Elsevier

Fluid Phase Equilibria

Volume 362, 25 January 2014, Pages 151-162
Fluid Phase Equilibria

Interfacial tension of binary mixtures exhibiting azeotropic behavior: Measurement and modeling with PCP-SAFT combined with Density Gradient Theory

https://doi.org/10.1016/j.fluid.2013.09.042Get rights and content

Highlights

  • Calculation of interfacial properties of binary mixtures having azeotropic behavior.

  • Combination of PCP-SAFT-EOS with Density Gradient Theory.

  • New experimental data related to the interfacial and surface tension, as well as mixtures densities.

  • Prediction of selective enrichment in the interphase.

Abstract

This work focuses on modeling and experimental investigation of temperature dependent interfacial properties of binary DMF/n-alkane (C7, C10, C12) mixtures. The systems consisting of solvents with very different polarity show azeotropic behavior. New experimental vapor–liquid and liquid–liquid interfacial tension data are provided between 298.15 and 328.15 K using the drop volume method. The Perturbed Chain Polar Statistical Associating Fluid Theory (PCP-SAFT) equation of state was combined with the Density Gradient Theory (DGT) to calculate phase equilibria and interfacial properties. Modeling results are in good agreement with the corresponding experimental data. Thereby, the binary parameter βij within the DGT framework does not equal one. Investigating density and concentration profiles in the interface revealed characteristic trends which are related to the azeotropic behavior of the mixtures.

Introduction

Mixing and/or separation of chemicals are two of the most common unit operations in the chemical industry. To design and efficiently operate e.g. an extraction column, information on interfacial properties are required to describe hydrodynamic behavior and mass transfer characteristics. The general aim is to replace time-consuming and costly experiments by reliable modeling of thermodynamic properties which include phase equilibria and interfacial tensions. The mixtures involved often consist of solvents exhibiting very different molecular interaction behavior resulting in the occurrence of azeotropes.

A major challenge concerning the calculation of thermodynamic properties is to properly capture the characteristic interaction behavior of the different pure components and their mixtures. A common strategy therefore is to equip mathematical models with expressions which explicitly account for individual interaction mechanisms. In this way, it is even possible to adequately represent complex azeotropic phenomena for instance. The Perturbed Chain Polar Statistical Association Fluid Theory (PCP-SAFT) equation of state is a prominent representative of the group of such models. Numerous publications demonstrate that the PCP-SAFT equation of state is well appropriate to model phase equilibria of mixtures exhibiting strong deviations from ideality including azeotropes (e.g. [1], [2], [3], [4], [5]). Regarding the calculation of interfacial properties, the Density Gradient Theory (DGT) is a widely used method besides the density functional theory for treating pure components and mixtures (e.g. [6], [7], [8], [9]).

Only a few studies dealing with the investigation of interfacial properties focus on polar components [10], [11], [12]. That is, DGT has not yet been extensively tested for mixtures consisting of components exhibiting very different polarity. Furthermore, the relation between vapor–liquid–liquid equilibrium (VLLE) behavior and interfacial properties has not been an explicit issue up to now.

Alkanes and N,N-dimethylformamide (DMF) are commonly used solvents in chemical processes. For the hydroformylation of long-chain olefins, the applicability of thermomorphic solvent (TMS) systems was studied in a previous work [13]. It was found that DMF/n-decane mixtures are suitable components of a TMS system for this reaction.

Linear alkanes (n-alkanes) belong to the group of apolar solvents whereas DMF exhibiting a gas phase dipole moment of 3.86 D [14] is a typical polar compound. Experimental vapor–liquid (V–L) interfacial tension data of DMF/alkane systems are rare. Wadewitz [15] measured V–L interfacial tensions of DMF/n-heptane mixtures at 288.15 K, 298.15 K, 308.15 K and 318.15 K using the pendant drop method. To the best of our knowledge, V–L interfacial tension data for mixtures of DMF and n-decane as well as DMF and n-dodecane have not been published yet. Concerning liquid–liquid (L–L) interfacial tension data of DMF/n-alkane mixtures the situation is similar. Literature data is available only for DMF/n-heptane and DMF/n-decane mixtures [16].

The objective of this work was on the one hand to provide additional experimental V–L and L–L interfacial tension data for DMF/n-alkane (C7, C10, C12) mixtures between 298.15 K and 328.15 K. The drop volume method was employed to measure interfacial tensions whereby densities were determined by a vibrating tube densitometer. On the other hand the calculation of these data was focused. Thereby, the DGT was applied in combination with the PCP-SAFT model. Investigation issues concerning the modeling included the selection of appropriate values for the binary parameter βij within the DGT framework. Furthermore, the relation between VLLE behavior and accumulation effects at the interface was studied.

Section snippets

Materials

Table 1 lists all chemicals used for the experiments in this work together with information about suppliers and purities. All chemicals were used without further purification and stored over molecular sieves for drying purposes.

Apparatus and measurement procedure

There exist numerous methods to measure interfacial tensions, in particular V–L interfacial tensions. The drop volume method is one of the standard techniques due to high accuracy and reproducibility of the measurements [17]. In case of V–L interfacial tension

Theory

In this work, the Perturbed Chain Polar Statistical Association Fluid Theory (PCP-SAFT) equation of state was combined with the Density Gradient Theory (DGT) to simultaneously calculate phase equilibria and interfacial properties.

Results

In the following, experimental V–L and L–L interfacial tension data as well as the corresponding densities are presented. Modeling results are introduced in the subsequent section.

Density data

As can be seen from Fig. 1a, experimental density data of fully miscible DMF/n-alkane mixtures from this work and from the literature [34] are consistent. Modeled and experimental data differ by less than 2.5% for all systems.

Regarding the results for the densities in the heterogeneous region where two equilibrium liquid phases coexist, Fig. 2 demonstrates that differences between experimental and modeled data are small (<3%). Furthermore, the trends nicely reveal the two phases becoming more

Conclusion

Due to large differences in the polarity of DMF and alkane molecules, DMF/n-alkane systems represent strongly non-ideal mixtures. The characteristic features include partial miscibility in the liquid state and azeotropic phenomena regarding VLLE phase behavior. In this work, the interfacial properties of DMF/n-alkane mixtures (C7, C10, C12) were investigated between 298.15 and 328.15 K focussing on the relation to the distinctive VLLE behavior of these systems. Besides providing new experimental

Acknowledgements

The authors thank the Deutsche Forschungsgemeinschaft (DFG) for financial support within the Sonderforschungsbereich/Transregio 63 “Integrated Chemical Processes in Liquid Multiphase Systems”.

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