Yun Xiong

Gas phase production of particles in reactive turbulent flows

Abstract Industrial production of carbon blacks and various oxide powders (silica, titania) is commercially carried out in turbulent conditions. A theoretical study was conducted to analyze the interplay of chemical reaction, Brownian and shear-induced coagulation in nonisothermal, highly turbulent flows, during gas phase production of ceramic powders. The dynamic behavior of these particles is modelled by approximating the particle size distribution by a lognormal function throughout the process. The validity of the lognormal approximation is investigated by comparing it to a detailed solution of the respective population balance equation. The effects of turbulent intensity, mass loading, process pressure, initial temperature and cooling rate on the polydispersity and average size of the product particles were investigated. It was found that initially, particle growth is controlled by chemical reaction and Brownian coagulation while later shear-induced coagulation becomes predominant. During the transition from Brownian to shear-induced coagulation the geometric standard deviation goes through a minimum. Thus, an optimal average particle diameter can be identified with respect to the polydispersity of the product powder. This optimal diameter decreases with increasing turbulent intensity and initial temperature, and with decreasing process pressure.

Formation of agglomerate particles by coagulation and sintering—Part II. The evolution of the morphology of aerosol-made titania, silica and silica-doped titania powders

Abstract A two-dimensional particle size distribution model is used to describe gas phase synthesis of titania and silica powders. By incorporating a coalescence term along with a particle sintering rate in the governing population balance equation, the model traces the evolution of both the volume and surface area of aerosol particles. The simultaneous calculation of particle volume and surface area distribution leads to a direct characterization of the primary particles (grains) as well as particle aggregates. The effects of residence time, temperature and particle material properties on powder morphology are investigated. Effective sintering rates for pure titania, silica and silica-doped titania are deduced by comparing grain sizes obtained by model simulations and those measured experimentally from specific surface area and microscopic analyses.

Role of particle evaporation during synthesis of lead oxide by aerosol decomposition

The role of product evaporation during lead monoxide (PbO) powder generation by aerosol decomposition (spray pyrolysis) was investigated at various temperatures in a flow reactor. Particles consisting of phase pure litharge and a mixture of litharge and massicot were formed with the dominant phase changing from litharge to massicot as the pyrolysis temperature was increased. Scanning electron microscopy showed particles produced at lower temperatures had a lumpy surface morphology and at higher temperatures appeared to be solid, indicated by the faceted surfaces and a plate-like morphology. Evaporative losses of PbO x , to the reactor walls were observed due to the substantial vapor pressure of PbO x . A simple model was developed that accounts for particle evaporation and mass transfer of lead oxide vapor to the reactor walls. This model suggested that the loss of lead oxide to the reactor walls was limited by diffusional transport of lead oxide vapor to the reactor walls.

The effect of ionic additives on aerosol coagulation

Abstract The effect of ionic additives on particle size characteristics during gas phase production of finepowders is investigated. In aerosol manufacture of particulate commodities, electrolytes are sprayed into the process to control the phase and size characteristics of the product powders. These electrolytes dissociate into their constitutive ions, some of which preferentially adsorb onto the particle surface and give rise to electrostatic repulsive forces between particles that affect particle-particle interactions, and hence, particle growth by coagulation. The size distribution of these particles depends on the strength of charging at the particle surface, which is related to the ionization of the electrolytes. A thermochemical equilibrium calculation is performed to estimate the charging ability of alkali metals (Na and K). The effects of ionic additives on the rate of aerosol coagulation are numerically evaluated using a sectional solution for the aerosol dynamics equation. It is found that ionic additives can result in narrow size distributions and small average particle sizes, in agreement with experimental observations in the literature. In addition, these distributions are narrower than those predicted by the self-preserving theory for coagulation.

Synthesis of Titania Powder by Titanium Tetrachloride Oxidation in an Aerosol Flow Reactor

Vapor phase synthesis of titania particles by oxidation of titanium tetrachloride (TiCI 4 ) was studied in an aerosol reactor between 1200 K and 1723 K. The effect of process variables (reactor residence time, temperature, reactant concentration) on powder size and phase characteristics was investigated using the differential mobility particle sizer, scanning electron microscopy and X-ray diffraction. The morphology of the particles remained unchanged under the process conditions investigated; titania particles were primarily anatase though the rutile weight fraction increased with increase in reactor temperature. The geometric number average diameter of the particles was between 0.13 µm and 0.35 [m and the geometric standard deviation of the particle size distribution was about 1.4. The average particle size increased with increasing temperature, TiCI 4 concentration and residence time. The observed changes in the particle size distribution were compared with those predicted by solving the aerosol dynamic equation by a sectional method and accounting for coagulation and first order chemical reaction.