## 3 Results and Discussion
for the molar ethanol/oil ratio = 11:1 showed that the amount of byproduct remained relatively high throughout the reaction. This could be partially provided due to contamination of the waste cooking oil with food debris, since only coarse particles could be filtered before usage. It is also known that used cooking or frying oil has a higher acid value which means it contains a higher amount of free fatty acids (FFA) than cooking oil [13]. It decreases the biodiesel yield of the reaction because FFAs can be saponified by the catalyst [14]. An excess amount of alcohol would counter this phenomenon by shifting the equilibrium of the saponification to the reactant's side and thereby favoring the alcoholysis of the FFA. Figures 9 and 10 show the direct comparison of FAEE yields for different types of oils. At a molar ethanol/oil ratio = 11:1 the original cooking oil provided a higher yield (96.6%) than the soybean oil (94.8%) at 8 min reaction time. Since the exact composition of the cooking oil is unknown it can only be assumed that the vegetable oils contained hold properties more appropriate for the  
Figure 8: FAEE yields in the batch reactor at ethanol-oil-ratios = 11:1 and 12:1 with waste cooking oil. Figure 9: FAEE yields for soybean oil, original cooking oil and waste cooking oil in a batch reactor at molar ethanol/oil ratio 11:1.  
transesterification. Using the waste cooking oil, the product yield was distinctively lower and a FAEE yield of 82.0% was obtained. Figure 10 shows the experimental results at a molar ethanol/oil ratio of 12:1. The soybean oil provides slightly lower maximum yields (96.7%) of FAEE than the original cooking oil (97.9%) and the waste cooking oil (97.5%), indicating that the mix of vegetable oils in the cooking oil is more suitable for alcoholysis than soybean oil. The increase of yields achieved can be described in a logarithmical function for the batch reactor experiments with soybean oil. By determining equations for this increase, the yields of the reactions after 4 min can be interpolated and then compared to the results in MX- and T-mixer reactor systems with soybean oil at the respectively optimal conditions from Table 5 (Table 8). Applying the calculated optimal reaction parameters for the MX-V reactor system to the experiment in the batch reactor leads to a higher product yield of 86.4%. The product yield of 90.8% at optimal reaction conditions in the T-V reactor system could not be exceeded in the batch mode under otherwise equal reaction conditions. In previous works, the maximum product yield of 93.8% FAEE using soybean oil in the MX-V reactor system was obtained at a reaction time of 11.3 min and molar ethanol/oil ratio = 6:1, the catalyst concentration 1 wt.-%, the flow rate 0.565 mL/min and the temperature 50 \({}^{\circ}\)C [14]. Interpolating the product yield of the reaction from a logarithmic equation depending on reaction time suggests the FAEE yield = 84.9% after 4 min. This result was reproduced using the same reactor system and reaction temperature but at a lower catalyst concentration of 0.86 wt.-%, higher molar ethanol/oil ratio of 12:1 and the higher flow rate = 2.6 mL/min. Using the T-V reactor system, the interpolated product yield of 43.9% obtained in [15] with a flow rate of 0.535 mL/min and otherwise equal experimental conditions in  
Figure 10: FAEE yields for soybean oil, original cooking oil and waste cooking oil in a batch reactor at a molar ethanol/oil ratio 12:1. \begin{table}
\begin{tabular}{c c c} \hline \hline
**n\({}_{\text{EOM}}\)/n\({}_{\text{oil}}\)** & **11:1** & **12:1** \\ \hline T-V & 90.8 & - \\ MX-V & - & 84.9 \\ Batch & 87.7 & 86.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Comparison of FAEE yields for the batch reactor and MX-V and T-V reactor system (interpolated) experiments with soybean oil after 4 min. the MX-V reactor system could be exceeded using a higher flow rate = 5.09 mL/min, a molar ethanol/oil ratio = 11:1 and catalyst concentration = 0.86 wt-%, leading to 90.8% FAEE.