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Ultrasound assisted biodiesel production in presence of dolomite catalyst
Fuel 180 (2016) 624–629
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Full Length Article
Ultrasound assisted biodiesel production in presence of dolomite catalyst Ibrahim Korkut ⇑, Mahmut Bayramoglu Gebze Technical University, Chemical Eng. Department, 41400 Kocaeli, Turkey
h i g h l i g h t s Dolomite is an alternative catalyst for ultrasound assisted biodiesel synthesis. High biodiesel yield is obtained with 20 kHz ultrasound. Suitable reaction temperature is determined as 60 °C. Best results are obtained with methanol/oil molar ratio of 9/1.
a r t i c l e
i n f o
Article history: Received 20 October 2015 Received in revised form 10 April 2016 Accepted 21 April 2016
Keywords: Ultrasound Dolomite CaO Biodiesel synthesis Heterogeneous catalyst
a b s t r a c t Ultrasound (US) assisted transesterification of canola oil in presence of heterogeneous catalysts calcined dolomite and CaO was investigated in comparison to each other. An US generator (200 W, 20 kHz) equipped with an horn type probe (19 mm) was used to study the effect of catalyst amount (3–7 wt.% of oil), methanol/oil molar ratio (4/1–15/1), ultrasound power (30–50 W), temperature (25–60 °C) and time (60–120 min) on US assisted biodiesel synthesis. Biodiesel yield reached over 97.4% for calcined dolomite at the end of 90 min and 95.5% for CaO at the end of 75 min. According to the results, US improved the transesterification reaction by reducing necessary time for high biodiesel yield, using calcined dolomite as well CaO as heterogeneous catalyst. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Biodiesel is renewable and environmentally friendly fuel which can be produced by the transesterification of triglycerides (TG) with a primary alcohol, commonly methanol, in the presence of a base or acid catalyst into fatty acid methyl ester (FAME) or fatty acid ethyl ester (FAEE) respectively. A large number of different heterogeneous catalysts have been synthesized and used for biodiesel production in the last few years [1–5]. Although heterogeneous catalysts are designated for higher activity and selectivity, they are not usually competitive with homogeneous ones due to especially slower physical/chemical steps of the reaction mechanism [6] which may be accelerated by various means such as the application of power US. As known, US is a sound wave with a frequency beyond the human audibility limits (approx. 20 kHz) [7,8]. In industry, US devices find wide applications, operating with frequencies up to several GHz. ⇑ Corresponding author. E-mail address: [email protected] (I. Korkut). http://dx.doi.org/10.1016/j.fuel.2016.04.101 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.
Biodiesel production occurs in a two-phase system (liquid/liquid) or a three-phase system (liquid/liquid/solid) depending on the catalyst type. In such multi-phase systems, US may accelerate various steps of the reaction by providing the mechanical/physical energy and supplying the required activation energy for initiating the reactions [8]. US have various physical and chemical effects on reaction systems; the chemical effect arises from the cavitation phenomena in the liquid bulk medium, generating various radicals such as OH, H during a transient implosive collapse of bubbles [8,9]. On the other hand, the physical effects occur by virtue of micro-turbulences generated due to radial motion of bubbles which leads to inmate mixing of the immiscible liquid/solid phases in the reaction medium. Thus, the interfacial region between oil and methanol intensively increases which results in faster reaction kinetics and higher reaction yield. The disperse phase droplets formed using cavitation are usually smaller size and more stable [8,10]. On the other hand, in the case of heterogeneous catalysis, US enhances mass transfer to or from catalyst surface by means of eddy jets created by micro-turbulences; in biodiesel synthesis, the solid catalyst is usually hydrophilic and mostly suspended in
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the methanol phase and due to the high viscosity of the oil phase the physical effects of ultrasonic cavitation are more effective in methanol phase [11]. Furthermore, US can also disintegrate the solid catalyst into smaller particles to create new active sites for the reaction [12]. Among various heterogeneous catalysts proposed for biodiesel production, CaO is well known for its high activity, high alkalinity and poor solubility in methanol which makes it an ideal solid base catalyst for biodiesel production [11]. In literature, there are many studies that investigate the silent or US assisted biodiesel production using CaO, giving detailed information about CaO activity [13– 17]. On the other hand, dolomite is an anhydrous carbonate mineral with the formula CaMg(CO3)2 [18]. Except some few studies, biodiesel production using dolomite is not thoroughly investigated and also there is not a detailed study on US assisted biodiesel production by transesterification using dolomite as solid catalyst. Thus, the aim of this work was to investigate the effects of ultrasound on the heterogeneously catalyzed transesterification reaction. For this purpose, two different catalysts namely dolomite and CaO were used and herewith the catalytic activities of CaO and Dolomite were compared under ultrasonic irradiation.
Fig. 1. Scheme of experimental setup.
Table 1 Variables and numeric values.
2. Experimental 2.1. Chemicals Commercial edible grade canola oil (mw: 876.6 g/mol) was obtained from market and was used without purification. Methanol (MerckÒ reagent of 99.9% purity) was used as received. CaCO3 (Daejung ChemicalsÒ extra pure) was used after calcination. Dolomite was obtained from Rizalar Mosaic Industry and Trade Limited Company located in Gebze/Kocaeli, Turkey. The origin of dolomite was Marmara Island of Turkey. Dolomite was used after calcination. The chemical composition of dolomite was 23.50% Ca, 12.07% Mg, 63.02% CO2 and 1.41% other components [18]. CaCO3 was calcined at 840 °C for 3 h. After calcination, the product CaO produced according to reaction (1) was pulverized in the porcelain mortar and CaO powder was calcined again at 840 °C for 60 min to prevent absorption of CO2 and humidity during the pulverization process. Then, CaO was cooled to the room temperature in a desiccator under vacuum. The same calcination procedure was applied also for dolomite according to reaction (2).
CaCO3 ! CaO þ CO2
ð1Þ
CaMgðCO3 Þ2 ! CaO þ MgO þ 2CO2
ð2Þ
2.2. Experimental setup The experimental setup used for calorimetric and transesterification experiments is presented in Fig. 1. An ultrasonic (US) generator (BANDELIN Ò HD 2200 SONOPULS, 200 W, 20 kHz) equipped with a horn type probe was used to deliver pulsed ultrasound with controllable power. All transesterification experiments were conducted using the same reaction volume (250 ml) to ensure replicable acoustic power density dissipated into the solution. In the experimental plan; catalyst amount (wt.% of oil), methanol/oil ratio (mol/mol), temperature (°C), ultrasound electric power (W) and time (min) were taken into account. Numerical values of these variables are shown in Table 1. 2.3. Calorimetric experiments Calorimetric method was used to evaluate the energetic efficiency of the US transducer [9]. For this purpose, various liquids
a
Variables
Low level
Catalyst amount (wt.% of oil) Methanol/oil ratio (mol/mol) Time (min) Ultrasound power (W) Temperature (°C)
3 4 60 30 25
4 6 75 35 45
High level 5a 9a 90a 40a 55a
6 12 105 45
7 15 120 50 60
Initial experimental conditions.
of different chemical compositions namely; water, methanol, canola oil and methanol–canola oil mixtures (methanol/oil molar ratios: 6/1–9/1–12/1) were used in the experiments. 250 ml liquid at 20 °C was poured into thermally isolated 300 ml three necked cylindrical reactor and during US irradiation the time–temperature data was monitored online using J type thermocouple via a data acquisition card (DAQ, model NIÒ USB 6009). The electric power supplied to the ultrasonic generator system, was also measured by a digital-wattmeter and recorded online by a computer (the net electric power used by the piezoelectric transducer, Welec, was then calculated by subtracting the electric power consumed by US system in stand-by condition (5.1 W) from the power display of the wattmeter). The ultrasonic power dissipated into the solution was calculated by Eq. (3).
W cal ¼ ðC m M þ C r Þ ðdT=dtÞ
ð3Þ
where Cm is the heat capacity of sonicated liquid, M is the mass of liquid, Cr is the heat capacity of reactor (glass) vessel and (dT/dt) is the time dependent change of temperature of the liquid. 2.4. Transesterification experiments Transesterification reactions were carried out in a 300 ml threenecked cylindrical glass reactor equipped with a reflux condenser. Known weights of methanol and catalysis were poured into the reactor and the mixture was stirred magnetically. Then, canola oil was added to the mixture. At the start of each run, the heterogeneous mixture was sonicated at continuous mode to generate an initial homogeneous mixture until the reaction mixture reached to the set temperature by the heating effect of ultrasound and hotplate heating as well. In all experiments, the heterogeneous mixture was sonicated at pulsed mode (pulse ratio 0.9) which is beneficial for low electrical energy consumption and for easier control of reactor temperature and also for cooling the transducer [9]. To maintain stable and uniform cavitational activity in the
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sonochemical reactor, magnetic stirring was applied at 600 rpm. Due to the heating effect of US in the reaction medium, cooling was applied by circulating water in the reactor jacket. In this way, the reaction temperature was kept at the set point within ±1 °C. At the end of the reaction, the solid catalyst was separated by centrifuge and then the liquid mixture was filtered by blue band filter paper under vacuum. After filtration, the excess methanol remaining in the heterogeneous liquid mixture was separated by rotary evaporator under vacuum (100 mbar). The product liquid was put into a separating funnel and was kept at room temperature for one day, to separate into an upper layer (biodiesel phase) and a lower layer (glycerol phase). The FAME content of the biodiesel phase was determined by AgilentÒ 6890 Gas Chromatogram (GC) equipped with a flame ionization detector (FID) and capillary column (CARBOWAX 20M) following European regulated procedure EN 14103. A typical biodiesel GC spectrum is given in Fig. 2.
Table 2 Calorimetric results. Sonicated liquid
Eq. (4)
Corr. coeff. R2
Water
Wcal = 0.763, Welec 6.40 Wcal = 0.937, Welec 15.24 Wcal = 0.615, Welec 5.71 Wcal = 0.659, Welec 2.15 Wcal = 0.677, Welec 7.20 Wcal = 0.645, Welec 7.20
0.9939
Canola oil Methanol Methanol/oil (molar ratio:6/1) mixture Methanol/oil (molar ratio:9/1) mixture Methanol/oil (molar ratio:12/1) mixture
0.9790 0.9939 0.9919 0.9918 0.9750
3.2. Transesterification results 3. Results and discussion 3.1. Calorimetric experimental results Calorimetric experiments were conducted in various liquids at different power levels. As seen in Table 2, a linear relationship exists between Wcal and Welec;
W cal ¼ a W elec b
ð4Þ
The slope, a, refers to the energetic efficiency of US transducer while (b/a) corresponds to the minimum Welec to overcome the acoustic threshold for generating cavitation bubbles. Furthermore, Table 2 reveals that both parameters of Eq. (4) depend on the chemical composition of the liquid which in turn affects relevant physical properties of the sonicated medium (such as viscosity, vapor pressure and heat capacity). An increase in the temperature reduces the viscosity of the medium facilitating the propagation and dissipation of US wave. Meanwhile, at higher temperatures, it is expected that vaporous cavities are formed due to the presence of the volatile components such as methanol, which reduces the bubble collapse intensity and hence decreases the cavitation effects [10]. Fig. 3a shows that as the temperature rises during the irradiation, the electric power applied to the system and consequently US power delivered to the reaction medium decrease due to varying properties of the liquid as function of temperature. For this reason, electric energy supplied to the US system was monitored during transesterification reactions. For a typical run conducted at isothermal reaction conditions, the electric power versus time is shown Fig. 3b. As seen, electrical power was hold constant during the reaction within the standard deviation (2 W).
3.2.1. Effect of catalyst loading The effect of catalyst loading on biodiesel yield was investigated between 3 wt.% of oil and 7 wt.% of oil at the following experimental conditions; methanol/oil molar ratio: 9/1, temperature: 55 °C, reaction time: 90 min and US power: 40 W. As seen in Fig. 4a, in the case of dolomite the yield increases up to 5% catalyst loading to reach a maximum value in accordance with the fact that the transesterification rate increases with increasing number of active sites [10]. On the other hand, in the case of CaO, 3% catalyst loading is suitable for high biodiesel yield. The sharp decrease of yield with higher catalyst loadings (not normally expected) may be due to the increasing shielding effect of catalyst particles on US propagation in reaction medium, which causes attenuated mass transfer rates around catalyst particles. Furthermore, high catalyst loading induces also saponification side-reaction, especially for the case of basic catalyst [19]. Mootabadi et al. investigated the effect of US at 20 kHz and 200 W on biodiesel production using CaO catalyst. They found 77.3% biodiesel yield under the conditions; catalyst loading: 3 wt.% of oil, methanol/oil molar ratio: 9/1, temperature: 65 °C, time: 60 min [12]. 3.2.2. Effect of methanol/oil molar ratio According to the stoichiometry of transesterification reaction, one mole of triglyceride requires three moles of methanol. In the case of heterogeneous catalyst, the reversible transesterification reaction completely proceeds even at the stoichiometric molar ratio of 3/1, because side reactions are not promoted and the catalyst remains in the system [20]. In this study, slightly higher methanol/oil ratio (4/1) was selected as the lower limit in the
Fig. 2. GC spectrum of biodiesel sample.
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Fig. 3. (a) Temperature and electric power versus time (1-electric, 2-temperature). (b) Electric power versus time (at constant temperature).
Fig. 4. Effect of catalyst amount on the biodiesel yield. (a) Dolomite. (b) CaO (Transesterification conditions; methanol/oil mol ratio: 9/1, US power: 40 W, temperature: 55 °C, time: 90 min).
experimental plan, because the transesterification reaction proceeds dominantly in the methanol phase in contact with hydrophilic catalyst surface. On the other hand, in the case of high methanol/oil ratios, due to viscosity dependent dispersive effect of US, the problem of low dispersion of oil in large volume of methanol phase becomes important (in very high ratios such as 24/1, dispersion of methanol in oil phase may also be incomplete). As a result, the interfacial area between reactant phases reduces and so does the reaction rate [21]. By considering this point, the high level for methanol/oil molar ratio was selected as 15/1. Thus, the effect of on the biodiesel yield was studied at five different levels. As seen in Fig. 5, for both catalysts, with increasing methanol/oil molar ratio, the biodiesel yield increases steadily up to a maximum value at 9/1 methanol/oil ratio and then decreases. Similar results were obtained by other researchers. Mootabadi et al. investigated the effect of methanol/oil molar ratio on US assisted transesterification reaction; for CaO catalyst, the highest biodiesel yield was achieved at 12/1 methanol/oil molar ratio, while for BaO and SrO catalysts, suitable methanol/oil ratio was found as 9/1 [12]. Choudhury et al. identified optimum methanol/oil molar ratio as 10/1 using CaO catalyst [13].
3.2.3. Effect of ultrasonic power The effect of Welec. on biodiesel yield was investigated between 30 W and 50 W. As seen in Fig. 6, for both catalysts the yield increases by virtue of increasing number of cavitation bubbles and the extent of acoustic streaming which enhance dispersion and mass transfer effects of US. Thus, various steps of the reaction namely; the diffusion of methanol into the oil phase, the adsorption of methanol on catalyst surface active sites and desorption of methyl ester and glycerol from the catalyst sites were enhanced. Meanwhile, at high acoustic power levels too many bubbles are generated at the tip of the horn leading to acoustic decoupling and bubble coalescence which decreases the number of active cavitation bubbles [9,10]. According to Fig. 6, suitable power level is 45 W for dolomite catalyst and 40 W for CaO catalyst. 3.2.4. Effect of temperature Fig. 7 shows the effect of reaction temperature on biodiesel yield. Temperature speeds the reaction rate and increases the FAME yield. On the other hand, as previously described in calorimetric experiment results, an increase in the temperature reduces the overall cavitational effect and for this reason lower temperature is
Fig. 5. Effect of methanol/oil molar ratio on the biodiesel yield. (a) Dolomite (Transesterification conditions; catalyst amount: 5 wt.% of oil, US power: 40 W, temperature: 55 °C, time: 90 min). (b) CaO (Transesterification conditions; catalyst amount: 3 wt.% of oil, US power: 40 W, temperature: 55 °C, time: 90 min).
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Fig. 6. Effect of electric power on the biodiesel yield. (a) Dolomite (transesterification conditions; catalyst amount: 5 wt.% of oil, methanol/oil molar ratio: 9/1, temperature: 55 °C, time: 90 min). (b) CaO (Transesterification conditions; catalyst amount: 3 wt.% of oil, methanol/oil molar ratio: 9/1, temperature: 55 °C, time: 90 min).
Fig. 7. Effect of temperature on the biodiesel yield. (a) Dolomite (Transesterification conditions; catalyst amount: 5 wt.% of oil, methanol/oil molar ratio: 9/1, US power: 45 W, time: 90 min). (b) CaO (Transesterification conditions; catalyst amount: 3 wt.% of oil, methanol/oil molar ratio: 9/1, US power: 40 W, time: 90 min).
Fig. 8. Effect of ultrasound irradiation time on the biodiesel yield. (a) Dolomite (Transesterification conditions; catalyst amount: 5 wt.% of oil, methanol/oil molar ratio: 9/1, US power: 45 W, temperature: 60 °C). (b) CaO (Transesterification conditions; catalyst amount: 3 wt.% of oil, methanol/oil molar ratio: 9/1, US power: 40 W, temperature: 60 °C).
Fig. 9. Reusability of catalysis in the transesterification of canola oil. (a) Dolomite (Transesterification conditions; catalyst amount: 5 wt.% of oil, methanol/oil molar ratio: 9/1, US power: 45 W, temperature: 60 °C, time: 90 min). (b) CaO (Transesterification conditions; catalyst amount: 3 wt.% of oil, methanol/oil molar ratio: 9/1, US power: 40 W, temperature: 60 °C, time: 75 min).
suggested for US applications. The optimum operating temperature is dictated by counteracting effects of temperature on cavitational phenomena and on the chemical reaction rate [10]. According to
Fig. 7, a reaction temperature of 60 ° C close to boiling point of methanol (64.7° C) is suitable for both catalysts to ensure high biodiesel yield. Similar finding was obtained by other researchers [13].
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3.2.5. Effect of time The ultrasonic irradiation time is an important reaction parameter which is closely related to the total energy consumption of US assisted biodiesel production process. As seen in Fig. 8, at the end of 90 min biodiesel yield reached 97.4% for dolomite and at the end of 75 min the yield reached 95.5% for CaO. Analytical results showed that 90 min was sufficient to achieve nearly constant (equilibrium yield) for both catalysts. As comparative purpose, it is worth to cite a research conducted without US application (conventional stirring) by Ilgen who obtained 91.8% biodiesel yield with dolomite catalyst at the following experimental conditions; catalyst loading: 3 wt.% of oil, methanol/oil molar ratio: 6/1, reaction temperature 65 ° C, reaction time: 180 min [18]. The great advantage of US application is easily noticeable. The electrical energy consumed by US transducer for per unit gram of biodiesel (Jelec) was calculated using Eq. (5)
J elec ¼
W elec t x moil 1000
ð5Þ
Here Welec is the time average electric power (W) supplied to the transducer, t is the time (s), x is the conversion of TG to biodiesel and moil is the weight of the oil (g). Calculated Jelec values for dolomite and CaO were 1.51 kJ/g and 1.14 kJ/g respectively and this indicates that the catalyst type is an important factor in terms of energy saving. 3.2.6. Reusability of catalyst The reusability of the solid catalyst is of primordial importance when compared with homogeneous catalysts from technical, environmental and economic points of view. In these respects, to investigate the reusability of dolomite and CaO catalysis, the catalyst powder recovered by filtration at the end of the reaction was mixed with known amount of methanol, poured in the reactor and ultrasonically irradiated for 15 min. To compensate for the catalyst lost during the washing and filtration steps, 5% fresh catalyst was added into the ultrasonic reactor. Then, the predetermined amount of oil was added and the reaction was restarted. As shown in Fig. 9a dolomite can be reused 4 times in transesterification only with a 7% biodiesel yield loss. Furthermore, Fig. 9b reveals that CaO can be reused 4 times in transesterification with 8.4% biodiesel yield loss only. For comparison, Mootabadi et al. found approximately 10% yield loss after 3 repeated use of CaO catalyst [12]. Finally, solid catalyst reusability, until final disposal or reactivation step, have obviously important impact on the energy (calcination/reactivation) consumption per mass of total biodiesel produced and will determine the net advantage of solid catalysts over homogeneous ones. This subject needs further detailed examination in the context of energy and exergy analysis of ‘‘heterogeneously catalyzed and US assisted biodiesel production process”. 4. Conclusion Ultrasound assisted heterogeneously catalyzed of canola oil was studied using dolomite and CaO as the catalysts to produce the biodiesel. FAME yield reached 97.4% for dolomite at the end of 90 min
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and 95.5% for CaO at the end of 75 min. Suitable operating conditions for dolomite catalyst are: catalyst loading: 5 wt.% of oil, methanol/oil ratio molar ratio: 9/1, temperature: 60 °C, US power: 45 W and time: 90 min. Similarly, suitable operating conditions for CaO catalyst are as follows: catalyst loading: 3 wt.% of oil, methanol/oil molar ratio: 9/1, temperature: 60 °C, US power: 40 W and time: 75 min. References [1] Refaat AA. Biodiesel production using solid metal oxide catalysts. Int J Environ Sci Technol 2011;8:203–21. [2] Chouhan APS, Sarma AK. Modern heterogeneous catalysts for biodiesel production: a comprehensive review. Renew Sustain Energy Rev 2011;15:4378–99. [3] Santacesaria E, Vicente GM, Di Serio M, Tesser R. Main technologies in biodiesel production: state of the art and future challenges. Catal Today 2012;195:2–13. [4] Borges ME, Díaz L. Recent developments on heterogeneous catalysts for biodiesel production by oil esterification and transesterification reactions: a review. Renew Sustain Energy Rev 2012;16:2839–49. [5] Semwal S, Arora AK, Badoni RP, Tuli DK. Biodiesel production using heterogeneous catalysts. Bioresour Technol 2011;102:2151–61. [6] Salamatinia B, Hashemizadeh I, Ahmad Zuhairi A. Alkaline earth metal oxide catalysts for biodiesel production from palm oil: elucidation of process behaviors and modeling using response surface methodology. Iranian J Chem Chem Eng (IJCCE) 2013;32:113–26. [7] Badday AS, Abdullah AZ, Lee KT, Khayoon MS. Intensification of biodiesel production via ultrasonic-assisted process: a critical review on fundamentals and recent development. Renew Sustain Energy Rev 2012;16:4574–87. [8] Ramachandran K, Suganya T, Nagendra Gandhi N, Renganathan S. Recent developments for biodiesel production by ultrasonic assist transesterification using different heterogeneous catalyst: a review. Renew Sustain Energy Rev 2013;22:410–8. [9] Korkut I, Bayramoglu M. Various aspects of ultrasound assisted emulsion polymerization process. Ultrason Sonochem 2014;21:1592–9. [10] Gole VL, Gogate PR. A review on intensification of synthesis of biodiesel from sustainable feed stock using sonochemical reactors. Chem Eng Process 2012;53:1–9. [11] Choudhury HA, Goswami PP, Malani RS, Moholkar VS. Ultrasonic biodiesel synthesis from crude Jatropha curcas oil with heterogeneous base catalyst: mechanistic insight and statistical optimization. Ultrason Sonochem 2014;21:1050–64. [12] Mootabadi H, Salamatinia B, Bhatia S, Abdullah AZ. Ultrasonic-assisted biodiesel production process from palm oil using alkaline earth metal oxides as the heterogeneous catalysts. Fuel 2010;89:1818–25. [13] Choudhury HA, Chakma S, Moholkar VS. Mechanistic insight into sonochemical biodiesel synthesis using heterogeneous base catalyst. Ultrason Sonochem 2014;21:169–81. [14] Kouzu M, Hidaka J-S. Transesterification of vegetable oil into biodiesel catalyzed by CaO: a review. Fuel 2012;93:1–12. [15] Esipovich A, Danov S, Belousov A, Rogozhin A. Improving methods of CaO transesterification activity. J Mol Catal A: Chem 2014;395:225–33. [16] Reyero I, Arzamendi G, Gandía LM. Heterogenization of the biodiesel synthesis catalysis: CaO and novel calcium compounds as transesterification catalysts. Chem Eng Res Des 2014;92:1519–30. [17] Kouzu M, Kasuno T, Tajika M, Sugimoto Y, Yamanaka S, Hidaka J. Calcium oxide as a solid base catalyst for transesterification of soybean oil and its application to biodiesel production. Fuel 2008;87:2798–806. [18] Ilgen O. Dolomite as a heterogeneous catalyst for transesterification of canola oil. Fuel Process Technol 2011;92:452–5. [19] Li H, Niu S, Lu C, Li J. Calcium oxide functionalized with strontium as heterogeneous transesterification catalyst for biodiesel production. Fuel 2016;176:63–71. [20] Tsuji T, Kubo M, Shibasaki-Kitakawa N, Yonemoto T. Is excess methanol addition required to drive transesterification of triglyceride toward complete conversion? Energy Fuels 2009;23:6163–7. [21] Kalva A, Sivasankar T, Moholkar VS. Physical mechanism of ultrasoundassisted synthesis of biodiesel. Ind Eng Chem Res 2009;48:534–44.
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Full Length Article
Ultrasound assisted biodiesel production in presence of dolomite catalyst Ibrahim Korkut ⇑, Mahmut Bayramoglu Gebze Technical University, Chemical Eng. Department, 41400 Kocaeli, Turkey
h i g h l i g h t s Dolomite is an alternative catalyst for ultrasound assisted biodiesel synthesis. High biodiesel yield is obtained with 20 kHz ultrasound. Suitable reaction temperature is determined as 60 °C. Best results are obtained with methanol/oil molar ratio of 9/1.
a r t i c l e
i n f o
Article history: Received 20 October 2015 Received in revised form 10 April 2016 Accepted 21 April 2016
Keywords: Ultrasound Dolomite CaO Biodiesel synthesis Heterogeneous catalyst
a b s t r a c t Ultrasound (US) assisted transesterification of canola oil in presence of heterogeneous catalysts calcined dolomite and CaO was investigated in comparison to each other. An US generator (200 W, 20 kHz) equipped with an horn type probe (19 mm) was used to study the effect of catalyst amount (3–7 wt.% of oil), methanol/oil molar ratio (4/1–15/1), ultrasound power (30–50 W), temperature (25–60 °C) and time (60–120 min) on US assisted biodiesel synthesis. Biodiesel yield reached over 97.4% for calcined dolomite at the end of 90 min and 95.5% for CaO at the end of 75 min. According to the results, US improved the transesterification reaction by reducing necessary time for high biodiesel yield, using calcined dolomite as well CaO as heterogeneous catalyst. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Biodiesel is renewable and environmentally friendly fuel which can be produced by the transesterification of triglycerides (TG) with a primary alcohol, commonly methanol, in the presence of a base or acid catalyst into fatty acid methyl ester (FAME) or fatty acid ethyl ester (FAEE) respectively. A large number of different heterogeneous catalysts have been synthesized and used for biodiesel production in the last few years [1–5]. Although heterogeneous catalysts are designated for higher activity and selectivity, they are not usually competitive with homogeneous ones due to especially slower physical/chemical steps of the reaction mechanism [6] which may be accelerated by various means such as the application of power US. As known, US is a sound wave with a frequency beyond the human audibility limits (approx. 20 kHz) [7,8]. In industry, US devices find wide applications, operating with frequencies up to several GHz. ⇑ Corresponding author. E-mail address: [email protected] (I. Korkut). http://dx.doi.org/10.1016/j.fuel.2016.04.101 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.
Biodiesel production occurs in a two-phase system (liquid/liquid) or a three-phase system (liquid/liquid/solid) depending on the catalyst type. In such multi-phase systems, US may accelerate various steps of the reaction by providing the mechanical/physical energy and supplying the required activation energy for initiating the reactions [8]. US have various physical and chemical effects on reaction systems; the chemical effect arises from the cavitation phenomena in the liquid bulk medium, generating various radicals such as OH, H during a transient implosive collapse of bubbles [8,9]. On the other hand, the physical effects occur by virtue of micro-turbulences generated due to radial motion of bubbles which leads to inmate mixing of the immiscible liquid/solid phases in the reaction medium. Thus, the interfacial region between oil and methanol intensively increases which results in faster reaction kinetics and higher reaction yield. The disperse phase droplets formed using cavitation are usually smaller size and more stable [8,10]. On the other hand, in the case of heterogeneous catalysis, US enhances mass transfer to or from catalyst surface by means of eddy jets created by micro-turbulences; in biodiesel synthesis, the solid catalyst is usually hydrophilic and mostly suspended in
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the methanol phase and due to the high viscosity of the oil phase the physical effects of ultrasonic cavitation are more effective in methanol phase [11]. Furthermore, US can also disintegrate the solid catalyst into smaller particles to create new active sites for the reaction [12]. Among various heterogeneous catalysts proposed for biodiesel production, CaO is well known for its high activity, high alkalinity and poor solubility in methanol which makes it an ideal solid base catalyst for biodiesel production [11]. In literature, there are many studies that investigate the silent or US assisted biodiesel production using CaO, giving detailed information about CaO activity [13– 17]. On the other hand, dolomite is an anhydrous carbonate mineral with the formula CaMg(CO3)2 [18]. Except some few studies, biodiesel production using dolomite is not thoroughly investigated and also there is not a detailed study on US assisted biodiesel production by transesterification using dolomite as solid catalyst. Thus, the aim of this work was to investigate the effects of ultrasound on the heterogeneously catalyzed transesterification reaction. For this purpose, two different catalysts namely dolomite and CaO were used and herewith the catalytic activities of CaO and Dolomite were compared under ultrasonic irradiation.
Fig. 1. Scheme of experimental setup.
Table 1 Variables and numeric values.
2. Experimental 2.1. Chemicals Commercial edible grade canola oil (mw: 876.6 g/mol) was obtained from market and was used without purification. Methanol (MerckÒ reagent of 99.9% purity) was used as received. CaCO3 (Daejung ChemicalsÒ extra pure) was used after calcination. Dolomite was obtained from Rizalar Mosaic Industry and Trade Limited Company located in Gebze/Kocaeli, Turkey. The origin of dolomite was Marmara Island of Turkey. Dolomite was used after calcination. The chemical composition of dolomite was 23.50% Ca, 12.07% Mg, 63.02% CO2 and 1.41% other components [18]. CaCO3 was calcined at 840 °C for 3 h. After calcination, the product CaO produced according to reaction (1) was pulverized in the porcelain mortar and CaO powder was calcined again at 840 °C for 60 min to prevent absorption of CO2 and humidity during the pulverization process. Then, CaO was cooled to the room temperature in a desiccator under vacuum. The same calcination procedure was applied also for dolomite according to reaction (2).
CaCO3 ! CaO þ CO2
ð1Þ
CaMgðCO3 Þ2 ! CaO þ MgO þ 2CO2
ð2Þ
2.2. Experimental setup The experimental setup used for calorimetric and transesterification experiments is presented in Fig. 1. An ultrasonic (US) generator (BANDELIN Ò HD 2200 SONOPULS, 200 W, 20 kHz) equipped with a horn type probe was used to deliver pulsed ultrasound with controllable power. All transesterification experiments were conducted using the same reaction volume (250 ml) to ensure replicable acoustic power density dissipated into the solution. In the experimental plan; catalyst amount (wt.% of oil), methanol/oil ratio (mol/mol), temperature (°C), ultrasound electric power (W) and time (min) were taken into account. Numerical values of these variables are shown in Table 1. 2.3. Calorimetric experiments Calorimetric method was used to evaluate the energetic efficiency of the US transducer [9]. For this purpose, various liquids
a
Variables
Low level
Catalyst amount (wt.% of oil) Methanol/oil ratio (mol/mol) Time (min) Ultrasound power (W) Temperature (°C)
3 4 60 30 25
4 6 75 35 45
High level 5a 9a 90a 40a 55a
6 12 105 45
7 15 120 50 60
Initial experimental conditions.
of different chemical compositions namely; water, methanol, canola oil and methanol–canola oil mixtures (methanol/oil molar ratios: 6/1–9/1–12/1) were used in the experiments. 250 ml liquid at 20 °C was poured into thermally isolated 300 ml three necked cylindrical reactor and during US irradiation the time–temperature data was monitored online using J type thermocouple via a data acquisition card (DAQ, model NIÒ USB 6009). The electric power supplied to the ultrasonic generator system, was also measured by a digital-wattmeter and recorded online by a computer (the net electric power used by the piezoelectric transducer, Welec, was then calculated by subtracting the electric power consumed by US system in stand-by condition (5.1 W) from the power display of the wattmeter). The ultrasonic power dissipated into the solution was calculated by Eq. (3).
W cal ¼ ðC m M þ C r Þ ðdT=dtÞ
ð3Þ
where Cm is the heat capacity of sonicated liquid, M is the mass of liquid, Cr is the heat capacity of reactor (glass) vessel and (dT/dt) is the time dependent change of temperature of the liquid. 2.4. Transesterification experiments Transesterification reactions were carried out in a 300 ml threenecked cylindrical glass reactor equipped with a reflux condenser. Known weights of methanol and catalysis were poured into the reactor and the mixture was stirred magnetically. Then, canola oil was added to the mixture. At the start of each run, the heterogeneous mixture was sonicated at continuous mode to generate an initial homogeneous mixture until the reaction mixture reached to the set temperature by the heating effect of ultrasound and hotplate heating as well. In all experiments, the heterogeneous mixture was sonicated at pulsed mode (pulse ratio 0.9) which is beneficial for low electrical energy consumption and for easier control of reactor temperature and also for cooling the transducer [9]. To maintain stable and uniform cavitational activity in the
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sonochemical reactor, magnetic stirring was applied at 600 rpm. Due to the heating effect of US in the reaction medium, cooling was applied by circulating water in the reactor jacket. In this way, the reaction temperature was kept at the set point within ±1 °C. At the end of the reaction, the solid catalyst was separated by centrifuge and then the liquid mixture was filtered by blue band filter paper under vacuum. After filtration, the excess methanol remaining in the heterogeneous liquid mixture was separated by rotary evaporator under vacuum (100 mbar). The product liquid was put into a separating funnel and was kept at room temperature for one day, to separate into an upper layer (biodiesel phase) and a lower layer (glycerol phase). The FAME content of the biodiesel phase was determined by AgilentÒ 6890 Gas Chromatogram (GC) equipped with a flame ionization detector (FID) and capillary column (CARBOWAX 20M) following European regulated procedure EN 14103. A typical biodiesel GC spectrum is given in Fig. 2.
Table 2 Calorimetric results. Sonicated liquid
Eq. (4)
Corr. coeff. R2
Water
Wcal = 0.763, Welec 6.40 Wcal = 0.937, Welec 15.24 Wcal = 0.615, Welec 5.71 Wcal = 0.659, Welec 2.15 Wcal = 0.677, Welec 7.20 Wcal = 0.645, Welec 7.20
0.9939
Canola oil Methanol Methanol/oil (molar ratio:6/1) mixture Methanol/oil (molar ratio:9/1) mixture Methanol/oil (molar ratio:12/1) mixture
0.9790 0.9939 0.9919 0.9918 0.9750
3.2. Transesterification results 3. Results and discussion 3.1. Calorimetric experimental results Calorimetric experiments were conducted in various liquids at different power levels. As seen in Table 2, a linear relationship exists between Wcal and Welec;
W cal ¼ a W elec b
ð4Þ
The slope, a, refers to the energetic efficiency of US transducer while (b/a) corresponds to the minimum Welec to overcome the acoustic threshold for generating cavitation bubbles. Furthermore, Table 2 reveals that both parameters of Eq. (4) depend on the chemical composition of the liquid which in turn affects relevant physical properties of the sonicated medium (such as viscosity, vapor pressure and heat capacity). An increase in the temperature reduces the viscosity of the medium facilitating the propagation and dissipation of US wave. Meanwhile, at higher temperatures, it is expected that vaporous cavities are formed due to the presence of the volatile components such as methanol, which reduces the bubble collapse intensity and hence decreases the cavitation effects [10]. Fig. 3a shows that as the temperature rises during the irradiation, the electric power applied to the system and consequently US power delivered to the reaction medium decrease due to varying properties of the liquid as function of temperature. For this reason, electric energy supplied to the US system was monitored during transesterification reactions. For a typical run conducted at isothermal reaction conditions, the electric power versus time is shown Fig. 3b. As seen, electrical power was hold constant during the reaction within the standard deviation (2 W).
3.2.1. Effect of catalyst loading The effect of catalyst loading on biodiesel yield was investigated between 3 wt.% of oil and 7 wt.% of oil at the following experimental conditions; methanol/oil molar ratio: 9/1, temperature: 55 °C, reaction time: 90 min and US power: 40 W. As seen in Fig. 4a, in the case of dolomite the yield increases up to 5% catalyst loading to reach a maximum value in accordance with the fact that the transesterification rate increases with increasing number of active sites [10]. On the other hand, in the case of CaO, 3% catalyst loading is suitable for high biodiesel yield. The sharp decrease of yield with higher catalyst loadings (not normally expected) may be due to the increasing shielding effect of catalyst particles on US propagation in reaction medium, which causes attenuated mass transfer rates around catalyst particles. Furthermore, high catalyst loading induces also saponification side-reaction, especially for the case of basic catalyst [19]. Mootabadi et al. investigated the effect of US at 20 kHz and 200 W on biodiesel production using CaO catalyst. They found 77.3% biodiesel yield under the conditions; catalyst loading: 3 wt.% of oil, methanol/oil molar ratio: 9/1, temperature: 65 °C, time: 60 min [12]. 3.2.2. Effect of methanol/oil molar ratio According to the stoichiometry of transesterification reaction, one mole of triglyceride requires three moles of methanol. In the case of heterogeneous catalyst, the reversible transesterification reaction completely proceeds even at the stoichiometric molar ratio of 3/1, because side reactions are not promoted and the catalyst remains in the system [20]. In this study, slightly higher methanol/oil ratio (4/1) was selected as the lower limit in the
Fig. 2. GC spectrum of biodiesel sample.
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Fig. 3. (a) Temperature and electric power versus time (1-electric, 2-temperature). (b) Electric power versus time (at constant temperature).
Fig. 4. Effect of catalyst amount on the biodiesel yield. (a) Dolomite. (b) CaO (Transesterification conditions; methanol/oil mol ratio: 9/1, US power: 40 W, temperature: 55 °C, time: 90 min).
experimental plan, because the transesterification reaction proceeds dominantly in the methanol phase in contact with hydrophilic catalyst surface. On the other hand, in the case of high methanol/oil ratios, due to viscosity dependent dispersive effect of US, the problem of low dispersion of oil in large volume of methanol phase becomes important (in very high ratios such as 24/1, dispersion of methanol in oil phase may also be incomplete). As a result, the interfacial area between reactant phases reduces and so does the reaction rate [21]. By considering this point, the high level for methanol/oil molar ratio was selected as 15/1. Thus, the effect of on the biodiesel yield was studied at five different levels. As seen in Fig. 5, for both catalysts, with increasing methanol/oil molar ratio, the biodiesel yield increases steadily up to a maximum value at 9/1 methanol/oil ratio and then decreases. Similar results were obtained by other researchers. Mootabadi et al. investigated the effect of methanol/oil molar ratio on US assisted transesterification reaction; for CaO catalyst, the highest biodiesel yield was achieved at 12/1 methanol/oil molar ratio, while for BaO and SrO catalysts, suitable methanol/oil ratio was found as 9/1 [12]. Choudhury et al. identified optimum methanol/oil molar ratio as 10/1 using CaO catalyst [13].
3.2.3. Effect of ultrasonic power The effect of Welec. on biodiesel yield was investigated between 30 W and 50 W. As seen in Fig. 6, for both catalysts the yield increases by virtue of increasing number of cavitation bubbles and the extent of acoustic streaming which enhance dispersion and mass transfer effects of US. Thus, various steps of the reaction namely; the diffusion of methanol into the oil phase, the adsorption of methanol on catalyst surface active sites and desorption of methyl ester and glycerol from the catalyst sites were enhanced. Meanwhile, at high acoustic power levels too many bubbles are generated at the tip of the horn leading to acoustic decoupling and bubble coalescence which decreases the number of active cavitation bubbles [9,10]. According to Fig. 6, suitable power level is 45 W for dolomite catalyst and 40 W for CaO catalyst. 3.2.4. Effect of temperature Fig. 7 shows the effect of reaction temperature on biodiesel yield. Temperature speeds the reaction rate and increases the FAME yield. On the other hand, as previously described in calorimetric experiment results, an increase in the temperature reduces the overall cavitational effect and for this reason lower temperature is
Fig. 5. Effect of methanol/oil molar ratio on the biodiesel yield. (a) Dolomite (Transesterification conditions; catalyst amount: 5 wt.% of oil, US power: 40 W, temperature: 55 °C, time: 90 min). (b) CaO (Transesterification conditions; catalyst amount: 3 wt.% of oil, US power: 40 W, temperature: 55 °C, time: 90 min).
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Fig. 6. Effect of electric power on the biodiesel yield. (a) Dolomite (transesterification conditions; catalyst amount: 5 wt.% of oil, methanol/oil molar ratio: 9/1, temperature: 55 °C, time: 90 min). (b) CaO (Transesterification conditions; catalyst amount: 3 wt.% of oil, methanol/oil molar ratio: 9/1, temperature: 55 °C, time: 90 min).
Fig. 7. Effect of temperature on the biodiesel yield. (a) Dolomite (Transesterification conditions; catalyst amount: 5 wt.% of oil, methanol/oil molar ratio: 9/1, US power: 45 W, time: 90 min). (b) CaO (Transesterification conditions; catalyst amount: 3 wt.% of oil, methanol/oil molar ratio: 9/1, US power: 40 W, time: 90 min).
Fig. 8. Effect of ultrasound irradiation time on the biodiesel yield. (a) Dolomite (Transesterification conditions; catalyst amount: 5 wt.% of oil, methanol/oil molar ratio: 9/1, US power: 45 W, temperature: 60 °C). (b) CaO (Transesterification conditions; catalyst amount: 3 wt.% of oil, methanol/oil molar ratio: 9/1, US power: 40 W, temperature: 60 °C).
Fig. 9. Reusability of catalysis in the transesterification of canola oil. (a) Dolomite (Transesterification conditions; catalyst amount: 5 wt.% of oil, methanol/oil molar ratio: 9/1, US power: 45 W, temperature: 60 °C, time: 90 min). (b) CaO (Transesterification conditions; catalyst amount: 3 wt.% of oil, methanol/oil molar ratio: 9/1, US power: 40 W, temperature: 60 °C, time: 75 min).
suggested for US applications. The optimum operating temperature is dictated by counteracting effects of temperature on cavitational phenomena and on the chemical reaction rate [10]. According to
Fig. 7, a reaction temperature of 60 ° C close to boiling point of methanol (64.7° C) is suitable for both catalysts to ensure high biodiesel yield. Similar finding was obtained by other researchers [13].
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3.2.5. Effect of time The ultrasonic irradiation time is an important reaction parameter which is closely related to the total energy consumption of US assisted biodiesel production process. As seen in Fig. 8, at the end of 90 min biodiesel yield reached 97.4% for dolomite and at the end of 75 min the yield reached 95.5% for CaO. Analytical results showed that 90 min was sufficient to achieve nearly constant (equilibrium yield) for both catalysts. As comparative purpose, it is worth to cite a research conducted without US application (conventional stirring) by Ilgen who obtained 91.8% biodiesel yield with dolomite catalyst at the following experimental conditions; catalyst loading: 3 wt.% of oil, methanol/oil molar ratio: 6/1, reaction temperature 65 ° C, reaction time: 180 min [18]. The great advantage of US application is easily noticeable. The electrical energy consumed by US transducer for per unit gram of biodiesel (Jelec) was calculated using Eq. (5)
J elec ¼
W elec t x moil 1000
ð5Þ
Here Welec is the time average electric power (W) supplied to the transducer, t is the time (s), x is the conversion of TG to biodiesel and moil is the weight of the oil (g). Calculated Jelec values for dolomite and CaO were 1.51 kJ/g and 1.14 kJ/g respectively and this indicates that the catalyst type is an important factor in terms of energy saving. 3.2.6. Reusability of catalyst The reusability of the solid catalyst is of primordial importance when compared with homogeneous catalysts from technical, environmental and economic points of view. In these respects, to investigate the reusability of dolomite and CaO catalysis, the catalyst powder recovered by filtration at the end of the reaction was mixed with known amount of methanol, poured in the reactor and ultrasonically irradiated for 15 min. To compensate for the catalyst lost during the washing and filtration steps, 5% fresh catalyst was added into the ultrasonic reactor. Then, the predetermined amount of oil was added and the reaction was restarted. As shown in Fig. 9a dolomite can be reused 4 times in transesterification only with a 7% biodiesel yield loss. Furthermore, Fig. 9b reveals that CaO can be reused 4 times in transesterification with 8.4% biodiesel yield loss only. For comparison, Mootabadi et al. found approximately 10% yield loss after 3 repeated use of CaO catalyst [12]. Finally, solid catalyst reusability, until final disposal or reactivation step, have obviously important impact on the energy (calcination/reactivation) consumption per mass of total biodiesel produced and will determine the net advantage of solid catalysts over homogeneous ones. This subject needs further detailed examination in the context of energy and exergy analysis of ‘‘heterogeneously catalyzed and US assisted biodiesel production process”. 4. Conclusion Ultrasound assisted heterogeneously catalyzed of canola oil was studied using dolomite and CaO as the catalysts to produce the biodiesel. FAME yield reached 97.4% for dolomite at the end of 90 min
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and 95.5% for CaO at the end of 75 min. Suitable operating conditions for dolomite catalyst are: catalyst loading: 5 wt.% of oil, methanol/oil ratio molar ratio: 9/1, temperature: 60 °C, US power: 45 W and time: 90 min. Similarly, suitable operating conditions for CaO catalyst are as follows: catalyst loading: 3 wt.% of oil, methanol/oil molar ratio: 9/1, temperature: 60 °C, US power: 40 W and time: 75 min. References [1] Refaat AA. Biodiesel production using solid metal oxide catalysts. Int J Environ Sci Technol 2011;8:203–21. [2] Chouhan APS, Sarma AK. Modern heterogeneous catalysts for biodiesel production: a comprehensive review. Renew Sustain Energy Rev 2011;15:4378–99. [3] Santacesaria E, Vicente GM, Di Serio M, Tesser R. Main technologies in biodiesel production: state of the art and future challenges. Catal Today 2012;195:2–13. [4] Borges ME, Díaz L. Recent developments on heterogeneous catalysts for biodiesel production by oil esterification and transesterification reactions: a review. Renew Sustain Energy Rev 2012;16:2839–49. [5] Semwal S, Arora AK, Badoni RP, Tuli DK. Biodiesel production using heterogeneous catalysts. Bioresour Technol 2011;102:2151–61. [6] Salamatinia B, Hashemizadeh I, Ahmad Zuhairi A. Alkaline earth metal oxide catalysts for biodiesel production from palm oil: elucidation of process behaviors and modeling using response surface methodology. Iranian J Chem Chem Eng (IJCCE) 2013;32:113–26. [7] Badday AS, Abdullah AZ, Lee KT, Khayoon MS. Intensification of biodiesel production via ultrasonic-assisted process: a critical review on fundamentals and recent development. Renew Sustain Energy Rev 2012;16:4574–87. [8] Ramachandran K, Suganya T, Nagendra Gandhi N, Renganathan S. Recent developments for biodiesel production by ultrasonic assist transesterification using different heterogeneous catalyst: a review. Renew Sustain Energy Rev 2013;22:410–8. [9] Korkut I, Bayramoglu M. Various aspects of ultrasound assisted emulsion polymerization process. Ultrason Sonochem 2014;21:1592–9. [10] Gole VL, Gogate PR. A review on intensification of synthesis of biodiesel from sustainable feed stock using sonochemical reactors. Chem Eng Process 2012;53:1–9. [11] Choudhury HA, Goswami PP, Malani RS, Moholkar VS. Ultrasonic biodiesel synthesis from crude Jatropha curcas oil with heterogeneous base catalyst: mechanistic insight and statistical optimization. Ultrason Sonochem 2014;21:1050–64. [12] Mootabadi H, Salamatinia B, Bhatia S, Abdullah AZ. Ultrasonic-assisted biodiesel production process from palm oil using alkaline earth metal oxides as the heterogeneous catalysts. Fuel 2010;89:1818–25. [13] Choudhury HA, Chakma S, Moholkar VS. Mechanistic insight into sonochemical biodiesel synthesis using heterogeneous base catalyst. Ultrason Sonochem 2014;21:169–81. [14] Kouzu M, Hidaka J-S. Transesterification of vegetable oil into biodiesel catalyzed by CaO: a review. Fuel 2012;93:1–12. [15] Esipovich A, Danov S, Belousov A, Rogozhin A. Improving methods of CaO transesterification activity. J Mol Catal A: Chem 2014;395:225–33. [16] Reyero I, Arzamendi G, Gandía LM. Heterogenization of the biodiesel synthesis catalysis: CaO and novel calcium compounds as transesterification catalysts. Chem Eng Res Des 2014;92:1519–30. [17] Kouzu M, Kasuno T, Tajika M, Sugimoto Y, Yamanaka S, Hidaka J. Calcium oxide as a solid base catalyst for transesterification of soybean oil and its application to biodiesel production. Fuel 2008;87:2798–806. [18] Ilgen O. Dolomite as a heterogeneous catalyst for transesterification of canola oil. Fuel Process Technol 2011;92:452–5. [19] Li H, Niu S, Lu C, Li J. Calcium oxide functionalized with strontium as heterogeneous transesterification catalyst for biodiesel production. Fuel 2016;176:63–71. [20] Tsuji T, Kubo M, Shibasaki-Kitakawa N, Yonemoto T. Is excess methanol addition required to drive transesterification of triglyceride toward complete conversion? Energy Fuels 2009;23:6163–7. [21] Kalva A, Sivasankar T, Moholkar VS. Physical mechanism of ultrasoundassisted synthesis of biodiesel. Ind Eng Chem Res 2009;48:534–44.