Elsevier

Powder Technology

Volume 140, Issue 3, 25 February 2004, Pages 217-227
Powder Technology

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

https://doi.org/10.1016/j.powtec.2004.01.015Get rights and content

Abstract

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<n<0.85. We also propose an improved granulation process in which dry powders are pre-wetted before introduction into the main granulating device. This scheme has the potential to produce larger shear forces during wetting and binder introduction and thereby improve homogeneity and consequently final granule properties.

Introduction

High shear mixer-granulators are used extensively in the pharmaceutical and chemical industries to produce agglomerates from fine powders. The process, called wet granulation, uses a liquid species (binder) to hold particles together while they are moving around in the granulating vessel. Strong granules are formed upon drying of the wet, so-called “green” agglomerates. Shear and normal forces are generated in the powder by rotating tools called impellers and choppers. Impellers are large metallic arms usually situated at the bottom of the vessel that rotate relatively slowly and induce an overall motion of the powder inside the granulator. Choppers are much smaller tools mounted on the side of the machine that rotate much faster and are there to disrupt the flow, disperse binder and disintegrate large chunks of material that may form.

To understand the granulation process in depth and to use this understanding in scale-up and control of the process, it is necessary to assess the magnitude of forces (stresses) generated in such mixers and the strength characteristics of granules as they form. There is a tight correlation between these two since agglomerates form and grow to a certain critical size that is limited by the equilibrium of these forces. A theory based on this premise [1], [2] proved to be very beneficial in understanding the process. We use the existence of the stress equilibrium in the present paper to indirectly measure shear forces in the machine by employing test particles of known yield strength.

Efforts to directly determine stresses in the agglomerator are numerous but the most often used one is by introducing into the moving powder a stationary arm with strain gauges mounted on it (see, for example, [3], [4], [5] and references within). Both the magnitude and the fluctuations of stresses in the machine as moving powder hits the stationary arm are recorded. Unfortunately, since the geometry of the powder flow is not well defined, the measured signal is machine- and powder-specific. An attempt to measure stresses in a granulator where the geometry is well known was attempted by Talu et al. [13] by using a Couette-type device and deploying shear and normal stress sensors on the inner wall of the unit. In all these attempts, however, only the frictional and impact component of stresses acting on the wall (or a stationary insert into the powder) are measured. Stresses on actual agglomerates moving in the bulk cannot be estimated this way. Furthermore, since the wet mass is usually very cohesive and sticky at and beyond the granulation point (the point at which granules start to grow in size), deposits may form around inserts in the vessel and on the walls. These distort the measurement of stresses using the direct method.

We present in the current paper a method to produce “test” particles (pellets) that have the properties of a Bingham fluid with low viscosity, i.e., well-defined yield strength with a low dependence on shear rate. These test particles were introduced into real granulators while granulating a wet mass of powder and binder. The condition under which the test granules deformed, broke or were layered on the existing granules was determined. This qualifies as a bona fide, albeit indirect method to measure stresses in the granulator.

Section snippets

“Test” particles and their yield strength

Play-Doh® behaves as a Bingham fluid, i.e., exhibits a well-defined yield strength. We found that different colors and batches of Play-Doh® have different yield strengths. Using this observation, test particles (pellets) with different yield strength were prepared by extrusion through a 1/4-in. hole and by cutting the samples into equal 6–7 mm cylinders. A picture of typical pellets is given in Fig. 1. The pellets have the shape of small squat cylinders with their height approximately equal to

Measurement of shear stresses in the granulator

The theoretical basis to measuring stresses in the moving powder from the knowledge of the yield strength of well-defined test particles (pellets) is work on the deformation and breakage of agglomerates in a shear field [12], [13]. A comparison of theoretical simulations and experiments performed with Play-Doh® is shown in Fig. 6. In the above studies, it was demonstrated both theoretically and experimentally that when shear stresses in the powder equal the yield strength of the granules,

Results and discussion

Initial results indicated that shear stress was most sensitive to the amount of binder present. Data is given in Fig. 9 and, as seen, the shear forces are very low for small amounts of binder. Under these conditions, the shear stress was too low to be detected (the pellets stayed intact) since the weakest pellet had a yield strength of at least 25 g/cm2. At higher liquid content, the shear forces increased to the yield strength of the test pellet and a measurement could be made as described

Granulation scale-up

A series of three different size granulators were used to assess how the shear stress scales with granulator size. Fig. 11 shows schematically the 2-l Fukae granulator and the Fielder 7.5 and 25 l mixers (PMA 10-25). The 7.5-l Fielder machine was obtained by using an insert (mask) inside the 10-l standard mixer. Geometric similarity between the machines was achieved from scaling the total powder mass M by the nominal volume of the machines V as indicated by the manufacturer (and shown in Fig. 11

Conclusions

We showed that using pellets with well-defined yield strength, one could indirectly measure shear forces in a granulating mass of wet powder. We found that shear forces in a “High-Shear” mixer-granulator of the vertical type (Fielder) are actually not always high. Shear forces in the dry mix appear to be very low, below the strength of calibrated particles and hence not measurable by the method proposed here. The mixer has the “potential” to produce high shear forces, but these forces are

Acknowledgements

The authors wish to thank Drs. Russell Lander and Charbel Haber for all their help in using the rheometer to measure test pellet properties and to Drs. Michael Gentzler and Russell Plank for fruitful discussions.

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