Wurster fluid bed

In a top-spray fluidised bed, air is introduced through a uniform air distribution plate. Compared to the particle motion in the bottom-spray fluidised bed, the fluidisation pattern is less controlled. Particular to the top-spray method, the nozzle is positioned above the particle bed, and coating liquid is sprayed coimtercurrently, or down, into the fluidising core particles. Compared to a Wurster fluid bed, it is more... 

Li L, Rasmuson A, Ingram A, et al PEPT study of particle cycle and residence time distributions in a Wurster fluid bed, AIChE J 61 756-768, 2015a. 

Li L, Remmelgas J, van Wachem BGM, et al Residence time distributions of different size particles in the spray zone of a Wurster fluid bed studied using DEM-CFD, Powder Technol 280 124-134, 2015b. 

Scotts technology (17) uses fluid-bed (Wurster column) technology to apply polymeric coatings to a number of fertilizer substrates including urea, potassium nitrate, potassium sulfate, and monoammonium phosphate (MAP). The coating material is appHed as a water-borne latex onto the fluidized substrate. As the substrate is fluidized with warm air (40—50°C), water is driven off and the latex coalesces into a continuous film around the fertilizer particle. The particular latex compositions used have selected glass-transition and blocking temperatures, which enable quick removal of the water before the soluble fertilizer core dissolves. This obviates the need to use precoats prior to the latex appHcation. 

When scaling-up the fluid-bed process, a major requirement is to produce fluidization behavior on the larger machines equivalent to that used on the scale that provided the basis for process development. To achieve this goal, and minimize attritional effects, the same air velocities for each scale of equipment are required. Thus, the overall increase in air volume required during scale-up will be related to the increase in area of the perforated base plate, and, in the case of the Wurster process, the open area of the partition plate immediately beneath each of the inner partitions. Such calculations are simplified when scaling-up from an 18" pilot scale machine to, say, a 32" machine, since the latter represents a three-multiple of the former, and thus would require a threefold increase in airflow. 

F. N. Christensen and P. Bertelsen. Qualitative description of the Wurster-based fluid-bed coating process. Drug Dev. Ind. Pharm. 23 451-463, 1997. 

Bottom-spray film coating is accomplished by means of the Wurster system, originally developed by Dr. D.E. Wurster (Wurster 1959). A scheme of the bottom-spray fluid bed is given in Figure 16.4. 

Draft Tube Designs and Spouted Beds A draft tube is often employed to regulate particle circulation patterns. The most common design is the Wurster draft tube fluid bed employed extensively in the pharmaceutical industry, usually for coating and layered growth applications. The Wurster coater uses a bottom positioned spray, but other variations are available (Table 20-47). 

The primary features that distinguish Wurster equipment from other fluid-bed equipment are the cylindrical partition located in the product chamber and the configuration of the air distributor plate, also... 

Figure 372. Schematic representations of the product handling sections of three fluidized bed coaters. (a) Top spray, (b) tangential spray (rotary fluid bed coater), (c) bottom spray (Wurster coating system)...

Bottom spraying, sometimes referred to as the Wurster process, involves the spraying of the tablets at the bottom of the fluid bed. In this setup, there is a column introduced into the fluid bed chamber, which ensures that tablets in the centre of the chamber are directed upward. At the top of the column, the tablets descend the chamber at the edge of the chamber from where they will be redirected to the spraying region. 

Fig. 6.2-86 SEM photographs of particles that were coated with fluidized bed coaters a) coated with ethylcellulose in a top-spray fluid-bed processor, b) coated with ethylcellulose in a Wurster apparatus, c) vacuum top-spray coated (retard) pellet, cut open,...

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