Ethyl esters from the single‐phase base‐catalyzed ethanolysis of vegetable oils

Abstract

The effects of alcohol/oil molar ratio, base concentration, and temperature on the single-phase base-catalyzed ethanolyses of sunflower and canola oils were determined. The use of tetrahydrofuran as co-solvent, as well as higher than usual alcohol/substrate molar ratios, prevented glycerol separation. This allowed each reaction to reach equilibrium rather than just steady-state conditions. High conversions of oil lowered the concentrations of MG and DG surfactants in the products, and thereby mitigated the formation of emulsions usually associated with ethanolysis reactions. An alcohol/oil molar ratio of 25:1, together with the necessary amount of cosolvent, gave optimal results. At this molar ratio, despite equilibrium being achieved, ethanolysis, unlike methanolysis, did not quite produce biodiesel-standard material, the MG content being approximately 1.5 mass%. For methanolysis and 1-butanolysis, the corresponding values were 0.6 and 2.0 mass%, respectively. The use of 1.4 mass% KOH (equivalent to 1.0 mass% NaOH) led to ethanolysis equilibrium within 6-7 min at 23°C rather than 15 min when only 1.0 mass% was used. At 60°C, equilibrium was reached within only 2 min. Soybean and canola oils behaved the same.Paper no. J10264 in JAOCS 80, 367-371 (April 2003). KEY WORDS:Base-catalyzed ethanolysis, biodiesel standards, canola and sunflower oils, one-phase transesterification.The base-catalyzed formation of methyl and ethyl esters (EE) of FA from vegetable oils (TG) is important for several reasons. For many years, these esters have been commercially available in several European countries as renewable diesel fuel substitutes. In the year 2000, these esters were designated as allowable substitute fuels under the U.S. Energy Policy Act (EPACT). Although it is easier to make methyl esters, in some jurisdictions, it may be more desirable to make ethyl esters because the ethanol can be derived from renewable starch sources such as corn.The base-catalyzed formation of ethyl esters is difficult compared to the production of methyl esters. Specifically, the formation of stable emulsions during ethanolysis is a problem (1). Methanol and ethanol are not miscible with TG at ambient temperatures, and the reaction mixtures are usually mechanically stirred to enhance mass transfer. During the course of these reactions, emulsions usually form. In the case of methanolysis, these emulsions quickly and easily break down to form a lower glycerol-rich layer and an upper methyl ester-rich layer. In ethanolysis, these emulsions are much more stable and severely complicate separation and purification of the ester. The emulsions are caused in part by the formation of the intermediate MG and DG, which have both polar hydroxyl groups and nonpolar hydrocarbon chains. Therefore, these intermediates are strong surface-active agents and are used as such in the food industry as emulsifiers. In alcoholysis reactions, the catalyst, usually either sodium or potassium hydroxide, is dissolved in the polar alcohol phase, into which TG must transfer in...

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