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Pervaporation of ethanol produced from banana waste
Waste Management xxx (2014) xxx–xxx
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Pervaporation of ethanol produced from banana waste Roger Hoel Bello a, Poliana Linzmeyer b, Cláudia Maria Bueno Franco b, Ozair Souza a,b,c, Noeli Sellin a,b,c, Sandra Helena Westrupp Medeiros a,b,c, Cintia Marangoni d,⇑ a
Chemical Engineering Department, University of Joinville Region (UNIVILLE), Joinville, SC, Brazil Sanitary and Ambient Engineering Department, University of Joinville Region (UNIVILLE), Joinville, SC, Brazil c Masters Program in Process Engineering, University of Joinville Region (UNIVILLE), Joinville, SC, Brazil d Federal University of Santa Catarina (UFSC), Campus Blumenau, Blumenau, SC, Brazil b
a r t i c l e
i n f o
Article history: Received 19 December 2013 Accepted 10 April 2014 Available online xxxx Keywords: Banana waste Bioethanol Biofuel Lignocellulosic residue Pervaporation Polydimethylsiloxane membrane
a b s t r a c t Banana waste has the potential to produce ethanol with a low-cost and sustainable production method. The present work seeks to evaluate the separation of ethanol produced from banana waste (rejected fruit) using pervaporation with different operating conditions. Tests were carried out with model solutions and broth with commercial hollow hydrophobic polydimethylsiloxane membranes. It was observed that pervaporation performance for ethanol/water binary mixtures was strongly dependent on the feed concentration and operating temperature with ethanol concentrations of 1–10%; that an increase of feed flow rate can enhance the permeation rate of ethanol with the water remaining at almost the same value; that water and ethanol fluxes was increased with the temperature increase; and that the higher effect in flux increase was observed when the vapor pressure in the permeate stream was close to the ethanol vapor pressure. Better results were obtained with fermentation broth than with model solutions, indicated by the permeance and membrane selectivity. This could be attributed to by-products present in the multicomponent mixtures, facilitating the ethanol permeability. By-products analyses show that the presence of lactic acid increased the hydrophilicity of the membrane. Based on this, we believe that pervaporation with hollow membrane of ethanol produced from banana waste is indeed a technology with the potential to be applied. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction One of the bases for the economic development of Brazil is agriculture, and in certain states, such as Santa Catarina, banana is widely cultivated as a commercial crop. Banana is one of the most consumed fruits in the world and it is commercially grown in about 120 countries. Currently, Brazil is the second largest producer (preceded by India) and is responsible for 7.5% of world production (about 7.2 million tons per annum, according to the Center of Socioeconomics and Agricultural Planning for the State of Santa Catarina). The State of Santa Catarina has approximately six thousand producers, being the fourth largest banana growing region in Brazil, with 663,892 tonnes of bunches of bananas produced per annum (ABIB, 2011). Commercial banana production generates a large proportion of waste; there are reports of 30% waste in Australia (Clarke et al., 2008) and Malaysia (Tock et al., 2010), and 25–50% in Central ⇑ Corresponding author. Address: Rua Pomerode, 710 Salto do Norte, Blumenau, SC 89065-300, Brazil. Tel.: +55 48 3721-6308. E-mail address: [email protected] (C. Marangoni).
and South America (Hammond et al., 1996). In countries like India, all kind of banana waste is considered an important urban waste because the fruit is used in all religious functions, festivals and in temples (Chanakya and Sreesha, 2011). According to Graefe et al. (2011), around 20–40% of the bananas produced do not meet export standards or even the quality demands of spot markets. In Brazil, particularly in the southern regions, it is estimated that for every 100 kg of harvested fruit, 46 kg are not used (EMBRAPA, 2006). Further, Souza et al. (2010) indicate that for every ton of bananas produced approximately 3 tons of pseudostem, 160 kg of stems, 480 kg of leaves and 440 kg of skins are generated. Fernandes et al. (2013) found that less than 10% of available biomass as waste (440 million tons) is designated to some application. Thus, an established commercial use for such residues, as well as generating extra remuneration for regional farmers, would help to reduce environmental pollution. Alternative uses for these discards have to be explored, and in this regard processing to produce ethanol is seen to have potential from both an environmental as well as an economic point of view. Many countries are investing in the development and use of biofuels as a way of reducing environmental impacts and ethanol
http://dx.doi.org/10.1016/j.wasman.2014.04.013 0956-053X/Ó 2014 Elsevier Ltd. All rights reserved.
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is one of the fuels that can be produced from various raw materials (España-Gamboa et al., 2011). The use of agricultural or agroindustrial waste is an interesting option in this context. Biofuel has been produced on a large scale in Brazil for three decades using sugarcane as feedstock (Soccol et al., 2010), however there are many criticisms of the practice and an ongoing debate about the ethical issue of using food (or land available for the cultivation of food) as an energy source (Sarkar et al., 2012; Swana et al., 2011). Lignocellulosic material does not play an intrinsic role in the food chain and this is a fundamental aspect that makes it an attractive alternative for ethanol production. Besides, the cost and availability of the feedstock are crucial and can contribute 65–70% to the total ethanol production costs (Kazi et al., 2010). In this sense, the substitution of biomass wastes for raw materials such as cane sugar, starch and corn for ethanol production is an alternative that has shown promising results (Dermibas, 2011; Mabee et al., 2011). Examples of this residual biomass include bagasse sugar cane, corn straw and fiber, wheat and rice straw, eucalyptus wood and crop wastes from commercial cultivation of fruits such as bananas, grapes and apples (Rivas-Cantu et al., 2013). Technologies for the conversion of biomass to ethanol are also under various stages of development. The use of these lignocellulosic residues requires some separation of cellulose and hemicellulose from lignin, followed by hydrolysis of sugars, and this bioconversion has been extensively studied using the different types of wastes. The potential yield of ethanol from lignocellulosics varies significantly between feedstocks, so many applications in alcoholic fermentation are reported in the literature with different wastes. Specifically in the case of ethanol from bananas, the few studies that have been published involve the use of the fruit, leaves and other waste such as the pseudostem. Tewari et al. (1986) reported the suitability of banana peel for alcohol fermentation. Hammond et al. (1996) presented ethanol yield (on a dry weight basis) from ripe bananas as higher than from most other agricultural commodities. Velásquez-Arredondo et al. (2010) investigated the acid hydrolysis of banana pulp and fruit and the enzymatic hydrolysis of flower stalk and banana skin, and the results obtained demonstrated a positive energy balance for the four production routes evaluated. The study by Graefe et al. (2011) presents results of a case study in Costa Rica and Ecuador which found that considerable amounts of ethanol could be produced from banana bunches that do not meet quality standards, as well as from which are partly left to rot in the fields. Oberoi et al. (2011) also demonstrated that banana peel could serve as an ideal substrate for the production of ethanol through simultaneous saccharification and fermentation. Hossain et al. (2011) evaluated bioethanol from rotten banana and concluded that this can be used in motor vehicle engines, producing low emissions, and thus it can be used as an environmental recycling process for waste management. Arumugam and Manikandan (2011) explore the potential application of pulp and banana peel wastes in bioethanol production using dilute acid pretreatment followed by enzymatic hydrolysis. Gonçalves Filho et al. (2013) evaluate the same techniques with banana tree pseudostem. Although the lignocellulosic material shows positive results, it still requires more research to be exploited on an industrial scale. Great efforts are being undertaken to improve ethanol productivity and reduce the overall production costs. According to Gaykawad et al. (2013), one of the ways to achieve these goals is to modify the configuration of the process and perform process integration. Traditionally, the recovery of ethanol by distillation is a challenge because of the high costs and energy expenditure required (Vane, 2008). Toward this end, membrane separation processes such as pervaporation have been used. The great interest in these processes is mainly because of features such as cost-effectiveness, high energy efficiency and environmental friendliness. Membrane
based separation technologies normally fulfill the criteria for sustainability and energy efficiency (Korelskiy et al., 2013). In addition to reducing the inhibition of ethanol in the production stage due to the possibility of its simultaneous use with fermentation (Lewandowska and Kujawski, 2007), this procedure could replace a concentration step that is required for recovery because of the presence of alcohol in small quantities in the broth (Nomura et al., 2002). As shown by Chovau et al. (2011), the composition of the fermentation broth influences the separation, and the use of different substrates leads to the need to reevaluate the process, even if it is already well established. Also, in multicomponent systems, the diffusivity of one component is influenced by the presence of others. The use of lignocellulosic biomass will not only affect feedstock pretreatment and fermentation process of the ethanol production but also the downstream processing (Gaykawad et al., 2013). In pervaporation, a liquid mixture is fed through a membrane. The mixture components permeate selectively through the membrane and vaporize on the other side of the membrane where low pressure is maintained. By this means, there is a selective removal of organic compounds from dilute aqueous solutions. There are several studies regarding ethanol pervaporation and they relate mainly to the use of different membranes. Specifically the pervaporation of ethanol from lignocellulosic residues is reported by Gaykawad et al. (2013) with barley straw and willow wood using commercial polydimethylsiloxane (PDMS) membranes. Zhang et al. (2012) studied the membrane fouling in pervaporation of ethanol from food waste after a flocculation–filtration pretreatment. Aroujalian and Raisi (2009) study the effects of various operating parameters such as feed temperature, permeate side pressure, and Reynolds number (volumetric flow rate) on the total flux, and ethanol selectivity of a porous membrane-based pervaporation process with 2% aqueous ethanol solutions, simulating an ethanol content from lignocellulosic residues. O’Brien et al. (2004) related an efficient system of coupled fermentation and pervaporation for ethanol from corn fiber hydrolisates. Studies of pervaporation in ethanol production hitherto have not used banana waste as a substrate for ethanol production. Thus the aim of this research is to evaluate if pervaporation can be used in the production of ethanol from banana and to investigate the effects of operating variables and of lignocellulosic biomass fermentation by-products on membrane performance for the recovery of ethanol by using pervaporation. 2. Materials and methods To investigate the membrane behavior, first model solutions of ethanol/water were separated in pervaporation experiments to characterize ethanol transfer across the hollow polydimethylsiloxane (PDMS) membrane and these results provided the reference for the broth experiments. In the first case, feed conditions (flow rate, temperature, ethanol composition) and permeate pressure were modified. Also, the time necessary to reach steady state was determined. Then, tests were performed with the fermentation broth produced using banana fruit waste as a substrate varying the ethanol feed mass fraction and feed flow rate. The presence of some byproducts was also studied. 2.1. Membrane The pervaporation unit used consisted of a removable permeation module made of polyvinyl chloride (PVC) of 0.2 m internal diameter containing 50 dense hollow polydimethylsiloxane (PDMS) membranes (di = 0.6 mm and de = 1 mm), 0.25 m in length,
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covering an area of 0.04 m2. This is a hydrophobic commercial membrane, with the membrane and module built by a Brazilian company (PAM-Membranas Selectivas). 2.2. Fermentation To obtain the feed from the process of pervaporation, 0.002 m3 of fermentation broth was produced, as described by Schulz (2010). Rejected banana fruit (Musa cavendishii) was used as lignocellulosic material (500 g L1 wet mass) and 20% (v/v) of inoculum (Saccharomyces cerevisiae). After a period of 10 h of fermentation, different ethanol mass fractions were obtained in broths for pervaporation tests. The total volume of the broth was centrifuged at 3800 min1 (3018g) for 1200 s in a refrigerated centrifuge and then stored at 277 K.
Table 2 Operating conditions used in the experiments with fermentation broth. Experiments with fermentation broth
Operating conditions
Effect of feed flow rate
Feed flow rate: 5.5 106 and 22.2 106 m3 s1 Feed mass fraction: 3 wt% Feed temperature: 295 K
Variation of feed weight fraction
Feed flow rate: 5.5 106 m3 s1 Feed mass fraction: 2, 2.5 and 3 wt% Feed temperature: 295 K
Eqs. (1)–(5), respectively (Pereira et al., 2006; Gaykawad et al., 2013; Sun et al., 2013):
yi =yj xi =xj
ð1Þ
W At
ð2Þ
J i ¼ J tot yi
ð3Þ
ai ¼ 2.3. Pervaporation The fermentation broth/standard solution was supplied with the aid of a gear pump from a reservoir positioned upstream from the membrane. A vacuum pump coupled to the permeate side of the assembly provided the pressure drop for vaporization of ethanol. The permeate vapor was directed to a condensation bath refrigerated with liquid nitrogen. The liquid retentate was recirculated to the feed tank. A summary of the operating conditions tested is presented in Table 1 for the standard mixture and in Table 2 for the fermentation broth. All experiments were carried out with condensation temperature of 77 K and permeate pressure lower than 667 Pa. The tests were carried out in duplicate, and the results presented here were consistent with the average of the same. Feed and permeate samples (upon reaching the steady-state condition), were collected for 1–2 during each test to quantify the concentrations of ethanol with a gas chromatograph (GC, Agilent model 6890, coupled with autosampler: Agilent model 7683) using a Hewlett–Packard HP-1 column with a length of 50 m, an external diameter of 0.32 mm, a stationary phase composed of 100% polydimethylsiloxane and a film thickness of 1.05 lm. 2.4. Parameters from the process evaluation The performance of the process of pervaporation has been expressed in terms of separation factor of the membrane (ai), the total permeate flux (Jtot), the ethanol permeate flux (Ji), enrichment factor (bi) and pervaporation separation index (PSI) according to
J tot ¼
bi ¼
yi xi
ð4Þ
PSI ¼ J tot ðai 1Þ
ð5Þ
where yi and yj are the weight fractions of ethanol and water in the permeate, respectively, and xi and xj are the weight fractions of ethanol and water in the feed, W is the mass (g) of the permeate, A is the effective area (m2) of the membrane, and t is the time interval (h) for pervaporation. The membrane flux was determined gravimetrically using a balance with an accuracy of 104 g by weighing the mass of permeate obtained during the collecting time. The enrichment factor represents the membrane capacity of concentrating the component i (Pereira et al., 2006) and is an interesting parameter to evaluate the process for multicomponent mixtures. Temperature dependency of the flux was analyzed using an Arrhenius-type equation:
Ea lnðJ i Þ ¼ J io exp RT
ð6Þ
Experiments with model solutions
Operating conditions
where Ji is the partial flux of the compound, Jio is the pre-exponential factor of the flux, R is the gas constant (J mol1 K1), T is the temperature (K) and Ea is the apparent activation energy (KJ mol1). Permeance was estimated according to Eq. (9), based on the molar flux of each component – ji (obtained from Eqs. (7) and (8)), and the selectivity of the membrane was calculated from the ratio between permeances (Eq. (10)).
Effect of ethanol feed weight fraction
Feed flow rate: 5.5 106 m3 s1
J im ¼ J tot yi
Table 1 Operating conditions used in the experiments with the model solutions.
Effect of feed flow rate
Effect of feed temperature
Effect of permeate pressure
Feed weight fraction: 1–12 wt% Feed temperature: 295 K Feed flow rate: 5.5 106 and 22.2 106 m3 s1 Feed weight fraction: 10 wt% Feed temperature: 295 K Feed flow rate: 5.5 106 m3 s1 Feed weight fraction: 3 wt% Feed temperature: 295, 300 and 303 K Feed flow rate: 5.5 106 m3 s1 Feed weight fraction: 3 wt% Feed temperature: 295 K Permeate pressure: <600, 2600 and 6000 Pa
ji ¼
mi mt
J im v Gi mi
Pi ji ¼ l ðxi ci Poi yi Pp Þ
ai=j ¼
Pi=l Pj=l
ð7Þ
ð8Þ
ð9Þ
ð10Þ
with mi and mt being the molecular weight of the component I and the mixture, respectively, tGi is the molar volume of gas, cI is the
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activity coefficient for each component, Poi is the vapor pressure and Pp is the total pressure in the permeate side. Reynolds number was calculated according to Schnabel et al. (1998). It was considered an aqueous solution, and because of this, viscosity and density used was the values described by the authors with temperature of 298 K (25 °C). 3. Results and discussion 3.1. Experiments with model fermentation broths 3.1.1. Effect of ethanol feed weight fraction Chovau et al. (2013) reported that the fermentation of lignocellulosic wastes produces, on average, 3–6 wt% of ethanol. Hence we analyzed the results for total and partial fluxes and separation and enrichment factors, as illustrated in Figs. 1 and 2, respectively, with an ethanol weight fraction range from 1% to 5% (values expected for a second-generation biomass) and then compared them with 9%, 10% and 12% (values related or even superior to first-generation biomass). Increasing alcohol concentration increases permeation fluxes of the mixture of ethanol and water resulting in higher total flux (Fig. 1). In fact increasing the amount of the component that has major affinity with the membrane in the feed stream leads to an increase in the flux of these components in the permeate as reported in literature (Ortiz et al., 2002; Sommer and Melin, 2005; Wu et al., 2005; Zereshki et al., 2011). A higher ethanol concentration in the feed produces a higher ethanol flux. Dobrak et al. (2010) reported similar results with polydimethylsiloxane composite membranes, where the permeate flux through the membrane increased with increasing concentration of ethanol in the feed, due to the swelling effects. The ethanol flux demonstrated an increase even with ethanol concentrations less than 5 wt% in the feed, but this increase is even greater at higher concentrations. As reported by Pereira et al. (2006), the permeate flux of an organic dilute feed solution is generally a linear function of concentration, and the water flux remains constant (dependent on the concentration of the organic component). This behavior is observed in this study when ethanol content in the feed is less than 5 wt%. However, water flux increases more than alcohol, resulting in a separation factor reduction as observed in Fig. 2. According to Mohammadi et al. (2005), this is a result of an enhanced diffusion of water into the membrane by the fact that increasing alcohol concentration increases membrane-free volume and simultaneously
Fig. 1. Effect of ethanol content in feed on total (j), ethanol (d) and (N) water fluxes.
Fig. 2. Effect of ethanol content in feed on separation (j) and enrichment (d) factors.
side chain mobility increases. Consequently, small-sized water cluster can permeate easily through the membrane-free volume. However, Van Baelen et al. (2004) reported that this behavior can be attributed to a dragging effect. In both cases, the final result is a decrease in separation factor. This also was observed by Zhou et al. (2011), Lai et al. (2012), Lee et al. (2012), who also analyzed pervaporation of alcohols in dilute mixtures with hollow polydimethylsiloxane membrane. 3.1.2. Effect of feed flow rate Table 3 compares the values for the average total and ethanol permeate mass fluxes, separation and enrichment factor of the polydimethylsiloxane membrane obtained in pervaporation of 10 wt% of alcoholic mixtures when the feed flow rate was varied. As expected, ethanol and total flux increased with feed flow rate as shown in Table 3 due to a decrease of concentration polarization. These results are in agreement with other research, even when different membranes were used (Jiraratananon et al., 2002; Zereshki et al., 2011). As the ethanol is the component having the greatest affinity with the polydimethylsiloxane membrane, a reduction of concentration polarization means that ethanol concentration near the membrane surface was close to the ethanol concentration in the bulk. As observed in relation to the partial flux of ethanol, an increase of feed flow rate can enhance the permeation rate of ethanol with the water remaining at almost the same value. This behavior is in agreement with studies considering the effect of Reynolds number in pervaporation of organic compounds (Liang et al., 2004) In fact, the lower flux was observed at the lowest value of the Reynolds number (feed flow = 5.5 106 m3 s1 – Reynolds number: 18.47; feed flow = 22.2 106 m3 s1 – Reynolds number = 73.90). The increase of feed flow rate enables better penetration of ethanol into the membrane as the driving force of ethanol increases. As consequence, the hydrophobic properties of the membrane also increase, and the process will be more selective for ethanol, as indicated by the separation factor that showed an increase around 30%. Li et al. (2004) also reported that with increasing flow rate both total flux and separation factor increase, using a similar membrane (polydimethylsiloxane supported by cellulose acetate) for a model solution with 5 wt% ethanol. The authors show values for the separation factor very close to that indicated in Table 4, with the same feed flow rate. Finally, an increase of PSI with feed flow rate indicated that better results will be obtained for the pervaporation system with higher flow rates, as observed in other works (Jiraratananon et al., 2002).
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R.H. Bello et al. / Waste Management xxx (2014) xxx–xxx Table 3 Results obtained for pervaporation experiments with model solutions (10 wt% ethanol) and flow rates of 5.5 and 22.2 106 m3 s1. Flow rate (m3 s1) 6
5.5 10 22.2 106
Total flux (g m2 h1)
Ethanol flux (g m2 h1)
Separation factor
Enrichment factor
PSI
8.07 8.22
3.34 3.92
6.32 8.26
4.13 4.72
43.07 61.05
The improvement in ethanol content in permeate, which occurs because of the effect of reduced concentration polarization at higher flow rates that reduces the transport resistance in the liquid boundary layer, results in a mass fraction of ethanol in permeate of 41.27 wt% with 5.55 106 m3 s1 and 47.24 wt% with 22.2 106 m3 s1. This is an interesting result when compared with the conventional ethanol recovery method, which is distillation. Standard purification involves a first distillation column, the ‘beer column’, where the top product consists of 37–40 wt% of ethanol (Chovau et al., 2013). The ethanol content observed was similar, indicating the potential of pervaporation as a concentration method for further distillation. 3.1.3. Effect of feed temperature The effect of the temperature on the total and partial fluxes for 3 wt% ethanol/water mixture is presented in Fig. 3. As related in other works (Li et al., 2004; Mohammadi et al., 2005; Dobrak et al., 2010; Luis et al., 2013) increasing temperature causes an increase in the total permeate flux due to the increase of diffusion rate of individual permeating molecules by the free volumes produced because of the thermal motions of polymer chains. As related by Pereira et al. (2006), a higher temperature stimulates the driving force due to an increase in vapor pressure and the activity coefficient of the permeating species (and their chemical potentials) as temperature grows. Fig. 3 illustrates that when temperature feed was tested from 295 to 300 K, total flux was increased by a factor of 1.18, water flux by 1.14 and ethanol flux by 1.5. This can also be observed from Fig. 4 which illustrates the effect of feed temperature in separation factor. Different effects were related in the literature, with some studies demonstrating that the separation factor increases when temperature is increasing and others showing the contrary. According to Zereshki et al. (2011), the main reason for the separation factor to be affected by temperature is because the diffusion and solubility of penetrating components changes significantly with temperature, but also depends on many factors such as different organics, different membrane-preparation method, and different supports of composite membranes and so on. The total and partial permeation flux is plotted in logarithmic scale as a function of the reciprocal temperature in Fig. 5 and results show that an Arrhenius type relationship exists between the fluxes and feed temperature, i.e. fluxes decrease with decreasing temperature. These results are in agreement with the literature (Zhou et al., 2011; Lai et al., 2012; Lee et al., 2012). The apparent activation energy could be calculated from the slope of the corresponding curve and Eq. (6) and the value is summarized in Table 4. The higher apparent activation energy value for ethanol flux indicates that it was more affected than water flux as the
temperature increased. These results are in agreement with the study by Shepherd et al. (2002) that used the same membrane to separate aroma compounds. Thus the separation factor increases with the increase of temperature. Similar results are presented by Dobrak et al. (2010). 3.1.4. Effect of permeate pressure Three variations of permeate pressure were investigated in the pervaporation system: 6000, 2600 and less than 600 Pa. The results obtained for the mass flow of permeate with respect to the different applied pressures in the process of pervaporation are shown in Table 5. Pereira et al. (2006) describe how when the permeate pressure grows, the chemical potential gradient across the membrane is reduced and, as a consequence, a reduction in transmembrane flux is observed. The same result was obtained in this work, with the higher effect observed when the vapor pressure in the permeate stream was close to the ethanol vapor pressure. The results presented in Table 5 show a decrease of 64% at a pressure of 2660 Pa and 95% at 6000 Pa in relation to a maximum of 5.85 g m2 h1 were obtained for the permeate flux at the system operating with a permeate pressure less than 600 Pa. Jiraratananon et al. (2002) found that in ethanol–water mixtures, as permeate pressure increases, the driving force for permeation of the ethanol molecules decreases, which results in a decrease in the mass flow of the permeate. 3.2. Experiments with fermentation broth 3.2.1. Effect of feed flow rate Tests were performed with the fermentation broth that contained about 3 wt% ethanol by weight in the feed mixture, and the results were compared with those obtained using the standard solution at the same ethanol weight fraction. The results are summarized in Table 6. It was observed that all of the parameters increased for the pervaporation of the broth at low flow rates compared with the model solutions. For the specific case of the ethanol produced from the
Table 4 Apparent activation energy for ethanol and water permeation estimated from experiments with ethanol (3 wt%) and water mixtures. Apparent activation energy (KJ mol1)
Value
Ethanol Water
83.23 62.64
Fig. 3. Temperature dependency on the total (j), ethanol (d) and water (N) fluxes for 3 wt% ethanol/water mixture.
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enrichment factor, since this parameter relates to the separation ability by the ratio of permeate and feed weight fractions. A reduction twice was observed. Also, when comparing the pervaporation of fermentation broth between different flow rates, all parameters decrease with the increase of feed flow. In these cases, the effect observed in the enrichment factor was more pronounced.
Fig. 4. Effect of feed temperature in separation factor for 3 wt% ethanol/water mixture.
Fig. 5. Arrhenius plot of total (j), water (N) and ethanol (d) fluxes vs. temperature for ethanol (3 wt%) and water mixtures.
lignocellulosic banana waste, with 5.5x106 m3 s1 of flow rate, an increase of about 28% in the enrichment factor was obtained, as well as a 3.5 times increase in ethanol flux, which resulted in a pervaporation index (PSI) double that achieved with the standard mixture (62.18 vs. 29.11). One important analysis consists of the enrichment factor. This parameter remained almost the same, indicating a possibility that there is no flux coupling mechanism. Shepherd et al. (2002) reported the same results using the similar membrane and studying multicomponent model mixtures. For the flow rate of 22.2 106 m3 s1, contrary observations were made. All performance parameters decrease for fermentation broth compared with model solutions. This can be a result of the presence of by-product that at this flow rate has an influence on the ethanol diffusion mechanism. As the flux was reduced by a factor of 0.25 and the separation factor was reduced by a factor of 0.43, it was concluded that the ethanol pervaporation was minimized. This behavior could be supported by the results of Table 5 Effect of permeate pressure in total permeate flux for 3 wt% ethanol/water mixtures. Pressure (Pa)
<600
2660
6000
Total flux (g m2 h1)
5.85
2.1
0.3
3.2.2. Variation of feed weight fraction Tests were performed with the fermentation broth of varying ethanol by weight in the feed mixture, and the results were compared with those obtained using the standard solution at the same ethanol weight fraction. Figs. 6–8 show the comparison of results obtained with the model mixture and fermentation broth for total flux, separation and enrichment factors and pervaporation index, respectively. The results of the parameters presented demonstrated an increase that was observed with the pervaporation of fermentation broth. The comparison between model solutions and broth shows similar results, i.e., when the feed ethanol weight fraction is increased, we observed an increase of flux, decrease of enrichment and separation factor (consequently an increasing of permeate ethanol weight fraction). More interesting, however, is the increase in pervaporation index (Fig. 8), which indicates that the pervaporation of the fermentation broth obtained from banana residue with PDMS membrane has a more pronounced increase with the increase of feed ethanol weight fraction than does the model solution. Figs. 9 and 10 show the comparison of results obtained with model mixture and fermentation broth for permeance and selectivity, respectively. According to Luis et al. (2013), these parameters enable better observation of the process efficiency once the effect of the driving force and operating conditions is eliminated when the parameters are calculated. Fig. 9 shows better results for broth pervaporation, and an increase of both ethanol and water flux as the feed ethanol weight fraction is increased, as observed in Fig. 6. However, the permeance analysis shows clearly that the ethanol flux increase has a major effect on the total flux increase, especially for fermentation broth. This indicates that by-products present in the multicomponent mixtures could be facilitating the ethanol permeability, and no coupling flux is observed. In fact, by-products of fermentation, such as carboxylic acids, aldehydes, alcohols and other salts, may influence the separation process, especially at lower ethanol feed mass fractions and flow rates. No glucose was observed in the broth used for the separation, when analyzed via high-performance liquid chromatography. Glucose has been shown to affect the performance of pervaporation, as reported by Chovau et al. (2011). In testing for selectivity, as observed before and with model solutions, the membrane shows more affinity for water, mainly at higher feed ethanol weight fractions. However, is important to notice that a value of selectivity equal to 1 represents the performance of a membrane with no intrinsic membrane selectivity, achieving the same separation as simple evaporation of the liquid into the vapor phase (Luis et al., 2013). In this work, selectivity shows values greater than 1, and higher for fermentation broth than for model solutions, which indicates that the effect of the membrane is to enrich the permeate with ethanol. 3.2.3. Influence of by-products Table 7 presents the average values of the by-products of the broth in the feed analyzed by HPLC. The results for the centrifuged broth were compared with those for the standard mixture to determine the influence of by-products on the permeate flux, the enrichment factor and the concentration of the permeate. According to Table 7, the glycerol concentration in the fermentation broth in the present study was 1.67% by weight. Chovau et al. (2011) have reported that the presence of glycerol in low con-
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R.H. Bello et al. / Waste Management xxx (2014) xxx–xxx Table 6 Comparison of the pervaporation carried out at different feed flow rates with ethanol/water model solutions and fermentation broth with 3 wt% of ethanol. Total flux (g m2 h1)
Ethanol flux (g m2 h1)
Enrichment factor
Separation factor
PSI
3.58 5.85
0.37 1.06
7.56 8.88
8.31 10.63
29.71 62.18
Flow rate 22.2 106 m3 s1 Ethanol/water 4.80 Broth 3.60
0.41 0.30
6.51 3.73
7.06 3.97
33.91 14.29
Experiment Flow rate 5.5 10 Ethanol/water Broth
6
3
m s
1
Fig. 6. Comparison of the effect of ethanol content in feed on total flux for model solutions (j) and fermentation broth (h).
Fig. 8. Comparison of the effect of ethanol content in feed in pervaporation index (PSI) for model solutions (j) and fermentation broth (h).
Fig. 7. Comparison of the effect of ethanol content in feed on separation (j) and enrichment (d) factors for model solutions (closed symbols) and fermentation broth (open symbols).
Fig. 9. Comparison of the effect of ethanol content in feed on permeance of ethanol (d) and water (N) for model solutions (closed symbols) and fermentation broth (open symbols).
centrations does not significantly affect the permeate flux parameter or the coefficient of enrichment. In addition, the effect of carboxylic acids on the process of pervaporation was studied, and the presence of lactic acid at a concentration of 10.9 mmol L1 was observed to increase both permeate and water flux; however, the coefficient of enrichment was reduced to 12.2%. As previously discussed, in tests where a flow rate of 5.5 106 m3 s1 was used, a comparison of the pervaporation of the fermentation broth and that of the standard mixture of ethanol and water indicates that the best results are achieved with the fermentation broth. Furthermore, in the specific case of ethanol produced from lignocellulosic banana waste, greater values were
observed for the pervaporation index than for the standard mixture. We believe that the observed increase in the pervaporation index was due to the presence of lactic acid in the broth. As a result of the low concentration of this acid observed in the fermentation broth at 0.75 mmol L1, a decrease was not observed in the enrichment factor of ethanol, as reported in the work of Chovau et al. (2011). Thus, because the polydimethylsiloxane membrane is hydrophobic, the presence of lactic acid increased the hydrophilicity of the membrane, which increased the flow of water, whereas the increase in the flow of ethanol can be attributed to the effect of membrane fouling. Bowen et al. (2007) have confirmed these statements in their studies.
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solutions. The process efficiency was evaluated by permeance and selectivity, showing that ethanol flux increase has a major effect on the total flux increase, especially for fermentation broth. In fact, in the specific case of ethanol produced from lignocellulosic banana waste, greater values were observed for the pervaporation index than for the standard mixture. This could due to by-products present in the multicomponent mixtures facilitating the ethanol permeability. By-products analysis show that the presence of lactic acid increased the hydrophilicity of the membrane, which increased the flow of water, whereas the increase in the flow of ethanol can be attributed to the effect of membrane fouling. Thus, it can be concluded that the pervaporation of ethanol produced from banana waste is indeed a technology with the potential to be applied. The results showed a very interesting performance, highlighting the better separation efficiency for fermentation broth in relation to model solutions. Fig. 10. Comparison of the effect of ethanol content in feed on selectivity for model solutions (j) and fermentation broth (h).
Table 7 By-products of the fermentation broth that may affect the process of pervaporation.
a
Byproduct
Average concentration (g L1)
Lactic acid Acetic acid Glucose Glycerol
0.068 NDª ND 1.67
Not detected.
Finally, in all experiments the flux values obtained (5 g m2 h1) were inferior to those described in the literature, such as those mentioned in work by Molina et al. (2002), who obtained 500 g m2 h1 at a concentration of 15% (wt) under similar operating conditions. This discrepancy indicates the need for optimization of the operating conditions. However, the values obtained here are consistent with the data provided by the membrane supplier. In this regard, the results are considered to be encouraging because the parameters show substantial increases in the recovery of ethanol produced by fermentation using pervaporation compared with using the standard mixture. 4. Conclusions The use of the pervaporation process for the recovery of ethanol from fermentation broth produced from lignocellulosic banana waste demonstrated a very attractive alternative to classical methods. When tests were carried out varying operational conditions with model solutions, it was observed behaviors related to reported in literature: a higher ethanol concentration in the feed produces a higher ethanol flux and decrease the separation factor; an increase of feed flow rate can enhance the permeation rate of ethanol with the water remaining at almost the same value; water and ethanol fluxes was increased with the temperature increase; and the higher effect in flux increase was observed when the vapor pressure in the permeate stream was close to the ethanol vapor pressure. One interesting point was that the ethanol flux demonstrated an increase even with ethanol concentrations less than 5 wt% in the feed (concentration expected in second generation ethanol), but this increase is even greater at higher concentrations, as expected. However, water flux increases more than alcohol, resulting in a separation factor reduction. It was observed that all of the parameters increased for the pervaporation of the broth at low flow rates compared with the model
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Pervaporation of ethanol produced from banana waste Roger Hoel Bello a, Poliana Linzmeyer b, Cláudia Maria Bueno Franco b, Ozair Souza a,b,c, Noeli Sellin a,b,c, Sandra Helena Westrupp Medeiros a,b,c, Cintia Marangoni d,⇑ a
Chemical Engineering Department, University of Joinville Region (UNIVILLE), Joinville, SC, Brazil Sanitary and Ambient Engineering Department, University of Joinville Region (UNIVILLE), Joinville, SC, Brazil c Masters Program in Process Engineering, University of Joinville Region (UNIVILLE), Joinville, SC, Brazil d Federal University of Santa Catarina (UFSC), Campus Blumenau, Blumenau, SC, Brazil b
a r t i c l e
i n f o
Article history: Received 19 December 2013 Accepted 10 April 2014 Available online xxxx Keywords: Banana waste Bioethanol Biofuel Lignocellulosic residue Pervaporation Polydimethylsiloxane membrane
a b s t r a c t Banana waste has the potential to produce ethanol with a low-cost and sustainable production method. The present work seeks to evaluate the separation of ethanol produced from banana waste (rejected fruit) using pervaporation with different operating conditions. Tests were carried out with model solutions and broth with commercial hollow hydrophobic polydimethylsiloxane membranes. It was observed that pervaporation performance for ethanol/water binary mixtures was strongly dependent on the feed concentration and operating temperature with ethanol concentrations of 1–10%; that an increase of feed flow rate can enhance the permeation rate of ethanol with the water remaining at almost the same value; that water and ethanol fluxes was increased with the temperature increase; and that the higher effect in flux increase was observed when the vapor pressure in the permeate stream was close to the ethanol vapor pressure. Better results were obtained with fermentation broth than with model solutions, indicated by the permeance and membrane selectivity. This could be attributed to by-products present in the multicomponent mixtures, facilitating the ethanol permeability. By-products analyses show that the presence of lactic acid increased the hydrophilicity of the membrane. Based on this, we believe that pervaporation with hollow membrane of ethanol produced from banana waste is indeed a technology with the potential to be applied. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction One of the bases for the economic development of Brazil is agriculture, and in certain states, such as Santa Catarina, banana is widely cultivated as a commercial crop. Banana is one of the most consumed fruits in the world and it is commercially grown in about 120 countries. Currently, Brazil is the second largest producer (preceded by India) and is responsible for 7.5% of world production (about 7.2 million tons per annum, according to the Center of Socioeconomics and Agricultural Planning for the State of Santa Catarina). The State of Santa Catarina has approximately six thousand producers, being the fourth largest banana growing region in Brazil, with 663,892 tonnes of bunches of bananas produced per annum (ABIB, 2011). Commercial banana production generates a large proportion of waste; there are reports of 30% waste in Australia (Clarke et al., 2008) and Malaysia (Tock et al., 2010), and 25–50% in Central ⇑ Corresponding author. Address: Rua Pomerode, 710 Salto do Norte, Blumenau, SC 89065-300, Brazil. Tel.: +55 48 3721-6308. E-mail address: [email protected] (C. Marangoni).
and South America (Hammond et al., 1996). In countries like India, all kind of banana waste is considered an important urban waste because the fruit is used in all religious functions, festivals and in temples (Chanakya and Sreesha, 2011). According to Graefe et al. (2011), around 20–40% of the bananas produced do not meet export standards or even the quality demands of spot markets. In Brazil, particularly in the southern regions, it is estimated that for every 100 kg of harvested fruit, 46 kg are not used (EMBRAPA, 2006). Further, Souza et al. (2010) indicate that for every ton of bananas produced approximately 3 tons of pseudostem, 160 kg of stems, 480 kg of leaves and 440 kg of skins are generated. Fernandes et al. (2013) found that less than 10% of available biomass as waste (440 million tons) is designated to some application. Thus, an established commercial use for such residues, as well as generating extra remuneration for regional farmers, would help to reduce environmental pollution. Alternative uses for these discards have to be explored, and in this regard processing to produce ethanol is seen to have potential from both an environmental as well as an economic point of view. Many countries are investing in the development and use of biofuels as a way of reducing environmental impacts and ethanol
http://dx.doi.org/10.1016/j.wasman.2014.04.013 0956-053X/Ó 2014 Elsevier Ltd. All rights reserved.
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is one of the fuels that can be produced from various raw materials (España-Gamboa et al., 2011). The use of agricultural or agroindustrial waste is an interesting option in this context. Biofuel has been produced on a large scale in Brazil for three decades using sugarcane as feedstock (Soccol et al., 2010), however there are many criticisms of the practice and an ongoing debate about the ethical issue of using food (or land available for the cultivation of food) as an energy source (Sarkar et al., 2012; Swana et al., 2011). Lignocellulosic material does not play an intrinsic role in the food chain and this is a fundamental aspect that makes it an attractive alternative for ethanol production. Besides, the cost and availability of the feedstock are crucial and can contribute 65–70% to the total ethanol production costs (Kazi et al., 2010). In this sense, the substitution of biomass wastes for raw materials such as cane sugar, starch and corn for ethanol production is an alternative that has shown promising results (Dermibas, 2011; Mabee et al., 2011). Examples of this residual biomass include bagasse sugar cane, corn straw and fiber, wheat and rice straw, eucalyptus wood and crop wastes from commercial cultivation of fruits such as bananas, grapes and apples (Rivas-Cantu et al., 2013). Technologies for the conversion of biomass to ethanol are also under various stages of development. The use of these lignocellulosic residues requires some separation of cellulose and hemicellulose from lignin, followed by hydrolysis of sugars, and this bioconversion has been extensively studied using the different types of wastes. The potential yield of ethanol from lignocellulosics varies significantly between feedstocks, so many applications in alcoholic fermentation are reported in the literature with different wastes. Specifically in the case of ethanol from bananas, the few studies that have been published involve the use of the fruit, leaves and other waste such as the pseudostem. Tewari et al. (1986) reported the suitability of banana peel for alcohol fermentation. Hammond et al. (1996) presented ethanol yield (on a dry weight basis) from ripe bananas as higher than from most other agricultural commodities. Velásquez-Arredondo et al. (2010) investigated the acid hydrolysis of banana pulp and fruit and the enzymatic hydrolysis of flower stalk and banana skin, and the results obtained demonstrated a positive energy balance for the four production routes evaluated. The study by Graefe et al. (2011) presents results of a case study in Costa Rica and Ecuador which found that considerable amounts of ethanol could be produced from banana bunches that do not meet quality standards, as well as from which are partly left to rot in the fields. Oberoi et al. (2011) also demonstrated that banana peel could serve as an ideal substrate for the production of ethanol through simultaneous saccharification and fermentation. Hossain et al. (2011) evaluated bioethanol from rotten banana and concluded that this can be used in motor vehicle engines, producing low emissions, and thus it can be used as an environmental recycling process for waste management. Arumugam and Manikandan (2011) explore the potential application of pulp and banana peel wastes in bioethanol production using dilute acid pretreatment followed by enzymatic hydrolysis. Gonçalves Filho et al. (2013) evaluate the same techniques with banana tree pseudostem. Although the lignocellulosic material shows positive results, it still requires more research to be exploited on an industrial scale. Great efforts are being undertaken to improve ethanol productivity and reduce the overall production costs. According to Gaykawad et al. (2013), one of the ways to achieve these goals is to modify the configuration of the process and perform process integration. Traditionally, the recovery of ethanol by distillation is a challenge because of the high costs and energy expenditure required (Vane, 2008). Toward this end, membrane separation processes such as pervaporation have been used. The great interest in these processes is mainly because of features such as cost-effectiveness, high energy efficiency and environmental friendliness. Membrane
based separation technologies normally fulfill the criteria for sustainability and energy efficiency (Korelskiy et al., 2013). In addition to reducing the inhibition of ethanol in the production stage due to the possibility of its simultaneous use with fermentation (Lewandowska and Kujawski, 2007), this procedure could replace a concentration step that is required for recovery because of the presence of alcohol in small quantities in the broth (Nomura et al., 2002). As shown by Chovau et al. (2011), the composition of the fermentation broth influences the separation, and the use of different substrates leads to the need to reevaluate the process, even if it is already well established. Also, in multicomponent systems, the diffusivity of one component is influenced by the presence of others. The use of lignocellulosic biomass will not only affect feedstock pretreatment and fermentation process of the ethanol production but also the downstream processing (Gaykawad et al., 2013). In pervaporation, a liquid mixture is fed through a membrane. The mixture components permeate selectively through the membrane and vaporize on the other side of the membrane where low pressure is maintained. By this means, there is a selective removal of organic compounds from dilute aqueous solutions. There are several studies regarding ethanol pervaporation and they relate mainly to the use of different membranes. Specifically the pervaporation of ethanol from lignocellulosic residues is reported by Gaykawad et al. (2013) with barley straw and willow wood using commercial polydimethylsiloxane (PDMS) membranes. Zhang et al. (2012) studied the membrane fouling in pervaporation of ethanol from food waste after a flocculation–filtration pretreatment. Aroujalian and Raisi (2009) study the effects of various operating parameters such as feed temperature, permeate side pressure, and Reynolds number (volumetric flow rate) on the total flux, and ethanol selectivity of a porous membrane-based pervaporation process with 2% aqueous ethanol solutions, simulating an ethanol content from lignocellulosic residues. O’Brien et al. (2004) related an efficient system of coupled fermentation and pervaporation for ethanol from corn fiber hydrolisates. Studies of pervaporation in ethanol production hitherto have not used banana waste as a substrate for ethanol production. Thus the aim of this research is to evaluate if pervaporation can be used in the production of ethanol from banana and to investigate the effects of operating variables and of lignocellulosic biomass fermentation by-products on membrane performance for the recovery of ethanol by using pervaporation. 2. Materials and methods To investigate the membrane behavior, first model solutions of ethanol/water were separated in pervaporation experiments to characterize ethanol transfer across the hollow polydimethylsiloxane (PDMS) membrane and these results provided the reference for the broth experiments. In the first case, feed conditions (flow rate, temperature, ethanol composition) and permeate pressure were modified. Also, the time necessary to reach steady state was determined. Then, tests were performed with the fermentation broth produced using banana fruit waste as a substrate varying the ethanol feed mass fraction and feed flow rate. The presence of some byproducts was also studied. 2.1. Membrane The pervaporation unit used consisted of a removable permeation module made of polyvinyl chloride (PVC) of 0.2 m internal diameter containing 50 dense hollow polydimethylsiloxane (PDMS) membranes (di = 0.6 mm and de = 1 mm), 0.25 m in length,
Please cite this article in press as: Bello, R.H., et al. Pervaporation of ethanol produced from banana waste. Waste Management (2014), http://dx.doi.org/ 10.1016/j.wasman.2014.04.013
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covering an area of 0.04 m2. This is a hydrophobic commercial membrane, with the membrane and module built by a Brazilian company (PAM-Membranas Selectivas). 2.2. Fermentation To obtain the feed from the process of pervaporation, 0.002 m3 of fermentation broth was produced, as described by Schulz (2010). Rejected banana fruit (Musa cavendishii) was used as lignocellulosic material (500 g L1 wet mass) and 20% (v/v) of inoculum (Saccharomyces cerevisiae). After a period of 10 h of fermentation, different ethanol mass fractions were obtained in broths for pervaporation tests. The total volume of the broth was centrifuged at 3800 min1 (3018g) for 1200 s in a refrigerated centrifuge and then stored at 277 K.
Table 2 Operating conditions used in the experiments with fermentation broth. Experiments with fermentation broth
Operating conditions
Effect of feed flow rate
Feed flow rate: 5.5 106 and 22.2 106 m3 s1 Feed mass fraction: 3 wt% Feed temperature: 295 K
Variation of feed weight fraction
Feed flow rate: 5.5 106 m3 s1 Feed mass fraction: 2, 2.5 and 3 wt% Feed temperature: 295 K
Eqs. (1)–(5), respectively (Pereira et al., 2006; Gaykawad et al., 2013; Sun et al., 2013):
yi =yj xi =xj
ð1Þ
W At
ð2Þ
J i ¼ J tot yi
ð3Þ
ai ¼ 2.3. Pervaporation The fermentation broth/standard solution was supplied with the aid of a gear pump from a reservoir positioned upstream from the membrane. A vacuum pump coupled to the permeate side of the assembly provided the pressure drop for vaporization of ethanol. The permeate vapor was directed to a condensation bath refrigerated with liquid nitrogen. The liquid retentate was recirculated to the feed tank. A summary of the operating conditions tested is presented in Table 1 for the standard mixture and in Table 2 for the fermentation broth. All experiments were carried out with condensation temperature of 77 K and permeate pressure lower than 667 Pa. The tests were carried out in duplicate, and the results presented here were consistent with the average of the same. Feed and permeate samples (upon reaching the steady-state condition), were collected for 1–2 during each test to quantify the concentrations of ethanol with a gas chromatograph (GC, Agilent model 6890, coupled with autosampler: Agilent model 7683) using a Hewlett–Packard HP-1 column with a length of 50 m, an external diameter of 0.32 mm, a stationary phase composed of 100% polydimethylsiloxane and a film thickness of 1.05 lm. 2.4. Parameters from the process evaluation The performance of the process of pervaporation has been expressed in terms of separation factor of the membrane (ai), the total permeate flux (Jtot), the ethanol permeate flux (Ji), enrichment factor (bi) and pervaporation separation index (PSI) according to
J tot ¼
bi ¼
yi xi
ð4Þ
PSI ¼ J tot ðai 1Þ
ð5Þ
where yi and yj are the weight fractions of ethanol and water in the permeate, respectively, and xi and xj are the weight fractions of ethanol and water in the feed, W is the mass (g) of the permeate, A is the effective area (m2) of the membrane, and t is the time interval (h) for pervaporation. The membrane flux was determined gravimetrically using a balance with an accuracy of 104 g by weighing the mass of permeate obtained during the collecting time. The enrichment factor represents the membrane capacity of concentrating the component i (Pereira et al., 2006) and is an interesting parameter to evaluate the process for multicomponent mixtures. Temperature dependency of the flux was analyzed using an Arrhenius-type equation:
Ea lnðJ i Þ ¼ J io exp RT
ð6Þ
Experiments with model solutions
Operating conditions
where Ji is the partial flux of the compound, Jio is the pre-exponential factor of the flux, R is the gas constant (J mol1 K1), T is the temperature (K) and Ea is the apparent activation energy (KJ mol1). Permeance was estimated according to Eq. (9), based on the molar flux of each component – ji (obtained from Eqs. (7) and (8)), and the selectivity of the membrane was calculated from the ratio between permeances (Eq. (10)).
Effect of ethanol feed weight fraction
Feed flow rate: 5.5 106 m3 s1
J im ¼ J tot yi
Table 1 Operating conditions used in the experiments with the model solutions.
Effect of feed flow rate
Effect of feed temperature
Effect of permeate pressure
Feed weight fraction: 1–12 wt% Feed temperature: 295 K Feed flow rate: 5.5 106 and 22.2 106 m3 s1 Feed weight fraction: 10 wt% Feed temperature: 295 K Feed flow rate: 5.5 106 m3 s1 Feed weight fraction: 3 wt% Feed temperature: 295, 300 and 303 K Feed flow rate: 5.5 106 m3 s1 Feed weight fraction: 3 wt% Feed temperature: 295 K Permeate pressure: <600, 2600 and 6000 Pa
ji ¼
mi mt
J im v Gi mi
Pi ji ¼ l ðxi ci Poi yi Pp Þ
ai=j ¼
Pi=l Pj=l
ð7Þ
ð8Þ
ð9Þ
ð10Þ
with mi and mt being the molecular weight of the component I and the mixture, respectively, tGi is the molar volume of gas, cI is the
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R.H. Bello et al. / Waste Management xxx (2014) xxx–xxx
activity coefficient for each component, Poi is the vapor pressure and Pp is the total pressure in the permeate side. Reynolds number was calculated according to Schnabel et al. (1998). It was considered an aqueous solution, and because of this, viscosity and density used was the values described by the authors with temperature of 298 K (25 °C). 3. Results and discussion 3.1. Experiments with model fermentation broths 3.1.1. Effect of ethanol feed weight fraction Chovau et al. (2013) reported that the fermentation of lignocellulosic wastes produces, on average, 3–6 wt% of ethanol. Hence we analyzed the results for total and partial fluxes and separation and enrichment factors, as illustrated in Figs. 1 and 2, respectively, with an ethanol weight fraction range from 1% to 5% (values expected for a second-generation biomass) and then compared them with 9%, 10% and 12% (values related or even superior to first-generation biomass). Increasing alcohol concentration increases permeation fluxes of the mixture of ethanol and water resulting in higher total flux (Fig. 1). In fact increasing the amount of the component that has major affinity with the membrane in the feed stream leads to an increase in the flux of these components in the permeate as reported in literature (Ortiz et al., 2002; Sommer and Melin, 2005; Wu et al., 2005; Zereshki et al., 2011). A higher ethanol concentration in the feed produces a higher ethanol flux. Dobrak et al. (2010) reported similar results with polydimethylsiloxane composite membranes, where the permeate flux through the membrane increased with increasing concentration of ethanol in the feed, due to the swelling effects. The ethanol flux demonstrated an increase even with ethanol concentrations less than 5 wt% in the feed, but this increase is even greater at higher concentrations. As reported by Pereira et al. (2006), the permeate flux of an organic dilute feed solution is generally a linear function of concentration, and the water flux remains constant (dependent on the concentration of the organic component). This behavior is observed in this study when ethanol content in the feed is less than 5 wt%. However, water flux increases more than alcohol, resulting in a separation factor reduction as observed in Fig. 2. According to Mohammadi et al. (2005), this is a result of an enhanced diffusion of water into the membrane by the fact that increasing alcohol concentration increases membrane-free volume and simultaneously
Fig. 1. Effect of ethanol content in feed on total (j), ethanol (d) and (N) water fluxes.
Fig. 2. Effect of ethanol content in feed on separation (j) and enrichment (d) factors.
side chain mobility increases. Consequently, small-sized water cluster can permeate easily through the membrane-free volume. However, Van Baelen et al. (2004) reported that this behavior can be attributed to a dragging effect. In both cases, the final result is a decrease in separation factor. This also was observed by Zhou et al. (2011), Lai et al. (2012), Lee et al. (2012), who also analyzed pervaporation of alcohols in dilute mixtures with hollow polydimethylsiloxane membrane. 3.1.2. Effect of feed flow rate Table 3 compares the values for the average total and ethanol permeate mass fluxes, separation and enrichment factor of the polydimethylsiloxane membrane obtained in pervaporation of 10 wt% of alcoholic mixtures when the feed flow rate was varied. As expected, ethanol and total flux increased with feed flow rate as shown in Table 3 due to a decrease of concentration polarization. These results are in agreement with other research, even when different membranes were used (Jiraratananon et al., 2002; Zereshki et al., 2011). As the ethanol is the component having the greatest affinity with the polydimethylsiloxane membrane, a reduction of concentration polarization means that ethanol concentration near the membrane surface was close to the ethanol concentration in the bulk. As observed in relation to the partial flux of ethanol, an increase of feed flow rate can enhance the permeation rate of ethanol with the water remaining at almost the same value. This behavior is in agreement with studies considering the effect of Reynolds number in pervaporation of organic compounds (Liang et al., 2004) In fact, the lower flux was observed at the lowest value of the Reynolds number (feed flow = 5.5 106 m3 s1 – Reynolds number: 18.47; feed flow = 22.2 106 m3 s1 – Reynolds number = 73.90). The increase of feed flow rate enables better penetration of ethanol into the membrane as the driving force of ethanol increases. As consequence, the hydrophobic properties of the membrane also increase, and the process will be more selective for ethanol, as indicated by the separation factor that showed an increase around 30%. Li et al. (2004) also reported that with increasing flow rate both total flux and separation factor increase, using a similar membrane (polydimethylsiloxane supported by cellulose acetate) for a model solution with 5 wt% ethanol. The authors show values for the separation factor very close to that indicated in Table 4, with the same feed flow rate. Finally, an increase of PSI with feed flow rate indicated that better results will be obtained for the pervaporation system with higher flow rates, as observed in other works (Jiraratananon et al., 2002).
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R.H. Bello et al. / Waste Management xxx (2014) xxx–xxx Table 3 Results obtained for pervaporation experiments with model solutions (10 wt% ethanol) and flow rates of 5.5 and 22.2 106 m3 s1. Flow rate (m3 s1) 6
5.5 10 22.2 106
Total flux (g m2 h1)
Ethanol flux (g m2 h1)
Separation factor
Enrichment factor
PSI
8.07 8.22
3.34 3.92
6.32 8.26
4.13 4.72
43.07 61.05
The improvement in ethanol content in permeate, which occurs because of the effect of reduced concentration polarization at higher flow rates that reduces the transport resistance in the liquid boundary layer, results in a mass fraction of ethanol in permeate of 41.27 wt% with 5.55 106 m3 s1 and 47.24 wt% with 22.2 106 m3 s1. This is an interesting result when compared with the conventional ethanol recovery method, which is distillation. Standard purification involves a first distillation column, the ‘beer column’, where the top product consists of 37–40 wt% of ethanol (Chovau et al., 2013). The ethanol content observed was similar, indicating the potential of pervaporation as a concentration method for further distillation. 3.1.3. Effect of feed temperature The effect of the temperature on the total and partial fluxes for 3 wt% ethanol/water mixture is presented in Fig. 3. As related in other works (Li et al., 2004; Mohammadi et al., 2005; Dobrak et al., 2010; Luis et al., 2013) increasing temperature causes an increase in the total permeate flux due to the increase of diffusion rate of individual permeating molecules by the free volumes produced because of the thermal motions of polymer chains. As related by Pereira et al. (2006), a higher temperature stimulates the driving force due to an increase in vapor pressure and the activity coefficient of the permeating species (and their chemical potentials) as temperature grows. Fig. 3 illustrates that when temperature feed was tested from 295 to 300 K, total flux was increased by a factor of 1.18, water flux by 1.14 and ethanol flux by 1.5. This can also be observed from Fig. 4 which illustrates the effect of feed temperature in separation factor. Different effects were related in the literature, with some studies demonstrating that the separation factor increases when temperature is increasing and others showing the contrary. According to Zereshki et al. (2011), the main reason for the separation factor to be affected by temperature is because the diffusion and solubility of penetrating components changes significantly with temperature, but also depends on many factors such as different organics, different membrane-preparation method, and different supports of composite membranes and so on. The total and partial permeation flux is plotted in logarithmic scale as a function of the reciprocal temperature in Fig. 5 and results show that an Arrhenius type relationship exists between the fluxes and feed temperature, i.e. fluxes decrease with decreasing temperature. These results are in agreement with the literature (Zhou et al., 2011; Lai et al., 2012; Lee et al., 2012). The apparent activation energy could be calculated from the slope of the corresponding curve and Eq. (6) and the value is summarized in Table 4. The higher apparent activation energy value for ethanol flux indicates that it was more affected than water flux as the
temperature increased. These results are in agreement with the study by Shepherd et al. (2002) that used the same membrane to separate aroma compounds. Thus the separation factor increases with the increase of temperature. Similar results are presented by Dobrak et al. (2010). 3.1.4. Effect of permeate pressure Three variations of permeate pressure were investigated in the pervaporation system: 6000, 2600 and less than 600 Pa. The results obtained for the mass flow of permeate with respect to the different applied pressures in the process of pervaporation are shown in Table 5. Pereira et al. (2006) describe how when the permeate pressure grows, the chemical potential gradient across the membrane is reduced and, as a consequence, a reduction in transmembrane flux is observed. The same result was obtained in this work, with the higher effect observed when the vapor pressure in the permeate stream was close to the ethanol vapor pressure. The results presented in Table 5 show a decrease of 64% at a pressure of 2660 Pa and 95% at 6000 Pa in relation to a maximum of 5.85 g m2 h1 were obtained for the permeate flux at the system operating with a permeate pressure less than 600 Pa. Jiraratananon et al. (2002) found that in ethanol–water mixtures, as permeate pressure increases, the driving force for permeation of the ethanol molecules decreases, which results in a decrease in the mass flow of the permeate. 3.2. Experiments with fermentation broth 3.2.1. Effect of feed flow rate Tests were performed with the fermentation broth that contained about 3 wt% ethanol by weight in the feed mixture, and the results were compared with those obtained using the standard solution at the same ethanol weight fraction. The results are summarized in Table 6. It was observed that all of the parameters increased for the pervaporation of the broth at low flow rates compared with the model solutions. For the specific case of the ethanol produced from the
Table 4 Apparent activation energy for ethanol and water permeation estimated from experiments with ethanol (3 wt%) and water mixtures. Apparent activation energy (KJ mol1)
Value
Ethanol Water
83.23 62.64
Fig. 3. Temperature dependency on the total (j), ethanol (d) and water (N) fluxes for 3 wt% ethanol/water mixture.
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R.H. Bello et al. / Waste Management xxx (2014) xxx–xxx
enrichment factor, since this parameter relates to the separation ability by the ratio of permeate and feed weight fractions. A reduction twice was observed. Also, when comparing the pervaporation of fermentation broth between different flow rates, all parameters decrease with the increase of feed flow. In these cases, the effect observed in the enrichment factor was more pronounced.
Fig. 4. Effect of feed temperature in separation factor for 3 wt% ethanol/water mixture.
Fig. 5. Arrhenius plot of total (j), water (N) and ethanol (d) fluxes vs. temperature for ethanol (3 wt%) and water mixtures.
lignocellulosic banana waste, with 5.5x106 m3 s1 of flow rate, an increase of about 28% in the enrichment factor was obtained, as well as a 3.5 times increase in ethanol flux, which resulted in a pervaporation index (PSI) double that achieved with the standard mixture (62.18 vs. 29.11). One important analysis consists of the enrichment factor. This parameter remained almost the same, indicating a possibility that there is no flux coupling mechanism. Shepherd et al. (2002) reported the same results using the similar membrane and studying multicomponent model mixtures. For the flow rate of 22.2 106 m3 s1, contrary observations were made. All performance parameters decrease for fermentation broth compared with model solutions. This can be a result of the presence of by-product that at this flow rate has an influence on the ethanol diffusion mechanism. As the flux was reduced by a factor of 0.25 and the separation factor was reduced by a factor of 0.43, it was concluded that the ethanol pervaporation was minimized. This behavior could be supported by the results of Table 5 Effect of permeate pressure in total permeate flux for 3 wt% ethanol/water mixtures. Pressure (Pa)
<600
2660
6000
Total flux (g m2 h1)
5.85
2.1
0.3
3.2.2. Variation of feed weight fraction Tests were performed with the fermentation broth of varying ethanol by weight in the feed mixture, and the results were compared with those obtained using the standard solution at the same ethanol weight fraction. Figs. 6–8 show the comparison of results obtained with the model mixture and fermentation broth for total flux, separation and enrichment factors and pervaporation index, respectively. The results of the parameters presented demonstrated an increase that was observed with the pervaporation of fermentation broth. The comparison between model solutions and broth shows similar results, i.e., when the feed ethanol weight fraction is increased, we observed an increase of flux, decrease of enrichment and separation factor (consequently an increasing of permeate ethanol weight fraction). More interesting, however, is the increase in pervaporation index (Fig. 8), which indicates that the pervaporation of the fermentation broth obtained from banana residue with PDMS membrane has a more pronounced increase with the increase of feed ethanol weight fraction than does the model solution. Figs. 9 and 10 show the comparison of results obtained with model mixture and fermentation broth for permeance and selectivity, respectively. According to Luis et al. (2013), these parameters enable better observation of the process efficiency once the effect of the driving force and operating conditions is eliminated when the parameters are calculated. Fig. 9 shows better results for broth pervaporation, and an increase of both ethanol and water flux as the feed ethanol weight fraction is increased, as observed in Fig. 6. However, the permeance analysis shows clearly that the ethanol flux increase has a major effect on the total flux increase, especially for fermentation broth. This indicates that by-products present in the multicomponent mixtures could be facilitating the ethanol permeability, and no coupling flux is observed. In fact, by-products of fermentation, such as carboxylic acids, aldehydes, alcohols and other salts, may influence the separation process, especially at lower ethanol feed mass fractions and flow rates. No glucose was observed in the broth used for the separation, when analyzed via high-performance liquid chromatography. Glucose has been shown to affect the performance of pervaporation, as reported by Chovau et al. (2011). In testing for selectivity, as observed before and with model solutions, the membrane shows more affinity for water, mainly at higher feed ethanol weight fractions. However, is important to notice that a value of selectivity equal to 1 represents the performance of a membrane with no intrinsic membrane selectivity, achieving the same separation as simple evaporation of the liquid into the vapor phase (Luis et al., 2013). In this work, selectivity shows values greater than 1, and higher for fermentation broth than for model solutions, which indicates that the effect of the membrane is to enrich the permeate with ethanol. 3.2.3. Influence of by-products Table 7 presents the average values of the by-products of the broth in the feed analyzed by HPLC. The results for the centrifuged broth were compared with those for the standard mixture to determine the influence of by-products on the permeate flux, the enrichment factor and the concentration of the permeate. According to Table 7, the glycerol concentration in the fermentation broth in the present study was 1.67% by weight. Chovau et al. (2011) have reported that the presence of glycerol in low con-
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R.H. Bello et al. / Waste Management xxx (2014) xxx–xxx Table 6 Comparison of the pervaporation carried out at different feed flow rates with ethanol/water model solutions and fermentation broth with 3 wt% of ethanol. Total flux (g m2 h1)
Ethanol flux (g m2 h1)
Enrichment factor
Separation factor
PSI
3.58 5.85
0.37 1.06
7.56 8.88
8.31 10.63
29.71 62.18
Flow rate 22.2 106 m3 s1 Ethanol/water 4.80 Broth 3.60
0.41 0.30
6.51 3.73
7.06 3.97
33.91 14.29
Experiment Flow rate 5.5 10 Ethanol/water Broth
6
3
m s
1
Fig. 6. Comparison of the effect of ethanol content in feed on total flux for model solutions (j) and fermentation broth (h).
Fig. 8. Comparison of the effect of ethanol content in feed in pervaporation index (PSI) for model solutions (j) and fermentation broth (h).
Fig. 7. Comparison of the effect of ethanol content in feed on separation (j) and enrichment (d) factors for model solutions (closed symbols) and fermentation broth (open symbols).
Fig. 9. Comparison of the effect of ethanol content in feed on permeance of ethanol (d) and water (N) for model solutions (closed symbols) and fermentation broth (open symbols).
centrations does not significantly affect the permeate flux parameter or the coefficient of enrichment. In addition, the effect of carboxylic acids on the process of pervaporation was studied, and the presence of lactic acid at a concentration of 10.9 mmol L1 was observed to increase both permeate and water flux; however, the coefficient of enrichment was reduced to 12.2%. As previously discussed, in tests where a flow rate of 5.5 106 m3 s1 was used, a comparison of the pervaporation of the fermentation broth and that of the standard mixture of ethanol and water indicates that the best results are achieved with the fermentation broth. Furthermore, in the specific case of ethanol produced from lignocellulosic banana waste, greater values were
observed for the pervaporation index than for the standard mixture. We believe that the observed increase in the pervaporation index was due to the presence of lactic acid in the broth. As a result of the low concentration of this acid observed in the fermentation broth at 0.75 mmol L1, a decrease was not observed in the enrichment factor of ethanol, as reported in the work of Chovau et al. (2011). Thus, because the polydimethylsiloxane membrane is hydrophobic, the presence of lactic acid increased the hydrophilicity of the membrane, which increased the flow of water, whereas the increase in the flow of ethanol can be attributed to the effect of membrane fouling. Bowen et al. (2007) have confirmed these statements in their studies.
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R.H. Bello et al. / Waste Management xxx (2014) xxx–xxx
solutions. The process efficiency was evaluated by permeance and selectivity, showing that ethanol flux increase has a major effect on the total flux increase, especially for fermentation broth. In fact, in the specific case of ethanol produced from lignocellulosic banana waste, greater values were observed for the pervaporation index than for the standard mixture. This could due to by-products present in the multicomponent mixtures facilitating the ethanol permeability. By-products analysis show that the presence of lactic acid increased the hydrophilicity of the membrane, which increased the flow of water, whereas the increase in the flow of ethanol can be attributed to the effect of membrane fouling. Thus, it can be concluded that the pervaporation of ethanol produced from banana waste is indeed a technology with the potential to be applied. The results showed a very interesting performance, highlighting the better separation efficiency for fermentation broth in relation to model solutions. Fig. 10. Comparison of the effect of ethanol content in feed on selectivity for model solutions (j) and fermentation broth (h).
Table 7 By-products of the fermentation broth that may affect the process of pervaporation.
a
Byproduct
Average concentration (g L1)
Lactic acid Acetic acid Glucose Glycerol
0.068 NDª ND 1.67
Not detected.
Finally, in all experiments the flux values obtained (5 g m2 h1) were inferior to those described in the literature, such as those mentioned in work by Molina et al. (2002), who obtained 500 g m2 h1 at a concentration of 15% (wt) under similar operating conditions. This discrepancy indicates the need for optimization of the operating conditions. However, the values obtained here are consistent with the data provided by the membrane supplier. In this regard, the results are considered to be encouraging because the parameters show substantial increases in the recovery of ethanol produced by fermentation using pervaporation compared with using the standard mixture. 4. Conclusions The use of the pervaporation process for the recovery of ethanol from fermentation broth produced from lignocellulosic banana waste demonstrated a very attractive alternative to classical methods. When tests were carried out varying operational conditions with model solutions, it was observed behaviors related to reported in literature: a higher ethanol concentration in the feed produces a higher ethanol flux and decrease the separation factor; an increase of feed flow rate can enhance the permeation rate of ethanol with the water remaining at almost the same value; water and ethanol fluxes was increased with the temperature increase; and the higher effect in flux increase was observed when the vapor pressure in the permeate stream was close to the ethanol vapor pressure. One interesting point was that the ethanol flux demonstrated an increase even with ethanol concentrations less than 5 wt% in the feed (concentration expected in second generation ethanol), but this increase is even greater at higher concentrations, as expected. However, water flux increases more than alcohol, resulting in a separation factor reduction. It was observed that all of the parameters increased for the pervaporation of the broth at low flow rates compared with the model
Acknowledgements The authors acknowledge the support received from the Universidade da Região de Joinville – Univille as well as all of the resources that allowed for the preparation of this work, and acknowledge the financial support that was received in the form of research grants. References ABIB – Associação Brasileira Indústrias Biomassa Brasil Biomassa E Energia Renovável. Inventário Residual Brasil – Wood Pellets - Briquete – Energia, 2011. E-book.
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Please cite this article in press as: Bello, R.H., et al. Pervaporation of ethanol produced from banana waste. Waste Management (2014), http://dx.doi.org/ 10.1016/j.wasman.2014.04.013