## 3 Results and Discussion
### Influence of Various Parameters on the Yield of Biodiesel  
The conditions used for the preparation of biodiesel in the continuous reactor were a reaction temperature of 60 \({}^{\circ}\)C, methanol/oil molar ratio of 16, catalyst bed height of 250 mm and pressure of 1 atm. The impurities and non-oil components of WFO were removed by filtration before use. Flow rate of reactant is one of the key parameters during the continuous transesterification carried out in the PBR. Residence time was controlled by changing the flow rate of the feedstock chemical [27]. The corresponding yield of biodiesel with the residence time is shown in Figure 4. The yield of methyl ester increased from 61.1 to 86.7% with an increase of residence time from 1 to 2 h. However, a slight increase in the yield was observed for an increase in residence time from 2 to 3 h. This might be because when the residence time is too long, the transesterification reaction between WFO and methanol could be reversible. The hydrolysis reaction of FAME can then be strengthened to generate more free fatty acid (FFA) and alcohol. From an economical point of view, it is therefore not necessary to spend a long time to increase the conversion of waste oil, when the yield of FAME has been nearly optimized to avoid saponification and hydrolysis reaction in transesterification reaction [28]. Figure 4: Effect of residence time on the yield of biodiesel in PBR. Figure 3: **(a)** SEM micrograph of activated carbon; **(b)** SEM micrograph of KOH/JS catalyst. The catalytic effect of reaction temperature on transesterification was investigated from the percentage of WFO conversion to methyl ester. From Figure 5, the yield of biodiesel was obviously raised from 66.8 to 85.6% with an increase in reaction temperature from 50 to 70 \({}^{\circ}\)C. The effect of reaction temperature on promoting transesterification can be explained by endothermic reaction aspects, in which higher temperature would lead to higher reaction rate [29,30]. The highest yield rate was obtained at the reaction temperature of 60 \({}^{\circ}\)C. However, when the reaction temperature continued to increase over 60 \({}^{\circ}\)C or above the boiling point of methanol, the conversion was decreased. Due to the fact that the evaporation of methanol leads to the reduced ratio of methanol/oil in the reaction, the result is a decline in the transesterification activity [31]. In general, the transesterification reaction temperature should be below the boiling point of alcohol in order to prevent the alcohol evaporation. The range of optimal reaction temperature may vary from 50 to 60 \({}^{\circ}\)C, depending upon the oils or fats used. The effect of methanol/oil molar ratio was investigated from 10 to 20. It is shown that the yield of biodiesel rapidly increases with increasing molar ratio of methanol/oil (Figure 6). Since biodiesel production by transesterification is a reversible reaction, the production yield could be elevated by introducing an excess amount of the reactant methanol to change the equilibrium. When the ratio is  
Figure 5: Effect of reaction temperature on the yield of biodiesel in PBR. Figure 6: Effect of methanol/oil molar ratio on the yield of biodiesel in PBR. less than 16, the molar ratio of methanol/oil has a significant effect on the catalytic activity. According to the [PERSON]'s principle, when the methanol amount is increased, the backward reaction is less favored over the forward reaction, and therefore higher product yield is observed [32]. The increase in transesterification became slower upon further increase in methanol/oil molar ratio to 18 and 20. Moreover, a higher molar ratio of methanol/oil could cause a separation problem during recycling [33]. Therefore, the optimal molar ratio of methanol/waste oil of 16 is preferable. The catalyst bed height in the PBR is associated with the residence time during continuous transesterification.