Thin film composite membranes: Does the porous support truly have negligible resistance?

Abstract

The permeation resistance of the porous support in a thin film composite (TFC) membrane is believed to be negligible compared with that of the thin polyamide film. Indeed, the resistance of a porous support, as determined from direct water permeation experiments as a function of transmembrane pressure (TMP), is only a fraction of the total resistance of a typical TFC membrane. However, the true resistance of the porous support in a TFC membrane, especially during high-pressure reverse osmosis (RO), could be significantly different from the value obtained via direct permeation measurement. Although the porous support is subjected to mechanical loading equivalent to the overall TMP, the effective TMP across the porous support that generates deformation can be significantly lower due to the internal pressure of the water within the pores. In this study we utilized a serial configuration, consisting of a TFC membrane on top of a representative porous support, to quantify the true resistance of the porous support under conditions of high mechanical loading but low permeation. Unsurprisingly, the results confirm that the porous support resistance appears negligible as compared with that of the TFC membrane under low TMP. However, after exposure to high TMP in the serial configuration, the porous support evidences up to 15X larger plastic strain and 28X higher resistance as compared to the same support tested separately under low TMP. At TMP = 8.3 MPa, the additional porous support beneath the TFC membrane increases the overall resistance by 31%. Using a resistance in-series model, the data suggest that the true resistance of the porous support could be as high as ~ 45% that of the thin film under the high-TMP but low-permeation condition encountered during high-pressure RO desalination.