Energy & Fuels 2004, 18, 425-437
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Simulation of the Effect of Metal-Surface Catalysis on the Thermal Oxidation of Jet Fuel T. Doungthip,‡ J. S. Ervin,*,†,‡ S. Zabarnick,†,‡ and T. F. Williams† University of Dayton Research Institute, 300 College Park, Dayton, Ohio 45469-0116, and Department of Mechanical and Aeronautical Engineering, University of Dayton, 300 College Park, Dayton, Ohio 45469-0210 Received April 24, 2003. Revised Manuscript Received July 28, 2003
Jet fuel is used for cooling in high-performance aircraft. Unfortunately, jet fuel reacts with dissolved O2 in the presence of heat to form unwanted surface deposits. Computational fluid dynamics that incorporates pseudo-detailed chemical kinetics with a wall reaction is used to simulate the effects of treated and untreated stainless-steel surfaces on the liquid-phase thermal oxidation of jet fuel in both isothermal and nonisothermal heated-tube experiments. A hydroperoxide decomposition reaction is used to represent the surface chemistry. The effects of a treated surface on thermal oxidation were modeled by adjusting the activation energy of the surface reaction. Nonisothermal heated-tube experiments that measure dissolved O2 are performed here, whereas isothermal flow experiments are performed elsewhere. Simulations of dissolved O2 consumption in the presence of treated and untreated surfaces, which include the wall reaction, agree reasonably well with the dissolved O2 measurements.
Introduction Jet fuel is used in military aircraft for cooling purposes before it is burned in the combustor. As fuel flows through the fuel system, an autoxidation chain that involves heteroatomic fuel species proceeds, which results in the reaction of dissolved O2 and the formation of oxidized products.1 Oxidized products may subsequently react to form surface deposits that reduce fuel flow and degrade heat-transfer effectiveness. Moreover, catastrophic engine failure could occur if these deposits impair the operation of close-tolerance valves. Thus, it is important to study liquid-phase fuel oxidation and the involved surface reactions. Computational fluid dynamics, together with chemical kinetics, can show aircraft engine designers how fluid dynamics and heat transfer influence fuel oxidation and the accompanying surface reactions. Three different types of chemical kinetic mechanisms have been used to simulate the thermal oxidation of jet fuel: global mechanisms, detailed mechanisms, and pseudo-detailed mechanisms.1,2 Generally, global kinetic mechanisms use one reaction to represent the thermal oxidation of jet fuel:
fuel + O2 f products
(1)
Although the thermal oxidation of jet fuel actually involves several reactions, the basic assumption of eq 1 is that the overall reaction of a mixture of compounds can be represented by one rate equation. This rate * Author to whom correspondence should be addressed. E-mail:
[email protected]. † University of Dayton Research Institute. ‡ Department of Mechanical and Aeronautical Engineering, University of Dayton. (1) Zabarnick, S. Energy Fuels 1998, 12, 547-553. (2) Ervin, J. S.; Zabarnick, S.; Williams, T. F. J. Energy Res. Technol. 2000, 122, 229-238.
equation consists of a rate constant multiplied by concentrations that have the same form as those in eq 2:
-
d[Ο2] ) k[RH][O2]n dt
(2)
In eq 2, the fuel is represented by a single compound RH. The rate constant k is represented by the product of an Arrhenius factor A and an activation-energy term. In addition, the order of the reaction is given by n. Because individual fuel samples are different, the factor A and the activation energy may have to be determined for different fuel samples. Although a global mechanism consists of relatively few reactions, a detailed kinetics mechanism may consist of hundreds of reactions to represent the thermal oxidation of jet fuel. Moreover, a detailed kinetics mechanism would include a multitude of species that exist within the fuel. These species and their concentrations will change for each fuel sample. Thus, a detailed mechanism is not practical for a study of the thermal oxidation of jet fuel that also uses computational fluid dynamics. An alternative approach that has been used to simulate the thermal oxidation of jet fuel is the use of a pseudo-detailed chemical kinetics mechanism. Pseudo-detailed chemistry represents the dominant chemistry and the behavior of classes of species within the fuel.1-4 Pseudo-detailed chemistry is midway in complexity between global and detailed kinetics mechanisms and was previously used with reasonable success in simulating the thermal oxidation of jet fuel. Because of their complexity, surface reactions have received little attention in previous computational studies of the liquid-phase thermal oxidation of jet fuel.1-4 (3) Zabarnick, S. Ind. Eng. Chem. Res. 1993, 32, 1012-1017. (4) Ervin, J. S.; Zabarnick, S. Energy Fuels 1998, 12, 344-352.
10.1021/ef030098d CCC: $27.50 © 2004 American Chemical Society Published on Web 02/28/2004
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Table 1. Present Pseudo-Detailed Chemical Kinetic Mechanism number
reaction
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
I f R• R• + O2 f RO2• RO2• + RH f ROOH + R• RO2• + RO2• f termination RO2• + AH f ROOH + A• AO2• + RH f AO2H + R• A• + O2 f AO2• AO2• + AH f AO2H + A• AO2• + AO2• f products R• + R• f R2 ROOH f RO• + •OH RO• + RH f ROH + R• • RO• f Rprime + carbonyl •OH + RH f H O + R• 2 RO• + RO• f termination • Rprime + RH f alkane + R• ROOH + SH f products
18
ROOH f RO• + •OH
Arrhenius factor A (mol, L, s) Bulk Reaction
Wall Reaction
Here, we attempt to include the effects of surface catalysis on autoxidation. Isothermal experimental studies of the surface effects on the thermal oxidation of jet fuel have shown that the use of tubing with a surface treatment can delay dissolved O2 consumption, relative to that of stainless steel.5 This surface treatment involved the chemical vapor deposition of a proprietary silica-based layer (Silcosteel tubing).6 The slower oxidation rate suggests that a surface that has been passivated by a surface treatment with an inert coating could delay or significantly reduce surface deposition. It is desirable to simulate the thermal oxidation of jet fuel using pseudo-detailed chemical kinetics and surface reactivity by including a wall reaction in the computational model. A goal of this work is to investigate if the surface-catalyzed decomposition of fuel hydroperoxides can be used to simulate the influence of surface type on the thermal oxidation of jet fuel. Studying the effects of a metal surface or a relatively inert surface on the oxidation of jet fuel is important because such research will ultimately assist the understanding of the surface deposition of jet fuel. Simulation Methodology Pseudo-detailed Chemical Kinetics. A pseudodetailed chemical kinetics mechanism used to simulate the liquid-phase oxidation of jet fuel was first proposed by Zabarnick.3 Table 1 shows the present chemical kinetics mechanism, which consists of 17 bulk reactions and 1 wall reaction. The rate constant (k) for each reaction can be represented in Arrhenius form:
( )
k ) A exp -
Ea RT
(3)
In eq 3, A is the pre-exponential factor, Ea the activation energy, R the universal gas constant, and T the absolute temperature. Reactions 1-4 and reaction 10 of Table 1 comprise a simple chain mechanism that involves the formation of (5) Jones, E. G.; Balster, L. M.; Balster, W. J. Energy Fuels 1996, 10, 813-836. (6) Silcosteel tubing, Restek Corp., Bellefonte, PA.
activation energy (kcal/mol)
1 × 10-7 3 × 109 3 × 109 3 × 109 3 × 109 3 × 105 3 × 109 3 × 109 3 × 109 3 × 109 1 × 1015 3 × 109 1 × 1016 3 × 109 3 × 109 3 × 109 3 × 109
0 0 10 0 5 10 0 6 0 0 42 10 15 10 0 10 16
1 × 109
37-42
free radicals that are due to hydrocarbon fuel oxidation. In reaction 1, species I is used to initiate the complex process that forms the free radical R• at a low reaction rate. Reaction 1 becomes negligible relative to other reactions after the chain begins. The single compound RH represents the bulk fuel and is assumed to have the chemical properties of a straight-chain alkane (such as n-dodecane). Reactions 5-9 in the table represent the antioxidant chemistry associated with the interception of an alkylperoxy radical by species AH. Species AH represents an antioxidant, such as butylated hydroxytoluene (BHT), that intercepts peroxy free radicals. Reactions 11-16 in Table 1 represent alkylhydroperoxide decomposition chemistry, which occurs at a sufficiently high temperature and has an important role in accelerated O2 consumption in the bulk fuel. Reaction 17 represents the reaction of fuel hydroperoxides with the hydroperoxide decomposing the species SH. SH is believed to include sulfur species (sulfides and disulfides, for example) that decompose fuel hydroperoxides into nonradical products. Previous simulations including only bulk reactions have shown that reaction 17 (from Table 1) can slow the oxidation rate when fuel hydroperoxides in the bulk fuel react with the SH species.4 In addition, previous simulations have demonstrated that AH and SH can act synergistically to slow the oxidation rate.1,4 For simplicity, the pseudo-detailed chemical kinetics mechanism uses the concentration of antioxidants AH and SH to differentiate fuel samples. Wall Reaction. Using a modified version of the pseudo-detailed chemical kinetics mechanism, Ervin and Zabarnick simulated the thermal oxidation of jet fuel that was flowing within stainless-steel tubes with reasonable success.4 In preliminary efforts during the current study, hydrocarbon oxidation on a passivated surface was simulated using the activation energy for the unimolecular hydroperoxide decomposition reaction assumed to occur in the bulk fuel. Unfortunately, agreement between experiment and simulation could not be obtained. In numerical simulations of the effects of different surfaces on the thermal oxidation of jet fuel, the use of surface reactions, rather than the arbitrary
Catalysis Effect on Thermal Oxidation of Jet Fuel
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modification of bulk reactions, is believed to be more representative of the actual chemistry. Simulations of reactions between a gas-phase species and a metal surface have commonly been performed through the use of Langmuir-Hinshelwood mechanisms, which have been reported to provide reasonable descriptions of the surface chemistry.7-12 These mechanisms consist of three types of reactions: the adsorption of molecules, the reaction involving adsorbed molecules (catalytic reaction), and the desorption of adsorbed molecules. A variable number of site species on the active surface is required with their use. Thus, the physical and chemical properties of the site species on the active surface, including their interactions with species in the fluid, are usually known.7-12 However, this is not the case for reactions between fuel hydroperoxides, which are known to have an important role in thermal oxidation, and the surface species of stainless-steel tubing. With the stainless-steel tubes used in the present heated-tube experiments, the active site species and their population are unknown. Although it is agreed that active metal ions (such as Fe, V, Mn, Ni, and Cu) that comprise the surface have an important role in the catalysis of fuel hydroperoxide decomposition, little is known about the actual reactions between fuel hydroperoxides and site species on metal surfaces.12 Moreover, adsorption and desorption processes on a real surface are extremely difficult, if not impossible, to model in detail in a computational fluid dynamics simulation.10,11 The present work does not consider the actual elementary surface reactions, which are unknown for the stainless steel and treated (inert) surfaces used in this study. Instead, for simplicity, the complex surface reactions are represented by the single reaction
ROOH f RO• + •OH
(4)
This hydroperoxide decomposition reaction is assumed to be catalyzed by an active surface and have a lower activation energy than the corresponding reaction in the bulk fuel. As a result, the rate of reaction of fuel hydroperoxide decomposition on the surface may be higher than the corresponding reactions in the bulk.5,13,14 In addition, the reactions between fuel hydroperoxides and the species that comprise the metal surfaces produce the free-radical species, RO• and •OH, which diffuse into the bulk liquid and react further, increasing the local free-radical pool.5,13,14 In the present study, a wall reaction (reaction 18, Table 1) that represents unimolecular alkylhydroper(7) Masel, R. I. Principle of Adsorption and Reaction on Solid Surfaces; Wiley: New York, 1996. (8) Anderson, A. W. Physical Chemistry of Surfaces; Wiley: New York, 1997. (9) Myers D. Surfaces Interface and Colloids, Principle and Application; Wiley: New York, 1999. (10) Coltrin, M. E.; Kee, R. J.; Rupley, F. M.; Meeks, E. Surface ChemkinsIII: A Fortran Package for Analyzing Heterogeneous Chemical Kinetics at a Solid-Surface-Gas-Phase Interface, Technical Report No. SAND96-8217, Sandia National Laboratory, Albuquerque, NM, 1996. (11) Stoltze, P. Prog. Surf. Sci. 2000, 65, 65-150. (12) Hiatt, R. J. Org. Chem. 1968, 33, 1416-1441. (13) Jones, E. G.; Balster, W. J.; Pickard, J. M. J. Eng. Gas Turbines Power 1996, 118, 286-291. (14) Jones, E. G.; Balster, W. J.; Rubey, W. A. Prepr. Am. Chem. Soc., Div. Pet. Chem. 1995, 40, 655-659.
oxide decomposition is appended to the pseudo-detailed chemical kinetics mechanism of Ervin and Zabarnick.4 The rate equation of the hydroperoxide decomposition reaction at the wall is given here as
-
( )
Ea d[ROOH] ) A exp [ROOH]R dt RT
(5)
In eq 5, R is the order of reaction for the hydroperoxide decomposition. In detailed studies of simple catalytic surface reactions, the order of the reaction is determined from the overall mechanism of the adsorption-surface reaction-desorption process.10,11 In this study, we do not have knowledge of these surface processes; therefore, the value of R was adjusted to study its influence on fuel oxidation. The choice of the activation energy in eq 5 is used to represent the effects of a stainless-steel or passive surface on the thermal oxidation of jet fuel that is flowing within a heated tube. Generally, the activation energy of a surface reaction is less than that of the corresponding bulk reaction.7,8 However, for simplicity, the activation energy in eq 5 for the passive surface is selected to be 42 kcal/mol and is equal to that of the corresponding bulk hydroperoxide decomposition reaction.1 Thus, flowing experiments that use a treated stainless-steel surface and a surrogate fuel can be performed to determine R and A for use in eq 5. In our approach, we assume that these values of R and A are equally valid for a stainless-steel surface. Although the Ea value of the fuel hydroperoxide decomposition reaction for a stainless-steel surface is unknown, it must be less than the Ea value of the more-passive Silcosteel surface. Experiments were performed to determine reasonable values of Ea for a stainless-steel surface, and the selection of Ea for stainless-steel surfaces is described later in this paper. Calibration of the Mechanism. In previous work, a “calibration” of the kinetics mechanism was performed by adjusting (within acceptable kinetics limits) the preexponential factors and activation energy of reactions 3 and 11 (from Table 1) until the chemistry model reasonably represented the thermal oxidation of a hydrotreated fuel on a stainless-steel surface.4 However, the hydrotreated fuel may likely have contained small concentrations of naturally occurring antioxidants that were not taken into consideration. It is important to make the mechanism independent of the initial antioxidant concentration and reduce the influence of the surface material via the use of an inert surface. In the present work, a “calibration” is performed using computational fluid dynamics, together with the pseudodetailed chemical kinetics mechanism, by simulating previous measurements of dissolved O2 consumption using treated tubes (Silcosteel, 2.16-mm inner diameter (ID) × 3.18-mm outer diameter (OD) and a heated length of 0.813 m) under isothermal flow conditions (wall temperature of 185 °C).15,16 In these experiments, the average residence time of the fuel (Exxsol D110, from Exxon-Mobil) in the heated tube was varied by adjusting the flow rate. Exxsol D110 is a hydrocarbon (15) Jones, E. G.; Balster, L. M. Energy Fuels 2000, 14, 640-645. (16) Handbook of Aviation Fuels Properties; Coordinating Research Council: Atlanta, GA, 1983; pp 22-34.
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Table 2. Source Term and Transport Coefficients Appearing in eq 7 Φ
ΓΦ
u v
µ + µt µ + µt
k
µ + (µt/σk)
µ + (µt/σ)
h Yi
(k/cp) + (µt/σh) FDi + (µt/σYi)
SΦ -(∂P/∂z) + + (∂/∂r)[(Γu(∂v/∂z)] + (Γu/r) (∂v/∂z) + Fg - (∂P/∂r) + (∂/∂z)[(Γv(∂u/∂r)] + (∂/∂r)[(Γv(∂v/∂r)] + (Γv/r) (∂v/∂r) + 2Γv(v/r2) low-Reynolds k-, G - F( + D) standard k-, G - F low-Reynolds k-, C1f1(G/k) - C2f2F(2/k) + E standard k-, C1G(/k) - C2F(2/k) 0 ω˘ i (∂/∂z)[Γu(∂u/∂z)]
a G ) µ {2[(∂u/∂z)2 + (∂v/∂r)2 + (v/r)2] + [(∂v/∂z) + (∂u/∂r)]2}; f ) 1.0; f )1 - 0.22 exp[-(Re/6)2]; E ) -2v(/y2) exp(-0.5y+); D ) 2vk/y2; t 1 2 C1 ) 1.44; C2 ) 1.92; C1 ) 1.35; and C2 ) 1.8.
solvent that is a combination of ∼50 wt % paraffins, ∼50 wt % cycloparaffin, and
