Energy Fuels 2010, 24, 1298–1304 Published on Web 11/25/2009
: DOI:10.1021/ef9010065
Microwave-Assisted Catalytic Transesterification of Camelina Sativa Oil Prafulla D. Patil, Veera Gnaneswar Gude, Lucy Mar Camacho, and Shuguang Deng* Chemical Engineering Department New Mexico State University, Las Cruces, New Mexico 88003 Received September 9, 2009. Revised Manuscript Received November 6, 2009
Catalytic conversion of Camelina Sativa oil to biodiesel through both conventional heating and microwave radiation was investigated. Three different types of catalysts: homogeneous catalysts (NaOH and KOH), heterogeneous metal oxide catalysts (BaO and SrO), and sol-gel derived catalysts (BaCl2/AA and SrCl2/ AA) were evaluated for their efficacy in biodiesel production. The following conditions were obtained for the catalysts based on the maximum biodiesel yield: potassium hydroxide/methanol to oil ratio of 1:9, catalyst concentration of 1% (w/w), and reaction time of 60 s; sodium hydroxide/methanol to oil ratio of 1:9, catalyst concentration of 0.5 wt %, and reaction time of 60 s; barium oxide/methanol to oil ratio of 1:9, catalyst concentration of 1.5% (w/w), and reaction time of 4 min; strontium oxide/methanol to oil ratio of 1:9, catalyst concentration of 2 wt %, and reaction time of 4 min. In the case of sol-gel derived catalysts, different catalyst loading rates in the range of 1-10 mmol/g were evaluated. Low biodiesel yields of 10-25% on the sol-gel derived catalysts were observed. On the basis of energy consumptions in the transesterification processes with both conventional heating and microwave-heating methods evaluated in this study, it was estimated that the microwave-heating method consumes less than 10% of the energy to achieve the same yield as the conventional heating method. The fuel properties of camelina biodiesel produced were compared with those of the regular diesel and found to be conforming to the American Society for Testing and Materials (ASTM) standards.
fuel which can be prepared from a range of organic feedstock, including new or waste vegetable oils, animal fats, and oilseed plants.6-8 Biodiesel has significantly lower emissions than petroleum-based diesel when it is burned, whether used in its pure form or blended with petroleum diesel. It does not contribute to a net rise in the level of atmospheric carbon dioxide with a minimal greenhouse effect.9,10 Well-known methods for biodiesel production are (1) pyrolysis,11,12 (2) microemulsions,13 (3) dilution,14 and (4) transesterification of oil to ester.13,15 Among these processes, transesterification has proven to be the simplest and economical route to produce biodiesel, with physical characteristics similar to fossil diesel, forming little or no deposits when used in diesel engines. Transesterification of vegetable oils is to simply reduce the viscosity of the vegetable oils. Transesterification is a process in which an alcohol (methanol or ethanol) in the presence of a catalyst (acid or alkali or enzyme) is used to chemically break the molecule of the vegetable oils or animal fats into methyl or
1. Introduction The world’s oil reserves are estimated to diminish by 2050 if the current rate of energy consumption persists.1,2 With addition to this concern, increasing population, rapid urbanization, and high living standards create pressing demands for alternative energy sources. Conventional energy sources are nonrenewable, and energy extraction from these sources as well as their usage cause pollution and adversely impacts the environment. Even developed countries are not able to meet the current fuel demands despite increasing the energy production manifold.2 Thus, it is crucial to develop energy production processes that are renewable, sustainable, and environmentally benign. Vegetable oils are recognized widely as promising alternatives to regular diesel fuel because they are renewable and environment-friendly and can be produced locally. Among the many vegetable crops, Camelina Sativa is a low-input crop with high per acre yield.3 Camelina Sativa oil is rich in Omega3 fatty acids4 and has positive energy balance for biodiesel production (net energy ratio = 1.47).5 Biodiesel is an abundant carbon-neutral renewable resource and a perfect replacement for a conventional diesel source. Biodiesel is a nontoxic, biodegradable, and renewable
(6) Ma, F.; Hanna, M. A. Bioresour. Technol. 1999, 70, 1–15. (7) Graboski, M. S.; McCormick, R. L. Prog. Energy Combust. Sci. 1998, 24, 125–164. (8) Schuchardt, U.; Sercheli, R.; Vargas, R. M. J. Braz. Chem. Soc. 1998, 9, 199–210. (9) Vicente, G.; Martinez, M.; Aracil, J. Bioresour. Technol. 2004, 92, 297–305. (10) Antolin, G.; Tinaut, F.; Briceno, Y.; Castano, V.; Perez, C.; Ramirez, A. Bioresour. Technol. 2002, 83, 111–114. (11) Maher, K. D.; Bressler, D. C. Bioresour. Technol. 2007, 98, 2351– 2368. (12) Hoekman, S. K. Renewable Energy 2009, 34, 14–22. (13) Sharma, Y. C.; Singh, B.; Upadhyay, S. N. Fuel 2008, 87, 2355– 2373. (14) Akoh, C. C.; Chang, S.; Lee, G.; Shaw, J. J. Agric. Food Chem. 2007, 55, 8995–9005. (15) Kusdiana, D.; Saka, S. Two-step preparation for catalyst-free biodiesel fuel production. Appl. Biochem. Biotechnol. 2004, 113-116, 781–791.
*To whom correspondence should be addressed. Telephone: þ1-575646-4346. Fax: þ1-575-646-7706. E-mail:
[email protected]. (1) Demirbas, A. Energy Sources, Part B 2009, 4, 212–224. (2) Saxena, R. C.; Adhikari, D. K.; Goyal, H. B. Renewable Sustainable Energy Rev. 2009, 13, 167–178. (3) Pilgeram, A. L.; Sands, D. C.; Boss, D.; Dale, N.; Wichman, D.; Lamb, P.; Lu, C.; Barrows, R.; Kirkpatrick, M.; Thompson, B.; Johnson, D. L. In Issues in New Crops and New Uses; Janick, J., Whipkey, A., Eds.; ASHS Press: Alexandria, VA, 2007; pp 129-131. (4) Leonard, C. INFORM 1998, 9, 830–838. (5) Putnam, D. H.; Budin, J. T.; Field, L. A.; Breene, W. M. In New Crops; Simon, J. E., Ed. Wiley: New York, 1993. r 2009 American Chemical Society
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New Jersey. Alumina-trisecondary butoxide, barium chloride (AR grade), and strontium chloride (AR grade) were obtained from Alfa Aesar. To test the physio-chemical properties of the oil, ethanol (95%, v/v), hydrochloric acid, and diethyl ether were procured from Fisher Scientific, New Jersey. Extra pure 99% methanol was purchased from Acros Organics. For gas chromatography/mass spectrometry (GC/MS) analysis, methyl heptadecanoate (C17), standard for GC, was purchased from Fluka, Milwaukee, WI. A domestic microwave oven with exiting power of 800 W was modified and fitted with a temperature reader and a watercooled reflux condenser. The interior bottom of the microwave unit was covered with aluminum foil to increase the effect of microwave radiation. Boiling stones were added to the reaction vessel to ensure uniform boiling of the reaction mixture. A ceramic stand to hold the sample vessel in the direction of the microwave source was placed inside the microwave reactor. Microwave-transparent reaction vessels made of borosilicate glass were used as sample vessels. Preliminary studies were conducted to evaluate the reproducibility of the biodiesel yield. The averages of the test results obtained for three repetitive tests were analyzed. After each test, the microwave reactor was allowed to cool and return to original conditions with adequate cooling and reaction interval. For the conventional heating method, all experiments for transesterification reaction were performed in a 250 mL roundbottom flask equipped with a water-cooled reflux condenser. A hot plate with magnetic stirrer arrangement was used for heating the mixture in the flask. The heating capacity of the hot plate was 500 W. After the transesterification reaction by conventional and microwave heating methods, a standard downstream procedure was followed in both cases. First, the reaction mixture was allowed to separate into two layers. The lower layer, which contained catalyst and glycerol, was drawn off. The crude methyl ester remained in the upper layer. The excess methanol in ester phase was distilled off under vacuum. Hot distilled water was sprayed over the surface of the ester and stirred gently to remove the entrained impurities and the glycerol. The lower layer was discarded, and the yellow colored layer (biodiesel) was separated. Washed biodiesel was then dried using sodium sulfate. 2.2. Catalyst Preparation. Initially, a modified Yoldas26-28 sol-gel process was used to synthesize a stable 2 M boehmite sol. During the process, the corresponding amount of aluminatrisecondary butoxide was hydrolyzed in 1 L of deionized water at a temperature of 75 °C. After dissolution, the solution was heated at 90 °C for an hour and the resulting precipitate was peptized with 1.0 M HNO3. The peptized sol was then refluxed at 90 °C and dried under atmospheric conditions to obtain a stable boehmite sol. Gelated spherical wet-gel spheres were obtained by passing the gel dropwise through an oil and ammonia solution. The obtained sol-gel activated alumina spheres were then calcined at 450 °C for 3 h. The sol-gel activated alumina (AA) catalyst was modified using two different chemicals, namely, barium oxide (BaO) and strontium oxide (SrO). For each of the two modifications, four different concentrations were prepared. To load the catalyst with BaO (samples A1-A4) or SrO (samples B1-B4), 5 g of the activated alumina were mixed with 50 mL of solution of BaCl2 or SrCl2 analytical grade containing concentrations of 1, 3, 5, and 10 mmol of strontium or barium per gram of activated alumina. The obtained solutions were shook for 24 h and filtered. The coated catalysts were then dried in a conventional oven at 60 °C for 24 h and calcined at 600 °C for 3 h.
Figure 1. Transesterification of triglycerides to yield alkyl esters and glycerin.
ethyl esters of the renewable fuel. The product of transesterification process is known as “biodiesel”. Transesterification of triglycerides to yield alkyl esters is shown in Figure 1. Transesterification of organic compounds to yield biodiesel can be achieved by the following methods: (1) conventional heating with acid, base catalysts, and cosolvents;16,17 (2) super- and subcritical methanol state without catalyst;15,18 (3) enzymatic method using lipases;19 and (4) microwave irradiation (with acid, base catalysts).20,21 Among these methods, the conventional heating method requires longer reaction times with higher energy inputs and losses to the ambient.21 The super and subcritical methanol state operates in expensive reactors at high temperatures and pressures resulting in higher energy inputs and higher production costs.22 The enzymatic method requires longer reaction times.14 Microwave-assisted transesterification, on the other hand, is an energy-efficient and a quick process to produce biodiesel from different feedstocks.20,21 Microwave-heating has been successfully applied to synthesize porous materials and supported catalyst in our previous research.23,24 Microwave-assisted transesterification of different feedstocks such as rapeseed oil, cotton seed oil, waste cooking oils, and C18 fatty acids has been reported by several researchers.16,19,20,25 In this study, microwave radiation was evaluated as a nonconventional heat source for tranesterification of Camelina Sativa oil. Three types of catalysts (two homogeneous catalysts, two heterogeneous metal oxide catalysts, and two sol-gel derived catalysts) were tested to assess their effect on biodiesel production. Microwave-assisted transesterification of Camelina oil to produce biodiesel was compared with conventionally heated transesterification. This paper includes process parametric evaluation, oil characterization, and fuel analysis of Camelina Sativa oil. 2. Experimental Section 2.1. Materials and Methods. Cold-pressed Camelina Sativa oil was obtained from Marx Foods Company, New Jersey. Heterogeneous metal oxide catalysts (SrO and BaO) were purchased from Alfa Aesar, Ward Hill, MA. Homogeneous catalysts NaOH and KOH flakes were procured from Acros Organics, (16) Azcan, N.; Danisman, A. Fuel 2007, 86, 2639–2644. (17) Patil, P. D.; Deng, S. Fuel 2009, 88, 1302–1306. (18) Patil, P. D.; Deng, S.; Rhodes, I.; Lammers, P. Fuel 2010, 89, 360– 364. (19) Roy, I.; Gupta, M. N. Curr. Sci. 2003, 85, 1685–1693. (20) Leadbeater, N. E.; Stencel, L. M. Energy Fuels 2006, 20, 2281– 2283. (21) Refaat, A. A.; El Sheltawy, S. T.; Sadek, K. U. Int. J. Environ. Sci. Technol. 2008, 5, 315–322. (22) Yin, J.; Xiao, M.; Song, J. Energy Convers. Manage. 2008, 49, 908–912. (23) Deng, S.; Lin, Y. S. Chem. Eng. Sci. 1997, 52, 1563–1575. (24) Deng, S.; Lin, Y. S. J. Mater. Sci. Lett. 1997, 16, 1291–1294. (25) Melo-Junior, C. A. R.; Albuquerque, C. E. R.; Fortuny, M.; Dariva, C.; Egues, S.; Santos, A. F.; Ramos, A. L. D. Energy Fuels 2009, 23, 580–585.
(26) Yoldas, B. E. Am. Ceram. Assoc. Bull. C 1975, 54, 289–290. (27) Deng, S.; Lin, Y. S. AIChE J. 1995, 41, 559–570. (28) Deng, S.; Lin, Y. S. AIChE J. 1997, 43, 505–514.
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3. Results and Discussion
Table 1. Fatty Acid Composition (% as Methyl Esters) in the Camelina Sativa Oil fatty acids
area (%)
capric acid (C10:0) lauric acid (C12:0) myristic acid (C14:0) palmitic acid (C16:0) stearic acid (C18:0) oleic acid (C18:1) linoleic acid (C18:2) linolenic acid (C18:3) arachidic acid (C20:0) gadoleic acid (C20:1)
3.5 0.4 5.2 8.7 3.5 20.6 5.1 30.5 3.8 15.7
This section elaborates on the experimental results obtained for three types of catalysts tested for the feasibility of biodiesel production using microwave radiation. Two homogeneous catalysts (potassium hydroxide, KOH, and sodium hydroxide, NaOH), two heterogeneous catalysts (barium oxide, BaO, and strontium oxide, SrO) and a set of sol-gel catalysts BaCl2 activated alumina (AA) coated and SrCl2 AA were tested. Process parameters such as methanol-to-oil ratio, catalyst concentration, and reaction time were optimized for each of the catalysts. Average of three test results with standard error bars are presented for each of the process parameters. Comparison in terms of energy requirements and biodiesel yields are made between microwave-assisted transesterifcation and conventionally heated transesterification. Finally, fuel properties of the biodiesel obtained from microwave-assisted transesterifcation were compared with the ASTM D-675133 standards for biodiesel. 3.1. Homogeneous and Heterogeneous Catalysts. 3.1.1. Effect of Methanol-to-Oil Ratio on Biodiesel Yield. One of the important variables affecting the ester yield is the molar ratio of alcohol to triglyceride. The stoichiometric ratio for transesterification requires 3 mol of alcohol and 1 mol of triglyceride to yield 3 mol of fatty acid alkyl esters and 1 mol of glycerol. However, transesterification is an equilibrium reaction in which a large excess of alcohol is required to drive the reaction to yield desired product.34 Methanol-to-oil ratios of 6:1, 9:1, 12:1, and 15:1 were tested for all the catalysts. For maximum biodiesel yield, a molar ratio of 6:1 was mostly used. In another study, the optimum ratio was reported as 10:1.21 In this study, for homogeneous catalysts (KOH and NaOH), a molar ratio of 9:1 was found to be effective with maximum biodiesel yields of 97% and 96% for KOH and NaOH catalysts, respectively, as shown in Figure 4. Similar results were obtained for heterogeneous catalysts, BaO and SrO, with maximum biodiesel yields of 92% and 98%, respectively, as shown in Figure 5. A comparable effect of heterogeneous metal oxide catalysts was observed during the conversion of Camelina Sativa oil in our previous studies.35,36 However, when the amount of methanol-to-oil molar ratio was increased over 9:1, excess methanol started to interfere in the separation of glycerin due to an increase in the solubility and resulted in lower biodiesel yield.37 3.1.2. Effect of Catalyst Concentration on Biodiesel Yield. Catalyst concentration is a critical factor to be determined in the transesterification process. High catalytic activity depends on the catalyst possessing strong basic sites.38 Low and high catalyst concentrations may result in undesired biodiesel yield as well as high production costs. In this study, four different catalyst amounts (0.5%, 1%, 1.5%, and 2%, w/w) were tested for each of the catalysts. For homogeneous catalysts as shown in Figure 6, 0.5% and 1% (w/w) of
2.3. Characteristics of Camelina Sativa Oil. The quality of oil is expressed in terms of the physicochemical properties, such as saponification value and acid value. The saponification and acid values of camelina oil were reported as 193.3 and 3.2 mg of KOH/g, respectively, corresponding to a free fatty acid (FFA) level of 1.58%. It has been reported that transesterification would not occur if the FFA content in the oil were above 3% (w/w)29 The FFA content in the oil was determined by a standard titrimetry method.30 Fatty acid composition (% as methyl esters) in the Camelina Sativa oil is given in Table 1. The Camelina Sativa oil contains a major proportion of esters of mono and polyunsaturated fatty acids. 2.4. Analysis of Camelina Biodiesel. For the quantification of reaction products, the samples were analyzed by a gas chromatography/mass spectrometry system incorporated with an Agilent 5975 C mass-selective detector (MSD) and an Agilent 7890 A gas chromatograph equipped with a capillary column (HP-5 MS, 5% phenyl methyl silox 30 m 250 μm 0.25 μm nominal). Methyl heptadecanoate (10.00 mg; internal standard) was dissolved in 1 mL of heptane to prepare the standard solution. Approximately 55 mg of crude methyl ester was dissolved in 1 mL of standard solution for GC analysis, and 1 μL sample was injected into the GC which uses helium as the carrier gas. The injection was performed in splitless mode. The parameters of the oven temperature program consist of start at 80 °C with 10 °C/min intervals up to 180 °C (1 min) and rise up to 255 °C with 15 °C/min intervals (2 min). The fatty acid methyl etser (FAME) content was calculated using the following equation: C ¼
ð
P
AÞ - AEI CEI VEI 100% AEI W
The biodiesel yield is apparently the same to that of the FAME content (%) calculated quantitatively by GC/MS.31 ΣA is the total peak area of methyl ester, AEI is the peak area of methyl heptadeconoate, CEI is the concentration (mg/mL) of standard solution (methyl heptadecanoate), VEI is the volume (mL) of standard solution (methyl heptadecanoate), and W is the weight (in milligrams) of sample. From GC/MS analysis, it can be noted that Camelina Sativa oil contains a major proportion of unsaturated fatty acids (71.9%). High levels of polyunsaturated fatty acids have a negative effect on some ester properties, such as iodine value and ash level, while other ester properties were found within specifications.32 Gas chromatogram for camelina biodiesel is shown in Figure 2. The mass spectrum (EI) of heptadecanoic acid and methyl ester, C17:0 (internal standard) in the NIST library obtained using GC/MSD is shown in Figure 3.
(33) ASTM (American Standards for Testing of Materials). 2003. (D189-01, D240-02, D4052-96, D445-03, D482-74, D5555-95, D6751-02, D93-02a, D95-990, D97-02). (34) Freedman, B.; Butterfield, R.; Pryde, E. J. Am. Oil Chem. Soc. 1986, 63, 1375–1380. (35) Patil, P. D.; Deng, S. Energy Fuels 2009, 23, 4619–4624. (36) Patil, P. D.; Gude, V. G.; Deng, S., Ind. Eng. Chem. Res. 2009, DOI: 10.1021/ie901146c. (37) Kim, J., II; Kang, B. S.; Kim, M. J.; Park, Y. M.; Kim, D. K.; Lee, J. S.; Lee, K. Y. Catal. Today 2004, 93, 315–320. (38) Dorado, M. P.; Ballesteros, E.; Mittelbach, M. Energy Fuels 2004, 18, 1457–1462.
(29) Van Gerpen, J.; Canakci, M. Am. Soc. Agric. Eng. 2001, 44, 1429–1436. (30) Paquot, C. Standard Methods for the Analysis of Oils, Fats, and Derivatives, Part 1, 6th ed.; Pergamon: New York, 1979 (31) Ma, F.; Clements, L. D.; Hanna, M. A. Trans. ASAE 1998, 41, 1261–1264. (32) Frohlich, A.; Rice, B. Ind. Crops Prod. 2005, 21, 25–31.
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Figure 2. GC chromatogram of camelina biodiesel.
Figure 3. Mass spectrum (EI) of heptadecanoic acid, methyl ester C17:0 in NIST library.
Figure 4. Effect of methanol to oil ratio on biodiesel yield for KOH and NaOH catalysts.
Figure 5. Effect of methanol to oil ratio on biodiesel yield for BaO and SrO catalysts.
catalyst concentrations were sufficient with high biodiesel yield of 95% and 96% for NaOH and KOH, respectively. It has been reported that the most desirable biodiesel properties were obtained using potassium hydroxide (KOH) as the
catalyst in many studies.19,21 The excess amount of sodium hydroxide catalyst leads to saponification resulting in lower biodiesel yield and lower biodiesel quality. At higher catalyst concentrations, the intensification of mass transfer becomes 1301
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Figure 6. Effect of catalyst concentration on biodiesel yield for KOH and NaOH catalysts.
more important than increasing the amount of catalyst. For heterogeneous catalysts, the optimum catalyst concentrations were determined to be 1.5% and 2% (w/w) for BaO and SrO with maximum biodiesel yields 94% and 95%, respectively. Compared to the homogeneous catalysts, these concentrations are 2-4 times higher. The influence of heterogeneous catalysts on the biodiesel yield is shown in parts a and b of Figure 7. However, an advantage associated with heterogeneous catalysts is that they can be recovered and reused several times.39 For homogeneous catalysts, the biodiesel separation from dissolved catalyst concentrations can be laborious. The reactivities of BaO and SrO catalysts were found to be quite different even though they belong to a similar classification (group IIA) in the periodic table. This is due to the reason that they possess different catalytic activity, basicity, leaching tendency, and specific surface area, which influence the transesterification of oil.40,41 3.1.3. Effect of Reaction Time on Biodiesel Yield. Effect of reaction time was tested for each of the catalysts to determine the optimum reaction time. The homogeneous catalysts resulted in high biodiesel yield with short reaction times. Potassium hydroxide showed a biodiesel yield in the range 96-98% while sodium hydroxide showed a yield in the range 90-93%, respectively, for the reaction times in the range of 30-60 s as shown in Figure 8. Biodiesel yield did not improve with a further increase in reaction time. High basic characteristics of these catalysts can be attributed to the result of high biodiesel yield in shorter reaction times. However, the reaction times may vary depending on the sample volume. For heterogeneous catalysts, the reaction times required are high as shown in parts a and b of Figure 7. The reaction time required to obtain decent biodiesel yield was 4 min for both BaO and SrO catalysts with biodiesel yields of 94% and 95%, respectively. This is one reason three different reaction times were tested to determine the optimum catalyst concentration. The reason for high reaction times can be explained as the inability of the microwaves to influence the solid materials in the same way as the liquid solvents. The microwaves have a poor ability to penetrate through solid materials. In the case of homogeneous catalysts, this is entirely different because the catalyst is completely dissolved in the solvent and the microwave effect is higher in liquid solvents resulting in higher biodiesel yield. From the information above, it can be
Figure 7. Effect of catalyst concentration and reaction time on biodiesel yield for BaO (a) and SrO (b) catalysts.
Figure 8. Effect of reaction time on biodiesel yield for homogeneous catalysts.
concluded that the homogeneous catalysts will, in general, require shorter reaction times compared to heterogeneous catalysts for microwave-assisted transesterification. 3.2. Sol-Gel Derived Catalysts. The modification of the sol-gel activated alumina (AA) catalyst was conducted using two different chemicals, namely, barium oxide (BaO) and strontium oxide (SrO). For each of the two
(39) Singh, A. K.; Fernando, S. D. Energy Fuels 2008, 22, 2067–2069. (40) Gotch, A. J.; Reeder, A. J.; McCormick, A. J. Undergrad. Chem. Res. 2008, 4, 58. (41) Tanabe, K.; Fukuda, Y. React. Kinet. Catal. Lett. 1974, 1, 21.
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Table 2. Effect of Sol-Gel Derived Catalyst Loadings on the Biodiesel Yielda catalyst
catalyst loading (mmol/g)
average reaction temp (°C)
biodiesel yield (%)
BaCl2 AA (A1) BaCl2 AA (A2) BaCl2 AA (A3) BaCl2 AA (A4) SrCl2 AA (B1) SrCl2 AA (B2) SrCl2 AA (B3) SrCl2 AA (B4)
1 3 5 10 1 3 5 10
76 74 76 76 81 80 81 83
12 22 24 27 10 16 16 20
Table 3. Comparison of Energy Consumptions for Biodiesel Production by Two Methods microwave heating
conventional heating
catalyst
energy dissipated (J/s)
800
500
time of reaction (s) % power emitted total energy required (kJ) ratio biodiesel yield (%)
60 100 48 1 98
1800 100 900 18.75 96
potassium hydroxide
time of reaction (s) % power emitted total energy required (kJ) ratio biodiesel yield (%)
60 40 19.2 1 80
900 100 450 23.4 76
sodium hydroxide
a
Methanol to oil ratio (1:9); catalyst amount (2% w/w); and reaction time (5 min).
Figure 10. Effect of power dissipation levels on biodiesel yield.
catalytic activities of alkaline earth metal oxides toward the transesterification are associated with their alkalinities.42 Besides, the specific surface area, acidity/basicity, and acid/ base sites, leaching tendencies in biodiesel, and selectivity toward the transesterification reaction are also the contributing factors of catalysts in biodiesel formation.40,41 From this analysis, it is imperative that high doping concentrations of barium oxide (BaO) and strontium oxide (SrO) may actually result in higher biodiesel yield if properly applied. Apart from this, benefits of using sol gel catalysts include (a) the catalyst material is recoverable and reusable and (b) capable of catalyzing the transesterfication at higher temperatures to yield higher quality biodiesel. 3.3. Microwave Effect. Microwave-assisted transesterification was compared with conventionally heated tranesterification in terms of biodiesel yield for different reaction times. It can be noted from parts a and b of Figure 9 that 30-60 s were sufficient for microwave heating while 30-60 min were required for conventional heating to achieve comparable biodiesel yields. This large difference in reaction time can be attributed to the limitations of conventional heating in which the energy is first utilized to increase the temperature of the reaction vessel and the higher temperature of the reaction vessel results in higher heat losses to the ambient. Energy requirements for the two methods are presented in Table 3. The energy required by the conventional method is found to be around 18 times greater than that by the
Figure 9. Comparison of reaction times and biodiesel yield by the microwave method (a) and conventional method (b).
modifications, four different concentrations of 1, 3, 5, and 10 mmol of strontium or barium per gram of activated alumina were tested. A slight increase in biodiesel yield was obtained by increasing the load concentration of barium or strontium in the sol-gel activated alumina catalyst (Table 2). For sol-gel catalysts, biodiesel yield was found to be low. The lower yield may be due to (1) low catalysts selectivity and low catalyst processing basic sites to transesterify camelina oil and (2) the heterogeneous metal catalyst generally requires high reaction temperatures that cannot be controlled and achieved with a domestic microwave unit. It suggests that the (42) Yan, S.; Lu, H.; Liang, B. Energy Fuels 2008, 22, 646–651.
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Table 4. Fuel Properties of Camelina Oil and Camelina Biodiesel properties
camelina oil
camelina biodiesel
regular diesel
biodiesel standards ASTM D 6751-02
testing method (ASTM)
specific gravity viscosity (mm2/s) at 40 °C calorific value (MJ/kg) cetane number pour point (°C)
0.91-0.92 14.03-15.12 44.50-46.50 35.16-36.25 -23 to -20
0.87-0.88 3.90-4.09 44.96-45.10 48.96-49.85 -10 to -8
0.85 2.6 42 46 -20
0.87-0.90 1.9-6.0
D4052 D445 D240 D613 D97
47 min -15 to 10
microwave method. Another test was conducted to control the power dissipation during the transesterification reaction. The microwave unit was equipped with a power control option which allows for 10 different levels of power dissipation. Different power dissipation levels (20%, 40%, 60%, 80%, and 100%) were tested and respective biodiesel yields are shown in Figure 10. For 40% power dissipation in the microwave method, the observed biodiesel yield was 80% in 1 min reaction time (Figure 10), and a similar yield of 76% (Figure 9b) was obtained for the NaOH catalyst in the conventional heating method in 15 min reaction time. The energy required for the microwave heating method is 23 times lower than the conventional method. These results suggest that proper power dissipation control will result in effective use of the microwave energy and further reduction in energy requirements. 3.4. Fuel Properties of Camelina Biodiesel. The fuel properties of biodiesel from Camelina Sativa oil with testing methods are given in Table 4. The viscosity of biodiesel from Camelina Sativa oil was comparable to regular diesel viscosity, i.e., 2.6 mm2/s. Hence, no hardware modifications are required for handling this fuel (biodiesel) in existing engines. The cetane number was estimated as 48.96-49.85 and found to be higher than ASTM biodiesel standards. A higher cetane number indicates a good ignition quality of fuel. The pour point of camelina biodiesel was found to be between -10 and -8 °C. This pour point might give rise to low running problems in the cold season, which could be overcome by the addition of suitable pour point depressants or by blending with diesel oil. Fuel consumption and vehicle operation with camelina ester are similar to rapeseed methyl ester.32 As shown in Table 4, most of the fuel properties of the Camelina methyl esters are quite comparable to those of ASTM biodiesel standards.
4. Conclusions Microwave-assisted transesterification of Camelina Sativa oil using several heterogeneous, homogeneous, and sol-gel derived catalysts was investigated for optimum reaction conditions. The catalysts evaluated in this study had a varying selectivity toward the transesterification reaction depending upon their acid/base-site strength, leaching tendency, and surface area. The homogeneous catalysts (KOH and NaOH) produced better results in shorter reaction times as compared to the heterogeneous catalysts (BaO and SrO) and the solgel derived catalysts. For the sol gel catalysts, a higher loading of active species (>10 mmol of catalyst/g of AA) is recommended in order to achieve a higher biodiesel yield. The fuel property values of the camelina biodiesel produced in this work are quite close to the ASTM biodiesel standards. The preliminary experimental study performed in this work has demonstrated that the microwave-heating method is energy-efficient and better than the conventional heating method. It is possible to significantly reduce the energy consumption in transesterification processes by using microwave-heating. Further studies on microwave-assisted transesterification kinetic at various conditions are necessary to elucidate the reaction mechanisms to determine the reaction rate constants and activation energies, which enable us to optimize and design energy-efficient transesterification processes for biodiesel production from various feedstocks. Acknowledgment. This project was partially supported by the New Mexico State University Office of Vice President for Research and the State of New Mexico through a New Mexico Technology Research Collaborative grant.
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