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Tablets & Capsules May 2017 21 Segregation during pile formation To assess how segregation occurs when the blend forms a pile, we need another segregation test protocol. To start, a thin slice of a bin or hopper is filled with material to form a pile. As the pile forms, a thin layer of material slides down the top of the pile's surface and cre- ates zones where the powders exhibit a lot of inter-parti- cle motion. There is a stagnant zone below this sliding zone. The fines could pass through the voids between coarse particles during sliding and get stuck in the void structure of the bulk material in the stagnant zone. This generally causes fines to accumulate at or near the top of the pile. In addition, air currents induced by the material freefalling into the vessel often carry fines to the vessel's edge, causing them to accumulate at the wall. Differences in the frictional characteristics of the par- ticles could cause some of them to slide faster down a pile, separating particles that have different repose angles. Differences in the coefficient of restitution of the particles could cause the more resilient particles to bounce farther down the pile, resulting in segregation due to the elastic properties of the particles. In a process, all these mechanisms can occur simultaneously. Therefore, the test protocol must simulate, to the extent possible, the expected behavior in the actual filling process. The protocol should provide data about the magnitude of segregation, its pattern, its intensity due to pile formation and, if possible, the root cause(s) of segre- gation ingredient by ingredient. Furthermore, by control- ling the flow rate and drop height into the test equip- ment, you can correlate data obtained from the test to real process behavior. Using the same spectroscopic tester [1] from the flu- idization test, a pile is formed in a slice model test cell, and the segregation pattern is observed from the side. Next, NIR spectral techniques are used to measure the concentration profiles of all the ingredients down the pile. When analyzing the example blend, which contains 10 percent API, we observe a significant amount of it seg- regates (figures 3 and 4). This segregation concentration profile is expressed as a function of a dimensionless radius, where 0.0 represents the top of the pile and 1.0 represents the bottom. The pile segregation data suggest that the API concentration is moderately high at the pile's top, decreases halfway down the pile, and becomes high again at the bottom. This curious relationship sug- gests multiple segregation mechanisms may be acting on the material during pile formation. The gycolate accumulates at the top of the pile, while its concentration is lower at the bottom of the pile. However, there is a slight increase near the bottom of the pile. The segregation behavior of the MCC and lactose, which are bulking agents, is interesting: When the concen- Figure 5 Typical patterns of segregation due to various mechanisms Angle-of-repose segregation Air-entrainment segregation Sifting segregation Coarse Coarse Fines Fines Fines