Fuel xxx (2012) xxx–xxx
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Estimation of the oxidation temperature of biodiesels from a limited number of chemical parameters Anderson dos Reis Albuquerque a,⇑, Jefferson Maul a, Jozemar Pereira dos Santos b, Iêda Maria Garcia dos Santos a, Antonio Gouveia de Souza a a b
LACOM, Departamento de Química, CCEN, Universidade Federal da Paraíba, Campus I, João Pessoa, PB, CEP 58059-900, Brazil Departamento de Estatística, CCEN, Universidade Federal da Paraíba, Campus I, João Pessoa, PB, CEP 58051-900, Brazil
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
" Experimental mixture design with
fatty acid methyl esters was performed to study oxidative stability. " The oxidation temperature (OT) was obtained applying non-isothermal PDSC. " A pseudo-molecular formula Ca Hb Hc H d He O2 ðOHÞf to represent the mixtures of FAME was developed. " A quadratic relationship for OT with H , H and (OH) was found.
a r t i c l e
i n f o
Article history: Received 14 September 2011 Received in revised form 21 July 2012 Accepted 25 July 2012 Available online 11 August 2012 Keywords: Biodiesel Mixture design Oxidative stability PDSC
a b s t r a c t Biodiesel is a fuel composed by fatty acid esters, usually methyl esters (FAME), have common structural features, as allylic hydrogens, bis-allylic hydrogens and secondary hydroxyl, that determinate its oxidative stability. In this study, the oxidation temperature (OT) of biodiesels formulated from a mixture design of methyl stearate, methyl oleate, methyl linoleate and methyl ricinoleate was determined by pressurized differential scanning calorimetry (PDSC). The generic representation Ca Hb Hc H d He O2 ðOHÞf for the mixtures was developed and their coefficients were used as parameters to describe the OT. A non-linear dependence of OT with the descriptors of allylic hydrogen Hc , bis-allylic hydrogen H d and secondary hydroxyls ðOHÞf were observed in the empirical domain Dexp = {(a; c, d, f) 2 R3 ; 0 6 c 6 4 and 0 6 d 6 2 and 0 6 f 6 1 and a = 19}, where the Ca parameter for chain length was kept fixed. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Biodiesel is a fuel consisting usually of a mixture of fatty acid methyl esters (FAME) from natural sources, such as animal fats, vegetable oils, frying oils and microalgae [1]. The fatty chains of these raw-materials vary significantly in relation to the amount of unsaturations, allylic hydrogens, bis-allylic hydrogens and hydroxyls, leading to different physicochemical properties among them [2–8], as oxidative stability. ⇑ Corresponding author. Tel./fax: +55 83 3216 7441. E-mail address:
[email protected] (A. dos Reis Albuquerque).
The oxidative stability is defined as the resistance to initiate autoxidation, photooxidation or thermoxidation. One form to observe the oxidative stability is evaluate the oxidation temperature (OT) from accelerated method, as thermal analysis. The knowledge of a function that predict the OT value is fundamental for quality control of biodiesel and to reduce the necessity of extensive analyses from each new proposed biodiesel composition developed by blending or by new raw-materials. Several studies have determined the oxidation susceptibility of fatty compounds as a function of their majority composition [7–10]. Classical works with oils, reported oxidation indices as a function of fatty chain composition. Cosgrove et al. [7] proposed
0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2012.07.056
Please cite this article in press as: dos Reis Albuquerque A et al. Estimation of the oxidation temperature of biodiesels from a limited number of chemical parameters. Fuel (2012), http://dx.doi.org/10.1016/j.fuel.2012.07.056
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A. dos Reis Albuquerque et al. / Fuel xxx (2012) xxx–xxx
a formula [0.02 (%oleic) + (%linoleic) + 2 (%linolenic)]/100 for soybean oil oxidation index. Fatemi and Hammond [9] determined a relationship between hydroperoxides formed at room temperature in purified olive and soybean oils as [(% oleic) + 10.3 (% linoleic) + 21.6 (% linolenic)]/100. In those formulas the representative fatty chains for the systems take into account the unsaturation degree. However, the hydroxylated chains have been considered an excellent alternative for blending with unstable biodiesels [11]. Thus, this component cannot be neglected in a general formula for oxidative stability prediction. A quantitative relation between 1H NMR signals and the OT, start temperature, oxidation active energy and rate constant of oxidation by differential scanning calorimetry (DSC) was determined by Adhvaryu et al. [6]. The advantage of this methodology is that no determination of chemical composition is needed, since the fatty chains oxidative stability are dominated by few and common molecular structures, such as allylic and bis-allylic hydrogens. Moser [12] determined the OT by PDSC at 1398.75 kPa, dry air and heating rate of 10 °C min1 for 24 fatty acids methyl and ethyl esters and observed structural trends that elucidated oxidative stability as the fatty acid chain length, the number of double bonds, stereochemistry and unsaturation position, and hydroxyl groups. In this study, the oxidation temperature (OT) of biodiesels formulated from a mixture design of methyl stearate (C18:0), methyl oleate (C18:1), methyl linoleate (C18:2) and methyl ricinoleate (C18:1, 12-OH) was obtained by pressurized differential scanning calorimetry (PDSC) and a generic representation Ca Hb Hc H d He O2 ðOHÞf for the mixtures was developed to describe a quantitative approach. The mixture design was evaluated using statistical procedures to observe the dependence of the OT with the descriptors of the generic representation.
2. Materials and methods 2.1. Mixture design The FAME stearate (S), oleate (O), linoleate (L) and ricinoleate (R) – all with purity >99.8%, Sigma–Aldrich – were used without further purification. The compounds were stored in amber glass container at 15 °C until utilization. The samples were divided into five groups labeled A–E to simplify the visualization of the matrix design. Group A contains the four unmixed FAME; group B 12 ternary mixtures at a ratio of 1:1:4 (mol/mol/mol); group C six binary mixtures 1:1 (mol/mol); group D four ternary mixtures 1:1:1 (mol/mol/mol); and group E contains a quaternary mixture 1:1:1:1 (mol/mol/mol/mol). Fig. 1 shows the geometric representation of the mixtures. The mixtures can be represented in terms of the proportion of the ith component in the mixture (xi), as (Sx1Ox2Lx3Rx4). All
mixtures labels and mass required for PDSC analysis (5.0 mg/analysis) are displayed in Table 1. The response function can be expressed in its canonical form as a low degree polynomial [13,14] (Eqs. (1)–(4)), where the terms x1, x2, x3 and x4 represents the molar fraction of the stearic, oleic, lin^ is the preoleic and ricinoleic, respectively, in the mixture. The y dictive dependent variable (oxidation temperature – OT) as a function of the factors xi using the regression coefficients bi, bij, bijk and dij. The models were evaluated as linear (Eq. (1)), quadratic (Eq. (2)), special cubic (Eq. (3)) and full cubic (Eq. (4)), considering a significance level of 5% (p-value
