Blade-granule bed stress in a cylindrical high shear granulator: Further characterisation using DEM

Abstract

In high shear granulation, the rotating impeller blades impart forces on the granules which are subsequently transmitted throughout the bed through inter-granule collisions. These affect the granule growth behaviour in several ways — the granules could deform, consolidate and/or break under stress. In this paper, the blade-granule bed normal stress in a cylindrical, vertical axis high shear granulator is characterised using Discrete Element Method (DEM) simulations. The simulations are compared and validated with the measured blade-bed stress and bed surface velocities from our previous studies [1,2]. The blade-bed stresses were previously measured using a novel, custom-built telemetric impeller pressure sensor system. Following experimental validation for impeller speeds up to 350rpm, higher impeller speeds up to 700rpm or Fr=3.88 are simulated to include flow regimes that can occur in most commercial granulators. Parameter study of particle properties, impeller geometries and granulator scales is also carried out. In our previous work, a blade-bed stress equation based on the blade and granule bed inertial forces with a correction factor, B1=K11vrel to account for bed fluidisation with increasing impeller speed, was proposed and validated with a range of dry granules [1]. Here, an improved correction factor, B2=K21Frrel0.52hwD0.3θ−0.5 is introduced to account for different granulator scales, blade widths and blade angles, with a fitted constant K2.