Molecular dynamic simulation of H2-CH4 binary hydrate growth induced by methane hydrate

Abstract

Hydrogen storage based on hydration is a promising technology, in which hydrogen molecules are captured in a three-dimensional cage structure formed by hydrogen bonds between water molecules. In this study, methane was selected as the accelerator guest molecule for hydrogen storage in hydrates, hydrogen-methane hydrate formation properties was investigated at 240 K and 110 MPa by molecular dynamic simulation. The combined TraPPE(CH4)-TIP4P/ice(H2O)-Diatomic(H2) force field was adopted. The molecular mechanism of the growth of CH4/H2 binary hydrates with different liquid methane concentrations (1.51–5.79 mol%) was explored, the effect of concentration on kinetic properties such as hydrate growth rate, hydrate hydrogen storage density, and properties related to cage occupancy and structural transformation were analysed. Any concentration of liquid-phase methane in this study increased hydrogen storage compared to a system without liquid methane. The growth rate of binary hydrates is related to the concentration and diffusion of liquid-phase methane: the higher the concentration of methane in liquid phase, the smaller the diffusion coefficient of hydrogen and water molecules in the solution, and the slower the growth rate of binary hydrates. The cage at the solid–liquid interface is preferentially occupied by hydrogen molecules, and the occupation process is gradually from the interface to the interior of the hydrate. At lower methane concentrations, the hydrate grows more uniformly along the interface. When the methane concentration is large, the hydrate not only grows along the interface, but also develops new nucleation sites in the liquid phase for growth, resulting in uneven growth of hydrate in the whole system. Methane acts as an accelerator to stabilize the cage while reducing diffusion. According to the hydrogen storage density of each system, it was found that hydrates have the best hydrogen storage efficiency when the concentration of methane in the liquid phase is 4.41 mol%. The transition from double-occupied cages to single-occupied cages indicated that the breakdown of the six-membered ring in the hydrate cage was the key to the passage of hydrogen molecules into and out of cages. This study will provide theoretical basis for the stable storage of H2 hydrate, and promote the industrial application of HHST technology.