Extraction of Thiophenic Sulfur Compounds from Model Fuel Using a

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Extraction of Thiophenic Sulfur Compounds from Model Fuel Using a Water-Based Solvent Biswajit Saha and Sonali Sengupta* Department of Chemical Engineering, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal 721302, India S Supporting Information *

ABSTRACT: Thiophene, benzothiophene, and dibenzothiophene were extracted from isooctane using water and aqueous solutions of hydrochloric acid, sulfuric acid, nitric acid, acetic acid, sodium hydroxide, ammonium hydroxide, and sodium chloride at different concentrations, as solvents or extractants. The sulfur compound in isooctane at a definite concentration comprises model fuel. It was observed that the extraction ability of water is enhanced by adding those solutes or reagents. The aim of this work is to establish water-based solutions as an economic extractant for desulfurization of model fuel containing thiophenic sulfur compounds, by choosing the appropriate concentration of those aqueous extractants and using them at the most suitable extractive condition. The best result was obtained from aqueous hydrochloric acid solution, whose maximum sulfur removal efficiency toward thiophene, benzothiophene, and dibenzothiophene was 50, 28.2, and 26.8%, respectively. The effect of operational parameters on extraction, such as solvent/model fuel volume ratio, temperature, stirring speed, extraction time, and extraction cycle, was explored. The extraction temperature of 50 °C and 2:1 solvent/model fuel volume ratio under stirring at 1000 rpm for 60 min were found to be the optimum operating conditions. Finally, liquid−liquid equilibria of the ternary mixture (thiophene + isooctane + aqueous HCl) were obtained at 40, 50, 60, and 70 °C at atmospheric pressure, and the equilibrium data were correlated with Othmer−Tobias and Hand correlations.

1. INTRODUCTION Exhaust gas from automobiles is one of the principal causes of air pollution. The modern world is very stringent to save the environment by abating the pollutant emission as a result of fuel combustion, specifically, the emission of SOx.1 The use of petroleum fuels, which contain an objectionable amount of sulfur, is limited by specific regulations posed by different countries. Not only polluting the environment, sulfur compounds in liquid fuels cause other unacceptable harms, such as corrosion to the refinery equipment, lowering the catalyst activity of the secondary processing of fuels in refinery, etc.2 Hence, the sulfur removal from petroleum fuels is adopted in a greater scale by all concerned countries, following strict rules in this matter.3 Although hydrodesulfurization (HDS) is the conventional method used in the refinery to remove sulfur from petroleum fuels, the process suffers from some prominent disadvantages, which include inability to crack refractory sulfur compounds, high severity of operation, and consumption of valuable hydrogen gas.4,5 Several alternative desulfurization methods to HDS are being developed, such as adsorptive, oxidative, extractive, membrane, biodesulfurization, etc. Although these methods are confined in the laboratory scale and still to be improved more to match the industry need by continuous research, most of them can overcome almost all drawbacks of HDS. Extractive desulfurization is a non-catalytic, low-severity process, which needs a suitable solvent to remove sulfur compounds from petroleum fuels economically without changing the chemical nature of the fuel cut. An appreciable amount of research work have been performed on extractive desulfurization using different kinds of organic solvents, such as © XXXX American Chemical Society

N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), furfural, sulfolane, methanol, etc.6−8 Most of the organic solvents are volatile, toxic, flammable, and costly. Ionic liquid is a better choice as an extraction solvent, and quite a number of studies have been performed on these.9−12 However, the ionic liquids show drawbacks, which include their higher cost compared to organic solvents; cost increases more if the number of extraction cycles or the volume of extractables increases, and its impact on the environment is still unknown.2 Hence, there is a great opportunity to broaden the area of research in finding out new eco-friendly and economic solvents for organic sulfur compound removal from fuel, and in this view, water was chosen as the medium of extraction in the present work. Model fuel is prepared by dissolving thiophene (TH), benzothiophene (BT), and dibenzothiophene (DBT) as the sulfur compounds separately in isooctane at different concentrations. The aqueous solutions of hydrochloric acid, sulfuric acid, nitric acid, acetic acid, sodium hydroxide, ammonium hydroxide, and sodium chloride at different weight percentages were taken as extraction solvents, and the extraction ability of those solvents along with distilled water was experimented. The main objective of this research work is to find out the extractability of water by the addition of different solutes, as mentioned above. The work also includes parametric studies with the best solvent and, finally, generation of liquid−liquid equilibria (LLE) data for the ternary mixture (TH + isooctane + HCl−water solution) at 40, 50, 60, and 70 Received: July 27, 2016 Revised: November 29, 2016 Published: December 5, 2016 A

DOI: 10.1021/acs.energyfuels.6b01842 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels °C at atmospheric pressure. The LLE data were correlated with Othmer−Tobias and Hand correlations.13

2. EXPERIMENTAL SECTION 2.1. Materials. Hydrochloric acid (35%), nitric acid (70%), sulfuric acid (98%), sodium chloride, acetic acid (99%), ammonium hydroxide (25%), and isooctane (99.5%) (boiling point = 99 °C) were procured from Merck India, Ltd., India. Sodium hydroxide from SRL, India, TH from Spectrochem, India, BT from Himedia, India, and DBT from Alfa Aesar, India, were purchased. 2.2. Methods. Three different model fuels were prepared by dissolving the desired amount (500 ppmw) of TH, BT, and DBT in isooctane. The extraction was performed in a 100 mL glass vessel fitted with a stirrer and condenser. The vessel was kept in a water bath whose temperature was maintained by a temperature controller cum indicator within the range of ±1 °C. The extraction experiment was conducted by mixing the extracting solvent and model fuel at the desired volume ratio at 50 °C under stirring at 1000 rpm for 60 min. The sample from the isooctane phase was collected at the end of the experiment after complete separation of the phases and analyzed in high-performance liquid chromatography (HPLC, PerkinElmer, series 200), with a reversed-phase Agilent SB C-18 column and a PerkinElmer series 200 ultraviolet/visible (UV/vis) detector set at 254 nm. The mobile phase used was 90% methanol in water. The same experimental procedure for extraction was followed for real fuel. The amount of total sulfur of real fuel after and before extraction was analyzed by a CHNS analyzer instrument (Vario Macro Cube Elementar). The “S removal efficiency (SR)” is defined by the following equation:

Figure 1. Effect of the solute concentration (wt %) on the extraction efficiency of aqueous solvents for TH extraction [initial sulfur concentration, 500 ppmw; stirrer speed, 1000 rpm; temperature, 50 °C; extraction cycle, 1; extraction period, 1 h; and solvent/model fuel ratio (v/v), 2:1].

SR = {(S0 − Sf )/S0}100% where S0 is the initial sulfur content (ppm) in model fuel and Sf is the final sulfur content (ppm) in raffinate oil after extraction.

3. RESULTS AND DISCUSSION 3.1. Performance of Pure Water toward Extraction of Sulfur Compounds. The extraction experiment was conducted with pure distilled water as the extractant, and the removal of TH, BT, and DBT was found to be 20, 0.2, and 0%, respectively. The results showed that the solubility of TH in water was significantly high compared to those of BT and DBT. 3.2. Efficiency of Different Aqueous Solvents with Various Solute Concentrations toward Sulfur Extraction. The previously mentioned aqueous solvents were prepared at different solute concentrations and used for extraction of TH, BT, and DBT from isooctane. The results of extraction for TH, BT, and DBT are shown in Figures 1, 2, and 3, respectively. It has been observed that the extraction efficiency of solvents has been increased to a significant extent compared to pure water. This may be because of the reason that, although water is a polar solvent, the addition of a little amount of inorganic acid, base, or salt to water results in the dissociation of water into hydrogen ions [H+] and hydroxyl ions [OH−] to a larger extent than water itself. TH is highly polar because its sulfur contains two non-bonding pairs of electrons. It may be hypothesized that, as soon as TH comes in contact with acidulated, alkalyated, or salt water, containing a large number of hydrogen ions [H+], sulfur becomes protonated. This protonation effect of those solvents is much more compared to that of pure water. Protonated sulfur or, as a whole, TH shows an intense polar effect and comes into the aqueous solvent easily, leaving the nonpolar solvent isooctane. This is the probable reason why those aqueous solvents are effective in extraction. Protonation

Figure 2. Effect of the solute concentration (wt %) on the extraction efficiency of aqueous solvents for BT extraction [initial sulfur concentration, 500 ppmw; stirrer speed, 1000 rpm; temperature, 50 °C; extraction cycle, 1; extraction period, 1 h; and solvent/model fuel ratio (v/v), 2:1].

Figure 3. Effect of the solute concentration (wt %) on the extraction efficiency of water for DBT extraction [initial sulfur concentration, 500 ppmw; stirrer speed, 1000 rpm; temperature, 50 °C; extraction cycle, 1; extraction period, 1 h; and solvent/model fuel ratio (v/v), 2:1].

of thiophenic sulfur by HCl in the gaseous phase has been proposed by some scientists.14 B


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nonpolar part of TH is smaller than that of BT and DBT. BT and DBT have benzene rings attached to the five-membered ring; hence, the nonpolar moiety of BT is more than that of TH, which is again more for DBT. The partition coefficient of TH, BT, and DBT in pure water was found to be 0.08, 0, and 0, respectively. 3.3. Effect of the Initial Sulfur Concentration on the Extraction Efficiency of Solvents. Panels a−c of Figure 4 show the effect of the initial sulfur concentration on the extraction efficiency of solvents. From the panels, it is noticed that, with the increase in the initial sulfur concentration from 300 to 1000 ppm, the SR of aqueous HCl was decreased from 69 to 32% for TH, from 35 to 15% for BT, and from 36 to 10% for DBT. Hence, we can show that, for TH, as an example, the concentrations of the raffinate phase (extracted model fuel) become 93, 250, 420, and 680 ppm for the initial TH concentration of 300, 500, 700, and 1000 ppm, respectively, in the model fuel. This may be because of the reason that, because the amount of solvent is not increased with the increase in the initial sulfur concentration, the removal efficiency of solvent was decreased, as expected. The same trend was also followed by other solvents. 3.4. Recovery of the Sulfur Compound from the Extract Phase (Solvent Phase). Sulfur compounds can be recovered from the extract phase. This has been experimented for DBT, where the extract phase, containing DBT in aqueous HCl, was neutralized by sodium carbonate and concentrated in a rotary vacuum evaporator at 60 °C. The remaining water was removed by sodium sulfate. DBT in the remains was solubilized in acetonitrile, and HPLC was performed. The HPLC result showed pure DBT in the acetonitrile phase. DBT was crystallized from the acetonitrile phase by evaporation. The same process was also applied for recovery of TH and BT. The recovery yield for DBT, BT, and TH is found to be 60, 60, and 70%. 3.5. Parametric Study of Extraction of TH, BT, and DBT Using an Aqueous Solution of Hydrochloric Acid as the Solvent. 3.5.1. Stirrer Speed Variation. Extraction of a component from one solvent to another needs intimate mixing; hence, the effect of the stirring speed on extraction of TH, BT, and DBT was studied by varying the stirrer speed from 400 to 1200 rpm. Figure 5 reports this effect, and it is observed that the removal of all three sulfur compounds was increased with increasing the stirrer speed up to 1000 rpm and, beyond that, no significant increase was observed.19,20 The highest sulfur removals were 50% for TH, 28.2% for BT, and 26.8% for DBT at 1000 rpm. Therefore, all of the successive experiments were carried out at 1000 rpm. 3.5.2. Solvent/Model Fuel Volume Ratio Variation. To study the effect of the solvent/model fuel volume ratio on the extraction of TH, BT, and DBT, a range of 0.2:1−4:1 was chosen for the experiment and the results of the experiments are presented in Figure 6. It has been observed that the highest sulfur removal reached up to 50% for TH, 28.2% for BT, and 26.8% for DBT at a 2:1 volume ratio of solvent/model fuel. When the solvent/model fuel volume ratio was increased from 0.2:1 to 2:1, the SR of the solvent for all three compounds was increased regularly but, upon a further increase in the volume ratio, no significant increase in sulfur removal was observed. It can be said that the extraction efficiency approaches a limit or optimum value with an increase in the solvent/model fuel volume ratio up to a certain point and, after that, it levels off.19 Because an excess volume of solvent contributes to a higher

It is observed from Figure 1 that the highest extraction recorded for TH was 52.6, 50, 47.2, 44.4, 40, 36, and 30.8% using sulfuric acid (15 wt %), hydrochloric acid (10 wt %), nitric acid (3.54 wt %), acetic acid (3.54 wt %), sodium chloride (10 wt %), ammonium hydroxide (10 wt %), and sodium hydroxide (10 wt %) in water as solvents, respectively. Figure 2 showed the highest extraction for BT as 28.2, 25.8, 23.6, 16.6, 14.8, 12, and 9.4% using hydrochloric acid (3.54 wt %), nitric acid (10 wt %), sodium hydroxide (8 wt %), acetic acid (3.54 wt %), sulfuric acid (25 wt %), ammonium hydroxide (3.54 wt %), and sodium chloride (10 wt %) in water as solvents, respectively. For DBT, it was found that, except aqueous hydrochloric acid, other solvents failed to extract the compound. Here, 3.54 wt % HCl in water solution showed the highest extraction of 26.8% DBT. Moreover, the solutes, which are used in the present experiments for preparation of different aqueous solvents, increased the polarity of the water; as a result, the dipole moment and the solubility parameter of the water were also increased, which, in turn, increased the solubility of the sulfur atom toward the more polar solvents.15−17 From the figures, it is observed that, with the increase in the concentration of solutes in water, the extraction efficiency of the solvents increases up to a certain point and, after that, any increase in the concentration of the solute does not help to increase the removal of sulfur compounds. A probable reason behind this behavior of the solvents may be that the concentration of the solutes showing the highest extraction is the optimum or saturation point for extraction. In comparison of the results of extraction, hydrochloric acid in water is chosen to be the best and common solvent for all three sulfur compounds. Acidulated water contains more hydrogen ions [H+] compared to alkylated or salt-dissolved water, because acids impart more H+ ion from its dissociation, and hence, acidic solution showed better performance than other aqueous solvents. The dissociation power of HCl or tendency to dissociate is more compared to H2SO4 and HNO3, which is revealed from the values of their dissociation constants Ka.18 Hence, HCl produces a better result than the other two acids. The optimum concentrations of hydrochloric acid in water were 10 wt % for TH extraction and 3.54 wt % for BT and DBT extractions. Hence, all of the successive experiments were carried out with the mentioned solvents. Table 1 shows the partition coefficients of sulfur compounds, TH, BT, and DBT, in four different solvents, aqueous HCl, Table 1. Partition Coefficient {KN, [mg(S) kg(Extractant)−1/ mg(S) kg(Oil)−1]} of Sulfur Compounds in Various Solvents (10 wt % Concentration) sulfur compound

water−HCl

TH BT DBT

0.335 0.128 0.121

water−HNO3 water−H2SO4 0.293 0.118 0

0.166 0.012 0

water−NaOH 0.149 0.10 0

aqueous HNO3, aqueous H2SO4, and aqueous NaOH, and it is clear that the partition coefficients of sulfur compounds are of much higher values for aqueous HCl solvent compared to the others. Moreover, TH shows the highest partition coefficient among the other two sulfur compounds, because the polar sulfur atom of TH is attached to the C5H4 ring. Hence, the C


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Figure 5. Effect of the stirrer speed on the extraction efficiency of aqueous HCl for TH, BT, and DBT [initial sulfur concentration, 500 ppmw; HCl weight percent in water, 10 (for TH) and 3.54 (for BT and DBT); temperature, 50 °C; extraction cycle, 1; extraction period, 1 h; and solvent/model fuel ratio (v/v), 2:1].

Figure 6. Effect of the solvent/model fuel volume ratio on the extraction efficiency of aqueous HCl for TH, BT, and DBT extractions [initial sulfur concentration, 500 ppmw; HCl weight percent in water, 10 (for TH) and 3.54 (for BT and DBT); temperature, 50 °C; extraction cycle, 1; extraction period, 1 h; and stirrer speed, 1000 rpm].

results of extraction of TH, BT, and DBT and variation of partition coefficients from 30 to 70 °C with aqueous HCl solution. It was observed that, when the extraction temperature was increased from 30 to 50 °C, the removal rates of sulfur compounds were increased. The reason for this may be that, with the increase in the extraction temperature, the viscosity of the solvent was decreased and, hence, the mobility of the solvent was increased; as a result, the mixing of the solvent and model fuel was very easy and more homogeneous within a shorter time, which, in turn, enhances the extraction efficiency for all three compounds.21,22 The highest sulfur removal reached up to 50% for TH, 28.2% for BT, and 26.8% for DBT at 50 °C, but with a further increase in the temperature, the extraction of all three compounds was decreased. One reason behind this decrease may be the loss of isooctane to some extent at a higher temperature, and another reason may be that, at a high temperature, the partition coefficient of the solvent/ model fuel system may not be that much sensitive toward the extraction temperature to be affected.5,19,23,24 Therefore, 50 °C

Figure 4. (a) Effect of the initial sulfur concentration on the extraction efficiency of solvents for TH [stirrer speed, 1000 rpm; solute weight percent in water, 10 (for TH); temperature, 50 °C; extraction cycle, 1; extraction period, 1 h; and solvent/model fuel ratio (v/v), 2:1]. (b) Effect of the initial sulfur concentration on the extraction efficiency of solvents for BT [stirrer speed, 1000 rpm; solute weight percent in water, 10 (for BT); temperature, 50 °C; extraction cycle, 1; extraction period, 1 h; and solvent/model fuel ratio (v/v), 2:1]. (c) Effect of the initial sulfur concentration on the extraction efficiency of aqueous HCl for DBT [stirrer speed, 1000 rpm; HCl weight percent in water, 10 (for DBT); temperature, 50 °C; extraction cycle, 1; extraction period, 1 h; and solvent/model fuel ratio (v/v), 2:1].

cost of extraction and recovery processes, the volume ratio was fixed to 2:1 for subsequent experiments. 3.5.3. Operating Temperature Variation. The temperature is an important factor affecting extraction. Figures 7 and 8 show D


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Figure 7. Effect of the temperature on the extraction efficiency of solvent for TH, BT, and DBT extractions [initial sulfur concentration, 500 ppmw; HCl weight percent in water, 10 (for TH) and 3.54 (for BT and DBT); solvent/model fuel ratio (v/v), 2:1; extraction cycle, 1; extraction period, 1 h; and stirrer speed, 1000 rpm].

Figure 9. Effect of the operating time on the extraction efficiency of solvent for TH, BT, and DBT extractions [initial sulfur concentration, 500 ppmw; HCl weight percent in water, 10 (for TH) and 3.54 (for BT and DBT); solvent/model fuel ratio (v/v), 2:1; extraction cycle, 1; extraction temperature, 50 °C; and stirrer speed, 1000 rpm].

3.5.5. Multiple Extractions. To investigate the extent of removal of sulfur compounds by extraction, multiple cycles of extraction with the aqueous HCl solvent has been tried and it was observed that the removal increased to a high extent when fresh solvent was used for each cycle. The results for repeated extraction of 500 ppm of TH, BT, and DBT in the isooctane phase are shown in Figure 10. It is observed from the figure that

Figure 8. Effect of the temperature on partition coefficients of TH, BT, and DBT extractions [initial sulfur concentration, 500 ppmw; HCl weight percent in water, 10 (for TH) and 3.54 (for BT and DBT); solvent/model fuel ratio (v/v), 2:1; extraction cycle, 1; extraction period, 1 h; and stirrer speed, 1000 rpm].

was selected as the optimum extraction temperature, which is economical also for industrial application for low energy consumption. 3.5.4. Extraction Period Variation. To determine the time needed to reach the maximum extraction efficiency, experiments were conducted at different periods of time ranging from 20 to 120 min. As shown in Figure 9, the SR was 10% for TH and 4% for both BT and DBT in 20 min and the highest SR reached up to 50, 28.2, and 26.8% for TH, BT, and DBT, respectively, at 60 min with aqueous HCl solution. This hike in removal may be related to prolonged agitation, which, in turn, increases the solubility of the sulfur compounds in the solvent as well as the mass transfer rate from the isooctane phase to the solvent phase. By further enhancement in the extraction period (beyond 60 min), the extraction was decreased, which indicates that the extraction process had reached equilibrium at 60 min. This behavior may be explained by the possibility of reverse migration of sulfur compounds from the solvent phase to the isooctane phase after 60 min.19

Figure 10. Effect of the number of extraction stages on the extraction efficiency of solvent for TH, BT, and DBT extractions [initial sulfur concentration, 500 ppmw; extraction period, 1 h; solvent/model fuel ratio (v/v), 2:1; HCl weight percent in water, 10 (for TH) and 3.54 (for BT and DBT); extraction temperature, 50 °C; and stirrer speed, 1000 rpm].

the extraction was increased from 50 to 70.3%, from 28.2 to 34%, and from 26.8 to 31% for TH, BT, and DBT, respectively, after 8 successive extraction cycles. It was also noted that the multiple extraction cycle has a profound effect on the extraction of TH compared to those of BT and DBT. 3.5.6. Reusability of the Solvent. Solvent reuse is an important factor for its industrial application and economy point of view. The reuse of the solvent aqueous HCl was studied here, and the results are presented in Figure 11. It is seen that the same solvent can be reused several times with a little decrease in extraction capability. After 5 consecutive E


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efficiency of solvent for TH is 20%, whereas for BT and DBT, that was 29 and 31%, respectively, after the fourth regeneration cycle. After that, a significant decrease was not observed. 3.6. Determination of Tie Lines of the Ternary Mixture. Collection of LLE data for a system in the extraction process is important in calibration and verification of the analytical model, for determining the extent of solubility of the solute in the extract and raffinate phases and also for the selection and design of extraction equipment.13,25 In the present work, determination of LLE data of the ternary mixture (TH + isooctane and aqueous HCl) had been performed by the extraction experiment of TH. Extraction was performed for TH varying its concentration, keeping other operating parameters constant. After the experiment, the aqueous and organic phases were separated by settling for 0.5 h, keeping the temperature constant. The samples from both of the phases were collected and analyzed in HPLC. Extraction experiments were conducted at four different temperatures, namely, 40, 50, 60, and 70 °C, and the results are shown in Tables 2−5. The results showed that the TH concentration at 40, 50, 60, and 70 °C varies from 0.013 to 0.048, from 0.011 to 0.044, from 0.012 to 0.0446, and from 0.0138 to 0.0446 in the raffinate phase, respectively, whereas it varies from 0.012 to 0.022, from 0.015 to 0.031, from 0.014 to 0.03, and from 0.01 to 0.03 in the extract phase, respectively. The mass fraction of all compounds (TH, isooctane, and solvent) of the extract and raffinate phases for 40, 50, 60, and 70 °C is plotted in a trianguler graph (showed in the Supporting Information). It has been assumed that isooctane and aqueous solvent phases are immiscible to each other, because solubility of isooctane in water is negligible.26 Hence, an assumption can be made that no water is going to the isooctane phase and vise versa. The solute distribution ratio (β) at those temperatures for this system is also reported in the tables.

Figure 11. Effect of the solvent reuse on the extraction efficiency of solvent for TH, BT, and DBT extractions [initial sulfur concentration, 500 ppmw; extraction period, 1 h; solvent/model fuel ratio (v/v), 2:1; extraction temperature, 50 °C; HCl weight percent in water, 10 (for TH) and 3.54 (for BT and DBT); and stirrer speed, 1000 rpm].

cycles, the TH removal decreased from 48 to 22%, starting from fresh to fifth time reused solvents. It is also observed that the extractability of the reused solvent for TH extraction was quite high compared to BT and DBT extractions. 3.5.7. Regeneration of the Solvent. The aqueous HCl solution recovered after 5 extraction cycles was stirred with activated charcoal powder (8.2 g/L solvent) for 1 h at room temperature for regeneration. Charcoal powder was filtered, and the regenerated solvent was analyzed in HPLC. It has been observed that the removal of TH, BT, and DBT from solvent is 52, 42, and 37%, respectively, in 1 h. This regenerated solvent was reused for desulfurization of all three sulfur compounds efficiently, which is shown in Figure 12. The regeneration process was repeated 5 times more after every fourth stage of extraction of sulfur compounds with the regenerated solvent without doing any regeneration in between the extraction of each stage. It has been observed that regenerated solvent works best for TH compared to BT and DBT because the drop in

β=

(TH)solvent (TH)hydrocarbon

3.6.1. Othmer−Tobias and Hand Correlations. Othmer− Tobias and Hand correlations13 were used to ascertain the reliability of the LLE experimental results. The high correlation factor (R2) and linearity of the plot indicate the degree of consistency of measured LLE.13 The Othmer−Tobias correlation is written as ⎛ 1 − X33 ⎞ ⎛ 1 − X11 ⎞ ln⎜ ⎟ = a + b ln⎜ ⎟ ⎝ X11 ⎠ ⎝ X33 ⎠

The Hand correlations is written as ⎛X ⎞ ⎛X ⎞ ln⎜ 21 ⎟ = a′ + b′ ln⎜ 23 ⎟ ⎝ X11 ⎠ ⎝ X33 ⎠

where X33 is the mass fraction of solvent in the extract/solvent phase, X11 is the mass fraction of isooctane in the raffinate/ isooctane phase, X21 is the mass fraction of TH in the isooctane/raffinate phase, and X23 is the mass fraction of TH in the solvent/extract phase. Plots of −ln[(1 − X33)/X33] versus −[(1 − X11)/X11] and −ln(X21/X11) versus −(X23/X33) at different temperatures are shown in Figures 13 and 14, respectively. The correlation coefficients were calculated from the slope and intercept of the plot and tabulated in Table 6. It represents the Othmer−Tobias (a and b) and Hand (a′ and b′)

Figure 12. Effect of the solvent regeneration on the extraction efficiency of solvent for TH, BT, and DBT extractions [initial sulfur concentration, 500 ppmw; extraction period, 1 h; solvent/model fuel ratio (v/v), 2:1; extraction temperature, 50 °C; HCl weight percent in water, 10 (for TH) and 3.54 (for BT and DBT); and stirrer speed, 1000 rpm]. F


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Table 2. Composition (Mass Fraction) of Experimental Tie Lines and Solute Distribution Ratio for the Ternary Mixture (TH + Isooctane and HCl−Water Solution) at 40 °C raffinate phase (hydrocarbon phase)

extract phase (solvent phase)

TH

isooctane

solvent

TH

isooctane

solvent

solute distribution ratio (β)

0.013 0.021 0.031 0.04 0.048

0.987 0.979 0.969 0.96 0.952

0 0 0 0 0

0.012 0.0158 0.016 0.0179 0.022

0 0 0 0 0

0.988 0.9842 0.984 0.9821 0.978

0.92 0.75 0.52 0.45 0.458



TH

isooctane

solvent

TH

isooctane

solvent


0.011 0.02 0.03 0.035 0.044

0.989 0.98 0.97 0.965 0.956

0 0 0 0 0

0.015 0.02 0.024 0.027 0.031

0 0 0 0 0

0.985 0.98 0.976 0.973 0.969

1.36 1.00 0.8 0.77 0.70


Figure 13. Othmer−Tobias plot of (TH + isooctane and HCl−water solution) ternary systems.


TH

isooctane

solvent

TH

isooctane

solvent


0.012 0.021 0.03 0.037 0.0446

0.988 0.979 0.97 0.963 0.9554

0 0 0 0 0

0.014 0.018 0.022 0.025 0.03

0 0 0 0 0

0.986 0.982 0.978 0.975 0.97

1.16 0.857 0.733 0.675 0.672

correlation coefficients at 40, 50, 60, and 70 °C. From the table, it is observed that the values of R2 for Othmer−Tobias and Hand correlations range from 0.951 to 0.994 and from 0.955 to 0.993, respectively, which reflect the linearity of the tie line data. 3.7. Extraction Efficiency of Solvents on Real Fuel. Table 7 showed the extraction efficiency of all ionic solvents on real fuels 1 and 2. It is observed that the sulfur removal efficiency for aqueous HCl solvent for real fuels 1 are 2 is 89.2 and 29.17%, respectively, and with aqueous HNO3 solvent, the efficiency is 82.1 and 34% for real fuels 1 are 2, respectively.

Figure 14. Hand plot of (TH + isooctane and HCl−water solution) ternary systems.

3.7.1. Multiple Extraction of Real Fuel. To investigate the extent of removal of sulfur compounds of real fuels by extraction, multiple cycles of extraction with aqueous HCl for real fuel 1 and aqueous HNO3 for real fuel 2 as a solvent were



TH

isooctane

solvent

TH

isooctane

solvent


0.0138 0.022 0.031 0.04 0.0446

0.9862 0.978 0.969 0.96 0.9554

0 0 0 0 0

0.01 0.015 0.016 0.0184 0.03

0 0 0 0 0

0.99 0.985 0.984 0.9816 0.97

0.724 0.681 0.516 0.46 0.672

G


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using this solvent. The optimum parameters chosen for this extraction are solvent/model fuel ratio of 2, extraction temperature of 50 °C, extraction time of 60 min, and stirrer speed of 1000 rpm. It was also observed that, for TH extraction, the efficiency of water−HCl solution was reduced from 50 to 40% only after 2 cycles starting from fresh solvent, but for BT and DBT extractions, this efficiency was quite low. Hence, water−HCl solution may be said to be an efficient extractant for TH extraction. The LLE data were collected at four temperatures, 40, 50, 60, and 70 °C, and the tie line data are correlated with Othmer−Tobias and Hand correlations with high correlation factors of 0.994 and 0.993, respectively.

Table 6. Correlation Coefficient and Correlation Factors for Othmer−Tobias and Hand Correlations at Different Temperatures Othmer−Tobias correlation

Hand correlation 2

temperature (°C)

a

b

R

a′

b′

R2

40 50 60 70

2.507 1.877 1.823 1.413

0.441 0.515 0.557 0.761

0.951 0.994 0.981 0.984

5.243 3.597 3.140 0.838

2.134 1.927 1.759 1.080

0.958 0.993 0.982 0.955

■

Table 7. Extraction Efficiency (%) of Solvents on Real Fuela solvent aqueous aqueous aqueous aqueous aqueous aqueous aqueous

HCl HNO3 H2SO4 CH3COOH NaOH NH4OH NaCl

real fuel 1b

real fuel 2c

89.2 82.1 78.2 78.7 83.6 83.7 80.4

29.17 34 29.17 24 30.2 25 27

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b01842. Fourier transform infrared spectroscopy (FTIR) analysis of the extract phase of aqueous HCl and aqueous NaOH for TH and BT, viscosity of isooctane at different temperatures, ternary plots of the system (isooctane + TH + aqueous HCl) at 40, 50, 60, and 70 °C, and description of the nature of contaminants present in crude oil HPLC calibration curves of TH, BT, and DBT (PDF)

Stirrer speed, 1000 rpm; temperature, 50 °C; extraction cycle, 1; extraction period, 1 h; solvent/model fuel ratio (v/v), 2:1; and solute concentration in water, 10 wt %. bReal fuel 1, gasoline; boiling range, 102−143 °C; specific gravity, 0.75; and total sulfur, 1.031 wt %. cReal fuel 2, light gas oil; boiling range, 170−336 °C; specific gravity, 0.84; and total sulfur, 1.73 wt %. a

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performed. The removals are very satisfactory when fresh solvent was used for each cycle. The feed sulfur contents of real fuels 1 and 2 are 1.031 and 1.73 wt %, respectively. It is observed that the extraction was increased from 89.2 to 97.2% and from 34 to 73.2% for real fuels 1 and 2, respectively, from the first cycle to after the eighth extraction cycle, which were also represented in a bar graph (showed in the Supporting Information). It was also noted that the multiple extraction cycle has a profound effect on the extraction of real fuel 2 compared to that of real fuel 1. 3.7.2. Properties of Real Fuels after and before Extraction. The physical properties of real fuels 1 and 2 before and after extraction are tabulated and shown in the Supporting Information. It is observed that a little improvement in flash and fire points after extraction have been observed for real fuel 1, whereas for real fuel 2, a small upgradation in the cetane number is noticed.

AUTHOR INFORMATION

Corresponding Author

*Telephone: +91-3222-283954. Fax: +91-3222-282250. E-mail: [email protected]. ORCID

Sonali Sengupta: 0000-0002-2748-1372 Notes

The authors declare no competing financial interest.

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REFERENCES

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4. CONCLUSION Extraction of TH, BT, and DBT from model fuel, comprising of isooctane and those sulfur compounds separately, was performed using pure water and a aqueous solution of hydrochloric acid, sulfuric acid, nitric acid, acetic acid, ammonium hydroxide, sodium hydroxide, and sodium chloride at different concentrations. Pure water showed a very poor performance toward the removal of TH and could not remove the other two compounds, whereas water−HCl solvent showed good activity for all three compounds, at 50, 28.2, and 26.8% for TH, BT, and DBT, respectively, compared to other solvents. The main reason behind this is the high partition coefficient of sulfur compounds in aqueous HCl. Parametric studies, such as determination of effects of solvent/model fuel volume ratio, extraction temperature, extraction time, and stirrer speed, for all three sulfur compounds were performed H


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Extraction of Thiophenic Sulfur Compounds from Model Fuel Using a

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