Nanofiltration Module Design and Operation

This chapter covers the design and operation of modules suitable for nanofiltration membranes. The roles of the module in housing and supporting the membrane and providing fluid management are described in the context of control of concentration polarization and fouling; relevant theory and mass transfer correlations are provided. The various modules, suitable for flat sheets, such as plate and frame and spiral wound, and cylindrical geometries, such as hollow fibers and tubes, are introduced and their characteristics are compared. A section is devoted to the widely used spiral wound module with details of feed channel spacers and modeling and optimization approaches. Strategies to improve concentration polarization control are reviewed for higher permeability membranes, including high shear and unsteady shear techniques, such as vibrations or two-phase flow. Systems incorporating multiple modules in series and parallel arrangements are described. Operations considered include diafiltration used to enhance separations. Reflux-recycle, combining nanofiltration with reverse osmosis, is shown to have the potential to achieve higher water recovery without significant energy penalty. Batch processing, particularly the closed-circuit approach with incrementing pressure to match the osmotic pressure, has the potential for high recovery and energy savings.