Energy & Fuels 1991,5, 281-282
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Communications Accelerated Hydroperoxide Formation in Jet Fuel at 65 "C in Capped and Vented Bottles
Sir: During long-term storage of jet fuel, autoxidation reactions lead to the formation of hydroperoxides. These species are detrimental to elastomers in aircraft fuel systems.'" A rapid method to determine a fuel's tendency to form hydroperoxides during storage is desired. Previous work has suggested that the mechanism of hydroperoxide formation changes above 65 0C.2 In that work, fuel samples were stressed in capped bottles at 43,65,80, and 100 "C to determine over what temperature range accelerated storage tests were applicable. If one assumed the Arrhenius relationship to be valid, then the correlation between hydroperoxides attained at the higher temperatures and those attained at 43 and 65 "C was particularly poor. Based on those findings, subsequently developed accelerated storage test methods4 were limited to 65 "C, and again in capped bottles. In both cases samples under stress remained capped except when an aliquot was removed for hydroperoxide analysis. The use of capped bottles limits the extent of fuel autoxidation by limiting the amount of oxygen available. It is reasonable, therefore, to assume that the limited amount of oxygen in a closed container could have an impact on the hydroperoxide levels attained in a reactive fuel under thermal stress. The purpose of this paper is twofold. The f i t is to show that accelerated storage of reactive fuels under these conditions, 65 "C in capped bottles, can suppress hydroperoxide formation. This in turn could lead one to incorrectly assess a fuel's oxidative tendency. The second purpose is to show that the apparent discrepancies in hydroperoxide concentrations attained at elevated temperatures could be due to oxygen starvation with concomitant cessation of autoxidation and not to mechanistic changes. It is not our intention to reiterate that limiting available oxygen can limit the extent of autoxidation. For the comparison between accelerated storage in capped and vented bottles, five fuels were used and included the following: three JP-5 blending stocks; coded Fuel no. 1, no. 2, and no. 3; Shale 11, a finished JP-5 jet fuel; and n-dodecane. The n-dodecane was treated with silica gel to remove polar species that may have influenced the hydroperoxide formation rates. This was done by adding 250 g of 100-200-mesh activated silica gel to 2 L of n-dodecane. The mixture was magnetically stirred for 6 h. Two liters of each sample was prefiltered through two Gelman type A/E glass fiber filters prior to accelerated aging. For each anticipated hydroperoxide analysis period, two 125-mL brown borosilicate glass bottles each containing a 100-mL sample were prepared. The duplicate samples were initially to be analyzed for hydroperoxide (1) Shertzer, R. H. Find Report NAPC-LR-78-20, Naval Air Propulsion Center, Trenton, NJ, 27 September 1978. (2) Hazlett, R. N.; Hall, J. M.; Nowack, C. J.; Tumer, L. Proceedings of the Conference on Long Term Stabilities of Liquid Fuels; Israel Institute of Petroleum and Energy: Tel Aviv, Israel, December 1983; pp B132-Bl48. (3) Nadler, C. Final Report NADC-80076-60,Naval Air Development Center, Warminster, PA, 20 May 1980. (4) Hall, J. M.; Hazlett, R. N. NRL Memorandum Report 5985, Naval Research Laboratory, Washington, DC, 21 May 1987.
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PEROXIDE NUMBER (ppm) 120
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4 5 6 7 S STRESS DURATION (Weeks)
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Fuel YZ
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Shale II JP-6
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Figure 1. Accelerated hydroperoxide formation at 65 "C in capped bottles. PEROXIDE NUMBER (ppm)
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6500 6000 4500
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-
0
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4 5 6 7 8 9 STRESS DURATION (Weeke)
+ Fuel #1
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Fuel # 2
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+ Fuel #3
11
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I
Figure 2. Accelerated hydroperoxide formation at 65 "C in vented
bottles.
concentration in duplicate every two weeks for 8 weeks. This was later modified and single samples were analyzed in duplicate for up to 12 weeks. Two sets of each fuel sample were prepared. The first set was left vented, but loosely covered with a cap to avoid contamination, and the second set remained tightly capped for the duration of the test. The samples were stressed in a Lab-Line Instruments, Inc., Environette oven, Catalog Number 702-X, at 65 "C for the duration of the test. The temperature was monitored with a Type K thermocouple connected to a Leeds and Northrup Speedomax 250 strip chart recorder. A Mettler DL-21 automatic titrator was used for hydroperoxide determinations. Analyses were performed according to ASTM D3703-85: the Standard Test Method for Peroxide Number of Aviation Turbine Fuels.5 The method was modified to determine the titration end points potentiometrically rather than colorimetrically. A Mettler DM 140-SC combined platinum ring electrode for redox reactions was used for the potentiometric titrations. (5) Standard Test Method for Peroxide Number of Aviation Turbine Fuela; ASTM D3703-85; American Society for Testing and Materials, Philadelphia, PA, 1989.
0887-0624/91/2505-0281$02.50/00 1991 American Chemical Society
Communications
282 Energy & Fuels, Vol. 5, No. 2, 1991 PEROXIDE NUMBER (ppm)
100 90
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70
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80 -
50 40 30
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Shale II JP-5
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5 6 7 8 9 1 0 STRESS DURATION (Weeks)
n-Dodecane
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Figure 3. Accelerated hydroperoxideformation at 65 "C in Shale
I1 JP-5and n-dodecane in vented bottles. PEROXIDE NUMBER (porn)
120 100 80
k
4
4 0
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-+ Fuel
2 3 4 STRESS DURATION (Weeks)
XI (vented)
Fuel Y1 (capped)
4- Fuel Y 2 (vented) 4-
Fuel # 2 (capped)
+
5
6
Fuel Y3 (vented) Fuel Y 3 (capped)
Figure 4. Accelerated hydroperoxide formation at 65 O C in Fuels no. 1, no. 2, and no. 3 during the first 6 weeks.
The results of accelerated aging at 65 "C in capped bottles are shown in Figure 1. It can be seen that four of the samples reached hydroperoxide concentrations between 65 and 105 ppm. The fifth sample, Shale I1 JP-5, continued slow hydroperoxide formation for the duration of the test. The results of accelerated aging at 65 "C in vented bottles are shown in Figure 2. It can be seen that three samples, Fuels no. 1, no. 2, and no. 3, produced extremely high concentrations of hydroperoxides under these conditions. In Figure 3 the rate of hydroperoxide formation in n-dodecane and Shale I1 JP-5 in vented bottles is shown with an expanded ordinate. In Figure 4 the formation of hydroperoxides in capped and vented bottles during the first 2 weeks for Fuel no. 1, and the first 6 weeks for Fuels no. 2 and no. 3, are compared. It can be seen that the hydroperoxide concentration differed in the capped and vented bottles within 2-4 weeks. Comparison of Figures 1and 2 shows that hydroperoxide formation in the capped bottles is significantly reduced. This is a result of the limitation of atmospheric oxygen. At 4 weeks Fuel no. 3 had the highest hydroperoxide concentration in both sets of fuels. It can be seen, however, that the concentration in the vented sample was nearly 8 times that of the capped sample. After 4 weeks, the hydroperoxide concentration in Fuel no. 3 continued to inc r e w in the vented bottle. In the capped bottle, however, the concentration of hydroperoxides remained relatively constant. This indicates that oxygen starvation has occurred.
In Figure 2 it can be seen that Fuels no. 1and no. 2 also exhibited a tendency to form high concentrations of hydroperoxides. Both of these fuels undergo a relatively slow rate of hydroperoxide formation for 6 weeks. This period of slow hydroperoxide formation is generally regarded as a fuel's induction period. After 6 weeks, both fuels rapidly form hydroperoxides. In the vented bottle, hydroperoxides are formed in Fuel no. 1 at a linear rate until 10 weeks. In the capped bottle, however, hydroperoxides formed in Fuel no. 1at much slower rate. Unlike Fuel no. 3, Fuel no. 1 slowly approaches what appears to be a limit of approximately 100 ppm. Similar characteristics are exhibited by Fuel no. 2. In the vented bottle this fuel rapidly forms hydroperoxides after 6 weeks. The hydroperoxide concentration continues to increase for the duration of the test. In the capped bottle, however, Fuel no. 2 forms hydroperoxides at a much slower rate finally reaching a concentration limit similar to that of Fuels no. 1and no. 3. These results indicate that accelerated aging in capped bottles not only limits the amount of hydroperoxides formed, but reduces the rate at which hydroperoxide formation occurs. Figure 3 shows the hydroperoxide formation for n-dodecane and Shale I1 JP-5 in vented bottles. Both fuels exhibited a relatively slow and linear hydroperoxide formation rate. When compared to Figure 1,it can be seen that Shale I1 exhibited similar characteristics in capped bottles. The Shale I1 sample reached a maximum hydroperoxide concentration of 28 ppm in the vented bottle. In the capped bottle the maximum concentration formed is approximately 18 ppm. In general, the amount of hydroperoxides formed in the capped bottles was approximately 60-7070 of the amount formed in vented bottles at each analysis interval for the Shale I1 fuel. This again shows that accelerated aging in capped bottles reduces the hydroperoxide formation rate. The rate of hydroperoxide formation in n-dodecane in capped bottles seemed to exhibit a periodicity. This was much less apparent in the vented samples. The maximum concentration of hydroperoxides formed was similar in both sets of samples. It is expected that after 12 weeks the hydroperoxide concentration in the capped n-dodecane sample, and in the other capped samples as well, would not significantly increase. This expectation is based on the limited amount of oxygen initially present in the various capped bottles, i.e., the ullage volume above the fuel sample, which is approximately 65-70 mL, and approximately 70 ppm dissolved oxygen in the fuel itself. The results reported here suggest that the fixed amount of oxygen in capped bottles is not sufficient to support autoxidation during accelerated storage of reactive fuels. It seems likely that the apparent mechanistic change reported earlier2 was a result of suppressed hydroperoxide formation due to oxygen starvation and not to mechanistic changes. It is recommended, therefore, that such tests be repeated with consideration given toward providing sufficient oxygen for the duration of the test. B. H. Black*
GEO- Centers, Znc. Ft. Washington, Maryland 20744 D. R. Hardy, E. J. Beal
CODE 6180, Naval Research Laboratory Washington, D. C. 20375-5000 Received January 23, 1991 Revised Manuscript Received February 21, 1991
