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Energy & Fuels 2008, 22, 2116–2120
Efficient Production of Biodiesel Using Radio Frequency Heating Shaoyang Liu, Yifen Wang,* Timothy McDonald, and Steven E. Taylor Biosystems Engineering Department, Auburn UniVersity, 200 Tom E. Corley Building, Auburn, Alabama 36849-5417 ReceiVed January 16, 2008. ReVised Manuscript ReceiVed March 11, 2008
Fast transesterification of canola oil and methanol for biodiesel production was achieved using radio frequency (RF) heating. The conversion rate of oil to biodiesel reached 97.3% with RF heating for 3 min, a NaOH concentration (based on oil) of 1.0%, and a methanol/oil molar ratio of 9:1. A central composite design (CCD) and response surface methodology (RSM) were employed to evaluate the impact of RF heating time, NaOH concentration, and molar ratio of methanol to oil on conversion efficiency. Experimental results showed that the three factors all significantly affected the conversion rate. NaOH concentration had the largest influence, with the effect being more pronounced at lower (0.2-0.6%, based on weight of oil) concentration. No evident interaction among the three factors was observed. RF heating efficiency was primarily related to the amount of NaOH and methanol. The scale of the experiment was increased by five times (from 20 to 100 g oil per batch) without decrease of the conversion rate, indicating the scale-up potential of RF heating for biodiesel production.
1. Introduction With the meteoric rise of petroleum prices in recent years, alternative energies have attracted a great deal of attention. Compared with other developing technologies, e.g. fuel cells and solar energy, biodiesel is an immediately applicable option for replacing petroleum-based diesel and, consequently, has become a focus in the alternative fuel arena.1–5 Biodiesel is generally produced from vegetable oils or animal fats by transesterification with methanol using either alkaline or acid catalysts, as shown in Scheme 1.4 It is nontoxic, biodegradable, and renewable. Since the carbon in the oil or fat originated mostly from carbon dioxide in the air, biodiesel is considered to contribute much less to global warming than fossil fuels.3 Moreover, its combustion produces lower carbon monoxide, hydrocarbon, and particulate matter emissions than petroleumbased diesel, although these benefits are somewhat offset by a slight increase of nitrogen oxide emissions.6,7 The application of biodiesel as an alternative fuel has been intensively studied. As early as the late 1980s, biodiesel was commercially used as a substitute for petroleum-based diesel.1 As of September 2007, there have been over 150 commercial biodiesel plants built in the United States (http://www.biodiesel.org/buyingbiodiesel/ producers_marketers/ProducersMap-Existing.pdf). Alkaline catalyzing transesterification is the most common method employed for industrial biodiesel production.3 Typical operation conditions are reaction temperatures of about 60 °C, * Corresponding author. (1) Zhang, Y.; Dube, M. A.; McLean, D. D.; Kates, M. Bioresour. Technol. 2003, 89, 1–16. (2) Zhang, Y.; Dube, M. A.; McLean, D. D.; Kates, M. Bioresour. Technol. 2003, 90, 229–240. (3) Gerpen, J. V. Fuel Process. Technol. 2005, 86, 1097–1107. (4) Marchetti, J. M.; Miguel, V. U.; Errazu, A. F. Renewable Sustainable Energy ReV. 2007, 11, 1300–1311. (5) Agarwal, A. K. Prog. Energy Combust. Sci. 2007, 33, 233–271. (6) Knothe, G.; Sharp, C. A.; Ryan, T. W. Energy Fuels 2006, 20, 403– 408. (7) Lebedevas, S.; Vaicekauskas, A.; Lebedeva, G.; Makareviciene, V.; Janulis, P.; Kazancev, K. Energy Fuels 2006, 20, 2274–2280.
molar ratios of methanol to oil of 6:1 or higher, and catalyst concentrations of ca. 1% (w/w).1,3,4 The reaction requires more than one hour to reach a conversion rate of 90% under these typical conditions.8,9 Many efforts have been made to speed up this reaction. Recently, microwave heating was studied as an alternative to conventional heating and results have indicated a dramatically reduced reaction time, to something on the order of several minutes for 95% conversion.10–12 Large amounts of catalyst (5% KOH)10,11 or additional solvent,12 however, were used in these studies. These requirements would increase the difficulties and costs of commercializing the process. Furthermore, the penetration ability of microwave energy into oil is poor due to its short wavelength. It typically can only penetrate a couple of centimeters into the reactants, which could possibly result in uneven heating in large reactor vessels and cause potential problems during scale-up. Radio frequency (RF) heating is a promising dielectric heating technology which has been widely used in the food and wood products industries.13–16 Both microwave heating and RF heating produce rapid heat generation through direct interaction between an electromagnetic field and an object.15 However, compared to microwave heating, RF heating systems are simpler to configure, have higher electricity to electromagnetic power conversion efficiency, and deeper penetration of RF energy into a wide array of materials. The penetration depth of RF energy at a frequency of 27.12 MHz can be generally as large as 1 order of magnitude greater than that of microwaves at frequen(8) Noureddini, H.; Zhu, D. J. Am. Oil Chem. Soc. 1997, 74, 1457– 1463. (9) Darnoko, D.; Cheryan, M. J. Am. Oil Chem. Soc. 2000, 77, 1263– 1267. (10) Leadbeater, N. E.; Stencel, L. M. Energy Fuels 2006, 20, 2281– 2283. (11) Barnard, T. M.; Leadbeater, N. E.; Boucher, M. B.; Stencel, L. M.; Wilhite, B. A. Energy Fuels 2007, 21, 1777–1781. (12) Hernando, J.; Leton, P.; Matia, M. P.; Novella, J. L.; Alvarez-Builla, J. Fuel 2007, 86, 1641–1644.
10.1021/ef800038g CCC: $40.75 2008 American Chemical Society Published on Web 05/01/2008
Production of Biodiesel Using RF Heating
Energy & Fuels, Vol. 22, No. 3, 2008 2117
Scheme 1
Table 1. Central Composite Design Matrix and Experimental Results
cies in the range of 915-2450 MHz.14,17 This results in more uniform heat distribution in larger objects. The premise of this study was, therefore, that RF heating could overcome the limitations of microwave heating in producing biodiesel, speeding up the reaction, and be more suitable for scale-up. In the present work, fast transesterification of canola oil and methanol for biodiesel production was achieved using RF heating. Response surface methodology (RSM)18,19 was employed to evaluate the effects of heating time, catalyst dose and molar ratio of methanol to oil. Canola oil was selected as feedstock because of its relatively high yield of oil per acre and potential for use in industrial production. 2. Experimental Section 2.1. Chemicals. Methanol and sodium hydroxide, both purchased from Fisher Scientific, were of analytic grade. Canola oil was purchased from a local grocery store. An average value of 879 was taken as the molecular weight of the oil.20 Chloroform-d (99.8%, contained 0.03% TMS) was purchased from Aldrich for nuclear magnetic resonance (NMR) analysis. All reagents were used as received. 2.2. Experimental Design. Response surface modeling, RSM, a mathematical and statistical technique for designing experiments, building models, and evaluating effects of independent variables, was employed in this study. Three factors, RF heating time, catalyst dose, and molar ratio of methanol to oil, were selected as the independent variables, and a central composite design (CCD) with five levels was performed (Table 1). The central values, step sizes, and ranges chosen were the following: RF heating time 2 min, step 0.5 min, 1-3 min range; NaOH concentration (w/w, based on oil) 0.6%, step 0.2%, 0.2-1.0% range; and methanol/oil molar ratio 7:1, step 1:1, with range 5:1-9:1. For developing the regression equation, the variables were coded according to eq 1: xi )
Xi - X/i ∆Xi
(1)
where xi is the coded value of the ith variable, Xi is the natural value of the ith variable, X/i is the central value of Xi in the investigated area, and ∆Xi is the step size. The experimental results were fitted using a polynomial quadratic equation in order to correlate the response variable to the independent variables. The general form of the polynomial quadratic equation is k
Y ) A0 +
∑ i)1
k
Bixi +
∑ i)1
k
Ciixi2 +
k
∑∑D xx
ij i j
(2)
i)1 j)1
(13) Piyasena, P.; Dussault, C.; Koutchma, T.; Ramaswamy, H. S.; Awuah, G. B. Crit. ReV. Food Sci. Nutr. 2003, 43, 587–606. (14) Wang, Y.; Wig, T. D.; Tang, J.; Hallberg, L. M. J. Food Eng. 2003, 57, 257–268. (15) Wang, Y.; Wig, T. D.; Tang, J.; Hallberg, L. M. J. Food Sci. 2003, 68, 539–544. (16) Wang, S.; Monzon, M.; Johnson, J. A.; Mitcham, E. J.; Tanga, J. PostharVest Biol. Technol. 2007, 45, 240–246. (17) Tang, J.; Wang, Y.; Chan, T. V. C. T. NoVel Food Processing Technologies; CRC Press: New York, 2005; pp 501-524. (18) Box, G. E. P.; Draper, N. R. Empirical Model-building and Response Surfaces; Wiley: New York, 1987. (19) Montgomery, D. C. Design and analysis of experiments; John Wiley & Sons Inc.: New York, 1997. (20) Tsevegsuren, N.; Verhe, R.; Verleyen, T.; Munkhjargal, B. J. Oleo Sci. 2001, 50, 781–785.
natural valuea
code value
run
X1
X2
X3
x1
x2
x3
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
1.5 2.5 2.0 1.5 1.5 2.0 1.5 2.5 2.0 2.5 2.5 2.0 3.0 2.0 2.0 2.0 2.0 2.0 2.0 1.0
0.8 0.4 0.6 0.8 0.4 0.6 0.4 0.8 0.6 0.8 0.4 0.6 0.6 0.6 0.6 0.6 0.2 0.6 1.0 0.6
6 8 7 8 8 7 6 6 7 8 6 7 7 9 7 7 7 5 7 7
-1 1 0 -1 -1 0 -1 1 0 1 1 0 2 0 0 0 0 0 0 -2
1 -1 0 1 -1 0 -1 1 0 1 -1 0 0 0 0 0 -2 0 2 0
-1 1 0 1 1 0 -1 -1 0 1 -1 0 0 2 0 0 0 -2 0 0
temperature
conversion rateb(%)
ramp final (°C/min) (°C) observed predicted 3.7 12.4 12.2 12.2 13.7 11.8 8.9 3.6 13.5 10.7 9.2 12.6 8.3 12.6 9.5 9.3 14.4 3.5 6.3 9.5
26.1 50.9 44.0 38.3 41.1 43.3 33.3 28.8 46.9 46.3 43.3 44.9 45.9 45.3 39.4 38.8 49.7 27.1 33.3 29.8
86.4 83.6 86.8 91.1 79.3 86.1 75.8 89.3 86.0 92.2 80.2 87.0 89.2 88.1 87.0 86.3 64.9 84.4 94.9 83.3
88.1 81.4 86.4 90.9 78.4 86.4 75.6 91.2 86.4 93.9 78.6 86.4 89.4 89.2 86.4 86.4 67.3 83.6 92.4 83.4
a X ) RF heating time (min), X ) NaOH concentration (%, w/w, 1 2 based on oil), X3 ) molar ratio of methanol to oil. b Conversion rate was calculated by (1 - (remaining oil after reaction/total oil before reaction)) × 100%.
where xi are the input variables, which influence the response variable Y, and A0, Bi, Cii, and Dij are the regression coefficients. Origin 7.0 (OriginLab Corp., USA) was used for the regression analysis. 2.3. RF Heating. An RF heating apparatus (SO6B; Strayfield Fastran, UK) was employed in this work. The distance between the two electrodes was fixed at 15 cm. A 150-mL conical flask with reflux condenser was used as reactor (Figure 1). Sodium hydroxide was dissolved in methanol before addition of canola oil. For each RSM experiment, 20 g of oil was used. The reactants were mixed with a magnetic stir bar in the vessel. The stirring started 2 min before the RF heating and lasted another 5 min after the RF heating to allow sufficient time for the heat absorbed during RF irradiation to be used in the transesterification reaction. The transesterification reaction time was the RF heating time plus 5 min. All experiments were initiated with oil temperatures at ambient conditions (ca. 20 °C). The temperature of the reactants was monitored using a fiber-optic sensor (ReFlex, Neoptix Inc., USA). After cessation of stirring, the reactants would separate into two distinct layerssan upper layer containing biodiesel and unreacted oil plus a lower layer containing glycerin. 2.4. Product Analysis. A 250 MHz NMR spectrometer (AVANCE II 250, Bruker, Germany) was used to record the 1H NMR spectra of the reaction products. A 0.2 mL aliquot of the upper layer (mixture of biodiesel and remaining oil) of the products was dissolved in 0.4 mL of chloroform-d for the analysis. Generally, gas chromatography (GC), as specified in the ASTM D6751 standard for biodiesel fuels (D6584 is the procedure itself)8,9 or high-performance liquid chromatography (HPLC)12 are employed for determining the completeness of the transesterification reaction. Using chromatographic equipment, amounts of remaining reactants (oil and methanol), intermediates (diglycerides and monoglycerides), and products (biodiesel and glycerol) can be comprehensively determined. This analysis, however, is time-consuming and expensive. For the purposes of this study, a fast and simple method of determining conversion efficiency was most important. NMR analysis fulfilled these requirements and has been shown to be accurate as well.21
3. Results and Discussion 3.1. NMR Analysis of Reaction Products. The 1H NMR spectra of canola oil, biodiesel, and a typical reaction product
2118 Energy & Fuels, Vol. 22, No. 3, 2008
Liu et al.
Figure 1. Photograph of RF heating apparatus. Table 2. ANOVA Analysis for Equation 3
model error total R2 ) 0.947
Figure 2. 1H NMR spectra of canola oil, biodiesel, and a typical reaction product (RF heating 2 min; NaOH concentration 0.6%; molar ratio of methanol to oil 7:1; conversion rate 86.8%).
degrees of freedom
sum of square
mean square
4 15 19
768.214 33.042 801.256
192.054 2.203
F-value
p-value
87.187
