Behavior and fluidization of the cohesive powders: agglomerates sizes approach

Abstract

-This work focuses on the fluidization of three types of TiO 2 powders: Anatase (99% TiO 2 ), Rutile 1 (95% TiO 2 and 5% Al) and Rutile 2 (96.5% TiO 2 and 3.5% Al and Si); the average diameters of the powders are 204 nm, 159 nm and 167 nm, respectively. These powders belong to group C of the Geldart classification and are characterized as cohesive powders with a non-free flow and a difficult fluidization. The fluidization of the powders was carried out in a glass column of 103 mm inner diameter and 1500 mm height. The experiments and analysis performed included measurements of the physical properties of the powders such as the particle size, density, specific surface area and the flow properties of the powders like the Hausner's index, the angle of repose, the angle of slide, consolidation and shearing (via shear cell testing). The results obtained with the nanometric TiO 2 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 TiO 2 powders have a free flow or intermediate-flow and a non-free-flow for higher strength intensities (consolidation and shearing). This behavior is related to the structure of the nanometric particles in the packed bed; the evolution of this structure is made up of individualized and spherical agglomerate shapes and is not perturbed by stresses of low intensities. Indeed, the latter seems to modify the structure of the powder (group C of Geldart classification) to acquire a behavior typical of group A, B or D in the Geldart classification. With high stress values, the individualized agglomerates are disintegrated and the powder is reduced to a more compact structure. The fluidization of TiO 2 powders seems to evolve in a more homogeneous way than the micronic powders. This behavior is related to the initial structure being made up of stable agglomerates. Thus, this fluidization is made by agglomerates with a gas velocity of 3×10 6 to 4.6×10 6 times the gas velocity for fluidizing the primary particles.A numerical approach based on a force balance in agglomerating fluidized beds was developed in order to estimate the agglomerates sizes.

Dustiness and Deagglomeration Testing: Interlaboratory Comparison of Systems for Nanoparticle Powders

“…For 1 mm particles, forces ranged from 0.06 to 2.05 nN. The dominant mechanism by which uncharged particles form agglomerates when stored as powders is direct contact (Turki and Fatah 2008), bonding individual particles by van der Waals (VDW) force. This interparticular force can be calculated as below (Hamaker 1937)…”

“…In contrast, higher drag forces created in the air jet system broke agglomerates down to the sizes for which associated drags become comparable to the interparticle binding force (as discussed in previous sections). Furthermore, the funnel setup-which resembles a fluidized bed-features another deagglomeration mechanism: collisions between flowing particles (Turki and Fatah 2008;van Ommen et al 2010). These conditions promoted sharper peaks in the size distribution of the aerosols generated.…”

Fluid dynamics and morphological characterization of soy protein isolate particles obtained by agglomeration in pulsed-fluid bed

“…This energy dissipation promotes an increase of bed cohesiveness, which depends on the strain rate, temperature, moisture content and chemical composition of the particles [10]. Furthermore, the fluid dynamic behavior can be affected by adhesion forces, which vary with the size distribution, morphology (shape) and physical properties (density, specific surface area and porosity) of the powder [17]. The soy protein isolate powder presented non-spherical particles with plastic behavior, once its main constituent is protein, and the fluidization regime showed to be cohesive with cracks and channeling behaviors, as a solid of Geldart's group C. However, during the agglomeration runs, this behavior changed with particle enlargement, and the powder became more free-flowing.…”

Vibration Assisted Handling of Dry Fine Powders

“…There are several ways to improve the handling of fine powders. Whereas chemists prefer to use additives [1] to reduce adhesion and cohesion in order to improve the flowability and avoid agglomeration, physicists prefer to use air flow [2] or mechanical vibration [3] to achieve the same effect. The use of additives leads to a permanent altering of powder characteristics.…”