Advanced Continuous Crystallizer Control

A variable speed positive displacement pump can minimize crystal attrition. The speed range must satisfy the flow requirements during start up and transients. Below a critical speed, crystals will settle out of the slurry. The minimum pump operating speed should be clamped above this rate for normal operation, and a line flush interlocked to stopping the pump for emergency shutdown. 

Having discussed many of the practical issues of operating and controlling continuous and batch industrial crystallizers in Section 

A considerable amount of academic research in the area of advanced crystallizer dynamics followed the development of the population balance crystallizer model (Randolph and Larson 1962 Hulbert and Katz 1964). Much of this work was primarily motivated by the occurrence of limit-cycle behavior in continuous 

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Advanced control strategies require a model that accurately represents the behavior of the process. Model identification involves determining an appropriate model structure, performing experiments, collecting data that allow identification of model parameters, and estimating the parameters. There are several ways to model crystallization processes, but a review of parameter estimation is beyond the scope of this chapter. A discussion of the most relevant methods of model identification for continuous crystallizers is given below. 

A clear-liquor advance operation is one in which the overflow liquid is continuously removed from the tank. The overflow is not actually clear but contains small crystals that have not settled in the overflow section. This method is a simple way of controlling the CSD, because the residence times of the clear mother-liquor overflow and crystal are different. The flows are separated and the excess nuclei are removed with the overflow. The nuclei in the overflow can be used, for example, in the following crystallization unit. The larger the clear-liquor flow for a given feed flow, the longer the residence time of the crystals in the product stream. 

Many liquid crystal phase transitions involve broken continuous symmetries in real space and their interactions on a molecular scale are short range [1]. As a result, fluctuations have long been known to be an important feature of liquid crystal phase transitions even weakly first order (discontinuous) ones. Compared to major advances in our understanding of fluctuation controlled second-order (continuous) phase transitions, relatively little is known about fluctuation phenomena (critical phenomena) at first-order phase transitions such as the nematic-isotropic transition. 

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