Prediction and control of crystal shape distribution in the presence of crystal growth modifiers

The crystal product quality, functionality and properties such as flow characteristics are greatly influenced by the crystal size and shape distributions. Hence, prediction and control of crystal size and shape distributions are important from the viewpoint of smooth operation and product quality control. The contribution of this paper is twofold: firstly, it describes the modelling and simulation aspects for prediction of crystal shape distribution in the presence of crystal growth modifiers (CGMs). Secondly, a control setup is proposed that manipulates the CGM concentration in the crystallizer in order to achieve the desired crystal shape. The effect of CGM on crystal shape is modelled using a morphological population balance model (PBM) in combination with Kubota-Mullin's model for the pinning mechanism of CGM molecules on the crystal face to take into account their possible hindering effect on crystal growth (Kubota and Mullin, 1995, Kubota, 2001). Subsequently, a simple feedback control configuration is implemented in the proposed control setup which manipulates the CGM concentration profile in the crystallizer. The control setup uses a hybrid continuous-batch operation of the crystallizer, in which the solid phase is operated in a batch mode, whereas the liquid phase is operated in a continuous mode maintaining a constant volume in the reactor. This hybrid operation allows the dynamic control of the CGM concentration during the batch. Using potassium dihydrogen phosphate (KDP) as a model system, the simulation results demonstrate that, in principle, the crystal shape distribution can be controlled by manipulating the CGM concentration profile.