Dynamic monitoring of glycine crystallisation with low power ultrasound reflection spectroscopy

Crystallisation processes are ubiquitous in the food and pharmaceutical industries and the development of process analytical technologies (PAT) for on-line, in situ monitoring is essential to ensure process efficiency and to optimise product quality. Current PAT, many of which are based on electromagnetic waves, have a range of limitations including an inability to accurately measure opaque solutions. Low power (<10 W m−2) pulsed acoustic techniques, such as ultrasound reflectance and velocimetry, have the benefit of being non-material altering, affordable, non-invasive and can study opaque systems without any dilution. Here, we present an improved in-situ ultrasound technique which is corroborated with optical turbidity in the measurement of the MSZW of glycine in water with good agreement. A Mann–Whitney U test was conducted, and no significant difference was found in the measurement of the MSZW between the two techniques. Density data were used with velocity measurements from the improved technique to calculate adiabatic compressibility of glycine solutions, which is an important and useful physical property when studying phase transitions. A frequency space analysis has been performed on the acoustic time domain measurements, and the data suggest the presence of scattering bodies in the metastable zone.