ZHAO Pu-jie, MA Cheng, WANG Ji-tong, QIAO Wen-ming, 2, LING Li-cheng

(1. State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai200237, China; 2. Key Laboratory of Specially Functional Polymeric Materials and Related Technology, Ministry of Education, East China University of Science and Technology, Shanghai200237,China)

Abstract: A two-stage desulfurization method for high-sulfur petroleum coke was developed using Na2CO3-promoted calcination, followed by ultrasonic-assisted chemical oxidation. Using a Na2CO3 to coke mass ratio of 1∶4 and an average particle size of 80 μm for the coke, the desulfurization efficiency of the coke reached a maximum of 67.2% using Na2CO3-promoted calcination at 900 ℃ for 2 h with a heating rate of 1℃ /min. The total desulfurization after the subsequent ultrasonic-assisted oxidation with nitric acid (65 wt%) reached 93.5% at 80 ℃ for 12 h using a nitric acid/coke ratio of 20 mL/g. The amounts of sulfur removed by the Na2CO3-promoted calcination were 73.4% of the thiophenic sulfur and 59.8% of the sulfoxide, with the total amounts removed after the ultrasonic-assisted oxidation being 93.6 and 93.3% respectively. Na2CO3 reacts with H2S and shifts the chemical equilibrium to remove more sulfur during the calcination. Chemical oxidation converts dibenzothiophenic compounds to water-soluble ones. These jointly increase the desulfurization efficiency of the coke.

Key words: Desulfurization; Na2CO3-promoted calcination; Ultrasonic oxidation; High-sulfur petroleum coke

1 Introduction

As an important by-product of heavy oil cracking during oil refining, petroleum coke is widely used as carbon electrode, graphite products, raw material of silicon carbide wear-resistant coating owing to its low volatility, high calorific value, low price and easy availability[1-4]. In recent years, the quality of petroleum coke has been significantly decreased because of using high sulfur petroleum, which consequently limits its use as carbon products. The sulfur contained in petroleum coke in industrial applications will eventually be discharged in the form of sulfur oxides, which will not only increase production costs but also pollute the environment[5]. In the air pollution prevention and control act in China in 2016, the sulfur content of petroleum coke is expected to be limited to lower than 3%. Therefore, it is of great value and significance to effectively reduce sulfur-containing compounds in high-sulfur petroleum coke for its utilization.

The sulfur species in petroleum coke is mostly organic sulfur[6]. To break the C-S bonds in these organic groups, a few methods have been developed, including high temperature calcination[7-8], wet chemical oxidation[9-11], dielectric gas desulfurization[12]and solvent extraction[13]. However, there is still a lack of a very economical way to achieve an efficient and cost-effective desulfurization for high-sulfur petroleum coke. Organic sulfur in petroleum coke has three main types of structures, namely, thiophene, benzothiophene, and dibenzothiophene[14]. It is difficult to remove thiophenic sulfur from high-sulfur petroleum coke, even when it is heated to 1 300 ℃, because of their high thermal stability. El-Kaddah and Ezz found that the desulfurization rate of high-sulfur petroleum coke reached 80% when it was calcined at a temperature of 1 400 ℃[15]. Jin Xiao carried out the oxidation treatment for high-sulfur petroleum coke through a self-made desulfurizer, and the results showed that the desulfurization rate of petroleum coke reached 50%[16]. Parmar demonstrated that the desulfurization rate of petroleum coke reached 31% in the range of 900-1 300 K under the atmosphere of steam[17]. Aly found that the desulfurization rate of petroleum coke reached 35% through soaking treatment with toluene[18]. Although sulfur-containing compounds in petroleum coke could be removed to some extent through the above mentioned methods, the damage for the quality of petroleum coke after desulfurization, the destruction of equipment and environmental pollution make it impossible to be applied in industrial applications. Therefore, these methods have certain limitations for petroleum coke desulfurization and their applications.

In this paper, a two-stage desulfurization method combining Na2CO3-promoted calcination and ultrasonic-assisted chemical oxidation was attempted to efficiently remove the sulfur-containing compounds in high-sulfur petroleum coke. The effect of the processing conditions such as calcination temperature, Na2CO3amount, holding time, particle sizes, and HNO3concentration, oxidation temperature, liquid-solid ratio on the desulfurization rate of high-sulfur petroleum coke were studied. The physicochemical properties of petroleum coke before and after desulfurization were compared.

2 Experimental

2.1 Desulfurization

High-sulfur petroleum coke (Qingdao, China) with particle sizes from 80 to 1 000 μm was used in this study. The proximate and ultimate analysis results of the petroleum coke are listed in Table 1. 3 g sample and different amounts of sodium carbonate were well mixed, placed in a constant-temperature zone of a vacuum tube and heated from room temperature to different final temperatures of 600-1 000 ℃ under inert atmosphere. Then, the sample was washed thoroughly with dilute hydrochloric acid, filtered and placed in a vacuum oven at 110 ℃ for drying. The sulfur content of samples before and after the treatment were measured, and the desulfurization rates were calculated. The treated samples were placed in a beaker, to which nitric acids with different concentrations were added, which was put into an ultrasonic cleaning machine for a further desulfurization with an acid oxidation method. After desulfurization, the samples were washed thoroughly with deionized water, filtered, and then placed in a vacuum oven at 110 ℃ for drying, the sulfur contents of dry samples were measured, and the desulfurization rates were calculated. Three samples at different stages were referred to as C0 (as-received petroleum coke), C1 (obtained after Na2CO3-promoted calcination), C2 (obtained after Na2CO3-promoted calcination and ultrasonic-assisted chemical oxidation).

Table 1 Proximate and ultimate analysis results of petroleum coke.

A: Ash;V: Volatile; FC: Fixed carbon;M: Moisture; ad: air-dried;*: by difference.

The total sulfur content in samples was measured using a high-frequency infrared carbon and sulfur analyzer (HSC-500, Shanghai, China). 20-50 mg samples were put in a crucible and 1-2 min was required to complete the analysis. The desulfurization rateη(%) was calculated using Eq.1:

η=(w0-w1)/w0×100

(1)

Wherew0represents sulfur content in petroleum coke before desulfurization andw1represents sulfur content in petroleum after desulfurization.

2.2 Characterization

The surface micromorphology of samples was observed under a field emission scanning electron microscope (SEM) (JEOL-7100F, JEOL, Tokyo, Japan). The microcrystalline structures of samples were obtained by X-ray diffraction patterns (XRD), which were recorded on a Rigaku D/Max2550 using the Cu (Kα) radiation (λ=0.154 06 nm) and 2θ/(°) ranging from 10° to 80°. The surface chemical compositions of samples were determined on the Axis Ultra DLD X-ray photo electron spectroscope (XPS). The chemical structures of samples were investigated by Fourier Transform Infrared Spectroscopy (FT-IR), using a scanning range from 4 000 to 400 cm-1and a ratio of coke to potassium bromide of 1∶150.

3 Results and discussion

3.1 Optimization of Na2CO3-promoted calcination

The Na2CO3-promoted calcination conditions were optimized byL2556orthogonal experiment using desulfurization rate as an evaluation index. The levels and factors of orthogonal experimental are listed in Table 2, and orthogonal test results are listed in Table 3. As observed from Table 2 and Table 3, the theoretical optimal experimental condition is A4B5C2D5E1, namely, the calcination temperature of 900 ℃, Na2CO3amount of 25 wt%, holding time for 2 h, and average particle size of 80 μm. The descending order of factors for the desulfurization rate of petroleum coke is A> B> C> D> E, and the calcination temperature exhibits the greatest influence on the desulfurization rate of petroleum coke, followed by the added amount of Na2CO3, holding time, particle size and heating rate. According to the above optimal condition, the desulfurization rate of high-sulfur petroleum coke reached 67.2% in the Na2CO3-promoted calcination process.

3.2 Optimization of ultrasonic-assisted chemical oxidation

The conditions of ultrasonic oxidation were optimized byL1645orthogonal experiment using the desulfurization rate as an evaluation index. The orthogonal experimental factors are listed in Table 4, and the results of orthogonal experiment are listed in Table 5. It can be obtained from Table 4 and Table 5 that the theoretical optimal experimental condition is A4B3C4D4E2, and the ultrasonic oxidation temperature at 80 ℃, the concentration of HNO3solution of 50 wt%, holding time for 12 h, average particle size of 80 μm, liquid-solid ratio of 20 mL/g. The descending order of factors for desulfurization rate of petroleum coke is D> E> B> A> C, namely, the particle size of petroleum coke has the greatest influence on the desulfurization rate of petroleum coke, followed by liquid-solid ratio, HNO3solution concentration, ultrasonic oxidation temperature and reaction time. According to the above optimal condition, the desulfurization rate of petroleum coke reached 93.5% through Na2CO3-promoted calcination combined with ultrasonic oxidation.

Table 2 Factors and levels of orthogonal experimental.

Table 3 The results of orthogonal experimental.

Table 4 Factors and levels of orthogonal experimental.

Table 5 The results of orthogonal experimental.

3.3 Characterization and analysis

3.3.1 FT-IR analysis of petroleum coke

Fig. 1 shows FT-IR spectra of three petroleum coke samples (C0, C1 and C2). As observed in Fig. 1, the C-H absorption peak at 3 074 cm-1on the thiophene ring of C1 and C2 disappears, which indicates that the thiophene ring in the C1 and C2 was destroyed. The thiophene characteristic peaks of C1 and C2 at 744 cm-1disappear, and is replaced by the stretching absorption peaks of the C-S bond at 620 cm-1, and the intensity of the C-S stretching peak of C1 is stronger than that of C2, which indicates that the thiophenic sulfur in the petroleum coke is converted to a more stable organic sulfur. But the subsequent ultrasonic oxidation treatment for the sample can effectively remove most of the more stable sulfur-containing compounds. The C-S characteristic absorption peaks of C1 and C2 at 863 cm-1are significantly decreased, and the C-S absorption peak intensity of C2 is weaker than that of C1, which is consistent with the results of desulfurization rate of petroleum coke (67.2% for C1 and 93.5% for C1). The above mentioned facts demonstrated that Na2CO3-promoted calcination combined with ultrasonic-assisted oxidation can effectively remove most of the organic sulfur in high-sulfur petroleum coke.

Fig. 1 FT-IR spectra of three petroleum coke samples.

3.3.2 XRD analysis of petroleum coke

Fig. 2 displays the XRD patterns of three petroleum coke samples. The crystallite parameters for three petroleum coke samples are listed in Table 6. It can be observed in Fig. 2 that the characteristic peaks of three samples are attributed to the crystal structure of graphite, demonstrating that the structure of graphite after desulfurization treatment has not been changed. The (002) peak shifts to the lower degree from C0 to C1 and C2, which illustrates that the layer spaces of graphite increase and the graphitic crystallinity decreases. The (002) peak intensity of C1 is slightly weaker than that of C0, which indicated that the crystal structure has been destroyed to some extent, the amorphous carbon content increases, grahene sheet orientation degree decreases. The intensity of the (002) peak has been weakened further through ultrasonic-assisted chemical oxidation, because the sulfur element of the petroleum coke is not only removed during the oxidation process, but also the carbon element on the carbon skeleton is removed at a small amount.

Fig. 2 XRD patterns of three petroleum coke samples.

Samples2θ002 /(°)d002/ nmLc /nmC025.620.347411.5C125.180.353468.8C224.660.360774.6

3.4 SEM images of petroleum coke

Fig. 3 shows SEM images of three petroleum coke samples. As observed in Fig. 3(a), the C0 exhibits no obvious cracks and pores and its particle gives sharp edges and corners, indicating a high density. However, a small amounts of small particles is attached on the surface of the C1, and local pore structure is formed inside the C1 (SC0=5.32 m2/g，SC1=19.4 m2/g), which is mainly due to the volatilization of light components in the petroleum coke such as sulfur-containing gas during the Na2CO3-promoted calcination. The loose particle surface, no sharp edges, and significant cracks are shown in Fig. 3(c), which is due to the fact that HNO3possibly enters the interior of the particles through petroleum coke pores or crystal defects and reacts with the sulfur-containing compounds during the ultrasonic oxidation.

Fig. 3 SEM images of three petroleum coke samples: (a) C0, (b) C1 and (c) C2.

3.5 XPS analysis

Fig. 4 exhibits XPS spectra of three samples. Five peaks are easily recognized in Fig.4, including S 2p (l62-167 eV), S 2s (223-232 eV), C 1s (282-288 eV), N 1s (395-404 eV), and O 1s (528-537 eV). As shown in Fig.4, the peak intensities of sulfur and nitrogen decrease with proceeding of desulfurization, but the peak intensity of oxygen increases, which is mainly due to the fact that the oxygen atoms are introduced into the petroleum coke during desulfurization[18].

Fig. 4 XPS spectra of petroleum coke samples: (a) C0, (b) C1, (c) C2, (d) S 2p of C0,

The peak fitting of XPS spectra are also shown in Fig. 4. XPSPEAK software was used to fit the peaks of sulfur element. Afterward, the sulfur species were identified from the peak positions with reference to previous literatures[19,20]. The contents of sulfur species in the three samples are displayed in Table 7.

As shown in panels d, e and f of Fig. 4 and Table 7, sulfur exists in the petroleum coke in the form of thiophenes (at 164.0-164.3 eV) and sulfoxides (at 165.0-165.3 eV). It can be obtained from Table 4 that the position of the characteristic peaks of petroleum coke before and after desulfurization has no obvious change. However, there is an additional peak appearing in the calcined sample (C1), which is assigned to sulfur sulfate, and the 2p photoelectron spectroscopy intensities of the sulfur in the samples (C1, C2) after desulfurization are lower than that of pristine sample (C0), indicating that the organic sulfur in the petroleum coke is effectively removed through the above mentioned desulfurization method. Furthermore, the removal rates of thiophenic sulfur and sulfoxide were 73.4% and 59.8% after Na2CO3-promoted calcination, and the total removal rates of thiophenic sulfur and sulfoxide were 93.6% and 93.3% after a further ultrasonic assisted oxidation treatment, respectively.

Table 7 Characteristic peaks and their assignments of three petroleum coke samples.

3.6 Desulfurization mechanism

The organic sulfur of petroleum coke is mainly thiophene sulfur, and it is difficult to be removed because of the stability of thiophene ring[21]. The C-S bond is the weakest one in the system of the thiophene and is preferentially broken during pyrolysis[22]. However, the destruction of C-S bonds in thiophene compounds requires higher energy because the bond energy of C-S bond is 156.64 kcal/mol. According to Attar[23], in pyrolysis, sulfur-containing compounds are cleaved to form free radicals, and these sulfur-containing radicals first abstract hydrogen atoms, and then decompose subsequently to form H2S and olefin. These hydrogen atoms come from hydrogen-containing compounds in petroleum coke (probably alkyl or hydro aromatic units), but the internal hydrogen atoms (3.40 wt%) of petroleum coke cannot meet sulfur-containing radicals. Thus, it is possible that large amounts of these radicals recombine to form more stable thiophene structures that are difficult to be removed. The addition of alkali metal compounds to petroleum coke can not only activate C-S bonds, but also react with hydrogen sulfide to avoid recombination of these free radicals to form more stable thiophenic sulfur compounds. However, the desulfurization for high-sulfur petroleum coke through adding alkali metal compounds is difficult to remove the thiophenic sulfur of complex structure. The sulfur atoms in the petroleum coke often exist in negative divalent, and these sulfur atoms contain two pairs of lonely electronic pairs, so their electronegativity is strong. The complex thiophene sulfur are partially oxidized in soluble state under the action of the oxidant, thus it could be further removed through chemical oxidation. The desulfurization mechanism of petroleum coke may be mainly carried out by (1)-(5).

Where A·represents ferrite free radicals.

4 Conclusions

The desulfurization method of Na2CO3-promoted calcination combined with ultrasonic-assisted chemical oxidation was developed to deeply remove the sulfur-containing compounds in high-sulfur petroleum coke. The desulfurization rate of petroleum coke reached the maximum value of 67.2% through Na2CO3-promoted calcination under the conditions of calcination temperature at 900 ℃, Na2CO3amount of 25%, holding time for 2 h, average particle size of 80 μm and heating rate of 1 ℃/min. The total desulfurization rate can reach 93.5% through further ultrasonic-assisted oxidation under the conditions of oxidation temperature at 80 ℃, HNO3concentration of 50 wt%, holding time for 12 h, average particle size of 80 μm and liquid-solid ratio of 20 mL/g. More definitely, the removal rates of thiophenic sulfur and sulfoxide reached 73.4% and 59.8% during the Na2CO3-promoted calcination, and the total removal rates of thiophenic sulfur and sulfoxide reached 93.6% and 93.3% after subsequent ultrasonic-assisted oxidation, respectively. It demonstrates that most of the organic

sulfur in petroleum coke can be effectively removed through Na2CO3-promoted calcination combined with ultrasonic-assisted chemical oxidation as revealed by the FTIR and XPS analysis, and the physicochemical properties of petroleum coke after desulfurization are slightly reduced.