超声强化对二甲苯结晶特性及调控机理实验

doi:10.16085/j.issn.1000-6613.2022-1899

摘要/Abstract

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

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

中图分类号:

TQ021.4

董佳宇, 王斯民. 超声强化对二甲苯结晶特性及调控机理实验[J]. 化工进展, 2023, 42(9): 4504-4513.

DONG Jiayu, WANG Simin. Experimental on ultrasound enhancement of para-xylene crystallization characteristics and regulation mechanism[J]. Chemical Industry and Engineering Progress, 2023, 42(9): 4504-4513.

图/表 15

表1 表面熵因子与晶体生长机理关系

表面熵因子f	生长机理	表面特性
f＜3	连续型生长	表面粗糙
3＜f＜5	二维扩散型生长	表面较光滑
f＞5	螺旋位错型生长	表面光滑

图1 实验装置图1—浊度仪；2—数显温度计；3—超声波发生器；4—双层夹套结晶器；5—恒温水浴装置；6—磁力搅拌器；7—搅拌子；8—计算机

图2 PX成核瞬间温度变化图

图3 有无超声时PX成核瞬间现象图

图4 临界成核参数变化图

表2 不同条件下临界晶核分子数

过饱和度	超声功率
过饱和度	0	44W	66W	88W
1.04	560	456	446	437
1.05	291	237	232	227
1.06	171	139	136	133
1.07	109	89	87	85
1.08	74	60	59	58

图5 PX溶解度及不同条件超溶解度曲线

图6 PX诱导期变化曲线图

图7 ln-2S与lntind关系图

表3 不同超声功率下界面张力值

功率/W	均相成核界面张力/mJ·m^-2	异相成核界面张力/mJ·m^-2
0	1.032	0.517
44	0.964	0.511
66	0.957	0.507
88	0.950	0.481

表4 均相成核及异相成核表面熵因子值

功率/W	均相成核表面熵因子	异相成核表面熵因子	均相f/异相f
0	0.368	0.184	2
44	0.344	0.182	1.89
66	0.342	0.181	1.89
88	0.339	0.172	1.97

图8 ln-2S与lnσ的关系图

表5 不同超声功率下各成核参数值

功率/W	斜率	lnσ= -2.75对应间距	成核级数
0	-5.77	6.163	5.77
44	-4.66	4.144	4.66
66	-4.57	3.620	4.57
88	-4.51	3.054	4.51

图9 成核级数及成核速率常数变化率曲线

图10 不同超声功率下PX分子扩散系数变化情况

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