Liquid–liquid equilibria for mixtures containing water, methanol, fatty acid methyl esters, and glycerol

Abstract

The liquid–liquid equilibrium (LLE) data, including tie-lines and phase boundaries, were measured for the ternary systems of water + methanol + methyl oleate, water + methanol + methyl linoleate, glycerol + methanol + methyl oleate, and glycerol + methanol + methyl linoleate at temperatures from 298.2 K to 318.2 K under atmospheric pressure. All the LLE data follow the Othmer-Tobias equation. Each ternary system behaves type-I LLE. The areas of two-liquid coexistence region decrease with increasing temperature. The experimental data were applied to test the validity of the UNIFAC model and its modified versions, including UNIFAC-LLE and UNIFAC-Dortmund. The LLE data were also correlated with the NRTL and the UNIQUAC models. The UNIQUAC model yielded better results.

Introduction

In recent years, biodiesel has been recognized as one of cleaner fuels produced from renewable resources [1], [2], [3]. The major components of the biodiesel are fatty acid methyl esters (FAME), which can be produced from oils with methanol via transesterification or from fatty acids with methanol via esterification. In these two reaction systems, the main products of transesterification: glycerol/FAME and those of esterification: water/FAME are all partially miscible. Typically it is necessary to separate the FAME-rich phase from the glycerol-rich or the water-rich phase by using a decanter in a biodiesel production process. To accurately calculate the phase compositions of each individual liquid phase in the process simulation, we need the LLE data to determine reliable model parameters. Since methanol is often in large excess in the transesterification and the esterification systems, the LLE data (especially the compositions of the coexistence phases) of methanol + glycerol + FAME and methanol + water + FAME systems are fundamentally important for the engineering applications. While some LLE data are available for methanol + glycerol + FAME, those of aqueous systems are still rather limited in literature. Actually FAME is a multicomponent mixture. Its constituent compounds depend on the sources of raw materials. Many investigators measured the phase equilibrium data by using real biodiesels produced from soybean oil [4], Jatropha curcas L. oil [5], or castor oil [6]. Some other research groups took methyl oleate as a model compound of FAME. Among several others, Chen et al. [7] measured the reaction and liquid–liquid distribution equilibria for the system containing oleic acid, methanol, methyl oleate, and water at 346 K. The LLE data of glycerol + methanol + methyl oleate at 333 K were reported by Negi et al. [8]. Andreatta et al. [9] conducted not only the LLE experiments at temperatures from 313 K to 353 K, but also the vapor–liquid–liquid equilibria (VLLE) measurements at 353–393 K for glycerol + methanol + methyl oleate. Nevertheless, the LLE data of the systems containing other major components of FAME, such as methyl linoleate or methyl palmitate, are still scarce in literature. In order to estimate the fugacity of the constituent compounds for those data of missing systems, a group contribution model, such as UNIFAC [10] was often adopted. The reliability of these predictive models is needed to be further verified with experimental phase equilibrium data.

The objective of this study is to measure the LLE data of water + methanol + methyl oleate, water + methanol + methyl linoleate, glycerol + methanol + methyl oleate, and glycerol + methanol + methyl linoleate at temperatures from 298.2 K to 318.2 K. These new experimental data were applied to test the validity of the UNIFAC model [10] and its modified versions, including the UNIFAC-LLE [11] and the UNIFAC-Dortmund [12], and were also correlated with the NRTL model [13] and the UNIQUAC model [14].