**2. Process overview**

252 Biodiesel – Feedstocks and Processing Technologies

the fatty acid alkyl esters of the common fatty acids in vegetable oils and/or animal fats. Therefore, the thermal cracking reaction reduces the acceptable primary alkyl ester content, as quantified and defined by the international standard for biodiesel (EN14103), and this is especially the case for the oils with a high polyunsaturated fatty acid content, such as

In addition, triglycerides are decomposed to fatty acids and some gaseous products within the temperature range of 350 - 450 °C, as shown in Fig. 9 (Lima et al., 2004; Marulanda et al., 2009). In the same manner, with thermal cracking at 300 - 350 °C, the fatty acids product can be esterified under supercritical conditions, but the alkyl ester content is also decreased. However, the small hydrocarbon molecules of the thermal cracking products could improve some fuel properties of biodiesel, such as viscosity,

> HO OH

O

O

O

heat (350 - 450 oC)

OH

O

Fig. 9. The thermal cracking reaction of triglycerides under supercritical conditions at

**1.4 The original reaction parameters and optimal conditions in the early scientific** 

The reaction parameters that were typically investigated in supercritical transesterification reactions are the temperature, pressure, alcohol to oil molar ratio and reaction time in batch

The extent of the reaction was reported in terms of the % alkyl ester content and the % conversion of triglycerides. The % alkyl esters content refers to the alkyl esters of the common fatty acids in the vegetable or animal oils/fats that can be identified by different analytical techniques, while the % triglycerides conversion implies the remaining triglyceride reactant that is converted to fuels. Note that the % alkyl esters content refers to the specified esters, which must not be less than 96.5%, in the International standard (EN14214) for biodiesel fuel. It should also be noted that a high alkyl esters content infers a high triglyceride conversion, but, in contrast, a high triglyceride conversion does not have to infer a high alkyl esters content because the triglycerides could have been converted by the

According to Table 1, the original optimal conditions, defined as yielding the highest extent of reaction as over 90% conversion or over 96% alkyl esters content, were within 300 – 350 ºC, 20 – 35 MPa, an alcohol to oil molar ratio of 40:1 – 42:1 and a reaction time of

O

O

O

O

O

soybean and sunflower oil.

density and cold flow properties.

temperature range of 350 - 450 °C

side reactions to other products.

and continuous reactors, and are summarized in Table 1.

**articles** 

#### **2.1 The effects of reaction parameters on the % conversion in supercritical transesterification**

Among the general operating parameters mentioned previously (temperature, pressure, alcohol to oil molar ratio and reaction time), the reaction temperature is the most decisive parameter for indicating the extent of the reaction. This is as a result of the accelerated chemical kinetics and changes to the alcohol's properties. For example, the rate constant of supercritical transesterification is dramatically enhanced some 7-fold as the temperature is increased from 210 to 280 ºC at 28.0 MPa and a 42:1 methanol to oil molar ratio (He et al., 2007a), whilst the degree of hydrogen bonding also suddenly drops as the temperature is increased from 200 to 300 ºC at 30.0 MPa (Hoffmann & Conradi, 1998). However, where a maximum alkyl ester content is required, that is for biodiesel production, the higher operating temperatures cause a negative effect on the proportion of alkyl esters obtained in the product due to the thermal cracking reaction. Indeed, the thermal cracking is the chemical limitation of supercritical transesterification, and this is discussed in Section 2.4.2.


**<sup>a</sup>** CT = Continuous reaction in a tubular vesicle.

**<sup>b</sup>**Reaction extents are expressed as the % triglyceride conversion (Con.), % methyl esters content (MEC) or the % ethyl esters content (EEC).

NR = not reported

Table 1. The original reaction parameters and optimal conditions of supercritical transesterification for various oil types and alcohols.

Transesterification in Supercritical Conditions 255

molar ratio, due to the insignificant changes in the alcohol concentration, but this

For the first-order model, the rate constants for each vegetable oil have a different temperature sensitivity, as noticed by the slope of Arrhenius' plot (Sawangkeaw et al., 2010). For example, the rate constants of rapeseed and soybean oil depend more strongly on the temperature than that for sunflower, palm and groundnut oils. The rate constants of saturated triglycerides were found to be faster than unsaturated triglycerides and slow down with increasing levels of double bonds in the triglyceride molecule (Rathore & Madras, 2007; Varma & Madras, 2006). However, saturated fatty acids have a slightly lower

On the other hand, a second-order kinetic model with respect to both the triglycerides and alcohol concentrations has also been proposed (Diasakou et al., 1998; Song et al., 2008). This divides the transesterification reaction into three steps; the reaction between a triglyceride and an alcohol that generates a diglyceride and an alkyl ester, the diglyceride and alcohol and finally the monoglyceride and alcohol. The concentration of the intermediates is then taken into account and the rate constants are found by mathematical model fitting. Thus, although more complex, the second-order kinetic model is more appropriate than the first-

The fact that the required optimal operating parameters can become milder with the addition of co-solvents has spurred much interest in the phase behavior of reactants during supercritical transesterification. The phase behavior of soybean oil-methanol with propane as a co-solvent was reported first (Hegel et al., 2007), followed by that for soybean oilmethanol and soybean oil-ethanol with carbon dioxide as the co-solvent (Anitescu et al., 2008). The study of the phase behavior of supercritical transesterification, when performed in a high-pressure view cell, revealed that the liquid - liquid (LL) alcohol - triglycerides mixture transforms to a vapor – liquid - liquid (VLL) phase equilibrium. The VLL equilibrium consists of two immiscible liquid phases (triglycerides and alcohol) and a vapor phase which mainly contains alcohol. Then, the VLL equilibrium changes to a vapor - liquid (VL) phase as a result of the triglycerides dissolving into the supercritical alcohol phase. Finally, the VL equilibrium merges to a one-phase supercritical at nearly the estimated

The transition temperature of the VLL to VL equilibriums decreases with increasing methanol to oil molar ratios (Anitescu et al., 2008; Hegel et al., 2007). For example, the reaction mixture of soybean oil and methanol are partially miscible up to temperatures close to 350 ºC at a methanol to oil molar ratio of 24:1, while the two liquid phases of soybean oil and methanol become completely miscible at 180 ºC and 157 ºC with a methanol to oil molar ratio of 40:1 and 65:1, respectively. For a soybean oil-ethanol mixture, it becomes a VL equilibrium at a lower temperature than that for the soybean oil-methanol due to the higher

The transition from a VL system to a one-phase supercritical system was observed near the estimated critical temperature of the mixture, as described in Section 1.1. At a methanol to oil molar ratio of 24:1, the critical temperature of the soybean oil-methanol mixture was 377 ºC where the transition temperature was reported to be higher than 350 ºC (Anitescu et al., 2008). Moreover, the transition temperature of the two-phase VL to a one-phase supercritical

solubility of soybean-oil in ethanol than in methanol (Anitescu et al., 2008).

increasingly becomes untrue as the alcohol to oil molar ratio decreases.

reactivity than the unsaturated fatty acids (Warabi et al., 2004).

critical point of mixture.

order model for reactions involving alcohol to oil molar ratios below 24:1.

**2.2.2 The phase behavior of reactants in supercritical transesterification** 

The reaction pressure also has a significant effect on the efficiency of the supercritical transesterification reaction below 20.0 MPa, but the effects tend to be negligible above 25.0 MPa (He et al., 2007a; He et al., 2007b), due to the fact that increasing the reaction pressure simultaneously increases both the density of the reaction mixture (Velez et al., 2010) and the degree of hydrogen bonding (Hoffmann & Conradi, 1998) at an otherwise constant temperature and alcohol to oil molar ratio. The transesterification conversion is enhanced with an increased reaction mixture density, due to the resulting increased volumetric concentration of alcohols and the residence time in a tubular reactor, which is commonly used to investigate the effect of pressure. On the other hand, the increasing degree of hydrogen bonding or alcohol cluster size weakens the nucleophilic strength of supercritical alcohols and so the reactivity of supercritical alcohol is reduced with increasing pressure. Thus, the desirable pressure for supercritical transesterification is in the range of 20.0 – 35.0 MPa.

From Table 1, the original alcohol to oil molar ratio for supercritical transesterification is in range of 40:1 – 42:1. The alcohol to oil molar ratio affects the supercritical transesterification efficiency strongly below 24:1, but its effect is then reduced with increasing alcohol to oil molar rations to plateau at over a 50:1 alcohol to oil molar ratio for methanol (He et al., 2007a) or 70:1 for ethanol (Silva et al., 2007) at 330 ºC and 20.0 MP. This is likely to be due to the fact that the operating temperature and pressure are much higher than the critical point of the reaction mixture, and are still located in the supercritical region.

As mentioned in Section 1.1, the critical point of the reaction mixture decreases with increasing alcohol to oil molar ratios. Thus, the optimal reaction temperature and pressure at a high alcohol to oil molar ratio is always milder than that at a low molar ratio. Nonetheless, the large amount of alcohol not only increases the required reactor volume but importantly it consumes a large amount of energy to heat the reactant and also to subsequently recover the excess alcohol. The energy for recycling excess alcohol might be minimized by a low temperature separation process, such as the use of a medium-pressure flash drum (Diaz et al., 2009), whereas the additional energy for heating the excess reactant alcohol cannot be avoided. Therefore, the use of assisting techniques, as described in Sections 3.2 – 3.4, have been introduced to decrease the alcohol to oil molar ratio whilst maintaining the transesterification conversion efficiency (and so the fatty acid alkyl ester content).

The effect of the reaction time on the transesterification conversion follows the general rate law. For example, the alkyl ester content increases gradually with reaction time and then remains constant after the optimal point (maximum conversion to alkyl esters) is reached. The optimal reaction time for supercritical transesterification at around 300 – 350 ºC varied between 4 to 30 minutes, depending upon the reactor size and type. Since the effect of residence time is directly related to the chemical kinetics of transesterification, the optimal reaction time at low temperatures is longer than that at high temperatures.

#### **2.2 The chemical kinetics and phase behavior in supercritical transesterification 2.2.1 The chemical kinetics of supercritical transesterification**

The chemical kinetics of supercritical transesterification is divided into three regions, that of the slow (<280 ºC), transition (280 - 330 ºC) and fast (>330 ºC) regions, and usually follows the first-order rate law with respect to the triglyceride concentration alone (He et al., 2007a; Kusdiana & Saka, 2001; Minami & Saka, 2006). Here, the reaction mechanism is merged into one overall step and the concentrations of all intermediates (mono- and diglycerides) are ignored. However, the first-order kinetic model is only suitable for a high alcohol to oil

The reaction pressure also has a significant effect on the efficiency of the supercritical transesterification reaction below 20.0 MPa, but the effects tend to be negligible above 25.0 MPa (He et al., 2007a; He et al., 2007b), due to the fact that increasing the reaction pressure simultaneously increases both the density of the reaction mixture (Velez et al., 2010) and the degree of hydrogen bonding (Hoffmann & Conradi, 1998) at an otherwise constant temperature and alcohol to oil molar ratio. The transesterification conversion is enhanced with an increased reaction mixture density, due to the resulting increased volumetric concentration of alcohols and the residence time in a tubular reactor, which is commonly used to investigate the effect of pressure. On the other hand, the increasing degree of hydrogen bonding or alcohol cluster size weakens the nucleophilic strength of supercritical alcohols and so the reactivity of supercritical alcohol is reduced with increasing pressure. Thus, the desirable

From Table 1, the original alcohol to oil molar ratio for supercritical transesterification is in range of 40:1 – 42:1. The alcohol to oil molar ratio affects the supercritical transesterification efficiency strongly below 24:1, but its effect is then reduced with increasing alcohol to oil molar rations to plateau at over a 50:1 alcohol to oil molar ratio for methanol (He et al., 2007a) or 70:1 for ethanol (Silva et al., 2007) at 330 ºC and 20.0 MP. This is likely to be due to the fact that the operating temperature and pressure are much higher than the critical point

As mentioned in Section 1.1, the critical point of the reaction mixture decreases with increasing alcohol to oil molar ratios. Thus, the optimal reaction temperature and pressure at a high alcohol to oil molar ratio is always milder than that at a low molar ratio. Nonetheless, the large amount of alcohol not only increases the required reactor volume but importantly it consumes a large amount of energy to heat the reactant and also to subsequently recover the excess alcohol. The energy for recycling excess alcohol might be minimized by a low temperature separation process, such as the use of a medium-pressure flash drum (Diaz et al., 2009), whereas the additional energy for heating the excess reactant alcohol cannot be avoided. Therefore, the use of assisting techniques, as described in Sections 3.2 – 3.4, have been introduced to decrease the alcohol to oil molar ratio whilst maintaining the transesterification

The effect of the reaction time on the transesterification conversion follows the general rate law. For example, the alkyl ester content increases gradually with reaction time and then remains constant after the optimal point (maximum conversion to alkyl esters) is reached. The optimal reaction time for supercritical transesterification at around 300 – 350 ºC varied between 4 to 30 minutes, depending upon the reactor size and type. Since the effect of residence time is directly related to the chemical kinetics of transesterification, the optimal

pressure for supercritical transesterification is in the range of 20.0 – 35.0 MPa.

of the reaction mixture, and are still located in the supercritical region.

conversion efficiency (and so the fatty acid alkyl ester content).

reaction time at low temperatures is longer than that at high temperatures.

**2.2.1 The chemical kinetics of supercritical transesterification** 

**2.2 The chemical kinetics and phase behavior in supercritical transesterification** 

The chemical kinetics of supercritical transesterification is divided into three regions, that of the slow (<280 ºC), transition (280 - 330 ºC) and fast (>330 ºC) regions, and usually follows the first-order rate law with respect to the triglyceride concentration alone (He et al., 2007a; Kusdiana & Saka, 2001; Minami & Saka, 2006). Here, the reaction mechanism is merged into one overall step and the concentrations of all intermediates (mono- and diglycerides) are ignored. However, the first-order kinetic model is only suitable for a high alcohol to oil molar ratio, due to the insignificant changes in the alcohol concentration, but this increasingly becomes untrue as the alcohol to oil molar ratio decreases.

For the first-order model, the rate constants for each vegetable oil have a different temperature sensitivity, as noticed by the slope of Arrhenius' plot (Sawangkeaw et al., 2010). For example, the rate constants of rapeseed and soybean oil depend more strongly on the temperature than that for sunflower, palm and groundnut oils. The rate constants of saturated triglycerides were found to be faster than unsaturated triglycerides and slow down with increasing levels of double bonds in the triglyceride molecule (Rathore & Madras, 2007; Varma & Madras, 2006). However, saturated fatty acids have a slightly lower reactivity than the unsaturated fatty acids (Warabi et al., 2004).

On the other hand, a second-order kinetic model with respect to both the triglycerides and alcohol concentrations has also been proposed (Diasakou et al., 1998; Song et al., 2008). This divides the transesterification reaction into three steps; the reaction between a triglyceride and an alcohol that generates a diglyceride and an alkyl ester, the diglyceride and alcohol and finally the monoglyceride and alcohol. The concentration of the intermediates is then taken into account and the rate constants are found by mathematical model fitting. Thus, although more complex, the second-order kinetic model is more appropriate than the firstorder model for reactions involving alcohol to oil molar ratios below 24:1.

#### **2.2.2 The phase behavior of reactants in supercritical transesterification**

The fact that the required optimal operating parameters can become milder with the addition of co-solvents has spurred much interest in the phase behavior of reactants during supercritical transesterification. The phase behavior of soybean oil-methanol with propane as a co-solvent was reported first (Hegel et al., 2007), followed by that for soybean oilmethanol and soybean oil-ethanol with carbon dioxide as the co-solvent (Anitescu et al., 2008). The study of the phase behavior of supercritical transesterification, when performed in a high-pressure view cell, revealed that the liquid - liquid (LL) alcohol - triglycerides mixture transforms to a vapor – liquid - liquid (VLL) phase equilibrium. The VLL equilibrium consists of two immiscible liquid phases (triglycerides and alcohol) and a vapor phase which mainly contains alcohol. Then, the VLL equilibrium changes to a vapor - liquid (VL) phase as a result of the triglycerides dissolving into the supercritical alcohol phase. Finally, the VL equilibrium merges to a one-phase supercritical at nearly the estimated critical point of mixture.

The transition temperature of the VLL to VL equilibriums decreases with increasing methanol to oil molar ratios (Anitescu et al., 2008; Hegel et al., 2007). For example, the reaction mixture of soybean oil and methanol are partially miscible up to temperatures close to 350 ºC at a methanol to oil molar ratio of 24:1, while the two liquid phases of soybean oil and methanol become completely miscible at 180 ºC and 157 ºC with a methanol to oil molar ratio of 40:1 and 65:1, respectively. For a soybean oil-ethanol mixture, it becomes a VL equilibrium at a lower temperature than that for the soybean oil-methanol due to the higher solubility of soybean-oil in ethanol than in methanol (Anitescu et al., 2008).

The transition from a VL system to a one-phase supercritical system was observed near the estimated critical temperature of the mixture, as described in Section 1.1. At a methanol to oil molar ratio of 24:1, the critical temperature of the soybean oil-methanol mixture was 377 ºC where the transition temperature was reported to be higher than 350 ºC (Anitescu et al., 2008). Moreover, the transition temperature of the two-phase VL to a one-phase supercritical

Transesterification in Supercritical Conditions 257

The original parameters to achieve a high transesterification conversion were a high temperature (330 – 350 ºC), high pressure (19 – 35 MPa) and high alcohol to oil molar ratio (1:40 – 1:42). Indeed, the high temperature and pressure requires both an expensive reactor and a sophisticate energy and safety management policy. As a result of the high alcohol to oil molar ratio a large energy consumption in the reactants pre-heating and recycling steps is required. Moreover, the high amount of alcohol in the biodiesel product retards the biodiesel-glycerol phase separation. Therefore, the use of those original parameters results in high capital costs, especially for the reactor and pump, being somewhat higher than the

To increase the technical and economical feasibility of supercritical transesterification, further studies are required to reduce the energy consumption and operating parameters of this process. For example, the integration of a heating and cooling system can improve (reduce) the energy demand. The experimental techniques that have demonstrated the ability to lower the original parameters for supercritical transesterification are illustrated in

The economical feasibilities of supercritical transesterification, compared with the conventional homogeneous catalytic methods, have been studied by computer simulation (van Kasteren & Nisworo, 2007). These studies usually employed the original parameters for transesterification in supercritical methanol (350 ºC, 20.0 MPa and 1:42 methanol to oil molar ratio) and the general parameters for the conventional catalytic methods, such as a reaction at 60 ºC, 0.1 MPa and a methanol to oil molar ratio of 9:1. With respect to chemical limitations, supercritical transesterification is limited by the operating temperature due to

Supercritical transesterification with the original reacting parameters is economically competitive compared to the conventional catalytic method especially when low-grade feedstocks are employed (van Kasteren & Nisworo, 2007). As expected, the supercritical transesterification has a larger capital cost, due to the required reacting and pumping systems, than the conventional catalytic method, but has no additional capital and operating

For a better economic feasibility, research into ways to reduce the high operating conditions and lower energy consumption are warranted. For example, supercritical transesterification is not economically feasible when the heating and cooling integration is not employed (Marchetti & Errazu, 2008), while it is a feasible method when heat integration and the presence of catalysts are applied (D'Ippolito et al., 2006; Glišic et al., 2009; van Kasteren & Nisworo, 2007). The addition of calcium oxide as a solid catalyst and the reduction of the alcohol to oil molar ratio significantly decreased the total energy demand and improved the economic feasibility as well (Glišic & Skala, 2009). Furthermore, the addition of propane as co-solvent also enhanced the economic feasibility of supercritical transesterification with methanol (van Kasteren & Nisworo, 2007). However, additional feasibility studies on other assisting techniques that

**2.4 The economical feasibilities and chemical limitations of supercritical** 

**2.4.1 The economical feasibilities of supercritical transesterification** 

lower the original parameters (see Sections 3.2 - 3.4) are still required.

costs on feedstock pre-treatment, product post-treatment and waste management.

the thermal cracking of the unsaturated fatty acids.

**2.3.2 The drawbacks of supercritical transesterification** 

novel catalytic methods.

sections 3.2 – 3.4.

**transesterification** 

could be reduced by the addition of gaseous co-solvents, such as carbon dioxide and propane. For instance, the addition of 24% by weight of propane decreased the transition temperature of soybean oil-methanol, at a methanol to oil molar ratio of 65:1, from 315 ºC to 243 ºC (Hegel et al., 2007). Therefore, the addition of gaseous co-solvents is able to reduce the original severe conditions due to their ability to lower the transition temperature from a VL system to a one-phase supercritical system.

#### **2.3 The advantages and drawbacks of supercritical transesterification**

Novel solid heterogeneous catalysts that catalyze the transesterification on acidic or basic surfaces instead of in solution have been proposed to overcome the drawbacks of the conventional homogeneous catalytic method, which in part are the same as the supercritical transesterification. The enzyme catalysts typically also allow for a very high selectivity on the alkyl ester products (Helwani et al., 2009; Lene et al., 2009).

#### **2.3.1 The advantages of supercritical transesterification**

The advantages of supercritical transesterification over the conventional homogeneous catalytic method are feedstock flexibility, higher production efficiency and it is more environmentally friendly. The feedstock quality is far less influential under supercritical conditions than with the heterogeneous catalytic method, whilst supercritical transesterification has a similar advantage with respect to the product separation as the novel catalytic methods, but it has a higher production efficiency than both novel catalytic methods. The feedstock flexibility is the most important advantage to consider for biodiesel production methods because the resultant biodiesel price strongly depends on the feedstock price (Kulkarni & Dalai, 2006; Lam et al., 2010). The free fatty acids and moisture in lowgrade feedstocks and hydrated ethanol pose a negative effect on the basic homogeneous and heterogeneous catalytic methods. Whereas, free fatty acid levels and moisture contents in the feedstock do not significantly affect supercritical transesterification with methanol or ethanol. Therefore, supercritical transesterification is more suitable for use with the lowgrade and/or the hydrated ethanol feedstocks (Demirbas, 2009; Gui et al., 2009; Kusdiana & Saka, 2004b; Vieitez et al., 2011). For example, the in-situ transesterification of wet algal biomass in supercritical ethanol gave a 100% alkyl ester yield (Levine et al., 2010).

Supercritical transesterification has a better production efficiency than the conventional catalytic method because it requires a smaller number of processing steps. For instance, the feedstock pretreatment to remove moisture and free fatty acids, and the post-production product treatment steps, such as neutralization, washing and drying, are not necessary. In addition, the rate of reaction under supercritical conditions is significantly faster than the conventional catalytic method, so that the supercritical transesterification requires a smaller reactor size for a given production output.

With respect to environmental aspects, supercritical transesterification does not require any catalysts or chemicals, whilst the waste from the pretreatment and post-treatment steps are also reduced, since those steps are not necessary, leading to the generation of insignificant waste levels. However, the distillation process to recover the excess alcohol requires a large amount of energy which reduces the environmentally friendly advantage of the process (Kiwjaroun et al., 2009). Thus to maintain an environmentally friendly advantage, lowenergy separation methods, such as medium pressure flash drum, must be applied to recover the excess alcohol (Diaz et al., 2009).

could be reduced by the addition of gaseous co-solvents, such as carbon dioxide and propane. For instance, the addition of 24% by weight of propane decreased the transition temperature of soybean oil-methanol, at a methanol to oil molar ratio of 65:1, from 315 ºC to 243 ºC (Hegel et al., 2007). Therefore, the addition of gaseous co-solvents is able to reduce the original severe conditions due to their ability to lower the transition temperature from a

Novel solid heterogeneous catalysts that catalyze the transesterification on acidic or basic surfaces instead of in solution have been proposed to overcome the drawbacks of the conventional homogeneous catalytic method, which in part are the same as the supercritical transesterification. The enzyme catalysts typically also allow for a very high selectivity on

The advantages of supercritical transesterification over the conventional homogeneous catalytic method are feedstock flexibility, higher production efficiency and it is more environmentally friendly. The feedstock quality is far less influential under supercritical conditions than with the heterogeneous catalytic method, whilst supercritical transesterification has a similar advantage with respect to the product separation as the novel catalytic methods, but it has a higher production efficiency than both novel catalytic methods. The feedstock flexibility is the most important advantage to consider for biodiesel production methods because the resultant biodiesel price strongly depends on the feedstock price (Kulkarni & Dalai, 2006; Lam et al., 2010). The free fatty acids and moisture in lowgrade feedstocks and hydrated ethanol pose a negative effect on the basic homogeneous and heterogeneous catalytic methods. Whereas, free fatty acid levels and moisture contents in the feedstock do not significantly affect supercritical transesterification with methanol or ethanol. Therefore, supercritical transesterification is more suitable for use with the lowgrade and/or the hydrated ethanol feedstocks (Demirbas, 2009; Gui et al., 2009; Kusdiana & Saka, 2004b; Vieitez et al., 2011). For example, the in-situ transesterification of wet algal

biomass in supercritical ethanol gave a 100% alkyl ester yield (Levine et al., 2010).

Supercritical transesterification has a better production efficiency than the conventional catalytic method because it requires a smaller number of processing steps. For instance, the feedstock pretreatment to remove moisture and free fatty acids, and the post-production product treatment steps, such as neutralization, washing and drying, are not necessary. In addition, the rate of reaction under supercritical conditions is significantly faster than the conventional catalytic method, so that the supercritical transesterification requires a smaller

With respect to environmental aspects, supercritical transesterification does not require any catalysts or chemicals, whilst the waste from the pretreatment and post-treatment steps are also reduced, since those steps are not necessary, leading to the generation of insignificant waste levels. However, the distillation process to recover the excess alcohol requires a large amount of energy which reduces the environmentally friendly advantage of the process (Kiwjaroun et al., 2009). Thus to maintain an environmentally friendly advantage, lowenergy separation methods, such as medium pressure flash drum, must be applied to

**2.3 The advantages and drawbacks of supercritical transesterification** 

the alkyl ester products (Helwani et al., 2009; Lene et al., 2009).

**2.3.1 The advantages of supercritical transesterification** 

VL system to a one-phase supercritical system.

reactor size for a given production output.

recover the excess alcohol (Diaz et al., 2009).
