气固流化床中颗粒涡的形成、演化及其调控的机制研究

Agglomerations and slags, caused by dynamic particle vortices, are important meso-scale structures in gas-solid fluidized beds. The behavior of these meso-scale structures exist extensively in numerous key industrial setups such as gas-phase polymerization, mineral fuel thermal conversion as well as granulation drying, and are of great significance in the operational reliability and security of these units. The project proposes to study formation and evolution of meso-scale particle vortex structures using novel experimental apparatus and methods. The self-designed polyethylene/aluminum composite microsphere particles with a core-shell structure are used as the tracking particles or fluidizing particles in the fluidized beds. The composite particles are heated by an electromagnetic induction pulse heating system to simulate heat release from reactions. Taking advantage of the highly cohesive feature of PE particle at high temperature, particles melt quickly and thus the dynamic particle vortex structures can be captured and sampled. Moreover, the sampled agglomerations will be analyzed by both Nuclear Magnetic Resonance and micro-Raman Spectrum for tomoscan and slice analysis. From the results of cross-sectional morphology, such as the distribution of crystallinity and voidage, it is feasible to backtrack the particle vortex formation and its development. In this project, acoustic emission technique will be used to measure the velocity/concentration fluctuation and the dynamic size of the agglomerates. Besides, effects of liquid flash evaporation and melting point variation on the regulation and control of particle agglomerations will be investigated. In combination with Discrete Element Method (DEM) simulations and wavelet analysis for the particle fluctuation parameters, the evolution law of agglomeration precursors, including particle vortex and wake structures, will be revealed explicitly. The ultimate objective is to establish a competitive evolution model for particle vortex meso-scale structures and to further give guidelines for scale-up and operation of industrial fluidized bed reactors.

由颗粒涡动态形成导致的颗粒团聚、结块结渣是气固流化床中重要的介尺度行为，广泛存在于气相聚合反应、矿物燃料热转化以及造粒干燥等关键工业装置中，直接操纵装置的安全可靠性。本申请拟采用创新的实验模拟体系和方法对颗粒涡介尺度结构形成及演化规律进行研究，设计具有核壳结构的聚乙烯/铝粉复合微球作为示踪颗粒或流化颗粒，通过电磁感应脉冲加热铝粉，模拟反应放热，利用聚乙烯受热易粘结的特性，快速“熔结”捕捉颗粒涡动态结构；采用核磁与显微拉曼光谱等技术对颗粒聚团样品进行断层扫描和切片分析，根据切面形态，孔隙率和结晶度的分布，追踪反演颗粒涡的形成与演化过程；采用声发射等技术在线检测颗粒聚团的动态尺寸和脉动量等，研究液体闪蒸和熔点改变对聚团的调控作用；联合离散元模拟和颗粒脉动参数的小波分析，研究颗粒聚团前驱体—涡旋/尾涡的演化规律，最终目标是建立颗粒涡介尺度结构的竞争演化模型，有效指导流化床工业反应器的放大和运行。

由颗粒涡动态形成导致的颗粒团聚、结块结渣是气固流化床中重要的介尺度行为，广泛存在于聚合反应、矿物燃料热转化以及造粒干燥等关键工业装置中，决定了工业装置的稳定运行、产品性能及其产能。为揭示流化床反应器中颗粒涡对催化剂细颗粒的作用机制，本项目研究了尾涡对不同粒径的颗粒具有选择性交换作用；采用计算流体力学模拟方法，研究了颗粒涡对壁面注入催化剂细颗粒的扩散混合行为的影响。本研究建立了气泡颗粒涡传热模型，匹配颗粒涡寿命、催化剂动力学和喷液参数，实现了颗粒涡内催化剂细颗粒的可控释放和传热强化，构建差异化环境中的交替聚合。通过在侧壁喷液流化床中建立电磁感应加热外覆石蜡的石墨颗粒的方法，模拟了液体存在下活性颗粒的反应放热过程，揭示了液桥力、固桥力及二者协同作用下的颗粒团聚流态化行为及转变规律。通过建立颗粒团聚力和破碎力与颗粒流态化行为的物理关联，提出基于颗粒团聚力与破碎力之比分布的流态化转变判据。本项目由此建立了聚团竞争演化模型，确定了多温区流化床的流体力学稳定操作域。进一步地，本项目耦合颗粒涡和颗粒聚团特征项，建立了多温区流化床的两区三相反应器模型及反应系统动态模型，揭示了反应器和反应系统的热稳定性特征及操作域，提出了放大过程热稳定性相似性的判据基于流化床反应器两相模型。选取Hopf分叉点为判据对催化剂进料速率和活化能操作空间进行划分，为持液气固流化床的流体力学稳定操作域和气液法流化床工业反应器的优化操作提供了指导。本项目形成的多温区流化床聚乙烯成套技术已成功应用于天津石化14万吨/年工业装置，成功生产热收缩膜等高端专用料，经济效益显著。本项目研究结果为气液法工艺中反应器的优化放大提供了重要的理论依据，有效地支撑了30万吨/年装置的反应器设计和工艺开发。