Monitoring the Dynamics of a Continuous Sonicated Tubular Cooling Crystallizer

For the first time, continuous ultrasound-assisted tubular crystallizer dynamics has been studied in pilot scale operation. It is vital to understand the process dynamics for continuous crystallizers as robust crystallization processes are developed. The mixing time using tracer impulses and experimental continuous crystallization runs to understand the performance of process variables, with focused beam reflectance measurement (FBRM), process video microscope measurements, and off-line particle size analytics, is reported. The model system was phthalic acid crystallization from water. The effects of ultrasound power and residence time on the crystallization dynamics and on the product crystal size distribution (CSD) were studied. The use of ultrasound was found to be vital in the crystallization operations in order to prevent pipe clogging. It was shown that an increase in ultrasound power improves mixing in tubular reactors, and it decreases the product CSD, most probably due to nucleation. Also, an increase in residence time decreases the CSD due to an increase in ultrasound irradiation time during crystal growth. The video-microscope-based relative backscatter index signal was more sensitive to crystallization startup in this system in comparison to FBRM chord lengths. The continuous crystallization process dead time was found to follow fluid mixing dynamics by monitoring relative backscatter index signals as the flow rate and US power were increased. The steady-state crystallization conditions were found to take more than 2 reactor residence times on the basis of continuous crystallization experiments.