Agglomeration of Particle Systems in Fluidised Beds: Low Temperature Granulation

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Author Last Name: 
G Tardos
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Research Area: 
Powder Flow
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United States

Executive Summary

The majority of previous granulation research has been of a mechanistic nature examining the effect of operating variables such as ‘fluid-bed excess gas velocity or spray characteristics on granule growth rate and morphology. Various mechanisms of granule growth have been identified and a general population balance similarity theory of granulation has been developed. However, little 4 m knowledge of granulation phenomena is provided by the above disparate, essentially macroscopic approaches alone, One must instead turn to a microscale consideration of interparticle cohesive forces in relation to the energetics of disruptive particle motion.

A brief description of the fluid-bed granulation process is presented in Chapter 2 with the aim of clarifying various competing mechanisms as well as establishing their respective controlling material parameters. Specifically, the present work seeks to explain differences in observed granulation morphology and growth rates in terms of the strength of a dynamically strained pendular bridge and differences in the viscous history of the involved binders. An extensive review of growth mechanisms, the effect of operating and material parameters, and granulation modeling has been given previously (Ennis et al., 1986).

Research emphasizing the importance of dynamic pendular bridge strength is outlined in Chapter 3. Completed results concerned with the strength of an axially strained bridge are given in Ennis et al.(1988). These as well as further preliminary results dealing with relative sphere shearing motion, surface roughness, and imperfect wetting are summarized in Chapter 4. An ad hoc solution of dynamic bridge strength based on the superposition of lubrication theory and circular approximation is presented. For small gap distance with sufficient bridge volume and in the limit of small Reynolds’ number, good agteemgnt between the experimental and present theoretical axial force response is observed indicating the importance of a capillary number Ca in determining pendular bridge strength. The present theoretical analysis is zeroeth order in capillary number and gap distance and hence, is expected to break down with increasing local inertial effects. Such inertial effects are governed by a modified Bond number and, in the limit of low Ca, lead to an increase in bridge strength due to an added mass effect, whereas in the limit of high Ca, lead to a reduced, shifted force response due to an insufficient rate of vorticity propagation. Preliminary investigations indicate that the present theoretical analysis extends to the cases of arbitrary (nearly touching) particle motion, imperfect solid wetting, and particles with only small scale surface roughness.

Initial granulation results and binder bridge measurements supporting the influence of viscosity are presented in Chapter 5. Typical industrial binder solutions such as 2.5 weight percent aqueous carboxymethylcellulose exhibit an exponential increase in pendular bridge strength due to solvent evaporation and, therefore, display a full range of capillary number behavior from an initial weak surface tension response to an extensive viscous reponse with an equivalent viscosity of the order of fifty poise. Liquid bridge measurements of binder solutions appear to adequately predict fluid-bed granule morphology indicating that growth is controlled by a combination of a viscous strengthening time constant and a final solid bridge strength. Such a time constant and final bridge strength are, in turn, related to the effect of binder concentration on solution viscosity and the molecular weight of the binder, respectively.

Lastly, a preliminary analysis of the competing processes of fluid-bed coalescence and breakage as well as an attempt at incorporating knowledge of these phenomona into a population balance framework is presented in Chapter 6. Two unknown cctnqtants related to coalescence and breakage should bear a fundamental relationship to the properities of the binder - namely, the viscous strengthening time constant and solid bridge strength, respectively.