Ultrasonic spectroscopy for in situ particle sizing and crystallization process monitoring

The application of ultrasonic techniques for crystal size distribution (CSD) and solid
volume concentration measurements in batch crystallization processes was
investigated in situ and on-line, with emphasis on the crystallization of the copper
sulphate pentahydrate crystallization system.
The fundamental crystal characteristics of copper sulphate pentahydrate including the
solubility, metastable zone width, and induction time were assessed. The kinetic
analyses carried out were independent of the acoustic measurement of the copper
sulphate pentahydrate and provided the research background for the acoustic study.
The potential of ultrasonic spectroscopy was demonstrated using an experimental
approach, focusing on the problem of cooling crystallization of copper sulphate
pentahydrate from an aqueous solution in a 20 litre scale reactor. The set up of the
physical properties matrix for the system allowed the ultrasound spectra to be
converted into the particle size and particle volume infonnation. Different cooling
conditions were optimized due to the limitations of the instrument. The overall growth
rate as a function of the relative solution supersaturation was determined from the
combination of particle size and solid concentration.
A new method derived from the Modified Urick equation focusing on the on-line
measurement of the crystal volume concentration without the phase separation was
built up, which was modified from the prototype applied in the non-soluble
oil-in-water system. The extended application on the copper sulphate pentahydrate
crystal volume concentration during the dynamic process was successfully developed.
The third generation prototype acoustic spectroscopy Alphasizer developed by
Malvern Ltd for on-line acoustic attenuation and velocity measurement was calibrated.
Particle size distribution was successfully calculated from the manually calibrated
attenuation spectra. The feedback closed loop supersaturation control based on
Alphasizer was designed. The control algoritluns and a corrununication system were
built up.