Analysis of Solid Deposits from Thermal Stressing of a JP-8 Fuel on

Thermal stressing of a JP-8 fuel was carried out in an isothermal flow reactor using nickel, stainless steel (316 and 304), Silcosteel, and glass-line...
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Ind. Eng. Chem. Res. 2001, 40, 596-603

Analysis of Solid Deposits from Thermal Stressing of a JP-8 Fuel on Different Tube Surfaces in a Flow Reactor Orhan Altin† and Semih Eser*,†,‡ Laboratory for Hydrocarbon Process Chemistry, The Energy Institute, 209 Academic Projects Building, and Department of Energy and Geo-Environmental Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802

Thermal stressing of a JP-8 fuel was carried out in an isothermal flow reactor using nickel, stainless steel (316 and 304), Silcosteel, and glass-lined stainless steel tubes at 500 °C wall temperature and 34 atm (500 psig) for 5 h at a liquid fuel flow rate of 1 mL/min. Different length segments along the sample tubes were analyzed to observe the deposit distribution throughout the test section. Temperature-programmed oxidation (TPO) analysis and SEM examination of the stressed tubes showed differences in the amount and nature of the solid deposits obtained on the different substrates. The activity of the tube surfaces toward carbon deposition decreases in the order nickel > SS 316 > SS304 > Silcosteel > glass-lined stainless steel. The catalytic activity of the metal surfaces was noted using TPO analysis in conjunction with SEM examination of the deposited tubes. 1. Introduction Because of high thermal loads in future advanced aircraft, jet fuel would be exposed to much higher temperatures, reaching as high as 540 °C, before combustion.1-3 At such high temperatures, both homogeneous and heterogeneous reactions of jet fuel can lead to solid deposition on metal surfaces.4,5 The formation of carbonaceous deposits on metal surfaces in the fuel system is, therefore, a major concern for the development of advanced aircraft. At high temperatures (>400 °C), nickel-based stainless steel surfaces deposit large amounts of carbonaceous deposits from jet fuel and related model compounds.6 Using inert coatings such as Silcosteel reduces carbon deposition from pyrolysis of fuels.7 Inert coatings have also been found to be useful in reducing carbon deposition in some industrial applications.8 Reactive species, such as radicals and unsaturated hydrocarbons that are usually formed in a fluid phase, react on the active sites of metal surfaces to produce carbon deposits.9 Coating of metal alloy surfaces might prevent diffusion of carbon into the base metals and, thus, inhibit the formation of metal carbides and/or filamentous carbon. Several authors have used temperature-programmed oxidation (TPO) to characterize carbon deposit on metal surfaces.10-12 Different peaks found in the TPO profiles have been attributed to differences in the nature of the carbon deposits. McCarty et al.13 used temperatureprogrammed surface reaction with hydrogen to characterize carbon deposits produced on an alumina-supported nickel methanation catalyst upon exposure to carbon monoxide at 277 °C. Based on the relative reactivity of the deposits toward hydrogen, the deposited * Author to whom correspondence should be addressed. Telephone: 814-863-1392. Fax: 814-865-3573. E-mail: sxe2@ psu.edu. † Laboratory for Hydrocarbon Process Chemistry. ‡ Department of Energy and Geo-Environmental Engineering.

carbon species were classified as chemisorbed carbon atoms (R) evolving around 190 °C, bulk nickel carbide, amorphous carbon (β) around 385 °C, filamentous carbon at 600 °C, encapsulating carbon at 690 °C, and crystalline graphite platelets at temperatures above 800 °C. The objective of this study is to investigate hightemperature solid deposition from a JP-8 fuel on different metal tubes, including nickel, stainless steel (304 and 316), and two coated tubes, Silcosteel and glasslined stainless steel, in a flow reactor. Different length segments of the tubes were analyzed by temperatureprogrammed oxidation in a multiphase carbon analyzer to determine the reactivity of the carbon deposits from the TPO profiles. Scanning electron microscopy was used to examine the morphology of the deposit along the length of the tubes in an effort to relate the reactivity and surface morphology of the deposits. 2. Experimental Section 2.1. Thermal Stressing of JP-8 Fuel. The flow reactor used in this study is a modified Chemical Data Systems (CDS) model 803 bench-scale reaction system. As shown schematically in Figure 1, the reactant fuel is pumped using a HPLC pump (Waters M600 A) to a preheater maintained at 250 °C in a valve oven. The preheated fuel is directed by an eight-port valve to pass through a reactor and a gas/liquid separator. A 1/4-in. (o.d.) and 20-cm tube reactor is jacket-heated isothermally to an external wall temperature of 500 °C. The reaction products then pass through a back-pressure regulator, which holds the system pressure at 34 atm (500 psi), and an air-cooled copper coil condenser to separate the liquid products. Noncondensable gases are vented to a fume hood. A liquid fuel flow rate of 1/mL min was maintained in all thermal stressing experiments with JP-8 fuel for a duration of 5 h. Prior to the introduction of the fuel, N2 gas was passed through the reaction system for 2 h with the preheater and reactor temperatures and system pressure maintained under the selected stressing conditions.

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Figure 1. Schematic flow diagram of fuel stressing system and the reactor detail. Table 1. Chemical Composition (wt %)14 of JP-8 Fuel Determined by GC/MS and Some Physical Properties Calculated Using SUPERTRAP and the Peng-Robinson Equation of State C7-C18 C6-C14 C6-C14 C10-C14 C10-C14 C9-C12 alkanes cyclohexanes aromatics Decalins tetralins indans 61.3

13.5 15.5 0.9 0.4 molecular weight: 167 (mean) boiling point: 205-300 °C (401-572°F) vapor pressure: 0.44 mmHg at 20 °C vapor density (air ) 1.0): 4.7 specific gravity (water ) 1.0): 0.81 viscosity: 8.0 cSt

1.4

The 316 stainless steel preheating section is approximately 2 mm in i.d. (1/8-in. o.d.) and 2 m in length. The fuel residence time in the preheater is approximately 6.3 min at a fuel flow rate of 1 mL/min. In the preheating section at 250 °C, the fuel is mostly in the liquid state with some low-boiling components in the vapor phase. In the reactor (500 °C external wall temperature and 34 atm pressure), the fuel can be assumed to be a supercritical fluid. The fuel residence time in the reactor part (4-mm i.d., 1/4-in. o.d., and 20cm length) was calculated to be 2.5 min at a fuel flow rate of 1 mL/min. Temperature measurements were made at four different points along the length of the reactor tubes at 5, 10, and 15 cm from the top of the reactor and at the end of the reactor. The end of the reactor tube was connected to a cross for measuring the bulk fuel temperature inside the reactor and at the outlet. The end section of the tube was insulated by a fiberglass wrapping. The bulk fuel temperatures were measured at a 1 mL/min JP-8 flow rate for each tube at 5 cm from the end of the reactor. Temperature measurements made along the length of the reactor were observed to change in a narrow range between 475 and 480 °C, when the outlet temperature of the tube wall was kept at 500 °C. A Reynolds number of 26.1 was calculated for the fuel flow in the reactor based on the physical properties of the JP-8 fuel calculated by SUPERTRAP software, using the Peng-Robinson equation of state. The chemical composition of the jet fuel sample, determined by a quantitative GC/MS method14 (Table 1), was used to determine its physical properties, also listed in Table 1.

Five different metal tubes were examined: SS 316, SS 304, nickel, Silcosteel (Restek Co., Bellefonte, PA), and glass-lined stainless steel (Alltech, Deerfield, IL). The tube dimensions were were 6-mm o.d., 4-mm i.d., and 20-cm length. Before the experiments, all of the tubes were washed with hexane and dried in air. 2.2. Analysis of Carbon Deposits. After the thermal stressing experiments, the tubes were cut into 2.5cm segments and analyzed for carbon deposits by using a LECO RC-412 multiphase carbon analyzer. Conventionally, the LECO RC-412 instrument has been used to measure the amount of deposition on metal surfaces.1,2 In the carbon analyzer, carbon in the deposit is oxidized to carbon dioxide by reaction with UHP O2 in a furnace and over a CuO catalyst bed. The product CO2 is quantitatively measured by a calibrated IR detector as a function of temperature in the furnace. In this study, we used temperature-programmed oxidation to assess the nature of the deposits from their reactivity during oxidation under selected heating conditions. The deposited tube segments were heated at a rate of 30 °C/ min in flowing O2 (750 mL/min) to a maximum temperature of 900 °C with a holding period of 6 min at the final temperature. Repeated experiments showed that the TPO profiles were reproducible with respect to individual peak positions and relative peak intensities. The total amount of deposition measured on different tube surfaces was reproducible to within (20 wt % of the deposit mass. To the best of our knowledge, this is the first attempt to use the carbon analyzer to extract information on the nature of the carbonaceous deposits from jet fuel decomposition, in addition to the amount of deposits. The carbon deposit morphologies were examined with an ISI-DS 130 dual-stage scanning electron microscope (SEM). To examine the morphology of the carbon deposits on the inner surface of the tubes, the tubes were cut longitudinally from the center. 3. Results and Discussion 3.1. Carbon Deposition on Different Tube Surfaces. Figure 2 shows the total mass of carbon deposits (in micrograms per square centimeter) collected on each tube. As expected, nickel gave the highest deposition,

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Figure 2. Total carbon deposit amount from JP-8 decomposition on different metal tubes at 500 °C and 34 atm (500 psig) with 1 mL/min JP-8 flow for 5 h.

Figure 3. Carbon deposition from JP-8 decomposition at 500 °C and 34 atm along the length of the tubes with 1 mL/min JP-8 flow for 5 h.

presumably because of its high catalytic activity.15 Stainless steel (SS) 316 produced more deposits than grade 304 SS under the same thermal stressing condition. The differences in chemical composition of the two alloys include the presence of molybdenum in SS 316 and higher Ni (2% higher) and lower Cr contents (2% lower) in SS 316 than in SS 304. Physical properties including grain size and surface roughness might also play a role in deposition on alloy surfaces. Figure 2 shows that the Silcosteel coating on the tubes inhibited carbon deposition from thermal decomposition of jet fuel. The glass-lined stainless steel, however, gave the smallest amount of carbon deposition. After thermal stressing of JP-8 on the glass-lined surface, the fuel did not show any color change. In contrast, the fuel became yellowish after stressing with the other tubes. This observation suggests that the tube surfaces might play an active role in initiating or accelerating the decomposition of jet fuel in addition to catalyzing solid deposition from reactive species. Figure 3 shows a plot of the carbon deposition on the metal tube surfaces versus the axial length of the test section from stressing of JP-8 fuel at a 500 °C wall temperature and 34 atm pressure for 5 h. For nickel, SS 316 and SS 304 the carbon deposition increases along the length of the tubes until a maximum is reached and then decreases. With these three tubes, the maximum deposition took place between 10 and 15 cm from the top of the reactor. For Silcosteel surfaces, two shallow maxima were observed, although the carbon deposition was very small. For the glass-lined surface, deposition was observed only between 5 and 12.5 cm of the test section. Marteney and Spadaccini16 obtained similar maxima in stressed tubes, and they stated that the decrease in deposit formation might be an indication of a depletion of oxygen in the fuel. The maxima seen in the deposit amounts before the end of the heated section

can be attributed to the depletion of reactive species that are precursors to the deposits. It is expected that the dissolved oxygen in the fuel would have been depleted in the preheater before the inlet of the reactor. It is interesting to note that the strongest maximum in the deposition profile was obtained with the nickel tube, the surface with the highest activity toward deposition. 3.2. Characterization of Carbonaceous Deposits. As mentioned previously, depending on the temperature on substrate surface, at least seven types of carbonaceous deposits were identified in the literature.13 The substrate temperature is a significant parameter having a major influence not only on the rate, but also on the type of carbonaceous deposition.17-19 At lower temperatures around 150 °C, thermal oxidative deposits result from the reactions of fuel components with dissolved oxygen (∼70 ppm).20 Other kinds of deposits occur at higher temperatures (>350 °C) and result from thermal/ catalytic cracking reactions of the fuel components, depending on the residence time. In this study, different kinds of solid carbon deposits were observed on different surfaces upon stressing of the same fuel under the same conditions. TPO experiments gave carbon burnoff profiles that consist of multiple peaks as a function of temperature. The different peaks observed in these plots can be attributed to the different natures of the deposits with respect to their oxidation reactivity under the constant heating conditions used in the carbon analyzer. Broadly, the peaks observed at high temperatures (>500 °C) in these plots can be assigned to less-reactive (or more structurally ordered) deposits produced most likely by catalytic reactions on active metal surfaces. The peaks at low temperatures, on the other hand, can be assigned to more-reactive (or relatively hydrogen rich and more amorphous) deposits. It is very likely that this kind of deposit results mainly from the secondary deposition processes, e.g., pyrolysis, on already-formed carbon deposits, such as platelets or filaments, produced by catalytic reactions. This process of sequential deposition of different kinds of carbonaceous solids is consistent with the mechanism proposed by Albright and Marek.17 One should also note that the low-temperature peaks seen in the TPO profiles might also result from catalytic oxidation of carbon deposits that contain finely dispersed metal particles, such as those found in fine carbon filaments. Observations from the TPO experiments and the SEM examinations of the deposits on each tube are presented and discussed below. Nickel. Figure 4 shows the CO2 evolution from the TPO of the JP-8 fuel deposits on different length segments of the nickel tube after 5 h at a 500 °C wall temperature and 34 atm (500 psig) pressure. As shown in Figure 2, under these conditions, the heaviest deposition was observed on the nickel surface. Four major “phases” of the deposit on the nickel surface can be identified for each 2.5-cm section with different deposition quantities. These phases are distinguished by differences in their oxidation reactivity upon being heated in flowing oxygen. The most-reactive phase would burn off at the lowest temperature, whereas the least-reactive deposit would burn off at the highest temperature as shown in the TPO profiles. The first phase shows a relatively small amount of carbon deposit that burns off at 150 °C. This phase probably consists of hydrogen-rich chemisorbed species that desorb and oxidize over CuO in the afterburner of the multiphase

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Figure 4. TPO profiles of the deposits collected on nickel tube segments from thermal stressing of JP-8 at 500 °C and 34 atm (500 psig) with 1 mL/min flow for 5 h.

carbon analyzer.21 The largest quantity of carbon deposits was observed in the second phase where carbon burnoff takes place at around 350 °C. The deposits oxidized at this temperature consist most likely of highly reactive amorphous carbon and fine filaments, similar to the deposits identified by McCarty and Wise22 as β-type deposits. The third and fourth phases show peaks at around 550 and 700 °C, respectively, in the TPO profiles. The lower reactivity of the carbon deposits in these phases can be attributed to their relatively ordered structures produced via surface-catalyzed heterogeneous reactions. All of the phases, except the fourth phase, go through a maximum on the segment of the 15-17.5-cm test section. This segment was just before the outlet section, where the fuel reached its maximum temperature along the length of the reactor. The fourth phase appears to increase monotonically across the reactor length, giving the maximum amount in the last segment, the 17.7-20-cm test section. The presence of different carbon phases on the nickel surface suggests that multiple deposition mechanisms occur during thermal stressing of jet fuel. High-temperature peaks (>500 °C) in the TPO profiles (Figure 4) indicate the presence of carbon deposits with relatively low reactivity. A high degree of structural order in solid carbons, i.e., the presence of large aromatic sheets aligned in a pre-graphitic or graphitic order would reduce the oxidation reactivity compared to that of amorphous carbon with no apparent structural order. The SEM micrographs of the deposited tube surfaces (Figure 5) did, in fact, show the presence of faceted crystallites in the deposits along the whole length of the reactor tubes. The formation of these crystallites can be explained by the catalytic activity of the nickel surface initiating the solid deposition. The presence of the high-temperature peaks in all of the tube segments, as shown in Figure 4, suggests that heterogeneous reactions are responsible for the initiation of deposition. The low-temperature peaks in the TPO profiles can be

Figure 5. SEM micrographs of carbon deposits from JP-8 decomposition at 500 °C and 34 atm (500 psig) with 1 mL/min flow for 5 h on nickel surface at (a) top (2.5-5 cm) and (b) center (12.5-15 cm) segments.

ascribed to the subsequent formation of more-reactive deposits (with less structural order), presumably through thermal deposition processes on the incipient deposits. After 5 h of reaction, the presence of different carbon phases can be attributed to sequential processes of deposition as a function of time as reported by Albright and Marek.17 These authors described carbon deposition on a stainless steel surface as a function of time in the following sequence: (1) formation of filamentous carbon on the clean surface, (2) thickening of filaments and collection of tar droplets, (3) creation of nonporous deposits away from the metal surface, and (4) creation of additional thickness of nonporous carbon by adsorption and reaction of microspecies during the thermal stressing of hydrocarbons. Figure 5 shows the SEM images of the deposits on the nickel surface. The deposits consist of crystallites with sharp edges, indicating the strong catalytic activity of the nickel surface. Most likely, these crystallites contain nickel carbide species. The sizes of the crystallites are between 5 and 10 µm at the center and 1 and 2 µm at the top (2.5-5 cm) and the bottom (17.5-20 cm) of the tube. The larger size of the crystallites at the center (12.5-15 cm) can be attributed to accumulation of the more deposits on the faceted crystallites. The large low-temperature peak for the center segments in the TPO profiles (Figure 4) also suggests the relatively high contents of the thermal deposits at the center. A similar morphology was reported by Graff and Albright23 on alonized Incoloy 800 from benzene thermal stressing. Transition metals such as iron, nickel, and

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Figure 6. TPO profiles of the deposits collected on SS 316 tube segments from thermal stressing of JP-8 at 500 °C and 34 atm (500 psig) with 1 mL/min flow for 5 h.

cobalt are known to catalyze the transformation of hydrocarbons into carbonaceous material at temperatures on the order of 500 °C. This catalytic property also constitutes the basis of the steam reforming of hydrocarbons on Ni catalyst surface.24 Stainless Steel 316. In contrast to the profiles of the deposited Ni tubes, the TPO profiles of the deposit on SS 316 (Figure 6) showed three major peak groups. The fourth phase that evolved at around 700 °C on the deposited Ni tube was absent in the SS 316 profile. The first and second phases evolve at around 150 and 400 °C, respectively, but they represent relatively small quantities of the deposit. The intensities of these lowtemperature peaks are much lower than those in the profiles of the deposited Ni. The largest contribution to the deposit formation comes from the third phase, which evolved at temperatures between 500 and 575 °C on the surface of each tube segment. This phase is believed to result from deposition catalyzed by iron and nickel. As for the morphology of the deposit on Ni tube, faceted crystallites were observed on the SS 316 surfaces, as shown by the SEM images (Figure 7). The crystallites are, however, much smaller than those on the Ni surface, with an average size of 0.2 µm. Filamentous and amorphous carbon structures were also observed on the center segment where heavy deposition took place (Figure 7). The presence of smaller crystallites, compared to those seen on the Ni tube, suggests a lower catalytic activity of the SS 316 surface compared to that of the Ni surface, as confirmed by the lower amount of deposition on the SS 316 surface detected by the TPO analysis (Figure 2). Stainless Steel 304. The TPO profiles in Figure 8 show that the carbon deposit phases on SS 304 are similar to those observed on SS 316, although the amount of deposition is lower than that on SS 316. There are three major phases that gave peaks at 150, 400, and 500600 °C burnoff temperatures, respectively. Low-intensity peaks that can be attributed to the fourth phase identified in the Ni tube profiles (evolving at 700 °C) are also seen in the profiles of the top and bottom segments of the reactor. As can be seen in Figures 6 and 8, there are significant shifts in the TPO peaks toward higher temperature with increasing reactor length, probably because of the increase in the temper-

Figure 7. SEM micrographs of carbon deposits from JP-8 decomposition at 500 °C and 34 atm (500 psig) with 1 mL/min flow for 5 h on SS 316 surface at at (a) top (2.5-5 cm) and (b) center (12.5-15 cm) segments.

Figure 8. TPO profiles of the deposits collected on SS 304 tube segments from thermal stressing of JP-8 at 500 °C and 34 atm (500 psig) with 1 mL/min flow for 5 h.

ature of the fuel flowing through the reactor. The TPO profiles in Figures 6 and 8 show that, as the temperature of the fuel increases along the reactor length, the deposit formation rate increases, whereas the reactivity

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Figure 10. TPO profiles of the deposits collected on Silcosteel tube segments from thermal stressing of JP-8 at 500 °C and 34 atm (500 psig) with 1 mL/min flow for 5 h.

Figure 9. SEM micrographs of carbon deposits from JP-8 decomposition at 500 °C and 34 atm (500 psig) with 1 mL/min flow for 5 h on SS 304 surface at at (a) top (2.5-5 cm) and (b) center (12.5-15 cm) segments.

of the deposit decreases (i.e., shifts to higher oxidation temperatures in TPO profiles). The dominant carbon deposit phase appears between 500 and 600 °C in every segment of the tube. Both the SS 316 and SS 304 tube surfaces appear to catalyze carbon deposition from the stressed jet fuel. The SEM micrographs in Figure 9 show the formation of faceted structures with different geometries containing needlelike and hexagonal structures. Highermagnification images show the agglomeration of crystallites to form larger particles of around 0.5 µm in size. Edwards and Atria20 observed filamentous carbon formation on the SS 304 surface during Decalin stressing at 550 °C and 48 atm (700 psi). Silcosteel. Under the conditions of thermal stressing used in this study, stainless steel tube walls showed catalytic activity, accelerating carbon deposition from the jet fuel. Inert coatings on metal surfaces can inhibit the catalysis of carbon deposition from jet fuel.7 Coated metal surfaces are effectively used for industrial applications to prevent carbon deposition.8 The TPO profiles in Figure 10 show that the Silcosteel coating inhibits the formation of deposits with a high degree of structural order, or low reactivity, in contrast to the results found for the SS 316 and SS 304 surfaces. The Silcosteel tube contains a silica-treated inner surface covered with a monolayer of a specific siloxane polymer to reduce surface activity. Each segment of the Silcosteel tube showed relatively low carbon deposition (Figures 3 and 10). The TPO profiles show mostly low-temperature peaks that evolve below 400 °C, indicating the

Figure 11. (a) SEM micrographs of carbon deposits on Silcosteel center segment (10-12.5 cm) and (b) magnified spherical particles from the same piece.

presence of relatively reactive, amorphous deposits. The last two segments of the tube section show peaks that evolve at 550 °C. These high-temperature peaks, which coincide with the third phase observed on the deposited SS 304 (Figure 8), result from the oxidation of a lessreactive deposit. Most likely, these deposits were formed on exposed metal surfaces because of the cracks on Silcosteel coating. The TPO profiles and SEM images (Figure 11) showing spherical deposit particles formed on the coated surface indicate that Silcosteel coating does not catalyze deposit formation. The diameter of the

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served to decrease in the order nickel > stainless 316 > and stainless 304. The formation of catalytic deposits on metal surfaces promotes secondary deposition, which proceeds via reactions of reactive species on incipient deposits. An inert coating such as Silcosteel or glass on metal surface effectively inhibits catalytic deposition and, thus, minimizes overall solid deposition from the thermal decomposition of jet fuel. Acknowledgment This work was funded by the Air Force Research Laboratory/Aero Propulsion and Power Directorate, Wright Patterson AFB. We thank Prof. H. H. Schobert of PSU for his support and Dr. T. Edwards of AFWL/ APPD for helpful comments and discussions. Literature Cited

Figure 12. TPO profiles of the deposits collected on glass-lined stainless steel tube segments from thermal stressing of JP-8 at 500 °C and 34 atm (500 psig) with 1 mL/min flow for 5 h.

spherical particles is around 1 µm. The cracks seen on the Silcosteel surface in the micrographs resulted from the cutting and bending of the tube for SEM analysis. Spherical deposit particles grow in size after the precursor species adsorbs on the surface and reacts with free radicals produced by thermal cracking of the fuel. A similar deposition sequence was also reported by Albright and Marek17 for deposit formation on a quartz coupon from toluene at 950 °C. Glass-Lined Stainless Steel. Quartz and Pyrex are generally considered to be noncatalytic surfaces with respect to carbon deposit formation during high- (>500 °C) and medium-temperature reactions of hydrocarbons. Surfaces that are silica or manganese-rich and coated with titanium, molybdenum, tantalum, and aluminum inhibit carbon formation.25,26 The TPO profiles of the stressed glass-lined tube in Figure 12 show no hightemperature peaks. This observation further indicates that high-temperature peaks observed on stressed metal surfaces result from the catalysis of carbon deposition by metal surfaces. Trace amounts of carbon phases on the glass-lined surface evolved at low temperatures of 150 and 275 °C. The TPO results for the original glasslined tube are also shown in Figure 12 as a blank experiment. The original glass-lined tube gave a higher carbon count than the other tests. It appears that the glass coating contains residual carbon, which is removed during preheating under a nitrogen atmosphere for 2 h at 500 °C before the thermal stressing experiments with the jet fuel 4. Conclusions The combined use of temperature-programmed oxidation in a multiphase carbon determinator and SEM examination of stressed tube surfaces provides useful information for the characterization of solid deposits in the elucidation of solid deposition mechanisms during thermal decomposition of jet fuel. The experimental results suggest that solid deposition is usually initiated by catalytic reactions on active metal surfaces and that the activity of a metal surface depends on its composition. The catalytic activity of metal surfaces was ob-

(1) Edwards, T.; Zabarnick, S. Supercritical Fuel Deposition Mechanisms. Ind. Eng. Chem. Res. 1993, 32, 3117. (2) Heneghan, S. P.; Martel, C. R.; Williams, T. F.; Ballal, D. R. Studies of Jet Thermal Stability in a Flowing System. Trans. ASME J. Eng. Gas Turb. Power. 1993, 115, 480. (3) Hazlett, R. N. Physicochemical Aspects of Aviation Fuel Thermal Stability. In Aviation Fuel: Thermal Stability Requirements; Kirklin, P. W., David, P., Eds.; ASTM STP 1138; ASTM: Philadelphia, PA, 1992. (4) Heneghan, S. P.; Zabarnick, S.; Ballal, D. R.; Harrison III, W. E. JP-8+100: The Development of High Thermal Stability Jet Fuel. Trans. ASME J. Eng. Gas Turbines Power. 1996, 118, 170. (5) Jones, E. G.; Balster, W. J. Surface Fouling in Aviation Fuel: Short- vs Long-Term Isothermal Tests. Energy Fuels. 1995, 9, 610. (6) 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. (7) Atria, J. V.; Cermignani, W.; Schobert, H. H. Nature of High-Temperature Deposits from n-Alkanes in Flow Reactor Tubes. ACS Pet. Chem. Div. Preprints 1996, 41 (2), p 493. (8) Szechy, G.; Luan, T.-C.; Albright, L. F. In Novel Production Methods for Ethylene, Light Hydrocarbons, and Aromatics; Albright, L. F., Crynes, B. L., Nowak, S., Eds.; Marcel Dekker Inc.: New York, 1992. (9) 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. (10) Larsson, M.; Hulten, M.; Blekkan, E. A.; Andersson, B. The Effect of Reaction Conditions and Time on Stream on the Coke Formed During Propane Dehydrogenation. J. Catal. 1996, 164, 44. (11) Lin, L.; Zhang, T.; Zang, J.; Xu, Z. Dynamic Process of Carbon Deposition on Pt and Pt-Sn Catalysts for Alkane Dehydrogenation. Appl. Catal. 1990, 67, 11. (12) Bacaud, R.; Charcosset, H.; Guenin, M.; Torrellas-Hidalgo, R.; Tournayan, L. Study of the Formation and Removal of Carbonaceous Deposits on Platinum-Ruthenium-Alumina Supported Bimetallic Catalysts. Appl. Catal. 1981, 1, 81. (13) McCarty, J. G.; Hou, P, Y.; Sheridan, D.; Wise, H. Reactivity of Surface carbon on Nickel Catalysts: Temperature Programmed Surface Reaction with Hydrogen and Water. In Coke Formation on Metal Surfaces; Albright, L. F., Baker, R. T. K., Eds.; ACS Symposium Series 202; American Chemical Society: Washington, D.C., 1982; Chapter 13, p 253. (14) Li, J. Metal Surface Effects on Deposit Formation from Thermally Stressed Jet Fuels and Model Compounds. Ph.D. Thesis, Pennsylvania State University, State College, PA, 1998. (15) Baker, R. T. K.; Harris, P. S. The Formation of Filamentous Carbon. In The Chemistry and Physics of Carbon; Walker, P. L., Jr., Thrower, P. A., Eds.; Marcel Dekker Inc.: New York, 1978. (16) Marteney, P. J.; Spadaccini, L. J. Thermal Decomposition of Aircraft Fuel. Trans. ASME J. Eng. Gas Turbines Power 1986, 108, 648.

Ind. Eng. Chem. Res., Vol. 40, No. 2, 2001 603 (17) Albright, L. F.; Marek, J. C. Mechanistic Model for Formation of Coke in Pyrolysis Units Producing Ethylene. Ind. Eng. Chem. Res. 1988, 27, 755. (18) Trimm, D. L. Fundamantal Aspects of the Formation and Gasification of Coke. In PyrolysissTheory and Industrial Practice; Albright, L. F., Crynes, B. L., Corcoran, W. H., Eds.; Academic Press: New York, 1983. (19) Baker, R. T. K.; Yates, D. J. C.; Dumenic, J. A. Filamentous Carbon Formation Over Iron Surfaces. In Coke Formation on Metal Surfaces; Albright, L. F., Baker, R. T. K., Eds.; ACS Symposium Series 202; American Chemical Society: Washington, D.C., 1982; Chapter 1, pp 1-22. (20) Edwards, T.; Atria, J. V. Thermal Stability of HighTemperature Fuels; ASME Report 97-GT-143; ASME: New York, 1997. (21) Querini, C. A.; Fung, S. C. Temperature-Programmed Oxidation Technique: Kinetics of Coke-O2 Reaction on Supported Metal Catalysts. Appl. Catal. A: Gen. 1994, 117, 53. (22) McCarty, J. G.; Wise, H. J. Hydrogenation of Surface Carbon on Alumina-Supported Nickel. J. Catal. 1979, 57, 406.

(23) Graff, J. M.; Albright, I. L. Coke Deposition from Acetylene, Butadiene, and Benzene Decompositions at 500-900 °C on Solid Surfaces. Carbon 1982, 20 (4), 319. (24) Froment, G. F. Coke Formation in the Thermal Cracking of Hydrocarbons. Rev. Chem. Eng. 1990, 16 (4), 293. (25) Baker, R. T. K.; Chludzinski, J. J. Jr. Filamentous Carbon Growth on Nickel-Iron Surfaces: The Effect of Various Oxide Additives. J. Catal. 1980, 64, 464. (26) Brown, D. E.; Clark, J. T. K.; Faster, A. l.; McCorroll, J. J.; Sims, M. L. Inhibition of Coke Formation in Ethylene Steam Cracking. In Coke Formation on Metal Surfaces; Albright, L. F., Baker, R. T. K., Eds.; ACS Symposium Series 202; American Chemical Society: Washington, D.C., 1982; Chapter 2, pp 23-44.

Received for review April 28, 2000 Revised manuscript received October 25, 2000 Accepted November 1, 2000 IE0004491