Optimization and Kinetic Study of Palmitic Acid Esterification with Subcritical Methanol via Response Surface Methodology

Biodiesel is a green, low-carbon, and renewable fuel with the potential to substitute fossil fuels. The effects of reaction temperature (175–290 °C), residence time (5–75 min), and molar ratio of methanol to palmitic acid (6:1–35:1) on the non-catalytic esterification of palmitic acid with methanol to produce biodiesel were investigated by using a batch reactor. Moreover, the reaction parameters were optimized by using the response surface methodology (RSM), and the reaction kinetics were analyzed. The results showed that the conversion rate of palmitic acid to methyl palmitate increased to 100% as the reaction temperature rose from 175 °C to 225 °C, slightly changed until 275 °C, and then decreased to 94.81% at 290 °C. The conversion rate increased with residence time and reached the maximum value of 94.93% at 60 min. With increasing the molar ratio, the conversion rate rose to a maximum value of 85.46% at 15:1 and subsequently decreased. RSM results indicated the relative influence of factors on the conversion rate as reaction temperature > residence time > molar ratio. The optimal reaction parameters were 224 °C, 26 min, and a molar ratio of 16:1, affording a palmitic acid conversion rate of 99.30%. The esterification reaction between methanol and palmitic acid follows the first-order kinetics model.