Ind. Eng. Chem. Res. 1993,32, 1012-1017
1012
Chemical Kinetic Modeling of Jet Fuel Autoxidation and Antioxidant Chemistry Steven Zabarnickt Applied Physics Division, KL-463, University of Dayton Research Institute, 300 College Park, Dayton, Ohio 45469-0140
Chemical kinetic modeling has been performed in order to simulate the chemical processes that occur during the autoxidation of jet fuels a t temperatures near 200 "C. Rate parameters are estimated for the elementary reactions that comprise the mechanism. The mechanism used treats the fuel and antioxidant species as single compounds. The model is able to reproduce the autoxidation chain mechanism that is responsible for oxygen removal in the fuel. The inhibition of oxygen removal by antioxidants is reproduced successfully by the model. Also,the model predicts that the thermal decomposition of alkyl hydroperoxides, even a t very small conversions, can play a crucial role in the oxidation mechanism. Introduction Aircraftjet fuelsare derived from a relativelyhigh boiling range (e.g., 205-300 "C for JP-8) distillation of crude oil often referred to as the kerosene fraction (Martel, 1987). These fuels are composed primarily of alkanes and cycloalkanes, with aromatics limited to 25% by volume (for JP-8). Autoxidation chemistry occurs both upon fuel storage at ambient temperatures and upon exposure to high temperatures in fuel lines during flight time. This exposure to high temperatures or "thermal stressing" results in the formation of oxidized soluble and insoluble products. The build-up of insolubles in aircraft fuel systems is of great concern because of the resulting possibility of fuel system failure. Fuels which tend not to form these insolubles are often referred to as "thermally stable" fuels. As present and future military aircraft will be subject to greater heat loads due to higher air speeds, the thermal stability problem is receiving a great deal of study. The thermal stability of a fuel differs from its "oxidative stability"-oxidative stability refers to the rate at which oxygen is consumed and oxidative products are formed, while thermal stability refers only to the formation of solid deposits. Recent work has demonstrated an apparent inverse relationship between oxidative and thermal stability for a variety of jet fuels (Heneghan and Harrison, 1992; Heneghan et al., 1992; Heneghan and Zabarnick, 1993). Recently, Heneghan and Zabarnick (1993)have proposed a chemical kinetic mechanism that attempts to account for this inverse behavior and yields interesting conclusions about the mechanism of antioxidant chemistry. In this work we have attempted a more quantitative study of these chemical kinetic mechanisms. Rate constant parameters are estimated and numerical modeling of the mechanism is performed in order to better understand the role that autoxidation and antioxidants play in fuel chemistry and thermal stability.
due to chemical reaction. The code was run on an HP 9000/730Unix RISC workstation. The following is input to the code: reaction mechanism with rate constante,initial concentrations for each species present in the mechanism, reaction time and time intervals for output, and various tolerances for the precision of the computation. The code outputs the individualspeciesconcentrations at each time interval. The version of REACT used here did not have the ability to read in reaction Arrhenius parameters (i.e., A-factors and activation energies), so a Unix "awk" code was written that converts a text file with Arrhenius parameters to the proper format required for REACT at the supplied reaction temperature. Jet fuels are composed of hundreds of compounds, with alkanes comprisinga large fraction. Thus, it is impractical to attempt to model the chemical changes of all components of the mixture. In order to perform this study we have chosen to model the bulk fuel as a single compound, RH, which has the chemical properties of a straight-chain alkane, such as n-dodecane. This single compound 'fuel" can also contain dissolved oxygen ( 0 2 ) and an antioxidant species (AH). Using the same formalism, Heneghan and Zabarnick (1993)have proposed the following mechanism for the autoxidation and antioxidant chemistry of jet fuels.
RO,' RO,'
formation of R'
(a)
R'+O,-RO,'
(b)
+ RH
+ E-mail:
[email protected].
RO,H
+ R'
+ RO,' =+ termination products RO,' + AH =+ ROzH + A' AO,' + RH A0,H + R'
Methodology The chemical kinetics modeling was performed using themodeling package REACT (Whitbeck, 1990). REACT is a relatively simple code that integrates the multiple differential equations that result from a chemical kinetics mechanism and yields the species concentration profiles versus time. It does not solve the energy equation, and therefore does not include energy production or removal
;d
AO,'
+ AH
-
A02H + A'
(C)
(d) (e)
(0
(h)
AO,' + AO,' products (i) R'is a hydrocarbon alkyl radical species, RH represents the bulk fuel as a single hydrocarbon compound, AH is an antioxidant species (i.e., a species with an easily abstractable hydrogen atom, also referred to in the literature
0888-5885/93/2632-1012$04.00/00 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No. 6, 1993 1013
as an inhibitor (Nixon, 1962; Waters, 1964)), and 02 represents the dissolved oxygen present in the liquid fuel. The alkyl radicals required to begin the mechanism are produced by an initiation step, reaction a. At present the reactions responsible for the initiation process in hydrocarbon oxidation are unknown. Emanuel et al. (1967)have concluded that the termolecular reaction, 2RH + 0,
-
2R'
+ H,02
is primarily responsible for R' radical initiation in the low-temperature liquid-phase oxidation of hydrocarbons. A bimolecular reaction has also been proposed, RH + 0, R'
+ HO,'
Benson and Nangia (1979) have pointed out that both of these reactions are too slow to account for initiation at temperatures less than 450 "C (with the former reaction being even slower than the latter), and have instead proposed an ion-pair mechanism that may play a role in the initiation process. Reaction b converts the R' radicals to RO2' radicals; this addition reaction is expected to have a near zero activation energy, and thus proceeds very rapidly. In reaction c, RO2' abstracts a hydrogen atom from a fuel molecule, generating a hydroperoxide, R02H, and another R' radical, thus propagating the chain. At sufficiently high temperatures, the hydroperoxide will decompose to yield the additional radicals, RO' + OH; we have included this pathway in the more complete mechanism discussed later. If the concentration of RO2' radicals is sufficient, the termination reaction, reaction d, can occur; this reaction can produce aldehydes, alcohols, and ketones, and regenerate oxygen by a disproportionation pathway. For ethylperoxy radicals, the reaction produces ethanol and acetaldehyde, 2C2H50,' * C,H50H + C2H40+ 0, These first four reactions (reactions a-d) constitute a simplified autoxidation mechanism for hydrocarbons in the liquid phase. We will demonstrate later through modeling that this mechanism displays the characteristics of a chain autoxidation. The remaining reactions (e-i) include the chemistry of an antioxidant molecule. The antioxidant species, AH, acts primarily by competing with RH (see reaction e), the fuel molecules, for Ron', thus preventing the reformation of R' radicals, which propagate the chain after forming RO2'. Reaction e generates a hydroperoxide and an antioxidant radical, A'. This antioxidant radical may react with oxygen (reaction g), in a similar manner to reaction b, generating a peroxy radical. This antioxidant peroxy radical can react with the fuel (reaction 0,an antioxidant molecule (reaction h), or with another antioxidant peroxy radical (reaction i). Reaction f regenerates an R' radical, we will show that this is very undesirable for antioxidant chemistry. Reaction h produces another antioxidant radical. Reaction i produces products that have been proposed to be precursors to deposit formation in fuels (Heneghan and Zabarnick, 1993). Table I lists the expanded reaction mechanism used for the modeling study of this paper. Reactions a-i above correspond directly to reactions 1-9 in Table I. We have added additional reactions to the above mechanism to take into account the thermal decomposition of alkyl hydroperoxides and the chemistry that occurs in the absence of oxygen. A t the highest temperatures of this study (220 "C), thermal decomposition of the alkyl hydroperoxides are expected to contribute to the mechanism due to the weakness of the RO-OH bond, which is
Table I. Reaction Mechanism for Chemical Kinetic Modeling Arrhenius activation reacA-factor energy tion (mol, L, and a) (kcdmol) no. reaction IbR' 1 x 10-1 0.0 1 R' + 0 2 * RO2' 3 x 109 0.0 2 RO2' + RH * ROzH R' 3 x 109 10.0 3 3x16 RO2' + RO2' termination 0.0 4 3 x 109 RO2' + AH 6 RO2H + A' 5.0 5 3x106 AO2' + RH AO2H + R' 10.0 6 A' + 0 2 * AO2' 3x16 0.0 I 3 x 109 AO2' + AH * AO2H + A' 5.0 8 3 x 109 AO2' + AO2' =* producta 0.0 9 R'+R'*R2 3X1@ 0.0 10 1 x 1016 RO2H 4 RO' + *OH 11 42.0 RO' + RH =) ROH + R' 3x16 10.0 12 RO' 4 &.he* + carbonyl 1 x 10'6 15.0 13 3 x 109 'OH + RH- H2O + R' 14 10.0 RO' + RO' =) RO' termination 3 x 109 0.0 15 3 x 109 &.be* + RH alkane + R' 10.0 16
-
+
-
=40 kcal/mol (Gray et al., 1967). We also wish to model the chemistry that occurs after the dissolved oxygen is consumed. Thus we need to include pathways that take into account reactions in the absence of dissolved oxygen. Reaction 10 in Table I is necessary for conditions where oxygen has been consumed;the alkyl radicals can no longer react with oxygen to form peroxy radicals (reaction 2), but they now can recombine producing the dimeric species, R2. These dimers have been observed in the autoxidation of octane and dodecane (Edwards and Zabarnick, 1992; Reddy et al., 1988). The cross-termination reaction, R' RO2' ROOR, may also play a role in the regime where oxygen is beginning to become depleted. We have chosen to leave out this reaction, as we are not concentrating on this regime. Alkyl hydroperoxide decomposition occurs via reaction 11,which forms alkoxy and hydroxyl radicals. These alkoxy and hydroxyl radicals can abstract H-atoms from the fuel to yield an alcohol and water, respectively (reactions 12 and 14). The alkoxy radicals can also decompose unimolecularly via reaction 13, yielding a smaller alkyl radical, %rime, and a carbonyl compound, i.e., a ketone or aldehyde. These alkoxy radicals can also self-terminate via reaction 15. The small alkyl radicals generated in reaction 13 can abstract H-atoms from the fuel to yield an alkane and another R' radical via reaction 16. Rprime has been included to differentiate these smaller alkyl radicals from those that are part of the primary autoxidation chain. The chemical kinetics modeling code, REACT, requires that rate constants be input for each reaction in the mechanism. We have estimated these rate constants using a combination of techniques: comparison with measured rate constants and Benson style (Benson, 1976) 'thermochemical kinetics" type of analysis. For some reactions the rate parameters were adjusted, and the effects of these adjustments were observed in the output. The rate constants, k, were estimated in Arrhenius form, i.e., k = A exp(EdRT); thus, an activation energy, E,, and an Arrhenius "A-factor" were estimated for each reaction. The rate constant parameters used for each reaction are also shown in Table I. We have assumed that the bimolecular reactions have Arrhenius A-factors of 3 X loe M-ls-l. This A-factor is close to the value M-' s-l) recommended by Benson (1976) for R'+ 0 2 and is similar to the average value of 2.2 X lo9 M-l s-l calculated by Moelwyn-Hughes (1947)for the average collisionfrequency of solution-phase bimolecular reactions. For the present purpose, it is reasonable to assume that the A-factors of these reactions are similar and"fast", as these are all simple bimolecular type reactions. For radical recombination
-
+
1014 Ind. Eng. Chem. Res., Vol. 32, No. 6, 1993
reactions, it is reasonable to assume that the activation energies are close to zero; therefore a zero activation energy was assigned to reactions 2, 4, 7, 9, 10, and 15. The activation energy of 10.0 kcal/mol chosen for reaction 3 was based on an average of similar H-atom abstraction reactions in the literature (Benson, 1976). In addition, as will be discussed later, in the presence of oxygen the chain carrier in autoxidation will be RO2'; a lower value for the activation energy of reaction 3 results in this reaction being fast enough for the R'radical concentration to be greater than the RO2' radical concentration. In order for an antioxidant molecule to compete with the fuel molecules for reaction with ROZ', reaction 5 must have a smaller activation energy than reaction 3. As antioxidants are usually added to fuels in very small amounts (
