Abstract:

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 introduced ultrasound into the PX crystallization process and analyzed 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 were investigated. The results showed that ultrasound reduced the metastable zone width and induction period of PX crystallization process, speeded up the crystal nucleation rate and prevented the explosion nucleation. When the supersaturation was below 1.055, the PX nucleation mechanism was heterogeneous nucleation, and when it was above 1.055, the PX nucleation mechanism was homogeneous nucleation. The crystal growth mechanism was continuous growth. In addition to lowering the critical nucleation radius and critical nucleation free energy of PX, ultrasound also reduced the surface entropy factor and interface tension that correlated to the two nucleation mechanisms and raised the system’s diffusion coefficient. When the ultrasonic power changed from 0 to 88W, the crystal nucleation rate constant increased by 22.4 times and the diffusion coefficient by 11.86 times, respectively. This study might enable future PX crystallization process improvements.

Key words: para-xylene, crystallization, classical nucleation theory, thermodynamics, dynamics, ultrasound enhancement

摘要：

对二甲苯（PX）是石油化工中最重要的芳烃原料之一。随着对PX的需求日益增长，迫切需要开发一种成本更低、效率更高的对二甲苯生产工艺。本文将超声引入到PX结晶过程，分析了不同工况下PX的超溶解度、介稳区宽度和诱导期等数据。基于经典成核理论，探究了PX的结晶特性和超声强化PX的作用机制。结果如下：超声减小了PX结晶过程的介稳区宽度和诱导期，加快了晶体成核速率，并避免了爆发成核。当过饱和度低于1.055时，PX成核机制为非均相成核；当过饱和度高于1.055时，PX为均相成核。晶体生长机制为连续生长。超声降低了PX的临界成核半径和临界成核自由能，也降低了两种成核机制对应的界面张力和表面熵因子，同时提高了体系的扩散系数。当超声功率从0变化到88W时，晶体成核速率常数增大了22.4倍，扩散系数增大了11.86倍。所得结论可为PX结晶工艺的改进提供参考。

关键词: 对二甲苯, 结晶, 经典成核理论, 热力学, 动力学, 超声强化