Monte-Carlo-Simulation of fluidized bed spray agglomeration

The processing of particulate materials by agglomeration is a widespread technology that facilitates the handling of intermediate products and allows for the fulfillment of end-user product specifications. Fluidized bed spray processes take advantage of the simultaneous agglomeration and drying in a single vessel by atomizing binder solutions into a bed of solid particles fluidized by a stream of heated gas. The particles experience coalescence by formation of liquid bridges that eventually solidify, resulting in stable blueberry-like aggregates. The present work focuses on the study of the formation of such agglomerates from the microscopic point of view by considering the single interactions which take place between the particles and the sprayed droplets within the fluidized bed. Those interactions which are considered to be most important are modeled individually and the obtained micro-models are introduced into a main model that is solved by a stochastic Monte Carlo method. The method follows an event-driven scheme with periodical regulation of the number of particles in the simulation box. This methodology enables to simulate of the agglomeration processes in a more straightforward way than the traditional population balances approach and allows for the simultaneous consideration of multiple physical mechanisms. The sensitivity of the model to changes of the most important characteristic quantities such as the interparticle collision time, the deposited droplet drying time or the droplet addition time is analyzed. The accuracy of the method in respect to the number of initial primary particles in the simulation box is evaluated, aiming at a compromise between the solution stability and the speed of the simulations. The main process parameters affecting the agglomeration behavior such as binder addition rate, superficial fluidization velocity, binder concentration, sprayed droplet size, equilibrium contact angle or gas inlet temperature are analyzed. A relatively simple agglomerate structure is proposed, based on coordination numbers and agglomerate porosity. This enables the comparison of simulation results with labscale agglomeration trials. The features of the model make possible the study of the influence of some individual mechanisms that are difficult to monitor experimentally in the fluidized bed, such as the droplet imbibition into the pores of the primary particles, the breakage mechanism or the solid crust formation on the deposited droplets. The effect of the loss of droplets during their flight from the injection point at the tip of the nozzle to the powder bed is additionally analyzed by a model that accounts for the drying of the sprayed droplet and the drag forces acting on it during the fall.