Efficient immobilization of acids into activated carbon for high durability and continuous desulfurization of diesel fuel

Fuel contaminants are the most critical issues in human health, cars' engines life and performance, and refinery equipment, and have an ultimate consecutive economic negative impact over time. Nowadays, sulfur is considered as one of the most dangerous contaminants in fuel, and different approaches have been carried out for desulfurization processing. In this study, new modified activated carbon (AC) for adsorptive diesel fuel desulfurization is carried out. To increase the robustness and effectiveness of a continuous adsorptive desulfurization (ADS) process, the study provides a new surface modification technique of the AC. Real diesel fuel (RDF, sulfur content 7510 ppm) and model diesel fuel (MDF, dibenzothiophene content 637 ppm) were used in a series of ADS tests carried out in a fixed‐bed adsorption column under atmospheric pressure and temperature. To achieve zero emissions of diesel fuel, the process parameters were optimized using the experimental findings. The AC and modified AC were tested via Brunauer–Emmett–Teller and Fourier Transform Infrared to evaluate the effect of the modification process on adsorbent characterization. The liquid hourly space velocity of the feedstock, the height of the absorber bed, and the kind of AC were the optimization parameters. In these circumstances, the ADS procedure was examined to address the impact of the feedstock flow rate on the effectiveness of ADS. The sulfur removal efficiency was 85.3% for RDF and 94% for MDF at 9 cm bed height, 8 h−1 LSHV, and 10 g bed weight, respectively. This is the first time that AC has been used to examine the stability of continuous ADS and to examine how acid immobilization affects sulfur removal. The investigation showed that the modified AC had a high rate of stability and was effective in the continuous ADS process. The spent modified AC was regenerated via a solvent extractive regeneration process to evaluate the durability of the ACs. The results proved that the regeneration performance of various solvents for modified activated carbon 1 decreases as follows: iso‐octane > ethanol > methanol > acetonitrile.


| INTRODUCTION
2][3][4][5] Disulfides, mercaptans, thiophenes (TH), and their derivatives, such as pentathiophene (BT) and dibenzothiophene (DBT), are typical organic sulfur compounds (OSCs) found in fuels.[8][9][10][11][12][13] Adsorptive desulfurization (ADS) is one of the alternatives to hydrodesulfurization (HDS) that has a high cost of production because it requires harsh working conditions.5][16][17][18][19][20] As a result, it is used to both produce fuels free of sulfur and lower the investment costs related to the HDS's challenging working conditions (high temperature and pressure).8][29] The limited implications of low cost, durability, and selectivity to absorb the most persistent sulfur component in the fuel make choosing an adsorbent difficult.Numerous desulfurization sorbents, including zeolites, [30][31][32][33] metal-natural frameworks, [34][35][36][37] and activated carbons (ACs), 1,[38][39][40][41][42] have been studied in the literature.Carbon-based adsorbents outperformed the other types of ADS sorbents in terms of performance thanks to a number of attributes, like, operational safety and cheap operating costs. 27,43,44oth the electrochemical use of ACs [45][46][47][48] and the adsorption of pollutants [49][50][51][52][53][54][55][56] have been studied.Additionally, ACs were recognized and widely employed as sulfur compound adsorbents due to their highprecision surface areas, high degrees of porosity, and superior adsorption potential.In terms of pore size distributions, [57][58][59] ACs have a wider range of pore size distributions compared with other sorbents.1][62][63] This is the main cause of why ACs had been desulfurization sorbents with significant strengths.In general, the selective adsorption of sulfur compounds on the surface of AC directly depends on the number of oxygen-containing complexes. 38,64,65The oxygen-containing complexes are mostly created using dry or wet oxidation methods via acid treatment.7][68] Farzin Nejad et al. 69 investigated the use of exhausted mesoporous carbons with regeneration by different solvents (toluene, ethanol, and acetonitrile).They found that the desulfurization efficiency of the regenerated adsorbents after the primary regeneration cycle varies as follows: toluene » ethanol ≈ acetonitrile.To regenerate AC exhausted through model diesel fuel (MDF), three approaches (thermal regeneration, ultrasonication, and solvent extraction) were studied by Li et al. 70 High durability of the AC was achieved for MDF.Solvent extraction outperformed the opposite strategies.However, excessive temperature treatment of the spent AC resulted in a substantial lack of porosity and the functional groups of the surface, which resulted in low regeneration performance.The fact that the acids are immobilized on the surface of these solid adsorbents and that the link between the adsorbent and the sulfur compounds is relatively weak are the reasons for the poor renderability, which must be taken into consideration.Thus, the heteropolyacids would be frequently leached out from the adsorbent.Amer et al. investigated the performance of two adsorbents (potassium permanganate and potassium phosphate/ AC) on the removal of sulfur compounds from gasoil.The results explained that potassium permanganate/ AC achieved maximum adsorption efficiency in comparison with potassium phosphate. 71Jasim et al. investigated the activity of a novel 10% Mn/nano-AC for the oxidative desulfurization process in a digital basket reactor.The results proved that the efficiency of sulfur removal was 94% under a temperature of 80°C, a time of 35 min, and a stirring rate of 750 rpm. 8n the present study, AC was treated with different acids in an attempt to functionalize it, aiming for a higher rate of sulfur removal by immobilization of different concentrations of mixed acids.AC is a costeffective material that can be derived from different industrial wastes, and can withstand high pressure and temperature.It is impervious to breakage, and is simple to regenerate, thus it has a long service life.This study develops a cost-effective continuous ADS process at a high rate of durability by using acid immobilization on AC.The study examined different acids (nitric, sulfuric, and nitric/sulfuric acids) to immobilize AC.These acids will prolong the ADS time cycle.The effect of several adsorption parameters (type of feedstock, bed height, catalyst weight, and types of acid) on the performance of the adsorption process was investigated.The best adsorption conditions were applied in the second part of the study to investigate the effect of the feedstock flow rate on the effectiveness of the ADS process in the fixed-bed adsorption column (FBAC).The spent AC was regenerated by the solvent extractive regeneration process.This study showed significant improvement in the activity of the modified activated carbon towards the desulfurization process under mild operating conditions.

| EXPERIMENTAL WORK
Different steps have been carried out in this study to synthesize and evaluate the modified AC that was treated with different acids in a scalable FBAC.The modified and bare materials were both assessed by using some useful techniques as labeled in the following sections.

| Chemicals
The feedstocks used in the present study are MDF (total sulfur content 9 ppm) obtained from Pendik and a real diesel fuel (RDF) (total sulfur content 7510 ppm) obtained from KAR Refinery.The physical properties are shown in Table 1.
In this study, DBT was used as a model sulfur compound in the MDF.DBT was purchased from Alfa Aesar, the United Kingdom.The specifications of the DBT were listed in Supporting Information Table A1.Nitric acid and sulfuric acid were used for modifying the AC.The acids were obtained from Sigma-Aldrich Company.The specifications of the acids used are summarized in Supporting Information Table A2.Methanol, ethanol, acetonitrile, and iso-octane were used as solvents for the solvent extractive regeneration process of AC.The specifications of these solvents are summarized in Supporting Information Table A3.AC with a particle size of 1-3 mm was purchased from Applichem GmbH Company.

| Preparation of MDF
Analytical grade DBT was used without further purification.An aliquot of 0.6 g of DBT was dissolved in 10 mL of benzene and vigorously shaken until DBT was completely dissolved.The later solution was added to 1 L of diesel fuel (sulfur content less than 9 ppm).The new solution was highly stirred for producing efficient dispersion of DBT in diesel fuel.The produced solution was used as an MDF in the present study.The concentration of sulfur compounds in that diesel fuel became 600 ppm approximately and the diesel fuel was tagged as mimic diesel fuel (MDF).The concentration of 600 ppm was confirmed by analyzing the MDF in the X-ray sulfur content analyzer, the specifications of the analyzer are shown in Supporting Information Table A4, which showed at 637 ppm.This concentration was used as the initial sulfur content in the MDF.

| Acid modification of AC
AC (1-3 mm particle size), purchased from Applichem GmbH Company, was milled and sieved in 45-60 meshes.To remove contaminants and obtain high purity, the AC particles were washed several times in hot distilled water before soaking in different concentrations of different acids.About 10 mL of acid was used for treating each gram of AC.The suspension was sonicated for 3 h at 50°C in, then filtered using filtration paper and thoroughly washed with distilled water for removing any impurities.AC was dried overnight at 110°C in an oven and kept in a container for adsorption experiments.Fresh and modified ACs were labeled according to the type and concentration of acid that was used for the modification process as shown in Table 2.All acid modification processes were followed by NaOH treatment.The modifications cause significant changes in the chemical properties while leaving the physical properties nearly unchanged.The addition of NaOH increases the content of hydroxyl groups, whereas the addition of the acids increases the amount of single-bonded oxygen functional groups, such as phenols, ethers, and lactones. 72

| Characterization of AC
Adsorption usually occurs on the adsorbent surface, the higher surface area leads to more absorptivity and productivity.The Brunauer-Emmett-Teller (BET) was used to measure the surface area of the modified AC.
The AC was treated with different acids to create active functional groups on the surface of AC, so Fourier Transform Infrared (FTIR) was used to characterize the functional groups.

| Surface area and pore-volume measurements
The BET surface area of the ACs was measured via an N 2 adsorption-desorption isotherm.Adsorption isotherms were measured using a BELSORP-max BET surface area analyzer at 77 K. Before BET analysis, the samples were degassed at 250°C for 6 h at 52 bar.

| Fourier transform infrared
FTIR test was conducted to investigate the acid sites of fresh and modified AC.A Nicolet IS50 Fourier Transform instrument was used to record the FTIR spectra of ACs (Thermal Scientific).The spectra of various AC samples were recorded in transmission mode on a KBr wafer containing 0.1 wt% carbon.FTIR spectra of samples were obtained in the 4000-400 cm −1 wavenumber region with a resolution of 4 cm −1 .

| Scanning electron microscopy (SEM)
The SEM test was conducted in a Zeiss-EM10C-100KV FESEM device to explain the nature of the fresh and modified AC as an adsorbent.This analysis was conducted at Sharif University of Technology, Iran.

| Analysis of diesel fuel samples
The sulfur content of diesel fuel feedstocks (RDF and MDF) and treated samples after adsorption experiments were evaluated using an energy-dispersive X-ray fluorescence (EDXRF) instrument (RX-360SH, Tanaka Scientific Ltd.) in accordance with ISO 9454 and ASTM D4294-03.After each experiment, a diesel sample of 10 mL was filtered using filtration paper for removing any particles and analyzed via EDXRF apparatus.Equation ( 1) is used to compute the percent conversion of sulfur compound: where C S in is the initial concentration of sulfur compound and C S out is the final concentration of sulfur compound.

| Experimental procedure
The experimental work went through several steps as follows.
The continuous adsorption of sulfur compounds was carried out in an FBAC unit as shown in Figure 1.
A column with a diameter of 3 cm, a length of 34 cm, and a capacity of 240.21 cm 3 was utilized in the adsorption process.The feedstock was stored in a feed tank connected to a syringe pump (IML Company).The net fuel flow is produced by using a dosing pump with two pumping efficiencies (20% and 100% pumping efficiency).
The dosing pump was connected to the top of the FBAC with a customized Swagelok fitting and polyterafluoroethylene tubing.A shower distributor was fitted to the top of the column to ensure uniform distribution of the feed throughout the AC bed.
The tubular adsorption column is made of Pyrex (Corning), with an inner diameter of 3 and 34 cm in length.The adsorption column is divided into three sections: the top and bottom contain ceramic balls (to ensure complete adsorbent wetting and reduce radial dispersion) and the middle section was loaded with the AC.Different volumes present of ceramic balls were used (10%, 20%, and 30% by volume).The sizes of the ceramic balls are 3-5 mm.The treated oil feedstock was delivered to a product storage tank.

| ADS process
The RDF and MDF were used as oil feedstocks to evaluate the ADS process on fresh and modified AC.
RDF was obtained from Pendik produced by OMV, with an initial sulfur concentration of 7510 ppm.RDF and MDF feedstocks were used in the ADS process in the FBAC.To conduct the ADS experiments, the oil feedstock was fed to the column using the dosing pump.The vertical downward FBAC was utilized in the continuous adsorption of sulfur compounds, with a flow rate of 1 mL s −1 .The column was gradually loaded with layers of different ACs heights (3-9 cm), corresponding to different AC weights (10, 7, and 4 g).
The discharge flow rate is controlled and the flow rate in the FBAC was kept constant to achieve steady-state conditions.The product samples were taken from the exit of the FBAC upon approaching steady-state conditions and prepared for the sulfur content analysis.To confirm the precision of desulfurization experimental results, the desulfurization runs were repeated twice for each run.The dosing pump was turned off after completion of the runs, and all open valves were set to off.Table 3 shows the experimental variables of the present study based on the full factorial experimental design.
The experiments were conducted at atmospheric pressure and temperature.The experiments were conducted using RDF or MDF at different values of AC and bed height parameters based on Table 3.After evaluating the best condition for the desulfurization process in FBAC (modified activated carbon 1 [AC-M1] at sorbent bed height = 9 cm), the impact of different liquid hourly space velocities (LHSVs) on the performance of the process using RDF and MDF was conducted under optimal conditions to achieve the maximum conversion.

| Regeneration of the spent adsorbent
In a batch regeneration system, sulfur compounds were removed by washing the spent AC with different solvents (methanol, ethanol, acetonitrile, and isooctane).About 10 mL of these solvents' solutions was shacked with 1 g of AC for 30 min at an ambient temperature.Then, the regenerated adsorbent is filtered and dried in an oven at 110 ± 0°C for 3 h and reused for the next adsorption cycle.The regeneration efficiency was calculated using Equation ( 2): Regeneration efficiency in cycle (%) = Adsorption capacity of regenerated AC in cycle Adsorption capacity of fresh AC 100%, ( where i is the number of the adsorption-regeneration cycle.

| RESULTS AND DISCUSSION
The modified AC was characterized and examined as follows.

| Adsorbent characterization
To characterize the modified AC, two important techniques were used as stated below.

| Surface area and pore-volume analysis
The pore volume, surface area, and pore size of the fresh and modified AC are summarized in Table 4.
The results showed that the BET surface area of fresh AC is 908 m 2 g −1 , the average pore volume is 0.5125 cm 3 g −1 , and the average pore diameter is 2.2 nm.Compared with fresh AC, these characteristics of the AC modified by 30% H 2 SO 4 (AC-M1) have been improved as the surface area of AC-M1 was found at 1151 m 2 g −1 , the average pore volume was 0.615 cm 3 g −1 , and the average pore diameter is 3.7 nm.Compared with the fresh AC, the surface area, average pore volume, and average pore diameter of AC-M1 were all shown to be significantly improved.In the other modified ACs, the surface areas were improved compared with the AC but the AC-M1 showed superior surface properties over the others as the modification with 30% H 2 SO 4 generated a higher number of active sites that can be utilized for adsorption of sulfur compounds.The enhancement of the adsorbent activity is synergistic when the adsorbent is acid modified. 58Most of the adsorbents employed in the ADS process for the removal of the aromatic sulfur compounds have a mesoporous structure (2-50 nm), which is necessary for the adsorption of sulfur species because the pores must be of the same size for adsorption to occur. 59The surface area and pore volume of AC-M1 are both large, which is expected to improve the activity of the adsorbent to remove the sulfur compounds.Also, the improvement in the adsorption efficiency of AC-M1 in comparison with the fresh AC can be attributed to the enhancement in the porosity of the adsorbent via acid modification.

| Fourier transform infrared
The FTIR spectra of fresh adsorbent (AC) and modified adsorbent (30% and 65% H 2 SO 4 /65% HNO 3 [AC-M6]) were recorded in the wavenumber range of 3600-400 cm −1 to diagnose the functional groups that present in AC and acid-modified ACs as shown in Figure 2. The number of characteristic peaks of the AC and acid-treated samples was observed in all samples, indicating a larger number of hydroxyl groups formed on the surface of the AC at different acid modification doses. 73,74The response of all the AC samples shows almost the same transmittance with a minor exception at the band of 3734 cm −1 .At this point, a weak sharp transmittance band was present in the spectra of AC and ACM-3-ACM-6 samples and faded in AC-M2, and then inverted in the AC-M1 sample.That peak may be ascribed to the isolated O─H groups.These results clearly show a more uninsulated O─H group present in the AC-M1 adsorbent that would strengthen the AC activity for a higher rate of sulfur removal.

| Scanning electron microscopy
The morphology of the fresh and best-modified AC (30% sulfuric acid [AC-M1]) was analyzed by using the SEM test.The images of the fresh and modified AC were explained in Figure 3.The results of the SEM analysis prove that the bright spots in the pictures refer to good modification of the adsorbent surface by acid.These results showed that there is a good distribution of acid over the adsorbent surface due to an efficient impregnation process.

| ADS of MDF and RDF
The MDF and RDF were both used as a source of sulfur contaminant materials and were evaluated at their tendencies of sulfur element extortion via bare and treated AC as illustrated below.

| Effect of the acid type on sulfur removal efficiency
The influence of the type of acid used for the pretreatment of AC samples on sulfur removal from the RDF and MDF was investigated using different acid combinations (shown in Table 6) and adsorbent bed heights (3, 6, and 9 cm).AC was modified using sulfuric acid, nitric acid, and sulfuric acid/nitric acid mixture at different concentrations: 30% H 2 SO 4 (AC-M1), 30% HNO 3 (AC-M2), 65% H 2 SO 4 (AC-M3), 65% HNO 3 (AC-M4), 30% H 2 SO 4 /30% HNO 3 (AC-M5), and 65% H 2 SO 4 /65% HNO 3 (AC-M6).Figures 4 and 5 depict that by increasing the strength of the acid solution, the sulfur removal efficiency was reduced in the following order: AC-M1 > AC-M2 > AC-M3 > AC-M4 > AC-M5 > AC-M6 > AC.This behavior can be attributed to the fact that increasing the acid intensity or its concentration may negatively affect the surface area of AC.The maximum removal efficiency of sulfur compounds from RDF and MDF using the fresh and modified ACs at 9 cm adsorbent bed height was 78.4%, 75.99%, 73.44%, 70.21%, 69.03%, 67.30%, and 65.10% for RDF and 88.70%, 87.13%, 84.14%, 81%, 78.02, 76.77%, and 73.47% using AC-M1, AC-M2, AC-M3, AC-M4, AC-M5, AC-M6, and AC, respectively.Compared with the fresh AC, all the acid-treated ACs gained better sulfur removal efficiency.This improvement attributes to surface modification by the generation of functional groups on the AC surface, which affects the adsorption capacity of sulfur.The modification of AC can also affect the kinetics of the ADS process and the surface acidity. 75xperimentally, the optimal sulfur adsorptive rate was obtained using AC modified by 30% sulfuric acid (AC-M1) (78.4% for RDF and 88.70% for MDF).It was reported that a high concentration of oxygen functional group in sulfuric acid resulted in a higher conversion rate of sulfur in petroleum fuels. 76The results showed that sulfuric acid treatment affects the adsorption of the OSCs because it enhances the oxygen surface group type of AC.Hence, the adsorption capacity of these compounds increases as the size of the carboxylic acid group increases. 77It can be concluded that the AC treated with sulfuric acid at a fairly low concentration improves the reaction efficiency between the generated hydroxyl radicals and the adsorbed OSCs, which was not previously reported for a continuous adsorption process of sulfur compounds from petroleum fuels.Also, the rapid enhancement in the ADS performance using AC-M1 can be attributed to the improvement in the adsorbent characterization, like, the porosity of adsorbent via acid modification.Table 5 shows this finding versus the published relevant works.

| Effect of adsorbent bed height on sulfur removal efficiency
The adsorbent bed height was changed to examine the impact of the amount of adsorbent in the bed.Different amounts of AC were loaded, the height of the packed bed was chosen, and the impact of the adsorbent weight on the desulfurization performance was investigated.The influence of adsorbent bed height on the adsorption capacity of sulfur compounds was investigated at three different adsorbent bed heights 3, 6, and 9 cm at LHSV = 15 h −1 and with different ACs.Figures 6 and 7 show the experimental results of sulfur removal from RDF and MDF at different adsorbent bed heights.When the bed height was increased from 3 to 9 cm, the sulfur removal efficiency increased from 46% to 65% (AC), 64% to 78% (AC-M1), 60% to 76% (AC-M2), 59% to 73% (AC-M3), 56% to 70% (AC-M4), 53% to 69% (AC-M5), 50% to 67% (AC-M6) for RDF and from 63% to 73% (AC), 78%  The desulfurization efficiency in FBAC was significantly improved via modification AC (enhanced the adsorbent surface and factionalized groups for more sulfur compounds removal).
to 89% (AC-M1), 76% to 87% (AC-M2), 75% to 84% (AC-M3), 73% to 81% (AC-M4), 72% to 78% (AC-M5), and 68% to 77% (AC-M6) for MDF.Experimentally, as the height of the adsorbent bed increases, a longer contact time between the adsorbate and the adsorbent occurred. 78An increase in the adsorbent dose leads to an increase in adsorption sites, which in role leads to an increase in the contact area, available for adsorption and complete saturation, between the fuel and the AC. 79Therefore, for all types of fresh and modified AC, the adsorption capacity of sulfur compounds in RDF and MDF increases with increasing bed height at a constant flow rate (LSHV).The best experimental conditions were obtained using AC-M1 at adsorption bed height = 9 cm, LSHV of 8 h −1 for RDF and MDF, as the sulfur removal was 78.40% for RDF and 88.70% for MDF.Saleh et al. studied the performance of the ADS process of sulfur-containing compounds on AC-manganese oxide nanocomposites in a fixed-bed adsorption column. 80By increasing the amount of adsorbent from 0.01 to 0.5 g, the desulfurization efficiency was improved.Yahya and Hussein 79 studied the adsorption capacity of heavy naphtha using a molecular sieve (13×) in a fixed-bed column.They studied the effect of adsorbent bed weight (15-30 g) on sulfur removal.The results showed that when the weight of the adsorbent bed increases, the breakthrough time increases and the adsorption sites increase.And increasing the contact time between the fuel and the adsorbent lead to increasing the adsorption capacity.

| Effect of feedstock LHSV on sulfur removal efficiency
The best-operating conditions for the desulfurization process from RDF and MOD in FBAC were obtained using the AC-M1 at sorbent bed height = 9 cm.The effect of LHSV on the adsorption capacity for enhancing the sulfur removal from RDF and MDF was conducted under optimal conditions to achieve the highest conversion.Therefore, the effect of the feedstock flow rate (LHSV) on sulfur removal efficiency in FABC was investigated at different levels (LHSV = 4, 8, ) at adsorbent bed height = 9 cm, atmospheric pressure, and temperature over the AC-M1.Figure 8 shows that the sulfur removal efficiency had been increased as the oil flow was reduced (LSHV = 75-8 h −1 ), the removal rate of sulfur compounds in RDF was increased from 56% to 85%, and the DBT removal rate of MDF was also increased from 61% to 94%.Proceeding with a lower oil flow rate of oxidation at LSHV = 4 h −1 the conversion of sulfur compounds is almost unchanged.The experimental results showed that the desulfurization performance of AC adsorbent strongly depends on the oil flow rate.At 5 mL s −1 , the residence time of the oil containing sulfur compounds in the fixed-bed adsorption tower is not long enough for high adsorption efficiency.In addition, the contact time between the adsorbate and the AC bed is very short, resulting in a decrease in the adsorption capacity of sulfur compounds.The more rapid saturation of the AC bed at higher oil flow rates may attribute to the increase in the oil flow rate, mixing enhancement, and the thickness of the oil film around the AC particle reduction, which decreases the film transfer resistance and thus an enhancement in the mass transfer rate. 79,81,82The decrease in oil flow resulted in a decrease in the sulfur content in RDF and MDF and an increase in the time.The maximum sulfur conversion (85%) for RDF and the maximum DBT conversion (94%) for MDF using AC modified with 30% sulfuric acid (adsorbent bed height = 9 cm; LHSV = 8 h −1 ; room temperature; and atmospheric pressure in FBAC).Gawande and Kaware investigated the performance of the ADS process using AC in an FBAC. 83The effect of oil flow rate on sulfur removal was studied in the range of 1.6-5.6 mL min −1 .Their results indicated that reducing oil flow led to enhancing desulfurization and longer breakthrough time.Shah et al. used acid-modified AC as an adsorbent to study the desulfurization efficiency of model oil, commercial kerosene, and diesel. 84The influence of contact time (10-50 min) on the performance of ADS was studied.It was observed that, due to the time required to reach the equilibrium state of DBT adsorption, the adsorption capacity of AC increased with the increase of contact time.

| Activity of the ADS process for desulfurization of RDF and MDF
The adsorption capacity of an adsorbent depends on several factors.Among those factors are the surface chemical properties of the adsorbent (such as surface functional groups and their density), as well as the physical properties of the adsorbent (such as the surface area, pore volume, and active site distribution). 85From Figures 9 and 10, the experimental results show that the adsorption capacity of sulfur compounds in the MDF is higher than that of RDF under all operating conditions.In addition, the best desulfurization efficiency is 85% for RDF and 94% for MDF under optimal conditions of bed height = 9 cm and LHSV = 8 h −1 .The reason for the difference in the desulfurization rate of both RDF and MDF is due to the fact that MDF contains only one type of sulfur The effect of LHSV on the desulfurization efficiency of RDF and MDF on AC-M1 at adsorption bed height = 9 cm and atmospheric conditions.AC, activated carbon; AC-M, modified activated carbon; LHSV, liquid hourly space velocity; MDF, model diesel fuel; RDF, real diesel fuel.
compound, but RDF contains different sulfur compounds, such as alkylated benzothiophenes (BT), DBT, and alkylated derivatives of DBT, like, 4,6dimethylbenzothiophenes. 86Therefore, the removal process of these compounds from RDF is more demanding compared with the removal of DBT from MDF under the same conditions and using the same adsorbent.Because RDF contains diverse sulfur compounds, the pores of the adsorbent tend to be occupied at a higher rate. 87The cetane index for MDF and RDF was 53.9 and 54 before the process and 54 and 54.1 after the adsorption process under the best conditions.The cetane index for MDF and RDF before and after the process showed insignificant changes in the properties of diesel fuel. 56,883 | Regeneration process of the spent adsorbent (AC-M1) The spent adsorbent (AC-M1) had been regenerated and evaluated as follows.

| Screening of regeneration solvents
In this study, various regeneration solvents were used to study the regeneration performance of AC-M1 in terms of RDF sulfur adsorption and MDF adsorption of DBT.
To regenerate AC-MI, iso-octane, ethanol, methanol, and cyclohexane were used as regeneration solvents.The regenerated adsorbent was used in only one ADS cycle  for RDF and MDF, and the obtained results were compared in choosing the most efficient solvent.As shown in Figure 11, the desulfurization efficiencies after one regeneration cycle are as follows: RDF of 85%, 82%, 79%, and 75%, for MDF of 94%, 93%, 91%, and 87% with iso-octane, ethanol, methanol, and cyclohexane, respectively.Therefore, with iso-octane as a regeneration solvent, the desulfurization efficiency was maximized.Figure 12 shows that the regeneration efficiency of AC-M1 is 99%, 96%, 92%, and 88% for RDF and 100%, 98%, 96%, and 92% for MDF using iso-octane, ethanol, methanol, and cyclohexane, respectively.Therefore, the regeneration performance of various solvents of AC-M1 decreases as follows: iso-octane > ethanol > methanol > acetonitrile.Sulfur compounds show very high solubility in isooctane, and their regeneration ability was higher than other tested solvents.The reason behind this behavior is that the diesel fuel that was used in this study is mainly composed of paraffin, so compared with ethanol, methanol, and acetonitrile, isooctane possesses higher solubility in diesel fuel.
Additionally, the extraction capacity of isooctane for paraffin, cycloparaffins, and aromatics is higher than that of methanol, ethanol, and acetonitrile.Cycloalkanes and aromatic hydrocarbons basically exist in the form of long-chain alkyl substituents.Therefore, isooctane has the highest extraction and solubility capacity for these compounds.Hence, using isooctane as a regeneration solvent, AC-M1 can be completely regenerated.
The activity of various solvents used in the regeneration process of spent adsorbents is shown in Table 6.

| Adsorption-desorption cycles
Among the four solvents which were used in this work, isooctane showed the best ability in the regeneration process of spent adsorbent (AC-M1) and was used in all four subsequent regeneration cycles.The regenerated adsorbent was reused several times for the adsorption process of sulfur compounds from RDF, and DBT from MDF under the best-operating conditions (adsorbent bed height = 9 cm and LHSV = 8 h −1 ), followed by regeneration.Figure 13 summarizes the desulfurization efficiency of the regenerated adsorbent after each cycle.AC-M1 experienced a marginal reduction in sulfur removal efficiency after four regeneration cycles.The reason for this behavior is due to the high stability of the adsorbent under optimal conditions.The slight decrease in the adsorption capacity of AC-M1 may be due to the loss of some active sites during the recovery process.The high activity of AC-M1 in terms of sulfur adsorption (80% for RDF and 92% for MDF) after four regeneration cycles.AC-M1 can be considered a promising adsorbent for large-scale desulfurization processes of diesel fuel at a low cost.Figure 14 (based on Equation 2) shows the regeneration efficiency of AC-M1 by isooctane after four cycles.The results showed that in the ADS process, the regeneration efficiencies using RDF and MDF are 94% and 98%, respectively.Iruretagoyena et al. 15 studied the efficiency of the ADS process using the HCl-modified AC system to sweeten an MDF, commercial kerosene, and commercial diesel fuel.The spent adsorbent was regenerated with various regeneration solvents, including methanol, acetonitrile, chloroform, and toluene.They found that toluene is the best solvent for regenerating spent AC and can be used in all subsequent regeneration cycles.The results showed that after four regeneration cycles, the adsorption efficiency of DBT on the acidmodified AC reached 90%.Thus, the present study enhanced the efficiency of sulfur removal and provides more durable AC for consecutive continuous desulfurization of diesel fuel.

| CONCLUSIONS
Continuous ADS of a real and an MDF was carried out in an FBAC using seven different types of AC (AC, AC-M1, AC-M2, AC-M3, AC-M4, AC-M5, and AC-M6), bed height (3, 6, and 9 cm), and LHSV (4, 8, 11, 15, 30, 45,  60, and 75 h −1 ), at ambient conditions.The data of FTIR spectra for AC-M1 adsorbent showed higher concentrations of hydroxyl groups that promoted a higher rate of sulfur removal from diesel fuel.The AC-MI showed the best desulfurization efficiency at 9 cm bed height, LSHV of 8 h −1 , and 10 g adsorbent dose, as the sulfur removal rates in RDF and MDF were 85% and 94%, respectively.Also, the results proved that the regeneration by isooctane through four regeneration cycles insignificantly affected the adsorption efficiency of the AC-MI adsorbent.Compared with previous relevant studies, a higher sulfur removal rate was achieved with only one pass.Although the steady-state sulfur removal efficiency was not the highest among the present studies, the present study provides insights into a cost-effective and highly durable ADS process with satisfactory sulfur removal and paved the way for future higher efficiency continuous ADS processes at economic scalability.The spent modified AC was regenerated using different solvents.The results showed that the regeneration activity for AC-M1 decreases as follows: iso-octane > ethanol > methanol > acetonitrile.

| 3667 F
I G U R E 2 FTIR spectra of fresh and modified ACs.AC, activated carbon; FTIR, Fourier Transform Infrared.F I G U R E 3 SEM for the AC adsorbent: (A, B) fresh AC adsorbent and (C, D) the best-modified AC adsorbent (30% sulfuric acid [AC-M1]).AC, activated carbon; SEM, scanning electron microscopy.

SulfurF
AC-M4 AC-M1 AC-M2 AC-M5 AC-M6 I G U R E 4 Effect of adsorbent type on sulfur removal efficiency for RDF at different bed heights, oil flow rate of 1 mL s −1 , and atmospheric conditions.AC, activated carbon; AC-M, modified activated carbon; BH, bed height; RDF, real diesel fuel.
The physical characteristics of diesel.
Pore volume, surface area, and pore size of the activated carbon.
The effect of the acid modification process on the ADS process in the present study and previous studies for sulfur removal.

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Effect of adsorbent bed height on adsorption capacity for RDF at different types of AC, LHSV = 15 h −1 , and atmospheric conditions.AC, activated carbon; LHSV, liquid hourly space velocity; RDF, real diesel fuel.
The performance of different regenerated solvents in the ADS process.
Recycling performance of AC-M1 in terms of regeneration efficiency for four consecutive cycles for RDF and MDF.AC, activated carbon; AC-M, modified activated carbon 1; MDF, model diesel fuel; RDF, real diesel fuel.