Impact of Applying Technique for Dual Frequency Sonication on Esterification

Prior to biodiesel production of a high free fatty acid (FFA) oil, esterification of the oil is recommended to reduce the FFA to less than 1%. In this work, dual frequency ultrasound-assisted esterification with three different techniques was investigated. Three wave applying techniques have been proposed with the same energy used during the process. Firstly, for technique A, 28 kHz (100 W) and 40 kHz (100 W) waves were concurrently applied for a total time of T min. Secondly, for technique B, 28 kHz (200 W) waves were firstly applied for a total time of T/2 min and followed by 40 kHz (200 W) for a total time of T/2 min. Lastly, for technique C, 28 kHz (200 W) and 40 kHz (200 W) waves were concurrently applied for a total time of T/2 min. Pertinent parameters, i.e., methanol to oil molar ratio, catalyst loading, and reaction time, were investigated. Response surface methodology (RSM) was used as a tool for optimization. The amount of final FFA and energy used during the process were the key indices to optimize the most suitable condition. The results revealed that technique C* gave the highest FFA conversion of 99.40% with specific energy consumption of 23.73 Wh/% for methanol to oil (M/O) molar ratio of 12:1, catalyst loading of 2.2%wt., and 40 min reaction time. However, when considering both final FFA less than 1% and least energy used, it was suggested that M/O ratio 15:1, catalyst loading of 1.5%wt., and 22.5 min reaction time could give as high as 94.5% conversion yield with 14 Wh/% for technique C.