气升循环分体式MBR的计算流体力学模拟与优化
文献类型：学位论文
| 作者 | 张晴 |
| 学位类别 | 硕士 |
| 答辩日期 | 2013-05 |
| 授予单位 | 中国科学院研究生院 |
| 授予地点 | 北京 |
| 导师 | 樊耀波 |
| 关键词 | 气升循环分体式膜生物反应器 计算流体力学 水力学优化 H循环管 airlift external circulation membrane bioreactor computational fluid dynamics hydraulics optimization H circulating pipe |
| 其他题名 | Simulation and Optimization of Airlift External Circulation Membrane Bioreactor Using CFD |
| 学位专业 | 环境工程 |
| 中文摘要 | 膜生物反应器（Membrane bioreactor, MBR）因出水水质好、占地面积少、剩余污泥少等优点在水污染控制与水回用领域得到广泛应用，但膜污染和运行能耗高仍是制约其发展的关键因素。MBR分为一体式和分置式两种，传统一体式MBR运行能耗低（0.6~2 kW·h/m3），但膜组件清洗维护较复杂；传统分置式MBR 膜清洗方便，但能耗较高（3~4 kW·h/m3）。基于此，本课题组开发的气升循环分体式MBR 结合了分体式和一体式MBR 的优点，具有运行能耗较低、膜组件清洗和维护方便的特点，并已有大量工程应用。但由于其结构复杂，反应器水力学条件不清楚，相关设计和运行参数常采用经验或半经验方法取得，致使该技术常难以在优化工作状态下运行，膜污染控制效率较低和运行能耗较高的问题仍制约着其推广应用。 为此，本研究基于计算流体力学（Computational Fluid Dynamics, CFD）方法，并辅以敏感性分析和响应面分析方法，以气升循环分体式MBR 为研究对象，系统展开了该MBR 水力学条件模拟与优化研究。主要包括膜单元曝气器布局、H 循环管尺寸及其他MBR 主要结构参数的优化，以及曝气强度等运行参数优化。取得如下主要结论： 1）揭示了MBR 膜污染和运行能耗高的内在流体力学原因： 在MBR 膜组件中存在着气水混合流冲刷不均匀现象，使得一定比例的膜组件和膜面积未能充分发挥有效作用。同时，膜面低流速边界层现象是导致膜污染加重，运行能耗升高的重要原因，且表明仅通过增大曝气强度控制膜污染的方法存在局限性。 2）膜单元曝气器布局优化研究结果表明，增加气液混合高度和曝气面积可提高反应器混合程度和膜冲刷强度及均匀性，有利于膜污染控制；而曝气器位置升高会使MBR 流场分布的均匀性下降； 3）气升循环分体式MBR 中H 循环管的作用是实现生物单元和膜单元之间的有效水力循环，具有必要性；H 循环管管径与系统循环量呈正相关关系，随H 循环管管径增大，MBR循环流量增大直至达到饱和；但H 循环管管径的利用效率是存在优化值的；推导得出H 循环管内流速计算公式为： 4）MBR 重要构型参数优化结果表明，膜组件摆放位置应以反应器截面形状为近似正方形为先，当反应器为类正方形布局，膜单元和生物单元为对称布局，膜单元和生物单元截面比为1:1.5，膜组件上升流占膜单元截面比为50%，挡板高度的设置保证溢流高度大于100mm 时，气升循环分体式MBR 的水力学条件较优。基于以上构型优化结果，课题组提出一种新型一体化A2/O‐MBR。 5）响应面分析得出本研究优化运行方案为：膜池曝气量4.67m3/h、生化池曝气量2.04m3/h、污泥粘度1.57mPa·s，相比经验运行方案，不仅能提高反应器水力学性能，还能减少膜单元曝气量、降低能耗。 本研究模拟结果经实验验证具有可靠性，研究结果可为低耗高效MBR 技术的设计和开发提供科学依据和技术支持。 |
| 英文摘要 | Membrane bioreactor (MBR) is now widely used for wastewater treatment and reuse, as it has significant benefits over other biological wastewater treatment processes, such as the smaller footprint and the better product quality. However, membrane fouling and high energy consumption remain the major disadvantages of this technology. There are two main MBR configurations, submerged MBR and external MBR (side-stream configuration). Of the external MBR, membrane cleaning or maintaining is easy, but high energy consumption (3~4 kW·h/m3) of the circulation pump made it less atrractive to users. While the submerged MBR become more applealing in wastewater treatment as its low operation cost (0.6~2 kW·h/m3), but the operation of membrane cleaning is inconvenient. In order to solve the conflict, a novel MBR named airlift external circulation membrane bioreactor (AEC-MBR) had been developed and used in diverse applications. AEC-MBR has the advantages of both the conventional side-stream MBR and the submerged MBR, with lower energy consumption and the membrane cleaning or maintaining could be done on line or only in the menbrane tank without interfering with aeration tank. However, the hydrodynamics of the system was complex and failed to understand, such as the distribution of mixture velocity, cross-flow velocity and shear stress at the membrane surface, even though it was of critical importance to the performance of the AEC-MBR system. For solving the fluid dynamics problems,Computational Fluid Dynamics (CFD) as a powerful tool was used in the research of AEC-MBR. In this study, CFD was implemented to study the hydrodynamic characteristics of AEC-MBR, and optimization was conducted through sensitivity analysis and response surface methodology (RSM). Four cases of MBRs with different diffuser configurations of membrane tank were simulated and the sensitivity analysis of their impacts on the velocity, shear stress, circulation rate of mixture in the MBR were presented. Meanwhile, the role of H circulating pipe and the effect of different diameters of H circulating pipe on circulation were modeled and analyzed. Furthermore, other important geometry parameters and operation parameters such as aeration intensity were also optimized in this thesis. The main conclusions were as follows: 1) The fluid velocity and wall shear stress were always higher at the central sheets and much lower at the side ones in membrane module. The non-uniformity of gas-liquid flow though the channels between the membrane sheets was one major reason to lose effective filtration area of membranes, to lose productivity of treated water and to result in high membrane fouling and energy consumption for MBR application. 2) Larger distance from diffusers to membrane modules (height of gas-liquid mixing zone) was helpful to improve the velocity and shear stress at the membrane surfaces for membrane fouling control; the distribution of shear stress at the membrane surface in the membrane tank with 3 diffusers was more uniform than that with 2 diffusers; higher position of the diffusers would result in more fluid dead zones under the diffusers in the membrane tank. 3) H circulating pipe was necessary and played an important role in the AEC-MBR, and that an increase in the diameter of H circulating pipe led to an increase in the mass flow rate between the membrane unit and the aeration tank. The mean flow velocity in H circulating pipe reached a maximum when D/L=22.2%. An equation was derived for determining the velocity of the circulation flow in the H circulating pipe as: 4) Hydrodynamics of tank could be more optimized with tank’s cross-section similar to square, with symmetrical layout of membrane unit and aeration unit, with cross-section ratio of membrane tank to aeration tank (Am/As) equalled to 1:1.5, with percentage of upstream zone in membrane tank reached 50%, and with the height of baffle keeping overflow height more than 100mm. Based on the above results, research group put forward a novel process of integrated A2/O-MBR. 5) The operational parameters optimized by RSM were that, aeration intensity of membrane unit and aeration tank was 4.67m3/h and 2.04m3/h respectively, while sludge viscosity was 1.57mPa·s. Compared to operating parameters based on empirical techniques (e.g. specific aeration demand), the proposed solution could not only improve the hydraulic performance, but also lower the operational energy consumption. The CFD model and modelling results in this study were successfully validated against experimental results. The results would provide technical support for design and development of MBR with low energy consumption and high efficiency, and the research method provide by this paper could be used as a reference for further hydraulic study on MBRs. |
| 公开日期 | 2014-10-24 |
| 源URL | [http://ir.rcees.ac.cn/handle/311016/7701]  |
| 专题 | 生态环境研究中心_水污染控制实验室 |
| 推荐引用方式 GB/T 7714 | 张晴. 气升循环分体式MBR的计算流体力学模拟与优化[D]. 北京. 中国科学院研究生院. 2013. |
入库方式： OAI收割
来源：生态环境研究中心
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