Investigation on minimum fluidization velocity in a modified Geldart's diagram

Fine or ultrafine powders (Geldart Group C) have attracted increasing attention in both industrial and academic scopes due to their very small primary particle size and large specific surface area. These Group C particles are usually regarded as cohesive and non-fluidizable because of strong interparticle forces. With the application of nanoparticle modulation technique, a new type of Group C⁺ particles with reduced cohesiveness has been proved to be able to fluidize. The minimum fluidization velocity (U_mf) is one of the most important parameters associated with a fluidized bed system, and it is generally defined as the superficial gas velocity at which the drag force of the upward moving gas counterbalances the particle weight in the bed. However, the incipient fluidization phenomenon for Group C⁺ particles showed considerable discrepancies with this definition because of their non-negligible cohesive forces. A number of U_mf values available in literatures for Group C⁺, A, and B particles were collected and reviewed in this study to provide a comprehensive scope on the U_mf by particle properties. More importantly, the effects of particle cohesion and gravity on the U_mf were qualitatively analyzed. The following conclusions were highlighted: the U_mf for Group C⁺ particles was controlled by the particle cohesive force, and exponentially increased with the cohesion index (σ*); while the U_mf for Group A and B particles was governed by the gravitational forces, and showed a power function with their gravitational forces, not involved in the interparticle forces. This study endeavors to advance the understanding of the U_mf in the fluidization of each group powders, nonetheless, more studies are still needed to enhance and deepen the current knowledge of the fluidization processes.