气升循环分体式MBR的CFD模拟及优化

Simulation and optimization of airlift external circulation membrane bioreactor using CFD

基于计算流体力学(CFD)方法,对气升循环分体式膜生物反应器(AL EC MBR)的关键结构参数与水力学参数的相关关系进行了模拟、优化和敏感性分析.研究结果表明,增加气液混合高度和曝气元件数量可提高混合液的混合程度及膜面流速、剪切力分布的均匀性,有利于膜污染控制;曝气器位置升高会使MBR流场分布的均匀性下降;混合液黏度的增加会降低混合液循环流速,但使膜组件中气液混合流的均匀度提高.在MBR膜组件中存在着混合液流速和剪切力分布中部区域高外部区域低的不均匀性,这种不均匀性是导致膜有效利用面积降低和水处理成本升高的重要流体力学因素.

Simulation and optimization of airlift external circulation membrane bioreactor （AL EC MBR） were performed using computational fluid dynamics （CFD） in this paper. Four cases of MBRs with different configurations were simulated and the sensitivity analysis of their impacts on the velocity, shear stress, circulation rate of mixture in the MBR were presented. The results showed that 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 of with 2 diffusers ~ higher position of the diffusers would result in more fluid dead zones under the diffusers in the membrane tank. It is also noted that higher viscosity of the mixture in the MBR will lower the circulation rate between the membrane unit and the aeration tank, but on the contrary, the distribution of air-liquid flowing over the membrane surfaces appeared more uniform. However, the fluid velocity and wall shear stress are 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 is one major reason to lose effective filtration area of membranes, to lose productivity of treated water and to result in high energy consumption for MBR application.