Measurement of residence time distributions and material tracking on three continuous manufacturing lines

Over the recent decade, benefits of continuous manufacturing (CM) of pharmaceutical products have been acknowledged widely. In contrast to batch processes, the product is not physically separated into batches in CM, which creates a few challenges. Product release is done for batches that should have a uniform quality over time, materials need to be tracked along the line, and locations to reject product must be established. To enable these, the residence time distributions (RTDs) of all unit operations must be known. In this paper, three CM tableting lines, each employing a different granulation technique, were investigated. The RTDs of their main unit operations were characterized, utilizing different measurement techniques successfully. All of these RTD measurement techniques could have been performed in any of the lines. The differences were related to the techniques themselves. Overall, external tracer with in-line Near-Infrared detection or color tracer with video recording proved most usable techniques, with few limitations. The RTDs for full lines were calculated by convoluting the unit operation RTDs, which enables material tracking through entire lines. The lines exhibited both truly continuous and quasi-continuous unit operations. Quasi-continuous unit operations divide the material stream into lots that can be utilized for tracking and rejection.