Fluidization and Study of the Nanometric powders behavior:Numerical Approach to estimate the Agglomerates sizes

摘要

This work focuses on the fluidization of Three types of TiO2 powders: Anatase (99% ofTiO2), Rutile 1 (95% of TiO2 and 5% of Al) and Rutile 2 (96.5% of TiO2 and 3.5% of Al andSi) the average diameters of the powders are 204 nm, 159 nm and 167 nm respectively.These powders belong to the group C of the Geldart [1] classification and arecharacterized as cohesive powders with a non-free flow and a difficult fluidization. Thefluidization of the powders was carried out in a glass column of 103 mm inner diameterand 1500 mm height. The experiments and analysis performed included measurements ofthe physical properties of the powders as the particle size, density, specific surface areaand the flow properties of the powders as the Hausner's index, the angle of repose, theangle of slide, consolidation and shearing (via shear cell testing). The results obtained withthe nanometric TiO2 powders show a more complex behavior than the micronic powders;with a low strength value (Hausner index, angle of repose and angle of slide) the TiO2powders have a free flow or intermediate-flow and a non-free-flow for higher strengthintensities (consolidation and shearing). This behavior is related to the structure of thenanometric particles in the packed bed, the evolution of this structure is made up ofindividualized and spherical agglomerates shape and is not perturbed by stresses of lowintensities. Indeed, the latter seems to modify the structure of the powder (group C ofGeldart classification) to acquire a typical behavior of group A, B or D in the Geldartclassification. With high stress values the individualized agglomerates are disintegratedand the powder is reduced to a more compact structure. The fluidization of TiO2 powdersseems to evolve more in a homogeneous way than the micronic powders. This behavior isrelated to the initial structure being made up of stable agglomerates. Thus, this fluidizationis made by agglomerates with a gas velocity of 3×106 to 4.6×106 times the gas velocity forfluidizing the primary particles. A numerical approach based on a force balance inagglomerating fluidized beds was developed in order to estimate the agglomerates sizes.