Issue link: https://www.e-digitaleditions.com/i/819216
24 May 2017 Tablets & Capsules Blender. The blender discharges in a funnel-flow pat- tern, and as indicated by the fluidization segregation test, if the gas attempting to leave the feed hopper vents upward through the blender, a concentration of glycolate may rise to the top of the material remaining in the blender (Figure 10). Due to this funnel-flow discharge, we expect the active flow channel to empty first, with mater- ial sloughing in from the sides as the emptying cycle nears its end. This suggests that the concentration of glycolate increases by about 25 to 35 percent during the blender emptying cycle. Fortunately, the fluidization test indi- cated that the API will not undergo this type of segrega- tion and should leave the blender with good uniformity. We now have all the information needed to fully understand segregation associated with this blend. Zones where material flow generates inter-particle motion will induce sifting segregation. Since 51.8 percent of the seg- regation occurring in the material is due to sifting, zones of high inter-particle motion should be avoided. There are two distinct zones high in inter-particle motion in typical tablet filling systems. One zone is due to the flow behavior as material periodically cascades down a pile in a thin layer. This will create a significant velocity gradi- ent across the flowing layer, and the sifting that occurs in this zone will result in fine particles accumulating at the top of any piles. Thus, filling the vessel without allowing piles to form or minimizing the piles will reduce sifting segregation. Angle-of-repose segregation will also be minimized by the reduction of piles. These steps will eliminate 60 percent of the segregation problem. Minimizing pile formation, however, will have no effect on air-entrainment segregation, which could be mitigated by filling the feed system or receiving bin in a way that reduces freefall. Designing the feed system or receiving vessel to create a velocity profile that remixes material at the hopper outlet could also help. Feed hopper. Next, let's consider the segregation pat- tern that may exist in the feed hopper above the tablet press. It typically has a funnel-flow design, so material will not flow along the hopper walls during discharge until material at the center discharges. Only then will material near the walls slough into the flow channel and discharge. The test for pile segregation suggests that a low concentra- tion of API will form at the center of the bin and a high concentration will form at the side (Figure 11). Therefore, the API concentration will be lower than normal when the discharge starts and higher than normal when it ends. Now, suppose that baffles or other devices were added to produce multiple piles in the feed hopper. That would change the segregation pattern relative to the active flow channel (Figure 12), creating a more uniform average API concen- tration during discharge. This demonstrates that the filling mode can greatly influence segregation in this location. As an alternative—or in addition—the press feed hop- per could be modified to induce flow along the walls (mass flow). This is often touted by solids flow practition- ers as the sole means of solving segregation issues, irre- spective of the segregation mechanism. But if we were to apply mass flow to our example material, the conical hop- per would need to be steeper than 25.6 degrees. Figure 13 shows the expected API concentration of the material as it discharges from the original funnel-flow feed hopper and from a hopper designed right at the mass-flow limit (25.6 degrees). With the original design, API concentration is Figure 11 Expected API concentration in feed hopper above press Active flow Stagnant zone Figure 13 Expected concentration of API as bin empties Figure 12 Expected API concentration in feed hopper above press after addition of baffles 0 20 40 60 80 100 Percentage empty Concentration of API (%) 11 10.5 10 9.5 9 8.5 8 7.5 7 Funnel flow Angle = 25.6 deg Angle = 20.0 deg Angle = 15.0 deg