Influence of the synthetic antioxidants, BHT and TBHQ, on soybean biodiesel corrosiveness and degradation
Isabella Pacifico Aquino1, Beatriz Peragine Celiberto1, Taís Pereira Alves1, Idalina Vieira Aoki2, Ricardo Belchior Torres1
1 - Chemical Engineering Department – Centro Universitário da FEI – São Bernardo do Campo – Brazil
2 - Chemical Engineering Department – Escola Politécnica da USP – São Paulo - Brazil
Biodiesel is a renewable, biodegradable and nontoxic alternative fuel produce from biomass [1,2]. In Brazil, the characterization of the Brazilian biodiesel quality must be made according to the specifications established by the National Agency of Petroleum, Natural Gas and Biofuels (ANP) that were published in the resolution number 7/2008 [3,4]. The biodiesel is quite similar to conventional diesel fuel in its main physical characteristics and for this reason it is considered an excellent alternative to fossil fuel used in diesel cycle engines. Just like fossil fuel, biodiesel are used in compression ignition engines . However, this biofuel differs in composition from petroleum diesel, whereas it has normally higher olefin content (poly-unsaturated) compared to diesel . Because of its chemical structure, biodiesel is more susceptible to oxidative and thermal degradation . Different vegetable oils are widely used for biodiesel production. For this reason, the oxidative stability of biodiesel is dependent on the amount of unsaturated fatty acids present in the feedstock. So, the biodiesel obtained from soybean oil is more sensitive to degradation than palm biodiesel as a result of its chemical composition that contains a higher content of unsaturated fatty acids. Biodiesel instability becomes a serious issue regarding its quality during long-term storage . Oxidation reactions are accelerated by oxygen, water, light, heat, and metallic ions . The biodiesel degradation leads to the formation of a variety of oxidation products such as peroxides and hydroperoxides. These compounds during the oxidations reactions are converted into aldehydes, ketones and acids that are volatile products of short chains , consequently, the oxidation mechanisms of biodiesel can increase its corrosive characteristics, and significantly change the physical and chemical properties as viscosity, density, polymer content and total acid number . In the engine, the fuel comes into contact with a wide variety of parts, including fuel pump, gaskets, fuel injectors, fuel filters, bearings, pistons, piston rings etc. Among them, the copper alloys present in some parts are the most attacked by biodiesel compared to ferrous and aluminum alloys . Therefore, biodiesel has been found to be more corrosive to automotive materials presents in the fuel circuit and more hygroscopic in nature than diesel [11,12], unless it is modified or treated with natural and synthetic antioxidants that inhibit oxidation process and improve the quality of biodiesel .
The biodiesel stability in the presence of antioxidants has been studied by many researchers. Synthetic antioxidants such as BHT (butyl-hydroxytoluene), BHA (butyl-hydroxyanisol) and TBHQ (tert-butyl-hydroquinone) have been evaluated as potential antioxidants in soybean oil biodiesel . Among the natural antioxidants, the vitamin E, ascorbic acid, caffeic acid, carotenoids and especially phenolic compounds were used. Sarin et al  reported that two types of antioxidants are known: chain breakers and hydroperoxide decomposers. The specification established by ANP for oxidation stability by the Rancimat method at 110 ºC in conformity with EN 14112 standard requires induction period of at least 6 h for the biodiesel. However, the ASTM D6751-09 standard that prescribes the required parameters for characterization of the American biodiesel quality reduced the induction period from 6 h to 3 h. It is worth highlighting that the limit of 6 h established by ANP Resolution nº 7/2008 still demands validation for production process via ethylic route.
The aim of this work is to evaluate the influence of the synthetic antioxidants, BHT e TBHQ, in the corrosion rate of copper and steel carbon immersed in soybean biodiesel. The oxidation reactions occasioned by metallic ions in the presence of antioxidants were also evaluated. All the experiments were conducted in pure biodiesel (B100) obtained from the transesterification reaction of refined soybean oil with ethanol in the presence of an alkaline catalyst. Antioxidants were added at 1000 ppm concentration. The study was made by the association of physico-chemical characterization and immersion tests. The biodiesel composition was analyzed by gas chromatography coupled to a mass spectrometry (GC-MS). The characterization of biodiesel quality and degradation was carried out according to the specifications required by the ANP together with infrared spectroscopy. Fuels were analyzed by total acid number, content water, density, viscosity and oxidation stability at 110 ºC. The characterization of corrosion behaviour was performed by weight loss measurements according to ASTM G1standard. Immersions tests were performed in the presence and absence of the antioxidants at room temperature without natural light incidence.Test samples carbon steel and cooper of 20 mm x 20 mm dimensions were used. The procedure performed in the weight loss measurements (ASTM G1) was the same reported by Aquino et al. . The whole exposure time was 120 h.
The average percent yield of ethyl esters obtained by transesterification reaction under the experimental conditions described was 99.7 % for soybean biodiesel. The results of physicochemical characterization for soybean biodiesel without additive showed that all analytical parameters are within specifications established by the ANP, except the induction period (3.5h). The low oxidation stability of soybean biodiesel is caused by the predominance of unsaturated fatty acids (alkyl chain) in its composition, since the oxidation and degradation reactions are favoured by high levels of unsaturated fatty acids . The induction period of soybean biodiesel increased from 3.5 h to 21 h and 19 h with addition of TBHQ and BHT (1000 ppm), respectively. However, the water content in the biodiesel increased after the addition of synthetic antioxidants. It was observed that the water content exceeds 700 ppm in the presence of BTH and TBHQ compared to 300 ppm for biodiesel without antioxidants. According to ANP, the water content limit for biodiesel is 500 ppm. This fact shows that biodiesel is out of specification regarding the water content in the presence of BHT and TBHQ at 1000 ppm concentration. Thus, the use of these antioxidants into biodiesel favours oxidative stability, but negatively affects water content.
Weight loss measurements according to ASTM G1 standard for copper and carbon steel samples in soybean biodiesel without antioxidants showed thickness loss of 5 µm/year and 0.4 µm/year, respectively, for 120 h of immersion. Antioxidants added into biodiesel caused different effects in the corrosion rate expressed as thickness loss. There is evidence that TBHQ retarded the corrosion process, while BHT accelerated the oxidation reactions of the metals studied. The kinetic study showed thickness loss of
4 µm/year and 7 µm/year for copper when immersed in biodiesel with TBHQ and BHT, respectively. Copper is less corrosion resistant than carbon steel. According to Almeida et al. , copper ions release is less intense in the biodiesel from recycled oil doped with TBQH. Based on this, TBHQ molecules can acts as a corrosion inhibitor by the adsorption process on the surface, leading to formation of a protective film layer . However, the oxidation stability of biodiesel at 100 ºC, in presence and absence of antioxidants, was significantly affected after copper corrosion, even for a short time of immersion (120 h). The IP of biodiesel decreases to approximately 0.05 h, regardless of the antioxidants employed to ensure higher oxidation stability. This indicates that the antioxidants do not impede biodiesel degradation when in contact with metallic ions. The water content, viscosity and density also exceeded the limit required by the ANP after contact with metallic ions, especially for copper. Therefore, the Cu2+ ions have a more strong catalytic effect on the degradation reactions compared to Fe2+ ions.
Keywords: Soybean, biodiesel, corrosion, antioxidant.
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