Measurement and prediction of liquid-liquid equilibria in ternary systems containing water, an organic component, and cyclohexanol
Graphical abstract
Introduction
Phase equilibrium data are the basis of the planning and operation of purification and separation processes of intermediates or end products. The understanding of liquid-liquid equilibrium (LLE) is especially important for extraction processes. In order to improve the experimental data base, the LLE of four ternary liquid mixtures were measured which have rarely been considered in the literature so far. The LLE were determined using the synthetic and the analytic methods [1] already successfully applied by our research group for mixtures of water + hydrocarbon + associating component, e.g. phenols [2] or amines [3].
In this paper, the phase equilibria of the following systems are presented:
System 1: water + heptane + cyclohexanol (CHOH) [LLE],
System 2: water + toluene + CHOH [LLE],
System 3: water + cyclohexane (CH) + CHOH [LLE],
System 4: water + cyclohexylamine (CHA) + CHOH [LLE],
System 5: toluene + CHOH [VLE].
The systematic choice of the ternary mixtures – three ternary systems containing water, CHOH and an aliphatic or an aromatic or a cycloaliphatic component (Systems 1–3) and one system with an associating component (System 4) – allows the influence of polarity on the LLE behaviour to be estimated. In order to determine the temperature dependence of the phase equilibrium, the measurements were carried out at 298.15 K and 323.15 K.
To evaluate prediction methods for phase equilibrium data, the LLE were predicted using the activity coefficient models UNIQUAC [4] and NRTL [5] as well as the equation of state proposed by Elliott, Suresh, and Donohue (ESD EOS) [6], [7]. In this way, the predictions of two classical activity coefficient models can be compared to those of an equation of state which takes into account attractive and repulsive forces and explicitly models associative interaction.
The predictions of the ternary systems are based solely on the use of binary interaction parameters. Some of the binary interaction parameters required for the prediction of the ternary systems had already been published [1], [3]. The binary interaction parameters for the systems water + CHOH, heptane + CHOH, toluene + CHOH, CH + CHOH, and CHA + CHOH were determined within this work. Since no experimental data were available for the binary toluene + CHOH system, the vapour-liquid equilibrium (VLE) was measured in a modified Röck and Sieg circulation still [8]. The measurement system's control algorithm was developed by our group [9] and has been successfully applied to a large number of systems containing associating components and hydrocarbons, e.g. water + toluene + aniline or water + toluene + CHA [10].
Section snippets
Material
The components heptane, toluene, CH, and CHOH were distilled over a bubble cap column at reduced pressures. CHA was used without further purification. Pre-treated water was purified in a Milli-Q Academic device, which produces ultrapure water of Type 1 (DIN ISO 3696) [11]. The organic components were stored over sodium sulphate. The commercial source, initial and final purities are given in Table 1.
Measurements
The LLE of the four ternary systems were determined by turbidity titration at atmospheric
Thermodynamic models
The LLE data in the ternary systems were predicted using the activity coefficient models UNIQUAC [4] and NRTL [5] and the equation of state proposed by Elliott, Suresh and Donohue (ESD) [6], [7]. Binary interaction parameters were determined by means of experimental binary VLE and (if applicable) LLE data.
In this study, the temperature dependence of the binary interaction parameters is assumed to be linear. It yields:with Cij = uij-ujj for UNIQUAC and Cij = gij-gjj for
System 1: water + heptane + CHOH
The experimental data for System 1 are listed in Table 5 and shown in Fig. 1. The binary water + heptane and water + CHOH subsystems are partially miscible. A miscibility gap in the ternary system of Type 2 was expected (classification of Treybal [16]) and verified experimentally. Due to the temperature dependence in the binary water + CHOH subsystem, the heterogeneous region decreases slightly with increasing temperature.
System 2: water + toluene + CHOH
The experimental results for System 2 are presented in Fig. 2 and Table 6
Conclusion
The LLE of the ternary systems of water + CHOH + an organic component (heptane, toluene, CH, or CHA) were determined experimentally at 298.15 K and 323.15 K. As expected, a Type 2 miscibility gap was determined for the systems containing heptane, CH, or toluene, and a Type 1 miscibility gap for the system containing CHA. The activity coefficient models UNIQUAC and NRTL and the ESD EOS were used to predict the LLE. The interaction parameters were determined using binary phase equilibrium data.
Acknowledgement
The financial support for this project by Deutsche Forschungsgemeinschaft (DFG, KL-2907/2-1) and by Bundesministerium für Bildung und Forschung (BMBF, 03FH041PX4) is gratefully acknowledged.
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