Energy & Fuels 1996, 10, 831-836
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Autoxidation of Aviation Fuels in Heated Tubes: Surface Effects E. Grant Jones* Innovative Scientific Solutions, Inc., 3845 Woodhurst Court, Beavercreek, Ohio 45430
Lori M. Balster and Walter J. Balster Systems Research Laboratories, An Operation of Calspan SRL Corporation, 2800 Indian Ripple Road, Dayton, Ohio 45440-3696 Received October 6, 1995. Revised Manuscript Received February 8, 1996X
Recently it has been reported that autoxidation of a Jet-A fuel which occurs during passage through heated stainless-steel tubing is accelerated by the tubing walls and that this effect is mitigated as the surface becomes fouled. To investigate the generality of this finding, we have studied the depletion of dissolved oxygen in 16 aviation fuels in a single-pass tubular heat exchanger. Experimental conditions of temperature and tube dimension were held constant, but the chemical composition of the inner wall surfaces was varied. Reaction was compared in commercial stainless-steel (304) tubes and in passivated or surface-treated (Silcosteel) tubes which are noted for their inert inner walls. The aviation fuels selected for study ranged from the highly thermally stable JPTS to an unstable Cu-doped Jet-A. The fuels contained a fixed amount of oxygen (air-saturated at room temperature) and were stressed under identical conditions at 185 °C during passage through 0.216-cm-i.d. tubing. Results reported herein show that autoxidation occurring as fuel flows through stainless-steel tubes is accelerated as compared to that occurring in treated tubes. The magnitude of this effect is fuel dependent, ranging from a low of 10-20% (barely detectable) to ∼75%. The role of surfaces in catalyzing aviation-fuel autoxidation in narrow-bore tubing and possible ramifications with regard to surface fouling in aircraft fuel lines are discussed.
Introduction Aviation fuels containing dissolved oxygen undergo autoxidation involving a complex series of free-radical chain reactions; this process can ultimately lead to formation of insolubles including solids and gums that collect on fuel linessa phenomenon known as fuel coking. Such surface fouling can reduce fuel flow, lower the efficiency of heat exchangers, alter nozzle dimensions, and cause hysteresis in servocontrols and flowdivider valves. Although all fuels undergo these deleterious reactions, the distribution of components in each fuel is unique, causing a different response to thermal stressing. Fuels containing trace quantities of heteroatoms such as S, N, and O show a propensity for surface fouling.1a Fouling problems are projected to become more severe with advanced aircraft since the airframe and engine will generate higher heat loads which must be dissipated by the fuel.2 Future concerns encompass both autoxidative and pyrolytic fuel degradation; the present study addresses only autoxidation. An understanding of the complex processes involved in the formation of surface and bulk insolubles in * Author to whom correspondence should be addressed. Tel: (513) 252-4264. X Abstract published in Advance ACS Abstracts, April 1, 1996. (1) Hazlett, R. N. Thermal Oxidation Stability of Aviation Turbine Fuels; ASTM Monograph 1; American Society for Testing and Materials: Philadelphia, 1991; (a) pp 79-84, (b) p 111, (c) p 73. (2) Edwards, T.; Anderson, S. D.; Pearce, J. A.; Harrison, W. E. High Temperature Thermally Stable JP FuelssAn Overview. AIAA Paper 92-0683, presented at the 30th Aerospace Sciences Meeting and Exhibit, Reno, NV, 6-9 January 1992.
0887-0624/96/2510-0831$12.00/0
aviation fuels can be gained through determination of the rate of reaction of dissolved oxygen and the kinetic parameters that describe this rate. Factors which may alter the rate of autoxidation are also important from a fundamental standpoint. The introduction of additives such as antioxidants potentially delays and slows autoxidation and enhances the performance of certain fuels. On the other hand, presence in the fuel of dissolved metals such as Cu can initiate autoxidation.1b Any mechanism whereby autoxidation is accelerated will be detrimental because of the potential for increasing the rate of surface fouling. For example, it was recently reported that the rate at which dissolved oxygen reacts at elevated temperatures (155-185 °C) in a particular Jet-A fuel, POSF-2827, is controlled in certain cases by the proximity and nature of nearby surfaces.3,4 The fact that surface fouling in this fuel correlates directly with autoxidation5,6 implies that the nature of the surface can impact both autoxidation and subsequent surface fouling. The onset of accelerated autoxidation or autocatalysis observed in this fuel was attributed to heterogeneous reactions occurring at the tubing walls; furthermore, this effect was reported to be greatly reduced or eliminated once the reaction tube surfaces became coated with deposits. Stainless-steel walls, it was argued, have active sites which accelerate (3) Jones, E. G.; Balster, W. J. Prepr. PapsAm. Chem. Soc., Div. Fuel Chem. 1994, 39 (3), 952-957. (4) Jones, E. G.; Balster, W. J. Energy Fuels 1995, 9, 610-615. (5) Jones, E. G.; Balster, W. J. Energy Fuels 1993, 7, 968-977. (6) Jones, E. G.; Balster, W. J. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1994, 39 (1), 78-81.
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oxidation, and these sites can become blocked by the passivating effects of deposits. Since this effect was observed for only one fuel, it was important to ascertain whether such findings apply for other fuels. If most fuels do, in fact, exhibit similar behavior, then fouling reactions would be exacerbated by conditions of high surface-to-volume ratio and the presence of clean metal surfacessprecisely those conditions that are desirable for certain components in aircraft fuel lines, namely, servocontrols, flow-divider valves, and nozzles. The goal of the current study was to investigate the generality of this previous finding by studying autoxidation of an array of fuels using experiments designed to emphasize surface effects; the premise here is that if wall interactions are significant in autoxidation, then the rate of autoxidation measured as fuel flows through clean stainless-steel tubes will differ from that measured as fuel flows through passivated tubes. For the latter case, we selected Silcosteel7 tubes reported to have high thermal stability and an inert interior. The extent to which Silcosteel tubing is inert for autoxidation reactions is unknown; however, for purposes of comparison, results for reaction in this tubing are considered to be representative of autoxidation occurring primarily in the bulk fuel (minimal surface activity), whereas results for reaction in stainless-steel tubing are considered to be representative of bulk autoxidation coupled with additional effects from possible surface interactions. A series of 16 aviation fuels was selected for studysa choice based primarily on their interest to the USAF and current availability. Both thermally stable and thermally unstable fuel candidates were used, all having been the subject of other USAF-funded research. The dissolved oxygen in each fuel was measured directly during autoxidation as the fuel passed through heated stainless-steel tubing and also through passivated tubing of the same dimensions. It is important to note that the current data were obtained from dissolved-oxygen measurements in fuel reacting in a single phase at high pressure rather than from oxygen-takeup measurements in the headspace above fuel. The empirical approach in the current study addresses differences in autoxidation which may be attributed directly to surface activity. Experimental Techniques All experiments were conducted using the Near-Isothermal Flowing Test Rig (NIFTR) which has been described in detail elsewhere.5 Reaction occurred as the fuel was pumped through 0.318 cm (0.125 in.) o.d., 0.216 cm (0.085 in.) i.d. tubing clamped tightly within a 81.3 cm (32 in.) heated Cu block (∼40 kg) heat exchanger maintained at 185 °C. The temperature range accessible by the techniques employed in this study is limited to 145-225 °C. The block temperature selected was midrange and provided test conditions under which the total dissolved oxygen in most fuels reacts within 20 min. System pressure was maintained at 2.3 MPa to ensure a single reaction phase. Stress duration, the residence time within the heated tube, was varied by changing the fuel flow rate and was calculated based on plug flow. A 15% correction to the stress duration has been applied to accommodate fuel expansion at 185 °C. At slow flow rates the fuel achieves block temperature in times that are short compared to the total residence time, and isothermal conditions apply. However, (7) Silcosteel tubing, Restek Corp., Bellefonte, PA.
Jones et al. under conditions of rapid flow rate, the system may depart significantly from isothermal. When the measured stress time is less than ∼1.5 min, the actual time that the fuel spends at temperature becomes significantly less than the calculated time. The resulting error is an overestimation of the dissolved oxygen, i.e., an underestimation of the extent of reaction. This error can be significant on an absolute basis but is of less concern for relative comparisons. Fuel entered the heated section saturated with respect to air at STP. The initial dissolved O2 concentration was equated to 100%, and the residual dissolved O2 following reaction was measured at the end of the tube. The absolute quantity of O2 dissolved in jet fuels at room temperature has been reported to be in the range 47-93 ppm (w/w), as measured using a GC-MS technique.8 In the current study the methodology employed for measuring dissolved O2 at elevated pressures utilizes GC detection as developed by Rubey and co-workers.9 Each fuel was studied during reaction in (1) 304 stainlesssteel (ss) tubing which was replaced after each test (i.e., daily), and (2) Silcosteel tubing (henceforth referred to as treated) of identical dimensions. The latter has an inert silica-treated inner surface covered with a monolayer of a specific siloxane polymer to further reduce surface activity. It has been demonstrated using a variety of S- and N-containing substances (chemical probes) and inverse chromatographic procedures that treated tubing provides much higher transport efficiency than conventional ss tubing.10 In the present study the treated tubing was used for only two experiments and then replaced to minimize surface changes arising from deposit buildup. Three of the 16 fuels selected for study were also evaluated for reaction in tubing that had previously been fouled by deposits. Fouling of the ss tube was achieved during earlier stressing of POSF-2827 fuel; based on the surfacecarbon burnoff of tubes exposed similarly, it was estimated that the fouled ss tube contained an approximately uniform deposit layer of 37 µg/cm2. Fouling of the treated tube was achieved by long-term exposure to heated fuel. Fuels were passed through a coarse (15 µm) filter for pump protection but were otherwise used as received. The treated tubing was cleaned using a methanol rinse in accordance with the manufacturer’s instructions; ss tubing was cleaned with BlueGold and ultrasound to remove oils and to ensure consistency with deposition experiments which require minimum background surface carbon. Results obtained from O2 measurements made in tubing (ss) cleaned by methanol rinse could not be distinguished from those made in tubing (ss) cleaned with BlueGold. Fuels of categories Jet-A, Jet-A-1, JP-5, JP-8, and JPTS as well as one Cu-doped fuel were studied and are summarized in Table 1; Columns 2-5 contain some general background information including breakpoint temperatures from the Jet Fuel Thermal Oxidation Test (JFTOT) and data on dissolved Cu and Fe for most fuels.
Results and Discussion Reaction in Treated Tubes. Figure 1 contains plots (closed circles) of residual dissolved O2 as a function of residence time for 16 fuels. The time required for complete conversion ranges from 1.5 to 13 min. With few exceptions, the curves show delay times or induction periods followed by reaction acceleration at higher conversion. This sigmoid behavior is characteristic of autocatalysis, i.e., self-initiation caused by dissociation of reaction products such as hydroperoxides. (8) Striebich, R. C.; Rubey, W. A. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1994, 39 (1), 47-50. (9) Rubey, W. A.; Striebich, R. C.; Tissandier, M. D.; Tirey, D. A.; Anderson, S. D. J. Chromatogr. Sci. 1995, 33, 433-437. (10) Anthony, L. J.; Holland, R. A. J. Noncryst. Solids 1990, 120, 82.
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Table 1. Fuel Properties and Autoxidation Summary fuel no.
class
2959 3084 3056 3119 2857 2922 2799 2985 2962 2963 2747 2827 2976 2934 2980 2926
Jet-A, Merox-treated Jet-A JP-8 Jet-A Jet-A, hydrotreated Jet-A, hydrotreated JPTS, thermally stable JP-5, hydrotreated JP-5 JP-5, Cu-doped 2962 JetA-1, hydrotreated Jet-A, straight run JPTS, thermally stable JP-8 Jet-A, Merox-treated Jet-A
JFTOT temp, K
total S, ppm
dissolved metals, ppba
time difference 50% conv, min
reduction (ss), %
Figure 1 ref
566 541
1652 527
Cu,
