【作者】 韩颖；

【导师】 冯连芳； 王嘉骏；

【作者基本信息】 浙江大学 ， 化学工程， 2013， 博士

【摘要】 聚合反应器技术的创新是基于对反应器中的物理传递过程(动量、热量和质量传递)和聚合反应过程的深入理解。通过实验手段解析聚合反应器中的传递和化学反应过程时,装置的建立需要耗费大量精力,实验工作量大,并且因为测量手段的限制,很难对温度分布和产品空间分布等物理量进行测量。通过计算流体力学(CFD)方法可获得非理想反应器中的浓度、速度、温度分布与化学反应过程规律,节省大量的人力物力。然而在烯烃聚合反应器中,体系的复杂性(多相流操作、介质粘度高)、传递与化学反应过程的耦合、聚合反应动力学的复杂性都使CFD模型的建立成为难题。因此对聚合反应器内的传递和化学反应过程进行模型化,促进研究方法论的进步,富有挑战性的同时又具有工业应用前景和十分重要的学术价值。论文针对烯烃聚合反应器,采用CFD方法建立耦合混合流动过程、热量传递过程和化学反应过程的CFD模型,并通过模拟研究取得了以下创新性结果：(1)针对气相搅拌流化床反应器,建立了基于颗粒动力学理论的双流体模型,并与多重参考坐标系的方法相结合,通过CFD模拟与实验相结合的方法揭示了搅拌桨转动对流态化的影响规律。径向流桨对流化床的床层压降影响不大,但足够大的搅拌速度可以显著减小压力脉动的幅值与气泡尺寸,提高流化质量。按照搅拌对流态化的影响程度不同,可将流化床由下至上分为三个区域：入口区,搅拌流态化区和自由流态化区。气体分布器的作用在入口区占优势,搅拌桨的转动可使搅拌流态化区的流态化质量得到明显改善。对自由流态化区而言,搅拌桨的作用不明显。(2)通过CFD模拟及对压力脉动的实验研究,揭示了Geldart D类颗粒在搅拌流化床中的流型转变规律。大粒径的Geldart D类颗粒出现了只有Geldart A类颗粒才具有的散式流态化现象。在搅拌桨的作用下,颗粒的最小流化速度基本不变,而最小鼓泡速度随搅拌桨转动而增加,散式流态化的气速操作范围随搅拌桨转速的增加而变大。由聚式流态化向散式流态化转变的过程是将“搅拌减小气泡尺寸”的量变转化为“无气泡化”的质变的过程。(3)通过考察工业尺度乙烯气相聚合流化床反应器内的温度分布与流态化和传热过程的相互关系,揭示反应器内温度分布不均匀性产生的根本原因。基于双流体模型与颗粒动力学理论,通过用户自定义编程(UDF)描述热量传递方程,对流动和传热规律进行考察,发现在流化床底部形成一对颗粒循环流,使气体分布器上方的颗粒主要沿径向运动,轴向方向上的混合质量较差,床层出现了较大的温度梯度。循环流使温度较低的颗粒沿循环流的交汇处上升,形成低温区。(4)在以中高粘度流体为介质的搅拌釜式反应器中,通过UDF建立有限速率／涡耗散-卷吸(FR/ED-E)微观混合模型,通过预测平行竞争反应的选择性考察微观混合情况。模拟结果表明,当搅拌槽中流体粘度较低、搅拌桨旋转速度较大以及进料位置处于搅拌桨的排出区时,微观混合质量好,副产物的选择性小。反应区域由于对流、卷吸、变形和扩散等过程而体积膨胀,同时由于化学反应的消耗而体积缩小,两因素共同作用,使反应区域的体积达到最大值后减小。模型参数通过实验值的回归得到,受流体粘度影响较大,而对于同一种非牛顿流体,在不同搅拌桨转速下,模型参数取相同的值即可得到与实验工作相符性较好的反应选择性。可利用此特性进行反应器的设计、放大和优化,从实验室中取得参数值,将其用于预测工业反应器中的传递与化学反应过程。更多还原

【Abstract】 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.更多还原