Two-Step Esterification Process of Palm Fatty Acid Distillate Using Soaking Coupled with Ultrasound: Process Optimization and Reusable Solid Acid Catalysts

In this study, high-intensity ultrasound (1000 W ultrasonic power and 18 kHz frequency) was employed in conjunction with heterogeneous acid catalysts to reduce the free fatty acid (FFA) content in palm fatty acid distillate (PFAD). The aim was to convert the FFAs in PFAD to methyl esters (MEs) through an esterification process. The use of Amberlyst-15 as a heterogeneous acid catalyst offers environmental advantages over homogeneous catalysts and the potential for reuse in the biodiesel production process. The ME purity in the esterified oil was optimized by varying four independent variables: methanol content (25-65 wt %), Amberlyst-15 loading (20-60 wt %), soaking time (30-130 min), and sonication time (30-630 s). Furthermore, the efficiency of reused solid acid catalysts in reducing FFA over multiple cycles was investigated. The results indicated that under optimal conditions of 36.73 wt % methanol content, 60 wt % Amberlyst-15 loading, 130 min soaking time, and 505 s sonication time at a reaction temperature of 60 °C, the ME purity reached 89.44 wt %. Moreover, the potential of the recovered solid catalyst to reduce FFA in fresh PFAD under optimal conditions was examined. It was found that recovered Amberlyst-15 could be reused for at least two cycles with ultrasound to achieve an ME purity of over 80 wt %.