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**12** 

*Thailand* 

**Transesterification in Supercritical Conditions** 

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

Triglyceride Alcohol Glycerol Ester Fig. 1. The overall transesterification reaction (R is a small alkyl group, R1, R2 and R3 are a

The reaction mechanism for supercritical transesterification has been proposed to be

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

OH

OH

+

R

R

R

O R1

O

O R2

O

O R3

O

OH

(see Section 1.1). The overall transesterification reaction is shown in Fig. 1.

+

R OH

R OH

R OH

somewhat alike the acidic-catalyzed reaction as described in Section 1.2.

O R1

O

O R2

O

O R3

O

**1. Introduction** 

fatty acid chain)

Somkiat Ngamprasertsith and Ruengwit Sawangkeaw *Fuels Research Center, Department of Chemical Technology,* 

*Center for Petroleum, Petrochemicals and Advance Materials,* 

*Faculty of Science, Chulalongkorn University,* 

*Chulalongkorn University,* 

Zhang, Y.; Dube, M. A.; McLean, D. D. & Kates, M. (2003). Biodiesel production from waste cooking oil: 1. Process design and technological assessment. *Bioresource Technology*, Vol. 89(1), pp. 1-16.
