Kinetics of Methane Hydrate Formation from Polycrystalline Deuterated Ice

Abstract

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 diffusioncontrolled. 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 rampingsa nonisothermal procedure that involves slowly increasing the sample temperature through the ice melting point.

Combined replacement and depressurization methane hydrate recovery method

“…This was in agreement with the findings by Hirohama et al (1996). Subsequently, in the experiments conducted by Wang et al (2002), the first stage lasted for 5 h; in this stage, the decomposition of the CH 4 hydrates and formation of CO 2 hydrates was fast. Moreover, Ota et al (2007) studied cage occupancy during replacement with in-situ laser Raman spectroscopy.…”

Methane clathrate hydrate FTIR spectrum

“…The outside methane molecule will have to overcome this barrier for the propagation of the methane into the bulk of the ice to proceed (e.g. Wang et al 2002).…”

A cometary nucleus model taking into account all phase changes of water ice: amorphous, crystalline, and clathrate

“…For the first stage, determined values between 3 × 10 −13 and 10 −12 mol m −2 Pa −1 s −1 for the formation rate of CO 2 clathrate from CO 2 gas and crystalline water ice at temperatures around 200 K. and Kuhs et al (2006) determined values of the kinetics parameter of methane clathrate formation between 245 and 272 K starting from hydrogenated and deuterated ices. They showed a temperature dependence with values ranging between about 9 × 10 −14 mol m −2 Pa −1 s −1 at 245 K and 5 × 10 −13 mol m −2 Pa −1 s −1 at 263 K. These authors and others (Wang et al 2002;Staykova et al 2003;Genov & Kuhs 2003) assumed that the value of the kinetics parameter of formation/dissociation of clathrates follows an Arrhenius-type function of temperature:…”