Pre-Treatment of Waste Cooking Oil using Continuous Microwave-Assisted Glycerolysis Reaction

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Pre-Treatment of Waste Cooking Oil using Continuous Microwave-Assisted Glycerolysis Reaction

Gopinathan M^1*, Fielza F², Kumaran P¹

1 Institute of Sustainable Energy, Universiti Tenaga Nasional, Kajang, Malaysia

2 College of Graduate Studies, Universiti Tenaga Nasional, Kajang, Malaysia

*Corresponding Author: [email protected]

Accepted: 15 December 2020 | Published: 31 December 2020

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Abstract: Waste cooking oil (WCO) can be an alternative feedstock for biodiesel production.

However, very often WCO containing high free fatty acid (FFA) with value above 3% that can reduce the quality and quantity of the biodiesel production is not suitable for direct alkali base transesterification. This research deployed glycerolysis reaction to reduce the FFA of WCO prior to transesterification reaction. Previous works on glycerolysis have reported that glycerolysis reaction required high temperature, longer reaction of time and high energy intense. This research paper reports improved glycerolysis reaction to effectively reduce FFA in WCO using continuous flow microwave irradiation. Initially, the experiment was conducted manually to determine the suitable microwave power for the reaction and found 440W microwave power able to reduce the FFA value the most. Subsequently, response surface methodology (RSM) has been used with three operating variables, namely molar ratio of oil to glycerol, pump speed and reaction time with a total of 17 experiment runs to determine the overall optimum parameters for microwave assisted glycerolysis of WCO. From this study, under the ideal conditions of 1:1 w/w oil to glycerol ratio, 70 rpm pump speed and 10 minutes reaction time, the FFA value has reduced from 3.95% to 0.34%.

Keywords: Pre-treatment, Waste Cooking Oil, Glycerolysis, Microwave Irradiation, Free Fatty Acids

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1. Introduction

Biodiesel is composed of long-chain fatty acid monoalkylesters obtained from vegetable oil and animal fats (Zhang, Dube, McLean, & Kates, 2003) (Singhabhandhu & Tezuka, 2010) (Hong, Jeon, Kim, & Lee, 2016) produced through transesterification. Biodiesel is regarded as a possible alternative to diesel and is a non-toxic and biodegradable fuel. It contains no sulphur or aromatic hydrocarbons, which will result to a reduction of sulphur oxide and carbon dioxide (Hong, Jeon, Kim, & Lee, 2016). Edible oils such as soybean oil, corn oil and canola oil has proven to be potential feedstock for the production of biodiesel but the cost of these edible oils is relatively high which could affect the biodiesel’s long-term commercial viability (Patil P.

D., Gude, Reddy, Muppaneni, & Shuguang, 2012). In this research, the use of a less expensive feedstock such as waste cooking oil (WCO) has been explored. However, WCO predominantly have high FFA value that could reduce the biodiesel conversion efficiency and is not suitable for direct alkali base transesterification. Glycerolysis would therefore serve as a promising pre- treatment method to decrease the amount of FFA in the oil. Glycerolysis is a well-known method that uses glycerol to form monoglycerides, diglycerides and triglycerides by reacting with FFA in WCO (Felizardo, et al., 2011) (Hayyan, Hashim, Mjalli, & Hayyan, 2013)

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(Binhayeeding, Klomklao, & Sangkharak, 2017). Glycerolysis has gained popularity for using less energy-intensive approaches than other methods of pre-treatment. Nevertheless, to boost the glycerolysis reaction, the glycerolysis process still needs a reasonably high temperature of above 200 ° C (Mićić, et al., 2017). Several researchers have indicated that the microwave- assisted method could increase the reaction of the process while significantly reducing the response time compared to the conventional method (Patil P. , Gude, Camacho, & Deng, 2010) (Motasemi & Ani, 2012) (Kostas, Beneroso, & Robinson, 2017) (Nayak & Vyas, 2019). Owing to the drawbacks of heat and mass transfer during the reaction, conventional heating methods need a longer reaction time with higher energy requirements (Encinar, Gonzalez, Martinez, Sanchez, & Pardal, 2012) (Maddikeri, Gogate, & Pandit, 2012) (Choedkiatsakul, Ngaosuwan, Assabumrungrat, Tabasso, & Cravotto, 2015). Microwave heating is a proven, cost-effective and energy-efficient way of speeding up chemical reactions. (Motasemi & Ani, 2012).

Therefore, it can also be a way to solve the costs of the manufacturing process by introducing microwave irradiation technology. (Groisman & Gedanken, 2008) (Motasemi & Ani, 2012) (Martinez-Guerra & Gude, 2015) (Wang, Xu, Okoye, Li, & Tian, 2018) (Sharma, Kodgire, &

Kachhwaha, 2020).

The present work based on the potential and efficiency of the use of microwave-assisted continuous glycerolysis reaction using used cooking oil without catalyst. In this analysis, process variables such as molar ratio of oil to glycerol, pump speed and reaction time were optimized using Response Surface Methodology (RSM), and the ideal condition was found to be 1:1 oil to glycerol ratio, 70 rpm pump speed and 10 minutes reaction time.

2. Materials and methods

2.1 Materials

Waste cooking oil (WCO) has been collected from food stalls in Universiti Tenaga Nasional (UNITEN) while the crude glycerol was bought from a biodiesel refinery plant. For this experiment, Figure 1(a) and Figure 1(b) below shows the condition of WCO and crude glycerol under room temperature.

2.2 Reactor details

The glycerolysis reaction was carried out in a modified rig in Biodiesel Research Lab, UNITEN and a 1000W conventional household microwave was used and modified for this experiment.

Figure 2 below shows the chemical reaction of glycerolysis process.

Figure 1: (a) WCO and (b) Crude Glycerol from Biodiesel Plant

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Figure 2: Chemical reaction of glycerolysis process (Chol, Dhabhai, Dalai, & Reaney, 2018)

2.3 Experimental procedure

WCO collected from local food outlets was preheated 3 times to remove the moisture prior to glycerolysis reaction. The temperature was set at 105°C, 110°C and 115°C and the mass reduction of WCO was recorded. Once constant mass of WCO was obtained, indicates that the moisture content in the oil has been completely eliminated. The FFA of WCO is measured using titration method and the initial FFA value of 3.95% was recorded. WCO and crude glycerol were then mixed with desired oil to glycerol ratio and heated and subsequently transferred to the feedstock tank. The stirring speed was kept constant. The mixture passed through the microwave chamber at desired pump speed. Samples were taken at a given interval in time. The reaction mixture was cooled to room temperature prior to the separation procedure.

To ensure that there was no glycerol in the WCO sample, the pre-treated WCO sample was sent to the centrifuge for further separation. The oil and glycerol were easily separated where heavier phase glycerol will be at the bottom layer and oil on the top. Figure 3 below shows the separation of oil and glycerol after centrifuge.

Figure 3: Separation of oil and glycerol after centrifuge

2.4 Sample analysis

To observe the FFA reduction in every experiment run, manual titration method was performed. In 0.9g of oil, 10 ml of isopropyl propanol and 7 drops of phenolphthalein were mixed together as a solvent and as an indicator. Then, drop by drop of sodium hydroxide will be added into the solution until it turns pink, indicating the neutralization of FFA.

2.5 Experimental design

RSM of Box Behnken Design (BBD) was used in this research to optimize the optimal conditions for reducing the FFA in the oil. BBD was developed using Design Expert 12 software. In this work, a total of 17 experiment runs, 3 variables, and 2 levels were carried out.

The variables and their levels are shown in

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Table 1 below, where they were chosen based on the experimental trials performed in the laboratory.

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Table 1: Variables for Box Behnken design for microwave-assisted glycerolysis using WCO Name Units Low High

A [Numeric] Molar ratio mol/mol 1 3 B [Numeric] Pump Speed RPM 60 80 C [Numeric] Reaction time min 10 40

3. Results and Discussion

3.1 Preliminary Experiment Optimization of Microwave Power

This experiment was conducted to determine the optimum condition of microwave power for the whole process. With a constant reaction time of 40 minutes, various microwave power levels (100W, 300W and 440W) with different molar ratios of oil to glycerol (1:1, 1:2 and 1:3) were carried out without catalysts. The effects of microwave power on the FFA value can be seen in Figure 4 below.

Figure 4: Effect of Microwave Power on FFA value

Higher temperatures have been shown to result in a greater reduction in FFA, obtained at 440W and 1:3 molar ratio. This is because high temperatures enhance the process reaction rate and increase the conversion of the reaction, similar to the results found in the previous article (Nguyen, et al., 2020).

3.2 Optimization by RSM

Upon obtaining the optimum microwave power via preliminary experiment, the effect of molar ratio of oil to glycerol, pump speed and reaction time was evaluated using RSM based Box Behnken method. In this method, three variables with two levels were employed for modelling and optimization of the process experiment. Random experiment runs were conducted to minimize the variability in the response. Table 2 shows the obtained results of RSM and the regression coefficients for FFA response were expressed as Equation 1 below,

Equation 1

FFA= 0.24008 - 0.0463875A + 0.0353125B - 0.02285C + 0.0543AC + 0.0552225A² + 0.0336475C²

where, A is the molar ratio of oil to glycerol, B is the pump speed and C is the reaction time.

0 0.5 1 1.5 2

Molar Ratio 1:1 Molar Ratio 1:2 Molar Ratio 1:3 1.9881

1.648

1.4649

0.3401 0.2878 0.2354

0.3139

0.2616 0.1831

FFA (%)

Effect of FFA Value on Different MW Power and Oil:

Glycerol Molar Ratio

100W Microwave Power 300W Microwave Power 440W Microwave Power

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Table 2: Results of RSM Run Factor 1

A: Molar Ratio (mol/mol)

Factor 2 B: Pump speed

(RPM)

Factor 3 C: Reaction Time

(min)

Initial FFA (%)

Response 1 FFA (%)

1 3 70 40

3.95%

0.3335

2 1 80 25 0.3923

3 2 70 25 0.2746

4 1 60 25 0.2746

5 3 80 25 0.2354

6 2 80 40 0.2550

7 2 70 25 0.2157

8 3 60 25 0.2354

9 2 70 25 0.3138

10 2 60 40 0.1373

11 2 60 10 0.2550

12 1 70 40 0.3139

13 2 80 10 0.3531

14 3 70 10 0.2354

15 2 70 25 0.1961

16 2 70 25 0.1961

17 1 70 10 0.4315

Significance of each individual, interaction and quadratic terms were determined by the F- value and probability p-value as shown in the Table 3 below. These results are essential in order to establish the consistency and reliability of the model and determine the impact of all reaction variables.

Table 3: Analysis of Variance (ANOVA)

Source Sum of Squares df Mean Square F-value p-value

Model 0.0655 9 0.0073 10.05 0.0030 significant

A-Molar ratio 0.0172 1 0.0172 23.77 0.0018

B-Pump speed 0.0100 1 0.0100 13.78 0.0075

C-Time 0.0042 1 0.0042 5.77 0.0473

AB 0.0035 1 0.0035 4.78 0.0650

AC 0.0118 1 0.0118 16.29 0.0050

BC 0.0002 1 0.0002 0.3404 0.5779

A2 0.0128 1 0.0128 17.73 0.0040

B2 0.0005 1 0.0005 0.6880 0.4342

C2 0.0048 1 0.0048 6.58 0.0372

Residual 0.0051 7 0.0007

Lack of Fit 0.0030 3 0.0010 1.98 0.2597 not significant

Pure Error 0.0020 4 0.0005

Cor Total 0.0706 16

Std. Dev. 0.0269 R² 0.9282

Mean 0.2768 Adjusted R² 0.8358

CV % 9.72 Predicted R² 0.2686

Adeq Precision 11.6266

Based on the obtained results, it can be noted that all terms in the proposed equation, including linear and quadratic terms, are significant, as indicated by the p-value values obtained, which are less than 0.05. The lower the p-value, the greater the significant variable and the larger the effect would be on the performance. Compared to the pure error, the model F-value was high (10.05), and the lack of fit was not significant (0.2597), which means that the model fit was good. In addition, the capability of the model was veriﬁed by determination coefficient (R²), adjusted R², and coefficient of variation (CV), where high R² indicated that the predicted model could explain 92.82% of the total variation. The CV was 9.72%, where a value that is less than

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10% proved that the experiments performed were accurate and effective. Thus, these ﬁndings showed that the predicted model was well-ﬁtted and can be used to predict a new experiment response. In Figure 5 to Figure 7, three-dimensional response surface graphs are shown.

Figure 5: Interaction of FFA, Pump speed and Molar Ratio

Figure 6: Interaction of FFA, Time and Molar Ratio

Figure 7: Interaction of FFA, Time and Pump speed

Apparently from the graphs above, there were significant interactions between the independent variables. With the optimum conditions obtained from ANOVA which is 1:1 oil to glycerol

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molar ratio, 70 rpm of pump speed and 10 minutes of reaction time, 3 sets of experiment runs have been conducted to observe the average value of FFA in the oil. Table 4 below shows the tabulated data of the 3 experiment runs, and it was found that the FFA value was able to reduce to 0.34% from 3.95% with the optimal conditions obtained.

Table 4: Readings of FFA Runs FFA (%) Average FFA (%)

1 0.3139

0.34 2 0.3661

3 0.34

In addition, using the same optimum parameters, 0.1% of zinc oxide was also used to observe the effect of FFA value between with and without catalyst. Previous article reported that 0.1%

of catalyst was found to be the optimum concentration value for glycerol esterification process (Garcia Martin, Ruiz, Garcia, Feng, & Mateos, 2019). From Figure 8 below, it can be seen that there is no significant difference in FFA value for both processes. Therefore, it was proven that the use of catalyst can be eliminated from the process, which can also be a solution for reducing the overall production costs.

Figure 8: Graph of FFA value with and without catalyst

4. Conclusion

In conclusion, pre-treatment of WCO using the continuous flow microwave reactor glycerolysis method is effective and efficient in reducing WCO's FFA below 1%, which is desirable for alkali-based transesterification. The glycerolysis reaction of continuous flow is more successful than the batch processing reactor stated by previous researchers. In addition, RSM was used to optimise the reaction using the BBD approach with selected 3 variables in a total of 17 experiment runs. Based on RSM, the ideal reaction condition obtained were 1:1 molar ratio of oil to glycerol, 10 minutes of reaction time and 70 rpm of pump speed. In addition, in this analysis, 0.1 percent zinc oxide was used to observe the difference between with and without a catalyst, and no substantial difference between the two processes was found.

It can therefore be concluded that microwave-assisted glycerolysis using waste cooking oil can reduce the overall production cost without any catalyst interference and provides a promising result in reducing the FFA value in the oil, where the initial FFA was reduced from 3.95 % to 0.34%.

0.34 0.34

0 0.1 0.2 0.3 0.4

With Catalyst Without Catalyst FFA %

FFA %

Effect of FFA Value with and without catalyst

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Acknowledgement

Authors would like to thank University Tenaga Nasional for funding this research through BOLD Grant (10436494/B/2019070) and UNITEN-IRMC management for their valuable supports, guidance and assistance throughout the research. Besides, the authors would like to thank UNITEN undergraduate students for their contribution in data collection for this study.

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