Influence of Reverse Solute Diffusion On the Performance of Pressure Retarded Osmosis

Pressure retarded osmosis (PRO) is an osmotically-driven membrane process and it has attracted increasing interest for extracting the renewable salinity-gradient energy. In a PRO process, the water transports from a less concentrated feed solution (FS) across the membrane to a more concentrated draw solution (DS). Meanwhile, the solute in the DS reversely diffuses into the FS. This study systematically investigated the reverse solute diffusion (RSD) on the performance of PRO. Several types of commercial osmotic membranes provided by HTI were tested in PRO to evaluate the RSD and the resulted PRO performance. In addition, a theoretical mathematical model was developed to simulate the RSD in PRO processes.

In the current study, the RSD was quantified with specific reverse solute flux (J_s/J_w). It was found that the J_s/J_w for all the membranes increased with increasing the applied pressure. However, the experimental (J_s/J_w) matched well with the simulated J_s/J_w at zero applied pressure (i.e., FO operation) but became much greater than the simulated J_s/J_w at greater applied pressures (i.e., PRO operation). The mismatch between the experimental J_s/J_w and simulated J_s/J_w (that assumed the A and B values of the membrane were constant under PRO operation) is due to the severe membrane deformation. It was observed that all the membranes tested under PRO operation underwent severe deformation when the spacers were employed in the FS flow channel to support the membrane. The deformed membrane had its separation properties changed, i.e., the B value substantially increased at higher DP. The substantially increased J_s/J_w in PRO drastically reduced the PRO water flux and the osmotic power output due to the RSD-enhanced internal concentration polarization. The results provide significant implications for PRO membrane design and process operation.