Measurement of granule layer thickness in a spouted bed coating process via optical coherence tomography

In this work a recently developed in-line measurement technique for monitoring of coating thickness was applied to a spray-coating process in a three-dimensional prismatic spouted bed. The layer thickness was quantified with an optical coherence tomography (OCT) sensor positioned in front of the observation window of the process chamber. The time-dependent layer thickness was measured directly and in real-time from the OCT images, with no chemometric calibration being needed. To validate the obtained data, additional off-line OCT measurements were performed with optical particle size measurements and micro-computed tomography (micro-CT). The off-line measurements were conducted with representative samples extracted at defined time points during the process. It is shown that OCT enables the correct layer thickness measurement on the scale of single particles. High variations in layer thickness were observed, which are probably caused by an inhomogeneous distribution of droplets on the particle surface and/or an insufficient spreading of the film. (C) 2019 Elsevier B.V. All rights reserved.