(95c) Design of Aerosol Coating Reactors

Nanoparticles are coated to make them compatible to their host matrices. With such core-shell particles, desirable properties of core (e.g. dielectric, scattering or opacifying performance) particles are conserved while modifying their surface by the shell material. For example, rutile TiO₂ particles are made by the ?chloride? or ?sulfate? process and are coated with silica, alumina and other oxides by wet impregnation processes to prevent the photocatalytic reaction of pigmentary TiO₂ with the organic host matrix in paints (Clark, 1975). Aerosol coating is attractive as it contributes to gas-phase synthesis of functionalized nanoparticles in one-step facilitating their economic manufacture. Here aerosol coating of TiO₂ core particles with SiO₂ shells is investigated by a combination of particle dynamic models and computational fluid dynamics, accounting for SiO₂ monomer generation and growth by coagulation and sintering. The evolution of the SiO₂ coating particle population is simulated distinguishing monomers and particles with the bimodal model of Jeong and Choi (2003). At industrial scale manufacture of nanoparticles, such aerosols can encounter high concentration dynamics resulting in much faster growth kinetics and even gelation (Buesser et al., 2009). To investigate the behavior of aerosol coating reactors (Teleki et al., 2008) the aerosol coating model of Buesser and Pratsinis (2010) has been combined with computational fluid dynamics to account for gas-phase reaction of coating shell precursor, core particle aerosol generation, flow and temperature fields that influence mixing of core and coating particle aerosol inside of the aerosol coating reactor (Teleki et al., 2009). The simulations are compared and validated with experimental data of nanometer thin SiO₂-coated flame-made rutile TiO₂ particles (Teleki et al. 2009). The surface of TiO₂ shows photocatalytic activity in the oxidation reaction of isopropanol where the rate is proportional to the uncoated TiO₂ surface area. Figure 1 compares the simulated fraction of uncoated core particles (red line) with the normalized photocatalytic activity of the silica coated titania particles (black line) of Teleki et al. (2009). The model shows that increasing the mixing of coating precursor and core particle aerosol by increasing N₂ flow rate leads to less uncoated TiO₂ particles in the product powder (red line), which is in agreement with the experimentally decreasing catalytic activity (black line). The influence of mixing flow rates and coating precursor concentration on coating thickness and uniformity of its distribution on the core particles are investigated. The causes for uncoated core particles are identified and discussed, showing that the model is suitable for optimization of aerosol coating reactors. Financial support from the Swiss National Science Foundation (SNF) grant # 200021-119946/1 is gratefully acknowledged. Clark, H. B., Ed. (1975). Titanium dioxide pigments. Treatise on Coatings, 3, Pigments Part I. New York, Marcel Dekker Jeong, J. I. & Choi, M. (2003). A simple bimodal model for the evolution of non-spherical particles undergoing nucleation, coagulation and coalescence. J. Aerosol Sci. 34: 965-976 Buesser, B., Heine, M.C., Pratsinis, S.E (2009), ?Coagulation of highly concentrated aerosols? J. Aerosol Sci., 40, 89-100 Buesser, B. and Pratsinis, S.E (2010), submitted