# Biodiesel and Biolubricant Production from Waste Cooking Oil: Transesterification Reactor Modeling
## 3 Results and Discussion
### WCO-TMP and TBHQ Addition  
The effect of the TBHQ addition on the oxidation stability of the WCO-TMP is shown in Figure 6. Clearly, an increasing linear trend was observed for the oxidation stability of the WCO-TMP with increasing addition of THBQ. This is the typical behavior observed for a variety of oil-based samples, including biodiesel and biolubricants produced from a wide range of vegetable oils, such as WCO [21; 52], a corn and sunflower oil mixture [57], cardoon oil [20], cottonseed oil [58], or safflower oil [59; 60] among others.  
Figure 5: TG (red line) and DTG (dashed black line) curves for WCO-TMP.  
Figure 6: Effect of the TBHQ concentration on the oxidation stability of the WCO-TMP biolubricant.  
Consequently, to prove the effectiveness of the antioxidant addition, 1000 ppm of TBHQ was considered as an interesting antioxidant concentration for further studies, where WCO-TMP was subjected to extreme oxidation conditions, as covered in previous experiments.  
Figure 7 shows the main effects of the antioxidant addition (TBHQ at 1000 ppm) on the viscosity and acidity, which are essential properties because the former is decisive for the suitable applicability of the resulting biolubricant, whereas the latter is an undesirable effect that should be avoided to prevent corrosion in equipment and facilities.  
Considering viscosity (see Figure 7a), the control samples (that is, without the TBHQ addition) had considerable increases in this parameter, especially after 300 min of extreme oxidation. This is normally because of oxidation processes, which can provoke undesirable effects, such as polymerization. As a consequence, an increase in viscosity, on account of the presence of compounds with a complex molecular structure, was found, as intermolecular interactions promote the resistance to the flow in the resulting product [18]. When TBHQ was added, the viscosity remained constant during the whole process, pointing out the effectiveness of this antioxidant for this purpose, as the antioxidant could have prevented the typical oxidation steps in free radical generation, that is, initiation, propagation, and termination [18].  
On the other hand, there was growth in the acidity of the control samples, whereas the acidity was constant when TBHQ was added, as shown in Figure 7b. Concerning the control samples, the abovementioned oxidation processes can equally produce free fatty acids [18], which could contribute to an increase in the acidity, which was delayed by adding TBHQ.  
As a result, it was demonstrated that the selected quantity of TBHQ, for its addition to WCO-TMP, was enough to preserve the main properties of the WCO-TMP during its oxidation. Also, it should be noted that low amounts of this antioxidant could enhance the quality of the final product obtained in this work. Previous research has pointed to the  
Figure 7: Effects of the optimal TBHQ addition on the WCO biolubricant during oxidation: (**a**) viscosity and (**b**) acidity number.  
same trend, with relatively low quantities of TBHQ or PG required to avoid the negative consequences of the oxidation of biodiesel [57; 59] and biolubricants [39].