## 3 Transesterification for Producing Biodiesel from Animal Fats
As previously mentioned, if an alkaline catalyst is used for transesterification and there is a high content of free fatty acids, the reaction efficiency is drastically reduced because of the reaction of such free fatty acids with the catalyst resulting in soap formation [80, 81]. This causes a loss of catalyst and ester product [82] and reduces the biodiesel yield to low levels, and therefore, the production costs increase [83]. The soap formation also makes glycerol separation and biodiesel purification difficult, which increases the cost of the resulting alkaline wastewater treatment [84]. The quality can also be affected due to side reactions producing unwanted products [85]. The free fatty acid content in animal fats is within the range of 5-30%, making a pretreatment necessary [26]. For an effective transesterification reaction, it is recommended that a limit in free fatty acids be equivalent to 1.0-1.5% [86], or an acid value below 2 mg KOH/g of oil [87]. There are different strategies in order to get such reduction in free fatty acids. For instance, waste pork fats containing free fatty acids within the range 4.9-13.5% were esterified for 4 h at 65 \({}^{\circ}\)C with 0.5 wt.% H\({}_{2}\)SO\({}_{4}\) or 1.0 wt.% p-toluene sulfonic acid, keeping a 6:1 methanol to fat molar ratio, and the acid value was reduced below 2 mg KOH/g [61]. Furthermore, the waste fat of rendered pork was reported to have a high acid value of 4.3 mg KOH/g, but it could be reduced down to 0.75 mg KOH/g through a standard titration pretreatment method using sulfuric acid, even though it can be corrosive [23]. A continuous esterification process that reduced the reactor cost and reaction time, was designed for pretreatment deacidification using an ion exchange resin reaching an acid value reduced to 0.89 mg KOH/g and a conversion rate as high as 99.26% [80]. Thus, pretreatments are necessary to remove the excess of water, free fatty acids and suspended solids of animal fats before transesterification. Some of such pretreatments for moisture reduction are heat drying, silica gel, calcium chloride or anhydrous sodium sulfate [88, 89, 90]. The excess of free fatty acids may be removed by neutralization and separation [78]. Finally, the suspended matter may be removed by filtration under vacuum or through cellulose filters [88, 90]. However, although the quality and yield of biodiesel are better, pretreatment steps result in additional costs [91]. An alternative process would be a two-step transesterification being the first step--an acid-catalyzed pretreatment to esterify the free fatty acids and thus, reducing their content--and the second step, where triacylglycerglycerols undergo transesterification with the alkaline catalyst [52]. Other alternatives to consider are heterogeneous acid and base catalysts that have better tolerance to high water content and free fatty acids and can be re-used [82]. Heterogeneous catalysts offer the possibility of recycling and reducing the biodiesel production steps. Basic zeolites, alkaline earth metal oxides and hydrolacites constitute some of the most used in recent literature. CaO is one of the most used catalysts and it has been loaded onto zeolite that offers a large surface area [15]. However, CaO can be poisoned when contacting water and carbon dioxide, making it not so attractive for industrial operation [92]. This catalyst could be recycled but the biodiesel yield is rapidly decreased down to 37.5% after its third use. Several nanocatalysts like nano CaO [48], activated carbon/CuFe\({}_{2}\)O\({}_{4}\) encapsulated with CaO [69] and KF/CaO nanocatalysts [93] have been reported at the laboratory scale. Such nanocatalysts show a higher catalytic efficiency, better rigidity, larger specific surface area and better resistance to saponification [69]. Heterogeneous solid acid catalysts are reported to give lower yields. However, they have some advantages in the case of animal fats because they are insensitive to the problem of