## 1 Introduction  
Animal byproducts generated in the European Union slaughterhouses represent nearly 17 million tons per year and, from them, 5 million tons inedible byproducts result from rendering and are mostly used for energy generation like biofuels and biodiesel [1; 2; 3]. After rendering byproducts, fat is obtained from beef tallow, mutton allow, pork lard and chicken fat [4; 5]. Such fat is majorly composed of triacylglyceredyns with fatty acids of 16 to 18 carbons. The most abundant saturated fatty acids are palmitic (16:0) and stearic (18:0) acids; the major monounsaturated fatty acid is oleic acid (18:1) and the most abundant polyunsaturated fatty acids are linoleic (18:2) and arachidonic (20:4) acids [6; 7]. Animal fat waste is also obtained from the meat processing industry and from recycled waste from the cooking business [8; 9] that are classified as yellow grease if the content of free fatty acids is lower than 15% by weight and brown grease when it is higher than 15% [10]. In 2019, more than 800 thousand tons of animal fats, equivalent to 6% of total feedstock, were used to produce biodiesel in the European Union [11; 12], while 8.4% of total feedstock was used in the case of the US, consisting of mainly 74 tons of poultry fat, 132 tons of tallow and 243 thousand tons of white grease [13]. Biodiesel produced from animal fats is cheaper than when made from vegetable oils. An additional advantage is that fossil CO\({}_{2}\) reduction is higher when using animal fat for biodiesel generation; nearly 80% CO\({}_{2}\) reduction may be reached for animal fat in comparison to 30% reduction when usingvegetable oil [14, 15]. The bioenergy demand is continuously increasing and in 2050 it is expected to reach 30% of the fuel consumed in the world for road transport [15, 16]. Research on biodiesel production is trying to maximize the yield and minimize the costs by using better catalysts that can be reused and improve the transesterification efficiency [17, 18]. Furthermore, the feedstock used as raw material for biodiesel production represents up to 80% of the total cost [19] and it explains its variability in different geographic areas depending on the climate and agriculture [20]. Total biodiesel world production has been increasing progressively year by year, reaching nearly 45 million tons in 2019 [12]. The European Union has the largest biodiesel production through its 202 plants producing more than 14 million tons of biodiesel in 2019 [11, 21]. More than 5.6 million tons of biodiesel were produced in the US in 2019 through its 91 plants [13, 22]. Nearly 80% of new diesel vehicles are prepared for B20 use that consists of fossil diesel blended with 20% biodiesel [13]. Transesterification through alkaline catalysis is the preferred process at industrial biodiesel production plants [23]. However, raw materials like animal fat that contain moisture and free fatty acids are troublesome for alkaline transesterification due to soap formation. Acid catalysis does not have such troubles, but the reaction is much slower than alkaline catalysis, needs a larger size reactor and requires a higher alcohol to fat molar ratio [24]. Heterogeneous catalysts are not sensitive to the presence of free fatty acids and moisture, can catalyze esterification and transesterification simultaneously, and can be separated from the reaction media. However, such solid catalysts tend to form three phases resulting in a reduced reaction rate and high energy consumption [25]. The simultaneous esterification and transesterification also occur with supercritical technology where high temperature and pressure conditions (i.e., \(>\)250 \({}^{\circ}\)C and 10 MPa) increase the solubility and reduce the mass transfer limitation resulting in good efficiency but with high energy consumption [26, 27, 28].