RAMAN STUDIES OF METHANE-ETHANE HYDRATE STRUCTURAL TRANSITION Ohno, Hiroshi; Strobel, Timothy A.; Dec, Steven F.; Sloan, E. Dendy; Koh, Carolyn A.
The inter-conversion of methane-ethane hydrate from metastable to stable structures was studied using Raman spectroscopy. To investigate factors controlling the inter-conversion, the rate of structural transition was measured at 59% and 93% methane in ethane. The observed slower structural conversion rate in the lower methane concentration atmosphere can be explained in terms of the differences in kinetics (mass transfer of gas and water rearrangement). Also, the effect of kinetic hydrate inhibitors, poly-N-vinylpyrrolidone (PVP) and polyethylene-oxide (PEO), on the hydrate metastability was investigated at 65% and 93% methane in ethane. PVP increased the conversion rate at 65% methane in ethane (sI is thermodynamically stable), but retarded the rate at 93% methane in ethane (sII is thermodynamically stable), indicating that the function of PVP depends on hydrate structure. PEO did not affect the structural transition considerably for either methane-ethane compositions.
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