On the analysis of FO mass transfer resistances via CFD analysis and film theory

• A comparative analysis of FO performance via CFD and the film model was carried out.
• RO-derived FO membrane permeability overestimates internal concentration polarization.
• FO improvements depend on permeability and hydrodynamics rather than support thickness.

The use of 2D finite element CFD model and the common film model (FM) for analysis of FO performance data was examined to assess the merits of conclusions derived from the above approaches regarding characterization of membrane and support layer resistances to water and salt permeation. The FM model leads to estimation of porous layer resistivity (K) that implies tortuosity/porosity ratio that is significantly higher than is physically attainable for typical membrane porous supports. Comparative FO data analysis with the 2D CFD model and FM approximation reveals that the latter approach overestimates (by an order of magnitude and greater) the significance of the permeation resistance of the FO membrane support layer. It is also shown that membrane (water and solute) permeabilities extracted from FO operation in the RO mode (FO/RO) are significantly higher than obtained from direct analysis of FO performance data via detailed 2D CFD modeling. The present analysis suggests that improvements in FO water permeability are more likely to emerge from developing higher permeability membrane skin and improved channel hydrodynamics (to decrease external concentration polarization) rather than from more permeable and thinner membrane support layer as has been asserted in various studies.