CONTROL STRATEGIES FOR BATCH CRYSTALLIZATION PROCESSES.

摘要

Batch crystallization is a key step in the production of pharmaceuticals, fine chemicals and food products. To date, only a few studies have been concerned with the question of open-loop strategies for control of the Crystal Size Distribution (CSD) and none, with feedback control strategies. To provide general guidelines for CSD control in batch systems, the author sought, in this study, to accomplish the following objectives: (1) the development of algorithms to calculate the operating condition sequence such that specific criteria or performance indices for the product CSD are satisfied (2) the identification of feedback control strategies to ensure attainment of a specified performance index (3) the design of estimators to overcome the limitations of currently-available on-line measurement instrumentation.The method developed in this study for identification of crystallizer control strategies may also be valuable for control of certain batch reactors such as emulsion polymerization or fermentation systems.Two approaches were adopted to achieve the first objective: the distributed system approach described in Chapter II and the lumped system approach in Chapter IV. The distributed approach involved solving the population balance equation by the method of characteristics. The algorithm derived from this approach permits calculation of the operating condition sequence such that a predetermined specific CSD will be obtained at the end of the crystallization process. The lumped approach is based on the transformed moment equations. Here, the Continuous Maximum Principle was applied to extremize various performance indices expressed in terms of the moments. A feedback control strategy has been developed (Chapter V) to maintain a specified performance index at its nominal value by a one-step iteration method. Finally, on-line measurement, estimation and identification studies, which can be considered a practical application of the distributed system approach, have been carried out in Chapter III.