VOLUME 20, NUMBER 4
JULY/AUGUST 2006
© Copyright 2006 American Chemical Society
Articles Technique for High-Viscosity Crude Oil Demetallization in the Liaohe Oil Field Bencheng Wu, Jianhua Zhu,* Juan Wang, and Changqi Jiang Faculty of Chemical Science & Engineering, China UniVersity of Petroleum, Beijing 102249, China ReceiVed June 27, 2005. ReVised Manuscript ReceiVed February 27, 2006
Demetallization of high-viscosity crude oil from the Liaohe oil field has been investigated by means of a SH-I electric desalting instrument. The high-viscosity crude oil which is similar to residual fractions of ordinary crude oil of both high-density and high-viscosity belongs to low-sulfur naphthenic crude oil, since it contains various metallic elements with calcium of 406.98 µg/g as the highest concentration, the rest being nickel, iron, sodium, manganese, etc. Research results demonstrate that under the proper conditions the demetallization agents MPTE, MPTF, and their mixtures could reduce the concentration of almost all metallic elements, e.g., calcium, iron, and manganese, in crude oil effectively. Moreover, the demetallization agents developed in this work are environmentally friendly because they contain no heteroatoms of P or S. Meanwhile, the important factors which may affect demetallization efficiency, such as the viscosity of the crude oil, the type and dosage of the demetallization agents, the acidity of their aqueous solution, etc., have been determined experimentally.
1. Introduction Naphthenic crude oil is a kind of rare crude oil resource in the world, that makes up less than 10% of the total crude oil reserves in the world and is mainly distributed in America, Europe, Asia, and the Pacific. China is relatively abundant in naphthenic crude oil with about 25% of the total world reserves, mainly existing in the Kelamayi oil field and the Liaohe oil field.1 The high-viscosity crude oil from the Shuguang zone in the Liaohe oil field is low-sulfur naphthenic crude oil with gum and asphaltene in a mass fraction of more than 50%, which is very difficult to process under atmospheric pressure. At present, part of the high-viscosity crude oil is used as the feedstock of the delayed coking plant and the the remainder is used as boiler fuel after emulsification processing. Although the phenomenon of catalyst poisoning does not exist in the delayed coking * To whom correspondence should be addressed. E-mail: rdcas@ cup.edu.cn. Tel.: +86-10-8973 3711. Fax: +86-10-8973 3419. (1) Zu, D. The new advance on production technology of lubricating base oil. Lubricating Oil (in Chinese) 2001, 16 (3), 2-6.
process, the high concentration of metallic elements in this type of crude oil will result in a higher amount of coke ashes which will degrade coke products and thus cut down the profit of the plant. The type and content of the metallic elements in the crude oil mainly depend on the geological conditions of the crude oil formation. Moreover, the application of enhanced oil recovery techniques may increase the content of metallic elements, especially calcium, in crude oil. Up to now, the calcium content in heavy oil from Kelamayi has exceeded 200 µg/g and in Liaohe heavy oil it has been up to 400 µg/g. Metallic elements in crude oil usually are in the form of inorganic salts, organic salts, metalloporphyrin, etc. The inorganic salts can be easily removed from crude oil by the ordinary pretreatment via electric desalting equipment. However, the metallic elements in the form of metalloporphyrin are difficult to remove since the decomposition temperature of porphyrin compounds is usually more than 250 °C.2 These metallic (2) Shi, Q.; Xu, M. Petroleum geochemistry (in Chinese); Press of the East China Petroleum Institute: 1980; pp 155-157.
10.1021/ef0501896 CCC: $33.50 © 2006 American Chemical Society Published on Web 05/27/2006
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Table 1. Physical and Chemical Properties of High-Viscosity Crude Oil in the Liaohe Oil Field item
data
F20 (g/cm3) υ80 (mm2/s) υ100 (mm2/s) Salt content (mg/L; based on NaCl) Acid value (mg KOH/g) ω(Water) (%) ω(Ash) (%) ω(N) (%) ω(S) (%) ω(Saturate) (%) ω(Aromatic) (%) ω(Resin) (%) ω(Asphaltene) (%) ω(Fe) (µg/g) ω(Ni) (µg/g) ω(Cu) (µg/g) ω(V) (µg/g) ω(Na) (µg/g) ω(Ca) (µg/g) ω(Mn) (µg/g)
0.9983 3661 823.9 9.4 3.95 1.7 0.20 0.71 0.45 20.43 22.05 48.22 9.30 46.34 110.90 0.63 1.91 10.37 406.98 7.80
elements are often removed through the hydrodemetallization technique, and chelating demetallization techniques have also been used as recorded in some of the literature.3 At the same time, the metallic elements in the form of organic salts are also difficult to remove; nevertheless, they could be transferred from the oil phase to the aqueous phase by using demetallization agents first and then removed by oil and water separation. Thus, the development of demetallization agents is critically important for removing metallic elements in the form of organic salts in crude oil. The research works on the chelating demetalization techniques for the demetallization of crude oil could be traced back to the 1980s, and the main demetallization agents were phosphoric acid, polyphosphoric acid, and salts4 thereof since they were easily obtained and cheaper. But, these demetallization agents could result in secondary pollution of water resources. According to the relative patents, EDTA5 and citric acid6 had better decalcifying performance, but their high cost were constraints to industrialization. Methanoic acid and acetic acid at a relatively lower price7,8 could also be used as the demetallization agents. However, their disadvantages are quite obvious due to larger dosage, lower efficiency, and stronger corrosion on equipment and pipelines of the process. At present, demetallization agents used in high-viscosity crude oil usually have an efficiency lower than 50% and cannot meet the demetallization requirement of crude oil in commercial processes. On the basis of many years of work in the laboratory, we have developed a series of demetallization agents successfully. In this work, the performance of these demetallization agents has been evaluated and the proper application conditions have been determined. (3) Lou, S.; Zhang, H.; Sha, O.; et al. Removal of metallic impurities from residual oil. Petrochem. Technol. (in Chinese) 2002, 31 (5), 357360. (4) Kukes, S. G.; Battiste, D. R. Demetallization of heavy oil with phosphorous acid. US Patent 4,522,702, 1985. (5) Reynolds, J. G.; Fenger, T. F. Decalcification of hydrocarbonaceous feedstocks using amino-carboxylic acid and salts thereof (in Chinese). CN Patent 86107286, 1987. (6) Reynolds, J. G. Decalcification of hydrocarbonaceous feedstocks using complex compounds (in Chinese). CN Patent 87105863A, 1987. (7) Reynolds, J. G.; Kramer, D. C. Demetalation of hydrocarbonaceous feedstocks using monobasic carboxylic acids and salts thereof. US Patent 4,988,433, 1990. (8) Xu, Z.; Fu, X.; Tan, L.; et al. Demetallization agents and the method of them for hydrocarbonaceous feedstocks (in Chinese). CN Patent 1431277A, 2003.
Figure 1. Structure of the electric desalting cylinder.
2. Experiments 2.1. Feedstocks, Demetallization Agents, and Analytical Instruments. The properties of the high-viscosity crude oil from the Liaohe oil field are showed in Table 1. From the table, it is clear that the density, viscosity, metal content, acid value, and N content of the oil sample are high while the yield of light fractions and S content are relatively lower. The crude oil is categorized to be low-sulfur naphthenic crude oil in accordance with the classification of the characteristic index of key components. On the basis of the evaluated results of the demulsifers, oilsoluble demulsifier P1, which was provided by the Liaohe Petrochemical Branch Company of PetroChina, was selected and diluted with analytical reagent alcohol. The demetallization agents developed in our prior works include two basic types, MPTE and MPTF. Both of them have mean molecular weights in the range of 100-180, are white or brown solids under atmospheric temperature and pressure, and contain some functional groups, e.g., carboxyl and hydroxyl, and no heteroatoms of P and S, but there is N in the MPTF type demetallization agent. The other demetallization agents are obtained by means of mixing MPTE and MPTF in a certain proportion or by adding ammonia to them. The concentration of demetallization agent was adjusted into 0.02 g/mL by distilled water, and the pH of the corresponding aqueous solution lies in the range of 0.5-6. On the basis of the China GB/T1884-2000 standard, the density of the oil sample is determined by the SY-II petroleum densimeter, which is made by the Shenyang Glass Meter Factory. The viscosity of the oil sample is measured with a DSY-004 kinetic viscosity apparatus, which is made in the Dalian Petroleum Instrument Factory, according to the China GB11137-89 standard. The content of all kinds of metallic elements in the oil sample are analyzed by a PE4300 optical emission spectrometer, made by the PE Company (USA) with the ASTM D4951 method. 2.2. Experimental Apparatus and Experimental Procedure. The SH-I electric desalting instrument was used in the demetallization experiments on the oil sample, and the structure of the electric desalting cylinder is shown in Figure 1. The experimental temperature, intensity of the electric field, reaction time, and sedimentation time were preset according to the specified experiment. The experimental procedures of demetallization for highviscosity crude oil are illustrated in Figure 2, which are described as follows: (1) Blended the high-viscosity crude oil with diesel as the feedstock of the desalting process. (2) Turned on the heating switch of the SH-I electric desalting instrument and set the temperature of the oil bath. (3) Added 50 g of feedstock, the specified amount of P1 demulsifier, the selected demetallization agent, and the water into the electric desalting cylinder, screwed down the upper cap and bottom cap of the electric desalting cylinder, put it into the oil bath of the SH-I electric desalting instrument, turned on the “preheating” button. Took out the electric desalting cylinder after 10 min and shook the cylinder 200 times by hand in order to mix the crude oil and demetallization agent well. (4) Put the electric desalting
Technique for Crude Oil Demetallization
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Figure 3. Influence of the diesel concentration on the viscosity of the oil sample.
Figure 2. Experimental procedure of demetallization for the oil sample.
cylinder into the oil bath again, turned on the “high voltage” button after the preheat finished, and subsequently, the feedstock in the electric desalting cylinder underwent a weak electric field, strong electric field, and free settling, respectively. (5) Took out the electric desalting cylinder from the oil bath, cooled it with water, then screwed off the bottom cap and opened the conical valve of the electric desalting cylinder, and discharged the water completely. Thus, the first-stage demetallization sample was made. (6) Repeated steps (3)-(5) and made the second-stage demetallization sample. 2.3. Demetallization Mechanism. In high-viscosity crude oil and residue, the calcium and iron elements in the form of carboxylate and carbolate9,10 could dissociate in aqueous solutionand reach ionic equilibrium as follows:
where R denoted an alkyl group and M denoted a metal element, such as Ca, Fe, etc. Due to the small equilibrium constant of the above equations, the solubility of carboxylate and carbolate usually is quite small in water. To enhanced the removal efficiency of the metallic elements, some measures must be taken to shift the ionic equilibrium to the right-hand side, thereby metallic elements in carboxylate and carbolate could be transferred into the aqueous phase as much as possible. From the principle of chemical equilibrium shift, decreasing the concentration of products could increase the dissociation degree of organic-metallic compounds in above equations. Therefore, it could shift the dissociation equilibrium into the right-hand side by increasing the concentration of H+ which could bond carboxyl and phenolic ions or by adding some agents which could form deposits or stable metal complexes with metallic ions. Both of the methods could decrease the concentration of metallic ions in the oil sample effectively. These effects could be catalogued into the strong acid effect, complexation effect, and precipitation effect. Guided by the above equilibrium shift principles and molecular design theory, we developed a series of demetallization agents which contain a -COOH group and included at least two kinds of the above-mentioned effects. Therefore, it is possible for a MPTE type demetallization agent, which has all of the above-mentioned effects, to have better performance for metal removal than other types of demetallization agents. (9) Hou, D.; Wang, X. Study on distribution and composition of calcium in some Chinese crudes. Acta Petrolei Sin. (Petroleum Processing Section; in Chinese) 2000, 16 (1), 54-59. (10) Liu, G.; Xu, X.; Gao, J. Study on the deferrization and desalting for crude oil. Energy Fuels 2004, 18 (4), 918-923
Figure 4. Influence of the diesel concentration on the density of the oil sample.
3. Results and Discussions 3.1. Visbreaking Experiments for the High-Viscosity Crude Oil. High-viscosity crude oil is difficult to process due to its high viscosity and high density. So, some diluting agents have been added into high-viscosity crude oil in order to reduce its viscosity and density. As we known, the lower viscosity of the sample could intensify the mixing of the demetallization agents with the oil sample and accelerate the transfer of the metallic elements from the oil phase into the aqueous phase, thus enhancing the demetallization efficiency. Meanwhile, from the Stokes law, the decrease in viscosity and density of the oil sample may speed up the separation of water from oil and enhance the demetallization efficiency in the experimental process. Thus, it is necessary for us to carry out visbreaking experiments to decrease the viscosity of the highly viscous crude oil by mixing diluting agents, such as diesel in certain proportions. To determine the proper proportion of diesel as the diluting agent, we have determined the kinetic viscosity ν100 and density F20 of the oil sample by mixing 5%, 10%, 15%, and 20% diesel (in mass fraction), respectively. In comparison with the pure crude oil sample, the measured viscosity and density data are shown in Figures 3 and 4, respectively. From Figure 3, the kinetic viscosity ν100 of the oil sample demonstrates remarkably the decreasing tendency with increases in the concentration of diesel. When the concentration of diesel is less than 10%, the ν100 of the oil sample decreases rapidly, e.g., ν100 of oil samples with 5% and 10% diesel are about onehalf and one-quarter of that of pure highly viscous crude oil sample, respectively. While the amount of added diesel is more than 10%, the ν100 of the sample changes moderately with a diesel concentration increase. From Figure 4, it is known that the F20 of the oil sample basically reduced linearly with an increase of the amount of diesel added. Although the kinetic viscosity and density of the oil samples decrease with an increase of the diesel concentration, large quantities of diesel added would be infeasible due to the lack of diesel resources and high cost. Therefore, the feasible amount
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Table 2. Demetallization Experimental Results for Feedstock I# ωD a (µg/g) sample No. I# I-1ac I-1bd I-2a I-2b
demetallization agent
first stage
MPTF-Ae MPTF-A MPTE MPTE
1920 1520 1760 1520
a
ηb (%)
ω (µg/g)
second stage
Ca
Fe
Mn
Ca
Fe
Mn
400
344.50 135.70 31.97 12.18 0
36.14 35.49 33.02 15.06 9.436
6.03 4.58 1.79 0 0
60.62 90.72 96.47 100
1.80 8.61 58.33 82.19
24.05 70.29 100 100
400 b
The mass fraction of added demetallization agent. Demetallization efficiency. c The “a” means first-stage processing. d The “b” means second-stage processing. e MPTF-A means the MPTF modified by NH3‚H2O. Table 3. Demetallization Experimental Results for Feedstock II# ωD (µg/g)
demetallization agent sample no. II# II-1 II-2 II-3 II-4 II-5 II-6 II-7 II-8 II-9 II-10 II-11 II-12 II-13 II-14 II-15 II-16
first stage MPTE MPTE MPTE MPTE MPTF MPTF MPTF MPTE MPTF MPT-1a MPT-1 MPT-2b MPT-3c MPTE-Ad MPTF-Ae MPT-1-Af
second stage MPTE MPTE MPTE MPTE MPTF MPTF MPTF MPTF MPTE MPT-1 MPT-1 MPT-2 MPT-3 MPTE-A MPTF-A MPT-1-A
first stage 1200 1280 1400 1600 1200 1400 1600 1400 1400 1600 1400 1400 1400 1600 1600 1600
ω (µg/g)
η (%)
second stage
Ca
Fe
Mn
Ca
Fe
Mn
300 420 400 530 300 400 600 400 400 600 400 400 400 530 600 600
362.1 214.7 53.09 48.56 8.32 176.0 99.40 16.15 14.64 4.58 117.4 179.4 24.28 8.43 210.9 100.7 155.6
38.13 21.8 9.81 5.29 8.31 39.98 16.39 49.3 0.649 0 15.58 11.32 0 0 26.05 34.61 61.27
7.16 3.96 0 0 0 5.59 1.39 1.53 0 0 0 0 0 0 2.89 4.88 5.03
40.72 85.34 86.59 97.70 51.39 72.55 95.54 95.96 98.74 67.58 50.45 93.29 97.67 41.76 72.19 57.04
42.83 74.27 86.13 78.21
44.75 100 100 100 21.94 80.56 78.62 100 100 100 100 100 100 59.61 31.80 29.70
57.02 98.30 100 51.27 70.31 100 100 31.68 9.23 60.69
a MPTE:MPTF ) 1:1 in mass. b MPTE:MPTF ) 3:1 in mass. c MPTE:MPTF ) 1:3 in mass. d MPTE modified by NH ‚H O. e MPTF modified by 3 2 NH3‚H2O. f MPTE: MPTF ) 1:1 in mass and modified by NH3‚H2O.
of added diesel should be controlled within 15% (in mass fraction). For convenience of narration, the oil samples mixed with 15% diesel are named as feedstock I#, and those with 10% diesel, as feedstock II#. 3.2. Demetallization experiments. Referring to previous works and the operating conditions of electric desalting equipment in a refinery, the demetallization temperature for highly viscous crude oil is set as 145 °C; the weak electric field intensity is 450 V/cm, and the acting time is 5 min; the strong electric field intensity is 1000 V/cm and the acting time is 10 min; the time of free settling is set as 30 min. The dosage of P1 type demulsifier is 50 µg/g, and the added water amount is about 8% of the oil sample mass for each stage. The demetallization experiments for the feedstock are carried out by means of a SH-I electric desalting instrument. 3.2.1. Demetallization Results for Feedstock I#. Following the experimental procedures described above, the demetallization experiments for feedstock I# are conducted by means of two types of demetallization agents. The experimental results are listed in Table 2. In view of the data in Table 2, it is evident that the removal efficiency of elements Ca, Fe, and Mn after second-stage processing is obviously higher than that of first-stage processing for both types of demetallization agents. For example, for the MPTF-A demetallization agent, the removal efficiency of Ca, Fe, and Mn is 60.62%, 1.80%, and 24.05% in first-stage processing, respectively, while the corresponding removal efficiency is 90.72%, 8.61%, and 70.29% after second-stage processing. For MPTE type demetallization agents, similar results could also be obtained. Except for sample I-1a, the removing efficiency for element Ca is more than 90% for both the MPTF-A and MPTE types of demetallization agent. For element Fe, there is only 8.61%
removal efficiency with the MPTF-A type demetallization agent with two stages of processing. For the MPTE type demetallization agent, the removal efficiency for element Fe is 58.33% and 82.19% with first-stage and second-stage processing, respectively. In general, the demetallization performance of the MPTE type demetallization agent is better than that of MPTF-A. 3.2.2. Demetallization Results for Feedstock II#. From the visbreaking results, we known that the higher the diesel concentration is, the lower the kinetic viscosity of oil sample is. But with an increase of added diesel, the processing cost of the highly viscous crude oil will rapidly increase. Comparied with feedstock I#, the processing cost for feedstock II# mixed with 10% diesel is reduced, while its viscosity is nearly twice as high as that of feedstock I#, which makes demetallization difficult leading to a demetallization efficiency of feedstock II# lower than that of feedstock I#. In view of the higher demetallization efficiency of second-stage processing than that of firststage processing, second-stage processing was adopted in demetallization experiments of feedstock II#. From Table 3, the demetallization efficiency illustrates the upward trend with a dosage increase both of MPTE and MPTF types of demetallization agents. When the dosage of the MPTE demetallization agent increases from 1500 to 2130 µg/g, the removal efficiency of element Ca changes from 40.72% to 97.70%; for element Fe, it changes from 42.83% to 78.21%, and it changes from 44.75% to 100% for element Mn. While the dosage of the MPTF type demetallization agent increased from 1500 to 2200 µg/g, the removal efficiencies of elements Ca and Mn changed in the range of 51.39-95.54% and 21.9480%, respectively. But for element Fe, the removal efficiency exhibits some abnormality with the type and dosage of demetallization agent. The phenomenon may be caused by the
Technique for Crude Oil Demetallization
Figure 5. Influence of the dosage of the demetallization agent on the total removal efficiency (ηT) of element Ca, Fe, and Mn: (1) MPTE; (2) MPTF.
corrosion of demetallization agents on the equipment. The reason may be need to be explained in a following study. From the Figure 5, is known that the removal of elements Ca, Fe, and Mn become much higher with an increase of the dosage of the demetallization agent. When the dosages both of the MPTE and MPTF types of demetallization agent are 1500 µg/g, the total removal efficiency for the three metallic elements is not high enough and is quite close for the different types demetallization agents. In the range of 1500-1700 µg/g, the demetallization efficiency of MPTE changes obviously. When the dosage of MPTE is exceed 1700 µg/g, the total demetallization efficiency rose slowly. For the MPTF type demetallization agent, it is seen that the total demetallization efficiency is basically monotonically ascending with dosage increases. In conclusion, the demetallization efficiency of MPTE is higher than that of MPTF due to its stronger acidity and better precipitation effect. Comparing with the demetallization efficiencies of the II-3, II-6, II-8, and II-9 samples in Table 3, it is founded that the efficiencies of demetallization agents mixed with different types of demetallization agents are better than that of a single demetallization agent. That may result from synergistic effects between different types of demetallization agents. On the basis of the analyses of data for the II-11, II-12, and II-13 samples, we know that the decalcifying efficiency of a mixture of MPTE and MTPF demetallization agents in a ratio of 1:1 is not high as expected and it is only 50.45%. But, for the MPTE and MTPF demetallization agents in a ratio of 3:1 or 1:3, the decalcifying efficiency is more than 90%. The demetallization efficiencies of the II-14, II-15, and II-16 samples in Table 3 are obtained by mixing MPTE and MPTF type demetallization agents in a ratio of 1:1 and modified with different amounts of aqueous ammonia. Although added aqueous ammonia could increase the pH of the demetallization agent aqueous solution and alleviate corrosion of the demetallization agents on the equipment and pipelines, the demetallization efficiencies are less than that of MPTE, MPTF, and their mixture in a ratio of 1:1 without ammonia modification. In view of the above facts, it is known that the use of ammonia was not benificial to the removal of metallic elements, this may be due to the higher pH of the demetallization agent aqueous solution inhibiting the acid effect of the demetallization agent. Therefore, to reduce corrosion of
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the demetallization agent aqueous solution on equipment and pipelines, the proper corrosion inhibitors would be used instead of added ammonia in the commercial process. Comparison of the demetallization effects of sample II-15 in Table 3 with sample I-1b in Table 2 shows that both of the oil samples are processed by use of the MPTF-A demetallization agent, and the former dosage of the demetallization agent is more than the latter, but with a different amount of diesel. The demetallization efficiency of sample II-15 is less than that of sample I-1b, i.e., the removal of the elements Ca, Fe, and Mn changes from 90.72%, 8.61%, and 70.29% to 72.19%, 9.23%, and 31.8%, respectively. Moreover, with comparison the demetallization effects of II-4 in Table 3 and I-2b in Table 2, a similar conclusion is also achieved, and the removal efficiencies of elements Ca and Fe varied from 100% and 82.19% to 97.70% and 78.21%, respectively. Therefore, it is evident that the higher viscosity of the oil sample would be disadvantageous toward removal of metallic elements. Furthermore, to achieve the higher demetallization efficiency, the demetallization agent must be well mixed with crude oil. When the high-viscosity crude oil is processed by using demetallization agents in electric desalting equipment in a refinery, more attention would be paid to decreasing the kinetic viscosity of the crude oil, and it would be very important for the crude oil be deeply pretreated to remove metallic elements in it. 4. Conclusions (1) The MPTE and MPTF type demetallization agents and their mixtures of certain proportions, as developed in our prior work, can remove elements Ca, Fe, and Mn in highly viscous crude oil effectively. (2) For highly viscous crude oil, the larger the amount of diesel added, the higher the demetallization efficiency. A lower viscosity crude oil sample would be favorable for the mixing of demetallization agents with the crude oil sample and reaction of demetallization agents with metallic elements. Therefore, a higher demetallization efficiency can be reached by the use of the same type and dosage of demetallization agents for crude oil with a lower viscosity. (3) The experimental results demonstrate that the acidity of the demetallization agent aqueous solution has a significant effect on the demetallization efficiency. The demetallization agent with stronger acidity usually has a higher demetallization efficiency. Modification of the demetallization agent by ammonia can decrease the acidity of the demetallization agent aqueous solution, but it would also deteriorate the demetallization performance of the demetallization agent, obviously. To prevent corrosion of the demetallization agent aqueous solution on equipment and pipelines, the proper corrosion inhibitors and/ or corrosion-resistant materials should be applied. Acknowledgment. The authors would like to thank the National Natural Science Foundation of China (Grant No. 20576075) and the Liaohe Petrochemical Branch Company for financial support of this work. EF0501896
