Energy Fuels 2009, 23, 5986–5994 Published on Web 11/03/2009
: DOI:10.1021/ef900683d
Removal of Dibenzothiophenes from Fuels by Oxy-desulfurization Sami H. Ali,* Dina M. Hamad, Bader H. Albusairi, and Mohamed A. Fahim Chemical Engineering Department, Post Office Box 5969, Kuwait University, Safat 13060, Kuwait City, Kuwait Received July 2, 2009. Revised Manuscript Received October 19, 2009
Removal of sulfur species from fuels is an increasingly critical environmental issue. Hydrodesulfurization (HDS) removes sulfur compounds, such as mercaptans and sulfides, from hydrocarbons; however, some sulfur-containing compounds are very difficult to remove and need deep desulfurization processes. In this study, another approach is explored, where dibenzothiophene can be removed from the HDS products by a liquid-phase process. Four different sulfur removal approaches are tested: oxidation, extraction, consecutive oxidation and extraction, and simultaneous oxidation and extraction. A detailed parametric experimental study was performed to select the best technique for the specified purpose of this investigation. n-Octane doped with dibenzothiophenes is used as a model fuel, which is extracted by polar solvents [N-methyl-2-pyrolidone (NMP), dimethyl formamide (DMF), and acetonitrile (ACN)]. These solvents are found to have a moderate capability of removing sulfur species. Applying oxidation in the liquid phase resulted in partial removal of the sulfur content, but most of the sulfur components are not removed from the fuel phase. However, applying both oxidation and extraction steps (either consecutively or simultaneously) resulted in dissolution of sulfur production in the solvent phase. NMP was found to be the best solvent (among the tested ones in this investigation) for the removal of oxidized sulfur species. This is related to the high capability of NMP (polar solvent) for the removal of oxidized (polar) sulfur species (sulfones).
Researchers have studied many methods of producing ultralow sulfur fuels. Some has investigated improving the catalyst used in the conventional HDS process.1-3 Others have applied different techniques, such as liquid-liquid extraction,1,2,4-6 adsorption,6-9 biodesulfurization,2,10,11 and oxy-desulfurization.2,4-6,11-42 These techniques are either applied directly to the oil that have high-sulfur content or applied to the
Introduction Heightened concerns for cleaner air and increasingly more stringent regulations on sulfur contents in transportation fuels will make desulfurization more important. The sulfur problem is becoming more serious in general, particularly for middle to heavy fuels, as the regulated sulfur content is becoming an order of magnitude lower. The chemistry of fuel processing has evolved significantly around the central issue of how to produce cleaner fuels in more efficient and environmentally friendly fashion. The removal of sulfur-containing compounds is currently achieved by hydrodesulfurization (HDS), a catalytic process operated at elevated temperatures (around 673.15-713.15 K) and pressures (in the range of 20-100 atm of H2) using conventional catalysts, such as Co-Mo/Al2O3 and Ni-Mo/ Al2O3, among other catalysts.1,2 The major drawbacks of hydrodesulfurization are high operating conditions (temperatures and pressures), high operating cost, and ineffective removal of refractory sulfur compounds. Although hydrodesulfurization is highly efficient in removing thiols, sulfides, and disulfides, it is less effective for refractory sulfur compounds, especially those containing functional groups that hinder the sulfur atom, such as alkyl-substituted dibenzothiophenes.1,2 For this reason, it limits the capabilities of this process to produce low to zero level sulfur fuel as regulated by environmental protection agencies, such as the United States Environmental Protection Agency (U.S. EPA). Accordingly, new approaches and techniques are necessary for making affordable ultra-low sulfur fuels.
(3) Song, C. Catal. Today 2003, 86 (1-4), 211–263. (4) Ramı´ rez-Verduzco, L. F.; et al. Catal. Today 2004, 98 (1-2), 289– 294. (5) Sampanthar, J. T.; et al. Appl. Catal., B 2006, 63 (1-2), 85–93. (6) Ito, E.; van Veen, J. A. R. Catal. Today 2006, 116 (4), 446–460. (7) Fukunaga, T.; et al. Catal. Today 2003, 84 (3-4), 197–200. (8) Kumagai, S.; et al. Fuel 2009, 88 (10), 1975–1982. (9) Meng, C.; et al. Catal. Today 2009, in press. (10) Li, W.; et al. Biochem. Eng. J. 2009, 44 (2-3), 297–301. (11) Villase~ nor, F.; et al. Fuel Process. Technol. 2004, 86 (1), 49–59. (12) Al-Shahrani, F.; et al. Appl. Catal., B 2007, 73 (3-4), 311–316. (13) Jiang, X.; et al. Fuel 2009, 88 (3), 431–436. (14) Mei, H.; et al. Fuel 2003, 82 (4), 405–414. (15) Otsuki, S.; et al. Energy Fuels 2000, 14 (6), 1232–1239. (16) Shiraishi, Y.; et al. Ind. Eng. Chem. Res. 2002, 41 (17), 4362–4375. (17) Zannikos, F.; et al. Fuel Process. Technol. 1995, 42 (1), 35–45. (18) Ramı´ rez-Verduzco, L. F.; et al. Pet. Sci. Technol. 2004, 22 (1), 129–139. (19) Ali, M. F.; et al. Fuel Process. Technol. 2009, 90 (4), 536–544. (20) Caero, L. C.; et al. Catal. Today 2005, 107-108, 564–569. (21) Campos-Martin, J. M.; et al. Green Chem. 2004, 6 (11), 557–562. (22) Cede~ no Caero, L.; et al. Catal. Today 2006, 116 (4), 562–568. (23) Collins, F. M.; et al. J. Mol. Catal. A: Chem. 1997, 117 (1-3), 397–403. (24) De Filippis, P.; Scarsella, M. Energy Fuels 2003, 17 (6), 1452– 1455. (25) Dehkordi, A. M.; et al. Fuel Process. Technol. 2009, 90 (3), 435– 445. (26) Di Giuseppe, A.; et al. Appl. Catal., B 2009, 89 (1-2), 239–245. (27) G omez-Bernal, H.; et al. Catal. Today 2009, 142 (3-4), 227–233. (28) Hulea, V.; et al. J. Catal. 2001, 198 (2), 179–186. (29) Ishihara, A.; et al. Appl. Catal., A 2005, 279 (1-2), 279–287. (30) Palomeque, J.; et al. J. Catal. 2002, 211 (1), 103–108. (31) Shiraishi, Y.; Hirai, T. Energy Fuels 2004, 18 (1), 37–40. (32) Tam, P. S.; et al. Ind. Eng. Chem. Res. 2002, 29 (3), 321–324.
*To whom correspondence should be addressed. Fax: (00965) 24839498. E-mail:
[email protected] and/or
[email protected]. (1) Babich, I. V.; Moulijn, J. A. Fuel 2003, 82 (6), 607–631. (2) Song, C.; Ma, X. Appl. Catal., B 2003, 41 (1-2), 207–238. r 2009 American Chemical Society
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desulfurized oil produced from the conventional HDS process for ultra-desulfurization. The used fuel types in these investigations were either real fuel, such as gasoline,3,9,36 kerosene,7,8,25,28,33 and diesel,2,3,5,10,11,14,27,37,39,41 or model fuel, such as n-octane,12,13,26,39,40 and n-hexadecane.20,22,42 As mentioned above, removal of refractory sulfur compounds is one of the limitations of the HDS process. Alternatively, oxy-desulfurization has shown better efficiency of removing these sulfur-containing compounds, especially if it is used with other separation process, such as adsorption23,29,33,36,40,41 or extraction.2,4,5,11-18,22,23,26,30-32,35,38,39 In this work, the oxidation of sulfur-containing compounds with extraction is of our interest. In the oxy-desulfurization process, sulfur-containing compounds are converted to their corresponding sulfones by oxidation using mainly hydrogen peroxide and either using a homogeneous catalyst, such as acetic acid12,16,18,19 or formic acid,15,24,25,34 or using different heterogeneous catalysts.4,5,12,13,20 As reported by Shiraishi et al.,16 oxidation alone can remove a substantial amount of sulfur-containing compounds fed to the process but not to the required limits by the new environmental regulations. Therefore, another separation process is applied on the organic phase to separate the remaining sulfones. Among tested separation processes, extraction is the most widely used separation process along with the oxidation. Extraction is used as a second step after oxidation and separation of the aqueous phase. Many solvents have been tested in the extraction, such as acetonitrile, N,N-dimethylformamide (DMF), N-methyl-2-pyrolidone (NMP), methanol, and others. Otsuki et al.15 compared between the DMF, acetonitrile, and methanol as solvents for extraction and found that DMF is the most effective under their experimental conditions but has the lowest oil recovery. Ramı´ rez-Verduzco et al.18 compared between methanol, acetonitrile, 2-ethoxyethanol, and γ-butyrolactone as solvent for extraction and found that γ-butyrolactone is the best, with a maximum of 76.4% sulfur removal for real diesel. Sampanthar et al.5 compared between acetonitrile, DMF, NMP, and methanol and found that NMP is the best solvent in term of sulfur removal at many conditions. In the consecutive oxidation/ extraction approach, the experiments were performed in the normal operating rage from 313 to 343 K and at 1 atm. The sulfur removal ranged from 65 to almost 100% depending upon the catalyst type, operating temperature, and feed composition, among other factors. Another approach for using extraction along with oxidation is to carry the oxidation and extraction at the same time (simultaneous). There are a few papers in the literature dealing with this approach. In Ramirez-Verduzco et al.,4 the oxidation reaction was carried out at 323 K and 1 atm, with hydrogen peroxide at 30 wt %, in a heterogeneous system with a WOx-ZrO2 catalyst at 15 wt % W. The extraction was performed with methanol, acetonitrile, 2-ethoxyethanol, or γ-butyrolactone as polar solvents. In this (33) (34) (35) (36) (37) (38) (39) (40) (41) (42)
process, almost 100% of sulfur removal is achieved for model diesel fuel and almost 72% of sulfur removal is achieved for real diesel. From all of the above-mentioned related literature of sulfur removal by oxy-desulfurization, it appears that no study in the literature dealt with comparing simultaneous versus consecutive approaches for sulfur removal from fuels. Therefore, this work is carried out for two main objectives: The first objective is to compare the conditions of extraction versus oxidation for sulfur removal. The other objective is to compare between the simultaneous and consecutive oxidation/extraction approaches. Accordingly, several experiments have been conducted for the removal of sulfur compounds (such as dibenzothiophenes) from a model fuel compound (represented by n-octane doped by sulfur species) at different conditions for the four approaches. The objective of these experiments is to carry a detailed parametric study by changing the following: stirring speed (200-1000 rpm), temperature (298.15-343.15 K), solvent (NMP, DMF, and ACN), solvent/model fuel ratio (DBT doped into n-octane) (0.0-3.0), oxidant (H2O2) to model fuel ratio (0.0-1.0), liquid catalyst type (acetic and formic acids), and its ratio to the oxidant (0.0-0.75), in addition to the form of sulfur compound [dibenzothiophene (DBT), dibenzothiophene dioxide (DBTDO), 4-methyldibenzothiophene (4-MDBT), 4,6-dimethyldibenzothiophene (4,6-DMDBT), and 4,6diethyldibenzothiophene (4,6-DEDBT)]. Reported Mechanism of Oxidation of DBT As reported in the literature, the oxidation of refractory sulfur compounds with hydrogen peroxide is known to take place over various catalytic systems, such as acetic acid, formic acid, metal-loaded Al2O3 catalysts, Ti-SiO2-based catalysts, W-V-TiO2 catalysts, WOx-ZrO2 catalysts, Ti-containing molecular sieves, polyoxometalate, and phosphotungstic acids.1,2,4,6,16,18,23,28,34,43 In this work, acetic acid is used and compared to formic acid. Dibenzothiophene (DBT), which is used as a model refractory sulfur compound, is oxidized by hydrogen peroxide in the presence of acetic acid as a catalyst to form dibenzothiophene sulfoxide (DBTO) and, ultimately, dibenzothiophene sulfone (DBTDO), as illustrated in the following reactions: acetic acid H O þ C H SO C12 H8 S þ H2 O2 s 12 8 f 2 ðDBTOÞ ðDBTÞ
ð1Þ
acetic acid H O þ C H SO C12 H8 SO þ H2 O2 s 12 8 2 f 2 ðDBTOÞ ðDBTDOÞ
ð2Þ
The overall oxidation reaction is given by acetic acid
C12 H8 S þ 2H2 O2 sf C12 H8 SO2 þ 2H2 O
ð3Þ
Peracetic acid (peroxyacetic acid), which is a selective oxidizing agent, is an equilibrium product generated from the reaction of hydrogen peroxide and acetic acid
Tao, H.; et al. Fuel 2009, 88 (10), 1961–1969. Te, M.; et al. Appl. Catal., A 2001, 219 (1-2), 267–280. Toteva, V.; et al. Fuel Process. Technol. 2009, 90 (7-8), 965–970. Wang, W.; et al. Fuel 2007, 86 (17-18), 2747–2753. Yan, X.-M.; et al. J. Fuel Chem. Technol. 2009, 37 (3), 318–323. Zhao, D.-S.; et al. Energy Fuels 2007, 21, 2543–2547. Zhao, D.-S.; et al. J. Fuel Chem. Technol. 2009, 37 (2), 194–198. Yu, G.; et al. Carbon 2005, 43 (11), 2285–2294. Yu, G.; et al. Front. Chem. Eng. China 2007, 1 (2), 162–166. Cede~ no-Caero, L.; et al. Catal. Today 2008, 133-135, 244–254.
CH3 COOH þ H2 O2 T CH3 COO-OH þ H2 O
ð4Þ
(43) Aidan, T.; et al. Trans. Mater. Res. Soc. Jpn. 1994, 18A, 391–395.
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The following is a schematic drawing representing the oxidation reaction of DBT into DBTO and DBTDO. Sulfoxide formation:
Figure 1. Experimental setup of the apparatus for oxidation/extraction.
Sulfone formation:
4-MDBT had a boiling point of 571 K and a melting point of 338 K. 4,6-Dimethyldibenzothiophene (C14H12S) was supplied by Aldrich. 4,6-Diethyldibenzothiophene (C16H16S) was supplied by Aldrich. Dibenzothiophene sulfone (C12H8O2S) was supplied by Aldrich. NMP (C5H9NO) was supplied by Fluka. NMP had a specific gravity of 1.032. Dimethylformamide [HCON(CH3)2] was supplied by Merck. Dimethylformamide (DMF) had a specific gravity of 0.94. Acetonitrile (C2H3N) was supplied by Fluka. Acetonitrile (ACN) had a specific gravity of 0.78. Hydrogen peroxide (H2O2) was supplied by Merck. Hydrogen peroxide had a specific gravity of 1.14 and a boiling point 363 K. It is used as an oxidizing agent. Acetic acid (CH3COOH) was supplied by Ajax chemicals. Acetic acid had a specific gravity of 1.05. It is used as a catalyst. Formic acid (HCOOH) was supplied by Merck. Formic acid had a specific gravity of 1.22. It is used as a catalyst. The aromatic compounds were stored under a 0.4 nm molecular sieve. All chemicals were used without further purification. Apparatus. The experimental apparatus used in this study for the oxidation/extraction experiments is shown in Figure 1. The apparatus consisted of a 60 cm3 glass cell with a water jacket to maintain a constant temperature. A temperature probe was inserted into the reactor to measure the temperature of the mixture with an accuracy of (0.5 K. The cell was connected to a Haake K15 water bath fitted with a Haake DC1 thermostat. The reactor was continuously stirred using a special stirring rod connected to a digitally controlled electrical motor. Chemical Analysis. The n-octane-rich and solvent-rich phases were analyzed using a Chrompack CP 9000 gas chromatograph (GC) equipped with an on-column injector, flame ionization detector (FID), and sulfur chemiluminescence detector (SCD) and a data processor system. GC-FID. The column initial temperature was maintained at 383 K for 2 min. The heating rate was 10 K/min. The final temperature of 673 K was maintained for 5 min. Helium was used as a carrier gas. The flow rate was maintained at 3 10-5 m3/ min. The injection temperature was 523 K. The detector temperature was 573 K. The temperature was controlled at (0.1 K. GC-SCD. A SCD (Chrompack 9001 CP) was used for detecting and measuring sulfur-containing compounds in different samples. This device was equipped with sievers 355 SCD with “flameless burner” at 1073 K, ozone excitation, detection with a photomultiplier tube (PMT), and filter at a wavelength between 250 and 480 nm. The column used was CB SIL 5 CB sulfur, 30 m long, with a 0.32 mm inner diameter and 4 μm thick layer. The initial oven temperature was kept at 353 K for 1 min. The heating rate was 20 K/min. The final temperature was maintained at 573 K for 10 min. Ultra-pure helium gas was used. Samples of 1 μL were used. The flow rate was maintained at 3 10-5 m3/min. The injection temperature was kept at 523 K, while the detector temperature was 573 K. The temperature was controlled at (0.1 K. Experimental Procedures. In this work, four sets of experimental procedures for removing sulfur compounds were
These reactions are homogeneous reactions occurring wherever peracetic acid is formed and DBT is present together because the peracetic acid is highly reactive; therefore, it will readily react with DBT that is present in the surrounding environment. Assuming first-order dependence of the rate of the reaction on the concentration of DBTs34 at excess amounts of hydrogen peroxide relative to DBTs, the kinetics can be expressed as ð7Þ -rDBT ¼ kDBT CDBT For cases of 4-MDBT, 4,6-DMDBT, and 4,6-DEDBT, the overall oxidation reactions are given by ð8Þ -r4-MDBT ¼ k4-MDBT C4-MDBT -r4;6-DMDBT ¼ k4;6-DMDBT C4;6-DMDBT
ð9Þ
-r4;6-DEDBT ¼ k4;6-DEDBT C4;6-DEDBT
ð10Þ
Experimental Section The main part of the experimental work in this study centers around desulfurization of fuel containing dibenzothiophenes by simultaneous oxidation and extraction of sulfone species formed. For this reason, four sets of experiments were conducted: the first is to study the oxidation of dibenzothiophenes in the presence of an oxidizing agent (hydrogen peroxide) and catalysts (acetic or formic acid). The second set of experiments was run to extract the oxidation products (sulfones) and the unreacted dibenzothiophenes (if any) remaining in model fuel. The last two sets were conducted to study how much dibenzothiophenes were removed by either two schemes: running oxidation followed by extraction (third set) or running simultaneously oxidation and extraction (fourth set). The last option was studied extensively. Chemicals. All chemicals have a purity of at least 99%. n-Octane (C8H18), supplied by Fluka, was used as a model fuel compound. n-Octane had a specific gravity of 0.702. DBT (C12H8S) was supplied by Aldrich. DBT had a melting point of 370 K. 4-MDBT (C13H10S) was supplied by Aldrich. 5988
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dibenzothiophene compounds was studied by varying stirring speed, reaction temperature, H2O2/model fuel volume ratio, acid type, and type of dibenzothiophene compound (see Table 1). Dibenzothiophene Removal by Extraction Only. The desired amount of solvent was adjusted, so that the solvent (NMP, DMF, or ACN) to model fuel (n-octane doped by DBT or DBTDO) volume ratio was set to 1:1. The prepared amounts of model fuel and solvent were heated, and once the required temperature was achieved (323.15 K), mixing of model fuel and solvent was performed at the assigned stirring speed (500 rpm) for 2 h and then stopped. Settling for 24 h was allowed, to obtain the two phases. Two samples of 0.3 mL, one from the upper layer and one from the bottom one, were then withdrawn. The two samples were analyzed by GC. Dibenzothiophene Removal by Consecutive Oxidation and then Extraction. In this experimental technique, the oxidation is performed first and then immediately followed by extraction by NMP, DMF, or ACN. The oxidation step procedure is exactly the same as the procedure used for oxidation only up to stopping the reaction (2 h of reaction time) and allowing for the phases to separation. These runs were performed for oxidizing DBT at 323.15 and 343.15 K, using a stirring speed of 500 rpm, solvent/model fuel ratio of 1:1, hydrogen peroxide/ model fuel ratio of 1:1, and acetic acid/hydrogen peroxide ratio of 0.5:1. The upper layer (oxidized model fuel) obtained from the oxidation step was then separated and mixed with the proper amount of solvent for another 2 h under the proper stirring speed. After the stirring stopped and the two phases were allowed to settle and separate for 24 h, two samples from the upper and bottom layer of 0.3 mL each were withdrawn and analyzed by GC. Dibenzothiophene Removal by Simultaneous Oxidation and Extraction. In this experimental technique, the oxidation and extraction were performed in one single step, where the oxidation reaction occurs in the presence of a solvent. The model fuel compound was prepared as mentioned earlier. Initially, n-octane, sulfur compound, catalyst, and a solvent were added together with the required ratios. Hydrogen peroxide was then added to the reactor at the desired temperature. The stirring speed was set to the desired value, remained for 2 h, and then stopped. After 24 h of settling and phase separation, two samples from the upper and bottom layers of 0.3 mL each were withdrawn and analyzed by GC. The influence of the stirring speed, temperature, solvent type, solvent/model fuel volume ratio, H2O2/model fuel volume ratio, acid type, acid/H2O2 ratio, and type of sulfur compound on this technique was studied carefully (see Table 1). Solubility of DBTDO in Different Phases. Solubility of DBTDO in different liquids (NMP, n-octane, and water) was measured at different temperatures ranging from 303.15 to 343.15 K. These measurements were performed to specify and compare between equilibrium amounts of dissolved DBTDO, which can be taken up by the different phases. This was performed by mixing an excess amount of DBTDO with each liquid phase separately at the desired temperature for 2 h, followed by 24 h of settling time. After that, samples were withdrawn and analyzed by GC.
Table 1. Parametric Study Schedule
approach
liquidliquid extraction
parameter stirring speed 500 (rpm) temperature 323.15 (K)
solvent solvent/ model fuel volume ratio H2O2/model fuel volume ratio
200 500 800 1000 303.15 323.15 343.15
NMP DMF acetonitrile 1.0
acid acid/H2O2 volume ratio
sulfur compound
oxidation
DBT DBTDO
consecutive simultaneous oxidation/ oxidation and extraction extraction 500
500 1000
323.15 343.15
298.15 303.15 313.15 323.15 343.15 NMP DMF acetonitrile 0.5 1.0 2.0 3.0 0.0 0.125 0.25 0.5 0.75 1.0 acetic formic 0.0 0.125 0.25 0.5 0.75 DBT 4-MDBT 4,6-DMDBT 4,6-DEDBT
NMP DMF acetonitrile 1.0
0.0 0.25 0.5 1.0
1.0
acetic formic 0.5
acetic 0.5
DBT DBT 4-MDBT 4,6-DMDBT 4,6-DEDBT
conducted. These procedures are oxidation only, extraction only, consecutive oxidation and then extraction, in addition to simultaneous oxidation and extraction. These experiments were performed at different conditions (as shown in Table 1). The model fuel used was prepared by doping n-octane with the required sulfur-containing compounds of dibenzothiophenes in the proper amounts to simulate fuel with 500 ppmw sulfur. Different sulfur-containing compounds were used for this purpose, namely, dibenzothiophene (DBT), 4-methyldibenzothiophene (4-MDBT), 4,6-dimethyldibenzothiophene (4,6-DMDBT), and 4,6-diethyldibenzothiophene (4,6-DEDBT). In the extraction runs, dibenzothiophene dioxide (DBTDO) was also prepared for the extraction runs. The variables considered in the four sets of experimental procedures (Table 1) are stirring speed, temperature, solvent type, solvent/model fuel ratio, oxidizing agent [hydrogen peroxide (H2O2)] to model fuel ratio, acid type, and acid/oxidizing agent ratio, as well as the type of sulfur-containing compound. Each measurement was repeated 3 times, and the reproducibilities were g99%. Dibenzothiophene Removal by Oxidation Only. The desired amount of oxidizing agent (hydrogen peroxide) was adjusted to model fuel (n-octane doped by DBT, 4-MDBT, 4,6-DMDBT, and 4,6-DEDBT) volume ratio. Afterward, the desired amount of acid and the desired values of reaction temperature were adjusted. The stirring speed was then adjusted, so that the amount of mixing required is determined by physical properties, such as viscosity, interfacial tension, and density differences, between the two phases. The prepared amount of model fuel was transferred to the reactor together with the weighted acetic or formic acid (catalyst). Then, hydrogen peroxide was added to the reactor at the desired temperature (zero time). During the 2 h reaction time, samples were withdrawn and analyzed by GC at the following times intervals: 10, 30, 60, 90, and 120 min. The influence of reaction parameters on the oxidation of
Results and Discussion Oxidation without Solvents. The removal of dibenzothiophene from model fuel depends upon the operating conditions of both oxidation and extraction. Factors affecting the oxidation of dibenzothiophene were carefully studied, including the effect of agitation, reaction temperature, oxidizing agent (hydrogen peroxide) to model fuel ratio, type of catalyst (type of organic acid), and the degree of hindrance of the sulfur compound. Figure 2 shows that DBT conversion by the use of the only oxidation step, which levels off at 500 rpm, while at higher stirring values (800 and 1000 rpm), 5989
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were identical. Thus, Figure 2 clearly shows that, at an agitation stirring speed of 500 rpm, mass transfer limitations were found to be insignificant. Therefore, the stirring speed was kept at a value of 500 rpm for all of the experiments. Ramirez-Verduzco et al.18 kept the value of the agitation stirring speed at 600 rpm for the desulfurization of middle distillates by the oxidation and extraction procedure using hydrogen peroxide and acetic acid as the oxidizing agent and catalyst. An increase of the reaction temperature of oxidation (from 303.15 to 343.15 K) was found to have a significant positive impact on the conversion of DBT to DBTDO for the case of using acetic acid as well as formic acid (both used as catalysts
in this study) (as shown in Figure 3). Indeed, other researchers found that increasing the temperature increased the conversion of DBT into DBTDO.34,41 Figure 3 also shows that formic acid had a somewhat higher catalytic activity for DBT oxidation than acetic acid. However, because of the fact that formic acid is less stable and hard to handle compared to acetic acid, the latter acid was selected for the activation of the DBT oxidation process. Ramirez-Verduzco et al.18 reached the same conclusion in their published work regarding activity, stability, and use of acidic versus formic acid. The conversion of DBT to DBTDO can reach approximately 100% at adequate conditions, as shown in Figure 3. The kinetics for DBT oxidation at different periods of time (0, 10, 30, and 60 min), at different temperatures (303.15, 323.15, and 343.15 K), and for different kinds of catalysts (acetic and formic acids) have been studied, as shown in Figure 4. The experimental data at 90 and 120 min have been excluded from the kinetic study because, at these times, the reaction is no longer kinetically controlled but is thermodynamically controlled, as observed from Figure 3. Assuming a first-order reaction kinetics, the pseudo-first-order oxidation reaction constants of DBT conversion were obtained using eq 7, as shown in Figure 4. The pseudo-firstorder oxidation reaction rate constants using acetic acid were found to be 0.016, 0.037, and 0.061 min-1 at 303.15, 323.15, and 343.15 K, respectively, while for formic acid, they were 0.019, 0.051, and 0.070 min-1, respectively. The obtained rate constants for DBT oxidation fit an Arrhenius-type relationship with reasonable accuracy (Figure 5). The activation energy and Arrhenius constant using acetic acid as the catalyst for oxidation were found to be 28.9 kJ/mol and 1607 min-1, respectively, while they were found to be equal to 28.6 kJ/mol and 1768 mim-1 for formic acid, respectively. Te et al.34 obtained the pseudo-first-order oxidation reaction rate constant for oxidation of DBT to be equal to 0.078 min-1 at 323.15 K using polyxymetalate catalyst Na3PW12O40. Moreover, Ishihara et al.29 obtained the activation energy for DBT and 4,6-DMDBT in light gas oil equal to 32 ( 1 kJ and equal to 28 ( 1 kJ in kerosene. These values are close to the obtained value in this work. Another factor influencing the kinetics of DBT oxidation is the degree of hindrance of the sulfur-containing compounds. This was investigated by comparing the kinetics of oxidation of different alkyl-substituted DBT compounds, DBT versus 4-MDBT, 4,6-DMDBT, and 4,6-DEDBT. Figure 6 shows clearly that the reactivity of the different dibenzothiophenes follows the following trend: 4,6-DEDBT
Figure 2. Effect of the stirring speed on the conversion of DBT in model fuel at 323.15 K using the only oxidation step, with H2O2/ model fuel ratio of 1:1 and acetic acid/H2O2 ratio of 0.5:1.
Figure 3. Effect of the temperature and type of acid on conversion of DBT in model fuel using the only oxidation step, with H2O2/model fuel ratio of 1:1 and acid/H2O2 ratio of 0.5:1.
Figure 4. Pseudo-first-order reactions of DBT in model fuel, with H2O2/model fuel ratio of 1:1 and acetic acid/H2O2 ratio of 0.5:1.
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Figure 5. Arrhenius plot for the reaction of DBT in model fuel using the only oxidation step, with H2O2/model fuel ratio of 1:1 and acid/H2O2 ratio of 0.5:1.
Figure 6. Effect of the degree of hindrance of different dibenzothiophenes on conversion using the only oxidation step at 323.15 K, with H2O2/ model fuel ratio of 1:1 and acetic acid/H2O2 ratio of 0.5:1.
> 4,6-DMDBT > 4-MDBT > DBT, which is in accordance with the corresponding values of electron densities of these compounds, as shown in Table 2. This latter finding is in accordance with the results reported by Otsuki et al.15 for the desulfurization of light gas oil and vacuum gas oil by the oxidation and extraction procedure using hydrogen peroxide and formic acid and by Al-Shahrani et al.12 for desulfurization of diesel using hydrogen peroxide and tungstate as the catalyst. However, in hydrodesulfurization units using solid catalysts, opposite trends for the reactivity of alkyl-substituted dibenzothiophene were obtained: DBT > monoalkylsubstituted DBT > dialkyl-substituted DBT > trialkyl-substituted DBT. Using the kinetic data collected for the different alkyl-substituted DBT oxidation reactions at 323.15 K, the pseudo-first-order oxidation reaction constants were obtained following eqs 7-10 for DBT, 4-MDBT, 4,6-DMDBT, and 4,6-DEDBT, as shown in Figure 7. The pseudo-first-order oxidation reaction constants at 323.15 K were found to be 0.037, 0.043, 0.050, and 0.069 min-1 for DBT, 4-MDBT, 4,6-DMDBT, and 4,6-DEDBT, respectively. These rate constant values for the oxidation reaction are in accordance with the corresponding values of electron densities reported in Table 2 and the obtained trend for alkyl-substituted DBT reactivity in this study, as reported previously (4,6-DEDBT > 4,6-DMDBT > 4-MDBT > DBT).
Table 2. Electron Density of Different Dibenzothiophenes sulfur compound DBT 4-MDBT 4,6-DMDBT
electron densitya
kib (min-1)
5.758 5.759 5.760
0.037 0.043 0.050
a Otsuki et al.15 b Pseudo-first-order reaction constants of dibenzothiophenes doped in n-octane at 323.15 K, with H2O2/model fuel ratio of 1:1 and acetic acid/H2O2 ratio of 0.5:1.
The effect of the ratio of the oxidizing agent (hydrogen peroxide) to model fuel (n-octane doped by DBT) on the conversion of DBT (using the only oxidation step) was found to be significant. When the hydrogen peroxide/model fuel ratio was increased from 0.0:1 to 0.25:1, the conversion of DBT increased from 0 to almost 69% over a 2 h period of time (Figure 8), and when this ratio was increased further, the conversion of DBT increased significantly.21,28,30,41 Figure 8 shows that, to achieve almost complete DBT conversion into DBTDO within a reasonable period of time and at a moderate temperature (323.15 K), the ratio of hydrogen peroxide/n-octane must be on the order of almost 1:1 (using an acetic acid/hydrogen peroxide ratio of 0.5:1). Extraction without Oxidation. Although DBT has converted to DBTDO, the sulfur has still not been completely removed from the model fuel. The solubility of DBTDO in n-octane is more than an order of magnitude higher than in 5991
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Figure 7. Pseudo-first-order reactions of different model fuels at 323.15 K, with H2O2/model fuel ratio of 1:1 and acetic acid/H2O2 ratio of 0.5:1.
Figure 8. Effect of the H2O2/n-octane feed ratio on the conversion of DBT in model fuel using the only oxidation step at 323.15 K, with an acetic acid/H2O2 ratio of 0.5:1. Table 3. Solubility of DBTDO in NMP, n-Octane, and Water Phases
at 323.15 K. It has to be noticed here that a higher temperature will enhance solvent miscibility in the octane phase (solvent loss), and therefore, it is avoided. Different studies found different solvents (among tested ones in this investigation) to be the most effective for the removal of sulfur with the extraction of oxidized or unoxidized sulfur species from different fuels at different conditions.4,5,15,17 Sampanthar et al.5 found that NMP is a more effective solvent for the removal of oxidized sulfur compounds from fuels than DMF and ACN. Consecutive/Simultaneous Oxidation and Extraction. In light of the previous results [the kinetics of oxidation of DBT (nonpolar compound) into DBTDO (polar compound) and the extraction of DBTDO and DBT], extraction was applied following the oxidation step (consecutive oxidation and extraction procedure). Extraction of sulfur contained in the n-octane-rich phase after almost completion of the oxidation of DBT into DBTDO using NMP, ACN, and DMF was investigated. Figure 10 shows that, at both studied temperatures (323.15 and 343.15 K), NMP was found to result in the highest removal of sulfur content of the model fuel followed by ACN and then DMF. The removal of the sulfur content in the model fuel increased from values of almost 60% using the only extraction without oxidation step (Figure 9) to almost 100% using oxidation followed by extraction steps, as shown in Figure 10. By proving that application of the extraction step (using polar solvents) following the conversion of DBT into a polar species (DBTDO), using an oxidation step, enhances
solubility of DBTDO (g/g) temperature (K) 303.15 313.15 323.15 343.15
NMP phase -2
3.14 10 3.21 10-2 3.27 10-2 3.38 10-2
n-octane phase -3
3.15 10 3.25 10-3 3.35 10-3 3.50 10-3
water phase 1.75 10-4 1.90 10-4 2.10 10-4 2.15 10-4
water, at a temperature in the range of 303.15-343.15 K. If a solvent such as NMP is used, the solubility of DBTDO is almost an order of magnitude higher than in n-octane. Solubility data of DBTDO in NMP, n-octane, and water at four different temperatures are given in Table 3. This results support the need of adding NMP or any other suitable solvent. From all of the above discussion, the removal of sulfone from the model fuel and the unreacted dibenzothiophene (if any) by the application of liquid-liquid extraction using different solvents (ACN, NMP, and DMF) is therefore investigated. Figure 9 shows that, at a temperature of 323.15 K, the removal of DBT by the different solvents was lower than 60% of the sulfur content; however, the removal capabilities of the solvents were more than 90% for DBTDO at the same conditions. Therefore, when DBT was converted into the corresponding polar compound (DBTDO) by means of oxidation, the capabilities of the tested solvents for sulfur removal enhance significantly. Figure 9 shows that using NMP resulted in the highest removal of DBTDO 5992
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significantly the removal of the sulfur content of the model fuel used, it was thought for ease of application purposes to combine both oxidation and extraction steps into one single step, using simultaneous oxidation and extraction. NMP was also found to result in the highest percentage removal of sulfur from the model fuel at all five tested temperatures (298.15, 303.15, 313.15, 323.15, and 343.15 K) followed by ACN and then DMF, as shown in Figure 11. Using this procedure (simultaneous oxidation and extraction) at the suitable conditions, it is obvious that almost all sulfur content in the model fuel compound could be removed.
Figure 9. Sulfur removal using liquid-liquid extraction for DBT and DBTDO in model fuel by different solvents at 323.15 K.
Even though it was found that the conversion of alkylsubstituted DBT compounds can be more easily transformed into oxidized species (polar species) than DBT, by the application of both oxidation and extraction simultaneously, it was found that the polar solvents are more capable of removing sulfur in the form of DBT than alkyl-substituted DBT species. Figure 12 shows that, at the studied temperatures, 323.15 and 343.15 K, sulfur removal using simultaneous oxidation and extraction increased following the trend: 4,6-DEDBT < 4,6-DMDBT < 4-MDBT < DBT. This order is due to the size of the produced sulfone, where DBTDO is smaller than 4,6-DMDBTDO; therefore, its extraction is easier. Thus, this factor would overcome the lower reactivity of DBT. Accordingly, the oxidation/extraction process can easily increase the percentage removal of DBT and 4,6-DMDBT, which are difficult to be removed by conventional HDS. The increase of the solvent/model fuel ratio resulted in the increase of the sulfur removal using the simultaneous oxidation and extraction procedure. Figure 13 shows again that NMP is the best solvent for the removal of DBT from model fuel using the simultaneous oxidation and extraction procedure, and when the ratio of NMP/model fuel is increased, an enhancement in the removal of the sulfur content is noticed, as found by Zannikos et al.17 This enhancement in the removal of the sulfur content from feed using this experimental procedure is also noticed when using ACN and DMF.
Figure 10. Sulfur removal using consecutive oxidation followed by extraction for DBT in model fuel by different polar solvents, with H2O2/model fuel ratio of 1:1 and acetic acid/H2O2 ratio of 0.5:1.
Figure 12. Effect of the degree of hindrance of different dibenzothiophenes in model fuel on sulfur removal using simultaneous oxidation and extraction with NMP, with H2O2/model fuel ratio of 1:1 and acetic acid/H2O2 ratio of 0.5:1.
Figure 11. Effect of the temperature on sulfur removal from model fuel using simultaneous oxidation and extraction with different polar solvents, with H2O2/model fuel ratio of 1:1 and acetic acid/H2O2 ratio of 0.5:1.
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Figure 15. Effect of the formic acid/H2O2 ratio on DBT conversion and sulfur removal from model fuel at 323.15 K using simultaneous oxidation and extraction, with H2O2/model fuel ratio of 1:1.
Figure 13. Effect of the solvent/model feed ratio on sulfur removal using simultaneous oxidation and extraction at 323.15 K, with H2O2/model fuel ratio of 1:1 and acetic acid/H2O2 ratio of 0.5:1.
Figure 16. Effect of the H2O2/model fuel ratio on DBT conversion and sulfur removal using simultaneous oxidation and extraction, with NMP at 323.15 K and acetic acid/H2O2 ratio of 0.5:1.
Figure 14. Effect of the acetic acid/H2O2 ratio on DBT conversion and sulfur removal from model fuel at 323.15 K using simultaneous oxidation and extraction, with H2O2/model fuel ratio of 1:1.
extraction procedure, as found in the case of the oxidation procedure alone. DBT conversion and sulfur removal increased significantly by increasing the hydrogen peroxide/model fuel ratio, as shown in Figure 16. The removal of sulfur was found to be the highest at a hydrogen peroxide/model fuel ratio of 1:1.
The change of the catalyst (acid) to oxidizing agent (hydrogen peroxide) ratio is found to be one of the important parameters that affect both the conversion of DBT into DBTDO and the removal of the sulfur content from the feed, as shown in Figures 14 and 15. It seems that an optimum ratio of acid to oxidizing agent exists for the removal of sulfur from the system while using a simultaneous oxidation and extraction procedure. A ratio of 0.5:1 of acetic acid/hydrogen peroxide seems to be the optimum ratio for sulfur removal (Figure 14), while a smaller ratio (0.25:1) seems to be the optimum in the case of using formic acid (Figure 15). Yu et al.41 found that by increasing the formic acid/hydrogen peroxide ratio for the deep desulfurization of diesel fuels by catalytic oxidation, DBT conversion increased. The decline in DBT conversion after reaching a certain ratio of organic acid to hydroxide (see Figures 14 and 15) is due to a decline in organic acid dissociation, which is necessary as the catalyst for the sulfur oxidation reaction. The decline in organic acid dissociation is a result of acid selfdimerization and/or association with water. Another factor contributing to this observation is the decline in the availability of the proton necessary for hydroxide dissociation to yield oxygen necessary for sulfur oxidation. This decline in proton availability is due to a decrease of the water concentration upon adding more organic acid. The effect of the ratio of hydrogen peroxide/model fuel on the conversion of DBT into DBTDO was also found to be a very important condition using the simultaneous oxidation and
Conclusions From all of the above, it can be concluded that, to remove almost all of the sulfur content from the model fuel compound composed of n-octane doped by DBT (nonpolar sulfur species) by the use of polar solvents, an oxidation step must applied simultaneously with the extraction step, where almost all DBT can be converted into DBTDO over a reasonable period of time at suitable conditions. For the tested conditions and chemicals used in this investigation, NMP was found to be the best solvent for the removal of the sulfur content from the model fuel when an oxidation step is used. The optimum conditions to operate the oxidation and extraction step of sulfur from a model fuel composed from n-octane doped by DBT using NMP seems to be a stirring speed of 500 rpm, temperature of 323.15 K, solvent/model fuel ratio of 1:1, oxidizing agent/model fuel ratio of 1:1, acetic acid as the acid to be used, and acetic acid/oxidizing agent ratio of 0.5:1. Examination of experimental results carried out in the simultaneous oxidation and extraction runs revealed that dibenzothiophene is oxidized and then preferably extracted by polar solvent. Also, it was found that the outcomes of consecutive versus simultaneous oxidation and extraction procedures are almost the same; however, it is clear that, because of the ease of the simultaneous procedure, it is more attractive to apply. 5994
