丙烯聚合建模研究:扩散作用的影响

MODELING OF PROPYLENE POLYMERIZATION: EFFECT OF DIFFUSION

提出了均匀分布多粒模型 (UMGM) ,用于研究单个聚丙烯粒子的增长过程。在不考虑催化剂多活性中心和失活的情况下 ,扩散作用能够在较大范围内解释丙烯聚合过程中分子量分布以及反应速率的变化。分析了扩散系数、催化剂的活性以及催化剂颗粒大小对反应的影响。仿真结果表明 ,扩散作用对高活性催化剂的影响更加显著 ,并且与催化剂粒子的大小有密切关系。本模型能够方便地扩展到多活性中心以及采用更加复杂的微观反应动力学方程 .与其他单粒子模型相比 ,UMGM模型参数物理意义明确 ,计算速度快 ,为工业反应器的建模和优化奠定了基础。

There are some puzzles in the polymerization of propylene on heterogeneous Ziegler-Natta catalysts. The broad molecular weight distribution (MWD) of polypropylene might be caused by two reasons. The first reason is the diffusional resistance by polymer around the active sites. The second reason is the presence of more than one kind of active sites on Ziegler-Natta catalyst. Both of the two reasons have been tested experimentally, but no general explanation has been generally accepted. A new kinetic model, namely Uniformly-distributed Multigrain Model (UMGM) is proposed to explain the broad MWD and reaction rate during the growth of a single catalyst-polymer particle. The model is based on experimental data and actual plant operation results. At the beginning stage of polymerization, a catalyst macroparticle breaks into thousands of uniformly-distributed primary particles. These microparticles are all identical in radius and uniformly-distributed in the growing macroparticle throughout the whole reaction. Monomers diffuse through polymer blank and react on the microparticles surface to form new polymer. Discussions are concentrated on diffusion effect and it is supposed that only one type of active site is distributed on catalyst surface. The model can account for multiple-active sites quite easily. Some simulation results are given to compare the physical and chemical reasons for broad MWD. The influences of catalyst's particle and subparticle size, catalyst's activity and monomer diffusion coefficient on polymerization are analyzed.The results show that the degree of polymerization (DP) and polydispersity index Q have strongly antisymmetric distribution under different diffusion coefficient D. D iffusion effects are more prominent with highly active catalyst. When catalyst radius and primary particle radius are small, diffusion resistance is not important. UMGM can be extended to multiple active sites and more complex polymerization kinetics. It lays a foundation for the modeling and optimization of actual industrial reactors.pposedthatonlyonetypeofac