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Novel Janus composite hollow fiber membrane-based direct contact membrane distillation (DCMD) process for produced water desalination
Journal of Membrane Science 597 (2020) 117756
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Journal of Membrane Science journal homepage: http://www.elsevier.com/locate/memsci
Novel Janus composite hollow fiber membrane-based direct contact membrane distillation (DCMD) process for produced water desalination Lusi Zou a, b, Pri Gusnawan a, b, Guoyin Zhang a, Jianjia Yu a, * a b
Petroleum Recovery Research Center, New Mexico Tech, Socorro, NM, 87801, United States Materials Engineering Department, New Mexico Tech, Socorro, NM, 87801, United States
A R T I C L E I N F O
A B S T R A C T
Keywords: Janus composite hollow fiber membrane Produced water Desalination DCMD
In this study, an innovative Janus composite hollow fiber membrane-based direct contact membrane distillation (DCMD) process was boosted for desalination of actual produced water with elevated total dissolved solids (TDS) of 154,220 mg/L. The Janus composite hollow fiber membrane (Janus-HFM) was characterized with a thin and hydrophobic polyvinylidene difluoride (PVDF) and superhydrophobic silica nanoparticles (Si-R) outer layer, and a thick porous and hydrophilic PVDF and polyethylene glycol (PEG) inner layer. Compared to the neat PVDF hollow fiber membrane, the Janus-HFM showed both increased permeate water flux and improved energy ef ficiency with more than 99.99% salt rejection. The long-term desalination performance of the Janus-HFM was investigated via 200 h of continuous DCMD experiment. The results showed that the permeate water flux declined from 25.41 kg/m2h to 15.21 kg/m2h, and the salt rejection was 98.4% at the end of the operation. It was found that both scales and dissolved organic matters induced foulants accumulated at the feed side of the membrane due to the complex composition of the produced water. Nonetheless, the desalination performance decline can be effectively inhibited from 40% to 9.7% with a simple physical cleaning process by using the permeate water that recovered in the DCMD process.
1. Introduction Produced water (PW) management is one of the most challenging issues in the oil and gas industry. Today, large volumes of PW is reinjected into the deep depleted aquifer for disposal. The PW treatment mainly involves the removal of organic matters and suspended partic ulate solids to minimize pore plugging and formation damage [1]. The PW treatment technologies used in the petroleum industry focus pri marily on physical separation, including the use of API separators, co alesces, or hydrocyclones. However, it is still challenging to transform the effluent from the physical separation to a resource for petroleum stimulation or other beneficial reuse due to the presence of dissolved organic matters and high-salinity in PW. Direct Contact Membrane Distillation (DCMD) has been considered as a promising technology for the treatment of high-salinity impaired water [2]. Compared to the conventional membrane-based pressure- driven separation process such as nanofiltration and reverse osmosis, the driving force of DCMD is the difference of vapor pressure across a porous hydrophobic membrane, which makes the DCMD process well tolerant of membrane fouling [3,4]. For example, Hussain et al. reported a
flat-sheet Polytetrafluoroethylene (PTFE) and polypropylene (PP) based MD for the treatment of PW from unconventional resources [5]. They found that MD is effective to desalinate the brines with the salt con centration up to 70,000 mg/L, and the permeate water flux is not sen sitive to salt concentration. Singh et al. demonstrate a series of DCMD desalination processes with a PP hollow fiber membrane module [6]. It was reported that the DCMD is feasible to treat different types of pro duced water to obtain high-quality permeate water and 80% water re covery. Macedonio et al. reported a laboratory-made PVDF hollow fiber membrane and a commercial PP membrane-based DCMD process to desalinate oilfield produced water [7]. They found that the DCMD process shows excellent rejections towards both total dissolved solids and dissolved carbon that present in the produced water. However, the conventional hydrophobic membrane is still chal lenged by the trade-off between water vapor flux and the significant loss of conductive heat. In DCMD, it has been found that only 17.7–42.1% of the total heat is effectively used as the latent heat for water vapor for mation. The majority of the heat gets lost by internal heat conduction through the polymeric membrane matrix because the heat transfer co efficient of polymers is much higher than that of the air. The DCMD cost
* Corresponding author. E-mail address: [email protected] (J. Yu). https://doi.org/10.1016/j.memsci.2019.117756 Received 26 August 2019; Received in revised form 13 December 2019; Accepted 14 December 2019 Available online 16 December 2019 0376-7388/© 2019 Elsevier B.V. All rights reserved.
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has been reported to vary from $0.3/m3 to $4.47/m3 for PW treatment [8–10]. To improve the energy efficiency of DCMD, different types of low-grade heat sources, including solar thermal energy, geothermal energy, waste heat or natural temperature gradient, have been used to make the DCMD as an economically viable large-scale desalination technology to be competitive with the commercially available reverse osmosis (RO) [11]. Besides, as a promising alternative, the design of innovative Janus composite hollow fiber membranes, to achieve both high mass transfer rate and low heat transfer rate, has attracted exten sive attention in recent years. Janus composite hollow fiber membrane (Janus-HFM) is character ized by distinct physical properties at the two surfaces, such as a hy drophobic surface and a hydrophilic surface [12]. In a typical Janus-HFM based DCMD desalination process, the hydrophobic sur face of the membrane functions as a barrier to prevent the direct contact between the hot feed solution and the cold permeate, but allow the water vapor to transport through the membrane pores. The existence of a thick and porous hydrophilic layer helps reduce the vapor transfer distance between the two sides of the membrane, and create additional conductive heat resistance in the DCMD process. The Janus-HFM has been prepared via different protocols that mainly focus on surface coating modification and co-extruding technique. The former method usually utilizes the neat conventional hydrophilic or hydrophobic membrane as the substrate. The Janus membrane is then obtained by surface modification, such as interfacial polymerization (IP) on the one surface of the membrane substrate [12–15]. For example, Yang et al. prepared a Janus-HFM by coating hydrophilic poly dopamine/polyethyleneimine onto the lumen side of a hydrophobic PP hollow fiber membrane. Ursino et al. prepared the Janus membrane through dip-coating and in-situ polymerization of perfluropolyether (PFPE) on a hydrophilic polyamide/polyethersulfone (PA/PES) com mercial membrane [13,14]. Most recently, Puranik et al. introduced a plasma polymerization method by depositing hydrophobic fluorosilox ane coating on the surface of different hydrophilic commercial PVDF membranes [16]. However, the properties of the above reported Janus membranes are highly determined by the structure of the commercially-available membrane substrate. It is difficult to change the thickness of the two distinct layers of the membrane. Furthermore, the process of dip-coating may also create a dense intermediate layer be tween the membrane substrate and the newly introduced hydrophilic or hydrophobic layer, which can sacrifice the permeate water flux of the membrane in DCMD [17]. The co-extruding of two different dope solutions through a tripleorifice spinneret is another well-known method to fabricate the JanusHFM. Because the properties of both the inner layer and outer layer of the membrane can be tuned with designed dope formulations, it pro vides more flexibility in the fabrication of Janus membranes [18,19]. Currently, most of the Janus structure was accomplished by tuning the hydrophobicity of the membrane materials. For example, Zhu and Feng et al. prepared hydrophobic/hydrophilic hollow fiber membranes by adding poly (vinyl alcohol) (PVA) to the PVDF dope solution. However, the hydrophobicity of the Janus membrane was restricted by the nature of the neat PVDF [20,21]. Bonyadi and Edwie et al. reported their pioneer work on the use of dual-layer hydrophobic/hydrophilic PVDF hollow fiber membranes for flux enhancement in DCMD. However, the fabrication of the Janus-HFM was largely based on the formulation of complex dope solutions, such as PVDF/PAN/cloisite NAþ/EG/NMP [22], and PVDF/PAN/hydrophilic clay particles/NMP [23]. The studies of compatibility among the different components may require extensive time and attention, which increases the cost for membrane fabrication. Therefore, the formulation of the simple and effective dope solution is still required to prepare the Janus composite hollow fiber membrane. Besides, it is worth to note that all the Janus membranes in the available literature aimed to desalinate the water with a relatively low salt concentration (TDS<35,000 mg/L). The desalination of highsalinity water (TDS>150,000 mg/L), such as oilfield produced water,
is never studied by the use of the Janus hollow fiber membrane. Thus, in this study, we fabricated a novel Janus composite hollow fiber mem brane by using a simple and effective dope solution, and investigated the Janus HFM based DCMD process for desalination of actual produced water with high salinity (TDS ¼ 154,220 mg/L). Specially, the JanusHFM was fabricated with a thin polyvinylidene fluoride (PVDF) and superhydrophobic silica nanoparticles (Si-R) outer layer, and a thick highly-porous hydrophilic PVDF and polyethylene glycol (PEG) inner layer. The Janus-HFM based DCMD experiments were performed by using actual produced water as the feed solution, and the desalination performance of the Janus-HFM was investigated in terms of permeate water flux, salt rejection and specific energy consumption. The longterm stability of the Janus-HFM was also evaluated through a physical cleaning process by using the permeate water received from the DCMD process. 2. Experimental 2.1. Materials Poly (vinylidene fluoride) (PVDF, Kynar® HSV900) was provided by the Arkema Inc. in powder form. Superhydrophobic Si-R nanoparticles were obtained from our previous work [24]. Polyethylene glycol (PEG, Mw 600), and N-methyl-2-pyrrolidone (NMP, >98%) were purchased from Sigma-Aldrich for the formulation of dope solution. Actual pro duced water was sampled from a production facility located in Carlsbad, New Mexico. The major composition of the produced water is listed in Table 1. 2.2. Preparation of PVDF/PEG and PVDF/Si-R dope solutions PVDF powder was directly dissolved in NMP solvent under me chanical stirring at ambient temperature. During the mixing process, polyethylene glycol (PEG) was added. The obtained homogeneous PVDF/PEG dope solution was degassed in a vacuum oven for 12 h. Similarly, PVDF/Si-R dope solution was formulated by dissolving Si-R nanoparticles in NMP to obtain a transparent brown NMP/Si-R solu tion. Then, PVDF powder was added to the NMP/Si-R solution under mechanical stirring to form the homogeneous PVDF/Si-R dope solution. The air bubbles in the dope solution were also removed in a vacuum oven before its further use. 2.3. Fabrication of Janus-HFM The Janus-HFM was fabricated through a co-extrusion method. Briefly, PVDF/PEG, PVDF/Si-R dope solutions and bore-fluid (NMP/ water) were simultaneously extruded through a triple-orifice spinneret with three ISCO syringe pumps. The membrane was formed in a water coagulation bath. The obtained Janus-HFM were first soaked in tap water for at least two days to allow the complete exchange between NMP and water, and then was sequentially transferred to methanol and hexane before the final drying process in a freeze dryer [25]. As a Table 1 Major composition of the produced water sampled from the Permian Basin.
2
Composition
Concentration (mg/L)
Sodium, Naþ Ammonium, NHþ 4 Potassium, Kþ 2þ Magnesium, Mg Calcium, Ca2þ Chloride, ClBromide, BrSulfate, SO24 Total Dissolved Solids (TDS) Non-Purgeable Organic Carbon (NPOC)
37,277 318 1,553 2,552 15,415 84,500 735 1,140 154,220 57.6
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Where Jw is the permeate water flux (kg/m2h), W is the mass of water received at the cold permeate side (kg); A is the effective membrane area (m2) and t is operating time (h). ΔH is the latent heat of water evapo ration (2257 kJ/kg), F is the mass flow rate of the feed solution (kg/s), Tf, in and Tf, out are the temperature of the feed solution at the inlet and outlet (K), respectively.
comparison, neat PVDF HFM was also fabricated with the similar method. The spinning parameters for the fabrication of hollow fiber membranes are listed in Table 2. The morphology of the Janus composite hollow fiber membrane was characterized with a high-resolution field emission scanning electron microscope (FESEM, JEOL 2010). The membrane sample was first frozen and fractured in liquid nitrogen for cross-sectional observation of the hollow fiber membrane. To examine the inner suface of the hollow fiber membrane, the membrane sample was first placed on an electrical adhesive tape and then carefully splited with a sharp surgical knife. The cross sections, along with both the inner surface and outer surface, were coated with carbon using a carbon sputter coater prior to FESEM char acterization. The contact angles of the silica particles were measured with a Contact Angle System equipped with SCA20 software.
3. Results and discussion 3.1. Characterization of the PVDF-PEG and PVDF-Si-R Janus-HFM Fig. 2 shows the cross-sectional and surface morphology of the Janus-HFM. It is obvious that the membrane exhibits an asymmetric structure with a thin and hydrophobic PVDF/Si-R outer layer and a thick and high-porous hydrophilic macrovoids-free PVDF/PEG inner layer, with the water contact angle of 137.6� and 56.2� at the outer surface and inner surface, respectively. In Fig. 2 (b), large numbers of pyriform-like macrovoids are formed beneath the PVDF/Si-R outer surface, attributing to the different chemical composition of the two dope solutions, and the similar phe nomenon has been found by previous researchers [20]. Fig. 3 shows the ternary phase diagram of the dope solution with the addition of super hydrophobic Si-R nanoparticles and hydrophilic PEG. It can be seen that both Si-R nanoparticles and PEG shift the binodal curve towards the PVDF/NMP axis, which indicates a reduced solvent power of the dope solution [29,30]. In the process of phase inversion, due to the presence of Si-R nanoparticles, the thermodynamic stability of the PVF/Si-R dope solution decreased, and an instantaneous liquid-liquid demixing occurs to form the macrovoids under the surface of the outer layer. From a dynamic point of view, the superhydrophobic property of the Si-R nanoparticles can effectively impede the water diffusion to the PVDF/Si-R dope, which hinders the exchange between NMP and water. Thus, instead of a liquid-liquid demixing, the semi-crystalline PVDF experiences a solid-liquid demixing, and thus forming a porous nodular structure at the outer layer of the Janus membrane [31], as shown in Fig. 2 (c). The interconnected macrovoids-free PVDF/PEG inner layer is mainly assigned by the increased dope viscosity due to the addition of PEG600 molecules to the PVDF dope solution, and also the existence of 70% NMP in the bore fluid. Similar membrane morphology was reported by Chung et al. [32]. It can be interesting to see that the pore size decreased gradually from an average of 2 μm at the inner surface to the average of 0.15 μm on the outer surface.
2.4. DCMD process for actual produced water desalination Janus composite hollow fiber membrane module was assembled through an epoxy-potting technique [26,27]. Briefly, 10 pieces of the Janus composite hollow fiber membranes were bundled together through a 1/4” stainless steel tubing. Both ends of the membrane bundle were sealed by epoxy. The effective length of the membrane is 15 cm. The apparatus of the Janus composite hollow fiber membrane based DCMD process was demonstrated in Fig. 1. The feed solution was first heated in a water bath, and then pumped into the shell side of the hollow fiber membrane; DI water was cooled and circulated at the lumen side of the membrane. The temperature was recorded with four digital tem perature sensors at the inlets (Tf, 1 and Tp, 1) and outlets (Tf, 2 and Tp, 2) of the membrane module. Two Masterflex peristaltic pumps were used to adjust flow rates for the feed solution and the cold DI water. The masses of the hot feed solution and cold permeate water were weighted through two digital scales equipped with a data acquisition system, to measure real-time fresh water harvest in the cold permeate side. Both electrical conductivity (EC) and total dissolved solids (TDS) in the permeate water were measured with a TDS/conductivity meter (Hach HQ40d), to evaluate salt rejection of the Janus composite hollow fiber membranes. The permeate water flux of the DCMD process was measured by recording the mass of water received at the cold permeate side according to Equation (1). The energy efficiency was evaluated by monitoring the feed temperatures at both the inlet and outlet, as described in Equation (2) [28],: Jw ¼
W A �t
EEð%Þ ¼
Jw ΔHA FCpðTf ; in Tf ; outÞ
(1)
3.2. Validation of the Janus-HFM based DCMD with 35,000 mg/L salt water as feed solution
(2)
Based on a conventional neat PVDF membrane, the desalination performance of the Janus-HFM DCMD process was validated by using 35,000 mg/L NaCl as feed solution, as shown in Fig. 4. It has to point out that the dimensions of the membrane modules were the same for the two hollow fiber membranes, and the operation conditions also remained the same for the DCMD experiment. Thus, the mass change on the permeate side is directly corresponding to the permeate water flux that listed in Table 3. The Janus-HFM produced more permeate water than the neat PVDF HFM, and the corresponding permeate water flux is 26.11 kg m-2 h-1 and 22.1 kg m-2 h-1, respectively. Table 3 also presents salt rejection and energy efficiency of the two membranes. Both membranes exhibited more than 99.99% salt rejec tion, attributing to the super-hydrophobicity of the PVDF membrane. However, the energy efficiency of the Janus-HFM is much higher than the neat PVDF HFM, which is contributed to the presence of a highporous hydrophilic PVDF/PEG inner layer in the Janus-HFM. The long-term stability of the Janus-HFM was evaluated with a continuous 82 h DCMD experiment. The temperature of the hot feed and the cold permeate was 85.1� C and 20� C, and the flow velocity was 0.15 m/s (129.6 ml/min) and 0.5 m/s (21.6 ml/min), respectively. During
Table 2 Parameters for fabrication of hollow fiber membranes. Parameters Outer Layer Dope Composition Inner Layer Dope Composition Bore fluid External coagulant Outer layer flow rate (mL/ min) Inner layer flow rate (mL/min) Bore flow rate (mL/min) Length of the air gap (cm) Take-up speed Membrane OD/ID (μm)
Value J-HFM
neat PVDF HFM
PVDF/Si-R/NMP (12/1/ 87) PVDF/PEG/NMP (12/6/ 82) NMP/water (70/30) Tap water 2
PVDF/NMP (12/ 88) PVDF/NMP (12/ 88)
2 2 5 free fall 805/584
3
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Fig. 1. Experimental setup of the Janus composite hollow fiber membrane based DCMD process.
Fig. 2. SEM image of the PVDF-PEG and PVDF-Si-R Janus-HFM.
the DCMD process, more than 6,066.72 g of clean water was recovered and the feed salinity was concentrated from 35,000 mg/L to 100,000 mg/L at the end of the 82 h operation. The permeate water flux and salt rejection are plotted in Fig. 5. It can be seen that the permeate water flux reduced from 33.50 kg m-2 h-1 to 30.89 kg m-2 h-1 at the end of the 82 h continuous operation, with the permeate water flux decline at around 7.8%. This should be caused by the increased feed salinity in the course of feed circulation. The water partial vapor pressure can be restrained due to the decreased water activity by the ion hydration in the feed solution [33]. During the 82 h DCMD process, the salt rejection remained at higher than 99.90%, indicating a good wetting resistance of the Janus composite hollow fiber membrane when 35,000 mg/L salt water was used as feed solution.
produced water was pretreated with a spin down sediment water filter to remove all the suspended solids with diameters larger than 50 μm. The pre-treated produced water was directly used in the DCMD process. Fig. 6 shows the water recovery during a 200 h continuous DCMD operation. It shows that the water recovery was 552 g at the first two 8-h operations, and then decreased at the time period of 16–24 h. The average water recovery was 311 g per 8 h during the time period of 80–200 h. Fig. 7 shows permeate flux and salt rejection as the function of operating time during the 200 h continuous desalination process. As indicated by the result of water recovery in Fig. 6, the permeate water flux decreased from 25.41 kg/m2h to 19.19 kg/m2h in the first 80 h operation, and then dropped down to 15.21 kg/m2h at the end of the 200 h operation. Besides, the salt rejection efficiency was >99% in the first 104 h operation, and then gradually reduced to 98.4% at the end of the DCMD process. The desalination performance decline of the JanusHFM can be attributed to the presence of high salinity in the feed so lution. As mentioned before, DCMD is a process that involves both water vapor transfer and heat transfer. The water vapor transfer caused an
3.3. Long-term Janus-HFM based DCMD process with actual PW as feed solution The long-term stability of the Janus-HFM DCMD process was eval uated by using actual produced water as feed solution. The original 4
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Fig. 3. Ternary phase diagram for PVDF/NMP/Water system with superhydrophobic Si-R nanoparticles and PEG as additives.
Fig. 5. Desalination performance of the Janus-HFM in 82 h of contin uous DCMD.
The concentrated produced water got supersaturated at the evaporation interface and thus promoted scale formation on the membrane surface. The produced water oversaturation can result in partial pore clogging of the membrane, and thus the permeate water flux declined. The occur rence of scale-induced pore clogging reduced the membrane hydro phobicity and caused the produced water migration into the big pores of the PVDF/Si-R layer, which resulted in the direct contact between permeate water and the produced water, and thus the salt rejection reduced [34,35]. The energy efficiency of the Janus-HFM DCMD desalination process was plotted in Fig. 8, which indicates the effective latent heat that applied to transfer water vapor through the pores of the membrane. The result shows that energy efficiency decreased from 58.16% to 28.68% during the 200 h continuous DCMD operation, and the declining trend was consistent with the permeate water flux in Fig. 7. The temperature profiles of the produced water at both inlet and outlet were also plotted in Fig. 8. It was found the inlet feed temperature has remained at 80 � C,
Fig. 4. Desalination performance of neat PVDF HFM and the Janus composite hollow fiber membrane (Janus-HFM). Table 3 Comparisons of desalination performance between neat PVDF HFM and the JHFM. Permeate flux (kg/m2h) Salt rejection (%) Energy efficiency (%)
neat PVDF HFM
J-HFM
22.1 99.99 55.64
26.11 99.99 72.45
increased salt concentration at the evaporation interface, and in the meantime, the feed solution temperature decreased at the interface due to the latent heat loss that accompanied with the vapor transportation. 5
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efficiency through the heat recovery or process design are beyond the topic of this study. 3.4. Fouling and wetting phenomenon in the 200 h of Janus-HFM DCMD process To identify the salt rejection during the 200 h continuous DCMD desalination process, ion chromatography was employed to measure the ion concentration at the permeate side. Fig. 9 (a) and (b) show the rejection efficiencies of five major cations and anions that present in the produced water. It can be seen that the rejection of all the five ions remained more than 99.99% in the first 24 h of operation; after that, the ion rejection started to decrease, and the decline trend increased when the operating time was more than 40 h. The mechanism of ions transportation in the DCMD process has not been well understood. The presence of large defects on the membrane surface has been reported to explain the salt permeation through the PVDF membrane in DCMD [36]. However, the Janus-HFM showed almost 100% salt rejection at the first 24 h operation, the decreased ion rejection in Fig. 9 should be more likely attributed to the partial mem brane pore wetting, which created water bridges in the pores, and allowed the passage of salts to the permeate side [37,38]. In this case, the salt transportation is governed by the salt concentration gradient across the membrane. Therefore, the ions of sodium and chloride un derwent significant rejection decline due to their significantly higher concentrations in the produced water as listed in Table 2. Compared to the monovalent ions of sodium and chloride, the divalent ions showed higher rejections of 98.89%, 98.91% and 99.54% for calcium, magne sium and sulfate, respectively. Fig. 10 show the outer surface morphology of both fresh membrane and the used membrane after 200 h of continuous DCMD. It is obvious that the feed side of the used membrane was covered by multi layers of particulate depositions. The chemical composition of these depositions was further identified by EDX, and all the peaks were analyzed with Genesis EDAX Genesis software. Compared to the fresh membrane in Fig. 10 (a), several new peaks appeared on the surface of the used membrane, as shown in Fig. 10 (b), mainly attributed to the elements of oxygen, irons, calcium, and magnesium. The signal of oxygen may be originated from hydroxide, sulfate or carbonate deposited on the membrane surface. The peak intensity of calcium was much higher than that of magnesium. By referring to the EDX spectrum of neat calcium carbonate in Ref. [39], calcium carbonate can be identified as one of the main scales on the surface of the used membrane. In contrast to the previous findings from Lokare et al. that the predominant scale from produced water was sodium chloride, the formation of calcium car bonate can be explained by the different composition of produced water in DCMD [10]. The accumulation of calcium carbonate on the mem brane surface can well explain the previous results in Fig. 7 that the desalination performance declined with the operating time in the 200 h of DCMD. As a unique characteristic of the oilfield PW, the presence of high content non-purgeable organic carbon (NPOC) is a big challenge for the membrane. At the end of the 200 h Janus-HFM DCMD process, the NPOC was significantly reduced from 57.6 mg/L to less than 2.0 mg/L. Attenuated total reflection-FTIR (ATR-FTIR) spectroscopy was used to examine the composition of organic deposition on the outer surface of the used membrane. Compare to the fresh membrane, Fig. 11 shows that three new peak bands were found on the outer surface of the used membrane, and the transmittance intensity are proportional to the operating time. The three peak bands, located at 1500-1700 cm-1, 28503000 cm-1, and 3200–3550 cm 1, are mainly originated from amide II I bands, symmetric and asymmetric starching of –CH2 group, and the stretching of O–H group, respectively [40]. These peaks should attribute to aliphatic hydrocarbons, aromatic hydrocarbons, and carbonates, which represent proteins, aliphatic hydrocarbon, and humic substances that are commonly detected in the oilfield produced water [40–42].
Fig. 6. Clean water recovery during 200 h of continuous DCMD by using actual produced water as the feed solution.
Fig. 7. Permeate flux and salt rejection of the Janus-HFM during 200 h of DCMD with actual produced water as feed solution.
Fig. 8. Energy efficiency of the Janus-HFM based DCMD process.
and the temperature at the outlet slightly decreased from 64.6 � C to 61.4� C, this may attribute to an intensified temperature polarization in the DCMD desalination process. It is worth to note that since heat re covery was not considered in DCMD, the further improvement of energy 6
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Fig. 9. Rejection of the major cations (a) and anions (b) during 200 h of continuous DCMD desalination process.
Fig. 10. Outer surface morphology and EDX of (a) fresh membrane and (b) the membrane after 200 h of continuous DCMD desalination operation.
Fig. 12 depicts the inner surface morphology of both the fresh membrane and the used membrane. It is obvious to see that the used membrane exhibits a super clean inner surface after 200 h of continuous operation. Since the amount of the permeated salts is small in the permeate water, the permeate water at the lumen side helps flush out all the salts from the inner layer, and avoid the formation of salt deposits on the inner surface.
3.5. Regeneration behavior of the Janus-HFM for actual PW desalination Membrane regeneration is a conventional approach to address the fouling problem in a typical DCMD desalination process. To study the membrane regeneration effect on the DCMD desalination performance, a 72 h DCMD experiment was conducted with physical cleaning of the membrane at an interval for every 12 h. The physical cleaning was achieved by using the permeate water received from the DCMD process 7
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without membrane regeneration. The water recovery started to decline at the end of 16 h of operation, and it decreased from 553.76 g to 327.91 g when the operating time increased from 16 h to 72 h; the recovery decline was 40% at the end of the DCMD operation. The significant decline of water recovery can be explained by the accumulated salt deposits on the outer surface of the membrane, which intensifies both scale fouling and membrane wetting. When physical cleaning was per formed, it shows in Fig. 13 (b) that the reduction of water recovery was small at the end of the 72 h operation. Fig. 14 plots both permeate water flux and salt rejection of the JanusHFM in the 72 h of DCMD operation along with the physical cleaning. It can be seen that the permeate water flux decreased from 26.6 kg/m2⋅h to 22.6 kg/m2⋅h, and the decline of permeate water flux significantly reduced from 40% to 10%. The promised permeate water recovery can be attributed to the low operating pressure used in the DCMD process, the hydrophobic interaction was weak between the membrane and the organic matters in the feed solution, which can be partially destroyed with the hydraulic force created in the physical cleaning process [24]. In addition, the membrane wetting was considerably inhibited by the prevention of scale accumulation on the membrane surface, and the salt rejection remained more than 99.81% until the end of the 72 h opera tion. Therefore, the permeate water from the DCMD process can be effectively used to maintain the desalination performance of the Janus-HFM, which helps reduce the demand for freshwater in the desalination process.
Fig. 11. ATR-FTIR of (a) fresh membrane and (b) the used membrane.
4. Conclusion
to flush the feed side of the membranes. The flow rate was 0.32 m/s (259.2 ml/min). Compared to the conventional physical cleaning pro cess with the usage of DI water, the use of permeate water is more realistic due to the fact that DI water is usually not available in the field. Based on an average salt rejection efficiency of 99.90% at the end of the first 24 h operation, the TDS of the permeate water is 150.0 mg/L. Fig. 13 (a) shows the water recovery in 72 h of continuous DCMD
A novel Janus composite hollow fiber membrane was fabricated comprising a thin hydrophobic PVDF/Si-R outer layer and a thick porous and hydrophilic PVDF/PEG inner layer for desalination of actual pro duced water with the TDS of 154,200 mg/L. In contrast to neat PVDF membrane, the Janus-HFM exhibited both increased permeate water flux and improved energy efficiency in the DCMD desalination process.
Fig. 12. Inner surface morphology of (a) fresh membrane and (b) the membrane after 200 h continuous DCMD desalination operation. 8
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Fig. 13. Clean water recovery during a 72 h continuous DCMD desalination process (a) without and (b) with membrane regeneration process.
In accordance with a 200 h continuous DCMD experiment, the JanusHFM showed a significant permeate water flux decline from 25.41 kg/ m2h to 15.21 kg/m2h, and the salt rejection was 98.4% at the end of the 200 h operation. It was confirmed by the SEM and FTIR that multi layers of particular depositions accumulated at the feed side of the membrane, including both scales and dissolved organics from the produced water. The membrane fouling-induced permeate water flux decline can be effectively inhibited from 40% to 10% through a simple physical cleaning process by using the permeate water received in the DCMD process.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Lusi Zou: Methodology, Investigation, Data curation, Writing original draft. Pri Gusnawan: Visualization, Investigation. Guoyin Zhang: Resources, Validation. Jianjia Yu: Conceptualization, Writing review & editing, Supervision, Funding acquisition.
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Yu, Experimental study of PES/ SiO 2 based TFC hollow fiber membrane modules for oilfield produced water desalination with low-pressure nanofiltration process, J. Ind. Eng. Chem. 44 (2016) 118–125. [28] J. Zhang, N. Dow, M. Duke, E. Ostarcevic, J.-D. Li, S. Gray, Identification of material and physical features of membrane distillation membranes for high performance desalination, J. Membr. Sci. 349 (2010) 295–303. [29] M.A. Alaei Shahmirzadi, A. Kargari, 9 - nanocomposite membranes, in: V.G. Gude (Ed.), Emerging Technologies for Sustainable Desalination Handbook, Butterworth-Heinemann, 2018, pp. 285–330. [30] C. Jao-Ming, W. Da-Ming, L. Fung-Ching, L. Juin-Yih, Formation and gas flux of asymmetric PMMA membranes, J. Membr. Sci. 109 (1996) 93–107. [31] T.-H. Young, L.-P. Cheng, D.-J. Lin, L. Fane, W.-Y. Chuang, Mechanisms of PVDF membrane formation by immersion-precipitation in soft (1-octanol) and harsh (water) nonsolvents, Polymer 40 (1999) 5315–5323. [32] L.-Y. Wang, W.F. Yong, L.E. Yu, T.-S. Chung, Design of high efficiency PVDF-PEG hollow fibers for air filtration of ultrafine particles, J. Membr. Sci. 535 (2017) 342–349. [33] M.E. Guendouzi, A. Dinane, A. Mounir, Water activities, osmotic and activity coefficients in aqueous chloride solutions atT¼ 298.15 K by the hygrometric method, J. Chem. Thermodyn. 33 (2001) 1059–1072. [34] S.J.W. Dah Y. Cheng, Composite Membrane for a Membrane Distillation System, in: US Patent, 4,419,242, 1983. [35] F. Edwie, T.-S. Chung, Development of hollow fiber membranes for water and salt recovery from highly concentrated brine via direct contact membrane distillation and crystallization, J. Membr. Sci. 421–422 (2012) 111–123. [36] Z. Yan, H. Yang, H. Yu, F. Qu, H. Liang, B. Van der Bruggen, G. Li, Reverse osmosis brine treatment using direct contact membrane distillation (DCMD): effect of membrane characteristics on desalination performance and the wetting phenomenon, Environ. Sci.: Water Research & Technology 4 (2018) 428–437. 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Fig. 14. Permeate flux and salt rejection of the Janus-HFM DCMD desalination process during a 72 h DCMD process with membrane regeneration.
Acknowledgements The authors would like to acknowledge the financial support from the Bureau of Reclamation (BOR) through the Desalination and Water Purification Research Program. The author also thanks Arkema Inc. for providing the Kynar® HSV900 material. Special thanks are given to Dr. Anthony Kennedy from the BOR for the valuable suggestion and help. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.memsci.2019.117756. References [1] J. Ochi, D. Dexheimer, P.V. Corpel, Produced-Water-Reinjection Design and Uncertainties Assessment, SPE-165138-PA, vol 29, 2014, pp. 192–203. [2] H. Cho, Y. Choi, S. Lee, J. Sohn, J. Koo, Membrane distillation of high salinity wastewater from shale gas extraction: effect of antiscalants, Desalin. Water Treat. 57 (2016) 26718–26729. [3] L. Li, L. Song, K.K. Sirkar, Desalination performances of large hollow fiber-based DCMD devices, Ind. Eng. Chem. Res. 56 (2017) 1594–1603. [4] K.L. Hickenbottom, T.Y. Cath, Sustainable operation of membrane distillation for enhancement of mineral recovery from hypersaline solutions, J. Membr. Sci. 454 (2014) 426–435. [5] J. Minier-Matar, A. Hussain, A. Janson, S. Adham, Treatment of produced water from unconventional resources by membrane distillation, in, International Petroleum Technology Conference. [6] D. Singh, P. Prakash, K.K. Sirkar, Deoiled produced water treatment using directcontact membrane distillation, Ind. Eng. Chem. Res. 52 (2013) 13439–13448. [7] F. Macedonio, A. Ali, T. Poerio, E. El-Sayed, E. Drioli, M. Abdel-Jawad, Direct contact membrane distillation for treatment of oilfield produced water, Separ. Purif. Technol. 126 (2014) 69–81. [8] N.A. Elsayed, M.A. Barrufet, M.M. El-Halwagi, An integrated approach for incorporating thermal membrane distillation in treating water in heavy oil recovery using SAGD, Journal of Unconventional Oil and Gas Resources 12 (2015) 6–14. [9] D. Singh, K.K. Sirkar, Desalination of brine and produced water by direct contact membrane distillation at high temperatures and pressures, J. Membr. Sci. 389 (2012) 380–388. [10] O.R. Lokare, S. Tavakkoli, S. Wadekar, V. Khanna, R.D. Vidic, Fouling in direct contact membrane distillation of produced water from unconventional gas extraction, J. Membr. Sci. 524 (2017) 493–501. [11] R. Ullah, M. Khraisheh, R.J. Esteves, J.T. McLeskey, M. AlGhouti, M. Gad-el-Hak, H. Vahedi Tafreshi, Energy efficiency of direct contact membrane distillation, Desalination 433 (2018) 56–67. [12] H.-C. Yang, W. Zhong, J. Hou, V. Chen, Z.-K. Xu, Janus hollow fiber membrane with a mussel-inspired coating on the lumen surface for direct contact membrane distillation, J. Membr. Sci. 523 (2017) 1–7.
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[40] X. Du, Z. Zhang, K.H. Carlson, J. Lee, T. Tong, Membrane fouling and reusability in membrane distillation of shale oil and gas produced water: effects of membrane surface wettability, J. Membr. Sci. 567 (2018) 199–208. [41] E.A. Bell, T.E. Poynor, K.B. Newhart, J. Regnery, B.D. Coday, T.Y. Cath, Produced water treatment using forward osmosis membranes: evaluation of extended-time performance and fouling, J. Membr. Sci. 525 (2017) 77–88.
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Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: http://www.elsevier.com/locate/memsci
Novel Janus composite hollow fiber membrane-based direct contact membrane distillation (DCMD) process for produced water desalination Lusi Zou a, b, Pri Gusnawan a, b, Guoyin Zhang a, Jianjia Yu a, * a b
Petroleum Recovery Research Center, New Mexico Tech, Socorro, NM, 87801, United States Materials Engineering Department, New Mexico Tech, Socorro, NM, 87801, United States
A R T I C L E I N F O
A B S T R A C T
Keywords: Janus composite hollow fiber membrane Produced water Desalination DCMD
In this study, an innovative Janus composite hollow fiber membrane-based direct contact membrane distillation (DCMD) process was boosted for desalination of actual produced water with elevated total dissolved solids (TDS) of 154,220 mg/L. The Janus composite hollow fiber membrane (Janus-HFM) was characterized with a thin and hydrophobic polyvinylidene difluoride (PVDF) and superhydrophobic silica nanoparticles (Si-R) outer layer, and a thick porous and hydrophilic PVDF and polyethylene glycol (PEG) inner layer. Compared to the neat PVDF hollow fiber membrane, the Janus-HFM showed both increased permeate water flux and improved energy ef ficiency with more than 99.99% salt rejection. The long-term desalination performance of the Janus-HFM was investigated via 200 h of continuous DCMD experiment. The results showed that the permeate water flux declined from 25.41 kg/m2h to 15.21 kg/m2h, and the salt rejection was 98.4% at the end of the operation. It was found that both scales and dissolved organic matters induced foulants accumulated at the feed side of the membrane due to the complex composition of the produced water. Nonetheless, the desalination performance decline can be effectively inhibited from 40% to 9.7% with a simple physical cleaning process by using the permeate water that recovered in the DCMD process.
1. Introduction Produced water (PW) management is one of the most challenging issues in the oil and gas industry. Today, large volumes of PW is reinjected into the deep depleted aquifer for disposal. The PW treatment mainly involves the removal of organic matters and suspended partic ulate solids to minimize pore plugging and formation damage [1]. The PW treatment technologies used in the petroleum industry focus pri marily on physical separation, including the use of API separators, co alesces, or hydrocyclones. However, it is still challenging to transform the effluent from the physical separation to a resource for petroleum stimulation or other beneficial reuse due to the presence of dissolved organic matters and high-salinity in PW. Direct Contact Membrane Distillation (DCMD) has been considered as a promising technology for the treatment of high-salinity impaired water [2]. Compared to the conventional membrane-based pressure- driven separation process such as nanofiltration and reverse osmosis, the driving force of DCMD is the difference of vapor pressure across a porous hydrophobic membrane, which makes the DCMD process well tolerant of membrane fouling [3,4]. For example, Hussain et al. reported a
flat-sheet Polytetrafluoroethylene (PTFE) and polypropylene (PP) based MD for the treatment of PW from unconventional resources [5]. They found that MD is effective to desalinate the brines with the salt con centration up to 70,000 mg/L, and the permeate water flux is not sen sitive to salt concentration. Singh et al. demonstrate a series of DCMD desalination processes with a PP hollow fiber membrane module [6]. It was reported that the DCMD is feasible to treat different types of pro duced water to obtain high-quality permeate water and 80% water re covery. Macedonio et al. reported a laboratory-made PVDF hollow fiber membrane and a commercial PP membrane-based DCMD process to desalinate oilfield produced water [7]. They found that the DCMD process shows excellent rejections towards both total dissolved solids and dissolved carbon that present in the produced water. However, the conventional hydrophobic membrane is still chal lenged by the trade-off between water vapor flux and the significant loss of conductive heat. In DCMD, it has been found that only 17.7–42.1% of the total heat is effectively used as the latent heat for water vapor for mation. The majority of the heat gets lost by internal heat conduction through the polymeric membrane matrix because the heat transfer co efficient of polymers is much higher than that of the air. The DCMD cost
* Corresponding author. E-mail address: [email protected] (J. Yu). https://doi.org/10.1016/j.memsci.2019.117756 Received 26 August 2019; Received in revised form 13 December 2019; Accepted 14 December 2019 Available online 16 December 2019 0376-7388/© 2019 Elsevier B.V. All rights reserved.
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has been reported to vary from $0.3/m3 to $4.47/m3 for PW treatment [8–10]. To improve the energy efficiency of DCMD, different types of low-grade heat sources, including solar thermal energy, geothermal energy, waste heat or natural temperature gradient, have been used to make the DCMD as an economically viable large-scale desalination technology to be competitive with the commercially available reverse osmosis (RO) [11]. Besides, as a promising alternative, the design of innovative Janus composite hollow fiber membranes, to achieve both high mass transfer rate and low heat transfer rate, has attracted exten sive attention in recent years. Janus composite hollow fiber membrane (Janus-HFM) is character ized by distinct physical properties at the two surfaces, such as a hy drophobic surface and a hydrophilic surface [12]. In a typical Janus-HFM based DCMD desalination process, the hydrophobic sur face of the membrane functions as a barrier to prevent the direct contact between the hot feed solution and the cold permeate, but allow the water vapor to transport through the membrane pores. The existence of a thick and porous hydrophilic layer helps reduce the vapor transfer distance between the two sides of the membrane, and create additional conductive heat resistance in the DCMD process. The Janus-HFM has been prepared via different protocols that mainly focus on surface coating modification and co-extruding technique. The former method usually utilizes the neat conventional hydrophilic or hydrophobic membrane as the substrate. The Janus membrane is then obtained by surface modification, such as interfacial polymerization (IP) on the one surface of the membrane substrate [12–15]. For example, Yang et al. prepared a Janus-HFM by coating hydrophilic poly dopamine/polyethyleneimine onto the lumen side of a hydrophobic PP hollow fiber membrane. Ursino et al. prepared the Janus membrane through dip-coating and in-situ polymerization of perfluropolyether (PFPE) on a hydrophilic polyamide/polyethersulfone (PA/PES) com mercial membrane [13,14]. Most recently, Puranik et al. introduced a plasma polymerization method by depositing hydrophobic fluorosilox ane coating on the surface of different hydrophilic commercial PVDF membranes [16]. However, the properties of the above reported Janus membranes are highly determined by the structure of the commercially-available membrane substrate. It is difficult to change the thickness of the two distinct layers of the membrane. Furthermore, the process of dip-coating may also create a dense intermediate layer be tween the membrane substrate and the newly introduced hydrophilic or hydrophobic layer, which can sacrifice the permeate water flux of the membrane in DCMD [17]. The co-extruding of two different dope solutions through a tripleorifice spinneret is another well-known method to fabricate the JanusHFM. Because the properties of both the inner layer and outer layer of the membrane can be tuned with designed dope formulations, it pro vides more flexibility in the fabrication of Janus membranes [18,19]. Currently, most of the Janus structure was accomplished by tuning the hydrophobicity of the membrane materials. For example, Zhu and Feng et al. prepared hydrophobic/hydrophilic hollow fiber membranes by adding poly (vinyl alcohol) (PVA) to the PVDF dope solution. However, the hydrophobicity of the Janus membrane was restricted by the nature of the neat PVDF [20,21]. Bonyadi and Edwie et al. reported their pioneer work on the use of dual-layer hydrophobic/hydrophilic PVDF hollow fiber membranes for flux enhancement in DCMD. However, the fabrication of the Janus-HFM was largely based on the formulation of complex dope solutions, such as PVDF/PAN/cloisite NAþ/EG/NMP [22], and PVDF/PAN/hydrophilic clay particles/NMP [23]. The studies of compatibility among the different components may require extensive time and attention, which increases the cost for membrane fabrication. Therefore, the formulation of the simple and effective dope solution is still required to prepare the Janus composite hollow fiber membrane. Besides, it is worth to note that all the Janus membranes in the available literature aimed to desalinate the water with a relatively low salt concentration (TDS<35,000 mg/L). The desalination of highsalinity water (TDS>150,000 mg/L), such as oilfield produced water,
is never studied by the use of the Janus hollow fiber membrane. Thus, in this study, we fabricated a novel Janus composite hollow fiber mem brane by using a simple and effective dope solution, and investigated the Janus HFM based DCMD process for desalination of actual produced water with high salinity (TDS ¼ 154,220 mg/L). Specially, the JanusHFM was fabricated with a thin polyvinylidene fluoride (PVDF) and superhydrophobic silica nanoparticles (Si-R) outer layer, and a thick highly-porous hydrophilic PVDF and polyethylene glycol (PEG) inner layer. The Janus-HFM based DCMD experiments were performed by using actual produced water as the feed solution, and the desalination performance of the Janus-HFM was investigated in terms of permeate water flux, salt rejection and specific energy consumption. The longterm stability of the Janus-HFM was also evaluated through a physical cleaning process by using the permeate water received from the DCMD process. 2. Experimental 2.1. Materials Poly (vinylidene fluoride) (PVDF, Kynar® HSV900) was provided by the Arkema Inc. in powder form. Superhydrophobic Si-R nanoparticles were obtained from our previous work [24]. Polyethylene glycol (PEG, Mw 600), and N-methyl-2-pyrrolidone (NMP, >98%) were purchased from Sigma-Aldrich for the formulation of dope solution. Actual pro duced water was sampled from a production facility located in Carlsbad, New Mexico. The major composition of the produced water is listed in Table 1. 2.2. Preparation of PVDF/PEG and PVDF/Si-R dope solutions PVDF powder was directly dissolved in NMP solvent under me chanical stirring at ambient temperature. During the mixing process, polyethylene glycol (PEG) was added. The obtained homogeneous PVDF/PEG dope solution was degassed in a vacuum oven for 12 h. Similarly, PVDF/Si-R dope solution was formulated by dissolving Si-R nanoparticles in NMP to obtain a transparent brown NMP/Si-R solu tion. Then, PVDF powder was added to the NMP/Si-R solution under mechanical stirring to form the homogeneous PVDF/Si-R dope solution. The air bubbles in the dope solution were also removed in a vacuum oven before its further use. 2.3. Fabrication of Janus-HFM The Janus-HFM was fabricated through a co-extrusion method. Briefly, PVDF/PEG, PVDF/Si-R dope solutions and bore-fluid (NMP/ water) were simultaneously extruded through a triple-orifice spinneret with three ISCO syringe pumps. The membrane was formed in a water coagulation bath. The obtained Janus-HFM were first soaked in tap water for at least two days to allow the complete exchange between NMP and water, and then was sequentially transferred to methanol and hexane before the final drying process in a freeze dryer [25]. As a Table 1 Major composition of the produced water sampled from the Permian Basin.
2
Composition
Concentration (mg/L)
Sodium, Naþ Ammonium, NHþ 4 Potassium, Kþ 2þ Magnesium, Mg Calcium, Ca2þ Chloride, ClBromide, BrSulfate, SO24 Total Dissolved Solids (TDS) Non-Purgeable Organic Carbon (NPOC)
37,277 318 1,553 2,552 15,415 84,500 735 1,140 154,220 57.6
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Where Jw is the permeate water flux (kg/m2h), W is the mass of water received at the cold permeate side (kg); A is the effective membrane area (m2) and t is operating time (h). ΔH is the latent heat of water evapo ration (2257 kJ/kg), F is the mass flow rate of the feed solution (kg/s), Tf, in and Tf, out are the temperature of the feed solution at the inlet and outlet (K), respectively.
comparison, neat PVDF HFM was also fabricated with the similar method. The spinning parameters for the fabrication of hollow fiber membranes are listed in Table 2. The morphology of the Janus composite hollow fiber membrane was characterized with a high-resolution field emission scanning electron microscope (FESEM, JEOL 2010). The membrane sample was first frozen and fractured in liquid nitrogen for cross-sectional observation of the hollow fiber membrane. To examine the inner suface of the hollow fiber membrane, the membrane sample was first placed on an electrical adhesive tape and then carefully splited with a sharp surgical knife. The cross sections, along with both the inner surface and outer surface, were coated with carbon using a carbon sputter coater prior to FESEM char acterization. The contact angles of the silica particles were measured with a Contact Angle System equipped with SCA20 software.
3. Results and discussion 3.1. Characterization of the PVDF-PEG and PVDF-Si-R Janus-HFM Fig. 2 shows the cross-sectional and surface morphology of the Janus-HFM. It is obvious that the membrane exhibits an asymmetric structure with a thin and hydrophobic PVDF/Si-R outer layer and a thick and high-porous hydrophilic macrovoids-free PVDF/PEG inner layer, with the water contact angle of 137.6� and 56.2� at the outer surface and inner surface, respectively. In Fig. 2 (b), large numbers of pyriform-like macrovoids are formed beneath the PVDF/Si-R outer surface, attributing to the different chemical composition of the two dope solutions, and the similar phe nomenon has been found by previous researchers [20]. Fig. 3 shows the ternary phase diagram of the dope solution with the addition of super hydrophobic Si-R nanoparticles and hydrophilic PEG. It can be seen that both Si-R nanoparticles and PEG shift the binodal curve towards the PVDF/NMP axis, which indicates a reduced solvent power of the dope solution [29,30]. In the process of phase inversion, due to the presence of Si-R nanoparticles, the thermodynamic stability of the PVF/Si-R dope solution decreased, and an instantaneous liquid-liquid demixing occurs to form the macrovoids under the surface of the outer layer. From a dynamic point of view, the superhydrophobic property of the Si-R nanoparticles can effectively impede the water diffusion to the PVDF/Si-R dope, which hinders the exchange between NMP and water. Thus, instead of a liquid-liquid demixing, the semi-crystalline PVDF experiences a solid-liquid demixing, and thus forming a porous nodular structure at the outer layer of the Janus membrane [31], as shown in Fig. 2 (c). The interconnected macrovoids-free PVDF/PEG inner layer is mainly assigned by the increased dope viscosity due to the addition of PEG600 molecules to the PVDF dope solution, and also the existence of 70% NMP in the bore fluid. Similar membrane morphology was reported by Chung et al. [32]. It can be interesting to see that the pore size decreased gradually from an average of 2 μm at the inner surface to the average of 0.15 μm on the outer surface.
2.4. DCMD process for actual produced water desalination Janus composite hollow fiber membrane module was assembled through an epoxy-potting technique [26,27]. Briefly, 10 pieces of the Janus composite hollow fiber membranes were bundled together through a 1/4” stainless steel tubing. Both ends of the membrane bundle were sealed by epoxy. The effective length of the membrane is 15 cm. The apparatus of the Janus composite hollow fiber membrane based DCMD process was demonstrated in Fig. 1. The feed solution was first heated in a water bath, and then pumped into the shell side of the hollow fiber membrane; DI water was cooled and circulated at the lumen side of the membrane. The temperature was recorded with four digital tem perature sensors at the inlets (Tf, 1 and Tp, 1) and outlets (Tf, 2 and Tp, 2) of the membrane module. Two Masterflex peristaltic pumps were used to adjust flow rates for the feed solution and the cold DI water. The masses of the hot feed solution and cold permeate water were weighted through two digital scales equipped with a data acquisition system, to measure real-time fresh water harvest in the cold permeate side. Both electrical conductivity (EC) and total dissolved solids (TDS) in the permeate water were measured with a TDS/conductivity meter (Hach HQ40d), to evaluate salt rejection of the Janus composite hollow fiber membranes. The permeate water flux of the DCMD process was measured by recording the mass of water received at the cold permeate side according to Equation (1). The energy efficiency was evaluated by monitoring the feed temperatures at both the inlet and outlet, as described in Equation (2) [28],: Jw ¼
W A �t
EEð%Þ ¼
Jw ΔHA FCpðTf ; in Tf ; outÞ
(1)
3.2. Validation of the Janus-HFM based DCMD with 35,000 mg/L salt water as feed solution
(2)
Based on a conventional neat PVDF membrane, the desalination performance of the Janus-HFM DCMD process was validated by using 35,000 mg/L NaCl as feed solution, as shown in Fig. 4. It has to point out that the dimensions of the membrane modules were the same for the two hollow fiber membranes, and the operation conditions also remained the same for the DCMD experiment. Thus, the mass change on the permeate side is directly corresponding to the permeate water flux that listed in Table 3. The Janus-HFM produced more permeate water than the neat PVDF HFM, and the corresponding permeate water flux is 26.11 kg m-2 h-1 and 22.1 kg m-2 h-1, respectively. Table 3 also presents salt rejection and energy efficiency of the two membranes. Both membranes exhibited more than 99.99% salt rejec tion, attributing to the super-hydrophobicity of the PVDF membrane. However, the energy efficiency of the Janus-HFM is much higher than the neat PVDF HFM, which is contributed to the presence of a highporous hydrophilic PVDF/PEG inner layer in the Janus-HFM. The long-term stability of the Janus-HFM was evaluated with a continuous 82 h DCMD experiment. The temperature of the hot feed and the cold permeate was 85.1� C and 20� C, and the flow velocity was 0.15 m/s (129.6 ml/min) and 0.5 m/s (21.6 ml/min), respectively. During
Table 2 Parameters for fabrication of hollow fiber membranes. Parameters Outer Layer Dope Composition Inner Layer Dope Composition Bore fluid External coagulant Outer layer flow rate (mL/ min) Inner layer flow rate (mL/min) Bore flow rate (mL/min) Length of the air gap (cm) Take-up speed Membrane OD/ID (μm)
Value J-HFM
neat PVDF HFM
PVDF/Si-R/NMP (12/1/ 87) PVDF/PEG/NMP (12/6/ 82) NMP/water (70/30) Tap water 2
PVDF/NMP (12/ 88) PVDF/NMP (12/ 88)
2 2 5 free fall 805/584
3
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Fig. 1. Experimental setup of the Janus composite hollow fiber membrane based DCMD process.
Fig. 2. SEM image of the PVDF-PEG and PVDF-Si-R Janus-HFM.
the DCMD process, more than 6,066.72 g of clean water was recovered and the feed salinity was concentrated from 35,000 mg/L to 100,000 mg/L at the end of the 82 h operation. The permeate water flux and salt rejection are plotted in Fig. 5. It can be seen that the permeate water flux reduced from 33.50 kg m-2 h-1 to 30.89 kg m-2 h-1 at the end of the 82 h continuous operation, with the permeate water flux decline at around 7.8%. This should be caused by the increased feed salinity in the course of feed circulation. The water partial vapor pressure can be restrained due to the decreased water activity by the ion hydration in the feed solution [33]. During the 82 h DCMD process, the salt rejection remained at higher than 99.90%, indicating a good wetting resistance of the Janus composite hollow fiber membrane when 35,000 mg/L salt water was used as feed solution.
produced water was pretreated with a spin down sediment water filter to remove all the suspended solids with diameters larger than 50 μm. The pre-treated produced water was directly used in the DCMD process. Fig. 6 shows the water recovery during a 200 h continuous DCMD operation. It shows that the water recovery was 552 g at the first two 8-h operations, and then decreased at the time period of 16–24 h. The average water recovery was 311 g per 8 h during the time period of 80–200 h. Fig. 7 shows permeate flux and salt rejection as the function of operating time during the 200 h continuous desalination process. As indicated by the result of water recovery in Fig. 6, the permeate water flux decreased from 25.41 kg/m2h to 19.19 kg/m2h in the first 80 h operation, and then dropped down to 15.21 kg/m2h at the end of the 200 h operation. Besides, the salt rejection efficiency was >99% in the first 104 h operation, and then gradually reduced to 98.4% at the end of the DCMD process. The desalination performance decline of the JanusHFM can be attributed to the presence of high salinity in the feed so lution. As mentioned before, DCMD is a process that involves both water vapor transfer and heat transfer. The water vapor transfer caused an
3.3. Long-term Janus-HFM based DCMD process with actual PW as feed solution The long-term stability of the Janus-HFM DCMD process was eval uated by using actual produced water as feed solution. The original 4
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Fig. 3. Ternary phase diagram for PVDF/NMP/Water system with superhydrophobic Si-R nanoparticles and PEG as additives.
Fig. 5. Desalination performance of the Janus-HFM in 82 h of contin uous DCMD.
The concentrated produced water got supersaturated at the evaporation interface and thus promoted scale formation on the membrane surface. The produced water oversaturation can result in partial pore clogging of the membrane, and thus the permeate water flux declined. The occur rence of scale-induced pore clogging reduced the membrane hydro phobicity and caused the produced water migration into the big pores of the PVDF/Si-R layer, which resulted in the direct contact between permeate water and the produced water, and thus the salt rejection reduced [34,35]. The energy efficiency of the Janus-HFM DCMD desalination process was plotted in Fig. 8, which indicates the effective latent heat that applied to transfer water vapor through the pores of the membrane. The result shows that energy efficiency decreased from 58.16% to 28.68% during the 200 h continuous DCMD operation, and the declining trend was consistent with the permeate water flux in Fig. 7. The temperature profiles of the produced water at both inlet and outlet were also plotted in Fig. 8. It was found the inlet feed temperature has remained at 80 � C,
Fig. 4. Desalination performance of neat PVDF HFM and the Janus composite hollow fiber membrane (Janus-HFM). Table 3 Comparisons of desalination performance between neat PVDF HFM and the JHFM. Permeate flux (kg/m2h) Salt rejection (%) Energy efficiency (%)
neat PVDF HFM
J-HFM
22.1 99.99 55.64
26.11 99.99 72.45
increased salt concentration at the evaporation interface, and in the meantime, the feed solution temperature decreased at the interface due to the latent heat loss that accompanied with the vapor transportation. 5
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efficiency through the heat recovery or process design are beyond the topic of this study. 3.4. Fouling and wetting phenomenon in the 200 h of Janus-HFM DCMD process To identify the salt rejection during the 200 h continuous DCMD desalination process, ion chromatography was employed to measure the ion concentration at the permeate side. Fig. 9 (a) and (b) show the rejection efficiencies of five major cations and anions that present in the produced water. It can be seen that the rejection of all the five ions remained more than 99.99% in the first 24 h of operation; after that, the ion rejection started to decrease, and the decline trend increased when the operating time was more than 40 h. The mechanism of ions transportation in the DCMD process has not been well understood. The presence of large defects on the membrane surface has been reported to explain the salt permeation through the PVDF membrane in DCMD [36]. However, the Janus-HFM showed almost 100% salt rejection at the first 24 h operation, the decreased ion rejection in Fig. 9 should be more likely attributed to the partial mem brane pore wetting, which created water bridges in the pores, and allowed the passage of salts to the permeate side [37,38]. In this case, the salt transportation is governed by the salt concentration gradient across the membrane. Therefore, the ions of sodium and chloride un derwent significant rejection decline due to their significantly higher concentrations in the produced water as listed in Table 2. Compared to the monovalent ions of sodium and chloride, the divalent ions showed higher rejections of 98.89%, 98.91% and 99.54% for calcium, magne sium and sulfate, respectively. Fig. 10 show the outer surface morphology of both fresh membrane and the used membrane after 200 h of continuous DCMD. It is obvious that the feed side of the used membrane was covered by multi layers of particulate depositions. The chemical composition of these depositions was further identified by EDX, and all the peaks were analyzed with Genesis EDAX Genesis software. Compared to the fresh membrane in Fig. 10 (a), several new peaks appeared on the surface of the used membrane, as shown in Fig. 10 (b), mainly attributed to the elements of oxygen, irons, calcium, and magnesium. The signal of oxygen may be originated from hydroxide, sulfate or carbonate deposited on the membrane surface. The peak intensity of calcium was much higher than that of magnesium. By referring to the EDX spectrum of neat calcium carbonate in Ref. [39], calcium carbonate can be identified as one of the main scales on the surface of the used membrane. In contrast to the previous findings from Lokare et al. that the predominant scale from produced water was sodium chloride, the formation of calcium car bonate can be explained by the different composition of produced water in DCMD [10]. The accumulation of calcium carbonate on the mem brane surface can well explain the previous results in Fig. 7 that the desalination performance declined with the operating time in the 200 h of DCMD. As a unique characteristic of the oilfield PW, the presence of high content non-purgeable organic carbon (NPOC) is a big challenge for the membrane. At the end of the 200 h Janus-HFM DCMD process, the NPOC was significantly reduced from 57.6 mg/L to less than 2.0 mg/L. Attenuated total reflection-FTIR (ATR-FTIR) spectroscopy was used to examine the composition of organic deposition on the outer surface of the used membrane. Compare to the fresh membrane, Fig. 11 shows that three new peak bands were found on the outer surface of the used membrane, and the transmittance intensity are proportional to the operating time. The three peak bands, located at 1500-1700 cm-1, 28503000 cm-1, and 3200–3550 cm 1, are mainly originated from amide II I bands, symmetric and asymmetric starching of –CH2 group, and the stretching of O–H group, respectively [40]. These peaks should attribute to aliphatic hydrocarbons, aromatic hydrocarbons, and carbonates, which represent proteins, aliphatic hydrocarbon, and humic substances that are commonly detected in the oilfield produced water [40–42].
Fig. 6. Clean water recovery during 200 h of continuous DCMD by using actual produced water as the feed solution.
Fig. 7. Permeate flux and salt rejection of the Janus-HFM during 200 h of DCMD with actual produced water as feed solution.
Fig. 8. Energy efficiency of the Janus-HFM based DCMD process.
and the temperature at the outlet slightly decreased from 64.6 � C to 61.4� C, this may attribute to an intensified temperature polarization in the DCMD desalination process. It is worth to note that since heat re covery was not considered in DCMD, the further improvement of energy 6
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Fig. 9. Rejection of the major cations (a) and anions (b) during 200 h of continuous DCMD desalination process.
Fig. 10. Outer surface morphology and EDX of (a) fresh membrane and (b) the membrane after 200 h of continuous DCMD desalination operation.
Fig. 12 depicts the inner surface morphology of both the fresh membrane and the used membrane. It is obvious to see that the used membrane exhibits a super clean inner surface after 200 h of continuous operation. Since the amount of the permeated salts is small in the permeate water, the permeate water at the lumen side helps flush out all the salts from the inner layer, and avoid the formation of salt deposits on the inner surface.
3.5. Regeneration behavior of the Janus-HFM for actual PW desalination Membrane regeneration is a conventional approach to address the fouling problem in a typical DCMD desalination process. To study the membrane regeneration effect on the DCMD desalination performance, a 72 h DCMD experiment was conducted with physical cleaning of the membrane at an interval for every 12 h. The physical cleaning was achieved by using the permeate water received from the DCMD process 7
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without membrane regeneration. The water recovery started to decline at the end of 16 h of operation, and it decreased from 553.76 g to 327.91 g when the operating time increased from 16 h to 72 h; the recovery decline was 40% at the end of the DCMD operation. The significant decline of water recovery can be explained by the accumulated salt deposits on the outer surface of the membrane, which intensifies both scale fouling and membrane wetting. When physical cleaning was per formed, it shows in Fig. 13 (b) that the reduction of water recovery was small at the end of the 72 h operation. Fig. 14 plots both permeate water flux and salt rejection of the JanusHFM in the 72 h of DCMD operation along with the physical cleaning. It can be seen that the permeate water flux decreased from 26.6 kg/m2⋅h to 22.6 kg/m2⋅h, and the decline of permeate water flux significantly reduced from 40% to 10%. The promised permeate water recovery can be attributed to the low operating pressure used in the DCMD process, the hydrophobic interaction was weak between the membrane and the organic matters in the feed solution, which can be partially destroyed with the hydraulic force created in the physical cleaning process [24]. In addition, the membrane wetting was considerably inhibited by the prevention of scale accumulation on the membrane surface, and the salt rejection remained more than 99.81% until the end of the 72 h opera tion. Therefore, the permeate water from the DCMD process can be effectively used to maintain the desalination performance of the Janus-HFM, which helps reduce the demand for freshwater in the desalination process.
Fig. 11. ATR-FTIR of (a) fresh membrane and (b) the used membrane.
4. Conclusion
to flush the feed side of the membranes. The flow rate was 0.32 m/s (259.2 ml/min). Compared to the conventional physical cleaning pro cess with the usage of DI water, the use of permeate water is more realistic due to the fact that DI water is usually not available in the field. Based on an average salt rejection efficiency of 99.90% at the end of the first 24 h operation, the TDS of the permeate water is 150.0 mg/L. Fig. 13 (a) shows the water recovery in 72 h of continuous DCMD
A novel Janus composite hollow fiber membrane was fabricated comprising a thin hydrophobic PVDF/Si-R outer layer and a thick porous and hydrophilic PVDF/PEG inner layer for desalination of actual pro duced water with the TDS of 154,200 mg/L. In contrast to neat PVDF membrane, the Janus-HFM exhibited both increased permeate water flux and improved energy efficiency in the DCMD desalination process.
Fig. 12. Inner surface morphology of (a) fresh membrane and (b) the membrane after 200 h continuous DCMD desalination operation. 8
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Fig. 13. Clean water recovery during a 72 h continuous DCMD desalination process (a) without and (b) with membrane regeneration process.
In accordance with a 200 h continuous DCMD experiment, the JanusHFM showed a significant permeate water flux decline from 25.41 kg/ m2h to 15.21 kg/m2h, and the salt rejection was 98.4% at the end of the 200 h operation. It was confirmed by the SEM and FTIR that multi layers of particular depositions accumulated at the feed side of the membrane, including both scales and dissolved organics from the produced water. The membrane fouling-induced permeate water flux decline can be effectively inhibited from 40% to 10% through a simple physical cleaning process by using the permeate water received in the DCMD process.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Lusi Zou: Methodology, Investigation, Data curation, Writing original draft. Pri Gusnawan: Visualization, Investigation. Guoyin Zhang: Resources, Validation. Jianjia Yu: Conceptualization, Writing review & editing, Supervision, Funding acquisition.
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Fig. 14. Permeate flux and salt rejection of the Janus-HFM DCMD desalination process during a 72 h DCMD process with membrane regeneration.
Acknowledgements The authors would like to acknowledge the financial support from the Bureau of Reclamation (BOR) through the Desalination and Water Purification Research Program. The author also thanks Arkema Inc. for providing the Kynar® HSV900 material. Special thanks are given to Dr. Anthony Kennedy from the BOR for the valuable suggestion and help. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.memsci.2019.117756. References [1] J. Ochi, D. Dexheimer, P.V. Corpel, Produced-Water-Reinjection Design and Uncertainties Assessment, SPE-165138-PA, vol 29, 2014, pp. 192–203. [2] H. Cho, Y. Choi, S. Lee, J. Sohn, J. Koo, Membrane distillation of high salinity wastewater from shale gas extraction: effect of antiscalants, Desalin. Water Treat. 57 (2016) 26718–26729. [3] L. Li, L. Song, K.K. Sirkar, Desalination performances of large hollow fiber-based DCMD devices, Ind. Eng. Chem. Res. 56 (2017) 1594–1603. [4] K.L. Hickenbottom, T.Y. Cath, Sustainable operation of membrane distillation for enhancement of mineral recovery from hypersaline solutions, J. Membr. Sci. 454 (2014) 426–435. [5] J. Minier-Matar, A. Hussain, A. Janson, S. Adham, Treatment of produced water from unconventional resources by membrane distillation, in, International Petroleum Technology Conference. [6] D. Singh, P. Prakash, K.K. Sirkar, Deoiled produced water treatment using directcontact membrane distillation, Ind. Eng. Chem. Res. 52 (2013) 13439–13448. [7] F. Macedonio, A. Ali, T. Poerio, E. El-Sayed, E. Drioli, M. Abdel-Jawad, Direct contact membrane distillation for treatment of oilfield produced water, Separ. Purif. Technol. 126 (2014) 69–81. [8] N.A. Elsayed, M.A. Barrufet, M.M. El-Halwagi, An integrated approach for incorporating thermal membrane distillation in treating water in heavy oil recovery using SAGD, Journal of Unconventional Oil and Gas Resources 12 (2015) 6–14. [9] D. Singh, K.K. Sirkar, Desalination of brine and produced water by direct contact membrane distillation at high temperatures and pressures, J. Membr. Sci. 389 (2012) 380–388. [10] O.R. Lokare, S. Tavakkoli, S. Wadekar, V. Khanna, R.D. Vidic, Fouling in direct contact membrane distillation of produced water from unconventional gas extraction, J. Membr. Sci. 524 (2017) 493–501. [11] R. Ullah, M. Khraisheh, R.J. Esteves, J.T. McLeskey, M. AlGhouti, M. Gad-el-Hak, H. Vahedi Tafreshi, Energy efficiency of direct contact membrane distillation, Desalination 433 (2018) 56–67. [12] H.-C. Yang, W. Zhong, J. Hou, V. Chen, Z.-K. Xu, Janus hollow fiber membrane with a mussel-inspired coating on the lumen surface for direct contact membrane distillation, J. Membr. Sci. 523 (2017) 1–7.
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