Molecular Selectivity of CH4–C2H6 Mixed Hydrates: A GCMC Study

Abstract

In this paper, we report the first grand canonical Monte Carlo simulation study aiming at characterizing the competitive trapping of CH4 and C2H6 molecules into clathrate hydrates under temperature conditions typical of those encountered at the surface of Titan. Various compositions of the fluid in contact with the clathrate phase have been considered in the simulations, including pure methane, pure ethane, and mixed fluids made of various methane/ethane ratios. The trapping isotherms obtained from the simulations clearly show that ethane molecules can be enclathrated at lower pressures than methane molecules. In addition, they provide evidence that the methane molecules can occupy both small and large cages of the clathrate lattice, whereas the ethane molecules have a strong preference for the large cages, in accordance with experimental conclusions. However, increasing the pressure may also lead to the trapping of ethane in the small cages of the clathrates, leading to a possible competition between methane and ethane molecules for these small cages at high pressure, if both molecules are concomitantly present in the fluid phase. The abovementioned features could strongly influence the composition of a mixed methane/ethane fluid phase in contact with the clathrate phase, which might thus be first impoverished in ethane before methane starts getting trapped into the clathrate. However, this conclusion strongly depends on the clathrate structure considered in the simulations.