Scholarly article on topic 'Microwave radiation improves biodiesel yields from waste cooking oil in the presence of modified coal fly ash'

Microwave radiation improves biodiesel yields from waste cooking oil in the presence of modified coal fly ash Academic research paper on "Chemical sciences"

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Abstract of research paper on Chemical sciences, author of scientific article — Yulin Xiang, Yukun Xiang, Lipeng Wang

Abstract This paper studied the effects of using modified coal fly ash as a catalyst to convert waste cooking oil (WCO) into biodiesel under microwave-strengthened action. Coal fly ash was modified with sodium sulphate and sodium hydroxide, and the obtained catalyst was characterized using FT-IR and X-ray diffraction (XRD). The experimental results showed that the modified coal fly ash catalyst improved biodiesel yields under the microwave-assisted system, and the maximum biodiesel yield from waste cooking oil reached 94.91% at a molar ratio of methanol to WCO of 9.67:1 with 3.99% wt% of modified coal fly ash catalyst (based on oil weight) at a 66.20°C reaction temperature. The reusability of the modified coal fly ash catalyst was excellent, and the conversion yield remained greater than 90% after the catalyst was reused 8 times. The produced biodiesel met the main parameters of the ASTM D-6751 and EN14214 standards.

Academic research paper on topic "Microwave radiation improves biodiesel yields from waste cooking oil in the presence of modified coal fly ash"

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Title: Microwave radiation improved biodiesel yields from waste cooking oil in presence of modified coal fly ash

Authors: Yulin Xiang, Yukun Xiang, Lipeng Wang

PII: DOI:

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S1658-3655(17)30068-7 http://dx.doi.org/doi:10.1016/j.jtusci.2017.05.006 JTUSCI 391

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Received date: Revised date: Accepted date:

6-9-2016

17-4-2017

11-5-2017

Please cite this article as: Yulin Xiang, Yukun Xiang, Lipeng Wang, Microwave radiation improved biodiesel yields from waste cooking oil in presence of modified coal fly ash (2010), http://dx.doi.org/10.1016/j.jtusci.2017.05.006

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Microwave radiation improved biodiesel yields from waste cooking oil in presence of modified coal fly ash

Yulin Xiang1*, Yukun Xiang2, Lipeng Wang1

1 College of Chemistry and Chemical Engineering, Yulin University, Yulin 719000 Shaanxi Province, China; 2 Yanshou No.1 Middle School, Harbin 150700 HeilongjiangProvince, China

Abstract: This paper studied the effects of modified coal fly ash as catalyst on the waste cooking oil (WCO) conversion into biodiesel under microwave strengthened action. The coal fly ash was modified with sodium sulfate and sodium hydroxide. The obtained catalyst was characterized by FT-IR and X-ray diffraction (XRD). Experimental results showed that the modified coal fly ash catalyst could improve biodiesel yields under microwave assisting system, and the maximum biodiesel yield from waste cooking oil reached 94.91% under a molar ratio of methanol to WCO of 9.67:1, a 3.99% wt% modified coal fly ash catalyst (based on oil weight), and a 66.20 °C reaction temperature. The reusability of the modified coal fly ash catalyst was well, and the conversion yield was still higher than 90% after the catalyst was used for 8 times repeatedly. The produced biodiesel met main parameters of the ASTM D-6751 and EN14214 standards.

Keywords: Biodiesel, Modified coal fly ash, Microwave assisting system, Waste cooking oil 1. Introduction

Biodiesel, a renewable diesel fuel, is obtained from oil or fat resources such as vegetable oils and domestic fats or waste cooking oil (WCO) using alcohol by means of a acidic or basic catalyst [1]. Researches showed that biodiesel properties (such as low emissions, carbon neutral, biodegradable and non-toxic) were superior to that of non-renewable fuel (diesel and petroleum) [2, 3]. However, the

* Corresponding author E-mail: yulinx@126.com; Phone: +8613720699281

biodiesel production costs are very high when raw materials come from food source (such as animal fat and edible oils). The WCO is produced after frying edible oils (such as soya bean, corn, sunflower oils and domestic fats) [4]. The physical and chemical properties of the WCO are changed during frying, but these changes would not affect biodiesel production [5]. Thus, WCO is considered a good alternative because it is inexpensive and prevents environmental pollution[6, 7].

The most common method to produce biodiesel is through transesterification of oil or fat with alcohol, and by-product is glycerine. A catalyst is used to promote the reaction yield and rate [8]. The catalyst may be heterogeneous or homogeneous. Previous report indicated that the homogeneous catalysts had some shortcomings, like complexities in the separation and purification of product and a huge wastewater production [9]. While heterogeneous catalysts have many advantages, like easier product separation, simpler operation, reusability, less problematic process due to the advantage of easy separation of catalysts from the products [10]. For a sustainable development, the heterogeneous catalysts from waste materials have been of recent interest in the search [11]. Mathiarasi et al. [12] utilized boiler ash as a solid catalyst for transesterification of soap nut oil, and the maximum yield of methyl ester of 89% was obtained using 3.5 wt% catalyst for a reaction time of 3 h. Viriya-empikul et al. [13] utilized waste shells of egg, golden apple snail and meretrix venus for the transesterification of palm oil, and a biodiesel yield of 90% was obtained using 10 wt% catalyst for a reaction time of 2 h. Nakatani et al. [14] utilized oyster shells for the transesterification of soybean oil and obtained a biodiesel yield of 73.8% using 25 wt% catalyst for a reaction time of 5 h. Suryaputra et al. [15] utilized waste capiz shells for the transesterification of palm oil and achieved a biodiesel yield of 93% by employing 3 wt% catalyst for a reaction time of 6 h. Gabriel et al. [1] utilized sea sand for the transesterification of cooking oil and achieved a highest biodiesel yield of 95.4% using 7.5wt% catalyst for a reaction time of 6 h.

Coal fly ash (CFA) produced from coal fired power plants, a problematic alkaline waste, is one such material with high levels of CaO and MgO [16-18]. The CFA use as a catalyst for transesterification of the WCO would not only reduce the cost from raw material but also control environment pollution. However, most of CFA are dumped into landfill. In addition, microwave could distinctly promote the rate of the transesterification reaction, and optimize process control. During the reaction, use of microwave could decrease the demand of methyl acetate and also need substantially lower reaction times [19-21]. Because microwave improved biodiesel production process has the potential to decrease the overall costs and energy consumption, there are many studies focused on using a microwave assisting method to increase the biodiesel yields from the WCO [22, 23]. Tangy et al. showed the use of microwave heating as a simple and fast way to produce biodiesel from a variety of renewable feedstock in batch mode [24]. Hong et al. improved the transesterification of waste cooking oil using microwave irradiation and achieved a biodiesel yield of 96.5% by employing 600 W microwave power for a reaction time of 6 min [25]. Gude et al. showed that the microwave effect on the transesterification reaction could be twofold: 1) enhancement of reaction by a thermal effect, and 2) evaporation of methanol due to the strong microwave interaction of the material [26].

Improving the biodiesel yields using a microwave assisting system and the modified CFA as a catalyst has not been reported. This paper investigated the biodiesel yields from the WCO with a microwave assisting system and modified CFA catalyst. The effects of molar ratio of methanol to oil, type and amount of catalyst, and reaction temperature are systematically researched. 2. Materials and methods 2.1. Materials

The CFA was obtained from Dabaodang, Shannxi, China. Sodium sulfate, sodium hydroxide and

methanol were of analytical reagent grade. The WCO was collected from a restaurant in Yulin, China. The WCO was filtrated to remove all suspended solid particles and food debris.

2.2. Catalyst preparation and characterization

The CFA was sieved (100 mesh) to remove impurities. Firstly, the modified CFA catalyst was synthesized by hydrothermal treatment under alkaline fusion condition. The procedure was as follows: The mass ratio of CFA and NaOH was 1:1.5 and the mixture was heated in 600 °C for 1.5 h. Secondly, the mixture was cooled and crushed, and placed in deionized water (liquid-solid ratio 10:1) and 35 wt% (powder to the solution mass ratio) of sodium sulfate was added. The mixture was stirred for 10 h at 60 C, then the mixture was washed, and calcined at 600 C for 1.5 h. The product was cleaned, filtered and dried at 90 C .

The X-ray diffraction (XRD) characterization of the CFA-derived catalyst was determined on a X-ray diffractometer (D/MAX-2400) using Cu ka radiation source carried out at 25mA and 30 kV over a 20 range from 10° to 60° with a step size of O.O2°(20) and a scan step time 0.5 s.

FT-IR spectra of the CFA before and after modified treatment were recorded using a Fourier Transform Spectrometer (IR Prestige-21). It is used to investigate the component changes of modified and untreated CFA. The wavenumber range of the spectrometer is 4000 to 500 cm-1 using 100 scans at 4 cm-1 resolution.

2.3. Transesterification of WCO

The transesterification reaction was carried out in a 100 ml reaction kettle. The three interfaces in the reaction kettle are used for the introductions of materials and electrical motor, and for injecting the temperature probe to control the reaction temperature. Fig. 1 shows the experimental schematic. The WCO volume was 10 ml in all experiments. After heating the WCO temperature up to 70 C, the modified CFA and methanol were introduced. In experiment process, the molar ratio of methanol to WCO were

modified between 3:1, 5:1, 7:1, 9:1 and 11:1, while the amount of the modified CFA were varied between 2%, 4%, 6%, 8% and 10% (wt/wt). and reaction temperatures were varied between 50, 60, 70, 80 and 90 °C. According to the initial research period, the reaction times was fixed at 6 min. After the reaction, the catalyst was separated from the products by centrifugation and the residual methanol was evaporated using vacuum evaporation.

Fig.1 Experimental setup.

2.4. Experimental design

Table 1 Variables and levels.

factors symbol -1.68 -1 coded levels 0 1 1.68

molar ratio of methanol to WCO X; 3:1 5:1 7:1 9:1 11:1

amount of the modified CFA (%) 2 4 6 8 10

reaction temperature (C) X3 50 60 70 80 90

The RSM was used to optimize the experiment process. The effects of the molar ratio of methanol to WCO (Xi), the amount of the modified CFA (X2) and the reaction time (X3) on the conversion yield of the WCO biodiesel (Y) were studied. Table 1 presents the ranges and the levels of the factors in RSM.

According to statistics theory, the three-factor Central Composite Design (CCD) experimental design consisted of 20 experimental runs. The experimental design is shown in Table 2.

Table 2 Results of CCD experimental design.

run factors response values (wt%)

Xi(coded) X2(coded) X3(coded) experimental predicted

1 5:1(-1) 8(1) 80(1) 59.50 58.92

2 3:1(-1.68) 6(0) 70(0) 58.85 59.97

3 7:1(0) 6(0) 70(0) 90.06 88.01

4 7:1(0) 6(0) 90(1.68) 62.02 60.32

5 9:1(1) 4(-1) 80(1) 84.25 82.09

6 5:1(-1) 4(-1) 60(-1) 60.02 61.1

7 7:1(0) 6(0) 50(-1.68) 76.99 77.03

8 11:1(1.68) 6(0) 70(0) 76.86 75.52

9 7:1(0) 6(0) 70(0) 89.27 89.13

10 5:1(-1) 4(-1) 80(1) 56.74 55.11

11 7:1(0) 2(-1.68) 70(0) 74.41 72.88

12 9:1(1) 8(1) 80(1) 47.55 47.68

13 7:1(0) 10(1.68) 70(0) 50.52 50.12

14 7:1(0) 6(0) 70(0) 88.54 89

15 7:1(0) 6(0) 70(0) 91.99 91.02

16 9:1(1) 4(-1) 60(-1) 90.97 90.56

17 7:1(0) 6(0) 70(0) 89.54 87.98

18 9:1(1) 8(1) 60(-1) 68.87 58.1

19 7:1(0) 6(0) 70(0) 90.14 87.01

20 5:1(-1) 8(1) 60(-1) 66.28 66.01

2.5. GC-MS analysis

Agilent 6890 GC /5973i MS was used to analyze the biodiesel composition. The GC operation conditions: hp-Innowax quartz capillary column (60 m ><0.25 mm*0.25 lm); capillary column temperature was initially raised by 10 °C/min from 70 °C to 160 °C, then raised by 5 °C/min from 160 °C to 230 °C, interface temperature was 260 °C; injector temperature was 260 °C, the diffluent ratio was 100:1, high purity helium carrier, gas flow rate was 1 mL/min (high purity helium carrier); and the injection volume was 0.2 ^L.

2.6. Biodiesel characterization

The fuel properties of self-made biodiesel was tested by the standard analysis methods of petroleum products, that is American Society for Testing and Materials [27]. The results were compared with the American and European standards of biodiesel [28, 29].

2.7. Statistical analysis

The design-expert 8.0.6 software was used to design the response surface. In order to minimize the systematic error, each experimental measurement was replicated 3 times. The differences were less than

5%, and the results were subjected to the ANOVA analysis using the Qrigin8.0 and Design-expert 8.0.6.

3. Results and discussion

3.1. Catalyst characterization

Fig.2 XRD image of CFA (modified, untreated and calcined CFA).

As shown in Fig.2 (untreated CFA), appearance of amorphous glassy phase in the CFA can be

observed through the broad peaks in the range of 18° to 46° 29. As indicated in Fig. 2 (calcined CFA), the peaks for calcined CFA at 600°C appeared at 29=21.12°, 25.23°, 50.08°, which were the S1O2

characteristic peaks, while the peaks appeared at 29=16.29°, 25.11°, 26.25°, 35.89°, 40.77°, which were the characteristic peaks for mullite(Al6Si2Oi3). The results indicated that the chief constituents in calcined CFA were SiO2 and mullite. Mullite could be broken down into activated aluminum silicate at high temperatures. The broad peaks appeared at 29=33°~36°, it indicated that the CFA contained numerous

vitreous body.

Fig.2 (modified CFA) depicts the XRD image of modified CFA. As can be seen, the numerous diffraction peaks of new crystalline phases appeared at the modified catalyst. Thus, we inferred that the

active ingredient had been loaded on the CFA and had formed new crystal phases with strong catalytic

activity [30]. The SiO2 characteristic peaks at the modified CFA still existed, but the characteristic peaks for mullite disappeared, this indicated that mullite had been decomposed. While the broad peaks of 20=33°~36° disappeared, this indicated that numerous vitreous body in the catalyst had converted into crystalline substances. Instead, the peaks appeared at 20=33.09°, 34.40°, 35.18°, which were the characteristic peaks for NaAlO2. While an extremely powerful peak appeared at 20=29.21°, which was the characteristic peaks for Na2SiO3. This was because that active compositions, namely NaOH and Na2SO4 in solution reacted with active products, namely SiO2 and Al2O3 from decomposed mullite and quartz. This is similar to the findings of Wang et al.(2010) [31].

4000 3000 2000 1500 1000 500

Wave number (cm-1)

Fig.3 IR image of modified CFA and untreated CFA.

The FTIR image of CFA with respect to treatment process Fig. 3 show major peaks at 1089, 799, 558 cm-1 in untreated CFA, which disappeared at modified CFA. According to Liu (2010) and Wang (2010) et al. [31, 32], the both peaks were the nonsymmetry flexible vibration band of Si/Al-O and the symmetry flexible vibration band of Si-O, respectively. After modified treatment, new bands appeared around 3458, 1630-1620, and 1391 cm-1. The observed changes in IR image indicated OH- and SO42- had been supported on the CFA.

90 80 70 60 ? 50 " 40 30 20 10 0

- ★ calcined CFA

/ ▼ modified CFA

J ^■k--

-<--<-<

0123456789 10 11 12 Time(min)

Fig.4 Effect of different catalysts on transesterification.

Three different catalyst types (untreated CFA, calcined CFA, and modified CFA) were examined for

their effectiveness under the identical experimental conditions (the molar ratio of methanol to WCO was 7:1, the catalyst amount 6% (wt/wt), and reaction temperature 70 °C. As shown in Fig. 4, the catalytic activities of the catalysts used in the transesterification reactions exhibit maximum ester conversions: namely untreated CFA 20.15%, calcined CFA 49.02%, and modified CFA 90.33%. From the Fig.4 it is clear that the highest biodiesel conversions of 90.33% was obtained when modified CFA was used as catalyst for the WCO transesterification, and the catalytic activity of modified CFA was first, then calcined CFA, and finally untreated CFA. The catalytic activity is a critical factor in the transesterification reaction. The catalyst possessing powerful basic sites more, catalytic activity higher [33, 34]. According to XRD and FTIR analysis, the modified CFA catalyst had basic and acidic active sites.

3.2. Response surface test 3.2.1. Regression model

The experimental results were fitted according to Eq. 1 as a quadratic polynomial regression equation.

k k k k • 2

r=/i0 + X fiiXi + v fisX? + £ £ ^XXj + s

f=l r=l f=l j=l

Where Y is the predicted response, fa is the intercept term, fa is the linear effect, fin is the square effect,

and Pij is the interaction effect; Xi and Xj are the variables, i and j are the index numbers for the pattern, and s is the error.

The relations of three effect factors (molar ratio of methanol to WCO, amount of the modified CFA and reaction temperature) to conversion yield of the WCO biodiesel was investigated by multiple regression analysis. According to the CCD experimental design, the results are shown in Table 2. The second order polynomial equation was gained to explain the conversion process. Eq. 2 represents the conversion yield as a function of the coded units. Y = 89.87 + 5.81Xj -6.59X2 -4.63X3 -8.48XX -2.25XX -

2.26X2X3 - 7.47Xj2 -9.37X22 - 6.89X32

Where Y is the predicted values for the conversion yield of the WCO biodiesel. X;, X2 and X3 are model terms that represent molar ratio of methanol to WCO, amount of the modified CFA and reaction temperature, respectively. The predicted conversion yields from different projects were determined by Eq. 2 (see Table 2). It can be seen that the predicted values are in good agreement with the experimental values. It indicates that the conversion yield of the WCO biodiesel is related to the selected variables in this study.

An analysis of variance (ANOVA) was carried out to gain sum of P values, F values, squares(SS), degrees of freedom (df) and mean squares (MS) by fitting the quadratic polynomial equation by the data of Table 2, as shown in Table 3.

Table 3 Variance analysis of the established regression model.

Source Sum of squares df Mean square F value Prob>F

Model 4314.64 9 479.40 136.57 < 0.0001significant

X1 461.50 1 461.50 131.47 < 0.0001

X2 592.56 1 592.56 168.80 < 0.0001

X3 293.18 1 293.18 83.52 < 0.0001

X1X2 574.94 1 574.94 163.79 < 0.0001

X1X3 40.41 1 40.41 11.51 0.0069

X2X3 40.85 1 40.85 11.67 0.0066

X12 804.00 1 804.00 229.04 < 0.0001

X22 1266.59 1 1266.59 360.82 < 0.0001

X32 683.32 1 683.32 194.66 < 0.0001

Residual 35.10 10 3.51

Lack of fit 28.28 5 5.66 4.14 0.0742 not significant

Pure error 6.82 5 1.36

Cor total 4349.74 19

R2=0.9919 R2Adj=0.9847 CV=2.54%

For conversion yield of the WCO biodiesel, the Model F-value of 137.57 implies that the model is significant. There is only a 0.01% chance that a "Model F-Value" this large could occur due to noise. Values of "Prob>F" less than 0.0500 indicate model terms are significant. In this case X;, X2, X3, XX2, X1X3, X2X3, X;2, X22, X32 are significant model terms. That values are greater than 0.1000 indicate the model terms are not significant. The "Lack of Fit F-value" of 4.14 implies the Lack of Fit is not significant relative to the pure error. Non-significant lack of fit is good. The "Pred R-Squared" of 0.9429 is in reasonable agreement with the "Adj R-Squared" of 0.9847. "Adeq Precision" measures the signal to noise ratio. A ratio greater than 4 is desirable. Adeq ratio of 33.129 indicates an adequate signal. Thus, the conversion yield of the WCO biodiesel model can be used to navigate the design space.

To verify the reliability of the model further, the normal plot of residuals, comparison of the actual and predicted values are described in Fig. 5. As seen in Figure 5, response values and experimental values of fit are good (0.05 <p<F). The points gathered around the diagonal line, and the value of R2 (0.9919) is close to R2Adj (0.9847). These are the cue to the goodness match between the predicted values and the experimental values, indicating excellent stability of the regression models.

Predicted vs. Actual Normal Plot of Residuals

Actual Internally Studentlzed Residuals

Fig.5 The predicted and true values (a), normal plot of residuals (b).

3.2.2. Response surfaces and contour plots

Response surface and contour plot are generally applied to determine the optimum values and

facilitate a better understanding for interactions between variables. The interactions between the variables

and responses are shown in Fig. 6.

Figure 6a showed the effects of molar ratio of methanol to WCO and amount of the modified CFA on

the conversion yield of the WCO biodiesel while reaction temperature was fixed at middle level. As we

can see from the figure, with molar ratio of methanol to WCO growing, conversion yield of the WCO

biodiesel increases. The variation of conversion yield begins to flatten after about 9:1. The conversion

yield initially increased and then decreased with the increase of amount of the modified CFA. Conversion

yield is significant when the amount of the modified CFA is lower (below 6%) and the molar ratio of

methanol to WCO is higher (above 7:1). According to Veera et al (2013) [34], the molar ratio of methanol

to oil is one important factor affecting biodiesel conversion yield. Transesterification is an equilibrium

reaction, and it's true that the equilibrium reaction is shifted toward the desired methylesters until

methanol is used in excess. The trend was similar to the previous study [35]. On the other hand, amount

of catalyst is also a critical factor for transesterification. Low catalyst amount can result in unsatisfactory

conversion yield; However, high catalyst amount can result in mass transfer resistance to be higher, which

causes the reaction rate to slow, therefore biodiesel conversion yield decreases [36]. Furthermore, high

catalyst amount may cause high costs.

Fig.6b showed the effects of molar ratio of methanol to WCO and reaction temperature on conversion

yield of the WCO biodiesel while amount of the modified CFA was kept at central level. As can be seen

from the Fig.6b, the contour plot submit well elliptical. This demonstrates that interaction of the two

factors is also remarkable. Moreover, with reaction temperature increasing, the conversion yield first

increases (from 50 °C to 70 °C) and then decreases (from 70 °C to 90 °C). When the reaction temperature was about 70 °C, the conversion yield achieved the maximum value for a given molar ratio of methanol to WCO, while the reaction temperature both higher and lower than 70 °C could both produce the decline of the conversion yield. Similar results were found by Hong et al. [25]. An increase in the temperature from 40 to 70 °C caused a significant increase in yield from 83.5% to 94.9%; when the temperature was further increased to 90 °C, the yield decreased from 94.9% to 85.3%. According to the previous study [20], the microwave output must not be too high, as it may lead to damage to organic molecules such as triglycerides, which are cleaved to FFA. Although higher temperature could cause a drastic decrease in viscosity of oil that is beneficial to increase the solubility of the oil in the methanol and enhance the contact between methanol and oil molecules, thus reaching a better conversion of triglycerides, higher temperature could also accelerate the saponification of triglycerides, and brought about a negative effect on the product yield [37, 38]. Furthermore, Patil et al. found that the energy required for the microwave heating method is 23 times lower than that required for the conventional method [21]. These results suggest that appropriate temperature control will result in effective use of microwave energy and reduce energy requirements, and the comprehensive analysis of energy requirements will be further studied in the future.

b molar ratio of methanol to WCO

Fig.6 Response surface curves (left) and contour plots (right) showing the effects of interactions

of the factors on conversion yield: (a) molar ratio of methanol to WCO and amount of the modified CFA, (b) the molar ratio of methanol to WCO and reaction temperature, (c) amount of

the modified CFA and reaction temperature.

The effect of amount of the modified CFA and reaction temperature on the conversion yield of the WCO biodiesel is presented in Fig. 6c. As seen here, interaction of amount of the modified CFA and reaction temperature is also significant. With increasing amount of the modified CFA, it is clear that the conversion yield initially increases (from 2% to 6%) and then decreases (from 6% to 10%). Similar results have been reported by other researcher [39]. When reaction temperature is about 66-70 °C, the conversion yield achieves the maximum value for a given amount of the modified CFA. The phenomenon was also consistent with that of Shao et al. (2016) [39]. From the contour plot, the conversion yield is considerable when amount of the modified CFA and reaction temperature is less than 6% and 70 C , respectively.

3.2.3. Optimization of WCO properties and model verification

Table 4. Verification of the Model.

run molar ratio of methanol to WCO amount of the modified CFA (%) reaction temperature (°C) predicted value(%) experimental value(%)

1 9.6:1 4.00 66 94.91 93.96

2 9.65:1 3.95 66.5 94.90 95.51

3 9.7:1 4.05 67 94.78 94.02

The standard deviations <5%

In the study, the optimal condition was obtained by using RSM. Optimal condition for the response at a molar ratio of methanol to WCO 9.67:1 was determined to be an amount of the modified CFA of 3.99% (wt/wt), and reaction temperature 66.20 °C. Under this optimal condition, the conversion yield of the WCO biodiesel was expected to be 94.91%. To confirm the adequacy of the model and the availability of the optimization process, three set experiments were performed near the optimum conditions. Each set experiment was repeated three times. Table 4 showed the results. As can be seen from the Table 4, the predicted and experimental values match well. The good agreement testifies the validity of the models for simulating the biodiesel production from the WCO using modified coal fly ash as catalyst under microwave assisting system. 3.3. Reusability of the modified CFA catalyst

A marked characteristic of the heterogeneous catalyst is recoverable [40]. Reusability of the modified CFA catalyst was tested for 8 cycles with 4 % catalyst (wt/wt), a methanol to WCO ratio of 10:1, a reaction temperature of 66 °C ( see Fig. 7). After each cycle, the catalyst was separated from the reaction mixture by filtration and washed with methanol to remove the adsorbed stains and recalcined at 600 C for 1 h for further use. The results show that a high WCO biodiesel conversion yield (above 90%) was realized for all the 8 experiments.

1 -------------

90 -■

^ 70 60

0123456789

Reuse number

Fig.7 Effect of repeated use of modified CFA catalyst on biodiesel conversion.

3.4. Physical properties of the WCO biodiesel Table 5 GC-MS analysis of biodiesel.

Retention time (min) Fatty acid methyl esters Corresponding acid Mass percent (%)

42.11 Methyl tetradecanoate C14:0 0.88

49.13 Methyl hexadecanoate C16:0 21.62

49.72 Methyl 9-hexadecenoate C16:1 1.93

55.61 Methyl octadecanoate C18:0 15.05

55.89 Methyl oleate C18:1 20.41

57.33 Methyl linoleate C18:2 36.77

58.81 Methyl linolenate C18:3 3.04

The mass percent of fatty acid methyl esters in prepared biodiesel is shown in Table 5. The produced biodiesel primarily contained ethyl hexadecanoate (21.62 wt%), methyl octadecanoate (15.05 wt%), methyl oleate (20.41 wt%), and methyl linoleate (36.77 wt%). In addition, lesser content fatty acid methyl esters such as methyl tetradecanoate (0.88 wt%), 9-hexadecenoic acid methyl ester (1.93 wt%), and methyl linolenate (3.04 wt%) were also detected.

The properties kinematic viscosity, density, acid value, flash point and moisture content of the self-made biodiesel were measured and compared with the EN14214 and ASTM D-6751 standards for biodiesel and showed in Table 6.

Table 6 Characterization of self-made biodiesel.

Test EN14214 ASTM D-6751 Self-made biodiesel

Kinematic viscosity (mm2/s) at 40 °C 3.50~5.00 1.9~6.0 4.77

Density (kg/m3) at 15 C 860~900 860~894 886

Acid value (mgKOH/g) ^0.5 ^0.5 0.41

Flash point (C) >120 > 120 181

Pour point (C) - - -4

Cloud point (C) - - 2

Cold filter plugging point (C) - - 2

Induction period (110 C, h) Min 6 Min 3 6.1

Calorific value (MJ/kg) - - 44.9

Moisture content (%) <0.05 <0.05 0.03

4. Conclusions

This study revealed that modified coal fly ash catalyst can increase WCO biodiesel yields under microwave strengthened action. Experimental results showed that the highest biodiesel yield of 94.91% was obtained under the conditions of a 9.67:1 molar ratio of methanol to WCO, a amount of the modified CFA of 3.99% (wt/wt), and a 66.20 °C reaction temperature. The experimental results agreed very well with the predicted values derived from the model, with an R2 value of 0.9919. The modified coal fly ash can catalyze the transesterification reaction with a yield above 90% after the catalyst was used for 8 times repeatedly indicating that the modified coal fly ash catalyst was recyclable and thermally stable. Main parameters of the obtained biodiesel met the EN14214 and ASTM D-6751 standards. Acknowledgement

The authors are grateful for the funding and support provided by the following projects: the Scientific Research Starting Foundation for high-level professionals in Yulin University of China (No. 12GK04), the Natural Sciences Special Foundation of Shaanxi Provincial Education Department in China (2013JK0880). References

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