Characterizing solids mixing in DEM simulations

In the production and processing of granular matter, solids mixing plays an important role. Granular materials such as sand, polymeric particles and fertilizers are processed in different apparatus such as fluidized beds, rotary kilns and spouted beds. In the operation of these apparatus mixing often plays an important role, as it helps to prevent formation of hot-spots, off-spec products and undesired agglomerates. DEM can be used to simulate these granular systems and provide insight in mixing phenomena. Several methods to analyse and characterize mixing on basis of DEM data have been proposed in the past, but there is no general consensus on what method to use. In this paper we discuss various methods that are able to give quantitative information on the solids mixing state in granular systems based on DEM simulations. We apply the different methods to full 3D DEM simulations of a fluidized bed at different operating pressures. The following analysis methods will be investigated: average height method, Lacey index, nearest neighbours method, partner distance method and the sphere radius method. It is found that some of these methods are grid dependent, are not reproducible, are sensitive to macroscopic flow patters and/or are only able to calculate overall mixing indices, rather than indices for each direction. We compare some methods described in literature and in addition propose two new methods, which do not suffer from the disadvantages mentioned above. We applied each of these aforementioned methods to full 3D discrete particle simulations (DPM) with 280·103 particles and we performed simulations for seven different operating pressures. We found that, mixing improves with operating pressure caused by increased porosity and the increased granular temperature of the particulate phase.