State of the art of biodiesel production under supercritical conditions

State of the art of biodiesel production under supercritical conditions

Progress in Energy and Combustion Science 63 (2017) 173203 Contents lists available at ScienceDirect Progress in Energy and Combustion Science jour...

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Progress in Energy and Combustion Science 63 (2017) 173203

Contents lists available at ScienceDirect

Progress in Energy and Combustion Science journal homepage: www.elsevier.com/locate/pecs

State of the art of biodiesel production under supercritical conditions TagedPObie Farobie, Yukihiko Matsumura* TagedPDivision of Energy and Environmental Engineering, Institute of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan

TAGEDPA R T I C L E

I N F O

Article History: Received 22 October 2016 Accepted 9 August 2017 Available online 29 August 2017 TagedPKeywords: Biodiesel Energy efficiency Empirical rate expression Supercritical fluid

TAGEDPA B S T R A C T

This paper reviews the current status of biodiesel production mainly under supercritical conditions. Various methods such as homogeneous acid- and alkali-catalyzed transesterification, heterogeneous acid and alkali-catalyzed transesterification, enzyme-catalyzed transesterification, and supercritical reactions have been employed so far to synthesize biodiesel. Herein, we review the reaction mechanisms and experimental results for these approaches. Recently, supercritical biodiesel production has undergone a vigorous development as the technology offers several advantages over other methods, including the fact that it does not require a catalyst, short residence time, high reaction rate, no pretreatment requirement, and applicability to a wide variety of feedstock. This technology was first designed for biodiesel production using methanol and ethanol. Biodiesel production without glycerol as a byproduct is attractive and has been achieved using supercritical methyl acetate and dimethyl carbonate (DMC). Most recently, biodiesel production in supercritical tert-butyl methyl ether (MTBE) has been developed also. In this review, supercritical biodiesel production will be discussed in detail. Empirical rate expressions are derived for biodiesel production in supercritical methanol, ethanol, methyl acetate, DMC, and MTBE in this study for the first time. These rate equations are critical to predicting biodiesel yields and to comparing the reaction behaviors in different solvents. Lastly challenges for improving energy recovery in supercritical biodiesel production and recommendations for future work are provided. © 2017 Elsevier Ltd. All rights reserved.

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodiesel production processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Homogeneous catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1. Homogeneous acid-catalyzed transesterification. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2. Homogeneous alkali-catalyzed transesterification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Heterogeneous catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. Heterogeneous acid-catalyzed transesterification. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2. Heterogeneous alkali-catalyzed transesterification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Enzyme-catalyzed biodiesel production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Supercritical biodiesel production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Microwave and ultrasound-assisted transesterification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Latest development in supercritical biodiesel production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Methanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Ethanol. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Methyl acetate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Dimethyl carbonate (DMC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Tert-butyl methyl ether (MTBE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Empirical rate expressions of supercritical biodiesel production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Factors affecting supercritical biodiesel production. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Temperature and reaction time. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Oil-to-reactant molar ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Corresponding author. E-mail address: [email protected] (Y. Matsumura).

http://dx.doi.org/10.1016/j.pecs.2017.08.001 0360-1285/© 2017 Elsevier Ltd. All rights reserved.

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5.3. Pressure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Challenges and recommendations for future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. Energy recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2. Excessive amount of reactants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3. Recommendations for future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concluding remarks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction

2. Biodiesel production processes

TagedPResearch on renewable energy has attracted a tremendous attention because of the challenges due to global climate change and environmental pollution problems associated with fossil fuel combustion. One of the most notable forms of renewable energy is biodiesel, which can be derived from the biomass including vegetable oil, animal fats, and microalgae. Biodiesel exhibits favorable biodegradability [1] and has lower particulate matter and CO emissions [2, 3, 4, 5], lower sulfur contents [6], and higher cetane numbers [7] than conventional diesel fuel. TagedPIn 1893, Rudolph Diesel operated his engines using vegetable oil (peanut oil) because petroleum was not available at that time. The direct application of vegetable oil was however problematic due to its high viscosity. Techniques are available to reduce the viscosity of vegetable oil, including dilution [8], microemulsion [9], pyrolysis [10], and transesterification [11]. In the dilution method, a vegetable oil is mixed with or diluted by a diesel fuel. It was reported in 1980 that Caterpillar Brazil Co. used a 10 vol% mixture of vegetable oil in diesel fuel to maintain total power without any adjustment to the engine [12]. A blend of 20% (vol) vegetable oil and 80% (vol) diesel fuel was also successfully used [12]. Another approach to reducing the viscosity of is microemulsion, in which butanol, hexanol, and octanol are usually used as solvents [13]. Pyrolysis of vegetable oil has also been employed at elevated temperatures in the presence of a catalyst for viscosity reduction. Schwab et al. [8] found that even though the viscosity of the pyrolyzed soybean oil (10.2 mm2/s at 37.8 °C) was higher than that of the ASTM specification range for diesel fuel, it was still acceptable for use in engines. TagedPAll methods just discussed are associated with problems due to carbon deposition and contamination. Transesterification is known to be the best technique for production of biodiesel due to its physical and chemical similarity with conventional diesel fuel. The process produces little or no deposits. Various transesterification processes are summarized in Fig. 1, each having its own advantages and disadvantages. Among them, supercritical biodiesel production is a promising route as it offers unique advantages over other methods, including the fact that it does require a catalyst and has a shorter reaction time. Supercritical biodiesel production can be applied to a wide variety of feedstock. No feedstock pretreatment is necessary, and separation and purification of products are relatively easy. As shown in Fig. 1, supercritical biodiesel production can use a variety of solvents ranging from methanol, ethanol, methyl acetate, dimethyl carbonate (DMC), or tert-butyl methyl ether (MTBE). TagedPThis review aims to provide a review on the latest advances in supercritical biodiesel production and a better understanding of the chemical processes by examining at a detailed level the mechanisms of triglycerides reactions with methanol, ethanol, methyl acetate, DMC, and MTBE. Empirical rate expressions are derived for reactions in these solvents for the first time. These rate equations are critical to predicting the product yield and to unraveling the reaction behaviors. Factors affecting supercritical biodiesel production are discussed in detail, including temperature, residence time, pressure, and oil-to-reactant molar ratio. Finally, the challenge for improving energy recovery and recommendations for future work are presented.

TagedPBiodiesel is mainly produced by reacting triglycerides with shortchain alcohols such as methanol and ethanol in a transesterification process (Fig. 1). These methods, along with their advantages and drawbacks, are discussed in what follows. 2.1. Homogeneous catalysts TagedPTwo types of homogeneous catalysts for biodiesel production are homogeneous acid catalysts and homogeneous alkali catalysts. TagedP2.1.1. Homogeneous acid-catalyzed transesterification TagedPSo far, hydrochloric acid (HCl) and sulfuric acid (H2SO4) have been the most commonly used catalysts for acid-catalyzed transesterification. Compared with an alkali-catalyzed process, the acid catalyst holds the advantages that it can catalyze esterification and transesterification simultaneously [14], that it is not sensitive to the presence of free fatty acid (FFAs) in the feedstock, and that it can be applied to low-cost lipid feedstock such as waste cooking oil [15]. Zhang et al. [16] reported that acid catalysis performed better when the amount of FFAs in the feedstock was greater than 1 wt%. TagedPFig. 2 shows the mechanism of homogeneous acid-catalyzed transesterification for biodiesel production using methanol. This mechanism involves the protonation of the carbonyl group, nucleophilic attack of methanol to produce a tetrahedral intermediate, followed by proton migration and intermediate breakdown. This sequence is repeated two more times to generate biodiesel and glycerol. TagedPEven though the acid catalyst is insensitive to the presence of FFAs, it requires more severe reaction conditions than alkali-

Fig. 1. Several methods to produce biodiesel.

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Fig. 2. Reaction mechanism for homogeneous acid-catalyzed transesterification of triglyceride.

TagedPcatalyzed transesterification, making it not suitable for industrial applications. Additionally, this process requires a high reaction temperature and high oil-to-alcohol ratio, takes a long time due to slow reaction rate, and can cause an environmental problem due to corrosion [14, 17]. Previous studies have shown that more that 90% conversion of waste cooking oil was achieved after 10 h reaction time, with addition of 4 wt% H2SO4 and using an oil-to-methanol molar ratio of 1:20 [18]. Freedman et al. [11] also investigated biodiesel production using H2SO4-catalyzed transesterification. They observed 99% oil conversion within 69 h reaction time with 1 mol% catalyst and an oil-to-methanol molar ratio of 1:30. These studies suggest that acid-catalyzed transesterification requires rather severe reaction conditions. TagedP2.1.2. Homogeneous alkali-catalyzed transesterification TagedPToday, biodiesel is generally produced using homogeneous alkali-catalyzed transesterification for several reasons. The catalyst is widely available; the transesterification process is performed

TagedP nder mild reaction conditions (low reaction temperature and under u atmospheric pressure); and high conversion can be achieved in time shorter than acid catalysis [19,20]. The most commonly used catalyst is sodium hydroxide (NaOH) and potassium hydroxide (KOH) [21,22]. Fukuda et al. [19] demonstrated that the homogeneous alkali-catalyzed transesterification reaction rate was 4000 times faster than that of acid-catalyzed transesterification under comparable conditions. The drawback of alkali-catalyzed transesterification is its sensitivity to the presence of FFAs in the feedstock. Ma and Hanna [23] concluded that the FFA content in vegetable oil must be less than 1 wt% for alkali-catalyzed transesterification. When the feedstock contains a large amount of FFA, saponification occurs as the result of an undesirable reaction between FFA and the alkali catalyst, as shown in Fig. 3, which deactivates the catalyst. The soap can also inhibit biodiesel purification of and reduce biodiesel yield [15,24]. TagedPAnother shortcoming of the alkali-catalyzed transesterification process is its sensitivity to the presence of water which can

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Fig. 3. Saponification as a result of reaction between oleic acid and sodium hydroxide.

Fig. 4. Hydrolysis of triglyceride to produce diglyceride and fatty acid.

TagedPhydrolyze triglycerides to form diglycerides and FFA as shown in Fig. 4. In industrial processes, the FFA and water content in the feedstock must be strictly controlled in order to obtain high-quality biodiesel. TagedPFig. 5 depicts the mechanism of homogeneous alkali-catalyzed transesterification process for biodiesel production using methanol. This mechanism involves the production of methoxide, the nucleophilic attack of methoxide to carbonyl group on triglycerides to form a tetrahedral intermediate, followed by intermediate breakdown and catalyst regeneration. The sequence is repeated two more times to produce biodiesel and glycerol. 2.2. Heterogeneous catalysts TagedPSimilar to homogeneous catalysis, transesterification using heterogeneous catalysts also has two types, namely heterogeneous

TagedP cid-catalyzed transesterification and heterogeneous alkali-cataa lyzed transesterification. Heterogeneous transesterification has some advantages over homogeneous transesterification. It is easy to separate the product from catalyst. The catalyst is reusable and there is no occurrence of saponification. Heterogeneous catalysts are also more environmentally friendly than homogeneous catalysts and can be used in batch or continuous-flow reactors [25,26]. TagedP2.2.1. Heterogeneous acid-catalyzed transesterification TagedPHeterogeneous acid-catalyzed transesterification has a lower activity. Nonetheless, this method has been applied in industrial processes because of its ability to catalyze esterification and transesterification processes simultaneously, low toxicity, less corrosiveness than homogeneous acid catalysts, insensitivity to the FFA content, elimination of the washing step of biodiesel, presence of a variety of acid sites with different Bronsted or Lewis acid strengths, easy

Fig. 5. Reaction mechanism for homogeneous alkali-catalyzed transesterification of triglyceride.

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TagedPseparation of the catalyst from the reaction medium, low product contaminant, and easy regeneration and recycling of the catalyst [15,27,28]. However, slow reaction rates and the occurrence of undesirable side reactions are some of its drawbacks. TagedPAcid catalysts utilized include zirconium dioxide (ZrO2), tin dioxide (SnO2), titanium dioxide (TiO2), zeolites, and sulfonic ionexchange resins including Amberlyst-15, Amberlyst-35, and NafionNR50. Among them, Nafion-NR50 exhibited a high selectivity for production of biodiesel and glycerol because of its acid strength [29]. The limitation is its high cost and lower activity compared to liquid acid catalysts [30]. TagedPZrO2 was used as heterogeneous acid catalyst due to its strong surface acidity. This catalyst is commonly used after impregnation in an acidic solution such as H2SO4 to form sulfated zirconia, SO42¡ZrO2 [31] or together with alumina (Al2O3) to form ZrO2Al2O3 or even with tungsten trioxide (WO3). The activity is different with or without impregnation. It was reported that when ZrO2 was used after impregnating with H2SO4, the yield of biodiesel from palm kernel oil and crude coconut oil was as high as 90.3% and 86.3%, respectively. Without impregnation, however, the yields drop to 64.5% and 49.3%, respectively [27]. Thus, modification of the metal oxide surface acidity was deduced as the key to obtaining high yields. Further, Jacobson et al. [14] demonstrated that combination of this catalyst with alumina and tungsten trioxide (ZrO2Al2O3WO3) could enhance its mechanical strength and acidity. TagedPFig. 6 shows a proposed reaction mechanism of SiO2/ZrO2-catalyzed transesterification using methanol. Initially, a carbonyl oxygen from triglyceride attacks zirconium to produce a reaction intermediate. The intermediate is then attacked by methanol, forming a new C O bond. The rearrangement in the intermediate produces biodiesel and regenerates the catalyst.

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TagedPSnO2 also has been used as a heterogeneous acid catalyst for biodiesel production. Studies are limited because of complications in catalyst preparation [32,33]. Similarly to ZrO2, SnO2 is impregnated also with a strong acid such as H2SO4 to form SO42¡SnO2. Furuta et al. [34] deduced that the acid strength of SO42¡SnO2 is higher than that of SO42¡ZrO2. Thus, for the esterification of n-octanoic acid with methanol below 150 °C, it was observed that SO42¡SnO2 has a better activity than SO42¡ZrO2. TagedPAnother heterogeneous acid catalyst used for biodiesel production is TiO2. Even though studies regarding utilization of this catalyst for biodiesel production are few, this metal oxide catalyst has attractive acidic properties. Chen et al. [35] reported that biodiesel yields from transesterification of cotton oil in SO42¡/TiO2 and SO42¡/ZrO2 was proportional to the specific surface area of the catalyst and biodiesel yields of 90% and 85% were obtained with SO42¡TiO2 at a specific surface area of 99.5 m2/g and SO42¡ZrO2 at 91.5 m2/g, respectively. Shortcomings of this catalyst include the use of fairly severe reaction conditions (high reaction temperature of 230 °C). de Almaeda et al. [36] found that a fatty acid methyl ester (FAME) yield of 40% was obtained after transesterification for 1 h at 120 °C using SO42¡TiO2. In another study, Peng et al. [37] found that the activity of SO42¡TiO2 catalyst could be enhanced by introducing a secondary metal oxide, SiO2 to generate SO42¡TiO2SiO2, which led to an increase in the specific surface area 258 m2/g. Optimum yield of biodiesel (90%) was achieved at 200 °C, with catalyst loading of 3 wt%, oil-to-methanol molar ratio of 1:9, and 3 h reaction time. TagedPZeolites have also been used as heterogeneous acid catalysts. The advantages of zeolites are that the catalysts can be obtained from natural sources; they are inexpensive; and their acid strength can be controlled by modifying the aluminosilicate framework. However, such catalysts have a low activity for transesterification due to steric hindrance that limits the diffusion of triglycerides into the

Fig. 6. Proposed reaction mechanism of SiO2/ZrO2-catalyzed transesterification (adapted from [184]).

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TagedPmicroporous structure [38]. Using zeolite Y75, Brito et al. [39] demonstrated a biodiesel yield of 26.6% from waste cooking oil at a temperature as high as 460 °C, 22 min reaction time, and an oil-tomethanol ratio of 1:6. Kusuma et al. [40] showed that KOH could be added to enhance the activity of zeolites. In their study the highest yield of biodiesel (95%) was obtained at 60 °C, using a purified natural zeolite impregnated with 100 g/100 mL KOH, a palm oil-to-methanol molar ratio of 1:7, 3 wt% catalyst, and 2 h reaction time. They also proposed a reaction mechanism (Fig. 7), which is almost the same as that of homogeneous alkali-catalyzed transesterification. Initially, the reaction between an active catalyst (K2O) site and methanol generates methoxide anion (CH3O¡). CH3O¡ then attacks the carbonyl carbon in triglyceride forming a tetrahedral intermediate, which rearranges to produce diglyceride anion and biodiesel. The formation of diglyceride is due to the reaction between H+ and diglyceride anion. It is possible that diglyceride anion also reacts with methanol to produce diglyceride and methoxide anion. TagedPSulfonic ion-exchange resins and sulfonic modified mesostructured silica have also been utilized for biodiesel production.

TagedP mberlyst-15, Amberlyst-35, Amberlyst-15 DRY, and Nafion SAC-13 A are common types of sulfonic ion-exchange [41,42]. The advantage is its excellent catalytic activity for esterification. The shortcomings include (1) low performance of the catalyst in transesterification, (2) the requirement of a very high oil-to-alcohol molar ratio, (3) low thermal stability of the catalyst, as it becomes unstable above 140 ° C, and (4) high reaction temperatures (150200 °C) [20]. TagedPVicente et al. [43] reported that a FAME yield of only 0.7% was obtained using Amberlyst-15 at atmospheric pressure and an oil-tomethanol molar ratio of 1:6 after 8 h reaction time. Dos Reis et al. [44] illustrated that to increase the biodiesel yield, the oil-to-methanol molar ratio should be increased. A yield of 80% was achieved at 60 °C and an oil-to-methanol molar ratio of 1:100 after 8 h reaction time. Common types of sulfonic modified mesostructured silica are propylsulfonic acid-modified SBA-15, arenesulfonic acid-modified SBA-15, and perfluorosulfonic acid-modified SBA-15 as shown in Fig. 8. According to Mbaraka and Shanks [45], these catalysts have large mesopores compared to the sulfonic ion-exchange resin, and the diffusion of reactants to the active site of the catalyst is

Fig. 7. Proposed reaction mechanism for transesterification of triglycerides using KOH-impregnated zeolite catalyst (adapted from [40]).

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Fig. 8. Organosulfonic acid-modified mesostructure silica (adapted from [185]).

TagedPimproved. Nonetheless, the use of this catalyst remains limited because of possible accumulation of organic matter on the catalyst surface that can inhibit the reaction. TagedP2.2.2. Heterogeneous alkali-catalyzed transesterification TagedPIt was thought that heterogeneous alkali-catalyzed transesterification is able to overcome the problems faced in homogeneous alkali-catalyzed transesterification. The use of a heterogeneous alkali catalyst affords several advantages: (1) the catalyst is reusable; (2) it is more environmentally benign than a homogeneous alkali catalyst; (3) it can be synthesized from inexpensive sources such as limestone or calcium hydroxide; (4) the amount of alkali waste water is reduced; and (5) the catalyst activity is almost the same as that of a homogeneous alkali catalyst under the same operating conditions [46]. However, heterogeneous alkali-catalyzed transesterification still faces some challenges: (1) the catalyst needs to be activated through a calcination process at high temperatures; (2) the reaction rate is slower than homogeneous alkali catalysis; and (3) side reactions may occur owing to the reaction between the catalyst and glycerol that generates calcium diglyceroxide [47], as shown in Fig. 9. TagedPSuitable heterogeneous alkali-catalysts include calcium oxide (CaO), magnesium oxide (MgO), strontium oxide (SrO), and hydrotalcites (Mg6Al2(OH)16CO3.4(H2O)). According to Zabeti et al. [48], CaO is the most attractive catalyst because of its small solubility in methanol and the strongest basic activity among all alkaline earth metal oxides. In a study reported by Kouzu et al. [49], a biodiesel

TagedP ield of 93% was obtained after 1 h transesterification at a methanol y reflux temperature of 64.7 °C with an oil-to-methanol ratio of 1:12 using a CaO catalyst from calcination of pulverized limestone (CaCO3) at 900 °C for 1.5 h. TagedPKazembe-Phiri et al. [50] achieved a biodiesel yield of 89% from ground nut oil after 2 h reaction time with an oil-to-ethanol molar ratio of 1:9 using 1 wt% CaO calcined at 900 °C for 1.5 h. Di Serio et al. [51] illustrated a FAME yield of only 20% from soybean oil at 100 °C. It is agreed in all previous studies that the activity of MgO is much lower than that of CaO. To enhance the activity of MgO, Xie et al. [52] demonstrated that hydrotalcite (Mg6Al2(OH)16CO34H2O) calcined at a high temperature resulted in a measurable activity. A FAME yield of >90% was observed by employing a Mg-Al oxide catalyst at a rather high reaction temperature [51]. TagedPFig. 10 displays the reaction mechanism for CaO-catalyzed transesterification of triglyceride using methanol as a reactant. This reaction mechanism is comprised of several steps: (1) the formation of methoxide anion through the proton abstraction from methanol by the basic site of CaO, (2) the subsequent attack of the carbon atom in the carbonyl group in triglyceride by the methoxide anion to generate the alkoxycarbonyl intermediate, (3) the rearrangement of the intermediate resulting in the generation of more stable compounds consisting of biodiesel and diglyceride anion, and (4) attack of the calcium hydroxide cation by the diglyceride anion forming diglyceride and CaO. It is seen that the catalyst is regenerated at the end. The reaction mechanism is repeated two more times until biodiesel and glycerol are obtained.

Fig. 9. The reaction between glycerol and calcium oxide forming calcium diglyceroxide and water.

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Fig. 10. Reaction mechanism for heterogeneous alkali-catalyzed transesterification of triglyceride.

2.3. Enzyme-catalyzed biodiesel production TagedPChemical-catalyzed transesterification is hampered by the need for wastewater treatment. It can be environmental unfriendly and difficult to recover glycerol from the catalyst. A green method employing enzyme as a catalyst, also known as a biocatalyst, has been devised for producing biodiesel. Figs. 11 and 12 compare an alkali catalyst and an enzyme in downstream operation. It is noticed that enzyme-catalyzed transesterification is superior to alkali-catalyzed transesterification due to its ability to eliminate the downstream operation required in alkali-catalyzed transesterification. In particular, for the case of alkali-catalyzed transesterification, several steps such as the evaporation of methanol and removal of saponified product are needed to recover glycerol as by-product. Meanwhile, for the case of enzyme-catalyzed process, the recovery of unreacted methanol and wastewater treatment are unnecessary. Only a simple concentration is required to recover glycerol. To remove the enzyme, the filtration and simple washing with distilled water are needed [19]. Thus, understandably the biodiesel product obtained by this method is of high purity. TagedPOther advantages of enzyme-catalyzed transesterification include (1) the absence of byproducts, (2) the use of moderate reaction temperatures (35‒45 °C), (3) easier separation and recovery of product, (4) the high selectivity of esterification-transesterification processes toward the substrate, (5) insensitivity to FFA and water contents in the feedstock, (6) the avoidance of side reactions such as saponification and hydrolysis, (7) lower oil-to-alcohol molar ratios required, (8) a good possibility of enzyme reusability, (9) the ability to convert FFA and triglyceride to biodiesel simultaneously, and (10) the favorable biodegradability of the enzyme catalyst [15,19, 53, 54, 55, 56, 57].

TagedPEnzyme-catalyzed transesterification also has its limitations, especially for industrial implementation [19, 21, 58, 59]. The problems include (1) the high production cost of enzyme, (2) longer reaction times compared to the alkali-catalyzed transesterification, (3) enzyme inhibition by methanol, (4) enzyme deactivation, and (5) limited enzyme regeneration that can lead to an extended operation time. TagedPDuring the past few years, researchers have utilized lipase produced from microorganisms such as fungi, bacteria, and yeasts. Lipase-producing fungi that were used in previous studies can be found in Christopher et al. [60]. Among the lipase-producing microorganisms explored, Candida sp., Pseudomonas sp., and Rhizopus sp. are the most frequently reported enzyme sources for biodiesel production [61]. Lipase from Candida antarctica, also known as Novozyme 435, was first used by Nelson et al. [62] using tallow as a feedstock. They found a biodiesel yield of 96% at 45 °C, a tallow-toalcohol molar ratio of 1:3, a reaction time of 16 h, and a stirring speed of 200 rpm. The addition of 6 mol% water relative to tallow could improve ester production when a secondary alcohol was employed. This can be easily understood because water is an essential to enhancing enzyme activity. However, the addition of a large amount of water reduces the yield of biodiesel since the hydrolysis of triglyceride generates FFA and diglyceride. It has been reported that Candida antarctica immobilized on an acrylic resin is the most useful lipase among extracellular enzymes employed for transesterification of vegetable oils using methanol as an acyl accpetor [62, 63, 64]. Novozyme 435, lipase from Pseudomonas cepacia (PS 30) has also been used to produce biodiesel, but its catalytic activity was low [62]. Nelson et al. [62] showed that the biodiesel yield from tallow using primary alcohols such as methanol, ethanol, and isobutanol is merely 14‒29%, whereas, by using isopropanol, a secondary alcohol, a biodiesel yield of 44% was obtained with a reaction time of

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Fig. 11. Biodiesel production using alkali-catalyzed transesterification process.

TagedP5 h, temperature at 45 °C, 0.34 mol/dm3 of triglyceride in hexane, an oil-to-methanol molar ratio of 1:3, 12.525 wt% enzyme, and a stirring speed of 200 rpm. Noureddini et al. [66] also reported a biodiesel yield of 67% from 10 g soybean oil using an immobilized enzyme from Pseudomonas cepacia at 35 °C, reaction time of 1 h, an oil-tomethanol molar ratio of 1:7.5, 475 mg enzyme, and 50 g kg¡1 water. Rhizopus oryzae is also one of the most commonly used lipase-producing microorganisms for biodiesel production. Chen et al. [67] found a biodiesel yield in the range of 8890% from enzymatic conversion of waste cooking oil at 40 °C, 1 atm, reaction time of 30 h, an oil-to-methanol molar ratio of 1:4, and immobilized lipase-to-oil weight ratio of 30%. TagedPImmobilized whole cell (intracellular enzyme) technique can be employed also to produce desirable results. A comparison of the steps involved in using extracellular enzyme and immobilized whole cell (intracellular enzyme) is presented in Fig. 13. The immobilized whole cell technique is more efficient than the extracellular enzyme technique which requires separation and purification of the enzyme before its immobilization. TagedPEnzyme immobilization is a vital part in enzyme-catalyzed biodiesel production. In this process, enzymes are attached physically to a solid support by adsorption [68], cross-linking [69], entrapment, and ion exchange [70]. The solid supports can be either natural or synthetic materials. According to Datta et al. [71], the support and technique selection is based on the natures of the substrate and the

TagedP nzyme, and the type of reaction. Compared to free enzymes, the e immobilized enzymes offers some advantages such as stabilization of enzyme owing to binding with the support, rapid termination of the enzyme-substrate reaction, and no contamination. TagedPMatsumoto et al. [72] developed a whole cell enzyme by immobilizing Rhizopus oryzae cells. It was reported a biodiesel content of 71% at 37 °C after 165 h reaction time. In that study, stepwise methanol addition to plant oil without solvent and water was employed. In another study, Ban et al. [73] reported a high biodiesel conversion of 90% to with stepwise addition of methanol with 15% water content using immobilized whole cell Rhizopus oryzae. They also found that several substrate related compounds, especially olive oil and oleic acid could enhance the methanolysis activity of the cells. Ban et al. [74] also examined the effectiveness of crosslinking treatment along with the addition of 0.1% glutaraldehyde in order to stabilize Rhizopus oryzae. They observed a conversion to biodiesel of only 50% without crosslinking, whereas, the conversion reached 7283% with cross-linking after six batch cycles. Fukuda and Kondo [75] reported that the reaction rate of biodiesel production using cells treated with lower alcohols increased 350600 times compared to untreated cells. TagedPEffect of alcohols on enzymatic activity for biodiesel production was also examined. Biodiesel production requires three moles of alcohol for each mole of triglyceride. The addition of excess alcohol is common for shifting chemical equilibrium towards biodiesel

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Fig. 12. Biodiesel production using enzymatic-catalyzed transesterification process.

TagedPformation as transesterification proceeds reversibly [76]. On the other hand, biodiesel yield was observed to decrease in enzymatic biodiesel production in high methanol concentrations possibly because the polar short chain of alcohol inhibits the enzyme activity and interferes with the separation of glycerol [76,77]. For this reason, enzymatic biodiesel production requires an alcohol-to-oil ratio lower than chemically-catalyzed biodiesel production [79]. It is common to add alcohol in an oil-to-alcohol molar ratio of 1:3 and 1:6 for enzymatic biodiesel production [78,80,81]. TagedPEffect of oil-to-methanol molar ratio has been examined for catalysis using immobilized lipase. Du et al. [82] reported the FAME yields of 75%, 92%, and 80% by methanolysis using immobilized lipase from Thermomyces lanuginosus, employing oil-to-methanol molar ratios of 1:3, 1:4, and 1:5, respectively, in a solvent-free system. In addition, the maximum yield of biodiesel (95%) was achieved by immobilized C. antarctica lipase at an oil-to-methanol molar ratio of 1:3 [63]. In another study, Garlapati et al. [83] revealed a maximum yield of biodiesel (92%) by methanolysis of Simarouba glauca oil using a fungal immobilized lipase and an oil-to-methanol molar ratio of 1:1. From these studies, it may be concluded that the yield of biodiesel is maximized by employing an oil-to-methanol molar ratio between 1:1 and 1:5. TagedPMethanol, ethanol, propanol, n-butanol, isopropanol, and isobutanol have also been used as the reaction media for enzyme-catalyzed transesterification. Among them, methanol is the most frequently used due to its high reactivity and low cost. It was shown by Nelson et al. [62] that short-chain alcohols inactivate enzymes more easily than longer aliphatic alcohols. To overcome the problem of inhibition caused by methanol addition in enzymatic biodiesel production, strategies have been devised that includes (1) stepwise addition of methanol [62,83,84] (2) the use of a solvent [86], and (3) the use of alternative acyl acceptors such as alkyl esters or longer chain alcohols [87]. TagedPStepwise addition of methanol has been examined in some detail. Shimada et al. [63] reported a biodiesel yield of 95% after 50 cycles of operation through stepwise addition of methanol. Shimada [85] obtained a conversion >90% to biodiesel via stepwise addition of methanol using waste cooking oil as the feedstock. Watanabe et al. [84] also observed an optimum yield of biodiesel (90%) in a two-step batch-wise addition of methanol and a three-step continuous addition of methanol. The yield was maintained even after 100 batches of operation. Similarly, a biodiesel yield of 97% was attained through

Fig. 13. Steps involved in enzyme-catalyzed biodiesel production for (a) extracellular enzyme and (b) immobilized whole cell (intracellular enzyme) (adapted from [186]).

TagedP three-step addition of 0.3 mol/dm3 equivalent of methanol using a plant oil as the feedstock [88]. TagedPUse of organic solvents such as hexane, n-heptane, tert-butanol, 1,4-dioxane, benzene, and chloroform also reduces methanol inhibition through protection of lipase from denaturation because of an increased solubility of methanol, the prevention of inhibition of lipase catalyzed reactions due to an increased solubility of glycerol, and the creation of single fluid phase [89,90]. Soumanou and Bornscheuer [91] studied the effect of organic solvents on the alcoholysis of sunflower oil using Pseudomonas fluorescens. It was found that the conversion to biodiesel reached 80% by adding n-hexane and petroleum ether. The addition of tert-butanol could also enhance biodiesel production. The yield reached 94% at 55 °C with an tert-butanol-oil volume ratio of 0.8:1 after 48 h of reaction time [92]. The addition of 2-ethyl-1-hexanol improved the conversion of rapeseed oil to biodiesel to 97% [93]. Alternative acyl acceptors to methanol, including isopropanol, tert-butanol, octanol, methyl acetate and ethyl acetate, were shown to lessen the inhibition effect of methanol [64,65,93,94]. 2.4. Supercritical biodiesel production TagedPAn exciting alternative to catalytic transesterification is supercritical biodiesel production. Proposed by Saka and Dadan [95], the approach has some definitive advantages over other methods. For example, it does not require a catalyst, has a high reaction rate

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TagedP(typically of the order of a few minutes), and allows for simultaneous triglyceride transesterification and FFA esterification, a higher production efficiency with a fewer number of processing steps (e.g., simpler separation and purification). In addition, supercritical processing is more tolerant to the presence of FFA and water and is applicable to a wide variety of feedstock and amenable to continuous operations. TagedPThe critical temperature and pressure of methanol is 239.2 °C and 8.09 MPa, respectively. Crossing the critical temperature and pressure, dramatic changes in the mass density of methanol to occur leading to changes its solubility and mass-transfer characteristics. In supercritical methanol, triglyceride and methanol become a single phase due to an increase in density of methanol and decrease of its dielectric constant. The increase in density leads to a diminishing polarity in methanol due to hydrogen bonding [95]. Thus, the nonpolar triglyceride is better dissolved in methanol under supercritical conditions to form a homogeneous phase. The solubility of triglyceride increases with increasing temperature and pressure as observed by Glisic and Skala [96]. TagedPThe phase behavior during methanolysis of triglyceride under subcritical and supercritical conditions has been studied by Glisic and Orlovic [97]. It was reported that during methanolysis, the distribution of methanol, triglyceride, biodiesel, and glycerol differed depending on the temperature and pressure. They divided the phase transition into three regimes as presented in Fig. 14. The first regime occurs below 170 °C and 1.5 MPa. The second regime corresponds to phase transition under the subcritical conditions (170220 °C and 1.55.0 MPa), and the last regime occurs around the critical point.

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TagedPThe reaction mechanism between triglycerides and methanol under supercritical conditions was first proposed by Kusdiana and Saka [98]. As methanol clusters break into free monomers at high temperatures and high pressures, they attack the carbon atom of the carbonyl group in triglycerides, resulting in an intermediate via the transfer of a methoxide moiety, as shown in Fig. 15. The next step is the rearrangement of the intermediate to generate more stable compounds, namely biodiesel and diglyceride. In a similar way, diglyceride react with another methanol molecule to form biodiesel and monoglyceride. The reaction between monoglyceride and methanol results in biodiesel and glycerol. TagedPSupercritical biodiesel production faces some limitations such as the need to use high oil-to-alcohol molar ratios and elevated temperature and pressure. To circumvent these problems the use of cosolvent and catalysts have been proposed. Kusdiana and Saka [99] proposed a two-step processing, in which triglycerides are first hydrolyzed in subcritical water to generate FFA and glycerol followed by esterification of FFA supercritically in lower oil-to-methanol molar ratios, as shown in Fig. 16. The two-step processing certainly reduces energy consumption due to the milder operating conditions (270 °C, 7 MPa) compared to one-step biodiesel production (350 °C, 2050 MPa) and a substantially increased oil-to-methanol molar ratio. TagedPCo-solvent has been proposed to reduce the severity of conditions needed for supercritical biodiesel production in methanol. Addition of a co-solvent can increase the mutual solubility between triglycerides and methanol. Consequently, biodiesel can be produced under milder conditions. Co-solvents that have been considered

Fig. 14. Phase distribution and composition as a result of reaction between methanol and triglycerides under different reaction conditions (adapted from [97]).

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Fig. 15. The proposed reaction mechanism between triglyceride and methanol under supercritical conditions (adapted from [98]).

TagedPinclude propane, carbon dioxide, ethane, n-butane, n-hexane, n-heptane, and tetrahydrofuran (THF). Cao et al. [100] reported that with a propane-to-methanol molar ratio of 0.1, the reaction temperature could be significantly reduced to 280 °C from 350 °C to obtain complete conversion of biodiesel. Imahara et al. [101] found the addition of CO2 as a co-solvent increased the yield of biodiesel in the methanolysis of canola oil. However, the yield decreased above 0.1 CO2/ methanol in the molar ratio. By employing a microtube reactor, Trentin et al. [102] investigated the effect of CO2 addition on biodiesel yield under supercritical conditions. They found the CO2-to-substrate mass ratio of 0.2:1 to be optimal. Tan et al. [103] reported that with an n-heptane-to-oil molar ratio of 0.2 n-heptane had a significant effect on palm oil conversion to biodiesel in supercritical methanol. n-Hexane as a co-solvent was studied by Muppaneni et al. [104] who showed a n-hexane-to-oil volume ratio of 0.2 to be optimal for the biodiesel yield. TagedPThe use of catalysts has been also proposed to improve biodiesel production under supercritical conditions. Demirbas [105] investigated the effect of CaO addition on the yield of biodiesel using sunflower oil as the feedstock. He reported that the biodiesel yield increased with CaO addition. In addition, the maximum biodiesel yield was observed within 6 min at 525 K, with an oil-to-methanol molar ratio of 1:41 and addition of 3 wt% CaO. Yoo et al., [106] examined biodiesel production in supercritical methanol by adding metal oxide catalysts such as SrO, CaO, ZnO, TiO2, and ZrO2. They

TagedP emonstrated the yield to be around 95% by adding 1 wt% ZrO2 at d 250 °C and with an oil-to-methanol molar ratio of 1:40 within 10 min reaction time. However, most of the heterogeneous catalysts are not stable under supercritical conditions. A few heterogeneous catalyst are stable under these conditions, such as zinc aluminate and zirconia supported metal oxide catalysts [107,108]. Due to undesirable stability of heterogeneous catalysts, the use of trace amount of homogeneous catalysts under subcritical conditions was also studied [109]. 2.5. Microwave and ultrasound-assisted transesterification TagedPAdvantages of microwave-assisted transesterification include a shorter reaction time and greater environmental friendliness than the conventional heating process. Constraints include high-energy consumption and all of the other limitations in catalytic transesterification. For these reasons, microwave-assisted transesterification has not been implemented in any commercial applications. Despite its many disadvantages, many laboratory-scale experiments has been reported recently. Leadbeater and Stencel [110] reported a maximum biodiesel yield of 98% at 323 K using 5% KOH or NaOH under 25 W microwave exit power and an oil-to-methanol molar ratio of 1:6 over a reaction time of 1 min. Similarly, Azcan and Danisman [111] used KOH as a catalyst, and observed a maximum yield of 93.7% using 1.0% KOH at 313 K, for 1 min reaction time. Zu et al. [112]

Fig. 16. Two-step processing for non-catalytic biodiesel production.

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TagedPobserved a 96% yield from yellow horn (Xanthoceras sorbifolia Bunge) oil at 60 °C, 500 W microwave power, an oil-to-methanol molar ratio of 1:6, and 1 wt% catalyst for 6 min. Acid catalysts such as H2SO4/C were used by Yuan et al. [113] as a catalyst in microwave irradiation. The highest biodiesel yield was found to be 94% at 338 K with an oilto-methanol molar ratio of 1:12, and 5 wt% of catalyst for 60 min. El Sherbiny et al. [114] compared biodiesel production using a conventional method and microwave irradiation and demonstrated a faster, more complete conversion (2 min) in microwave-assisted transesterification than the conventional method (60 min). Recently, direct, microwave-assisted conversion was shown to produce biodiesel from microalgae Nannochloropsis sp. at the yield of 38%, which is higher than the sonication method yielding 21%, both with 5 min of reaction time [115]. TagedPUltrasound-assisted transesterification processes have been proposed, though studies remain to be limited. Key drawbacks include the use of a tremendous amount of the chemical catalyst, this method remains commercially infeasible. Stavarache et al. [116] used low-frequency ultrasound (2840 kHz) in biodiesel production from a vegetable oil and found the yield as large as 98% at 25 °C, 40 kHz, an oil-to-methanol molar ratio of 1:6, 0.5% NaOH, and 20 min reaction time. Transesterification of palm oil was examined by Mootabadi et al. [117] using the ultrasound method with oxides of alkali-earth metals as the catalysts. They noticed that the yield can reach 95% with an oil-to-methanol molar ratio of 1:15 and 60 min reaction time, when BaO and SrO were used as the catalysts. 3. Latest development in supercritical biodiesel production TagedPAs will be demonstrated below, supercritical biodiesel production is one of the most promising techniques due to its many advantages, including a large reaction rate, a short reaction time, and a small sensitivity to water and FFA. Supercritical processing also eliminated the need for catalyst use. A wide range of reactants and reaction media have been explored, including methanol, ethanol,

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TagedP ethyl acetate, and DMC. The major product and by-product m obtained from each reactant are summarized in Fig. 17. In what follows, we shall provide a detailed discussion of the results, reaction mechanisms and kinetics from available studies. 3.1. Methanol TagedPBiodiesel production in supercritical methanol was proposed first by Kusdiana and Saka [118]. Using a batch-type reactor, they demonstrated that supercritical reaction could overcome many problems encountered in chemically- and enzyme-catalyzed transesterification. Measured yield data of rapeseed oil are provided in Table S1 of the Supplementary Material. Using six vegetable oils (cottonseed, hazelnut kernel, poppy seed, rapeseed, safflower seed, and sunflower seed), Demirbas [119] further demonstrated that non-catalytic biodiesel production in supercritical methanol was superior over conventional methods in terms of reaction time required to produce biodiesel. Biodiesel yield was observed to increase with an increase in the temperature and residence time (Table S2 of the Supplementary Materials). In addition, biodiesel yield was also found to increase with an increase in the oil-tomethanol molar ratio from 1:1 to 1:41. TagedPKusdiana and Saka [120] elucidated the reaction behavior of FFAs in supercritical methanol. FFA was found to be converted into biodiesel at 350 °C with a yield of over 95%. Hence, a wide variety of feedstock high in FFA contents including waste cooking oil and waste lard could be converted to biodiesel under a non-catalytic condition. TagedPKusdiana and Saka [98] noted that the presence of water did not have a notable effect on biodiesel yield. In contrast, a certain amount of water in the system could enhance biodiesel formation as water could hydrolyze triglyceride into FFA and glycerol. FFA then reacts with methanol to yield biodiesel. Obviously, bio-methanol prepared from wood gasification has been demonstrated to be a viable source for methanol [121]. Since bio-methanol could contain traces of

Fig. 17. Reaction of biodiesel production under various supercritical reactants.

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TagedPethanol, 1-butanol, methyl formate, water, and diisopropyl ether, they confirmed that all but diisopropyl ether can participate in the reactions biodiesel conversion. TagedPA large number of feedstock has been examined. Using a batch reactor, Song et al. [122] investigated bleached and deodorized (RBD) palm oil in supercritical methanol. They found that the biodiesel production rate increased dramatically under supercritical conditions, but the FAME yield decreased above 350 °C due to thermal decomposition of RBD palm oil and/or biodiesel (see Table S3 of the Supplementary Materials). Tan et al. [123] determined that biodiesel yield exceeded 70% from palm oil in supercritical methanol within 20 min reaction time (Table S4 of the Supplementary Materials). Kasim et al. [124] reported the transesterification of rice bran, a low cost feedstock, in supercritical methanol at 30 MPa, 300 °C and 5 min reaction time and found a rather low yield (51%). The Jatropha curcas L. seed has also been used by Lim et al. [125] as a feedstock via supercritical reactive extraction. They observed an oil extraction efficiency of 105% and a FAME yield of 104 wt% at 300 °C, 240 MPa, 2.5 mL/g of n-hexane to seed ratio, and 10.0 mL/g methanol-to-solid ratio. Using the response surface method, Patil et al. [126] optimized the single-step conversion of wet algal biomass (Inoculum Nannochloropsis sp., CCMP1776), which is composed of »90% water, in supercritical methanol and found the maximum yield to occur at 255 °C, 25 min reaction time and a wet algae-to-methanol (wt/v) ratio of 1:9 (Table S5 of the Supplementary Materials). Anikeev and Yakovleva [127] investigated biodiesel production from sunflower, rapeseed, cottonseed, and camelina oils supercritical methanol, and found that neither product yield nor the conversion is sensitive to feedstock type. TagedPSamniang et al. [128] compared biodiesel production from two non-edible oils: (Calophyllum inophyllum L.) and crude jatropha. Yields reached 90% and 85%, respectively at 320 °C, 15 MPa and an oil-to-methanol molar ratio of 1:40. A comparison of the yield data is shown in Table S6 of the Supplementary Material. TagedPDemirbas [129] found that the supercritical methanol transesterification was superior to base-catalysis of low-grade feedstock such as waste cooking oil and lard, because under the supercritical condition FFA in the waste cooking oil was transesterified simultaneously and the process was insensitive to water and FFA. In addition, using a batch-type reactor, Shin et al. [130] noted that the biodiesel yield reached 90% from waste lard at 335 °C, 20 MPa, an oil-to-methanol molar ratio of 1:45 and an agitation speed of 500 rpm within 15 min reaction time (see Table S7 of the Supplementary Materials). Lee et al. [131] investigated waste canola oil in supercritical methanol under relatively moderate conditions (240270 °C and 10 MPa) and reported that the biodiesel yield reached 102% at 270 °C, 10 MPa, an oil-to-methanol weight ratio of 2:1 with a reaction time of 45 min. Recently, in optimizing biodiesel production from waste vegetable oil in supercritical methanol, Ghoreishi and Moein [132] showed a biodiesel yield of 95 at 271.1 °C, 23.1 MPa, an oil-to-methanol molar ratio of 1:33.8 within about 20 min reaction time. Results from aforementioned studies are compiled into Fig. 18, and as expected, the yield increases with an increase in reaction time. The results of Tan et al. [123] are excluded from the plot because they are inconsistent with others. TagedPContinuous, supercritical flow reactor has been demonstrated for biodiesel conversion. He et al. [133] found a maximum biodiesel yield 77% from soybean oil at 310 °C, 35 MPa, and an oil-to-methanol molar ratio of 1:40 after 25 min residence time (See Table S8 of the Supplementary Materials). Similarly, using a vertical tubular reactor, Zhou et al. [134] obtained a biodiesel yield as high as 92% at 375 °C and 15 MPa, a fixed oil-to-methanol molar ratio of 1:40 after 23.3 min reaction time. Macaira et al. [59] carried out transesterification of sunflower oil using a continuous reactor in supercritical methanol with CO2 as a co-solvent. They deduced that the reaction

Fig. 18. Effect of residence time on biodiesel yield in a batch reactor using supercritical methanol.

TagedP f non-catalytic, supercritical biodiesel production was 20 times o faster than conventional methods under comparable conditions, and the addition of a co-solvent enhance the rate further. They demonstrated that a yield of 88% could be achieved at 200 °C after only 2 min reaction time. TagedPUsing a continuous reactor, Tsai et al. [135] found that from waste cooking oil the effect of CO2 addition was negligible, in contrast to the findings of Macaira et al. [59]. The yield of biodiesel reached 90% at 573.2 K within 13 min (Table S9 of the Supplementary Materials). TagedPRecently, an interesting study was reported that used a spiral reactor [136] for biodiesel production in supercritical methanol. The reactor was shown to be effective for transesterification with the ability to recover heat. Complete conversion to biodiesel from canola oil was achieved at 350 °C and 20 MPa within 10 min reaction time (Table S10 of the Supplementary Materials). TagedPFig. 19 shows the effect of temperature and residence time on biodiesel yield in continuous flow reactors. At 10 MPa, the biodiesel yield increases with increases in temperature and residence time. However, the biodiesel yield at 32 MPa increases as temperature is increased from 280 to 320 °C, but it decreases thereafter. TagedPMechanistically, the transesterification of triglyceride under supercritical methanol proceeds through three consecutive reaction steps, as shown in Fig. 20. First, the reaction between triglyceride and methanol results in FAME and diglyceride generation. As an intermediate compound, diglyceride reacts further with methanol to generate FAME and monoglyceride. Finally, the reaction between monoglyceride and methanol yields FAME and glycerol. TagedPHe et al. [137] examined the reaction kinetics of soybean oil conversion to biodiesel in supercritical methanol. The overall activation energy and activation volume were measured to be 56 kJ/mol and ¡206 cm3/mol, respectively. Similarly, Varma and Madras [138] also determined the activation energy of reactions to be 35.0 and 46.5 kJ/ mol for castor and linseed oils, respectively, from 200 to 350 °C at 20 MPa. Recently, Choi et al. [139] deduced the activation energy, entropy of activation, and reaction activation volume to be 81.4 kJ/ mol, ¡175.35 J/(mol-K), and ¡233.29 cm3/mol, respectively, for palm olein oil in supercritical methanol at 350 °C, 35 MPa, and an oil-to-methanol molar ratio of 1:40 over a residence time of 20 min.

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TagedPsupercritical methanol (300400 °C and 20 MPa) using a batch reactor, and reported the 45.9, 31.2, and 79.5 kJ/mol for the reactions of triglyceride to diglyceride (k1), diglyceride to monoglyceride (k2), and monoglyceride to glycerol (k3), respectively. He et al. [137] measured the activation energies of k1, k2 and k3 to be 59.4, 58.6, and 67.4 kJ/mol, respectively, also using a batch reactor operating at 28 MPa and between 240 and 280 °C. 3.2. Ethanol

Fig. 19. Effect of temperature and residence time on biodiesel yield in continuous flow reactor using supercritical methanol (oil-to-methanol oil ratio 1:40).

TagedPDetermination of the rate parameters can be complicated by the type of reactor used in the study. The batch-type reactor has difficulties in treating the heating-up and cooling-down of the reactor. Fig. 21 presents the Arrhenius plots of rate constants reported for the three reaction stages during supercritical transesterification in methanol. The activation energy reported for the forward reactions by Farobie and Matsumura [136] are higher than those obtained by Kusdiana and Saka [118] and He et al. [137]. Kusdiana and Saka [118] determined the rate constants of rapeseed oil conversion in

TagedPEthanol can be derived from biomass via fermentation. It is therefore of interest to explore biodiesel production in supercritical ethanol. Silva et al. [140] investigated the conversion of soybean oil to biodiesel in sub- and supercritical ethanol in a tubular reactor operating from 473 to 648 K and pressure from 7 to 20 MPa, and with an oil-to-ethanol molar ratio from 1:10 to 1:100. Under subcritical conditions, the conversion to fatty acid ethyl ester (FAEE) was low. Yet in supercritical ethanol a biodiesel yield as high as 80% was observed at 623 K, 20 MPa, and an oil-to-ethanol molar ratio of 1:40 within 15 min reaction time. Vieitez et al. [141] reported continuous biodiesel production from soybean oil in supercritical ethanol with water addition at 350 °C, 20 MPa, and an oil-to-ethanol molar ratio of 1:40. The results show biodiesel yields of 78% and 68% without and with 10 wt% of water addition, respectively (Table S11 of the Supplementary Materials). Clearly, the presence of water has a negative effect on process efficiency. TagedPGui et al. [142] presented the results of optimized supercritical ethanol process by varying temperature (300400 °C), the ethanolto-oil molar ratio (550), and reaction time (230 min). Using a systematic, response surface approach, they reported an optimum yield of biodiesel of 79% at 349 °C, an oil-to-ethanol molar ratio of 1:33 and reaction time of 30 min. Quite significantly, direct conversion of the 3rd generation feedstock, i.e., microalgae Nannochloropsis salina,

Fig. 20. Global reaction steps of biodiesel production under supercritical methanol and ethanol conditions.

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TagedPOne of the advantages of supercritical transterification over chemical and enzyme-catalyzed processes is the easiness in carrying out the reaction in a continuous manner. Da Silva [144] showed continuous production of biodiesel from soybean oil using microtube and tubular reactors at 523598 K, 1020 MPa, and the oil-to-ethanol molar ratio of 1:101:40 with CO2 added as co-solvent. The microtube reactor with an inner diameter of 0.76 mm gave higher biodiesel yields than the tubular reactor with an inner diameter of 3.2 mm. They attributed this to the mass transfer phenomenon. In the microtube reactor, mass transfer is enhanced due to the faster flow through the reactor than that for the tubular reactor, no such flow can be obtained. CO2 as a co-solvent appeared to offer little improvements to the yield. Relevant data are reported in Table S12 of the Supplementary Materials. TagedPFig. 22(a) and (b) show the biodiesel yield at 20 MPa and the oilto-ethanol molar ratios of 1:20 and 1:40, respectively, as a function of temperature and residence time. For both ratios, the yields increase with both temperature and residence time as expected. For the ratio of 1:20, the results of Silva et al. [140] are in a good agreement with those of Da Silva et al. [144]. At 375 °C, the yield decreases after 10 min reaction time. Lastly, the yields are higher for the 1:40 ratio than the 1:20 ratio, as can be seen in the figure. TagedPTrentin et al. [102] investigated continuous biodiesel production in supercritical ethanol using a microtube reactor, but found no significant difference in results from those of Da Silva [144].

Fig. 21. Comparison of Arrhenius plots of the rate constants for the transesterification reaction in supercritical methanol. The data are taken from Farobie and Matsumura [177].

TagedPwithout pretreatment was reported by Reddy et al. [143]. They confirmed that the highest yield of biodiesel to be 67% at 265 °C, using the dry algae-to-ethanol (w/v) ratio of 1:9 over 20 min reaction time.

Fig. 22. Effect of temperature and residence time on biodiesel yield in continuous reactor using supercritical ethanol at (a) oil-to-ethanol molar ratio of 1:20 and (b) oilto-ethanol molar ratio of 1:40.

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TagedPAdditionally, Vieitez et al. [145] reported continuous production of biodiesel from castor oil and reported a yield reaching 74% at 573 K and 20 MPa with 5 wt% water addition, and a feedstock flow rate of 0.8 mL/min. TagedPThe effects of FFA, co-solvent, and catalyst addition on biodiesel production in supercritical ethanol have also been examined previously. In a continuous supercritical ethanol process, Vietiez et al. [146] found that FFA addition enhanced the conversion efficiency of soybean, rice bran, and high oleic sunflower oils to biodiesel. As a co-solvent, n-hexane was found to increase biodiesel yields from camelina [104] and palm oils [147] under a reduced severity in operating conditions. In these same studies, optimization showed that for camelina oil, the maximum yield could be achieved at 295 °C, an oil-to-ethanol molar ratio of 1:45, an n-hexane-to-oil ratio of 0.2% (v/v), and over 20 min reaction time, whereas for palm oil the optimal conditions were 300 °C, an oil-to-ethanol molar ratio of 1:33, and an n-hexane-to-oil ratio of 0.4% (v/v) over 30 min reaction time. TagedPSupercritical transterification in ethanol appears to be less efficient than in methanol. Using rapeseed oil in the batch mode reactor, Warabi et al. [148] established that in supercritical methanol the yield of biodiesel reaches 100% within 15 min reaction time, but it required 45 min in supercritical ethanol. Tan et al. [103] observed that supercritical methanol leads to a conversion of about 85% over a shorter reaction time than supercritical ethanol with a yield of 79%. Similarly, Vieitez [149] found that the conversion to biodiesel by methanolysis was higher than by ethanolysis, as did Santana et al. [150] who noted that supercritical methanol gave a biodiesel yield of 90% in 2 min whereas in supercritical ethanol a yield of 80% was achieved in 6 min. Similar results were also reported very recently by Kiss et al. [151].

189

TagedPTriglyceride conversion to biodiesel in supercritical ethanol also proceeds in a three reversible reaction steps nearly identically to that in supercritical methanol as shown in Fig. 20. Kinetically, Velez et al. [152] proposed a first-order kinetic model and determined the rate parameters for sunflower oil conversion to FAEE in supercritical ethanol using a continuous mode reactor. The activation energy of the overall reaction was reported to be 67.6 kJ/mol. Santana et al. [150] derived the activation energy values for the forward and reverse reactions of the three step processes as E1 ¼ 104.8, E1 ¼ 98.3, E2 ¼ 91.7, E2 ¼ 58.6, E3 ¼ 126.0, and E3 ¼ 140.0 kJ/mol (see Fig. 20). In another study and using a tubular reactor in a continuous mode, Silva et al. [140] obtained an activation energy of 78.7 kJ/mol for transesterification of soybean oil in supercritical ethanol. Activation energies in supercritical ethanol are larger than those in supercritical methanol, confirming that ethanol had a lower reactivity than methanol. 3.3. Methyl acetate TagedPProposed by Saka and Isayama [153], the major reasoning behind using methyl acetate is its transesterification byproduct, triacetin which is of a higher value than glycerol. The global reaction steps of biodiesel production in supercritical methyl acetate is presented in Fig. 23. Similarly to the methanol route, transesterification of triglyceride in supercritical methyl acetate also proceeds in three reaction steps. In the first stage, reaction between triglyceride and methyl acetate results in FAME and monoacetin diglyceride, which reacts with another methyl acetate molecule to form FAME and diacetin monoglyceride. FAME and triacetin are generated in the last reaction step. Overall, one triglyceride molecule reacts with three methyl

Fig. 23. Global reaction steps of biodiesel production under supercritical methyl acetate conditions.

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TagedPacetate to generate three biodiesel molecules and one triacetin. The reaction is reversible and it requires a large amount of methyl acetate to obtain complete conversion to biodiesel [154]. TagedPSaka and Isayama [153] investigated transesterification of rapeseed oil in supercritical methyl acetate in the temperature range of 270380 °C for a reaction time of 10120 min, a fixed pressure of 20 MPa, and an oil-to-methyl acetate molar ratio of 1:42. The biodiesel yield data are presented in Table S13 of the Supplementary Materials. Biodiesel was produced at the yield of 105% with triacetin as the byproduct without glycerol formation. Moreover, triacetin in biodiesel appear to affect very little the drop point and oxidation characteristics of the biodiesel. TagedPTan et al. [155] conducted transesterification of palm oil in supercritical methyl acetate using a batch-type tube reactor and optimized the operation with respect to the yield. The yield of biodiesel was 98% at 399 °C with an oil-to-methyl acetate molar ratio of 1:30 and over a reaction time of 59 min. In another study [156], the same authors noted that the highest yield of biodiesel (99%) could be obtained at 400 °C and 22 MPa with an oil-to-methyl acetate molar ratio of 1:30, and over a reaction time of 60 min (Table S14 of the Supplementary Materials). TagedPOptimization was also carried out by Goembira and Saka [157] who examined four key factors affecting the yield, namely, temperature, reaction time, pressure, and the oil-to-methyl molar ratio. They found that for rapeseed oil in supercritical methyl acetate the optimal condition was 350 °C, 20 MPa, 54 min reaction time, and an oilto-methyl acetate molar ratio of 1:42. Under this condition, the FAME yield was 97 wt% and the triacetin yield was 8.8 wt% (see Table S15 of the Supplementary Materials for additional data). TagedPCampanelli et al. [158] also investigated biodiesel synthesis from soybean, sunflower, J. curcas, and waste soybean oil using the same approach. They observed that the composition of feedstock did not significantly affect the yield. A complete conversion to biodiesel was observed at 345 °C, 20 MPa, an oil-to-methyl acetate of 1:42, and reaction time of 50 min for all feedstock considered. Data from their study is summarized in Table S16 of the Supplementary Materials. TagedPThe FFA content in the feedstock affects the biodiesel yield. Using a tubular packed bed reactor, Dona et al. [159] studied soybean and macauba oils in supercritical methyl acetate and reported that the yield was merely 44% for soybean at 350 °C and an oil-to-methyl acetate molar ratio of 1:5 over 45 min reaction time. In comparison, macauba oil achieved a maximum biodiesel yield of 83% at 325 °C, and an oil-to-methyl acetate molar ratio of 1:5 for 45 min reaction time. The difference is possibly caused by the FFA content which is significantly higher in macauba oil. The authors suggested that FFA reacted with methyl acetate to generate FAME and acetic acid following the reaction as presented in Fig. 24. They also concluded that the acetic acid generated could act as an acid catalyst to enhance transesterification. Relevant data of FAME yields are summarized in Table S17 of the supplementary materials. Fig. 25 presents the FAME yield as a function of temperatures and residence time. Biodiesel

Fig. 24. Reaction between fatty acid and methyl acetate.

Fig. 25. Effect of temperature and residence time on biodiesel yield under supercritical methyl acetate conditions at 20 MPa.

TagedPyields increased by increasing temperature from 300 °C to 380 °C due to the fact that higher reaction temperature would result in the higher reaction rates, which eventually correspond to the higher biodiesel yields. However, in some cases, particularly at the temperatures above 338 °C, the biodiesel yield was initially increased to a certain extent, but it was decreased over a prolonged reaction time. Niza et al. [160] elucidated the different reaction behaviors in supercritical methanol and methyl acetate using J. curcas oil. They reported that the maximal yield of biodiesel was 89% in supercritical methanol at 358 °C, 27 min reaction time, and an oil-to-methanol molar ratio of 1:44, whereas the same yield was 72% in supercritical methyl acetate at 400 °C, 32 min reaction time, and an oil-to-methanol molar ratio of 1:50. The difference was attributed to the reactivity and mutual solubility. The thermal stability of biodiesel production in supercritical methyl acetate was examined by the same researchers from 330 to 420 °C [161]. They observed a reduction in thermal stability of poly-unsaturated methyl linoleate with increasing temperature from 330 to 420 °C, while the thermal decomposition of methyl oleate occurred above 390 °C. Thermal decomposition mechanism of biodiesel under supercritical conditions were studied in details by Lin et al. [162], Nan et al. [163], and Liu et al. [164]. They found that thermal decomposition of biodiesel involves isomerization, polymerization (Diels-Alder reaction), and pyrolysis reactions in which these reactions occur in the temperature ranges of 275400 °C, 300425 °C, and >350 °C, respectively. TagedPThe reaction kinetics of transesterification in supercritical methyl acetate was examined in only one study. Campanelli et al. [158] examined the reaction kinetics of oil conversion to biodiesel from 300 to 345 °C and found the overall activation energy to be 373, 349, 364, and 369 kJ/mol for soybean, sunflower, J. curcas, and waste soybean oil, respectively. TagedPEffect of additives in supercritical methyl acetate transterification were reported recently by Goembira and Saka [165]. They demonstrated that the addition of acetic acid and water could enhance the

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TagedPbiodiesel yield, whereas the addition of oleic acid and methanol did not affect the yield. At 300 °C, a higher yield was obtained when aqueous acetic acid was employed as an additive. In addition, the highest biodiesel yield was obtained at 300 °C, 20 MPa, 45 min reaction time by adding 10 wt% aqueous acetic acid. Thus, additives could enhance biodiesel production via the supercritical methyl acetate route. TagedPOther carboxylate esters have also been used as reactants as well as reaction media for supercritical biodiesel production [154]. Among the 12 carboxylate esters studied, methyl acetate gave the highest yield of biodiesel at 98%. Clearly, the shorter chain in methyl acetate resulted in a higher reactivity with triglyceride. 3.4. Dimethyl carbonate (DMC) TagedPSupercritical DMC presents yet another route to producing biodiesel. In this route, biodiesel and glycerol dicarbonate are obtained. In contrast to supercritical methanol, ethanol, and methyl acetate, the global reaction steps in supercritical DMC proceeds via a twostep reversible reaction process as depicted in Fig. 26. The reaction between triglyceride and DMC produces FAME and fatty acid glycerol carbonate (FAGC). Next, FAGC reacts with another DMC to generate the second FAME molecule and glycerol dicarbonate. TagedPStudies pertaining to biodiesel production in supercritical DMC are still limited. Ilham and Saka [166] were the first to investigate biodiesel production from rapeseed oil in supercritical DMC using a batch-type reactor (5 cm3 vessel) made of Inconel alloy-625. They observed that the reaction produced FAME, glycerol carbonate and citramalic acid. A conversion to biodiesel of about 94% (w/w) was achieved at 350 °C, 20 MPa, and an oil-to-dimethyl carbonate

191

TagedP olar ratio of 1:42 after 12 min reaction time. They noted that m FFA could react with DMC to generate FAME and glyoxal as presented in Fig. 27. TagedPIlham and Saka [167] also proposed a two-step supercritical DMC method to produce biodiesel from J. curcas oil under a milder condition under which triglyceride was first hydrolyzed in subcritical water and subsequently esterified in supercritical DMC as shown in Fig. 28. The optimum condition to produce FFA from triglyceride under subcritical water was found to be at 270 °C, 27 MPa and over 25 min reaction time, while in supercritical DMC alone, the condition was given as 300 °C, 9 MPa and 15 min, under which 97% biodiesel yield was obtained. TagedPTan et al. [168] investigated the effect of key factors such as temperature, reaction time, and oil-to-DMC molar ratio in biodiesel production from palm oil. They found that a 91% yield of biodiesel was attained under the optimal condition at 380 °C, 30 min, and an oilto-DMC molar ratio of 1:39. In another study by Ilham and Saka [169], the optimal condition was found to be 300 °C, 20 MPa, 20 min reaction time, and oil-to-DMC molar ratio of 1:42, with a 97% yield. The different optimal conditions could be attributed to the different experimental conditions of which the effect of pressure was neglected for the study by Tan et al [168]. TagedPCoconut oil has been converted to biodiesel in supercritical DMC in an atmospheric-pressure tubular reactor, as reported by Kwon et al. [170]. A nearly complete conversion (98% yield) to biodiesel was achieved in a short reaction time of 12 min at 365450 °C. Relevant data are summarized in Table S18 of the Supplementary Materials. TagedPBiodiesel yield in supercritical DMC is shown in Fig. 29 as a function of reaction time and temperature using data reported by Ilham

Fig. 26. Global reaction steps of biodiesel production under supercritical dimethyl carbonate conditions.

Fig. 27. Reaction between fatty acid and dimethyl carbonate.

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Fig. 28. Global reaction steps of two-step supercritical dimethyl carbonate method for biodiesel production.

TagedPand Saka [169]. For all cases, the biodiesel yield increased with temperature and residence time.

3.5. Tert-butyl methyl ether (MTBE) TagedPAnother interesting route to biodiesel is by using MTBE. Proposed by Farobie et al. [171], supercritical transterification in MTBE, produces FAME and glycerol tert-butyl ether (GTBE). It was reported that GTBE improves the biodiesel quality by increasing its cetane number, reducing particulate matter and CO emissions, and decreasing the cloud point (CP) [172,173]. TagedPThe global reaction steps of biodiesel production in supercritical MTBE also comprises of three consecutive, reversible reactions, as shown in Fig. 30. Triglyceride reacts first with MTBE to produce diglyceride mono-tert-butyl ether (DGE) which reacts with MTBE to

TagedP enerate monoglyceride di‒tert-butyl ether (MGE). Finally, MGE g reacts with MTBE to yield FAME and GTBE. TagedPThe effects of temperature and residence time on biodiesel yield in supercritical MTBE is shown in Fig. 31 with data taken from Farobie and Matsumura [174], which is the only study that evaluated these effects. Again, for all the cases, the biodiesel yield increases with increases in temperature and residence time. TagedPFig. 32 presents a comparison of FAME yields from canola oil in supercritical methanol (375 °C and 15 MPa) [134], ethanol (350 °C and 20 MPa) [140], methyl acetate (350 °C and 20 MPa) [159], and MTBE (400 °C and 10 MPa) [171] in a continuous-mode reactor. Kinetically, conversion in supercritical methanol is the fastest among the reactants compared. The conversion in supercritical MTBE is notably lower than methanol despite the higher temperature and the fact that supercritical MTBE gives the yield almost the same as in methanol over longer reaction times. Interestingly, the yield in supercritical MTBE is larger than methyl acetate probably because of the low polarity in MTBE as compared to methyl acetate, leading to a better miscibility of MTBE with oil. The better miscibility of MTBE circumvents the mass transfer problem of more polar compounds. 4. Empirical rate expressions of supercritical biodiesel production TagedPIt is well known that transesterification consists of a three-step reversible reaction. Modifications of the model have been proposed by considering the order and reversibility of the reactions, and ten commonly used kinetic models have been evaluated [175]. However, in supercritical transterification, methanol is used in excess stoichiometrically. The forward reactions became dominant and each reaction step can be regarded as an irreversible reaction. If we assume that methanolysis of triglycerides (TG) is the rate-controlling step of the overall reaction, the transesterification reaction of TG can be defined by:

Fig. 29. Biodiesel yield obtained in supercritical dimethyl carbonate. The data are taken from Ilham and Saka [166].

TG þ 3MeOH  !  3FAME  þ  GL

ð1Þ

dCTG rTG ¼  ¼ kCa TG Cb MeOH dt

ð2Þ

where, ‒rTG is the rate of TG consumption [mol/(dm3 s)]. As 1 mol of TG reacts with 3 mol of methanol to produce 3 mol of FAME and by ignoring the intermediates, the production rate of FAME can be

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193

Fig. 30. Scheme of the global reaction steps in transesterification of triglyceride in supercritical MTBE.

TagedPexpressed as: dC dCTG ¼ 3kCa TG Cb MeOH rFAME ¼  FAME ¼ 3 dt dt

ð3Þ

TagedPIf we assume that the loss by decomposition of TG or FAME is negligible, the conversion of TG would be the same as the mole ratio between FAME produced and TG loaded, i.e., CTG CTG CFAME =3 CFAME X ¼ 1 ¼ 1 ¼ CTG;0 CTG;0 3CTG;0

ð4Þ

TagedPIntroducing the activation energy (Ea) and pre-exponential factor (A), Eq. (2) becomes:   dCTG Ea a C TG Cb MeOH ¼ A exp  ð6Þ RT dt TagedPIntegration Eq. (6) yields: Zt CTG ðtÞ ¼ CTG;0

  Ea a C TG Cb MeOH dt Aexp  RT

ð7Þ

0

TagedPSince the molecular weight of TG is almost three times that of FAME, conversion can be related to the mass yield of FAME as shown below:

ð5Þ

TagedPData were collected from Kusdiana and Saka [118], Demirbas [119], Shin et al. [130], and Farobie and Matsumura [136], all at 20 MPa. Experimental data were then fitted to the calculated values by the least squared errors (LSE) method. Fig. 33 shows the result of the fitting between the experimental data and the calculated value. The temperature dependence of the rate constant is then modeled

Fig. 31. Biodiesel yield obtained in supercritical MTBE at 20 MPa. The data are taken from Farobie and Matsumura [177].

Fig. 32. Comparison of FAME yield in supercritical methanol, ethanol, methyl acetate, and MTBE.

CFAME m =MwFAME ¼ FAME 3CTG;0 3mTG;0 =MwTG m =3MwFAME mFAME ¼ FAME  ¼ mass yield of FAME mTG;0 =MwTG mTG;0



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Fig. 33. Result of fitting between experimental data with the calculated value for supercritical methanol.

TagedPby the Arrhenius equation as shown in Fig. 34. The rate equation for biodiesel production in supercritical methanol was determined and may be expressed as: dCTG ¼f½0:5179 ðmol dm3 Þ0:3887 s1 g dt   19:60kJ=mol CTG 1:2411 CMeOH 0:1476  exp  RT

ð8Þ

TagedPUsing the same procedure, the experimental data of Silva et al. [140], Da Silva et al. [144], and Farobie and Matsumura [176] (all at 20 MPa and oil-to-ethanol molar ratio of 1:40) were fitted as shown in Fig. 35. The rate equation for biodiesel production in supercritical ethanol may be modeled by: dCTG ¼ð2:5  1012 ðmol dm3 Þ0:126 s1 Þ dt   175:98 kJ=mol CTG 1:0 CEtOH 0:126  exp  RT

Fig. 35. The result of fitting between experimental data with the calculated value for supercritical ethanol.

ð9Þ

TagedPTo derive the rate equation for methyl acetate, the data reported by Saka and Isayama [153], Campanelli et al. [158], and Goembira and Saka [157] (all at 20 MPa and oil-to-methyl acetate molar ratio

TagedPof 1:42) were used. Fig. 36 shows the result of the fitting and the rate equation may be expressed as: dCTG ¼ð5:4  1022 ðmol dm3 Þ0:65 s1 Þ dt   305:92 kJ=mol CTG 1:5 CMeAc 0:15  exp  RT

ð10Þ

TagedPData from the previous work of Ilham and Saka [169] (20 MPa and oil-to-DMC molar ratio of 1:42) was used to generate the empirical rate expressions of biodiesel production in supercritical DMC. Fig. 37 shows the result of the fitting between the experimental data and the calculated values for supercritical DMC. The empirical rate expressions for biodiesel production in supercritical DMC may be modeled by:   dCTG 41:64 kJ=mol CTG 1:1 CDMC 0:1 ¼ ð3:8541ðmol dm3 Þ0:2 s1 Þ exp  RT dt ð11Þ TagedPEmpirical rate expressions for biodiesel production under supercritical MTBE conditions was derived from the data of Farobie and Matsumura [177] (20 MPa and oil-to-MTBE molar ratio of 1:40). Fig. 38 shows the result of the fitting and the rate equation may be expressed as: dCTG ¼ð1:24  104 ðmol dm3 Þ0:155 s1 Þ dt   82:15 kJ=mol CTG 1:05 CMTBE 0:105  exp  RT

ð12Þ

TagedPTo compare the rate behaviors in supercritical methanol, ethanol, methyl acetate, dimethyl carbonate, and MTBE, a pseudo first-order reaction may be assumed:   dCTG Ea CTG ¼ A exp  ð13Þ RT dt

Fig. 34. Arrhenius plot for oil conversion to biodiesel in supercritical methanol.

TagedPThe equation is roughly valid for reactions in which the reactant (e.g., methanol) is in excess. Indeed the rate equations (8) thru (12)

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195

Fig. 38. The result of fitting between experimental data with the calculated value for supercritical MTBE. The data are taken from Farobie and Matsumura [177].

Fig. 36. The result of fitting between experimental data with the calculated value for supercritical methyl acetate.

TagedPshow fairly weak dependency of the reaction rate on the concentration of the reactant but they are close to the first order for the oil concentration. The results are shown in Table 1 and Fig. 39. The reaction rate constants for supercritical 1-propanol and 1-butanol are also plotted under the reaction condition of 300 °C [145]. As can be observed, activation energies follow the order of: methanol  DMC < ethanol < MTBE < methyl acetate. TagedPAs shown in Fig. 39, the reaction rate of biodiesel synthesis in methanol is the highest, owing possibly to its small molecular size, which enables the oxygen atom from methanol to readily attack the carbon atom of the carbonyl functional group from triglycerides. The second highest reaction rate is for supercritical DMC. The two methoxyl groups in DMC increase the reaction probability for it to attach triglyceride to generate fatty acid methyl ester. The reactivity in MTBE is almost the same as in ethanol. The reactivity of MTBE

cTagedP ould be caused by its decomposition into the methoxy and t-butyl radicals under the temperature and pressure of the reaction. The tbutyl radical may have a high concentration due to its stability due to the hyperconjugation effect, leading to appreciable radical reactions in the overall reaction process. Finally, the lowest reactivity is observed for methyl acetate, which may be attributed to the necessity of breaking the methyl ketone-methoxy bond in order for the reaction to occur, and the methyl ketone radical is not a stable intermediate.

5. Factors affecting supercritical biodiesel production 5.1. Temperature and reaction time TagedPReaction time, together with temperature, plays an important role in determining the reaction kinetics of supercritical oil conversion to biodiesel. In all reports pertaining to supercritical methanol and ethanol, oil conversion to biodiesel mainly increased with reaction temperature and time. It was owing to an increase in the reaction rate constant based on Arrhenius equation and a change of alcohol properties at high temperatures and high pressures. The effect of temperature on biodiesel yield in supercritical methanol was first investigated by Kusidana and Saka [118]. They studied the effect of temperature over the temperature and pressure ranges of 200 °C/7 MPa to 487 °C/105 MPa. They reported that at 200 and 230 °C, the biodiesel yield was relatively low owing to the subcritical state of methanol. A complete conversion of rapeseed oil to biodiesel was found at 350 °C after 4 min obtaining Table 1 Comparisons of empirical rate expressions for biodiesel production under supercritical conditions using first order reaction. Supercritical condition Methanol Ethanol Methyl Acetate Dimethyl carbonate

Fig. 37. The result of fitting between experimental data with the calculated value for supercritical dimethyl carbonate. The data are taken from Ilham and Saka [166].

MTBE

Equation

  kJ=mol ¡ (0.254 s¡1) £ exp  17:2 RT C TG   80:7 kJ=mol dCTG 3 ¡1 ¼ ¡ (8.30 £ 10 s ) £ exp  C TG RT dt   278:2 kJ=mol dCTG 20 ¡1 ¼ ¡ (1.10 £ 10 s ) £ exp  C TG RT dt   60:2 kJ=mol dCTG 2 ¡1 C ¼ ¡ (1.95 £ 10 s ) £ exp  TG RT dt  kJ=mol dCTG ¼ ¡ (1.59 £ 104 s¡1) £ exp  82:6 RT ) C TG dt dCTG ¼ dt

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rTagedP atio (42), a complete conversion was apparent with methyl ester yield of 95%. However, methanol molar ratio more than 50 gave no benefits. Similar results were reported by Varma and Madras [138], Song et al. [122], He et al. [133], Goembira and Saka [157], and Tan et al. [156]. TagedPClearly, oil-to-alcohol molar ratio affects chemical equilibrium, and higher molar amount of alcohol for the same amount of triglyceride drives the chemical equilibrium to the product side. Considering that the reaction should be completed with respect to triglyceride so that separation of reactant can be considered only for the alcohol and not for triglyceride, the oil-to-alcohol molar ratio of 1:40 is recommended. Note that adding more alcohol cannot improve the yield once complete conversion is achieved, and it will make separation of alcohol from the product more energy-consuming. 5.3. Pressure

TagedP95% of biodiesel. At 400 °C, a complete conversion needed only 2 min. However, thermal decomposition of biodiesel occurred at temperatures above 400 °C. Similar results were reported by Rathore and Madras (2007) [178], Imahara et al. [179], and Lin et al. [180]. TagedPAt higher temperatures, usually above 400 °C, thermal cracking of triglyceride also occurred as reported by Marulanda et al. [181]. Fig. 40 shows the products of thermal cracking of triglyceride under supercritical conditions. TagedPConsidering decomposition of structure required for the biodiesel, the reaction temperature around 350 °C is recommended for the biodiesel production using supercritical alcohol. Reaction rates of isomerization, polymerization, and DielsAlder reaction should be determined in addition to the biodiesel production rates derived in this review to find the detailed optimum conditions.

TagedPPressure also plays an important role on biodiesel production under supercritical conditions since it affects the supercritical fluid properties including hydrogen bond intensity, density, and viscosity. Shin et al. [130] investigated the effect of pressure on biodiesel content in a batch reactor at a fixed temperature of 335 °C and oil-tomethanol molar ratio of 1:45. It was reported that an increase in pressure from 15 to 20 MPa resulted in a higher biodiesel yield, but it was relatively constant at pressures above 20 MPa. Similar results were reported by Goembira and Saka [157] and Ilham and Saka [169]. The pressure effect on supercritical biodiesel production using various reactants is presented in Table 2. TagedPConsidering the fact that the reaction to produce of biodiesel does not change the molar number of the molecules (4 mol to 4 mol), the effect of pressure on chemical equilibrium should not be observed. Its effect should be more on the chemical reaction rate due to the higher molecular density and homogenization of the phase due to the miscibility of supercritical alcohol with oil. As reported by Anitescu et al. [182], the pressure could affect the mixing of the reaction mixtures. A homogeneous reaction phase could be achieved if the pressure is appropriately selected. Because too high pressure results in higher cost of the reactor, pressure around 20 MPa is recommended for biodiesel production using supercritical alcohols.

5.2. Oil-to-reactant molar ratio

6. Challenges and recommendations for future work

TagedPStoichiometrically, transesterification of triglyceride requires three molecules of methanol. However, since the transesterification process is reversible, an enormous amount of methanol is needed to achieve a complete conversion. The effect of oil-to-methanol molar ratio was first studied by Kusdiana and Saka [120] in the range of 1:3.51:42. It was reported that at the lower methanol molar ratio (6 or less), incomplete conversion to biodiesel was observed resulting in lower yield of biodiesel. However, at higher methanol molar

6.1. Energy recovery

Fig. 39. The comparison of Arrhenius plots for biodiesel production in supercritical methanol, dimethyl carbonate, MTBE, ethanol, 1-propanol, 1-butanol, and methyl acetate.

TagedPDespite the many advantages of supercritical biodiesel production, this technology is still facing many challenges. One such challenge is the energy efficiency of the process. As we discussed in Section 5.1., the desired temperature for supercritical transesterification is 350 °C. The conversion reactions are largely endorthermic with reaction enthalpy of 39.89 kJ/mol for supercritical methanol.

Fig. 40. Products obtained after thermal cracking of triglyceride under supercritical conditions.

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Table 2 Effect of pressure on biodiesel yield under various supercritical reactants. Reactant

Pressure [MPa]

Temperature [°C]

Residence time [min]

Molar ratio

Yield

Reference

Methanol

8.7 10.5 12.5 15.5 21 25 36 10 20 30 40 10 20 30 40 10 20 30 40 5 10 15 20 30 5 10 20 40 10 20 30

280 280 280 280 280 280 280 300 300 300 300 300 300 300 300 300 300 300 300 350 350 350 350 350 300 300 300 300 300 300 300

30 30 30 30 30 30 30 20 20 20 20 20 20 20 20 20 20 20 20 30 30 30 30 30 30 30 30 30 30 30 30

1:42 1:42 1:42 1:42 1:42 1:42 1:42 1:42 1:42 1:42 1:42 1:42 1:42 1:42 1:42 1:42 1:42 1:42 1:42 1:42 1:42 1:42 1:42 1:42 1:42 1:42 1:42 1:42 1:40 1:40 1:40

56.13 63.87 76.89 81.76 86.55 90.92 91.51 76.03 77.86 80.46 82.75 71.45 73.28 76.03 78.17 66.41 68.09 66.41 67.79 0 9.85 19.37 84.31 85.71 39.58 71.66 94.04 94.04 46.96 50.55 57.49

[137]

Ethanol

Propanol

Butanol

Methyl acetate

Dimethyl carbonate

MTBE

TagedPOne possible solution to improving the energy efficiency is to use a spiral reactor design. This reactor is composed of a parallel tube heat exchanger and a high temperature transesterification reactor as shown in Fig. 41. It was reported that the spiral reactor was superior to conventional reactor in terms of heat recovery. In addition, the effectiveness of this reactor was also been investigated for biodiesel production using supercritical MTBE. Our research group found that the spiral reactor performed well for biodiesel production using supercritical MTBE, affording a higher FAME yield compared to the conventional flow reactor for the same residence time [174]. Moreover, the spiral reactor employed here was effective for biodiesel production using supercritical MTBE due to the successful recovery of the heat [183]. TagedPEnergy analysis for biodiesel production under supercritical conditions has been carried out by Farobie and Matsumura [183], comparing conventional and spiral reactors shown in Fig. 42. Here, analysis for biodiesel production in supercritical methanol is shown

[148]

[148]

[148]

[157]

[169]

[187]

TagedPas an example. Analyses in supercritical ethanol and MTBE can be found elsewhere [43, 69]. TagedPEnergy required to heat the reactant stream to the described reaction temperature (Qheating) for supercritical methanol was calculated to be 1.095 MJ/d using Eq. (14).   ð14Þ Qheating ¼ moil;c Cp;oil þ mMeOH;c Cp;MeOH DTc where moil,c represents the mass flow rate of oil (for canola oil, moil,c ¼ 0.5976 kg/d), Cp,oil is the specific heat of canola oil (1.913 kJ/ (kg K)), DTc is the temperature difference of the reactant stream from the inlet to the reactor center (310 °C), mMeOH is the mass flow rate of methanol (0.9518 kg/d) in the cold, and Cp,MeOH is the specific heat of methanol (2.51 kJ/(kg K)). TagedPThe recovered heat from the system (Qrecovery) was calculated to be 0.9361 MJ/d using Eq. (15).   ð15Þ Qrecovery ¼ mBDF;h Cp;BDF þ mGL;h Cp;GL þ mMeO;h Cp;MeOH DTh

Fig. 41. Design of spiral reactor and its temperature profile taken from [176].

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Fig. 42. Schematic diagram of biodiesel production using (a) spiral reactor and (b) conventional flow reactor.

TagedPwhere mBDF,h is mass flow rate of biodiesel fuel produced (0.6038 kg/ d), Cp,BDF is specific heat of biodiesel fuel (1.80 kJ/(kg K)), DTh is the temperature difference from the reactor center to the outlet (271 ° C), mGL,h is the mass flow rate of glycerol produced (0.0605 kg/d), Cp,GL is the specific heat of glycerol (2.41 kJ/(kg- K)), mMeOH,h is the mass flow rate of unreacted methanol (0.8851 kg/d), and Cp,MeOH is the specific heat of methanol (2.51 kJ/(kg K)). TagedPThe total energy supply for biodiesel production in the spiral reactor was determined by subtracting recovered heat from the heat required to achieve reaction temperature, and was calculated to be merely 0.1589 MJ/d. Obviously, the energy recovered in the conventional reactor is zero, and the energy supply is therefore several factors larger than that of the spiral reactor. TagedPThe calculation of energy analysis for biodiesel production in a spiral reactor was not only evaluated for supercritical methanol but also for supercritical ethanol and MTBE. Fig. 43 shows the comparisons of energy efficiency calculated for supercritical methanol, ethanol, and MTBE. Energy efficiency (he) and heat recovery efficiency (Qeff) were determined using Eqs. (16) and (17), respectively. Please

Fig. 43. Comparison of energy efficiencies for the production of biodiesel in supercritical MTBE, ethanol, and methanol.

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agedPT TagedP(1)

TagedPnote that the energy efficiency and heat recovery efficiency presented in this figure are fractional values, not in % units.

he ¼

Ep Ef

Qeff ¼

Qrecovery Qheating

ð16Þ

TagedP(2)

ð17Þ

TagedP(3)

TagedPHere, he represents energy efficiency, Ep represents the total energy in the products [MJ/d], Ef represents the total energy in the feedstock [MJ/d], 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]. TagedPAs can be seen, energy efficiency and heat recovery efficiency of the spiral reactor were 0.98 and 0.85, respectively, and they were the same for all processes. The difference of total energy in the feedstock and product among three reactants could be attributed to the difference of optimum conditions. Again, the optimum conditions for the supercritical biodiesel production using methanol, ethanol, and MTBE were achieved within 10, 30, and 30 min, respectively [177].

TagedP(4)

TagedP(5) 6.2. Excessive amount of reactants TagedPAnother challenge of supercritical biodiesel production is an excessive amount of reactants required to achieve the complete conversion of oil to biodiesel. As we mentioned in Section 5.2., the optimum oil-to-reactant molar ratio for supercritical biodiesel production is 1:40. This amount is 5 times larger than the conventional biodiesel production using homogeneous alkali-catalyzed transesterification. The excessive amount of reactant results in high cost needed for biodiesel production process. The subsequent reactant recovery also needs high energy consumption. An alternative way to overcome this severity is by employing integration process of supercritical biodiesel production and reactant recovery at the same time. TagedPBesides above challenges, 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, coking in the reactor, the impurities such as metal content and intermediate compounds of diglyceride and monoglyceride, the engine evaluation, and the cost of this technology. Since supercritical biodiesel production requires high temperature and high pressure, the safety issue is still in debate. In addition, the coking in the reactor may occur due to the formation of char and tarry material. The impurities such as heavy material and intermediate compounds of diglyceride and monoglyceride should be recovered in order to obtain high biodiesel quality. TagedPFrom 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 include the use of integration process and utilization of waste or non-edible oil. To quantitatively prove the applicability of this technology, the technical issues including engine evaluation and economic feasibility are required for further study.

Biodiesel production in supercritical methyl acetate, DMC, and MTBE enable generation of value-added byproducts. The application of these byproducts requires further investigation. There is a lack of economic analysis on supercritical biodiesel production. Such an analysis would help to bridge the laboratory studies with real-world application. Since the supercritical technology requires high temperature, methods to reduce the severity of conditions, e.g., a two-step supercritical methyl acetate, DMC, or MTBE method for biodiesel production should be studied in more detail. Another method to reduce the severity of condition is to add co-solvents, e.g., nhexane. Since the critical pressure and temperature of n-hexane is much lower than those of alcohols, methyl acetate, DMC, and MTBE, the addition of the co-solvent would help to achieve the higher biodiesel yields under milder conditions. The use of cosolvents can also reduce the amount of reactants required to achieve high yields. Biodiesel production using supercritical method requires an excessive amount of reactants to achieve the complete conversion of oil to biodiesel. In order to make the process efficient and less reactant consuming, a rational integration process of reactant recovery is required. Global reaction steps for supercritical biodiesel production using methanol, ethanol, methyl acetate, DMC, and MTBE have been alluded. However, there is still a need for understanding the detailed reaction mechanisms and models for reactor scaling up purpose.

7. Concluding remarks TagedPBiodiesel production processes are reviewed in this paper. It is shown that supercritical biodiesel production offers many advantages over conventional methods, including large reaction rate, no catalyst requirement, and insensitivity to feedstock. Economical and technical hurdles remain but significant progress has been made through experiments with various reactants such as methanol, ethanol, methyl acetate, DMC, and MTBE in a variety of reactor types, including batch-type reactor, continuous flow reactor, and spiral reactor. Reaction rates for biodiesel production in supercritical methanol, ethanol, methyl acetate, DMC, and MTBE were compared for the first time in this review. Overall activation energies of 17.2, 80.7, 278.2, 60.2, and 82.6 kJ/mol were determined for biodiesel production in supercritical methanol, ethanol, methyl acetate, DMC, and MTBE, respectively, assuming pseudo first-order reaction kinetics. Supplementary materials TagedPSupplementary material associated with this article can be found, in the online version, at doi:10.1016/j.pecs.2017.08.001.

TagedP

TagedP

TagedP

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6.3. Recommendations for future work TagedPEven though biodiesel production under supercritical conditions has enormous benefits, its feasibility in commercial applications remains debatable. Laboratory scale analyses, tests and finding are not always directly indicative of potential for technology success at the industrial scale. Given this consideration, we make the following recommendations for future work:

199

TagedP

TagedP

TagedP

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