Experimental Measurements for Equilibrium Conditions of Carbon Dioxide Hydrate Mixtures with Each Additive of 1,3-Dioxane, Acetamide, Cyclopentanol, Cyclopentanone, or 1,3,5-Trioxane

Gas hydrate research studies have received much attention in recent years. In addition to the availability of naturally existing methane hydrates as a new energy source, carbon dioxide hydrates also offer the potential for carbon capture and sequestration. This study reports the equilibrium temperatures and pressures at the dissociation points where three phases of hydrate (H), liquid water (L-w), and vapor (V) coexisted for carbon dioxide hydrates with various additives. The equilibrium conditions for carbon dioxide with pure water and each additive of 1,3dioxane (0.10, 0.15, and 0.20 mass fractions), acetamide (0.10 and 0.20 mass fractions), cyclopentanol (0.08 mass fraction), cyclopentanone (0.05 mass fraction), or 1,3,5-trioxane (0.15 mass fraction) were experimentally measured in the pressure range of 1.5-3.3 MPa using the isochoric method. Additives of 1,3dioxane, cyclopentanol, cyclopentanone, or 1,3,5-trioxane presented a promotion effect on the formation of carbon dioxide hydrates. The maximum promotion result under isobaric conditions was 7.8 K with the addition of 0.20 mass fraction of 1,3-dioxane. The addition of acetamide showed an inhibition effect, with a maximum inhibition result of 5 K at 0.20 mass fraction. Comparisons of the promotion or inhibition effect for various additives are illustrated. To simulate seawater conditions, equilibrium conditions for carbon dioxide with brine and each additive of acetamide (0.10 and 0.20 mass fractions) or 1,3-dioxane (0.20 mass fraction) were also reported. This study employed the Clausius-Clapeyron equation to estimate possible hydrate structures for systems with various additives. The estimated hydrate structures were in good agreement with the promotion or inhibition effect of carbon dioxide hydrate mixtures with various additives.