Apriori design of a continuous fine-powder blending module is currently quite difficult due to potentially complex granular flow behavior that impacts all performance aspects: 1) ingredient feeder fluctuations 2) maximum mixer inlet flow, or 'choke point' 3) temporal mixer efficiency, for damping of inlet concentration fluctuations 4) spatial mixer efficiency, for reduction of the length scale of segregation at the inlet to a level that is below the final scale of scrutiny.

The performance of two horizontal, tubular continuous mixers was experimentally assessed for a moderately cohesive pharmaceutical powder. Colorimetry was used to determine the axial Peclet number by the response to a step input for a wide range of impeller speeds [0.2<Fr<6] and flow rates. An intriguing correlation was obtained and found to be useful for evaluating mixer designs. For this class of mixer, axial dispersion is fast, as compared to rotating drums, and radial/spatial mixing is highly efficient. Overall module performance was found to be predictable with simple white-noise transfer function analysis, given experimentally determined feeder variances, mixer residence time and an axial dispersion coefficient.