Esterification of free fatty acids with supercritical methanol for biodiesel production and related kinetic study

Non-catalytic esterification of free fatty acids (oleic acid) with methanol for biodiesel production was studied in the reaction temperature range of 220-280 °C. To optimize the reaction conditions, the influence of the process parameters on biodiesel yield was studied with response surface methodology (RSM). It was found that biodiesel yield decreases at an exorbitant temperature. Thus, a reaction temperature below 260 °C is appropriate. At a temperature of 260 °C, with a methanol to oleic acid molar ratio of 20/1 and a reaction time of 60 min, the biodiesel yield can reach about 95%. In addition, a simplified approach to implementing a kinetic model for the esterification reaction was developed. The kinetic parameters were analyzed and discussed, and the experimental results were found to fit a first-order kinetic rate law. The rate constant of esterification in supercritical methanol increases significantly in comparison to that of subcritical conditions, and the apparent activation energy is higher than that of subcritical conditions.