(26f) Using Powder Rheology to Assess the Effect of Microcrystalline Cellulose Sphere Size on Performance in a Wurster Fluid-Bed Coater

A fluid-bed coater with a Wurster cylinder is a widely used unit operation for pellet coating. It is a circulating process and doesn't contain a fluid-bed in the traditional sense. Instead, a Wurster cylinder-containing (two-compartment) design is intended to reduce weight gain variability in comparison to a one-compartment design.

A Wurster cylinder contains four distinct zones: the coating zone, the expansion chamber, the downflow bed zone, and the horizontal transport zone. Individual particles are propelled into the coating zone via a high velocity air flow where droplets are sprayed from a nozzle to coat the particles. Once through the Wurster cylinder, the particles settle in the expansion chamber and into the down-flow bed. Between the bottom part of down-flow bed zone and coating zone is the horizontal transport zone. The horizontal transport zone is analogous, to some extent, to pneumatic conveying. The fluidised-bed region is only present within the horizontal transport zone and reduces the friction between particles, helping to convey them to the coating zone. The adjustable position of the Wurster cylinder (the distance between the air distribution disk and the bottom edge of the Wurster cylinder) controls powder flow rate.

The main objective of this study [1] was to investigate the impact of the particle size (d50 ranging from 94 to 424µm) of microcrystalline cellulose (MCC) spheres on the flow behaviour within a Wurster cylinder containing fluid-bed coater. Standard pharmacopeia methods were employed, as well as measuring dynamic and bulk powder flow properties via powder rheology (FT4 Powder Rheometer®, Freeman Technology Ltd, UK). Samples were also measured using alternate powder flow characterisation techniques the results from which are compared to powder rheology.

Specific powder rheology methods are proposed to characterise powder behaviour in the down-flow bed, horizontal transport and coating zones. Thereby providing an understanding of what powder properties could deliver optimum performance.

This study also underpins the requirement for multivariate characterisation when assessing powder flow. Meaning that several characterisation methodologies are required, the results from which can be correlated with process ranking to find the most influential parameters on process behaviour. Rather than relying on single number characterisation to describe behaviour across all processes, a multivariate approach using powder rheology allows for a direct investigation of a powder’s response to various process conditions.

[1] Mohylyuk et al., Journal of Drug Delivery Science and Technology, 54, 101320, 2019