A discrete element study of moisture dependent particle-particle interaction during granulation in a spout fluidized bed

Spout fluidized beds find widespread application in the process industry in granulation processes, in which efficient contacting between large particles, droplets and gas is of paramount importance. However, detailed understanding of the complex behavior of these systems is lacking. In this paper we study the effect of the inter-particle interaction on the bed dynamics, by investigating the bed height, pressure drop and vertical particle velocity as function of the variable restitution coefficient, which varies in time and space as function of the moisture content due to the particle-droplet interaction. This is done computationally, by using the extended discrete element model (DEM) which describes the dynamics of the continuous gas-phase and the discrete particles and droplets. The objective of this work is to gain insight in the effect of the variable restitution coefficient on the flow behavior of spout fluidized beds at different flow regimes using DEM. The three flow regimes comprise the intermediate / spout-fluidization regime (B1), spouting-withaeration regime (B2) and the jet-in-fluidized-bed regime (B3). The simulation results with variable restitution coefficients were compared to simulations with constant restitution coefficients reported by Van Buijtenen et al. (2007). The trend of increasing average bed height with decreasing restitution coefficient is also valid for the variable restitution coefficient. However, the average bed height is larger for the variable restitution coefficient for all flow regimes. This is also observed for the pressure drop, showing a lower value compared to the constant restitution coefficient. These results suggest a significant influence of the variable restitution coefficient on the bed dynamics, since the variable restitution coefficient provides regions in the bed with particles having different collision properties. The presence of these distinctive regions causes different behavior of the bed dynamics, which is also shown in the time-averaged vertical particle velocity. The velocity in the spout region for the variable restitution coefficient is lower than for the constant restitution coefficient for case B1 and B2. However, for case B3 these regions are less pronounced, due to the larger mixing capacity caused by the larger interaction between the spout channel and the bubbles in the annulus region. As a result, the particle velocity for the variable restitution coefficient complies with the particle velocity for the constant restitution coefficient. These findings reveal the significant impact of the influence of the variable restitution coefficient on the dynamics of the bed, which is clearly different compared to the constant restitution coefficient. This is due to the presence of distinctive regions with different restitution coefficient, which can only be simulated when the dependency of the moisture content on the restitution coefficient is accounted for. Currently, only the wetting process on the particles has been simulated without evaporation and crystallization of the deposited granulate solution, which are phenomena that are very important in the granulation process. It is therefore desirable to further improve the discrete element model, by solving mass and energy balances for the particles and the gas phase