Molecular Simulation Of The Mechanism Of Hydrate Nucleation And Kinetic Hydrate Inhibitors

Clathrate hydrates are crystalline solid compounds composed of guest molecules and polyhedral cages of water.The guest molecules are stable in water cages at high pressure and low temperature.Clathrate hydrates are not only a potential energy source,but also can easily form in oil pipelines during transportation and production,also in blowout preventers causing clogging,which threaten the oil & gas industrial safety.There is an urgent need to explore the micro-mechanism of hydrate nucleation and inhibitors,in order to prevent the formation of hydrate more efficiently.As a kind of the high-efficiency and low-dose additives,kinetic hydrate inhibitors(KHIs)have great potential in the future.However,the research on the mechanism of KHIs is still in the laboratory test stage,and the cognition of KHIs is still very poor.In this thesis,molecular dynamics(MD)simulation method is used to reveal the mechanism of hydrate kinetics inhibitors from two aspects: the natural nucleation process of hydrate;KHIs' action on the growth interface of hydrate.In precious studies on hydrate nucleation,hydrate nucleation was regarded as random processes without a fixed water-substructure evolution pathway.In time scale,the induction times of hydrate nucleation also are random.Moreover,within the milder environmental conditions,the uncertainty of induction time of hydrate is higher.Therefore,the past research did not have a suitable theory to explain and describe these phenomena quantitatively.The inadequate understanding of the mechanism of hydrate nucleation seriously affects the efficient design of inhibitors.Not only that,the ways of KHIs' adsorption and inhibition at the hydrate interface have been controversial in the past studies,and the mechanism of KHIs is still lacking.Therefore,this paper will explore these issues.Through the calculation and statistics of a large number of data,a stable evolution path of water substructure is found.It is proved that the evolution pathway of hydrate is driven by the Collision-Pumping effect of guest molecules,which innovates the previous hypothesis that water plays a dominant role in nucleation.According to the proposed theory,the theoretical formula of the hydrate nucleation induction time is derived,which is consistent with the simulation results.These provide new ideas and theoretical ways to control hydrate nucleation more reasonably:(1)to control the evolution of water substructure;(2)to control the collision intension of guest molecules in solution.Thus,the mechanism of KHIs,prolonging the induction time of hydrate nucleation,is revealed.The simulation proved that KHIs can limit the diffusion of methane molecules to water by bonding with a certain size of dissolved molecules cluster,which can reduce the guest molecules' collision probability.During the process of hydrate growth,it is proved that the adsorption groups of KHIs are hydrophobic carbon heterocyclic rings,rather than the hydrophilic groups of KHIs previously reported.The hydrophilic groups of KHIs are the inhibiting groups that destroy the hydrate growth.