Continuous Biodiesel Production Catalyzed by Trace-Amount Alkali under Methanol Subcritical Conditions

Abstract

Biodiesel is a promising alternative biofuel, but the treatment of wastewater containing alkali catalyst and glycerol discharged from the washing unit operation increases the cost significantly. Here a novel continuous biodiesel production process with trace-amount alkali as the catalyst under the methanol subcritical condition was proposed and investigated. The optimal operation conditions of preliminary batch reaction experiment are temperature of 200 °C, catalyst concentration of 513 mg/kg, reaction time of 38 min, molar ratio of methanol to oil of 11.9:1, and system pressure of 1.5 MPa. The one-step conversion of raw oil in a batch reactor can reach up to 85.5%. The alkali residue in the biodiesel product can be further reduced by removing the methanol and washing by glycerol instead of by acid or water. The optimal weight ratio of glycerol to biodiesel is 1.5:1, and the residual alkali in the final ester product is about 4.6 mg/kg. The simulation and bench scale continuous experiment based on the optimal batch operation parameters confirmed that the biodiesel produced by the process is qualified well up to the Chinese standard of biodiesel, and the K + concentration in biodiesel was less than 3.0 mg/kg. The economic evaluation showed that this new process is more economically feasible than the traditional processes.