Kinetics of methane hydrate formation from polycrystalline deuterated ice

The kinetics of methane hydrate formation was investigated by in-situ time-of-flight neutron powder diffraction. Samples were prepared from deuterated ice particles (< 0.25 mm) and transformed to clathrate hydrate by pressurizing the system with methane gas. The rates of sI methane hydrate formation were measured in-situ under isothermal conditions with a methane pressure of 1000 psi (6.9 MPa). Kinetic data were analyzed in terms of a shrinking core model, including possible contributions of nucleation, methane diffusion, and interface reaction. The data support the hypothesis that methane hydrate formation reaction from ice particles is diffusion-controlled. The reaction starts quickly at the nucleation stage, which propagates to form a hydrate layer that covers the ice particle. Further reaction is limited by the growth of the hydrate layer and inward diffusion of methane molecules through the hydrate layer to the unreacted ice core. The reaction rate at the interface between hydrate and unreacted ice particle is fast compared to that of methane diffusion. The conversion of ice particle to methane hydrate follows Arrhenius behavior, from which an activation energy of 14.7(5) kcal/mol was derived. Complete transformation of ice to methane hydrate was achieved through temperature ramping-a nonisothermal procedure that involves slowly increasing the sample temperature through the ice melting point.