Numerical study of a hybrid multi-stage vacuum membrane distillation and pressure-retarded osmosis system

In this study, we introduce a hybrid system that integrates a multi-stage vacuum membrane distillation (MVMD) with pressure-retarded osmosis (PRO). The MVMD system employs a recycling flow scheme (MVDM-R) for the continuous production of both distillate water and highly concentrated brine. The concentrated brine that is produced from the MVMD-R system is then used as a draw solution for power generation in the PRO system. We theoretically assessed the distillate and power production of the MVMD-R-PRO system with respect to inlet feed flow rate and recycling flow ratio in the MVMD-R system. When the inlet feed flow rate is constant, the production of distilled water increases slightly, with a decrease in the recycling flow. The maximum possible brine concentration from the MVMD-R system is 1.9 M NaCl at an inlet feed flow rate of 3 kg/min and a 90% recycling flow. A maximum power density of 9.7 W/m2 is achieved when river water is used as a feed solution in the PRO system at feed and draw solution flow rates of 0.5 kg/min and a constant hydraulic pressure difference. © 2015 Elsevier B.V. All rights reserved.

Fig. 8. Effect of the recycling flow ratio on (a) mean permeate flux, (b) discharged brine concentration, and 0 (c) discharged brine flow with respect to inlet feed flow rate in the MVMD-R system (Tf,in = 80 °C, vf = 3–12 kg/min, RR = 5–90%, Pv = 4 kPa). the discharged brine flow decreases linearly with an increasing recycling flow ratio. Highly concentrated brine discharged from the MVMD-R system is provided as a draw solution to the PRO system with an inlet draw flow rate of 0.5 kg/min. As seen in Fig. 8(c), the draw flow rate requirements in the PRO system cannot be attained for the following operating conditions of the MVMD-R system: (i) 80, 85 and 90% recycling flow ratios at In this study, we proposed a hybrid multi-stage VMD with a recycling flow scheme and a PRO system to produce both fresh water and power. The MVMD system employs a recycling flow scheme (MVDM-R) for the continuous production of both distillate water and highly concentrated brine. The concentrated brine produced from the MVMD-R system is then provided as a draw solution to the PRO system for power generation. The distillate and power production of the MVMD-R-PRO system are theoretically assessed with respect to the inlet feed flow rate and the recycling flow ratio in the MVMD-R system. When the inlet feed flow rate is kept constant, the production of distilled water increases slightly with a decrease in the recycling flow ratio. In addition, the discharged brine concentration increases with an increase in the recycling flow ratio, while its value decreases with an increase in the feed flow rate at a fixed recycling flow ratio. The discharged brine concentration increases dramatically when the recycling flow ratio is approximately 90%. The maximum possible brine concentration from the MVMD-R system is 1.9 M NaCl at a feed flow rate of 3 kg/min and a recycling flow of 90%. The maximum power density of 9.7 W/m2 is achieved when river water is used as a feed solution in the PRO system at feed and draw solution flow rates of 0.5 kg/min and a constant hydraulic pressure difference of 1300 kPa. Nomenclature Aw Water permeability coefficient [m2/s/kPa] Bs Salt permeability coefficient [m/s] B′ Membrane distillation coefficient [kg/m2/s/pa] c Salt concentration [g/kg] Cp Specific heat [J/mol K] dh Hydraulic diameter [m] di Inside diameter of a fiber [m] do Outside diameter of a fiber [m] ds Inside diameter of a shell [m] h Convective heat transfer coefficient [W/m2 K] ΔH Enthalpy of water evaporation [J/kg] Jv Local permeate flux from MVMD-R system [kg/m2 h] Jw Water flux from PRO system [m/s] k External concentration polarization coefficient [m/s] K Internal concentration polarization coefficient [s/m] L Module length [m] m: Mass flow rate [kg/min] M Molecular weight [g/mol] N Number of fibers [−] P Pressure [Pa] ΔP Hydraulic pressure difference [Pa] Q Heat energy [W] R Ideal gas constant [J/mol K] r Mean pore size [μm] T Temperature [K] w Mass fraction in liquid phase [−] x Molar fraction in liquid phase [−] v Velocity [m/s] V Molar volume [m2/mol] z Axial coordinate for a hollow fiber [m] Dimensionless numbers Nu Nusselt number [−] Pr Prandtl number [−] Re Reynolds number [−] Greek letters δ Membrane thickness [m] ε Membrane porosity [m] μ Kinematic viscosity [Pas] θ Angle between the direction of flow and the hollow fiber axis [θ] ρ Density [kg/m3] ϕ Packing density [%] τ Tortuosity [−] πD,b Osmotic pressure of the bulk draw solution [kPa] πF,b Osmotic pressure of the bulk feed solution [kPa] Subscripts b Bulk p Permeate side f Feed side D Draw solution F Feed solution s Salt w Water m Membrane or mean C Circulation pump v Vacuum or VMD h Heat Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MSIP) (2014R1A2A2A01006899).