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