Solvent extraction of thiophene from n-alkanes (C7, C12, and C16) using the ionic liquid [C8mim][BF4]

doi:10.1016/j.jct.2008.01.025

The Journal of Chemical Thermodynamics

Volume 40, Issue 6, June 2008, Pages 966-972

https://doi.org/10.1016/j.jct.2008.01.025 Get rights and content

Abstract

In the last years, new strict environmental regulations to reduce sulfur content in liquid fuels have been established. Thiophene derivates can be considered as the key substances to be separated from liquid fuel oils. This paper reports the ability of the ionic liquid 1-methyl-3-octylimidazolium tetrafluoroborate to act as solvent in the (liquid + liquid) extraction of thiophene from aliphatic hydrocarbons. Tie-line data have been determined for ternary systems containing the ionic liquid, thiophene, and some n-alkanes at T = 298.15 K. Extraction process has been analyzed by means of thiophene distribution ratio and selectivity. The solute distribution coefficient decreases and the selectivity increases as the chain length of n-alkane increases. The use of 1-methyl-3-octylimidazolium tetrafluoroborate as potential solvent for separation of thiophene from n-alkanes is feasible using the necessary quantity of solvent. A correlation of the equilibrium data reported here has also been made, using the NRTL activity coefficient model, in order to facilitate their use in simulation and design processes.

Introduction

Over the past few years, new strict environmental regulations are being imposed on oil refineries to reduce sulfur level for liquid fuels. The S-limit will be reduced in USA to a maximum of 15 ppm by 2010 and to 10 ppm in Europe by 2009. To date, desulfurization is obtained by heterogeneous catalyzed hydrodesulfurization (HDS). Refineries with hydrotreaters are likely to achieve production of ultra low sulfur products by modifying catalysts and operating conditions. Variations in feedstock sulfur content and the amount of cracked stock have a big influence in the process and operation conditions. Nonetheless, a two-stage deep desulfurization process will be required by most, if not all refiners, to achieve legislative requirements. The second stage requires substantial modification of the desulfurization process, drastic higher pressure and increase of hydrogen rate, or retrofitted processes with new units [1]. That leads to an expensive process in terms of high operating cost and significant investment of capital.

New technologies are under development that could reduce the cost of desulfurization. Among them [2]: extraction, precipitation, oxidative desulfurization, selective adsorption, biodesulfurization, using bacteria as catalyst, etc. All these processes, considered individually or in combination with HDS, may be new economically viable solutions for desulfurization.

Extractive desulfurization is based on the fact that sulfur-containing compounds are more soluble than hydrocarbons in an appropriate solvent. In this way, remarkable solvation ability and negligible vapour pressure of ionic liquids (ILs) make them potential solvents of great interest. Some works can be found on bibliography which analyze the use of these compounds as extraction agents to desulfurizate [3], [4], [5], [6], [7], [8], [9] or combining oxidation and extraction [10], [11], [12], [13]. The engineering aspects of RTIL desulphurization have received particular attention from Andreas Jess et al. [14], [15], [16], [17], [18] who regard extraction with RTILs as best deployed as a final desulphurization stage following conventional catalytic hydrodesulfurization. Promising results encourage further research on this subject.

Due to compounds with aromatic rings make sulfur removal more difficult, thiophene and thiophene-derivates can be considered as the key substances that need to be separate from liquid fuel-oils. The objective of this work is to analyze the viability of using the IL 1-methyl-3-octylimidazolium tetrafluoroborate ([C₈mim][BF₄]) as solvent in the extraction of thiophene from its mixtures with aliphatic hydrocarbons, and to study the effect of the n-alkane chain length in this separation. For this reason, liquid liquid equilibrium (LLE) data for the ternary systems [C₈mim][BF₄] + thiophene + n-alkane (C₇, C₁₂, and C₁₆) were determined at T = 298.15 K. Solute distribution ratio (β) and selectivity (S) values were also determined from the tie-line data, to establish the feasibility of using [C₈mim][BF₄] for the separation of (thiophene + alkane) binary mixtures. The effect of n-alkane chain length upon these parameters was analyzed. In order to facilitate their use in simulation and design processes, reported phase equilibrium data were correlated using the NRTL activity coefficient model [19].

Section snippets

Chemicals

n-Heptane was supplied by Fluka with a quoted purity >0.995 mass fraction. The thiophene, n-dodecane, and n-hexadecane were obtained from the Aldrich Chemical Company with quoted purities of 0.995, 0.99, and 0.99 mass fraction, respectively. Gas chromatography (GC) analysis did not detect any appreciable peak of impurities. The IL, [C₈mim][BF₄], was synthesized in our laboratory by ion metathesis of 1-methyl-3-octylimidazolium bromide ([C₈mim][Br]) with sodium tetrafluoroborate in aqueous

(Liquid + liquid) equilibrium

(Liquid + liquid) equilibrium data for the ternary systems [C₈min][BF₄] + thiophene + n-heptane, [C₈min][BF₄] + thiophene + n-dodecane, and [C₈min][BF₄ + thiophene + n-hexadecane were obtained experimentally at T = 298.15 K by analysis of phases at equilibrium. TABLE 3, TABLE 4, TABLE 5 give the experimental compositions for the tie-lines of the three systems. The phase diagrams of these systems are shown in FIGURE 1, FIGURE 2, FIGURE 3. All systems presented here are type II since [C₈min][BF₄] is partially

Conclusions

In this work, the ability of the ionic liquid 1-methyl-3-octylimidazolium tetrafluoroborate to act as solvent in the (liquid + liquid) extraction of thiophene from aliphatic hydrocarbons was analyzed. To this aim, phase equilibria of different ternary systems involving [C₈min][BF₄], thiophene, and n-alkanes (n-heptane, n-dodecane, or n-hexadecane) present in liquid fuel-oils were obtained experimentally at temperature of 298.15 K. The solute distribution coefficient decreases and the selectivity

Acknowledgements

The authors are grateful to the Ministerio de Educación y Ciencia (Spain) for financial support under Project CTQ2006-07687. L. Alonso is grateful to the same Ministerio for financial support via “Juan de la Cierva” Programme. M. Francisco also wants to thank them for the award of the FPI grant with reference BES-2007-16693 through the same project.

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Solvent extraction of thiophene from n-alkanes (C7, C12, and C16) using the ionic liquid [C8mim][BF4]

Abstract

Introduction

Section snippets

Chemicals

(Liquid + liquid) equilibrium

Conclusions

Acknowledgements

Fuel

Fluid Phase Equilibr.

Green Chem.

Energy Fuels

Ind. Eng. Chem. Res.

Green Chem.

Green Chem.

Ind. Eng. Chem. Res.

AIChE J.

Green Chem.

Energy Fuels

Solvent extraction of thiophene from n-alkanes (C₇, C₁₂, and C₁₆) using the ionic liquid [C₈mim][BF₄]