## 3 Results and Discussion
Figure 2: (**a**) SCE (supercritical ethanol); (**b**) SCM (supercritical methanol) based conversion for 1:6 molar ratios of oil to alcohol at different retention time. The liquid product composition percent in terms of carbon number for SCM and SCE are demonstrated in Tables 2-5. In addition, the products have been classified as glycerol, non-esters and esters. From the tables, it can be inferred that C17:0, C18:2, C18:3 and C19:1 are the major components of transesterification of WVO using SCM while C18:0, C20:1, C20:2 and C20:3 are the major components while using SCE. This distinctly shows the effect of addition of one extra carbon from ethanol to the product. It is difficult to completely characterize the reaction products because of the numerous reaction paths and the variety of reaction products. The basic mechanism of transesterification of triglycerides with alcohol along with the mechanism of thermal decomposition of triglycerides is discussed by [PERSON] et al. [37]. Transesterification consists of a series of forward and reversible reactions. It can be inferred from the tables that the presence of glycerol is significant in using SCM while glycerol is almost absent while using SCE as solvent. The absence of glycerol in SCE results into slightly higher amount of products in the biodiesel range. The presence of glycerol in the solution helps to drive the equilibrium towards reactant, lowering the yield of esters [38]. Methanol and ethanol are not miscible with triglycerides at ambient temperature, and the reaction mixtures are usually mechanically stirred to enhance mass transfer. Emulsions usually form during this course of reaction. In the case of methanolysis, these emulsions quickly and easily break down forming a lower glycerol rich layer and upper methyl ester rich layer. In ethanolysis, these emulsions are more stable and severely complicate the separation and purification of esters [39]. The absence of glycerol in using SCE will strengthen its usability compared to SCM resulting into increased ester content. Tables 2 and 3 show the indifference of transesterification temperature (250, 270 and 290 \({}^{\circ}\)C) on the product composition. Although the conversion increased on increasing the transesterification temperature from 250 to 290 \({}^{\circ}\)C, no variation was seen on ester composition. The prominent series of forward and backward reaction during supercritical transesterification lead into the complex product combination. Furthermore, in Tables 4 and 5, it can be seen that no significant variation in ester composition occurred on varying WVO to alcohol ratio. Transesterification temperature of 250 \({}^{\circ}\)C and molar ratio of 1:6 is sufficient to obtain the maximum possible conversion. The fact that the critical temperatures of alcohols (\(T_{\text{c}}\) = 239.45 \({}^{\circ}\)C and 240.75 \({}^{\circ}\)C; \(P_{\text{c}}\) = 80.9 and 61.4 bar; \(\rho_{\text{c}}\) = 0.272 g/cm\({}^{3}\) and 0.276 g/cm\({}^{3}\) for methanol and ethanol, respectively) are similar is due to the products obtained in terms of esters are similar, with a difference of one carbon. The similarity, with some exceptions, is due to the resemblance between the two alcohols in terms of chemical characteristics. The trend of the products obtained is similar to that of the results obtained for transesterification of waste pig fat in supercritical alcohols by [PERSON] et al. [40]. The results obtained in Tables 2-5 demonstrate that no significant variation was obtained on ester composition on increasing the transesterification temperature and retention time. However, from Figures 2 and 3, it can be concluded that the conversion increased with increase in transesterification temperature, retention time and molar ratio. Combining, these two results it can be concluded that transesterification temperature of 250 \({}^{\circ}\)C, retention time of 0 min and molar ratio of 1:6 is enough to obtain maximum possible ester composition even though the conversion is much less than 100%.