Search

In order to obtain powdered food products of good handling and dissolution properties, single food particles are agglomerated and transformed into a particle network. This paper analyses the importance of well known particle adhesion mechanisms with a focus on aspects of mechanical and rheological material properties, which are strongly linked with the processing conditions, e.g. time and moisture content. The physical state of the material (amorphous or crystalline) and the dominating adhesion principle (Van der Waals forces, liquid bridges, viscoelastic bridges or solid bridges) are defining the process parameters (temperature, moisture, velocity, pressure) necessary to establish a proper connection between the particles and to form stable agglomerates. For dynamic agglomeration processes, e.g. fluid bed agglomeration, it could be shown, that the impact energy dissipation is detrimental to avoid a rebound of impacting particles and thus a sustainable contact creation. The impact energy can be absorbed by the particle body itself, liquid droplets or layers on the surface of impacting particles. Especially the energy dissipation capacity of amorphous material is strongly depending on temperature and moisture content. For agglomeration processes with limited particle motion, e.g. compaction or sintering, energy dissipation effects are of less importance. More important is the creation of contact area by plastic or viscoelastic deformation. Once stable contacts have been created various mechanisms can lead to maturation/stabilization of the particle contact (sintering, concentration or re-crystallization). Different factors affecting the maturation/stability of agglomerates such as the physical state of the material, mechanical and rheological properties and their relation to the selected process conditions are discussed.

The fluid, particle and collision dynamics inside a fluidised bed granulator are described in detail using coupled DEM-CFD simulations. Three different granulator configurations are compared in terms of their particle–particle interactions, using criteria such as the average particle velocity, angular velocity, average particle–particle and particle–wall collision velocity and the collision frequency. From these measures the agglomeration probability, the breakage and growth rate and the agglomerate strength can be assessed. Depending on the required product properties the results highlight advantages of specific granulator geometries. The top-spray granulator is the simplest device and is often used for large-scale granulation processes in the food and fertilizer industries, but wetting intensity and growth rate are relatively low compared to the other configurations investigated in this work. The spouted bed has the most intensive gas–liquid–solid contact and produces compact and dense agglomerates. Both the agglomeration probability and the breakage rate are very high. The Wurster-coater achieves the fastest growth rate for the applied process conditions and is characterised by homogeneous cyclic particle wetting and a low breakage rate. The simulation results are confirmed by lab scale batch agglomeration experiments using maltodextrin DE21, a model substance for amorphous food powders. It can be concluded that the DEM model offers large potential for process intensification by investigating the effect of geometry modifications on the fluid and particle dynamics and on the homogeneity of particle wetting. This is shown exemplarily for fluidised bed agglomeration.