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Tablets & Capsules May 2017 25 low when discharging begins and grows higher when the hopper is nearly empty. For this feed hopper and material, funnel flow is a very bad choice. But a feed hopper at the mass-flow limit also results in a low API concentration at the beginning of discharge. Mass flow—flow at the walls—is important, but it may not be sufficient to solve segregation in a feed process. In this case, preventing seg- regation is all about velocity profile control. The velocity within a mass-flow device or vessel must match the segregation pattern in the vessel. For example, if segregation in a feed device is present from top to bot- tom, it would be illogical to induce perfect plug flow (a type of mass flow with a uniform velocity profile) in it because that would preserve the segregation pattern in the discharge. The material on the bottom would empty first followed by the material at the top. That's why a non- plug-flow velocity profile is sometimes wanted, so long as all the material flows upon discharge. The non-plug-flow velocity will tend to blend material in different parts of the feed device, and the right mass flow velocity during discharge will help maintain uniformity. Indeed, some- times both modes of segregation prevention—blending and uniform flow—are needed to minimize segregation. To illustrate this, Figure 13 also plots the expected API concentration profiles of material leaving the feed hopper if its mass-flow cone has a slope of 20 and 15 degrees. Figure 14 shows the velocity profiles for four cones of different angles and the profile of the 25.6-degree cone is very steep, almost as steep as the funnel-flow profile. But the velocity profile in the 15- and 20-degree mass-flow Figure 14 Expected velocity profiles in feed hopper 20-degree mass-flow cone 15-degree mass-flow cone Funnel-flow bin (orignial hopper) 25.6-degree mass-flow hopper (at mass flow limit)