**Transesterification in Supercritical Conditions**

Somkiat Ngamprasertsith and Ruengwit Sawangkeaw

*Fuels Research Center, Department of Chemical Technology, Faculty of Science, Chulalongkorn University, Center for Petroleum, Petrochemicals and Advance Materials, Chulalongkorn University, Thailand* 

### **1. Introduction**

246 Biodiesel – Feedstocks and Processing Technologies

Zhang, Y.; Dube, M. A.; McLean, D. D. & Kates, M. (2003). Biodiesel production from waste

Vol. 89(1), pp. 1-16.

cooking oil: 1. Process design and technological assessment. *Bioresource Technology*,

The transesterification or biodiesel production under supercritical conditions (supercritical transesterification) is a catalyst-free chemical reaction between triglycerides, the major component in vegetable oils and/or animal fats, and low molecular weight alcohols, such as methanol and ethanol, at a temperature and pressure over the critical point of the mixture (see Section 1.1). The overall transesterification reaction is shown in Fig. 1.

Fig. 1. The overall transesterification reaction (R is a small alkyl group, R1, R2 and R3 are a fatty acid chain)

The reaction mechanism for supercritical transesterification has been proposed to be somewhat alike the acidic-catalyzed reaction as described in Section 1.2.

Since the actual feedstocks are not composed solely of triglycerides, especially the low-grade feedstocks, but are also contaminated with water and free fatty acids, some side reactions also take place under supercritical conditions (see Section 1.3). For example, the esterification of free fatty acids with alcohols increases the fatty acid alkyl ester content in the biodiesel product, while the thermal cracking of unsaturated fatty acids decreases the esters content.

The earlier research on supercritical transesterification mostly employed methanol as the reacting medium and reacting alcohol at the same time due to the fact that it has the lowest critical point and the highest activity (Warabi et al., 2004). Ethanol is also an interesting candidate because it can be industrially produced from renewable sources in many countries nowadays. However, other supercritical mediums, such as methyl acetate (Saka & Isayama, 2009) and dimethyl carbonate (Ilham & Saka, 2009; Tan et al., 2010b), have also

Transesterification in Supercritical Conditions 249

Soybean oil Palm kernel oil

Soybean oil Palm kernel oil

0 5 10 15 20 25 30 35 40 45 50

**Methanol to oil molar ratio**

0 5 10 15 20 25 30 35 40 45

**Methanol to oil molar ratio**

Fig. 3. The estimated critical pressure of soybean oil-methanol (Hegel et al., 2008) and palm

Below the critical point of mixture, transesterification can take place in the presence of acidic or basic catalysts. Thus, the reaction mechanisms of transesterification are divided into acid and base catalyzed paths, as summarized elsewhere (Meher et al., 2006). The reaction mechanism of supercritical transesterification is somewhat similar to the acid catalyzed path in that the hydrogen bond of the alcohol is weakened at high temperatures (Hoffmann & Conradi, 1998). However, whilst the acid-catalyzed transesterification reaction is much slower than the basecatalyzed one at ambient conditions, the supercritical transesterification is much faster and achieves complete conversion of triglycerides to esters rapidly because the chemical kinetics

The reaction mechanism of supercritical transesterification, as shown in Fig. 4, was proposed by the analogues between the hydrolysis of esters in supercritical water (Krammer

Fig. 2. The estimated critical temperature of soybean oil-methanol (Hegel et al., 2008) and

200

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

kernel oil-methanol (Bunyakiat et al., 2006) mixtures.

are dramatically accelerated under supercritical conditions.

**1.2 The reaction mechanism of supercritical transesterification** 

**Critical pressure(MPa)**

palm kernel oil-methanol (Bunyakiat et al., 2006) mixtures.

250

300

350

**Critical temperature (oC)**

400

450

500

been used to produce biodiesel, but these are not be described in this chapter since the chemical reaction involved is not a transesterification reaction. The reaction parameters and optimal conditions for supercritical transesterification are summarized in Section 1.4.

#### **1.1 The definition of supercritical transesterification**

A pure substance ordinarily exists in a solid, liquid or gaseous state, depending on the temperature and pressure. For example, methanol is in a liquid state at ambient temperature (and pressure) and changes to a gaseous state above its boiling point. A gaseous substance can be compressed to a liquid state when a pressure above the boiling point is applied. Until the critical temperature is reached, a gaseous substance cannot be compressed to the liquid state. In the same manner, a compressed liquid substance cannot be heated to a gaseous state at its critical pressure.

Above its inherent critical temperature and pressure, the substance becomes a supercritical fluid, which is a non-condensable dense fluid. In the supercritical state, the density is generally in a range between 20 – 50% of that in the liquid state and the viscosity is close to that in the gaseous state. In other words, the molecules in the supercritical fluid have high kinetic energy like a gas and high density like a liquid. Therefore, the chemical reactivity can be enhanced in this state.

The critical point of any transesterification reaction mixture is mostly calculated by the critical properties of the alcohols and the vegetable oils and/or animal fats. However, the critical properties of vegetable oils and/or animal fats cannot be experimentally measured because they thermally decompose before the critical point is reached. In addition, the molecular structure of vegetable oils and/or animal fats is impossible to know because the exact distribution of the fatty acids chain in triglycerides mixture is unknown.

Therefore to estimate the critical properties of vegetable oils and/or animal fats, their molecular is assumed to be a simple triglyceride (tripalmitin, triolein, etc.) or pseudotriglycerides (Espinosa et al., 2002), with the proportion of such different simple triglycerides reflecting the actual overall fatty acid composition in the feedstock. The type of simple triglycerides or the pseudo-triglycerides are thus defined by their actual fatty acid profile in the vegetable oils and/or animal fats. For instance, soybean oil has linoleic acid as the major fatty acid, and so it is usually assumed to be trilinolein. Next, the critical properties of the simple triglycerides or the pseudo-triglycerides are estimated by the Fedor and Lydersen group contribution method (Poling et al., 2001), or similar. After the critical properties of each of the triglycerides are estimated, the critical point of the mixture can be estimated by mixing rules, such as the Lorentz-Berthelot-type (Bunyakiat et al., 2006) and the group-contribution with associated mixing rules (Hegel et al., 2008).

For example, the critical temperature and pressure of soybean oil-methanol and palm kernel oil-methanol mixtures are illustrated in Figs. 2 and 3, respectively.

From Figs. 3 and 4, it is clear that the critical point of the reaction mixture depends on the alcohol to oil molar ratio, so the selected alcohol to oil molar ratio will reflect the operating temperature and pressure, as described in Section 2.1. For a high transesterification conversion (triglyceride to alkyl ester) at a constant methanol to oil molar ratio, the operating temperature and pressure have to be approximately 1.5- to 2.0-fold over the critical point of the reaction mixture. For example, the optimal conditions at a methanol to oil molar ratio of 42:1 is 350 °C and 20 MPa, respectively. Therefore, the definition of supercritical conditions is the temperature and pressure above the critical point of the reaction mixture, which is calculated from the critical properties of the vegetable oils and/or animal fats and the alcohols.

been used to produce biodiesel, but these are not be described in this chapter since the chemical reaction involved is not a transesterification reaction. The reaction parameters and

A pure substance ordinarily exists in a solid, liquid or gaseous state, depending on the temperature and pressure. For example, methanol is in a liquid state at ambient temperature (and pressure) and changes to a gaseous state above its boiling point. A gaseous substance can be compressed to a liquid state when a pressure above the boiling point is applied. Until the critical temperature is reached, a gaseous substance cannot be compressed to the liquid state. In the same manner, a compressed liquid substance cannot be heated to a gaseous

Above its inherent critical temperature and pressure, the substance becomes a supercritical fluid, which is a non-condensable dense fluid. In the supercritical state, the density is generally in a range between 20 – 50% of that in the liquid state and the viscosity is close to that in the gaseous state. In other words, the molecules in the supercritical fluid have high kinetic energy like a gas and high density like a liquid. Therefore, the chemical reactivity can

The critical point of any transesterification reaction mixture is mostly calculated by the critical properties of the alcohols and the vegetable oils and/or animal fats. However, the critical properties of vegetable oils and/or animal fats cannot be experimentally measured because they thermally decompose before the critical point is reached. In addition, the molecular structure of vegetable oils and/or animal fats is impossible to know because the

Therefore to estimate the critical properties of vegetable oils and/or animal fats, their molecular is assumed to be a simple triglyceride (tripalmitin, triolein, etc.) or pseudotriglycerides (Espinosa et al., 2002), with the proportion of such different simple triglycerides reflecting the actual overall fatty acid composition in the feedstock. The type of simple triglycerides or the pseudo-triglycerides are thus defined by their actual fatty acid profile in the vegetable oils and/or animal fats. For instance, soybean oil has linoleic acid as the major fatty acid, and so it is usually assumed to be trilinolein. Next, the critical properties of the simple triglycerides or the pseudo-triglycerides are estimated by the Fedor and Lydersen group contribution method (Poling et al., 2001), or similar. After the critical properties of each of the triglycerides are estimated, the critical point of the mixture can be estimated by mixing rules, such as the Lorentz-Berthelot-type (Bunyakiat et al., 2006) and

For example, the critical temperature and pressure of soybean oil-methanol and palm kernel

From Figs. 3 and 4, it is clear that the critical point of the reaction mixture depends on the alcohol to oil molar ratio, so the selected alcohol to oil molar ratio will reflect the operating temperature and pressure, as described in Section 2.1. For a high transesterification conversion (triglyceride to alkyl ester) at a constant methanol to oil molar ratio, the operating temperature and pressure have to be approximately 1.5- to 2.0-fold over the critical point of the reaction mixture. For example, the optimal conditions at a methanol to oil molar ratio of 42:1 is 350 °C and 20 MPa, respectively. Therefore, the definition of supercritical conditions is the temperature and pressure above the critical point of the reaction mixture, which is calculated from the critical properties of the vegetable oils and/or

exact distribution of the fatty acids chain in triglycerides mixture is unknown.

the group-contribution with associated mixing rules (Hegel et al., 2008).

oil-methanol mixtures are illustrated in Figs. 2 and 3, respectively.

optimal conditions for supercritical transesterification are summarized in Section 1.4.

**1.1 The definition of supercritical transesterification** 

state at its critical pressure.

be enhanced in this state.

animal fats and the alcohols.

Fig. 2. The estimated critical temperature of soybean oil-methanol (Hegel et al., 2008) and palm kernel oil-methanol (Bunyakiat et al., 2006) mixtures.

Fig. 3. The estimated critical pressure of soybean oil-methanol (Hegel et al., 2008) and palm kernel oil-methanol (Bunyakiat et al., 2006) mixtures.

#### **1.2 The reaction mechanism of supercritical transesterification**

Below the critical point of mixture, transesterification can take place in the presence of acidic or basic catalysts. Thus, the reaction mechanisms of transesterification are divided into acid and base catalyzed paths, as summarized elsewhere (Meher et al., 2006). The reaction mechanism of supercritical transesterification is somewhat similar to the acid catalyzed path in that the hydrogen bond of the alcohol is weakened at high temperatures (Hoffmann & Conradi, 1998). However, whilst the acid-catalyzed transesterification reaction is much slower than the basecatalyzed one at ambient conditions, the supercritical transesterification is much faster and achieves complete conversion of triglycerides to esters rapidly because the chemical kinetics are dramatically accelerated under supercritical conditions.

The reaction mechanism of supercritical transesterification, as shown in Fig. 4, was proposed by the analogues between the hydrolysis of esters in supercritical water (Krammer

Transesterification in Supercritical Conditions 251

OH O R1 H

OH O R1 R

Fatty acid Alcohol Ester Water

Fig. 7. The esterification reaction under supercritical conditions (R is an alkyl group and R1

In actuality, the hydrolysis reaction or the presence of water and free fatty acids do not affect the final alkyl ester content obtained for supercritical transesterification (Kusdiana & Saka, 2004b), because the alcohols have a much higher reactivity than water at the optimal point of supercritical transesterification. For example, the chemical rate constant for the transesterification of rapeseed oil is approximately 7-fold higher than that for the hydrolysis

Secondly, the thermal cracking of unsaturated fatty acids, especially the polyunsaturated fatty acids, occurs at temperatures over 300 °C and reaction times over 15 min (Quesada-Medina & Olivares-Carrillo, 2011). An example of the thermal cracking of a palmitic, oleic

O

O

heat (300 - 350 oC)

O

O

O

O

O

O

O

O

O

O

temperature range of 300 - 350 °C and a reaction time of over 15 min.

Fig. 8. The thermal cracking reaction of a triglyceride under supercritical conditions at a

The triglyceride product from the thermal cracking reaction can be transesterified afterwards under supercritical conditions to alkyl esters. However, these alkyl esters are not

Ester Water Fatty acid Alcohol

Fig. 6. The hydrolysis reaction of alkyl esters under supercritical conditions (R is an alkyl

O

O

<sup>+</sup> <sup>R</sup>

<sup>+</sup> <sup>H</sup>

OH

OH

O R1

group and R1 is a fatty acid chain)

O R1

O

<sup>+</sup> <sup>H</sup>

<sup>+</sup> <sup>R</sup>

of soybean oil (Khuwijitjaru et al., 2004; Kusdiana & Saka, 2001).

and linoleic acid based triglyceride is illustrated in Fig. 8.

O

R

H

is a fatty acid chain)

& Vogel, 2000) and the transesterification of triglycerides in supercritical methanol (Kusdiana & Saka, 2004b). It is assumed that the alcohol molecule (in this case methanol) directly attacks the carbonyl carbon of the triglyceride because the hydrogen bond energy is lowered; which would allow the alcohol to be a free monomer. In the case of methanol, the transesterification is completed via transfer of a methoxide moiety, whereby fatty acid methyl esters and diglycerides are formed. Consequently, the diglyceride reacts with other methanol molecules in a similar way to form the methyl ester and monoglyceride, the later of which is further converted to methyl ester and glycerol in the last step. The same process is applicable to other primary alkyl alcohols, such as ethanol.

Fig. 4. The proposed reaction mechanism of transesterification in supercritical methanol (R' is a diglyceride group and R1 is a fatty acid chain).

#### **1.3 The side reactions in the supercritical conditions**

Firstly, the hydrolysis reaction of alkyl esters and triglyceride can take place at over 210 °C (Khuwijitjaru et al., 2004) and over 300 °C (W. King et al., 1999), respectively, in present of water and pressure above 20 MPa. The overall hydrolysis reaction of triglycerides and alkyl esters is shown in Figs. 5 and 6, respectively, with the principal products of the hydrolysis reaction being the respective fatty acids which are subsequently converted to alkyl esters under the supercritical condition by the esterification, as illustrated in Fig. 7.

Fig. 5. The overall hydrolysis reaction of triglycerides under supercritical conditions (R is an alkyl group and R1, R2 and R3 are fatty acid chains)

& Vogel, 2000) and the transesterification of triglycerides in supercritical methanol (Kusdiana & Saka, 2004b). It is assumed that the alcohol molecule (in this case methanol) directly attacks the carbonyl carbon of the triglyceride because the hydrogen bond energy is lowered; which would allow the alcohol to be a free monomer. In the case of methanol, the transesterification is completed via transfer of a methoxide moiety, whereby fatty acid methyl esters and diglycerides are formed. Consequently, the diglyceride reacts with other methanol molecules in a similar way to form the methyl ester and monoglyceride, the later of which is further converted to methyl ester and glycerol in the last step. The same process

Fig. 4. The proposed reaction mechanism of transesterification in supercritical methanol (R'

Firstly, the hydrolysis reaction of alkyl esters and triglyceride can take place at over 210 °C (Khuwijitjaru et al., 2004) and over 300 °C (W. King et al., 1999), respectively, in present of water and pressure above 20 MPa. The overall hydrolysis reaction of triglycerides and alkyl esters is shown in Figs. 5 and 6, respectively, with the principal products of the hydrolysis reaction being the respective fatty acids which are subsequently converted to alkyl esters

Triglyceride Water Glycerol Fatty acid Fig. 5. The overall hydrolysis reaction of triglycerides under supercritical conditions (R is an

OH

OH

+

H

H

H

O R1

O

O R2

O

O R3

O

OH

under the supercritical condition by the esterification, as illustrated in Fig. 7.

OH

OH

OH

is applicable to other primary alkyl alcohols, such as ethanol.

is a diglyceride group and R1 is a fatty acid chain).

O R1

O

O R2

+

H

H

H

O

O R3

alkyl group and R1, R2 and R3 are fatty acid chains)

O

**1.3 The side reactions in the supercritical conditions** 

Fig. 6. The hydrolysis reaction of alkyl esters under supercritical conditions (R is an alkyl group and R1 is a fatty acid chain)

Fig. 7. The esterification reaction under supercritical conditions (R is an alkyl group and R1 is a fatty acid chain)

In actuality, the hydrolysis reaction or the presence of water and free fatty acids do not affect the final alkyl ester content obtained for supercritical transesterification (Kusdiana & Saka, 2004b), because the alcohols have a much higher reactivity than water at the optimal point of supercritical transesterification. For example, the chemical rate constant for the transesterification of rapeseed oil is approximately 7-fold higher than that for the hydrolysis of soybean oil (Khuwijitjaru et al., 2004; Kusdiana & Saka, 2001).

Secondly, the thermal cracking of unsaturated fatty acids, especially the polyunsaturated fatty acids, occurs at temperatures over 300 °C and reaction times over 15 min (Quesada-Medina & Olivares-Carrillo, 2011). An example of the thermal cracking of a palmitic, oleic and linoleic acid based triglyceride is illustrated in Fig. 8.

Fig. 8. The thermal cracking reaction of a triglyceride under supercritical conditions at a temperature range of 300 - 350 °C and a reaction time of over 15 min.

The triglyceride product from the thermal cracking reaction can be transesterified afterwards under supercritical conditions to alkyl esters. However, these alkyl esters are not

Transesterification in Supercritical Conditions 253

5 – 30 min, for both methanol and ethanol. These parameters are referred to as the original supercritical transesterification parameters, and have been employed to study the effects of each parameter, the chemical kinetics, the phase behavior and the economical feasibility of the process. Since the original parameters are elevated conditions, innovative techniques are

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

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

supercritical transesterification, and this is discussed in Section 2.4.2.

**Alcohol: oil (mol: mol)** **Reaction time (min)**

Methanol 350 NR 41:1 5 100-mL / Batch 95% MEC (Demirbas, 2002)

& Ethanol 400 20 40:1 30 8-mL / Batch 97% Con. (Madras et al., 2004)

Palm kernel Methanol 350 19 42:1 7 - 15 251-mL / CT 95% MEC (Bunyakiat et al., 2006)

Groundnut Methanol 400 20 50:1 30 11-mL / Batch 95% Con. (Rathore & Madras,

Palm kernel Methanol 350 20 42:1 30 250-mL / Batch 95% MEC (Sawangkeaw et al.,

Rapeseed Methanol 350 45 42:1 4 5-mL / Batch 98% MEC (Saka & Kusdiana,

Rapeseed Methanol 350 20 42:1 30 200-mL / CT 87% MEC (Minami & Saka, 2006) Soybean Methanol 350 20 42:1 30 250-mL / Batch 95% MEC (Yin et al., 2008a)

Castor Ethanol 300 20 40:1 NR 42-mL / CT 75% EEC (Vieitez et al., 2011) Soybean Ethanol 350 20 40:1 15 42-mL / CT 80% Con. (Silva et al., 2007) Sunflower Ethanol 280 NR 40:1 5 100-mL / Batch 80% EEC (Balat, 2008)

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

Table 1. The original reaction parameters and optimal conditions of supercritical

**Reactor (size / type) a** **Extent of** 

**reactionb Ref** 

2007)

2007)

2001)

**(MPa)** 

proposed to reduce these original parameters.

**2. Process overview**

**Oil type Alcohol T (oC) <sup>P</sup>**

Coconut &

Hazelnut kernel & Cottonseed

Palm and

Sunflower Methanol

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

transesterification for various oil types and alcohols.

or the % ethyl esters content (EEC).

NR = not reported

**transesterification** 

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 soybean and sunflower oil.

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, density and cold flow properties.

Fig. 9. The thermal cracking reaction of triglycerides under supercritical conditions at temperature range of 350 - 450 °C

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

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 and continuous reactors, and are summarized in Table 1.

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 side reactions to other products.

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 5 – 30 min, for both methanol and ethanol. These parameters are referred to as the original supercritical transesterification parameters, and have been employed to study the effects of each parameter, the chemical kinetics, the phase behavior and the economical feasibility of the process. Since the original parameters are elevated conditions, innovative techniques are proposed to reduce these original parameters.
