Article pubs.acs.org/EF
Oxidation Stability of Diesel/Biodiesel Fuels Measured by a PetroOxy Device and Characterization of Oxidation Products Kenza Bacha,*,†,‡ Arij Ben-Amara,§ Axel Vannier,† Maira Alves-Fortunato,§ and Michel Nardin‡ †
Renault, 78280 Guyancourt, France Institut de Sciences des Matériaux de Mulhouse (IS2M), 68057 Mulhouse, France § IFP Energies Nouvelles (IFPEN), 92852 Rueil-Malmaison, France ‡
S Supporting Information *
ABSTRACT: In the present work, the oxidation stability of diesel, rapeseed (RME), and soybean (SME) fatty acid methyl esters (FAME) and a blend of diesel with 10% (v/v) RME (B10−RME) was studied. Fuel samples were aged in the PetroOxy test device from 383 to 423 K at 7 bar. Experiments were conducted in oxygen excess, and the global kinetic constants were determined. The global kinetic constants for diesel, B10−RME, and RME at 383 K were 7.92 × 10−6, 2.78 × 10−5, and 8.87 × 10−5 s−1, respectively. The oxidation products formed at different stages of the oxidation were monitored by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis−differential thermal analysis (TGA−DTA), and gas chromatography/ mass spectrometry (GC/MS). The impact of the FAME nature and level of blending on the kinetic rate constant and the oxidation products was investigated. Results show that RME oxidation forms C19 epoxy as the main oxidation product, in addition to a methyl ester FAME derivative and short-chain oxidation products, such as alkane, alkene, aldehydes, ketones, alcohols, and acids with a carbon number up to C11. The overall amount of oxidation products increases with a higher degradation time. The DTA profile suggests that higher molecular weight products are formed at an advanced level of oxidation. For all highly oxidized fuels, a similar DTA peak was obtained at a temperature of around 573 K, which may suggest the formation of products having similar molecular weights for both diesel and FAME.
1. INTRODUCTION Oxidation stability refers to the tendency of fuels to react with oxygen at temperatures near ambient. It describes the relative susceptibility of the fuel to degradation by oxidation.1 Nowadays, for diesel/biodiesel fuel applications, oxidative stress can occur during logistics or on board vehicles, causing the formation of insolubles and deposits or mechanical component failure, involving injector blockage, filter plugging, and pump wear2.3 Although several developments have been made recently toward the understanding of diesel/biodiesel oxidation and deposit formation pathways, it is crucial to develop a systemic approach for fuel evolution monitoring during its life cycle. The most approved pathway for hydrocarbon (RH) oxidation proceeds through free-radical reactions.4−6 Figure 1 illustrates a simplified mechanism for RH oxidation that evolves in three major steps. First, the initiation step R1a can be triggered by either thermal decomposition of the hydrocarbon RH or its reaction with a chemical initiator, such as metal ion or ultraviolet light. If the fuel already contains hydroperoxides, those can decompose and contribute to the initiation step by reactions R1b and R1c. Second, the propagation step occurs through reactions R2, R3a, and R3b, forming hydroperoxides and polyperoxides. Those further decompose to form carbonyl oxidation products7 through reaction R4, which involves several reactions. Finally, the termination step can occur by the reactions R5, R6, and R7 with the recombination of free radicals and the formation of stable products. Being composed of hundreds of different hydrocarbon molecules, real fuels could oxidize in various ways.8 Figure 1 presents a widely © XXXX American Chemical Society
documented kinetic mechanism used for a global kinetic understanding assuming that the hydrocarbon components can be represented by a single species RH, which decomposes with a global oxidation rate equivalent to the real fuel. The rate constants for initiation, propagation, and termination are noted Ri, kp, and kt, respectively.8,6 Several parameters influence the kinetics of oxidation of commercial diesel and biodiesel fuels, such as the oxygen concentration, temperature, light, presence of natural antioxidant additives, water, or residues of the transesterification process, such as methanol or glycerides.9 In the case of fatty acid methyl esters (FAME), the chemical structure of the FAME directly impacts their reactivity.7,10,11 Several methods have been developed to monitor the oxidation stability of commercial diesel and biodiesel fuels. Most used tests are based on the measurement of fuel properties after accelerated degradation under a standard defined condition. These tests include the measurement of the induction period (IP), acid number, kinematic viscosity, peroxides, and insolubles.1,12 In the current work, the measurement of the IP by a PetroOxy test was used to follow the oxidation kinetics and the formed oxidation products. This technique requires a small amount of sample and a short time of analysis allowing for fast and highly reproducible results. It provides flexibility in the operating conditions, knowing that we carried out several tests at different temperatures, and it offers Received: March 3, 2015 Revised: May 24, 2015
A
DOI: 10.1021/acs.energyfuels.5b00450 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Figure 1. Simplified mechanism of hydrocarbon oxidation.4 6 using the same fuel sample with a renewed oxygen atmosphere at 423 K. For example, ΔP60% corresponds to 6 successive PetroOxy tests. This controlled aging procedure allowed us to study fuel evolution at different oxidation stages, with ΔP from 10 to 60%. Multicycle tests were not explored for the kinetic study, because the initial composition is radically altered. The measurement uncertainty of PetroOxy follows a linear relationship with IP: τIP (min) = 0.011IP (min) + 0.80. This correlation is based on repeatability tests measured on 26 fuel samples and different temperatures measured in the present study and by other groups.15 2.2. Fourier Transform Infrared Spectroscopy−Attenuated Total Reflectance (FTIR−ATR). FTIR was performed with a Bruker spectrometer IFS66. The spectra were recorded using a horizontal ATR cell, with ATR diamond monoreflection (Golden Gate), covering a 600−4000 cm−1 spectral range. Each FTIR−ATR spectrum is in the average of 100 scans using air as a reference at a resolution of 4 cm−1, and all spectra were collected at ambient temperature. 2.3. Thermogravimetric Analysis−Differential Thermal Analysis (TGA−DTA). The TGA−DTA method is widely employed for determining the thermal stability and the thermo-oxidative behavior of biodiesel.18−20 The TGA was performed with Mettler-Toledo TGA 851e. A quantity of 10−20 mg of fuel was used for optimum exposure with the aluminum support and oxygen. The oxygen flow rate was set to 100 mL/min, and the temperature ranged from ambient to 773 K with a slope of 10 K/min. Figure 2 presents the change in the sample weight within the TGA cell as a function of the temperature for fresh and oxidized RME. The onset of mass loss (Tonset) indicates the resistance of the sample to thermal oxidative stress and can be correlated to the oxidizability of the sample,21 while its derivative (DTA) allows the determination of the inflection points (Tevent 1, Tevent 2, Tevent 3, etc.) corresponding to evaporation and/or oxidation of its components.19,22 This indicates the oxidation progress. For example, Tonset and Tevent 1 decrease for oxidized RME compared to fresh RME, which can be associated with the lower molecular weight or higher reactivity of the oxygenated products
the possibility to get in a kinetic regime with excess oxygen in the cell to study fuel kinetic oxidation and also to perform successive oxidation runs on the same sample. The first part of this study was dedicated to the oxidation kinetics of a conventional diesel fuel (B0), diesel/FAME mixture (B10−RME), and two FAME [rapeseed methyl ester (RME) and soybean methyl ester (SME)]. These fuels were oxidized into the PetroOxy apparatus. In addition to the IP, a PetroOxy test allowed for the monitoring of oxygen consumption (−d[O2]/dt), which was used to determine global rate constants of oxidation, using a simple kinetic model. The second part was devoted to the characterization by Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) of the oxidation products formed at different oxidation stages. Gas chromatography/mass spectrometry (GC/MS) was used to identify the oxidation products formed following RME oxidation. To provoke severe oxidation, the fuels were oxidized in a PetroOxy apparatus in successive cycles. Rapeseed and soybean are widely used in the European and U.S. markets13,14
2. EXPERIMENTAL SECTION 2.1. PetroOxy Device. A PetroOxy test has been widely used for fuel stability studies,15,16 and its detailed description is provided elsewhere.16 This test allows for the determination of the IP defined by the time necessary to reach 10% of pressure drop measured in the test cell (ΔP) reflecting oxygen consumption (−d[O2]/dt) and, hence, the oxidation of the fuel as a function of time. Tests were conducted from 383 to 423 K at 7 bar. At the IP, oxygen remains sufficiently in excess within the test cell,17 which allows for the use of PetroOxy results for the evaluation of global oxidation kinetics. Successive oxidation cycles were performed for the formation of oxidation products. Those consisted of reaching 10% of ΔP “n” successive times, with n from 1 to B
DOI: 10.1021/acs.energyfuels.5b00450 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
acids of RME are 2 times higher than SME. The fatty methyl ester composition of RME and SME highlighted in Table 2 suggests a high
Table 2. FAME Composition for SME and RME composition
SME (%, m/m)
RME (%, m/m)
C16:0 C16:1 C18:0 C18:1 C18:2 C18:3
10.1 0.1 4.2 26.4 50.6 7.4
4.4 0.3 1.8 60.7 19.6 9.8
content of monounsaturated allylic compounds for RME compared to SME mainly composed of polyunsaturated compounds. Those are characterized by the presence of bisallylic sites that have higher reactivity24 and, therefore, a higher oxidizability compared to RME.16,25,26 Table 3 shows an inventory of fuel samples and test conditions used in the two studies of kinetic and oxidation product characterization. The labels designate samples selected for the oxidation product characterization study.
Figure 2. TGA and DTA curves for fresh (f) and oxidized (o) RME. formed.23 Two additional peaks (Tevent 2 and Tevent 3) were observed for oxidized RME that may result from the higher molecular weight products formed during the oxidation (e.g., dimers and polymers). The intensity of the peaks reflects the amount of the leaving product. Together, these parameters were employed to compare the oxidation stability of different fuels and evaluate the progress of oxidation. 2.4. Gas−Liquid Chromatography/Mass Spectrometry (GC/ MS). GC/MS analyses were performed with an Agilent GC 7890N gas chromatograph equipped with a J&W column (60 m × 250 μm × 0.25 μm) under the following temperature program: 50 °C (for 3 min), 5 °C min−1 until 70 °C, then 3 °C min−1 until 325 °C, and 325 °C (for 35 min). Samples were introduced into the column via a split injector (0.2 μL) at 340 °C. The GC was coupled to an Agilent 5975C mass selective detector, and spectra data were acquired over a mass range of 20−600 amu. 2.5. Fuels. The fuels used for the present work are an ultralowsulfur diesel fuel (B0) provided by Total ACS, and RME and SME, provided by ASG, containing no added additives. Both FAME were used during the month following their production. They were conserved in a refrigerator at 5 °C to minimize the oxidation. Table 1 presents a selection of physical and chemical properties of the abovementioned fuels. All of the samples have levels of density, viscosity, water content, and acid number that stand within the limits of the specification. Note that the level of the acid number and free fatty
3. RESULTS AND DISCUSSION 3.1. Kinetic Study. The simplified oxidation mechanism given in Figure 1 was used to calculate the global kinetic rate constant k from the different oxidation steps. From the propagation step, the rate of oxygen consumption follows the following relation: −d[O2 ]/dt = k p[ROO•][RH]
(1)
The rate of initiation equals the rate of termination at steady state; therefore, Ri can be written as Ri = 2kt[ROO•]2. Equation 1 then becomes −d[O2]/dt = −d[RH]/dt = (Ri/2kt)1/2kp[RH] = k[RH], where k = (Ri/2kt)1/2kp. k is assumed to follow an Arrhenius formalism k = Ae−(Ea/RT), with Ea being the activation energy and A being the pre-exponential factor of the global kinetic constant of the oxidation reaction. Using eq 1, the
Table 1. Selected Physical and Chemical Properties of RME, SME, and B0 specification FAME parameter density at 15 °C viscosity at 40 °C water content oxidation stability at 110 °C oxidation stability acid number iodine number linolenic acid methyl ester methyl ester >4 double bonds monoglyceride diglyceride triglyceride total contamination metals (Na and K) metals (Ca and Mg) free fatty acids sulfur content
RME
specification B0
SME
minimum
maximum
B0
minimum
maximum
unit
method
883.3 4.463 116 6.6
884.9 4.118 120 5.6
860 3.5
900 5 500
825 2
845 4.5 200
0.39 113 9.8 4. The amount of carbonyl products formed during oxidation seems to be higher for RME compared to SME. This result is in line with the tendency obtained for the kinetic rate constant, but DTA results indicate that products with a higher molecular weight are formed for the SME-6 sample, suggesting that C18:2 oxidation lead to the formation of a higher level of oligomer material. GC/MS was used to identify the oxidation products of RME at different levels of oxidation. The oxidation of C18:2 and C18:3 occurred first, followed by C18:1 at a high oxidation degree. The main products formed were epoxy C19, FAME derivative, and short-chain oxidation products (C2−C11), such alkane, alkene, acids, alcohols, aldehydes, and ketones. The oxidation products are so complex that much of these results can be regarded as only qualitative accounts. The findings of this work can be a step forward for the development of key strategies for fuel-quality monitoring during logistics and on-board utilization.
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ASSOCIATED CONTENT
S Supporting Information *
Evolution of the pressure and temperature in the PetroOxy cell for B0 at 423 K, FTIR functions associated with the peak wavenumber, and distillation curve of diesel fuel (B0) (PDF) and evolution of relative percentage of RME (alkane and alkene, aldehyde and alcohol, acids and ketones, FAME derivative, and epoxy) oxidation products identified with GC/ MS as a function of the oxidation cycle and Tonset, Tevent 1, and Tevent 2 of B0, B10−RME, RME, and SME samples as a function of the oxidation cycle (PDF). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.5b00450.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the laboratory of IFPEN hosting the research on accelerated fuel oxidation, IS2M for the provision of their analytical platform (FTIR and TGA), and Renault for financial contribution, as well as Pascal Hayrault from IFPEN and Simon Gree and Gautier Shrodj from IS2M for their technical support and David Gerard from Renault for project monitoring.
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DOI: 10.1021/acs.energyfuels.5b00450 Energy Fuels XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.energyfuels.5b00450 Energy Fuels XXXX, XXX, XXX−XXX
