Numerical investigation of screw design influence on screw feeding in a roller compactor

Roller compaction refers to a dry granulation process where fine particulate feed is fed to the counter rotating rolls of a roller compactor to form ribbons which are further milled to produce free flowing agglomerates. For the continuous production of ribbons, there needs to be an adequate supply of powder by the screw to the rolls without any interruptions. In general, screws used in roller compactors are designed to convey powders of all types (cohesive, bulky, compressible, etc.), whereby usage of different screw designs for different powder types may be avoided. However, using such single screw type roller compactors for poor flowing powders may be challenging. On the other hand, the selection of the right screw for a given powder can only be done based on a combination of prior experience and trial-and-error experimentation. Empirical correlations exist to predict the draw down rate of screw feeders depending on their design, however, these correlations assume that there is continuous supply of powder by the screw, which limits its application to free-flowing powders only. To address this, in this study numerical simulations are performed based on discrete element method (DEM) to investigate the impact of screw design on the powder supply to rolls for cohesive and poorly flowing powders. The geometry considered includes a hopper, horizontal feeding screw below the hopper, and two counter-rotating rolls at the end of the screw. Two different screw designs are investigated where the main difference between them is the pitch length. The influence of scraper speed is investigated. Additionally, the influence of material attribute such as cohesion is studied. For both designs, the simulation results calculated include the rate of powder supply by the screw, velocity of particles in the screw etc. The simulation results of powder supply rate are also compared with results obtained based on empirical correlation. Overall, this simulation approach helps in selecting appropriate screw design for the given cohesive powder