Coating of particulate materials prevents them from caking and provides a delayed release of the active substance. When thin polymer shells are applied, it is very important to determine the process parameters that ensure the coating uniformity. In this work the quality of the coating process was evaluated by the degree of coverage, which is equal to the fraction of the total surface area of the particles coated by the protective shell. The paper presents a mathematical model, which allows to predict the degree of coverage of particles in a multistage fluidized bed apparatus operated in the continuous mode. The process chamber of each stage was virtually divided into two compartments: drying zone and spraying zone. In the spraying zone the coating is formed, in the drying zone the film solidification and mixing of newly incoming particles takes place. An algorithm for the numerical solution of model equations for multistage apparatus was developed. The algorithm allows to calculate the particle distribution function by degree of coverage. Influence of the number of stages in the unit, flow rate of film-forming agent solution, flow rate of treated material and circulation speed of particles through the spraying zone on coating uniformity was shown. It was found that at the outlet of the four-stage apparatus practically all encapsulated particles have 100% degree of coverage.
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