Segregation of powders during gravity-driven flow through ducts

Abstract

During certain pharmaceutical operations, such as encapsulation and tableting, problems arise due to the handling, transport, and storage of powders. Considerable effort is often expended to blend or mix several different species of powders (of various sizes, shapes and densities), only to have the mixture de-mix or segregate when transported. A well-mixed system may segregate during transport to the tablet press and this may lead to pills that do not fall within Food and Drug Administration specifications. At present there is a lack of understanding of the relationship between the collective motion of the particles, and the particle properties, boundary conditions and the history of the system. The goal of this work is investigate the physical mechanisms of segregation during gravity-driven flow of powders. Sedimentation or dropping experiments have been carried out in vertical ducts (or pipes) for different particle systems and various process conditions. Glass beads and pharmaceutical excipients have been used as the initial test materials. The glass beads of different size were dyed different colors to allow one to observe the flow and segregation. The experiment involved dropping a fixed amount of powder in a pipe with a sealed bottom end. Once the powder had come to rest at the bottom of the pipe, it was analyzed for segregation as a function of height. The degree of segregation was quantified using sieve analysis of the material. During a vertical drop, the air drag force influences the particles according to their size, density and shape, causing different settling velocities, which ultimately leads to a distribution of settling velocities in the mixture. The results of these experiments are being used to develop an understanding of how the different magnitudes of the air drag and gravity forces can lead to segregation of powders during vertical drops.