Study of the Effect of the Degree of Overcooling During the Formation of Hydrates of a Methane-Propane Gas Mixture on the Equilibrium Conditions of Their Decomposition

A study is made of the effect of the initial degree of supercooling on the equilibrium conditions of decomposition of hydrates of a model gas mixture comprised of 95.66 mol % CH4 + 4.34 mol. % C3H8. To ensure a high degree of conversion of water to hydrate, 0.1 wt. % sodium dodecyl sulfate (SDS) solution was used as the liquid phase in experiments performed in an RCS6 Sapphire Rocking Cell instrument. The hydrates were produced by cooling under isochoric conditions in a GHA350 gas hydrate autoclave and under isochoric-isothermic conditions on the RCS6 instrument. The hydrated samples that were obtained were decomposed by heating under isochoric conditions at the rate of 0.2 K/h. Analysis of the decomposition curves indicates that, depending upon the initial conditions, both mixed methane-propane hydrates of various compositions and methane hydrate are formed when the test gas mixture is hydrated. It is demonstrated that the proportion of the CH4 hydrate which is formed increases with an increase in the initial degree of overcooling during hydrate formation. The curve depicting the equilibrium hydrate decomposition conditions for the gas mixture has a more complex configuration than the simple exponential curve proposed earlier. The data that is obtained points to the existence of a multiplicity of equilibrium conditions for the hydrate decomposition of gas mixtures, with the exact conditions in a specific case depending on the degree of overcooling of the system during hydrate formation.