Wurster coater

If back-flow of particles into the spout is avoided by enclosing the spout with a concentric pipe and the spray liquid is a hot melt or a saturated solution, an excellent coater ( Wurster coater, see Section 10.1) is obtained. 

Fig. 5.13 Sketch of the material processing section of a bottom spray fluidized bed coater (Wurster coating system) [B.71]...

Draft Tube Designs and Spouted Beds A draft tube is often employed to regnlate particle circnlation patterns. The most common design is the Wurster draft tnbe flnidbea employed extensivelvin the pharmacentical industry, nsnally for coating and layered growtJi applications. The Wurster coater nses a bottom positioned spray, bnt other variations are available (Table 20-47). 

Processes based on fluidized bed coating have been developed (49). In this process, the bioactive agent is dissolved in an organic solvent along with the polymer. This solution is then processed through a Wurster air suspension coater apparatus to form the final microcapsule product. A solvent partition technique based on continuous injection of a polymer-drug solution into flowing mineral oil has been reported (50). 

Spouted beds Wurster coaters Moderate (layered) 50 tonne continuous cohesive powders, good for coating applications detergents Batch pharmaceuticals, agricultural chemicals, nuclear wastes... 

Fig. 4. Diagram of a Bottom Spray (Wurster) Coater. (Courtesy Glatt Air Techniques, Inc.)...

Fig. 4 Air distributer or orifice plate of a Wurster coater. (From Ref. l)...

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)...

Figure 374. Schematic representations of three different configurations of Wurster coaters for use in coating (a) tablets, (b) coarse granules, and (c) fine powders ...

Fig. 10.30 Schematic explanation of Wurster coater scale-up (courtesy Aeromatic-Fielder, Bubendorf Switzerland).

Fig. 10,31 View into the coating section of a (precision) Wurster coater featuring ten tubes (courtesy Aeromatic-Fielder, Bubendorf, Switzerland).

Fig. 6.2-83 Principle of the Wurster coater (right) and outline of an apparatus (left) (courtesy Glatt, Binzen, Germany)...

Fig. 6.2-85 Sketches of the different chamber configurations of single tube Wurster coaters as used for a) tablets, b) coarse granules, c) fine particles [B.48]...

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,...

The stochastic movement in low-density fluidized beds is used for coating relatively small particles with narrow particle size distribution. Many of the newer designs use some modification of the Wurster coater [B.48, B.97] (Fig. 5.13). 

During studies of particle and gas flow patterns in traditional Wurster coaters it was found that flow patterns were dominated by the particles rather than the gas. This explains why sometimes, in spite of the well defined flow, uniform coating is difficult to control. To overcome this problem, a precision Wurster coater was developed [B.97]. 

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