A novel method of quantifying the coating progress in a three-dimensional prismatic spouted bed

An optical method of experimentally quantifying the coating progress in a laboratory prismatic spouted bed is presented.

Microcrystalline cellulose particles (Cellets) with size d_p = 0.57 mm are used as core material. Waterborne coating suspensions with methylene blue as a dye are used and sprayed via a two-fluid nozzle in bottom-spray configuration. A high-speed camera with a color filter is positioned in front of the observation window of the three-dimensional plant and records the flow pattern in the process chamber with a certain frequency. With increasing surface coverage, the particles become bluer and darker.

The change in brightness is detected by digital image analysis. In the implemented algorithm, the spherical particles are detected, cut from the background and normalized regarding their lighting and area. The detection of coated pixels on the single particles allows the calculation of the coating fraction and coating uniformity after a Kalman filter has been applied to reduce noise.

It is shown that the method can be used to determine the time needed to reach the maximum blue value indicating a complete coating of the particle bed. Besides the minimum coating duration, information on the spraying performance (e.g., the time point of a possibly occurring blockage of the nozzle) can be obtained from the data.

Two exemplary experiments are presented, showing a connection of the slope of the coating fraction and the uniformity of the coating of the particles in the apparatus that results in the recommendation of a slow liquid injection for obtaining consistent coating layers as desired in many applications. More on quantifying the coating process in a prismatic spouted bed