Oxidative stability of biodiesels produced from vegetable oils having different degrees of unsaturation
Introduction
Biodiesel refers to the monoalkyl esters of the long-chain fatty acids derived from renewable lipid sources. The name ‘biodiesel’ is used for a variety of ester based oxygenated fuels from renewable biological sources, and these fuels can be used in compression ignition engines with little or no modifications [1].
The main advantages of using biodiesel is that it is biodegradable, can be used without modifying existing engines and produces less harmful gas emissions, such as sulfur oxide. On a lifecycle basis, biodiesel reduces net carbon dioxide emissions by 78% when compared to conventional diesel fuel [2]. However, despite its many advantages, the nature of biodiesel makes it more sensitive to oxidation than petroleum diesel during long-term storage. The sensitivity varies, depending on the raw material, the presence of naturally occurring antioxidants and the storage conditions [3].
Biodiesel having high concentrations of unsaturated fatty acid esters, such as linoleates and linolenates, are especially susceptible to oxidation [4]. The higher the degree of unsaturation, the more susceptible the molecule is to degradation, both thermal and oxidative, which forms products that can lead to the formation of deposits and obstruction of the fuel injection system of the engine [5].
The oxidative stability of biodiesel is mainly related to the number of bis-allylic methylene groups adjacent to the double bond and not to the total number of double bonds expressed by the iodine value [6]. Thus, polyunsaturated methyl esters are more vulnerable to oxidation than monounsaturated esters because they contain more allylic methylene configurations [3]. Hence, the relative oxidation rates for unsaturated esters are ordered as linolenates > linoleates > oleates [7].
The oxidation of biodiesel begins with the removal of a hydrogen from a carbon atom to produce a carbon free radical. If diatomic oxygen is present, the subsequent reaction to form a peroxy radical is extremely rapid. The peroxy free radical is not as reactive as the carbon free radical but is sufficiently reactive to quickly abstract a hydrogen from a carbon to form another carbon radical and hydroperoxide. The new carbon free radical can then react with diatomic oxygen to continue the propagation cycle. This chain reaction terminates when two free radicals react with each other to yield stable products, such as aldehydes, short-chain carboxylic acids, insoluble gums and sediments [8]. The use of antioxidants and their functional mechanisms have been widely studied, and the antioxidants can be classified as primary antioxidants, synergists, oxygen removers, chelating agents and mixed antioxidants. The primary antioxidants promote the removal or inactivation of the free radicals formed during the initiation or propagation of the reaction through the donation of hydrogen atoms to these molecules, thereby interrupting the chain reaction [9].
The objective of this study is to analyze the influence of the content of unsaturated fatty acids on the properties of biodiesel, such as the kinematic viscosity, cold filter plugging point and oxidative stability, and to study the influence of adding synthetic antioxidants on the oxidative stability of açaí, passion fruit and linseed biodiesel fuels.
Section snippets
Materials
The vegetable oils were obtained from local companies. Cupuaçu fat and açaí oil were purchased from Extratos Vegetais Ativos LTDA. The passion fruit oil was purchased from Cooperativa Agrícola Mista de Tomé-açú (CAMTA) and the linseed oil from Acrilex.
The reagents used in the transesterification were as follows: methyl alcohol (99.9%) and potassium hydroxide (85.0%) were purchased from Nuclear; sulfuric acid (95–97%) was obtained from Merck and sodium sulfate (99.9%) was obtained from Synth.
Other
Fatty acid composition of the vegetable oils
Cupuaçu fat contains predominantly saturated fatty acids (51.6%), mainly stearic acid (33.0%), whereas the açaí, passion fruit and linseed oils present mainly unsaturated fatty acids, with more than one half composed of oleic acid (52.9%). Passion fruit oil is composed of 85.8% unsaturated fatty acids, mainly linoleic acid (52.7%), see Table 1. The content of unsaturated fatty acids in the oils directly influences the physical–chemical properties of the biodiesel, mostly with regard to the
Conclusion
This study examined the influence of fatty acid composition on the physical–chemical properties of biodiesels. The elevated kinematic viscosity values, cold filter plugging point and oxidative stability were due to the presence of saturated fatty acids. The PG antioxidant was shown to be more efficient than BHA and TBHQ. It was verified that, when antioxidants are added to methyl esters with low oxidation rates, the oxidative stability sharply increases.
Acknowledgements
The authors would like to thank the Chemistry Graduate Program of Federal University of Pará, the Catalyst and Oleo-chemistry Laboratory (LCO) and the Fuel Research and Analysis Laboratory (LAPAC).
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