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Studies on the effect of antioxidants on the long-term storage and oxidation stability of Pongamia pinnata (L.) Pierre biodiesel
Fuel Processing Technology 99 (2012) 56–63
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Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Studies on the effect of antioxidants on the long-term storage and oxidation stability of Pongamia pinnata (L.) Pierre biodiesel Asir Obadiah a, Ramanujam Kannan a, Alagunambi Ramasubbu b, Samuel Vasanth Kumar a,⁎ a b
Department of Chemistry, Karunya University, Coimbatore, Tamilnadu, India Department of Chemistry, Govt. Arts College (Autonomous), Coimbatore, Tamilnadu, India
a r t i c l e
i n f o
Article history: Received 10 February 2011 Received in revised form 15 December 2011 Accepted 8 January 2012 Available online 13 March 2012 Keywords: Pongamia oil Biodiesel Oxidation stability Storage stability Antioxidant
a b s t r a c t This study investigates the impact of various synthetic phenolic antioxidants on the oxidation stability and storage stability of Pongamia (karanja) biodiesel (PBD). The results of Rancimat experiments show that the induction point (IP) increased substantially on adding certain antioxidants to the Pongamia biodiesel. The study reveals pyrogallol (PY) to be the best antioxidant to show the best improvement in the oxidative stability of PBD, the induction time being enhanced to 34.35 h at a PY concentration of 3000 ppm at 110 °C. The storage stability studies were carried out according to the ASTM standard procedures 1) ASTM D4625 at 30 °C/50 weeks and 2) ASTM D4625 at 43 °C/12 weeks by adding different antioxidants like BHT, BHA, PY, GA and TBHQ. The effectiveness of these five antioxidants on PBD was examined at varying loading levels during the storage period. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Biodiesel has attracted attention in recent years as a renewable fuel with less pollutant emissions compared to mineral diesel on its combustion [1,2]. Fatty acid alkyl esters, especially FAME, are the most commonly used liquid biofuel or biodiesel. Biodiesel based on vegetable oils has advantages over petroleum diesel, but it also possesses some drawbacks such as poor oxidative stability and the like [3]. One of the major drawbacks for the quality of biodiesel and its widespread commercialization is its oxidation stability. Unlike petroleum diesel fuel, the nature of biodiesel makes it more susceptible to oxidation or autoxidation during long term storage. Storage conditions like exposure to water and exposure to oxygen, which are naturally present in the ambient air, influence the rate of oxidation. The biodiesel stability generally depends upon the fatty acid composition of the parent oil. Unsaturated fatty acids are significantly more reactive to oxidation than saturated compounds. With respect to longchain FAMEs, polyunsaturated fatty esters are approximately twice as reactive to oxidation as monounsaturated esters. This is attributed to the fact that these unsaturated fatty acid chains contain the most reactive sites, which are particularly susceptible to free-radical attack. While bis-allylic methylenes are much more susceptible to oxidation, biodiesel stability can also depend upon the presence of allylic methylenes in the hydrocarbon chain [4]. ⁎ Corresponding author at. Department of Chemistry, School of Science and Humanities, Karunya University, Coimbatore-641 114, Tamilnadu, India. Tel.: + 91 422 2614480; fax: + 91 422 2615615. E-mail address: [email protected] (S.V. Kumar). 0378-3820/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2012.01.032
The bis-allylic configurations, where the central methylene group is activated by the two double bonds (i.e., \CH_CH\CH2\CH_CH\), react with oxygen via the autoxidation mechanism, with the radical chain reaction steps of initiation, propagation, chain branching, and termination. During these reaction steps, several products can be formed, such as peroxides and hydro peroxides, low molecular weight organic acids, aldehydes and keto compounds, alcohols, as well as high molecular-weight species (dimers, trimers, and cyclic acids) via polymerization mechanisms. The use of antioxidant additives can help slow the degradation process and improve fuel stability up to a point [5–8]. Fuel properties degrade during long-term storage as follows: (i) oxidation or autoxidation from contact with ambient air; (ii) thermal or thermal-oxidative decomposition from excess heat; (iii) hydrolysis from contact with water or moisture in tanks and fuel lines; or (iv) microbial contamination from migration of dust particles or water droplets containing bacteria or fungi into the fuel [9]. Monitoring the effects of autoxidation on biodiesel fuel quality during long-term storage presents a significant concern for biodiesel producers, suppliers, and consumers [10]. Therefore, especially, engine and injection pump producers insisted on the parameter of oxidation stability which was finally fixed at a minimum limit of a 6-hour induction period at 110 °C [11]. The method adopted for determination of the oxidation stability is the so called Rancimat method which is commonly used in the vegetable oil sector. Especially high contents of unsaturated fatty acids, which are very sensitive to oxidative degradation, lead to very low values for the induction period. Thus, even the conditions of fuel storage directly affect the quality of the product. Several studies showed that the quality of biodiesel over a longer
A. Obadiah et al. / Fuel Processing Technology 99 (2012) 56–63
period of storage strongly depends on the tank material as well as on contact to air or light. Increase in viscosities and acid values and decreases in induction periods could be observed [12,13] during such storage. To retard oxidative degradation and to guarantee a specific stability, it becomes necessary to find appropriate additives for biodiesel. Simkovsky et al. studied the effect of different antioxidants on the induction period of rapeseed oil methyl esters at different temperatures but did not find significant improvements [14]. Schober and mittlebach tested the influence of the antioxidant TBHQ on the PV of soybean oil methyl esters during storage and found good improvement of stability [15]. Canakci et al. described the effect of the antioxidants TBHQ and α-tocopherol on fuel properties of methyl soyate and found beneficial effects on retarding oxidative degradation of the sample. [16]. Das et al. described the effect of commercial antioxidants used in kharanja biodiesel for storage stability [17]. Most recently Karavalakis et al. described the effect of synthetic phenolic antioxidants used for storage stability and oxidative stability. The storage stability of different biodiesel blends with automotive diesel treated with various phenolic antioxidants was investigated over a storage time of 10 weeks [18]. During the previous studies, numerous methods for assessing the oxidation status of biodiesel have been investigated, including acid value, density, and kinematic viscosity. The peroxide value may not be suitable because, after an initial increase, it decreases due to secondary oxidation reactions, although the decrease likely affects only samples oxidized beyond what may normally be expected. Thus there is the possibility of the fuel undergoing relatively extensive oxidation but displaying an acceptable peroxide value. The peroxide value is also not included in biodiesel standards. Acid value and kinematic viscosity however are two facile methods for rapid assessment of biodiesel fuel quality as they continuously increase with deteriorating fuel quality [19]. The aim of the present study is to investigate the oxidative stability and storage stability of Pongamia pinnata oil methyl ester. The Rancimat procedure for oxidation stability and the ASTM procedure for storage stability have been used in this study. Using different antioxidants in different concentrations, the fuel properties such as acid value (AN) and kinematic viscosity (KV) of Pongamia Bio-diesel (PBD) were determined at a regular period of time. 2. Materials and methods P. pinnata biodiesel used for the study was purchased from Bannari Institute of Technology, Tamilnadu, India. The antioxidants such as butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), tert-butylhydroxyquinone (TBHQ), Gallic acid (GA) and pyrogallol (PY) were purchased from Sigma-Aldrich. The characteristics of Pongamia biodiesel methyl ester are given in Table 1. 2.1. Storage condition All the biodiesel samples of volume 200 ml were stored in open Borosil glass bottles of 250 ml capacity and kept indoors, at a room temperature of 30 °C and 42 °C. The sample is exposed to air under daylight condition. 50 ml of space in the bottle is occupied by air. That container is fully opened for air contact. Room humidity is 41% to 72%, day time humidity is low but night time humidity is high. Every week the samples were taken for analysis. 2.2. Determination of oxidative stability Oxidative stability (OS) of biodiesel sample was studied with a Rancimat 873 instrument (Metrohm, Switzerland). In the Rancimat procedure the sample was heated at a constant temperature with an excess airflow, which passed through a conductivity cell filled with distilled water. During this oxidation process volatile acids
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Table 1 Properties of PBD (Pongamia biodiesel). Properties
Pongamia biodiesel
Standard method
Iodine value Peroxide value Kinematic viscosity (at 40 °C) Acid value Saponification value Water content Carbon residue value Cloud point Pour point Calorific value Oxidative stability Free glycerol Total glycerol Ester content C16:0 C18:0 C18:1 C18:2 C18:3 C20:0 C22:0
92.75 g iodine/100 g oil EN 14214 5.88(meq/kg) – 5.96 cSt ASTM-D 445
120 max. – 1.9–6 cSt
0.467(mg KOH/g) 174.86 mg KOH/g oil 0.014% 0.049% 4 to63 − 3 to 0 34.778 MJ/kg 3.17 h 0.015% mass 0.21% mass 97.07% 0.86% 13.57% 68.69% 9.81% 1.49% 1.18% 1.47%
0.5 max. – Max. 0.05% Max. 0.05% – – – 6h 0.02% max. 0.24% max. 96.5% – – – – – – –
ASTM-D 664 – ASTM-D 2709 ASTM-D 4530 ASTM-D 2500 ASTM-D 2500 – EN 14214 ASTM-D 6584 ASTM-D 6584 EN 14103 – – – – – – –
Standard value
were formed and conductivity increased at an end and the period up to this point is called “induction period”. The induction period of PBD was determined without antioxidant but with different antioxidants (BHA, BHT, TBHQ, PY and GA) under different concentration of the antioxidants (500, 1000, 2000, and 3000 ppm) at 110 °C. This method of determination was followed by Das et al. [17] Dinkov et al. [20] Karavalakis et al. [21] Knothe [22] and Tang et al. [23] in their earlier works. 2.3. Evaluation of storage stability To evaluate the storage stability, the ASTM procedures D4625 — 50 weeks and D4625 — 12 weeks were carried out. In the ASTM D4625 — 50 week procedure, the KV and AN values were determined at 30 °C over a period of 50 weeks. The measurements were carried out every week. In the ASTM D4625 — 12 method, the KV and AN values were monitored at 43 °C over a period of 12 weeks at regular intervals. The storage container was 250 ml capacity biodiesel exposed in 4:1 ratio i.e. 200 ml of biodiesel and 50 ml free space. Room humidity was 41% to 72%, day time humidity is low but night time humidity is high. 3. Results and discussion The Study on karanja oil methyl ester was carried out by L.M. Das et al. [17] by using three antioxidants namely butylated hydroxy anisole, butylated hydroxy toluene and propyl gallate. They varied the load level of the three antioxidants from 100 to 1000 ppm and noticed decrease in peroxide values as the concentration of antioxidants increased. But peroxide value was not listed as a parameter in the biodiesel fuel specification. Whereas, viscosity and acid value were among the specifications listed within PS121 (Provisional fuel standard guideline for biodiesel) and are known to be affected by the autoxidation of biodiesel. Therefore in our study we monitored the changes in viscosity and acid value to understand the oxidative stability of Pongamia biodiesel. The work of Dunn [15] examines the effects of oxidation under controlled accelerated conditions on fuel properties of methylsoyate. This study employed only TBHQ and α-tocopherol as antioxidants which were found to have beneficial effects on retarding oxidative degradation of methylsoyate biodiesel. George Karavalakis et al. [21] in
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A. Obadiah et al. / Fuel Processing Technology 99 (2012) 56–63 Table 2 Induction time of PBD with different antioxidants at different concentration.
Fig. 1. Chemical structures of various phenolic antioxidants.
their study evaluated the impact of biodiesel concentration in diesel fuel on the stability of the final blend. They have also discussed the effect of sulfur content in the base diesel on the oxidation stability of the blend (Fig. 2). Our study is different from all the other works conducted earlier in that our work confines to the study of oxidative stability and storage stability of P. pinnata biodiesel. The effect of five different antioxidants on the oxidative stability and storage stability of P. pinnata biodiesel have been evaluated using standard rancimat procedure (EN 14214) and recommended ASTM procedures (i) D4625 — 12 weeks and (ii) D4625 — 50 weeks. 3.1. Determination of oxidation stability The Rancimat test is the specified standard method for oxidative stability for biodiesel in accordance with EN 14112 [24]. The absolute difference between two independent single test results did not exceed the repeatability limit of the EN 14112 method. The IP for 100% biodiesel (B100) specified in ASTM D6751-05 is not less than 3 h [25]. The induction period of Pongamia biodiesel without addition of antioxidant is 0.33 h. So different antioxidants at four different concentration levels were used to improve the oxidation stability. Table 2 shows the induction period of Pongamia biodiesel
Fig. 2. Oxidation stability measurements of PBD treated with the various phenolic antioxidants.
Sl. no.
Antioxidants
1 2 3 4 5
BHT BHA TBHQ GA PY
Without antioxidants induction time (h)
Induction time (h) 500 ppm
1000 ppm
2000 ppm
3000 ppm
0.33 0.33 0.33 0.33 0.33
0.80 0.76 0.86 0.67 2.86
1.76 1.70 1.54 0.76 4.99
3.14 2.96 2.91 0.82 25.02
4.88 5.02 6.19 0.88 34.35
using different antioxidants at different concentration. The study reveals Pyrogallol (PY) to be the best antioxidant which showed the best improvement in the OS of Pongamia biodiesel that is 33.45 h at a concentration of 3000 ppm at 110 °C. The study also reveals that with TBHQ the oxidation stability of biodiesel is enhanced to 6.19 h at a concentration of 3000 ppm at 110 °C. All the other antioxidants used make small improvement in the induction period. When investigating phenolic antioxidants, it was found that their antioxidative capabilities bear a relationship to the number of phenol groups occupying 1, 2 or 1, 4 positions in an aromatic ring, as well as to the volume and electronic characteristics of the ring substituent present. Generally, the active hydroxyl group can provide protons that inhibit the formation of free radicals or interrupt the propagation of free radical and thus delay the rate of oxidation. The effectiveness of PY, TBHQ, GA, can be explained based on their molecular structure. These additives possess two OH groups attached to the aromatic ring, while both BHT and BHA possess one OH group attached to the aromatic ring. Thus, based on their electro-negativities, TBHQ and PY offer more sites for the formation of a complex between free radical and antioxidant radical for the stabilization of the ester chain. Another contributing factor for the poor antioxidant performance of both BHT and BHA is their relatively low volatility, which under the operating conditions of the Rancimat method will lead to loss of additives during the early part of the test [18]. The improvement in the induction time using different antioxidants with different concentrations is presented in Table 2. 3.2. Evaluation of storage stability 3.2.1. Kinematic viscosity (KV) During storage, the viscosity of the methyl esters increases by the formation of more polar, oxygen containing molecules and also by the formation of oxidized polymeric compounds that can lead to the formation of gums and sediments that clog filters [6]. In ASTM D4625 — 50 procedure, the KV and AN values were determined at 30 °C over a period of 50 weeks at regular intervals. The kinematic viscosity of Pongamia biodiesel at the initial stage at 30 °C was 5.9 mm 2/s. When the biodiesel was left by itself for duration of 50 weeks in an open to air condition, the oxidation process started and the KV value rose to an enormously high value of 13.3 mm 2/s which is an indication that storage stability of biodiesel is a serious problem. On employing antioxidants to retard the oxidation process during storage it is found in Fig. 3 (ASTM 4625 50 weeks at 30 °C) that the antioxidants definitely improve the storage stability (Fig. 3). All the five antioxidants tested showed that at a concentration of 3000 ppm, they are able to substantially retard the oxidation process during a 50 week storage period at 30 °C and improve the storage stability of the biodiesel. Antioxidants PY, BHA and TBHQ seem to have a better effect on the storage stability of PBD than the other antioxidants over a 50 week period at a concentration of 3000 ppm. In the other accelerated method namely ASTM D4625 at 12 weeks at 43 °C it is found again that in all the five antioxidants a concentration of 3000 ppm
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Fig. 3. Change of kinematic viscosity of PBD with time. Panels a to e indicate the variation of kinematic viscosity with different antioxidants at different concentrations. Panels f to i indicate the variation of kinematic viscosity with different antioxidants at a particular concentration of antioxidant (ASTM 4625 at 30 °C at 50 weeks).
of the antioxidants suppresses the oxidation and improves the storage stability of the biodiesel (Fig. 4). In fact it is found in Fig. 4 that the KV values are maintained as that of the standard value of KV as prescribed by ASTM for biodiesel, when the concentration of the five antioxidants used is 3000 ppm. At a concentration of 3000 ppm PY, TBHQ all improves the storage stability of biodiesel and maintains the KV value around the standard value of 5.9 mm 2/s (Fig. 3). The change of kinematic viscosity of the biodiesel was plotted for every week of storage with a particular antioxidant at four different concentration (Fig. 3 a to e). The change of kinematic viscosity of the biodiesel was also plotted for every week of storage with a particular concentration of all the five antioxidants (Fig. 3 f to i). The graph plotted brings out clearly that at a
concentration of 3000 ppm, each of the five antioxidants (BHT, BHA, GA, TBHQ and PY) improves the storage stability of the biodiesel to a very large extent (Fig. 1 a to e). In the graph plotted (Fig. 1 f to i) it is distinctively seen that at a particular concentration, a particular antioxidant improves the storage stability to a maximum. At a concentration of 500 ppm, 1000 ppm, 2000 ppm and 3000 ppm PY is shown to be the best antioxidant. From Fig. 4 for the ASTM 4625 at 43 °C — 12 week procedure it is seen clearly, that Butylated hydroxy toluene and pyrogallol are the best antioxidant at any concentration between 500 ppm to 3000 ppm (Fig. 4 f to i). The ASTM D4625 at 43 °C 12 – week method, again proves distinctively that the antioxidant concentration of 3000 ppm is the optimum concentration between 500 ppm and 3000 ppm for the
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A. Obadiah et al. / Fuel Processing Technology 99 (2012) 56–63
Fig. 4. Change of kinematic viscosity of PBD with time. Panels a to e indicate the variation of kinematic viscosity with different antioxidants at different concentrations. Panels f to i indicate the variation of kinematic viscosity with different antioxidants at a particular concentration of antioxidant (ASTM 4625 at 43 °C at 12 weeks).
highest improvement in storage stability of the biodiesel sample. Previously Das et al. studied that during storage the viscosity of biodiesel increases due to the formation of oxidized compounds. Viscosity of biodiesel samples slightly increased with increasing storage time. The standard value of viscosity in ASTM limit is 1.9–6.0 mm 2/s at 40 °C and after addition of antioxidants, i.e. PrG, BHT and BHA at loading 1000 ppm the viscosity was found to remain at 5.3 mm 2/s [17]. 3.2.2. Acid value (AN) The acid value (AN) of biodiesel samples also increased with increasing storage time as a result of hydrolysis of fatty acid methyl esters (FAME) to fatty acids (FA). The specification limit of 0.5 mg KOH/g exceeded when the methyl ester (ME) samples were not
exposed to daylight but kept open to air over a storage time of 350 days (50 weeks) [22,24]. The acid value of Pongamia biodiesel initially was 0.44 mg KOH/g but when the biodiesel was stored in an open to air condition and kept for 50 weeks it was found to undergo oxidation and the AN rose up to a very high value of 6 mg KOH/g. From Fig. 5, it is evident that the addition of antioxidants to retard the oxidation is found to be effective with all the five antioxidants when the antioxidant concentration is 3000 ppm. In the other accelerated procedure of ASTM D4625 at 12 weeks/43 °C, it is clearly established that a concentration of 3000 ppm of all the five antioxidants greatly improves the storage stability of the biodiesel sample. The AN is brought down substantially to the standard AN of biodiesel samples. From Fig. 6 it is also evident that the antioxidants PY, BHA, TBHQ and BHT have greater effect on
A. Obadiah et al. / Fuel Processing Technology 99 (2012) 56–63
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Fig. 5. Change of Acid value viscosity of PBD with time. Panels a to e indicate the variation of acid value with different antioxidants at different concentrations. Panels f to i indicate the variation of acid value with different antioxidants at a particular concentration of antioxidant (ASTM — 30 °C at 50 weeks).
the storage stability of Pongamia biodiesel over a storage period of 12 weeks at 43 °C. The change of acid value of the biodiesel was plotted for every week of storage with a particular antioxidant in four different concentration (Fig. 5 a to e). The change of acid value of the biodiesel was also plotted for every week of storage with a particular concentration of all the five antioxidants. The graph plotted brings out clearly that a concentration of 3000 ppm of each of the five antioxidants (BHT,BHA, GA, TBHQ and PY) improves the storage stability of the biodiesel to a very large extent (Fig. 5 a to e). In the graph plotted (Fig. 5 f to i) it is distinctively seen that at a specific concentration, only, a particular antioxidant improves the storage stability to a large extent. At a concentration of 500 ppm, 1000 ppm, 2000 ppm, and 3000 ppm Pyrogallol and TBHQ are shown to be the best antioxidant. In the accelerated test ASTM D4625 at 43 °C — 12 weeks, the graph plotted clearly
proves that pyrogallol is the best antioxidant at any concentration between 500 ppm to 3000 ppm (Fig. 6 f to i). Again it is evident that the antioxidant concentration of 3000 ppm is the optimum concentration between 500 ppm to 3000 ppm for the best improvement in storage stability of the biodiesel sample (Fig. 6 a to e).
4. Conclusion Biodiesel, which consists of monoalkyl esters of long-chain fatty acids made from biolipids, generally suffers from less oxidative stability. In this study, we investigated the oxidative stability and storage stability in an open to air storage condition of Pongamia biodiesel. The oxidative stability of PBD decreased i.e. the kinematic viscosity and acid value increased with increase in storage time
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Fig. 6. Change of acid value of PBD with time. Panels a to e indicate the variation of acid value with different antioxidants at different concentrations. Panels f to i indicate the variation of acid value with different antioxidants at a particular concentration of antioxidant (ASTM 4625 at 43 °C at 12 weeks).
of the biodiesel. The results of this study can be summarized as follows: All the samples were stored under open air and not exposed to day light condition. The effect of Pyrogallol as an antioxidant for PBD is found to be the best as it increased the induction time from0.33 h to 34.35 h, when the concentration of pyrogallol was 3000 ppm. KV and AN values are good indicators of storage stability of biodiesel. They increase on increase of storage time. A concentration of 3000 ppm of antioxidants like BHT, TBHQ, PY, GA, and BHA has a beneficial effect on the storage stability of Pongamia biodiesel. Acknowledgments This research was financially supported by the Department of Science and Technology, New Delhi, India (DST/TSG/AF/2006/22). The authors thank the Management and Administration of Karunya
University for their support and help. The authors are also grateful to Dr. R.B.N. Prasad of the Indian Institute of Chemical Technology for his kind help and support. References [1] Y.C. Sharma, B. Singh, J. Korstad, Application of an efficient nonconventional heterogeneous catalyst for biodiesel synthesis from Pongamia pinnata oil, Energy & Fuels 24 (2010) 3223–3231. [2] Y.C. Sharma, B. Singh, S.N. Upadhyay, Advancements in development and characterization of biodiesel: a review, Fuel 87 (2008) 2355–2373. [3] S.K. Loha, S.M. Chew, Y.M. Choo, Oxidative stability and storage behavior of fatty acid methyl esters derived from used palm oil, JAOCS 83 (11) (2006) 947–952. [4] G. Karavalakis, S. Stournas, Impact of antioxidant additives on the oxidation stability of diesel/biodiesel blends, Energy & Fuels 24 (2010) 3682–3686. [5] S. Schober, M. Mittelbach, The impact of antioxidants on biodiesel oxidation stability, European Journal of Lipid Science and Technology 106 (2004) 382–389. [6] O. Dunn, Effect of antioxidants on the oxidative stability of methyl soyate (biodiesel), Fuel Processing Technology 86 (2005) 1071–1085.
A. Obadiah et al. / Fuel Processing Technology 99 (2012) 56–63 [7] K. Ryu, The characteristics of performance and exhaust emissions of a diesel engine using a biodiesel with antioxidants, Bioresource Technology 101 (2010) 578–582. [8] H. Tang, R.C.D. Guzman, S.O. Salley, K.Y. Simon Ng, The oxidative stability of biodiesel: effects of FAME composition and antioxidant, Lipid Technology 20 (2008) 249–252. [9] A. Bouaid, M. Martinez, J. Aracil, Production of biodiesel from bioethanol and Brassica carinata oil: oxidation stability study, Bioresource Technology 100 (2009) 2234–2239. [10] R.L. McCormick, S.R. Westbrook, Storage stability of biodiesel and biodiesel blends, Energy & Fuels 24 (2010) 690–698. [11] P. Bondioli, A. Gasparoli, L.D. Bella, S. Tagliabue, Evaluation of biodiesel storage stability using reference methods, European Journal of Lipid Science and Technology 104 (2002) 777–784. [12] M. Mittelbach, S. Gangl, Long term stability of biodiesel made from rapeseed and used frying oil, Journal of the American Oil Chemists' Society 78 (2001) 573–577. [13] P. Bondioli, A. Gasparoli, A. Lanzani, E. Fedeli, S. Veronese, M. Sala, Storage stability of biodiesel, Journal of the American Oil Chemists' Society 72 (1995) 699–02. [14] N.M. Simkovsky, A. Ecker, Effect of antioxidants on the oxidative stability of rapeseed oil methyl esters, Erdöl Erdgas Kohle 115 (1999) 317–318. [15] R. Dunn, Effect of oxidation under accelerated conditions on fuel properties of methyl soyate (biodiesel), Journal of the American Oil Chemists' Society 79 (2002) 915–920.
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[16] M. Canakci, A. Monyem, J.V. Gerpen, Accelerated oxidation processes in biodiesel, Transactions of ASAE 42 (1999) 1565–1572. [17] L.M. Das, D.K. Bora, S. Pradhan, M.K. Naik, S.N. Naik, Long-term storage stability of biodiesel produced from Karanja oil, Fuel 88 (2009) 2315–2318. [18] G. Karavalakis, D. Hilari, L. Givalou, D. Karonis, S. Stournas, Storage stability and ageing effect of biodiesel blends treated with different antioxidants, Energy 36 (2011) 369–374. [19] G. Knothe, Analysis of oxidized biodiesel by 1H-NMR and effect of contact area with air, European Journal of Lipid Science and Technology 108 (2006) 493–500. [20] R. Dinkov, G. Hristov, D. Stratiev, V.B. Aldayri, Effect of commercially available antioxidants over biodiesel/diesel blends stability, Fuel 88 (2009) 732–737. [21] G. Karavalakis, S. Stournas, D. Karonis, Evaluation of the oxidation stability of diesel/biodiesel blends, Fuel 89 (2010) 2483–2489. [22] G. Knothe, Some aspects of biodiesel oxidative stability, Fuel Processing Technology 88 (2007) 669–677. [23] H. Tang, R.C.D. Guzman, K.Y. Simon Ng, S.O. Salley, Effect of antioxidants on the storage stability of soybean-oil-based biodiesel, Energy & Fuels 24 (2010) 2028–2033. [24] J. Xin, H. Imahara, S. Saka, Kinetics on the oxidation of biodiesel stabilized with antioxidant, Fuel 88 (2009) 282–286. [25] A. Sarin, R. Arora, N.P. Singh, M. Sharma, R.K. Malhotra, Influence of metal contaminants on oxidation stability of Jatropha biodiesel, Energy 34 (2009) 1271–1275.
Contents lists available at SciVerse ScienceDirect
Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Studies on the effect of antioxidants on the long-term storage and oxidation stability of Pongamia pinnata (L.) Pierre biodiesel Asir Obadiah a, Ramanujam Kannan a, Alagunambi Ramasubbu b, Samuel Vasanth Kumar a,⁎ a b
Department of Chemistry, Karunya University, Coimbatore, Tamilnadu, India Department of Chemistry, Govt. Arts College (Autonomous), Coimbatore, Tamilnadu, India
a r t i c l e
i n f o
Article history: Received 10 February 2011 Received in revised form 15 December 2011 Accepted 8 January 2012 Available online 13 March 2012 Keywords: Pongamia oil Biodiesel Oxidation stability Storage stability Antioxidant
a b s t r a c t This study investigates the impact of various synthetic phenolic antioxidants on the oxidation stability and storage stability of Pongamia (karanja) biodiesel (PBD). The results of Rancimat experiments show that the induction point (IP) increased substantially on adding certain antioxidants to the Pongamia biodiesel. The study reveals pyrogallol (PY) to be the best antioxidant to show the best improvement in the oxidative stability of PBD, the induction time being enhanced to 34.35 h at a PY concentration of 3000 ppm at 110 °C. The storage stability studies were carried out according to the ASTM standard procedures 1) ASTM D4625 at 30 °C/50 weeks and 2) ASTM D4625 at 43 °C/12 weeks by adding different antioxidants like BHT, BHA, PY, GA and TBHQ. The effectiveness of these five antioxidants on PBD was examined at varying loading levels during the storage period. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Biodiesel has attracted attention in recent years as a renewable fuel with less pollutant emissions compared to mineral diesel on its combustion [1,2]. Fatty acid alkyl esters, especially FAME, are the most commonly used liquid biofuel or biodiesel. Biodiesel based on vegetable oils has advantages over petroleum diesel, but it also possesses some drawbacks such as poor oxidative stability and the like [3]. One of the major drawbacks for the quality of biodiesel and its widespread commercialization is its oxidation stability. Unlike petroleum diesel fuel, the nature of biodiesel makes it more susceptible to oxidation or autoxidation during long term storage. Storage conditions like exposure to water and exposure to oxygen, which are naturally present in the ambient air, influence the rate of oxidation. The biodiesel stability generally depends upon the fatty acid composition of the parent oil. Unsaturated fatty acids are significantly more reactive to oxidation than saturated compounds. With respect to longchain FAMEs, polyunsaturated fatty esters are approximately twice as reactive to oxidation as monounsaturated esters. This is attributed to the fact that these unsaturated fatty acid chains contain the most reactive sites, which are particularly susceptible to free-radical attack. While bis-allylic methylenes are much more susceptible to oxidation, biodiesel stability can also depend upon the presence of allylic methylenes in the hydrocarbon chain [4]. ⁎ Corresponding author at. Department of Chemistry, School of Science and Humanities, Karunya University, Coimbatore-641 114, Tamilnadu, India. Tel.: + 91 422 2614480; fax: + 91 422 2615615. E-mail address: [email protected] (S.V. Kumar). 0378-3820/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2012.01.032
The bis-allylic configurations, where the central methylene group is activated by the two double bonds (i.e., \CH_CH\CH2\CH_CH\), react with oxygen via the autoxidation mechanism, with the radical chain reaction steps of initiation, propagation, chain branching, and termination. During these reaction steps, several products can be formed, such as peroxides and hydro peroxides, low molecular weight organic acids, aldehydes and keto compounds, alcohols, as well as high molecular-weight species (dimers, trimers, and cyclic acids) via polymerization mechanisms. The use of antioxidant additives can help slow the degradation process and improve fuel stability up to a point [5–8]. Fuel properties degrade during long-term storage as follows: (i) oxidation or autoxidation from contact with ambient air; (ii) thermal or thermal-oxidative decomposition from excess heat; (iii) hydrolysis from contact with water or moisture in tanks and fuel lines; or (iv) microbial contamination from migration of dust particles or water droplets containing bacteria or fungi into the fuel [9]. Monitoring the effects of autoxidation on biodiesel fuel quality during long-term storage presents a significant concern for biodiesel producers, suppliers, and consumers [10]. Therefore, especially, engine and injection pump producers insisted on the parameter of oxidation stability which was finally fixed at a minimum limit of a 6-hour induction period at 110 °C [11]. The method adopted for determination of the oxidation stability is the so called Rancimat method which is commonly used in the vegetable oil sector. Especially high contents of unsaturated fatty acids, which are very sensitive to oxidative degradation, lead to very low values for the induction period. Thus, even the conditions of fuel storage directly affect the quality of the product. Several studies showed that the quality of biodiesel over a longer
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period of storage strongly depends on the tank material as well as on contact to air or light. Increase in viscosities and acid values and decreases in induction periods could be observed [12,13] during such storage. To retard oxidative degradation and to guarantee a specific stability, it becomes necessary to find appropriate additives for biodiesel. Simkovsky et al. studied the effect of different antioxidants on the induction period of rapeseed oil methyl esters at different temperatures but did not find significant improvements [14]. Schober and mittlebach tested the influence of the antioxidant TBHQ on the PV of soybean oil methyl esters during storage and found good improvement of stability [15]. Canakci et al. described the effect of the antioxidants TBHQ and α-tocopherol on fuel properties of methyl soyate and found beneficial effects on retarding oxidative degradation of the sample. [16]. Das et al. described the effect of commercial antioxidants used in kharanja biodiesel for storage stability [17]. Most recently Karavalakis et al. described the effect of synthetic phenolic antioxidants used for storage stability and oxidative stability. The storage stability of different biodiesel blends with automotive diesel treated with various phenolic antioxidants was investigated over a storage time of 10 weeks [18]. During the previous studies, numerous methods for assessing the oxidation status of biodiesel have been investigated, including acid value, density, and kinematic viscosity. The peroxide value may not be suitable because, after an initial increase, it decreases due to secondary oxidation reactions, although the decrease likely affects only samples oxidized beyond what may normally be expected. Thus there is the possibility of the fuel undergoing relatively extensive oxidation but displaying an acceptable peroxide value. The peroxide value is also not included in biodiesel standards. Acid value and kinematic viscosity however are two facile methods for rapid assessment of biodiesel fuel quality as they continuously increase with deteriorating fuel quality [19]. The aim of the present study is to investigate the oxidative stability and storage stability of Pongamia pinnata oil methyl ester. The Rancimat procedure for oxidation stability and the ASTM procedure for storage stability have been used in this study. Using different antioxidants in different concentrations, the fuel properties such as acid value (AN) and kinematic viscosity (KV) of Pongamia Bio-diesel (PBD) were determined at a regular period of time. 2. Materials and methods P. pinnata biodiesel used for the study was purchased from Bannari Institute of Technology, Tamilnadu, India. The antioxidants such as butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), tert-butylhydroxyquinone (TBHQ), Gallic acid (GA) and pyrogallol (PY) were purchased from Sigma-Aldrich. The characteristics of Pongamia biodiesel methyl ester are given in Table 1. 2.1. Storage condition All the biodiesel samples of volume 200 ml were stored in open Borosil glass bottles of 250 ml capacity and kept indoors, at a room temperature of 30 °C and 42 °C. The sample is exposed to air under daylight condition. 50 ml of space in the bottle is occupied by air. That container is fully opened for air contact. Room humidity is 41% to 72%, day time humidity is low but night time humidity is high. Every week the samples were taken for analysis. 2.2. Determination of oxidative stability Oxidative stability (OS) of biodiesel sample was studied with a Rancimat 873 instrument (Metrohm, Switzerland). In the Rancimat procedure the sample was heated at a constant temperature with an excess airflow, which passed through a conductivity cell filled with distilled water. During this oxidation process volatile acids
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Table 1 Properties of PBD (Pongamia biodiesel). Properties
Pongamia biodiesel
Standard method
Iodine value Peroxide value Kinematic viscosity (at 40 °C) Acid value Saponification value Water content Carbon residue value Cloud point Pour point Calorific value Oxidative stability Free glycerol Total glycerol Ester content C16:0 C18:0 C18:1 C18:2 C18:3 C20:0 C22:0
92.75 g iodine/100 g oil EN 14214 5.88(meq/kg) – 5.96 cSt ASTM-D 445
120 max. – 1.9–6 cSt
0.467(mg KOH/g) 174.86 mg KOH/g oil 0.014% 0.049% 4 to63 − 3 to 0 34.778 MJ/kg 3.17 h 0.015% mass 0.21% mass 97.07% 0.86% 13.57% 68.69% 9.81% 1.49% 1.18% 1.47%
0.5 max. – Max. 0.05% Max. 0.05% – – – 6h 0.02% max. 0.24% max. 96.5% – – – – – – –
ASTM-D 664 – ASTM-D 2709 ASTM-D 4530 ASTM-D 2500 ASTM-D 2500 – EN 14214 ASTM-D 6584 ASTM-D 6584 EN 14103 – – – – – – –
Standard value
were formed and conductivity increased at an end and the period up to this point is called “induction period”. The induction period of PBD was determined without antioxidant but with different antioxidants (BHA, BHT, TBHQ, PY and GA) under different concentration of the antioxidants (500, 1000, 2000, and 3000 ppm) at 110 °C. This method of determination was followed by Das et al. [17] Dinkov et al. [20] Karavalakis et al. [21] Knothe [22] and Tang et al. [23] in their earlier works. 2.3. Evaluation of storage stability To evaluate the storage stability, the ASTM procedures D4625 — 50 weeks and D4625 — 12 weeks were carried out. In the ASTM D4625 — 50 week procedure, the KV and AN values were determined at 30 °C over a period of 50 weeks. The measurements were carried out every week. In the ASTM D4625 — 12 method, the KV and AN values were monitored at 43 °C over a period of 12 weeks at regular intervals. The storage container was 250 ml capacity biodiesel exposed in 4:1 ratio i.e. 200 ml of biodiesel and 50 ml free space. Room humidity was 41% to 72%, day time humidity is low but night time humidity is high. 3. Results and discussion The Study on karanja oil methyl ester was carried out by L.M. Das et al. [17] by using three antioxidants namely butylated hydroxy anisole, butylated hydroxy toluene and propyl gallate. They varied the load level of the three antioxidants from 100 to 1000 ppm and noticed decrease in peroxide values as the concentration of antioxidants increased. But peroxide value was not listed as a parameter in the biodiesel fuel specification. Whereas, viscosity and acid value were among the specifications listed within PS121 (Provisional fuel standard guideline for biodiesel) and are known to be affected by the autoxidation of biodiesel. Therefore in our study we monitored the changes in viscosity and acid value to understand the oxidative stability of Pongamia biodiesel. The work of Dunn [15] examines the effects of oxidation under controlled accelerated conditions on fuel properties of methylsoyate. This study employed only TBHQ and α-tocopherol as antioxidants which were found to have beneficial effects on retarding oxidative degradation of methylsoyate biodiesel. George Karavalakis et al. [21] in
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A. Obadiah et al. / Fuel Processing Technology 99 (2012) 56–63 Table 2 Induction time of PBD with different antioxidants at different concentration.
Fig. 1. Chemical structures of various phenolic antioxidants.
their study evaluated the impact of biodiesel concentration in diesel fuel on the stability of the final blend. They have also discussed the effect of sulfur content in the base diesel on the oxidation stability of the blend (Fig. 2). Our study is different from all the other works conducted earlier in that our work confines to the study of oxidative stability and storage stability of P. pinnata biodiesel. The effect of five different antioxidants on the oxidative stability and storage stability of P. pinnata biodiesel have been evaluated using standard rancimat procedure (EN 14214) and recommended ASTM procedures (i) D4625 — 12 weeks and (ii) D4625 — 50 weeks. 3.1. Determination of oxidation stability The Rancimat test is the specified standard method for oxidative stability for biodiesel in accordance with EN 14112 [24]. The absolute difference between two independent single test results did not exceed the repeatability limit of the EN 14112 method. The IP for 100% biodiesel (B100) specified in ASTM D6751-05 is not less than 3 h [25]. The induction period of Pongamia biodiesel without addition of antioxidant is 0.33 h. So different antioxidants at four different concentration levels were used to improve the oxidation stability. Table 2 shows the induction period of Pongamia biodiesel
Fig. 2. Oxidation stability measurements of PBD treated with the various phenolic antioxidants.
Sl. no.
Antioxidants
1 2 3 4 5
BHT BHA TBHQ GA PY
Without antioxidants induction time (h)
Induction time (h) 500 ppm
1000 ppm
2000 ppm
3000 ppm
0.33 0.33 0.33 0.33 0.33
0.80 0.76 0.86 0.67 2.86
1.76 1.70 1.54 0.76 4.99
3.14 2.96 2.91 0.82 25.02
4.88 5.02 6.19 0.88 34.35
using different antioxidants at different concentration. The study reveals Pyrogallol (PY) to be the best antioxidant which showed the best improvement in the OS of Pongamia biodiesel that is 33.45 h at a concentration of 3000 ppm at 110 °C. The study also reveals that with TBHQ the oxidation stability of biodiesel is enhanced to 6.19 h at a concentration of 3000 ppm at 110 °C. All the other antioxidants used make small improvement in the induction period. When investigating phenolic antioxidants, it was found that their antioxidative capabilities bear a relationship to the number of phenol groups occupying 1, 2 or 1, 4 positions in an aromatic ring, as well as to the volume and electronic characteristics of the ring substituent present. Generally, the active hydroxyl group can provide protons that inhibit the formation of free radicals or interrupt the propagation of free radical and thus delay the rate of oxidation. The effectiveness of PY, TBHQ, GA, can be explained based on their molecular structure. These additives possess two OH groups attached to the aromatic ring, while both BHT and BHA possess one OH group attached to the aromatic ring. Thus, based on their electro-negativities, TBHQ and PY offer more sites for the formation of a complex between free radical and antioxidant radical for the stabilization of the ester chain. Another contributing factor for the poor antioxidant performance of both BHT and BHA is their relatively low volatility, which under the operating conditions of the Rancimat method will lead to loss of additives during the early part of the test [18]. The improvement in the induction time using different antioxidants with different concentrations is presented in Table 2. 3.2. Evaluation of storage stability 3.2.1. Kinematic viscosity (KV) During storage, the viscosity of the methyl esters increases by the formation of more polar, oxygen containing molecules and also by the formation of oxidized polymeric compounds that can lead to the formation of gums and sediments that clog filters [6]. In ASTM D4625 — 50 procedure, the KV and AN values were determined at 30 °C over a period of 50 weeks at regular intervals. The kinematic viscosity of Pongamia biodiesel at the initial stage at 30 °C was 5.9 mm 2/s. When the biodiesel was left by itself for duration of 50 weeks in an open to air condition, the oxidation process started and the KV value rose to an enormously high value of 13.3 mm 2/s which is an indication that storage stability of biodiesel is a serious problem. On employing antioxidants to retard the oxidation process during storage it is found in Fig. 3 (ASTM 4625 50 weeks at 30 °C) that the antioxidants definitely improve the storage stability (Fig. 3). All the five antioxidants tested showed that at a concentration of 3000 ppm, they are able to substantially retard the oxidation process during a 50 week storage period at 30 °C and improve the storage stability of the biodiesel. Antioxidants PY, BHA and TBHQ seem to have a better effect on the storage stability of PBD than the other antioxidants over a 50 week period at a concentration of 3000 ppm. In the other accelerated method namely ASTM D4625 at 12 weeks at 43 °C it is found again that in all the five antioxidants a concentration of 3000 ppm
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Fig. 3. Change of kinematic viscosity of PBD with time. Panels a to e indicate the variation of kinematic viscosity with different antioxidants at different concentrations. Panels f to i indicate the variation of kinematic viscosity with different antioxidants at a particular concentration of antioxidant (ASTM 4625 at 30 °C at 50 weeks).
of the antioxidants suppresses the oxidation and improves the storage stability of the biodiesel (Fig. 4). In fact it is found in Fig. 4 that the KV values are maintained as that of the standard value of KV as prescribed by ASTM for biodiesel, when the concentration of the five antioxidants used is 3000 ppm. At a concentration of 3000 ppm PY, TBHQ all improves the storage stability of biodiesel and maintains the KV value around the standard value of 5.9 mm 2/s (Fig. 3). The change of kinematic viscosity of the biodiesel was plotted for every week of storage with a particular antioxidant at four different concentration (Fig. 3 a to e). The change of kinematic viscosity of the biodiesel was also plotted for every week of storage with a particular concentration of all the five antioxidants (Fig. 3 f to i). The graph plotted brings out clearly that at a
concentration of 3000 ppm, each of the five antioxidants (BHT, BHA, GA, TBHQ and PY) improves the storage stability of the biodiesel to a very large extent (Fig. 1 a to e). In the graph plotted (Fig. 1 f to i) it is distinctively seen that at a particular concentration, a particular antioxidant improves the storage stability to a maximum. At a concentration of 500 ppm, 1000 ppm, 2000 ppm and 3000 ppm PY is shown to be the best antioxidant. From Fig. 4 for the ASTM 4625 at 43 °C — 12 week procedure it is seen clearly, that Butylated hydroxy toluene and pyrogallol are the best antioxidant at any concentration between 500 ppm to 3000 ppm (Fig. 4 f to i). The ASTM D4625 at 43 °C 12 – week method, again proves distinctively that the antioxidant concentration of 3000 ppm is the optimum concentration between 500 ppm and 3000 ppm for the
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Fig. 4. Change of kinematic viscosity of PBD with time. Panels a to e indicate the variation of kinematic viscosity with different antioxidants at different concentrations. Panels f to i indicate the variation of kinematic viscosity with different antioxidants at a particular concentration of antioxidant (ASTM 4625 at 43 °C at 12 weeks).
highest improvement in storage stability of the biodiesel sample. Previously Das et al. studied that during storage the viscosity of biodiesel increases due to the formation of oxidized compounds. Viscosity of biodiesel samples slightly increased with increasing storage time. The standard value of viscosity in ASTM limit is 1.9–6.0 mm 2/s at 40 °C and after addition of antioxidants, i.e. PrG, BHT and BHA at loading 1000 ppm the viscosity was found to remain at 5.3 mm 2/s [17]. 3.2.2. Acid value (AN) The acid value (AN) of biodiesel samples also increased with increasing storage time as a result of hydrolysis of fatty acid methyl esters (FAME) to fatty acids (FA). The specification limit of 0.5 mg KOH/g exceeded when the methyl ester (ME) samples were not
exposed to daylight but kept open to air over a storage time of 350 days (50 weeks) [22,24]. The acid value of Pongamia biodiesel initially was 0.44 mg KOH/g but when the biodiesel was stored in an open to air condition and kept for 50 weeks it was found to undergo oxidation and the AN rose up to a very high value of 6 mg KOH/g. From Fig. 5, it is evident that the addition of antioxidants to retard the oxidation is found to be effective with all the five antioxidants when the antioxidant concentration is 3000 ppm. In the other accelerated procedure of ASTM D4625 at 12 weeks/43 °C, it is clearly established that a concentration of 3000 ppm of all the five antioxidants greatly improves the storage stability of the biodiesel sample. The AN is brought down substantially to the standard AN of biodiesel samples. From Fig. 6 it is also evident that the antioxidants PY, BHA, TBHQ and BHT have greater effect on
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Fig. 5. Change of Acid value viscosity of PBD with time. Panels a to e indicate the variation of acid value with different antioxidants at different concentrations. Panels f to i indicate the variation of acid value with different antioxidants at a particular concentration of antioxidant (ASTM — 30 °C at 50 weeks).
the storage stability of Pongamia biodiesel over a storage period of 12 weeks at 43 °C. The change of acid value of the biodiesel was plotted for every week of storage with a particular antioxidant in four different concentration (Fig. 5 a to e). The change of acid value of the biodiesel was also plotted for every week of storage with a particular concentration of all the five antioxidants. The graph plotted brings out clearly that a concentration of 3000 ppm of each of the five antioxidants (BHT,BHA, GA, TBHQ and PY) improves the storage stability of the biodiesel to a very large extent (Fig. 5 a to e). In the graph plotted (Fig. 5 f to i) it is distinctively seen that at a specific concentration, only, a particular antioxidant improves the storage stability to a large extent. At a concentration of 500 ppm, 1000 ppm, 2000 ppm, and 3000 ppm Pyrogallol and TBHQ are shown to be the best antioxidant. In the accelerated test ASTM D4625 at 43 °C — 12 weeks, the graph plotted clearly
proves that pyrogallol is the best antioxidant at any concentration between 500 ppm to 3000 ppm (Fig. 6 f to i). Again it is evident that the antioxidant concentration of 3000 ppm is the optimum concentration between 500 ppm to 3000 ppm for the best improvement in storage stability of the biodiesel sample (Fig. 6 a to e).
4. Conclusion Biodiesel, which consists of monoalkyl esters of long-chain fatty acids made from biolipids, generally suffers from less oxidative stability. In this study, we investigated the oxidative stability and storage stability in an open to air storage condition of Pongamia biodiesel. The oxidative stability of PBD decreased i.e. the kinematic viscosity and acid value increased with increase in storage time
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Fig. 6. Change of acid value of PBD with time. Panels a to e indicate the variation of acid value with different antioxidants at different concentrations. Panels f to i indicate the variation of acid value with different antioxidants at a particular concentration of antioxidant (ASTM 4625 at 43 °C at 12 weeks).
of the biodiesel. The results of this study can be summarized as follows: All the samples were stored under open air and not exposed to day light condition. The effect of Pyrogallol as an antioxidant for PBD is found to be the best as it increased the induction time from0.33 h to 34.35 h, when the concentration of pyrogallol was 3000 ppm. KV and AN values are good indicators of storage stability of biodiesel. They increase on increase of storage time. A concentration of 3000 ppm of antioxidants like BHT, TBHQ, PY, GA, and BHA has a beneficial effect on the storage stability of Pongamia biodiesel. Acknowledgments This research was financially supported by the Department of Science and Technology, New Delhi, India (DST/TSG/AF/2006/22). The authors thank the Management and Administration of Karunya
University for their support and help. The authors are also grateful to Dr. R.B.N. Prasad of the Indian Institute of Chemical Technology for his kind help and support. References [1] Y.C. Sharma, B. Singh, J. Korstad, Application of an efficient nonconventional heterogeneous catalyst for biodiesel synthesis from Pongamia pinnata oil, Energy & Fuels 24 (2010) 3223–3231. [2] Y.C. Sharma, B. Singh, S.N. Upadhyay, Advancements in development and characterization of biodiesel: a review, Fuel 87 (2008) 2355–2373. [3] S.K. Loha, S.M. Chew, Y.M. Choo, Oxidative stability and storage behavior of fatty acid methyl esters derived from used palm oil, JAOCS 83 (11) (2006) 947–952. [4] G. Karavalakis, S. Stournas, Impact of antioxidant additives on the oxidation stability of diesel/biodiesel blends, Energy & Fuels 24 (2010) 3682–3686. [5] S. Schober, M. Mittelbach, The impact of antioxidants on biodiesel oxidation stability, European Journal of Lipid Science and Technology 106 (2004) 382–389. [6] O. Dunn, Effect of antioxidants on the oxidative stability of methyl soyate (biodiesel), Fuel Processing Technology 86 (2005) 1071–1085.
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