On the numerical implementation of the Van der Waals force in Soft-Sphere Discrete Element Models for gas-solid fluidization

Attractive inter-particle forces, such as Van Der Waals forces (VDW), are known to strongly influence the fluidization behavior of particles classified as Geldart C-type, and recent literature suggests that VDW forces may also become important for Geldart A and B type particles at elevated temperatures. Discrete Element Models (DEM) using a soft-sphere collision model have been developed to investigate the effects of particle–particle and particle-gas interactions on the fluidization behavior in great detail. However, it is shown that inconsistent results may be obtained with soft-sphere based DEM, depending on the numerical implementation of the VDW forces, particularly related to the calculation of the VDW forces for overlapping particles during particle collisions, as is usually done in the literature. In this work we present a new numerical approach that allows the calculation of the VDW force for particles that are in contact in a physically sound way, corresponding with theoretical arguments given by Rietema et al. (1993). We also provide a generic protocol to determine the relevant parameters based on our new approach, which will allow future work to simulate gas-solid fluidized beds with attractive inter-particle forces in a soft-sphere DEM framework.