Kinetic Modeling of the ExtractionâOxidation Coupling Process for the

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Kinetic Modelling of Extraction-oxidation Coupling Process for the Removal of Dibenzothiophene Zongjing Lu, Erliang Guo, Hua Zhong, Yajie Tian, Yue Yao, and Shuxiang Lu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01552 • Publication Date (Web): 23 Aug 2016 Downloaded from http://pubs.acs.org on August 28, 2016

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Energy & Fuels

Kinetic

Modelling

of

Extraction-oxidation

Coupling

Process for the Removal of Dibenzothiophene Zongjing Lu1, , Erliang Guo1, Hua Zhong1, Yajie Tian2, Yue Yao1, Shuxiang Lu1,* 1

College of Chemical Engineering and Materials Science, Tianjin University of Science

and Technology, Tianjin 300457, China 2

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072,

China * Corresponding author e-mail address: [email protected]

1

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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 oxidant, acetonitrile as extractant and Mo/γ-Al2O3 as catalyst was performed in a batch reactor. In order to study this complex heterogeneous system, 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 extraction process and catalytic oxidation reaction, as the terms involving the intrinsic resistance of the chemical reaction are no more important than mass transfer resistance.

Keywords: Desulfurization; Kinetics; Extraction; Oxidation; Mass transfer

1. INTRODUCTION Recently, desulfurization from fuel oils has become an increasingly important subject worldwide due to 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, respectively1-4. 2


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The traditional hydrodesulfurization (HDS) technology demands severe operating conditions such as high temperature and high hydrogen pressures5. Another disadvantage of HDS is hardly enough to acquire ultraclean oils, due to the observably low hydrogenated reactivity of the heterocyclic sulfur compounds, such as dibenzothiophene (DBT) and its derivatives4-6. To overcome the disadvantages of HDS, varied methods such as adsorptive desulfurization7, extractive desulfurization (EDS), oxidative desulfurization (ODS)5,6,8, and biodesulfurization8 are being investigated world over to produce ultra clean fuels. Among these techniques, ODS is regarded as a potential method to obtain ultralow-sulfur fuel oils9-15. In ODS process, DBT and its derivatives are oxidized to sulfones in mild conditions

16-19

and then the products are removed by adsorption, decomposition or

solvent extraction7,20. Heterogeneous catalysts attract a great deal of attention owing to their recycling use in the ODS system4,5,13, especially Mo-based catalysts with high oxidation activity and high selectivity21-24. Aqueous H2O2 as a green and cheap oxidant is used for ODS9,22,23. However, organosulfur compounds in oil can not fully contact with H2O2 in water. To make sure H2O2, oil and catalyst mixed enough, violent stir or ultraphonic is necessary10. Unfortunately, a continuous ODS process for industrial applications is not established because of disadvantages related to product separation and system economics4. Meanwhile, products adsorption and metal leaching make catalysts rapidly deactivated 11. A new method of the extraction-oxidation desulfurization(EODS) has been 3


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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 consumption25,26. In our recent EODS study4, Mo/γ-Al2O3 catalysts have been evaluated, showing prolonged catalyst life and high oxidation activity for DBT and its derivatives in model fuel. Acetonitrile is 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 interphases2,18. Because of the use of 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, a 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, the extraction-oxidation dibenzonthiophene in a triphasic model fuel/acetonitrile/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 couple with chemical reaction was evaluated and discriminated. Simultaneously, the corresponding kinetic parameters were obtained.

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2. MATERIALS AND METHODS 2.1. Materials γ-Al2O3 used as the catalyst carriers, dibenzothiophene(DBT, analytical grade) chosen as model sulfur compound and (NH4)6Mo7O24·4H2O were supplied by Adamas Reagent Co., Ltd. n-Octane (analytical grade), hydrogen peroxide 30 wt.% solution in water used as oxidant and acetonitrile(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 previously4. Simply, γ-Al2O3 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 during 5 h at 773 K. The nitrogen adsorption isotherms were acquired by an AUTOSORB-IQ (Quantachrome Instruments, United States) apparatus. Mo content loading on the Mo/γ-Al2O3

catalyst

was

obtained

by

ICP−OES(Shimadzu

ICPE-9000).

The

characterization results revealed that Mo/γ-Al2O3 catalyst with surface area 231 m2/g and pore volume 0.3665 cm3/g was synthesized with a 16 wt.% molybdenum.

2.3. Experimental methods The model oil with a concentration of 320 ppmw sulfur was prepared by dissolving DBT into n-octane. The experiments of desulfurization, either by simultaneous extraction 5


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and oxidation or by sole oxidation were performed in a 100 mL three-necked 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. 76 µL of 30 wt.% H2O2(molar ratio of H2O2/sulfur=7 which ensure 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 GC-FID. To obtain intrinsic reaction rate constants and to examine the effects of extraction process and chemical reaction on the performances of the EODS system, sole oxidation experiments were done with the similar operating sequence of EODS. Model solution was prepared by dissolving dibenzothiophene in acetonitrile at a concentration of 320 ppmw 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 done in GC-FID equipped with a HP-5 capillary column (30 m×0.25mm×0.32µm)

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3. RESULTS and DISCUSSION In the extraction-oxidation process for desulfurization, 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 extractant phase via a liquid-solid interface into the catalyst. On the other hand, H2O2 moves from extractant to the catalyst through a liquid-solid interface. Subsequently, the active peroxomolybdate species are generated by nucleophilic attack of H2O2 on the catalyst surface13. In this scheme, H2O2 will not exist in model oil because H2O2 is nonmiscible in this phase18. After that, the peroxomolybdate species react with DBT oxidized first to sulfoxide(DBTO) and then to the corresponding sulfone(DBTO2) on the catalytic surface. Finally, DBTO2 is transmitted from the catalytic into extractant and keeps in the polar extractant phase, owing to their similar polarity. As DBT in 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 into 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 character18.

3.1. Mass transfer and catalytic oxidation reaction The

concentration

profile

of

the

liquid-liquid-solid

extraction-oxidation

desulfurization process is shown in Figure 2. The liquid-liquid and liquid-solid mass transfer and liquid-solid catalytic oxidation reactions affect kinetics of the EODS system. 7


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Catalytic oxidation reactions and mass transfer of reactants and products in the EODS system are analyzed.

Figure 1. Process of extraction-oxidation desulfurization.

Figure 2. Concentration profiles of dibenzothiophene and sulfone during the extraction-oxidation of DBT with the peroxomolybdate species. 3.1.1. Reactant mass transfer H2O2 mass transfer In the extraction-oxidation desulfurization process, addition of H2O2 aqueous

8


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solution to this system is miscible in the polar solvents phase and is transmitted to the catalyst active sites rapidly, then a surface perxo-molybdenum species above-mentioned 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. Based on two-film theory, DBT mass transfer in the liquid-liquid-solid phases is described. DBT transfer from oil to oil-extractant interface DBT concentration decreases from CDBT,O to CiDBT,O, where CDBT,O and C i DBT,O stand the concentrations of DBT in model fuel and in the model fuel section of oil-extractant interface, respectively(mol·m-3). DBT transmission rate in model oil is presented as: i JDBT,O =kL,O (CDBT,O -CDBT,O )

(1)

where JDBT, is mass transfer rate of DBT transmitted out of model oil (mol·m-3·min-1), kL,O is mass transfer coefficient of DBT in model oil (m-2·min-1), aL is the liquid-liquid interfacial area between model oil and extractant (m2). DBT transfer from oil-extractant interface to extractant DBT concentration decreases from C i DBT,E to CDBT,E, where C i DBT,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). DBT transmission rate in extractant is expressed as:

9


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i JDBT,E =kL,E (CDBT,E -CDBT,E )

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(2)

where JDBT,E is mass transfer rate of DBT transmitted into extractant (mol·m-3·min-1), kL,E is mass transfer coefficient of DBT in extractant(m-2·min-1). Assuming DBT is partition equilibrium at the oil-extractant interface, CiDBT,E can be given : i i CDBT,E =KO,E CDBT,O

(3)

where KO,E is the partition coefficient at the interface of the oil-extractant phases. Substituting this value of C i DBT,E into Eq. (2), DBT transmission rate in extractant is also presented as: i JDBT,E =kL,E (KO,E CDBT,O -CDBT,E )

(4)

Likewise, considering DBT transfer from extractant to catalyst: s JDBT,S =ke CDBT,E -CDBT,E

(5)

where JDBT,S is mass transfer rate of DBT transmitted out of extractant(mol·m-3·min-1), ke is mass transfer coefficient of DBT moving from extractant to catalyst(m-2·min-1), Se is the outer surface area of catalyst(m2), C s DBT,E is DBT concentration in the outer surface of catalyst((mol·m-3). DBT transfer from the outer surface to the inner surface of catalyst: s JDBT,C =ki (CDBT,E -CDBT,C )

(6)

where JDBT,C is mass transfer rate of DBT transferred into 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 catalyst(m2), CDBT,C is DBT concentration in the catalyst(mol·m-3). 3.1.2. Catalytic oxidation reaction 10


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In liquid-phase catalytic reaction, acetonitrile is used as 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, acetonitrile 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 products2. Compared with acetonitrile, slight amount of H2O can be negligible. Thus, the physical adsorption of H2O on the Mo/γ-Al2O3 catalyst also is neglected17. DBT adsorbed on the catalyst is primarily physical adsorption13,14, so DBT adsorption on the Mo/γ-Al2O3 catalyst is much weaker than the chemisorption of H2O2. As a result, the 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. Stoichiometric equation(i) of this oxidation process represents the rebirth of the catalyst, followed by stoichiometric equation(ii) and (iii) which lead to the sulfoxide and sulfone23. The peroxomolybdate species Mo(O2) with the oxidation activity are regenerated, where Mo=O group interacts with H2O2 to undergo a water molecule lost on the alumina surface. The 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 chemical reaction 14,23. 11


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Based on the above discussion, the second reaction step(ii) is the controlling step. According to the reaction mechanism13,14,23, the reaction rate is expressed as:

Figure 3. The proposed sequence involved in the chemical reaction. r=

k1 k2 CMo CH2 O2 CDBT,C dCDBT,C = (7) dt k-1 +k1 CH2 O2 +k2 CDBT,C +k3 CDBTO,C

where k1, k-1, k2 and k3 represent the reaction rate constant, CMo, CH2O2, CDBTO,C denote the concentration of internal surface of the catalyst, H2O2 and DBTO, respectively. Under the initial rate conditions, the reaction formula (iii) could be ignored. The Eq. (7) is simplified into the Eq. (8) i i k1 k2 CMo CiH2 O2 CDBT,C ∆CDBT ,C ri = = (8) i ∆t k-1 +k1 CiH2 O2 +k2 CDBT,C

According to the experimental condition and reaction mechanism as mentioned above, so that k-1+k1CiH2O2>>k2CiDBT,C, the Eq. (8) is further simplified into the Eq. (9) r=kCDBT,C

(9)

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

k=

k1 k2 CMo CH2 O2 (10) k-1 +k1 CH2 O2

3.1.3. Product mass transfer It is well known that the reaction products of DBTO2 and H2O, once form in the

12


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surface of catalyst, diffuse from liquid-solid interface into the acetonitrile phase and remain in acetonitrile phase due to their high polar affinity18. Thus, the influence of product mass transfer on the extraction-oxidation desulfurization process should be very small and can be ignored.

3.2. Exclusion of catalytic diffusion limitations To properly study 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 particle size29. As shown in Figure 4, it has been found that removal rates of DBT, either by single pot extraction-oxidation or by sole oxidation, are changed hardly by different particle diameters 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 extraction-oxidation system or sole oxidation system. Therefore, the particle size of catalyst was controlled at less than 0.090-0.125 mm in the kinetic experiment.

13


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

Figure 5. Influence of mixing speed on DBT-removal efficiency. Reaction condition: temperature of 333 K; n(H2O2)/n(S)=7; C0=320 µg/g; t=60 min; catalyst dosage=0.2 g. Concerning the liquid-solid film diffusion limitation from extractant to the catalyst contribution, different rotating speeds indicate whether the removal rate of DBT is affected by the external diffusion limitations30. Figure 5 reveals the influence of rotating 14


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speed on the desulfurization rate by EODS or sole oxidation. It is also acquired that the desulfurization rate increased with raising the agitation speed up to 600 r/min and attained a stable condition when the rotating speed is beyond this value. Under 600 r/min, extraction-oxidation system and sole oxidation system have no the external mass transfer limitation. Therefore, the rotating speed with 600 r/min is used in the kinetic experiment. In

the case of

eliminating internal

and

external

diffusion

limitations,

CDBT,C=C S DBT,E=CDBT,E, Eq. (9) becomes

r=kCDBT,E

(11)

3.3. Kinetic Equation Before mass balance equation established for the EODS process, assumptions are presented: this process is carried out in ideally mixed-isothermal batch reactor, i.e., the reaction heat is ignored. Under typical process conditions, as 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 inserting Eqs. (1), (4), (11) in Eq. (12) and eliminating the concentration parameters of C i DBT,E and CDBT,E, the total rate combining mass transfer and oxidation reaction is

r=

CDBT,O (13) 1 1 1 KO,E kL,E aL + kL,O aL + KO,E k CDBT,O r= (14) 1 1 kL + kr,O where kr,O is the reaction rate constant in oil phase(kr,O=KO,Ek) and the inverse of kLaL 15


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Page 16 of 26

represents the overall mass transfer resistance(the detailed derivation of Eq. (13) in Supporting Information); that is:

1 1 1 = + (15) kL KO,E kL,E aL kO,L aL

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 extraction process, catalytic oxidation reaction is adequately fast, which makes DBT concentration close to zero in extractant, i.e. CDBT,E=0. Based on our previous study4, 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, rate is determined by catalytic oxidation reaction alone and the influence of extraction process is negligible, so that kLaL>>kr,O. Eq. (14) can be simplified as or

r=kr,O CDBT,O

(16)

r=kCDBT,E

(11)

3.3.3. Extraction process and chemical reaction control When the reaction is intermediate, rate is jointly determined by extraction process and catalytic oxidation reaction. Both chemical reaction resistance and mass transfer resistance can not be neglected. Eq. (14) can be expressed as

r=kT CDBT,O

(17)

where kT is the total rate constant considering extraction process and catalytic oxidation 16


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reaction; that is:

1 1 1 = + (18) kT kL kr,O

3.4. Kinetic Parameters

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

Figure

7.

Pseudo-first-order

kinetic

plots

for


of

(a)

dibenzothiophene and sole oxidation of (b) dibenzothiophene at different temperatures. Based on 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 either by EODS or by sole oxidation were done at four temperatures, respectively. The results are shown in Figure 6(a) and (b), respectively. A raise temperature from 303 K to 333 K results in an obvious increase in 17


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16Temperature(K) 17 18 303 K 19 20 313 K 21 323 K 22 333 K 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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the reaction rates for both of EODS and sole oxidation. The conversion of DBT for both systems was complete 20 min at 333 K. The Mo/γ-Al2O3 catalyst exhibits high catalytic activity for both EODS and sole oxidation, which is meaningful for industrial application. Table 1 Rate constants and partition coefficients of sulfur removal at different temperatures. DBT removal by EODS -1

kT(min ) (1.69±0.01)×10-2 (4.22±0.05)×10-2 (9.17±0.21)×10-2 (2.02±0.03)×10-1

DBT removal by sole oxidation

Correlation factor R2 0.9991 0.9979 0.9956 0.9982

k(min-1) (3.71±0.05)×10-2 (9.09±0.14)×10-2 (1.94±0.06)×10-1 (3.99±0.06)×10-1

Correlation factor R2 0.9952 0.9992 0.9980 0.9981

KO,E

kLaL(min-1)

kr,O(min-1)

1.55 1.32 1.21 1.04

2.39×10-2 5.84×10-2 1.72×10-1 3.57×10-1

5.75×10-2 1.20×10-1 2.35×10-1 4.15×10-1

The kinetic data of either EODS or sole oxidation process was fitted well to the first-order rate equation Eq. (19) and equation Eq. (20), which were obtained from Eq. (17) and Eq. (11), respectively. C0 ln =kT t (19) Ct CE,0 ln =kt (20) CE,t where kT and k are the first-order rate constants of EODS and sole oxidation, respectively(min-1). C0 is the initial DBT concentration and Ct is DBT concentration after t minutes of reaction in oil phase. CE,0 is the initial DBT concentration and CE,t is DBT concentration after t minutes of reaction in the model solution, respectively. As shown in Figure 7(a) and (b), 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,

18


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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, k value is about twice 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. kr,O value is slight higher than kLaL value at the same temperature, which also indicates that EODS system is co-controlled by extraction process and catalytic oxidation reaction, as the terms involving the intrinsic resistance of the chemical reaction are no more important than the liquid-liquid resistance. Due to change the distribution of DBT in model oil and ACN with an increase in temperature, the values of the liquid-liquid partition coefficient KO,E for EODS decrease slowly. The similar trend was reported elsewhere26. Figure 8 shows the corresponding Arrhenius plots. The calculated apparent activation energy is Ea1=69.0 kJ/mol for DBT removal by extraction-oxidation process and the calculated intrinsic activation energy is Ea2=66.2 kJ/mol for DBT removal by sole oxidation. As the liquid-liquid film resistance does influence the kinetic process of EODS, the two values activation energies differ very slightly. Simultaneously, this result suggests that the chemical reaction mechanism of the extraction-oxidation for DBT carried out on Mo/γ-Al2O3 catalyst is no essentially different from that of sole oxidation.

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Energy & Fuels

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Figure

8.

Arrhenius

plots

for

dibenzothiophene

Page 20 of 26

removal

by

simultaneous

extraction-oxidation process or by sole oxidation process.

4. CONCLUSIONS Extraction-oxidation desulfurization using H2O2, acetonitrile and Mo/γ-Al2O3 catalyst is a novel technology for industrial application. The extraction-oxidation kinetics of DBT in the triphasic oil/acetonitrile/catalyst system was studied in the temperature region 303-333 K. Since H2O2 and Mo/γ-Al2O3 have a rather high hydrophilic character, most of chemical reaction of EODS should occur in the ACN atmosphere. In order to study the heterogeneous system, mass transfer on each phase and chemical reaction in extractant were individually assessed. Base on the detailed analysis of the EODS system, this work developed a kinetic model considering mass transfer couple with chemical reaction based on two-film theory, Eley-Rideal mechanism and a nucleophilic path reaction. The kinetic model indicated that the extraction-oxidation

20


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Energy & Fuels

desulfurization for model fuel was a first order kinetic law, which was in good accordance with the experimental data. By comparing the kinetic parameters values of EODS with the corresponding kinetic parameters of sole oxidation, these experimental data indicates 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 extraction process and catalytic oxidation reaction. The calculated apparent activation energy is Ea1=69.0 kJ/mol for DBT removal by extraction-oxidation process and the calculated intrinsic activation energy Ea2=66.2 kJ/mol for DBT removal by sole oxidation.

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Energy & Fuels

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Acknowledgments This research was supported by the National Undergraduate Innovation and Entrepreneurship Training Project (Nos. 201410057043), Tianjin College Science & Technology Developing Fund (No. 20140502), and Tianjin Research Program of Application Foundation and Advanced Technology (Nos. 14JCZDJC40600). 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, 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. 22


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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: Environ. 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 Dibenzothiophenes Using Tert-butyl Hydroperoxide. Appl. Catal. A: Gen. 2003, 253, 91. (13) Garcia-Gutierrez, J. L.; Fuentes, G. A.; Hernández-Terán, M. E.; García, P.; Murrieta-Guevara, F.; Jiménez-Cruz, F. Ultra-deep Oxidative Desulfurization of Diesel Fuel by the Mo/Al2O3-H2O2 System: The Effect of System Parameters on Catalytic Activity. Appl. Catal. A: Gen. 2008, 334, 366. (14) Zhu, W. S.; Zhu, G. P.; Li, H. M.; Chao, Y. H.; Zhang, M.; Du, D. L.; Wang, Q.; Zhao, Z. Catalytic Kinetics of Oxidative Desulfurization with Surfactant-type Polyoxometalate-based Ionic Liquids. Fuel Process. Technol. 2013, 106, 70. (15) Huang, D.; Zhai, Z.; Lu, Y. C.; Yang, L. M.; Luo, G. S. Optimization of Composition of a Directly Combined Catalyst in Dibenzothiophene Oxidation for Deep Desulfurization. Ind. Eng. Chem. 23


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Res. 2007, 46, 1447. (16) Sengupta, A.; Kamble, P. D.; Basu, J. K.; Sengupta, S. Kinetic Study and Optimization of Oxidative Desulfurization of Benzothiophene Using Mesoporous Titanium Silicate-1 Catalyst. Ind. Eng. Chem. Res. 2011, 51, 147. (17) Stencel, J. M.; Makovsky, L. E.; Sarkus, T. A.; De Vries, J.; Thomas, R.; Moulijn, J. A. Raman Spectroscopic Investigation of the Effect of H2O on the Molybdenum Surface Species in MoO3/Al2O3 Catalysts. J. Catal. 1984, 90, 314. (18) Ramírez-Verduzco, L. F.; De los Reyes, J. A.; Torres-García, E. Solvent Effect in Homogeneous and Heterogeneous Reactions to Remove Dibenzothiophene by an Oxidation-extraction Scheme. Ind. Eng. Chem. Res. 2008, 47, 5353. (19) Fang, D. W.; Wang, Q.; Liu, Y.; Xia, L. X.; Zang, S. L. High-efficient Oxidation–extraction Desulfurization Process by Ionic Liquid 1-butyl-3-methyl-imidazolium Trifluoroacetic Acid. Energy Fuels. 2014, 28, 6677. (20) Tang, X. D.; Hu, T.; Li, J. J.; Wang, F.; Qing, D. Y. Desulfurization of Kerosene by the Electrochemical Oxidation and Extraction Process. Energy Fuels. 2015, 29, 2097. (21) Baran, R.; Averseng, F.; Millot, Y.; Onfroy, T.; Casale, S.; Dzwigaj, S. Incorporation of Mo into the Vacant T-Atom Sites of the Framework of BEA Zeolite as Mononuclear Mo Evidenced by XRD and FTIR, NMR, EPR, and DR UV–Vis Spectroscopies. J. Phys. Chem. C. 2014, 118, 4143. (22) Zhu, W. S.; Li, H. M.; Jiang, X.; Yan, Y. S.; Lu, J. D.; Li, L. N.; Xia, J. X.; Commercially Available Molybdic Compound-catalyzed Ultra-deep Desulfurization of Fuels in Ionic Liquids. Green Chem. 2008, 10, 641. 24


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(23) Al-Ajlouni, A. M.; Daiafla, T. M.; El-Khateeb, M. New Nitrophenyl-substituted Polyperoxotungstate Catalyst: A More Active and Selective for the Oxidation of Sulfides by Hydrogen Peroxide. J. Mol. Catal. A: Chem. 2007, 275, 139. (24) Ishihara, A.; Dumeignil, F.; Wang, D. H.; Li, X. G.; Arakawa, H.; Qian, E. W.; Inoue, S.; Muto, A.; Kabe, T. Investigation of Sulfur Behavior on CoMo-based HDS Catalysts Supported on High Surface Area TiO2 by 35S Radioisotope Tracer Method. Appl. Catal. A: Gen. 2005, 292, 50. (25) Ram í rez-Verduzco, L. F.; Torres-Garc í a, E.; G ó mez-Quintana, R.; Gonz á lez-Peña, V.; Murrieta-Guevara, F. Desulfurization of Diesel by Oxidation/extraction Scheme: Influence of the Extraction Solvent. Catal. Today. 2004, 98, 289. (26) Maity, U.; Basu, J. K.; Sengupta, S. Performance Study of Extraction and Oxidation–extraction Coupling processes in the Removal of Thiophenic Compounds. Fuel Process. Technol. 2014, 121, 119. (27) Lu, S.; Wang, L.; Wang, Y.; Mi, Z. Kinetic Model of Gas-liquid-liquid Reactive Extraction for Production of Hydrogen Peroxide. Chem. Eng. Technol. 2011, 34, 823. (28) Moore, J. W.; Pearson, R. G. Kinetics and mechanism, 3rd ed.; John Wiley & Sons: 1961. (29) Marra, L.; Wolbers, P. F.; Gallucci, F.; van Sint Annaland, M. Development of a RhZrO2 Catalyst for Low Temperature Autothermal Reforming of Methane in Membrane Reactors. Catal. Today. 2014, 236, 23. (30) Leveneur, S.; de Araujo Filho, C. A.; Estel, L.; Salmi, T. Modeling of a Liquid–liquid–solid Heterogeneous Reaction System: Model System and Peroxyvaleric Acid. Ind. Eng. Chem. Res. 2011, 51, 189.

Figure Captions 25


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Page 26 of 26

Figure 1. Process of extraction-oxidation desulfurization. Figure 2. Concentration profiles of dibenzothiophene and sulfone during the extraction-oxidation of DBT with the peroxomolybdate species. Figure 3. The proposed sequence involved in the chemical reaction. Figure 4. Influence of mesh number on DBT-removal efficiency. Reaction condition: temperature of 333 K; n(H2O2)/n(S)=7; C0=320 µg/g; t=60 min; catalyst dosage=0.2 g. Figure 5. Influence of mixing speed on DBT-removal efficiency. Reaction condition: temperature of 333 K; n(H2O2)/n(S)=7; C0=320 µg/g; t=60 min; catalyst dosage=0.2 g. Figure 6. Influence of temperature and time on the sulfur removal by(a) the extraction-oxidation reaction and (b) sole oxidation. Figure

7.

Pseudo-first-order

kinetic

plots

for


of

(a)

dibenzothiophene and sole oxidation of (b) dibenzothiophene at different temperatures. Figure

8.

Arrhenius

plots

for

dibenzothiophene

removal

by

simultaneous

extraction-oxidation process or by sole oxidation process. Table Captions Table 1 Rate constants and partition coefficients of sulfur removal at different temperatures.

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