Formation of Porous Gas Hydrates from Ice Powders:  Diffraction Experiments and Multistage Model

Abstract

Gas hydrates grown at gas−ice interfaces were examined by electron microscopy and found to have a submicron porous structure. In situ observations of the formation of porous CH4- and CO2-gas hydrates from deuterated ice Ih powders were made, using time-resolved neutron diffraction on the high-flux diffractometer D20 (ILL, Grenoble) at different pressures and temperatures. For the first time neutron diffraction experiments were also performed with methane in hydrogenated samples. The isotopic differences between H2O and D2O are found insignificant concerning the clathrate formation kinetics. At similar excess fugacities, the reaction of CO2 was distinctly faster than that of CH4. The transient formation of the CO2-hydrate crystal structure II was also observed in coexistence with the usual type-I hydrate reaching a maximum of 5% after 5 h of the reaction at 272 K. A phenomenological model for the kinetics of the gas hydrate formation from ice powders is developed with special account of sample consolidation effects. It describes the initial stage (I) of hydrate film spreading over the ice surface and the two subsequent stages which are limited (II) by the clathration reaction at the ice−hydrate interface and (III) by the gas and water transport (diffusion) through the hydrate shells surrounding the shrinking ice cores. Comparable activation energies are found for stage II of the CH4−hydrate formation in deuterated and hydrogenated ice with 8.1 and 9.5 kcal/mol, respectively. In the case of a diffusion-limited clathration (stage III) the activation energy in D2O-ice powders can be estimated as 14.3 kcal/mol.

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

“…Kuhs et al (2006) calculated E a and find a value (92.8 kJ mol −1 ) two times greater than the ones of Staykova et al (2003) and Genov et al (2004) for the formation of CH 4 and CO 2 clathrates respectively. In the case of CO 2 -clathrate, Genov et al (2004) estimated the activation energies to be 5.5 kJ mol −1 at low temperatures and 31.5 kJ mol −1 above 220 K, indicating that water molecule mobility plays a considerable role in the clathration reaction.…”

“…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:…”

A laboratory investigation into the seismic velocities of methane gas hydrate‐bearing sand

“…It can also be noted that the gradients of the pore pressure curves of specimens H3-2 and H5-2 are markedly different to that of specimen H40 (from when the methane gas pressure is locked off). Barrer and Edge [1967] showed that the rate of hydrate nucleation depends on the surface area of the icehydrate interface while recent studies have shown that ice grains with a typical diameter of 40-80 mm formed hydrate at temperatures as low as À43°C Staykova et al, 2002Staykova et al, , 2003] with the growth ratedependent on the gas pressure and temperature. In our tests, it has been assumed that ice forms at grain contacts (due to capillarity and surface tension) and then infills the pore space as the water content increases.…”

The Abundance of Atmospheric CO2 in Ocean Exoplanets: a Novel CO2 Deposition Mechanism

“…The experiments were conducted in the temperature range of 193 K-272 K, which fully covers our temperature range of interest. We adopt the theory of Staykova et al (2003) and Genov et al (2004) in order to derive the mole fraction of ice Ih converted to CO 2 clathrate hydrate, α(t). This parameter is a complicated function of the pressure of CO 2 , temperature and of the ice morphology.…”

“…We further assume that S w = 1. The initial grain size is important in determining the rate of conversion from ice Ih into SI CO 2 clathrate hydrate (Staykova et al 2003;Genov et al 2004). The smaller the initial ice Ih grain size is the faster is the conversion.…”