Evaluation of an in-line NIR spectroscopic method for the determination of the residence time in a tablet press

In the current study, the ability to use in-line NIR inside the feed frame of a tablet press to monitor the residence time distribution inside the tablet press was investigated. Pulse-response experiments were performed. In-line measurements inside the feed frame were compared to measurements on the actual produced tablets and the influence of different tableting parameters on the concentration profiles were studied. Turret speed had a major influence on the concentration profiles and RTD parameters. The in-line and off-line concentration profiles did not align with each other as spiking material was still detected in the tablets when no spiking material was observed in the powder blend by the in-line measuring method. The cause of this mismatch between the in-line and off-line curves was the in-line measuring position. By adjusting the position of the NIR probe to the bottom of the feed frame instead of at the top of the feed frame, a perfect alignment between in-line and off-line concentration curves was acquired. In-line measurements at the bottom of the feed frame could, if optimisation to avoid probe fouling is possible, be ideal for real-time measurement and control of the blend concentration during tableting. When probe fouling cannot be avoided, the correlation between the in-line measured concentration at the top of the feed frame and the concentration of the produced tablets should be investigated and modelled to enable real-time measurement and control based on in-line measurements.