静电流化床的流场结构及过程强化的研究

Frictional electrification of the insulating particles can represent a serious problem for gas-solids fluidized bed reactors at very low humidity. Since electrostatic fluidization can be stabilized by the presence of an imposed electric field, which is of great importance to intensification of heat and mass transfer and prevention of electrostatic hazard for many highly important processes, such as, coal gasification and synthetic resin production. Toward to innovation of new process for gas phase ethylene polymerization fluidized bed, the objective of this work is to tackle three important problems experimentally by means of XPS, Faraday cages, high precise balance, electrostatic probes and acoustic emission measurement, and theoretically by CFD simulation. Firstly charge segregation dependence on particle size will be obtained for triboelectrically charged powder material in consideration of adsorption on particle surface in order to enlarge or alleviate electrostatic charge artificially. Secondly, under the new circumstance of hydro- and electro-dynamics, criterion of fluidized particle classification, flow pattern transition, bubble movement will be re-examined comparably with and without imposed DC/AC electric fields. Effect of imposed field on the agglomeration of fines near the wall region of fluidized beds will be mainly focused on with aim for problem solving. Finally, a methodology will be conceived to approach a possibility for electrostatic fluidization of a powder system with broader size distribution, the coarser particles having lower charge-to-mass ratios are mainly controlled by bubbling fluidization, meanwhile fines having higher charge-to-mass ratios are controlled by electro-mechanics within one fluidized bed. Based on the cooperation of these two mechanisms, stabilization and adjustability of electrostatic fluidized bed are needed to be illustrated quantitatively in this work to give a theoretical basis for process intensification in electrostatic fluidized bed. The success of this work will advance our fundamental understanding and provide direct evidence for utilization of electrostatics and innovation of new processes with highly efficiency.

绝缘粉体在流化床中摩擦起电产生严重的危害。静电流态化工程利用外加电场的作用使带静电颗粒均匀稳定地流态化，是煤气化、合成树脂等重要化工过程强化热质传递、消除静电危害的新途径。本申请课题以新型的气相法聚乙烯流化床反应器为研究与应用背景，利用XPS、Faraday筒、精密天平、以及动态粉体静电-声发射在线联合测量等手段和CFD模拟计算手段，探索静电荷在粒径分散的同质聚合物粉体中的转移、分布的规律，以及化学吸附质对动态粉体静电增强或抑制的调变规律；研究新的受力环境下颗粒分类、流型转变、气泡运动规律及其受外加直流/交流电场的影响规律；研究壁面边界层静电场的调变规律，指导壁面防颗粒静电团聚技术的开发；研究外加电场控制的高荷质比细粉均匀分散悬浮机制与流体力学控制的低荷质比的粗颗粒流态化机制之间的协调，提高流化床稳定性和可调控性。为静电特性利用、新工艺开发提供普适性的科学理论指导。

为探明静电及其危害的形成机理，系统探索通过外加电场等措施控制静电“化害为利”、“以电治电”的科学方法，本项目从动态颗粒的静电荷分布规律及调控手段、常规静电流化床中静电场-流场的耦合作用机制、外加电场的静电流化床中静电场-外加电场-流场的耦合作用机制、多场耦合过程的CFD模拟等方面开展研究，获得了5项创新性成果。（1）通过弧形/环形感应电极等多种检测手段，测量了流化床内的静电分布信号，发现了静电信号与流化床内的流体力学行为具有对应关系，首次建立了颗粒荷质比、颗粒速度以及结块的在线检测方法，其中指型静电在线检测仪顶替进口产品、在6套聚乙烯流化床反应器成功工业应用。（2）基于颗粒摩擦荷电、电晕荷电过程的理论分析，结合流体力学特征参数分析，分别建立了流化床原位电极电晕消电模型和描述细颗粒对床内静电分布规律的数学模型，提出了相应的消电方法。特别是所提出的基于细粉管控的流化床静电控制技术成功应用于45万吨/年多区循环流化床聚丙烯反应器的优化控制，成效显著。（3）通过添加微量抗静电剂，建立了流化床静电的单因素实验方法。定量揭示了静电场-流场耦合作用下，颗粒静电力对气泡生长和颗粒运动的抑制作用。研究成果得到国内外同行高度关注。（4）通过考察不同电场方式（交流/直流、顺流/错流、向心/离心）、不同场强的外加电场对静电流化床中气泡、聚团、颗粒运动以及床层粘壁的作用规律，发现库仑力与极化力的重要竞争作用机制，为交流电场抑制粉体粘壁的工业应用提供了理论依据。（5）建立了多场耦合的CFD模拟方法，包括静电场与流场耦合的CFD-DEM模拟方法以及静电场-外加电场-流场耦合的欧拉双流体模拟方法。分别定量描述了由静电力主导的颗粒团聚对流化床稀相段细颗粒扬析的抑制作用，错流的外加直流电场对静电流化床中气泡的破碎作用，计算结果与实际情况符合。本项目的研究成果以及衍生的检测技术已取得显著社会经济效益，并获得多项国家及省部级奖项。