Mixed hydrates of methane and water-soluble hydrocarbons modeling of empirical results

The hydrate disappearance conditions (H - L-W - V--> L-W - V transition) in the water - methane - water-soluble hydrocarbon system were measured and modeled. The hydrocarbons in the experiments are tetrahydrofuran (THF), 1,3-dioxolane and tetrahydropyran, and THF, 1,3-dioxolane and acetone in the modeling study. Experiments and calculations, which correspond qualitatively and quantitatively very well, show a large reduction in the equilibrium pressure upon addition of the hydrocarbons. The minimum pressure is about 5-6 mol % hydrocarbon relative to water, independent of temperature and applied hydrocarbon, with the composition almost equal to the ratio between the number of large cages and water molecules in structure II hydrates (5.88%). Based on this observation, the hydrates are assumed to be structure II methane - hydrocarbon mixed hydrates. The model is also used to calculate the occupancies of the hydrate cavities. No experimental data are available to compare the predicted values, but the results help understand observed phenomena better.