Secondary Nucleation of α-Glycine Induced by Fluid Shear Investigated Using a Couette Flow Cell

A Couette cell flow device was designed, and an experimental procedure was developed to enable a quantitative study of the effects of fluid shear on secondary nucleation using a fixed seed crystal under controlled supersaturation, temperature, and flow conditions. This approach excludes mechanical impact, which is often considered to be the principal source of secondary nucleation, for example, through crystal attrition. We found that secondary nucleation rates of α-glycine in aqueous solutions induced by fluid shear were very significant and about 6 orders of magnitude higher than primary nucleation rates at the same conditions. Secondary nucleation rates per seed crystal were found to be about 1 order of magnitude lower compared with the magnetically stirred vials investigated previously, where a single seed crystal was freely moving, and thus, its mechanical impacts could not be ruled out. Computational fluid dynamics was used to calculate the wall shear stress along the surface of fixed seed crystals placed in the Couette cell gap at rotation rates between 100 and 600 rpm investigated here. This approach allows relating the secondary nucleation rate to the wall shear stress so that quantitative models can be developed to capture the effects of fluid shear on secondary nucleation kinetics. Such models will then facilitate scale-up and transfer of secondary nucleation kinetics between various equipment used in industrial crystallization processes.

The table of contents graphic shows the nucleation rates recorded in small, magnetically agitated vials compared with those from the Couette flow cell in which a seed was held in place and exposed to laminar shear. Secondary nucleation rates are much higher than primary nucleation rates in both cases, which indicates the significance of secondary nucleation induced by fluid shear.