Pressure-retarded membrane distillation for simultaneous hypersaline brine desalination and low-grade heat harvesting

Journal of Membrane Science 597 (2020) 117765

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Pressure-retarded membrane distillation for simultaneous hypersaline brine desalination and low-grade heat harvesting Ziwen Yuan a, Yanxi Yu a, Li Wei a, Xiao Sui a, Qianhong She b, c, **, Yuan Chen a, * a

School of Chemical and Biomolecular Engineering, The University of Sydney, Darlington, NSW, 2006, Australia School of Civil and Environmental Engineering, Nanyang Technological University, 639798, Singapore c Singapore Membrane Technology Centre, Nanyang Environment & Water Research Institute, Nanyang Technological University, 637141, Singapore b

A B S T R A C T

Membrane distillation (MD), an emerging pre-concentration technology for zero liquid discharge to manage industrial wastewater, has become increasingly attractive when the low-grade waste heat is utilized as its driving force. Pressure-retarded membrane distillation (PRMD) – a derivative of MD by applying a hydraulic pressure lower than the membrane liquid entry pressure at the cold permeate side – has the potential to further recover the low-grade heat in terms of electricity, which is an added benefit for the use of conventional MD. Here, we for the first time experimentally evaluated the feasibility of using PRMD for simultaneous desalination of hypersaline water and harvesting of low-grade heat and explored the mechanisms governing the experimental PRMD performance. Using a com­ mercial membrane, NaCl solution at the concentration as high as 4 M (233.76 g L 1) is desalinated with the salt rejection >99.9% and additional energy is generated in PRMD at the same time. We showed that the membrane operated with its active layer facing a cold solution orientation in PRMD exhibits better mechanical stability at higher applied pressures, which is essential for achieving higher power output. However, in contrast to the other orientation that is used in conventional membrane distillation (MD) processes, the coupled effects of internal temperature polarization (ITP) and internal concentration polarization (ICP) lower effective driving force and membrane permeability, which significantly compromises the water vapor flux and peak power density. These results indicate that unlike MD processes, controlling internal scaling and fouling are critical for PRMD and our findings can serve as useful guides for creating high-performance PRMD membranes in practical environmental applications.

1. Introduction In past decades, membrane-based technologies have been increas­ ingly used in various water industries, such as seawater/brackish water desalination, water purification, and wastewater reclamation, to sustain clean and fresh water supply [1–3]. Among all the membrane processes, membrane distillation (MD) is an emerging non-isothermal membrane separation process driven by the partial vapor pressure gradient across a hydrophobic membrane [4]. Compared with other membrane processes, MD has a couple of advantages, such as a theoretically complete rejec­ tion of non-volatile components, mild operating conditions and rela­ tively small areal footprint [5–7]. Additionally, MD is capable of desalinating hypersaline waters with total dissolved solids up to 360 g L 1 [8], which is an outstanding advantage over reverse osmosis (RO) with a maximum feed salinity of around 80 g L 1 [6]. In recent years, an ambitious industrial wastewater management strategy, i.e., zero liquid discharge (ZLD), has received considerable interest. Only solid waste is produced after the wastewater treatment process in ZLD, which not only eliminates the environmental impact of the discharged liquid waste but

also alleviates the pressure on ecosystems resulting from freshwater withdrawal [9–11]. Due to its insensitivity and high tolerance to feed salinity, MD has been proposed as one of the most promising technol­ ogies for realizing ZLD [12–16]. However, compared with desalination processes such as RO and electrodialysis, MD is inherently less energy efficient since large amount of energy is consumed for liquid-vapor phase change during the sepa­ ration process which hinders its widespread application. Fortunately, rather than high quality energy (e.g., electricity and mechanical en­ ergy), the energy sources of MD can be low-grade thermal energy (temperature < 130 � C) [17–19], which is world-widely available from industrial plants, geothermal wells or solar thermal collectors [20–22]. Moreover, a derivative of MD, i.e., pressure-retarded membrane distil­ lation (PRMD), was proposed recently to extend the application and further improve the low-grade thermal energy utilization efficiency of MD [23,24]. PRMD applies a hydraulic pressure (lower than the liquid entry pressure (LEP) of the membrane) on the cold permeate. Thus, the transmembrane vapor from the hot side condenses and is pressurized at the cold side, which can be used to generate electric power by

* Corresponding author. ** Corresponding author. School of Civil and Environmental Engineering, Nanyang Technological University, 639798, Singapore. E-mail addresses: [email protected] (Q. She), [email protected] (Y. Chen). https://doi.org/10.1016/j.memsci.2019.117765 Received 17 October 2019; Received in revised form 16 December 2019; Accepted 18 December 2019 Available online 24 December 2019 0376-7388/© 2019 Elsevier B.V. All rights reserved.

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2. Materials and methods

depressurizing it through a hydro-turbine (as illustrated in Fig. 1a). Straub and Elimelech reported that the heat-to-electricity energy con­ version efficiency of PRMD could reach 4.1% with 60 � C heat source [25]. Considering that most of the low-grade heat energy is contained in impure water, such as wastewater from industrial process [21,26] and saline water from geothermal wells [27,28], PRMD is potential to utilize such untapped low-grade heat for simultaneous freshwater production and electric power generation, which integrates the features of both pressure-retarded osmosis (PRO) and MD. Despite the potential of PRMD, no experimental studies have demonstrated its feasibility in treating hypersaline water, which is essential for ZLD. In recent studies, PRMD was applied for low-grade waste heat recovery using low salinity water as feed, and the deforma­ tion of polymetric membrane critically affects the performance of PRMD [29,30], similar to PRO [31]. Due to the deterioration of the membrane properties (i.e., pore size, porosity, and thickness) under elevated pres­ sures, deformed membranes have lower permeability and higher tem­ perature polarization in PRMD, resulting in the decline of water vapor fluxes [29]. Thus, improving the mechanical stability of membranes would be the first critical step in realizing practical applications of PRMD. In addition, we found that it is preferential to use an asymmetric MD membrane with its active layer facing the pressurized cold water in PRMD to maintain the long-term membrane mechanical stability and performance reversibility [29,32]. This preferred membrane orientation in PRMD (i.e., active-layer-facing-cold solution (AL-CS) orientation) is opposite to the preferred orientation used in conventional MD and the heat and mass transfer behaviors in PRMD would be significantly different. However, no study has used hypersaline feedwater in PRMD and its associated mechanisms such as temperature polarization (TP) and concentration polarization (CP) on the influences of PRMD perfor­ mance have not been well understood. Here, we for the first time evaluated the feasibility of using PRMD for simultaneous freshwater production from hypersaline water and lowgrade heat recovery by experiments. We also comprehensively investi­ gated the associated mass and heat transports with different feed con­ centrations and understood their impact on the performance of PRMD. The unique internal polarization effects in PRMD were further explored. Potential strategies to improve the performance of PRMD by controlling the internal scaling and fouling and optimizing membrane designs were also proposed.

2.1. Chemicals and membrane All the feed (hot) solutions for the PRMD tests were prepared by dissolving sodium chloride (NaCl, Sigma-Aldrich) in the deionized (DI) water (Millipore) and their concentrations ranged from 0.5 to 4.0 M. DI water was used as the cold solution at permeate side. A commercial flatsheet asymmetric PTFE membrane (Pall Corporation, PTF002LH0P) was used in this study. It is composed of a thin and hydrophobic PTFE active layer with high liquid entry pressure (�15 bar), and a thick and hy­ drophilic support layer made of non-woven polyester (PET). Thus, liquid can penetrate the hydrophilic support layer and reach to the surface of the hydrophobic active layer, and the water-vapor interface of PRMD is on the surface of active layer. The properties of the commercial mem­ brane were characterized and listed in Table 1. Note that the membranes obtained from different batches may have some differences in their properties (i.e. thickness, porosity, contact angle, etc.), we used the same batch of membrane for all the experiments in this study. 2.2. Lab-scale PRMD setup and performance measurements The details of the cross-flow lab scale PRMD setup was described in our previous study [29]. It has a configuration similar to that used in DCMD, except that a gear pump was used to provide hydraulic pressures at the permeate side (Fig. S1). For all the tests, three tricot-type spacers were employed in the feed channel to support the membrane against the hydraulic pressure applied in the permeate side. The membranes were tested in a membrane cell with either of the two orientations: the active-layer-facing-hot-solution (AL-HS) or the active-layer-facing-coldsolution (AL-CS). The PRMD performance was evaluated in terms of water vapor flux (Jw), salt rejection and power density (W). The mea­ surement methods and other experimental details were described in the Supplementary Note 1. 2.3. Theoretical models Theoretical models for using asymmetric membrane in PRMD were developed from the existing mechanisms for DCMD and have been used in previous PRMD studies [23,24,29]. However, since salt was added in the feed solution to evaluate the desalination performance of PRMD in this study, the effects of feed concentration on the mass and heat transfer

Fig. 1. (a) A schematic illustration of using pressure-retarded membrane distillation (PRMD) system to achieve both water purification and electric energy gen­ eration from low-grade heat resources. (b) Specific energy production (SEP) (with per m3 of produced fresh water, solid line) and water vapor flux change (J/J0 where J is the water vaper flux at different applied pressures and J0 is the vapor flux when the applied pressure is zero) against the hydraulic pressure applied in the cold solution in PRMD. For the theoretic SPE calculation, it is assumed that the head loss in the PRMD system is zero and the efficiency of pumps and energy recovery devices is 100%. Other conditions: temperature of cold solution Tc, ¼ 20 � C, temperature of hot solution TH ¼ 60 � C, 4 M NaCl feed solution.

2

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Table 1 Properties of commercial membrane. Nominal pore size (μm)a

Overall thickness (μm)b

Layers of the asymmetric membrane

Material

Thickness (μm)b

Tortuosityc

Porosity (%)b

Contact angle (� )b

0.03

180.7 � 14.2

Active layer Support layer

PTFE Non-woven polyester

30.4 � 8.8 150.3 � 6.5

1.15 1.70

75.1 � 6.1 28.7 � 0.2

131.2 � 5.2 54.5 � 2.7

a

Provided by the manufacturer. The properties were measured at least 5 times with five specimens; Since the active layer of the commercial membrane can be peeled off from the support layer, the properties of the active layer and support layer can be characterized separately. Details of characterization methods are described in Supplementary Note 1. c Tortuosity was estimated by the Bruggeman equation: τ ¼ ε 0:5 [33,34]. b

in PRMD should be considered. The water vapor flux (Jw) of PRMD can be determined by multiplying the vapor permeability of the membrane (Bw) by the vapor pressure difference (△Pv) across the membrane: Jw ¼ Bw ΔPv ¼ Bw ðPv;H

Pv;C Þ

membrane substrate (i.e. the hydrophilic support layer). The theoretical models and calculation procedures for TP and CP in PRMD were developed using asymmetric membrane based on previous studies on MD and osmotic processes (i.e., forward osmosis (FO) and PRO) [23,31, 41–44], and are described in details in Supplementary Note 2. The severity of TP and CP is evaluated by a temperature polarization coef­ ficient (θ) and concentration polarization coefficient (β), which are given by Refs. [4,44]:

(1)

where Pv,H and Pv,C are the vapor pressure on the surface of liquid-vapor interface at the hot and cold side, respectively. Based on Eq. (1), Bw can be estimated by dividing the experimental results of Jw by the effective driving force ΔPv of PRMD. Alternative, Bw can be calculated theoretically using the structural properties of mem­ brane, including the effective porosity (ε), tortuosity (τ), thickness (δ) and average pore radius (r) [24,35–37]: Bw ¼

1 Rm

Rm ¼

τδ RT 1 pa þ ε M Dkw PD0wa

Dkw ¼

2r 3

rffiffiffiffiffiffiffiffi 8RT πM

TH;m TH;b

β¼

CH;m CH;b

TC;m TC;b

(6) (7)

where T is temperature and C is solute concentration. The subscripts H and C represent the hot feed and cold permeate solution, respectively. The subscripts m and b represent the liquid-vapor interface (the surface of the membrane) and the bulk solution, respectively. A small θ value indicates a severe TP, while severe CP is characterized by high β value. In PRMD the applied pressure in the cold side will do additional work on the condensed permeate. Therefore, the theoretic energy generation is the product of the applied pressure (Ph) and the volume of condensed permeate (Vp) in the cold side, as expressed in Eq. (8) below.

(2) �

θ¼

� (3)

(4)

where Rm is the total resistant of membrane. Dkw is the Knudsen diffusion coefficient of vapor and D0Wa is the diffusion coefficient of water in the air.; M is the molecular weight of water; R is the gas constant; pa is the partial pressure of the air entrapped in pores; P is the total pressure inside pores; T is the temperature. The calculated Bw of the membrane used in this study was determined as 318.3 L m 2 h 1 bar 1 (using the structure parameters listed in Table 1). The revised Antoine and Kelvin equation was used to describing partial vapor pressure of pure water (Pv,0) on temperature (T) [23,38], which takes into account the osmotic pressure of dissolved species in the feed solution and the applied hydraulic pressure at the permeate side [39,40]: � � ðPh ΠÞVm Pv ðT; C; Ph Þ ¼ Pv;0 ðTÞexp (5) Rg T

E ¼ Ph Vp

(8)

Assuming that in the PRMD system the hydraulic pressure in the feed side is zero, the head loss in the whole system is zero, and the efficiency of all the pumps and energy recovery devices is 100%, the theoretic specific energy production (SEP) (i.e., energy production per unit vol­ ume of produced fresh water) is equivalent to the applied hydraulic pressure in the cold side, as expressed below. SEP ¼

E ¼ Pc Vp

(9)

3. Results and discussion 3.1. Theoretical analysis of PRMD for both saline water desalination and low-grade heat energy recovery

where Vm is the molar volume of water, Ph is the hydraulic pressure applied to the solution. Π is the osmotic pressure caused by dissolved species at the concentration of C. It should be noted that the T and Π here refers to the exact temperature and osmotic pressure on the liquidvapor interface. The temperature and concentration differences between the bulk solution and the liquid-vapor interface are known as temper­ ature polarization (TP) and concentration polarization (CP), respec­ tively. Furthermore, the polarization effects can be divided into external and internal polarization effects. The heat transfer and solute diffusion resistance of the boundary layer (the transition layer between bulk so­ lution and membrane surface) can induce external temperature polari­ zation (ETP) and external concentration polarization (ECP), respectively, while the internal polarization effects including internal temperature polarization (ITP) and internal concentration polarization (ICP) are resulted from the heat and mass transfer resistance of the

As illustrated in Fig. 1a, PRMD is similar to DCMD that can desalinate hypersaline brine. Compared to MD, the major advantage of PRMD is that it can simultaneously convert the heat into a mechanical energy that can be harvested in terms of electricity during the desalination process. Fig. 1b further shows the theoretic analysis of specific energy production (SEP) and vapor flux change against the applied hydraulic pressure in PRMD. Here, we assume that an ideal membrane with high LEP (>50 bar) and strong mechanical strength was used in the analysis (i.e., all the membrane properties were unchanged within the range of operating pressures) to evaluate the theoretical performance of PRMD in simultaneous desalination and heat harvesting. According to Eq. (9), the theoretic SEP increases linearly with the applied hydraulic pressure on the permeate side. As shown in Fig. 1b, the energy obtained by applying 10, 20 and 50 bar hydraulic pressure was 0.278, 0.556 and 1.390 kWh 3

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when producing 1 m3 freshwater by PRMD, respectively. Interestingly, Fig. 1b also shows that the theoretic water vapor flux within the range of the applied pressure (0–50 bar) in PRMD was nearly unchanged. This is because the hydraulic pressure within the range of 50 bar cannot significantly reduce the effective driving force of PRMD for a given membrane with fixed membrane properties, noting that a temperature difference of 40 � C (with heat sink of 20 � C) is equivalent of a vapor pressure of more than 2900 bar according to the Antoine and Kelvin equations [38,45] and Eq. (5). As shown in Fig. 1b, applying up to50 bar hydraulic pressure difference in PRMD to generate electric power will only theoretically result in less than 1% reduction in water vapor flux for a 60 � C hot solution with NaCl concentration of 4 M and a 20 � C cold solution compared to the conventional DCMD process under the same conditions. The theoretical analysis in Fig. 1 indicates that the increase in the applied pressure to increase the energy production will not influence the theoretic performance of freshwater production in PRMD. This further demonstrates the advantage of the PRMD process for simultaneous water and energy production over the conventional MD process only for water production. Previous study also showed that higher heat-toelectricity conversion efficiencies can be achieved in PRMD by applying higher operating pressures [23]. Obviously, the hydraulic pressure applied (Ph) in the cold solution is a critical factor determining the energy production in PRMD. We note that the maximum Ph that can be applied in PRMD is strongly dependent on the membrane properties (i.e., Ph should be lower than LEP that is related to membrane pore size, hydrophobicity, roughness, etc.) but independent of solution properties. This theoretical behavior of the PRMD is different from the scenario of PRO for osmotic energy harvesting, in which the applied hydraulic

pressure (and thus the energy production) is constrained by the osmotic pressure difference between the draw and feed solutions [46,47]. In addition, water flux in PRO decreases significantly with the increase in the applied hydraulic pressure due to the decreased effective driving force. The theoretically stable vapor flux in PRMD also demonstrates the advantage of PRMD over hybrid PRO and MD processes for heat re­ covery [48]. Despite the theoretical advantages of PRMD over MD and PRO, PRMD may be influenced by other factors in practical experi­ mental operations, which will be studied in the subsequent sections. 3.2. Experimental performance of PRMD for simultaneous desalination and power generation The performance of PRMD for simultaneous desalination and power generation was tested in the experimental PRMD system (Fig. 2) using a variety of NaCl solutions with concentrations ranging from 0.5 to 4 M as feed solutions. Highly concentrated feed solutions (i.e., 2 M and 4 M NaCl solutions) were used to investigate the PRMD performance in hy­ persaline brine treatment and to reveal the factors affecting the PRMD performance. The duration for each measurement of PRMD performance was 30 min after we observed stable water flux to avoid membrane wetting or rupture under hydraulic pressure in long-tern operation (as shown Fig. S2 in Supplementary Note 3). Although the water flux for all the feed solutions decreased with the increase in the applied hydraulic pressure, the salt rejection maintains above 99.9% over all the applied hydraulic pressures from 0 to 8 bar for all the feed solutions (Fig. 2a, and the details of the conductivity changes of the cold permeate are shown in Fig. S3 in Supplementary Note 3). This indicates that PRMD shares the same advantage with traditional MD, which is capable of desalinating

Fig. 2. Experimental results of PRMD performance in terms of (a,c) water vapor flux and salt rejection, and (b,d) power density as a function of the applied pressure (0–8 bar) on the permeate side at different feed concentrations (0.5–4 M NaCl) with the (a,b) AL-CS membrane orientation and (c,d) AL-HS orientation. Note that the selected feed solution can be used to simulate seawater (0.5 M NaCl) and various hypersaline brines (NaCl concentration > 0.5 M). Other conditions: temperature in the bulk cold solution Tc,b ¼ 20 � C, temperature in the bulk hot solution TH,b ¼ 60 � C, cross flow rate Q ¼ 0.3 L min 1. 4

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hypersaline waters [8,49]. It should be noted that a typical RO process is unable to desalinate saline water with an equivalent NaCl concentration higher than 2 M NaCl due to the constraints of osmotic pressure and membrane mechanical stability [6,50]. Furthermore, PRMD can convert heat energy into mechanical energy via a pressurized cold solution, which can be harvested to generate electricity. As shown in Fig. 2b, a peak power density of 0.66 � 0.05 W m 2 is obtained under the applied pressure of 4 bar with a 0.5 M NaCl feed solution. These results demonstrate experimentally that both hypersaline water desalination and low-grade heat energy recovery can be achieved simultaneously in PRMD. The PRMD performance results shown in Fig. 2a and b were measured in the AL-CS membrane orientation. The PRMD performance in the other membrane orientation (i.e., active-layer-facing-hot-solution (AL-HS)) was also tested. As shown in Fig. 2c and d, employing this traditional MD membrane orientation (AL-HS) in PRMD would result in a dramatic decrease in water vapor flux with the increase of hydraulic pressure, and the membrane would completely lost its separation per­ formance under a pressure of only 2 bar. This result is consistent with previous studies [29,32] and can be ascribed to the mechanical insta­ bility of this orientation under hydraulic pressure. The vulnerable active layer is directly impressed on the feed spacers in AL-HS orientation, which is more prone to be damaged under the applied hydraulic pres­ sure from the permeate side (The rupture of membrane in this orienta­ tion is shown in Fig. S4 in Supplementary Note 4). We also note that water vapor flux exhibited significant decrease with the increasing hydraulic pressure even in the AL-CS orientation where membrane is more mechanically stable. According to Fig. 2a, increasing the hydraulic pressure from 0 to 8 bar resulted in 77.5%– 87.3% of decrease in water vapor fluxes. This water vapor flux re­ ductions with the increase of hydraulic pressure observed in experi­ ments are much more severe than the that in theoretical prediction shown in Fig. 1b, where the applied hydraulic pressures in the experi­ ments (0–8 bar) can only result in less than 0.15% flux reduction. The disparity between the experimental and theoretical results was pre­ dictable and could be ascribed to the pressure-induced deformation of the polymetric membrane, which was also reported in previous studies [29,30]. The deformed membrane, which could not maintain its original structure properties (i.e., pore size, porosity, and thickness), has reduced vapor permeability and can lead to increased temperature polarization in PRMD, resulting in the observed decreased water vapor fluxes (detailed explanation is presented in Supplementary Note 5 and has been also reported in our previous study [29]). Although the membrane in the AL-CS orientation exhibited better mechanical stability than that in the AL-HS orientation under higher applied pressures, its performance is poor under no or low applied pressures where membrane deformation is not significant. According to Fig. 2a and c, the water vapor fluxes and power densities obtained in ALCS orientation at 0–1.0 bar were 21.6%–64.2% lower than those observed in AL-HS orientation. Also, PRMD performance in the AL-CS orientation is more susceptible to the concentration of feed solution than the AL-HS orientation when membrane deformation is not signif­ icant. For example, increasing feed concentration from 0 to 2 M and 4 M could result in 40.4% and 57.5% water vapor flux reduction respectively in the AL-CS orientation at the applied pressure was 0.5 bar, while the value for the AL-HS orientation was only 18.7% and 40.7%, respectively (the flux reduction for both orientation in PRMD as a function of NaCl concentration of feed solution was calculated and shown in Fig. S7 in Supplementary Note 6). These results suggest that there is a trade-off between the membrane mechanical stability and performance at elevated concentrations in the AL-CS orientation. To the best of our knowledge, this is the first time this phenomenon has been reported. It is essential to operate the membrane under high applied pressures in PRMD to achieve a high power output (also refer to Fig. 1b) [25]. The AL-CS orientation is preferred due to its mechanical stability. Further, the feed concentration is also expected to affect the PRMD performance

significantly in this mechanically stable membrane orientation. Thus, it is interesting to understand the underlying mechanisms related to temperature polarization, concentration polarization as well as mem­ brane property change in PRMD, which are further analyzed in the subsequent sections. 3.3. PRMD performance reduction for treating hypersaline brine To further understand the underlying mechanisms for the reduction of PRMD water vapor flux at an elevated feed salinity, here we sys­ tematically analyzed PRMD performance at different feed concentra­ tions without applying pressures. Under these conditions, the potential impact of membrane deformation is excluded by operating the mem­ brane under zero applied pressure, since the impact of membrane deformation has been discussed and studied in above section and our previous study [29]. 3.3.1. Loss of effective driving forces in PRMD due to temperature polarization (TP) and internal concentration polarization (ICP) Fig. 3a illustrates the temperature and concentration profiles across the asymmetric membrane in the AL-CS orientation in PRMD. First, the temperature at the liquid-vapor interface (i.e., the support layer-active layer interface) may decrease significantly due to internal temperature polarization (ITP), while the concentration of salt at the liquid-vapor interface may increase significantly due to internal concentration po­ larization (ICP). The temperature and concentration at the liquid-vapor interface on the hot side were estimated by the theoretical models described in section 2.3 using the membrane structure parameters listed in Table 1 and the experimentally measured water vapor fluxes shown in Fig. S8 in Supplementary Note 6 (the detailed calculation methods are described in Supplementary Note 2). As shown in Fig. 3b, the temper­ ature polarization coefficient and the temperature at the liquid-vapor interface do not change significantly with the increase of feed concen­ tration. However, the net temperature drop from the bulk feed to the interface is more than 15 � C. The significant temperature drop in the ALCS orientation can be ascribed to the high heat transfer resistance of the polyester support layer with a low thermal conductivity [29,51,52]. In comparison, since feed solution could directly contact with the hydro­ phobic active layer and evaporate on the liquid-vapor interface in AL-HS orientation, temperature change on the feed side could be only attrib­ uted to external temperature polarization (ETP) (Fig. 4a). Thus, the net temperature drop on the feed side in AL-HS orientation is only ~ 2 � C (Fig. 4b). Further, the mass transport in PRMD with AL-CS orientation is affected by ICP, occurring inside the unstirred thick support layer. The evaporation of water causes the ICP after the water vapor goes through the membrane, salts accumulate at the liquid-vapor interface. Fig. 3c shows that the salt concentration at the liquid-vapor interface increases substantially. Although concentration polarization factor decreases at higher feed concentrations, indicating less severe ICP, ICP is still sig­ nificant even at high feed concentrations. For example, when the bulk feed concentration is at 4 M, the NaCl concentration at the liquid-vapor interface reaches 6.3 M, exceeding the saturation concentration of NaCl aqueous solution [8]. On the other hand, only external concentration polarization (ECP) occurs in the AL-HS orientation because the resis­ tance for solute back-diffusion away from the liquid-vapor interface would only come from the boundary layer when the hydrophobic active layer is facing the hot solution, which is similar to ECP in conventional MD, as shown in Fig. 4a. The ECP can be minimized by improving hy­ drodynamic conditions, such as using spacers and increasing the cross-flow velocity [42,53]. As shown in Fig. 4c, the salt concentration increase near the liquid-vapor interface is not significant. Overall, our results suggest that the ICP is a unique phenomenon in PRMD in the AL-CS orientation, which influences the PRMD performance detrimentally. Severe polarization effects would lead to the loss of driving forces in 5

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Fig. 3. (a) Schematic diagram of water vapor and heat transport in PRMD with the asymmetric membrane in the AL-CS orientation; The profiles illustrate the external temperature polarization (ETP) on both sides of the membrane and the internal temperature polarization (ITP) and internal concentration polarization (ICP) on the feed side. (b) Temperature in the bulk solution (TH,b) and at the liquid-vapor interface (TH,m) on hot side and temperature polarization coefficient (θ), (c) concentration of NaCl solution at the liquid-vapor interface on hot side (CH,m) and concentration polarization coefficient (β), (d) driving force (ΔPv) in PRMD, and (e) contribution of temperature polarization (TP) and concentration polarization (CP) effects on the driving force reduction as a function of the bulk feed solution concentration (0–4.0 M NaCl) in PRMD. Other conditions: effective applied hydraulic pressure ΔP ¼ 0 bar, temperature in the bulk cold solution Tc,b ¼ 20 � C, temperature in the bulk hot solution TH,b ¼ 60 � C, cross flow rate Q ¼ 0.3 L min 1, and the AL-CS orientation.

PRMD because its driving forces depend on the temperature and solute concentration at the liquid-vapor interface [24] (Eqs. (1) and (5)). Fig. 3d shows that the ideal driving force, which was calculated using the NaCl concentration and temperature in the hot and cold bulk solu­ tions without considering any polarization effects, slightly reduces with the increase in the bulk feed solution concentration due to the increase in the osmotic pressure on the feed side [39,40,42]. In comparison, the effective driving force, which was calculated by taken into consideration of both TP and CP, declines significantly to less than 25% of the ideal driving forces in the AL-CS orientation (Fig. 3d), while the effective driving force in the AL-HS orientation is around 40% of the ideal driving

force (Fig. 4d). Further analysis shows that more than 90% of the driving force loss for both orientations can be attributed to TP (Figs. 3e and 4e). These results suggest that TP, is the critical limiting factor of PRMD performance because PRMD has the thermally-driven nature of MD [54–56]. More severe internal temperature polarization (ITP) could be observed in AL-CS orientation as the loss of driving force due to ITP is more than 8.0 � 10 2 bar in this orientation (Fig. 3e) compared to ~ 6.0 � 10 2 bar in AL-HS orientation (Fig. 4e). Compared with the negligible effect of CP in the AL-HS orientation (only less than 0.03 � 10 2 bar driving force is attributed to ECP as shown in Fig. 4e), CP causes sig­ nificant driving force loss due to the unique ICP effect in the AL-CS 6

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Fig. 4. (a) Schematic diagram of water vapor and heat transport in PRMD with the asymmetric membrane in the AL-HS orientation; The profiles illustrate tem­ perature polarization (TP) on both sides of the membrane and the external concentration polarization (ECP) on the feed side. (b) Temperature in the bulk solution (TH,b) and at the liquid-vapor interface (TH,m) on hot side and temperature polarization coefficient (θ), (c) concentration of NaCl solution at the liquid-vapor interface on hot side (CH,m) and concentration polarization coefficient (β), (d) driving force (ΔPv) in PRMD, and (e) contribution of temperature polarization (TP) and con­ centration polarization (CP) effects on the driving force reduction as a function of the bulk feed solution concentration (0–4.0 M NaCl) in PRMD. Other conditions: effective applied hydraulic pressure ΔP ¼ 0 bar, temperature in the bulk cold solution Tc,b ¼ 20 � C, temperature in the bulk hot solution TH,b ¼ 60 � C, cross flow rate Q ¼ 0.3 L min 1, and the AL-HS orientation.

orientation. Besides, the contribution of ICP to the loss of effective driving force increases from 0 to 0.84 � 10 2 bar with the increase of the feed concentration from 0 to 2 M. (Fig. 3e). Interestingly, further increasing the feed concentration to 4 M results in a slight decline of the contribution by ICP to 0.66 � 10 2 bar. This could be due to the over­ saturation of the NaCl solution at the liquid-vapor interface (Fig. 3c).

both membrane orientations calculated using the water vapor flux without applying pressure from Fig. 2a and c (more specific experi­ mental data are shown in Fig. S8) divided by the effective partial vapor pressure driving force across the membrane from Figs. 3d and 4d, respectively. At the feed concentrations from 0 to 1 M, Bw for both membrane orientations remain approximately constant at 314.2 � 8.2 L m 2 h 1 bar 1, which statistically equal to the theoretical vapor permeability coefficient of the membrane (318.3 L m 2 h 1 bar 1, the black dash line in Fig. 5a) calculated using the structural parameters of the membrane (Eqs. (2)–(4)). However, Bw for AL-CS orientation (blue

3.3.2. Loss of vapor permeability of membrane due to ICP-induced crystallization in PRMD Fig. 5a shows the actual membrane permeability coefficient (Bw) for 7

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Fig. 5. (a) Vapor permeability coefficient (Bw) as a function of the concentration of bulk feed solution (0–4.0 M NaCl) in PRMD with AL-CS orientation (blue spheres) and AL-HS orientation (red circles). (b,c) Photographs (left, scale bars 1 cm) and SEM images (right, scale bars 100 μm) of the membranes after the PRMD tests with AL-CS orientation in feed solution of (b) 0.5 M NaCl and (c) 4.0 M NaCl. Other conditions: effective applied hydraulic pressure ΔP ¼ 0 bar, temperature in the bulk cold solution Tc,b ¼ 20 � C, temperature in the bulk hot solution TH,b ¼ 60 � C, cross flow rate Q ¼ 0.3 L min 1. (d) Schematic illustration of loss of membrane permeability due to crystallization of NaCl within the support layer. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

spheres in Fig. 5a) decreases drastically to 263.8 � 15.9 L m 2 h 1 bar 1 and 169.7 � 18.3 L m 2 h 1 bar 1 when the feed concentration in­ creases to 2 and 4 M, respectively. This is presumably caused by the crystallization of NaCl solution within the support layer since the NaCl concentration is significantly elevated due to the ICP effect (Fig. 3c). To verify this speculation, we further characterized the membranes after PRMD tests with AL-CS orientation. After tested with 0.5 M NaCl feed solution, the membrane exhibited a relatively clean and flat surface without noticeable morphological changes compared to the pristine membrane, and only a few small NaCl crystals (<1 μm) can be observed (Fig. 5b). In contrast, after tested with 4.0 M NaCl feed, a distinct membrane morphological change was observed with plenty of small protrusions on the membrane surface (highlighted by white dash circles in Fig. 5c). In addition, the SEM image in Fig. 5c shows that many large NaCl crystals (with the diameter ranging from 10 to 60 μm) were densely trapped into the pores among the polyester fiber of the support layer. Here, it should be noted that most of these large NaCl crystals should be generated during the PRMD process since only a few and much smaller NaCl crystals could be observed on surface the membrane sample after solely immerged in 4 M NaCl solution (Fig. S9 and Supplementary Note 7). As aforementioned, the NaCl solution may reach saturation at the liquid-vapor interface because of the severe ICP in PRMD (Fig. 3c).

Besides, the reduced temperature caused by ITP would further reduce the solubility of NaCl at the liquid-vapor interface. Thus, the crystalli­ zation of the saturated NaCl solution within the confined support layer, particularly at the liquid-vapor interface, would block the permeate pathway and reduce effective membrane area, which results in the loss of membrane permeability (Fig. 5d). The loss of membrane permeability was also found for the bulk feed NaCl concentration of 2 M (Fig. 5a), although the NaCl concentration at the liquid-vapor interface was still undersaturated (Fig. 3c). It should be noted that the calculated NaCl concentration at the liquid-vapor interface is an average value, assuming homogeneous membrane structural properties of the whole support layer. The structure of a real membrane is not homogenous, which may lead to various NaCl concentrations at different local locations at the liquid-vapor interface. Thus, some NaCl crystals are formed locally within the support layer. Further, the membrane water permeability coefficient B is un­ changed in the AL-HS orientation for all the feed solutions at different concentrations after PRMD tests (red circles in Fig. 5a), which suggests that the ICP-induced crystallization is a unique mechanism for PRMD in the AL-CS orientation.

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4. Conclusions and implications

Acknowledgments

We demonstrated that PRMD has the potential to help to realize a ZLD process in wastewater management because of its distinctive capability in simultaneously producing fresh water from hypersaline brine and harvesting low-grade heat energy. The commercial PEFE membrane used in PRMD can desalinate NaCl solution at the concen­ tration as high as 4 M (233.76 g L 1) with the salt rejection >99.9% and generate a theoretic specific energy of 0.22 kWh m 3 at an operating pressure of 8 bar for used in this study. However, PRMD performance would be compromised when the AL-CS orientation is used to obtain better mechanical stability. Severe ITP and ICP resulted from the addi­ tional heat and mass transfer resistances in the membrane support layer would reduce the effective driving force for PRMD. We also found that the ICP-induced crystallization takes place in the support layer when hypersaline brines are used, which would significantly reduce the membrane permeability. This implies that the PRMD operated in the ALCS orientation is prone to performance deterioration under fouling or scaling conditions when the feed water contains foulants or scaling precursor ions. The inevitable polarization effects and potential fouling/ scaling issues will impair performance of PRMD in treatment of hyper­ saline brine containing extremely high concentration of salts (i.e., TDS > 200 g L 1) and complicated dissolved components. Thus, one of the future research directions of PRMD can focus on optimization of the operation conditions and process design to minimize the polarization effects and scaling/fouling issues. For example, enhancing the turbu­ lence in the feed channel by increasing the cross-flow rate or using specially designed feed channel spacers can help to alleviate the polar­ ization effects in PRMD; Integrating pretreatment process (i.g., coagu­ lation, filtration and acidification) can partially remove foulants or scaling precursor ions in feed solution to control fouling/scaling, and combining post-treatment processes (i.g., crystallization, thermal evap­ oration) with PRMD can ultimately achieve real ZLD application. Further, the findings in this study provide guides for developing novel PRMD membranes. Improving mechanical stability of membrane is the first critical step to minimize the membrane deformation under applied pressure. New membrane material with higher LEP without significant sacrifice of the membrane permeability and mechanical strength can substantially enhance the energy generation in PRMD due to the higher hydraulic pressure allowed for the operation. In addition, developing an asymmetry membrane by adding a hydrophilic support layer along with the porous hydrophobic active layer is a practically feasible strategy to improve the mechanical stability of the PRMD membrane. Apart from robust mechanical strength to support the membrane against deformation under hydraulic pressures, the hydro­ philic support layer is also expected to have lower resistance to the heat and mass transfer of the hot feed solution before it reaches to the surface of hydrophobic active layer, which can alleviate the internal polariza­ tion effects in PRMD. For example, increasing the porosity or reducing the tortuosity and thickness of the support layer can reduce its mass transfer resistance, while the low heat transfer resistance of support layer can be achieved by applying high thermal conductive materials.

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Declaration of interest statement All authors declare that there are no conflicts of interest in this work. CRediT authorship contribution statement Ziwen Yuan: Conceptualization, Validation, Methodology, Formal analysis, Writing - original draft, Writing - review & editing. Yanxi Yu: Validation, Methodology, Formal analysis. Li Wei: Validation, Meth­ odology, Formal analysis. Xiao Sui: Validation, Methodology. Qian­ hong She: Conceptualization, Formal analysis, Writing - review & editing. Yuan Chen: Conceptualization, Formal analysis, Writing - re­ view & editing, Funding acquisition. 9

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