Bottom spray processing

The bottom-spray process, commonly called the Wurster process and the only one specifically designed as a coating process... 

The principle and possible applications of the three basic processing options (top-spray, bottom-spray or tangential-spray) of the batch-fluidised bed are discussed below (Jones 1994). Both the top- and bottom-spray methods are characterised by a conical product container and expansion chamber, resulting in a more vigorous fluidisation pattern and a decrease in velocity as the particles move upward in the expansion chamber. 

In contrast to the coating pans, small particles such as beads, pellets, granules, and powders are generally coated using the fluidized bed or air suspension method, which utilizes a carrier gas to keep the cores in motion. The high air current makes this technique more efficient at water removal. The bottom spray technique is one of the most common fluidized bed application methods and a schematic of this process is shown in Fig. 4A. A perforated distribution base plate allows sufficient air into the product container to force the particles up into a cylindrical or slightly conical coating chamber, known as a Wurster insert. These inserts have been recently improved to keep... 

Substrates with poor fluidization characteristics such as larger particles and/or particles of higher density are difficult to coat with the top spray mode, and the bottom spray mode should be preferred. With the tangential spray process, the product temperatures have to be maintained lower than with the top spray technique in order to avoid adherence of the coated particles to the product container. The tangential spray... 

FIG. 21-170 Niro Precision Granulation process. Agglomeration in bottom-spray draft tube fluid-bed granulation. Scale-up is accomplished by replicating draft tube geometry. Courtesy Niro Fharma Systems.)... 

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. 10.27 Sketches of the material processing sections of three fluidized bed coaters [B.42]. (a) Top spray, (b) tangential spray (rotating disc fluidized bed coater), (c) bottom spray (Wurster coating system).

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

Bottom spray This process is known as the Wurster process/bottom spray when the spray nozzle is located at the bottom of the fluidized bed of particles. As the particles move upward through the perforated bottom plate and pass the nozzle area, they are encapsulated by the coating material. Although it is a time-consuming process, the multilayer coating procedure helps in reducing particle defects. 

In a spray-coating process, an inert soluble core composed of sugars, salts, polymers, or a combination of these are flnidized within a fluid bed and an enzyme solution is sprayed on the core to make a continuous film on the surface. The process can be carried out either in a top spray mode, that is, where the spray nozzles are placed in the top of the fluid bed chamber or as a bottom spray... 

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