(303d) Experiments and Simulations of Cohesionless Particles with Varying Roughness in a Bladed Mixer

Many industrial processes involving particulate systems employ the use of cylindrical mixers mechanically agitated by an impeller. Bladed mixers are used in industrial pharmaceutical processes, such as wet granulation, agitated drying and tablet press operations. Although common in industry, many bladed mixer processes are not fully understood. The lack of process understanding can lead to operational problems and can cause complications during scale-up. At the root of these issues is the lack of information on flow kinematics. Particle velocities affect the rates of mixing, heat and mass transfer and, in the case of wet granulation, the rates of agglomerate formation. Improved knowledge on how particle motion develops in bladed mixers could provide the basis for rational process design and could assist in the development of first-principle based models.

The kinematics of cohesionless particle flows in a bladed mixer was studied experimentally using Particle Image Velocimetry and computationally using the discrete element method. The discrete element simulations were able to reproduce the surface velocities, granular temperature profiles and mixing kinetics observed experimentally. Procedures for roughening of glass bead surfaces via coating were developed and the effect of surface roughness was studied experimentally and computationally. Increasing particle surface roughness led to the development of less uniform flows inside the mixer and to increased dilation of the particle bed. Systems composed of rougher particles experienced increased radial and vertical velocities as well as higher particle diffusivities. Cylinder wall roughness was also shown to significantly influence particle velocities in bladed mixers. Rough cylinder walls led to more pronounced particle velocity fluctuations near the top surface and to an increase in granular temperature. The effect of increasing blade speed was also studied. Two distinct flow regimes were observed as blade speed was increased. At low rotational speeds, the flow occurs in the quasi-static regime where particle surface velocities are linearly proportional to the tip speed of the blades. In this regime, the rotational speed of the blades provides the time scale for momentum transfer and for the mixing process. At higher rotational speeds, the intermediate regime is encountered where particle surface velocities are no longer proportional to the tip speed of the blades. This regime is characterized by enhanced radial and vertical particle velocities as well as faster mixing kinetics.