Metastable structure II methane hydrate: A bridge to obtain high methane uptake in the presence of thermodynamic promoter

Abstract

The addition of thermodynamic promoters is a promising and perhaps the only effective method to reduce hydrate formation conditions. However, this is achieved at the cost of the gas storage capacity reduction since the thermodynamic promoter will occupy some hydrate cages. To date, there is no effective method that can simultaneously limit this negative effect and obtain high water-to-hydrate conversion. In this work, we determine the methane (CH4) hydrate formation mechanism in the presence of a thermodynamics promoter, Cyclopentane (CP), and find that structure II (Str.II) CH4 hydrate, as a metastable intermediate structure of the reaction, plays a quite significant role in encapsulating CH4 molecules in some 51264 cages (Str.II hydrates) and facilitating structure I CH4 hydrate formation, thereby increasing gas uptake significantly. A CH4 uptake, as high as 141.21 mmol/molH2O, is obtained from this work. Relative to the acknowledged limit of CH4 uptake of Str.II hydrates formed commonly in the presence of same mole fractions of thermodynamics promoter (8(Promoter)·16(CH4)·136H2O)), the highest increase in CH4 uptake is up to 232.75 %. These results would stimulate the development of the hydrate reaction theory and provide a new direction for promoter design.