Hydrate thermal dissociation behavior and dissociation enthalpies in methane-carbon dioxide swapping process

The swapping of methane with carbon dioxide in hydrate has been proposed as a potential strategy for geologic sequestration of carbon dioxide and production of methane from natural hydrate deposits. However, this strategy requires a better understanding of the thermodynamic characteristics of CH4 and CO2 hydrate as well as (CH4 + CO2) or (CH4 + CO2 + N2) mixed hydrates (since (CO2 + N2) gas mixture is often used as the swapping gas), along with the thermal physics property changes during gas exchange. In this study, a high pressure micro-differential scanning calorimetry (HP μ-DSC) was performed on synthesized gas hydrates to investigate the dissociation behavior of various hydrates. The hydrate dissociation enthalpies were determined by both μ-DSC measurement and Clapeyron equation. For the single guest molecule hydrate system, the average dissociation enthalpies of CH4 hydrate and CO2 hydrate measured by integrating the endothermic peak area are 55.01 kJ·mol−1 and 58.96 kJ·mol−1, respectively, which are very close to the values calculated by Clapeyron equation. However, in the multicomponent guest hydrates system, the μ-DSC measured dissociation enthalpies of the (CH4 + CO2) binary hydrates and (CH4 + CO2 + N2) ternary hydrates are a little higher than that of Clapeyron equation, it was found that their dissociation enthalpies are located between the limiting values of pure CH4 hydrate and CO2 hydrate, increasing with the mole fraction of CO2 in hydrate phase. By monitoring the heat flow changes with the μ-DSC apparatus, it was observed that there was no noticeable dissociation or reformation process of hydrate occurring in the CH4 − CO2/(CO2 + N2) swapping, which indicates that most CH4 hydrate forms (CH4 + CO2) or (CH4 + CO2 + N2) mixed hydrates directly instead of dissociating into liquid water or ice first. The dissociation equilibrium data obtained from the endothermic thermograms of the mixed hydrates after CO2 and (CO2 + N2) swapping demonstrates that about 66% and 85% of CH4 in hydrate phase are replaced, respectively.