Energy analysis for the production of biodiesel in a spiral reactor using supercritical tert-butyl methyl ether (MTBE)

Energy analysis for the production of biodiesel in a spiral reactor using supercritical tert-butyl methyl ether (MTBE)

Bioresource Technology 196 (2015) 65–71 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/b...

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Bioresource Technology 196 (2015) 65–71

Contents lists available at ScienceDirect

Bioresource Technology journal homepage: www.elsevier.com/locate/biortech

Energy analysis for the production of biodiesel in a spiral reactor using supercritical tert-butyl methyl ether (MTBE) Obie Farobie, Yukihiko Matsumura ⇑ 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  Energy analyses for supercritical

MTBE biodiesel production in a spiral reactor.  Net energy ratio (NER) of 0.92 and energy efficiency of 0.98 were obtained.  The production of biodiesel in a spiral reactor is an energy-efficient process.  Utilization of spiral reactor improves the energy requirement for the process.

g r a p h i c a l a b s t r a c t Pressure pump 1, Heat to attain reaction 0.02% Pressure pump 2, temperature, 6.23% 0.08%

GTBE, 17.14%

MTBE, 18.26%

Net energy ratio (NER) = 0.92 Energy efficiency = 0.98

Energy input

Canola oil, 75.40%

Biodiesel, 82.86%

Heat loss Biodiesel

Oil

Energy output

Recovered heat GTBE MTBE MTBE

a r t i c l e

i n f o

Article history: Received 4 June 2015 Received in revised form 14 July 2015 Accepted 16 July 2015 Available online 21 July 2015 Keywords: Biodiesel Energy balance Energy efficiency Spiral reactor Supercritical fluid

a b s t r a c t In this study, energy analysis was conducted for the production of biodiesel in a spiral reactor using supercritical tert-butyl methyl ether (MTBE). This study aims to determine the net energy ratio (NER) and energy efficiency for the production of biodiesel using supercritical MTBE and to verify the effectiveness of the spiral reactor in terms of heat recovery efficiency. The analysis results revealed that the NER for this process was 0.92. Meanwhile, the energy efficiency was 0.98, indicating that the production of biodiesel in a spiral reactor using supercritical MTBE is an energy-efficient process. By comparing the energy supply required for biodiesel production between spiral and conventional reactors, the spiral reactor was more efficient than the conventional reactor. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction The search for energy alternatives has been driven by the increasing demand of fuel, depletion of fossil fuels, and issues of climate change. Biodiesel is one of the more notable forms of renewable energy as it exhibits better biodegradability (Zhang et al., 1998) as well as lower particulate matter, CO, and unburned hydrocarbon (Nabi et al., 2006; Canakci, 2007; Kegl, 2008) as well as a higher cetane number (Can, 2014) and lower sulfur content (Moser and Vaughn, 2010). ⇑ 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.2015.07.049 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.

Biodiesel is mainly produced by transesterification of triglycerides (TGs) with short-chain alcohols such as methanol and ethanol. Thus far, numerous methods have been employed for producing biodiesel, namely homogeneous acidand alkali-catalyzed transesterification (Vicente et al., 2004; Ye et al., 2010), heterogeneous-acid- and alkali-catalyzed transesterification (Semwal et al., 2011; Kazembe-Phiri et al., 2010), enzymatic -catalyzed transesterification (Hama and Kondo, 2013; Ranganathan et al., 2008), as well as under non-catalytic supercritical conditions (Saka and Kusdiana, 2001; Lim and Lee, 2013). Among these methods, the last one is the most outstanding one as it exhibits several advantages, such as high yield of biodiesel within a short residence time, easier separation, no generation of

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waste water, no requirement of catalyst, and applicability to various feedstocks (Kusdiana and Saka, 2001; Goembira et al., 2012). A new route for producing biodiesel using supercritical tert-butyl methyl ether (MTBE) has been developed (Farobie et al., 2014). By this method, fatty acid methyl esters (FAME) and glycerol tert-butyl ether (GTBE) were obtained. In previous studies, GTBE has been reported to exhibit a positive effect when combined with diesel fuel, which can improve its quality, as GTBE can enhance the cetane number, reduce the particulate matter and CO, and decrease the cloud point of diesel fuel (Klepácˇová et al., 2007; Frusteri et al., 2009). As the production of biodiesel under supercritical conditions requires high temperature and high pressure, an appropriate technology with the possibility of heat recovery is needed. Hence, in a previous study, a spiral reactor has been proposed for biodiesel production (Farobie et al., 2015). This spiral reactor comprised two parts: a heat exchanger and the reactor. The heat exchanger consists of a pair of tubes placed side-by-side and connected to each other. On the other hand, the reactor is a single tube, which is thermally controlled by an electric heater. As compared to the conventional flow reactor, the spiral reactor is reported to exhibit good performance for biodiesel production, affording a higher FAME yield at the same residence time. In addition, the spiral reactor is effective for the production of biodiesel using MTBE under high-temperature and high-pressure conditions because of successful heat recovery (Farobie and Matsumura, 2015b). However, a thorough energy analysis of the production of biodiesel in the spiral reactor using supercritical MTBE has not been sufficiently conducted, because of which there could be uncertainties in the effectiveness of spiral reactor technology for the production of biodiesel using supercritical MTBE. Thus, an appropriate analysis of biodiesel is imperative for the development of its production by employing this new method using supercritical MTBE. Several previous studies have attempted to investigate energy analysis for the production of biodiesel derived from various feedstocks such as palm oil, waste cooking oil, and microalgae by the conventional method (Kamahara et al., 2010; Mohammadshirazi et al., 2014; Xu et al., 2011; Chowdhury et al., 2012; Khoo et al., 2013). However, to the best of authors’ knowledge, energy analysis for the production of biodiesel using supercritical MTBE has not been reported. Hence, this new process needs to be evaluated in terms of energy analysis for the production of biodiesel using supercritical MTBE. This study aims to determine the net energy ratio (NER) and energy efficiency for the production of biodiesel using supercritical MTBE and to verify the effectiveness of the spiral reactor in terms of heat recovery efficiency. For this purpose, the energy balance for the production of biodiesel using supercritical MTBE was calculated. In addition, the energy efficiency between conventional and spiral reactors was compared, as well as the energy efficiency of the new route (using supercritical MTBE) for the production of biodiesel using supercritical methanol and supercritical ethanol.

2. Methods 2.1. Biodiesel production The experimental apparatus and a detailed schematic of the spiral reactor have been reported in the previous study (Farobie et al., 2015). In brief, the apparatus consists of a pump, the spiral reactor, thermocouples, heat transfer cement, a ceramic micro heater, a thermal insulator, a filter, and a back-pressure regulator. This reactor was made of a stainless-steel tubing (SS316) with outer and inner diameters of 3.17 and 2.17 mm, respectively. The reactor is composed of a heat exchanger, which comprises a pair of tubes

placed side-by-side in spiral formation, and the reactor is composed of single-insulated tubing. Thermocouples were used to measure the temperature inside the spiral reactor. The lengths of the heat exchanger and reactor were 2.5 and 10.0 m, respectively. First, feedstocks consisting of canola oil and MTBE were fed into the spiral reactor at the desired temperature. Second, the pressure was increased to 10 MPa using the back-pressure regulator. To ensure a steady-state condition, the feedstocks were fed into the system for 1 h before samples were collected. Finally, the products obtained were collected after passing through the filter and back-pressure regulator.

2.2. Analysis The products were analyzed by a gas chromatograph (GC-390B; GL Sciences) equipped with an MET-Biodiesel column (Sigma Aldrich, 28668-U) and a flame-ionization detector. Argon was used as the carrier gas. The detailed explanation of this analysis has been reported previously (Farobie et al., 2014). The FAME yields from the experiments were calculated by dividing the moles of the FAME product by the moles of the fatty acid groups in the initial TGs.

2.3. Calculation of mass and energy balance Under the steady-state condition, the mass of feedstock must be equal to the mass of product. Eq. (1) represents the general equation for the calculation of mass balance.

Massin þ Massgenerated ¼ Massout þ Massconsumed

ð1Þ

The calculation of mass balance is based on the optimum yield of biodiesel obtained (complete conversion to biodiesel was attained at 385 °C, 10 MPa, with an oil-to-MTBE molar ratio of 1:40, and at a residence time of 20 min). The energy efficiency (ge) of biodiesel production using supercritical MTBE was determined using Eq. (2). Meanwhile, the NER of this process was adapted from a previous study by Fore et al. (2011), as shown in Eq. (3).

ge ¼

Ep Ef

NER ¼

ð2Þ

Ep Ei

ð3Þ

Here, Ep represents the total energy in the products (MJ/d), Ef represents the total energy in the feedstock (MJ/d), and Ei represents the total primary energy inputs, including energy from feedstock, heat required to attain the reaction temperature, and pressure pump (MJ/d). In this study, the calculation of energy balance exclusively depends on the biodiesel production step; thus, inputs from steps that are not included in the biodiesel production system, such as steps of cultivation and oil extraction, are not included in this system. Fig. 1 shows the system boundary for the production of biodiesel in a spiral reactor using supercritical MTBE. In addition, the energy balance calculation is also based on the optimum yield of biodiesel obtained in this study. Table 1 shows the lower heating values (LHVs) of canola oil, MTBE, biodiesel, and GTBE obtained from previous studies.

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Canola oil

Heat to attain temperature Pressure Pump 1 (oil)

GTBE

Transesterification using supercritical MTBE

Pressure Pump 2 (MTBE)

Biodiesel Remaining MTBE

MTBE

Fig. 1. System boundary for the production of biodiesel using supercritical MTBE.

3. Results and discussion 3.1. Mass and energy balance As mentioned previously, the calculation of mass balance is based on the optimum condition of the production of biodiesel using supercritical MTBE. The complete conversion of canola oil to FAME was attained at 385 °C, 10 MPa, with an oil-to-MTBE molar ratio of 1:40, and a residence time of 20 min. When 0.2252 and 0.7399 kg/d of canola oil and MTBE, respectively, were fed into the system, 0.2288 and 0.0578 kg/d of biodiesel and GTBE were obtained, respectively, and 0.6785 kg/d of the remaining MTBE was obtained. Fig. 2(a) shows the result of the mass balance calculation for the production of biodiesel using supercritical MTBE. As observed, the total mass of the feedstock is the same as the total mass of the product, which is expected. In the case of the energy balance calculation, the energy input for the production of biodiesel using supercritical MTBE was obtained from the LHVs of canola oil and MTBE, heat required to attain the reaction temperature, and pressure pumps 1 and 2. Meanwhile, the total energy output was calculated from the LHVs of biodiesel, GTBE, and the remaining MTBE. The energies of canola oil and MTBE, as obtained from their LHVs, were 8.8939 and 2.1544 MJ/d, respectively. In addition, to produce the optimum yield of biodiesel, the energy required to attain the reaction temperature was calculated using Eq. (4) to be 0.7344 MJ/d.

Q heating ¼ moil C p;oil DT þ mMTBE C p;MTBE DT

ð4Þ

Here, moil represents the mass flow rate of canola oil (0.2252 kg/d), Cp,oil represents the specific heat of canola oil (1.913 kJ/(kg K)), DT represents the temperature difference between the inlet and outlet of the cold tube of the heat exchanger (321 °C), mMTBE represents the mass flow rate of MTBE (0.7399 kg/d), and Cp,MTBE represents the specific heat of MTBE (2.51 kJ/(kg K)). Moreover, the energies from pressure pumps 1 and 2 were calculated using Eq. (5) to be 0.0025 and 0.0100 MJ/d, respectively.

Wt ¼ v DP

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Here, v represents the volumetric flow rate of canola oil for pump 1 and of MTBE for pump 2 (m3/s), and DP represents the pressure increase (Pa). Fig. 2(b) shows the result of the energy balance calculation for the production biodiesel using supercritical MBTE. In addition, Table 2 summarizes energy-related feedstock inputs, energy caused by the heat required to attain the reaction temperature, and various energy outputs in the biodiesel production system. From this table, the total energy feedstock (Ef) consisting of canola oil and MTBE was determined to be 11.0483 MJ/d. In addition, taking into account the energy from canola oil and MTBE as feedstocks, heat to attain the reaction temperature, and pressure pumps, the total energy input (Ei) was 11.80 MJ/d. Meanwhile, the total energy product (Ep) consisting of energy from biodiesel and GTBE was 10.87 MJ/d. Hence, by employing Eqs. (2) and (3), the energy efficiency and NER obtained were 0.98 and 0.92, respectively, indicating that the production of biodiesel in a spiral reactor using supercritical MTBE is an energy-efficient process. Fig. 3(a) shows the percentage of energy consumed during the production of biodiesel using supercritical MTBE. As can be seen, the highest contribution to energy consumption for the production of biodiesel using supercritical MTBE was from canola oil (75.40%) and MTBE (18.26%). Their contribution amounted to approximately 93.66% of the total energy inputs. The highest contribution of energy input from oil as a feedstock was in good agreement with the result obtained from the study of Mohammadshirazi et al. (2014); they have reported that oil contributes to the highest share of energy input (77.08%) for biodiesel production. The second contribution toward energy input is from MTBE, caused by the fact that high amounts of MTBE are required for biodiesel production. Several studies have reported that it is common to add a reactant in excess of the stoichiometric requirement during transesterification under supercritical conditions to ensure a complete reaction (Silva et al., 2007; Trentin et al., 2011; Santana et al., 2012). Moreover, analysis has shown that high temperature and high pressure do not significantly contribute to energy consumption. This result was in good agreement with the result obtained from the study of Glišic´ et al. (2009). They have reported that the production of biodiesel under supercritical conditions, i.e., at higher temperature and higher pressure, does not significantly increase the total energy consumption for biodiesel production. Fig. 3(b) shows the percentage of energy produced throughout the production of biodiesel using supercritical MTBE. The highest contribution toward energy production was from biodiesel (82.86%) followed by GTBE (17.14%). The energy output–input ratio is one of the important indicators for maintaining the energy efficiency for the production of biodiesel under supercritical conditions. An energy efficiency of 0.98 for the production of biodiesel using supercritical MTBE indicates that the process is energetically efficient. In addition, an NER of 0.92 indicates that for every megajoule of energy consumed to produce biodiesel, 0.92 MJ of energy was obtained.

ð5Þ 3.2. Comparison of energy efficiency between spiral and conventional reactors

Table 1 Lower heating values used for energy balance calculation. Material Canola oil MTBE Biodiesel GTBE

Lower heating value (LHV)

Reference 1

7700 cal g1

39.49 MJ kg 35.1 MJ kg1 39.369 MJ kg1 32.2168 MJ kg1

Roy et al. (2013) Raman et al. (2014) Roy et al. (2013) Frusteri et al. (2012)

It should be stated here that even though the effectiveness of spiral reactor for biodiesel production has been previously studied by the same authors, the comparison of energy efficiency between spiral and conventional reactors for biodiesel production using supercritical MTBE has not been reported. To compare the energy efficiency between spiral and conventional reactors, the energy supply required for the production of biodiesel in both reactors was determined. The calculation was

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(a)

GTBE 0.0578 kg/d Canola oil 0.2252 kg/d

Transesterification using supercritical MTBE

MTBE 0.0614 kg/d

Biodiesel 0.2288 kg/d Remaining MTBE 0.6785 kg/d

8.8939 MJ/d

(b)

0.7344 MJ/d

Heat to attain temperature

0.0025 MJ/d

Pressure Pump 1 (oil)

0.0100 MJ/d

1.8626 MJ/d

Canola oil GTBE

9.0069 MJ/d

Transesterification using supercritical MTBE

Biodiesel Remaining MTBE

Pressure Pump 2 (MTBE)

MTBE 23.8151 MJ/d 2.1544 MJ/d

Fig. 2. (a) Mass balance and (b) energy balance calculation of the production of biodiesel using supercritical MTBE.

Table 2 Details of the energy balance calculation for the production of biodiesel using supercritical MTBE. Material

Input Canola oil MTBE Heat required to attain reaction temperature Pressure pump 1 Pressure pump 2

Unit

Quantity Lower heating value (MJ kg1)

kg/d 0.2252 kg/d 0.0614

39.4900 35.1000

Energy (MJ/d)

8.8939 2.1544 0.7344 0.0025 0.0100

Total energy input, Ei (MJ/d)

11.80

Output Biodiesel GTBE

kg/d 0.2288 kg/d 0.0578

Total energy product, Ep (MJ/d)

10.87

Net energy ratio (NER) Energy efficiency (ge)

0.92 0.98

39.3690 32.2168

9.0069 1.8626

performed on the basis of the schematics in Figs. 4 and 5 for the spiral and conventional reactors, respectively. Qheating was calculated using Eq. (4) to be 0.7344 MJ/d. In addition, Qrecovery by the spiral reactor was determined using Eq. (6) to be 0.6273 MJ/d.

Q recov ery ¼ mBDF C p;BDF DT þ mGTBE C p;GTBE DT þ mMTBE C p;MTBE DT

ð6Þ

Here, Qrecovery represents the energy from the recovered heat (MJ/d), mBDF represents the mass flow rate of biodiesel fuel (0.2288 kg/d), Cp,BDF represents the specific heat of biodiesel fuel (1.80 kJ/(kg K)), DT represents the temperature difference between the inlet and outlet of the hot tube of the heat exchanger (279 °C), mGTBE represents the mass flow rate of GTBE (0.0578 kg/d), and Cp,GTBE represents the specific heat of GTBE (2.31 kJ/(kg K)), mMTBE represents the mass flow rate of the remaining MTBE (0.6785 kg/d), and Cp,MTBE represents the specific heat of MTBE (2.51 kJ/(kg K)). The total energy supply for the production of biodiesel in a spiral reactor using supercritical MTBE was determined by subtracting Qrecovery from Qheating, as shown in Eq. (7). The energy supply (Qsupply) was calculated to be 0.1071 MJ/d.

Q supply ¼ Q heating  Q recov ery

ð7Þ

Here, Qsupply represents the energy supply for the production of biodiesel using supercritical MTBE (MJ/d), Qheating represents the energy required to attain the reaction temperature (MJ/d), and Qrecovery represents the energy from the recovered heat (MJ/d). The efficiency of heat recovery of the spiral reactor employed here was calculated using Eq. (8) to be 85.4%.

Q eff ¼

Q recov ery  100% Q heating

ð8Þ

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(a) MTBE, 18.26 %

Heat to aain reacon temperature, 6.23%

Pressure pump 1, Pressure pump 2, 0.02% 0.08%

Canola oil, 75.40%

(b)

GTBE, 17.14%

Biodiesel, 82.86% Fig. 3. (a) Percentage of energy input and (b) percentage of energy output for the production of biodiesel using supercritical MTBE.

Energy supply

Heat loss

Oil Biodiesel

Recovered heat

GTBE

MTBE

MTBE Fig. 4. Schematic of the production of biodiesel in a spiral reactor using supercritical MTBE.

Here, Qeff represents the heat recovery efficiency (%), Qrecovery represents the energy from the recovered heat (MJ/d), and Qheating represents the energy required to attain the reaction temperature (MJ/d). For the production of biodiesel in a conventional flow reactor using supercritical MTBE, the energy supply of this system was only taken from the energy required to attain the reaction

temperature, as shown in Fig. 5, because there is no heat recovered for the production of biodiesel in a conventional reactor. The energy supply for the production of biodiesel in a conventional reactor using supercritical MTBE was calculated to be 0.7344 MJ/d, which is significantly higher than that in a spiral reactor (0.1071 MJ/d). Thus, the spiral reactor is confirmed to be more efficient than the conventional reactor.

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Energy supply

Heat loss

Biodiesel Oil

GTBE MTBE

MTBE Fig. 5. Schematic of the production of biodiesel in a conventional reactor using supercritical MTBE.

SCM

0.855 0.855 0.854

Heat recovery efficiency (Qeff)

SCE SCMTBE

0.98 0.98 0.98

Energy efficiency ( e)

24.93

Total energy in the product (Ep) [MJ/d]

8.31

Total energy in the feedstock (Ef) [MJ/d]

8.51

10.87

25.45 11.05

0.00

5.00

10.00

15.00

20.00

25.00

30.00

Fig. 6. Comparison of energy efficiencies for the production of biodiesel using supercritical methanol (SCM), supercritical ethanol (SCE), and supercritical MTBE (SCMTBE). Table 3 Comparison of net energy ratio (NER) for supercritical methanol (SCM), supercritical ethanol (SCE), and supercritical MTBE (SCMTBE). Material

Supercritical methanol (SCM)

Supercritical ethanol (SCE)

Mass flow Lower heating Energy rate (kg/d) value (MJ kg1) (MJ/d)

Mass flow Lower heating Energy Mass flow Lower heating Energy rate (kg/d) value (MJ kg1) (MJ/d) rate (kg/d) value (MJ kg1) (MJ/d)

Input Canola oil 0.5976 Reactant 0.0667 Heat required to attain the reaction temperature Pressure pump 1 Pressure pump 2 Total energy input (Ei) (MJ/d)

26.58

Output Biodiesel Byproduct

0.6038 0.0605

39.4900 27.7308

23.5992 0.2066 1.8495 0.0126 1.0950 0.0133 0.0240

39.4900 27.7308

8.98

39.3690 19.2000

23.7722 0.2033 1.1609 0.0160

Supercritical MTBE (SCMTBE)

8.1602 0.2252 0.3490 0.0614 0.4544 0.0046 0.0110

39.3690 19.2000

8.0018 0.2288 0.3066 0.0578

24.93

8.31

10.87

Net energy ratio (NER)

0.94

0.93

0.92

The energy efficiency of this process was also compared with those obtained using supercritical methanol and supercritical ethanol. Again, the energy analysis calculation was based on the optimum conditions for the production of biodiesel from supercritical methanol (SCM), supercritical ethanol (SCE), and supercritical MTBE (SCMTBE). The optimum conditions for the

8.8939 2.1544 0.7344 0.0025 0.0100

39.3690 32.2168

9.0069 1.8626

11.80

Total energy product (Ep) (MJ/d)

3.3. Comparison of the new route for the production of biodiesel using supercritical MTBE with those using supercritical methanol and supercritical ethanol

39.4900 35.1000

production of biodiesel using supercritical methanol were 350 °C, 20 MPa, and a residence time of 10 min. Meanwhile, the optimum conditions for the production of biodiesel using supercritical ethanol were 350 °C, 20 MPa, and at a residence time of 30 min (Farobie and Matsumura, 2015a). Fig. 6 shows the result obtained for the energy efficiency calculated for SCM, SCE, and SCMTBE. As can be seen, the heat recovery efficiency of the spiral reactor as well as energy efficiencies for all processes were the same, i.e., 0.85 and 0.98, respectively. Even though different reactants were used to produce biodiesel in the same spiral reactor, the heat recovery

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efficiency of the spiral reactor as well as energy efficiency of the system were the same. The NER values of SCM, SCE, and SCMTBE were compared, and Table 3 shows these results. The NER values of SCM, SCE, and SCMTBE were 0.94, 0.93, and 0.92, respectively, indicating no significant difference between them. We would like to add some discussion on applicability and limitations of the results at the end of manuscript. The preceding results gave a baseline estimation of the net energy ratio and energy efficiency of biodiesel production in a spiral reactor using supercritical MTBE. This finding can be applied for another route of biodiesel production, i.e., supercritical methanol, ethanol, methyl acetate, and dimethyl carbonate. Even though biodiesel production in a spiral reactor using supercritical MTBE have enormous benefits as discussed previously, there are still several limitations that need to be addressed before this technology could play a vital role in industrial applications. Those include safety issue during the operation process and its cost. Since supercritical MTBE technology requires high temperature and high pressure, the safety issue is still in debate. In addition, from economic point of view, this technology requires high cost which involved expensive expenditure in furnaces and high pressure pumps. Furthermore, the cost to fabricate a huge spiral reactor for commercialization purposes will be enormous. Some strategies to reduce the cost as well as to minimize risk include the use of small-scale continuous biodiesel production, integration of process, and utilization of waste or non-edible oil. To quantitatively prove the applicability of this technology, the technical and economic feasibility are required for further study.

4. Conclusion Energy analysis for the production of biodiesel in a spiral reactor using supercritical MTBE was conducted. The novelty of this work highlights the investigation of thorough energy analysis for biodiesel production in the spiral reactor using supercritical MTBE which has not been previously reported. A net energy ratio (NER) of 0.92 and an energy efficiency of 0.98 were obtained, indicating that the production of biodiesel in a spiral reactor using supercritical MTBE is an energetically efficient process. The use of spiral reactor for the production of biodiesel using supercritical MTBE improves the energy requirement for the process from 0.7344 MJ/d to 0.1071 MJ/d caused by heat recovery. Heat recovery efficiency and energy efficiency values obtained for supercritical methanol, ethanol, and MTBE using the same spiral reactor were similar. Acknowledgement The authors would like to acknowledge Hiroshima University for financial support in the form of a research grant. References 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. Canakci, M., 2007. Combustion characteristics of a turbocharged DI compression ignition engine fueled with petroleum diesel fuels and biodiesel. Bioresour. Technol. 98, 1167–1175. Chowdhury, R., Viamajala, S., Gerlach, R., 2012. Reduction of environmental and energy footprint of microalgal biodiesel production through material and energy integration. Bioresour. Technol. 108, 102–111. Farobie, O., Matsumura, Y., 2015a. A comparative study of biodiesel production using methanol, ethanol, and tert-butyl methyl ether (MTBE) under supercritical conditions. Bioresour. Technol. 191, 306–311.

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