Natural gas hydrates are solid, non-stoichiometric compounds of small gas molecules and water. They form when the constituents come into contact at low temperature and high pressure. The physical ...properties of these compounds, most notably that they are non-flowing crystalline solids that are denser than typical fluid hydrocarbons and that the gas molecules they contain are effectively compressed, give rise to numerous applications in the broad areas of energy and climate effects. In particular, they have an important bearing on flow assurance and safety issues in oil and gas pipelines, they offer a largely unexploited means of energy recovery and transportation, and they could play a significant role in past and future climate change.
Despite the industrial implications and worldwide abundance of gas hydrates, the formation mechanism of these compounds remains poorly understood. We report direct molecular dynamics simulations of ...the spontaneous nucleation and growth of methane hydrate. The multiple-microsecond trajectories offer detailed insight into the process of hydrate nucleation. Cooperative organization is observed to lead to methane adsorption onto planar faces of water and the fluctuating formation and dissociation of early hydrate cages. The early cages are mostly face-sharing partial small cages, favoring structure II; however, larger cages subsequently appear as a result of steric constraints and thermodynamic preference for the structure I phase. The resulting structure after nucleation and growth is a combination of the two dominant types of hydrate crystals (structure I and structure II), which are linked by uncommon 5¹²6³ cages that facilitate structure coexistence without an energetically unfavorable interface.
This work reviews major hydrocarbon hydrate advances in flowline applications of 25 international hydrate organizations. After a review of hydrate history and the current state-of-the-art, four ...conclusions were drawn: (1) engineers must take risks and cannot always afford the luxury to await scientific developments, (2) industry is more likely than academia to suggest hydrate needs and solutions, (3) the best hydrate blockage prevention practices are evolving and (4) a stepwise conceptual model can be proposed for a transient restart flowline hydrate blockage.
Significant factors controlling gas hydrate growth in water and water-in-oil dispersions have been tested. In particular, the influence of shear rate, presence of oil, and thermodynamic driving force ...(represented by pressure supersaturation) on hydrate growth rates is included. Formation rates in water show some discrepancy compared to previous work, which is likely caused by differences in the apparatus geometries. A model is proposed for growth of hydrate in oil, in which a hydrate shell forms on a water droplet, followed by additional conversion of the water core to hydrate.
Clathrate hydrate film growth has been investigated at the hydrocarbon/water interface for cyclopentane and methane hydrate, using video microscopy combined with gas consumption measurements. Hydrate ...formation was characterized by the film thickness, propagation rate across the hydrocarbon/water interface, and gas consumption. The film formation processes of cyclopentane and methane hydrate were measured over the temperature range of 260–273
K and pressure range of atmospheric to 8.3
MPa. Hydrate formation was initiated by the propagation of a thin, porous film across the hydrocarbon/water interface. This thickening rate was strongly dependent on the hydrate former solubility in the aqueous phase, in the absence and presence of hydrate. The methane hydrate film thickness began at about
5
μ
m
and grew to a final thickness (20–
100
μ
m
) which increased with subcooling. The cyclopentane hydrate film thickness began at about
12
μ
m
and grew to a final thickness (15–
40
μ
m
) which again increased with subcooling. The hydrate film grew into the water phase. Gas consumption indicated that the aqueous phase supplied hydrate former during the initial hydrate growth, and the free gas supplied the hydrate former for film thickening.
Clathrate hydrates have steadily emerged as an important field in the areas of flow assurance, energy storage and resource, and environment. To better understand the role of hydrates in all of these ...areas, knowledge developed in laboratory experiments must be effectively transferred to address the challenges related to hydrate formation, dissociation, agglomeration, and stability. This paper highlights the recent hydrate literature focusing on the thermodynamics, kinetics, structural properties, particle properties, rheological properties, and molecular mechanisms of formation. The foundation for continued understanding and development of hydrates in engineering practice will rely on laboratory measurements utilizing traditional and innovative tools capable of probing time-dependent and time-independent properties.
The size of droplets in emulsions is important in many industrial, biological, and environmental systems, as it determines the stability, rheology, and area available in the emulsion for physical or ...chemical processes that occur at the interface. While the balance of fluid inertia and surface tension in determining droplet size under turbulent mixing in the inertial subrange has been well established, the classical scaling prediction by Shinnar half a century ago of the dependence of droplet size on the viscosity of the continuous phase in the viscous subrange has not been clearly validated in experiment. By employing extremely stable suspensions of highly viscous oils as the continuous phase and using a particle video microscope (PVM) probe and a focused beam reflectance method (FBRM) probe, we report measurements spanning 2 orders of magnitude in the continuous phase viscosity for the size of droplets in water-in-oil emulsions. The wide range in measurements allowed identification of a scaling regime of droplet size proportional to the inverse square root of the viscosity, consistent with the viscous subrange theory of Shinnar. A single curve for droplet size based on the Reynolds and Weber numbers is shown to accurately predict droplet size for a range of shear rates, mixing geometries, interfacial tensions, and viscosities. Viscous subrange control of droplet size is shown to be important for high viscous shear stresses, i.e., very high shear rates, as is desirable or found in many industrial or natural processes, or very high viscosities, as is the case in the present study.
Recent advances and challenges in energy and environmental applications are discussed. Gas hydrates are shown to be fascinating Euclidean solids, with the potential, for significant impact on world ...energy and the environment. For energy production, it is clear that more investment needs to be made for long-term field production tests. Better detection tools are required since the main bottom simulating reflectors (BSR) technique is often unreliable. Improved fundamental understanding of the nucleation, growth and agglomeration processes would enhance the development of floe assurance tools. This is also important to the development of energy storage materials, which require effective synthesis and stabilities technologies.
Fundamentals and applications of gas hydrates Koh, Carolyn A; Sloan, E Dendy; Sum, Amadeu K ...
Annual review of chemical and biomolecular engineering,
07/2011, Letnik:
2
Journal Article
Recenzirano
Fundamental understanding of gas hydrate formation and decomposition processes is critical in many energy and environmental areas and has special importance in flow assurance for the oil and gas ...industry. These areas represent the core of gas hydrate applications, which, albeit widely studied, are still developing as growing fields of research. Discovering the molecular pathways and chemical and physical concepts underlying gas hydrate formation potentially can lead us beyond flowline blockage prevention strategies toward advancing new technological solutions for fuel storage and transportation, safely producing a new energy resource from natural deposits of gas hydrates in oceanic and arctic sediments, and potentially facilitating effective desalination of seawater. The state of the art in gas hydrate research is leading us to new understanding of formation and dissociation phenomena that focuses on measurement and modeling of time-dependent properties of gas hydrates on the basis of their well-established thermodynamic properties.
Interest in describing clathrate hydrate formation mechanisms spans multiple fields of science and technical applications. Here, we report findings from multiple molecular dynamics simulations of ...spontaneous methane clathrate hydrate nucleation and growth from fully demixed and disordered two-phase fluid systems of methane and water. Across a range of thermodynamic conditions and simulation geometries and sizes, a set of seven cage types comprises approximately 95% of all cages formed in the nucleated solids. This set includes the ubiquitous 5(12) cage, the 5(12)6(n) subset (where n ranges from 2-4), and the 4(1)5(10)6(n) subset (where n also ranges from 2-4). Transformations among these cages occur via water pair insertions/removals and rotations, and may elucidate the mechanisms of solid-solid structural rearrangements observed experimentally. Some consistency is observed in the relative abundance of cages among all nucleation trajectories. 5(12) cages are always among the two most abundant cage types in the nucleated solids and are usually the most abundant cage type. In all simulations, the 5(12)6(n) cages outnumber their 4(1)5(10)6(n) counterparts with the same number of water molecules. Within these consistent features, some stochasticity is observed in certain cage ratios and in the long-range ordering of the nucleated solids. Even when comparing simulations performed at the same conditions, some trajectories yield swaths of multiple adjacent sI unit cells and long-range order over 5 nm, while others yield only isolated sI unit cells and little long-range order. The nucleated solids containing long-range order have higher 5(12)6(2)/5(12) and 5(12)6(3)/4(1)5(10)6(2) cage ratios when compared to systems that nucleate with little long-range order. The formation of multiple adjacent unit cells of sI hydrate at high driving forces suggests an alternative or addition to the prevailing hydrate nucleation hypotheses which involve formation through amorphous intermediates.
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