Kinetic Modeling of the Extraction–Oxidation Coupling Process for the

Article pubs.acs.org/EF

Kinetic Modeling of the Extraction−Oxidation Coupling Process for the Removal of Dibenzothiophene Zongjing Lu,† Erliang Guo,† Hua Zhong,† Yajie Tian,‡ Yue Yao,† and Shuxiang Lu*,† †

College of Chemical Engineering and Materials Science, Tianjin University of Science and Technology, Tianjin 300457, People’s Republic of China ‡ School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People’s Republic of China S Supporting Information *

ABSTRACT: Extraction−oxidation desulfurization (EODS) technology would be a potential industrial application for achieving ultralow-sulfur fuel oils. A combined extraction−oxidation system for dibenzothiophene removal from model fuel with H2O2 as an oxidant, acetonitrile as an extractant, and Mo/γ-Al2O3 as a catalyst was performed in a batch reactor. To study this complex heterogeneous system, the chemical reaction and mass transfer on each phase were individually assessed. Kinetics of the EODS system for dibenzothiophene in model fuel was developed, comprising the chemical kinetic mechanism and mass transfer effects. The kinetic parameters of the kinetic model, such as the kinetic constants and apparent activation energy, were determined. The EODS system in the experimental condition is determined by both the extraction process and catalytic oxidation reaction, because the terms involving the intrinsic resistance of the chemical reaction are no more important than mass transfer resistance.

1. INTRODUCTION Recently, desulfurization from fuel oils has become an increasingly important subject worldwide as a result of environmental reasons and stricter regulations on sulfur content. According to the environmental protection laws of the European Union and the United States, the sulfur concentration in fuel oils should not exceed 10 and 15 ppmw, respectively.1−4 The traditional hydrodesulfurization (HDS) technology demands severe operating conditions, such as high temperature and high hydrogen pressures.5 Another disadvantage of HDS is hardly enough to acquire ultraclean oils as a result of the observably low hydrogenated reactivity of the heterocyclic sulfur compounds, such as dibenzothiophene (DBT) and its derivatives.4−6 To overcome the disadvantages of HDS, varied methods, such as adsorptive desulfurization,7 extractive desulfurization (EDS), oxidative desulfurization (ODS),5,6,8 and biodesulfurization,8 are being investigated the world over to produce ultraclean fuels. Among these techniques, ODS is regarded as a potential method to obtain ultralow-sulfur fuel oils.9−15 In the ODS process, DBT and its derivatives are oxidized to sulfones in mild conditions16−19 and then the products are removed by adsorption, decomposition, or solvent extraction.7,20 Heterogeneous catalysts attract a great deal of attention, owing to their recycling use in the ODS system,4,5,13 especially Mo-based catalysts with high oxidation activity and high selectivity.21−24 Aqueous H2O2 as a green and cheap oxidant is used for ODS.9,22,23 However, organosulfur compounds in oil cannot fully contact H2O2 in water. To make sure H2O2, oil, and catalyst mixed enough, violent stirring or ultraphonics is necessary.10 Unfortunately, a continuous ODS process for industrial applications is not established because of disadvantages related to product separation and system economics.4 © 2016 American Chemical Society

Meanwhile, product adsorption and metal leaching make catalysts rapidly deactivated.11 A new method of extraction−oxidation desulfurization (EODS) has been considered as the potential way to solve these problems. The EODS system can reduce the unit operation, overcome the equilibrium limitations, improve conversion, and reduce in raw materials, energy, and solvent consumption.25,26 In our recent EODS study,4 Mo/γ-Al2O3 catalysts have been evaluated, showing prolonged catalyst life and high oxidation activity for DBT and its derivatives in model fuel. Acetonitrile (ACN) is an optimal extractant because reagents (DBT and H2O2) can be extracted and reaction products (sulfone and H2O) can be dissolved; besides, its low surface tension enhances mass transfer at the liquid−liquid and liquid−solid interphases.2,18 Because of the use of an extractant, the EODS system contains two liquid phases (fuel oil and extractant) and a solid phase (catalyst particles). The EODS system takes place in a triphasic mixture, including physical and chemical steps. Mass transfer and chemical reaction play important roles in the kinetic aspect of EODS. Although our previous study is beneficial to ascertain the optimal operating conditions of EODS, further research is necessary to investigate this process in detail that involves physical and chemical issues and develop a theoretical model that can represent the kinetics of EODS. In this work, extraction−oxidation dibenzonthiophene in a triphasic model fuel/ACN/catalyst system was analyzed in detailed to establish a kinetic model for the EODS system. From the experimental data, the kinetic model considering mass transfer coupled with chemical reaction was evaluated and Received: June 28, 2016 Revised: August 19, 2016 Published: August 23, 2016 7214

DOI: 10.1021/acs.energyfuels.6b01552 Energy Fuels 2016, 30, 7214−7220

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Energy & Fuels discriminated. Simultaneously, the corresponding kinetic parameters were obtained.

2. MATERIALS AND METHODS 2.1. Materials. γ-Al2O3 used as the catalyst carriers, DBT (analytical grade) chosen as the model sulfur compound, and (NH4)6Mo7O24·4H2O were supplied by Adamas Reagent Co., Ltd. n-Octane (analytical grade), 30 wt % hydrogen peroxide solution in water used as the oxidant, and ACN (analytical grade) selected as a polar solvent for extraction in the EODS system were obtained from Tianjin Jiangtian Technology Co., Ltd. 2.2. Catalyst Preparation. Mo/γ-Al2O3 catalysts were prepared by incipient wetness impregnation as described previously.4 Simply, γAl2O3 was impregnated in (NH4)6Mo7O24·4H2O aqueous solution. The impregnated carriers were stirred for 30 min, dried at 303 K for 12 h, and then calcined in air for 5 h at 773 K. The nitrogen adsorption isotherms were acquired by an AUTOSORB-IQ (Quantachrome Instruments, Boynton Beach, FL) apparatus. Mo content loading on the Mo/γ-Al2O3 catalyst was obtained by inductively coupled plasma optical emission spectrometry (ICP−OES, Shimadzu ICPE-9000). The characterization results revealed that the Mo/γ-Al2O3 catalyst with a surface area of 231 m2/g and pore volume of 0.3665 cm3/g was synthesized with 16 wt % molybdenum. 2.3. Experimental Methods. The model oil with a concentration of 320 ppmw of sulfur was prepared by dissolving DBT into n-octane. The experiments of desulfurization, by either simultaneous extraction and oxidation or sole oxidation were performed in a 100 mL threenecked round-bottom flask, and the reaction temperature was controlled by a water bath. In the extraction−oxidation experiment, a certain amount of Mo/γ-Al2O3 catalyst, model fuel, and ACN (volume ratios of ACN/fuel = 1:1) were mixed together. A total of 76 μL of 30 wt % H2O2 (molar ratio of H2O2/sulfur = 7, which ensures that H2O2 was in excess) was put into the reactor. A three-phase system was stirred vigorously at a constant speed for 2 h, and samples from the upper model fuel phase were taken at desired time intervals. Samples from the oil phase with sulfides were analyzed in gas chromatography−flame ionization detector (GC−FID). To obtain intrinsic reaction rate constants and examine the effects of the extraction process and chemical reaction on the performances of the EODS system, sole oxidation experiments were performed with the similar operating sequence of EODS. Model solution was prepared by dissolving DBT in ACN at a concentration of 320 ppmw of sulfur. Sole oxidation experiments were performed in a flask with desired amounts of model solution and catalyst at a particular temperature and required stirring speed. Hydrogen peroxide was mixed as an oxidizing agent to this mixture. During the sole oxidation, the mass transfer resistance between phases of model oil and extractant was eliminated. Samples at different intervals of time were withdrawn to be analyzed in GC−FID. The analysis of samples was performed in GC−FID equipped with a HP-5 capillary column (30 m × 0.25 mm × 0.32 μm).

Figure 1. Process of EODS.

(DBTO2) on the catalytic surface. Finally, DBTO2 is transmitted from the catalyst to the extractant and stays in the polar extractant phase, owing to their similar polarity. As DBT in the extractant is oxidized to sulfone and the extraction equilibrium of DBT between model fuel and extractant is destroyed, DBT is continuously transmitted from model fuel to extractant until DBT is totally extracted into the polar extractant phase. It is important to mention that the interface of the Mo/γ-Al2O3 catalyst is covered by the extractant, because Mo/γ-Al2O3 has a rather high hydrophilic character.18 3.1. Mass Transfer and Catalytic Oxidation Reaction. The concentration profile of the liquid−liquid−solid EODS process is shown in Figure 2. The liquid−liquid and liquid−

Figure 2. Concentration profiles of DBT and sulfone during the extraction−oxidation of DBT with the peroxomolybdate species.

solid mass transfer and liquid−solid catalytic oxidation reactions affect kinetics of the EODS system. Catalytic oxidation reactions and mass transfer of reactants and products in the EODS system are analyzed. 3.1.1. Reactant Mass Transfer. 3.1.1.1. H2O2 Mass Transfer. In the EODS process, the addition of H2O2 aqueous solution to this system is miscible in the polar solvent phase and is transmitted to the catalyst active sites rapidly and then surface perxo-molybdenum species mentioned above are formed at the initial time of the process. Thus, the reactant H2O2 transfer from the polar solvent to the catalyst can be neglected. Thus, the mass transfer resistance of H2O2 transferring from the polar solvent to the catalyst can be neglected. On the basis of two-film theory, DBT mass transfer in the liquid−liquid−solid phases is described.

3. RESULTS AND DISCUSSION In the extraction−oxidation process for desulfurization, the extraction process and catalytic oxidation reaction take place simultaneously, as depicted in Figure 1. Initially, DBT is transmitted from model oil into extractant through a liquid− liquid interface and then diffuses from the extractant phase via a liquid−solid interface into the catalyst. On the other hand, H2O2 moves from the extractant to the catalyst through a liquid−solid interface. Subsequently, the active peroxomolybdate species are generated by nucleophilic attack of H2O2 on the catalyst surface.13 In this scheme, H2O2 will not exist in model oil because H2O2 is non-miscible in this phase.18 After that, the peroxomolybdate species react with DBT oxidized first to sulfoxide (DBTO) and then to the corresponding sulfone 7215

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Energy & Fuels 3.1.1.2. DBT Transfer from Oil to the Oil−Extractant Interface. The DBT concentration decreases from CDBT,O to CiDBT,O, where CDBT,O and CiDBT,O stand for the concentrations of DBT in model fuel and in the model fuel section of the oil− extractant interface, respectively (mol m−3). The DBT transmission rate in model oil is presented as i JDBT,O = kL,OaL(C DBT,O − C DBT,O )

comparison to ACN, a slight amount of H2O can be negligible. Thus, the physical adsorption of H2O on the Mo/γ-Al2O3 catalyst also is neglected.17 DBT adsorbed on the catalyst is primarily physical adsorption;13,14 therefore, DBT adsorption on the Mo/γ-Al2O3 catalyst is much weaker than the chemisorption of H2O2. As a result, catalytic oxidation desulfurization over Mo/γ-Al2O3 should conform the Eley− Rideal mechanism, where chemisorbed H2O2 and molecular DBT participated in the chemical process. On the other hand, an accepted reaction sequence is proposed in Figure 3.

(1)

where JDBT is the mass transfer rate of DBT transmitted out of model oil (mol m−3 min−1), kL,O is the mass transfer coefficient of DBT in model oil (m−2 min−1), and aL is the liquid−liquid interfacial area between model oil and extractant (m2). 3.1.1.3. DBT Transfer from the Oil−Extractant Interface to Extractant. The DBT concentration decreases from CiDBT,E to CDBT,E, where CiDBT,E and CDBT,E represent the concentrations of DBT in the extractant section of the liquid−liquid interface and in extractant, respectively (mol m−3). The DBT transmission rate in the extractant is expressed as i JDBT,E = kL,EaL(C DBT,E − C DBT,E)

Figure 3. Proposed sequence involved in the chemical reaction.

(2)

where JDBT,E is the mass transfer rate of DBT transmitted into the extractant (mol m−3 min−1) and kL,E is the mass transfer coefficient of DBT in the extractant (m−2 min−1). Assuming DBT is partition equilibrium at the oil−extractant interface, CiDBT,E can be given i i C DBT,E = K O,EC DBT,O

Stoichiometric equation i of this oxidation process represents the rebirth of the catalyst, followed by stoichiometric equations ii and iii, which lead to sulfoxide and sulfone.23 The peroxomolybdate species Mo(O2) with the oxidation activity are regenerated, where the MoO group interacts with H2O2 to undergo a water molecule lost on the alumina surface. Sulfoxide is generated by the nucleophilic attack of DBT on Mo(O)2 to form sulfoxide and MoO. Subsequently, DBTO is further oxidized by Mo(O)2 to form DBTO2. In the reaction mechanism, an oxygen transfer from Mo(O)2 to DBT is the rate-controlling step of the chemical reaction.14,23 On the basis of the above discussion, the second reaction step (ii) is the controlling step. According to the reaction mechanism,13,14,23 the reaction rate is expressed as

(3)

where KO,E is the partition coefficient at the interface of oil− extractant phases. When this value of CiDBT,E is substituted into eq 2, the DBT transmission rate in the extractant is also presented as i JDBT,E = kL,EaL(K O,EC DBT,O − C DBT,E)

(4)

Likewise, considering DBT transfer from the extractant to catalyst s JDBT,S = keSe(C DBT,E − C DBT,E )

r=

(5)

dt

=

k1k 2CMoC H2O2C DBT,C k −1 + k1C H2O2 + k 2C DBT,C + k 3C DBTO,C (7)

where JDBT,S is the mass transfer rate of DBT transmitted out of the extractant (mol m−3 min−1), ke is the mass transfer coefficient of DBT moving from the extractant to catalyst (m−2 min−1), Se is the outer surface area of the catalyst (m2), and CsDBT,E is the DBT concentration in the outer surface of the catalyst (mol m−3). 3.1.1.4. DBT Transfer from the Outer Surface to the Inner Surface of the Catalyst. s JDBT,C = k iSi(C DBT,E − C DBT,C)

dC DBT,C

where k1, k−1, k2, and k3 represent the reaction rate constants and CMo, CH2O2, and CDBTO,C denote the concentrations of the internal surface of the catalyst, H2O2, and DBTO, respectively. Under the initial rate conditions, the reaction formula iii could be ignored. Equation 7 is simplified to eq 8. ri =

(6)

i ΔC DBT,C

Δt

=

i k1k 2CMoC Hi 2O2C DBT,C i k −1 + k1C Hi 2O2 + k 2C DBT,C

(8)

According to the experimental condition and reaction mechanism, as mentioned above, so that k−1 + k1CiH2O2 ≫ k2CiDBT,C, eq 8 is further simplified to eq 9

where JDBT,C is the mass transfer rate of DBT transferred into the catalyst (mol m−3 min−1), ke is the diffusion coefficient of DBT in the catalyst (m−2 min−1), Si is the inner surface area of the catalyst (m2), and CDBT,C is the DBT concentration in the catalyst (mol m−3). 3.1.2. Catalytic Oxidation Reaction. In the liquid-phase catalytic reaction, ACN is used as the aportic solvent and has major impact on the reaction and reaction kinetics. According to Moore et al.,28 a nucleophilic reaction occurs more activity in aprotic solvents than in the atmosphere of protic solvents. Besides, ACN is chosen as a suitable solvent, owing to the extraction and dissolution ability of products and the performance of low surface tension, which is in favor of the chemical stripping and mass transfer of products.2 In

r = kC DBT,C

(9)

where k is the apparent rate constant of the reaction in the catalyst (min−1); that is k=

k1k 2CMoC H2O2 k −1 + k1C H2O2

(10)

3.1.3. Product Mass Transfer. It is well-known that the reaction products of DBTO2 and H2O, once form in the surface of the catalyst, diffuse from the liquid−solid interface to the 7216

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Energy & Fuels ACN phase and remain in the ACN phase as a result of their high polar affinity.18 Thus, the influence of product mass transfer on the EODS process should be very small and can be ignored. 3.2. Exclusion of Catalytic Diffusion Limitations. To properly study the extraction process and chemical reaction in the EODS system, liquid−solid film and pore diffusion limitations over the catalyst should be excluded before establishing the kinetic equations of the extraction−oxidation. The influence of pore diffusion limitation is powerfully connected with the particle size.29 As shown in Figure 4, it has

Figure 5. Influence of the mixing speed on DBT-removal efficiency. Reaction conditions: temperature, 333 K; n(H2O2)/n(S), 7; C0, 320 μg/g; t, 60 min; and catalyst dosage, 0.2 g.

conditions, because H2O2 is always in excess, the impact of H2O2 decomposition is negligible. When the extraction− oxidation process is at state steady, the mass balance of DBT can be written as JDBT,O = JDBT,E = r (12) By insertion of eqs 1, 4, and 11 into eq 12 and elimination of the concentration parameters of CiDBT,E and CDBT,E, the total rate combining mass transfer and oxidation reaction is

Figure 4. Influence of the particle size on DBT removal efficiency. Reaction conditions: temperature, 333 K; n(H2O2)/n(S), 7; C0, 320 μg/g; t, 60 min; and catalyst dosage, 0.2 g.

r=

been found that removal rates of DBT, by either single-pot extraction−oxidation or sole oxidation, are changed hardly by different particle diameters of the catalyst in the case of a particle size smaller than 0.090−0.125 mm (120−170 mesh); that is to say, the influence of pore diffusion limitation can be ignored for the extraction−oxidation system or sole oxidation system. Therefore, the particle size of the catalyst was controlled at less than 0.090−0.125 mm in the kinetic experiment. Concerning the liquid−solid film diffusion limitation from the extractant to the catalyst contribution, different rotating speeds indicate whether the removal rate of DBT is affected by the external diffusion limitations.30 Figure 5 reveals the influence of the rotating speed on the desulfurization rate by EODS or sole oxidation. It is also acquired that the desulfurization rate increased with raising the agitation speed to 600 revolutions per min (rpm) and attained a stable condition when the rotating speed is beyond this value. Under 600 rpm, the extraction−oxidation system and sole oxidation system have no external mass transfer limitation. Therefore, the rotating speed with 600 rpm is used in the kinetic experiment. In the case of eliminating internal and external diffusion limitations, CDBT,C = CsDBT,E = CDBT,E, eq 9 becomes r = kC DBT,E

r=

C DBT,O 1 K O,EkL,EaL

+

1 kL,OaL

+

1 K O,Ek

(13)

C DBT,O 1 kLaL

+

1 k r,O

(14)

where kr,O is the reaction rate constant in the oil phase (kr,O = KO,Ek) and the inverse of kLaL represents the overall mass transfer resistance (the detailed derivation of eq 13 is in the Supporting Information); that is 1 1 1 = + kLaL K O,EkL,EaL k O,LaL (15) In the complexity of the EODS system, the rate-controlling step may be one or several steps. Several possible conditions are discussed below. 3.3.1. Extraction Process Control. If rate is determined by the extraction process, the catalytic oxidation reaction is adequately fast, which makes the DBT concentration close to zero in extractant; i.e., CDBT,E = 0. On the basis of our previous study,4 the CDBT,E/CDBT,O ratio was kept near the fixed constant in the whole process. Therefore, mass transfer of DBT is not a single rate-determining step. 3.3.2. Chemical Reaction Control. If chemical reaction is infinitely slow, the rate is determined by the catalytic oxidation reaction alone and the influence of the extraction process is negligible, so that kLaL ≫ kr,O. Equation 14 can be simplified as

(11)

3.3. Kinetic Equation. Before mass balance equation established for the EODS process, assumptions are presented: this process is carried out in an ideally mixed isothermal batch reactor; i.e., the reaction heat is ignored. Under typical process

r = k r,OC DBT,O

(16)

or 7217

DOI: 10.1021/acs.energyfuels.6b01552 Energy Fuels 2016, 30, 7214−7220

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Energy & Fuels

Figure 6. Influence of the temperature and time on the sulfur removal by the (a) extraction−oxidation reaction and (b) sole oxidation.

Figure 7. Pseudo-first-order kinetic plots for (a) extraction−oxidation and (b) sole oxidation of DBT at different temperatures.

Table 1. Rate Constants and Partition Coefficients of Sulfur Removal at Different Temperatures DBT removal by EODS kT (min−1)

temperature (K) 303 313 323 333

(1.69 (4.22 (9.17 (2.02

r = kC DBT,E

± ± ± ±

0.01) 0.05) 0.21) 0.03)

× × × ×

DBT removal by sole oxidation k (min−1)

correlation factor R2 −2

10 10−2 10−2 10−1

0.9991 0.9979 0.9956 0.9982

(3.71 (9.09 (1.94 (3.99

± ± ± ±

0.05) 0.14) 0.06) 0.06)

(18)

where kT is the total rate constant considering the extraction process and catalytic oxidation reaction; that is 1 1 1 = + kT kLaL k r,O

10 10−2 10−1 10−1

0.9952 0.9992 0.9980 0.9981

KO,E 1.55 1.32 1.21 1.04

kLaL (min−1) 2.39 5.84 1.72 3.57

× × × ×

−2

10 10−2 10−1 10−1

kr,O (min−1) 5.75 1.20 2.35 4.15

× × × ×

10−2 10−1 10−1 10−1

The kinetic data of either the EODS or sole oxidation process were fitted well to the first-order rate eqs 20 and 21, which were obtained from eqs 18 and 17, respectively

(17)

3.3.3. Extraction Process and Chemical Reaction Control. When the reaction is intermediate, the rate is jointly determined by the extraction process and catalytic oxidation reaction. Both chemical reaction resistance and mass transfer resistance cannot be neglected. Equation 14 can be expressed as r = k TC DBT,O

× × × ×

correlation factor R2 −2

⎛C ⎞ ln⎜ 0 ⎟ = k Tt ⎝ Ct ⎠

(20)

⎛ C E,0 ⎞ ⎟⎟ = kt ln⎜⎜ ⎝ C E, t ⎠

(21)

where kT and k are the first-order rate constants of EODS and sole oxidation, respectively (min−1), C0 is the initial DBT concentration, Ct is DBT concentration after t minutes of reaction in the oil phase, CE,0 is the initial DBT concentration, and CE,t is the DBT concentration after t minutes of reaction in the model solution. As shown in panels a and b of Figure 7, plots of ln(C0/Ct) or ln(CE,0/CE,t) versus reaction time obtained by the experimental data confirm the first-order kinetics of EODS or sole oxidation. From Table 1, the rate constants kT, k, kLaL, and kr,O increase with the increasing reaction temperature, which implies that the total resistance, the liquid−liquid film resistance, and the chemical reaction resistance decrease with the increasing reaction temperature. At the same temperature investigated, the k value is about twice the kT value, which can be explained by the fact that the liquid− liquid film resistance and the chemical reaction resistance jointly influence the kinetic process of EODS and the EODS system is co-controlled by mass transfer and chemical reaction.

(19)

3.4. Kinetic Parameters. On the basis of a rather high hydrophilic character of H2O2 and Mo/γ-Al2O3, most of the chemical reaction of EODS should occur in the ACN atmosphere. To obtain kinetic parameters, the experiments of desulfurization by either EODS or sole oxidation were performed at four temperatures. The results are shown in panels a and b of Figure 6. A raised temperature from 303 to 333 K results in an obvious increase in the reaction rates for both EODS and sole oxidation. The conversion of DBT for both systems was complete after 20 min at 333 K. The Mo/γAl2O3 catalyst exhibits high catalytic activity for both EODS and sole oxidation, which is meaningful for industrial application. 7218

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Energy & Fuels The kr,O value is slightly higher than the kLaL value at the same temperature, which also indicates that the EODS system is cocontrolled by the extraction process and catalytic oxidation reaction, because the terms involving the intrinsic resistance of the chemical reaction are no more important than the liquid− liquid resistance. As a result of the change of the distribution of DBT in model oil and ACN with an increase in the temperature, the values of the liquid−liquid partition coefficient KO,E for EODS decrease slowly. A similar trend was reported elsewhere.27 Figure 8 shows the corresponding Arrhenius plots.

process and catalytic oxidation reaction. The calculated apparent activation energy is Ea1 = 69.0 kJ/mol for DBT removal by the extraction−oxidation process, and the calculated intrinsic activation energy is Ea2 = 66.2 kJ/mol for DBT removal by sole oxidation.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b01552. Detailed derivation of eq 13 (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was supported by the National Undergraduate Innovation and Entrepreneurship Training Project (201410057043), the Tianjin College Science & Technology Developing Fund (20140502), and the Tianjin Research Program of Application Foundation and Advanced Technology (14JCZDJC40600).

■

Figure 8. Arrhenius plots for DBT removal by the simultaneous extraction−oxidation process or sole oxidation process.

REFERENCES

(1) De Filippis, P.; Scarsella, M.; Verdone, N. Oxidative Desulfurization I: Peroxyformic Acid Oxidation of Denzothiophene and Dibenzothiophene. Ind. Eng. Chem. Res. 2010, 49, 4594. (2) Capel-Sanchez, M. C.; Perez-Presas, P.; Campos-Martin, J. M.; Fierro, J. L. G. Highly Efficient Deep Desulfurization of Fuels by Chemical Oxidation. Catal. Today 2010, 157, 390. (3) Zhang, M.; Zhu, W. S.; Xun, S. H.; Li, H. M.; Gu, Q. Q.; Zhao, Z.; Wang, Q. Deep Oxidative Desulfurization of Dibenzothiophene with POM-based Hybrid Materials in Ionic Liquids. Chem. Eng. J. 2013, 220, 328. (4) Tian, Y. J.; Yao, Y.; Zhi, Y. H.; Yan, L. J.; Lu, S. X. Combined Extraction−oxidation System for Oxidative Desulfurization (ODS) of a Model Fuel. Energy Fuels 2015, 29, 618. (5) Hasan, Z.; Jeon, J.; Jhung, S. H. Oxidative Desulfurization of Benzothiophene and Thiophene with WOx/ZrO2 Catalysts: Effect of Calcination Temperature of Catalysts. J. Hazard. Mater. 2012, 205− 206, 216. (6) Dhir, S.; Uppaluri, R.; Purkait, M. K. Oxidative Desulfurization: Kinetic Modeling. J. Hazard. Mater. 2009, 161, 1360. (7) Srivastav, A.; Srivastava, V. C. Adsorptive Desulfurization by Activated Alumina. J. Hazard. Mater. 2009, 170, 1133. (8) Song, C. S.; Ma, X. L. New Design Approaches to Ultra-clean Diesel Fuels by Deep Desulfurization and Deep Dearomatization. Appl. Catal., B 2003, 41, 207. (9) Shen, C.; Wang, Y. J.; Xu, J. H.; Luo, G. S. Oxidative Desulfurization of DBT with H2O2 Catalysed by TiO2/porous Glass. Green Chem. 2016, 18, 771. (10) Gonzalez, L. A.; Kracke, P.; Green, W. H.; Tester, J. W.; Shafer, L. M.; Timko, M. T. Oxidative Desulfurization of Middle-distillate Fuels Using Activated Carbon and Power Ultrasound. Energy Fuels 2012, 26, 5164. (11) Chica, A.; Corma, A.; Dómine, M. E. Catalytic Oxidative Desulfurization (ODS) of Diesel Fuel on a Continuous Fixed-bed Reactor. J. Catal. 2006, 242, 299. (12) Wang, D. H.; Qian, E. W.; Amano, H.; Okata, K.; Ishihara, A.; Kabe, T. Oxidative Desulfurization of Fuel Oil: Part I. Oxidation of

The calculated apparent activation energy is Ea1 = 69.0 kJ/mol for DBT removal by the extraction−oxidation process, and the calculated intrinsic activation energy is Ea2 = 66.2 kJ/mol for DBT removal by sole oxidation. Because the liquid−liquid film resistance does influence the kinetic process of EODS, the two values of activation energies differ very slightly. Simultaneously, this result suggests that the chemical reaction mechanism of extraction−oxidation for DBT carried out on the Mo/γ-Al2O3 catalyst is not essentially different from that of sole oxidation.

4. CONCLUSION EODS using H2O2, ACN, and Mo/γ-Al2O3 catalyst is a novel technology for industrial application. The extraction−oxidation kinetics of DBT in the triphasic oil/ACN/catalyst system was studied in the temperature region of 303−333 K. Because H2O2 and Mo/γ-Al2O3 have a rather high hydrophilic character, most of the chemical reaction of EODS should occur in the ACN atmosphere. To study the heterogeneous system, mass transfer on each phase and chemical reaction in the extractant were individually assessed. On the basis of the detailed analysis of the EODS system, this work developed a kinetic model considering mass transfer coupled with chemical reaction based on the two-film theory, Eley−Rideal mechanism, and a nucleophilic path reaction. The kinetic model indicated that EODS for model fuel was a first-order kinetic law, which was in good accordance with the experimental data. By comparison of the kinetic parameter values of EODS to the corresponding kinetic parameters of sole oxidation, these experimental data indicate that the liquid−liquid film resistance and the chemical reaction resistance jointly influence the kinetic process of EODS and the kinetic control step of EODS is co-controlled by the extraction 7219

DOI: 10.1021/acs.energyfuels.6b01552 Energy Fuels 2016, 30, 7214−7220

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DOI: 10.1021/acs.energyfuels.6b01552 Energy Fuels 2016, 30, 7214−7220