**3. Process improvements**

The process improvements to the supercritical transesterification can be divided into three routes; the addition of the co-solvents, the use of catalysts and process modifications. The general goal, to reduce the original parameters altogether, is the most challenging aspect for supercritical transesterification. The reduced parameters are 270 – 300 °C, 15 – 20 MPa and an alcohol to oil molar ratio of 24:1 – 35:1.

#### **3.1 The chronological development of supercritical transesterification**

In 1998, non-catalytic transesterification of soybean oil at the near-critical point of methanol (230 ºC, 6.2 MPa and a 27:1 methanol to oil molar ratio) was invented as an alternative method to produce biodiesel, but this method obtained only an 85% methyl ester content after over 10 hours (Diasakou et al., 1998). In 2001, the pioneering transesterification of rapeseed oil in supercritical methanol at 350 ºC, 45 MPa and a 42:1 methanol to oil molar ratio, attaining a high methyl ester content (98%) after only 4 min was reported (Kusdiana & Saka, 2001; Saka & Kusdiana, 2001). Transesterification in supercritical methanol has evolved continuously since 2001.

In 2002, the transesterification of cottonseed, hazelnut kernel, poppy seed, safflower and sunflower derived oils in supercritical methanol were evaluated, with a nearly complete transesterification reaction being found for all of the vegetable oils (Demirbas, 2002). Meanwhile, the effect of water and free fatty acids (Kusdiana & Saka, 2004b), the catalytic effect of a metal reactor for supercritical transesterification with methanol (Dasari et al.,

To fulfill the international standard of biodiesel (EN14214), which requires over 96.5% esters content, thermal cracking of polyunsaturated fatty acids is a serious obstacle. At over 300 ºC and a reaction time of over 15 min, the methyl linoleate content in biodiesel decreases by approximately 10% compared with the level in the feedstock (Quesada-Medina & Olivares-Carrillo, 2011). Whereas, the % recovery of biodiesel samples which are prepared from various vegetable oils remains constant after exposure with supercritical methanol at 270 ºC over 40 min (Imahara et al., 2008). Therefore, the 96.5% alkyl esters content requirement for biodiesel cannot be achieved when an operating temperature of over 300 ºC and a reaction

To prevent this thermal degradation, the suggested temperature for supercritical transesterification is below 300 ºC, and preferably 270 ºC. However, the required reaction time to nearly complete transesterification conversion at a 42:1 alcohol to oil molar ratio is then significantly longer at more than 90 min (Minami & Saka, 2006). This prolonged reaction time might cause a decline in the production efficiency obtained by supercritical transesterification, but it could be shortened by the use of assisting methods, as discussed in

On the other hand, the gradual heating technique in a tubular reactor has been demonstrated to avoid the thermal cracking of unsaturated fatty acids and shorten the reaction time at the same time (He et al., 2007b). For instance, when the reaction mixture is heated in a tubular reactor gradually from 100 ºC at the inlet to 320 ºC at the outlet, the biodiesel product obtained after 25 min of reaction time has an over 96% methyl ester

The process improvements to the supercritical transesterification can be divided into three routes; the addition of the co-solvents, the use of catalysts and process modifications. The general goal, to reduce the original parameters altogether, is the most challenging aspect for supercritical transesterification. The reduced parameters are 270 – 300 °C, 15 – 20 MPa and

In 1998, non-catalytic transesterification of soybean oil at the near-critical point of methanol (230 ºC, 6.2 MPa and a 27:1 methanol to oil molar ratio) was invented as an alternative method to produce biodiesel, but this method obtained only an 85% methyl ester content after over 10 hours (Diasakou et al., 1998). In 2001, the pioneering transesterification of rapeseed oil in supercritical methanol at 350 ºC, 45 MPa and a 42:1 methanol to oil molar ratio, attaining a high methyl ester content (98%) after only 4 min was reported (Kusdiana & Saka, 2001; Saka & Kusdiana, 2001). Transesterification in supercritical methanol has

In 2002, the transesterification of cottonseed, hazelnut kernel, poppy seed, safflower and sunflower derived oils in supercritical methanol were evaluated, with a nearly complete transesterification reaction being found for all of the vegetable oils (Demirbas, 2002). Meanwhile, the effect of water and free fatty acids (Kusdiana & Saka, 2004b), the catalytic effect of a metal reactor for supercritical transesterification with methanol (Dasari et al.,

**3.1 The chronological development of supercritical transesterification** 

**2.4.2 The chemical limitation of supercritical transesterification** 

time of over 15 min are employed.

Sections 3.2 – 3.4.

content (He et al., 2007b).

**3. Process improvements** 

an alcohol to oil molar ratio of 24:1 – 35:1.

evolved continuously since 2001.

2003; Kusdiana & Saka, 2004a) and the reactivity of supercritical alcohols were all reported (Warabi et al., 2004). In 2004, the first supercritical transesterification of sunflower oil with ethanol and supercritical carbon dioxide in the presence of a lipase enzyme were investigated in a batch reactor (Madras et al., 2004). However, during 2001 – 2005, the maximum alkyl ester contents were generally observed at nearly the same reaction conditions as that reported earlier by the Japanese pioneers (Kusdiana & Saka, 2001; Saka & Kusdiana, 2001).

In 2005, carbon dioxide and propane were introduced as co-solvents to obtain milder operating parameters for the supercritical transesterification with methanol (Cao et al., 2005; Han et al., 2005). Then, the two-step supercritical process (Minami & Saka, 2006) was demonstrated to reduce those operating parameters. In the following years, various catalysts were employed to assist the supercritical transesterification to achieve the maximum alkyl esters content but at milder operating conditions (Demirbas, 2007; Wang et al., 2008; Wang et al., 2007; Wang & Yang, 2007; Yin et al., 2008b). The continuous production of biodiesel in supercritical methanol was reported in 2006 (Bunyakiat et al., 2006) (Minami & Saka, 2006) and 2007 (He et al., 2007b). Therefore, the research focus on the reduction of the elevated operating conditions and continuous process has been ongoing since 2005.

In 2007, the gradual heating technique was introduced to limit or prevent thermal cracking of the unsaturated fatty acids and so prevent the reduction in the final methyl esters content obtained (He et al., 2007b). At the same time, the effect of using co-solvents to reduce the viscosity of vegetable oils was successfully investigated (Sawangkeaw et al., 2007). Supercritical transesterification in ethanol was studied in a continuous reactor in 2008 (Vieitez et al., 2008). In 2009, carbon dioxide was applied to supercritical transesterification with ethanol to reduce the operating conditions (Bertoldi et al., 2009). From 2007 to 2010, numerous additional studies, such as vapor-liquid equilibria of binary systems (Anitescu et al., 2008; Fang et al., 2008; Shimoyama et al., 2008; Shimoyama et al., 2009; Tang et al., 2006), phase behavior of the reaction mixture (Glišic & Skala, 2010; Hegel et al., 2008; Hegel et al., 2007), thermal stability of unsaturated fatty acids in supercritical methanol (Imahara et al., 2008) and process simulation and economic analysis (Busto et al., 2006; D'Ippolito et al., 2006; Deshpande et al., 2010; Diaz et al., 2009; van Kasteren & Nisworo, 2007) were reported, leading to a better understanding of the supercritical transesterification process.

#### **3.2 The addition of co-solvents**

The co-solvents that have been used in supercritical transesterification are liquid cosolvents, such as hexane and tetrahydrofuran (THF), and gaseous co-solvents, such as propane, carbon dioxide (CO2) and nitrogen (N2). Both types of co-solvents have different purposes and advantages that will be presented accordingly.

The liquid co-solvents are added into the supercritical transesterification reaction to reduce the viscosity of the vegetable oils, which might otherwise pose some pumping problems in a continuous process (Sawangkeaw et al., 2007). Since hexane is the conventional solvent for vegetable oil extraction, it could be possible to combine the supercritical transesterification after the extraction process using hexane for both. Additionally, THF improves the solubility of alcohols in the triglyceride and so forms a single phase mixture, allowing a single highpressure pump to be employed to feed the reaction mixture into the reactor. A small amount of liquid co-solvent, up to ~20% (v/v) of hexane in vegetable oil, neither affects the

Transesterification in Supercritical Conditions 261

be poisoned by the presence of water and free fatty acids. Therefore, further studies on

The two-step process is based on firstly a hydrolysis reaction in subcritical water to obtain fatty acid products and then secondly the transesterification and esterification reactions in supercritical alcohol to form the alkyl esters product. The two-step process reduces the optimal operating parameters successfully since the hydrolysis and esterification reactions reach complete conversion at a lower temperature than the transesterification reaction does (Minami & Saka, 2006). Nonetheless, the two-step process is more complicated than the single-step process. For example, the process has high-pressure reactors that connect in series with a high-pressure water-glycerol-fatty acid phase separator. Furthermore, the glycerol-water stream, which is contaminated by trace amounts of fatty acids, requires more separation units. Although a distillation tower is the simplest separation unit for handling

The high-temperature process involves increasing the operating temperature to 400 to 450 °C (Marulanda et al., 2009; Marulanda et al., 2010), so that the operating pressure, methanol to oil molar ratio and reaction time for complete conversion are reduced to 10.0 MPa, 6:1 and 4 min, respectively. As expected, the unsaturated fatty acids are partially consumed by thermal degradation but the oxidation resistance or storage stability of the product might be enhanced. Under these conditions it was reported that triglyceride and glycerol convert to oxygenate liquid fuel with a conversion of up to 99.5%. The glycerol dehydration both increases the fuel yield by up to 10% and reduces the amount of glycerol by-products (Aimaretti et al., 2009). By using the high-temperature process, the simultaneous conversion of triglyceride, free fatty acids and glycerol to liquid fuel is an alternative option that will increase the feasibility and profitability of supercritical

In this section, the process prospective is split into two on the basis of the operating temperature since the temperature is the key parameter and chemical limitation for supercritical transesterification. The low-temperature approach aims to produce biodiesel that fulfills the 96.5% alkyl esters content requirement for biodiesel, while the hightemperature approach proposes an alternative method to synthesize the biofuel from a

The term "Low-temperature approach" defines supercritical transesterification within a temperature range of 270 – 300 ºC so as to avoid the thermal degradation of unsaturated fatty acids and to maximize the alkyl esters content in the product. Without the assistance of any co-solvent, catalyst or other process modification techniques, the low-temperature approach employs a high pressure, a high alcohol to oil molar ratio and a long reaction time to achieve the >96.5% alkyl esters content required for biodiesel composition by the international standard. However, with the assisting techniques, as mentioned in Sections 3.2 – 3.4, the optimal conditions of low-temperature approach generally involve 20 – 30 MPa, an

using low-grade feedstocks with basic heterogeneous catalysts are still required.

the glycerol-water stream, it consumes a large amount of energy to operate.

**3.4 The process modifications** 

transesterification.

**4. Process prospective** 

triglyceride-base biomass in supercritical conditions.

**4.1 The low-temperature approach** 

transesterification conversion nor lowers the original operating parameters. Whereas, an excess amount of hexane shows a negative effect on the final obtained alkyl esters content due to dilution and obstruction of the reactants (Tan et al., 2010a).

The addition of gaseous co-solvents to the supercritical transesterification reaction aims to reduce the original operating parameters. Due to the fact that the critical properties of gaseous co-solvents are much lower than alcohol and triglycerides, the addition of a small amount of gaseous co-solvents dramatically decreases the critical point of the reaction mixture allowing the use of milder operating parameters. For example, 0.10 mole of CO2 or 0.05 mole of propane per mole of methanol lowers the reaction temperature and methanol to oil molar ratio to 280 °C and 1:24, respectively (Cao et al., 2005; Han et al., 2005). Furthermore, it was reported that the addition of N2 improved the oxidation stability and reduced the total glycerol content in the biodiesel product (Imahara et al., 2009). Gaseous cosolvents have the advantage of easier separation from the product than the liquid cosolvents. For instance, they can be separated from the biodiesel product by expansion without using additional energy at the end of the transesterification process, unlike the liquid co-solvents that typically need to be recovered by distillation.

#### **3.3 The use of catalysts**

The homogeneous acidic and basic catalysts, such as H3PO4, NaOH and KOH, have been applied to supercritical transesterification to obtain milder operating conditions (Wang et al., 2008; Wang et al., 2007; Yin et al., 2008b). However, despite the milder operating conditions and faster rate of reaction obtained compared to the catalyst-free process, the addition of homogeneous catalysts is not an interesting idea because the problem of subsequent catalyst separation and waste management still remain, the same situation as with the conventional homogeneous catalytic process. The use of solid heterogeneous catalysts might enhance the technical and economical feasibility of using supercritical transesterification as a result of the ease of separation of the catalysts. However, the acidic and basic heterogeneous catalysts have different characteristics and advantages, as will be discussed below.

The acidic heterogeneous catalysts, such as WO3/ZrO2, zirconia-alumina, sulfated tin oxide and Mg–Al–CO3 hydrotalcites, have been evaluated in the supercritical transesterification process (Helwani et al., 2009). However, despite the presence of the catalysts, the chemical kinetics of the acidic heterogeneous catalysts at atmospheric pressure were slower than the catalyst-free process. For example, the transesterification of soybean oil in supercritical methanol at 250 °C and a 40:1 methanol to oil molar ratio in the presence of WO3/ZrO2 as catalyst still takes 20 hours to attain a 90% conversion level (Furuta et al., 2004). However, the acidic catalysts are less sensitive to moisture and free fatty acid content than the basic catalysts and so they could be appropriate for low-grade feedstocks.

Alternatively, basic heterogeneous catalysts, such as CaO (Demirbas, 2007) MgO (Demirbas, 2008) and nano-MgO (Wang & Yang, 2007), have been applied to supercritical transesterification to reduce the original operating conditions. These catalysts have the ability to catalyze the transesterification reaction at the boiling point of alcohols and are stable at supercritical conditions. As expected, the rate of reaction at the supercritical conditions is faster than that at lower temperatures. For example, the CaO catalyst takes over 180 min to reach over 95% conversion at 65 °C (Liu et al., 2008), but only 10 min to reach complete conversion at 250 °C (Demirbas, 2007). Unfortunately, the basic catalysts can be poisoned by the presence of water and free fatty acids. Therefore, further studies on using low-grade feedstocks with basic heterogeneous catalysts are still required.

#### **3.4 The process modifications**

260 Biodiesel – Feedstocks and Processing Technologies

transesterification conversion nor lowers the original operating parameters. Whereas, an excess amount of hexane shows a negative effect on the final obtained alkyl esters content

The addition of gaseous co-solvents to the supercritical transesterification reaction aims to reduce the original operating parameters. Due to the fact that the critical properties of gaseous co-solvents are much lower than alcohol and triglycerides, the addition of a small amount of gaseous co-solvents dramatically decreases the critical point of the reaction mixture allowing the use of milder operating parameters. For example, 0.10 mole of CO2 or 0.05 mole of propane per mole of methanol lowers the reaction temperature and methanol to oil molar ratio to 280 °C and 1:24, respectively (Cao et al., 2005; Han et al., 2005). Furthermore, it was reported that the addition of N2 improved the oxidation stability and reduced the total glycerol content in the biodiesel product (Imahara et al., 2009). Gaseous cosolvents have the advantage of easier separation from the product than the liquid cosolvents. For instance, they can be separated from the biodiesel product by expansion without using additional energy at the end of the transesterification process, unlike the

The homogeneous acidic and basic catalysts, such as H3PO4, NaOH and KOH, have been applied to supercritical transesterification to obtain milder operating conditions (Wang et al., 2008; Wang et al., 2007; Yin et al., 2008b). However, despite the milder operating conditions and faster rate of reaction obtained compared to the catalyst-free process, the addition of homogeneous catalysts is not an interesting idea because the problem of subsequent catalyst separation and waste management still remain, the same situation as with the conventional homogeneous catalytic process. The use of solid heterogeneous catalysts might enhance the technical and economical feasibility of using supercritical transesterification as a result of the ease of separation of the catalysts. However, the acidic and basic heterogeneous catalysts have different characteristics and advantages, as will be

The acidic heterogeneous catalysts, such as WO3/ZrO2, zirconia-alumina, sulfated tin oxide and Mg–Al–CO3 hydrotalcites, have been evaluated in the supercritical transesterification process (Helwani et al., 2009). However, despite the presence of the catalysts, the chemical kinetics of the acidic heterogeneous catalysts at atmospheric pressure were slower than the catalyst-free process. For example, the transesterification of soybean oil in supercritical methanol at 250 °C and a 40:1 methanol to oil molar ratio in the presence of WO3/ZrO2 as catalyst still takes 20 hours to attain a 90% conversion level (Furuta et al., 2004). However, the acidic catalysts are less sensitive to moisture and free fatty acid content than the basic

Alternatively, basic heterogeneous catalysts, such as CaO (Demirbas, 2007) MgO (Demirbas, 2008) and nano-MgO (Wang & Yang, 2007), have been applied to supercritical transesterification to reduce the original operating conditions. These catalysts have the ability to catalyze the transesterification reaction at the boiling point of alcohols and are stable at supercritical conditions. As expected, the rate of reaction at the supercritical conditions is faster than that at lower temperatures. For example, the CaO catalyst takes over 180 min to reach over 95% conversion at 65 °C (Liu et al., 2008), but only 10 min to reach complete conversion at 250 °C (Demirbas, 2007). Unfortunately, the basic catalysts can

due to dilution and obstruction of the reactants (Tan et al., 2010a).

liquid co-solvents that typically need to be recovered by distillation.

catalysts and so they could be appropriate for low-grade feedstocks.

**3.3 The use of catalysts** 

discussed below.

The two-step process is based on firstly a hydrolysis reaction in subcritical water to obtain fatty acid products and then secondly the transesterification and esterification reactions in supercritical alcohol to form the alkyl esters product. The two-step process reduces the optimal operating parameters successfully since the hydrolysis and esterification reactions reach complete conversion at a lower temperature than the transesterification reaction does (Minami & Saka, 2006). Nonetheless, the two-step process is more complicated than the single-step process. For example, the process has high-pressure reactors that connect in series with a high-pressure water-glycerol-fatty acid phase separator. Furthermore, the glycerol-water stream, which is contaminated by trace amounts of fatty acids, requires more separation units. Although a distillation tower is the simplest separation unit for handling the glycerol-water stream, it consumes a large amount of energy to operate.

The high-temperature process involves increasing the operating temperature to 400 to 450 °C (Marulanda et al., 2009; Marulanda et al., 2010), so that the operating pressure, methanol to oil molar ratio and reaction time for complete conversion are reduced to 10.0 MPa, 6:1 and 4 min, respectively. As expected, the unsaturated fatty acids are partially consumed by thermal degradation but the oxidation resistance or storage stability of the product might be enhanced. Under these conditions it was reported that triglyceride and glycerol convert to oxygenate liquid fuel with a conversion of up to 99.5%. The glycerol dehydration both increases the fuel yield by up to 10% and reduces the amount of glycerol by-products (Aimaretti et al., 2009). By using the high-temperature process, the simultaneous conversion of triglyceride, free fatty acids and glycerol to liquid fuel is an alternative option that will increase the feasibility and profitability of supercritical transesterification.

#### **4. Process prospective**

In this section, the process prospective is split into two on the basis of the operating temperature since the temperature is the key parameter and chemical limitation for supercritical transesterification. The low-temperature approach aims to produce biodiesel that fulfills the 96.5% alkyl esters content requirement for biodiesel, while the hightemperature approach proposes an alternative method to synthesize the biofuel from a triglyceride-base biomass in supercritical conditions.

#### **4.1 The low-temperature approach**

The term "Low-temperature approach" defines supercritical transesterification within a temperature range of 270 – 300 ºC so as to avoid the thermal degradation of unsaturated fatty acids and to maximize the alkyl esters content in the product. Without the assistance of any co-solvent, catalyst or other process modification techniques, the low-temperature approach employs a high pressure, a high alcohol to oil molar ratio and a long reaction time to achieve the >96.5% alkyl esters content required for biodiesel composition by the international standard. However, with the assisting techniques, as mentioned in Sections 3.2 – 3.4, the optimal conditions of low-temperature approach generally involve 20 – 30 MPa, an

Transesterification in Supercritical Conditions 263

The authors would like to acknowledge the financial support from Postdoctoral Fellowship (Ratchadaphiseksomphot Endowment Fund) and the Thai Government Stimulus Package 2 (TKK2555), under the Project for Establishment of Comprehensive Center for Innovative Food, Health Products and Agriculture. We also express thanks to Dr. Robert Douglas John Butcher from the Publication Counseling Unit, Faculty of Science, Chulalongkorn

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alcohol to oil molar ratio of 24:1 and a reaction time over 30 min. The biodiesel product, which typically exceeds the 96.5% alkyl esters content of the international standard for biodiesel (EN14214), can be used as biodiesel.

For future research involving the low-temperature approach, the use of low-grade feedstocks and/or heterogeneous catalysts are very interesting topics. Alternatively, studies on scale up continuous reactors which are more suitable for an industrial scale are required. These have been successfully evaluated in lab-scale tubular reactors (Bunyakiat et al., 2006; He et al., 2007b; Minami & Saka, 2006), but an evaluation on a scaled-up reactor is presently lacking. An optimal reaction time to achieve over 96.5% alkyl esters content is the most important finding for the low-temperature approach studies because it corresponds with reactor sizing and reflects on the economical feasibility.

#### **4.2 The high-temperature approach**

The high-temperature approach uses supercritical transesterification at temperatures over 400 ºC, as described in Section 3.4. Even though the mono-alkyl esters content in the product from the high-temperature process is always lower than the biodiesel specification value of 96.5%, it can be proposed as an alternative biofuel that would require further studies on engine testing and fuel properties itself. Improved fuel properties, such as the viscosity and density of the biofuel product, from the hightemperature approach have been proposed (Marulanda et al., 2009). Furthermore, the operating temperature and pressure used in the high-temperature approach are close to those for catalytic hydrocracking in conventional petroleum refining, so it has a high possibility that it can be realized in an industrial scale.

Since the high-temperature approach, as recently initiated, has evaluated the triglycerides found in soybean oil (Anitescu et al., 2008) and chicken fat (Marulanda et al., 2009; Marulanda et al., 2010) only, then additional research into other triglycerides are needed. In addition, studies on the economical feasibility and environmental impact are also required. Indeed, the complete fuel properties need examining along with engine testing for the biofuel product for the high-temperature approach (Basha et al., 2009). On the other hand, the fine studies on the reactions pathways and/or chemical kinetics are also attractive works to better understand the high-temperature approach.
