Replacement of CH4 in the hydrate by use of liquid CO2

The dynamics of CH4 replacement in the CH4 hydrate with saturated liquid CO2 at 273.2 K was measured with a high pressure optical cell. The results showed that CH4 in the hydrate gradually moved to the liquid CO2 phase while CO2 in the liquid phase penetrated into the hydrate from the quantitative analysis. The decomposing process of the CH4 hydrate during the replacement was analyzed with in situ Raman spectroscopy, which allowed us to distinguish the cage structure of the CH4 hydrate and discuss the microscopic view of the replacement in the hydrate. It was found that the decomposition of the medium cage (M-cage) in the CH4 hydrate proceeded faster than that of the small cage (S-cage). The observed rate difference could be related to the stability of the S-cage in the CH4 hydrate or the re-formation tendency of CH4 and water molecules in the S-cage after decomposing the hydrate structure, whereas the guest molecule exchange of CH4 with CO2 could occur in the M-cage. Based on the experimental data, we developed a kinetic model for calculation of the CH4 remaining in the hydrate considering the decomposition rate difference between the M-cage and S-cage in the CH4 hydrate. The results indicate that the driving force could be the fugacity difference between the fluid phase and the hydrate phase for the replacement process. (c) 2004 Elsevier Ltd. All rights reserved.