Biodiesel Production in Supercritical Methanol Using a Novel Spiral Reactor

Biodiesel Production in Supercritical Methanol Using a Novel Spiral Reactor

Available online at www.sciencedirect.com ScienceDirect Procedia Environmental Sciences 28 (2015) 204 – 213 The 5th Sustainable Future for Human Sec...

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Available online at www.sciencedirect.com

ScienceDirect Procedia Environmental Sciences 28 (2015) 204 – 213

The 5th Sustainable Future for Human Security (SustaiN 2014)

Biodiesel production in supercritical methanol using a novel spiral reactor Obie Farobiea, Yukihiko Matsumurab* a

Department of Mechanical Science and Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima, 739-8527 Japan. Surfactant and Bioenergy Research Center (SBRC), Bogor Agricultural University, Jl. Pajajaran No.1, Kampus IPB Baranangsiang, Bogor, 16144 Indonesia. b Division of Energy and Environmental Engineering, Institute of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima, 739-8527 Japan. a

Abstract Biodiesel was produced via non-catalytic transesterification in supercritical methanol using a novel spiral reactor. This spiral reactor could serve as a heat exchanger, thus it provided the advantage of being able to recover the heat. Transesterification was carried out at 270‒400 °C, a pressure of 20 MPa, oil-to-methanol molar ratio of 1:40, and reaction time of 3‒30 min. Using this technique, a complete conversion of fatty acid methyl ester (FAME) (100 wt%) was obtained in a short reaction time of 10 min at 350 °C and oil-to-methanol molar ratio of 1:40 under a reactor pressure of 20 MPa. The result revealed that biodiesel yield conducted in spiral reactor is higher than that in batch reactor at the same reaction conditions. The kinetic model of canola oil conversion to biodiesel in supercritical methanol was also determined. © 2015 The Authors. Published by Elsevier B.V This is an open access article under the CC BY-NC-ND license © 2015 The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Sustain Society. Peer-review under responsibility of Sustain Society Keywords:biodiesel;methanol;spiral reactor;supercritical fluid

1. Introduction Nowadays, many countries around the world have placed focus on the development and application of renewable energy technologies as a consequence of the depletion of global fossil fuel reserves and to mitigate environmental

* Corresponding author. Tel.: +81-(0)-82-424-7561; fax: +81-(0)-82-422-7193. E-mail address: [email protected]

1878-0296 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Sustain Society doi:10.1016/j.proenv.2015.07.027

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damage. Biodiesel, which is ordinarily produced by transesterification of vegetable oils, animal fats, or waste oils with short chain alcohol, has been considered as one of the most promising renewable energy sources as a conventional diesel fuel substitute owing to its numerous comparative advantages such as low carbon monoxide, particulate matter, and unburnt total hydrocarbon emission1. Homogeneous alkali-catalyzed transesterification is mainly used to produce biodiesel in developing countries since these catalyst are widely available. However, this method has drawbacks such as low free fatty acid and water content requirements, soap formation, longer reaction time, lower reaction rates, and strict reaction conditions. In order to circumvent the problem, a new breakthrough of non-catalytic biodiesel production under supercritical conditions has been proposed by Saka and Dadan2. This technology promises a lot of advantages such as no catalyst requirement, no waste water generated, easier separation, and higher reaction rates. However, this technology still remains the drawback regarding heat recovery that has been difficult for commercial application. Therefore, a novel spiral reactor, which is composed of a parallel tube heat exchanger where heat is recovered and high-temperature transesterification reactor where the reaction mainly takes place, is proposed. The objective of this study is to develop a novel spiral reactor for transesterification reaction between canola oil and methanol under supercritical conditions. For this purpose, the effect of temperature and reaction time on FAME yields was investigated. In addition, the comparison between batch and spiral reactor in terms of temperature effect on biodiesel yield was also evaluated. Nomenclature A Ea k R t T

pre-exponential factor activation energy reaction rate constant universal molar gas constant residence time temperature

2. Material and Methods 2.1. Reagents and materials All of the chemicals were used in this study without further treatment. The chemical reagents used was methanol (99.0%) purchased from Nacalai Tesque, Inc., Kyoto, Japan. The standard compounds used in this study were the same with those reported in our previous paper3. 2.2. Experimental Biodiesel production was carried out in the spiral reactor that was schematically illustrated in Fig. 1. The spiral reactor was made of stainless-steel tubing (SS316) with an inner diameter of 2.17 mm. Thermocouples were equipped to measure the flow temperature. The length of the heat exchanger part was 2.5 m and that of the reactor portion was 10 m. This spiral reactor was buried in heat transfer cement with a cartridge heater in the center of the spiral reactor. First, the feedstocks consisted of canola oil and methanol were fed to the reactor using a high-pressure pump. Subsequently, the pressure was increased to 20 MPa using a back-pressure regulator. The samples were collected after achieving steady state. The effluent was cooled down, removed from the reactor, and depressurized with a back-pressure regulator.

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Fig. 1. Experimental apparatus and a detail schematic of spiral reactor.

In this investigation, biodiesel production in supercritical methanol was carried out in temperature range of 270‒400 °C at a pressure of 20 MPa. The oil-to-methanol molar ratio used in this study was fixed at 1:40. The transesterification reaction was carried out over 3 to 30 min. Further specific details of the experimental conditions are shown in Table 1. The feedstock used in this study was canola oil. Table 1. Experimental conditions for biodiesel production in supercritical methanol. Type of condition

Experimental range

Feedstock

Canola oil

Reaction time

3‒30 min

Oil-to-methanol molar ratio

1:40

Reactor type

Spiral reactor

Temperature

270‒400 °C

Pressure

20 MPa

Reactor length

10 m

Residence time was determined using Eq. 1, taking into consideration the density of canola oil and methanol at the reaction temperature, mass flow rate of canola oil and methanol, and the reactor volume. reactor volume [dm3 ]

Residence time [min] = mass flow rate of oil [g/min] density of oil [g/dm3 ]

+

mass flow rate of methanol [g/min] density of methanol [g/dm3 ]

(1)

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2.3. Analytical methods Gas chromatography analyses were carried out to determine FAME yield, intermediate compounds concentration, i.e. diglyceride and monoglyceride, and glycerol concentration. Gas chromatography (GC-390B; GL Sciences) was installed with Supelco MET-Biodiesel w/int. 2 m guard column, Sigma Aldrich and equipped with a flameionization detector (FID). Argon was used as the carrier gas. The technical detail of GC analysis procedure was reported in our previous paper3. Content of products from the experimental results was calculated by dividing the weight of product by weight of initial triglyceride (TG) as shown in Eq.2. In addition, product yield was calculated using calibration curve on the basis of peak area. Yield [wt%] =

weight of product [g] weight of initial triglyceride [g]

× 100

(2)

3. Results and Discussion 3.1. Biodiesel yield in batch versus spiral reactors To investigate the characteristics of spiral reactor, the comparison between batch and spiral reactor was evaluated. Fig. 2 reveals the comparison between batch and spiral reactors in terms of temperature effect on FAME yield. FAME yield conducted in spiral reactor was much higher than that in batch reactor4 at the same reaction conditions. This temperature effect could be attributed to the heating and cooling rates employed in the experiments. 100 90

FAME yield [wt%]

80 70 60 50 40

30

Spiral reactor (this study) Batch reactor (Tan et al., 2010)

20 10 0 250

270

290

310

330

350

370

390

410

430

450

Temperature [°C] Fig. 2. Comparison between batch and spiral reactor in supercritical methanol (experimental conditions: 20 MPa, oil-to-methanol molar ratio of 1:40, and reaction time of 20 min). .

3.2. Effect of temperature and time on FAME yield With the pressure and oil-to-methanol molar ratio being fixed at 20 MPa and 1:40, respectively, effect of temperature on FAME yield was investigated as shown in Fig. 3. The yield of FAME significantly increased with a change in reaction temperature from 270 to 400 °C. This result was consistent with the previous work of Rathore and

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Madras5, who reported that the conversion of oil to FAME in supercritical methanol increased with the increase in temperature from 200 to 400 °C.

110

FAME yield [%]

100

270 °C 300 °C

90

350 °C

80

400 °C

70 60 50 40 30 20 10 0 0

3

5

10

15

20

25

30

Residence time [min] Fig. 3. Effect of temperature on FAME yield (experimental conditions: 20 MPa, oil-to-methanol molar ratio of 1:40).

Reaction time also played an important role in supercritical methanol biodiesel production since it could determine the reaction rate constant. Expectedly, a longer reaction time allowed the transesterification reaction of canola oil to proceed a complete conversion and result in a higher FAME yield. Fig. 4 shows the effect of reaction time on FAME yield and triglyceride consumption treated at 270 and 300 °C. At 270 °C, the FAME yield increased from 9 wt% to 87 wt%, whereas at 300 °C, it significantly increased from 23 wt% to 96 wt% with an increase in reaction time from 3 to 30 min. At both 270 and 300 °C, the triglyceride consumption in proportional to DG and MG production. As expected, high concentration of DG and MG were observed at the beginning of reaction, demonstrating the evident formation and consumption of both intermediate compounds as reaction takes place.

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100

(a) 270 °C

90 80

FAME

80

70

70 TG

60

Yield [wt%]

Yield [wt%]

(b) 300 °C

90

FAME

50 40 DG

30

50 40 DG TG

30

20

20

MG

10

GL 5

10

15

20

25

GL

MG

10

0 0

60

0

30

0

5

10

Reaction time [min]

15

20

25

30

Reaction time [min]

Fig. 4. Effect of reaction time on FAME concentration and triglyceride consumption at (a) 270 °C and (b) 300 °C (experimental conditions: 20 MPa, oil-to-methanol molar ratio of 1:40).

Fig. 5 shows the effect of reaction time on FAME concentration and triglyceride consumption treated at 350 and 400 °C. At 350 °C, a complete conversion of FAME was noted at a reaction time of 10 min. FAME yields of about 68 and 100 wt% were obtained after transesterification reaction times of 3 and 10 min, respectively. It is to be noted that 350 °C is the optimum temperature to achieve the highest yield of FAME in supercritical methanol. This result was in a good agreement with the previous works of Kusdiana and Saka6 and Madras et al.7 who reported that 350 °C was the best temperature to carry out transesterification of oil in supercritical methanol. At 400 °C, a complete conversion of canola oil to FAME was observed at the initial reaction time of 3 min. The FAME yield was relatively constant after 3 min reaction. Overall, the TG consumption was favored with increasing temperature and reaction time. The trend of by-product glycerol concentration increased proportionally with the increasing FAME concentration and triglyceride consumption. 110

90 80

80

70

70

60 50 40 30

DG MG

TG

90

FAME

TG

20

(b) 400 °C

100

Yield [wt%]

Yield [wt%]

110

(a) 350 °C

100

GL

60 50 40 30 DG MG

20 10

10

FAME

GL

0

0 0

5

10

15

20

Reaction time [min]

25

30

0

5

10

15

20

25

Reaction time [min]

Fig. 5. Effect of reaction time on FAME concentration and triglyceride consumption at (a) 350 °C and (b) 400 °C (experimental conditions: 20 MPa, oil-to-methanol molar ratio of 1:40).

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3.3. Kinetic model The mechanism of transesterification reaction of canola oil with methanol proceeds 3 steps of reversible reactions as shown in Fig. 6. Firstly, triglyceride reacts with methanol to generate FAME and diglyceride (DG). Secondly, the reaction between intermediate DG with methanol produces FAME and monoglyceride (MG). Lastly, the intermediate compound MG then reacts with methanol to generate FAME and glycerol.

Fig. 6. Mechanism of transesterification reaction of triglyceride with methanol.

Each reaction shown above is deduced to follow the second-order kinetics; and the rate of change in concentration, hence, can be expressed by the differential rate equations shown in Eqs. 3‒8. d[TG]

= – k1 [TG] [MeOH] + k-1 [FAME] [DG]

dt d[DG]

= k1 [TG] [MeOH] – k-1 [FAME] [DG] – k2 [DG] [MeOH] + k-2 [FAME] [MG]

dt d[MG]

= k2 [DG] [MeOH] – k-2 [FAME] [MG] – k3 [MG] [MeOH] + k-3 [FAME] [GL]

dt d[FAME] dt

= k1 [TG] [MeOH] – k-1 [FAME] [DG] + k2 [DG] [MeOH] – k-2 [FAME] [MG] + k3 [MG] [MeOH] – k-3 [FAME] [GL]

d[methanol] dt

d[GL] dt

= – k1 [TG] [MeOH] + k-1 [FAME] [DG] – k2 [DG] [MeOH] + k-2 [FAME] [MG] – k3 [MG] [MeOH] + k-3 [FAME] [GL] = k3 [MG] [MeOH] – k-3 [FAME] [GL]

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where, [TG] is triglyceride concentration [mol dm-3], [DG] is diglyceride concentration [mol dm-3], [MG] is monoglyceride concentration [mol dm-3], [MeOH] is methanol concentration [mol dm-3], [FAME] is fatty acid methyl ester or biodiesel concentration [mol dm-3], [GL] is glycerol concentration [mol dm-3], ki is reaction rate constants [dm3 mol-1 min-1], and t is residence time [min]. The reaction rate constants were than calculated using the non-linear regression with the least-squares-error (LSE) method (i.e., the difference between the experimental and calculated values) as a criterion to fit the model with the experimental data. Fig. 7 shows a parity plot comparing the calculated (lines) and experimental FAME concentration. The model was able to reproduce the trends of most of the FAME concentration in terms of variation in the temperature and residence time as shown in this figure and as suggested by the high R2(coefficient of determination) values. 0.8

Calculated FAME concentration [mol dm-3]

R² = 0.9815 0.7 0.6

0.5 0.4 0.3 0.2 0.1 0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Experimental FAME concentration [mol dm-3] Fig. 7. Comparison of calculated and experimental FAME concentration in supercritical methanol using a spiral reactor (experimental conditions: 270‒400 °C, 20 MPa, oil-to-methanol molar ratio of 1:40)

The corresponding reaction rate constants were calculated based on the results of biodiesel yield as a function of temperature and reaction time. The values of corresponding reaction rate constants are presented in Table 2. Table 2. Kinetic parameters obtained from the second order model for biodiesel production in supercritical methanol. Kinetic parameter 3

-1

-1

[dm mol min ]

Denotation

Temperature [°C] 300

350

400

k1

TG → DG

1.33 × 10-2

270

4.28 × 10-2

2.34 × 10-1

9.08 × 10-1

k-1

DG → TG

2.50 × 10-3

6.50 × 10-3

2.16 × 10-2

5.90 × 10-2

-2

-2

-1

3.81 × 10-1

k2

DG → MG

1.70 × 10

3.88 × 10

k-2

MG → DG

3.00 × 10-2

9.00 × 10-2

2.80 × 10-1

9.24 × 10-1

k3

MG → GL

1.59 × 10

-2

-2

-1

7.39 × 10-1

k-3

GL → MG

5.00 × 10-2

8.00 × 10-2

1.00 × 10-1

5.61 × 10

6.00 × 10-2

1.50 × 10 1.86 × 10

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Having determined the corresponding reaction rate constants, the temperature dependence reaction rate constant is modeled by Arrhenius equation as presented in Eq. 12. The Arrhenius plots of the individual rate constants for the transesterification reaction of canola oil in supercritical methanol are presented in Fig. 8. The logarithm of the overall reaction rate constants was linear with the inverse temperatures as expected, demonstrating that the transesterification reaction of canola oil to biodiesel in supercritical methanol follows the Arrhenius equation. ki (T) = Ae-Ea/RT

(3)

where, ki = reaction rate constant [dm3mol-1min-1], A = Pre-exponential factor[dm3mol-1min-1], Ea = Activation energy [kJmol-1], R = Universal molar gas constant [kJmol-1K-1], T = Temperature [K].

Fig. 8. Arrhenius plots of the individual reaction rate constant for the transesterification reaction of canola oil to FAME in supercritical methanol using spiral reactor (experimental conditions: 270‒400 °C, 20 MPa, oil-to-methanol molar ratio of 1:40)

Table 3 shows the activation energy (Ea) and pre-exponential factor (A) for the detailed kinetic analysis of biodiesel production in supercritical methanol. Activation energies of canola oil conversion to biodiesel in supercritical methanol were determined between 16.32 and 99.02 kJ mol -1. The activation energies obtained in this study are consistent with the values obtained by Varma and Madras8. They determined activation energy of castor and linseed oil conversion in SCM (200‒350 °C and 20 MPa) using batch reactor, obtaining activation energies of 35.00 and 46.50 kJ mol-1 for castor and linseed oil, respectively. Table 3. Activation energies and pre-exponential factors. Reaction rate constant

Reaction direction

Activation energy,

Pre-exponential factor,

[dm3 mol-1 min-1]

Ea [kJ mol-1]

A [dm3 mol-1 min-1]

k1

TG → DG

99.02

4.50 × 107

0.99

k-1

DG → TG

73.49

3.06 × 104

0.99

5

0.99

R2

k2

DG → MG

73.73

2.10 × 10

k-2

MG → DG

78.36

1.10 × 106

0.99

k3

MG → GL

87.18

4.20 × 106

0.99

k-3

GL → MG

16.32

1.85

0.99

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4. Conclusion Biodiesel production in supercritical methanol using a novel spiral reactor was successfully investigated. In the spiral reactor case, FAME yield increased with temperature compared to the batch reactor case that could be attributed to the heating and cooling rates. The temperature and reaction time effect of FAME yield were examined, and the result showed that the yield of FAME increased with temperature and reaction time. The highest yield of biodiesel (100 wt%) can be obtained by transesterification of canola oil in supercritical methanol in a short reaction time of 10 min at 350 °C and oil-to-methanol molar ratio of 1:40 under a reactor pressure of 20 MPa. The secondorder kinetic model for canola oil conversion to FAME in supercritical methanol was proposed, and a good agreement between experimental and predicted values was observed. The activation energies determined in this work for the conversion of TG to DG, DG to TG, DG to MG, MG to DG, MG to GL, and GL to MG were about 99.02, 73.49, 73.73, 78.36, 87.18, and 16.32 kJ mol-1, respectively.

Acknowledgements The writer (OF) would like to acknowledge Japan-Indonesia Presidential Scholarship (JIPS), The World Bank Institute for supporting his study in Japan.

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