Energy Fuels 2010, 24, 3179–3182 Published on Web 04/23/2010
: DOI:10.1021/ef100101m
Biodiesel Production from Subcritical Methanol Transesterification of Soybean Oil with Sodium Silicate Jian-Zhong Yin,*,† Zhen Ma,† Da-Peng Hu,† Zhi-Long Xiu,‡ and Tong-Hua Wang§ †
State Key Laboratory of Fine Chemicals, School of Chemical Machinery, Dalian University of Technology, Dalian 116012, PR China, ‡School of Biology Science and Technology, Dalian University of Technology, Dalian 116024, PR China, and § School of Chemical Engineering, Dalian University of Technology, Dalian 116012, PR China Received September 12, 2009. Revised Manuscript Received April 2, 2010
Biodiesel from supercritical methanol reaction is a high-efficiency method of noncatalysis. Due to high temperature and pressure, this technology has been limited to practical applications. To improve the operation conditions, developing a mild method has become a new trend for biodiesel production. In this paper, the transesterification of soybean oil in subcritical methanol was studied with a small amount of catalyst of sodium silicate (Na2SiO3) to obtain biodiesel. Na2SiO3 as a catalyst can improve the transesterification reaction of soybean oil in supercritical methanol. The variables affecting the fatty acid methyl ester (FAME) yield during the transesterification reaction, such as the reaction temperature, catalyst content, reaction rate, and the molar ratio of soybean oil to methanol, were investigated and compared with those of noncatalyst experiments. The test results show that a FAME yield of 95.6% was achieved when the reaction was performed with a catalyst content of 0.5 wt % at 220 °C for 30 min.
A number of studies have focused on using lipase catalysis for biodiesel production. This enzyme-catalyzed biodiesel synthesis can overcome most problems of conventional chemical processes, but the high catalyst cost is the main obstacle to the use of enzymes in the commercial production of biodiesel.7-9 Recently, Saka and Kusdiana first proposed that biodiesel may be prepared from vegetable oil via noncatalytic transesterification with supercritical methanol (scMeOH).10 In this process, the transesterification reaction can be performed in a shorter time (just several minutes) and with a much simpler purification procedure. It was found to be suitable for such low-grade wastes with high free fatty acids and water in oils and fats.10 Moreover, it has better environmental benefits since it is a reaction without catalyst.11,12 However, the synthesis of biodiesel in supercritical methanol has drawbacks such as high cost of apparatus and energy consumption due to the high-temperature and high-pressure conditions, which are not viable in the large industry practice. To improve the operation condition of this new process, Yin et al.13,14 used a small amount of potassium hydroxide (KOH) catalyst to enhance the supercritical transesterification reaction process and reported that only 0.1 wt % of KOH in the reaction could lower the reaction temperature to 160 °C and reduce the molar ratio of methanol to oil from 42 to 24. Moreover, the supercritical transesterification in the presence of calcium oxide
1. Introduction As fossil fuels supply is limited and energy demand continues to rise, research is increasingly directed toward alternative renewable fuels.1 Biomass is considered as a renewable source at zero emission as it fixes CO2 from the atmosphere through photosynthesis. The replacement of fossil fuel with biomass can contribute to the reduction of the emission of CO2 reducing the burning of the fossil carbon. Biodiesel, defined as the monoalkyl esters of fatty acids derived from triglycerides by transesterification with alcohols, has recently attracted considerable attention due to its environmental benign and the fact that it comes from renewable resources.2,3 Application of this energy can not only significantly reduce the pollution generated from petroleum-base diesel oil but also lessen the dependence on petroleum. Commercially, biodiesel is mainly produced by the transesterification of vegetable oil using a homogeneous acid or basic catalyst, such as H2SO4, NaOH, or KOH.4,5 However, in this process, removal of these catalysts and purification of glycerol is very difficult and a large amount of wastewater is simultaneously produced,6,7 which brings extra cost to the final product. Heterogeneous catalysts could improve the synthesis approaches by avoiding the assistant processing costs related to homogeneous catalysts.2 *To whom correspondence should be addressed. Telephone: þ86411-39893695. E-mail:
[email protected]. (1) Demirbas, A. Energy Convers. Manage. 2009, 50, 14–34. (2) Ma, F.; Hanna, M. A. Bioresour. Technol. 1999, 70, 1–15. (3) Pinnarat, T.; Savage, P. E. Ind. Eng. Chem. Res. 2008, 47, 6801–6808. (4) Freedman, B.; Pryde, E. H.; Mounts, T. L. J. Am. Oil Chem. Soc. 1984, 61 (10), 1638–1643. (5) Aksoy, H. A.; Kahraman, I.; Karaosmanoglu, F.; Civelekoglu, H. J. Am. Oil Chem. Soc. 1988, 65, 936–938. (6) Demirbas, A. Prog. Energy Combust. Sci. 2005, 31, 466–487. (7) Narasimharao, K.; Lee, A.; Wilson, K. J. Biobased Mater. Bioenergy 2007, 1, 19–30. r 2010 American Chemical Society
(8) Chen, X.; Du, W.; Liu, D. H.; Ding, F. J. Chem. Technol. Biotechnol. 2008, 83, 71–76. (9) Nie, K. L.; Xie, F.; Wang, F.; Tan, T. W. J. Mol. Catal. B: Enzym. 2006, 43, 142–147. (10) Saka, S.; Kusdiana, D. Fuel 2001, 80, 225–231. (11) Kusdiana, D.; Saka, S. Appl. Biochem. Biotechnol. 2004, 115, 781–791. (12) Demirbas, A. Bioresour. Technol. 2008, 99, 1125–1130. (13) Yin, J. Z.; Xiao, M.; Song, J. B. Energy Convers. Manage. 2008, 49, 908–912. (14) Yin, J. Z.; Xiao, M.; Wang, A. Q.; Xiu, Z. L. Energy Convers. Manage. 2008, 49, 3512–3516.
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Energy Fuels 2010, 24, 3179–3182
: DOI:10.1021/ef100101m
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(CaO) and sodium hydroxide (NaOH) were investigated by Demirbas15 and Wang et al.16 It was indicated that the methyl ester (biodiesel) yield was greatly improved even when a little CaO or NaOH was added. Sodium silicate is a solid base material. It can not be dissolved in the reaction medium, and it acts as a heterogeneous catalyst in transesterification. The aim of this study was focused on the conversion of soybean oils to biodiesel via transesterification reaction employing Na2SiO3 as catalyst in supercritical and subcritical methanol conditions. The catalytic efficiency of sodium silicate in this reaction was studied according to the methyl ester yield. The influence of various reaction parameters, such as molar ratio of methanol to soybean oil, catalyst content, reaction temperature, and reaction time on the methyl ester yield were discussed.
Table 1. Physical Properties of Soybean Oil properties
soybean oil
acid value, mg KOH/g saponification value, mg KOH/g molecular weight, g/mol
0.397 191.143 880.8
fatty acid compositions, % palmitic acid oleic acid linoleic acid linolenic acid stearic acid
11.5 29.1 54.0 0.8 4.6
Table 2. Calculated Critical Properties of Oil and Methanol Mixture at Various Compositions molar ratio of methanol to oil
2. Experimental Section
properties
6
12
24
36
42
2.1. Materials. Refined soybean oil from Shanghai Fulingmen Food Co., Ltd. was used as reactant. Methanol was obtained from Shenyang Lianbang reagent factory (analytical grade). Sodium silicate (Na2SiO3, analytical grade) was purchased from Tianjin Bodi Chem. Co., Ltd. Na2SiO3 was ground and calcined in a muffle furnace at 400 °C for 4 h before the transesterification reaction. All other chemicals were obtained commercially and were of analytical grade. 2.2. Experimental Procedure. 2.2.1. Measurements of Physical Properties for Soybean Oil. The physical properties of reactants are the basic data for chemical reactions. So, before transesterification running, the acid value (AV), saponification value (SV), and fatty acid compositions of soybean oil were measured according to the standard of the American Oil Chemical Society.17 The molecular weight (Mw) of soybean oil was calculated by the formula as follows,
Tc (°C) Pc (MPa)
426.01 3.181
370.45 4.556
323.97 5.847
302.23 6.456
295.27 6.649
Mw ¼
at 60 °C for 30 min to remove methanol and then settled for about 2 h in order to accomplish phase separation. The upper phase was the methyl ester and the lower one was glycerin. Several reactions were conducted in triplicate, and the variation in the conversion to biodiesel was less than (2%. 2.3. Analysis. The reaction samples were analyzed by gas chromatography (GC, Agilent 6890, FID). The GC was equipped with a HP-5 capillary with dimensions 30 m 0.32 mm 0.25 μm. Sample volumes were 0.05 μL, the carrier gas was nitrogen, and the GC sample was separated in a constant flow mode with a flow rate of 9.0 mL/min. A split injector was used with a split ratio of 40 and a temperature of 270 °C. The FID (flame ionization detector) was operated at 270 °C. The oven temperature was initially held at 160 °C, then it was elevated to 225 °C at a rate of 3 °C/min and further elevated to 260 °C at a rate of 15 °C/min, then held for 3 min. Total run time of this method was 27 min. The calibration curve of peak area and the quantity of biodiesel was linear. The properties of Na2SiO3 were characterized and tested by XRD and BET. The results show that Na2SiO3 is not a kind of porous material (solid base), and the BET is just about 3.
3 1000 56:1 SV - AV
The tested and calculated results of properties are listed in Table 1. 2.2.2. Transesterification Reaction. A 250 mL autoclave made of SUS316L stainless steel, equipped with a magnetic stirrer (50-1500 rpm) and internal cooling system, was used as the reactor (R-32-520/250 Dalain Tongda Reactor Co., Ltd., China). The pressure and temperature were monitored in real time up to maximum values of 32 MPa and 520 °C, respectively. The reactor was controlled with a given amount of soybean oil and liquid methanol with different ratios. Then, the reactor was heated with an external electrical furnace (maximum power is 1.5 kW) to the desired temperature while the liquid solution was stirred at a constant speed of 300 rpm. The operation temperature (180-260 °C) of the reactor was measured with a thermocouple and automatically controlled by a controller at (5 °C for a set time. The pressure (2-12 MPa) of the reactor was indicated by a pressure sensor (DG1300-B2-B-2-60, ( 0.1 MPa, Guangzhou Shengnashi Pressure Instrumental Co., Ltd., China). After a certain reaction time, the reactor was moved into an ice-water bath to stop the reaction quickly. Then, the autoclave was opened and the contents of it were poured into a collector. The catalyst was calcined at 400 °C for 4 h before use. After the transesterification was complete, the samples were recovered by simple decantation. The remaining catalyst in the reactor was used to catalyze the next batch of transesterification. The reaction mixture was evaporated with a rotary evaporator
3. Results and Discussion Table 1 shows the physical properties of soybean oil. It can be found out that the soybean oil has high oleic acid and linoleic acid content. The miscibility of triglycerides and methanol is rather poor due to their dissimilarity in size and polarity, and they form two liquid phase upon their initial introduction into reactors. The phase behavior of the reaction mixture is crucial for the reaction process. To know the phase state of the reactants, the critical properties of the mixture of methanol and oil are calculated by Lorentz-Berthelot-type mixing rules,18 and the parameter values are shown in Table 2. The critical point is the point at which fluid starts to enter the supercritical state and is no longer condensable by raising the pressure alone. In this state, the molecules are gas-like, with high kinetic energy, and liquid-like, with high density.19-21 It is indicated that at a molar ratio of methanol to oil of 6, the critical temperature (Tc) and pressure (Pc) of the mixture (18) Bunyakiat, K.; Makmee, S.; Sawangkeaw, R.; Ngamprasertsith, S. Energy Fuels 2006, 20, 812–817. (19) Imahara, H.; Xin, J.; Saka, S. Fuel 2009, 88, 1329–1332. (20) Patil, P. D.; Gude, V. G.; Deng, S. Energy Fuels 2010, 24 (2), 746–751. (21) Marulanda, V. F.; Anitescu, G.; Tavlarides, L. L. Energy Fuels 2010, 24, 253–260.
(15) Demirbas, A. Energy Convers. Manage. 2007, 48, 937–941. (16) Wang, L. Y.; He, H. Y.; Xie, Z. F.; Yang, J. C.; Zhu, S. L. Fuel Process. Technol. 2007, 88, 477–481. (17) Methods Cd 3d-63, Cd 3b-73, and C3 2-66; American Oil Chemical Society: Champaign, IL, 1997.
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Figure 2. Effect of reaction temperature on methyl ester yield. Molar ratio of methanol to soybean oil 36; reaction time 30 min; 1-3 MPa.
Figure 1. Effect of the molar ratio of methanol to soybean oil on the methyl ester yield with 1 wt % Na2SiO3. Soybean oil 60 g, reaction time 30 min.
system were 426.01 °C and 3.181 MPa, respectively. As the methanol content in mixture system increases, the critical temperature decreases, while the critical pressure increases. As can be seen, all experiments in this study were performed under subcritical methanol condition except the comparison runs of 350 °C and 12 MPa. 3.1. Effect of Molar Ratio of Methanol to Soybean Oil on the Methyl Ester Yield. The molar ratio of methanol to soybean oil is one of the most important variables affecting the methyl ester yield. Theoretically, the transesterification reaction is an equilibrium reaction. A large amount of methanol was used to shift the reaction equilibrium to the right side and produce more methyl ester. To determine the optimal molar ratio of methanol to oil, the reactions were carried out at a fixed sodium silicate content of 1 wt % and fixed reaction time of 30 min. As shown in Figure 1, for molar ratios of methanol to oil ranging from 12 to 42, with the molar ratio of methanol to oil increased, the methyl ester yield evidently increased at 220 and 260 °C. When the molar ratio of methanol to oil increased to 36, the methyl ester yield reached 99%. Further increasing the molar ratio of methanol to oil, the methyl ester yield was not changed obviously. However, effect of the molar ratio of methanol to oil on the methyl ester yield was not significant at the temperature of 180 °C. This result may be due to that the solubility of oil in MeOH phase was low at lower temperature. 3.2. Effect of Reaction Temperature on the Methyl Ester Yield. Figure 2 presents the variations of the methyl ester yield with the reaction temperature. The transesterification reactions were carried out at a fixed molar ratio of methanol to oil of 36 and fixed reaction time of 30 min under different temperatures (180-260 °C). The mass ratio of sodium silicate to oil ranged from 0 to 1 wt %. It was observed that increasing the reaction temperature had a great influence on methyl ester yield. Compared with noncatalyst transesterification in supercritical (subcritical) methanol, the methyl ester yield increased obviously even when only adding a small amount of sodium silicate. Moreover, the methyl ester yield reached above 90% as the temperature increased from 180 to 220 °C. However, when the temperature was further increased from 240 to 260 °C, the FAME yield was only increased a little. So, the optimum temperature was suggested to be 220 °C in our runs. 3.3. Effect of the Sodium Silicate (Na2SiO3) Content on the Methyl Ester Yield. To discuss the effect of the sodium
Figure 3. Effect of the sodium silicate content on methyl ester yield. Reaction temperature 220 °C; molar ratio of methanol to soybean oil 36; reaction time 30 min; 3 MPa.
silicate content on methyl ester yield, the transesterification reactions were conducted at the temperature of 220 °C, with the molar ratio of methanol to oil of 36, and the reaction time of 30 min. The results of the experiment are shown in Figure 3. It can be affirmed that Na2SiO3 can evidently increase the methyl ester yield that derived from soybean oil at 220 °C even when only a little Na2SiO3 (0.25 wt % of the oil) is added. And a high yield of 95.6% was obtained with a Na2SiO3 content of 0.5 wt %, whereas 3 wt % Na2SiO3 was needed for conventional catalysis method under atmospheric pressure condition (see Table 3). The catalyst could be recovered by the deposition and high speed centrifugal separation method. The same method was also reported by Guo et al.22 Then the particles were washed several times by methanol and water. Finally, they were dried in air bath at 70 °C for 24 h. The catalytic activity of the Na2SiO3 can remain at more than 90% after 3 or 4 times of being reused under atmospheric conditions.22 3.4. Compared with the Conditions of Different Methods for Biodiesel Production. To estimate the technical feasibility of this method, we list the some different methods for biodiesel production in Table 3. Compared to the supercritical transesterification method (350 °C, 12 MPa,), it was evident that (22) Guo F.; Peng Z. G.; Dai J. Y.; Xiu Z. L. Fuel Process. Technol. 2010, 91, 322-328.
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Table 3. Optimize Condition of Different Methods for Biodiesel Production No.
temperature (°C)
1 222 313,14
220 60 350
pressure (MPa)
molar ratio of methanol-to-oil
3.0 atmospheric 12
36 7.5 42
the temperature of transesterification in subcritical methanol with Na2SiO3 could be improved effectively. For supercritical methanol transesterification, the optimum molar ratio of MeOH to oil is 42:1.13,14 The advantages of using a lot of MeOH are that the reaction can be performed under a single phase state because the oils become soluble with supercritical methanol. The yield of FAME production and reaction rate should be enhanced obviously. The effect of high molar ratio of MeOH to oil was mainly due to three reasons. First, higher pressure can be reached at the same temperature for the supercritical method. The methanol is in the liquid phase and is easily soluble with oil. Second, a higher concentration of reactant is in favor of the performance of the transesterification reaction. Finally, higher oil solubility in MeOH can be obtained under higher density of MeOH. Moreover, compared with the transesterification reactions that were catalyzed by Na2SiO3 at atmospheric pressure (obtained from literatures and proved by our experiments), both the catalyst content and the reaction time were decreased for the transesterification under subcritical methanol conditions. An economic study showed that the total investment and operating costs under high pressure conditions are less than those of the conventional method with acid or basic catalysts.3 3.5. Effect of Reaction Time on the Methyl Ester Yield. Figure 4 shows the effect of reaction time on the methyl ester yield. Initially, the methyl ester yield increased rapidly, and after reacting for 30 min, the methyl ester yield changed slowly. Especially when above the temperature of 220 °C, the methyl ester yield almost achieved its maximum value. However, when the temperature was lower (180 °C), the methyl ester yield was only 82.4% after running for 30 min. So the transesterification reaction at lower temperature may require more reaction time to achieve higher methyl ester yield. Compared with the catalysis method (see Table 3), the reaction time could be shortened from about 60 min to 30 min. This is in favor of decreasing the biodiesel production cost and enhancing the production efficiency for the large scale industry practice.
catalyst Na2SiO3 0.5 wt % 3 wt % none
time (min)
yield (%)
30 60 10
95.6 99.0 95.4
Figure 4. Effect of reaction time on methyl ester yield. Molar ratio of methanol to soybean oil 36; sodium silicate content 0.5 wt %; 1-3 MPa.
4. Conclusion The effects of solid catalyst Na2SiO3 added to subcritical methanol on biodiesel production were studied in a batch-type reactor. The results show that sodium silicate has high catalytic activity for the transesterification of soybean oil under our experimental conditions. It can be used to enhance the supercritical methanol transesterification reaction process and to improve the operating conditions remarkably. With only a little amount of sodium silicate as the catalyst (Na2SiO3/oil = 0.5 wt %), a 95.6% yield of methyl esters was obtained in 30 min at a reaction temperature of 220 °C and a molar ratio (methanol/oil) of 36:1. In contrast, above 3 wt % of catalyst is required for the sodium silicate catalyzed reaction method under atmospheric pressure (If 0.5 wt % of catalyst is used under this condition, a yield of only about 20% of methyl esters is achieved). Acknowledgment. This work was financial supported by National Natural Science Foundation of China (20976026, 20976028), National High Technology Research and Development Project of China (863 Project, 2007AA02Z208).
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