# Biodiesel and Biolubricant Production from Waste Cooking Oil: Transesterification Reactor Modeling
## 1 Introduction
Accordingly, great interest has arisen on this subject, with multiple studies devoted to the valorization of WCO for biodiesel and biolubricant production. Specifically, transesterification of WCO methyl esters with several alcohols to produce biodiesel [25] or biolubricants has been studied, offering a similar performance compared to those of standard products  
Figure 1: Primary energy consumption in Spain, expressed in percentages: (**a**) in general; (**b**) according to different renewable energy sources [3]. found on the market [26]. Another study focused on transesterification of WCO fatty acid methyl esters with trimethyllopropane (TMP), observing high yields (above 85%) and requiring the addition of antioxidants to improve the oxidation stability [27]. Furthermore, heterogeneous catalysts, like hydrotalcite loaded with potassium carbonate, were applied to the transesterification of WCO FAMEs with TMP, showing high FAME conversion rates (above 95%) and requiring the reactivation of the catalyst through calcination [28]. In this sense, the use of lipase in esterification processes was also effective, with yields and selectivity values above 80% [29]. Similarly, a magnetic cross-linked enzyme aggregate of Eversa was used in the enzymatic synthesis of biolubricants by transesterification of WCO with several alcohols, such as isoamyl alcohol. As a result, maximum yields of around 90% were achieved [30]. Other chemical routes (hydrolysis, dehydration/ketonization, Friedel-Crafts acylation/alkylation, and mild hydrotreatment) are possible for biolubricant production from WCO, as pointed out by some authors [31]. Other works are devoted to the optimization of biolubricants based on WCO, which is an important step for the possible implementation of this technology at an industrial scale. Some studies focused on the optimization of the esterification of free fatty acids from WCO with octanol, using Novozyme 435 as a catalyst. Thus, [PERSON]'s design method was selected, offering high conversion (95%) under the optimized reaction conditions [32]. In a similar way, the effect of water on hydrolysis, as well as those of other parameters, such as the octanol molar ratio, temperature, stirring rate, or catalyst amount, were determined in WCO hydrolysis and esterification with octanol to produce biolubricants, with a total conversion for this purpose [33]. Finally, the possible implementation of this technology in industrial plants has been accomplished in some works. For instance, biolubricant production from WCO through transesterification with ethylene glycol was studied, simulating the process at an industrial scale, with encouraging results that prove the possible applicability of this process [34]. Equally, the optimization and simulation of biolubricant production through transesterification of a mixture of FAMEs (from animal fats and waste cooking oil) with ethylene glycol were carried out, obtaining high yields (above 90%) and indicating that this process could be applied at an industrial scale. For this purpose, a Box-Behnken design was carried out [35]. It should be noted that WCO, in general, presents a variable composition depending on culinary habits. For instance, WCO in Europe might vary compared to WCO in Asia (where other vegetable oils, such as palm oil, can be the majority). Consequently, the fatty acid content of WCO might vary. For instance, oleic acid (from 0.8 to 18%), palmitic acid (from 0.4 to 43%), or linoleic acid (from 0.1 to around 34%) compositions present a wide range depending on the kind of sample [36]. This fact points out the requirement of a thorough characterization of WCO, especially the fatty acid composition, to understand the properties of the raw material used in this process and, subsequently, the design of plants based on these sources.