Effect of Potassium Acetate on Coke Growth during Light Naphtha

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Effect of Potassium Acetate on Coke Growth during Light Naphtha Thermal Cracking Wang Zhiyuan,† Xu Hong,*,† Luan Xiaojian,† Hou Feng,† and Zhou Jianxin‡ † ‡

State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China Jiangsu Key Laboratory of Process Enhancement and New Energy Resource Equipment Technology, Nanjing University of Technology, Nanjing 210009, China ABSTRACT: Potassium acetate was used as a coking inhibitor to reduce coking during light naphtha cracking on a Cr25Ni35 alloy specimen that had already been used for 8 years. The effects of the mass concentration of potassium acetate on the morphology and structure of coke were investigated by high-resolution scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. The results show that the oxide scale formed on the inner surface of the cracking tube after 8 years of service is mainly composed of (Fe, Ni, Cr) spinels and the needlelike intermetallic compound of Cr and Fe. Changes of the surface conditions accelerate catalytic coking. The amount of coke decreases with increasing mass concentration of potassium acetate in a 1-h cracking period. The amount of coke was found to decrease by about 60% when the mass concentration was 400 ppm. The diameters of filamentous coke were about 100, 60, 45, and 35 nm when the mass concentrations of potassium acetate were 0, 100, 200, and 400 ppm, respectively. However, the gasification reaction was found to have little effect on the length of catalytic coke. The gasification reaction catalyzed by potassium acetate removes the noncatalytic coke surrounding the filamentous coke, and filamentous cokes at different concentrations are carbon nanofibers with a solid structure. Coke is mainly composed of amorphous carbon.

1. INTRODUCTION Cr25Ni35 alloy is widely used in the thermal cracking of hydrocarbon because of its excellent high temperature strength. However, coke formed on the inner surface of a Cr25Ni35 tube is a serious problem because it decreases the heat-transfer coefficient and blocks the tube. Meanwhile, carburization leads to tube failure, which comes from carbon atoms dissolving in the tube materials. The coke must be periodically burned off using an oxygen and steam mixture. However, decoking reduces ethylene production and the life of the furnace tube. In the initial stage of thermal cracking, the coking mechanism is mainly catalytic coking with a high rate,1,2 and filamentous coke is the typical morphology of catalytic coke.1,2 The active metal particle is at the top of filamentous coke. Moreover, amorphous carbon is always present during thermal cracking. The catalytic activity decreases when the metal particle is covered with coke, at which point the coking mechanism changes from catalytic coking to pyrolytic coking, and the coking rate gradually decreases to a steady-state value.1,2 The methods to reduce coke formation include pretreatment of feedstocks, alteration of the inner surface chemistry of the cracking tube,3,4 and addition of coking inhibitors.5,6 For example, the carbon
steam reaction can be accelerated by alkali metal elements.7
10 The addition of alkali metal compounds is a promising means to reduce coking in ethylene steam cracking. Adding small amounts of alkali metal compounds to the feedstock for coking inhibition in an industrial furnace was reported in ref 11. When naphtha was used as the feedstock, the operation period was prolonged from 40 to 180 days, and the ethylene and propylene yields were increased by about 2%. However, few reports in the literature have been interested in the effect of alkali metal on the morphology and structure of coke growth during ethylene cracking. r 2011 American Chemical Society

In this article, light naphtha with a boiling range from 35 to 120 °C was used for coking on a Cr25Ni35 specimen that had already been used for 8 years. Different mass concentrations of potassium acetate were added to reduce coking. The effects of the mass concentration of potassium acetate on the morphology and structure of coke formation were investigated.

2. EXPERIMENTAL SECTION A schematic diagram of the experimental apparatus is shown in Figure 1. The experiments were conducted in a tubular reactor made of quartz (25 mm in diameter and 300 mm in length) that was placed vertically inside an electrical furnace. Light naphtha with a boiling range of 35
120 °C was used for coking. The density at 20 °C was 669.8 kg/m3, and the mean molecular weight was 97. The detailed composition of light naphtha used in the experiments is presented in Table 1. Specimens with a size of 10 mm  10 mm  3 mm were cut from an 8-year-old Cr25Ni35 furnace tube and rinsed in acetone by ultrasonic wave. Then, the specimens were suspended in the center of the tube reactor with quartz wire. Potassium acetate was weighed using a microbalance with a precision of 0.1 mg. Subsequently, potassium acetate was dissolved indeionized water. When the reactor had been heated to the required temperature, deionized water and light naphtha were introduced into the vaporizing furnace at 200
300 °C by a metering pump. The flow rates of steam and light naphtha were 42 and 120 mL/h, respectively. The mass concentrations of potassium acetate relative to naphtha were 0, 100, 200, and 400 ppm. Received: January 21, 2011 Accepted: July 15, 2011 Revised: May 12, 2011 Published: July 16, 2011 10292

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Figure 1. Schematic diagram of the experimental apparatus.

The inlet and outlet temperatures of the reactor tube were 600 and 850 °C, respectively, and the inlet and outlet pressures of the reactor tube were both the atmospheric pressure during cracking tests because of the low gas flow rate. According to our previous coking tests, 1 h is the desired time for coke formation in our experimental tests. Therefore, the power was turned off after 1 h, and the reactor was allowed to cool to room temperature in nitrogen gas. The specimens before and after coking experiment were weighed with a microbalance (0.1-mg accuracy). Under the same cracking conditions, three coking tests were repeated, and the mean value of mass of coke deposited on the specimen for three trials was employed as the final result. High-resolution scanning electron microscopy (HRSEM) was used in the morphology observations of specimens and coke. In addition, transmission electron microscopy (TEM) was used to investigate the microstructure of the cokes. The coke samples were previously dispersed in an ethanol solution by ultrasonic wave, and the suspension was then deposited on a copper grid covered with a porous carbon membrance. Raman spectra of cokes were measured with a laser Raman spectrometer, using the 514.5-nm line of an argon ion laser at room temperature. The incident laser power was 2 MW with a resolution of 4 cm
1, and spectra were collected for wavenumbers ranging from 100 to 2000 cm
1.

3. RESULTS AND DISCUSSION 3.1. Eight-Year-Old Cr25Ni35 Specimen. Cr25Ni35 alloy, which contains 25 wt % chromium and 35 wt % nickel, exhibits excellent high-temperature behavior during thermal cracking. However, furnace tubes have to be replaced after the properties of such steels deteriorate with increasing operating time. The service life depends highly on decoking times, which are controlled by the coking rate on the inner surface of the cracking tube.12 Figure 2a shows an SEM image of an 8-year-old Cr25Ni35 specimen. The surface is rough and jagged. In addition, it was

Table 1. Composition of Light Naphtha Used in the Experiments content (%) carbon number

n-paraffin

isoparaffin

naphthenes

aromatics

5

2.84

1.74

0.67

6

9.08

22.98

5.22

0.02

7

3.77

17.23

10.51

0.35

8

1.06

9.96

11.08

0.32 17.07

1.56 53.47

0.29 27.78

9 total

0.37

found that many needlelike particles existed on the surface of specimen (see Figure 2b). In a steam cracking furnace, the cracking tube not only is subjected to high temperature, but is also in contact with variable reducing and oxidizing atmospheres, which lead to the formation of an oxide scale on the inner surface.13,14 Some microcracks formed in the oxide scale can be seen, which result from periodic cold/heat fatigue or heat vibration for the cracking tube.13 In addition, filamentous coke easily nucleates and grows at the microcracks. The growth of filamentous coke further disrupts the oxide scale and makes exposes the bare substrate metal to cracking gas, which could lead to more serious catalytic coking.13,14 Parts c and d of Figure 2 show EDS and XRD results, respectively, for the oxide scale. As seen in Figure 2c, the oxide scale contains mainly Cr, O, Fe, Mn, Si, Ni, and C elements. From Figure 2d, it is found that the oxide scale is mainly composed of (Fe, Ni, Cr) spinel structures and the needlelike phase, which is an intermetallic compound of Cr and Fe.15 The reduction of (Fe, Ni, Cr) spinels during cracking results in the formation of fine (Fe, Ni) particles, which accelerates coke formation.16 In addition, the needlelike intermetallic compound of Cr and Fe roughens the surface and increases the surface area for coke deposition. Therefore, the change of the 10293

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Figure 2. Characterization of the oxide scale formed on the inner surface of an 8-year-old Cr25Ni35 cracking tube: (a) SEM image of the oxide scale, (b) SEM image of the needlelike phase, (c) EDS spectrum of the oxide scale, and (d) XRD spectrum of the oxide scale.

surface chemical conditions results in accelerated catalytic coking for the 8-year-old Cr25Ni35 alloy.4 3.2. Coke Characteristics. Figure 3 presents the amount of coke deposited on the surface of specimen in a 1-h cracking period as a function of the mass concentration of potassium acetate. The amount of coke decreases with increasing mass concentration, with the amount of coke being decreased by 60% for a mass concentration of 400 ppm. The addition of potassium acetate has a positive effect on decreasing the coking rate, indicating good anticoking properties for potassium acetate. Figure 4 shows SEM images of cokes formed on 8-year-old Cr25Ni35 specimens in a 1-h cracking period at different mass concentrations of potassium acetate. When the cracking time is 1 h, the morphologies of the cokes show significant evidence of filamentous coke, and bright points are found at the filament tips or in the filaments. Typically, catalytic coke is formed during the initial stage of coking and presents the morphology of coke filaments. The catalytic influence of the wall material steadily decreases with time, and further coke growth occurs through a radical mechanism at a constant coking rate. With increasing mass concentration, the diameter of these interwoven coke filaments decreases, and the surface of the filamentous coke becomes smoother. High-magnification images of the filamentous cokes show that their diameters were about 100, 60, 45, and 35 nm when the mass concentration was 0, 100, 200, and 400 ppm, respectively. When the concentration was 400 ppm, the coke was characterized by TEM because the filamentous coke was indistinguishable by HRSEM. Light naphtha pyrolysis is a free-radical reaction in nature. Ethylene, propylene, butane, butylene, and aromatics are produced

during the pyrolysis, and Olefins and diolefins are thought to be precursors for coking.17 Catalytic coke formation on metals involves surface reaction, diffusion, and precipitation of carbon.1,18 Initially, a hydrocarbon molecule is chemisorbed onto the metal crystallite, and it is converted to carbon by a surface reaction. Then, carbon atoms dissolve in and diffuse through the metal particles. Metal particles are lifted from the surface because of the tension caused by the accumulation of carbon atoms in the metal crystals. These metal particles can act as catalytic sites and lead to the growth of filamentous coke.1,19 Meanwhile, carbon precipitation can give rise to structural deficiencies in the carbon lattice, thereby creating reactive carbon centers along the filament skin. Hydrocarbon radicals or molecules in the gas phase are incorporated at these reactive sites. Coke precursors in the gas phase react with the coke surface through radical reactions, and lateral growth of the filament begins,18,19 which gives rise to the coarse skin of filamentous coke (see Figure 4a). Therefore, filaments are excellent collection sites for the precursors of pyrolytic coke and promote its formation. Meanwhile, potassium is highly mobile above 500 °C, and it can spread on the surface of filamentous coke and diffuse into the coke.20 The migration of potassium into the coke is through the formation of potassium intercalate-like structures in condensed aromatic layers of carbon.21 The intrinsic mobility of the potassium compound leads to good contact with coke, which provides a high gasification rate. The gasification reaction between steam and coke catalyzed by potassium acetate eliminates pyrolytic coke surrounding the filamentous coke. Therefore, the diameter of filamentous coke decreases with increasing mass concentration of potassium acetate, and the surface of filamentous coke becomes smoother. However, the 10294

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Industrial & Engineering Chemistry Research gasification reaction has little effect on the length of the coke filaments (see Figure 4). This indicates that potassium could inhibit the formation of pyrolytic coke, whereas it would not poison the metal particle at the tip of filamentous coke. Therefore, according to our study, potassium acetate has little effect on the growth of catalytic coke.

Figure 3. Amount of coke deposited on the surface of the specimen in a 1-h cracking period as a function of the mass concentration of potassium acetate.

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Figure 5 shows images of filamentous coke for potassium acetate mass concentrations of 0 and 400 ppm. When the mass concentration was 0 ppm, amorphous carbon was observed on the outer surface of the filaments. When the mass concentration was 400 ppm, filamentous coke with a solid structure was observed. In the latter case, most of the amorphous carbon was removed because of the catalytic gasification reaction of potassium acetate. These filamentous cokes exhibit a cylindrical cross section with small amounts of amorphous carbon on their outer surface (Figure 5c). The appearance of the solid structure is related to the relative rates of nucleation of carbon filaments and diffusion of carbon through the catalytic metal particle and to differences in the diffusion path lengths on the metal/carbon interface.22 When the rate of nucleation is low compared to that of diffusion, filamentous coke without a solid structure is formed.22 Figure 5d shows a coke filament with a metal particle at the filament tip, and the metal particle is covered with carbon. Raman spectroscopy is an effective method for studying carbon materials. Figure 6 shows Raman spectra of coke formation in a 1-h cracking period at different mass concentrations of potassium acetate. The spectra mainly show two Raman bands at 1350 cm
1 (D band) and 1580 cm
1 (G band). The G band is assigned to the in-plane carbon
carbon stretching vibrations of graphite layers, which indicates original graphite features.23,24 Upon the introduction of disorder in the graphite structure, the existing bands broadened, and additional bands appeared at about 1350 cm
1 (D band) and 1605 cm
1 (D0 band). The D band is attributed to structure imperfect of graphite.23 Highly disordered carbon shows very broad Raman bands, and the

Figure 4. SEM images of coke formed at different mass concentrations of potassium acetate in a 1-h cracking period: (a) 0, (b) 100, (c) 200, and (d) 400 ppm. Insets: High-magnification images of filamentous coke. 10295

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Figure 5. TEM images of filamentous coke: (a) filamentous coke for 0 ppm potassium acetate, (b) filamentous coke for 400 ppm potassium acetate, (c) cylindrical form of filamentous coke for 400 ppm potassium acetate, (d) filamentous coke with a metal particle at the filament tip.

different mass concentrations of potassium acetate. These ratios (ID/IG > 2) indicate a low graphitization degree for the filamentous coke in our study. This means that coke formed at different mass concentrations of potassium acetate is mainly composed of amorphous carbon.

Figure 6. ID/IG ratios of coke formed in a 1-h cracking period at different mass concentrations of potassium acetate. Inset: Raman spectra of the same samples.

intensity of D band increases when carbon becomes highly disordered.25 D and G bands are also sensitive to the type of carbon bonding, that is, sp2 or sp3 hybridization.26 The intensity ratio of the D and G bands (ID/IG) can be regarded as an index of the crystalline order of graphite.23
26 The individual G and D peaks from our spectra were fitted with Lorentzian lines, and the results for the ID/IG ratios are given in Figure 6. It was found that the ratios do not change notably at

4. CONCLUSIONS Alkali metal compounds can catalyze the gasification reaction of carbon and steam. Therefore, addition of alkali metal compounds is a promising means to reduce coke formation in steam cracking for ethylene production. In this work, different mass concentrations of potassium acetate were added to light naphtha to reduce the coking on a Cr25Ni35 alloy specimen that had already been used for 8 years, and the effect of potassium on the coke growth was studied. The following conclusions can be reached: (1) The oxide scale on the inner surface of the 8-year-old Cr25Ni35 alloy tube was rough and contained some microcracks. This oxide scale was mainly composed of (Fe, Ni, Cr) spinels and a needlelike intermetallic compound of Cr and Fe. Moreover, the needlelike intermetallic compound roughened the surface and increased the surface area for coke deposition. The change of the surface conditions was found to accelerate the catalytic coking. (2) The amount of coke formed in a 1-h cracking period decreased with increasing mass concentration of potassium acetate. For a mass concentration of 400 ppm, the amount of coke decreased by 60%, and potassium acetate acting as the coking inhibitor exhibited good anticoking properties. 10296

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Industrial & Engineering Chemistry Research (3) For a cracking time of 1 h, filamentous coke formed on the 8-year-old Cr25Ni35 alloy specimen. The gasification reaction catalyzed by potassium acetate removed the noncatalytic coke surrounding the filamentous coke. The diameter of the filamentous coke was found to decrease and the surface to become smoother with increasing mass concentration of potassium acetate. However, the gasification reaction had little effect on the length of catalytic coke. The diameters of filamentous coke were about 100, 60, 45, and 35 nm when the mass concentration of potassium acetate was 0, 100, 200, and 400 ppm, respectively. (4) Filamentous cokes at different concentrations are carbon nanofibers with a solid structure. The change in the mass concentration of potassium acetate has little effect on the structure of filamentous coke. Cokes formed at different mass concentrations of potassium acetate were found to be mainly composed of amorphous carbon.

’ AUTHOR INFORMATION Corresponding Author

*Fax: + 86-21-64253810. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by Sinopec Yangzi Petrochemical Company Ltd., Nanjing, P.R China. This work was supported by Shu Guang Fellowship project supported by Shanghai Municipal Education Commission and Shanghai Education Development Foundation, China and Sinopec Yangzi Petrochemical Company Ltd., Nanjing, P.R China. ’ REFERENCES (1) Reyniers, M.-F. S. G.; Froment, G. F. Influence of metal surface and sulfur addition on coke deposition in the thermal cracking of hydrocarbons. Ind. Eng. Chem. Res. 1995, 34, 773. (2) Leftin, H. P.; Newsome, D. S. Coking rates in a laboratory pyrolysis furnace: Liquid petroleum feedstocks. Ind. Eng. Chem. Res. 1987, 26, 1003. (3) Zhou, J. X.; Xu, H.; Liu, J. L.; Qi, X. G.; Zhang, L.; Jiang, Z. M. Study of anti-coking property of SiO2/S coating deposited by APCVD technique. Mater. Lett. 2007, 61, 5087. (4) Zychlinski, W.; Wynns, K. A.; Ganser, B. Characterization of material samples for coking behavior of HP40 material both coated and uncoated using naphtha and ethane feedstock. Mater. Corros. 2002, 53, 30. (5) Chan, K. Y. G.; Inal, F.; Senkan, S. Suppression of coke formation in the steam cracking of alkanes: Ethane and propane. Ind. Eng. Chem. Res. 1998, 37, 901. (6) Kumar, S. Triethyl phosphate additive-based fouling inhibition studies. Ind. Eng. Chem. Res. 1999, 38, 1364. (7) Wigmans, T.; G€oebel, J. C.; Moulijn, J. A. The influence of pretreatment conditions on the activity and stability of sodium and potassium catalysts in carbon
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