Elsevier

Fluid Phase Equilibria

Volume 388, 25 February 2015, Pages 142-150
Fluid Phase Equilibria

Separation of benzene and thiophene with a mixture of N-methyl-2-pyrrolidinone (NMP) and ionic liquid as the entrainer

https://doi.org/10.1016/j.fluid.2015.01.005Get rights and content

Highlights

  • NMP and ionic liquid (IL) mixture were proposed for the separation of benzene and thiophene by extractive distillation.

  • Isobaric VLE data were measured for benzene and thiophene system using NMP and NMP + IL mixture as entrainers.

  • UNIFAC model was extended to benzene–thiophene–NMP and benzene–thiophene–NMP–IL systems.

Abstract

In this study, a mixture of N-methyl-2-pyrrolidinone (NMP) and ionic liquid (IL) was proposed as the entrainer for the separation of benzene and thiophene by extractive distillation. [EMIM]+[BF4] might be a suitable IL for investigating the selectivity and capacity together using the COSMO-RS model. The experimental vapor–liquid equilibrium (VLE) results indicated that the addition of IL did not obviously improve the relative volatility of benzene to thiophene; however, the content of NMP in the vapor phase could be significantly reduced using the combination of NMP and [EMIM]+[BF4] in comparison to the benchmark solvent NMP. Moreover, the UNIFAC model was extended and the corresponding interaction parameters were obtained by correlating the ternary (benzene + thiophene + NMP) and quaternary (benzene + thiophene + NMP + [EMIM]+[BF4]) VLE equilibrium data obtained in this work at ambient pressure.

Introduction

Benzene is an important intermediate in the organic chemical industry. Unfortunately, thiophene is a common impurity in benzene, which causes serious impacts on the quality of benzene [1], [2]. As the quality requirements for benzene product improve, the content of thiophene allowed in the benzene product is getting lower and lower [3]. Therefore, separating thiophene from benzene is essential.

Currently, methods to remove thiophene from coking benzene include the sulphuric acid method, catalytic hydrogenation method, selective adsorption method, solvent extraction method, extractive distillation method, and azeotropic distillation method [4], [5], [6], [7], [8], [9]. Among these methods, extractive distillation is one of the most effective and is applicable for the separation of mixtures with close boiling points. In this work, extractive distillation was applied to the separation of benzene and thiophene because the relative volatility of benzene to thiophene is close to one. In extractive distillation, a solvent, called the entrainer, is added to the mixture to increase the relative volatility of the components to be separated. To ensure an economical process, selecting an effective and appropriate entrainer is a key step. At present, five types of entrainers are used in extractive distillation, i.e., liquid solvents, solid salts, mixtures of liquid solvents and solid salts, hyperbranched polymers, and ionic liquids (ILs) [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. The comparison among these entrainers has been summarized in our previous publication [23]. Traditionally, N-methyl-2-pyrrolidinone (NMP) has been used commercially as an entrainer in the extractive distillation process [24], [25], [26], but NMP is a volatile organic compound, which is inevitably entrained in the product stream. In recent years, ILs have been applied to extractive distillation processes for separating azeotropic or close-boiling mixtures [27], [28] because of the unique advantages of these ILs over classical entrainers, such as negligible vapor pressure, wide range of the liquid phase, salt effect, chemical stability and non-flammability. The non-volatile character of ILs can prevent its presence in distillate streams, and allow the easy recovery of these solvents. Aiming to enhance the benzene/thiophene separation and reduce the volatile loss of NMP, a physical mixture of organic solvent (NMP) and IL was proposed as the entrainer in extractive distillation in this work.

This work is aimed at solving the following issues: (1) screening the potential ILs with respect to selectivity and capacity using predictions from the COSMO-RS model in the COSMOthermX package (version C30_1301); (2) measuring the vapor–liquid equilibrium (VLE) data for the benzene and thiophene system using single NMP and NMP + IL mixed entrainers at 101.3 kPa; (3) extending the applicability of the UNIFAC model based on the experimental data; and (4) comparing the volatility of single NMP with that of the mixed entrainers.

Section snippets

Materials

Benzene, thiophene, NMP and IL were purchased from the chemical market as summarized in Table 1. Prior to the experiment, the IL was pretreated to remove traces of water and volatile impurities using a vacuum rotary evaporator at 333.2 K for 12 h. The water content was kept at less than 400 ppm after drying. Other chemicals were used directly.

Apparatus and procedure

The VLE data for the ternary system of benzene + thiophene + NMP and the quaternary system of benzene + thiophene + NMP + IL were measured at atmospheric pressure

Extension of the UNIFAC parameter matrix

The UNIFAC model was adopted to describe the thermodynamic equilibrium of the benzene and thiophene system. In this work, the IL was decomposed into several functional groups in a similar way as proposed by Lei et al. [31], [32], [33], [34], Kim et al. [35], [36], Breure et al. [37]. Meanwhile, the benzene molecule was decomposed as six ACH groups, whereas thiophene and NMP were each treated as one separate group.

The activity coefficient as a function of temperature and composition can be

Screening of ILs for benzene and thiophene separation

The selectivity (or separation factor) is used to directly evaluate the various entrainers, which is defined asS12=γ1γ2=y1/x1y2/x2P2sP1swhere γ1 and γ2 are the activity coefficients at finite concentrations of the key components to be separated, respectively; xi and yi represent the mole fractions of component i in the liquid and vapor phases, respectively; and Pis represents the vapor pressure of pure component i and can be obtained by the Antoine equation [43], [44], and these parameters are

Conclusions

In this work, a mixture of organic solvent (NMP) and IL was proposed for the separation of benzene and thiophene by extractive distillation. It was found that [EMIM]+[BF4] might be a suitable IL for investigating the selectivity and capacity together using the COSMO-RS model. The VLE data of benzene and thiophene at 101.3 kPa were measured using only NMP and a mixture of NMP + [EMIM]+[BF4] as the entrainers. On this basis, the familiar UNIFAC model was extended to the benzene–thiophene–NMP and

Acknowledgements

This work was financially supported by the National Nature Science Foundation of ChinaNos. 21476009, 21406007 and U1462104, and by the Chinese Universities Scientific Fund (ZY1401).

References (52)

  • J. Liao et al.

    Fuel Process. Technol.

    (2014)
  • Y. Zeng et al.

    Sep. Purif. Technol.

    (2012)
  • Z. Lei et al.

    Fluid Phase Equilib.

    (2002)
  • Z. Lei et al.

    Chem. Eng. J.

    (2002)
  • Z. Lei et al.

    Chem. Eng. J.

    (2002)
  • Z. Lei et al.

    Comput. Chem. Eng.

    (2002)
  • A. Marciniak

    Fluid Phase Equilib.

    (2010)
  • A. Pereiro et al.

    J. Chem. Thermodyn.

    (2012)
  • Y. Kim et al.

    Fluid Phase Equilib.

    (2005)
  • J.E. Kim et al.

    Fluid Phase Equilib.

    (2011)
  • B. Sander et al.

    Fluid Phase Equilib.

    (1983)
  • M. Kleiber

    Fluid Phase Equilib.

    (1995)
  • J. Palgunadi et al.

    Thermochim. Acta

    (2009)
  • U. Domańska et al.

    J. Chem. Thermodyn.

    (2009)
  • S.A. Kozlova et al.

    J. Chem. Thermodyn.

    (2009)
  • P. Yan et al.

    Fluid Phase Equilib.

    (2010)
  • E. Olivier et al.

    J. Chem. Thermodyn.

    (2010)
  • T.M. Letcher et al.

    J. Chem. Thermodyn.

    (2005)
  • R. Kato et al.

    J. Chem. Thermodyn.

    (2005)
  • A. Takahashi et al.

    Ind. Eng. Chem. Res.

    (2002)
  • P. Sui et al.

    Sci. Adv. Mater.

    (2013)
  • Z. Kang et al.

    Ind. Eng. Chem. Res.

    (2009)
  • J. Liao et al.

    Mod. Chem. Ind.

    (2009)
  • J. Weitkamp et al.

    J. Chem. Soc.

    (1991)
  • J. Chang et al.

    J. Fuel Chem. Technol.

    (2013)
  • J. Liao et al.

    Sep. Sci. Technol.

    (2012)
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