|Nelson Saksono||Department of Chemical Engineering, University of Indonesia, Kampus Baru UI Depok, West Java, 16424, Indonesia|
|Adream Bais Junior||Department of Chemical Engineering, University of Indonesia, Kampus Baru UI Depok, West Java, 16424, Indonesia|
|Ratih Anditashafardiani||Department of Chemical Engineering, University of Indonesia, Kampus Baru UI Depok, West Java, 16424, Indonesia|
Plasma electrolysis is a process of electrolysis that uses a DC current to excite electrons in the electrolyzed solution. The method is very prolific in producing hydroxyl radical (OH•), which is then used to react with methanol and form a methoxyl radical (CH3O•). Methoxyl radical is used to break the bond of triglycerides to form methyl ester (biodiesel) and glycerol. The purpose of this study is to obtain a good quality and quantity of biodiesel by examining the effect of anode depth with a constant contact area where the anode is the spot of plasma formed. The solution used contains Refined, Bleached, and Deodorized Palm Oil and methanol with a molar ratio of 1:24 and a concentration of KOH 1%-wt. The variations of anode depth are 0.5 cm, 1.5 cm, and 3.5 cm below the surface of the solution, with 5 mm as the constant contact area. The results of this research show an improvement in efficiency, as indicated by yield, and the energy consumption of biodiesel synthesis with increasing depth of the anode. The maximum yield was reached at an anode depth of 3.5 cm, which produced 96.09% as a biodiesel yield with 0.039%-vol water content, 0.138 as the acid number, and a specific energy requirement of 0.909 kJ/ml.
Anode depth; Anodic plasma; Biodiesel; Plasma electrolysis
The application of plasma electrolysis has proven to be effective in the production of valuable products, such as hydrogen, with a low consumption of energy (Saksono et al., 2016). The method has also been proven to degrade complex organic waste, such as phenol (Saksono et al., 2015). The application of the technology to biodiesel production is an interesting topic to explore due to the fact that biodiesel is attracting global attraction as an alternative fuel type owing to its biodegradability and environmental friendliness (Haron et al., 2017). The synthesis of biodiesel using the plasma electrolysis method offers a breakthrough because it can accelerate the rate of synthesis and reduce specific energy consumption, compared to the conventional methods (Saksono et al., 2018).
Conventional biodiesel synthesis using Crude Palm Oil (CPO) generally employs the transesterification, or alcoholysis, reaction method of triglycerides using a homogeneous catalyst in the form of an acid catalyst (H2SO4, HCl) and alkaline (NaOH, KOH). It is most common for alkaline catalysts to be used in the production process of biodiesel plants due to the speed and ease of the process. However, the process also has the disadvantage of forming significant volumes of water and soap from a sapling reaction, which results in a decrease in the quality of the catalyst used. In addition, the method is characterized by its slow reaction rate and a tendency for the reaction to sometimes stop prior to 100% conversion into a product in the form of biodiesel (Boocock et al., 1998). It is possible to overcome this weakness by using an acid catalyst. However, while acid catalysts can negate the saponification reaction, they also require complex and corrosive conditions, which also require a long separation time. In addition, the acid and base catalysts need to be processed first, thereby entailing additional costs.
Plasma, on the other hand, produces a large amount of methoxyl radical (CH3O•) (Zong et al., 2009) that will react with triglycerides to form biodiesel. Plasma can be either cathodic or anodic, depending on the electrode where the plasma is formed. In general, plasma is more stable and easily formed in the cathode than the anode because the cathode emits more secondary electrons (Bruggeman & Leys, 2009). Plasma electrolysis is a process of electrolysis that uses a DC current to form electric sparks as a result of the electrons undergoing plasma excitation in the electrolyzed solution (Saito et al., 2015).
Initially, methanol is mixed with potassium hydroxide to produce the species shown in Equation 1 (Lotero et al., 2005). In the plasma electrode, the high-energy electrons produced by the plasma will break apart water molecule into radicals and transform methoxyl ions into methoxyl radicals (Zong et al., 2009). Meanwhile, on the other electrode, where there is no plasma, a conventional electrolysis reaction will take place that produces ions (Kozáková, 2011). Using anodic plasma, the reactions denoted in Equations 2 and 3 occur in the anode, while the reaction in Equation 4 takes place in the cathode.
The methoxyl radicals produced by those reactions will attack the triglycerides in palm oil and produce methyl esters (Lotero et al., 2005). In general, the mechanism consists of three stages as shown in Figure 1 (Lee et al., 2009). The first stage is a nucleophilic alkoxide attack producing a tetrahedral intermediate. The second stage is the formation of alkyl ester and anion diglyceride. The third stage is the regeneration of the active species that will react with the second molecule of the other alcohol, followed by the recovery of the base catalyst.
Figure 1 Hypothesis of reaction mechanisms (Lee et al., 2009)
In previous research (Saksono et al., 2018), the synthesis of biodiesel using plasma electrolysis has been proven to produce biodiesel products. This research aims to increase the effectivity of the plasma electrolysis method to produce biodiesel. The main objective of this study was to observe the effect of the depth of the anode (which is partially coated by quartz glass) in the solution, where the plasma formation takes place in the anode. The contact area between the anode and solution will remain constant, even when the anode depth is increased; this is done by using a quartz glass coating, which will be explained further in the next section. The effect of anodic plasma depth with a constant anode contact area is crucial because a deeper anode position will affect the amount of energy consumed (Bismo et al., 2013).
This research has observed the effect of anode depth on biodiesel synthesis effectivity, with the results relating to the yield and specific energy requirement of biodiesel. It was found that the greater the depth of the anode, the higher the yield achieved, with the highest yield of 96.09% achieved at an anode depth of 3.5 cm. At the same time, the specific energy requirement of biodiesel also increases in line with anode depth, with the lowest specific energy requirement for biodiesel synthesis of 0.809 kJ/mL recorded at a depth of 0.5 cm. Based on the quantity of methyl ester content, kinematic viscosity, density, and acid number, this biodiesel research is qualified according to SNI 7182:2015. The water content conforming with SNI 7182:2015 occurs only with biodiesel produced at an anode depth of 3.5 cm.
This research was partially funded by Hibah Penelitian Dasar Kemenristekdikti with Contract No. NKB-1654/UN2.R3.1/HKP.05.00/2019. The authors declare no competing interests or any conflicts of financial interests.
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