Solid Deposits from Thermal Stressing of n-Dodecane and Chinese

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Energy & Fuels 2009, 23, 356–365

Solid Deposits from Thermal Stressing of n-Dodecane and Chinese RP-3 Jet Fuel in the Presence of Several Initiators Guozhu Liu, Yongjin Han, Li Wang, Xiangwen Zhang,* and Zhentao Mi Key Laboratory of Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin UniVersity, Tianjin 300072, People’s Republic of China ReceiVed August 13, 2008. ReVised Manuscript ReceiVed October 19, 2008

Solid depositions from thermal stressing of a jet fuel model compound, n-dodecane (Dod), is studied in the presence of three initiator additives, 1-nitropropane (NP), triethylamine (TEA), and 3,6,9-triethyl-3,6,9-trimethyl1,4,7-triperoxonane (TEMPO), to show the role of initiators in the carbon deposition because of thermal cracking of jet fuels. It is found that the thermal cracking rate of Dod is enhanced by the initiators in the following order: TEMPO > NP > TEA and that TEMPO and TEA remarkably inhibit the formation of pyrolytic deposit by 30-50% at a similar conversion level. Temperature-programmed oxidation (TPO) of deposits indicates that reactive deposition is improved slightly by TEMPO and TEA but the less reactive deposits are reduced because of possible radical scavenging or the hydrogen donation effect, resulting from the decomposition of TEMPO and TEA. Scanning electron microscopy (SEM) shows that TEA and NP also have a significant effect on the deposit morphologies. Those results are also observed in the thermal stressing of Chinese RP-3 jet fuel with initiators.

Introduction As modern aircraft flight speed increases up to higher Mach regimes, aerodynamic heating becomes increasingly severe, which makes thermal management of the vehicles and engines become one of the crucial issues in the development of hypersonic aircrafts. In this case, jet fuel is usually used not only for its conventional role as a energy source of heat through combustion but also as a “heat sink” (or a regenerative coolant) to remove waste heat from various aircraft subsystem and components, such as engine lubricating oil, environmental control system, electrical system, air frame, etc.1-6 The required cooling capacity (also heat sink) of liquid hydrocarbon fuels is partially due to their significant sensible heat sink as well as endothermic reactions, resulting from chemical decompositions at that high temperature.7,8 Considering that the sensible heat sink capacities change slightly for different hydrocarbon fuels, a practical way to obtain enough cooling capability for >5 Mach flight is to acquire more heat sink from * To whom correspondence should be addressed. Telephone/Fax: 8622-2789-2340. E-mail: [email protected]. (1) Edwards, T.; Maurice, L. Q. Surrogate mixtures to represent complex aviation and rocket fuels. J. Propul. Power 2001, 17 (2), 461–466. (2) Edwards, T. Liquid fuels and propellants for aerospace propulsion: 1903-2003. J. Propul. Power 2003, 19 (6), 1089–1107. (3) Edwards, T. Cracking and deposition behavior of supercritical hydrocarbon aviation fuels. Combust. Sci. Technol. 2006, 178 (1-3), 28. (4) Edwards, T. Advancements in gas turbine fuels from 1943 to 2005. J. Eng. Gas Turbines Power 2007, 129 (1), 8. (5) Huang, H.; Spadaccini, L. J.; Sobel, D. R. Fuel-cooled thermal management for advanced aeroengines. J. Eng. Gas Turbines Power 2004, 126 (2), 284–293. (6) Maurice, L. Q.; Lander, H.; Edwards, T.; Harrison, W. E. Advanced aviation fuels: A look ahead via a historical perspective. Fuel 2001, 80 (5), 747–756. (7) Andersen, P. C.; Bruno, T. J. Thermal decomposition kinetics of RP-1 rocket propellant. Ind. Eng. Chem. Res. 2005, 44 (6), 1670–1676. (8) Widegren, J. A.; Bruno, T. J. Thermal decomposition kinetics of the aviation turbine fuel Jet A. Ind. Eng. Chem. Res. 2008, 47 (13), 4342– 4348.

the endothermic reactions of fuels, such as the cracking of chain hydrocarbons. Furthermore, the cracking of hydrocarbons also produces a mixture of hydrogen and smaller hydrocarbons, such as ethylene and propylene, in a heat exchanger/reactor prior to combustion, which provides another extra advantage for shortening the ignition delay time of cracked jet fuel in the hypersonic combustion.1,3,5,9 The formation of carbonaceous deposits from decomposition of jet fuel is another crucial concern in the development of advanced aircraft when the jet fuel is used as a heat sink.3,10-20 Because of high thermal loads in future advanced aircraft, jet (9) Puri, P.; Ma, F.; Choi, J. Y.; Yang, V. Ignition characteristics of cracked JP-7 fuel. Combust. Flame 2005, 142 (4), 454–457. (10) Altin, O.; Eser, S. Characterization of carbon deposits from jet fuel on Inconel 600 and Inconel X surfaces. Ind. Eng. Chem. Res. 2000, 39 (3), 642–645. (11) 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 (2), 589–595. (12) 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 (2), 596–603. (13) Altin, O.; Eser, S. Pre-oxidation of Inconel alloys for inhibition of carbon deposition from heated jet fuel. Oxid. Met. 2006, 65 (1-2), 75–99. (14) Altin, O.; Rudnick, L. R. Deposit formation caused by thermal stressing of petroleum and coal-derived jet fuels on Inconel 718 and fuel pre-treatment effect. Abstr. Pap. Am. Chem. Soc. 2004, 227, U1071–U1071. (15) Andresen, J. M.; Strohm, J. J.; Mathews, J. P.; Song, C. S. Modeling of coke formation from napthenic jet fuel in the pyrolytic regime. Abstr. Pap. Am. Chem. Soc. 2001, 222, U460–U460. (16) Balster, L. M.; Corporan, E.; DeWitt, M. J.; Edwards, J. T.; Ervin, J. S.; Graham, J. L.; Lee, S. Y.; Pal, S.; Phelps, D. K.; Rudnick, L. R.; Santoro, R. J.; Schobert, H. H.; Shafer, L. M.; Striebich, R. C.; West, Z. J.; Wilson, G. R.; Woodward, R.; Zabarnick, S. Development of an advanced, thermally stable, coal-based jet fuel. Fuel Process. Technol. 2008, 89 (4), 364–378. (17) Ervin, J. S.; Ward, T. A.; Williams, T. F.; Bento, J. Surface deposition within treated and untreated stainless steel tubes resulting from thermal-oxidative and pyrolytic degradation of jet fuel. Energy Fuels 2003, 17 (3), 577–586. (18) 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.

10.1021/ef800657z CCC: $40.75  2009 American Chemical Society Published on Web 12/08/2008

Deposits from Thermal Cracking of Jet Fuels

fuel would be exposed to such high temperatures (>650 °C) that both homo- and heterogeneous reactions of jet fuel can lead to solid deposition on metal surfaces as fuel flows through the fuel system. First, trace species within the heated fuel react with dissolved O2 to form autoxidation deposits. At relatively higher fuel temperatures, the dissolved O2 is depleted and the pyrolytic reaction becomes dominant (at temperatures >450 °C). Pyrolytic reactions change the composition of the major fuel species and produce surface deposits and other undesirable products. Rapid accumulation of solid deposits on various components of the fuel system, including valves, flow tubes, and nozzles, could cause catastrophic failure of the aircraft engine, resulting from filter plugging, fouling of various valves, and other problems.3 Consequently, it is very important to study the influence of the initiators on deposition behaviors occurring under both thermal oxidative and pyrolytic fuel degradation. In a practical point of view, it is almost impossible for the thermal cracking of jet fuel to provide the necessary endotherm at high Mach flight because thermal cracking only occurs at a useful rate at very high temperatures.21 Cracking reactions of hydrocarbons could also occur in the presence of a zeolite catalyst via the carbenium ion mechanism, which has a higher cracking activity at relative temperatures. However, the catalytic cracking has several fatal drawbacks: the zeolite catalyst exhibits high selectivity for coke and low selectivity for olefins compared to thermal cracking, thus deactivating rapidly by accumulation of coke, and the catalyst must be applied in very thin layers to reduce the impact of the catalyst in heat transfer and pressure drop within the tube.21 This makes the design and construction of the reactor very complex and brings many difficulties for regeneration of the catalyst.5,20 Some researchers noted that thermal cracking of a hydrocarbon molecule proceeds via the chain reaction of free radicals generated from C-C bond cleavage, in which free-radical generation is a typical ratelimited step of this process. Therefore, the addition of an initiator that rapidly produces a mount of free radicals would result in an increase in the overall thermal cracking rate, excluding the described disadvantages of thermal and catalytic cracking. Wickham et al.21 first investigated the promoting effect of an initiator in the thermal cracking of heptane under supercritical conditions and observed that only 2 wt % initiator accelerates the cracking rate of heptane up to 200% with the similar reaction mechanism and product distribution as the thermal cracking under supercritical conditions (773.15 K and 6.8 MPa). Wickham et al.22 further applied laboratory bench- and pilot-scale calorimetric tests to directly measure heat sink capacity of JP-7 and model compound n-decane with or without initiator and found that the initiator (without a given name or structure) increases the heat sink (from 25 to 550 °C, under 550 psi) from 766 to 849 Btu/lb for n-decane and from 732 to 791 Btu/lb for JP-7 fuel at liquid hourly space velocity (LHSV) of 1000 h-1. Since then, Wang et al.23-26 reported thermal cracking of heptane initiated by triethylamine (TEA) and tributlyamine (TBA) in a flowing reactor and indicated that both TEA and (19) Guel, O.; Rudnick, L. R.; Schobert, H. H. Effect of the reaction temperature and fuel treatment on the deposit formation of jet fuels. Energy Fuels 2008, 22 (1), 433–439. (20) Huang, H.; Spadaccini, L. J. Coke removal in fuel-cooled thermal management systems. Ind. Eng. Chem. Res. 2005, 44 (2), 267–278. (21) Wickham, D. T.; Engel, J. R.; Hitch, B. D.; Karpuk, M. E. Initiators for endothermic fuels. J. Propul. Power 2001, 17 (6), 1253–1257. (22) Wickham, D. T.; Engel, J. R.; Rooney, S.; Hitch, B. D. Additives to improve fuel heat sink capacity in air/fuel heat exchangers. J. Propul. Power 2008, 24 (1), 55–63. (23) Wang, Z.; Guo, Y.; Lin, R. Effect of triethylamine on the cracking of heptane under a supercritical condition and the kinetic study on the cracking of heptane. Energy ConVers. Manage. 2008, 49 (8), 2095–2099.

Energy & Fuels, Vol. 23, 2009 357 Scheme 1. Structure Formulations of Initiators

TBA are effective in promoting the cracking rate of heptane and improving the heat sink of hydrocarbons. Those works confirmed that initiative thermal cracking (novel item defining the thermal cracking of hydrocarbons in the presence of initiators) is a promising and feasible technology to improve the heat sink of jet fuel under supercritical conditions. However, to our best knowledge, all of the studies do not address the role of initiators in the formation of both autoxidative and pyrolytic deposits during thermal stressing under supercritical conditions, regardless of its crucial importance in the development of the active cooling system and thermal management system. Jet fuels include hundreds of aliphatic and aromatic hydrocarbons, among which major components are normal alkanes, branched alkanes, cycloalkanes, and aromatics. n-Dodecane (Dod), a major component of some petroleum-derived jet fuels (JP-7, JP-8, and RP-3), is one of the most important surrogates usually chosen for normal alkanes1,27-30 and selected as a jet fuel model compound in this work. In the literature, nitromethane, ditertbutyl peroxide (DTBP), and TEA were used in the thermal cracking of hydrocarbons as the initiators because of lower bond dissociation energy (BDE) of C-N and O-O bonds. It was a pity that the DTBP initiator was not suitable for hydrocarbon fuels because of little solubility and nonagitation process. Likewise, it was also impossible to use nitromethane because of its lower flash point and thermal instability. A cyclic triperoxy initiator 3,6,9-triethyl-3,6,9trimethyl-1,4,7-triperoxonane (TEMPO, see Scheme 1), compared to DTBP, has better thermal stability and solubility in the hydrocarbons up to 40 wt %. 1-Nitropropane (NP), with a higher flash point and stability than nitromethane, was also selected as an initiator. Therefore, in this work, we choose TEA, NP, and TEMPO as the initiative additives for the supercritical thermal cracking of Dod. The objective of this paper is to study the effect of initiators on the solid deposit behavior from the thermal stressing of a jet fuel model compound, Dod, in the presence of three initiators, (24) Wang, Z.; Lin, R. Theoretical study on the reaction route for the major liquid product from pyrolysis of triethylamine. J. Anal. Appl. Pyrolysis 2008, 81 (2), 205–210. (25) Wang, Z.; Lin, R.; Fang, W.; Li, G.; Guo, Y.; Qin, Z. Triethylamine as an initiator for cracking of heptane. Energy 2006, 31 (14), 2437–2454. (26) Wang, Z.; Lin, R.; Guo, Y.; Li, G.; Fang, W.; Qin, Z. Tributylamine as an initiator for cracking of heptane. Energy ConVers. Manage. 2008, 49 (6), 1584–1594. (27) Huber, M. L.; Laesecke, A.; Perkins, R. Transport properties of n-dodecane. Energy Fuels 2004, 18 (4), 968–975. (28) Huber, M. L.; Smith, B. L.; Ott, L. S.; Bruno, T. J. Surrogate mixture model for the thermophysical properties of synthetic aviation fuel S-8: Explicit application of the advanced distillation curve. Energy Fuels 2008, 22 (2), 1104–1114. (29) Lemmon, E. W.; Huber, M. L. Thermodynamic properties of n-dodecane. Energy Fuels 2004, 18 (4), 960–967. (30) Lenhert, D. B.; Miller, D. L.; Cernansky, N. P. The oxidation of JP-8, Jet-A, and their surrogates in the low and intermediate temperature regime at elevated pressures. Combust. Sci. Technol. 2007, 179 (5), 845– 861.

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Table 1. Specification Properties of Chinese RP-3 Jet Fuel Used in This Work

Table 2. Conditions for Thermal Cracking of Dod with and without Initiators

properties

values

parameters

values

density (kg/m3) critical pressure (MPa) critical temperature (°C) boiling range, initial point (°C) 10% (°C) 50% (°C) 90% (°C) dry point (°C) n-paraffins (%) iso-paraffins (%) clyl-paraffins (%) aromatics (%) sulfur content (ppm)

0.7958 2.390a 372.35a 135 162.5 182.7 213.3 230 57.72 18.56 15.2 8.52 65

temperature (°C) pressure (MPa) reaction time (min) reactor length × o.d. × i.d. (mm) additive concentration (wt %) superficial liquid hourly space velocity (h-1)

620-670 4.0 180 900 × 3 × 2 2.0 42.5

a Experimental measurement by Sun et al. (Sun, Q. M.; Mi, Z. T.; Zhang, X. W. Determination of critical properties (tc, Pc) of endothermic hydrocarbon fuels RP-3 and simulated JP-7. J. Fuel Chem. Technol. 2006, 34 (4), 466-470. )

Figure 1. Schematic diagram of the experimental apparatus for supercritical cracking of jet fuels.

NP, TEA, and TEMPO. The solid deposit profiles for Dod with or without initiators are compared as well as the wall temperature profiles and conversions. The natures and morphologies of solid deposits are further characterized by temperatureprogrammed oxidation (TPO) and scanning electron microscopy (SEM) to show the role of the initiator in the formation of the pyrolytic deposit, respectively. Thermal stressing of Chinese RP-3 jet fuel under supercritical conditions is also performed to provide some useful information on the influences of the initiator on the real jet fuels. Experimental Section Materials. Dod, with 99.5% purity, is obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). TEA, with 99.0% purity, is purchased from Tianjin Chemical Reagent Company. NP, with 98.0% purity, is provided by Alfa Aesar. A cyclic trifunctional peroxide initiator, TEMPO (41% solution in aromatics free mineral spirit), is purchased from Aacros Organics. All of the chemicals were used as received. The jet fuels used in civil aviation nowadays in China is RP-3 (renamed Chinese number 3 Jet Fuel). RP-3 jet fuel used in this work was refined by the Daqing Refining Factory of SINOPEC Co. Table 1 describes the typical properties of jet fuel used in this work. Thermal Stressing of Fuels in the Flowing Reactor. Figure 1 presents the schematic diagram of a flowing reactor used in this work. The flow reactor apparatus consists of a SSI IV high-pressure liquid chromatography pump, a preheater, a three-zone furnace with

a heated zone of 90 cm in length, and a 304 stainless-steel tube (3 mm o.d., 2 mm i.d.). Before each experiment, nitrogen sparging is performed for 2 h to remove the dissolved oxygen in the fuel. The fuel blended with the initiator is then pumped into the reactor at a rate of 3 mL/min through a filter to remove any sediment. The fuel is heated to 250 °C in the preheater with a superficial resistance time of ca. 20 s. The system is maintained at 4 MPa to guarantee the fuel in a supercritical phase. After the thermal stressing, the fuel is quenched by a counterflow heat exchanger and then passed through a filter to remove any solids formed in the fuel. Each thermal stressing experiment is performed for 3 h to obtain sufficient deposit buildup for analysis. Table 2 summarizes experimental conditions for thermal cracking of Dod with and without initiators. It should be noted that the temperature of the three-zone furnace is controlled by the following methodology: the heating powers of three heating zones are input by the same methodology, and the thermal couple for the temperature controlling point is put at the middle heating zone, which leads to the wall temperature decreasing at x > 45 cm because of the significant heat loss in the third heating zone. All of the deposition data (scaled by the tube surface area) is plotted as a function of the wall temperature, excluding points of decreasing wall temperature before the outlet of the furnace. The wall temperature of the reaction tube is measured using type-K thermocouples bound at 5 cm intervals along the tube. Because the outer diameter of 304 SS tube used is only 3 mm, it is almost impossible to directly measure the bulk temperature of fuel. Therefore, we also provided the wall temperature profile for each experimental run and referred the reaction temperature corresponding to the maximum temperature in the wall temperature profile of the tube for the sake of convenience in the following discussions. In addition, after the removal of dissolved oxygen with nitrogen sparging, there is still 4.5 ppm oxygen (determined by HIOXY-R oxygen sensor, Ocean Optics, Inc.) in the fuels, which leads to the minimal thermal oxidation deposits collected in the 5-10 cm segment of reaction tubes. Characterization of Carbon Deposition. To determine carbon deposition profiles, the reactor tubing is washed 3 times with pentane to remove any residual fuel after the completion of the thermal stressing. Afterward, it is cut into 2 cm pieces, washed again with pentane, and dried in a vacuum oven at 140 °C overnight to ensure the complete removal of the residual pentane. The samples are then analyzed using a GXH-1050 carbon analyzer (Beijing Junfang Institute of Physical and Chemical Technology, China), which quantifies carbon deposition (precision of 1 ppm) by measuring carbon dioxide. In the carbon analyzer, the carbon deposit is oxidized to carbon dioxide with flowing UHP O2 (750 mL/min) over a CuO catalyst bed in a furnace kept at 750 °C. The product CO2 is quantitatively measured by a calibrated IR detector and used to calculate the amount of deposit presented in units of µg/cm2 (milligrams of carbon per square centimeter). The total amount of deposition measured on different tube surfaces is reproducible within 5 wt % deviation of the deposit mass. According to Eser et al.,31 TPO is also used as an important technique to characterize the nature and amount of the carbonaceous deposits from their reactivity during oxidation under selected heating conditions. The deposited tube segments are heated in the furnace at a rate of 30 °C/min from a starting temperature of 100 °C to a (31) Eser, S.; Venkataraman, R.; Altin, O. Utility of temperatureprogrammed oxidation for characterization of carbonaceous deposits from heated jet fuel. Ind. Eng. Chem. Res. 2006, 45 (26), 8956–8962.


Energy & Fuels, Vol. 23, 2009 359 Table 3. Comparison of Initiator Performance

Figure 2. Deposits from thermal stressing of pure Dod under 4.0 MPa for 3 h (9, Xd ) 34.97%, Tp ) 638 °C, Wd ) 38.59 mg; b, Xd ) 39.65%, Tp ) 648 °C, Wd ) 60.96 mg; 2, Xd ) 47.25%, Tp ) 670 °C, Wd ) 89.29 mg).

maximum temperature of 750 °C, with a holding period of 6 min at the final temperature. The GXH-1050 analyzer signal is recorded and plotted as a function of the furnace temperature to give a TPO profile. The TPO analysis of carbon deposits usually gives multiple CO2 peaks that can evolve over a temperature range of 200-700 °C. The different peaks observed in these plots can be attributed to differences in the nature of the carbon deposits.31 Repeated experiments showed that the TPO profiles are reproducible with respect to individual peak positions and relative peak intensities. To examine the morphology of the carbon deposits on the inner surface of the tubes, the tubes are cut longitudinally from the center and then examined with an XL30E scanning electron microscope (SEM, Philips, Inc.). Gas and Liquid Product Analyses. The cracking products are quenched in a water-cooled heat exchanger, and the liquid and gas products are collected for analysis. The liquid products are analyzed quantitatively by Agilent 6890N/5975 inert gas chromatography (GC)-mass selective detector (MSD) equipped with a HP-5 MS column (30 m × 0.25 mm × 0.25 µm). High-purity helium (99.999%) is used as a carrier gas, with a flow rate of 1 mL/min. The column temperature is first kept at 60 °C for 3 min and then programmed to 200 °C at 5 °C/min and to 280 °C at 10 °C/min, with a final isothermal period of 10 min at 280 °C. The transfer line temperature is 280 °C, and quadrupole MSD temperature is kept at 150 °C. The mass range and the scan rate are 50-400 amu and 5.12 scans/s. The gaseous products are collected by the water displacement method and analyzed quantitatively using a SP-3420 GC equipped with a flame ionization detector (FID) and an Al2O3/S capillary column (50 m × 0.53 mm × 0.50 µm) from Zhongke Kaidi Chemical Technolgy Co., Ltd. (Lanzhou, China). The flame ionization detector (FID) and oven temperature are kept at 200 and 80 °C, respectively. The gaseous products are identified and quantified using standard gas mixtures. Uncertainties associated with the measurement of gas composition were less than 1.0%. The gas mass could be calculated from average molecular weight. Errors in the mass balance of the reactant and products before and after the reaction were less than 1.5%. The Dod conversion (Xd) is defined as the mole fraction of the reactant converted. Selectivity of liquid-phase reaction products is expressed as the number of moles obtained per 100 mol of the reactant converted. Selectivity of gaseous products is expressed as the mole number of a certain compound per 100 mol of gas products. Reproducibility tests show that the relative uncertainties for the yields of major products, except the C5 and C6 products, are lower than 1.5%.

Results and Discussion Solid Deposit Profiles for Thermal-Stressed Dod. Figure 2 plots wall temperature and corresponding carbon deposition profiles along the axial length of tubes after 3 h of thermal

compounds

T (°C)

XDa

WDb

MDc

pure Dod pure Dod pure Dod Dod + TEA Dod + TEA Dod + TEA Dod + TEMPO Dod + TEMPO Dod + TEMPO Dod + NP Dod + NP Dod + NP

638 648 670 628 656 669 636 646 653 623 648 663

34.97 39.65 47.25 33.53 45.51 53.78 42.69 48.59 55.88 35.13 45.79 54.53

38.59 60.96 89.29 28.23 59.58 84.05 28.78 43.08 55.88 97.61 108.27 141.59

1.99 3.93 5.68 1.77 3.98 6.18 1.84 2.61 4.49 7.10 7.11 10.49

a Conversion of Dod (%). b Overall amount of deposits (mg). c Peak value of solid deposit profile curves (mg/cm2).

stressing of Dod under 4.0 MPa pressure at different wall temperature profiles. Table 3 presents Dod conversion and the overall deposit amount under the described conditions. At the peak temperature of 638 °C, the conversion of Dod is 34.97%, the carbon deposition increases along the length of the tube until reaches a maximum of 1.98 mg/cm2 at 45 cm from the inlet and then decreases gradually, and the total amount of carbon deposition is 38.59 mg. At 648 °C, the Dod conversion increases up to 39.65% and produces a total deposit amount of 60.96 mg with a similar profile curve but a maximum of 3.93 mg/cm2 as a result of the temperature rise. When the temperature rises to 670 °C, the Dod conversion further reaches up to 47.25% and gives a total deposit amount of 89.29 mg and a similar profile curve with a maximum of 5.68 mg/cm2, shifting at 50 cm from the inlet of the deposit. Therefore, it is evident that both the total solid deposit amounts and their profiles are noticeably dependent upon reactor wall profiles. Edwards and Zabarnick32 observed two deposit peaks ascribed to thermal oxidative and pyrolytic deposits in the supercritical thermal cracking of JP-7 (100-650 °C) under 69 atm, wherein the pyrolytic deposit also coherently increased or decreased with the increasing or decreasing wall temperature. However, in this work, the dissolved oxygen in the fuel would be depleted in the preheater before the inlet of the reactor. Therefore, the maximum 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 because of the lower wall temperature. Ervin et al.17,33 observed similar results in the thermal stressing of Jet-A (wall temperature of 400-700 °C). Solid Deposit Profiles for Thermal-Stressed Dod with Initiators. Figure 3 presents the solid deposit profiles from 3 h of thermal stressing of Dod with 2 wt % TEA under 4 MPa, as well as the corresponding wall temperature profiles. Table 3 presents Dod conversion with different initiators and overall deposit amount under the corresponding conditions. In comparison to the pure Dod, it is easy to find that reaction temperatures are reduced because of the promoting effect of TEA. For example, the conversion of Dod in the presence of TEA reaches 33.53% at 628 °C, which is lower than the reaction temperature of 638 °C for pure Dod of about 10 °C. At 669 °C, the Dod conversion reaches up to 53.78%, which is obviously higher than 47.25% for pure Dod at 670 °C. Moreover, the solid deposit increases gradually to reach a maximum at 45-50 cm and then decreases as a result of (32) Edwards, T.; Zabarnick, S. Supercritical fuel deposition mechanisms. Ind. Eng. Chem. Res. 1993, 32 (12), 3117–3122. (33) Ervin, J. S.; Zabarnick, S.; Williams, T. F. One-dimensional simulations of jet fuel thermal-oxidative degradation and deposit formation within cylindrical passages. J. Energy Resour. Technol. 2000, 122 (4), 229– 238.


Figure 3. Deposits from thermal stressing of Dod with 2 wt % TEA for 3 h (2, Xd ) 33.53%, Tp ) 628 °C, Wd ) 28.23 mg; b, Xd ) 45.51%, Tp ) 656 °C, Wd ) 59.58 mg; 9, Xd ) 53.78%, Tp ) 669 °C, Wd ) 84.05 mg).

Figure 4. Deposits from thermal stressing of Dod with 2 wt % TEMPO for 3 h (2, Xd ) 42.69%, Tp ) 636 °C, Wd ) 28.78 mg; b, Xd ) 48.59%, Tp ) 646 °C, Wd ) 43.08 mg; 9, Xd ) 55.88%, Tp ) 653 °C, Wd ) 62.21 mg).

decreasing wall temperature, which is similar with the pure Dod. It is interesting that the positive effect of TEA in reducing the total amount of carbon deposit is also observed. For instance, at 669 °C, the total amount of carbon deposit is 84.05 mg, which is lower than that of 89.29 mg for pure Dod at 670 °C, regardless of it high conversion, as mentioned above. Further discussion on the initiator on the deposits will be given in the following sections. Figure 4 presents the solid deposit profiles with their corresponding wall temperature profiles for the thermal stressing of Dod with 2 wt % TEMPO under 4 MPa. Evidently, the promoting effect of TEMPO on the Dod cracking rates is greater than that of TEA, which is proven by the fact that the conversion of Dod with TEMPO is 48.59% at 646 °C, while with TEA, the Dod conversion only reaches 45.51% at 656 °C and, at 653 °C, the conversion of Dod with TEMPO reaches up to 55.88%, which is obviously higher than 47.25% for pure Dod at 670 °C, and 53.78% for Dod with 2 wt % TEA at 669 °C. Therefore, TEMPO is a better initiator than TEA in enhancing the thermal cracking rate of Dod. For each deposit profile, the solid deposit generally varies in the similar trends with the pure Dod and Dod with TEA, but both the maximum and the total amount of carbon deposit are significantly reduced compared to the pure Dod. For instance, at 648 °C, the deposit maximum and deposit total amount for the pure Dod are 3.9 mg/cm2 and 60.96 mg with conversion of 39.65%, but at 646 °C, the deposit maximum and deposit total amount for Dod with TEMPO are 2.3 mg/cm2 and 43.08 mg with Dod conversion of 48.59%. This convincingly shows that TEMPO has better performance than TEA in both promoting the Dod cracking rate and reducing the total amount of carbon deposit. Therefore, it is likely that the N atom

Liu et al.

Figure 5. Deposits from thermal stressing of Dod with 2 wt % NP for 3 h (b, Xd ) 35.13%, Tp ) 623 °C, Wd ) 97.61 mg; 2, Xd ) 45.79%, Tp ) 648 °C, Wd ) 108.27 mg; 9, Xd ) 54.53%, Tp ) 663 °C, Wd ) 141.59 mg).

in the TEA and NP has a special role in the formation of the deposits when the fuel has been deoxygenated. Figure 5 presents the solid deposit profiles from thermal stressing of Dod with 2 wt % NP as well as the corresponding wall temperature profiles under 4 MPa. At 648 °C, the Dod conversion reaches 45.79% because of the acceleration effect of NP. As compared to 48.59% at 646 °C with TEMPO, it is found that the promoting effect of NP is lower than TEMPO. At 663 °C, the conversion of Dod with NP reaches 54.53% compared to 53.78% at 669 °C with TEA, which indicates that the promoting effect of NP is greater than TEA. However, the solid deposits are also sharply improved by NP as a result of conversion enhancement. For instance, at 623 °C, the total deposit amount is 97.61 mg with the conversion of 35.13%, which approaches the deposit level for the pure Dod at 670 °C. In conclusion, the promoting effects of initiators on the thermal cracking rates of Dod vary with the initiators types and decrease in following the order: TEMPO > NP > TEA > pure Dod. However, it seems that the effect of the initiators on the carbon deposits changes greatly in the following order: NP > pure Dod >TEA > TEMPO. Carbon Deposit at a Similar Conversion Level. We also note that the conversion of Dod with 2 wt % NP (at 648 °C) is 45.79%, which is close to pure Dod conversion of 47.25% at 670 °C, 48.59% with 2 wt % TEMPO at 646 °C, and 45.51% with 2 wt % TEA at 656 °C, as shown in Table 3. In this respect, we further compare the wall temperature and deposit profiles to show the effect of additives on the carbon deposit behaviors, as shown in Figure 6. In general, the reaction temperatures were sharply reduced after 45 cm from the inlet, which shows that introduction of the initiator improves the thermal cracking rate and thus provides equivalent heat sink at relative reaction temperatures. The results in Figure 6 also indicate that, at similar conversion of Dod (45.51-48.59%), the carbon deposits decrease by the following order: Dod + 2 wt % NP > pure Dod > Dod + 2 wt % TEA > Dod + 2 wt % TEMPO, wherein the total deposit of Dod with TEMPO (43.08 mg) is only 48% of the pure Dod (89.29). Therefore, TEMPO is the best choice for the application as a coolant additive because of it remarkable enhancement on the thermal cracking rates, as well as its special role in inhibiting the deposit. In brief, the presence of TEMPO and TEA as initiators not only substantially improves the supercritical thermal cracking rates of Dod by their promoting effect but also effectively reduces the carbon deposit because of the lower cracking temperature to achieve the same conversion. This fact indicates that the initiative thermal cracking is a promising technology to provide more chemical heat sink attributed to the faster


Figure 6. Carbon deposit from thermal stressing of Dod with different initiators under 4 MPa for 3 h (9, pure Dod, Xd ) 47.25%, Tp ) 670 °C, Wd ) 89.29 mg; 2, Dod + TEMPO, Xd ) 48.59%, Tp ) 646 °C, Wd ) 43.08 mg; b, Dod + TEA, Xd ) 45.51%, Tp ) 656 °C, Wd ) 59.58 mg; 1, Dod + NP, Xd ) 45.79%, Tp ) 648 °C, Wd ) 108.27 mg).

cracking rate of hydrocarbons while inhibiting the formation of the carbon deposit. However, the question is how the initiators influence the deposit behavior. Edwards and Liberio34 briefly examined the effects of autoxidation on pyrolysis. They found that, for certain fuels, if dissolved oxygen was not removed from the fuel, the subsequent pyrolytic deposit was less than observed if the fuel was sparged with nitrogen. This phenomenon was observed for Exxsol D80 (a solvent consisting of n-alkanes), Jet A, and decalin. They suggested that the oxygenated compounds formed in the autoxidative phase of the reactions were able to act as radical scavengers or hydrogen donors, thus reducing pyrolytic deposit when dissolved oxygen was present in the initial fuel by reducing propagation rates. Roan and Boehman35 also observed that the pyrolytic deposit formed in a flow reactor for a saturated light cycle oil with dissolved oxygen removed via a nitrogen sparge was significantly higher than that with dissolved oxygen present. Therefore, initiators may have different influences on the thermal oxidative and pyrolytic deposit, i.e., improving the autoxidative deposit as a result of higher oxygenated compound concentrations because of initiator addition but possibly reducing pyrolytic deposit ascribed to its role as radical scavengers or hydrogen donors. In the following sections, TPO characterization of the deposit on different tube segments is carried out to further clarify the role of the initiator in the deposit formation. TPO Characterization of Carbon Deposits. As mentioned in the literature previously, several types of carbonaceous deposits were identified depending upon the TPO CO2 peak temperature. Obviously, for the 5-10 cm segment, the only observed peak in the TPO curves is mainly assigned to the thermal oxidative deposits because it is impossible for the fuels to crack at such low temperatures of ca. 250-350 °C. However, for the 20-25, 40-45, and 60-65 cm segments, the peaks observed for TPO curves should be mainly assigned to the pyrolytic deposition, which were classified into less-reactive deposits and reactive deposits. Less-reactive deposits at high temperatures (with lower H/C ratio), might be produced most likely by catalytic reactions on active metal surfaces. The peaks at low temperatures, on the other hand, could be assigned to more-reactive (or relatively hydrogen-rich and more amorphous) (34) Edwards, T.; Liberio, P. The relation between oxidation and pyrolysis in fuels heated to ∼500 °C (1100 °F). Prepr. Pap.-Am. Chem. Soc., DiV. Pet. Chem. 1998, 43 (3), 353–365. (35) Roan, M. A.; Boehman, A. L. The effect of fuel composition and dissolved oxygen on deposit formation from potential JP-900 basestocks. Energy Fuels 2004, 18 (3), 835–843.

Energy & Fuels, Vol. 23, 2009 361

deposits, resulting mainly from the thermal oxidative deposits and secondary deposition processes on already formed carbon deposits produced by catalytic reactions.10 Therefore, it is very likely that this kind of deposit is distinguished by differences in their oxidation reactivity and fuel temperature using TPO.31 Figure 7 shows the CO2 evolution from the TPO of deposits on different length segments of the 304 SS tube from thermal stressing of the Dod (with and without initiators) under 4 MPa. In general, two major TPO peaks of the deposit on the tube surface are identified in every 5.0 cm segment. The first peak shows a relatively small amount of carbon deposit that burns off at 290-300 °C, and the peak area changes slightly at different axial position. The second TPO peaks present at 500-600 °C, and the peak area considerably changes with increasing reactor length, which is a typical character of pyrolytic deposit that resulted from the improvement of the reaction temperature. Altin and Eser11,12 also observed the similar results in the thermal stressing of JP-8 over 304 SS tube. Figure 7a presents the TPO curves for the deposits from the thermal stressing of the pure Dod at different positions. For the 5-10 cm segment of tube, only the thermal oxidative deposit is observed because of the low fuel temperature, at which the pyrolytic reaction could not occur any more. With the increase of the tube length and fuel temperature, the pyrolytic deposit is gradually produced at the 20-25 cm segment (wall temperature of 500 °C) and sharply increases at the 40-45 cm segment (wall temperature of 660 °C). Both less-reactive deposits and reactive deposits are observed in the pyrolytic regime. Furthermore, there are also significant shifts in the TPO peaks toward higher temperature with increasing reactor length as a result of increasing deposit. For instance, for 20-25 cm, the pyrolytic peak in the TPO profile is located at 500 °C, which shifted to 550 °C for 60-65 cm and to 600 °C for 40-45 cm because of the incensement of deposits. It may be possible that peak position shifting is an artifact because of the higher total amount of deposit, which will be further discussed incorporating SEM results in the following section. Parts b-d of Figure 7 depict TPO of the deposits from thermal stressing of Dod with TEA, NP, and TEMPO on different length segments. As shown in part b-d of Figure 7, there are also two major TPO peaks of the deposit on the tube surface that can be identified for each 5.0 cm section with different initiators and there are also significant shifts in the TPO peaks toward higher temperature as a result of increasing deposit. In comparison to pure Dod, thermal oxidative deposit at 5-10 cm segments increases slightly but the pyrolytic deposit decreases sharply by TEMPO and TEA, which verifies the speculation as mentioned before. However, the pyrolytic deposit is slightly improved by NP. Conclusively, TPO results show that the addition of initiators slightly improves the oxidative deposit in the following order: TEA > TEMPO > NP but sharply reduced the pyrolytic deposit in the following order: TEMPO > TEA> NP. Morphologies of Carbon Deposits. To obtain more information on the effect of initiators on the deposit reactivity with oxygen, we also examine the morphologies of carbon deposits by SEM. Figure 8 shows the SEM images of the deposits at the same position of tube (45 cm) resulting from thermal stressing of Dod with or without initiators. Figure 8a shows the SEM images of the 304SS tube after as a reference of blank tube. Figure 8b shows the SEM images of the deposits from the thermal stressing of the pure Dod. The deposits consist of amorphous deposit, sphere deposits, and plate deposit. The diameter of the sphere deposit is between 0.1 and 1 µm. The


Liu et al.

Figure 7. TPO profiles of deposits from Dod with different initiators at a similar conversion level for 3 h (a, pure Dod, Xd ) 47.25%, Tp ) 670 °C; b, Dod + TEA, Xd ) 45.51%, Tp ) 656 °C; c, Dod + TEMPO, Xd ) 48.59%, Tp ) 646 °C; d, Dod + NP, Xd ) 45.79%, Tp ) 648 °C).

size of the plate deposit is about 1-2 µm, attributed to the accumulation of more deposits. By addition of TEA, there are many circular plate deposits of 1 µm appearing over the amorphous deposit, as shown in Figure 8c. This fact implies that the TEA initiator may change the formation mechanism of deposits and, thus, leads to deposits with different morphologies. Figure 8d illuminates the deposits resulted from Dod with 2 wt % TEMPO. It seems that there is a amount of amorphous and sphere deposits over the surface of reactor and that the sphere deposits randomly adhere to the surface as shown in the SEM micrograph. Figure 8e shows clearly peanut-like carbon deposits (or short filamentous carbon deposits) formed in the thermal stressing of Dod with 2 wt % NP. The filaments are approximately 2 µm in length and 0.5 µm in diameter, which leads to the large specific area of carbon deposits and thus high reactivity in the oxygen flow. In this sense, NP almost completely changes the morphology of the deposit resulting from thermal stressing of the Dod. To summarize, the initiator is an additive having a particular influence on not only the thermal cracking and deposit rate but also the morphology of carbonaceous deposition. Therefore, it is reasonable to infer that the formation mechanism of the pyrolytic deposit via the condensation of aromatics rings should be effectively delayed by the radicals from the decomposed initiators. Much more research would be required to make any conclusive statements about the difference in deposit formation mechanisms with different initiators. Gas and Liquid Product Comparison. Figure 9 shows the gas product distribution of Dod with or without initiator at a similar conversion level. The major gaseous products from both thermal and initiative thermal cracking of Dod are methane, ethane, propane, propylene, and butylene. Only a small quantity of butane, pentane, and pentylene are observed in the gas phase because most of them dissolve in the liquid phase. Among all of the gaseous products, the yields of hydrogen and methane are always lower than those of ethane and propane and the yield

of ethylene is always lower than that of propylene. Yu and Eser36 also observed the similar results and ascribed the lowest yields of ethylene to the particular reaction mechanism under the supercritical conditions. It is also noted that the initiator type also slightly influences the gaseous composition. Wang et al.25,26 also reported that cracking of n-heptanes with TEA produced more C4 compounds in contrast to TEA, which also shows the possible effect of initiator types on the product distributions because of the different products of initiator decompositions. The liquid product distributions (carbon number > 6) are also compared at a similar conversion level to show the effect of additives on the liquid product selectivity. Parts a and b of Figure 10 present the production of n-alkane and 1-alkene distributions from the thermal cracking of Dod with and without initiators. For the Dod with or without initiators, both n-alkane and alkene product yields decrease with the increasing carbon number. It is also interesting to observe that there are no remarkable differences in the liquid-phase product distributions with the addition of initiators, considering the slight influences of conversion and temperature on the selectivity and the possible analysis errors of GC. These results further confirm that initiative thermal cracking of Dod occurs by the same mechanism as thermal cracking, as shown by Wickham et al.21 Solid Deposit from Thermal-Stressed RP-3 Jet Fuel with and without Initiators. We also carry out the thermalstressing experiments of a real jet fuel, Chinese RP-3 jet fuel, to show the role of initiators in improving the thermal cracking rate and controlling the deposit formation. The gaseous product yield (YG) is defined to characterize the thermal cracking rate of jet fuels by moles of gaseous products per gram of jet fuel because jet fuel is a complex of thousands of hydrocarbons. Figure 11 illuminates the deposit behaviors of RP-3 jet fuel at different temperatures. When the reaction temperature rises from (36) Yu, J.; Eser, S. Thermal decomposition of C10-C14 normal alkanes in near-critical and supercritical regions: Product distributions and reaction mechanisms. Ind. Eng. Chem. Res. 1997, 36 (3), 574–584.



Figure 8. SEM micrographs of carbon deposits from Dod with different initiators at a similar conversion level for 3 h (a, blank tube; b, pure Dod, Xd ) 47.25%, Tp ) 670 °C; c, Dod + TEA, Xd ) 45.51%, Tp ) 656 °C; d, Dod + TEMPO, Xd ) 48.59%, Tp ) 646 °C; e, Dod + NP, Xd ) 45.79%, Tp ) 648 °C).

651 to 662 °C, YG increases from 5.93 to 11.99 mmol/g and the total solid deposit increases from 36.71 to 50.51 mg. Unlike the deposit of Dod, the deposit maxima presents at 55 cm, which may be ascribed to the composition difference between jet fuel and Dod, i.e., the influence of iso-paraffins, cycloparaffins, and aromatics on the thermal cracking rate of jet fuels at different wall temperature profiles. Of course, the presence of heterogeneous atoms in the jet fuel, such as sulfur, may be attributed to this difference.

Figure 12 presents the deposit behaviors of RP-3 jet fuel with different initiators at 651 °C. Roughly speaking, the addition of initiators almost does not change the wall temperature profiles at all. At the same wall temperature, TEMPO produces a gaseous products yield of 8.27 mmol/g, while TEA and NP only give their gaseous product yields of 7.14 and 7.89 mmol/g, respectively. Additionally, the deposit produced by TEMPO is only 2.90 mg/cm2, which is slightly higher than that of jet fuel but significantly lower than the 4.0 and 6.0 mg/cm2 for TEA and


Figure 9. Gas product selectivity of Dod with or without initiators under 4 MPa (pure Dod, Xd ) 47.25%, Tp ) 670 °C; Dod + TEMPO, Xd ) 48.59%, Tp ) 646 °C; Dod + TEA, Xd ) 45.51%, Tp ) 656 °C; Dod + NP, Xd ) 45.79%, Tp ) 648 °C).

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Figure 11. Carbon deposit from thermal stressing of RP-3 jet fuel (b, YG ) 5.93 mmol/g, Tp ) 651 °C, Wd ) 36.71 mg; 2, YG ) 7.89 mmol/ g, Tp ) 658 °C, Wd ) 41.62 mg; 9, YG ) 11.99 mmol/g, Tp ) 662 °C, Wd ) 50.51 mg).

Figure 12. Carbon deposition from thermal stressing of RP-3 jet fuel at Tp ) 651 °C (9, RP-3, YG ) 5.93 mmol/g, Wd ) 36.71 mg; b, RP-3 + TEMPO, YG ) 8.27 mmol/g, Wd ) 46.31 mg; 2, RP-3 + TEA, YG ) 7.14 mmol/g, Wd ) 53.27 mg; 1, RP-3 + NP, YG ) 7.89 mmol/g, Wd ) 81.09 mg).

Figure 10. Liquid alkane and alkene product distributions from thermal cracking of n-dodecnae with and without initiators (9, pure Dod, Xd ) 47.25%, Tp ) 670 °C; 1, Dod + TEMPO, Xd ) 48.59%, Tp ) 646 °C; b, Dod + TEA, Xd ) 45.51%, Tp ) 656 °C; 2, Dod + NP, Xd ) 45.79%, Tp ) 648 °C).

NP. The results agree very well with the phenomena observed in the thermal stressing of Dod: that the initiator has a determinative effect on its performance enhancement and the formation of the deposit. Evidently, it is reasonable for better initiation by TEMPO because of the relative strength of the O-O bond, potentially making it applicable for pyrolytic conditions. However, this compound will most likely be very bad for oxidative deposition because it will increase the reactivity rate and potentially increase the deposition level as a result of the higher oxygenated compound concentrations resulting from TEMPO addition. Morphologies of Carbon Deposits from RP-3 Jet Fuel. Figure 13 presents the SEM images of the deposits resulting

from the thermal stressing of RP-3 jet fuel with or without initiators. The deposits from RP-3, as shown in Figure 13a, consist of amorphous deposits and 0.1-1 µm sphere deposits. Ervin et al. also observed both amorphous and sphere deposits on the treated SS 316 surface in the pyrolytic stage during the thermal stressing of Jet-A. By addition of TEA, amorphous deposits are significantly reduced but the circular plate deposit of 1 µm and their congeries of 2-3 µm appears, as shown in Figure 13b. Figure 13c illuminates the deposits resulted from Dod with 2 wt % TEMPO. It seems that the amorphous deposit completely coats the tube surface and that some sphere deposits are randomly attached on the surface as shown in the SEM micrograph. Figure 13d shows clearly peanut-like carbon deposits (short filamentous carbon deposits) having high specific area during the thermal stressing of Dod with 2 wt % NP. The filaments that are approximately 2 µm in length and 0.5 µm in diameter appear to be formed over a carbonaceous layer from the secondary deposition process. Unobvious differences between deposit morphologies from RP-3 jet fuel and Dod further imply the specific role of the initiator in the formation of deposits. Concluding Remarks Solid deposition behaviors from thermal stressing of Dod are studied using a flowing reactor in the presence of three initiative additives, TEA, NP, and TEMPO, under the supercritical conditions. It is found that, at the similar temperature profiles, 2 wt % TEMPO not only remarkably improves the conversion



Figure 13. SEM micrographs of carbon deposits from RP-3 with different initiators at a similar conversion level for 3 h (a, RP-3, YG ) 5.93 mmol/g; b, RP-3 + TEMPO, YG ) 8.27 mmol/g; c, RP-3 + TEA, YG ) 7.14 mmol/g; d, RP-3 + NP, YG ) 7.89 mmol/g).

of Dod because of the promoting effect of initiators on the thermal cracking reactions but also inhibits 50% pyrolytic deposit. The similar trends are also observed for TEA. TPO and SEM characterization on the deposit at different axial positions of the reaction tube shows that the initiators only remarkably inhibit the pyrolytic deposit to a certain degree, resulting from the radical scavengers or hydrogen donors effect of free radicals produced from the decomposition of initiators, and that the autoxidative deposit, attributed to the residual oxygen after nitrogen sparging, is significantly improved as a result of accelerating thermal oxidation and higher concentration of the polar oxygenated compounds. SEM shows that TEA and NP also have a significant effect on the deposit morphologies. Those results are very interesting and promising in providing a solution for the thermal management of hypersonic aircrafts by solving the tradeoff effect between heat sink and solid deposit for the thermal cracking. In summary, the role of the initiator can be ascribed to four aspects: enhancement of the thermal cracking rate of Dod by

accelerating the initiation step, inhibiting the formation of pyrolytic deposits by delaying the aromatic ring condensation with radical pool effect, regulating the deposit morphology with an unknown mechanism, and improving the thermal oxidation deposits by introducing the polar oxygenated species and accelerating the initiation step of the thermal oxidation reaction. Further work on the role of the initiator in inhibiting pyrolytic deposit and controlling the deposit morphologies is under way in our laboratory to provide more information on the deposit mechanism in the presence of initiators. Acknowledgment. The authors gratefully acknowledge the financial support from the China Postdoctoral Science Foundation (20060400191), the Innovative Research Project of Petrochina Co. Ltd, and the Programme of Introducing Talents of Discipline to Universities (B06006). EF800657Z

Solid Deposits from Thermal Stressing of n-Dodecane and Chinese

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