Modeling And Simulation Of Olefin Polymerization Reactor Based On Computational Fluid Dynamics Method

In-depth understanding of process of mass transfer, momentum transfer, heat transfer and polymerization is necessary for the development of polymerization reactor. The experimental work consumes a large amount of manpower when it is used to investigate process characteristic. Still, it is difficult to obtain relevant physical quantities such as spatial distributions of temperature and components, which can be predicted with the computational fluid dynamic method. In the polymerization reactor, the coupling between transport process and complex polymerization kinetics makes it very difficult to develop CFD models, especially when the process is operated based on multiphase flow or high-viscosity fluid. Therefore, modeling physical and polymerization processes in the polymerization reactors is challenge and valuable in theory and industrial applications.This thesis models the mixing, heat transfer and reaction process in three typical polymerization reactors with the application of CFD method, and achieves the following innovative results:1. The Eulerian-Eulerian two fluid model based on the kinetic theory of granular flow is coupled with the multiple reference frame method to investigate the fluidization performance in an agitated fluidized bed. The radial type impeller hardly has any effect on the pressure drop. Large enough agitation speed can reduce the bubble size and the amplitude of pressure fluctuation, as a result, the fluidization performance is improved. According to the effect of agitation of frame impeller, the fluidized bed can be divided into three zones:inlet zone, agitated fluidization zone and free fluidization zone. The fluidization in the inlet zone is dominated by the gas distribution. The agitation of frame impeller improves the fluidization performance in the agitated fluidization zone. For the free fluidization zone where no impeller exists, the effect of agitation can be ignored.2. The effect of the agitation of impeller on the transition of flow pattern is further studied. The large diameter Geldart D particles perform the transition to particulate fluidization, which commonly happens for Geldart A particles. As the agitation speed increases, the minimum fluidizing velocity keeps constant while the minimum bubbling velocity goes up gradually. The upper limit of gas velocity for particulate fluidization increases with the increasing agitation speed. The agitation of impeller forces the particles to move into the bubbles, reducing the bubble size. Bubbles vanish as soon as the agitation is strong enough.3. The relationship among distributions of temperature, volume fraction and velocity of both phases in an industrial fluidized bed reactor for ethylene polymerization is investigated. The user defined function (UDF) helps to finish establishing the numerical model based on the two fluid model and the KTGF. At the bottom of the fluidized bed, there exist a low temperature zone and a region with large temperature gradient. The reason for this uneven distribution of reactor temperature is revealed. There are a pair of particle flow recirculation above the gas distributor, which force the particles here to move in the radial direction instead of the axial direction. As a result, a large temperature gradient is formed here. In addition, particles with low temperature rise along the intersection of the two circulation flow, which results in the formation of the low temperature zone.4. The micromixing in a viscous stirred tank is investigated with CFD numerical simulation, by predicting the product selectivity of a parallel competitive reaction system. To include the micromixing effect on the sub grid scale, a new finite rate/eddy dissipation-engulfment (FR/ED-E) model is established, which can predict the reaction process in high-viscosity fluid more accurately. Better micromixing quality prefers the low-viscosity fluid, the large agitation speed and the feeding location in the discharge area. The product selectivity and reaction rate are also impacted by the path of the reaction zone. The volume of reaction zone expands due to the fluid convection, engulfment, deformation and diffusion, and shrinks due to the consumption of chemical reaction, which results in a maximum volume of reaction zone. The model parameter is sensitive to the fluid viscosity but is kept constant when the agitation speed is changed. This characteristic can be used in the design, scale-up and optimization of reactor. The value of model parameter can be measured in the lab scale reactor and then used in the industrial scale one.