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Sorption properties of organic solvents in PEBA membranes
Journal of Membrane Science 206 (2002) 341–349
Sorption properties of organic solvents in PEBA membranes Yanwei Cen, Claudia Staudt-Bickel, Rüdiger N. Lichtenthaler∗ Applied Thermodynamics, Institute of Physical Chemistry, University of Heidelberg, Im Neuenheimer Feld 253, D-69120 Heidelberg, Germany Received 2 April 2001; received in revised form 4 September 2001; accepted 12 September 2001
Abstract Commercially available polyether block amides (PEBAs) are important hydrophobic membrane polymers for the separation of organics from water. Therefore, in this study the sorption properties are reported for 21 solvents (water, alcohols, alkanes, aromatics, chlorinated hydrocarbons and ethers) in PEBA membranes. The sorption isotherms have been determined at different temperatures for solvent vapor activities up to 0.98. For all solvents it was found that the sorption is independent of temperature in the range between 30 and 80 ◦ C. Two theoretical models have been tested to describe the experimental sorption isotherms. It was found that the Flory–Huggins theory gives only a qualitative description of the sorption isotherms, whereas the UNIQUAC model is able to describe the experimental data very well, i.e. nearly quantitatively. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Solubility and partitioning; PEBA membrane; Flory–Huggins theory; UNIQUAC model
1. Introduction The separation of liquid mixtures by pervaporation is determined by the specific interactions between the permeating components and the membrane material. Based on the solution-diffusion model, the mass transport through polymeric membranes can be described by a three step process: the sorption of the components on the feed side, the diffusion of the components through the membrane and the desorption of the components on the permeate side of the membrane. In pervaporation, the separation process is due to differences in solubility and diffusivity of the feed components in the membrane material. Usually the component with the ∗ Corresponding author. Tel.: +49-6221-545204; fax: +49-6221-544255. E-mail address: [email protected] (R.N. Lichtenthaler).
highest solubility and the largest diffusion coefficient in the polymeric material permeates preferentially [1]. The polyether block amide (PEBA) membranes used in this study for sorption and pervaporation experiments are hydrophobic membranes which were developed at GKSS Research Center (Geesthacht, Germany). PEBAs have been investigated by many authors because they show high selectivity for aromatic hydrocarbons and their derivatives, such as phenol and aniline [2–4]. In order to understand and describe the good separation characteristics of PEBA membranes achieved in pervaporation, the sorption and diffusion properties of each permeating components have to be determined. In this work, the sorption characteristics of different solvents in PEBA membranes are reported. The pervaporation characteristics of PEBA membranes as expected according to the sorption data are discussed.
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2. Experimental 2.1. Apparatus For the measurement of vapor sorption in polymeric materials up to a vapor activity of 0.98, a sorption apparatus has been developed using quartz spring balances as shown schematically in Fig. 1. The apparatus consists of eight quartz springs placed in glass tubes which are located in a water bath (WB), a solvent reservoir (VD) and a condenser (C), whose temperatures are kept at T1, T2 and T3, respectively, with three thermostats (T 1 < T 2 < T 3). The temperature T1 and T2 can be measured precisely with a digital thermometer (TA). In order to avoid activity differences of solvent vapor in the apparatus, the vapor can be circulated from the evaporator VD passing the membrane samples which are suspended from the springs. The driving force for the circulation of the vapor in the apparatus is the pressure
difference between VD and C caused by the temperature difference. The membrane samples suspended from the quartz springs are thermostated at temperature T1 which can be varied between 10 and 90 ◦ C (±0.02 ◦ C). Before the experiment is started the apparatus is evacuated. The solvent is filled into VD through the valve V5. This valve can also be used to remove the solvent after finishing the sorption measurements. As soon as the desired temperature T2 is reached, valve V4 is opened and the membrane samples are exposed to solvent vapor at the corresponding vapor pressure. The changes in elongation of the springs is measured as a function of time by a digital cathetometer until saturation of the membrane samples has been reached. Then valve V4 is closed, the temperature T2 is changed and another equilibrium vapor pressure is established. The next sorption step can be started by opening valve V4 again. With this procedure, the membrane samples can be exposed to solvent activities varying between 0 and 0.98.
Fig. 1. Schematics of the quartz spring balance.
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2.2. Materials The polymeric PEBA membranes are prepared from a PEBA which is commercially available under the trade name PEBAX (Atochem) [2]. PEBAX is produced by a polycondensation reaction of a dicarboxylic polyamide and a polyether diol. The reaction of the rigid polyamide (hard segments) and the amorphous polyether (soft segments) in the presence of heat, vacuum and a catalyst yields the PEBA with the general formula [5]:
In this structure, PA represents the polyamide blocks and PE represents the polyether blocks. The PEBA is a linear, nonbranched polymer containing PA and PE blocks. The membranes investigated in this study have been kindly provided by the GKSS Research Center (Geesthacht, Germany). Information about the particular PE and/or PA blocks which have been used for these membranes unfortunately is not available. The membrane type PEBA-E (extruded) is prepared by melt extrusion of PEBAX and the membrane type PEBA-C (solution casting) is prepared by a solution casting method. The membrane thickness was 25 m for the PEBA-E and 30 m for the PEBA-C, respectively. In order to remove traces of solvent left from the membrane production, all membranes were soaked in ethanol at room temperature for 48 h and dried in vacuum before starting sorption experiments. In this study, the sorption isotherms of 21 common solvents have been determined including water, alcohols, alkanes, aromatics, chlorinated hydrocarbons and ethers. All solvents were obtained from standard supply sources with a minimum purity of 99% and used without further purification.
3. Results and discussion Fig. 2 shows the sorption isotherms of trichloroethylene in PEBA-E at 60 ◦ C. Thereby, the solvent activity ai is plotted versus weight fraction wi of the solvent absorbed in the membrane which is defined as follows: mi wi = (1) mi + m M
Fig. 2. Sorption (䊏) and desorption (䊉) isotherms of trichloroethylene in PEBA-C at 60 ◦ C.
In this expression, mi is the mass of solvent i absorbed in the membrane and mM the mass of the polymer. The solvent activity is given as follows: (Bi − Vi )(Pi − Pi0 ) Pi Pi ai = 0 exp (2) ≈ 0 RT Pi Pi At low pressure ai is nearly equal to the ratio of the vapor pressure Pi of the solvent dissolved in the membrane at temperature T and the vapor pressure Pi0 of the pure solvent at the same temperature. The exponential term corrects for non-ideal behavior of the vapor and usually is close to unity (Bi is the second virial coefficient, Vi the molar liquid volume of the solvent at temperature T and R the universal gas constant). If necessary, all these data can be obtained from standard reference books. At pressures smaller than 1 bar the vapor was treated as an ideal gas [6,7]. Sorption measurements were performed by stepwise increasing the vapor activity from 0 to 0.98, while the increasing amount of solvent absorbed in the membrane samples was measured. After the highest activity was reached, the vapor activity was decreased stepwise, i.e. a desorption experiment was performed in order to reveal possible changes in the membrane morphology due to solvent absorption. As shown in Fig. 2, the sorption and desorption isotherms of trichloroethylene in PEBA-C do not show any significant differences. However, as shown in Fig. 3 slight differences of the sorption and desorption isotherms of
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Fig. 3. Sorption (䊏) and desorption (䊉) isotherms of aniline in PEBA-C at 60 ◦ C.
Fig. 4. Sorption isotherms of n-hexane in PEBA-C-1 (䊏), PEBA-C-2 (䊉) and a PEBA-E (䉱) sample at 60 ◦ C.
aniline in PEBA-C are obvious. At a given activity of solvent vapor, the desorption leads to a higher weight fraction of solvent in the membrane than the sorption. This phenomenon is also widely observed in gas sorption [8,9]. The reason for this behavior is probably that solvent molecules are captured in the polymer matrix during desorption due to kinetic effects. From all solvents investigated in this study, aniline shows the largest difference between sorption and desorption isotherms, which might be due to the low volatility of this substance. However, even for this substance the differences in the sorbed amount during sorption and desorption, respectively, are rather small and nearly within the experimental error. Repetition of the sorption of any solvent after the desorption steps and after having removed the solvent completely by applying vacuum gave the same sorption isotherm within experimental error as before. This shows that the sorption behavior of the polymer films is not affected by previous sorption/desorption processes indicating that the morphology of the films has not been changed. Fig. 4 shows the sorption isotherms of n-hexane in three different membrane samples, whereas only PEBA-E is really a membrane produced in a different way, because the two samples PEBA-C-1 and PEBA-C-2 are the same type of membrane. No significant difference between the sorption isotherms for the latter two membrane samples was found and the same behavior was observed with all other solvents
investigated. For all the solvents slight differences in the sorption isotherms between the PEBA-C and the PEBA-E membranes were found which, however, are of some significance only at high solvent activities (a > 0.8). In general, manufacturing processes for membranes can lead to different sorption characteristics. In a recent study, e.g. investigating gas sorption it was found that casting membrane films using the same polymer but different solvents can lead to significant differences in sorption behavior [10]. The results presented here indicate that the procedure of preparing the different PEBA films is not significantly affecting their sorption behavior for the solvents investigated. In order to find out the effect of temperature on the sorption properties, the temperature at which the experiments were performed was varied between 30 and 80 ◦ C. Fig. 5 shows the sorption isotherms of ethanol in PEBA-E for three different temperatures. Within experimental error all data can be represented by the same spline curve. Corresponding experiments with other solvents not included in this paper, e.g. toluene, in the temperature range between 30 and 70 ◦ C reveal the same results. Therefore, the vapor sorption behavior of the PEBA membranes investigated in general seems to be independent of temperature at least in the temperature range covered. But this should not lead to the wrong conclusion that the transmembrane flux also will be independent of temperature. According to Eq. (3) the flux can be regarded as the product of the
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Fig. 5. Sorption isotherms of ethanol in PEBA-C at three different temperatures: 40 ◦ C (䊉); 60 ◦ C (䊏); and 80 ◦ C (䉱).
solubility coefficient S and the diffusion coefficient D. As the diffusion coefficient, especially in rubbery polymeric materials like PEBA, is very sensitive to the changes in temperature it is obvious that the flux will depend on temperature too. With the quartz spring apparatus used in this study, unfortunately, the dependence of the sorption process on time cannot be measured and therefore diffusion coefficients could not be determined. Fig. 6 shows the sorption isotherms of different alcohols and water in the PEBA-C membrane at 60 ◦ C. Whereas methanol and ethanol are absorbed in the
Fig. 6. Sorption isotherms of water (䊏), methanol (䉬), ethanol (䊐), n-propanol (䊉) and n-butanol (䉱) in PEBA-C at 60 ◦ C.
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Fig. 7. Sorption isotherms of n-hexane (䊏), n-heptane (䊉), iso-octane (䉱), cyclohexane (䉬), cyclohexene (䉫), benzene (䊐) and toluene (䊊) in PEBA-C at 60 ◦ C.
membrane material up to 10 wt.%, only 0.2 wt% of water is absorbed. The sorption isotherms of n-propanol and n-butanol reveal much higher solubilities than those of water, methanol and ethanol. The maximum sorption of n-butanol reaches 55 wt.%. Compared to the water sorption of 0.2% found for the PEBA membranes, it can be concluded that the polymeric material used for the membrane is highly hydrophobic and especially suitable for the separation of organics from water. Fig. 7 shows the sorption isotherms of different aliphatic and aromatic hydrocarbons in the PEBA-C membrane at 60 ◦ C. The sorption isotherms of n-hexane, n-heptane and iso-octane are similar. At high solvent vapor activity all three of them reach a maximum sorption of approximately 20 wt.%. Compared to the linear alkanes, the cycloalkanes cyclohexane and cylcohexene are absorbed much stronger. The maximum for the cyloalkanes is 40 wt.%. With aromatics (benzene and toluene), the highest sorption was reached compared to other solvents investigated in this study. Fig. 8 shows the sorption isotherms of five other solvents at 60 ◦ C. Three of them are halogenated hydrocarbons (chloroform, carbon tetrachloride and trichloroethylene). Generally, all halogenated hydrocarbons show an extremely high solubility in the membrane. During the experiments, strong swelling
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only absorbed in this type of membrane material by 30 wt%. From the results presented it is obvious that there are large differences in solubilities for organics and water. It was expected that also large differences in permeability will appear for the various solvents. According to the solution-diffusion model, the permeability Pi of a component i depends on its solubility Si and diffusivity Di in the membrane polymer, as follows: Pi = Si Di
Fig. 8. Sorption isotherms of chloroform (䊏), CCl4 (䊉), trichloroethylene (䉱), ethylacetate (䊐) and acetone (䉬) in PEBA-C at 60 ◦ C.
of the PEBA membrane material was observed. Ethylacetate shows a maximum sorption of 40 wt.%, whereas with acetone only a maximum sorption of 20 wt.% is reached. In Fig. 9 the sorption isotherms of aniline, tetrahydrofurane (THF), 1,4-dioxane, ethyl-tert-butylether (ETBE) and methyl-tert-butylether (MTBE) are shown. Among them, THF, 1,4-dioxane and aniline showed the highest sorption. Their maximum solubility is between 45 and 55 wt.%. MTBE and ETBE are
Fig. 9. Sorption isotherms of THF (䊏), 1,4-dioxane (䊉), MTBE (䉬), ETBE (䊐) and aniline (䉱) in PEBA-C at 60 ◦ C.
(3)
Due to the fact that the halogenated hydrocarbons as well as aniline show an extremely high solubility in the PEBA membrane compared to the solubility of water, it is expected that this type of membrane can be used for efficiently removing the organic components from aqueous feed streams. This has been verified with experimental investigations [11]. 3.1. Model calculations The differences for the solubility of the solvents used in this study reflect the different interactions between solvent and membrane polymer, which can be described with thermodynamic theories or models, such as the Hildebrand solubility parameters, the Flory–Huggins theory and the UNIQUAC model [12]. The simple Flory–Huggins theory developed for the description of the thermodynamic properties of polymer solutions was tested to describe the experimental sorption isotherms. For a binary system of a polymer p and a solvent i, the Flory–Huggins equation for the chemical potential can be given as follows [13]: µi 1 + χ φp2 = ln ai = ln(1 − φp ) + φp 1 − RT Pn (4) In this equation, Pn is the degree of polymerization of the membrane polymer, φ p the volume fraction of the polymer in the binary solution and χ the so-called Flory–Huggins interaction parameter, representing the difference in intermolecular interaction between the polymer segments and solvent molecules, depending on the polymer structure, solvent and temperature. The Flory–Huggings interaction parameter χ can be obtained for each system by fitting the experimental sorption isotherms with Eq. (4). The parameters
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Table 1 Flory–Huggins interaction parameters χ of PEBA with water and some alcohols according to Eq. (3) at 60 ◦ C Substances
χ
Water Methanol Ethanol n-Propanol n-Butanol
3.48 1.6 1.27 0.92 0.8
obtained for alcohols in the PEBA membrane are listed in Table 1. Using these parameters, the sorption isotherms can be described, but only qualitatively for different alcohols. Calculated and experimental data show significant deviations at higher activities as shown in Fig. 10. For example, the solubility at an activity close to 1 cannot be described adequately with the Flory–Huggins theory, which is obvious from Fig. 10. Actually, the Flory–Huggins interaction parameters are not concentration independent when calculated from the sorption data using Eq. (4). Fig. 11 shows an example how the Flory–Huggins parameter depends on the activity of the solvent vapor the membrane is exposed to. With increasing activity of the solvent vapor, which correlates with an increasing concentration of solvent absorbed by the membrane, the interaction parameter χ decreases. This explains why
Fig. 11. The dependence of Flory–Huggins interaction parameter χ on the activity of ethanol vapor for the system ethanol in PEBA at 60 ◦ C.
the Flory–Huggins theory is not describing the sorption isotherms very well when using the interaction parameter (χ ) independent of concentration. The UNIQUAC model originally proposed by Abrams and Prausnitz [14] is widely used for the description of liquid–liquid and vapor–liquid equilibria. The general equation for the activity ai of a mixture with n components is given by [13] n z Θi φi ln ai = ln φi + qi ln + li + xj lj 2 φi xi j =1 n n Θj τij
n −qi ln Θj τij + qi − qi k=1 Θk τkj j =1
j =1
(5) where li defined as follows: li = 21 z(ri − qi ) − (ri − 1)
(6)
and the volume fraction φ i and the surface fractions Θi and Θi are defined as follows: x i ri φi = n (7) j =1 xj rj Fig. 10. Comparison of experimental sorption data for water (䊏), methanol (䉬), ethanol (䊐), n-propanol (䊉), n-butanol (䉱) in the PEBA-C at 60 ◦ C with calculated results using the Flory–Huggins theory (—).
x i qi Θi = n i=1 xj qj
(8)
xi q Θi = n i j =1 xj qj
(9)
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where τ kj , τ ij and τ j i are the binary interaction parameters with i, j and k ranging from 1 to n. Overall, n (n − 1) τ -parameters are necessary for describing a n-component mixture with the UNIQUAC model. The parameters ri and qi are dimensionless for the relative molecular size and surface of component i related to the size and surface of a CH2 segment in polyethylene, respectively. The coordination number z is a number between 6 and 12 usually set equal to 10 [13]. The equation is a modified version of the original UNIQUAC model extended by an additional term containing the parameter Θi (effective surface of the molecule) which is needed in the case of systems containing molecules which form hydrogen bonds. For a binary system membrane polymer M and solvent i, the UNIQUAC equation reduces as follows: z ri Θi ln ai = ln φi + q1 ln + φ M li − l M 2 φi rM −qi ln(Θi + ΘM τMi ) τMi τiM qi − +ΘM + Θ τ τ Θi + ΘM ΘM Mi i iM (10) Using this equation, the sorption isotherms can be described very well. The binary parameters τ i M and τ Mi can be obtained by fitting the equation to the sorption isotherms. In Table 2, these binary parameters of the solvents investigated are listed together with the dimensionless pure component structural parameters ri and qi . Fig. 12 shows, as an example, the theoretical calculation with UNIQUAC model and the experimental sorption isotherms of alcohols in the PEBA membrane at 60 ◦ C. Compared to the calculated sorption isotherms with Flory–Huggins theory in Fig. 11, the description of sorption isotherms with the UNIQUAC model is much better. The binary UNIQUAC parameters determined from the solubility data of a pure solvent in PEBA are not limited to describe the sorption isotherms of such binary systems. As shown by Enneking et al. [15] the model can also be used for a rather good prediction of the solubility of a solvent mixture in polymeric membranes knowing only the binary parameters of all the pure solvents constituting the mixture.
Table 2 The system-specific parameters of the UNIQUAC model at 60 ◦ C for various solvents (subscript i) and the polymer PEBA (subscript M) Substance
ri a
qi a
τiM
τiM
Water Methanol Ethanol n-Propanol n-Butanol Benzene Cyclohexane Cyclohexene Aniline PEBA
0.920 1.431 2.106 2.780 3.454 3.188 4.046 3.814 3.717 qM /rM = 0.85b
1.400 1.432 1.972 2.512 3.052 2.400 3.240 3.027 2.816
0.599 1.093 1.336 1.746 1.821 0.974 1.033 0.898 0.271
0.254 0.157 0.156 0.129 0.175 0.791 0.715 0.862 1.968
a
[5]. [15] (for polymers the absolute values of qM and rM are large numbers depending on the molecular weight which is not the case for the ratio qM /rM . In the model calculations for polymeric components only this ratio is relevant and therefore it is sufficient to know it). b
Fig. 12. Comparison of experimental sorption data for water (䊏), methanol (䉬), ethanol (䊐), n-propanol (䊉), n-butanol (䉱) in the PEBA-C at 60 ◦ C with calculated results using the UNIQUAC model (—).
4. Conclusion The sorption isotherms of various solvents in two types of PEBA membranes, PEBA-E and PEBA-C manufactured in different ways from the same bulk polymer, show a slight difference in the sorption behavior. Between 40 and 80 ◦ C, the sorption isotherms
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cannot be distinguished from each other for the same solvent/membrane system which implies that the influence of temperature on the amount of solvent absorbed in PEBA can be neglected in this temperature range. The solubilities of each solvent in the PEBA membranes vary strongly from 0.2 to 70 wt.%. Halogenated hydrocarbons, aromatic hydrocarbons and aniline show extremely high solubilities in the membrane polymer. Cyclic hydrocarbons, alkanes and higher alcohols are absorbed very well in the polymer, followed by different ethers, ethanol and methanol. Compared to most of the organic solvents, the sorption of water is extremely small (around 0.2 wt.%). From this difference it is obvious that PEBA membranes are strongly hydrophobic. Therefore, it should be possible to use such membranes to remove organics, especially halogenated hydrocarbons, aromatics and aniline from waste or process water efficiently. The experimental data obtained by vapor sorption experiments can be described with thermodynamic models. The widely used Flory–Huggins theory can describe the sorption isotherms only qualitatively. The prediction from this theory using an interaction parameter χ independent of composition shows large deviations from the experimental results especially at high solvent activities. This clearly indicates that χ cannot be regarded as constant as in the original Flory–Huggins theory, but depends on the composition. The UNIQUAC model, however, has been proven to describe the sorption isotherms of different solvents in the PEBA membranes very well with the binary model parameters being independent of composition. Furthermore, as shown by Enneking et al. [15] the latter model has been used successfully to predict the solubility of solvent mixtures in PEBA membranes knowing only the binary parameters of all the pure solvents constituting the mixture. Acknowledgements The authors are grateful for financial support of the Bundesministerium für Forschung und Technologie (BMBF) of Germany.
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References [1] H.E.A. Brüschke, G.F. Tusel, R. Rautenbach, Pervaporation membranes: application in the chemical process industry, ACS Symp. Ser. Nr. 281 (1985) 467–478. [2] K.W. Böddeker, G. Bengtson, E. Bode, Pervaporation of low volatility aromatics from water, J. Membr. Sci. 53 (1990) 143–158. [3] K.W. Böddeker, G. Bengtson, H. Pingel, Pervaporation of isomeric butanols, J. Membr. Sci. 54 (1990) 1–12. [4] K.W. Böddeker, G. Bengtson, Selective pervaporation of organics from water, in: R.Y.M. Huang (Ed.), Pervaporation Membrane Separation Processes, Elsevier, Amsterdam, 1991 (Chapter 10). [5] J.R. Flesher, Pebax polyether block amide: a new family of engineering thermoplastic elastomers, in: Proceedings of the Symposium on the History of High Performance Polymers at the ACS Meeting in New York, April 1986, pp. 401– 408. [6] J. Gmehling, U. Onken, Vapor–liquid equilibrium data collection, Aqueous-Organic Systems Chemistry Data Series, Vol. 1, Part 1, DECHEMA, 1991. [7] J. Gmehling, U. Onken, W. Arlt, Vapor–liquid equilibrium data collection, Aliphatic Hydrocarbons C4–C6 Chemistry Data Series, Vol. 1, Part 6a, DECHEMA, 1980. [8] H. Funke, Sorption von Reingasen in Polymeren und an Zeolithen bei Drücken bis zu 70 bar, Dissertation, Universität Heidelberg, 1991. [9] G.K. Fleming, W.J. Koros, Dilation of substituted polycarbonates caused by high-pressure carbon dioxide sorption, J. Polym. Sci. Part B: Polym. Phys. 28 (1990) 1137– 1152. [10] B. Schiewe, C. Staudt-Bickel, A. Vuin, G. Wegner, Gas separation for product enrichment in microreactor technology, Chem. Phys. Chem. 2 (2001) 211–218. [11] K. Meckl, R.N. Lichtenthaler, Hybrid process using pervaporation for the removal of organics from process and waste water, J. Membr. Sci. 113 (1996) 81–86. [12] Y. Cen, Untersuchungen zur Pervaporation, Sorption und Diffusion niedermolekularer Stoffe in Polymermembranen, Dissertation, Universität Heidelberg, 1993. [13] J.M. Prausnitz, E. de Gomes Azevedo, R.N. Lichtenthaler, Molecular Thermodynamics of Fluid-Phase Equilibria, 3rd Edition, Prentice Hall, Upper Saddle River, NJ, USA, 1999. [14] D.S. Abrams, J.M. Prausnitz, Statistical thermodynamics of liquid mixtures: a new expression for the excess Gibbs energy of partly or completely miscible mixtures, AIChE J. 21 (1975) 116–128. [15] L. Enneking, A. Heintz, R.N. Lichtenthaler, Sorption equilibria of the ternary mixture benzene/cyclohexene/ cyclohexane, J. Membr. Sci. 115 (1996) 161–170.
Sorption properties of organic solvents in PEBA membranes Yanwei Cen, Claudia Staudt-Bickel, Rüdiger N. Lichtenthaler∗ Applied Thermodynamics, Institute of Physical Chemistry, University of Heidelberg, Im Neuenheimer Feld 253, D-69120 Heidelberg, Germany Received 2 April 2001; received in revised form 4 September 2001; accepted 12 September 2001
Abstract Commercially available polyether block amides (PEBAs) are important hydrophobic membrane polymers for the separation of organics from water. Therefore, in this study the sorption properties are reported for 21 solvents (water, alcohols, alkanes, aromatics, chlorinated hydrocarbons and ethers) in PEBA membranes. The sorption isotherms have been determined at different temperatures for solvent vapor activities up to 0.98. For all solvents it was found that the sorption is independent of temperature in the range between 30 and 80 ◦ C. Two theoretical models have been tested to describe the experimental sorption isotherms. It was found that the Flory–Huggins theory gives only a qualitative description of the sorption isotherms, whereas the UNIQUAC model is able to describe the experimental data very well, i.e. nearly quantitatively. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Solubility and partitioning; PEBA membrane; Flory–Huggins theory; UNIQUAC model
1. Introduction The separation of liquid mixtures by pervaporation is determined by the specific interactions between the permeating components and the membrane material. Based on the solution-diffusion model, the mass transport through polymeric membranes can be described by a three step process: the sorption of the components on the feed side, the diffusion of the components through the membrane and the desorption of the components on the permeate side of the membrane. In pervaporation, the separation process is due to differences in solubility and diffusivity of the feed components in the membrane material. Usually the component with the ∗ Corresponding author. Tel.: +49-6221-545204; fax: +49-6221-544255. E-mail address: [email protected] (R.N. Lichtenthaler).
highest solubility and the largest diffusion coefficient in the polymeric material permeates preferentially [1]. The polyether block amide (PEBA) membranes used in this study for sorption and pervaporation experiments are hydrophobic membranes which were developed at GKSS Research Center (Geesthacht, Germany). PEBAs have been investigated by many authors because they show high selectivity for aromatic hydrocarbons and their derivatives, such as phenol and aniline [2–4]. In order to understand and describe the good separation characteristics of PEBA membranes achieved in pervaporation, the sorption and diffusion properties of each permeating components have to be determined. In this work, the sorption characteristics of different solvents in PEBA membranes are reported. The pervaporation characteristics of PEBA membranes as expected according to the sorption data are discussed.
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2. Experimental 2.1. Apparatus For the measurement of vapor sorption in polymeric materials up to a vapor activity of 0.98, a sorption apparatus has been developed using quartz spring balances as shown schematically in Fig. 1. The apparatus consists of eight quartz springs placed in glass tubes which are located in a water bath (WB), a solvent reservoir (VD) and a condenser (C), whose temperatures are kept at T1, T2 and T3, respectively, with three thermostats (T 1 < T 2 < T 3). The temperature T1 and T2 can be measured precisely with a digital thermometer (TA). In order to avoid activity differences of solvent vapor in the apparatus, the vapor can be circulated from the evaporator VD passing the membrane samples which are suspended from the springs. The driving force for the circulation of the vapor in the apparatus is the pressure
difference between VD and C caused by the temperature difference. The membrane samples suspended from the quartz springs are thermostated at temperature T1 which can be varied between 10 and 90 ◦ C (±0.02 ◦ C). Before the experiment is started the apparatus is evacuated. The solvent is filled into VD through the valve V5. This valve can also be used to remove the solvent after finishing the sorption measurements. As soon as the desired temperature T2 is reached, valve V4 is opened and the membrane samples are exposed to solvent vapor at the corresponding vapor pressure. The changes in elongation of the springs is measured as a function of time by a digital cathetometer until saturation of the membrane samples has been reached. Then valve V4 is closed, the temperature T2 is changed and another equilibrium vapor pressure is established. The next sorption step can be started by opening valve V4 again. With this procedure, the membrane samples can be exposed to solvent activities varying between 0 and 0.98.
Fig. 1. Schematics of the quartz spring balance.
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2.2. Materials The polymeric PEBA membranes are prepared from a PEBA which is commercially available under the trade name PEBAX (Atochem) [2]. PEBAX is produced by a polycondensation reaction of a dicarboxylic polyamide and a polyether diol. The reaction of the rigid polyamide (hard segments) and the amorphous polyether (soft segments) in the presence of heat, vacuum and a catalyst yields the PEBA with the general formula [5]:
In this structure, PA represents the polyamide blocks and PE represents the polyether blocks. The PEBA is a linear, nonbranched polymer containing PA and PE blocks. The membranes investigated in this study have been kindly provided by the GKSS Research Center (Geesthacht, Germany). Information about the particular PE and/or PA blocks which have been used for these membranes unfortunately is not available. The membrane type PEBA-E (extruded) is prepared by melt extrusion of PEBAX and the membrane type PEBA-C (solution casting) is prepared by a solution casting method. The membrane thickness was 25 m for the PEBA-E and 30 m for the PEBA-C, respectively. In order to remove traces of solvent left from the membrane production, all membranes were soaked in ethanol at room temperature for 48 h and dried in vacuum before starting sorption experiments. In this study, the sorption isotherms of 21 common solvents have been determined including water, alcohols, alkanes, aromatics, chlorinated hydrocarbons and ethers. All solvents were obtained from standard supply sources with a minimum purity of 99% and used without further purification.
3. Results and discussion Fig. 2 shows the sorption isotherms of trichloroethylene in PEBA-E at 60 ◦ C. Thereby, the solvent activity ai is plotted versus weight fraction wi of the solvent absorbed in the membrane which is defined as follows: mi wi = (1) mi + m M
Fig. 2. Sorption (䊏) and desorption (䊉) isotherms of trichloroethylene in PEBA-C at 60 ◦ C.
In this expression, mi is the mass of solvent i absorbed in the membrane and mM the mass of the polymer. The solvent activity is given as follows: (Bi − Vi )(Pi − Pi0 ) Pi Pi ai = 0 exp (2) ≈ 0 RT Pi Pi At low pressure ai is nearly equal to the ratio of the vapor pressure Pi of the solvent dissolved in the membrane at temperature T and the vapor pressure Pi0 of the pure solvent at the same temperature. The exponential term corrects for non-ideal behavior of the vapor and usually is close to unity (Bi is the second virial coefficient, Vi the molar liquid volume of the solvent at temperature T and R the universal gas constant). If necessary, all these data can be obtained from standard reference books. At pressures smaller than 1 bar the vapor was treated as an ideal gas [6,7]. Sorption measurements were performed by stepwise increasing the vapor activity from 0 to 0.98, while the increasing amount of solvent absorbed in the membrane samples was measured. After the highest activity was reached, the vapor activity was decreased stepwise, i.e. a desorption experiment was performed in order to reveal possible changes in the membrane morphology due to solvent absorption. As shown in Fig. 2, the sorption and desorption isotherms of trichloroethylene in PEBA-C do not show any significant differences. However, as shown in Fig. 3 slight differences of the sorption and desorption isotherms of
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Fig. 3. Sorption (䊏) and desorption (䊉) isotherms of aniline in PEBA-C at 60 ◦ C.
Fig. 4. Sorption isotherms of n-hexane in PEBA-C-1 (䊏), PEBA-C-2 (䊉) and a PEBA-E (䉱) sample at 60 ◦ C.
aniline in PEBA-C are obvious. At a given activity of solvent vapor, the desorption leads to a higher weight fraction of solvent in the membrane than the sorption. This phenomenon is also widely observed in gas sorption [8,9]. The reason for this behavior is probably that solvent molecules are captured in the polymer matrix during desorption due to kinetic effects. From all solvents investigated in this study, aniline shows the largest difference between sorption and desorption isotherms, which might be due to the low volatility of this substance. However, even for this substance the differences in the sorbed amount during sorption and desorption, respectively, are rather small and nearly within the experimental error. Repetition of the sorption of any solvent after the desorption steps and after having removed the solvent completely by applying vacuum gave the same sorption isotherm within experimental error as before. This shows that the sorption behavior of the polymer films is not affected by previous sorption/desorption processes indicating that the morphology of the films has not been changed. Fig. 4 shows the sorption isotherms of n-hexane in three different membrane samples, whereas only PEBA-E is really a membrane produced in a different way, because the two samples PEBA-C-1 and PEBA-C-2 are the same type of membrane. No significant difference between the sorption isotherms for the latter two membrane samples was found and the same behavior was observed with all other solvents
investigated. For all the solvents slight differences in the sorption isotherms between the PEBA-C and the PEBA-E membranes were found which, however, are of some significance only at high solvent activities (a > 0.8). In general, manufacturing processes for membranes can lead to different sorption characteristics. In a recent study, e.g. investigating gas sorption it was found that casting membrane films using the same polymer but different solvents can lead to significant differences in sorption behavior [10]. The results presented here indicate that the procedure of preparing the different PEBA films is not significantly affecting their sorption behavior for the solvents investigated. In order to find out the effect of temperature on the sorption properties, the temperature at which the experiments were performed was varied between 30 and 80 ◦ C. Fig. 5 shows the sorption isotherms of ethanol in PEBA-E for three different temperatures. Within experimental error all data can be represented by the same spline curve. Corresponding experiments with other solvents not included in this paper, e.g. toluene, in the temperature range between 30 and 70 ◦ C reveal the same results. Therefore, the vapor sorption behavior of the PEBA membranes investigated in general seems to be independent of temperature at least in the temperature range covered. But this should not lead to the wrong conclusion that the transmembrane flux also will be independent of temperature. According to Eq. (3) the flux can be regarded as the product of the
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Fig. 5. Sorption isotherms of ethanol in PEBA-C at three different temperatures: 40 ◦ C (䊉); 60 ◦ C (䊏); and 80 ◦ C (䉱).
solubility coefficient S and the diffusion coefficient D. As the diffusion coefficient, especially in rubbery polymeric materials like PEBA, is very sensitive to the changes in temperature it is obvious that the flux will depend on temperature too. With the quartz spring apparatus used in this study, unfortunately, the dependence of the sorption process on time cannot be measured and therefore diffusion coefficients could not be determined. Fig. 6 shows the sorption isotherms of different alcohols and water in the PEBA-C membrane at 60 ◦ C. Whereas methanol and ethanol are absorbed in the
Fig. 6. Sorption isotherms of water (䊏), methanol (䉬), ethanol (䊐), n-propanol (䊉) and n-butanol (䉱) in PEBA-C at 60 ◦ C.
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Fig. 7. Sorption isotherms of n-hexane (䊏), n-heptane (䊉), iso-octane (䉱), cyclohexane (䉬), cyclohexene (䉫), benzene (䊐) and toluene (䊊) in PEBA-C at 60 ◦ C.
membrane material up to 10 wt.%, only 0.2 wt% of water is absorbed. The sorption isotherms of n-propanol and n-butanol reveal much higher solubilities than those of water, methanol and ethanol. The maximum sorption of n-butanol reaches 55 wt.%. Compared to the water sorption of 0.2% found for the PEBA membranes, it can be concluded that the polymeric material used for the membrane is highly hydrophobic and especially suitable for the separation of organics from water. Fig. 7 shows the sorption isotherms of different aliphatic and aromatic hydrocarbons in the PEBA-C membrane at 60 ◦ C. The sorption isotherms of n-hexane, n-heptane and iso-octane are similar. At high solvent vapor activity all three of them reach a maximum sorption of approximately 20 wt.%. Compared to the linear alkanes, the cycloalkanes cyclohexane and cylcohexene are absorbed much stronger. The maximum for the cyloalkanes is 40 wt.%. With aromatics (benzene and toluene), the highest sorption was reached compared to other solvents investigated in this study. Fig. 8 shows the sorption isotherms of five other solvents at 60 ◦ C. Three of them are halogenated hydrocarbons (chloroform, carbon tetrachloride and trichloroethylene). Generally, all halogenated hydrocarbons show an extremely high solubility in the membrane. During the experiments, strong swelling
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only absorbed in this type of membrane material by 30 wt%. From the results presented it is obvious that there are large differences in solubilities for organics and water. It was expected that also large differences in permeability will appear for the various solvents. According to the solution-diffusion model, the permeability Pi of a component i depends on its solubility Si and diffusivity Di in the membrane polymer, as follows: Pi = Si Di
Fig. 8. Sorption isotherms of chloroform (䊏), CCl4 (䊉), trichloroethylene (䉱), ethylacetate (䊐) and acetone (䉬) in PEBA-C at 60 ◦ C.
of the PEBA membrane material was observed. Ethylacetate shows a maximum sorption of 40 wt.%, whereas with acetone only a maximum sorption of 20 wt.% is reached. In Fig. 9 the sorption isotherms of aniline, tetrahydrofurane (THF), 1,4-dioxane, ethyl-tert-butylether (ETBE) and methyl-tert-butylether (MTBE) are shown. Among them, THF, 1,4-dioxane and aniline showed the highest sorption. Their maximum solubility is between 45 and 55 wt.%. MTBE and ETBE are
Fig. 9. Sorption isotherms of THF (䊏), 1,4-dioxane (䊉), MTBE (䉬), ETBE (䊐) and aniline (䉱) in PEBA-C at 60 ◦ C.
(3)
Due to the fact that the halogenated hydrocarbons as well as aniline show an extremely high solubility in the PEBA membrane compared to the solubility of water, it is expected that this type of membrane can be used for efficiently removing the organic components from aqueous feed streams. This has been verified with experimental investigations [11]. 3.1. Model calculations The differences for the solubility of the solvents used in this study reflect the different interactions between solvent and membrane polymer, which can be described with thermodynamic theories or models, such as the Hildebrand solubility parameters, the Flory–Huggins theory and the UNIQUAC model [12]. The simple Flory–Huggins theory developed for the description of the thermodynamic properties of polymer solutions was tested to describe the experimental sorption isotherms. For a binary system of a polymer p and a solvent i, the Flory–Huggins equation for the chemical potential can be given as follows [13]: µi 1 + χ φp2 = ln ai = ln(1 − φp ) + φp 1 − RT Pn (4) In this equation, Pn is the degree of polymerization of the membrane polymer, φ p the volume fraction of the polymer in the binary solution and χ the so-called Flory–Huggins interaction parameter, representing the difference in intermolecular interaction between the polymer segments and solvent molecules, depending on the polymer structure, solvent and temperature. The Flory–Huggings interaction parameter χ can be obtained for each system by fitting the experimental sorption isotherms with Eq. (4). The parameters
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Table 1 Flory–Huggins interaction parameters χ of PEBA with water and some alcohols according to Eq. (3) at 60 ◦ C Substances
χ
Water Methanol Ethanol n-Propanol n-Butanol
3.48 1.6 1.27 0.92 0.8
obtained for alcohols in the PEBA membrane are listed in Table 1. Using these parameters, the sorption isotherms can be described, but only qualitatively for different alcohols. Calculated and experimental data show significant deviations at higher activities as shown in Fig. 10. For example, the solubility at an activity close to 1 cannot be described adequately with the Flory–Huggins theory, which is obvious from Fig. 10. Actually, the Flory–Huggins interaction parameters are not concentration independent when calculated from the sorption data using Eq. (4). Fig. 11 shows an example how the Flory–Huggins parameter depends on the activity of the solvent vapor the membrane is exposed to. With increasing activity of the solvent vapor, which correlates with an increasing concentration of solvent absorbed by the membrane, the interaction parameter χ decreases. This explains why
Fig. 11. The dependence of Flory–Huggins interaction parameter χ on the activity of ethanol vapor for the system ethanol in PEBA at 60 ◦ C.
the Flory–Huggins theory is not describing the sorption isotherms very well when using the interaction parameter (χ ) independent of concentration. The UNIQUAC model originally proposed by Abrams and Prausnitz [14] is widely used for the description of liquid–liquid and vapor–liquid equilibria. The general equation for the activity ai of a mixture with n components is given by [13] n z Θi φi ln ai = ln φi + qi ln + li + xj lj 2 φi xi j =1 n n Θj τij
n −qi ln Θj τij + qi − qi k=1 Θk τkj j =1
j =1
(5) where li defined as follows: li = 21 z(ri − qi ) − (ri − 1)
(6)
and the volume fraction φ i and the surface fractions Θi and Θi are defined as follows: x i ri φi = n (7) j =1 xj rj Fig. 10. Comparison of experimental sorption data for water (䊏), methanol (䉬), ethanol (䊐), n-propanol (䊉), n-butanol (䉱) in the PEBA-C at 60 ◦ C with calculated results using the Flory–Huggins theory (—).
x i qi Θi = n i=1 xj qj
(8)
xi q Θi = n i j =1 xj qj
(9)
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where τ kj , τ ij and τ j i are the binary interaction parameters with i, j and k ranging from 1 to n. Overall, n (n − 1) τ -parameters are necessary for describing a n-component mixture with the UNIQUAC model. The parameters ri and qi are dimensionless for the relative molecular size and surface of component i related to the size and surface of a CH2 segment in polyethylene, respectively. The coordination number z is a number between 6 and 12 usually set equal to 10 [13]. The equation is a modified version of the original UNIQUAC model extended by an additional term containing the parameter Θi (effective surface of the molecule) which is needed in the case of systems containing molecules which form hydrogen bonds. For a binary system membrane polymer M and solvent i, the UNIQUAC equation reduces as follows: z ri Θi ln ai = ln φi + q1 ln + φ M li − l M 2 φi rM −qi ln(Θi + ΘM τMi ) τMi τiM qi − +ΘM + Θ τ τ Θi + ΘM ΘM Mi i iM (10) Using this equation, the sorption isotherms can be described very well. The binary parameters τ i M and τ Mi can be obtained by fitting the equation to the sorption isotherms. In Table 2, these binary parameters of the solvents investigated are listed together with the dimensionless pure component structural parameters ri and qi . Fig. 12 shows, as an example, the theoretical calculation with UNIQUAC model and the experimental sorption isotherms of alcohols in the PEBA membrane at 60 ◦ C. Compared to the calculated sorption isotherms with Flory–Huggins theory in Fig. 11, the description of sorption isotherms with the UNIQUAC model is much better. The binary UNIQUAC parameters determined from the solubility data of a pure solvent in PEBA are not limited to describe the sorption isotherms of such binary systems. As shown by Enneking et al. [15] the model can also be used for a rather good prediction of the solubility of a solvent mixture in polymeric membranes knowing only the binary parameters of all the pure solvents constituting the mixture.
Table 2 The system-specific parameters of the UNIQUAC model at 60 ◦ C for various solvents (subscript i) and the polymer PEBA (subscript M) Substance
ri a
qi a
τiM
τiM
Water Methanol Ethanol n-Propanol n-Butanol Benzene Cyclohexane Cyclohexene Aniline PEBA
0.920 1.431 2.106 2.780 3.454 3.188 4.046 3.814 3.717 qM /rM = 0.85b
1.400 1.432 1.972 2.512 3.052 2.400 3.240 3.027 2.816
0.599 1.093 1.336 1.746 1.821 0.974 1.033 0.898 0.271
0.254 0.157 0.156 0.129 0.175 0.791 0.715 0.862 1.968
a
[5]. [15] (for polymers the absolute values of qM and rM are large numbers depending on the molecular weight which is not the case for the ratio qM /rM . In the model calculations for polymeric components only this ratio is relevant and therefore it is sufficient to know it). b
Fig. 12. Comparison of experimental sorption data for water (䊏), methanol (䉬), ethanol (䊐), n-propanol (䊉), n-butanol (䉱) in the PEBA-C at 60 ◦ C with calculated results using the UNIQUAC model (—).
4. Conclusion The sorption isotherms of various solvents in two types of PEBA membranes, PEBA-E and PEBA-C manufactured in different ways from the same bulk polymer, show a slight difference in the sorption behavior. Between 40 and 80 ◦ C, the sorption isotherms
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cannot be distinguished from each other for the same solvent/membrane system which implies that the influence of temperature on the amount of solvent absorbed in PEBA can be neglected in this temperature range. The solubilities of each solvent in the PEBA membranes vary strongly from 0.2 to 70 wt.%. Halogenated hydrocarbons, aromatic hydrocarbons and aniline show extremely high solubilities in the membrane polymer. Cyclic hydrocarbons, alkanes and higher alcohols are absorbed very well in the polymer, followed by different ethers, ethanol and methanol. Compared to most of the organic solvents, the sorption of water is extremely small (around 0.2 wt.%). From this difference it is obvious that PEBA membranes are strongly hydrophobic. Therefore, it should be possible to use such membranes to remove organics, especially halogenated hydrocarbons, aromatics and aniline from waste or process water efficiently. The experimental data obtained by vapor sorption experiments can be described with thermodynamic models. The widely used Flory–Huggins theory can describe the sorption isotherms only qualitatively. The prediction from this theory using an interaction parameter χ independent of composition shows large deviations from the experimental results especially at high solvent activities. This clearly indicates that χ cannot be regarded as constant as in the original Flory–Huggins theory, but depends on the composition. The UNIQUAC model, however, has been proven to describe the sorption isotherms of different solvents in the PEBA membranes very well with the binary model parameters being independent of composition. Furthermore, as shown by Enneking et al. [15] the latter model has been used successfully to predict the solubility of solvent mixtures in PEBA membranes knowing only the binary parameters of all the pure solvents constituting the mixture. Acknowledgements The authors are grateful for financial support of the Bundesministerium für Forschung und Technologie (BMBF) of Germany.
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