Mechanisms of synergistic inhibition of NaCl and glycine mixtures on methane hydrate formation: Experimental and molecular dynamic simulation

Abstract

Composite inhibitors exhibit significant performance in inhibiting hydrate nucleation and growth and reducing hydrate production, however, their working processes and mechanisms need to be further clarified. Herein, the effects of NaCl-glycine mixture on methane hydrate formation and the underlying mechanisms were investigated through experimental and molecular dynamic simulation. The results showed that in addition to changing the phase equilibrium conditions of the hydrate, NaCl can inhibit hydrate growth owing to the salt resistance of water clathrate structures. Glycine, which is a promising new inhibitor, possesses both kinetic and thermodynamic inhibition capabilities. The ratio C of water molecules disturbed by the mixture of NaCl and glycine decreased to below 0.733, and the hydrate phase equilibrium pressure increased to above 3.919 MPa, which effectively inhibited hydrate nucleation and growth. Moreover, during the hydrate growth stage, NaCl reduced the solubility of methane and glycine promotes the formation of methane nano bubbles, associated with the accelerated diffusion and high degrees of disorder of water molecules in the composite inhibitor solution, which further hindered hydrate growth. A higher glycine dosage led to stronger synergistic inhibition. When the mixture consists of 5.0 wt% glycine and 3.5 wt% NaCl, the induction time was 10.69 times longer than that of pure water, and correspondingly, the methane consumption decreased by 51.35%, with the average methane consumption rate dropping by 75.38%.