Fluidization XVII
A New Model for Triboelectric Charging of Polarizable Particles in DEM
When non-conductive particles are fluidized, the frequent particle-particle and particle-wall contacts tend to generate electric charge transfer (tribocharging) and charge accumulation on the particles and possibly on the unit walls. This, in turn, causes attractive forces on the particles and can eventually lead to the formation of particle layers stuck on the walls, such as in the âsheetingâ phenomenon occurring in fluid-bed polymerization reactors, with subsequent process inefficiency and costly plant shutdowns. The origin of such processes is not fully understood, as for example particles with same sign charges are intuitively expected to repel. However, it is also well known that electrically interacting surfaces induce charge polarization on the opposite surface that, in turn, generate a force contribution that is always attractive. Consequently, in certain conditions this induced polarization contribution can overcome the repulsive force of same sign charged particles, leading to overall attractive forces between particles of polarizable (dielectric) materials. This counterintuitive behavior may explain, at least in part, why similar particles tend to stick to the column walls in industrial fluidized bed units. With its ability to track contacts in detail, CFD-DEM has been recently coupled to tribocharging models to study electric charge build up in fluidized beds. However, current models attribute a net, lumped charge to each particle which is then evolved according to its contacts. So, these models account only for repulsive interactions between particles with same sign charges and generally fail to predict complex phenomena observed in practice. In this contribution, a new model for tribocharging and electrostatic interactions in DEM is presented in which interactions including both net and polarization induced charges are accounted for so that interactions can lead to a much wider variety of behaviors. Applications are illustrated and discussed for binary and multiparticle systems.