Visual observations of CH 4 + CO 2 hydrate crystal growth formed at the gas/liquid interface and in liquid water presaturated with a mixed gas have been made. The compositions of the CH 4 + CO 2 gaseous mixture were 40 : 60 and 30 : 70 for the gas/liquid interface observations, 30 : 70 and 70 : 30 for water saturated with the guest gas. The feed gas compositions of the CH 4 and CO 2 gaseous mixture were 40 : 60 and 30 : 70 for the gas/liquid interface observations, or 30 : 70 and 70 : 30 for liquid water. The crystal morphology of the CH 4 + CO 2 hydrate observed in both feed gas compositions was similar. This may be ascribed to the fact that the molar ratios of CO 2 to CH 4 in the liquid phase ranged from 90 : 10 to 97 : 3 due to the greater solubility of CO 2 in water. These results suggest that the crystal morphology of the CH 4 + CO 2 hydrate may be controlled by the guest composition in the liquid phase, not by the feed gas composition. As the system subcooling increased, the shape of the hydrate crystals changed from polygons to sword-like or dendrites. The implications for the process design of the hydrate-based technologies are discussed based on the observations.

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Owing to its strong oxidizing power, ozone (O 3 ) is utilized in various industrial processes and commercial activities, such as the decontamination of air and water, disinfection of medical instruments and hospital equipment, sterilization of perishables, and bleaching of organic compounds. However, ozone in the gaseous state reacts with itself and rapidly decomposes to oxygen (O 2 ). Owing to this reaction, it is generally considered that ozone cannot be stored and transported like other industrial gases.In 1964, McTurk and Waller [1] presented the idea of storing ozone in the form of a clathrate hydrate, a crystalline solid compound framed by interlinked cages made up of hydrogen-bonded water molecules, in which the ozone molecules could be separated from each other by the cage walls and thus prevented from mutually interacting to cause the ozone-to-oxygen reaction. In this pioneering study, the authors experimentally demonstrated the formation of an sII O 3 + CCl 4 double hydrate from pure ozone and carbon tetrachloride (CCl 4 ) and showed ten sets of four-phase (hydrate + O 3 -rich gas + CCl 4 -rich liquid + H 2 O-rich liquid) equilibrium pressure-temperature data, in which the CCl 4 served as the "help-guest substance" for decreasing the hydrate-forming pressure. However, they reported no actual test of preserving the enclathrated ozone. No subsequent study on ozone-containing hydrates has been reported in the literature for more than 40 years. In the patent application document released in 2007, Masaoka et al.[2] stated that they formed an O 3 + O 2 hydrate by spraying water into a reactor charged with an O 3 + O 2 gas mixture (containing O 3 at a mole fraction of about 5 %) and maintained at 13 MPa and À25 8C. Based on an analysis of the gas released from this hydrate while being decomposed, they estimated the ozone content in the hydrate to be 2.3 g L À1 (ca. 0.2 % in mass fraction). They also described that the hydrate could be preserved for 10 days in a closed container conditioned at 13 MPa and À25 8C without causing a substantial loss of its ozone content.More recently, we showed that hydrates formed from an O 3 + O 2 gas mixture and CCl 4 or xenon (Xe) and cooled to around À20 8C can preserve ozone at a mass fraction on the order of 0.1 % for over 20 days under atmospheric pressure. [3] It should be noted that such in-hydrate ozone concentrations are higher than the typical ozone concentration in "ozonated water" for disinfection use by three orders of magnitude. Besides the ozone preservation tests, we measured the fourphase equilibrium for the O 3 + O 2 + CCl 4 hydrate-forming system (O 3 mole fraction in the gas phase = 6.9 AE 0.8 %) in the temperature range from 2.4 to 4

Owing to its strong oxidizing power, ozone (O 3 ) is utilized in various industrial processes and commercial activities, such as the decontamination of air and water, disinfection of medical instruments and hospital equipment, sterilization of perishables, and bleaching of organic compounds. However, ozone in the gaseous state reacts with itself and rapidly decomposes to oxygen (O 2 ). Owing to this reaction, it is generally considered that ozone cannot be stored and transported like other industrial gases.In 1964, McTurk and Waller [1] presented the idea of storing ozone in the form of a clathrate hydrate, a crystalline solid compound framed by interlinked cages made up of hydrogen-bonded water molecules, in which the ozone molecules could be separated from each other by the cage walls and thus prevented from mutually interacting to cause the ozone-to-oxygen reaction. In this pioneering study, the authors experimentally demonstrated the formation of an sII O 3 + CCl 4 double hydrate from pure ozone and carbon tetrachloride (CCl 4 ) and showed ten sets of four-phase (hydrate + O 3 -rich gas + CCl 4 -rich liquid + H 2 O-rich liquid) equilibrium pressure-temperature data, in which the CCl 4 served as the "help-guest substance" for decreasing the hydrate-forming pressure. However, they reported no actual test of preserving the enclathrated ozone. No subsequent study on ozone-containing hydrates has been reported in the literature for more than 40 years. In the patent application document released in 2007, Masaoka et al.[2] stated that they formed an O 3 + O 2 hydrate by spraying water into a reactor charged with an O 3 + O 2 gas mixture (containing O 3 at a mole fraction of about 5 %) and maintained at 13 MPa and À25 8C. Based on an analysis of the gas released from this hydrate while being decomposed, they estimated the ozone content in the hydrate to be 2.3 g L À1 (ca. 0.2 % in mass fraction). They also described that the hydrate could be preserved for 10 days in a closed container conditioned at 13 MPa and À25 8C without causing a substantial loss of its ozone content.More recently, we showed that hydrates formed from an O 3 + O 2 gas mixture and CCl 4 or xenon (Xe) and cooled to around À20 8C can preserve ozone at a mass fraction on the order of 0.1 % for over 20 days under atmospheric pressure. [3] It should be noted that such in-hydrate ozone concentrations are higher than the typical ozone concentration in "ozonated water" for disinfection use by three orders of magnitude. Besides the ozone preservation tests, we measured the fourphase equilibrium for the O 3 + O 2 + CCl 4 hydrate-forming system (O 3 mole fraction in the gas phase = 6.9 AE 0.8 %) in the temperature range from 2.4 to 4

This paper aims at presenting a computational scheme to thermodynamically simulate a continuous multistage operation for separating, by forming clathrate hydrates, carbon dioxide (CO2) and hydrogen sulfide (H2S) from a low-quality natural gas and at showing the stage-to-stage changes in the gas-phase composition, the crystallographic structure and composition of the formed hydrate, and the gas/aqueous-liquid/hydrate equilibrium temperature (the higher temperature limit for hydrate formation). The paper first describes the fundamental concept and algorithm of the computational scheme and then applies the scheme to the processing of a specific natural gas modeled as a CH4 + C2H6 + C3H8 + N2 + H2S + CO2 mixture. It is demonstrated that the optimum number of stages should be determined by finding a compromise between the improving removal of CO2 and H2S and increasing losses of combustible substances, particularly C2H6 and C3H8, from the residual gas with an increasing number of stages.