Journal of Membrane Science 208 (2002) 233–245
Effect of DGDE additive on the morphology and pervaporation performances of asymmetric PSf hollow fiber membranes H.A. Tsai a,b , M.J. Hong b , G.S. Huang b , Y.C. Wang c , C.L. Li c , K.R. Lee c , J.Y. Lai b,∗ a
b
Department of Textile Engineering, Nanya Institute of Technology, Chung Li 32034, Taiwan The Research and Development Center for Membrane Technology, Department of Chemical Engineering, Chung Yuan University, Chung Li 32023, Taiwan c Department of Chemical Engineering, Nanya Institute of Technology, Chung Li 32034, Taiwan Received 4 February 2002; received in revised form 6 April 2002; accepted 11 June 2002
Abstract In this article, the effect of co-solvent (N-methyl-2-pyrrolidone (NMP)/diethylene glycol dimethyl ether (DGDE)) of the dope solution on the morphology, mechanical property and the pervaporation performance of the polysulfone (PSf) hollow fiber membranes prepared via a wet spinning process were investigated. Water was employed as the external coagulant, 70 wt.% NMP in water was used as bore liquid. The rate of solvent–nonsolvent exchanged during membrane formation was characterized by Fourier transform infrared (FT-IR) microscopy. The addition of DGDE in the dope solution decreases the exchange rate of solvent–nonsolvent during the membrane formation process, resulting in the suppression of the macrovoids formation. The tensile stress, elongation at break and Young’s modulus of the PSf hollow fiber membranes increased with increasing the DGDE content. In addition, the effects of DGDE content, feed composition and feed solution temperature on the pervaporation performances were also investigated. The permeation rate and separation factor towards water for a 90 wt.% aqueous ethanol solution through the asymmetric PSf hollow fiber membrane with a NMP/DGDE (8/2) ratio of co-solvent system were 172.7 g/m2 h and 23.9, respectively. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Polysulfone; DGDE; Macrovoids; FT-IR microscopy; Pervaporation
1. Introduction Pervaporation is an emerging membrane separation process to separate the azeotropic mixtures, close boiling point mixtures, isomers or heat-sensitive mixtures. In pervaporation process, the liquid mixture to be separated is placed in contact with one side of membrane and the vapor phase permeate is removed by condensing and collecting from the other side. The pervapo∗ Corresponding author. Tel.: +886-3-4563-672; fax: +886-3-4563-672. E-mail address:
[email protected] (J.Y. Lai).
ration driving force can be created by applying either vacuum pump or an inert purge on the down-stream to maintain the permeate vapor pressure lower than the partial pressure of the feed liquid. From the application point of view, asymmetric flat membrane and hollow fiber membrane are of great interest using in the pervaporation process. Polymeric membranes have been well established in a wild variety of industrial applications since the development of asymmetric type membrane by Loeb and Sourirajan [1]. The wet phase inversion method is the most widely used for the preparation of asymmetric membranes [2,3]. In addition, hollow fiber membranes possess many
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advantageous characteristics than the flat ones such as: (1) the membrane packing density (i.e. membrane area per unit module volume) can be much higher than that of the flat membranes; (2) the hollow fiber membranes are self-supporting; (3) the hollow fiber membranes themselves form the vacuum vessel if the shell-fed mode of operation is used [4]. Unfortunately, only a few literatures have been found on the systematic study on pervaporation of aqueous organic mixtures through hollow fiber membranes [5,6] despite their wide acceptance in other separation processes such as gas separation and ultrafiltration process [7–13]. Hachisuka et al. [14] have reported that an asymmetric polyimide gas separation flat membrane can be obtained by using diethylene glycol dimethyl ether (DGDE) as the co-solvent of N-methyl-2-pyrrolidinone (NMP). Byun et al. [15] have also used the NMP and DGDE binary mixture solvent systems to prepare the sulfonated polysulfone (PSf) and sulfonated poly(ether sulfone) asymmetric flat membrane to separate the water–butanol solution by pervaporation operation. Recently, Kim et al. [16] have added DGDE and acetic acid as nonsolvent additive in PEI/NMP casting solution to prepare PEI asymmetric flat nanofiltration membrane. All of them have shown that the macrovoids structure could be suppressed by adding DGDE in the casting solution. It is well known that thermodynamic and kinetic parameters of the phase inversion process influence the structure formation. Thermodynamics is often demonstrated by coagulation value or ternary phase diagram that were obtained by titrating the coagulant into polymer solution until it became turbid. The solvent–nonsolvent exchange rate was one of the kinetics factors attracting most attention. It determined the path on the phase diagram during membrane formation. Strathman et al. [17] has developed an optical microscopy technique to observe the membrane formation process. At the optical microscopy analysis technique, the coagulant penetrated into the casting solution when they were brought into contact. The penetration fronts were observed and analyzed to determine the time dependence of the penetration distance of coagulant. In this technique, the outflow of solvent cannot be observed. Therefore, another powerful equipment (FT-IR microscopy) will be used in this work to measure the composition change at any spot during the membrane formation.
The controlling factors for hollow fiber spinning are not only complicated, but also quite different from preparing flat one. For example, there are two coagulants that can induce different phase separation, the viscosity of spinning dope solution is greater than the casting solution of flat membrane, there may be different stresses that resulted from different dope extrusion rate, viscosity etc. to induce molecular orientation during hollow fiber spinning. The PSf possess very good chemical and thermal stability. The purpose of this article attempts to prepare PSf hollow fiber membrane by using the wet spinning process to explore the relationship between DGDE addition and pervaporation performances on aqueous ethanol solution. The relationship between the dope solution composition and the membrane morphology was investigated. Furthermore, the kinetic of membrane formation was characterized by FT-IR microscopy. Our focus is to realize the effect of DGDE content in dope solution on PSf hollow fiber membranes morphology and separation performances. Moreover, the effect of ethanol concentration and feed solution temperature on pervaporation performances was also investigated. 2. Experimental 2.1. Materials Polysulfone (Udel P-3500) used in this study was supplied from AMOCO Performance Products Inc., USA. NMP and DGDE were of reagent grade and used without further purification. Water was used as the external coagulant. NMP and water mixture solution (NMP/H2 O (7/3)) were used as bore liquid. According to the predetermined amounts of each component, PSf, NMP and DGDE were mixed in a flask under agitation to form a homogenous polymer solution and then was stored at 30 ◦ C for at least 1 day. The homogenous polymer solution was then poured into the dope tank of spinning frame and kept at 30 ◦ C overnight. 2.2. Fabrication of PSf hollow fibers PSf hollow fibers were fabricated by a wet spinning process. The spinning dope was extruded under a pressure of 0.05 MPa through a spinneret. The dimensions of this spinneret were 1.2 and 0.6 mm for outer diameter (o.d.) and inner diameter (i.d.), respectively.
H.A. Tsai et al. / Journal of Membrane Science 208 (2002) 233–245 Table 1 Process parameters and spinning conditions Process parameters/spinning conditions
Value
Spinning solution Polymer concentration (by weight) (wt.%) Solvent system
PSf/NMP:DGDE 30
Spinning temperature (◦ C) Spinneret o.d./i.d. (mm) Spinning solution pressure (MPa) Bore liquid Air gap distance (cm) External coagulant External coagulation bath temperature (◦ C) Drying procedure
NMP/DGDE: (10/0) to (6/4) 30 1.2/0.6 0.05 NMP/H2 O (7/3) 0 H2 O 30 3 days in water 2 h in methanol 1 day air dry
The bore liquid was introduced by gravity force. The degassed homogenous polymer solution and bore liquid were extruded through the spinneret die at 30 ◦ C to form a nascent hollow fiber and then entering into the coagulation bath of water. There were no external elongation stresses except gravity was applied to the nascent hollow fibers. The solidified PSf hollow fibers were stored in fresh water for at least 3 days and then in methanol for 2 h to remove the residual solvent and then air-dried for at least 24 h at room temperature. Table 1 reveals the detailed process parameters and spinning conditions. 2.3. SEM observation of hollow fibers The cross-section of PSf hollow fibers was observed with a scanning electron microscopy (Hitachi Model S4700). The fiber samples were immersed in liquid nitrogen to fracture and then sputtered with Pt. 2.4. Module fabrication and pervaporation tests The hollow fiber module for pervaporation test is consisted of five fibers. The fiber bundles were plotted in 5-min rapid solidified epoxy resin binder. The effective length of every hollow fiber for pervaporation was 10 cm. A traditional pervaporation process was used in this study [18], besides instead the membrane cell with hollow fiber module. The feed solution was pumped
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into the shell side of the module and the permeates came out from the lumen of the fibers. The permeation rate was determined by: W At where P, W, A, and t represent the permeation rate (g/m2 h), weight of permeate (g), the effective hollow fiber area base on the outside diameter (o.d.) of fiber (m2 ), and operation time (h), respectively. A vacuum pump maintained the down-stream pressure at 3–5 mmHg. The permeation rate was determined by measuring the weights of permeate. The compositions of the feed solutions and permeates were measured by gas chromatography (GC China Chromatography 8700). The separation factor was calculated from: P =
αH2 O/EtOH =
YH2 O /YEtOH XH2 O /XEtOH
where XH2 O , XEtOH are the weight fractions of water and ethanol in the feed, and YH2 O , YEtOH are the weight fractions of water and ethanol in the permeate. 2.5. Determination of the coagulation value The coagulation value was determined by a titration method at 30 ◦ C. Two grams of PSf polymer was dissolved in a 100 g pure or mixed solvent to form the polymer solution. The coagulant (water) was added to the polymer solution slowly while being stirred intensively until the homogeneous polymer solution turbid. The coagulation value was defined as that the amount of coagulant added to the polymer solution resulting in the polymer solution turbid. 2.6. Determination of the surface tension The surface tension of solution was measured by using FACE Automatic Interfacial Tensiometer (Model PD-VP, KYOWA Interface Science Co., Ltd.) with Pendant Drop Method. The solution droplet was pushed out from a tip of thin needle. Surface tension of the liquid can be detected based on the fundamental equation as below. The PD-V software can detect the required dimensions de and ds automatically and apply equation to lead surface tension: γ =
gρ(de)2 H
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where γ is the surface tension, g the gravitational constant, ρ the liquid density, 1/H the correction factor depending on the results of de/ds, de the maximum diameter of droplet, ds the diameter of droplet at the place of de value above the droplet tip. 2.7. Determination of the dope viscosity The viscosity of PSf polymer solutions were obtained at 30 ◦ C with the HAAKE Rotational Rheometer (RS-100, Germany) at the shear rate of 10 s−1 . 2.8. Mechanical properties measurement Tensile stress and elongation at break of the fiber were measured by using tensile test machine (Instron 5544 model) at a crosshead speed of 50 mm/min, with a clamp distance of 3 in. The initial Young’s modulus was calculated in the range of 0.5–1.0% tensile strain. 2.9. FT-IR microscopy analysis A drop of polymer solution was placed between two salt plate (CaF2 ) with a 15 m PTFE space. The two salt plates were clamped together by liquid cell then put it onto the microscopy stage. Nonsolvent was introduced from the empty end after the sample interface was positioned in the spectrometer. The nonsolvents entered the space between two salt plates due to capillary action then introduced phase separation. Absorption profile versus time was measured by autoimage and timebase software. FT-IR microscopy was acquired using Perkin-Elmer Spectrum One FT-IR link AutoIMAGE system microscopy, purchased from Perkin-Elmer with mercury cadmium telluride (MCT) detector. The resolution is 4 cm−1 wave number and scanning time is about 1.1 s for every spectrum.
3. Results and discussion 3.1. Effect of DGDE content on the hollow fiber membrane morphologies The composition of co-solvent (NMP/DGDE) of the dope solution is a parameter, which strongly influences the hollow fiber membrane morphologies. The
SEM pictures of the cross-section, outer edge and inner edge of PSf hollow fiber membranes are shown in Figs. 1–3, respectively. It shows that the macrovoids of the hollow fiber membrane were suppressed with increasing the DGDE content in the dope solution. Similar results were observed by the other researchers [14–16]. Hachisuka et al. [14] showed that the size and number of the macrovoids in flat membrane decreases with increasing the DGDE content in the casting solution. These phenomena might be due to the fact that the viscosity of the PSf dope solution decreases with increasing the DGDE content in the co-solvent (NMP/DGDE) system. However, Doi and Hamanaka [19] and Cabasso et al. [20,21] suggested that the decrease of the viscosity of dope solution could enhance the formation of macrovoids. In order to further investigate the above trend, the effect of co-solvent compositions on the viscosity of the PSf dope solution was shown in Fig. 4. It shows that the viscosity of PSf dope solution decreases with the DGDE content increases in the co-solvent (NMP/DGDE) system. Therefore, the viscosity of the PSf dope solution is not the major factor leading to the macrovoid formation. In addition, the cloud point curve corresponding to the position of binodal curve in a ternary phase diagram can be characterized as the thermodynamic properties of membrane formation and can be determined with simple turbidity measurements. In order to obtain the cloud point curve of various solvent composition dopes, one should prepare different composition and different PSf concentration dope to be titrated. As in the high PSf concentration combined with high content of DGDE polymer solution, the dope was slightly turbid and difficult to distinguish whether it is phase demixing. The coagulation value that presented another measurement of nonsolvent tolerance in polymer solution can be carried out since the polymer solution was used in diluted solution and easy to distinguish whether it is phase demixing. The coagulation value decreases with the DGDE content in the PSf dope solution increases, as shown in Table 2. This indicates that DGDE is a poor solvent compared with NMP for the PSf polymer. Hence, the polymer solution is phase separation easy as increasing the DGDE content in the dope solution. In general, the case of easy phase separation system results in the membrane with macrovoids structure. However, as shown in Figs. 1 and 2, it can be observed that the macrovoids was
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Fig. 1. Effect of DGDE content on the morphologies of PSf hollow fiber membranes. NMP/DGDE ratio: (A) 10/0; (B) 9/1; (C) 8/2; (D) 7/3; and (E) 6/4.
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Fig. 2. Effect of DGDE content on the outer edge layer morphologies of PSf hollow fiber membranes. NMP/DGDE ratio: (A) 10/0; (B) 9/1; (C) 8/2; (D) 7/3; (E) 6/4.
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Fig. 3. Effect of DGDE content on the inner edge layer morphologies of PSf hollow fiber membranes. NMP/DGDE ratio: (A) 10/0; (B) 9/1; (C) 8/2; (D) 7/3; (E) 6/4.
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H.A. Tsai et al. / Journal of Membrane Science 208 (2002) 233–245 Table 3 The surface tension of solution Solution NMP NMP/DGDE NMP/DGDE NMP/DGDE NMP/DGDE Water
Fig. 4. The viscosity of PSf (30 wt.%)/NMP:DGDE solution.
suppressed as the DGDE content increased (the coagulation value decreased). It means that the thermodynamics could not dominate the membrane formation in this case. Strathman et al. [17] suggested that a finger-like structure was formed when the rate of nonsolvent inflow was faster than the rate of solvent outflow. However, when the former is lower than that of the latter, a sponge-like membrane is formed. Hachisuka et al. [14–16] reported that if the NMP was dissolved in water easily, it cannot form a sharp interface between the NMP and water. Therefore, by using NMP solvent system, the porous skin layer can be formed as immersed the nascent membrane into water bath. There existed a sharp interface between the DGDE and water, since the lower miscibility between DGDE and water. This suggests that DGDE can inhibit the water entering into the nascent membrane and the dense skin layer and sponge-like sublayer were formed. The surface tension of a series of co-solvent pairs was shown Table 2 The coagulation valuea of PSf/NMP:DGDE/water system Ratio of NMP/DGDE
Coagulation value (g)
10/0 8/2 6/4 4/6 2/8 0/10
8.2 7.6 7.0 6.4 3.2 1.6
a
Measured at 30 ◦ C.
(9/1) (8/2) (7/3) (6/4)
Surface tension, γ (mN/m)
γ solution–water
41.4 38.4 37.6 36.2 34.9 72.8
31.4 34.4 35.2 36.6 37.9 –
in Table 3. It reveals that the surface tension difference between co-solvent solution and water increases with increasing DGDE content. That is, the interface between co-solvent solution and water will be formed and can reduce the solvent–nonsolvent exchange rate as increasing DGDE content in the PSf dope solution. The membrane formation situation of the outer edge layer region of the wet spun PSf hollow fiber was similar to the flat membrane. Once the nascent hollow fiber membrane was extruded from spinneret tip, it immersed into water coagulation bath immediately. Adding DGDE in PSf dope solution can retard the solvent–nonsolvent exchange rate, thus the dense skin layer was formed and macrovoids were suppressed in the outer layer region. Consequently, adding DGDE in PSf dope solution, a sharp interface between the DGDE and water was formed, resulting in the solvent–nonsolvent exchange rate decreased. It is a key point to affect the membrane morphology during the membrane formation. 3.2. Mechanical properties of PSf hollow fiber In order to investigate the relationship between the mechanical properties of PSf hollow fiber membrane and the DGDE content in the dope solution, the mechanical properties of PSf hollow fiber was measured by using tensile test machine (Instron 5544 model) at a crosshead speed of 50 mm/min with a clamp distance of 3 in., as shown in Table 4. It shows that the tensile stress at break and Young’s modulus of PSf hollow fiber membrane increased with the DGDE in the dope solution increased (e.g. 3.67 and 320.7 MPa for PSf/NMP hollow fiber membrane; 4.50 and 381.4 MPa for PSf/NMP:DGDE (9/1) hollow fiber membrane, respectively). These phenomena might be due to the fact that the macrovoids was suppressed and the dense skin layer was formed for the PSf/NMP:DGDE (9/1)
H.A. Tsai et al. / Journal of Membrane Science 208 (2002) 233–245 Table 4 Effect of DGDE content on the mechanical properties of PSf hollow fiber Ratio of NMP/DGDE
Tensile stress at break (MPa)
10/0 9/1 8/2 7/3 6/4
3.67 4.50 4.69 4.70 5.53
± ± ± ± ±
0.26 0.32 0.21 0.40 0.48
Elongation at break (%) 97.7 103.8 107.9 107.5 117.4
± ± ± ± ±
4.6 4.6 4.8 7.4 7.8
Young’s modulus (MPa) 320.7 381.4 396.3 414.6 430.0
± ± ± ± ±
49.4 36.3 32.7 26.0 17.4
hollow fiber membrane. Thus, the tensile stress and Young’s modulus increased as adding DGDE in dope solution. Furthermore, the tensile stress and Young’s modulus increased with increasing DGDE content. It might be due to the fact that the dense skin layer thickness increased with increasing DGDE content. The increasing in elongation at break might be due to the fact that the macrovoids structure was suppressed to sponge-like structure as adding DGDE in dope solution, resulting in the elongation of membrane increased. 3.3. FT-IR microscopy analysis Strathman et al. [17] has developed a penetration technical to observe the membrane formation process. It can only observe the coagulant inflow the polymer solution by using optical microscopy, but the outflow of solvent cannot be observed. Fourier transform infrared (FT-IR) spectroscopy with the microscopy is a well-established technique in diffusion studies. It can measure the composition change during membrane formation. The absorbance for each component can be extracted at different times. On the base of Beer’s law state that absorbance of a band is directly related to its concentration in the sample and absorbance profiles are equivalent to concentration profiles for each component [22]. Therefore, the absorbance profiles trend is as well as concentration profile. It is difficult to introduce water into the liquid cell by syringe due to capillary action of water. Hence, ethanol was utilized as the coagulant of the PSf polymer solution to analyze FT-IR microscopy measurement (see Figs. 5 and 6). The assignments were summarized as follows: the band at 1199.3 cm−1 was the C–O asymmetric stretching vibration for
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DGDE, the band at 1401.7 cm−1 for NMP (aliphatic amide) and 1046.9 cm−1 for ethanol (saturated primary alcohol C–O stretching vibration) were chosen for quantitative analysis for each substance. Fig. 5 shows that the coagulation medium absorbance increases with increasing the time of the dope solution contact with coagulant of PSf/NMP, PSf/DGDE and PSf/DGDE:NMP (6/4) systems. Compared with the PSf/NMP system, a lower coagulation medium absorbance in the PSf/DGDE and PSf/DGDE:NMP (6/4) systems were obtained. It means that the concentration of ethanol diffused into DGDE content PSf polymer solution is lower than that of the NMP system i.e. the DGDE can form a barrier to inhibit coagulation medium diffuse into the polymer solution. Furthermore, the data in Fig. 6 reveals that the DGDE absorbance in the PSf/DGDE system decreased faster than that of the NMP in the PSf/NMP system. Similar trends were observed in the co-solvent (NMP/DGDE) system. This suggests that the outflow rate of DGDE was faster than that of the NMP during the membrane formation process caused the polymer concentration at the interface of nascent membrane and coagulant raised fast. Thus, the skin layer was formed and inhibited the coagulant inflow. Consequently, the addition of DGDE decreases the exchange rate of solvent and nonsolvent, resulting in the suppression of the macrovoids formation. 3.4. Effect of DGDE additive on the pervaporation performance The effect of DGDE additive on the pervaporation performances of a 90 wt.% aqueous ethanol solution through the DGDE added hollow fiber membrane is shown in Fig. 7. The permeation rate decreases dramatically and the water-permselectivity increases with increasing the ratio of NMP/DGDE in the dope solution. These phenomena might be due to the fact that the macrovoids in the PSf hollow fiber was suppressed by introducing DGDE in dope solution (see Figs. 1–3). Thus, the permeation rate decreases dramatically as adding DGDE in the dope solution. Moreover, the dense skin layer of PSf hollow fiber was formed as adding DGDE in the dope solution, resulting in the separation factor increased. The product of total permeation rate and separation factor has been defined as the permeation separation
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Fig. 5. Time dependent of the normalized absorbance at 1046.9 cm−1 (EtOH), analyzed by FT-IR microscopy. (䉫) PSf/NMP, () PSf/DGDE, (䊊) PSf/DGDE:NMP (6/4).
index (PSI), which is a measure of the separation ability of the membrane. The optimum pervaporation results were obtained by the hollow fiber membrane prepared with NMP/DGDE (8/2) solvent systems, giving a permeation rate of 172.7 g/m2 h, separation factor 23.9 and 4.1 × 103 PSI value. 3.5. Effect of feed composition on the pervaporation performance Since the NMP/DGDE (8/2) solvent ratio is the optimum content of DGDE in this system. The effect of feed composition on the pervaporation performances of the PSf/NMP:DGDE (8/2) hollow fiber at 25 ◦ C are shown in Fig. 8. It shows that an increase in the feed ethanol concentration results in an increase in the permeation rate. However, a minimum separation factor obtained at 50 wt.% feed ethanol concentration. These phenomena might be due to the
fact that the PSf hollow fibers were easily swollen at the high ethanol concentration. When the ethanol concentration in the feed is higher, the amorphous regions of the membrane are more swollen. Hence, the polymer chain becomes more flexible, thus decreasing the energy required for diffusive transport through the membrane, resulting in permeation rate increases and separation factor decreases as increasing the ethanol concentration in the feed solution (feed ethanol concentration < 50 wt.%). Moreover, the water molecules can easily diffuse through the PSf hollow fiber matrix than ethanol because the interaction between the water and the PSf hollow fiber membranes is very weak and the molar volume of water is small. Once the water molecules are incorporated into the PSf hollow fiber membranes, they can easily diffuse through the PSf hollow fiber membranes, especially in the case of higher swollen. Thus, the separation factor increases with the ethanol
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Fig. 6. Time dependent of the normalized absorbance at 1401.7 cm−1 (NMP) and absorbance at 1199.3 cm−1 (DGDE), analyzed by FT-IR microscopy. (䉫) PSf/NMP, () PSf/DGDE, (䊊) PSf/DGDE:NMP (6/4) for NMP, (䊉) DGDE/NMP (6/4) for DGDE.
Fig. 7. Effect of DGDE content on pervaporation performances.
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Fig. 8. Effect of feed composition on pervaporation performances.
concentration in the higher concentrated feed solution (feed ethanol concentration > 50 wt.%). 3.6. Effect of feed solution temperature on pervaporation performance The effect of feed solution temperature on the pervaporation performance of 90 wt.% aqueous ethanol solution through the PSf (30 wt.%)/NMP:DGDE (8/2) hollow fiber membrane is shown in Fig. 9. It shows that the permeation rate increases and the separation factor decreases with the feed solution temperature in-
creases. These phenomena might be due to the fact that an increase in the swelling of the PSF hollow fiber membrane matrix at higher temperature results in an increase in the free volume, frequency and amplitude of the PSf polymer chain motions. Thus, the permeation of the permeating molecules and the associated molecules through the PSf hollow fiber membrane become easier, resulting in an increase of the total permeation rate. Additionally, the increase of swelling of the hollow fiber matrix at higher temperature results in an increase of the transport of ethanol molecules along with water, thereby reduces the separation factor.
Fig. 9. Effect of temperature on the pervaporation performances.
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4. Conclusion In this study, the PSf hollow fiber membrane was prepared by using the DGDE/NMP co-solvent system. The effects of co-solvent on the morphology, mechanical property and separation performance were investigated. The addition of DGDE decreases the exchange rate of solvent and nonsolvent, resulting in the suppression of the macrovoids formation. The tensile stress at break and Young’s modulus of the PSf hollow fiber membrane increases with the DGDE in the dope solution increases. Adding DGDE in PSf dope solution, a sharp interface between the DGDE and water was formed, resulting in the solvent–nonsolvent exchange rate decreased. It is a key point to affect the membrane morphology during the membrane formation. The permeation rate and separation factor towards water for a 90 wt.% aqueous ethanol solution through the asymmetric PSf hollow fiber membrane with a NMP/DGDE (8/2) ratio of co-solvent system were 172.7 g/m2 h and 23.9, respectively.
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