Hydrates production with gaseous CO2/C3H8 and CH4/C3H8 (90/10 vol%) mixtures and definition of the role of propane during CO2/CH4 replacement processes

Within the range of temperatures suitable for CO₂/CH₄ replacement, propane can form hydrates at widely milder conditions than those required for methane and carbon dioxide hydrates. Recent studies proved that the addition of minor quantities of propane to carbon dioxide strongly enhances replacement process, both in terms of methane recovery and carbon dioxide storage. At the same time, the capture of propane remains limited, even if the thermodynamic conditions are widely suitable for its capture. This study experimentally provided a coherent explanation of such a process. Hydrates were formed and dissociated on a lab-scale reactor with binary mixtures containing 90 vol% of CO₂/CH₄ and 10 vol% of C₃H₈; the results were compared with the phase equilibrium conditions of the pure species, obtained with the same procedure. The promoting/inhibiting effect of propane on the forming system was quantified and explained in terms of typology of hydrate structure formed and cage occupancy. Based on the results achieved in this study, the replacement mechanism, in presence of propane, was finally characterized and compared with the results obtained when replacement was carried out with the same procedure but with pure carbon dioxide and CO₂/N₂ mixtures.