Characteristics of pellet flow in a Wurster coater draft tube utilizing piezoelectric probe

The main objective of this study was to develop a piezoelectric probe for measuring the impact rate of particles to enable characterization of the shape of particle number density profiles and characteristic particle impact frequencies inside the draft tube of a Wurster coating chamber. Pellets from 800 μm to 900 μm were subjected to fluidization in a conventional and swirl Wurster coating chambers. Fast Fourier Transform (FFT) analysis of the raw signal obtained at four heights and three different radial distances from the draft tube centerline was performed for experiments in a conventional and a swirl Wurster coating chamber at different process parameters. Power spectra revealed three characteristic frequency bands for both chambers: 1–5 Hz, 5–7 Hz, and 12–14 Hz. Particle number density profiles demonstrated an evident difference between both chambers. In the conventional Wurster coater, the location of the maximum number of particle impacts was close to the tube center, whereas in the swirl Wurster coater the particle impacts were distributed rather uniformly. The difference in particle number density profiles for both process chambers can help to understand the difference in performance of both Wurster coater designs.

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► Piezoelectric probe to evaluate particle impact rate within the coater draft tube
► Three characteristic particle impact frequencies have been identified
► 1–5 Hz impact frequency band correlates with particle bed height fluctuations
► Particle number density profiles differ for two coating chamber designs