Simulations of fluidized fine powders

During the past few decades, several studies have been conducted to understand the behaviour of powders in vibrated and gas-solid fluidized beds. In fact, vibration and fluidization of these powders pose various challenges due to the complexity of the microscopic processes such as agglomeration. This thesis introduces an innovative technique of incorporating the agglomeration and deagglomeration phenomena in the simulation of the behaviour of fine powders. More precisely, in order to predict the behaviour of cohesive powders in vibrated or gas fluidized beds, the agglomeration and deagglomeration processes are modelled as the formation and destruction of interparticle bonds during particle collisions. The two parameters which characterize these phenomena are the cohesive energy barrier and the cohesivity of the powder.

For this simulation, Molecular Dynamics is used for vibrated beds while Stokesian Dynamics is employed to describe gas-solid fluidized beds.

Two-dimensional direct simulations are performed using 300 spheres 2.99 mm in diameter in a trapezoidal container vibrated vertically at an amplitude of 2.5 mm and 20 Hz frequency as preliminary conditions. Under non-cohesive conditions, the results are in agreement with those found in the literature. Simulation results reveal two aggregate populations, one with uniform size aggregates and another population with mufti-sized aggregates. The former aggregates were more prevalent in weakly cohesive powders while the latter in highly cohesive powders. Interesting macroscopic bed behaviour such as alternating cycles of agglomeration and deagglomeration were also observed.

Simulations of gas-solid fluidized beds are carried out using the Stokesian Dynamics method in two dimensions. In order to experimentally validate the use of this simulation type, the bed characteristics and the motion of particles obtained by Stokesian Dynamics in the case of FCC powder (Fluid Cracking Catalyst) are compared with experimental results obtained by the radioactive particle tracking method, fiber-optic method and those found directly in the literature.

The agglomeration-deagglomeration model is used in conjunction with the Stokesian Dynamics method to simulate weakly and strongly cohesive powders. Here again, simulation of weakly cohesive powders indicate the formation of small, uniform size aggregates which are held together by weak bonds. Fluidization of strongly cohesive powders causes the complete agglomeration of the fine particles to form of strong, large size agglomerates which caused complete or partial defluidization of the bed depending on the parameters that are introduced. Additional simulation results indicate that higher values of the minimum collisional energy and lower cohesivity of the powder aids in improving the fluidizability of cohesive powders.

To improve the gas fluidizability of strongly cohesive powders, the following three techniques (already used in experiments reported in the literature) are used: (a) gas velocity much higher than the minimum fluidization velocity; (b) vibration-assisted fluidization and (c) tapered fluidizer. The influence of these techniques on the bed microscopy such as particle motion and the formation and destruction of agglomerates during collisions are studied qualitatively. In accordance with the literature, the three techniques improve the fluidizability of a highly cohesive powder. Moreover, for a simultaneous improvement of the fluidizability and restrained particle entrainment, a combination of the fluidization at gas velocity higher than the minimum fluidization condition coupled with an external vibration of the bed is suggested. Finally, a cohesive powder diagram is provided to explain the interactions of the parameters of the agglomeration-deagglomeration model with the operating conditions.