Sonocrystallization and crystallization with gassing of adipic acid

Sonocrystallization is widely used to control nucleation and, therewith, improve the product quality. For this purpose, power ultrasound is turned on within the metastable zone during the crystallization process. But the mechanism by which power ultrasound induces nucleation is still unclear. Efforts have been made to identify the mechanism. It is assumed that nucleation is induced due to the bubble surface provided acting itself as nucleation center so that the mechanism seems to be a heterogeneous one. To prove this assumption, gassing has been investigated to induce nucleation during batch cooling crystallization. The gas bubbles are just expanding not collapsing. This study focuses on the investigation of the crystallization behavior of adipic acid during sonocrystallization and, furthermore, during the crystallization with gassing. During sonocrystallization experiments the insonation period, the ultrasonic frequency and the initial supersaturation ratio were varied. The results show that the metastable-zone width can be reduced and the crystal-size distribution, which turns out to be bimodal, can be narrowed the longer the insonation period and the higher the initial supersaturation ratio is. The crystals formed were identified as agglomerates, whereas the agglomeration can be reduced by power ultrasound. The gassing experiments demonstrated that the crystallization behavior of adipic acid can be affected similar to the application of power ultrasound. It can be concluded that gassing is an alternative to power ultrasound.