Second-Pass Vessel Arrangement and Separation-Limited Operation in Two-Pass Seawater Reverse Osmosis Under Progressive Fouling: Implications for Electrolytic Hydrogen Production.

Seawater reverse osmosis (SWRO) supplies the purification chain of electrolytic hydrogen plants, yet its long-term behaviour under fouling is rarely resolved at train level. This work presents a dynamic, quasi-steady-state model of a two-pass SWRO train and compares three second-pass architectures under progressive first-pass fouling, represented by time-evolving water- and solute-permeability coefficients. At each of 51 instants over a 500-day horizon, the operating point is re-optimized to minimize specific energy consumption (SEC), subject to a product limit of 20 mg·L-1 set by the downstream polishing stage rather than the electrolyzer. Two share-membrane types, element count and installed area, differ only in vessel arrangement; the third is the industrial reference. Arrangement alone changes SEC by 36.9-41.6% across three fouling scenarios, 8.38 against 11.46 kWh·m-3 under severe fouling, widening to 71% at fixed production. Fouling constrains the system through separation, not hydraulics: the technically admissible operating window widens as membranes degrade, whereas the window satisfying the product specification closes. The energy penalty of compliance begins at days 310 and 170, respectively, accumulating about twice as fast thereafter in the former. Energy recovery reduces SEC by 63-64% without altering the ordering.