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Influence of oil quality on biodiesel purification by ultrafiltration
Journal of Membrane Science 496 (2015) 242–249
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Influence of oil quality on biodiesel purification by ultrafiltration Maria Carolina Sérgi Gomes a,n, Pedro Augusto Arroyo b, Nehemias Curvelo Pereira b a b
Federal University of Technology of Paraná-UTFPR, Rua Marcílio Dias, 635, CEP 86812-460 Apucarana, PR, Brazil Department of Chemical Engineering, State University of Maringá, Av. Colombo 5790, Bloco D90, CEP 87020-900 Maringá, PR, Brazil
art ic l e i nf o
a b s t r a c t
Article history: Received 19 May 2015 Received in revised form 31 July 2015 Accepted 4 September 2015 Available online 8 September 2015
In the present study, biodiesel was produced by ethylic transesterification of soybean and canola oils using sodium hydroxide as a catalyst. It was evaluated the influence of oil quality on the biodiesel and glycerol separation by ultrafiltration. The experiments were carried out with tubular a-Al2O3/TiO2 membranes with average pore diameter of 0.05 mm and 20 kDa, varying the transmembrane pressure and the concentration of the feed mixture. The comparison among the use of degummed soybean oil, refined soybean oil, crude canola oil, and refined canola oil, demonstrated that free fatty acid presented in the oils influence the formation of droplets containing glycerol. The separation was efficient when reaction mixture was produced from degummed soybean oil and crude canola oil, both with a higher acidity value. The highest free fatty acid content in the crude canola oil, not only favored the formation of a dispersed phase containing glycerol, which was retained by the membrane, but also resulted in the lowest flux decline rates. The ultrafiltration was efficient in removing glycerol, since the highest glycerol content in the permeate was 0.013 wt%. This novel refining process of biodiesel showed the advantage of not requiring previous decantation to separate the two phases obtained after transesterification and the reduction in the amount of water used in the washing steps. The properties of the biodiesel produced, which were evaluated, meet the ANP biodiesel standards required for marketing. & 2015 Elsevier B.V. All rights reserved.
Keywords: Ultrafiltration Ceramic membranes Transesterification Biodiesel Glycerol
1. Introduction Biodiesel is considered a renewable fuel, as vegetable oil and animal fat are the main raw materials for its production. It has higher flash point than mineral diesel, ensuring greater safety in use and also presents appropriate viscosity for burning in diesel engines. It is biodegradable, non-toxic and has excellent lubricity, providing longer life to components of engines [1]. The most common method for producing biodiesel is the transesterification, a reaction between a triacylglycerol, from vegetable oils or animal fats, and a short chain alcohol producing alkyl esters of fatty acids and glycerol as a co-product [2,3]. Biodiesel has similar physicochemical properties to that of mineral diesel and can be used in engines without modification [3]. The potential use of a raw material in biodiesel production depends on several factors, such as availability, cost, storage properties and performance as fuel [4]. Given the high biodiversity, large territory, climate diversity and soil conditions, Brazil contains different oil sources for biodiesel production including soybean, canola, castor, babassu, cotton, palm and sunflower [5]. Since Brazil has a well stablished soybean production, presently n
Corresponding author. E-mail address: [email protected] (M.C.S. Gomes).
http://dx.doi.org/10.1016/j.memsci.2015.09.004 0376-7388/& 2015 Elsevier B.V. All rights reserved.
most of biodiesel produced in the country used soybean oil [5,6]. Canola presents a high percentage of seed oil, 34–40%, which is approximately twice the value found in soybeans and correspond to 84% of all raw materials used in global biodiesel production [7,8]. Regarding the alcohol used in the process, in Brazil, the use of ethyl route has a strategic importance, since the availability of raw materials and technology allows an economically viable production of ethanol by fermentation processes, resulting in a cheaper product than the methanol. In biodiesel production, the use of vegetable oils and ethanol derived from sugarcane makes the process completely independent from petroleum, providing environmental benefits and generating a socio-economic development program [9,10]. After transesterification, the final mixture is mainly composed of alkyl esters of fatty acids, residual alcohol, glycerol, catalyst, mono-, di- and triglycerides. These and other contaminants in the biodiesel can cause operational and environmental problems and must be separated so that esters can be used as fuel [9,11]. The quality of B100 biodiesel in Brazil is specified by the National Agency of Petroleum, Natural Gas and Biofuels (ANP), based on the American ASTM D6751 and European EN14214 standards, with some modifications to meet the Brazilian raw material requirements [12]. One of the most important parameters for quality control of
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biodiesel is the amount of free glycerin, which has a maximum allowed limit of 0.02%. A high concentration of free glycerin can result from the separation of glycerin, causing problems during storage and in the fuel injection system [9,13]. Thus, besides controlling all the parameters influencing the reaction of biodiesel production, the stage of glycerol separation is very important to achieve a quality product, free of impurities and no risk of corrosion to the engine. Conventional purification procedures use a large volume of water, which varies depending on the impurities present, and provides the generation of a large amount of effluent that must be properly treated and disposed [14]. Studies in the literature [15–19] indicate that distinct methods have been applied for glycerol removal and biodiesel purification. Among the alternative methods of separation, the process with membranes has many advantages and good prospects for use in the separation and purification of biodiesel. The use of membranes can provide high quality and purity biodiesel, and environmental and economic advantages for reducing the amount of water used and eliminating the use of adsorbents. Apart from the reduction of costs related to effluent treatment, studies indicate that the use of membranes in the processing of biodiesel provides reduced power consumption, enabling the application of this process in industrial scale [20]. Regarding the use of membranes for biodiesel production, the published studies have focused on the use of membrane reactors to improve conversion into esters [21–23] and the use of membranes in the purification step, after separation by decantation [18,24–28]. Direct use of micro- and ultrafiltration after transesterification for the separation of phases, without previous decantation, was studied in previous works developed by our research group. The first experiments [29] were carried out with microfiltration membranes using synthetic mixtures of biodiesel, glycerol, and ethanol. They were evaluated the effects of the membrane porosity, transmembrane pressure, and the ethanol concentration in the mixture. The results indicated the influence of ethanol on behavior of the emulsion and the potential of applying ceramic membranes in the separation of glycerol and biodiesel. Afterwards [30], the experiments were performed using the reactional mixture produced by ethylic transesterification of degummed soybean oil. Considering the emulsion behavior of the mixture produced in the transesterification of degummed soybean oil, it was developed a methodology of addition of acidified water, aiming to destabilize this emulsion and improve the retention of glycerol by the membrane. The results showed the key role of both ethanol and water in the formation of agglomerates in the dispersed phase, since the glycerol retention after adding water was significant, with glycerol mass content in the permeate below 0.02%, indicating the efficiency of the used methodology. Continuing the study [31], it was evaluated the influence of the amount of acidified water added on the separation of glycerol and biodiesel using micro and ultrafiltration ceramic membranes. The results showed that the amount of acidified water added influences the emulsion properties and, consequently, the distribution of agglomerate size containing the glycerol. The current work aimed at improving knowledge on the behavior of ultrafiltration with ceramic membranes in separating glycerol from biodiesel using different feedstock. To this end, it was evaluated the influence of oil quality on ultrafiltration of the reaction mixture produced after ethyl esterification of vegetable oils with different characteristics. The separation of glycerol was performed using ultrafiltration membranes, and the best operating condition was evaluated in terms of permeate flux and quality of the product.
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2. Experimental section 2.1. Production of biodiesel The reaction mixtures needed for the experiments were prepared by alkaline ethyl transesterification of vegetable oils. They were used degummed soybean oil and crude canola oil donated by Cocamar (Maringá, Paraná State, Brazil), and commercial refined soybean and canola oils. The anhydrous ethanol (99.4% purity) was donated by Cocafé, Astorga, Paraná State, Brazil and sodium hydroxide (NaOH) was purchased from Biotec. The previous results of the study of the yield of esters as a function of reaction parameters [30] indicate that the reaction temperature for obtaining a high ester yield depends on the acidity of the oil. In this way, when used refined oils, the reaction temperature was 45 °C, and when used crude and degummed oils, the transesterification was carried out at 30 °C. For all oils, the oil: alcohol molar ratio used was 1:7.5 and the amount of catalyst was 1% of mass of oil. The reaction was prepared in a 2 L batch reactor and the reaction time was 1 h. The reactor contents were mixed using a mechanical stirrer. 2.2. Ultrafiltration runs The experimental equipment consisted of a micro- and ultrafiltration pilot unit UF NETZSCH, Pomerode, Santa Catarina State, Brazil, model 027.06-1C1/07-0005/AI, operating in cross-flow conditions. A detailed description of the experimental unit was presented in a previous study [30]. The hydrophilic ceramic membranes used in the experiments were made of tubular α-Al2O3/TiO2 (Shumacher GmbHTi 01070), 250 mm long, 7 mm in diameter, and 0.005 m2 filtration area, purchased from Andritz, Pomerode, Santa Catarina State, Brazil. The tests were carried out with ultrafiltration membranes of 0.05 mm and 20 kDa. All ultrafiltration experiments were performed at 50 °C. In the first step, ultrafiltration of the mixture obtained from refined canola oil was evaluated. For all tests, before ultrafiltration of the mixture, the addition of acidified water (0.5% HCl) was evaluated in the mass concentrations varying from 0% to 20% in relation to the total mixture mass, under transmembrane pressure of 0.5–2.0 bar. As previously discussed [31], addition of water affords the formation of dispersed phase containing water, glycerol, catalyst, salts and other water-soluble substances, distinct from the continuous phase rich in ethyl esters and unreacted oil. Next, ultrafiltration experiments were run with reaction mixtures produced with refined soybean, degummed soybean, refined canola and raw canola oils. The experiments were performed only with the 0.05 mm membrane, under pressure of 1.0 bar, with prior addition of 10% acidified water. Finally, it was evaluated the separation of glycerol from the mixture produced with raw canola oil. In these experiments were used the membranes of 0.05 mm and 20 kDa, under pressures of 1.0, 2.0 and 3.0 bar and water concentration of 10%. Approximately four liters of the mixture were poured into the feed tank of the module for each run. After heating to 50 °C, the mixture was pumped into the membrane and the pressure was set to begin the permeate flux. All the runs were undertaken with a flow rate of 700 L/h, corresponding to a tangential velocity of about 8 m/s. The permeate was collected and the concentrate was completely recirculated to the feed tank. The performance of the glycerol separation process by ceramic membranes were characterized in terms of glycerol rejection, stabilized permeate flux, percentage of flux decline and flux with pure water after membrane cleaning. The reaction mixture obtained after adding acidified water was
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Table 1 Physicochemical characteristics of the vegetable oils used. Vegetable oil
Viscosity at 40 °C (mm2/s)
Density at 20 °C (g/cm3)
Moisture (%)
Acidity (%)
Degummed soybean Refined soybean Crude canola Refined canola
32.55 32.01 32.22 36.39
0.9156 7 0.0007 0.91227 0.0007 0.9105 7 0.0007 0.91347 0.0007
0.1277 0.004 0.090 7 0.003 0.1137 0.004 0.098 7 0.004
0.87 70.01 0.43 70.05 2.40 70.06 0.2470.03
directly fed to the micro- and ultrafiltration module, substituting the steps of settling and washing with water by micro- or ultrafiltration. Thus the use of membranes for glycerol separation enables the reduction in the number of steps in relation to the conventional treatment, in addition to a lower consumption of water.
of 1% NaOH and 1% citric acid at 70 °C and then with deionized water at the same temperature. It was evaluated the membrane hydraulic permeability so that a cleaning parameter could be set. After each regeneration cycle, the permeate flux was measured with deionized water, ensuring the reproducibility of the tests.
2.3. Analytical methods
3. Results and discussion
The viscosity at 40 °C was determined with a Brookfield model DV-III digital rheometer. A 25-mL pycnometer with a coupled thermometer was used to determine the density at 20 °C. The acidity was determined according to the American Oil Chemical Society (AOCS). The moisture content was determined by the Karl Fished method using an Analyzer apparatus, model Moisture Control KF-1000. The calorific value was determined using a Parr 6200 calorimeter. All analyses were performed in triplicate. The fatty acid profile of the oil and the quantification of the ethyl esters were done by gas chromatography with Varian model CP-3800 coupled with a flame ionization detector (FID) and a 30 mx0.25 mm capillary column specific for fatty acid separation, BP-X70. The carrier gas used was helium, in a split rate of 1:10. The analysis was performed with a column temperature program starting at 140 °C, heating up to 250 °C at 5 °C/min. The detector temperature was kept at 220 °C and that of the injector at 260 °C. The internal standard used was 99% methyl tricosanoate, purchased from Sigma-Aldrich. The free glycerol content of the permeate was determined by volumetric method based on the official AOCS methodology for the analysis of free glycerol in oils and fats (Ca 14-56) [32]. Each sample was analyzed three times and the averages are reported. The titration method with sodium periodate allows fast and accurate determination of glycerol in oils and fats at a low cost compared with chromatographic methods [9,32,33]. When glycerol concentration in the permeate was too high, i.e., when there was permeation of the dispersed phase through the membrane, titration method could not be applied and it was only performed a volumetric quantification of the percentage of the dispersed phase containing glycerol.
3.1. Characteristics of the oils
2.4. Membrane cleaning process Although membrane processes are extensively employed in the industry, the major disadvantage of their application is the fouling of the membranes, which imposes the need for frequent cleaning. Therefore, an improvement of the cleaning procedure may have a significant influence on the overall process efficiency [34]. Current membrane cleaning technologies include hydraulic, chemical, and mechanical methods. Ultrasound is an effective technique for cleaning a variety of surfaces, which has been used for cleaning fouled membranes or for increasing permeate flux of water through membranes [35]. The basic principle of operation is that ultrasound removes particles from the surface by causing particle movement in or near membrane [36]. In this work, the membrane cleaning was performed in ultrasonic bath (Ultra Cleaner 800, UNIQUE) at 40 kHz, with solutions
Table 1 lists the physicochemical characteristics of the four types of oil used in the experiments: degummed soybean oil, refined soybean oil, refined canola oil and crude canola oil. Vegetable oils exhibited similar density and viscosity and in accordance with the values in the literature [7,37]. The degummed soybean oil and crude canola oil showed acidity values of 0.87% and 2.4%, respectively. These percentages are higher than the recommended by Freedman et al. [38] for alkaline transesterification. Nevertheless, as studies have shown good results for this reaction with the use of vegetable oils with up to 3% acidity [39] and given the possibility of reducing the costs of treating the raw material, oils were not pre-treated. For all oils, the moisture content was below 0.5%, which is the maximum value recommended for the alkaline transesterification is not impaired [38]. 3.2. Ultrafiltration in the separation of biodiesel and glycerol produced by ethyl transesterification of refined canola oil Ultrafiltration of the mixture produced from refined canola oil, whose acidity was the lowest of all evaluated oils, was not efficient in the separation of glycerol. Considering the stoichiometry of the reaction, the amount of glycerol in feed stream varied from 6% to 7%, depending on the quantity of water added. For all the conditions evaluated, the dispersed phase containing glycerol permeated through the membrane, and the percentage of this phase in the permeate varied from 1% to 44% (Table 2). These results are justified by the absence of acidity in the oil, which prevented the agglomeration of the dispersed phase in the final mixture. Saleh et al. [26] studied biodiesel purification using membrane process and demonstrated that at lower soap concentrations, the increasing amount of water promotes the generation of larger glycerol rich particles. In this present work, soaps formed in the reaction, when there are free fatty acids in the oil, along with salts from the neutralization with acidified water, contribute to the formation of a dispersed phase containing glycerol, which can be retained by the membrane. Fig. 1 illustrates the samples of permeates in all conditions evaluated, indicating the presence of two phases. The lower phase is the dispersed phase containing glycerol, and the upper phase is formed mainly by ethyl esters. Permeate flux curves over filtration time, produced from ethyl transesterification of refined canola oil, using membranes of 0.05 mm and 20 kDa are presented in Figs. 2 and 3, respectively. It is interesting to observe a flux increasing in the beginning of the filtration for most of the runs. Probably, until the phase
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450
Table 2 Concentration of the dispersed phase in the permeate – ultrafiltration of refined canola oil. Pressure (bar)
Concentration of acidified water (% mass)
Dispersed phase (% mass)
1 2 3 4 5 6 7 8 9 10 11 12 13 14
0.5 1.0 1.0 1.0 2.0 2 2.0 0.5 0.5 1.0 1.0 2.0 2.0 2.0
10 1 3 10 0 5 10 3 10 1 10 0 10 20
1.7 31 8.4 12 34 18 22 4 5 3 1 3 10 44
0.05 0.05 0.05 0.05 0.05 0.05 0.05 20 20 20 20 20 20 20
400
1.0 bar - 1% 1.0 bar - 3% 1.0 bar - 10% 2.0 bar - 0% 2.0 bar - 5% 2.0 bar - 10%
350 2
Permeate Flux (kg/h.m )
Sample Membrane (mm/kDa)
245
300 250 200 150 100 50 0 0
10
20
30
40
50
60
70
80
90
100
Time (min) Fig. 2. Permeate flux in the ultrafiltration of biodiesel from refined canola oil for all conditions evaluated. 0.05 mm membrane, T ¼ 50 °C.
0.5 bar - 3% 0.5 bar - 10% 1.0 bar - 1% 1.0 bar - 10% 2.0 bar - 0% 2.0 bar - 10% 2.0 bar - 20%
240
180
2
Permeate Flux (kg/h.m )
210
150 120 90 60 30 0 0
10
20
30
40
50
60
70
80
90
100
Time (min) 8
9
10
11
12
13
14
Fig. 1. Samples of ultrafiltration permeates of the refined canola oil.
initially retained on the membrane surface is removed by the convective flux and a concentration profile is established, which is the transient period, there is an increase in the flux. After this initial period, due to the size of glycerol agglomerates and the pressure used, permeation occurs through the membrane pores, and consequently the fouling, which reduces the flux.
Fig. 3. Permeate flux in the ultrafiltration of biodiesel from refined canola oil for all conditions evaluated. 20 kDa membrane, T ¼50 °C.
According to Fig. 2, the 0.05 mm membrane showed very high flux rates, so that, in some tests, the minimum volume required in the feed tank was achieved before the default time of 95 min. The use of 2.0 bar pressure promoted a sharp decline in flux for all concentrations of water, as well as a high glycerol content in the permeate (Table 1). Although the lower pressure, 1.0 bar, has caused a reduction of membrane fouling, there was glycerol permeation for the three concentrations of water evaluated, and the lowest value of glycerol phase in the permeate was 8.4%, with acidified water concentration of 3%. Given the same concentration of water, 10%, increased pressure of 1.0 to 2.0 bar provided increased glycerol content in the permeate from 12 to 22% in the 0.05 mm membrane and 1 to 10% in the 20 kDa membrane. On the other hand, at constant pressure, it is observed that in 0.05 mm membrane, there was an increased retention of glycerol with higher concentration of water compared to the lowest concentration studied. As for the 20 kDa membrane, increased retention of glycerol with increasing concentration of water was only verified at 1.0 bar pressure. At 2.0 bar pressure, increasing the concentration of water promoted a reduction of glycerol retention,
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and with the maximum concentration of water evaluated, it was obtained the highest content of glycerol phase in the permeate, 44%. Compared to other results, it is observed that the addition of 20% water caused a great increase in permeation of glycerol, probably indicating inversion of the emulsion. From these results, it was concluded that for the two membranes at the same concentration of water, the increased pressure favors the permeation of glycerol. The 0.05 mm membrane showed an average value of glycerol phase in permeate greater than 20 kDa membrane, suggesting that glycerol agglomerates have sizes near the pore diameter of the 0.05 mm membrane. As the best retention of glycerol in the permeate was obtained with the 20 kDa membrane at 1.0 bar pressure and 10% water, additional tests were performed by reducing the pressure to 0.5 bar. However, even with the reduction of pressure, for the two water concentrations tested, there was no glycerol retention by the membrane. 3.3. Influence of the type of oil used in the separation To evaluate the influence of the type of oil used in membrane separation, ultrafiltration tests were run with reaction mixtures obtained from refined and degummed soybean oils and from crude and refined canola oils. Thus, it was possible to compare the characteristics of ultrafiltration of the biodiesel produced from oil with a higher acidity (degummed soybean and crude canola) and biodiesel produced from refined oil, which has low acidity. Tests were performed at 50 °C at a pressure of 1.0 bar with prior addition of 10% acidified water, and the curves of permeate flux over time are presented in Fig. 4. According to the results, an increase is observed in the flux for refined oils in the 10 initial minutes, followed by a constant decline over time. As already discussed previously, glycerol permeation provides fouling increasing. The curve for degummed soybean oil shows a typical behavior of membrane separation processes, with concentration polarization and fouling. The fouling is caused by external clogging of pores, but without glycerol permeation, according to the percentage of glycerol in the permeate listed in Table 3. In turn, for the crude canola oil, the decline of flux over time is minimum, indicating that the retained phase does not cause clogging of pores. The presence of fatty acids in the oil and hence, soaps and salts formed in the reaction, favor the formation of
Fig. 4. Permeate flux in the ultrafiltration of biodiesel from soybean and biodiesel from canola. Membrane of 0.05 mm, P¼ 1.0 bar, 10% acidified water.
Table 3 Comparison of the permeate flux and glycerol concentration of the ultrafiltration with different vegetable oils studied. Raw material
Permeate flux (kg/h m2)
Glycerol in the permeate (%)
Degummed soybean Refined soybean Crude canola Refined canola
69.7 166.1 101.1 121.4
0.0187 0.001 12 0.013 70.003 12
agglomerates containing glycerol and water-soluble substances. Such agglomerates are likely much larger than the membrane pores, thus retained more easily. Comparing the refined soybean oil with degummed soybean oil and refined canola oil with crude canola oil, it can be concluded that the formation of the dispersed phase is favored by the presence of fatty acids in the oil, allowing retention by the membrane. However, the agglomeration of this phase forms a layer on the membrane surface that provides an additional resistance to the flux and thus a lower value of the stabilized permeate flux compared to the corresponding refined oil. Importantly, as shown in Table 3, the percentage of glycerol in the permeate was the same, 12%, both for refined soybean oil and refined canola oil. These results indicate that probably the separation of glycerol by ultrafiltration depends more on the acidity of the oil used in the reaction than on its origin. Furthermore, the higher acidity of the crude canola oil provided the best retention of the dispersed phase and, consequently, of glycerol when compared to the degummed soybean oil. Samples of the permeates obtained from the ultrafiltration of mixtures produced from vegetable oils are shown in Fig. 5. As these samples passed through the refining process that removed substances providing color to the oil, the permeates obtained from the mixtures produced from refined oils exhibited a lighter color compared to permeates of degummed soybean and crude canola oils. In addition, it is possible to identify the dispersed phase in the permeate of refined oils, in which the separation was not efficient.
Degummed soybean
Refined soybean
Crude canola
Refined canola
Fig. 5. Permeate obtained in the ultrafiltration of mixtures of the vegetable oils studied.
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Table 4 Permeate flux and glycerol concentration in the permeate in the ultrafiltration of the mixture produced from crude canola oil.
160
2
Permeate Flux (kg/h.m )
140
100 80 60
1.0 bar 2.0 bar 3.0 bar
40
0
0
10
20
30
40
50
60
70
80
90
100
Time (min) Fig. 6. Permeate flux over time under different pressures in the ultrafiltration of the mixture produced from crude canola oil. 0.05 mm membrane, T ¼50 °C, 10% acidified water.
3.4. Ultrafiltration of biodiesel produced from crude canola oil: influence of membrane pore diameter and transmembrane pressure In order to examine the influence of acidity on the separation of glycerol, ultrafiltration tests were performed with the mixture produced from the transesterification of crude canola oil, whose acidity is 2.4%, that is ten-fold higher than the acidity of the refined canola oil. For the production of the reaction mixture, transesterification was carried out at 30 °C and 1:7.5, which was the best condition when using an oil with high acidity [30]. The variation of permeate flux along time for the membranes of 0.05 mm and 20 kDa are presented in Figs. 6 and 7, respectively. For all conditions evaluated, the concentration factor ranged from 1.3 to 1.4. For both membranes at the three pressures examined, the stabilized permeate fluxes were close, ranging from 89 to 108 kg/h m2. Furthermore, the highest stabilized permeate flux was obtained under the pressure of 2.0 bar. Meantime, higher rates of flux decline were observed at this pressure, i.e. 24% for the 0.05 mm membrane and 26% for the 20 kDa membrane. The increase in pressure from 1.0 to 2.0 bar caused an increase in the permeate flux. However, the pressure of 3.0 bar showed a lower stabilized 140 120 2
Membrane (mm)
Pressure (bar)
Permeate flux (kg/h m2)
Glycerol (%)
0.05 0.05 0.05 20 kDa 20 kDa 20 kDa
1.0 2.0 3.0 1.0 2.0 3.0
101.1 108.4 97.8 89.4 96.4 90.5
0.0137 0.003 0.0127 0.005 0.0137 0.007 0.0117 0.008 0.0117 0.004 0.0137 0.008
120
20
Permeate Flux (kg/h.m )
247
100 80 60
1.0 bar 2.0 bar 3.0 bar
40 20 0 0
10
20
30
40
50
60
70
80
90
100
Time (min) Fig. 7. Permeate flux over time under different pressures in the ultrafiltration of the mixture produced from crude canola oil. 20 kDa membrane, T ¼ 50 °C, 10% acidified water.
permeate flux in relation to the pressure of 2.0 bar. In accordance with Chakrabarty et al. [40] and Yi et al. [41], the increased flux with increasing pressure is due to driving force across the membrane. Nevertheless, this increase is not linear, since the appearance of additional resistances, such as the compaction of the phase retained on the membrane surface, which reduce the flux. The ultrafiltration of the crude canola oil, with membranes of 0.05 mm and 20 kDa, was effective in retaining glycerol, once the values of glycerol in the permeate varied from 0.011% to 0.013% under the conditions evaluated (Table 4). Besides that, the flux curves versus time showed a high initial flux, which remained almost constant throughout the operation, that is, the decline in flux over time was small and ranged from 14% to 26%. These results indicate that the retained phase did not cause membrane fouling. Probably, this reduction of fouling is due to the larger size of glycerol agglomerates, which are formed in the presence of free fatty acids in the oil used. This retention of the dispersed phase was confirmed by glycerol analysis (Table 4), since its highest concentration in the permeate was 0.013%. Comparing the results of ultrafiltration of crude canola biodiesel with the results of degummed soybean biodiesel presented in a previous study [31], it was observed that the largest amount of free fatty acids in the crude canola oil favors the formation of the dispersed phase containing glycerol and thus the removal by the membrane. Yi et al. [41] investigated the influence of acidity on the ultrafiltration of oil-in-water emulsions and concluded that the clusters of oil retained when used an acidic solution had a larger size compared to ultrafiltration of an alkaline solution. The authors explained that under acidic conditions, there is a reduction of the electrostatic repulsive forces between the clusters, causing them to bind to each other, forming larger agglomerates and thus reducing membrane fouling. Similar results of fouling reduction, given the formation of larger clusters, were also obtained by Hesampour et al. [42] when treated oily emulsions by ultrafiltration. Also, this formation of larger clusters when used the mixture produced from crude canola oil provided not only a greater retention of glycerol by the membrane, but also the stabilization of fluxes at higher values (Fig. 7 and Table 4). Therefore, regarding the lowest rate of flux decline over time and the quality of permeate achieved, the pressure of 1.0 bar is the most recommended for separation of glycerol and biodiesel produced by ethyl transesterification of the crude canola oil through ultrafiltration. Wang et al. [25] employed ultrafiltration with 0.1 mm ceramic membrane for biodiesel purification after separation of the phases by settling, and concluded that the smaller reduction of flux along time (16.7%) was obtained at a pressure of 1 5 bar with a stabilized permeate flux of 300 L/h m2. 3.5. Membrane cleaning Before
starting
the
experiments,
the
membranes
were
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characterized with deionized water at pressures of 1.0, 2.0 and 3.0 bar. The hydraulic permeability values of the membranes of 0.05 mm and 20 kDa were respectively 893 and 474 kg/h.m2 bar. The average values of permeate flux at 25 °C and 1.0 bar used to assess the cleaning of the membranes after each cleaning cycle, for the membranes of 0.05 mm and 20 kDa, were 867 and 450 kg/h m2, respectively. The methodology for membrane cleaning was efficient, and after each washing cycle, the initial flux of the membranes was restored with deionized water. Additionally, the cleaning in ultrasonic bath reduced the water consumption compared to washing the membrane in the module. Considering a complete washing cycle, corresponding to 6 washes with sodium hydroxide solution, 6 washes with citric acid and 6 rinses with deionized water, the total volume of water used is approximately 4.5 l. If the same washing was performed in the micro-and ultrafiltration module, the minimum water consumption would be about 15 l, considering an alkaline wash, an acid wash and three rinses with deionized water, since the volume necessary for the recirculation in the module is 3 l. 3.6. Characteristics of the biodiesel produced The permeate of the mixture obtained from crude canola oil using the 20 kDa membrane, at a pressure of 1.0 bar and 10% acidified water was subjected to evaporation under reduced pressure in a rotary evaporator at 70 °C and 600 mmHg for 30 min and then characterized for specific mass, kinematic viscosity, ester content, acidity, moisture, free glycerol content and calorific value. These results were compared to those obtained using degummed soybean oil [31] and are listed in Table 5 together with the values specified for the B100 biodiesel in Brazil [12]. The permeates showed viscosities within the specified range. The canola biodiesel had a higher average viscosity than the soybean biodiesel, which is a characteristic related to the fatty acid profile of the oil. Although ultrafiltration has removed most of the moisture, since the initial concentration of water was 9.1% and the permeate before being evaporated showed an average moisture content of 0.45%, the vacuum evaporation treatment was not sufficient to reduce the moisture in the final product to less than 0.05%. It is possible that using a lower evaporating pressure for more than 30 min is adequate to reach the specified moisture value. The temperature should not exceed 70 °C, given the possibility of polymerization of the esters. In the case of canola biodiesel, the high acidity can be reduced if the crude oil, which had an acidity of 2.4%, receives a preTable 5 Characteristics of the biodiesel produced from degummed soybean oil and crude canola oil using ultrafiltration to remove glycerol. Unit
Limit
Specific mass at 20 °C Kinematic viscosity at 40 °C Ester content, mín Free glycerol, max Moisture, max Acidity, max Calorific value
kg/m3
850–900 874
866
mm /s
3.0–6.0
4.38
4.85
% massa
96.5
97.2
97.5
% massa
0.02
0.014
0.011
976 0.48 38154
1670 1.26 39575
2
mg/kg 500 mg KOH/g 0.5 kJ/kg 35000
Soybean biodiesel (Permeate) GOMES et al. [31]
Canola biodiesel (Permeate)
Characteristic
treatment to reduce its acidity to around 0.8%, which was the acidity of the degummed soybean oil. As discussed earlier, it is important to have a certain acidity in the oil so that the methodology developed in this work is adequate to separate glycerol using membranes. The calorific value of all biodiesel samples was above the minimum required by the specification, 35,000 kJ/kg. The average calorific values of the vegetable oil and of the biodiesel produced from it were similar. The calorific value of degummed soybean oil was 39,230 kJ/kg and of the degummed soybean biodiesel was 38,154 kJ/kg. The calorific value of crude canola oil was 39,381 kJ/ kg and increased to 39,575 kJ/kg in the crude canola biodiesel. These values, as well as variations between oil and biodiesel were similar to values reported by Lee et al. [7] in the transesterification of soybean and canola oils. The ester content of both soybean biodiesel and canola biodiesel was higher than 97.0% in all samples, indicating that the reaction conditions used were appropriate for the transesterification of each oil. In other words, the permeates showed mass percentage of esters greater than the minimum required for marketing as biodiesel, 96.5%. Likewise, in all tests with the reaction mixture directly ultrafiltered, the percentage of glycerol was below the value specified by the ANP for free glycerol, which is 0.02% in mass. Compared to the works in the literature using membrane technology in the purification of biodiesel [18,24–28], the methodology used herein has the advantage of eliminating the step of separation by settling and provide a biodiesel with a very low content of glycerol after a single processing step. Taking into account that steps usually employed of settling and washing can be replaced by membrane filtration and that this process has provided lower content of glycerol than those presented in the literature, this study demonstrates a great prospect for improving the separation and purification of biodiesel.
4. Conclusions Ceramic ultrafiltration membranes are used for the purification of biodiesel produced from ethyl transesterification of vegetable oils with different characteristics, and enable to evaluate the influence of the acidity of the oil used on the membrane separation step. The ultrafiltration separation of glycerol from the reaction mixture obtained from refined canola oil is not effective in any condition evaluated. The comparison between the mixtures obtained by transesterification of degummed soybean oil, refined soybean oil, crude canola oil and refined canola oil demonstrate that the efficiency of membrane separation of glycerol depends on the acidity present in the oil. The highest percentage of acid in the oil contributes to the agglomeration of the glycerol after addition of acidified water in the final reaction mixture. Accordingly, the ultrafiltration of reaction mixtures produced from degummed soybean oil and crude canola oil is efficient in the removal of glycerol. On the other hand, when used refined oils, the agglomerates in the emulsion show a diameter smaller than the membrane pore size, so that the glycerol permeates through the membrane. The efficiency of the ultrafiltration of the mixture obtained from crude canola oil, which has the highest acidity, indicates that the formation of dispersed phase clusters not only causes a greater retention of glycerol by the membrane, but also the stabilization of fluxes at higher rates. This process of biodiesel purification by ultrafiltration has the advantage of eliminating the step of separation by settling and results in a biodiesel with very low content of glycerol after a
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single processing step, and reduce the amount of water used for washing. Biodiesel produced using the methodology developed, both from degummed soybean oil and crude canola oil, showed values of ester content, glycerol content, viscosity, density and calorific value within the range specified by the ANP for marketing.
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Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Influence of oil quality on biodiesel purification by ultrafiltration Maria Carolina Sérgi Gomes a,n, Pedro Augusto Arroyo b, Nehemias Curvelo Pereira b a b
Federal University of Technology of Paraná-UTFPR, Rua Marcílio Dias, 635, CEP 86812-460 Apucarana, PR, Brazil Department of Chemical Engineering, State University of Maringá, Av. Colombo 5790, Bloco D90, CEP 87020-900 Maringá, PR, Brazil
art ic l e i nf o
a b s t r a c t
Article history: Received 19 May 2015 Received in revised form 31 July 2015 Accepted 4 September 2015 Available online 8 September 2015
In the present study, biodiesel was produced by ethylic transesterification of soybean and canola oils using sodium hydroxide as a catalyst. It was evaluated the influence of oil quality on the biodiesel and glycerol separation by ultrafiltration. The experiments were carried out with tubular a-Al2O3/TiO2 membranes with average pore diameter of 0.05 mm and 20 kDa, varying the transmembrane pressure and the concentration of the feed mixture. The comparison among the use of degummed soybean oil, refined soybean oil, crude canola oil, and refined canola oil, demonstrated that free fatty acid presented in the oils influence the formation of droplets containing glycerol. The separation was efficient when reaction mixture was produced from degummed soybean oil and crude canola oil, both with a higher acidity value. The highest free fatty acid content in the crude canola oil, not only favored the formation of a dispersed phase containing glycerol, which was retained by the membrane, but also resulted in the lowest flux decline rates. The ultrafiltration was efficient in removing glycerol, since the highest glycerol content in the permeate was 0.013 wt%. This novel refining process of biodiesel showed the advantage of not requiring previous decantation to separate the two phases obtained after transesterification and the reduction in the amount of water used in the washing steps. The properties of the biodiesel produced, which were evaluated, meet the ANP biodiesel standards required for marketing. & 2015 Elsevier B.V. All rights reserved.
Keywords: Ultrafiltration Ceramic membranes Transesterification Biodiesel Glycerol
1. Introduction Biodiesel is considered a renewable fuel, as vegetable oil and animal fat are the main raw materials for its production. It has higher flash point than mineral diesel, ensuring greater safety in use and also presents appropriate viscosity for burning in diesel engines. It is biodegradable, non-toxic and has excellent lubricity, providing longer life to components of engines [1]. The most common method for producing biodiesel is the transesterification, a reaction between a triacylglycerol, from vegetable oils or animal fats, and a short chain alcohol producing alkyl esters of fatty acids and glycerol as a co-product [2,3]. Biodiesel has similar physicochemical properties to that of mineral diesel and can be used in engines without modification [3]. The potential use of a raw material in biodiesel production depends on several factors, such as availability, cost, storage properties and performance as fuel [4]. Given the high biodiversity, large territory, climate diversity and soil conditions, Brazil contains different oil sources for biodiesel production including soybean, canola, castor, babassu, cotton, palm and sunflower [5]. Since Brazil has a well stablished soybean production, presently n
Corresponding author. E-mail address: [email protected] (M.C.S. Gomes).
http://dx.doi.org/10.1016/j.memsci.2015.09.004 0376-7388/& 2015 Elsevier B.V. All rights reserved.
most of biodiesel produced in the country used soybean oil [5,6]. Canola presents a high percentage of seed oil, 34–40%, which is approximately twice the value found in soybeans and correspond to 84% of all raw materials used in global biodiesel production [7,8]. Regarding the alcohol used in the process, in Brazil, the use of ethyl route has a strategic importance, since the availability of raw materials and technology allows an economically viable production of ethanol by fermentation processes, resulting in a cheaper product than the methanol. In biodiesel production, the use of vegetable oils and ethanol derived from sugarcane makes the process completely independent from petroleum, providing environmental benefits and generating a socio-economic development program [9,10]. After transesterification, the final mixture is mainly composed of alkyl esters of fatty acids, residual alcohol, glycerol, catalyst, mono-, di- and triglycerides. These and other contaminants in the biodiesel can cause operational and environmental problems and must be separated so that esters can be used as fuel [9,11]. The quality of B100 biodiesel in Brazil is specified by the National Agency of Petroleum, Natural Gas and Biofuels (ANP), based on the American ASTM D6751 and European EN14214 standards, with some modifications to meet the Brazilian raw material requirements [12]. One of the most important parameters for quality control of
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biodiesel is the amount of free glycerin, which has a maximum allowed limit of 0.02%. A high concentration of free glycerin can result from the separation of glycerin, causing problems during storage and in the fuel injection system [9,13]. Thus, besides controlling all the parameters influencing the reaction of biodiesel production, the stage of glycerol separation is very important to achieve a quality product, free of impurities and no risk of corrosion to the engine. Conventional purification procedures use a large volume of water, which varies depending on the impurities present, and provides the generation of a large amount of effluent that must be properly treated and disposed [14]. Studies in the literature [15–19] indicate that distinct methods have been applied for glycerol removal and biodiesel purification. Among the alternative methods of separation, the process with membranes has many advantages and good prospects for use in the separation and purification of biodiesel. The use of membranes can provide high quality and purity biodiesel, and environmental and economic advantages for reducing the amount of water used and eliminating the use of adsorbents. Apart from the reduction of costs related to effluent treatment, studies indicate that the use of membranes in the processing of biodiesel provides reduced power consumption, enabling the application of this process in industrial scale [20]. Regarding the use of membranes for biodiesel production, the published studies have focused on the use of membrane reactors to improve conversion into esters [21–23] and the use of membranes in the purification step, after separation by decantation [18,24–28]. Direct use of micro- and ultrafiltration after transesterification for the separation of phases, without previous decantation, was studied in previous works developed by our research group. The first experiments [29] were carried out with microfiltration membranes using synthetic mixtures of biodiesel, glycerol, and ethanol. They were evaluated the effects of the membrane porosity, transmembrane pressure, and the ethanol concentration in the mixture. The results indicated the influence of ethanol on behavior of the emulsion and the potential of applying ceramic membranes in the separation of glycerol and biodiesel. Afterwards [30], the experiments were performed using the reactional mixture produced by ethylic transesterification of degummed soybean oil. Considering the emulsion behavior of the mixture produced in the transesterification of degummed soybean oil, it was developed a methodology of addition of acidified water, aiming to destabilize this emulsion and improve the retention of glycerol by the membrane. The results showed the key role of both ethanol and water in the formation of agglomerates in the dispersed phase, since the glycerol retention after adding water was significant, with glycerol mass content in the permeate below 0.02%, indicating the efficiency of the used methodology. Continuing the study [31], it was evaluated the influence of the amount of acidified water added on the separation of glycerol and biodiesel using micro and ultrafiltration ceramic membranes. The results showed that the amount of acidified water added influences the emulsion properties and, consequently, the distribution of agglomerate size containing the glycerol. The current work aimed at improving knowledge on the behavior of ultrafiltration with ceramic membranes in separating glycerol from biodiesel using different feedstock. To this end, it was evaluated the influence of oil quality on ultrafiltration of the reaction mixture produced after ethyl esterification of vegetable oils with different characteristics. The separation of glycerol was performed using ultrafiltration membranes, and the best operating condition was evaluated in terms of permeate flux and quality of the product.
243
2. Experimental section 2.1. Production of biodiesel The reaction mixtures needed for the experiments were prepared by alkaline ethyl transesterification of vegetable oils. They were used degummed soybean oil and crude canola oil donated by Cocamar (Maringá, Paraná State, Brazil), and commercial refined soybean and canola oils. The anhydrous ethanol (99.4% purity) was donated by Cocafé, Astorga, Paraná State, Brazil and sodium hydroxide (NaOH) was purchased from Biotec. The previous results of the study of the yield of esters as a function of reaction parameters [30] indicate that the reaction temperature for obtaining a high ester yield depends on the acidity of the oil. In this way, when used refined oils, the reaction temperature was 45 °C, and when used crude and degummed oils, the transesterification was carried out at 30 °C. For all oils, the oil: alcohol molar ratio used was 1:7.5 and the amount of catalyst was 1% of mass of oil. The reaction was prepared in a 2 L batch reactor and the reaction time was 1 h. The reactor contents were mixed using a mechanical stirrer. 2.2. Ultrafiltration runs The experimental equipment consisted of a micro- and ultrafiltration pilot unit UF NETZSCH, Pomerode, Santa Catarina State, Brazil, model 027.06-1C1/07-0005/AI, operating in cross-flow conditions. A detailed description of the experimental unit was presented in a previous study [30]. The hydrophilic ceramic membranes used in the experiments were made of tubular α-Al2O3/TiO2 (Shumacher GmbHTi 01070), 250 mm long, 7 mm in diameter, and 0.005 m2 filtration area, purchased from Andritz, Pomerode, Santa Catarina State, Brazil. The tests were carried out with ultrafiltration membranes of 0.05 mm and 20 kDa. All ultrafiltration experiments were performed at 50 °C. In the first step, ultrafiltration of the mixture obtained from refined canola oil was evaluated. For all tests, before ultrafiltration of the mixture, the addition of acidified water (0.5% HCl) was evaluated in the mass concentrations varying from 0% to 20% in relation to the total mixture mass, under transmembrane pressure of 0.5–2.0 bar. As previously discussed [31], addition of water affords the formation of dispersed phase containing water, glycerol, catalyst, salts and other water-soluble substances, distinct from the continuous phase rich in ethyl esters and unreacted oil. Next, ultrafiltration experiments were run with reaction mixtures produced with refined soybean, degummed soybean, refined canola and raw canola oils. The experiments were performed only with the 0.05 mm membrane, under pressure of 1.0 bar, with prior addition of 10% acidified water. Finally, it was evaluated the separation of glycerol from the mixture produced with raw canola oil. In these experiments were used the membranes of 0.05 mm and 20 kDa, under pressures of 1.0, 2.0 and 3.0 bar and water concentration of 10%. Approximately four liters of the mixture were poured into the feed tank of the module for each run. After heating to 50 °C, the mixture was pumped into the membrane and the pressure was set to begin the permeate flux. All the runs were undertaken with a flow rate of 700 L/h, corresponding to a tangential velocity of about 8 m/s. The permeate was collected and the concentrate was completely recirculated to the feed tank. The performance of the glycerol separation process by ceramic membranes were characterized in terms of glycerol rejection, stabilized permeate flux, percentage of flux decline and flux with pure water after membrane cleaning. The reaction mixture obtained after adding acidified water was
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Table 1 Physicochemical characteristics of the vegetable oils used. Vegetable oil
Viscosity at 40 °C (mm2/s)
Density at 20 °C (g/cm3)
Moisture (%)
Acidity (%)
Degummed soybean Refined soybean Crude canola Refined canola
32.55 32.01 32.22 36.39
0.9156 7 0.0007 0.91227 0.0007 0.9105 7 0.0007 0.91347 0.0007
0.1277 0.004 0.090 7 0.003 0.1137 0.004 0.098 7 0.004
0.87 70.01 0.43 70.05 2.40 70.06 0.2470.03
directly fed to the micro- and ultrafiltration module, substituting the steps of settling and washing with water by micro- or ultrafiltration. Thus the use of membranes for glycerol separation enables the reduction in the number of steps in relation to the conventional treatment, in addition to a lower consumption of water.
of 1% NaOH and 1% citric acid at 70 °C and then with deionized water at the same temperature. It was evaluated the membrane hydraulic permeability so that a cleaning parameter could be set. After each regeneration cycle, the permeate flux was measured with deionized water, ensuring the reproducibility of the tests.
2.3. Analytical methods
3. Results and discussion
The viscosity at 40 °C was determined with a Brookfield model DV-III digital rheometer. A 25-mL pycnometer with a coupled thermometer was used to determine the density at 20 °C. The acidity was determined according to the American Oil Chemical Society (AOCS). The moisture content was determined by the Karl Fished method using an Analyzer apparatus, model Moisture Control KF-1000. The calorific value was determined using a Parr 6200 calorimeter. All analyses were performed in triplicate. The fatty acid profile of the oil and the quantification of the ethyl esters were done by gas chromatography with Varian model CP-3800 coupled with a flame ionization detector (FID) and a 30 mx0.25 mm capillary column specific for fatty acid separation, BP-X70. The carrier gas used was helium, in a split rate of 1:10. The analysis was performed with a column temperature program starting at 140 °C, heating up to 250 °C at 5 °C/min. The detector temperature was kept at 220 °C and that of the injector at 260 °C. The internal standard used was 99% methyl tricosanoate, purchased from Sigma-Aldrich. The free glycerol content of the permeate was determined by volumetric method based on the official AOCS methodology for the analysis of free glycerol in oils and fats (Ca 14-56) [32]. Each sample was analyzed three times and the averages are reported. The titration method with sodium periodate allows fast and accurate determination of glycerol in oils and fats at a low cost compared with chromatographic methods [9,32,33]. When glycerol concentration in the permeate was too high, i.e., when there was permeation of the dispersed phase through the membrane, titration method could not be applied and it was only performed a volumetric quantification of the percentage of the dispersed phase containing glycerol.
3.1. Characteristics of the oils
2.4. Membrane cleaning process Although membrane processes are extensively employed in the industry, the major disadvantage of their application is the fouling of the membranes, which imposes the need for frequent cleaning. Therefore, an improvement of the cleaning procedure may have a significant influence on the overall process efficiency [34]. Current membrane cleaning technologies include hydraulic, chemical, and mechanical methods. Ultrasound is an effective technique for cleaning a variety of surfaces, which has been used for cleaning fouled membranes or for increasing permeate flux of water through membranes [35]. The basic principle of operation is that ultrasound removes particles from the surface by causing particle movement in or near membrane [36]. In this work, the membrane cleaning was performed in ultrasonic bath (Ultra Cleaner 800, UNIQUE) at 40 kHz, with solutions
Table 1 lists the physicochemical characteristics of the four types of oil used in the experiments: degummed soybean oil, refined soybean oil, refined canola oil and crude canola oil. Vegetable oils exhibited similar density and viscosity and in accordance with the values in the literature [7,37]. The degummed soybean oil and crude canola oil showed acidity values of 0.87% and 2.4%, respectively. These percentages are higher than the recommended by Freedman et al. [38] for alkaline transesterification. Nevertheless, as studies have shown good results for this reaction with the use of vegetable oils with up to 3% acidity [39] and given the possibility of reducing the costs of treating the raw material, oils were not pre-treated. For all oils, the moisture content was below 0.5%, which is the maximum value recommended for the alkaline transesterification is not impaired [38]. 3.2. Ultrafiltration in the separation of biodiesel and glycerol produced by ethyl transesterification of refined canola oil Ultrafiltration of the mixture produced from refined canola oil, whose acidity was the lowest of all evaluated oils, was not efficient in the separation of glycerol. Considering the stoichiometry of the reaction, the amount of glycerol in feed stream varied from 6% to 7%, depending on the quantity of water added. For all the conditions evaluated, the dispersed phase containing glycerol permeated through the membrane, and the percentage of this phase in the permeate varied from 1% to 44% (Table 2). These results are justified by the absence of acidity in the oil, which prevented the agglomeration of the dispersed phase in the final mixture. Saleh et al. [26] studied biodiesel purification using membrane process and demonstrated that at lower soap concentrations, the increasing amount of water promotes the generation of larger glycerol rich particles. In this present work, soaps formed in the reaction, when there are free fatty acids in the oil, along with salts from the neutralization with acidified water, contribute to the formation of a dispersed phase containing glycerol, which can be retained by the membrane. Fig. 1 illustrates the samples of permeates in all conditions evaluated, indicating the presence of two phases. The lower phase is the dispersed phase containing glycerol, and the upper phase is formed mainly by ethyl esters. Permeate flux curves over filtration time, produced from ethyl transesterification of refined canola oil, using membranes of 0.05 mm and 20 kDa are presented in Figs. 2 and 3, respectively. It is interesting to observe a flux increasing in the beginning of the filtration for most of the runs. Probably, until the phase
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450
Table 2 Concentration of the dispersed phase in the permeate – ultrafiltration of refined canola oil. Pressure (bar)
Concentration of acidified water (% mass)
Dispersed phase (% mass)
1 2 3 4 5 6 7 8 9 10 11 12 13 14
0.5 1.0 1.0 1.0 2.0 2 2.0 0.5 0.5 1.0 1.0 2.0 2.0 2.0
10 1 3 10 0 5 10 3 10 1 10 0 10 20
1.7 31 8.4 12 34 18 22 4 5 3 1 3 10 44
0.05 0.05 0.05 0.05 0.05 0.05 0.05 20 20 20 20 20 20 20
400
1.0 bar - 1% 1.0 bar - 3% 1.0 bar - 10% 2.0 bar - 0% 2.0 bar - 5% 2.0 bar - 10%
350 2
Permeate Flux (kg/h.m )
Sample Membrane (mm/kDa)
245
300 250 200 150 100 50 0 0
10
20
30
40
50
60
70
80
90
100
Time (min) Fig. 2. Permeate flux in the ultrafiltration of biodiesel from refined canola oil for all conditions evaluated. 0.05 mm membrane, T ¼ 50 °C.
0.5 bar - 3% 0.5 bar - 10% 1.0 bar - 1% 1.0 bar - 10% 2.0 bar - 0% 2.0 bar - 10% 2.0 bar - 20%
240
180
2
Permeate Flux (kg/h.m )
210
150 120 90 60 30 0 0
10
20
30
40
50
60
70
80
90
100
Time (min) 8
9
10
11
12
13
14
Fig. 1. Samples of ultrafiltration permeates of the refined canola oil.
initially retained on the membrane surface is removed by the convective flux and a concentration profile is established, which is the transient period, there is an increase in the flux. After this initial period, due to the size of glycerol agglomerates and the pressure used, permeation occurs through the membrane pores, and consequently the fouling, which reduces the flux.
Fig. 3. Permeate flux in the ultrafiltration of biodiesel from refined canola oil for all conditions evaluated. 20 kDa membrane, T ¼50 °C.
According to Fig. 2, the 0.05 mm membrane showed very high flux rates, so that, in some tests, the minimum volume required in the feed tank was achieved before the default time of 95 min. The use of 2.0 bar pressure promoted a sharp decline in flux for all concentrations of water, as well as a high glycerol content in the permeate (Table 1). Although the lower pressure, 1.0 bar, has caused a reduction of membrane fouling, there was glycerol permeation for the three concentrations of water evaluated, and the lowest value of glycerol phase in the permeate was 8.4%, with acidified water concentration of 3%. Given the same concentration of water, 10%, increased pressure of 1.0 to 2.0 bar provided increased glycerol content in the permeate from 12 to 22% in the 0.05 mm membrane and 1 to 10% in the 20 kDa membrane. On the other hand, at constant pressure, it is observed that in 0.05 mm membrane, there was an increased retention of glycerol with higher concentration of water compared to the lowest concentration studied. As for the 20 kDa membrane, increased retention of glycerol with increasing concentration of water was only verified at 1.0 bar pressure. At 2.0 bar pressure, increasing the concentration of water promoted a reduction of glycerol retention,
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and with the maximum concentration of water evaluated, it was obtained the highest content of glycerol phase in the permeate, 44%. Compared to other results, it is observed that the addition of 20% water caused a great increase in permeation of glycerol, probably indicating inversion of the emulsion. From these results, it was concluded that for the two membranes at the same concentration of water, the increased pressure favors the permeation of glycerol. The 0.05 mm membrane showed an average value of glycerol phase in permeate greater than 20 kDa membrane, suggesting that glycerol agglomerates have sizes near the pore diameter of the 0.05 mm membrane. As the best retention of glycerol in the permeate was obtained with the 20 kDa membrane at 1.0 bar pressure and 10% water, additional tests were performed by reducing the pressure to 0.5 bar. However, even with the reduction of pressure, for the two water concentrations tested, there was no glycerol retention by the membrane. 3.3. Influence of the type of oil used in the separation To evaluate the influence of the type of oil used in membrane separation, ultrafiltration tests were run with reaction mixtures obtained from refined and degummed soybean oils and from crude and refined canola oils. Thus, it was possible to compare the characteristics of ultrafiltration of the biodiesel produced from oil with a higher acidity (degummed soybean and crude canola) and biodiesel produced from refined oil, which has low acidity. Tests were performed at 50 °C at a pressure of 1.0 bar with prior addition of 10% acidified water, and the curves of permeate flux over time are presented in Fig. 4. According to the results, an increase is observed in the flux for refined oils in the 10 initial minutes, followed by a constant decline over time. As already discussed previously, glycerol permeation provides fouling increasing. The curve for degummed soybean oil shows a typical behavior of membrane separation processes, with concentration polarization and fouling. The fouling is caused by external clogging of pores, but without glycerol permeation, according to the percentage of glycerol in the permeate listed in Table 3. In turn, for the crude canola oil, the decline of flux over time is minimum, indicating that the retained phase does not cause clogging of pores. The presence of fatty acids in the oil and hence, soaps and salts formed in the reaction, favor the formation of
Fig. 4. Permeate flux in the ultrafiltration of biodiesel from soybean and biodiesel from canola. Membrane of 0.05 mm, P¼ 1.0 bar, 10% acidified water.
Table 3 Comparison of the permeate flux and glycerol concentration of the ultrafiltration with different vegetable oils studied. Raw material
Permeate flux (kg/h m2)
Glycerol in the permeate (%)
Degummed soybean Refined soybean Crude canola Refined canola
69.7 166.1 101.1 121.4
0.0187 0.001 12 0.013 70.003 12
agglomerates containing glycerol and water-soluble substances. Such agglomerates are likely much larger than the membrane pores, thus retained more easily. Comparing the refined soybean oil with degummed soybean oil and refined canola oil with crude canola oil, it can be concluded that the formation of the dispersed phase is favored by the presence of fatty acids in the oil, allowing retention by the membrane. However, the agglomeration of this phase forms a layer on the membrane surface that provides an additional resistance to the flux and thus a lower value of the stabilized permeate flux compared to the corresponding refined oil. Importantly, as shown in Table 3, the percentage of glycerol in the permeate was the same, 12%, both for refined soybean oil and refined canola oil. These results indicate that probably the separation of glycerol by ultrafiltration depends more on the acidity of the oil used in the reaction than on its origin. Furthermore, the higher acidity of the crude canola oil provided the best retention of the dispersed phase and, consequently, of glycerol when compared to the degummed soybean oil. Samples of the permeates obtained from the ultrafiltration of mixtures produced from vegetable oils are shown in Fig. 5. As these samples passed through the refining process that removed substances providing color to the oil, the permeates obtained from the mixtures produced from refined oils exhibited a lighter color compared to permeates of degummed soybean and crude canola oils. In addition, it is possible to identify the dispersed phase in the permeate of refined oils, in which the separation was not efficient.
Degummed soybean
Refined soybean
Crude canola
Refined canola
Fig. 5. Permeate obtained in the ultrafiltration of mixtures of the vegetable oils studied.
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Table 4 Permeate flux and glycerol concentration in the permeate in the ultrafiltration of the mixture produced from crude canola oil.
160
2
Permeate Flux (kg/h.m )
140
100 80 60
1.0 bar 2.0 bar 3.0 bar
40
0
0
10
20
30
40
50
60
70
80
90
100
Time (min) Fig. 6. Permeate flux over time under different pressures in the ultrafiltration of the mixture produced from crude canola oil. 0.05 mm membrane, T ¼50 °C, 10% acidified water.
3.4. Ultrafiltration of biodiesel produced from crude canola oil: influence of membrane pore diameter and transmembrane pressure In order to examine the influence of acidity on the separation of glycerol, ultrafiltration tests were performed with the mixture produced from the transesterification of crude canola oil, whose acidity is 2.4%, that is ten-fold higher than the acidity of the refined canola oil. For the production of the reaction mixture, transesterification was carried out at 30 °C and 1:7.5, which was the best condition when using an oil with high acidity [30]. The variation of permeate flux along time for the membranes of 0.05 mm and 20 kDa are presented in Figs. 6 and 7, respectively. For all conditions evaluated, the concentration factor ranged from 1.3 to 1.4. For both membranes at the three pressures examined, the stabilized permeate fluxes were close, ranging from 89 to 108 kg/h m2. Furthermore, the highest stabilized permeate flux was obtained under the pressure of 2.0 bar. Meantime, higher rates of flux decline were observed at this pressure, i.e. 24% for the 0.05 mm membrane and 26% for the 20 kDa membrane. The increase in pressure from 1.0 to 2.0 bar caused an increase in the permeate flux. However, the pressure of 3.0 bar showed a lower stabilized 140 120 2
Membrane (mm)
Pressure (bar)
Permeate flux (kg/h m2)
Glycerol (%)
0.05 0.05 0.05 20 kDa 20 kDa 20 kDa
1.0 2.0 3.0 1.0 2.0 3.0
101.1 108.4 97.8 89.4 96.4 90.5
0.0137 0.003 0.0127 0.005 0.0137 0.007 0.0117 0.008 0.0117 0.004 0.0137 0.008
120
20
Permeate Flux (kg/h.m )
247
100 80 60
1.0 bar 2.0 bar 3.0 bar
40 20 0 0
10
20
30
40
50
60
70
80
90
100
Time (min) Fig. 7. Permeate flux over time under different pressures in the ultrafiltration of the mixture produced from crude canola oil. 20 kDa membrane, T ¼ 50 °C, 10% acidified water.
permeate flux in relation to the pressure of 2.0 bar. In accordance with Chakrabarty et al. [40] and Yi et al. [41], the increased flux with increasing pressure is due to driving force across the membrane. Nevertheless, this increase is not linear, since the appearance of additional resistances, such as the compaction of the phase retained on the membrane surface, which reduce the flux. The ultrafiltration of the crude canola oil, with membranes of 0.05 mm and 20 kDa, was effective in retaining glycerol, once the values of glycerol in the permeate varied from 0.011% to 0.013% under the conditions evaluated (Table 4). Besides that, the flux curves versus time showed a high initial flux, which remained almost constant throughout the operation, that is, the decline in flux over time was small and ranged from 14% to 26%. These results indicate that the retained phase did not cause membrane fouling. Probably, this reduction of fouling is due to the larger size of glycerol agglomerates, which are formed in the presence of free fatty acids in the oil used. This retention of the dispersed phase was confirmed by glycerol analysis (Table 4), since its highest concentration in the permeate was 0.013%. Comparing the results of ultrafiltration of crude canola biodiesel with the results of degummed soybean biodiesel presented in a previous study [31], it was observed that the largest amount of free fatty acids in the crude canola oil favors the formation of the dispersed phase containing glycerol and thus the removal by the membrane. Yi et al. [41] investigated the influence of acidity on the ultrafiltration of oil-in-water emulsions and concluded that the clusters of oil retained when used an acidic solution had a larger size compared to ultrafiltration of an alkaline solution. The authors explained that under acidic conditions, there is a reduction of the electrostatic repulsive forces between the clusters, causing them to bind to each other, forming larger agglomerates and thus reducing membrane fouling. Similar results of fouling reduction, given the formation of larger clusters, were also obtained by Hesampour et al. [42] when treated oily emulsions by ultrafiltration. Also, this formation of larger clusters when used the mixture produced from crude canola oil provided not only a greater retention of glycerol by the membrane, but also the stabilization of fluxes at higher values (Fig. 7 and Table 4). Therefore, regarding the lowest rate of flux decline over time and the quality of permeate achieved, the pressure of 1.0 bar is the most recommended for separation of glycerol and biodiesel produced by ethyl transesterification of the crude canola oil through ultrafiltration. Wang et al. [25] employed ultrafiltration with 0.1 mm ceramic membrane for biodiesel purification after separation of the phases by settling, and concluded that the smaller reduction of flux along time (16.7%) was obtained at a pressure of 1 5 bar with a stabilized permeate flux of 300 L/h m2. 3.5. Membrane cleaning Before
starting
the
experiments,
the
membranes
were
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characterized with deionized water at pressures of 1.0, 2.0 and 3.0 bar. The hydraulic permeability values of the membranes of 0.05 mm and 20 kDa were respectively 893 and 474 kg/h.m2 bar. The average values of permeate flux at 25 °C and 1.0 bar used to assess the cleaning of the membranes after each cleaning cycle, for the membranes of 0.05 mm and 20 kDa, were 867 and 450 kg/h m2, respectively. The methodology for membrane cleaning was efficient, and after each washing cycle, the initial flux of the membranes was restored with deionized water. Additionally, the cleaning in ultrasonic bath reduced the water consumption compared to washing the membrane in the module. Considering a complete washing cycle, corresponding to 6 washes with sodium hydroxide solution, 6 washes with citric acid and 6 rinses with deionized water, the total volume of water used is approximately 4.5 l. If the same washing was performed in the micro-and ultrafiltration module, the minimum water consumption would be about 15 l, considering an alkaline wash, an acid wash and three rinses with deionized water, since the volume necessary for the recirculation in the module is 3 l. 3.6. Characteristics of the biodiesel produced The permeate of the mixture obtained from crude canola oil using the 20 kDa membrane, at a pressure of 1.0 bar and 10% acidified water was subjected to evaporation under reduced pressure in a rotary evaporator at 70 °C and 600 mmHg for 30 min and then characterized for specific mass, kinematic viscosity, ester content, acidity, moisture, free glycerol content and calorific value. These results were compared to those obtained using degummed soybean oil [31] and are listed in Table 5 together with the values specified for the B100 biodiesel in Brazil [12]. The permeates showed viscosities within the specified range. The canola biodiesel had a higher average viscosity than the soybean biodiesel, which is a characteristic related to the fatty acid profile of the oil. Although ultrafiltration has removed most of the moisture, since the initial concentration of water was 9.1% and the permeate before being evaporated showed an average moisture content of 0.45%, the vacuum evaporation treatment was not sufficient to reduce the moisture in the final product to less than 0.05%. It is possible that using a lower evaporating pressure for more than 30 min is adequate to reach the specified moisture value. The temperature should not exceed 70 °C, given the possibility of polymerization of the esters. In the case of canola biodiesel, the high acidity can be reduced if the crude oil, which had an acidity of 2.4%, receives a preTable 5 Characteristics of the biodiesel produced from degummed soybean oil and crude canola oil using ultrafiltration to remove glycerol. Unit
Limit
Specific mass at 20 °C Kinematic viscosity at 40 °C Ester content, mín Free glycerol, max Moisture, max Acidity, max Calorific value
kg/m3
850–900 874
866
mm /s
3.0–6.0
4.38
4.85
% massa
96.5
97.2
97.5
% massa
0.02
0.014
0.011
976 0.48 38154
1670 1.26 39575
2
mg/kg 500 mg KOH/g 0.5 kJ/kg 35000
Soybean biodiesel (Permeate) GOMES et al. [31]
Canola biodiesel (Permeate)
Characteristic
treatment to reduce its acidity to around 0.8%, which was the acidity of the degummed soybean oil. As discussed earlier, it is important to have a certain acidity in the oil so that the methodology developed in this work is adequate to separate glycerol using membranes. The calorific value of all biodiesel samples was above the minimum required by the specification, 35,000 kJ/kg. The average calorific values of the vegetable oil and of the biodiesel produced from it were similar. The calorific value of degummed soybean oil was 39,230 kJ/kg and of the degummed soybean biodiesel was 38,154 kJ/kg. The calorific value of crude canola oil was 39,381 kJ/ kg and increased to 39,575 kJ/kg in the crude canola biodiesel. These values, as well as variations between oil and biodiesel were similar to values reported by Lee et al. [7] in the transesterification of soybean and canola oils. The ester content of both soybean biodiesel and canola biodiesel was higher than 97.0% in all samples, indicating that the reaction conditions used were appropriate for the transesterification of each oil. In other words, the permeates showed mass percentage of esters greater than the minimum required for marketing as biodiesel, 96.5%. Likewise, in all tests with the reaction mixture directly ultrafiltered, the percentage of glycerol was below the value specified by the ANP for free glycerol, which is 0.02% in mass. Compared to the works in the literature using membrane technology in the purification of biodiesel [18,24–28], the methodology used herein has the advantage of eliminating the step of separation by settling and provide a biodiesel with a very low content of glycerol after a single processing step. Taking into account that steps usually employed of settling and washing can be replaced by membrane filtration and that this process has provided lower content of glycerol than those presented in the literature, this study demonstrates a great prospect for improving the separation and purification of biodiesel.
4. Conclusions Ceramic ultrafiltration membranes are used for the purification of biodiesel produced from ethyl transesterification of vegetable oils with different characteristics, and enable to evaluate the influence of the acidity of the oil used on the membrane separation step. The ultrafiltration separation of glycerol from the reaction mixture obtained from refined canola oil is not effective in any condition evaluated. The comparison between the mixtures obtained by transesterification of degummed soybean oil, refined soybean oil, crude canola oil and refined canola oil demonstrate that the efficiency of membrane separation of glycerol depends on the acidity present in the oil. The highest percentage of acid in the oil contributes to the agglomeration of the glycerol after addition of acidified water in the final reaction mixture. Accordingly, the ultrafiltration of reaction mixtures produced from degummed soybean oil and crude canola oil is efficient in the removal of glycerol. On the other hand, when used refined oils, the agglomerates in the emulsion show a diameter smaller than the membrane pore size, so that the glycerol permeates through the membrane. The efficiency of the ultrafiltration of the mixture obtained from crude canola oil, which has the highest acidity, indicates that the formation of dispersed phase clusters not only causes a greater retention of glycerol by the membrane, but also the stabilization of fluxes at higher rates. This process of biodiesel purification by ultrafiltration has the advantage of eliminating the step of separation by settling and results in a biodiesel with very low content of glycerol after a
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single processing step, and reduce the amount of water used for washing. Biodiesel produced using the methodology developed, both from degummed soybean oil and crude canola oil, showed values of ester content, glycerol content, viscosity, density and calorific value within the range specified by the ANP for marketing.
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