CFD-DEM Simulation of the Fluidization Behavior of Particles during Coating in a New Developed Spouted Bed Apparatus

For various products, e.g. in the pharmaceutical, chemical and food industry the coating of solid particles with different substances is an important production step. For this purpose, spouted beds or Wurster coaters are often used (Grace und Epstein 2011; Heinrich et al. 2015). To control and optimize these processes, it is essential to know and describe the particle kinematics and dynamics as well as the growth of the coating layer on the particles. An essential tool to obtain these data are numerical simulations of the multiphase flow with the computational fluid dynamics (CFD) coupled with the discrete element method (DEM). With CFD, the flow field of the gas in the process apparatus can be determined by the Eulerian-Lagrangian approach, which treats the fluid phase as a continuum. In DEM, the interactions are calculated for each single particle based on models describing physical properties of particles and their mechanical behavior during slow, fast and repeated stressing (Antonyuk et al. 2015; Breuninger et al. 2018; Salikov et al. 2015).

In this work, we studied the particle dynamics during a coating process in a self-developed spouted bed apparatus, consisting of a conical and a cylindrical part, which is operating at negative pressure. The apparatus consists of an annular gap through which the gas enters with a high velocity. The base plate can be adjusted by a fine threat in height and thus the gap height can be varied. With the help of a two-fluid nozzle the liquid was injected into the apparatus. As coating substance, water with coloring agent was used. The coating process was calculated during 3D CFD-DEM simulation in order to obtain additional information about the fluidization behavior of the particles and the influence of the injected liquid on the particles.

For the calculation of the particles contacts, a novel contact elastic-plastic model was implemented. In the model, the restitution coefficient depends on the load level and number of collisions due to contact hardening. The drying rate of the water droplets for the process parameters was determined by experimental investigations. For that, droplets were applied on a substrate with a known contact angle and the time to complete evaporation was measured under different humidities, temperatures and gas velocities. The size distribution of the droplets formed by the used nozzle were obtained by static light scattering. Furthermore, the wetting of the particles with the injected droplets was implemented in the simulation and determined. The additional attractive forces due to the existence of liquid layers after droplet deposition were taken into account based on the model of Tsuazawa et al. (Tsunazawa et al. 2016).

The results were used to determine the boundary conditions under which a successful fluidization and coating of particles in the self-developed spouted bed can be realized. With the help of the simulations the particle movement in this spouted bed was analyzed. Another focus was on the behavior of the particles during the coating process as well as the different contact scenarios (particle-particle or particle-wall contact). The numerical results were compared with the conducted experiments.

References

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Heinrich, Stefan; Dosta, Maksym; Antonyuk, Sergiy (2015): Multiscale Analysis of a Coating Process in a Wurster Fluidized Bed Apparatus. In: Mesoscale Modeling in Chemical Engineering Part I, Bd. 46: Elsevier (Advances in Chemical Engineering), S. 83–135.

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