## 3 Chemical Properties of Waste Cooking Oils and Purification Process
### Transesterification of Waste Cooking Oil  
The transesterification reaction is the reaction between the treated oil and alcohol, mostly methanol due to availability and cost, in the presence of a catalyst, as presented in Figure 8. In addition, the reaction using methanol is often considered faster than ethanol, and also, separation is more difficult when ethanol is used for the transesterification reaction [107; 108]. The types of catalysts used during this process could be homogenous, heterogenous, or enzymatic [109]. The efficiency of a homogeneous catalyst is high with low reaction time, low temperature requirement, and high yield, as presented in Table 5, but due to the homogeneous nature, it requires several rinses to remove traces of soap in the oil. This may introduce moisture and impurities in the oil, consequently affecting the quality of the oil as an insulating liquid and possibly corroding the component of the transformer. This may be the reason for the high conductivity of fatty acid methyl ester reported by Refs. [101; 106]. The common examples of homogeneous catalysts are NaOH and KOH. However, the heterogeneous catalyst does not dissociate in the oil, therefore providing easy separation and no possibility of corrosion on equipment components. Due to the possibility of reuse, the heterogeneous catalyst has cost benefits over the homogeneous catalyst [109; 110]. The significant disadvantage of heterogeneous catalysts is the high reaction temperature and duration. Examples of heterogeneous catalysts are CaO, MgO, BaO, SrO, titanium oxide, and zinc oxide. It is to be mentioned that base heterogeneous catalysts have gained high applicability due to their high activity and stability; however, they require high reaction temperatures, which may hinder their application for industrial purposes. The metal-doped catalysts are now finding application in the replacement of base heterogeneous catalysts (TiO\({}_{2}\)-MgO, SO\({}_{2}\)\({}^{2-}\)/ZrO\({}_{2}\)/SiO\({}_{2}\)) [95] due to their low reaction time and excellent yield, as presented in Table 5. The enzyme-based catalyst is also used in the transesterification reaction. The advantage of this catalyst is that it requires no difficult separation of products, it can be reused, and it wastes no water, consequently reducing the cost and environmental pollution [111; 112]. In addition, this reaction is not affected by the high free fatty acid content of the base oil. However, these catalysts are expensive, which may eventually affect the production cost during transesterification. Due to strenuous activity in the adoption of catalysts in the transesterification process (washing of biodiesel, separation of by-products, etc. ), the application of the supercritical approach (supercritical alcohols) has surfaced as an alternative, as it requires no application of catalysts, is not affected by FFA values, and rapidly transforms oil into FAME. In addition, due to low reaction time, it requires low energy consumption [95; 113]. The comparative overview of transesterification methods during fatty acid methyl ester production is presented in Table 6.  
Based on the data presented in Table 5, it is evident that the percentage yield varies irrespective of the reaction type. This indicates that, apart from the reaction type, the reaction conditions, including the process variables, also play a crucial role. Therefore, optimizing these reaction parameters is essential for achieving a high percentage yield during fatty acid methyl ester production. A summary of the key factors that can affect the percentage yield during FAME production is outlined in Table 7[20; 95]. Among the crucial parameters that affect the yield of FAME during the transesterification process is the  
Figure 8: Transesterification reaction. reaction temperature, as it significantly influences the reaction kinetics [114; 115; 116]. This can be attributed to the reduction in oil viscosity at elevated temperatures, which consequently enhances the reaction rate. The investigation reported in Ref.