Application of process modelling tools in the scale-up of pharmaceutical crystallisation processes

Crystallisations are frequent process steps in the manufacture of active pharmaceutical ingredients (APIs). They are the primary means of intermediate or product formation and separation to achieve the desired purity and form. These unit operations are complex processes which are difficult to control due to the interlinked chemical and physical effects. For example, chemical aspects such as salt and polymorph concerns are in the forefront of process research, but physical effects manifesting themselves on scale-up, due to equipment :influences, can be equally important for the successful outcome of a campaign. Several operational parameters, such as temperature or impeller speed, need to be understood and controlled to achieve constant desupersaturation, consistent narrow particle size distribution around the desired mean, minimal attrition, and homogeneous growth conditions. This paper focuses on the equipment influence on crystallisations; relating it to first principles with respect to heat and momentum transfer, analysing it with computational fluid dynamics (CFD), and demonstrating its process impact using examples from recent development work. Dynamic process modelling and CFD are state-of-the-art engineering tools to identify process requirements and match them with equipment capabilities. The work reported here demonstrates how a semiquantitative application of these tools can lead to a controllable, robust process in an existing plant despite the time and resource limitations usually encountered in the industry.