Desulfurization of liquid fuel via extraction with imidazole-containing deep eutectic solvent

2.1 Chemicals

A list of chemicals used in this work is presented in Table 1. The table also shows their specifications alongside their suppliers. All chemicals were used as procured without further purification. However, the hygroscopic ones among them were dried overnight in a vacuum oven before usage.

2.2 DES synthesis and characterization

The imidazole containing DES was synthesized in accordance with the literature-reported procedures [18], [19], [20]. TBAB salt was mixed with imidazole in a 1:1 mole ratio inside a tight screw-capped bottle and inserted into an incubator shaker (Brunswick Scientific Model INNOVA 40R, Eppendorf, Hamburg, Germany) operated under atmospheric pressure at 270 rpm and 80°C. The mixture was shaken until a clear homogenous solution was obtained with no visible precipitate.

The resulting solvent was characterized by performing the following: differential scanning calorimetry (DSC) on a DSC machine from TA Instruments, New Castle, DE, USA (model Q20, appropriately calibrated with indium metal), electrospray ionization mass spectroscopy (ESI-MS) on an Agilent Triple Quad LC-MS machine (6460 series), Santa Clara, CA, USA and Fourier transform infrared spectroscopy (FT-IR) on a PerkinElmer FT-IR spectrometer (Frontier model), Waltham, MA, USA. Sample pellets were formed in KBr matrix before the FT-IR spectra were recorded.

Similarly, the effects of temperature on other basic physical properties of the solvent were further investigated: density using Anton Paar DMA4500M, (Graz, Austria) density meter, conductivity using Jenway conductivity meter model 4520 (Staffordshire, UK), viscosity using Anton Paar RheolabQc viscometer, and pH using Thermo Scientific (Waltham, MA, USA) 3 star pH bench top meter. The estimated standard uncertainties in measurements of these properties by the above-mentioned equipment are as listed in Table 2.

2.3 Extraction of sulfur compounds

As with our previous works [4], [17], simulated fuel consisting of n-decane (29.79%), cyclohexane (29.79%), iso-octane (29.79%), and toluene (10.63%) was used throughout the progress of this work. A standard solution of fuel containing 500 mg of sulfur compounds (thiophene and DBT) per 1 kg of fuel was prepared by dissolving 0.05 g of each of the sulfur compounds in 99.9 g of the simulated fuel.

Liquid-liquid extraction experiments were performed by mixing the synthesized DES samples and the prepared fuel standard (in the required proportion) inside closed cap vials and inserted into the blocks of temperature-controlled thermomixers (Ditabis; Model MKR 13 and MHR 23). The thermomixers work in a temperature range of −16°C to 105°C with 0.1°C accuracy. A similar setup was used for the desulfurization of commercial diesel obtained from two commercial fuel stations in Oman. To ascertain the reproducibility of the work, experiments were made in duplicates, and the experimental errors were less than 1% for both thiophene and DBT.

2.4 Chemical analyses of raffinate

Monitoring of the concentration of sulfur compounds in the simulated fuel was conducted by performing high-performance liquid chromatography (HPLC) on the fuel sample before and after extraction experiments using an Agilent 1260 infinity series HPLC machine with specifications shown in Table 3. The table also shows the details of experimental conditions of an internally developed method used for the analyses.

Therefore, the extraction efficiency of the solvent was evaluated as follows:

where C₀ is the initial thiophene/DBT concentration in fuel (ppmw) and C₁ is the final thiophene/DBT concentration in the raffinate phase (ppmw) after extraction with the DES.

Considering the fact that commercial diesel oils may contain various sulfur compounds other than thiophene and DBT, the monitoring of the concentration of sulfur and sulfur compounds in the oil samples was performed using an energy-dispersive X-ray fluorescence machine from Rigaku^® (model number Rigaku NEX QC⁺) with an internal standard test method. The equipment complies with ISO 13032 for the measurement of ultralow sulfur in diesel fuel. However, the DES extraction efficiency was evaluated using Equation 1, but in this case, the concentration terms are the concentration of the total sulfur present in the diesel before and after extraction and not the concentration of individual sulfur compound, as it was the case with the simulated fuel.

3.1 DES characterization

The formation of the imidazole containing DES was established via the formation of a homogenous and colorless solution at synthesis temperature. The inflexions observed on the DSC thermograms of the DES, shown in Figure 1, suggest that the resulting solvent exhibits a glass transition (T_g) at –66°C, cold crystallization (T_c) at –26°C, and melting (T_m) at +15°C.

Figure 1:

DSC thermograms of DES (T_g, glass transition temperature; T_c, cold crystallization temperature; T_m, melting temperature).

The ESI-MS and the FT-IR spectra of the DES are as shown in Figures 2 and 3. The spectra in show the possible complex ions formed from the ionization of the DES, whereas the spectra in show the possible associations involved in the formation of the DES, starting with pure quaternary salt to freshly synthesized DES. Thus, the peak at 242 m/z in corresponds to the quaternary cation of [CH₃(CH₂)₃]₄N⁺. Similarly, the energy bands around 2962 and 2857 cm⁻¹ in Figure 3 could be associated with the sp³ C–H stretching and sp³ C–H bending in the butyl ligands of the quaternary salt, respectively. The energy bands at approximately 1518 and 1478 cm⁻¹ could also be associated with C=C and C=N bonds found in the imidazole molecule. In addition, the appearance of energy band at 1060 cm⁻¹ in the spectra of the DES could be attributed to a C–N bond in the imidazole ring.

Figure 2:

Electrospray ionization mass spectroscopy (ESI-MS) spectra of DES.

Figure 3:

FT-IR spectra of quaternary salt, fresh DES, and DES after extraction.

The basic physical properties (viscosity, density, pH, and conductivity) of the DES, which are considered significant to its sulfur extraction performance, were measured, and their variation with temperature was studied. The results show that changes in the temperature of the solvent produced a wide change in its viscosity and conductivity values as compared with the narrow change it produced in its density and pH, as shown in Figures 4 and 5, respectively. For instance, when the temperature of the solvent was varied from 30°C to 80°C, the viscosity decreased from 1293.00 to 50.00 cP and the conductivity increased from 7 to 1347 μS/cm. In the same context, both the density and the pH decreased from 1.0630 to 1.0305 g/cm³ and from 8.83 to 7.86, respectively. However, these properties exhibited by the imidazole-containing DES, particularly viscosity and density, favor its usage for extraction purpose when compared with the physical properties commonly exhibited by their alcohol-based [21], [22], sugar-based [23], [24], and acid-based [25] counterparts.

Figure 4:

Effect of temperature on the viscosity and conductivity of the DES.

Figure 5:

Effect of temperature on the density and pH of the DES.

The major sulfur extraction mechanism by the DES is likely via the formation of liquid clathrate because of the π-π interaction between the unsaturated bonds of sulfur compounds and the imidazole ring of the DES [26], [27].

An important criterion that could make or mar the application of any solvent in the EDS process is the ability of the solvent to dissolve other hydrocarbon components of the fuel in addition to the sulfur compounds. For this reason, HPLC was conducted to determine the solubility of the fuel in the DES and that of the DES in the fuel after extraction. No trace of the DES was found in the raffinate phase, thus suggesting that the DES is insoluble in the fuel. However, some amount of toluene was detected in the extract phase with a solubility of 2.7 wt%. Although this value is higher than the ones reported with FeCl₃-based DESs [17], it is still much lower than the ones reported with methylpyridinium ILs [28], hence suggesting also that the fuel has low solubility in the DES.

3.2 Effect of extraction variables on the solvent’s extraction efficiency

The study of the effect of extraction time on the solvent’s desulfurization efficiency, shown in Figure 6, showed that the efficiency increases rapidly within the first 30 min. A marginal increase was however observed between 60 and 90 min of the extraction time. The efficiency subsequently became constant for as long as 180 min, thus suggesting that extraction equilibrium has been achieved between the solvent and the fuel phases. Consequently, an extraction time of 95 min was fixed in subsequent extraction experiments.

Figure 6:

Effect of extraction time on the desulfurization ability of the DES (extraction conditions: 30°C, 0.5 solvent mass fraction, and 500 ppm starting concentration of each sulfur compound in fuel).

Similarly, the effect of temperature on the solvent’s desulfurization efficiency, shown in Figure 7, showed that the efficiency slightly decreased with increase in the extraction temperature. This could be attributed to the observed increase in evaporation of the fuel components at higher temperatures. Consequently, it increases the concentration of the sulfur compounds in the raffinate phase of the fuel. Therefore, the evaluated extraction efficiency of Equation 1 at lower temperatures would be higher than that evaluated at higher temperatures. To achieve less evaporation of fuel components and higher efficiency therefore, 30°C was fixed as extraction temperature in subsequent extraction experiments.

Figure 7:

Effect of extraction temperature on the desulfurization ability of the DES (extraction conditions: 95 min, 0.5 solvent mass fraction, and 500 ppm starting concentration of each sulfur compound in fuel).

However, in a clear contrast to that of extraction temperature, varying the solvent mass fraction as an extraction variable resulted in a significant change on the solvent’s desulfurization efficiency as shown in Figure 8. In essence, when the solvent mass fraction was varied from 0.2 to 0.8, the solvent’s desulfurization efficiency changed from 34% to 90% and from 21% to 78% for DBT and thiophene, respectively. This suggests that the solvent’s desulfurization efficiency is more sensitive to variation in the solvent’s mass fraction than it is on the duo of extraction time and temperature. Nevertheless, considering the significance of minimizing solvent consumption in any extraction process for economic reasons among others, the solvent mass fraction was fixed at 0.5 in subsequent extraction experiments.

Figure 8:

Effect of solvent mass fraction on the desulfurization ability of the DES (extraction conditions: 95 min, 30°C, and 500 ppm starting concentration of each sulfur compound in fuel).

In the same vein, varying the initial concentration of sulfur compounds in fuel (C₀) from 500 to 2500 ppm produced a somewhat constant trend on the desulfurization efficiency of the solvent as shown in Figure 9, thus suggesting that the desulfurization efficiency is relatively independent of C₀. This performance is particularly encouraging toward the application of the solvent in the industry where fuels of varying sulfur concentrations are found.

Figure 9:

Effect of sulfur starting concentration in fuel on the desulfurization ability of the DES (extraction conditions: 95 min, 30°C, and 0.5 solvent mass fraction).

The effect of repetitive extraction with the same solvent on the desulfurization efficiency of the solvent, shown in Figure 10, shows that the solvent loses its sulfur removal efficiency as the number of extractions increases. This observed phenomenon could be attributed to the fact that the amount of extracted sulfur compounds needed to achieve extraction equilibrium decreases because of the presence of residual sulfur compounds in the extract phase, which would accumulate during the extraction period. Although this phenomenon does not favor the continuous usage of the solvent, especially in large capacity, removing the residual sulfur compounds from the extract phase after each extraction cycle via solvent regeneration would greatly enhance their reusability.

Figure 10:

Effect of repetitive usage of DES on its extraction efficiency (extraction conditions: 95 min, 30°C, and 0.5 solvent mass fraction and 500 ppm initial sulfur concentration in fuel).

3.3 Desulfurization of commercial diesel

For better practical picture of the desulfurization performance of the DES, commercial-grade diesel fuels obtained from two commercial fuel stations in Oman were desulfurized using the new solvent in a similar way as the ones conducted on the simulated fuel. The measured basic physical properties of the diesel fuels are as shown in Table 4. The results showed that the solvent exhibited 47% and 48% total sulfur removal from the diesel fuels from station A and station B, respectively, as shown in Figure 11.

Figure 11:

Desulfurization performance of DES with commercial diesel (extraction conditions: 95 min, 30°C, and 0.5 solvent mass fraction).

For comparison, the desulfurization performance of the DES on the simulated fuel was evaluated on the basis of total sulfur removal. The result, shown in Figure 11, showed that the DES removed 62% of the total sulfur present in the simulated fuel. The reason for the observed higher sulfur removal from the simulated fuel than that from the commercial diesel fuel could be attributed to the presence of various hydrocarbon compounds in the diesel fuel. These are not present in the simulated fuel. In addition, the presence of heavier alkyl-substituted thiophenic sulfur compounds such as 4,6-dimethyl DBT contributes to this behavior. These compounds are more difficult to desulfurize than DBT and thiophene [29].

3.4 Deep desulfurization of fuel

The ultimate target for this work is to lower the sulfur content of liquid fuels to below the environmentally set limits (<50 ppmw). However, going by the studies conducted in this work, achieving this target would require a very high solvent mass fraction (>0.8) that could be economically unreasonable to implement especially in large scale operations. For this reason, a multistage batch extraction process was used to achieve the target and was conducted in such a way that fresh solvent was added to the raffinate of the previous stage. By this procedure, the concentrations of both DBT and thiophene in the simulated fuel were reduced to less than 10 and 40 ppmw, respectively, after four extraction stages, as shown in Figure 12. Also by this procedure, the total sulfur content of the commercial diesel was reduced to less than 15 ppmw in five extraction stages, as shown in Figure 13.

Figure 12:

Deep desulfurization of simulated fuel.

Figure 13:

Deep desulfurization of commercial diesel.

The reason behind achieving the deep desulfurization of the simulated fuel in lower number of stages than that of the commercial diesel is possibly due to the presence of other hydrocarbons and heavier alkyl-substituted thiophenic compounds in the commercial diesel. These are not present in the simulated fuel, thus affecting the sulfur extraction efficiency of the DES as earlier identified. However, the overall deep desulfurizations of both fuels in this work were achieved in lower number of stages than those achieved with FeCl₃-based DESs [17] because of the higher sulfur extraction efficiency exhibited by the imidazole containing DES as compared with its FeCl₃-based counterparts.

3.5 Solvent regeneration

The regeneration of the DES was successfully conducted via the following procedure: the separation of the raffinate and extract phases from each other, followed by the dissolution of the extract phase in water. The resulting solution was then filtered twice using nitrocellulose membrane filters of pore sizes 0.45 and 0.22 μm, respectively. Thereafter, the solvent was recovered as the residue in the distillation of the filtrate solution, performed in a rotary evaporator operated under vacuum. The solvent was subsequently dried for 4 h in an oven operated at 80°C.

Figure 14 a shows the extraction performance of the DES after various regeneration cycles. It can obviously be said from the figure that the sulfur extraction ability of the solvent was effectively regained after extraction with the simulated fuel. By this procedure, the solvent was successfully used in five successive extractions without significant decrease in both its DBT and thiophene removal efficiencies. By this procedure also, the solvent was equally regenerated after extraction with the commercial diesel, as shown in Figure 14B. The FT-IR spectra of the DES, shown in Figure 15, suggest that the chemical structure of the recovered solvent was no different from that of the fresh solvent and, therefore, remains intact after each regeneration cycle.

Figure 14:

Effect of solvent regeneration on the desulfurization ability of the DES: (A) simulated fuel, (B) commercial diesel.

Figure 15:

FT-IR of regenerated DESs at various regeneration cycles.