Evaluation of controlled cooling for seeded batch crystallization incorporating dissolution

A process control strategy to obtain large product crystals in batch cooling crystallization is investigated through simulation studies. Optimal cooling profiles, which incorporate crystal dissolution by temperature raise, are obtained for various seeding temperatures, with the potassium nitrate/water system as an example. When dissolution of crystals is allowed, sensitivity of the resulting average crystal size to the seeding temperature may be diminished, but the open-loop controlled cooling is still vulnerable to operational perturbations such as shifts in solubility and feed concentration. Feedback control may be a prerequisite to improve the reproducibility of the product crystal size distribution. Feedback control schemes based on the second moment (mu(2)-control) and concentration measurement (C-control) are introduced. The reference tracking control manipulates the crystallizer temperature so that the sensor measurement closely follows the optimal trajectory obtained from the off-line optimization calculation. The early stage control scheme is proposed in which the mu(2) feedback control is applied only at the early stage of the batch, while the rest of the batch is left open-loop. Robustness of the control schemes is evaluated through extensive simulation studies, and it is found that the early stage mu(2) control is capable of providing robust performance despite its simpler implementation. (C) 2012 Elsevier Ltd. All rights reserved.