Product design in spray fluidised beds

Modern fluidised bed processes provide industry and research with the opportunity of obtaining tailor-made physical properties and application characteristics of solid products. Common to all processes is that fluids are sprayed in fluidised beds to obtain defined particle structures or build functional layers of particles.

Figure 1 is a summary of some process versions of this type. After a discussion of fundamental conditions of these processes, the author presents different product forms.

The processes on which most applications are based can be characterised by different combinations of processing solid and/or liquid input materials and auxiliary substances that may be required. Starting from this premise, the most frequent processes are referred to as agglomeration, spray granulation, coating, encapsulation and powder layering.

Fluidised powders or mixtures of powders are bonded to each other in agglomeration processes in that the injection of liquid in the fluid bed builds stable bonds between fine particles. The liquid injected can be water or liquid binder which reinforces the adhesive forces if the powders lack inherent adhesive capacity.
In contrast with this, spray granulation involves exclusively liquid raw materials (solutions, suspensions, melts, emulsions, etc.) which are sprayed onto fluidised particles. The solvent (e. g., water) evaporates on the surface of the particles and the remaining solid material sticks, causing the particle to increase in size shell by shell. The so called granulation cores which this process needs are obtained either by spray-drying part of the spray liquid in the fluid bed machine or by returning some crushed particle material from the end product.

Due to the homogeneous and very dense particle structure, this process is also excellent for encapsulation of liquids. In this process a primary or active material distributed very evenly is embedded in a matrix liquid, e.g., in the form of an emulsion. The following process in which particles form by spray granulation produces compact particles which contain the primary or active substance distributed evenly in their volume

Another principle illustrated in Figure 1 is coating. By spraying one or several solids-containing liquids on fluidisable particles, these particles are provided with one or several solid shells. Functional layers (e.g., for retarding, taste masking, etc.) can be built around starter particles with this process.
A very special process is powder layering. Similar to coating, one or several shells are produced by finely dispersed powder and a binder and attached to larger particles. This can be a very energy saving method because only little evaporation capacity is required.

Today, these particle-producing processes are integrated in different machines, which are illustrated in Figure 2. Depending on process option, batch (WSG, GPCG, Rotor) or continuous machines (GF, AGT, ProCell) are available. The configuration and dimensions of the suitable machine are defined by the requirements of the application to obtain a product with specific properties, the variability of the process versions to be implemented, the frequency of product change and also the production capacity, plus a number of fringe conditions.

Flexibility and universal use are among the many advantages of fluid-bed machines. This means that different types of product can be made in one machine. The conditions under which the particles are shaped can be systematically controlled by setting the process parameters. For example, a smooth transition from agglomeration to spray granulation processes is possible. This can be done by changing the ratio of solids (e. g., powder, crystal, dust, …) and liquid feedstock. In this way, product characteristics, such as bulk density, porosity and particle shape can be defined. These changes of physical parameters have an immediate effect on the application properties of the products made. For example, products produced by agglomeration dissolve readily and can be used as instant products due to their large specific surface. Particles obtained by spray granulation, on the other hand, have a minimum specific surface, which is of advantage for substances that should be less hygroscopic.

Some mechanisms involved in the formation of particles in a fluidised bed are illustrated in Figure 3. Products of different shape can be produced in one machine merely by changing the process parameters.

Every application consists of individual mechanisms which are defined by the fluid bed (right side in Figure 3) and the injection of liquid (left side in Figure 3).
For example, most fluidised-bed applications work in what is called the bubbling regime, i.e., in which the fluidisation of the particles in the process space is very strong and contact between particles is intensive.

Liquid is injected in the fluidized bed of particles. Depending on the type of particles, the liquid penetrates the pores of the particles, liquid bridges build or the liquid binds to their surface.

Under ideal conditions for the intensive exchange of heat and matter drying and solidification can be highly efficient processes while being affected by the setting of defined process conditions.

While the particles are being built in the process chamber, bond and shear forces act in opposition to each other. The injection of liquid generates liquid pressure which causes bonding forces to build among the particles. On the other hand, the intensive movement of the particles generates shear forces in the fluid bed. If the shear forces dominate, the destruction of the liquid bridges prevents the agglomeration of particles. In this case, the injected liquid spreads and solidifies on the surface of the particles. In this way the granules grow in the same way as with spray granulation. If the bonding forces dominate this process, larger porous aggregates can be formed due to the combination of small particles.
This makes the spray fluid bed a very efficient and flexible method, by varying the different process parameters and choosing the feedstock components and recipes, resp., to systematically define the product properties.

The fluid bed technology provides ample opportunity for the development and production of innovative products. Detailed knowledge of the properties of the substances and the related process and technological know-how form an inseparable unit.