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332 Fluidization, Solids Handling, and Processing

When dealing with particle and powder systems, often there are many physical material parameters that are unknown. In analyzing problems in the operation or design of such equipment, it should be understood that these parameters may have to be estimated using known testing procedures or previous operating data. There also exists a large body of literature which focuses on case studies of coating and granulation processes, this literature is not cited with great frequency in this chapter since the purpose here is to generalize theories and experimental findings rather than to catalog them.

The chapter is split into two main sections. The first section (Sec. 2) covers the area of fluidized bed coating. Processes utilizing this technology are refereed to as film coating techniques. Film coating occurs when a continuous thin film of material, often a polymer, is applied evenly over the surface of the particles comprising the bed. The film is added little by little to the particles through repeated exposure to a liquid spray in the bed. The reasons for coating the particles are varied and include: the controlled release of the active ingredient in the particle e.g., enteric or sustained release; to provide a moisture barrier; to improve the appearance of the product; to mask the taste of the product; and to control the attrition of the product. The approach presented here is to first consider some of the microscopic phenomena such as droplet formation in the atomizing nozzle and evaporation rates of the liquid drops. The modelling of the process is covered next and two approaches are considered. First, the coating process is modeled using a population balance approach then it is considered from a probabilistic or stochastic standpoint, the results from the two approaches are compared. Finally, equipment design, availability and scale-up considerations are discussed.

The second section (Sec. 3) covers the area of granulation of fine particles in fluidized beds. First, microscopic considerations such as the wetting and spreading rates of the liquid binder and the critical binder/powder ratio are considered. Next an overview of the fundamentals of the strength of the pendular liquid bridge which holds two particles together is given. By characterizing the forces holding particles together, estimates of critical parameters affecting the growth of granules can be made. The strength of granules produced from agglomeration processes is then discussed and methods to evaluate granule strength are given. The modeling of the granulation process using a population balance approach is covered next and rate expressions for nucleation, coalescence and breakage are given. Finally, the topic of unwanted agglomeration in fluidization processes is discussed.

Coating and Granulation 333

It is the hope of the authors that this chapter will be of value to the practicing technologist and also will provide a basis for focused research in the many areas of this field which is still required.

2.0COATING OF PARTICLES IN FLUIDIZED BEDS

2.1Introduction

The term fluidized bed coating can refer to one of two fundamentally different types of processes. Figure 1 shows a general overview of fluidized bed coating. It can be seen from this figure that the objects to be coated can either be the bed material or solid objects which are immersed into a bed of coating material. The most common application of the second type of coating process is in the treatment of metal objects. For example, consider a fluidized bed comprised of fine polymer particles fluidized by an inert gas at a temperature below the melting point of the polymer. If a piece of hot metal, above the melting point of the polymer, were to be immersed into the fluidized bed, then the polymer would start to melt onto the surface of the metal. By carefully controlling the temperature, the time that the metal is immersed, and the level of fluidization in the bed, it is possible to deposit a smooth, coherent polymer film of a desired thickness onto the metal surface. This type of coating has several advantages, namely that no solvent is required in the coating process and that objects with complicated shapes can be coated easily. The coating of objects other than the particles comprising the fluidized bed, is not covered further in this chapter. Several references are provided (Ranney, 1971, Landrock, 1965 and 1967) on this subject for the reader.

The remainder of this section covers processes in which the particles comprising the fluidized bed are to be coated. The basic principle of fluidized bed particle coating and granulation operations is to inject a fine liquid spray into a bed of fluidized particles. The spray consists of a solute which acts as a coating medium, and a solvent in which the solute is dissolved or slurried. The liquid spray impinges on the fluidized particles and spreads on the surface of the bed solids. As the solvent evaporates, the solute remains on the surface of the particle. In agglomeration processes, the solute is a binder which tends to cause individual particles or groups of particles to stick together. The action of the fluidized bed causes these particle agglomerates to