Energy efficient crystallization of paracetamol using pulsed ultrasound

This work studies the use of pulsed ultrasound during cooling crystallization of paracetamol. The effect of the pulse time on the nucleation temperature, crystal size and shape was evaluated and compared to silent conditions and continuous sonication. Most work is performed in a batch crystallizer, though some preliminary data in a recirculation configuration is also provided. In both setups, the nucleation temperature increased by at least 8 degrees C when ultrasound was applied compared to the non-sonicated case. When ultrasound is switched on more than 10% of the time, a similar nucleation temperature as with continuous treatment is obtained. At this minimal pulse setting, a bubble population, consisting of both oscillating and dissolving bubbles, is present in the vessel at all times. The pulse threshold can be validated using bubble dissolution calculations, and its existence leads to a vast reduction in ultrasonic energy consumption compared to continuous sonication. Finally, this work shows that the final particle size of paracetamol can be controlled in the batch setup by the pulse conditions, without affecting the crystal shape. The recirculation system shows a similar response, although further validation is recommended. (C) 2017 Elsevier B.V. All rights reserved.