Agglomeration of Particle Systems in Fluidized Beds: Low Temperature Granulation

Publication Reference: 
08-09
Author Last Name: 
Tardos
Authors: 
G Tardos B J Ennis R Pfeffer
Report Type: 
FRR
Research Area: 
Powder Flow
Publication Year: 
1992
Publication Month: 
03
Country: 
United States

Executive Summary

Agglomeration or granulation, as the name implies, is a process by which larger (millimeter or fractions of millimeter in diameter) granules are .produced from fine (micron sized) powders in a mechanical agitator such as a drum, pen or high shear mixer or in a fluidized bed. Particle growth in these devices is facilitated by the use of a binder, i.e., a sticky fluid, a solution or a melt which upon dispersion in the powder mass and subsequent solidification at the interstices between particles generates stable granules. While powder size increase (agglomeration or granulation) is a widely used unit operation, few underlying physical principles describing the phenomena have been drawn. Successful granulation operation is therefore a largely haphazard undertaking. The present research attempts to lay a rational foundation to describe the mechanics of granulation by examining the process at the level of particle-binder-particle interaction, at the so-called microlevel.

The ultimate goal of the present work was to build a granulation model to predict granule size and growth rates from first principles using the properties of the powder, the binder and the characteristics of the mixer. It was discovered early in the project that liquid (binder) bridges formed between moving solid particles are the key to understanding of the many different processes taking place during granulation. It was also found that the study of these bridges, although attempted as far as their behavior with regard to surface tension effects is concerned, is not sufficiently developed and hence, a basic study of viscous effects in moving liquid bridges was undertaken. Furthermore, the phenomenon of particle coalescence and growth was studied using the theory of viscous liquid bridges developed earlier and regimes of granulation were defined in which both the growth rate and the limiting particle (granule) size were calculated. Three such regimes were identified, each characterized by a different dependence of the growth rate on such parameters as particle size, binder viscosity and surface tension and other parameters of lesser importance. All these quantities were incorporated in a dimensionless so-called Stokes (or Reynolds) number, characteristic values of which in turn delimit the different regimes.

Finally, the existence of the different growth and granule consolidation regimes was tested by experiments specially designed to isolate the important phenomena in question for each regime. As it clearly appears from the present work, the theory of coalescence regimes as presented above is only a framework which provides us with some basic insight into the phenomena of granule growth and consolidation but is not in fact a comprehensive model of granulation (aithough an attempt was made, to incorporate the above findings into a theory of defluidization of fluidized beds). One of the major practical achievements of the present work was the development of an instrument to characterize binders used in granulation and to measure binder strengthening times. These measured characteristics were then used during pilot scale fluid bed and drum granulation experiments to predict limiting granule diameters.

The present work did not provide a final solution for granulation theory but rather opened the field, presented a preliminary general framework and established the important lines of inquiry to be followed in the .-future. First and most important, is the measurement and/or prediction of shear forces in a mixer. This is essential since the knowledge of these forces is key to developing a comprehensive granule growth model by equating the disruptive and cohesive forces in the device. Introduction of particle and granule breakage into the overall theory through fracture mechanics is also paramount especially in such devices in which some drying of the granules occurs such as a fluid bed granulator or where the disruptive forces are very high such as in high shear mixers.