Continuum modelling of particle flows in high shear granulation

Doctoral thesis, 2015

High shear granulation is an important process in the pharmaceutical industry. The aim of the process is to produce granules with specific properties, like size and hardness, from powder mixtures. The properties of the granules are determined by the flow field in the mixer. The most common approach taken to modelling the flow in a mixer includes tracking the forces on each individual particle and resolving each occurring collision. This gives detailed information, but the computational cost restricts this use to small-scale equipment. Continuum modelling of particle flows means that averages are made to form a continuous flow rather than tracking individual entities. The problem that arises in this procedure is correctly describing the transfer rates of mass and momentum in the system. The focus of the research in this thesis is on evaluating previously used continuum models, and finding and developing new approaches. The connection between flow field information and the evolution of particle properties is also studied through the development of a compartment model. Results show that the continuum model currently being used has a promising parameterization for describing the overall effect on a flow field caused by particle property changes that occur during granulation. The model is; however, not capable of adequately resolving the flow field in the important regions close to the walls and the impeller of the vessel where the particle volume fraction is high. A rheology-like model is used to improve the dense granular flow regions, while the theory for the more dilute parts is improved via kinetic theory models modified for inelasticity and improved for its validity in the transition region to dense flows.