Spouted beds have shown great applicability for particles which are difficult to handle in classical fluidized beds, as e.g. very small or big or cohesive ones. This research project is focused on the experimental and numerical investigation of three-dimensional prismatic spouted beds in laboratory and pilot plant scale. The laboratory apparatus ProCell 5 (Glatt, Germany) with different particles is used for coating purposes and the droplet distribution is investigated numerically in order to predict the change of process conditions. CFD-DEM simulations allow the detailed flow description on the scale of single particles as for example the calculation of circulation frequencies, residence times and surface coverage with coating agent. In both experiments and simulations the pressure drop signal is recorded and analyzed by means of Fourier transformation in order to validate the simulations and to quantify the spouting stability.
The investigated apparatus ProCell 5 (Glatt, Germany) with the geometry of process chamber and the two-fluid nozzle in bottom-spray configuration is shown in Fig. 1.
Besides the experimental part of the project, the spouted bed process is simulated. Therefore, a computational fluid dynamics (CFD) and a discrete element method (DEM) approach are used and combined. For the description of the fluid phase the CFD tool OpenFOAM® is used. The determination of the particle forces and velocities is carried out with LIGGGHTS®. With regard to a feasible simulation time, the simulations are parallelized and a coarse-grain approach is used in order to handle a high amount of particles. Regarding the coating of the granules, a spray zone is implemented and the residence time distribution of the particles in the zone is determined. The liquid injection is performed in a post-processing step after the resource-intensive simulations as the liquid loading during the investigated coating process is very low. A sketch of droplet injection in post-processing step is shown in Fig. 2.
The spouting stability could be increased by inserting two parallel draft plates into the process chamber. By running several simulations with different configurations, two parallel plates (60 mm x 10 mm x 20 mm) with a horizontal distance of 45 mm and a distance in vertical direction from middle profile of 10 mm were found to improve best the spouting stability In Fig. 3 the pressure drop fluctuations and corresponding power plots for the original and optimized apparatus are shown. As can be seen the fluctuations are more homogeneous and the main frequency is increased in the optimized chamber. For understanding of the stabilizing effect, circulation frequencies are calculated for different process conditions. Fig. 4 shows that the circulation frequency distribution is broader and more shifted to higher frequencies with increasing gas flow rate. A broader distribution indicates a more inhomogeneous flow pattern. In case of draft plates, the circulation frequency is decreased and the distribution is slimmer compared to the original apparatus.
The change in flow pattern influences the droplet deposition and therefore the coating layer distribution. In order to validate the coating quality in situ during experiments, a digital image analysis algorithm was implemented, which detects the color change of particles coated with a suspension with dye.
Besides the investigation of the laboratory apparatus, the continuously operated ProCell 25 is investigated in this research project. A novel method for residence time measurements has been developed in order to compare the influence of different process conditions on the dispersion in the apparatus.