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Performance of a nanofiltration membrane for removal of ethanol from aqueous solutions by pervaporation
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Separation and Purification Technology 60 (2008) 54–63
Performance of a nanofiltration membrane for removal of ethanol from aqueous solutions by pervaporation Adrian Verhoef a,∗ , Alberto Figoli b , Bram Leen a , Ben Bettens a , Enrico Drioli b , Bart Van der Bruggen a a
Department of Chemical Engineering, Laboratory for Applied Physical Chemistry and Environmental Technology, Katholieke Universiteit Leuven, W. de Croylaan 46, B-3001 Leuven, Belgium b Research Institute on Membrane Technology (ITM-CNR), c/o University of Calabria, via P. Bucci, Cubo 17/C, 87030 Rende, Cosenza, Italy Received 30 December 2006; received in revised form 10 July 2007; accepted 26 July 2007
Abstract In this study, the performance of a hydrophobic nanofiltration membrane (SolSep 3360) for treating alcoholic solutions by pervaporation is investigated and compared to a conventional pervaporation membrane (PV 1070, Sulzer Chemtech and Pervatech PDMS). Both binary ethanol/water mixtures and common multicomponent mixtures (alcoholic beverages) are examined. The experiments were performed at feed ethanol concentrations up to 50 vol% and at temperatures up to 45 ◦ C. The effects of feed ethanol content and temperature were studied in terms of: (1) fluxes and permeances of individual components, and (2) separation factor, enrichment factor and selectivity of ethanol to water. Using permeance and selectivity instead of flux and separation/enrichment factor allows the effects on performance evaluation of operating conditions, such as temperature and swelling, to be decoupled. In this way the contribution by nature of the membrane to separation performance can be clarified and quantified. In addition, previous analyses indicate that the aqueous activity coefficient and the saturated vapour pressure play an important role when evaluating the membrane performance in terms of permeance and selectivity. This is confirmed by this study. Furthermore, it was found that multicomponent alcoholic beverages behave in exact the same manner as binary ethanol/water mixtures. Using a nanofiltration membrane for pervaporation purposes is a suitable possibility, because of the higher fluxes and permeances, while remaining a good separation factor and selectivity. The difference between nanofiltration and pervaporation membranes is explained by the influence of swelling, making the membrane more dense, and the different interactions between permeating molecules and the membrane. © 2007 Elsevier B.V. All rights reserved. Keywords: Pervaporation; Nanofiltration membrane; Alcoholic solutions; PDMS membranes
1. Introduction In the beginning of the 20th century, Kober [1] first observed that a membrane could efficiently separate two liquid chemicals mixed together, by applying a vacuum on the other side of the membrane, resulting in a gradient of chemical potential. In reaction to this gradient, the components of the mixture penetrate into the membrane and evaporate on the other side. Kober named this phenomenon pervaporation. Separation is ensured by differ-
∗
Corresponding author. Tel.: +32 16 32 23 64; fax: +32 16 32 29 91. E-mail addresses: [email protected] (A. Verhoef), [email protected] (A. Figoli). 1383-5866/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2007.07.044
ences in solvents sorption affinity and diffusion coefficients in the membrane. Industrially, pervaporation is particularly useful to separate mixtures hard to separate by conventional techniques, such as distillation or extraction. Examples are azeotropic mixtures, such as alcohol/water, or chemical products with close boiling points, such as acetic acid/water. For every membrane process, a good membrane must be found, in order to obtain optimal separation. Many studies prove the membrane has significant influence. The selectivity and performance of the membrane is not only determined by obvious features such as thickness, porous or dense nature of the top layer [2–5], pore size and geometry, and porosity [2,3,6,7], but also by less obvious material properties such as glass
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Nomenclature A F J l Mw m p S t x x y y
membrane area (m2 ) membrane permeability (kg m−1 h−1 bar−1 ) flux (kg m−2 h−1 ) membrane thickness (m) molar mass (kg/mol) permeate mass (kg) partial pressure (bar) ideal membrane selectivity measurement time (s) feed weight fraction feed molar fraction permeate weight fraction permeate molar fraction
Greek letters α separation factor β enrichment factor γ activity coefficient Subscripts i component i j component j tot total parameter Superscripts p permeate sat saturated vapour phase
transition temperature [2,8], composition [2,9,10], hydrophobicity/hydrophilicity [2,3,6,11,12] and membrane surface charge [2,13]. For each membrane process, optimal membrane properties are determined separately in terms of structure and material. For pervaporation, rather thick membranes with a dense nature are used, in view of chemical stability. In contrast, nanofiltration membranes can be dense or nanoporous and are as thin as possible, because the flux through the membrane is inversely proportional to its thickness. In literature, specific membranes are reported to be suitable for both nanofiltration and pervaporation [14,15]. Both polymeric and inorganic membrane materials were used for this purpose. Polymers are a commonly used material for membranes. However, upon wetting, they swell, altering the structure of the membrane [12]. Swelling occurs because a solvent enters and passes through the membrane, due to a chemical potential gradient. This increases permeability, but decreases selectivity, since another component in the feed mixture can benefit from the now available free volume inside the membrane, and permeate as well [16]. This property can be used as an advantage. The swelling phenomenon can make the structure of a polymeric micro- or nanoporous nanofiltration membrane more dense [2]. In this study, the effect of swelling is examined to see whether a hydrophobic nanofiltration membrane can be used for separating alcohol/water mixtures by pervaporation. In order to investigate
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the behaviour of the membrane with common multicomponent solutions, alcoholic beverages were used for pervaporation experiments. Since polymeric materials have a lower cost, the use of nanofiltration membranes in pervaporation can be expected to broaden the application range of membrane processes in industry. 2. Materials and methods 2.1. Membranes All membranes in this study have a PDMS (poly dimethyl siloxane)-based top layer. This is a silicon elastomer with a typical hydrophobic character. Three different membranes were used: SolSep 3360 is a hydrophobic nanofiltration membrane manufactured by SolSep BV (Apeldoorn, the Netherlands); Pervatech PDMS is a dense hydrophobic pervaporation membrane manufactured by Pervatech BV (Enter, the Netherlands); and PV 1070 is a zeolite-filled, dense pervaporation membrane manufactured by Sulzer Chemtech (Neunkirchen, Germany). Experiments were performed on the SolSep 3360 nanofiltration membrane. These results were compared to those of the two other traditional pervaporation membranes. The results for the Pervatech membrane were obtained by Ranieri et al. [17] on the same set-up. 2.2. Chemicals Measurements were performed with ethanol/water mixtures. The ethanol concentration varies up to 50 vol%. Ethanol was of analytical purity (>99.8%) and was obtained from Carlo Erba. In this study, it was also tested if common multicomponent mixtures show identical pervaporation behaviour as ethanol/water mixtures. To investigate this, alcoholic beverages were measured. Lager beer (Beck’s, 5 vol% alcohol), white wine (Marino, Le Contrade 2004, 11.5 vol% alcohol) and gin (Argia Gin, extra dry, 38 vol% alcohol) were used as common multicomponent mixtures containing ethanol. These beverages were chosen since they do not contain too many impermeable components, to prevent fouling [18], and are not too viscous, in order to avoid fluid mechanical phenomena such as concentration polarisation. Earlier research [19,20] showed that having a non-viscous feed not necessarily avoids concentration polarisation, but slow sorption of permeants in the membrane ensures that permeation fluxes are independent of feed membrane concentration. The relatively low separation factor in this work indicates this is the case. 2.3. Pervaporation experiments In the pervaporation system, schematically shown in Fig. 1, the feed temperature is controlled by a thermostat (T). A recirculation pump circulates the feed through the set-up at a flow rate of 0.7 l/min. The pervaporation module contains a circular flat sheet membrane with a membrane area of 56.74 cm2 .
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Fig. 1. Schematic diagram of the pervaporation set-up.
At the permeate side, a vacuum is maintained below 5 mbar by a vacuum pump, and the permeate pressure is measured with a manometer (P). Two condensers are used, cooled with liquid nitrogen. The permeate is collected in the first condenser, and the second protects the vacuum pump. Several experiments are performed at different concentrations up to 50 vol% and temperatures below 50 ◦ C, to avoid problems with the membrane module, such as blocking of the cold trap. For each experiment, at least three measurements of about 1 h are performed and averaged to calculate fluxes. After the completion of each experiment, the permeate collected inside the cold trap was warmed up to room temperature. The permeate was then weighed to determine the total flux. For each measurement, samples are taken from feed and permeate. The concentration of ethanol in these samples is determined by means of gas chromatography. The apparatus used is an Agrilent Technologies 6890N. The capillary column consists of a stainless steel Porapack Q 80/20s mesh coating. The furnace temperature is kept constant at 120 ◦ C, N2 is used as carrier gas and the pressure is held constant at 1.29 bar. The detector used is a TCD detector.
Here, [Fi /l] is the membrane permeability Fi divided by the membrane thickness l, also called the permeance; x i the mole fraction in the feed; γ i the activity coefficient; psat i the saturated vapour pressure; y i the permeate mole fraction and pp is the permeate pressure. The activity coefficients were calculated using the UNIFAC equation; the saturated vapour pressures using the Clausius–Clapeyron equation [21]. These two equations give a driving force based on molar fractions, but are used since no correct equations based on weight fractions are available to calculate these parameters. Probably this difference will not lead to large errors in the calculations. The ideal membrane selectivity Si/j is now defined as the ratio of the permeances: Si/j =
[Fi / l] [Fj / l]
(4)
The separation factor α is determined by Eq. (5). Here, xi is the feed concentration of the preferentially permeating component (ethanol in this study), and yi the permeate concentration of this component. α=
yi /(1 − yi ) xi /(1 − xi )
(5)
The enrichment factor β is calculated by dividing the permeate concentration by the feed concentration, as illustrated in Eq. (6). yi β= (6) xi For the membranes in this study, i means ethanol, and j means water, since ethanol is the preferentially permeating component. Attention must be paid that, although the driving force is based on molar fractions, all parameters used in this research are calculated in mass units.
2.4. Theoretical analysis of the performance parameters 3. Results and discussion Pervaporation, as well as other membrane separation processes, can be characterized by performance parameters like flux and selectivity, which are indicators of the ability of the process for the extraction of a chosen component. In this study, the membrane efficiency and selectivity is defined by the separation factor α, the enrichment factor β and the ideal membrane selectivity S. Total fluxes were derived from measurements using Eq. (1), where m is the mass of the permeate, A is the membrane area and t is the measurement time. m Jtot = (1) At Partial fluxes were determined by Eq. (2), with xi the weight fraction of component i in the membrane. Ji = Jtot xi
(2)
From these partial fluxes, the permeance can be calculated from Eq. (3). Fi Ji (3) = sat l (xi γi pi − yi pp )
3.1. Effects of feed composition on fluxes and permeances of ethanol and water Fig. 2a illustrates the variations of total flux, partial ethanol flux and partial water flux for a binary system at 33 ◦ C, whereas Fig. 2b shows the corresponding ethanol and water permeance versus feed water concentration at 33 ◦ C. Fig. 2a indicates that the partial ethanol flux increases rapidly with feed ethanol concentration. The flux versus feed ethanol concentration relationship can be elaborated by the interaction between polymer molecules in the membrane and permeating molecules. In the SolSep 3360 membrane, the selective layer is made of poly dimethyl siloxane (PDMS), a silicon elastomer with a typical hydrophobic character. Thus, it is non-polar and experiences strong interactions with non-polar compounds. In an ethanol/water mixture, ethanol is the least polar component, and will therefore have more interaction with the PDMS membrane. At higher ethanol concentrations in the feed, more ethanol molecules are in contact with the selective layer of the membrane. Therefore, more ethanol molecules are sorbed in the membrane, causing a greater degree of swelling in the top
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Fig. 2. (a) Total flux (Jtot ), ethanol flux (JEtOH ) and water flux (JH2 O ) vs. feed ethanol composition at 33 ◦ C for SolSep 3360. (b) Ethanol permeance (FEtOH ) and water permeance (FH 2 O ) vs. feed ethanol composition at 33 ◦ C for SolSep 3360.
layer. Consequently, more ethanol molecules are able to pass through the ethanol-swollen membrane and ethanol permeation flux increases as the feed ethanol concentration increases. At the same time however, because of this swelling, water is also able to permeate through the membrane, because of drag effects or possibly as a coupling unit of alcohol and water. This causes the water flux to remain constant, while a decrease would be expected, since the ethanol flux has increased. At low ethanol concentrations, the water flux is even higher than the ethanol flux. This can be explained by the fact that at low ethanol concentrations the difference in polarity of water and ethanol is counterbalanced by the larger amount of water molecules present. Therefore, the water flux will be higher than the ethanol flux. The permeance versus feed ethanol concentration plot (Fig. 2b) is not fully in agreement with the previous analysis for ethanol. When comparing the permeance and flux increment percentages with increasing feed concentration for both water and ethanol, it can be seen that for water these are comparable. However, for ethanol the increment percentage permeance is lower than that of flux. When looking at the formula for permeance (Eq. (3)), the denominator shows that the effect of fugacity difference as the driving force is significantly decoupled from the
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Fig. 3. (a) Ethanol flux (JEtOH ) vs. feed composition at 23, 33 and 44 ◦ C for SolSep 3360. (b). Water flux (JH2 O ) vs. feed composition at 23, 33 and 44 ◦ C for SolSep 3360.
overall membrane transport. The fugacity difference can be written in permeant-specific terms. Therefore, permeance exhibits more accurate permeant-specific transport properties with feed concentration. This explains the difference in increment percentages. 3.2. Effects of temperature on fluxes and permeances of ethanol and water Trends of ethanol and water fluxes versus feed ethanol concentration as a function of temperature are presented in Fig. 3a and b, respectively, for 23, 33 and 44 ◦ C. Both ethanol and water flux increase with temperature. This phenomenon can be explained by a thermally induced increased motion of the polymer chains and hence an expansion of the free volume. In addition, the increased thermal motions of the permeating molecules also promote their diffusion. The combination of these effects brings about the rapid increase in permeation flux. In Fig. 4a and b, respectively, variations as a function of temperature in ethanol and water permeances versus feed ethanol concentration are given for 23, 33 and 44 ◦ C. The effects of temperature on ethanol and water permeances are quite opposite to what is expected and to the permeance versus temperature
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Fig. 4. (a) Ethanol permeance (FEtOH ) vs. feed composition at 23, 33 and 44 ◦ C for SolSep 3360. (b). Water permeance (FH 2 O ) vs. feed composition at 23, 33 and 44 ◦ C for SolSep 3360.
relationship observed in most gas separation membranes, i.e. permeance does not increase with temperature. The highest permeance is found for the system at 23 ◦ C, followed by the system at 33 ◦ C, and then 44 ◦ C (when extrapolating to higher feed ethanol concentrations). The negative dependence of ethanol and water permeances on temperature may arise from the fact that permeance is defined as permeant flux divided by permeant driving force. The driving force combines two temperature dependent factors: γ i and psat i , which are external factors outside the membrane. This is independent from the fact whether they are based on molar or weight fractions. As shown in Table 1, the values of ethanol and water activity coefficients are quite close at different temperatures. Thus, psat i plays a more important role than the activity coefficient. Mathematically, a higher
temperature results in a higher psat i , a larger denominator of Eq. (3) (since yi pp usually is negligible), and a smaller permeance. The predicted order of permeance versus temperature for both ethanol and water will follow the order 23 ◦ C > 33 ◦ C > 44 ◦ C. In addition, the negative and positive temperature dependence of γ i and psat i , respectively, may also counterbalance each other’s effect on permeance. Thermodynamically, the indifference of permeance to temperature at lower feed ethanol concentrations indicates that there is a high degree of counterbalance of the negative temperature effect on sorption and the positive temperature effect on diffusion. An increase in temperature tends to reduce sorption, but increase diffusion. However, the degree of counterbalance is reduced at high ethanol concentrations, possibly because of swelling. In the past, similar approaches have been used. Wijmans and Baker [20] are the first to use permeances, called ‘pressurenormalized fluxes’, for performance analysis to quantify the contribution of the vapour liquid equilibrium. Ten and Field [22] used a similar approach. Although that is more complicated, it can be reduced to the equations used in this research. These analyses demonstrate the importance of taking the combined effects of activity coefficient and saturation vapour pressure into consideration when analysing the temperature effect on membrane performance. The external factors seriously affect the apparent membrane performance of membranes in pervaporation. Without using the permeance as a membrane performance parameter, these effects may be simply overlooked from the flux plots. Permeance may reflect the true separation potential of the membrane because flux comprises the effects of external operational factors, such as temperature and swelling, and the intrinsic membrane properties. 3.3. Separation and enrichment factors versus selectivity of ethanol to water Fig. 5a–c illustrates, respectively, the separation factor, enrichment factor and selectivity of ethanol to water for the SolSep 3360 membrane at different temperatures. Both separation and enrichment factor decrease with an increase in feed ethanol concentration. Increasing feed ethanol content enhances membrane swelling, resulting in an enlarged interstitial space between the polymer chains and declined separation performance. The selectivity (defined as the ratio of the permeances) shows a tendency completely different from that of the separation and enrichment factor: with increasing feed ethanol concentration, the selectivity also increases. This can also be concluded from
Table 1 A comparison of ethanol and water activity coefficients at different temperatures (ethanol concentration = 10, 30 or 50 vol%), calculated with help of the UNIFAC equation [21] Activity coefficient
23 ◦ C 33 ◦ C 44 ◦ C
Ethanol (10%)
Water (90%)
Ethanol (30%)
Water (70%)
Ethanol (50%)
Water (50%)
5.6716 5.6070 5.5385
1.0046 1.0045 1.0044
3.5829 3.5758 3.5660
1.0348 1.0340 1.0333
2.5794 2.5874 2.5938
1.0835 1.0819 1.0804
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Fig. 6. Permeate ethanol concentration ([EtOH]) vs. feed ethanol concentration at 23, 33 and 44 ◦ C for SolSep 3360.
rate permeant-specific transport properties with feed ethanol concentration. Since the SolSep 3360 membrane is a nanofiltration membrane, under non-swollen conditions, the interstitial spaces in and between the polymer chains are larger than in a pervaporation membrane. Upon swelling, the membrane free volume is altered. This alteration causes lower separation and enrichment factors because of drag and coupling effects. However, also a more dense membrane structure [3] is formed. Therefore, the permeant-specific transport properties of a highly swollen nanofiltration membrane will be comparable to those of a dense PDMS pervaporation membrane. This explains why an increased feed ethanol concentration (and thus increased swelling) results in a small increment in membrane selectivity, which is the ratio of selectivities of the components. Fig. 5a and b shows no temperature dependence for both separation and enrichment factor. This is in agreement with the results obtained for the permeate ethanol concentration versus the feed ethanol concentration at different temperatures, as depicted in Fig. 6. With the given definitions for both factors (Eqs. (5) and (6)), this equal tendency comes as no surprise, since the separation factor can be written as the ratio of enrichment factors of both components: α=
Fig. 5. (a) Separation factor (α) vs. feed ethanol concentration at 23, 33 and 44 ◦ C for SolSep 3360. (b) Enrichment factor (β) vs. feed ethanol concentration at 23, 33 and 44 ◦ C for SolSep 3360. (c) Selectivity (S) vs. feed ethanol concentration at 23, 33 and 44 ◦ C for SolSep 3360.
the plot of the permeances versus feed ethanol concentration, as shown in Fig. 2b, since water permeance remains constant and ethanol permeance increases. From the influence of feed composition on permeances, it was concluded that the effect of fugacity difference as the driving force is significantly decoupled from the overall membrane transport, thereby causing permeance to exhibit more accu-
yi /yj βi = xi /xj βj
(7)
For the selectivity no distinct temperature dependence can be seen (Fig. 5c), though this is not as definite as for the separation and enrichment factors. This small difference can be explained by looking at the definitions of selectivity (Eq. (4)) and separation factor (Eq. (5)). With the help of some general assumptions, very common in chemical engineering, a correlation between the two parameters can be derived. If a vacuum is applied at the permeate side of the membrane and all permeate is condensed immediately, the permeant concentration near the membrane surface can be written as the permeate flux. Then the separation factor can be rewritten (Eq. (8)) as a combination of both membrane transport properties (ethanol and water permeances) and component properties (activity coefficients and saturated vapour pressures of both ethanol and water). The selectivity is mainly dominated by
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membrane transport properties (Eq. (9)). α= =
JEtOH x H2 O × xEtOH JH 2 O p γEtOH psat x H2 O [FEtOH / l] (xEtOH EtOH − yEtOH p ) × × xEtOH [FH2 O / l] (xH γ psat − yH pp ) 2 O H2 O H 2 O 2O
=S×
S=
factor and selectivity. However, neither separation and enrichment factor, nor the selectivity, show significant temperature dependence.
Mw,H2 O γEtOH psat EtOH Mw,EtOH γH2 O psat H2 O
[FEtOH / l] [FH2 O / l]
3.4. Common multicomponent mixtures
(8)
(9)
Since the downstream pressure is normally very low compared to the upstream pressure, it may be neglected, and the separation factor can be simplified to the selectivity multiplied by the ratio of the activity coefficient and saturated vapour pressure divided by the molecular mass of ethanol to water. With the temperature variation, the ratio of the activity coefficient and saturated vapour pressure of ethanol will change accordingly and counterbalance temperature dependence of the permeances. Therefore, there is a small difference in temperature dependence of the separation
With the SolSep 3360 membrane it was also tested whether common multicomponent systems behave in a similar way to binary ethanol/water mixtures. Fig. 7a–d gives the results for both the partial fluxes (Fig. 7a and b) and permeances (Fig. 7c and d) for ethanol and water. Fig. 8a–c shows the results for the separation factor, enrichment factor and selectivity, respectively. In the past, Lipnizki showed that the presence of impermeable components in the feed can change the selectivity of the membrane [18]. As can be seen in this research, the tested alcoholic beverages behave exactly in the same way as the binary ethanol/water mixtures. Apparently, the unknown extra components in the mixtures have no influence on the pervaporation properties or counterbalance each others effects, ensuring these mixtures behave like regular ethanol/water mixtures.
Fig. 7. (a) Ethanol flux vs. feed ethanol concentration of ethanol/mixtures and common multicomponent mixtures at different temperatures for SolSep 3360. (b) Water flux vs. feed ethanol concentration of ethanol/mixtures and common multicomponent mixtures at different temperatures for SolSep 3360. (c) Ethanol permeance vs. feed ethanol concentration of ethanol/mixtures and common multicomponent mixtures at different temperatures for SolSep 3360. (d) Water permeance vs. feed ethanol concentration of ethanol/mixtures and common multicomponent mixtures at different temperatures for SolSep 3360.
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Fig. 8. (a) Separation factor vs. feed ethanol concentration of ethanol/mixtures and common multicomponent mixtures at different temperatures for SolSep 3360. (b) Enrichment factor vs. feed ethanol concentration of ethanol/mixtures and real multicomponent mixtures at different temperatures for SolSep 3360. (c) Selectivity vs. feed ethanol concentration of ethanol/mixtures and real multicomponent mixtures at different temperatures for SolSep 3360.
3.5. Comparison with pervaporation membranes In the past, different studies on nanofiltration membranes [23–25] showed that whether convection or diffusion is the most important transport mechanism, the solute transport rate is influ-
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enced by the effective pore size (in case of convection) or the available free volume (in case of diffusion) and the effective solute size. When looking at molecule size, it can be noticed that both water and ethanol are relatively small molecules. Therefore the effect of sterical hindrance will have a minor influence. By using a swollen nanofiltration membrane for pervaporation purposes, the pore size of the membrane is altered. The effect of this change on the pervaporation performance can be checked when comparing the results of the PDMS nanofiltration membrane with results of PDMS pervaporation membranes. Therefore, the results of the SolSep 3360 nanofiltration membrane are compared to the results of two PDMS pervaporation membranes, namely PV 1070 and Pervatech PDMS. For these membranes, pervaporation experiments were performed with binary mixtures of varying ethanol concentrations and at different temperatures. For the fluxes, similar tendencies are found for all three membranes, although the order of magnitude differs. At higher temperatures, the fluxes increase and at higher feed ethanol concentrations, the partial ethanol fluxes increase, whereas the water fluxes remain constant. One difference can be found for the pervaporation membranes, where the break-even point for dominant partial flux (at lower feed ethanol concentrations the water flux dominates) lies at higher feed ethanol concentrations than for the SolSep 3360 nanofiltration membrane. This indicates stronger interactions between ethanol molecules and membrane for the nanofiltration membrane than for the pervaporation membranes. Another difference is found in the order of magnitude of the fluxes. For the SolSep 3360 membrane, fluxes in the range of 0–3.5 kg m−2 h−1 were found, whereas the fluxes of the PV 1070 membrane staid below 0.3 kg m−2 h−1 . This can be explained by the fact that SolSep 3360 is a nanofiltration membrane with larger free volume, but also because it has a more recent and advanced chemical structure. The SolSep 3360 membranes fluxes are nearly twice as large as the fluxes through the Pervatech membrane at low temperatures. However, at higher temperatures this difference has almost completely vanished. Apparently, at large temperatures the thermally induced movement of polymer chains in the pervaporation membrane and the better diffusion of the permeant counterbalance the advantages of the larger pore size of the nanofiltration membrane. For the permeances, similar differences in order of magnitude were found. The permeances of the SolSep 3360 membrane are several factors larger than of the PV 1070 membrane, for the same reasons as explained above. For the Pervatech membrane the order of magnitude of the permeances is smaller than for the SolSep 3360 membrane at lower temperatures, just as for the fluxes. At higher temperatures, the permeances approach the same order of magnitude. Although the permeances show the same increasing tendency with feed ethanol concentration, a striking difference when comparing the permeances of the Pervatech and the SolSep 3360 membrane, is the fact that for the SolSep 3360 membrane the ethanol permeance dominates, while for the Pervatech membrane an opposite behaviour is observed. Since permeance shows more accurate permeant-specific transport properties, it appears
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water has more interaction with the Pervatech membrane than with the SolSep 3360 membrane. The obtained higher water fluxes than ethanol fluxes at low feed ethanol concentration also manifest this. When looking at the temperature influence, it was found the ethanol permeance of the PV 1070 membrane hardly changes, whereas it was expected it would decrease. Possibly, the swelling has no effect here on the degree of counterbalance between the negative temperature effect on sorption and the positive temperature effect on diffusion. For the Pervatech membrane, with increasing feed ethanol concentration a decreasing separation factor is found, while the selectivity shows an increasing tendency, just as for the SolSep 3360 membrane. The selectivity of the PV 1070 membrane shows the same trend. For this membrane however, constant separation and enrichment factors are reported at low temperatures. Apparently, the swelling of the membrane has no influence on the separation and enrichment factor of a pervaporation membrane under these conditions. At higher temperatures, the separation and enrichment factors decrease, as do they in the SolSep 3360 membrane. Where there is little or no temperature dependence for the SolSep 3360 membrane for separation and enrichment factors and selectivity, the PV 1070 membrane shows larger values for all three parameters at higher temperatures. This is probably because the ethanol permeance for this membrane is higher than expected, resulting in a larger ratio in Eqs. (8) and (9).
4. Conclusion This research confirms findings of earlier studies, that plots of flux and permeance versus feed ethanol content may give different results and conclusions on effects of processing conditions on separation performance. Using permeance and selectivity, instead of flux and separation and enrichment factor, can significantly decouple the effect of operating conditions on performance evaluation, while clarifying and quantifying the contribution by the nature of the membrane to the separation performance. Aqueous activity coefficient and saturated vapour pressure appear to play an important role when using permeance and selectivity in the performance analysis. Furthermore, it was found that the membrane performance in terms of permeance and selectivity, as well as in terms of flux, separation and enrichment factors, is similar when alcoholic beverages are used as feed instead of synthetic solutions. Apparently, the extra components in these mixtures have no influence on the pervaporation properties, or their influences are counterbalanced. Compared to two PDMS-based pervaporation membranes, both fluxes and permeances are larger in the nanofiltration membrane, while the separation factor and selectivity are comparable or even better. The difference between the nanofiltration and pervaporation membranes can be explained by the influence of swelling and the different interactions between permeating molecules and the membrane. This makes a nanofiltration membrane very useful for pervaporation.
Acknowledgments The Research Council of the K.U. Leuven is gratefully acknowledged for financial support (OT/2002/33 and OT/2006/37). SolSep and Pervatech are thanked for kindly supplying membrane samples. The Socrates program is also gratefully acknowledged for the grant given to Bram Leen for performing the experimental work at ITM-CNR, ITALY. References [1] P.A. Kober, Pervaporation, perstillation and percrystallization, J. Am. Chem. Soc. 39 (5) (1917) 944–948. [2] B. Van der Bruggen, J.C. Jansen, A. Figoli, J. Geens, K. Boussu, E. Drioli, Characteristics and performance of a ‘universal’ membrane suitable for gas separation, pervaporation and nanofiltration applications, J. Phys. Chem. B 110 (28) (2006) 13799–13803. [3] B. Van der Bruggen, J.C. Jansen, A. Figoli, J. Geens, D. Van Baelen, E. Drioli, C. Vandecasteele, Determination of parameters affecting transport in polymeric membranes: parallels between pervaporation and nanofiltration, J. Phys. Chem. B 108 (35) (2004) 13273–13279. [4] J.P.G. Villaluenga, M. Khayet, P. Godino, B. Seoane, J.I. Mengual, Analysis of the membrane thickness effect on the pervaporation separation of methanol/methyl tertiary butyl ether mixtures, Sep. Purif. Technol. 47 (1–2) (2005) 80–87. [5] X.-P. Wang, Y.-F. Feng, Z.-Q. Shen, Pervaporation properties of a threelayer structure composite membrane, J. Appl. Polym. Sci. 75 (6) (2000) 740–745. [6] J. Caro, M. Noack, P. K¨olsch, Chemically modified ceramic membranes, Microporous Mesoporous Mater. 22 (1–3) (1998) 321–332. [7] S.S. Madaeni, The effect of surface characteristics on RO membrane performance, Desalination 139 (1–3) (2001) 371. [8] J. Meier-Haack, W. Lenk, S. Berwald, T. Rieser, K. Lunkwitz, Influence of thermal treatment on the pervaporation separation properties of polyamide6 membranes, Sep. Purif. Technol. 19 (3) (2000) 199–207. [9] S. Ray, S.K. Ray, Effect of copolymer type and composition on separation characteristics of pervaporation membranes—A case study with separation of acetone–water mixtures, J. Membr. Sci. 270 (1–2) (2006) 73–87. [10] S.V. Satyanarayana, A. Sharma, P.K. Bhattacharya, Composite membranes for hydrophobic pervaporation: study with the toluene–water system, Chem. Eng. J. 102 (2) (2004) 171–184. [11] S.V. Satyanarayana, P.K. Bhattacharya, Pervaporation of hydrazine hydrate: separation characteristics of membranes with hydrophilic to hydrophobic behaviour, J. Membr. Sci. 238 (1–2) (2004) 103–115. [12] S.I. Semenova, H. Ohya, K. Soontarapa, Hydrophilic membranes for pervaporation, an analytical review, Desalination 110 (3) (1997) 251–286. [13] J. Meier-Haack, T. Rieser, W. Lenk, D. Lehmann, S. Berwald, S. Schwarz, Effect of polyelectrolyte complex layers on the separation properties and the fouling behavior of surface and bulk modified membranes, Chem. Eng. Technol. 23 (2) (2000) 114–118. [14] J. Liu, W.K. Teo, C.H. Chew, L.M. Gan, Nanofiltration membranes prepared by direct microemulsion copolymerization using poly(ethylene oxide) macromonomer as a polymerizable surfactant, J. Appl. Polym. Sci. 77 (12) (2000) 2785–2794. [15] J. Sekuli´c, J.E. ten Elshof, D.H.A. Blank, A microporous titania membrane for nanofiltration and pervaporation, Adv. Mater. 16 (17) (2004) 1546–1550. [16] B. Barri`ere, L. Leibler, Permeation of a solvent mixture through an elastomeric membrane—The case of pervaporation, J. Polym. Sci. Part B 41 (2) (2003) 183–193. [17] R. Ranieri, A. Figoli, A. Criscuoli, E. Drioli, La pervaporazione e i membrane contactors come sistemi alternative per la determinazione dell’alcool etilico in soluzioni idroalcoliche, Master Thesis, ITM-CNR, Rende (CS), Italy, 2005.
A. Verhoef et al. / Separation and Purification Technology 60 (2008) 54–63 [18] F. Lipnizki, S. Hausmanns, R.W. Field, Influence of impermeable components on the permeation of aqueous 1-propanol mixtures in hydrophobic pervaporation, J. Membr. Sci. 228 (2) (2004) 129–138. [19] P. Cot´e, C. Lipski, Mass transfer limitations in pervaporation for water and wastewater treatment, in: Proceedings of the 3rd Conference on Pervaporation Processes in Chemical Industry, 1988, pp. 449–462. [20] J.G. Wijmans, R.W. Baker, A simple predictive treatment of the permeation process in pervaporation, J. Membr. Sci. 79 (1) (1993) 101– 113. [21] VLECalc v1.1. http://www.softlookup.com/display.asp?id=3122 (accessed August 2006). [22] P.K. Ten, R.W. Field, Organophilic pervaporation: an engineering science analysis of component transport and the classification of behaviour with
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reference to the effect of permeate pressure, Chem. Eng. Sci. 55 (8) (2000) 1425–1445. [23] L. Braeken, R. Ramaekers, Y. Zhang, G. Maes, B. Van der Bruggen, C. Vandecasteele, Influence of hydrophobicity on retention in nanofiltration of aqueous solutions containing organic compounds, J. Membr. Sci. 252 (1–2) (2005) 195–203. [24] J. Geens, A. Hillen, B. Bettens, B. Van der Bruggen, C. Vandecasteele, Solute transport in non-aqueous nanofiltration: effect of membrane material, J. Chem. Technol. Biotechnol. 80 (12) (2005) 1371–1377. [25] J. Geens, K. Peeters, B. Van der Bruggen, C. Vandecasteele, Polymeric nanofiltration of binary water–alcohol mixtures: influence of feed composition and membrane properties on permeability and rejection, J. Membr. Sci. 255 (1–2) (2005) 255–264.
Separation and Purification Technology 60 (2008) 54–63
Performance of a nanofiltration membrane for removal of ethanol from aqueous solutions by pervaporation Adrian Verhoef a,∗ , Alberto Figoli b , Bram Leen a , Ben Bettens a , Enrico Drioli b , Bart Van der Bruggen a a
Department of Chemical Engineering, Laboratory for Applied Physical Chemistry and Environmental Technology, Katholieke Universiteit Leuven, W. de Croylaan 46, B-3001 Leuven, Belgium b Research Institute on Membrane Technology (ITM-CNR), c/o University of Calabria, via P. Bucci, Cubo 17/C, 87030 Rende, Cosenza, Italy Received 30 December 2006; received in revised form 10 July 2007; accepted 26 July 2007
Abstract In this study, the performance of a hydrophobic nanofiltration membrane (SolSep 3360) for treating alcoholic solutions by pervaporation is investigated and compared to a conventional pervaporation membrane (PV 1070, Sulzer Chemtech and Pervatech PDMS). Both binary ethanol/water mixtures and common multicomponent mixtures (alcoholic beverages) are examined. The experiments were performed at feed ethanol concentrations up to 50 vol% and at temperatures up to 45 ◦ C. The effects of feed ethanol content and temperature were studied in terms of: (1) fluxes and permeances of individual components, and (2) separation factor, enrichment factor and selectivity of ethanol to water. Using permeance and selectivity instead of flux and separation/enrichment factor allows the effects on performance evaluation of operating conditions, such as temperature and swelling, to be decoupled. In this way the contribution by nature of the membrane to separation performance can be clarified and quantified. In addition, previous analyses indicate that the aqueous activity coefficient and the saturated vapour pressure play an important role when evaluating the membrane performance in terms of permeance and selectivity. This is confirmed by this study. Furthermore, it was found that multicomponent alcoholic beverages behave in exact the same manner as binary ethanol/water mixtures. Using a nanofiltration membrane for pervaporation purposes is a suitable possibility, because of the higher fluxes and permeances, while remaining a good separation factor and selectivity. The difference between nanofiltration and pervaporation membranes is explained by the influence of swelling, making the membrane more dense, and the different interactions between permeating molecules and the membrane. © 2007 Elsevier B.V. All rights reserved. Keywords: Pervaporation; Nanofiltration membrane; Alcoholic solutions; PDMS membranes
1. Introduction In the beginning of the 20th century, Kober [1] first observed that a membrane could efficiently separate two liquid chemicals mixed together, by applying a vacuum on the other side of the membrane, resulting in a gradient of chemical potential. In reaction to this gradient, the components of the mixture penetrate into the membrane and evaporate on the other side. Kober named this phenomenon pervaporation. Separation is ensured by differ-
∗
Corresponding author. Tel.: +32 16 32 23 64; fax: +32 16 32 29 91. E-mail addresses: [email protected] (A. Verhoef), [email protected] (A. Figoli). 1383-5866/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2007.07.044
ences in solvents sorption affinity and diffusion coefficients in the membrane. Industrially, pervaporation is particularly useful to separate mixtures hard to separate by conventional techniques, such as distillation or extraction. Examples are azeotropic mixtures, such as alcohol/water, or chemical products with close boiling points, such as acetic acid/water. For every membrane process, a good membrane must be found, in order to obtain optimal separation. Many studies prove the membrane has significant influence. The selectivity and performance of the membrane is not only determined by obvious features such as thickness, porous or dense nature of the top layer [2–5], pore size and geometry, and porosity [2,3,6,7], but also by less obvious material properties such as glass
A. Verhoef et al. / Separation and Purification Technology 60 (2008) 54–63
Nomenclature A F J l Mw m p S t x x y y
membrane area (m2 ) membrane permeability (kg m−1 h−1 bar−1 ) flux (kg m−2 h−1 ) membrane thickness (m) molar mass (kg/mol) permeate mass (kg) partial pressure (bar) ideal membrane selectivity measurement time (s) feed weight fraction feed molar fraction permeate weight fraction permeate molar fraction
Greek letters α separation factor β enrichment factor γ activity coefficient Subscripts i component i j component j tot total parameter Superscripts p permeate sat saturated vapour phase
transition temperature [2,8], composition [2,9,10], hydrophobicity/hydrophilicity [2,3,6,11,12] and membrane surface charge [2,13]. For each membrane process, optimal membrane properties are determined separately in terms of structure and material. For pervaporation, rather thick membranes with a dense nature are used, in view of chemical stability. In contrast, nanofiltration membranes can be dense or nanoporous and are as thin as possible, because the flux through the membrane is inversely proportional to its thickness. In literature, specific membranes are reported to be suitable for both nanofiltration and pervaporation [14,15]. Both polymeric and inorganic membrane materials were used for this purpose. Polymers are a commonly used material for membranes. However, upon wetting, they swell, altering the structure of the membrane [12]. Swelling occurs because a solvent enters and passes through the membrane, due to a chemical potential gradient. This increases permeability, but decreases selectivity, since another component in the feed mixture can benefit from the now available free volume inside the membrane, and permeate as well [16]. This property can be used as an advantage. The swelling phenomenon can make the structure of a polymeric micro- or nanoporous nanofiltration membrane more dense [2]. In this study, the effect of swelling is examined to see whether a hydrophobic nanofiltration membrane can be used for separating alcohol/water mixtures by pervaporation. In order to investigate
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the behaviour of the membrane with common multicomponent solutions, alcoholic beverages were used for pervaporation experiments. Since polymeric materials have a lower cost, the use of nanofiltration membranes in pervaporation can be expected to broaden the application range of membrane processes in industry. 2. Materials and methods 2.1. Membranes All membranes in this study have a PDMS (poly dimethyl siloxane)-based top layer. This is a silicon elastomer with a typical hydrophobic character. Three different membranes were used: SolSep 3360 is a hydrophobic nanofiltration membrane manufactured by SolSep BV (Apeldoorn, the Netherlands); Pervatech PDMS is a dense hydrophobic pervaporation membrane manufactured by Pervatech BV (Enter, the Netherlands); and PV 1070 is a zeolite-filled, dense pervaporation membrane manufactured by Sulzer Chemtech (Neunkirchen, Germany). Experiments were performed on the SolSep 3360 nanofiltration membrane. These results were compared to those of the two other traditional pervaporation membranes. The results for the Pervatech membrane were obtained by Ranieri et al. [17] on the same set-up. 2.2. Chemicals Measurements were performed with ethanol/water mixtures. The ethanol concentration varies up to 50 vol%. Ethanol was of analytical purity (>99.8%) and was obtained from Carlo Erba. In this study, it was also tested if common multicomponent mixtures show identical pervaporation behaviour as ethanol/water mixtures. To investigate this, alcoholic beverages were measured. Lager beer (Beck’s, 5 vol% alcohol), white wine (Marino, Le Contrade 2004, 11.5 vol% alcohol) and gin (Argia Gin, extra dry, 38 vol% alcohol) were used as common multicomponent mixtures containing ethanol. These beverages were chosen since they do not contain too many impermeable components, to prevent fouling [18], and are not too viscous, in order to avoid fluid mechanical phenomena such as concentration polarisation. Earlier research [19,20] showed that having a non-viscous feed not necessarily avoids concentration polarisation, but slow sorption of permeants in the membrane ensures that permeation fluxes are independent of feed membrane concentration. The relatively low separation factor in this work indicates this is the case. 2.3. Pervaporation experiments In the pervaporation system, schematically shown in Fig. 1, the feed temperature is controlled by a thermostat (T). A recirculation pump circulates the feed through the set-up at a flow rate of 0.7 l/min. The pervaporation module contains a circular flat sheet membrane with a membrane area of 56.74 cm2 .
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A. Verhoef et al. / Separation and Purification Technology 60 (2008) 54–63
Fig. 1. Schematic diagram of the pervaporation set-up.
At the permeate side, a vacuum is maintained below 5 mbar by a vacuum pump, and the permeate pressure is measured with a manometer (P). Two condensers are used, cooled with liquid nitrogen. The permeate is collected in the first condenser, and the second protects the vacuum pump. Several experiments are performed at different concentrations up to 50 vol% and temperatures below 50 ◦ C, to avoid problems with the membrane module, such as blocking of the cold trap. For each experiment, at least three measurements of about 1 h are performed and averaged to calculate fluxes. After the completion of each experiment, the permeate collected inside the cold trap was warmed up to room temperature. The permeate was then weighed to determine the total flux. For each measurement, samples are taken from feed and permeate. The concentration of ethanol in these samples is determined by means of gas chromatography. The apparatus used is an Agrilent Technologies 6890N. The capillary column consists of a stainless steel Porapack Q 80/20s mesh coating. The furnace temperature is kept constant at 120 ◦ C, N2 is used as carrier gas and the pressure is held constant at 1.29 bar. The detector used is a TCD detector.
Here, [Fi /l] is the membrane permeability Fi divided by the membrane thickness l, also called the permeance; x i the mole fraction in the feed; γ i the activity coefficient; psat i the saturated vapour pressure; y i the permeate mole fraction and pp is the permeate pressure. The activity coefficients were calculated using the UNIFAC equation; the saturated vapour pressures using the Clausius–Clapeyron equation [21]. These two equations give a driving force based on molar fractions, but are used since no correct equations based on weight fractions are available to calculate these parameters. Probably this difference will not lead to large errors in the calculations. The ideal membrane selectivity Si/j is now defined as the ratio of the permeances: Si/j =
[Fi / l] [Fj / l]
(4)
The separation factor α is determined by Eq. (5). Here, xi is the feed concentration of the preferentially permeating component (ethanol in this study), and yi the permeate concentration of this component. α=
yi /(1 − yi ) xi /(1 − xi )
(5)
The enrichment factor β is calculated by dividing the permeate concentration by the feed concentration, as illustrated in Eq. (6). yi β= (6) xi For the membranes in this study, i means ethanol, and j means water, since ethanol is the preferentially permeating component. Attention must be paid that, although the driving force is based on molar fractions, all parameters used in this research are calculated in mass units.
2.4. Theoretical analysis of the performance parameters 3. Results and discussion Pervaporation, as well as other membrane separation processes, can be characterized by performance parameters like flux and selectivity, which are indicators of the ability of the process for the extraction of a chosen component. In this study, the membrane efficiency and selectivity is defined by the separation factor α, the enrichment factor β and the ideal membrane selectivity S. Total fluxes were derived from measurements using Eq. (1), where m is the mass of the permeate, A is the membrane area and t is the measurement time. m Jtot = (1) At Partial fluxes were determined by Eq. (2), with xi the weight fraction of component i in the membrane. Ji = Jtot xi
(2)
From these partial fluxes, the permeance can be calculated from Eq. (3). Fi Ji (3) = sat l (xi γi pi − yi pp )
3.1. Effects of feed composition on fluxes and permeances of ethanol and water Fig. 2a illustrates the variations of total flux, partial ethanol flux and partial water flux for a binary system at 33 ◦ C, whereas Fig. 2b shows the corresponding ethanol and water permeance versus feed water concentration at 33 ◦ C. Fig. 2a indicates that the partial ethanol flux increases rapidly with feed ethanol concentration. The flux versus feed ethanol concentration relationship can be elaborated by the interaction between polymer molecules in the membrane and permeating molecules. In the SolSep 3360 membrane, the selective layer is made of poly dimethyl siloxane (PDMS), a silicon elastomer with a typical hydrophobic character. Thus, it is non-polar and experiences strong interactions with non-polar compounds. In an ethanol/water mixture, ethanol is the least polar component, and will therefore have more interaction with the PDMS membrane. At higher ethanol concentrations in the feed, more ethanol molecules are in contact with the selective layer of the membrane. Therefore, more ethanol molecules are sorbed in the membrane, causing a greater degree of swelling in the top
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Fig. 2. (a) Total flux (Jtot ), ethanol flux (JEtOH ) and water flux (JH2 O ) vs. feed ethanol composition at 33 ◦ C for SolSep 3360. (b) Ethanol permeance (FEtOH ) and water permeance (FH 2 O ) vs. feed ethanol composition at 33 ◦ C for SolSep 3360.
layer. Consequently, more ethanol molecules are able to pass through the ethanol-swollen membrane and ethanol permeation flux increases as the feed ethanol concentration increases. At the same time however, because of this swelling, water is also able to permeate through the membrane, because of drag effects or possibly as a coupling unit of alcohol and water. This causes the water flux to remain constant, while a decrease would be expected, since the ethanol flux has increased. At low ethanol concentrations, the water flux is even higher than the ethanol flux. This can be explained by the fact that at low ethanol concentrations the difference in polarity of water and ethanol is counterbalanced by the larger amount of water molecules present. Therefore, the water flux will be higher than the ethanol flux. The permeance versus feed ethanol concentration plot (Fig. 2b) is not fully in agreement with the previous analysis for ethanol. When comparing the permeance and flux increment percentages with increasing feed concentration for both water and ethanol, it can be seen that for water these are comparable. However, for ethanol the increment percentage permeance is lower than that of flux. When looking at the formula for permeance (Eq. (3)), the denominator shows that the effect of fugacity difference as the driving force is significantly decoupled from the
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Fig. 3. (a) Ethanol flux (JEtOH ) vs. feed composition at 23, 33 and 44 ◦ C for SolSep 3360. (b). Water flux (JH2 O ) vs. feed composition at 23, 33 and 44 ◦ C for SolSep 3360.
overall membrane transport. The fugacity difference can be written in permeant-specific terms. Therefore, permeance exhibits more accurate permeant-specific transport properties with feed concentration. This explains the difference in increment percentages. 3.2. Effects of temperature on fluxes and permeances of ethanol and water Trends of ethanol and water fluxes versus feed ethanol concentration as a function of temperature are presented in Fig. 3a and b, respectively, for 23, 33 and 44 ◦ C. Both ethanol and water flux increase with temperature. This phenomenon can be explained by a thermally induced increased motion of the polymer chains and hence an expansion of the free volume. In addition, the increased thermal motions of the permeating molecules also promote their diffusion. The combination of these effects brings about the rapid increase in permeation flux. In Fig. 4a and b, respectively, variations as a function of temperature in ethanol and water permeances versus feed ethanol concentration are given for 23, 33 and 44 ◦ C. The effects of temperature on ethanol and water permeances are quite opposite to what is expected and to the permeance versus temperature
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Fig. 4. (a) Ethanol permeance (FEtOH ) vs. feed composition at 23, 33 and 44 ◦ C for SolSep 3360. (b). Water permeance (FH 2 O ) vs. feed composition at 23, 33 and 44 ◦ C for SolSep 3360.
relationship observed in most gas separation membranes, i.e. permeance does not increase with temperature. The highest permeance is found for the system at 23 ◦ C, followed by the system at 33 ◦ C, and then 44 ◦ C (when extrapolating to higher feed ethanol concentrations). The negative dependence of ethanol and water permeances on temperature may arise from the fact that permeance is defined as permeant flux divided by permeant driving force. The driving force combines two temperature dependent factors: γ i and psat i , which are external factors outside the membrane. This is independent from the fact whether they are based on molar or weight fractions. As shown in Table 1, the values of ethanol and water activity coefficients are quite close at different temperatures. Thus, psat i plays a more important role than the activity coefficient. Mathematically, a higher
temperature results in a higher psat i , a larger denominator of Eq. (3) (since yi pp usually is negligible), and a smaller permeance. The predicted order of permeance versus temperature for both ethanol and water will follow the order 23 ◦ C > 33 ◦ C > 44 ◦ C. In addition, the negative and positive temperature dependence of γ i and psat i , respectively, may also counterbalance each other’s effect on permeance. Thermodynamically, the indifference of permeance to temperature at lower feed ethanol concentrations indicates that there is a high degree of counterbalance of the negative temperature effect on sorption and the positive temperature effect on diffusion. An increase in temperature tends to reduce sorption, but increase diffusion. However, the degree of counterbalance is reduced at high ethanol concentrations, possibly because of swelling. In the past, similar approaches have been used. Wijmans and Baker [20] are the first to use permeances, called ‘pressurenormalized fluxes’, for performance analysis to quantify the contribution of the vapour liquid equilibrium. Ten and Field [22] used a similar approach. Although that is more complicated, it can be reduced to the equations used in this research. These analyses demonstrate the importance of taking the combined effects of activity coefficient and saturation vapour pressure into consideration when analysing the temperature effect on membrane performance. The external factors seriously affect the apparent membrane performance of membranes in pervaporation. Without using the permeance as a membrane performance parameter, these effects may be simply overlooked from the flux plots. Permeance may reflect the true separation potential of the membrane because flux comprises the effects of external operational factors, such as temperature and swelling, and the intrinsic membrane properties. 3.3. Separation and enrichment factors versus selectivity of ethanol to water Fig. 5a–c illustrates, respectively, the separation factor, enrichment factor and selectivity of ethanol to water for the SolSep 3360 membrane at different temperatures. Both separation and enrichment factor decrease with an increase in feed ethanol concentration. Increasing feed ethanol content enhances membrane swelling, resulting in an enlarged interstitial space between the polymer chains and declined separation performance. The selectivity (defined as the ratio of the permeances) shows a tendency completely different from that of the separation and enrichment factor: with increasing feed ethanol concentration, the selectivity also increases. This can also be concluded from
Table 1 A comparison of ethanol and water activity coefficients at different temperatures (ethanol concentration = 10, 30 or 50 vol%), calculated with help of the UNIFAC equation [21] Activity coefficient
23 ◦ C 33 ◦ C 44 ◦ C
Ethanol (10%)
Water (90%)
Ethanol (30%)
Water (70%)
Ethanol (50%)
Water (50%)
5.6716 5.6070 5.5385
1.0046 1.0045 1.0044
3.5829 3.5758 3.5660
1.0348 1.0340 1.0333
2.5794 2.5874 2.5938
1.0835 1.0819 1.0804
A. Verhoef et al. / Separation and Purification Technology 60 (2008) 54–63
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Fig. 6. Permeate ethanol concentration ([EtOH]) vs. feed ethanol concentration at 23, 33 and 44 ◦ C for SolSep 3360.
rate permeant-specific transport properties with feed ethanol concentration. Since the SolSep 3360 membrane is a nanofiltration membrane, under non-swollen conditions, the interstitial spaces in and between the polymer chains are larger than in a pervaporation membrane. Upon swelling, the membrane free volume is altered. This alteration causes lower separation and enrichment factors because of drag and coupling effects. However, also a more dense membrane structure [3] is formed. Therefore, the permeant-specific transport properties of a highly swollen nanofiltration membrane will be comparable to those of a dense PDMS pervaporation membrane. This explains why an increased feed ethanol concentration (and thus increased swelling) results in a small increment in membrane selectivity, which is the ratio of selectivities of the components. Fig. 5a and b shows no temperature dependence for both separation and enrichment factor. This is in agreement with the results obtained for the permeate ethanol concentration versus the feed ethanol concentration at different temperatures, as depicted in Fig. 6. With the given definitions for both factors (Eqs. (5) and (6)), this equal tendency comes as no surprise, since the separation factor can be written as the ratio of enrichment factors of both components: α=
Fig. 5. (a) Separation factor (α) vs. feed ethanol concentration at 23, 33 and 44 ◦ C for SolSep 3360. (b) Enrichment factor (β) vs. feed ethanol concentration at 23, 33 and 44 ◦ C for SolSep 3360. (c) Selectivity (S) vs. feed ethanol concentration at 23, 33 and 44 ◦ C for SolSep 3360.
the plot of the permeances versus feed ethanol concentration, as shown in Fig. 2b, since water permeance remains constant and ethanol permeance increases. From the influence of feed composition on permeances, it was concluded that the effect of fugacity difference as the driving force is significantly decoupled from the overall membrane transport, thereby causing permeance to exhibit more accu-
yi /yj βi = xi /xj βj
(7)
For the selectivity no distinct temperature dependence can be seen (Fig. 5c), though this is not as definite as for the separation and enrichment factors. This small difference can be explained by looking at the definitions of selectivity (Eq. (4)) and separation factor (Eq. (5)). With the help of some general assumptions, very common in chemical engineering, a correlation between the two parameters can be derived. If a vacuum is applied at the permeate side of the membrane and all permeate is condensed immediately, the permeant concentration near the membrane surface can be written as the permeate flux. Then the separation factor can be rewritten (Eq. (8)) as a combination of both membrane transport properties (ethanol and water permeances) and component properties (activity coefficients and saturated vapour pressures of both ethanol and water). The selectivity is mainly dominated by
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membrane transport properties (Eq. (9)). α= =
JEtOH x H2 O × xEtOH JH 2 O p γEtOH psat x H2 O [FEtOH / l] (xEtOH EtOH − yEtOH p ) × × xEtOH [FH2 O / l] (xH γ psat − yH pp ) 2 O H2 O H 2 O 2O
=S×
S=
factor and selectivity. However, neither separation and enrichment factor, nor the selectivity, show significant temperature dependence.
Mw,H2 O γEtOH psat EtOH Mw,EtOH γH2 O psat H2 O
[FEtOH / l] [FH2 O / l]
3.4. Common multicomponent mixtures
(8)
(9)
Since the downstream pressure is normally very low compared to the upstream pressure, it may be neglected, and the separation factor can be simplified to the selectivity multiplied by the ratio of the activity coefficient and saturated vapour pressure divided by the molecular mass of ethanol to water. With the temperature variation, the ratio of the activity coefficient and saturated vapour pressure of ethanol will change accordingly and counterbalance temperature dependence of the permeances. Therefore, there is a small difference in temperature dependence of the separation
With the SolSep 3360 membrane it was also tested whether common multicomponent systems behave in a similar way to binary ethanol/water mixtures. Fig. 7a–d gives the results for both the partial fluxes (Fig. 7a and b) and permeances (Fig. 7c and d) for ethanol and water. Fig. 8a–c shows the results for the separation factor, enrichment factor and selectivity, respectively. In the past, Lipnizki showed that the presence of impermeable components in the feed can change the selectivity of the membrane [18]. As can be seen in this research, the tested alcoholic beverages behave exactly in the same way as the binary ethanol/water mixtures. Apparently, the unknown extra components in the mixtures have no influence on the pervaporation properties or counterbalance each others effects, ensuring these mixtures behave like regular ethanol/water mixtures.
Fig. 7. (a) Ethanol flux vs. feed ethanol concentration of ethanol/mixtures and common multicomponent mixtures at different temperatures for SolSep 3360. (b) Water flux vs. feed ethanol concentration of ethanol/mixtures and common multicomponent mixtures at different temperatures for SolSep 3360. (c) Ethanol permeance vs. feed ethanol concentration of ethanol/mixtures and common multicomponent mixtures at different temperatures for SolSep 3360. (d) Water permeance vs. feed ethanol concentration of ethanol/mixtures and common multicomponent mixtures at different temperatures for SolSep 3360.
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Fig. 8. (a) Separation factor vs. feed ethanol concentration of ethanol/mixtures and common multicomponent mixtures at different temperatures for SolSep 3360. (b) Enrichment factor vs. feed ethanol concentration of ethanol/mixtures and real multicomponent mixtures at different temperatures for SolSep 3360. (c) Selectivity vs. feed ethanol concentration of ethanol/mixtures and real multicomponent mixtures at different temperatures for SolSep 3360.
3.5. Comparison with pervaporation membranes In the past, different studies on nanofiltration membranes [23–25] showed that whether convection or diffusion is the most important transport mechanism, the solute transport rate is influ-
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enced by the effective pore size (in case of convection) or the available free volume (in case of diffusion) and the effective solute size. When looking at molecule size, it can be noticed that both water and ethanol are relatively small molecules. Therefore the effect of sterical hindrance will have a minor influence. By using a swollen nanofiltration membrane for pervaporation purposes, the pore size of the membrane is altered. The effect of this change on the pervaporation performance can be checked when comparing the results of the PDMS nanofiltration membrane with results of PDMS pervaporation membranes. Therefore, the results of the SolSep 3360 nanofiltration membrane are compared to the results of two PDMS pervaporation membranes, namely PV 1070 and Pervatech PDMS. For these membranes, pervaporation experiments were performed with binary mixtures of varying ethanol concentrations and at different temperatures. For the fluxes, similar tendencies are found for all three membranes, although the order of magnitude differs. At higher temperatures, the fluxes increase and at higher feed ethanol concentrations, the partial ethanol fluxes increase, whereas the water fluxes remain constant. One difference can be found for the pervaporation membranes, where the break-even point for dominant partial flux (at lower feed ethanol concentrations the water flux dominates) lies at higher feed ethanol concentrations than for the SolSep 3360 nanofiltration membrane. This indicates stronger interactions between ethanol molecules and membrane for the nanofiltration membrane than for the pervaporation membranes. Another difference is found in the order of magnitude of the fluxes. For the SolSep 3360 membrane, fluxes in the range of 0–3.5 kg m−2 h−1 were found, whereas the fluxes of the PV 1070 membrane staid below 0.3 kg m−2 h−1 . This can be explained by the fact that SolSep 3360 is a nanofiltration membrane with larger free volume, but also because it has a more recent and advanced chemical structure. The SolSep 3360 membranes fluxes are nearly twice as large as the fluxes through the Pervatech membrane at low temperatures. However, at higher temperatures this difference has almost completely vanished. Apparently, at large temperatures the thermally induced movement of polymer chains in the pervaporation membrane and the better diffusion of the permeant counterbalance the advantages of the larger pore size of the nanofiltration membrane. For the permeances, similar differences in order of magnitude were found. The permeances of the SolSep 3360 membrane are several factors larger than of the PV 1070 membrane, for the same reasons as explained above. For the Pervatech membrane the order of magnitude of the permeances is smaller than for the SolSep 3360 membrane at lower temperatures, just as for the fluxes. At higher temperatures, the permeances approach the same order of magnitude. Although the permeances show the same increasing tendency with feed ethanol concentration, a striking difference when comparing the permeances of the Pervatech and the SolSep 3360 membrane, is the fact that for the SolSep 3360 membrane the ethanol permeance dominates, while for the Pervatech membrane an opposite behaviour is observed. Since permeance shows more accurate permeant-specific transport properties, it appears
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water has more interaction with the Pervatech membrane than with the SolSep 3360 membrane. The obtained higher water fluxes than ethanol fluxes at low feed ethanol concentration also manifest this. When looking at the temperature influence, it was found the ethanol permeance of the PV 1070 membrane hardly changes, whereas it was expected it would decrease. Possibly, the swelling has no effect here on the degree of counterbalance between the negative temperature effect on sorption and the positive temperature effect on diffusion. For the Pervatech membrane, with increasing feed ethanol concentration a decreasing separation factor is found, while the selectivity shows an increasing tendency, just as for the SolSep 3360 membrane. The selectivity of the PV 1070 membrane shows the same trend. For this membrane however, constant separation and enrichment factors are reported at low temperatures. Apparently, the swelling of the membrane has no influence on the separation and enrichment factor of a pervaporation membrane under these conditions. At higher temperatures, the separation and enrichment factors decrease, as do they in the SolSep 3360 membrane. Where there is little or no temperature dependence for the SolSep 3360 membrane for separation and enrichment factors and selectivity, the PV 1070 membrane shows larger values for all three parameters at higher temperatures. This is probably because the ethanol permeance for this membrane is higher than expected, resulting in a larger ratio in Eqs. (8) and (9).
4. Conclusion This research confirms findings of earlier studies, that plots of flux and permeance versus feed ethanol content may give different results and conclusions on effects of processing conditions on separation performance. Using permeance and selectivity, instead of flux and separation and enrichment factor, can significantly decouple the effect of operating conditions on performance evaluation, while clarifying and quantifying the contribution by the nature of the membrane to the separation performance. Aqueous activity coefficient and saturated vapour pressure appear to play an important role when using permeance and selectivity in the performance analysis. Furthermore, it was found that the membrane performance in terms of permeance and selectivity, as well as in terms of flux, separation and enrichment factors, is similar when alcoholic beverages are used as feed instead of synthetic solutions. Apparently, the extra components in these mixtures have no influence on the pervaporation properties, or their influences are counterbalanced. Compared to two PDMS-based pervaporation membranes, both fluxes and permeances are larger in the nanofiltration membrane, while the separation factor and selectivity are comparable or even better. The difference between the nanofiltration and pervaporation membranes can be explained by the influence of swelling and the different interactions between permeating molecules and the membrane. This makes a nanofiltration membrane very useful for pervaporation.
Acknowledgments The Research Council of the K.U. Leuven is gratefully acknowledged for financial support (OT/2002/33 and OT/2006/37). SolSep and Pervatech are thanked for kindly supplying membrane samples. The Socrates program is also gratefully acknowledged for the grant given to Bram Leen for performing the experimental work at ITM-CNR, ITALY. References [1] P.A. Kober, Pervaporation, perstillation and percrystallization, J. Am. Chem. Soc. 39 (5) (1917) 944–948. [2] B. Van der Bruggen, J.C. Jansen, A. Figoli, J. Geens, K. Boussu, E. Drioli, Characteristics and performance of a ‘universal’ membrane suitable for gas separation, pervaporation and nanofiltration applications, J. Phys. Chem. B 110 (28) (2006) 13799–13803. [3] B. Van der Bruggen, J.C. Jansen, A. Figoli, J. Geens, D. Van Baelen, E. Drioli, C. Vandecasteele, Determination of parameters affecting transport in polymeric membranes: parallels between pervaporation and nanofiltration, J. Phys. Chem. B 108 (35) (2004) 13273–13279. [4] J.P.G. Villaluenga, M. Khayet, P. Godino, B. Seoane, J.I. Mengual, Analysis of the membrane thickness effect on the pervaporation separation of methanol/methyl tertiary butyl ether mixtures, Sep. Purif. Technol. 47 (1–2) (2005) 80–87. [5] X.-P. Wang, Y.-F. Feng, Z.-Q. Shen, Pervaporation properties of a threelayer structure composite membrane, J. Appl. Polym. Sci. 75 (6) (2000) 740–745. [6] J. Caro, M. Noack, P. K¨olsch, Chemically modified ceramic membranes, Microporous Mesoporous Mater. 22 (1–3) (1998) 321–332. [7] S.S. Madaeni, The effect of surface characteristics on RO membrane performance, Desalination 139 (1–3) (2001) 371. [8] J. Meier-Haack, W. Lenk, S. Berwald, T. Rieser, K. Lunkwitz, Influence of thermal treatment on the pervaporation separation properties of polyamide6 membranes, Sep. Purif. Technol. 19 (3) (2000) 199–207. [9] S. Ray, S.K. Ray, Effect of copolymer type and composition on separation characteristics of pervaporation membranes—A case study with separation of acetone–water mixtures, J. Membr. Sci. 270 (1–2) (2006) 73–87. [10] S.V. Satyanarayana, A. Sharma, P.K. Bhattacharya, Composite membranes for hydrophobic pervaporation: study with the toluene–water system, Chem. Eng. J. 102 (2) (2004) 171–184. [11] S.V. Satyanarayana, P.K. Bhattacharya, Pervaporation of hydrazine hydrate: separation characteristics of membranes with hydrophilic to hydrophobic behaviour, J. Membr. Sci. 238 (1–2) (2004) 103–115. [12] S.I. Semenova, H. Ohya, K. Soontarapa, Hydrophilic membranes for pervaporation, an analytical review, Desalination 110 (3) (1997) 251–286. [13] J. Meier-Haack, T. Rieser, W. Lenk, D. Lehmann, S. Berwald, S. Schwarz, Effect of polyelectrolyte complex layers on the separation properties and the fouling behavior of surface and bulk modified membranes, Chem. Eng. Technol. 23 (2) (2000) 114–118. [14] J. Liu, W.K. Teo, C.H. Chew, L.M. Gan, Nanofiltration membranes prepared by direct microemulsion copolymerization using poly(ethylene oxide) macromonomer as a polymerizable surfactant, J. Appl. Polym. Sci. 77 (12) (2000) 2785–2794. [15] J. Sekuli´c, J.E. ten Elshof, D.H.A. Blank, A microporous titania membrane for nanofiltration and pervaporation, Adv. Mater. 16 (17) (2004) 1546–1550. [16] B. Barri`ere, L. Leibler, Permeation of a solvent mixture through an elastomeric membrane—The case of pervaporation, J. Polym. Sci. Part B 41 (2) (2003) 183–193. [17] R. Ranieri, A. Figoli, A. Criscuoli, E. Drioli, La pervaporazione e i membrane contactors come sistemi alternative per la determinazione dell’alcool etilico in soluzioni idroalcoliche, Master Thesis, ITM-CNR, Rende (CS), Italy, 2005.
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