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Application of supported ionic liquid membranes (SILMs) for biobutanol pervaporation
Separation and Purification Technology xxx (2015) xxx–xxx
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Application of supported ionic liquid membranes (SILMs) for biobutanol pervaporation P. Rdzanek a,⇑, S. Heitmann b, A. Górak a, W. Kamin´ski a a b
´ ska 213, 90-924 Łódz´, Poland Lodz University of Technology, Faculty of Process and Environmental Engineering, ul. Wólczan TU Dortmund University, Laboratory of Fluid Separations, Emil-Figge-Str. 70, 44227 Dortmund, Germany
a r t i c l e
i n f o
Article history: Received 30 September 2014 Received in revised form 26 February 2015 Accepted 9 March 2015 Available online xxxx Keywords: Pervaporation n-Butanol Poly(ether block amide) (PEBA) Silicone
a b s t r a c t n-Butanol is a promising future biofuel. It can be produced in the so-called Acetone–Butanol–Ethanol (ABE) fermentation. The main drawback of this kind of fermentation is the toxicity of n-butanol towards the production strains. The concentration of butanol in the fermentation broth does not usually reach 2 wt.%. Biobutanol can be recovered continuously from the fermentation broth using pervaporation (PV), thus overcoming toxicity of n-butanol against microorganisms. To improve the separation efficiency of the membranes, supported ionic liquid membranes (SILMs) are used, in which ionic liquid (IL) is immobilized in the active layer of PV membranes. Two ionic liquids, namely trihexyl(tetradecyl)phosphonium tetracyanoborate (P6,6,6,14 tcb) and 1-hexyl-3-methylimidazolium tetracyanoborate (Im6,1 tcb) are immobilized by inclusion in a polyether block amide (PEBA) polymer matrix, which was covered by an additional silicone layer. Pervaporation experiments for these membranes were carried out at 37 °C using quaternary ABE mixtures with n-butanol concentrations up to 3 wt.% in the feed solution. It was found that permeate fluxes as well as selectivity of the SILMs can be influenced by immobilization of different ILs in the membranes. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction A large number of studies are focused on the production of sustainable and renewable liquid biofuels from biomass [1–3]. Biobutanol is one of the biofuels that is currently receiving considerable attention in the scientific and industrial community. Butanol is very promising when used as a gasoline additive, especially when compared to ethanol. The energy density of gasoline (32 MJ/L) is similar, to butanol (29.2 MJ/L). Butanol is less corrosive than ethanol and can be blended with petrol at any ratio without the need to modify the engine [4]. Biobutanol production belongs to important engineering challenges and the production technology is not as advanced as for bioethanol. The main problem in biobutanol production is the butanol toxicity to the clostridia microorganisms, which limits butanol concentration in fermentation broth to less than 2 wt.% [5]. This affects the process efficiency and makes the overall cost of butanol production higher compared to costs of other fuels (bioethanol, biodiesel or biomethane). By integrating a separation and a fermentation process the biobutanol can be continuously remove from fermentation broth [6–8]. ⇑ Corresponding author. E-mail address: [email protected] (P. Rdzanek).
In Table 1 are presented advantages and disadvantages of alternative separation processes for biobutanol purification like distillation, gas-stripping, adsorption, etc. [9–11]. Liquid–liquid extraction is promising method for butanol recovery from fermentation broth. Conventional solvents like oleyl alcohol (OA) [12] or novel components such as ionic liquids [13] as extractants can be used. However, extraction turned to be not suitable for industrial applications because of high costs of advanced solvents. Also efficient solvent recovery is challenging. Pervaporation can overcome these drawbacks and is considered as alternative to extraction. The effective use of pervaporation for butanol separation depends to a large extent on the properties of the membrane used [14]. The membrane permeability is mainly determined by the solubility and diffusivity of the permeating component in the membrane [15,16]. Selection of a suitable component forming the active part of the membrane is a crucial issue. This element improves the selectivity for n-butanol in the membrane. Several methods of membrane modification that are used to improve biobutanol separation are reported in the literature [17,18]. One of the most promising methods combining extraction with pervaporation is IL immobilization in the membrane [19]. Supported ionic liquid membranes (SILMs) are hybrid materials, in which two functionalities are integrated: an improved solubility of ionic liquids and higher selectivity of membrane.
http://dx.doi.org/10.1016/j.seppur.2015.03.024 1383-5866/Ó 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: P. Rdzanek et al., Application of supported ionic liquid membranes (SILMs) for biobutanol pervaporation, Separ. Purif. Technol. (2015), http://dx.doi.org/10.1016/j.seppur.2015.03.024
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Table 1 Advantages (+), disadvantages ( ) of butanol recovery processes [9–11]. Processes
Characteristics
Advantages/ Disadvantages (+)/( )
Distillation
High energy demand Large wastewater streams Large amounts of water
Gas-stripping
Simple design Use of fermentation gases Low selectivity Foam formation
+ +
Adsorption
Variety of adsorbent materials/ high selectivity Fouling Very low adsorbent capacity for butanol
+
Membrane reactor
Poor mechanical strength Leakage of cells from matrices
Extraction
Low energy demand Large amount of solvent required Toxicity issues
+
Pervaporation
Selective separation at low butanol Concentrations Energy efficient Small amounts of IL Simple and flexible experimental setup Membrane costs and properties
+
1.5% and 3 wt.% in the feed. Temperature and concentrations were chosen to represent the conditions in a conventional ABE fermentation. 2. Experimental
+ + +
Since amounts of IL, immobilised in the membrane is low, operation costs of biobutanol recovery from fermentation broth using SILMs are considerably lower than those using liquid–liquid extraction. Immobilization of ILs in membranes offers an interesting opportunity to produce membranes with different properties. ILs immobilization in the active layer of membranes may improve separation properties of conventional polymeric pervaporation membranes. Stable immobilization of IL is crucial for technical applications of SILMs. Elution of IL from the membrane remains still a challenge, preventing large scale applications of SILMs. Usually, ionic liquids [20] are defined as non-flammable, nonvolatile and chemically stable and often used in separation processes [21] as tuneable solvents. Basic properties of ILs have been summarized in the literature [22,23]. In last decades, the ILs appear to be an expanding group of high-potential compounds due to unique thermodynamic properties. Different ILs have been studied for the biobutanol recovery. Kohoutova et al. [24] used benzyl-3butylimidazolium tetrafluoroborate ILs. Heitmann et al. [19] immobilized in membranes 3 different types of ionic liquids 1-decyl-3-methylimidazolium tetracyanoborate (Im10,1 tcb), trihexyltetradecylphosphonium tetracyanoborate (P6,6,6,14 tcb) and 1-decyl 3-methylimidazolium tris(pentafluoroethyl) trifluorophosphate (Im10,1 fap). They achieved permeate flux of 560 g/(m2 h) and concentration of n-butanol in permeate of 55 wt.%. Izak et al. [25,26] investigated membranes with 1-ethenyl-3-ethyl-imidazolium hexafluorophosphate and tetrapropyl-ammonium tetracyanoborate ionic liquids which significantly improved the diffusion coefficients of 1-butanol in comparison with the neat PDMS membrane. Mai et al. [27] examine membrane with a [Tf2N] as ILs and the enrichment factor of butanol was three times higher compare to pure PDMS. The aim of this study was to examine recovery of n-butanol through pervaporation from four-component mixture using SILMs. We produced and investigated double layers membranes with IL immobilized in the active layer. Pervaporation process was conducted at 37 °C for the four component mixture of Acetone–Butanol–Ethanol–Water with n-butanol concentration of
2.1. Materials Two different ionic liquids, namely trihexyl(tetradecyl)phosphonium tetracyanoborate (P6,6,6,14 tcb) and 1-hexyl-3-methylimidazolium tetracyanoborate (Im6,1 tcb) (Fig. 1) provided by Merck KGaA were used to immobilized ILs in the membranes. Polysulfone membranes PS20 (Sepro Membranes Inc, pore size > 0.10 lm) and polypropylene (PP) membranes (Accurel PP, Membrana GmbH) were used as a support. Poly(ether block amide) (PEBA 2533) were kindly supplied by PRO-plast Kunststoff GmbH, Darmstadt. The one-component, fast-cure silicone (TSE 399C) purchased from Momentive Performance Materials. For preparing ABE mixtures acetone (99.9%), ethanol (99.5%) and n-butanol (99.9%) was acquired from Sigma Aldrich, Germany or Merck Schuchard OHF, Hohenbrunn. Acetic acid (96%), n-heptane (>99%) and acetonitrile (99.9%) were purchased from VMR International GmbH, Darmstadt. 2.2. Membrane preparation We produced SILMs in two different arrangements (1) and (2) of membrane. In both arrangements two membranes were placed on top of each other in the membrane cell. The difference in those two approaches is the silicone layer orientation presented in Fig. 2. The membranes with and without IL were prepare the same way. The arrangement (1) consists of two separated layers, PEBA + IL and silicon layer oriented to the feed side. Therefore, both membranes active layer (PEBA + IL, silicone) are oriented to the feed side. For the arrangement (2) PEBA + IL is located to feed side while the silicon layer is oriented to the permeate side. This causes silicone layer directly contacts the PEBA + IL layer. For arrangements (1) and (2), the PEBA was dissolved in acetic acid with ratio of 20:80 (wt.%/wt.%) in glass vials. The vials were placed in an oven at 80 °C for 24 h. 30 wt.% of IL was added to the solution after dissolution of PEBA in the acetic acid. Two ILs were taken into consideration. Selection of ILs were based on liquid–liquid extraction results [13,28, 29] of n-butanol from water solutions (Fig. 3). Subsequently, the homogeneous PEBA-acetic acid-IL mixture was poured onto the support membrane which was polysulfone membranes. A homogeneous active layer was produced by coating the polysulfone PS20 membranes using a coating knife. The membranes were left for 24 h at room temperature to let the acetic acid evaporate. The thickness of the coating layer were varied between 9 and 13 lm. The silicon coating was produced either on PS20 in arrangement (1) or on PP membrane in arrangement (2). In the latter case it was important that the porous support layer oriented to the feed side does not influence the separation efficiency of SILMs, therefore porous PP membrane was used as support instead of PS20 membranes. To minimize the mass transfer resistance the silicone layer needs to be as thin as possible. Silicone was mixed with heptane in ratio of 10:90 (wt.%/wt.%) to reduce the thickness of the silicone layer. Then heptane–silicone mixture was poured out into the membrane and placed in a vertical position. The average thickness of coating was about 5 lm. The membranes were left for 24 h at room temperature to let the heptane evaporate and to allow curing of the silicone. As an example SEM image of membrane structure PS20 as a support with immobilized P6,6,6,14 tcb is presented in Fig. 4. The
Please cite this article in press as: P. Rdzanek et al., Application of supported ionic liquid membranes (SILMs) for biobutanol pervaporation, Separ. Purif. Technol. (2015), http://dx.doi.org/10.1016/j.seppur.2015.03.024
P. Rdzanek et al. / Separation and Purification Technology xxx (2015) xxx–xxx
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Fig. 1. Ionic liquids used for membrane preparation.
SB,W = DB / DW
Fig. 2. Arrangement of double layer membranes.
600
2.3. Pervaporation process
500
Manufactured membranes were cut in circular samples with a diameter of 13 cm and mounted into the module, which is the heart of pervaporation plant (Fig. 5). Flat circular membrane module (Helmholz–Zentrum Geesthacht) has an effective membrane area of 104 cm2. Feed mixtures were aqueous solutions of three organic components acetone, butanol and ethanol in a 3:6:1 wt.% ratio with n-butanol concentrations of 1.5 and 3 wt.%. The experiments were carried out continuously in a quasi-steady state conditions. The pervaporation operating conditions are the feed flow rate 30 L/h, the permeate side pressure 10 mbar and the feed temperature 37 °C. Feed samples were taken at the beginning and at the end of each experiment. The permeate (in average 2.2 g) was collected in a cooled trap using liquid nitrogen and the retentate was recycled to the feed vessel. Because the permeate volume collected is small compared to the total feed volume, a quasi-steady state in the experiments can be assumed. The permeate mass collected in a specific time was evaluated by weighing the cooling trap before and after each experiment with and without permeate. To measure the concentration of components in the permeate acetonitrile was added to the sample as a tracer. The composition of the permeate was analyzed by gas chromatography (Shimadzu 14a) using flame ionization detection at 250 °C. The injection volume was 5 lL. The
400 300
Im₆,₁ tcb
200
P₆,₆,₆,₁₄ tcb
100 0
Oleyl alcohol
0
1
2
3
4
DB = wB,IL /wB,aq or wB,OA /wB,aq Fig. 3. Distribution coefficient and selectivity for biobutanol extraction from aqueous solutions [28–31].
top is the PEBA + IL coating and the bottom one is the PS20 support.
Fig. 4. SEM images of membrane coated with a mixture of PEBA and P6,6,6,14 tcb.
Fig. 5. Scheme of the pervaporation laboratory plant.
Please cite this article in press as: P. Rdzanek et al., Application of supported ionic liquid membranes (SILMs) for biobutanol pervaporation, Separ. Purif. Technol. (2015), http://dx.doi.org/10.1016/j.seppur.2015.03.024
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chromatograph was equipped with a capillary column (FSInnopeg-FFAP). All concentrations were determined threefold to exclude an influence of the sample preparation on the analytical results. The mean analytical error concerning concentration measurement was assessed to be ±2.3%.
3. Results and discussion Two different arrangements of SILMs were tested in the experimental setup [32]. The membranes with IL were weighted before and after PV experiments to confirm if the IL was leached from the membrane. In all cases gravimetric experiments confirm that the immobilization of IL was stable. Results obtained with double layer SILMs produced according to arrangement (1) are shown in Figs. 6 and 7. Fig. 6 shows partial permeate fluxes of butanol, acetone and ethanol for two ILs. Fig. 7A shows concentrations of butanol in permeate and Fig. 7B total permeate flux. The concentrations of butanol in permeate were slightly higher for membranes containing P6,6,6,14 tcb, while for Im6,1 tcb-based membranes the total permeate flux was higher. If we follow the selectivities and solubilities, measured in extraction (Fig. 3), we would expect that the mass fraction of n-butanol in permeate should be higher for the P6,6,6,14 tcb than for the Im6,1 tcb. However, the partial fluxes of butanol, acetone and ethanol for different ILs are almost the same but the permeate fluxes of water are much higher for Im6,1 tcb than for P6,6,6,14 tcb (Fig. 6A and B). Consequently, differences in the total permeate fluxes and concentrations for the SILMs containing different ILs are observed (Fig. 7). Fig. 6C presents partial fluxes for the membranes without ILs which are similar to the partial fluxes for membranes with ILs. Understanding of the mass transfer through the SILM in arrangement (2) layers is more complicated then in arrangement (1). The partial fluxes of butanol, acetone, and ethanol were different for different ILs (Fig. 8). The butanol permeate fluxes increase when using Im6,1 tcb, but the acetone and ethanol fluxes were slightly higher for P6,6,6,14 tcb.
250
In Fig. 8C the butanol flux for the membranes arrangement (2) without ILs is presented. In that case the butanol fluxes are significantly smaller for the membrane without immobilized IL. Fig. 9A shows concentrations of butanol in permeate and Fig. 9B displays the total permeate flux. While comparing arrangements (1) and (2), differences in partial fluxes of components were observed for P6,6,6,14 tcb, while partial fluxes for arrangements (1) and (2) for the Im6,1 tcb were almost the same. The partial flux of butanol for the arrangement (2) for P6,6,6,14 tcb was 170 g/m2 h while it was 220 g/m2 h for the (1) arrangement. Differences in total flux for both arrangement result from different water flux, which is higher for the membrane with Im6,1 tcb. Table 2 shows the differences in water fluxes for both arrangements. The water fluxes based on calculation for 3 wt.% of butanol in feed are significant lower for the arrangement (2). Therefore, comparing Figs. 7A and 9A show a slightly (10–20%) higher butanol concentration in the permeate. When a hydrophilic IL like Im6,1 tcb is immobilized it can cause an increase of the water flux.
4. Conclusions Two different arrangements of SILMs were produced and tested in pervaporation plant. It was shown that immobilization of ILs can influence the permeation properties of pervaporation membranes dependent on the properties of the immobilized ILs. Dependent on the arrangement for immobilization partial permeate fluxes and – connected to this – permeate concentration of a component can be influenced by choosing different ILs. The butanol concentration in permeate (after pervaporation separation with immobilized IL for arrangement (2)) was 10 time higher compare to feed. The total permeate flux for first arrangement was about 0.80 kg/ m2 h while for second 0.70 kg/m2 h. The butanol partial flux for (1) arrangement for Im6,1 tcb was 0.210 kg/m2 h and for P6,6,6,14 tcb was 0.220 kg/m2 h. For the (2) arrangement, the butanol partial flux for Im6,1 tcb was 0.210 kg/m2 h and for P6,6,6,14 tcb was 0.170 kg/m2 h. We cannot observed differences in partial flux for
250
A Ji [g/(m2h)]
Ji [g/(m2h)]
B
200
200 150 100
150 100
50
50
0 0
0.01
0.02
0 0.00
0.03
0.01
wi,Feed [g/g]
0.02
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wi,Feed [g/g]
250
C
Ji [g/(m2h)]
200 150 100 50 0 0.00
0.01
0.02
0.03
0.04
wi,Feed [g/g] Fig. 6. Partial permeate fluxes of ABE, containing 30% of Im6,1 tcb (A) or P6,6,6,14 tcb (B), without IL (C).
Please cite this article in press as: P. Rdzanek et al., Application of supported ionic liquid membranes (SILMs) for biobutanol pervaporation, Separ. Purif. Technol. (2015), http://dx.doi.org/10.1016/j.seppur.2015.03.024
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0.4
900
A
B
750
Jtotal [g/(m2h)]
wBuOH,Perm [g/g]
0.3 0.2 30 % Im₆,₁ tcb 30 % P₆,₆,₆,₁₄ tcb Without IL
0.1 0.0 0.00
0.01
0.02
0.03
600 450 300 30 % Im₆,₁ tcb 30 % P₆,₆,₆,₁₄ tcb Without IL
150 0 0.00
0.04
0.01
0.02
0.03
0.04
wBuOH,Feed [g/g]
wBuOH,Feed [g/g]
Fig. 7. Concentrations of butanol in permeate (A) and total permeate flux (B).
250
A
200
200
150
150
J i [g/(m2h)]
Ji [g/(m2h)]
250
100 50
B
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wi,Feed [g/g]
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0.03
wi,Feed [g/g] 250
C
Ji [g/(m2h)]
200 150 100 50 0 0.00
0.01
0.02
0.03
wi,Feed [g/g] Fig. 8. Partial permeate fluxes of ABE, containing 30% of Im6,1 tcb (A) or P6,6,6,14 tcb (B), without IL (C).
A
800
0.3
0.2
30 % Im₆,₁ tcb 30 % P₆,₆,₆,₁₄ tcb Without IL
0.1
0 0.00
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wBuOH,Feed [g/g]
Jtotal [g/(m2h)]
wBuOH,Perml [g/g]
0.4
B
600 400 30 % Im₆,₁ tcb 30 % P₆,₆,₆,₁₄ tcb Without IL
200 0 0.00
0.01
0.02
0.03
0.04
wBuOH,Feed [g/g]
Fig. 9. Concentrations of butanol in permeate (A) and total permeate flux (B).
arrangement (1), but when the membrane PS20 were replaced for PP membrane we observed differences for partial fluxes for ILs for arrangement (2).
The arrangement (1) bears an additional mass transfer resistance caused by the porous support material, which separates the silicone layer and the PEBA-IL layer – so the impact of the IL immobilization can be seen more clearly for arrangement (2). An
Please cite this article in press as: P. Rdzanek et al., Application of supported ionic liquid membranes (SILMs) for biobutanol pervaporation, Separ. Purif. Technol. (2015), http://dx.doi.org/10.1016/j.seppur.2015.03.024
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Table 2 Water fluxes for arrangements (1) and (2). Arrangement
IL
Water fluxes (g/m2 h)
(1)
Im6,1 tcb
476 454 472
P6,6,6,14 tcb
373 305 386
Without IL
332 285 327
Im6,1 tcb
325 328 325
P6,6,6,14 tcb
232 229 227
Without IL
221 229 221
(2)
influence of the IL-selectivity on the solvent-to-water selectivity is observed for the double layer SILMs produced in this work. Thanking into account share of water flux in total flux we concluded that the arrangement (2) is more suitable for the butanol separation because the water flux is smaller. The selectivity for the membranes with and without IL is almost the same but the butanol flux is higher for the membrane with IL for arrangement (2). It is supposed that the interactions between the polymer, IL and feed cause that partial fluxes are similar or different. One main obstacle to be overcome is elution of ILs into the liquid feed solution. So far an additional polymer (silicone) coating was necessary to able a stable immobilization. It causes additional mass transfer resistance and therefore this coating must be sufficiently thin. Acknowledgements The research was financed from the National Science Center funds Granted on the basis of decision 2012/07/B/ST8/03379. All ionic liquids used in this work were kindly supplied by Merck KGaA, Darmstadt. References [1] X. Chuang, Z. Xin-Qing, L. Chen-Guang, C. Li-Jie, B. Feng-Wu, Prospective and development of butanol as an advanced biofuel, Biotechnol. Adv. 31 (2013) 1575–1584, http://dx.doi.org/10.1016/j.biotechadv.2013.08.004. [2] J. Yu-Sin Jang, M. Alok, C. Changhee, L. Joungmin, L. Sang Yup, Butanol production from renewable biomass by clostridia, Bioresour. Technol. 123 (2012) 653–663, http://dx.doi.org/10.1016/j.biortech.2012.07.104. [3] N. Qureshi, T.C. Ezeji, Butanol, a superior biofuel production from agricultural residues (renewable biomass): recent progress in technology, Biofuels Bioproduct Biorefining 2 (2008) 319–330, http://dx.doi.org/10.1002/bbb.85. [4] S.S. Merola, C. Tornatore, L. Marchitto, G. Valentino, F.E. Corcione, Experimental investigations of butanol–gasoline blends effects on the combustion process in a SI engine, Int. J. Energy Environ. Eng. 3 (2012) 1–14. [5] E.M. Green, Fermentative production of butanol—the industrial perspective, Curr. Opin. Biotechnol. 22 (2011) 337–343, http://dx.doi.org/10.1016/ j.copbio.2011.02.004. [6] N. Abdehagh, F.H. Tezel, J. Thibault, Separation techniques in butanol production: challenges and developments, Biomass Bioenergy 60 (2014) 222–246, http://dx.doi.org/10.1016/j.biombioe.2013.10.003. [7] L.M. Vane, A review of pervaporation for product recovery from biomass fermentation processes, J. Chem. Technol. Biotechnol. 80 (2005) 603–629, http://dx.doi.org/10.1002/jctb.1265.
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Kragl, Separation properties of supported ionic liquid–polydimethylsiloxane membrane in pervaporation process, Desalination 241 (2009) 182–187, http://dx.doi.org/ 10.1016/j.desal.2007.12.050. [26] P. Izák, W. Ruth, Z. Fei, P.J. Dyson, U. Kragl, Selective removal of acetone and butan-1-ol from water with supported ionic liquid–polydimethylsiloxane membrane by pervaporation, Chem. Eng. J. 139 (2008) 318–321, http:// dx.doi.org/10.1016/j.cej.2007.08.001. [27] N.L. Mai, S.H. Kim, S.H. Ha, H.S. Shin, Y.M. Koo, Selective recovery of acetone– butanol–ethanol from aqueous mixture by pervaporationn using immobilized ionic liquid polydimethylsiloxane membrane, Korean J. Chem. Eng. 30 (2013) 1804–1809, http://dx.doi.org/10.1007/s11814-013-0116-6. [28] F. Santangelo, A. Wentink, A. Górak, W.R. Pitner, M.M. Schulte, Purification of biofuels using ionic liquids, in: Proceedings of ISEC 2008 – International Solvent Extraction Conference, Tucson, AZ, United States, 2008, pp. 931–936. [29] M. Stoffers, A. Górak, Continuous multi-stage extraction of n-butanol from aqueous solutions with 1-hexyl-3-methylimidazolium tetracyanoborate, Sep. Purif. Technol. 120 (2013) 415–422, http://dx.doi.org/10.1016/ j.seppur.2013.10.016. [30] K. Kraemer, A. Harwardt, R. Bronneberg, W. Marquardt, Separation of butanol from acetone–butanol–ethanol fermentation by a hybrid extraction– distillation process, Comput. Chem. Eng. 35 (2011) 949–963, http:// dx.doi.org/10.1016/j.compchemeng.2011.01.028. [31] F. Santangelo, Ionic liquids as extraction solvents for biobutanol purification, PhD thesis, TU Dortmund University, 2013. [32] S. Heitmann, Membrane-assisted downstream processing for biobutanol purification, PhD thesis, TU Dortmund University, 2015.
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Application of supported ionic liquid membranes (SILMs) for biobutanol pervaporation P. Rdzanek a,⇑, S. Heitmann b, A. Górak a, W. Kamin´ski a a b
´ ska 213, 90-924 Łódz´, Poland Lodz University of Technology, Faculty of Process and Environmental Engineering, ul. Wólczan TU Dortmund University, Laboratory of Fluid Separations, Emil-Figge-Str. 70, 44227 Dortmund, Germany
a r t i c l e
i n f o
Article history: Received 30 September 2014 Received in revised form 26 February 2015 Accepted 9 March 2015 Available online xxxx Keywords: Pervaporation n-Butanol Poly(ether block amide) (PEBA) Silicone
a b s t r a c t n-Butanol is a promising future biofuel. It can be produced in the so-called Acetone–Butanol–Ethanol (ABE) fermentation. The main drawback of this kind of fermentation is the toxicity of n-butanol towards the production strains. The concentration of butanol in the fermentation broth does not usually reach 2 wt.%. Biobutanol can be recovered continuously from the fermentation broth using pervaporation (PV), thus overcoming toxicity of n-butanol against microorganisms. To improve the separation efficiency of the membranes, supported ionic liquid membranes (SILMs) are used, in which ionic liquid (IL) is immobilized in the active layer of PV membranes. Two ionic liquids, namely trihexyl(tetradecyl)phosphonium tetracyanoborate (P6,6,6,14 tcb) and 1-hexyl-3-methylimidazolium tetracyanoborate (Im6,1 tcb) are immobilized by inclusion in a polyether block amide (PEBA) polymer matrix, which was covered by an additional silicone layer. Pervaporation experiments for these membranes were carried out at 37 °C using quaternary ABE mixtures with n-butanol concentrations up to 3 wt.% in the feed solution. It was found that permeate fluxes as well as selectivity of the SILMs can be influenced by immobilization of different ILs in the membranes. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction A large number of studies are focused on the production of sustainable and renewable liquid biofuels from biomass [1–3]. Biobutanol is one of the biofuels that is currently receiving considerable attention in the scientific and industrial community. Butanol is very promising when used as a gasoline additive, especially when compared to ethanol. The energy density of gasoline (32 MJ/L) is similar, to butanol (29.2 MJ/L). Butanol is less corrosive than ethanol and can be blended with petrol at any ratio without the need to modify the engine [4]. Biobutanol production belongs to important engineering challenges and the production technology is not as advanced as for bioethanol. The main problem in biobutanol production is the butanol toxicity to the clostridia microorganisms, which limits butanol concentration in fermentation broth to less than 2 wt.% [5]. This affects the process efficiency and makes the overall cost of butanol production higher compared to costs of other fuels (bioethanol, biodiesel or biomethane). By integrating a separation and a fermentation process the biobutanol can be continuously remove from fermentation broth [6–8]. ⇑ Corresponding author. E-mail address: [email protected] (P. Rdzanek).
In Table 1 are presented advantages and disadvantages of alternative separation processes for biobutanol purification like distillation, gas-stripping, adsorption, etc. [9–11]. Liquid–liquid extraction is promising method for butanol recovery from fermentation broth. Conventional solvents like oleyl alcohol (OA) [12] or novel components such as ionic liquids [13] as extractants can be used. However, extraction turned to be not suitable for industrial applications because of high costs of advanced solvents. Also efficient solvent recovery is challenging. Pervaporation can overcome these drawbacks and is considered as alternative to extraction. The effective use of pervaporation for butanol separation depends to a large extent on the properties of the membrane used [14]. The membrane permeability is mainly determined by the solubility and diffusivity of the permeating component in the membrane [15,16]. Selection of a suitable component forming the active part of the membrane is a crucial issue. This element improves the selectivity for n-butanol in the membrane. Several methods of membrane modification that are used to improve biobutanol separation are reported in the literature [17,18]. One of the most promising methods combining extraction with pervaporation is IL immobilization in the membrane [19]. Supported ionic liquid membranes (SILMs) are hybrid materials, in which two functionalities are integrated: an improved solubility of ionic liquids and higher selectivity of membrane.
http://dx.doi.org/10.1016/j.seppur.2015.03.024 1383-5866/Ó 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: P. Rdzanek et al., Application of supported ionic liquid membranes (SILMs) for biobutanol pervaporation, Separ. Purif. Technol. (2015), http://dx.doi.org/10.1016/j.seppur.2015.03.024
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Table 1 Advantages (+), disadvantages ( ) of butanol recovery processes [9–11]. Processes
Characteristics
Advantages/ Disadvantages (+)/( )
Distillation
High energy demand Large wastewater streams Large amounts of water
Gas-stripping
Simple design Use of fermentation gases Low selectivity Foam formation
+ +
Adsorption
Variety of adsorbent materials/ high selectivity Fouling Very low adsorbent capacity for butanol
+
Membrane reactor
Poor mechanical strength Leakage of cells from matrices
Extraction
Low energy demand Large amount of solvent required Toxicity issues
+
Pervaporation
Selective separation at low butanol Concentrations Energy efficient Small amounts of IL Simple and flexible experimental setup Membrane costs and properties
+
1.5% and 3 wt.% in the feed. Temperature and concentrations were chosen to represent the conditions in a conventional ABE fermentation. 2. Experimental
+ + +
Since amounts of IL, immobilised in the membrane is low, operation costs of biobutanol recovery from fermentation broth using SILMs are considerably lower than those using liquid–liquid extraction. Immobilization of ILs in membranes offers an interesting opportunity to produce membranes with different properties. ILs immobilization in the active layer of membranes may improve separation properties of conventional polymeric pervaporation membranes. Stable immobilization of IL is crucial for technical applications of SILMs. Elution of IL from the membrane remains still a challenge, preventing large scale applications of SILMs. Usually, ionic liquids [20] are defined as non-flammable, nonvolatile and chemically stable and often used in separation processes [21] as tuneable solvents. Basic properties of ILs have been summarized in the literature [22,23]. In last decades, the ILs appear to be an expanding group of high-potential compounds due to unique thermodynamic properties. Different ILs have been studied for the biobutanol recovery. Kohoutova et al. [24] used benzyl-3butylimidazolium tetrafluoroborate ILs. Heitmann et al. [19] immobilized in membranes 3 different types of ionic liquids 1-decyl-3-methylimidazolium tetracyanoborate (Im10,1 tcb), trihexyltetradecylphosphonium tetracyanoborate (P6,6,6,14 tcb) and 1-decyl 3-methylimidazolium tris(pentafluoroethyl) trifluorophosphate (Im10,1 fap). They achieved permeate flux of 560 g/(m2 h) and concentration of n-butanol in permeate of 55 wt.%. Izak et al. [25,26] investigated membranes with 1-ethenyl-3-ethyl-imidazolium hexafluorophosphate and tetrapropyl-ammonium tetracyanoborate ionic liquids which significantly improved the diffusion coefficients of 1-butanol in comparison with the neat PDMS membrane. Mai et al. [27] examine membrane with a [Tf2N] as ILs and the enrichment factor of butanol was three times higher compare to pure PDMS. The aim of this study was to examine recovery of n-butanol through pervaporation from four-component mixture using SILMs. We produced and investigated double layers membranes with IL immobilized in the active layer. Pervaporation process was conducted at 37 °C for the four component mixture of Acetone–Butanol–Ethanol–Water with n-butanol concentration of
2.1. Materials Two different ionic liquids, namely trihexyl(tetradecyl)phosphonium tetracyanoborate (P6,6,6,14 tcb) and 1-hexyl-3-methylimidazolium tetracyanoborate (Im6,1 tcb) (Fig. 1) provided by Merck KGaA were used to immobilized ILs in the membranes. Polysulfone membranes PS20 (Sepro Membranes Inc, pore size > 0.10 lm) and polypropylene (PP) membranes (Accurel PP, Membrana GmbH) were used as a support. Poly(ether block amide) (PEBA 2533) were kindly supplied by PRO-plast Kunststoff GmbH, Darmstadt. The one-component, fast-cure silicone (TSE 399C) purchased from Momentive Performance Materials. For preparing ABE mixtures acetone (99.9%), ethanol (99.5%) and n-butanol (99.9%) was acquired from Sigma Aldrich, Germany or Merck Schuchard OHF, Hohenbrunn. Acetic acid (96%), n-heptane (>99%) and acetonitrile (99.9%) were purchased from VMR International GmbH, Darmstadt. 2.2. Membrane preparation We produced SILMs in two different arrangements (1) and (2) of membrane. In both arrangements two membranes were placed on top of each other in the membrane cell. The difference in those two approaches is the silicone layer orientation presented in Fig. 2. The membranes with and without IL were prepare the same way. The arrangement (1) consists of two separated layers, PEBA + IL and silicon layer oriented to the feed side. Therefore, both membranes active layer (PEBA + IL, silicone) are oriented to the feed side. For the arrangement (2) PEBA + IL is located to feed side while the silicon layer is oriented to the permeate side. This causes silicone layer directly contacts the PEBA + IL layer. For arrangements (1) and (2), the PEBA was dissolved in acetic acid with ratio of 20:80 (wt.%/wt.%) in glass vials. The vials were placed in an oven at 80 °C for 24 h. 30 wt.% of IL was added to the solution after dissolution of PEBA in the acetic acid. Two ILs were taken into consideration. Selection of ILs were based on liquid–liquid extraction results [13,28, 29] of n-butanol from water solutions (Fig. 3). Subsequently, the homogeneous PEBA-acetic acid-IL mixture was poured onto the support membrane which was polysulfone membranes. A homogeneous active layer was produced by coating the polysulfone PS20 membranes using a coating knife. The membranes were left for 24 h at room temperature to let the acetic acid evaporate. The thickness of the coating layer were varied between 9 and 13 lm. The silicon coating was produced either on PS20 in arrangement (1) or on PP membrane in arrangement (2). In the latter case it was important that the porous support layer oriented to the feed side does not influence the separation efficiency of SILMs, therefore porous PP membrane was used as support instead of PS20 membranes. To minimize the mass transfer resistance the silicone layer needs to be as thin as possible. Silicone was mixed with heptane in ratio of 10:90 (wt.%/wt.%) to reduce the thickness of the silicone layer. Then heptane–silicone mixture was poured out into the membrane and placed in a vertical position. The average thickness of coating was about 5 lm. The membranes were left for 24 h at room temperature to let the heptane evaporate and to allow curing of the silicone. As an example SEM image of membrane structure PS20 as a support with immobilized P6,6,6,14 tcb is presented in Fig. 4. The
Please cite this article in press as: P. Rdzanek et al., Application of supported ionic liquid membranes (SILMs) for biobutanol pervaporation, Separ. Purif. Technol. (2015), http://dx.doi.org/10.1016/j.seppur.2015.03.024
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Fig. 1. Ionic liquids used for membrane preparation.
SB,W = DB / DW
Fig. 2. Arrangement of double layer membranes.
600
2.3. Pervaporation process
500
Manufactured membranes were cut in circular samples with a diameter of 13 cm and mounted into the module, which is the heart of pervaporation plant (Fig. 5). Flat circular membrane module (Helmholz–Zentrum Geesthacht) has an effective membrane area of 104 cm2. Feed mixtures were aqueous solutions of three organic components acetone, butanol and ethanol in a 3:6:1 wt.% ratio with n-butanol concentrations of 1.5 and 3 wt.%. The experiments were carried out continuously in a quasi-steady state conditions. The pervaporation operating conditions are the feed flow rate 30 L/h, the permeate side pressure 10 mbar and the feed temperature 37 °C. Feed samples were taken at the beginning and at the end of each experiment. The permeate (in average 2.2 g) was collected in a cooled trap using liquid nitrogen and the retentate was recycled to the feed vessel. Because the permeate volume collected is small compared to the total feed volume, a quasi-steady state in the experiments can be assumed. The permeate mass collected in a specific time was evaluated by weighing the cooling trap before and after each experiment with and without permeate. To measure the concentration of components in the permeate acetonitrile was added to the sample as a tracer. The composition of the permeate was analyzed by gas chromatography (Shimadzu 14a) using flame ionization detection at 250 °C. The injection volume was 5 lL. The
400 300
Im₆,₁ tcb
200
P₆,₆,₆,₁₄ tcb
100 0
Oleyl alcohol
0
1
2
3
4
DB = wB,IL /wB,aq or wB,OA /wB,aq Fig. 3. Distribution coefficient and selectivity for biobutanol extraction from aqueous solutions [28–31].
top is the PEBA + IL coating and the bottom one is the PS20 support.
Fig. 4. SEM images of membrane coated with a mixture of PEBA and P6,6,6,14 tcb.
Fig. 5. Scheme of the pervaporation laboratory plant.
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chromatograph was equipped with a capillary column (FSInnopeg-FFAP). All concentrations were determined threefold to exclude an influence of the sample preparation on the analytical results. The mean analytical error concerning concentration measurement was assessed to be ±2.3%.
3. Results and discussion Two different arrangements of SILMs were tested in the experimental setup [32]. The membranes with IL were weighted before and after PV experiments to confirm if the IL was leached from the membrane. In all cases gravimetric experiments confirm that the immobilization of IL was stable. Results obtained with double layer SILMs produced according to arrangement (1) are shown in Figs. 6 and 7. Fig. 6 shows partial permeate fluxes of butanol, acetone and ethanol for two ILs. Fig. 7A shows concentrations of butanol in permeate and Fig. 7B total permeate flux. The concentrations of butanol in permeate were slightly higher for membranes containing P6,6,6,14 tcb, while for Im6,1 tcb-based membranes the total permeate flux was higher. If we follow the selectivities and solubilities, measured in extraction (Fig. 3), we would expect that the mass fraction of n-butanol in permeate should be higher for the P6,6,6,14 tcb than for the Im6,1 tcb. However, the partial fluxes of butanol, acetone and ethanol for different ILs are almost the same but the permeate fluxes of water are much higher for Im6,1 tcb than for P6,6,6,14 tcb (Fig. 6A and B). Consequently, differences in the total permeate fluxes and concentrations for the SILMs containing different ILs are observed (Fig. 7). Fig. 6C presents partial fluxes for the membranes without ILs which are similar to the partial fluxes for membranes with ILs. Understanding of the mass transfer through the SILM in arrangement (2) layers is more complicated then in arrangement (1). The partial fluxes of butanol, acetone, and ethanol were different for different ILs (Fig. 8). The butanol permeate fluxes increase when using Im6,1 tcb, but the acetone and ethanol fluxes were slightly higher for P6,6,6,14 tcb.
250
In Fig. 8C the butanol flux for the membranes arrangement (2) without ILs is presented. In that case the butanol fluxes are significantly smaller for the membrane without immobilized IL. Fig. 9A shows concentrations of butanol in permeate and Fig. 9B displays the total permeate flux. While comparing arrangements (1) and (2), differences in partial fluxes of components were observed for P6,6,6,14 tcb, while partial fluxes for arrangements (1) and (2) for the Im6,1 tcb were almost the same. The partial flux of butanol for the arrangement (2) for P6,6,6,14 tcb was 170 g/m2 h while it was 220 g/m2 h for the (1) arrangement. Differences in total flux for both arrangement result from different water flux, which is higher for the membrane with Im6,1 tcb. Table 2 shows the differences in water fluxes for both arrangements. The water fluxes based on calculation for 3 wt.% of butanol in feed are significant lower for the arrangement (2). Therefore, comparing Figs. 7A and 9A show a slightly (10–20%) higher butanol concentration in the permeate. When a hydrophilic IL like Im6,1 tcb is immobilized it can cause an increase of the water flux.
4. Conclusions Two different arrangements of SILMs were produced and tested in pervaporation plant. It was shown that immobilization of ILs can influence the permeation properties of pervaporation membranes dependent on the properties of the immobilized ILs. Dependent on the arrangement for immobilization partial permeate fluxes and – connected to this – permeate concentration of a component can be influenced by choosing different ILs. The butanol concentration in permeate (after pervaporation separation with immobilized IL for arrangement (2)) was 10 time higher compare to feed. The total permeate flux for first arrangement was about 0.80 kg/ m2 h while for second 0.70 kg/m2 h. The butanol partial flux for (1) arrangement for Im6,1 tcb was 0.210 kg/m2 h and for P6,6,6,14 tcb was 0.220 kg/m2 h. For the (2) arrangement, the butanol partial flux for Im6,1 tcb was 0.210 kg/m2 h and for P6,6,6,14 tcb was 0.170 kg/m2 h. We cannot observed differences in partial flux for
250
A Ji [g/(m2h)]
Ji [g/(m2h)]
B
200
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150 100
50
50
0 0
0.01
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C
Ji [g/(m2h)]
200 150 100 50 0 0.00
0.01
0.02
0.03
0.04
wi,Feed [g/g] Fig. 6. Partial permeate fluxes of ABE, containing 30% of Im6,1 tcb (A) or P6,6,6,14 tcb (B), without IL (C).
Please cite this article in press as: P. Rdzanek et al., Application of supported ionic liquid membranes (SILMs) for biobutanol pervaporation, Separ. Purif. Technol. (2015), http://dx.doi.org/10.1016/j.seppur.2015.03.024
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0.4
900
A
B
750
Jtotal [g/(m2h)]
wBuOH,Perm [g/g]
0.3 0.2 30 % Im₆,₁ tcb 30 % P₆,₆,₆,₁₄ tcb Without IL
0.1 0.0 0.00
0.01
0.02
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150 0 0.00
0.04
0.01
0.02
0.03
0.04
wBuOH,Feed [g/g]
wBuOH,Feed [g/g]
Fig. 7. Concentrations of butanol in permeate (A) and total permeate flux (B).
250
A
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150
150
J i [g/(m2h)]
Ji [g/(m2h)]
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C
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200 150 100 50 0 0.00
0.01
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wi,Feed [g/g] Fig. 8. Partial permeate fluxes of ABE, containing 30% of Im6,1 tcb (A) or P6,6,6,14 tcb (B), without IL (C).
A
800
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30 % Im₆,₁ tcb 30 % P₆,₆,₆,₁₄ tcb Without IL
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200 0 0.00
0.01
0.02
0.03
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wBuOH,Feed [g/g]
Fig. 9. Concentrations of butanol in permeate (A) and total permeate flux (B).
arrangement (1), but when the membrane PS20 were replaced for PP membrane we observed differences for partial fluxes for ILs for arrangement (2).
The arrangement (1) bears an additional mass transfer resistance caused by the porous support material, which separates the silicone layer and the PEBA-IL layer – so the impact of the IL immobilization can be seen more clearly for arrangement (2). An
Please cite this article in press as: P. Rdzanek et al., Application of supported ionic liquid membranes (SILMs) for biobutanol pervaporation, Separ. Purif. Technol. (2015), http://dx.doi.org/10.1016/j.seppur.2015.03.024
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Table 2 Water fluxes for arrangements (1) and (2). Arrangement
IL
Water fluxes (g/m2 h)
(1)
Im6,1 tcb
476 454 472
P6,6,6,14 tcb
373 305 386
Without IL
332 285 327
Im6,1 tcb
325 328 325
P6,6,6,14 tcb
232 229 227
Without IL
221 229 221
(2)
influence of the IL-selectivity on the solvent-to-water selectivity is observed for the double layer SILMs produced in this work. Thanking into account share of water flux in total flux we concluded that the arrangement (2) is more suitable for the butanol separation because the water flux is smaller. The selectivity for the membranes with and without IL is almost the same but the butanol flux is higher for the membrane with IL for arrangement (2). It is supposed that the interactions between the polymer, IL and feed cause that partial fluxes are similar or different. One main obstacle to be overcome is elution of ILs into the liquid feed solution. So far an additional polymer (silicone) coating was necessary to able a stable immobilization. It causes additional mass transfer resistance and therefore this coating must be sufficiently thin. Acknowledgements The research was financed from the National Science Center funds Granted on the basis of decision 2012/07/B/ST8/03379. All ionic liquids used in this work were kindly supplied by Merck KGaA, Darmstadt. References [1] X. Chuang, Z. Xin-Qing, L. Chen-Guang, C. Li-Jie, B. Feng-Wu, Prospective and development of butanol as an advanced biofuel, Biotechnol. Adv. 31 (2013) 1575–1584, http://dx.doi.org/10.1016/j.biotechadv.2013.08.004. [2] J. Yu-Sin Jang, M. Alok, C. Changhee, L. Joungmin, L. Sang Yup, Butanol production from renewable biomass by clostridia, Bioresour. Technol. 123 (2012) 653–663, http://dx.doi.org/10.1016/j.biortech.2012.07.104. [3] N. Qureshi, T.C. Ezeji, Butanol, a superior biofuel production from agricultural residues (renewable biomass): recent progress in technology, Biofuels Bioproduct Biorefining 2 (2008) 319–330, http://dx.doi.org/10.1002/bbb.85. [4] S.S. Merola, C. Tornatore, L. Marchitto, G. Valentino, F.E. Corcione, Experimental investigations of butanol–gasoline blends effects on the combustion process in a SI engine, Int. J. Energy Environ. Eng. 3 (2012) 1–14. [5] E.M. Green, Fermentative production of butanol—the industrial perspective, Curr. Opin. Biotechnol. 22 (2011) 337–343, http://dx.doi.org/10.1016/ j.copbio.2011.02.004. [6] N. Abdehagh, F.H. Tezel, J. Thibault, Separation techniques in butanol production: challenges and developments, Biomass Bioenergy 60 (2014) 222–246, http://dx.doi.org/10.1016/j.biombioe.2013.10.003. [7] L.M. Vane, A review of pervaporation for product recovery from biomass fermentation processes, J. Chem. Technol. Biotechnol. 80 (2005) 603–629, http://dx.doi.org/10.1002/jctb.1265.
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Please cite this article in press as: P. Rdzanek et al., Application of supported ionic liquid membranes (SILMs) for biobutanol pervaporation, Separ. Purif. Technol. (2015), http://dx.doi.org/10.1016/j.seppur.2015.03.024