Thermal dissociation behavior and dissociation enthalpies of methane-carbon dioxide mixed hydrates.

Replacement of methane with carbon dioxide in hydrate has been proposed as a strategy for geologic sequestration of carbon dioxide (CO(2)) and/or production of methane (CH(4)) from natural hydrate deposits. This replacement strategy requires a better understanding of the thermodynamic characteristics of binary mixtures of CH(4) and CO(2) hydrate (CH(4)-CO(2) mixed hydrates), as well as thermophysical property changes during gas exchange. This study explores the thermal dissociation behavior and dissociation enthalpies of CH(4)-CO(2) mixed hydrates. We prepared CH(4)-CO(2) mixed hydrate samples from two different, well-defined gas mixtures. During thermal dissociation of a CH(4)-CO(2) mixed hydrate sample, gas samples from the head space were periodically collected and analyzed using gas chromatography. The changes in CH(4)-CO(2) compositions in both the vapor phase and hydrate phase during dissociation were estimated based on the gas chromatography measurements. It was found that the CO(2) concentration in the vapor phase became richer during dissociation because the initial hydrate composition contained relatively more CO(2) than the vapor phase. The composition change in the vapor phase during hydrate dissociation affected the dissociation pressure and temperature; the richer CO(2) in the vapor phase led to a lower dissociation pressure. Furthermore, the increase in CO(2) concentration in the vapor phase enriched the hydrate in CO(2). The dissociation enthalpy of the CH(4)-CO(2) mixed hydrate was computed by fitting the Clausius-Clapeyron equation to the pressure-temperature (PT) trace of a dissociation test. It was observed that the dissociation enthalpy of the CH(4)-CO(2) mixed hydrate lays between the limiting values of pure CH(4) hydrate and CO(2) hydrate, increasing with the CO(2) fraction in the hydrate phase.