Continuous production of biodiesel using supercritical fluids: A comparative study between methanol and ethanol

Continuous production of biodiesel using supercritical fluids: A comparative study between methanol and ethanol

Fuel Processing Technology 102 (2012) 110–115 Contents lists available at SciVerse ScienceDirect Fuel Processing Technology journal homepage: www.el...

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Fuel Processing Technology 102 (2012) 110–115

Contents lists available at SciVerse ScienceDirect

Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc

Continuous production of biodiesel using supercritical fluids: A comparative study between methanol and ethanol Aline Santana a, José Maçaira b, M. Angeles Larrayoz a,⁎ a b

Department of Chemical Engineering, ETSEIB, Universitat Politècnica de Catalunya, 08028, Barcelona, Spain LEPAE, Department of Chemical Engineering, University Porto — Faculty of Engineering, Rua Dr. Roberto Frias, s/n 4200‐465 Porto, Portugal

a r t i c l e

i n f o

a b s t r a c t

Article history: Received 17 February 2012 Received in revised form 13 April 2012 Accepted 17 April 2012 Available online 18 May 2012 Keywords: Biodiesel Supercritical fluids Ethyl esters Vegetable oil Kinetic equations Solid acid catalyst

Biodiesel was produced from vegetable oil (triglycerides) by transesterification with supercritical ethanol and carbon dioxide as cosolvent in the presence of solid acid catalyst. The objective of this work was to evaluate transesterification kinetics for biodiesel production from vegetable oil under supercritical conditions. Experimental investigation was carried out with vegetable oil and ethanol at molar ratio of 1:25, temperature between 150 and 200 °C, reaction time from 2 to 10 min and pressure around 200 bar in a continuous reactor. The biodiesel products were analyzed by gas chromatography. The effects of methanol to ethanol to temperature and reaction time towards biodiesel yield are discussed in detail. From this study, it was found than an optimum biodiesel yield of 80% can be attained at a relatively short reaction time around 6 min using supercritical condition with ethanol and carbon dioxide as cosolvent. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Biodiesel is a renewable and environmentally friendly energy that can be produced from a range of organic feedstock including fresh or waste vegetable oils, animal fats, and oilseed plants. It is also an important substitute for petroleum diesel. The resulting biodiesel is quite similar to conventional diesel fuel in terms of its main characteristics, unlike petrodiesel, it is biodegradable and does not contribute to global warming due to the closed carbon cycle [1–4]. Biodiesel is produced by the transesterification of triglycerides (TG) (usually vegetable oils). TG reacts with an alcohol in the presence of a strong acid or base, producing a mixture of fatty acids alkyl esters and glycerol [5,6]. Chemically, biodiesel is called a methyl ester if the alcohol used is methanol. If ethanol is used, it is called an ethyl ester. The overall process is a sequence of three consecutive and reversible reactions, in which di‐ and monoglycerides (DG and MG) are formed as intermediates [6]. The stoichiometric reaction requires 1 mol of a triglyceride and 3 mol of the alcohol and glycerol (G) is the side product. The three reactions are consecutive and reversible. However, an excess of the alcohol is used to increase the yields of the alkyl esters and to allow its phase separation from the glycerol formed. The transesterification global reaction can be represented by Eq. (1). catalyst



TriglycerideðTGÞ þ 3 Alcohol ↔ RCOOR þ glycerol ⁎ Corresponding author. E-mail address: [email protected] (M.A. Larrayoz). 0378-3820/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2012.04.014

ð1Þ

Transesterification consists of a sequence of three consecutive reversible reactions. The first step is the conversion of triglycerides to diglycerides, followed by the conversion of diglycerides to monoglycerides, and finally monoglycerides into glycerol, yielding one ester molecule from each glyceride at each step. The reactions are reversible, although the equilibrium lies towards the production of fatty acid esters and glycerol [7–10]. The actual mechanism of the transesterification reaction consists of sets of equilibrium reactions in series and all of the reactions are reversible [6,11,12]. Alcohol provides the alkyl group that substitutes the fatty fraction of triglyceride. Shortchain alcohols such as methanol, ethanol, and butanol are the most frequently employed. The selection of the alcohol is based on cost and performance consideration. From an environmental point of view, ethyl ester utilization is also more advantageous than the utilization of methyl esters. Ethanol can be produced from agricultural renewable resources, thereby attaining total independence from petroleum-based alcohols. Also, ethanol, as an extraction solvent, is preferable to methanol because of its much higher dissolving power for oils. For this cause, ethanol is sometimes used as a suitable alcohol for the transesterification of vegetables oils. The fuel qualities of alkyl esters have received varying evaluations in terms of alcohol used. Huber et al. [13] and Saraf and Thomas [14] commented that higher or branched alcohols can produce biodiesel with better fuel characteristics. In contrast, Tyson [15] reported that methyl ester and ethyl ester are similar in heat content, but ethyl ester formed by transesterification reaction is slightly less viscous than methyl ester. Therefore, producing ethyl esters rather than methyl esters is of considerable interest, because, in addition to the entirely agricultural nature of the

A. Santana et al. / Fuel Processing Technology 102 (2012) 110–115

ethanol, the extra carbon atom provided by the ethanol molecule slightly increases the heat content and the cetane number [16]. The reaction can be alkali catalyzed, acid catalyzed or enzyme catalyzed and chemically catalyzed processes have proved to be more practical because of the short reaction times and low cost compared with enzyme catalyzed [17]. Base-catalyzed transesterification involves stripping the glycerin from the fatty acids with a catalyst such as sodium or potassium hydroxide, and replacing it with an anhydrous alcohol, usually methanol. The resulting raw product is then centrifuged and washed with water to cleanse it of impurities. This yields methyl or ethyl ester (biodiesel), as well as a smaller amount of glycerol, a valuable by-product used in making soaps, cosmetics, and numerous other products [18]. An alternative method for the production of biodiesel is to use heterogeneous (solid) catalysts in the transesterification process. Heterogeneous (solid) catalysts have the general advantage of easy separation from the reaction medium and reusability. Heterogeneous catalysis is thus considered to be a green process. The process requires neither catalyst recovery nor aqueous treatment steps: the purification steps of products are then much more simplified and very high yields of methyl esters, close to the theoretical value, are obtained [19]. Glycerin is directly produced with high purity levels (at least 98%) and is exempt from any salt contaminants [20,21]. Supercritical fluid (SCF) has received a special attention as a new reaction filed due to its unique properties [22–25]. Supercritical alcohol can form a single phase in contrast to the two phase nature of oil/ alcohol mixture at ambient condition. This is due to a decrease in dielectric constant of alcohol at supercritical state. Our research group has been using methanol supercritical (SCM) and carbon dioxide (CO2) as cosolvent [26]. The addition of cosolvent in combination with supercritical conditions seems to be an efficient means to reduce significantly the operating conditions. Just a few works are available in the open literature regarding the use of cosolvents in the supercritical transesterification, such as the use CO2 [26–30]. With these considerations, and as a continuation of previous works [26], we carried out a study on the transesterification process of vegetable–sunflower-based oil utilizing supercritical ethanol (SCE) and carbon dioxide as cosolvent with solid acid catalyst, in order to characterize the ethyl esters obtained for their applications as fuels in internal combustion engines. The aim of this work was to experimentally investigate how the temperature and reaction time affect on biodiesel

111

yield vegetable oil supercritical methanol (SCM) and ethanol (SCE) conditions using CO2 as cosolvent. 2. Experimental section 2.1. Material description The vegetable–sunflower-based oil (S5007) used in the experiments was from Sigma Aldrich (Barcelona, Spain). The mixture ethanol/CO2 1:3 molar ratio was supplied by Abello Linde S. A. (Barcelona, Spain). The solvents, standards and reagents used in the derivatization step required for the chromatography analysis were supplied by Sigma Aldrich. A commercial catalyst (Nafion® SAC-13) was purchased from Sigma Aldrich. 2.2. Supercritical ethanol transesterification method The continuous flow experimental set up is shown in Fig. 1. The catalytic supercritical transesterification was carried out in continuous mode in a fixed bed titanium reactor (152 mm of length and internal diameter of 15.5 mm) which can sustain high temperature and pressure needed in supercritical treatment as reported by Maçaira et al. [26]. The mixture of ethanol and CO2 is liquefied in a refrigerator containing ethylene glycol, before being pumped with a diaphragm pump (Dosapro Milton Roy, maximum flow 4.17 L/h). The oil was pumped using a HPLC pump (Gilson 305 pump). The three fluids are mixed in a static mixer (Kenics® model 37-04-065, Chemineer, U.K.; 200 mm length and 2.5 mm inner diameter). The reactant mixture was preheated to the desired operating temperature before entering the reactor. The titanium reactor was heated by an electrical heating jacket and monitored by two thermocouples directly connected at the inlet and outlet of the reactor. The reaction temperature was controlled with a precision better than 5 °C. Pressure is measured in the inlet and outlet of the reactor, to check for pressure drops in the fixed bed reactor. Samples were collected periodically in a glass vial placed in the reactor outlet after reaching the steady state condition. The stationary states of the reactor were determined after 30 min and two samples were collected for each experiment and the average error was below 1%.

Fig. 1. Scheme of the experimental installation for the continuous production of biodiesel: 1 — synthetic oil; 2 — oil piston pump; 3 — line valve; 4 — check valve; 5 — CO2 bottle; 6 — line valve; 7 — check valve; 8 — CO2/ethanol mixture bottle; 9 — mixture line valve; 10 — mixture check valve; 11 — compressor; 12 — pressure regulator; 13 — line valve; 14 — cooling device; 15 — mixture pump; 16 — purge valve; 17 — pressure regulator; 18 — check valve; 19 — safety valve; 20 — static mixer; 21 — pre-heater; 22 — rupture disk; 23 — fixed bed reactor; 24 — expansion needle-valve; 25 — sample collector; 26 — alcohol collector; 27 — flowmeter; 28 — line valve.

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2.3. Analytical methods

written as described in Eqs. (6)–(11).

The amount of ethyl ester was determined by a Shimadzu 2010 gas chromatography (GC). The GC was equipped with a FID detector and a capillary column (Teknokroma SupraWax-280 with dimensions 30 m × 0.32 mm × 0.25 μm). Helium was used as carrier gas with the initial oven temperature at 120 °C held for 1 min and increased to 250 °C, hold 11 min at 10 °C/min. The injection and detector temperatures were 250 °C with a split ratio of 1:10. Methyl heptadecanoate was used as internal standard. Compounds were quantified upon analysis following the standard UNE-EN 14103 [31]. The free and total glycerol and mono-, di- and triglycerides quantitation was performed in a Shimadzu 2010 gas chromatography that was equipped with a cold on-column injection and a Teknokroma TRB-Biodiesel fused silica column with an internal diameter of 0.32 mm and 10 m of length with helium as the carrier gas. This method was based on European standard EN 14105 [32].

d½TG ¼ −k1 ½TG½A þ k2 ½DG½E dt

ð6Þ

d½DG ¼ k1 ½TG½A−k2 ½DG½E−k3 ½DG½A þ k4 ½MG½E dt

ð7Þ

d½MG ¼ k3 ½DG½A−k4 ½MG½E−k5 ½MG½A þ k6 ½GL½E dt

ð8Þ

d½GL ¼ k5 ½MG½A−k6 ½GL½E dt

ð9Þ

dðEÞ ¼ k1 ½TG½A−k2 ½DG½E þ k3 ½DG½A−k4 ½MG½E dt þ k5 ½MG½A−k6 ½GL½E

ð10Þ

d½A −d½E ¼ dt dt

3. Kinetics model Several kinetic mechanisms have been proposed by different researchers for the transesterification of vegetable oil in literature; pseudo-first order and second order [6], second order [11] and pseudo-second order [33]. This kinetics study was based on our previous work [26]. Transesterification of vegetable oils with alcohol follow a multiple scheme, consisting of a number of consecutive and reversible reactions. TG is converted stepwise to DG, MG, E and finally glycerol (GL) as described in the following equations. k1

TG þ A ⇄ DG þ E

where k1 to k6 are reaction rate constants; TG, DG MG, GL, A, and E are the concentrations in weight percent of TG, DG, MG, GL, alcohol, and esters in a reaction mixture. In our kinetic studies, MATLAB was used to solve the ordinary equation system and simultaneously fitting the kinetic model to the experimental results. 4. Results and discussions The selection of these experimental conditions was based on previous experimental results by Maçaira et al. [26], which showed that, under selected parameters, it was possible to achieve a high conversion of vegetable–sunflower-based oil to biodiesel with free glycerol content below than established in the European quality standard for commercial biodiesel, EN14214 [34]. Table 1 shows the operating conditions in each run and the content (mass percentage) of each sample in terms of fatty acid ethyl esters (FAEEs).

ð2Þ

k2

k3

DG þ A ⇄ MG þ E

ð3Þ

k4

k5

MG þ A ⇄ GL þ E

4.1. Effect of temperature and reaction time

ð4Þ

k6

According to previous study, Maçaira et al. [26] reported experiments under SCM and carbon dioxide as cosolvent, at temperature 200 °C and the reaction time of 2 min, the yield of fatty methyl ester (FAME) was around of 90%. In this study, the effect of temperature and reaction time on the conversion of oil to ethyl esters was investigated. The oil to ethanol molar ratio was kept 1:25, catalyst mass of 9 g and the pressure fixed at 200 bar, varying the temperature from 150 to 200 °C and reaction time from 2 to 10 min. Fig. 2 shows the effect of reaction time on the FAEE yield in our experiments. When the reaction temperature was higher than 180 °C,

The overall reaction is given as catalyst

TG þ 3A ⇄ 3E þ GL

ð11Þ

ð5Þ

where, TG is triglyceride, DG is diglyceride, MG is monoglyceride, A is ethanol and E is ethyl ester. The governing set of differential equations characterizing the stepwise reactions involved in the trasesterification triglyceride can be

Table 1 Experimental conditions of transesterification experiments and results (weight and composition of obtained ester and glycerol phase). Temperatura

Space time (min)

Solvent (g/min)

oil (ml/min)

MG (wt.%)

DG (wt.%)

TG (wt.%)

FAEEs (wt.%)

total glycerol (wt.%)

150

2.2 4.5 6.5 8.8 2.2 4.5 6.5 8.8 2.2 4.5 6.5 8.8

3.6 2.02 1.28 0.96 3.6 2.02 1.28 0.96 3.6 2.02 1.28 0.96

0.242 0.131 0.089 0.065 0.242 0.131 0.089 0.065 0.242 0.131 0.089 0.065

0.8 1.4 0.9 1.6 3.1 3.4 2.7 2.4 2.9 2.2 1.4 1.2

9.6 7.4 7.1 6.2 11.2 8.5 7.8 6.7 8.4 6.2 4.9 3.9

42.8 34.6 30.4 23.7 20.4 18.9 15.8 14.7 17.5 16.7 14.3 11.8

48.5 55.8 59.4 62.5 58.6 62.5 66.9 69.4 67.9 69.7 78.9 79.5

5.9 5.1 4.4 3.8 4.5 4.1 3.5 3.2 3.7 3.2 2.6 2.05

180

200

Relative standard deviation for MG, DG, TG, FAEEs and glycerol for each process was less than 1%.

A. Santana et al. / Fuel Processing Technology 102 (2012) 110–115

80

is a three step series of reversible reactions, so the conversion of triglycerides is not equal to the FAEE content in each sample, due to the formation of intermediate products. As indicated in Fig. 3, the supercritical transesterification reaction at 200 °C and 200 bar produced about 80% FAEE, with 90% of triglyceride converted to FAEE. The triglycerides concentration decreased as the reaction proceeded. The concentration of diglycerides and monoglycerides increased until approximately 6 min of the reaction before decreasing and finally reaching equilibrium.

70

FAEE yield / wt %

113

60 50 40 30 20

4.2. Kinetics study 10 0 0

2

4

6

8

10

Space Time / min 150ºC

180ºC

200ºC

Fig. 2. Variations of FAEE yield with time at different temperatures. Pressure = 200 bar, catalyst mass = 9 g mass and ethanol/oil molar ratio = 25.

the FAEE yield increased with the increase of reaction time. The highest yield was around 80% in 200 °C with a space time of 4 min. It was clear that the conversion increased with the residence time inside the reactor, and the higher the temperature, the higher the conversion. In the initial stages of the reaction, production of ethyl esters was rapid, and the rate diminished and finally reached equilibrium in about 6 min. Silva et al. [35] reported esters yield of 80 wt.% for the continuous mode transesterification of soybean oil in supercritical ethanol at 350 °C using an oil to ethanol molar ratio of 1:40 and with a reaction time of 15 min. Vieitez et al. [36] studied the supercritical ethanolysis of castor oil in a batch-type tube reactor in supercritical conditions at 300 °C 200 bar and 20 min of reaction, the ethyl ester yield achieved a maximum ester content of 74.2 wt.%. Gui et al. [37] reported that ester yields for the batch-mode transesterification of palm oil in supercritical ethanol using oil to ethanol of 1:33 at 350 °C was 79.2 wt.% and reaction time of 29 min. In this study the reactions were performed at lower temperatures and reaction time due to addition of a cosolvent (CO2), which allows a reduction in operating conditions in order to improve the system. The yields of ethyl esters obtained in this study with a shorter reaction time were similar than the conversions reported in these references. The increase in ethyl ester concentration was followed by an increase of glycerol, it was liberated from the triglyceride molecules. This is because the transesterification of vegetable oil into ethyl esters

The kinetics of heterogeneously catalyzed ethanolysis has been rarely studied. Li et al. [38] reported the first-order kinetics with respect to all acyglycerols at the temperature range of 140–200 °C. The variation with reaction time of triglycerides and the consequent transesterification products, monoglyceride, diglyceride, glycerol, ethyl ester and ethanol was determined for transesterification reaction, as shown in Fig. 4. The triglyceride initial concentration continuously decreases, and at the end of the process is approached value close to zero, while mono- and diglyceride increased. The diglyceride concentration increased rapidly during the first reaction minute reaching a maximum and after that slowly decreased, however it did not reach zero at the end of the trasesterification process. The monoglyceride concentration slowly increased during the process and its concentration was not completed at the end of the reaction time. The accumulation of glycerol, which is a by‐product of the transesterification reaction, is directly related to the advancement to the reaction. The free glycerol content in the biodiesel samples was performed by GC-on-column injector, following the EN14105 [32]. The purification process was not required due to the free glycerol content in all of the experiments was below the imposed maximum limit of 0.020% for the free glycerol, following the EN14124 [36]. As shown in Fig. 4, the model prediction and experimental data presented in this study exhibit some deviation for the transesterification profile. The deviation between model and experimental data of glycerol and FAEEs were more significant, but this fact did not change the overall good prediction of the model for these operation conditions. 4.2.1. Rate constants The effect of reaction temperature on the kinetics of transesterification reactions of vegetable oil to ethyl esters was carried out at a 7.5 7

Concentration / mol.dm-3

100 90 80

yield / wt %

70 60 50 40 30 20 10

1.2 6.5 1.0 6

0.8

5.5

0.6 0.4

5

0.2

4.5 4

0.0 0

0 2.2

4.5

6.5

8.8

Space Time / min TG conversion

Ethanol concentration / mol.dm-3

1.4

2

4

6

8

Space Time / min Monoglycerides

Diglycerides

Triglycerides

FAEEs

Glycerol

Ethanol

FAEE

Fig. 3. Comparison between the triglyceride conversion and the FAEEs content in each sample obtained at 200 °C, 200 bar, catalyst mass of 9 g, and ethanol/oil ratio of 25.

Fig. 4. Content of ethyl esters, triglycerides, diglycerides, monoglycerides, glycerol and ethanol in the products obtained at 180 °C, 200 bar, catalyst mass of 9 g, and ethanol/ oil ratio of 25.

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Table 2 Reaction rate constants. Temperature (°C) 150 180 200

Table 4 Critical data for pure components.

Rate Constants (L mol− 1 min− 1) k1

k2

k3

k4

k5

k6

0.134 2.121 2.698

5.310 33.971 101.640

0.324 1.323 5.400

10.935 33.105 63.584

4.090 47.705 177.230

3.905 79.141 243.930

constant pressure of 200 bar and at different temperatures range 150–200 °C. Based on the values biodiesel yield and regarding them a as the function of reaction temperature and time (see Fig. 2), the corresponding rate constants were calculated. The apparent rate constants different reaction temperatures are listed in Table 2. Reaction rates almost always increase with temperature for elementary irreversible reactions but multiple and reversible reactions occasionally exhibit an optimal temperature with respect to the yield of a desired product. All the reaction rate constants increased with temperature. The reverse reaction of transformation of monoglycerides and FAEEs into triglycerides and alcohol was the less affected by temperature, being almost insensitive to temperature. The same reaction but in the forward direction (diglyceride to monoglyceride) was enhanced by temperature as was verified to all the other reactions (Table 2). 4.2.2. Energy of activation With the apparent rate constants at different temperatures determined, the apparent activation energy for the transesterification reaction was calculated through the Arrhenius formula (Eq. (12)).   E 0 k ¼ k exp a RT

ð12Þ

where k is the reaction rate constant, k 0 is the factor of frequency, Ea is the energy of activation, R is the universal molar gas constant, and T is the temperature. Table 2 shows the values for the rate constant (k) obtained at each temperature (see Fig. 2). Expectedly, the increase in the reaction temperature has caused a corresponding improvement on the reaction rate, especially at range of 180–200 °C. The activation energies are illustrated in Table 3. Activation energies between 58.63 and 139.95 kJ/mol were determined (see Table 3). Disakou et al. [39] calculated the activation energy for the transesterification of soybean oil at 220–235 °C. Activation energy was about 117 kJ/mol for the transformation of TG to DG and 128 kJ/mol for that of DG to MG. Song et al. [40] investigated the transesterification of RBD palm oil at temperature of 200–400 °C and the activation energy was 105 kJ/mol. Silva et al. [35] reported the activation energies of 92.9 to 78.7 kJ/mol for the transesterifiction of soybean oil in compressed ethanol. These studies were close to the value in this work. This supercritical study was carried out with catalyst while the other studies of supercritical transesterification presented in the literature are carried out without catalyst. Catalytic

Components

Molecular weight (kg kmol− 1)

Critical temperature, Tc (°C)

Critical pressure, Pc (bar)

Acentric factor, ω

Reference

Ethanol Methanol CO2

46 32 44

241 240 31

61 80 74

0.644 0.566 0.225

[41,42] [41,42] [41,43]

reactions have a lower rate-limiting free energy of activation than corresponding uncatalyzed reaction, resulting in higher reaction rate at same temperature. 4.3. Comparison of supercritical ethanolysis versus methanolysis using carbon dioxide as cosolvent In the previous investigation of the process used for the biodiesel production by methanolysis using CO2 as cosolvent under supercritical condions and SAC-13 as catalyst was analyzed [26]. One of the objectives in this paper is investigates and comparison of the reaction performance of vegetable–sunflower-based oil transesterification under SCM and SCE. The critical properties and other parameters of the pure components are listed in Table 4. To ensure that supercritical alcohol conditions were reached, the operating pressure must be higher than the critical pressure of methanol and ethanol which are 250 bar and 200 bar respectively. Methanol is the most used alcohol in the biodiesel production due to its suitable physical–chemical properties, low cost and easy phase separation. Although ethanol is currently more expensive, its advantages are much superior because of its dissolving power in vegetable oils and low toxicity compared to methanol. The greatest number of vegetable oil ethanolysis researches was done in the South America countries, especially in Brazil, which is one of the greatest producers of ethanol from biomass in the world. The transesterification using ethanol as solvent was carried out at the same operating conditions of the previous work using methanol [26] for the comparison purposes. Table 5 shows the experiments using methanol and ethanol in the supercritical transesterification. Good yields, higher than 90 and 80% were obtained using methanol and ethanol, respectively. The lower yield values in the case of FAEE can be attributed to the problems in the purification step due to the higher inter-solubility of the mixture. The type of alcohol employed to conduct the reactions can also affect the activation energy [29]. In fact, these authors found 35 and 55 kJ/mol for the catalytic alcoholysis of vegetable oil in SCM and SCE, respectively (see Table 5). The higher activation energy implies lower reaction rate. In this study, the activation energies observed for the transesterification using SCE are lower than our previous study [26], which was the transesterification using SCM. 5. Conclusion The supercritical transesterification of vegetable oil using ethanol and carbon dioxide as cosolvent with solid acid catalyst was conducted at temperatures of 150, 180 and 200 °C, reaction time from 2

Table 3 Activation energies and preexponential factors. Reaction direction

Rate constants (L/mol min)

Ea (kJ/mol)

k0 (L/mol.min)

R2

TG → DG DG → TG DG → MG MG → DG MG → GL GL → MG

k1 k2 k3 k4 k5 k6

104.82 98.30 91.69 58.63 125.99 139.95

28.0 29.61 24.85 19.06 37.25 41.25

0.91 0.99 0.98 0.99 0.99 0.98

Notes: Ea — activation energy; k0 — pre exponential constant; R2 — correlation coefficient.

Table 5 Optimum conditions and yields of SCM and SCE reactions.

Reaction time (min) Reaction temperature (°C) Pressure (bar) Yields (%) Ea (kJ/mol)

SCM

SCE

2 200 250 90 35

6 200 200 80 55

A. Santana et al. / Fuel Processing Technology 102 (2012) 110–115

to 9 min, oil to ethanol from 1:25 and pressure at 200 bar. The biodiesel production using these conditions obtained the highest yield at a temperature of 200 °C, reaction time of 4 min, a molar ratio of 1:25 and a pressure of 200 bar. Supercritical transesterification process has been shown to be able to produce biodiesel by using methanol and ethanol. Both alcohols using supercritical conditions obtained a high yield of biodiesel with a short reaction time compared to the conventional biodiesel process. The supercritical technology does not require the steps of purification and separation. Supercritical transesterification using ethanol and CO2 as cosolvent could reach almost the same conversion in a higher reaction time and with lower pressure compared with the SCM and CO2 as cosolvent media. Comparing the two methods (SCM and SCE) in term of optimum yield, the performance of SCM obtained higher ester contents than SCE with values of 90% and 80%, respectively, with a reaction time around 2 min for SCM and 6 min for SCE. The reaction rate of the both methods was 20 times faster than conventional process. Acknowledgments The authors would like to acknowledge Spanish Ministry of Science, Technology and Innovation (Grant no.. ENE 2009‐14502) for the financial support given. References [1] J. Sheehan, V. Cambresco, J. Duffield, M. Garboski, H.H. Shapouri, An overview of biodiesel and petroleum diesel life cycles, A report by US Department of Agrigulture and Energy, 1998, pp. 1–35. [2] G. Antolin, F. Tinaut, Y. Briceno, V. Castano, C. Perez, A. Ramirez, Optimization of biodiesel production by sunflower oil transesterification, Bioresource Technology 83 (2002) 111–114. [3] G. Vicent, M. Martinez, J. Aracil, Integrated biodiesel production: a comparison of different homogeneous catalysts systems, Journal of Bioresource Technology 92 (2004) 297–305. [4] N. Khan, M.A. Warith, G. Luk, A comparison of acute toxicity of biodiesel, biodiesel blends, and diesel on aquatic organisms, Journal of the Air & Waste Management Association 57 (2007) 286–296. [5] H.J. Wright, J.B. Segur, H.V. Clark, S.K. Coburn, E.E. Langdon, E.N. DuPuis, A report on ester interchange, Oil Soap 21 (1944) 145–148. [6] B. Freedman, R.O. Butterfield, E.H. Pryde, Transesterification kinetics of soybean oil, Jounal Americal Oil Chemists’ Society 63 (1986) 1375–1380. [7] Y. Zhang, M.A. Dubé, D.D. McLean, M. Kates, Biodiesel production from waste cooking oil: 1. Process design and technological assessment, Bioresource Technology 89 (2003) 1–16. [8] F. Ma, M.A. Hanna, Biodiesel production: a review, Bioresource Technology 70 (1999) 1–15. [9] L.C. Meher, D. Vidya Sagar, S.N. Naik, Technical aspects of biodiesel production by transesterification — a review, Renewable and Sustainable Energy Reviews 10 (2006) 248–268. [10] J. Van Gerpen, Biodiesel processing and production, Fuel Processing Technology 86 (2005) 1097–1107. [11] H. Noureddini, D. Zhu, Kinetics of transesterification of soybean oil, Applied Engineering in Agriculture 74 (1997) 1457–1463. [12] D. Darnoko, M. Cheryan, Kinetics of palm oil transesterification in a batch reactor, Journal of the American Oil Chemists' Society 77 (2000) 1263–1267. [13] G.W. Huber, S. Iborra, A. Corma, Synthesis of transportation fuels from biomass: chemistry, catalysts, and engineering, Chemical Review 106 (2006) 4044–4098. [14] S. Saraf, B. Thomas, Influence of feedstock and process chemistry on biodiesel quality, Process Safety and Environmental Protection 85 (2007) 360–364. [15] K.S. Tyson, Biodiesel Handling and Use Guidelines, National Renewable Energy Laboratory, NREL/TP-580-30004, Golden, September 2001. [16] G. Vicente, M. Martinez, J. Aracil, Optimisation of integrated biodiesel production. Part I. A study of the biodiesel purity and yield, Bioresource Technology 98 (2007) 1724–1733.

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