Diffusion Model for Gas Replacement in an Isostructural CH<inf>4</inf>-CO<inf>2</inf> Hydrate System

© 2017 American Chemical Society. Guest exchange in clathrates is a complex activated phenomenon of the guest-host cage interaction on the molecular-scale level. To model this process, we develop a mathematical description for the nonequilibrium binary permeation of guest molecules during gas replacement based on the microscopic "hole-in-cage-wall" diffusive mechanism. The transport of gas molecules is envisaged as a series of jumps between occupied and empty neighboring cages without any significant lattice restructuring in the bulk. The gas exchange itself is seen as two-stage swapping initiated by almost instantaneous formation of a mixed hydrate layer on the hydrate surface followed by a much slower permeation-controlled process. The model is constrained by and validated with available time-resolved neutron diffraction data of the isostructural CH 4 guest replacement by CO 2 in methane hydrate, a process of possible importance for the sequestration of CO 2 with concomitant recovery of CH 4 in marine gas hydrates. (Graph Presented)

Gas replacement in clathrate hydrates during CO2 injection - Kinetics and micro-structural mechanism

The replacement process in of pure sI methane clathrate powders exposed to CO2 have been quantitatively followed by means of neutron diffraction at conditions relevant to sedimentary matrixes of continental margins. The exchange of methane with CO2 within a crystalline lattice of gas hydrates is seen as a two-step process of (1) a fast interfacial reaction (2) followed by much slower diffusion-limited transport. Copyright © 2013 by The International Society of Offshore and Polar Engineers (ISOPE)

Kinetics of CO2 hydrate formation from water frost at low temperatures: Experimental results and theoretical model

The gas hydrate growth from frostlike powders composed of micrometer-sized ice particles does not start with hydrate shell formation, because the initial hydrate film thickness established in earlier work exceeds the ice particle dimensions. In this limiting case, the ice grains are directly consumed by a growing nucleus created on the particle surface. The conventional Johnson-Mehl-Avrami-Kolmogorov (JMAK) model,(1)which considers (re-) crystallization reactions phenomenologically in terms of the constituent nucleation and subsequent growth processes, cannot be directly applied to the hydrate formation from frost due to the assumption of an infinitely large domain of crystallization. We present here a modified approach to account for the small particle sizes of the starting material and extend the existing theory of gas hydrate formation from monodisperse ice powders(3-5)to the low-temperature and low-ice-particle-size limit. This approach may also prove to be very useful for applying chemical reactions starting on the surface of nanomaterials. In situ neutron scattering was used to obtain the experimental degree of transformation as a function of temperature between 185 and 195 K. The data were analyzed with the modified JMAK model constrained by information from cryo-SEM and BET measurements. Based on the obtained activation energies for hydrate nucleation and growth, an estimate is given for the probability of formation of CO2 hydrates at conditions relevant for Mars; a direct reaction of CO2 gas with water frost is considered to be very unlikely on the Martian surface under current conditions. © 2011 American Chemical Society

Kinetics of CO2-hydrate formation from ice powders: Data summary and modeling extended to low temperatures

The shrinking-core model of the formation of gas hydrates from ice spheres with a well-defined geometry gives experimental access to the gas permeation in bulk hydrates. Here we report on results obtained for CO2 clathration experiments in the temperature range from 185 to 272 K, extending earlier work to much lower temperature conditions. The activation energy deduced from the permeation coefficients changes its value from ∼46 kJ/mol at higher temperatures to ∼19 kJ/mol below 225 K. We compare our results with published molecular dynamics simulation as well as nuclear magnetic resonance studies and provide arguments that the rate limiting process at lower temperatures is the cage-to-cage jumping of CO2 molecules via a "hole-in-the-cage" mechanism involving extrinsic water vacancies in cage walls. The rate-limiting process at higher temperatures can be explained by the temperature-dependent creation of intrinsic water-vacancy-interstitial pairs. The results obtained for CO2-hydrate are compared to earlier results for CH4-hydrate formation. The permeation of CO2 molecules through bulk hydrate is found to be about three times faster when compared to the CH4 case. This explains the faster clathration reaction of CO2-hydrate in comparison to CH4-hydrate. © 2013 American Chemical Society

Kinetics of CO2-hydrate formation from ice powders: Data summary and modeling extended to low temperatures

The shrinking-core model of the formation of gas hydrates from ice spheres with a well-defined geometry gives experimental access to the gas permeation in bulk hydrates. Here we report on results obtained for CO2 clathration experiments in the temperature range from 185 to 272 K, extending earlier work to much lower temperature conditions. The activation energy deduced from the permeation coefficients changes its value from ∼46 kJ/mol at higher temperatures to ∼19 kJ/mol below 225 K. We compare our results with published molecular dynamics simulation as well as nuclear magnetic resonance studies and provide arguments that the rate limiting process at lower temperatures is the cage-to-cage jumping of CO2 molecules via a "hole-in-the-cage" mechanism involving extrinsic water vacancies in cage walls. The rate-limiting process at higher temperatures can be explained by the temperature-dependent creation of intrinsic water-vacancy-interstitial pairs. The results obtained for CO2-hydrate are compared to earlier results for CH4-hydrate formation. The permeation of CO2 molecules through bulk hydrate is found to be about three times faster when compared to the CH4 case. This explains the faster clathration reaction of CO2-hydrate in comparison to CH4-hydrate. © 2013 American Chemical Society

Kinetics of CO2-hydrate formation from ice powders: Data summary and modeling extended to low temperatures

The shrinking-core model of the formation of gas hydrates from ice spheres with a well-defined geometry gives experimental access to the gas permeation in bulk hydrates. Here we report on results obtained for CO2 clathration experiments in the temperature range from 185 to 272 K, extending earlier work to much lower temperature conditions. The activation energy deduced from the permeation coefficients changes its value from ∼46 kJ/mol at higher temperatures to ∼19 kJ/mol below 225 K. We compare our results with published molecular dynamics simulation as well as nuclear magnetic resonance studies and provide arguments that the rate limiting process at lower temperatures is the cage-to-cage jumping of CO2 molecules via a "hole-in-the-cage" mechanism involving extrinsic water vacancies in cage walls. The rate-limiting process at higher temperatures can be explained by the temperature-dependent creation of intrinsic water-vacancy-interstitial pairs. The results obtained for CO2-hydrate are compared to earlier results for CH4-hydrate formation. The permeation of CO2 molecules through bulk hydrate is found to be about three times faster when compared to the CH4 case. This explains the faster clathration reaction of CO2-hydrate in comparison to CH4-hydrate. © 2013 American Chemical Society

Diffusion Model for Gas Replacement in an Isostructural CH<inf>4</inf>-CO<inf>2</inf> Hydrate System

© 2017 American Chemical Society. Guest exchange in clathrates is a complex activated phenomenon of the guest-host cage interaction on the molecular-scale level. To model this process, we develop a mathematical description for the nonequilibrium binary permeation of guest molecules during gas replacement based on the microscopic "hole-in-cage-wall" diffusive mechanism. The transport of gas molecules is envisaged as a series of jumps between occupied and empty neighboring cages without any significant lattice restructuring in the bulk. The gas exchange itself is seen as two-stage swapping initiated by almost instantaneous formation of a mixed hydrate layer on the hydrate surface followed by a much slower permeation-controlled process. The model is constrained by and validated with available time-resolved neutron diffraction data of the isostructural CH 4 guest replacement by CO 2 in methane hydrate, a process of possible importance for the sequestration of CO 2 with concomitant recovery of CH 4 in marine gas hydrates. (Graph Presented)

Guest Migration Revealed in CO<inf>2</inf> Clathrate Hydrates

© 2015 American Chemical Society. The shrinking-core model of the formation of gas hydrates from ice spheres with well-defined geometry gives experimental access to the gas permeation in bulk hydrates which is relevant to their use as energy storage materials, their exploitation from natural resources, as well as to their role in flow assurance. Here we report on a new approach to model CO2 clathration experiments in the temperature range from 230 to 272 K. We develop a comprehensive description of the gas permeation based on the diffusion along the network of polyhedral cages, some of them being empty. Following earlier molecular dynamics simulation results, the jump from a cage to one of its empty neighbors is assumed to proceed via a "hole-in-cage-wall" mechanism involving water vacancies in cage walls. The rate-limiting process in the investigated temperature range can be explained by the creation of water-vacancy-interstitial pairs. The gas diffusion leads to a time-dependent cage filling which decreases across the hydrate layer with the distance from the particle surface. The model allows a prediction of the time needed for a complete conversion of ice spheres into clathrate as well as the time needed for a full equilibration of the cage fillings. The findings essentially support our earlier results obtained in the framework of a purely phenomenological permeation model in terms of the overall transformation kinetics, yet it provides for the first time insight into the cage equilibration processes. The diffusion of CO2 molecules through bulk hydrate is found to be about three to four times faster in comparison with the CH4 case

Gas replacement in clathrate hydrates during CO2 injection - Kinetics and micro-structural mechanism

The replacement process in of pure sI methane clathrate powders exposed to CO2 have been quantitatively followed by means of neutron diffraction at conditions relevant to sedimentary matrixes of continental margins. The exchange of methane with CO2 within a crystalline lattice of gas hydrates is seen as a two-step process of (1) a fast interfacial reaction (2) followed by much slower diffusion-limited transport. Copyright © 2013 by The International Society of Offshore and Polar Engineers (ISOPE)

Gas replacement in clathrate hydrates during CO2 injection - Kinetics and micro-structural mechanism

The replacement process in of pure sI methane clathrate powders exposed to CO2 have been quantitatively followed by means of neutron diffraction at conditions relevant to sedimentary matrixes of continental margins. The exchange of methane with CO2 within a crystalline lattice of gas hydrates is seen as a two-step process of (1) a fast interfacial reaction (2) followed by much slower diffusion-limited transport. Copyright © 2013 by The International Society of Offshore and Polar Engineers (ISOPE)