A novel retentate spacer geometry for improved spiral wound membrane (SWM) module performance

Abstract

The investigated novel retentate-spacer design comprises spherical nodes symmetrically connected with cylindrical filaments, thus forming a net-type structure with parallelogram unit-cells. Key feature of this configuration is the contact points (or small contact-regions) of nodes with bounding membranes, instead of filament-membrane contact lines characterizing the conventional unwoven-filament geometries; undesirable contact lines create “dead-flow” zones, that promote reduced mass-transfer (i.e. increased concentration polarization-CP) and fouling. Direct numerical simulations of the flow field and mass transfer, complemented by pressure drop and mass-transfer coefficient measurements, reveal the advantages of novel spacers. The flow field, and local shear-stress and mass-transfer-coefficient distributions on membranes, are rather uniform. The novel spacers, considering also the extensive flow constrictions between filaments and membranes, exhibit broader distributions of these time-averaged transport parameters, which extend to higher values (compared to conventional spacers), with insignificant percentage of zero values; a fairly uniform shear-stress distribution is also identified on spacer filaments. Thus, significant benefits are expected in mitigating CP and fouling phenomena in membrane operation. Friction losses show increased dependence on cross-flow velocity (compared to conventional designs), as they are affected more by flow-shearing at the membrane surfaces, than by form-drag. Useful pressure drop and mass-transfer correlations are obtained.