## 4 Results and Discussion
### Reactor Simulation and Analysis of the Effect of the Feed Oil Changes on the System  
Three abrupt feed oil changes were simulated in order to analyze the effect of these disturbances on the reactor in continuous operation. The changes were simulated at the 500 th, 2000 th, and 3500 th min, at which the system is operating in a steady state. While fed with palm oil at 500 min of simulation, at which point the reactor is operating in a steady state, the feed oil is replaced by mustard oil. At minute 2000, when the system is again operating in a steady state, the oil in the feed stream is replaced again, this time with sunflower oil. Finally, the reaction tank is fed with palm oil once more at minute 3500. Figure 4 shows the behavior of the ester concentrations after the oil changes, omitting the first transitional period when the state of the system moves from initial conditions to its first steady state. After a feed oil change, the esters from the previous oil begin to decrease due to the interrupted glyceride supply, while, in contrast, the esters from the new oil begin to be formed. As the rate at which the first esters leave the tank is higher than the formation rate of the new esters, a decrease in the total quantity of esters is observed after the feed oil changes (green curve), with the highest concentration in the new post-change steady state obtained with palm oil. Figure 3: Experimental vs. simulation data for the transesterification reactions for different vegetable oils. Mass fraction of methyl esters during the transesterification reaction of the following: (**a**) palm oil (60 \({}^{\circ}\)C, 1 wt% catalyst), (**b**) mustard oil (60 \({}^{\circ}\)C, 1 wt% catalyst), and (**c**) sunflower oil (35 \({}^{\circ}\)C, 0.5 wt% catalyst) [42; 43]. The concentration of ester during the steady obtained with a particular oil mainly depends on the net reaction rate of the components and the triglyceride concentration in the input stream (see Equations (16)-(25)). However, the quantity of triglycerides has a greater influence than the kinetic parameter on the rate of reaction. For this reason, palm oil, which contains a higher proportion of triglycerides and lower kinetic constants than sunflower oil [42; 43], was observed in higher concentrations during the steady state. Regarding mustard oil, palm oil has almost two times more triglycerides than mustard oil (see Table 3). This fact causes the esters produced with palm oil to be about two times the esters produced with mustard oil (see Figure 4). These results agree with those obtained for Issariyaku and Dalai in a batch system [42]. In the practice, ester molarity is not a useful indicator of ester production, while the mass fraction is a more convenient indicator because it is used to determine ester quality in the ester phase. Figure 5 presents the effect of the oil changes on the mass fraction, where, after the first change (at minute 500), the steady state conditions under which the reactor operates is disturbed. For a short period after the change, the mass fraction of the ester increases slightly; however, after a minute, it starts to rapidly decrease up to a minimum value of 46.96% by minute 505.40, causing a small peak in the mass fraction response. This initial peak is caused by the lower concentration of glycerides in the mustard oil. The ester mass fraction then increases until reaching a new steady value of 55.56% at minute 19.14 post-feed oil change. Similar results are obtained during the subsequent feed oil changes (except the initial small peak observed), wherein an initial decrease occurs after the change, followed by a different steady state mass fraction. In contrast to the molarity observed, the highest mass fraction was obtained with sunflower oil after the second change (at minute 2000), while the lowest mass fraction was obtained with the mustard oil. Moreover, the results show that the mass fraction changed considerably after each oil change.