Experimental and Simulation Study for the Dissociation Behavior of Gas Hydrates-Part Ⅱ: sⅡ Mixed Gas Hydrates

摘要

This study is an extension of our research on the dissociation behavior of sⅠ CH_4 hydrates (Part Ⅰ). In this study, investigations of the dissociation of binary and multicomponent structure Ⅱ hydrates were carried out. A novel approach involving in situ and ex situ Raman spectroscopic measurements together with molecular dynamics simulations were applied for a systematic assessment of the hydrate dissociation and gas release behavior due to the destabilization of hydrate cavities. The hydrate decomposition was induced by thermal stimulation to mimic the warming of oceans and atmosphere brought by climate changes. The interactions between the released gas, aqueous phase, and hydrates were described. The results demonstrated that in the vicinity of the hydrate surface, the liquid phase was oversaturated with the gas molecules and some unstable partial hydrate cavities were also formed. Consequently, the release of gas was temporarily stopped or very slow. Throughout the process, the hydrate underwent decomposition-reformation-continuing decomposition until the crystal disappeared. A faster breakdown of small cavities (5~(12)) was recorded, leading to an increase in the large-to-small cavity ratio during dissociation. Moreover, the results indicated a faster release of CH_4 molecules compared to C_3H_8 over the dissociation process of the sⅡ CH_4-C_3H_8 hydrate. This could be due to the higher diffusivity of CH_4 molecules from the hydrate surface to the gas phase as well as its lower potential to stabilize the cavities compared to C_3H_8. The release of CH_4 molecules was also faster compared to CO_2 and C_2H_6 molecules in the sⅡ mixed hydrate, leading to changes in the hydrate composition throughout the process.