In situ Raman spectroscopic observation of the temperature-dependent partition of CH4 and CO2 during the growth of double hydrate from aqueous solution

Sequestration of CO2 in hydrate form within deep oceanic sediments, to simultaneously replace methane trapped in submarine hydrate, has been considered as an efficient method for CO2 reduction. Consequently, research efforts are underway to deeply understand the properties of CH4 and CO2 hydrates. In this work, CH4 + CO2 double hydrate crystals were grown from aqueous solution in a capillary high-pressure optical cell. The quantitative relationships between Raman intensity and dissolved gas (i.e., CH4 and CO2) concentration in water and in hydrate were established. The partition coefficients of CH4 and CO2 between the hydrate and aqueous phases were measured at 5 constant temperatures from 275.15 to 293.15 K. The equilibrium concentrations of dissolved CH4 and CO2 decrease with decreasing temperatures, indicating more dissolved CH4 and CO2 will be transferred from the aqueous to the hydrate phase at lower temperature. With decreasing temperature, the molality of CO2 in hydrate increases and the molality of CH4 in 51262 cavities decreases, while the sum of the molality of CH4 in 51262 cavities and molality of CO2 in hydrate keeps constant, indicating that CO2 molecules can replace more methane molecules under lower temperature only by occupying the 51262 cavities.