Forage turnip, sunflower, and soybean biodiesel obtained by ethanol synthesis: Production protocols and thermal behavior

Fuel 89 (2010) 3725–3729

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Forage turnip, sunflower, and soybean biodiesel obtained by ethanol synthesis: Production protocols and thermal behavior C.M. Soares a, L.C.V. Itavo a, A.M. Dias a, E.J. Arruda b, A.A.S.T. Delben c, S.L. Oliveira c, L.C.S. de Oliveira d,* a

Programa de Mestrado em Biotecnologia, Universidade Católica Dom Bosco, Av. Tamandaré, 6000, Jardim Seminário, 79117-900 Campo Grande, MS, Brazil Faculdade de Ciências Exatas e Tecnologia, Universidade Federal da Grande Dourados, Rodovia Dourados-Itahum, Km 12, 79804-970 Dourados, MS, Brazil c Departamento de Física, Universidade Federal de Mato Grosso do Sul, Cidade Universitária, s.n, 79070-900 Campo Grande, MS, Brazil d Departamento de Química, Universidade Federal de Mato Grosso do Sul, Cidade Universitária, s.n., 79070-900 Campo Grande, MS, Brazil b

a r t i c l e

i n f o

Article history: Received 5 February 2010 Received in revised form 14 June 2010 Accepted 15 July 2010 Available online 25 July 2010 Keywords: Vegetable oils Ethanol Biodiesel Thermal analysis

a b s t r a c t In this work it is reported a detailed investigation of the effect of different production protocols based on alkaline ethanolysis on conversion yield of forage turnip, soybean, sunflower, and castor oil into the respective biodiesel. Parameters such as catalyst contents, reaction times and temperatures were evaluated. Additionally, it was also investigated the relationship between the conversion yield and the chemical composition of the fatty acids in the feedstock. Conversion yields ranging between 70% and 100% point out the viability of the production of biodiesel using ethanol. Based on thermal analysis, sequential steps of weight loss were observed indicating that biodiesel undergoes oxidative thermal decomposition with the elimination of different portions of the molecules in each step. Besides, the energies released by the samples during thermal decomposition were determined. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Biodiesel are esters that can be obtained from a wide range of oils (edible and nonedible) and animal fats [1] by transesterification of fatty alcohol in the presence or absence of catalyst [2]. More than 95% of the oils and fats are made up of triglycerides, which are esters formed from one glycerol molecule and three fatty acids. Triglycerides are insoluble in water and may be either solid or liquid at room temperature depending on the feedstock. Besides triglycerides, fats and oils may contain small amounts of mono and diglycerides (emulsifiers), free fatty acids, tocopherols (antioxidants), sterols, and fat soluble vitamins [3]. The chemical composition and concentration of fatty acids in soybean, sunflower, castor, and forage turnip (Raphanus sativus L.) oil reported in Table 1 plays an important role in the biodiesel production. Structural differences such as unsaturation level and chain length lead to different viscosity and density values as well as rates of conversion of vegetable oil into esters. Brazil is not self-contained in methanol, but it is in ethanol [4]. In this context, the choice of ethanol for production of biodiesel proves to be advantageous, besides it is environmentally sustainable, not raising many concerns regarding its toxicity [1]. The concentration of ethanol used in alkaline transesterification of vegetable oils is one of the determining factors in the separation of * Corresponding author. Tel.: +55 67 3345 3576; fax: +55 67 3345 3552. E-mail address: [email protected] (L.C.S. de Oliveira). 0016-2361/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2010.07.024

glycerin from the oil. An alcohol to oil ratio below 6:1 makes difficult the separation of glycerin, while higher ratios do not contribute to an increase in conversion rate of oil into esters [5]. In turn, basic catalysts such as hydroxides and alkoxides of alkali metals have been widely used in industry for the production of biodiesel because they lead to a relatively fast transesterification reaction, are less corrosive as compared to the acid catalysts, and can be easily removed from the reaction medium. On the other hand, despite the good conversion yields, reactions catalyzed by acids such as sulfuric and hydrochloric acids have been abandoned because they are slow and require temperatures above 100 °C for periods longer than 3 h [6–9]. Besides biodiesel production, the quality of the biodiesel after production needs to be assessed. Acidity and presence of contaminants are some of the parameters to be monitored in order to guarantee that the biodiesel follows the specifications recommended by the national regulatory agencies for biodiesel [10,11]. High acidity levels make the biodiesel inadequate because of the corrosive effect on engine components [12]. The acidity is related to partial hydrolysis of glycerides and depends on the nature and quality of the raw material. The presence of contaminants in biodiesel such as glycerides can be evaluated by thin layer chromatography (TLC). In this technique glycerol and other contaminants might be identified in a chromatographic plate because compounds with different polarities present different levels of fixation on a thin layer of adsorbent, visible when revealed with iodine vapor or in ultraviolet chambers [3,9,13].

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Table 1 Composition, chemical structure, and unsaturation level of the main fatty acids found in soybean, sunflower, forage turnip and castor oil.

a

Fatty acid

Structure

Unsaturation levela

Soybean

Sunflower

Forage turnip

Castor

Myristic Palmitic Stearic Arachidic Behenic Lignoceric

CH3(CH2)12CO2H CH3(CH2)14CO2H CH3(CH2)16CO2H CH3(CH2)18CO2H CH3(CH2)20COOH CH3(CH2)22COOH

– – – – – –

<0.5 7.0–14.0 1.4–5.5 <1.0 <0.5 –

<0.5 3.0–10.0 1.0–10 <1.5 <1.0 <0.5

6 7.9–10.0 2.2–3.1 0.97–8.2 14.1 –

– 0.9–1.5 1.4–2.1 – – –

Palmitoleic Oleic Linoleic Linolenic Arachidonic Erucic Nervonic Vaccenic Ricinoleic

CH3(CH2)5CH@CH(CH2)7CO2H CH3(CH2)7CH@CH(CH2)7CO2H CH3(CH2)4CH@CH(CH2)CH@CH(CH2)7CO2H CH3(CH2CH@CH)3(CH2)7CO2H CH3(CH2)3(CH2CH@CH)4(CH2)3CO2H CH3(CH2)11CH@CH(CH2)7CO2H CH3(CH2)7CH@CH(CH2)13COOH CH3(CH2)5CH@CH(CH2)9CO2H CH3(CH2)4CH@CH(CH2)10CO2H

16:1 D9 18:1 D9 18:2 D9,12 18:3 D9,12,15 20:4 D5,8,11,14 22:1 D9 24:1–D15 18:1–D11 18:1 D12

<0.5 19.0–30.0 44.0–62.0 4.0–11.0 <1.0 – – – –

<1.0 14.0–35.0 55.0–75.0 <0.3 <0.5 <0.5 <0.5 – –

– 4.5–29.1 4.5–16.3 12.7 – 1.2–16.3 – 1.4 –

– 3.1–5.9 2.9–6.5 – – – – – 80.5–91.0

Nomenclature indicates: [numbers of carbon: number of unsaturation–D(number

of the carbon with a double bond)

Thermal analysis techniques have also been applied in the analysis of physical and chemical properties of esters, so they play key roles for quality control of biodiesel. Techniques such as thermogravimetry (TG), derivative thermogravimetry (DTG) and differential scanning calorimetry (DSC) have been used to evaluate thermal stability and heat capacity of biodiesel [14,15]. In this paper a systematic study on the effect of several production protocols on conversion yield of different oils (soybean, sunflower, castor and forage turnip) into their respective biodiesel, using anhydrous ethanol, is presented. Moreover, a detailed investigation of the thermal behavior of the biodiesel samples is reported. 2. Experimental methods Ethyl esters were prepared using refined soybean oil (Soya brand), refined sunflower oil (Salada brand), crude castor oil which was purchased from local traders, and crude forage turnip oil. The oil content in forage turnip is about 35% and as far as we know it is not commercially available so that it was obtained by cold extrusion of seeds provided by the MS Foundation [16]. The ethyl esters were produced by adapting the methodology proposed by Sanli and Canakci [5]. The production protocols of

].

the biodiesel samples, shown in Table 2, were found for the soybean and sunflower oil and then, they were taken as reference to find the protocols for the castor and furnage turnip oil. The reactions were performed under constant agitation in an Erlenmeyer flask. An ethanol to oil ratio of 6:1 (w/w) was used in all experiments. Potassium hydroxide was chosen as alkaline catalyst because it has a good solubility in ethanol under stirring at room temperature [5]. The alkoxide was first prepared by dissolving the catalyst in ethanol. The reaction time was recorded from the addition of the oil to the alkoxide. The mixtures with different KOH concentrations were stirred between 20 and 240 min either at room temperature or 50 °C (Table 2). After stirring, the solutions were allowed to stand in separating funnels and two phases were observed, one containing mainly ethyl esters and other consisting of glycerin. To eliminate the excess of ethanol in the phase rich in ethyl esters, it was heated at 60 °C and stirred at 60 rpm during 30 min. Then, the samples were again placed in separating funnels and washed three to five times with distilled water 3:1 (v/v) at room temperature and intervals of 20 min. To better evaluate the conversion of the vegetable oils into the respective ethyl esters, the transesterification reactions were monitored by TLC [3,9,13]. As stationary phase were used plates of sil-

Table 2 Conversion yield and acid values of the esters obtained by means of transesterification reaction under different protocols and 6:1 ethanol to oil (w/w). Biodiesel

Sunflower

Soybean

Sample

KOH

Reaction

Yield

Acid value

% (w/w)

Time (min)

Temperature (°C)

(%)

(mg KOH/g)

1 2 3 4 5 6 7 8

0.5 0.5 1 1 1 1 1.5 2

30 30 20 80 80 240 (4 h) 30 80

Room temperature 50 50 Room temperature 50 50 Room temperature 50

– – 92 86.5 90 98.7 85.5 93.4

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

9 10 11 12 13 14 15 16

0.5 0.5 1 1 1 1 1.5 2

30 30 20 80 80 240 (4 h) 30 80

Room temperature 50 50 Room temperature 50 50 Room temperature 50

– – 93 87 92.7 100 84.5 94

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

Forage turnip

17

1

80

50

Castor

18 19 20

1 1 1

80 90 240 (4 h)

55 40 Room temperature

70 – – –

0.19 – – –

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ica gel ALUGRAM Sil G, from Macherey–Nagel, and as mobile phase was used a mixture of eluent: hexane, glacial acetic acid, and ethyl acetate at a ratio of 9.5:0.3:0.2 mL, respectively. The determination of acid value of the esters was carried out in triplicate following the method reported by Sharma et al. [1]. The thermogravimetric (TG/DTG) and calorimetric (DSC) curves were obtained in a Shimadzu TA-50 with heating rate of 20 °C/min and temperature range of 23–650 °C. All measurements were carried out using an uncovered platinum crucible containing a sample mass of about 8.6 mg, in synthetic air atmosphere and flow of 100 mL/min. It is worth to point out that these parameters are similar to the ones reported in Ref. [17].

3. Results and discussion The results show that the alkaline transesterification reaction of soybean, sunflower, and forage turnip oil strongly depends on catalyst content as well as on reaction temperature and stirring time. The transesterification reactions of soybean oil and sunflower using 0.5% (w/w) of KOH did not allow the separation of glycerin, even heating the solution at 50 °C and stirring (experiment nos. 1 and 2, 9 and 10 – Table 2). This proportion of catalyst may have increased the content of mono and diglycerides in the solutions, which act as emulsifying agents preventing phase separation [18]. In the experiment nos. 3–8 and 11–16 (Table 2) with soybean and sunflower oil using between 1% and 2% (w/w) of KOH, the conversion yields were between 84.5% and 100%. The obtained yields are promising because it has been reported that transesterification of vegetable oils using ethanol are challenging [5]. The increase of KOH content became the mixture darker, the glycerin visually thicker and the wash water more turbid, thick and milky in appearance, indicating saponification. These features suggest the degradation of components of the fatty acids [19]. The stirring time also plays an important role on the conversion of the oils into esters. After 20 min of agitation (experiment nos. 3 and 11 – Table 2), even reaching a conversion of 92%, it was detected by TLC the presence of contaminants likely mono and/or diglycerides. Based on that, the stirring time was extended to 80 and 240 min in order to obtain biodiesel samples with higher purity. The retention factor (Rf’s) of the samples prepared under 80 min of stirring or longer was about 0.5 cm, higher than the 0.3 cm on the chromatographic plate of crude oils, confirming the efficiency of conversion of the fatty acids into ethyl esters. The transesterification reactions of soybean and sunflower oil at times longer than 80 min of agitation at room temperature (experiments nos. 4 and 12, Table 2) led to reasonable conversion yields, nevertheless when the solution was heated to 50 °C (experiments nos. 5, 6, 8, 13, 14, and 16 – Table 2), the conversion yields were over 90%.

0.03

139.8 278.5 603.5

0.28 89.66 6.26

Sunflower

31.3 176.2 280.8 28.0 196.8 303.9 355.8

176.2 280.8 602.8 196.8 303.9 355.8 602.8

3.35 87.51 6.51 4.26 86.70 4.05 1.46

Forage turnip

0.03

200

300

400

500

Temperature (°C)

600

-0.06 700

80

Weight (%)

0.03

100

Sunflower

3.0 3.1

Fornage Turnip

80 0.00

60 40

-0.03

0.00 60 40

-0.03

20

0 0

100

200

300

400

500

Temperature (°C)

600

-0.06 700

0 0

100

200

300

400

500

Temperature (°C)

Fig. 1. TG/DTG curves of soybean, sunflower, and forage turnip biodiesel in synthetic air atmosphere.

600

-0.06 700

o

100

4.7

Final

23.3 139.8 278.5

dm/ dT(%/ C)

0

Residue (%)

Initial

20

0

Weight loss (%)

Soybean

o

20

Temperature (°C)

dm/ dT(%/ C)

-0.03

o

40

dm/ dT(%/ C)

Weight (%)

80 60

Biodiesel

100

Soybean

0.00

Table 3 Temperature range, weight losses, and residues of the thermal decomposition of soybean, sunflower, and forage turnip biodiesel.

Weight (%)

100

The acidity of the esters obtained from soybean, sunflower and forage turnip oil did not exceed 0.19 mg KOH/g as reported in Table 2, which is the standard established by the National Agency of Petroleum, Natural Gas and Biofuels (ANP) [11]. These findings reveal that the production protocols allow the production of biodiesel with low concentration of free fatty acids. The oil conversion into esters also depends on the composition of the fatty acids in the feedstock. The high concentrations and structural similarity of the oleic and linoleic acid present in soybean and sunflower oil (Table 1) should lead to similar conversion yields for these oils. In turn, despite the forage turnip oil also has significant concentrations of oleic and linoleic acids in its constitution, it exhibits a lower conversion yield as compared to the soybean and sunflower oil. This result might be attributed to a lower concentration of linoleic acid as compared to the one found in the soybean and sunflower oil (Table 1) or to the use of crude (unrefined) forage turnip oil. The refining process of the crude oil, which eliminates compounds such as colloidal substances, proteins, dyes, hydrocarbons, inorganic molecules, and free fatty acids, should improve the oil conversion into biodiesel. The ricinoleic acid, the main compound of the castor oil (Table 1), was not converted into esters, regardless of its good solubility in ethanol. This finding may be attributed to high stability and viscosity of this oil due to the double bond at carbon 9 as well as a hydroxyl at carbon 12 (Table 1) [6,10,20]. On other hand, this behavior might also be related to high acid value of the castor oil which may lead to the neutralization of part of the catalyst, and consequently reducing the ethoxide content available to assist the transesterification reaction [21,22]. TG/DTG curves of soybean, sunflower, and forage turnip biodiesel are shown in Fig. 1. The data are summarized in Table 3 and reveal at least three sequential steps of weight loss at temperature range of 27–650 °C, indicating that biodiesel samples underwent an oxidative thermal decomposition with the elimination of different fractions of molecules in each step. A slow weight loss is ob-

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2

6

Exo up

Exo up

Sunflower Heat Flow (W)

Heat Flow (W)

4

2

Soybean

1

0

-1

0

-2

-2 0

100

200

300

400

500

600

700

0

100

o

200

300

400

500

600

700

o

Temperature ( C)

Temperature ( C)

Fig. 2. DSC curves of soybean and sunflower biodiesel in synthetic air atmosphere.

Table 4 Temperatures and heat flow obtained from DSC curves for soybean and sunflower biodiesel. Biodiesel

Temperature (°C)

Heat flow (J/g)

Ti

Tp

Soybean

178.2 305.0 400.2

200.0 357.8 433.5

44.88 103.85 63.62

Sunflower

181.8 283.8 401.8

202.6 354.1 435.9

65.28 419.63 201.76

Ti: Initial temperature; Tp: Peak maximum temperature.

served at the beginning of the TG/DTG curves of sunflower and forage turnip biodiesel, which might be associated to evaporation of remaining water molecules or even residue of ethanol. Then a second weight loss, faster than the first one, starting at temperatures between 139 °C and 196 °C and average loss of 88% occurred in all biodiesel samples. This effect is attributed to thermal decomposition of the pure esters [17,20]. Additionally, the curves show the presence of residues with weights ranging between 3.0% and 3.7%, perhaps leftovers of catalyst used in the synthesis of the biodiesels. The thermal decomposition in steps might affect the engine performance because it is wanted fast and spontaneous explosions with complete burning of the fuel at a single time under air atmosphere, so traces of mono and/or diglyceride as well as residues induce multiple burning [23]. DSC curves of soybean and sunflower biodiesel show successive endo and exothermic peaks, as shown in Fig 2. The results are in accordance with the weight losses observed in the TG/DTG curves and corroborate the statement that oxidative thermal decompositions took place, leading the removal of different molecules in each step. The DSC curve of sunflower biodiesel show initially an exothermic peak followed by an endothermic peak at temperature of about 178 °C and then two exothermal ending the decomposition process. In turn, the DSC curve for soybean biodiesel exhibit an endothermic peak starting at around 181 °C followed by exothermal similar to ones observed in sunflower biodiesel, although at different temperatures and heat flows, as indicated in Table 4. Finally, Table 4 shows that energies of approximately 420 J/g and 202 J/g were released by burning sunflower biodiesel at temperature ranges of 283–389 and 402–460 °C, respectively. Such energies are significantly higher than ones released by the soybean biodiesel. 4. Conclusions In this work, alkaline ethanolysis was used for biodiesel production from different vegetable oils. Although it has not been widely used in industrial processes, it leads to good conversion of the fatty

acids of soybean and sunflower oils into esters. The crude forage turnip oil does not exhibit high oil conversion rate, however it is expected that its refining could contribute for the increase of the conversion yield. The acid values of the biodiesel samples meet standards established by the National Agency of Petroleum, Natural Gas and Biofuels (ANP). An efficient transesterification reaction of vegetable oil into biodiesel is important to assure the quality of the fuel. In fact, the injection of esters occur when the combustion chamber is already at 600 °C or 800 °C, hence according to the data obtained, a thermal decomposition of the biodiesel shall take place significantly faster, and sunflower biodiesel should release the largest amount of energy. Finally, residues of water, ethanol, and catalyst suggest that the procedures currently used for obtaining biodiesel might be improved. Acknowledgments Financial support from CNPq and FUNDECT are gratefully acknowledged. We also thank Prof. Dr. Luiz H. Viana and Profa. Dra. Simone P. Favaro for allowing us to work in their laboratories at the earliest stage of this research as well as MS Foundation for providing crude forage turnip oil. References [1] Sharma YC, Singh B, Upadhyay SN. Advancements in development and characterization of biodiesel: a review. Fuel 2008;87:2355–73. [2] Meher LC, Dharmagadda VSS, Naik SN. Optimization of alkali-catalyzed transesterification of Pongamia pinnata oil for production of biodiesel. Bioresour Technol 2006;97:1392–7. [3] Ubhayasekera SJKA, Dutta PC. Sterols and oxidized sterols in feed ingredients obtained from chemical and physical refining processes of fats and oils. J Am Oil Chem Soc 2009;86:595–604. [4] Cerqueira Leite RC de, Leal MRLV. O biocombustível no Brasil. Novos Estud. – CEBRAP [online] 2007;78:15–21. [5] Sanli H, Canakci M. Effects of different alcohol and catalyst usage on biodiesel production from different vegetable oils. Energy Fuels 2008;22:2713–9. [6] Suarez PAZ, Meneghetti SMP, Meneghetti MR, Wolf CR. Transformation of triglycerides into fuels, polymers and chemicals: some applications of catalysis in oleochemistry. Quím Nova 2007;30:667–76. [7] Neto PRC, Rossi L, Zagonel GF, Ramos LP. The utilization of used frying oil for the production of biodiesel. Quím Nova 2000;23:531–7. [8] Schuchardt ULF, Sercheli R, Vargas RM. Transesterification of vegetable oils: a review. J Braz Chem Soc 1998;9:199–210. [9] Ferrari RA, Oliveira VS, Scabio A. Biodiesel de soja–taxa de conversão em ésteres etílicos, caracterização físicoquímica e consumo gerador de energia. Quim Nova 2005;28:19–23. [10] Conceição MM, Candeia RA, Silva FC, Bezerra AF, Fernandes V, Souza AG. Thermoanalytical characterization of castor oil biodiesel. Renew Sust En Rev 2007;11:964–75. [11] National Agency of Petroleum, Natural Gas and Biofuels ANP) – Regulations. http://nxt.anp.gov.br/NXT/gateway.dll/leg/resolucoes_anp/2008/ mar%C3%A7o/ranp%207%20-%202008.xml?f=templates$fn=documentframe.htm$3.0$q=$x=$nc=6637. [accessed 01.29.10]. [12] Dorado MP, Ballesteros E, Arnal JM, Gómez J, López Gimenez FJ. Testing waste olive oil methyl ester as a fuel in a diesel engine. Em & Fuels 2003;17:1560–5.

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