The Effect Of Silica Surface Or Electric Field On Gas Hydrate Formation And Dissociation:MD Simulation

Natural gas hydrate,abundant in the permafrost and marine sediment environment,has been considered as an ideal energy resource,due to its high energy density and cleaning.Simultaneously there are series of applications based on hydrate technology,such as mixture gas separation,gas storage and desalination.Currently studies on gas hydrates have been attracting all over the world.Natural gas hydrate formation and decomposition environment contains various media,such as sand and clay,but it still has been a debatable point about the effect mechanism of media on hydrate formation.Besides that,the hydration process is crystallizing by water molecules and gas molecules,electric field could affect the dynamic behavior of water–a polar molecule.So,the paper explored the formation and dissociation of gas hydrates at the molecular level to study the effect of surface or electric fields on the hydration process.In this work,silica surface and 3.5 wt%NaCl solution were chosen to simulate the marine environment for hydrate nucleation.Methane,carbon dioxide,and CH₄/CO₂ mix gases were chosen as guest molecules.At 250 K and 50 MPa,molecular dynamics simulations were preformed to study the nucleation process of gas hydrate near silica surface through changing the size of surface and solution-gas phase.Simulation results showed that the presence of silica surface reduced the solubility of gases in water and affected the gas hydrate nucleation.CO₂ hydrate nucleation near silica surface would not happen because silica surface caused CO₂ solubility in water below the critical aqueous CO₂ concentration?0.6 mole fraction?for hydrate nucleation.For all of mix gases systems in the paper,methane hydrate formation made CO₂ hydrate nucleation happen,even though aqueous CO₂ concentration was below 0.6.At the same time CO₂ would also accelerate the methane solubility in water.Results revealed that there was a free water layer?at least 0.3 nm?from the silica surface.In other words,gas hydrate nucleation did not happen on the silica surface directly.Finally,the paper tried to apply external electric field to natural gas hydrate exploitation,molecular dynamics simulations were employed to evaluate the effect of external static and oscillation electric fields on methane hydrate formation/dissociation at 260 K and 10 MPa.For static e-field,the diploe moments of water molecules were more saturate,and the alignment of water molecules was more inclined to e-field direction with the intensity increasing.Methane hydrate dissociated when the applied intensity was over 1.5 v/nm,the“ice-like”structure of water molecules occurred in the presence of 2.0 v/nm.Cosine oscillation electric fields have double effects,they could promote both methane hydrate growth and dissociation.the presence of 2.45 GHz e-field of more than 1.5 v/nm or300 GHz e-field of more than 2.0 v/nm would accelerate methane hydrate dissociation.300GHz/1.5v nm^-1,1 THz/1v nm^-1,1 THz/1.5 v nm^-11 and 1 THz/2.0 v nm^-1 in this paper accelerated the diffusion of water and benefited methane hydrate formation.