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polyethersulfone blend membrane for water desalination using vacuum membrane distillation
Desalination 346 (2014) 30–36
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Performance of a newly developed titanium oxide nanotubes/polyethersulfone blend membrane for water desalination using vacuum membrane distillation H. Abdallah a, A.F. Moustafa b, Adnan AlHathal AlAnezi c,⁎, H.E.M. El-Sayed d,1 a
Chemical Engineering and Pilot Plant Department, Engineering Research Division, National Research Center, El Buhouth St, Dokki, Giza, Egypt Environmental Screening — Environmental Management Unit, Beni Suef Governorates, Egypt c Department of Chemical Engineering Technology, College of Technological Studies, The Public Authority for Applied Education and Training (PAAET), P.O. Box 42325, Shuwaikh 70654, Kuwait d Mechanical Engineering Department, Engineering Research Division, National Research Center, El Buhouth St, Dokki, Giza, Egypt b
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• Fabrications of (TNTs-PES) blend membrane by immersion precipitation. • Performance of (TNTs-PES) membrane was investigated for desalination using VMD. • The developed (TNTs-PES) membrane was superior in terms of salt rejection. • The permeate flux was significantly affected by the time of the VMD experiment. • At optimum conditions the permeate flux reached 15.2 kg/m2h, where the salt rejection was 98%.
a r t i c l e
i n f o
Article history: Received 9 February 2014 Received in revised form 30 April 2014 Accepted 2 May 2014 Available online 24 May 2014 Keywords: Titanium oxide nanotubes Polyethersulfone Vacuum membrane distillation Desalination
a b s t r a c t The present paper introduces a comprehensive study of the performance of newly developed titanium oxide nanotubes (TNTs) incorporated into a Polyethersulfone (PES) blend membrane for desalination using vacuum membrane distillation (VMD) process. The study examines the effect of different operating conditions. The results showed a maximum salt rejection of 98% and a permeate flux of 15.2 kg/m2 h at 7000 ppm feed salt concentration for the TNTs–PES membrane at a temperature of 65 °C and a vacuum pressure of 300 mbar with feed flow rate of 11 mL/s. A comparison between the performance of the developed TNTs-PES membrane, and commercial Polytetrafluoroethylene (PTFE) membrane was performed at different feed salt concentrations. The achieved results showed a significant improvement in the performance of the new membrane compared to the commercial PTFE membrane, where the salt rejection reached 99.3% at feed concentration 3000 ppm and 96.7% at 35,000 ppm using the new membrane, compared to salt rejection of up to 90.6% at 3000 ppm and 62.5% at 35,000 ppm using PTFE membrane. The dense TNTs layer formed on the top surface of the TNTs-PES blend membrane is considered a selective layer that prevents salt passage through the membrane. The decline in permeate flux may be overcome by membrane washing every hour. © 2014 Elsevier B.V. All rights reserved.
⁎ Corresponding author. Tel.: +965 55509994; fax: +965 22314430. E-mail addresses: [email protected] (H. Abdallah), [email protected] (A.A. AlAnezi). 1 Currently a Visiting Scholar at Civil and Environmental Engineering Department, University of Windsor, 401 Sunset Avenue, Windsor, ON N9B 3P4, Canada.
http://dx.doi.org/10.1016/j.desal.2014.05.003 0011-9164/© 2014 Elsevier B.V. All rights reserved.
1. Introduction Membrane technology is considered a very effective separation method, particularly in the area of water/wastewater treatment and
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water desalination. The application of nanoparticles or nanotubes to polymeric membranes has attracted the attention of many researchers recently. Moreover, the family of inorganic nanotubes has been expanded extensively from carbon nanotubes to sulfides [1], nitrides [2] and oxides [3]. One-dimensional nanostructured titanium oxide is of great interest for possible applications in high effect solar cells [4], photocatalysts [5,6], gas sensors [7], molecular straws [8] and semiconductor devices due to its nanotubular structure, high specific surface area, ion-changeable ability, and size-dependent properties. Currently, methods developed for fabricating titanium oxides-based nanotubes, include the assisted-template method [9,10], the sol–gel process [11], electrochemical anodic oxidation [12,13], and hydrothermal treatment [14,15]. The latter process is of great interest due to its ability to yield very low-dimensional, well-separated crystallized nanotubes, and a pure-phase structure [16]. During membrane fabrication, nanoparticles and nanotubes have recently shown significant potential in improving polymeric membrane performance. The remarkable effect of the addition of nanoparticles/ nanotubes during membrane preparation may be attributed to the interactions between nanoparticle surfaces and polymer chains and/or solvents, which lead to the production of desirable structured membranes. These modifications of membrane structure result in favorable selectivity, permeability, and satisfactory performance in ultrafiltration and nano-filtration membranes [17]. Moreover, the nanoparticle functional groups and their hydrophilic properties were found to be able to control membrane fouling phenomena, which is a major drawback of membrane separation technology [18–20]. There are two major ways of incorporating nanoparticles into polymeric membranes; (i) assembling engineered nanoparticles on the surface of the porous membranes, where these are deposited/coated on top of the membrane surface [21–33], or (ii) blending them with polymeric casting solution, wherein the nanoparticles are dispersed uniformly in the membrane solution by one of the well-known methods; melt, solvent, sol–gel mixing and in-situ grafting [34–40]. Polyethersulfone (PES) is a widely used polymer in membrane preparation, and is now rapidly becoming the material of choice for membrane applications. Its advantages include being a high performance engineering thermoplastic, given its high glass transition temperature, good mechanical properties, and excellent thermal and chemical stability. However, the hydrophobicity of PES limits its application, especially in the field of water desalination [41,42]. Thus, the introduction of nanoparticles to PES membranes is very useful in overcoming many of the disadvantages. Among the different operational membrane techniques, namely micro-, ultra-, or nano-filtration, and reverse osmosis, dialysis, etc., membrane distillation (MD) is considered one of the most popular techniques utilized for seawater desalination applications. The idea of MD is based on separating two components present in phase equilibrium; vapor/liquid or liquid/liquid. The main driving force for such separation is the vapor pressure gradient resulting from a temperature difference between hot feed (salt water) and cold permeate (pure water). MD provides many advantages over other distillation separation methods. These advantages include; the (i) capability to produce ultra-pure water, regardless of the salt concentration in the feed stream, (ii) lower operating cost, (iii) the unique feature of the possibility of achieving complete rejection of non-volatile substances, and (iv) commercially long life with saline solutions [43–45]. Among the four wellknown modes of operation of MD, direct contact membrane distillation (DCMD), air gap membrane distillation (AGMD), sweep gas membrane distillation (SGMD), and vacuum membrane distillation (VMD) [46–49], the latter configuration is utilized in the current study. In this study, titanium nanotubes synthesized using the hydrothermal method were blended into PES during polymeric membrane preparation in order to enhance its performance. VMD operational techniques were utilized in desalination experiments. The performance of the developed TNTs-PES blend membrane was tested in terms of permeate flux
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and percent salt rejection at different operating conditions. In addition, the performance of the developed TNTs-PES blend membrane was compared with the well-known commercial Polytetrafluoroethylene (PTFE) flat-sheet membrane at different feed salt concentrations. 2. Experimental work 2.1. Materials Polyethersulfone (PES), Ultrason E 6020P, molecular weight of 58,000 g/mol and glass transition temperature of Tg = 225 °C and N-Methylpyrrolidone as a solvent were purchased from BASF chemical company. TiO2 nano powder, NaOH, HCl, acetonitrile, and tetramethylsiloxane were purchased from Sigma Aldrich Company. Commercial Polytetrafluoroethylene (PTFE) flat-sheet membranes were supplied by Millipore Corporation. The specifications of the supplied membrane are as follows; diameter of 4.7 cm, thickness of 120 μm, porosity of 75%, and pore diameter of 0.2 μm. Synthetic salt solutions were prepared with different concentrations using distilled water and commercial NaCl, where the commercial NaCl used as a food salt, and contains 98.5% sodium chloride and 70–30 ppm potassium iodine. 2.2. Synthesis of titanium oxide nanotubes The approach of self-organized synthesis using the hydrothermal method is applied in the present study for the preparation of the titanium oxide nanotubes. TiO 2 nanopowder was dispersed in 150 mL of 10 M NaOH and stirred for 15 min. The dispersion was then transferred to a Teflon lined autoclave (KH-300) and heated at 150 °C for 16 h. The white precipitate obtained was washed with 1 M HCl first, then with distilled water, and finally dried at 80 °C for 4 h. A detailed description of the preparation method is described elsewhere [50,51]. 2.3. Titanium oxide nanotubes membrane fabrication For the preparation of the titanium oxide nanotube solution, 5 wt.% of tetramethylsiloxane was added first to the acetonitrile solvent, then 5 wt.% of the titanium oxide nanotube powder was added under constant stirring for 3 h. For the preparation of the polymer casting solution, 18 wt.% of PES was dissolved first in the N-Methylpyrrolidone (NMP) solvent and then added to 9% of the previously prepared nanotube solution under constant stirring for 6 h at room temperature. The prepared polymer solution was cast on a smooth flat glass plate by a casting knife with uniform speed, where the membrane thickness was sustained at 0.2 ± 0.01 mm. Subsequently, the polymer solution on the glass plates was solidified by immediate immersion in a coagulation bath containing distilled water. The membrane was spontaneously released from the plate and kept for 24 h to ensure complete removal of solvent from the membrane. Finally, the membranes were dried by placing them between two sheets of filter paper for 24 h at room temperature. 2.4. Desalination experiments A flat sheet membrane module supplied by Millipore was utilized in the desalination experiments. The effective area of the membrane in the module was determined to be 17.34 cm2. A one liter jacketed mixer vessel was filled with different concentrations of previously prepared feed salt solutions. The salt solution in the mixer feed vessel was heated using a circulation water bath to the jacket, which was controlled by a thermostat to be kept in the range of (25–65 °C). The feed temperature inside the mixer feed vessel was continuously monitored using a thermometer fitted to the feed vessel. The saline solution was continuously fed to the membrane module from the jacketed feed mixer by a peristaltic pump. The permeate water vapor was drawn from the membrane module to a chiller condenser with the aid of a vacuum pump connected
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to the condenser to apply vacuum pressure. The difference in temperature and pressure between the feed side and the outside of the membrane (main driving forces) makes water vaporize and pass through the membrane pores. The permeate water produced is collected from the bottom of the condenser. A concentrated solution recycle stream from the membrane module is returned to the feed vessel on a continuous basis. The condensed permeate is weighed by a double precision balance at predetermined time periods. A simple flow diagram of the vacuum membrane distillation system is given in Fig. 1. 3. Results and discussion 3.1. Characterization of the synthesized titanium oxide nanotubes The prepared nanotubes were characterized using X-ray diffraction (XRD) analysis. The XRD spectrum (Brukurd 8 advance, CuK, target with secondary mono chromator λυ = 40, mA = 40, Germany) was applied to identify the phase formation and crystal size. The morphology and geometry of the prepared TiO2 nanotubes were deduced using transmission electron microscopy (TEM) analysis (JEOL-2010, Japan). Images from both XRD and TEM are shown in Figs. 2 through 4. The XRD pattern was used for the phase identification of raw TiO2 powder, which was used for preparation of the nanotubes. It was found that the raw material is of a pure rutile phase of TiO2 nanopowder with a crystal size of about 93 nm and is highly crystalline, as shown in Fig. 2. The phase of prepared titanium oxide nanotubes was obtained from XRD patterns as shown in Fig. 3. This illustrates that the prepared nanotubes are a mixture of different sodium titanium oxides and titanium oxide nanotubes. Fig. 4 presents the TEM image of the obtained nanotubes. It can be deduced from the figure that there is a large quantity of prepared nanotubes with complete rolling and uniform inner and outer diameters along the length of the nanotubes. Moreover, the obtained titanium nanotubes appeared as a layered structure with good crystallinity. In addition, the nanotubes have a narrow size distribution with diameter of around 1.67 nm for inner diameter, 8.35 nm for outer diameter and length of about 141 nm. 3.2. Nanotubes membrane characterization The morphology of the membrane was characterized by a scanning electron microscope (SEM) (JEOL 5410, Japan) operating at 20 kV. Cross sectional samples were prepared by fracturing the membrane under liquid nitrogen. The dried samples were coated by gold sputtering to provide electrical conductivity. SEM images of the cross section of the asymmetric membranes are presented in Fig. 5(a, b). Fig. 5a indicates
Fig. 2. XRD patterns of commercial rutile phase TiO2 nano-powder.
the SEM view of a pure PES cross section, where the spongy structure revealed the porous nature of the membrane. Fig. 5b shows the SEM view of the TNTs-PES blend membrane. It is clear from the figure that the top surface appears as a dense layer (was not existing in the first figure) given the presence of the titanium nanotubes. This plays an important role in salt rejection, as will be explained later. It may also be noted that another highly porous layer appears at the bottom (glass side) of the section [51]. 3.3. Effect of operating parameters 3.3.1. Effect of feed temperature The effect of different feed temperatures (25, 45, 55 and 65 °C) on VMD permeate flux and salt rejection of TNTs/PES membrane was studied under the following operating conditions: 7000 ppm feed concentration, 300 mbar vacuum pressure and 11 mL/s feed flow rate. It is obvious from Figs. 6 and 7 that the feed temperature had a remarkable effect on permeate flux and salt rejection, respectively. As shown, the permeate flux and salt rejection increased linearly with feed temperature. This behavior is mainly due to the exponential dependence of water vapor pressure on temperature, according to the Antoine equation. Therefore, increasing the feed temperature increases the temperature difference, which consequently enhances the vapor pressure difference (i.e. the driving force for mass transfer) and increases the permeate dilution as a result of the higher recovery rate. 3.3.2. Effect of feed flow rate The feed flow rate is one of the most important parameters affecting the VMD performance. Therefore, in order to study the effect of feed flow
Fig. 1. Flow diagram of VMD process.
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Fig. 3. XRD patterns of prepared titanium nanotubes, (1) Na2Ti3O7, (2) Na2Ti6O16, (3) Na2Ti9O19, (4) Ti3O5.
rate on the performance of the developed TNTs-PES blend membrane, several experiments were performed under the following operating conditions: 7000 ppm feed concentration, 300 mbar vacuum pressure, and 65 °C feed temperature. As shown in Figs. 8 and 9, increasing the feed flow rate from 8 to 28 mL/s, increased the permeate flux and salt rejection linearly, until they reached a maximum value of 15.2 kg/m2 h and 98%, respectively, at 11 mL/s. This increase is attributed to the rise in Reynolds number, which causes enhanced mixing of the flow in the channels due to turbulence. The turbulent flow reduces the thickness of both the temperature and concentration boundary layers, which means lower boundary layer resistance. Moreover, it is clear that, the permeate flux and salt rejection are significantly reduced to a minimum value of 10.4 kg/m2 h and 97.2%, respectively at a maximum feed flow rate of 28 mL/s. This reduction can be referred to the vapor pressure reduction. It is clear that the permeate flux increases to an asymptotic value with increasing the feed flow rate until it approaches a certain limit, where the effect of the feed flow rate leads to a decrease in permeate flux due to a decrease in membrane surface temperature [46, 47]. Moreover, lower boundary layer resistance can permit the salt to pass though the membrane, which in turn decreases the salt rejection. Therefore, it is worth noting that increasing the feed flow rate further has a negative influence on permeate flux and salt rejection; hence, the effective way is to optimize the feed flow rate to achieve high values of permeate flux and salt rejection.
Fig. 4. TEM image of prepared titanium nanotubes.
Fig. 5. SEM cross section of (a) pure PES membrane, (b) TNTs-PES blend membrane.
3.3.3. Effect of vacuum pressure The vacuum pressure at the permeate side plays a major role in the VMD performance, because the boundary layer resistance and the heat conduction across the membrane is negligible due to the very low pressure on the permeate side. In Figs. 10 and 11, the effect of the vacuum pressure on the permeate flux and salt rejection, at 7000 ppm feed concentration, 11 mL/s feed flow rate, and 65 °C feed temperature, is presented. It is worth mentioning that both permeate flux and salt rejection increase with decreasing vacuum
Fig. 6. Effect of feed temperature on permeate flux at 7000 ppm salt solution 300 mbar vacuum pressure and 11 ml/s feed flow rate.
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Fig. 7. Effect of feed temperature on salt rejection at 7000 ppm feed concentration, 300 mbar vacuum pressure and 11 ml/s feed flow rate.
pressure at the permeate side. This is attributed to the significant increase in the vapor pressure difference (i.e. the driving force) between the feed and permeate side. 3.3.4. Effect of feed concentration The effect of feed concentration on the permeate flux and the percentage rejection were investigated in VMD. The experiments were performed for different feed concentrations (3000, 5000, 7000, 10,000 and 35,000 ppm) at 11 mL/s feed flow rate, 300 mbar vacuum pressure and 65 °C feed temperature, as shown in Figs. 12 and 13, respectively. The results show that increasing the feed salt concentration decreases the permeate flux and salt rejection due to the reduction in water vapor pressure difference (i.e. driving force). Additionally, the reduction can be attributed to the effects of temperature and concentration polarization, which increased the thickness of the boundary layer, and consequently reduced the evaporation driving force. 3.4. Effect of experiment time on the permeates flux and feed salt rejection In order to study the possible effects of experiment time on the performance of the TNTs/PES blend membrane during VMD, permeate flux as well as salt rejection for different time intervals up to 120 min were measured. The experiments were performed at 7000 ppm feed salt concentration, 65 °C feed temperature and 300 mbar vacuum pressure. Fig. 14 shows a bar chart of the results, which indicates a decline in the permeate flux with time. As time goes on, a scale layer of salts is formed on the membrane surface, which is known as fouling. This layer acts as a barrier and increases the resistance to flow due to blocking of the membrane pores. The maximum flux reached was about 15.2 kg/m2 h after 40 min of operation. After that, the permeate flux declined. It was noted that the salt rejection (SR) increased to 98% after 40 min and then became constant and was not further affected by time. It should be pointed out here that fouling during the VMD
Fig. 8. Effect of feed flow rate on permeate flux at 7000 ppm feed concentration, 300 mbar vacuum pressure, and 65 °C feed temperature.
Fig. 9. Effect of feed flow rate on salt rejection at 7000 ppm feed concentration, 300 mbar vacuum pressure, and 65 °C feed temperature.
process may be overcome by membrane washing, which is recommended here in every hour. 3.5. Comparison between commercial PTFE membrane and developed TNTs-PES blend membrane A series of saline water VMD experiments were performed using hydrophobic Polytetrafluoroethylene (PTFE) flat sheet membrane and the prepared TNTs-PES membrane. In order to investigate the performance of the VMD process, the permeate water flux was measured for both membranes at different feed concentration of (3000, 5000, 7000, 10,000 and 35,000 ppm) at operating conditions of 300 mbar vacuum pressure, 65 °C feed temperature and 11 mL/s feed flow rate for the membrane area of 17.34 cm2. The comparison between the two membranes was evaluated in terms of permeate flux and percentage salt rejection, as indicated in Table 1. The results indicate that the permeate flux of the TNTs-PES blend membrane was higher than that of the PTFE membrane at low feed salt concentration (lower than 10,000 mg/L). Whereas, the PTFE membrane permeate flux was higher than that of the TNTs-PES blend membrane at high feed salt concentration (≥10,000 ppm). The authors' explanation of these results is that the developed TNTs-PES blend membrane may be considered a hydrophobic/hydrophilic membrane. This means it can absorb water in both liquid and vapor phases faster than the hydrophobic PTFE membrane, which can only absorb water in the vapor state from the feed side. As the feed solution becomes more concentrated, more water will be diffusing from the bulk feed solution to the membrane surface due to the formation of the titanium oxide nanotubes dense layer on the top surface of the membrane. Thus, the salt polarization effect tends to limit the driving force for water permeation, which decreases the permeate flux (this is not the case for the PTFE membrane). On the other hand, the TNTs-PES blend membrane provides higher percentage salt
Fig. 10. Effect of vacuum pressure on permeate flux at 7000 ppm feed concentration, 11 ml/s feed flow rate, and 65 °C feed temperature.
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Fig. 11. Effect of vacuum pressure on % salt rejection at 7000 ppm feed concentration, 11 ml/s feed flow rate, and 65 °C feed temperature.
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Fig. 14. Performance of the developed TNTs-PES blend membrane with time at 7000 ppm feed salt concentration, 65 °C temperature, and 300 mbar vacuum pressure.
Table 1 Comparison between PTFE membrane and prepared TNTs-PES membrane.
Fig. 12. Effect of feed salt concentration on permeate flux at 300 mbar vacuum pressure, 11 mL/s feed flow rate and 65 °C feed temperature.
rejection over the whole range of feed concentrations. Again, the dense TNT layer formed on the top surface of the TNTs-PES blend membrane is considered a selective layer that prevents salt passage through the membrane.
4. Conclusions Introducing titanium oxide nanotubes during polymeric membrane fabrication has resulted in significant improvements in the membrane performance in terms of permeate flux and salt rejection. Vacuum membrane distillation (VMD) for water desalination experiments were carried out in the present study at different operating conditions.
Feed concentration ppm
PTFE
3000 5000 7000 10,000 35,000
21.5 18.4 12.7 10.2 7.20
TNTs-PES
Permeate flux (kg/m2 h)
PTFE
TNTs-PES
Salt rejection (%) 24.9 21.8 15.2 8.00 5.50
90.6 88.2 79.4 68.6 62.5
99.3 98.6 98.0 97.8 96.7
The results showed a maximum salt rejection of 98% and a permeate flux of 15.2 kg/m2 h at the optimum conditions of 65 °C feed temperature, 7000 ppm feed salt concentration, 300 mbar vacuum pressure, and 11 mL/s feed flow rate. The permeate flux was found to be significantly affected by the time of the VMD experiment. The formation of salt scales that blocked the membrane pores hindered the flow of fluids across the membrane. After 40 min of operation, the permeate flux was declining after it reached a value of 15.2 kg/m2 h. On the other hand, the salt rejection reached 98% at 40 min of operation and remained constant after that until the end of the experiment (120 min). The developed TNTs-PES blend membrane was superior to the commercial PTFE hydrophobic membrane in terms of salt rejection. Employing the developed TNTs-PES blend membrane in VMD using a feed concentration of 3000 ppm provided a higher salt rejection of 99.3% and higher permeate flux of 24.9 kg/m2 h, compared to 90.6% and 21.5 kg/m2 h in the case of the PTFE membrane, therefore using the developed new membrane in a high salt feed concentration of 35,000 ppm provided 96.7% salt rejection compared to 62.5% in the case of the PTFE membrane but the permeate flux of the new membrane declined at increasing feed concentration to 5.5 kg/m2 h compared to 7.2 kg/m2 h using the PTFE membrane, where the dense layer formed on the top surface of the developed TNTs-PES blend membrane limited the permeate flux at feed salt concentrations higher than 7000 ppm. From the above findings, the authors recommend the possibility of using the developed TNTs-PES blend membrane in nanofiltration (NF) and reverse osmosis (RO) operational regimes in addition to VMD operation, due to the presence of a double hydrophobic/hydrophilic property. References
Fig. 13. Effect of salt concentration on salt rejection at 300 mbar vacuum pressure, 11 mL/s feed flow rate, and 65 °C feed temperature.
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Desalination journal homepage: www.elsevier.com/locate/desal
Performance of a newly developed titanium oxide nanotubes/polyethersulfone blend membrane for water desalination using vacuum membrane distillation H. Abdallah a, A.F. Moustafa b, Adnan AlHathal AlAnezi c,⁎, H.E.M. El-Sayed d,1 a
Chemical Engineering and Pilot Plant Department, Engineering Research Division, National Research Center, El Buhouth St, Dokki, Giza, Egypt Environmental Screening — Environmental Management Unit, Beni Suef Governorates, Egypt c Department of Chemical Engineering Technology, College of Technological Studies, The Public Authority for Applied Education and Training (PAAET), P.O. Box 42325, Shuwaikh 70654, Kuwait d Mechanical Engineering Department, Engineering Research Division, National Research Center, El Buhouth St, Dokki, Giza, Egypt b
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• Fabrications of (TNTs-PES) blend membrane by immersion precipitation. • Performance of (TNTs-PES) membrane was investigated for desalination using VMD. • The developed (TNTs-PES) membrane was superior in terms of salt rejection. • The permeate flux was significantly affected by the time of the VMD experiment. • At optimum conditions the permeate flux reached 15.2 kg/m2h, where the salt rejection was 98%.
a r t i c l e
i n f o
Article history: Received 9 February 2014 Received in revised form 30 April 2014 Accepted 2 May 2014 Available online 24 May 2014 Keywords: Titanium oxide nanotubes Polyethersulfone Vacuum membrane distillation Desalination
a b s t r a c t The present paper introduces a comprehensive study of the performance of newly developed titanium oxide nanotubes (TNTs) incorporated into a Polyethersulfone (PES) blend membrane for desalination using vacuum membrane distillation (VMD) process. The study examines the effect of different operating conditions. The results showed a maximum salt rejection of 98% and a permeate flux of 15.2 kg/m2 h at 7000 ppm feed salt concentration for the TNTs–PES membrane at a temperature of 65 °C and a vacuum pressure of 300 mbar with feed flow rate of 11 mL/s. A comparison between the performance of the developed TNTs-PES membrane, and commercial Polytetrafluoroethylene (PTFE) membrane was performed at different feed salt concentrations. The achieved results showed a significant improvement in the performance of the new membrane compared to the commercial PTFE membrane, where the salt rejection reached 99.3% at feed concentration 3000 ppm and 96.7% at 35,000 ppm using the new membrane, compared to salt rejection of up to 90.6% at 3000 ppm and 62.5% at 35,000 ppm using PTFE membrane. The dense TNTs layer formed on the top surface of the TNTs-PES blend membrane is considered a selective layer that prevents salt passage through the membrane. The decline in permeate flux may be overcome by membrane washing every hour. © 2014 Elsevier B.V. All rights reserved.
⁎ Corresponding author. Tel.: +965 55509994; fax: +965 22314430. E-mail addresses: [email protected] (H. Abdallah), [email protected] (A.A. AlAnezi). 1 Currently a Visiting Scholar at Civil and Environmental Engineering Department, University of Windsor, 401 Sunset Avenue, Windsor, ON N9B 3P4, Canada.
http://dx.doi.org/10.1016/j.desal.2014.05.003 0011-9164/© 2014 Elsevier B.V. All rights reserved.
1. Introduction Membrane technology is considered a very effective separation method, particularly in the area of water/wastewater treatment and
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water desalination. The application of nanoparticles or nanotubes to polymeric membranes has attracted the attention of many researchers recently. Moreover, the family of inorganic nanotubes has been expanded extensively from carbon nanotubes to sulfides [1], nitrides [2] and oxides [3]. One-dimensional nanostructured titanium oxide is of great interest for possible applications in high effect solar cells [4], photocatalysts [5,6], gas sensors [7], molecular straws [8] and semiconductor devices due to its nanotubular structure, high specific surface area, ion-changeable ability, and size-dependent properties. Currently, methods developed for fabricating titanium oxides-based nanotubes, include the assisted-template method [9,10], the sol–gel process [11], electrochemical anodic oxidation [12,13], and hydrothermal treatment [14,15]. The latter process is of great interest due to its ability to yield very low-dimensional, well-separated crystallized nanotubes, and a pure-phase structure [16]. During membrane fabrication, nanoparticles and nanotubes have recently shown significant potential in improving polymeric membrane performance. The remarkable effect of the addition of nanoparticles/ nanotubes during membrane preparation may be attributed to the interactions between nanoparticle surfaces and polymer chains and/or solvents, which lead to the production of desirable structured membranes. These modifications of membrane structure result in favorable selectivity, permeability, and satisfactory performance in ultrafiltration and nano-filtration membranes [17]. Moreover, the nanoparticle functional groups and their hydrophilic properties were found to be able to control membrane fouling phenomena, which is a major drawback of membrane separation technology [18–20]. There are two major ways of incorporating nanoparticles into polymeric membranes; (i) assembling engineered nanoparticles on the surface of the porous membranes, where these are deposited/coated on top of the membrane surface [21–33], or (ii) blending them with polymeric casting solution, wherein the nanoparticles are dispersed uniformly in the membrane solution by one of the well-known methods; melt, solvent, sol–gel mixing and in-situ grafting [34–40]. Polyethersulfone (PES) is a widely used polymer in membrane preparation, and is now rapidly becoming the material of choice for membrane applications. Its advantages include being a high performance engineering thermoplastic, given its high glass transition temperature, good mechanical properties, and excellent thermal and chemical stability. However, the hydrophobicity of PES limits its application, especially in the field of water desalination [41,42]. Thus, the introduction of nanoparticles to PES membranes is very useful in overcoming many of the disadvantages. Among the different operational membrane techniques, namely micro-, ultra-, or nano-filtration, and reverse osmosis, dialysis, etc., membrane distillation (MD) is considered one of the most popular techniques utilized for seawater desalination applications. The idea of MD is based on separating two components present in phase equilibrium; vapor/liquid or liquid/liquid. The main driving force for such separation is the vapor pressure gradient resulting from a temperature difference between hot feed (salt water) and cold permeate (pure water). MD provides many advantages over other distillation separation methods. These advantages include; the (i) capability to produce ultra-pure water, regardless of the salt concentration in the feed stream, (ii) lower operating cost, (iii) the unique feature of the possibility of achieving complete rejection of non-volatile substances, and (iv) commercially long life with saline solutions [43–45]. Among the four wellknown modes of operation of MD, direct contact membrane distillation (DCMD), air gap membrane distillation (AGMD), sweep gas membrane distillation (SGMD), and vacuum membrane distillation (VMD) [46–49], the latter configuration is utilized in the current study. In this study, titanium nanotubes synthesized using the hydrothermal method were blended into PES during polymeric membrane preparation in order to enhance its performance. VMD operational techniques were utilized in desalination experiments. The performance of the developed TNTs-PES blend membrane was tested in terms of permeate flux
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and percent salt rejection at different operating conditions. In addition, the performance of the developed TNTs-PES blend membrane was compared with the well-known commercial Polytetrafluoroethylene (PTFE) flat-sheet membrane at different feed salt concentrations. 2. Experimental work 2.1. Materials Polyethersulfone (PES), Ultrason E 6020P, molecular weight of 58,000 g/mol and glass transition temperature of Tg = 225 °C and N-Methylpyrrolidone as a solvent were purchased from BASF chemical company. TiO2 nano powder, NaOH, HCl, acetonitrile, and tetramethylsiloxane were purchased from Sigma Aldrich Company. Commercial Polytetrafluoroethylene (PTFE) flat-sheet membranes were supplied by Millipore Corporation. The specifications of the supplied membrane are as follows; diameter of 4.7 cm, thickness of 120 μm, porosity of 75%, and pore diameter of 0.2 μm. Synthetic salt solutions were prepared with different concentrations using distilled water and commercial NaCl, where the commercial NaCl used as a food salt, and contains 98.5% sodium chloride and 70–30 ppm potassium iodine. 2.2. Synthesis of titanium oxide nanotubes The approach of self-organized synthesis using the hydrothermal method is applied in the present study for the preparation of the titanium oxide nanotubes. TiO 2 nanopowder was dispersed in 150 mL of 10 M NaOH and stirred for 15 min. The dispersion was then transferred to a Teflon lined autoclave (KH-300) and heated at 150 °C for 16 h. The white precipitate obtained was washed with 1 M HCl first, then with distilled water, and finally dried at 80 °C for 4 h. A detailed description of the preparation method is described elsewhere [50,51]. 2.3. Titanium oxide nanotubes membrane fabrication For the preparation of the titanium oxide nanotube solution, 5 wt.% of tetramethylsiloxane was added first to the acetonitrile solvent, then 5 wt.% of the titanium oxide nanotube powder was added under constant stirring for 3 h. For the preparation of the polymer casting solution, 18 wt.% of PES was dissolved first in the N-Methylpyrrolidone (NMP) solvent and then added to 9% of the previously prepared nanotube solution under constant stirring for 6 h at room temperature. The prepared polymer solution was cast on a smooth flat glass plate by a casting knife with uniform speed, where the membrane thickness was sustained at 0.2 ± 0.01 mm. Subsequently, the polymer solution on the glass plates was solidified by immediate immersion in a coagulation bath containing distilled water. The membrane was spontaneously released from the plate and kept for 24 h to ensure complete removal of solvent from the membrane. Finally, the membranes were dried by placing them between two sheets of filter paper for 24 h at room temperature. 2.4. Desalination experiments A flat sheet membrane module supplied by Millipore was utilized in the desalination experiments. The effective area of the membrane in the module was determined to be 17.34 cm2. A one liter jacketed mixer vessel was filled with different concentrations of previously prepared feed salt solutions. The salt solution in the mixer feed vessel was heated using a circulation water bath to the jacket, which was controlled by a thermostat to be kept in the range of (25–65 °C). The feed temperature inside the mixer feed vessel was continuously monitored using a thermometer fitted to the feed vessel. The saline solution was continuously fed to the membrane module from the jacketed feed mixer by a peristaltic pump. The permeate water vapor was drawn from the membrane module to a chiller condenser with the aid of a vacuum pump connected
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to the condenser to apply vacuum pressure. The difference in temperature and pressure between the feed side and the outside of the membrane (main driving forces) makes water vaporize and pass through the membrane pores. The permeate water produced is collected from the bottom of the condenser. A concentrated solution recycle stream from the membrane module is returned to the feed vessel on a continuous basis. The condensed permeate is weighed by a double precision balance at predetermined time periods. A simple flow diagram of the vacuum membrane distillation system is given in Fig. 1. 3. Results and discussion 3.1. Characterization of the synthesized titanium oxide nanotubes The prepared nanotubes were characterized using X-ray diffraction (XRD) analysis. The XRD spectrum (Brukurd 8 advance, CuK, target with secondary mono chromator λυ = 40, mA = 40, Germany) was applied to identify the phase formation and crystal size. The morphology and geometry of the prepared TiO2 nanotubes were deduced using transmission electron microscopy (TEM) analysis (JEOL-2010, Japan). Images from both XRD and TEM are shown in Figs. 2 through 4. The XRD pattern was used for the phase identification of raw TiO2 powder, which was used for preparation of the nanotubes. It was found that the raw material is of a pure rutile phase of TiO2 nanopowder with a crystal size of about 93 nm and is highly crystalline, as shown in Fig. 2. The phase of prepared titanium oxide nanotubes was obtained from XRD patterns as shown in Fig. 3. This illustrates that the prepared nanotubes are a mixture of different sodium titanium oxides and titanium oxide nanotubes. Fig. 4 presents the TEM image of the obtained nanotubes. It can be deduced from the figure that there is a large quantity of prepared nanotubes with complete rolling and uniform inner and outer diameters along the length of the nanotubes. Moreover, the obtained titanium nanotubes appeared as a layered structure with good crystallinity. In addition, the nanotubes have a narrow size distribution with diameter of around 1.67 nm for inner diameter, 8.35 nm for outer diameter and length of about 141 nm. 3.2. Nanotubes membrane characterization The morphology of the membrane was characterized by a scanning electron microscope (SEM) (JEOL 5410, Japan) operating at 20 kV. Cross sectional samples were prepared by fracturing the membrane under liquid nitrogen. The dried samples were coated by gold sputtering to provide electrical conductivity. SEM images of the cross section of the asymmetric membranes are presented in Fig. 5(a, b). Fig. 5a indicates
Fig. 2. XRD patterns of commercial rutile phase TiO2 nano-powder.
the SEM view of a pure PES cross section, where the spongy structure revealed the porous nature of the membrane. Fig. 5b shows the SEM view of the TNTs-PES blend membrane. It is clear from the figure that the top surface appears as a dense layer (was not existing in the first figure) given the presence of the titanium nanotubes. This plays an important role in salt rejection, as will be explained later. It may also be noted that another highly porous layer appears at the bottom (glass side) of the section [51]. 3.3. Effect of operating parameters 3.3.1. Effect of feed temperature The effect of different feed temperatures (25, 45, 55 and 65 °C) on VMD permeate flux and salt rejection of TNTs/PES membrane was studied under the following operating conditions: 7000 ppm feed concentration, 300 mbar vacuum pressure and 11 mL/s feed flow rate. It is obvious from Figs. 6 and 7 that the feed temperature had a remarkable effect on permeate flux and salt rejection, respectively. As shown, the permeate flux and salt rejection increased linearly with feed temperature. This behavior is mainly due to the exponential dependence of water vapor pressure on temperature, according to the Antoine equation. Therefore, increasing the feed temperature increases the temperature difference, which consequently enhances the vapor pressure difference (i.e. the driving force for mass transfer) and increases the permeate dilution as a result of the higher recovery rate. 3.3.2. Effect of feed flow rate The feed flow rate is one of the most important parameters affecting the VMD performance. Therefore, in order to study the effect of feed flow
Fig. 1. Flow diagram of VMD process.
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Fig. 3. XRD patterns of prepared titanium nanotubes, (1) Na2Ti3O7, (2) Na2Ti6O16, (3) Na2Ti9O19, (4) Ti3O5.
rate on the performance of the developed TNTs-PES blend membrane, several experiments were performed under the following operating conditions: 7000 ppm feed concentration, 300 mbar vacuum pressure, and 65 °C feed temperature. As shown in Figs. 8 and 9, increasing the feed flow rate from 8 to 28 mL/s, increased the permeate flux and salt rejection linearly, until they reached a maximum value of 15.2 kg/m2 h and 98%, respectively, at 11 mL/s. This increase is attributed to the rise in Reynolds number, which causes enhanced mixing of the flow in the channels due to turbulence. The turbulent flow reduces the thickness of both the temperature and concentration boundary layers, which means lower boundary layer resistance. Moreover, it is clear that, the permeate flux and salt rejection are significantly reduced to a minimum value of 10.4 kg/m2 h and 97.2%, respectively at a maximum feed flow rate of 28 mL/s. This reduction can be referred to the vapor pressure reduction. It is clear that the permeate flux increases to an asymptotic value with increasing the feed flow rate until it approaches a certain limit, where the effect of the feed flow rate leads to a decrease in permeate flux due to a decrease in membrane surface temperature [46, 47]. Moreover, lower boundary layer resistance can permit the salt to pass though the membrane, which in turn decreases the salt rejection. Therefore, it is worth noting that increasing the feed flow rate further has a negative influence on permeate flux and salt rejection; hence, the effective way is to optimize the feed flow rate to achieve high values of permeate flux and salt rejection.
Fig. 4. TEM image of prepared titanium nanotubes.
Fig. 5. SEM cross section of (a) pure PES membrane, (b) TNTs-PES blend membrane.
3.3.3. Effect of vacuum pressure The vacuum pressure at the permeate side plays a major role in the VMD performance, because the boundary layer resistance and the heat conduction across the membrane is negligible due to the very low pressure on the permeate side. In Figs. 10 and 11, the effect of the vacuum pressure on the permeate flux and salt rejection, at 7000 ppm feed concentration, 11 mL/s feed flow rate, and 65 °C feed temperature, is presented. It is worth mentioning that both permeate flux and salt rejection increase with decreasing vacuum
Fig. 6. Effect of feed temperature on permeate flux at 7000 ppm salt solution 300 mbar vacuum pressure and 11 ml/s feed flow rate.
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Fig. 7. Effect of feed temperature on salt rejection at 7000 ppm feed concentration, 300 mbar vacuum pressure and 11 ml/s feed flow rate.
pressure at the permeate side. This is attributed to the significant increase in the vapor pressure difference (i.e. the driving force) between the feed and permeate side. 3.3.4. Effect of feed concentration The effect of feed concentration on the permeate flux and the percentage rejection were investigated in VMD. The experiments were performed for different feed concentrations (3000, 5000, 7000, 10,000 and 35,000 ppm) at 11 mL/s feed flow rate, 300 mbar vacuum pressure and 65 °C feed temperature, as shown in Figs. 12 and 13, respectively. The results show that increasing the feed salt concentration decreases the permeate flux and salt rejection due to the reduction in water vapor pressure difference (i.e. driving force). Additionally, the reduction can be attributed to the effects of temperature and concentration polarization, which increased the thickness of the boundary layer, and consequently reduced the evaporation driving force. 3.4. Effect of experiment time on the permeates flux and feed salt rejection In order to study the possible effects of experiment time on the performance of the TNTs/PES blend membrane during VMD, permeate flux as well as salt rejection for different time intervals up to 120 min were measured. The experiments were performed at 7000 ppm feed salt concentration, 65 °C feed temperature and 300 mbar vacuum pressure. Fig. 14 shows a bar chart of the results, which indicates a decline in the permeate flux with time. As time goes on, a scale layer of salts is formed on the membrane surface, which is known as fouling. This layer acts as a barrier and increases the resistance to flow due to blocking of the membrane pores. The maximum flux reached was about 15.2 kg/m2 h after 40 min of operation. After that, the permeate flux declined. It was noted that the salt rejection (SR) increased to 98% after 40 min and then became constant and was not further affected by time. It should be pointed out here that fouling during the VMD
Fig. 8. Effect of feed flow rate on permeate flux at 7000 ppm feed concentration, 300 mbar vacuum pressure, and 65 °C feed temperature.
Fig. 9. Effect of feed flow rate on salt rejection at 7000 ppm feed concentration, 300 mbar vacuum pressure, and 65 °C feed temperature.
process may be overcome by membrane washing, which is recommended here in every hour. 3.5. Comparison between commercial PTFE membrane and developed TNTs-PES blend membrane A series of saline water VMD experiments were performed using hydrophobic Polytetrafluoroethylene (PTFE) flat sheet membrane and the prepared TNTs-PES membrane. In order to investigate the performance of the VMD process, the permeate water flux was measured for both membranes at different feed concentration of (3000, 5000, 7000, 10,000 and 35,000 ppm) at operating conditions of 300 mbar vacuum pressure, 65 °C feed temperature and 11 mL/s feed flow rate for the membrane area of 17.34 cm2. The comparison between the two membranes was evaluated in terms of permeate flux and percentage salt rejection, as indicated in Table 1. The results indicate that the permeate flux of the TNTs-PES blend membrane was higher than that of the PTFE membrane at low feed salt concentration (lower than 10,000 mg/L). Whereas, the PTFE membrane permeate flux was higher than that of the TNTs-PES blend membrane at high feed salt concentration (≥10,000 ppm). The authors' explanation of these results is that the developed TNTs-PES blend membrane may be considered a hydrophobic/hydrophilic membrane. This means it can absorb water in both liquid and vapor phases faster than the hydrophobic PTFE membrane, which can only absorb water in the vapor state from the feed side. As the feed solution becomes more concentrated, more water will be diffusing from the bulk feed solution to the membrane surface due to the formation of the titanium oxide nanotubes dense layer on the top surface of the membrane. Thus, the salt polarization effect tends to limit the driving force for water permeation, which decreases the permeate flux (this is not the case for the PTFE membrane). On the other hand, the TNTs-PES blend membrane provides higher percentage salt
Fig. 10. Effect of vacuum pressure on permeate flux at 7000 ppm feed concentration, 11 ml/s feed flow rate, and 65 °C feed temperature.
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Fig. 11. Effect of vacuum pressure on % salt rejection at 7000 ppm feed concentration, 11 ml/s feed flow rate, and 65 °C feed temperature.
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Fig. 14. Performance of the developed TNTs-PES blend membrane with time at 7000 ppm feed salt concentration, 65 °C temperature, and 300 mbar vacuum pressure.
Table 1 Comparison between PTFE membrane and prepared TNTs-PES membrane.
Fig. 12. Effect of feed salt concentration on permeate flux at 300 mbar vacuum pressure, 11 mL/s feed flow rate and 65 °C feed temperature.
rejection over the whole range of feed concentrations. Again, the dense TNT layer formed on the top surface of the TNTs-PES blend membrane is considered a selective layer that prevents salt passage through the membrane.
4. Conclusions Introducing titanium oxide nanotubes during polymeric membrane fabrication has resulted in significant improvements in the membrane performance in terms of permeate flux and salt rejection. Vacuum membrane distillation (VMD) for water desalination experiments were carried out in the present study at different operating conditions.
Feed concentration ppm
PTFE
3000 5000 7000 10,000 35,000
21.5 18.4 12.7 10.2 7.20
TNTs-PES
Permeate flux (kg/m2 h)
PTFE
TNTs-PES
Salt rejection (%) 24.9 21.8 15.2 8.00 5.50
90.6 88.2 79.4 68.6 62.5
99.3 98.6 98.0 97.8 96.7
The results showed a maximum salt rejection of 98% and a permeate flux of 15.2 kg/m2 h at the optimum conditions of 65 °C feed temperature, 7000 ppm feed salt concentration, 300 mbar vacuum pressure, and 11 mL/s feed flow rate. The permeate flux was found to be significantly affected by the time of the VMD experiment. The formation of salt scales that blocked the membrane pores hindered the flow of fluids across the membrane. After 40 min of operation, the permeate flux was declining after it reached a value of 15.2 kg/m2 h. On the other hand, the salt rejection reached 98% at 40 min of operation and remained constant after that until the end of the experiment (120 min). The developed TNTs-PES blend membrane was superior to the commercial PTFE hydrophobic membrane in terms of salt rejection. Employing the developed TNTs-PES blend membrane in VMD using a feed concentration of 3000 ppm provided a higher salt rejection of 99.3% and higher permeate flux of 24.9 kg/m2 h, compared to 90.6% and 21.5 kg/m2 h in the case of the PTFE membrane, therefore using the developed new membrane in a high salt feed concentration of 35,000 ppm provided 96.7% salt rejection compared to 62.5% in the case of the PTFE membrane but the permeate flux of the new membrane declined at increasing feed concentration to 5.5 kg/m2 h compared to 7.2 kg/m2 h using the PTFE membrane, where the dense layer formed on the top surface of the developed TNTs-PES blend membrane limited the permeate flux at feed salt concentrations higher than 7000 ppm. From the above findings, the authors recommend the possibility of using the developed TNTs-PES blend membrane in nanofiltration (NF) and reverse osmosis (RO) operational regimes in addition to VMD operation, due to the presence of a double hydrophobic/hydrophilic property. References
Fig. 13. Effect of salt concentration on salt rejection at 300 mbar vacuum pressure, 11 mL/s feed flow rate, and 65 °C feed temperature.
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