Polyazole Hollow Fiber Membranes for Direct Contact Membrane

Mar 8, 2013 - Finally, the hollow fiber membranes were tested for DCMD. ... and hydrophilic membrane surfaces in a submerged membrane bioreactor...
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Polyazole Hollow Fiber Membranes for Direct Contact Membrane Distillation Husnul Maab, Ahmad Al Saadi, Lijo Francis, Sara Livazovic, Noreddine Ghafour, Gary L. Amy, and Suzana P. Nunes* Water Desalination and Reuse Center, King Abdullah University of Science and Technology (KAUST), 23955-6900 Thuwal, Kingdom of Saudi Arabia ABSTRACT: Porous hollow fiber membranes were fabricated from fluorinated polyoxadiazole and polytriazole by a dry−wet spinning method for application in desalination of Red Sea water by direct contact membrane distillation (DCMD). The data were compared with commercially available hollow fiber MD membranes prepared from poly(vinylidene fluoride). The membranes were characterized by electron microscopy, liquid entry pressure (LEP), and pore diameter measurements. Finally, the hollow fiber membranes were tested for DCMD. Salt selectivity as high as 99.95% and water fluxes as high as 35 and 41 L m−2 h−1 were demonstrated, respectively, for polyoxadiazole and polytriazole hollow fiber membranes, operating at 80 °C feed temperature and 20 °C permeate.

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INTRODUCTION Membrane technology is becoming predominant in different sectors of water desalination and reuse. Seawater reverse osmosis has been delivering drinking water to countries in the Middle East for decades, although through thermal processes. Thermal desalination is still important in countries with an abundance of fossil fuel. But the need for sustainability is driving a constant process optimization to save energy and minimize environmental impact. The idea of process intensification, by combining different membrane technologies or integrating emerging membrane processes into more conventional industrial technology, has been advocated by Drioli et al.1 for a long time. Membrane distillation (MD) has gained more interest in the last years with the possibility of hybridization with thermal desalination or operation with solar power. Aspects of MD technology have been reviewed by different authors,2−4 and different MD configurations have been explored. In the direct contact membrane distillation (DCMD), the water to be desalinated or purified is brought to temperatures typically as high as 80 °C and separated from the cold permeate by a porous hydrophobic membrane (Figure 1). A great part of the work published until now has used flat-sheet commercial membranes based on polytetrafluorethylene (PTFE) (e.g., Gore) and polypropylene (PP) (e.g., Celgard, Accurel).5,6. However hollow fibers are gaining attention and are being explored by different groups.2,7−15 Because PTFE and PP are not soluble at room temperature, at least in most common solvents, these polymers could not be used for phase inversion hollow fiber manufacture by solvent exchange and coagulation in water. Poly(vinylidene fluoride) (PVDF) has been the polymer of choice of many groups for hollow fibers applied to MD.2,7−15 It is soluble in polar solvents like dimethyl acetamide, dimethyl formamide, and N-methyl pyrrolidone, and it is more hydrophobic than other polymers frequently used for membranes like polysulfone and polyacrylonitrile and cellulose derivatives. Efforts have been focused on (i) optimization of fiber morphology by integration of additives, change of solvent, © XXXX American Chemical Society

Figure 1. Membrane distillation.

and polymer concentration,2,7−15 (ii) manufacture of dual-layer fibers, 12−15 and (iii) development of innovative fiber designs.16,17 Except for blending of surface modifying polymeric or olygomeric additives,18 practically no efforts have been dedicated to develop new polymers tailored for the needs of MD. We recently19 reported the development of new hydrophobic polymers and their manufacture into flat-sheet membranes for MD. They are based on fluorinated polyoxadiazole and polytriazole. Polyazoles are known for their thermal stability, higher than that of PVDF, and by choosing the right backbone, we were able to make membranes with hydrophobicity close to that of PTFE but soluble in a polar solvent and therefore with very good processability in solution. We prepared flat-sheet membranes by phase inversion Special Issue: Enrico Drioli Festschrift Received: January 5, 2013 Revised: March 7, 2013 Accepted: March 8, 2013

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RO water for three days, changing the water every day to remove any residual solvent. 2.5. Hollow Fiber Modules. To test the hollow fiber membranes performance in direct contact membrane distillation, permeation modules were prepared by fixing five hollow fibers with a length of 20 cm in each module. The two ends of the hollow fiber bundles were glued with epoxy resin and were left overnight for curing before testing. In this way, the obtained surface area for each module was 25 cm2. 2.6. Membrane Morphology. The membrane surface and cross-section morphologies were investigated by field emission scanning electron microscopy (FESEM) on a Nova Nano FEI microscope. The samples were sputter coated with gold using a K575X Emitech equipment. For the cross sections, the membranes were previously fractured in liquid nitrogen. 2.7. Pore Diameter, Liquid Entry Pressure (LEP), and Liquid Water Permeability Measurements. A Poroflux 1000 capillary flow porometer was used to measure the pore size distribution, liquid entry pressure (LEP), and liquid water permeability. For pore size distribution determination, the membrane is wetted with perfluoroether (to fill all the pores) and fixed in the sample holder. Nitrogen is pressed into the lumen of the hollow fiber membranes, and when the pressure overcomes the capillary action of the liquid inside the pore, the liquid flows out of the pore. Liquid Entry Pressure (LEP) is the minimum pressure at which the membrane can withstand prior to the flow of permeate. The pressure is then automatically increased to remove the wetting liquid from all pores of the membranes. By measuring the percent nitrogen flow at each applied pressure, the pore size distribution is calculated. Nitrogen flow was also measured through a dry membrane (without being exposed to any wetting liquid). For liquid water permeability tests, a water tank was placed in between the nitrogen pressure control and the hollow fiber holder, and water was pressurized at 5 bar into the lumen. Three to five hollow fibers, each with a length of 20 cm, were assembled into a module for the characterizations. Only the initial flux was measured. 2.8. Direct Contact Membrane Distillation (DCMD) Experiments. Figure 3 shows the DCMD setup, and Table 1 shows the optimized experimental conditions used for the DCMD experiments. The DCMD setup consists of two parts: part one is connected to the heating system for controlling the feedwater temperature, while part two is connected to a chiller

and electrospun membranes. In this paper, we have taken advantage of the polymer processability and are now producing the first hollow fiber membranes manufactured from fluorinated polyoxadiazoles and fluorinated polytriazoles. We tested their performance in DCMD setups and compared it to PVDF hollow fibers prepared in our lab.

2. EXPERIMENTAL SECTION 2.1. Materials. 4,4′-(Hexafluoroisopropylidene)bis(benzoic acid) (CAS 117-47-7) (HFA) (99% Aldrich), hydrazine sulfate (HS) (CAS 10034-93-2) (98% Aldrich), N-methyl pyrrolidone (NMP) (99% Aldrich), polyphosphoric acid (PPA) (ca. 84% as phosphorus pentaoxide, Alfa Aesar), and aniline from Sigma Aldrich were used as received. Poly(vinylidene fluoride) (PVDF) was purchased from Alfa Aesar. 2.2. Synthesis of Polyoxadiazole and Polytriazole. Fluorinated polyoxadiazole and polytriazole were synthesized according to previously reported procedure.19 The polyoxadiazole synthesis was based on the reaction of polyphosphoric acid (PPA) with hydrazine sulfate (HS) at 100 °C for 1 h, and further continued at 160 °C with addition of HFA. For polytriazole, aniline was added to the reactive medium 1 h after the addition of HFA at 180 °C. 2.3. Preparation of Dope Solutions. Dope solutions were prepared by dissolving 18 wt % fluorinated polyoxadiazole, polytriazole, or PVDF in N-methyl-2-pyrrolidone (NMP) under constant stirring at 80 °C for 24 h. The homogeneous dope solutions were cooled to room temperature before spinning and maintained without stirring for 24 h. 2.4. Preparation of Hollow Fiber Membranes. The dope solution was transferred to the stainless tank of the hollow fiber machine (Samwon Ams Co., LTD, Figure 2) and was

Figure 2. Hollow fiber machine.

forced to the spinneret by a Gear Pump (from Kawasaki) with a speed controller and nitrogen pressure of 1 bar. The bore fluid was a mixture of organic solvent (NMP), and reverse osmosis (RO) desalinated water in the ratio of 80/20 and was fed to the spinneret by a flash pump 100 digital HPLC (from Laballicane). The used spinneret was a double orifice with 0.5 mm inner diameter and 0.9 mm outer diameter. The air gap was 20 cm. The optimized flow rates for the bore fluid and dope solution were both 6 mL/min, maintained throughout the process. No additional extension force was applied to the nascent hollow fibers (free fall take-up). The fibers were spun at room temperature and humidity of 65%. The coagulation bath was RO water. Once formed, the hollow fibers were stored in

Figure 3. Bench scale setup for direct contact membrane distillation (DCMD). B

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Table 1. DCMD Experimental Conditions for Hollow Fiber Performance Test condition/parameter

temperature condition

hot feed temp (inlet)/°C cold permeate temp (inlet)/°C hot feed temp (outlet)/°C cold permeate temp (outlet)/°C Δt (outlet)/°C

40.1 20.1

50.5 20.1

60.3 20.2

70.1 20.3

78.5 20.5

39.5 21.3

49.7 21.3

58.8 22.1

68.5 22.7

77.1 23.5

18.2

28.4

36.7

45.8

53.9

feed flow rate (L/min) permeate flow rate (L/min) pressure on hot feed side pressure on cold permeate side membrane effective area

1 1 0.35 bar 0.21 bar 25 cm2

(cooling loop) to condense the permeate vapors. Red Sea water was used as a hot feed solution (highlighted by red color) and was circulated through the shell side of the membrane, while the cold permeate (highlighted by blue color) was circulated through the lumen of the hollow fibers. The flow rates at feed and permeate side were maintained at 1 L/min in the steady state. The salt concentrations in the feed and permeate sides were monitored with a conductivity meter (Oakton Eutech Instruments). The feed and distillate streams flowed counter-currently, and the MD flux and salt rejection (% SR) were calculated according to eqs 1 and 2. MD Flux = W /(AΔt )

(1)

%SR = (Cf − Cp)/Cf

(2)

Figure 4. FESEM images of cross section and outer and inner surfaces of fluorinated polyoxadiazole, fluorinated polytriazole, and PVDF hollow fibers.

hollow fiber membranes measured by capillary flow porosimetry using Porefil as wetting liquid are shown in the Figure 5. A

where W is the amount of distilled water collected during the membrane distillation experiment in time (Δt); A is the membrane surface area; Cf is the salt concentration in the hot feed seawater; and Cp is the permeate salt concentration.

3. RESULTS AND DISCUSSION 3.1. Hollow Fiber Morphology. The hollow fiber morphology was examined by FESEM. Micrographs of the fiber cross section and internal and external surfaces are shown in Figure 4. The fibers are asymmetric with finger-like macrovoids extending from both inner and outer walls of the hollow fiber. Nanopores can be seen both on the external and internal surfaces of the hollow fibers. The internal surfaces have additionally regular structures in the micrometer range, which are formed as a consequence of the presence of 80 wt % NMP in the bore fluid. The high content of NMP delays the phase inversion process. Fibers prepared only with water in the bore fluid had a much less porous internal surface with a denser skin and lower water flux in MD experiments. The PVDF hollow fiber membranes had smaller pore diameters when compared to the polyazoles hollow fibers. 3.2. Pore Diameter, Liquid Entry Pressure (LEP), and Water Flux Measurements. We recently reported 19 flatsheet membranes for membrane distillation with porosity as high as 70% and pore size diameter ranging from 50 nm to higher than 1.0 μm. The highest pore sizes and porosity were obtained for membranes prepared by electrospinning. Flatsheet membranes prepared by phase inversion from fluorinated polyoxadiazole and polytriazole had mean flow pore sizes (MFP) around 50 and 80 nm. Pore diameter distributions of all

Figure 5. Pore diameters measured by porosimetry for hollow fibers manufactured from PVDF, fluorinated polyoxadiazole, and fluorinated polytriazole.

very narrow distribution of the pore size can be observed for PVDF hollow fibers with a MFP value of 55 nm, while a broad pore size distribution can be seen for the fluorinated polyoxadiazole and polytriazole hollow fibers. For polyoxadiazole hollow fibers, the pore diameters vary from 70 to 130 nm, while for polytriazole hollow fibers they vary from 110 to 170 nm. The LEP values, determined on Porolux 1000, were 2.1 and 2.0 bar for fluorinated polyoxadiazole and fluorinated polytriazole hollow fibers, respectively. For PVDF hollow fibers, the LEP value is also close to 2 bar. All LEP values were measured with perfluoroether Porefil (surface tension 16 mN/ m, according to POROLUX). Water has a surface tension of 72 mN/m at room temperature (63 mN/m, at 80 °C),20 much higher than Porefil. The membranes are hydrophobic with large water−surface contact angles, as reported before for flat-sheet films.19 Large surface tension and large contact angles contribute to large LEP values. The LEP values (here measured using Porefil) are expected to be even higher if water would be C

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the “wetting” liquid. The LEP values in the case of these membranes are therefore much higher than the inlet pressure of about 0.35 bar that was maintained on the hot feed side throughout the MD experiments. So there is no chance of leakage of water molecules, and the DCMD fluxes were obtained only from the vapors transport from the hot feed side to cold permeate side. The nitrogen permeation through dry membranes was measured as 78, 82, and 76 m3 m−2 h−1 bar−1, respectively, for fluorinated polyoxadiazole, fluorinated polytriazole, and PVDF. This confirms that polytriazole membranes have (7%) higher porosity as well as larger pores (according to Figure 5). On the other side, while the porosity of polyoxadiazole and PVDF membranes are practically the same (same nitrogen flux), the pores of the tested PVDF membranes are smaller. The hydrodynamic fluxes of liquid water through the membranes were 85, 98, and 110 L m−2 The following use of keep-together tags is outside the context of a table.h−1 bar−1 for fluorinated polyoxadiazole, fluorinated polytriazole, and PVDF, respectively. They were measured at a pressure difference of 5 bar, which is higher than the LEP values for all membranes. Taking in consideration only polyoxadiazole and polytriazole, this is in perfect agreement with the porosity values and the measured pore size. However, the liquid water permeability is higher for PVDF than for fluorinated polyoxazole membranes, even though the pores are smaller. The reason is probably the higher hydrophobicity of the polyazole membranes. We recently reported the contact angles for flat-sheet porous membranes19 based on fluorinated polytriazole, polyoxadiazole, and PVDF as being 103°, 120°, and 88°, respectively, when prepared by phase inversion. The nitrogen permeation (and porosity) varies only 2.5% between PVDF and fluorinated polyoxadiazole, while the hydrophobicity of the latter is much higher. The LEP values reflect the liquid penetration in the largest pores of the membrane. It might be that although the pressure at which the hydrodynamic liquid permeation was measured is higher than LEP, water could not penetrate the smallest pores of the highly hydrophobic polyoxadiazole membranes. 3.3. Membrane Distillation. DCMC flux values for the hollow fiber membranes are shown in Figure 6, comparing the performances of fluorinated polyoxadiazole, polytriazole, and PVDF. While the hydrodynamic liquid water flux was higher for PVDF membranes than for those made of fluorinated

polytriazole and polyoxadiazole, the (vapor) water permeation under DCMC operation depends mainly on the porosity and pore size, and it was therefore higher for fluorinated polytriazole (high nitrogen permeation and high pore sizes in Figure 5) and polyoxadiazole than for PVDF. During the DCMD experiments, the cold permeate and Red Sea hot feed were circulated counter-currently through the lumen and shell sides of the hollow fiber membranes, respectively. The inlet temperature of cold permeate was kept constant at 20 °C, while different experiments were done with different hot feed temperatures adjusted at 40, 50, 60, 70, and 80 °C. Small heat losses, causing temperature variation smaller than 3 °C, could be detected from the inlet to outlet in both sides of the membrane, as shown in Table 1. This is due to the fact that the membrane is not a perfect thermal insulator; however, the variation is acceptable for MD operation. The trans-membrane water permeation flux under MD operation achieved 35 and 41 L m−2 h−1 for polyoxadiazole and polytriazole hollow fibers, respectively, when the inlet temperature of the hot feed was 80 °C. The experimental conditions were the same during the DCMD experiments for all hollow fibers, yet the difference in flux can be attributed to the higher porosity and larger pore sizes of polytriazole fibers as mentioned above. Salt selectivities as high as 99.95% in DCMD experiments were confirmed for all hollow fibers. Fluorinated polyoxadiazole and polytriazole hollow fibers manufactured by using pure water as bore fluid showed a poorer performance in DCMD experiments. The water fluxes were respectively below 25 and 15 L m−2 h−1 with feed temperature 80 °C and permeate 20 °C. This reflects the lower porosity, particularly in the lumen of the fiber. At the same feed and permeate temperatures, the water flux of flat-sheet polyoxadiazole and polytriazole membranes manufactured by phase inversion19 were 25 and 28 L m−2 h−1, respectively. This confirms the superiority of the optimized hollow fibers. Furthermore, hollow fibers modules are known for their advantageous area/volume compared to flat-sheet modules. By achieving good performances with hollow fibers, the perspectives of manufacturing compact modules for largescale operation are now much better. Although we reported even better performance for electrospun membranes based on the same materials tested in similar experimental conditions, phase inversion is a well-established membrane manufacture process easy to upscale. The phase inversion membranes, at least at this stage of optimization, seem to be also more robust. It is also important to mention that the mechanical and thermal stability of fluorinated polyazoles are much better than of PVDF, as reported previously.19 This combined with higher hydrophobicity allows the operation under much more extreme conditions than reported here. The high mechanical stability of polyazoles is also promising in terms of resistance to creep. While PVDF can be susceptible to deformation under long mechanical stress, polyazoles are much less prone to it. Previous measurements19 at room temperature show that fluorinated polytriazole or polyoxadiazole films needed stresses practically 4 times larger than for PVDF to have 10% deformation. The water flux values obtained for PVDF hollow fibers are in the same range reported for single-layer membranes by leading groups in the field.15 We believe that further optimization of the polyazole membranes is possible, bringing even higher values compared to PVDF. Strategies like manufacture of duallayer hollow fibers, which have been demonstrated to be

Figure 6. Water fluxes in DCMD experiments for fluorinated polyoxadiazole, fluorinated polytriazole, and PVDF hollow fibers. D

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effective for PVDF13,14 should bring also be promising in the case of polyazole membranes.

(13) Edwie, F.; Teoh, M. M.; Chung, T. S. Effects of additives on dual-layer hydrophobic, hydrophilic PVDF hollow fiber membranes for membrane distillation and continuous performance. Chem. Eng. Sci. 2012, 68, 567−578. (14) Wang, P.; Teoh, M. M.; Chung, T. S. Morphological architecture of dual-layer hollow fiber for membrane distillation with higher desalination performance. Water Res. 2011, 45, 5489−5500. (15) Edwie, F.; Chung, T. S. Development of hollow fiber membranes for water and salt recovery from highly concentrated brine via direct contact membrane distillation and crystallization. J. Membr. Sci. 2012, 421−422, 111−123. (16) Yang, X.; Wang, R.; Fane, A. G. Novel designs for improving the performance of hollow fiber membrane distillation modules. J. Membr. Sci. 2011, 384, 52−62. (17) Wang, P.; Chung, T. S. Design and fabrication of lotus-root-like multi-bore hollow fiber membrane for direct contact membrane distillation. J. Membr. Sci. 2012, 421−422, 361−374. (18) Khayet, M.; Matsuura, T.; Mengual, J.; Qtaishat, M. Design of novel direct contact membrane distillation membranes. Desalination 2006, 192, 105−111. (19) Maab, H.; Francis, L.; Al Saadi, A.; Aubry, C.; Ghafour, N.; Amy, G. L.; Nunes, S. P. Synthesis and fabrication of nanostructured hydrophobic polyazole membranes for low-energy water recovery. J. Membr. Sci. 2012, 423−424, 11−19. (20) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed.; Wiley-Interscience: New York, 1997.

4. CONCLUSIONS Hollow fiber membranes were fabricated for the first time from fluorinated polyoxadiazole and polytriazole and were also tested under direct contact membrane distillation operation. The membranes were fully characterized in terms of porosity. Salt selectivity as high as 99.95% was confirmed in DCMD tests, operating with a seawater feed at different temperatures. The performance of polyazole hollow fibers was demonstrated to be better than PVDF hollow fibers prepared under similar conditions. The water fluxes were also higher than those reported before for flat-sheet phase inversion membranes based on the same polymers.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the KAUST central workshop for the fabrication of the test setups and cells and Dr. Jan Roman Pauls for help with the hollow fiber module design.

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REFERENCES

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dx.doi.org/10.1021/ie400043q | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX