Seeding in Situ the Cooling Crystallization of Adipic Acid using Ultrasound

This study investigates the use of a short ultrasonic burst at the beginning of a cooling crystallization to achieve a growth dominated process and produce crystals with regular habit without the requirement of inoculation of seeds. The results of conventionally seeded crystallization experiments were compared with unseeded experiments where a short ultrasonic burst was applied before the beginning of cooling and was stopped a few minutes after the visible detection of the nucleation onset. A 10% of seed loading was chosen for conventionally seeded experiments, providing a unimodal size distribution and regular habit in the final product. Unseeded experiments where adipic acid was initially crystallized under insonation produced crystals with irregular habit and widespread size distributions. However, a short ultrasonic burst applied at the beginning of the cooling crystallization of adipic acid offers comparable advantages compared to conventional seeding, in terms of size distribution and a regular crystal shape, when approximately 20–40 wt % of solute initially crystallizes under insonation. Furthermore, the use of a short ultrasonic burst offers the advantage of regular hexagonal particle shapes in the final product, with reduced elongation than after conventional seeding, especially when cooling proceeds uninterrupted after the insonation is stopped. Increasing the sonication power provides more regular crystal habits and narrowed particle size distributions. Finally, a short ultrasonic burst applied exclusively at the beginning of the cooling was compared with an intermittent insonation regularly operated throughout the cooling. The results show that the same duration of an ultrasonic burst leads to smaller particles when distributed throughout the cooling.