(30c) Experimental and Numerical Investigation of a Three-Dimensional Prismatic Spouted Bed with Liquid Injection

Particles which are difficult to handle in fluidized beds due to their broad size distribution or surface properties can be fluidized in spouted beds. Particles are entrained and accelerated in the so-called spout zone by the fluidization gas, which enters the process chamber via two adjustable slots. After passing the spout region, the particles fall along the apparatus’ walls back into the annulus zone. Spouted beds are well applicable for processes with liquid injection, e.g. for spray granulation or coating, due to the more structured flow pattern and the resulting improved heat, mass and momentum transfer compared to classical bubbling fluidized beds. In order to avoid the release of the coated core material, the layer has to be dense and uniform without unnecessary material waste.

Experiments were carried out in a ProCell 5 granulator of the company Glatt (Germany). γ-Al₂O₃ particles were used as core material. A transparent replica of the apparatus with a high speed differential pressure detector (PD-23/8666.1, Keller, Germany) was installed in order to measure the pressure drop fluctuations and to investigate the spout pattern in the three dimensions. The pressure drop signal was analyzed with Fourier transform. For the determination of main frequencies of the system, the power spectrum distribution was used. A coating suspension with blue dye was injected via a two-fluid nozzle into the ProCell 5 apparatus. A high speed camera was positioned in front of the observation window to record the coating process. The progress of liquid injection and thereby the coating quality was traced with digital image analysis. Simulations were performed with a CFD-DEM approach by coupling OpenFOAM® and LIGGGHTS® [1]. Injection of droplets and contact detection with particles was conducted in a post-processing step as the liquid loading in the system during the investigated coating process is low. In order to simulate a high number of particles, a coarse-graining approach [2] was used, in which several particles are represented by parcels.

By running several experiments and simulations with varying gas volume flow rates, a regime map for the given apparatus and the used particles was obtained by Fourier transform of the pressure drop signal indicating the regimes of stable and instable spouting. Additionally, experiments with polydisperse systems (γ-Al₂O₃ particles with 0.656 mm, 1.8 mm, 3.0 mm) were performed. It was shown that the amount of the fraction with small size in the mixture has an influence on the stability range. Salikov et al. [3] could increase the spouting stability of a pseudo two-dimensional prismatic spouted bed 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 of the regarded three-dimensional ProCell 5 apparatus [4].

For investigation of the liquid injection, the spray zone above the nozzle was calibrated for the simulations by measuring the spray pattern of the nozzle at different heights above it. By means of a self-written post-processing tool, the trajectories of particles were analyzed and the residence times in the spray zone were determined. The time dependent surface coverage distribution for different process conditions was recorded and compared with the experimental results obtained by digital image analysis. The circulation frequency was found to be reduced in case of the chamber with inserted draft plates and as a consequence thereof the coating progress is slower but more homogeneous than in the original apparatus. By means of the experimentally obtained images, the duration and the minimum amount of coating suspension for getting homogeneous and dense coating layers was determined for different process conditions.

References

[1] C. Goniva, C. Kloss, N.G. Deen, J.A.M. Kuipers, S. Pirker, Influence of rolling friction on single spout fluidized bed simulation, Particuology 10 (2012) 582–591.

[2] C. Bierwisch, T. Kraft, H. Riedel, M. Moseler, Three-dimensional discrete element models for the granular statics and dynamics of powders in cavity filling, Journal of the Mechanics and Physics of Solids 57 (2009) 10–31.

[3] V. Salikov, S. Heinrich, S. Antonyuk, V.S. Sutkar, N.G. Deen, J.A.M. Kuipers, Investigations on the spouting stability in a prismatic spouted bed and apparatus optimization, Advanced Powder Technology 26 (2015) 718–733.

[4] S. Pietsch, S. Heinrich, K. Karpinski, M. Müller, M. Schönherr, F.K. Jäger, CFD-DEM modeling of a three-dimensional prismatic spouted bed, Powder Technology 316 (2017) 245–255.