Temperature control strategies for batch crystallisation : dynamic optimisation and experimental approaches

Abstract

The relevant advantages of seeding versus self-nucleating operation in commercial batch cooling crystallisation have been debated for many decades, especially in the Australian dairy industry. Auto-nucleated batch crystallisation is still commonly practised in the industry because it can be initiated at the highest possible supersaturation above the Secondary Nucleation Threshold (SNT) concentration. The focus of this paper is to use α lactose monohydrate as a case study to develop the optimal cooling strategies for initial supersaturation well above the SNT concentration. Dynamic optimisation, previously performed on the batch model to find the cooling temperature profile for pure lactose syrup at 39° Brix, 60° C, was revised for a pure concentrated feed at 48° Brix, 60° C with a condition that the solution supersaturation was not constrained. A new approach was developed, treating the initial equivalent diameter as an optimised variable to re-evaluate the growth rate constant of crystals, using the same nonlinear least square method applied for low concentrated syrup. With the new approach, the models closely predict the crystallisation performance of pure lactose syrup in self-nucleated and seeded batches. The prediction is not as good for impure lactose syrup due to the non-lactose impurities. Seeded batch cooling crystallisation gives better control of crystal size within the desired range than auto-nucleated batches for pure and impure feeds