9 – Control of crystallization processes

Publisher Summary This chapter discusses some of the successful, currently practiced industrial control strategies and reviews the current understanding of relating process control technology to the requirements of automating crystallization processes. The effectiveness of the control strategy is not determined by the computational power of the control system or the user-friendliness of the operator interface, but by the ability to achieve and maintain the conditions in the process that meet the process objectives. Some of the process variables must be controlled by the configuration of the equipment, which is an important consideration during the mechanical design. The principal variables that must be controlled in crystallization are the solution supersaturation, the crystal surface area available for growth, and the nucleation rate. These are affected by multiple interacting secondary variables, which may be divided into two categories—equipment design variables and dynamic variables affecting the crystallization. Three additional considerations are required to successfully control crystallization process—local conditions rather than bulk, instantaneous rates of change rather than mean values that control the relative rates of nucleation, and crystal growth. Further, the response of the system to control changes is history dependent. Spontaneous nucleation that accompanies an excursion beyond the supersaturation metastable limit dramatically affects the surface area available for crystal growth and influences the product crystal size distribution (CSD) of a continuous process for several residence times or the final CSD of a batch process.

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