Energy & Fuels 1995,9, 610-615
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Surface Fouling in Aviation Fuel: Short- vs LongTerm Isothermal Tests E. Grant Jones* and Walter J. Balster Systems Research Laboratories, Inc., A Division of Space Industries International, Inc., 2800 Indian Ripple Road, Dayton, Ohio 45440-3696 Received January 16, 1995. Revised Manuscript Received March 20, 1995@
The rate of surface fouling has been studied using the aviation fuel POSF-2827 under nearisothermal reaction conditions during flow through heated stainless-steel (304) tubing. We previously reported rates of fouling stainless-steel walls as determined from measurements of surface carbon collected over a 6-h integration period. The goal of the current study was to complete similar experiments but with the integration time being extended to include contributions from deposition on previously fouled surfaces. In this paper, results of short-term (6 h) and long-term (> 70 h) tests are compared. Differences in both deposition rate and its stresstime dependence are explained in terms of the changes in the rate of consumption of dissolved oxygen that result from surface interactions. The nature of the surface is shown to be important in controlling both oxygen reaction and deposition.
Introduction Problems associated with surface fouling of fuel lines, nozzles, and heat exchangers caused by thermooxidative stressing of aviation fuel have been d0cumented.l Heat loads from both the airframe and the engine that must be dissipated via the fuel are increasing with improvements in aircraft performance. Since the extent of surface fouling depends strongly upon the particular aviation fuel, it is important to be able to assess each fuel in a simple accelerated laboratory test that simulates the thermooxidative stress experienced in aircraft fuel lines. Although the Jet-Fuel Thermal-Oxidation Test (JFTOT) has traditionally been used, subjectivity in assessing lacquer color is considerable,2 and surface temperatures up to 260 "C result in reactions taking place over a broad temperature range. Over the past 3 years, we have been studying fuels using a simpler dynamic test in which fuel is stressed in a single-pass heat exchanger (stainless-steel tube) under well-defined near-isothermal chemical-oxidation conditions. The combination of very slow fuel flow rates (70-h duration) at a series of temperatures within the autoxidation regime. The (6) Marteney, P. J. Thermal Decomposition of JP-5 in Long Duration Tests. Report NAPC-PE-201C, United Technologies Research Center on contract with Naval Air Propulsion Center, Trenton, NJ, June 1989. (7) Kendall, D. R.; Houlbrook, G.; Clark, R. H.; Bullock, S. P.; Lewis, C. The Thermal Degradation of Aviation Fuels in Jet Engine Injector Feed Arms. Part 1-Results from a Full Scale Rig. Paper 87-IGTC-49, presented at the International Gas Turbine Congress, Tokyo, October 1987. (8)Clark, R. H.; Stevenson, P. A. The Thermal Degradation of Aviation Fuels in Jet Engine Injector Feed-Arms: Results from a HalfScale Rig. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1990,35 (41, 1302-1314. (9) Kamin, R. A.; Nowack, C. J.; Darrah, S. Thermal Stability Measurements Using the Fiber Optic Modified Jet Fuel Thermal Oxidation Tester. In Proceedings of the 3rd International Conference on Stability and Handling of Liquid Fuels; Institute of Petroleum: London, November 1988; p 240.
Jet-A aviation fuel chosen for study, POSF-2827, is a representative fuel that passes the JFTOT criterion, meets all USAF’ specifications,and, unlike hydrotreated fuels which cause minimal surface fouling, creates measurable deposits during 6-h tests, making it an ideal candidate for experimental study. Surface and bulk fouling in this fuel has already been reported in detail at 185 O C I O J 1 and in general over the temperature range 155-225 O C . 1 2 Furthermore, deposition in this fuel has been shown t o have a direct correlation with oxygen consumption; this offers distinct advantages for data interpretation.
Experimental Section POSF-2827 fuel has a JFTOT breakpoint of 266 “C and a sulfur level of 0.079% (w/w). This fuel falls into a category described by Kendall and Mills13where the presence of sulfur compounds tends to inhibit oxidation but at the expense of increased insoluble f0rmati0n.l~ Data were collected using the Near-Isothermal Flowing Test Rig (NIFTR) which has been described previouslylOJ1and will be briefly outlined here. The schematic diagram in Figure 1 shows a single-pass heat exchanger with 0.125-in.-o.d., 0.085in.4.d. commercial stainless-steel(304) tubing clamped tightly in a heated copper block. Fuel is passed through the system at a slow flow rate and a pressure of 2.3 MPa. Currently, two channels in the copper block permit a second pass for doubling the reaction time. Tubes were initially cleaned ultrasonically with a 10% Blue Gold s01ution.l~ Dissolved oxygen was measured using a 32-in. tube as a reaction cell; stress duration was changed by varying the flow rate. Deposition rates were measured in a separate set of uninterrupted experiments a t a (10)Jones, E. G.; Balster; W. J.; Post, M. E. J.Eng. Gas Turb. Power 1996,117, 125-131. (11)Jones, E. G.; Balster, W. J . Energy Fuels 1993, 7 , 968-977. (12) Jones, E. G.; Balster, W. J. Prepr.-Am. Chem. Soc., Diu. Pet. Chem. 1994,39 (l), 78-81. (13) Kendall, D. R.: Mills, J. S. Znd. Eng. - Chem. Prod. Res. Deu. 1986, 25,360-366. (14) Jones. E. G.: Balster. W. J.:Anderson. S. D. PreDr.-Am. Chem. SOC.,Diu. Pet. Cheh. 1992,’37 (Zj, 393-402. (15) Blue Gold. Modern Chemical, Jacksonville, AR.
Jones and Balster
185°C 0 24hr + 30hr,2parwr
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Figure 2. Deposition rate vs stress duration for POSF-2827 fuel at 185 "C for a series of experimental test times and a fuel-flow rate of 0.25 mumin.
fmed fuel-flow rate; reaction time or stress duration was determined from the flow rate, tube dimensions, and location along the tube axis. The calculated times were reduced by 15%t o compensate for fuel expansion. At the end of each test the heated tube was cut into 2 4 . sections, and the quantity of carbon in the deposits was determined from surface-carbon burnoff (LECO RC-412). Deposition rates were based upon the amount of carbon in each 2-in. segment, the stress time within that segment, and the total amount of fuel passed. Each segment provides one data point, representing the rate averaged over the entire test time (Le., minimum of 6 h and maximum of 144 h). The rate is expressed in units of micrograms of carbon per unit stress time (h) within a tube section per unit volume (mL) of fuel passed through the system.l' The rate has not been corrected for fuel expansion. The heater blocks used were 32, 34, and 36 in. in length. For longer test times the spatial distribution of the deposition broadened, frequently making it impossible to collect the complete profile at that flow rate during a single pass through the heat exchanger. Two attempts were made to collect a complete profile; in the first the fuel was passed back through a second tube within the NIFTR to double the path length and reaction time, and in the second the fuel-flow rate was reduced to extend the stress duration accordingly within a single pass. In dynamic isothermal experiments it is important to distinguish stress duration from experimental test time. Stress duration is the reaction time at temperature, and the experimental test time (along with the fuel-flow rate) determines the total quantity of fuel passed through the system. In the present study the total quantity of fuel ranged from 25 to 2160 mL. Obviously, "long-term" has a different connotation for isothermal tests and for tests in larger rigs where 2000 to 50 000 L of fuel may be used per test.'s8 Two in-line Agmembrane filters (0.45 and 0.2 pm) collected the nonadhering bulk insolubles. Experiments were conducted at 155, 165, 175, and 185 "C. In each case the initial fuel was saturated with respect to air, with oxygen being measured as 64 ppm (w/w).16
Results and Discussion Deposition Profiles. The deposition rates averaged for 6, 24, 30, 32,48, 69, and 144 h are shown in Figure (16) Striebich, R. C.; Rubey, W. A. Prepr.-Am. Chem. SOC.,Diu. Pet. Chem. 1994,39 (11, 47-50.
40
Figure 3. Deposition rate vs stress duration for POSF-2827 fuel at 175 "C for a series of experimental test times and fuel-
flow rates (in parentheses).
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Figure 4. Deposition rate vs stress duration for POSF-2827
fuel at 165 "C for a series of experimental test times and fuelflow rates (in parentheses).
2 as a function of stress duration at 185 "C for a fixed fuel-flow rate of 0.25 mumin. The following changes in surface deposition are observed with extension of integration time: (1) the profile broadens with significant tailing, (2) the maximum in the deposition rate is reduced, and (3) the center of the deposition peak shifts to slightly longer stress times. Also, the profiles appear to approach a common shape at the longest experimental times. Extended reaction time in the 30-and 69-h experiments was achieved by a second pass of the fuel through the heat exchanger; note the discontinuity from the external loop. Results of similar experiments at 175, 165, and 155 "C are presented in Figures 3, 4, and 5, respectively. Flow rates (in parentheses) and experimental test times are shown on the figures. Similar trends are observed; however, the reaction times have been extended to permit completion of the deposition processes at lower temperatures. In making a comparison of rates which are averaged over different experimental times, one must take into
Surface Fouling in Aviation Fuel
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Energy & Fuels, Vol. 9, No. 4, 1995 613
155°C
185°C
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Figure 5. Deposition rate vs stress duration for POSF-2827 fuel at 155 "C for a series of experimental test times and fuelflow rates (in parentheses). account that the baseline arising from background carbon on the tube drops as the experimental time is extended. The major difference between 6-h tests and those of much longer duration concerns the nature of the tube surface. Initially, deposition occurs on cleaned stainless-steel tube surfaces. As the experiment time is extended, this surface gradually becomes completely carbonaceous coated. For example, at the end of the 69-h test at 185 "C, the surface density at the maximum is 75 pg cm-2. If it is assumed that the deposit has a density of unity and is distributed uniformly, the thickness is 0.75 pm. Deposition should be constant d e r complete coverage of the initially cleaned stainlesssteel surfaces; based on Figure 2 the deposition rate becomes constant after 30 h or at -30 pg cm-2 coverage. The observed variation in deposition rate is related to surface changes, the dominant factor being the rate of oxygen consumption, as shown below. OKygen Loss. The residual dissolved oxygen for this fuel has been reportedll as a function of reaction time in stainless-steel tubes. The decrease in dissolved oxygen for reaction at 185 "C is shown in Figure 6; the accelerated reaction rate a t higher conversion is attributed to autocatalysis. The 6-h deposition profile displays a maximum when the dissolved oxygen drops to zero. Also shown in Figure 6 is the oxygen loss measured using a tube that had been fouled in a previous fuel-stressing experiment and was estimated to have a uniform deposit containing 37 pg cm-2 of carbon. The initial rates of oxygen loss measured in the two tubes are similar. The acceleration in oxygen reaction is either no longer present or greatly diminished in importance once the surface becomes coated with deposits. These results indicate that autocatalysis in POSF-2827 fuel is a surface-related phenomenon. The rate measured with the fouled tube remains constant over a wide conversion range, indicating a predominantly oxygen-non-limited reaction.1° After 8 min the rate slows as oxygen is further depleted and the source of the deposits is reduced. This time corresponds t o the location of the maximum in the shifted deposition profile obtained in long-term tests at 185 "C. Similar behavior is observed at 155 "C (cf. Figure 7).
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Figure 6. Residual dissolved oxygen vs stress duration for POSF-2827 fuel at 185 "C. Measurements made in (1) cleaned stainless-steeltubing and (2) stainless-steeltubing fouled with 37 pglcm2 of carbonaceous deposit. 10
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Figure 7. Residual dissolved oxygen vs stress duration for POSF-2827 fuel at 155 "C. Measurementsmade in (1) cleaned stainless-steeltubing and (2) stainless steel tubing fouled with 37 pg/cm2 of carbonaceous deposit.
Short- and long-term data sets differ as a result of the presence of deposits. If the cleaned stainless-steel tubing is viewed as the norm, then oxygen consumption is perturbed (i.e., slowed) by deposits. If the coated tube which has been somewhat passivated is viewed as the norm-an alternative and preferable view-then faster autoxidation observed in the stainless steel arises from active sites associated with the stainless-steel surface. Deposition in short-term (6-h) tests over the temperature range 155-225 "C correlates with oxygen loss measured in cleaned stainless-steel tubes;12 similarly, deposition in long-term tests is expected to correlate with oxygen loss measured in tubes that have been passivated by deposits. The higher deposition rate observed in 6-h experiments results from the faster oxygen loss. The reduction in surface deposition rate, the broadening of the deposition profile, and the shift
Jones and Balster
614 Energy & Fuels, Vol. 9, No. 4, 1995 Table 1. Insolubles (Average from Long-Term Tests, PglmL) insolubles temp ("C)
bulk
surface
total
185
2.7 f 1.0 2" l.Bb 2.4 f 0.5 2.1" 2.4 f 0.3 2.7" 2.2 f 0.6 1.5"
2.6 f 0.6 3" 3.2b 5.9 f 0.4 5.6a 7.3 f 0.2 6.2" 6.8 f 2.6 7.3"
5.3 & 1.2 50 5.0b 8.3 f 0.1 7.7" 9.7 f 0.6 8.ga 9.0 f 2.1 8.8"
175 165 155
6-h tests from ref 12. 144-h test.
in the maximum in the long-term experiments are manifestations of slower autoxidation. Quantification of Insolubles. Table 1summarizes the bulk and surface insolubles averaged a t each temperature for the extended-duration tests up to 72 h. Shown for comparison are the reported12 6-h averages and values for the longest test (144 h). It should be noted that in earlier 6-h tests, contributions from a single 0.45-pm filter were used, while the more recent long-term tests include additional bulk contributions from a second filter (0.20pm). The surface contributions were evaluated by summing the total surface carbon or integrating the area under the complete deposition profiles. In each case an estimated baseline contribution was subtracted. Within the reproducibility of the data, the short- and long-term results are in agreement, the latter displaying the same inverse temperature dependence as reported in the 6-h tests.12 Implications. The findings from these simple tests have important implications for POSF-2827 fuel and possible significance with regard t o the general thermal oxidative stability of fuels. Autocatalysis. This process has been attributed t o thermal dissociation of an oxidation product such as hydroperoxide.ll The present results indicate that the accelerated autoxidation observed in POSF-2827 fuel is heterogeneous. Carbonaceous deposits passivate the surface by blocking active sites, thereby reducing or eliminating autocatalysis. Quantity of Surface Insolubles. The quantity of surface insolubles can be used to rank fuels based on thermal oxidative stability. Our evaluations are predicated on accelerated short-term tests in which deposition processes have progressed to completion. When the entire profile of deposit is captured on the heated tube, the total deposit is independent of deposition rate, i.e., autocatalysis which may increase the deposition rate does not alter the total amount of surface carbon. If the basis for estimating surface deposits were the quantity formed within a fixed stress time t , then fuel evaluation would be based on the area under the deposition profile up to t and would be rate dependent. Clearly, fuels such as POSF-2827, which react slowly with oxygen at low conversion, would be ranked differently if a low-conversion (Le., fixed reaction time) criterion rather than a complete-conversion criterion were used. The current results indicate that quantification based on longer-term tests has distinct advantages, the primary one being the increased signal-to-noiseratio in the data. Also, initial surface catalytic effects occurring at
the shortest test times are mitigated by averaging over longer test times. Fuel-Evaluation Techniques. Techniques such as the JFTOT and Hot Liquid Process Simulator (HLPS)which make use of clean aluminum and stainless-steel surfaces, respectively, should be sensitive to the nature of the surface and also the manner in which oxygen is consumed. For example, Clark et al.17reported that a metal deactivator (MDA) functions optimally on clean metal surfaces but does not reduce deposition once a lacquer layer forms. The vulnerability of JFTOT results in short-term tests has been discussed by Clark and Bishop.2 The initial surface-passivation effects of MDA were reported to mask problems detectable only during extended test times. We conjectured in a preliminary report on some of these datal8 that the short-term benefit of MDA might be surface passivation which delays oxygen consumption by reducing surface catalysis. Reduced deposition or an improved JFTOT result might then be explained in terms of MDA slowing oxygen consumption rather than hindering adherence to the surface. Since that report we have measured the oxygen consumption in POSF2827 fuel containing the DuPont metal deactivator N,"disalicylidene-1,2-propanediaminepresent at 2 mg/L and have found that MDA slows the rate of oxygen consumption by preventing auto~atalysis.'~ Comparison with Results from Large Rigs. Investigators at Shell,8 using large rigs, have found common behavior among several fuels and additives. Weight vs time profiles indicate a three-stage process, consisting of induction (barely measurable deposition), take-off (constant deposition rate), and tail-off (reduced deposition rate) regions. They attributed induction to the time required t o cover virgin metal with a complete lacquer layer that can provide different chemical reactivity and more rapid subsequent deposition. It is questionable whether any attempt should be made to compare results from the current isothermal experiments with those from the larger rigs. Different fuels and very different test conditions make detailed comparisons difficult; however, some general comments can be made. It is clear that we do not observe increased deposition rates at longer test times for POSF-2827 fuel. We attribute the apparent higher rates in the short-term (6 h) tests solely to catalyzed oxygen consumption. That is, if the oxygen reaction rate were constant, the deposition-rate profile would be independent of test time. If the concept of induction period is applicable to POSF-2827 fuel, then our experiments characterized by a constant deposition rate must relate to either the induction or the postinduction period. After a 30-h test, the region of the tubes where deposition rates maximize should be totally covered with carbonaceous deposits, and subsequent deposition should occur at a constant rate. This is based on calculated deposit thickness, the constancy of observed deposition-rate profiles, and visual inspection. If we were to relate these results t o the Shell-group three-stage process, then our rates (6144 h average) would pertain to the second or constantrate stage and any induction period would be < 6 h. (17jClark, R. H.; Delargy, K. M.; Heins, R. J. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1990,35 (41, 1223-1232. (18)Jones, E. G.; Balster, W. J. Prepr. Pup.-Am. Chem. SOC.,Diu. Fuel Chem. 1994,39 (3j, 952-957. (19)Jones, E. G.; Balster, W. J. manuscript in preparation.
Surface Fouling in Aviation Fuel
According to Clark and Wolveridge,20it is preferable to collect data in this region rather than in the first stage. JFTOT standard operation relates to the first stage. We would not expect a third stage in the current isothermal experiments. Nature o f the Surface. The existence of virgin stainless-steel surfaces either in aircraft fuel lines or in the current experiments is highly unlikely. The term "cleaned in the present context is used to describe not only the initial state of the tube prior to fuel and heat exposure but also, in a nebulous way, surfaces early in the experiments. It is anticipated that adsorption or chemisorption of fuel components will always occur. For example, with POSF-2827 fuel it has been observed, using both X P S and Auger spectroscopy, that a thin sulfur-rich coating forms within the first 15 min of stressing at 185 0C.21 It is not clear at present whether this layer results from an inherent fuel component or an oxidized product. In either event, surface activity ascribed to cleaned stainless steel may be associated with this layer. Carbonaceous deposits eventually mask details of the initial surface. ~~
(20) Clark, R. H.; Wolveridge, P. E. Induction Periods: Are You Measuring the Right Rate? Presented at the Coordinating Research Council Meeting, Alexandria, VA, 21-22 April 1993. (21) Jones, E. G.; Balster, W. J.; Kauffman, R. E. Surface DepositsSulfur Aspects. Presented at the Symposium on Sulfur Chemistry of Jet Fuels, Wright Laboratory, Aero Propulsion and Power Directorate, Wright-Patterson Air Force Base, OH, 6 April 1994.
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Conclusions The dependence of the surface deposition rate of POSF-2827 fuel on isothermal stress duration over the temperature range 155-185 "C has been found to be a function of experimental test time. In short-term (6 h) tests, initial deposition is governed by the autocatalytic consumption of oxygen. For long-term ('70 h) tests during which the initial stainless-steel surface becomes covered with deposits, the deposition profile is governed by a slower loss of oxygen. Oxygen loss in cleaned stainless-steel tubes is accelerated as a result of surface interactions. After deposits build up, active sites necessary for autocatalysis are removed. In dynamic experiments with POSF-2827 aviation fuel, the effect of surface interactions on both dissolvedoxygen consumption and subsequent surface deposition rates has been shown to be very significant. These evaluations must be extended t o other fuels in order to assess the generality of the current findings.
Acknowledgment. This work was supported by Wright Laboratory, Aero Propulsion and Power Directorate, Wright-Patterson Air Force Base, Ohio, under USAF' Contract No. F33615-90-C-2033. The authors thank Mr. Tim Gootee for conducting the surface-carbon analyses, Mrs. Marian Whitaker for editorial assistance, and Dr. James M. Pickard for valuable discussions. EF9500090
