Pre-esterification of high acidity animal fats to produce biodiesel: A kinetic study

Abstract

Biodiesel is generally produced through transesterification with base catalysts, using vegetable oils as raw materials. These vegetable oils can be expensive, being replaced for other alternatives like animal fats. However, animal fats usually present high free fatty acid (FFA) levels, which make the use of base catalysts difficult. Pre-esterification of these raw materials is an effective pre-treatment to reduce FFA levels as free fatty acids react with methanol to produce methyl esters and water. In this research work, the esterification of animal fats was studied, obtaining kinetic parameters through a simplified model. This model, with 1.5 reaction order with respect to FFA, only takes into account the direct reaction of esterification chemical equilibrium, proving that the reaction rate of the inverse reaction is negligible. The reaction rate coefficient, which presents a linear correlation between methanol and the catalyst (H2SO4), was adjusted to temperature through the Arrhenius equation, obtaining the activation energy and the pre-exponential factor. A thorough experimentation has been carried out in order to determine the influence of catalyst concentration, methanol/fat ratio and reaction times corresponding to esterification and transesterification. Reaction times for esterification and transesterification longer than 120 min were enough to obtain biodiesel yields above 97%.