Effective extraction of benzene and thiophene by novel deep eutectic solvents from hexane / aromatic mixture at different temperatures
Graphical abstract
Introduction
The separation of aromatics from aliphatics mixtures is a challenge for refinery and petrochemical industries. However, an efficient method for extraction of aromatics with a weight fraction of less than 20% at normal temperature and pressure has not been yet introduced [[1], [2], [3]]. It is recommended to minimize aromatic contains (such as benzene or thiophene) in refinery and petrochemical products due to their negative health effects and possibility of ground water sources contamination with aromatic hydrocarbons [3,4]. According to the standards EN 228 and ASTM D1319, the maximum permitted benzene concentration in gasoline was 1% (v/w), which revised to 0.62% (v/w) in 2011 by EPA.
According to the standard (ASTM 5623), many of sulfur compounds are odorous and can cause corrosion for equipments, have the potential to inhibit or damage the catalysts, and have negative impact on product quality. Sulfur compounds in the fuels contribute to emissions of sulfur dioxide which is the main source of the acid rains.
Due to the azeotropic behavior of the aromatic-aliphatic mixtures, separation of these mixtures can occur near their boiling point by increasing the third component like N-formylmorpholine (NFM). NFM changes the relative volatility of the azeotrope mixtures. NFM extraction process continues with extractive distillation to recover the NFM [5]. In general, the specification for selection of an appropriate solvent for extractive distillation are: high selectivity or high ability to change equilibrium point of the vapor-liquid mixture [5], high-capacity to solve certain amount of solute [5], low volatility due to prevent solvent evaporation with distillation [6], high separation capacity [7], easily separable from the mixture, and available in low price [8], no toxicity [9], chemical stability [10], without corrosive effects on the distillation tower [11,12], high boiling and low freezing point, and finally low viscosity to increase distillation tower trays output [13,14].
At present, in the petrochemical industries, sulfolan or NFM solvents are using for the separation of aromatic from aliphatic mixtures. There are some problems in the use of these solvents for extraction: first of all, these solvents are as known toxic substances and environmental pollutant [15]. Secondly, the extraction process efficiency is not complete and at least (1–5) % of aromatic compounds impurity remains at the end of the extraction. NFM also work at non ambient pressure and temperature which consume a lot of energy. On the basis of mentioned disadvantages finding an echo friendly solvent which can work in ambient temperature and pressure is one of the research priorities [15].
In the recent decade, ionic liquids have been used as solvents for separation of aromatic – aliphatic mixtures [3,4,[16], [17], [18], [19]]. Application of these neoteric solvents was growing rapidly till deep eutectic solvents (DESs) were introduced as a novel generation of green solvents. During last year, Fluid Phase Equilibria published a special issue on DESs [20]. The editors of that special issue justify its publication due to the need of correcting some prevalent misunderstanding about DESs. In fact DES should be not classified as low cost ionic liquids (ILs), because they do not have the ILs properties [20]. DESs are not pseudo pure components, and they should be defined as binary or ternary mixtures [20]. Deep eutectic solvents with high purity can be prepared. They are nonreactive with water and most of them are biodegradable with low toxicity to mammals. Furthermore, their low prices pave the way for expanding their applications in the liquid – liquid extraction [3,4,21].
In this work, some choline chloride based deep eutectic solvent (DES) in combination with several industrial grade hydrogen bond donors including, mono-ethylene glycol (MEG), di-ethylene glycol (MEG), tri-ethylene glycol (MEG), mono-ethanolamine (MEA), di-ethanolamine (DEA), and tri-ethanolamine (TEA) were prepared. The experimental liquid - liquid equilibria for several systems containing (n-hexane + benzene + DES), (n-hexane + thiophene + DES) were measured at T = (298.15–318.15) K and atmospheric pressure. The calculated LLE results were correlated by NRTL and UNIQUAC models. The Hansen solubility parameters were used to evaluate the solutes miscibility in the studied DESs.
Section snippets
Materials
Choline chloride was used as hydrogen bond acceptor. The (mono-, di- and tri-) ethylene glycol, and (mono-, di- and tri-) ethanolamine were used as hydrogen bond donor. Ethanol was used for crystallization of choline chloride. A summary of the chemicals specification used in this work are listed in Table 1.
Apparatus and procedure
The experimental LLE data were determined at T = (298.15–318.15) K and atmospheric pressure. The equilibrium cell method was used for determination of LLE data. In this method certain amounts
Solubility results
The high solubility of aromatics and low solubility (or non-solubility) of aliphatic (non-aromatics) in the DESs indicates applicability of this type of green solvents for liquid-liquid extraction. DESs viscosity and the test temperature are two effective parameters on the solubility test results.
Solubility of benzene, thiophene and n-hexane in the studied DESs were determined at T = 298.15 K and atmospheric pressure via turbidity test method [23]. Table 2 shows the results of turbidity test
Conclusions
In this work, applicability of two series of DESs based on (mono-, di- and tri-) ethylene glycols and ethanolamines, as novel extracting agents for separation of benzene/thiophene from n-hexane were studied. Solubility test were made due to understand efficiency of the studied DESs for this separation.
The studied DESs show better solubility for benzene and thiophene. LLE data of the studied systems {n-hexane (1) + benzene (2) + DES (3)} and {n-hexane (1) + thiophene (2) + DES (3)} were
Acknowledgement
The authors would like to express their gratitude to University of Tabriz Research Council for the financial support of this research.
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