# Biodiesel and Biolubricant Production from Waste Cooking Oil: Transesterification Reactor Modeling
## 1 Introduction  
Because of the pollution derived from traditional industrial practices, along with the depletion of energy sources, such as oil, there is an increasing environmental concern, which is pointed out in different policies, like Sustainable Development Goals [1]. The environmental impact of fossil fuels has provoked the reactions of international agencies with different agendas, such as the transition toward renewable energies (so-called climate technologies, such as wind energy, solar power, or hydropower) through decarbonization, as promoted by the United Nations' Framework Convention on Climate Change, which claims that there is a lot of work to do to complete this energy transition, according to the UN Climate Change Conference in Baku [2]. There is still a global dependence on petrol-based industries, requiring further efforts for the implementation of green technologies. Global renewable sources represent 30% of the energy consumption in the electricity sector,with room for improvement in the heating and transportation sectors [1]. Spain is, equally, in an initial transition stage, increasing the production and consumption of renewable energies (at around 20%). However, as observed in Figure 1, oil is still the most consumed primary energy source in this country (Figure 1a). Thus, increased efforts should be made to reduce oil dependence. According to Figure 1b, different renewable energy sources seem to play important roles in Spain, such as wind energy, photovoltaic and thermal solar energy, or biomass, whereas other sources, like seawater, geothermal energy, biogas, or biofuels, could considerably contribute to a greener transition, as there is room for improvement in these sectors. Regarding biofuels and other bioproducts, they can constitute a sound alternative for those derived from petrol, with biodiesel and biolubricants playing important roles. Specifically, a wide range of vegetable oils (such as soybean [4; 5], rapeseed [6; 7], corn [8; 9], or palm oils [10; 11]) or animal fats [12; 13; 14] have been used for the synthesis of these products through different methods, such as transesterification or epoxidation among others [15; 16]. The role of waste cooking oil in Spain could be promising, as its reuse is considerable in the service sector (above 70%), whereas it is reduced in households (5%). In total, around 350 million liters of WCO are generated yearly [17]. Concerning transesterification, it is an interesting process for biodiesel and biolubricant production, as double transesterification with methanol (or ethanol) and a superior alcohol (such as trimethylolpropane, TMP, or pentaerythnitol, PE) could be the starting point for the implementation of a biorefinery. Thus, different valuable products (such as biodiesel, glycerol, or biolubricants) could be produced from natural sources, like vegetable oils, animal fats, or even microalgae, with intermediate products that can be reused. That is the case for methanol, which is generated in transesterification to produce biolubricants and can be reused in the first transesterification of fatty acids to obtain fatty acid methyl esters [18; 19]. Even though the products obtained in this biorefinery context have very interesting and sustainable properties, such as high biodegradability or low environmental impact during their production, some challenges should be overcome, like low oxidation stability because of the presence of double bonds in their final molecular structures. Different measures can be taken to avoid this problem, like the selection of vegetable oils with high oleic and palmitic acid contents or the addition of antioxidants, like propyl gallate (PG) [20], tert-butylhydroquinone (TBHQ) [21; 22], or natural antioxidants/green additives [23; 24]. Different kinds of waste can be used for biolubricant production, as in the case of waste cooking oil (WCO), which is a waste with difficult environmental management.