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Resolving Filling and Dosing Issues with Powder Rheology: Impact of Segregation on Wall Friction and Powder Flow of Pharmaceutical Products

Particulate matter often tends to segregate (or demix) by size, volume, density, shape and other properties. This usually unwanted effect can be problematic when, for example, excipient and active pharmaceutical ingredients demix, but can also affect how the powders flow. A change of flow properties can pose a challenge during processing and filling. Therefore considering the change of wall friction caused by segregation can be crucial for processing parameters.

Introduction

Segregation of powders with different particle sizes or different compounds is a constant problem in industrial processes concerning bulk solids and dosing. It is generally either of a mechanical nature or a product of fluidization (e.g. via pneumatic transport).

The mechanical effect is called granular convection, or more commonly, the Brazil nut effect, in which mechanical agitation of a granular medium segregates the particles, such that the largest particles end up on top. The opposite effect is observed in fluidized systems such as pneumatic conveyance systems, where larger particles are not carried in the airstream, so that segregation leaves them on the bottom of the system.

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