A Holistic Approach for Model-based Control of Crystal Size, Shape and Purity in Integrated Batch and Continuous Crystallization-Wet Milling Systems

Executive Summary
This report summarizes the main achievements during the year 2021 of the project with the aim of developing process systems engineering approaches for improved crystal size, shape, and purity control during crystallization processes. The successful crystallization process and system design requires an interdisciplinary effort, which ranges from population balance model (PBM) development of the system concept, through efficient implementation of model equations to soft-sensor development, which is required for the model predictive control (MPC) design as well. This report gives a deeper insight into these interdisciplinary development efforts, which also highlights the achievable improvements enabled by the combination of process modeling, high performance process simulation and optimization.
This year, the focus was on system integration both in simulation and experimentally. A one-dimensional (1D) model was used for process intensification and integration. A model for a cascade of multi-stage continuous crystallizers connected to a downstream wet mill and two ideal classifiers were developed and the effect of recycle on crystal size and purity was studied. An attainable region was found for crystal size by optimizing the process for feed, recycle and mill ratio and temperatures of the three crystallizers. A model was developed for tracking impurity in crystals and effect of recycle on impurity was investigated. Prior experience was used to correlate one dimensional (1D) crystallization model outcomes to bulk properties to use the 1D model for fast optimization of the process to obtain large crystals with short aspect ratio. This work poses an example to what must be considered and learned from experimental data to formulate process optimization routines or 2D model for more accurate shape control. Preliminary experiments were done on a small-scale integrated process including two crystallizers, a wet mill and use of recycle. A model for real classification process including separation efficiency was also proposed to later implement in the system and investigate its effect both through simulation and experiments.
Achieved Deliverables
1. Development of the overall model of the integrated system consisting of the continuous crystallization cascade, wet mill, ideal classifiers and recycle
2. Investigation of the effect of recycle on purity
3. Investigation of the effect of recycle on crystal size
4. Investigation of model-based crystallization design with milling for shape optimization
5. Preliminary experiment results for continuous crystallization with wet mill and recycle