(167b) Modelling and Measurement of Granule Friability

Pneumatic Conveyance systems are used within pharmaceutical manufacturing sites to transport granular materials. Granule attrition can occur during the dilute phase transfer of materials which can reduce the particulate d50 by as much as 50% (Chapelle et al 2004). Under-lubrication can occur post pneumatic conveying due to a large increase in the granule surface area that can produce tablet elegance issues. Using an early phase friability model to understand the mechanical effects of conveying upon granules, will reduce granule robustness issues upon scale-up.

The particle size distribution (PSD) of a powder system is routinely measured by laser diffraction using benchtop equipment such as the Malvern Mastersizer (Malvern Instruments, Malvern, UK). The apparatus is provided with a dry powder feeder. The powder under test is fed using a vibrating feeder then suspended by a jet of compressed air. Increasing the air jet pressure has been shown to produce a proportional increase in volumetric surface area. It is hypothesized that more friable granules would have a larger change in surface area with jet pressure, and from this a quantitative measure of the granule friability has been developed.

Granule samples have been sized using the Malvern Mastersizer producing a linear increase in volumetric surface area between feed air jet pressures of 0.5 to 2 barg. The particle size data has been evaluated using a breakage model to evaluate the breakage rate constant Si. The breakage rate constant can be correlated with the volumetric surface area, with both showing a linear increase in relation to change in air jet pressure.

Fluent 6.1 computational fluid dynamics software has been used to develop a 2-D model for solid and gas movement within the Malvern Mastersizer. This has been used to determine the shear rate within the dry powder feeder. By running the model using the equivalent air jet pressures, a linear correlation between increasing shear rate and air jet pressures has been found. It is now possible to correlate the observed changes in particle size with the shear forces within the system. Consequently, the friability metric can be scaled to a production scale powder transfer system, and can be used to assess the suitability of a granule for larger scale processing.

Chappelle, P., Abou-Chakra, N., Christakis, N., Bridle, I., Patel, M.K., Baxter, J., Tuzun, U., Cross, M. (2004) Powder Technology, 143-144 321-330.