(600d) Analysis of H2-Clathrate Hydrate Formation Kinetics with a Phase Diffusion Model

Hydrogen hydrates, which are clathrate compounds that contain guest hydrogen molecules, are expected to be one of the possible hydrogen storage methods in future energy systems. The use of mainly water to store the hydrogen at low pressures (5 to 10 MPa) with suitable promoters makes this method competitive with other methods, since it is safe, less expensive and good for the environment. Additionally, recovery of 100% of the hydrogen stored in the hydrogen hydrate is simple through pressure release.

In the absence of promoters, hydrogen hydrates form at super high pressures (220 MPa, -24 oC) in H2O-H2 mixtures1). Use of a small amount of organic additive such as tetrahydrofuran (THF) can reduce this pressure (5 MPa, 7oC) with THF stabilizing the cage structure2). Since H2-THF binary hydrates have been discovered, thermodynamics of the hydrates have been studied in terms of phase equilibria and storage capacity (occupancy). However, few data exist on hydrogen hydrate formation kinetics, which is important for practical processes. In this study, we report on binary H2-THF clathrate hydrate formation kinetics with a pressure decay method at temperatures from 265 to 276 K for stoichiometric promoter concentrations and for particle sizes between 200 and 1400 ìm.

A high pressure cell (130 cm3) with windows was used to allow both visual observation and Raman spectroscopy to be performed. In each experiment, hydrate and hydrogen gas were loaded into the cell. Hydrogen hydrate was made in the cell and the amount of consumed hydrogen was calculated by a pressure decay method with material balance on hydrogen. Consumed hydrogen was confirmed by Raman spectroscopy to be included in the hydrogen cage. Two promoters are considered, THF and cyclopentane.

For THF hydrates, hydrogen was rapidly consumed in the initial stage when the hydrogen molecules seemed to absorb surface area of particles with water molecules making hydrate cages. Initial formation rate increased for smaller particle sizes, higher pressures and lower temperatures. After the initial absorption stage, consumption rate became slow and finally the amount of hydrogen changed little, which can be attributed to the diffusion of hydrogen into the hydrate solid phase.

A hydrogen delocalization model and a proposed hydrogen hydrate phase diffusion (HHPD) model were used to analyze the formation mechanisms. The HHPD model assumes that the H2-promoter hydrate phase is formed due to hydrogen adsorption onto the particle surface that is followed by subsequent diffusion of hydrogen into the clathrate hydrate. The HHPD model could express the kinetics quantitatively at the experimental conditions studied. Values of the hydrogen diffusion coefficient in the clathrate hydrate estimated from the bulk data and the phase thickness in the HHPD model were agreed well with the literature.

1) W. L. Mao, et al., Science, 297, 2247(2002). 2) L. J. Florusse, et al., Science, 306, 469(2004).