Experimental determination and modeling of structure II hydrates in mixtures of methane plus water plus 1,4-dioxane

Hydrate phase equilibrium conditions were measured with a Cailletet apparat us in the pressure range 2 < 14 MPa. The investigated 1,4-dioxane concentra tions were 1, 2, 5, 7, 10, 20 and 30 mol% relative to water. The results sh ow that adding 1,4-dioxane up to concentrations of 6 mol%, about the stoich iometric ratio of large sII cages to water (1/17), reduced the equilibrium pressure of hydrate formation. Adding I,4-dioxane beyond 6 mol% caused a sl ow increase of the equilibrium pressures. The hydrate phase equilibria data were modeled as equilibrium between a liquid phase of water and 1,4-dioxan e, with a small amount of methane, and a sII hydrate of 1,4-dioxane and met hane. The chemical potential of the hydrate phase was described using the v an der Waals and Platteeuw theory. Activity coefficients of the liquid phas e were calculated by a van Laar relation, based on literature 1,4-dioxane water VLE data. The predicted equilibrium pressures calculated were within 5% of the data up to a concentration of 20 mol% 1,4-dioxane relative to wa ter. (C) 1999 Elsevier Science B.V. All rights reserved.