Catalytic Cracking and Coking of Supercritical n-Dodecane in

Aug 18, 2010 - Thermal or catalytic cracking of n-dodecane, a model compound of hydrocarbon fuels, was experimentally investigated in the stainless st...
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Ind. Eng. Chem. Res. 2010, 49, 8977–8983

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Catalytic Cracking and Coking of Supercritical n-Dodecane in Microchannel Coated with HZSM-5 Zeolites Fanxu Meng, Guozhu Liu,* Shudong Qu, Li Wang, Xiangwen Zhang, and Zhentao Mi Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin UniVersity, Tianjin 300072, China

To improve the cracking rate of supercritical hydrocarbon fuels, HZSM-5 coating was prepared over the inner surface of microchannels reactors by the washcoating method. Thermal or catalytic cracking of n-dodecane, a model compound of hydrocarbon fuels, was experimentally investigated in the stainless steel microchannel coated with HZSM-5 coatings under supercritical conditions (p > 4 MPa, T > 450 °C). It is found that cracking of n-dodecane was enhanced more than 100% by HZSM-5 coatings in 30 min reaction durations at 525 and 550 °C despite gradual deactivations of catalytic activities due to conversion coke from pyrolysis and acid-catalyzed reaction. Cokes deposited on the microchannels with and without HZSM-5 coating were characterized by scanning electron microscopy (SEM) and temperature-programmed oxidation (TPO), indicating that HZSM-5 coating effectively reduces formation of filamentous cokes and its damage on the micro channel surface by shielding surface metals when the temperature is higher than 600 °C. Introduction The development of advanced cooling technologies becomes one of the essential issues for future advanced aircrafts to remove heat loadings from the high temperature components of advanced aircraft.1,2 Liquid hydrocarbon fuels are taken as an ideal coolant of advanced cooling techniques by absorbing the heat through both physical heating (sensible heat-absorbing) and chemical reaction (cracking and dehydrogenation). In the fuel system, the pressure is above 4 MP; thus hydrocarbon fuels will reach the supercritical phase when the temperature of fuel hydrocarbon is heated to 400 °C. Recently, much work has been done to investigate the hydrocarbon cracking and coking process under supercritical condition, as well as various catalysts, to improve cracking of hydrocarbons, such as coated HZSM-5 over micro channel3 and highly dispersed (or soluble) nano-HZSM-5 catalysts in fuels.4 As far as the HZSM-5 coatings are concerned, two possible advantages could be the cracking reaction enhancement in the presence of HZSM-5 regardless of the possible deactivation of zeolite coating, and the filamentous coke inhibition due to the shielding of surface metals (such as Ni) of heat exchanger micro channels. Several authors have reported that catalysts (metal and zeolite) are helpful to enhance endothermic reaction. Nixon et al. used selective dehydrogenation of MCH on a platinum/ alumina catalyst to obtain sufficient cooling.5,6 Cooper et al. carried out the catalytic cracking of JP-10 utilizing common industrial zeolite pellets in a benchtop reactor.6 Sicard et al. presented catalytic cracking reactions of a model fuel (ndodecane) under supercritical conditions in a stirred batch reactor.7 Zeolite coating over the inner micro channels is considered as a practical technique to improve the heat sink of hydrocarbon for aircraft applications without remarkably improving the pressure drop and thermal resistance of heat exchanges. Spadaccini et al. first prepared SAPO-34 and Y zeolite coating, obtaining above 700 Btu/lbm endotherm by a mixture of normal alkanes at 650 °C and 2.4 MPa (supercritical conditions), and they conclude that catalyst* To whom correspondence should be addressed. Fax: +86 22 27402604. E-mail: [email protected].

coated surface is effective and essential to the practical aircraft application of endothermic fuels in flight-weight heat exchangers.8,9 Grill et al. prepared catalyst zeolite Y and ZSM-5 coatings on stainless steel through hydrothermal treatment, but however did not report results on catalytic cracking over zeolite coating.10 Up to now, little work has been done on the deactivation and coking behavior of zeolite coating for catalytic cracking of supercritical hydrocarbons as a function of stream time regardless of its potential influence on the future applications. The coke deposits may form in the microchannel of heat exchangers, on the inside surfaces of fuel system, leading to system failure11–16 and catalyst deactivation.17 Eser et al.12,13,18–20 investigated coke from thermal stressing of JP-8 fuel or Jet A fuels on different metal tubes and suggested that coke from thermal decomposition of jet fuel is closely related to metal surfaces. Usually, active metals (e.g., Ni, Fe) initiate catalytic reaction and generate metal carbides and produce filamentous carbon, called the filamentous coke mechanism. This metal catalytic coke can result in both system fouling and weaken the strength of metal surface of substrate contacted by the fuel, which is a serious problem for thin walled micro channel exchanger for advanced aircraft. To reduce this type of coke from pyrolysis of fuels, additives such as hydrogen donors and organic selenides21 and inert coating such as silcosteel and glasslined stainless steel13,22 are used. Considering the inert composition of zeolite (mainly Si, Al, O), ZSM-5 coating can accordingly play a role of inert coating just like silcosteel. Figure 1 shows the different coke formation mechanism over the bare tube surface and ZSM-5 coated surface. ZSM-5 coating can create a barrier between the catalytic stainless steel surface and high temperature fuels, preventing the carbide and filamentous coke formation. In this sense, zeolite coatings in the micro channels exchanger may be ideal candidates to enhance the cracking of hydrocarbon fuels and to inhibit the metal catalytic coke. Additionally, catalytic cracking occurs at lower temperature on the same heating environments or the same conversion level as thermal pyrolysis, which possibly reduces coke formation because coke formation is tightly connected with reaction temperature.23 What results these opposite behaviors may cause are still unclear and should be well worth investigation.

10.1021/ie101158w  2010 American Chemical Society Published on Web 08/18/2010

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Figure 1. Coke formation mechanism over bare tube surface and zeolite coated surface.

In this work, catalytic cracking of n-dodecane under supercritical conditions (4 MPa, 525-600 °C) was studied in the micro channel reactor (ø3 × 0.5 mm SS304 tubes) coated with commercial HZSM-5 catalysts as compared to bare tube. Results on the catalytic cracking performance varied with temperature and run length. Additionally, cokes from cracking of n-dodecane at the micro channels coated with and without HZSM-5, respectively, were characterized by scanning electron microscopy (SEM) and temperature-programmed oxidation (TPO) to relate the surface morphology and coke nature. Experimental Section Materials. n-Dodecane was used as reactant for it can represent the typical structure of n-paraffin in significant quantity in operational hydrocarbon jet fuels. 304 stainless steel tubes (ø3 mm × 0.5 mm) were used as the cooling micro channel for the heat exchanger due to its middle activity toward coke formation between rich nickel tube and inert silcosteel. HZSM-5 coating was prepared by the washcoating method. The HZSM-5 zeolite (average particle size of around 5 µm, commercially obtained from Catalyst Plant of Nankai University) was calcined for 2 h at 450 °C before used. A commercial suspension of 23 wt % colloidal silica was used as binder to enhance zeolite crystal attachment on stainless steel tube substrates after the necessary pretreatment. In the second step, a stable slurry containing 20 wt % HZSM-5 and colloidal silica water suspension and alcohols as solvent was homogeneously dispersed by ball-milling. Next, this suspension was used to coat HZSM-5 coating onto the tube by washcoating, forming a uniform wall-coated layer. This washcoating step could be repeated either for a specified number or until a specified thickness (loading) was reached. Finally, the prepared zeolite coatings were dried overnight at room temperature and then calcined at 600 °C for 2 h. Catalytic Cracking Tests. The catalytic cracking runs of n-dodecane in the presence of ZSM-5 coating were carried out on the flowing reactor apparatus described in our previous article.3 n-Dodecane was pumped with an HPLC pump, and the flow rate was measured by an electronic balance online. The as-prepared zeolite coated tube was used as the reactor.

The wall temperature of the zeolite-coated tube reactor was controlled directly by direct current (DC) power and kept constant through the reaction. Wall temperatures were measured by K-type thermocouples, and the pressure was adapted by a backpressure valve. Operating temperatures, defined as the outlet wall temperature, from 525 to 600 °C were examined. The flow rate and pressure were kept at 10 mL/min and 4 MPa. The cracking product, in the end, passed a condenser and then flowed into a gas-liquid separator. After the end of the reaction, the tubes were cooled under a nitrogen flow in the reactor. Analysis. Gas chromatograph was used for reaction-product identification and analysis. The liquid products were analyzed by a HP4890 gas chromatography, and the gas samples were identified by a SP3420 with an FID and an Al2O3/S capillary column (50 m × 0.53 mm). The surfaces of HZSM-5 coatings and stainless steel tubes before and after reaction were characterized by scanning electron microscopy (SEM, FEI, Nanosem 430 field emission gun scanning electron microscope) to observe the coating morphology. The stressed tubes were cut into 2.5 cm segments and then analyzed with temperature-programmed oxidation (TPO) from 100 to 800 °C with a heating rate of 25 °C/min by measuring the CO2. Results and Discussion Characterization of Zeolite Coatings. There are several works reporting the preparation of zeolite coatings on the inner surface of micro channels employing washcoating methods.24–27 In this work, the repeated washcoating procedure was performed until a loading amount of 2.0 mg/cm2 was obtained. Figure 2 gives SEM images of as-prepared HZSM-5 coatings. It clearly shows that the surface of HZSM-5 coating on stainless steel tube is very smooth and compact with few drawbacks from the SEM top view of fresh HZSM-5 coating and that the thickness of the coating is approximately 16 µm according to the side view of the SEM images. Catalytic Cracking Activities of HZSM-5 Coatings. Conversion of n-dodecane is defined as an index representing the catalytic cracking activity of HZSM-5 zeolite coating. Figure 3 compares the average conversion (reaction time is 30 min) of

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Figure 2. Morphologies of HZSM-5 coating (top (left) and side (right) views).

Figure 3. Average conversion of n-dodecane obtained by thermal and catalytic cracking carried out from 525 to 600 °C (conditions: 4 MPa, 10 mL/min, 30 min).

n-dodecane in the stainless tubes with and without HZSM-5 zeolite coatings in the temperature range from 525 to 600 °C. At the temperature from 525 to 575 °C, the conversion of n-dodecane for the bare tube coated with HZSM-5 coating is generally higher than that of the bare tube. Especially, at 525 °C, the conversion of n-dodecane is only 6.1% for the bare tube, whereas with HZSM-5 coating it reaches 16.1%, which is approximately equal to that at 550 °C for bare tube. This is significant evidence of the catalytic activity of the HZSM-5 coating at moderate operating temperature. Contrary to the performance obtained below 575 °C, the conversion of ndodecane in the presence of the HZSM-5 coating is generally lower than that of the bare tube at relative high operating temperature. Usually, for catalytic cracking of n-dodecane, high temperature is helpful to obtain high conversion but reduces the activity of zeolite catalysts due to its more rapid deactivation due to cokes. The conversion of n-dodecane, at 600 °C, which is 55.7% for the bare tube, decreases to 51.6% with the HZSM-5 catalysts coating. As the operating temperature increases, thermal pyrolysis is enhanced, while more rapid deactivation of zeolite catalysts is further deteriorated as stream time length. These behaviors both lead to more improvement of n-dodecane conversion with thermal pyrolysis over bare tube as compared to catalytic cracking over HZSM-5 coating. However, even if the catalysts totally deactivated, the conversion with HZSM-5 coating should not be lower than bare tube if it has no catalytic effect. So, the active metal on bare tube catalytic may give way to an improvement of n-dodecane conversion. Such metal catalytic behavior will cause filamentous coke formation, which will be discussed by SEM and TPO later. The later discussion would provide evidence that metal catalytic really exists and acts as a major factor increasing the conversion over bare tube.

Figure 4. Conversion with HZSM-5 at various operating temperature as a function of time of stream (conditions: 4 MPa, 10 mL/min, 525-600 °C).

Conversion with HZSM-5 as a function of stream time at different temperature is depicted in Figure 4 to investigate the deactivation process as contact time increases between reactant and catalysts coating. Obviously, in all cases the conversion gradually drops with extension of stream time. Notably, for conversion decrease curves at 525 and 550 °C, even at the end of the runs (30 min), the conversion obtained with HZSM-5 catalysts coating is still higher than that with bare tube at the same temperature, respectively (conversion in thermal pyrolysis stays approximately constant and is close to the average conversion in Figure 3), whereas, at 575 and 600 °C, the conversion with HZSM-5 catalysts coating rapidly falls below that with bare tube (32.1% at 575 °C and 55.7% at 600 °C, respectively), resulting in the lower average conversion in catalytic cracking than that in thermal pyrolysis. The initial high conversion over HZSM-5 coating is evidence of the catalytic cracking activity of the catalysts coating at higher temperature, although this activity with HZSM-5 decreases rapidly due to deactivation after relative shorter stream time, which becomes much shorter as the temperature goes up. Notably, the reason why conversion with bare tube is higher than that with HZSM-5 coating may be the metal catalytic effect. Atria and Schobert investigated Norpar-13 (C12-C15 n-alkanes mixture) cracked over SS 304 tubes at a flow rate 12 mL/min, 5 MPa, and 600-620 °C maximum wall temperature and found a large amount of catalytic filamentous coke.22 For our experimental conditions, in view of the overall conversion and conversion as a function of stream time length, it can be found that at the point of 575 °C, the metal catalytic becomes significant and is consequently helpful for the conversion with bare tube. Accordingly, it can be expected that filamentous coke may become

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Figure 5. Total coke amount by thermal and catalytic cracking from n-dodecane ranging from 525 to 600 °C (conditions: 4 MPa, 10 mL/min, 30 min).

obvious above 575 °C in our work, which actually is supported by later analysis and discussion. Coke is generally the main cause of deactivation of HZSM-5 catalysts coating. It can poison the active sites and block the access of the reactant to the active sites.28 Coke is unavoidably generated during hydrocarbon fuels cracking.1,23 HZSM-5 has a slow rate of coking, and the partial, moderate regeneration ability of the supercritical reaction medium in our experiment conditions may help remove coke.29 Figure 5 shows the coke amount deposited on the reaction tube after a 30 min experiment run. It is observed that the coke amount increases with temperature and the coke amount increases sharply at 600 °C for bare tube and HZSM-5 coated tube, respectively. Higher coke amount is due to higher conversion and the behavior that at temperature as high as 600 °C, the coke forms much more rapidly. Interestingly, the coke amount obtained with HZSM-5 coating is higher than that with bare tube below 575 °C. It should be explained as follows: (1) at moderate temperature HZSM-5 generally gives higher cracking activity and produces more coke due to higher conversion, and (2) coating provides holes between zeolite particles and zeolite has pores, resulting in easier coke condensation and absorbing ability. At 600 °C, HZSM-5 leads to lower coke amount than does bare tube. This may be ascribed to bare tube producing more metal catalytic coke, which is hard to remove by supercritical regeneration. SEM and TPO Analysis of Cokes. Several origins can interpret the coke formation: oxidative and absorbing cokes, conversion cokes by hydrocarbon catalytic cracking/thermal pyrolysis (including cokes occur in catalytic site or condense from gas phase precursors), and metal catalytic cokes.11,12,18,30,31 Morphology and coke nature over bare stainless steel tube and zeolite coating are discussed, respectively, by use of SEM and TPO in previous work.18,30,31 For instance, metal catalytic coke whose filamentous appearance is usually observed is visibly distinguished from other amorphous ones in morphology. Besides, the reactivity of the coke to the oxygen deceases in the order oxidative and absorbing saturated coke > conversion coke > metal catalytic coke, which is easily observed in the peaks of TPO profiles. The formation of filamentous coke can be described in the following steps:15,16,32,33 metal carbide initially forms when the active metal (e.g., Ni, Fe) is contacted with hydrocarbon fuels at high temperature; accordingly, this metal carbide provides a lower energy pathway for the fuel decomposition reaction and then decomposes to carbon and the catalytically active metals, which are again available to form a carbide compound by reacted with fuel; the precipitated nonreactive carbon causes

stress in the metal structure and removes the active metal eventually; and as more carbon is deposited, a carbon filament with a metal particle on top is formed. This schematic can be described as Figure 1. The filamentous coke occurs over a heat exchanger wall of the stainless steel tube for hypersonic aircraft travel, which naturally has considerable nickel and iron exposed to hydrocarbon fuels under high temperature. It can be expected that the HZSM-5 coating prevents contact between the fuel and the active metal surface and effectively inhibits the formation of coke by the filamentous mechanism. Thus, coke formed in such a coated tube can be primarily attributed to the cracking reaction on acid active sites/pores of zeolite and condensation mechanism. Heavy secondary reaction occurs usually on acid active sites or inside pores of zeolite, generally producing coke and then leading to deactivation of acid zeolite catalysts used in hydrocarbon process. Our HZSM-5 catalysts coating inevitably generates coke from n-dodecane cracking from 525-600 °C, although HZSM-5, due to its low density of acid sites, has a slow rate of coking. This cracking coking should be enhanced as conversion increases. On the other hand, in the condensation process, high molecular weight compounds (coke precursors) are generated due to polymerization in the hydrocarbon cracking reaction. These compounds then condense on the tube walls and eventually grow into coke of graphitic carbon with large networks of aromatic rings.32 Notably, the condensation process happens during both catalytic cracking over HZSM-5 catalysts coating and thermal pyrolysis over bare tube. The coke formation schematic over bare tube surface and ZSM-5 coated surface is depicted in Figure 1. Figure 6 shows the SEM graphs of coke formed from n-dodecane stressing over bare tube and HZSM-5 coating from 525 to 600 °C, respectively. In Figure 6A1 and A2, no obvious filamentous coke is observed, whereas in Figure 6A3, filamentous coke is seen among the coke layer, after the bare tube is exposed to n-dodecane for 30 min at 575 °C operating temperature. Moreover, the stainless steel surface is completely covered with filamentous coke, after it is exposed to n-dodecane for 30 min at 600 °C operating temperature, shown in Figure 6A4. The appearance of filamentous corresponds to the beginning of conversion with bare tube overweight that with HZSM-5 catalysts coating, probably indicating that the metal catalytic behavior has become significant. The coke obtained on HZSM-5 coating seemed very amorphous as compared to the pyrolytic filament coke. There is no filamentous coke observed on the HZSM-5 coating even at 600 °C. The coated HZSM-5 catalysts coating created a barrier from the catalytic stainless steel surface, eliminating filament formation. This process has been described in Figure 1. Active metal can not contact with the fuel, and consequently no metal carbide forms. This key step of filamentous coke formation is inhibited, and the filamentous coke is eliminated accordingly. The TPO profiles shown in Figure 7 indicate that on the bare and HZSM-5 coated stainless steel tube there are mainly three types of coke that exhibit different reactivity toward oxygen, providing three CO2 evolution peaks ranging from 250 to 450 °C, from 450 to 650 °C, and above 700 °C, respectively. The lower temperature peak can be attributed to oxidative and absorbing saturated coke; the middle one is due to conversion coke from pyrolysis and acid-catalyzed reaction; the higher temperature one should result from the more ordered filamentous coke. In Figure 8, the TPO profiles (B1-B3) obtained with HZSM-5 from 525 to 575 °C indicate more coke amount than that (A1-A3) with bare tube. The increase is mainly described

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Figure 6. SEM top view of (A1-A4) bare and (B1-B4) HZSM-5 coated tube after reaction at various temperatures from 525 to 600 °C, respectively.

to the middle peaks by conversion coke. This conversion coke is brought by the higher conversion and easier condensation/ absorbing ability (pores, active sites) with HZSM-5 zeolite catalysts coating, responding to the coke amount results in Figure 5. Bare tube, on the other hand, leads to lower coking rate for lower conversion and has smoother surface, which is not as easy to accumulate coke precursors. Notably, the Figure 7A3 profile by bare tube stressed at 575 °C has a more obvious filamentous coke peak than those at 525 and 550 °C, whereas the Figure 7B3 profile by HZSM-5 coated tube stressed at 575 °C shows no filamentous coke peak corresponding to the SEM view in Figure 6A3. It can be more obviously seen in Figure 8A4 (the carbon signal in profile A4 is much higher than that in A1-A3) that the Figure 7A4 profile by bare tube stressed at 600 °C has a significant filamentous coke peak. This is consistent

with the SEM graph in Figure 6A4, as a considerable amount of filamentous cokes is generated under this condition. The profile in Figure 7A4 also appears to have a much higher conversion of coke. This can be explained in that the tube leads to much higher conversion and coking rate at 600 °C; besides, considerable filamentous coke provides an absorbing and condensing place for the coke precursors in the thermal pyrolysis reaction. On the other hand, in the Figure 7B4 profile by the HZSM-5 coated tube stressed at 600 °C, still there appears no peak by filamentous coke, but there is a high conversion coke peak. To investigate in detail the surface damage by metal catalytic reaction, the SS304 tubes were characterized by SEM after being burned in TPO analysis. Figure 8A shows that the tube surface became porous and rough as compared to Figure 8C, which is

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on remaining Inconel surfaces after coke burning. The ZSM-5 coating on the tube surface in Figure 8B has been scratched off after burning. Notably, the cracks shown in Figure 8B are mainly due to the corrosive pretreatment depicted in Figure 8D. As compared to considerable holes on the bare tube surface in Figure 8A, the stainless steel surface in Figure 8B shows little holes. This can demonstrate that little filamentous is generated for zeolite coating suppression, or active metals will be removed from the surface, leaving only holes after filamentous coke is burnt off. Responding to this, Figure 6 really shows no filamentous coke but amorphous ones among zeolite coating. Therefore, it is concluded that zeolite coating plays a significant role in inhibiting filamentous coke formation in the hydrocarbon cracking at temperature higher than 600 °C. Conclusion

Figure 7. TPO profiles of the coke deposited on microchannel with and without zeolite coatings (A/B representing bare/zeolite coated tube, numbers 1-4 representing 525, 550, 575, and 600 °C, respectively).

a fresh bare tube surface. This is possibly due to carbide formation and migration of metal particles as the filamentous coke formation schematic described above. Active metals are removed from the surface and a few holes are left, indicating filamentous coke may lead to obvious weakness to the stainless steel surface. Similar damage was observed by Altin and Eser34

The present work established that zeolite coating on micro channel can enhance the cracking of hydrocarbon fuels and inhibit the metal catalytic coking. The cracking conversion of n-dodecane was obviously improved (from 6.1% to 16.1% and from 12.5% to 21.9%) by HZSM-5 coating in 30 min duration at 525 and 550 °C, respectively. At 575 and 600 °C, conversion with HZSM-5 catalysts coating rapidly falls below that with bare tube due to rapid deactivation and metal catalysis in bare tube. HZSM-5 coating also produced more cokes that lead to the rapid deactivation from 525 and 575 °C. At 600 °C, filamentous occurs obviously over bare tube and the conversion with HZSM-5 catalysts coating rapidly falls below that with bare tube due to rapid deactivation and metal catalysis in bare tube. SEM and TPO characterization on the cokes from cracking of n-dodecane at bare and HZSM-5 coated SS 304 tube indicates that HZSM-5 coating effectively eliminates the formation of filamentous cokes by shielding the filamentous coking mechanism.

Figure 8. Surface SEM images of (A) bare and (B) HZSM-5 coated tube after burning of coke at 500 mL/min O2 under 900 °C.

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Acknowledgment Financial support from the National Natural Science Fund of China (Grant No. 90916022) is gratefully acknowledged. SEM characterization was supported by the Centre for Analysis and Measurement of Tianjin University. Literature Cited (1) Edwards, T. Cracking and deposition behavior of supercritical hydrocarbon aviation fuels. Combust. Sci. Technol. 2006, 178, 307–334. (2) Edwards, T. Advancements in gas turbine fuels from 1943 to 2005. J. Eng. Gas Turbines Power 2007, 129, 13–20. (3) Meng, F.; Liu, G.; Wang, L.; Qu, S.; Zhang, X.; Mi, Z. Effect of HZSM-5 coating thickness upon catalytic cracking of n-dodecane under supercritical condition. Energy Fuels 2010, 24, 2848–2856. (4) Bao, S.; Liu, G.; Zhang, X.; Wang, L.; Mi, Z. New method of catalytic cracking of hydrocarbon fuels using a highly dispersed nanoHZSM-5 catalyst. Ind. Eng. Chem. Res. 2010, 49, 3972–3975. (5) Lander, H.; Nixon, A. C. Endothermic fuels for hypersonic vehicles. J. Aircr. 1971, 8, 200–207. (6) Cooper, M.; Shepherd, J. E. Experiments studying thermal cracking, catalytic cracking, and pre-mixed partial oxidation of JP-10. 39th AIAA/ ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Huntsville, July 20-23, 2003; pp AIAA 2003-4687. (7) Sicard, M., M. G.; Raepsaet, B.; Ser, F. Comparison between thermal and catalytic cracking of a model endothermic fuel. 15th AIAA International Space Planes and Hypersonic Systems and Technologies Conference, Dayton, OH, 2008; pp AIAA 2008-2622. (8) Sobel, D. R.; Spadaccini, L. J. Hydrocarbon fuel cooling technologies for advanced propulsion. J. Eng. Gas Turbines Power 1997, 119, 344– 351. (9) Huang, H.; Sobel, D. R.; Spadaccini, L. J. Endothermic heat-sink of hydrocarbon fuels for scramjet cooling. 38th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Indianapolis, IN, July 7-10, 2002; pp AIAA 2002-3871. (10) Grill, M.; Sicard, M.; Ser, F.; Potvin, C.; Dje´ga-Mariadassou, G.; Ruren Xu, Z. G. J. C.; Wenfu, Y. Preparation of zeolite Y and ZSM-5 coatings for cracking fuel in a cooling system for hypersonic vehicles. Stud. Surf. Sci. Catal. 2007, 170, 258–266. (11) Gascoin, N.; Gillard, P.; Bernard, S.; Bouchez, M. Characterisation of coking activity during supercritical hydrocarbon pyrolysis. Fuel Process. Technol. 2008, 89, 1416–1428. (12) 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. (13) 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. (14) 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, 433–439. (15) Wickham, D. T.; Atria, J. V.; Engel, J. R.; Hitch, B. D.; Karpuk, M. E. Formation of carbonaceous deposits in a model jet fuel under pyrolysis conditions. Abstr. Pap. Am. Chem. Soc. 1998, 216, 065-PETR. (16) Wickham, D. T.; Engel, J. R.; Karpuk, M. E. Additives to prevent filamentous coke formation in endothermic heat exchangers. Abstr. Pap. Am. Chem. Soc. 2000, 220, 58-PETR.

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ReceiVed for reView November 26, 2009 ReVised manuscript receiVed July 24, 2010 Accepted August 4, 2010 IE101158W