Isothermal vapor–liquid equilibria for binary mixtures of benzene, toluene, m-xylene, and N-methylformamide at 333.15 K and 353.15 K

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Abstract

Isothermal vapor–liquid equilibrium (VLE) at 333.15 K and 353.15 K for four binary mixtures of benzene + toluene, benzene + N-methylformamide, toluene + m-xylene and toluene + N-methylformamide have been obtained at pressures ranged from 0 kPa to 101.3 kPa. The NRTL, UNIQUAC and Wilson activity coefficient models have been employed to correlate experimental pressures and liquid mole fractions. The non-ideal behavior of the vapor phase has been considered by using the Soave–Redlich–Kwong equation of state in calculating the vapor mole fraction. Liquid and vapor densities were also measured by using two vibrating tube densitometers. The Pxy diagram and the activity coefficient indicate that two mixtures of benzene + toluene and toluene + m-xylene were close to the ideal solution. However, two mixtures containing N-methylformamide present a large positive deviation from the ideal solution. The excess Gibbs energy in the benzene + toluene mixture is negative indicates that it is an exothermic system.

Introduction

Vapor–liquid equilibrium (VLE) data are essential for engineering design of separation processes and unit operations. It is useful for the extension of thermodynamic models commonly applied for petrochemical related industries. Such information can be obtained experimentally or adopted from generalized methods to calculate properties of multi-component mixtures. Accurate densities of vapor and liquid phases are critical and important to result in a proper size in the process design of many separation equipments. For an ideal system, it is relatively easy to estimate vapor–liquid equilibrium. However, most liquid systems of industrial interest show deviations from the ideal behavior. Raal and Muhlbauer describe with detail a few commonly used liquid phase activity coefficient models [1]. Klara et al. [2], Huang et al. [3], Gultekin [4] successfully correlate their PTxy data of benzene + toluene and toluene + N-formylmorpholine (NFM) mixtures with the NRTL, UNIQUAC and Wilson models. In their studies, binary interaction parameters of mixtures were obtained to predict phase behavior by examining experimental data for thermodynamic consistency. Santiago et al. [5] correlates the VLE data by a few activity coefficient models for a ternary system of ethanol + 2-methyl-1-butanol + calcium chloride. Simoni et al. [6] models the L–L equilibria of ionic liquid systems with NRTL, electrolyte-NRTL and UNIQUAC models. These liquid-activity models with optimally fitted binary parameters have then been employed to predict phase behaviors of the VLE and LLE systems. Densities of the benzene + toluene mixture have been analyzed using refractive index measurements by Sumer and Thompson [7]. To our knowledge, isothermal VLE data for a system of N-methylformamide + toluene have not been found in the literature.

In this study, experimental Px data were measured at 333.15 K and 353.15 K under pressure ranged from 0 kPa to 101.3 kPa for four binary mixtures of benzene + toluene, benzene + N-methylformamide, toluene + m-xylene and toluene + N-methylformamide by using the isothermal bubble-point pressure method. These experimental data (P, xi) were then used to obtain vapor mole fraction (yi), activity coefficient (γi) and the system excess molar Gibbs free energies (GE). Finally, the phase equilibrium calculation was carried out by adopting the Soave–Redlich–Kwong (SRK) equation of state [8] for the vapor phase and the Wilson [9], NRTL [10] and UNIQUAC [11] activity coefficient models for the liquid phase.

Section snippets

Chemicals

De-ionized water was prepared using a Milli-Q purification system and 99.99% nitrogen was obtained from Air Product Co. (Taiwan). Both are used for calibration of pressure and temperature related constants for two densitometers. All the chemicals were high-purity grade purchased from a local supplier, such as, benzene (Fluka, ≥99.5% pure), toluene (Sigma, ≥99% pure), m-xylene (Alfa Aesar, ≥99% pure) and N-methylformamide (Sigma, ≥99% pure) used as reagents without further purification.

Apparatus and procedures

The

Pxy phase diagram and density data

The pressure related constants of the two densitometers (KpL, KpV) were calibrated using water and nitrogen. Vapor and liquid densities for each experiment were calculated using the following equations:KPL,V=ρH2OρN2τH2O2τN22ρL=ρH2O+KPL[(τL)2(τH2O)2]ρV=ρN2+KPV[(τV)2(τN2)2]

For density measurement, two pressure-dependent calibration constants of the vapor and the liquid densitometer were determined at constant temperature, using water and nitrogen as calibrated fluids for all investigated

Conclusions

Vapor–liquid equilibria of four binary mixtures of benzene, toluene, m-xylene and N-methylformamide have successfully been obtained at 333.15 K and 353.15 K in this study. Experimental data of phase densities, liquid mole fractions and system pressures were determined using the bubble-point method coupled with two vibration densitometers. Three activity coefficient models have been employed to correlate the experimental data and the results are satisfactory. The calculated activity coefficient

Acknowledgments

The authors would like to thank the National Science Council of the Republic of China, Taiwan for financially supporting this research (NSC97-2622-E005-011-CC1). This work is also supported in part by the ministry of Education, Taiwan, ROC under the ATU plan and China Petroleum Company and National Chung Hsing University project (EEA9715003).

References (14)

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