Automatic Correction for Window Fouling of near Infrared Probes in Fluidised Systems

Near infrared (NIR) spectroscopy is a versatile, non-invasive and non-destructive tool that is often used for process monitoring in the pharmaceutical industry. Often, equipment window fouling or probe fouling of in-situ NIR probes occurs, leading to biased spectra and wrong interpretations (e.g. process-state estimation). Physical counter measures, including self-cleaning probes and geometrical considerations, are called for. This paper presents a mathematical solution to the problem of window fouling for an NIR-monitored process: by determining the distance to the particles, we established which part of the signal was missing owing to the coating accumulation on the probe window. The proposed approach is illustrated with the example of hot-melt coating in a fluidised bed, during which coating buildup on substrate particles was monitored despite window fouling.