Elsevier

Fuel Processing Technology

Volume 132, April 2015, Pages 133-138
Fuel Processing Technology

Influence of synthetic antioxidants on the oxidation stability of biodiesel produced from acid raw Jatropha curcas oil

https://doi.org/10.1016/j.fuproc.2014.12.003Get rights and content

Highlights

  • Jatropha curcas biodiesel was produced from a raw high free fatty acid oil.

  • Acid esterification allowed the reduction of the free fatty acid content from 18%w/w to 0.8%w/w.

  • Biodiesel produced presented very low oxidation stability (IP = 1.37 h).

  • Effectiveness of antioxidants: PY > PG > BHT > TBHQ.

  • Predictive models were established and using around 200 ppm of PY it is possible to achieve an IP of 8 h.

Abstract

In the present work, Jatropha curcas biodiesel was produced from a high free fatty acid raw oil (AV = 35.36 mg KOH g 1) containing 76.5% w/w of unsaturated fatty acids. The production route consisted of a two-step method, using acid esterification, followed by conventional alkali methanolysis. Biodiesel was characterized in agreement with EN 14214:2014 and a study on the use of 4 synthetic antioxidants was conducted. The high free fatty acid content of the oil could be reduced to 0.8% w/w by acid esterification. A good product quality was generally observed but the very low oxidation stability, corresponding to an induction period (IP) of 1.37 h, was the highest concern. Statistically significant predictive models, which related each antioxidant concentration with the IP, were obtained. Pyrogallol (PY) showed the best results, being estimated that the use of 204 ppm in biodiesel could increase its IP to the limit imposed by the quality standard (8 h). The following rank, in terms of effectiveness, was obtained: PY > propyl gallate (PG) > butylated hydroxytoluene (BHT) > tert-butyl hydroquinone (TBHQ). In agreement, the stabilization factors (F), considering the use of 204 ppm of antioxidant, were: 5.84 for PY, 4.06 for PG, 1.85 for BHT and 0.85 for TBHQ.

Introduction

Biodiesel production has increased significantly in the last years; in fact, in 2011, biodiesel production in the world reached around 404 000 barrels per day, almost 12 times higher than in 2003, showing that the market accepts biodiesel as a viable substitute of fossil diesel [1].

About 95% of the biodiesel production plants use food vegetable oils as raw material [2]. When considering alternative non-food crops (second generation crops), jatropha (Jatropha curcas L.) is one of the most promising, because it grows very easily in adverse soil conditions where food plants have difficulties to grow and presents one of the highest oil yields compared to other non-edible oil plants (1590 kg oil/ha, 35–40 wt.% of the seed) [2], [3]. Most of jatropha is cultivated in Asia, Africa and Central and South America [4], [5], [6], [7], [8].

The most used process for biodiesel production, due to the higher simplicity and lower cost, is the transesterification reaction between the oil and an alcohol (usually methanol), in the presence of an alkali catalyst, to produce biodiesel and the by-product glycerol [9]. For an effective alkali transesterification reaction, a low amount of free fatty acids (FFA) on the feedstock is required (usually less than 1 wt.%) [10]; this means that if a high FFA feedstock is available, it needs to be pretreated before proceeding to the transesterification process. In order to reduce the FFA content of the oil, the acid esterification of the FFA with methanol is the most used pretreatment process because it allows the production of methyl esters from the acids present [11], [12], [13], taking advantage of all the feedstock towards biodiesel production (Eq. (1)).R1COOH+CH3OHacidR1COOCH3+H2O

Oxidation of biodiesel is a major concern, occurring mostly due to air exposure and being highly promoted by the presence of unsaturated fatty acids, since the double bonds offer a high level of reactivity with oxygen. For instance, methyl or ethyl linoleate (C18:2) reacts close to 40 times faster than oleate (C18:1) [14]. The work performed on the chemistry of oxidation reports mostly the primary and secondary oxidation [15]. The primary oxidation is a free-radical chain reaction that might be represented as shown in Fig. 1 [14], [15]. The initiator (I) is mostly likely a free radical that results from the decomposition of hydroperoxides present [14]. On the secondary oxidation, hydroperoxides (which are reactive molecules), which result from primary oxidation, decompose readily to form a number of stable products such as aldehydes, ketones and hydroxyl fatty acids, the last being responsible for the increased acidity of the product [14]. An increase of the viscosity generally indicates the presence of higher molecular weight materials formed by oxidative polymerization [15].

In fact, when oxygen is present, oxidation cannot be completely prevented or reversed; in agreement, the methods used to overcome this problem work on the inhibition of the reactions, therefore delaying or significantly slowing down the accumulation of oxidized products [14]. Such inhibitors are known as antioxidants, being generally chain breakers (free radical terminators) or hydroperoxide decomposers [15]. Chain breakers are the most used and they work by removing the reactive radicals produced during the initiation and propagation steps of the primary oxidation. The two most common are phenolic and amine-type of antioxidants; however, in what concerns biodiesel applications, mostly phenolic antioxidants are used [15]. Antioxidants used to control lipid oxidation can be natural or synthetic and there are several natural and synthetic phenols that might compete, even under low concentrations, with the triacylglycerol molecule as hydrogen donor. Consequently, stabilized radicals are produced which are not able to initiate or propagate the oxidation reactions, therefore increasing the oxidation stability of the product [14], [15].

Different parameters might be used to access the oxidation stability of biodiesel, namely: Iodine Value, Anisidine Value, Peroxide Value, Oxidation Stability Index and Induction Period (IP). The European Standard on biodiesel quality, EN 14214:2014, adopted the accelerated oxidation test (EN 14112:2003) for the determination of the oxidation stability in terms of the IP (“time which passes between the moment when the measurement is started and the moment when the formation of oxidation products rapidly begins to increase”). A minimum IP of 8 h is required according to this standard to ensure biodiesel quality. Most biodiesel, being produced from oils with significant amounts of unsaturated fatty acids (e.g. soybean, rapeseed and sunflower oil) cannot fulfill the requirements; palm oil is an exception since it is not as rich in unsaturated fatty acids [15]. The use of different raw materials (with variable fatty acid composition), the application of an ethylic or methylic route for the transesterification (that can lead to products with different properties, namely the acid value, viscosity and water content) and the adoption of different purification processes (e.g. water washing (more common), membranes, resins, distillation) will influence the oxidation stability of the biodiesel product [14], [16]. The oxidation stability might also be improved by the use of raw material blends (for instances blending jatropha oil with palm oil) [17] and blends with fossil fuel (biodiesel + petrodiesel) [18], [19].

Among the most used synthetic antioxidants to improve biodiesel oxidation stability are: pyrogallol (PY), propyl gallate (PG), tert-butyl hydroquinone (TBHQ), butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT) [15]; the most studied natural antioxidants are the natural phenolic compounds—tocopherols (α, δ and γ tocopherol), that can be obtained from the refining of vegetable oils; carnosic acid, obtained for instances from rosemary; and, sesamol, present in sesame oil [14], [15].

In a review by Jain and Sharma [15], the effectiveness of different antioxidants towards the improvement of oxidation stability of various types of biodiesel (e.g. from rapeseed, sunflower, soybean, palm, tallow, and frying oils) is reported, using concentrations ranging from 200 to 7000 ppm. In general, the synthetic PY and TBHQ presented the best results and synthetic antioxidants always performed better than the natural ones (α, δ and γ tocopherol). The mentioned studies included the use of: (i) edible and non-edible oils and fats to produce biodiesel with different properties through conventional alkali transesterification [20]; (ii) commercial biodiesel originated from refined oils and waste oils also produced by alkali transesterification [21], [22], [23]; and, (iii) synthetic methyl esters produced by blending pure methyl esters in the same proportions as presented in natural esters [22]. The variability of the results reveals the need to conduct dedicated studies when considering the use of antioxidants for biodiesel obtained from different raw materials, also considering different antioxidant concentrations.

From the literature review, there is clearly a lack of studies on the oxidation stability of biodiesel derived from non-edible oils such as jatropha. In the study by Sarin et al. [24], the oxidation stability of biodiesel obtained from low free fatty acid J. curcas oil was found to be 3.95 and a minimum of 200 ppm of BHT was required to achieve an IP of 6 h (previous limit imposed by EN 14214) [24]. Jain and Sharma [19] evaluated the oxidation stability of jatropha biodiesel by mixing it with diesel and/or by using synthetic antioxidants and found that around 100 ppm of PY was the optimum amount of antioxidant for the pure biodiesel (initial IP = 3.27 h, considering a final IP of 6 h) whereas 50 ppm would be required for a B30 blend with diesel (30 wt.% biodiesel). No studies were found on the evaluation of biodiesel production from acid raw J. curcas oil as well as on the oxidation of the derived biodiesel.

In agreement with what was previously stated, the objective of the present work was to evaluate the influence of the most effective and used synthetic antioxidants, namely PY, PG, TBHQ and BHT, on the oxidation of biodiesel obtained from acid raw J. curcas L. oil. For that purpose, biodiesel was synthetized directly from raw oil using a two-step process (acid esterification followed by alkali transesterification), purified, characterized according to EN14214 and after the oxidation stability studies were conducted considering the use of four synthetic antioxidants at different concentrations (from 100 to 2500 ppm).

Section snippets

Materials

Raw J. curcas oil was purchased from PT Pura Green Energy, Indonesia. Methanol was supplied by VWR (brand AnalaR NORMAPUR), sulfuric acid 97% was supplied by VWR (Merck, EMSURE® ISO) and sodium hydroxide powder 97% was supplied by Sigma Aldrich (reagent grade). Sodium sulfate anhydrous pro analysis was supplied by Merck KGaA. Regarding the synthetic antioxidants, the commercial Baynox Plus®, which has as active ingredient butylated hydroxytoluene (BHT), was from LANXESS, propyl gallate was from

Oil characterization

The initial acid value of J. curcas oil was 35.36 mg KOH/g sample (around 18% w/w of free fatty acids), iodine value was 99.3 g I2/100 g and the water content was 0.252% w/w. Taking into account the very high acid value, a pre-treatment was required in order to enable biodiesel production through alkali methanolysis. The iodine value relates to oil composition and indicates the fulfillment of the biodiesel standard that imposes a value < 120 g I2/100 g. Since acid esterification was performed as

Conclusions

Biodiesel was produced from high free fatty acid raw J. curcas oil (around 18% w/w) using acid esterification followed by conventional alkali transesterification. The product showed generally a good quality and the very low oxidation stability (Induction Period of 1.37 h) was the highest concern, being very far from the standard limit of 8 h, imposed by EN 14214:2014.

The study on the use of 4 synthetic antioxidants allowed obtaining statistically significant predictive models, which considered a

Acknowledgments

Supriyono thanks the Erasmus Mundus Action 2 (Lotus Project) scholarship. J. M. Dias thanks the FCT for the fellowship SFRH/BPD/73809/2010.

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