A comparative study of biodiesel production using methanol, ethanol, and tert-butyl methyl ether (MTBE) under supercritical conditions

Bioresource Technology 191 (2015) 306–311

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A comparative study of biodiesel production using methanol, ethanol, and tert-butyl methyl ether (MTBE) under supercritical conditions Obie Farobie a, Yukihiko Matsumura b,⇑ a b

Department of Mechanical Science and Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan Division of Energy and Environmental Engineering, Institute of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan

h i g h l i g h t s  Biodiesel production in supercritical MeOH, EtOH, and MTBE were compared.  Biodiesel yield increased with reaction time and temperature for all cases.  Supercritical methanol gave the highest biodiesel yield under the same reaction conditions.  At 350 °C, 20 MPa, 10, 30, and 30 min gave the optimum yield for MeOH, EtOH, and MTBE, respectively.  The second-order kinetic model expressed good agreement with experimental data.

a r t i c l e

i n f o

Article history: Received 12 March 2015 Received in revised form 26 April 2015 Accepted 27 April 2015 Available online 14 May 2015 Keywords: Biomass Biodiesel MTBE Spiral reactor Supercritical fluids

a b s t r a c t In this study, biodiesel production under supercritical conditions among methanol, ethanol, and tert-butyl methyl ether (MTBE) was compared in order to elucidate the differences in their reaction behavior. A continuous reactor was employed, and experiments were conducted at various reaction temperatures (270–400 °C) and reaction times (3–30 min) and at a fixed pressure of 20 MPa and an oil-to-reactant molar ratio of 1:40. The results showed that under the same reaction conditions, the supercritical methanol method provided the highest yield of biodiesel. At 350 °C and 20 MPa, canola oil was completely converted to biodiesel after 10, 30, and 30 min in the case of – supercritical methanol, ethanol, and MTBE, respectively. The reaction kinetics of biodiesel production was also compared for supercritical methanol, ethanol, and MTBE. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Biodiesel, which comprises fatty acid alkyl esters, has been recognized as one of the most promising biofuels because it offers several advantages over petro-diesel, such as a relatively higher combustion efficiency (Kuti et al., 2013), fuel properties comparable to that of conventional diesel fuel (Silitonga et al., 2013), lower particulate matter and CO emissions (Knothe et al., 2006; Tat et al., 2007), and a higher cetane number (Can, 2014). During the past few years, numerous methods have been proposed for producing biodiesel by using diverse feedstocks, including edible and non-edible oil (Qian et al., 2010), animal fats (Encinar et al., 2011), waste cooking oil (Wen et al., 2010), algae (Komolafe et al., 2014), and even insects (Manzano-Agugliaro et al., 2012).

⇑ Corresponding author. Tel.: +81 (0)82 424 7561; fax: +81 (0)82 422 7193. E-mail address: [email protected] (Y. Matsumura). http://dx.doi.org/10.1016/j.biortech.2015.04.102 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.

Biodiesel is typically produced by the reaction of triglyceride with short-chain alcohols such as methanol or ethanol, and this is widely recognized as the transesterification reaction. In order to accelerate this reaction, various types of catalysts have been used in biodiesel production processes: acids (Wu et al., 2014; Fu et al., 2013), bases (Kawashima et al., 2008; Olutoye and Hameed, 2013), and enzymes (Hama and Kondo, 2013; Christopher et al., 2014). Acid catalysis is applied in esterification of free fatty acids, but homogeneous base-catalyzed transesterification is the most commonly used method worldwide for commercial biodiesel applications because the catalyst is inexpensive and widely available (Tan and Lee, 2011; Demirbas, 2011). However, this process presents several constraints such as the generation of undesirable products, requirement of low free fatty acid (FFA) and water content, and long reaction times. Moreover, separating the product in the downstream step is complicated. To circumvent this problem, a new approach of producing biodiesel by using the supercritical MTBE method has been developed in the authors’

O. Farobie, Y. Matsumura / Bioresource Technology 191 (2015) 306–311

laboratory at Hiroshima University (Farobie et al., 2014), and this method yields glycerol-tert butyl ether (GTBE) as a by-product. Compared with other methods, this method enables biodiesel production within shorter reaction times, is less sensitive to the FFA and water content of feedstock, and offers a higher added-value in the form of the by-product GTBE. However, for evaluating the utility of the proposed supercritical MTBE method, the reaction behavior of biodiesel production in supercritical MTBE, methanol, and ethanol must be directly compared, which remains to be done. Moreover, biodiesel was previously produced in supercritical MTBE at 10 MPa. In order to elucidate the differences in the reaction behavior relative to supercritical methanol and ethanol, biodiesel must be produced in supercritical MTBE at 20 MPa. The use of methanol and ethanol for biodiesel production under supercritical conditions has been widely compared by using batch and continuous-flow reactors. One of the earliest of these studies was conducted on biodiesel production in a batch reactor by Warabi et al. (2004), who observed that the highest yield of alkyl esters (almost 100%) was achieved within 15 and 45 min in the case of supercritical methanol and ethanol, respectively. Tan et al. (2010) also determined that supercritical methanol was superior to supercritical ethanol in terms of biodiesel yield and reaction time; the supercritical methanol reaction time required for achieving >80% yield of biodiesel was only 20 min, whereas supercritical ethanol only produced a 65% yield in 23 min. By contrast, Madras et al. (2004) determined that biodiesel conversion in supercritical ethanol was higher than that in supercritical methanol under the same reaction conditions; these investigators also used a batch-mode reactor in their study. Santana et al. (2012) reported that in a continuous-flow reactor, the ester yields of supercritical methanol and ethanol were 90% and 80%, respectively, and the reaction times for these alcohols were around 2 and 6 min, respectively. All of the aforementioned studies provide insights into relative biodiesel production in supercritical methanol and ethanol. However, these have not been compared with biodiesel production in supercritical MTBE. Comparing biodiesel production in supercritical methanol, ethanol, and MTBE is vital for understanding their reaction behaviors. Thus, in this study, the aim was to elucidate the differences in the reaction behavior of methanol, ethanol, and MTBE in the production of biodiesel under supercritical conditions by examining the temperature and residence-time effects on biodiesel yields and reaction kinetics.

307

of alcohol or MTBE wr [kg/s], the density of oil qo [kg/m3] and that of alcohol or MTBE qr [kg/m3] at a certain reaction temperature and pressure, and the reactor volume V [m3]:

s ¼ wo qo

V þ wq r

ð1Þ

r

2.2. Analytical methods Reaction products were analyzed using a gas chromatograph (GC) (GC-390B; GL Sciences) equipped with a flame-ionization detector and a MET-Biodiesel column featuring an integrated 2-m guard column (Sigma Aldrich, 28668-U). The details of the analytical methods have been reported previously (Farobie et al., 2014). Biodiesel yields were calculated from the experimental results by dividing the moles of product biodiesel by the moles of the fatty acid group in the initial triglyceride (TG), as shown in Eq. (2). Furthermore, the product concentration was calculated by using a calibration curve obtained based of peak area.

Product yield ¼

mol of product biodiesel mol of fatty acid group in initial TG

ð2Þ

2.3. Reagents and materials All chemicals used in this study were of high purity and were used without further treatment and purification. The canola oil feedstock used in this experimental work was produced by a commercial manufacturer (J-Oil Mills, Tokyo, Japan). Methanol (99.0%), ethanol (99.5%), and MTBE (99.5%) were purchased from Nacalai Tesque, Inc. (Kyoto, Japan). The following standard compounds were purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan): fatty acid methyl esters (FAME): methyl oleate (min. 60.0%), methyl linoleate (min. 98.0%), methyl linolenate (min. 95.0%), methyl palmitate (min. 97.0%), and methyl stearate (min. 95.0%); and fatty acid ethyl esters (FAEE): ethyl oleate (99.9%), ethyl linoleate (min. 97.0%), ethyl linolenate (min. 95.0%), ethyl palmitate (min. 95.0%), and ethyl stearate (min. 90.0%). Triolein (99.9%), diolein (99.9%), and monoolein (min. 40.0%) standards were purchased from Nacalai Tesque, Inc., Sigma–Aldrich, Co. (Japan), and Tokyo Chemical Industry Co., Ltd., respectively. To prepare GC standard solutions, analytical grade tricaprin and n-hexane were used.

2. Methods 3. Results and discussion 2.1. Experimental 3.1. Effect of temperature on biodiesel yield Biodiesel production in supercritical methanol, ethanol, and MTBE was compared in the temperature range of 270–400 °C under a fixed pressure of 20 MPa. The oil-to-reactant molar ratio used in this study was fixed at 1:40. The transesterification reaction was conducted for 3–30 min. The details of the reactor employed here are presented elsewhere together with the features of the reactor (Farobie and Matsumura, 2015). Briefly, feedstock consisting of canola oil and the reactant (methanol, ethanol, or MTBE) was fed into the reactor by using a high-pressure pump, and then the pressure was increased to 20 MPa by using a back-pressure regulator. The reactor temperature was raised to the desired value, and the samples were collected after steady state was achieved. The obtained products were removed from the reactor after being passed through the filter and the back-pressure regulator. Residence time s [s] was determined using Eq. (1), which accounts for the mass flow rate of oil wo [kg/s], the mass flow rate

Fig. 1 shows the effect of temperature on biodiesel yields in supercritical methanol, ethanol, and MTBE at the same residence time of 15 min. In all cases, biodiesel yield increased with temperature. This result agrees well with the results obtained using previous supercritical biodiesel-production methods (Varma et al., 2010; Zhou et al., 2010; Biktashev et al., 2011; Velez et al., 2012; Tsai et al., 2013; Goembira and Saka, 2013). Biodiesel production in the supercritical methanol method was superior to the production in the supercritical ethanol and MTBE methods. This could be attributed to methanol being the smallest among these molecules, which enables the oxygen atom from methanol to readily attack the carbon atom of the carbonyl functional group from triglycerides. This process results in intermediate compound via transfer of a methoxide moiety. This intermediate compound is then rearranged to generate more stable compounds, namely biodiesel and diglyceride.

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3.2. Effect of reaction time on product yield SCM

1.00

SCE SCMTBE

Yield [mol/mol]

0.80

0.60

0.40

0.20

0.00 270

300

350

400

Temperature [°C] Fig. 1. Effect of temperature on biodiesel yield in supercritical methanol (SCM), supercritical ethanol (SCE), and supercritical MTBE (SCMTBE) (experimental conditions: 20 MPa, 15 min, oil-to-reactant molar ratio of 1:40).

Intriguingly, at a low temperature of 270 °C, biodiesel yield in supercritical MTBE was higher than that in supercritical ethanol. This could be because at low temperature, the effect of solubility might be stronger than the steric effect; the polarity of MTBE is lower than that of ethanol, and thus its miscibility with oil is higher than that of ethanol. However, at P300 °C, the biodiesel yield in supercritical ethanol was higher than that in supercritical MTBE because MTBE is structurally bulkier than ethanol.

100

TG(SCE)

90

110

(a) 270 °C

90

FAME(SCM)

80

70

Yield [wt%]

TG(SCM)

60

FAME(SCM)

50 40

FAME(SCMTBE)

30

FAEE(SCE)

10

FAEE(SCE)

TG(SCMTBE)

70 60

FAME(SCMTBE)

TG(SCE)

50 40

TG(SCM)

30

GTBE(SCMTBE) GL(SCM) GL(SCE)

20

20

GTBE(SCMTBE)

GL(SCE)

GL(SCM)

10

0

0 0

5

10

15

20

25

30

35

0

Reaction time [min] 110

110

(c) 350 °C

100

FAME(SCM)

Yield [wt%]

FAME(SCMTBE)

50

TG(SCMTBE)

40 30

TG(SCE)

20

GL(SCM)

10

30

35

FAME(SCM) FAME(SCMTBE)

50 40

GL(SCM) GL(SCE) TG(SCM) TG(SCE)

GTBE(SCMTBE)

10

TG(SCMTBE)

0 5

25

60

20

0 0

20

70

30

GTBE(SCMTBE) GL(SCE)

TG(SCM)

10

15

Reaction time [min]

FAEE(SCE)

80

70 60

10

90

FAEE(SCE)

80

5

(d) 400 °C

100

90

Yield [wt%]

(b) 300 °C

100

TG(SCMTBE)

80

Yield [wt%]

The reaction behavior in biodiesel production in supercritical methanol, ethanol, and MTBE was further compared by examining the effect of residence time. Fig. 2 shows the changes in feedstock triglyceride, the final biodiesel product, and byproduct yields at 270, 300, 350, and 400 °C. In all cases, long reaction times allowed the completion of transesterification and thus increased biodiesel yields, as expected. First, at 270 °C – which is just above the critical temperature of methanol, ethanol, and MTBE – biodiesel yields were low for both supercritical ethanol and MTBE even after 30 min of reaction, whereas a large biodiesel yield was obtained in supercritical methanol after 25 min. Under this condition, biodiesel yields of 72.6, 18.8, and 19.9 wt% were obtained within 20 min for supercritical methanol, ethanol, and MTBE, respectively. At 300 °C, more canola oil was converted to biodiesel than at 270 °C in all cases. A considerable amount of oil converted to biodiesel within 15 min in supercritical methanol and provided a biodiesel yield of 96.5 wt%. This result closely agreed with the work of Warabi et al. (2004), who reported almost complete conversion into biodiesel in supercritical methanol at 300 °C within 15 min. By contrast, biodiesel yields of only 37.2 and 28.9 wt% were obtained within 15 min of reaction in supercritical ethanol and MTBE, respectively. At 350 °C, canola oil was completely converted to biodiesel within 10, 30, and 30 min in the case of supercritical methanol, ethanol, and MTBE, respectively. In the supercritical methanol case, biodiesel yields were approximately 68.3 and 97.7 wt% after 3 and 5 min, respectively, and within 10 min, transesterification was

15

20

25

Reaction time [min]

30

35

0

5

10

15

20

Reaction time [min]

25

30

35

Fig. 2. Effect of reaction time on triglyceride (TG) consumption and biodiesel yield in supercritical methanol, ethanol, and MTBE at (a) 270 °C, (b) 300 °C, (c) 350 °C, and (d) 400 °C (experimental conditions: 20 MPa, oil-to-reactant molar ratio of 1:40).

309

O. Farobie, Y. Matsumura / Bioresource Technology 191 (2015) 306–311 Table 1 Reaction rate constants obtained from the second order model (experimental conditions: 270–400 °C, 20 MPa, reactant-to-oil molar ratio of 40:1). Rate constants [dm3 mol1 min1]

Supercritical methanol 270 °C

300 °C

350 °C

400 °C

270 °C

300 °C

350 °C

400 °C

270 °C

300 °C

350 °C

400 °C

k1 k1 k2 k2 k3 k3

0.0133 0.0015 0.0180 0.0300 0.0165 0.0280

0.0428 0.0035 0.0428 0.0900 0.0661 0.0420

0.2338 0.0165 0.1505 0.2800 0.1857 0.1050

0.9985 0.0590 0.3810 0.9242 0.5094 0.1800

0.0024 0.0080 0.0080 0.0680 0.0085 0.0488

0.0068 0.0208 0.0210 0.1737 0.0372 0.0840

0.0883 0.0738 0.0580 0.7363 0.0880 0.1876

0.9085 0.2690 0.2810 1.6242 0.3439 0.3000

0.0052 0.0022 0.0157 0.0350 0.0148 0.0283

0.0095 0.0086 0.0374 0.1015 0.0520 0.0458

0.0390 0.0300 0.1341 0.3200 0.1255 0.1070

0.4760 0.0800 0.3546 1.0150 0.5388 0.1800

Supercritical ethanol

0 -1

ln k [dm3 mol-1 min-1]

almost complete and thus almost all of the canola oil was converted to biodiesel. Conversely, biodiesel yields were about 59.6 and 22.9 wt% after 5 min for supercritical ethanol and MTBE, respectively; a substantial change of the oil into biodiesel was observed after 20 min, and the oil completely was converted to biodiesel in the case of both supercritical ethanol and MTBE after reaction for 30 min. Here, 350 °C was found to be optimal for transesterification under supercritical conditions, which again agrees well with the results of previous studies (Kusdiana and Saka, 2001; Silva et al., 2007; Madras et al., 2004). At 400 °C, in supercritical methanol, canola oil was completely converted to FAME within the first 3 min. However, in supercritical ethanol and MTBE, the transesterification of canola oil was complete after 10 min and yielded biodiesel at 100 wt%. Overall, increasing the reaction temperature favored triglyceride consumption. Moreover, at 270–350 °C, supercritical methanol yielded the highest amount of the by-product glycerol (GL), whereas at 400 °C, almost the same by-product yields were measured for supercritical methanol, ethanol, and MTBE.

Supercritical MTBE

-2 -3 -4

k1, SCM k-1, SCM k1, SCE k-1, SCE k1, SCMTBE k-1, SCMTBE

-5 -6 -7 0.001

0.0012

0.0014

0.0016

0.0018

0.002

0.0018

0.002

0.0018

0.002

1/T [K-1] 1 0

d½TG ¼ k1 ½TG½RA ORB  þ k1 ½FAAE½DGRB  dt

ð3Þ

d½DGRB  ¼ k1 ½TG½RA ORB   k1 ½FAAE½DGRB   k2 ½DGRB ½RA ORB  dt þ k2 ½FAAE½MGRB  ð4Þ

-1 -2 -3 -4 -5

k2, SCM k-2, SCM k2, SCE k-2, SCE k2, SCMTBE k-2, SCMTBE

-6 0.001

0.0012

0.0014

0.0016

1/T [K-1] 0 -1

ln k [dm3 mol-1 min-1]

Lastly, the reaction kinetics of the transesterification reaction of oil in supercritical methanol, ethanol, and MTBE were compared. The transesterification reaction of oil with MTBE and the differential rate equations obtained for supercritical MTBE have been reported in the previous paper (Farobie et al., 2014). However, the rate equation for biodiesel decomposition must be eliminated because temperatures up to 400 °C were used here and no decomposition product was observed. The transesterification reaction of triglyceride with methanol, ethanol, or MTBE was assumed to occur in three reversible reaction steps. Firstly, intermediate compound of diglyceride (DG) for supercritical methanol and ethanol or diglyceride mono tert-butyl ether (DGE) for supercritical MTBE was generated. These intermediate compounds further react to generate monoglyceride (MG) for supercritical methanol and ethanol or monoglyceride di tert-butyl ether (MGE) for supercritical MTBE. Finally, by-product glycerol (GL) for supercritical methanol and ethanol, or glycerol tert-butyl ether (GTBE) for supercritical MTBE was produced. The reaction rate constants were determined by using nonlinear regression with the least squares of error (LSE) method (i.e., determination of the difference between experimental and calculated values) as a criterion to fit the model with the experimental data. Because each of these reactions is assumed to follow second-order kinetics, the rate of change in concentration can be expressed using the differential rate equations shown in Eqs. (3–8).

ln k [dm3 mol-1 min-1]

3.3. Comparison of reaction kinetics

-2 -3 -4 -5

k3, SCM k-3, SCM k3, SCE k-3, SCE k3, SCMTBE k-3, SCMTBE

-6 0.001

0.0012

0.0014

0.0016

1/T [K-1] Fig. 3. Arrhenius plots for supercritical methanol (SCM), supercritical ethanol (SCE), and supercritical MTBE (SCMTBE) (experimental conditions: 20 MPa, oil-to-reactant molar ratio of 1:40).

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Table 2 Activation energies and pre-exponential factors (experimental conditions: 270–400 °C, 20 MPa, oil-to-reactant molar ratio of 1:40). Rate constants [dm mol

1

min

1

k1 k1 k2 k2 k3 k3

]

Supercritical methanol

Supercritical ethanol

Activation energy, Ea [kJ mol1]

Pre-exponential factor, A [dm3 mol1 min1]

Activation energy, Ea [kJ mol1]

Pre-exponential factor, A [dm3 mol1 min1]

Activation energy, Ea [kJ mol1]

Pre-exponential factor, A [dm3 mol1 min1]

100.99 86.81 71.77 78.36 77.28 44.98

6.85  107 3.13  105 1.47  105 1.10  106 5.50  105 5.73  102

141.28 81.32 80.31 75.52 81.64 43.05

7.03  1010 5.25  105 4.09  105 1.32  106 7.29  105 6.98  102

103.68 82.32 73.28 77.24 79.84 44.16

3.39  107 2.19  105 1.78  105 3.43  106 7.76  105 5.00  102

d½MGRB  ¼ k2 ½DGRB ½RA ORB   k2 ½FAAE½MGRB   k3 ½MGRB ½RA ORB  dt ð5Þ þ k3 ½FAAE½GLRB  d½FAAE ¼ k1 ½TG½RA ORB   k1 ½FAAE½DGRB  þ k2 ½DGRB ½RA ORB  dt  k2 ½FAAE½MGRB  þ k3 ½MGRB ½RA ORB   k3 ½FAAE½GLRB  ð6Þ

d½RA ORB  ¼  k1 ½TG½RA ORB  þ k1 ½FAAE½DGRB   k2 ½DGRB ½RA ORB  dt þ k2 ½FAAE½MGRB   k3 ½MGRB ½RA ORB  þ k3 ½FAAE½GLRB 

ð7Þ

d½GLRB  ¼ k3 ½MGRB ½RA ORB   k3 ½FAAE½GLRB  dt

ð8Þ

[mol dm3], [FAAE] is fatty acid alkyl ester or biodiesel concentration [mol dm3], [GLRB] is glycerol concentration for supercritical methanol and ethanol or glycerol tert-butyl ether concentration for supercritical MTBE [mol dm3], ki is reaction rate constants [dm3 mol1 min1], and t is residence time [min]. The corresponding reaction rate constants were once again calculated by using nonlinear regression with the LSE method, and these values are shown in Table 1. Overall, the rate constants calculated for the forward reaction in supercritical methanol were the highest, followed by those calculated for supercritical MTBE and ethanol. The Arrhenius plots of the individual rate constants for the transesterification reaction of oil in supercritical methanol, ethanol, and MTBE are compared in Fig. 3. The logarithms of the overall reaction rate constants were linear with inverse temperatures, demonstrating that the transesterification reaction in supercritical methanol, ethanol, and MTBE followed the Arrhenius behavior. Table 2 shows the activation energy (Ea) and pre-exponential factor (A) calculated from the detailed kinetic analysis of biodiesel production in supercritical methanol, ethanol, and MTBE. The activation energies of oil conversion to biodiesel were determined to range between 44.98 and 100.99 kJ mol1, 43.05 and 141.28 kJ mol1, and 44.16 and 103.68 kJ mol1 for supercritical

0

-2

-2

-2

-4 -6

-8

-10 -12

k1, SCM, This work k-1, SCM, This work k1, Kusdiana and Saka (2001) k-1, Kusdiana and Saka (2001) k1, He et al. (2007) k-1, He et al. (2007)

-14 0.0014 0.0015 0.0016 0.0017 0.0018 0.0019

(a) ln k [dm3 mol-1 min-1]

6 5 4 3 2 1 0 -1 -2 -3 -4 -5 -6 -7 -8 0.0014 0.0016 0.0018

(b)

-8 -10 -12

k2, SCM, This work k-2, SCM, This work k2, Kusdiana and Saka (2001) k-2, Kusdiana and Saka (2001) k2, He et al. (2007) k-2, He et al. (2007)

0.002

-4 -6 -8 -10 -12

k-1, SCE, This work k1, Santana et al. (2012) k-1, Santana et al. (2012)

0.0022 0.0024 0.0026

6 5 4 3 2 1 0 -1 -2 -3 -4 -5 -6 -7 -8 0.0014 0.0016 0.0018

k-2, SCE, This work k2, Santana et al. (2012) k-2, Santana et al. (2012)

1/T [K-1]

0.002

1/T [K-1]

k2, SCE, This work

0.002

k3, SCM, This work k-3, SCM, This work k3, Kusdiana and Saka (2001) k-3, Kusdiana and Saka (2001) k3, He et al. (2007) k-3, He et al. (2007)

-14 0.0014 0.0015 0.0016 0.0017 0.0018 0.0019

1/T [K-1]

k1, SCE, This work

1/T [K-1]

-6

-14 0.002 0.0014 0.0015 0.0016 0.0017 0.0018 0.0019

1/T [K-1]

0.002

-4

ln k [dm3 mol-1 min-1]

0

ln k [dm3 mol-1 min-1]

0

ln k [dm3 mol-1 min-1]

ln k [dm3 mol-1 min-1]

where, [TG] is triglyceride concentration [mol dm3], [DGRB] is diglyceride concentration for supercritical methanol and ethanol or diglyceride mono tert-butyl ether concentration for supercritical MTBE [mol dm3], [MGRB] is monoglyceride concentration for supercritical methanol and ethanol or monoglyceride di tert-butyl ether concentration for supercritical MTBE [mol dm3], [RAORB] is reactant concentration, in this case methanol, ethanol, or MTBE

Supercritical MTBE

ln k [dm3 mol-1 min-1]

3

0.0022 0.0024 0.0026

6 5 4 3 2 1 0 -1 -2 -3 -4 -5 -6 -7 -8 0.0014 0.0016 0.0018

k3, SCE, This work

k-3, SCE, This work k3, Santana et al. (2012) k-3, Santana et al. (2012)

0.002

0.0022 0.0024 0.0026

1/T [K-1]

Fig. 4. Comparison of Arrhenius plots of the rate constants for the transesterification reaction in (a) supercritical methanol (SCM) and (b) supercritical ethanol (SCE).

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methanol, ethanol, and MTBE, respectively. The activation energy for the forward reaction in supercritical methanol was the lowest, and this was followed by that determined for supercritical MTBE and ethanol; the result implies that the activated intermediate was most stable in the case of supercritical methanol. Intriguingly, the activation energies of supercritical MTBE calculated here were close to those of supercritical methanol and not supercritical ethanol. In Fig. 4, the activation energies obtained in this work are compared with those measured in previous studies. In the supercritical methanol case, the activation energies for the forward reaction were higher than the values obtained by Kusdiana and Saka (2001) and He et al. (2007). Kusdiana and Saka (2001) determined the activation energy of rapeseed oil conversion in supercritical methanol (300–400 °C and 20 MPa) by using a batch reactor, and these energies were 45.86, 31.24, and 79.49 kJ mol1 for the conversion reactions of triglyceride to diglyceride, diglyceride to monoglyceride, and monoglyceride to glycerol, respectively. Furthermore, He et al. (2007) calculated the activation energies for the conversion reactions of triglyceride to diglyceride, diglyceride to monoglyceride, and monoglyceride to glycerol to be approximately 59.38, 58.64, and 67.38 kJ mol1, respectively, by using a batch reactor and the conditions of 28 MPa and 240– 280 °C. The differences between the models of previous studies and this study might be attributed to the distinct types of apparatus used (batch reactor versus continuous reactor). Batch type reactor has difficulty in treatment of reaction during heating up and cooling down of the reactor. In the case of supercritical ethanol, the activation energies obtained were close to those obtained by Santana et al. (2012), who determined that the activation energies for the transesterification of sunflower oil that ranged from 59 to 140 kJ mol1 at 150–200 °C and 20 MPa by using a fixed-bed reactor in the continuous mode. 4. Conclusions Reaction behaviors of biodiesel production in supercritical methanol, ethanol, and MTBE were compared by investigating the effects of temperature and residence time, and analyzing the reaction kinetics. The results showed that the supercritical methanol method provided the highest biodiesel yields under the same reaction conditions. At 350 °C and 20 MPa, the optimal biodiesel yield was achieved after 10, 30, and 30 min in biodiesel production in supercritical methanol, ethanol, and MTBE, respectively. The reaction parameters for the conversion of oil to biodiesel in supercritical methanol, ethanol, and MTBE were also determined. Acknowledgements OF is so glad to express his deep gratitude to Japan-Indonesia Presidential Scholarship (JIPS), the World Bank Institute for PhD scholarship. References Biktashev, Sh.A., Usmanov, R.A., Gabitov, R.R., Gazizov, R.A., Gumerov, F.M., Gabitov, F.R., Abdulagatov, I.M., Yarullin, R.S., Yakushev, I.A., 2011. Transesterification of rapeseed and palm oils in supercritical methanol and ethanol. Biomass Bioenergy 35, 2999–3011. Can, Ö., 2014. Combustion characteristics, performance and exhaust emissions of a diesel engine fueled with a waste cooking oil biodiesel mixture. Energy Convers. Manage. 87, 676–686. Christopher, L.P., Kumar, H., Zambare, V.P., 2014. Enzymatic biodiesel: challenges and opportunities. Appl. Energy 119, 497–520.

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