Pollutant Critical Adhesion Force-based Numerical Simulation For The Dynamic Ultrafiltration Process

Membrane separation technology is a frontier in the water treatment field, with high separation efficiency, no secondary pollution, high selectivity, low energy consumption and so on. However, during the long-term operation, the colloid particle and macromolecular substances in the feed solution were absorbed into membrane hole and deposited on membrane surface by physical chemistry or mechanical effect, which cause the formation of forming the cake layer, concentration polarization phenomenon and membrane fouling. Membrane fouling increased membrane permeability resistance.As a result, the membrane flux was reduced, and the application of membrane separation technology in water treatment field was limited. Therefore, studying the mechanism of fouling and understanding the fouling formation process is the core to solve the fouling problem effectively. Generally, the study of mechanism of fouling is mostly qualitative or semi-quantitative because of the restriction of the size of membrane module and experimental method.The visualization of internal flow can be achieved by numerical simulation based on Computational Fluid Dynamics(CFD). The parameter range will not be constrained,like that happens in experimental test. In addition, quantitative research and optimizing design of membrane module can be carried out as well. The existed numerical simulation of fouling just focused on the specific problem, such as flow field,concentration field, flux and shear force, and investigated the changes of fouling under different operating conditions. But there is no any work that takes into account the flow conditions, pollutant state and flux together.Hollow fiber ultrafiltration membrane fouling process was studied by COMSOL Multiphysics 4.3, a finite element simulation software in the paper. After, consideringthe influence of flow shear force, pollutant critical adhesion force and deformed geometry, the dynamic model of retaining pollution was simulated. A two-dimensional(2D) axisymmetric model of hollow fiber membrane was established,aiming at the dynamic problem of fouling accumulation on ultrafiltration membrane surface. The influence of flow shear force and pollutant critical adhesion was considered and the concept of effective shear force was put forward. The dynamic model of retaining pollution was established, which coupled free and porous media flow, the transport of diluted species, deformed geometry. Grid velocity at the pollutant interface was related to the normal accumulation of pollutants and the tangential shear. The feasibility of the effective shear stress was put forward and verified in this fouling model. The pollutants interception and the shear modified parameter of numerical model were determined by a dead-end and a cross-flow filtration experiments.In this paper, the major research is to learn how the different hydraulic conditions impact the membrane fouling. Based on this model, the influece of feed modes(continuous flow and pulsatile flow) on membrane fouling was investigated. The study showed that the flux decline and the flow shear rate distributio fitted experimental data well when c1 was 3.7m3/mol and c2 was 1×10-11 m. Nevertheless, compared to continuous flow, pulsatile flow could provide a higher effective shear rate and was beneficial to scouring foulant on the membrane surface. Besides, pulsatile flow could also reduce membrane fouling and improve the efficiency of membrane separation.