6609r 2010 American Chemical Society pubs.acs.org/EF

Energy Fuels 2010, 24, 6609–6615 : DOI:10.1021/ef1011929Published on Web 11/18/2010

Transesterification of Soybean and Rice Bran Oil with Ethanol in a Continuous-Flow

Microwave-Assisted System: Yields, Quality, and Reaction Kinetics

Beatrice G. Terigar, Sundar Balasubramanian, Marybeth Lima, and Dorin Boldor*

Department of Biological and Agricultural Engineering, Louisiana State University and LSU AgCenter Baton Rouge,Louisiana 70803, United States

Received September 3, 2010. Revised Manuscript Received October 30, 2010

The use ofmicrowaves as a heatingmedium to drive the transesterification of vegetable oils into ethyl esterswas investigated. A continuous-flow microwave-assisted transesterification of soybean and rice bran oilwas carried out in the presence of ethanol, using a molar ratio alcohol/oil of 5:1, and 0.6%w/w of sodiumhydroxide as catalyst. The reactions were performed at atmospheric pressure, using a flow rate of 100mL/minat temperatures of 50 and 73 �C for different reaction times (1, 5, and 10 min). At all tested parameters,the ethyl ester biodiesel produced conformed with ASTM specifications, with conversion rates between96.7 and 99.3%, indicating a good quality biodiesel. The results indicated that microwave heating can beused effectively to obtain high ethyl esters of fatty acids (biodiesel) yields at lower alcohol/oil ratios,effectively reducing the reaction time, even under continuous-flow reactions due to a reduction inactivation energy and an increase in Arrhenius pre-exponential factor. The results indicate that thisheating process has potential to be scaled up for use in the industry to produce good quality biodiesel usingethanol as the alcohol of choice.

1. Introduction

Biodiesel, a renewable fuel composed ofmonoalkyl (methylor ethyl) esters of long chain fatty acids from plant oils oranimal fats, is an alternative to conventional petroleum diesel.1

Worldwide, there is increasing interest inbiodiesel as a renewabletransportation fuel and blending agent, to displace petroleumdiesel, to lower net global warming gas emissions from thetransportation sector, and to reduce carcinogenic particulatematter emissions.2

Using vegetable oils as a substitute for diesel fuel has someadvantages: oil is portable, has high heat content, is readilyavailable, and is renewable. However, the direct use of oil ininternal combustion engines is restricted due to its highviscosity, which results in poor fuel atomization, incompletecombustion, and carbon deposition on the injector and valveseats, which causes serious engine fouling.3 Other constraintslimiting the use of vegetable oil directly are its low volatility,content of polyunsaturated carbon chains, deterioration dueto oxidation, and limited availability.

To overcome these barriers, processes such as pyrolysis(thermal cracking),4 microemulsification in diesel fuel,5

dilution,6 and transesterification2 have been developed. Withpyrolysis, resulting products have positive characteristics suchas lowviscosity, higher cetane number, acceptable amounts of

sulfur, water, and sediments, and acceptable copper corrosionvalues, but are unacceptable in terms of their ash contents,carbon residue, and pour points. Similarly, microemulsionshave lower viscosities but result in sticking of the injectorneedles, heavy carbon deposits, and incomplete combustionduring tests.7

Transesterification (or alcoholysis) of vegetable oils andfats can be performed, with orwithout catalysts (base, acid, orenzyme), as shown in Figure 1 using primary and secondarymonohydric aliphatic alcohols having 1-8 carbon atoms ascosolvents.8 Because the reaction is reversible, excess alcoholis used to shift the equilibrium to the product side. The mostfrequently used alcohols are ethanol and methanol, withmethanol being preferred at the industrial scale due to itslow cost and its physical and chemical advantages (polar andshortest chain alcohol). Ethanol, also used as an extractionsolvent, is preferable to methanol because of its superiorsolubility for oil which results in ethyl esters with increasedheat content and cetane number, lower cloud point, and thecharacteristics of an entirely renewable, agricultural-basedfeedstock for biodiesel.9

The major obstacles in the commercialization of biodiesel,in comparison to petroleum-based diesel fuel, are its high costof production, feedstock cost and limited availability, plantsize, and low value of glycerine byproduct.10,11 Research hasbeen performed on a wide range of feedstocks to decreasebiodiesel production costs, including soybean oil,12 rapeseed

*Towhom correspondence should be addressed.Mail: 149 EBDoranBuilding, Department of Biological and Agricultural Engineering,Louisiana State University Agricultural Center, Baton Rouge, LA 70803,USA. Phone: 1-225-578-7762. Fax: 1-225-578-3492. E-mail: dboldor@agcenter.lsu.edu.(1) Ma, F. R.; Hanna, M. A. Bioresour. Technol. 1999, 70, 1–15.(2) Van Gerpen, J. Fuel Process. Technol. 2005, 86, 1097–1107.(3) Altin, R.; Cetinkaya, S.; Yucesu, H. S. Energy Convers. Manage.

2001, 42, 529–538.(4) Demirbas, M. F. Energy Educ. Sci. Technol. 2008, 22, 59–67.(5) Lin, C. Y.; Lin, S. A. Fuel 2007, 86, 210–217.(6) Awang, R.; May, C. Y. J. Oil Palm Res. 2008, 20, 571–576.

(7) Ziejewski, M.; Kaufman, K. R.; Schwab, A. W.; Pryde, E. H.J. Am. Oil Chem. Soc. 1984, 61, 1620–1626.

(8) Fukuda,H.;Kondo,A.; Noda,H. J. Biosci. Bioeng. 2001, 92, 405–416.

(9) Demirbas, A. Energy Sources, Part B 2008, 3, 177–185.(10) Srivastava, A.; Prasad, R. Renewable & Sustainable Energy Rev.

2000, 4, 111–133.(11) Duffield, J. A. Inhalat. Toxicol. 2007, 19, 1029–1031.(12) Zhou, W. Y.; Boocock, D. G. B. J. Am. Oil Chem. Soc. 2006, 83,

1041–1045.

6610

Energy Fuels 2010, 24, 6609–6615 : DOI:10.1021/ef1011929 Terigar et al.

oil,13 canolaoil,14 rice branoil,15 sunfloweroil,16 pumpkinoil,17

coconut andpalmoil,18 castor oil,19waste cookingoil,20 animalfat,4 and algae oil.21 Nonedible oils such as rubber, jatropha,karanja, and tobacco, with high free fatty acids (FFA) contentsalso have been investigated as potential feedstocks for biodieselin an attempt to bring down production costs.22 Increasingly,investigators are studying the production of biodiesel usingcontrolled microwave heating for accelerating synthetic or-ganic reactions. Using amicrowave apparatus, it is possible toperformreactionsmore efficiently and rapidlywith reductionsin the quantity of byproduct formed and reduced energy con-sumption.23 Several microwave-assisted transesterificationmethods from different oil-based crops have been reported.23-28

Most of the literature available focuses on the use ofmethanolas the solvent. The current study focuses on investigating thefeasibility of using ethanol for the production of biodieselfrom soybean and rice bran oil with the assistance of micro-waves in a continuous reaction system, with identification ofreaction kinetics in such a system.

Published literature highlights the efficiency of microwavetreatment and the benefit of usingbase-catalyzed transesterifi-cation over acid-catalyst transesterification. A base-catalyzedtransesterification (with KOH or NaOH) process is usedcommercially because the reaction can take place at relativelylow temperatures (60 or 70 �C) andpressures (1 atm)withhighconversion yields (98%), and involves direct conversion withno intermediate steps. Also, alkali-based transesterificationoccurs at a faster rate than with acid catalysts.10

Although many publications focus on the use of sealedvessels usingmicrowaveheating,24,29,30 it is alsopossible touse

an open-vessel microwave system for transesterification. Useof an open-vessel system allows for a continuous reaction,making the process faster. Using this procedure, researchersachieveda98%conversion tomethyl esters biodiesel in 1min,31

showing the efficiency of using microwave heating for thetransesterification reaction.

The objectives of this studywere to investigate a continuoustransesterificationprocesswith ethanol assisted bymicrowaveheating of soybean and rice bran oils at different target tem-peratures (50and73 �C) ina fully instrumentedand controlledopen-vessel continuousmicrowave reactor at reaction times of1, 5, and 10minand todetermine thepertinent reactionkineticparameters for a base catalyst.

2. Experimental Section

2.1. Materials. Soybean oil (SB) was purchased from a localgrocery store (Walmart Inc., Bentonville, AR), and rice bran oil(RB) was purchased from Honest Foods (San Bruno, CA). Thesolvent, 200 proof, ACS/US grade ethanol, was purchased fromPharmco-AAPER (Shelbyville, KY). NaOH flakes, KOH, andHCl were procured from Fisher Scientific (Pittsburgh, PA).Standards of fatty acids ethyl esters (FAEE) were purchasedfrom Sigma Aldrich (St. Louis, MO).

2.2. Procedures.Before the reaction, 0.6%NaOH catalyst (byweight of oil) was dissolved in ethanol. This mixture and the oil,at a 5:1 stoichiometric ratio (oil/ethanol) were transferred into aclosed feed tank reactor and stirred vigorously for approxi-mately 15min. The microwave-induced transesterification reac-tions were conducted using a commercially available, fullyinstrumented, batch-type microwave system (ETHOS EMicro-wave Extraction System, Milestone Inc., Monroe, CT) whichwas modified for continuous operation (Figure 2). The setupincluded a 2-L cylindrical Teflon beaker (SPI Supplies, WestChester, PA) of o.d. 12 cm and height 21 cm, which acted as theprocessing reactor in the center of the microwave applicationchamber. Two Teflon tubes with inner diameters of 0.953 cmwere inserted through the top wall into the beaker to serve asinflow and outflow conduits. These tubes were connected toflexible PET tubing using quick disconnect fittings to facilitateeasy dismantling and cleaning. The reaction mixture waspumped into themicrowave chamber (into the reactor) at a con-stant flow rate of 100 mL/min using a peristaltic pump (model7518, Cole-Parmer Instrument Co., Barrington, IL) and waspumped out using another peristaltic pump operating at ahigher flow rate (150mL/min). The different inflow and outflowflow rates helped to ensure a continuous operation of the systemwhile maintaining a constant sample level in the reactor. Theconstant sample level in the reactor was maintained by placingthe outflow Teflon tube at specific positional heights in thereactor, thereby controlling the residence time (Figure 2). Thereaction mixture was heated to 50 and 73 �C under constantstirring using magnetic stirrers, with residence times of 1, 5, and10 min. Temperature was monitored with a fiber optic probeand automatically controlled by the microwave control system.After completion of the treatment, the biodiesel mixture wasquenched with 1 M HCl to stop the reaction, and the crudeglycerinwas separated gravitationally for 20 h (phase separationwas observed within 10 min), followed by water washing of thebiodiesel phase (6% v/v) in three steps until the ester layer wasclear. The samples were then centrifuged in a vacuum centrifugeevaporator (CentriVap Console Labconco, Kansas City, MO)running for 2-4 h to evaporate the excess ethanol.

2.3. Quality Analysis. The products were analyzed for cloudpoint (ASTM D 2500), flash point (ASTM D 93), kinematic

Figure 1. Transesterification of triglycerides with alcohol.

(13) Jeong, G. T.; Park, D. H. Appl. Biochem. Biotechnol. 2006, 131,668–679.(14) Kulkarni, M. G.; Dalai, A. K.; Bakhshi, N. N. J. Chem. Technol.

Biotechnol. 2006, 81, 1886–1893.(15) Einloft, S.; Magalhaes, T. O.; Donato, A.; Dullius, J.; Ligabue,

R. Energy Fuels 2008, 22, 671–674.(16) Georgogianni, K. G.; Kontominas, M. G.; Pomonis, P. J.;

Avlonitis, D.; Gergis, V. Fuel Process. Technol. 2008, 89, 503–509.(17) Schinas, P.; Karavalakis, G.; Davaris, C.; Anastopoulos, G.;

Karonis, D.; Zannikos, F.; Stournas, S.; Lois, E. Biomass Bioenergy2009, 33, 44–49.(18) Kansedo, J.; Lee, K. T.; Bhatia, S. Biomass Bioenergy 2009, 33,

271–276.(19) Da Silva, N. D.; Maciel,M. R.W.; Batistella, C. B.; Filho, R.M.

Appl. Biochem. Biotechnol. 2006, 130, 405–414.(20) Al-Widyan, M. I.; Tashtoush, G.; Abu-Qudais, M. Fuel Process.

Technol. 2002, 76, 91–103.(21) Chisti, Y. Biotechnol. Adv. 2007, 25, 294–306.(22) Patil, P. D.; Deng, S. G. Fuel 2009, 88, 1302–1306.(23) Hernando, J.; Leton, P.; Matia, M. P.; Novella, J. L.; Alvarez-

Builla, J. Fuel 2007, 86, 1641–1644.(24) Azcan, N.; Danisman, A. Fuel 2008, 87, 1781–1788.(25) Breccia, A.; Esposito, B.; Fratadocchi, G. B.; Fini, A.

J. Microwave Power Electromagn. Energy 1999, 34, 3–8.(26) Mazzocchia, C.; Modica, G.; Kaddouri, A.; Nannicini, R. C. R.

Chim. 2004, 7, 601–605.(27) Perin, G.; Alvaro, G.; Westphal, E.; Viana, L. H.; Jacob, R. G.;

Lenardao, E. J.; D’Oca, M. G. M. Fuel 2008, 87, 2838–2841.(28) Zu, Y. G.; Zhang, S.; Fu, Y. J.; Liu, W.; Liu, Z. G.; Luo, M.;

Efferth, T. Eur. Food Res. Technol. 2009, 229, 43–49.(29) Leadbeater, N. E.; Barnard, T. M.; Stencel, L. M. Energy Fuels

2008, 22, 2005–2008.

(30) Zhang, S.; Zu,Y.G.; Fu,Y. J.; Luo,M.; Zhang,D.Y.; Efferth, T.Bioresour. Technol. 2010, 101, 931–936.

(31) Barnard, T. M.; Leadbeater, N. E.; Boucher, M. B.; Stencel,L. M.; Wilhite, B. A. Energy Fuels 2007, 21, 1777–1781.

6611

Energy Fuels 2010, 24, 6609–6615 : DOI:10.1021/ef1011929 Terigar et al.

viscosity at 40 �C (ASTMD 445), acid number (ASTMD 974),oxidation stability index (OSI) (EN 14112), free and totalglycerin (ASTMD 6584), and free fatty acid composition, usingtheir respective ASTM and EN standard procedures. The fattyacid ethyl ester compositions were determined by gas chroma-tography (Varian 450-GC) coupled with a Varian 240-MS iontrap mass spectrometer (Varian Inc. USA). A Varian FactorFour capillary columnWAXms (30 m� 0.25 mm i.d., 0.25 μm)was used at 255 �Cwith helium at 1 mL/min as the carrier gas, asplit injector at 270 �C with a split ratio of 1:20, a detectortemperature of 270 �C, and with a total running time of 22 min.Components were identified using a standard fatty acid ethylester (FAEE) mix (Supelco, Bellefonte, PA).

2.4. Statistical Analysis. The experiments were performed intriplicate and the mean conversion yields for each experimentalcondition along with their standard deviations were computed.A two-way ANOVA using Proc Mixed 2 � 5 factorial (version9.1, SAS Institute, Inc., Cary, NC) was used to test significantdifferences in conversion yields among temperature (50 and73 �C) and reaction time (1, 5, and 10 min). Two-way ANOVAusing Proc Multiple comparison tests were performed by usingTukey adjustment to determine the significant differencebetween treatments. All significant values were expressed asp e 0.05.

2.5. Reaction Kinetics.Reaction rate constants and activationenergies for the time and temperature tested were calculatedassuming that the reaction proceeds according to a first-orderreaction with the final conversion of 100% (eq 1).32 The reactionrate constant was determined by using the Arrhenius relation-ship (eq 2).32

CðtÞ ¼ Cf 3 ð1- e- k 3 tÞ ð1Þk ¼ A 3 e

-Ea=RT ð2ÞWhere C(t) = concentration of product at time t (%); Cf =

final concentration assumed to be 100% (complete conversion);k=reaction rate constant (s-1); t= time (s); A=pre-expo-

nential/frequency factor, describing themolecularmotion of thereactants (s-1);Ea=activation energy (J/mol);R=gas constant(8.3144 J/mol K); and T = temperature (K).

The reaction rate constant k was determined in SigmaPlot10.0 (Systat Software, San Jose, CA) by curve fitting eq 1 to thedata for both types of oils and the two temperature investigated.Activation energy Ea and pre-exponential factor A were deter-mined by solving eq 2 analytically for the two temperatures andreaction rate constants at each temperature investigated.

3. Results and Discussion

3.1. Transesterification Reaction Conversion Rate. Thetransesterification conversion rates increasedwith increasingtemperature and time of exposure. No significant differencesamong conversion rates for SB biodiesel were found amongdifferent times at the same temperature. Significant differ-ences were observed between the conversion rates obtainedat 50 and 73 �C at each reaction time (Table 1). The highestconversion rate was obtained at 73 �C and 10 min treatment(99.25%), whereas the lowest conversion was at 50 �C for 1minreaction (96.67%). For RB biodiesel, the only significant differ-ence was found between the lowest and the highest temperatureand time treatment (Table 1). Conversions as high as 99.34%were achieved, while the lowest conversion rate was 98.35%.

Similar results were found in the literature33 where a studyon biodiesel conversion using soybean oil showed a 98%conversion rate after 5 min when using microwave-assistedreaction with methanol at a 1:6 oil to solvent ratio and usingNaOH as a catalyst. In contrast, using conventional heatingmethods at a reaction temperature of 65 �C for the transes-terification of rice bran oil usingmethanol (9:1) as the solventand NaOH (at 0.75%) as catalyst, investigators reported34

Figure 2. Schematic representation of the laboratory-scale continuous-microwave transesterification system setup.

Table 1. Free Glycerin, Total Glycerin, and Conversion Rates for SB and RB Biodiesel at Studied Temperatures and Reaction Times

temperature time (min)

SB biodiesel RB biodiesel conversion (%)

free glycerin % mass total glycerin % mass free glycerin % mass total glycerin % mass SB RB

50 �C 1 0.03a,b( 0.01 0.23a( 0.01 0.02b( 0.01 0.21a( 0.01 96.67a 98.35a,b

5 0.02b( 0.01 0.19a,b( 0.02 0.01b( 0.01 0.19a,b( 0.01 97.46a 98.59b

10 0.01a,b( 0.01 0.18a,b( 0.01 0.02b( 0.01 0.18a,b( 0.02 97.79a 98.88b

73 �C 1 0.02b( 0.01 0.17a,b( 0.02 0.01b( 0.01 0.17a,b( 0.02 98.87a,b 99.18b

5 0.02b( 0.01 0.17a,b( 0.01 0.01b( 0.01 0.16a,b( 0.01 98.89a,b 99.22b

10 0.01a,b( 0.00 0.16a( 0.01 0.01b( 0.00 0.16a,b( 0.01 99.25a,b 99.34a,b

ASTM D6751 max 0.03 max 0.25 max 0.03 max0.25

aNumbers from the same column, significantly different from each other. bNumbers from same the column, not significantly different from eachother.

(32) Fogler, H. Elements of Chemical Reaction Engineering, 3rd ed.;Prentice Hall: Upper Saddle River, NJ, 1999.

(33) Leadbeater, N. E.; Stencel, L. M. Energy Fuels 2006, 20, 2281–2283.

(34) Sinha, S.; Agarwal, A. K.; Garg, S. Energy Convers. Manage.2008, 49, 1248–1257.

6612

Energy Fuels 2010, 24, 6609–6615 : DOI:10.1021/ef1011929 Terigar et al.

that about 80% conversion of triglycerides to methyl estersoccured during the first 5 min while it took more than anhour for 93-98% conversion. In another study on the tran-sesterification of soybean oil (using CaO as catalyst) andcanola oil (using KOH as a catalyst) at 65 �C with methanol(12:1molar ratio) as the solvent, it took about 1.5 h to obtain86%conversionwhile usinghotwater as theheatingmedium.35

Microwave heating is a rapid process and the current exper-iments indicate high conversion rates at times as short as1min. The efficiencyofmicrowave-assisted transesterificationstems from the unique dielectric properties of themixtures ofpolar and ionic components of vegetable oil, solvent, andcatalyst. Rapid and efficient heating as a result of rapidlychanging electric fields is observed upon microwave irradia-tion because microwaves interact with the sample on amolecular level. The changing electric fields induce rotationin polar molecules, resulting in efficient intermolecular mix-ing and agitation, which increases the chances of an alcoholmolecule to encounter an oil molecule and a catalyst. As aresult of this mixing, a more conducive atmosphere forgreater interaction among molecules is formed, resulting inmore rapid biodiesel formation. Microwaves accelerate thetransesterification reaction process, resulting in higher prod-uct yield in lesser time. These results are encouraging,indicating a significant reduction in process time withmicro-wave-assisted heating.

3.2. Total and Free Glycerin. Glycerin is a byproduct ofbiodiesel production, and if not removed from the finalproduct can result in fuel separation, material incompatibil-ity, engine deposits, and engine durability concerns. For anideal transesterification reaction, the amount of total resid-ual glycerin should not exceed 0.25% w/w (ASTM D6751).In this study, total glycerin content in SB and RB biodieselwas below the ASTM stipulated level for all combinations

tested. The maximum total glycerin values obtained for theSB and RB biodiesel were 0.23% and 0.21%, respectively,registered at the lowest process temperature (50 �C) andreaction time (1 min). The higher total glycerin concentra-tion observed could be due to the low conversion of oil intoethyl esters which is observed in the lower conversion rate inTable 1 at 50 �C and 1 min. No significant differences werefound in total glycerin concentration as a function of reac-tion time at 73 �C, the lowest value being 0.16% at 10 minreaction time. For RB biodiesel, significant differences werefound between the glycerin contents present in the biodieselprocessed at 1 and 5 min reaction times at 50 �C andcompared with the biodiesel obtained at a reaction time of10 min processed at 73 �C. No significant differences werefound among reaction times at 73 �C.

Total and free glycerin contents were found to be depen-dent on the reaction temperature and exposure time tomicrowave irradiation (Table 1). For SB derived biodiesel,significant differences in free glycerin values were foundbetween the results at 50 �C and 1 min (0.03%) and thoseat 10 min regardless of reaction temperature (0.01%). Theother values for free glycerin did not significantly changewithin the process parameters. For RB biodiesel, the freeglycerin concentration did not show any significant changesbetween treatments.

3.3. Fatty Acid Ethyl Esters (FAEE) Composition. For SBno significant changes were detected between the reactions at50 and 73 �C (Table 2). Additionally, reaction time did notinfluence the FAEE concentration. For RB, no significantchanges were found in FAEE among the parameters studied(Table 3). The biodiesels derived from both oils containedmore than 82% total unsaturated FAEE content, with ahigher percentage of polyunsaturated fatty acids thanmono-unsaturated fatty acids. Also, the biodiesel contained moreshort-chain fatty acids (less than C20) than long chain fattyacids (C20-C22). This is typical with any commerciallyavailable biodiesel derived from vegetable oil,36 indicating

Table 2. FAEE of SB Biodiesel at Studied Temperatures and Reaction Times

50 �C 73 �C

1 min 5 min 10 min 1 min 5 min 10 min

myristic (14:0) 0.04 ( 0.03 0.02( 0.01 0.07( 0.02 0.05( 0.01 0.06( 0.01 0.05( 0.01palmitic (16:0) 8.39( 0.19 7.32( 0.01 8.19( 0.57 8.03( 0.02 8.42 ( 0.24 8.45( 0.37stearic (18:0) 3.51( 0.44 3.65 ( 0.20 3.59( 0.15 3.55( 0.18 3.76( 0.09 3.43( 0.01oleic (18:1) 25.08( 0.45 22.99( 1.18 23.64( 2.54 25.29( 0.13 25.26( 0.24 25.42 ( 0.07linoleic (18:2) 55.70( 0.05 57.64( 1.19 56.79 ( 2.31 55.42( 0.33 55.41( 0.36 55.09( 0.36linolenic (18:3) 6.89( 0.16 8.00( 0.11 7.21( 0.13 7.36( 0.30 6.59( 0.26 7.33( 0.05eicosanoic (20:0) 0.03( 0.03 0.05( 0.01 0.04( 0.06 0.02 ( 0.02 0.07( 0.03 0.02( 0.01eicosenoic (20:1) 0.14 ( 0.10 0.15( 0.04 0.27( 0.09 0.03( 0.01 0.17( 0.23 0.02( 0.03docosanoic (22:0) 0.20( 0.11 0.13( 0.04 0.16( 0.04 0.20( 0.02 0.20 ( 0.02 0.16( 0.01docosenoic (22:1) 0.06( 0.02 0.04 ( 0.02 0.04( 0.01 0.05( 0.02 0.06( 0.01 0.04( 0.01

Table 3. FAEE of RB Biodiesel at Studied Temperatures and Reaction Times

50 �C 73 �C

1 min 5 min 10 min 1 min 5 min 10 min

myristic (14:0) 0.23( 0.01 0.22( 0.03 0.23 ( 0.02 0.24 ( 0.01 0.20 ( 0.02 0.22 ( 0.02palmitic (16:0) 14.53( 0.28 16.09( 1.51 13.79 ( 0.81 14.26 ( 0.10 14.05 ( 0.45 14.55 ( 0.20stearic (18:0) 2.03( 0.23 1.94 ( 0.23 1.98 ( 0.15 1.98 ( 0.30 1.82 ( 0.04 1.93 ( 0.07oleic (18:1) 40.82( 0.53 40.25( 0.99 42.41 ( 0.25 41.39 ( 1.34 42.87 ( 0.17 42.05 ( 0.04linoleic (18:2) 40.26( 0.45 39.76( 0.01 39.42 ( 1.11 40.02 ( 0.28 39.11 ( 0.20 39.35 ( 0.37linolenic (18:3) 1.62( 0.06 1.42( 0.13 1.61 ( 0.05 1.57 ( 0.14 1.42 ( 0.06 1.43 ( 0.08eicosanoic (20:0) 0.07( 0.01 0.05( 0.01 0.05 ( 0.01 0.07 ( 0.01 0.04 ( 0.01 0.09 ( 0.03eicosenoic (20:1) 0.16 ( 0.08 0.04( 0.06 0.21 ( 0.07 0.23 ( 0.02 0.24 ( 0.08 0.11 ( 0.09docosanoic (22:0) 0.16( 0.02 0.17( 0.01 0.18 ( 0.02 0.13 ( 0.01 0.15 ( 0.04 0.16 ( 0.02docosenoic (22:1) 0.12( 0.01 0.06 ( 0.09 0.13 ( 0.02 0.11 ( 0.01 0.10 ( 0.01 0.10 ( 0.02

(35) Liu, X. J.; He, H. Y.; Wang, Y. J.; Zhu, S. L.; Piao, X. L. Fuel2008, 87, 216–221.

6613

Energy Fuels 2010, 24, 6609–6615 : DOI:10.1021/ef1011929 Terigar et al.

the suitability of using this method to obtain high qualityFAEE.

3.4. Cloud Point. The cloud point of biodiesel is tempera-ture and time dependent (Figure 3). Acceptable values forcloud point should not be higher than 10 �C, according toASTM D 2500; the cloud points of all samples tested wereconsidered acceptable. For both SB andRBbiodiesels, cloudpoint values decreased with increasing reaction temperatureand time of reaction. Significant differences in the valueswere found for SB biodiesel between the two temperatures atall exposure times. However, significant differences were notdetected at 73 �C among the exposure times studied. For RBbiodiesel, significant differences were observed between thetwo temperatures (50 and 73 �C) and between reaction timeswith higher values observed at lower temperatures and shortertimes.

These results are in contrast to the results observed duringthe transesterification of rapeseed oil using conventionalheating methods and using methanol as the solvent with dif-ferent catalysts, reaction temperatures, and catalyst concen-trations.37 For the same catalyst (KOH), catalyst concen-tration (1%), and methanol/oil molar ratio (6:1), varyingreaction temperatures (35, 50, and 65 �C) did not produceany difference in the cloud point of the derived rapeseedmethyl ester (-3 �C). However, varying the catalyst ormolarratio or catalyst concentration did produce changes in thecloud point of the derived rapeseed methyl ester.

3.5. Flash Point. The flash point temperatures, the mini-mum values at which fuels will ignite (flash) under specifiedconditions, are shown in Figure 4. A higher value indicateslower volatility and, therefore, better safety in storage. Forboth biodiesel feedstocks, this parameter was influenced byreaction temperature and time. Higher flash point valueswere observed at 73 �C, and increased with increasing re-action time. For SB biodiesel, the flash point measured at50 �Cwas significantly lower than the one measured at 73 �Cat all reaction times studied. Also, shorter reaction times(1 and 5min) gave significantly lower flash point values thanthose obtained at 10 min of exposure. Similar trends wereobserved for RB biodiesel. All values for this parameter metASTM standard specifications (minimum 130 �C), even atthe lowest temperature and shortest times.

A higher flash point is given by increasing the number oflonger chains due to more ethanol groups substituting a CHgroup in the reaction. This also gives good informationabout the reaction conversion. Less volatiles are producedat higher reaction temperatures and times, indicating thatbetter conversion rates are given at these points, not only bythe longer chains but also by stronger bonds present in thestructure of the biodiesel. Differences in the flash points ofrice bran and soybean biodiesel can be explained by thedifferent fatty acid compositions of these oils, especially thehigher oleic acid content of rice bran oil which requires moreenergy to replace the CH group.

3.6.Viscosity.Kinematic viscosityaffects injector lubricationand fuel atomization. Biodiesel fuels generally have improvedlubricity.38 However, their higher viscosity tends to form largerdroplets on injection, which can cause poor combustion andincreased exhaust smoke. In our study, the biodiesel from thetwo different sources did not exceed viscosity specifications,although the ethyl esters formed at 50 �C were at the limit ofthe values specified in the ASTM standards (1.9-6.0 cSt).The viscosity decreased significantly with higher reactiontemperature and time (Figure 5). Higher viscosity of biodie-sel is usually due to the possible presence of unreacted fattyacid chains or glycerol after downstream processing (lowertemperatures yielded more glycerol, Table 1). Also, viscositytends to increase with chain length and with increasing degreeof saturation.39

With conventional heating methods (9:1 oil/alcohol ratio,0.75% NaOH, 1 h), it was reported that the viscosity of ricebran oil methyl esters was not influenced by reaction temper-atures between 50 and 65 �C.34 In the case of using ethanol,the viscosity can be influenced by reaction temperature andtime due to the higher number of carbon atoms in the ethylester chain. The longer ethyl ester chain results in longermolecular relaxation time which is proportional to the viscos-ity.40,41 At the same time, viscosity as a physical property istemperature dependent, decreasing with increasing reactiontemperature and time, especially inmicrowave-assisted reactions

Figure 3. Cloud point results for soybean and rice bran biodiesel atdifferent temperature and reaction times (error bars representstandard deviation values).

Figure 4. Flash point results for soybean and rice bran biodiesel atdifferent temperature and reaction times (error bars representstandard deviation values).

(36) Lin, C. Y.; Li, R. J. Fuel Process. Technol. 2009, 90, 130–136.(37) Rashid, U.; Anwar, F. Fuel 2008, 87, 265–273.

(38) Demirbas, A. Energy Pol. 2007, 35, 4661–4670.(39) Falk, O.;Meyer-Pittroff, R.Eur. J. Lipid Sci. Technol. 2004, 106,

837–843.(40) Suzuki, A.; Masuko, M.; Wakisaka, K. Tribology Int. 2002, 35

(1), 55–63.(41) Vasan, S. T.; Sannaninganavar, F.M.; Ayachit, N. H.; Deshpande,

D. K. J. Mol. Liq. 2007, 135 (1-3), 115–119.

6614

Energy Fuels 2010, 24, 6609–6615 : DOI:10.1021/ef1011929 Terigar et al.

where volumetric heating is higher due to continuous ex-posure to molecular mixing.

3.7. Acid Number. Acid values for all samples tested werewithin ASTM standard specifications (maximum of 0.7 mgKOH/g), and can be found in Figure 6. Acid values for SBand RB biodiesel generally decreased with increasing reac-tion temperature and time. This decrease was significant forSB at 50 �C, but not at 73 �C. No significant difference wasfound between acid values at the two temperatures at 10 minof exposure time for SB and RB biodiesel. For RB biodiesel,the only significant differencewas found at 50 �Cbetween the1 and 5 min exposure times.

Acid number, which measures the acids in the fuel, couldemanate from two sources: (i) catalysts utilized in theproduction of the biodiesel that are not completely removedin the production process, and (ii) degradation of oil byoxidation. Since the samples were analyzed immediatelyafter washing, it is unlikely that the samples had time tooxidize. Therefore, the higher acid values were probably theconsequence of some small quantities of unreacted catalyst,or some leftover acid from the quenching step.

3.8. Oxidative Stability Index (OSI). The OSI results areshowed in Figure 7. Although lower stability indexes werenoticed at lower temperatures and reaction times, the valuespresented in Figure 7 are within ASTM specifications

(minimum 3 h). Temperature influences the oxidation rate,with significantly higher stability being registered at highertemperatures for both biodiesel sources. For RB biodiesel, alinear increase with exposure time was observed at bothtemperatures, with significant differences between the val-ues. For SB biodiesel, the only significant difference wasobserved between 1 and 5 min of reaction time. No signifi-cant differences were found between 5 and 10 min reactiontimes, for both temperatures used for SB transesterificationreaction. At reaction higher temperatures, the conversion ofoil into biodiesel occurred at a faster rate, producing moreFAEEs from the oils. The addition of more ethyl groups tothe oil chain at higher reaction temperature resulted in higheroxidative stability of the derived biodiesel, as evident inFigure 7. Besides the presence of air, various other factorsinfluence the oxidation process of biodiesel including thepresence of light, elevated temperatures, the presence ofsome extraneous materials, peroxides and the fatty acidprofile, which are generally the consequence of inadequatestorage.

3.9. Reaction Kinetics. The values for the reaction ratekinetic parameters (k,Ea,A) determined from eqs 1 and 2 arepresented in Table 4. A higher value for A indicates a higherreaction rate, as its value is largely associated with thefrequency of the vibrating molecules present.42 Microwavesinduce an increase in the molecular vibrations due to theoscillatory electric field, resulting in an increased value ofA and ultimately of the reaction rate.42

The activation energy of soybean oil is higher than that ofthe rice bran oil, an effect observed particularly well at 50 �C(rice bran oil had a significantly higher conversion rate). Athigher temperature (73 �C) the conversions are almost the

Figure 5. Kinematic viscosity results for soybean and rice branbiodiesel at different temperature and reaction times (error barsrepresent standard deviation values).

Figure 6. Acid number for soybean and rice bran biodiesel atdifferent temperature and reaction times (error bars representstandard deviation values).

Figure 7. Oxidative stability indexes for soybean and rice branbiodiesel at different temperature and reaction times (error barsrepresent standard deviation values).

Table 4. FAEE of RB Biodiesel at Studied Temperatures and Reac-

tion Times

parameter soybean oil rice-bran oil

50 �C 73 �C 50 �C 73 �C

k (s-1) 0.0567 0.0747 0.0684 0.0800Ea (J/mol) 11,147 6,334A (s-1) 3.5932 0.7228

(42) Lidstrom, P.; Tierney, J.; Wathey, B.; Westman, J. Tetrahedron2001, 57 (45), 9225-9283.

6615

Energy Fuels 2010, 24, 6609–6615 : DOI:10.1021/ef1011929 Terigar et al.

same,with the activation energy playing less of a role as nearlycomplete conversion was observed for both the oils. It isproposed that in case of microwaves, rice bran oil has slightlyhigher reaction rates than soybean oil due to its higher ratio ofoleic acid (with one double bond) to linoleic acid (with twodouble bonds). The presence of the single double bond bendsthe molecule only once in a single direction, whereas thedouble bonds in the linoleic acid (predominant in the soybeanoil) bend it twice, reducing the molecule’s gyration radius andsubsequently the chance of reaction. As such, the rice bran oilinteracts better with the microwave field. Regardless of thesesmall differences in reaction rates, at all parameters investi-gated and for both oils, the reaction rate was observed toincrease at the very beginning of the reaction (first minute).The tremendous increase in the reaction rate in the firstminuterepresents the major advantage of microwaves over the con-ventional heating methods, as when using conventional heat-ing, a very slow reaction rate is usually observed at thebeginning for a significant longer time (15 min) followed bya sudden increase in the rate of reaction.43,44

4. Conclusions

Continuous-flow microwave heating compares favorablywith conventional methods mentioned in literature. The

biodiesel produced in this study met ASTM requirements atall conditions tested. Conversion rates ranged from 96.7 to99.3% for biodiesel derived from soybeanoil and from98.4 to99.3% for that derived from rice bran oil.Microwave-assistedproduction of biodiesel was achieved with higher yields, alower alcohol/oil ratio (5:1molecular ratio), and shorter timescompared to conventional methods. Reaction rate constantsranged from 0.0567 to 0.0800 s-1, whereas activation energieswere 11147 and 6334 J/mol for soybean and rice bran ethylesters, respectively. Pre-exponential factors were 3.593 and0.723 s-1 for soybean and rice bran oil ethyl esters, respec-tively. Soybean has slightly lower conversion rate than ricebran oil at lower temperatures, but this difference becomesnegligible at higher temperatures. Future studies will focus onscaling-up the microwave-assisted biodiesel production pro-cess and the potential economic implications of introducingthe process in the industry.

Acknowledgment. We acknowledge the LSU AgCenter andthe Biological andAgricultural EngineeringDepartment for theirsupport of this project. Funding for this researchwas provided byLouisiana Board of Regents Industrial Ties Research Subpro-gram (LABOR-ITRS Award LEQSP (2007-10)-RD-B-01), U.S.Department of Transportation SunGrant Initiative (Award AB-5-61770.LSU1), and the Louisiana Soybean and Grain Researchand Promotion Board. Assistance was provided by Javed Iqbal(sample analysis), Dr. Claudia Leonardi (statistical analysis) andPranjaliMuley (reaction kinetics). Publishedwith the approval ofthe Director of Louisiana Agricultural Experiment Station asManuscript 2010-232-5130.

(43) Noureddini, H.; Zhu, D. J. Am. Oil Chem. Soc. 1997, 71 (11),1457-1463.(44) Freedman, B.; Butterfield, R. O. J. Am. Oil Chem. Soc. 1986, 63

(10), 1375–1380.