Computer modelling of the mode of action of gas hydrate kinetic inhibitors
This thesis reports an investigation of two families of potential gas hydrate inhibitor: quaternary amine carboxylates and sulphonates. Molecular dynamics simulations of the inhibitors have been performed both in liquid water and at the surface of a thin hydrate film under natural gas. Trajectories were analysed to determine inhibitor effects upon the structure of the water and the stability of the gas hydrate lattice. TIP conditions for the thin film study were determined from the calculated stability of a structure II gas hydrate under pressure. All three studies were analysed using radial distribution functions, time correlation functions, and order parameters to probe the underlying structure. To complement the computer modelling study, an experimental investigation of one of the amine sulphonates, N,N ,N-tributylammonium-l-(3-propylsulphonate), and one established inhibitor, polyvinylpyrrolidone was performed. These measured the effect of the inhibitor on nucleation time, crystal growth and morphology for both tetrahydrofuran and ethane hydrate. Results from the liquid water simulations indicate that bulk water is largely unaffected by the inhIbitors, while solvated water shows increased short- and long-range structure. Hydrate pressures were determined from the thin film simulations. A transition to a high-pressure phase was also found at around 20 kbar. These results are consistent with experimental data. Pressures in the middle of the hydrate stability zone were adopted for the subsequent inhibitor simulations. Inhibitor behaviour at the interface showed signs of relaxation of the gas hydrate water network, while for solvated water there was a contraction in the water network for the carboxylates head group. The experimental investigation revealed the amine sulphonate to be a more effective kinetic inhibitor of hydrate formation than polyvinylpyrrolidone. This was achieved by delaying nucleation, along with some growth modifying properties.