## 2 General Aspects of Biodiesel  
The American Society for Testing and Materials (ASTM International) with the standard D6751 defines to biodiesel as \"a fuel comprised of mono-alkyl esters of long chain fatty acids derived from vegetable oils or animal fats, designated B100\" [21]. The transesterification process is the method used to transform vegetable oils and animal fats into the esters that form the biodiesel fuel. In a general sense, transesterification is the reaction between the triglycerides (contained in the oils and fats) and an acyl-acceptor that may be carboxylic acids (acidolysis), alcohols (alcoholysis), or another ester (interesterification) [9]. However, most of the literature uses the term transesterification as synonymous with alcoholysis and, therefore, the word transesterification will be used to refer to alcohols in the present work. In the practice, only transesterification (alcoholysis) and interestricification are used to produce biodiesel, the transesterification being the usual process used in the industry to produce the biodiesel fuel. Different alcohols can be used in the transesterification process such that: ethanol, methanol, propanol, butanol, and others. However, the most suitable alcohol for use in biodiesel production is methanol in terms of both practicality and cost [22]. Methanol is cheaper, more reactive, and the fatty acid methyl esters (FAME) derived from methanol are more volatile than the fatty acid ethyl esters (FAEE) derived from ethanol [9]. Furthermore, FAEE have slightly higher viscosities and slightly lower cloud and pour points than the corresponding FAME, and methyl esters produced slightly higher power and torque than ethyl esters [23]. The main parameters affecting transesterification reaction are the catalyst, the methanol to oil molar ratio, the mixing, the reaction temperature, the residence time and the feedstock state. Typically, in the transesterification process, there are some forms to accelerate the reaction rates, namely the use of a catalyst, a supercritical alcohol, or a co-solvent. The use of a catalyst is the most common way to produce biodiesel. The transesterification catalyst can be divided into three categories: homogeneous, heterogeneous and the ezymatic (biocatalyst). Both homogeneous and heterogeneous catalysts can be acid or basic. Moreover, immobilized or soluble lipases are used as biocatalysts in the biodiesel transesterification. Homogeneous basic catalysts are the most used in the biodiesel production because they are inexpensive and effective. The stoichiometry of the transesterification defines that the alcohol-to-oil molar ratio must be 3:1; however, a higher ratios are needed in real scenarios to raise the miscibility reactants and, consequently, the possibility of their contact for improving the final product yield [24]. In the literature, it has been reported that the formation of alkyl esters is adequate induced by an alcohol-to-oil molar ratio greater than or equal to 6:1 [25]. Agitation is a vital parameter in the biodiesel production process because it has an important influence on the reaction rate, on the mixing of reactants that are not miscible, and on the mass transfer. Intensities of agitation higher than 600 rpm produced acceleration of the reaction rate and, practically, eliminate the initial delay caused by reactant immiscibility the mass transfer in laboratory scale experiments. Temperature is an important factor in the transesterification process. Higher temperatures improve the transesterification process by reduction of the mass transfer effect and the higher energy state to the molecules [26]. The required residence time is directly dependent on other parameters that enhance the rate of transesterification reaction, such as mixing intensity, residence time distribution (RTD), and temperature [24]. Regarding feedstock state, the critical factors that influence the transesterification process are the content free fatty acids, moisture and water in the feedstock, and its composition.