Experimental Verification of Methane-Carbon Dioxide Replacement in Natural Gas Hydrates Using a Differential Scanning Calorimeter

摘要

The methane (CH_4) - carbon dioxide (CO_2) swapping phenomenon in naturally occurring gas hydrates is regarded as an attractive method of CO_2 sequestration and CH_4 recovery. In this study, a high pressure microdifferential scanning calorimeter (HP μ-DSC) was used to monitor and quantify the CH_4 - CO_2 replacement in the gas hydrate structure. The HP μ-DSC provided reliable measurements of the hydrate dissociation equilibrium and hydrate heat of dissociation for the pure and mixed gas hydrates. The hydrate dissociation equilibrium data obtained from the endothermic thermograms of the replaced gas hydrates indicate that at least 60% of CH_4 is recoverable after reaction with CO_2, which is consistent with the result obtained via direct dissociation of the replaced gas hydrates. The heat of dissociation values of the CH_4 + CO_2 hydrates were between that of the pure CH_4 hydrate and that of the pure CO_2 hydrate, and the values increased as the CO_2 compositions in the hydrate phase increased. By monitoring the heat flows from the HP μ-DSC, it was found that the noticeable dissociation or formation of a gas hydrate was not detected during the CH_4 - CO_2 replacement process, which indicates that a substantial portion of CH_4 hydrate does not dissociate into liquid water or ice and then forms the CH_4 + CO_2 hydrate. This study provides the first experimental evidence using a DSC to reveal that the conversion of the CH_4 hydrate to the CH_4 + CO_2 hydrate occurs without significant hydrate dissociation.