Kinetics of CeO2 Surface Area Reduction in a Mixture of HCI, H2O, and O2

Surface area reduction of a cerium dioxide powder is studied at 900 K in a HCI, H₂O, and O₂ atmosphere. The rate of surface area decrease is determined as a function of the surface chloride content and of the partial pressure of hydrogen chloride. It is shown that particle growth occurs due to the surface diffusion of cerium and hydroxyl ions, which results from hydrogen chloride adsorption. Adsorbed chloride ions react to give gaseous chlorine. The rate-determining step of the process of initial coarsening is hydrogen chloride fixation at the surface of cerium dioxide particles.     

Light-Scattering Cross Section of Hydrophilic and Hydrophobic Silica Aggregates

The aggregation dynamics of solid particles in liquid media is currently determined by optical-based particle sizing methods. Because it can be used in situ and applied to a wide particle size range, turbidimetry is acknowledged as one of the best methods for this characterization. Although much work has been done on aggregation, some aspects are less known and require additional experimental and theoretical research. This is particularly the case of aggregation of hydrophobic particles. Corresponding aggregates are three-phase objects (solid-liquid-gas) the morphology and optical properties of which are not known. The present work rests on the turbidimetric study of hydrophilic and hydrophobic silica samples in stirred aqueous solutions. Modeling involves different aspects: aggregate morphology, aggregate optical properties, and aggregation dynamics. This article particularly emphasizes the second aspect. Fractal-like models are proved to be representative of the aggregate morphology even at small size. Light-scattering cross section of the aggregates is calculated from their averaged projected area; effective refractive index is proved to be a good parameter for modeling the optical properties of both hydrophilic and hydrophobic aggregates. Classical models of porous aggregate formation (Kusters theory) are used for describing the aggregation dynamics.     

Kinetics of Anatase TiO2 Surface Area Reduction in a Mixture of HCI, H2O, and O2: II, Quantitative Modeling

From the experimental rate for surface area reduction of anatase TiO₂, expressed versus the partial pressure in H₂O and the surface content of chloride ion, a kinetic model is proposed in which two parallel mechanisms are involved. Two ways of material transport appear to contribute to the surface area loss: in the vapor phase and at the surface of the oxide. The related rate-determining steps are attributed to the formation of the volatile compound Ti(OH)₂Cl₂ in the first case, and to the diffusion of hydroxyl ions in the second one. At high partial pressure of water, the rate is enhanced by surface interactions between water molecules and chlorine-containing species.     

Modelling of a CO2-gas jet into liquid-sodium following a heat exchanger leakage scenario in Sodium Fast Reactors

Sodium-cooled Fast Reactors (SFRs) represent one of the most promising technologies in the context of generation IV nuclear power reactors. In order to avoid a reaction between sodium and water when Rankine cycles are employed, the concept of Brayton cycles using supercritical CO₂ (SCCBCs) is being investigated as alternative energy conversion cycle. However, an accidental scenario must be evaluated, since a leakage inside the CO₂-sodium heat exchanger would cause a reactive underexpanded CO₂-into-sodium jet, which in turn could lead to mechanical and thermal problems. A two-fluid approach has been investigated for the modelling of the two-phase jet: according to flow maps, mist flow has been assumed at the leak exit, where high gas volume fraction and high interfacial slip velocity exist, and bubbly flow has been assumed for lower gas volume fraction and slip velocity. An interfacial friction model has been developed. Droplet and bubble diameters have been estimated following literature experimental results and using critical Weber number. For the drag coefficients, consistent correlations have been developed. A two-phase mixture turbulence model has been added. The interfacial friction approach has been implemented into the two-fluid model of the CFD software Ansys Fluent 14.0. 3D numerical simulations of gas-into-water jets have been performed for vertical upward jets and optical probe technique has been employed for the experimental measurement of void fraction inside an underexpanded N₂-into-water jet: numerical results agree with experimental results in terms of axial and radial void fraction profile. The two-fluid approach presented here will be the base for the implementation of a chemical reaction model, in order to account for the exothermic chemical reaction between the CO₂ and the sodium.     

Light-Scattering Cross Section of Hydrophilic and Hydrophobic Silica Aggregates

The aggregation dynamics of solid particles in liquid media is currently determined by optical-based particle sizing methods. Because it can be used in situ and applied to a wide particle size range, turbidimetry is acknowledged as one of the best methods for this characterization. Although much work has been done on aggregation, some aspects are less known and require additional experimental and theoretical research. This is particularly the case of aggregation of hydrophobic particles. Corresponding aggregates are three-phase objects (solid-liquid-gas) the morphology and optical properties of which are not known. The present work rests on the turbidimetric study of hydrophilic and hydrophobic silica samples in stirred aqueous solutions. Modeling involves different aspects: aggregate morphology, aggregate optical properties, and aggregation dynamics. This article particularly emphasizes the second aspect. Fractal-like models are proved to be representative of the aggregate morphology even at small size. Light-scattering cross section of the aggregates is calculated from their averaged projected area; effective refractive index is proved to be a good parameter for modeling the optical properties of both hydrophilic and hydrophobic aggregates. Classical models of porous aggregate formation (Kusters theory) are used for describing the aggregation dynamics.     

Kinetics of Anatase TiO2 Surface Area Reduction in a Mixture of HCl, H2O, and O2: l, Experimental Study

Changes in the surface area of anatase TiO₂ were investigated at 690 K as a function of the gas composition in mixtures of hydrogen chloride, water vapor, and oxygen. The chlorine content on the catalyst support was not dependent on the annealing time, which indicates that the adsorption equilibrium is reached very quickly. The kinetics of surface area were obtained as a function of partial pressure of HCI (from 0.1 to 40 kPa), water vapor (from 0.05 to 10 kPa), and oxygen (from 0.4 to 20 kPa). A mathematical expression for the rate of surface area loss was obtained which includes the partial pressure of water and the chlorine content at the surface of anatase.     

Precipitation dynamics of zinc sulfide multiscale agglomerates

Homogeneous precipitation of zinc sulfide from thioacetamide decomposition in presence of zinc sulfate in acid solution results in the formation of four‐scale agglomerates. We present here experimental results relative to the monitoring of several physicochemical (pH, electrical conductivity, concentration in sulfide ions, turbidity) and morphological (agglomerate shape and size) parameters throughout the precipitation process. From these parameters essential characteristics of the precipitation process can be determined, especially the supersaturation level and the precipitated product mass. From this information and with the help of microphotographs of samples withdrawn at different stages of the precipitation, it is possible to formulate a new mechanism of zinc sulfide precipitation. In particular, the nature of the different agglomeration scales is elucidated as well their succession in time. © 2009 American Institute of Chemical Engineers AIChE J, 2009