ABE fermentation products recovery methods—A review

Renewable and Sustainable Energy Reviews 48 (2015) 648–661

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

ABE fermentation products recovery methods—A review Anna Kujawska a,n, Jan Kujawski b, Marek Bryjak b, Wojciech Kujawski a a b

Nicolaus Copernicus University in Toruń , Faculty of Chemistry, 7 Gagarina Str., Toruń, Poland Wroclaw University of Technology, Chemical Faculty, 27 Wybrzeże Wyspiańskiego Str., Wrocław, Poland

art ic l e i nf o

a b s t r a c t

Article history: Received 25 February 2014 Received in revised form 11 March 2015 Accepted 3 April 2015 Available online 29 April 2015

Butanol has a great potential as a biofuel and to date a lot of research has been done both in terms of more efficient butanol production as well as in developing product recovery methods. Many of them deal with separation techniques which can be used for selective recovery of acetone, n-butanol and ethanol from model solutions and fermentation broths. This work is a review of techniques used for ABE recovery, such as distillation, adsorption, gas stripping, liquid–liquid extraction, pertraction, membrane distillation, sweeping gas pervaporation, thermopervaporation and vacuum pervaporation. Advantages and disadvantages of using particular methods, examples of applications and integrated processes are also described. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Butanol production Separation techniques Butanol recovery Pervaporation

Contents 1.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 648 1.1. n-Butanol as a biofuel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 648 1.2. ABE fermentation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 649 2. Separation techniques for ABE fermentation products recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 650 2.1. Distillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 650 2.1.1. Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 651 2.1.2. Gas stripping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 651 2.1.3. Liquid–liquid extraction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 652 2.1.4. Pertraction (membrane extraction). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 653 2.1.5. Reverse osmosis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 653 2.1.6. Membrane distillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 653 2.1.7. Pervaporation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 654 2.2. Integration of n-butanol fermentation with various removal techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 658 3. Final remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 659 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 659 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 659

1. Introduction 1.1. n-Butanol as a biofuel The biofuels production is nowadays one of the main approaches in developing a sustainable economy [1]. According to

n

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http://dx.doi.org/10.1016/j.rser.2015.04.028 1364-0321/& 2015 Elsevier Ltd. All rights reserved.

data presented by U.S. Energy Information Administration [2] total World biofuels production in 2001 was equal to 54,511 m3 day
1, whereas in 2011 production of biofuels reached a value of 304,587 m3 day
1 and 302,290 m3 day
1 in 2012. This means that production of biofuels increased during 10 years by more than five times. n-Butanol has been in production since 1916, mostly as a solvent feedstock. Nowadays, after decades of stagnation, new applications for n-butanol are becoming popular, such as a fuel enhancer and basic feedstock for chemical industry [3]. n-Butanol

A. Kujawska et al. / Renewable and Sustainable Energy Reviews 48 (2015) 648–661

Nomenclature ABE AGMD Ai, Aj Bi, Bj DCMD Dp EBA ETBE IL Jt MAVS MD MPCs MTBE NMP

acetone, n-butanol, ethanol air gap membrane distillation molar or weight fractions of components i and j in permeate [g g
1] or [mol mol
1] molar or weight fractions of components i and j in feed [g g
1] or [mol mol
1] direct contact membrane distillation pore diffusion coefficient [m2 s
1] expanded bed adsorption 2-ethoxy-2-methylpropane (ethyl tert-butyl ether) ionic liquid total flux in pervaporation [g m
2 h
1] membrane assisted vapour stripping membrane distillation mesoporous carbons 2-methoxy-2-methylpropane (methyl tertbutyl ether) n-methyl-2-pyrrolidone

PDMS PDMSM PE PEBA PP PPO PSf PSI PTMSP RO SGMD SGPV TAME TAEE THF TPV VMD VOC VPV

β

649

poly(dimethylsiloxane) poly(dimethylsilmethylene) polyethylene poly(ether block amide) poly(propylene) poly(dimethylphenyleneoxide) polysulfone Pervaporation Separation Index [kg m
2 h
1] poly[1-(trimethylsilyl)-1-propyne] reverse osmosis sweeping gas membrane distillation sweeping gas pervaporation 2-methoxy-2-methylbutane (tert-amyl methyl ether) 2-ethoxy-2-methylbutane (tert-amyl ethyl ether) tetrahydrofuran thermopervaporation vacuum membrane distillation volatile organic compounds vacuum pervaporation separation factor in pervaporation [–]

Table 1 Comparison of fuels [5–8]. Parameter
1

Energy density [MJ L ] Vapour pressure at 20 1C [kPa] Motor octane number Boiling point [1C] Freezing point [1C] Air-fuel ratio Density at 20 1C [g mL
1]

Petrol

Diesel

Methanol

Ethanol

n-Butanol

32 0.7–207 81–89 27–225 o
60 14.6 0.74–0.80

39–46 o0.07 – 180–343
40 to
9.9 – 0.829

16 12.8 97–104 64.5–65
97 to
97.6 6.5 0.787

19.6 7.58 102 78–78.4
114 to
114.5 9.0 0.785

27–29.2 0.53 78 117–118
89.3 to
89.5 11.2 0.810

is 4-carbon alcohol that can be produced by fermentation of the biomass [4]. Comparing with methanol and ethanol (Table 1), nbutanol is a more complex alcohol, possessing several advantageous characteristics: higher heating value, lower volatility, less ignition problems, higher viscosity and is safer for distribution [4]. Moreover, n-butanol can be blended with petrol at any ratio. Furthermore, using butanol as a fuel enables reduction of NOx emission and soot creation in exhaust gases [3,4]. Ni and Sun [9] listed eleven Chinese companies producing nbutanol by Clostridium strains. Moreover, two other companies were under construction, one was designed and another two are planned. Such significant development of ABE production plants is attributed to a very high n-butanol demand in China [9]. In 2012, Saudi Kayan, Sadara Chemical and Saudi Acrylic Acid Company (SAAC) started to cooperate in organization of a new company— The Saudi Butanol Company (Saudi Arabia). The company will be the first one producing n-butanol in the Middle East. The projected plant will be situated at Tasnee Petrochemicals Complex in Jubail Industrial City (Saudi Arabia). The factory will be operated by Tasnee and start of n-butanol production is planned in 2015. The designed capacity of the plant is 330,000 t of n-butanol and 11,000 t of iso-butanol annually [10]. SGBio Renewable (joint venture of GranBio (Brazil) and Rhodia (Belgium)) plan to build a biomass-based n-butanol installation in Brazil [11]. Biobutanol will be produced from bagasse and sugar cane straw. A pilot plant is planned to be built in 2015 to test their technologies, and based on the results of that research, a commercial scale plant will be built later on [12]. In VITO Company (Belgium) has been opened pilot installation for demonstration of acetone–butanol–ethanol fermentation integrated with in situ butanol removal technology by

pervaporation [13]. The project realised within this study is called Demonstration of In Situ Product Recovery (ISPR) to improve the fermentation processes productivity (DemoProBio). The demonstrated pilot plant will be operated in 50 and 150 L scale to assess energy requirements and process efficiency in ABE fermentation products recovery [13]. The biggest biobutanol facilities in the world, in operation or under construction, are marked in Fig. 1. 1.2. ABE fermentation n-Butanol can be produced during fermentation process performed by bacteria strain. The most popular strains used in nbutanol production are Clostridium strains such as Clostridium acetobutylicum, Clostridium saccharobutylicum, Clostridium beijerinckii and Clostridium saccharoperbutylacetonicum [14–16]. The comprehensive description of metabolism during acetone–butanol–ethanol fermentation can be found elsewhere [16,17]. The main problem associated with the ABE fermentation by bacteria is the self-inhibition of the process due to n-butanol toxicity to the culture. Mentioned toxicity of solvent to the culture and nutrient depletion during long time fermentation processes are two main factors caused premature termination of the fermentation [14]. Otherwise, bacterial n-butanol fermentation could be more efficient due to gene modification of bacteria already used in n-butanol production or by utilization of another bacteria strain, more tolerant to produced product. Moreover, fermentation process could be improved if new and cheaper substrates, such as hydrolysed lignocellulosic biomass, would be utilized. Additionally, reduction of the by-products production and

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A. Kujawska et al. / Renewable and Sustainable Energy Reviews 48 (2015) 648–661

Fig. 1. Butanol production plants in the world.

Table 2 Azeotropes during distillation of ABE–water mixture [21,54,55]. System

Azeotrope

Temperature of azeotrope [1C]

Water—nbutanol Water— ethanol

Heterogeneous 91.7–92.4

38.0

Homogeneous

4.4

78.1

Water content in azeotrope [wt%]

found that fermentation intermediates did not affect n-butanol concentration in the removed condensate stream [18]. Maddox et al. [20] tested possible inhibitory effects of high salt and sugar concentrations on the ABE fermentation process. Authors [20] found out that sugar concentrations up to 200 g L
1 can be fermented, however that salt concentrations greater than 30 g L
1 negatively affect bacterial growth.

2. Separation techniques for ABE fermentation products recovery application of advanced and sterile fermentation or downstream technology could positively affect fermentation process efficiency [3]. The highest n-butanol productivity by the strain fermentation reported in literature is 3.0 wt% [4]. Qureshi and Blaschek [15] described that C. beijerinckii BA101 strains can produce solvent in the concentration range of 27– 29 g L
1. Properties of bacteria were examined in terms of impact of fermentation substrates and products inhibition to solvents production performance. It was shown that supplementing the fermentation medium with sodium acetate enhances production of solvents up to 33 g L
1. Pervaporation was combined with fedbatch reactors and applied as a separation technique for fermentation products recovery. Solvents production after applying recovery technique was 165 g L
1 [15]. Mariano et al. [18] described vacuum fermentation as a simple technique during which the desired product is removed due to vacuum applied directly to bioreactor. Products boil off at the temperature of fermentation and in subsequent step are recovered by condensation. Using this approach the low concentration of fermentation products is maintained in bioreactor during fermentation process and toxic impact of products to the microbes is minimalized. The technique was already successfully used in ethanol fermentation [19]. Mariano and co-workers [18] performed a research using vacuum fermentation to produce ABE fermentation products. Authors evaluated the technical feasibility of vacuum technique application in acetone–butanol–ethanol fermentation as well as the influence of vacuum on the fermentation performed by C. beijerinckii and products recovery rate. The performance of vacuum process was higher than for standard batch fermentation (production of 106.0 g vs. 80.6 g of n-butanol, and 132.4 g vs. 110.1 g total ABE, respectively). Moreover, application of vacuum for product recovery from fermentation resulted in a decrease of fermentation time, maximal utilization of glucose, and superior cell growth, and greater acetone, butanol, ethanol production comparing with control batch fermentation. It was also

Several techniques have been suggested for acetone, butanol and ethanol recovery from fermentation broth: distillation [21– 24], adsorption [1,25–27], freeze crystallization [28], gas stripping [14,20,29–32], liquid–liquid extraction [33,34], pertraction [35], reverse osmosis [28], membrane distillation [36–38], thermopervaporation [39], sweeping gas pervaporation [40] and vacuum pervaporation [41–52]. 2.1. Distillation Distillation is one of the well-known separation techniques in which separation occurs due to the difference of volatilities of separated components. When a mixture containing substances of various volatilities is brought to boiling, the composition of the vapours released will be different than content of solvents in the boiling liquid. There are several possible modes of distillation: continuous flash distillation, batch distillation, fractional distillation and steam distillation [53]. Aqueous acetone–butanol–ethanol mixture is a complex system, in which water–organic azeotropic mixtures can be formed during distillation (Table 2). Thanks to heterogeneous water-n-butanol azeotrope, a simple two-column distillation system can be used and no additional compound has to be added to separate mixture [55]. Conventional distillation for recovery of ABE fermentation products from batch fermentator was described by Roffler et al. [22]. Acetone, nbutanol and ethanol are heated to 100 1C by heat exchange and are removed from the broth by stream of vapours [22]. Obtained vapours contain about 70 wt% of water and 30 wt% of AcO, BuOH and EtOH. Subsequently, vapours are separated in series of four distillation columns. In the first column, operating at 0.7 atm pressure, about 99.5 wt% of acetone is removed, whereas residual, bottoms products of the first column are transported to so-called ethanol column, operating at 0.3 atm pressure. 95 wt% ethanol is

A. Kujawska et al. / Renewable and Sustainable Energy Reviews 48 (2015) 648–661

obtained in this step. Subsequently, the bottom products of ethanol column and overhead streams are redirected to decanter, where water and n-butanol are separated. The water phase, with ca. 9.5 wt% of n-butanol content, is transported to water stripper, whereas the n-butanol rich phase (with ca. 23 wt% water content) is redirected to n-butanol stripper. In the n-butanol stripper 99.7 wt% n-butanol is obtained [22]. Mariano et al. [23] described the flash fermentation, i.e. continuous fermentation with integrated ABE recovery. In this design, a bioreactor worked at atmospheric pressure, whereas broth was continuously circulated to a vacuum chamber. In the vacuum chamber acetone, n-butanol and ethanol were boiled off and condensed. Proposed technology allowed to produce 30– 37 g L
1 of n-butanol [23]. Luyben [24] reported pressure-swing azeotropic distillation as a method to separate compounds. In this technique two columns, operating at various pressures, were used for separation of liquid mixtures. In such module no additional compounds were required. The high-purity product streams were removed, whereas the streams of the composition near azeotropic one, were recycled. It was also pointed out that higher pressure changes can markedly shift the azeotrope composition [24]. During n-butanol recovery by distillation, most of the energy consumption originates from the evaporation of the water in the feed. Additionally, a binary n-butanol–water azeotrope is obtained at 92.7 1C. Conversion of a feed containing 20 g L
1 n-butanol into an azeotropic mixture at 1 atm leads to a selectivity of 72 [56]. Matsumura et al. [21] provided analysis of energy requirement for n-butanol recovery by distillation. n-Butanol concentration before distillation was 0.5 wt%. Energy requirement to obtain 99.9 wt% of n-butanol by distillation was 79.5 MJ kg
1. Vane et al. [57] described that energy requirement to produce 99.5 wt % n-butanol is 14.5 MJ kg
1 by traditional distillation-decanter method. Green [58] claimed that distillation is a robust and proven process to be used for ABE fermentation products recovery, but this technique is energy intensive. It was stated that to produce 1 t of solvents, about 12 t of steam is necessary. The author [58] suggested that improvements can be made to traditional distillation but investigation of nonconventional methods should be also performed to reduce ABE production costs. Distillation is nowadays the most popular technique used in industry for ABE fermentation product recovery. However, this technique possesses several disadvantages such as high investment costs, high energy consumption and low selectivity [28]. Due to this fact other products recovery techniques are being investigated.

2.1.1. Adsorption Adsorption is described as a process in which particles from a liquid or gas mixture are preferentially attached on a solid surface [59]. Levario and co-workers [25] investigated adsorption of ethanol and n-butanol on mesoporous carbons (MPCs) with surface areas ranging from 500 to 1300 m2 g
1. It was found that nbutanol was adsorbed more efficiently compare to ethanol on each of tested mesoporous carbons. It was also found that capacity of alcohol adsorbtion increased with an increase of adsorbents surface area. Moreover, applied mesoporous carbons were thermally and chemically stable during performed measurements [25]. Lin et al. [1] applied macroporous adsorption resin (KA-I) with a crosslinked polystyrene framework as adsorbent for n-butanol removal from acetone-n-butanol–ethanol–water quaternary mixture at various concentrations of organics. Ratio of organic compounds equal to 3:6:1 of acetone, n-butanol and ethanol, respectively was maintained constant. KA-I resin selectively adsorbed n-butanol, whereas acetone and ethanol were less adsorbed compounds. It was reported that increase of temperature enhanced adsorption capacity

651

and rate of n-butanol removal [1]. The effective pore diffusion coefficients (Dp) at 10 1C and 37 1C were 0.251  10
10 m2 s
1 and 4.31  10
10 m2 s
1, respectively. Additionally, the results obtained fitted well with the Langmuir isotherm equation [1]. Authors found that effective Dp is temperature dependent, but uninfluenced by initial n-butanol content. The total maximal amount of n-butanol adsorbed per mass of wet resin up to saturation of KA-I resin was found to be 139 mg g
1 at 10 1C and 304 mg g
1 at 37 1C [1]. Some other adsorbents reported in literature with n-butanol adsorption capacities are: high silica zeolite CBV28014 (116.0 mg g
1) [26], ZSM-5 (160.8 mg g
1) [60] and silicalite (97.0 mg g
1) [61]. Sharma and Chung [27] described development of a new zeolite to be utilized in preparation of mixed matrix membranes. The authors [27] also presented adsorption potentials of the mentioned materials during n-butanol recovery. The highest obtained capacity towards butanol adsorption of MEL6 zeolite type material was 222.24 mg g
1 at 30 1C. Oudshoorn et al. [26] investigated n-butanol adsorption by three various commercial high silica zeolites (CBV28014, CBV811C300 and CBV901) in the presence of ethanol and acetone in aqueous mixtures and fermentation broth. The surface areas of the silica zeolites were equal to 400, 620 and 700 m2 g
1, whereas pore volumes of tested particles were equal to 0.19, 0.24 and 0.50 cm3 g
1, respectively. CBV901 possesses the highest adsorption capacity for n-butanol among all zeolites tested in this study, whereas CBV28014 has the highest affinity towards n-butanol at the organic component content in water below 2 g L
1. It was also found that compounds were competitively adsorbed following the order: n-butanol 4acetone4ethanol. Wiehn et al. [62] applied expanded bed adsorption (EBA) method for the in situ removal of BuOH from ABE fermentation broth. Macroporous hydrophobic poly(styrene-codivinylbenzene) resin was used in this study as butanol adsorbent. After 38.5 h of process 27.2 g L
1 and 40.7 g L
1 of butanol and total solvents were produced, respectively. Efficiency of total solvent production was improved in expanded bed adsorption method 2.3-fold, compared to traditional batch fermentation. At the same time, butanol production was increased 2.2-fold. Authors [62] recovered ca. 81% of butanol from fermentation broth using EBA technique recovery. Liu et al. [63] used KA-I cross-linked polystyrene framework resin to recover butanol from fermentation broth. KA-I was chosen due to its good adsorbing properties towards butanol, butyrate, and acetone and high selectivity. The authors [63] developed an operation combining biofilm reactor with simultaneous product recovery by the KA-I resin. Obtained solvent productivity in such a module was 1.5 g L
1 h
1 and yield of solvent production was 0.33 g g
1. It was also shown that co-adsorption of acetone by the KA-I resin caused improvement of the fermentation process performance [63]. Although adsorbents used in adsorption technique possess high selectivity towards butanol over water [26], there are several problems during ABE fermentation products recovery by adsorption. One of them are difficulties in desorption of organic compound previously adsorbed on the sorbent—several separation methods should be used to realise this process. Additionally, bacteria can adhere to the adsorbent and decrease the adsorption efficiency, especially if the adsorbent is recycled [64,65].

2.1.2. Gas stripping Gas stripping is a separation method which enables selective removal of volatile components from ABE fermentation broth [14,29–32]. In this technique gas is sparged into the fermentor and volatiles are condensed and subsequently recovered from the condenser. Application of this technique is possible due to the volatile properties of the ABE.

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Ezeji et al. [29] tested the influence of various parameters, such as presence of acetone and ethanol, gas recycle rate and bubble size, on performance of ABE recovery from fermentation broth. It was found that application of sparger gas stripping mode resulted in creation of overmuch amounts of foam in bioreactor, which caused necessity of addition of more antifoam to compare with impeller module. The consequence of antifoam addition is reduced production of the fermentation products, which is explained as a toxic effect of antifoam to microbes. Authors found that gas recycle rates of 80 cm3 s
1 and constant gas stripping rate of 0.058 h
1 are sufficient to maintain the nbutanol concentration below toxic levels during the run of the ABE fermentation. Ezeji and co-workers [29] demonstrated that bubble sizes below 5.0 mm did not affect the stripping rate of nbutanol under the experimental conditions used in this study. It was also found that presence of acetone, and ethanol had no influence on n-butanol removal rate. Additionally, gas bubble size in diameter ranging from 0.5 mm to 5.0 mm was recommended to obtain good mass transfer and to overcome problems with overmuch foam creation in gas stripping process. Ezeji and co-workers [14] examined impact of gas-stripping on the in situ recovery of ABE fermentation products directly from batch reactor. An integrated batch fermentation experiment produced total ABE of 23.6 g L
1 and it was higher than for the nonintegrated process (17.7 g L
1). The authors [14] proved that gasstripping intensifies the selective recovery of acetone, butanol and ethanol from the fermentation broth and encourages effective assimilation of acids produced by the culture for conversion into solvents. Also it was shown that acids were not removed from the fermentation broth during gas stripping and the bacteria strain was not negatively affected by this removal method. Park et al. [30] employed a gas-phase-continuous immobilized cell reactor–separator concept (ICRS) for n-butanol production from ABE fermentation. Authors compared two modes of gas stripping reactor: immobilized cell reactor (ICR) and immobilized cell reactor separator (ICRS). In ICRS mode greater glucose consumption rate and higher n-butanol productivity could be obtained. The average glucose conversion was improved by 54.7% (from 19.63 to 30.36 g L
1) due to application of the product separation method [30]. Qureshi and Blaschek [31] reported that the adoption of gas stripping allows reducing n-butanol inhibition and due to this fact the application of gas stripping coupled with fermentation broth results in improving total solvent productivity and yield. Enhanced yield of fermentation can be obtained thanks to the fact that gas stripping does not remove intermediate products of the ABE production process. In conclusion, gas stripping removing nbutanol (and thus reducing fermentation product toxicity) can be performed within the fermentor without any negative influence on bacterial culture. Moreover, concentrated sugar solutions can be used during gas stripping coupled with fermentor [31]. Setlhaku et al. [32] tested properties of fermentor containing C. acetobutylicum ATCC 824 strain coupled with gas stripping set-up. Experiments were carried out at 35 1C, whereas ABE vapours were collected at
2 1C using 50:50 vol% of ethyl glycol–water mixture. Experiments were performed at a gas (nitrogen) circulation rate in the range of 4.8–6.6 L min
1. Authors obtained maximum performance of gas stripping equal to 72.9 g L
1 of acetone, n-butanol and ethanol at a third fed-batch fermentation. After 272 h of fermentation gas stripping was started and at that time glucose and butyric acid concentration in the reactor were equal to 0.2 and 1.7 g L
1, respectively. Liao et al. [66] tested influence of agitation speed, flow rate and type of non-polar gases on the performance of gas stripping. Stripping rate of butanol was proportional to butanol content in feed and a decrease in butanol selectivity was observed with the

increasing butanol concentrations up to 0.01 g cm
3. It was explained in terms of thermodynamics that more inert gas was dissolved at higher butanol concentration in feed. Higher quantity of gas dissolved in the solution resulted in a decrease of butanol activity. The authors [66] concluded that the best way for improving butanol recovery with gas stripping method is to perform process at high superficial velocity of gas bubbles, what results in the lower resistance on the liquid side. Among tested gases (N2, O2 and CO2), nitrogen was recommended as the best one for butanol recovery with gas stripping method (mass transfer coefficient equal to 17.4  106 s
1) [66]. The gas-stripping process possesses several advantages over other removal techniques, such as a simplicity and low cost of operation and its efficiency is not disturbed by fouling or clogging due to the presence of biomass [14]. Moreover, gases produced during the fermentation (CO2 and H2) can be used for ABE products recovery by gas stripping. Furthermore, only volatile products are removed from fermentation broth and due to this fact the reaction intermediates (acetic acid and butyric acid) are not removed from the fermentation broth and are converted almost entirely into ABE [20]. One of disadvantages is that tiny bubbles, produced in gas stripping, create excessive amounts of foam in a bioreactor. Such a process results in the necessity of addition of an antifoam agent, which can be toxic to bacteria. This, in turn, results in overall lower productivity of ABE fermentation [29].

2.1.3. Liquid–liquid extraction Liquid–liquid extraction is a method used to extract a dissolved substance from liquid mixture in a certain solvent, by another solvent [67]. Eckert and Schügerl [33] described application of continuously operated membrane bioreactor combined with a four-stage mixer–settler cascade in n-butanol recovery. BuOH was selectively extracted from the cell-free cultivation medium by butyric acid saturated n-decanol, and the n-butanol-free medium was re-fed into the reactor. Under steady-state conditions n-butanol concentration of 8 g L
1 and n-butanol productivity of 0.51 g L
1 h
1 were obtained. Unfortunately, the addition of ndecanol to the reactor strongly reduces the fermentation process productivity due to the poisoning of the cells. Due to this, contact of the cells with the n-decanol phase should be eliminated. Authors [33] checked mass of the cells before and after the experiment. It was shown that the mass of cells decreased after extraction. Such a phenomenon caused productivity decrease of ABE in the second cycle of experiments. Application of good extractants, such as n-decanol, to ABE fermentation broth for direct removal of n-butanol can cause destruction of bacteria strains in fermenter. To overcome this negative impact on bacteria, Evans and Wang [34] used combination of toxic decanol and nontoxic oleyl alcohol. Authors convinced that up to 40 vol% decanol in oleyl alcohol is nontoxic to bacteria growth. Increase of intermediate fermentation products concentration was observed at higher pH. At constant pH value equal to 4.5 an increase in n-butanol production with addition of decanol was observed. Approximately 90 mM of n-butanol was produced in system without addition of decanol, ca. 150 mM n-butanol at 0.3 vol% of decanol and approximately 40 mM of n-butanol were obtained when 0.4 vol% of decanol was present in system [34]. Kurkijärvi et al. [68] applied non-biocompatible solvents (1heptanol, 1-octanol and 1-decanol) during continuous extraction of ABE fermentation products in dual extraction process with solvent regeneration. Distribution coefficients of butanol recovery, obtained during experiments performed at 37 1C, were equal to 11.26, 9.95 and 7.17 for 1-heptanol, 1-octanol and 1-decanol, respectively. The authors [68] claimed that with this method the

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energy consumption of the ABE fermentation product recovery can be lowered to less than 4 MJ kg
1. Kurkijärvi and Lehtonen [69] described a dual extraction method utilizing petrol components as extraction solvents in ABE fermentation. This dual extraction method contains two extraction columns. In the first column nonbiocompatible solvents were utilized to extract effectively ABE products, whereas in the second column traces of the toxic solvent were removed from the broth to make it biocompatible. After the extractions the fermentation broth was recycled back to the reactor, thanks to that unfermented nutrients, reaction intermediates, and remaining products could be reutilized. To avoid migration of microbes to extraction column, immobilization and filtration steps were added to the process. The authors [69] claimed that product mixture of this process (ABE removed from broth and extractants) could be utilized as a petrol additive without purification steps. Simulation performed in this study showed that ETBE and MTBE were the most effective solvents for butanol recovery, followed by TAME and TAEE. However, ABE concentration in the end product was low (7.6 kg of butanol in 477.4 kg total amount of product, i.e. less than 16 g kg
1) [69]. Stoffers and Gorak [70] tested efficiency of butanol recovery by ionic liquid, 1-hexyl-3-methylimidazolium tetracyanoborate, during continuous multi-stage extraction in mixer–settler unit. The authors [70] obtained selectivity of butanol recovery towards water in the range of 48–89, whereas distribution coefficient for the tested system was 5.2–6.5. Moreover, extraction model, based on NRTL parameters from ternary mixture experimental data, was proposed. Based on the results it was stated that in an equilibrium approach of the multi-stage extraction model there is no need to model individual stage efficiency [70]. Comparing to other separation techniques, high capacity of the extractant and high selectivity of n-butanol/water separation can be obtained. The main disadvantage of using direct extraction in fermentation products recovery is the creation of emulsions and the extractant fouling. Such phenomena can result in problems with phase separation and consequently in significant contamination of aqueous streams with chemicals [71,72].

2.1.4. Pertraction (membrane extraction) Pertraction can be described as a liquid–liquid extraction technique in which a porous membrane is placed between the two phases [73]. Pertraction is a membrane process based on the same separation mechanism as extraction [74], where both extraction and stripping of the solute are realized in one unit [75]. Membrane extraction requires the installation of membrane area, which separates extracting liquid from the extractant. Grobben et al. [35] applied in-line solvent recovery for direct removal of acetone, n-butanol and ethanol from potato waste. Authors used C. acetobutylicum DSM 1731 strain to produce ABE broth. Fermentation broth was coupled with two modes of solvents recovery: direct pertraction and microfiltration combined with pertraction. Pertraction was performed using polypropylene fibre membranes and a mixture of 50:50 (vol%) of oleyl alcohol and decane was pumped through the fibres. In the second tested mode a cylindrical separation chamber containing a rotating cartridge equipped with polysulphone microfiltration membrane was applied. Compared to standard fermentation, application of pertraction resulted in increased productivity of ABE by 60% to 1.0 g L
l h
1, whereas the product yield based on dry weight was improved from 0.13 g g
1 to 0.23 g g
1. Experiments with fermentation coupled with microfiltration and pertraction showed that the initial ABE recovery through the membrane (0.55 g L
l h
1) was greater than the ABE productivity (0.38 g L
1 h
1). Such efficiency of the process allowed maintaining n-butanol concentration below the toxic level for a extended period of time comparing with standard fermentation.

653

Total production of ABE was equal to 27 g L
1 and product yield, based on quantity of consumed sugars, was equal to 32 wt% and was higher than for the control fermentation broth. Qureshi et al. [73] investigated pertraction mode coupled directly with fermentor at 35 1C. Silicone membrane as selective boundary and oleyl alcohol as extractant were used. Butanol production in the first cycle was 8.89 g L
1, whereas in the second operation cycle butanol productivity of 10.29 g L
1 was obtained. Although recovery of butanol was efficient, acetone removal from fermentation broth was poor (1.62 g L
1 in the first cycle) [73]. In another work Qureshi and Maddox [76] used oleyl alcohol as an extractant for recovery of acetone, n-butanol and ethanol. The solvents were produced by Clostridium strains from lactose. Butanol productivity was equal to 2.89 g L
1, whereas acetone and ethanol production efficiencies were 3.25 and 1.87 g L
1, respectively. Pertraction possesses some limitations such as lower masstransfer coefficients compared with liquid–liquid extraction and instability of hollow fibre modules in contact with solvent [71]. On an industrial scale, problems with extraction of membrane solvent may occur, due to the relatively high viscosity of extractants. Such difficulties resulted in pressure losses and mass transfer limitations in the solvent phase [72]. The major advantage of the pertraction method is that dispersion of the extractant in the solvent phase is unnecessary. Using membrane pertraction it is possible to connect selective membrane properties with the capacity of extractant [72]. Application of membrane as a barrier in pertraction mode minimizes passage of extractant into the aqueous phase and alleviates some common problems of the liquid–liquid extraction process, such as toxicity of extractant to the cells [73]. 2.1.5. Reverse osmosis Reverse osmosis (RO) is a membrane based technology commonly applied in desalination of water and production of potable water [77]. In RO semi-permeable membranes separate a feed solution into two streams: permeate (purified water) and concentrate (solution with salts and retained compounds) [77]. Polyamide membranes were described as good materials for BuOH recovery in RO (rejection rate r85%). Garcia et al. [78] obtained rejection rates in the range of 98% and the optimal rejection of BuOH in the ferment liquor occurred at recoveries of 20–45%. Flux varied in the range 0.05–0.60 dm3 m
2 min
1 [78]. Ito et al. [79] patented a method to separate highly pure butanol from a butanol-containing solution. The method assumes that in the first step of separation a nanofiltration of fermentation broth is performed. In a subsequent step, the filtered solution is sent to reverse osmosis module. Retentate contains two phases system enriched in butanol. The last step is recovery of butanol rich phase. This technique allows to obtain butanol–water mixture containing 80% of BuOH. Diltz et al. [80] utilized reverse osmosis (RO) method for a posttreatment of an anaerobic fermentation broth. Experiments were performed at 25 1C using six organic model compounds: ethanol, butanol, butyric acid, lactic acid, oxalic acid, and acetic acid. Efficiency of butyric acid, lactic acid, and butanol rejection was greater than 99% at a pressure of 5515.8 kPa, whereas acetic acid, ethanol and oxalic acid were rejected with efficiency in the range of 79–92% at a pressure of 5515.8 kPa. The rejection of organic components was improved when the fermentation broth was used as the feed stream, comparing with RO experiments performed for each component individually [80]. 2.1.6. Membrane distillation Membrane distillation (MD) is a process in which a microporous, hydrophobic membrane is applied to separate aqueous

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solutions at different temperatures [81]. Membrane distillation process is similar to conventional distillation: requires heating of the feed solution in order to obtain the necessary latent heat of vaporization and MD is based on the vapour/liquid equilibrium [82]. The temperature difference between both sides of microporous membrane results in a vapour pressure difference. Thanks to that vapour molecules are transported through the membrane from higher vapour pressure to lower vapour pressure side of the membrane. Membrane used in membrane distillation process should be highly porous (of porosity higher than 70%). Moreover, membrane wetting cannot occur and only vapours should be transported through the pores of the membrane [81]. Five various membrane distillation modes are described in literature: direct contact (DCMD), vacuum (VMD), air gap (AGMD), sweeping gas (SGMD) and osmotic (OMD) membrane distillation [36–38,82–87]. Most of membranes used in MD are manufactured from highly hydrophobic polymers, like polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyethylene (PE) and polypropylene (PP) [36,37,81,82,86]. Gryta et al. [86] tested properties of batch fermentation producing ethanol with membrane distillation recovery method. The authors used porous capillary polypropylene (PP) membranes to separate volatile organic compounds from fermentation broth, which was supposed to increase productivity/efficiency of fermentation process. The efficiency of fermentation broth combined with membrane distillation was 0.4–0.51 (g EtOH)/(g of sugar) and the production rate of 2.5–4 (g EtOH)/dm3 h. Banat and Al-Shannag [36] investigated recovery of acetone, nbutanol and ethanol from aqueous solutions by air gap membrane distillation with PVDF membrane. The authors found out that nbutanol was the most effectively removed compound. It was also shown that temperature, air gap width and compounds concentration affect the flux and selectivity of compounds recovery. Fluxes of compounds increase with feed temperature increase. Selectivity of acetone and ethanol recovery also increase with temperature. n-Butanol selectivity decreased from 5.8 to 2.4 as the coolant temperature increased from 10 1C to 30 1C. The best results were obtained for the lowest tested cooling temperature. According to mass transfer equations, air gap width has also a significant impact on membrane distillation process efficiency. Decrease of air gap width increases the transport but lowers selectivity of separation. Banat et al. [37] applied air-gap membrane distillation for separation of ethanol–water mixture by PVDF membranes. At 50 1C the highest obtained flux was 8.7  10
4 kg m
2 s
1, whereas selectivity remained between 2.5 and 3.1, within tested concentration range 0.83–10.2 wt% of EtOH. In that work [37] a mathematical model of transport was also proposed and experimental data was used to evaluate the accuracy of the model. The model including effect of temperature and concentration polarization fitted well the experimental data. Cooling liquid flow rate had no influence on obtained fluxes during MD experiments. It was also found that an increase of feed temperature results in increased flux as well as higher selectivity for all tested concentrations. The permeate flux was inversely proportional to the air gap width [37]. Rom et al. [87] developed vacuum membrane distillation model using AspenPlus software on the basis of the dusty gas model. The experimental data obtained for poly(propylene) (PP) membrane of 0.2 μm pore size in contact with water–butanol mixture were used as source of data for the determination of the component permeance and for extrapolation of data for model. It was found that implementation of the generated permeance functions in the programming code resulted in a unit operation of the programme. Authors [87] concluded that model showed good correlation with experimentally obtained results.

2.1.7. Pervaporation Pervaporation (PV) is a membrane separation technique for separation of binary or multicomponent liquid mixtures [88]. Transport through membrane occurs owing to the difference in chemical potentials between both sides of the membrane [89,90]. The difference in chemical potentials can be created by temperature difference (thermopervaporation—TPV), application of a sweep gas on the permeate side (sweep gas pervaporation—SGPV) and pressure difference (vacuum pervaporation—VPV) between both sides of the membrane. 2.1.7.1. Thermopervaporation (TPV). Thermopervaporation (TPV) is the least studied mode of pervaporation. Feed mixture is in direct and continuous contact with the membrane selective layer, whereas permeate is condensed on a cold wall at the atmospheric pressure [91,92]. Transport in TPV can be facilitated by increasing temperature difference and decreasing the distance between the membrane and the cold wall [39,93]. Franken et al. [91] proposed polysulfone (PSf) membrane to be used for ethanol recovery by thermopervaporation. Total flux obtained during experiments with PSf membrane was equal to 14.4 g m
2 h
1, whereas separation factor was 10, at 16.5 1C difference and 35 wt% of ethanol in feed [91]. Borisov et al. [39] investigated recovery of n-butanol by thermopervaporation using poly(1-trimethylsilyl-1-propyne) (PTMSP) membranes. Authors used in their experiments plateand-frame flowthrough module with an air gap. Application of thermopervaporation allows to decrease the dimension of the separation units; and to increase the condensation temperature of the permeate. Thanks to the mentioned advantages it is possible to reduce energy consumption of the separation process. Kujawska et al. [94] tested intrinsic properties of two commercially available PDMS based membranes (Pervap 4060 and Pervatech) in thermopervaporative recovery of acetone, butanol and ethanol from model aqueous solutions. Authors [94] obtained an increase of organic component transport and selectivity coefficient with increase of feed temperature during TPV experiments with water–acetone and water–butanol mixtures. Permeance of ethanol through both membranes was comparable, whereas significantly higher water transport was obtained during TPV experiments with Pervatech. Such a difference was attributed to different membrane preparation conditions [94]. 2.1.7.2. Sweeping gas pervaporation (SGPV). Sweeping gas pervaporation (SGPV) is pervaporation mode in which the permeant partial pressure on the permeate side is decreased by sweeping out the vapours with an inert gas stream. Hollow fibres are applied in SGPV, which allows obtaining a large surface area per volume ratio [95]. Nii et al. [95] developed a mass-transfer model for SGPV through polymeric hollow fibre (HF) membranes. Basing on the solution–diffusion–evaporation theory it was possible to assess the permeation rate of alcohol across the membrane. Diffusional resistance through the gas boundary film was accounted for in the model [95]. The model was applied to test properties of a rubbery hollow fiber membrane module by sweeping gas pervaporation at 304 K in contact with binary aqueous mixtures of ethanol and isopropanol. PDMS hollow fibre membranes were used in SGPV experiments and nitrogen was applied as the sweeping carrier gas. It was found that water permeation occurred independently of the alcohol permeation. The calculation of water flux using proposed model was provided and it was found that at lower gas velocity model did not fit well. Moreover, it was found that ethanol flux did not vary at liquid flow rates in the range of 100–500 cm3 min
1. It was concluded that liquid film resistance did not occur under the experimental conditions [95].

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Plaza et al. [40] applied sweeping gas membrane pervaporation for n-butanol recovery. Membranes prepared by gelation of an ionic liquid-1-butyl-3-methylimidazoliumhexafluorophosphate ([bmim][PF6]) in the pores of polytetrafluoroethylene (PTFE) hollow fibres were used in experiments. Partial flux of n-butanol was equal to 1300 g h
1 m
2 at 500 ppm n-butanol content in feed. Authors did not observe losses of IL during SGPV experiments, but membrane selectivity decreased after several hours of SGPV process. 2.1.7.3. Vacuum pervaporation (VPV). In vacuum pervaporation mode a driving force is created by vacuum on the permeate side of the membrane [88]. Liu et al. [41] provided a list of membranes used for ABE fermentation products recovery by vacuum pervaporation: poly (dimethyl siloxane) (PDMS), PDMS filled with silicate, ethylene propylene diene rubber (EPDR), styrene butadiene rubber (SBR), poly(methoxy siloxane) (PMS) and poly[-1-(trimethylsilyl)-1propyne] (PTMSP). PDMS can be also applied in sweep gas pervaporation mode as well as porous propylene (PP) and porous polytetrafluoroethylene (PTFE). Vane [42] described poly(dimethylsiloxane) (PDMS) membranes as the most popular separation barrier used in recovery of alcohols from water. PDMS is an elastomeric material which can be utilized for fabrication of hollow fibers, unsupported sheets and thin layer supported membranes. Separation factor for PDMS membranes in water–ethanol pervaporation is in the range of 4.4 to 10.8. Such broad range is a result of performance parameters for a given polymer and separation conditions. Reported separation factors of butanol recovery from n-butanol–water mixture for poly(dimethylsiloxane) also cover a fairly broad range, from 20 to 60, which is much wider than that of ethanol–water system [42,43]. Several PDMS membranes modified with octadecyldiethoxymethylsilane (M1) and poly(dimethylsiloxane) integrated with PTFE (M2) or PP (M3) support have been tested by Vane [42]. Selectivity coefficients for M1–M3 membranes were equal to 16.3, 14.0 and 12.6, respectively, in water–ethanol separation [42]. Other polymeric membranes have been also tested by pervaporation for selective recovery of ABE fermentation products or model solutions. Application of these membranes will be described more in detail in this section. To compare apparent properties of various membranes, separation factor (β) defined by Eq. (1) is used [96]:

β¼

Ai =Aj Bi =Bj

ð1Þ

where Ai, Aj are molar or weight fractions of component i and component j in permeate, respectively. Bi and Bj are molar or weight fractions of components in feed. Rozicka et al. [43] tested properties of PDMS based commercial membranes (Pervatech, Pervap 4060 and PolyAn) in contact with binary aqueous mixtures of acetone, n-butanol and ethanol at 25 1C. Authors obtained the best n-butanol recovery from water-nbutanol mixture during vacuum pervaporation with Pervap 4060 membrane. In this work apparent and intrinsic membrane properties were discussed in detail. It was shown that membranes are selective towards organic compounds; however, considering intrinsic membranes properties it was found that all tested PDMS membranes transport n-butanol the most selectively among all organic solvents used in this study [43]. Niemisto et al. [44] investigated properties of composite PDMS on polyacrylonitrile support membrane (Pervatech) for recovery of acetone, n-butanol and ethanol from binary, ternary and quaternary model aqueous solutions at 42 1C. At 3.5 wt% of n-butanol in feed separation factor was equal to 22 during PV experiments performed for n-butanol-water mixture, whereas at 3.23 wt%

655

BuOH content in acetone–butanol–ethanol–water mixture, separation factor was also equal to 22. It was also pointed out that organic compounds and water fluxes do not change significantly comparing binary, ternary or quaternary mixtures [44]. Kujawski et al. [97] tested properties of PDMS based membranes (Pervap 1070 and Pervatech) in recovery of acetone, butanol and ethanol from binary and quaternary aqueous mixtures by pervaporation. Separation factor for butanol removal from its binary aqueous mixture at 1 wt% of organic component in feed was equal to 27 and 40 for Pervap 1070 and Pervatech membranes, respectively. Membrane with the best pervaporative efficiency during VPV with binary water–butanol mixture (Pervatech) was chosen for next pervaporation measurements with quaternary aqueous mixtures at 65 1C. Separation factor of organics recovery from quaternary mixture was equal to 24.5, 27.6 and 8.5 for acetone, butanol and ethanol, respectively. The authors [97] performed also a simulation of batch pervaporation process. It was found that the needed to recover a given amount of organics is dependent on the feed volume to membrane area ratio. Moreover, it was claimed that longer duration of batch pervaporation process results in a more diluted permeate, what was explained by the decrease of organic components fluxes with the duration of batch pervaporation whereas water flux is practically constant. Liu et al. [45] tested properties of in situ crosslinked polydimethylsiloxane/brominated polyphenylene oxide (c-PDMS/ BPPO) membrane for n-butanol recovery by pervaporation. During PV experiments with PDMS/BPPO membrane in contact with nbutanol–water mixture total flux of 220 g m
2 h
1 and separation factor of 35 were obtained. Fouad and Feng [46] tested separation properties of a silicalitefilled PDMS (Pervap 1070) composite membrane adapted to remove n-butanol from dilute aqueous solutions containing nbutanol up to 0.5 wt% by vacuum pervaporation. Authors found out that water flux increased linearly with n-butanol content in feed. In the same experiment it was also shown that silicate fillers exhibit strong affinity to n-butanol particles because of increasing n-butanol flux. At 0.3 wt% of n-butanol in feed and 25 1C feed temperature BuOH flux of around 5 g m
2 h
1 and separation factor (β) of ca. 18 were obtained, whereas at 65 1C n-butanol flux was equal to around 16 and β was equal to ca. 10. Li et al. [47] tested properties of tri-layer PDMS composite membrane in contact with n-butanol by pervaporation. The tested membrane consisted of PDMS active layer and dual support layers of high porosity polyethylene (PE) and high mechanical stiffness perforated metal (PDMS/PE/Brass). With the feed solution of 2 wt% n-butanol in water at 37 1C, the PDMS/PE/Brass support composite membrane confers a total flux of 132 g h
1 m
2 and a separation factor of 32. It was also shown that the increase of the PDMS layer thickness results in improvement of separation factor values and decline of the total flux [47]. García et al. [48] tested efficiency of n-butanol recovery from its water salt solution by pervaporation. During VPV experiments the following commercially available membranes were applied: membrane with a selective layer composed of polysiloxane polymer (CELFA) and PERTHESE membrane of selective layer consisting of silicone elastomer with dimethyl and methyl vinyl siloxane copolymers. At tested concentration range (0–1.36 wt%) and at 40 1C separation factors of n-butanol recovery equal to ca. 56 and 39 were obtained for CELFA and PERTHESE membrane, respectively. Total fluxes obtained at 40 1C for PERTHESE and CELFA membranes were equal to 34 g m
2 h
1 and 366 g m
2 h
1, respectively. Liu et al. [41] found out that selectivity of poly(ether block amide) (PEBA2533) membrane during pervaporation in contact with water–ABE systems at 23 1C follows the order of n-butanol4 acetone 4ethanol. At 5 wt% of organic compound content in

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feed fluxes obtained for n-butanol, acetone and ethanol, were equal to 42.2 g m
2 h
1, 21.4 g m
2 h
1 and 13.1 g m
2 h
1, respectively. Separation factor was equal to 5.9, 3.3 and 2.5 for n-butanol, acetone and ethanol, respectively [41]. Tan et al. [49] tested n-butanol recovery by pervaporation in contact with composite membranes. The membranes were prepared by incorporation of ZSM-5 zeolite into poly(ether-blockamide) (PEBA). The effect of various content of zeolite in PEBA on pervaporation performance was tested. Inclusion of ZSM-5 zeolite results in decrease of the activation energy of n-butanol flux through the composite membrane. The authors [49] tested properties of PEBA membranes filled with 2 wt%, 5 wt% and 10 wt% of ZSM-5 in their structure. The best transport and selective properties were obtained for the membrane with 5% addition of zeolite filler to the membrane. In the work of Tan et al. [49] investigation of membrane performances at various temperatures was also presented. Obviously it was found that the highest fluxes were obtained at the highest temperature tested (45 1C in this work). Vrana et al. [50] used polytetrafluoroethylene flat sheet membranes to be used in pervaporation in contact with model nbutanol–water mixture and with model solutions of ABE products. Authors tested influence of feed temperature (in the range of 30– 55 1C) on pervaporation process performance in contact with binary and quaternary mixtures, finding that fluxes increase with the increase of feed temperature during experiments with nbutanol–water and ABE–water mixtures. Separation factor (β) obtained during PV with ABE–water solution also increased at higher feed temperature in whole tested temperature range, whereas the highest β value was reached during PV experiments at 45 1C (β ¼ 12.9) in contact with binary mixture and at 50 1C and 55 1C separation factor was equal to 9.9 and 5.2, respectively. It has been also pointed out that in contact with n-butanol–water system lower separation factors were obtained than in contact with ABE–water system, which was attributed to the fact that the presence of the ABE mixture enhances the flux and selectivity of the PTFE membranes [50]. Claes et al. [51] investigated properties of laboratory made silicasupported poly[1-(trimethylsilyl)-1-propyne] (PTMSP) membranes filled with silica in contact with binary aqueous mixtures of ethanol and n-butanol. The highest performance of vacuum pervaporation process was reached for PTMSP membrane containing 25 wt% of silica in its structure. During VPV experiments with 25 wt% silica filled PTMSP membrane in contact with ethanol–water (5 wt% of EtOH in feed) mixture, 9500 g m
2 h
1 flux and separation factor of 18.3 were obtained. Whereas during VPV measurements in contact with n-butanol–water (5 wt% of BuOH in feed) system, flux of 9500 g m
2 h
1 and separation factor of 104 were found. Tong et al. [52] tested properties of hydroxyterminated polybutadiene-based polyurethaneurea (HTPB-PU) by pervaporation in contact with dilute aqueous solutions of acetone and n-butanol. The increase of n-butanol separation factor value with increasing feed concentration was observed, however the reverse tendency was obtained for acetone. Authors [52] pointed out that the separation efficiency of the ternary mixture was better than that of the binary mixture at the same organic component content in feed. Such a phenomenon was attributed to the permeant–permeant and permeant–membrane interactions. The pervaporation performance for the fermentation broth was better comparing with the model solutions (ternary system) at similar feed composition [52]. Wei et al. [98] tested influence of PDMS chains length on the performance of PDMS/ceramic composite membranes for pervaporative recovery of ethanol from its aqueous solutions. The PDMS/ceramic composite membrane prepared using PDMS of the highest molecular weight possesses superior performance than membrane fabricated using poly(dimethylsiloxane) of lower molecular weight. The total flux and the separation factor of a

PDMS/ceramic (PDMS layer of 5 mm) composite membrane were 1600 g m
2 h
1 and 8.9, respectively, during VPV experiments performed at 40 1C feed temperature and 5 wt% of ethanol content in the feed solution [98]. Fadeev et al. [99] tested properties of PTMSP membranes during butanol recovery from aqueous solutions and ABE fermentation broth. Influence of feed temperature on VPV process performance was tested. The highest selectivity of PTMSP membrane in butanol recovery was obtained at feed temperature equal to 37 1C (β ¼135). In a subsequent paper, Fadeev et al. [100] tested also effectiveness of butanol recovery from diluted aqueous solutions by vacuum pervaporation. The authors [100] observed decline of flux through PTMSP membrane with duration of VPV experiments what was attributed to compaction of the membrane structure [100]. In another work Fadeev et al. [101] tested properties of PTMSP based membrane during pervaporative recovery of ethanol from model aqueous solutions and yeast fermentation broth. During VPV experiments performed in contact with model ethanol–water mixture separation factor of 9 and total permeate flux equal to ca. 320 g m
2 h
1 were obtained. Deterioration of PTMSP membrane performance in the presence of yeast fermentation broth was observed [99,101]. Borisov et al. [102] tested properties of poly[1-(trimethylsilyl)-1-propyne] membrane filled with poly(dimethylsilmethylene) (PDMSM) in pervaporative recovery of butanol. PTMSP/PDMSM modified membranes demonstrated better butanol/water pervaporation selectivity and permeability than native PTMSP membranes. Authors [102] claimed that introduction of 1.2 wt% of PDMSM into poly[1-(trimethylsilyl)-1propyne] membrane structure results in increasing permeate flux up to 75% and separation factor to 67% comparing with native PTMSP membranes. Improved pervaporative properties of PTMSP/ PDMSM modified membranes in butanol recovery were attributed to higher hydrophobicity of filled membranes [102]. Dubreuil et al. [103] utilized PTMSP membranes during pervaporative recovery of n-butanol directly from fermentation broth. Drop of permeate flux through PTMSP membrane during VPV process was obtained due to occurrence of significant membrane fouling by fermentation process intermediates. In order to diminish negative impact of fermentation bioproducts on pervaporation process performance, the upstream nanofiltration was applied. It was shown that the pretreatment of the fermentation mixture resulted in the improvement of separation factor by the factor 4 and increase of total permeate flux from 90 g m
2 h
1 (without pretreatment) to 370 g m
2 h
1 (with nanofiltration). Xue et al. [104] tested properties of PDMS–PVDF composite membranes in recovery of butanol from aqueous model mixtures and fermentation broth. Authors [104] observed a minor diminution of butanol separation factor during recovery of the component from quaternary aqueous mixture comparing with results performed with binary water–butanol mixture. It was attributed to preferential dissolution and competitive permeation of acetone and ethanol through the membrane. During pervaporation experiments with fermentation broth, butanol content in permeate and flux of the organic component maintained at a steady level within the range of 139.9–154.0 g L
1 and 13.3–16.3 g m
2 h
1, respectively. Kujawa et al. [105] tested properties of surface hydrophobized alumina and titania ceramic membranes during pervaporation of water–butanol mixture. Membranes surfaces were modified by grafting with 1H,1H,2H,2H-perfluorooctyltriethoxysilane and due to this membrane properties were changed from hydrophilic to hydrophobic. Modified membranes selectively transported butanol from its aqueous mixture (separation factor equal to 2). Concise summary of various membranes and conditions discussed above along with the pervaporation performances is presented in Table 3.

A. Kujawska et al. / Renewable and Sustainable Energy Reviews 48 (2015) 648–661

657

Table 3 Comparison of various hydrophobic membranes performance in pervaporative recovery of acetone, n-butanol and ethanol. Membrane Binary model mixtures Acetone–water HTPB-PU PDMS (Pervatech) PDMS (Pervap 4060) PDMS (PolyAn) n-Butanol–water ZSM-5 zeolite-filled PEBA (5% of zeolite) ZSM-5 zeolite-filled PEBA (5% of zeolite) 25 wt% silica filled PTMSP PDMS/PE/Brass c-PDMS/BPPO PTFE HTPB-PU Silicalite-filled PDMS Silicalite-filled PDMS PDMS (Pervatech) PDMS (Pervap 4060) PDMS (PolyAn) PDMS (Pervatech) Hydrophobic zeolite filled PDMS (Pervap 1070) PDMS (Pervatech) PTMSP/PDMSM PTMSP PTMSP PTMSP PDMS–PVDF Surface modified ceramic Ethanol–water 25 wt% silica filled PTMSP PTMSP PDMS/ceramic composite membrane PDMS (Pervatech) PDMS (Pervap 4060) PDMS (PolyAn) Ternary model mixtures Acetone-n-butanol–water HTPB-PU Quaternary model mixtures PTFE PDMS (Pervatech)

Pervatech (PDMS)

PDMS–PVDF

Organic solvent content in feed [wt%]

T [1C]

Organic flux [g m
2 h
1]

Separation factor [–]

Ref.

ca. 0.5 2 2 2

40 25 25 25

ca. 1 344 431 649

ca. 14 29 66 40

[52] [43] [43] [43]

4.3 2.5 5 2 5 1.25 (v./v.) ca. 1 0.3 0.3 2 2 2 3.5 1 1 2 2 1 1 1.5 (initial concentration) 1

35 45 50 37 40 40 40 25 65 25 25 25 42 65 65 25 37 25 70 37 35

719.3 569 9500 132 (total flux) 220 (total flux) 170 (total flux) ca. 2 ca. 5 ca. 16 112 224 202 ca. 950 ca. 100 ca. 750 120 ca. 800 (total flux) 20 413 31.5 ca. 50

33.3 30.7 104 32 35 8.5 ca. 11 ca. 18 ca. 10 10 36 11 22 40 27 128 135 52 70 17 2

[49] [49] [51] [47] [45] [50] [52] [46] [46] [43] [43] [43] [44] [97] [97] [102] [99] [100] [100] [104] [105]

5 6 5 2 2 2

50 30 40 25 25 25

9500 320 (total flux) 1600 (total flux) 75 61 152

18.3 9 8.9 7 10 7

[51] [101] [98] [43] [43] [43]

ca. 0.5 (AcO) ca. 1 (BuOH)

40

ca. 1.5 (AcO) ca. 2 (BuOH)

ca. 15 (AcO) ca. 12 (BuOH)

[52]

1.25 (v./v.) 1.54 (AcO) 3.23 (BuOH) 0.43 (EtOH) 1.60 (total organics in feed, ratio 3:6:1)

40 42

980 (total flux) ca. 480 (AcO) ca. 800 (BuOH) ca. 80 (EtOH) ca. 350 (AcO) ca. 750 (BuOH) ca. 50 (EtOH) 18 (AcO) 32 (BuOH) 1 (EtOH)

9.5 22 (AcO) 22 (BuOH) 6 (EtOH) 24.5 (AcO) 27.6 (BuOH) 8.5 (EtOH) 15 (AcO) 14 (BuOH) 3 (EtOH)

[50] [44]

0.5 (AcO) 1.1 (BuOH) 90 (total flux) 8 (AcO) 20 (BuOH) 0.5 (EtOH) 16 (ABE)

15.3 (AcO) 13.7 (BuOH) 24 (BuOH) 23 (AcO) 14 (BuOH) 5 (EtOH) 25 (ABE)

[52]

0.75 (AcO) 1.5 (BuOH) 0.25 (EtOH)

Fermentation broth HTPB-PU

65

37

PTMSP PDMS–PVDF

0.5 (AcO) 1.1 (BuOH) – –

37 37

Silicone

6.0 (ABE)

–

Total flux (Jt) and separation factor (β) can be combined into the so called Pervaporation Separation Index (PSI), according to Eq. (2). PSI ¼ J t β
1




ð2Þ

Isopsines i.e. lines corresponding to constant values of PSI, were used to compare various membranes efficiency in butanol recovery from its model aqueous solutions. In Fig. 2 dashed and solid lines correspond to PSI equal to 10 kg m
2 h
1 and 20 kg m
2 h
1, respectively. Membranes of the best performance in butanol recovery were PDMS and ZSM-5 zeolite-filled PEBA. Process separation index in between 10 and 20 kg m
2 h
1 was obtained for two PDMS

[97]

[104]

[103] [104]

[106]

membranes, PDMS filled by hydrophobic zeolite and PTMSP/PDMSM membrane. It has to be pointed out that the PTMSP/PDMSM membrane possess the highest separation factor value; however, flux through the membrane is not impressive. Most of membranes presented in Fig. 2 possess PSI value lower than 10 kg m
2 h
1. Efficiency of ABE recovery by pervaporation process can by reduced by fouling. The fouling is described as adsorption of macromolecules on the surface and inside the membrane [99,107]. The mentioned phenomenon results in reduction of flux and due to this caused drop in membrane performance [99]. To reduce negative impact of the fouling macromolecules should be removed from the feed before pervaporation or membrane cleaning procedure should

658

A. Kujawska et al. / Renewable and Sustainable Energy Reviews 48 (2015) 648–661

Fig. 2. Comparison of various membranes performance in n-butanol recovery by VPV; solid isopsine line corresponds to PSI equal to 20 kg m
2 h
1 and dashed isopsine line to PSI equal to 10 kg m
2 h
1.

be applied [99,108]. Membranes after rinsing could restore its previous properties [108]. 2.2. Integration of n-butanol fermentation with various removal techniques Müller and Pons [109] tested properties of microporous (PTFE, PP) and nonporous (silicone) membranes in pervaporation coupled to alcoholic fermentation. Such solution allowed to obtain improved productivity of ABE fermentation products. In terms of long time operation process, microporous membranes can undergo clogging of the pores, which would result in reduction of porous membrane properties. During pervaporation experiments gradual loss of the hydrophobicity of the polypropylene membrane was observed, whereas silicone material does not suffer from this problem [109]. Mulder et al. [110] proposed multistage set-up to be used for continuous removal of ethanol from fermentation broth. The rig consisted of fermenter directly coupled with ultrafiltration (UF) mode. Permeate of 5 to 10 wt% ethanol content from UF was directly transported to VPV rig, where permeate of around 40 wt% was obtained. The permeate was transmitted to the second VPV mode, in which product liquid of 95 wt% was obtained. Unfortunately, no practical application of proposed solution was described. Authors [110] investigated properties of cellulose acetate (CA), polysulfone (PSf) and poly(dimethylphenyleneoxide) (PPO) membranes by vacuum pervaporation in contact with water–ethanol (50 wt% of organic compound in feed) system at 20 1C. Separation factor towards water equal to 12.3, 3.0 and 9.3 for CA, PSf and PPO membranes, respectively, was obtained. Vane and Alvarez [111] described the separation of n-butanol– water and ABE–water solutions using a combination of unit operations such as: vapour stripping, vapour compression, and vapour permeation membrane separation. Such procedure was termed the membrane assisted vapour stripping (MAVS). In the MAVS process volatile compounds are removed from the broth in a stripping column and subsequently the vapours are adiabatically compressed. Such procedure allows raising the pressure of the stream and maintains it in the vapour phase. In a subsequent step the compressed vapour stream is separated into solvent- and water–rich vapour streams with a vapour permeation membrane unit. The water–rich vapour stream of permeate from the

membrane is returned to the stripping column to diminish the reboiler heat requirement [111]. Qureshi et al. [106] described production of ABE from concentrated whey/lactose solutions and removal of acetone, butanol, and ethanol by pervaporation technique. The whey/lactose as a fermentation substrate was chosen due to the fact that it requires less upstream processing than other substrates for ABE fermentation and it is commercial dairy industry by-product. 211 g L
1 of lactose was used to obtain total ABE productivity of 0.43 g L
1 h
1 and total amount of ABE in the reactor of 79.0 g (acetone 4.8 g, butanol 72.4 g, and ethanol 1.8 g). Silicone membrane was applied during pervaporative recovery of fermentation products. ABE separation factor was equal to ca. 25 and total ABE flux was ca. 16 g m
2 h
1 at 6 wt% of total organics in feed. It was concluded that pervaporation allowed to selectively recover ABE fermentation products and it minimized diffusion of water through the membrane and due to this significantly less energy is necessary for product recovery comparing with gas stripping [106]. Xue et al. [112] applied two-stage gas stripping method to perform recovery of butanol directly from ABE fermentation broth in a fibrous bed bioreactor. The first-stage of gas stripping was coupled directly with fermentation broth. The aim of first stage was to mitigate inhibition of the ABE fermentation products towards bacteria cells, whereas the second stage gas stripping allowed for further concentration of solvents. Influence of several parameters (butanol concentration, temperature of feed, gas flow rate, cooling temperature) on stripping gas process efficiency was tested. The optimal conditions chosen for two-stage process given in that study were 37 1C of fermentation broth in the first stage, 55 1C feed temperature for the second stage and gas flow rate equal to 1.6 L min
1. After two-stage gas stripping process, the composition of the final product was equal to 515.3 g L
1, 139.2 g L
1 and 16.6 g L
1 of butanol, acetone, and ethanol, respectively. It was claimed that such a method allows to reduce total energy consumption of the butanol recovery process [112]. Chen et al. [113] investigated butanol recovery using intermittent permeating–heating–gas stripping method integrated with ABE fed-batch fermentation. During solvents recovery, performed at 70 1C, 290 g L
1 of glucose was utilized, 106.27 g L
1 of ABE and 66.09 g L
1 of butanol were produced. During the removal process a highly concentrated condensate containing ca. 15% (w/v) butanol, 4% (w/v) acetone, and o1% (w/v) ethanol was received, due to

A. Kujawska et al. / Renewable and Sustainable Energy Reviews 48 (2015) 648–661

this highly concentrated butanol solution (ca. 70% (w/v)) was obtained after phase separation. The authors [113] concluded that the integrated fermentation process with periodic nutrient supplementation allowed to maintain a stable productivity and high butanol yield for an extended period of time.

3. Final remarks Development of efficient approach for butanol production is a very fast developing area. One of promising approaches is butanol production and recovery from renewable resources. However, a lot of variables have to be taken into account to determine profitability of butanol production. Qureshi and Manderson [114] performed cost analysis of renewable resources bioconversion into ethanol. Recovery of ethanol by pervaporation was examined and costs of VPV process were compared with those for distillation method. The authors [114] found that application of membrane recovery to a production plant of capacity 58.6  106 L year
1 of ethanol that utilizes a continuous wood hydrolysate fermentation process allow to reduce cost of ethanol production from $0.52/L (distillative recovery) to $0.46/L (membrane recovery). An increase of membrane flux by a factor of 5 allows reducing this price to $0.42/L. Membrane prices has significant impact on ethanol production costs; however, application of larger plants allows to obtain only slightly lower ethanol prices ($0.40/L). One can assume that similar solutions applied to n-butanol recovery, including shift from distillation towards more energy-efficient methods will decrease n-butanol production price, making it even more attractive and economically competitive as a biofuel. There is a need for cheaper feedstocks, improved ABE fermentation process performance and more sustainable operation methods for solvents recovery [58]. Especially that the price of feedstock contribute up to 79% of ABE solvents production costs and additionally, the feedstock price depends strongly on market prices fluctuations [58]. This renders the need for the possibility of converting plants to use cheaper fermentation feedstocks [58]. There are negative and positive aspects of integrated n-butanol fermentation set-up with separation techniques [115]. Practical application of a combined system will be possible if the integrated process is microbial friendly, scalable, non-foulable, and enhances n-butanol productivity. Application of the various separation techniques like adsorption, gas stripping, liquid–liquid extraction, perstraction and pervaporation diminishes n-butanol toxicity towards fermentation broth and allows to obtain increased productivity. Among all mentioned techniques only gas stripping has increased yield of the combined process. There are also limitations of n-butanol recovery methods when combined with fermentation [115]. Application of adsorption causes loss of nutrients to adsorbent, clogging, and loss of fermentation intermediate products. Gas stripping technique is limited by a low n-butanol stripping rate, whereas during liquid–liquid extraction not only the extractant used can be toxic to cells, but also formation of precipitate layer, emulsion and loss of fermentation intermediate products can occur. During pertraction process loss of intermediate fermentation products to extractant phase and membrane fouling can take place. Application of pervaporation can cause losses of fermentation intermediate products. Moreover, the membrane fouling is also possible [115]. Recovery techniques allow efficient removal of ABE fermentation products, although industrial applications of the techniques are still not so popular. More effort should be paid to commercialise recovery techniques in industrial applications.

659

Acknowledgements This work was financially supported by the Grant number N N209 761240 founded by Polish Ministry of Science and Higher Education. Authors would like to kindly thank Dr. Maciej Kujawski for his help with the text editing.

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