Effect of the Parameter

Effect of the Ethanol to Oil Molar Ratio

Figure 3a shows the effect of ethanol to oil molar ratio and reaction time for the bio-jet fuel production. It is shown, from the ANOVA analysis (Table 5), that ethanol to oil molar ratio has the positive effect for bio-jet fuel production (*p* value < 0.05). Increasing the ethanol to oil molar ratio from 1:6 to 1:9.36, the bio-jet fuel yield increases until an optimal point and then decreases. Similar trends have been reported in several studies [46]. As stated before, the stoichiometry of the transesterification requires three moles of alcohol to react with one mole of triglyceride. However, since it involves a reversible reaction, so, an excessive amount of alcohol is usually needed to shift the reaction equilibrium toward the production of biodiesel. In the work done by Encinar et al. [47], the conventional alkalicatalyzed transesterification was reported to be incomplete for the molar ratio of methanol to Cynara oil that was less than 4.05. As expected, the methyl esters yield increases with the methanol molar ratio and achieve an optimal yield at a molar ratio of 5.67. However, for the methanol molar ratio higher than 5.67, the methyl esters yield drops. This result tallies with

current work. This phenomenon is observed because the separation and recovery process of glycerol has been interfered by the high alcohol molar ratio [48]. The high amount of ethanol, in this case, has increased the solubility of the glycerol in the ester phase and shifts the reaction equilibrium towards the reactant side. As the result, biodiesel yield decreases as well as bio-jet fuel.

**Figure 3.** Three-dimensional response for the interaction on bio-jet fuel yield between (**a**) reaction time and ethanol to oil molar ratio, (**b**) microwave power and reaction time and, (**c**) ethanol to oil molar ratio and microwave power.
