黏结剂对甲醇制烯烃中单颗粒催化剂内部反应过程的影响

Effects of binders on internal reaction process of single particle catalyst in methanol to olefin

【目的】以甲醇制烯烃(methanol to olefins, MTO)为代表的非均相催化反应在现代化学工业中占据着重要的地位,研究MTO过程中单颗粒催化剂的反应、传质与传热过程,有助于系统理解催化反应机理、优化催化剂设计,实现工艺升级。【方法】建立单颗粒催化剂颗粒模型、反应动力学模型以及传质与传热模型;采用COMSOL Multiphysics 6.0软件模拟MTO反应过程,分别由二氧化硅、氧化铝和高岭土作为黏结剂,建立3种SAPO-34分子筛催化剂颗粒模拟模型;分析3种催化剂颗粒内甲醇扩散和温度传导过程,分析黏结剂对酸性位点、多甲基苯和多甲基萘的浓度分布的影响,将催化剂颗粒内部的传热、传质过程与化学反应进行耦合。【结果】甲醇分子向催化剂颗粒内部的扩散过程中,在t=100 s时甲醇浓度分布基本稳定,从颗粒边缘处向中部、核心处甲醇浓度依次减小;以二氧化硅为黏结剂的颗粒在中心处、边缘处的热力学温度分别为656、 627 K,颗粒内部的甲醇扩散速度最快;在MTO反应过程中,3种颗粒内部的热力学温度均为由中心向边缘处递减;随着MTO反应时间的增加,3种催化剂颗粒的酸性位点浓度平均值逐渐减小,多甲基苯、多甲基萘的浓度平均值逐渐增大;以二氧化硅为黏结剂的颗粒边缘处的的酸性位点浓度降幅最大,然后依次是颗粒的中部和核心处,颗粒内多甲基萘的分布更均匀。【结论】以二氧化硅为黏结剂的催化剂颗粒内部温度最高、温升最快,有效促进了MTO反应的进行,有利于颗粒内分子筛的充分利用和多甲基苯和多甲基萘的均匀分布。

Objective Heterogeneous catalytic reactions,such as Methanol to olefins(MTO),play an important role in the modern chemical industry.The study of reaction kinetics,mass and heat transfer of single-particle catalyst in the MTO process is helpful to under-standing catalytic reaction mechanism,optimizing catalyst design and realizing process upgrading.Methods The particle model,reaction kinetics model and mass and heat transfer model of single-particle catalyst were estab-lished.COMSOL Multiphysics6.0software was used to simulate the MTO reaction process,and the SAPO-34zeolite catalyst particle model was established with silica,alumina and kaolin as binders,respectively.The methanol diffusion and temperature conduction processes within the catalyst particles were analyzed.The influences of binders on the concentration distribution of acid sites,polymethylbenzene and polymethylnaphthalene were studied,and the heat and mass transfer in catalyst particles were coupled with the chemical reaction.Resuits and Discussion The methanol quickly diffuses into the catalyst particles at the initial stage,and the methanol concentra-tion decreases from the edge of the particle to the middle and core of the particle at 100seconds(s)due to the large concentra-tion difference between inside and outside the catalyst particles,during the diffusion process of methanol molecules into the cata-lyst particles.The highest methanol concentration is foundinside the particles with silica as a binder,so the binder silica has the greatest influence on the diffusion of methanol in the catalyst particles.During the MToreaction process,the thermodynamic temperature distribution inside the three kinds of particles decreases from the center to the edge.The reaction temperature inparticles with silica as the binder is the highest,and the thermodynamic temperature within the particles reaches the highest val-ues at 100s,which are656K at the center and 627K at the edge of the particles,respectively.The usage of silica as the binder for catalyst particles is more conducive to promoting the reaction within the particles.With the increase of MTO reaction time,the average acidic sites concentration of the three catalyst particles gradually decreases,and the particles with silica as the binder have the fastest decreasing rate,and the acidic sites concentration at the edge of the catalyst particles decreases the most.The average polymethylbenzene concentration of the three catalyst particles increases gradually,with the lowest average polymethyl-benzene concentration in the particles used silica-based binder and the fastest polymethylbenzene concentration at the edge.The concentration of polymethylnaphthalene within the three catalyst particles increasesgradually,and the distribution of polymethy1-naphthalene within the particles is more uniform using silica as the binder.Conclusion Conclusion The mass and heat transfer processes inside catalyst particles formed by three kinds of binders and the influence of their chemical reaction processes are studied by numerical simulation.It is found that the MT0 reaction in silica particles is more fully carried out,which exhibits more utilization efficiency of acid sites and more uniform carbon deposition distribution,compared with other particles.