Yang Fluidization, Solids Handling, and Processing

Coating and Granulation 365

3.0GRANULATION OF FINE POWDERS IN FLUIDIZED BEDS

3.1Introduction

Granulation is a size enlargement operation by which a fine powder is agglomerated into larger granules in order to produce a specific size and shape, to improve flowability and appearance and to reduce dustiness. Granulation, as a unit operation, is used by a host of industries ranging from detergents, to food products, to agricultural chemicals such as pesticides and fertilizers and to pharmaceutical products such as tablets. During granulation, a liquid such as a solution (binder) or a melt is pumped, poured or atomized onto an agitated bed of different powders contained in a mixer whose main role is to provide shearing forces in the powder mass. As solvent evaporates from the binder (liquid) or the melt thickens, the powder particles stick together and, as interparticle bridges and capillary regions strengthen, larger granules of the original powder are left behind. These granules are further consolidated by the forces in the mixer and, upon final solidification of the binder or melt, strong agglomerates are formed.

Binder granulation, as it is known in industry, can be achieved in different types of mixers ranging from rotating drums and pans to high shear mixers and fluidized and spouted beds. The present review is focused on fluidized bed granulation which is in some ways different from other types of mixer granulation in that the gas supplied to produce powder agitation through fluidization also causes binder evaporation and cooling (or heating) of the powder. In addition, particle size increase is associated in a fluidized bed with many changes in fluidization characteristics, the most important of which are the mixing properties of the bed. These interacting phenomena make fluid bed granulation by far the most complex. At the same time it is also the most versatile, allowing drying and cooling operations to be carried out simultaneously with size increase (agglomeration).

Several excellent reviews of fluid bed granulation were published by Nienow and coworkers (Nienow, 1983, Nienow and Rowe, 1985 and Nienow, 1994) in which the operation is described in detail. Heat and mass balances on the system are presented and a granule growth model is proposed. The advantages and disadvantages of the operation are discussed. The present approach differs from the above work by looking at the microscale

366 Fluidization, Solids Handling, and Processing

processes of granule formation, growth and breakage and by studying the behavior of liquid bridges between agglomerating particles. This view is in some ways more powerful in that it generates an overall picture of binder granulation. Furthermore, specific phenomena such as particle coalescence and layering of fines or the deformation and breakage of wet granules are components which result directly from the models at the microscale.

Fluid bed granulation equipment is, in general, not different from industrial fluidized bed coating devices except the spraying zone occupies a larger portion of the bed and also gas velocities used are somewhat smaller. Granulation equipment and some of their main characteristics were described in some detail in the previous section, see Figs. 2 and 9, and are not repeated here. The tendency in present industrial granulation is oriented toward the use of “combination” equipment, for example the high shear-mechanical fluidized bed type of the kind presented in Fig. 9b. These devices are very versatile and allow, in addition to very effective particle growth and coating, drying, heating and grinding operations to occur over a very short operation period.

3.2Microscopic Phenomena

The process by which fine powder particles stick together in a shearing mass of powder to form large granules which posses, in the end, enough strength to survive in the granulator is quite complex (Ennis et al. 1990a,b, 1991; Ennis, 1990; and Tardos et al. 1991, 1993, and 1997). Figure 12 depicts some more common ways in which agglomerate growth may occur: nucleation is defined (Fig. 12a), as the sticking together of primary particles due to the presence of a liquid binder on the solid surface. Coalescence, on the other hand (Fig. 12b), is the process by which two larger agglomerates combine to form a granule. Other modes of granule growth are layering of a binder coated granule by small seed particles (Fig. 12c) and the capture of fines by a partially filled binder droplet (Fig. 12d).

It is instructive to simplify the above picture somewhat and consider the coalescence or sticking of two particles schematically shown in Fig. 13. One can assume that due to shear forces in the mixer, a fluidized bed in the present case, the two particles posses a relative velocity U0 which ensures collision at some point on their trajectory and possible sticking under appropriate conditions. It is essential that some binder be present at the point of contact, as depicted in the figure. From this simplified picture, allmechanisms

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amounts of liquid, a situation is reached where enough of these liquid bridges are formed to create a continuous network of interconnected particles which extends throughout the bulk: this is called the pendular state. Many properties of the bulk powder show a significant change as this state is reached, a good example being the yield strength of the powder which shows a drastic increase. Values of the so called saturation, S, defined as the fraction of voids filled by the liquid to the total amount of voids in the dry solid, of the order of 20–30% are characteristic of this state.

In the other extreme, when all available voids in the bulk are filled with liquid, the so called capillary state is reached and under these conditions the saturation is S = 1. Between these limits, an intermediate, or so called funicular state is achieved, for which the saturation value is 0.3 < S < 1.0. Rumpf (1975), among many others, studied the yield strength of a bulk powder as it moved from the pendular state through the funicular and finally to the capillary state by mixing into the bulk ever increasing amounts of liquid. It was found that the yield strength increases significantly at the transition between the pendular and funicular and again before the onset of the capillary state with a subsequent decrease upon formation of a wet cake or slurry. In an effort to control granulation in fluidized beds and high shear mixers, Lauenberger (1979), Schaefer (1988) and Schaefer et al. (1990a,b) introduced a shear measuring device (head) into a fluid bed granulator and also measured the torque and the power consumption of a high shear mixer while granule growth took place. These authors were able to generate curves of these quantities as a function of liquid saturation by taking samples of powder at different times during the granulation run. The increase of both the shear forces in the fluid bed as well as the torque and power consumption of the mixers as a function of saturation was found to take place at the transition between the pendular and funicular states and again between the funicular and the capillary states. This resembled, not surprisingly, the yield strength dependence of the powder on saturation, as illustrated above. This indicates that liquid bridges between particles critically influence the behavior of the bulk and the granulation characteristics of a powder-binder system and that the appropriate amount of binder can be determined by measuring one of these quantities.

The instrument and the procedure to determine the critical binder:powder ratio were developed (Tardos, 1994) based on the above observations. The instrument consists essentially of a low shear mixer, schematically represented in Fig. 14, in which the powder is slowly agitated and the binder is

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Another important point has to be made for the case of fluid bed granulation, namely that binder introduction is usually concentrated in the so called “spray zone” and that the process of growth is not instantaneous or even rapid. Hence, agglomeration and granule growth will occur in the granulator while binder is introduced and the prevailing growth mechanisms will be locally determined by the amount of liquid present in the spray zone or in the bed. This is an essential difference as this process is compared to, for example, high shear granulation where the bed of powder is much more homogeneous.

Table 3 gives critical binder:powder ratios for a few powder-binder systems again given as percentage of dry powder. One can see that these ratios are higher for DBT than for water and also that large and porous particles such as the sodium carbonate and the agglomerated Sipernat (Sipernat is an activated silica powder produced by the Degussa Corporation) take in large amounts of liquid. Very flaky and nonporous powders such as dicalite, on the other hand, exhibit a very low critical ratio. In fact, for this powder, the transition takes place almost directly from the pendular to the capillary state and therefore, the critical ratio is quite low. Data in Table 3 is given for both water and oil (DBT); a water-based binder such as a low concentration solution will behave mostly like water while an oily surfactant binder or melt will behave like DBT. For binders used in a specific case, the above measurement has to be repeated and the actual critical ratios obtained.

Table 3. Critical Binder/Powder Ratios for Some Selected PowderLiquid Systems

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Characterizing Binder Wetting Properties and the Impact on Granulation. Introduction of at least the critical amount of binder into the granulator is only a first and necessary step to achieve proper granule growth. As can easily be inferred from the simplified picture of granule growth depicted in Fig. 13, the liquid binder layer present at the point of contact between two particles needs to have, at the moment of collision, a certain depth to insure sticking instead of rebound. If the layer is too thin and/or the liquid is adsorbed under the granular surface into pores, its influence will be negligible, granule growth will not occur and the binder will be lost for the process.

Figure 16 schematically depicts the process of surface wetting and liquid penetration into the pores of a particle. Under normal conditions of granulation, the binder is introduced in the bed as a spray of small droplets. These droplets impact on the solid surface and deposit on it as described in the chapter on coating: assuming that solid-liquid contact angles are such that surface spreading can take place, the binder droplet will flatten and cover an ever increasing area of the surface. At the same time however, liquid will penetrate into the surface pores of the granule and will be unavailable on the surface to ensure sticking to another granule. Both surface wetting and spreading are necessary since a non-wetting liquid will either not adhere to the surface at all or will be present on or cover a very small area, thus the number of collisions which yield growth is limited. Penetration into surface pores is also required in order to give the surface the malleable, plastic property required for large scale deformation as granules form by coalescence (Fig. 12b).

Methods of characterizing wetting include spreading of drops on powder surfaces, using a contact angle goniometer, and penetration of drops into powder beds, using the Washburn test. The first approach considers the ability of the fluid drop to spread across the powder and involves the measurement of the contact angle of a drop on a compact of the powder of interest (Aulton and Banks, 1979). The contact angle is a measure of the affinity of the fluid for the solid and it is given by the Young-Dupré equation, or

where γ sv, γ sl, γ lv are the solid-vapor, solid-liquid and liquid-vapor interfacial energies, respectively, and θ is the contact angle measured through the