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Monitoring the crystallization from solution of a reactive dye by ultrasound

Ultrasound has potential to be used as a process monitoring tool alongside other techniques, such as pH, conductivity and dielectric measurements, tomography and optical turbidity. This paper outlines the development of a small volume test cell designed to monitor crystallization from solution in real time using low power ultrasound. The associated software is described and a series of experiments that have been carried out on a crystallizing orange reactive dye (BASF plc) are presented. Ultrasound has been used to monitor the crystallization process and the measurements yield data that separate into two distinct groups. These correspond to whether the dye is stirred or unstirred whilst crystallizing. Optical microscopy shows that well-dispersed crystals or extended networks of closely associated crystals are formed respectively. It was not possible to determine the principal physical constants of the crystallizing material due to its nature, so detailed theoretical modelling of the crystallization could not be achieved. Nevertheless, the phenomenological results indicate that the technique has potential to be incorporated into rugged instrumentation for monitoring crystallization in an industrial process environment.