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Stress measurements in high-shear granulators using calibrated "test" particles: application to scale-up

journal contribution
posted on 2004-02-25, 00:00 authored by G I Tardos, Karen HapgoodKaren Hapgood, O O Ipadeola, J N Michaels
Granulation is a process by which fine powders are agglomerated into larger particles using a liquid binder. In high-shear granulation the powder-binder mix experiences intense agitation inside a mixing vessel as binder is dispersed and granules form and strengthen under the influence of shear and compacting forces in the device. It is an implicit assumption that in a "high shear" mixer, large forces are transmitted to the powder and this results in a short and efficient granulation process. Owing to these desirable characteristics, high-shear granulation was adopted by several industries including pharmaceuticals and detergents where the process is used almost exclusively. In the work reported here, we attempt to measure shear forces in a moving powder inside a mixer-granulator. The method is based on previous numerical simulations [Powder Technology 110 (2000) 59] and experiments [Journal of Fluid Mechanism 347 (1997) 347] where we showed that at equilibrium between stresses in the mixer and the yield strength of the particles, granules attain a characteristic elongated shape. The measuring method adopted is indirect in the sense that pellets with well-defined mechanical properties were used to interrogate forces inside the granulating vessel at the point where they attain their characteristic elongated shape. We subsequently used the condition of equal shear forces in the device as a scale-up criterion so as to preserve the magnitude of stresses at both scales and thereby to expose forming granules to similar forces in both the small- and large-scale machines. We found that shear forces in a "High-Shear" mixer-granulator with a vertical axis (Fielder) are actually not always high. The mixer has the potential to produce high shear forces but these forces are transmitted to the powder mass only if the powder is sufficiently cohesive or becomes cohesive due to binder addition. Shear forces in the granulator are strongly wet-mass-dependent and they increase rapidly as soon as a "granulation limit" is achieved, i.e., at the point where granules start to form in the shearing powder mass. We found that granulators with geometrically similar bowls can be scaled to generate comparable shear forces by decreasing the impeller rotational speed of the large machine by the factor (D/d)n, where D and d are the impeller diameters of the large and small machine, respectively, and n is a scaling index that depends on impeller geometry but not on wet mass properties. For the equipment studied in this work, the coefficient n was obtained as 0.80

History

Journal

Powder technology

Volume

140

Issue

3

Pagination

217 - 227

Publisher

Elsevier

Location

Amsterdam, The Netherlands

Start date

2012-01-01

End date

2120-01-01

ISSN

0032-5910

Language

eng

Publication classification

C Journal article; C1.1 Refereed article in a scholarly journal

Copyright notice

2004, Elsevier