# Biological Methods in Biodiesel Production and Their Environmental Impact
## 2 Production of Biodiesel by Chemical Transesterification Process  
The biodiesel can be produced from different vegetable and animal fats in the conventional transesterification process (also called alcoholysis) and purification [6]. Obtained fatty acid alkyl esters (FAAE) are used as biofuels and components of diesel oil, and they are characterized by lower viscosity, higher fluidity, and better low-temperature properties in comparison with pure vegetable oil. It is commonly said that fatty acid esters are carbon-neutral fuels because the carbon in compounds emitted in the exhaust gases was captured by plants from the atmosphere via the photosynthesis process. The carriers of a new alkyl part of esters are different alcohols--commonly methanol and ethanol (used in the industrial production) [7]. Depending on which alcohols are used as a product, we obtain fatty acid methyl esters (FAME) or fatty acid ethyl esters (FAEE). The most commonly used esters in the automotive industry are FAME. They are obtained by displacement of glycerin by methanol in a process similar to hydrolysis. The simple version of this reaction can be represented by the general equation:  
\[(\text{RCO})_{3}\text{O}_{3}\text{R}^{\prime}+3\text{R}^{\prime}\text{OH}=3 \text{RCOOR}^{\prime\prime}+\text{R}^{\prime}(\text{OH})_{3}\]  
The reaction products are three moles of new fatty acid esters (biodiesel) and one mole of glycerol for one mole of completely converted triacylglycerenol. The process is more complex because it consists of a number of reversible reactions that follow one another: conversions of triglycerides to diglycerides, of diglycerides to monoglycerides, and of monoglycerides to glycerol [8]. Alcoholysis reaction is an equilibrium reaction. To shift its course in favor of the products, one of the reactants (usually alcohol) should be used in excess. Another way is to carry out the reaction in stages and to collect a by-product (glycerol) after each stage. The molar ratio of alcohol to triglyceride is one of the most important parameters which influence the yield of biodiesel. According to the equation, to obtain three moles of fatty acid ester and one mole of glycerin, we need three moles of alcohol and one mole of glyceride. The molar ratio depends on the type of catalyst used during the reaction. For example, to obtain the same ester yield for a given reaction time during diesel production from butanol and soybean oil, a 30:1 ratio of BuOH to soybean oil is required for acid catalysts and only a 6:1 ratio for alkali catalysts [7]. Higher molar ratios of substrates result in higher conversion of oil and shorten the conversion time. Another important parameter is the type of catalyst. Transesterification can be catalyzed by different catalysts: alkalis (e.g., sodium hydroxides, potassium hydroxides, sodium and potassium alkoxides), acids (e.g., sulfuric acid, hydrochloric acid, sulfonic acids), ion exchange resins, supercritical fluids, and biocatalysts (lipases). Catalysis can be carried out both homogeneously and heterogeneously. The triglycerides and alcohols used during alkali-catalyzed alcoholysis should be substantially anhydrous because the presence of water promotes the formation of soap. Only 0.3% of water in the mixture lowers the yield of transesterification, causes an increase in the viscosity of the reaction mixture, and hinders the separation of the biodiesel [9]. Additionally, the content of free fatty acids should be low, and if it is higher than 2% _w/w_, the pretreatment by alcoholysis with an acid catalyst or the acid-catalyzed transesterification is recommended [10]. The time and reaction yield in the alkali-catalyzed process are varied and depend on the kind of catalyst and the process temperature. For most of the catalysts used in this transesterification, the temperature is about 60-65 \({}^{\circ}\)C, but there are catalysts where the temperature is higher, for example 70 \({}^{\circ}\)C for a K\({}_{2}\)CO\({}_{3}\)/MgO catalyst or 100 \({}^{\circ}\)C for a KOH/MgO catalyst.