Adverse Impact of Feed Channel Spacers on the Performance of

Mar 15, 2012 - Department of Thermal Systems, Korea Institute of Machinery and ... the action of the high hydraulic pressure on the feed channel space...
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Adverse Impact of Feed Channel Spacers on the Performance of Pressure Retarded Osmosis Yu Chang Kim†,‡ and Menachem Elimelech†,* †

Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520-8286, United States Department of Thermal Systems, Korea Institute of Machinery and Materials, Daejeon 305-343, Republic of Korea

‡

S Supporting Information *

ABSTRACT: This article analyzes the influence of feed channel spacers on the performance of pressure retarded osmosis (PRO). Unlike forward osmosis (FO), an important feature of PRO is the application of hydraulic pressure on the high salinity (draw solution) side to retard the permeating flow for energy conversion. We report the first observation of membrane deformation under the action of the high hydraulic pressure on the feed channel spacer and the resulting impact on membrane performance. Because of this observation, reverse osmosis and FO tests that are commonly used for measuring membrane transport properties (water and salt permeability coefficients, A and B, respectively) and the structural parameter (S) can no longer be considered appropriate for use in PRO analysis. To accurately predict the water flux as a function of applied hydraulic pressure difference and the resulting power density in PRO, we introduced a new experimental protocol that accounts for membrane deformation in a spacer-filled channel to determine the membrane properties (A, B, and S). PRO performance model predictions based on these determined A, B, and S values closely matched experimental data over a range of draw solution concentrations (0.5 to 2 M NaCl). We also showed that at high pressures feed spacers block the permeation of water through the membrane area in contact with the spacer, a phenomenon that we term the shadow effect, thereby reducing overall water flux. The implications of the results for power generation by PRO are evaluated and discussed.

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INTRODUCTION Pressure-retarded osmosis (PRO), an osmotically driven membrane process, has the potential to produce electric power sustainably through the exploitation of natural salinity gradients, such as seawater (as a high salinity solution) and river water (as a low salinity solution).1−3 In PRO, seawater must be pressurized, thereby retarding the permeating flow. However, the applied hydraulic pressure difference should be lower than the osmotic pressure difference. Owing to osmosis, water permeates through the membrane and mixes with the pressurized seawater. Power is then generated by releasing the pressurized, diluted seawater through a hydro-turbine. The osmotic power that can be generated per unit membrane area is equal to the product of the water flux through the membrane and the hydraulic pressure applied to the high salinity solution. Water flux in osmotically driven membrane processes is governed by the membrane transport properties.4−8 In PRO, a specialized high-performance membrane is required for conversion of osmotic energy because the high salinity solution is pressurized. However, most previous studies on PRO have been performed using reverse osmosis (RO) membranes with dense and thick porous support layers because there was no appropriate membrane for PRO.1,3,9−16 When used in PRO, RO membranes induce severe internal concentration polarization (ICP), which dramatically reduces the water flux.11,14,17−19 To reduce ICP effects, a cellulose-based forward © 2012 American Chemical Society

osmosis (FO) membrane with a thin and hydrophilic support layer has been developed and commercialized.20 This commercial FO membrane was also employed in recent laboratory-scale PRO studies instead of RO membranes.2,8,21 Recently, flat-sheet and hollow-fiber thin-film composite PRO membranes with thin and highly porous support layers have been developed for potential use in the PRO process.8,22 The major limitation of the membranes discussed above is their ability to withstand the hydraulic pressure that is necessary for PRO systems. A recent laboratory-scale PRO study using the cellulose-based commercial FO membrane indicated that the maximum applied hydraulic pressure that the membrane could withstand was approximately 9.7 bar.2 This hydraulic pressure difference is lower than the operating pressure which can theoretically generate maximum power density when seawater and river water are used as the salinity gradient resources. Flat-sheet membranes for PRO need a feed channel spacer to maintain the feed channel geometry. Feed channel spacers are used to improve mass transfer near the membrane surface, although they also inevitably increase the pressure drop along Received: Revised: Accepted: Published: 4673

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Figure 1. (a) Comparison of laboratory-scale crossflow membrane test cells for RO, FO, and PRO. A porous frit is used on the permeate side of the RO membrane instead of the permeate spacer that is used in commercial RO modules. It is possible to carry out RO and FO experiments in the corresponding cells without a spacer in the feed channel; however, in the PRO cell, a spacer in the feed channel must be used. The membrane active layer faces the feed solution in the RO and FO cells, while in the PRO cell it faces the draw solution. (b) Spacer with two layers of strands (top). Concave-shaped (middle) and convex-shaped (bottom) deformed membranes after PRO experiments at 12.5 bar.

the channel.23−32 However, in addition to increasing the pressure drop along the membrane channel, the feed channel spacers in contact with the membrane may also deform the membrane under the high hydraulic pressure in PRO. To date, however, the impact of feed spacers on membrane deformation and resulting PRO performance has not been explored. In this study, we investigate the influence of feed channel spacers on PRO performance under high hydraulic pressure difference. PRO performance experiments using a commercial FO membrane were carried out in a laboratory-scale crossflow PRO test cell over a range of draw solution salinities. To predict the water flux as a function of applied hydraulic pressure difference and resulting PRO power density, we developed a new approach to determine the membrane transport properties (water and salt permeability coefficients) in a PRO test cell with a spacer-filled channel. The impact of applied hydraulic pressure and feed channel spacers on membrane deformation and resulting PRO performance are analyzed and discussed.

bench-scale experimental PRO unit. The crossflow test cell had symmetric channels on both sides of the membrane. The effective membrane surface area was 20.02 cm2. A variable speed gear pump (Cole-Parmer, Vernon Hills, IL) was used to circulate the feed solution (deionized (DI) water) in a closed loop at a flow rate of 0.5 L/min. A high-pressure pump (Hydracell pump, Wanner Engineering, Inc., Minneapolis, MN) was used to circulate the draw (NaCl) solution in a closed loop at a flow rate of 0.5 L/min. A pulsation dampener was installed on the pump discharge to convert the pulsing draw solution flow to a continuous flow by absorbing peak pressures.34 We adjusted the flow rate with a bypass valve (Swagelok, Solon, OH) connected to the high-pressure pump and a backpressure valve (Swagelok, Solon, OH) on the outlet of the draw solution. The combination of the bypass and backpressure valves allowed fine control over a wide range of inlet draw pressures. A water bath (Neslab, Newington, NH) was used to maintain both the feed and draw solutions at a desired temperature (20 ± 0.5 °C). The weight of the feed solution was recorded in a data-logging program to determine the water permeate flow. Appropriate Determination of Membrane Transport Properties for PRO. DI water and NaCl solution were used as feed solutions in RO mode to determine pure water permeability coefficient (A) and salt permeability coefficient (B), respectively. RO experiments are usually conducted in an RO test cell to determine the membrane transport properties (A and B) and salt rejection (R).2,6−8,35−41 However, in such experiments, the membrane will not deform in the RO test cell because the membrane in the permeate channel is supported by a porous frit (part a of Figure 1). Hence, the A, B, and R values of the membrane (obtained in an RO test) would not be the same as those of a deformed membrane (obtained in a PRO test). Accordingly, we devised a method to determine the A and B suitable for PRO as shown in Figure S3 of the Supporting Information. To accurately predict the membrane water flux and resulting power density in PRO, water and salt permeability coefficients were obtained through a modified RO experiment. The modified RO experiment was performed in a PRO test cell to simulate the PRO conditions. The feed and draw solution channels of the PRO test cell were used as the permeate and feed channels of the RO test cell, respectively. The pure water permeate is usually measured by a flowmeter in an RO test cell,

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MATERIALS AND METHODS FO Membrane. A flat-sheet, cellulose-based FO membrane obtained from Hydration Technology Innovations (HTI) was used in our PRO tests. With significantly reduced thickness to minimize ICP, the FO membrane is reinforced by an embedded polyester mesh.33 The flat-sheet FO membrane coupon was loaded in a PRO test cell resulting in a compartment of two flow channels. The active layer of the membrane faced the high salinity solution (draw solution), and the support layer of the membrane faced the low salinity solution (feed solution) because the active layer side facing the draw solution was pressurized for the PRO process. Feed-Channel Spacers. Spacers are essential in PRO test cells to keep the membrane separated from the channel wall to maintain the flow channel at the feed side when the draw side is pressurized. Spacers are employed on both channels to support the membrane and maintain the channel geometry in PRO experiments. Accordingly, we used commercial mesh-type spacers composed of two levels of filaments forming diamond-type spacer geometry.26,27 We tested three commercially available feed channel spacers (Figure S1 of the Supporting Information) to investigate the impact of feed spacer geometry on permeate water flux in PRO. Bench-Scale Experimental PRO Setup. Figure S2 of the Supporting Information shows a schematic diagram of our 4674

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water flux between test cells with spacer-filled channels and empty channels.26,27,33 Water flux dramatically improved by inserting a spacer in the feed channels in RO26,27 and FO,33 because the spacer enhanced mass transfer near the membrane surface. In the crossflow PRO test cell used in the current work, the feed and draw solutions flowed on both sides of the membrane. However, unlike FO, which operates without pressure difference between the feed and draw channels, an important feature of PRO is that hydraulic pressure is applied on the draw solution side (part a of Figure 1) to convert the osmotic flow to electric energy via a turbine. Notably, because there is a large space in the channel inlet and outlet regions of conventional FO test cells, the membrane is not supported at those regions as depicted in Figure S4 of the Supporting Information. For this reason, in our PRO mode tests, part of the membrane was ruptured in the channel outlet region at high hydraulic pressure difference. Remarkably, even an RO membrane with a thick support structure was ruptured around 9.5 bar of hydraulic pressure difference in the PRO test cell (data not shown), although commercial spiral wound RO membrane modules usually operate in the range of 60 to 70 bar in RO systems. Because membranes have been tested at low hydraulic pressure difference to avoid damage, there have been no experimental results with PRO test cells performed over 9.7 bar.2,8 To avoid membrane deformation and eventual rupture at the inlet and outlet regions of the feed channel, we redesigned the channel geometry to have small inlet and outlet regions (6 mm × 26 mm) as depicted in Figure S4 of the Supporting Information. With this newly designed PRO test cell, the membrane was not ruptured even at approximately 15 bar of pressure difference. For our PRO experiments, we tested the membrane up to 12.5 bar as the maximum hydraulic pressure difference because the flow rate could not be controlled using our backpressure and bypass valves at 15 bar. Net-type spacers (spacer A, Figure S1 of the Supporting Information) were placed on both sides of the membrane to simulate spiral wound modules. Water Flux and Projected Power Density at Various Draw Solution Salinities. For the PRO process, there may be several types of salinity gradient resources, such as seawater (∼35 000 ppm)/fresh water, brine from seawater RO plants (∼70 000 ppm)/fresh water,10,12,14 and brine from an FO desalination process (∼140 000 ppm)42/fresh water. Accordingly, we used concentrations of 0.5, 1, and 2 M NaCl as draw solutions and DI water as a feed solution to simulate each of these salinity gradient resources. Experimentally measured water flux (JW) data obtained with the redesigned PRO test cell and the corresponding projected power density (W) as a function of hydraulic pressure difference for the three draw solution concentrations are presented in Figure 2. Theoretically, as the hydraulic pressure increases, the water flux decreases linearly and power density shows a quadratic function curve with a maximum point.2,14 The power density is equal to the product of the water flux and the applied hydraulic pressure at the draw solution side. The membrane power density increases with increasing hydraulic pressure difference but reaches a maximum value (Wmax) when the applied hydraulic pressure difference (ΔP) is approximately half of the osmotic pressure difference across the membrane (Δπ); that is, Wmax = A Δπ2/4 at ΔP = Δπ/2, with A being the water permeability coefficient. The open circles on the horizontal axes

but here we used a digital balance in a PRO test cell (Figure S2 of the Supporting Information). Thus, it is required to circulate DI water in the feed channel of the PRO test cell (used as the permeate channel in this experiment) to measure the volume and concentration change of the circulating solution (initially DI water). The water permeability coefficient (A) was determined from the measured pure water flux over a range of applied hydraulic pressures (2.47 to 12.14 bar). Initially, the loaded membrane was compacted with DI water at an applied hydraulic pressure difference (ΔP) of 12.14 bar until the permeate flux reached a steady state (after ∼12 h). Next, the water permeate rate was measured at applied pressure differences ranging from 12.14 to 2.47 bar in approximately 3.23 bar decrements. The water flux (JW) at each applied pressure difference was calculated by dividing the water permeate rate by the membrane area. Water permeability (A) was then determined by dividing the water flux by the applied pressure, A = JW/ΔP.2,6−8,35−41 The modified RO experiment was also performed to determine B with 0.05 M NaCl feed solution under hydraulic pressure differences (ΔP) of 6.02, 9.08, and 12.14 bar for 1 h. The salt permeability coefficient (B) was determined using B = JW exp(−JW/k)(1−R)/R, with k being the feed channel mass transfer coefficient.24 This equation to determine B accounts for concentration polarization.6,7,38 The salt rejection (R) of the membrane was calculated by measuring the conductivities of the bulk feed (CF) and permeate (CP) solutions. To determine R (= 1 − CP/CF) and JW, we used a 0.05 M NaCl feed solution and measured the volume and concentration change of the circulating permeate solution (initially DI water) at predetermined time intervals. The concentration of the permeate (CP) was calculated based on the mass of the permeating salt (MPS) and volume of permeating water (VPW,i) during predetermined time intervals. MPS is obtained from the measured concentrations of the circulating solution (CC,i and CC,i+1) and volume of permeating water (VPW,i) at predetermined time intervals using: CP =

MPS VPW,i

CC,iVC,i + MPS = CC,i + 1(VC,i + VPW,i)

(1) (2)

where VC,i is the initial volume of the circulating solution. PRO Tests. For PRO experiments, NaCl solutions (0.5, 1, and 2 M NaCl) and deionized (DI) water were used as draw and feed solutions, respectively. Osmotic pressures of the draw solutions were calculated using a commercial software program from OLI Systems, Inc. (Morris Plains, NJ). The initial volumes of feed and draw solutions were 3 and 6 L, respectively. During the PRO experiments, the draw concentration decreased and the feed concentration increased because the two solutions were recycled. The weight of the feed solution was recorded every 0.5 min and the average water flux was calculated over 120 data points (i.e., 60 min) after it had stabilized. The applied hydraulic pressure differences (ΔP) were 0.50, 2.92, 6.16, 9.40, and 12.40 bar.

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RESULTS AND DISCUSSION Feed Channel Requirements for a PRO Test Cell. Most previous RO and FO bench-scale studies were performed using a crossflow test cell loaded with a flat-sheet membrane coupon. These studies showed a significant difference in the measured 4675

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The results in Figure 2 also demonstrate higher water fluxes for higher draw solution concentrations. Notably, for the experiments using 0.5 M NaCl draw solution (part a of Figure 2), we observed that the peak power density was obtained at an applied hydraulic pressure below Δπ/2. For experiments using 1 and 2 M NaCl draw solutions, we were unable to confirm the peak power density because we were not able to carry out the experiments at applied hydraulic pressures exceeding 12.5 bar. However, whereas the experimental water fluxes increased with draw solution concentrations, they were much lower than the expected values considering the ideal flux reversal points for each draw solution (open circles on the horizontal ΔP axis). Previous PRO model analyses attributed reduced water fluxes to internal concentration polarization as well as external concentration polarization and reverse salt diffusion.2,8 However, because we used DI water as a feed solution and spacers as turbulence promoters in our experiments, the effect of ICP and ECP on the water flux was probably not as significant. As we report in the following subsections, the lowerthan-expected water fluxes were also a result of membrane deformation and the shadow effect caused by the feed spacers. Membrane Deformation Against Feed Channel Spacers. Generally, membrane channel spacers suppress concentration polarization and improve mass transfer over the membrane surface.27 In PRO, the role of a feed channel spacer is to support the membrane against the high hydraulic pressure difference. When we examined the membrane surface after PRO tests, we observed significant membrane deformation by the feed channel spacer which could impact membrane performance. Specifically, concave and convex shape deformations were observed on the membrane active and support layers, respectively, (part b of Figure 1) because the spacer was inserted in the feed channel and hydraulic pressure was applied on the draw channel side. The feed channel spacer is an essential element of the PRO test cell. It is possible to conduct RO or FO experiments without spacers in the feed channel of the test cells; however, PRO experiments must be conducted with a spacer in the feed channel of the PRO test cell as depicted in part a of Figure 1. In RO test cells, a porous frit is used on the low-pressure permeate side instead of a spacer, but a mesh-type spacer should be used in the feed channel of the PRO test cell to act as a mechanical stabilizer for the channel geometry. The mesh-type spacer used in the experiments described in Figure 2 (spacer A in Figure S1 of the Supporting Information) has a two-level structure and an open space between adjacent strands.25,26 The membrane area that was unsupported over the open space had to withstand the high pressure applied on the draw solution side. At low pressure difference, the membrane was deformed to contact with one layer of filaments, while at high pressure difference, it was deformed to contact both layers of filaments. The pressurized draw solution stream compressed the feed solution channel formed by the spacer, which became narrower, resulting in an increase in flow resistance and pressure drop along the membrane channel on the feed side (Figure S5 of the Supporting Information). The bulging of the membrane in the PRO test cell under pressure on the draw solution subjected the membrane to a tensile stress. As the tension increased, the active layer would begin to crack and the support layer would be stretched, finally resulting in rupture. Small cracks could be seen on the active layer of the deformed membrane after PRO experiments at 12.5 bar (Figure S6 of the Supporting Information). As will be discussed later, these harsh

Figure 2. Experimental water flux (JW) and corresponding projected power density (W) as a function of applied hydraulic pressure difference (ΔP). The projected power densities are calculated based on the water flux results. Results are shown for three draw solution concentrations: (a) 0.5 M NaCl, (b) 1 M NaCl, and (c) 2 M NaCl. DI water was used as the feed solution in all experiments. Both feed and draw solution flow rates were held at 0.5 L/min and temperature was fixed at 20 °C. The open circle on the horizontal axis of each graph represents the osmotic pressure (π) of the NaCl draw solutions used as determined by the OLI Stream Analyzer software. Error bars represent one standard deviation.

of Figure 2 represent ΔP = Δπ, which are also referred to as the flux reversal pressures. As the concentration difference between the draw and feed solutions increases, so does the hydraulic pressure difference necessary to generate maximum power density. However, even though the 1 and 2 M NaCl draw solutions generated high concentration difference, the PRO experiments could not be performed at hydraulic pressures over 12.5 bar due to the previously discussed limitations of the PRO test cell. At the hydraulic pressure difference of 12.5 bar, the water fluxes were measured to be 2.75, 10.87, and 22.74 Lm2−h−1 (0.76 × 10−6, 3.02 × 10−6, and 6.32 × 10−6 m/s) for the 0.5, 1, and 2 M NaCl draw solutions, respectively. The corresponding projected power densities (W) calculated on the basis of the water flux results (at 12.5 bar) were 0.95, 3.81, and 7.89 W/m2. 4676

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The measured PRO water fluxes and corresponding projected power densities at ΔP = 12.5 bar for the three spacers are compared in Figure 4. In agreement with our hypothesis, the results demonstrate that PRO experiments utilizing spacers with larger openings yielded higher water fluxes. The membranes with all spacers were significantly deformed and the feed channel spacers were imprinted on the membrane surface at high hydraulic pressure difference (ΔP) as demonstrated in Figure S7 of the Supporting Information. Concave and convex shapes were observed on the active and support layers, respectively, because the spacer was in the feed channel and hydraulic pressure was applied on the draw solution side. Membrane Transport Properties in Spacer-Filled PRO Channels. PRO membrane performance depends on the transport properties of the membrane active layer (i.e., water and salt permeability coefficients, A and B) and the support layer structural parameter (S).6−8 These three parameters control the membrane water flux and the resulting PRO power density. In addition, the channel mass transfer coefficient (k) indirectly influences the water flux through its impact on external concentration polarization. As described in the previous sections, spacers under PRO testing induce membrane deformation and block available membrane area for water permeation, which would influence membrane performance. Accordingly, if a PRO experiment for measuring the water flux is performed with a spacer-filled channel, A, B, and S should also be determined using a spacer-filled channel under similar PRO operating conditions. Conventional methods for determining A and B involve the use of an RO test cell, while the structural parameter S is determined using an FO test cell.6,8 Hence, these parameters determined from conventional methods will not be representative of those relevant to PRO where membrane deformation takes place. To circumvent this problem, we introduced a new experimental protocol to determine A, B, and S in a PRO test cell as described in the Materials and Methods section. Part a of Figure 5 demonstrates that the water permeability coefficient (A) measured under our new procedure was not constant because water flux increased nonlinearly with increasing hydraulic pressure difference. The resulting average A value was 1.23 L m−2h−1bar−1, which is higher than that reported in previous studies with the same membrane (Table 1). The observed behavior suggests that the structure of the asymmetric membrane used and the resulting water permeability was influenced by the pressure difference in the PRO

conditions in the PRO tests likely resulted in structural changes of the active and support layers of the membrane, thereby affecting membrane performance. Reduction of Permeate Water Flux in PRO due to Spacer Shadow Effect. In addition to membrane deformation, the spacers can obstruct osmotic flow and adversely impact the water permeation rate in PRO as illustrated in Figure 3. In this proposed mechanism, the spacer strand exerts resistance to water permeation at high hydraulic pressure difference (ΔP). As the hydraulic pressure difference increases, the membrane surface is compressed against the spacer mesh, which results in increased contact area between the membrane and spacer strand, and hence, reduced available membrane area for water permeation. In other words, the spacer strand blocks or shadows the membrane for water permeation. We refer to this phenomenon as the shadow effect. Interestingly, a somewhat similar effect occurs in reverse electrodialysis, another process to generate energy from salinity gradients, because nonconductive spacers block the ionic transport.43,44 To investigate our proposed shadow effect mechanism, we compared the PRO water fluxes for three spacers of different geometries (Figure S1 of the Supporting Information). The mesh configuration of the tested spacers was either rhombus (spacers A and B) or square (spacer C). The rhombus-type spacers have a two-level structure, whereas the square-type spacer has a one-level structure. According to the manufacturer’s specifications, the opening sizes of spacers A, B, and C were 0.06 in × 0.06 in, 0.22 in × 0.22 in, and 0.035 in × 0.05 in, respectively. However, the actual opening size of spacer C seemed to be smaller than the specified size.

Figure 3. Schematic illustration of the shadow effect induced by the feed channel spacer and its impact on PRO permeate water flux. The spacer strand provides additional resistance to water permeation at high hydraulic pressure differences (ΔP). As the hydraulic pressure is increased, the area in contact with the spacer strand also increases, reducing the available membrane area for water permeation.

Figure 4. Comparison of the measured water flux and projected power density for three different spacers for: (a) 0.5 M, (b) 1 M, and (c) 2 M NaCl draw solutions. DI water was used as the feed solution in all experiments. Both feed and draw flow rates were held at 0.5 LPM and temperature was 20 °C in all experiments. The hydraulic pressure difference (ΔP) was approximately 12.5 bar in all experiments. Error bars represent one standard deviation. 4677

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permeability coefficient were determined to be 76% and 2.62 L m−2h−1, respectively. For our experiments in the PRO test cell with the deformed membrane, the salt rejection was lower and the salt permeability coefficient was higher than those reported in previous studies. Specifically, the salt rejection of the FO membrane used in this study has been reported to be around 95%6,37,48 and the salt permeability coefficients reported in previous studies were much lower than those observed in our study as summarized in Table 1. The mass transfer coefficients (k) in the spacer-filled channel were determined to be 8.62 × 10−5, 8.62 × 10−5, and 8.63 × 10−5 m/s for the 0.5, 1, and 2 M NaCl solutions, respectively. The k values were identical even though the draw solution concentrations increased. Details on the determination of k are given in the Supporting Information. A recent study has established that, when the effects of ECP, ICP, and reverse salt diffusion are incorporated, the resulting water flux equation for PRO is expressed by8

JW Figure 5. (a) Water flux (JW) and water permeability coefficient (A) as a function of applied hydraulic pressure difference (ΔP) during the modified RO experiment with DI water feed solution. The slope of the dotted line represents the water permeability coefficient of the undeformed membrane as obtained from a conventional RO membrane test cell. (b) Salt permeability coefficient (B) and salt rejection (R) as a function of applied hydraulic pressure difference (ΔP) during the modified RO experiment with 0.05 M NaCl feed solution. All experiments were performed with solutions at a fixed temperature of 20 °C. Experiments were carried out with spacer A (Figure S1 of the Supporting Information).

⎡ ⎤ JW ⎢ π D,bexp − k − π F,bexp(JW K ) ⎥ = A⎢ − ΔP ⎥ ⎢ 1 + B ⎡⎢exp(J K ) − exp − JW ⎤⎥ ⎥ W JW ⎣ k ⎦ ⎣ ⎦

( )

( )

(3)

where πD,b and πF,b are the osmotic pressures of the bulk draw and feed solutions, respectively, k is the mass transfer coefficient, K is the solute resistivity of the membrane support layer, and D is the diffusion coefficient of the draw solute. From eq 3, we can see that the solute resistivity (K) behaves as an exponential deterrent of the permeate water flux. Therefore, a smaller K value would result in better membrane performance. Here, the resistance to diffusion (K) can be expressed as tsτ D−1ε−1 (ts is the support layer thickness, τ the tortuosity, D the diffusion coefficient of the draw solute, and ε the porosity). Instead of an FO test for determining K, we applied the PRO test data to the water flux equation (eq 3) for PRO and solved numerically for K. Specifically, the solute resistivity (K) was determined by substituting the property values (A, B, and k) determined above and the water flux values (JW) obtained from the PRO experimental results in eq 3 and then solving the equation numerically. We used three water flux data values obtained at hydraulic pressure differences of 0.50, 3.00, and 6.15 bar. The resulting average values of K were calculated to be 4.68 × 105, 4.92 × 105, and 3.98 × 105 s/m for the 0.5, 1, and 2 M NaCl draw solutions, respectively. The structural parameter (S = KD) was then calculated resulting in S values of 689, 730, and 602 μm for the 0.5, 1, and 2 M NaCl draw solutions, respectively. Comparison of Measured and Predicted PRO Performance. The PRO water flux model (eq 3) was solved numerically to determine the theoretical water flux (JW) over a

cell. We note that unlike our results with the PRO test cell that exhibit higher water permeability, A, at higher pressure difference (part a of Figure 5), RO test cells yield A values that are independent of the applied pressure.2 Deformation of the membrane also influenced the salt rejection (R) and salt permeability coefficient (B) as shown in part b of Figure 5. Dense selective membranes, like RO and FO, are constrained by the permeability-selectivity trade-off, where an increase in water permeability is accompanied by an increase in salt permeability.45−47 Part b of Figure 5 indicates that the salt permeability coefficient was not constant and increased with an increase of the water permeability coefficient. At hydraulic pressure differences (ΔP) of 6.02, 9.08, and 12.14 bar, water fluxes were 4.64, 8.42, and 13.39 L m−2h−1 and salt rejections were 71.6, 79.8, and 76.5%, respectively. The corresponding salt permeability coefficients were 1.81, 2.08, and 3.95 L m−2h−1. The resulting average salt rejection and

Table 1. Reported Transport Properties of the Commercial FO Membrane Used in This Study and Corresponding Experimental Conditions ref

water permeability A (L m−2h−1bar−1)

salt permeability B (L m−2h−1)

structural parameter S (μm)

effective membrane area (cm2)

operating temperature (°C)

use of spacer

2 6 36 38 40 this work

0.68 0.36 0.80 0.44 1.19 1.23

0.40 0.32 0.61 0.27 0.92 2.62

678 595 400 481 720 689

18.75 20.02 60 20.02 60 20.02

25 25 23 20 23 20

yes no yes no yes yes

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Figure 6. Modeled and experimental water flux (JW) and the respective projected power density (W) as a function of applied hydraulic pressure difference (ΔP). (a) Modeling results for the indicated draw solutions. (b−d) Experimental and modeling results for 0.5, 1.0, and 2.0 M NaCl draw solutions. The osmotic pressures (π) of the 0.5, 1, and 2 M NaCl draw solutions were 22.75, 46.75, and 100.44 bar, respectively, as determined by OLI Stream Analyzer software. These osmotic pressures are indicated by open circles on the horizontal (applied pressure) axes of each graph. For each graph, symbols (open squares and circles) represent experimental water fluxes and the corresponding projected power densities, respectively, and lines represent model results. All experiments were performed with solutions at a fixed temperature of 20 °C.

Implications. Our results demonstrate for the first time that the feed channel spacer directly influences the water flux in PRO and, hence, the resulting power density. Therefore, the selection of appropriate spacers and novel spacer and module design would be of paramount importance in PRO osmotic power generation. Further investigations using spiral-wound modules are also needed for better understanding and optimization of the PRO process because the operating conditions (flow rate and pressure drop) of laboratory PRO test cells are markedly different from those of spiral-wound modules. Furthermore, the behavior and performance of fullscale membrane processes are quite different than those observed in small, laboratory-scale membrane devices.49

range of applied pressures. The corresponding power densities were then calculated as the product of the water flux and hydraulic pressure difference (ΔP):8 W = JW ΔP

(4)

Part a of Figure 6 presents the model results based on the values of A, B, k, and S determined in the previous section in the spacer-filled channel. The maximum power densities for each draw solution concentration (0.5, 1, and 2 M) were calculated to be 1.65, 4.81, and 17.7 W/m2, respectively. Ideally, maximum power density is obtained when the hydraulic pressure difference is half of the osmotic pressure difference. However, the optimal hydraulic pressure difference was significantly lower than the ideal hydraulic pressure difference due to the detrimental effects of ICP, ECP, reverse salt flux, and the negative impacts of the feed channel spacer as discussed before. Parts b−d of Figure 6 compare the experimental water flux and the corresponding projected power density results (symbols) with model predictions (lines). The model results, obtained with the parameters derived from a spacer-filled channel, closely matched the experimental data. These results indicate that membrane deformation resulting from the feed channel spacer was one of the PRO performance-limiting phenomena along with ECP, ICP, and reverse salt diffusion across the membrane. At low pressure difference, good agreement was observed between actual and predicted water flux values, but at high pressure difference, there was a noticeable deviation between experimental results and model predictions. As the draw solution concentration increased, the deviation also increased (part d of Figure 6), likely due to inaccuracies of calculating the osmotic pressure from the van’t Hoff equation.

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ASSOCIATED CONTENT

S Supporting Information *

Details on the determination of the mass transfer coefficient (k), photographs and specifications of the three spacers used in PRO experiments to investigate the spacer shadow effect, a schematic diagram of the bench-scale PRO experimental unit, a schematic illustration of a new protocol for measuring membrane transport properties for PRO, a cross-sectional view of the feed channel of a bench-scale test cell, relationship between feed solution hydraulic pressure drop and draw solution applied hydraulic pressure, SEM images of the deformed membrane after a PRO experiment at 12.5 bar, and photographs of membrane deformation and spacer imprint on the membrane surface at high hydraulic pressure difference. This material is available free of charge via the Internet at http://pubs.acs.org. 4679

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], Phone: (203) 4322789. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Dr. Yu Chang Kim was supported by the Postdoctoral Program of the Korea Institute of Machinery and Materials (KIMM). Professor Elimelech acknowledges the support of the World Class University (WCU) Program (Case III) through the National Research Foundation of Korea and the Ministry of Education, Science and Technology (R33-10046).

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