Efficient Impurity Removal from Model FCC Fuel in Millireactors Using Deep Eutectic Solvents

Abstract

The goal of strict fuel quality regulations is to decrease the levels of sulfur, nitrogen, and aromatic chemicals in gasoline, thereby enhancing environmental safety. Due to the high costs of hydrodenitrification and hydrodesulfurization, many studies are looking for alternative fuel-purifying processes. The straightforward extraction approach using deep eutectic solvents (DESs) has proven to result in the removal of impurities and the enhancement of gasoline quality. Seven DESs were employed in a batch extraction process to purify the model fuel. The TbabFa-0 solvent was chosen for extraction in millireactors with different lengths, volume flows, and solvent ratios. In the millireactor, a slug regime and a laminar flow pattern were established for every process condition. For the chosen process conditions, the diffusion coefficient, volumetric mass transfer coefficient, and distribution ratio were determined. Better separation of all three key components was achieved during extraction in a millireactor using TbabFa-0. The efficiency of extraction with regenerated solvent was lowered by a maximum of 8%, showing the possibility of performing extraction in a millireactor with solvent recirculation.

1. Introduction

The rapid expansion of the oil and transportation industries needs the most efficient use of the existing fossil fuel supply. Simultaneously, tight environmental standards compel the production of cleaner fuel with extremely low sulfur content. The combustion of fossil fuels causes the release of harmful pollutants such as SOx, NOx, and COx. Emissions cause environmental contamination and harm human health [1]. The current sulfur content in the European Union, the United States, and China is capped at 10 parts per million (ppm) [2]. These regulations regarding the environment will become more difficult to meet in the future as crude oil quality deteriorates, with increasing amounts of sulfur and aromatic content, as well as acid and heavy crudes [3]. Therefore, alternative methods of removing sulfur, nitrogen, and aromatic compounds are investigated with the goal of replacing the conventional fuel purification processes, called hydrodesulphurization and hydrodenitrification. The initiative for the replacement of conventional processes was the reduction in significant costs and the achievement of deep desulfurization and denitrification. In response, an environmentally friendly and cost-effective extraction process using green solvents was investigated [2].

Extractive desulfurization is a desirable approach for fuel purification, especially if it is performed with ecologically acceptable solvents. It is typically carried out at atmospheric pressure and low temperatures and does not require extra energy input into the system. Despite many advantages, it is important to consider the potential drawbacks that might arise during the implementation of this methodology. Removing compounds containing S and N heteroatoms may lead to a reduction in the number of saturated olefins, which are essential constituents in the petroleum blend used in commercial motor vehicles. Furthermore, the efficacy of the extraction process is affected by the solubility of the compounds requiring elimination. Consequently, the selection of an ideal solvent is critical to the success of desulfurization [4].

Numerous studies using deep eutectic solvents (DESs) were carried out in an effort to replace hazardous volatile organic solvents, which are still commonly used in the industry today [5,6,7,8]. DESs are distinguished by the fact that they are inexpensive, nonvolatile, non-hazardous, simple to produce, and regenerable. Furthermore, these solvents offer the benefit of enabling processes to be conducted at room temperature and pressure conditions, thus decreasing purification costs and fulfilling the requirements of green chemistry [9,10,11]. However, DESs’ high viscosity—which might cause mixing difficulties during extraction—is one of its key disadvantages.

Millistructured reactors can be used to solve the issues that often arise when viscous liquids are mixed. In general, millireactors have significant advantages over conventional reactors in terms of energy efficiency, reaction rate and yield, safety, reliability, and on-site production [12,13] The millireactor’s high surface-to-volume ratio enables efficient phase interaction, leading to improved fuel purification and reduced solvent usage. A continuous flow of solvents is guaranteed by the use of accurate piston pumps. Mass transfer occurs at the moment of first contact and continues across the millireactor and phase boundary. The shape of the millimixer, which is often a Y- or T-shape, determines the flow patterns in the millireactor. This is particularly evident in multiphase systems, where the flow pattern, except for the geometry of the mixer, depends on the velocities of different phases [14].

Seven DESs were used in a batch liquid extraction to purify the model, fluidized catalytic cracking gasoline, in order to investigate the potential of removing toluene, sulfur, and nitrogen compounds from the fuel. DESs with various features and physicochemical properties were examined, enabling us to identify the factors that have the greatest effect on the extraction process. The DES that yielded the most favorable results was used in a fused filament fabrication (FFF) millireactor, and its results were compared with those obtained during the bulk extraction procedure.

2. Materials and Methods

2.1. Materials

Table 1. List and specifications of used chemicals.

Chemicals	M, gmol−1	Purity, %	Manufacturer
Betaine	117.15	98.0	Lach-Ner
Choline chloride	139.62	99.0	Acros Organics
Citric acid	119.12	95.00	Lach-Ner
Ethylene glycol	62.07	99.96	Lach-Ner
Formic acid	46.03	98.0	Lach-Ner
D-fructose	180.16	99.00	Lach-Ner
Glycerol	92.09	99.70	BDH Prolabo
Isooctane	114.23	99.8	BDH Prolabo
Malic acid	134.09	99.0+	Acros Organics
n-heptane	100.21	99.0	Carlo Erba
n-hexane	86.18	≥95.0	Carlo Erba
Nile red	318.37	-	Acros Organics
Pyridine	79.10	≥99.0	Acros Organics
Tetrabutylammonium bromide	322.36	99.0+	Acros Organics
Thiophene	84.14	99.0+	Acros Organics
Toluene	92.14	99.96	Lach-Ner

lists the chemicals used for the DESs and the model fuel preparation. The chemicals were used without further purification except for hygroscopic components (choline chloride, ethylene glycol, and malic acid) that were dried for 8 h at 60 °C in a vacuum oven. The content of water in other chemicals was less than 1%.

2.2. Preparation of Deep Eutectic Solvents and Model Fuel

DESs were produced by combining hydrogen bond acceptor (HBA) and hydrogen bond donor (HBD) in a specified molar ratio (

Table 2. Compositions and abbreviations of DESs used in this study.

DES	HBA	HBD	wH2O %	Molar Ratio
BGly-0	Betaine	Glycerol	0	1:3
ChCit-30	Choline chloride	Citric acid	30	2:1
ChEg-0		Ethylene glycol	0	1:2.5
ChGly-0		Glycerol	0	1:2
ChMa-30		Malic acid	30	1:1
MaFruGly-30	Malic acid	Fructosa/glycerol	0	1:1:1
TbabFa-0	Tetrabutylammonium bromide	Formic acid	0	1:1

). The materials were mixed in the IKA RV 10 basic rotating evaporator at a temperature of 60 °C until a uniform and clear liquid was obtained. After the formation of a transparent liquid without any crystalline substances, 30% by weight of water (w) was added to DESs ChMa and ChCit in order to decrease the viscosity.

The model fuel was prepared by the gravimetric method, containing six components (

Table 3. Composition of the model fuel [7].

Components	w, wt%
n-Hexane	26
n-Heptane	26
Isooctan	26
Pyridine	6
Tiophen	6
Toluene	10

).

2.3. Physicochemical Properties of Deep Eutectic Solvents

The Brookfield DV-III Ultra Programmable Rheometer and spindle SC4-21T (Middleborou, MA, USA) were used for the determination of the DESs’ viscosities. Densities were measured with a Mettler Toledo Densito 30PX densitometer (Columbus, OH, USA). pH was measured by a WTW inoLab pH/Cond 740 multi-meter (London, UK). The polarities of DESs were estimated using Nile red as a solvatochromic probe, as described in [15]. The absorption spectra of the dye were acquired with Shimadzu spectrophotometer UV-1280 (Kyoto, Japan). The surface tension of all DESs was measured using a Data Physics Instruments goniometer, OCA 20 (Stuttgart, Germany).

2.4. Batch Extraction

The solvent that was most appropriate for liquid extraction was determined using the IKA KS 3000i control shaker (Staufen, Germany). To attain equilibrium, the mixture of the DES and FCC fuel was mixed for 24 h at 25 °C and 200 min⁻¹. After shaking, the mixture was left for phase separation. The concentration of key components was determined by gas chromatography [16].

2.5. Chromatographic Analysis

The concentration of the model fuel components after extraction was evaluated using gas chromatography (GC). The GC used was a Shimadzu GC-2014 (Kyoto, Japan) equipped with an FID detector and a Zebron Phase ZB-1 GC column (30 m × 0.53 mm × 1.50 mm). The GC solution Software (Shimadzu, Kyoto, Japan) program settings for the analyses used in this study were identical to those previously reported in the literature [16].

The desulfurization efficiency was calculated using Equation (1).

$$\epsilon ={\displaystyle \frac{X_{\mathrm{F}}-X_{\mathrm{R}}}{X_{\mathrm{F}}}}$$

(1)

X represents the mass ratio for feed or raffinate phase.

2.6. Continuous Extraction in Millireactor

Three millireactors were manufactured using FFF technology and made from polylactic acid (PLA). These millireactors were employed for continuous extraction, with each having a different length but an equal inlet section. Figure 1 and

Table 4. Characteristics of millireactor.

Material	PLA
Pipe inner diameter, mm	2.5
Pipe length, mm	300, 500, 900

, respectively, display the experimental setup and characteristics of the milliextractor. The milliextractor received fuel and DESs continuously from two syringe pumps through a 120°-angled Y-shaped inlet, which ensured phase contact [17].

At flow rates of 100 to 1000 mL/min and at solvent ratios of 0.5, 1, and 2, continuous extraction in millireactors of various lengths was provided. (Table 4). For defining the flow pattern in the milliextractor, the Weber, Reynolds, and capillary numbers were calculated according to the following equations:

$$\mathrm{We}={\displaystyle \frac{\rho ·v^2·L}{\sigma }}$$

(2)

$$\mathrm{Re}={\displaystyle \frac{\rho ·v·d}{\mu }}$$

(3)

$$\mathrm{Ca}={\displaystyle \frac{\mu ·v}{\sigma }}$$

(4)

Symbols in the equations represent as follows:

ρ—density, kg∙m⁻³

v—velocity, m∙s⁻¹

L—length of millireactor, m

σ—surface tension

μ—dynamic viscosity, Pa∙s

The retention τ time in the millireactor (

Table 5. The retention time τ at different volume flow and millireactor lengths for solvent ratio S = 1.

QR, μL∙min	1000	800	400	400	200	400	200	100	200	100	100
QE, μL∙min	668	535	267	267	134	267	134	67	134	67	67
L, cm	50	50	30	50	30	90	50	30	90	50	90
τ, min	1.60	1.84	2.21	3.68	4.41	6.62	7.35	8.82	13.24	14.71	26.47

) was calculated according to the following equation:

$$\tau ={\displaystyle \frac{V}{Q_{\mathrm{R}}+Q_E}}$$

(5)

where V represents volume of the millireactor and Q_R and Q_E represent volume flow for the raffinate and extract phases.

2.7. Volumetric Mass Transfer Coefficient Calculation

In order to quantify the performance of the extraction process, the distribution ratio (β) and volumetric mass transfer coefficient were calculated (K_r∙a). The distribution ratio determines the quantity of the dissolved component in two liquid phases under equilibrium conditions and is determined from the formula

$${\beta }_i={\displaystyle \frac{X_{\mathrm{Ri}}}{Y_{\mathrm{Ei}}}}$$

(6)

where X and Y are the mass ratios in the raffinate and extract phases and subscript i denotes the solute.

The volumetric mass transfer coefficient is used to evaluate the efficiency of the extraction procedures. For the millireactor system, it may be obtained by calculating the driving force for the raffinate phase using known mass ratio and distribution ratios.

$$K_{\mathrm{R}}·a=Q_{\mathrm{R}}·{\displaystyle \frac{\left(X_{\mathrm{F}}-X_{\mathrm{R}}\right)}{V·\mathsf{\Delta }{X}_{\ln }}}$$

(7)

$$\mathsf{\Delta }{X}_{\mathrm{LN}}={\displaystyle \frac{\left(X_{\mathrm{R}}-\frac{Y_{\mathrm{E}}}{\beta }\right)-\left(X_{\mathrm{F}}-\frac{Y_{\mathrm{F}}}{\beta }\right)}{\ln \frac{\left(X_{\mathrm{R}}-\frac{Y_{\mathrm{E}}}{\beta }\right)}{\left(X_{\mathrm{F}}-\frac{Y_{\mathrm{F}}}{\beta }\right)}}}$$

(8)

2.8. Diffusion Coefficients

For the estimation of the diffusion coefficient, D different equations were applied (

Table 6. Correlations for diffusion coefficients.

Correlation	Formula	References
Wilke–Chang	$D_{AB}={\displaystyle \frac{7.4·{10}^{-8}{\left({\phi }_B{M}_B\right)}^{1/2}T}{{\mu }_B{V}_A^{0.6}}}$	[18]
Stokes–Einstein	$D_{AB}={\displaystyle \frac{9.96·{10}^{-16}T}{{{\mu }_B·V}_A^{1/3}}}$	[19,20]
Siddiqi–Lucas	$D_{AB}=2.98·{10}^{-7}·{\displaystyle \frac{T}{V_A^{0.5473}{\mu }_B^{1.026}}}$	[21,22]
Polson	$D_{AB}={\displaystyle \frac{9.4·{10}^{-15}T}{{{\mu }_B·M}_A^{1/3}}}$	[19,20]
Scheibel	$D_{AB}={\displaystyle \frac{8.2·{10}^{-8}T}{{\mu }_B{V}_A^{1/3}}}\left[1+{\left({\displaystyle \frac{3V_B}{V_A}}\right)}^{2/3}\right]$	[22,23]

).

T, M, and φ denote the values of the temperature, molar mass, and volume ratio. Indexes A and B represent components A and B.

2.9. Regeneration of DES

In order to evaluate the feasibility of solvent reuse, TbabFa-0 was regenerated by evaporation in a vacuum evaporator (Laborota 4000 Heidolph) at low pressure and at 60 °C for two and a half hours, or until all of the extracted components (thiophene, pyridine, and toluene) were removed. TbabFa-0 was selected for millireactor extraction and regeneration since it was the best of the tested DESs in terms of extracting all three components from the model fuel. To determine the effectiveness of the regeneration process, samples of the regenerated DES and pure DES were analyzed using Fourier-transformed infrared spectroscopy. The analysis was carried out using a Vertex 70 FTIR (Billerica, MA, USA) spectrophotometer equipped with a platinum detector.

3. Results and Discussion

3.1. Batch Extraction

To determine suitable solvents for the model fuel purification, batch extraction of thio-phene, pyridine, and toluene was carried out using seven DESs produced by mixing various HBAs and HBDs (Table 2). In order to prove the formation of DESs, FTIR analysis was performed on all generated DESs, as well as the components involved in their production, that can be found in the Supplementary Material, Figures S1–S7. The FTIR analysis confirms the existence of a eutectic system by showing changes in characteristic peaks when the spectrum of DESs is compared to that of their individual parts. This shift usually suggests the formation of strong hydrogen bonds between the donor and acceptor molecules. Figures S1–S7 show shifts in frequency, or peak broadening, for every DES. Furthermore, the absence of any new peaks in the provided FTIR analysis indicates a lack of chemical interaction among the DES components [24,25]. To achieve equilibrium, the extraction procedure was carried out on a laboratory shaker for 24 h at a temperature of 25 °C.

The physical properties of the DESs vary according to their composition (HBA and HBD), and their values are shown in

Table 7. Physicochemical properties of used solvents.

DES	ρ, g cm−3	μ, Pa s	pH	ENR kcal mol−1	σ, mN m−1
BGly-0	1.237	2.2415	8.83	51.37	49.11
ChCit-30	1.196	0.0168	0.93	49.70	41.26
ChEg-0	1.119	0.0356	4.14	51.24	49.29
ChGly-0	1.986	0.3510	6.13	51.11	52.26
ChMa-30	1.191	0.0179	0.67	49.52	52.48
MaFruGly-30	1.295	0.0361	0.92	49.73	57.74
TbabFa-0	1.077	1.0874	0.94	51.22	40.99
FCC	0.720	0.0046		-	21.3

E_NR represents polarity of solvents.

. The density, viscosity, pH, polarity, and surface tension of DESs have been measured to determine the impact of the DESs’ physical properties on extraction efficiency. The densities of all DESs are higher than that of the model fuel, resulting in easy phase separation after extraction. A key variable that affects the hydrodynamic conditions during mixing in a batch process or the type of flow pattern in channels during a continuous operation is the viscosity of the applied DESs [26]. The used DESs show a broad range of viscosity, from 0.0168 Pa·s to 2.2415 Pa·s, which may impact hydrodynamic conditions and in turn extraction efficiency. The acidity of the DESs has significant effects on extraction efficiency; hence, DESs with varying acidities were used. DESs prepared with acid have a pH value below one that is high acidic, while ChEg-0 is slightly acidic due to the presence of hydroxyl groups in ethylene glycol, and BGly-0 is mildly basic (pH value 8.50). The most polar DES (ChMa-30 49.52 kcal mol⁻¹) and the least polar DES (BGly-0 51.37 kcal mol⁻¹) vary in polarity by 1.85 kcal mol⁻¹, which is not significant. Generally, the high polarity of DESs impacts their capacity to separate various components. As the compounds extracted from the fuel differ significantly in polarity, it is expected that this will affect the extraction efficiency. Regardless of differences in the physicochemical characteristics of the applied DESs, pyridine, with the highest polarity of the three extracted components [27], is the easiest to separate from the fuel. Applying DESs with a pH lower than one almost completely removes pyridine. Extraction with acidic DESs prepared with choline chloride as a HBA (ChMA and CHCIT) achieved the highest efficiency values for pyridine removal. DESs that contain the –COOH group enhance hydrogen bonding interaction between the DES and pyridine due to increased dipole strength, and it is already known that the hydrogen bond between the nitrogen compound and the DES is the main driving force for mass transfer in the extraction process [22].

Also, even though they had a high viscosity, DESs made without water and DESs that were less acidic and polar worked better for extracting thiophene (Table 7 and Figure 2). The presence of water in DESs has a negative effect on the extraction of thiophene, while on the other hand, the addition of water has no effect on the extraction of pyridine from the model fuel [4]. Also, HBAs have a strong influence on the extraction of thiophene from the model fuel, i.e., for this specific purification process, tetrabutilammonium bromide is more favored as HBA than choline chloride or betaine. The formation of hydrogen bonds between the extracted component and the DES is the main driving force for the extractive desulfurization, as pointed out by Li et al. [28].

TbabFa-0 was the only DES tested that was effective in removing toluene from the model fuel, while the other DESs failed to provide any results in the extraction procedure. Extraction efficiency with TbabFa-0 was around 23%. This research demonstrates that HBA have a greater impact on extraction than HBD. DESs with formic acid as a HBD have one C=O bond, which may enhance the extraction process due to the π-π interaction between C=O bound to TbabFa-0 and toluene [29,30].

Although the extraction efficiency could be limited by the physicochemical properties of solvents (viscosity, surface tension, and polarity of the DES), results of the batch extractions show that solvent selection and their affinity to dissolve some components play a key role in the extraction efficiency (Figure 2).

By summarizing the above results from the batch extractions, conducted with different solvents, the most suitable solvent for the extraction process was TbabFa-0. Therefore, the experiments in the millireactor were performed using TbabFa-0.

3.2. Extraction in Millireactor

The concept of minimizing chemical processes has multiple advantages and has gained significant interest in the extraction field. Using microreactors or millireactors in the extraction process has many benefits, such as the ability to lower the amount of reagents needed and keep a steady laminar flow at high shear rates, which can ensure the suitable flow. Additionally, the high surface-to-volume ratio of narrow channels can result in exceptional efficiency for heat and mass transfer [31].

TbabFa-0 was selected for further research to investigate the potential for enhancing the extraction of thiophene, pyridine, and toluene from the model fuel using millireactors. This was due to its capacity to separate all three components from the fuel during batch extraction, thereby providing a potential for improvement in all three components. The hydrodynamic conditions, optimal retention time, and flow regime were all characterized in millireactors. The effect of the DES-to-feed mass ratio and the millireactor length on extraction performance was investigated.

3.2.1. The Flow Behavior

The flow behavior was determined by observation during experiments (Figure 3) and confirmed by values of dimensionless parameter groups such as the capillary, Weber, and Reynolds numbers (

Table 8. Values of Reynolds, capillary, and Weber numbers of the phases.

Phase	Re	Ca	We	Flow Pattern
FCC	2.68–1.34·101	1.45·10−5–7.27·10−5	1.09·10−4–2.73·10−3	Slug
TbabFa-0	7.17·10−4–3.59·10−3	3.01·10−2–1.50·10−1	4.03·10−5–1.01·10−3	Slug

).

During all the millireactor experiments, a slug flow regime occurs when the capillary and Weber numbers for both phases are low, namely at Ca < 0.01 and We < 1 [32,33]. The measured values of Ca = 1.5·10⁻¹–1.5·10⁻⁵ and We = 4.03·10⁻⁵–2.7·10⁻³ show that surface tension is stronger than viscous and inertial forces in the systems that were studied. This results in the formation of a slug flow regime inside the millireactor that ensures a uniform interfacial surface and thus efficient mass transfer [34].

The Reynolds numbers in examined process conditions were less than 100 (ranging from 7.17·10⁻⁴ to 2.68), indicating laminar flow. All of the aforementioned facts support the observation of a slug flow for all process conditions in the experiments (Figure 3). The movement of fluid elements within the narrow channel generates internal vortices that enhance mass transfer [17,32]. As a result, the slug flow regime in the millireactor is expected to offer superior mass transfer compared to other flow regimes [35].

3.2.2. Influence of Retention Time on Extraction Efficiency

The retention time in the continuous extraction process was varied by modifying the flow rates of solvents and by changing millireactor lengths (Table 5). Figure 4 shows the extraction efficiency of pyridine, thiophene, and toluene as a function of retention time in a defined slug flow regime. Extending the retention time from 1.6 to 26.5 min improved the extraction efficiency. Shorter retention times led to lower extraction efficiency, as the retention time became insufficient for achieving stationary conditions. Equilibrium was attained after 14 min for all extracted components. This retention time was achieved in a reactor with a length of 50 cm and at a volumetric flow rate of 100 μL/min for the FCC gasoline and 66 μL/min for TbabFa-0 (S = 1). Since the used apparatus could not achieve lower flow rates, extending the millireactor’s length is a more effective way to increase the retention time. Specifically, Figure 4 demonstrates a trend in increasing of the extraction process efficiency for all three components, irrespective of the way used to achieve the retention time.

The results show that, similar to batch extraction, TbabaFa-0 extraction is more successful in extracting pyridine, followed by thiophene and toluene. More than 96% of the pyridine, 75% of the toluene, and 14% of the toluene were removed after 14 min of extraction in the millireactor. The extraction efficiency of pyridine, thiophene, and toluene was increased by 11.0%, 54.5%, and 37.6%, respectively, as compared to batch extraction. Furthermore, this improvement was achieved in a much shorter time period. The microfluidic system exhibits superior performance, particularly when slug flow exists, as evidenced by studies that compared batch and extraction in microchannels for various systems [34,35,36,37]. A similar pattern may be seen in the case of a millireactor, where the dimensions are such that they maintain the flow and distribution of forces inside the reactor. This enhancement is attributed to the internal circulations that are characteristic of the slug flow, which significantly improve convection within each liquid phase [35]. In addition to the more intense mixing, the millireactor’s higher pressure (approximately 2 bars) [38] in comparison to the batch reactor may be another contributing factor.

3.2.3. Impact of Solvent Ratio on Extraction Efficiency

Despite the relatively inexpensive and simple production of DESs, the industrial processes of extractive desulfurization and denitrification prefer to minimize the amount of selective solvent (DES) to achieve optimal results [28]. Figure 5 presents the extraction results for a single key component using the DES TbabFa-0 at different solvent ratios. Increasing the amount of DES enhances the extraction of all key components. To identify the optimal solvent ratio, the specific mass of the extracted component was measured at various DESs-to-feed mass ratio (

Table 9. The specific mass of the extracted key component m_s for different DES-to-feed mass ratios.

S, kg kg−1		0.5	1	2
Key Components	DES	ms, gkc kgDES−1
Thiophene	TbabFa-0	38.38	31.26	19.97
Pyridine		75.44	51.41	27.59
Toluene		26.86	24.33	21.48

). The results indicate that the highest DES-to-feed mass ratio resulted in the lowest specific removal of the key component, while the lowest mass ratio provided the most effective removal per kilogram of solvent used. Although a larger amount of DES improves extraction efficiency, the results do not justify increasing the DES-to-feed mass ratio. The DES-to-feed mass ratio depends on the importance of the purification process, the cost of the secondary solvent, and the specific application [39].

3.2.4. Volumetric Mass Transfer Coefficient and Diffusion Coefficient

Figure 6 shows the volumetric mass transfer coefficients, K_r·a, for the raffinate phase in extraction with TbabFa-0 at a DES-to-feed mass ratio of one. As the retention time increases, the volumetric mass transfer coefficients for all key components increase until they reach a stationary condition. The removal of pyridine yielded the highest volumetric mass transfer coefficients, whereas toluene yielded the lowest volumetric mass transfer coefficients, which is in coincidence with extraction efficiency results. The volumetric mass transfer coefficient reflects both the efficiency of mass transfer (through the mass transfer coefficient) and the extent of the interfacial area available for that transfer. In this case, the mass transfer coefficient is the primary factor determining the value of the volumetric mass transfer coefficient, as surface tension forces outweigh inertial forces at low flow rates, resulting in a stable slug flow regime and a constant interfacial area. As the efficiency of extraction and subsequent mass transfer increases with retention time (Figure 6), the volumetric mass transfer coefficient increases.

The diffusion coefficient is crucial to the design and development of DES liquid extraction because it affects the efficiency process and is the primary mechanism of mass transfer through liquids. Therefore, different equations were used to calculate the diffusion coefficients during the extraction of key components using TbabFa-0 (Table 6). The diffusion coefficient values that were obtained are lower (

Table 10. Diffusion coefficient calculated for extraction of pyridine, thiophene, and toluene for extraction with TbabFa-0.

Correlation	DAB, m2 s−1
	Thiophene	Pyridine	Toluene
Wilke–Chang	1.15∙10−12	1.12∙10−12	9.50∙10−13
Stokes–Einstein	3.94∙10−12	3.88∙10−12	3.54∙10−12
Siddiqi–Lucas	3.76∙10−13	3.66∙10−13	3.15∙10−13
Polson	3.75∙10−13	3.83∙10−13	3.64∙10−13
Scheibel	1.33∙10−12	1.27∙10−12	1.02∙10−12

) than the literature values for mass transfer in liquid–liquid systems (D = 10⁻⁹–10⁻¹⁰ m²∙s⁻¹) [40]. It is known that the order of magnitude of the diffusion coefficient in liquid extraction with DESs is typically lower for many solutes than in conventional solvents [32]. DESs often have significantly higher viscosities than traditional organic solvents or water. Higher viscosity leads to lower diffusion coefficients because the solute molecules encounter more resistance as they move through the solvent. Furthermore, DESs have a complex hydrogen bonding network, which can further slow down the movement of solute molecules, reducing the diffusion coefficient compared to conventional solvents [41].

The diffusion coefficient, depending on the correlation used, takes into account the solvent’s molar mass, the key component’s molar volume, the solvent’s viscosity, and the temperature. All of the correlations used give approximately similar diffusion coefficient values (within the same order of magnitude), except for the Stokes–Einstein correlation, which gives one order of magnitude higher values. The Stokes–Einstein correlation can be used in the systems where the dissolved key component’s molecule diameter is smaller than the solvent molecule’s diameter [42,43]. However, this is not the case in the tested systems. Therefore, we can conclude that this correlation is not appropriate for the tested systems. Upon comparing the diffusion coefficients of the key components, we observe that thiophene exhibits marginally higher values than pyridine and toluene across all correlations. However, Polson’s correlation, the sole one that considers the molar mass of the extracted component, yields the highest diffusion coefficient value for pyridine, followed by thiophene, and the lowest for toluene. This calculation aligns with the experimental data from the extractions. The obtained diffusion coefficient values suggest that the equations for estimating the diffusion coefficient must consider the affinity of DESs for the dissolution of specific key components.

3.2.5. Reuse of Regenerated DES

From an ecological and economic point of view, the possibility of regeneration and reuse of DESs in the extraction process is a significant advantage compared to conventional desulfurization and denitrification procedures. The results of extracting thiophene and pyridine from the model fuel using a fresh and a regenerated DES (TbabFa-0) demonstrated that the extraction ability of the DES did not significantly change, indicating its successful regeneration. The largest reduction in extraction efficiency for all components was determined to be less than 8%, indicating the potential for the effective use of regenerated DESs (

Table 11. Comparison of extraction efficiency of fresh and regenerated TbabFa-0.

TbabFa-0	ε, %
	Thiophene	Pyridine	Toluene
Fresh	86.6	48.9	22.59
Regenerated	79.7	46.5	20.86
Deviation, %	7.97	4.91	7.66

).

The amount of regeneration rounds without significant DES performance reduction varies per instance. Some researchers have successfully regenerated DESs several times without losing any qualities, while others have observed somewhat fewer cycles. This is dependent on a number of factors, including the kind of DES, the process conditions under which it is used, the kind of contamination, and the regeneration method. The literature describes effective DES regeneration utilizing various approaches. These methods included rinsing with water [44], rinsing with water followed by a distillation procedure [45], and rinsing with diethyl ether followed by a heating procedure at 105 °C [46]. Although the method of solvent regeneration used in this work is carried out at a high temperature and reduced pressure, it is very simple and quick to implement.

FTIR analysis was employed for a comparison between the fresh, after-extraction, and regenerated DES. The peak characteristics of Tbab-Fa-0 appear in all three spectra. The maxima of the absorption bands are located at the same wavenumbers as those of the fresh DES (Figure 7). The FTIR spectra are same in the functional group region, exhibiting absorption maxima at 2958 cm⁻¹ and 2878 cm⁻¹, which correspond to C-H stretching. The absorption maximum at 2652 cm⁻¹ corresponds to O-H stretching, the absorption maximum at 1743 cm⁻¹ to C=O stretching, the vibrations at 1465 cm⁻¹ to C-H bending, the absorption maximum at 1165 cm⁻¹ to C-O stretching, and the absorption maximum at 879 cm⁻¹ in the fingerprint region to C-H bending. After extraction, the FTIR spectra (Figure 7) for DESs show some extra peaks that correspond to the removed components (1595, 1581, 1437, 990, 708, 668, and 603 cm⁻¹). A couple of these peaks (990 and 708) can also be found in the FTIR spectra of the regenerated DES. This indicates that certain removed components remain even after DES regeneration. The peak at 990 cm⁻¹ indicates that toluene was not completely removed from the regenerated DES, while the peak at 708 cm⁻¹ is expected for toluene.

The effective and easy regeneration of the DES allows it to be reused in industrial processes, resulting in cost savings [47]. However, determining the number of cycles necessary for the effective removal of impurities is crucial in order to determine the ideal regeneration frequency, as the prolonged usage of a DES lowers its capacity.

4. Conclusions

A batch extraction was conducted with seven DESs to determine the most suitable DES for the purification of the FCC model fuel in a millireactor. Thiophene extraction is more successful with DESs that are prepared without water, less acidic, and less polar (TbabFa-0, ChEg-0, ChGly-0, and BGly-0). Pyridine, a more polar component, is extracted more effectively by more acidic and polar solvents that contain water (ChMa-30, ChCit-30, and MaFruGly-30). The DES TbabFa-0 was the only solvent that successfully extracted toluene.

A slug flow regime was observed in all of the millireactor experiments, resulting in superior outcomes when compared with batch extraction. The extraction efficacy of all key components was enhanced as a result of a longer millireactor (50 and 90 cm) and a higher DES-to-feed mass ratio. The Polson correlation yielded the best agreement between the estimated diffusion coefficient values and the extraction efficiency results. The structure of the regenerated DES did not change according to FTIR analysis, while the extraction efficiency of the regenerated TbabFa-0 was slightly lower than that of the fresh DES. These facts suggest that a circular fuel purification process could be implemented in a millireactor through the use of a straightforward method of DES regeneration.