## 2 Biodiesel Synthesis Processes
### Criteria for Alcohol Selection  
While dealing with high FFA content, double step acid-base catalyzed method is used. Esterification of FFA by acid followed by transesterification. Conventionally, in biodiesel production, esterified products are separated from water produced and then further proceeded [14].  
Transesterification is recognized as alcoholysis in which three moles of alcohol reacts with triglycerides in oil to produce biodiesel as main product and glycerin as byproduct and reaction rate is supported by catalyst [15]  
1. Alcohol such as methanol, ethanol, propanol, butanol, pentanol and even isopropanol can be used. Higher alcohols are avoided because they are more sensitive to the contamination of water and the reaction is inhibited. Hence, methanol and ethanol are frequently; used.
2. When compared, methanol is more effective than ethanol and it provides a high biodiesel yield in less time duration. It helps to purify the product efficiently by separating the glycerin, which is byproduct of a biodiesel process. A most relevant factor for choosing methanol is its low cost. It was found that 6:1 alcohol to oil ratio is the basic requirement for single step reaction completion. More than 1.6 times excess of alcohol is necessary to complete the transesterification [16]. Nevertheless, methanol usage is exposed to few drawbacks, for example it is produced from natural gas (CH\({}_{4}\)) that increases its toxicity. Therefore, the production of one of the reactants of non-renewable origin makes the perception of green fuel ambiguous. A pressure vessel is mandatory when methanol is used as solvent in order to tolerate the boiling temperature, especially at 65 \({}^{\circ}\)C temperature for 1-2 h of reaction time. Reaction temperature directly influences the rate of reaction, product viscosity, reaction equilibrium, kinetics and mass transfer limitations.  
On the other hand, ethanol is produced from vegetable feedstock for example beet, potato, cassava, and sugar cane. Therefore, ethanol is non-toxic and could be the best alternate for methanol in biodiesel synthesis reactions. However, ethanol is not preferred due to several drawbacks such as it is required in excess with less reactivity, long reaction duration/time and high optimization temperature and high operating cost. Moreover, product purification is also a challenging task due to emulsion formation. These issues can be resolved by conducting the reaction in various steps. Hence, the amount of alcohol needed to complete the process can be reduced. In each step, alcohol and the catalyst are added, and glycerol is removed by water washing. By increasing the amount of reactant and removing the product, the reaction will proceed in forward direction at a high speed with maximum yield [17]. * In the case of high moisture content in alcohol, acids such as hydrochloric acid and sulfuric acids (0.87-2.5 M) are preferable for transesterification. * Diglycerides, in reaction with alcohol, disperse in the oil phase and result in yield reduction. Therefore, a co-solvent can be used that limits this mass transfer. To enhance oil and alcohol miscibility, Tetrahydrofuran and Methyl tert-butyl ether have proven effective; tetrahydrofuran is best co-solvent as compared to Methyl tert-butyl ether. Various investigations have been conducted for methanol with co-solvent, for example, di-ethyl ether, acetone, di-isopropyl ether, dichlorobenzene with 1:0.5 to 1:2 mole/mole ratios. * A vacuum-based heat exchanger is used at the last stage to remove excess water from biodiesel. Both biodiesel and glycerin are present in the reaction mixture in the form of layers and physical separation required to recover them. Glycerol, being the unwanted byproduct, is removed from the main product (biodiesel). The glycerol regeneration occurs when it is neutralized by the acid addition. It is further used for various purposes after purification [16; 18].