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. (author)