Abstract
One of the most crucial processes in the chemical processing industry is crystallization, which has the major challenge of producing solid products with the required purity and properties. The standard crystallization methods have various processing limitations; hence, research into alternative methods like ultrasonic-assisted crystallization has been at the forefront. The present work investigates the application of low-frequency ultrasound irradiation (40 kHz) for improving the cooling crystallization of ascorbic acid. The impact of ultrasonic irradiation on crystal size, shape, and induction time has been studied, and a comparison with the conventional method has been made. The standing time had a minor effect on particle size on average, while the initial temperature had a far more significant impact. Photographic analysis using image-analysis software indicated that when ultrasound was applied to the saturated solutions, the resulting crystals were smaller and more uniform in size and shape. The resultant crystal grows predominantly in a single direction, resembling thin prisms. The use of ultrasound was also shown to reduce the induction time significantly. This ultrasonic impact is stronger at lower initial temperatures. Indeed, the smallest induction time of 300 s was obtained for sonicated solutions at an initial temperature of 65°C and ultrasonic power of 250 w. When different levels of ultrasonic power dissipation were tested, it was established that the time required for the first nuclei to appear decreased as the power increased. There was, nevertheless, a marginal impact on average crystal size.
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