Comparison of free fatty acid conversion yields for esterification assisted by single- and dual-frequency sonication

Abstract

Crude organic oils mostly contain high amounts of free fatty acids (FFAs), which are not ready to be transesterified into biodiesel. Using esterification, FFAs must be reduced to be less than 1% prior to transesterification. This research aimed to compare the FFA conversion yields of the esterification of crude palm oil (CPO), with 11.3% FFAs, under single- and dual-frequency sonication. Single frequencies of 28 kHz and 40 kHz and a dual frequency of 28 + 40 kHz were investigated. The methanol to oil molar ratio (M/O), catalyst loading, and time were examined to optimise the FFA conversion. Composite central design (CCD), analysis of variance (ANOVA), and response surface methodology (RSM) were applied for the experimental design, quadratic equation, and optimisation, respectively. The results revealed that esterification with a dual frequency gave a higher conversion than that of single frequencies. A quadratic model was proposed for CPO esterification assisted by dual-frequency sonication. The highest conversion, 98.0%, was achieved under the following simplified optimised conditions: M/O molar ratio of 12:1, catalyst amount of 2.3%, and reaction time of 85 min. Moreover, regarding energy efficiency, a 12:1 M/O molar ratio, 2% of the catalyst, and a reaction time of 70 min were recommended, which provided a FFA conversion of 97.9%. In conclusion, the FFA conversion was improved by 1.12–3.28% using dual-frequency sonication and, importantly, the reaction time was obviously shortened. Thus, these improvements led to less energy consumption during the process.