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2. Transesterification Vegetable oil is pre-treated with catalyst to form esters of free fatty acids (FFA) in order to eliminate saponification reaction. The reaction useful when raw material contains high content of FFA. Reaction is described in figure 1 and the factors affecting the transesterification reaction are also given below in equation 1 [9]. ( 1 ) 2.1. Water content Water content in waste cooking oil reduces the amount of ester formation and…show more content… Azeotrope and water are formed when ethanol is used, making the separation of alcohol from water difficult during distillation. Some cases involve addition of i-butanol or t-butanol as solvent to avoid the inhibition of lipase by glycerol or methanol. 2.3. Catalyst type Researchers have used various catalysts for the production of alkyl esters. It can be concluded that fastest catalyst is NaOH and among the catalysts used. Raw materials with higher FFA content, aid catalysts are used for transesterification. The problems related to the usage of homogeneous catalysts are soap formation and recovery of catalyst from final product. Research has shown that heterogeneous catalyst is not affected by FFA and moisture present in raw material. Enzyme catalytic reaction is slower than other catalytic reactions used for transesterification. The separation is easier for enzyme catalysts but preparation of enzyme catalyst is critical [10], [9]. 2.4. Alcohol to oil ratio Three moles of alcohol and one mole of triglyceride are needed to produce three moles of alkyl esters. Alcohol to oil ratio has positive effect on production of biofuel. Product formation rate increases as the concentration of reactant increases. Increase in molar ratio…show more content… Temperature There is significant effect of temperature on transesterification. Increase in temperature increases biodiesel yield and reaction rate. Temperature shouldn’t exceed the boiling point of alcohol to avoid evaporation of alcohol. Viscosity of biodiesel will increase if temperature is maintained below 50°C. Waste cooking oil is heated up to 120°C in order to remove water and then cooled up to 60°C. Highest biodiesel yield was recorded at 65°C for waste cooking oil by using KOH catalyst. 2.7. Reaction time Reaction time depends upon the availability of reactants and 99% of yield can be obtained if reaction is carried out for longer period. Reaction time has an impact on the properties of biodiesel. The reaction time should be optimized in order to maximize yield and minimize the production cost. 2.8. pH It isn’t the major factor while using acid or base catalyst but it can be considered during the usage of lipase as catalyst because enzyme can be decomposed at lower or higher pH values. 2.9. Stirrer speed Proper mixing of reactants is necessary for completion of transesterification reaction. Agitation increases the rate of reaction as well as diffusion of reactants and catalyst into each other. Optimizing the stirrer speed is necessary to obtain maximum yield. 2.10. Catalyst concentration High temperature conditions are required for the conversion of waste cooking oil into renewable fuel without using catalyst. Biodiesel yield increases with the increase in