Fouling of poly[-1-(trimethylsilyl)-1-propyne] membranes in pervaporative recovery of butanol from aqueous solutions and ABE fermentation broth

Journal of Membrane Science 173 (2000) 133–144

Fouling of poly[-1-(trimethylsilyl)-1-propyne] membranes in pervaporative recovery of butanol from aqueous solutions and ABE fermentation broth A.G. Fadeev a , M.M. Meagher a,∗ , S.S. Kelley b , V.V. Volkov c a

Biological Process Development Facility, University of Nebraska-Lincoln, 143 Filley Hall, Lincoln, NE 68583-0919, USA b National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO 80401-3393, USA c Topchiev Institute of Petrochemical Synthesis RAS, 29 Leninsky Prospekt, Moscow 117912, Russia Received 6 July 1999; received in revised form 14 February 2000; accepted 15 February 2000

Abstract Poly[1-(trimethylsilyl)-1-propyne] (PTMSP) dense films were evaluated for n-butanol recovery from aqueous solutions and acetone–butanol–ethanol (ABE) fermentation broth. Flux decline through PTMSP dense films due to polymer compaction under vacuum conditions, relaxation in alcohol and alcohol/water mixtures, and membrane contamination are discussed. Flux decline of a PTMSP film during pervaporation of 20 g/l BuOH/water mixture was fitted to the following function, y=At−b . PTMSP films change their geometry when exposed to alcohol and alcohol/water mixtures and then dried. As a result of the relaxation process, polymer film becomes thicker and denser, effecting membrane performance. PTMSP films that were treated with 70% iso-propanol/water show linear flux decline versus pervaporation time. Strong lipid adsorption seems to occur on the membrane surface when fermentation broth is used as a feed causing flux decline within short period of time. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Pervaporation; Fouling; n-butanol; PTMSP

1. Introduction Fermentation is an attractive method for production of chemicals and fuels from renewable resource. Unfortunately, fermentative products are present at low concentrations and conventional separation methods are inefficient and energy consuming. Therefore, development of economical processes for product recovery from fermentation broth is necessary. In ∗ Corresponding author. Tel.: +1-402-472-2342; fax: +1-402-472-1693. E-mail address: [email protected] (M.M. Meagher)

contrast to the ethanol (EtOH) fermentation, the final concentration of n-butanol (BuOH) in the fermentation broth is 3–4 times lower (15–20 g/l) [1], which makes recovery of BuOH by distillation cost prohibitive. Energy required for BuOH recovery by distillation at concentration <5 g/l exceeds the energy content of the recovered product [2]. Until the 1950s, all BuOH was produced via a fermentative process. Later, butanol bio-production started to decline because less expensive BuOH became available by chemical synthesis [3]. For a fermentative process to become economically attractive, an efficient butanol recovery process must be devel-

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oped. A significant number of approaches have been tested to recover BuOH from aqueous streams including extraction, adsorption, gas stripping, vacuum distillation, reverse osmosis, perstraction, membrane distillation, and pervaporation. None of these technologies offers significant advantages over others [4]. Considering pervaporation, there is currently no polymeric membrane available, that has flux and selectivity high enough to make the technology cost-effective. Use of a silicalite-filled polydimethylsiloxane (PDMS) membrane [5] provided noticeably improved BuOH separation over pure PDMS membrane with separation factors over 100, but low fluxes still restrict industrial application. Another attractive polymer for n-butanol recovery is poly[1-(trimethylsilyl)-1-propyne] (PTMSP) [6]. Due to its high permeability the polymer has been extensively studied for pervaporation of ethanol/water solutions [7–17]. Surprisingly, very little data is provided on BuOH recovery using PTMSP membranes [18] even though the polymer is more selective to BuOH than EtOH. Membrane fouling was observed with ethanol fermentation broth (Clostridium thermohydrosulfuricum) as a feed [11]. The authors suggested that the polymer did not withstand organic acids present in the fermentation broth. It was later reported, that for pervaporative separation of Saccharomyces cerevisiae and Zymomonas mobilisfermentations, PTMSP membrane flux decline occurs primarily due to polymer free volume contamination with big organic molecules [19], such as ethylacetate, iso-propanol, 3-methyl-1-butanol, 2 methyl-1-butanol. The bulky molecules have diffusion coefficient much lower than that of ethanol and restrict total flux through the membrane. The objective of this research was to investigate PTMSP flux decline during the pervaporative recovery of BuOH from aqueous solutions and fermentation broth.

2. Experimental 2.1. Membrane preparation Poly[1-tirmethylsilyl)-1-propyne] was synthesized according to Masuda [6] using TaCl5 and NbCl5 catalysts. The reaction was carried out at room tempera-

ture and yields were about 85%. The reaction was terminated with a toluene/methanol mixture after 12 h. Polymer was dissolved in toluene and precipitated in methanol several times, filtered and air-dried. Polymer films, 15–32 mm thick, were fabricated by spreading a PTMSP solution in toluene (2–5 wt.%) on a glass plate using a casting knife. The polymer solution was air-dried at room temperature for 3 days. Polymer films were peeled off the glass by applying deionized water at the polymer/glass interface. A 75 mm thick polydimethylsiloxane (PDMS) membrane (MEM-100TM ) was purchased from MemPro. Corp. (www.mempro.com). 2.2. ABE fermentation Clostridium acetobutylicum ATCC 824 [5] was grown in batch culture. A spore suspension (3–6 ml) was heat shocked for 3.5 min at 75◦ C and cooled for 1 min at 1–0.5◦ C prior to inoculating 2 l of media containing yeast extract (5 g/l), tryptone (5 g/l), sodium acetate (5 g/l), glucose (60 g/l). Fermentation was carried out in anaerobic chamber at 37◦ C for 3–4 days. Final BuOH concentration was 4–7 g/l. 2.3. Pervaporation Pervaporation experiments were performed on a custom stainless steel pervaporation setup (Fig. 1). Feed was circulated through the membrane cell with a peristaltic pump at 1.2–1.3 l/min. Feed temperature was controlled with a heat exchanger in the feed line and heating tape around the feed vessel. Membrane was supported by a sintered porous metal plate (Pall Co.) incorporated into high density 1/4 in. polyethylene plate. The membrane was sealed between the polyethylene plate and upper cell manifold using a buna O-ring providing an active membrane area of 109 cm2 . Vacuum directly under the sintered porous metal plate was maintained at lower than 0.01 Torr using KTC-112 (Kinney Vacuum Co.) oil vacuum pump and monitored with McLeod vacuum gauge. Vacuum traps were cooled with dry ice or dry ice/ethanol mixture. There were two vacuum traps placed between vacuum pump and permeate trap to prevent any possible contamination of PTMSP membrane with vacuum oil. Flux was determined by weighting the collected

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Fig. 1. Schematic representation of pervaporation system. (1) liter fermentor (feed tank); (2) peristaltic pump; (3) heat exchanger; (4) membrane cell; (5) membrane; (6) metal porous support; (7) vacuum gauge; (8) throttling valve; (9) vacuum valve; (10) permeate trap; (11) dry ice/ethanol bath; (12) vapor trap; (13) vacuum pump suction.

permeate. Feed and permeate were analyzed by gas chromatography using Shimadzu 17A GC with Stabilwax 15 ft, 0.53 ID capillary column (Restek Corp.) and flame ionization detector. Permeate consisting of two phases was diluted with DI water to a homogeneous solution prior to GC analysis. Separation factor was calculated according the following equation α=

[CBuOH /CH2 O ]permeate [C BuOH /C H2 O ]feed

where C is concentration (g/l), C represents a mean concentration. Feed composition was changing during the experiment and mean concentrations were used to estimate separation factor.

a paper towel and placed on the 37◦ C metal surface to dry. After total solvent evaporation, geometric measurements were taken with a micrometer and ruler. Change in polymer volume was calculated based on the measurements. v=

Vo Vn

where Vo is a volume of fresh PTMSP film, Vn is polymer volume after ‘swell-dry’ cycle. Polymer sample volume was estimated as follows V =L×W ×T where L, W and T are the polymer sample length, width, and thickness, respectively.

2.4. Relaxation procedure 3. Results Polymer strips with dimensions of 100 mm×30 mm were cut from a polymer film having a thickness of 32 ␮m. Each sample was weighted on an analytical balance with a sensitivity 0.1 mg and then immersed into either an alcohol or alcohol/water mixture. When swollen (after 3–5 min), the film was wiped dry with

Temperature dependencies of flux and selectivity of fresh cast PTMSP, NbCl5 synthesized, dense film (16 ␮m) and PDMS membrane (75 ␮m) are presented in Fig. 2. Pervaporation measurements were conducted using 500 ml of 20 g/l BuOH/water mixture. Change

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Fig. 2. Temperature dependencies of flux and separation factor of PTMSP membrane (16 ␮m) compared to PDMS (75 ␮m) membrane in BuOH/water pervaporation. (䉱 PDMS flux; 䊏 PTMSP flux; 4 PDMS separation factor; 䊐 PTMSP separation factor).

in BuOH concentration during each of the experiments did not exceed 3 g/l. Flux through both membranes exponentially increased with temperature and flux through the PTMSP film was about 10 times higher regardless of the feed temperature. Selectivity of the PDMS membrane increased linearly with temperature and permeate compositions were 41% BuOH at 26◦ C and 51% of BuOH at 62◦ C. Selectivity of PTMPS membrane reached a maximum value (135) at 37◦ C. Membrane selectivity values at 26 and 62◦ C were close, 80 and 84, respectively. The lowest permeate composition was 62% BuOH at 62◦ C and highest was 72% BuOH at 37◦ C. The performance of a 15 ␮m PTMSP dense film with a 20 g/l BuOH/water mixture is presented in Fig. 3. The film was cast from polymer synthesized using TaCl5 catalyst and was ‘on the shelf’ for approximately 0.5 year prior to the experiment. The initial flux was 376 g/m2 h, and both, flux and selectivity decreased over time PTMSP fouling was modeled with a simple semi-empirical model, which has been widely used for reverse osmosis [20]

J = J0 t n ,

n < 0.0

where J is the fouled flux, J0 the initial flux and t represents time. The model was chosen because PTMSP has a porosity similar to RO membranes, ≤1 nm [21–23]. Fitting the flux decline data in Fig. 3 resulted in the following equation: J = 557t−0.30 . To investigate short-time membrane fouling in the presence of fermentation broth, the membrane was subjected to ABE fermentation broth two times during the pervaporation run (Fig. 3). In the first case 500 ml of fermentation broth was circulated through the membrane cell for 6 h without a vacuum applied to the membrane. Next, the membrane and feed lines were washed with 0.5 M NaOH solution and rinsed with DI water at room temperature. The membrane was then evaluated with a 20 g/l BuOH/water solution and the flux dropped from 205.8 to 129.0 g/m2 h and the permeate composition decreased from 543 to 353 g/l of BuOH. After approximately 15 h of operation using a BuOH/water mixture, the flux and separation factor reached their respective trend lines. At this point the feed was switched to ABE fermentation broth again

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Fig. 3. Effect of membrane exposure to ABE fermentation broth on pervaporation of BuOH/water mixture (20 g/l, 500 ml volume) through PTMSP (15 ␮m) at 23◦ C. (䉱 flux; 4 BuOH in permeate).

and the broth was circulated across the membrane cell for about an hour under vacuum. In this case the fermentation broth was washed from membrane cell with just DI water and no subsequent NaOH rinse. Again the flux and permeate compositions changed from 171 to 131 g/m2 h and 520 to 407 g/l, respectively, and gradually increased to the trend lines (Fig. 3). The effect of swelling and shrinking on fresh cast PTMSP film was evaluated (polymerization catalyst was NbCl5 ) (Fig. 4). A new membrane was evaluated with a 20 g/l BuOH/water solution and the flux decline equation was J = 488t−0.32 curve. The film was then removed from the membrane cell and immersed into 100% BuOH for 2–3 min and then into DI water. Film shrinkage occurred in 1–2 min as the butanol was extracted by the water. The film reduced in size and became wrinkled as a result of the non-uniform shrinkage. The film was mounted into the membrane cell and flux decline was monitored using 20 g/l BuOH (Fig. 4). Initial permeability of the film was three times higher, 1209 g/m2 h, but it declined t2 times faster (J = 1156t−0.60 ) compared to untreated film (Fig. 4). After 10–15 h of operation time the flux

and selectivity of the film were similar to the untreated membrane. To understand the influence of PTMSP shrinkage on membrane properties, polymer films were treated with different alcohols and alcohol/water mixtures. Swollen films were air dried on a 37◦ C surface until constant weight. Change of polymer film geometry after immersion into ethanol and butanol are presented in Fig. 5. As a result of the procedure, polymer film became smaller in dimensions, its thickness noticeably increased, and volume of the polymer sample slightly decreased. Volume change was 1.3 and 2.4% when film was treated with EtOH and BuOH, respectively, while thickness of the polymer increased by 9 and 12% at the same time. Pervaporation properties of PTMSP film kept in 70% i-PrOH for several days were investigated (Fig. 6). Flux decline was linearly dependent on the pervaporation time when 20 g/l BuOH/water mixture used as a feed. After 12 h feed was switched to ABE fermentation broth adjusted to 20 g/l BuOH concentration. Data on long-term membrane fouling is shown in Fig. 6. As soon as BuOH/water feed

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Fig. 4. Pervaporation of BuOH/water mixture (20 g/l, 500 ml volume) using fresh and immersed in BuOH PTMSP membrane (15 ␮m) at 23◦ C. (䉬 PTMSP flux; 䊉 PTMSP after BuOH, flux; 䉫: PTMSP selectivity; 䊊: PTMSP after BuOH, selectivity).

Fig. 5. Change in PTMSP geometry due to ‘swell-dry’ procedure using EtOH and BuOH. (4 thickness; 䉫 volume; 䊐 length; 䊉 width).

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Fig. 6. Pervaporation of BuOH/water mixture (20 g/l, 500 ml volume) and ABE fermentation broth using pre-swollen PTMSP membrane (15 ␮m) at 23◦ C. (䉬 flux; 4 BuOH in permeate).

mixture was switched to ABE broth, the flux decline was non-linear. After 20 h in the presence of fermentation broth the flux gradually leveled off at about 100 g/m2 h. The effect of long chain fatty acids, sodium salts of stearic and palmitic acid, PTMSP performance was evaluated (Table 1). First, a baseline condition was determined using the standard 20 g/l BuOH. Next, sodium stearate and palmitate were added at 0.5 mM/l, respectively, to 20 g/l BuOH, followed by pervaporation for 2 h. The system was rinsed several times with Table 1 Effect of stearate and palmitate on PTMSP performancea Step Feed

Flux % BuOH Separation (g/m2 h) (permeate) factor

1 2

587 32

20 g/l BuOH 20 g/l BuOH with 0.5 mM/l sodim stearate and 0.5 mM/l sodim palmitate 20 g/l BuOH

3 a

Feed temperature was

37◦ C.

23

48.6 8.6

13.1

46.3 4.6

7.4

DI water and a final flux measurement was taken with the standard 20 g/l BuOH. After addition of the fatty acids the flux dropped from 560 to 32 g/m2 h (Table 1). The water rinse did not recover any significant amount of the flux or selectivity. PTMSP films were analyzed by FTIR-ATR spectroscopy. Spectra of as-cast film, film used in BuOH/water separation only (both sides), and film soaked in ABE fermentation broth were generated (Fig. 7). Spectra of the film soaked in ABE fermentation broth and of the film used in BuOH/water separation (feed side) had low intensity picks in the range of 1625–1775 cm−1 . There was no absorption observed in the range for as-cast film and the film used for BuOH/water separation only (vacuum side).

4. Discussion Low flux rate is a common drawback of most pervaporation membranes. PTMSP attracts a lot of attention as it is one of the most permeable polymers known [24]. As shown in Fig. 2, PTMSP outperforms PDMS

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Fig. 7. FTIR-ATR spectra of PTMSP membranes.

in both flux and selectivity. Selectivity reached a maximum at 37◦ C, a similar observation was reported by Masuda et al. [7,13] for a 10% ethanol/water. They observed a maximum separation factor at approximately 30◦ C and attributed this to a change in membrane swelling condition. At this time we do not have an explanation why 37◦ C is the optimum, except that it is the ideal temperature for integrating directly to fermentation. Others have also observed the decline of PTMSP membrane flux and selectivity during pervaporation of EtOH/water [8,10,12,14]. Masuda et al. [13] reported no change in PTMSP membrane permeability using a 10 wt.% ethanol/water feed over 40 h at 30◦ C and 0.1 mm Hg pressure. PTMSP flux decline has been more extensively studied during gas permeation [25–29]. Flux decline was attributed to a loss of non-equilibrium free volume by the polymer or loss in polymer free volume interconnectivity due to polymer relaxation, oxidation, and contamination. Robeson et al. [25] cleaned the feed gas stream by first passing the stream through a packed adsorption column filled with PTMSP and observed a very slow linear flux decline, which was

thought to be due to free volume relaxation. Robeson et al. results suggest that significant flux decline within a short period of time was caused by some components of the feed contaminating the PTMSP membrane. The same two factors, membrane contamination and loss of free volume, will also effect the pervaporation of ogranics from aqueous solutions. Membrane contamination is very difficult to control due to PTMSP high sensitivity to contaminants. On the other hand, plasticizing action of contaminants may facilitate polymer free volume relaxation. This interdependence makes the PTMSP flux decline phenomenon very difficult to investigate. Deterioration of membrane properties in the presence of contaminates should be attributed to the polymer’s open porosity. In reverse osmosis, where membranes have a microporosity (<1 nm), comparable to PTMSP, special pretreatment of the feed is required to minimize membrane fouling [30]. To address this issue, PTMSP flux decline was fitted to a power function to discriminate between differences in flux decline due to changes in polymer morphology or contamination.

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According to Robeson et al. [25], permeability of a 5 ␮m PTMSP film gradually declined to 75% of the initial value over a 6-months period. It was expected that the 6-month old PTMSP membrane used in this study would have aged resulting in a decreased flux rate. In contrast, the initial flux values and the flux decline kinetics of both the old (376 g/m2 h, y = 557x−0.30 , Fig. 3) and new (333 g/m2 h, y = 488x−0.32 , Fig. 4) films were very similar, showing that 6-month shelf life did not affect the initial membrane performance. Interaction between polymer and permeating molecules is much stronger in pervaporation than in gas permeation. Therefore, a higher degree of free volume loss (polymer compaction) within shorter periods of time should be anticipated. One also should always keep in mind the possibilities of membrane contamination originating from feed mixture or even from sealing material [31]. Higher flux of PTMSP films immersed in alcohol and then dried, suggests that PTMSP samples with more open structure were obtained [25]. It was noted that the open porosity obtained in this way collapses over time faster than that formed during film casting. As shown experientially for methanol immersed film, very high initial gas (oxygen and nitrogen) permeability declined and reached permeability of untreated film after 1 month [25]. In our studies, film immersed into BuOH for a few minutes and then into DI water to remove BuOH from the film, showed t2 times faster decline of initial permeability (y = 1156x−0.60 ) which reached values of untreated film in about 20 h of pervaporation time. The result is very similar to the one reported by Robeson et al. [25], indicating that densification of PTMSP during pervaporation occurs much faster as compared to gas permeation. Beside more open PTMSP structure BuOH ‘swelldry’ procedure causes significant structural change in polymer film. Non-uniform film shrinkage may reflect polymer film stress during the casting procedure. As shown in Fig. 5, films became smaller in dimension, thicker and more dense. This result contradicts higher permeability of ‘swelled-dried’ film. More dense and thicker film should have lower permeability. Redistribution of free volume within the film is likely to occur during ‘swell-dry’ treatment, resulting in a loss of some portion of the free volume (BuOH being a better PTMSP solvent causes higher free vol-

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ume loss) and the rest should be comprised of fewer, but bigger pores. This explanation is supported by a lower initial selectivity of treated film (Fig. 4). Some pores apparently have dimensions too big to provide separation based on an exclusion mechanism [22,32]. The pores gradually collapse over time leading to higher separation factors. Formation of bigger pores could be related to fast rate of alcohol removal from the polymer. When swollen polymer film is transferred from BuOH to water most of the shrinkage occurs within first 10–15 s. When polymer was treated with 70% iso-propanol/ water mixture without following solvent removal step, initial high permeability was not observed, Fig. 6, and flux decline of the film was linear over time investigated. These data indicate that increase in permeability of solvent-immersed PTMSP films is caused by a fast solvent removal. Conditioning of PTMSP film in alcohol/water causes some minor changes in polymer free volume distribution leading to linear mode of flux decline. Similar changes in polymer free volume should occur when fresh cast PTMSP film is brought in contact with the BuOH feed. Relatively high and selective alcohol absorption by the polymer [13] could induce structural changes in polymer even in diluted alcohol solutions. PTMSP membrane conditioning in alcohol/water seems to be an important step to more stable membrane performance. Non-linear flux decline of PTMSP membrane should be related to a fast loss of some of the polymer free volume formed during the casting procedure. Polymer conditioning in alcohol/water may promote more uniform free volume distribution. Linear dependence of flux decline versus time under pervaporation conditions for PTMSP film relaxed in alcohol/water, Fig. 6, may reflect gradual membrane compaction. Indeed, direct measurements of membrane thickness showed that part of the membrane subjected to vacuum became thinner (14–15 ␮m) compared to the thickness (16 ␮m) of the membrane part which was extending beyond the cell sealing. Tanimura et al. [16] investigated separation properties of PTMSP in pervaporation and reverse osmosis using alcohol/water solutions. He did not observe flux decline in reverse osmosis studies and suggested that flux decline during pervaporation might be associated with vacuum condition on the permeated side.

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Fig. 8. Linear permeation rate decline vs. pervaporation time. (䉬 Volkov et al. [15]; 䊏 data from Fig. 6).

Flux decline during EtOH/water pervaporation can be described by a linear function (Fig. 8) [14]. Compared to BuOH/water pervaporation, a similar but less pronounced linear flux decline is seen. BuOH is a better solvent than ethanol (higher plasticising action) for PTMSP causing PTMSP compaction under pervaporation. Flux declines faster through thinner PTMPS films (about 20 ␮m) than it does through thicker ones (50 ␮m) during pervaporation [33]. A more steep pressure gradient across a thinner membrane causing faster membrane compaction seems to be a logical explanation if membrane contamination does not occur. Similar observation was made during gas permeation studies using 5 and 50 ␮m PTMSP films [25]. Based on resistance in series model, the authors concluded that permeability would drop faster for thinner films. When fermentation broth was applied to the membrane, flux decline was modeled to the following equation, y = 1857x−0.84 , which shows that flux declined t1.5 times faster than the flux of BuOH treated membranes, suggesting membrane contamination. Since the presence of media, glucose and fermentation products such as n-butanol, ethanol, acetone, acetic and

butyric acids did not significantly affect membrane performance (data not shown), it was concluded that fouling components are biomolecules produced during the fermentation adsorbing on the membrane surface. Internal membrane fouling of the membrane was disregarded. The reason was the fact that there were no components other than ethanol, butanol, acetic and butyric acids observed in the permeate when ABE broth was used as a feed. Any component that is able to penetrate into PTMSP membrane should eventually appear on the vacuum side of the membrane and be present in the permeate. The data shown in Fig. 3 also support the idea of membrane surface contamination. Drop in membrane flux after short exposure of the membrane to fermentation broth suggests membrane contamination was gradually removed by fresh batches of 20 g/l BuOH/water feed. This suggests that some of the components of the fermentation broth form an adsorption layer on the membrane surface restricting flux through the membrane. This is supported by the fact that the contamination occurs within a short period of time and is independent of pervaporation conditions. Washing the membrane with DI water only did not

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restore the membrane. The presence of BuOH appears to facilitate the removal of the contaminants from the membrane, indicating that the adsorbed compounds are not soluble in water. Among the biomolecules which are likely to be present in fermentation broth and are soluble in alcohols but not in water are lipids. Pervaporation of BuOH is the presence of the Na salt of two fatty acids, stearate and palmitate, resulted in a dramatic flux loss (Table 1). It is suggested that changes in membrane properties were due to the preferential adsorption of fatty acids to the membrane surface blocking BuOH and water from entering the membrane. Stearic and palmitic acids are the major building blocks of such lipids as acylglycerols, phospholipids and steroids. Palmitic is also one of the most frequently found fatty acids in bacteria [34]. Cellular membranes begin to loose their integrity when butanol concentration reaches 10 g/l [35]. Disruption of cellular membrane makes lipids from membrane cells more available. Data presented in Fig. 6 were obtained with ABE broth containing 20 g/l BuOH, which was at the maximal limit of toxicity to the microorganism.Bearing in mind that only 0.001 M/l sodium stearate in the feed caused more than 90% flux decline, even trace amounts of lipids in the feed would cause noticeable flux decline through PTMSP membrane. Similar to experiments with ABE fermentation broth, washing the membrane with DI water did not restore membrane separation properties, implying a high stability of the adsorbed layer. According to FTIR data in Fig. 7 film exposed to ABE fermentation broth was contaminated with ABE components having carbonyl groups. Some contamination was found on a feed side of the film used in BuOH/water pervaporation. The contamination was less pronounced. Total removal of contamination from films used in alcohol BuOH/water was achieved by soaking contaminated films in excess of ethanol. Similar cleaning procedure applied to ABE exposed film caused significant, but not entirely complete removal of the contaminates.

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posed to 100% BuOH, followed by fast removal of the butanol have more open structure resulting in a very high initial flux value, which declines rapidly reaching values of an untreated film after about 20 h of pervaporation. Exposure of PTMSP film with alcohol/water mixtures does not cause a dramatic change in polymer morphology and showed a linear flux decline versus pervaporation time. Studies indicate that conditioning the PTMSP membrane in an alcohol/water mixture with a composition similar to pervaporation feed is an important step in achieving better membrane performance. Exposure to fermentation broth causes dramatic flux decline of PTMSP films. That fouling phenomenon may be caused by a strong adsorption of lipids on the membrane surface. Very low concentrations of stearate and palmitate (0.0005 M/l each) caused a 10-fold reduction in flux and selectivity. In order to retain high membrane performance these contaminants must be removed from the feed or an effective membrane cleaning procedure must to be developed. Even suffering severe flux and selectivity decline in the environment of ABE fermentation broth PTMSP dense films still show at least four-fold higher flux than commercial PDMS membrane at the same selectivity after 30 h of operation time, Figs. 2 and 6. PDMS membrane could be ultimately a better choice for continuous operation times extending to months and years. However, periodic membrane cleaning is a normal practice for most of membranes applications. Progress is being made in developing fast and efficient PTMSP membrane cleaning treatment, that would allow to maintain PTMSP properties on high levels.

Acknowledgements This work was funded by the National Corn Growers Association in conjunction with the Nebraska Corn Board and National Renewable Energy Laboratory. References

5. Conclusions Flux decline of toluene-cast PTMSP films during pervaporation of 20 g/l BuOH follows an exponential time-dependent model, y=At−b . PTMSP films ex-

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