Temperature-Dependent Structural Transitions in Methane–Ethane Mixed Gas Hydrates

Abstract

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 (72–74 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 solid–solid 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