【作者】 王伟文；

【导师】 李鑫钢；

【作者基本信息】 天津大学 ， 化学工程， 2010， 博士

【摘要】 本文围绕宽筛分细硅粉颗粒的流化特性及传热过程,从理论、实验和数值模拟三个方面对有机硅单体合成流化床反应器开展系统研究。采用CFD模拟软件FLUENT,运用基于颗粒动力学理论的欧拉气固多相流模型和PC-SIMPLE算法,在考虑气体与颗粒、颗粒组分以及组分内颗粒间的相互作用的情况下,建立了流化床内气固多相流动的数学模型,并确定了初始条件与边界条件。在此基础上,评判了流化床外型对流化特性的影响,结果表明:对于宽筛分细颗粒流化床,静床床面位于扩大段下端的异径流化床内部气泡尺寸较小、颗粒的扬析少、流场稳定。模拟比较了自由床、垂直沉浸床、水平三角形排列沉浸床、水平正方形排列沉浸床的流化特性。模拟结果表明:对宽筛分硅粉颗粒,整个系统的最小流化速度大为降低,粒径较大的颗粒不容易流化,粒径较小的颗粒容易流化;垂直沉浸管式流化床内的大气泡被指管破碎、气泡上升速度减缓、聚并现象减弱,床内径向返混程度减弱,增大垂直管管间距可降低壁面处的固含率和密相床层高度;水平沉浸管式流化床内有一定数量的管道被“气穴”包围,管间距缩小后“气穴”增多;水平三角形排列沉浸管比水平正方形排列沉浸管床层高度低,“气穴”少。采用压力脉动法研究了宽筛分细硅粉颗粒的流化特性,拟合出了压力脉动标准方差与气速的关系式,通过拟合公式计算和分析得到的最小流化速度与理论值吻合较好。针对有机硅单体合成流化床进气分布不均、取热不均、能耗和物耗高等弊端,以模拟和实验结果为依据,提出了开发均热直流型流化床反应器的新思路:采用均布的单管程指管取热、双锥进导流式气分布器和硅粉直接回床技术。经工业应用表明:均热直流型流化床反应器密相段床层温度相同,真正实现了均匀取热、进气分布均匀和直接回床顺利畅通。对有机硅单体合成反应的传热过程进行了对流传热系数的计算和校核,提出了适合于流化床强传热过程的有内热源的传热膜模型,推导出了气膜厚度的计算式。气膜厚度计算值与文献实验值基本吻合,说明本文建立的公式和模型可用于反应器内浓相段气膜厚度的计算。由推导出的气膜厚度表达式可知:气膜厚度δ主要与单位体积反应热q.和气体导热系数k的比值有关。该结论对强放热反应传热过程理论研究十分有益,对类似工业反应器的设计有重要指导意义。更多还原

【Abstract】 A fluidized bed, which is applied in the synthesis of organosilicon monomer, is studied in theory, experiment and numerical simulation to investigate the fluidization characteristics and heat transfer of Wide Particle Size Distribution (abbr: WPSD) of fine silicon powders.This paper adopts CFD, Eulerian model of particle dynamics theory and PC-SIMPLE numerical method to set a model of gas-solid multiphase flow under considering the interaction between gas and particles, one particle component and another component, and particle and particle in one component, to ascertain the initial condition and boundary condition and evaluate the influence of the configuration of fluidized bed on efficiency. The results show that, for the fluidized bed with WPSD of fine particles, there are small bubbles size, little particle raising phenomenon and steady flow fields in the static bed layer located at the bottom of the enlargement section of the fluidized bed.Compared the fluidization characteristics of the free bed, the vertical immersion bed, the immersion bed with triangular level arrangement (abbr: IBTLA) and the immersion bed with square level arrangement (abbr: IBSLA) respectively, the results of the numerical simulation indicate that the minimum fluidization velocity substantially decreased for WPSD, and the smaller particles are easier to fluidize than the larger ones; that the bubbles are broken in bed because of the figure-pipes and raise slower, and the phenomenon of the bubble coalescence is weakened as well as the degree of radial back mixing. And the results also show that the solid fraction closed to the wall decrease with the increase of the space between the vertical pipes, the height of the dense phase section of the bed has the same trend. In addition, some pipes are surrounded by the cavitations in the level of immersion tube fluidized bed. While, the cavitations increase with the decrease of the distance between pipes. The height of the bed layer of IBTLA is lower than the one of IBSLA and the cavitations in IBTLA is also less than in IBSLA.The pressure pulsation method is applied to study the fluidization characteristic of WPSD of silicon powders, and to draw out the relationship function between the pressure pulsation Standard Deviation and the gas velocity. The results show that the value of the minimum fluidization velocity is in agreement with the theoretical value.In order to overcome the following disadvantages, such as, uneven heat absorption and uneven air distribution of inlet in the fluidized bed reactors which applied in synthesis of organosilicon monomer. A new thought, developing a new type of the fluidized bed with once-through flow and uniform heat absorption, is proposed based on the simulation and experiment results. In this reactor, the following technologies are applied: heat absorption through symmetrical once-through figure pipe; the air diversion distributor with double cones; silicon powders directly back to bed. The industrial application of the new type of reactor show that there exist the same temperature of dense phase section in the new type of fluidized bed, and this reactor realize the uniform heat absorption.The convective heat transfer coefficient of the synthesis of organosilicon monomer is calculated and verified. In addition, the model of the fluidized bed with inner heat source, which is suitable for strong heat transfer process, is set up and the calculation function about the thickness of gas film is also proposed. And the calculated value of gas film thickness is in good agreement with the experimental values in the relative literature, which shows that the function and model set up in this paper could be applied in the calculation of gas film thickness in dense phase section. It could be speculated from the function that Gas film thicknessδmainly is relevant with the ratio of the unit volume reaction heat q. to the thermal conductivityk . The conclusion should be beneficial to the research of heat transfer process in the strongly exothermic reaction and be significant for designing related industrial reactors.更多还原