Dynamics and scale-up of cohesionless and cohesive granular flows in a bladed mixer
Creators: name: Boonkanokwong, Veerakiet
Publisher: name: No Publisher Supplied
Year: 2018

description: Manufacture of commercial products such as chemicals, cosmetics, foods, and pharmaceutical solid dosage forms often involves particle processing. Compared to fluid processing, our fundamental understanding of solids processing lags behind, and therefore problems such as attrition, segregation, and agglomeration still occur during these processing steps. Moreover, the roles of material properties, equipment configurations, and process parameters on the flow behaviors of particulate systems remain unclear. In this dissertation, cohesionless (dry) and cohesive (wet) granular flows in a bladed mixer were studied using both computational and experimental techniques to obtain better understanding of the flow behaviors and mixing performance. First, the effect of the number of impeller blades used in an agitated mixer was examined via discrete element method (DEM) numerical simulations varying from one to four blades. It was found that granular temperature, particle diffusivity, and mixing rate in the 2- and 3-bladed mixers were larger than those in the 1- and 4-bladed cases. This resulted from a larger magnitude of the tangential component of the blade-particle contact forces and a great extent of dilation of the granular bed in the 2- and 3-bladed mixers. Next, scale-up of cohesionless and cohesive granular systems to a larger, industrially relevant scale was accomplished in the DEM simulations. Scaling-up systems composed of cohesionless monodisperse spherical particles in 2- and 4-bladed mixers when increasing the mixer diameter to particle diameter (D/d) ratio from 63 to 90 revealed that changing the system size had insignificant impact on the granular flow behaviors and mixing kinetics. Scale-up of non-cohesive granular systems based on the number of impeller blades (2 and 4 blades) used in the agitated mixer could be scaled by the diameter of the mixer and the rotational speed of the impeller blades within the range from D/d = 63 to 90. Although there was an impact of cohesion that caused some differences between system sizes, it was found that wet granular flows in bladed mixers could be scaled by the diameter of the mixer and the tip speed of the impeller blades within the range of D/d ratio from 75 to 100. Additionally, experimental measurements of the agitation torque exerted on a particle bed and the power draw for the motor driving the impeller blades in a mixing process were conducted to investigate the impact of particle properties and blade geometry as a function of the blade rotation rate. It was found that the torque exerted on a granular bed and the power consumption were a strong function of the impeller blade configuration, the position of the blades in a deep granular bed, the fill height of the glass beads, and the size and friction coefficient of the particles. It was observed that the time-averaged torque and power consumption for different particle sizes qualitatively scaled with particle diameter. A scale-up relationship for a deep granular bed was developed: the time-averaged torque and average adjusted power consumption scaled with the square of the material fill height.