IMPACT BEHAVIOUR OF PARTICLES WITH LIQUID FILMS: ENERGY DISSIPATION AND STICKING CRITERIA

The normal impact of spherical particles (elastic glass, elastic-plastic Al2O3) and non-spherical dominantly plastic agglomerates of amorphous maltodextrin on a wall with a liquid layer was studied. The objective was to investigate the effects of thickness (0.1-1 mm) and viscosity of the liquid layer (1-250 mPas) as well as of the impact velocity (1-3.0 m/s) of the granule on the restitution coefficient. The restitution coefficient was measured by using a free-fall apparatus. In the presence of a liquid layer, the higher the viscosity and thickness of the liquid layer the more the energy dissipated during impact and the smaller the critical thickness needed for the sticking of the particle. The measured restitution coefficients were compared with experiments performed without liquid layer. In contrast to the dry restitution coefficient, due to viscous losses at lower impact velocity, higher energy dissipation was obtained. A rational explanation for the effects obtained was given by establishing and numerically solving the force and energy balances for particles impacting on a liquid layer. The model takes into account forces acting on the particle, which includes viscous forces, surface tension and capillary forces, contact forces due to deformation of the wall, drag forces, buoyancy and gravity. A good agreement between simulations and experiments has been achieved. The results are essential for estimating the adhesion probability during agglomeration processes and crusting on equipment surfaces

DEM Study of Fluidized Bed Dynamics During Particle Coating in a Spouted Bed Apparatus

A novel process for coating of spherical aerogel particles in a spouted bed is suggested. Using Discrete Element Method the influence of the density and restitution coefficient of experimentally coated aerogels on the fluidized bed dynamics in the developed apparatus was described

DEM-CFD MODELLING OF A FLUIDIZED BED SPRAY GRANULATOR

Coupled DEM-CFD simulations have been performed to study the hydrodynamics of a Wurster granulator on the scale of individual particles. Based on extensive material tests, the collision behaviour of dry Gamma -Al2O3 particles is identified and incorporated into the model. The effect of process parameters like air flow rate and geometry details like the Wurster position is studied. Based on a physical description of the material properties, an effective tool for design and scale-up of a Wurster granulator is obtained

DEM-CFD Modeling of a Bubbling Fluidized Bed and a Wurster Coater

Coupled DEM-CFD simulations were performed to study the fluid and particle dynamics of a fluidized bed granulator on the micro-scale. In a first study, wetting of the particles is estimated based on the residence time distribution inside a conical 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. For a small simulation system, heat and mass transfer laws were resolved to calculate the moisture content of the individual particles An effective modelling tool for design of a fluidized bed spray granulator is obtained

Direct numerical simulations of collision dynamics of wet particles

Fluidized beds involving liquid injection have a wide industrial application ranging from physical operation, like agglomeration and coating, to chemical processes including catalytic oxidization, fluid catalytic cracking, condensed-mode polyethylene (1). The injection of the liquid results in wet particles, which behave completely different from dry particles and hence lead to much more complicated hydrodynamics of fluidized beds (2). Nevertheless, a fundamental description of the dynamics of wet particles is predominantly missing, which however is crucial for prediction of fluidization behavior effecting on the product quality. Despite significant investigation, experimental studies of wet collisions under actual fluidization condition (e.g., low particle velocity, thin liquid layer) are virtually impossible to perform and control. Direct numerical simulations can complement experiments by providing quantitative predictions of the micro-mechanical collisional behaviour of one or more particles with well-defined and easy-controlled system parameters. Jain et al. (3) demonstrated that the experimentally observed phenomena of collision between a particle and a wet wall can be reproduced by a hybrid model combining the volume of fluid (VOF) method and the immersed boundary method (IBM). Such simulations will be extended in this work to investigate the effects of liquid layer thickness, impact velocity, particle size and surface tension on the wet restitution coefficient () under normal collisions as well as oblique collisions. The motion of a solid particle is described by the IBM (Figure 1), which enforces a no-slip condition at the particle surface. Whereas, the VOF (Figure 2) describes the motion of the gas-liquid interface by a piece-wise linear reconstruction of the interface. Please click Additional Files below to see the full abstract

Numerical Investigation of the Particle Dynamics in a Rotorgranulator Depending on the Properties of the Coating Liquid

In the pharmaceutical industry, the coating of particles is a widely used technique to obtain desired surface modifications of the final product, e.g., controlled release of the active agents. The production of round, coated particles is particularly important, which is why fluidized bed rotor granulators (FBRG) are often used for this process. In this work, Computational Fluid Dynamics (CFD) coupled with the Discrete Element Method (DEM) is used to investigate the wet particle dynamics, depending on the properties of the coating liquid in a FBRG. The DEM contact model was extended by liquid bridge model to account for capillary and viscous forces during wet contact of particles. The influence of the relative contact velocity on the maximum length of the liquid bridge is also considered in the model. Five different cases were compared, in which the particles were initially wetted, and the liquid loading as well as the surface tension and viscosity of the liquid were changed. The results show that increasing viscosity leads to a denser particle bed and a significant decrease in particle rotational velocities and particle motion in the poloidal plane of the FBRG. Reducing the liquid loading and surface tension results in increased particle movement