Resolving Powder Flow Problems

Interphex, New York, NY (Apr. 25)-Problems caused by the presence and movement of air in pharmaceutical powders are increasing, and many times these problems can be avoided. James K. Prescott, senior consultant at Jenike & Johanson (Tyngsboro, MA, www.jenike.com) reviewed these problems in his presentation, “Interstitial Air Effect on Powder Flow,” at the Wednesday conference session at Interphex.

“Pinpointing the cause of flow-stoppage problems and how they relate to how air can move through powders can be quite counterintuitive and quite difficult to understand,” said Prescott.

Prescott defined two-phase flow, which relates to the concept that powders integrate the characteristics of a solid (particles) and of a gas (interstitial air or air between the particles). These two phases affect compressibility, density (bulk and tapped) measurements, and permeability (a function of bulk density). The correlations among these parameters become important especially as manufacturers make observations related to container filling and the powder settlement.

Some of the implications related to settlement include the observation of “ant hills,” or “volcanoes,” sloshing of the powder, or liquid-like parameters. Other implications include overestimation of bulk containers, drums, or blender capacities. In addition, materials that are not deaerated might “flood.” As Prescott observed, the biggest problem is that the pressure drop resulting from settlement can lead to more-coarse materials present at the bottom of a container and more-fine materials moving to the top, which affects content uniformity and the quality of the product.

Prescott also reviewed flow patterns. In particular, funnel flow is more likely to yield flow-pattern problems such as material being stationary at the sides of the container and causing rat holes in the middle. Mass-flow patterns, however, do not cause rat holes and result in a consistent feed at a uniform density. Segregation also is generally minimized with mass-flow patterns. “The downside to mass flow, however, is that you need a smooth enough and deep enough hopper,” said Prescott.

Prescott finished his presentation with an overall approach to reducing powder-flow problems, including implementing mass flow for better powder control, possibly needing larger outlets, being aware of pressures and vacuums, slowing down the process (which, as he acknowledged would be a costly solution), adjusting the particle size if possible, and adding air permeation.