(232c) Enhanced Fluidization of Cohesive Particles by Surface Modification

Fluidization has been widely used in many powder processes because of its continuous powder handling ability and good gas-solid contacting. This results in high heat and mass transfer coefficients and high rates of reaction. However, in a conventional gravity-driven fluidized bed, particles having an average diameter smaller than about 30 microns are extremely difficult to fluidize and generally will form cracks, channels or “rat holes” or even lift as a solid plug when exposed to the fluidizing gas. Nonetheless, there are many applications that can benefit from being able to fluidize such cohesive particles, and therefore, fluidization of cohesive particles has been an active area of research for number of years. Past reports on this topic indicate that the fluidization quality of cohesive powders can be improved by using external aids such as vibration, magnetic field, and acoustic field. However, there are some disadvantages with each method.

In this work, we first examine the fundamental reasons for the difficulty in fluidizing Class C particles by considering various terms that influence the boundary between Geldart class A and C. It can be easily shown that the transition between A and C is governed by relative magnitudes of forces such as the apparent weight of the particle, cohesion force, etc. In previous work, it was shown that one term that can be manipulated is the “particle body force” through application of centrifugal field, which will increase the “body force” and thus shift the A-C transition curve on Gerdart's map to the left, indicating that now much smaller particles may be fluidized. Based on that, fluidization of otherwise cohesive (class “C”) particle have been shown to occur in a rotating fluidized bed. However, in this work, we examine another approach that can be employed to decrease the relative magnitude of the cohesive force as compared to the body force through surface modification. This is accomplished through dry particle coating where very fine fumed silica nanoparticles are attached to the surface of otherwise cohesive powders to create nanoscale roughness. The weight percentage of additive is changed between 0.025% and 1%, which allows for a variation of the interparticle attractive force. The results show how the agglomerate size and bed expansion during fluidization are influenced by the amount of coated powders.