Highly Robust Thin-Film Composite Pressure Retarded Osmosis (PRO

Jun 17, 2013 - The practical application of pressure retarded osmosis (PRO) technology for renewable blue energy (i.e., osmotic power generation) from...
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Highly Robust Thin-Film Composite Pressure Retarded Osmosis (PRO) Hollow Fiber Membranes with High Power Densities for Renewable Salinity-Gradient Energy Generation Gang Han, Peng Wang, and Tai-Shung Chung* Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117602 S Supporting Information *

ABSTRACT: The practical application of pressure retarded osmosis (PRO) technology for renewable blue energy (i.e., osmotic power generation) from salinity gradient is being hindered by the absence of effective membranes. Compared to flat-sheet membranes, membranes with a hollow fiber configuration are of great interest due to their high packing density and spacer-free module fabrication. However, the development of PRO hollow fiber membranes is still in its infancy. This study aims to open up new perspectives and design strategies to molecularly construct highly robust thin film composite (TFC) PRO hollow fiber membranes with high power densities. The newly developed TFC PRO membranes consist of a selective polyamide skin formed on the lumen side of well-constructed Matrimid hollow fiber supports via interfacial polymerization. For the first time, laboratory PRO power generation tests demonstrate that the newly developed PRO hollow fiber membranes can withstand trans-membrane pressures up to 16 bar and exhibit a peak power density as high as 14 W/m2 using seawater brine (1.0 M NaCl) as the draw solution and deionized water as the feed. We believe that the developed TFC PRO hollow fiber membranes have great potential for osmotic power harvesting.

1. INTRODUCTION Renewable blue energy production has attracted rapid attention due to the explosive increase in energy demand and the depletion of fossil fuel resources in addition to the global trend toward environmental sustainability.1 Salinity-gradient energy (i.e., osmotic power) generated from the mixing of solutions with different salinities via pressure retarded osmosis (PRO) represents a high potential source of renewable energy.2−7 The latest estimation of osmotic power generated from the mixing of river water and seawater alone is approximately 1600 TW h per year.6,7 More energy can be generated when high salinity reverse osmosis (RO) retentate is purposely mixed with recycled water. Not only can this osmotic energy lower the overall energy consumption for the RO process, but it can also solve the disposal problem of RO retentate.4,5 In a typical PRO process, water is osmotically drawn from a low-salinity feed to a pressurized high-salinity solution across a semipermeable membrane due to the water chemical potential gradient. The continuous water influx into the high pressure compartment provides the driving force to run the hydroturbine for electricity generation. Mathematically, the power density is a product of the trans-membrane hydraulic pressure and the water permeation flux across the membrane.3,6 Based on Statkraft’s analyses on commercially viable PRO processes, the power density should be larger than 5 W/m2 for flat membranes in order to lower the capital cost and footprint even for modest PRO plants.6,7 However, the current PRO technology is hindered by the absence of effective PRO © 2013 American Chemical Society

membranes with outstanding mechanical strength and power density.8−11 Traditional RO membranes for seawater desalination and commercially available cellulose triacetate (CTA) forward osmosis (FO) membranes all showed low power densities less than 4.0 W/m2 due to the severe concentration polarization and relatively low intrinsic water permeability of CTA, respectively.8,11−13 Recently, some novel flat-sheet thin-film composite (TFC) PRO membranes have been developed and reported.14−17 However, few studies have been devoted to the engineering design of PRO hollow fiber membranes for osmotic power generation. Compared with flat membranes, membranes with a hollow fiber configuration are of great interest because of their high packing density and ease of module fabrication. Most importantly, hollow fiber modules may not require spacers between the membranes.18 Not only could this minimize membrane deformation and structure parameter enhancement owing to unavoidable spacer− membrane interactions under high-pressure PRO operations, but also eliminate the extra energy consumption for water transport through the spacers.18,19 Chou et al. reported one TFC-PRO hollow fiber membrane, but their hollow fibers can only withstand a hydraulic pressure of less than 9 bar.20 Received: Revised: Accepted: Published: 8070

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Figure 1. Control of the phase inversion process with the aid of (a)21 dope-solvent coextrusion technology employing a dual-layer spinneret; and (b)22 dual-bath coagulation technology using a single-layer spinneret.

Figure 2. SEM micrographs of different bulk and surface morphologies of HF-1 (left) and HF-3 (right) hollow fiber supports.

effectively control the phase inversion during membrane formation and obtain the desirable membrane structure and morphology.21−23 Table S1 summarizes the detailed spinning conditions of all hollow fiber supports. The detailed procedures for membrane post-treatment and module fabrication are described in the Supporting Information (SI). 2.2. Interfacial Polymerization of TFC-PRO Hollow Fiber Membranes. As shown in Figure S1, the polyamide selective layer was formed on the inner surface of the fabricated hollow fiber supports by interfacial polymerization between mphenylenediamine (MPD) and trimesoyl chloride (TMC). The detailed specification of the experimental setup and preparation steps is disclosed in the SI. 2.3. Membrane Characterizations. Membrane morphology was observed by a field-emission scanning electron microscope, while surface hydrophilicity was evaluated by water contact angle measurements on the inner membrane surface using a Contact Angle Geniometer.21 Membrane porosity of the hollow fiber support was measured using a standard protocol described by Han et al. and Sukitpaneenit et al.17,21 Fiber mechanical properties were determined by an Instron tensiometer at a constant elongation rate of 10 mm

Theoretically, the preferable operating trans-membrane pressure during the mixing of river water and seawater (or seawater brine) in PRO is at about 13.5 bar (or higher) in order to generate the maximal energy. Therefore, the objectives of this work are to develop novel TFC-PRO hollow fiber membranes with favorable robustness and high power density for osmotic power production. By effectively controlling the phase inversion during membrane formation, the hollow fiber supports were designed to possess different dimensions and morphologies. Polyamide selective skin was formed on the lumen side of the hollow fibers via a simple and highly reproducible interfacial polymerization. The PRO performance of the newly developed TFC hollow fibers was evaluated via a lab scale PRO setup to demonstrate their potential for osmotic power generation.

2. EXPERIMENTAL METHODS 2.1. Fabrication of Highly Robust Hollow Fiber Supports. The hollow fiber membrane supports were prepared via a dry-jet wet phase inversion spinning process. As illustrated in Figure 1, both dope-solvent coextrusion and dual-bath coagulation technologies were explored during spinning to 8071

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min−1. Pure water permeability (PWP) of the hollow fiber supports was measured using a lab-scale filtration unit described in SI and previously.21,24 The pore size, pore size distribution, and molecular weight cutoff (MWCO) of the hollow fiber supports were measured by solute rejection experiments using polyethylene glycol (PEG) as a neutral rejection probe as described in SI.25 Mass transport characteristics of the TFCPRO hollow fiber membranes, including the intrinsic water permeability (A), the salt rejection rate (R), and the salt permeability (B), were characterized by testing the membranes under the RO mode via a lab scale circulating RO filtration apparatus following the method described elsewhere.17,21 2.4. PRO Tests. Figure S2 shows the schematic diagram of the lab scale PRO setup for hollow fiber membrane tests. A hydraulic pressure was applied on the draw solution and increased to predetermined values. NaCl was used as the solute to simulate the water sources of both draw and feed solutions. The detailed experimental setup, operating conditions, and the determination of water and salt flux and membrane structure parameters are described in the SI and elsewhere.21,26,27

support and reduce the ICP effects; however, the HF-3 membrane is expected to possess higher membrane strength due to its connected outer surface. Table S2 summarizes the basic characteristics of the as-spun hollow fiber supports including the mean pore size, pure water permeability (PWP), molecular-weight cutoff (MWCO), porosity, and water contact angle. Figure S4 shows that all spun hollow fibers possess a small mean pore size less than 7 nm. This pore feature is favorable for the formation of a continuous and homogeneous polyamide layer in the subsequent interfacial polymerization.21,27 The HF-2 membrane has a smaller pore size distribution than HF-1 mainly due to the effect of gravity elongation because the former has a larger air gap distance during spinning than the latter. The PWP of each developed hollow fiber support is higher than 245 L/(m2·bar·h) mainly due to its high overall porosity (72.4− 79.0%). In order to investigate the stability of the hollow fiber supports in high pressure PRO tests, the evolution of their PWP as a function of ΔP was explored using the PRO setup. As shown in Figure 3a, the normalized PWP of each developed

3. RESULTS AND DISCUSSION 3.1. Characteristics of the Hollow Fiber Supports. The newly developed TFC-PRO hollow fiber membranes are designed with an inner polyamide selective layer and a porous outer surface to reduce ICP. Figure 2 and Figure S3 show the typical morphologies of the hollow fiber supports fabricated from different protocols. The fibers were designed to have an inner diameter ranging from 460 to 820 μm (Table S2). A similar cross-section structure consisting of small elongated and tear-shape macrovoids in the middle of the cross-section is observed for all hollow fibers. This is mainly due to an instantaneous de-mixing induced by the non-solvent-rich (70 wt % water) bore fluid. A thick open-cell sponge-like layer (larger than 40 μm) was observed underneath the membrane inner surface for all three fibers, which is critical for interfacial polymerization to form a robust TFC-PRO membrane.28 Credit to the fast phase inversion induced by the water-rich bore fluid, all as-spun fibers have smooth inner surfaces with uniformly distributed small pores. This help form a less defective polyamide layer with a high water permeability and salt rejection during interfacial polymerization.16,17,28 Interestingly, the outer surfaces and cross-section morphologies underneath the outer surfaces of the hollow fibers are varied with different fabrication methods. HF-1 and HF-2 membranes fabricated via the dope-solvent coextrusion technology exhibit a quite similar outer surface with fully open porous rough morphology. This is due to the fact that a pure NMP solvent was fed at the outer channel of the tri-orifice spinneret during spinning that not only reduces the polymer concentration at the outer surface of the nascent fiber, but also delays the phase inversion in the air-gap region prior to entering the water coagulant (Figure 1). However, HF-3 membrane fabricated from a dual-bath coagulation technology exhibit a porous outer surface with disconnected big pores. The isopropyl alcohol (IPA) rich coagulant in the first bath reduces the solvent−nonsolvent demixing rate, which provides time for the nuclei formation and surface pore formation. However, the IPA/water mixture has stronger coagulation strength than the NMP used in the dope-solvent coextrusion process. Thus, a relative thick outer surface with discontinuous big pores was formed.23 All aforementioned fibers have porous cross-section morphologies that facilitate water and salt transportation in the

Figure 3. (a) Variations of the normalized pure water permeability (PWP) as a function of hydraulic pressure; and (b) the “critical pressure” of the hollow fiber supports (the pressure represented by the filled symbol in (a)).

hollow fiber support decreases with an increase in ΔP due to membrane compaction, but relatively slightly.16,17 However, HF-2 and HF-3 exhibit much smaller reductions compared to HF-1 at equal pressure. This may be due to the fact that HF-2 has a smaller fiber dimension and HF-3 has a continuous outer surface morphology. As a result, they have better membrane mechanical strength and anticompaction ability. Interestingly, it is worth noting that the PWP suddenly changed to increase when the hydraulic pressure is beyond a 8072

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Figure 4. Typical morphology of TFC-PRO hollow fiber membranes.

Table 1 summarizes the intrinsic transport properties of the developed TFC-PRO hollow fiber membranes in terms of pure

certain value. This is possibly due to the microstructural changes caused by the greatly increased membrane surface area and reduced thickness induced by the high pressure in the lumen side. In this study, the pressure at which the PWP begins to increase is defined as “critical pressure” to quantitatively characterize the hollow fiber membrane overall robustness. It is believed that the polyamide selective layer of the TFC hollow fibers with an inner selective layer will form significant defects when the applied hydraulic pressure is larger than the defined “critical pressure”. The newly developed hollow fiber supports show excellent “critical pressures”, particularly for HF-2 and HF-3. They are 13 and 16 bar, respectively (Figure 3(b)). To the best of our knowledge, these values are superior to other reported PRO hollow fibers in terms of “critical pressure” or burst pressure.5,8,9,15,20 In order to investigate the sources of membrane robustness, the mechanical properties of the newly developed hollow fiber supports was measured in terms of Young’s modulus, tensile strength, and elongation at break, as summarized in Table S3. Since the hollow fiber membrane under high pressure PRO processes will be subjected to several forces together such as compaction, expansion, bending, and shear forces, membrane toughness was also estimated by integrating the stress−strain curve. Interestingly, HF-3 shows the highest membrane toughness, followed by HF-2 and then HF-1. This toughness order seems to be consistent with their critical pressures and PWP stability under PRO tests. In summary, the newly developed hollow fiber membrane supports show desirable characteristics of being supports for TFC-PRO membranes. 3.2. Characteristics of TFC-PRO Hollow Fiber Membranes. A selective polyamide skin was formed on the inner surface of the newly developed hollow fiber supports via interfacial polymerization as depicted in Figure S1. Figure 4 shows the surface and cross-sectional morphologies of the fabricated TFC-PRO hollow fiber membranes. A typical thin layer of “ridge-and-valley” morphology has been attached onto the inner surfaces of the supports. The estimated thickness of the selective polyamide layer varies from 100 to 200 nm. The water contact angle of the inner surface is lowered from about 80.5° to about 48.6° after interfacial polymerization. This increase in surface hydrophilicity also confirms the successful formation of the polyamide selective skin. The hydrophilic thinner polyamide layer is crucial to achieve high water permeation and potentially high power generating efficiency.

Table 1. Transport Properties and Structural Parameters of TFC-PRO Hollow Fiber Membranes

membrane

water permeability, A [L/(m2· bar·hr)]

salt rejection (200 ppm @ 1 bar)

salt permeability, B (L m−2 h−1)

Km (×105 s m−1)

S (×10−4 m)

TFC-HF1 TFC-HF2 TFC-HF3

1.40 1.70 1.90

89.20% 88.50% 87.80%

0.13 0.41 0.48

6.82 5.04 5.24

9.87 7.45 7.76

water permeability (A), salt permeability coefficient (B), and membrane structure parameter (S). TFC-HF1 and TFC-HF2 membranes made of hollow fiber supports from the dopesolvent coextrusion technology exhibit a water permeability of 1.40 and 1.70 L/(m2·bar·hr) with relatively small salt permeability of 0.13 and 0.41 L m−2 h−1, respectively. The TFC-HF3 membrane made of a support from the dual-bath coagulation technology show the highest water permeability of 1.90 L/(m2·bar·hr) with a salt permeability of 0.48 L m−2 h−1. Compared with the commercial HTI FO membranes and some reported TFC membranes, the newly developed TFC-PRO hollow fiber membranes possess much higher water permeability.3,8,9 The FO performance (ΔP = 0) of the TFC-PRO hollow fibers was evaluated using deionized water as the feed and 1 M NaCl as the draw solution under the PRO mode. As summarized in Table S4, a water flux as high as 30−38 LMH with a salt reverse flux of 8−10 gMH can be achieved. Comparing with other reported FO membranes, the newly developed TFC-PRO hollow fiber membranes possess remarkably high water fluxes and relatively low salt leakage.4,5,8,9 This superior performance may be attributed to the inherent characteristics of the hollow fiber membrane configuration in addition to the well-structured polyamide selective layer. Since membrane structural parameter (S) plays a determining role on membranes’ PRO performance,13,18 Table 1 indicates that these TFC-PRO membranes have reasonably small S in the range of 776−987 μm. The TFC-HF3 membrane made of a support from the dual-bath coagulation exhibits a relatively smaller structure parameter, and the order of structure 8073

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Figure 5. Water flux and power density of the developed TFC-PRO hollow fiber membranes with seawater brine (1 M NaCl) as draw solution, and fresh water as feed solution.

Figure 6. Schematic of membrane properties variations of the TFC-PRO hollow fiber membranes during PRO operations.

parameter of these TFC-PRO membranes follows: TFC-HF2 ≈ TFC-HF3 < TFC-HF1. 3.3. Implications of Developed TFC-PRO Hollow Fibers for Osmotic Power Generation. The applicability and power output of the newly developed TFC-PRO hollow fibers were evaluated using different synthetic brine as draw solutions and several water sources as feed solutions. Table S5 lists the details of synthetic solutions for PRO tests where NaCl is the model solute. Figure 5(a) compares the water flux (Jw) as

a function of hydraulic pressure difference (ΔP) across the TFC-PRO hollow fiber membranes using 1.0 M NaCl synthetic brine as the draw solution and deionized water as the feed. As ΔP rapidly increases, Jw exhibits a nearly linear decrease due to the reduced driving force and membrane compaction. At the same ΔP, TFC-HF3 and TFC-HF2 show substantially higher water fluxes than the TFC-HF1 membrane. This could be attributed to their higher water permeability (A) and smaller membrane structure parameter (S) (Table 1). 8074

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Figure 7. Power density of the TFC-HF3 PRO hollow fiber membranes with seawater brine (1 M NaCl) as draw solution, and river water and wastewater brine as feed solutions.

An interesting phenomenon was observed that the water flux suddenly begins to increase when ΔP reaches a certain value, which is contrary to the theoretical prediction. This may be due to the unique hollow fiber configuration and the property changes in the polyamide selective layer induced by the high pressure water flow in the lumen side. As illustrated in Figure 6(a), the hollow fiber support and polyamide layer will be expanded by the increased ΔP in the lumen side because of the highly porous and polymeric nature, resulting in an enlarged surface area and reduced thickness. Therefore, the effective channel dimension and length for water transport through the TFC layer may be slightly enlarged and shortened respectively due to the stretching and thinning of the polyamide layer. However, the polyamide selective layer may experience irreversible changes, and minor defects are formed before physically breaking away from the hollow fiber support when ΔP is beyond a certain value, as illustrated in Figure 6(b). If we define the “critical pressure” for TFC-PRO hollow fiber membranes with an inner polyamide selective layer as the pressure (ΔP) at which the water flux begins to increase sharply. It is interesting to note that this “critical pressure” is quite similar and closely related to the afore-defined “critical pressure” for their corresponding hollow fiber supports (Figure 3), such as 16 bar for TFC-HF3, 12 bar for TFC-HF2, and 10 bar for TFC-HF1 membranes. In addition, the TFC-PRO hollow fibers with an inner polyamide selective layer could be prestabilized using a pressure below the “critical pressure”. As shown in Figure 5(c), after being stabilized by rapidly

increasing ΔP to 16 bar, the TFC-HF3 membrane shows a higher and stabilized water flux as confirmed by the hysteresis tests. Figure 5(b) and (d) plot power density (W) as a function of ΔP; the newly developed TFC-PRO hollow fibers can withstand a ΔP as high as 10−16 bar with a stable power density up to 6−14 W/m2 when using 1 M NaCl synthetic brine as the draw solution and fresh water as the feed. Particularly, TFC-HF3 membrane exhibits the highest W of 14 W/m2 at 16 bar, which is attributed to its superior mechanical properties (Table S3) and highest water permeability (Table 1 and Table S4). In addition, the power output is very stable and repeatable confirmed by the 1.5 hysteresis tests under a hydraulic pressure varying from 0 to 16 bar. To the best of our knowledge, this PRO performance (i.e., operating pressure of 16 bar and power density of 14 W/m2) outperform all other PRO hollow fibers and most flat-sheet membranes reported in the literature.8,9,14−17 The prestabilized TFC-HF3 PRO membrane was further evaluated using synthetic river water (10 mM NaCl) and wastewater (40 mM NaCl) as feeds. As shown in Figure 7, a slightly reduction in power density is observed with an increase in feedwater salinity. For example, power density drops from 14.0 W/m2 to 11.5 W/m2 and 9.5 W/m2 at 16 bar when replacing fresh water by synthetic river water and wastewater, respectively. This reduction is caused by the combinative effects of reduced osmotic driving force and enhanced ICP effects. However, the obtained power density is still superior to most 8075

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Table 2. Comparison of the PRO Membrane Performance salty water Seawater brine (1.0 M NaCl)

fresh water

Seawater (3.5 wt % NaCl)

Tap water Tap water River water (10 mM NaCl) Waste water brine (40 mM NaCl) River water (10 mM NaCl) Waste water brine (40 mM NaCl) DI water

60 g/L NaCl (1.03 M NaCl) π = 101.3 bar π = 25.3 bar π = 78.0 bar π = 81.1 bar

DI water Water Water Water Water with 0.2% formaldehyde

Seawater brine (1.0 M NaCl)

operation pressure, P (bar)

power density, W (W/m2)

12 16 16 16

8.9 14.0 11.0 9.2

TFC-hollow fiber membrane

Current work

8.4 5.1

11 6.2

TFC-hollow fiber membrane

20

TFC flat sheet membrane TFC flat sheet membrane HTI flat sheet membrane FRL composite membrane Permasep B-10 membranes

16 15 29 26 30 12 31

12 10 9.72 19.25 12.16 40.53 40.53

other reported values as compared in Table 2. Clearly, the newly developed TFC-PRO hollow fiber membranes possess encouraging power density which is much higher than the required value of 5 W/m2 estimated by Statkraft to make PRO commercially viable. 32−34 Future works will focus on membrane module design together with membrane fouling and long-term behavior.

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reference

REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

PRO process; materials; hollow fiber spinning (Table S1) and post-treatment and module fabrication; interfacial polymerization (Figure S1); pressure retarded osmosis setup (Figure S2) and operating conditions and performance evaluation; membrane characterization; additional SEM images (Figure S3) of the supports; protocols for physical and mass transport characterizations of membranes; pore size distribution profiles (Figure S4 and Table S2); membrane mechanical strength (Table S3); forward osmosis performance of the hollow fibers (Table S4); and synthetic feed solutions for PRO tests (Table S5).This material is available free of charge via the Internet at http://pubs.acs.org.

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2.85 2.6 5.06 1.56 0.35 3.12 3.27

membrane

AUTHOR INFORMATION

Corresponding Author

*Tel: +65-65166645; fax: +65-67791936; E-mail: chencts@nus. edu.sg. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research grant is supported by the Singapore National Research Foundation under its Environmental & Water Technologies Strategic Research Programme and administered by the Environment & Water Industry Programme Office (EWI) of the PUB under the project entitled “Membrane development for osmotic power generation, Part 1. Materials development and membrane fabrication” (1102-IRIS-11-01) and NUS grant number of R-279-000-381-279. Special thanks are due to Dr. Panu Sukitpaneenit, Dr. Sui Zhang, Miss Xue Li, and Miss Xiuzhu Fu for their valuable suggestions and kind help. 8076

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(16) Li, X.; Zhang, S.; Fu, F. J.; Chung, T. S. Deformation and reinforcement of thin-film composite (TFC) polyamide-imide (PAI) membranes for osmotic power generation. J. Membr. Sci. 2013, 434, 204−217. (17) Han, G.; Zhang, S.; Li, X.; Chung, T. S. High performance thin film composite pressure retarded osmosis (PRO) membranes for renewable salinity-gradient energy generation. J. Membr. Sci. 2013, 440, 108−121. (18) Sivertsen, E.; Holt, T.; Thelin, W.; Brekke, G. Pressure retarded osmosis efficiency for different hollow fibre membrane module flow configurations. Desalination 2013, 312, 107−123. (19) Kim, Y. C.; Elimelech, M. Adverse impact of feed channel spacers on the performance of pressure retarded osmosis. Environ. Sci. Technol. 2012, 46, 4673−4681. (20) Chou, S.; Wang, R.; Shi, L.; She, Q.; Tang, C. Y.; Fane, A. G. Thin-film composite hollow fiber membranes for pressure retarded (PRO) process with high power density. J. Membr. Sci. 2012, 389, 25− 33. (21) Sukitpaneenit, P.; Chung, T. S. High performance thin-film composite forward osmosis hollow fiber membranes with macrovoifree and highly porous structure for sustainable water production. Environ. Sci. Technol. 2012, 46, 7358−7365. (22) Widjojo, N.; Chung, T. S.; Krantz, W. B. A morphological and structural study of Ultem/P84 copolyimide dual-layer hollow fiber membranes with delamination-free morphology. J. Membr. Sci. 2007, 294, 132−146. (23) Wang, P.; Chung, T. S. Design and fabrication of lotus-root-like multi-bore hollow fiber membrane for direct contact membrane distillation. J. Membr. Sci. 2012, 421−422, 361−374. (24) Su, J.; Yang, Q.; Teo, J. F.; Chung, T. S. Cellulose acetate nanofiltration hollow fiber membranes for forward osmosis process. J. Membr. Sci. 2010, 355, 36−44. (25) Singh, S.; Khulbe, K. C.; Matsuura, T.; Ramamurthy, P. Membrane characterization by solute transport and atomic force microscopy. J. Membr. Sci. 1998, 142, 111−127. (26) Loeb, S.; Mehta, G. D. A two-coefficient water transport equation for pressure-retarded osmosis. J. Membr. Sci. 1979, 4, 351− 362. (27) Han, G.; Chung, T. S.; Toriida, M.; Tamai, S. Thin-film composite forward osmosis membranes with novel hydrophilic supports for desalination. J. Membr. Sci. 2012, 423−424, 543−555. (28) Yip, N. Y.; Tiraferri, A.; Phillip, W. A.; Schiffman, J. D.; Elimelech, M. High performance thin-film composite forward osmosis membrane. Environ. Sci. Technol. 2010, 44, 3812−3818. (29) Achilli, A.; Cath, T. Y.; Childress, A. E. Power generation with pressure retarded osmosis: an experimental and theoretical investigation. J. Membr. Sci. 2009, 343, 42−52. (30) Loeb, S.; Hessen, F. V.; Shahaf, D. Production of energy from concentrated brines by pressure retarded osmosis. II. Experimental results and projected energy costs. J. Membr. Sci. 1 1976, 249−269. (31) Mehta, G. D.; Loeb, S. Performance of Permasep B-9 and B-10 membranes in various osmotic regions and at high osmotic pressures. J. Membr. Sci. 1978, 4, 335−349. (32) Thorsen, T.; Holt, T. WO Pat. 2003, 047733 A1. (33) Thorsen, T.; Holt, T. US Pat. 2009, 7,566,402 B2. (34) Thorsen, T.; Holt, T. US Pat. 2009, 0008330 A1.

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