Continuum modeling of particle flows in high shear granulation

Licentiate thesis, 2012

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 modeling the flow 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 modeling 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 to correctly describe the transfer rates of mass and momentum in the system. The focus of the present work is on evaluating the previously used continuum model and investigating other possible techniques that have not yet been used in this application. Results show that the continuum model currently being used has a promising parameterization for describing the overall effect on the flow field due to the 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 where the particle volume fraction is high. When solving these regions, the theory used at present suffers a strong spatial resolution dependence on the solution. The present theory is developed for low particle loadings. A critical review was made to investigate available modifications to the modeling framework for high volume fraction granular flows. Although there is no full solution to the problem the investigation shows that promising improvements to the theory are available.