Liquid–liquid equilibria of lactam containing binary systems

https://doi.org/10.1016/j.fluid.2008.02.008Get rights and content

Abstract

In this work, we report a study on the liquid–liquid phase equilibria for N-methyl-2-pyrrolidone (NMP) + hydrocarbon (n-hexane, n-octane, n-decane, cyclohexane, cyclooctane, 2-methylpentane, 3-methylpentane, isooctane, 2,2-dimethylbutane and 2,3-dimethylbutane) and N-cyclohexyl-2-pyrrolidone (NCP) + water binary mixtures at ambient pressure. The studied systems, except NCP + water, present miscibility curves with upper critical solutions temperatures (UCSTs). The results for NMP + hydrocarbon mixtures are correlated using two different approaches: (i) NRTL and UNIQUAC semiempirical models and (ii) molecular-based equations of state (statistical associating fluid theory (SAFT) and perturbed-chain statistical associating fluid theory (PC-SAFT)). Accurate correlations are inferred for NRTL and UNIQUAC models whereas for SAFT and PC-SAFT EOS complex mixing rules are required to obtain accurate results.

Introduction

N-Methyl-2-pyrrolidone (NMP) is the lactam of the 4-methylaminobutyric acid. It is a thermal and chemically stable polar compound with a very weak base character and with powerful solvent abilities [1]. Due to its unique physical and chemical properties, NMP is used for many industrial processes [2]. In the petrochemical industry, it is applied for the recovery of pure aromatics, what is of great economical importance because of the large use of these compounds in several areas, or for the production of butadiene. It is also used for the desulfuration of natural or synthetic gases. NMP is also a powerful selective solvent for the separation of polar and non-polar compounds [3]. For the plastics industry, it is used as a solvent for the production of thermoresistant polymers such as polyethersulfone or polyamides because NMP is a very good solvent for natural and synthetic plastics. NMP is also involved in processes within the electronic equipment manufacture, surface coatings, cleaning and agrochemistry. NMP is a volatile organic compound, VOC, but it has a low vapour pressure, 0.373 kPa at 330 K [4], and thus the atmospheric pollution rising from its use is clearly lower than with other VOCs; hence, it has replaced other solvents with less favourable toxicological profiles or with ozone-depleting ability. With regard to the toxicological properties of NMP, the main drawback of this molecule is its teratogenic ability but besides this, no remarkable effects have been described [5]. On the other side, NMP is non-toxic for aquatic life and can be readily biodegraded [6], thus it has a more favourable environmental profile than other VOCs.

N-Cyclohexyl-2-pyrrolidone, NCP, is a high boiling, high flash point polar aprotic solvent for many industrial applications, and in particular for different extraction processes. The mixed solvent NCP/water has been applied successfully for the extraction of aromatics from hydrocarbon streams. It is also interesting because contains a substantial apolar region as well as a peptide bond-like moiety; therefore, this solvent provides a useful model for protein interiors [7]. Moreover, it has showed no teratogenic ability and a favourable toxicological profile [8].

Among the different technological applications of NMP and NCP, liquid solvent extraction is of remarkably great importance. The overall performance of an extractive solvent is directly related with its solvency and selectivity properties, thus the accurate knowledge of liquid–liquid equilibria for selected systems is a clear technological need. Hence, our previous studies on the thermodynamic properties of mixtures containing lactams [9], [10] are extended to the liquid–liquid phase equilibria in this work. The objectives are (i) to obtain accurate phase equilibria data for selected systems and (ii) to study the performance of several models to correlate the obtained results.

In order to perform a systematic research, the selected second compound for NMP containing binary mixtures is always a hydrocarbon, but properly selected to analyze several structural features on phase equilibria. Thus, NMP + lineal, +cyclic and +ramified hydrocarbons are studied. For NCP, we report here results for the NCP + water mixture because of its technological relevance. Experimental data are fitted to a simple empirical equation to calculate critical properties [11], [12].

Data modelling is carried out according to two different approaches: (i) excess free energy models and (ii) molecular-based equations of state. Hence, non-random two liquid, NRTL, model [13], and the universal quasi-chemical, UNIQUAC, equation [14], are applied. Molecular-based equations of state, EOS, are considered because these EOS have deep molecular-level foundations, through statistical mechanics; thus, they are clearly superior to other available models. Within this approach, statistical associating fluid theory, SAFT [15], [16], and perturbed-chain statistical associating fluid theory, PC-SAFT [17], [18], are selected considering their wide use both in industry and academia, because of their reliability, accuracy and computational simplicity for complex systems like those studied in this work.

Section snippets

Experimental

All reagents were obtained from Fluka and Aldrich Chemical Companies and used without further purification, Table 1. Millipore water (Milli-Q, resistivity 18.2  cm) was used in all the experiments. The solvents were degassed with ultrasound and kept out of light over Fluka 0.3 nm molecular sieves before use. It is well known the effect of water impurities on phase equilibria, mainly in the vicinity of the critical region, thus, it was measured by Karl–Fischer titration. Purity was measured by

Results and discussion

The purity and physical properties of pure compounds are reported in Table 1, good agreement with literature values may be inferred. Table 2, Table 3 list the liquid–liquid equilibria experimental results for the investigated mixtures. All the systems show an upper critical solution temperature, UCST, except the NCP + water mixture, Fig. 1, Fig. 2, Fig. 3, Fig. 4, Fig. 5. Liquid–liquid equilibria coexistence curves with UCST show a rather flat shape in the vicinity of the critical points and are

Modelling

Liquid–liquid equilibria data are commonly described in the literature according to Gibbs energy models. Liquid immiscibility rises from the highly non-ideal character of the involved systems. As it is well known, deviations from ideal mixing behaviour can be accounted by a correction factor like the activity coefficient, γiG=i=1nxiGi+RTi=1nxiln(xi)+RTi=1nxiln(γi)where G is Gibbs energy of a non-ideal mixture, Gi is the Gibbs energy of pure components, xi is the molar fraction of

Concluding remarks

Liquid–liquid equilibria for the NMP + hydrocarbons and NCP + water binary mixtures are measured at ambient pressure. Our results are in good agreement with experimental values when available and small deviations may be justified considering the different water content of the samples. For the NMP + hydrocarbon mixtures remarkable effect rising from the size and shape of the involved hydrocarbon may be inferred. The mixture NCP + water shows a complex behaviour wit a LCST at low NCP mole fractions

Acknowledgements

The financial support by Junta de Castilla y León, Project BU020A07, and Ministerio de Educación y Ciencia, Project CTQ2005-06611/PPQ, (Spain), is gratefully acknowledged.

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