Crystallization kinetics of lactose recovered at sub-zero temperatures: A population balance model combining mutarotation, nucleation and crystal growth

Crystallization kinetics of lactose at sub-zero temperatures are modelled in this paper using a population balance model. There are two competing effects on the crystallization kinetics at low temperatures. On the one hand, mass transport and the rate of both mutarotation and nucleation are reduced, reducing the crystallization rate. On the other hand, supersaturation is increased, which increases nucleation and crystal growth and thereby increases the crystallization rate. To explore this phenomenon, the crystal growth of alpha-lactose monohydrate at sub-zero conditions was modelled, including the effects of mutarotation and nucleation. The model is compared with experiments. It was found that stirring did not have a significant influence, indicating that the process was not limited by mass transfer. Mutarotation had a significant effect; the model showed fast depletion of alpha-lactose; at high lactose concentrations, the mutarotation of beta-lactose could not keep up with this depletion.