Guest Column: Powder Characterization Techniques For Hopper Design

By Tim Freeman, Freeman Technology

In coming editorials, I plan to take a step back from the application-focused approach of recent months and look at some of the broadest issues surrounding powder characterization in the pharmaceutical industry. As manufacturing comes under increasing scrutiny, I'm going to be exploring whether the sector has the powder characterization tools it needs, in place, to rise to new challenges. However, before I start to consider these bigger questions, I'd like to discuss a topic that provides a useful link: hopper design. Hopper design is clearly a very specific application, but for many the associated shear testing methodologies are their first — and for some, only — introduction to the world of powder characterization.

Getting powders to flow consistently from hoppers, at a controlled rate, is an issue for many powder processors. Frequently encountered problems include: erratic flow/stoppages, flooding (uncontrolled flow), segregation, and funnel flow/ratholing (flow through the core of the hopper with an outer stagnant layer). Successful hopper operation relies on an efficient match between the in-process material and certain attributes of the hopper: material of construction, half angle (the steepness of incline of the hopper walls), and outlet size. Smoother materials, steeper half angles, and larger outlet sizes all tend to promote flow.