Basics of the Transesterification Reaction

Four methods to reduce the high viscosity of vegetable oils to enable their use in common diesel engines without operational problems such as engine deposits have been investigated: blending with petrodiesel, pyrolysis, microemulsification (cosolvent blending), and transesterification (1). Transesterification is by far the most common method and will be dealt with in this chapter. Only the transesterification reaction leads to the products commonly known as biodiesel, i.e., alkyl esters of oils and fats. The other three methods are discussed in Chapter 10. The most commonly prepared esters are methyl esters, largely because methanol is the least expensive alcohol, although there are exceptions in some countries. In Brazil, for example, where ethanol is less expensive, ethyl esters are used as fuel. In addition to methanol and ethanol, esters of vegetable oils and animal fats with other low molecular weight alcohols were investigated for potential production and their biodiesel properties. Properties of various esters are listed in the tables in Appendix A. Table 1 of this chapter contains a list of C1–C4 alcohols and their relevant properties. Information on vegetable oils and animal fats used as starting materials in the transesterification reaction as well as on resulting individual esters and esters of oils and fats appears in Appendix A. In addition to vegetable oils and animal fats, other materials such as used frying oils can also be suitable for biodiesel production; however, changes in the reaction procedure frequently have to be made due to the presence of water or free fatty acids (FFA) in the materials. The present section discusses the transesterification reaction as it is most commonly applied to (refined) vegetable oils and related work. Alternative feedstocks and processes, briefly indicated here, will be discussed later. The general scheme of the transesterification reaction was presented in the introduction and is given here again in Figure 1. Diand monoacylglycerols are formed as intermediates in the transesterification reaction. Figure 2 qualitatively depicts conversion vs. reaction time for a transesterification reaction taking into account the intermediary diand monoacylglycerols. Actual details in this figure, such as the final order of concentration of Ch4.1(Biodiesel)(26-41)(Final) 6/6/05 3:38 PM Page 26

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