Qualitative Description of the Wurster-Based Fluid-Bed Coating Process

Abstract

The Wurster-based fluid-bed coating process is often treated as just another fluid-bed coating process. However, there are significant differences between the two types of fluid-bed coatings. The Wurster-based coating process does not contain any fluid-bed regions in the traditional sense, as it is a circulating fluid-bed process. Four different regions within the equipment can be identified: the upbed region, the expansion chamber, the downbed region, and the horizontal transport region. The size of these regions is determined by the dimensions of the coating apparatus. Part of the upbed region constitutes the coating zone where the spray mist hits the substrate (the material that is going to be coated). The coating process consists of three phases: the start-up phase, the coating phase, and the drying/cooling phase. During the coating phase, several processes take place simultaneously. They are: atomization of the film solution/suspension, transport of the film droplets to the substrate, adhesion of the droplets to the substrate, film formation, the coating cycle of the substrate, and the drying of the film. When discussing the coating process, it is important to consider properties of the substrate. Key properties of the substrate determine important process properties such as bed expansion, bubble properties, slug properties, and spouting. The most important properties of the substrate are the density of the particles, their diameter, and their stickiness. The process characteristics are very different in each of the four regions. The upbed region is the most difficult to control. It is here that the most sensitive processes in relation to the coating occur. The product flow in the upbed region is a dilute vertical pneumatic conveying. The pneumatic conveying is controlled by the upbed fluidization air rate. Slugging is a frequent problem with the flow in this region for dense and large substrates. The air flow is the combined air flows of the fluidization air and the nozzle air. Air and substrate velocities are not uniform across the upbed. The velocities at the center are significantly higher than those along the walls. There is a risk that substrate might fall down along the partition wall, and that clusters of particles might form in the upbed at certain processing conditions. The particle terminal velocity in the upbed is limited by the height of the expansion chamber. Unfortunately, the particle terminal velocity cannot readily be calculated and must be measured, if attrition of the substrate is a problem. The product concentration in the mist region of the upbed region must be high enough to secure adherence of all spray droplets to a substrate particle. The air velocity in the expansion region must be well below the minimum fluidization velocity. It is the expansion in diameter that secures the drop in air velocity, when the air moves out of the partition and into the expansion region. The downbed region is a slightly expanded bed where the air rate is below the minimum fluidization velocity. This is the region where sticking is most likely to occur, since the movement is gentle and the particles are in close contact with one another. Actually, the product is only slightly expanded over a loosely packed powder. The substrate moves into the upbed via the horizontal transport region. The air flow through this region is very complex. Air rate measurements and pressure drop calculations suggest that some of the air flows from the downbed bottom plate, through the horizontal transport region, and into the upbed region. The Wurster-based coating process is very different from the top-spray coating process, and optimization should be treated from a completely different angle. A stepwise process optimization procedure is suggested involving optimizing the product circulation, adjusting the spray rate, and ensuring that the droplet size of the spray falls within specified limits. The product circulation is optimized by the choice of the correct bottom plate configuration of the downbed and the upbed regions. In conclusion, the Wurster-based coating process is a highly complex process, and sufficient attention should be directed toward optimization of the process.