Microwave-Assisted Transesterification of Waste Cooking Oil for Biodiesel Production

Abstract

Waste cooking oil (WCO) is considered the most promising biodiesel feedstock despite its drawback of high free fatty acid (FFA). In this study, microwave-assisted transesterification of WCO is carried out in the presence of potassium hydroxide supported on carbonized coconut shell (KOH/CS) catalyst. The effect of reaction temperature on the yield of fatty acid methyl esters (FAME) is studied. Conventional transesterification is also performed for comparison. The results show that reaction temperature of 80 °C is optimum for FAME production. The properties of the produced biodiesel satisfy the criteria according to ASTM D6751. Acid-catalysed esterification of WCO before transesterification leads to higher production of FAME due to reduction of FFA. At 80 °C, reaction time of 40 min, alcohol to oil ratio of 12:1 and 5 wt.% catalyst, a FAME production of 91.3% is achieved. At 65 °C, reaction time of 90 min, alcohol to oil ratio of 12:1 and 5 wt.% catalyst, conventional transesterification results in slightly higher FAME production (92.1%). However, conventional transesterification using WCO without esterification as feedstock yields lower FAME. Pretreatment of feedstock with esterification is found to be more beneficial and applicable in the case of biodiesel production through conventional transesterification, allowing production of FAME with higher yields. This furthermore indicates that although microwave heating decreases the reaction time, it does not necessarily lead to increased FAME. The production of FAME depends on not only the type of transesterification process but also the type of feedstock used. Overall, the proposed methodology allows the use of high FFA content feedstock. However, this will need careful selection of feedstock as well as additional treatment prior to transesterification process; otherwise the yield may be reduced.