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Engineering, United States

Chakraborty S.N.,Center for Hydrate Research | Grzelak E.M.,Center for Hydrate Research | Barnes B.C.,Center for Hydrate Research | Wu D.T.,Center for Hydrate Research | And 2 more authors.
Journal of Physical Chemistry C | Year: 2012

Molecular simulation of clathrate hydrate has provided significant advancements in our understanding of hydrate properties and formation. In this work, we report the application of Voronoi tessellation to characterize the structuring of water and guest molecules forming hydrates. Tessellation of perfect sI and sII hydrate reveals positions of Voronoi vertices similar to the oxygen atoms of enclathrating water molecules. Applying tessellation to a simulation trajectory of hydrate formation, and using a further selection criteria based on polyhedra volume and coordination number, we identify numbers and types of cagelike polyhedra. Voronoi analysis of this type results in similar numbers of identified cages but with differing topologies. However, once nearest neighbor methanes are also enclathrated, the topologies of the Voronoi polyhedra approach that of the actual water cages. Since only methane coordinates are required, Voronoi tessellation is a fast and simple tool that can be used as an order parameter to identify the structuring of molecules when studying hydrates in simulations. © 2012 American Chemical Society. Source

Boxall J.A.,Center for Hydrate Research | Koh C.A.,Center for Hydrate Research | Sloan E.D.,Center for Hydrate Research | Sum A.K.,Center for Hydrate Research | And 2 more authors.
Langmuir | Year: 2012

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. © 2011 American Chemical Society. Source

Jensen L.,Technical University of Denmark | Thomsen K.,Technical University of Denmark | Von Solms N.,Technical University of Denmark | Wierzchowski S.,Center for Hydrate Research | And 7 more authors.
Journal of Physical Chemistry B | Year: 2010

Monte Carlo simulation methods for determining fluid- and crystal-phase chemical potentials are used for the first time to calculate liquid water-methane hydrate-methane vapor phase equilibria from knowledge of atomistic interaction potentials alone. The water and methane molecules are modeled using the TIP4P/ice potential and a united-atom Lennard-Jones potential, respectively. The equilibrium calculation method for this system has three components, (i) thermodynamic integration from a supercritical ideal gas to obtain the fluid-phase chemical potentials, (ii) calculation of the chemical potential of the zero-occupancy hydrate system using thermodynamic integration from an Einstein crystal reference state, and (iii) thermodynamic integration to obtain the water and guest molecules chemical potentials as a function of the hydrate occupancy. The three-phase equilibrium curve is calculated for pressures ranging from 20 to 500 bar and is shown to follow the Clapeyron behavior, in agreement with experiment; coexistence temperatures differ from the latter by 4-16 K in the pressure range studied. The enthalpy of dissociation extracted from the calculated P-T curve is within 2% of the experimental value at corresponding conditions. While computationally intensive, simulations such as these are essential to map the thermodynamically stable conditions for hydrate systems. © 2010 American Chemical Society. Source

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