Experimental and numerical study of TiO2 nanoparticle evolution in a diffusion flame reactor

Abstract

The spatiotemporally resolved formation and growth of TiO₂ nanoparticles synthesized in a pilot-scale diffusion flame reactor is investigated experimentally and numerically. More specifically, the detailed nanoparticle morphology, size and polydispersed size distribution information along the centerline of the flame are obtained using a custom-designed thermophoretic sampling device and a semi-automated TEM analysis software. Quantitative data on crucial parameters, particularly the spatial evolution of primary particle size distribution (PPSD) and aggregate (or agglomerate) size distribution (ASD) are reported simultaneously, providing the experimental data for developing or validating numerical models and methods. The characterization of the TiO₂ products synthesized at lab-scale (10 g/h) and pilot-scale (200 g/h) has also been performed. The results indicate that monodisperse spherical nanoparticles (∼ 1–3 nm) formed via multiple mechanisms are the primary characteristic of particle growth in the early stage near the burner. These single particles subsequently evolve into mature products in the end. Then, an advanced LES-bivariate sectional method (LES-BiSe) is used to simulate the spatiotemporally resolved formation and growth of TiO₂ nanoparticles in the diffusion flame. The employed model possesses the capability to simultaneously predict the size, morphology, as well as polydispersed PPSD and ASD of nanoparticles. Quantitative comparisons of spatially resolved PPSD, ASD, primary particle and aggregate diameters, as well as the average primary particle number per aggregate (or agglomerate) demonstrate satisfactory agreement with the experimental results. The differences between experiment and simulation are also thoroughly discussed. Owing to the pronounced inhomogeneous field information and the particle transport mixing in flame, the pilot-scale flame exhibits significantly broader size distributions compared to an ideal homogeneous system or a small-scale flame. Furthermore, once having access to the full information regarding the spatial evolution of particle morphology and polydisperse size distributions, a comprehensive spatial identification of various particle dynamic events is explored to discern their competitive influences on particle evolution.