Fluidized bed spray granulation is an important and widely spread process for the production of granular solids. While particles are fluidized by heated air, a solid-containing liquid is sprayed onto them. The liquid spreads on the particle surface and evaporates due to the hot air, forming a solid shell around the core. Due to the good heat, mass and momentum transfer caused by the high contact area between gas and solid phase, the fluidized bed process offers the possibility to handle a great variety of products. This allows the production of particles with highly distinct properties. By variation of the process parameters the granule properties can be adjusted specifically to the consumers' needs. Moreover, the material of the primary particles and the coating liquid as well as the apparatus design, including for instance the nozzle configuration, influence the granulation process and therefore the granule properties.
Another mechanism that might take place in a fluidized bed with liquid injection is agglomeration due to the formation and solidification of liquid bridges between particles. Depending on the application and the desired particle properties, granulation or agglomeration can be preferred. The agglomeration mechanism allows fast particle growth and the produced agglomerates usually have a good flowability and rehydration behaviour. For example, a fast rehydration is desired for instant food powders. However, agglomerates tend to have a low particle strength and break easily. Granules and coated particles on the other hand can be designed to have a compact structure, so they are able to withstand high mechanical stress. Furthermore, a uniform coating with a defined microstructure allows the controlled release of active substances, which is important for pharmaceuticals and fertilizers.
The aim of this project is to investigate the influence of the process conditions, determined by process and ambient variables, the design of the apparatus and the material properties of the core particles and coating liquid on the product properties. These properties include among others the particle size and strength as well as the surface morphology. The surface morphology is determined by the roughness and the microstructure of the coating, which includes the shell porosity and thickness. As shown in Figure 2, the particles develop a significantly different surface structure if the granulation process is carried out under varied process conditions.
The granulation process is carried out on laboratory scale in the ProCell5 system (Glatt, Germany), which can be operated as fluidized or spouted bed. During the coating of particles, the spreading of the droplets on the particle surface strongly depends on the wettability, which can be evaluated by the contact angle between a solid surface and a liquid droplet. To measure this contact angle, the sessile drop method can be used.
To analyze the product particles regarding their mechanical properties, the particle strength is measured in a single particle compression tester (TextureAnalyzer XT) and a pneumatic impact gun. Furthermore, imaging techniques allow the investigation of the granule’s microstructure, including surface roughness and shell porosity. For the characterization of the particle surface, a laser scanning microscope and a scanning electron microscope (SEM) can be used. The shell porosity of coated particles can be measured viaX-ray micro-tomography in a µCT device.
In addition to the experimental investigation of the influence of the process conditions on the product properties, simulations are performed. The coupling of Computational Fluid Dynamics (CFD) and the Discrete Element Method (DEM) allows the description of flow and transport phenomena during the fluidized bed spray granulation. By simulating the fluid and particle dynamics in the apparatus, the process can be optimized while the experimental effort is reduced.