Fatty acids methyl esters (FAME) have been prepared under microwave irradiation, using homogeneous catalysis, either in batch or in a flow system. The quality of the biodiesel obtained has been confirmed by GC analysis of the isolated product. While the initial experiments have been performed in a small scale laboratory batch reactor, the best experiment has been straightforward converted into a stop-flow process, by the use of a microwave flow system. Compared with conventional heating methods, the process using microwaves irradiation proved to be a faster method for alcoholysis of triglycerides with methanol, leading to high yields of FAME

Hernando, J.

Letón García, Pedro

Matía Martín, Ma de la Paz

Novella Robisco, José Luis

Álvarez-Builla Gómez, Julio

Matía Martín, María de la Paz

e_Buah - Biblioteca Digital de la Universidad de Alcalá

www.fuelfirst.comFuel 86 (2007) 1641–1644Short CommunicationBiodiesel and FAME synthesis assisted by microwaves:Homogeneous batch and flow processesJ. Hernando, P. Leton, M.P. Matia, J.L. Novella, J. Alvarez-Builla *Planta Piloto de Quı́mica Fina, Universidad de Alcala, 28871 Alcala de Henares, Madrid, SpainReceived 8 September 2006; received in revised form 31 October 2006; accepted 2 November 2006Available online 30 November 2006AbstractFatty acids methyl esters (FAME) have been prepared under microwave irradiation, using homogeneous catalysis, either in batch orin a flow system. The quality of the biodiesel obtained has been confirmed by GC analysis of the isolated product. While the initial exper-iments have been performed in a small scale laboratory batch reactor, the best experiment has been straightforward converted into astop-flow process, by the use of a microwave flow system. Compared with conventional heating methods, the process using microwavesirradiation proved to be a faster method for alcoholysis of triglycerides with methanol, leading to high yields of FAME. 2006 Elsevier Ltd. All rights reserved.Keywords: Biodiesel; FAME; Microwaves1. IntroductionThe transesterification of natural triglycerides (e.g. oilsand fats) is employed to obtain fatty acid methyl esters(FAME) which are key reagents in the chemical industry[1–4]. The FAME are the raw materials for the productionof long chain carboxylic acids, detergents, alternative fuelsfor diesel engines (biodiesel) and mono and diglycerides,employed as additives for foods, cosmetics and pharmaceu-ticals [5]. Biodiesels are biodegradable and non-toxic andhave lower CO and hydrocarbon emissions than petro-leum-based diesel when burned [6,7]. They present, how-ever, other technical challenges such as low cloud pointsand elevated NOx emissions [8].The most commonly used technology for triglyceridestransesterification is based on the use of batch processes,in which a basic homogeneous catalyst is used (NaOH orNaOMe) and, at the end of the reaction, the catalyst is neu-tralised [9,10, as recent examples]. A continuous transeste-0016-2361/$ - see front matter  2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.fuel.2006.11.003* Corresponding author.E-mail address: julio.alvarez@uah.es (J. Alvarez-Builla).rification process would be a good opportunity to reduceproduction costs [11].Several processes are currently employed for homoge-neous catalytic transesterification, although these processessuffer from the presence of by-products that reduce selec-tivity and increase the separation time between products,while recently heterogeneous catalysts have been claimedto produce cleaner processes. In any of those situations,when the reaction is carried out under microwaves, transe-sterification is efficiently activated, with short reactiontimes, and as a result, a drastic reduction in the quantityof by-products and a short separation time is obtained,and all with a reduced energy consumption.Several examples of microwave irradiated transesterifi-cation methods have been published in the literature, byLoupy [1] and Mazzocchia [12], but they have made useof batch laboratory ovens, which do not allow to studythe process at a higher scale. Similar results have beendescribed by adapting domestic ovens to use them as flowsystems [13,14]. A recent paper from Leadbeater describesthe use of two batch laboratory microwave apparatus, onemonomode to perform micro experiments, the othermultimode able to irradiate flasks up to 5 L [15]. Some1642 J. Hernando et al. / Fuel 86 (2007) 1641–1644microwave flow prototypes have been described in recentpatents [16,17].2. Experimental method2.1. ReagentsTests of transesterification were performed either withcommercial rapeseed, or soybean oil from Aceites AlbertS.A. (Valencia, Spain). Methyl t-butyl ether (MTBE),sodium hydroxide (98%), phosphoric acid (85%) and meth-anol (98%) were purchased from Aldrich. An average valueof 884 was taken as a molecular weight of the oil.2.2. ApparatusAll batch microwave tests have been conducted with aCEM Explorer PLS, while the flow microwave experimentshave been performed with a CEM Voyager [14], both singlemode operating systems, working at 2.45 GHz, with apower programmable from 1 to 300 W. Stirring was per-formed at 400 rpm, with a magnetic nucleus, because theapparatus cannot modulate the speed, not to emulsifyand disperse the methanol in oil, but solely to make themixture temperature homogeneous. Tests were conductedwith the ratio methanol/oil of 9/1, 18/1 and 30/1.2.3. Experimental procedures and analysis2.3.1. Microwaves batch testsIn a standard CEM 10 ml pressurised reaction vial,NaOH (0.013–0.04 g, 0.325–1 mmol) was dissolved inMeOH (1.33–2.66 mL, 1.05–2.1 g, 32.77–2.655 mmol), then,the corresponding oil (2.50–3.26 mL, 2.30–3.0 g, 1.327–3.394 mmol) and methyltertbutyl ether (MTBE) (2.72 mL)were added. The vial was tapered with a cap and irradiatedat 60 C for 1–60 min, with stirring (400 rpm). After allowingthe mixture to cool to room temperature, H3PO4 (85%,0.0375–0.1553 g, 0.022–0.068 mmol) was added. The solidformed was filtered off and the organic solvents wereremoved by vacuum distillation. The residue was treated inan Eppendorf 5804R centrifuge, to remove the glycerollayer: Then, the sample was extracted with 1 mL of water,dried with Na2SO4, weighed and analysed.2.3.2. Conventional heating testsIn a 50 mL round-bottomed flask, with magnetic stir-ring, NaOH (0,13 g, 3.25 mmol) was dissolved in MeOH(5.52 ml, 4.37 g, 136.27 mmol), then, the correspondingoil (10.87 mL, 10 g, 11.31 mmol) and MTBE (11.8 mL)were added. The mixture was heated to 60 C for 1 min,with stirring (600 rpm). After allowing the mixture to coolto room temperature, H3PO4 (85%, 0.22 mL, 0.375 g,3.25 mmol) was added. The solid formed was filtered offand the organic solvents were removed by vacuum distilla-tion. The residue was treated in an Eppendorf 5804R cen-trifuge, to remove the glycerol layer: then, the sample wasextracted with 10 mL of water, dried with Na2SO4, weighedand analysed.2.3.3. Microwaves flow testsTwo stock solutions were prepared: one (A) containingoil (800 mL, 736 g, 0.832 mol) and MTBE (672 mL), theother (B) containing NaOH (9.6 g, 0.24 mol) dissolved inMeOH (404.5 mL, 320 g, 10 mol). The apparatus, workingwith a cell of 50 ml in stop-flow regime, runs sequentialseries of operations, repeating the optimal batch procedure.Initially, 34 mL of A (2.89 mL/s) and 9 mL of B (2.99 mL/s)were pumped into the flow cell; then, the cell was irradiated,with stirring (400 rpm), at 60 C for 1 min. The mixture wasallowed to cool for 3 min, and then, the sequence startedagain. The mixture was treated as indicated in the batchtests, to get an aliquot of it analysed.2.3.4. AnalysisThe products were analysed by a FID gas chromato-graph (Varian CP 3800) using the official standard methods[19]. The analysis of FAME 2 was made using a column cpmax 52 CB, 30 m long, with ID of 0.32 mm and 0.25 lmfilm. Temp. ramp starting with 75 C, then heating 16 C/min up to 140 C, and then 15 C/min up to 300 C. Theanalysis of glycerides 3, 4, 5 and glycerol 6 was made usinga column W COT fused silica 10 m long, ID of 0.32 mmand 0.10 lm film. Temperature ramp starting at 50 C,then heating 20 C/min up to 180 C, then heating 10 C/min up to 230 C and finally 15 C/min up to 370 C. Inboth methods, hydrogen was used as carrier at 80 KPa,and the injection volume was 1 lL.3. Results and discussion3.1. Batch testsThe overall transesterification process (Scheme 1) wasperformed as a set of operations as indicated in Scheme 2.To obtain a homogeneous system, the reagents – oil andmethanol – were mixed with MTBE as a safe solvent whichcan be easily recovered and reused. Then, the mixture wasirradiated with microwaves, acidified with phosphoric acid,and distilled to eliminate the volatiles – mainly methanoland MTBE – and then, the residue was centrifugated andwashed with water, to eliminate the glycerol, and theremaining biodiesel was dried and analysed.As it can be seen in Table 1, experiments 13 and 14,made either with rapeseed or soybean oil, respectively,where allowed to obtain biodiesel within standards, interms of the contents of contaminants 3, 4, 5 and glycerol 6.No substantial differences were obtained with the differentorigin of the oil. The best results were obtained when thecorresponding oil (2.3 g), NaOH, (30 mg), MeOH(1.27 mL), MTBE (2.1 mL) were placed in a sealed tubewith magnetic stirring. The mixture was heated for 1 minat 60 C. The temperature was fixed, so the irradiationReagents Microwaves CentrifugationDistillationAcidBiodieselVolatilesGCRe gents Micr aves Centrif tionDisti lationAcBio ieselVol ilesGBIODIESEL:OVERALL PROCESSScheme 2.Table 1Batch tests performed with microwaves to obtain biodieselEntry no. Mode Oil (g) NaOH (g) MeOH (mL) tc (min) Yield (%) Other components3 (%) 4 (%) 5 (%) 6 (%)1 MW 3.0a 0.018 0.83 1 17 0.50 1.81 35.53 0.042 MW 5 8 0.37 1.28 42.95 0.053 MW 3.0a 0.03 0.83 1 92 7.22 3.66 2.84 0.0534 MW 5 92 9.74 2.01 1.17 0.115 MW 3.0a 0.04 0.83 1 95 2.26 0.46 0.24 0.00056 MW 5 94 1.36 0.37 0.19 0.047 MW 3.0a 0.018 1.66 1 92 6.19 0.46 0.56 0.058 MW 5 92 1.92 0.01 0.02 0.069 MW 2.3a 0.03 1.27 1 95 0.95 0.03 0.0006 0.0310 MW 5 95 0.89 0.02 0.0003 0.0311 MW 2.3a 0.03 1.27 1d 94 0.91 0.02 0.004 0.03612 MW 5d 92 1.29 0.03 0.01 0.15413 MW 2.3d 0.03 1.27 1 97 0.48 0.03 0.012 0.00714 MW 2.3b 0.03 1.27 1 95 0.73 0.05 0.02 0.000815 MW 2.3b 0.03 1.27 1d 95 1.09 0.02 0.008 0.03616 CH 10a 0.13 5.52 5 84 2 0.60 0.30 0.2017 30 87 1.5 0.35 0.20 0.15EN 14214 <0.80 <0.20 <0.20 <0.02Notes: In the process, the corresponding oil is mixed with methanol and NaOH. MW, experiments performed with microwaves, 2.1 mL of MTBE wereadded and the mixture irradiated at 60 C. CH, experiments performed by conventional heating, 9.1 mL of MTBE were added and the mixture heated at60 C, for the time indicated (t). Yield and composition of the mixture, as obtained by GC.a Rapeseed oil.b Soybean oil.c Reaction time in minutes.d Power Max was used, to cool externally the reaction tube.OOOR3O OR1OR2CH3OH OHOHOHCH3OORNaOHOOOR5R4R6         FAME2 R = R1, R2, R3MTBEMW, 60˚ C1Glycerol 6       R4      R5       R63     H        H        COR34     H      COR2   COR35   COR1 COR2   COR3Scheme 1.J. Hernando et al. / Fuel 86 (2007) 1641–1644 1643energy was automatically controlled by the apparatus tokeep the system within the established temperature limits.Then, the mixture followed the process as indicated inScheme 2.Table 2Flow tests performed with microwaves to obtain biodieselEntry no. (no. cycles) Oil (g) NaOH (g) MeOH (mL) t (min) 2 Yield (%) Other components3 (%) 4 (%) 5 (%) 6 (%)18 (3) 18.4b 0.24 10.16 1 96 0.71 0.06 0.0010 0.00819 (17) 18.4b 0.24 10.16 1 97 0.60 0.03 0.019 0.016EN 14214 <0.80 <0.20 <0.20 <0.02Notes: In the process, every cycle was performed as follows: the corresponding oil was mixed with methanol and sodium hydroxide, in the quantitiesindicated in the table, and 16.8 mL of MTBE. Then, the mixture was pumped through the flow cell, with a useful volume of 50 mL, and heated for the timeindicated (t). Yield and composition of the biodiesel, as obtained by GC.a Rapeseed oil.b Soybean oil.c Time of irradiation at 60 C, in min.1644 J. Hernando et al. / Fuel 86 (2007) 1641–1644To compare microwave and conventional heating, twoexperiments (16–17) are included in Table 1, in which heat-ing to 60 C with an oil bath, with stirring (600 rpm) wasperformed, reproducing the conditions of the experiments13 and 14. The samples reached an equilibrium after30 min, (87% FAME) and no improvement in yield wasobtained after this period.3.2. Flow testsA CEM Voyager flow system [18] was used to show howthe optimal batch method could be scaled up to be con-verted into a production method. The volumes and quanti-ties of the experiment 14 in Table 1 were multiplied by 8, tobe reproduced in the flow cell of 80 mL – useful 50 mL –under a stop-flow regime. Homogeneous phase is the bestchoice to easily reproduce a method in the system. An ali-quot of the mixture was used to follow Scheme 2. The com-position of the biodiesel obtained was analysed as indicatedin experimental, according present standards [19]. As it canbe seen in Table 2, soybean oil was used to reproduce theexperiment 14 of Table 1. Two experiments were made,one with 3 cycles (exp. 18, 55 ml of oil treated) and theother with 17 cycles (exp. 19, 312,8 mL of oil treated). Inboth experiments, the biodiesel obtained was under thequality standards (Table 2).4. ConclusionAs a conclusion, microwave irradiation has revealed fas-ter and cleaner than conventional heating in the synthesisof FAME (biodiesel) from triglycerides, even in the pres-ence of homogeneous catalysts. The use of modern com-mercial devices allows to straightforward transfer of theprocess from batch to stop-flow. This opens the way todevelop microwave procedures for a big scale biodieselproduction.AcknowledgementAuthors acknowledge E. Ruiz and A.I. Cañadas fortechnical assistance in the experiments.References[1] Loupy A, Petit A, Ramdani M, Yvanaeff C, Majdoub M, Labiad B,et al. Can J Chem 1993;71:90.[2] Silver RS, Hasenhuettl GL. US Pat. 5,596,085.[3] Granbreg EP, Schafermeyer RG, Letton JA. PCT/US Pat. 96/07799.[4] Ahn E, Koncar M, Mittelbach M, Marr R. Separ Sci Tech1982;30:2021.[5] Vicente G, Martinez M, Aracil J. Bioresour Technol 2004;92:297 andliterature cited therein.[6] Boehman AL. Fuel Process Technol 2005;86:1057.[7] Knothe G, Sharp CA, Ryan TV. Energ Fuel 2006;20:403.[8] Szybist JP, Kirby SR, Boheman AL. Energ Fuel 2005;19:1484.[9] Vicente G, Martinez M, Aracil J. J Am Oil Chem Soc 2005;86:1057.[10] Eerée T, Peter S, Weidner E. Ind Eng Chem Res 2005;44:9535.[11] Serio MD, Ledda M, Cozzolino M, Minutillo G, Tesser R,Santacesaria E. Ind Eng Chem Res 2006;45:3009.[12] Mazzocchia C, Modica G, Nannicini R, Kaddouri A. CR Chimie2004;7:601.[13] Saiffudin N, Chua KH. Malaysian J Chem 2004;6:77.[14] Silva FC, Ferreira VF, Souza MCBV. Quim Nova 2006;29:376.[15] Leadbeater NE, Stencel LM. Energ Fuel 2006;20:2281.[16] Breccia FA WO2003014272 A3. Chem Abs 138:172236.[17] Portnoff MA, Purta DA, Nasta MA, Zhang J, Pourarian F. US 2005/0274065 A1. Chem Abs 144:24718.[18] Microwave CEM Explorer (batch) and CEM Voyager (flow) reactorswere used in the project. The temperature was fixed at 60 C, so theirradiation energy was limited by the temperature control. The flowsystem works in stop-flow regime, thus facilitating the reproductionof the optimal batch conditions in the flow system.[19] Norms EN14214 and EN14103 describe biodiesel requirements andofficial tests.

Fuel

English

Biodiesel and FAME synthesis assisted by microwaves: homogeneous batch and flow processes

Biblioteca Digital de la Universidad de Alcalá

www.fuelﬁrst.com
Fuel 86 (2007) 1641–1644Short Communication
Biodiesel and FAME synthesis assisted by microwaves:
Homogeneous batch and ﬂow processes
J. Hernando, P. Leton, M.P. Matia, J.L. Novella, J. Alvarez-Builla *
Planta Piloto de Quı´mica Fina, Universidad de Alcala, 28871 Alcala de Henares, Madrid, Spain
Received 8 September 2006; received in revised form 31 October 2006; accepted 2 November 2006
Available online 30 November 2006Abstract
Fatty acids methyl esters (FAME) have been prepared under microwave irradiation, using homogeneous catalysis, either in batch or
in a ﬂow system. The quality of the biodiesel obtained has been conﬁrmed by GC analysis of the isolated product. While the initial exper-
iments have been performed in a small scale laboratory batch reactor, the best experiment has been straightforward converted into a
stop-ﬂow process, by the use of a microwave ﬂow system. Compared with conventional heating methods, the process using microwaves
irradiation proved to be a faster method for alcoholysis of triglycerides with methanol, leading to high yields of FAME.
 2006 Elsevier Ltd. All rights reserved.
Keywords: Biodiesel; FAME; Microwaves1. Introduction
The transesteriﬁcation of natural triglycerides (e.g. oils
and fats) is employed to obtain fatty acid methyl esters
(FAME) which are key reagents in the chemical industry
[1–4]. The FAME are the raw materials for the production
of long chain carboxylic acids, detergents, alternative fuels
for diesel engines (biodiesel) and mono and diglycerides,
employed as additives for foods, cosmetics and pharmaceu-
ticals [5]. Biodiesels are biodegradable and non-toxic and
have lower CO and hydrocarbon emissions than petro-
leum-based diesel when burned [6,7]. They present, how-
ever, other technical challenges such as low cloud points
and elevated NOx emissions [8].
The most commonly used technology for triglycerides
transesteriﬁcation is based on the use of batch processes,
in which a basic homogeneous catalyst is used (NaOH or
NaOMe) and, at the end of the reaction, the catalyst is neu-
tralised [9,10, as recent examples]. A continuous transeste-0016-2361/$ - see front matter  2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.fuel.2006.11.003
* Corresponding author.
E-mail address: julio.alvarez@uah.es (J. Alvarez-Builla).riﬁcation process would be a good opportunity to reduce
production costs [11].
Several processes are currently employed for homoge-
neous catalytic transesteriﬁcation, although these processes
suﬀer from the presence of by-products that reduce selec-
tivity and increase the separation time between products,
while recently heterogeneous catalysts have been claimed
to produce cleaner processes. In any of those situations,
when the reaction is carried out under microwaves, transe-
steriﬁcation is eﬃciently activated, with short reaction
times, and as a result, a drastic reduction in the quantity
of by-products and a short separation time is obtained,
and all with a reduced energy consumption.
Several examples of microwave irradiated transesteriﬁ-
cation methods have been published in the literature, by
Loupy [1] and Mazzocchia [12], but they have made use
of batch laboratory ovens, which do not allow to study
the process at a higher scale. Similar results have been
described by adapting domestic ovens to use them as ﬂow
systems [13,14]. A recent paper from Leadbeater describes
the use of two batch laboratory microwave apparatus, one
monomode to perform micro experiments, the other
multimode able to irradiate ﬂasks up to 5 L [15]. Some
1642 J. Hernando et al. / Fuel 86 (2007) 1641–1644microwave ﬂow prototypes have been described in recent
patents [16,17].
2. Experimental method
2.1. Reagents
Tests of transesteriﬁcation were performed either with
commercial rapeseed, or soybean oil from Aceites Albert
S.A. (Valencia, Spain). Methyl t-butyl ether (MTBE),
sodium hydroxide (98%), phosphoric acid (85%) and meth-
anol (98%) were purchased from Aldrich. An average value
of 884 was taken as a molecular weight of the oil.
2.2. Apparatus
All batch microwave tests have been conducted with a
CEM Explorer PLS, while the ﬂow microwave experiments
have been performed with a CEM Voyager [14], both single
mode operating systems, working at 2.45 GHz, with a
power programmable from 1 to 300 W. Stirring was per-
formed at 400 rpm, with a magnetic nucleus, because the
apparatus cannot modulate the speed, not to emulsify
and disperse the methanol in oil, but solely to make the
mixture temperature homogeneous. Tests were conducted
with the ratio methanol/oil of 9/1, 18/1 and 30/1.
2.3. Experimental procedures and analysis
2.3.1. Microwaves batch tests
In a standard CEM 10 ml pressurised reaction vial,
NaOH (0.013–0.04 g, 0.325–1 mmol) was dissolved in
MeOH (1.33–2.66 mL, 1.05–2.1 g, 32.77–2.655 mmol), then,
the corresponding oil (2.50–3.26 mL, 2.30–3.0 g, 1.327–
3.394 mmol) and methyltertbutyl ether (MTBE) (2.72 mL)
were added. The vial was tapered with a cap and irradiated
at 60 Cfor 1–60 min,with stirring (400 rpm).After allowing
the mixture to cool to room temperature, H3PO4 (85%,
0.0375–0.1553 g, 0.022–0.068 mmol) was added. The solid
formed was ﬁltered oﬀ and the organic solvents were
removed by vacuum distillation. The residue was treated in
an Eppendorf 5804R centrifuge, to remove the glycerol
layer: Then, the sample was extracted with 1 mL of water,
dried with Na2SO4, weighed and analysed.
2.3.2. Conventional heating tests
In a 50 mL round-bottomed ﬂask, with magnetic stir-
ring, NaOH (0,13 g, 3.25 mmol) was dissolved in MeOH
(5.52 ml, 4.37 g, 136.27 mmol), then, the corresponding
oil (10.87 mL, 10 g, 11.31 mmol) and MTBE (11.8 mL)
were added. The mixture was heated to 60 C for 1 min,
with stirring (600 rpm). After allowing the mixture to cool
to room temperature, H3PO4 (85%, 0.22 mL, 0.375 g,
3.25 mmol) was added. The solid formed was ﬁltered oﬀ
and the organic solvents were removed by vacuum distilla-
tion. The residue was treated in an Eppendorf 5804R cen-
trifuge, to remove the glycerol layer: then, the sample wasextracted with 10 mL of water, dried with Na2SO4, weighed
and analysed.2.3.3. Microwaves ﬂow tests
Two stock solutions were prepared: one (A) containing
oil (800 mL, 736 g, 0.832 mol) and MTBE (672 mL), the
other (B) containing NaOH (9.6 g, 0.24 mol) dissolved in
MeOH (404.5 mL, 320 g, 10 mol). The apparatus, working
with a cell of 50 ml in stop-ﬂow regime, runs sequential
series of operations, repeating the optimal batch procedure.
Initially, 34 mL of A (2.89 mL/s) and 9 mL of B (2.99 mL/s)
were pumped into the ﬂow cell; then, the cell was irradiated,
with stirring (400 rpm), at 60 C for 1 min. The mixture was
allowed to cool for 3 min, and then, the sequence started
again. The mixture was treated as indicated in the batch
tests, to get an aliquot of it analysed.2.3.4. Analysis
The products were analysed by a FID gas chromato-
graph (Varian CP 3800) using the oﬃcial standard methods
[19]. The analysis of FAME 2 was made using a column cp
max 52 CB, 30 m long, with ID of 0.32 mm and 0.25 lm
ﬁlm. Temp. ramp starting with 75 C, then heating 16 C/
min up to 140 C, and then 15 C/min up to 300 C. The
analysis of glycerides 3, 4, 5 and glycerol 6 was made using
a column W COT fused silica 10 m long, ID of 0.32 mm
and 0.10 lm ﬁlm. Temperature ramp starting at 50 C,
then heating 20 C/min up to 180 C, then heating 10 C/
min up to 230 C and ﬁnally 15 C/min up to 370 C. In
both methods, hydrogen was used as carrier at 80 KPa,
and the injection volume was 1 lL.
3. Results and discussion
3.1. Batch tests
The overall transesteriﬁcation process (Scheme 1) was
performed as a set of operations as indicated in Scheme 2.
To obtain a homogeneous system, the reagents – oil and
methanol – were mixed with MTBE as a safe solvent which
can be easily recovered and reused. Then, the mixture was
irradiated with microwaves, acidiﬁed with phosphoric acid,
and distilled to eliminate the volatiles – mainly methanol
and MTBE – and then, the residue was centrifugated and
washed with water, to eliminate the glycerol, and the
remaining biodiesel was dried and analysed.
As it can be seen in Table 1, experiments 13 and 14,
made either with rapeseed or soybean oil, respectively,
where allowed to obtain biodiesel within standards, in
terms of the contents of contaminants 3, 4, 5 and glycerol 6.
No substantial diﬀerences were obtained with the diﬀerent
origin of the oil. The best results were obtained when the
corresponding oil (2.3 g), NaOH, (30 mg), MeOH
(1.27 mL), MTBE (2.1 mL) were placed in a sealed tube
with magnetic stirring. The mixture was heated for 1 min
at 60 C. The temperature was ﬁxed, so the irradiation
Reagents Microwaves CentrifugationDistillation
Acid
BiodieselVolatiles
GC
Re gents Micr aves Centrif tionDisti lation
Ac
Bio ieselVol iles
G
BIODIESEL:
OVERALL PROCESS
Scheme 2.
Table 1
Batch tests performed with microwaves to obtain biodiesel
Entry no. Mode Oil (g) NaOH (g) MeOH (mL) tc (min) Yield (%) Other components
3 (%) 4 (%) 5 (%) 6 (%)
1 MW 3.0a 0.018 0.83 1 17 0.50 1.81 35.53 0.04
2 MW 5 8 0.37 1.28 42.95 0.05
3 MW 3.0a 0.03 0.83 1 92 7.22 3.66 2.84 0.053
4 MW 5 92 9.74 2.01 1.17 0.11
5 MW 3.0a 0.04 0.83 1 95 2.26 0.46 0.24 0.0005
6 MW 5 94 1.36 0.37 0.19 0.04
7 MW 3.0a 0.018 1.66 1 92 6.19 0.46 0.56 0.05
8 MW 5 92 1.92 0.01 0.02 0.06
9 MW 2.3a 0.03 1.27 1 95 0.95 0.03 0.0006 0.03
10 MW 5 95 0.89 0.02 0.0003 0.03
11 MW 2.3a 0.03 1.27 1d 94 0.91 0.02 0.004 0.036
12 MW 5d 92 1.29 0.03 0.01 0.154
13 MW 2.3d 0.03 1.27 1 97 0.48 0.03 0.012 0.007
14 MW 2.3b 0.03 1.27 1 95 0.73 0.05 0.02 0.0008
15 MW 2.3b 0.03 1.27 1d 95 1.09 0.02 0.008 0.036
16 CH 10a 0.13 5.52 5 84 2 0.60 0.30 0.20
17 30 87 1.5 0.35 0.20 0.15
EN 14214 <0.80 <0.20 <0.20 <0.02
Notes: In the process, the corresponding oil is mixed with methanol and NaOH. MW, experiments performed with microwaves, 2.1 mL of MTBE were
added and the mixture irradiated at 60 C. CH, experiments performed by conventional heating, 9.1 mL of MTBE were added and the mixture heated at
60 C, for the time indicated (t). Yield and composition of the mixture, as obtained by GC.
a Rapeseed oil.
b Soybean oil.
c Reaction time in minutes.
d Power Max was used, to cool externally the reaction tube.
OO
O
R3
O O
R1
O
R2
CH3OH OHOH
OH
CH3 O
O
R
NaOH
OO
O R5
R4R6
         FAME
2 R = R1, R2, R3
MTBE
MW, 60˚ C
1
Glycerol 6
       R4      R5       R6
3     H        H        COR3
4     H      COR2   COR3
5   COR1 COR2   COR3
Scheme 1.
J. Hernando et al. / Fuel 86 (2007) 1641–1644 1643energy was automatically controlled by the apparatus to
keep the system within the established temperature limits.Then, the mixture followed the process as indicated in
Scheme 2.
Table 2
Flow tests performed with microwaves to obtain biodiesel
Entry no. (no. cycles) Oil (g) NaOH (g) MeOH (mL) t (min) 2 Yield (%) Other components
3 (%) 4 (%) 5 (%) 6 (%)
18 (3) 18.4b 0.24 10.16 1 96 0.71 0.06 0.0010 0.008
19 (17) 18.4b 0.24 10.16 1 97 0.60 0.03 0.019 0.016
EN 14214 <0.80 <0.20 <0.20 <0.02
Notes: In the process, every cycle was performed as follows: the corresponding oil was mixed with methanol and sodium hydroxide, in the quantities
indicated in the table, and 16.8 mL of MTBE. Then, the mixture was pumped through the ﬂow cell, with a useful volume of 50 mL, and heated for the time
indicated (t). Yield and composition of the biodiesel, as obtained by GC.
a Rapeseed oil.
b Soybean oil.
c Time of irradiation at 60 C, in min.
1644 J. Hernando et al. / Fuel 86 (2007) 1641–1644To compare microwave and conventional heating, two
experiments (16–17) are included in Table 1, in which heat-
ing to 60 C with an oil bath, with stirring (600 rpm) was
performed, reproducing the conditions of the experiments
13 and 14. The samples reached an equilibrium after
30 min, (87% FAME) and no improvement in yield was
obtained after this period.
3.2. Flow tests
A CEM Voyager ﬂow system [18] was used to show how
the optimal batch method could be scaled up to be con-
verted into a production method. The volumes and quanti-
ties of the experiment 14 in Table 1 were multiplied by 8, to
be reproduced in the ﬂow cell of 80 mL – useful 50 mL –
under a stop-ﬂow regime. Homogeneous phase is the best
choice to easily reproduce a method in the system. An ali-
quot of the mixture was used to follow Scheme 2. The com-
position of the biodiesel obtained was analysed as indicated
in experimental, according present standards [19]. As it can
be seen in Table 2, soybean oil was used to reproduce the
experiment 14 of Table 1. Two experiments were made,
one with 3 cycles (exp. 18, 55 ml of oil treated) and the
other with 17 cycles (exp. 19, 312,8 mL of oil treated). In
both experiments, the biodiesel obtained was under the
quality standards (Table 2).
4. Conclusion
As a conclusion, microwave irradiation has revealed fas-
ter and cleaner than conventional heating in the synthesis
of FAME (biodiesel) from triglycerides, even in the pres-
ence of homogeneous catalysts. The use of modern com-
mercial devices allows to straightforward transfer of the
process from batch to stop-ﬂow. This opens the way todevelop microwave procedures for a big scale biodiesel
production.
Acknowledgement
Authors acknowledge E. Ruiz and A.I. Can˜adas for
technical assistance in the experiments.
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Biodiesel and FAME synthesis assisted by microwaves: homogeneous batch and flow processes

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