Isobaric vapor-liquid equilibrium for binary system of methyl caprate + methyl laurate at 2, 4 and 6 kPa

Highlights

• The VLE data for binary system of methyl caprate and methyl laurate were measured.

• The Wilson, NRTL and UNIFAC models were used to reproduce the experimental values.

• The COSMO-SAC model was applied to predict the VLE of the binary system.

Abstract

In this work, we measured the isobaric vapor-liquid equilibrium (VLE) data for the binary system of methyl caprate (1) + methyl laurate (2) at 2,4 and 6 kPa with a modified Othmer still. The point consistency test of Van Ness test method was applied to check the thermodynamic consistency of all the experimental values, which guaranteed the reliability of our measurements. Afterwards, the experimental data were regressed by two activity coefficient models (Wilson, NRTL). Furthermore, we made the data prediction with the group contribution method (UNIFAC) and the COSMO-SAC model. The results of all the regressions and predictions are satisfying and reasonable, while the results provided by the NRTL model are relatively better than the other three models, with the root mean square deviations of temperature and vapor-phase mole fraction less than 0.33 K and 0.0069, respectively.

Introduction

Methyl caprate and methyl laurate are two significant vegetable-based fatty acid esters which typically exist in the biodiesel production [1], [2], [3]. However, the fine separation of them from the biodiesel could generate higher added values. In recent years, they are enjoying widely application in the chemical industry, such as in the preparation of food, pharmaceuticals, cosmetics [4], solvents, plasticizers [5], detergents, surfactants, lubricants [6], and antibacterial agents [7]. Furthermore, new applications of them have been exploited now. Leah C. Liston et al. [8] and Yaghoob Farnam et al. [9] have revealed the novel use of methyl laurate as the phase change materials in concrete pavements. For another, they have also been used as raw materials to produce higher alcohols [10], [11].

To the best of our knowledge, the physicochemical properties and the VLE data are essential information for the fine separation and better application of them. However, the related study is far from adequate. Currently, quite a few researches relevant to their physicochemical properties have been reported. Monika Zarska et al. [12] measured the speed of sound and the densities of methyl caprate and methyl laurate. Based on the experimental results, a function has been established for the calculation of their densities, isobaric thermal expansivities and isentropic compressibilities. By determining the densities and viscosities of seven ethyl esters and eight methyl esters, reliable models for the computation of the densities and the viscosities of biodiesel fuel were proposed by Maria Jorge Pratas et al. [13]. Using the pulse-heating method, Nikitin et al. [14] obtained the critical temperatures and pressures of methyl esters C_nH_2nO₂CH₃(n = 6, 7, 8, 9, 10, 11, 12), and the equations for the calculation of the critical temperatures and pressures were hereby developed. In addition, Xiangyang Liu et al. [3] determined the isobaric molar heat capacities of methyl caprate and methyl laurate within the temperatures from 300 K to 380 K and at pressures from 0.1 MPa to 4.25 MPa.

All the above researches have provided important basic information about their physicochemical properties, which is of great help for their industrial application. Nevertheless, the works concerning the phase equilibrium behavior are scarce. It is known to all that the phase equilibrium behavior is a significant property for their fine separation in the field of distillation. Only few works have been revealed. The work by N. Bureau et al. [15] reported the vapor pressure of some heavy fatty acid esters including methyl laurate and the work by Troy A. Scott et al. [16] presented the vapor pressure of both methyl caprate and methyl laurate. As for the vapor-liquid equilibrium for binary system of methyl caprate and methyl laurate, we have only located the work by Rose et al. [17], in which 3 to 6 VLE data points at 30, 40, 50 and 100 mmHg were covered and are now provided in the Supplementary Material (Table S1). None of the data were checked or regressed by any thermodynamic models. Regretfully, obvious mistakes also existed in the data. Hence, the industrial cannot use these data as a guide for the vacuum distillation directly. For more complete research and considering the significance of the VLE data in the vacuum distillation, here in this work, the isobaric VLE data for binary system of methyl caprate and methyl laurate at 2, 4 and 6 kPa were measured with a modified Othmer still. First, the Van Ness test [18] was employed to check the thermodynamic consistency of the obtained data. Then, the experimental values were regressed with the Wilson and the Nonrandom two-liquid (NRTL) activity coefficient models. Furthermore, the UNIFAC model was applied to make a data prediction. Finally, the newly established COSMO-SAC model was also adopted for the prediction of the VLE data of the binary system. The activity coefficients of each material were acquired from the COSMO-SAC model. And based on the calculated activity coefficients, the composition of vapor-phase at each point was achieved.