Experimental and computational study of a high speed pin mixer via PEPT, visualization and CFD

Abstract

Continuous high speed pin mixers have been successfully implemented industrially to mix high fractions of powders into highly viscous non-Newtonian liquids. However, they have not been described in the literature yet. This work presents a multi-method approach, consisting of conventional torque and throughput measurements to calculate average residence times and dimensionless Reynolds/power numbers for suspension mixing, Positron Emission Particle Tracking (PEPT) to gain information on particle trajectories and velocities in the investigated opaque model system, high speed film visualization and CFD simulations. By the combination of these methods, valuable information on the influence of variable process parameters, particularly on the influence of pin configuration, pin shape and inclination of semi-cylindrical pins on mixing mechanisms and efficiency, could be gathered for a broad range of viscosities and powder fractions in Newtonian and non-Newtonian fluids. Flow patterns such as a “Split-and-Recombine” convection pattern at the pins, axial mixing mechanisms and beneficial process conditions to reduce particle sedimentation caused by centrifugal forces in the mixer were retrieved from the results. The PEPT methodology was successfully implemented for a high-shear continuous mixing process of suspensions for the first time. As the combined methodology shows reasonable and reliable results that were also applicable at industrial scale, the work provides sound basis for further research development of measurement techniques for other mixing processes, especially in opaque systems.