1232
Energy & Fuels 1997, 11, 1232-1236
Catalysis of Jet-A Fuel Autoxidation by Fe2O3 James M. Pickard* Kinetica, Inc., COS 4214, Mound Advanced Technology Center, 720 Mound Avenue, Miamisburg, Ohio 45343
E. Grant Jones Innovative Scientific Solutions, Inc., 2786 Indian Ripple Road, Dayton, Ohio 45440-3638 Received April 8, 1997. Revised Manuscript Received August 18, 1997X
Experiments on the kinetics of O2 depletion in air-saturated (65 ppm O2) Jet-A (POSF-2827) fuel containing 4 ppm Fe2O3 were conducted with a near-isothermal flowing test rig (NIFTR) using passivated heat-exchanger tubing over the range 418-468 K. The kinetic data are consistent with an oxidation mechanism involving dissociation of an Fe2O3-hydroperoxide adduct (formed by adsorption of hydroperoxide on the Fe2O3 surface) and subsequent H-atom abstraction by surface-adsorbed radicals. Data analysis yielded the following rate parameters: log(kap/M-1 s-1) ) (11.68 ( 0.46) - (27.68 ( 0.94)/θ and log(kviiKeq/M-1 s-1) ) (9.20 ( 0.75) - (18.6 ( 1.5)/θ (where kap is the apparent rate constant, kvii is the rate constant for the dissociation of the Fe2O3hydroperoxide adduct, Keq is the equilibrium constant for adsorption of hydroperoxide on Fe2O3, and θ ) 2.303RT kcal mol-1, where R is the ideal-gas-law constant and T is absolute temperature). Analysis of the surface insolubles for neat and doped fuel indicated that both the maximum and the magnitude of the surface deposition decreased for the Fe2O3 catalysis of the autoxidation and suggested that precursors for surface deposition are scavenged by Fe2O3 particles.
Introduction Insoluble-gum and varnish formation that accompanies fuel autoxidation is a major factor contributing to engine downtime in modern aircraft.1 Problems in fuel lines result from fouling or coking of heat-exchanger surfaces used for system cooling, of servo mechanisms used to control fuel distribution, and of fuel injectors that control combustion performance. Methods employed to reduce surface fouling include additional fuel processing at the refinery, introduction of antioxidants to slow or retard autoxidation, and introduction of dispersants to reduce agglomeration of insolubles. The fouling problem can be aggravated if reactions are accelerated. Autoxidation is catalyzed homogeneously by cations of dissolved metals such as Cu, which can dissociate hydroperoxides,2 or heterogeneously by reactions on hot surfaces such as stainless steel3,4 or oxidized steel.5 Particulates present in the fuel may also act as catalysts for autoxidation. Surveys have identified the iron oxides Fe2O3, hematite (red), and Fe3O4, magnetite (black), at sizes 0-10 µm as the two primary aircraft turbine fuel contaminants, with hematite being the major component.6 Over the past 5 years, near-isothermal flowing test rigs (NIFTR) have been used to study the phenomenological kinetics of fuel autoxidation and surface deposiX Abstract published in Advance ACS Abstracts, October 1, 1997. (1) Hazlett, R. N. Thermal Oxidation Stability of Aviation Turbine Fuels; ASTM Monograph 1; American Society for Testing and Materials: Philadelphia, 1991. (2) Walling, C. Free Radicals in Solution; John Wiley and Sons, Inc.: New York, 1957; p 427. (3) Jones, E. G.; Balster, W. J. Energy Fuels 1995, 9, 610-615. (4) Jones, E. G.; Balster, L. M.; Balster, W. J. Energy Fuels 1996, 10, 831-836. (5) Johnson, R. K.; Monita, C. M. Jet Fuel Stability and Effect of Fuel-System Materials. Technical Report AFAPL-TR-68-20; Air Force Aero Propulsion Laboratory, Wright-Patterson Air Force Base: Dayton, OH, 1968.
S0887-0624(97)00057-1 CCC: $14.00
tion. In a typical experiment air-saturated fuel is passed through a heat exchanger under flow and temperature conditions that provide well-defined reaction or residence times. Disappearance of dissolved oxygen and the formation of bulk and surface insolubles are monitored as a function of reaction time. In an actual aircraft, fuel experiences flow and temperature distributions that are very difficult to study in the laboratory. The major advantage of the NIFTR technique is the application of a simple laboratory experiment to simulate some of the conditions of thermal oxidative stress occurring in fuel lines. Recently, we published7 NIFTR data on O2 depletion for a Jet-A fuel (POSF-2827). In this study we used heat-exchanger tubing passivated with the Silcosteel process8 to eliminate the influence of catalysis on the wall of the stainless steel tubing. The following mechanism was proposed for fuel autoxidation:
RH + O2 f R• + •O2H
(i)
R• + O2 f RO2•
(ii)
RH + RO2• f RO2H + R•
(iii)
2RO2• f P1 + P2
(iv)
RO2H + XH f X1 + X2
(v)
where RH is a hydrocarbon, R• is a radical, RO2• is a (6) SAE International Aerospace Information Reports AIR4023, Rev. A; AIR4246, Rev. A; Society of Automotive Engineers, International: Warrendale, PA, 1994. (7) Pickard, J. M.; Jones, E. G. Energy Fuels 1996, 10, (5), 10741077. (8) Silcosteel tubing is available commercially from Restek Corp., Bellefonte, PA.
© 1997 American Chemical Society
Catalysis by Fe2O3
Energy & Fuels, Vol. 11, No. 6, 1997 1233
peroxy radical, RO2H is hydroperoxide, P1 and P2 are molecular species formed by termination, XH is a natural inhibitor or retarder, and X1 and X2 are molecular products from reaction of hydroperoxide with inhibitor. The initiation reaction for neat fuel corresponds to reaction i. Reactions ii and iii are chain propagation, and reaction iv is the termination reaction. Most of the intermediate hydroperoxide is depleted by reaction v; RO2H depletion by dissociation is negligible. In this paper we employ the same NIFTR technique with passivated tubing to investigate and quantify the effect of Fe2O3 particles in catalyzing autoxidation of POSF-2827 fuel. This work will demonstrate that autoxidation with added Fe2O3 is autocatalytic and that the observed autocatalysis arises from
RO2H + Fe2O3 a [Fe2O3RO2H]
(vi, -vi)
[Fe2O3RO2H] f [(Fe2O3)1/2RO•] + [(Fe2O3)1/2HO•] (vii) RH + [(Fe2O3)1/2RO•] f 0.5Fe2O3 + ROH + R• (viii) RH + [(Fe2O3)1/2HO•] f 0.5Fe2O3 + H2O + R•
(ix)
In reaction vi, -vi the bracketed term represents an intermediate formed by adsorption of RO2H on the surface of Fe2O3. Radical production is assumed to arise from subsequent H-atom abstraction from RH by the surface-adsorbed radicals [(Fe2O3)1/2RO•] and [(Fe2O3)1/2HO•] formed in reaction vii. This reaction series is similar to that proposed by Mukherjee and Graydon9 in the oxidation of tetralin catalyzed by the oxides of Ni, Mn, and Cu. Taylor10,11 has also postulated a similar mechanism for catalytic oxidation of tetralin on polymeric surfaces. The global kinetic parameters obtained are compared with those for neat POSF-2827 fuel measured in earlier work using both virgin stainless steel12 and passivated tubing.7 The results are discussed with reference to the conventional mechanism expected for fuel autoxidation and the ramifications for surface fouling. Experimental Section POSF-2827 is a representative straight-run fuel of average thermal stability. It has a thermal breakpoint temperature of 539 K from the jet-fuel thermal oxidation test (JFTOT; ASTM D 3241), specific gravity of 0.8072, and a total sulfur content of 0.079% (w/w). Fe2O3 was obtained from Mach I, Inc. as Nanocat Superfine dispersed 2% (w/v) in mineral oil. The reported particle size was 3 nm. Fuel doping was accomplished by adding the appropriate volume of Fe2O3 suspension to achieve final concentrations ranging from 4 to 12 ppm. Agglomerates were reduced by a 30 min treatment in a commercial ultrasound (FS28, Fisher Scientific) prior to each experiment. Oxidation experiments were conducted using an 81 cm NIFTR that was described in detail earlier.3 Reaction occurred at elevated temperature as the fuel was pumped at 2.3 MPa through 0.318 cm o.d., 0.216 cm i.d. Silcosteel tubing clamped (9) Mukherjee, A.; Graydon, W. F. J. Phys. Chem. 1967, 71, 42324240. (10) Taylor, W. F. Catalysis 1970, 16, 20-26. (11) Taylor, W. F. J. Phys. Chem. 1970, 74, 2250-2256. (12) Pickard, J. M.; Balster, W. J.; Jones, E. G. Manuscript in preparation.
Figure 1. Oxygen depletion vs time for fuel doped with 4 ppm Fe2O3. tightly within a Cu block. Silcosteel tubing has an inert silicatreated inner surface covered with a monolayer of a proprietary siloxane polymer to further reduce surface activity.8 It has been demonstrated using a series of aviation fuels that these surfaces have much less influence on autoxidation than similar stainless steel tubes.4 Stress duration, the residence time within the heated tube, was varied by changing the fuel flow rate and was calculated on the basis of plug-flow. Relative dissolved O2 concentrations in stressed and unstressed fuel were measured based on the GC methods of Rubey and coworkers13 and were calibrated based on the independent GCMS measurement14 of 65 ppm (w/w) for air-saturated fuel. O2 depletion data collected over the temperature range 418-468 K were converted to moles per liter (M) from the fuel density within the specified temperature range. The deposition rate and the quantity of surface and bulk insolubles were evaluated using surface-carbon burnoff of 5.1 cm tubing segments and a 0.45 µm in-line Ag membrane filter. Background surface carbon from tubing segments and filters was 20 and 40 µg, respectively.
Results and Discussion Autoxidation. Data Treatment and Kinetics. Plots of oxygen depletion versus time over the temperature range 418-468 K for the POSF-2827 fuel doped with 4 ppm Fe2O3 are shown in Figure 1. The observed O2 depletion may be analyzed as a perturbation to the mechanism previously proposed for fuel autoxidation that occurs using passivated heat-exchanger surfaces. Application of the steady-state principle with the assumption of second-order termination yields the conventional expression for oxidation,
-
[ ]
Ri d[O2] ) kiii dt 2kiv
1/2
[RH]
(1)
where Ri is the rate of initiation, kiii is the rate constant for propagation, and kiv is the rate constant for termination. Additional possible termination reactions that might be expected to occur in the presence of Fe2O3 consist of reactions x and xi (13) Rubey, W. A.; Striebich, R. C.; Tissandier, M. D.; Tirey, D. A.; Anderson, S. D. J. Chromatogr. Sci. 1995, 33, 433-437. (14) Striebich, R. C.; Rubey, W. A. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1994, 39 (1), 47-50.
1234 Energy & Fuels, Vol. 11, No. 6, 1997
Pickard and Jones
2R• f R2
(x)
R• + RO2• f RRO2
(xi)
where R2 and RRO2 are molecular termination products. However, with high [O2], reaction iv is still the major termination step, since [RO2•] . [R•] and the rate constants for reactions ii and iii are of such magnitude that kii > kiii at steady state.7 The assumption of a steady state for reactions vii-ix yields Ri ≈ 2kviiKeq[RO2H]γ, where Keq is the equilibrium constant for reaction vi, -vi and γ is the effective concentration of the finely dispersed Fe2O3. Reaction vi is proportional to the surface area of Fe2O3; the use of concentration instead of surface area is justified by the small particle size and dispersion of the Fe2O3. With the relations [O2] ) [O2]o(1 - R), [RO2H] ) R[O2]o, and [RH] ) [RH]o(1 λR) {where R is the extent of reaction and λ is [O2]o/ [RH]o}, eq 1 may be rearranged to obtain
[
]
kviiKeqγ dR ) 21/2kiii dt 2kiv
[RH]o
1/2
[O2]o1/2
1/2
R (1 - λR)
(2)
Figure 2. Temperature dependence of Fe2O3-catalyzed autoxidation: Z1, Arrhenius plot; Z2, γ.
By defining u ) R1/2, eq 2 may be rearranged to obtain
[RH]o du ) kap(γ/2)1/2 λ[(λ-1/2)2 - u2] dt [O2]o1/2
(3)
where
kap ) kiii
[ ] kviiKeq 2kiv
1/2
is the apparent rate constant. Integration of eq 3 yields
u2 ) λ-1
[
ekap(2γ[RH]o)
ekap(2γ[RH]o)
1/2t
1/2t
]
-1
2
(4)
+1
Replacing u2 ) 1 - [O2]/[O2]o in eq 4 yields
[
[O2] ) [O2]o 1 - λ-1
(
ekap(2γ[RH]o)
1/2t
kap(2γ[RH]o)1/2t
e
)]
-1
2
(5)
+1
Values of kap and γ were evaluated from nonlinear leastsquares fits using the O2 loss data at each temperature given in Figure 1. Regression values of γ exhibited only a marginal temperature dependence; therefore, these data were fixed with the assumption that Eγ ) 0 ( 1 kcal mol-1. With identities Z1 ) log(kap) and Z2 ) log(γ), a least-squares analysis led to
log(kap/M-1 s-1) ) (11.68 ( 0.46) -
(27.89 ( 0.94) θ (6)
and
log(γ/M-1) ) (-4.75 ( 0.15) -
(0 ( 1) θ
(7)
where the error estimates are one standard deviation and θ ) 2.303RT kcal mol-1 (R is the ideal-gas-law constant and T is absolute temperature). Figure 2 is an Arrhenius plot of these data. Discussion. The plots in Figure 1 exhibit an S-shaped O2-versus-time dependence. For comparison purposes,
Figure 3. Oxygen depletion vs time at 458 K (185 °C) for neat and Fe2O3 (4 ppm)-treated fuel.
O2 depletion data at 458 K for neat fuel from previous work and for Fe2O3-doped POSF-2827 are illustrated in Figure 3. These data clearly illustrate a slower reaction and the absence of any S-shaped O2 dependence for the neat fuel. The S-shape dependence for the plots in Figure 1 is consistent with autocatalysis arising from the heterogeneous decomposition of RO2H on the surface of Fe2O3. Integral O2-loss fits based on eq 5 using the average rate parameters summarized by eqs 6 and 7 for the experimental data are illustrated by the solid lines in Figure 1. The value of Eap ) 27.9 ( 0.9 kcal mol-1 is similar to the result, E ) 27.0 kcal mol-1, observed previously for neat fuel subjected to autoxidation with a virgin stainless steel heat-exchanger tube.12 From eq 3, one obtains
kap ) kiii
[ ] kviiKeq 2kiv
1/2
(8)
In previous work15 we determined log(kiii/(2kiv)1/2/M-1/2 s-1/2) ) (7.08 ( 0.27) - (18.6 ( 0.5)/θ and log(ki/s-1) )
Catalysis by Fe2O3
Energy & Fuels, Vol. 11, No. 6, 1997 1235
Figure 4. Surface deposition on inert tubing at 458 K (185 °C) in neat and Fe2O3-treated fuels. Data were collected for 72 h experiments.
Figure 5. Bulk and total insolubles formed at 458 K (185 °C) in neat and Fe2O3-treated fuels. Data were collected for 6 h experiments in stainless steel tubing.
(11.2 ( 0.5) - (36.6 ( 0.9)/θ. The value for kiii/(2kiv)1/2 may be combined with the average value of kap from eq 6 using eq 8 to obtain
log(kviiKeq/M-1 s-1) ) (9.20 ( 0.75) -
(18.6 ( 1.5) θ (9)
The magnitude of log[(AviiAvi/A-vi)/M-1 s-1] ) 9.20 ( 0.75 is much smaller than the normal value expected for a unimolecular dissociation of hydroperoxide. This is consistent with the premise that RO2H decomposes on the surface of Fe2O3. The value of Evii + ∆Hvi,-vi ) 18.6 kcal mol-1 is 18 kcal mol-1 smaller than Ei ) 36.6 ( 0.9 kcal mol-1 determined for neat fuel oxidized with passivated heat-exchanger tubing. These observations are consistent with the fact that Fe2O3 is reported to be a powerful decomposition catalyst in peroxide systems (even at small concentrations).16 (15) Jones, E. G.; Balster, W. J.; Vonada, M. R.; Pickard, J. M. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1994, 39 (1), 10-13. (16) Bretherick, L. Reactive Chemical Hazards Handbook, 3rd ed.; Butterworth: London, 1985; p 1159.
Surface and Bulk Fouling. Surface insolubles have been measured for neat and doped fuel on inert tubes at 185 °C. The results for 72 h experiments in an extended-length NIFTR (183 cm) are shown in Figure 4. Deposition from neat fuel has a broad profile maximizing after 600 s of stressing. Doping with 4 ppm Fe2O3 shifts the maximum in surface deposition to the reaction-time interval 0-200 s. These findings are consistent with the measured autoxidation of neat and doped fuel (see Figure 3). The amount of carbonaceous insolubles collected on the tubing walls and on the inline filter has been measured in 6 h experiments in stainless steel tubing for neat and doped fuel. The results shown in Figure 5 indicate that the total quantity of insolubles remains approximately constant; however, doping seems to increase the bulk contribution at the expense of the surface. Fe2O3 particles provide additional surface area for capturing insoluble gums. Thus, a significant fraction of the insolubles that reach the tubing walls in neat fuel will encounter and adhere to Fe2O3 particles in the treated fuel. Most of these are swept through the system and filtered to be counted as bulk insolubles. Kauffman17 has also investigated the impact of Fe2O3 on POSF-2827 fuel using steel wire in sealed ampules at 210 °C. On the basis of Auger measurements, he reported deposit thicknesses to decrease from 900 to 80 nm for an increase in Fe2O3 concentration of 12-50 ppm. This observation was attributed to the surface deposition-inhibiting capabilities of Fe2O3. In view of the current findings concerning catalyzed autoxidation and deposition, we would ascribe Kauffman’s results to the same effect as that in Figure 5, namely, scavenging of deposition precursors by Fe2O3 particles. These results suggest that the catalytic activity of Fe2O3 would degrade rapidly under the current stress conditions because of the adherence of gums, analogous to passivation effects as deposits accrue on active stainless steel surfaces.4 Conclusions The kinetics of O2 depletion in air-saturated POSF2827 aviation fuel containing 4 ppm Fe2O3 has been studied using passivated heat-exchanger tubing to minimize surface chemical effects at the walls. Results have been described in terms of an autocatalytic mechanism where the apparent rate constant kap is given by
log(kap/M-1 s-1) ) (11.68 ( 0.46) -
(27.89 ( 0.94) θ
Initiation becomes dominated by catalytic dissociation of hydroperoxides on Fe2O3 surfaces (reaction vi), with the rate constant given by
log(kviiKeq/M-1 s-1) ) (9.20 ( 0.75) -
(18.6 ( 1.5) θ
Surface fouling measured in neat and treated fuel is consistent with the observed changes in autoxidation. In addition to their role as an organic hydroperoxide decomposition catalyst, Fe2O3 particles provide surfaces for capturing insoluble gums. Although this (17) Kauffman, R. E. Trans. ASME: J. Eng. Gas Turbines Power 1997, 119, 322-327.
1236 Energy & Fuels, Vol. 11, No. 6, 1997
effect tends to reduce wall deposits, the concomitant increase in bulk filterables leads to no net reduction in total carbonaceous insolubles. Acknowledgment. This work was sponsored by Wright Laboratory, Aero Propulsion and Power Directorate, Wright-Patterson Air Force Base, OH, under
Pickard and Jones
USAF Contract No. F33615-95-C-2507. We acknowledge valuable discussions with Messrs. Robert E. Kauffman, University of Dayton Research Institute, and Steven D. Anderson, United States Air Force. We thank Ms. Lori M. Balster for performing the O2 measurements and Mrs. Marian Whitaker for editorial assistance. EF970057F
