Closed circuit desalination series no. 8: record saving of RO energy by SWRO-CCD without need of energy recovery

Abstract

A theoretical model is developed for comparing reverse osmosis (RO) energy and permeates quality of closed of circuit desalination (CCD) and conventional plug flew desalination (PFD) under the same conditions. The application of the theoretical model is illustrated by a comparison between a CCD unit with two modules each of four elements and a single module conventional PFD unit with eight elements in the context of ocean water (3.5%) desalination with 50% recovery at 25°C under average flux of 13 LMH and 85% efficiency of feed pressurizing pumps using the same membrane elements (SWC6). This model analysis reveals savings of RO energy by CCD compared with conventional PFD as function of its absolute energy conversion (AEC) efficiency (in bracket) as followed: 9.8% (95%), 13.4% (90%), 17.2% (85%), 20.6% (80%), 23.8% (75%), and 28.7% (70%). Most large and modern conventional seawater RO plants operate with AEC efficiency in the range of 70–80% and therefore, the actual RO energy saved by CCD compared with such conventional techniques is found in the respective range of 28.7–20.6%. Despite the large difference in RO energy revealed by the model analysis between the compared techniques, both produce about the same quality permeates. CCD is a continuously staged and pressure-boosted consecutive sequential technology which operates with near AEC efficiency without need for energy recovery (ER) from brine, and the low RO energy requirements by this technique manifest the average diagonal sequential pressure rise as function of increased sequential recovery under conditions of fixed flow rates of pressurized feed and permeate. According to the model analysis, CCD of ocean water (3.5%) at 13 LMH with 50% recovery proceeds with 1.625 kWh/m3 compared with 1.962 kWh/m3 by conventional techniques with AEC efficiency of 85%. The model analysis energy of 1.625 kWh/m3 for ocean water (3.5%) CCD at 13 LMH with 50% recovery agrees with extrapolated energy for ocean water (1.60–1.70 kWh/m3) from experimental results (2.0–2.1 kWh/m3) received for the Mediterranean water (4.1%) under the same conditions. A pressure-volume work model for high pressure pump (HP) (85% eff.) in CCD revealed under infinitesimally small permeation of near zero flux conditions the theoretical minimum energies 1.40 and 1.25 kWh/m3 for the Mediterranean (4.1%) and ocean (3.5%) water, respectively. Extrapolation of experimental CCD energies (HP + CP) to near zero flux revealed 1.44 and 1.29 kWh/m3 for the Mediterranean and ocean water, respectively. The difference between the extrapolated (HP + CP) and theoretical minimum (HP) revealed the minor circulation pump (CP) energy contributions 0.04 and 0.05 kWh/m3 in CCD of the Mediterranean (2.78%) and ocean (3.87%) water sources, respectively.