Experimental Measurements for Equilibrium Conditions of Carbon Dioxide Hydrate Mixtures with Each Additive of 2-Methoxyethyl Ether, 1,1,3,3-Tetramethylguanidine, 4-Methylmorpholine, 1-Ethyl-3-Methylimidazolium Chloride, Urea, and 1,3-Cyclohexanebis(methylamine)

Due to decarbonization issues, there is growing concern about greenhouse gases, especially carbon dioxide (CO₂). Gas hydrate research is a potential solution to energy needs and CO₂ storage tasks. Naturally occurring methane hydrates provide natural gas availability, while CO₂ hydrates are also receiving widespread attention for carbon capture, storage, and pipeline flow assurance. This study measured the thermodynamic equilibrium temperatures and pressures at the dissociation points for CO₂ hydrate mixtures containing various additives. These results provide fundamental data on the thermodynamic phase boundaries of CO₂ hydrate mixtures where three phases of hydrate (H), liquid water (L_w), and vapor (V) coexisted. The equilibrium conditions for CO₂ with pure water and various additives including 2-methoxyethyl ether (0.20 and 0.30 mass fractions), 1,1,3,3-tetramethylguanidine (0.20 and 0.30 mass fractions), 4-methylmorpholine (0.10 and 0.20 mass fractions), 1-ethyl-3-methylimidazolium chloride (0.10, 0.20, and 0.30 mass fractions), urea (0.10, 0.20, and 0.30 mass fractions), and 1,3-cyclohexanebis(methylamine) (0.10 and 0.20 mass fractions) were experimentally measured in the pressure range of 1.54–3.58 MPa using the isochoric and temperature cycling method. These additives all had an inhibition effect on the formation of CO₂ hydrates, and the maximum average inhibition effect of each additive on the equilibrium temperature ranged from 5 to 11 K. To simulate seawater conditions, equilibrium conditions for CO₂ hydrate mixtures with three of these additives are also reported. The Clausius–Clapeyron equation was used to estimate possible hydrate structures. The results show that the CO₂ hydrate mixtures exhibited possible sI structures after adding each additive in this study.