Energies, Vol. 18, Pages 6004: Valorization of Rambutan Seed Waste into Biodiesel via Non-Catalytic Supercritical Ethanol and Ethyl Acetate

Energies, Vol. 18, Pages 6004: Valorization of Rambutan Seed Waste into Biodiesel via Non-Catalytic Supercritical Ethanol and Ethyl Acetate

Energies doi: 10.3390/en18226004

Authors:
Manita Kamjam
Doonyapong Wongsawaeng
Ruengwit Sawangkeaw
Wirasinee Supang
Peter Hosemann
Phachirarat Sola
Somkiat Ngamprasertsith

Rambutan seed waste from fruit processing remains underutilized, while conventional biodiesel routes face high feedstock costs and food-versus-fuel concerns. This study investigated a novel catalyst-free process for biodiesel production from rambutan seed waste using supercritical ethanol and ethyl acetate as renewable reactants to valorize fruit by-products. Batch reactions on the semi-solid fraction of rambutan seed oil (RSO) were conducted at 15 MPa to evaluate the effects of temperature (275–375 °C), reactant-to-oil molar ratio (20:1–40:1), and reaction time (15–50 min) on fatty acid ethyl ester (FAEE) yield. Under optimal conditions, FAEE yields of 59.92 and 41.92% were obtained using ethanol (350 °C, 40:1, 30 min) and ethyl acetate (350 °C, 30:1, 40 min), respectively. However, severe conditions degraded unsaturated esters, revealing a conversion–stability trade-off. The ethanol system exhibited faster reaction kinetics and lower activation energy than ethyl acetate. Applying the optimized ethanol-based conditions to the liquid fraction of RSO, which contained a lower proportion of saturated fatty acids, resulted in a markedly improved FAEE yield of 94.16%. This study demonstrated a catalyst-free supercritical route for converting rambutan seed waste into biodiesel, advancing waste-to-energy strategies and circular bioeconomy.