Thermodynamic optimization of Multistage Pressure Retarded Osmosis (MPRO) with variable feed pressures for hypersaline solutions

Abstract

Salinity gradient processes, such as Forward Osmosis and Pressure Retarded Osmosis, have been proven to be promising technologies for reducing the energy consumption in water treatment processes, for energy production, and for energy recovery. Based on the thermodynamic concepts, specifically Gibbs' Free Energy of Mixing, the concentration of the draw solution plays an important role in determining whether the selected salinity gradient process is economically feasible or not. An increase in the salinity of a draw solution does not only increase the osmotic pressure difference between the draw and feed solutions, but also allows a higher hydraulic pressure to be applied on the draw solution, which together greatly increases the volumetric flux of the draw solution per single pass when PRO is used. Even though higher power densities can be achieved by applying higher hydraulic pressures on the draw solution, this requires greater mechanical stability of the membrane to be able to withstand these higher hydraulic pressures. In order to increase the mechanical stability of the membranes, generally, thicker support layers can be applied, which have a direct negative impact on membrane permeability. Therefore, there is a limitation to the salinity of the draw solution which can be used in the PRO processes. This being dependent on the concentration of the hypersaline solution and hence overall hydraulic pressure, necessitating the use of an ultra-thick support layer for maximum energy production and/or recovery. In this theoretical and simulative optimization of the PRO process, we achieved the optimum energy recovery from a hypersaline solution (TDS ~ 300,000 mg/l) by using a multistage PRO (MPRO) system which included implementing variable applied feed pressures to each stage. The results showed that the volumetric flow rate of the hypersaline draw solution increased by up to a factor of 10 during the MPRO process in single pass, and the concentration of the hypersaline draw solution diluted up to 10× accordingly. Energy conversion efficiency of osmotic pressure to hydraulic pressure was found to be around 10% without variable feed pressure and approximately 20% with variable feed pressure.