449249 Conditioning Membranes for Improved PRO Performance
Maximizing energy production from PRO starts with designing membranes that are able to withstand PRO conditions. An appropriately designed PRO membrane exhibit high water permeance to limit resistance to water flux, excellent salt rejection to maintain the osmotic driving force, low structure parameter to minimize internal concentration polarization (ICP), and robust strength to withstand the required hydraulic pressure.
There are multiple trade-offs for making PRO membranes, however. As is typical with many membrane processes, increasing selectivity to reduce ICP will generally result in lower water permeance. Making low structural parameter membranes (thin, highly porous support layers) make them less tolerant to pressure. A low structural parameter may also make the membrane susceptible to severe compaction (which would ultimately enhance ICP). These tradeoffs make approaching membrane performance from a membrane design standpoint challenging.
We have recently identified a method of conditioning an existing membrane prior to PRO testing that resulted in improved overall PRO performance (higher water flux, lower salt flux, and higher power density). By conditioning the membrane under 150 psi pressure (essentially in RO) for 2 hours prior to testing under relevant PRO conditions, water flux increases by 17% and salt flux drops by an astonishing 85%. We believe this low pressure RO treatment may “heal” defects in the membrane through modest fouling caused by particles and organics present in any membrane system. The fouling results in a slightly lower permeance, which mitigates some of the water flux improvements in PRO, the lower salt flux lessens ICP. The results suggests that better utilization of osmotic pressure is possible in PRO systems when the membranes can be conditioned under pressure.
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