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Deep oxidative desulfurization of model fuel using ozone generated by dielectric barrier discharge plasma combined with ionic liquid extraction
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JIEC-1661; No. of Pages 6 Journal of Industrial and Engineering Chemistry xxx (2013) xxx–xxx
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Deep oxidative desulfurization of model fuel using ozone generated by dielectric barrier discharge plasma combined with ionic liquid extraction Cunhua Ma a,b, Bin Dai b,*, Ping Liu b, Na Zhou b, Aijun Shi b, Lili Ban b, Hongwei Chen b a
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China School of Chemistry and Chemical Engineering, Shihezi University, Key Laboratory for Green Processing of Chemical Engineering of Xinjiang Bintuan, Xinjiang, Shihezi 832003, PR China
b
A R T I C L E I N F O
Article history: Received 24 June 2013 Accepted 2 November 2013 Available online xxx Keywords: Oxidative desulfurization Model fuel DBD plasma Ozone concentration Ionic liquid
A B S T R A C T
The dielectric barrier discharge (DBD) is often used to prepare ozone. In this study, a novel room temperature oxidative desulfurization method involving ozone oxidation produced in the DBD reactor combined with ionic liquid (IL) [BMIM]CH3COO ([BMIM]Ac) extraction was developed. The method was suitable for the deep removal of sulfur (S)-containing compounds from model fuel. By this desulfurization technology, 4,6-dimethyldibenzothiophene (4,6-DMDBT), dibenzothiophene (DBT), benzothiophene (BT) and thiophene (TS) were efficiently removed. Normally, the removal of TS and BT from fuel is highly difficult. However, using the proposed method of this study without any catalyst, the removal rate of TS and BT reached 99.9%. When TiO2/MCM-41 was used as a catalyst, the S-removal of DBT and 4,6-DMDBT increased to 98.6 and 95.2%, respectively. The sulfur removal activity of the four sulfur compounds decreased in the order of TS > BT >> DBT > 4,6-DMDBT. ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Sulfur (S)-containing compounds are undesirable in diesel fuel because the emission of SOx gases after combustion significantly pollutes the environment and poses risks to human health [1,2]. In recent years, numerous regulations limiting the S-content (<10 ppm) to substantially low levels have been introduced in many countries [3–5]. Thus, a low-cost, high-efficiency, easyoperation, and environment friendly method of achieving ultradeep desulfurization of diesel fuel must be developed [6]. Diesel fuel is predominantly a mixture of 75% aliphatic hydrocarbons and 25% aromatic hydrocarbons [7]. Thiol, sulfide, thiophene (TS), and its derivatives such as benzothiophene (BT), dibenzothiophene (DBT), and 4,6-dimethyldibenzothiophene (4,6DMDBT) are classified as organic S-compounds. BT and its derivatives are more difficult to remove than the others because of their steric hindrance structure. Various desulfurization techniques such as hydrodesulfurization [5,8,9], extractive desulfurization [10,11], biodesulfurization
* Corresponding author. Tel.: +86 0 993 2058176; fax: +86 0 993 2057270. E-mail address: [email protected] (B. Dai).
[12,13], adsorptive desulfurization [14–19], and oxidative desulfurization (ODS) [20–23] have been extensively investigated. ODS method means that S-compounds in the fuels firstly are oxidized into corresponding sulfones and/or sulfoxides, then sulfones and/ or sulfoxides are extracted to remove from oil phase using suitable extractants, and ODS method combined with extraction is considered to be one of the most promising alternative methods for deep desulfurization because of its ability to remove BT and its derivatives effectively compared with other processes [24–26]. The oxidation of S-compounds in fuel is a crucial step to desulfurization. The oxidant used most often in ODS is hydrogen peroxide [27]. However, hydrogen peroxide can decompose into water, forming an oil–water biphasic system that affects the fuel quality and results in the difficult recovery of the oil phase. If a gas is used as an oxidizing agent in ODS, oil–water biphasic problems cannot exist. DBD plasma, which is formed in a strong electric field with low energy and high efficiency, is regarded as a new and promising technology [28,29]. The DBD plasma has been widely used in catalyst modification [30], organic synthesis [31,32], treatment of exhaust gases and wastewater [33,34], and other fields [35–37]. Especially, fresh ozone can be conveniently and continuously produced by DBD plasma in presence of air or oxygen at
1226-086X/$ – see front matter ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2013.11.005
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atmospheric pressure [38]. Ozone is a very strong oxidant and can be used to oxidize the organic sulfur compounds in fuel. In this study, we described a room-temperature oxidative desulfurization system, which attempted to produce ozone generated by wire-cylinder DBD reactor as oxidant to oxidize TS, BT, DBT, and 4,6-DMDBT to their corresponding sulfones and/or sulfoxides. The resulting compounds were then extracted by the IL [BMIM]CH3COO ([BMIM]Ac). Key factors such as the oxidation time, amount of IL, air flow rate, discharge voltage, extraction time, and extraction temperature that influenced the efficiency of oxidation and extraction were thoroughly investigated.
2. Experimental 2.1. Materials and methods TS, BT, DBT, 4,6-DMDBT, and IL were purchased from J&K Chemical Ltd. and used without further purification. Model diesel was prepared by dissolving T, BT, DBT, and 4,6-DMDBT in n-octane to provide a solution with S-contents of 500, 500, 107, and 94 ppm, respectively. TiO2/MCM-41 catalyst was synthesized according to a reported method [39]. 2.2. Experimental apparatus Fig. 1 is the schematic of the experimental apparatus for ODS process. The apparatus was mainly composed of a plasma generator (CTP-2000 K, Nanjing Suman electronic Co., Ltd.), a DBD reactor, a mass flow controller, an air compressor, an oscilloscope, and ODS portion. The DBD reactor was, wire-cylinder structure (inner diameter: 19 mm), made of 2 mm thick quartz glass, which was served as a dielectric barrier. Outside of the quartz glass, a layer wire mesh was surrounded as low voltage (LV) electrode, and inside of the quartz glass, a 3 mm thick round steel rod was installed as high voltage (HV) electrode with a discharge gap of about 8 mm between the two electrodes. A variable voltage transformer was connected to the plasma generator to provide the input voltage. Air controlled using mass flow controller was forced into the discharge gap at a certain flow rate, and a digital storage oscilloscope (RIGOL DS1102E) was used to measure the discharge voltage. The discharge voltage was varied from 13.4 to 18.6 kV (peak-to-peak value) with a frequency of 14.3 kHz to change the
concentration of ozone generated, and the concentration of ozone was detected according to CJ/T3028.2–94 (Chinese Standard). 2.3. Oxidative and extractive desulfurization procedure Desulfurization experiments were carried out in 10 mL twonecked flasks equipped with a condenser. Fresh ozone was prepared by DBD plasma, in which air was used as the feed gas. About 3 mL of model oil was added to the flask (for the model oil consisting of DBT or 4,6-DMDBT, 0.01 mg of TiO2/MCM-41 catalyst was added). Under magnetic stirring, O3 was injected into the system. After a specific time, the oxidation reaction was terminated, and 0.25 g of IL was added to extract the oxidative products BTO and/or BTO2. Then, the upper oil phase was periodically collected and analyzed by microcoulometry (WK2D, Jiangsu Jiangfen Electroanalytical Instrument Co., Ltd., China); the detection limit of the instrument was 0.2 mg L1 and relative standard deviation (RSD) of this method was within 2%, respectively. 3. Results and discussion 3.1. The example waveform of the voltage at DBD plasma reactor The example waveform of the voltage was recorded by the digital storage oscilloscope and shown in Fig. 2 at the wire-cylinder DBD plasma reactor when the peak voltage was 16.4 kV. When the discharge voltage applied to the DBD reactor reached the breakdown voltage of the air, a large number of filamentous micro-discharges were generated between the two electrodes, in the meantime blue luminous phenomenon could be observed clearly in the quartz tube. The micro-discharges were randomly distributed on the surface of the inner rod electrode and randomly sprawled to another location in a remarkably short time. Obviously, the waveform of the voltage of DBD was a sinusoidal waveform. 3.2. Effect of the discharge voltage and air flow rate on ozone concentration Fig. 3 shows the effect of the discharge voltage and the air flow rate on the ozone concentration generated in the wire-cylinder DBD reactor. Mok et al., [40] have reported that the generation
Fig. 1. The schematic of the experimental apparatus for ODS process.
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3.3. Effect of the ozone oxidation time on BT removal Initially, model oil consisting of BT was chosen as a model to explore the oxidative desulfurization method (Fig. 4). S-removal did not increase even with 90 min extraction time when only [BMIM]Ac was introduced and ozone oxidation was not performed. Thus, BT was almost insoluble in [BMIM]Ac, and the desulfurization system was ineffective in the absence of ozone oxidation. However, the S-removal rate significantly increased with the oxidation time followed by [BMIM]Ac extraction. Fig. 4 shows that about 70% S-removal was achieved with 10 min oxidation time, and 99.9% was achieved with 30 min oxidation time. The oxidative products of BT, BTO and/or BTO2, were easily extracted by the IL [BMIM]Ac compared with BT. This was because the stronger polarity of BTO and/or BTO2 than that of initial Scompounds, they were extracted more easily by strongly polar IL according to the theory of ‘‘similarity and intermiscibility’’. Thus, ozone oxidation played an important role in the desulfurization system. Based on this result, a desulfurization system containing both ozone oxidation and IL extraction was subsequently used.
Fig. 2. The example waveforms of the voltage at the plasma reactor.
mechanism of ozone in DBD reactor is consisted of two steps: the oxygen molecule is collided by the energetic electrons (e) produced during the electrical discharge and dissociated into the oxygen atom, and the ozone is generated based on the collision by the oxygen atom and the oxygen molecule. The specific reaction steps are expressed by Eqs. (1) and (2): O2 þ e ! O þ O þ e
(1)
O þ O2 ! O3
(2)
As can be seen, the ozone concentration increased with the increasing discharge voltage but fell beyond 19 kV. However, the ozone content decreased with the increasing air flow rate. The causes of the decline were the decomposition of the generated ozone that might include the collision decomposition with the excess electron produced by strong discharge voltage or with the oversupply impurity in air by large air flow rate, and the heat decomposition [41] as shown in Eq. (3). (When the air flow rate was below 90 mL min1 or discharge voltage was less than 14.6 kV, the ozone could not be detected). O3
e or impurity or heat
!
O2 þ O
(3)
Fig. 3. Effect of the discharge voltage and air flow rate on ozone concentration. Experimental condition: frequency value = 14.3 kHz.
3.4. Optimization of the air flow rate and discharge voltage on desulfurization of model fuel The air flow rate introduced into the DBD reactor was an important factor in the process of ODS. As shown in Fig. 5, with increased air flow rate, S-removal initially increased, formed a plateau, and finally decreased. When the air flow rate was less than 90 mL min1, the input quantity of air in unit time was inadequate, which resulted in less active oxidant such as ozone when air discharged in the reactor. Similarly, when the air flow rate was too large (more than 300 mL min1), the generated ozone would be decomposed into the oxygen atom and the oxygen molecule by collision with the excess impurity in air as indicated in Eq. (3). However, within the moderate air flow of 90–300 mL min1, sufficient air input quantity was provided, which supplied an excellent condition for S-removal. Consequently, the S-compounds in the model oil were almost completely oxidized to BTO and/or BTO2. Thus, 150 mL min1 was chosen as the optimal air flow rate in subsequent investigations. The discharge voltage applied to the DBD reactor was another key factor that influenced desulfurization rate in the process of ODS. Fig. 6 shows that the desulfurization efficiency of BT increased with increased discharge voltage. A higher discharge voltage resulted in more ozone (Fig. 3) produced in the discharge region, which induced complete oxidation. As shown in Fig. 6, the
Fig. 4. Effect of the oxidation time on sulfur removal of BT. Experimental conditions: 3 mL model oil (S-content 500 ppm), m(oil)/m(IL) = 8, frequency value = 14.3 kHz; discharge voltage = 17.2–18.6 kV; air flow rate = 150 mL min1, extraction temperature = 25 8C.
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Fig. 5. Effect of air flow rate on sulfur removal of BT. Experimental conditions: 3 mL model oil (S-content 500 ppm), m(oil)/m(IL) = 8, frequency value = 14.3 kHz, discharge voltage = 17.2–18.6 kV, oxidation time = 30 min, extraction temperature = 25 8C, extraction time: 1 h.
Fig. 7. Effect of different ILs on BT removal. Experimental conditions: frequency value = 14.3 kHz, discharge voltage = 17.2–18.6 kV, air flow rate = 150 mL min1, oxidation time = 30 min, extraction temperature = 25 8C, m(oil)/m(IL) = 8, extraction time = 1 h.
desulfurization rate reached 99.9% when the discharge voltage was between 17.2 and 18.6 kV.
In the industrial application of ILs, the IL dosage is vital. To determine the effect of the mass ratios of model oil to [BMIM]Ac, various ratios of model oil to IL were used for S-removal, and the results are shown in Fig. 9. S-removal exceeded 99.5% when the m(oil)/m(IL) ratio changed from 2 to 9. With gradually increased m(oil)/m(IL) to 18 and 32, the corresponding S-removal slightly decreased to 98.9% and 97.8%, respectively. Further increased m(oil)/m(IL) remarkably decreased S-removal, which demonstrated that a small amount of [BMIM]Ac was better for S-removal in the proposed desulfurization system. In this work, m(oil)/m(IL) ratios between 2 and 9 were selected. Regarding the recyclability of ILs, this will be the subject of our future research.
3.5. Optimization of the IL extraction process to improve BT removal Ionic liquids (ILs) with different structures have different extraction abilities. Four kinds of imidazolium-based ILs with different anions, i.e., [BMIM]PF6, [BMIM]HSO4, [BMIM]H2PO4, and [BMIM]Ac, were selected as extractants for comparison. The results are summarized in Fig. 7. Compared with ILs containing the anions PF6, HSO4, and H2PO4, whose desulfurization efficiencies were all less than 90%, [BMIM]Ac had a better extraction ability than the others and enhanced the S-removal to 99.9%. This finding indicated that the type of anion played an important role in the desulfurization system. Therefore, [BMIM]Ac was chosen as an extractant in this research. The effect of the extraction temperature on BT removal was also investigated. After BT was oxidized to BTO and/or BTO2, [BMIM]Ac was added to this system as an extraction agent. Fig. 8 shows the Sremoval of BT as a function of the extraction temperature. With increasing temperature, S-removal slightly decreased from 99.9% at 25 8C to 95.9% at 80 8C. This result was due to the decreased viscosity of IL with increased temperature, which resulted in the reduced solubility of BTO and/or BTO2 in IL and increased solubility of BTO and/or BTO2 in model oil. The insensitivity of S-removal to the extraction temperature is desirable in industrial applications because it can ensure that desulfurization proceeds at ambient conditions with minimal energy consumption requirement.
Fig. 6. Effect of discharge voltage on sulfur removal of BT. Experimental conditions: 3 mL model oil (S-content 500 ppm), m(oil)/m(IL) = 8, frequency value = 14.3 kHz, air flow rate = 150 mL min1, oxidation time = 30 min, extraction temperature = 25 8C, extraction time = 1 h.
3.6. Comparison of S-removal in model fuel containing different Scompounds To evaluate the performance of O3/[BMIM]Ac desulfurization system on the removal of different S-compounds, four kinds of model diesel fuels containing TS, BT, DBT, and 4,6-DMDBT were examined under the same conditions. As shown in Fig. 10, after oxidizing for 30 min by ozone, the removal of TS and BT both reached 99.9% in the presence of O3/[BMIM]Ac. However, DBT and 4,6-DMDBT were barely removed under the same conditions as those for TS and BT. Interestingly, when 0.01 g of TiO2/MCM-41 was added to the system, 98.6% and 95.2% S-removal rates were achieved for DBT and 4,6-DMDBT, respectively. Regarding the catalyst TiO2/MCM-41 and its catalytic mechanism for desulfurization, this will be discussed in follow-up studies.
Fig. 8. Effect of the extraction temperature on sulfur removal of BT. Experimental conditions: frequency value = 14.3 kHz, discharge voltage = 17.2–18.6 kV, air flow rate = 150 mL min1, oxidation time = 30 min, m(oil)/m(IL) = 8, extraction time = 1 h.
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Fig. 9. Effect of the m(oil)/m(IL) on sulfur removal of BT. Experimental conditions: frequency value = 14.3 kHz, discharge voltage = 17.2–18.6 kV, air flow rate = 150 mL min1, oxidation time = 30 min, extraction temperature = 25 8C, extraction time = 1 h.
5
Fig. 11. The IR spectra of BT and its oxidized products (KBr, cm1).
4. Conclusions The oxidation activity of the four S-compounds decreased in the order of TS, BT >> DBT > 4,6-DMDBT, inconsistent with previous findings [42–44]. The present research involved a gas–oil biphasic or gas–oil–solid triphasic system in the presence of ozone, whereas the previous studies involved oil–water biphasic or oil–water– solid triphasic systems using H2O2 as an oxidant in the absence of a gas phase. This difference may be one of the possible reasons for the discrepant activity orders. 3.7. Proposed oxidative mechanism of the initial sulfur compounds The IR spectra of BT and its oxidized products are shown in Fig. 11. The peak at 700 cm1 represented the stretching vibration of C–S in the conjugated system of BT. In the oxidized product BTOx, the intensity decreased and new characteristic peaks appeared, which were attributed to the asymmetric and symmetric stretching vibrations of O5 5S5 5O at 1283 and 1117 cm1. A peak belonging to the S5 5O vibration appeared at 1073 cm1, indicating that BT was oxidized to BTO and/or BTO2. The oxidative reaction equation can be described as follows:
An effective room temperature desulfurization system for the removal of TS, BT, DBT, and 4,6-DMDBT from model diesel fuel was developed. After 30 min of ozone oxidization at an air flow rate of 150 mL min1 and discharge voltage of 17.2–18.6 kV, followed by extraction with [BMIM]Ac for 1 h, the S-removal of model oil containing TS and BT reached 99.9%. When TiO2/MCM-41 was used as a catalyst, DBT and 4,6-DMDBT removal reached 98.6% and 95.2%, respectively. This novel technology provides excellent desulfurization efficiency for model diesel fuel, especially for TS and BT, which are difficult to remove using other oxidative desulfurization methods. The oxidation activity of the four Scompounds decreased in the order of TS, BT >> DBT > 4,6-DMDBT. Acknowledgements This work was financially supported by the Program for Changjiang Scholars and Innovative Research Team in University (Grant No. IRT1161) and the National Natural Science Foundation of China (No. 21063012). References
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Deep oxidative desulfurization of model fuel using ozone generated by dielectric barrier discharge plasma combined with ionic liquid extraction Cunhua Ma a,b, Bin Dai b,*, Ping Liu b, Na Zhou b, Aijun Shi b, Lili Ban b, Hongwei Chen b a
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China School of Chemistry and Chemical Engineering, Shihezi University, Key Laboratory for Green Processing of Chemical Engineering of Xinjiang Bintuan, Xinjiang, Shihezi 832003, PR China
b
A R T I C L E I N F O
Article history: Received 24 June 2013 Accepted 2 November 2013 Available online xxx Keywords: Oxidative desulfurization Model fuel DBD plasma Ozone concentration Ionic liquid
A B S T R A C T
The dielectric barrier discharge (DBD) is often used to prepare ozone. In this study, a novel room temperature oxidative desulfurization method involving ozone oxidation produced in the DBD reactor combined with ionic liquid (IL) [BMIM]CH3COO ([BMIM]Ac) extraction was developed. The method was suitable for the deep removal of sulfur (S)-containing compounds from model fuel. By this desulfurization technology, 4,6-dimethyldibenzothiophene (4,6-DMDBT), dibenzothiophene (DBT), benzothiophene (BT) and thiophene (TS) were efficiently removed. Normally, the removal of TS and BT from fuel is highly difficult. However, using the proposed method of this study without any catalyst, the removal rate of TS and BT reached 99.9%. When TiO2/MCM-41 was used as a catalyst, the S-removal of DBT and 4,6-DMDBT increased to 98.6 and 95.2%, respectively. The sulfur removal activity of the four sulfur compounds decreased in the order of TS > BT >> DBT > 4,6-DMDBT. ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Sulfur (S)-containing compounds are undesirable in diesel fuel because the emission of SOx gases after combustion significantly pollutes the environment and poses risks to human health [1,2]. In recent years, numerous regulations limiting the S-content (<10 ppm) to substantially low levels have been introduced in many countries [3–5]. Thus, a low-cost, high-efficiency, easyoperation, and environment friendly method of achieving ultradeep desulfurization of diesel fuel must be developed [6]. Diesel fuel is predominantly a mixture of 75% aliphatic hydrocarbons and 25% aromatic hydrocarbons [7]. Thiol, sulfide, thiophene (TS), and its derivatives such as benzothiophene (BT), dibenzothiophene (DBT), and 4,6-dimethyldibenzothiophene (4,6DMDBT) are classified as organic S-compounds. BT and its derivatives are more difficult to remove than the others because of their steric hindrance structure. Various desulfurization techniques such as hydrodesulfurization [5,8,9], extractive desulfurization [10,11], biodesulfurization
* Corresponding author. Tel.: +86 0 993 2058176; fax: +86 0 993 2057270. E-mail address: [email protected] (B. Dai).
[12,13], adsorptive desulfurization [14–19], and oxidative desulfurization (ODS) [20–23] have been extensively investigated. ODS method means that S-compounds in the fuels firstly are oxidized into corresponding sulfones and/or sulfoxides, then sulfones and/ or sulfoxides are extracted to remove from oil phase using suitable extractants, and ODS method combined with extraction is considered to be one of the most promising alternative methods for deep desulfurization because of its ability to remove BT and its derivatives effectively compared with other processes [24–26]. The oxidation of S-compounds in fuel is a crucial step to desulfurization. The oxidant used most often in ODS is hydrogen peroxide [27]. However, hydrogen peroxide can decompose into water, forming an oil–water biphasic system that affects the fuel quality and results in the difficult recovery of the oil phase. If a gas is used as an oxidizing agent in ODS, oil–water biphasic problems cannot exist. DBD plasma, which is formed in a strong electric field with low energy and high efficiency, is regarded as a new and promising technology [28,29]. The DBD plasma has been widely used in catalyst modification [30], organic synthesis [31,32], treatment of exhaust gases and wastewater [33,34], and other fields [35–37]. Especially, fresh ozone can be conveniently and continuously produced by DBD plasma in presence of air or oxygen at
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atmospheric pressure [38]. Ozone is a very strong oxidant and can be used to oxidize the organic sulfur compounds in fuel. In this study, we described a room-temperature oxidative desulfurization system, which attempted to produce ozone generated by wire-cylinder DBD reactor as oxidant to oxidize TS, BT, DBT, and 4,6-DMDBT to their corresponding sulfones and/or sulfoxides. The resulting compounds were then extracted by the IL [BMIM]CH3COO ([BMIM]Ac). Key factors such as the oxidation time, amount of IL, air flow rate, discharge voltage, extraction time, and extraction temperature that influenced the efficiency of oxidation and extraction were thoroughly investigated.
2. Experimental 2.1. Materials and methods TS, BT, DBT, 4,6-DMDBT, and IL were purchased from J&K Chemical Ltd. and used without further purification. Model diesel was prepared by dissolving T, BT, DBT, and 4,6-DMDBT in n-octane to provide a solution with S-contents of 500, 500, 107, and 94 ppm, respectively. TiO2/MCM-41 catalyst was synthesized according to a reported method [39]. 2.2. Experimental apparatus Fig. 1 is the schematic of the experimental apparatus for ODS process. The apparatus was mainly composed of a plasma generator (CTP-2000 K, Nanjing Suman electronic Co., Ltd.), a DBD reactor, a mass flow controller, an air compressor, an oscilloscope, and ODS portion. The DBD reactor was, wire-cylinder structure (inner diameter: 19 mm), made of 2 mm thick quartz glass, which was served as a dielectric barrier. Outside of the quartz glass, a layer wire mesh was surrounded as low voltage (LV) electrode, and inside of the quartz glass, a 3 mm thick round steel rod was installed as high voltage (HV) electrode with a discharge gap of about 8 mm between the two electrodes. A variable voltage transformer was connected to the plasma generator to provide the input voltage. Air controlled using mass flow controller was forced into the discharge gap at a certain flow rate, and a digital storage oscilloscope (RIGOL DS1102E) was used to measure the discharge voltage. The discharge voltage was varied from 13.4 to 18.6 kV (peak-to-peak value) with a frequency of 14.3 kHz to change the
concentration of ozone generated, and the concentration of ozone was detected according to CJ/T3028.2–94 (Chinese Standard). 2.3. Oxidative and extractive desulfurization procedure Desulfurization experiments were carried out in 10 mL twonecked flasks equipped with a condenser. Fresh ozone was prepared by DBD plasma, in which air was used as the feed gas. About 3 mL of model oil was added to the flask (for the model oil consisting of DBT or 4,6-DMDBT, 0.01 mg of TiO2/MCM-41 catalyst was added). Under magnetic stirring, O3 was injected into the system. After a specific time, the oxidation reaction was terminated, and 0.25 g of IL was added to extract the oxidative products BTO and/or BTO2. Then, the upper oil phase was periodically collected and analyzed by microcoulometry (WK2D, Jiangsu Jiangfen Electroanalytical Instrument Co., Ltd., China); the detection limit of the instrument was 0.2 mg L1 and relative standard deviation (RSD) of this method was within 2%, respectively. 3. Results and discussion 3.1. The example waveform of the voltage at DBD plasma reactor The example waveform of the voltage was recorded by the digital storage oscilloscope and shown in Fig. 2 at the wire-cylinder DBD plasma reactor when the peak voltage was 16.4 kV. When the discharge voltage applied to the DBD reactor reached the breakdown voltage of the air, a large number of filamentous micro-discharges were generated between the two electrodes, in the meantime blue luminous phenomenon could be observed clearly in the quartz tube. The micro-discharges were randomly distributed on the surface of the inner rod electrode and randomly sprawled to another location in a remarkably short time. Obviously, the waveform of the voltage of DBD was a sinusoidal waveform. 3.2. Effect of the discharge voltage and air flow rate on ozone concentration Fig. 3 shows the effect of the discharge voltage and the air flow rate on the ozone concentration generated in the wire-cylinder DBD reactor. Mok et al., [40] have reported that the generation
Fig. 1. The schematic of the experimental apparatus for ODS process.
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3.3. Effect of the ozone oxidation time on BT removal Initially, model oil consisting of BT was chosen as a model to explore the oxidative desulfurization method (Fig. 4). S-removal did not increase even with 90 min extraction time when only [BMIM]Ac was introduced and ozone oxidation was not performed. Thus, BT was almost insoluble in [BMIM]Ac, and the desulfurization system was ineffective in the absence of ozone oxidation. However, the S-removal rate significantly increased with the oxidation time followed by [BMIM]Ac extraction. Fig. 4 shows that about 70% S-removal was achieved with 10 min oxidation time, and 99.9% was achieved with 30 min oxidation time. The oxidative products of BT, BTO and/or BTO2, were easily extracted by the IL [BMIM]Ac compared with BT. This was because the stronger polarity of BTO and/or BTO2 than that of initial Scompounds, they were extracted more easily by strongly polar IL according to the theory of ‘‘similarity and intermiscibility’’. Thus, ozone oxidation played an important role in the desulfurization system. Based on this result, a desulfurization system containing both ozone oxidation and IL extraction was subsequently used.
Fig. 2. The example waveforms of the voltage at the plasma reactor.
mechanism of ozone in DBD reactor is consisted of two steps: the oxygen molecule is collided by the energetic electrons (e) produced during the electrical discharge and dissociated into the oxygen atom, and the ozone is generated based on the collision by the oxygen atom and the oxygen molecule. The specific reaction steps are expressed by Eqs. (1) and (2): O2 þ e ! O þ O þ e
(1)
O þ O2 ! O3
(2)
As can be seen, the ozone concentration increased with the increasing discharge voltage but fell beyond 19 kV. However, the ozone content decreased with the increasing air flow rate. The causes of the decline were the decomposition of the generated ozone that might include the collision decomposition with the excess electron produced by strong discharge voltage or with the oversupply impurity in air by large air flow rate, and the heat decomposition [41] as shown in Eq. (3). (When the air flow rate was below 90 mL min1 or discharge voltage was less than 14.6 kV, the ozone could not be detected). O3
e or impurity or heat
!
O2 þ O
(3)
Fig. 3. Effect of the discharge voltage and air flow rate on ozone concentration. Experimental condition: frequency value = 14.3 kHz.
3.4. Optimization of the air flow rate and discharge voltage on desulfurization of model fuel The air flow rate introduced into the DBD reactor was an important factor in the process of ODS. As shown in Fig. 5, with increased air flow rate, S-removal initially increased, formed a plateau, and finally decreased. When the air flow rate was less than 90 mL min1, the input quantity of air in unit time was inadequate, which resulted in less active oxidant such as ozone when air discharged in the reactor. Similarly, when the air flow rate was too large (more than 300 mL min1), the generated ozone would be decomposed into the oxygen atom and the oxygen molecule by collision with the excess impurity in air as indicated in Eq. (3). However, within the moderate air flow of 90–300 mL min1, sufficient air input quantity was provided, which supplied an excellent condition for S-removal. Consequently, the S-compounds in the model oil were almost completely oxidized to BTO and/or BTO2. Thus, 150 mL min1 was chosen as the optimal air flow rate in subsequent investigations. The discharge voltage applied to the DBD reactor was another key factor that influenced desulfurization rate in the process of ODS. Fig. 6 shows that the desulfurization efficiency of BT increased with increased discharge voltage. A higher discharge voltage resulted in more ozone (Fig. 3) produced in the discharge region, which induced complete oxidation. As shown in Fig. 6, the
Fig. 4. Effect of the oxidation time on sulfur removal of BT. Experimental conditions: 3 mL model oil (S-content 500 ppm), m(oil)/m(IL) = 8, frequency value = 14.3 kHz; discharge voltage = 17.2–18.6 kV; air flow rate = 150 mL min1, extraction temperature = 25 8C.
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Fig. 5. Effect of air flow rate on sulfur removal of BT. Experimental conditions: 3 mL model oil (S-content 500 ppm), m(oil)/m(IL) = 8, frequency value = 14.3 kHz, discharge voltage = 17.2–18.6 kV, oxidation time = 30 min, extraction temperature = 25 8C, extraction time: 1 h.
Fig. 7. Effect of different ILs on BT removal. Experimental conditions: frequency value = 14.3 kHz, discharge voltage = 17.2–18.6 kV, air flow rate = 150 mL min1, oxidation time = 30 min, extraction temperature = 25 8C, m(oil)/m(IL) = 8, extraction time = 1 h.
desulfurization rate reached 99.9% when the discharge voltage was between 17.2 and 18.6 kV.
In the industrial application of ILs, the IL dosage is vital. To determine the effect of the mass ratios of model oil to [BMIM]Ac, various ratios of model oil to IL were used for S-removal, and the results are shown in Fig. 9. S-removal exceeded 99.5% when the m(oil)/m(IL) ratio changed from 2 to 9. With gradually increased m(oil)/m(IL) to 18 and 32, the corresponding S-removal slightly decreased to 98.9% and 97.8%, respectively. Further increased m(oil)/m(IL) remarkably decreased S-removal, which demonstrated that a small amount of [BMIM]Ac was better for S-removal in the proposed desulfurization system. In this work, m(oil)/m(IL) ratios between 2 and 9 were selected. Regarding the recyclability of ILs, this will be the subject of our future research.
3.5. Optimization of the IL extraction process to improve BT removal Ionic liquids (ILs) with different structures have different extraction abilities. Four kinds of imidazolium-based ILs with different anions, i.e., [BMIM]PF6, [BMIM]HSO4, [BMIM]H2PO4, and [BMIM]Ac, were selected as extractants for comparison. The results are summarized in Fig. 7. Compared with ILs containing the anions PF6, HSO4, and H2PO4, whose desulfurization efficiencies were all less than 90%, [BMIM]Ac had a better extraction ability than the others and enhanced the S-removal to 99.9%. This finding indicated that the type of anion played an important role in the desulfurization system. Therefore, [BMIM]Ac was chosen as an extractant in this research. The effect of the extraction temperature on BT removal was also investigated. After BT was oxidized to BTO and/or BTO2, [BMIM]Ac was added to this system as an extraction agent. Fig. 8 shows the Sremoval of BT as a function of the extraction temperature. With increasing temperature, S-removal slightly decreased from 99.9% at 25 8C to 95.9% at 80 8C. This result was due to the decreased viscosity of IL with increased temperature, which resulted in the reduced solubility of BTO and/or BTO2 in IL and increased solubility of BTO and/or BTO2 in model oil. The insensitivity of S-removal to the extraction temperature is desirable in industrial applications because it can ensure that desulfurization proceeds at ambient conditions with minimal energy consumption requirement.
Fig. 6. Effect of discharge voltage on sulfur removal of BT. Experimental conditions: 3 mL model oil (S-content 500 ppm), m(oil)/m(IL) = 8, frequency value = 14.3 kHz, air flow rate = 150 mL min1, oxidation time = 30 min, extraction temperature = 25 8C, extraction time = 1 h.
3.6. Comparison of S-removal in model fuel containing different Scompounds To evaluate the performance of O3/[BMIM]Ac desulfurization system on the removal of different S-compounds, four kinds of model diesel fuels containing TS, BT, DBT, and 4,6-DMDBT were examined under the same conditions. As shown in Fig. 10, after oxidizing for 30 min by ozone, the removal of TS and BT both reached 99.9% in the presence of O3/[BMIM]Ac. However, DBT and 4,6-DMDBT were barely removed under the same conditions as those for TS and BT. Interestingly, when 0.01 g of TiO2/MCM-41 was added to the system, 98.6% and 95.2% S-removal rates were achieved for DBT and 4,6-DMDBT, respectively. Regarding the catalyst TiO2/MCM-41 and its catalytic mechanism for desulfurization, this will be discussed in follow-up studies.
Fig. 8. Effect of the extraction temperature on sulfur removal of BT. Experimental conditions: frequency value = 14.3 kHz, discharge voltage = 17.2–18.6 kV, air flow rate = 150 mL min1, oxidation time = 30 min, m(oil)/m(IL) = 8, extraction time = 1 h.
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Fig. 9. Effect of the m(oil)/m(IL) on sulfur removal of BT. Experimental conditions: frequency value = 14.3 kHz, discharge voltage = 17.2–18.6 kV, air flow rate = 150 mL min1, oxidation time = 30 min, extraction temperature = 25 8C, extraction time = 1 h.
5
Fig. 11. The IR spectra of BT and its oxidized products (KBr, cm1).
4. Conclusions The oxidation activity of the four S-compounds decreased in the order of TS, BT >> DBT > 4,6-DMDBT, inconsistent with previous findings [42–44]. The present research involved a gas–oil biphasic or gas–oil–solid triphasic system in the presence of ozone, whereas the previous studies involved oil–water biphasic or oil–water– solid triphasic systems using H2O2 as an oxidant in the absence of a gas phase. This difference may be one of the possible reasons for the discrepant activity orders. 3.7. Proposed oxidative mechanism of the initial sulfur compounds The IR spectra of BT and its oxidized products are shown in Fig. 11. The peak at 700 cm1 represented the stretching vibration of C–S in the conjugated system of BT. In the oxidized product BTOx, the intensity decreased and new characteristic peaks appeared, which were attributed to the asymmetric and symmetric stretching vibrations of O5 5S5 5O at 1283 and 1117 cm1. A peak belonging to the S5 5O vibration appeared at 1073 cm1, indicating that BT was oxidized to BTO and/or BTO2. The oxidative reaction equation can be described as follows:
An effective room temperature desulfurization system for the removal of TS, BT, DBT, and 4,6-DMDBT from model diesel fuel was developed. After 30 min of ozone oxidization at an air flow rate of 150 mL min1 and discharge voltage of 17.2–18.6 kV, followed by extraction with [BMIM]Ac for 1 h, the S-removal of model oil containing TS and BT reached 99.9%. When TiO2/MCM-41 was used as a catalyst, DBT and 4,6-DMDBT removal reached 98.6% and 95.2%, respectively. This novel technology provides excellent desulfurization efficiency for model diesel fuel, especially for TS and BT, which are difficult to remove using other oxidative desulfurization methods. The oxidation activity of the four Scompounds decreased in the order of TS, BT >> DBT > 4,6-DMDBT. Acknowledgements This work was financially supported by the Program for Changjiang Scholars and Innovative Research Team in University (Grant No. IRT1161) and the National Natural Science Foundation of China (No. 21063012). References
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