Methane Hydrate Formation in the Salty Water Confined in Clay Nanopores: A Molecular Simulation Study

Abstract

A sound knowledge of the effects of clay surfaces and salt ions on CH4 hydrate formation is vital to understand the formation of natural gas hydrate resources and develop hydrate-based technologies, such as seawater desalination. Herein, microsecond simulations are conducted to investigate CH4 hydrate formation from fresh water and NaCl solution in kaolinite nanopores and the outside bulk phase. Simulation results show that the gibbsite surface shows strong affinity for water and salt ions, while the siloxane surface exhibits certain affinity for CH4 and hence affects hydrate formation. With two siloxane surfaces in the nanopore, hydrate formation is severely inhibited by the significantly lowered aqueous CH4 concentration, caused by surface adsorption of CH4 to form nanobubbles; salt ions greatly enhance such an inhibitory effect by promoting surface nanobubble formation and suppressing CH4 dissolution from nanobubbles. However, with one siloxane surface and one gibbsite surface presented in the nanopore, no nanobubble forms on the siloxane surface, no matter salt ions are present or not, and the inhibitory effects caused by the siloxane surface are not observed. In contrast, gibbsite surfaces are more beneficial to hydrate formation, and salt ions have little effect as most ions adsorb to the surface. Hydrate formation outside of the nanopores is obviously inhibited by salt ions, especially at high salt concentrations. Some interesting phenomena are observed, such as stochastic formation of large sI and sII hydrate domains, which then blocks the nanopore throat and hinders the mass transfer of CH4 and water, and sustained growth of large hydrate solids outside of the nanopore causes decomposition of small hydrate solids in the nanopore. Additionally, densification of salt ions in solution and formation of ion-containing hydrate cages are systematically analyzed to figure out the surface effects of kaolinite on the desalination process. These molecular insights are of scientific and engineering significance to sustainable chemistry related to exploitation of natural gas hydrate and hydrate-based seawater desalination technology.