Membrane-Based Technologies for Sustainable Production of Power Menachem Elimelech Department of Chemical & Environmental Engineering Yale University Water-Energy Nexus Symposium, January 29, 213, Exhibition Center, Tel-Aviv
Energy of Mixing G sep + G mix Gibbs Free Energy of Mixing, G mix G LC c 1 RT mix, V M ln cm clc ln clc chc ln chc
Energy from Natural Salinity Gradients Annual Global River Discharge 1 : ~37,3 km 3? metoffice.gov.uk 1 Dai & Trenberth, J. Hydrometeorology 22, 3 (6), 66 687
G mix, (kwh/m 3 ) V RW V RW Gibbs Free Energy of Mixing, G mix G RW c 1 RT mix, V M ln cm crw ln crw csw ln csw c molar concentration volume fraction of RW # of electrolyte ions 1 R gas constant 9 T absolute temperature River Water Seawater 1. =.1.8.6.4.2....2.4.6.8 1. = V RW /(V RW +V SW ) 3. G mix is theoretical maximum > actual extractable Work 2. 1. G mix, (kj/l)
Pressure Retarded Osmosis (PRO) Power Generation Low concentration W Extract useful work by controlled mixing High concentration PdV Semi-permeable membrane < P <
PRO: A Membrane-Based Process +
Schematic of PRO System Active Layer Draw Solution Support Layer Feed Solution Diluted draw PRESSURE EXCHANGER Semipermeable Membrane Energy High concentration draw TURBINE Diluted draw Low concentration feed Concentrated feed MEMBRANE MODULE
Theoretical Energy (kwh/m 3 ) Water Height (m) Energy of Mixing from Natural Salinity Gradients 16 15 14 13 12 11 1 9 8 SWRO Brine Salt Dome Great Salt Lake Dead Sea 55 5 45 4 35 3 1 Sea Water 3 Saline Solutions For river water seawater mixing: G mix ~.8 kwh/m 3
Power density, W PRO Water Flux and Power Density J w Water Flux: A m P Water flux, J w Power Density: W J w P P
Power density, W (W/m 2 ) Water flux, J w (L m -2 h -1 ) RO Membranes Perform Poorly in PRO Commercial RO polyamide thin-film composite membrane 24 16 TFC-RO 8 16 m 6 5 1 15 2 25 4 Target: 5 W/m 2 Skilhagen Desal. & Water Treatment 21, 15 (1-3), 271-278 2 5 1 15 2 25 P (bar) Draw: Seawater Feed: River water
Support Layer Structure Controls Internal Concentration Polarization Solute resistance to diffusion, K t s D Structural parameter of porous support, S Tortuosity, Large S results in severe ICP Porosity, Thickness, t s D = draw solute diffusivity Commercial RO Membrane
Power density, W (W/m 2 ) Water flux, J w (L m -2 h -1 ) Commercial Cellulose Acetate FO Membrane has Limitations Commercial FO cellulose triacetate asymmetric membrane 24 16 CTA-FO TFC-RO 8 m 8 6 5 1 15 2 25 Limitations Low intrinsic water permeability Low salt rejection Narrow operable ph range 4 2 5 1 15 2 25 P (bar) Draw: Seawater Feed: River water
Redesigning the Thin-Film Composite (TFC) Membrane Thin Porous Non-tortuous Chemically Robust High Salt Selectivity High Water Permeability PET fabric Polysulfone (PSf) Partly enmeshed with PET Thin low-density polyester fabric (PET) Phase Separation 4 μm Polyamide (PA) Support layer 4 μm 2 μm Interfacial Polymerization 8-1 μm TFC membrane
Role of Solvent in Phase Separation More favorable solvent More thermodynamically stable Rapid precipitation Finger-like structure Less favorable solvent Less thermodynamically stable Delayed precipitation Sponge-like structure NMP (N-methyl pyrrolidinone) DMF (dimethylformamide)
Increase Membrane Permeability on the Expense of Membrane Selectivity Post-treatment of polyamide selective layer Membrane Chlorine Alkaline permeability LP Low permeability MP Medium permeability HP High permeability No post-treatment 1, ppm NaOCl @ ph 7., 6 mins 2, ppm NaOCl @ ph 7., 12 mins Yip et al. Environ. Sci. Technol. 211, 45, 436 4369.1 M NaOH, 16 h.1 M NaOH, 62 h
Water permeability, A (L m -2 h -1 bar -1 ) Solute permeability, B (L m -2 h -1 ) Structural parameter, S (m) Membrane Characteristics 8 6 Water permeability, A Solute permeability, B Structural parameter, S 6 5 4 n = 3 RO experiments DI water flux at 25 psi 4 2 3 2 1 FO experiments Draw:.5 M NaCl Feed: DI water Water flux, J w, reverse salt flux, J s All tests conducted at 25 C LP MP HP TFC-PRO membrane Yip et al. Environ. Sci. Technol. 211, 45, 436 4369
Power density, W (W/m 2 ) Water flux, J w (L m -2 h -1 ) Water permeability, A (L m -2 h -1 bar -1 ) Solute permeability, B (L m -2 h -1 ) Structural Parameter, S (m) Water permeability, A (L m -2 h -1 bar -1 ) Solute permeability, B (L m -2 h -1 ) Structural Parameter, S (m) Water permeability, A (L m -2 h -1 bar -1 ) Solute permeability, B (L m -2 h -1 ) Structural Parameter, S (m) TFC-PRO Membrane Performance 8 5 8 5 LP MP HP 4 4 6 6 8 6 5 4 4 3 2 4 3 2 4 3 2 2 1 2 1 2 1 6 4 2 A B S Membrane Characteristic Parameters Draw: Seawater Feed: Riverwater Brackish Water A B S Membrane Characteristic Parameters A B S Membrane Characteristic Parameters 6 4 2 12 5 1 15 2 25 5 1 15 2 25 5 1 15 2 25 12 1 1 8 8 6 6 4 4 2 2 5 1 15 2 25 5 1 15 2 25 5 1 15 2 25 P (bar) P (bar) P (bar)
PRO Water Flux J w Water Flux: J Jw A m J PS w w D, b exp Salt Flux: Jk F, b exp s BC m D A J S dc x s J 1 B exp w exp w J J wc D k w dx ICP Boundary Layer: x C C F, b t s C t s C F, P x Performance-Limiting Phenomena: x m 1.External Concentration Polarization ECP Boundary Layer: 2.Internal Concentration z C Polarization C D, m 3.Reverse Salt z Permeation C C D, b Yip et al. Environ. Sci. Technol. 211, 45, 436 4369
Salt Permeability, B (L m -2 h -1 ) Permeability-Selectivity Tradeoff 1 8 6 4 2 Hand-cast SW3 Fitting to eq R 2 =.873 Chlorine-alkaline membrane post-treatment Handcast membranes - 3 modifications (LP, MP, HP) Commercial seawater RO membranes (Filmtec SW3) - 7 modifications (IVII) 2 4 6 8 1 Water Permeability, A (L m -2 h -1 bar -1 ) Yip et al. Environ. Sci. Technol. 211, 45, 1273 1282 Water-Salt Permeability Tradeoff Relation: B 2 L RT M w 3 A 3
Structural Parameter, S (m) Optimal Active Layer Maximizes W peak Membrane properties A, B, and S determine power density 1, NaCl Permeability, B (L m -2 h -1 ) Power.11 Density Eqn: W 5..85 2.87 6.81 13.3 D,b = J 26.14 bar (Seawater) w P F,b =.45 bar (River Water) Increase Increase permeability permeability Increase Decrease power density power density Water-Salt Permeability 1, PRO Water Flux Eqn: Tradeoff Relation 3 J 3 w J ws 2 D, b exp k F, b exp L RT 3 D B A J w A 1. P 7.5 M 1 J S J w 1 B w exp exp w J w D k 12.5 Peak Power Density, W peak (W/m 2 ) 2.5 5. 7.5 1. 12.5 2.5 1 15. 2 4 6 8 1 Water Permeability, A (L m -2 h -1 bar -1 ) 15. 17.5 Yip et al. Environ. Sci. Technol. 211, 45, 1273 1282
Limitations of Open-Loop PRO with Natural Waters Environmental impacts of locating PRO plants at estuaries and bays Need extensive pretreatment and fouling control measures; fouling may be inevitable Limited effective driving force for river water seawater system Low power density and hence large capital cost (need a large membrane area)
How Much Energy We Can Extract? Seawater-River Water PRO System SW: 6 mm (35 g/l) NaCl RW: 1.5 mm ( 88 mg/l) NaCl G mix =.75 kwh/m 3 Practical Constraints in Operation: actual = 6% W actual =.45 kwh/m 3 River Water Losses due to inefficiencies: ~2% Seawater Pressure Retarded Osmosis Power Generation W =.35 kwh/m 3 Pre-treatment ~.15 kwh/m 3 W ~.2 kwh/m 3 = Frictional Losses G mix + Unutilized Energy + Extractable Work Yip, N.Y. and Elimelech, M. "Thermodynamic and Energy Efficiency Analysis of Power Generation from Natural Salinity Gradients by Pressure Retarded Osmosis", Environmental Science & Technology, 46, 212, 523 5239.
Mixed solution Low concentration High concentration Closed Loop PRO with Synthetic Solutions: The Heat Engine Heat Thermal Separation PRO Energy
Closed Loop Ammonia-Carbon Dioxide Osmotic Heat Engine McGinnis, R.L., McCutcheon, J.R. and Elimelech, M. "A Novel Ammonia-Carbon Dioxide Osmotic Heat Engine for Power Generation", Journal of Membrane Science, 35, 27, 13-19.
High Embedded Energy in NH 3 - CO 2 Draw Solution
High Power Densities with Osmotic Heat Engine
The Promise: Concluding Remarks A lot of energy is contained in natural and synthetic saline solutions The Challenges: Need to develop robust, high performance, low cost membranes Overcome fouling when using natural waters The Prospects: Engineers are well experienced with membranebased separation systems
Acknowledgments Research group at Yale Funding: National Science Foundation, Office of Naval Research