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Food powders often exhibit hydrophobic surface areas. To understand the effect of corresponding wettability variations on particle floating, which critically influences powder reconstitution, we studied floating of glass beads after creating surface heterogeneity by spraying cocoa butter. Force-position curves were recorded to characterise contact line pinning.\ud \ud \ud \ud Floating of particles with surface heterogeneities comprised a dynamic phase, which to our knowledge has not been reported previously. Measured forces varied due to contact line pinning/depinning. The results indicate that the dynamic floating phase is caused by gradual depinning. Steady-state floating is reached once pinning is sufficiently pronounced.\ud \ud \ud \ud We also demonstrate that surface heterogeneities complicate theoretical predictions of particle floating. The use of sessile drop contact angles for modelling was identified as an important source of error, as weak pinning points can affect contact angles but are oftentimes overcome during floating. Moreover, local pinning effects are not necessarily represented by a sessile drop.

In this paper, a hybrid computational technique for the Direct Numerical Simulation of hydrodynamics and (convective) mixing in complex two-phase systems is presented. The hybrid Immersed Boundary and Volume of Fluid method is based upon a single-fluid approach in combination with Henry’s law for the concentration jump. The convective species fluxes are evaluated using PLIC-based geometrical advection, which is consistent with the advection of the local fluid volume fractions. Complex geometries can be accounted for and are treated with the Immersed Boundary method. Following the verification for single phase and two-phase systems, the model was used to investigate the influence of the container geometry on the mixing of a tracer due to tilting of the container. For both cylindrical and rectangular containers, an increasing tilting angle enhances the convective motion and consequently the mixing efficiency. The cylindrical geometry was found to possess a better mixing efficiency than the rectangular geometry. Keywords: Direct numerical simulation, Volume of fluid method, Immersed boundary method, Mass transfer, Henry’s law, Contact line dynamics

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.

Coupled DEM–CFD simulations have been performed to study the fluid and particle dynamics in a fluidized bed spray granulator on the scale of individual particles. The aim of this study is to develop a model of a fluidized bed granulator by combining the gas and particle dynamics with a simple model of particle wetting. Based on material tests, the collision behavior of γ-Al 2 O 3 particles was characterized and incorporated into the contact model. For two different granulator configurations, a bubbling fluidized bed with top-spray injection and a Wurster-coater, wetting of the particle surface is estimated based on the residence time distribution inside a biconical spray zone. The effect of the geometry of the apparatus on the homogeneity of wetting is analyzed in order to understand the performance and specificity of different granulator configurations. Compared to the top-spray granulator, the Wurster-coater has a narrow residence time distribution of the particles in the spray zone, which corresponds to more homogeneous particle wetting. For the Wurster-coater, the effect of process parameters like air flow rate and geometry details like the position of the Wurster tube is studied using the model. A stable circulating fluidization regime is established by keeping the jet velocity above 100 m/s and the gap distance between bottom plate and Wurster tube below 15 times the particle diameter Based on a description of the physical material properties, an effective modeling tool for design and scale-up of a fluidized bed spray granulator is obtained. Modeling the interactions of the individual particles, a step forward is taken towards a description of the micro-processes in granulation, which also considers the material properties.