Pressure drop in spacer-filled RO feed and permeate channels: Correlation with TMP-induced membrane deformation

In reverse osmosis (RO), membrane deformation under transmembrane pressure (TMP) can impact the feed and permeate channel geometries and thus the performance of spiral-wound elements. In this study, a flow cell was used to evaluate the pressure drop in feed and permeate channels, filled with seawater (SW) and brackish water (BW) spacers, at TMP from 5 to 40 bar alternating between high (pressurized) and low (relaxed) TMP. Membrane deformation was measured in-situ by Optical Coherence Tomography. The results revealed that pressure drop along the channels depends on TMP and its history. High TMP correlated with decreased pressure drop in the feed (channel expansion) and increased in permeate (channel shrink), and generated a permanent effect even in later relaxed conditions. Pressure drop changes with increasing TMP were ascribed to decreased inertial losses in feed and increased frictional losses in permeate, and characterized numerically by pressure loss coefficients. The loss coefficients varied at different rates function of TMP in pressurized and relaxed conditions because of reversible/irreversible membrane deformation. However, the changes in loss coefficients became similar when expressed function of membrane deformation. This allowed to describe by single sets of parameters function of membrane deformation: (i) the linear decline of inertial loss coefficients in feed; (ii) the non-linear (BW) or linear (SW) increase of frictional coefficients in permeate channel. By this method, one could obtain pressure loss parameters usable in projection software, for improving the design of NF, RO and (ultra)high-pressure RO spiral wound elements.