Limitations of sensors for the on-line measurement of solid-phase properties have restricted the development and implementation of improved design, monitoring, and control methods for crystallizers and other particulate processes. Impage analysis has recently become a popular technique to monitor crystal size and shape in crystallization processes. Plummer and Kausch (1995) used image analysis to measure the real-time crystal size density (CSD) of polyoxymethylene as it crystallizes on a static stage under a microscope. Monnier et al. (1997) use image analysis to measure the final CSDs of adipic acid in water crystallization. Puel et al. (1997) use image analysis to measure twocharacteristic lengths of hydroquinone crystals as a function of time. The technique is not yet fully automated, however, and the crystal slurry is manually sampled and placed under the microscope. In addition, imaging requires an operator to occasionally interrupt automatic sizing and discard measurements before taking more samples. In their review, Braatz and Hasebe (2001) discuss the recently available in situ Lasentec particle and vision measurement (PVM) system. This sensor provides two-dimensional images of crystals in randomorientation, however, the authors state that the quality of the images limits the ability of imaging software to automatically identify individual particles but is suitable for qualitative troubleshooting. Image analysis has not yet been demonstrated to automatically monitor in real time the size and shape characteristics of crystals in a suspension crystallizer. The strong advantage of image analysis in the preceding applications, however, is that it requires almost no assumptions about particle size or shape and does not require inversion of a signal and inference of the resultant particle size or shape density. Although image analysis is a direct observation technique, it is a two-dimensional measurement and care must be taken if one is to infer a three-dimensional shape.
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