Structural predictions for clathrate hydrates of binary mixtures of ethane, propane, and argon

Recently, Maekawa [T. Maekawa, Fluid Phase Equilib. 243 (2006) 115] reported on experimental phase equilibrium data for binary mixtures of ethane (C2H6), propane (C3H8), and noble gases (Ar, Kr, Xe). Structural transitions for the Xe–C3H8 and Kr–C2H6 systems were found, but not for the Ar–C2H6 system. Presented here are phase equilibria predictions using a mathematical method in which the van der Waals–Platteeuw statistical mechanical model with the Lennard-Jones and Devonshire approximation is solved directly for the intermolecular potential between the guest molecules and the host molecules. This method is applicable for hydrates in which the Langmuir constants can be computed, either using experimental data or from ab initio data. This method was used to predict existing mixed hydrate phase equilibrium data without any fitting parameters for hydrates from ethane, propane, and argon mixtures. In the ethane–argon hydrate system, a structural transition from structure II to I was predicted to occur for ethane vapor mol fractions below yeth,waterfree = 0.320 at 273.5 K and yeth,waterfree = 0.319 at 276.5 K. These conditions lie outside of the range of conditions considered by Maekawa [T. Maekawa, Fluid Phase Equilib. 243 (2006) 115] thus explaining the inability to find structure transitions for the Ar–C2H6 system. This result, when examined in context with the work of Maekawa, illustrates the power in pairing theoretical modeling with the design of experiments.