ethanol mixture through lithiated polysulfone membrane

ethanol mixture through lithiated polysulfone membrane

Journal of Membrane Science 193 (2001) 59–67 Pervaporation separation water/ethanol mixture through lithiated polysulfone membrane Shih-Hsiung Chen∗ ...

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Journal of Membrane Science 193 (2001) 59–67

Pervaporation separation water/ethanol mixture through lithiated polysulfone membrane Shih-Hsiung Chen∗ , Rey-May Liou, Ching-Shan Hsu, Dong-Jong Chang, Kuang-Chang Yu, Chia-Yuan Chang Department of Environmental Engineering and Health, Chia-Nan University of Pharmacy and Science, Tainan 717, Taiwan ROC Received 19 December 2000; received in revised form 23 April 2001; accepted 30 April 2001

Abstract For dehydrating water/ethanol mixture by pervaporation, a lithiated polysulfone membrane was prepared. The separation performance of water and ethanol strongly depend on the degree of lithiation of polysulfone (PSF) membrane. The water permeation rate decreased and separation factor increased with increasing the degree of lithiation of polysulfone membrane upto 0.75. Beyond the degree of substitution 0.75, the permeation rate increased and separation factor decreased with increasing the substitution. The effect of lithiation on separation performance was due to the improvement of diffusion selectivity lithiated membrane. It was found that the diffusion selectivity was the dominant contribution to overall permselectivity. The diffusion difference between permeates through lithiated membrane was the dominant factor for separating water/ethanol mixture. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Pervaporation; Lithiation; Polysulfone; Water/ethanol separation

1. Introduction Mostly water-permselective pervaporation membranes focus on separation based on solubility selectivity rather than mobility selectivity [1–5]. Generally, high water-permselective membranes can be achieved by increasing either the sorption ratio of water to ethanol or the diffusion ratio of water to ethanol. For better sorption ratio, a hydrophilic moiety was introduced into the polymer chain to enhance the water solubility, although it was always accompanied by excessive swelling due to the hydration of hydrophilic group in membrane [6,7]. For better diffusion ratio, the hydrophobic moiety introduced into polymer ∗

Corresponding author. Tel.: +886-6-2660028; fax: +886-6-2667323. E-mail address: [email protected] (S.-H. Chen).

chain to improve the diffusion difference between water and ethanol. However, the hydrophobic moiety introduced into polymer chain always results in a low permeation rate of modified membranes. Thus, it has been concluded that a balance of hydrophilic to hydrophobic moiety area in membranes must be incorporated into the membrane [8]. Hydrophilic polymers such as PAA, PVA and chitosan [9–12], were first considered for the preparation of pervaporation membranes. However, it was found that the rich hydrophilic moiety in the polymer chain induced excessive swelling during the pervaporation process, because the greater free volume lead to decreased selectivity of the pervaporation membrane. Therefore, to avoid the excessive swelling, a modified hydrophilic membrane was developed using, polymer crosslinking and blending to modify the hydrophilic material [13–18]. However, this modified technology

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not only reduced the excessive swelling of the hydrophilic membrane but also reduced the water permeation rate because of the decrease in free volume of the membrane. It was usually found that the enhancement of separation performance of modified hydrophilic membrane actually did not increase the sorption ratio of water to ethanol. On the contrary, introducing a hydrophilic moiety into the polymer matrix mostly lead to an increase in diffusion selectivity of water to ethanol when the membrane was under the optimum swelling. Therefore, it can be seen that diffusion selectivity usually plays an important role in the transport mechanism of modified pervaporation membranes. The purpose of this study was to prepare lithiated polysulfone membrane to separate water from water/ethanol mixture. In order to understand the influence of lithiation on the packing density of polysulfone (PSF) membrane, we measured the degrees of swelling with different degrees of lithiation and analyzed sorption content in the modified membrane. The permeation rate and degree of swelling was measured independently. Glass transition temperature (Tg ) measurements analyses were used characterize the changes in the modified membranes. In addition, the relationship between microstructure change of lithiated membranes and pervaporation properties were discussed.

and immersed into distilled water for 12 h. The wet membrane was put into a vacuum oven for 24 h before sorption and pervaporation measurement. 2.3. Pervaporation experiment A preparation process as described in a previous report [23] was used. To avoid the time dependence on permeation properties of lithiated membrane, all of lithiated membranes were swelled in feed solution for 12 h before permeation and sorption measurements. It had been studied that the polymer chain of pervaporation membrane was sufficiently swelled in feed solution by above treatment before pervaporation tests. In pervaporation, the feed solution was in direct contact with the membrane and kept at 25◦ C; the effective membrane area was 10.2 cm2 and the permeation rate was determined by measuring the weight of permeate. The compositions of the feed solution, permeates, solution adsorption in the membranes were measured by gas chromatography (GC China Chromatography). The separation factor, ␣A/B , was calculated from αA/B =

YA /YB XA /XB

Udel® Polysulfone P-3500 was obtained from Amoco Performance Products. Merck Chemical Co. supplied chloride sulfonic acid and ethanol.

where XA , XB and YA , YB are the weight fractions of A and B in the feed and permeate, respectively (A being the faster permeating species). The permeation properties was showed as a unit of permeation rate (g/cm2 -h) which was usually used in pervaporation papers. For comparison, the separation performance with different materials, the PSI value of pervaporation membrane was used. The PSI value was calculated by the product of permeation rate and separation factor.

2.2. Membrane preparation

2.4. Contact angle measurements

The pure polysulfone membrane was prepared from a casting solution of polysulfone in chloroform. The lithiated polysulfone membranes were prepared from a casting solution of direct lithiation method [20–22] by adding appreciable amounts of n-butyl lithium in tetrahydrofuran at 0◦ C. The polymer solution was kept at room temperature and it was cast onto a glass plate to a predetermined thickness of 350 ␮m using a Gardner Knife at room temperature. The membrane was dried at room temperature for 30 min, then, peeled off

The contact angle of water was measured with a FACE contact angle meter CA-D type (Kyowa Interface Science Co. Ltd.).

2. Experimental 2.1. Materials

2.5. Swelling measurement The degree of swelling of lithiated membranes were determined in distilled water and in aqueous ethanol solution at 25◦ C. The weight of dry membrane (Wdry ) was first determined. After equilibrium with

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water or ethanol solution, the fully swollen membrane was wiped with tissue paper and weighed. Since the ethanol evaporated very fast, it is difficult to read the real weight directly. The weight of the membrane was measured every 5 s and plotted as a function of time for 30 s after it wiping dry. The weight at zero time could be extrapolated and was taken as swollen weight (Wwet ) of the membrane. The degree of swelling was calculated by following equation: Degree of swelling (%) =

Wwet − Wdry × 100 Wdry

2.6. Sorption measurements The membrane was immersed in the ethanol/water mixture for 24 h at 25◦ C, then blotted between tissue paper to remove excess solvent and placed in the left half of a twin tube set-up. The system was evacuated to 10−3 Torr after immersing the left tube in liquid for 30 min. Then, the tube was heated in hot water for 30 min and the right tube was cooled in liquid nitrogen. The composition of the condensed liquid in the right tube was determined by GC. The separation factor of sorption was calculated by sorption selectivity =

Yw /Ye Xw /Xe

where, Xe , Xw and Ye , Y w are the weight fraction of ethanol and water in the feed and membranes, respectively.

3. Results and discussion 3.1. Degree of substitution and physical property of lithiated membrane The lithiated polysulfone membrane was prepared by lithiation as reported previously [20–22], and the degree of lithiation was determined by elementary analysis. Fig. 1 showed the effect of the mole ratio of n-butyl lithium to polysulfone repeat unit on the degree of lithiation substitution on polysulfone repeat unit. It can be seen that the degree of substitution increased with the increase of the mole ratio. This result indicates that there is more than one lithium substituted into polysulfone unit, this is similar to the

Fig. 1. Effect of the mole ratio of n-butyl lithium to polysulfone repeats unit on degree of lithiation substitution on polysulfone repeats unit.

report by Guiver and co-workers [19] and Guiver and Apsimon [21].The stability of lithiated membranes was identified by ICP-AES (Inductively Coupling Plasma-Atomic Emission Spectrometer) measurements. It was indicated that the lithium content in the membranes before pervaporation test was almost the same as the content after pervaporation test. The ICP-AES measurements strongly supported that the lithium content in modified membrane did not change, no matter what the lithiated membrane had been — pervaporation tested or not. It was indicated that the pervaporation performance may be affected by lithium content in lithiated membranes. To explain the effect of packing density on the pervaporation performance of lithiation membranes, the effect of degree of substitution of lithiated membranes on glass transition temperature (Tg ) is shown in Fig. 2. It can be seen that the glass transition temperature of lithiated membrane slightly decreased with increasing degree of substitution, indicating that flexible polymer chain was expected for the lithiated membrane. Generally, it is expected that flexible polymer chain for a lithiated membrane lead to increase in permeation flux and a decrease in diffusion selectivity. On the other hand, the change of polymer packing in ethanol solution was also discussed to distinguish the dominant factor in transport in lithiated membranes.

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Fig. 2. Effect of degree of substitution of lithiated membranes on glass transition temperature.

Fig. 3 shows the effect of degree of lithiation on the degree of swelling for a 90 wt.% ethanol solution. It can be seen that the degree of swelling slightly increased with an increase in the degree of lithiation. The degree of swelling increased with increasing the degree of substitution. It was indicated that the

Fig. 3. Effect of degree of substitution of lithiated membranes on degree of swelling at 90 wt.% ethanol solution.

Fig. 4. Effect of degree of substitution on water contact angles of the lithiated membranes.

flexibility of polymer chain in the lithiated membrane would be increased in the lithiation in 90 wt.% ethanol solution. On the other hand, the result of Tg measurement indicated concluded that lithiation of polysulfone had a flexible structure. However, the lithiated substitution on polysulfone should be due to matrix and membrane with poor hydrophilicity generally own a lower degree of swelling. Hydrophobic matrices have lower swelling for polar components. However, the swelling test of the lithiated membrane showed the contrary result, as shown in Fig. 3, possibly because some of the lithiated substitution of polysulfone carboxylated with CO2 before the properties was measured. The carboxylation of polysulfone was easily performed with CO2 , as reported by Breitbach et al. [20]. Therefore, the degree of swelling-increase may be because of the carboxylation enhanced hydrophilicity of modified polysulfone. In order to determine the hydrophilic change, contact angle measurements were made. Fig. 4 shows the water contact angles of the lithiated membranes at room temperature. The contact angle decreased with increasing the degree of substitution of lithium in modified membranes. This decrease in water contact angle of modified membranes implies that the polar surface was obtained by increasing the hydrophilicity. The increase of hydrophilicity gives evidence that the

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carboxylation possibly existed during the period of membrane formation because the lithium substitution on polysulfone was not a hydrophilic group. 3.2. Effect of substitution degree on pervaporation properties The pervaporation performance of lithiated membranes for the feed 90 wt.% ethanol solutions was shown in Fig. 5 as a function of degree of lithiation. It can be seen that the water flux decreased with increasing the degree of lithiation upto 0.75 and then, increased with further increase of the degree of lithiation. The separation factor increased with the degree of lithiation upto 0.75 and then decreased. The pervaporation separation index (PSI) is the product of the total permeation rate and separation factor, and it is a good index for determining the performance of a pervaporation membrane. Fig. 6 shows the relationship of PSI of lithiated membranes by degree of lithiation. It can be seen that the PSI value increased as the degree of substitution increases upto 0.75 and then decreased. Thus, it is proposed that the higher substitution degree leads to a poor membrane formation, and therefore the PSI value decreased with increasing degree lithiation, when the degree of substitution was larger than 0.75. The poor membrane formation would lead to some defects in the membrane and it would lose the selectivity of water to ethanol and, also lowering the PSI value

Fig. 6. The relationship between degree of lithiation substitution and PSI value of lithiated membranes.

of the modified membrane. Fig. 5 shows the effect of lithiation substitution on permeation rate of lithiated membrane, indicating that the permeation rates were almost in the range of 350–550 g/m2 -h. The PSI values were contributed by both permeation rate and separation factor, and the small difference in the permeation rate indicates that the change of PSI value was due to the separation difference from modified membranes. To further distinguish which reason caused the change of PSI, the sorption and diffusion properties should be discussed relative to the degree of lithiation of modified membranes. The diffusion selectivity (Sd ) can be defined as the ratio of permeation selectivity (Sp ) and sorption selectivity (Ss ) S p = Sd S s

Fig. 5. Pervaporation performance of lithiated membranes for the feed 90 wt.% ethanol solution.

Fig. 7 shows the relationship between sorption selectivity and degree of substitution, and Fig. 8 shows the relationship between diffusion selectivity and degree of substitution. It can be seen that the sorption selectivity decreased with increasing degree of substitution. However, the separation factor first increased with the increasing degree of lithium substitution upto 0.75, and then decreased, indicating that the dominant influence on separation factor was diffusion selectivity. It also can be seem that the diffusion selectivity

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Fig. 7. The relationship between sorption selectivity and degree of substitution of lithiated membranes.

decreased when the degree of substitution was higher than 0.75, implying that a looser packing density of lithiated membrane was formed. This indicates that a poorer membrane formation was formed when the degree of lithium substitution higher than 0.75.

Fig. 8. The relationship between diffusion selectivity and degree of substitution of lithiated membranes.

Fig. 9. Effect of ethanol composition in feed on permeation flux and separation factor of lithiated membranes with degree substitution of 0.75.

3.3. Effect of feed concentration on pervaporation properties The effect of ethanol composition in feed on permeation flux and separation factor of lithiated membranes with degree substitution of 0.75 is shown in Fig. 9. It can be seen that the permeation flux increased with increasing the ethanol composition in feed, although the separation factor also increased with ethanol composition in feed. Generally, the increases of permeation rate were usually accompanied by a low separation factor because of the looser membrane structure when the permeant swelled the polymer membrane. Thus, we propose that some polar–polar interactive force between ethanol and the membrane should affect the transport mechanism of permeants in the permeant transport through the membrane. Because the separation factor was not dependent only on the sorption selectivity or the diffusion selectivity, we propose that either sorption selectivity or diffusion selectivity would be the dominant factor for the increase of separation factor with increasing the ethanol composition in feed. To further distinguish the transport mechanism, the sorption and swelling test with different ethanol compositions in the feed were made. Fig. 10 shows the effect of ethanol composition in feed on the degree of swelling of lithiated membrane as the ethanol

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Fig. 10. Effect of ethanol composition in feed on the degree of swelling of lithiated membrane as various ethanol compositions in feed.

composition in feed increases. Using a lithiated membrane with degree of substitution of 0.75, it can be seen that the degree of swelling increased as the ethanol composition in feed increased. The good swelling properties also implied that the membrane structure would be looser due to the swelling of polymer chain in higher ethanol concentration. A lower packing density of membrane usually leads to decrease diffusion selectivity in pervaporation process. Fig. 11 shows the relationship between ethanol concentration in feed and sorption selectivity of lithiated membrane with degree of substitution 0.75, indicating that the sorption selectivity increased with increasing ethanol concentration in the feed solution. Therefore, we propose that the increase of separation factor be affected by the increment of sorption selectivity. Obviously, Fig. 12 shows the effect of ethanol composition on diffusion selectivity of lithiated membrane with degree of substitution 0.75. It can be seen that the diffusion selectivity decreased with increasing the ethanol composition in feed. It was respected that high degree of swelling introduced flexible polymer chain in lithiated membranes in higher ethanol concentration. Both of water and ethanol easy transport through membrane. The swelling effect on membrane structure on separation performance of the lithiated membrane was strongly affected by ethanol concentration.

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Fig. 11. Relationship between ethanol concentration in feed and sorption selectivity of lithiated membrane with degree of substitution 0.75.

Fig. 12. Effect of ethanol composition in feed on diffusion selectivity of lithiated membrane with degree of substitution 0.75.

3.4. Effect of operating temperature on pervaporation properties Fig. 13 shows the effect of feed temperature on permeation flux and separation factor of lithiated

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4. Conclusion Lithiation successfully improved the pervaporation performance of polysulfone membranes and the transport mechanism of the lithiated membrane may behave as diffusion control. It was also indicated that the separation performance of lithiated membrane was strongly affected by ethanol concentration, and that diffusion selectivity decreased and sorption selectivity increased with increasing ethanol composition in feed. The permeation flux is strongly dependent on the operating temperature and, the separation factor decreased with increasing the operating temperature. References Fig. 13. Effect of feed temperature on permeation flux and separation factor of lithiated membrane with 90 wt.% ethanol in feed.

membrane with 90 wt.% ethanol in feed solution. It can be seen that the permeation flux increased with increasing feed temperature, although the separation factor decreased with increasing the operating temperature. It is well known that the decrease of separation factor was due to the increase of molecular motion with increasing feed temperature. The increase in permeation flux with operating temperature in feed was due to the increase in membrane swelling. Generally, the amount of gas sorption in solid membrane typically decreased with increasing temperature. However, the feed solution contact with lithiated membrane in pervaporation operation and the feed solution was a liquid form. The feed solution was sorbed as liquid form into membrane and then vaporized from the down stream of lithiated membrane. Generally, the amount of sorption was determined by the solubility of lithiated membrane. The amount of liquid sorbed into solid membrane typically increased with increasing temperature. On the other hand, the chain mobility of polymer m76bn membrane also increased with increasing operating temperature. A swelling membrane could be formed by increasing the chain mobility and permeate sorption in membrane. Therefore, the increase in permeation flux with increased temperature is due to the swelling. The low packing density of lithium membrane indicated that the increment of ethanol flux was relatively higher than that of water flux.

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