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Methanol removal from organic mixtures by pervaporation using polypyrrole membranes
j o u r n a l of MEMBRANE SCIENCE ELSEVIER
Journal of Membrane Science 117 (1996) 303-309
Methanol removal from organic mixtures by pervaporation using polypyrrole membranes Ming Zhou, Michel Persin, Jean Sarrazin * Laboratoire des Mat£riaux et Procdd£s Membranaires, UMR CNRS 5635, ENSCM, 8, rue de l'Ecole Normale, 34053 Montpellier Cedex 1, France
Received 18 July 1995; revised 21 March 1996; accepted 22 March 1996
Abstract Conducting polymer composite membranes with a separation layer of polypyrrole doped with hexafluorophosphate (PF6) and p-toluenesulfonate (CH3C6H4SO3), were examined for the removal of methanol from organic solvents (toluene, IPA, MTBE and acetonitrile) by pervaporation. In all the cases, both membranes displayed preferential permeation of methanol. Selectivities and permeation rates as functions of methanol content were studied. The highest selectivity to methanol over toluene, accompanying an acceptable methanol flux, was obtained with the membrane doped by PF6. Keywords: Conducting polymer; Methanol; Organic/organic separation; Pervaporation; Polypyrrole
1. Introduction The separations of organic/organic mixtures through pervaporation is a very promising but challenging area. Although the organic/organic separations made by Binning [1] in the 1950s are historically considered as one significant landmark, commercial systems designed for organic/organic separations have not yet come to the marketplace. The development of membranes for this purpose seems to be very difficult. For the dehydration and organic removal from aqueous solution, the membrane materials are distinct, respectively, glassy hydrophilic and rubber organophilic. But no such criterion exists for the choice of organic/organic separation membranes. As the number of potential organic/organic
* Corresponding author.
pairs is large, the requirements of membranes may be quite different according to the mixtures to be treated. Consequently, except the basic stability and insolubility, it is impossible to establish universal criteria for the material choice. Methanol, though less polar than water, can be considered as rather polar among organic solvents. A membrane developed for dehydration normally can then also transport methanol fairly well [2]. The removal of methanol from organic mixtures [3,4], especially from synthesis mixtures of octane enhancers, including DMC (dimethyl carbonate), MTBE (methyl tert-butyl ether) and ETBE (ethyl tert-butyl ether) [5-13], is increasingly becoming a focus of interest in current research relating to organic/organic separation. Owing to its non-toxic and non-pollutant properties, MTBE has gained great attention as a gasoline additive (octane enhancer) to replace lead additives. Its production increased very
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M. Zhou et al. / Journal of Membrane Science 117 (1996) 303-309
rapidly in the 1980s and will stay at a high rate in the 1990s [12]. It is usually synthesized by the reaction of isobutene and methanol. In the product stream both the methanol/MTBE and methanol/isobutene form azeotropes which are difficult to separate by distillation. In our previous studies on conducting polypyrrole (PPy) membranes, high pervaporating rates of polar, hydrophilic and small molecular size substances were found [14]. It seemed that such a kind of membrane was promising for the methanol removal from some organic solvents. Attempts were then made using PPy composite membranes for such a separation, and are reported in this paper.
2. Experimental
Two types of PPy composite membranes were prepared by oxidation at a controlled potential, directly on the stainless steel meshes as described in our earlier papers [14,15], using a Ag + (0.1 M AgNO3)/Ag reference electrode. PPy doped with hexafluorophosphate (PF6) was obtained at 1.0 V in acetonitrile (containing 0.1 M pyrrole, 0.1 M Bu4NPF6 and 1 wt% H20) and PPy doped with p-toluenesulfonate (CH3C6H4SO 3) was prepared at 0.8 V in water (containing 0.1 M pyrrole, 0.1 M CH3C6H4SO3Na). The thickness, determined from scanning electron microscopy after breaking the membrane in liquid nitrogen, was about 10 and 20 p~m, respectively. Pervaporation of m e t h a n o l / t o l u e n e , methanol/IPA, methanol/MTBE and methanol/acetonitrile were performed using a GFT laboratory unit with a home-made cell [16]. After electrodeposition, membranes were left a few days drying in air at room temperature. The membrane was then installed into the pervaporation cell, the polymer layer facing the liquid. For each pervaporation experiment, the membrane was swollen by the liquid mixture by keeping it overnight in contact with the feed liquid, which was circulating in the cell. A vacuum was then applied, fractions of permeate were collected at regular interval (1 to 4 h depending on the permeation rate, until permeate flux and composition were stahilised and a steady state was reached, which usually occurred within 1
day. The feed could then be changed to perform the following experiment. Three samples of membranes, i.e. two PPy-PF and one PPy-PTS were used in this work. The two PPy-PF membranes had been previously used for the pervaporation of a series of solvents [14]. The PPy-PF sample used for the separation of methanol/toluene mixtures reported here had been employed in the pervaporation of cyclohexane (! day), MEK (1 day), acetonitrile (4 days), pyridine (l day), toluene (1 day), IPA (1 day) and water/IPA (5 days), water/ethanol (2 days), water/MEK (2 days). After 6 days of test of methanol/toluene, it was found to have leakage. The PPy-PF sample used for the separation of methanol/IPA had been also employed in the former research for the pervaporation of n-heptane (2 days), cyclohexane (1 day), methanol/n-heptane (1 day), pyrrole (1 day), methanol (1 day) and methanol/IPA (12 days). After 18 days of usage, the membrane remained undamaged. The PPy-PTS membrane was used sequentially for cyclohexane (1 day), toluene (1 day), methanol/toluene (10 days), methanol/MTBE (9 days), methanol/acetonitrile (9 days), and remained undamaged. Solvents used were: acetonitrile (sds, ref. no, 060516, > 99.5%, analytical grade, water _< 0.05%); methanol (sds, ref. no. 930521, > 99.8%, analytical grade, water < 0.05%, ethanol < 0.02%); 2-propanol (IPA) (sds, ref. no. 950521, > 99.7%, analytical grade, water < 0.1%); toluene (sds, ref. no. 710521, > 99.3%, analytical grade, water < 0.02%, benzene < 0.05%); methyl tert-butyl ether (MTBE) (Fluka, ref. no. 20256, puriss, p.a.; > 99.5%, water<
0.05~). The concentrations of permeates were analysed by gas chromatography.
3. Results and discussion
3.1. PPy-PF membrane PPy-PF membranes were used to separate methanol/toluene and methanol/IPA mixtures. Methanol is miscible with both IPA and toluene in all ranges, and an azeotrope forms in
M. Zhou et al. / Journal of Membrane Science 117 (1996) 303-309 100
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methanol/toluene with 72.4 wt% methanol at 63.7°C [17]. These two systems were chosen because our previous results [14] indicated a great permeation difference for pure solvents between methanol and the two solvents in question. The separation of these systems through PPy-PF membranes was carried out over the whole concentration range to evaluate the permeation performance. It can be observed from the separation diagram (Fig. 1) that the membranes are selective to methanol over IPA and toluene. However the performances of the membranes were quite different, as can be found clearly in Figs. 2 and 3, where the pervaporation selectivities and fluxes are presented,
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Separation diagram of m e t h a n o l / I P A and methanol/toluene mixtures by pervaporation using a P P y - P F membrane at 5 8 . 7 ° C .
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selectivity of methanol/toluene mixtures through a P P y - P F brane at 5 7 . 5 ° C
mem-
selectivity of methanol/IPA mixtures through a P P y - P F brane at 5 7 . 5 ° C
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as a function of methanol concentration, for methanol/toluene and methanol/IPA, respectively, through the PPy-PF membranes. One part of the difference has its origin in the fact that the membrane samples, though identical after synthesis, have their properties modified by their use - this phenomenon was revealed for example by the fluxes observed for pure methanol - and that the histories of the two samples were not identical before they were employed for the studies reported here, since the latter are in keeping with the general pattern of a first approach of a large screening of the behaviour of polypyrrole based membranes toward organic solvents in pervaporation. The observed differences are also related to the solvents investigated, and although a direct quantitative comparison of the two pairs could be ambiguous, the role of the feed mixture composition can be discussed separately for each pair of solvents separately. In the case of methanol/toluene, a rather high selectivity was found at low methanol content, e.g. the separation factor of methanol over toluene was 590 at a composition of 5% methanol. With the increase of methanol content the selectivity quickly decreased, but the partial flux of methanol increased as usually expected. As for the flux of methanol at low concentration, values of 230 and 280 g / m 2 h were obtained for mixtures of methanol at 5 and 10%, respectively. On the other hand, the flux of toluene stayed at a very low level (6-7.5 g / m 2 h) over the whole concentration range.
306
M. Zhou et aL / Journal of Membrane Science 117 (1996) 303-309
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In the case of methanol/IPA, anomalous permeation characteristics was observed (Fig. 3), namely, for methanol, both the permeability and selectivity over IPA changed in the same direction with methanol concentration in the feed mixtures. The usual "trade-off" between selectivity and permeability did not appear in the methanol/IPA-PPy-PF system. To describe the flux coupling, the permeation ratio 0MeOH of methanol was calculated and is illustrated in Fig. 4. 0MeOH is defined as below [18,19]: 0MeOH ~
200
JMeOH 0 JMeOH X CMeOH
where JM~OH is the partial permeation flux of 0 methanol in a binary mixture, JMeOH the permeation flux of pure methanol and CM~OH the weight fraction of methanol in the feed mixture. It was found that at low methanol content toluene has an enhancing effect to the permeation of methanol since 0M~OH> 1 when the methanol concentration is less than 40%; whereas the presence of IPA retards the methanol transport through the PPy-PF membrane since 0MeOH< 1 over the whole concentration of the methanol/IPA mixture. Because of this effect, the partial flux of methanol remains at an acceptable level when the methanol/toluene feed mixture is becoming poor in methanol.
40
Three mixture systems containing methanol were tested using the same sample of PPy-PTS membrane.
80
0
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Methanolcontentin feedmixture(wt.%) Fig. 5. Influence of feed composition on partial permeability and selectivity of methanol/toluene mixtures through a PPy-PTS membrane at 57.5°C.
Azeotropes exist in all three mixture systems, i.e. methanol/toluene, methanol/MTBE and methanol/acetonitrile.
3.2.1. Methanol/toluene and methanol / MTBE Compared with the PPy-PF membrane, PPy-PTS showed a poorer selectivity for methanol/toluene mixtures (Fig. 5). The permeate compositions for the methanol/toluene and methanol/MTBE systems are displayed in Fig. 6. Considering the process to which the pervaporation of methanol/MTBE is to be applied, the very high selectivity of membrane to methanol over MTBE doesn't seem to be indispensable; the flux, on the other hand, is more important 100 8O
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Methanol content in feed mixture (wt.%) Fig. 6. Separation diagram of methanol/toluene and methanol/MTBE mixtures by pervaporation using a PPy-PTS membrane at 57.5°C and 50°C, respectively.
M. Zhou et al. / Journal of Membrane Science 117 (1996) 303-309 100
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for production capacity. From Fig. 7, we can find that in the range of low methanol content, the partial flux of methanol is not high enough. Nevertheless, in view of the industrial use of a membrane of this type, a study of flux increase by means of a membrane thickness decrease could be of interest.
3.2.2. Methanol / acetonitriIe Methanol and acetonitrile form an azeotrope with composition of 19 wt% methanol at 63.5°C [17]. Pervaporation of pure components indicated very close values of permeation rates for methanol and acetonitrile, i.e. 950 and 900 g / m 2 h at 50°C, respectively. That is to say, the PPy-PTS membrane showed an almost equal affinity to methanol and acetonitrile.
,~ 40
I,,,I,, 60
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Methanol content in feed mixture (wt.%) Fig. 8. Separation diagram of methanol/acetonitrile mixtures by pervaporation using a PPy-PTS membrane at 50°C.
to the distillation azeotrope and is therefore termed as "permazeotropic concentration". In our case, however, because the slope of the pervaporation curve near the permazeotropic crossing-point is very close to that of the diagonal, the existence of a permazeotrope and its definite position are to be further confirmed. Fig. 9 shows the influence of the feed mixture composition on the pervaporation flux. A synergistic effect was found, with respect to the flux, when the methanol content in the feed was higher than 20%; actually, the permeation ratios of both methanol 0MeOH and acetonitrile 0MeCN were then higher than 1 (Fig. 10).
4
3.2.3. Pervaporation characteristics The permeate composition as a function of feed mixture is depicted in the separation diagram (Fig. 8). The membrane shows a small selectivity for methanol over acetonitrile according to expectation. The highest value of the separation factor OLmethanol/acetonitrile, obtained with mixture of 5 wt% methanol, was 2.7. At a concentration of 20 wt% methanol (near to the azeotropic composition), O~methanol/acetonitrile decreased to 2.2. It should also be noted that when the methanol content in the feed reached 90 wt%, the pervaporate seems to possess exactly the same composition as the feed mixture. This characteristic concentration shows an analogy
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308
M. Zhou et al. / Journal of Membrane Science 117 (1996) 303-309
"~
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, , ,
of pure components at different temperatures demonstrated that the activation energies of some common solvents through the PPy membranes are low [14] and differences between them cannot be very large. Therefore, within the studied temperature range (from 30 to 63.5°C, corresponding flux 650-1470 g / m 2 h), the selectivity for this system showed no significant change, the methanol content in the permeate varying over a very small range from 34 to 36.5%.
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3.2.4. Temperature effect Pervaporation was performed with a mixture containing 20 wt% methanol at different temperatures. As illustrated in Fig. 11, the total and partial permeation rates increased with the temperature following an Arrhenius type equation with a regression coefficient of over 0.99. The overall activation energy of pervaporation is 4.76 Kcal/mol, and the individual activation energies of methanol and acetonitrile obtained from this system are 4.52 and 4.89 Kcal/mol, respectively. The influence of temperature on the selectivity depends on the relative values of the activation energies and probably the relationship between flux coupling and temperature as well. The measurement 10 4
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4. Conclusions Polypyrrole films doped by different anions may be used as pervaporation membranes for the separations of organic pairs based on the differences of polarity, molecular size and hydrophilicity. The present work on methanol removal from organic solvents indicates that these membranes display a good selectivity and an acceptable permeation rate under some circumstances. Nevertheless, the phenomena appear to be complex, and although some studies have been performed by other authors [20-22] on the interactions between methanol and polypyrroles, further investigations are necessary to understand the mechanism of pervaporation in such media.
References [1] R.C. Binning, R.J. Lee, J.F. Jennings and E.C. Martin, Separation of liquid mixtures by permeation, Ind. Eng. Chem., 53 (1961) 45-53. [2] R. Rautenbach, S. Klatt and J. Vier, State of the art of pervaporation - 10 Years of industrial PV, Proc. 6th Int. Conf. Pervaporation Processes Chem. Ind., 27-30 September, Ottawa, Canada, 1992, pp. 2-15. [3] H.E,A. Briischke, W.H. Schneider, H. Scholz and H. Steinhauser, Removal of methanol from organic mixtures, in Proc. 6th Int. Conf. Pervaporation Processes Chem. Ind., 27-30 September, Ottawa, Canada, 1992, pp. 423-429. [4] H.C. Park, R.M. Meertens, M.H.V. Mulder and C.A. Smolders, Pervaporation of alcohol-toluene mixtures through polymer blend membranes of poly(acrylic acid) and poly(vinyl alcohol), J. Membrane Sci., 90 (1994) 265-274. [5] M.S.K. Chen, G.S. Markiewicz and K.G. Venugopal, Development of membrane pervaporation TRIM TM process for methanol recovery from C H 3 O H / M T B E / C 4 mixtures, AIChE Symp. Ser., 85 (1989) 82-88. [6] B.A. Farnand and S.H. Noh, Pervaporation as an alternative process for the separation of methanol from C a hydrocarbons
M. Zhou et al. / Journal of Membrane Science 117 (1996) 303-309
[7]
[8]
[9]
[10]
[11]
[12]
[13]
in the production of MTBE and TAME, AIChE Symp. Ser., 85 (1989) 89-92. V.M. Shah, C.R. Bartels, M. Pasternak and J. Reale, Opportunities for membranes in the production of octane enhancers, AIChE Symp. Ser., 85 (1989) 93-97. M. Pasternak, Separation of organic liquids such as mixtures of methanol and dimethyl carbonate or methanol and methyl tert-butyl ether, US Pat., 5,238,573. M. Pasternak, Preferential removal of methanol from organic solutions by Nafion membranes containing organic or metal counter-ions, Macromol. Rep., A30 (Suppls. 1 and 2) (1993) 47-54. F. Doghieri, A. Nardella, G.L. Sarti and C. Valentini, Pervaporation of methanol-MTBE mixtures through modified poly(phenylene oxide) membranes, J. Membrane Sci., 91 (1994) 283-291. I. Noezar, Q.T. Nguyen, R. Clement and J. N6el, High performance polymer blend membranes for alcohol-ether separation, Proc. 7th Int. Conf. Pervaporation Processes Chem. Ind., 26 February-1 March, Reno, NV, 1995, pp. 45-51. C. Streicher, P. Kremer, V. Thomas, A. Hubner and G. Ellinghorst, Development of new pervaporation membranes, systems and processes to separate alcohols/ethers/hydrocarbons mixtures, Proc. 7th Inter. Conf. Pervaporation Processes Chem. Ind., 26 February-I March, Reno, NV, 1995, pp. 297-309. H.-H. Schwarz, R. Apostel, K. Richau and D. Paul, Membranes from surfactants for separation of polar organic liquids, Proc. 7th Int. Conf. Pervaporation Processes Chem. Ind., 26 February-1 March, Reno, NV, 1995, pp. 374-382.
309
[14] M. Zhou, M. Persin, W. Kujawski and J. Sarrazin, Electrochemically synthesised polypyrrole membranes for the separation of organic mixtures by pervaporation, Proc. 7th Int. Conf. Pervaporation Processes Chem. Ind., 26 February-1 March, Reno, NV, 1995, pp. 193-205. [15] M. Zhou, M. Persin and J. Sarrazin, Electrodeposition of membrane-oriented conducting poly (pyrrole, thiophene) on stainless steel meshes, J. Appl. Electrochem., in press. [16] M. Zhou, M. Persin, W. Kujawski and J. Sarrazin, Electrochemical preparation of polypyrrole membranes and their application in ethanol-cyclohexane separation by pervaporation, J. Membrane Sci., 108 (1995) 89-96. [17] R.C. Weast and M.J. Astle (Eds.), Handbook of Chemistry and Physics, CRC Press, West Palm Beach, FL, 1979. [18] E. Drioli, S. Zhang and A. Basile, On the coupling effect in pervaporation, J. Membrane Sci., 81 (1993) 43-55. [19] R.Y.M. Huang and X. Feng, Pervaporation of water/ethanol mixtures by an aromatic poly(ether imide) membrane, Sep. Sci. Technol., 27 (1992) 1583-1597. [20] D. Blackwood and M. Josowicz, Work function and spectroscopic studies of interactions between conducting polymers and organic vapors, J. Phys. Chem., 95 (1991) 493-502. [21] P. Topart and M. Josowicz, Characterization of the interaction between poly(pyrrole) films and methanol vapor, J. Phys. Chem., 96 (1992) 7824-7830. [22] P. Topart and M. Josowicz, Transcient effects in the interaction between polypyrrole and methanol vapor, J. Phys. Chem., 96 (1992) 8662-8666.
Journal of Membrane Science 117 (1996) 303-309
Methanol removal from organic mixtures by pervaporation using polypyrrole membranes Ming Zhou, Michel Persin, Jean Sarrazin * Laboratoire des Mat£riaux et Procdd£s Membranaires, UMR CNRS 5635, ENSCM, 8, rue de l'Ecole Normale, 34053 Montpellier Cedex 1, France
Received 18 July 1995; revised 21 March 1996; accepted 22 March 1996
Abstract Conducting polymer composite membranes with a separation layer of polypyrrole doped with hexafluorophosphate (PF6) and p-toluenesulfonate (CH3C6H4SO3), were examined for the removal of methanol from organic solvents (toluene, IPA, MTBE and acetonitrile) by pervaporation. In all the cases, both membranes displayed preferential permeation of methanol. Selectivities and permeation rates as functions of methanol content were studied. The highest selectivity to methanol over toluene, accompanying an acceptable methanol flux, was obtained with the membrane doped by PF6. Keywords: Conducting polymer; Methanol; Organic/organic separation; Pervaporation; Polypyrrole
1. Introduction The separations of organic/organic mixtures through pervaporation is a very promising but challenging area. Although the organic/organic separations made by Binning [1] in the 1950s are historically considered as one significant landmark, commercial systems designed for organic/organic separations have not yet come to the marketplace. The development of membranes for this purpose seems to be very difficult. For the dehydration and organic removal from aqueous solution, the membrane materials are distinct, respectively, glassy hydrophilic and rubber organophilic. But no such criterion exists for the choice of organic/organic separation membranes. As the number of potential organic/organic
* Corresponding author.
pairs is large, the requirements of membranes may be quite different according to the mixtures to be treated. Consequently, except the basic stability and insolubility, it is impossible to establish universal criteria for the material choice. Methanol, though less polar than water, can be considered as rather polar among organic solvents. A membrane developed for dehydration normally can then also transport methanol fairly well [2]. The removal of methanol from organic mixtures [3,4], especially from synthesis mixtures of octane enhancers, including DMC (dimethyl carbonate), MTBE (methyl tert-butyl ether) and ETBE (ethyl tert-butyl ether) [5-13], is increasingly becoming a focus of interest in current research relating to organic/organic separation. Owing to its non-toxic and non-pollutant properties, MTBE has gained great attention as a gasoline additive (octane enhancer) to replace lead additives. Its production increased very
0376-7388/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S 0 3 7 6 - 7 3 8 8 ( 9 6 ) 0 0 0 9 1 - 9
304
M. Zhou et al. / Journal of Membrane Science 117 (1996) 303-309
rapidly in the 1980s and will stay at a high rate in the 1990s [12]. It is usually synthesized by the reaction of isobutene and methanol. In the product stream both the methanol/MTBE and methanol/isobutene form azeotropes which are difficult to separate by distillation. In our previous studies on conducting polypyrrole (PPy) membranes, high pervaporating rates of polar, hydrophilic and small molecular size substances were found [14]. It seemed that such a kind of membrane was promising for the methanol removal from some organic solvents. Attempts were then made using PPy composite membranes for such a separation, and are reported in this paper.
2. Experimental
Two types of PPy composite membranes were prepared by oxidation at a controlled potential, directly on the stainless steel meshes as described in our earlier papers [14,15], using a Ag + (0.1 M AgNO3)/Ag reference electrode. PPy doped with hexafluorophosphate (PF6) was obtained at 1.0 V in acetonitrile (containing 0.1 M pyrrole, 0.1 M Bu4NPF6 and 1 wt% H20) and PPy doped with p-toluenesulfonate (CH3C6H4SO 3) was prepared at 0.8 V in water (containing 0.1 M pyrrole, 0.1 M CH3C6H4SO3Na). The thickness, determined from scanning electron microscopy after breaking the membrane in liquid nitrogen, was about 10 and 20 p~m, respectively. Pervaporation of m e t h a n o l / t o l u e n e , methanol/IPA, methanol/MTBE and methanol/acetonitrile were performed using a GFT laboratory unit with a home-made cell [16]. After electrodeposition, membranes were left a few days drying in air at room temperature. The membrane was then installed into the pervaporation cell, the polymer layer facing the liquid. For each pervaporation experiment, the membrane was swollen by the liquid mixture by keeping it overnight in contact with the feed liquid, which was circulating in the cell. A vacuum was then applied, fractions of permeate were collected at regular interval (1 to 4 h depending on the permeation rate, until permeate flux and composition were stahilised and a steady state was reached, which usually occurred within 1
day. The feed could then be changed to perform the following experiment. Three samples of membranes, i.e. two PPy-PF and one PPy-PTS were used in this work. The two PPy-PF membranes had been previously used for the pervaporation of a series of solvents [14]. The PPy-PF sample used for the separation of methanol/toluene mixtures reported here had been employed in the pervaporation of cyclohexane (! day), MEK (1 day), acetonitrile (4 days), pyridine (l day), toluene (1 day), IPA (1 day) and water/IPA (5 days), water/ethanol (2 days), water/MEK (2 days). After 6 days of test of methanol/toluene, it was found to have leakage. The PPy-PF sample used for the separation of methanol/IPA had been also employed in the former research for the pervaporation of n-heptane (2 days), cyclohexane (1 day), methanol/n-heptane (1 day), pyrrole (1 day), methanol (1 day) and methanol/IPA (12 days). After 18 days of usage, the membrane remained undamaged. The PPy-PTS membrane was used sequentially for cyclohexane (1 day), toluene (1 day), methanol/toluene (10 days), methanol/MTBE (9 days), methanol/acetonitrile (9 days), and remained undamaged. Solvents used were: acetonitrile (sds, ref. no, 060516, > 99.5%, analytical grade, water _< 0.05%); methanol (sds, ref. no. 930521, > 99.8%, analytical grade, water < 0.05%, ethanol < 0.02%); 2-propanol (IPA) (sds, ref. no. 950521, > 99.7%, analytical grade, water < 0.1%); toluene (sds, ref. no. 710521, > 99.3%, analytical grade, water < 0.02%, benzene < 0.05%); methyl tert-butyl ether (MTBE) (Fluka, ref. no. 20256, puriss, p.a.; > 99.5%, water<
0.05~). The concentrations of permeates were analysed by gas chromatography.
3. Results and discussion
3.1. PPy-PF membrane PPy-PF membranes were used to separate methanol/toluene and methanol/IPA mixtures. Methanol is miscible with both IPA and toluene in all ranges, and an azeotrope forms in
M. Zhou et al. / Journal of Membrane Science 117 (1996) 303-309 100
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methanol/toluene with 72.4 wt% methanol at 63.7°C [17]. These two systems were chosen because our previous results [14] indicated a great permeation difference for pure solvents between methanol and the two solvents in question. The separation of these systems through PPy-PF membranes was carried out over the whole concentration range to evaluate the permeation performance. It can be observed from the separation diagram (Fig. 1) that the membranes are selective to methanol over IPA and toluene. However the performances of the membranes were quite different, as can be found clearly in Figs. 2 and 3, where the pervaporation selectivities and fluxes are presented,
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F i g . 3. Influence of feed composition on partial permeability and
Separation diagram of m e t h a n o l / I P A and methanol/toluene mixtures by pervaporation using a P P y - P F membrane at 5 8 . 7 ° C .
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selectivity of methanol/toluene mixtures through a P P y - P F brane at 5 7 . 5 ° C
mem-
selectivity of methanol/IPA mixtures through a P P y - P F brane at 5 7 . 5 ° C
mem-
as a function of methanol concentration, for methanol/toluene and methanol/IPA, respectively, through the PPy-PF membranes. One part of the difference has its origin in the fact that the membrane samples, though identical after synthesis, have their properties modified by their use - this phenomenon was revealed for example by the fluxes observed for pure methanol - and that the histories of the two samples were not identical before they were employed for the studies reported here, since the latter are in keeping with the general pattern of a first approach of a large screening of the behaviour of polypyrrole based membranes toward organic solvents in pervaporation. The observed differences are also related to the solvents investigated, and although a direct quantitative comparison of the two pairs could be ambiguous, the role of the feed mixture composition can be discussed separately for each pair of solvents separately. In the case of methanol/toluene, a rather high selectivity was found at low methanol content, e.g. the separation factor of methanol over toluene was 590 at a composition of 5% methanol. With the increase of methanol content the selectivity quickly decreased, but the partial flux of methanol increased as usually expected. As for the flux of methanol at low concentration, values of 230 and 280 g / m 2 h were obtained for mixtures of methanol at 5 and 10%, respectively. On the other hand, the flux of toluene stayed at a very low level (6-7.5 g / m 2 h) over the whole concentration range.
306
M. Zhou et aL / Journal of Membrane Science 117 (1996) 303-309
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In the case of methanol/IPA, anomalous permeation characteristics was observed (Fig. 3), namely, for methanol, both the permeability and selectivity over IPA changed in the same direction with methanol concentration in the feed mixtures. The usual "trade-off" between selectivity and permeability did not appear in the methanol/IPA-PPy-PF system. To describe the flux coupling, the permeation ratio 0MeOH of methanol was calculated and is illustrated in Fig. 4. 0MeOH is defined as below [18,19]: 0MeOH ~
200
JMeOH 0 JMeOH X CMeOH
where JM~OH is the partial permeation flux of 0 methanol in a binary mixture, JMeOH the permeation flux of pure methanol and CM~OH the weight fraction of methanol in the feed mixture. It was found that at low methanol content toluene has an enhancing effect to the permeation of methanol since 0M~OH> 1 when the methanol concentration is less than 40%; whereas the presence of IPA retards the methanol transport through the PPy-PF membrane since 0MeOH< 1 over the whole concentration of the methanol/IPA mixture. Because of this effect, the partial flux of methanol remains at an acceptable level when the methanol/toluene feed mixture is becoming poor in methanol.
40
Three mixture systems containing methanol were tested using the same sample of PPy-PTS membrane.
80
0
100
Methanolcontentin feedmixture(wt.%) Fig. 5. Influence of feed composition on partial permeability and selectivity of methanol/toluene mixtures through a PPy-PTS membrane at 57.5°C.
Azeotropes exist in all three mixture systems, i.e. methanol/toluene, methanol/MTBE and methanol/acetonitrile.
3.2.1. Methanol/toluene and methanol / MTBE Compared with the PPy-PF membrane, PPy-PTS showed a poorer selectivity for methanol/toluene mixtures (Fig. 5). The permeate compositions for the methanol/toluene and methanol/MTBE systems are displayed in Fig. 6. Considering the process to which the pervaporation of methanol/MTBE is to be applied, the very high selectivity of membrane to methanol over MTBE doesn't seem to be indispensable; the flux, on the other hand, is more important 100 8O
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tO0
Methanol content in feed mixture (wt.%) Fig. 6. Separation diagram of methanol/toluene and methanol/MTBE mixtures by pervaporation using a PPy-PTS membrane at 57.5°C and 50°C, respectively.
M. Zhou et al. / Journal of Membrane Science 117 (1996) 303-309 100
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for production capacity. From Fig. 7, we can find that in the range of low methanol content, the partial flux of methanol is not high enough. Nevertheless, in view of the industrial use of a membrane of this type, a study of flux increase by means of a membrane thickness decrease could be of interest.
3.2.2. Methanol / acetonitriIe Methanol and acetonitrile form an azeotrope with composition of 19 wt% methanol at 63.5°C [17]. Pervaporation of pure components indicated very close values of permeation rates for methanol and acetonitrile, i.e. 950 and 900 g / m 2 h at 50°C, respectively. That is to say, the PPy-PTS membrane showed an almost equal affinity to methanol and acetonitrile.
,~ 40
I,,,I,, 60
80
100
Methanol content in feed mixture (wt.%) Fig. 8. Separation diagram of methanol/acetonitrile mixtures by pervaporation using a PPy-PTS membrane at 50°C.
to the distillation azeotrope and is therefore termed as "permazeotropic concentration". In our case, however, because the slope of the pervaporation curve near the permazeotropic crossing-point is very close to that of the diagonal, the existence of a permazeotrope and its definite position are to be further confirmed. Fig. 9 shows the influence of the feed mixture composition on the pervaporation flux. A synergistic effect was found, with respect to the flux, when the methanol content in the feed was higher than 20%; actually, the permeation ratios of both methanol 0MeOH and acetonitrile 0MeCN were then higher than 1 (Fig. 10).
4
3.2.3. Pervaporation characteristics The permeate composition as a function of feed mixture is depicted in the separation diagram (Fig. 8). The membrane shows a small selectivity for methanol over acetonitrile according to expectation. The highest value of the separation factor OLmethanol/acetonitrile, obtained with mixture of 5 wt% methanol, was 2.7. At a concentration of 20 wt% methanol (near to the azeotropic composition), O~methanol/acetonitrile decreased to 2.2. It should also be noted that when the methanol content in the feed reached 90 wt%, the pervaporate seems to possess exactly the same composition as the feed mixture. This characteristic concentration shows an analogy
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308
M. Zhou et al. / Journal of Membrane Science 117 (1996) 303-309
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, , ,
of pure components at different temperatures demonstrated that the activation energies of some common solvents through the PPy membranes are low [14] and differences between them cannot be very large. Therefore, within the studied temperature range (from 30 to 63.5°C, corresponding flux 650-1470 g / m 2 h), the selectivity for this system showed no significant change, the methanol content in the permeate varying over a very small range from 34 to 36.5%.
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Fig. 10. Flux coupling of methanol/acetonitrile mixtures through a PPy-PTS membrane by pervaporation.
3.2.4. Temperature effect Pervaporation was performed with a mixture containing 20 wt% methanol at different temperatures. As illustrated in Fig. 11, the total and partial permeation rates increased with the temperature following an Arrhenius type equation with a regression coefficient of over 0.99. The overall activation energy of pervaporation is 4.76 Kcal/mol, and the individual activation energies of methanol and acetonitrile obtained from this system are 4.52 and 4.89 Kcal/mol, respectively. The influence of temperature on the selectivity depends on the relative values of the activation energies and probably the relationship between flux coupling and temperature as well. The measurement 10 4
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IO00/T (K-l) Fig. 11. Temperature effect on permeation rate of methanol/acetonitrile mixture containing 20 wt% methanol through a PPy-PTS membrane.
4. Conclusions Polypyrrole films doped by different anions may be used as pervaporation membranes for the separations of organic pairs based on the differences of polarity, molecular size and hydrophilicity. The present work on methanol removal from organic solvents indicates that these membranes display a good selectivity and an acceptable permeation rate under some circumstances. Nevertheless, the phenomena appear to be complex, and although some studies have been performed by other authors [20-22] on the interactions between methanol and polypyrroles, further investigations are necessary to understand the mechanism of pervaporation in such media.
References [1] R.C. Binning, R.J. Lee, J.F. Jennings and E.C. Martin, Separation of liquid mixtures by permeation, Ind. Eng. Chem., 53 (1961) 45-53. [2] R. Rautenbach, S. Klatt and J. Vier, State of the art of pervaporation - 10 Years of industrial PV, Proc. 6th Int. Conf. Pervaporation Processes Chem. Ind., 27-30 September, Ottawa, Canada, 1992, pp. 2-15. [3] H.E,A. Briischke, W.H. Schneider, H. Scholz and H. Steinhauser, Removal of methanol from organic mixtures, in Proc. 6th Int. Conf. Pervaporation Processes Chem. Ind., 27-30 September, Ottawa, Canada, 1992, pp. 423-429. [4] H.C. Park, R.M. Meertens, M.H.V. Mulder and C.A. Smolders, Pervaporation of alcohol-toluene mixtures through polymer blend membranes of poly(acrylic acid) and poly(vinyl alcohol), J. Membrane Sci., 90 (1994) 265-274. [5] M.S.K. Chen, G.S. Markiewicz and K.G. Venugopal, Development of membrane pervaporation TRIM TM process for methanol recovery from C H 3 O H / M T B E / C 4 mixtures, AIChE Symp. Ser., 85 (1989) 82-88. [6] B.A. Farnand and S.H. Noh, Pervaporation as an alternative process for the separation of methanol from C a hydrocarbons
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[7]
[8]
[9]
[10]
[11]
[12]
[13]
in the production of MTBE and TAME, AIChE Symp. Ser., 85 (1989) 89-92. V.M. Shah, C.R. Bartels, M. Pasternak and J. Reale, Opportunities for membranes in the production of octane enhancers, AIChE Symp. Ser., 85 (1989) 93-97. M. Pasternak, Separation of organic liquids such as mixtures of methanol and dimethyl carbonate or methanol and methyl tert-butyl ether, US Pat., 5,238,573. M. Pasternak, Preferential removal of methanol from organic solutions by Nafion membranes containing organic or metal counter-ions, Macromol. Rep., A30 (Suppls. 1 and 2) (1993) 47-54. F. Doghieri, A. Nardella, G.L. Sarti and C. Valentini, Pervaporation of methanol-MTBE mixtures through modified poly(phenylene oxide) membranes, J. Membrane Sci., 91 (1994) 283-291. I. Noezar, Q.T. Nguyen, R. Clement and J. N6el, High performance polymer blend membranes for alcohol-ether separation, Proc. 7th Int. Conf. Pervaporation Processes Chem. Ind., 26 February-1 March, Reno, NV, 1995, pp. 45-51. C. Streicher, P. Kremer, V. Thomas, A. Hubner and G. Ellinghorst, Development of new pervaporation membranes, systems and processes to separate alcohols/ethers/hydrocarbons mixtures, Proc. 7th Inter. Conf. Pervaporation Processes Chem. Ind., 26 February-I March, Reno, NV, 1995, pp. 297-309. H.-H. Schwarz, R. Apostel, K. Richau and D. Paul, Membranes from surfactants for separation of polar organic liquids, Proc. 7th Int. Conf. Pervaporation Processes Chem. Ind., 26 February-1 March, Reno, NV, 1995, pp. 374-382.
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[14] M. Zhou, M. Persin, W. Kujawski and J. Sarrazin, Electrochemically synthesised polypyrrole membranes for the separation of organic mixtures by pervaporation, Proc. 7th Int. Conf. Pervaporation Processes Chem. Ind., 26 February-1 March, Reno, NV, 1995, pp. 193-205. [15] M. Zhou, M. Persin and J. Sarrazin, Electrodeposition of membrane-oriented conducting poly (pyrrole, thiophene) on stainless steel meshes, J. Appl. Electrochem., in press. [16] M. Zhou, M. Persin, W. Kujawski and J. Sarrazin, Electrochemical preparation of polypyrrole membranes and their application in ethanol-cyclohexane separation by pervaporation, J. Membrane Sci., 108 (1995) 89-96. [17] R.C. Weast and M.J. Astle (Eds.), Handbook of Chemistry and Physics, CRC Press, West Palm Beach, FL, 1979. [18] E. Drioli, S. Zhang and A. Basile, On the coupling effect in pervaporation, J. Membrane Sci., 81 (1993) 43-55. [19] R.Y.M. Huang and X. Feng, Pervaporation of water/ethanol mixtures by an aromatic poly(ether imide) membrane, Sep. Sci. Technol., 27 (1992) 1583-1597. [20] D. Blackwood and M. Josowicz, Work function and spectroscopic studies of interactions between conducting polymers and organic vapors, J. Phys. Chem., 95 (1991) 493-502. [21] P. Topart and M. Josowicz, Characterization of the interaction between poly(pyrrole) films and methanol vapor, J. Phys. Chem., 96 (1992) 7824-7830. [22] P. Topart and M. Josowicz, Transcient effects in the interaction between polypyrrole and methanol vapor, J. Phys. Chem., 96 (1992) 8662-8666.