The hydrate phase equilibrium behaviors of tetrahydrofuran (THF) + CH 4 , THF + CO 2 , CH 4 + CO 2 , and THF + CO 2 + CH 4 were investigated over wide ranges of temperature, pressure, and concentration. The dissociation conditions of THF + CH 4 and THF + CO 2 hydrates were shifted to lower pressures and higher temperatures from the dissociation boundaries of pure CH 4 and pure CO 2 hydrates. X-ray diffraction results revealed that the CH 4 + CO 2 and THF + CO 2 + CH 4 hydrates prepared from a CH 4 /CO 2 (50:50) gas mixture formed structure I and II clathrate hydrates, respectively. Raman measurements provided detailed information regarding the cage occupancy of CH 4 and CO 2 molecules encaged in the hydrate frameworks. For the CH 4 + CO 2 hydrates, the concentrations of CO 2 in the hydrate phase were higher than those in the vapor phase. In contrast, for the THF + CO 2 + CH 4 hydrates, the concentrations of CO 2 in the hydrate phase were lower than those in the vapor phase.

Three-phase equilibria for the carbon dioxide + methane + water system were obtained by employing the isobaric temperature search method. Based on these isobaric hydrate equilibrium studies, the ternary hydrate, water-rich liquid, and vapor equilibrium lines generated at different compositions of carbon dioxide and methane were all located between two three-phase equilibrium lines of simple hydrates formed by a single guest component. The upper quadruple points where the four phases hydrate, water-rich liquid, CO2-rich liquid, and vapor coexist were measured for the composition range of 100−82.50 mol % carbon dioxide. Below 82.50 mol % carbon dioxide, the upper quadruple points do not exist because none of the components in the vapor phase, neither methane nor carbon dioxide, is able to liquefy at these conditions. In addition, two-phase equilibria of vapor and hydrate were also determined at the three different pressures 20, 26, and 35 bar. Judging from the resulting T − x diagram, the concentration of carbon dioxide in the hydrate phase was found to be higher than 90 mol % when the corresponding equilibrium vapor-phase composition was more than 40 mol % carbon dioxide. The carbon dioxide concentration and relative selectivity over methane in the hydrate phase appeared to increase with decreasing pressure.

The solubility of carbon dioxide in monoethanolamine (MEA) + ethylene glycol (EG) + water and monoethanolamine (MEA) + poly(ethylene glycol) (PEG) + water has been measured at 313.2 K and at partial pressure ranges of carbon dioxide up to 2500 kPa. The concentrations of aqueous mixtures are 15.3 mass % MEA + 15.3 mass % EG, 15.3 mass % MEA + 42.3 mass % EG, 15.3 mass % MEA + 15.3 mass % PEG, and 15.3 mass % MEA + 42.3 mass % PEG. In each case, the solubility was represented as functions of partial pressures of carbon dioxide.

Landfill gas (LFG), which is primarily composed of CH(4), CO(2), and N(2), is produced from the anaerobic digestion of organic materials. To investigate the feasibility of the storage and transportation of LFG via the formation of hydrate, we observed the phase equilibrium behavior of CO(2)-CH(4)-N(2) mixture hydrates. When the specific molar ratio of CO(2)/CH(4) was 40/55, the equilibrium dissociation pressures were gradually shifted to higher pressures and lower temperatures as the mole fraction of N(2) increased. X-ray diffraction revealed that the CO(2)-CH(4)-N(2) mixture hydrate prepared from the CO(2)/CH(4)/N(2) (40/55/5) gas mixture formed a structure I clathrate hydrate. A combination of Raman and solid-state (13)C NMR measurements provided detailed information regarding the cage occupancy of gas molecules trapped in the hydrate frameworks. The gas storage capacity of LFG hydrates was estimated from the experimental results for the hydrate formations under two-phase equilibrium conditions. We also confirmed that trace amounts of nonmethane organic compounds do not affect the cage occupancy of gas molecules or the thermodynamic stability of LFG hydrates.

The potential of hydroquinone (HQ) clathrates to selectively separate/capture CO2 from mixtures of CO2 and H2 gas is investigated. Selective CO2 enclathration within cages of a HQ framework, from mixtures of various concentrations of the two gases, are identified using 13C nuclear magnetic resonance (NMR) and Raman spectra. Spectroscopic results indicate that CO2 molecules from the gas mixture are exclusively accommodated into the cages of HQ clathrates and that the H2 molecules are thereby concentrated in the remaining gas phase. Quantitative evidence is presented by deconvolution of NMR peaks and an elemental analyzer that CO2 molecules can be captured into the clathrate compound even at the partial CO2 fugacity of 0.38 MPa in the gas mixtures tested. Storage capacity of 53–64.4 L of CO2/kg of HQ for the HQ clathrate with full conversion makes a HQ-clathrate-based process viable for pre-combustion CO2 separation.