Effects of Gas Compositions on Tuning the Kinetics of CH₄/CO₂ Hydrates with 1,3-Dioxolane: Implication for Hydrate-Based Biogas Storage

A key to the application of hydrate-based biogas storage (primary components as CH₄ and CO₂) lies in elucidating the mechanism of the slow kinetics and seeking solutions to enhance the kinetics. In this study, the low-toxicity 1,3-dioxolane (DIOX) was employed to promote CH₄/CO₂ hydrate formation at various DIOX concentrations (<i>C</i>_DIOX = 5.56 mol %, 3.00 mol %, and 1.00 mol %) under different gas mixture compositions (CH₄/CO₂ = 71.7 mol %/28.3 mol %, 47.5 mol %/52.5 mol %, and 23.9 mol %/76.1 mol %). The phase equilibria, cage occupancy, kinetics, and resulting morphology of CH₄/CO₂ + DIOX mixed hydrates were acquired for analysis. The thermodynamic promotion of DIOX diminishes as the CO₂ composition increases and <i>C</i>_DIOX decreases. An increase in the CO₂ composition reduces the occupancy of CH₄ in the sII hydrate 5¹²6⁴ cages. Additionally, CO₂ dissolution in DIOX solution weakens the fractionation of the gas mixture due to hydrate formation. At <i>C</i>_DIOX = 5.56 mol % and 3.00 mol %, two distinct stages of hydrate growth were identified based on morphology observation: (a) initial slow gas uptake due to the formation of hydrate film at the gas-liquid interface and (b) enhanced gas uptake stage due to the rupture of the hydrate film. Increasing the CO₂ composition, employing solutions with a memory effect, and moderately reducing <i>C</i>_DIOX can expedite hydrate film rupture and shorten the period of hydrate film formation. The findings offer insights into technical-feasible strategies to enhance the kinetics of CH₄/CO₂ hydrates for biogas and CO₂-rich containing natural gas storage.