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Crystallization is an essential solid-liquid separation technique which is widely used in the industry such as in pharmaceuticals, pigments, food, plastics, semi-conductors and others due to its capability to produce high quality of crystals. The final specification of the crystal product usually is given in terms of crystal size distribution (CSD), shape and purity. However achieving the target CSD for the case of batch seeded cooling crystallization is always a challenge due to the limited control on the production of fine crystals. Generally crystallization process is controlled to be operated in the metastable zone where the crystal particles will grow until the end of the operation. If the crystallization process is operated beyond the metastable concentration, excessive nucleation is expected due to the high supersaturation. As a consequence the target crystal product is able to achieve but there is unnecessary amount of fine crystals by the end of the operation. This unnecessary nucleation can be reduced by incorporating the crystallization process with dissolution phenomena where the temperature of the solution is controlled so that the solution is operated in the undersaturated condition. This condition is enabling the grown crystal particles to dissolve back into solution. Thus reducing the amount of fine crystals in the overall CSD. Therefore the objective of this study is to develop and evaluate controlled cooling for batch seeded crystallization of potassium nitrate incorporating dissolution phenomena. The mathematical model for potassium nitrate crystallization is developed and simulated in MATLAB software where it is validated against experimental data. The optimization algorithm is then developed in order to determine the set-point trajectory for closed­ loop control. Based on this optimal trajectory a closed-loop control is proposed to maintain the crystallization operation at its set-point using two types of controller which consists of Proportional (P) and Proportional-Integral (PI) controllers. Two important criteria are measured in the closed-loop simulation which are the performance of PI controller to follow the given set-point trajectory and the achievement of target CSD. Based on the closed-loop control it is shown that the PI control is more superior than P control by maintaining the operation at its set-point and successfully achieved the desired CSD. In addition the production of fines particle is greatly reduced indicating this new optimal trajectory is capable of producing the target CSD and minimizes the effects of nucleation.

Evaluation of controlled cooling for seeded batch crystallization incorporating dissolution