- Email: [email protected]
Separation of ethanol from ethanol–water mixture and fermented sweet sorghum juice using pervaporation membrane reactor
Desalination 271 (2011) 88–91
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Separation of ethanol from ethanol–water mixture and fermented sweet sorghum juice using pervaporation membrane reactor P. Kaewkannetra a,b, N. Chutinate a,b, S. Moonamart a,b, T. Kamsan c, T.Y. Chiu d,⁎ a
Department of Biotechnology, Faculty of Technology, Khon Kaen University, Khon Kaen 40002, Thailand Research Centre of Fermentation for Value Added Agricultural Product (FerVAAP), Faculty of Technology, Khon Kaen University, Khon Kaen 40002 Thailand Department of Chemical Engineering, Faculty of Engineering, Khon Kaen University, Khon Kaen, 40002 Thailand d AECOM Design Build, Barnsley, Sheffield, S75 3DL, UK b c
a r t i c l e
i n f o
Article history: Received 23 October 2010 Received in revised form 3 December 2010 Accepted 6 December 2010 Available online 5 January 2011 Keywords: Ethanol–water mixture Sweet sorghum juice Pervaporation Membrane reactor
a b s t r a c t Ethanol is an important renewable energy that can be produced from byproducts of agricultural products such as cassava, corn, potato, sorghum and sugar cane etc. via chemical and bio-fermentation processes. Pervaporation, an effective and economical membrane technology, has been proven to substitute distillation process for ethanol separation. In this work, the separation of two types of mixtures; ethanol–water mixture and fermented sweet sorghum were investigated using cellulose acetate membranes. The pervaporation performances of the two mixtures were carried out under various operating conditions such as ethanol concentrations (10%, 15%, 20% and 30%), operating times (15, 30, 45 and 60 min) and temperatures (50 °C, 60 °C and 70 °C). The permeate fluxes, selectivity and percent separation declined significantly under all operating conditions in the presence of sweet sorghum fermentation broth when compared to the binary mixture. Preliminary economic analysis shows that cost of producing 1 l of ethanol from the broth is about 1.0 $/l which is about 1.2 times higher than from the pure binary system and approximately 5 times higher than the minimum production cost of 0.2 $/l reported by Wasewar and Pangarkar (2006) [20]. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The conversion of biomass into an energy source by fermentation processes to produce chemicals and fuels such as bioethanol, replacing the use of fossil energy resources is receiving substantial interests. This is due to the increasing petroleum prices and concerns over climate change. One potential biomass of increasing interest is sweet sorghum because it can be cultivated in almost all temperate and tropical climate areas and is the only crop providing grain and stem that can be used as substrates for the production of sugar, alcohol, syrup, fodder, fuel, bedding, roofing, fencing and paper [1]. The recovery and purification of ethanol from fermentation broth generally involves distillation which is energy intensive and complex. It has been estimated to consume more than half of the total energy in the production of alcohol by fermentation [2]. To make bioethanol processes competitive with fossil energy resources, production costs must be reduced. An alternative non-distillation technique for in-situ ethanol recovery from fermentation is pervaporation [2–5]. Perva-
⁎ Corresponding author. Tel.: + 44 122 622 4176; fax: + 44 1226 224 488. E-mail address: [email protected] (T.Y. Chiu). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.12.012
poration is a membrane separation process based on the difference in solubility and diffusivity of the components to be separated through a dense membrane. It is not limited by the relative volatility of the components and requires less energy. The complexity of fermentation broths, which are multicomponent mixtures that contain a variety of byproducts, results in several anecdotal observations regarding its impact, both positive and negative, on the pervaporation performances [6–10]. To complicate matters, several studies have indicated no significant effect of fermentation broth on pervaporation performance [9,11,12]. To the authors’ knowledge, there has been no work undertaken to elucidate the effect of sweet sorghum fermentation broth on pervaporation performances although ethanol production from these biomass is ever increasing. The main objective of this study was to investigate the effect of sweet sorghum fermentation broth on the pervaporation performances using commercial cellulose acetate membranes. Pervaporation studies were carried out using ethanol–water mixture as the control study prior to separating ethanol from fermented sweet sorghum broth The effects of operating temperature, ethanol concentrations and pervaporation period also were investigated and evaluated in terms of permeate flux, separation factor and pervaporation separation index.
P. Kaewkannetra et al. / Desalination 271 (2011) 88–91
2. Materials and methods 2.1. Preparation of ethanol–water and fermented sweet sorghum juice mixtures All chemicals used in this study were of analytical grade (SC Scientific and Merck Chemical, Bangkok, Thailand). For the first ethanol–water mixture was prepared from pure 99.9% ethanol (analytical grade) and mixed with deionized water to the desired ethanol concentrations (10, 15, 20 and 30%) while the second mixture of fermented sweet sorghum juice was obtained after 3 days batch fermentation of sweet sorghum juice using a pure culture of Saccharomyces cerevisiae. 2.2. Experimental setup and procedure A 500 ml bench scale Pyrex membrane reactor was assembled with the pervaporation system. The pervaporation system consisted of a membrane reactor on a magnetic and hot plate stirrer, air tube, vacuum unit, condenser and the vacuum pump was connected in permeate side of the system (see Fig. 1). The flat sheet cellulose acetate membranes, kindly received from Research Centre of Nanomaterial, Faculty of Science, Khon Kaen University, Thailand, were placed in a circular plate and frame permeation cell. The membranes were 3.6 cm in diameter and had a surface area of 10.2 cm2. The nominal pore size of the membranes was 0.45 μm and membrane thickness was approximately 120 μm. The mixtures were pumped into the membrane from the stirred feed tank via a peristaltic pump. A vacuum pump was fixed to maintain a permeate pressure of 50.8 mm Hg. The temperature of the feed liquid mixture was heated and kept constant at the desired temperatures (50 °C, 60 °C and 70 °C) using a hot plate magnetic stirrer to heat the stirred tank feed reactor. Pervaporation was carried out for periods of 15, 30, 45 and 60 min. Each pervaporation experiment was repeated several times and the average value is reported here. All measurements show good reproducibility as the errors never exceed 5%.
89
The flux, J, is calculated as J=
W A×t
ð1Þ
where W, A and t are the mass feed pass through membrane (kg), surface area of membrane (m2) and pervaporation time (h) respectively. The selectivity (α) of the membrane, or also termed as separation coefficient, for ethanol (E) relative to water (Wat) is defined as α=
YE = YWat XE = XWat
ð2Þ
where Yi and Xi are the mass fractions of component i in the permeate vapour and feed liquid respectively. The effectiveness of separation is determined equally by the membrane selectivity and permeation properties. The parameter which accounts for these two factors is the so-called “pervaporation separation index”—calculated as the ratio of the total permeate flux ( J) and the separation coefficient (α) as ð3Þ
PSI = Jα
The percent separation, also termed as separation efficiency %, was calculated as 100 × {(EtOH in feed) − (EtOH in permeate)} / (EtOH in feed). The swelling measurements were carried out by immersing dried membrane in water–ethanol mixtures of different compositions. After 24 h the membranes were removed, pressed between a tissue paper and weighed. This procedure was continued until no further weight increase was observed. The overall solubility, Soloverall, is calculated from the weight of the swollen and the dry membrane sample and is expressed in units of grams of sorbed mixture per gram of dry membrane using the expression Soloverall = ðmM −mD Þ = mD
ð4Þ
where mM and mD are the mass of the swollen membrane and mass of the dry membrane, both in grams respectively.
2.3. Analytical methods
3. Results and discussions
Permeate and feed concentrations were measured off-line before and after pervaporation. The volumes of the mixtures were weighed using analytical balance (BP 2215, Sertorius, Germany) and ethanol concentration was collected and analyzed using gas chromatography (GC) (14 B-Shimazu, Japan).
3.1. Effect of ethanol concentrations
2.4. Evaluation of pervaporation performances Pervaporation performances were evaluated in terms of flux, separation factor (α), pervaporation separation index (PSI) and percent separation.
The effects of feed ethanol concentration on separation efficiency and permeate flux are shown in Table 1. As the ethanol concentration increases, the permeate flux increases but decreased separation efficiency at the highest ethanol concentration (see Fig. 2). This anomaly is due to the increased swelling of the cellulose acetate polymer in high ethanol concentrations. In swollen membrane, the distance between polymer chains increase that will promote ethanol and water pass across the membrane freely, leading to the decrease of membrane selectivity [13,14]. Diffusion behaviour of water and small molecules in Polymer gel can be interpreted by free volume theory as well as NMR spectroscopy [15,16]. At higher ethanol concentrations, similar water permeation flux is achieved compared to
Table 1 Effect of ethanol concentrations on pervaporation system at 50.8 mm Hg; 60 °C; 30 min.
Fig 1. Schematic of pervaporation system.
Ethanol concentration (%)
Jtotal (kg/m2 h)
α
PSI (kg/m2 h)
Separation efficiency %
Overall solubility (weight fraction)
10 15 20 30
3.4 5.9 6.8 19.1
11.0 9.7 8.4 4.5
37.5 57.9 57.3 81.2
51.9 48.5 31.1 18.9
0.130 0.135 0.152 0.164
90
P. Kaewkannetra et al. / Desalination 271 (2011) 88–91
4
25
3
J (kg/m2hr)
J (kg/m2hr)
20
15
10
2
1 5
0
0 5
10
15
20
25
30
35
0
10
20
30
40
50
60
70
Pervaporation time (mins)
Ethanol concentrations (%) Fig 2. Influence of ethanol feed concentration for the binary feed mixture ethanol/water on permeation fluxes (▲: total permeation flux; □: ethanol permeation flux; ■: water permeation flux).
Fig 3. Influence of operating time for the sweet sorghum fermentation broth on permeation fluxes (▲: total permeation flux; □: ethanol permeation flux; ■: water permeation flux).
ethanol permeation flux. This is in agreement with other workers [17,18] who studied the separation of ethanol–water mixtures by pervaporation.
the polymer chain bigger, and the kinetic energy of permeate are increasing, which make the diffusion easier. Because the coupling effect, the ethanol relative concentration are increased, and the separate selectivity are notable decreased with temperature increasing.
3.2. Effect of operating time The membrane durability is a critical factor in commercial applications. The long-term membrane operating stability was, therefore, investigated in this study. Table 2 presents the data for the operating time effect on the pervaporation separation of a 15% ethanol/water solution at 60 °C. The data indicated that both the permeation rate and the selectivity remained unchanged during the operation for 60 min at 60 °C. This finding illustrates that the cellulose acetate membrane exhibited membrane durability during the pervaporation separation at high operating temperature. 3.3. Effect of temperature It is evident in Table 3 that with the increasing temperature, the separation factors and PSI decrease whilst the permeate fluxes increases. According to the solution–diffusion mechanism [19], the increasing of temperature makes the solubility on the surface of membrane and the diffusion rate in the membrane increase. From the microstructure analysis, the free volume of the membranes increases under higher temperature, which makes the interspaces between
Table 2 Effect of operation time on pervaporation at 50.8 mm Hg; 60 °C; 15% ethanol–water. Time (min)
Jtotal (kg/m2 h)
α
PSI (kg/m2 h)
Separation efficiency %
15 30 45 60
11.4 5.9 6.4 6.2
6.8 9.7 6.8 7.3
77.5 57.9 43.5 45.3
47.8 48.5 44.5 47.8
Table 3 Effect of temperature pervaporation at 50.8 mm Hg, 30 min, 15% ethanol–water. Temp (° C)
Jtotal (kg/m2 h)
α
PSI (kg/m2 h)
Separation efficiency %
50 60 70
5.0 5.9 6.9
14.2 9.7 7.0
71.0 57.9 48.3
53.4 48.5 35.3
3.4. Effect of fermented sweet sorghum juice broth on pervaporation performances The concentration of ethanol within the fermentation broth is at 14.6%. Generally, there is significant deterioration in membrane flux and selectivity when pervaporation was carried out using the fermentation broth. This results from the presence of high yeast cell concentrations and residual byproducts within the fermented ethanol. The yeast cell concentration in the broth was approximately 9.0 cells ml− 1 whilst residual soluble solids of 50 g l− 1. This result was also observed with other workers [2,8] who attributed the decline in membrane flux to the presence of yeast cells as well as other byproducts within the fermented ethanol. Ethanol selectivity decreases in the presence of the broth. The permeation flux for ethanol seems to be affected to a greater extent than water permeation (see Fig. 3). The pervaporation performances in the fermentation broth followed the similar trend observed with ethanol–water mixture studies under the changing temperatures although at the highest temperature of 70 °C, the separation percentage was almost halved. However, as the operation time increases for pervaporation, both fluxes and selectivity declines (Table 4). Following Wasewar and Pangarkar [20], a preliminary sensitivity analysis of the pervaporation performance was carried out. The cost of producing ethanol per liter in the ethanol–water mixture as well
Table 4 Effect of various operating conditions on the pervaporation performance in fermentation broth. Operating conditions
Jtotal (kg/m2 h)
α
PSI (kg/m2 h)
Separation efficiency %
Time (min) 15 30 45 60
2.7 2.3 1.4 1.5
6.6 6.3 3.6 2.3
17.8 14.5 5.0 3.5
32.7 31.3 22.3 17.3
Temperatures (° C) 50 60 70
1.2 1.4 4.2
9.3 3.6 2.2
11.2 5.0 9.2
38.3 22.3 15.7
P. Kaewkannetra et al. / Desalination 271 (2011) 88–91
Direct production cost ($/l)
1.1
91
Acknowledgements The authors would like to gratefully acknowledge the Research Centre of Fermentation for Value Added Agricultural Products (FerVAAP) for facilities and financial contribution and P. Kaewkannetra also would like to sincerely acknowledge the JSPS-NRCT on Asian Core Program for scientist exchange and travel support.
1
0.9
References
0.8 0
10
20
30
40
50
60
70
80
90
PSI (kg/m2hr) Fig 4. Influence of pervaporation performances on production costs (□: binary feed mixture ethanol/water; ■: sweet sorghum fermentation broth).
as from sweet sorghum broth has been estimated based on PSI (see Fig. 4). The PSI was chosen as the overall determining pervaporation parameter since it relates to both flux and separation factor. Generally, membrane having higher flux requires less pervaporation membrane area and hence lowers production costs whilst membrane having higher separation factor gives higher concentration in permeates and requires less energy for further enrichment in the concentration and hence less production cost. Fig. 4 shows that as PSI increases the cost of producing ethanol decreases. The cost of producing 1 l of ethanol from the broth is about 1.2 times higher than from the pure binary system and approximately 5 times higher than the minimum production cost of 0.2 $/l reported elsewhere [16]. This implies that the cellulose acetate membrane for the separation of ethanol from sweet sorghum fermentation broth is not commercially viable and further investigations in the development of membranes for high flux and separation factor are required for the separation of ethanol from sweet sorghum fermentation broth. 4. Conclusion The pervaporation performances under the various operating conditions using ethanol–water mixtures and subsequently, sweet sorghum fermentation broth were investigated. In the binary mixture, increasing ethanol concentrations and temperature resulted in lowered selectivity but increased fluxes. This is attributed to the swelling of the cellulose acetate membrane structure. Extending the operating times, however, did not adversely affect the flux and selectivity of the membrane. Direct comparisons of pervaporation performances were made at 15% ethanol–water since it closely resembles the ethanol concentration found in the sweet sorghum fermentation broth. In the sweet sorghum fermentation broth, both permeate flux and selectivity decreased significantly with percent separation of up to half compared to the binary mixture under all operating conditions which makes pervaporation using cellulose acetate membrane undesirable.
[1] A.K. Rajvanshi, N. Nimbkar, Sweet sorghum R&D at the Nimbkar Agricultural Research Institute (NARI), 20058 http://nariphaltan.virtualave.net/sorghum.htm. [2] S.I. Nakao, F. Saitoh, T. Asakura, K. Toda, S. Kimura, Continuous ethanol extraction by pervaporation from a membrane bioreactor, J. Membr. Sci. 30 (1987) 273–287. [3] K. Matsumoto, H. Ohya, Separation of ethanol from culture broth by pervaporation with hydrophobic porous membrane, Proc. 3rd World Congress of Chemical Engineering, 1986, pp. 843–846. [4] Y. Mori, T. Inaba, Ethanol production from starch in a pervaporation membrane bioreactor using Clostridium thermohydrosulfuricum, Biotechnol. Bioeng. 36 (1990) 849. [5] W.J. Groot, C.E. van der Oever, N.W.F. Kossen, Pervaporation for simultaneous product recovery in the butanol/isopropanol batch fermentation, Biotechnol. Lett. 6 (1984) 709–714. [6] T. Ikegami, H. Yanagishita, D. Kitamoto, K. Haraya, T. Nakane, H. Matsuda, N. Koura, T. Sano, Production of highly concentrated ethanol in a coupled fermentation/ pervaporation process using silicalite membranes, Biotechnol. Technol. 11 (1997) 921–924. [7] D.J. O'Brien, J.C. Craig Jr., Ethanol production in a continuous fermentation/ membrane pervaporation system, Appl. Microbiol. Biotechnol. 44 (1996) 699–704. [8] T. Ikegami, H. Yanagishita, D. Kitamoto, K. Haraya, T. Nakane, H. Matsuda, N. Koura, T. Sano, Highly concentrated aqueous ethanol solutions by pervaporation using silicalite membrane—improvement of ethanol selectivity by addition of sugars to ethanol solution, Biotechnol. Lett. 21 (1999) 1037–1041. [9] S.L. Schmidt, M.D. Myers, S.S. Kelley, J.D. McMillan, N. Padukone, Evaluation of PTMSP membranes in achieving enhanced ethanol removal from fermentations by pervaporation, Appl. Biochem. Biotechnol. 63–65 (1997) 469–482. [10] M. Nomura, T. Bin, S.-I. Nakao, Selective ethanol extraction from fermentation broth using a silicalite membrane, Sep. Purif. Technol. 27 (2002) 59–66. [11] N. Qureshi, M.M. Meagher, J. Huang, R.W. Hutkins, Acetone butanol ethanol (ABE) recovery by pervaporation using silicalite–silicone composite membrane from fedbatch reactor of Clostridium acetobutylicum, J. Membr. Sci. 187 (2001) 93–102. [12] N. Qureshi, H.P. Blaschek, Fouling studies of a pervaporation membrane with commercial fermentation media and fermentation broth of hyper-butanol-producing Clostridium beijerinckii BA101, Sep. Sci. Technol. 34 (1999) 2803–2815. [13] P. Schaetzel, C. Vanclair, Q.T. Naguyen, G. Mo, Mass Transfer in Pervaporation: The Total Free Volume Model, Proceeding Euro Membrane, Jerusalem, Israel, 20008 September. [14] V.V. Volkov, A.O. Malakhov, V.S. Khotimsky, N.A. Plate, Pervaporation and Sorption of Associating Fluids in Nanopores of Poly [1-Trimethylsilyl-1Propyene], Euro Membrane, Jerusalem Israel, 20008 September. [15] S. Matsukawa, I. Ando, Study of self-diffusion of Molecules in a polymer gel by pulsed-gradient spin-echo 1H NMR. 2. Intermolecular hydrogen–bond interaction between the probe polymer and network polymer in N, N-dimethylacrylamideacrylic acid copolymer gel systems, Macromolecules 30 (1997) 8310–8313. [16] S. Matsukawa, H. Yasunaga, C. Zhao, S. Kuroki, H. Kurosu, I. Ando, Diffusion Processes in Polymer Gels as Studied by Pulsed Field- Gradient Spin-Echo NMR Spectroscopy, Elsevier, Amsterdam, 1999, pp. 995–1044. [17] K.M. Song, W.H. Hong, Dehydration of ethanol and isopropanol using tubular type cellulose acetate membrane with ceramic support in pervaporation process, J. Membr. Sci. 123 (1997) 27–33. [18] H. Schissel, R.A. Orth, Separation of ethanol–water mixtures by pervaporation through thin, composite membranes, J. Membr. Sci. 17 (1984) 109–120. [19] P. Shao, R.Y.M. Huang, Polymeric membrane pervaporation, J. Membr. Sci. 287 (2007) 162–179. [20] V. Pangarkar, K. Wasewar, Intensification of recovery of ethanol from fermentation broth using pervaporation: Economical evaluation, Chem. Biochem. Eng. Q. 20 (2006) 135–145.
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Separation of ethanol from ethanol–water mixture and fermented sweet sorghum juice using pervaporation membrane reactor P. Kaewkannetra a,b, N. Chutinate a,b, S. Moonamart a,b, T. Kamsan c, T.Y. Chiu d,⁎ a
Department of Biotechnology, Faculty of Technology, Khon Kaen University, Khon Kaen 40002, Thailand Research Centre of Fermentation for Value Added Agricultural Product (FerVAAP), Faculty of Technology, Khon Kaen University, Khon Kaen 40002 Thailand Department of Chemical Engineering, Faculty of Engineering, Khon Kaen University, Khon Kaen, 40002 Thailand d AECOM Design Build, Barnsley, Sheffield, S75 3DL, UK b c
a r t i c l e
i n f o
Article history: Received 23 October 2010 Received in revised form 3 December 2010 Accepted 6 December 2010 Available online 5 January 2011 Keywords: Ethanol–water mixture Sweet sorghum juice Pervaporation Membrane reactor
a b s t r a c t Ethanol is an important renewable energy that can be produced from byproducts of agricultural products such as cassava, corn, potato, sorghum and sugar cane etc. via chemical and bio-fermentation processes. Pervaporation, an effective and economical membrane technology, has been proven to substitute distillation process for ethanol separation. In this work, the separation of two types of mixtures; ethanol–water mixture and fermented sweet sorghum were investigated using cellulose acetate membranes. The pervaporation performances of the two mixtures were carried out under various operating conditions such as ethanol concentrations (10%, 15%, 20% and 30%), operating times (15, 30, 45 and 60 min) and temperatures (50 °C, 60 °C and 70 °C). The permeate fluxes, selectivity and percent separation declined significantly under all operating conditions in the presence of sweet sorghum fermentation broth when compared to the binary mixture. Preliminary economic analysis shows that cost of producing 1 l of ethanol from the broth is about 1.0 $/l which is about 1.2 times higher than from the pure binary system and approximately 5 times higher than the minimum production cost of 0.2 $/l reported by Wasewar and Pangarkar (2006) [20]. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The conversion of biomass into an energy source by fermentation processes to produce chemicals and fuels such as bioethanol, replacing the use of fossil energy resources is receiving substantial interests. This is due to the increasing petroleum prices and concerns over climate change. One potential biomass of increasing interest is sweet sorghum because it can be cultivated in almost all temperate and tropical climate areas and is the only crop providing grain and stem that can be used as substrates for the production of sugar, alcohol, syrup, fodder, fuel, bedding, roofing, fencing and paper [1]. The recovery and purification of ethanol from fermentation broth generally involves distillation which is energy intensive and complex. It has been estimated to consume more than half of the total energy in the production of alcohol by fermentation [2]. To make bioethanol processes competitive with fossil energy resources, production costs must be reduced. An alternative non-distillation technique for in-situ ethanol recovery from fermentation is pervaporation [2–5]. Perva-
⁎ Corresponding author. Tel.: + 44 122 622 4176; fax: + 44 1226 224 488. E-mail address: [email protected] (T.Y. Chiu). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.12.012
poration is a membrane separation process based on the difference in solubility and diffusivity of the components to be separated through a dense membrane. It is not limited by the relative volatility of the components and requires less energy. The complexity of fermentation broths, which are multicomponent mixtures that contain a variety of byproducts, results in several anecdotal observations regarding its impact, both positive and negative, on the pervaporation performances [6–10]. To complicate matters, several studies have indicated no significant effect of fermentation broth on pervaporation performance [9,11,12]. To the authors’ knowledge, there has been no work undertaken to elucidate the effect of sweet sorghum fermentation broth on pervaporation performances although ethanol production from these biomass is ever increasing. The main objective of this study was to investigate the effect of sweet sorghum fermentation broth on the pervaporation performances using commercial cellulose acetate membranes. Pervaporation studies were carried out using ethanol–water mixture as the control study prior to separating ethanol from fermented sweet sorghum broth The effects of operating temperature, ethanol concentrations and pervaporation period also were investigated and evaluated in terms of permeate flux, separation factor and pervaporation separation index.
P. Kaewkannetra et al. / Desalination 271 (2011) 88–91
2. Materials and methods 2.1. Preparation of ethanol–water and fermented sweet sorghum juice mixtures All chemicals used in this study were of analytical grade (SC Scientific and Merck Chemical, Bangkok, Thailand). For the first ethanol–water mixture was prepared from pure 99.9% ethanol (analytical grade) and mixed with deionized water to the desired ethanol concentrations (10, 15, 20 and 30%) while the second mixture of fermented sweet sorghum juice was obtained after 3 days batch fermentation of sweet sorghum juice using a pure culture of Saccharomyces cerevisiae. 2.2. Experimental setup and procedure A 500 ml bench scale Pyrex membrane reactor was assembled with the pervaporation system. The pervaporation system consisted of a membrane reactor on a magnetic and hot plate stirrer, air tube, vacuum unit, condenser and the vacuum pump was connected in permeate side of the system (see Fig. 1). The flat sheet cellulose acetate membranes, kindly received from Research Centre of Nanomaterial, Faculty of Science, Khon Kaen University, Thailand, were placed in a circular plate and frame permeation cell. The membranes were 3.6 cm in diameter and had a surface area of 10.2 cm2. The nominal pore size of the membranes was 0.45 μm and membrane thickness was approximately 120 μm. The mixtures were pumped into the membrane from the stirred feed tank via a peristaltic pump. A vacuum pump was fixed to maintain a permeate pressure of 50.8 mm Hg. The temperature of the feed liquid mixture was heated and kept constant at the desired temperatures (50 °C, 60 °C and 70 °C) using a hot plate magnetic stirrer to heat the stirred tank feed reactor. Pervaporation was carried out for periods of 15, 30, 45 and 60 min. Each pervaporation experiment was repeated several times and the average value is reported here. All measurements show good reproducibility as the errors never exceed 5%.
89
The flux, J, is calculated as J=
W A×t
ð1Þ
where W, A and t are the mass feed pass through membrane (kg), surface area of membrane (m2) and pervaporation time (h) respectively. The selectivity (α) of the membrane, or also termed as separation coefficient, for ethanol (E) relative to water (Wat) is defined as α=
YE = YWat XE = XWat
ð2Þ
where Yi and Xi are the mass fractions of component i in the permeate vapour and feed liquid respectively. The effectiveness of separation is determined equally by the membrane selectivity and permeation properties. The parameter which accounts for these two factors is the so-called “pervaporation separation index”—calculated as the ratio of the total permeate flux ( J) and the separation coefficient (α) as ð3Þ
PSI = Jα
The percent separation, also termed as separation efficiency %, was calculated as 100 × {(EtOH in feed) − (EtOH in permeate)} / (EtOH in feed). The swelling measurements were carried out by immersing dried membrane in water–ethanol mixtures of different compositions. After 24 h the membranes were removed, pressed between a tissue paper and weighed. This procedure was continued until no further weight increase was observed. The overall solubility, Soloverall, is calculated from the weight of the swollen and the dry membrane sample and is expressed in units of grams of sorbed mixture per gram of dry membrane using the expression Soloverall = ðmM −mD Þ = mD
ð4Þ
where mM and mD are the mass of the swollen membrane and mass of the dry membrane, both in grams respectively.
2.3. Analytical methods
3. Results and discussions
Permeate and feed concentrations were measured off-line before and after pervaporation. The volumes of the mixtures were weighed using analytical balance (BP 2215, Sertorius, Germany) and ethanol concentration was collected and analyzed using gas chromatography (GC) (14 B-Shimazu, Japan).
3.1. Effect of ethanol concentrations
2.4. Evaluation of pervaporation performances Pervaporation performances were evaluated in terms of flux, separation factor (α), pervaporation separation index (PSI) and percent separation.
The effects of feed ethanol concentration on separation efficiency and permeate flux are shown in Table 1. As the ethanol concentration increases, the permeate flux increases but decreased separation efficiency at the highest ethanol concentration (see Fig. 2). This anomaly is due to the increased swelling of the cellulose acetate polymer in high ethanol concentrations. In swollen membrane, the distance between polymer chains increase that will promote ethanol and water pass across the membrane freely, leading to the decrease of membrane selectivity [13,14]. Diffusion behaviour of water and small molecules in Polymer gel can be interpreted by free volume theory as well as NMR spectroscopy [15,16]. At higher ethanol concentrations, similar water permeation flux is achieved compared to
Table 1 Effect of ethanol concentrations on pervaporation system at 50.8 mm Hg; 60 °C; 30 min.
Fig 1. Schematic of pervaporation system.
Ethanol concentration (%)
Jtotal (kg/m2 h)
α
PSI (kg/m2 h)
Separation efficiency %
Overall solubility (weight fraction)
10 15 20 30
3.4 5.9 6.8 19.1
11.0 9.7 8.4 4.5
37.5 57.9 57.3 81.2
51.9 48.5 31.1 18.9
0.130 0.135 0.152 0.164
90
P. Kaewkannetra et al. / Desalination 271 (2011) 88–91
4
25
3
J (kg/m2hr)
J (kg/m2hr)
20
15
10
2
1 5
0
0 5
10
15
20
25
30
35
0
10
20
30
40
50
60
70
Pervaporation time (mins)
Ethanol concentrations (%) Fig 2. Influence of ethanol feed concentration for the binary feed mixture ethanol/water on permeation fluxes (▲: total permeation flux; □: ethanol permeation flux; ■: water permeation flux).
Fig 3. Influence of operating time for the sweet sorghum fermentation broth on permeation fluxes (▲: total permeation flux; □: ethanol permeation flux; ■: water permeation flux).
ethanol permeation flux. This is in agreement with other workers [17,18] who studied the separation of ethanol–water mixtures by pervaporation.
the polymer chain bigger, and the kinetic energy of permeate are increasing, which make the diffusion easier. Because the coupling effect, the ethanol relative concentration are increased, and the separate selectivity are notable decreased with temperature increasing.
3.2. Effect of operating time The membrane durability is a critical factor in commercial applications. The long-term membrane operating stability was, therefore, investigated in this study. Table 2 presents the data for the operating time effect on the pervaporation separation of a 15% ethanol/water solution at 60 °C. The data indicated that both the permeation rate and the selectivity remained unchanged during the operation for 60 min at 60 °C. This finding illustrates that the cellulose acetate membrane exhibited membrane durability during the pervaporation separation at high operating temperature. 3.3. Effect of temperature It is evident in Table 3 that with the increasing temperature, the separation factors and PSI decrease whilst the permeate fluxes increases. According to the solution–diffusion mechanism [19], the increasing of temperature makes the solubility on the surface of membrane and the diffusion rate in the membrane increase. From the microstructure analysis, the free volume of the membranes increases under higher temperature, which makes the interspaces between
Table 2 Effect of operation time on pervaporation at 50.8 mm Hg; 60 °C; 15% ethanol–water. Time (min)
Jtotal (kg/m2 h)
α
PSI (kg/m2 h)
Separation efficiency %
15 30 45 60
11.4 5.9 6.4 6.2
6.8 9.7 6.8 7.3
77.5 57.9 43.5 45.3
47.8 48.5 44.5 47.8
Table 3 Effect of temperature pervaporation at 50.8 mm Hg, 30 min, 15% ethanol–water. Temp (° C)
Jtotal (kg/m2 h)
α
PSI (kg/m2 h)
Separation efficiency %
50 60 70
5.0 5.9 6.9
14.2 9.7 7.0
71.0 57.9 48.3
53.4 48.5 35.3
3.4. Effect of fermented sweet sorghum juice broth on pervaporation performances The concentration of ethanol within the fermentation broth is at 14.6%. Generally, there is significant deterioration in membrane flux and selectivity when pervaporation was carried out using the fermentation broth. This results from the presence of high yeast cell concentrations and residual byproducts within the fermented ethanol. The yeast cell concentration in the broth was approximately 9.0 cells ml− 1 whilst residual soluble solids of 50 g l− 1. This result was also observed with other workers [2,8] who attributed the decline in membrane flux to the presence of yeast cells as well as other byproducts within the fermented ethanol. Ethanol selectivity decreases in the presence of the broth. The permeation flux for ethanol seems to be affected to a greater extent than water permeation (see Fig. 3). The pervaporation performances in the fermentation broth followed the similar trend observed with ethanol–water mixture studies under the changing temperatures although at the highest temperature of 70 °C, the separation percentage was almost halved. However, as the operation time increases for pervaporation, both fluxes and selectivity declines (Table 4). Following Wasewar and Pangarkar [20], a preliminary sensitivity analysis of the pervaporation performance was carried out. The cost of producing ethanol per liter in the ethanol–water mixture as well
Table 4 Effect of various operating conditions on the pervaporation performance in fermentation broth. Operating conditions
Jtotal (kg/m2 h)
α
PSI (kg/m2 h)
Separation efficiency %
Time (min) 15 30 45 60
2.7 2.3 1.4 1.5
6.6 6.3 3.6 2.3
17.8 14.5 5.0 3.5
32.7 31.3 22.3 17.3
Temperatures (° C) 50 60 70
1.2 1.4 4.2
9.3 3.6 2.2
11.2 5.0 9.2
38.3 22.3 15.7
P. Kaewkannetra et al. / Desalination 271 (2011) 88–91
Direct production cost ($/l)
1.1
91
Acknowledgements The authors would like to gratefully acknowledge the Research Centre of Fermentation for Value Added Agricultural Products (FerVAAP) for facilities and financial contribution and P. Kaewkannetra also would like to sincerely acknowledge the JSPS-NRCT on Asian Core Program for scientist exchange and travel support.
1
0.9
References
0.8 0
10
20
30
40
50
60
70
80
90
PSI (kg/m2hr) Fig 4. Influence of pervaporation performances on production costs (□: binary feed mixture ethanol/water; ■: sweet sorghum fermentation broth).
as from sweet sorghum broth has been estimated based on PSI (see Fig. 4). The PSI was chosen as the overall determining pervaporation parameter since it relates to both flux and separation factor. Generally, membrane having higher flux requires less pervaporation membrane area and hence lowers production costs whilst membrane having higher separation factor gives higher concentration in permeates and requires less energy for further enrichment in the concentration and hence less production cost. Fig. 4 shows that as PSI increases the cost of producing ethanol decreases. The cost of producing 1 l of ethanol from the broth is about 1.2 times higher than from the pure binary system and approximately 5 times higher than the minimum production cost of 0.2 $/l reported elsewhere [16]. This implies that the cellulose acetate membrane for the separation of ethanol from sweet sorghum fermentation broth is not commercially viable and further investigations in the development of membranes for high flux and separation factor are required for the separation of ethanol from sweet sorghum fermentation broth. 4. Conclusion The pervaporation performances under the various operating conditions using ethanol–water mixtures and subsequently, sweet sorghum fermentation broth were investigated. In the binary mixture, increasing ethanol concentrations and temperature resulted in lowered selectivity but increased fluxes. This is attributed to the swelling of the cellulose acetate membrane structure. Extending the operating times, however, did not adversely affect the flux and selectivity of the membrane. Direct comparisons of pervaporation performances were made at 15% ethanol–water since it closely resembles the ethanol concentration found in the sweet sorghum fermentation broth. In the sweet sorghum fermentation broth, both permeate flux and selectivity decreased significantly with percent separation of up to half compared to the binary mixture under all operating conditions which makes pervaporation using cellulose acetate membrane undesirable.
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