Multiscale Analysis on CH₄–CO₂ Swapping Phenomenon Occurred in Hydrates

Abstract

Swapping CH4 with CO2 in hydrocarbon hydrates is renowned as an eco-friendly method for potential energy production and climate change mitigation. To fully understand the CH4–CO2 replacement process, in situ Raman spectroscopy combined with long-term macroscopic measurements were carried out by putting CH4 hydrates into gaseous CO2 at temperatures ranging from 273.2 to 281.2 K. A kinetic model based on gas diffusion theory was modified. Results showed that the gas swapping process started with surface reaction followed by gas diffusion in hydrate phase. At hydrate surface, the average occupancy of CH4 decreased without dramatic fluctuations, suggesting that the replacements were not simply composed of a series of dissociation and reformation process. A rise in temperature was found to effectively increase the gas swapping rates and strengthen the gas diffusion in hydrate phase. In situ Raman results together with gas consumption data were fitted by the kinetic model. The calculated gas-hydrate interfacial area revealed the foamy nature of hydrates. Fitting parameters representing the gas diffusion coefficient were of the same order of magnitude as the water mobility in hydrate phase, which implied that the replacements were limited by the mobility of water molecules.