Closed-circuit PRO series no. 6: advanced PRO membranes and technologies
Avi Efraty · Desalination and Water Treatment · 2017
Recent advances in pressure-retarded osmosis (PRO) membranes are viewed and analyzed in the context of the closed-circuit PRO technology (CC-PRO) of near absolute energy conversion efficiency without need of an energy recovery device. The performance of an extended list of advanced PRO membranes in different salinity gradients is analyzed by means of their readily available permeability coefficient (A), actual/ideal forward osmosis flux ratio (), and draw/permeation flow ratio (), and the results of this approach reveal consistent results with rigorous theoretical model calculations and experimental data. CC-PRO simulations on the basis of A, , and illustrate the application this approach for the projection of flux, power density (PD), and net electric power density (NEPD) dependence on applied pressure for various advance PRO membranes with emphasis of their maximum PD and NEPD prospects and the distinction between these terms. NEPD takes into account the power demand of the auxiliary pumps in the PRO system and, therefore, represents the actual power availability of such systems. Experimental PD results derived from small lab-bench systems are shown to be consistent with the A- CC-PRO simulated projections of high flow ratio ( > 40), which correlate to maximum PD of negative NEPD and, therefore, inappropriate for an effective PRO operation. The relationship of PD and NEPD is exemplified for the seawater brine and river water like salinity gradient system using an effective PRO membrane with = 0.447 and the simulated CC-PRO results revealed maximum PD of 21.8 W/m 2 at 23 bar and = 40 under which conditions NEPD is (-)31.8 W/m 2 ; whereas maximum NEPD of 10.0 W/m 2 is attainable at 26 bar and = 3. Apart from the application of the simple A- method for power projections of advanced thin film composites PRO membranes, this study also appraises the PRO technology as a standalone or in hybrid forms for practical applications in different salinity gradients.