Methylic and ethylic biodiesels from pequi oil (Caryocar brasiliense Camb.): Production and thermogravimetric studies

Fuel 136 (2014) 10–18

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Methylic and ethylic biodiesels from pequi oil (Caryocar brasiliense Camb.): Production and thermogravimetric studies Tiago Almeida Silva a, Rosana Maria Nascimento de Assunção a, Andressa Tironi Vieira b, Marcelo Firmino de Oliveira b, Antonio Carlos Ferreira Batista a,⇑ a b

Faculdade de Ciências Integradas do Pontal, Universidade Federal de Uberlândia, Rua Vinte, 1600, Ituiutaba, MG CEP 38304-402, Brazil Departamento de Química, Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, Universidade de Sao Paulo, Av. Bandeirantes, 3900, Ribeirão Preto, SP CEP 14040-901, 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

 A new optimized route for pequi

biodiesel production.  An interesting proposal for an

alternative fuel to diesel.  A contribution to the chemistry of

biofuels.

a r t i c l e

i n f o

Article history: Received 14 April 2014 Received in revised form 15 July 2014 Accepted 15 July 2014 Available online 27 July 2014 Keywords: Biodiesel Pequi Transesterification Thermogravimetric analysis

a b s t r a c t This research reports on the synthesis of methylic and ethylic biodiesels from pequi oil using the transesterification reaction process via alkaline homogeneous catalysis. Thermogravimetric studies helped to evaluate the thermal stability of the biodiesels. A methanol/ethanol 20:80 (w/w) alcoholic solution furnished the ethylic biodiesel and allowed for straightforward optimization of the biodiesel–glycerin phase separation. Classic physicochemical analyses, gas chromatography–mass spectrometry (GC–MS), infrared spectroscopy (FT-IR), and proton nuclear magnetic resonance (1H NMR) aided characterization of the synthesized biodiesel samples. Thermogravimetric analysis (TGA) provided information about the thermal decomposition kinetics of pequi biodiesels. The biodiesels obtained here had satisfactory thermal stability and qualified as potential substituents of conventional mineral diesel. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Current predictions of declining oil reserves have guided the research for alternative energy sources [1]. Environmental ⇑ Corresponding author. Address: Rua Vinte, 1600, Bairro Tupã, Ituiutaba, MG CEP 38304-402, Brazil. Tel.: +55 (34) 3269 2195; fax: +55 (34) 3268 4828. E-mail address: fl[email protected] (A.C.F. Batista). http://dx.doi.org/10.1016/j.fuel.2014.07.035 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.

problems related to fossil fuel burning, such as the greenhouse effect, acid rain, and damage to the ozone layer, have also boosted the demand for new energy sources. In this context, biodiesel, a biofuel that can substitute mineral diesel oil, has received increasing attention in recent years [2,3]. The American Society for Testing and Materials (ASTM) has defined biodiesel as alkyl esters bearing a long-chain carboxylic acid, obtained from renewable sources such as vegetable or animal fat oils [4]. Compared with mineral

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T.A. Silva et al. / Fuel 136 (2014) 10–18

diesel, biodiesel offers many advantages [5]: it emits lower levels of CO2 and particulate material and originates from renewable sources. In addition, it is easy to adapt biodiesel to diesel engines, dismissing the need to modify the entire existing fleet that runs on diesel oil [6]. Transesterification consists in reacting a vegetable oil or animal fat (triacylglycerides) with short chain alcohols in the presence of a catalyst [5] to obtain a mixture of glycerol esters, as illustrated in Fig. 1. This process has found wide application in biodiesel production. It can separate the glycerine molecule from the alkyl chains, thereby reducing the viscosity of the vegetable oil [5]. Alkaline homogeneous catalysis constitutes the main catalytic pathway to perform the transesterification reaction, a process that is well established in the industry, especially in the case of sodium and potassium alkoxides [5]. In Brazil, biodiesel production relies on soybean oil and animal fat. According to the Monthly Bulletin of the Biodiesel ANP (Petroleum, Natural Gas, and Biofuel National Agency), together, these two sources of raw materials account for almost 95% of the domestic biodiesel production [7]. Although the National Program for Biodiesel Production and Use (NPPB) in Brazil has advocated biodiesel production from various sources, so as not to impact food production, the Brazilian biodiesel production still depends on a very small number of raw materials [8,9]. Rathmann et al. [8] have analyzed the future of biodiesel and have come to the conclusion that only raw materials diversification will allow for biodiesel production in the B12 form (diesel with 12% biodiesel) in the coming decades. Therefore, it is essential to adopt more effective political and economic initiatives that will ensure investments in biodiesel production from various sources. This will guarantee that the producing regions will develop from both a social and economic standpoint while avoiding endangering food production [8,9]. Pequi (Caryocar brasiliense Camb.) is a typical fruit of the Brazilian cerrado, which includes the states of Pará, Mato Grosso, Goiás, the Federal District, São Paulo, Minas Gerais, and Paraná, as well as the northeastern states Piauí, Ceará, and Maranhão [10,11]. Its main fatty acid constituents are palmitic acid (31.4%), stearic acid (2.2%), oleic acid (47.3%), linoleic acid (15.6%), myristic acid (0.1%), palmitoleic acid (0.7%), vaccenic acid (1.7%), linolenic acid (0.7%), arachidic acid (0.1%), and gondoic acid (0.1%) [12]. In the context of biodiesel production, Borges et al. [13] recently presented an initial study on the synthesis and characterization of methylic and ethylic biodiesel from pequi oil, which showed that this oil is a potential source of raw material to produce biodiesel with satisfactory physicochemical properties. The present research aimed to synthesize methylic and ethylic biodiesels from pequi oil using the transesterification reaction via alkaline homogeneous catalysis; to characterize the synthesized biodiesel samples by physicochemical analyses, gas chromatography–mass spectrometry (GC–MS), infrared spectroscopy (FT-IR), proton nuclear magnetic resonance (1H NMR), and thermogravimetric analysis (TGA); and to investigate the thermal decomposition kinetics of these biodiesels.

2. Experimental 2.1. Materials and reagents Pequi oil samples (from Guiscond & Silva LTDA) were acquired in the local commerce. Methanol, ethanol, and potassium hydroxide (Synth), both P.A. grade, were also employed. All the other solvents used in this work were also P.A. analytical grade. 2.2. Biodiesel samples synthesis and physicochemical characterization Samples of methylic and ethylic biodiesel pequi oil were synthesized by transesterification via alkaline homogeneous catalysis, using a 6:1 alcohol/oil molar ratio and a catalyst concentration of 1% w/w. For the calculation of the 6:1 alcohol/oil ratio was used the weighted mass of the pequi oil, obtained from the molar weight and relative percentage of each fatty acid constituent of the pequi oil. The quantities of alcohol and catalyst mass required for reaction with 100 g of oil were added to the reactor (reflux system); the mixture stirred for 15 min, to obtain the corresponding alkoxide. After preparation of the alkoxide, 100 g of oil was added to the reactor, and the reaction system was kept under stirring for 40 min at 40 °C. After reaction completion, the product was transferred to a separatory funnel, to separate the biodiesel and glycerol phases. The crude biodiesel phase was subjected to four successive washings with a volume of water approximately equal to a third of the biodiesel volume, to achieve pH 7. Subsequently, the biodiesel was dried with silica and filtered under a vacuum system. The samples of methylic biodiesel and ethylic biodiesel from pequi oil were named as MBP and EBP. Note: an alcoholic methanol/ethanol mixture 20:80 (w/w) was used to prepare the ethylic biodiesel. Such mixture is necessary to optimize the biodiesel/glycerin phase separation process. Refined biodiesel samples were subjected to physicochemical characterization, namely density at 20 °C, kinematic viscosity at 40 °C, acidity index, oxidative stability at 110 °C, and refractive index at 40 °C; these parameters were obtained according to the following established official standard methods: AOAC 920 212, ASTM D 445 modified, ASTM D 664, and EN 14112, respectively. The viscosity was obtained using a Cannon–Fenske viscosimeter (mm2 s2 constant k = 0.2482 at 40 °C). The 873 Biodiesel Rancimat equipment (Metrohm) was used to determine the oxidative stability. An Abbe refractometer was employed to measure the refractive index. All the physicochemical analyses were performed in triplicate (n = 3). 2.3. GC–MS analysis Chromatographic analysis of the biodiesel samples was conducted on an HP gas chromatograph model 5890 GC series II equipped with an HP1 column (100% dimethyl polysiloxane) with length of 30 m and internal diameter of 0.2 mm. The mobile phase consisted of N2 and H2 (30 L min1), and air (300 L min1); the injection volume was 0.5 mL; the injector temperature was set at

O R1 C

O O

O O C

R2 C O

O

Catalyst

HO

R3 + 3RO

H Alcohol

O

R3 C OR R2 C OR O OH + R1 C OR

HO Glycerol

Triacylglicerides Fig. 1. General transesterification reaction scheme.

Esters - Biodiesel

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T.A. Silva et al. / Fuel 136 (2014) 10–18

4

2

(a)

3

6

60

Pequi oil Soybean oil Canola oil Corn oil Sunflower oil

Intensity (× 10 )

−1

Conductivity (μS.cm )

80

6.67 h

9.56 h

7.26 h

40

14.45 h 4.44 h

20

2

3 5

1 21 1

22 23 Time (min)

24

Oxidative stability

0 0

2

4

6

8

10

12

14

0

16

5

Time (h)

10

15

Fig. 2. Oxidative stability of pequi, soybean, canola, corn, and sunflower oils.

30

35

40

(b)

2

3

4 1

Yield (%) Refined biodiesel

96 ± 1 80 ± 1

74 ± 4 57 ± 2

6

Crude biodiesel

Intensity (×10 )

Methylic Ethylic

25

6

Table 1 Reaction yield obtained for transesterification via the methylic and ethylic routes. Route

20

Time (min)

200 °C; and a temperature ramp from 80 to 200 °C was used for the analysis. The mass spectra of the main chromatographic peaks were monitored with a mass spectrometer HP model 5988A coupled to the gas chromatograph.

2

3 1

21

5

22 23 Time (min)

7 24

0

2.4. FT-IR analysis

5 Infrared spectra were obtained using an infrared spectrometer Perkin Elmer Spectrum RX I FT-IR System, working in the spectral range 4500–450 cm1. The samples aliquots were added between two KBr pellets suitable for analysis.

10

15

20

25

30

35

40

Time (min) Fig. 3. Gas chromatograms obtained for (a) MBP and (b) EBP.

(27–600 °C), with a nitrogen or synthetic air flow of 30 L min1, using mass samples of approximately 6 mg.

2.5. 1H NMR analysis 1

H RMN spectra were recorded on an NMR spectrometer Bruker Avance DRX400 400 MHz. Samples (approximately 150 lL) were diluted in 500 lL of deuterated chloroform (CDCl3). The 1H NMR spectra were acquired in the following conditions: spectral window of 20.69 ppm, number of transients equal to 16, and fid size of 64 k.

2.7. Thermal decomposition kinetics Below is the general kinetic equation adopted to describe thermal transformations occurring at a linear heating rate:

2.6. Thermogravimetric analysis

dx dx ¼ kðTÞf ðxÞ ! b ¼ AeEa =RT f ðxÞ dt dT

Oil and biodiesel samples were submitted to thermogravimetric analysis (TGA) in inert nitrogen and synthetic air oxidizing atmosphere, using a Shimadzu equipment model DTG-60H. The thermogravimetric curves (TG curves) were obtained for three different heating rates (10, 20, and 40 K min1) in the range 300–873 K

where x is the reaction degree; x ¼ m0  m=m0  mf (m is the sample mass at time t or temperature T); m0 and mf are the sample mass at the beginning and end of mass loss, respectively; t is the time; k(T) is the speed constant described by the Arrhenius equation (k(T) = AeEa =RT , where A is the pre-exponential factor, Ea is the acti-

ð1Þ

Table 2 Physicochemical properties of the samples of methylic (MBP) and ethylic (EBP) pequi biodiesel samples (n = 3). Property

Unity

Pequi oil

MPB

EBP

Brazil ANP-07/2008

EU-EN 14214

EUA-ASTM D6751

Acidity índex Density, 20 °C Kinematic viscosity, 40 °C Oxidative stability, 110 °C Refractive index, 40 °C

mg of KOH/g kg m3 mm2 s1 h –

2.2 ± 0.1 905.5 ± 0.7 35 ± 1 14.4 ± 0.3 1.4652 ± 0.0002

0.47 ± 0.06 869.69 ± 0.07 4.27 ± 0.03 4.5 ± 0.3 1.4463 ± 0.0005

0.57 ± 0.05 862.3 ± 0.3 4.54 ± 0.02 4.95 ± 0.05 1.4403 ± 0.0004

0.50 max 850–900 3.0–6.0 6.0 –

0.50 max – 3.5–5.0 6.0 –

0.50 max – 1.9–6.0 3.0 –

13

T.A. Silva et al. / Fuel 136 (2014) 10–18 Table 3 Retention time (Tr) and respective compounds in the chromatograms obtained for MBP and EBP, identified by mass spectrometry. Peak

1 2 3 4 5 6 7

MBP

0

x

Compound

Tr (min)

Compound

20.946 21.292 22.808 22.975 23.142 – –

Methyl Methyl Methyl Methyl Methyl – –

21.233 21.933 22.792 22.917 23.425 23.550 23.783

Methyl palmitate Ethyl palmitate Methyl linoleate Methyl oleate Ethyl linoleate Ethyl oleate Ethyl esthereate

dx A ¼ gðxÞ ¼ f ðxÞ b

Z

ð3Þ



Tr (min)

vation energy, and R is the universal gas constant); f(x) is the reaction model; T is the temperature; and b is the heat rate. Integration of Eq. (1) gives:

Z

A Ea  0:00484  exp 1:052  b RT



Application of the natural logarithm of Eq. (3) affords:

EBP

palmitoleate palmitate linoleate oleate esthereate

gðxÞ ¼



lnðbÞ ¼ ln

 AEa Ea 1  5:331  1:052   RgðxÞ R T

ð4Þ

Thus, the activation energy can be easily calculated by the slope of the line ln(b) vs. 1/T, constructed from the temperature values for each heating rate at different degrees of conversion. The activation energies were calculated for degrees of conversion in the range 10–90%. 3. Results and discussion 3.1. Pequi oil properties

T

eEa =RT dT

ð2Þ

0

Among the mathematical approaches used to calculate and study kinetic parameters, the method of Ozawa–Flynn–Wall stands out. Therefore, this method was selected to study the activation energy of the biodiesel samples during the decomposition process. For Ozawa–Flynn–Wall, Doyle approximation is used to integrate in temperature, which converts Eq. (2) to:

The pequi oil used in this research presented the following properties: acidity index = (2.2 ± 0.1) mg of KOH/g of oil; density at 20 °C = (905.5 ± 0.7) kg m3; kinematic viscosity at 40 °C = (35 ± 1) mm2 s1; oxidative stability at 110 °C = (14.4 ± 0.3) h; refractive index at 40 °C = 1.4652 ± 0.0002. Information about the acidity index of the oil allows for selection of the most suitable catalyst. Vegetable oils with acidity index up to 6 mg of KOH/g are generally susceptible to transesterification via alkaline homogeneous catalysis [14]. Samples with acidity index higher than

Fig. 4. IR spectrum obtained for MBP.

Fig. 5. IR spectrum obtained for EBP.

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T.A. Silva et al. / Fuel 136 (2014) 10–18

a

b 3C

H( ) n

b ( )n

g

d

O

f

c O

e

d

g

CH3

f

b

a

e g d

c

Fig. 6. 1H NMR spectrum obtained for MBP.

a

H3C

b ( )n

b ( )n

g

d g

O

f

c e

d

O

CH3

b

f

a

g

e d

c

Fig. 7. 1H NMR spectrum obtained for EBP.

6 mg of KOH/g are not suitable for this route, because parallel neutralization reactions can occur between the catalyst and the free fatty acids (FFA), resulting in emulsions (soap) and reducing the

Table 4 H NMR spectra signals for MBP and EBP.

1

Chemical shift (d, ppm)

Corresponding proton

0.9 1.3–1.4 1.6–1.7 2.0–2.1

a = terminal group protons – linear chain CH3 b = methylenic group protons – linear chain CH2A c = methylenic group protons ACH2A with carbon b d = methylenic group protons ACH2A adjacent to the unsaturated carbon e = methylenic group protons ACH2A with carbon a g = protons attached to unsaturated carbons

2.3 5.3–5.5

process yield. The pequi oil used in this study had an acidity index lower than 6 mg of KOH/g, so it was possible to use the alkaline transesterification reaction to produce biofuel. For comparison purposes, Fig. 2 illustrates the oxidative stabilities of pequi and other vegetable oils commonly used to produce biodiesel. Compared with soybean, corn, canola, and sunflower oils, pequi oil exhibited better oxidative stability, justified by its higher levels of natural antioxidants such as carotenoids. This property is quite interesting for biodiesel production; indeed, low oxidative stability has been a frequent problem concerning biodiesel. An oil sample rich in antioxidants should therefore culminate in more chemically stable biodiesel samples. The refractive index of pequi oil was close to the values recommended by ANVISA (National Health Surveillance Agency, Brazil) for soybean (1.466–1.470) and sunflower (1.467–1.469) oils [15].

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T.A. Silva et al. / Fuel 136 (2014) 10–18

(a)

100 80

0.0 -0.5

MBP EBP Oil

40

−1

60

DTG (% K )

Mass (%)

-1.0 -1.5

20 -2.0 0 300

400

500

600

700

800

900

-2.5

Temperature (K)

(b)

100 80

0.0 -0.5

MBP EBP Oil

40

-1.0 -1.5

20

−1

60

DTG (% K )

Table 1 summarizes the mass yield results obtained for the alkaline transesterification of pequi oil, calculated for the crude and refined biodiesel. The methylic route yield was higher than that of the ethylic route, because methanol is more reactive than ethyl alcohol. Biodiesel refinement led to 25% lower yield. However, the final yields were quite satisfactory considering that part of the mass reduction stemmed from removal of contaminants such as remaining glycerin, unreacted alcohol, and catalyst. Table 2 lists the data on the physicochemical characterization of methylic and ethylic pequi biodiesel samples as well as the official limits for each physicochemical property according to Brazilian (ANP-07/2008), European (EN 14214), and American (ASTM D6751) standards. The acidity index is one of the most important physicochemical properties of biodiesel. Biodiesel is corrosive due to the presence of FFAs, which constitutes a serious problem regarding biodiesel storage and use in engines [16,17]. Acidity may also indicate the degree of biodiesel aging during the storage step—this parameter increases when biodiesel esters undergo hydrolysis, because this process generates FFAs [16]. Because this parameter is of utmost importance, all the three agencies consider a maximum acidity value of 0.5 mg of KOH/g for biodiesel. The methylic biodiesel sample acidity index fell below this limit, whereas the ethylic biodiesel samples presented values slightly above this limit. Density is another important parameter to indicate biodiesel quality, because it can change due to impurities or even adulteration [18]. The density values obtained for the methylic and ethylic biodiesel samples (869.69 ± 0.07 and 862.3 ± 0.3 kg m3, respectively) were in accordance with the official standards. Another important physicochemical property in a fuel species is its kinematic viscosity [18]. The viscosity is closely related to the chemical composition of the material under study [18]. The presence of triacylglycerides confers high viscosity to vegetable oils, which can harm the injection system of diesel engines. Transesterification aims to reduce this parameter. In this work, the viscosity of pequi oil decreased from (35 ± 1) mm2 s1 to (4.27 ± 0.03) mm2 s1 and (4.54 ± 0.02) mm2 s1 for the methylic and ethylic biodiesel samples, respectively, which represented a reduction of almost 90% in the kinematic viscosity of the oil and met the official standards for quality control. Official standard agencies also establish a minimum value of 6 h for biodiesel oxidative stability. The biodiesel samples prepared herein did not reach this value. However, the obtained values were promising, because they were superior to those of other types of biodiesel reported in the literature [19]. Alternatively, antioxidants can help to correct biodiesel oxidative [14]. Although the refractive index directly relates to the structure and chemical composition of the analyzed sample, the three regulation agencies have not yet adopted it as a biodiesel quality standard. The biodiesel samples possessed lower refractive index value than pequi oil, demonstrating that transesterification, especially triacylglyceride esters conversion, modified the chemical composition of the sample oil. In fact, such esters deviate light in different ways. Fig. 3 shows the chromatograms obtained during the GC–MS analysis of methylic and ethylic pequi biodiesels. Table 3 presents the compounds with the higher relative intensity peaks in the mass spectrum, as enumerated in Fig. 3. The results agreed with the literature data on the composition of pequi oil fatty acids; palmitate, stearate, oleate and linoleate methyl and ethyl esters were the main fatty acids products in the oil [10]. Analysis of ethylic biodiesel revealed a mixture of methylic and ethylic

esters. This was justified by the fact that an alcoholic methanol/ethanol solution was used to synthesize the ethylic biodiesel sample. FT-IR and 1H NMR analyses of the synthesized biodiesel samples confirmed that the catalytic process efficiently converted the triacylglycerides into esters. Figs. 4 and 5 depict the IR absorption spectra of the pequi methylic and ethylic biodiesel samples. The strong absorption bands in the region of 2850–3000 cm1 correspond to CAH bond stretching. The intense absorption band in the region of 1735–1750 cm1 refers to the carbonyl group C@O bond stretching. The absorption band near 1465 cm1 stems from symmetrical angular deformation of the CAH bond of methylenes (ACH2A)n. The absorption band near 1375 cm1 concerns the symmetrical angular deformation of the CAH bond of the terminal CH3 group. The strong absorption band near 1170 cm1 results from CAOAC bond stretching. The average absorption band near 720 cm1 is relative to the asymmetric deformation of the angular CAH bond groups of methylenes (ACH2A)n. All the bands are consistent with the features expected for fatty acid esters. Figs. 6 and 7 bring the 1H NMR spectra of the methylic and ethylic biodiesel samples. A set of signals that are specific to each type of biodiesel (methylic or ethylic form) attested to the conversion of triacylglycerides into their methylic or ethylic esters. The methylic ester displayed a strong singlet at d 3.7 ppm, relative to methoxyl ester protons (f@AOACH3). As for ethylic esters, a quartet at d 4.1–4.2 ppm evidenced methylenic protons from the ethoxylated group (f@AOACH2ACH3). These results completely agreed with literature data [18,20,21].

Mass (%)

3.2. Synthesis and characterization of biodiesel from pequi oil

-2.0 0 200

400

600

800

1000

1200

-2.5

Temperature (K) Fig. 8. TG-DTG curves obtained in: (a) inert and (b) oxidant atmosphere for MBP, EPB, and pequi oil. Heating rate of 10 K min1.

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T.A. Silva et al. / Fuel 136 (2014) 10–18

Table 4 also lists additional signs that commonly emerge in the H NMR spectra of vegetable oils—they refer to protons that belong to parts of the triacylglyceride molecule that remain unchanged during the transesterification reaction. The main difference between the 1H NMR spectra of vegetable oils and esters lies on the presence of double doublets in the region of d 4.2 ppm for vegetable oil samples, which is characteristic of glycerol methylene protons.

1

3.3. Thermogravimetric analysis of pequi biodiesels Fig. 8 contains the thermogravimetric curves (TG-DTG) obtained for pequi oil, methylic pequi biodiesel, and ethylic/ methylic pequi biodiesel under inert nitrogen atmosphere and synthetic airflow. In both atmospheres, pequi oil was more thermally stable than the corresponding biodiesel samples. The fact that vegetable oils typically consist of triacylglycerides with higher molecular weight could account for the higher thermal stability of the biodiesel samples as compared with pequi oil [14]. Pequi oil underwent mass loss associated with triacylglycerides volatilization and/ or pyrolysis in inert atmosphere [22–24]. The onset of pequi oil mass loss under inert atmosphere was around 200 °C (473 K). However, in oxidizing atmosphere, three mass loss events were associated with triacylglycerides volatilization and/or combustion [22–24]. Weight loss started around 177 °C (450 K), demonstrating that oxygen acted in the combustion of triacylglycerides in oxidiz-

100

ing atmosphere, accelerating the thermal decomposition of the oil. The methylic (MBP) and ethylic (EBP) biodiesel samples behaved similarly, because they had similar chemical composition (Table 3). For both employed atmospheres and the two types of biodiesel, there was a common mass loss event associated with volatilization and/or pyrolysis (inert atmosphere) and/or combustion (oxidizing atmosphere) of esters consisting mainly of palmitic, oleic, linoleic and stearic acids, which are the major constituents of biodiesel samples (Table 3). The mass loss began around 110 °C (383 K) in both inert and oxidizing atmosphere, for both biodiesel samples. No official rules about the thermal stability of biodiesel exist at the moment. However, according to Kivevele et al. [25], biodiesel which remains stable up to 150 °C in oxidizing atmosphere can be considered as thermally stable. From this point of view, the pequi biodiesel samples prepared here did not present adequate thermal stability, however it should be noted that a lower decomposition temperature indicate the higher volatility of the produced biofuel, an important feature for its application in diesel motors [26]. Thus, the thermal stability of the pequi biodiesels was satisfactory. 3.4. Pequi biodiesel thermal decomposition kinetics Treatment of the TG data using the method of Ozawa–Flynn–Wall aided evaluation of the thermal decomposition kinetics of pequi biodiesel samples. This method is useful to treat thermogravimet-

Mass loss

(a)

100 80 −1

10 K min −1 20 K min −1 40 K min

Mass (%)

60 40

Mass (%)

80

20 0

Mass loss

(b)

−1

10 K min −1 20 K min −1 40 K min

60 40 20 Conversion

Conversion

300

0

400

500

600

700

800

300

400

Temperature (K)

100

(c)

600

700

800

Temperature (K)

Mass loss

100

(d)

Mass loss

80 −1

10 K min −1 20 K min −1 40 K min

60 40 20

Mass (%)

80 Mass (%)

500

−1

10 K min −1 20 K min −1 40 K min

60 40 20

Conversion

0

0

300

400

500

600

700

Temperature (K)

800

Conversion

300

400

500

600

700

800

Temperature (K)

Fig. 9. TG curves for MBP decomposition and conversion in (a) inert and (b) oxidizing atmosphere, and EBP decomposition in (c) inert and (d) oxidizing atmosphere. Heating rates of 10, 20, and 40 K min1.

17

T.A. Silva et al. / Fuel 136 (2014) 10–18

(a)

10% 20% 30% 40% 50% 60% 70% 80% 90%

ln β

3.2

2.8

3.6

2.8

2.4

2.4

0.0018 0.0019 0.0020 0.0021 0.0022 0.0023

0.0018 0.0019 0.0020 0.0021 0.0022 0.0023

−1

−1

1/T (K )

1/T (K )

(c)

10% 20% 30% 40% 50% 60% 70% 80% 90%

ln β

3.2

2.8

2.4

3.6

10% 20% 30% 40% 50% 60% 70% 80% 90%

(d)

3.2 ln β

3.6

10% 20% 30% 40% 50% 60% 70% 80% 90%

(b)

3.2

ln β

3.6

2.8

2.4

0.0018 0.0019 0.0020 0.0021 0.0022 0.0023 −1

0.0018 0.0019 0.0020 0.0021 0.0022 0.0023 −1

1/T (K )

1/T (K )

Fig. 10. Linear regression for 10–90% conversion based on the Ozawa–Flynn–Wall method for MPB in (a) inert and (b) oxidizing atmosphere and for EBP in (c) inert and (d) oxidizing atmosphere. Heating rates of 10, 20, and 40 K min1.

ric data because it is applicable even if the mechanism of the target reaction is unknown. Additionally, it has successfully provided thermal decomposition kinetic data for biodiesels [23]. This

Table 5 Correlation coefficients (R) and activation energies (Ea) obtained by the Ozawa– Flynn–Wall method. Sample

Conversion (%)

Inert atmosphere R

Ea (kJ mol

Oxidizing atmosphere 1

)

R

Ea (kJ mol1)

MBP

10 20 30 40 50 60 70 80 90

0.761 0.998 0.995 0.993 0.991 0.982 0.988 0.985 0.982

87.79 64.80 67.10 67.44 67.77 71.01 68.53 67.91 67.91

0.995 0.988 0.991 0.996 0.997 0.993 0.997 0.993 0.999

67.76 71.25 69.50 67.88 66.99 67.73 68.72 68.67 68.45

EBP

10 20 30 40 50 60 70 80 90

0.969 0.959 0.964 0.959 0.966 0.976 0.98 0.985 0.994

79.04 77.77 79.95 80.51 83.40 84.67 88.22 91.55 96.18

0.992 0.9997 0.9999 0.999 0.999 0.998 0.999 0.998 0.999

73.04 77.30 81.56 80.25 80.37 80.89 80.32 81.50 83.36

method furnished the activation energy (Ea) involved in the thermal decomposition of the MBP and EBP samples at three different heating rates (10, 20, and 40 K min1), with conversion percentages ranging from 10% to 90%. Fig. 9 shows the thermogravimetric curves and the corresponding decomposition obtained for biodiesel samples in inert and oxidizing atmosphere. Fig. 10 presents the linear regressions ln(b) vs. 1/T obtained for each sample type and atmosphere. Table 5 displays the Ea values obtained from the inclination of each straight conversion rate as well as the correlation coefficient (R) of each curve. The Ea for the thermal decomposition of methylic pequi biodiesel in inert atmosphere ranged from 64.80 to 87.79 kJ mol1, with a mean value of 70.03 kJ mol1. In oxidizing atmosphere, Ea lay between 67.73 and 71.25 kJ mol1, with a mean value of 68.56 kJ mol1. Thus, the thermal decomposition of pequi methylic biodiesel required almost the same Ea irrespective of the atmosphere and heat treatment temperature (Fig. 11A). For the ethylic/methylic biodiesel sample, Ea in inert atmosphere ranged from 77.77 to 96.18 kJ mol1, with a mean value of 84.59 kJ mol1. In oxidizing atmosphere, Ea lay from 73.04 to 83.36 kJ mol1 with a mean value of 79.84 kJ mol1. Hence, oxidizing atmosphere lowered the energy barrier required for EBP thermal decomposition (especially for 50% conversion, Fig. 11B). In other words, the sample was more susceptible to degradation in oxygen-rich atmosphere. Comparing the decomposition kinetics of both biodiesel types, EBP demanded the highest Ea for both atmospheres. Thus, EBP demonstrated to be a good substituent for mineral diesel fuel, due to its greater thermal stability.

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100 Inert atmosphere

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Conversion (%) Fig. 11. Comparison of Ea obtained in (a) inert and (b) oxidizing atmosphere for MBP and EBP.

4. Conclusions The pequi oil is a feasible raw material to obtain methylic and ethylic biodiesels by alkaline transesterification. Decomposition kinetic studies indicated that the resulting methylic/ethylic constitutes a potential substituent for mineral diesel fuel, because it is more thermally stable. Acknowledgements The authors are grateful to Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG) and to Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for financial support. Additionally, the authors would like to thank Dr. Cynthia Maria de Campos Prado Manso for revising and editing the text. References [1] Silva TA, Batista ACF, Vieira AT. A brief discussion of general aspects of biodiesel production. Braz Geog J: Geosci Hum Res Med 2012;3(1):79–88.

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