Rapid palm-biodiesel production assisted by a microwave system and sodium methoxide catalyst
Introduction
The main advantages of using biodiesel are renewability, lower exhaust gas emissions, and biodegradability. Previous studies found that when biodiesel is used as an alternative fuel in diesel engines it can reduce emissions of hydrocarbons (HC), carbon monoxide (CO), sulfur oxide (SO2), PM (particle matter), polycyclic aromatic hydrocarbon (PAH), and polychlorinated dibenzo-p-dioxin/dibenzofuran (PCDD/F) [1], [2], [3], [4], [5], [6]. The major obstacle to biodiesel commercialization is its high cost, which is approximately 1.5 times more than petroleum diesel fuel due to the cost of vegetable oil [7]. One of the best ways to reduce the cost of biodiesel is to increase biodiesel yields. The reaction temperature, reaction time, catalyst amount, and alcohol to oil ratio are important parameters in biodiesel production. Some of the catalytic reactions used are alkali catalyzed, acid catalyzed and enzymatic transesterifications, of which the first achieves the best results. Biodiesel can be produced at a lower temperature with an alkali catalyst, while acid catalysts require a higher temperature and longer reaction time [8], [9]. Mootabadi et al. [10] investigated the ultrasonic-assisted transesterification of palm oil in the presence of alkaline earth metal oxide catalysts (CaO, SrO and BaO). At optimum conditions, 60 min was required to achieve 95% palm-biodiesel yield compared to 2–4 h with conventional stirring. In addition, the palm-biodiesel yields achieved in 60 min increased from 5.5% to 77.3% (CaO), 48.2% to 95.2% (SrO), and 67.3% to 95.2 (BaO). Suryaputra et al. [11] used waste Capiz shell as raw material for catalyst production for palm-biodiesel preparation. The maximum yield of biodiesel was 93%, obtained at 6 h of reaction time and 3 wt% of amount of CaO catalyst. Soetaredjo et al. [12] investigated KOH/bentonite as a catalyst for biodiesel production. The highest yield of palm-biodiesel over KOH/bentonite catalyst was 90.7% under KOH/bentonite 1:4, a reaction time of 3 h, 3% catalyst, methanol to oil ratio of 6, and a reaction temperature of 60 °C. However, the conventional heating of a sample has a few significant drawbacks, such as heterogenic heating of the surface, limitations dependent on the thermal conductivity of materials, specific heat, and density when compared to microwave irradiation [13], [14], and thus many research groups have recently focused on the latter approach.
Previous studies indicated that microwave-assisted chemical reactions are better than other synthetic techniques, and that microwave heating systems can increase the reaction rate, product yields, and purity of products [15], [16], [17], [18], [19]. Azcan and Danisman [15] carried out microwave assisted transesterification of cottonseed oil in the presence of methanol and potassium hydroxide (KOH), and found that a 7 min reaction time, 333 K temperature and 1.5% catalyst to oil ratio were the optimum reaction parameters for microwave heating. Similar results were found for conventional heating, but with 30 min of reaction time, and the biodiesel yield was in the range of 89.5–92.7%, while rapeseed oil was converted to biodiesel by transesterification using microwave heating, with a yield of 88.3–93.7% [16]. Suppalakpanya et al. [17] concluded that the optimum reaction parameters for the transesterification process aided by microwave heating are a molar ratio of palm oil to ethanol of 1:8.5, 1.5 wt% of KOH/oil, a reaction time of 5 min and a microwave power of 70 W. Suppalakpanya et al. [18] also concluded that a molar ratio of free fatty acid (FFA) to ethanol of 1:24 with 4 wt% of H2SO4/FFA, a microwave power of 70 W, and a reaction time of 60 min are the optimum reaction parameters for the transesterification process aided by microwave heating with a palm-biodiesel yield of 80%. Koberg et al. [19] found microwave irradiation could improve the biodiesel yield, and that the optimum yield of cooked oil biodiesel is 99.8% with microwave irradiation of 1100 W and SrO catalyst. The microwave heating method for the transesterification reaction has been shown to be more energy-efficient than using a conventional heating approach.
Previous studies indicate that palm-biodiesel is cheaper than both soybean-biodiesel and corn-biodiesel [20]. The physico-chemical properties of palm-biodiesel meet the requirements for diesel engine combustion, and are comparable with those of other biodiesels, such as soybean and rapeseed oils [20], [21]. Palm oil production has higher production yield (4.2 MT ha−1) compared to cottonseed (0.2 MT ha−1), peanut (0.3 MT ha−1), soybean (0.4 MT ha−1), sunflower seed (0.5 MT ha−1) and rapeseed (0.7 MT ha−1) [22], [23]. Palm oil is the major oil produced in the world followed by soybean, rapeseed and sunflower oil, and this indirectly helps to lower price of palm oil biodiesel [23], [24]. Indonesia and Malaysia are main palm oil producing countries [23], [25]. The life cycle cost for a 50 ktons palm biodiesel production plant with an operating period of 20 years is $665 million, yielding a payback period of 3.52 years [26]. These results indicate that palm-biodiesel has a greater potential for commercial applications than other biodiesels. Lots of studies focused on sodium methoxide catalyst, microwave heating system and palm-biodiesel [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [27], [28], [29]. However, no study focused on using sodium methoxide catalyst with a microwave heating system to improve the yields of palm methyl ester. Additionally, the effects of different catalysts, amount of catalyst, reaction time, molar ratio of methanol to oil, and microwave power are also assessed.
Section snippets
Transesterification procedures
The acid value of palm oil was less than 2 mg of KOH g−1. The methanol, sodium hydroxide (NaOH), and sodium methoxide (CH3ONa) were high-performance liquid chromatography (HPLC) grade. The experimental setup is shown in Fig. 1. A microwave synthesis reactor (NN-S235, Panasonic Co., Ltd., Taiwan), equipped with a mechanical stirrer and a condenser (LC-10, Hi-point Co., Ltd, Taiwan) was used for microwave reactions. The stirrer was operated at 600 rpm with a magnetic nucleus. Various catalysts (CH3
Comparing the yield using microwave and conventional heating
The conventional heating was carried out at 65 °C, with a methanol to oil molar ratio of 6 and 0.75 wt% CH3ONa. The microwave heating was carried out at 750 W, with a methanol to oil molar ratio of 6, and 0.75 wt% CH3ONa. As shown in Table 1, the methyl ester yield increased with reaction time for conventional heating, and the maximum yield of methyl ester from palm oil with conventional heating was 98.3%. Similar result was found by Noiroj et al. [32]. The 25 wt% KOH/Al2O3 and 10 wt% KOH/NaY
Conclusion
Lots of studies focused on sodium methoxide catalyst, microwave heating system and palm-biodiesel. However, no study focused on using sodium methoxide catalyst with a microwave heating system to improve the yields of palm methyl ester. The experimental results indicate that the reaction time was reduced significantly and the yield of palm methyl ester was improved due to the use of microwave heating and CH3ONa. Microwave heating can achieve better performances compared with conventional
Acknowledgments
This research was supported by the National Science Council of Taiwan under grant NSC 98-2221-E-110-017. The authors gratefully acknowledge the contributions of Professor Houng-Yung Chen’s group, Institute of Marine Biology, National Sun Yat-sen University, for helping with the yield analysis.
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