Thermodynamic behavior and spectroscopic properties of CO and C3H8 mixed gas hydrates: Implications for hydrate-based gas separation

Abstract Gas mixtures containing carbon monoxide (CO) and propane (C3H8) are encountered in a variety of chemical processes such as the incomplete combustion of fossil fuels, syngas production, and steel plants. Here we investigate the thermodynamic stability and spectroscopic properties of CO-C3H8 mixed gas hydrates for potential applications related to hydrate-based gas separation (HBGS). The CO-C3H8 mixed gas hydrates were formed from CO/C3H8 gas mixtures at pressures of 0.5–1.4 MPa and 274 K. Three-phase (H-Lw-V) equilibrium conditions of the CO-C3H8 mixed gas hydrates were shifted to higher pressure and lower temperature regions with increasing CO concentration in the vapor phase. X-ray diffraction patterns of the CO-C3H8 mixed gas hydrates suggest the formation of cubic Fd3m structures (sII hydrate). Sold-state 13C cross polarization/magic angle spinning NMR and Raman spectroscopy demonstrated the preferential occupation behavior of CO and C3H8 molecules in the hydrate cages. Thermodynamic predictions using the predictive Soave□Redlich□Kwong model with the UNIFAC group contribution method were in excellent agreement with the experimental phase equilibria and the cage occupancy analysis by 13C NMR and Raman spectroscopy. The compositions of the CO-C3H8 mixed gas hydrates were examined by gas measurements after hydrate dissociations, which were very close to the 13C NMR analysis results and theoretical predictions. The recovery ratio of C3H8 was always higher than 1 and gradually increased with decreasing C3H8 concentration in the vapor phases. These results indicate that the HBGS process can effectively be used for selectively separating CO and C3H8 from CO/C3H8 gas mixtures.