Long a thermodynamic curiosity, supercritical fluids (SCFs) have gradually gained ground in today’s life, generating an increasing number of new, efficient processes in diverse industrial sectors and fueling active R&D programs. Indeed, the versatility of SCFs allows them to serve a wide variety of applications. The list includes not only food processing, biofuel production, extraction of biomolecules marketable as medicines, cosmetics and nutraceuticals, but also emerging technologies for the production of electrical power, based on supercritical or transcritical thermodynamic cycles. This jointly authored article will provide a review of important applications covered by our laboratories in the agri-food, chemical and energy sectors. We will then try to detect recent trends and outline future prospects.

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.