ZSM-5分子筛结构对苯烷基化反应性能影响的数值模拟研究

doi:10.11949/0438-1157.20231130

摘要/Abstract

摘要：

以ZSM-5分子筛催化苯和乙烯烷基化为体系，建立并验证了一种三维各向异性扩散-反应数学模型，该模型考虑了分子筛形貌、孔结构和扩散各向异性的影响。模拟发现当分子筛颗粒体积和abc轴尺寸保持不变时，改变分子筛形貌并不会显著影响其表观活性，因而该情况下无须调控分子筛形貌；而当分子筛颗粒体积不变时，仅缩短b轴尺寸才能显著降低扩散阻力，提高催化剂表观活性，因而合成ZSM-5分子筛时缩短b轴尺寸最有利于扩散和反应。在分子筛颗粒中引入大孔可显著提高分子筛催化剂利用率和表观活性，保持大孔孔隙率不变时，减小大孔孔径更有利。存在最优大孔孔隙率以平衡扩散阻力和活性材料含量，当分子筛粒径4 μm、大孔孔径300 nm时，最优大孔孔隙率0.16，对应最高表观反应速率80.5 mol/(m³·s)。另外，由于分子筛内扩散各向异性的影响，当大孔取向平行于c轴时最有利。结果可为苯和乙烯烷基化ZSM-5分子筛催化剂乃至其他分子筛催化剂的设计优化提供一定的理论指导。

关键词: 苯烷基化, ZSM-5分子筛, 扩散-反应模型, 孔道, 扩散各向异性, 颗粒形貌

Abstract:

This paper uses ZSM-5 zeolite to catalyze the alkylation of benzene and ethylene as a system to establish and verify a three-dimensional anisotropic diffusion-reaction mathematical model. This model takes into account the influence of zeolite morphology， pore structure and diffusion anisotropy. The simulation results show that changing zeolite particle shape would not significantly affect the apparent activity when the zeolite particle volume and sizes in a-, b-, and c-axis keep unchanged, indicating modifying zeolite particle shape is unnecessary in this case. When the particle volume remains constant, only shortening the size in b-axis could importantly reduce diffusion resistance and then enhance apparent catalytic activity, indicating that decreasing the size in b-axis is most favorable for diffusion and reaction in the preparation of ZSM-5 zeolites. When the particle size is 4 μm and the large pore diameter is 300 nm, the optimal large pore porosity that balances diffusion resistance and active material content is 0.16, corresponding to a maximum apparent reaction rate of 80.5 mol/(m³·s). Besides, due to the influence of diffusion anisotropy, introducing the macropores being parallel to c-axis is most favorable. These results provide some theoretical guidance for the optimal design of ZSM-5 zeolite catalysts for benzene alkylation with ethylene and other zeolite catalysts.

Key words: benzene alkylation, ZSM-5 zeolite, diffusion-reaction model, pore, diffusion anisotropy, particle shape

中图分类号:

TQ 021.4

黄静茹, 陈佳轩, 张群锋, 阮晋, 朱来, 叶光华, 周兴贵. ZSM-5分子筛结构对苯烷基化反应性能影响的数值模拟研究[J]. 化工学报, 2024, 75(7): 2544-2555.

Jingru HUANG, Jiaxuan CHEN, Qunfeng ZHANG, Jin RUAN, Lai ZHU, Guanghua YE, Xinggui ZHOU. Effect of ZSM-5 zeolite structure on the reaction performance of benzene alkylation: a computational study[J]. CIESC Journal, 2024, 75(7): 2544-2555.

图/表 16

图1 网格数量对催化剂效率因子的影响

Fig.1 The effect of mesh number on catalyst effectiveness factor

图2 模型计算结果与文献数据比较

Fig.2 Comparison of the calculated results with the ones in the reference

表1 模拟所用参数

Table 1 Parameters used in simulations

参数	数值
温度T/K	653
压力P/bar	5
苯和乙烯摩尔比	5∶1
单胞密度/(mol·m^-3)	309.50
正反应速率常数k₁/s^-1	696
逆反应速率常数k_-1/s^-1	0.137
乙烯扩散系数D_E,0/(m²·s^-1)	99.5×10^-13
苯扩散系数D_B,0/(m²·s^-1)	2.0×10^-13
乙苯扩散系数D_E+B,0/(m²·s^-1)	1.6×10^-13

表2 不同形状ZSM-5分子筛颗粒的尺寸参数

Table 2 Structural parameters of the ZSM-5 particles with different shapes

条件	球形	立方形	药片形	棺形
颗粒体积一致
颗粒体积一致	l=2000 nm L=V/S=333 nm	m=1612 nm h=1612 nm l=1612 nm L=V/S=269 nm	m=1612 nm h=1612 nm l=1874 nm n=486 nm L=V/S=288 nm	m=1612 nm h=1612 nm l=2077 nm n=465 nm L=V/S=281 nm
颗粒表面积一致
颗粒表面积一致	l=2000 nm L=V/S=333 nm	m=1447 nm h=1447 nm l=1447 nm L=V/S=241 nm	m=1498.5 nm h=1498.5 nm l=1742 nm n=435.5 nm L=V/S=268 nm	m=1481 nm h=1481 nm l=1908 nm n=427 nm L=V/S=258 nm

图3 不同形状ZSM-5分子筛颗粒的效率因子（η）和表观反应速率（Rapp）

Fig.3 Effectiveness factors (η) and apparent reaction rates (Rapp) of ZSM-5 zeolite particles with different morphologies

图4 颗粒表面积一致时不同形状ZSM-5分子筛颗粒内部浓度分布（切块为各形状的1/8）

Fig.4 Concentration distributions of ethylene and ethylbenzene in the ZSM-5 particles with different morphologies when the particle surface area is consistent (the displayed parts are 1/8 of each shape)

图5 不同a、b和c轴长度（a×b×c）ZSM-5分子筛颗粒的效率因子（η）和表观反应速率（Rapp）

Fig.5 Effectiveness factors (η) and apparent reaction rate (Rapp) of ZSM-5 zeolite particles with different a, b, c-axis lengths (a×b×c)

图6 不同a、b和c轴长度（a×b×c）ZSM-5分子筛颗粒内部浓度分布（切块为各形状的1/8）

Fig.6 Concentration distributions of ethylene and ethylbenzene in the ZSM-5 particles with different a, b, c-axis lengths (a×b×c) (the displayed parts are 1/8 of each shape)

图7 分子筛颗粒粒径对立方形ZSM-5分子筛颗粒效率因子（η）和表观反应速率（Rapp）的影响

Fig.7 The effects of particle size on effectiveness factor (η) and apparent reaction rate (Rapp) of cubic ZSM-5 zeolite particles

图8 不同粒径的立方形ZSM-5分子筛颗粒内部浓度分布（切块为各形状的1/8）

Fig.8 Concentration distributions of ethylene and ethylbenzene in the cubic ZSM-5 particles with different particle sizes (the displayed parts are 1/8 of each shape)

图9 立方形ZSM-5分子筛颗粒中大孔孔径对效率因子（η）和表观反应速率（Rapp）的影响

Fig.9 The effects of macropore size on effectiveness factor (η) and apparent reaction rate (Rapp) of cubic ZSM-5 zeolite particles

图10 具有不同大孔孔径的立方形ZSM-5分子筛颗粒内部浓度分布（切面垂直于孔道方向并位于颗粒中间位置）

Fig.10 Concentration distributions of ethylene and ethylbenzene in the cubic ZSM-5 particles with different macropore sizes (the displayed planes are perpendicular to macropore direction and in the middle of the zeolite particles)

图11 立方形ZSM-5分子筛颗粒中大孔孔隙率对效率因子（η）和表观反应速率（Rapp）的影响

Fig.11 The effects of macroporosity on effectiveness factor (η) and apparent reaction rate (Rapp) of cubic ZSM-5 zeolite particles

图12 具有不同大孔孔隙率的立方形ZSM-5分子筛颗粒内部浓度分布（切面垂直于孔道方向并位于颗粒中间位置）

Fig.12 Concentration distributions of ethylene and ethylbenzene in the cubic ZSM-5 particles with different macroporosities (the displayed planes are perpendicular to macropore direction and in the middle of the zeolite particles)

图13 立方形ZSM-5分子筛颗粒中大孔取向对效率因子（η）和表观反应速率（Rapp）的影响

Fig.13 The effects of macropore orientations on effectiveness factor (η) and apparent reaction rate (Rapp) of cubic ZSM-5 zeolite particles

图14 具有不同大孔取向的立方形ZSM-5分子筛颗粒内部浓度分布（切块为各形状的1/8）

Fig.14 Concentration distributions of ethylene and ethylbenzene in the cubic ZSM-5 particles with different macropore orientations (the displayed parts are 1/8 of each shape)

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