QUALITATIVE DESCRIPTION OF THE WURSTER-BASED FLUID-BED COATING PROCESS

The Wurster-based fluid-bed coating process is often treated as just a nother fluid-bed coating process. However, there are significant diffe rences between the two types of fluid-bed coatings. The Wurster-based coating process does not contain any fluid-bed regions in the traditio nal sense, as it is a circulating fluid-bed process. Four different re gions within the equipment can be identified: the upbed region, the ex pansion chamber, the downbed region, and the horizontal transport regi on. The size of these regions is determined by the dimensions of the c oating apparatus. Part of the upbed region constitutes the coating zon e 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 dr oplets to the substrate, adhesion of the droplets to the substrate, fi lm formation, the coating cycle of the substrate, and the drying of th e film. When discussing the coating process, it is important to consid er properties of the substrate. Key properties of the substrate determ ine important process properties such as bed expansion, bubble propert ies, slug properties, and spouting. The most important properties of t he substrate are the density of the particles, their diameter, and the ir 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 coat ing occur. The product flow in the upbed region is a dilute vertical p neumatic conveying. The pneumatic conveying is controlled by the upbed fluidization air rate. Slugging is a frequent problem with the flow i n this region for dense and large substrates. The airflow is the combi ned airflows of the fluidization air and the nozzle air. Air and subst rate velocities are not uniform across the upbed. The velocities at th e center are significantly higher than those along the walls. There is a risk that substrate might fall down along the partition wall, and t hat clusters of particles might form in the upbed at certain processin g conditions. The particle terminal velocity in the upbed is limited b y the height of the expansion chamber. Unfortunately, the particle ter minal velocity cannot readily be calculated and must be measured, if a ttrition of the substrate is a problem. The product concentration in t he mist region of the upbed region must be high enough to secure adher ence of all spray droplets to a substrate particle. The air velocity i n the expansion region must be well below the minimum fluidization vel ocity. It is the expansion in diameter that secures the drop in air ve locity, when the air moves out of the partition and into the expansion region. The downbed region is a slightly expanded bed where the air r ate is below the minimum fluidization velocity. This is the region whe re sticking is most likely to occur, since the movement is gentle and the particles are in close contact with one another. Actually, the pro duct is only slightly expanded over a loosely packed powder. The subst rate moves into the upbed via the horizontal transport region. The air flow through this region is very complex. Air rate measurements and pr essure drop calculations suggest that some of the airflows from the do wnbed 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 pr ocedure is suggested involving optimizing the product circulation, adj usting the spray rate, and ensuring that the droplet size of the spray falls within specified limits. The product circulation is optimized b y the choice of the correct bottom plate configuration of the downbed and the upbed regions. In conclusion, the Wurster-based coating proces s is a highly complex process, and sufficient attention should be dire cted toward optimization of the process.