(284d) Controlled Manipulation of the Size and Shape of Needle-like Crystals in a Cyclic Process

Crystallization from solution leads to a wide range of particle sizes and shapes, which has an impact on various downstream processes. This emphasizes the fact that there is a need to develop processes and operating strategies to obtain equant shaped crystals with a narrow distribution. Experimental studies in the literature have often dealt with the manipulation of size, but not shape, due to the lack of commercially available devices capable of characterizing both the size and the shape of crystals. Recent advances in online size and shape monitoring¹, have enabled understanding fundamental phenomena and providing a means to explore complex size and shape modification processes. For instance, the cyclic 3-stage process, involving a growth, a breakage, and a dissolution stage, has been modeled and experimentally validated. Its purpose is to transform a needle-like seed population to equant shaped particles.^2,3 This process was previously operated at predefined operating conditions, and depending on the choice of these conditions and of the number of cycles, final product populations with different average aspect ratios (defined as the ratio of the average particle length to the average particle width) were obtained. However, operating a process in this fashion would not guarantee consistent product quality in terms of the final particle size and shape distribution (PSSD) due to the presence of unmodeled phenomena and process disturbances. This issue can be circumvented by integrating feedback controllers into the overall process.

The availability of a quantitative, online size and shape monitoring tool, the µ-DISCO,^1,4 and three control strategies that have been developed recently for the three stages of the process, enable operating the entire 3-stage process in a fully automated and controlled fashion. The controller for the growth stage drives seed populations to predefined target orthants under growth-dominated conditions.^5,6 The controller for the breakage stage reduces the average length of the particles – and thereby their aspect ratio – by subjecting them to breakage in a rotor-stator wet mill.⁷ Finally, the controller for the dissolution stage dissolves a certain fraction of the total solid volume by altering the temperature of the process, with the goal of dissolving fines.⁸ The robust performance of these controllers are successfully demonstrated in thorough experimental campaign.

These controllers are then implemented within the context of the 3-stage process to drive needle-like seed populations to different targets in the size and shape space. Experimental outcomes reveal that the process control strategy manages to repeatedly reach the same final PSSD. The experimental campaign clearly underlines the robustness of the controllers, which helped eliminating the need to specify operating conditions for individual stages. It is worth noting all the control laws operate by solely relying on the online monitoring capabilities of the µ-DISCO and on solubility data. To conclude, this study highlights the effectiveness of simple model-free feedback control strategies exploiting online monitoring to address issues related to manipulation of particle shape.

References

1. Rajagopalan, A. K.; Schneeberger, J.; Salvatori, F.; Bötschi, S.; Ochsenbein, D. R.; Oswald, M. R.; Pollefeys, M.; Mazzotti, M. A Comprehensive Shape Analysis Pipeline for Stereoscopic Measurements of Particulate Populations in Suspension. Powder Technol. 2017, 321, 479–493.
2. Salvatori, F.; Mazzotti, M. Manipulation of Particle Morphology by Crystallization, Milling, and Heating Cycles - A Mathematical Modeling Approach. Eng. Chem. Res. 2017, 56 (32), 9188–9201.
3. Salvatori, F.; Mazzotti, M. Manipulation of Particle Morphology by Crystallization, Milling, and Heating Cycles: Experimental Characterization. Eng. Chem. Res. 2018, 57 (45), 15522–15533.
4. Bötschi, S.; Rajagopalan, A. K.; Morari, M.; Mazzotti, M. An Alternative Approach to Estimate Solute Concentration: Exploiting the Information Embedded in the Solid Phase. Phys. Chem. Lett. 2018, 9 (15), 4210–4214.
5. Bötschi, S.; Rajagopalan, A. K.; Morari, M.; Mazzotti, M. Feedback Control for the Size and Shape Evolution of Needle-like Crystals in Suspension. I. Concepts and Simulation Studies. Growth Des. 2018, 18 (8), 4470–4483.
6. Rajagopalan, A. K.; Bötschi, S.; Morari, M.; Mazzotti, M. Feedback Control for the Size and Shape Evolution of Needle-like Crystals in Suspension. II. Cooling Crystallization Experiments. Growth Des. 2018, 18 (10), 6185–6196.
7. Rajagopalan, A. K.; Bötschi, S.; Morari, M.; Mazzotti, M. Feedback Control for the Size and Shape Evolution of Needle-like Crystals in Suspension. III. Wet Milling. Growth Des. 2019.
8. Bötschi, S.; Rajagopalan, A. K.; Morari, M.; Mazzotti, M. Feedback Control for the Size and Shape Evolution of Needle-like Crystals in Suspension. IV. Modeling and Control of Dissolution. submitted for publication.