Elsevier

Bioresource Technology

Volume 91, Issue 3, February 2004, Pages 283-287
Bioresource Technology

Reactivity of triglycerides and fatty acids of rapeseed oil in supercritical alcohols

https://doi.org/10.1016/S0960-8524(03)00202-5Get rights and content

Abstract

A catalyst-free biodiesel production method with supercritical methanol has been developed that allows a simple process and high yield because of simultaneous transesterification of triglycerides and methyl esterification of fatty acids. From these lines of evidence, we expected that similar results would be attained with the use of various alcohols by the supercritical treatment. However, it still remains unclear which type of reaction, transesterification or alkyl esterification, is faster. This parameter would be important in designing the optimum reaction conditions of the supercritical alcohol method. Therefore, we studied the effect of transesterification of triglycerides and esterification of fatty acids in rapeseed oil. Reaction temperature was set at 300 °C, and methanol, ethanol, 1-propanol, 1-butanol or 1-octanol was used as the reactant. The results showed that transesterification of triglycerides (rapeseed oil) was slower in reaction rates than alkyl esterification of fatty acids for any of the alcohols employed. Furthermore, saturated fatty acids such as palmitic and stearic acids had slightly lower reactivity than that of the unsaturated fatty acids; oleic, linoleic and linolenic.

Introduction

Most energy that the world is using is derived from unrenewable fossil fuel that has a great impact on environments. Research is, therefore, oriented for alternative energy. Biomass is one of its candidates, because biomass energy has some advantageous in reproduction, cyclic and carbon neutral properties. Biodiesel fuel is one example of biomass energy, and it is generally made of methyl esters of fatty acids produced by the transesterification reaction of triglycerides with methanol with the help of a catalyst (Clark et al., 1984).

In Japan, we have been using biodiesel fuel for about 220 garbage trucks in Kyoto city since 1997. This effort was recently extended to the municipal busses with a 20% blend of biodiesel fuel with petroleum diesel fuel. Currently, the annual consumption in Kyoto city is about 1500 ton of waste oil (Saka, 2001).

The use of a catalyst, either alkaline or acid catalyst, results in a more complicated process. Particularly, in the case of an alkaline catalyst, a removal of both the catalyst and the saponified product is necessary after the reaction. This conventional method not only makes the process longer, but also results in lower yield because vegetable oil contains mainly triglycerides with free fatty acids in a minor quantity, the latter being unable to be converted to methyl esters (Fukuda et al., 2001).

A two step reaction process that involves acid-catalyzed methyl esterification and subsequent alkaline-catalyzed transesterification have been, therefore, studied to increase the yield from free fatty acids (Haas and Bloomer, 2000; Frohlich et al., 2001; Boocock, 2002). Saka and Kusdiana (2001), on the other hand, developed a new method for biodiesel production with supercritical methanol. It was demonstrated in this work that fatty acids present in the vegetable oil are successfully converted to methyl esters. Two types of reactions exist in this method for methyl esters formation; transesterification of triglycerides and methyl esterification of fatty acids. Thus, a higher yield can be obtained than that produced by the alkaline-catalyzed method (Kusdiana and Saka, 2001a, Kusdiana and Saka, 2001b). In addition, because of a catalyst-free process, separation and purification becomes much simpler in this supercritical methanol treatment. From these lines of evidence, we then expected that similar results would be attained with the use of various alcohols by the supercritical treatment.

Although an extensive work has been done on supercritical methanol treatment with vegetable oil (Saka and Kusdiana, 2001; Kusdiana and Saka, 2001a, Kusdiana and Saka, 2001b), it still remains unclear which type of reaction, transesterification and alkyl esterification, is faster in the reaction rate. This parameter would be important in designing the optimum reaction condition of the supercritical alcohol method. Therefore, we studied in this paper the reactivity of transesterification of triglycerides and alkyl esterification of fatty acids in the supercritical alcohol process.

Section snippets

Methods

Rapeseed oil was used as a vegetable oil, while palmitic acid, stearic acid, oleic acid, linoleic acid, and linolenic acid were used as free fatty acids. All were purchased from Nakalai Tesque, Japan. Since rapeseed oil consists of 98.5% of triglycerides and only 1.5% of free fatty acids, rapeseed oil was assumed in this study to be triglycerides. As a reactant, several kinds of alcohols; methanol, ethanol, 1-propanol, 1-butanol and 1-octanol, were used.

The batch-type supercritical biomass

Results and discussion

To evaluate the reaction of triglycerides and fatty acids in supercritical treatment of alcohol, five kinds of alcohols were selected; methanol, ethanol, 1-propanol, 1-buthanol and 1-octanol. Reaction temperature was set at 300 °C in the batch-type reaction system. The HPLC system was used to analyze the conversion of the samples to fatty acid alkyl esters.

Concluding remarks

The reactivity on transesterification of triglycerides and alkyl esterification of fatty acids was investigated under supercritical alcohol treatment. The result obviously demonstrated that the alkyl esterification had higher reaction rates compared to the transesterification. This result proved that in the supercritical alcohol treatment, free fatty acids present in vegetable oil would be completely converted to the alkyl esters under the treatment of transesterification.

Acknowledgements

This work was done in the COE program in the 21 Century of “Establishment of COE on Sustainable-Energy System”, and Grant-in-Aid for Scientific Research (B) (2) (13556058, 2001. 4-2003. 3) from the Ministry of Education, Science, Sports and Culture, Japan.

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