NUCLEATION PATHWAYS OF GAS HYDRATES FROM MOLECULAR DYNAMICS SIMULATIONS

Study of hydrate nucleation is of current interest for many practical applications. In particular the understanding of the underlying mechanisms of these phenomena may aid the design of more efficient low dosage antiagglomerants and kinetic inhibitors of natural gas hydrates. The time and length scales (10 -9 s and 10 -9 m) in these phenomena make them difficult to study experimentally. Molecular simulations are suitable for bringing to light the hydrate formation and growth mechanisms. In the literature there are several reports on the spontaneous formation of stable hydrate cages under several instances. In all cases the conditions are out of the range for many practical applications, namely, very high pressure (from 500 to 2000 bar). In most of cases amorphous clusters containing structural units of sI and sII hydrates are observed. Recently we reported a simulation approach which allows simulating hydrate formation at moderate subcooling. Our work produces structures with higher structural order in much lower computational times. In this work hydrate formation in water-methane, water-propane, and watermethane-propane mixtures is studied by means of molecular dynamics simulations within P and T ranges of practical interest. The incipient formation of cages, nucleation, and sustained growth stages of hydrates are observed in the course of our simulations. We monitor the different geometries of the cages formed and describe the structures produced. Significant differences are observed between the methane and propane hydrates. The methane-propane mixture produces hydrates structures with elements reminiscent from the two pure components.