1Kinetics on the Oxidation of Biodi

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Kinetics on the Oxidation of Biodiesel Stabilized with Antioxidant
Jiayu Xin, Hiroaki Imahara, Shiro Saka
Graduate School of Energy Science, Kyoto University, Kyoto, Japan
Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japan
Corresponding author, Tel: +81-75-753-4738; Fax: +81-75-753-4738. E-mail address:
saka@energy.kyoto-u.ac.jp (S. Saka).
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Abstract
Oxidation stability of safflower biodiesel stabilized with propyl gallate whose concentration
spreads from 0-5,000ppm was studied by Rancimat method at temperatures from 100-120
o
C. It
was consequently demonstrated that the induction period of biodiesel increases with the increase
of antioxidant concentration and decreases with increase of temperature. Kinetics on its oxidation
was described by the first order rate law with an accuracy higher than 0.98. The reaction rate of
propyl gallate consumed in safflower biodiesel obtained from the experiment fits well with
Arrhenius equation and the activation energy obtained from Arrhenius equation was 97.02kJ/mol.
Logarithm of induction periods determined by Rancimat method with various antioxidant
concentrations shows a linear relation with temperatures. It was, consequently, found that the
Rancimat method for the oxidation stability determination shows an approximate correlation
between storage stability and Rancimat induction period. The Rancimat method cannot directly
measure the overall storage stability of fuels, since other conditions such as presence of water,
microbial contamination, storages conditions etc. would affect fuel quality during storage.
Keywords: kinetics; biodiesel; oxidation stability; antioxidant; propyl gallate
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1. Introduction
Biodiesel consists of long-chain fatty acid methyl esters (FAME) obtained from vegetable
oils such as rapeseed oil, palm oil, soybean oil, sunflower oil, peanut oil, as well as animal fats
and used cooking oils [1-3], which are renewable and their utilization is carbon-neutral and low
exhaust emission. With the increase of environment protection consciousness and decrease of
petroleum reserves, more and more biodiesel is being used in many countries such as Germany,
France, Italy, USA and so on. It was, therefore, reported that production capacity of biodiesel in
EU in 2006 was 6,069,000 tones, and this value will reach 10,000,000 tones by 2010 [4].
However, biodiesel has lower oxidation stability compared with petroleum diesel because
biodiesel has high content of unsaturated methyl esters, especially poly-unsaturated methyl esters
easily oxidized such as methyl linoleate (C18:2) and methyl linolenate (C18:3), which lead to the
formation of decomposed compounds such as acids, aldehydes, esters, ketones, peroxides and
alcohols. These products not only affect the properties of biodiesel, but also bring the problems of
engine operation [5]. As a result, the European Committee for Standardization established a
standard (EN 14214) for biodiesel in 2003, which requires that biodiesel must reach a minimum
induction period of 6 h as tested by Rancimat method at 110
o
C.
Many researchers have paid attentions on the factors that affect the oxidation stability of
biodiesel [6-15]. It was, then, found that, kinematic viscosity, acid value, ester content and
peroxide value of biodiesel deteriorate along with oxidation time, at the end of Rancimat induction
period, and that all of these parameters could not meet specification of the FAME or oils and fats.
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At the same time, the other research theme, prolonging the induction period of biodiesel by adding
the antioxidant seems to be promising, which ensures high biodiesel oxidation stability. The effect
of synthetic antioxidants such as pyrogallol, 3,4,5-trihydroxybenzoic acid propyl ester (propyl
gallate), tert-butyl hydroquinone, tert-butyl hydroxyanisole, tert-butyl methyl-phenol and so on as
well as natural antioxidants (tocopherols) has been studied on enhancing oxidation stability of
biodiesel [16-19]. The results show that easily oxidized biodiesel can reach the oxidation stability
specification by adding the antioxidant. The stability of biodiesel enhanced by adding the
antioxidant has been widely studied, but on the kinetics of oxidation has not been reported yet.
Therefore, the purpose of this study was to establish an oxidation reaction law of the
antioxidant preventing oxidation of biodiesel. Rancimat method test which can accelerate
oxidation was carried out for safflower biodiesel stabilized with propyl gallate at various
temperatures. Furthermore, the Rancimat method for long term storage stability of biodiesel
determination was also evaluated.
2. Materials and Methods
2.1. Materials
To obtain induction period readily and study the effect of antioxidant on stabilizing biodiesel
in a wide range, susceptibly oxidized biodiesel that has poor oxidation stability and high
unsaturated methyl ester content is a good candidate. Thus the safflower biodiesel was used in this
study, which was prepared by alkali-catalyzed method [20] from safflower oil (Nacalai Tesque;
Kyoto, Japan). Key properties of safflower biodiesel and European specification standard are
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shown in Table 1. Palm and rapeseed biodiesel were also prepared by alkali-catalyzed method
from palm oil and rapeseed oil (Nacalai Tesquel; Kyoto, Japan).
In our previous experiment, one of the most widely used antioxidant, butylated
hydroxytoluene (BHT) was found to have high vapor pressure, thus a part of BHT vaporized at
high temperature. To avoid a change of the antioxidant concentration due to vaporization, propyl
gallate was used as antioxidant. As a chain-breaking radical scavenging antioxidant, propyl gallate
is widely used in food, petroleum chemistry and polymer [21]. Propyl gallate was purchased from
Sigma, Japan.
Standard free fatty acid methyl esters such as methyl palmitate (C16:0), methyl stearate (C18:0),
methyl oleate (C18:1), methyl linoleate (C18:2) and methyl linolenate (C18:3) were purchased from
Nacalai Tesque Inc., Japan as standards whit their purities being higher than 99%. α-tocopherol,
β-tocopherol and γ-tocopherol were bought from Supelco. Inc., Bellefonte, Pennsylvania, USA.
2.2. Methods
Oxidation stability of safflower biodiesel samples with different antioxidant blends was
studied according to EN 14112 [22] in Rancimat equipment model 743 (Metrohm, Herisau,
Switzerland), which was operated under the following conditions: air flow rate, 10L/h, 3g
biodiesel sample was placed at a heating block with temperature set from 100-120
o
C, the vapors
discharged to a flask containing 0.06L distilled water and the conductivity change was recorded by
a computer simultaneously. The induction periods of biodiesel samples with antioxidant
concentrations from 0-5,000ppm were determined.
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The chemical composition of safflower biodiesel was determined by a high-performance
liquid chromatography (HPLC) (Shimadzu, LC-10AT) which consists of the column (Cadenza
CD-C18, 25cm in length ×4.6mm in inner diameter, Imtakt Co.) and refractive index detector
(Shimadzu, RID-10A) operated at 40
o
C with 1.0mL/min flow rate of methanol as a carrier solvent,
and the peak identification was made by comparing the retention time between the sample and the
standard compound. Tocopherol content was also determined by the HPLC with the column
(Asahipak ODP-50 6D, 15cm in inner length ×6mm diameter, Shodex Co.) and fluorescence
detection (excitation 298nm, emission 325nm, RF-10AXL, Shimadzu) operated at 30
o
C with
1.0mL/min flow rate of methanol as a carrier solvent, and the peak identification was made by
comparing the retention time between the sample and the standard compound.
3. Results and Discussion
3.1. Composition and natural antioxidant content of safflower biodiesel
Safflower biodiesel contains 89.1% unsaturated methyl esters (C18:1, C18:2, C18:3). As we know
that the oxidation stability of unsaturated methyl esters decreases according to the order of C
18:3,
C18:2, C18:1, and its relative rates reported in the literature [23] are 1 for C18:1, 41 for C18:2 and 98
for C18:3. However, safflower biodiesel contains 104ppm tocopherol (78ppm α-tocopherol, 3ppm
β-tocopherol and 23ppm γ-tocopherol) as a natural antioxidant. Therefore, it affects oxidation
stability of the biodiesel, because of lower natural antioxidant content and high unsaturated ester
composition, the induction period of safflower biodiesel is only 0.86h, which is far below the
specification of EN14112 to satisfy the requirements. Therefore, additional antioxidant to the
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biodiesel is inevitable.
3.2. Effect of temperature and antioxidant concentration on oxidation stability
Figure 1 shows the influence of antioxidant concentration on the oxidation stability of
safflower biodiesel for the test temperatures as set at 100, 105, 110, 115 and 120
o
C. We can see
clearly that the higher the temperature, the shorter the induction period. In addition, the higher the
propyl gallate concentration, the longer the induction periods. The effect of propyl gallate
concentration on the induction period is more evident when the antioxidant concentration is less
than 1,000ppm. Changes of induction period with the concentration of propyl gallate can be
illustrated by the slop of the curve. With the increase of propyl gallate concentration, the slope
becomes smaller.
3.3. Kinetics on oxidation of biodiesel as stabilized with propyl gallate
Oxidation reaction of organic compound is a chain reaction process and rather complicated,
consisting of numerous elementary steps [24]. The process is an autoxidation process if chain
propagation reaction is faster than chain termination; if the antioxidant is active and its
concentration is high enough, chain propagation will be broken through the reaction of transfer
hydrogen atom from antioxidant to intermediate peroxyl radicals [25].
The curves in Fig. 1 appear to be