Issue link: https://www.e-digitaleditions.com/i/922724
Tablets & Capsules January 2018 29 Several scaling rules have been proposed for scaling impeller speed in high-shear granulators. Some authorities advocate scaling with constant tip speed, while others suggest scaling with a constant Froude number (the ratio of centrifugal to gravitational forces in the granulator). Work by other researchers suggests scaling using constant shear stresses inside the granulator [3]. The researchers propose scaling using this relationship: ND n where N is the impeller rotational speed, D is the impeller diameter, and n is a constant (found in this work to be equal to 0.85, although this value is likely to be system- and formulation-dependent). Chopper speed The role of the high-shear granulator's chopper (if so equipped) isn't clearly defined. Depending on the bulk mixture's properties, it can either grow and densify agglomerates or have a role in granule breakage [4]. Increasing the granulator bowl's size will decrease the frequency of interaction between the granules and chopper. Scaling the chopper to have the same tip speed in this case may be a good idea, as the granules will experience similar impact forces. However, if other granulator geometrically similar to—that is, with the same height-to-diameter ratio—as the machine you're scaling up from. Choosing geometrically similar granulators generally means that scaling the impeller speed becomes a simpler process. Impeller speed Scaling impeller speed isn't a trivial exercise. The effect of impeller speed on granule size depends largely on your formulation's properties and on the process's dominant granulation mechanisms. In some cases, increasing impeller speed will lead to higher granule growth; in other cases, higher impeller speed leads to greater breakage rates and, hence, smaller granule size distribution. One of the most important things to consider is the flow pattern within the granulator. Most granulators should be run in the roping regime, where the powder bed moves in a toroidal flow pattern within the granulator (Figure 2a) [2]. If the impeller speed is too low, the granulator will operate in the bumping regime, where the surface of the powder bed moves very slowly, bumping in phase with the impeller blades' passing (Figure 2b). The roping regime is important for good mixing and liquid distribution within the granulator and has a major effect on the nuclei size. Figure 2 Flow regimes in high-shear granulators [2] b. Bumping regime (low impeller speed) a. Roping regime (high impeller speed) Choosing geometrically similar granulators make scaling the impeller speed a simpler process. Allendale, New Jersey 201-962-8200 • www.kraemerus.com Andre.Petric@kraemerus.com The nutraceutical and pharmaceutical industries rely upon Krämer to polish and convey capsules with the highest efficiency. T o learn more about how to IMPROVE the EFFICIENCY of your capsule filling lines, contact Kraemer U.S. Capsule Polishing Solutions from the Global Leader