An examination of sonocrystallization kinetics of l-glutamic acid

The power ultrasound effects, the sonocrystallization kinetics and mechanism are investigated for cooling crystallization of l-glutamic acid (LGA) from aqueous solution.

Sonocrystallization experiments involving slow and crash cooling have been undertaken for the metastable zone width and induction time measurement. LGA nucleation kinetics was extracted using Nývlt’s method. The results revealed that application of ultrasound can effectively narrow the metastable zone width, significantly reducing the induction time, and accelerate the nucleation rate. The calculated critical nucleus size and interfacial tension suggested that ultrasound reduces the nucleation energy barrier to allow crystallization to occur readily. These effects became more obvious with the increase of ultrasound power.

The pressure upon the collapsing cavitation bubble was calculated along with the nucleation rateunder the collapsing pressure. In order to identify the mechanism, an approach was developed in the literature for calculating the ultrasound-induced nuclei number which was employed to establish the inter-relationship between the cavitation number and nucleation event. Whilst the theoretical calculation did not fully match the experimental measurement, the total induced nuclei number was found to be proportional to the cavitation issue; therefore, it still provides a potentially credible mechanism for illustrating the sonocrystallization process.

Studies on seeded crystal growth in the ultrasound field indicated that the effect of ultrasound irradiation on LGA growth depends on the supersaturation. The ultrasound increased the growth rate at low supersaturation, while it appeared to have no effect at high supersaturation. The corresponding growth mechanism is believed to be the 2-D nucleation growth. A population balance model was applied for the seeded growth process to predict the dynamic evolution of the particle size distributions that are validated by experimental measurements.

The influence of operating conditions on LGA polymorphism was also studied. Investigation of the ultrasound effect on polymorphism suggested that ultrasound favours the precipitation of the stable β-form by improving the surface nucleation of the β-form and hence increasing the transformation rate. The analysis of the LGA crystals produced proved that the variation of ultrasonic power and insonation interval can be utilizedto manipulate the particle size distribution and crystal morphology.