(220b) Effect of Reaction Mechanism On the Multiscale Modeling of Titanium Dioxide Nanoparticles

Titanium dioxide (TiO2) nanoparticles are manufactured in flame reactors by injection of precursor (TiCl4) into a pre-existing flame where it oxidizes to form nanoparticles. The flame configuration, dependent on fuel and oxidizer flow rates and position, is important as it leads to particles with different sizes as well as morphology (as depicted in Fig. 3 of [2]). The reaction mechanism for TiCl4 oxidation can be represented with different degrees of complexity. Most models for TiO2 production use a one-step reaction[3]. Recently, West et al.[4] proposed a detailed reaction mechanism for TiCl4 oxidation containing 30 species and 66 reactions. It was found that the choice of the reaction mechanism will have a strong effect on particle evolution. The two mechanism predict very different locations in the flame for particle nucleation as seen from Fig. 1.

In this work we study the effect of the two reaction mechanisms (one-step and detailed) on different flame configurations as given by Pratsinis et al.[2]. The nanoparticle evolution is tracked with the help of a population balance equation (PBE), which describes the evolution of the particle size distribution (PSD). The evolution of PSD due to nucleation, surface growth and aggregation can be represented by a single variable (e.g. particle volume) but the inclusion of the sintering effects require an additional variable, surface area. Here a bi-variate PBE is solved with the help of conditional quadrature method of moments (CQMOM)[1] to track the evolution of nanoparticles in flame reactors. The inclusion of this detailed particle evolution model in a flow configuration helps us predict the product properties, as described by the two reactions mechanisms, and discuss their relative merits.

Figure 1: Evolution of number density (solid) and temperature (dashed) in PFR.

References :

[1] Janine Chungyin Cheng and Rodney O. Fox. Kinetic modeling of nanoprecipitation using CFD coupled with a population balance. Ind. Eng. Chem. Res., ISCRE Special Issue, 2010.

[2] Sotiris E. Pratsinis, Wenhua Zhu, and Srinivas Vemury. The role of gas mixing in flame synthesis of titania powders. Powder Technol., 86(1):87--93, 1996.

[3] Patrick T. Spicer, Olivier Chaoul, Stavros Tsantilis, and Sotiris E. Pratsinis. Titania formation by TiCl4 gas phase oxidation, surface growth and coagulation. J. Aerosol Sci., 33:17--34, 2002.

[4] Richard H. West, Raphael A. Shirley, Markus Kraft, C. Franklin Goldsmith, and William H. Green. A detailed kinetic model for combustion synthesis of titania from TiCl4. Combust. Flame, 156(9):1764--1770, 2009.