Energy-exergy analysis of seawater reverse osmosis plants

In this paper, a seawater reverse osmosis desalination plant with various energy recovery systems is studied using exergy analysis. These energy recovery devices include turbines and pressure exchangers as well as infinite area based single and two-stage pressure retarded osmosis units. The appropriate exergetic efficiency definition for such systems is mentioned. The effect of pump and turbine efficiency, salinity, temperature and mass ratio is studied using a validated program. In this regard, modified Van't Hoff constants for a large range of seawater surface temperatures are also determined. The best efficiency was obtained using the pressure exchanger for all systems investigated. Use of pressure retarded osmosis units as energy recovery devices provided efficiencies nearly equal to or less than the hydro-turbine. Thus, for the range investigated, it does not seem to be a viable energy recovery method for reverse osmosis units with seawater feed since constraints such as concentration polarization and finite area would further decrease performance. © 2016 Elsevier B.V. All rights reserved.

Different ERD for a SWRO desalination plant are explored using first and second-law analysis. Besides the common ERD, these included single and two-stage PRO as well as PRO coupled with a PX. The reverse osmosis plant performed the best when the PX was used as an energy recovery device. Effect of temperature, salinity, mass ratio as well as turbine and pump isentropic efficiencies was explored. Salinity had the greatest effect on second-law efficiency with absolute variation of ~24% when the PX was used. The specific energy consumption was affected the most by pump isentropic efficiency with absolute variation of 0.4 kWh/m3 when the PX was used. For the factors investigated with their ranges, all PRO configurations considered had exergetic efficiencies lower than or almost equal to the turbine even though effectiveness was taken as unity for the PRO modules. Therefore, PRO does not seem to be an appropriate energy recovery method for the range investigated. Nomenclature b molality (mol/kg) ERD energy recovery device(s) _ m mass flow rate (kg/s) P pressure (kPa) MR mixing ratio R recovery ratio RO reverse osmosis S salinity (g/kg) SEC specific energy consumption (kWh/m3) T temperature (°C) _ V volumetric flow rate (m3/s) x specific exergy (kJ/kg) _ X exergy (kW) _ W power requirement (kW) Greek symbols ε effectiveness η efficiency (%) ρ density (kg/m3) ϕ osmotic coefficient β constant in Eq. (B.1) λ constant in Eq. (B.1) Subscripts act actual B brine err error f feed i entering is isentropic l least min minimum o exiting p permeate pp pump pred predicted px pressure exchanger R0 based on infinitesimal recovery ratio Rg0 based on finite recovery ratio t turbine II second-law Acknowledgment The authors acknowledge support from the King Fahd University of Petroleum and Minerals (KFUPM) through the Center for Clean Water and Clean Energy at MIT and KFUPM under Project Number R15-CW-11. Syed Zubair also acknowledges support from KFUPM through the project IN151001.