Supercritical fluid technology in biodiesel production

2.4.1 Catalytic method

Vegetable oils can be involved in the transesterification, by heating them with an alcohol and a catalyst [4]. Catalysts used in biodiesel production are divided in two general categories: homogenous and heterogeneous types. If the catalyst remains in the same (liquid) phase as that of the reactants during transesterification, it is defined as homogeneous catalytic transesterification. By contrast, if the catalyst remains in a separate phase (i.e., solid, immiscible liquid, or gaseous) from that of the reactants, the process is called heterogeneous catalytic transesterification. In this process, the selection of a suitable catalyst is an important factor in maximizing the oil conversion yield at mild reaction conditions, and thus reducing the biodiesel production cost [4].

Commercial biodiesel is currently produced by transesterification using a homogenous catalyst solution. A factor affecting the selection of the catalyst type is the amount of FFAs present in the oil [4]. For oils having a lower amount of FFAs, a base-catalyzed reaction gives a better conversion in a relatively short time, while for higher FFAs containing oils, acid-catalyzed esterification followed by transesterification is more convenient.

Numerous studies have been performed employing various oils, different alcohols (methanol, ethanol, butanol), as well as different catalysts, including homogeneous ones, such as sodium hydroxide (NaOH) [8], potassium hydroxide (KOH) [9, 10], and sulfuric acid [11], and heterogeneous ones such as lipases, calcium oxide (CaO) [12–14] and magnesium oxide (MgO) [15].

2.4.1.1 Homogeneous catalyst

There are two kinds of homogenous catalysts, namely base and acid ones [3]. The homogenous transesterification process requires a high purity of the raw materials, and a separation of the catalyst, byproduct, and product at the end of the reaction. Both requirements contribute to the high the cost of biodiesel. The general form of a homogeneous catalytic transesterification process can be seen on a process flow diagram of Figure 3.

Figure 3

Homogenous catalytic transesterification process.

a. Base catalyst Biodiesel is normally produced using a homogeneous base catalyst, such as alkaline metal alkoxides and hydroxides, as well as sodium carbonate (Na₂CO₃) or potassium carbonate (K₂CO₃) [1–4]. There are some advantages in using homogeneous base catalysts: mild operation conditions, high conversion in a minimal time, high catalytic activity, and wide availability as well as being cheap catalysts.

In general, base catalytic transesterification processes are carried out at low temperatures and pressures (60–65°C and 0.14–0.42 MPa), with low catalyst concentrations (0.4–2 wt% NaOH or KOH in oil, in a transesterification with methanol) [1–3]. In addition, the typical process conditions for homogeneous base catalysis are mild, involving stoichiometric alcohol/oil molar ratio (or slightly higher), reaction at alcohol reflux temperature, atmospheric pressure or small overpressure, and low reaction times (typically 1 h).

Some drawbacks of this process are the sensitivity to the purity of the reactants, the FFAs content, as well as the amount of water in the oil sample [1, 3]. When the feedstocks contain significant amounts of FFAs and water content, they cannot be converted into biodiesels without soap formation. FFAs react with the catalyst to produce soaps that inhibit the separation of biodiesel from glycerin, which requires more wastewater from purification. Because the presence of water leads to saponification, the base catalyst is consumed in producing soap, which reduces the catalyst efficiency. The soap causes an increase in viscosity, formation of gels, thus reducing the ester yield, and makes the separation of glycerol difficult. Therefore, side reactions such as saponification and hydrolysis must be kept to a minimum.

Several strategies were recently investigated to enhance the advantages of base-catalyzed transesterification processes, with the purpose of seeking a new way to reduce the biodiesel cost: Berchmans et al. [9] devised the reaction of a mixture of Jatropha curcas and waste food oil, with methanol and KOH. At a mixing speed of 900 rpm, a methanol to oil mole ratio of 6:1, and a KOH concentration of 1 wt% of oil, the total oil conversion reached 96% within <10 min.

The process intensification is another wide opportunity to increase the profitability of the process, by investigating the most suitable reactor configurations: with the same catalyst (KOH), Santacesaria et al. [10] developed a simple instrument to test the characteristics of the transesterification between soybean oil and methanol in microchannels of different sizes. With experimental conditions of a methanol to oil ratio of 6:1, KOH concentration of 2 wt%, and a temperature of 60°C, they obtained a 95% conversion with the tubular reactor filled with stainless steel ribbon wool in a very short residence time, equal to 3 min, thus highlighting the importance of the involved phase mixing phenomena, and increasing the productivity per reactor volume.

Finally, other alcohols are also widely investigated in the literature, whose advantages over methanol could be their bio-derived source, like ethanol from biomasses, although the price is inevitably higher. Velickovic et al. [8] evaluated the effects of temperature, ethanol to oil molar ratio, and catalyst loading in the process of converting sunflower oil via ethanolysis with NaOH. According to the results, the maximum FA ethyl ester (FAEE) purity was 98.6% after 5 min under the optimal conditions, being a reaction temperature of 74.6°C, an ethanol to oil molar ratio of 12:1, and an amount of catalyst of 1.25 wt%.

b. Acid catalyst An alternative way of processing triglycerides for biodiesel production is to use an acid catalyst [4]. Acid catalytic transesterification of biodiesel can economically compete with base catalytic processes using virgin oil, especially when the former uses low-cost feedstock. Sulfuric acid, hydrochloric acid, and sulfonic acid are normally employed as acid catalysts. Acid-catalyzed transesterification starts by mixing the oil directly with the acidified alcohol, so that separation and transesterification occur in a single step, with the alcohol acting both as the solvent and as the esterification reagent. The use of excess alcohol reduces the time required for the homogeneous acid catalyzed reaction. Hence, Bronsted acid catalyzed transesterification requires a high catalyst concentration and a higher molar ratio to reduce the reaction time.

One advantage of homogeneous acid catalysts (over alkali ones) is their low susceptibility to the presence of FFA in the feedstock. However, homogeneous acid catalytic transesterification is especially sensitive to water concentration: as little as 0.1 wt% water in the reaction mixture is able to affect ester yields in transesterification of vegetable oil with methanol, with the reaction almost completely inhibited at 5 wt% water concentration. Other drawbacks of this process are the equipment corrosion, more waste from neutralization, difficulty to recycle, formation of secondary products, higher reaction temperature, long reaction times, relatively slow reaction rate, weak catalytic activity, and elaborate process engineering.

Wang et al. [11] studied the transesterification between methanol and waste cooking oil in the presence of sulfuric acid. A 10 h lasting reaction led to a 90% conversion of the feedstock at a temperature of 95°C, an amount of sulfuric acid equal to 4%, and a methanol to oil ratio of 16:1. The major obstacle of using acid catalyst is related to equipment, which has to withstand the acidity of the reacting medium.

2.4.2 BIOX solvent process

This is an innovative process for the synthesis of biodiesel from high FFA feedstock studied and developed by the University of Toronto in Canada [4, 29] (Figure 4). This patented process was acquired by BIOX and a commercial plant was built in Hamilton, Ontario, Canada, in 2006. Firstly, this process converts the FFAs through acid esterification, and then the triglycerides to biodiesel with a base catalyzed transesterification. By the addition of a co-solvent, the BIOX concept is a single phase, continuous process, which works at atmospheric pressures and near-ambient temperatures, and has a residence time in the two-steps of the reaction of <90 min, rather than the several hours or even days under conventional processes, reaching almost complete conversion of the feedstock. The co-solvent is then recycled and reused continuously in the process. The BIOX process handles not only grain-based feedstock, but also waste cooking greases and animal fats in order to create the competitive method for biodiesel production.

Figure 4

BIOX process.

The role of the co-solvent is to form the homogeneous phase in order to reduce the need for extensive agitation, multiple batch reactor, as well as extensive reaction time, therefore, saving energy and reducing the operating cost.

2.4.3 Microwave-assisted method

Microwave irradiation is a well-established method of improving the production of biodiesel by reducing the reaction time and gaining high yields. It enhances the reaction rate and makes the separation process easier in comparison with conventional heating [30]. Lertsathapornsuk et al. [31] prepared FAEE from waste frying palm oil with a household 800 W modified microwave oven. The best conversion yield (over 97%) was obtained by applying 3.0 wt% NaOH as a catalyst, with an ethanol to oil molar ratio of 12:1 for 0.5 min. Mazzocchia et al. [32] converted the triglycerides in rapeseed oil to FAME in the presence of montmorillonite, by using the microwave irradiation heating technique. Firstly, it was observed that the microwave technique decreased the reaction time significantly, and secondly montmorillonite was proven to be a promising catalyst for microwave-assisted production of biodiesel. It can work at a temperature of about 170°C and can lower the alcohol to oil molar ratio to 9:1.

Suppalakpanya and co-workers [33] investigated the production of ethyl ester from the esterified crude palm oil with 1.7 wt% FFA employing microwave heating. Optimum reaction parameters for the transesterification process have been identified: a molar ratio of oil to ethanol of 1:8.5, 1.5 wt% of KOH/oil, a reaction time of 5 min and a microwave power of 70 W. This research yielded 85 wt% and its ester content was 98.1 wt%; its fuel properties were within the limits prescribed by American standards for methyl ester.

A two-step transesterification process of kenaf seed oil catalyzed by KOH, was studied by Rathana et al. [34]. Biodiesel produced by this research possesses acceptable fuel properties used for diesel engines. The experimental results also indicate that a conversion level of 94% was obtained at the optimum conditions: 0.55% KOH catalyst, 4.5:1 methanol to oil molar ratio, 57.5°C and 6.5 min reaction time.

Besides the traditional base catalyst, a study on application of a solid catalyst for biodiesel production was carried out by Zhang et al. [35]. An efficient microwave-assisted transesterification technique was developed to prepare biodiesel from yellow horn (Xanthoceras sorbifolia Bunge) oil with a heteropolyacid catalyst, namely Cs_2.5H_0.5PW₁₂O₄₀. The highest yield of 96.22% was obtained with the following optimal reaction conditions: temperature of 60°C, ratio of methanol to oil of 12:1, and 1 wt% catalyst for 10 min. Koberg et al. [36] evaluated the capability of producing biodiesel from Nannochloropsis microalgae, with the aid of SrO catalyst. It was found that a direct transesterification reaction using microwave irradiation resulted in a higher biodiesel yield of 37.1% in 5 min, while only 20.9% yield can be obtained in the same reaction time by using the ultrasound-assisted (UA) technique. This work also indicated that the use of a unique biotechnology-based environmental system utilizing flue gas from a coal burning power station for cultivating microalgae reduced the cost of algae production significantly.

Jianxun and co-workers [37] synthesized a solid base catalyst (K₂SiO₃/C) capable of microwave absorption, which was used for the transesterification of soybean oil under microwave radiation. The K₂SiO₃/C catalyst was prepared by an impregnation method that loaded K₂SiO₃ on carbon particles followed by drying at 120°C. The maximum yield of biodiesel was 96.5% after 150 min reaction time at 65°C, with a methanol to oil ratio of 30:1 and a catalyst concentration of 24 wt%.

Chen et al. [38] reported the microwave-assisted transesterification of waste cooking oil in the presence of methanol and CH₃ONa catalyst. The experimental results showed that an increase of reaction time from 1 to 3 min caused a significant increase in biodiesel yield, which decreased with a further increase to 6 min. The optimal reaction conditions are 0.75 wt% catalyst, a methanol to oil ratio of 6, a reaction time of 3 min, and a microwave power of 750 W.

Kim et al. [39] studied the microwave-assisted esterification of FFA with a heterogeneous catalyst, namely sulfated zirconia (S–ZrO₂), in order to demonstrate the microwave energy efficiency. In the reaction condition of 5 wt% sulfated zirconia and a ratio of oil to methanol of 20:1 at 60°C, more than 90% conversion of the esterification was achieved in 20 min by microwave heating, while the same reaction needed 130 min with conventional heating. The authors also concluded that the electric energy consumed by microwave heating was only 67% of the minimum conventional estimated heat energy due to the significant reduction in reaction time.

Recently, calcium oxide was intensively utilized in various techniques for biodiesel production including microwave-assisted process. Hsiao et al. [40] examined the effect of nanopowder CaO on the transesterification of soybean oil assisted by microwave irradiation. The reaction conditions that could reach a 96.6% of conversion, were a methanol to oil molar ratio of 7:1, 3.0 wt% catalyst, a reaction temperature of 65°C, and a reaction time of 60 min. Kamath et al. [41] investigated two-step biodiesel synthesis using microwave irradiation. Crude karanja oil, with high FFA (8.8%), was reduced to 1.1% by pretreating the oil with methanol (33.83 w/w% methanol–oil ratio) employing H₂SO₄ as a catalyst (3.73 w/w%) and irradiating for 190 s at 180 W. The pretreated oil was used to prepared biodiesel via alkali catalyzed transesterification. The optimal parameters were a methanol to oil ratio of 33.4, a KOH concentration of 1.33 wt%, and a microwave irradiation time of 150 s at 180 W. The predicted FAME yield of 91.4% was validated experimentally.

2.4.4 UA method

Various studies implemented UA transesterification to reactions catalyzed by acid, base or enzymes, in the case of different kinds of feedstock, such as oil-bearing materials (seeds, rice, bran), waste cooking oils, fish oils, and animal fats [42]. In UA processes, batch and flow reactors with indirect or direct sonication are used. The former is performed in a reaction vessel (usually Erlenmeyer or a round-bottom flask) immersed in an ultrasonic cleaning bath and the latter is performed in a reaction vessel with an ultrasonic processing probe, known as horn, sonotrode or sonoprobe. Two sonication modes are used, either pulse (on-off cycles) or continuous.

UA transesterification is among the alternative and innovative biodiesel production processes. The use of UA transesterification has been already proved to be very effective for intensification of biodiesel production processes [42]. Despite the large potential for UA transesterification, further studies are needed to overcome considerable technical and process limitations. Several different approaches have been used in the last decade to improve the UA biodiesel production, such as the optimization of process factors using statistical methods, modeling the process kinetics, the application of in situ UA transesterification processes, the use of phase transfer catalysts, the use of a continuous process instead of batch ones, and the development and application of novel types of ultrasonic reactors.

One of the latest modifications is the combination between ultrasounds and microwaves, which leads to synergistic effects [43]. This approach improved process chemistry, yield, and reduction in the process time by about 15 times. Esterification processing was employed to reduce the initial acid value of the oil from 18.9 mg to 1.7 mg of KOH/g oil with the purpose of avoiding a saponification reaction during the alkaline transesterification. The molar ratio and catalyst concentrations have been optimized for the esterification and transesterification stages for microwave, ultrasound, and sequential approaches. It was observed that the optimum molar ratio required for esterification is 1:4, 1:3, and 1:2 for the ultrasound, microwave, and sequential approach, respectively, whereas for transesterification, the optimum ratio in the same order is 1:6, 1:6, and 1:4. The reaction time for esterification and transesterification using ultrasound alone was 60 min and 20 min, respectively, and it reduced to only 15 min and 6 min, respectively, for the sequential approach. The energy required for processing and separation was reduced significantly compared with individual operation.

2.4.5 Non-catalytic process

In a catalytic reaction to produce biodiesel through transesterification, several post-reaction processes, such as purification of the esters, separation, and recovery of residual reactants and catalysts are involved [2, 4, 44]. These processes render the production of biodiesel transesterification system complicated, thus giving a reason to investigate the production of biodiesel from triglycerides via non-catalytic reactions. There are mainly two non-catalytic transesterification processes which will be discussed in detail.

The use of supercritical fluids (SCFs) is an innovative approach to be used in various industrial applications [2, 4, 44]. Numerous research works concerning biodiesel synthesis via SCFs have been investigated and the summary of all current SCF techniques will be discussed in the next part of this review.

3.2.1 SCF methanol

Supercritical methanol (SCMe), the critical parameters of which are critical temperature (T_c)=239.4°C, critical pressure (P_c)=8.1 MPa, and ρ_c=0.276 g.cm^-3, was one of the first SCFs evaluated for the production of biodiesel. The promising results obtained with supercritical alcohols, have led to many studies on the effects of various reactions parameters on FAMEs yield.

In early studies of SCMe, Saka and Kusdiana [45] and Kusdiana and Saka [46] investigated the conversion of rapeseed oil into biodiesel at 350°C and 400°C, at a pressure of 45–60 MPa, and with a fixed methanol to oil ratio of 42:1. It was stated that, at a temperature of 350°C, after 4 min of treatment with SCF methanol, the yield of FAME was found to be higher than that of the common method with a base catalyst.

Madras et al. [47] conducted experiments (with sunflower oil employed as feedstock) over the temperature range of 200–400°C, at a constant pressure of 20 MPa. The highest FAME yield was 96% at the following optimal conditions: temperature 400°C, methanol to oil ratio 40:1, reaction time of 40 min.

The main drawback of SCF is the cost of equipment, due to the harsh operation conditions at high temperatures and high pressure. Some authors have employed a catalyst for SCMe, with the ultimate purpose of gaining milder conditions, so as to reduce the expenditure for the whole process.

Wang et al. [48] carried out the transesterification reaction of rapeseed oil with SCMe in the presence of NaOH, without soap formation. Various parameters of the reaction were studied to obtain the optimal conditions for the process. A high methyl ester yield (up to 98%) was reported at a temperature of 220°C after 13 min of reaction, at a pressure of 5.6 MPa and 0.8 wt% NaOH.

In another study, by Yin et al. [49], a small amount of KOH was added to the reaction under SCMe and the yield of FAME reached 98% in 10 min at 160°C, with a methanol to oil ratio of 24:1. This yield is obviously more efficient than that of the traditional alkali-catalyzed path which produced 6% FAME with 1% catalyst at a temperature of 260°C. In order to decrease the disadvantage of homogenous catalyst, the same authors [50] studied the synthesis of biodiesel from soybean oil in the presence of a heterogeneous catalyst tri-potassium phosphate (K₃PO₄) in SCF methanol. The outcome obtained displayed high catalytic activity of K₃PO₄ towards the transesterification of soybean oil. The highest yield of FAME was 95.6% under the following conditions: temperature of 220°C, methanol to oil ratio of 24:1, and 1 wt% catalyst.

Tateno and Sasaki [51] reported the use of MgO for SCMe with sunflower oil and obtained the highest yield of FAME of 91% at 300°C, with a methanol to oil ratio of 39.6:1 and a catalyst concentration of 1.29 wt%. Recently, Wang and Yang [52] investigated the variables affecting the conversion of soybean oil into biodiesel in the support of nano-MgO. The reaction was completed within 10 min, at a temperature of 260°C and a methanol to oil ratio of 36:1.

Yoo et al. [53] examined the synthesis of biodiesel using SCMe with some metal oxide catalysts (included CaO, SrO, ZnO, TiO₂, and ZrO₂). ZnO was chosen as the optimal catalyst for transesterification of rapeseed oil in SCF methanol; the yield was around 95% at 250°C and 15 MPa after 10 min. This study was believed to be more profitable than any other process, including the alkali-catalyzed process and non-catalytic supercritical process. Shin et al. [54] combined Cs-doped heteropolyacid catalyst with SCMe to investigate an efficient and environmentally friendly method for biodiesel synthesis. This study discovered that Cs_2.5PW₁₂O₄₀ exhibited highest stability in SCF methanol and the content of obtained FAME was 92% under the optimal conditions of 260°C, 20 MPa, and a methanol to oil ratio of 40:1 after 40 min reaction in the presence of 3% catalyst.

In an attempt to decrease the dependence on agriculture-based feedstock, Shin et al. [55] carried out biodiesel preparation from waste lard, which is available at restaurants after cooking. The highest yield of FAME was reached at 89.9% under the following optimal conditions: temperature of 335°C, a methanol to oil ratio of 45:1, pressure of 20 MPa, and a time reaction of 15 min. It is concluded that waste lard can be seen as an alternative source for the synthesis of biodiesel, thus replacing the expensive refined vegetable oil, and the SCMe with this feedstock should be considered as a promising process for the biofuel industry.

The obvious advantage of using SCFs is that they are capable of tuning and controlling the solvent polarity through the adjustment of temperature and pressure. Indeed, the dielectric constant of methanol decreases from 34 to 5 upon isobaric heating from 25°C to 260°C at 20 MPa [44]. The hydrogen bond network existing in liquid methanol is broken under SC conditions, leading to a fluid in which only small oligomers exist. Reduction of the polarity and hydrogen bonding can support a much stronger direct nucleophilic attack by methanol on the carbonyl group. Furthermore, a reduced dielectric constant may even lead to a single oil/alcohol phase, increasing the reaction rate. These facts explain the differences observed when the reaction is performed either under subcritical or supercritical conditions, since the reaction kinetics is highly dependent on the homogeneity of the reaction mixture. The solubility of reactants and transesterification products (homogeneous or heterogeneous) in the alcohol under various pressure-temperature-molar fraction (P-T-x) conditions is a keystone for rapid and complete transesterification reactions.

Anitescu and Bruno [56] and Anitescu et al. [57] studied the liquid-vapor-SC phase transitions for the binary/ternary systems (soybean oil and alcohol at different oil:ethanol molar ratios), by using a view cell attached to the outlet of the reactor, which was heated from 26°C to 400°C at constant volume/density. With two liquid phases in ambient conditions, the system showed increasing homogeneity with increasing temperature and pressure, until a single SC phase was reached. Under these conditions, the conversion of triglyceride to FAMEs was very rapid compared to that observed under subcritical conditions. The reaction in continuous flow conditions yields above 98% conversion, with a single clear phase at moderate pressures (10–20 MPa), high temperatures (375–400°C), low methanol ratios (3:1–6:1), and short reaction times (<3 min). These results are in contrast with those reported by Hegel et al. [58], Pereda et al. [59], and Velez et al. [60], who observed that high conversion can be obtained, in most cases, even when operating in the two-phase region. In this case, oil transesterification could mainly occur in the light supercritical phase, where the oil and the monoglycerides and diglycerides are partially soluble and the concentration of methanol is high. Moreover, the light-phase transport properties favor a higher reaction rate. The increased methanol/oil ratio leads to a homogeneous supercritical phase by increasing the system temperature while working at moderate pressures of 10–15 MPa, since an increase in the methanol/oil ratio decreases the T_c of the system. Although a high methanol ratio favors the formation of a homogeneous supercritical phase while also driving the reaction to completion, it also increases costs and energy consumption.

An additional advantage of the use of supercritical alcohols in biodiesel synthesis, is the high tolerance towards possible feedstock contaminants, mainly FFAs and water. In supercritical alcohols, esterification of the FFAs to the corresponding FAAEs takes place simultaneously to oil transesterification, yielding higher amounts of biodiesel. Thus, low cost feedstock, usually containing high portions of FFA/oil, can be used in the process without any additional pretreatment. Regarding water content, transesterification reactions catalyzed under acid or alkaline conditions are highly sensitive to the presence of water, as hydrolysis of the corresponding esters is favored in the presence of water. In contrast, the non-catalytic supercritical process can be performed even with high water contents.

3.2.2 SCF ethanol

The selection of the alcohol is based on the consideration of cost and performance. Also, the application of ethyl ester is more beneficial than that of methyl esters if being based on environmental considerations. Ethanol can be prepared from agricultural renewable feedstock, thereby gaining total independence from petroleum-based alcohols. Moreover, ethanol, as an extraction solvent, is preferable to methanol, because of its much higher dissolving power for oils. Therefore, ethanol is sometimes used as a suitable alcohol for the transesterification of vegetables oils. Most research into vegetable oil ethanolysis has been performed in the South America countries, especially in Brazil, which is one of the greatest producers of ethanol from biomass in the world.

The fuel qualities of alkyl esters depend on the alcohol being used for transesterification. Some authors stated that higher or branched alcohols can generate biodiesel with better fuel characteristics. Some studies reported that methyl ester and ethyl ester are similar in heat content, but ethyl ester is slightly less viscous than methyl ester. Therefore, producing ethyl esters rather than methyl esters is of great interest, due to the entirely agricultural nature of the ethanol; moreover, the extra carbon atom provided by the ethanol molecule slightly increases the heat content and the cetane number.

Madras et al. [47] reported the highest conversion of sunflower oil of SCF ethanol compared to methanol when performing experiments over the temperature of (200–400°C) at a constant pressure of 20 MPa, a methanol to oil ratio of 40:1, and a reaction time of 40 min. The reason of higher yield of FAEE can be explained by the solubility of the oil in the system. In the supercritical state, there is only single phase in the mixture of reactants. The solubility of ethanol is closer to that of oil than of methanol; therefore, the conversion is higher in ethanol compared to methanol.

An optimization study on ethanol as the SCF was carried out by Gui and Lee in 2009 [61]. The reaction was performed in a batch-type tubular reactor, with palm oil employed as feedstock for a specific duration ranging from 2 to 30 min; the temperature was screened by a proportional-integral-derivative controller. This study focused on the effects of the individual parameters during the reaction. It was reported that an FAEE yield of 83.1 wt% was achieved at a temperature of 349°C, an ethanol to oil ratio of 33, and a reaction time of 29 min. The yields, obtained at the investigated reaction conditions in this work, are lower than those of other researchers, which is crucial in terms of economic viability to commercialize this process.

In a new approach to reduce the intensity of the operating conditions, Santana et al. [62] employed a co-solvent (CO₂) along with ethanol to prepare biodiesel under a supercritical state, in a fixed-bed reactor. The highest yield of biodiesel was recorded at a temperature of 200°C, a reaction time of 4 min, a molar ratio of 1:25, and a pressure of 20 MPa. The addition of CO₂ is the main reason for lowering of the temperature, and can lead to a reduction of the operating costs.

A research team at Oregon State University (US) [63] made a breakthrough for continuous production of biodiesel with tin-based catalysts with supercritical ethanol and methanol. The catalyst was applied to 50–250 µm stainless steel plasma powder, then tested with six different kinds of oil. With all type of oils, the conversion was >90% under the following operating parameters: temperature of 305°C, pressure of 17.3 MPa, ethanol to oil ratio of 30:1, and residence time of 8 min.

3.2.3 SCF methyl acetate

A new process using methyl acetate to prevent the production of glycerol as a byproduct has been recently studied under supercritical conditions (Figure 5). The supercritical methyl acetate (SCMA) non-catalytic method converts triglycerides into FAMEs and triacetin, instead of glycerol. In addition, it is believed that there were no adverse effects on the main fuel characteristics when the molar ratio of methyl oleate to triacetin was 3:1, which corresponds to the theoretically derived molar ratio from the transesterification reaction of rapeseed oil with methyl acetate. Moreover, the addition of triacetin improved the cold flow properties and has high oxidation stability. Therefore, by defining biodiesel fuel (BDF) as a mixture of methyl oleate and triacetin, the yield of SCMA can reach 105% of BDF compared with the conventional process. However, this value is lower than the theoretical maximum recovery (125% yield), due to the breakdown of some unsaturated FFMEs at the high operating temperatures. Despite this fact, the SCMA method not only improves the quality of the BDF, but also minimizes the separation and purification steps, and therefore the energy required. Moreover, triacetin is itself a valuable compound that can be used in the cosmetic and food industry, or as an additive to petroleum-based fuel.

Figure 5

Transesterification between triglyceride and methyl acetate.

The first study on the application of methyl acetate as SCF for biodiesel production was proposed by Saka and Isayama [64]. The conditions were set at temperatures of 270–380°C, 20 MPa of pressure, a reaction time ranging from 10 to 120 min, and the methyl acetate to oil ratio equal to 42:1. It is reported that the highest yield of BDF was 105% (at 350°C and 20 MPa), which is still higher than the maximum conventional recovery of BDF (100%) when measured as FAME only.

Campanelli et al. [65] employed various feedstocks for biodiesel synthesis via SCMA. The process proved to be appropriate for edible, non-edible or waste oils, regardless of the FFA content. The author carried out experiments under different conditions: the reaction temperature ranged between 300°C and 450°C and the pressure between 10 MPa and 30 MPa; the methyl acetate to oil ratio varied between 25:1 and 59:1. Complete conversion was achieved after 50 min at 345°C, 20 MPa, and with methyl acetate:oil molar ratio equal to 42:1, with the highest yield of BDF being equal to 106%.

In order to obtain a deeper understanding into the SCMA, Tan et al. [66] investigated various factors, including temperature, pressure, reaction time, and the molar ratio, which have a strong influence on the reaction performance. It is critical to thoroughly examine these effects to acknowledge the prospect of SCMA for biodiesel synthesis at the industry-scale. This study showed a 99 wt% BDF yield, achieved at 400°C, a molar ratio of 30:1, and a reaction time of 60 min. More importantly, the SCMA process demonstrated a high tolerance to impurities normally found in oils/fats, such as water and FFA. This allows utilizing inexpensive triglycerides, rather than refined oils.

Goembira et al. [67] used various supercritical carboxylate esters to convert triglycerides into FAMEs and triacetin, without the presence of a catalyst. The experiments were performed with a molar ratio of carboxylate ester to oil of 42:1 at 350°C in a batch-reaction vessel. The highest product yield was obtained with SCMA, 97.7 wt% when considering a mixture of both FAMEs and triacetin as BDF. Lower reactivity was shown by other carboxylate esters, which seems to be related to the length of alkyl chains in the esters, e.g., longer alkyl chains in both alcohol and acyl moieties of carboxylate esters gave lower product yields.

3.2.4 SCF dimethyl carbonate

Other authors have studied the use of supercritical dimethyl carbonate (SCDMC) instead of methanol SCDMC (T_c=274.9°C, P_c=4.6 MPa). In this way, triglycerides, as well as FAs, are successfully converted to FA methyl esters (FAMEs). Besides, with this methodology, another valuable compound, namely glycerol carbonate, is obtained as a secondary product, instead of the undesirable glycerol, and a weak acid, such as citramalic acid, as the main byproduct. Glycerol carbonate has a higher commercial value than glycerol, and has a greater potential for industrial application. Hence, this methodology could have a large impact on biodiesel economics, as two high-value products are produced in a single-step reaction, leading to increased cost effectiveness, even if high temperatures and pressures are required. The FAMEs produced by this method are obtained at higher yields than with SCMe, and satisfy the international standards for use as BDF. In conclusion, many works have been done to demonstrate the huge potential of SCDMC to prepare biodiesel economically.

Tan and co-workers [68] carried out the optimization study of SCDMC by using response surface methodology analysis. It was reported that the optimum conditions for SCDMC were 380°C for the reaction temperature, a dimethyl carbonate to oil ratio of 39:1, and 30 min of reaction time, which achieved a 91% yield of biodiesel.

Ilham and Saka [69] reached 97 wt% of FAME yield for the SCDMC at 300°C, 20 MPa, 20 min, and a 42:1 molar ratio of dimethyl carbonate to oil. This work also investigated the fuel properties of FAMEs to ensure that they are satisfactory as biodiesels. The results demonstrated that the FAME from this method met all requirements for international biodiesel standards of Japan, Europe, and the US, except for the oxidation stability. However, this drawback can be eliminated by the addition of an antioxidant or by employing feedstocks with low levels of unsaturated FAs.