A systematic multivariate analysis of the supercritical synthesis of soy biodiesel using 92.8% w/w hydrated ethanol

The transesterification of vegetable oils under supercritical conditions has been identified as a viable alternative for efficient biodiesel synthesis and has the potential for industrial applications. In the present study, a multivariate analysis was used to optimize the supercritical transesterification of soybean oil and hydrated ethanol (92.8% w/w). The following variables were evaluated: reaction time, temperature, ethanol/oil molar ratio and the ratio between the reagent volume and reactor volume. The reactions were conducted according to a Doehlert design using four variables in a reactor specifically designed for this purpose. Glass windows positioned at the ends of the reactor allowed for the monitoring of the phases in the system at both the critical point and the supercritical environment. Both a mathematical linear regression model and response surface were created and used to determine the optimal synthesis conditions. An ester content of greater than 97.3% which was estimated using the optimized experimental conditions (320 °C, 50 min, ethanol/oil 49:1, reagent to reactor volume ratio of 60% and pressure lower than 20 MPa) indicated that biodiesel synthesis can be performed with high yields using hydrated bioethanol under supercritical conditions. The effects of temperature on polyunsaturated esters containing two and three carbon double bonds were also evaluated. The advantages of a supercritical synthetic route are that it requires neither catalysts nor washing water during the purification stages, produces purer glycerin than conventional processes and permits the re-use of ethanol without preliminary dehydration, which simplifies the overall process and has a low environmental impact. - Highlights: • A multivariate study was used to optimize the supercritical transesterification. • The use of soybean oil and hydrated ethanol was performed with high contents of FAEE. • Yield of 97.3% was predicted in the optimal point, clean glycerin is also produced. • Innovative images of phase evolution obtained inside the reactor were taken. • Structural modifications of unsaturated esters were observed at t ≥ 325 °C.