Modelling of solids motion in high shear mixer granulator

摘要

Granulation is one of the important unit operations in the chemical, pharmaceutical, detergent, and nuclear industries for enhancing the flowability, appearance, strength and other physical and mechanical properties of particulate solids. As the granule structure depends on the prevailing stress and strain status given other conditions, knowledge of the macroscopic flow field is crucial. The objective of this study is to model the solids motion in a conical frustum-shaped vertical high shear mixer granulator with Computational Fluid Dynamics (CFD). The simulation is based on the continuum model of dense-gas kinetic theory (Gidaspow et al., 1992) with consideration of inter-particle friction force at dense condition (Schaeffer, 1987). The solids motion obtained from the simulation is then validated against the experimental results of Positron Emission Particle Tracking (PEPT) technique (Ng et al., 2007). The mixer granulator has a vertical shaft, to which four sets of impellers are attached at different elevations. The shaft is operated at 5.8 Hz, which corresponds to the top impeller tip speed of 4.1 m/s. The PEPT results show that particles circulated in both the horizontal and vertical directions. The period of horizontal circulation is short and is a fraction of second, whereas that of the vertical circulation takes seconds. There is a dominant solids motion in the tangential direction while less motion is seen in the axial and radial directions. In general, the Eulerian based continuum model captures the main features of solids motion in high shear mixer granulator. Qualitative similarities are seen in the dominating solids motions (the tangential velocity) of both experiments and simulations. However, over-predictions are seen in the velocities magnitudes which indicate the necessity to improve the constitutive relations of dense granular materials as a continuum.