Elsevier

Fluid Phase Equilibria

Volume 456, 25 January 2018, Pages 109-115
Fluid Phase Equilibria

Liquid-liquid equilibrium data for extractive desulfurization using 1-butyl-3-methyl imidazolium thiocyanate, n-alkane and thiophene

https://doi.org/10.1016/j.fluid.2017.10.017Get rights and content

Abstract

The experimental liquid – liquid equilibrium (LLE) data for three ternary mixtures containing an ionic liquid (IL) (1-butyl-3-methyl imidazolium thiocyanate [Bmim][SCN]), alkanes (n-hexane, n-octane and n-decane) and sulfur compound (thiophene) have been measured experimentally. Distribution and selectivity coefficients have been calculated using experimental data. The LLE data have been obtained at 25 °C and ambient pressure. The experimental LLE data of ternary mixtures have been compared with our previous works. It has been shown that the IL with higher molecular weight of cation has higher solute distribution and lower selectivity coefficients. It has been found that the solubility decreases as the cation length decreases, meanwhile ILs cation chain length has a remarkable effect on solubility of alkanes. Moreover, the experimental data have been demonstrated that the selectivity decreases as alkane chain length of alkane decreases while the solubility of alkanes in the ILs increases, hence the removal costs of aromatic sulfur components will be increased. The results show that [Bmim][SCN] has the lowest alkane solubility and highest selectivity coefficient, in the system containing n-decane and thiophene. A comparative study reveals that compared to other ILs, [Bmim][SCN] is a more suitable solvent. Finally, the experimental data have been correlated utilizing non-randomness two liquid (NRTL) model. The binary energy parameters were obtained, it can be seen that the model can correlate the experimental data efficiently.

Introduction

Aromatic sulfur compounds such as thiophene and thiophene derivative compounds (benzothiophene, dibenzothiophene and their derivatives) are the major challenge in deep desulfurization processes. Hydro desulfurization process (HDS) as a well-known and most efficient sulfur removal process cannot remove aromatic sulfur compounds at normal operating conditions. In this regard usually severe conditions such as high pressure, temperature are required for deep desulfurization. The high level of hydrogen consumption at high pressure and temperature along with using improved, high reactive and high selective catalysts make HDS an expensive and hazardous process [1], [2]. In this regard, various new methods such as extractive, oxidative, adsorptive, bio and extractive oxidative desulfurization [3] have been developed to remove sulfur compounds. Among various methods extractive desulfurization using ionic liquids (ILs) as solvent received more attentions [4], [5], [6], [7], [8], [9]. ILs have some unique properties which are created by the combination of certain anions and cations [1]. The flexibility in using specific anion and cation in IL structure brings legendary features for them as solvent compared to the other conventional organic solvents. ILs are non-volatile solvents with high thermal stability, these features introduce them as environmental friendly solvent. Desulfurization using ILs is a simple process which can be run at low temperature and pressure [8], [9]. A suitable IL for desulfurization process must be highly selective to aromatic sulfur compounds while fuels have low solubility in it [10]. Previous works show that methylpyridinium and pyridinium based ILs are stronger solvents and have more distribution coefficients in extractive desulfurization. According to mentioned studies, the distribution coefficients of ILs varies in the following order: dimethylpyridinium > methylpyridinium > pyridinium ≈ imidazolium ≈ pyrrolidium, while the imidazolium based ILs are more selective compared to pyridinium based [11], [12], [13]. Besides selectivity, distribution coefficients and solubility concerns, viscosity and density are important as well. It is obvious that ILs with lower viscosity and density are more attractive from operational cost point of view beside that they show higher sulfur removal in extractive desulfurization process [2]. In many researches the effect of anion, cation and cation chain length on desulfurization process were studied [5], [6], [10], [14], [15], [16]. Asumana et al. studied four low-viscosity ILs based on the dicyanamide anion, they showed that the imidazolium ILs are more efficient than others. The result indicated that as the cation length of the imidazolium ILs decreases the distribution coefficient increases [17]. In another work, Mokhtarani et al. studied on cation chain length and its effect on selectivity, they showed that [Bmim][NO3] compared to [Omim][NO3] with higher cation length is more selective in sulfur removal process although more solvent is required [18]. Domanska et al. studied the effects of anion type on selectivity, they evaluated different anions such as [BF4], [EtSO4], [FAP], [MeSO3], [MP], [N(CN)2], [NTf2] and [SCN] and found that those ILs which have [MP], [N(CN)2] and [SCN] anions are more sulfur selective [10]. Wilfred et al. studied 18 types of ILs based on imidazolium as cation and different anions. Among all of them, ILs with [C8H17SO4], [N(CN)2] and [SCN] as anions showed the highest extraction ability [2]. These studies revealed that [SCN] has a good potential as anion.

The present research is in the line of our previous works on sulfur removal of alkanes [6], [7]. In this work one IL has been adopted based on the physical and chemical properties. In our previous work selectivity of [Hmim][SCN] and [Omim][SCN] were studied [6], the results showed that as the cation length decreases the selectivity toward organic sulfur compounds increase while solubility of alkane decreases. In this regard, we decided to focus on equilibrium behavior of an IL with shorter length of the cation. In this work 1-butyl-3- methyl imidazolium thiocyanate [Bmim][SCN] is used for extraction of thiophene from normal alkanes. Liquid-Liquid equilibrium data of ternary mixtures are measured experimentally. Finally, the measured experimental data are correlated using the NRTL model.

Section snippets

Chemicals and materials

[Bmim][SCN] has been synthesized in Chemistry and Chemical Engineering Research Center of Iran (CCERCI). This IL has been prepared from [Bmim][Cl] according to the procedure reported in the literature [19], [20]. The structure of the synthesized IL has been checked using the spectroscopy of nuclear magnetic resonance (NMR) and was consistent with literature value [21], [22]. The purified IL has been dried and for 24 h at 70 °C under vacuum, then was stored under argon gas in a sealed bottle.

Results and discussion

The experimental data for three ternary systems of [Bmim][SCN](1)+ n-hexane(2)+ thiophene(3), [Bmim][SCN](1)+ n-octane(2)+ thiophene(3) and [Bmim][SCN](1)+ n-decane(2)+ thiophene(3) were obtained at 25 °C under ambient pressure (Table 3). The selectivity and the distribution coefficient values have been calculated using measured experimental data. This table shows that for all data when concentration of thiophene decreases, the selectivity coefficient and solute distribution values increases;

Modeling

Thermodynamic modeling of the experimental data is very important from process engineering point of view as the model parameters can be used in thermodynamic package of engineering softwares. There are two approaches for modeling of systems containing ILs, using EOS and excess Gibbs functions. Usually excess Gibbs function are more applicable and popular in LLE, however during recent years considerable efforts have been paid to the EOSs as consistence tools for thermodynamic modeling [25]. In

Conclusion

This research in fact is the continuation of our previous work [6] for desulfurization of cyano-based IL. Our new experimental data indicated more improvement on selectivity and fuel solubility parameters compared to our previous study. Liquid-liquid equilibrium data for extractive desulfurization process were obtained. Distribution of thiophene between alkanes (n-hexane, n-octane and n-decane) and [Bmim][SCN] was obtained at 25 °C and ambient pressure. The selectivity and the distribution

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