The effect of the ladder-type spacers configuration in NF spiralwound modules on the concentration boundary layers disruption

The laminar flow structure and concentration distribution in a narrow rectangular channel simulating the feed channel of nanofiltration (NF) spiral-wound modules are investigated. The continuity, Navier–Stokes equations and the solute continuity equation are solved by the control volume formulation. To validate the numerical predictions, NF permeation experiments with an aqueous solution of sodium chloride (2 g/l) at 25°C were performed in a laboratory cell with a rectangular feed channel (2 mm height × 30 mm width × 20 cm length) filled with a ladder-type spacer with a transverse inter-filament distance of 3.8 mm. The numerical results show that the average concentration polarization for the membrane wall with adjacent transverse filaments is independent of the distance to the channel inlet, while for the membrane wall without adjacent filaments the average concentration polarization increases with the channel length. This is due to the fact that in the first case the concentration boundary layer is periodically disrupted by the transverse filaments while in the second case the concentration boundary layer grows continuously along the channel length. The experimental results of the NaCl apparent rejection coefficients are compared to the model predictions, the agreement being good. These results clearly establish how crucial the spacers configuration is in the optimization of the spiral wound module efficiency.

vp — x y — — The concentration polarization profiles in the two membranes walls of the feed channel of a NF spiral-wound module with ladder-type spacers are very different. The average concentration polarization for the membrane with adjacent transverse filaments is independent of the distance to the channel inlet, while for the membrane without adjacent filaments the average concentration polarization increases with the channel length. This is explained by the periodic disruption of the concentration boundary layer in the first case and the growth of the concentration boundary layer along the channel in the second case. The analysis of the flow structure and solute concentration distribution shows that the performance of the spiral-wound module can be enhanced if additional transverse filaments are placed adjacent to each membrane of the feed channel. These results clearly establish how crucial the spacers configuration is in the optimization of the spiral wound module efficiency.