Fluidized spray zone

Figure 7. Sheltering effect of particles in the spray zone of a fluidized bed coater.

It is far from certain that such a high-voidage spray zone can exist within a fluidized bed a spray zone, with a submerged nozzle, would require a jet to be blown in the dense phase by the atomising air. Work by Rowe et al. (1979) and by Smith and Nienow (1982), using X-ray... 

Cheng, X. X., and Turton, R. (2000), The prediction of variability occurring in fluidized bed coating equipment. II The role of nonuniform particle coverage as particle pass through the spray zone, Pharm. Dev. Technol., 5, 323-332. 

A spouted bed process is similar to the Wurster process. The principal difference is that an air velocity differential needed to properly circulate particles through the nozzle spray zone is created by a tapered coat chamber design rather than an air distribution pattern created by the fluidization plate. An illustration of a spouted bed is provided in Figure 7.5. Process ciir entering through the fluidization plate passes upward past the nozzle, Ccurying particles with it as they feed in from the periphery of the plate and lower chamber. Spray is directed upward with the direction of pmticle flow. Vertical increase in cross-sectional area reduces the air and particle velocity to allow pmticles... 

Most of the discussion in this chapter has focused on theoretical principles of operation. In actual practice, various other limiting concerns are realized. Fluidization plates may not always be ideal, and parameters may require adjustment to achieve coating under less favorable conditions. The most basic requirement in all cases is that all particles routinely circulate through the spray zone during processing so that they will be coated. Electrostatic forces between particles, or particles and equipment surfaces, are a common concern. These forces can promote agglomeration or prevent particles from circulating properly. Formulation or parameter adjustments are typically employed to overcome or minimize electrostatic concerns, but they are not always successful. 

Fig. 7.47 Two-compartment model of a fluidized bed with a spraying zone and a drying zone.

Certainly, coating can also be conducted by spraying from the top on a conventional fluidized bed or (and more easily) by spraying from the bottom in a spouted bed, if the operating conditions are appropriately selected for the material to be processed. However, assuming that agglomeration takes place in the spray zones of these two units, it may be concluded from the data listed in Tab. 4.8 that it would be faster in the ProCell, as the frequency of collisions between particles is much higher in the spray zone of this equipment (namely,... 

When the fluidized bed consists of non-porous particles, solvent in the feed liquid is evaporated in a well-defined zone close to the spray nozzle and from fhe surface of the bed particles with which it inevitably comes into contact. No permanent gas jet or void exists in this region particle motion is not well ordered and no regular coating of parficles wifh feed solution takes place. The random and intense contact between particles and liquid results in agglomeration. Even if the mass and heat balances for the bed as a whole have been satisfied, both... 

A primary difference is that a higher fluidization velocity is desired resulting in the need for an expanded deceleration zone, hence the extended conical expansion chamber. Additionally, continuous fluidization is desireable from both economical and functional points of view. For this reason, the filter housing is enlarged and designed to shake fines back into the batch without stopping fluidization or spraying. 

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