Particle Coating in a Wurster Type Bed
Abstract: The Wurster bed process is frequently used for film coating. In the Wurster bed, particles circulate in the equipment and are sprayed with a liquid which forms a coating when dry. The procedure is repeated until the desired characteristics of the coating layer are obtained. The complex process involves zones of both high and low particle concentrations, as well as high and low velocities. To gain a better understanding of the mechanisms controlling the quality of the coating and how optimisation and scale-up of the process can be improved, both experimental and theoretical studies were made. To study the coating formation and investigate how the coating is affected by the spray and drying velocities, a new device for coating a single levitated particle under controlled conditions was designed and tested. The particle was levitated and droplets were sprayed from below, as in the Wurster process. Coating with a solution of HydroxyPropylCellulose (HPC) and a latex dispersion was investigated in the single particle coating device, and the influence of operating conditions was studied. Temperature, gas humidity and droplet frequency can be varied and are well defined. The experimental results show that both increased gas moisture content and increased droplet frequency give a smoother coating. The solvent in the coating liquid and the temperature also influenced the coating structure. A ring pattern was observed when the particle was coated with the dispersion. A model was developed to study the mechanism influencing the coating structure at the single particle level. Knowledge of the particle movement in the bed is needed in order to couple the single particle coating to the particle and gas dynamics in the process equipment. To study the particle movement, a laboratory Wurster bed of plexiglass, to allow optical access to the flow, was constructed. A technique was developed to track the particle in the laboratory scale bed. A small amount of particles are marked with fluorescence and the marked particles are tracked with a high-speed video camera. With this technique, particles can be followed in the zones with low particle concentration and near the walls; with image analysis, particle trajectories and velocities can be calculated. The technique can be used for the characterization of differences in process dynamics due to variation in operating conditions. The distribution of the maximum height of the particle trajectories was studied for selected parameters of particle loading, jet air velocity and position of the Wurster tube. A model of the Wurster bed coater, which combines the particle and gas dynamics with the wetting and drying, was developed. The model can be used to predict the influence of the operating conditions and geometric dimensions of the equipment. To simulate particle and gas motion, the Eulerian-Eulerian approach was used. The simulation indicates where the drying takes place in the equipment and how different process conditions influence the drying. The influence of spray rate, temperature and moisture content in the fluidised air was investigated. Results show that most of the drying takes place in the Wurster tube under typical operating conditions.
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