Thermodynamic, structural, and kinetic studies of cyclopentane + CO2 hydrates: Applications for desalination and CO2 capture

Abstract

This study examines the thermodynamic, structural, and kinetic characterizations of cyclopentane (CP) + CO2 hydrates in the presence of NaCl for their potential applications in desalination and CO2 capture. Powder X-ray diffraction (PXRD) demonstrated that CP molecules fully occupied the large (51264) cages of CP + CO2 hydrate (structure II), whereas approximately 62% of the small (5 1 2) cages were captured by CO2 molecules. To determine accurate hydration numbers and dissociation enthalpies of CP and CP + CO2 hydrates, cage occupancy of guest molecules revealed by Rietveld refinement of PXRD patterns was combined with integrated areas of endothermic heat-flow curves obtained by a high-pressure micro-differential scanning calorimeter (HP μ-DSC). The addition of CP contributed to a significant stabilization of CO2 hydrate due to CP molecules’ occupation of the large (51264) cages and a subsequent structural transformation to sII. The shift in the equilibrium curves of CO2 + CP + NaCl hydrates to lower-temperature and higher-pressure regions was more significant at higher NaCl concentrations. The cage-filling behavior of guest molecules and the growth patterns revealed by collected time-dependent Raman spectra indicated that the presence of NaCl retarded the growth rate of CO2 + CP hydrate. The experimental results covering hydrate phase equilibria, dissociation enthalpies, hydration numbers, guest distributions, and enclathration behaviors of CO2 + CP hydrates form fundamental reference data for understanding cage-specific guest occupation and growth behaviors of gas hydrates in the presence of salts. The results will also be useful when designing hydrate-based desalination and CO2 capture technologies using CP.