Risks of Powder Flow Obstruction in Hopper and Bin Discharge in Solid Dosage Form Manufacture should be Predicted Under The Active Stress State.

Discharge of powder from a hopper or bin is a common operation in solid dosage form manufacture. Powder flow obstruction during hopper/bin discharge, such as arching or ratholing, remains an outstanding risk and cannot be reliably diagnosed using the existing flow function coefficient-based method. In this study, we showed that the major principal stress (σ) at the bin outlet is required for an accurate prediction of powder flow obstruction risks. We noted that powder is susceptible to flow obstruction when the unconfined yield strength exceeds the stress facilitating powder failure. We presented a complete model to calculate the stress conditions and subsequently predict flow obstruction risks in hopper/bin discharge based on this criterion. The method was experimentally verified by hopper/bin discharge experiments encompassing 10 powder blends and 2 equipment systems. Importantly, we showed that the active stress state assumption should be employed for the powder flow obstruction prediction because σ is high and powder is more susceptible to flow obstruction. Prediction under the passive stress state can lead to significant under-estimation of flow obstruction risks. Therefore, the hopper design protocol, which assumes the passive stress state in arching prediction, should not be indiscriminately used toward pharmaceutical powder flow applications.