Biodiesel oxidative stability

Falk, O. R. Meyer-Pittroff. The effect of fatty acid composition on biodiesel oxidative stability. Eur. J. Lipid Sci. TechnoL 2004, 106, 837-843. 

Schober, S. and Mittelbach, M. 2004. The impact of antioxidants on biodiesel oxidation stability. Eur. J. Lipid Sci. Technol. 106 382-389. 

Wazilewski, W.T., Bariccatti, R.A., Martins, G.I., Secco, D., Melegari de Souza, S.N., Rosa, H.A., Chaves, L.I., 2013. Study of the methyl crambe (Crambe abyssinica Hochst) and soybean biodiesel oxidative stability. Ind. Crops Prod. 43, 207-212. 

Knothe G. Some aspects of biodiesel oxidative stability. Fuel Process Technol. 2007 88 669-677. 

This prompted us to investigate the possibility of selectively hydrogenate highly unsaturated oils, unsuitable for the production of Biodiesel, in order to improve their oxidative stability while keeping the cold properties. 

Hydrotreating has been proposed by Arbokem Inc. in Canada as a means of converting Grade Tall Oil into biofuels and fuel additives. However, this process is a hydrogenation process which produces hydrocarbons rather than biodiesel. Recently a process for making biodiesel from crude tall oil has been proposed. It relies on the use of an acid catalysts or of an acyl halide for the esterification reaction, but no information is given on the properties of this fuel, particularly concerning the oxidative stability. 

The Biodiesel Stability (BIOSTAB) project, supported by the European Commission, was initiated in 2001 to establish clear criteria and analytical methods for the monitoring biodiesel fuel stability (Various, 2003 Prankl, 2002). The resulting unified method, EN 14112 (Anon., 2003c) established a means for measuring oxidative stability utilizing the Rancimat or oxidation stability instruments. This test method was essentially developed from standards employed in the fats and oils industry to measure isothermally the induction period for oxidation of fatty derivatives. At present, both biodiesel fuel standards ASTM D 6751 (Anon., 2007a) and EN 14214 (Anon., 2003b) include an oxidative stability specification based on measurement by method EN 14112. 

Although there are numerous publications on the effect of natural and synthetic antioxidants on the stability of oils and fats used as food and feed, until recently relatively little publicly available information was available on the effect of antioxidants on the oxidative stability of biodiesel. One of the earliest studies reporting of the effects of antioxidants on biodiesel was that of Du Plessis et aL (1985), which examined storage stability of sunflower oil methyl esters (SFME) at various temperatures for 90 d. Effects of air temperature, presence of light, addition of TBHQ (see Figure 1.1) and contact with steel were evaluated by analysis of free fatty acid content, PV, kinematic viscosity, anisidine value, and induction period. Addition of TBHQ delayed oxidation of samples stored at moderate temperatures (<30°C). In contrast, under unfavorable (50°C) conditions, TBHQ was ineffective. 

Conversely, SFME exhibited relatively poor improvement in oxidative stability with the use of antioxidants, presumably due to the higher concentrations of linoleic acid methyl esters in sunflower oil in comparison to the other biodiesel samples evaluated by the authors. Therefore, a good correlation was found between the improvement in oxidative stability as measured by OSI when antioxidants are used and the fatty acid composition of the biodiesel sample (Mittelbach and Schober, 2003). 

Stavinoha and Kline (2001) adapted ASTM method D 6186 (Oxidation Induction Time of Lubricating Oils by Pressure Differential Scanning Calorimetry [P-DSC]) for analyzing the oxidative stability of SME treated with antioxidants. This report concluded that isothermal P-DSC analysis is suitable for screening the effectiveness of antioxidants for treating biodiesel. 

Dunn, R. 0.2005b. Effect of Antioxidants on the Oxidative Stability of Methyl Soyate (Biodiesel). Fuel Proc. Technol., 86,1071-1085. 

Dunn, R. O. 2006a. Oxidative Stability of Biodiesel by Dynamic Mode Pressurized-Differential Scanning Calorimetry (P-DSC). Trans. ASABE, 49,1633-1641. 

Stavinoha, L. L., and Kline, K. S. 2001. Oxidation Stability of Methyl Soyates—Modified ASTM D5304 and D 6186 for Biodiesel B100. In Report, Oxidation Stability of Methyl Soyates—Modified ASTM D 5304 and D 6186 for Biodiesel B100. Warren MI U.S. Army, TACOM, TARDEC, National Automotive Center. 

Though FAME has limited oxidation stability, they remain a valid alternative for diesel. Conventional diesel fuel has a boiling range of 180-340 °C, with a composition of n-alkanes, cycloalkanes, alkyl benzenes, and polyaromatic compounds. Fossil diesels have a CN in the range 40-100. FAME has properties that are close to all these basic diesel properties. FAME can also easily blend with fossil diesel at any level due to their similar solvent behavior the viscosity of fossil diesel and biodiesel are also in the same range. 

Operation of the Biodiesel Cost Optimizer is fast and easy, making it possible to make large sets of simulations in a short time. This helps better understanding the different value of the different fatty acid methyl esters. It will quickly become clear, when using the Biodiesel Cost Optimizer, that oleic acid methyl esters are the preferential FAME in every biodiesel formula. Oleic acid methyl esters bring a relatively high oxidation stability (50h or more), combined with a more than acceptable CN of around 56, and excellent melting point at -19 °C. Unfortunately, pure oleic acid methyl esters are not available in the market. 

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