Dynamics of Fine Powders

Although much research has been done on special aspects of powders, the dynamic behaviour of powders has failed to draw much attention. This is surprising because reliable scaling-up of powder-handling operations seems impossible as long as the dynamics have not been solved. Especially, transfer processes such as heat and mass transfer, as well as mixing and flow of powders, depend strongly on scaling-up and hence on the dynamics. This book is an account of more than 20 years' research on powders carried out at the Technical University of Eindhoven in the Netherlands. This research started with the discovery that in homogeneous fluidization of fine powders the fluidized particles stay in permanent contact with each other, thus refuting the then general assumption of free floating of fluidized particles. Indeed theoretical analyses of the stability of gas-fluidized beds which were based on this assumption failed to predict the possibility of stable homogeneous fluidization, thus contradicting experimental evidence of homogeneous fluidisation at higher bed expansion. The logical conclusion was that, owing to the permanent contact between the particles, interparticle forces would exist which had to be introduced in the momentum equations of the theoretical analysis and which were responsible for the stabilization. Later on the discovery that the viscosity of the fluidization gas has a remarkable effect on the maximum possible bed expansion made it clear that a gas-fluidized system should be considered as a two-system of fluidized particles and the fluidization gas. A third important discovery was that interparticle forces are enhanced by absorption of gas to the surface of the solid particles, thus increasing the maximum possible homogeneous bed expansion at higher gas pressures. The increased insight into the behaviour of gas-fluidized powders and the effect of the nature of the gas phase on this behaviour suggested also that in other powder-handling operations such as grinding and mixing, the nature of the gas phase might have a similar strong effect on the behaviour of the powder during operation. This idea was confirmed by an extensive research programme on grinding of powders in ball mills at various gas pressures and with various gasses. Also the effect of the gas phase on the rate of mixing in a rotating drum was studied. This was the first systematic research programme on the effect of the gas phase in powder-handling operations known in the literature. The Dynamics of Fine Powders will be of special interest to chemical engineers specialized in powder technology, fluid flow and transfer processes.