Phase equilibria of methanol–triolein system at elevated temperature and pressure
Introduction
Study on the alcholysis of oils with simple alcohols to produce fatty acid esters, which are important intermediates, surfactants, lubricants as well as an alternative fuel derived from renewable resource [1], [2], is of great importance. Usually, the reactions take place in the presence of an acid or an alkaline catalyst [3]. Recently, some researchers reported non-catalytic alcoholysis of vegetable oils in supercritical methanol [4], [5].
The miscibility of triglycerides and methanol are rather poor due to their dissimilarity in size and polarity, and they form two liquid phases upon their initial introduction into reactors. One factor of particular importance in the alcoholysis process is the degree of mixing between the alcohol and triglyceride phases [6]. In other words, the phase behavior of the reaction mixture is crucial for the reaction process. Boocock et al. [7] reported that the initial concentration of oil in the methanol is only about 3.7 g l−1 (can be neglectable in mole fraction) at 303.2 K in the absence of co-solvent, whereas the miscibility for methanol and oil system could be enhanced by addition of some simple ethers, such as tetrahydrofuran and diethyl ether. A literature survey indicated that the phase equilibrium data of triglycerides and methanol reported are very limited, especially at elevated pressures.
Triolein, one of the typical triglycerides, is the most abundant component in many plant oils, such as canola, soybean, sunflower seed, etc. In this work, we studied the phase behavior of methanol–triolein system at different temperatures and pressures. The Peng–Robinson [8] equation of state (PR-EOS) was used to correlate the experimental data.
Section snippets
Materials
Anhydrous methanol (A.R. grade, >99.5%) was purchased from Beijing Chemical Reagent Company. The triolein (99.0 wt.%) was provided by Aldrich Co., it was distillated in vacuum of 1.33 kPa, 333 K for 2 h in order to dehumidify before it was used in experiments.
Appararus and procedures
The apparatus and procedures of this work was similar to that used previously [9]. The schematic diagram of the apparatus is shown in Fig. 1. It consisted mainly of a volume-variable view cell, a constant temperature air bath, a pressure
Modeling of the data
The PR-EOS was used to correlate the experimental data. The equation can be expressed as following [8]:
For a pure component i, the parameters ai and bi in the PR-EOS are the function of the critical temperature, critical pressure and acentric factor of the component. To model the molecular interactions between components i and j, the binary interaction parameters (ka,ij, kb,ij) are introduced through the mixing rules as follows [10]:
Results and discussion
In this work, all the experiments were carried out in the two phase region, and the experimental data are listed in Table 2. Calculation was performed in the whole pressure and temperature ranges studied. Table 3 lists the interaction parameters obtained by the best fit of the phase equilibrium data of methanol–triolein binary system. The calculated values at different conditions are also listed in Table 2. Maximum relative deviations between the experimental and calculated mole fraction of
Conclusions
There are two phases in methanol and triolein system in ranges of 6.0–10.0 MPa and 353.2–463.2 K. The miscibility of the system can be enhanced effectively by increasing temperature, and at a higher pressure the miscibility is more sensitive to temperature at a fixed pressure. The Peng–Robinson equation of state can be used to calculate the phase compositions. The maximum relative deviation between the calculated and the experimental mole fraction of methanol in the triolein-rich phase and
Acknowledgement
The authors would like to thank the financial support from Petrochemical Corporation of China (Sinopec).
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