(225c) Catastrophic Failure of Agitated Filter-Driers Due to Triboelectric Charging of Certain APIs

Agitated filter driers, typically consisting of a dished vessel with a perforated plate, and jacketed walls, bottom dish, and impeller, are commonly used in the process of solid API isolation. This work will present the investigation of the physical processes underlying a catastrophic failure during the drying of a certain API at the pilot plant scale. Prior to the failure, the powder resistance to flow would increase dynamically during the course of the agitation resulting in an increase in the agitator torque (as measured by the hydraulic pressure required to maintain the rotational speed set point). Eventually, this increase in flow resistance would lead to a solid body-like rotation of the powder bed, translating the agitatorâ??s mechanical energy, through the non-slip contact at the dryer bottom, into shear on the filter cloth sufficient to tear it out of its retaining o-ring.

Additional experimentation at scale showed that this dynamic increase in flow resistance was repeatable across multiple batches, and was also evident in other APIs and intermediates. Further investigation identified tribo-electric charging as the underlying cause for this behavior. Characterization of the dependence of the phenomenon on the process parameters and cake properties allowed for the development of a procedure for the early detection of potential â??problemâ? compounds at the lab-scale.