Structural Transition of the Methane–Ethane Mixture Hydrate in a Hydrate/Water/Hydrocarbon Three-Phase Coexistence System: Effect of Gas Concentration

Abstract

Hydrate formation of a natural gas mixture is fascinating. Whereas both pure methane and ethane form a structure I (sI) hydrate, their mixture may form a structure II (sII) hydrate at certain compositions. Here, we investigated the underlying mechanisms of the methane–ethane mixture hydrate structural transition using an sII-hydrate–water–hydrocarbon three-phase interface system. The results indicate that sII hydrate formation is a function of methane concentration with a maximum at a mole concentration of 83–90%. In contrast, a significant amount of the sI hydrate forms at concentrations below 75% or equal to 100%. In addition, we unveil the mechanism of the rarely discussed structure conversion from sII to sI. The 15-hedron (mainly 51263) cages, which may originate from both aqua and the 16-hedron (51264) cage transition, play a vital role as a transitional bridge in the conversion at the transition interface. Owing to the specific structure of 15-hedron cages, the 14-hedron (51262) cages can grow, and thus, the sI hydrate grows. Furthermore, the methane occupancy in the 14-hedron cages and the concentration for the energetically favorable formation of the sII structure were quantified based on the formation energies for hydrates with methane and ethane molecules trapped inside. We show that the adsorption preference of guest molecules at the interfaces or cages can be attributed to the difference between the formation energy prediction and the reaction process results. Our findings have both academic and engineering significance for sustainable chemistry related to flow assurance, gas separation, gas transportation, and natural hydrate development in reservoirs with mixed gas hydrate structures.

Molecular behavior of hybrid gas hydrate nucleation: separation of soluble H2S from mixed gas

“…Wu et al 39 reported a mixed methane/tetrahydrofuran system, and the induction time of the CH 4 /THF mixed guest system was found to be significantly shorter than that of pure CH 4 and pure THF systems. Pang et al 40 built a system containing methane and ethane. The results indicated that sII hydrate formation was a function of methane concentration with a maximum mole concentration of 83–90%.…”

Molecular Origins of Deformation in Amorphous Methane Hydrates

“…Besides the 5 12 water cages above, 5 12 6 2 , 5 12 6 3 , 5 12 6 4 , 4 1 5 10 6 2 , 4 1 5 10 6 3 , and 4 1 5 10 6 4 water cages are also identified in amorphous methane hydrates. Such water cages were also observed in the nucleation of gas hydrates ,− and their deformation process. ,, By comparison, it is found that 5 12 , 5 12 6 2 , 5 12 6 3 , and 4 1 5 10 6 2 water cages are primary water cages in amorphous methane hydrates as shown in Supporting Information Figures S4–S7. This indicates that these primary water cages play an important role in mechanical properties of amorphous methane hydrates.…”

The Development of Clathrate Hydrate Science

“…There are some double hydrates, whose largest guest on its own would form str.I, that form str.I or str.II depending on the guest ratio, e.g., Kr/Xe, where pure Xe would form str.I and pure Kr would form str.II. Even more intriguing is the case of the CH 4 /C 2 H 6 double hydrate, where both guests on their own form str.I but for some ratios the double hydrate is str.II. , Note that in both cases the two guests can fit into 5 12 , 5 12 6 2 , and 5 12 6 4 cages. These cases point to the subtle interplay of the different cage–guest interactions that contribute to the overall stability of str.I or str.II and determine which is the more stable for different compositions.…”

“…Even more intriguing is the case of the CH 4 /C 2 H 6 double hydrate, where both guests on their own form str.I but for some ratios the double hydrate is str.II. 163,164 Note that in both cases the two guests can fit into 5 12 , 5 12 Another interesting aspect of this size issue is the observation that if the guest molecule has conformational flexibility it may adapt a conformation that allows it to fit more easily into the space of the cage. The lowest-energy conformation of the molecule in the free state may be such that it would encounter strong repulsive forces with the cage walls (put simply, too large to fit), but a higher energy conformation may have much weaker interactions.…”