Phase Equilibria of Methanol–Triolein System at Elevated Temperature And

Fluid Phase Equilibria 239 (2006) 8–11

Phase equilibria of methanol–triolein system at elevated temperature and pressure Zhong Tang a,∗, Zexue Du a, Enze Min a, Liang Gao b, Tao Jiang b, Buxing Han b a Research Institute of Petroleum Processing, Sinopec, Beijing 100083, China b Institute of Chemistry, Chinese Academy of Science, Beijing 100080, China Received 21 January 2005; received in revised form 12 September 2005; accepted 9 October 2005 Available online 21 November 2005

Abstract The phase behavior of methnol–triolein system was determined experimentally at 6.0, 8.0 and 10.0 MPa in the temperature range of 353.2–463.2 K. The results demonstrated that the miscibility of the system was rather poor at low temperature, and the miscibility could be improved by increasing temperature. At a higher pressure, the miscibility was more sensitive to temperature as pressure was ﬁxed. The critical temperature, critical pressure, and acentric factor of triolein were estimated and the experimental data were correlated using the Peng–Robinson equation of state. The calculated data agreed reasonably with the experimental results. © 2005 Elsevier B.V. All rights reserved.

Keywords: Phase equilibrium; Methanol; Triolein; Equation of state; Calculation; High pressure

1. Introduction triglycerides and methanol reported are very limited, especially at elevated pressures. Study on the alcholysis of oils with simple alcohols to pro- Triolein, one of the typical triglycerides, is the most abun- duce fatty acid esters, which are important intermediates, sur- dant component in many plant oils, such as canola, soybean, factants, lubricants as well as an alternative fuel derived from sunﬂower seed, etc. In this work, we studied the phase behavior renewable resource [1,2], is of great importance. Usually, the of methanol–triolein system at different temperatures and pres- reactions take place in the presence of an acid or an alkaline sures. The Peng–Robinson [8] equation of state (PR-EOS) was catalyst [3]. Recently, some researchers reported non-catalytic used to correlate the experimental data. alcoholysis of vegetable oils in supercritical methanol [4,5]. The miscibility of triglycerides and methanol are rather poor 2. Experimental due to their dissimilarity in size and polarity, and they form two liquid phases upon their initial introduction into reactors. 2.1. Materials One factor of particular importance in the alcoholysis process is the degree of mixing between the alcohol and triglyceride Anhydrous methanol (A.R. grade, >99.5%) was pur- phases [6]. In other words, the phase behavior of the reaction chased from Beijing Chemical Reagent Company. The triolein mixture is crucial for the reaction process. Boocock et al. [7] (99.0 wt.%) was provided by Aldrich Co., it was distillated in reported that the initial concentration of oil in the methanol is vacuum of 1.33 kPa, 333 K for 2 h in order to dehumidify before − only about 3.7 g l 1 (can be neglectable in mole fraction) at it was used in experiments. 303.2 K in the absence of co-solvent, whereas the miscibility for methanol and oil system could be enhanced by addition of 2.2. Appararus and procedures some simple ethers, such as tetrahydrofuran and diethyl ether. A literature survey indicated that the phase equilibrium data of 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 ∗ Corresponding author. Tel.: +86 10 82363812; fax: +86 10 62311290. cell, a constant temperature air bath, a pressure gauge, and two

Table 1 Predicted physical properties of triolein

a Tb (K) 879.9 a Tc (K) 954.1 a Pc (MPa) 0.3602 ωb 1.6862 3 VL (m /kmol) [16] 0.9717 a Estimated by the method of Dohrn and Brunner [13,14]. b Acentric factor estimated according to Han and Peng [17].

ing the piston of the view cell. The masses of the samples were determined by a balance (OHAUS) with a resolution of 0.1 mg. Fig. 1. Schematic diagram of the experimental apparatus. (1) Volume-variable The methanol in the sample bombs was removed in a vacuum view cell, (2) pressure gauge, (3) thermalcouple, (4) temperature controllor, (5) oven at 323.2 K. Then the sample bombs were weighed again and constant temperature oven, (6) discharge tank and (7 and 8) up and bottom phase sample bombs. the masses of the triolein, which was nonvolatile, in the sample bombs were known. The compositions of the two phases were sample bombs (both with 0.5 ml volume). There was a magnetic easily calculated from the masses of methanol and triolein. The stirrer in the view cell, and its volume could be changed in the experiment at the same condition was carried out once again range from 23 to 50 cm3 by moving the piston. The temperature and the mean values of two sets of compositions were obtained. of the air bath was controlled by a XS/A-1 temperature con- Then, the experiment at another pressure was conducted. In this troller (Beijing Tianchen Automatic Instrument Co.), and the work, the equilibrium was conformed by the fact that the com- accuracy of temperature measurement was better than ±0.2 K. positions of the phases were independent of equilibration time. The pressure gauge was composed of a pressure transducer It was estimated the mole fraction of methanol determined could (FOXBORO/ICT Model 93) and an indicator. Its accuracy was be accurate to 0.001. ±0.025 MPa in the pressure range of 0–20 MPa. In the experiment, suitable amounts of methanol and triolein 3. Modeling of the data were loaded into the view cell. The system was equilibrated at ﬁxed temperature and pressure with stirring for 2 h. The stir- The PR-EOS was used to correlate the experimental data. rer was then stopped, and the system was stabilized for about The equation can be expressed as following [8]: 20 min. The samples of the up phase and bottom phase were RT a taken separately using the sample bombs. During the sampling p = − (1) process, the pressure of the system remained constant by mov- v − b v(v + b) + b(v − b)

Table 2 xexp yexp xcal ycal Experimental ( 1 , 1 ) and calculated ( 1 , 1 ) compositions in bottom phase and up phase in methanol (1)–triolein (2) system at different temperatures and pressures P (MPa) T (K) Mole fraction in bottom phase Mole fraction in up phase xexp xcal a yexp ycal a 1 1 D (%) 1 1 D (%) 6.0 353.2 0.186 0.180 3.2 0.998 0.999 0.1 6.0 393.2 0.566 0.532 6.0 0.996 0.997 0.1 6.0 413.2 0.676 0.668 1.2 0.996 0.995 0.1 6.0 433.2 0.730 0.729 0.1 0.993 0.992 0.1 6.0 443.2 0.749 0.750 0.1 0.990 0.986 0.4 6.0 453.2 0.746 0.746 0.0 0.986 0.977 0.9 6.0 463.2 0.777 0.778 0.1 0.988 0.970 1.8 8.0 353.2 0.107 0.111 3.7 1.000 0.999 0.1 8.0 393.2 0.433 0.451 4.2 0.999 0.999 0.0 8.0 413.2 0.592 0.601 1.5 0.995 0.997 0.2 8.0 433.2 0.690 0.690 0.0 0.992 0.993 0.1 8.0 443.2 0.732 0.731 0.1 0.984 0.987 0.3 8.0 453.2 0.733 0.733 0.0 0.980 0.981 0.1 8.0 463.2 0.791 0.790 0.1 0.961 0.975 1.4 10.0 353.2 0.070 0.068 2.9 1.000 1.000 0.0 10.0 393.2 0.343 0.390 13.7 0.999 0.999 0.0 10.0 413.2 0.596 0.590 0.1 0.998 0.997 0.1 10.0 433.2 0.687 0.686 0.1 0.991 0.991 0.0 10.0 443.2 0.773 0.773 0.0 0.988 0.987 0.1 10.0 453.2 0.777 0.777 0.0 0.982 0.981 0.1 10.0 463.2 0.810 0.810 0.0 0.958 0.974 1.6 a D stands for the relative deviation between the experimental and calculated data. 10 Z. Tang et al. / Fluid Phase Equilibria 239 (2006) 8–11

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]: √ a = xixj aiaj(1 − ka,ij) (2) bi + bj b = xixj (1 − kb,ij) (3) 2 The binary interaction parameters were obtained using Dem- ing algorithm [11]. The objective function is as follows: T m − T c 2 Pm − Pc 2 Q = + σ σ k T k P xm − xc 2 ym − yc 2 + 1 1 + 1 1 σ σ (4) Fig. 2. Phase diagram of methanol–triolein binary system at 6.0 MPa. k x1 k y1

The quality of the regressed data was assessed using the average absolute deviation (AAD) between the measured and calculated data: N = 1 |d |× AAD (%) N i 100 (5) i=1 where di is the error between the experimental and calculated values and N is the number of data points. For the calculation of a and b, the critical parameters and acentric factor of the pure components are required. Some group contribution methods recommended by Joback, Ambrose (referred to Reid et al. [12]) had been used to predict the critical parameters based on the boiling points of the pure components. However, the normal boiling point of triolein cannot be determined experimentally due to its thermal decomposition at temperatures even below its boiling point. Furthermore, the predicted boiling points of triglycerides with group contribution Fig. 3. Phase composition diagram of methanol–triolein system at 8.0 MPa. methods mentioned above are unreasonable because of their molecular weight and structure. Therefore, we estimated the critical parameters in this work according to the method reported by Dohrn and Brunner [13,14], as it requires only the liquid molar volume at 298 K (VL), and vapor pressure data as the input data. Some vapor pressure data of triolein are available in the open literature [15,16]. Extrapolative and predictive method was used to estimate the vapor pressure at the low temperatures at which the data are not available due to the difﬁculty to carry out vapor pressure measurements. The group contribution method reported by Han and Peng [17] was used to estimate the acentric factor of triolein. Table 1 lists the predicted critical properties of triolein used in this work.

4. 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 Fig. 4. Phase composition diagram of methanol–triolein system at 10.0 MPa. Z. Tang et al. / Fluid Phase Equilibria 239 (2006) 8–11 11

Table 3 Acknowledgement Binary interaction parameters for the PR-EOS for the system methanol (1)–triolein (2) The authors would like to thank the financial support from System Methanol–triolein Petrochemical Corporation of China (Sinopec). ka,12 0.0289 kb,12 −0.0109 a References AADx (%) 1.8 b AADy (%) 0.4 [1] F. Ma, M.A. Hanna, Biodiesel production: a review, Bioresour. Technol. a Average relative deviation between experimental and calculated mole frac- 70 (1999) 1–15. tion of methanol in the bottom phase. [2] A. Srivastava, R. Prasad, Triglycerides-based diesel fuels, Renewable b Average relative deviation between experimental and calculated mole frac- Sustainable Energy Rev. 4 (2000) 111–133. tion of methanol in the up phase. [3] B. Freedman, R.O. Butterfield, E.H. Pryde, Transesterification kinetics of soybean oil, JAOCS 63 (10) (1986) 1377–1380. and temperature ranges studied. Table 3 lists the interaction [4] S. Saka, D. Kusdiana, Biodiesel fuel from rapeseed oil as prepared in parameters obtained by the best fit of the phase equilibrium supercritical methanol, Fuel 80 (2001) 225–231. [5] G. Madras, C. Kolluru, R. Kumar, Synthesis of biodiesel in supercritical data of methanol–triolein binary system. The calculated values fluids, Fuel 83 (2004) 2029–2033. at different conditions are also listed in Table 2. Maximum [6] M.A. Dasari, M.J. Goff, G.J. Suppes, Noncatalytic alcoholysis kinetics relative deviations between the experimental and calculated of soybean oil, JAOCS 80 (2003) 189–192. mole fraction of methanol in the bottom phase and up phase [7] D.G.B. Boocock, S.K. Konar, H. Sidi, Phase diagram for are 13.7% and 1.8%, respectively, and the average relative oil/methanol/ether mixtures, JAOCS 73 (10) (1996) 1247–1251. [8] D.Y. Peng, D.B. Robinson, A new two-constant equation of state, Ind. deviations are 1.8% and 0.4%, respectively. Eng. Chem. Fundam. 15 (1976) 59–64. The experimental and calculated data at 6.0, 8.0 and 10.0 MPa [9] L. Gao, Z.S. Hou, H.F. Zhang, J. He, Z.M. Liu, X.G. are also shown in Figs. 2–4. It can be seen from the figures that at Zhang, B.X. Han, Critical parameters of hexane + carbon monox- all the pressures the solubility of methanol in triolein and that of ide + hydrogen and hexane + methanol + carbon monoxide + hydrogen triolein in methanol increase with increasing temperature, i.e., mixtures in hexane-rich region, J. Chem. Eng. Data 46 (2001) 1635– 1637. the miscibility of the two compounds is enhanced as temperature [10] J.M. Prausnitz, R.N. Lichtenthaler, E.G. De Azevedo, Molecular Ther- rises. It is interesting to notice that at the higher pressures the madynamics of Fluid-Phase Equilibria, Prentice-Hall, 1986. miscibility is more sensitive to temperature. [11] Aspen Technology Inc., Aspen Plus Version 10.1: User Guide, Cam- The data in Table 2 and Figs. 2–4 indicate that the PR-EOS bridge, USA, 1999. can be used to calculate the phase equilibrium data. Therefore, [12] R.C. Reid, J. Prausnitz, B. Poling, The Properties of Gases and Liquids, fourth ed., McGraw Hill, New York, 1987. using the equation and the parameters in Tables 1 and 3,wecan [13] R. Dohrn, G. Brunner, Correlations of pure-component parameters of predict the phase equilibrium data at some other conditions. the Peng–Robinson equation of state, in: Proceedings of Second Inter- national Symposium on Supercritical Fluids, Boston, 1991, pp. 471– 5. Conclusions 478. [14] R. Dohrn, G. Brunner, An estimation method to calculate Tb, Tc, Pc and ␻ from the liquid molar volume and the vapor pressure, in: Pro- There are two phases in methanol and triolein system in ceedings of the Third International Symposium on Supercritical Fluids, ranges of 6.0–10.0 MPa and 353.2–463.2 K. The miscibility of Strasbourg, France, 1994, pp. 241–248. the system can be enhanced effectively by increasing tempera- [15] E.S. Perry, W.H. Weber, B.F. Daubert, Vapor pressure of phlegmatic ture, and at a higher pressure the miscibility is more sensitive liquids. I. Simple and mixed triglycerids, J. Am. Chem. Soc. 71 (1949) to temperature at a fixed pressure. The Peng–Robinson equa- 3720–3726. [16] M.W. Formo, E. Jungermann, F.A. Norris, N.O.V. Sonntag, in: D. Swern tion of state can be used to calculate the phase compositions. (Ed.), Bailey’s Industry Oil and Fat Products, vol. 1, fourth ed., John The maximum relative deviation between the calculated and the Wiley & Sons, New York, 1979 (Chapter 3). experimental mole fraction of methanol in the triolein-rich phase [17] B.X. Han, D.Y. Peng, A group-contribution correlation for predicting the and methanol-rich phase are 13.7% and 1.8%, respectively, and acentric factors of organic compounds, Can. J. Chem. Eng. 71 (1993) the average relative deviations are 1.8% and 0.4%. 332–334.