Inhibition Effect of APCVD Titanium Nitride Coating on Coke Growth

Article pubs.acs.org/IECR

Inhibition Effect of APCVD Titanium Nitride Coating on Coke Growth during n‑Hexane Thermal Cracking under Supercritical Conditions Shiyun Tang,† Shuang Gao,‡ Shengwang Hu,‡ Jianli Wang,*,† Quan Zhu,‡ Yaoqiang Chen,† and Xiangyuan Li‡ †

Key Laboratory of Green Chemistry & Technology of the Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064, People’s Republic of China ‡ College of Chemical Engineering, Sichuan University, Chengdu 610065, People’s Republic of China ABSTRACT: In this work, titanium nitride (TiN) coating was used as a passive layer to inhibit metal catalytic coking during hydrocarbon fuel cracking on the microchannel inner surface of stainless steel 304 (SS304) tubes. In order to obtain an inert and effective passive coating, TiN coating was prepared in SS304 tubes with 2 mm inside diameter and 700 mm length by atmospheric pressure chemical vapor deposition (APCVD) using a TiCl4−H2−N2 system. The coating’s thickness, phase composition, morphology, and chemical composition were investigated by metalloscopy, X-ray diffraction, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX), respectively. Characterization results indicated that TiN coating had a relatively complete cubic-phase crystal form with a N/Ti ratio of 1:1, presenting small star-shaped crystals on the whole. The inhibition effects of TiN coating on the morphologies and amounts of coke were studied by SEM and EDX after nhexane thermal cracking at 600 °C and 3.3 MPa for 20 min. Under these conditions, different contributions to carbon deposition were discussed including oxidative reactions and pyrolysis of n-hexane. Along the axial length of the bare tube, stunted and clubbed cokes formed by autoxidation near the distance of 100 mm; granular metal carbides and filamentous cokes formed by metal catalysis near the distances of 350 and 600 mm, respectively. However, no morphologies of carbon deposits on a TiNcoated tube surface were observed after thermal cracking of n-hexane at 600 °C and 3.3 MPa for 20 min. At distances of 100, 350, and 600 mm away from the tube inlet, the carbon atomic percentages of coke in these three areas were 27.28%, 58.04%, and 99.69% for the bare tube, larger than those of 5.76%, 15.73%, and 30.66% for the TiN-coated tube, respectively. The results showed that the inhibition effect of APCVD TiN coating on coke growth is superior to that of other coatings (e.g., alumina coating). The reason is that TiN coating not only creates a barrier between the hydrocarbon fuels and metal surface to inhibit related catalytic coke formation but also minimizes carbon deposits by absorbing C atoms.

1. INTRODUCTION In recent years, liquid hydrocarbon fuels have attracted much interest in the development of advanced aircraft because they serve as not only the propellant but also the ideal coolants to resolve the problem of thermal management by removing the waste heat from aircrafts with physical and chemical heat sinks (sensible heat-absorbing and cracking heat-absorbing).1−3 In the industrial design, hydrocarbon fuels flow around the wall of the engine and absorb amounts of harmful heat before running into the combustion chamber, and the accompanying chemical cracking produces a lot of small-molecule products with better combustion performance. In order to absorb the most heat as far as possible, very long and thin tubes are adopted to increase the contact areas for heat transfer. Usually, the fuel tubes have only 1−2 mm of inside diameter, leading to a serious problem: a lot of solid carbon deposits (also known as coke) are formed from hydrocarbon fuels on the metal surface, which not only worsen the heat-transfer process by sharply increasing heat resistances but also weaken the substrate, thereby resulting in a catastrophe by accumulating and plugging the flow lines.4,5 To solve the problem of coke growth during hydrocarbon fuel pyrolysis, tremendous efforts have been devoted to analyzing the mechanism of coke formation in the thermalcracking process. It is generally accepted that the formation of carbon deposition is mainly assigned to three paths: thermal © 2014 American Chemical Society

oxidation, condensation of aromatic compounds, and metal catalytic coking,6 wherein metal catalytic coking is the main cause of the large amount of surface deposits. The formation of solid deposition on metal surfaces has been extensively studied in the last 3 decades. Typically, Li and Eser studied the effects of different metal surfaces [Ni, Cu, and stainless steel (SS)] on deposit formation from thermally stressed dodecane and suggested that Ni-based SS surfaces deposit larger amounts of carbon deposits than the others at high temperatures (>400 °C).7 Albright and Marek reported that the Ni and Fe that catalyze coke formation are often incorporated in the coke formed on SS surfaces and affect the morphology of the coke.8 Graff and Albright investigated coke formation from the decomposition of acetylene, butadiene, and benzene on Incoloy 800, aluminized Incoloy 800, and Vycor glass surfaces at 500− 900 °C. Their results showed that filamenteous coke, which contains Ni and Fe, was formed only on Incoloy 800 surfaces.9 In addition, Altin and Eser investigated the cracking of JP-8 fuel in an isothermal flow reactor and observed that the activity of Received: Revised: Accepted: Published: 5432

June 14, 2013 September 29, 2013 March 7, 2014 March 7, 2014 dx.doi.org/10.1021/ie401889p | Ind. Eng. Chem. Res. 2014, 53, 5432−5442

Industrial & Engineering Chemistry Research

Article

Therefore, the objective of this work is to prepare TiN coating on the microchannel inner surface of SS304 tubes by the APCVD method and then to investigate their inhibition effect on coke growth during n-hexane thermal cracking under supercritical conditions. The supercritical conditions mentioned here are that n-hexane is in a supercritical state before cracking. This work is arranged as follows: First, the experimental method and devices are introduced including the preparation of TiN coating and thermal cracking of n-hexane in the Experimental Section. Second, the quality and characteristics of TiN coating and coke deposits are detected and discussed to assess the inhibition effect of TiN on coke growth, accompanied by analysis of the variation of the fuel and wall temperature along the reaction tube and cracking products. Finally, the conclusion is drawn in the last section.

the tube surfaces toward carbon deposition decreased in the order of nickel > SS316 > SS304 > silcosteel > glass-lined SS.10 Using inert coatings to reduce the carbon deposition from the cracking of fuels proves to be a promising method. Recently, Ram Mohan and Eser successfully deposited alumina, zirconia, tantalum oxide, and platinum on AISI304 foils by metal−organic chemical vapor deposition (MOCVD) to alleviate coking due to thermal oxidation in thermal stressing of Jet A (350 °C and 3.5 MPa, 1 mL/min for 5 h) and reported that the effectiveness of the coatings in mitigating carbon deposition was found to decrease in the following order: platinum > Ta2O5 > alumina from acetylacetonate > ZrO2 > alumina from aluminum trisecondary butoxide > AISI304.11 Similarly, Liu et al. prepared a series of alumina coatings with various thicknesses (318−1280 nm) in SS321 tubes (2 mm i.d.) by MOCVD using aluminum trisecondary butoxide and evaluated the anticoking performance of the MOCVD alumina coatings during thermal cracking of Chinese RP-3 jet fuel under supercritical conditions (inlet temperature, 575 °C; outlet temperature, 650 °C; pressure, 5 MPa). Their results indicated that the anticoking performance increased from 37% to 69% as the thickness of the alumina coatings changed from 318 to 1280 nm.12 However, the oxide coatings they prepared by the MOCVD method are essentially amorphous instead of crystalline, and the surfaces of the MOCVD alumina coatings present coordinatively unsaturated Lewis acid sites and strong Brønsted acid sites, which are beneficial to the formation of carbonaceous solids.11 It is generally acknowledged that alumina coatings can be deposited in amorphous, metastable, and stable crystalline phases, but amorphous and metastable phases transforming to the thermodynamically stable corundum phase (α-Al2O3, the hardest phase and chemically very inert) often requires substrate temperatures above 1000 °C. The range of deposition temperatures reported in the literature reviews by Maruyama and Arai, Huntz et al., and Pranhan et al. for amorphous films was between 250 and 550 °C.13−15 The thermodynamically stable α phase was grown at temperatures between 900 and 1200 °C, as reported by Bahlawane et al. and Müller et al.16,17 Moreover, it is worth pointing out that the alumina coatings will generate a lot of cracks and delamination by the MOCVD method as their thickness increases.12 They explained that there existed a critical thickness during deposition of the thin alumina coatings owing to internal stress at the outer side of the film, and it was elaborated in detail by Haanappel et al. about 1 μm during deposition of the thin alumina films on AISI304.18 In this work, the main task is to develop a more effective method to prepare inert coating on the inner surface of a SS microchannel with controllable thickness. Atmospheric pressure chemical vapor deposition (APCVD) is a widely used method for depositing thin and high-quality coatings with well-defined chemical composition and structural uniformity. The main advantage derived by an APCVD process is the resulting uniform, adherent, and reproducible coatings.19−21 Nitrides exhibit excellent hard and wear-resistant properties, together with high melting points and good chemical resistance. Among the nitrides, titanium nitride (TiN) coatings are extensively used as hard coatings, for wear resistance, as diffusion barriers, and for optical applications. However, few reports in the literature focus on the inhibition effect of TiN coating on coke growth during hydrocarbon fuel cracking.

2. EXPERIMENTAL SECTION 2.1. Materials. Titanium tetrachloride (TiCl4) with a purity of 99% and n-hexane with a purity of 97% were purchased from Shanghai Zhanyun and Chengdu Kelong Chemical Reagent Company, respectively. Their physical properties are described in Table 1. The elemental composition of SS304 substrates Table 1. Physical Properties of TiCl4 and n-Hexane Used in This Work TiCl4

n-hexane

property

value

property

molecular weight (mean) (g/mol) density (at 20 °C) (g/cm3) melting point (°C) boiling point (°C) vapor pressure (at 21 °C) (kPa) critical temperature (°C)

189.71 1.726 −24.1 136.4 1.33

molecular weight (mean) (g/mol) density (at 20 °C) (g/cm3) melting point (°C) boiling point (°C) critical temperature (°C)

value

0.66 −94.3 69 234.8

358

critical pressure (MPa)

3.09

86.2

used in this study was detected by energy-dispersive X-ray spectroscopy (EDX), and its weight and atomic percentage are given in Table 2. Table 2. Elemental Composition of SS304 Substrates in Weight and Atomic Percentage element

wt %

atom %

Cr Fe Ni total

19.62 72.53 7.85 100.00

20.84 71.77 7.39 100.00

2.2. Preparation of APCVD TiN Coating. The CVD experiments were conducted in a horizontal and tubular hotwall reactor with a uniform temperature zone of 1000 mm at the atmospheric pressure. Figure 1 shows a schematic diagram of the CVD experimental apparatus. The device is mainly composed of four systems: a gas supply system, a Ti supply system, a sedimentary system, and a tail gas treatment system. In a typical CVD process, reactant gases are transported to the substrate surface, where thermal deposition occurs. In this study, TiN coatings were deposited on the microchannel inner surface of SS304 tubes with 2 mm inside diameter and 700 mm length by means of the APCVD method using TiCl4 as the precursor and hydrogen as the carrier gas. Before entering the 5433

dx.doi.org/10.1021/ie401889p | Ind. Eng. Chem. Res. 2014, 53, 5432−5442


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Figure 1. Schematic diagram of the APCVD apparatus: (I) gas supply system; (II) Ti supply system; (III) sedimentary system; (IV) tail gas treatment system; (1) drying tube; (2) needle valve; (3) gas flowmeter; (4) TiCl4 evaporator; (5) heater band; (6) temperature controller; (7) reaction chamber; (8) ball valve; (9) purification plant.

Figure 2. Flow reactor device for thermal-cracking experiments with fuels: (1) N2 cylinders; (2) relieve valve; (3) drying tube; (4) needle valve; (5) storage tanks; (6) high-performance liquid chromatograph; (7) filter; (8) nonreturning valve; (9) mass flowmeter; (10) pressure gauge; (11) thermocouple; (12) transformer; (13) condenser; (14) back-pressure valve; (15) gas/liquid separator; (16) ball valve; (17) liquid collector; (18) wet type gas flowmeter; (19) gas chromatograph.

deposition chamber, N2 was introduced as a mixer to mix with H2 and TiCl4 completely. Usually, TiN coating was produced under high temperature (>900 °C), but such a high temperature has a certain influence on the substrate. Therefore, the deposition temperature was fixed at 800 °C, and the deposition time was 2 h in this work. The basic chemical equation for the preparation of TiN coating is as follows:

metalloscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and EDX, respectively. To measure the thickness of TiN coating by metalloscopy after embedding it in resin and then cutting it into slices, a cross section of 100 mm away from the front end of the TiN-coated tube was typically chosen. XRD identifies the phase composition of the analyzed coatings if they are crystalline and polycrystalline. Amorphous coatings cannot be analyzed by this method because of their random arrangement of atoms in the coating. An XRD experiment of TiN coating was carried out on a DX-2500 rotating-anode X-ray diffractometer (Dandong Fangyuan Instruments Co., Ltd., China) using Cu Kα (λ = 0.15406 nm) radiation. The tube voltage and current were 40 kV and 100 mA, respectively. The X-ray diffractogram was recorded at 0.03°/s intervals in the range of 20−90°. The inner surface morphologies of the bare and TiN-coated tubes were analyzed by SEM (Hitachi S-4800, Japan), and the chemical composition of deposited films was analyzed using EDX (Oxford IE-250, Germany) along with SEM. It is worth pointing out that several different parts of a sample surface were analyzed under lower magnification by EDX, and the average value was obtained by means of selecting and measuring at least three different areas on the sample surface. 2.4. Thermal-Cracking Experiments and Analysis of Carbon Deposits. To evaluate the coking performance of TiN coating, thermal cracking runs of n-hexane were carried out using a self-made supercritical cracking device, which is shown in Figure 2. The cracking experiments were conducted in SS304 TiN-coated and uncoated tube (bare tube) reactors, which have 2 mm inside diameter and 700 mm length. The reactor was

2TiCl4(g) + 4H 2(g) + N2(g) → 2TiN(s) + 8HCl(g)

Prior to deposition, all substrates were pretreated using a standard cleaning procedure: soaked in acid (1% HCl) for 10 min, rinsed with acetone five times, dried in argon for 30 min, cleaned with ultrasonic wave for 20 min in a deionized water bath, and then dried at 120 °C in an oven. Next, the substrates were typically placed at the center of the isothermal zone that extended to 1000 mm. TiCl4, with an equilibrium vapor pressure of 1.33 KPa at 21 °C, was carried by purified H2, and the feed rate of TiCl4 was adjusted by changing the flow rate of H2. In this work, deposition was carried out at a substrate temperature of 800 °C using the carrier gas H2, reactant N2, and dilution gas H2 with flow rates of 1200, 1400, and 1000 mL/min, respectively. The total gas flow rate was kept constant at 3600 mL/min. After deposition, the substrates were kept at atmospheric conditions and subsequently analyzed after cooling. 2.3. Characterization of TiN Coating. In order to identify its physical and chemical properties, the deposited TiN coating was analyzed. The thickness, phase composition, morphology, and chemical composition were investigated in this study using 5434



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heated by direct current power to 600 °C at a rate of 100 °C/ min, with a back-pressure control valve keeping the system pressure constant at 3.3 MPa. These conditions for cracking are higher than the critical point of 234.8 °C and 3.09 MPa for nhexane. Therefore, the supercritical conditions mentioned in this work are that n-hexane is in the supercritical state before cracking. The fuel outlet temperatures were controlled at 600 °C and measured by K-type thermocouples. Meanwhile, the wall temperatures were also measured by K-type thermocouples welded on the outside surface of the tube. At the end of the reactor, the cracking products were first cooled in the condenser and then flowed into a gas/liquid separator through the back-pressure control valve. The bare and TiN-coated tube samples were washed in nhexane and dried in high-purity N2 for 1 h before the experiment. First, high-purity N2 gas was passed through the reaction system for 10 min to wipe the air inside. Then, nhexane was pumped at the required flow rate of 40 mL/min with a high-performance liquid chromatography pump. The residence time of the fuel in the reactor was 3.3 s under the normal temperature and a pressure of 3.3 MPa, and the pyrolysis reaction time was 600 °C for 20 min. In order to reduce the experimental errors in the material balance, gaseous products and liquid samples were collected and measured at the same time using a wet type gas flowmeter and a precision electronic balance at an interval of 5 min, respectively. In the process of the experiment, gas chromatography (GC) was adopted to analyze the gas products online, and GC−mass spectrometry was used for the liquid product analysis after the experiment. Particularly, the gas products were analyzed by a GC-2000 III (Shanghai Institute of Computing Technology, China), using a flame ionization detector and an Al2O3/S capillary column (0.53 mm × 50 m, 5 μm). After thermalcracking experiments, the residual fuel in the reactors was removed by high-purity N2 gas and dried in an oven at 120 °C for 1 h. After the n-hexane thermal-cracking experiment, both bare and TiN-coated tubes were cut into 2-cm segments and three split segments were sampled100, 350, and 600 mm along the axial length of bare or coated tubes (see Figure 7)and the corresponding morphologies and microzone amounts of carbon deposits were characterized by SEM and EDX. The total amounts of carbon deposits for the last two segments were obtained after thermal cracking of n-hexane at 600 °C and 3.3 MPa for 20 min by temperature-programmed oxidation (TPO) in a CO2 IR analyzer (GXH-1050; Beijing Junfang Research Institute).22 In the IR analyzer, carbon in the deposit was oxidized to CO2 by high-purity O2 in a furnace and over a Pt/ Al2O3 oxidation catalyst bed at 300 °C (CO was completely converted CO2). The coking-deposited tube samples were loaded in a quartz reactor with flowing O2 (600 mL/min) and then heated from 70 to 900 °C at a rate of 6 °C/min with a holding period of 5 min at the final temperature. The CO2 produced was measured quantitatively by an IR detector.

Figure 3. Metallographic cross-sectional images of the APCVD TiN coating (conditions: deposition temperature, 800 °C; TiCl4 temperature, 21 °C; total flow rate, 3.6 L/min).

Considering the actual industrial conditions (the coating does not affect the fuel delivery), the best coating thickness is less than 0.02 mm.24 In this work, the thickness of TiN coating is more than 8 um, which is enough to cut off the catalytic coke growth effect of the metal substrate. The crystal-phase structure of TiN coating was analyzed by XRD and is shown in Figure 4. It can be seen that TiN coating

Figure 4. XRD pattern of the APCVD TiN coating (conditions: deposition temperature, 800 °C; TiCl4 temperature, 21 °C; total flow rate, 3.6 L/min).

has a series of obvious diffraction peaks, and the peaks are relatively narrow and acute. Halo peaks or other signs of amorphous phases were not observed in the TiN-coated sample. This implies that the crystal-phase structure of TiN coating is relatively complete and has a high crystallinity. In general, the (111) and (220) reflections at 2θ angles of 36.6° and 61.6° are more pronounced than the (200), (311), and (222) peaks at 42.6°, 73.8°, and 77.7°. In contrast to the standard TiN XRD card (JCPDS standard: PDF 38-1420), the diffraction patterns show the TiN coating structure with apparent differences concerning the preferred orientation at (111) and (220) crystal faces, and this is closely related to the preparation methods and experimental conditions.25,26 Moreover, these diffraction peaks reflect the typical NaCl-type structure of the TiN coating with the face-centered-cubic lattice

3. RESULTS AND DISCUSSION 3.1. Characterization of TiN-Coated Substrates. Metalloscopy was used to measure the coating thickness.23 Figure 3 displays the thickness of the fracture cross section of a TiNcoated tube by metalloscopy. It is clear that TiN coating has a clear border with the substrate and uniform thickness. The average thickness of the coating is about 8.89 μm, from which the deposition rate is estimated at about 4.45 μm/h. 5435



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Furthermore, it can be seen that elemental analysis of the Ti/ N ratio is close to 1:1 at different parts of the coated tube inner surface, which is consistent with the XRD results. In other words, the composition of TiN coating is relatively uniform, and it is a nonstoichiometric compound. This nonstoichiometry illustrates the presence of pyramidal and tetrahedral coordination on the surface due to coordinative unsaturation. Numerous studies show that the composition of TiN is nonstoichiometric from the CVD method.27 According to work by Sundgren et al., the x value in nonstoichiometric TiNx can change from 0.6 to 1.2, while its cubic-phase structure will not change.28 In order to take the effect of TiN coating on heat transfer into account, supercritical thermal cracking of n-hexane in a microchannel flow reactor was adopted to monitor the fuel and tube wall temperatures. Figure 7 shows a schematic diagram of the electric heat tube reactor for the supercritical cracking experiment, and the wall temperatures of the reaction tube are sampled at three points: 100, 350, and 600 mm along the axial length of bare or coated tubes. The system pressure and temperature of the fuel are very important for solid carbon deposits on the reaction tube inner surface, which will be discussed below in detail. Figure 8 presents inlet/outlet and wall temperature (Tw) profiles along the axial length of bare or TiN-coated tube during n-hexane thermal cracking. It is worth pointing out that the measurement method for the bulk fuel temperature is similar to that of Liu et al.29 Obviously, the wall and fluid temperature profiles are almost the same for the bare and TiNcoated tubes, as shown in Figure 8. This demonstrates that the thermal conductive performance of TiN coating is excellent. A possible reason for this result is that the thermal conductivity of TiN [29.31 W/(m·K) at room temperature] is only a little higher than that of the SS304 tube [12.1 W/(m·K) at room temperature] and the coating is too thin to considerably improve thermal transfer resistances. Furthermore, there is a good compactness and adhesion of TiN coating on SS304 because the linear expansion coefficient of TiN is 9.4 × 10−6 K−1, which is close to 16.0 × 10−6 K−1 of the SS304 substrate. This implies that there is a good combination between TiN coating and the SS304 substrate, which is beneficial to heat transfer. It is interesting to note that the temperature of the wall and fluid increased simultaneously along the oil inlet to the outlet, and the wall temperatures of bare and TiN-coated tubes are both much higher than the corresponding fluid temperature (the difference is about 220 °C). Also, from Figure 8, it is found that the fluid temperature is not exactly a linear relationship along with the tube length: Within the scope of the 0−250 mm length, the temperature profile seems to be a linear profile because of the simple physical heating process, while after the

structure (its cell parameter is 0.4242 nm), and the phase composition of TiN coating has a Ti/N ratio of 1:1, as expected for TiN. This is also supported by the fact that the golden color of the coated samples is typical due to stoichiometric TiN. Nevertheless, because of the strong penetration of X-rays, some additional reflections related to the SS304 substrate such as Cr2Ni3 appear. It becomes obvious that TiN coating by the APCVD method in this study is a pure TiN phase. Figure 5 shows the SEM top-view morphology of TiN coating deposited onto the tube inner surface by the APCVD

Figure 5. SEM top-view of the APCVD TiN coatings (conditions: deposition temperature, 800 °C; TiCl4 temperature, 21 °C; total flow rate, 3.6 L/min): (A) bare tube; (B) TiN coating.

method. It is apparent that the microstructures of the inner surface of the TiN-coated tube are significantly different from those of the bare tube. As shown in Figure 5, TiN coating is homogeneous and dense, and it presents small star-shaped crystals on the whole. It can be observed that the sizes of the star-shaped crystals are about 2 μm (Figure 5B). This could be attributed to three factors: the reaction temperature, the concentration of the precursor, and the properties of the surface of the substrate. In addition, the inner surface of the bare tube shows a lot of cracks and small metal particles, which will be the best active sites of catalytic coking, as shown in Figure 5A. Therefore, TiN coating completely covers the cracks and small metal particles in the rough inner surface of the bare tube, and it provides a very good protective layer to prevent the substrate from coking effectively. EDX analysis can provide important information for a film depth below 1 mm, while it is difficult to detect elements of low atomic number such as H. Some elements contaminating the surface of the film could also be detected by the EDX detector, while this method is considered to be a near-surface elemental analysis technique. Similar to SEM, the tubes were cut into 2cm segments, and three split sample points were chosen: 100, 350, and 600 mm along the axial length of bare or coated tubes. EDX investigations of the TiN-coated tube inner surfaces show pronounced contents of the elements Ti and N without any other elements, as outlined in Figure 6. The results show that the prepared TiN coating in this work is very pure.

Figure 6. EDX spectra of the APCVD TiN coating: (A) front end, 100 mm; (B) middle, 350 mm; (C) back end, 100 mm. 5436



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Figure 7. Schematic diagram of the electric heat tube reactor for n-hexane thermal cracking (conditions: temperature, 600 °C; pressure, 3.3 MPa; flow rate, 40 mL/min).

very low carbon atomic percentage of less than 0.03%, the deposits are really through carbonaceous deposition, and EDX analysis gives around 27.28% carbon atomic percentage in Figure 9B1. Similarly, a large number of small granular particles appear on the inner surface of the SS304 substrate in Figure 9B2, and the detected carbon content on the surface is approximately 58.04% by EDX analysis. There is a possibility that these small granular particles belong to metal carbides, such as Fe3C, Cr3C, and Ni3C. The evidence is the following three points. First, the wall temperature is 548 °C here, so metal catalytic cracking of n-hexane can occur. Altin and Eser reported that the metal catalytic coking mechanism has an intimate relationship with the wall of the cooling channel because of catalysis of the active particles (Fe and Ni) at the bare metal surface, and the catalytic reactions formed metal carbides (Ni3C and Fe3C) in the initial reaction period.31,32 Second, the deposits consist of crystallites with sharp edges, indicating that the strong catalytic activity of the metal surface and these crystallites contain metal carbide species; this is in agreement with the literature of Altin and Eser.10 Last but not least, microarea elemental analysis of a single particle by EDX shows that component elements of the deposits are Fe, Cr, and Ni, in addition to C. As can be seen from Figure 9B3, there are a lot of filamentous cokes on the surface of SS304, and the bright points are found at the filament tips or in the filaments. This can be attributed to severe cracking of n-hexane in this area, which corresponds to a high wall temperature of 745 °C. According to the viewpoint of DeWitt et al., these bright points on the filamentous tips are most likely metallic atoms extracted from the tube, which catalyze filamentous growth.33 There are many reports about the filamentous carbon. Typically, Kuvshinov et al. investigated the growth of catalytic filamentary carbon granules formed upon methane decomposition over nickel catalysts and proposed the physical model of granule formation.34 Recently, Xu et al. studied the effect of potassium acetate on coke growth during light naphtha thermal cracking and found that the cokes were carbon nanofibers with a solid structure and were mainly composed of amorphous carbon.35 As a consequence, under this experimental condition, filamentous carbon deposition from thermal cracking of nhexane is consistent with these literature reports. An interesting but not surprising result is that no morphologies of carbon deposits on the TiN-coated tube surface were observed and the star-shaped crystals of TiN coating were not significantly changed, as seen in group C in Figure 9. It is found that the carbon contents in the atomic percentage on the TiN coating surface after thermal-cracking experiments are approximately 5.76%, 15.73%, and 30.66% at the distance away from the inlet of 100, 350, and 600 mm by

Figure 8. Heat-transfer performance of bare and TiN-coated tubes by thermal cracking of n-hexane (conditions: temperature, 600 °C; pressure, 3.3 MPa; flow rate, 40 mL/min).

250 mm length, the temperature profile becomes more complex because of the phase change and fuel pyrolysis reaction. 3.2. Inhibition Effect of APCVD TiN Coating on Coke Growth. To obtain more information on the mechanism of inhibiting coke growth for APCVD TiN coating, the morphologies of the carbon deposits were characterized by SEM and the carbon atomic percentages of cokes were detected by EDX. Figure 9 presents SEM images of the carbon deposits obtained from different parts of the bare and TiN-coated tubes after thermal cracking of n-hexane under supercritical conditions. Among them, groups A−C are SEM photographs for the inner surface of SS304 bare, SS304 coked, and TiN coked tubes, respectively. Furthermore, groups 1−3 were magnified 100000, 50000, and 10000 times, which correspond to the positions of 100, 350, and 600 mm along the axial length of different types of reaction tubes, respectively. As shown in group A of Figure 9, a lot of small metal particles and cracks are visible on the inner surface of the bare tube. In Figure 9B1, a lot of stunted and clubbed cokes are on the position of 100 mm away from the inlet of the bare reactor. Correspondingly, the outer wall and fluid temperatures are 308 and 83 °C, respectively, as shown in Figure 8. According to the viewpoint of Mohan and Eser, autoxidation of jet fuel, which causes the formation of oxygenated products, is predominant when the reaction temperature is less than 260 °C.30 This means that these stunted and clubbed cokes are formed by thermal oxidation because n-hexane cracking will not occur at this temperature. Moreover, compared to Figure 9A1 with a 5437



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Figure 9. SEM images of different parts and types of tubes. Group A (SS304): (A1) 100 mm; (A2) 350 mm; (A3) 600 mm. Group B (SS304 after experiment): (B1) 100 mm; (B2) 350 mm; (B3) 600 mm. Group C (TiN coating after experiment): (C1) 100 mm; (C2) 350 mm; (C3) 600 mm (conditions: temperature, 600 °C; pressure, 3.3 MPa; flow rate, 40 mL/min; cracking time, 20 min).

Figure 10. SEM images of solid carbon on the inner surface of TiN-coated tubes with different cracking times: (A) 40 min; (B) 60 min; (C) 80 min (conditions: temperature, 600 °C; pressure, 3.3 MPa; flow rate, 40 mL/min).

number of spherical carbon deposits accumulated and aggregated after thermal cracking for 80 min (see Figure 10C). Additionally, the carbon contents in atomic percentage on the TiN coating surface also had a regular increasing tendency: 30.66% for 20 min, 83.47% for 40 min, 88.37% for 60 min, and 99.76% for 80 min. In particular, as shown in Figure 10C, those carbon deposits have structures similar to that of carbon black, consisting of clusters of spherules with a more homogeneous size distribution, which fits well with the literature of Alonso-Morales et al.36 This structure indicates that reactions in the gas phase are responsible for the formation of solid carbons. In brief, the spherical or filamentous solid carbons depended on whether the solid deposits were formed in the gas phase or over the metal wall. Table 3 summarizes the carbon atomic percentages of the Ti, N, and C elements for the TiN-coated tube via EDX after nhexane thermal cracking at 600 °C and 3.3 MPa for different cracking times. It can be seen from the table that, as the atomic percentage of C increases from 5.76% to 83.47% in the coating, the atomic percentage of Ti (and N) decreases monotonically from 44.38% to 16.53% (from 49.87% to 0%). Because the N

EDX elemental analysis, respectively. On the one hand, because of the higher carbon content, these carbons must be inside the coating instead of carbon pollution. On the other hand, the carbon content increases gradually; this is consistent with the severity of n-hexane thermal cracking increasing along the reactor tube. In order to gain further information about the inhibition mechanism of coke growth of TiN coating, carbon deposition experiments by thermal cracking of n-hexane under the same conditions (temperature, 600 °C; pressure, 3.3 MPa; flow rate, 40 mL/min) were supplemented in the TiN-coated tubes for different cracking times. Figure 10 shows representative examples of SEM images of solid carbon on the inner surface of TiN-coated tubes with different cracking times. In general, the growth of carbon deposits shows regular changes with increasing cracking time: the star-shaped crystals presented no obvious change after thermal cracking for 20 min (see Figure 9C3), the star-shaped crystals disappeared and the surface became smooth after thermal cracking for 40 min (see Figure 10A), some spherical carbon deposits began to emerge after thermal cracking for 60 min (see Figure 10B), and a large 5438



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Tmax. Moreover, the different Tmax values means that there are four different reactivities and structure carbon deposits forming in the process of n-hexane thermal cracking, and the quantities of carbon deposits at unit area are 8900, 1350, 5440, and 1450 μg/cm2, respectively. The total amount of carbon deposits reaches 17120 μg/cm2, so many cracking carbon deposits are expected to bring great harm as mentioned before. Furthermore, according to Alonso-Morales et al., hydrogenrich structurally disordered deposits were oxidized at lower temperature, while hydrogen-lean structurally ordered deposits were oxidized at higher temperature.22 Therefore, the hydrogen contents of the four different active carbon deposits were also not the same and reduce as Tmax increases. Because of a detection limit of 200 ppm, carbon deposition on TiN coating was not detected by TPO. This result further confirms that TiN coating has a good effect on the inhibition of coke growth. 3.3. Analysis Gas and Liquid Products of n-Hexane Cracking. The cracking gas yield and conversion are the key parameters to characterize the cracking severity of hydrocarbons. In this study, the gas yield is defined as the ratio of the gaseous mass to the initial mass of the reactant fuel. The conversion is defined as the ratio of the mass fraction of fuel reacted to the feed. Therefore, the gas yield and conversion of n-hexane cracking are 11.2% (gas flow rate is 2.6 L/min) and 56.6% (liquid products flow rate is 21.2 g/min) for the bare tube when the cracking temperature is 600 °C, and the corresponding values are 14.0% (gas flow rate is 3.2 L/min) and 58.4% (liquid products flow rate is 18.7 g/min) for the TiN-coated tube. Figure 12 presents the distribution of gas

Table 3. Carbon Atomic Percentages of the Ti, N, and C Elements for the TiN-Coated Tube after n-Hexane Thermal Cracking at 600 °C and 3.3 MPa with Different Cracking Times 20 min for the TiN-coated tube

40 min for the TiNcoated tube

60 min for 80 min for the TiNthe TiNcoated coated tube tube

element

100 mm

350 mm

600 mm

600 mm

600 mm

600 mm

C N Ti

5.76 49.87 44.38

15.73 51.19 33.08

30.66 32.28 37.06

83.47 0 16.53

88.37 0 11.63

99.76 0 0.24

atoms finally disappear from TiN coating, this may imply that the N atoms are replaced by C atoms in TiN coating under the experimental conditions. We will discuss the inhibition mechanism of coke growth for TiN coating in detail in the last section. TPO is a useful and extensive technique to study the amounts and structures of solid carbons, such as nanotubes and nanofibers, anthracites, and other carbon materials.37,38 This technique is based on the fact that the reaction of solid carbons with molecular oxygen takes place at specific active sites, such as at structural defects or C atoms on the edges of graphene layers; thus, we can relate the oxygen reactivity of solid carbons to their structures. The temperature oxidation profiles of the solid carbon deposits obtained by pyrolysis of n-hexane are compiled in Figure 11. It can be observed that the reactivity

Figure 11. Oxidation profiles of solid carbon deposits obtained by pyrolysis of n-hexane (conditions: temperature, from 70 to 900 °C; Pt/Al2O3 oxidation catalyst temperature, 300 °C; O2 flow rate, 600 mL/min).

Figure 12. Distribution of the gas products of n-hexane cracking (conditions: temperature, 600 °C; pressure, 3.3 MPa; flow rate, 40 mL/min).

curve can be deconvoluted into four individual curves, corresponding to maximum oxidation temperatures (Tmax) of 624, 695, 765, and 828 °C, respectively. Therein the first two Tmax values of carbon deposits are consistent with the literature.12 Around 630 °C, the carbon deposits resulted from aromatic condensation coking. Around 690 °C, the carbon deposits belonged to the metal catalytic cokes. The latter two Tmax values of carbon deposits with a high degree of graphitization are reported in the literature,22 and the atomic C/H of the solid carbon products increased with increasing

products of n-hexane cracking. Table 4 summarizes the characterization data of the liquid products obtained in thermal cracking of n-hexane. As far as the gas components are concerned, the most abundant components in the gas products are methane and ethene for both the bare and TiN-coated tubes. It can be observed that there are very few differences in the gas and liquid products for bare and TiN-coated tubes on the whole. Therefore, the n-hexane cracking reactions in the bulk phase have little relation with the property of TiN coating, 5439



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has no catalytic action. Therefore, there are no Cr atoms in the metal particles, which are the bright points at the top of the filamentous carbon by SEM. After that, the metal particles are lifted from the surface, which is caused by the accumulation of C atoms in the metal crystallite, leading to the filamentous coke lateral growth.33,35,41 It is worth pointing out that the surface of the SS304 substrate is rough under a microscope; there are a lot of cracks and small particles (see Figure 5A), and it is wellknown that different parts show different catalytic activities. Therefore, from Figure 14a, it is found that carbon deposits present the microstructure with different lengths and sizes. Figure 14b shows homogeneity of TiN coating with a golden color. Because the TiN and TiC lattice parameters are very close (0.4230 and 0.4238 nm, respectively), a N atom is often replaced by C atoms at any proportion to form a solid solution. It is true that the N element in TiN coating is gradually replaced by C atoms because of the thermal cracking reaction, leading to the formation of TiNC or even TiC. Of course, this replacement is also nonstoichiometric and forms Ti0.10C0.90 ∼ Ti0.18C0.82 by EDX elemental analysis. Meanwhile, the color of the coating changes gradually from golden to gray (see Figure 13). If the thermal cracking reaction continues to proceed, when N atoms in TiN coating are completely replaced by C atoms, there will be coke formation on the surface of the spherical carbon deposit in the manner of gas-phase coke growth, and the color of the coating will be completely changed into gray from the outside to the inside, as shown in Figure 14d. In conclusion, one can state that, compared with other coatings, TiN coating not only creates a barrier between the hydrocarbon fuels and the metal surface, inhibiting related catalytic coke formation, but also minimizes carbon deposits by absorbing C atoms.

Table 4. Comparison of the Liquid Products from Thermal Cracking of n-Hexane for the Bare and TiN-Coated Tubes compound

bare tube (wt %)

TiN-coated tube (wt %)

uncracked fuel paraffin olefin arene

43.40 29.65 26.43 0.52

41.56 32.06 24.41 1.97

while the inhibition effect on coke growth on the inner surface of the fuel tube mainly depends on the inert property of the coating. 3.4. Possible Mechanisms of the Inhibition Effect of APCVD TiN Coating on Coke Growth. It has been reported that aluminide, silicide, and boride coatings have certain effects in terms of reducing coke.39,40 In this study, the mechanisms of inhibiting coking are slightly different between TiN coating and other reported coatings. Figure 13 presents a photograph of the

Figure 13. Photograph of the TiN-coated reactor after thermal cracking of n-hexane for different cracking times (conditions: temperature, 600 °C; pressure, 3.3 MPa; flow rate, 40 mL/min).

4. CONCLUSION Hydrocarbon fuel thermal cracking can give rise to the formation of solid carbon deposits on the surface of metal substrates, and the coating method is used to inhibit metal catalytic coking and proved to be promising during thermal cracking. Therefore, TiN coating with appropriate thickness (more than 8 μm) was deposited on the microchannel inner surface of SS304 tubes using the APCVD method. Under the conditions of this experiment, TiN coating has a relatively complete cubic-phase crystal form with a N/Ti ratio of 1:1 and presents small star-shaped crystals on the whole. The surface coking performance of the bare tube is tremendously eliminated by TiN coating. Characterization of the carbon deposits by SEM and EDX shows different morphologies and amounts along the axial length of the bare tube from n-hexane thermal cracking at 600

TiN-coated reactor after thermal cracking of n-hexane at 600 °C and 3.3 MPa for different cracking times. It is clearly shown that the color of TiN coatings is significantly changed after the experiment. Figure 14 shows a schematic representation of the simplified mechanism for the inhibition effect with and without TiN coating on coke growth during n-hexane thermal cracking. Figure 14a presents filamentous coke growth on the surface of the bare tube, and this process can be described as follows: Initially, hydrocarbon molecules are chemisorbed onto the metal crystallite surface, convert to carbon through reactions on the surface, and finally form metal carbides (Ni3C, Fe3C, and Cr3C), wherein these Ni3C and Fe3C can decompose into C and Fe atoms or an atomic cluster, thus forming a catalytic cycle. However, once Cr3C forms, it is difficult to decompose into Cr and C atoms; therefore, it is generally believed that Cr

Figure 14. Schematic representation of the simplified mechanism for the inhibition effect of APCVD TiN coating on coke growth during n-hexane cracking with (a) filamentous formation of coke on the bare tube, (b) TiN coating covering the surface of the bare tube, (c) carbon inserted into TiN coating, and (d) spherical coke formation. 5440



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°C and 3.3 MPa for 20 min. Along the axial length of the bare tube, stunted and clubbed cokes formed by autoxidation near the distance of 100 mm; granular metal carbides and filamentous cokes formed by metal catalysis near the distances of 350 and 600 mm, respectively. No morphologies of carbon deposits on the TiN-coated tube surface were observed after thermal cracking of n-hexane at 600 °C and 3.3 MPa for 20 min. At distances of 100, 350, and 600 mm away from the tube inlet, the carbon atomic percentages of 5.76%, 15.73%, and 30.66% for the TiN-coated tube in these three areas were less than those of 27.28%, 58.04%, and 99.69% for the bare tube, respectively. The results show that the inhibition effect of APCVD TiN coating on coke growth is superior to that of other coatings (e.g., alumina coating) during n-hexane thermal cracking under supercritical conditions. The reason is that TiN coating not only creates a barrier between the hydrocarbon fuels and the metal surface to inhibit related catalytic coke formation but also minimizes carbon deposits by absorbing C atoms.

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the Mitigation of Fouling from Heated Jet Fuel. Ind. Eng. Chem. Res. 2011, 50, 7290−7304. (12) Yang, C.; Liu, G.; Wang, X.; Jiang, R.; Wang, L.; Zhang, X. Preparation and Anticoking Performance of MOCVD Alumina Coatings for Thermal Cracking of Hydrocarbon Fuels under Supercritical Conditions. Ind. Eng. Chem. Res. 2012, 51, 1256−1263. (13) Maruyama, T.; Arai, S. Aluminum Oxide Thin Films Prepared by Chemical Vapor Deposition from Aluminum acetylacetonate. Appl. Phys. Lett. 1992, 60, 322−323. (14) Huntz, A. M.; Andrieux, M.; Herdst-Ghysel, M.; Huet, C.; Vahlas, C.; Samelor, D.; Sovar, M.; Gleizes, A. Dehydration and Phase Transformations of MOCVD Processed Alumina coatings Investigated by Temperature Induced Deflection of Dissymmetrical Samples. Proc.Electrochem. Soc. (EUROCVD-15, Bochum, Germany) 2005, 09, 1037−1044. (15) Pranhan, S. K.; Reucroft, P. J.; Ko, Y. Crystallinity of Al2O3 Films Deposited by Metalorganic Chemical Vapor Deposition. Surf. Coat. Technol. 2004, 176, 382−384. (16) Bahlawane, N.; Blittersdorf, S.; Kohse-Höinghaus, K.; Atakan, B.; Müller, J. Investigating of CVD Processes to Perform Dense αAlumina Coating on Superalloys. J. Electrochem. Soc. 2004, 151, C182− 186. (17) Müller, J.; Schierling, M.; Zimmermann, E.; Neuschütz, D. Chemical Vapor Deposition of Smooth α-Al2O3 Films on Nickel Base Superalloys as Diffusion Barriers. Surf. Coat. Technol. 1999, 120, 16− 21. (18) Haanappel, V. A. C.; Van Corbach, H. D.; Fransen, T.; Gellings, P. J. Cracking and Delamination of Metal Organic Vapour and Deposited Alumina and Silica Films. Mater. Sci. Eng. A 1993, 167, 179−185. (19) Binions, R.; Piccirillo, C.; Parkin, I. P. Tungsten Doped Vanadium Dioxide Thin Films Prepared by Atmospheric Pressure Chemical Vapour Deposition from Vanadyl Acetylacetonate and Tungsten Hexachloride. Surf. Coat. Technol. 2007, 201, 9369−9372. (20) Nable, J. C.; Nosheen, S.; Suib, S. L.; Galasso, F. S. Atmospheric Pressure Chemical Vapor Deposition of Titanium Nitride on Metals. Surf. Coat. Technol. 2006, 200, 2821−2826. (21) Nasution, I.; Velasco, A.; Kim, H. J. Atmospheric Pressure Chemical Vapor Deposition Mechanism of Al2O3 Film from AlCl3 and O2. J. Cryst. Growth 2009, 311, 429−434. (22) Alonso-Morales, N.; Gilarranz, M. A.; Heras, F.; Rodriguez, J. J.; Eser, S. Oxidation Reactivity and Structure of LDPE-Derived Solid Carbons: A Temperature-Programmed Oxidation Study. Energy Fuels 2013, 27, 1151−1161. (23) Yuan, L.; Liang, G. Z.; Xie, J. Q.; Li, L.; Guo, J. Preparation and Characterization of Poly(urea−formaldehyde) Microcapsules Filled with Epoxy Resins. Polymer 2006, 47, 5338−5349. (24) Park, C. S.; Kim, J. G.; Chun, J. S. The Effects of Reaction Parameters on the Deposition Characteristics in Al2O3 CVD. J. Vacuum Sci. Technol. A 1983, 1, 1820−1824. (25) Gil, E.; Park, J. B.; Oh, J. S.; Yeom, G. Y. Characteristics of SiOx Thin Films Deposited by Atmospheric Pressure Chemical Vapor Deposition as a Function of HMDS/O2 Flow Rate. Thin Solid Films 2010, 518, 6403−6407. (26) Vallejo, B.; Gonzalez-Manas, M.; Martınez-Lopez, J.; Morales, F.; Caballero, M. A. Characterization of TiO2 Deposited on Textured Silicon Wafers by Atmospheric Pressure Chemical Vapour Deposition. Sol. Energy Mater. Sol. Cells 2005, 86, 299−308. (27) Lin, X. X.; Zhao, G. L.; Wu, L. Q.; Duan, G. F.; Han, G. R. TiNx Thin Films for Energy-saving Application Prepared by Atmospheric Pressure Chemical Vapor Deposition. J. Alloys Compd. 2010, 502, 195−198. (28) Sundgren, J. E.; Johansson, B. O.; Rockett, A.; Barnett, S. A.; Greene, J. E. TiNx (0.6 < x < 1.2): Atomic arrangements, Electronic structure and recent results on crystal growth and physical properties of epitaxial layers. AIP Conf. Proc. 1986, 149, 95−115. (29) Jiang, R.; Liu, G.; Zhang, X. Thermal Cracking of Hydrocarbon Aviation Fuels in Regenerative Cooling Microchannels. Energy Fuels 2013, 27, 2563−2577.

AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: +86-28-85418451. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grants 91016002 and J1103315), Outstanding Young Scholar Fund of Sichuan University (Grant 2013SCU04A05), and Science and Technology Support Program of Sichuan Province (Grant 2012GZ0006).

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REFERENCES

(1) Nixon, A. C.; Henderson, H. T. Thermal Stability of Endothermic Heat-sink Fuels. Ind. Eng. Chem. Prod. Res. Dev. 1966, 5, 87−92. (2) Edwards, T. Liquid Fuels and Propellants for Aerospace Propulsion: 1903−2003. J. Propul. Power 2003, 19, 1089−1107. (3) Sobel, D. R.; Spadaccini, L. J. Hydrocarbon Fuel Cooling Technologies for Advanced Propulsion. J. Eng. Gas Turbines Power 1997, 119, 344−351. (4) 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, 773−785. (5) Eser, S.; Venkataraman, R.; Altin, O. Deposition of Carbonaceous Solids on Different Substrates from Thermal Stressing of JP-8 and Jet A Fuels. Ind. Eng. Chem. Res. 2006, 45, 8946−8955. (6) Edwards, T.; Zabarnick, S. Supercritical Fuel Deposition Mechanism. Ind. Eng. Chem. Res. 1993, 32, 3117−3122. (7) Li, J.; Eser, S. Carbonaceous Deposit Formation on Metal Surfaces from Thermally Stressed Dodecane. Carbon ‘95, Extended Abstracts; 22nd Biennial Conference on Carbon; American Carbon Society: San Diego, CA, 1995; p 314. (8) Albright, L. F.; Marek, J. C. Coke Formation during Pyrolysis: Roles of Residence Time, Reactor Geometry, and Time of Operation. Ind. Eng. Chem. Res. 1988, 27, 743−751. (9) Graff, M. J.; Albright, L. F. Coke Deposition from Acetylene, Butadiene, and Benzene Decompositions at 500−900 °C on Solid Surfaces. Carbon 1982, 20, 319−330. (10) Altin, O.; Eser, S. Analysis of Solid Deposits from Thermal Stressing of a JP-8 Fuel on Different Tube Surfaces in a Flow Reactor. Ind. Eng. Chem. Res. 2001, 40, 596−603. (11) Ram Mohan, A.; Eser, S. Effectiveness of Low-Pressure Metal− Organic Chemical Vapor Deposition Coatings on Metal Surfaces for 5441



Article

(30) Mohan, A. R.; Eser, S. Analysis of Carbonaceous Solid Deposits from Thermal Oxidative Stressing of Jet-A Fuel on Iron- and NickelBased Alloy Surfaces. Ind. Eng. Chem. Res. 2010, 49, 2722−2730. (31) Altin, O.; Eser, S. Characterization of Carbon Deposits From Jet Fuel on Inconel 600 and Inconel X Surfaces. Ind. Eng. Chem. Res. 2000, 39, 642−645. (32) Altin, O.; Eser, S. Analysis of Carboneceous Deposits From Thermal Stressing of a JP-8 Fuel on Superalloy Foils in a Flow Reactor. Ind. Eng. Chem. Res. 2001, 40, 589−595. (33) DeWitt, M. J.; Edwards, T.; Shafer, L.; Brooks, D.; Striebich, R.; Bagley, S. P.; Wornat, M. J. Effect of Aviation Fuel Type on Pyrolytic Reactivity and Deposition Propensity under Supercritical Conditions. Ind. Eng. Chem. Res. 2011, 50, 10434−10451. (34) Kuvshinov, G. G.; Mogilnykh, Yu. I.; Kuvshinov, D. G.; Yermakov, D. Yu.; Yermakova, M. A.; Salanov, A. N.; Rudina, N. A. Mechanism of Porous Filamentous Carbon Granule Formation on Catalytic Hydrocarbon Decomposition. Carbon 1999, 37, 1239−1246. (35) Wang, Z. Y.; Xu, H.; Luan, X. J.; Hou, F.; Zhou, J. X. Effect of Potassium Acetate on Coke Growth during Light Naphtha Thermal Cracking. Ind. Eng. Chem. Res. 2011, 50, 10292−10297. (36) Alonso-Morales, N.; Gilarranz, M. A.; Heras, F.; Eser, S.; Rodriguez, J. J. Influence of Operating Variables on Solid Carbons Obtained by Low-Density Polyethylene Pyrolysis in a Semicontinuous Fast Heating Quartz Reactor. Energy Fuels 2009, 23, 6102−6110. (37) Perez-Cabero, M.; Rodriguez-Ramos, I.; Guerrero-Ruiz, A. Characterization of Carbon Nanotubes and Carbon Nanofibers Prepared by Catalytic Decomposition of Acetylene in a Fluidized Bed Reactor. J. Catal. 2003, 215, 305−316. (38) Arenillas, A.; Cuervo, S.; Dominguez, A.; Menendez, J. A.; Rubiera, F.; Parra, J. B.; Merino, C.; Pis, J. J. Effects of Oxidative Treatments with Air and CO2 on Vapour Grown Carbon Nanofibres (VGCNFs) Produced at Industrial Scale. Thermochim. Acta 2004, 423, 99−106. (39) Jazayeri, S. M.; Karimzadeh, R. Experimental Investigation of Initial Coke Formation over Stainless Steel, Chromium, and Iron in Thermal Cracking of Ethane with Hydrogen Sulfide as an Additive. Energy Fuels 2011, 25, 4235−4247. (40) Zhou, J. X.; Xu, H.; Liu, J. L.; Qi, X. G.; Zhang, L.; Jiang, Z. M. Study of Anti-coking Property of SiO2/S Composite Coatings Deposited by Atmospheric Pressure Chemical Vapor Deposition. Mater. Lett. 2007, 61, 5087−5090. (41) Blank, V. D.; Alshevskiy, Y. L.; Belousov, Y. A.; Kazennov, N. V.; Perezhogin, I. A.; Kulnitskiy, B. A. TEM studies of Carbon Nanofibres Formed on Ni Catalyst by Polyethylene Pyrolysis. Nanotechnology 2006, 17, 1862−1866.

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Inhibition Effect of APCVD Titanium Nitride Coating on Coke Growth

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