Enhancement of Biodiesel Production from High-Acid-Value Waste Cooking Oil Via a Microwave Reactor Using a Homogeneous Alkaline Catalyst †
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energies Article Enhancement of Biodiesel Production from High-Acid-Value Waste Cooking Oil via a Microwave Reactor Using a Homogeneous Alkaline Catalyst † Ming-Chien Hsiao 1,2, Peir-Horng Liao 1, Nguyen Vu Lan 3 and Shuhn-Shyurng Hou 2,4,* 1 Department of Environmental Engineering, Kun Shan University, Tainan 71070, Taiwan; [email protected] (M.-C.H.); [email protected] (P.-H.L.) 2 Green Energy Technology Research Center, Kun Shan University, Tainan 71070, Taiwan 3 Faculty of Mechanical Engineering, Ho Chi Minh City University of Technology and Education, Ho Chi Minh City 70000, Vietnam; [email protected] 4 Department of Mechanical Engineering, Kun Shan University, Tainan 71070, Taiwan * Correspondence: [email protected]; Tel.: +886-6-205-0496 † This article is an extended version of our paper presented in 2019 IEEE International Conference on Applied System Innovation, 11–15 April 2019, Fukuoka, Japan. Abstract: In this study, low quality oils (waste cooking oils) with high acid value (4.81 mg KOH/g) were utilized as the feedstocks for a transesterification reaction enhanced by additional microwave power and the use of an NaOH catalyst. The kinetics of the transesterification reaction under different reaction times and temperatures was studied. It was found that in the microwave-assisted transesterification reaction, the optimum conditions under a microwave power of 600 W were as follows: an NaOH catalyst of 0.8 wt %, a 12:1 molar ratio of methanol to oil, a reaction time of 2 min, and a reaction temperature of 65 ◦C. The conversion of waste cooking oil into biodiesel reached 98.2% after this short reaction time. This result conformed to 96.5% of the standard value of Taiwan CNS 15072. In addition, with increases in the reaction temperature from 55 to 65 ◦C, Citation: Hsiao, M.-C.; Liao, P.-H.; the reaction rate constant increased from 0.635 to 2.396 min−1, and the activation energy required for Lan, N.V.; Hou, S.-S. Enhancement of the transesterification reaction was 123.14 kJ/mole. Biodiesel Production from High-Acid-Value Waste Cooking Oil Keywords: biodiesel; microwave; waste cooking oil; transesterification; high acid value via a Microwave Reactor Using a Homogeneous Alkaline Catalyst. Energies 2021, 14, 437. https://doi. org/10.3390/en14020437 1. Introduction Received: 25 November 2020 In the present world, a shortage of energy resources has been foreseen. Fossil fuels Accepted: 11 January 2021 are gradually being replaced by sustainable fuels, one of which is biodiesel. One of the Published: 15 January 2021 most promising methods for sustainable fuels is to convert vegetable oils and other raw materials that mainly contain triglycerides (TGs) and free fatty acids (FFA) into biodiesel [1]. Publisher’s Note: MDPI stays neu- The use of homogeneous catalysts to produce biodiesel has a high conversion rate and a fast tral with regard to jurisdictional clai- reaction, but due to the following reasons, it has some disadvantages in the production of ms in published maps and institutio- biodiesel: (1) the catalyst has to be neutralized when the reaction is completed and cannot nal affiliations. be reused; (2) continuous processing methods are limited; (3) it is technically difficult to remove the catalyst; (4) cleaning and separating products and catalysts will produce a lot of wastewater; and (5) reactant purity and free fatty acids (FFA) content affect the process quite sensitively. Therefore, high-quality feedstocks (for example, refined vegetable Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. oils (RVOs)) are required to prevent undesirable saponification and hydrolysis reactions, This article is an open access article or additional reaction processes are required to reduce FFA content [1–3]. distributed under the terms and con- The major disadvantage of biodiesel production at present is the high production ditions of the Creative Commons At- cost [4], which mainly comes from the cost of RVOs. Using waste cooking oil (WCO) tribution (CC BY) license (https:// (an inexpensive or sometimes even free raw material) to produce biodiesel is one of the creativecommons.org/licenses/by/ ways to solve this problem. Furthermore, the reuse of WCO is also a solution to waste 4.0/). oil treatment [5,6]. Accordingly, using low-cost feedstocks like high-acid-value oil [4] Energies 2021, 14, 437. https://doi.org/10.3390/en14020437 https://www.mdpi.com/journal/energies Energies 2021, 14, 437 2 of 11 and WCO [7,8] can greatly reduce the production costs related to biodiesel production. However, compared with new oil, WCO has a high FFA content, and therefore requires more extensive transesterification conditions [9]. In general, an alkaline process can produce high-yield biodiesel within 0.5–1 h. How- ever, in this process, only refined vegetable oils with a low acid value (below 2 mg KOH/g) can be utilized as reactants. When the treated oil contains FFA of 2 mg KOH/g or more, an acidic process is preferably used because it is simple. In fact, low-cost oils such as WCO and waste fats are the promising desirable feedstocks, but their FFA content is high. Using alkaline catalysts, these materials are easy to produce soap, which makes it difficult to separate biodiesel from the glycerol fraction [10]. This problem can be avoided by using an acidic catalyst, where the acids catalyze FFA esterification to generate fatty acid methyl esters (FAMEs), thereby enhancing the yield of biodiesel. Unfortunately, according to [11,12], despite the help of an acidic catalyst, the reaction time can still be as long as 9 h even if methanol is refluxed, and a high catalyst concentration of about 5 wt % and a high methanol-to-oil molar ratio of about 40:1 are used. The conversion rate of FAME is as high as 97% at 95 ◦C. Microwave irradiation is an excellent method that can be used to activate polar molecules and ions. This method is thus also very suitable for vegetable oil, which contains methanol and potassium hydroxide, because they have both polar and ionic components. Accordingly, during microwaving, energy is absorbed by these molecules due to ionic movement and dipole rotation. Molecular friction caused by rapid rotation generates heat. It has been proven that microwave-assisted chemical reactions can lead to better synthesis performance as compared to other methods in terms of increasing the reaction rate [13–16]. As reported by [17–19], microwave irradiation increases the reaction rate, thereby achieving high biodiesel yield more quickly. In our previous studies, it was shown that the transesterification of soybean oil can be improved by using an alkali catalyst via ultrasonic mixing and microwave heating methods [20]. Microwave-assisted irradiation also led to dramatic enhancement of the transesterification efficiency of soybean oil used to synthesize biodiesel [21]. More recently, a modified CaO catalyst was used to improve the transesterification reaction of WCO in a microwave reactor [9]. The reaction time for the synthesis of modified CaO was found to be greatly reduced by microwave heating. When the transesterification reaction was optimized under the following conditions, the conver- sion rate of biodiesel was 98.2%: a catalyst loading of 4 wt %, a methanol/oil molar ratio of 8:1, a reaction temperature of 65 ◦C, and a reaction time over a 75 min period. Additionally, compared with the traditional water bath typically used in the transesterification reaction, the microwave-assisted method is a more energy-efficient option [9,22]. Factors including the amount of catalyst [22–25], the methanol-to-oil molar ratio [26,27], the reaction temperature [28,29], and the reaction time [30,31], significantly influence the transesterification of WCO. Thus, investigation of these important factors is conducted in this research, where low-value oil (high-acid-value WCO) is converted to biodiesel under microwave-assisted conditions. Little attention has been paid to the use of advanced microwave technology to produce biodiesel using low-quality oil (WCO) with high acid value as a raw material. This article is of interest as it combines the use of an alternative energy to the microwave method of biodiesel synthesis with the reuse of oils containing a high acidity index. This study also emphasizes the use of a small amount of alkaline catalyst to avoid saponification in the process. Especially, the transesterification kinetics of high-acid-value WCO was also performed under different reaction times and reaction temperatures. 2. Chemicals, Equipment and Methods 2.1. Chemicals In this study, the waste cooking oil (WCO) used was gathered from an instant fried chicken shop in Tainan, Taiwan, and its FFA content was about 2.41 wt %. The catalyst used was sodium hydroxide (purity: 98%) obtained from Mallinckrodt Co., Ltd., St. Louis, Energies 2021, 14, x FOR PEER REVIEW 3 of 11 2. Chemicals, Equipment and Methods 2.1. Chemicals Energies 2021, 14, 437 3 of 11 In this study, the waste cooking oil (WCO) used was gathered from an instant fried chicken shop in Tainan, Taiwan, and its FFA content was about 2.41 wt %. The catalyst used was sodium hydroxide (purity: 98%) obtained from Mallinckrodt Co., Ltd., the MO,United USA, States and theof America, alcohol used and wasthe alcohol methanol used (purity: was methanol 99.8%) acquired (purity: from 99.8%) Shimakyu acquired0s Purefrom Chemicals Shimakyu Co.,′s Pure Ltd., Chemicals Osaka, Japan. Co., Ltd., Acetate Japan. and Acetate Methyl and laurate Methyl were laurate supplied were fromsup- Tokyoplied Chemicalfrom Tokyo Industry Chemical Co., Industry Ltd., Tokyo, Co., Ltd., Japan. Japan. 2.2.2.2. Equipment Equipment TheThe reactions reactions were were performed performed using using a a microwave microwave heating heating system, system, as as shown shown in in Figure Figure1.