Separation of Ethylene Glycol−Water Mixtures Using Sulfonated Poly

Ind. Eng. Chem. Res. 2002, 41, 2957-2965

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Separation of Ethylene Glycol-Water Mixtures Using Sulfonated Poly(ether ether ketone) Pervaporation Membranes: Membrane Relaxation and Separation Performance Analysis R. Y. M. Huang,* Pinghai Shao, X. Feng, and W. A. Anderson Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1

Pervaporation separation of ethylene glycol aqueous solutions was carried out using sulfonated poly(ether ether ketone) homogeneous membranes. Membrane relaxation in separation processes was observed and alleviated by heat treatment. The heat-treated pervaporation membranes experience further relaxation in the separation process by the swelling interactions of the feed mixtures before steady-state transport is reached. Membrane performance is investigated in terms of sorption and pervaporation separation. The preferential sorption and diffusion of water in the membranes were observed. Membrane performance can be interpreted by the modified solution-diffusion model, which takes into account the plasticization interaction of the transporting species in the membrane. Some parameters in this model were estimated by fitting the related experimental data. The benefit of using pervaporation for ethylene glycol dehydration is discussed in terms of energy consumption. 1. Introduction Pervaporation has been successfully applied in industries as an energy-efficient separation technology.1-3 Of all of the pervaporation applications, pervaporation dehydration shares the biggest market.4,5 Well-known applications in this category are the dehydration of ethanol, 2-propanol, and other chemical solvents.6-8 The dehydration of ethylene glycol is a relatively new topic that has attracted some attention from researchers9-14 for the importance of this separation. Ethylene glycol is synthesized from the hydrolysis of ethylene oxide.15 To suppress the formation of the byproducts (e.g., diethylene glycol and triethylene glycol), excess water is added to the reactor. The equilibrium composition of ethylene glycol in the product flow is around 15% (w/w),12 and it can be enriched to 7080% by distillation. Further concentration of ethylene glycol by distillation becomes economically infeasible because of the lower water content in the vapor phase. Pervaporation is suitable for the further removal of water from this product stream because of its high separation efficiency.11 In addition, ethylene glycol is widely used as an antifreeze or deicing agent in many industries.16-18 The discharge of the used ethylene glycol can cause environmental problems, and therefore, recycling of ethylene glycol is necessary and/or mandatory. Some polymer materials have been tried by early researchers for the separation of ethylene glycol-water mixtures, including chitosan-polysulfone composite membranes.11 surface cross-linked chitosan-poly(ether sulfone) composite membranes,12 and cross-linked poly(vinyl alcohol)-poly(ether sulfone) composite membranes.9,10 In this work, sulfonated poly(ether ether ketone) (SPEEK) membranes were used for this purpose because polyelectrolyte membranes usually show a higher affinity for water than for alcohols.19,20 Membrane relaxation was observed during the pervaporation separation. The homogeneous membranes were thus * Correspondingauthor.E-mail: [email protected].

heat-treated to facilitate the relaxation process at 80 °C. The separation performance of the membranes was analyzed based on the modified solution-diffusion model,27 considering the plasticization interaction between penetrants and the membrane. 2. Experimental Section 2.1. Materials. The samples of SPEEK of different sulfonation degrees (SD ) 62.9 and 79.2%) were prepared in our laboratory. The concentrated sulfuric acid was used as a sulfonating agent, and the reaction was carried out at 55 °C. SD can be controlled by varying the reaction time. The details in this aspect were described in our previous paper.30 Solvent dimethylformamide (DMF) was purchased from Aldrich Chemicals, while ethylene glycol (class IIIB) was provided by Fisher Scientific. 2.2. Membrane Preparation. Samples of SPEEK of different SDs were dissolved in DMF to formulate 10% (w/v) polymer solutions, and undissolved impurities were removed by filtration. The homogeneous polymer solutions were then cast on glass plates with a casting knife. The casting thicknesses were kept at 100, 150, and 200 µm, respectively. Solvent evaporation of the liquid films was carried out in an oven at 60 °C for 3 days. The resulting membranes were peeled off in a water bath. The thicknesses of the membranes corresponding to the three casting knife gaps (100, 150, and 200 µm) were 10, 15, and 22 µm, respectively. All of these membranes were air-dried. 2.3. Heat Treatment of the Membrane. The heat treatment of the air-dried membranes was carried out in an oven at 80 °C for 1-8 h. The heat-treated membrane was then cooled to room temperature over 30 min. 2.4. Fourier Transformation Infrared Spectrometry (FTIR) Measurement. The air-dried or heattreated membrane sample was sandwiched into a special folder and fixed with transparent tape for the measurement of infrared light transmission using a BOMEN, MB series FTIR spectrometer.

10.1021/ie010926v CCC: $22.00 © 2002 American Chemical Society Published on Web 05/17/2002

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2.5. Sorption. Vacuum-dried membrane samples were immersed in various ethylene glycol aqueous solutions for 2 days at two temperatures (20 and 32 °C). These membranes were taken out from their respective solutions to remove the liquid attached to membrane surfaces. The surface-cleaned membranes were immediately sandwiched between two sheets of dried tissues, which act as supports to prevent the loss of the membrane surface due to the surface contact and to provide ample passage for the transfer of the desorbed species. The sandwiched membrane roll was then inserted into to a cylindrical glass chamber which was attached to a vacuum pump to conduct desorption for 1 h. The desorbed vapor was condensed in a glass tube using liquid nitrogen. The composition of the collected liquid was analyzed by gas chromatography (GC; Hewlett-Packard series II). The oven, detector, and injector chamber were all set at 200 °C to prevent condensation of ethylene glycol. The partition coefficient of species can be estimated based on sorption data, and the partition coefficient (K) of water is defined as follows:

Cmembr K) ) Cfeed WsorbY/CTX WsorbY/FH2O + Wsorb(1 - Y)/FEG + Wmembr/FSPEEK

(1)

where Cmembr and Cfeed represent concentrations of water in the membrane and feed phases, respectively, CT represents the total mass concentration of the feed mixture, X and Y represent water contents in the feed and sorbed mixture, respectively, Wmembr and Wsorb represent the weight of dry membrane and the mixture sorbed in the membrane, respectively, and FH2O, FEG, and FSPEEK represent the density of water, ethylene glycol, and SPEEK (SD ) 79.2%), respectively. In this work, the density of SPEEK is assumed to be the same as that of PEEK, which is 1.32 g/cm3. 2.6. Pervaporation. Membrane samples were cut into disk form to be fitted into a separation cell to perform the pervaporation tests. The effective membrane area was 12.56 cm2. The downstream side of the membrane was maintained at 3 mmHg absolute pressure. Permeate is condensed and collected in a glass tube using liquid nitrogen and analyzed with GC. The weight of permeate was determined by a digital electronic balance. The permeation rate and separation factor of the membrane is defined respectively as follows:

J ) Q/A∆t

(2)

YW/YE XW/XE

(3)

R)

where Q is the weight of permeates collected in time interval ∆t, A is the effective membrane area, X and Y represent mass fractions in the feed and permeate, respectively, and subscripts W and E represent water and ethylene glycol, respectively. 3. Results and Discussion 3.1. Membrane Relaxation and Heat Treatment. Figure 1 presents two types of membrane permeation behavior. For membrane B, which experienced no heat

Figure 1. Permeation behaviors of membranes with or without heat treatment.

treatment, the total permeation rate increases before the feed temperature reached around 50 °C (the accumulative running time is about 3 h as specified in Figure 1). When temperature increased further but below around 60 °C, the total permeation rate went down. When the temperature surpassed 60 °C, the normal temperature dependence of the permeation rate was restored. The decline in the permeation rate in the temperature range of 50-60 °C can be attributed to the structure relaxation of the membrane. The homogeneous membrane was made by solvent evaporation from a thin polymer solution. As the concentration of the polymer solution increases during solvent evaporation, the polymer chains become more and more restricted in terms of mobility. Generally speaking, there exists heterogeneity in the microstructure of the membranes formed by solvent evaporation. If this type of membrane is heat-treated, the vibration of chain segments can be intensified, and the equilibrium location of the polymer chains can be regulated by this intensified vibration through the release of stresses resulting from the solvent evaporation phase. This remolded polymer network becomes more uniform and denser. Similar phenomena had been observed by Koros et al.21,22 and by Paul et al.23,24 in gas permeation tests using glassy polymers, where the term “physical aging” was used to describe this behavior. The structural change is dynamically taking place in the pervaporation process, noticeably starting from around 45 °C, thus making the total permeation rate deviate from the dotted lines (represented by B′ in Figure 1) that would have been followed. When the temperature is over 60 °C, the polymer network tends to be more or less stabilized. The decrease in the permeation resistance of the polymer network due to the higher flexibility of the polymer segments at elevated temperatures overcomes the decline in the total permeation rate due to the further relaxation of the membrane. As a result, a net increase in the total permeation rate of membrane B was observed. Membrane A was not heat-treated, and compared with membrane B, the relaxation of membrane A is much slower; part of the reason might be ascribed to its larger thickness. Membrane C, which was heattreated for 3 h at 80 °C prior to its permeation test, demonstrates conventional permeation behavior. It seems that the influence of the membrane relaxation in this case is insignificant.

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Figure 2. Dependence of the total permeation rate on the heat treatment time length.

Figure 4. Dependence of the total permeation rate on the operation time length.

Figure 5. Partition coefficient of water versus the feedwater composition.

Figure 3. FTIR spectra of the membranes with and without heat treatment.

Figure 2 shows the dependence of the total permeation rate on the heat treatment time at 80 °C. The permeation rate was measured in the first hour of operation after the membrane was mounted in the separation cell. There is a relatively sharp decrease of the total permeation rate in the first 3 h of heat treatment. Membranes with heat treatment over 3 h (from 3 to 8 h in this test) show no substantial losses in the permeation rate, showing that 3 h for heat treatment at 80 °C is sufficient for the heat relaxation of the membrane. Some researchers25,26 had reported that the heat treatment could initiate the chemical cross-linking of the membrane materials, which can also compact the polymer chains, thus reducing the permeability of the membrane. SPEEK has the potentially active sulfonic acid groups in the backbone of PEEK, and the sulfonation reaction can further take place to form the crosslink bonds in an inter- and/or intramolecular manner. To exclude this possibility, we analyzed the heat-treated and non-heat-treated SPEEK membrane samples using FTIR. The transmission spectra of FTIR are presented in Figure 3. No new absorption peak was observed, suggesting that there is no occurrence of cross-linking in the membrane material after 8 h of heat treatment

at 80 °C. Interestingly, there is a shift of peaks toward low wavelength in the spectrum of the heat-treated membrane, implying that the polymer matrix is more densely packed after heat treatment, and the vibration of the related groups needs a higher energy. It can thus be concluded that the decline in the permeation rate is attributed to the membrane relaxation or the physical aging of the membranes. The decay of the total permeation rate with operation time in pervaporation tests is described in Figure 4, for a membrane that was heat-treated for 3 h at 80 °C prior to its pervaporation test. Membrane relaxation is still observed in the first 3 h of operation. This further relaxation is caused by the plasticization interaction of permeating species with the polymer chains. The presence of small molecules in the polymer network diminishes the inter- and/or intramolecular interactions of the polymer, enabling the self-adjustment of the polymer chains. This kind of polymer chain adjustment cannot be completed under mild heat treatment (e.g., 80 °C in this case). The heat treatment of membranes at elevated temperatures will be reported in a separate paper. 3.2. Partition Coefficient of Water. The partition coefficient (K) of water in SPEEK membranes (SD ) 79.2%) at 32 °C, which was obtained from the related sorption data specified in eq 1 , is plotted against the feedwater content in Figure 5. One can see that the partition coefficient decreases as the water content in the feed increases. Generally speaking, the sorption behavior of a species in the membrane is nonideal. The ideal sorption of water by the membrane is restricted to a very dilute water concentration range. As the water content in the feed increases, the membrane shows less preference to water. This sorption behavior was interpreted in our previous paper.19 SPEEK membranes

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Figure 6. Sorption behaviors of the SPEEK membrane at different temperatures.

Figure 7. Dependence of the permeation rate on the water content in the feed.

consist of two environments: the hydrophilic environment (the polar sulfonic groups) and the hydrophobic environment (the membrane matrix except for sulfonic groups). Water dissolves only in the hydrophilic environment, which is adsorbed around the polar sulfonic groups to form the so-called “water clusters” based on the strong electrostatic interaction. The cluster grows as the water content in the feed increases. This increased water cluster size undermines the electrostatic interaction between sulfonic groups (the nucleus of the cluster) and water molecules in the outermost parts of the clusters as a result of further sorption from the feed mixture. Thus, relatively more ethylene glycol can be sorbed into the cluster based on the hydrogen bond interaction, which is less water selective than the electrostatic interaction. This is why a relatively sharp decrease of the partition coefficient of water is observed when the feedwater content increases up to about 60% (w/w). When the feedwater content increases further, we can speculate that the size of the water cluster in the membrane has become so large that the sorption process in this stage is basically governed by hydrogen bond interaction rather than by electrostatic interaction. As a result, a relatively stable partition coefficient of water in the membrane is shaped in this feedwater content range. This analysis also suggests that water can be more efficiently removed from dilute water content systems. On the basis of this knowledge, one can understand why pervaporation technology has found wide applications in the dehydration of organic solvents, removal of VOC(s) from industrial effluents, and aroma recovery from fruit juices, etc., where the main species in the removed streams are the minor components in their respective feed mixtures. 3.3. Membrane Sorption Behaviors. The sorption experiments were carried out over wide ranges of feed composition using the SPEEK membrane of SD ) 79.2% at two temperatures (20 and 32 °C). The sorption behavior of this membrane is depicted in Figure 6, which shows that the SPEEK membrane has an excellent affinity for water. The water content was significantly enriched in the membrane phase. The preferential sorption of water by the membrane is the basis for the pervaporation separation of water from ethylene glycol aqueous solutions. The effect of temperature on the water content of the sorbed mixture is also shown in Figure 6. The sorption process is usually accompanied by negative enthalpy changes. Lower

temperatures favor the sorption of both water and ethylene glycol in SPEEK membranes. The water content in the sorbed mixtures decreases with an increase in the feed temperature, showing that the absolute change in enthalpy before and after sorption of ethylene glycol is larger than that of water. 3.4. Permeation Behavior. The total permeation rate of the membrane and the partial permeation rate of ethylene glycol are plotted against the feed composition in Figure 7. Compared to the total permeation rate, the permeation rate of ethylene glycol is negligibly small, showing the semipermeable property of the SPEEK membrane to water. It is clear that the permeation rate of water increases in an exponential manner as the water content in the feed increases. According to the modified solution-diffusion mechanism,27 which considers the plasticization effect of the dissolved species, the diffusion coefficient of the permeating species is, in general, exponentially dependent on local concentrations of each species. For a binary system of an aqueous ethylene glycol solution, the diffusion coefficient of either species can be written as

D ) D0 exp(ξCW + ψCE)

(4)

where CW and CE represent the local concentrations of water and ethylene glycol in the membrane and ξ and ψ are the plasticization coefficients of water and ethylene glycol, respectively. In this case, the plasticization effect of ethylene glycol can be neglected for its dilute concentration in the membrane; therefore, the diffusivity of water is only determined by the water content in the membrane.

DW ) DW0 exp(ξCW)

(5)

By introduction of the partition coefficient (K) of water between the membrane phase and the feed solution, the diffusion coefficient at this interface can be written as

DW ) DW0 exp(ξKCTX)

(6)

where X is the water content in the feed and CT is the total mass concentration of an aqueous ethylene glycol solution. The densities of water (1.00 g/cm3) and ethylene glycol (1.12 g/cm3) are quite close, so the total mass concentration as a constant makes sense. Based on Fick’s first law, we can obtain the permeation rate (J)

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Figure 9. Dependence of the water content in the permeate on the feedwater composition. Figure 8. Relationship between the logarithmic total permeation rate and the feedwater content. Table 1. Slope and Intercept of the Line and the Obtained Plasticization and Diffusion Coefficients at Infinite Dilutiona slope ξKCT

ξ (cm3/g)

intercept ln(DW0/ξδ)

DW0 (cm2/s)

4.48

5.56

-14.14

8.84 × 10-9

a

K ) 0.76, CT ) 1.06 g/cm3, and δ ) 2.2 × 10-3 cm.

of water through the membrane, assuming the water concentration in downstream side of the membrane is zero.

J)

DW0 [exp(ξKCTX) - 1] ξδ

(7)

where δ is the thickness of the membrane. Equation 8 can be obtained by brief mathematical transformation of eq 7.

(

ln J +

) ( )

DW0 DW0 ) ln + ξKCTX ξδ ξδ

(8)

To test and elucidate this relationship, the logarithmic total permeation rate (ignoring the DW0/ξδ term on lefthand side of eq 8) is plotted against the feedwater content as shown in Figure 8. A good linear relationship is observed in the range of 60-93.6% (w/w), suggesting the validity of the exponential dependence of diffusivity on local concentrations of the species. According to Figure 5, the partition coefficient K can be taken as a constant (the average is approximately 0.76) in the range of 60-93.6% (w/w). We can, therefore, expect a linear relation between the logarithmic total permeation rate and the feedwater content X in this designated range as shown in Figure 8. When the feedwater content goes down to the range of 20-60% (w/w), the partition coefficient of water in this range increases sharply. We can thus see that the three data in this range are over the line. When the feedwater content is below 20% (w/ w), DW0/ξδ (see the intercept of the line in Figure 8) is comparable to the permeation rate; in this case, it cannot be ignored, and therefore, the bigger deviations of the two data from the linear relation are thus observed. The slope and intercept of the line are summarized in Table 1, through which both the plasticization coefficient and the diffusion coefficient of water at infinite dilution can be estimated. 3.5. Water Content in the Permeate. Figure 9 shows the dependence of the water content in the

Figure 10. Theoretical comparison of the energy consumption of pervaporation with distillation.

permeate versus the feedwater content at 32 °C. Over the wide range of feed composition, the permeate features a high water percentage of over 99.6% (w/w). It is interesting that the feed mixtures of around 5% (w/w) water content can be dehydrated quite efficiently by this membrane, and the purity of the removed water can be over 98% (w/w). This is one of the advantages of using pervaporation membranes for solvent dehydration. For the purpose of comparison with traditional distillation technology in separation efficiency, the vapor-liquid equilibrium line (at a constant pressure of 730 mmHg) is also presented in Figure 9. There is a big gap between the water purity of the two removed streams especially when the feedwater content is relatively low (say, less than 20%). High water purity in the removed water mixture means a lower energy consumption and less waste of ethylene glycol. To illustrate how the pervaporation technology saves energy in the dehydration of ethylene glycol aqueous solutions, the energy consumption (P) based on the heat requirement for evaporation for the removal of 1 kg water from feed mixtures was estimated, which is shown in Figure 10.

P ) ∆HW +

(

)

1 - 1 ∆HE YW

(9)

where ∆HW and ∆HE represent the evaporation heat (kcal/kg) of water and ethylene glycol, respectively. There is only a slight increase in the energy consump-

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Figure 11. Dependence of separation factors on the feedwater content.

tion by pervaporation as the feedwater content decreases because of the consistently high purity of water in the permeate. However, in the case of distillation, we can see a very sharp increase in this aspect because of the lower separation efficiency of distillation. The separation by distillation is based on the relative volatility between water and ethylene glycol. As the concentration of ethylene glycol increases, more ethylene glycol evaporates into the vapor phase; thus, additional energy is consumed. It is clear in Figure 10 that the benefit of applying pervaporation technology for dehydration of ethylene glycol becomes significant when the water content in the feed is significantly low. It should also be pointed out that this simple comparison was based only on the theoretical energy consumption due to the liquid-vapor phase change at a constant pressure. Many other factors such as cooling of distillation, thermodynamic heat effectiveness, and capital cost are not considered in this comparison, all of which are quite important for the economic evaluation of these two separation technologies. 3.6. Separation Factors. The separation factors of both permeation and sorption processes at 32 °C are illustrated in Figure 11 over different ranges of the feedwater content. As discussed previously, water is preferentially sorbed in the SPEEK membranes. Comparing the two separation factors presented in Figure 11, one can realize that preferential transport of water also took place in the membrane. This outcome agrees with the general estimation based on the size difference of the transporting species. The size of water is much smaller than that of ethylene glycol. According to solution-diffusion theory, the separation factor of pervaporation is the product of the diffusion selectivity and sorption selectivity:

Rperm ) RdiffRsorp

(10)

Based on this understanding, it is now possible to estimate the contribution of the diffusion to the total permselectivity of the membrane. The diffusion selectivity is obtained by conversion using eq 10 and is illustrated in Figure 12. It decreases as the water content in the feed increases. This behavior results from the swelling of the membrane by water. Generally speaking, a swollen membrane shows reduced diffusion selectivity due to the enhanced free volume in the membrane matrix.

Figure 12. Dependence of the diffusion selectivity on the feedwater content.

To obtain the plasticization coefficient of water to ethylene glycol and the diffusivity of ethylene glycol at infinite dilution from the relationship between the diffusion selectivity and the feedwater content, a brief mathematical derivation is provided here. Because it is known that the concentration of ethylene glycol in the membrane phase is negligibly small, its plasticization effect on its own diffusion coefficient is also ignored. Theoretically, the ratio of the permeation rate of water to ethylene glycol can be described as

JW DW0 exp(ξCW) dCW/dδ ) JE DE0 exp(γCW) dCE/dδ

(11)

By integration, eq 11 can be written as

JW DW0 exp{(ξ - γ)CSW} - 1 ) JE DE0 (ξ - γ)CSE

(12)

where CSW and CSE represent the concentrations of water and ethylene glycol, respectively, at the upstream surface of the membrane. By expansion of the term exp{(ξ - γ)CSW} into a series, eq 12 becomes

JW DW0 CSW ) × JE DE0 CSE

{

[(ξ - γ)KCT]2 2 (ξ - γ)KCT X+ X + ... 1+ 2! 3!

}

(13)

In reference to the definitions of permeation and sorption selectivity, eq 14 is equivalent to eq 13.

Rperm ) Rsorp

{

DW0 × DE0

[(ξ - γ)KCT]2 2 (ξ - γ)KCT X+ X + ... 1+ 2! 3!

}

(14)

Therefore, the rigorously derived diffusion selectivity is like the following:

Rdiff )

{

DW0 × DE0

1+

[(ξ - γ)KCT]2 2 (ξ - γ)KCT X+ X + ... 2! 3!

}

(15)

Ind. Eng. Chem. Res., Vol. 41, No. 12, 2002 2963 Table 2. Estimated Quantities by the Second-Order Polynomial Fitting

Table 4. Activation Energies of Water in Two SPEEK Membranes of Different SDs

ξ-γ

γ (cm3/g)

ξ/γ

DW0/DE0

DE0 (cm2/s)

-3.67

9.23

0.60

80.5

0.11 × 10-9

Table 3. Some Related Parameters (Plasticization Coefficients of Water and Diffusion Coefficients at Infinite Dilution) in the Literature membrane Nafion H+ Nafion (CH3)4N+ Nafion (C4H9)4N+ Nafion (C8H17)4N+ aromatic polyamidesa SPEEKb

a

plasticization coefficient

diffusion coefficient at infinite dilution

0.3128 3.7028 9.5528 8.4328 0.51-9.0029

10-9 cm2/s

°C)19

7.0 (ethanol at 26 7.8 (ethanol at 30 °C)19 9.3 (ethanol at 40 °C)19

The plasticizer is ammonia. b SD ) 79.2%.

Figure 13. Arrhenius relationships between the total permeation rate and the absolute temperature.

Fitting the data of diffusion selectivity to eq 15 by retaining the first three terms of the series, we can obtain DW0/DE0 and the difference in plasticization coefficient ξ - γ, and thus the diffusion coefficient of ethylene glycol at infinite dilution and the plasticization coefficient of water to ethylene glycol γ can be estimated, all of which are listed in Table 2. Clearly, the diffusion coefficient of ethylene glycol is more significantly enhanced by membrane swelling caused by water because the ratio ξ/γ of the plasticization coefficients is less than 1. This is the theoretical interpretation of why a swollen membrane shows reduced diffusion selectivity. The plasticization coefficients of water in modified Nafion and ammonia in aromatic polyamides and the diffusion coefficients of ethanol at infinite dilution in the SPEEK membrane (SD ) 79.2%) obtained by inverse GC (IGC) are summarized in Table 3 for comparison. Showing that all of the parameters estimated are reasonable, the big difference between the diffusion coefficient of ethylene glycol and those of water and ethanol may be attributed to its bigger molecular size. 3.7. Permeation Activation Energy. The logarithmic total permeation rates were plotted against the temperature reciprocals for two membranes in Figure 13. The typical linear relationships were obtained. The feedwater content is the same at 20% (w/w) for the two membranes. The permeation activation energies of water in these two membranes are listed in Table 4. It is known that the permeation activation energy Eaperm

membrane

SD (%)

activation energy (kcal/mol)

A B

62.9 79.2

15.08 5.71

Table 5. Effect of Temperature on the Water Percentage in the Permeate at a Feed Water Percentage of 20% (w/w) temp (°C)

membrane (% SD)

membrane thickness (µm)

water percentage in the permeate [% (w/w)]

32 40 50 60 70 32 40 50 60 70 32 40 50 60 70

62.9

15

79.2

10

79.2

22

99.62 ( 0.06 99.85 ( 0.01 99.73 ( 0.01 99.20 ( 0.02 99.39 ( 0.05 99.44 ( 0.01 99.65 ( 0.02 99.66 ( 0.03 99.80 ( 0.01 99.70 ( 0.02 99.50 ( 0.15 99.72 ( 0.06 99.38 ( 0.15 99.65 ( 0.05 99.73 ( 0.02

is the sum of the sorption enthalpy change ∆Hsorp and diffusion activation energy Eadiff; thus, the big difference in permeation activation energy of the two membranes can be attributed to their difference in the diffusion activation energy. It is generally accepted that water molecules jump from one sulfonic group to another, and thus the diffusion of water across the membrane is directly affected by the density of this group. On the other hand, membranes with a higher SD are more swollen, and membranes with increased free volume certainly favor the transport process. 3.8. Effect of Temperature. The effect of temperature on the water percentage in the permeate is shown in Table 5 Over the temperature range (32-70 °C) studied, no significant changes in the water content of the permeate were observed. This behavior is also found in the dehydration of 2-propanol aqueous solutions using the same membrane as that in this work.19 Part of the reason may be that the water content in the membrane phase is reduced for the unfavorable sorption of water at higher temperatures. The SPEEK membrane is highly sensitive to the water content in the membrane phase; the membrane swelling at higher temperatures is thus suppressed. The less swollen SPEEK membrane demonstrated a high diffusion selectivity, which, to some extent, counteracted the lower sorption selectivity at higher temperatures. The ratio of the selectivity of diffusion to that of sorption is illustrated in Figure 14 to justify this interpretation. Clearly, the ratio is larger than 1 when the feedwater content is less than about 60% (w/w), implying that the membrane performance is dominated by the diffusion behavior of the species in the membrane. The influence of sorption selectivity is of less weight in this case. Therefore, the water content in the permeates appears to be insensitive to temperature. For the membranes tested, the water content in the permeate is always over 99% (w/w), suggesting that SPEEK is a very promising material for pervaporation dehydration of ethylene glycol-water systems.

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Ind. Eng. Chem. Res., Vol. 41, No. 12, 2002 P ) energy consumption (kcal/kg) Q ) weight of the permeate collected (g) X ) mass percentage in the feed Y ) mass percentage in the permeate Greek Letters R ) separation factor δ ) thickness of the membrane (cm) φ ) plasticization coefficient of ethylene glycol to water diffusivity (cm3/g) γ ) plasticization coefficient of water to ethylene glycol diffusivity (cm3/g) ξ ) plasticization coefficient of water to water diffusivity (cm3/g)

Figure 14. Ratio of the selectivity of diffusion to that of sorption versus the feedwater content.

4. Conclusions Membrane relaxation was observed in the permeation process, resulting in a reduced permeation rate. Heat treatment was found to be able to facilitate the relaxation process. However, the heat-treated membrane (3 h at 80 °C) can undergo further relaxation in the permeation process because of the plasticization effect of water molecules in the membrane matrix. In the case of dehydration of ethylene glycol aqueous solutions, the membrane relaxation process comes to an end in nearly 3 h of permeation. The SPEEK membranes demonstrated both preferential sorption and preferential transport for water molecules. The permeation of transporting species can be explained by the modified solution-diffusion mechanism. The diffusion coefficients vary exponentially with local concentrations of the transporting species. Because of the preferential sorption of water in membranes, the plasticization effect of ethylene glycol can be reasonably ignored. The partition coefficient of water was obtained by sorption experiments, which decreases as the feedwater content increases. The plasticization coefficient of water and diffusion coefficients of water and ethylene glycol at infinite dilution were estimated. The water content in the permeate is much higher than that in the vapor phase in equilibrium with feed mixtures. The advantage of pervaporation dehydration over distillation becomes significant when the feedwater percentage is significantly low. The effect of temperature on the water content in the permeate is insignificant, and the permeation flux of the membrane increases exponentially as the operation temperature goes up, suggesting that SPEEK(s) are very good membrane materials for dehydration of ethylene glycol aqueous solutions. Acknowledgment Financial support for this research project from the Natural Sciences and Engineering Research Council (NSERC) of Canada is gratefully acknowledged. Nomenclature A ) effective membrane area (m2) C ) concentration of the species (g/cm3) D ) diffusion coefficient (cm2/s) Ea ) activation energy (kcal/mol) ∆H ) enthalpy change (kcal/mol or kcal/kg) J ) total permeation rate (g/m2‚h) K ) partition coefficient ∆t ) operation time interval (h)

Subscripts S ) surface W ) water E ) ethylene glycol T ) total 0 ) concentration at infinite dilution

Literature Cited (1) Tusel, G. F.; Bruschke, H. E. A. Use of pervaporation systems in the chemical industry. Desalination 1985, 53, 327338. (2) Fleming, H. L. Consider membrane pervaporation. Chem. Eng. Prog. 1992, 88, 45-52. (3) Brano, J. L.; Fair, J. R.; Humphrey, J. L.; Martio, C. L.; Seibert, A. F.; Joshi, S. Fluid mixtures separation technologies for cost reduction and process improvement; Noyes Publications: Park Ridge, NJ, 1986. (4) Asada, T. In Pervaporation Membrane Separation Processes; Huang, R. Y. M., Ed.; Elsevier: Amsterdam, The Netherlands, 1991. (5) Neel, J. In Pervaporation Membrane Separation Processes; Huang, R. Y. M., Ed.; Elsevier: Amsterdam, The Netherlands, 1991. (6) Fleming, H. L. Membrane pervaporation: separation of organic/aqueous mixtures. Sep. Sci. Technol. 1990, 25, 1239. (7) Rautenbach, R.; Klatt, S.; Vier, J. State of the art of pervaporation, -10 years of PV. Sixth International Conference on Pervaporation Processes in the Chemical Industry, Englewood, NY, 1992. (8) Maeda, Y.; Kai, M. In Pervaporation Membrane Separation Processes; Huang, R. Y. M., Ed.; Elsevier: Amsterdam, The Netherlands, 1991. (9) Chen, F. R.; Chen, H. F. Pervaporation separation of ethylene glycol-water mixtures using cross-linked PVA-PES composite membranes. Part I. Effect of membrane preparation conditions on pervaporation performances. J. Membr. Sci. 1996, 109, 247-256. (10) Chen, F. R.; Chen, H. F. A diffusion model of the pervaporation separation of ethylene glycol-water mixtures through crosslinked poly(vinyl alcohol) membrane. J. Membr. Sci. 1997, 139, 201-209. (11) Feng, X.; Huang, R. Y. M. Pervaporation with chitosan membranes. I. Separation of water from ethylne glycol by a chitosan/polysulfone composite membrane. J. Membr. Sci. 1996, 116, 67-76. (12) Nam, S.; Lee, Y. M. Pervaporation of ethylene glycol-water mixtures, I. Pervaporation performance of surface cross-linked chitosan membranes. J. Membr. Sci. 1999, 153, 155-162. (13) Bartels, C. B.; Reale, J. Dehydration of glycols. U.S. Patent 4,802,988, 1989. (14) Jehle, W.; Staneff, Th.; Wagner, B.; Steinwandel, J. Separation of glycol and water from coolant liquid by evaporation, reverse osmosis and pervaporation. J. Membr. Sci. 1995, 102, 9-19. (15) Forkner, W. M.; Robson J. H.; Snellings, W. M. Glycol. In Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed.; Kroschwitz, J. I., Howe-Grant, M., Eds.; Wiley: New York, 1994; Vol. 12. (16) Greek, B. F. Ethylene glycol supplies will be sufficient to meet rising demand. Chem. Eng. News 1991, 69, 11.

Ind. Eng. Chem. Res., Vol. 41, No. 12, 2002 2965 (17) Randall, P. M.; Gavaskar, A. R. Evaluation of filtration and distillation methods for recycling automotive coolant. J. Air Waste Manage. Assoc. 1993, 43, 463-468. (18) Rincon, C.; Ortiz de Zarate, J. M.; Mengual, J. I. Separation of water and glycols by direct contact membrane distillation. J. Membr. Sci. 1999, 158, 155-165. (19) Huang, R. Y. M.; Shao, P.; Feng, X.; Burns, C. M. Pervaporation separation of water/2-propanol mixture using sulfonated poly(ether ether ketone) (SPEEK) membranes: Transport mechanism and separation performance. J. Membr. Sci. 2001, 192, 115-127. (20) Huang, R. Y. M.; Shao, P.; Feng, X.; Burns, C. M. Measurement of partition, diffusion coefficients of solvents in polymer membranes using rectangular thin- channel column inverse gas chromatography (RTCCIGC). J. Membr. Sci. 2001, 188, 205-218. (21) Pfromm, P. A.; Koros, W. J. Accelerated physical aging of thin glassy polymer films: Evidence from gas transport measurements. Polymer 1995, 36, 2379. (22) Rezac, M. E.; Pfromm, P. H.; Castello, L. M.; Koros, W. J. Aging of thin polyimide-ceramic and polycarbonate-ceramic composite membranes. Ind. Eng. Chem. Res. 1993, 32, 192-1926. (23) Rezac, M. E. Update on the aging of a thin polycarbonateceramic composite membrane. Ind. Eng. Chem. Res. 1995, 34, 3170-3172. (24) McCaig, M. S.; Paul, D. R. Effect of film thickness on the change in gas permeability of a glassy polyarylate due to physical aging, Part I. Experimental observations. Polymer 2000, 4, 629637.

(25) Shieh, J. J. Synthesis and Preparation of Novel Polymer Membranes and Their Applications in the Pervaporation Membrane Separation of Alcohol-Water Systems and the Development of a New Mass Transport Model for the Pervaporation Process. Ph.D. Dissertation, University of Waterloo, Ontraio, Canada, 1996. (26) Masamitsn, S.; Norihiko, N.; Masahiro, T. Photoassisted thermal cross-linking of polymers having imino sulfonate units. React. Funct. Polym. 1998, 37, 147-154. (27) Brun, J.-P.; Larchet, C.; Melet, R.; Bulvestre, G. Modelling of the pervaporation binary mixtures through moderate swelling, nonreacting membranes. J. Membr. Sci. 1985, 23, 257. (28) Kusumocahyo, S. P.; Sudoh, M. Dehydration of acetic acid by pervaporation with charged membranes. J. Membr. Sci. 1999, 161, 77-83. (29) Seemnova, S. I.; Ohya, H.; Smirnov, S. Physical transitions in polymers plasticized by interacting penetrants. J. Membr. Sci. 1997, 136, 1-11. (30) Huang, R. Y. M.; Shao, P.; Burns, C. M.; Feng, X. Sulfonation of poly (ether ether ketone): Kinetic study and characterization. J. Appl. Polym. Sci. 2001, 82, 2651-2660.

Received for review November 13, 2001 Revised manuscript received March 18, 2002 Accepted March 23, 2002 IE010926V