(743d) Understanding Particle Scale Effect of Mixture Segregation

One of the challenges in pharmaceutical, food, and chemical product development is the ability to relate particle scale properties to behaviors in complex processes. A key issue in processing powders in these industries is mixture segregation, which is responsible for nearly one third of all processing issues with these types of powders. The trouble with solving segregation effects by product design is that the effects are driven by various mechanisms that can act at the same time. Each mechanism, or cause, of segregation is due to a characteristic of the particles that make up a mixture. For example, sifting segregation is induced by differences in particle size. Angle of repose segregation is induced by differences in friction between particles. Rebound segregation is due to differences in coefficients of restitution. Air entrainment segregation is facilitated by particle size differences and true particle densities. Experiments can be constructed to isolate each mechanism, but it is difficult to combine the effects of these individual mechanisms to predict overall segregation because one must determine how much of each mechanism is active in the mixture. This issue is the focus of this paper. Key particle scale parameters are selected for each mechanism of segregation, and these key parameters were measured for each mixture. The segregation intensity of multiple mixtures was then measured and this was combined through proposed segregation models to relate segregation to particle scale properties. Segregation flux velocity functions were determined and then used to estimate the expected variance each mechanism has on the total segregation in the system. This allowed separation of each mechanism on a component-by-component basis, so that the formulator knows what to change about the various components in the system so as to mitigate the segregation of the mixture.