CCD Series No-14: SWRO-CCD under fixed-pressure and variable flow compared with fixed-flow and variable pressure conditions

Abstract

A theoretical model on the basis of the conceptual principles of closed circuit desalination (CCD) is explored in the present study for the performance assessment of the single-element seawater desalination reverse osmosis (SWRO) unit ME (E = SWC6-MAX) of near absolute energy efficiency without an energy recovery device (ERD) under fixed-pressure (FP) of variable flow compared with fixed-flow (FF) of variable pressure conditions. Performance simulations of same feed source (3.2% NaCl), sequence duration (9.01 min), recovery (60.3%), average flux (14.9 lmh), concentrate recycling flow (4.0 m3/h), number of CCD cycles (10), and cycle period (0.90 min/cycle) under FP (65 bar) compared with FF (37.1–71.3 bar) conditions revealed the respective specific energy values (SE) of 2.177 and 1.792 kWh/m3 with average permeate Total Dissolved Salts (TDS) of 574 and 464 ppm. The average SE sequential progression under FF conditions (1.234–1.792 kWh/m3) compared with that under FP conditions (2.140–2.177 kWh/m3) reveals the declined percent energy-saving mode of the former as function of increased recovery (R) in the order: 32.2% (R = 40%); 30.1% (R = 45%); 26.4% (R = 50%), 21.9% (R = 55%); and 17.7% (R = 60,3%). Since conventional SWRO desalination of Ocean seawater (~35,000 ppm TDS equivalent to ~3.2 NaCl) is normally being carried out with ~45% recovery, average flux near 15 lmh and an applied pressure around 65 bar using ERD, the findings of the current study suggest RO energy saving of 30.1% before accounting for ERD energy losses if the desalination is performed with SWRO-CCD under FF conditions. This conclusion is supported by the reported SE (2.46 kWh/m3) for the conventional SWRO desalination plant in Australia compared with the extrapolated SE value for Ocean water (~1.70 kWh/m3) from normalized experimental SWRO-CCD data with Mediterranean seawater. The average TDS of permeate projections during the batch sequence progression under FP (110–574 ppm) compared with FF (291–454 ppm) conditions reveals higher quality permeates by the former up to 54% recovery and thereafter, the preference of the latter mode. The observed inversion takes place due to rapidly declined flux under the FP conditions towards the end of the batch sequence which creates a fast rise in TDS per cycle manifested also by the average TDS.