Methyl Ester Derived from Used Frying Oil by Two-Step Chemical Process

Utilization of used frying oil as a raw material for biodiesel production process is of interest due to the higher price and the future depletion of petroleum diesel. However, used frying oil contains large amount of free fatty acid (FFA) which adversely affects alkaline transesterification reaction, a commonly used method for biodieseil production. The FFA in the oil is therefore needed to be firstly reduced by acid esterification to a certain value (less than 1%) prior to the transesterification process. This research proposes biodiesel (methyl ester) production by two-step chemical process; sulfuric acid catalyzed esterification followed by potassium hydroxide catalyzed transesterification. Influences of reaction temperature, reaction time, methanol to oil molar ratio and amount of catalyst on FFA content of the product obtained by esterification as well as yield and properties of methyl ester (ME) obtained by transesterification were experimentally investigated. The experimental results showed that increases in reaction temperature, methanol to oil molar ratio, and reaction time as decreases in quantity of catalyst led to the faster reduction of FFA content. The FFA content could be reduced from the original 2.87% to the minimum 0.30% when the esterification process was performed at reaction temperature of 60°C, molar ratio of 18:1, catalyst amount of 0.5 %wt, and reaction time of 120 minutes. Percentage yield of ME could be enhanced by increasing reaction temperature and quantity of catalyst, and decreasing methanol to oil molar ratio and reaction time. The yield of ME was maximized (93.125%) when the transesterification was reacted at temperature of 60 °C, molar ratio of 3:1, catalyst amount of 1%wt and reaction time of 60 minutes. Flash point and kinematic viscosity of the ME undergoing the two stage chemical reaction were in the acceptable ranges of the ASTM standard. Flash point and viscosity of the ME tended to decrease with increasing reaction temperature, catalyst quantity, molar ratio and reaction time.