Comparison of flow structures in spacer-filled flat and annular channels*

Spacers are designed to create directional changes in the flow through membrane modules. Such secondary flow structures reduce concentration polarization and membrane fouling. Obviously, type of the spacer used in membrane modules strongly influences the resulting flow and therefore performance of the module. In this work, we have modeled detailed fluid dynamics of spacer filled channels by using the ‘unit cell’ approach. The validated computational fluid dynamics (CFD) model was used to evaluate performance of certain spacer shapes and compare the resulting fluid dynamics in flat and curved channels. The simulations show that fluid flow behavior in a spacerfilled flat and a spiral channel was not significantly different. This means that pressure drops and mass transfer coefficients measured in flat channels may provide adequate guidelines, which would also be valid for spiralwound modules. The results presented in this work will have significant implications for identifying improved spacers with higher propensities to reduce fouling in membrane modules.

The 3D CFD models based on unit cell approach adopted in this work appear to be promising for understanding influence of spacer shapes on resulting flow and mixing in membrane modules. The flow in spacer filled flat and annular channels is complex and requires use of significantly large number of computational cells to ensure that results are independent of number of computational cells. It is therefore very difficult to carry out detailed 3D simulations of a full membrane module. However a ‘unit cell’ approach can be used. Fluid dynamics of spacer filled flat and curved channels was not significantly different since the secondary flows created by the spacers are much stronger than those influenced by low values of torsion and curvature of the channel. Results obtained with flat modules could therefore be used to estimate performances of spiral wound membrane modules with spacers.