Journal of Membrane Science 373 (2011) 29–35
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Pervaporation separation of ethanol/ETBE mixture using poly(lactic acid)/poly(vinyl pyrrolidone) blend membranes S. Zereshki a,b , A. Figoli c,∗ , S.S. Madaeni b,∗∗ , F. Galiano c , E. Drioli c,d a
Chemical Engineering Department, Kermanshah University of Technology, Kermanshah 67178, Iran Chemical Engineering Department, Razi University, Kermanshah 67149, Iran Institute on Membrane Technology, ITM-CNR, Rende (CS) 87030, Italy d Hanyang University, WCU Energy Engineering Department, Seoul 133791, South Korea b c
a r t i c l e
i n f o
Article history: Received 3 December 2010 Received in revised form 31 January 2011 Accepted 22 February 2011 Available online 1 March 2011 Keywords: Pervaporation Poly(lactic acid) Poly(vinyl pyrrolidone) Ethanol Ethyl tert-butyl ether (ETBE)
a b s t r a c t The pervaporation properties of poly lactic acid (PLA), a natural source polymer, were studied in a polar/non-polar case study. PLA/PVP (poly vinyl pyrrolidone) blend membranes were prepared containing different PVP contents and evaluated in ethanol/ethyl tert-butyl ether azeotropic separation. The swelling and mechanical properties of the membranes were investigated. SEM cross-sectional images showed a porous structure at higher PVP concentrations. The degree of swelling as well as the permeation ﬂux (0.05–1.36 kg/m2 h) gradually increased when the PVP amount increased to 21 wt%. On the other hand, the EtOH separation factor initially raised to 16 using 3 wt% PVP and then decreased to 3 using 21 wt% PVP. The continuously decreasing water contact angle from 74◦ in PLA to 54◦ conﬁrmed higher hydrophilicity and EtOH afﬁnity at higher PVP contents. However, the more porous morphology and the plasticization effect resulted in selectivity decrease. This was also in agreement with the observed mechanical behavior of the blends. At higher EtOH concentrations and PVP contents, the elastic modulus of the membranes decreased, contrary to the membrane elongation. © 2011 Elsevier B.V. All rights reserved.
1. Introduction During the past decades, the environmental pollution concerns became more important all around the world. The former tetra alkyl lead derivatives used as fuel octane enhancers have been banned from gasoline, according to the stringent regulations (i.e., Clean Air Act of 1990, Kyoto protocol). Two main groups of new fuel oxygenators have been gradually introduced to avoid lead dissemination and to limit the aromatic (benzene) content in gasoline: alcohols (e.g., methanol and ethanol) and ethers (e.g., methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), and tert-amyl methyl ether (TAME)). The latter group has several advantages such as low heat of vaporization and low water sensitivity compared to alcohols which makes them preferred by the reﬁners. Ethers have been used in gasoline for over 30 years at various levels up to 15%. They could be blended at the reﬁnery and efﬁciently transported to the marketplace using the current distribution infrastructure and the existing vehicle ﬂeet.
∗ Corresponding author. Tel.: +39 0984 492027; fax: +39 0984 402103. ∗∗ Corresponding author. Tel.: +98 831 4274535; fax: +98 831 4274542. E-mail addresses: a.ﬁ[email protected]
(A. Figoli), [email protected]
(S.S. Madaeni). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.02.031
MTBE was the most used oxygenated octane enhancer for years particularly in the US. But it found to be a water esthetic quality contaminator by affecting the taste and odor of drinking water. The groundwater analysis across the USA ﬁnally suggested moving to another oxygenator. MTBE has a relatively high solubility in water (43–54 g/L) and little afﬁnity for soil. On the other hand, ETBE has lower water solubility (<26 g/L) and higher partition coefﬁcient so that the hydrologic cycle is not affected by ETBE signiﬁcantly . Furthermore, this biofuel is much better biodegradable than MTBE thanks to the presence of a hydrogen atom in ␤-position to the ether group [2,3]. It has been also shown that ETBE has better antiknock properties than MTBE . ETBE is the preferred oxygenator in Europe which was ﬁrstly used in France (1992) and it is expected that its demand will increase in the early future. ETBE is usually obtained from the reaction of a large excess of ethanol with isobutene to ensure a high isobutene conversion toward ether production . The obtained EtOH/ETBE mixture is difﬁcult to separate by conventional technologies like distillation because of azeotrope formation (20/80 wt%). Ternary (azeotropic) distillation is an efﬁcient but very expensive mean to break the azeotrope which combines three distillation towers with high cost in terms of time and energy. Pervaporation process could be used to break the azeotrope in a hybrid distillation-pervaporation system or even alone to produce pure ETBE. There would be a certain energy
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saving as well as the other general advantages of the membrane processes. Most of the studies on EtOH/ETBE separation have been performed by a French research team in Nancy University. Almost all their studied membranes were based on cellulose acetate [6–10]. Polyamideimide [5,11] and poly pyrrolidinone based membranes  were also studied in the same research center. Ortiz et al.  used the commercial PERVAP 2256® membrane from Sulzer Chemtech. The basic development of this membrane was done by GFT with the help provided by different partners amongst whom were the Institut Franc¸ais du Pétrole (French Petroleum Institute) and the above mentioned laboratory in Nancy. Sulzer Chemtech acquired the GFT membrane system business in 1997. Billard et al. used cellulose acetate butyrate as the linear polymer and the polyethylene glycol dimethacrylate (PEGDMA) as the monomer to be photopolymerized to form the s-IPN (semiinterpenetrating polymer networks) membrane. They obtained a selectivity of 11.8–21 with a ﬂux of 2.1–1.4 kg/m2 h at 40 ◦ C . The ﬂux for s-IPN membranes made of different cellulose esters and methacrylate network was 6.5–0.02 kg/m2 h with the permeate EtOH concentration of 75 wt% to higher purities of EtOH . Cellulose based blends (PVP-PVAc/CA [8,9] and PAA/CAP ) represented similar results with low ﬂux at higher selectivities. The permeation ﬂux and selectivity of graft copolymers CA-g-PMDEGMA with different architectures  were 0.23–1.09 kg/m2 h and 28–329, respectively. Pyrrolidinone-based membranes (PVP/PTMA) were able to efﬁciently separate EtOH/ETBE azeotrope with high ﬂux (20.21–6.06 kg/m2 h at 50 ◦ C) . While the results were not outstanding for polyamideimides (ﬂux 1.07–0.57 kg/m2 h, permeate EtOH concentration 63–65 wt%) , they were better for poly(urethane-amide-imide) (PUAI) block copolymers (ﬂux 8.24–0.035 kg/m2 h, permeate EtOH concentration 49–91 wt%) . Faujasite NaY membranes prepared by Kita et al.  showed a high selectivity of 1200 and a ﬂux of 0.2 kg/m2 h for a 10 wt% mixture at 50 ◦ C. In general, because of the nature of the separating components, hydrophilic polymers particularly cellulose based membranes were extensively studied. The chemical stability and strength of the membranes were also focused in the literature. In spite of the acceptable reported results, challenging the new polymers and materials in order to include some other advantages is always of researchers’ interest. Poly(lactic acid) (PLA) is a natural source polymer with very few pervaporation background studies [16,17]. It is often studied in gas separation applications e.g. for packaging [18–22]. PLA could be produced by lactic acid polycondensing or lactide ring-opening . It was formerly rather expensive with low availability so that it was mainly used in medical applications. Recently, economical production of high molecular weight PLA using new techniques from renewable resources has made it available to a broader ﬁeld of applications [24,25]. Therefore, PLA is considered to be a suitable replacement for numerous petroleum-based polymers in the future . The green aspects and environmental and health risks of the used polymers (PLA and PVP), solvent (chloroform), and feed (EtOH and ETBE) are reported in Table 1 [27–30] in comparison with the polymers, solvents, and feeds commonly employed. Note that in spite of using chloroform as the solvent for literature comparison purposes, PLA is easily soluble in many other less toxic solvents. In the current work, pervaporation properties of PLA/PVP blend membranes are investigated for EtOH/ETBE azeotropic separation as a case study. PVP was used as an additive for further structural control and tailoring the hydrophilicity and permeability of the membranes.
2. Materials and methods 2.1. Materials PLA polymer granules were supplied by Cargill-Dow Inc. (USA) with the trade name of Nature Green® 2100D. This highly crystalline type is mostly consisted of the L co-monomer containing less 1.47 ± 0.2% of the D co-monomer. Low molecular weight PVP (Luviskol K17, Mw = 9000) was obtained from BASF (USA). The polymers were dried in a vacuum oven before use. Chloroform (99%) and ethanol (99.5%) were purchased from Carlo Erba Reagenti (Italy). The solvent was stabilized with 0.6–1% ethanol against toxic gas formation. ETBE (>95%) was purchased from TCI Europe (Belgium). The chemicals were used as received from supplier. 2.2. Membrane preparation The polymer casting solution was prepared by homogeneously dissolving the polymers in chloroform so that a 7.5 wt% dope were obtained. The mixing was performed on a magnet-stirrer for at least 12 h at room temperature. The casting solutions were prepared for a total of eight different PLA/PVP ratios in addition to the pure PLA. The weight percent of PVP was gradually increased from 0 to 1.6 wt% with 0.2 wt% intervals. After 15 min ultrasonic treatment and 2 h of degassing, polymer ﬁlms were cast on glass plates. The wet thickness of the ﬁlms was adjusted at 120 m on the manual casting knife. The prepared ﬁlms were immediately moved to an oven where the solvent was evaporated at 40 ◦ C for 48 h. The obtained membranes were detached in distilled water and dried. With respect to the PVP content of the membranes they were named as M0 to M8. The thickness of the prepared dense transparent membranes was 9 ± 4 m, approximately, as measured using a digital micrometer (Mahr, 40E, Germany) with the accuracy of ±4 m and conﬁrmed by SEM micrographs (Cambridge Stereoscan 360, Cambridge Instruments, UK). As the PVP content of the membranes increased, the transparency decreased to some extent though. This could be an indication of partial demixing of the two polymers, as conﬁrmed later by the SEM analysis. The structural modiﬁcations of the membranes were studied using these micrographs. 2.3. Swelling experiments The swelling behavior of the membranes was studied in different concentrations of EtOH/ETBE mixtures. The mixtures include pure EtOH and pure ETBE as well as their 33 wt% and 66 wt% solutions. Few pieces of each membrane with the minimum area of 5 cm2 were weighed using a digital balance (Gibertini, Crystal 500, Italy) with the accuracy of 10−6 kg. The membranes were immersed in the prepared solutions for 72 h at 30 ◦ C to reach swelling equilibrium. Finally, they were quickly taken out and reweighed after removing the liquid drops from their surface using tissue papers. The degree of swelling (DS) for each membrane type was calculated based on the weight of the membrane before (Wd ) and after (Ws ) swelling as follows: DS(%) =
Ws − Wd × 100 Wd
2.4. Mechanical properties The mechanical properties of the membranes were studied on a Zwick/Roell universal testing machine, single column model Z2.5, equipped with a 50 N maximum load cell (BTC-FR2.5TN-D09, Germany). The reported properties include the tensile (Young’s) modulus, the maximum stress and the elongation (strain) at break
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Table 1 Environmental and health risks comparison and molecular properties of the used solvents and the polymers [27–30]. Solvent or polymer
Vapor pressure (mmHg)
PLA PVP CA CTA
n/a 40–100 >5 >3.2
EtOH ETBE MeOH
7 5 5.6
44 152 100
– – – –
Solubility parameters (MPa1/2 )
Molar volume (10−3 m3 /kmol)
No risks. No adverse health effect. May act as a respiratory or eye irritant. –
Highly ﬂammable. Irritant. Highly ﬂammable. Slight irritant. Highly ﬂammable. Toxic by inhalation, ingestion or skin absorption. Highly ﬂammable. Possible carcinogen. May be harmful by inhalation, ingestion or through skin contact. Skin irritant. Probable carcinogen. Irritant. Inhalation and ingestion are harmful. Highly ﬂammable. Irritant. Harmful to eyes. May be harmful by inhalation, ingestion or skin absorption. Irritant. Harmful by inhalation, ingestion or skin contact. May act as a carcinogen. Exposure may result in foetal death. Skin, eye and respiratory system irritant. May cause harm to the unborn child. Highly ﬂammable. Severe irritant. May be harmful by ingestion, inhalation or through skin contact. Highly ﬂammable. Irritant. Probable carcinogen. Toxic.
15.8 n/a 15.1
8.8 n/a 12.3
19.4 n/a 22.3
26.5 14.8 29.6
59.1 136.5 40.7
Note: LD50: lethal dose 50%, oral-rat; vapor pressures are presented at 20 ◦ C (except ETBE at 25 ◦ C); used polymers and solvents are shown in bold; n/a: data not available.
for the membranes before and after soaking in EtOH/ETBE mixtures. The tests were performed on at least three 1 cm × 5 cm strips of each membrane. The soaking conditions were the same as the swelling conditions described before.
This parameter is the combination of the quantity and the quality aspects of a pervaporation process performance, considering both the selectivity and the amount of product at the same time. 3. Results and discussion
2.5. Pervaporation experiments 3.1. Membrane characterization The experimental pervaporation apparatus is described elsewhere . The azeotropic EtOH/ETBE mixture was used as the feed in all the experiments. The feed stream was continuously ﬂowing over the membrane surface with a rate of 50 L/h at the constant temperature of 30 ± 0.01 ◦ C. The effective area of the used round membrane was 60.7 cm2 . After reaching the steady state conditions, the permeated vapor was collected using liquid nitrogen under a vacuum of 4 ± 1 mbar. It was weighed and analyzed using an Abbe 60 type direct reading refractometer (60/DR, Bellingham + Stanley Ltd., UK) at 25 ◦ C. The permeation separation factor of EtOH was calculated from the following Eq. (2): ˛=
yEtOH /yETBE xEtOH /xETBE
where yi and xi are weight fractions of the component i in permeate and feed, respectively. The total permeation ﬂux was also calculated based on the weight of the collected permeate: J=
where Q is the weight of the permeate (kg), A is the membrane area (m2 ) and t is the operating time (h). In order to neutralize the effect of membrane thickness on the calculated ﬂux when comparing membranes, all the ﬂuxes are normalized to a membrane thickness of 10 m. The pervaporation separation index (PSI) is also calculated based on the normalized ﬂux: PSI = J(˛ − 1)
The general characterizations of the studied membrane are similar to what was previously reported by the authors . The effect of the PVP content on the structure of the blend membranes is shown in Fig. 1 for M0, M2, M4, M6 and M8. Introducing PVP into the casting solution resulted in a cellular structure. The average size of the pores increased at higher PVP amounts to some extent. The decrease in water contact angle of the membranes is the other effect of the PVP addition. The contact angle decreased from 74◦ in M0 to 72◦ in M1, then smoothly to 54◦ in M8 as an indication of the increasing hydrophilicity of the blends. The PVP existence in the structure of the membranes was also conﬁrmed by FTIR-ATR studies. A new peak was appeared at 1655 cm−1 which originates the carbonyl stretching vibration in the pyrrolidone group of PVP. Details on the characterization studies could be found in the above mentioned reference. 3.2. Swelling results Fig. 2 represents the swelling results in terms of the degree of swelling of each membrane type at different EtOH/ETBE concentrations. The effect of the PVP content of the blend membranes on the swelling trend is clearly shown. Regardless of the mixture concentration, the membranes with higher PVP percents swelled more. Furthermore, higher degrees of swelling were observed at high EtOH concentrations. The swelling degree sharply increased from 13% in M0 to 20% in M1 initially and then gradually to 29% in M8 for the membranes soaked in pure EtOH. A similar trend of lower swelling degrees
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Fig. 1. Cross sectional SEM image of M0, M2, M4, M6 and M8.
was observed in pure ETBE with the corresponding values of 7%, 8% and 15%. Evidently, all the membranes, in particular the membranes with higher PVP content, preferentially absorb EtOH. PVP is a hygroscopic polymer with strong interaction with polar molecules. Therefore, because of the polar character of EtOH it may be favorably separated by PVP containing blends. Note that the dipole moment of alcohols is greater than that of ethers which results in greater water solubility or miscibility for alcohols. In general, a solvent-polymer pair with close solubility parameters is considered to be more favorable for pervaporation application. Pervaporation is based on the well-known solutiondiffusion mechanism. Therefore, the permeating component(s) should dissolve in the polymeric structure of the membrane before passing through it by diffusion. Obviously, very close solubility parameters would even result in dissolving the membrane in the solvent. The polar contribution of the solubility parameters of PVP and EtOH are rather high. Consequently, the solubility parameter of PVP (32.5 MPa1/2 ) is more close to EtOH (26.5 MPa1/2 ) than that of ETBE (14.8 MPa1/2 ). This value is 21.7 MPa1/2 for PLA polymer which is also closer to EtOH (Table 1). It moves toward the EtOH value by increasing the PVP content of the blend membranes. Solubility parameter of the blend is a linear function of composition which is the volume fraction for each polymer . Thus, it could be obtained summing up volume fraction times the solubility parameter values. The density of PLA and PVP are around 1.29 and 1.25 g/cm3 , respectively. Therefore, the solubility parameter of the membranes increases from 21.7 MPa1/2 in M0 to 24.7 MPa1/2 in M8. Apart from the exact values, the solubility parameter of the blends approaches more to EtOH. Furthermore, the cellular structure of the blends physically improves the solvent absorption and membrane swelling at high PVP contents and EtOH concentrations.
3.3. Mechanical strength results The results of the mechanical tests for M0, M1, M3, M5 and M7 are summarized in Table 2. The untreated membranes tested before soaking are compared to the membranes soaked in different EtOH/ETBE mixtures (0, 33, 66 and 100 wt%). Although the results may not follow a clear trend at certain points, an overall trend in the measured mechanical parameters is evident. In general the Young’s (elastic) modulus and the maximum stress decreased with increasing the PVP content of the blends. On the contrary, the elongation of the membranes followed a slightly increasing trend and mainly remained constant over a range. The results are less meaningful for the middle concentrations though. Similar trends were observed for the membranes immersed in high EtOH concentration. Lower elastic modulus and stress values and higher strain values were measured for pure EtOH treated membranes compared to the other concentrations. As reported in Table 2, the Young’s modulus decreased from 2224 N/mm2 in untreated M0 to 301 N/mm2 in pure EtOH treated M7. The corresponding values were 51 N/mm2 and 21 N/mm2 for the maximum stress. For the same two conditions, elongation at break drastically increased from 6% to 266%. The morphological change in the structure of the blend membranes is an explanation for the observed mechanical behavior. The obtained closed cellular structure physically reduces the stress and increases the strain of the tested membranes. The changes in the mechanical strength are even more apparent at high PVP contents due to the further structural change. As discussed before on the swelling results (Section 3.2), the small EtOH molecules dissolve in the polymer causing the membranes to swell. Therefore, it could be the second explanation for the observed mechanical strength behavior, particularly at high EtOH concentrations. Because of the plasticization effect the polymeric chain-chain interactions are reduced in the swollen membrane. As a result, the elongation of the membrane strips increased opposite to their strength because of sliding polymeric chains over each other. 3.4. Pervaporation results
Fig. 2. Degree of swelling of the membranes in different EtOH concentrations.
The EtOH separation factors and total permeation ﬂuxes are represented in Fig. 3 for all the membrane compositions at azeotropic concentration. The pervaporation experiments were carried out several times particularly for the low PVP content blends. An increasing ﬂux trend was observed when the PVP content of the membranes increased. A ﬂux of around 0.05 kg/m2 h for M0 gradually increased to around 1.35 kg/m2 h using M8. This behavior is in agreement with the swelling results. As the PVP amount increased, the tendency for absorbing higher amounts of the feed components increased. Furthermore, the porous structure of the high PVP content blends enables the membranes to absorb larger amount of liquid feed. The effective membrane thickness which is the mass transfer barrier is lower in such membranes.
178 248 305 265 266 29 19 30 19 21 958 444 200 262 301
221 194 55 263 51 31 22 32 25 37 892 514 920 550 1281 283 228 324 297 214 33 24 37 26 21 703 680 619 442 484 7 5 7 7 6 47 36 46 26 36 1342 1334 1349 1291 1397 6 37 12 3 11 51 38 46 28 36 2224 1858 1835 1266 1646 M0 M1 M3 M5 M7
Max stress (N/mm2 ) Emod (N/mm2 ) Max stress (N/mm2 ) Emod (N/mm2 ) Max stress (N/mm2 )
Emod (N/mm2 ) -Break (%) Max stress (N/mm2 ) Emod (N/mm2 )
Table 2 Mechanical properties of the membranes at different treatment conditions.
Emod (N/mm2 )
Max stress (N/mm2 )
S. Zereshki et al. / Journal of Membrane Science 373 (2011) 29–35
Fig. 3. Total permeation ﬂux and separation factor of membranes calculated for azeotropic concentration.
The separation factor sharply increased from M0 to M1 (from 7 to 16). In other words, both the selectivity and the ﬂux initially increase while usually a trade-off exists between these two parameters . Increasing the PVP content gradually reduced the selectivity to even lower than M0 in M8 (3.7). Because of the nature of PVP and the solubility parameters as discussed in swelling section, high amounts of PVP should improve the EtOH separation. However, taking the morphological change into account, the EtOH selectivity may be affected by the facilitated bulk permeation of both the feed components. The EtOH absorption effect of PVP could not compensate the high feed ﬂux through the larger cavities any more. Therefore, an optimum membrane composition is usually considered according to the maximum separation factor obtained. If higher selectivity is more important than the product volume, 2–3 wt% PVP in the membrane blend results in a better separation. The selectivity decrease started earlier than what is reported in the literature for ethanol/cyclohexane PV case using similar blends [17,31]. Cyclohexane is a completely non-polar component compared to EtOH and the PVP blends does not even swell in pure cyclohexane. On the other hand, the membranes swell in ETBE to some extent as shown in the swelling results. Therefore, the separation performance is more sensitive to the membrane morphology in EtOH/ETBE separation. The PVP-based membranes studied by Touchal et al.  exhibited the same selectivity decrease while having a non-porous structure. Because of the increasing trend of permeation ﬂux they concluded that the membrane afﬁnity for EtOH does not decrease when the PVP content is increased. Therefore, this behavior could be ascribed to either the higher EtOH afﬁnity in the main polymer compared to PVP (which is not the case here), or a plasticizing effect induced by the PVP content increase, leading to a coupling transport phenomenon between the two permeants. Pervaporation separation index (PSI) combines the selectivity and the ﬂux of a PV separation, as shown in Fig. 4. This ﬁgure suggests using 11–13 wt% PVP (M4–M5) to get a selectivity-ﬂux balanced separation. Plotting the separation factors and ﬂuxes on a Robeson like plot (Fig. 5) shows an overall comparison between the performances of the blends. Obviously, three almost distinct regions are distinguishable over the plot. The lowest PVP content (M1) is located in the high selectivity zone. However, the ﬂux is rather low in this zone. The highest PVP content (M8) represents the highest ﬂux but lowest separation factor. The M4 and M5 membranes are the two topmost points in the moderate performance zone.
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the chemical related industries depends on using green materials, chemicals, and technologies. Acknowledgments The Ministry of Science, Research and Technology of Iran is gratefully acknowledged for the ﬁnancial support given to Sina Zereshki for performing the experimental work at ITM-CNR in Italy. The authors are also thankful to Dr. Silvia Simone, Dr. Gianluca Di Proﬁo, and Dr. Mariano Davoli for their kind cooperation for mechanical tests, FTIR analysis, and SEM pictures. References
Fig. 4. Pervaporation separation index of the membranes for azeotropic concentration.
Fig. 5. Separation factors as a function of membrane ﬂuxes.
4. Conclusion The application of PLA/PVP blend membranes in EtOH/ETBE pervaporation was discussed. It was shown that PVP could be used to control the morphology and the hydrophilicity of the membranes. Moreover, the possibility of using PLA in pervaporation applications was demonstrated. All the studied membranes selectively separated EtOH from ETBE. Compared to the literature, the obtained separation factors were not satisfactory. However, the ﬂuxes were well in the range. In addition, the pervaporation performance could be improved by preparing composite PLA membranes for further ﬂux increase. Up to 3 wt% PVP in the membrane structure was detected as optimum amount resulting in the highest observed selectivity. The mechanical properties of PLA studied after soaking in feed mixtures suggest it as a favorable membrane material. The mechanical strength of the membranes decreased a bit at higher EtOH concentrations. But they were strong enough at the studied concentration. Future substitution of such natural source polymers and gradually phasing out of petroleum based polymers would be a saving step for the environment. The sustainable development of
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