Concentration polarisation and flow structure within nanofiltration spiral-wound modules with ladder-type spacers

The flow structure and the solute concentration distribution of an aqueous solution inside a slit filled with ladder-type spacers are investigated. The geometry is asymmetric: the upper-wall is impermeable and the lower-wall is semi-permeable. The total continuity, the Navier–Stokes and the solute continuity equations are numerically solved through the control-volume approach. Two different cases are simulated: case 1––filaments adjacent to the membrane; case 2––filaments adjacent to the impermeable wall. Results are validated against experimental values acquired in a laboratory cell operating with NaCl and sucrose solutions. The slit (2 × 30 × 200 mm) is filled with spacers having transversal filaments placed every 3.8 mm. Results show that the average concentration polarisation for case 1 does not depend on the distance to the inlet as filaments disrupt periodically the concentration boundary layer and, therefore, the concentration polarisation is controlled. Although shear stresses exhibit a cleansing effect for case 2, the concentration polarisation increases uninterruptedly as the concentration boundary layer grows continuously across the channel length. Results demonstrate that the spacersʹ position and filamentsʹ configuration determine the flow structure and the mass transfer efficiency.