Agglomerate TiO 2 Aerosol Dynamics at High Concentrations Martin C. Heine, Sotiris E. Pratsinis* (Received: 8 September 2006; accepted: 12 March 2007) DOI: 10.1002/ppsc.200601076 In memoriam of Professor Dr. Brian Scarlett 1 Introduction Titanium dioxide is one of the most important ceramic materials. In 2000 about 4 million tons of titania pigment were produced world wide, about 60 % of those in aero- sol reactors by combustion of TiCl 4 [1]. Typically its applications are in paints, papers, plastics, inks and even food or toothpaste. High surface area titania with pri- mary particle size below 50 nm is transparent and so it is used in cosmetics and sunscreens as a thickener and for ultraviolet (UV) light absorption. Additionally it is an effective photocatalyst under UV light that is used to degrade organics in wastewater and industrial effluents. Aerosol reactors are routinely used to manufacture na- noscale commodities resulting in products of high purity. In these reactors product primary and agglomerate particle sizes can be controlled by reactant concentra- tion and particle residence time at high temperatures [2]. Their scale-up has been studied systematically by variation of precursor, oxidant and fuel flow rates [3]. Population balance models like monodisperse [4] and one- [5] or two-dimensional sectional [6] have been developed, revealing the details of particle growth accounting for chemical reaction, particle formation, coagulation and sintering. For most applications the product specific surface area (SSA) and crystallinity are of key importance (e.g. pig- ments, catalysis), especially for nanosized powders. However, particle morphology, surface chemistry and degree of agglomeration can be equally important for final product performance [2]. Industrial aerosol reac- tors are usually operated at high precursor concentra- tions, close to stoichiometry, to achieve high reactor volume yields [1]. Then flame temperatures have to be 56 Part. Part. Syst. Charact. 24 (2007) 56–65 * M.C. Heine, S.E. Pratsinis, Particle Technology Laboratory, Institute of Process Engineering, Department of Mechanical and Process Engineering, ETH Zurich, 8092 Zürich (Switzer- land). E-mail: pratsinis@ptl.mavt.ethz.ch Abstract Primary and agglomerate particle dynamics are investi- gated for aerosol synthesis of titania at high solids con- centrations that are typically used for its industrial scale manufacture. Particle formation and growth are simu- lated accounting for chemical reaction, coagulation and sintering. Process conditions are chosen so that the re- sulting primary and hard-agglomerate sizes are compar- able 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-ag- glomerate 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. Keywords: aerosol dynamics, coagulation and sintering, fumed titania, hard- and soft-agglomerates http://www.ppsc-journal.com © 2006 WILEY-VCH Verlag GmbH & Co. KGaA,Weinheim