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Sulfur removal in coal tar pitch by oxidation with hydrogen peroxide catalyzed by trichloroacetic acid and ultrasonic waves
Fuel 90 (2011) 3456–3460
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Sulfur removal in coal tar pitch by oxidation with hydrogen peroxide catalyzed by trichloroacetic acid and ultrasonic waves Li’e Jin a, Qing Cao a,b,⇑, Jinpin Li b, Jinxiang Dong b a b
Institute of Chemistry and Chemical Engineering, Taiyuan University of Technology, P.O. Box 030024, Shanxi, PR China Technology Center of Biomass Renewable Energy of Shanxi Province, Taiyuan University of Technology, P.O. Box 030024, Shanxi, PR China
a r t i c l e
i n f o
Article history: Received 7 May 2011 Received in revised form 14 June 2011 Accepted 15 June 2011 Available online 16 July 2011 Keywords: Sulfur removal Coal tar pitch Catalysis Ultrasonic waves
a b s t r a c t A procedure for the desulfurization of coal tar pitch (CTP) by oxidation with hydrogen peroxide (H2O2) was developed, in which trichloroacetic acid (TCA) was used as a catalyst combined with ultrasonic waves. For comparison, the effects of H2O2 combined with different catalysts on sulfur removal were also investigated. The oxidative system composed of H2O2 and TCA is highly effective for sulfur removal from CTP. The reaction conditions such as type of solvent used, temperature, and CTP-to-TCA ratio considerably influence sulfur removal when the same oxidant is used. The desulfurization efficiency for CTP with 0.9 wt.% sulfur content reaches 91.1 wt.% at a xylene-to-CTP volume ratio of 2.5, a CTP-to-TCA mass ratio of 0.5, an ultrasonic treatment duration of 60 min, a reaction temperature of 70 °C, and with an extraction liquid containing methanol and sodium hydroxyl. The experiment confirms that the addition of surface active agent has no beneficial effect on sulfur removal. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Coal tar pitch (CTP) is a complex substance with high carbon content. It is an excellent binding material because of its strong stickability, and is also used as paving material. One of its more valuable applications is in the preparation of carbonaceous mesophase, which is a precursor for the production of carbon–carbon composites such as carbon fiber [1], needle coke [2–4], and fillers for liquid chromatography [5]. However, the presence of sulfur affects the formation of the mesophase sphere when sulfur content exceeds 0.7% [6]. Sulfur also causes ‘‘puffing’’ through its evolution at the graphitization stage [7–9] and irreversible electric capacity loss [10]. In the aluminum-refining industry and electric steel process, CTP with low sulfur content has been proposed for anode fabrication using coke obtained either through delayed coking or horizontal chamber coking. Therefore, sulfur removal from the parent material, or CTP, is necessary. The sulfur content in CTP ranges from 0.6 to 1.2 wt.%, values that considerably influence its use as a new advanced carbon material. Sulfur in CTP exists mainly in the form of organosulfur compounds. Two methods for removing such compounds are currently adopted. One is hydrodesulfurization (HDS), conducted at high temperatures (320–380 °C) and pressures (3–7 MPa), over sulfided CoMo or NiMo catalysts. The other is oxi⇑ Corresponding author at: Institute of Chemistry and Chemical Engineering, Taiyuan University of Technology, P.O. Box 030024, Shanxi, PR China. Tel./fax: +86 3516014476. E-mail address: [email protected] (Q. Cao). 0016-2361/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2011.06.047
dative desulfurization (ODS), through which organic sulfur may be converted into more polar sulfones and/or sulfoxides; these converted compounds can be readily extracted by a polar solvent [11]. Generally, S-containing aromatic compounds such as benzothiophenes, dibenzothiophenes, and 4,6-dimethyldibenzothiophene are the substrates most resistant to HDS. These organic sulfur compounds can be removed by ODS. In addition, the conditions required for HDS are milder than that required for ODS, so that the latter has drawn considerable interest. To date, many studies on organic sulfur removal for diesel and jet fuels have been published, but very little is known about sulfur removal from CTP. The properties of CTP are completely different from those of liquid hydrocarbon fuels. Not only is the composition more complex, but the unsaturated degree and viscosity are also higher than those of liquid hydrocarbon fuels. In particular, the S-containing compounds in CTP are more diversified [12], making S removal from CTP more difficult than that in any other liquid hydrocarbon fuel described in literature. To date, a number of oxidants such as NO2 [13], tert-butyl-hydroperoxide [14], and hydrogen peroxide (H2O2) have been investigated for use in ODS processes. Among these, H2O2 is widely used because it is environmentally friendlier and produces no residue. Typically, H2O2 is used in the presence of a catalyst such as formic acid [15], heteropolyacid [16,17], polyoxometalates [18], TS-1 [19], and CF3COOH [20]. In the current work, H2O2 was selected as the oxidant. The influence of catalysts and reaction conditions on desulfurization of CTP was also investigated.
3457
L. Jin et al. / Fuel 90 (2011) 3456–3460 Table 2 Influence of solvents DMF and Xylene on sulfur removal.
2. Experimental 2.1. Materials
Volume ratio of solvent to CTP
The CTP used in the experiment is a medium pitch, and its properties are shown in Table 1. All reagents used in this study, such as trichloroacetic acid (TCA), xylene, methanol, monochloroacetic acid (CH2ClCOOH), and N,N-dimethylformamide (DMF), were analytically pure. The concentrations of hydrogen peroxide (H2O2) solution and acetic acid were 30% and 36%, respectively.
0.25 0.5 1.0 2.0 2.5 3.0
2.2. Instrument and analysis Elemental analysis of the sample was performed using an Elementar Vario EL (Germany) at Beijing University. The determination of sulfur content in coal tar pitch was carried out according to the national standard GB/T387–90 (P.R. China). Desulfurization efficiency (DE) is calculated as follows:
DE ¼ ½ðc1 c2 Þ=c1 100%
ð1Þ
where c1 represents the sulfur content in the raw CTP and c2 denotes the remaining sulfur content in CTP after sulfur removal. The ultrasonic wave generator (FS-300pv, 20 kHz, 300 W) used in the experiment was manufactured by Shanghai ShengXi Ultrasonic Instrument Company, China. 2.3. Method First, the CTP was grinded into fine powder with particle sizes of less than 0.25 mm (60-mesh sieve). Then, 10 g of the CTP powder was placed into a 300 mL beaker. Subsequently, 25 mL xylene, 40 mL H2O2, and 20 g TCA were added into the beaker. The mixture was then treated by ultrasonic waves in a water bath at 70 °C for 1 h. The mixture was held at this temperature for about 40 min until lamination occurred, in which the solid and liquid phases were separated. Then, the liquid was decanted. The remaining CTP was washed two to three times with 50 mL of a mixed solution composed of 50 mL methanol and 50 mL sodium hydroxide (0.5 wt.%). The solid and liquid phases were separated by vacuum filtration. The desulfurized CTP was dried and analyzed for remaining sulfur content. 3. Results and discussion
DMF
Xylene
Remanent S (%)
DE
Remanent S (%)
DE
0.64 0.48 0.33 0.29 0.26 0.22
29 47 63 68 71 76
0.61 0.24 0.21 0.17 0.08 0.09
32 73 77 81 91 90
shown in Fig. 1. The extraction effect of the mixed solution is better than that of methanol for the oxidized S-containing compounds because the presence of sodium hydroxide increases the polarity of the mixture and enhances the extractability of S-containing compounds. 3.2. Influence of surfactant on sulfur removal Qiu et al. [21] reported that amphiphilic hexadecyl trimethyl ammonium chloride is more favorable for wrapping the large molecules of dibenzothiophene around the catalytic center and for forming stable emulsion droplets. It has higher conversion rates than does dodecyl trimethyl ammonium chloride. Jiang et al. [22] used quaternary ammonium surfactant-type decatungstate as a catalyst and reported the influence of carbon chain length on exhibiting the best catalytic performance. In theory, increasing the contact area and contact time of S-containing compounds and oxidants aids sulfur removal. For this reason, the effect of three kinds of surface active agents, namely cetanol, sodium stearate, and polyoxyethylene octylphenol ether-10 (M = 646), which act as dispersants and emulsifiers in sulfur removal, was investigated. The amount of surface active agent added is 0.5 wt.% of CTP mass; the other conditions employed were the same as those described in Section 2.3. Fig. 2 shows that the addition of the surface active agents has no positive effect on sulfur removal regardless of whether one type of surface active agent or a mixture of two surface active agents is applied. These surface active agents increase the dispersion of CTP; thus, they also enhance the emulsification of oxidized S-containing compounds in the organic phase. This phenomenon has no beneficial effect on the phase transfer of the oxidized S-containing compounds from the organic to the aqueous phase. This phenomenon also influences the extraction of polar S-
3.1. Influence of solvents on sulfur removal
N
C
H
S
Oa
C/H
1.23
92.06
4.95
0.90
0.86
1.55
By difference.
0.6 0.5 0.4 0.3 0.2 0.1
Table 1 Elemental analysis of raw CTP.
a
0.7
Sulfur content wt%
Given that the dispersibility and solubility of CTP during reaction have an important influence on the contact between oxidants and S-containing compounds, the role of DMF and xylene as thinners in sulfur removal were investigated at the same volumes and oxidative conditions. The results in Table 2 show that as a solvent, xylene is more suitable than DMF for sulfur removal. When the organic sulfurs are oxidized, they are converted into polar S-containing compounds, which easily transfer into a polar solvent phase. In the experiment, the mixture of methanol and 0.5 wt.% sodium hydroxide was used as an extracting agent, whose volume accounts for half of the mixture volume. The effect of different volume ratios of the extracting agent on sulfur removal from CTP is
0.0
Methanol Methanol+0.5 wt% sodium hydroxy 1
2
3
4
5
Volume ratio of extracting agent to CTP Fig. 1. Influence of volume ratio of extracting agent to CTP on sulfur removal.
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L. Jin et al. / Fuel 90 (2011) 3456–3460 Table 3 Effect of the different oxidative systems on the sulfur removal.
Remanent sulfur content %
1.0
0.8
0.6
Types of oxidative system
Remanent sulfur (wt.%)
DE (wt.%)
Recovery rate (wt.%)
H2O2/acetic acid H2O2/formic acid H2O2/CH2ClCOOH H2O2/DTA H2O2/HClO4 H2O2/Fe2+
0.75 0.66 0.64 0.08 0.83 0.81
16.7 26.7 28.9 91.1 7.8 10.0
67 73 12.4 160 84 80
0.4
Cl 0.2
Cl
O O O
Cl
OH
O
O
Cl
Cl
+ HO2+
Cl
0.0 TEO
TES
TOS
TE
TO
TS
--
Fig. 2. Comparison of effect of sulfur removal after adding different surface active agents using the same condition as Section 2.3. Letters E, O, and S represent cetanol, polyoxyethylene octylphenol ether and sodium stearate respectively. Letters EO, OS, and ES represent the combination of T, E or S whose mass fraction accounts for 0.5% of mass of CTP.
containing compounds. The concentration of surfactants doesn’t make an obvious difference since the concentration of O, E or S is relatively higher than that of EO, OS or ES in the system.
3.3. Comparison of the effect of different oxidative systems on sulfur removal The S-containing compounds in CTP are organosulfur compounds, which include numerous types of thiofuran, benzothiofuran, or thienyl-containing derivatives. The extent to which refractory sulfur compounds are removed is related to the structural complexity of sulfur-contained compounds [23]. Dai et al. [24] reported that the oxidizability of H2O2 can be intensified by ferrous ion (Fe2+), in which Fenton reactions take place. In the current work, iron dichloride was added in the reactive system. Perchloric acid (HClO4) was also investigated as an accelerator for H2O2. Table 3 shows the comparison of the effect of different oxidative systems on sulfur removal via the method described in Section 2.3. The catalyst amount added was 0.12 mol for all the catalysts (TCA, formic acid, acetic acid, Fe2+, and accelerator HClO4). HClO4 exhibits strong acidity and oxidizability, but its effect on sulfur removal is inferior to those of the other organic acids. During the experiment, the addition of Fe2+ causes a marked increment of CTP viscosity. This increase prevents reactive free radicals, such as O-, OH produced by Fe2+, from accessing the S-containing species. Thus, Fe2+ imposes a negative effect on sulfur removal. Although the four organic acids all form peroxoic acids with H2O2, which produces some reactive species, they exhibit different roles in sulfur removal. Only TCA performs excellently in sulfur removal. Among the peroxoic acids formed by the organic acids with H2O2, the O–OOH bond weakens and causes breakage in the presence of chloride; it also produces stronger oxidative species of HOþ 2 because of the strong induced effect of chloride atom, as depicted in Fig. 3. For comparison, the effect of CH2ClCOOH as a catalyst on sulfur removal was also explored. The acidic strengths of the catalysts follow the order TCA > CH2ClCOOH > formic acid > acetic acid. The effect of these catalysts on sulfur removal is consistent with their acidic strengths. Another reason for the excellent effect of TCA on sulfur removal is that TCA has the highest boiling point among the four organic acids. This characteristic of TCA inhibits the volatilization losses observed in acetic and formic acid. The
Fig. 3. Induced effect caused by chloride atom.
influence of different amounts of TCA on desulfurization efficiency (DE) is shown in Fig. 4. The optimal amount for TCA is 0.12 mol. 3.4. Reaction temperature Fig. 5 illustrates the influence of temperature on DE when TCA is used as catalyst. As seen from the figure, the values of DE at 70 and 80 °C are 91.1% and 91.5%, respectively. Given that the higher temperature also accelerates the rate of decomposition of H2O2, the value of DE at 90 °C is smaller than that at 70 and 80 °C. The values of DE at 70 and 80 °C are close; thus, 70 °C was the temperature chosen. 3.5. Influence of H2O2 volume on sulfur content The influence of the quantity of H2O2 as an oxidant on DE was investigated at 0.12 mol TCA and 70 °C. The results are shown in Fig. 6. As the volume of H2O2 increases, the values of DE also increase. When the volume of H2O2 exceeds 40 mL, however, the value of DE no longer changes. 3.6. Ultrasonic time Compared with using surfactants, ultrasonic treatment can significantly improve reaction efficiency under phase transfer conditions [25,26]. It enables the easy formation of microbubbles and 100 90 80 70 60
DE
T
50 40 30 20 0.00
0.05
0.10
0.15
0.20
0.25
n (TCA) / mol Fig. 4. Influence of moles of TCA on DE.
0.30
0.35
L. Jin et al. / Fuel 90 (2011) 3456–3460
3459
ultrasonic time on DE values is shown in Fig. 7. The DE value increases with the extension of ultrasonic time. When ultrasonic time exceeds 60 min, however, the value of DE no longer increases. The longer the ultrasonic treatment, the better the dispersion of CTP. The CTP is well distributed, but the effect of the increase in ultrasonic time on DE is imperceptible.
92
90
DE
88
4. Conclusions 86
The composition of CTP is more complex than that of any other liquid hydrocarbon fuel, such as diesel and gas. The S-containing compounds in CTP are complex and diversified. Therefore, sulfur removal from CTP is highly challenging work. Given that CTP has high adhesive force even at high temperatures, using the adsorbents reported in literature does not allow for sulfur removal [27,28]. Compared with the other catalysts examined, TCA, used in combination with H2O2 and ultrasonic waves at atmospheric pressure, is a highly effective catalyst for the oxidation of the refractory sulfur compounds. The sulfur content is reduced to 0.05–0.10 wt.% after coupling with extraction by polar solvent. This sulfur concentration satisfies the requirement for precursors for mesophase material. The duration of ultrasonic treatment and temperature impose significant influence on sulfur removal from CTP. Chloride content increases by 15.8 wt.% during desulfurization.
84
82 60
65
70
75
80
85
90
Temperature / ºC Fig. 5. Influence of reaction temperature on DE.
100
90
80
Acknowledgments
DE
70
We wish to thank the key scientific and technological projects in Shanxi Province (20110321039-02) and Program for the Top Science and Technology Innovation Teams of Higher Learning Institutions of Shanxi (TSTIT) for financial support.
60
50
References 40 10
20
30
40
50
V (H2O2) /mL Fig. 6. Influence of H2O2 volume on DE.
100 90 80
DE
70 60 50 40 30 0
10
20
30
40
50
60
70
80
Ultrosonic time /min Fig. 7. Influence of ultrasonic time on DE.
helps improve the liquid–liquid interface area through emulsification. It is also an environment friendly approach. The influence of
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Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Sulfur removal in coal tar pitch by oxidation with hydrogen peroxide catalyzed by trichloroacetic acid and ultrasonic waves Li’e Jin a, Qing Cao a,b,⇑, Jinpin Li b, Jinxiang Dong b a b
Institute of Chemistry and Chemical Engineering, Taiyuan University of Technology, P.O. Box 030024, Shanxi, PR China Technology Center of Biomass Renewable Energy of Shanxi Province, Taiyuan University of Technology, P.O. Box 030024, Shanxi, PR China
a r t i c l e
i n f o
Article history: Received 7 May 2011 Received in revised form 14 June 2011 Accepted 15 June 2011 Available online 16 July 2011 Keywords: Sulfur removal Coal tar pitch Catalysis Ultrasonic waves
a b s t r a c t A procedure for the desulfurization of coal tar pitch (CTP) by oxidation with hydrogen peroxide (H2O2) was developed, in which trichloroacetic acid (TCA) was used as a catalyst combined with ultrasonic waves. For comparison, the effects of H2O2 combined with different catalysts on sulfur removal were also investigated. The oxidative system composed of H2O2 and TCA is highly effective for sulfur removal from CTP. The reaction conditions such as type of solvent used, temperature, and CTP-to-TCA ratio considerably influence sulfur removal when the same oxidant is used. The desulfurization efficiency for CTP with 0.9 wt.% sulfur content reaches 91.1 wt.% at a xylene-to-CTP volume ratio of 2.5, a CTP-to-TCA mass ratio of 0.5, an ultrasonic treatment duration of 60 min, a reaction temperature of 70 °C, and with an extraction liquid containing methanol and sodium hydroxyl. The experiment confirms that the addition of surface active agent has no beneficial effect on sulfur removal. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Coal tar pitch (CTP) is a complex substance with high carbon content. It is an excellent binding material because of its strong stickability, and is also used as paving material. One of its more valuable applications is in the preparation of carbonaceous mesophase, which is a precursor for the production of carbon–carbon composites such as carbon fiber [1], needle coke [2–4], and fillers for liquid chromatography [5]. However, the presence of sulfur affects the formation of the mesophase sphere when sulfur content exceeds 0.7% [6]. Sulfur also causes ‘‘puffing’’ through its evolution at the graphitization stage [7–9] and irreversible electric capacity loss [10]. In the aluminum-refining industry and electric steel process, CTP with low sulfur content has been proposed for anode fabrication using coke obtained either through delayed coking or horizontal chamber coking. Therefore, sulfur removal from the parent material, or CTP, is necessary. The sulfur content in CTP ranges from 0.6 to 1.2 wt.%, values that considerably influence its use as a new advanced carbon material. Sulfur in CTP exists mainly in the form of organosulfur compounds. Two methods for removing such compounds are currently adopted. One is hydrodesulfurization (HDS), conducted at high temperatures (320–380 °C) and pressures (3–7 MPa), over sulfided CoMo or NiMo catalysts. The other is oxi⇑ Corresponding author at: Institute of Chemistry and Chemical Engineering, Taiyuan University of Technology, P.O. Box 030024, Shanxi, PR China. Tel./fax: +86 3516014476. E-mail address: [email protected] (Q. Cao). 0016-2361/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2011.06.047
dative desulfurization (ODS), through which organic sulfur may be converted into more polar sulfones and/or sulfoxides; these converted compounds can be readily extracted by a polar solvent [11]. Generally, S-containing aromatic compounds such as benzothiophenes, dibenzothiophenes, and 4,6-dimethyldibenzothiophene are the substrates most resistant to HDS. These organic sulfur compounds can be removed by ODS. In addition, the conditions required for HDS are milder than that required for ODS, so that the latter has drawn considerable interest. To date, many studies on organic sulfur removal for diesel and jet fuels have been published, but very little is known about sulfur removal from CTP. The properties of CTP are completely different from those of liquid hydrocarbon fuels. Not only is the composition more complex, but the unsaturated degree and viscosity are also higher than those of liquid hydrocarbon fuels. In particular, the S-containing compounds in CTP are more diversified [12], making S removal from CTP more difficult than that in any other liquid hydrocarbon fuel described in literature. To date, a number of oxidants such as NO2 [13], tert-butyl-hydroperoxide [14], and hydrogen peroxide (H2O2) have been investigated for use in ODS processes. Among these, H2O2 is widely used because it is environmentally friendlier and produces no residue. Typically, H2O2 is used in the presence of a catalyst such as formic acid [15], heteropolyacid [16,17], polyoxometalates [18], TS-1 [19], and CF3COOH [20]. In the current work, H2O2 was selected as the oxidant. The influence of catalysts and reaction conditions on desulfurization of CTP was also investigated.
3457
L. Jin et al. / Fuel 90 (2011) 3456–3460 Table 2 Influence of solvents DMF and Xylene on sulfur removal.
2. Experimental 2.1. Materials
Volume ratio of solvent to CTP
The CTP used in the experiment is a medium pitch, and its properties are shown in Table 1. All reagents used in this study, such as trichloroacetic acid (TCA), xylene, methanol, monochloroacetic acid (CH2ClCOOH), and N,N-dimethylformamide (DMF), were analytically pure. The concentrations of hydrogen peroxide (H2O2) solution and acetic acid were 30% and 36%, respectively.
0.25 0.5 1.0 2.0 2.5 3.0
2.2. Instrument and analysis Elemental analysis of the sample was performed using an Elementar Vario EL (Germany) at Beijing University. The determination of sulfur content in coal tar pitch was carried out according to the national standard GB/T387–90 (P.R. China). Desulfurization efficiency (DE) is calculated as follows:
DE ¼ ½ðc1 c2 Þ=c1 100%
ð1Þ
where c1 represents the sulfur content in the raw CTP and c2 denotes the remaining sulfur content in CTP after sulfur removal. The ultrasonic wave generator (FS-300pv, 20 kHz, 300 W) used in the experiment was manufactured by Shanghai ShengXi Ultrasonic Instrument Company, China. 2.3. Method First, the CTP was grinded into fine powder with particle sizes of less than 0.25 mm (60-mesh sieve). Then, 10 g of the CTP powder was placed into a 300 mL beaker. Subsequently, 25 mL xylene, 40 mL H2O2, and 20 g TCA were added into the beaker. The mixture was then treated by ultrasonic waves in a water bath at 70 °C for 1 h. The mixture was held at this temperature for about 40 min until lamination occurred, in which the solid and liquid phases were separated. Then, the liquid was decanted. The remaining CTP was washed two to three times with 50 mL of a mixed solution composed of 50 mL methanol and 50 mL sodium hydroxide (0.5 wt.%). The solid and liquid phases were separated by vacuum filtration. The desulfurized CTP was dried and analyzed for remaining sulfur content. 3. Results and discussion
DMF
Xylene
Remanent S (%)
DE
Remanent S (%)
DE
0.64 0.48 0.33 0.29 0.26 0.22
29 47 63 68 71 76
0.61 0.24 0.21 0.17 0.08 0.09
32 73 77 81 91 90
shown in Fig. 1. The extraction effect of the mixed solution is better than that of methanol for the oxidized S-containing compounds because the presence of sodium hydroxide increases the polarity of the mixture and enhances the extractability of S-containing compounds. 3.2. Influence of surfactant on sulfur removal Qiu et al. [21] reported that amphiphilic hexadecyl trimethyl ammonium chloride is more favorable for wrapping the large molecules of dibenzothiophene around the catalytic center and for forming stable emulsion droplets. It has higher conversion rates than does dodecyl trimethyl ammonium chloride. Jiang et al. [22] used quaternary ammonium surfactant-type decatungstate as a catalyst and reported the influence of carbon chain length on exhibiting the best catalytic performance. In theory, increasing the contact area and contact time of S-containing compounds and oxidants aids sulfur removal. For this reason, the effect of three kinds of surface active agents, namely cetanol, sodium stearate, and polyoxyethylene octylphenol ether-10 (M = 646), which act as dispersants and emulsifiers in sulfur removal, was investigated. The amount of surface active agent added is 0.5 wt.% of CTP mass; the other conditions employed were the same as those described in Section 2.3. Fig. 2 shows that the addition of the surface active agents has no positive effect on sulfur removal regardless of whether one type of surface active agent or a mixture of two surface active agents is applied. These surface active agents increase the dispersion of CTP; thus, they also enhance the emulsification of oxidized S-containing compounds in the organic phase. This phenomenon has no beneficial effect on the phase transfer of the oxidized S-containing compounds from the organic to the aqueous phase. This phenomenon also influences the extraction of polar S-
3.1. Influence of solvents on sulfur removal
N
C
H
S
Oa
C/H
1.23
92.06
4.95
0.90
0.86
1.55
By difference.
0.6 0.5 0.4 0.3 0.2 0.1
Table 1 Elemental analysis of raw CTP.
a
0.7
Sulfur content wt%
Given that the dispersibility and solubility of CTP during reaction have an important influence on the contact between oxidants and S-containing compounds, the role of DMF and xylene as thinners in sulfur removal were investigated at the same volumes and oxidative conditions. The results in Table 2 show that as a solvent, xylene is more suitable than DMF for sulfur removal. When the organic sulfurs are oxidized, they are converted into polar S-containing compounds, which easily transfer into a polar solvent phase. In the experiment, the mixture of methanol and 0.5 wt.% sodium hydroxide was used as an extracting agent, whose volume accounts for half of the mixture volume. The effect of different volume ratios of the extracting agent on sulfur removal from CTP is
0.0
Methanol Methanol+0.5 wt% sodium hydroxy 1
2
3
4
5
Volume ratio of extracting agent to CTP Fig. 1. Influence of volume ratio of extracting agent to CTP on sulfur removal.
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L. Jin et al. / Fuel 90 (2011) 3456–3460 Table 3 Effect of the different oxidative systems on the sulfur removal.
Remanent sulfur content %
1.0
0.8
0.6
Types of oxidative system
Remanent sulfur (wt.%)
DE (wt.%)
Recovery rate (wt.%)
H2O2/acetic acid H2O2/formic acid H2O2/CH2ClCOOH H2O2/DTA H2O2/HClO4 H2O2/Fe2+
0.75 0.66 0.64 0.08 0.83 0.81
16.7 26.7 28.9 91.1 7.8 10.0
67 73 12.4 160 84 80
0.4
Cl 0.2
Cl
O O O
Cl
OH
O
O
Cl
Cl
+ HO2+
Cl
0.0 TEO
TES
TOS
TE
TO
TS
--
Fig. 2. Comparison of effect of sulfur removal after adding different surface active agents using the same condition as Section 2.3. Letters E, O, and S represent cetanol, polyoxyethylene octylphenol ether and sodium stearate respectively. Letters EO, OS, and ES represent the combination of T, E or S whose mass fraction accounts for 0.5% of mass of CTP.
containing compounds. The concentration of surfactants doesn’t make an obvious difference since the concentration of O, E or S is relatively higher than that of EO, OS or ES in the system.
3.3. Comparison of the effect of different oxidative systems on sulfur removal The S-containing compounds in CTP are organosulfur compounds, which include numerous types of thiofuran, benzothiofuran, or thienyl-containing derivatives. The extent to which refractory sulfur compounds are removed is related to the structural complexity of sulfur-contained compounds [23]. Dai et al. [24] reported that the oxidizability of H2O2 can be intensified by ferrous ion (Fe2+), in which Fenton reactions take place. In the current work, iron dichloride was added in the reactive system. Perchloric acid (HClO4) was also investigated as an accelerator for H2O2. Table 3 shows the comparison of the effect of different oxidative systems on sulfur removal via the method described in Section 2.3. The catalyst amount added was 0.12 mol for all the catalysts (TCA, formic acid, acetic acid, Fe2+, and accelerator HClO4). HClO4 exhibits strong acidity and oxidizability, but its effect on sulfur removal is inferior to those of the other organic acids. During the experiment, the addition of Fe2+ causes a marked increment of CTP viscosity. This increase prevents reactive free radicals, such as O-, OH produced by Fe2+, from accessing the S-containing species. Thus, Fe2+ imposes a negative effect on sulfur removal. Although the four organic acids all form peroxoic acids with H2O2, which produces some reactive species, they exhibit different roles in sulfur removal. Only TCA performs excellently in sulfur removal. Among the peroxoic acids formed by the organic acids with H2O2, the O–OOH bond weakens and causes breakage in the presence of chloride; it also produces stronger oxidative species of HOþ 2 because of the strong induced effect of chloride atom, as depicted in Fig. 3. For comparison, the effect of CH2ClCOOH as a catalyst on sulfur removal was also explored. The acidic strengths of the catalysts follow the order TCA > CH2ClCOOH > formic acid > acetic acid. The effect of these catalysts on sulfur removal is consistent with their acidic strengths. Another reason for the excellent effect of TCA on sulfur removal is that TCA has the highest boiling point among the four organic acids. This characteristic of TCA inhibits the volatilization losses observed in acetic and formic acid. The
Fig. 3. Induced effect caused by chloride atom.
influence of different amounts of TCA on desulfurization efficiency (DE) is shown in Fig. 4. The optimal amount for TCA is 0.12 mol. 3.4. Reaction temperature Fig. 5 illustrates the influence of temperature on DE when TCA is used as catalyst. As seen from the figure, the values of DE at 70 and 80 °C are 91.1% and 91.5%, respectively. Given that the higher temperature also accelerates the rate of decomposition of H2O2, the value of DE at 90 °C is smaller than that at 70 and 80 °C. The values of DE at 70 and 80 °C are close; thus, 70 °C was the temperature chosen. 3.5. Influence of H2O2 volume on sulfur content The influence of the quantity of H2O2 as an oxidant on DE was investigated at 0.12 mol TCA and 70 °C. The results are shown in Fig. 6. As the volume of H2O2 increases, the values of DE also increase. When the volume of H2O2 exceeds 40 mL, however, the value of DE no longer changes. 3.6. Ultrasonic time Compared with using surfactants, ultrasonic treatment can significantly improve reaction efficiency under phase transfer conditions [25,26]. It enables the easy formation of microbubbles and 100 90 80 70 60
DE
T
50 40 30 20 0.00
0.05
0.10
0.15
0.20
0.25
n (TCA) / mol Fig. 4. Influence of moles of TCA on DE.
0.30
0.35
L. Jin et al. / Fuel 90 (2011) 3456–3460
3459
ultrasonic time on DE values is shown in Fig. 7. The DE value increases with the extension of ultrasonic time. When ultrasonic time exceeds 60 min, however, the value of DE no longer increases. The longer the ultrasonic treatment, the better the dispersion of CTP. The CTP is well distributed, but the effect of the increase in ultrasonic time on DE is imperceptible.
92
90
DE
88
4. Conclusions 86
The composition of CTP is more complex than that of any other liquid hydrocarbon fuel, such as diesel and gas. The S-containing compounds in CTP are complex and diversified. Therefore, sulfur removal from CTP is highly challenging work. Given that CTP has high adhesive force even at high temperatures, using the adsorbents reported in literature does not allow for sulfur removal [27,28]. Compared with the other catalysts examined, TCA, used in combination with H2O2 and ultrasonic waves at atmospheric pressure, is a highly effective catalyst for the oxidation of the refractory sulfur compounds. The sulfur content is reduced to 0.05–0.10 wt.% after coupling with extraction by polar solvent. This sulfur concentration satisfies the requirement for precursors for mesophase material. The duration of ultrasonic treatment and temperature impose significant influence on sulfur removal from CTP. Chloride content increases by 15.8 wt.% during desulfurization.
84
82 60
65
70
75
80
85
90
Temperature / ºC Fig. 5. Influence of reaction temperature on DE.
100
90
80
Acknowledgments
DE
70
We wish to thank the key scientific and technological projects in Shanxi Province (20110321039-02) and Program for the Top Science and Technology Innovation Teams of Higher Learning Institutions of Shanxi (TSTIT) for financial support.
60
50
References 40 10
20
30
40
50
V (H2O2) /mL Fig. 6. Influence of H2O2 volume on DE.
100 90 80
DE
70 60 50 40 30 0
10
20
30
40
50
60
70
80
Ultrosonic time /min Fig. 7. Influence of ultrasonic time on DE.
helps improve the liquid–liquid interface area through emulsification. It is also an environment friendly approach. The influence of
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