Liquid–liquid phase equilibrium studies of organic–aqueous medium during biodiesel synthesis

Abstract

This work investigates the liquid–liquid phase equilibrium of reaction mixtures arising from the ethanolysis of vegetable oil. Knowledge of equilibrium phase distribution of key components in such biphasic systems is essential for guiding the design and optimisation of a counter-current extractive reactor for lipasic biodiesel production. Waste cottonseed frying oil was reacted with (to different conversion levels) in the presence of lipase B (Novozym 435). The reaction mixtures were then combined with aqueous solutions of ethanol (5, 15 and 25%v/v) at three different volume fractions (0.20, 0.35 and 0.50) and allowed to equilibrate at 318K, followed by compositional analysis of the resultant organic phase (mainly composed of glycerides and fatty acid ethyl esters) and aqueous phase (largely water and ethanol). Distribution coefficients of ethanol and water peaked at moderate reaction extents (40–50%) and could be described by a decaying sinusoidal expression (with respect to extent of reaction) symptomatic of the dynamics for two interacting mixing tanks-in-series implicating species migration between the organic and aqueous phases. Equilibrium aqueous glycerol concentration profiles also exhibited double maxima at reaction extents of 57% and 95% with the magnitude increasing at higher organic phase volume fraction. Among the operating variables studied, reaction extent had the most significant influence on phase equilibrium possibly due to changes in the hydrophilicity of the reaction medium occasioned by compositional variation in the course of the reaction. In particular, a linear relationship was found between global and organic phase molar ratio of ethanol-to-fatty acid residues. Consequently, the findings of this study would permit a realistic determination of the rate-composition envelope as a function of the organic volume fraction along the extractive reactor column and hence, improved biodiesel recovery efficiency.