Investigation and modeling of the mechanisms involved in batch cooling crystallization and polymorphism through efficient use of the FBRM
Barthe, Stephanie Cecile
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Batch crystallization is used widely in the production of high-value added species. It is widely recognized that product properties, some of which may be related directly to the utility of the drug, and downstream processes, such as tableting, are influenced by crystal morphology, size, and shape. The ability to observe on-line the evolution of the population density and detect a polymorphic transformation would constitute a major asset in understanding crystallizer operation and the phenomena that influence product quality. Focused-beam reflectance measurement (FBRM) is among the process analytical technologies (PAT) that hold promise for enhanced monitoring of pharmaceutical crystallization. It is based on scattering of laser light and provides a methodology for on-line monitoring of a representation of the crystal population in either batch or continuous crystallization systems. Properly installed, the FBRM allows on-line determination of the chord-length density, which is a complex function of crystal geometry and is statistically related to the population density. A model based on the geometry of the crystal was therefore established to relate both densities and thus enable computation of the population density from a measured chord length density. The evolution of the population density as a function of time leads to the estimation of the supersaturation and therefore allows the determination of the systems kinetics. From there, the population balance can be solved. Paracetamol is a common substance which exhibit polymorphism and is mainly used as an analgesic and antipyretic drug. The developed model was here applied to batch cooling crystallization of paracetamol from ethanol solutions; this system was used to explore the utility of FBRM data in detection of the polymorphic transformations. As different shapes generate different chord length densities, a transition from one polymorphic form with one specific crystal habit to another can be tracked through the FBRM. The purpose of the present study is to use the FBRM to monitor the evolution of the crystallization process, develop a model describing the evolution of the process, and monitor polymorphic transformation. The end results would be the possibility to implement a better control of the crystallization process that would ensure that downstream processing and product quality meet expectations.