydrate cages with CH4. The oscillation amplitude of CH4, C2H6, and CO2 in the hydrate cages are affected by the interactions between gas molecules, with the dominant interaction changing with the gas composition. The oscillation frequency in the rotational motion curves of CO2 is suppressed by the weak hydrogen bonding between CO2 and cage H2O molecules.