Preparation and Anticoking Performance of MOCVD Alumina Coatings

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Preparation and Anticoking Performance of MOCVD Alumina Coatings for Thermal Cracking of Hydrocarbon Fuels under Supercritical Conditions Caihua Yang, Guozhu Liu,* Xuqing Wang, Rongpei Jiang, Li Wang, and Xiangwen Zhang Key Laboratory of Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, P.R.China ABSTRACT: For advanced thermal management technology of next-generation aircraft, hydrocarbon fuel cooling technology using endothermic cracking reactions is taken as a promising approach to removing heat loading but with a fatal drawback of forming carbonaceous deposits. To develop an effective anticoking technique to resolve this problem, a series of alumina coatings with various thicknesses (318
1280 nm) were prepared in stainless steel 321 tubes (2-mm i.d.) by metal
organic chemical vapor deposition (MOCVD) using aluminum tri-sec-butoxide. X-ray diffraction characterization showed that the prepared MOCVD alumina coatings were essentially amorphous. The anticoking performances of the MOCVD alumina coatings were evaluated using thermal cracking of Chinese RP-3 jet fuel under supercritical conditions (inlet temperature, 575 °C; outlet temperature, 650 °C; pressure, 5 MPa). The results showed that the anticoking performance increased from 37% to 69% as the thickness of the alumina coatings increased from 318 to 1280 nm. Further characterizations of the cokes with temperature-programmed oxidation and scanning electron microscopy indicated that the MOCVD alumina coatings were favorable for depressing metal catalysis cokes over the tube surface, as well as aromatic condensation cokes from bulk cracked fuel.

1. INTRODUCTION Liquid hydrocarbon fuels have attracted much interest in developing advanced aircraft because they not only serve the conventional role of propellant but also are ideal coolants to resolve the problem of thermal management by removing the waste heat from aircrafts with the physical and chemical heat sink of hydrocarbon fuels.1
4 In advanced aircraft, the fuel system is generally operated with a pressure of 3.4
6.9 MPa; thus, hydrocarbon fuels will reach the supercritical phase when the temperature reaches 400 °C.2,5 It is practical to obtain higher heat sinks (above 3.0 MJ/kg) through cracking reactions of supercritical hydrocarbon fuels because the physical heat sink is usually limited to less than 2.5 MJ/kg.2,5
8 However, a byproduct of carbonaceous deposits is inevitably generated accompanied by the cracking of jet fuel, which would result in catastrophic failure of thin microchannel heat exchangers for advanced aircraft, which is taken as one of the uncertain issues in utilizing hydrocarbon fuels as the heat sink for advanced cooling.9
11 Generally, the formation of coke is a complex chemical and physical process in the thermal stressing of jet fuels, wherein the chemical mechanisms involved include thermal oxidation, condensation of aromatic compounds, and metal catalytic coking.12
14 The thermal oxidative deposition ascribed to dissolved oxygen in the fuel occurs at low temperatures (below 480 °C) and has been successfully resolved by fuel deoxygenation and additives.15,16 At relatively higher temperatures (above 480 °C), pyrolytic deposition becomes dominant, including polyaromatic condensation in the bulk fuel and metal catalysis in the flow pass walls. Hydrogen-transfer reactions of alkylbenzenes in the fuel and pyrolytic dehydrogenation reactions contribute to ring formation and ring growth of the condensation mechanism. Hydrogen donors are effective in reducing the r 2011 American Chemical Society

hydrogen-abstraction reactions by terminating the propagation of the radicals produced from the thermal decomposition of fuel.17 Unlike the condensation mechanism, the metal catalytic coking mechanism has an intimate relationship with the wall of the cooling channel, which has a high coking rate in the initial reaction period because of the catalysis of active particles (Fe, Ni) at the bare metal surface.18
20 Usually, nickel and iron particles initiate catalytic reactions, forming metal carbides (Ni3C, Fe3C) rapidly and producing filamentous carbon through a series of reactions, which not only worsen the heat-transfer process by sharply improving heat resistances but also weaken the substrate. Previous works clearly showed that these filaments on different substrate surfaces can be effectively controlled by preventing the formation of metal carbides. Wickham et al. demonstrated that an organic selenide could be an effective additive to reduce filamentous carbon under supercritical conditions and that the amount of carbon deposition decreased by 90% owing to the formation of very stable iron and nickel selenide on metal surfaces.21 Similarly, Guo et al. reported that hydrogen donors and organic selenides possibly have a synergistic effect in inhibiting coking formation by retarding thermal cracking and reducing both the thermal oxidative and pyrolytic carbonaceous deposits caused by surface catalysis.22 In addition, Eser et al. observed that solid deposits were reduced by nearly 90% in a silcosteel reactor relative to that in stainless steel 316 tubes during supercritical thermal stressing of JP-8 (500 °C and 3.4 MPa, 1 mL/min for 5 h).19 Ervin et al. found that pyrolytic Received: September 1, 2011 Accepted: December 20, 2011 Revised: December 15, 2011 Published: December 20, 2011 1256

dx.doi.org/10.1021/ie201978c | Ind. Eng. Chem. Res. 2012, 51, 1256–1263

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Table 1. Physical Properties of RP-3 Chinese Jet Fuel Used in this Work property

value

density (15 °C) (kg/m ) 3

794.7

ASTM distillation (°C) IBP

162.2

10%

187.0

20%

195.6

50%

209.7

90%

229.2

FBP

239.5

method GB/T 1884 D2887

PONA (wt %)

SH/T 0606-2005

normal paraffins cycloparaffin

48.4 51.1

aromatics

0.5

total sulfur (ppm)

rc3. As the reaction proceeds, however, the surface is gradually covered by coke, and the coking rate gradually decreases and finally tends to a steady-state value. For the coated tube, rc drops sharply because almost all of the metal active sites are covered by inert materials, that is, [θ] is reduced and approximately equal to zero. Hence, for the coated tube, (rc1 + rc2) , rc3. Thus, we obtain 0

0

0

rc ¼ rc3 ¼ k3 ½Pl 

ð4Þ

where rc0 and rc30 represent the total coking rate and the rate of pathway 3, respectively; k03 represent the rate constant of pathway 3. Equation 3 well explains the high formation rate of filament cokes. However, the formation rate of aromatic consendation cokes on the bare tube (rc3) is significantly higher than that on the coated tube (r0c3), as shown in Figure 6, indicating that the bare tube surfaces might catalyze the coke reaction of pathway 3 (rc3 in eq 3). To show the possible influence of the alumina coating on the aromatic condensation coke, the coke precursors in the cracked fuels were analyzed. Figure 8 presents an HPLC chromatogram of the aromatics compounds from thermal cracking of Chinese RP-3 in bare and coated tubes. In fact, we detected many 1261

dx.doi.org/10.1021/ie201978c |Ind. Eng. Chem. Res. 2012, 51, 1256–1263

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Figure 8. HPLC chromatogram of aromatic compounds from the thermal cracking of Chinese RP-3 in bare and coated tubes. Identified compounds, from left to right, are benzene, toluene, naphthalene, ethylbenzene, 1,3-dimethylbenzene, 1,2-dimethylbenzene, acenaphthalene, 2-methylnaphthalene, 1,3,5-trimethylbenzene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, and benzo[a]anthracene.

Table 3. Comparison of Typical Precursor from Thermal Cracking of Chinese RP-3 in Bare and Coated Tubes concentration (mg/L) compound

bare tube

505-nm coated tube

benzene

6401.2

6556.8

toluene

30923.4

32055.7

naphthalene ethylbenzene

1756.6 46593.6

1762.3 47249.7

m-xylene

100562.0

101082.3

p-xylene

38139.1

39323.8

acenaphthylene

4600.5

4785.1

2-methylnaphthalene

7420.5

7339.3

acenaphthene

1020.0

1007.6

aromatic compounds including benzene, toluene, xylene, phenanthrene, anthracene, fluoranthene, pyrene, benzo[a]anthracene, and so on. Further analysis of several precursors is presented in Table 3. Obviously, at a given cracking degree, the precursor concentrations were almost completely identical considering the possible analysis errors. The coking rate from the aromatic condensation reaction, rc3, decreases gradually with increasing thickness of the coating. Therefore, the coking rate constant for the coated tube, k03, is less than that for the bare tube, k3. In view of the previous conclusion that the catalysis of the metal surface decreases as the coating thickness increases, one can infer that the metal wall also promotes coke formation from aromatic condensation in addition to the filamentous coke from the metal. To summarize, catalytic coke is mainly formed during the initial stage of coking. The catalytic influence of the wall material steadily decreases with time, and further coke growth occurs through a radical mechanism at a constant coking rate. Catalytic coke is somewhat rigid and branchlike in structure, creating a rough surface to promote pyrolytic coke formation and accumulation; that is, filamentous coke promotes the deposition of pyrolytic coke because it collects coking precursors like a filter.13,14 For instance, some intermediates such as aromatic hydrocarbons can be filtered by filamentous coke. These intermediates are finally turned into coke by surface migration,

accumulation, and dehydrogenation reactions. In this regard, the alumina coating creates a barrier between the hydrocarbon and the metal surface, inhibiting catalytic coke formation, as well as the subsequent pyrolytic coke formation that normally builds around the catalytic coke sites. Much more research is required to make any conclusive statements about the interaction of the deposit formation mechanisms.

4. CONCLUSIONS Alumina coatings with controllable thicknesses (318
1280 nm, 0.16
0.60 mg/cm2) were deposited on the inner surface of stainless steel tubes using the MOCVD method. Essentially, the coatings had an amorphous structure. The surface morphology of the bare tube was dramatically improved by the aluimina coatings, and the uniformity of the inner tube surface was enhanced by increasing the coating thickness. The behavior of solid deposits obtained during the thermal cracking of Chinese RP-3 jet fuel in both bare and coated SS321 tubes was studied at an inlet temperature of 575 °C, an outlet temperature of 650 °C, and a pressure of 5 MPa for 30 min. To be specific, the coking inhibition rate increased from 37% to 69% by depressing the catalytic activity of the metal surface as the coating thickness was increased from 318 to 1280 nm. Meanwhile, the reactor wall temperature profile had a vital impact on the carbon deposit profile, and the solid deposit coherently increased with increasing wall temperature. TPO and SEM characterization on the coke indicated that the MOCVD alumina coatings inhibited not only metal catalytic cokes but also aromatic condensation cokes. It should be noted that the jet fuel used in this study contains a very low amount (0.5%) of aromatics, which are strong coking precursors. Therefore, for other jet fuels containing more aromatics, the coking behavior in the untreated tube might be so bad that the coatings could possibly exhibit even better anticoking performance, although this possibility to needs to be verified experimentally. ’ AUTHOR INFORMATION Corresponding Author

*Tel./Fax: +86-22-27892340. E-mail: [email protected]. 1262

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