Determination of the dynamic metastable limit for α-lactose monohydrate crystallization

Abstract

In lactose refining, crystal size distributions of lactose crystals can be controlled by two major kinetic steps: nucleation and growth. Nucleation is strongly related to the metastable zone width (MSZW), defined as the area between the saturation curve and labile zone. Inside the MSZW, crystal growth is fastest with least secondary nucleation. This study aimed to refine the metastable limit (ML) developed earlier for lactose crystallization, making it suitable for designing an industrial operation. The ML under continuous stirring and seeded conditions was measured experimentally. The microscopy data showed that the temperature of first nuclei detection was independent of the agitation speed. However, the spectroscopy data showed that, after the production of the first nuclei, the extent of secondary nucleation increased rapidly as the agitation speed increased. Thus, to effectively avoid secondary nucleation and promote growth, a lactose crystallization process should operate at minimal agitation rate, in the upper ML recommended in this study.