## 3 Results and Discussion  
The conversion to alkyl esters obtained from the transesterification of WVO in SCM and SCE is shown in Figure 2. There was an increase in conversion with respect to retention time and temperature. The highest conversion obtained was approximately 99% at 290 \({}^{\circ}\)C, and 60 min retention time. Temperature plays a critical part in supercritical alcohol transesterification reaction for biodiesel production. Liquid alcohol is a polar solvent and has hydrogen bonding between OH oxygen and OH hydrogen developing alcohol clusters. As the degree of hydrogen bonding reduces with increasing temperature, the polarity of alcohol would decline in supercritical state. This means that supercritical alcohol has a hydrophobic nature with the lower dielectric constant. As a result, non-polar triglycerides can be well solvated with supercritical alcohol to form a single phase [31]. The conversion obtained for lower holding time is lower for SCE for all transesterification temperatures. However, the conversion for SCE abruptly increases for higher holding time resulting into comparatively higher conversion than SCM. Increasing both reaction retention time and temperature had favorable influence on the conversion. Study by [PERSON] and [PERSON] exhibit that, at the supercritical region, the degree of hydrogen bonding of ethanol is somewhat weaker than in methanol [32]. As the temperature accelerates, the degree of hydrogen bonding lowers, demonstrating the possibility that ethanol can work as a better solvent at supercritical conditions. In consonance with the results acquired in this work, the work by [PERSON] at al. [33] also features conversion in supercritical ethanol is higher compared to supercritical methanol during transesterification of sunflower oil. As a result, the solubility parameter of ethanol is closer to the solubility parameter of the oil. According to the work by [PERSON], the activation energies, computed from the slope of the regressed lines of the Arrhenius plot are 3 and 2 kJ/mol for transesterification of sunflower oil using SCM and SCE, respectively, reflecting the higher effectiveness of SCE over SCM. Although the cited work also features the yield obtained using SCM is higher than that achieved using SCE, the difference is quite negligible [34]. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multicolumn{8}{c}{**FA**} & \multirow{2}{*}{**FFA**} \\ \cline{2-2} \cline{4-7}  & **13:2** & **16:1** & **18:1** & **18:2** & **18:3** & **Others** \\ \hline Virgin Oil & 6.49 & 9.86 & 2.41 & 13.58 & 58.48 & 9.19 & 0.28 \\ WVO & 3.38 & 9.89 & 2.41 & 36.70 & 43.21 & 4.42 & 3.15 \\ \hline \hline \end{tabular} Note: FA are in area % and FFA is in wt.%.  
\end{table} 
Table 1: FA (fatty acid) and FFA (free fatty acid) contents of virgin oil and WVO (waste vegetable oil). Ethanol used as a solvent in biodiesel synthesis is comparatively less toxic than methanol. In Figure 3, it can be inferred that the conversion increases with increase in retention time as well as with higher molar ratio. For all molar ratios, the maximum conversion obtained for both SCM and SCE is approximately 99% for all molar ratios. In supercritical alcohol reaction, extreme amount of alcohol is used to shift the equilibrium to generate more biodiesel [35]. Although excessive amounts of alcohol can boost the reaction rate, high concentration of alcohol in the reaction mixture can suppress transesterification reaction. Moreover, the purification process of biodiesel turns into energy intensive process because of the amount of alcohol in the product mixture [36]. Hence, the molar ratio should be kept at optimum in supercritical alcohol transesterification reaction. Figure 3: (**a**) SCE and (**b**) SCM based conversion at 290 \({}^{\circ}\)C for three different molar ratios of oil to alcohol at different retention time.