Discrete element method simulation of a conical screen mill: A continuous dry coating device

Abstract

A conical screen mill or a comil is a frequently used device for powder delumping. It has been recently shown as a promising continuously operating device for powder dry coating. In order to examine the influence of the operating parameters of comil, which has rather complex geometry, discrete element method (DEM) modeling was carried out and the residence time distribution (RTD) and mean residence time (MRT) were computed. The simulation results show that MRT increases with increasing impeller speeds, an outcome that is counter-intuitive. Limited experimental investigation showed a qualitative agreement with corresponding simulation results. Simulation results also showed that MRT increases with decreasing feed rates, decreasing open areas, and decreasing screen hole sizes. In addition to computing the RTD and MRT, the relationship between average collision number (ACN), number of particles inside the transition zone (region above the conical part), and MRT was examined indicating that higher impeller speed leads to longer MRT and larger ACNs. In contrast, the equilibrium number of particles inside the comil decreases with increasing feed rate, open area, and screen hole size, therefore, leading to a decrease in both MRT and ACNs. Such analysis shows that increasing impeller speed forces more particles to stay in the transition zone, leading to increased MRT; hence increased MRT may not always lead to improved dry coating. Overall, the results demonstrate the relationship between MRT and various operating parameters, while shedding light on the impact of ACNs on MRT, hence providing better understanding of the complex dynamic process and the selection of experimental conditions.