Molecular Dynamics Simulations Of Synergistic Inhibition On Clathrate Hydrates

It is significant to research clathrate hydrates not only due to the great potential theypossess as a prominent energy source, but also due to the problems they give rise to thepetroleum industry during the production, processing and transportation of natural gas and oil.Furthermore, it need to promote the decompostion of natural gas and water duringexploitation of clathrate hydrates in the ocean. It is related to hydrate inhibitors for solvingthese problems.In this paper, two types of structures on clathrate hydrates (type structure of methaneclathrate hydrate and type I structure of propane and methane clathrate hydrates) have beeninvestigated. Dynamic simulation is a powerful tool to study the interaction between hydrateand inhibitor molecular, inhibition mechanism. Synergistic kinetic inhibition between twoinhibitors is able to save the cost for injection by decreasing requirement for hydrateinhibition. Hydrate thermodynamic inhibitor (glycerol) and hydrate kinetic inhibitor (PVPand PVCap) are employed as models of clathrate hydrates formation inhibitors in this work.Molecular dynamics (MD) simulations were applied to investigate a series of relevance fortwo clathrate hydrates dissociation and inhibition. The analysis results through Monte Carlo,Quantum chemistry and MD simulations are as follows:At lower pressure conditions, adsorption of methane molecules in the empty clathratehydrate is very sensitive to change of temperature, while under high pressure conditions, theadsorption of it is not obvious to the change of temperature. The adsorption of methanemolecules in the clathrate hydrate is physical adsorption. The formation of clathrate hydratesis likely to be that water molecules at a suitable temperature and pressure conditions, firstlyform a cage-like structure in the form of hydrogen bond, and then gas molecules enter intothe cage structure through the adsorption effect.The formation of metastable (supercooled) water during methane hydrate dissociationbetween253K and273K were obtained for the first time. It is consistent with the directoptical evidences of experiment. As the temperature increases, the diffusion of methane hydrates are not the trend of a linear increase, but there has been a slight fluctuation.Under pressure of5atm conditions, clathrate hydrates are not decomposited when thetemperature is in the range of193K to213K. However, clathrate hydrates configuration isdestroyed gradually with the increase in temperature. The higher temperature is favorable forthe decomposition of methane gas and water.PVP monomer as a surfactant and kinetic hydrate inhibitor can be favorably adsorbedonto the hydrate surface due to its affinity for the water–hydrate interface. The nucleationprocess gets delayed in the PVP system. However, a glycerol molecule has three hydroxylgroups forming H–bonds with water molecules in hydrate or liquid phase, which suppressedwater molecules participating in forming hydrate structures. The mechanism is mainly thatthe binding of inhibitor molecules to the hydrate crystal surface retards further growth andcrystallization.Through MD simulations, we obtained the possibility of synergistic kinetic inhibitioneffect by mixing both glycerol and PVP in aqueous phase. The management of hydrate riskcan be achieved by delaying hydrate onset and retarding hydrate crystal growth at theglycerol and PVP concentrations of40wt%and1.39wt%, respectively. Such a procedure isable to save the cost for glycerol injection by decreasing glycerol requirement for hydrateinhibition.The synergistic kinetic inhibition effect is most obvious at the glycerol and PVCapconcentrations of30wt%and1.71wt%, respectively.