On the mechanics of granulation.
Item
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Title
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On the mechanics of granulation.
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Identifier
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AAI9108099
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identifier
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9108099
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Creator
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Ennis, Bryan Jerard.
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Contributor
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Adviser: Gabriel I. Tardos
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Date
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1990
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Language
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English
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Publisher
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City University of New York.
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Subject
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Engineering, Chemical | Engineering, Agricultural | Engineering, Materials Science
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Abstract
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The agglomeration of fine powders and other granular media is not a new problem, yet few underlying physical principles describing the phenomenon have been drawn. Successful granulation operation is largely a haphazard undertaking. The present research attempts to lay a rational foundation describing the mechanics of granulation by examining the process from a microlevel perspective. The strength of a dynamic pendular liquid bridge joining two relatively moving particles is seen to either control or have a major influence on the phenomena of granule coalescence, granule consolidation, and granule attrition. For the case of two particles approaching one another along their line of centers, the dynamic strength of the bridge is found to be given by a simple superposition of capillary and viscous lubrication forces as shown experimentally and justified theoretically by the method of matched asymptotic expansions. The influence of fluid inertia, wettability, power law behavior, particle surface roughness, and particle shearing motion are also discussed. Since the viscous contribution to bridge strength is singular in the particle gap distance, it typically dominates and allows us to neglect the capillary contribution. A dimensionless Stokes' number St{dollar}\sb{lcub}\rm v{rcub}{dollar} is then introduced which measures the level of relative kinetic energy between two colliding granules in relation to the viscous dissipation brought about by the bridge. The Stokes' number provides a convenient classification of granulation regimes. For the case of St{dollar}\sb{lcub}\rm v{rcub} \to 0{dollar}, the probability of coalescence is independent of particle kinetic energy and binder viscosity. In this regime, binder viscosity controls the rate of granule consolidation and, hence, also influences granule attrition by controlling final granule voidage. For the case where the maximum St{dollar}\sb{lcub}\rm v{rcub}{dollar} is of the order of St{dollar}\sbsp{lcub}\rm v{rcub}{lcub}*{rcub}{dollar}, increases in binder viscosity increase coalescence rate as traditionally expected. Here St{dollar}\sbsp{lcub}\rm v{rcub}{lcub}*{rcub}{dollar} is a critical Stokes' number being a known function of the volume of binder deposited on the bed. Finally for large St{dollar}\sb{lcub}\rm v{rcub}{dollar}, only granule coating is possible. Granulation and defluidization experiments supporting this simple classification of granulation regimes are presented. Recent attempts to apply fracture mechanics to the attrition of solidified granules are reviewed and applied to the binders of the present work. Last, some implications regarding successful granulation operation are drawn.
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Type
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dissertation
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Source
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PQT Legacy CUNY.xlsx
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degree
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Ph.D.
