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Preparation and applying the membranes with carbon black to pervaporation of toluene from the diluted aqueous solutions
Separation and Purification Technology 57 (2007) 507–512
Preparation and applying the membranes with carbon black to pervaporation of toluene from the diluted aqueous solutions Dorota Panek ∗ , Krystyna Konieczny 1 Silesian University of Technology, Faculty of Energy and Environmental Engineering, Institute of Water and Wastewater Engineering, ul. Konarskiego 18, 44-100 Gliwice, Poland
Abstract The application of pervaporation (PV) to the removal of volatile organic (VOC) from the aqueous solutions has become very interesting in the last few years. It is caused by the increasing level of the compounds such as petrochemical solvents (benzene, toluene, and xylenes) or chlorinated solvents (trichloroethylene or tetrachloroethylene) in the natural environment. The aim of this work is to apply the membranes with carbon black as a filler to pervaporation (PV) of toluene from the diluted aqueous solution. In the research polydimethylsiloxane (PDMS) composite membranes, PDMS and PEBA (polyether-block-polyamide) membranes filled with carbon black were used. The membranes were prepared in a laboratory scale especially for this purpose. Both PEBA and PDMS membranes show excellent properties to separate toluene from the water, however, toluene solubility in the PDMS membrane was better, it could be observed that the removal of this compound from the feed was better when this membrane was used. In consequence the process selectivity increases and better concentration of toluene on the permeate side can be obtained. The influence of the membrane filler on the total flux, toluene flux and process selectivity was also determined. In some cases total flux as well as toluene flux decrease for the membranes with the filler comparing to those without the filler. © 2006 Elsevier B.V. All rights reserved. Keywords: Pervaporation; Toluene; Polydimethylosiloxane membrane; Polyether-b-polyamide membrane; Filler
1. Introduction In the last few years pervaporation has become an intensive developing separation technique [1–3]. Referred to as a so-called “clean technology” it is now a promising separation method for azeotropes, close boiling point substances, isomers or compounds sensitive to high temperatures. The separation of the mixtures using PV can be divided into three fields [2]:
• flux density: JV =
Vr Sm t
(1)
• separation factor: α=
Ya/Yb Xa/Xb
(2)
• enrichment factor: • dehydration of aqueous–organic mixtures; • removal of trace volatile organic compounds (VOC) from aqueous solution; • separation of organic–organic solvent mixtures. The basic factors which describe the effectiveness of the process are (Eqs. (1)–(3)):
∗ 1
Corresponding author. Tel.: +48 32 237 29 81. E-mail address: [email protected] (D. Panek). Tel.: +48 32 237 29 81.
1383-5866/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2006.10.011
β=
Ya Xa
(3)
where Vr is the amount of the permeate, Sm the membrane area unit, t the time unit, a the component of the mixture preferentially transported through the membrane, b the second component of the mixture, X the concentration of the component a or b in the feed, and Y is the concentration of the component a or b in the permeate. The application of PV to the VOC removal from the waters has become very interesting. It is the result of an increasing level of these substances in the natural environment. Man-made VOCs are generated by several sources such as municipal wastes,
508
D. Panek, K. Konieczny / Separation and Purification Technology 57 (2007) 507–512
traffic or industrial and agricultural activity [1]. Very dangerous is the presence of an increasing amount of petrochemical solvents, especially benzene, toluene, xylenes or chlorinated solvents such as trichloroethylene or tetrachloroethylene. However, the concentration of these agents is too low to remove them economically with conventional ways. Not long ago air stripping and/or activated carbon treatments were employed for this task, however, the former is susceptible to fouling—it merely turns a water pollution problem into an air pollution issue, while the latter needs costly regeneration steps and may not be suitable for VOCs that are easily displaced by other organic compounds [1]. As compared to these methods, pervaporation does not emit additional pollution, as well as does not need expensive regeneration steps. Additionally, concentrated permeate can be used again [4–6]. To achieve high selectivity or high flux for specific compounds the modifications of membranes such as filling, grafting or coating have been performed [1]. The most common is adding fillers, especially high aspect ratio fillers. It improves physical properties such as increased stiffness or reduced creep and a variety of other purposes: improves thermal stability, high voltage resistance, electrical conductivity, radiation shielding and optical and aesthetic effects [7]. In many cases, filled polymeric membranes show permeabilities much lower than the conventional unfilled membranes, and hence can serve as barriers for oxygen, water and other solutes [7]. In the area of organophilic pervaporation membranes mostly tree types of fillers have been used: zeolite [8–11], silicate [8,12] and carbon sieve [13–16]. The aim of this work was to apply dense polymer membranes: polydimethylsiloxane (PDMS) composite membranes, PDMS and PEBA (polyether-block-polyamide) membranes filled with carbon black for the removal of volatile organic compounds (VOC). Such membranes were prepared in the laboratory scale especially for this purpose. Toluene, as the representative of VOC was chosen. 2. Experimental 2.1. Pervaporation experimental setup The experimental set-up, used for the pervaporation measurements is shown schematically in Fig. 1.
Fig. 1. Toluene pervaporation process set-up: (1) feed tank; (2) gear pump; (3) pervaporation cell; (4) vacuum sensor; (5) cooling trap; (6) cooling trap with nitrogen; (7) vacuum pump.
A module of the membrane area of 100 cm2 was used. The feed tank of the capacity of 2 dm3 was filled with water solution of toluene. The toluene concentration was about 200 ppm. The temperature of the feed was kept at the temperature of 25 ◦ C by the thermostat. The retentate was being recycled to the feed tank and the permeate was being collected in the cooling traps. The permeate was cooled down by liquid nitrogen (−196 ◦ C). The pressure on the permeate side was about 1 mbar. It was generated with a vacuum pump and was controlled by a digital sensor. Before the pump an additional cooling trap with liquid nitrogen was installed to prevent diffusion of the oil from the pump to the permeate and vice versa. The overall flow above the membrane was constant and amounted to 1770 cm3 /min. Each experiment lasted 5 h. Every hour a sample was taken from the feed tank and the concentration of the toluene content was determined by UV analysis (λ = 261, 2 nm). Because of a very small amount of the permeate, in case of PDMS and PEBA membranes without the support one cooling trap during whole experiment was applied. In case of composite PDMS membrane it was exchanged together with taking of the sample. The concentration of toluene in the permeate, followed by the sample dilution with water, was determined in the way similar to the feed samples. The permeate mass was determined before the dilution using a balance. Each experiment was repeated five times and the results were averaged to minimize the errors. 2.2. Preparation of the membranes To determine the influence of the filling on the course of the process we applied membranes made solely from polymer film, and membranes with the filling. Membranes without support as well as composite membranes were also tested. Almost all of the investigated membranes were prepared by the authors in a laboratory scale, with the exception of the composite membrane PDMS which was made and offered for investigation by GKSS/Forschungszentrum Geesthacht, Germany. The quantity of carbon black was the same for all of the membranes and amounted to 15 wt.%. 2.2.1. Preparation of the PDMS membrane without filling and without support The first stage of PDMS membrane casting consisted in the preparation of polymer solution (PDMS Wacker Silicon Dehesive 940) together with cross-linking agent (V24 manufactured by the firm Wacker) in the solvent (isooctane). The solution was subjected to homogenisation using a magnetic stirrer for about 12 h. After that time, cross-linking catalyst (complex of platinum and 1,1,3,3-tetramethyle-1,3-divinyledisiloxane) in isooctane was added to the mixture. The whole was further stirred for about 30 min. The mixture prepared in that way was deposited onto a Teflon plate and left for about 12 h to let the solvent evaporate completely. Due to low resistance of PDMS material and possible damage to the membrane during the testing, a 400 m thick membrane was prepared. The measurement was effected using a micrometer screw and it is an average of 20 points from the membrane area (100 cm2 ).
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2.2.2. Preparation of PDMS membrane with carbon filler and without support As in the case of membrane without filling the process started with the preparation of polymer solution (PDMS Wacker Silicon Dehesive 940) together with the cross-linking agent (V24 manufactured by Wacker) in isooctane as solvent. The solution was placed on a magnetic stirrer and was being stirred until homogeneous solution was obtained. At the same time the mixture of carbon in isooctane was prepared. The prepared polymer solution was added to the carbon slurry and the whole was mixed for further 12 h. After that time, the cross-linking catalyst (platinum complex with 1,1,3,3-tetramethyle-1,3-divinyledisiloxane) was being added to the mixture. The whole was further mixed for about 2 h. Than the mixture was poured onto a glass plate covered with Teflon and left for about 12 h until the solvent evaporated completely. The prepared membrane was 266 m thick (the measurement was carried out with a micrometer screw and it is an average of 20 points from the membrane area (100 cm2 )). 2.2.3. Preparation of composite PDMS membrane with carbon filler The preparation process of polymer mixture with carbon black was carried out in the same way as in the case of membrane without support (Section 2.2.2). The membrane was cast using the immersion method by depositing the film of formerly prepared mixture onto the support made up by polyacrylonitryle (PAN). The thickness of that membrane was determined at GKSS, by method measuring the flux of gases permeating through it: oxygen and nitrogen, and it was calculated from the empirically determined dependencies. It equaled 3 m and was an average of 20 points from the membrane area (100 cm2 ). 2.2.4. Preparation of PEBA membrane without the filler and without support The preparation of the membrane started with the preparation of polymer solution (PEBAX 4033) in the mixture of solvents of 1-butanol and t-butanol in mass ratio 1:2.5. The solution was heated under the reflux condenser until the boiling point (ca. 120 ◦ C) and maintained at this temperature until total dissolution of polymer. After that time the temperature was reduced to 95 ◦ C and heating was continued for another 1 h. The homogeneous mixture was deposited onto a formerly prepared glass plate heated to 60 ◦ C. After 1 h, the heating was switched off and the membrane was left in such conditions for about 12 h. The thickness of membrane prepared in such a way and measured with a micrometer screw was 75 m (an average of 20 points from the membrane area (100 cm2 )). 2.2.5. Preparation of PEBA membrane with carbon filler without support The polymer solution in solvents was prepared in the same way as in Section 2.2.4. At the same time the mixture of carbon black in 1-butanol was prepared. The mixture was placed on a magnetic stirrer for about 1 h and then in an ultrasonic bath where stirring was continued for another 30 min. As soon as both solutions were ready the carbon mixture was slowly poured into the polymer solution and with T = 95 ◦ C being maintained,
509
continuing the stirring for about 30 min. Then the casting of membrane was taking place onto the formerly prepared glass plate heated to 60 ◦ C. After 1 h the heating of the glass was switched off and the membrane was left in such conditions for about 12 h. The thickness of membrane prepared in this way and measured with a micrometer screw was 75 m (an average of 20 points from the membrane area (100 cm2 )). 3. Results and discussion 3.1. Discussion of results involving the influence of the filler on the course of toluene pervaporation for PEBA membrane The first membranes subjected to testing were polyether-bpolyamide membranes with carbon filler and without the filler. They were characterized by the same thickness, and therefore they could be directly compared. The results of the comparison are presented in Figs. 2 and 3. The presented in Fig. 2 change of toluene concentration in the feed versus time for the membrane with filler and without filler indicates that the filling of membrane plays an insignificant role regarding the rate of toluene removal. For both membranes the run of curves is very similar, and the obtained, final removal ratio is comparable. A distinctive difference can be observed when comparing the permeate flux, toluene flux, selectivity and enrichment factor obtained for these membranes. They were presented in Fig. 3(a)–(d). The total permeate flux for the membrane with the filler is higher by 50% as compared to the membrane made of the polymer alone. The toluene flux changes in the analogous way: from the value 4.61 g/m2 h for the membrane without the filler to 5.64 g/m2 h for the membrane with carbon, so the increase of toluene pervaporation flux is by about 20%. Hence, the increase of permeate flux is principally made up by the increase of pervaporating water, which results in the drop of both separation index and enrichment factor by 25% and 20%, respectively. The phenomenon involving the increase of both permeate flux and toluene flux for the membrane with the filler is unexpected. It could have been the result of the PEBA structure and of the preparation process of the membrane. The grains of carbon black may have been sealed insufficiently by the polymer during the
Fig. 2. Changes of toluene concentration in the feed as time function for PEBA membranes with the filler and without the filler of the thickness of 75 m.
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Fig. 3. Comparison of (a) permeate flux, (b) toluene flux, (c) selectivity and (d) enrichment factor for PEBA membranes with the filler and without the filler of the thickness of 75 m.
formation of membrane and as a result free spaces were formed, whereby the permeation of both water and toluene was faster. The application of carbon black of smaller granulation as the filler might have improved the properties of this membrane. 3.2. Discussion of results involving the influence of the filler on the course of toluene pervaporation for composite PDMS membrane
Fig. 4. Changes of toluene concentration as time function, for composite PDMS membranes with the filler and without the filler of the thickness 3 m.
The next membranes subjected to testing were composite polydimethylsiloxane membranes. Their thickness was about 3 m. The characteristics of these membranes, as in the case of PEBA membranes, were defined basing on the total permeate flux, toluene flux, selectivity index and enrichment factor. The
Fig. 5. Comparison of (a) permeate flux, (b) toluene flux, (c) selectivity and (d) enrichment factor for composite PDMS membranes with the filler and without the filler of the thickness of 3 m.
D. Panek, K. Konieczny / Separation and Purification Technology 57 (2007) 507–512
results involving the tests are presented in Figs. 4 and 5(a)–(d). As in the case of PEBA membranes, the run of curves involving concentration changes in time, independent of the applied membrane, is almost identical (Fig. 5). Therefore, the application of membranes with the filler is senseless when our objective is solely to remove the pollution from water, without the necessity to concentrate it. However, the filling exerts a very strong influence on the total permeate flux. For the PDMS membrane its value dropped by 58% as compared to the membrane without filling (Fig. 5a). But the toluene flux remained the same (Fig. 5b). As a result of such a run of the experiment, both the separation index and enrichment factor rise by about 130%. The fact that there is no drop of toluene flux as compared to a considerable drop of total permeate flux has probably resulted from a similar hydrophobic character of the filler and toluene—its dissolution and diffusion is running faster than the dissolution and diffusion of polar water. The application of the filler in the case of PDMS membrane has therefore a totally different effect than in the case of PEBA membrane. It results from different properties, both chemical and mechanical, of the polymer itself. During the formation of membrane the grains of carbon must have been tightly sealed in the polymer structure, whereby it remained compact and without free spaces. 3.3. Discussion of testing results involving the influence of filling on the course of toluene pervaporation of PDMS membrane without support The last membranes subjected to tests were the PDMS membranes without carrier films. Due to low mechanical strength of PDMS material, the prepared membranes were characterized by a relatively high thickness—266 m for the membrane with the filler and 400 m for the membrane without the filler. The differences in the thickness of the tested membranes are slightly
511
Fig. 6. Changes of toluene concentration as time function, for PDMS membranes with the filler (400 m) and without the filler (266 m).
hindering the comparative analysis, although some conclusions can be drawn. The results of experiments obtained for these membranes are presented in Figs. 6 and 7(a)–(d). Basing on Fig. 6, we can observe that independent of filling and thickness of PDMS membrane, the curves involving the drop of toluene concentration in the feed in the function of time have almost identical run. The rate of toluene removal is the same for both membranes. It has also been confirmed by a similar toluene flux obtained for both membranes (Fig. 7b). However, a distinctive difference can be observed in the value of total permeate flux and it is strongly combined with its separation index and enrichment factor (Fig. 7(a), (c) and (d)). For the membrane with the filler the permeate flux is higher by about 90%, but in view of almost invariable toluene flux we can say that it is effected by the rise of water flux. Such a result of testing may have been effected by a considerably lower thickness of the membrane with the filler—PDMS membrane of the thickness 400 m is in practice impermeable for water. Unfortunately, as mentioned above, a direct comparison of these membranes is difficult. Basing on the testing results for PDMS composite membranes we can only presume that the
Fig. 7. Comparison of (a) permeate flux, (b) toluene flux, (c) selectivity and (d) enrichment factor for PDMS membranes with the filler (400 m) and without the filler (266 m).
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results obtained for membranes without a carrier film with the filler and without the filler of the similar thickness would have been analogous. 4. Conclusion All membranes investigated within the scope of the present paper show very good separation properties. The removal of toluene after one testing cycle was about 80% for PDMS membranes and about 70% for PEBA membranes, independent of the fact whether the filler was applied or not. The filler has no influence, or a minute one, on the flux of toluene. But it has a relevant influence on the total permeate flux. In the case of PEBA membranes the application of carbon black as a filler in the membrane resulted in the rise of permeate flux, and hence the deterioration of separation indexes and enrichment factors. A reverse situation took place in the case of PDMS membranes; here the filling reduced the permeation (mainly the permeation of water) improving efficiency indexes of the process. It probably resulted from the differences in closing mechanism of carbon black grains in the structure of polymers—more flexible structure of PDMS made it possible to block carbon black tightly whereas in the case of PEBA free spaces were created whereby the permeation of particles was easier. Also the yield of the process was different for particular membranes. The best results with this respect were obtained for composite PDMS membranes due to their low thickness. And the best concentration of permeate was obtained for PDMS membrane without a carrier film and filler—in this case the permeate in 75% consisted of toluene. References [1] M. Peng, L.M. Vane, S.X. Liu, Recent advances in VOC’s removal from water by pervaporation, J. Hazard. Mater. B 98 (2003) 69–90.
[2] B. Smitha, D. Suhanya, S. Sridhar, M. Ramakrishna, Separation of organic–organic mixtures by pervaporation—a review, J. Membr. Sci. 241 (2004) 1–21. [3] S. Sain, S. Dincer, O.T. Savascyi, Pervaporation of methanol–methyl acetate binary mixtures, Chem. Eng. Process. 37 (1998) 203–206. [4] M. Mulder, Basic Principles of Membrane Technology, Kluwer Academic Publishers, Dordrecht, 1996, ISBN 0-7923-4247-X. [5] W. Kujawski, M. Waczy´nski, Perwaporacja – membranowa technika separacyjna, Przemysł Chemiczny 75 (1996) 326–328. [6] M. Bodzek, J. Bohdziewicz, K. Konieczny, Techniki membranowe w ´ askiej, Gliwice 1997, ochronie s´rodowiska, Wydawnictwo Politechniki Sl˛ ISBN 83-85718-97-4 S. [7] Kumar, J.N. Shah, S.B. Sawant, J.B. Joshi, V.G. Pangarkar, Permeation in filled membranes: role of solute filter interactions, J. Membr. Sci. 134 (1997) 225–233. [8] M.V. Chandak, Y.S. Lin, W. Ji, R.J. Higgins, Sorption and diffusion of VOCs in DAY zeolite and silicate-filled PDMS membranes, J. Membr. Sci. 133 (1997) 231–243. [9] S.A. Netke, S.B. Sawant, J.B. Joshi, V.G. Pangarkar, Sorption and permeation of acetic acid through zeolite filled membrane, J. Membr. Sci. 107 (1995) 23–33. [10] H. Jang, O.T. Nguyen, Z. Ping, Y. Long, Y. Hirata, Desorption and pervaporation properties of zeolite-filled Poly(dimethylsiloxane) membranes, Mater. Res. Innovat. 5 (2001) 101–1096. [11] J.P. Boom, I.G.M. P¨unt, H. Zwijnenberg, R. de Boer, D. Bargeman, C.A. Smolders, H. Strathmann, Transport through zeolite filled polymeric membranes, J. Membr. Sci. 138 (1998) 237–258. [12] S.Y. Lu, C.P. Chiu, H.Y. Huang, Pervaporation of acetic acid/water mixtures through silicate filled polydimethylsiloxane membranes, J. Membr. Sci. 176 (2000) 159–167. [13] L. Li, Z. Xiao, Z. Zhang, S. Tan, Pervaporation of acetic acid/water mixtures through carbon molecular sieve-filled PDMS membranes, Chem. Eng. J. 97 (2004) 83–86. [14] F. Peng, Z. Jiang, Ch. Hu, Y. Wang, H. Xu, J. Liu, Removing benzene from aqueous solution using CMS-filled PDMS pervaporation membranes, Sep. Purif. Technol. 48 (2006) 229–234. [15] J.M. Duval, B. Folkers, M.H.V. Moulder, G. Desgrandchamps, C.A. Smolders, Separation of a toluene/ethanol mixture by pervaporation using active carbon-filled polymeric membranes, Sep. Sci. Technol. 29 (1994) 357–373. [16] W. Ji, S.K. Sikdar, S.T. Hwang, Sorption, diffusion and permeation of 1,1,1trichloroethane through adsorbent-filled polymeric membranes, J. Membr. Sci. 103 (1995) 243–255.
Preparation and applying the membranes with carbon black to pervaporation of toluene from the diluted aqueous solutions Dorota Panek ∗ , Krystyna Konieczny 1 Silesian University of Technology, Faculty of Energy and Environmental Engineering, Institute of Water and Wastewater Engineering, ul. Konarskiego 18, 44-100 Gliwice, Poland
Abstract The application of pervaporation (PV) to the removal of volatile organic (VOC) from the aqueous solutions has become very interesting in the last few years. It is caused by the increasing level of the compounds such as petrochemical solvents (benzene, toluene, and xylenes) or chlorinated solvents (trichloroethylene or tetrachloroethylene) in the natural environment. The aim of this work is to apply the membranes with carbon black as a filler to pervaporation (PV) of toluene from the diluted aqueous solution. In the research polydimethylsiloxane (PDMS) composite membranes, PDMS and PEBA (polyether-block-polyamide) membranes filled with carbon black were used. The membranes were prepared in a laboratory scale especially for this purpose. Both PEBA and PDMS membranes show excellent properties to separate toluene from the water, however, toluene solubility in the PDMS membrane was better, it could be observed that the removal of this compound from the feed was better when this membrane was used. In consequence the process selectivity increases and better concentration of toluene on the permeate side can be obtained. The influence of the membrane filler on the total flux, toluene flux and process selectivity was also determined. In some cases total flux as well as toluene flux decrease for the membranes with the filler comparing to those without the filler. © 2006 Elsevier B.V. All rights reserved. Keywords: Pervaporation; Toluene; Polydimethylosiloxane membrane; Polyether-b-polyamide membrane; Filler
1. Introduction In the last few years pervaporation has become an intensive developing separation technique [1–3]. Referred to as a so-called “clean technology” it is now a promising separation method for azeotropes, close boiling point substances, isomers or compounds sensitive to high temperatures. The separation of the mixtures using PV can be divided into three fields [2]:
• flux density: JV =
Vr Sm t
(1)
• separation factor: α=
Ya/Yb Xa/Xb
(2)
• enrichment factor: • dehydration of aqueous–organic mixtures; • removal of trace volatile organic compounds (VOC) from aqueous solution; • separation of organic–organic solvent mixtures. The basic factors which describe the effectiveness of the process are (Eqs. (1)–(3)):
∗ 1
Corresponding author. Tel.: +48 32 237 29 81. E-mail address: [email protected] (D. Panek). Tel.: +48 32 237 29 81.
1383-5866/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2006.10.011
β=
Ya Xa
(3)
where Vr is the amount of the permeate, Sm the membrane area unit, t the time unit, a the component of the mixture preferentially transported through the membrane, b the second component of the mixture, X the concentration of the component a or b in the feed, and Y is the concentration of the component a or b in the permeate. The application of PV to the VOC removal from the waters has become very interesting. It is the result of an increasing level of these substances in the natural environment. Man-made VOCs are generated by several sources such as municipal wastes,
508
D. Panek, K. Konieczny / Separation and Purification Technology 57 (2007) 507–512
traffic or industrial and agricultural activity [1]. Very dangerous is the presence of an increasing amount of petrochemical solvents, especially benzene, toluene, xylenes or chlorinated solvents such as trichloroethylene or tetrachloroethylene. However, the concentration of these agents is too low to remove them economically with conventional ways. Not long ago air stripping and/or activated carbon treatments were employed for this task, however, the former is susceptible to fouling—it merely turns a water pollution problem into an air pollution issue, while the latter needs costly regeneration steps and may not be suitable for VOCs that are easily displaced by other organic compounds [1]. As compared to these methods, pervaporation does not emit additional pollution, as well as does not need expensive regeneration steps. Additionally, concentrated permeate can be used again [4–6]. To achieve high selectivity or high flux for specific compounds the modifications of membranes such as filling, grafting or coating have been performed [1]. The most common is adding fillers, especially high aspect ratio fillers. It improves physical properties such as increased stiffness or reduced creep and a variety of other purposes: improves thermal stability, high voltage resistance, electrical conductivity, radiation shielding and optical and aesthetic effects [7]. In many cases, filled polymeric membranes show permeabilities much lower than the conventional unfilled membranes, and hence can serve as barriers for oxygen, water and other solutes [7]. In the area of organophilic pervaporation membranes mostly tree types of fillers have been used: zeolite [8–11], silicate [8,12] and carbon sieve [13–16]. The aim of this work was to apply dense polymer membranes: polydimethylsiloxane (PDMS) composite membranes, PDMS and PEBA (polyether-block-polyamide) membranes filled with carbon black for the removal of volatile organic compounds (VOC). Such membranes were prepared in the laboratory scale especially for this purpose. Toluene, as the representative of VOC was chosen. 2. Experimental 2.1. Pervaporation experimental setup The experimental set-up, used for the pervaporation measurements is shown schematically in Fig. 1.
Fig. 1. Toluene pervaporation process set-up: (1) feed tank; (2) gear pump; (3) pervaporation cell; (4) vacuum sensor; (5) cooling trap; (6) cooling trap with nitrogen; (7) vacuum pump.
A module of the membrane area of 100 cm2 was used. The feed tank of the capacity of 2 dm3 was filled with water solution of toluene. The toluene concentration was about 200 ppm. The temperature of the feed was kept at the temperature of 25 ◦ C by the thermostat. The retentate was being recycled to the feed tank and the permeate was being collected in the cooling traps. The permeate was cooled down by liquid nitrogen (−196 ◦ C). The pressure on the permeate side was about 1 mbar. It was generated with a vacuum pump and was controlled by a digital sensor. Before the pump an additional cooling trap with liquid nitrogen was installed to prevent diffusion of the oil from the pump to the permeate and vice versa. The overall flow above the membrane was constant and amounted to 1770 cm3 /min. Each experiment lasted 5 h. Every hour a sample was taken from the feed tank and the concentration of the toluene content was determined by UV analysis (λ = 261, 2 nm). Because of a very small amount of the permeate, in case of PDMS and PEBA membranes without the support one cooling trap during whole experiment was applied. In case of composite PDMS membrane it was exchanged together with taking of the sample. The concentration of toluene in the permeate, followed by the sample dilution with water, was determined in the way similar to the feed samples. The permeate mass was determined before the dilution using a balance. Each experiment was repeated five times and the results were averaged to minimize the errors. 2.2. Preparation of the membranes To determine the influence of the filling on the course of the process we applied membranes made solely from polymer film, and membranes with the filling. Membranes without support as well as composite membranes were also tested. Almost all of the investigated membranes were prepared by the authors in a laboratory scale, with the exception of the composite membrane PDMS which was made and offered for investigation by GKSS/Forschungszentrum Geesthacht, Germany. The quantity of carbon black was the same for all of the membranes and amounted to 15 wt.%. 2.2.1. Preparation of the PDMS membrane without filling and without support The first stage of PDMS membrane casting consisted in the preparation of polymer solution (PDMS Wacker Silicon Dehesive 940) together with cross-linking agent (V24 manufactured by the firm Wacker) in the solvent (isooctane). The solution was subjected to homogenisation using a magnetic stirrer for about 12 h. After that time, cross-linking catalyst (complex of platinum and 1,1,3,3-tetramethyle-1,3-divinyledisiloxane) in isooctane was added to the mixture. The whole was further stirred for about 30 min. The mixture prepared in that way was deposited onto a Teflon plate and left for about 12 h to let the solvent evaporate completely. Due to low resistance of PDMS material and possible damage to the membrane during the testing, a 400 m thick membrane was prepared. The measurement was effected using a micrometer screw and it is an average of 20 points from the membrane area (100 cm2 ).
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2.2.2. Preparation of PDMS membrane with carbon filler and without support As in the case of membrane without filling the process started with the preparation of polymer solution (PDMS Wacker Silicon Dehesive 940) together with the cross-linking agent (V24 manufactured by Wacker) in isooctane as solvent. The solution was placed on a magnetic stirrer and was being stirred until homogeneous solution was obtained. At the same time the mixture of carbon in isooctane was prepared. The prepared polymer solution was added to the carbon slurry and the whole was mixed for further 12 h. After that time, the cross-linking catalyst (platinum complex with 1,1,3,3-tetramethyle-1,3-divinyledisiloxane) was being added to the mixture. The whole was further mixed for about 2 h. Than the mixture was poured onto a glass plate covered with Teflon and left for about 12 h until the solvent evaporated completely. The prepared membrane was 266 m thick (the measurement was carried out with a micrometer screw and it is an average of 20 points from the membrane area (100 cm2 )). 2.2.3. Preparation of composite PDMS membrane with carbon filler The preparation process of polymer mixture with carbon black was carried out in the same way as in the case of membrane without support (Section 2.2.2). The membrane was cast using the immersion method by depositing the film of formerly prepared mixture onto the support made up by polyacrylonitryle (PAN). The thickness of that membrane was determined at GKSS, by method measuring the flux of gases permeating through it: oxygen and nitrogen, and it was calculated from the empirically determined dependencies. It equaled 3 m and was an average of 20 points from the membrane area (100 cm2 ). 2.2.4. Preparation of PEBA membrane without the filler and without support The preparation of the membrane started with the preparation of polymer solution (PEBAX 4033) in the mixture of solvents of 1-butanol and t-butanol in mass ratio 1:2.5. The solution was heated under the reflux condenser until the boiling point (ca. 120 ◦ C) and maintained at this temperature until total dissolution of polymer. After that time the temperature was reduced to 95 ◦ C and heating was continued for another 1 h. The homogeneous mixture was deposited onto a formerly prepared glass plate heated to 60 ◦ C. After 1 h, the heating was switched off and the membrane was left in such conditions for about 12 h. The thickness of membrane prepared in such a way and measured with a micrometer screw was 75 m (an average of 20 points from the membrane area (100 cm2 )). 2.2.5. Preparation of PEBA membrane with carbon filler without support The polymer solution in solvents was prepared in the same way as in Section 2.2.4. At the same time the mixture of carbon black in 1-butanol was prepared. The mixture was placed on a magnetic stirrer for about 1 h and then in an ultrasonic bath where stirring was continued for another 30 min. As soon as both solutions were ready the carbon mixture was slowly poured into the polymer solution and with T = 95 ◦ C being maintained,
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continuing the stirring for about 30 min. Then the casting of membrane was taking place onto the formerly prepared glass plate heated to 60 ◦ C. After 1 h the heating of the glass was switched off and the membrane was left in such conditions for about 12 h. The thickness of membrane prepared in this way and measured with a micrometer screw was 75 m (an average of 20 points from the membrane area (100 cm2 )). 3. Results and discussion 3.1. Discussion of results involving the influence of the filler on the course of toluene pervaporation for PEBA membrane The first membranes subjected to testing were polyether-bpolyamide membranes with carbon filler and without the filler. They were characterized by the same thickness, and therefore they could be directly compared. The results of the comparison are presented in Figs. 2 and 3. The presented in Fig. 2 change of toluene concentration in the feed versus time for the membrane with filler and without filler indicates that the filling of membrane plays an insignificant role regarding the rate of toluene removal. For both membranes the run of curves is very similar, and the obtained, final removal ratio is comparable. A distinctive difference can be observed when comparing the permeate flux, toluene flux, selectivity and enrichment factor obtained for these membranes. They were presented in Fig. 3(a)–(d). The total permeate flux for the membrane with the filler is higher by 50% as compared to the membrane made of the polymer alone. The toluene flux changes in the analogous way: from the value 4.61 g/m2 h for the membrane without the filler to 5.64 g/m2 h for the membrane with carbon, so the increase of toluene pervaporation flux is by about 20%. Hence, the increase of permeate flux is principally made up by the increase of pervaporating water, which results in the drop of both separation index and enrichment factor by 25% and 20%, respectively. The phenomenon involving the increase of both permeate flux and toluene flux for the membrane with the filler is unexpected. It could have been the result of the PEBA structure and of the preparation process of the membrane. The grains of carbon black may have been sealed insufficiently by the polymer during the
Fig. 2. Changes of toluene concentration in the feed as time function for PEBA membranes with the filler and without the filler of the thickness of 75 m.
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Fig. 3. Comparison of (a) permeate flux, (b) toluene flux, (c) selectivity and (d) enrichment factor for PEBA membranes with the filler and without the filler of the thickness of 75 m.
formation of membrane and as a result free spaces were formed, whereby the permeation of both water and toluene was faster. The application of carbon black of smaller granulation as the filler might have improved the properties of this membrane. 3.2. Discussion of results involving the influence of the filler on the course of toluene pervaporation for composite PDMS membrane
Fig. 4. Changes of toluene concentration as time function, for composite PDMS membranes with the filler and without the filler of the thickness 3 m.
The next membranes subjected to testing were composite polydimethylsiloxane membranes. Their thickness was about 3 m. The characteristics of these membranes, as in the case of PEBA membranes, were defined basing on the total permeate flux, toluene flux, selectivity index and enrichment factor. The
Fig. 5. Comparison of (a) permeate flux, (b) toluene flux, (c) selectivity and (d) enrichment factor for composite PDMS membranes with the filler and without the filler of the thickness of 3 m.
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results involving the tests are presented in Figs. 4 and 5(a)–(d). As in the case of PEBA membranes, the run of curves involving concentration changes in time, independent of the applied membrane, is almost identical (Fig. 5). Therefore, the application of membranes with the filler is senseless when our objective is solely to remove the pollution from water, without the necessity to concentrate it. However, the filling exerts a very strong influence on the total permeate flux. For the PDMS membrane its value dropped by 58% as compared to the membrane without filling (Fig. 5a). But the toluene flux remained the same (Fig. 5b). As a result of such a run of the experiment, both the separation index and enrichment factor rise by about 130%. The fact that there is no drop of toluene flux as compared to a considerable drop of total permeate flux has probably resulted from a similar hydrophobic character of the filler and toluene—its dissolution and diffusion is running faster than the dissolution and diffusion of polar water. The application of the filler in the case of PDMS membrane has therefore a totally different effect than in the case of PEBA membrane. It results from different properties, both chemical and mechanical, of the polymer itself. During the formation of membrane the grains of carbon must have been tightly sealed in the polymer structure, whereby it remained compact and without free spaces. 3.3. Discussion of testing results involving the influence of filling on the course of toluene pervaporation of PDMS membrane without support The last membranes subjected to tests were the PDMS membranes without carrier films. Due to low mechanical strength of PDMS material, the prepared membranes were characterized by a relatively high thickness—266 m for the membrane with the filler and 400 m for the membrane without the filler. The differences in the thickness of the tested membranes are slightly
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Fig. 6. Changes of toluene concentration as time function, for PDMS membranes with the filler (400 m) and without the filler (266 m).
hindering the comparative analysis, although some conclusions can be drawn. The results of experiments obtained for these membranes are presented in Figs. 6 and 7(a)–(d). Basing on Fig. 6, we can observe that independent of filling and thickness of PDMS membrane, the curves involving the drop of toluene concentration in the feed in the function of time have almost identical run. The rate of toluene removal is the same for both membranes. It has also been confirmed by a similar toluene flux obtained for both membranes (Fig. 7b). However, a distinctive difference can be observed in the value of total permeate flux and it is strongly combined with its separation index and enrichment factor (Fig. 7(a), (c) and (d)). For the membrane with the filler the permeate flux is higher by about 90%, but in view of almost invariable toluene flux we can say that it is effected by the rise of water flux. Such a result of testing may have been effected by a considerably lower thickness of the membrane with the filler—PDMS membrane of the thickness 400 m is in practice impermeable for water. Unfortunately, as mentioned above, a direct comparison of these membranes is difficult. Basing on the testing results for PDMS composite membranes we can only presume that the
Fig. 7. Comparison of (a) permeate flux, (b) toluene flux, (c) selectivity and (d) enrichment factor for PDMS membranes with the filler (400 m) and without the filler (266 m).
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results obtained for membranes without a carrier film with the filler and without the filler of the similar thickness would have been analogous. 4. Conclusion All membranes investigated within the scope of the present paper show very good separation properties. The removal of toluene after one testing cycle was about 80% for PDMS membranes and about 70% for PEBA membranes, independent of the fact whether the filler was applied or not. The filler has no influence, or a minute one, on the flux of toluene. But it has a relevant influence on the total permeate flux. In the case of PEBA membranes the application of carbon black as a filler in the membrane resulted in the rise of permeate flux, and hence the deterioration of separation indexes and enrichment factors. A reverse situation took place in the case of PDMS membranes; here the filling reduced the permeation (mainly the permeation of water) improving efficiency indexes of the process. It probably resulted from the differences in closing mechanism of carbon black grains in the structure of polymers—more flexible structure of PDMS made it possible to block carbon black tightly whereas in the case of PEBA free spaces were created whereby the permeation of particles was easier. Also the yield of the process was different for particular membranes. The best results with this respect were obtained for composite PDMS membranes due to their low thickness. And the best concentration of permeate was obtained for PDMS membrane without a carrier film and filler—in this case the permeate in 75% consisted of toluene. References [1] M. Peng, L.M. Vane, S.X. Liu, Recent advances in VOC’s removal from water by pervaporation, J. Hazard. Mater. B 98 (2003) 69–90.
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