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Amorphous Teflons AF as organophilic pervaporation materials
Journal of Membrane Science 216 (2003) 241–256
Amorphous Teflons AF as organophilic pervaporation materials Transport of individual components A.M. Polyakov, L.E. Starannikova, Yu.P. Yampolskii∗ A.V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Leninsky Pr., 29, 119991 Moscow, Russia Received 9 July 2002; received in revised form 7 January 2003; accepted 31 January 2003
Abstract Permeation and sorption of various organic liquids (chlorinated hydrocarbons, lower alcohols, hydrocarbons, etc.) in amorphous copolymers of 2,2-bis-trifluoromethyl-4,5-difluoro-1,3-dioxole and tetrafluoroethylene (amorphous Teflons AF) were studied in the temperature range 5–95 ◦ C. Based on permeation rate and solubility coefficients, the diffusion coefficients of organic penetrants in AF copolymers AF were estimated. It was shown that the copolymer with a higher content (87%) of the dioxole component (AF2400), as distinguished by a larger free volume, is much more permeable to liquids than the copolymer AF1600 containing 65% of the dioxole comonomer. In contrast, solubility of organic compounds in both copolymers is hardly affected by their composition. The effects of penetrant parameters, such as critical volume (Vc ) and critical temperature (Tc ) were analyzed. The variation in permeation rates of liquids and gases through these materials are consistent with a mobility (diffusion) controlled mechanism for mass transfer. A trade-off between the Arrhenius parameters for permeability and the van’t Hoff parameters for solubility is demonstrated for pervaporation. On this basis, novel correlations are obtained that enable a determination of the activation energy for permeation EP and the enthalpy of sorption HS from the permeability (P) and solubility (S) coefficients. Long term tests (up to 12 months) showed excellent time stability of the transport parameters, a property unexpected and quite important for a high flux, high free volume pervaporation material. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Pervaporation; Sorption; Diffusion; Chloromethanes; Teflons AF
1. Introduction Pervaporation is one of the oldest membrane process [1]. However, systematic studies of separation of liquid mixtures by means of pervaporation started as late as in 1950–1960s [2,3], whereas the practical implementation of pervaporation was accomplished only in 1980s thanks to the development of the industrially produced GFT membrane [4]. A classification of various pervaporation processes has been proposed ∗ Corresponding author. Tel.: +7-95-9554210; fax: +7-95-2302224. E-mail address: [email protected] (Yu.P. Yampolskii).
based on the aims of separation [5]. According to it and depending on the target products of water–organic mixtures, one can speak of hydrophilic or hydrophobic pervaporation, where the permeate flux is enriched with either water or organic components, respectively. These types of pervaporation have attracted more attention of researchers. However, there are numerous problems and a wide variety of industrial streams where different mixtures of liquid organic compounds must be subjected to separation. This kind of separation process is known as organic–organic, or target-organophilic pervaporation. Because of importance of separation processes and due to the high efficiency of pervaporation compared to traditional
0376-7388/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0376-7388(03)00077-2
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Nomenclature C D D0 DAV Ea ED EP HC HS HV F FFV J J0 l m p ps P R S S0
SAV T Tc Tg Vc x
the concentration of organic component in the polymer and solubility (g/g) the diffusion coefficient (cm2 /s) pre-exponential factor in Arrhenius dependence of D (cm2 /s) the apparent diffusion coefficient (cm2 /s) the apparent activation energy of flux (kJ/mol) the activation energy of diffusion (kJ/mol) the activation energy of permeation (kJ/mol) the enthalpy of condensation (kJ/mol) the enthalpy of sorption (kJ/mol) the enthalpy of evaporation (kJ/mol) the surface area of membrane (m2 ) fractional free volume (%) permeate flux (kg/(m2 h)) pre-exponential factor in Arrhenius dependence of J (kg/(m2 h)) the film thickness (m) the mass of permeate (kg) the vapor pressure (cmHg) the saturated vapor pressure (cmHg) the permeability coefficient (1 Barrer = 1 × 10−10 cm3 (STP) cm/(cm2 s cmHg)) universal gas constant (J/(mol K)) the solubility coefficient (cm3 (STP)/(cm3 cmHg)) pre-exponential factor in van’t Hoff dependence of S (cm3 (STP)/(cm3 cmHg)) the apparent solubility coefficient (cm3 (STP)/(cm3 cmHg)) temperature (◦ C) critical temperature (◦ C) glass transition temperature (◦ C) critical volume (cm3 /mol) coordinate along the diffusion process
Greek letters α ideal selectivity τ time interval (h)
processes such as adsorption, distillation and extraction, the most developed process is hydrophilic pervaporation [6,7]. Two other types of pervaporation (hydrophobic and target-organophilic pervaporation) are also distinguished by higher energy efficiency, possibility due to the use of compact equipment and low inertia [8,9] in comparison to alternative processes. These have not found substantial practical development. Although the effects of process conditions on pervaporation parameters have been studied in detail [8–10], the principles for selection of membrane materials for specific separation problems are still a relevant and insufficiently developed problem [11,12]. In the formulation of separation principles one has to take into account the specific transport and sorption parameters needed for the separation and possible strong effects of the liquid on the membrane material. All this makes a prognostication of membrane behavior on the of separation a difficult and still unsolved problem. Just as in the case of other membrane materials, pervaporation materials should satisfy several requirements: high transport parameters (permeability and permselectivity), stability in contact with the separated liquid mixture, thermal stability, and stable transport and mechanical characteristics over time. An analysis of the literature indicates that, in spite of the diversity of available polymeric materials, only a few have been studied as membrane materials for hydrophobic and target-organophilic pervaporation [5–10]. In this regard, a consideration of novel classes of polymeric materials should extend the potential applications of this type. So the objective of this work was to study pervaporation properties of the copolymers of 2,2-bis-trifluoromethyl-4,5-difluoro1,3-dioxole and tetrafluoroethylene (amorphous Teflons AF of DuPont Co.) having the following chemical structure:
These copolymers have been studied in detail as materials for gas separating membranes [13–15]. They reveal extremely high gas permeability and are
A.M. Polyakov et al. / Journal of Membrane Science 216 (2003) 241–256 Table 1 Properties of Teflons AF [14,17,18] Property
AF2400
AF1600
Content of dioxol component (mol%) Crystallinity Tg (◦ C) FFV (%) Density (g/cm3 ) Permeability O2 (Barrer)a
87 None 240 33.4 1.75 1140
65 None 160 30.0 1.82 170
None C 6 F6 , C6 H5 CF3 , FC-72, etc.
None C 6 F6 , C6 H5 CF3 , FC-72, etc.
26.4 7.9
26.9 17.1
Solubility Common organic solvents Perfluorinated solvents
Mechanical properties Stress at break (MPa) Elongation at break (%) a
1 Barrer = 1 × 10−10 cm3 (STP) cm/(cm2 s cmHg).
characterized by very large free volume: the average radii of free volume elements in them are about 5–6 Å, that is, much larger than those in other polymers [16]. The latter result implies that liquid permeation rates in these polymers would be also high. It is also worth noting that these copolymers exhibit several properties important for pervaporation: insolubility in most organic solvents (except perfluorinated) solvents, excellent chemical and thermal stability, very good film forming properties. Some of the characteristics of amorphous Teflons AF are presented in Table 1. In this work permeability and sorption of chloromethanes (CH2 Cl2 , CHCl3 , CCl4 ) and some other liquid organic compounds in Teflons AF were studied. 2. Background In pervaporation, the specific permeate flux J (kg/(m2 h)) can be determined using the formula: m (1) J = Fτ where m is the mass of permeate collected for time interval τ and F is the surface area of the film. Using this flux, permeability coefficients of a liquid penetrant can be found via the equation: Jl P = (2) ps
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where ps is the saturated vapor pressure at the experimental temperature and l is the film thickness. It is well known that mass transfer in pervaporation, just as in gas permeation, can be described by the solution-diffusion mechanism [12]. In this model the permeability coefficient P is a function of the diffusion coefficient D, which characterizes the rate of mass transfer under a certain gradient of concentration, and solubility coefficient S, which determines the driving force (concentration gradient) in the membrane: P = DS
(3)
Depending on the physical state of a material (i.e. glassy or rubbery) the variation of the permeability coefficients and selectivity for a penetrant series can be governed by changes in either D or S. So the transport process can be categorized in terms of mobility or solubility controlled mass transfer [19,20]. In the case of mobility controlled mass transfer the variation in the P values for a penetrant series are similar to those of the diffusion coefficients for the same penetrants, that is, permeability decreases when the size of diffusing molecule increases. For solubility controlled mass transfer, both permeability and solubility coefficients increase with the size and mass of the penetrant. In pervaporation processes, the conditions on the upstream and downstream sides of membranes differ markedly even at steady state because one surface of the membrane is in contact with the organic liquid while the other side is in contact with vapor of low pressure or activity. The non-linear character of the sorption isotherms results in essentially non-linear penetrant concentration profile across the membrane. In addition, the diffusion coefficient of the liquid penetrant depends on the penetrant concentration and, hence, in an indirect way, on the location across the membrane. Therefore, a solution of Fick’s first law gives: J = −D(c)
dc dx
(4)
where c is a complex function of x [11]. Integration of Eq. (4) results in: 1 J = D(c) dc (5) l
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In this way, a solution of the equation can only be obtained provided experimental sorption isotherms and concentration dependencies of the diffusion coefficients are known over the whole range of activity from 0 to 1. The methods for solution of the equation for the flux J are well described in the literature [21,22]. For liquids (p/ps is equal to unity), the apparent solubility coefficients S can be defined as: C S= ps
(6)
where C is the concentration of the organic component in the polymer. The effects of temperature on pervaporation rate are described by the Arrhenius equation: Ea J = J0 exp − (7) RT where Ea is the apparent activation energy of flux. For the solution-diffusion mechanism of pervaporation, the apparent activation energies of permeation should depend on several factors. The temperature changes cause a variation in the driving force for the process because of changes in the saturated vapor pressure and, therefore, the chemical potential and concentration of a penetrant across the membrane. The van’t Hoff equation can be used to describe temperature dependence of the solubility coefficient: HS S = S0 exp − (8) RT where HS is the enthalpy change due to transfer of penetrant from its liquid phase to the polymer (enthalpy of dissolution in the terms of solution thermodynamics). Finally, temperature effects for diffusion coefficients are defined by the Arrhenius equation: ED D = D0 exp − (9) RT where ED is the activation energy for diffusion.
3. Experimental 3.1. Film casting Dense films of AF2400 and AF1600 were prepared by casting solutions of the polymers (1.5 mass%) in
octafluorotoluene onto the surface of cellophane in the bottom of cylindrical cups. The time for evaporation of the solvent was about 7–9 days. Then the films were kept in vacuum at 40 ◦ C until constant weight was achieved. In this manner films with a thickness of 5–40 m were obtained. 3.2. Pervaporation experiments The experiments were carried out using the set up shown in Fig. 1. The separation cell was provided with a mixer and a jacket. A film with a working surface area of 27.3 cm2 was mounted over a porous metal support and sealed by an O-ring made of Teflon. The separation was performed in the temperature range 5–95 ◦ C with the deviations from the isothermal regime being within 0.5 ◦ C. Residual pressure in the downstream part of the cell was about 267 Pa (2 Torr). The permeate vapor was condensed in a trap cooled with liquid nitrogen. The quantity of permeate collected for a certain time interval was determined by weighing and in this way the specific permeate flux J (kg/(m2 h)) was determined using Eq. (1). 3.3. Sorption measurement The sorption of liquid permeates was studied using a gravimetric method. Samples of dense films with a thickness of 50–80 m and an initial mass of 0.1–0.2 g were equilibrated in the liquid organic solutions at temperatures of 5–80 ◦ C. Then the samples taken from the liquids were weighed using an analytical balance after removal of liquid droplets with filter paper.
4. Results and discussion 4.1. The effects of nature and properties of penetrants on their permeation rates through and sorption in Teflons AF Pervaporation and sorption of organic liquids having different chemical structure, molecular mass and size were studied in amorphous Teflons AF. Table 2 presents several physical properties [23] of the organic liquids studied, together with the observed permeability (Jl) and solubility (C) in the AF copolymers. It
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Fig. 1. Schematic of the pervaporation apparatus. I—Membrane cell; II—thermostats; III—montejus; IV—compressor; V—cool traps; VI—air drier (CaCl2 ); VII—vacuum pump; TI—thermometer; PI—vacuumeter.
can be seen that permeability of the studied penetrants varies over a substantially wider range than the solubility. Just as it has been observed for gas permeability [14] the copolymer with the larger content of the dioxole component and exhibiting the higher free volume (AF2400) displays larger liquid permeability. Solubility of organic liquids also increases with an increase in the content of the dioxole comonomer. However, these differences are less than those observed for gases and vapors [15,24,25]. The results of this work on liquid permeation rates, as well as those published earlier for gas permeation [13–15,24,25], indicate that permeability coefficients of Teflons AF decrease, when the size of penetrant, as expressed, e.g. by its critical volume Vc , increases. Such behavior is typical for glassy polymers. Fig. 2 shows the correlations of permeability coefficients
of gases, vapors and liquids with the critical volume of the penetrant. For liquid permeation, the permeability coefficients were calculated using Eq. (2). Similar correlations are shown for a conventional glassy polymer, polysulfone (PSF); a glassy polymer with extremely large free volume, poly(trimethylsilyl propyne) (PTMSP); a rubber, polydimethylsiloxane (PDMS); and, a perfluorinated polymer, polytetrafluoriethylene (PTFE). It is seen that, at comparable critical volumes of penetrants, the permeation rate of the liquid through Teflons AF is higher than for gases and vapors. The dependence of permeability on penetrant size is weaker for liquids than for gases. It can be noted that, for many penetrant-polymer systems, permeability is an increasing function of penetrant activity. Liquid permeation in pervaporation processes proceeds at the maximum activity (i.e. p/ps = 1).
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Table 2 Physicochemical properties of organic liquids and their transport and sorption parameters in Teflons AF (25 ◦ C) Compound
CH2 Cl2 CHCl3 CCl4 H2 O CH3 OH C2 H5 OH (CH3 )2 CO C 6 H6 C6 H 5 F C6 H12
Tc (K)
510 536.6 556.4 647 513.2 516.3 509.1 562.1 560.1 553.2
Vc (cm3 /mol)
193 240 276 56 118 167 211 260 271 308
At this activity, polymers are swollen and plasticized, leading to increased permeability relative to what would be obtained for infinite dilution gas permeation experiments. This might be partially why, at a given penetrant size, permeability of liquids is higher than can be extrapolated from gas permeability. In order to evaluate the contribution of the solubility coefficient to the permeability coefficient during per-
Jl (kg m/(m2 h))
C (g/100 g polymer)
AF2400
AF1600
AF2400
AF1600
28.30 11.91 1.76 0.97 0.98 0.67 6.02 2.86 5.24 0.36
1.10 0.21 0.08 0.07 – – – 0.72 0.57 –
5.15 9.11 16.94 – 1.84 2.81 2.43 4.65 – 6.75
4.36 8.14 11.90 – 0.54 1.12 2.38 4.35 – 4.21
vaporation, we compared the sorption data for gases, vapors, and liquids in the polymers studied. The results for gas and vapor sorption were taken from Bondar et al. [24] and Alentiev et al. [25]. The solubility coefficients for the gases and vapors were measured by pressure decay and gas chromatographic methods and used to estimate the infinite dilution conditions, i.e. the initial parts of the sorption isotherm at activity p/ps close
Fig. 2. Dependence of permeability coefficient P (Barrer) on critical volume (PDMS—40 ◦ C [26]; PTMSP—23 ◦ C [27]; PTFE—25 ◦ C [28,29]; PSF—23 ◦ C [30]).
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Fig. 3. Solubility coefficients S (cm3 (STP)/(cm3 cmHg)) vs. the square of critical temperature (PDMS—40 ◦ C [26]; PTFE—25 ◦ C [28,29]).
to zero. For liquids (p/ps equal to unity), the solubility coefficients were calculated using Eq. (6). The correlation of the solubility coefficients of gases, vapors and liquids in the copolymers AF with Tc2 , (Tc is the critical temperature of solutes) is shown in Fig. 3. For gases and vapors, such correlations have been demonstrated previously [24,25,31,32]. For comparison, the correlations are shown for various solutes in PDMS and PTFE [26,28,29]. It is seen that, in agreement with previous results [24], solubility coefficients correlate with critical temperature squared and not with Tc , a more common dependence. The reasons for this have been discussed earlier [24]. It is evident that solubility coefficients of various compounds in Teflons AF are comparable with those in PDMS and much higher than in PTFE. The solubility coefficients in AF2400 are higher than those in AF1600. For all the polymers considered, the solubility coefficients increase linearly (in semi-logarithmic scale) with Tc2 . In the case of Teflons AF2400 and AF1600, this dependence can be described by the following equations, respectively: log SAF2400 = 9 × 10−6 Tc2 − 2.10,
R 2 = 0.93 (10)
log SAF1600 = 8 × 10−6 Tc2 − 2.23,
R 2 = 0.96 (11)
It is an unexpected result that the solubility coefficients of gases and liquids, that is for solutes at infinite dilution conditions (activity close to zero) and at activity equal to 1, in both AF Teflons, are described by similar equations. Sorption isotherms in glassy polymers are known to be strongly non-linear over a wide range of variation of the activity. This was particularly true for gases and vapors [24,25] in AF Teflons. A possible explanation of the difference in behavior may be related to the appearance of inflection points in the sorption isotherms of glassy polymers at sufficiently large activity. Fig. 4 shows, schematically, the shape of sorption isotherms in glassy polymers according to the results of [33–35]. The apparent solubility coefficient is defined as the ratio Ci /pi . In glassy polymers, sorption isotherms at relatively small pressure are concave to the pressure axis, so this ratio first decreases with increases in pressure. In this figure this is illustrated by a decrease in S from point 1 to 2. However, above some pressure pinf , the rate of increase in the solubility coefficients starts to accelerate as is illustrated in Fig. 4
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A.M. Polyakov et al. / Journal of Membrane Science 216 (2003) 241–256 Table 3 Apparent diffusion coefficients (D) of organic liquids in amorphous Teflons AF (25 ◦ C) Compound
CH2 Cl2 CHCl3 CCl4 CH3 OH C2 H5 OH (CH3 )2 CO C 6 H6 C6 H12
Fig. 4. Sorption isotherms for an extended range of vapor activity p/ps : variation of apparent solubility coefficient with activity.
for point 3. So at activity p/ps = 1 the apparent solubility coefficient can be rather similar to the infinite dilution solubility coefficients of gases. Experiments devised to prove the validity of this concept are now in progress. Assuming that the steady state penetrant concentration across the membrane decreases linearly from C at the upstream side to zero at the downstream side, one can estimate the mean diffusion coefficient from: P DAV = (12) SAV where SAV = 21 S
(13)
It should be noted that such approximate estimates are common in pervaporation studies (see, e.g. [11]). The diffusion coefficients found in this way for several penetrants are presented in Table 3. It can be seen that diffusion coefficients of chloromethanes decrease when the number of chlorine atoms in the molecule increases, that is when its size increases. The D values found are larger in AF2400, which is characterized by a greater free volume [14,16], than in AF1600. A comparison of the variation of the permeability and diffusivity values for penetrants series (Tables 2 and 3) shows that these two quantities change in a similar way. This can be considered as to be a manifestation of the mobility selectivity of perfluorinated copolymers in pervaporation. This result is rather unexpected. It is known that low permeable glassy poly-
D (×108 ) (cm2 /s) AF2400
AF1600
195.0 42.2 3.38 16.0 7.09 76.7 19.0 1.61
8.57 0.79 0.22 – – – 4.93 –
mers, such as polyimides [36], polyacrylonitrile [37] and cellulose [11] display mobility selectivity during pervaporation. On the other hand, solubility selectivity is a characteristic of rubbers, in particular PDMS [11,26]. It has been noted though, that the same behavior is observed for polymers with an extra large free volume, e.g. PTMSP [6]. As PTMSP and AF2400 are close in fractional free volume and size of free volume elements [16], one might expect similar pervaporation behavior for these two materials. The differences in selectivity trends of these polymers might imply that the free volume micro-structure in them is different. This conclusion is consistent with the results of the studies for vapor transport in these polymers [13,27]. The permeability coefficient was measured for film samples of a fixed thickness. It is known that the diffusion and permeability coefficients of gases and liquids can depend on film thickness [38,39]. In this regard, the permeability coefficient measured can be considered as a characteristic of the material only after demonstration of its independence of film thickness (or inverse proportionality of the flux and l). Hence, permeability was studied for several films of AF2400 and AF1600 with thicknesses in the range 5–35 m. Fig. 5 shows an example of the dependencies obtained. Inverse proportionality of the flux and film thickness indicates that the permeability coefficients are independent of the thickness l and, indeed, can be used to characterize the pervaporation performance of amorphous Teflons AF. The transport parameters of Teflons AF were compared with the literature data reported for the known pervaporation materials. Such comparisons
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Fig. 5. Reciprocal fluxes of chloromethanes vs. film thickness.
are common for materials used for gas separation membranes but are more complicated in the case of pervaporation, since the transport parameters of a polymer strongly depend on the liquid penetrant. The reported transport parameters for chloromethanes (Table 4) indicate that permeability of AF2400 is significantly lower than that of PDMS, one of the most permeable known pervaporation materials. However, AF2400 is much more permselective. On the other hand, the permeability of AF2400 is markedly higher
than polyethylene which has been studied in detail as a material for pervaporation. The AF1600 copolymer seems to show less attractive properties because of its low permeability. 4.2. Time stability of permeation rates Amorphous Teflon AF2400 is a material with a large free volume. This seems to be a reason for the high permeation rates of liquid penetrants in the films cast
Table 4 Permeability’s of chloromethanes in amorphous Teflons AF and other polymers Polymer
AF2400 AF1600 PDMS PE a
T (◦ C)
25 25 40 25
Jl (kg m/(m2 h)) CH2 Cl2
CHCl3
CCl4
28.3 1.1 1810 8.2
11.9 0.21 1980 17
1.76 0.08 961 8.6
Ideal selectivity α (CH2 Cl2 /CCl4 ) = J CH2 Cl2 /JCCl4 .
α (CH2 Cl2 /CCl4 )a
Reference
16 14 1.9 0.95
This work [26] [40]
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from AF2400. It is known that high free volume materials, in particularly PTMSP [41], can exhibit physical aging which results in a rapid decline in permeability over time. Therefore, pervaporation behavior of films cast from AF2400 was studied using continuous experiments with a duration of several months. Two different types of experiments were performed. In the
first series, films were periodically put in the pervaporation cell and the rate of permeation of different organic compounds was measured; the duration of these runs was about 1 year (see, Fig. 6A). In another series, the films were kept in the cell in permanent contact with the liquid phase and downstream vacuum for a period of up to 3 months. Periodically, measurements
Fig. 6. Time dependence of permeability Jl in AF2400. A—periodic regime; B—continuous regime.
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of permeation rates were carried out (Fig. 6B). The results presented in this figure show that essentially no reduction in permeation rates was observed even over a time frame of as much as 1 year. 4.3. Temperature effects The effects of temperature on the pervaporation rate were analyzed in terms of the Arrhenius Eq. (7). This equation holds for pervaporation of chloromethanes through the copolymers AF as Fig. 7 indicates. The pervaporation process proceeds according to the solution-diffusion mechanism, hence, the apparent activation energies of permeation should depend on several factors. The temperature changes cause a variation in the driving force of the process because of changes in the saturated vapor pressure and, therefore, the chemical potential and concentration of a penetrant across the membrane. The van’t Hoff Eq. (8)
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can be used to describe temperature dependence of the solubility coefficient, whereas temperature effects on the diffusion coefficients can be described by Eq. (9). The Ea values that are useful for practical application of pervaporation processes gives little information about the process mechanism. Feng and Huang [42] have noted that, because pervaporation is a process including a phase transition, one has to account for it: in calculating the permeability coefficient in this process the specific flux Jl should be divided by the saturated vapor pressure of a penetrant. In such an approach, the Ea value is replaced by EP = Ea − HV , where HV is the enthalpy of evaporation of the penetrant. In this case, the intrinsic activation energy EP characterizes the processes that take place inside the membrane. The intrinsic activation energy is an analogue to the gas permeability coefficient. Therefore EP can be presented as a sum of the activation energy
Fig. 7. Temperature dependence of permeability (kg m/(m2 h)). AF2400: 1—CH2 Cl2 ; 2—CHCl3 ; 3—CCl4 . AF1600: 4—CH2 Cl2 ; 5—CHCl3 ; 6—CCl4 .
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Table 5 Activation energies of permeation and enthalpies of sorption for organic liquids in amorphous Teflons AF Compound
CH2 Cl2 CHCl3 CCl4 H2 O CH3 OH C2 H5 OH (CH3 )2 CO C 6 H6 C 6 H5 F C6 H12 a
HC a (kJ/mol)
Ea (kJ/mol) AF2400
AF1600
11.9 12.4 21.4 14.4 26.8 17.1 13.9 17.4 15.3 13.5
31.6 33.7 31.8 39.8 – – – 19.6 35.3 –
30.1 31.8 32.3 1.9 37.5 42.3 30.9 33.8 34.6 32.9
HS (kJ/mol)
EP (kJ/mol) AF2400
AF1600
AF2400
AF1600
−18.2 −19.4 −10.9 12.5 −10.7 −25.2 −17 −16.4 −19.3 −19.4
1.5 1.9 −0.5 37.9 – – – −14.2 0.7 –
−39.3 −37 −37 – −44.6 −48.4 −35.8 −35.1 – −35.2
−40.6 −36.7 −38.8 – −51.7 −50.1 −32.4 −34.2 – −42.2
According to [44].
for diffusion ED and the enthalpy of sorption HS . Note that, in contrast to gas separation [43], the HS value does not contain a contribution accounting for the phase transition, that is, the enthalpy of condensation HC = −HV . In this work, experimental values of Ea and HS were obtained in the range 5–80 ◦ C. These parameters together with the EP values are given in Table 5.
Both copolymers have comparable values of HS , whereas the apparent activation energies Ea through AF1600 are noticeably larger than those found for AF2400. There is no doubt that this difference is induced by higher energy barriers for diffusion in the case of AF1600. The apparent activation energies of chloromethanes in AF1600 are virtually the same, while those values measured in the AF2400 films are
Fig. 8. Temperature dependence of permeability Jl (kg m/(m2 h)): 1—AF2400; 2—AF1600; permeability coefficient P (Barrer): 3—AF2400; 4—AF1600; and saturated vapor pressure ps (cmHg), 5 of CH2 Cl2 .
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as follows: Ea (CCl4 ) > Ea (CHCl3 ) Ea (CH2 Cl2 ). Interestingly, the intrinsic activation energies EP through AF2400 are negative. This observation shows that, in this polymer, temperature affects sorption more than it affects diffusion. So, the intrinsic permeability coefficient should decrease when temperature increases. The activation energies EP of chloromethanes in AF1600 are close to zero. These statements are further illustrated in Fig. 8. It can be noted that the negative activation energies of gas permeation have been observed for several polymers with a large free volume [25,45,46].
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The nature of these phenomena is likely to be related to unusually low energy barriers for diffusion. It has been shown [47–49] that a linear correlation exists between the parameters of the Arrhenius dependencies of gas diffusion and permeability coefficients in rubbers and glassy polymers (a so-called compensation effect). A useful consequence of such behavior enables determination of activation energies using the P and D values. In this way it is possible to characterize the process over a wide temperature range using only limited experimental data [49]. It was of interest
Fig. 9. Correlations of pre-exponential factor P0 (Barrer) and activation energy of permeability EP (kJ/mol), pre-exponential factor S0 (cm3 (STP)/(cm3 cmHg)) and enthalpy of sorption HS (kJ/mol) in amorphous Teflons.
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to check the existence of such an effect in the case of pervaporation. Fig. 9 shows that there is a satisfactory correlation between the parameters of van’t Hoff (ln S0 and HS ) and Arrhenius (ln P0 and EP ) equations observed for pervaporation of organic liquids through Teflons AF. On the basis of the compensation effect, one can calculate the activation energies in AF2400 and AF1600 using the following formulas, respectively: AF2400: 8.433 − ln P EP = (14) , R 2 = 0.96 (1/RT) − 0.455 HS =
0.829 − ln S , (1/RT) − 0.367
AF1600: 5.180 − ln P , EP = (1/RT) − 0.434 HS =
0.967 − ln S , (1/RT) − 0.422
R 2 = 0.85
R 2 = 0.97 R 2 = 0.95
(15)
(16) (17)
where P is the permeability coefficient and S is the solubility coefficient at temperature T. It will be interesting to check the parameters for the linear dependencies of the compensation effect for other pervaporation materials. If they do not differ significantly, such an approach would provide the ability to evaluate the transport and thermodynamic parameters during pervaporation at different temperatures based on limited experimental information carried out at a single temperature.
2. The permeability coefficients of liquids decrease when the critical volume Vc of the penetrants increases, which is typical for mass transfer through glassy polymers. However, this dependence for liquids is weaker than that for gases. 3. A correlation is observed between the solubility coefficients of liquid organic compounds in Teflons AF and the square of the critical temperature of the solutes. 4. The variations in the permeability and diffusion coefficients in the considered series of penetrants demonstrate mobility selectivity for the pervaporation of organic compounds in perfluorinated polymers AF. 5. An analysis of the parameters for the temperature dependencies of permeability and solubility coefficients of the perfluorinated polymers studied indicates that, while the enthalpies of sorption HS are similar in Teflons AF2400 and AF1600, the apparent activation energies of pervaporation Ea through AF1600 are markedly larger than those in the case of AF2400. Such differences are due to energy barriers for diffusion. 6. A compensation effect was demonstrated between the parameters of the Arrhenius and van’t Hoff dependencies for permeability and sorption of liquids in glassy polymers. Based on this effect, a correlation is proposed for evaluation of the EP and HS values using the permeability and solubility coefficients. 7. Excellent time stability (over a period of up to 1 year) was demonstrated for organic pervaporation through amorphous Teflons AF.
5. Conclusions Acknowledgements 1. A study of permeation of several individual organic compounds through amorphous Teflons AF indicated that the copolymer with the larger content of perfluorodioxole component and a greater free volume (AF2400) is much more permeable for liquids. Solubility of organic liquids also increases when the content of perfluorodioxole is larger. However, the effect of the chemical composition of copolymers is much larger for permeability. These observations are in agreement with the results of earlier studies of permeation and sorption of gases and vapors in Teflons AF.
This work was partly supported by the 2002 ARC Linkage International Grant LXO218588 of Australia; the authors are grateful to Referee 3 for interesting and encouraging suggestions in reviewing of the manuscript. References [1] P.A. Kober, Pervaporation, J. Am. Chem. Soc. 39 (1917) 944. [2] R.C. Binning, R.J. Lee, J.F. Jennings, E.C. Martin, Separation of liquid mixtures, Ind. Eng. Chem. 53 (1961) 45.
A.M. Polyakov et al. / Journal of Membrane Science 216 (2003) 241–256 [3] V.V. Kuznetsov, V.A. Malyusov, Separation of organic liquids using cellophane, Khim. Prom. 5 (1962) 345. [4] H.E.A. Bruschke, Mehrschichtige Membran und ihre Verwendung zur Trennung von Flˆussigkeitsgemischen nach dem Pervaporationsverfahren, German Patent DE 3220570 A1, 1983. [5] F. Lipnizki, S. Hausmanns, P.-K. Ten, R.W. Field, G. Laufenberg, Organophilic pervaporation: prospects and performance, Chem. Eng. J. 73 (1999) 113. [6] V.V. Volkov, Separation of liquids by pervaporation through polymeric membranes, Russ. Chem. Bull. 43 (1994) 187. [7] S.I. Semenova, H. Ohya, K. Soontarapa, Hydrophilic membranes for pervaporation: an analytical review, Desalination 110 (1997) 251. [8] Yu.I. Dytnerskii, Membrane Processes of Separation of Liquid Mixtures, Khimiya, Moscow, 1975. [9] R.Y.M. Huang, Pervaporation Membrane Separation Processes, Elsevier, Amsterdam, 1991. [10] J. Neel, P. Aptel, R. Clement, Basic aspects of pervaporation, Desalination 53 (1985) 297. [11] C.-M. Bell, F.J. Gerner, H. Strathmann, Selection of polymers for pervaporation membrane, J. Membr. Sci. 36 (1988) 315. [12] C. Staudt-Bickel, R.N. Lichtenthaler, Pervaporation thermodynamic properties and detection of membrane polymers, Polym. Sci. 36 (1994) 1628. [13] I. Pinnau, L.G. Toy, Gas and vapor transport properties of amorphous perfluorinated copolymer membranes based on 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole/tetrafluoroethylene, J. Membr. Sci. 109 (1996) 125. [14] A.Yu. Alentiev, Yu.P. Yampolskii, V.P. Shantarovich, S.M. Nemser, N.A. Plate, High transport parameters and free volume of perfluorodioxole copolymer, J. Membr. Sci. 126 (1997) 123. [15] T.C. Merkel, V. Bondar, K. Nagai, B.D. Freeman, Yu.P. Yampolskii, Gas sorption poly(2,2-bis(trifluromethyl)-4,5difluoro-1,3-dioxole-co-tetrafluroethyelene), Macromolecules 32 (1999) 8427. [16] V.P. Shantarovich, I.B. Kevdina, Yu.P. Yampolskii, A.Yu. Alentiev, Positron annihilation lifetime study of high and low free volume glassy polymers: effects of free volume sizes on the permeability and permselectivity, Macromolecules 33 (2000) 7453. [17] P.R. Resnick, W.H. Buck, in: J. Scheirs (Ed.), Modern Fluoropolymers, Wiley, New York, 1997, p. 397. [18] P. Colaianna, G. Brinati, V. Arcella, Amorphous Perfluoropolymers, US Patent 5,883,177 (1999). [19] Yu.P. Yampolskii, S.G. Durgaryan, N.S. Nametkin, Permeation and diffusion of hydrocarbons in poly(vinyltrimethyl silane), Vysokomol. Soed. 20B (1978) 632. [20] B. Freeman, I. Pinnau, Separation of gases using solubilityselective polymers, Trends Polym. Sci. 5 (1997) 167. [21] A. Heintz, A. Funke, R.N. Lichtenthaler, Pervaporation membrane separation processes, in: R.Y.M. Huang (Ed.), Pervaporation Membrane Separation Processes, Elsevier, Amsterdam, 1991, p. 279. [22] M. Mulder, Basic Principles of Membrane Technology, Kluwer Academic Publishers, Dordrecht, 1996.
255
[23] R.C. Reid, J.M. Prausnitz, T.K. Sherwood, The Properties of Gases and Liquids, McGraw-Hill, New York, 1977. [24] V.I. Bondar, B.D. Freeman, Yu.P. Yampolskii, Sorption of gases and vapors in an amorphous perfluorodioxole copolymer, Macromolecules 32 (1999) 6163. [25] A.Yu. Alentiev, V.P. Shantarovich, T.C. Merkel, V.I. Bondar, B.D. Freeman, Yu.P. Yampolskii, Gas and vapor sorption, permeation, and diffusion in glassy amorphous Teflon AF1600, Macromolecules 35 (2002) 9513. [26] I. Blume, P.J.F. Schwering, M.H.V. Mulder, C.A. Smolders, Vapour sorption and permeation properties of poly(dimethylsiloxane) films, J. Membr. Sci. 61 (1991) 85. [27] I. Pinnau, L.G. Toy, Transport of organic vapours through poly(1-trimethylsilyl-1-propyne), J. Membr. Sci. 116 (1996) 199. [28] R.A. Pasternak, M.V. Christensen, J. Heller, Diffusion and permeation of oxygen, nitrogen, carbon dioxide, and nitrogen dioxide through polytetrafluoroethylene, Macromolecules 3 (1970) 366. [29] N. Yi-Yan, R.M. Felder, W.J. Koros, Selective permeation of hydrocarbon gases in poly(tetrafluoroethylene) and poly(fluoroethylene-propylene) copolymer, J. Appl. Polym. Sci. 25 (1980) 1755. [30] Permeability and Other Film Properties, Plastic Design Library, New York, 1995, pp. 283–284. [31] S.A. Stern, J.T. Mullhaupt, P.J. Gareis, The effect of pressure on the permeation of gases and vapors through polyethylene. Usefulness of the corresponding states principle, AIChE J. 15 (1969) 64. [32] M.S. Suwandi, S.A. Stern, Transport of heavy organic vapors through silicone rubber, J. Polym. Sci., Polym. Phys. 11 (1973) 663. [33] J.S. Chiou, Y. Maeda, D.R. Paul, Gas and vapor sorption in polymers just below Tg , J. Appl. Polym. Sci. 30 (1985) 4019. [34] Y. Kamiya, T. Hirose, K. Mizoguchi, Y. Naito, Gravimetric study of high pressure sorption of gases in polymers, J. Polym. Sci., Polym. Phys. Ed. 24 (1986) 1525. [35] V.I. Bondar, Y.P. Yampolskii, S-shaped sorption isotherms: effects of nature of polymers and gases, in: Proceedings of the Euromembrane’95, vol. V.II (II-141), University of Bath, Bath, 1995. [36] K.I. Okamoto, T. Semoto, K. Tanaka, Application of pervaporation to phenol-acetone condensation reaction, in: Proceedings of the Congress on Membranes and Membrane Processes, Chicago, IL, NAMS 1, 1990, p. 347. [37] M.H.V. Mulder, J.O. Hendrikman, H. Hegeman, C.A. Smolders, Ethanol–water separation by pervaporation, J. Membr. Sci. 16 (1983) 269. [38] I. Pinnau, M.W. Hellums, W.J. Koros, Gas transport through homogeneous and asymmetric polyestercarbonate membranes, Polymer 32 (1991) 2612. [39] S.M. Shishatskii, Yu.P. Yampolskii, K.-V. Peinemann, Effects of film thickness on density and gas permeation parameters of glassy polymers, J. Membr. Sci. 112 (1996) 275. [40] R.Y.M. Huang, V.J.C. Lin, Separation of liquid mixtures using polymer membrane. I. Permeation of binary organic mixtures through polyethylene, J. Appl. Polym. Sci. 12 (1968) 2615.
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A.M. Polyakov et al. / Journal of Membrane Science 216 (2003) 241–256
[41] A.G. Fadeev, Ya.A. Selinskaya, S.S. Kelly, M.M. Meagher, E.G. Litvinova, V.S. Khotimsky, V.V. Volkov, Extraction of butanol from aqueous solutions by pervaporation through poly(trimethylsilyl propyne), J. Membr. Sci. 186 (2001) 205. [42] X. Feng, R.Y.M. Huang, Estimation of activation energy for permeation in pervaportaion process, J. Membr. Sci. 118 (1991) 127. [43] Yu.P. Yampolskii, N.E. Kaliuzhnyi, S.G. Durgaryan, Thermodynamics of sorption in glassy poly(vinyltrimethyl silane), Macromolecules 19 (1986) 846. [44] D.R. Stull, E.F. Westrum, G.S. Sinke, The Chemical Thermodynamics of Organic Compounds, Wiley, New York, 1969.
[45] T. Masuda, Y. Iguchi, B.Z. Tang, T. Higashimura, Diffusion and solution of gases in substituted polyacetylene membranes, Polymer 29 (1988) 2041. [46] L.E. Starannikova, V.V. Teplyakov, Gas permeability of poly[1-(trimethylsilyl)-1-propyne]: evaluation of experimental data and calculation methods, Polym. Sci. A 39 (1997) 1142. [47] G.J. Van Amerongen, Diffusion in elastomers, Rubber Chem. Technol. 37 (1969) 1065. [48] D.W. van Krevelen, Properties of Polymers, Elsevier, Amsterdam, 1976. [49] Yu. Yampolskii, S. Shishatskii, A. Alentiev, K. Loza, Correlations with and prediction of activation energies of gas permeation and diffusion in glassy polymers, J. Membr. Sci. 148 (1998) 59.
Amorphous Teflons AF as organophilic pervaporation materials Transport of individual components A.M. Polyakov, L.E. Starannikova, Yu.P. Yampolskii∗ A.V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Leninsky Pr., 29, 119991 Moscow, Russia Received 9 July 2002; received in revised form 7 January 2003; accepted 31 January 2003
Abstract Permeation and sorption of various organic liquids (chlorinated hydrocarbons, lower alcohols, hydrocarbons, etc.) in amorphous copolymers of 2,2-bis-trifluoromethyl-4,5-difluoro-1,3-dioxole and tetrafluoroethylene (amorphous Teflons AF) were studied in the temperature range 5–95 ◦ C. Based on permeation rate and solubility coefficients, the diffusion coefficients of organic penetrants in AF copolymers AF were estimated. It was shown that the copolymer with a higher content (87%) of the dioxole component (AF2400), as distinguished by a larger free volume, is much more permeable to liquids than the copolymer AF1600 containing 65% of the dioxole comonomer. In contrast, solubility of organic compounds in both copolymers is hardly affected by their composition. The effects of penetrant parameters, such as critical volume (Vc ) and critical temperature (Tc ) were analyzed. The variation in permeation rates of liquids and gases through these materials are consistent with a mobility (diffusion) controlled mechanism for mass transfer. A trade-off between the Arrhenius parameters for permeability and the van’t Hoff parameters for solubility is demonstrated for pervaporation. On this basis, novel correlations are obtained that enable a determination of the activation energy for permeation EP and the enthalpy of sorption HS from the permeability (P) and solubility (S) coefficients. Long term tests (up to 12 months) showed excellent time stability of the transport parameters, a property unexpected and quite important for a high flux, high free volume pervaporation material. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Pervaporation; Sorption; Diffusion; Chloromethanes; Teflons AF
1. Introduction Pervaporation is one of the oldest membrane process [1]. However, systematic studies of separation of liquid mixtures by means of pervaporation started as late as in 1950–1960s [2,3], whereas the practical implementation of pervaporation was accomplished only in 1980s thanks to the development of the industrially produced GFT membrane [4]. A classification of various pervaporation processes has been proposed ∗ Corresponding author. Tel.: +7-95-9554210; fax: +7-95-2302224. E-mail address: [email protected] (Yu.P. Yampolskii).
based on the aims of separation [5]. According to it and depending on the target products of water–organic mixtures, one can speak of hydrophilic or hydrophobic pervaporation, where the permeate flux is enriched with either water or organic components, respectively. These types of pervaporation have attracted more attention of researchers. However, there are numerous problems and a wide variety of industrial streams where different mixtures of liquid organic compounds must be subjected to separation. This kind of separation process is known as organic–organic, or target-organophilic pervaporation. Because of importance of separation processes and due to the high efficiency of pervaporation compared to traditional
0376-7388/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0376-7388(03)00077-2
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Nomenclature C D D0 DAV Ea ED EP HC HS HV F FFV J J0 l m p ps P R S S0
SAV T Tc Tg Vc x
the concentration of organic component in the polymer and solubility (g/g) the diffusion coefficient (cm2 /s) pre-exponential factor in Arrhenius dependence of D (cm2 /s) the apparent diffusion coefficient (cm2 /s) the apparent activation energy of flux (kJ/mol) the activation energy of diffusion (kJ/mol) the activation energy of permeation (kJ/mol) the enthalpy of condensation (kJ/mol) the enthalpy of sorption (kJ/mol) the enthalpy of evaporation (kJ/mol) the surface area of membrane (m2 ) fractional free volume (%) permeate flux (kg/(m2 h)) pre-exponential factor in Arrhenius dependence of J (kg/(m2 h)) the film thickness (m) the mass of permeate (kg) the vapor pressure (cmHg) the saturated vapor pressure (cmHg) the permeability coefficient (1 Barrer = 1 × 10−10 cm3 (STP) cm/(cm2 s cmHg)) universal gas constant (J/(mol K)) the solubility coefficient (cm3 (STP)/(cm3 cmHg)) pre-exponential factor in van’t Hoff dependence of S (cm3 (STP)/(cm3 cmHg)) the apparent solubility coefficient (cm3 (STP)/(cm3 cmHg)) temperature (◦ C) critical temperature (◦ C) glass transition temperature (◦ C) critical volume (cm3 /mol) coordinate along the diffusion process
Greek letters α ideal selectivity τ time interval (h)
processes such as adsorption, distillation and extraction, the most developed process is hydrophilic pervaporation [6,7]. Two other types of pervaporation (hydrophobic and target-organophilic pervaporation) are also distinguished by higher energy efficiency, possibility due to the use of compact equipment and low inertia [8,9] in comparison to alternative processes. These have not found substantial practical development. Although the effects of process conditions on pervaporation parameters have been studied in detail [8–10], the principles for selection of membrane materials for specific separation problems are still a relevant and insufficiently developed problem [11,12]. In the formulation of separation principles one has to take into account the specific transport and sorption parameters needed for the separation and possible strong effects of the liquid on the membrane material. All this makes a prognostication of membrane behavior on the of separation a difficult and still unsolved problem. Just as in the case of other membrane materials, pervaporation materials should satisfy several requirements: high transport parameters (permeability and permselectivity), stability in contact with the separated liquid mixture, thermal stability, and stable transport and mechanical characteristics over time. An analysis of the literature indicates that, in spite of the diversity of available polymeric materials, only a few have been studied as membrane materials for hydrophobic and target-organophilic pervaporation [5–10]. In this regard, a consideration of novel classes of polymeric materials should extend the potential applications of this type. So the objective of this work was to study pervaporation properties of the copolymers of 2,2-bis-trifluoromethyl-4,5-difluoro1,3-dioxole and tetrafluoroethylene (amorphous Teflons AF of DuPont Co.) having the following chemical structure:
These copolymers have been studied in detail as materials for gas separating membranes [13–15]. They reveal extremely high gas permeability and are
A.M. Polyakov et al. / Journal of Membrane Science 216 (2003) 241–256 Table 1 Properties of Teflons AF [14,17,18] Property
AF2400
AF1600
Content of dioxol component (mol%) Crystallinity Tg (◦ C) FFV (%) Density (g/cm3 ) Permeability O2 (Barrer)a
87 None 240 33.4 1.75 1140
65 None 160 30.0 1.82 170
None C 6 F6 , C6 H5 CF3 , FC-72, etc.
None C 6 F6 , C6 H5 CF3 , FC-72, etc.
26.4 7.9
26.9 17.1
Solubility Common organic solvents Perfluorinated solvents
Mechanical properties Stress at break (MPa) Elongation at break (%) a
1 Barrer = 1 × 10−10 cm3 (STP) cm/(cm2 s cmHg).
characterized by very large free volume: the average radii of free volume elements in them are about 5–6 Å, that is, much larger than those in other polymers [16]. The latter result implies that liquid permeation rates in these polymers would be also high. It is also worth noting that these copolymers exhibit several properties important for pervaporation: insolubility in most organic solvents (except perfluorinated) solvents, excellent chemical and thermal stability, very good film forming properties. Some of the characteristics of amorphous Teflons AF are presented in Table 1. In this work permeability and sorption of chloromethanes (CH2 Cl2 , CHCl3 , CCl4 ) and some other liquid organic compounds in Teflons AF were studied. 2. Background In pervaporation, the specific permeate flux J (kg/(m2 h)) can be determined using the formula: m (1) J = Fτ where m is the mass of permeate collected for time interval τ and F is the surface area of the film. Using this flux, permeability coefficients of a liquid penetrant can be found via the equation: Jl P = (2) ps
243
where ps is the saturated vapor pressure at the experimental temperature and l is the film thickness. It is well known that mass transfer in pervaporation, just as in gas permeation, can be described by the solution-diffusion mechanism [12]. In this model the permeability coefficient P is a function of the diffusion coefficient D, which characterizes the rate of mass transfer under a certain gradient of concentration, and solubility coefficient S, which determines the driving force (concentration gradient) in the membrane: P = DS
(3)
Depending on the physical state of a material (i.e. glassy or rubbery) the variation of the permeability coefficients and selectivity for a penetrant series can be governed by changes in either D or S. So the transport process can be categorized in terms of mobility or solubility controlled mass transfer [19,20]. In the case of mobility controlled mass transfer the variation in the P values for a penetrant series are similar to those of the diffusion coefficients for the same penetrants, that is, permeability decreases when the size of diffusing molecule increases. For solubility controlled mass transfer, both permeability and solubility coefficients increase with the size and mass of the penetrant. In pervaporation processes, the conditions on the upstream and downstream sides of membranes differ markedly even at steady state because one surface of the membrane is in contact with the organic liquid while the other side is in contact with vapor of low pressure or activity. The non-linear character of the sorption isotherms results in essentially non-linear penetrant concentration profile across the membrane. In addition, the diffusion coefficient of the liquid penetrant depends on the penetrant concentration and, hence, in an indirect way, on the location across the membrane. Therefore, a solution of Fick’s first law gives: J = −D(c)
dc dx
(4)
where c is a complex function of x [11]. Integration of Eq. (4) results in: 1 J = D(c) dc (5) l
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In this way, a solution of the equation can only be obtained provided experimental sorption isotherms and concentration dependencies of the diffusion coefficients are known over the whole range of activity from 0 to 1. The methods for solution of the equation for the flux J are well described in the literature [21,22]. For liquids (p/ps is equal to unity), the apparent solubility coefficients S can be defined as: C S= ps
(6)
where C is the concentration of the organic component in the polymer. The effects of temperature on pervaporation rate are described by the Arrhenius equation: Ea J = J0 exp − (7) RT where Ea is the apparent activation energy of flux. For the solution-diffusion mechanism of pervaporation, the apparent activation energies of permeation should depend on several factors. The temperature changes cause a variation in the driving force for the process because of changes in the saturated vapor pressure and, therefore, the chemical potential and concentration of a penetrant across the membrane. The van’t Hoff equation can be used to describe temperature dependence of the solubility coefficient: HS S = S0 exp − (8) RT where HS is the enthalpy change due to transfer of penetrant from its liquid phase to the polymer (enthalpy of dissolution in the terms of solution thermodynamics). Finally, temperature effects for diffusion coefficients are defined by the Arrhenius equation: ED D = D0 exp − (9) RT where ED is the activation energy for diffusion.
3. Experimental 3.1. Film casting Dense films of AF2400 and AF1600 were prepared by casting solutions of the polymers (1.5 mass%) in
octafluorotoluene onto the surface of cellophane in the bottom of cylindrical cups. The time for evaporation of the solvent was about 7–9 days. Then the films were kept in vacuum at 40 ◦ C until constant weight was achieved. In this manner films with a thickness of 5–40 m were obtained. 3.2. Pervaporation experiments The experiments were carried out using the set up shown in Fig. 1. The separation cell was provided with a mixer and a jacket. A film with a working surface area of 27.3 cm2 was mounted over a porous metal support and sealed by an O-ring made of Teflon. The separation was performed in the temperature range 5–95 ◦ C with the deviations from the isothermal regime being within 0.5 ◦ C. Residual pressure in the downstream part of the cell was about 267 Pa (2 Torr). The permeate vapor was condensed in a trap cooled with liquid nitrogen. The quantity of permeate collected for a certain time interval was determined by weighing and in this way the specific permeate flux J (kg/(m2 h)) was determined using Eq. (1). 3.3. Sorption measurement The sorption of liquid permeates was studied using a gravimetric method. Samples of dense films with a thickness of 50–80 m and an initial mass of 0.1–0.2 g were equilibrated in the liquid organic solutions at temperatures of 5–80 ◦ C. Then the samples taken from the liquids were weighed using an analytical balance after removal of liquid droplets with filter paper.
4. Results and discussion 4.1. The effects of nature and properties of penetrants on their permeation rates through and sorption in Teflons AF Pervaporation and sorption of organic liquids having different chemical structure, molecular mass and size were studied in amorphous Teflons AF. Table 2 presents several physical properties [23] of the organic liquids studied, together with the observed permeability (Jl) and solubility (C) in the AF copolymers. It
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Fig. 1. Schematic of the pervaporation apparatus. I—Membrane cell; II—thermostats; III—montejus; IV—compressor; V—cool traps; VI—air drier (CaCl2 ); VII—vacuum pump; TI—thermometer; PI—vacuumeter.
can be seen that permeability of the studied penetrants varies over a substantially wider range than the solubility. Just as it has been observed for gas permeability [14] the copolymer with the larger content of the dioxole component and exhibiting the higher free volume (AF2400) displays larger liquid permeability. Solubility of organic liquids also increases with an increase in the content of the dioxole comonomer. However, these differences are less than those observed for gases and vapors [15,24,25]. The results of this work on liquid permeation rates, as well as those published earlier for gas permeation [13–15,24,25], indicate that permeability coefficients of Teflons AF decrease, when the size of penetrant, as expressed, e.g. by its critical volume Vc , increases. Such behavior is typical for glassy polymers. Fig. 2 shows the correlations of permeability coefficients
of gases, vapors and liquids with the critical volume of the penetrant. For liquid permeation, the permeability coefficients were calculated using Eq. (2). Similar correlations are shown for a conventional glassy polymer, polysulfone (PSF); a glassy polymer with extremely large free volume, poly(trimethylsilyl propyne) (PTMSP); a rubber, polydimethylsiloxane (PDMS); and, a perfluorinated polymer, polytetrafluoriethylene (PTFE). It is seen that, at comparable critical volumes of penetrants, the permeation rate of the liquid through Teflons AF is higher than for gases and vapors. The dependence of permeability on penetrant size is weaker for liquids than for gases. It can be noted that, for many penetrant-polymer systems, permeability is an increasing function of penetrant activity. Liquid permeation in pervaporation processes proceeds at the maximum activity (i.e. p/ps = 1).
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Table 2 Physicochemical properties of organic liquids and their transport and sorption parameters in Teflons AF (25 ◦ C) Compound
CH2 Cl2 CHCl3 CCl4 H2 O CH3 OH C2 H5 OH (CH3 )2 CO C 6 H6 C6 H 5 F C6 H12
Tc (K)
510 536.6 556.4 647 513.2 516.3 509.1 562.1 560.1 553.2
Vc (cm3 /mol)
193 240 276 56 118 167 211 260 271 308
At this activity, polymers are swollen and plasticized, leading to increased permeability relative to what would be obtained for infinite dilution gas permeation experiments. This might be partially why, at a given penetrant size, permeability of liquids is higher than can be extrapolated from gas permeability. In order to evaluate the contribution of the solubility coefficient to the permeability coefficient during per-
Jl (kg m/(m2 h))
C (g/100 g polymer)
AF2400
AF1600
AF2400
AF1600
28.30 11.91 1.76 0.97 0.98 0.67 6.02 2.86 5.24 0.36
1.10 0.21 0.08 0.07 – – – 0.72 0.57 –
5.15 9.11 16.94 – 1.84 2.81 2.43 4.65 – 6.75
4.36 8.14 11.90 – 0.54 1.12 2.38 4.35 – 4.21
vaporation, we compared the sorption data for gases, vapors, and liquids in the polymers studied. The results for gas and vapor sorption were taken from Bondar et al. [24] and Alentiev et al. [25]. The solubility coefficients for the gases and vapors were measured by pressure decay and gas chromatographic methods and used to estimate the infinite dilution conditions, i.e. the initial parts of the sorption isotherm at activity p/ps close
Fig. 2. Dependence of permeability coefficient P (Barrer) on critical volume (PDMS—40 ◦ C [26]; PTMSP—23 ◦ C [27]; PTFE—25 ◦ C [28,29]; PSF—23 ◦ C [30]).
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Fig. 3. Solubility coefficients S (cm3 (STP)/(cm3 cmHg)) vs. the square of critical temperature (PDMS—40 ◦ C [26]; PTFE—25 ◦ C [28,29]).
to zero. For liquids (p/ps equal to unity), the solubility coefficients were calculated using Eq. (6). The correlation of the solubility coefficients of gases, vapors and liquids in the copolymers AF with Tc2 , (Tc is the critical temperature of solutes) is shown in Fig. 3. For gases and vapors, such correlations have been demonstrated previously [24,25,31,32]. For comparison, the correlations are shown for various solutes in PDMS and PTFE [26,28,29]. It is seen that, in agreement with previous results [24], solubility coefficients correlate with critical temperature squared and not with Tc , a more common dependence. The reasons for this have been discussed earlier [24]. It is evident that solubility coefficients of various compounds in Teflons AF are comparable with those in PDMS and much higher than in PTFE. The solubility coefficients in AF2400 are higher than those in AF1600. For all the polymers considered, the solubility coefficients increase linearly (in semi-logarithmic scale) with Tc2 . In the case of Teflons AF2400 and AF1600, this dependence can be described by the following equations, respectively: log SAF2400 = 9 × 10−6 Tc2 − 2.10,
R 2 = 0.93 (10)
log SAF1600 = 8 × 10−6 Tc2 − 2.23,
R 2 = 0.96 (11)
It is an unexpected result that the solubility coefficients of gases and liquids, that is for solutes at infinite dilution conditions (activity close to zero) and at activity equal to 1, in both AF Teflons, are described by similar equations. Sorption isotherms in glassy polymers are known to be strongly non-linear over a wide range of variation of the activity. This was particularly true for gases and vapors [24,25] in AF Teflons. A possible explanation of the difference in behavior may be related to the appearance of inflection points in the sorption isotherms of glassy polymers at sufficiently large activity. Fig. 4 shows, schematically, the shape of sorption isotherms in glassy polymers according to the results of [33–35]. The apparent solubility coefficient is defined as the ratio Ci /pi . In glassy polymers, sorption isotherms at relatively small pressure are concave to the pressure axis, so this ratio first decreases with increases in pressure. In this figure this is illustrated by a decrease in S from point 1 to 2. However, above some pressure pinf , the rate of increase in the solubility coefficients starts to accelerate as is illustrated in Fig. 4
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A.M. Polyakov et al. / Journal of Membrane Science 216 (2003) 241–256 Table 3 Apparent diffusion coefficients (D) of organic liquids in amorphous Teflons AF (25 ◦ C) Compound
CH2 Cl2 CHCl3 CCl4 CH3 OH C2 H5 OH (CH3 )2 CO C 6 H6 C6 H12
Fig. 4. Sorption isotherms for an extended range of vapor activity p/ps : variation of apparent solubility coefficient with activity.
for point 3. So at activity p/ps = 1 the apparent solubility coefficient can be rather similar to the infinite dilution solubility coefficients of gases. Experiments devised to prove the validity of this concept are now in progress. Assuming that the steady state penetrant concentration across the membrane decreases linearly from C at the upstream side to zero at the downstream side, one can estimate the mean diffusion coefficient from: P DAV = (12) SAV where SAV = 21 S
(13)
It should be noted that such approximate estimates are common in pervaporation studies (see, e.g. [11]). The diffusion coefficients found in this way for several penetrants are presented in Table 3. It can be seen that diffusion coefficients of chloromethanes decrease when the number of chlorine atoms in the molecule increases, that is when its size increases. The D values found are larger in AF2400, which is characterized by a greater free volume [14,16], than in AF1600. A comparison of the variation of the permeability and diffusivity values for penetrants series (Tables 2 and 3) shows that these two quantities change in a similar way. This can be considered as to be a manifestation of the mobility selectivity of perfluorinated copolymers in pervaporation. This result is rather unexpected. It is known that low permeable glassy poly-
D (×108 ) (cm2 /s) AF2400
AF1600
195.0 42.2 3.38 16.0 7.09 76.7 19.0 1.61
8.57 0.79 0.22 – – – 4.93 –
mers, such as polyimides [36], polyacrylonitrile [37] and cellulose [11] display mobility selectivity during pervaporation. On the other hand, solubility selectivity is a characteristic of rubbers, in particular PDMS [11,26]. It has been noted though, that the same behavior is observed for polymers with an extra large free volume, e.g. PTMSP [6]. As PTMSP and AF2400 are close in fractional free volume and size of free volume elements [16], one might expect similar pervaporation behavior for these two materials. The differences in selectivity trends of these polymers might imply that the free volume micro-structure in them is different. This conclusion is consistent with the results of the studies for vapor transport in these polymers [13,27]. The permeability coefficient was measured for film samples of a fixed thickness. It is known that the diffusion and permeability coefficients of gases and liquids can depend on film thickness [38,39]. In this regard, the permeability coefficient measured can be considered as a characteristic of the material only after demonstration of its independence of film thickness (or inverse proportionality of the flux and l). Hence, permeability was studied for several films of AF2400 and AF1600 with thicknesses in the range 5–35 m. Fig. 5 shows an example of the dependencies obtained. Inverse proportionality of the flux and film thickness indicates that the permeability coefficients are independent of the thickness l and, indeed, can be used to characterize the pervaporation performance of amorphous Teflons AF. The transport parameters of Teflons AF were compared with the literature data reported for the known pervaporation materials. Such comparisons
A.M. Polyakov et al. / Journal of Membrane Science 216 (2003) 241–256
249
Fig. 5. Reciprocal fluxes of chloromethanes vs. film thickness.
are common for materials used for gas separation membranes but are more complicated in the case of pervaporation, since the transport parameters of a polymer strongly depend on the liquid penetrant. The reported transport parameters for chloromethanes (Table 4) indicate that permeability of AF2400 is significantly lower than that of PDMS, one of the most permeable known pervaporation materials. However, AF2400 is much more permselective. On the other hand, the permeability of AF2400 is markedly higher
than polyethylene which has been studied in detail as a material for pervaporation. The AF1600 copolymer seems to show less attractive properties because of its low permeability. 4.2. Time stability of permeation rates Amorphous Teflon AF2400 is a material with a large free volume. This seems to be a reason for the high permeation rates of liquid penetrants in the films cast
Table 4 Permeability’s of chloromethanes in amorphous Teflons AF and other polymers Polymer
AF2400 AF1600 PDMS PE a
T (◦ C)
25 25 40 25
Jl (kg m/(m2 h)) CH2 Cl2
CHCl3
CCl4
28.3 1.1 1810 8.2
11.9 0.21 1980 17
1.76 0.08 961 8.6
Ideal selectivity α (CH2 Cl2 /CCl4 ) = J CH2 Cl2 /JCCl4 .
α (CH2 Cl2 /CCl4 )a
Reference
16 14 1.9 0.95
This work [26] [40]
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from AF2400. It is known that high free volume materials, in particularly PTMSP [41], can exhibit physical aging which results in a rapid decline in permeability over time. Therefore, pervaporation behavior of films cast from AF2400 was studied using continuous experiments with a duration of several months. Two different types of experiments were performed. In the
first series, films were periodically put in the pervaporation cell and the rate of permeation of different organic compounds was measured; the duration of these runs was about 1 year (see, Fig. 6A). In another series, the films were kept in the cell in permanent contact with the liquid phase and downstream vacuum for a period of up to 3 months. Periodically, measurements
Fig. 6. Time dependence of permeability Jl in AF2400. A—periodic regime; B—continuous regime.
A.M. Polyakov et al. / Journal of Membrane Science 216 (2003) 241–256
of permeation rates were carried out (Fig. 6B). The results presented in this figure show that essentially no reduction in permeation rates was observed even over a time frame of as much as 1 year. 4.3. Temperature effects The effects of temperature on the pervaporation rate were analyzed in terms of the Arrhenius Eq. (7). This equation holds for pervaporation of chloromethanes through the copolymers AF as Fig. 7 indicates. The pervaporation process proceeds according to the solution-diffusion mechanism, hence, the apparent activation energies of permeation should depend on several factors. The temperature changes cause a variation in the driving force of the process because of changes in the saturated vapor pressure and, therefore, the chemical potential and concentration of a penetrant across the membrane. The van’t Hoff Eq. (8)
251
can be used to describe temperature dependence of the solubility coefficient, whereas temperature effects on the diffusion coefficients can be described by Eq. (9). The Ea values that are useful for practical application of pervaporation processes gives little information about the process mechanism. Feng and Huang [42] have noted that, because pervaporation is a process including a phase transition, one has to account for it: in calculating the permeability coefficient in this process the specific flux Jl should be divided by the saturated vapor pressure of a penetrant. In such an approach, the Ea value is replaced by EP = Ea − HV , where HV is the enthalpy of evaporation of the penetrant. In this case, the intrinsic activation energy EP characterizes the processes that take place inside the membrane. The intrinsic activation energy is an analogue to the gas permeability coefficient. Therefore EP can be presented as a sum of the activation energy
Fig. 7. Temperature dependence of permeability (kg m/(m2 h)). AF2400: 1—CH2 Cl2 ; 2—CHCl3 ; 3—CCl4 . AF1600: 4—CH2 Cl2 ; 5—CHCl3 ; 6—CCl4 .
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Table 5 Activation energies of permeation and enthalpies of sorption for organic liquids in amorphous Teflons AF Compound
CH2 Cl2 CHCl3 CCl4 H2 O CH3 OH C2 H5 OH (CH3 )2 CO C 6 H6 C 6 H5 F C6 H12 a
HC a (kJ/mol)
Ea (kJ/mol) AF2400
AF1600
11.9 12.4 21.4 14.4 26.8 17.1 13.9 17.4 15.3 13.5
31.6 33.7 31.8 39.8 – – – 19.6 35.3 –
30.1 31.8 32.3 1.9 37.5 42.3 30.9 33.8 34.6 32.9
HS (kJ/mol)
EP (kJ/mol) AF2400
AF1600
AF2400
AF1600
−18.2 −19.4 −10.9 12.5 −10.7 −25.2 −17 −16.4 −19.3 −19.4
1.5 1.9 −0.5 37.9 – – – −14.2 0.7 –
−39.3 −37 −37 – −44.6 −48.4 −35.8 −35.1 – −35.2
−40.6 −36.7 −38.8 – −51.7 −50.1 −32.4 −34.2 – −42.2
According to [44].
for diffusion ED and the enthalpy of sorption HS . Note that, in contrast to gas separation [43], the HS value does not contain a contribution accounting for the phase transition, that is, the enthalpy of condensation HC = −HV . In this work, experimental values of Ea and HS were obtained in the range 5–80 ◦ C. These parameters together with the EP values are given in Table 5.
Both copolymers have comparable values of HS , whereas the apparent activation energies Ea through AF1600 are noticeably larger than those found for AF2400. There is no doubt that this difference is induced by higher energy barriers for diffusion in the case of AF1600. The apparent activation energies of chloromethanes in AF1600 are virtually the same, while those values measured in the AF2400 films are
Fig. 8. Temperature dependence of permeability Jl (kg m/(m2 h)): 1—AF2400; 2—AF1600; permeability coefficient P (Barrer): 3—AF2400; 4—AF1600; and saturated vapor pressure ps (cmHg), 5 of CH2 Cl2 .
A.M. Polyakov et al. / Journal of Membrane Science 216 (2003) 241–256
as follows: Ea (CCl4 ) > Ea (CHCl3 ) Ea (CH2 Cl2 ). Interestingly, the intrinsic activation energies EP through AF2400 are negative. This observation shows that, in this polymer, temperature affects sorption more than it affects diffusion. So, the intrinsic permeability coefficient should decrease when temperature increases. The activation energies EP of chloromethanes in AF1600 are close to zero. These statements are further illustrated in Fig. 8. It can be noted that the negative activation energies of gas permeation have been observed for several polymers with a large free volume [25,45,46].
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The nature of these phenomena is likely to be related to unusually low energy barriers for diffusion. It has been shown [47–49] that a linear correlation exists between the parameters of the Arrhenius dependencies of gas diffusion and permeability coefficients in rubbers and glassy polymers (a so-called compensation effect). A useful consequence of such behavior enables determination of activation energies using the P and D values. In this way it is possible to characterize the process over a wide temperature range using only limited experimental data [49]. It was of interest
Fig. 9. Correlations of pre-exponential factor P0 (Barrer) and activation energy of permeability EP (kJ/mol), pre-exponential factor S0 (cm3 (STP)/(cm3 cmHg)) and enthalpy of sorption HS (kJ/mol) in amorphous Teflons.
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to check the existence of such an effect in the case of pervaporation. Fig. 9 shows that there is a satisfactory correlation between the parameters of van’t Hoff (ln S0 and HS ) and Arrhenius (ln P0 and EP ) equations observed for pervaporation of organic liquids through Teflons AF. On the basis of the compensation effect, one can calculate the activation energies in AF2400 and AF1600 using the following formulas, respectively: AF2400: 8.433 − ln P EP = (14) , R 2 = 0.96 (1/RT) − 0.455 HS =
0.829 − ln S , (1/RT) − 0.367
AF1600: 5.180 − ln P , EP = (1/RT) − 0.434 HS =
0.967 − ln S , (1/RT) − 0.422
R 2 = 0.85
R 2 = 0.97 R 2 = 0.95
(15)
(16) (17)
where P is the permeability coefficient and S is the solubility coefficient at temperature T. It will be interesting to check the parameters for the linear dependencies of the compensation effect for other pervaporation materials. If they do not differ significantly, such an approach would provide the ability to evaluate the transport and thermodynamic parameters during pervaporation at different temperatures based on limited experimental information carried out at a single temperature.
2. The permeability coefficients of liquids decrease when the critical volume Vc of the penetrants increases, which is typical for mass transfer through glassy polymers. However, this dependence for liquids is weaker than that for gases. 3. A correlation is observed between the solubility coefficients of liquid organic compounds in Teflons AF and the square of the critical temperature of the solutes. 4. The variations in the permeability and diffusion coefficients in the considered series of penetrants demonstrate mobility selectivity for the pervaporation of organic compounds in perfluorinated polymers AF. 5. An analysis of the parameters for the temperature dependencies of permeability and solubility coefficients of the perfluorinated polymers studied indicates that, while the enthalpies of sorption HS are similar in Teflons AF2400 and AF1600, the apparent activation energies of pervaporation Ea through AF1600 are markedly larger than those in the case of AF2400. Such differences are due to energy barriers for diffusion. 6. A compensation effect was demonstrated between the parameters of the Arrhenius and van’t Hoff dependencies for permeability and sorption of liquids in glassy polymers. Based on this effect, a correlation is proposed for evaluation of the EP and HS values using the permeability and solubility coefficients. 7. Excellent time stability (over a period of up to 1 year) was demonstrated for organic pervaporation through amorphous Teflons AF.
5. Conclusions Acknowledgements 1. A study of permeation of several individual organic compounds through amorphous Teflons AF indicated that the copolymer with the larger content of perfluorodioxole component and a greater free volume (AF2400) is much more permeable for liquids. Solubility of organic liquids also increases when the content of perfluorodioxole is larger. However, the effect of the chemical composition of copolymers is much larger for permeability. These observations are in agreement with the results of earlier studies of permeation and sorption of gases and vapors in Teflons AF.
This work was partly supported by the 2002 ARC Linkage International Grant LXO218588 of Australia; the authors are grateful to Referee 3 for interesting and encouraging suggestions in reviewing of the manuscript. References [1] P.A. Kober, Pervaporation, J. Am. Chem. Soc. 39 (1917) 944. [2] R.C. Binning, R.J. Lee, J.F. Jennings, E.C. Martin, Separation of liquid mixtures, Ind. Eng. Chem. 53 (1961) 45.
A.M. Polyakov et al. / Journal of Membrane Science 216 (2003) 241–256 [3] V.V. Kuznetsov, V.A. Malyusov, Separation of organic liquids using cellophane, Khim. Prom. 5 (1962) 345. [4] H.E.A. Bruschke, Mehrschichtige Membran und ihre Verwendung zur Trennung von Flˆussigkeitsgemischen nach dem Pervaporationsverfahren, German Patent DE 3220570 A1, 1983. [5] F. Lipnizki, S. Hausmanns, P.-K. Ten, R.W. Field, G. Laufenberg, Organophilic pervaporation: prospects and performance, Chem. Eng. J. 73 (1999) 113. [6] V.V. Volkov, Separation of liquids by pervaporation through polymeric membranes, Russ. Chem. Bull. 43 (1994) 187. [7] S.I. Semenova, H. Ohya, K. Soontarapa, Hydrophilic membranes for pervaporation: an analytical review, Desalination 110 (1997) 251. [8] Yu.I. Dytnerskii, Membrane Processes of Separation of Liquid Mixtures, Khimiya, Moscow, 1975. [9] R.Y.M. Huang, Pervaporation Membrane Separation Processes, Elsevier, Amsterdam, 1991. [10] J. Neel, P. Aptel, R. Clement, Basic aspects of pervaporation, Desalination 53 (1985) 297. [11] C.-M. Bell, F.J. Gerner, H. Strathmann, Selection of polymers for pervaporation membrane, J. Membr. Sci. 36 (1988) 315. [12] C. Staudt-Bickel, R.N. Lichtenthaler, Pervaporation thermodynamic properties and detection of membrane polymers, Polym. Sci. 36 (1994) 1628. [13] I. Pinnau, L.G. Toy, Gas and vapor transport properties of amorphous perfluorinated copolymer membranes based on 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole/tetrafluoroethylene, J. Membr. Sci. 109 (1996) 125. [14] A.Yu. Alentiev, Yu.P. Yampolskii, V.P. Shantarovich, S.M. Nemser, N.A. Plate, High transport parameters and free volume of perfluorodioxole copolymer, J. Membr. Sci. 126 (1997) 123. [15] T.C. Merkel, V. Bondar, K. Nagai, B.D. Freeman, Yu.P. Yampolskii, Gas sorption poly(2,2-bis(trifluromethyl)-4,5difluoro-1,3-dioxole-co-tetrafluroethyelene), Macromolecules 32 (1999) 8427. [16] V.P. Shantarovich, I.B. Kevdina, Yu.P. Yampolskii, A.Yu. Alentiev, Positron annihilation lifetime study of high and low free volume glassy polymers: effects of free volume sizes on the permeability and permselectivity, Macromolecules 33 (2000) 7453. [17] P.R. Resnick, W.H. Buck, in: J. Scheirs (Ed.), Modern Fluoropolymers, Wiley, New York, 1997, p. 397. [18] P. Colaianna, G. Brinati, V. Arcella, Amorphous Perfluoropolymers, US Patent 5,883,177 (1999). [19] Yu.P. Yampolskii, S.G. Durgaryan, N.S. Nametkin, Permeation and diffusion of hydrocarbons in poly(vinyltrimethyl silane), Vysokomol. Soed. 20B (1978) 632. [20] B. Freeman, I. Pinnau, Separation of gases using solubilityselective polymers, Trends Polym. Sci. 5 (1997) 167. [21] A. Heintz, A. Funke, R.N. Lichtenthaler, Pervaporation membrane separation processes, in: R.Y.M. Huang (Ed.), Pervaporation Membrane Separation Processes, Elsevier, Amsterdam, 1991, p. 279. [22] M. Mulder, Basic Principles of Membrane Technology, Kluwer Academic Publishers, Dordrecht, 1996.
255
[23] R.C. Reid, J.M. Prausnitz, T.K. Sherwood, The Properties of Gases and Liquids, McGraw-Hill, New York, 1977. [24] V.I. Bondar, B.D. Freeman, Yu.P. Yampolskii, Sorption of gases and vapors in an amorphous perfluorodioxole copolymer, Macromolecules 32 (1999) 6163. [25] A.Yu. Alentiev, V.P. Shantarovich, T.C. Merkel, V.I. Bondar, B.D. Freeman, Yu.P. Yampolskii, Gas and vapor sorption, permeation, and diffusion in glassy amorphous Teflon AF1600, Macromolecules 35 (2002) 9513. [26] I. Blume, P.J.F. Schwering, M.H.V. Mulder, C.A. Smolders, Vapour sorption and permeation properties of poly(dimethylsiloxane) films, J. Membr. Sci. 61 (1991) 85. [27] I. Pinnau, L.G. Toy, Transport of organic vapours through poly(1-trimethylsilyl-1-propyne), J. Membr. Sci. 116 (1996) 199. [28] R.A. Pasternak, M.V. Christensen, J. Heller, Diffusion and permeation of oxygen, nitrogen, carbon dioxide, and nitrogen dioxide through polytetrafluoroethylene, Macromolecules 3 (1970) 366. [29] N. Yi-Yan, R.M. Felder, W.J. Koros, Selective permeation of hydrocarbon gases in poly(tetrafluoroethylene) and poly(fluoroethylene-propylene) copolymer, J. Appl. Polym. Sci. 25 (1980) 1755. [30] Permeability and Other Film Properties, Plastic Design Library, New York, 1995, pp. 283–284. [31] S.A. Stern, J.T. Mullhaupt, P.J. Gareis, The effect of pressure on the permeation of gases and vapors through polyethylene. Usefulness of the corresponding states principle, AIChE J. 15 (1969) 64. [32] M.S. Suwandi, S.A. Stern, Transport of heavy organic vapors through silicone rubber, J. Polym. Sci., Polym. Phys. 11 (1973) 663. [33] J.S. Chiou, Y. Maeda, D.R. Paul, Gas and vapor sorption in polymers just below Tg , J. Appl. Polym. Sci. 30 (1985) 4019. [34] Y. Kamiya, T. Hirose, K. Mizoguchi, Y. Naito, Gravimetric study of high pressure sorption of gases in polymers, J. Polym. Sci., Polym. Phys. Ed. 24 (1986) 1525. [35] V.I. Bondar, Y.P. Yampolskii, S-shaped sorption isotherms: effects of nature of polymers and gases, in: Proceedings of the Euromembrane’95, vol. V.II (II-141), University of Bath, Bath, 1995. [36] K.I. Okamoto, T. Semoto, K. Tanaka, Application of pervaporation to phenol-acetone condensation reaction, in: Proceedings of the Congress on Membranes and Membrane Processes, Chicago, IL, NAMS 1, 1990, p. 347. [37] M.H.V. Mulder, J.O. Hendrikman, H. Hegeman, C.A. Smolders, Ethanol–water separation by pervaporation, J. Membr. Sci. 16 (1983) 269. [38] I. Pinnau, M.W. Hellums, W.J. Koros, Gas transport through homogeneous and asymmetric polyestercarbonate membranes, Polymer 32 (1991) 2612. [39] S.M. Shishatskii, Yu.P. Yampolskii, K.-V. Peinemann, Effects of film thickness on density and gas permeation parameters of glassy polymers, J. Membr. Sci. 112 (1996) 275. [40] R.Y.M. Huang, V.J.C. Lin, Separation of liquid mixtures using polymer membrane. I. Permeation of binary organic mixtures through polyethylene, J. Appl. Polym. Sci. 12 (1968) 2615.
256
A.M. Polyakov et al. / Journal of Membrane Science 216 (2003) 241–256
[41] A.G. Fadeev, Ya.A. Selinskaya, S.S. Kelly, M.M. Meagher, E.G. Litvinova, V.S. Khotimsky, V.V. Volkov, Extraction of butanol from aqueous solutions by pervaporation through poly(trimethylsilyl propyne), J. Membr. Sci. 186 (2001) 205. [42] X. Feng, R.Y.M. Huang, Estimation of activation energy for permeation in pervaportaion process, J. Membr. Sci. 118 (1991) 127. [43] Yu.P. Yampolskii, N.E. Kaliuzhnyi, S.G. Durgaryan, Thermodynamics of sorption in glassy poly(vinyltrimethyl silane), Macromolecules 19 (1986) 846. [44] D.R. Stull, E.F. Westrum, G.S. Sinke, The Chemical Thermodynamics of Organic Compounds, Wiley, New York, 1969.
[45] T. Masuda, Y. Iguchi, B.Z. Tang, T. Higashimura, Diffusion and solution of gases in substituted polyacetylene membranes, Polymer 29 (1988) 2041. [46] L.E. Starannikova, V.V. Teplyakov, Gas permeability of poly[1-(trimethylsilyl)-1-propyne]: evaluation of experimental data and calculation methods, Polym. Sci. A 39 (1997) 1142. [47] G.J. Van Amerongen, Diffusion in elastomers, Rubber Chem. Technol. 37 (1969) 1065. [48] D.W. van Krevelen, Properties of Polymers, Elsevier, Amsterdam, 1976. [49] Yu. Yampolskii, S. Shishatskii, A. Alentiev, K. Loza, Correlations with and prediction of activation energies of gas permeation and diffusion in glassy polymers, J. Membr. Sci. 148 (1998) 59.