Macro and microscopic CH4-CO2 replacement in CH4 hydrate under pressurized CO2

CH4-CO2 replacement in CH4 hydrate with high pressure CO2 was studied with in-situ laser Raman spectroscopy at 273.2 K and at initial pressures of 3.2, 5.4, and 6.0 MPa. Replacement rates increased with increasing pressures up to 3.6 MPa and did not change at higher pressures (similar to 6.0 MPa). These results showed that the replacement rates were dependent on pressure and phase conditions with the driving force being strongly related to fugacity differences of the two guest components between fluid and hydrate phases. When CH4 hydrate was contacted with CO2 under flow conditions, in-situ Raman measurements of the hydrate phase showed differences of cage decomposition rates between the Medium-cage (M-cage) and the Small-cage (S-cage) in the CH4 hydrate with decomposition of the M-cage being faster than that of the S-cage. The van der Waals-Platteeuw model was applied to the measurements of the transient data and it is shown that the theory allows estimation of occupancies of each component during replacement. (c) 2007 American Institute of Chemical Engineers.