Dynamic evolution investigation on crystal size and morphology of sodium sulfate based on a combination of ultrasonic and image on-line testing

Crystallization is widely used in the fine chemical processes of purification and separation, where the crystal size variation, together with the crystallization temperature, plays a crucial role. An online measurement system for the simultaneous characterization of crystal size and other morphological parameters was studied, in which an ultrasonic spectrometer and image analysis was effectively combined for the monitoring and understanding of key parameters during the entire crystallization of sodium sulfate solution. The change in crystallization temperature, crystal size, and morphology at different cooling rates and stirring rates were analyzed, which shows that the cooling rate plays a dominant role in the variation of the critical time of crystal precipitation and average crystal size. In the middle stage of the crystallization, the median diameter change obtained by the ultrasonic and image analysis is close together, with a relative deviation of less than 10%. While the aspect ratio is mainly distributed in the range of 1-1.6, the crystal size has a relatively stable growth rate. An ultrasonic spectrometer can be used to measure particle size distribution of higher concentrations compared with image analysis. The combination actually provides an effective complement to implementing straightforward testing during the relatively complete crystallization process.