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An electro-thermal braid-reinforced PVDF hollow fiber membrane for vacuum membrane distillation
Journal of Membrane Science 591 (2019) 117359
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
An electro-thermal braid-reinforced PVDF hollow fiber membrane for vacuum membrane distillation
T
Liang Song, Qinglin Huang*, Yan Huang, Ruifan Bi, Changfa Xiao State Key Laboratory of Separation Membranes and Membrane Processes/ National Center for International Joint Research on Separation Membranes, Department of Material Science and Engineering, Tianjin Polytechnic University, Tianjin, 300387, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Electro-thermal Temperature polarization (TP) Vacuum membrane distillation (VMD) PVDF Specific energy consumption (SEC)
In membrane distillation (MD) process, the unfavorable effect due to latent heat of vaporization is the creation of a temperature difference between the feed bulk and the membrane surface, which is called temperature polarization (TP). This phenomenon causes a decrease of membrane surface temperature, and ultimately, induces a loss in driving force for mass transportation. In this study, a kind of electrical heating wire (nichrome resistance wire, NRW) was introduced into braid-reinforced polyvinylidene fluoride (PVDF) hollow fiber membrane (HFM), aiming to suppress the TP in MD process by localized heating the membrane surface. Results showed that the PVDF/NRW HFM surface temperature increased from room temperature to 70 °C by applying a pretty low direct current (0.15 A). In comparison to the membrane without electricity applied, the electro-thermal membrane (a current of 0.15 A) showed up not only about 2.5 times enhancement in permeate flux, but also the lowest specific energy consumption (SEC), while the salt rejection maintained above 99.0%. Meanwhile, owing to the electro-thermal effect, the value of temperature polarization factor (TPF) was higher than 100%, which demonstrated the effective suppression of temperature polarization.
1. Introduction Membrane distillation (MD) is an emerging nonisothermal separation technology that combines membrane technology with conventional distillation process. In MD, a microporous hydrophobic membrane is in contact with a heated aqueous solution on feed side. The hydrophobic character of membrane prevents the mass transfer in liquid phase and creates a vapor-liquid interface at the entrance of each pore where water evaporates. Vapors diffuse across the membrane, and condense on the opposite side of the system [1–3]. Vacuum membrane distillation (VMD), a branch of MD, utilizes the steam pressure difference between two sides of membrane by vacuuming the cold side to accumulate steam and condense it into liquid [4]. Compared with other separation technologies, MD is able to treat highly concentrated salt solution, which has higher salt rejection, lower energy consumption, and moderate operation conditions [5,6]. However, MD performance is offset by temperature polarization (TP), a phenomenon intrinsically related to water evaporation. There is a temperature difference between the bulk and membrane surface where vapor-liquid transition occurs, and the evaporation lowers the temperature at the interface of bulkmembrane, which further decreases the driving force of the process [7–9]. This phenomenon could weaken MD performance, such as
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decrease of permeate flux and increase of energy consumption. In addition, membrane materials also transfer heat to the permeate side via heat conduction [1,10–14]. There are lots of research works so far have been focused on the hydrophobic or super-hydrophobic modification of PVDF membranes to improve MD performance [15–17]. However, less research works on TP influence in MD process were reported, although some of which were conducted on exploring a photothermal way to suppress the influence of TP. For example, Politano incorporated Ag NPs into microporous polyvinylidene fluoride (PVDF) membranes and made a use of its thermoplasmonic characters to increase membrane surface temperature by UV irradiation, which overcame the TP effectively [18]. Likewise, Wu coated SiO2/Au nanoshells on PVDF membrane and used it in direct contact membrane distillation under simulated sun irradiating, ended up a 33.0% promotion in permeate flux [19]. Also, there were approaches by optimizing membrane process to solve TP issue. Alsaadi designed a novel flash-feed VMD module so as to prevent the liquid feed stream from contacting the membrane surface, and increased the temperature polarization coefficient (TPC) near to 0.9–0.97 [20]. Thomas et al. prepared different feed channel spacers called triply periodic minimal surface by 3D printing, which exhibited 60% higher water flux and 63% higher overall film heat transfer coefficient than the
Corresponding author. No. 399 West Binshui Road, Xiqing District, Tianjin Polytechnic University, 300387, Tianjin, PR China. E-mail address: [email protected] (Q. Huang).
https://doi.org/10.1016/j.memsci.2019.117359 Received 25 May 2019; Received in revised form 19 July 2019; Accepted 7 August 2019 Available online 08 August 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.
Journal of Membrane Science 591 (2019) 117359
L. Song, et al.
Fig. 1. Schematic illustration of the preparation of electro-thermal PVDF/NRW HFMs.
applied current. Then, NRW and FEP yarns were braided into hollow tube by a two-dimensional braiding machine [26] (Fig. 2). The braiding parameters were shown in Table 2.
commercial spacer [21]. However, few studies have introduced Joule heaters in MD process [22]. Nichrome is an oldest documented form of resistance heating alloy, which consists of 80% nickel and 20% chromium. Nichrome has many excellent performances including corrosion resistance, low cost of manufacture, and stability at high temperatures [23–25]. Herein, as Fig. 1 illustrated, poly(tetrafluoroethylene-cohexafluoropropylene) (FEP) yarns and nichrome resistance wire (NRW) were braided into hollow tube via a two-dimensional machine. Then, a PVDF casting solution was homogeneously coated on the outer surface of the braided tube, and the electro-thermal hollow fiber membrane (PVDF/NRW HFMs) were obtained finally. Effects of membrane's electro-thermal performance on temperature polarization factor (TPF) and energy consumption during VMD process were investigated.
2.3. Preparation of electro-thermal PVDF/NRW HFM
2. Experimental
The electro-thermal braid-reinforced PVDF/NRW HFM was fabricated by a dry-wet spinning process which was shown in Fig. 3 [27,28]. The preparation process consisted of 3 steps. First, a homogenous PVDF/SiO2 casting solution was prepared on the basis of our previous study [29]. Second, the PVDF solution was uniformly coated on the outer surface of FEP/NRW braided tube. Third, the prepared HFMs were stored in distillated water for at least 48 h to remove the residual solvents. Finally, the electro-thermal braid-reinforced PVDF/NRW HFMs were obtained. The compositions of PVDF casting solution and spinning parameters were shown in Table 3.
2.1. Materials
2.4. Characterization
PVDF (Solef 6010) was purchased from Solvay Chemical Industry Co., Ltd. N,N-dimethylacetamide (DMAc) was purchased from Tianjin Kermel Chemical Reagents Co., Ltd (Tianjin, China). Nano scale SiO2 particles were purchased from Jibisheng Technology Industry Co., Ltd. (Guangzhou, China). FEP fibers were fabricated in our lab by melt spinning method (FEP: 5100-J, Du Pont), while nichrome resistance wire (NRW, Cr20Ni80) was purchased from Jiangyin Ruiqi Technology Co., Ltd. (Jiangsu China). The parameters of the FEP fiber and NRW were tabulated in Table 1.
Morphology: The membrane morphologies were observed by scanning electron microscopy (SEM TM3030, Hitachi) and confocal laser scanning microscope (CLSM, Zeiss CSM700, Zeiss). Hydrophobicity: Water contact angle (WCA) measurements were performed using an optical contact angle meter (model DSA100, KRUSS) by the sessile drop method of water drops, at room temperature of 25 °C and relative humidity of 30–40%. The droplet was left on the membrane surface for 30s before recording the contact angle data. To minimize the experimental error, the contact angle was measured at five random locations for each sample and the average value was reported. Liquid entry pressure (LEP): LEP value of membrane was measured via a lab-scale device at room temperature as described in literatures [30,31]. The LEP value was confirmed by testing five membrane samples’ and the average value was reported. Porosity and pore size: The gravimetric method was used to assess membrane porosity (ε), which was calculated from the weight of liquid immersed in the membrane pores. n-Butyl alcohol was used as the wetting liquid. Membrane mean pore size was measured by capillary flow porometry (Porolux 1000, POROMETER). Porosity and pore size were confirmed by testing five membrane samples’ and the average value was reported. Electro-thermal performance: Electro-thermal performance of PVDF/ NRW HFMs were measured via infrared thermography (FLIR E40, FLIRSystems, Inc.)
2.2. Preparation of FEP/NRW braided tube First, NRW was homogenously coated with FEP melt to prevent a direct contact with brine solution, in case of the electrolysis when Table 1 Parameters of the FEP fiber and NRW. Characteristic of FEP
Value
Linear density (tex) Maximum use temperature (°C) Breaking strength (cN·dtex−1) Boiling water shrinkage (%) Single fiber diameter (μm) Water contact angle (°) Resistance of solvent
70 tex/24f 200 8.0 ± 0.4 ≤20.5 43.2 ± 2.0 115.0 ± 3.5 Excellent
Characteristic of NRW
Value
Resistance (Ω·m−1) Diameter (mm) Maximum use temperature (°C) Tensile strength (MPa)
138.8 ± 0.5 0.35 ± 0.02 1200 650
2.5. VMD experiments Fig. 4 showed the schematic diagram of experimental VMD set-up with a DC Power Supply (MCH-K305D). An electro-thermal PVDF/NRW HFM module with an effective area of 20.17 cm2 was immersed in the 2
Journal of Membrane Science 591 (2019) 117359
L. Song, et al.
Fig. 2. Schematic illustration of FEP/NRW braided tube preparation by a two-dimensional braiding machine. Table 2 Braiding parameters of braided tube. Characteristic
Braiding pitch (mm)
Braiding speed (r·min−1)
Braiding spindles
Outer diameter of braided tube (mm)
Value
1.0 ± 0.3
500
24
1.86 ± 0.01
feed solution (3.5 wt% NaCl aqueous solution), which was heated up and kept at a stable temperature by a hot water bath with accuracy of ± 1 °C. During VMD process, permeate flux, salt rejection as well as energy consumption were investigated respectively under different feed bulk temperatures (30 °C, 50 °C, 70 °C) and currents (Table 4). Permeate flux: The permeate flux Jw was evaluated by Eq. (1)
Jw =
V T ×A
Table 3 Compositions of PVDF casting solution and spinning parameters.
(1)
Value
Dope composition (PVDF/SiO2/DMAc) (wt%) Casting solution temperature (°C) Air gap (mm) Take up speed (m·min−1) Water coagulation temperature (°C) Spinneret inner diameter (mm)
15/3/82 70 ± 5 75 ± 5 0.72 ± 0.10 20 ± 5 2.24
−1
where Jw is permeate flux (L·m ·h ), V is volume of the liquid collected in the flask (L), T is operating time (h), A is the effective membrane surface area (m2). Salt rejection: The conductivity values of permeate and feed solution were measured by a conductivity meter, which was also used to 2
Characteristic
calculated the salt concentration of permeate by a linear relationship described elsewhere [31,32]. The salt rejection (R) was calculated by Eq. (2):
Fig. 3. Schematic illustration of dry-wet spinning process for preparation of PVDF/NRW HFM. 3
Journal of Membrane Science 591 (2019) 117359
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Fig. 4. A schematic diagram of the experimental VMD set-up.
braided tube. The cross-section morphology of electro-thermal PVDF/ NRW HFM showed a typical finger-like pores structures owing to the double diffusion during the dry-wet spinning process (Fig. 6a–b) [33]. In addition, it can be found that some PVDF casting solution infiltrated into the gaps between fibers in the braided tube owning to capillary effect, which would improve the interfacial bonding strength between the braided tube and PVDF layer (Fig. 6b–c). Other membrane performances in terms of porosity, LEP, WCA and mean pore size were tabulated in Table 5, respectively.
Table 4 The relation of voltage and current. Voltage (V)
0
7.0 ± 0.3
14.0 ± 0.3
21.0 ± 0.3
Current (A)
0
0.05
0.10
0.15
Cp ⎞ R = ⎜⎛1 − ⎟ Cf ⎠ ⎝
(2)
where Cp and Cf is the concentration of the permeate and feed bulk, respectively. For each membrane sample, permeate flux and salt rejection were first measured without electricity and then applying current. Specific energy consumption (SEC): SEC represents the energy consumed in kW·h to produce one L of distilled water. The individual energy consumption including the vacuum pump, water bath and the DC Power supply was monitored by three digital power meters respectively (HUA194E-3T4, Huawei Instrument and Meter Co., Ltd.). In addition, there were two temperature probes (Elitech, RC-4HC), one was placed at the feed bulk center and the other was adhered to the membrane surface.
3.2. Electro-thermal performance The electro-thermal performance of PVDF/NRW HFM was carefully investigated in air condition (Fig. 7). It can be clearly seen that the temperature of PVDF/NRW HFM raised significantly with the increase of applied current. When the current value came to 0.15 A, the temperature reached above 70 °C within 3 min. However, the voltage was only about 21.0 V (see Table 4), suggesting pretty good localized electro-thermal and energy-efficient performances. Furthermore, the PVDF/NRW HFM exhibited a decent electrothermal ability under feed solution as well (Fig. 8). Compared with the results measured in air, the temperature of PVDF/NRW HFM was several-degrees higher than it when the feed solution temperature reached 50 °C, which may due to the heat preservation of feed solution. Specifically, when the feed solution temperature was 70 °C, this phenomenon was more pronounced.
3. Results and discussion 3.1. Membrane characterizations
3.3. Membrane performance in VMD
Fig. 5 showed the digital photographs and outer surface morphology of FEP/NRW braided tube while Fig. 6 showed the morphologies of the electro-thermal PVDF/NRW HFM. It can be seen that the NRW was uniformly braided into FEP braided tube (Fig. 5c), and the PVDF casting solution was homogeneously coated on the outer surface of FEP/NRW
To further evaluate the electro-thermal performance of PVDF/NRW HFM in VMD process, the permeate flux and salt rejection under different currents and feed bulk temperatures were investigated for 4 h
Fig. 5. Digital photographs of FEP (a) and FEP/NRW braided tube (c), and SEM image of NRW in FEP braided tube (b). 4
Journal of Membrane Science 591 (2019) 117359
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Fig. 6. The characteristic morphology and structure of the electro-thermal PVDF/NRW HFM: (a–c) cross-section, (d) NRW in FEP fibers, and (e) outer surface.
increased current improved the energy consumption, while finally kept the VMD process energy efficient.
Table 5 Performances of electro-thermal PVDF/NRW HFM. Characteristic
Value
Outer diameter (mm) Inner diameter (mm) LEP (MPa) Porosity (%) Mean pore size (nm) WCA (°)
2.14 ± 0.05 1.01 ± 0.04 0.35 ± 0.04 36.9 ± 2.8 107.3 ± 3.5 110.8 ± 0.4
3.4. Temperature polarization factor In VMD process, a TPF is commonly used to quantify the extent of TP phenomenon at the membrane surface, which is defined as [18,36]:
TPF (%) =
T‾ fm T‾ fb
× 100 (3)
where T‾ fm is the average value of temperature at the membrane surface, T‾ fb is the average value of feed bulk temperature. For conventional VMD
(Fig. 9). Fig. 9a,b,c indicated that, with the increase of feed bulk temperature, the electrical consumption for NRW (DC) became lower and lower than the one for heating feed solution (water bath) and vacuum pump (W + V) owing to the low current and voltage needed. Fig. 9d indicated clearly that there was positive effect on permeate flux with increase of applied current. Compared to a no-current condition, a nearly 2.5 times increase of permeate flux was recorded when the current value was 0.15 A with a feed bulk of 50 °C or 70 °C. These results suggested that the electro-thermal effect of NRW resulted in the raising temperature which accelerated the water evaporation at the bulk-membrane interface [34,35]. As for salt rejection, there was no obvious effects of electro-thermal on the salt rejection, which all maintained above 99.0%. To evaluate the energy consumption of VMD, specific energy consumption (SEC) was used to represent the energy consumed in kW·h to produce 1 L distilled water. It can be seen from Fig. 9f, when the feed bulk temperature was 30 °C, and the applied current was 0 A or 0.05 A, the SEC values cannot be measured because of the extremely low permeate flux. However, the SEC values were not very low although it decreased a lot with the increase of feed bulk temperature, owing to electrical heating the feed solution (water bath) and vacuum pump were the bulk of the total energy consumption (Fig. 9a, b, c). Thus, we analyzed the SEC with the same feed bulk temperature (for example 70 °C), it can be found that, the highest current of 0.15 A resulted in a lowest SEC value of 11.86 kW·h·L−1. These results indicated that the electro-thermal effect improved the permeate flux, although the
process, the membrane surface temperature is usually lower than the feed bulk (T fm < T fb, TPF < 1), which means that the value of TPF was lower than 100% (Fig. 10a). In this work, NRW was introduced into braid-reinforced membrane aiming to suppress the TP phenomenon by heating the membrane surface. When a current applied, the NRW was able to heat up the membrane surface (Figs. 7 and 8), which would speed up the evaporate rate of water at the bulk-membrane interface during VMD process. Thus, membrane permeate flux increased and the temperature polarization was suppressed effectively, which would bring about a higher membrane surface temperature than feed bulk (T fm > T fb, TPF > 1) (Fig. 10b). The temperature of feed bulk and membrane surface was tested every 10 min by two high-accuracy temperature probes. When there was no current applied in membrane during VMD process, the TPFs were about 99.0%, 98.4% and 98.1% according to the bulk temperature of 30 °C, 50 °C and 70 °C respectively (Fig. 11). However, the thermal energy produced by Joule heater NRW reversed the polarization effects, leading to an higher interface temperature than the feed bulk, and the TPF higher than 100%. Hereto, the TP was successfully suppressed by introducing NRW into membrane. 4. Conclusions In summary, we proposed a novel method of suppressing the temperature polarization by introducing an electro-thermal NRW into Fig. 7. The IR images of electro-thermal PVDF/ NRW HFM after applying current (air condition).
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Journal of Membrane Science 591 (2019) 117359
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Fig. 8. The images of electro-thermal PVDF/NRW HFM in feed solution with different applied current, (a): the digital image, and (b): the IR images.
Fig. 9. Energy consumption of water bath and vacuum pump (W + V), direct current (DC), (a) 30 °C, (b) 50 °C, (c) 70 °C, and (d) Permeate flux, (e) Salt rejection, (f) SEC of whole VMD process (Vacuum pressure: −0.04 MPa, 3.5 wt% NaCl).
Fig. 10. Schematic of (a) conventional VMD and (b) VMD with electro-thermal PVDF/NRW HFM.
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Journal of Membrane Science 591 (2019) 117359
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Fig. 11. The temperature difference between feed bulk and membrane surface, and the corresponding TPF value. Feed bulk temperature (a) 30 ̊C, (b) 50 ̊C, (c) 70 ̊C.
braid-reinforced PVDF HFM. It was demonstrated that electro-thermal effect of NRW significantly raised the surface temperature of PVDF/ NRW HFM. The electro-thermal effect promoted the permeate flux of all the PVDF/NRW HFMs. When the applied current was 0.15 A and the feed bulk temperature was 70 °C, the VMD permeate flux of PVDF/NRW HFM went up to 2.5 times higher than the one without current, while kept a relatively low SEC value. Meanwhile, the value of TPF was higher than 100%, which demonstrated the effective suppression of TP.
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An electro-thermal braid-reinforced PVDF hollow fiber membrane for vacuum membrane distillation
T
Liang Song, Qinglin Huang*, Yan Huang, Ruifan Bi, Changfa Xiao State Key Laboratory of Separation Membranes and Membrane Processes/ National Center for International Joint Research on Separation Membranes, Department of Material Science and Engineering, Tianjin Polytechnic University, Tianjin, 300387, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Electro-thermal Temperature polarization (TP) Vacuum membrane distillation (VMD) PVDF Specific energy consumption (SEC)
In membrane distillation (MD) process, the unfavorable effect due to latent heat of vaporization is the creation of a temperature difference between the feed bulk and the membrane surface, which is called temperature polarization (TP). This phenomenon causes a decrease of membrane surface temperature, and ultimately, induces a loss in driving force for mass transportation. In this study, a kind of electrical heating wire (nichrome resistance wire, NRW) was introduced into braid-reinforced polyvinylidene fluoride (PVDF) hollow fiber membrane (HFM), aiming to suppress the TP in MD process by localized heating the membrane surface. Results showed that the PVDF/NRW HFM surface temperature increased from room temperature to 70 °C by applying a pretty low direct current (0.15 A). In comparison to the membrane without electricity applied, the electro-thermal membrane (a current of 0.15 A) showed up not only about 2.5 times enhancement in permeate flux, but also the lowest specific energy consumption (SEC), while the salt rejection maintained above 99.0%. Meanwhile, owing to the electro-thermal effect, the value of temperature polarization factor (TPF) was higher than 100%, which demonstrated the effective suppression of temperature polarization.
1. Introduction Membrane distillation (MD) is an emerging nonisothermal separation technology that combines membrane technology with conventional distillation process. In MD, a microporous hydrophobic membrane is in contact with a heated aqueous solution on feed side. The hydrophobic character of membrane prevents the mass transfer in liquid phase and creates a vapor-liquid interface at the entrance of each pore where water evaporates. Vapors diffuse across the membrane, and condense on the opposite side of the system [1–3]. Vacuum membrane distillation (VMD), a branch of MD, utilizes the steam pressure difference between two sides of membrane by vacuuming the cold side to accumulate steam and condense it into liquid [4]. Compared with other separation technologies, MD is able to treat highly concentrated salt solution, which has higher salt rejection, lower energy consumption, and moderate operation conditions [5,6]. However, MD performance is offset by temperature polarization (TP), a phenomenon intrinsically related to water evaporation. There is a temperature difference between the bulk and membrane surface where vapor-liquid transition occurs, and the evaporation lowers the temperature at the interface of bulkmembrane, which further decreases the driving force of the process [7–9]. This phenomenon could weaken MD performance, such as
*
decrease of permeate flux and increase of energy consumption. In addition, membrane materials also transfer heat to the permeate side via heat conduction [1,10–14]. There are lots of research works so far have been focused on the hydrophobic or super-hydrophobic modification of PVDF membranes to improve MD performance [15–17]. However, less research works on TP influence in MD process were reported, although some of which were conducted on exploring a photothermal way to suppress the influence of TP. For example, Politano incorporated Ag NPs into microporous polyvinylidene fluoride (PVDF) membranes and made a use of its thermoplasmonic characters to increase membrane surface temperature by UV irradiation, which overcame the TP effectively [18]. Likewise, Wu coated SiO2/Au nanoshells on PVDF membrane and used it in direct contact membrane distillation under simulated sun irradiating, ended up a 33.0% promotion in permeate flux [19]. Also, there were approaches by optimizing membrane process to solve TP issue. Alsaadi designed a novel flash-feed VMD module so as to prevent the liquid feed stream from contacting the membrane surface, and increased the temperature polarization coefficient (TPC) near to 0.9–0.97 [20]. Thomas et al. prepared different feed channel spacers called triply periodic minimal surface by 3D printing, which exhibited 60% higher water flux and 63% higher overall film heat transfer coefficient than the
Corresponding author. No. 399 West Binshui Road, Xiqing District, Tianjin Polytechnic University, 300387, Tianjin, PR China. E-mail address: [email protected] (Q. Huang).
https://doi.org/10.1016/j.memsci.2019.117359 Received 25 May 2019; Received in revised form 19 July 2019; Accepted 7 August 2019 Available online 08 August 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.
Journal of Membrane Science 591 (2019) 117359
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Fig. 1. Schematic illustration of the preparation of electro-thermal PVDF/NRW HFMs.
applied current. Then, NRW and FEP yarns were braided into hollow tube by a two-dimensional braiding machine [26] (Fig. 2). The braiding parameters were shown in Table 2.
commercial spacer [21]. However, few studies have introduced Joule heaters in MD process [22]. Nichrome is an oldest documented form of resistance heating alloy, which consists of 80% nickel and 20% chromium. Nichrome has many excellent performances including corrosion resistance, low cost of manufacture, and stability at high temperatures [23–25]. Herein, as Fig. 1 illustrated, poly(tetrafluoroethylene-cohexafluoropropylene) (FEP) yarns and nichrome resistance wire (NRW) were braided into hollow tube via a two-dimensional machine. Then, a PVDF casting solution was homogeneously coated on the outer surface of the braided tube, and the electro-thermal hollow fiber membrane (PVDF/NRW HFMs) were obtained finally. Effects of membrane's electro-thermal performance on temperature polarization factor (TPF) and energy consumption during VMD process were investigated.
2.3. Preparation of electro-thermal PVDF/NRW HFM
2. Experimental
The electro-thermal braid-reinforced PVDF/NRW HFM was fabricated by a dry-wet spinning process which was shown in Fig. 3 [27,28]. The preparation process consisted of 3 steps. First, a homogenous PVDF/SiO2 casting solution was prepared on the basis of our previous study [29]. Second, the PVDF solution was uniformly coated on the outer surface of FEP/NRW braided tube. Third, the prepared HFMs were stored in distillated water for at least 48 h to remove the residual solvents. Finally, the electro-thermal braid-reinforced PVDF/NRW HFMs were obtained. The compositions of PVDF casting solution and spinning parameters were shown in Table 3.
2.1. Materials
2.4. Characterization
PVDF (Solef 6010) was purchased from Solvay Chemical Industry Co., Ltd. N,N-dimethylacetamide (DMAc) was purchased from Tianjin Kermel Chemical Reagents Co., Ltd (Tianjin, China). Nano scale SiO2 particles were purchased from Jibisheng Technology Industry Co., Ltd. (Guangzhou, China). FEP fibers were fabricated in our lab by melt spinning method (FEP: 5100-J, Du Pont), while nichrome resistance wire (NRW, Cr20Ni80) was purchased from Jiangyin Ruiqi Technology Co., Ltd. (Jiangsu China). The parameters of the FEP fiber and NRW were tabulated in Table 1.
Morphology: The membrane morphologies were observed by scanning electron microscopy (SEM TM3030, Hitachi) and confocal laser scanning microscope (CLSM, Zeiss CSM700, Zeiss). Hydrophobicity: Water contact angle (WCA) measurements were performed using an optical contact angle meter (model DSA100, KRUSS) by the sessile drop method of water drops, at room temperature of 25 °C and relative humidity of 30–40%. The droplet was left on the membrane surface for 30s before recording the contact angle data. To minimize the experimental error, the contact angle was measured at five random locations for each sample and the average value was reported. Liquid entry pressure (LEP): LEP value of membrane was measured via a lab-scale device at room temperature as described in literatures [30,31]. The LEP value was confirmed by testing five membrane samples’ and the average value was reported. Porosity and pore size: The gravimetric method was used to assess membrane porosity (ε), which was calculated from the weight of liquid immersed in the membrane pores. n-Butyl alcohol was used as the wetting liquid. Membrane mean pore size was measured by capillary flow porometry (Porolux 1000, POROMETER). Porosity and pore size were confirmed by testing five membrane samples’ and the average value was reported. Electro-thermal performance: Electro-thermal performance of PVDF/ NRW HFMs were measured via infrared thermography (FLIR E40, FLIRSystems, Inc.)
2.2. Preparation of FEP/NRW braided tube First, NRW was homogenously coated with FEP melt to prevent a direct contact with brine solution, in case of the electrolysis when Table 1 Parameters of the FEP fiber and NRW. Characteristic of FEP
Value
Linear density (tex) Maximum use temperature (°C) Breaking strength (cN·dtex−1) Boiling water shrinkage (%) Single fiber diameter (μm) Water contact angle (°) Resistance of solvent
70 tex/24f 200 8.0 ± 0.4 ≤20.5 43.2 ± 2.0 115.0 ± 3.5 Excellent
Characteristic of NRW
Value
Resistance (Ω·m−1) Diameter (mm) Maximum use temperature (°C) Tensile strength (MPa)
138.8 ± 0.5 0.35 ± 0.02 1200 650
2.5. VMD experiments Fig. 4 showed the schematic diagram of experimental VMD set-up with a DC Power Supply (MCH-K305D). An electro-thermal PVDF/NRW HFM module with an effective area of 20.17 cm2 was immersed in the 2
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Fig. 2. Schematic illustration of FEP/NRW braided tube preparation by a two-dimensional braiding machine. Table 2 Braiding parameters of braided tube. Characteristic
Braiding pitch (mm)
Braiding speed (r·min−1)
Braiding spindles
Outer diameter of braided tube (mm)
Value
1.0 ± 0.3
500
24
1.86 ± 0.01
feed solution (3.5 wt% NaCl aqueous solution), which was heated up and kept at a stable temperature by a hot water bath with accuracy of ± 1 °C. During VMD process, permeate flux, salt rejection as well as energy consumption were investigated respectively under different feed bulk temperatures (30 °C, 50 °C, 70 °C) and currents (Table 4). Permeate flux: The permeate flux Jw was evaluated by Eq. (1)
Jw =
V T ×A
Table 3 Compositions of PVDF casting solution and spinning parameters.
(1)
Value
Dope composition (PVDF/SiO2/DMAc) (wt%) Casting solution temperature (°C) Air gap (mm) Take up speed (m·min−1) Water coagulation temperature (°C) Spinneret inner diameter (mm)
15/3/82 70 ± 5 75 ± 5 0.72 ± 0.10 20 ± 5 2.24
−1
where Jw is permeate flux (L·m ·h ), V is volume of the liquid collected in the flask (L), T is operating time (h), A is the effective membrane surface area (m2). Salt rejection: The conductivity values of permeate and feed solution were measured by a conductivity meter, which was also used to 2
Characteristic
calculated the salt concentration of permeate by a linear relationship described elsewhere [31,32]. The salt rejection (R) was calculated by Eq. (2):
Fig. 3. Schematic illustration of dry-wet spinning process for preparation of PVDF/NRW HFM. 3
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Fig. 4. A schematic diagram of the experimental VMD set-up.
braided tube. The cross-section morphology of electro-thermal PVDF/ NRW HFM showed a typical finger-like pores structures owing to the double diffusion during the dry-wet spinning process (Fig. 6a–b) [33]. In addition, it can be found that some PVDF casting solution infiltrated into the gaps between fibers in the braided tube owning to capillary effect, which would improve the interfacial bonding strength between the braided tube and PVDF layer (Fig. 6b–c). Other membrane performances in terms of porosity, LEP, WCA and mean pore size were tabulated in Table 5, respectively.
Table 4 The relation of voltage and current. Voltage (V)
0
7.0 ± 0.3
14.0 ± 0.3
21.0 ± 0.3
Current (A)
0
0.05
0.10
0.15
Cp ⎞ R = ⎜⎛1 − ⎟ Cf ⎠ ⎝
(2)
where Cp and Cf is the concentration of the permeate and feed bulk, respectively. For each membrane sample, permeate flux and salt rejection were first measured without electricity and then applying current. Specific energy consumption (SEC): SEC represents the energy consumed in kW·h to produce one L of distilled water. The individual energy consumption including the vacuum pump, water bath and the DC Power supply was monitored by three digital power meters respectively (HUA194E-3T4, Huawei Instrument and Meter Co., Ltd.). In addition, there were two temperature probes (Elitech, RC-4HC), one was placed at the feed bulk center and the other was adhered to the membrane surface.
3.2. Electro-thermal performance The electro-thermal performance of PVDF/NRW HFM was carefully investigated in air condition (Fig. 7). It can be clearly seen that the temperature of PVDF/NRW HFM raised significantly with the increase of applied current. When the current value came to 0.15 A, the temperature reached above 70 °C within 3 min. However, the voltage was only about 21.0 V (see Table 4), suggesting pretty good localized electro-thermal and energy-efficient performances. Furthermore, the PVDF/NRW HFM exhibited a decent electrothermal ability under feed solution as well (Fig. 8). Compared with the results measured in air, the temperature of PVDF/NRW HFM was several-degrees higher than it when the feed solution temperature reached 50 °C, which may due to the heat preservation of feed solution. Specifically, when the feed solution temperature was 70 °C, this phenomenon was more pronounced.
3. Results and discussion 3.1. Membrane characterizations
3.3. Membrane performance in VMD
Fig. 5 showed the digital photographs and outer surface morphology of FEP/NRW braided tube while Fig. 6 showed the morphologies of the electro-thermal PVDF/NRW HFM. It can be seen that the NRW was uniformly braided into FEP braided tube (Fig. 5c), and the PVDF casting solution was homogeneously coated on the outer surface of FEP/NRW
To further evaluate the electro-thermal performance of PVDF/NRW HFM in VMD process, the permeate flux and salt rejection under different currents and feed bulk temperatures were investigated for 4 h
Fig. 5. Digital photographs of FEP (a) and FEP/NRW braided tube (c), and SEM image of NRW in FEP braided tube (b). 4
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Fig. 6. The characteristic morphology and structure of the electro-thermal PVDF/NRW HFM: (a–c) cross-section, (d) NRW in FEP fibers, and (e) outer surface.
increased current improved the energy consumption, while finally kept the VMD process energy efficient.
Table 5 Performances of electro-thermal PVDF/NRW HFM. Characteristic
Value
Outer diameter (mm) Inner diameter (mm) LEP (MPa) Porosity (%) Mean pore size (nm) WCA (°)
2.14 ± 0.05 1.01 ± 0.04 0.35 ± 0.04 36.9 ± 2.8 107.3 ± 3.5 110.8 ± 0.4
3.4. Temperature polarization factor In VMD process, a TPF is commonly used to quantify the extent of TP phenomenon at the membrane surface, which is defined as [18,36]:
TPF (%) =
T‾ fm T‾ fb
× 100 (3)
where T‾ fm is the average value of temperature at the membrane surface, T‾ fb is the average value of feed bulk temperature. For conventional VMD
(Fig. 9). Fig. 9a,b,c indicated that, with the increase of feed bulk temperature, the electrical consumption for NRW (DC) became lower and lower than the one for heating feed solution (water bath) and vacuum pump (W + V) owing to the low current and voltage needed. Fig. 9d indicated clearly that there was positive effect on permeate flux with increase of applied current. Compared to a no-current condition, a nearly 2.5 times increase of permeate flux was recorded when the current value was 0.15 A with a feed bulk of 50 °C or 70 °C. These results suggested that the electro-thermal effect of NRW resulted in the raising temperature which accelerated the water evaporation at the bulk-membrane interface [34,35]. As for salt rejection, there was no obvious effects of electro-thermal on the salt rejection, which all maintained above 99.0%. To evaluate the energy consumption of VMD, specific energy consumption (SEC) was used to represent the energy consumed in kW·h to produce 1 L distilled water. It can be seen from Fig. 9f, when the feed bulk temperature was 30 °C, and the applied current was 0 A or 0.05 A, the SEC values cannot be measured because of the extremely low permeate flux. However, the SEC values were not very low although it decreased a lot with the increase of feed bulk temperature, owing to electrical heating the feed solution (water bath) and vacuum pump were the bulk of the total energy consumption (Fig. 9a, b, c). Thus, we analyzed the SEC with the same feed bulk temperature (for example 70 °C), it can be found that, the highest current of 0.15 A resulted in a lowest SEC value of 11.86 kW·h·L−1. These results indicated that the electro-thermal effect improved the permeate flux, although the
process, the membrane surface temperature is usually lower than the feed bulk (T fm < T fb, TPF < 1), which means that the value of TPF was lower than 100% (Fig. 10a). In this work, NRW was introduced into braid-reinforced membrane aiming to suppress the TP phenomenon by heating the membrane surface. When a current applied, the NRW was able to heat up the membrane surface (Figs. 7 and 8), which would speed up the evaporate rate of water at the bulk-membrane interface during VMD process. Thus, membrane permeate flux increased and the temperature polarization was suppressed effectively, which would bring about a higher membrane surface temperature than feed bulk (T fm > T fb, TPF > 1) (Fig. 10b). The temperature of feed bulk and membrane surface was tested every 10 min by two high-accuracy temperature probes. When there was no current applied in membrane during VMD process, the TPFs were about 99.0%, 98.4% and 98.1% according to the bulk temperature of 30 °C, 50 °C and 70 °C respectively (Fig. 11). However, the thermal energy produced by Joule heater NRW reversed the polarization effects, leading to an higher interface temperature than the feed bulk, and the TPF higher than 100%. Hereto, the TP was successfully suppressed by introducing NRW into membrane. 4. Conclusions In summary, we proposed a novel method of suppressing the temperature polarization by introducing an electro-thermal NRW into Fig. 7. The IR images of electro-thermal PVDF/ NRW HFM after applying current (air condition).
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Fig. 8. The images of electro-thermal PVDF/NRW HFM in feed solution with different applied current, (a): the digital image, and (b): the IR images.
Fig. 9. Energy consumption of water bath and vacuum pump (W + V), direct current (DC), (a) 30 °C, (b) 50 °C, (c) 70 °C, and (d) Permeate flux, (e) Salt rejection, (f) SEC of whole VMD process (Vacuum pressure: −0.04 MPa, 3.5 wt% NaCl).
Fig. 10. Schematic of (a) conventional VMD and (b) VMD with electro-thermal PVDF/NRW HFM.
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Fig. 11. The temperature difference between feed bulk and membrane surface, and the corresponding TPF value. Feed bulk temperature (a) 30 ̊C, (b) 50 ̊C, (c) 70 ̊C.
braid-reinforced PVDF HFM. It was demonstrated that electro-thermal effect of NRW significantly raised the surface temperature of PVDF/ NRW HFM. The electro-thermal effect promoted the permeate flux of all the PVDF/NRW HFMs. When the applied current was 0.15 A and the feed bulk temperature was 70 °C, the VMD permeate flux of PVDF/NRW HFM went up to 2.5 times higher than the one without current, while kept a relatively low SEC value. Meanwhile, the value of TPF was higher than 100%, which demonstrated the effective suppression of TP.
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