Production of biodiesel from castor oil using sub and supercritical ethanol: Effect of sodium hydroxide on the ethyl ester production

•We investigated biodiesel production with ethanol under sub and supercritical conditions.•Using low concentration of NaOH it was possible to obtain high conversions.•Pseudo first order model kinetic represents the overall transesterification reaction.

The biodiesel obtained by the reaction of castor oil with ethanol under sub and supercritical condition (200–350 °C at endogenous pressure) using small amounts of sodium hydroxide (NaOH) as catalyst was studied. The influence of the operating variables such as temperature, reaction time, ethanol:oil molar ratio and catalyst concentration (when used) on the reaction yield was evaluated using a statistical design of experiments along with the surface response methodology for both the catalytic and non-catalytic processes. It was observed that using only 0.1% wt of NaOH for the catalytic process, amount smaller than the necessary to neutralize the castor oil, it was possible to obtain high yields. The maximum ethyl ester yield reached was 98.9% for the catalytic process and 56.2% for the non-catalytic process. Additionally, the reaction kinetic was determined and it was found that a pseudo first order model represents the overall reaction. Activation energies were calculated from the Arrhenius equation for both processes.

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