Particulate materials are used in a wide variety of industries and applications, such as catalysts, pharmaceuticals and chemicals. In many cases, these materials are cohesive due to the size of the particles or presence of solvents and binders, which can significantly affect their behavior during processing and thus the final product quality. Thermal treatment of these materials is often carried out in rotating drums, kilns or dryers. Therefore, it is of high interest to understand the mixing and heating behavior of particulate materials in rotary drums to optimize the process and reduce energy expenditure. Depending on the material properties and process conditions, different flow regimes are observed in rotating drums. In the slipping regime, particles slip at the wall, and there is very little mixing. In the rolling regime, where the particles flow down the surface and there is good mixing. The roughness of the drum and the cohesiveness of the particles are two key factors that can influence the flow regime and thus mixing, heat transfer, and temperature uniformity in a rotary drum. The roughness of the drum is influenced by the variety of industries that use this type of equipment. In addition, the type of material handled will also depend on the application and therefore cohesion will play an important role. Previous studies have been made on flow of cohesive materials, as well as the implications of surface roughness on heat transfer for non-cohesive materials. However, many questions remain on the effect of both cohesion and surface roughness on the flow regime, heat transfer and mixing of particles.
In this study, heating times were obtained based on Discrete Element Method (DEM) simulations in order to evaluate how cohesion and wall friction impact the flow regime, heating and mixing behavior, which can be used to predict large-scale operations. For these simulations, the Hertz-Mindlin + JKR model was used to account for cohesion. Based on this study, we were able to observe that increasing the cohesion levels resulted in higher heating times. Additionally, the slipping and rolling regime were observed for different surface roughness, which is consistent with previous results for non-cohesive materials. The influence of cohesion on thermal time was maintained in the different regimes, but cohesion impeded mixing in the transition phase from slipping to the rolling regime. Furthermore, the impact of baffles on mixing and heating of materials with low cohesion was studied. Consistent with the results observed for non-cohesive materials, baffles enhanced heat transfer significantly in the slipping regime (low frictions), that were later correlated to the kinetic energy of the bed. Finally, surface plots were created to summarize those findings and examine how they can be used to predict the behavior in practical operations, depending on the roughness of the drum and the level of cohesion of the material. In summary, this study gives insightful information on how both wall friction and cohesion play a role on the flow regime, mixing and heat transfer in a rotating drum.
