Elsevier

Fluid Phase Equilibria

Volume 363, 15 February 2014, Pages 248-262
Fluid Phase Equilibria

Prediction and measurement of phase equilibria for the extraction of BTX from naphtha reformate using BMIMPF6 ionic liquid

https://doi.org/10.1016/j.fluid.2013.11.034Get rights and content

Abstract

New Liquid–Liquid Equilibrium (LLE) data for three ternary systems namely; hexane + benzene + BMIMPF6, heptane + toluene + BMIMPF6, octane + o-xylene + BMIMPF6, and a fourth system consisting of synthetic naphtha reformate which is composed of hexane/heptane/octane + benzene/toluene/o-xylene + BMIMPF6 were obtained. The LLE were determined at T = 298.15 K and atmospheric pressure. Effect of the number of substituted methyls attached to aromatic rings was evaluated in terms of selectivity and extractive capacity of the ionic liquid BMIMPF6. The results showed a decrease of the extracted amount of aromatics by the ionic liquid as the increase of the number attached methyl groups in the corresponding aromatic systems. Thus, the extraction capability of aromatics by BMIMPF6 was in the order of benzene > toluene > o-xylene, either present in individual ternary systems or as mixture in one pseudo ternary system. Five thermodynamics models were used to correlate satisfactorily the LLE data for the four studied ternary systems. These are T-K-Wilson, Dortmund UNIFAC, ASOG, NRTL, and UINQUAC. Different strategies and modifications are made to the group contribution models UNIFAC and ASOG. These modifications showed an improved prediction compared to their original expressions. However, among all models considered in this study, the UINQUAC model gave the best predictions.

Introduction

Aromatics are a major sector within the petrochemical industry. The key products are Benzene, Toluene, and Xylene which are known as BTX. These aromatics are the raw material of the most important petrochemical intermediates which are used in the production of polymers, solvents, resins, paints, polishes, polyesters, plastics, rubber, fiber, pharmaceutical and food processing agents. Currently petroleum refining is the main source for BTX production. Reformate from catalytic reforming of naphtha is rich in aromatics as well as aliphatic hydrocarbons. The separation of aromatics BTX from aliphatic hydrocarbon mixtures is challenging since these hydrocarbons have boiling points in a close range and several combinations form azeotropes [1]. The conventional processes for the separation of these aromatic and aliphatic hydrocarbons mixtures are liquid–liquid extraction, extractive distillation and azeotropic distillation, however; liquid–liquid extraction is more applicable than the other two [2]. Many organic solvents such as sulfolane, dimethylsulfoxide, N-methylpyrrolidone, N-formylmorpholine, methyl carbonate were used to extract aromatics from multicomponent hydrocarbon mixture [3]. The liquid–liquid extraction by sulfolane showed high recovery of aromatic compounds along with a good balance of solvent properties. These solvents are highly effective and offer high yields, however, the disadvantage of these solvents is due to their toxic and pollutant effects. Additionally, at lower aromatics content (<20 wt.%), the extraction process by sulfolane is less efficient as it requires further stages to purify the extract and raffinate phases [4]. Therefore, the improvement of aromatic separation by liquid–liquid extraction has meant the necessity to search for alternative solvents, which allow an effective separation and cause less damage to the environment instead of the existing ones.

Ionic liquids (ILs) have emerged as novel alternatives to traditional solvents because of their nonvolatile nature, low melting point, wide liquid range (over 300 °C), extremely low vapor pressure, good thermal stability, non-flammable, high density (usually >1.0 g/mL); and more importantly, they are considered as green solvents, and hence, environmentally safe [5], [6]. Furthermore, extraction by ILs is favorable because of the low operating temperature (room temperature), rapid phase separation, and an easy IL regeneration for reuse compared to the conventional organic solvents. Consequently, researchers around the world directed their attention to ILs based solvents which resulted in a considerable number of publications reporting Liquid–Liquid equilibrium (LLE) data for systems composing of an ionic liquid, aromatic and an aliphatic hydrocarbon [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. Among all the ILs investigated so far, only few posses both a higher selectivity and a higher extractive capacity than any conventional solvent, such as sulfolane [4], [17]. In addition, some of the ILs are greatly affected by water and other impurities resulting in a negative modification of their physical properties [18]. Up to our knowledge, all the reported studies in the literature consider only single aromatic compound or single aliphatic compound regarding aromatic extraction by ILs. However, the effect of the presence of more than one aromatic compound and more than one aliphatic compound on the overall extraction is needed to be addressed.

The knowledge of LLE data for the ternary systems (aliphatic hydrocarbons + aromatic hydrocarbons + ILs) is essential to evaluate the feasibility of the ILs as extractive solvents. In this paper, we are concerned about the application of IL for the extraction of BTX aromatic compounds from their corresponding linear alkanes mixtures. The ionic liquid used in this work was synthesized in our lab, which is constituted by the combination of 1-butyl-3-methyl imidazolium cation and hexafluorophasphate anion [PF6].

Three ternary systems and one mixed system containing all three systems were studied and they are: system (1); hexane + benzene + BMIMPF6, system (2); heptane + toluene + BMIMPF6, system (3); octane + o-xylene + BMIMPF6, and system (4) is a mixture of all compounds; hexane/heptane/octane + benzene/toluene/o-xylene + BMIMPF6, which is a representation of naphtha reformate.

The first aim of this work is to experimentally investigate the separating capability of the IL solvent to extract each aromatic compound and also to analyze its performance toward each aromatic compound in the presence of the other aromatic and aliphatic compounds. Hence new LLE data for the four systems at 298 K and atmospheric pressure were experimentally studied together with the calculation of the selectivity and extractive capacity of the IL. The second aim of this work is to model these systems with different kind of thermodynamics models to observe their performance in predicting the experimental LLE data. Five models were considered in this study which range from simple thermodynamic models to more sophisticated ones. These are T-K-Wilson [19], Dortmund UNIFAC [20], ASOG [21], NRTL [22], and UINQUAC [23]. These models were used to predict the tie-line compositions of the ternary systems as well as the selectivity and capacity of IL.

Section snippets

Chemicals

List of the used chemicals in this study and their specifications are summarized in Table 1.

Physiochemical properties

DensoMeter DDM 2911 was used to carry out IL density measurements. The apparatus was calibrated with air and water at ambient temperature. Infrared spectra (IR in acetonitrile) were recorded on Nicolet iS10 (FT-IR) spectrometer in transmittance mode for IL. 1H and 13C NMR spectra were carried out in CDCl3 and CD3OD on NMR Bruker DPX 400 MHz spectrometer, Switzerland. Refractometer Koehler K27550 was used

Synthesis and structure characterization of the BMIMPF6

Synthesis of the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate, BMIMPF6, was reported in diverse literatures using different synthetic conditions [26], [27], [28], [29], [30], [31]. In the present investigation, we have synthesized the ionic liquid BMIMPF6 by refluxing equimolar amounts of 1-methyl-imidazole and 1-chlorobutane in acetonitrile under dry conditions. After working up and purification, structure of the resulting 1-butyl-3-methyl-imidazolium chloride was confirmed by

Liquid–liquid equilibria models:

To design, simulate, or upgrade extraction equipments, it is essential to have a thermodynamic model that describes the Liquid–Liquid Equilibria (LLE) data. In LLE, the compositions of the two phases can be calculated using the activity coefficient approach as:γiExiE=γiRxiRwhere xiE and xiR are the mole fraction of component i in the extract phase and the raffinate phase, respectively. γiE and γiR are their corresponding activity coefficients. Therefore, representation of activity coefficients

Conclusion

New Liquid–Liquid Equilibrium (LLE) data for the extraction of BTX aromatics from synthetic naphtha reformate by 1-butyl-3-methyl imidazolium hexafluorophasphate (BMIMPF6) ionic liquid were obtained at 298 °K and atmospheric pressure. Four systems were considered in this study and they are: hexane + benzene + BMIMPF6, heptane + toluene + BMIMPF6, octane + o-xylene + BMIMPF6, and hexane/heptane/octane + benzene/toluene/o-xylene + BMIMPF6. The later system is a mixture of all previous systems which is a

Acknowledgments

This work was supported by Kuwait University, Research Grant No. EC 01/12. The authors express their gratitude to the General Facility lab GE03/08 in the College of Engineering & Petroleum – Kuwait University for sample analysis. We also would like to thank the General Facility lab GS01/01 in the college of Science – Kuwait University for NMR measurements.

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