High FFA Oils: A Comparative Analysis of Biodiesel Production via Direct Transesterification and Sequential Esterification-Transesterification Processes

In this study, biodiesel production was carried out using a single-step (transesterification) and a two-step (esterification + transesterification) process with Pistacia terebinthus oil (TO), which has a high free fatty acid (FFA) content. TO, containing 5.8% FFA, was converted into biodiesel using single-step (TOTB100) and two-step (TOETB100) methods, and the effects of FFA content on biodiesel yield were analyzed using analytical characterization techniques. According to the Gas Chromatography-Mass Spectrometry (GC-MS) results, the oleic acid content in the TO sample was determined to be 48.4%, while the total Fatty Acid Methyl Ester (FAME) content in TOETB100 biodiesel was calculated as 95.7%. Fourier Transform Infrared Spectroscopy (FT-IR) analysis revealed a characteristic peak at 1438.1 cm⁻¹, indicating FAME formation in the TOETB100 biodiesel. Proton Nuclear Magnetic Resonance (¹H NMR) and Carbon Nuclear Magnetic Resonance (¹³C NMR) analyses showed that the conversion efficiency of TOTB100 biodiesel was lower (60.74%) compared to that of TOETB100 biodiesel (78.01%). As a result, it was determined that the two-step method provides higher conversion efficiency and FAME content compared to the single-step method in biodiesel production from oils with high FFA content. These findings suggest that the two-step chemical process is a more suitable method for biodiesel production from oils with high FFA content.