Extraction desulfurization process of fuels with ionic liquids

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Highlights

  • Extraction of thiophene and benzothiophene from heptane.

  • The ternary (liquid + liquid) equilibria using ILs.

  • High selectivity and solute distribution ratio for the extraction of sulfur compounds form alkanes.

  • [BMPYR][TCM] was proposed as entrainer for the separation process.

Abstract

In this work, we studied the applicability of three ionic liquids (ILs) in the extraction of thiophene, or benzothiophene from heptane at T = 308.15 K and ambient pressure. Experimental data for (liquid + liquid) phase equilibrium (LLE) were obtained for five ternary systems. The 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate, [BMPYR][CF3SO3], 1-butyl-1-methylpyrrolidinium tricyanomethanide, [BMPYR][TCM] and 1-hexyl-3-methylimidazolium tetracyanoborate, [HMIM][TCB] were used for the desulfurization process. The [BMPYR][CF3SO3] showed much better selectivity than [HMIM][TCB] in the extraction of thiophene from heptane and all of them showed excellent results in terms of benzothiophene selectivity and distribution ratio compared to what is currently published for different ILs. Chromatography analysis showed that IL was not present in the heptane layer. This eliminates the process of the separation of the solvent from the raffinate layer. The data obtained have been correlated with the non-random two liquid NRTL model. The experimental tie-lines and the phase composition in mole fraction in the ternary systems were calculated with an average root mean square deviation (RMSD) of 0.005.

Introduction

Some petroleum processes, such as naphtha steam cracking, need the removal of low level sulfur compounds such as thiophene, benzothiophene, methyldibenzothiophenes, 4,6-dibenzothiophenethiols, thioethers, and disulfides. From industrial point of view and according to strict regulations regarding sulfur content in diesel fuel in the USA and Europe [1], [2], the content of total sulfur in European gasoline and diesel fuels must be at a maximum concentration level of 10 · 10−6 [2]. Thus, the emission of sulfur from petrol and diesel oils, which is linked to acid rain phenomena, plays a crucial role in pollution problems of large conglomerates. Commercially, the hydrodesulfurization (HDS) process is the established method used in some industrial technologies to remove organic sulfur compounds from fuels. However, to achieve low sulfur targets with current HDS technology, higher temperature, higher pressure, larger reactor volumes, and more active catalysts are needed [3]. Therefore, (liquid + liquid) extraction (LLE) is proposed for deep desulfurization, because of mild operation conditions and due to its simplicity. Extraction desulfurization with ionic liquids (ILs) has the potential for alternative and future complementary technology for deep desulfurization [4], [5], [6], [7]. These new organic solvents are generally not volatile and non-flammable and revealed large intermolecular interaction mainly via hydrogen bonding. Ionic liquids (ILs) are already known as very good entrainers in many processes. The application of some ILs as LLE solvents for petrol desulfurization has already been reported in the literature [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16].

The 1-alkylpyrrolidinium-based ILs with different anions [13] have been recently studied in our laboratory in ternary LLE (IL + thiophene + heptane) at T = 298.15 K. The highest selectivity (Smax = 133.4) with high solute distribution ratio (β = 3.47) was found for 1-butyl-1-methylpyrrolidinium tricyanomethanide [BMPYR][TCM] [13].

Attractive extraction parameters were presented as well for 1-ethyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}imide, [EMIM][NTf2], [9], 1-ethyl-3-methylimidazolium acetate, [EMIM][OAc] [10] and 1-butyl-1-methylpyrrolidinium tetracyanoborate [BMPYR][TCB] [13]. The very good entrainers for the extraction of sulfur compounds from model fuels were ILs with tricyanomethanide anion (i.e. 1-butyl-3-methylimidazolium tricyanomethanide, [BMIM][TCM], or 1-ethyl-3-methylimidazolium tricyanomethanide, [EMIM][TCM] [15]), and pyridynium-based ILs with dicyanamide anion [N(CN)2] and thiocyanate anion [SCN] [7]. The most important are the results of measurements made on model fuel with different ILs, which confirm the tricyanomethanide, dicyanamide and thiocyanate anions as a very effective [7].

The analysis made by us few years ago for activity coefficients at infinite dilution for the selectivity for separation of thiophene from hexane showed that 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate, [BMPYR][CF3SO3] IL revealed high selectivity at infinite dilution [17].

The imidazolim-based ILs with bis{(trifluoromethyl)sylfonyl}imide were widely tested in that particular extraction process [6], [7], [8], [9], [10]. To our best knowledge, the 1-hexyl-3-methylimidazolium tetracyanoborate, [HMIM][TCB] IL was not measured in ternary LLE.

The current work represents the continuation of our systematic study on desulfurization of model fuels. We report new experimental ternary LLE data for three ILs, which we expected to show high selectivity for the extraction of sulfur compounds: 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate, [BMPYR][CF3SO3], 1-butyl-1-methylpyrrolidinium tricyanomethanide, [BMPYR][TCM] and 1-hexyl-3-methylimidazolium tetracyanoborate, [HMIM][TCB].

The ternary systems {IL (1) + thiophene, or benzothiophene (2) + heptane (3)} were measured at T = 308.15 K and ambient pressure. The [BMPYR][TCM] was measured only in one ternary mixture {IL (1) + benzothiophene (2) + heptane (3)} because ternary mixture with thiophene was published by us earlier [13]. The experimental tie lines for the five new ternary mixtures have been correlated with NRTL equation [18], [19]. From the experimental data the extraction selectivity and the solute distribution ratio were determined and discussed.

Section snippets

Experimental

The ILs studied: [BMPYR][CF3SO3], [BMPYR][TCM] and [HMIM][TCB] were purchased from IoLiTec (the structure, name, abbreviation and molecular mass are presented in table 1) and all solvents used from Merck or Sigma Aldrich (the origins of the chemicals, CAS numbers, purity, water content and densities are listed in table 1S in the Supporting Material). The samples of ILs were dried for 24 h at T = 300 K under reduced pressure to remove volatile impurities and trace amounts of water. Thiophene and

Results and discussion

Table 3 shows the equilibrium compositions of the experimental tie-line ends in five ternary systems {IL (1) + thiophene, or benzothiophene (2) + heptane (3)} at T = 308.15 K and ambient pressure. Analysis of samples with gas chromatography showed that the IL which is the entrainer has not been found in the raffinate (heptane) layer after the LLE experiment. This achievement is very important from an industrial point of view. The “solvent lost” should not be expected. In the binary {IL (1) + heptane

Data correlation

The ternary LLE data measured in this study were correlated (the tie-line correlation) using the well-known non-random liquid equation, NRTL [18]. The equations and algorithms used for the calculation of the compositions in both phases followed the method described by Walas [19]. The objective function F(P), was used to minimise the difference between the experimental and calculated compositions:F(P)=i=1nx2Iexp-x2Icalc(PT)2+x3Iexp-x3Icalc(PT)2+x2IIexp-x2IIcalc(PT)2+x3IIexp-x3IIcalc(PT)2,where P

Conclusions

The (liquid + liquid) phase equilibrium data were measured in this study for the extraction of thiophene, or benzothiophene from heptane using three ILs at temperature T = 308.15 K at ambient pressure using a GC for the composition analysis. As a conclusion from all the data presented in this work, it is clear that all ILs showed superior results compared to many findings published earlier. It has been demonstrated that [BMPYR][TCM] achieved the best separation efficiency. Thus [BMPYR][TCM] can be

Acknowledgement

This work has been supported by the project of National Science Center in Poland 011/01/B/ST5/00800.

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