1 Department of Chemical and Biochemical Engineering, Technical University of Denmark
The scope of this work is to investigate the synthesis of aluminum oxide particles in flames from the combustion of an aluminum alkoxide precursor.A general introduction to particles formation in the gas phase is presented with emphasis on the mechanisms that control the particle morphology after the initial formation of the monomers.Among those are Brownian coagulation of particles and the coalescence mechanism where collided particles fuse together thereby forming a sphere. Some existing models in the field are presented and compared.Two different burner systems are used in this study: A diffusion flame burner and a premixed burner with a precursor jet. The experimental setups and results are shown and discussed in detail. Alumina powder with specific surface area between 45 m2/g and 190 m2/g was obtained.Temperature and flow fields of the flame processes are analysed by numerical simulations (Computational Fluid Dynamics) where the fundamental equation for flow, heat- and mass transfer are solved numerically in computational domains similar to the real systems.A model describing the particle dynamics in the flame is coupled with the flow-field information in order to compute effluent particle characteristics, e.g. primary particle size and aggregate size.The accuracy of the model depends highly on the expression for the characteristic coalescence time for aggregates. The results indicate that the large variations in particle size and crystallinity are too complex to be described by a single activation energy expression.Furthermore, the model is validated by comparison with experimental data of the flame synthesis of titania by combustion of TiCl4 previously presented by Pratsinis et al. (1996).The combination of particle dynamics and CFD simulations has proved to be an efficient method for the analysis of particle formation in flames. Good results for a wide range of operating conditions were obtained. Therefore, the method should be useful as a tool for the optimization and/or design of flame processes for particle production.
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Technical University of Denmark, Department of Chemical Engineering, 1999