Bentonite-enhanced biodiesel production by NaOH-catalyzed transesterification: Process optimization and kinetics and thermodynamic analysis

Fuel xxx (2016) xxx–xxx

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Full Length Article

Bentonite-enhanced biodiesel production by NaOH-catalyzed transesterification: Process optimization and kinetics and thermodynamic analysis Lian Wu, Tengyou Wei, Zijun Lin, Yun Zou, Zhangfa Tong ⇑, Jianhua Sun Guangxi Colleges and Universities Key Laboratory of New Technology and Application in Resource Chemical Engineering, School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China

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


The use of bentonite produced an

important enhancement to the transesterification.
Response surface methodology was applied to optimize the process parameters.
The optimal conditions led to a high FAME yield of 98.56%.
Kinetics and thermodynamic parameters of the transesterification were determined.
The activation energy (Ea) was 31.03 kJ mol1, DG > 0, DH > 0 and DS < 0.

a r t i c l e

i n f o

Article history: Received 9 March 2016 Received in revised form 11 May 2016 Accepted 13 May 2016 Available online xxxx Keywords: Biodiesel Transesterification Bentonite Process optimization Kinetics and thermodynamic analysis

a b s t r a c t The use of bentonite as a water adsorbent to remove moisture from the biodiesel synthesis reaction system could produce an important enhancement to the transesterification process. The influence of concentration of NaOH, methanol/oil molar ratio, amount of bentonite, agitation rate, reaction temperature and reaction time was investigated. Based on the results of single factor experiments, response surface methodology central composite design (RSMCCD) was applied to optimize the process parameters of the bentonite-enhanced transesterification reaction. The maximum FAME yield of 98.56% was achieved under the optimized conditions of 7.47:1 methanol/oil molar ratio, 0.77 wt% concentration of NaOH, 1.76 wt% amount of bentonite and 57.8 °C reaction temperature. This study also examined the kinetics and evaluated the thermodynamic activation parameters for the bentonite-enhanced transesterification reaction, and the following results are obtained: the activation energy (Ea) was 31.03 kJ mol1, DG > 0, DH > 0 and DS < 0. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction

⇑ Corresponding author. E-mail address: [email protected] (Z. Tong).

The severe environmental problems associated with burning fossil fuels, especially the increase in global warming and air pollution, has prompted many researchers to investigate the possibility of using alternative sources of energy. Biodiesel, chemically

http://dx.doi.org/10.1016/j.fuel.2016.05.065 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Wu L et al. Bentonite-enhanced biodiesel production by NaOH-catalyzed transesterification: Process optimization and kinetics and thermodynamic analysis. Fuel (2016), http://dx.doi.org/10.1016/j.fuel.2016.05.065

2

L. Wu et al. / Fuel xxx (2016) xxx–xxx

defined as the alkyl monoesters of long chain fatty acids derived from renewable feedstocks, such as vegetable oils and animal fats [1,2], has attracted great interest as a replacement for petroleumbased diesel fuel that does not require major modifications, with only a small decrease in performance [3]. Biodiesel is better than traditional diesel in several respects, including faster biodegradability, renewability, lower toxicity, high cetane number, good lubricity and higher flash point [1,3,4]. In addition, expanded use of biodiesel would contribute to rural development, because it would increase the demand for vegetable oils, which would promote the development of agriculture associated with oil-bearing crops [5]. However, the high cost of biodiesel is a major impediment to its commercialization [6]. It has been found that feedstock alone represents 75% of the overall biodiesel production cost [5]. Therefore, selecting an inexpensive raw material source is important if biodiesel is to become economically viable. But even if a suitable feedstock is selected, other aspects of the production cost must be further reduced by improving the downstream production process in order to make biodiesel products more competitive relative to fossil fuels. In general, the synthesis of biodiesel is carried out by a transesterification reaction, and homogeneous base-catalyzed transesterification is the technique most often used for industrial production of biodiesel [7,8]. The homogeneous alkaline catalysts, such as NaOH, CH3ONa and KOH have been widely used for producing biodiesel due to their high catalytic activity [9]. The most commonly used alcohol for the synthesis of biodiesel is methanol (CH3OH), primarily because of its low cost and wide availability [10]. So the main molecular component of biodiesel is fatty acid methyl esters (FAME). Water has significant negative effects on this type of catalytic transesterification process [11]. Specifically, water can accelerate the hydrolysis of esters, mainly the triglycerides, which produces free fatty acids (FFA). The FFA thus formed partially consume the homogeneous alkaline catalyst, leading to the formation of soaps and producing more water [7,12]. This vicious circle leads to a very low reaction conversion and biodiesel yield. Moreover, the saponification reaction also occurs between the catalyst and glycerin esters (triglycerides, diglycerides, monoglycerides) and FAME, leading to the increase of the soaps, which makes the posttreatment (separation and purification) of biodiesel more difficult and costly through an energy-intensive process that generates significant amounts of wastewater [13]. Water in the reaction system also shifts the hydroxide/alkoxide equilibrium (Eq. (1)) toward the formation of the hydroxide [14,15], while the transesterification reaction is catalyzed by methoxide ions as a true catalyst [16], thus lowering the catalytic activity of the catalyst. In this case, the extent of the transesterification reaction will decrease.

CH3 OH þ OH $ CH3 O þ H2 O

ð1Þ

Because the homogeneous base-catalyzed transesterification requires a feedstock with water concentration <0.06 wt% and acid value <1 mg KOH/g [11,13,17], pretreatment to deacidify and dehydrate the raw materials has been widely applied in industry. However, such pretreatments unavoidably lead to an increase in production costs, including energy consumption. Our previous work [7] focused on one method for removing water during the homogeneous base-catalyzed transesterification process. In this work, we present a new reaction system; i.e., bentonite-enhanced transesterification reaction system, that has significant advantages. Bentonite was proved to enhance the transesterification reaction, because it can rapidly remove water from the reaction system, which promotes the transformation of NaOH

to the catalytically active methoxide species. In addition, the main side reactions; i.e., hydrolysis and saponification of esters, are significantly inhibited. As a result, the concentration of soap in the crude biodiesel is also reduced, which was beneficial for the post-treatment of the crude biodiesel. In the present paper, the bentonite-enhanced transesterification reaction was adopted to synthesize FAME with refined soybean oil as the feedstock. The effects of concentration of NaOH, methanol/oil molar ratio, amount of bentonite, agitation rate, reaction temperature and reaction time on the FAME yield were investigated. The optimization of four main process variables for the bentonite-enhanced transesterification process was obtained using response surface methodology central composite design (RSMCCD). A mathematical model to predict the biodiesel yield was formulated and validated. In addition, experiments were performed to determine the kinetic and thermodynamic parameters. 2. Materials and methods 2.1. Materials Refined soybean oil (acid value of 0.5 mg KOH/g) was obtained from Greatocean Oils & Grains Industries Co., Ltd., Fangchenggang, Guangxi, China. Bentonite was purchased from Shanghai No. 4 Reagent & H.V. Chemical Co., Ltd., Shanghai, China and was further dried at 200 °C to constant weight. It is a kind of silicate layered and porous structural material with montmorillonite content >75%, average particle size of 37 lm and specific surface area of 21 m2/g. Sodium hydroxide, anhydrous methanol, hydrochloric acid and anhydrous sodium sulfate, purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China, were all analytical grade reagents and used without further purification. The gas chromatography reference standards for methyl palmitate, methyl stearate, methyl oleate, methyl linoleate and methyl linolenate were all purchased from Sigma–Aldrich (China). 2.2. Reaction procedure In accord with procedures used in our previous work [7], the reactions were carried out in a 500 mL 3-neck flask reactor equipped with a mechanical stirrer, thermostatic water bath, condenser and sampling outlet. Firstly, 250 g soybean oil and a desired amount of bentonite were placed into the reactor, heated up to the reaction temperature and then stirred for 0.5 h at a constant stirring speed. A solution of the established concentration of NaOH in methanol was prepared. The resulting NaOH/methanol solution was added to the reactor and the transesterification reaction continued for a prescribed time. After the reaction was completed, the phases were separated by centrifugation. The upper layer, containing methyl esters, was further purified by evaporation under vacuum in a rotary evaporator at 65 °C to remove the residual methanol, the resulting solid soap was removed by centrifugation, and then the oil phase was weighed, namely the weight of biodiesel. After that, 2 g samples of the obtained biodiesel were washed with deionized water to remove the remaining catalyst. The residual water was removed by adding a small amount of anhydrous sodium sulfate, followed by centrifugation. The methyl ester contents of the samples were determined by gas chromatography as described in earlier work [7,18]. The FAME yield was evaluated through the Eq. (2).

FAME yield ¼ FAME content from GC analysis 

weight of biodiesel weight of soybean oil

ð2Þ

Please cite this article in press as: Wu L et al. Bentonite-enhanced biodiesel production by NaOH-catalyzed transesterification: Process optimization and kinetics and thermodynamic analysis. Fuel (2016), http://dx.doi.org/10.1016/j.fuel.2016.05.065

L. Wu et al. / Fuel xxx (2016) xxx–xxx

For the kinetic study, 3 mL samples were removed from the reaction mixture at prescribed times (1, 2, 3, 4, 5, 6, 8, 10, 12, 15, 20, 25, 30, 40, 50 and 60 min) and quenched immediately with 1 mL 0.5 mol/L of hydrochloric acid (HCl) and immediately cooled to 0 °C in an ice batch [16,19]. All of the samples mixture were centrifuged at 8000 rpm for 10 min. The recovered methyl ester phases were further purified by water washing as described above and analyzed by gas chromatography. The experiments were repeated twice, and the average value for the measured data was used. 2.3. Analytical methods The FAME content was determined by gas chromatography using an Agilent USA gas chromatograph 7820A equipped with a flame ionization detector (FID) and a capillary column (Agilent HP-5, 30 m  0.32 mm  0.25 lm). The injector and detector temperatures were set at 280 °C and 290 °C, respectively. The flow rate of N2 was 0.4 mL/min with the split ratio of 50:1. The GC oven temperature program was as follows: initial temperature of 150 °C for 1 min, heating up to 200 °C at 20 °C/min, then heating up to 230 °C at 1.8 °C/min with a holding time of 1 min, and finally heating up to 260 °C at 10 °C/min. The analysis of the sample solution was carried out by injecting 1 lL of the sample solution into the GC. 3. Results and discussion

3

methanol [13], and the catalyst concentration based on total mass decreased with the increase of methanol/oil molar ratio, which led to the decrease of the reaction rate and conversion. The experimental results (runs 1–4, Table 1) show that the highest FAME yield 97.45% was reached when the methanol/oil molar ratio was 7.5:1. Because of the adsorption of methanol by bentonite, which was shown in our previous study [7], the optimal ratio was higher than 6:1 which is a widely accepted optimal ratio in the traditional homogeneous base-catalyzed transesterification reaction [20–22]. 3.1.2. Effect of concentration of NaOH Usually, the increase of concentration of NaOH would increase the reaction rate and conversion. However, the excess of NaOH could lead to increased saponification [23,24]. The effect of the concentration of NaOH in the presence of bentonite was studied by comparing the results from experiments 3, 5–7 and 18 in Table 1. The highest FAME yield was 98.22%, which was obtained with 0.7 wt% NaOH. As can be seen, the absence of catalyst (NaOH) hindered the formation of FAME, which proved that the bentonite by itself did not promote to the transesterification. The run without bentonite (run 8, Table 1) was adopted to compare with the experiment 6. It turned out that, with the NaOH concentration of 0.7 wt %, the presence of bentonite exercised a positive effect in FAME yield. The suitable concentration of NaOH and amount of bentonite are the keys to improving the FAME yield of the transesterification reaction. This behavior was also observed by our previously study [7].

3.1. Single factor experiments The process variables studied were: methanol/oil molar ratio (4.5:1–9:1), concentration of NaOH (0.6–0.9 wt%), agitation rate (500–800 rpm), amount of bentonite (0–4 wt%) and reaction temperature (50–65 °C). The oil type (soybean), alcohol type (methanol) and reaction time (60 min) were fixed as common parameters in all experiments. The specific values of the parameters used in the study are shown in Table 1, along with the FAME yield for each experiment. 3.1.1. Effect of methanol/oil molar ratio Methanol/oil molar ratio strongly affects the FAME yield. Excess methanol increases the conversion of oil, since transesterification is an equilibrium reaction. However, excess methanol can also increase the energy consumption needed for the recovery of

Table 1 Conditions and results of single factor experiment. Run

MeOH/ oil

Catalyst (wt%)

Amount of bentonite (wt%)

T (°C)

A.R. (rpm)

FAME yield (wt%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

4.5:1 6:1 7.5:1 9:1 7.5:1 7.5:1 7.5:1 7.5:1 7.5:1 7.5:1 7.5:1 7.5:1 7.5:1 7.5:1 7.5:1 7.5:1 7.5:1 7.5:1

0.9 0.9 0.9 0.9 0.6 0.7 0.8 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 None

2.0 2.0 2.0 2.0 2.0 2.0 2.0 None 1.0 3.0 4.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0

55 55 55 55 55 55 55 55 55 55 55 50 60 65 55 55 55 55

600 600 600 600 600 600 600 600 600 600 600 600 600 600 500 700 800 600

89.55 ± 0.38 93.68 ± 0.20 97.45 ± 0.25 96.15 ± 0.62 95.81 ± 0.40 98.22 ± 0.48 97.87 ± 0.35 92.65 ± 0.28 97.38 ± 0.68 96.52 ± 0.45 87.45 ± 0.50 94.38 ± 0.27 98.05 ± 0.36 97.58 ± 0.55 92.65 ± 0.52 98.15 ± 0.34 98.35 ± 0.45 –

3.1.3. Effect of amount of bentonite The amount of bentonite is a vital factor for improving the FAME yield. Therefore, the effect of the amount of bentonite was studied by varying this parameter in the range 0–4.0 wt% (runs 6 and 8–11, Table 1). The presence of a small amount of bentonite (1.0 wt%) produced an increase of 4.73% of FAME yield. Increasing the added bentonite to 2.0 wt% led to a greater FAME yield. However, excessive amounts of bentonite led to a decrease of the final FAME yield in the separation process due to the generation of a gelatinous substance that formed when the soap and glycerol were adsorbed on the bentonite. 3.1.4. Effect of reaction temperature The effect of reaction temperature on FAME yield was studied by varying the temperature in the range 50–65 °C (runs 6 and 12–14, Table 1). Temperature has a great effect on the kinetics and equilibrium constants of the reaction system. A higher reaction temperature can lower the viscosities of oils and improve mass transfer and reactivity between oil and methanol [3,13]. However, the increased temperature will also increase the saponification of esters, which will reduce the FAME yield [23,24]. Furthermore, the reaction temperature should be lower than the boiling point of methanol in order to avoid the vaporization of methanol. The results show that 55 °C is the optimal reaction temperature. 3.1.5. Effect of agitation rate In the initial stage of the transesterification reaction, the reactants form a two-phase liquid system due to their immiscibility [3]. The reaction is diffusion-controlled, with poor diffusion between the phases resulting in a slow rate [25]. Although the generated FAME can improve the mutual solubility between oil and methanol, it is necessary to stir the reaction mixture to achieve high reaction rate. The effect of agitation rate on FAME yield was studied by varying it in the range 500–800 rpm (runs 6 and 15– 17, Table 1). As can be seen from the results, the influence of agitation rate had little significant impact above 600 rpm.

Please cite this article in press as: Wu L et al. Bentonite-enhanced biodiesel production by NaOH-catalyzed transesterification: Process optimization and kinetics and thermodynamic analysis. Fuel (2016), http://dx.doi.org/10.1016/j.fuel.2016.05.065

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3.2. Optimization of reaction conditions by response surface methodology

Table 3 Full factorial central composite design matrix for biodiesel production. Run

3.2.1. Experimental design and results The central composite design (CCD) was utilized to find the optimal conditions for maximizing the FAME yield. The main operating parameters; i.e., methanol/oil molar ratio (X1), concentration of NaOH (X2), amount of bentonite (X3) and temperature (X4), were chosen as independent variables, and the FAME yield was the dependent variable. Table 2 shows the choice of experimental range and levels of independent variables for biodiesel production, which were based on the results of the single factor experiment shown in Table 1. The other operating parameters were as follows: reaction rate of 600 rpm and reaction time of 60 min. The Design Expert 8.0.6 software (STAT-EASE Inc., Minneapolis, USA) was used for the regression and graphical analysis of the data. The response factor (FAME yield) was correlated to the selected parameters using a full quadratic model which is expressed as Eq. (3) [26–28].

Y ¼ b0 þ

k k X XX X bj xj þ bij xi xj þ bjj x2j þ e j¼1

ð3Þ

j¼1

i
where Y is the predicted response; xi and xj are the coded independent variables or factors; b0, bj, bij and bjj are constant coefficients; e is random error. 30 runs were performed in a randomized order. The experimental points in both coded and uncoded values are presented in Table 3, along with the experimental and predicted values for FAME yield at the specific design points. 3.2.2. Development of regression model equation The data obtained were then analyzed by analysis of variance (ANOVA) for fitting second-order response surface model by least square method and to assess the goodness of fit. The significance of each parameter, which was evaluated by the probability value (P value) listed in Table 4. At 95% confidence level, the P values less than 0.05 indicate significant effects of those parameters [6]. Based on the coded parameters, the quadratic regression model with determined coefficients is given in Eq. (4):

Yield ¼ 98:17  0:49X 1 þ 1:27X 2  1:33X 3 þ 0:91X 4 þ 0:28X 1 X 2  0:27X 1 X 3 þ 0:22X 1 X 4 þ 0:41X 2 X 3  0:26X 2 X 4 þ 0:15X 3 X 4  2:62X 21  0:72X 22  1:97X 23  0:61X 24

ð4Þ

The P values presented in Table 4 clearly indicated highly significant of the model and insignificant lack of fit (LOF), which indicated the high significance of the fitted model showing the reliability of the regression model for predicting the FAME yield [26]. These statistical tests showed that the selected model was satisfactory for predicting the FAME yield within the range of the experimental variables studied. The suitability of the model was also tested using the determination coefficient (R2) and adjusted determination coefficient (R2Adj). The values of R2 and R2Adj were both found to be very high,

Table 2 Experimental range and levels of independent process variables for biodiesel production. Variable

Methanol/oil molar ratio Concentration of NaOH (wt%) Amount of bentonite (wt%) Temperature (°C)

Symbol

X1 X2 X3 X4

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Independent variables

FAME yield (%)

X1

X2

X3

X4

Experimental

Predicted

1(6) +1(9) 1(6) +1(9) 1(6) +1(9) 1(6) +1(9) 1(6) +1(9) 1(6) +1(9) 1(6) +1(9) 1(6) +1(9) 2(4.5) +2(10.5) 0(7.5) 0(7.5) 0(7.5) 0(7.5) 0(7.5) 0(7.5) 0(7.5) 0(7.5) 0(7.5) 0(7.5) 0(7.5) 0(7.5)

1(0.6) 1(0.6) +1(0.8) +1(0.8) 1(0.6) 1(0.6) +1(0.8) +1(0.8) 1(0.6) 1(0.6) +1(0.8) +1(0.8) 1(0.6) 1(0.6) +1(0.8) +1(0.8) 0(0.7) 0(0.7) 2(0.5) +2(0.9) 0(0.7) 0(0.7) 0(0.7) 0(0.7) 0(0.7) 0(0.7) 0(0.7) 0(0.7) 0(0.7) 0(0.7)

1(1) 1(1) 1(1) 1(1) +1(3) +1(3) +1(3) +1(3) 1(1) 1(1) 1(1) 1(1) +1(3) +1(3) +1(3) +1(3) 0(2) 0(2) 0(2) 0(2) 2(0) +2(4) 0(2) 0(2) 0(2) 0(2) 0(2) 0(2) 0(2) 0(2)

1(50) 1(50) 1(50) 1(50) 1(50) 1(50) 1(50) 1(50) +1(60) +1(60) +1(60) +1(60) +1(60) +1(60) +1(60) +1(60) 0(55) 0(55) 0(55) 0(55) 0(55) 0(55) 2(45) +2(65) 0(55) 0(55) 0(55) 0(55) 0(55) 0(55)

92.88 ± 0.42 91.25 ± 0.68 93.81 ± 0.28 94.08 ± 0.35 89.20 ± 0.40 86.50 ± 0.53 92.95 ± 0.75 91.43 ± 0.52 93.96 ± 0.62 93.19 ± 0.30 95.01 ± 0.54 95.51 ± 0.63 91.24 ± 0.48 90.32 ± 0.34 93.65 ± 0.55 92.86 ± 0.70 88.50 ± 0.31 86.42 ± 0.45 92.65 ± 0.25 97.45 ± 0.56 92.65 ± 0.35 87.45 ± 0.61 93.42 ± 0.40 97.58 ± 0.48 98.38 ± 0.33 98.17 ± 0.40 98.23 ± 0.32 98.01 ± 0.58 97.78 ± 0.29 98.45 ± 0.64

92.42 90.97 94.09 93.76 89.19 86.67 92.49 91.08 94.02 93.47 94.66 95.23 91.38 89.75 93.64 93.14 88.67 86.72 92.75 97.81 92.94 87.62 93.9 97.56 98.17 98.17 98.17 98.17 98.17 98.17

Table 4 Analysis of variance (ANOVA) for response surface quadratic model. Source of variation

Sum of squares

Degrees of freedom

Mean square

F value

P value Prob > F

Model X1 X2 X3 X4 X1X2 X1X3 X1X4 X2X3 X2X4 X3X4 X21 X22 X23 X24 Residual Lack of Fit Pure error Total

374.24 5.72 38.41 42.51 20.09 1.25 1.16 0.81 2.64 1.06 0.34 188.16 14.28 106.63 10.18 2.45 2.14 0.30 376.69

14 1 1 1 1 1 1 1 1 1 1 1 1 1 1 15 10 5 29

26.73 5.72 38.41 42.51 20.09 1.25 1.16 0.81 2.64 1.06 0.34 188.16 14.28 106.63 10.18 0.16 0.21 0.061

163.95 35.10 235.55 260.71 123.24 7.69 7.09 4.97 16.20 6.51 2.10 1154.04 87.61 653.98 62.43

<0.0001 <0.0001 <0.0001 <0.0001 <0.0001 0.0142 0.0178 0.0415 0.0011 0.0222 0.1680 <0.0001 <0.0001 <0.0001 <0.0001

3.53

0.0884

C.V. = 0.43%, R2 = 0.9935, R2Adj = 0.9874, Predicted R2 = 0.9661.

which were 0.9935 and 0.9874, respectively, thus showing an excellent correlation between the independent variables and advocating a high significance of the model [29]. On the other hand, the coefficient of variation is relatively low, which revealed a better precision and reliability of the experimental results of the fitted model [30].

Range and their levels 2

1

0

1

2

4.5 0.5 0 45

6 0.6 1 50

7.5 0.7 2 55

9 0.8 3 60

10.5 0.9 4 65

3.2.3. Effects of transesterification process variables The results of variance analysis (ANOVA) in Table 4 revealed that all the linear terms and quadratic terms have large effect on the FAME yield due to their high F values and their correspondingly low P values. In addition, all of the interaction terms, except X3X4

Please cite this article in press as: Wu L et al. Bentonite-enhanced biodiesel production by NaOH-catalyzed transesterification: Process optimization and kinetics and thermodynamic analysis. Fuel (2016), http://dx.doi.org/10.1016/j.fuel.2016.05.065

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L. Wu et al. / Fuel xxx (2016) xxx–xxx

(the interaction term for the amount of bentonite with temperature), also affected the biodiesel yield significantly. Since the higher the F value (or the lower the P value) for a parameter the more significant is the parameter and hence it reflects the relative importance of the term associated with that parameter [26]. So the concentration of NaOH and the amount of bentonite are the two most important variables for the bentonite-enhanced transesterification reaction, as shown by they are having the lowest P values and the highest F values among the other variables. The 3D response curves show the effects of the independent variables on the response. The maximum predicted values were indicated by the surface confined in the smallest ellipse in the contour diagram, and elliptical contours were obtained when the interaction between the independent variables was significant [27]. Fig. 1a–f shows the 3D response curves related to the effect of two variables on the FAME yield. The curved nature of 3D

response curves in Fig. 1a–e indicated relatively significant interactions of methanol/oil molar ratio with concentration of NaOH, methanol/oil molar ratio with amount of bentonite, methanol/oil molar ratio with temperature, concentration of NaOH with amount of bentonite and concentration of NaOH with temperature. On the other hand, a weaker interaction was observed between amount of bentonite and temperature as represented in Fig. 1f. 3.2.4. Optimization process Since the fitted model (Eq. (4)) offered a good estimate to the experimental conditions, it was employed to predict the optimum conditions for obtaining a maximum FAME yield by using the Design expert software numerical optimization tool. The optimal conditions occurred when the methanol/oil molar ratio was 7.47:1, concentration of NaOH was 0.77 wt%, amount of bentonite was 1.76 wt% and reaction temperature was 57.8 °C. The predicted

Cata

b FAME yield (%)

FAME yield (%)

a

X3=2 wt% X4=55 °C

lyst

con cent ra t i o

n (w

t%)

Met

han

o

ratio olar m l i l/o

X2=0.7 wt% X4=55 °C

Ben tonit e ad ditio n am

c FAME yield (%)

(%) FAME yield

(wt% )

Met

han

o

ratio

d

X2=0.7 wt% X3=2 wt%

Tem p

eratu

re (°

C)

Met

hano

Ben

X1=7.5:1 X4=55 °C

toni

ratio olar m l i l/o

e

te ad

ditio

n am

oun

t (w

t%)

Cata

c lyst

onc

ation entr

(wt%

)

)

f (% FAME yield

FAME yield (%)

ount

ar mol l/oil

X1=7.5:1 X3=2 wt%

Tem p

erat

ure

(°C )

c lyst Cata

o

on ( trati ncen

wt%

)

X1=7.5:1 X2=0.7 wt%

Tem p

erat

ure

(°C )

te toni Be n

) (wt% ount m a tion addi

Fig. 1. Response surface contours for interaction on FAME yield.

Please cite this article in press as: Wu L et al. Bentonite-enhanced biodiesel production by NaOH-catalyzed transesterification: Process optimization and kinetics and thermodynamic analysis. Fuel (2016), http://dx.doi.org/10.1016/j.fuel.2016.05.065

6

L. Wu et al. / Fuel xxx (2016) xxx–xxx

FAME yield under these conditions was 99.04%, while the experimental value was 98.56%.

4

3.3. Kinetics and thermodynamic analysis

3

TG þ CH3 OH $ DG þ FAME

ð5Þ

DG þ CH3 OH $ MG þ FAME

ð6Þ

MG þ CH3 OH $ GL þ FAME

ð7Þ

-ln (1-x)

3.3.1. Kinetics model The reaction scheme of the transesterification of triglycerides (TG) with methanol involves three consecutive, reversible reactions in which diglycerides (DG) (Eq. (5)), monoglycerides (MG) (Eq. (6)) and glycerol (GL) (Eq. (7)) are formed. In each step a molecule of FAME is formed, with an overall three molecules of FAME and one molecule of glycerol being formed upon completion of the reaction.

2

1

0

0

5

10

15

20

t (min) Fig. 3. Determination of kinetic rate constants on the basis of the pseudo first-order kinetic model.

The rate constant of the overall reaction was determined without considering intermediate steps in this work. The pseudo firstorder kinetic model was used, based on the following assumptions: (a) The effect of mass transfer was negligible because the agitation rate was set at 600 rpm. (b) The free fatty acid neutralization was not significant, since the soybean oil is refined and the free fatty acid content was negligible. (c) The saponification reaction was insignificant and catalyst concentration was constant. (d) The effects of reverse reaction and intermediate reactions were ignored. The equation was as follows:

imental data displayed a linear trend. After that, the conversion remained nearly constant, and experimental points did not show the same linear trend. The results of regression analysis of the data plotted in Fig. 3, which include the pseudo kinetic constants (k) and R2 coefficient, are shown in Table 5. The high values of R2 show that the data are a good fit to the pseudo first-order kinetic model [3]. The activation energy (Ea) can be calculated from the Arrhenius equation (Eq. (9)).

lnð1  XÞ ¼ kt

k ¼ A expðEa =RTÞ

ð8Þ

where X is the extent of conversion of TG at any time t, which was defined as the total moles of FAME produced divided by total mol of triglycerides, and k is the rate constant which can be obtained from the slope of the curve of ln(1  X) versus t. 3.3.2. Determination of kinetics and thermodynamic parameters The kinetics of the transesterification reaction was studied at 40, 50, 60 and 65 °C, and the other experimental conditions were according to the optimal conditions described in Section 3.2.4. Fig. 2 shows the effect of reaction temperature on conversion of soybean oil. The reaction rate increased with the increase of temperature and the reaction temperature showed positive influence during the first 30 min of reaction, after that the final conversion was similar for all reactions. Fig. 3 is the graphical representation of Eq. (8) corresponding to each reaction, which shows that during the first 20 min the exper-

100

conversion (%)

80 40 °C 50 °C 60 °C 65 °C

60 40

where k is the rate constant, A is the frequency factor, R is the universal gas constant and T is the absolute temperature. Eq. (9) could be linearized as shown in Fig. 4, from which the Ea can be determined from the slope of the curve. The value of Ea was 31.03 kJ mol1. This value is relatively small compared with the range of activation energies i.e. 30.92–83.80 kJ mol1 obtained for homogeneous catalyzed transesterification of soybean oil [31– 33]. These data are evidence that bentonite has greatly improved the catalytic activity of NaOH. Thermodynamic characteristics of the bentonite-enhanced transesterification reaction system were calculated using Eyring– Polanyi equation (Eq. (10)) based on the rate constants.

lnðk=TÞ ¼ DH=RT þ lnðkb =hÞ þ DS=R

ð10Þ

where k is the rate constant at temperature T, DH and DS are the changes in enthalpy and entropy of activation for the reaction system, respectively, kb is the Boltzmann constant, h is the Planck constant and R is the universal gas constant. The Eyring–Polanyi plot for the bentonite-enhanced transesterification reaction is shown in Fig. 5. DH and DS were determined from the slope and y-intercept of the curve, respectively. The Gibbs energy change (DG) was determined by Eq. (11). The value of DH, DS and DG for the bentonite-enhanced transesterification reaction are shown in Table 6.

DG ¼ DH  T DS

ð11Þ

Table 5 Regression of model parameters and correlations under different temperatures.

20 0

ð9Þ

0

10

20

30 40 t (min)

50

60

Temperature (K)

k (min1)

R2

313.15 323.15 333.15 338.15

0.0850 0.1168 0.1648 0.2082

0.99 0.98 0.99 0.98

Fig. 2. Effect of the temperature on conversion of soybean oil.

Please cite this article in press as: Wu L et al. Bentonite-enhanced biodiesel production by NaOH-catalyzed transesterification: Process optimization and kinetics and thermodynamic analysis. Fuel (2016), http://dx.doi.org/10.1016/j.fuel.2016.05.065

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L. Wu et al. / Fuel xxx (2016) xxx–xxx

[34,35]. Many previous studies have reported similar trends for thermodynamic parameters. For example, Encinar et al. [3] reported on the transesterification of rapeseed oil using diethyl ether as a co-solvent at 20–40 °C. Sarve et al. [35] investigated the transesterification of the non-edible Schleichera triguga oil using heterogeneous catalyst (Ba(OH)2) under ultrasound condition at 30–60 °C. Nautiyal et al. [36] studied the biodiesel production from Spirulina platensis algae biomas, using single stage extraction-transesterification process at 35–75 °C. The thermodynamic parameters reported in these studies are shown in Table 7.

-1.4 -1.6

lnk

-1.8 -2.0 -2.2 -2.4 -2.6 0.0029

4. Conclusions

0.0030

0.0031 1/T (K-1)

0.0032

Fig. 4. Activation energy determination for the transesterification process according to Arrhenius equation (best fit: ln k = 3732.3/T + 9.4311; R2 = 0.98).

-7.4

ln (k/T)

-7.6 -7.8 -8.0 -8.2 0.0029

0.0030

0.0031

0.0032

1/T (K-1) Fig. 5. Determination of the changes in enthalpy and entropy of activation for the transesterification reaction, using the Eyring–Polanyi equation (best fit: ln(k/T) = 3407.1/T + 2.6462; R2 = 0.98).

The bentonite-enhanced transesterification is found to be non-spontaneous, endothermic and endergonic in nature; i.e., the values of DH and DG are all positive. The negative value of DS indicates that the transition state has a higher degree of ordered geometry/alignment than do the reactants in the ground state [34]. The final conversion of TG was increased by the increasing temperature, which indicates that heat input is required to bring the reactants to the transition state in order to form the products

Bentonite as a water adsorbent can rapidly remove water from the transesterification reaction system, which promote the transformation of NaOH to the catalytically active methoxide species. Therefore, a high oil conversion can be achieved at a relatively low concentration of NaOH, producing a high FAME yield and a low likelihood of the undesirable saponification reaction that would result from excessive amounts of NaOH. CCD-RSM was successfully applied to study the effects of the key operating parameters on the bentonite-enhanced transesterification reaction. According to the ANOVA results, concentration of NaOH and amount of bentonite are the two most significant factors examined. In addition, the interaction of methanol/oil molar ratio – concentration of NaOH, methanol/oil molar ratio – temperature, concentration of NaOH – amount of bentonite and amount of bentonite – temperature were found to have significant effects on the FAME yield. The maximum FAME yield, as predicted by quadratic polynomial model, was expected to be 99.04% under the optimal reaction conditions of 7.47:1 methanol/oil molar ratio, 0.77 wt% concentration of NaOH, 1.76 wt% amount of bentonite and 57.8 °C temperature. These optimal reaction conditions were validated with actual FAME yield in 98.56%. The bentonite-enhanced transesterification reaction followed a pseudo-first order kinetic model, and the rate constants at several temperatures were determined. In addition, the activation energy and the thermodynamic properties of the reaction; i.e., DG, DH and DS, were also determined. The relatively low value of activation energy (31.03 kJ mol1) and high values of kinetic constants indicated a very fast reaction rate, which could be attributed to the enhancement of the transformation of NaOH to the catalytically active methoxide species due to the rapidly removal of water from the reaction system. The positive values of DG and DH and the negative value of DS showed that the bentonite-enhanced transesterification was a non-spontaneous, endothermic and endergonic reaction.

Table 6 Thermodynamic parameters of the bentonite-enhanced transesterification reaction system. Thermodynamic parameters

DH (kJ mol1)

DS (kJ mol1 K1)

DG (kJ mol1) 313.15 K

323.15 K

333.15 K

338.15 K

Values

28.33

0.18

83.30

85.05

86.81

87.69

Table 7 Thermodynamic parameters involved in biodiesel production. Source

DG (kJ mol1)

DH (kJ mol1)

DS (kJ mol1 K1)

Refs.

Soybean oil Rapeseed oil Schleichera triguga oil Spirulina platensis algae biomass

83.30–87.69 75.26–79.06 82.44–85.55 92.71

28.33 19.59 50.62 16.35

0.18 0.19 0.11 0.23

Present work [3] [35] [36]

Please cite this article in press as: Wu L et al. Bentonite-enhanced biodiesel production by NaOH-catalyzed transesterification: Process optimization and kinetics and thermodynamic analysis. Fuel (2016), http://dx.doi.org/10.1016/j.fuel.2016.05.065

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L. Wu et al. / Fuel xxx (2016) xxx–xxx

Acknowledgements The authors gratefully acknowledge financial support for this research from the National Natural Science Foundation of China (Grant Nos. 21576055, 21076046 and 21166004). The authors are grateful to Dr. Donald G. Barnes, Guest Professor of Environmental Science at South China University of Technology, for his helpful discussions and writing assistance.

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Please cite this article in press as: Wu L et al. Bentonite-enhanced biodiesel production by NaOH-catalyzed transesterification: Process optimization and kinetics and thermodynamic analysis. Fuel (2016), http://dx.doi.org/10.1016/j.fuel.2016.05.065