Continuous production of ethanol-based biodiesel under subcritical conditions employing trace amount of homogeneous catalysts

- High yield of FAEEs biodiesel was achieved under subcritical conditions with trace amount of catalysts.
- The three-step kinetic model fit the data well.
- Water and FFA content benefitted the acid reactions and decreased the yield for base reactions.
- Reaction pressure can be dropped to 80 bar without decreasing the reaction yield.

Syntheses of ethanol based biodiesel fuel fatty acid ethyl esters (FAEEs) from corn oil were conducted at subcritical ethanol conditions (T: 175-225 °C, P: 80-200 bar) with the presence of trace amount of catalysts (0.1 wt% sulfuric acid, 0.05 and 0.1 wt% potassium ethoxide, based on oil mass). Experiments were conducted in a continuous tubular 316SS reactor. Potassium ethoxide showed a stronger catalytic ability than sulfuric acid at the same temperature of 225 °C, as the reaction yield reached 95% in five minutes with potassium ethoxide while only 25% yield was achieved for sulfuric acid. A three-step partially reversible model was determined to best fit the kinetic data for both catalytic reactions. To investigate the effect of water and free fatty acids (FFA) impurities on reaction yield under subcritical conditions, 10 vol% water (based on ethanol volume) and 30 wt% oleic acid (based on oil mass) were added to the reaction systems. For sulfuric acid, the reaction yield benefitted from the presence of these impurities. On the contrary, for reactions catalyzed by potassium ethoxide, the yield decreased from 96.8% to 15.3% and 2.6% when adding 10 vol% water and 30 wt% oleic acid, respectively. A pressure range of 80-200 bar at 225 °C was investigated to study the pressure effect on reaction yield. The reactions at these pressures were observed to occur in the homogeneous phase, and the yields at all pressures were in the range of 94-96%, which implies that when the reactions are performed in the homogeneous state, pressure will have little effect on the yield.

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