Novel liquid–liquid equilibrium data for six ternary systems containing IL, hydrocarbon and thiophene at 25 °C
Introduction
Due to the detrimental effects of fuel's sulfur compounds on environment as well as human health, tremendous effort has been focused to produce clean fuels, i.e. ultra-deep desulfurized diesel and gasoline. During the last decade, a plethora of research has been developed for this purpose to comply with environmental regulations [1], [2], [3], [4], [5].
The conventional hydrodesulfurization (HDS) process has been highly effective for reduction of sulfur levels of thiol, sulfide and disulfide compounds. The reaction needs high temperatures of 350 °C and high pressure of around 30 bar. This means a high-energy consumption and high cost for industrial equipment [2]. Moreover, aromatic sulfur compounds like thiophene, benzothiophene (BT), dibenzothiophene (DBT), and their related derivatives have great resistance to HDS. In this regard their hydrogenation requires higher energy and hydrogen consumption while the reactivity and selectivity of the catalysts should be improved [3]. Therefore the excessive hydrogenation is usually inevitable in order to reduce the sulfur content to a much lower level (<10 ppm), leading to a decrease of octane number [4].
Liquid – liquid extraction has been widely used in industrial processes such as aromatic separation and purification because of their operational simplicity and mild extraction conditions (low pressure and temperature) [5]. Since the application of usual organic solvents such as sulfolane causes significant environmental contaminations, during recent years the ILs has been introduced as useful substitute for liquid–liquid extraction of sulfur compounds [6], [7]. ILs are considered as substances less harmful to the environment. An ideal IL for the separation of thiophene requires high solubility of sulfur compound and low solubility of an alkane in the IL [8].
The high interest in ILs is due to the many possible combinations of cations and anions, leading to fine-tuning of their characteristics like density, viscosity, melting point, hydrophobicity and solubility power which is more important [2]. Wilfred and colleagues [3] have found that ILs with higher specific volume showed higher desulfurization efficiency, it has been explained as lower densities have more free space between ions for accommodating DBT molecules, moreover lower density ILs may have a higher chance for interaction between the cations and electron rich sulfur compounds. This leads to an easy prediction of the desulfurization performance considering their densities. In other words, the physical properties of an IL has a very important role in prediction of capacity for a particular application [9].
ILs like 1-octyl-3-methylimidazolium bis[trifluoromethylsulfonyl]-imide, [Omim][NTf2] and 1-octyl-3-methylimidazolium tetrafluoroborate [Omim][BF4] have presented encouraging results in extraction of thiophene from aliphatic hydrocarbons with encouraging results [10], [11]. They showed that when chain length of the alkane increases, the distribution coefficient decreases while the selectivity increases.
Usually utilizing equilibrium data, solute distribution ratio (related to the amount of solvent needed for performing extraction) and selectivity (related to the separation power of the solvent to achieve the aimed separation) are used for performance evaluation of solvents [12].
Nie et al. [13] investigated the effect of alkyl group length of alkylphosphate ionic liquids and found that longer alkyl substitute shows higher solute distribution coefficient. Their results showed that for one specific IL sulfur removal selectivity is dependent on the molecular structure of the S-compounds with the following order: DBT > BT > thiophene > 3-methylthiophene, and the capacity of the IL for removal of aromatic S-compounds depends on the structure and size of anions and cations in ILs [13].
The pyridinium based IL ([hmmpy][NTf2]) was investigated by Francisco et al. [14]. They studied solute distribution and selectivity for systems containing thiophene and aliphatic compounds. The results showed that [hmmpy][NTf2] has the best performance at low thiophene concentrations but large amount of solvent is needed for desulfurization. Moreover high alkane solubility was obtained which causes loss of fuel and consequently higher recycling costs.
Mokhtarani et al. [15] showed that [Bmim][NO3]] is a better solvent than [Omim][NO3] for desulfurization in studied systems although more solvent is required. Domanska et al. [8] investigated the effects of anions such as tetrafluoroborate [BF4], [NTf2], [FAP], methylsulphonate [MeSO3] and ethylsulphate [EtSO4], [N(CN)2], [SCN] and [MP] on selectivity. They found that just [SCN], [N(CN)2] and [MP] shows significant effect on selectivity. In other work, Domanska and wlazlo [16] demonstrated that ILs such as 1-butyl-4-methylpyridinium-dicyanamide[bmPY][DCA], 1-butyl-1-methylpyrrolidinium- dicyanamide[bmPYR][[DCA] and N-butyl-N-methylmorpholinium tricyanomethanide[bmMOR][TCM] indicate high selectivities (>200) and high solute distribution ratio (>1.2) for desulfurization process.
Considering mentioned researches we think imidazolium-based ILs with the thiocyanate anion [SCN] will create satisfactory results. In this regard to extract thiophene from aliphatic hydrocarbons (n-hexane, n-octane and n-decane), 1-Octyl-3-methylimidazolium and 1-Hexyl-3-methylimidazolium based ILs, with thiocyanate [SCN] as anion were studied. The thiocyanate ILs were chosen in the current study as convenient type of ILs due to their halogen-free nature along with lower density and viscosity.
In present work, liquid–liquid equilibria (LLE) data for the ternary systems of [Hmim][SCN] + alkanes + thiophene and [Omim][SCN] + alkanes + thiophene are investigated. The removal of thiophene are studied with different types of n-alkane as a fuel model and the chain length of the cation of IL on extraction of thiophene from alkane are also measured. Finally, nonrandom two-liquid (NRTL) model was employed for correlation of LLE data.
Section snippets
Chemicals and materials
[Hmim][SCN] and [Omim][SCN] were synthesized by Chemistry and Chemical Engineering Research Center of Iran (CCERCI) laboratory. These ILs were prepared using [Hmim][Cl] and [Omim][Cl] based on the procedures reported in the literature. Purified ILs were characterized by 1H NMR spectroscopy, and the results were consistent with literature values [17], [18], [19], [20]. The storage of ILs was under vacuum for 24 h at 343.15 K and stored in a bottle under argon gas. The water content of the
Results and discussion
The experimental data for six ternary systems containing [Omim][SCN](1)+ n-hexane(2)+ thiophene(3), [Omim][SCN](1)+ n-octane(2)+ thiophene(3), [Omim][SCN](1)+ n-decane(2)+ thiophene(3), [Hmim][SCN](1)+ n-hexane(2)+ thiophene(3), [Hmim][SCN](1)+ n-octane(2)+ thiophene(3) and [Hmim][SCN](1)+ n-decane(2)+ thiophene(3) were determined at 25 °C under ambient pressure (Table 3, Table 4). The Results show that for all systems when thiophene concentration increases, the selectivity and distribution
Thermodynamic modeling
The NRTL model was employed to correlate the experimental data. In the NRTL model [25] the activity coefficients are defined as:x stands for mole fraction, R is the gas constant, T refers to absolute temperature and α refers to non-randomness parameter, set a 0.3. Energy parameters were adjusted through minimizing the following objective function:Where m is the number of tie
Conclusion
Extraction of thiophene from alkane (n-hexane, n-octane and n-decane) was performed using [Hmim][SCN] and [Omim][SCN] as the solvent at 298.15 K and ambient pressure. The selectivity and the solute distribution coefficient values were calculated using experimental data. It is found that selectivity increase as the alkane chain length increases, while alkane solubility in the ILs and the thiophene distribution coefficients decrease. Our results showed that cation chain length has a remarkable
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