In Situ X-ray Diffraction Measurements of the Self-Preservation Effect of CH₄ Hydrate

Abstract

In situ observations of CH4 hydrate dissociation using X-ray diffraction were carried out at atmospheric pressure and at both 168 and 189 K. Dissociation rates of the hydrate and the rate of transformation into hexagonal ice were measured using time-resolved energy-dispersive X-ray diffraction. The dissociation of CH4 hydrate had an initially fast regime followed by slower dissociation. Thus, the data support a previously suggested two-step process. In addition, we observed dynamic behavior of the X-ray diffraction intensities of ice Ih, which implies a transient crystal structure at the beginning of the dissociation. Our analyses indicates that the first step, which lasted several tens of minutes, was the formation of an ice Ih layer around the CH4 hydrate, and the second step was relatively slow because the CH4 had to diffuse through the thickening ice layer. This second step determined the hydrate lifetime. The resulting diffusion coefficients were estimated at 2.2 × 10-11 m2/s at 189 K and 9.6 × 10-12 m2/s at 168 K.

Methane Hydrate Pellet Transport Using the Self-Preservation Effect: A Techno-Economic Analysis

“…Self-preservation, or anomalous preservation [1,[11][12][13][14][15][16], is a kinetic anomaly in which thermodynamically unstable clathrate hydrates, after an initial vigorous decay liberating free gas, continue to dissociate at rates even up to several orders of magnitude slower than what could be expected. It plays a pivotal role for the NGH-based gas transport technology [17][18][19][20], as it allows gas storage with low boil off rates at very moderate P/T conditions.…”

Modeling the Self-Preservation Effect in Gas Hydrate/Ice Systems

“…9,12) The most popular explanation of the self-preservation effect assumes a kinetic barrier for methane diffusion through the ice film of the surface of gas hydrate particles. 4,5,10,13) Large-scale nanosecond classical molecular dynamics (MD) calculations support this assumption (the formation of an ice layer inhibits the decomposition of gas hydrate). 14) However, as mention about, similar self-preservation effect was never observed in the experimental studies of the mixed methaneethane hydrate, 12) which does not support the ''kinetic'' approach of self-preservation effect.…”

“…3) This term describes the ability of methane hydrate to resist dissociation at temperatures higher than the equilibrium temperature of decomposition. The effect of incomplete dissociation of methane hydrate has been the subject of many experimental studies in the last years (see, for example [4][5][6][7][8][9][10][11][12][13] ). The experiments show the anomalous preservation of methane hydrate at temperatures below 273 K (ice Ih melting point) under ambient pressure with simultaneous formation of ice phase at temperatures above 242 K (beginning of methane hydrate dissociation).…”

Kinetics of Methane Hydrate Formation Catalyzed by Iron Oxide and Carbon under Intense Stirring Conditions

“…Presently, many researchers are planning to develop storage and transportation systems for natural gas (methane) using gas hydrate according to this property. [3][4][5][6] Recently, the effects of liquid additives of some kind of surfactants (sodium dodecyl sulfate, linear alkyl benzene sulfonate and cetyl trimethyl ammonium bromide, 7) alkylpolyglucoside, sodium dodecyl benzene sulfonate and potassium oxalate monohydrate 8) ) on the hydrate formation and the storage capacity have been studied. In the case of solid catalyst, an activated carbon for the storage capacity has been studied by A. Celzard et al 9) and H. Najibi et al 10) In this study, we are working on the development of a catalyst for hydrate formation.…”