Experimental Investigation of p‑Xylene Crystallization Characteristics and Ultrasound Enhancement Mechanism

Para-xylene (PX) is one of the most crucial aromatic raw materials in petrochemical industry. It is urgent to develop a more low-cost and efficient PX production process due to the growing demand for PX. This paper introduces ultrasound into the PX crystallization process and analyzes the data of supersolubility, metastable zone width, and induction period of PX under different working conditions. Based on the classical nucleation theory and the cluster coalescence model, the PX crystallization characteristics and the mechanism of ultrasound enhancement are investigated. The results can be concluded as follows. Ultrasound significantly reduces the metastable zone width and induction period of the PX crystallization process, speeds up the crystal nucleation rate, and prevents the explosion nucleation. When the supersaturation is above 1.055, the PX nucleation mechanism is homogeneous nucleation, and when it is below 1.055, the PX nucleation mechanism is heterogeneous nucleation. The crystal growth mechanism is continuous growth. The PX-dominated cluster at the mesoscale contains 14 monomers. The formation period of clusters is accelerated by ultrasound, and the critical nucleation radius and critical nucleation free energy of PX are both dramatically decreased. The system’s diffusion coefficient is improved by ultrasound, which also lowers the interfacial tension and the surface entropy factor corresponding to the two nucleation mechanisms. When the ultrasonic power changes from 0 to 88 W, the crystal nucleation rate constant increases by 22.4 times and the diffusion coefficient by 11.86 times. Theoretically, this study might enable future PX crystallization process improvements.