(273g) Predicting Minimum Fluidization and Bubbling Velocities for Geldart Type A Particles

                The fluidization behavior of Geldart A particles is investigated using soft-sphere discrete particle simulations.  The inclusion of cohesive forces permits ready distinction between the minimum fluidization velocity (U_mf) and the minimum bubbling velocity (U_mb) that are typically observed with these type of particles. The simulation predictions are consistent with experimental observations detailed in fluidization textbooks (e.g. Kunii and Levenspiel, Fluidization Engineering). Specifically, the predictions demonstrate a spike in pressure drop near U_mf as well as the formation of small voids that do not coalesce into fast bubbles and greatly increase the bed expansion until reaching U_mb.  The results also clearly show the onset of bubbling coincides with the breaking of cohesive links between particles once the bed has expanded considerably.  This occurs at a velocity U_mb that can be two to three times larger than U_mf. Quantitatively, the predicted values of these design-critical velocities depend on the size and geometry of the system as well as other considerations, such as polydispersity.

             While cohesive van der Waals forces are easy to incorporate in a discrete particle model, the present simulations predict a very sticky bed which cannot be fluidized when realistic values of the Hamaker constant are employed.  This issue was resolved by considering the effects of asperities on the particle’s surface, which tend to diminish the strength of cohesive forces. Finally, an explanation is proposed to address the observed discrepancies between the results of this study and those of previous publications studying systems under similar conditions.