Temperature-Dependent Structural Transitions in Methane Ethane Mixed Gas Hydrates

A thermodynamic interpretation of the interconversion between structures I and II occurring in methane (CH4) + ethane (C2H6) mixed gas hydrates is of great importance from both fundamental and applied perspectives. The present study experimentally confirms the predicted temperature dependence of structural changes in the lower transition region (7274 mol % of CH4 balanced with C2H6) of the CH4 + C2H6 + H2O system. The measurements of phase equilibria and Raman spectra, at the macroscopic and microscopic levels, respectively, reveal the phase transition point at which the structural rearrangements occur. The isothermal data reported here clearly demonstrate significant changes of transition behavior from sII inhibition to sII promotion in accordance with increased equilibrium temperatures. This solidsolid transition trend may be dictated by the peculiar structural feature of the CH4 + C2H6 mixed gas hydrates on the basis of the comprehensive experimental and theoretical data published previously. The predominance of CH4 over C2H6 in cage occupancy may lead to a change in guest molecules playing a dominant role in determining the preferential hydrate structure.