| Development of polyolefin science and technology comes from the deep understanding and analysis in the combined process of physical transport phenomena (mass, momentum and heat) and chemical reaction. The gas phase ethylene polymerization is referred to the complex gas solid two-phase flow and reaction process, involving the spatial and temporal multiscale phenomena. Fluidized bed reactor (FBR) is commonly used for industrial polyethylene (PE) production due to its simple construction and effective heat and mass transfer. The transport characteristics, reaction status and polymer quality are all controlled by two phase properties and the reactor hydrodynamic behaviors. Therefore, in the gas phase polymerization reactors, following the principles of multiphase flow and chemical reaction, the coupled CFD-Population Balance Model (PBM) has been used to investigate the gas-solid two phase flow, mixing and reaction, which is helpful in mastering the process of ethylene polymerization engineering, and developing new products and production technology. Not only does the work show great potential application in industry and important academic values, it also contributes to promoting the development of novel PE production technology into the directions of new type, high efficiency and energy saving.The thesis explores the gas phase ethylene polymerization process combined with the non-pelletizing PE production process (NPPP) (a more energy-efficient and environment friendly PE production process technology) and bimodal PE process (production of new PE products), respectively, and two commercial PE production processes (UNIPOL PE process and Borstar Bimodal PE process) have been covered. The ethylene polymerization and growth of single polymer particle in the mesoscale, the gas-solid two phase flow, mixing and reaction state in the reactor scale, and the process of industrial PE production in the multiscale have been investigated through combination of experiment, modeling and simulation. The hybrid model coupling multigrain model and multi-active sites model has been used to explore the effects of single particle growth on the polymerization behaviors and polymer particle morphology. The thesis focuses on PE production technologies, and the multiscale modeling, simulation and experimental methods have been applied to explore the complicated behaviors in industrial reactors by coupling the transport phenomena and the ethylene polymerization reaction. The following conclusions have been drawn:(1) The hybrid model (coupling four-active sites chemical model and the multigrain model) can visually describe the single PE particle morphology and property in gas phase ethylene polymerization process. Whether or not the particle voidage effect should be considered in the model is determined by the catalyst particle size, and evident concentration and temperature gradients are observed in polymer particle under NPPP, which can be improved by the tendem reactor process. It can provide references for further understanding of ethylene polymerization process and polymer properties in NPPP.(2) Through CFD modeling and simulation of a pilot-plant FBR (UNIPOL PE process), the core-annulus flow structure in the bed presents thicker core region and thinner annulus area due to scale up effect. The bed expanding section (BES) can reduce polymer particle velocity and improve particle flow behaviors. When the reactor is operated under NPPP, gas velocity of 0.90 m·s-1 is optimized to reach steady fluidization state but the bed becomes sensitive.(3) Due to the effect of ethylene polymerization and gas-solid heat/mass transfer, the bed expansion height and values of solid holdups are obviously increased. The core-areas of core-annulus flow structures are broader than that of the cold-model. The axial particle velocities in the core-region decrease along the bed radial direction. In addition, the particle breakage leads to a decrease in polymer particle size, and the fluid dynamics also plays a crucial role in the dynamic evolutions of polymer particle size distributions (PSDs).(4) Based on the 3D Eulerian-Eulerian two-fluid model coupled with a PBM under TPPP (Geldart B) and NPPP (Geldart D), it is found that the polymer particles observably concentrated on the bed inlet region for the effects of Geldart D particles and superfical gas velocity. Moreover, obvious vortexes and large bubbles can be observed in the bed height direction, which can be improved by optimizing the operating gas velocity and adjusting the gas distribution plate. The FBR performance is also explored by 3D CFD-PBM model. Temperature distribution shows an uneven zone at the top of the reactor, and it needs much higher region for fully uniform distributed temperature under NPPP. The solid flow patterns present different flow behaviors in two processes. For NPPP, thicker annulus area can be predicted and a lower bed expansion height is clarified. Cluster can be clearly observed in transient flow, which means that the bed undergoes a more complicated operation condition. Meanwhile, it could offer guidances for the design and operation of industrial reactor, and the knowledge of multiscale and multiphase can be used to guide the new polymerization process development, reduce energy consumption and improve the product productivity and quality.(5) In Borstar Biomodal PE process, the polymer PSDs and the particle morphology in the homopolymerization loop reactor and the copolymerization FBR show obvious differences, and a relationship between the polymer particle size and molecular weight distribution (MWD) has been obtained by modeling and simulation. Moreover, the particle voidage in the FBR is larger than that in the loop reactor, and the polymer particle surface roughness in the loop reactor is better. |
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