Molecular dynamics simulation of the effects of different salts on methane hydrate formation: an analysis of NaCl, KCl and CaCl2

Abstract

The formation of hydrates in seawater is of great significance for hydrate prevention and control and seawater desalination. Molecular dynamics simulations were performed to investigate the spontaneous methane hydrate formation in 3.5wt % (seawater salinity) NaCl, 3.5wt % KCl and 3.5wt % CaCl2 solution, respectively. Results indicated that salt inhibited the formation of hydrates, and the strength of inhibition followed the sequence of 3.5wt % CaCl2 > 3.5wt % NaCl > 3.5wt % KCl. Salt inhibited nucleation and growth of hydrates by suppressing methane dissolution and reducing the number of tetrahedral water molecules in the methane hydration layer. It was observed that Na+, K+ and Cl- can replace the water molecules in the hydrate cage to participate in the formation of the cage. Compared with the pure water system and KCl system with a fast hydrate growth rate, the randomness of hydrate crystallization was smaller in the NaCl system and CaCl2 system with slow hydrate growth rate.