Agglomerate TiO₂ Aerosol Dynamics at High Concentrations

Abstract

Primary and agglomerate particle dynamics are investigated for aerosol synthesis of titania at high solids concentrations that are typically used for its industrial scale manufacture. Particle formation and growth are simulated accounting for chemical reaction, coagulation and sintering. Process conditions are chosen so that the resulting primary and hard‐agglomerate sizes are comparable with commercial product specifications. Neglecting aerosol polydispersity, the evolution of the diameter of primary particles, hard‐ and soft‐agglomerates along with the agglomerate effective volume fraction are investigated. During synthesis of nanostructured titania (e.g. for catalysts and cosmetics) the effective soft‐agglomerate volume fraction can exceed 30% within 100 s residence time indicating that a transition takes place from dilute to concentrated aerosol dynamics. At these conditions, classic Smoluchowski theory may no longer describe agglomerate coagulation and particles may affect fluid flow and heat transfer in industrial aerosol reactors. Furthermore, this could lead to restructuring and fragmentation of the product powder.

Extension of the Smoluchowski Theory to Transitions from Dilute to Dense Regime of Brownian Coagulation: Triple Collisions

“…The γ values calculated at different fractional volumes are plotted in Figure Langevin dynamics simulations performed by Heine and Pratsinis (2007a). A good agreement for the transition from dilute to dense regime at f ≤ 10% reasonably confirms the validity of the new approach.…”

“…Such highly concentrated suspensions are well known in emulsions, where the droplet volume fraction may vary from zero to almost one (Bibette et al 1999) but can also be observed in aerosols. For example, a transition from dilute to dense (or concentrated) particle dynamics may take place during flame aerosol synthesis of nanostructured, fractal-like carbon black, fumed silica (Heine and Pratsinis 2006) or titania (Heine and Pratsinis 2007a) particles at industrially relevant conditions. Even though in such processes the particle volume fraction, f , is only 0.001-0.01%, depending on the process temperature, fractal-like silica agglomerates form and grow to occupy more than 10% of the gas volume during typical reactor residence times in the absence of restructuring or fragmentation (Heine and Pratsinis 2007a).…”

Coagulation of highly concentrated aerosols

“…In contrast, agglomerate size and structure are affected and can influence the rheological properties of suspensions of these particles (Heine & Pratsinis, 2006, 2007a. The influence of high particle concentration on coagulation and even gelation has been discussed in several theoretical studies over the past years.…”

“…Corrections to that have been developed for van der Waals forces, charged or non-spherical particles (Friedlander, 2000) and other particle interactions (Seinfeld & Pandis, 2006) without altering, however, its basic theoretical framework: establishment of a steady-state Fickian flux to the colliding particle surface. By definition, the Smoluchowski equation cannot describe coagulation at high particle concentrations (effective volume fraction above 1%) that are encountered in large scale aerosol synthesis of fumed silica and titania (Heine & Pratsinis, 2006, 2007a as well as alumina, carbon black or soot (Sorensen, Hageman, Rush, Huang, & Oh, 1998).…”

Aerosol Science and Technology: History and Reviews

“…To minimize fusion or necking of the TiO 2 particles and formation of large hard agglomerates (or aggregates), their temperature has to be reduced to less than 600ºC (Mezey, 1966). Titania particles leaving the oxidizer (flame aerosol reactor) are typically soft agglomerates, several micrometers in diameter, consisting of smaller (about 500 nm) hard agglomerates (aggregates) that contain even smaller (about 200 nm) primary particles (Heine & Pratsinis, 2007). As a result, special milling (grinding) is used to break up the soft agglomerates before feeding them to the finishing plant where their surfaces are coated and conditioned for suspension in paints or compounding in polymer or paper matrixes.…”