Application of CFD in Membrane Technique

Ding, Wei

Recently the use of low pressure membrane filtration systems, i.e. ultra- and microfiltration membrane filters, increases rapidly in the water technology field. The performance of a membrane filtration system is strongly influenced and limited by its design. German membrane manufacturers inge AG and Microdyn-Nadir GmbH designed special membrane filtration systems recently: a ultrafiltration membrane filtration system built by series of in/out capillary modules and a submerged microfiltration membrane filtration system built by series of pillow-shaped membrane plates. Both of these two new designs are as compact and flexible as possible. However a compact and flexible design can be in trouble with the performance of the filtration system. There is a need to do the optimization between the compact, flexible design and the performance of the filtration system. Computational Fluid Dynamics (CFD) involves the solution of governing laws of fluid dynamics numerically. CFD models can be used to analyze different influencing parameters of a hydraulic system like the membrane filtration system. These influencing parameters can be geometry parameters or operation conditions. In the conventional CFD simulations, membrane filtration systems are considered on the microscopic scale and the CFD models were well developed. However on the macroscopic scale, the conventional CFD models have extremely high time and computer hardware strength requirement. In this work, with help of several methods, a new macroscopic scale membrane simulation tool will be developed and introduced. A Calibration-Library-Interpolation method is employed which successfully solves the conflict between accuracy and time/hardware strength requirement of the simulation. The CFD tool becomes simple, easy, highly flexible and economical with the help of this method compared with the traditional CFD models. The scale method is also applied in this work due to the special dimension problem of membranes. The CFD simulation is based on the discretized meshes of geometry. The mesh quality determines the performance of the simulation. Equilateral geometry mesh has the best quality. The pillow-shaped membrane plate has a thickness of millimeter-range, a length and a height of meter-range and the capillary membrane has a diameter of millimeter-range and a length of meter-range. If the membrane geometry is discretized with high quality mesh, then the mesh number will be too large to be simulated and the time and hardware strength requirement will be too high. The scale method can be applied here to solve the problem of the high time/hardware strength requirement of simulation caused by the high quality mesh. In some certain systems, like in the microfiltration system built by Microdyn-Nadir, because of the pressure difference at different sides of membrane plate, the geometry of membrane plate can be compressed. This scale model can solve the problem caused by this kind of geometry change like compression too. The pillow-shaped membrane plate consists of two membrane sheets and one spacer to support and prevent the sticking of these two sheets. The spacer is built by porous fiber networks, which has very complex geometry which is difficult to simulate with traditional CFD tools. For simplification, porous media approximation method based on Modified Navier Stokes equation is employed in this work instead of complex geometry simulation. After the validation by experiments, this CFD tool is very useful to help the designer of the membrane system. It can be used to find the optimal solution between the design and performance of membrane system. This new CFD model can be also applied for other membrane systems that the systems are built up of identical substructures with just a slight adaptation. Using the new CFD model saves time and money as expensive tests don't have to be conducted.

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Ding, Wei: Application of CFD in Membrane Technique. 2013.

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