Energy & Fuels 1997, 11, 1303-1308
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Aviation Fuel Recirculation and Surface Fouling E. Grant Jones,* Walter J. Balster, and Lori M. Balster Innovative Scientific Solutions, Inc., 2786 Indian Ripple Road, Dayton, Ohio 45440-3638 Received June 23, 1997. Revised Manuscript Received September 10, 1997X
Surface and bulk fouling of four Jet-A fuels has been studied at 185 °C. Results are reported for two experimental configurations. In the first, fuel is stressed in a single-pass heat exchanger designed to simulate the application of aviation fuel as a heat sink for cooling aircraft component systems. In the second, fuel is stressed as above, cooled, reoxygenated, and then restressed in another pass through the heat exchanger. The second setup is designed to simulate recirculation of stressed fuel to the fuel tanks and its subsequent use for on-board cooling. Total insolubles per unit volume of fuel are found to be the same for the two experimental setups; however, prestressed fuels tend to undergo more rapid surface fouling upon reheating. The origin of this effect is shown to be chemical changes that occur during the initial stressing. Antioxidant depletion and formation of catalytic products increase the rate of autoxidation in three of the four fuels. Also, the presence of bulk insolubles circumvents delays in deposition usually observed in isothermal stressing of neat fuels.
Introduction In modernsparticularly militarysaircraft, jet or turbine fuel that is produced from the refining of crude oil has several important functions. In addition to providing combustion energy, it is the major source of cooling for component systems such as the airframe, engine lubricants, hydraulics, and gear boxes.1 Cooling applications can subject fuel to severe thermal oxidative stress prior to combustion, leading to the undesirable buildup of deposits on heat-exchanger and other sensitive fuel-line surfaces such as nozzles and servo-controls. Surface fouling at temperatures below 300 °C is dominated by autoxidative fuel degradation and is dependent upon temperature, exposure time, concentration of dissolved O2, and distribution and abundance of individual fuel components. Pyrolytic fuel degradation becomes more important at temperatures above 400 °C. Fouling problems in both of these temperature regimes and attempts to ameliorate them have been recently summarized by Hazlett.2 Because greater heat dissipation will be required in future fighter aircraft, more severe surface-fouling problems are anticipated. Hydrotreatment of fuel at the refinery coupled with the introduction of additives can enhance fuel thermal stability. Surface treatment can minimize catalytic effects, and finally, efficient fuel management can reduce stress temperatures and limit exposure time at temperature. Combustion and heat dissipation needs must be accommodated during all phases of a flight ranging from maximum thrust to idle descent. Under the former condition most of the fuel is * To whom correspondence should be addressed. Telephone: (937) 252-4264. E-mail:
[email protected]. X Abstract published in Advance ACS Abstracts, October 15, 1997. (1) Edwards, T.; Anderson, S. D.; Pearce, J. A.; Harrison, W. E., III. High-Temperature JP Fuels--An Overview. AIAA 30th Aerospace Sciences Meeting and Exhibit, Reno, NV, January 6-9, 1992; AIAA Paper No. 92-0683. (2) Hazlett, R. N. Thermal Oxidation Stability of Aviation Turbine Fuels; American Society for Testing and Materials: Philadelphia, 1991.
S0887-0624(97)00091-1 CCC: $14.00
needed for combustion; however, under the latter condition heat loads for the fuel may be high at a time when combustion requirements are minimal. Under certain conditions most of the thermally stressed fuel may be returned to on-board storage tanks. An increasingly larger fraction of prestressed fuel is utilized as the flight progresses. Recently, deleterious effects of fuel recirculation have been reported in a JP-8 fuel, POSF-2980, by Binns and co-workers.3-6 Such experiments were conducted under conditions of turbulent flow, bulk fuel temperatures of 163 °C, and wetted-wall temperatures of 204 °C based on design criteria established for aviation-fuel systems. The chemical products of prestressing are the most likely source of problems associated with fuel recirculation. For example, free-radical initiators such as hydroperoxides can accelerate autoxidation. Also, insolubles formed during the prestressing/cooling cycle may contribute to fouling under subsequent stressing. The goal of the current study was to apply isothermal test methods to assist in the understanding of autoxidative surface fouling that occurs as a result of recirculation. This approach involves an extremely simple simulation of complicated on-board fuel management. No attempt is made to duplicate a specific flight scenario. Welldefined chemical-reaction conditions are used to investigate surface fouling from neat and prestressed fuels. Dissolved O2 depletion and surface fouling are tracked as a function of reaction time at 185 °C for neat fuel (3) Dieterle, G. L.; Binns, K. E. Evaluation of High Thermal Stability Fuels for Future Aircraft. 30th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Indianapolis, IN, June 27-29, 1994; AIAA Paper No. 94-3171. (4) Binns, K. E.; Dieterle, G. L.; Williams, T. In Proceedings of the 5th International Conference on Stability and Handling of Liquid Fuels; Giles, H. N., Ed.; U.S. Department of Energy: Washington, DC, 1995; Vol. 1, pp 407-422. (5) Dieterle, G. L.; Binns, K. E. Extended Duration Thermal Stability Test of Improved Thermal Stability Jet Fuels. International Gas Turbine and Aeroengine Congress and Exposition, Houston, Texas, June 5-8, 1995; ASME Paper No. 95-GT-69. (6) Binns, K. E.; Dieterle, G. L. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1996, 41 (2), 457-460.
© 1997 American Chemical Society
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Figure 1. Schematic of NIFTR in recirculation configuration. Table 1. Fuel Properties fuel no.
class
JFTOT break point, K
total S, ppm
dissolved metals,a ppb
3166 3084 3119 2827b
Jet-A Jet-A Jet-A Jet-A
547 541 (fails) 539
700 527 1000 790
Cu, 7; Fe, 9 Cu, 35; Fe, 1.8 µg/mL), these Jet-A fuels have average to below-average thermal stability and, thus, are considered to be good candidates for recirculation studies. Experiments are conducted at 185 °C using single-pass heat exchangers, called NIFTRs (near-isothermal flowing test rigs). The basic experiments for deposition and oxidation measurements using the NIFTR apparatus have been described in detail previously7 and are reviewed here only briefly. A new recirculation configuration and related O2 measurements are described in greater detail. Four tests have been used. The (7) Jones, E. G.; Balster, L. M.; Balster, W. J. Energy Fuels 1996, 10, 509-515.
first two involve measurements of surface and bulk insolubles; the last two relate to O2 depletion. A total system pressure of 2.3 MPa is used to ensure a single reaction phase. Standard Single-Pass Test. Fuel, initially saturated with respect to air at room temperature, is pumped at 0.25 mL/ min through stainless steel (304) tubing (183 cm long, 0.216 cm i.d., 0.318 cm o.d.) maintained at constant wall temperature within a Cu-block heat exchanger. Because of the slow fuel flow rate, reaction occurs isothermally, and reaction time or stress duration can be calculated at each position along the tube, assuming plug flow. The full block provides 22 min of stressing during which all of the dissolved O2 is depleted. Fuel, after leaving the heated region, cools and passes through two in-line Ag-membrane filters (0.45 and 0.20 µm). After a 72 h period and the passage of 1.08 L of fuel, the tube is removed, rinsed with heptane, and cut into 5.1 cm sections. Each segment is treated in a vacuum oven for 24 h and then evaluated using a LECO RC-412 surface-carbon analyzer. The quantity of carbon in the surface insolubles is summed along the tube, and the result is expressed as total surface carbon in micrograms per milliliter of fuel as a function of residence time in minutes. Plotted data for this test represent an average of two to three separate experiments. Total bulk insolubles are quantified similarly from total carbon on the filters. Recirculation Double-Pass Test. The new test configuration is shown schematically in Figure 1. This is a variation on the single-pass test setup whereby pump 1 passes fuel through the heat exchanger to provide the initial stressing. After exiting the heated block, fuel is cooled for 1 min, mixed with an equal flow of unstressed fuel from pump 2, and directed back through a second channel in the heat exchanger. The second pass simulates the use of prestressed (and reoxygenated) fuel as a heat sink. Since the stressed fuel contains no residual O2, the resulting blend contains 50% stressed fuel and a dissolved O2 concentration that is 50% of the airsaturated level. At the higher flow rate (0.5 mL/min), the recirculated fuel blend experiences a maximum of 11 min of stressing. At the exit of the heated block, fuel cools and passes through in-line filters. After a 72 h period during which 1.08 L of neat fuel have passed through the first tube and 2.16 L
Fuel Recirculation and Surface Fouling
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Figure 2. Surface-deposition profiles at 185 °C. of the prestressed fuel blend through the second tube, tubes and filters are evaluated for surface carbon as described above. Autoxidation of Neat Fuel. Autoxidation occurs as fuel passes through a fixed length (81.3 cm) of heated tubing within a Cu-block heat exchanger. Reaction time within the heated tube is varied by changing the flow rate. Fuel enters the heated section saturated with respect to air at room temperature. The initial dissolved O2 concentration is equated to 100%, and the residual dissolved O2 following reaction is measured at the end of the tube using GC detection, as developed by Rubey and co-workers.8 In one series of experiments using reduced (50%) initial O2 concentration, fuel is sparged for 30 min with a 1/9 blend of O2/N2 prepared using a Porter CM 4 interface module and F200 thermal mass flow controllers. For reducing surface catalysis in autoxidation experiments, tubing passivated by the Silcosteel process9 is used. Autoxidation of Prestressed Fuel. Depletion of O2 in the recirculated blend is tracked by diverting the blended fuel into a second heater block (81.3 cm), as shown in Figure 1, and changing the residence time by adjusting the system flow rate. These measurements are much less accurate than those discussed above for neat fuels. Through the establishment of (8) Rubey, W. A.; Striebich, R. C.; Tissandier, M. D.; Tirey, D. A.; Anderson, S. D. J. Chromatogr. Sci. 1995, 33, 433-437. (9) Silcosteel tubing, Restek Corporation, Bellefonte, PA.
equal, progressively higher flow rates at pumps 1 and 2, the initial dissolved O2 entering the second block is maintained at 50%, and reaction time in the sampling block is correspondingly reduced. Data can be collected only at flow rates where the initial fuel from pump 1 experiences 100% O2 conversion before blending. This places an upper limit on the flow rate from pumps 1 and 2 and, thus, a lower limit on reaction time in the sampling block. As a result, data collection is limited to several data points at high conversion and an additional point at t ) 0.
Results and Discussion Surface Fouling and Autoxidation of Neat Fuel. The total surface carbon measured for neat (airsaturated) fuels is represented by closed circles in Figure 2. Totals range from 3.5 to 5.5 µg/mL, and deposition tracks reasonably well with the corresponding autoxidation data given by the closed circles in Figure 3. POSF-2827, the slowest oxidizing fuel, has an extended deposition region and the highest thermal stability of the four fuels studied. The concentration of hydroperoxides remains low in this fuel.7 As a result, the rate of autoxidation is approximately constant and independent of conversion. Autoxidation of the other three fuels is characterized by acceleration at higher
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Figure 3. Autoxidation profiles at 185 °C.
conversion. This autocatalytic behavior, typical of a major class of fuels,10 is caused by self-initiation from thermalsand, in addition, possible metal-catalyzeds dissociation of hydroperoxides. Surface Fouling of Recirculated Fuel. Before entering into a discussion of surface fouling from recirculated fuel, it is important to note that in the current test configuration, recirculated fuel is actually a 1/1 blend of neat fuel and fuel that has been stressed for 22 min at 185 °C. Since all of the O2 has been consumed in 22 min of stressing, the resultant blend contains only 50% of the O2 present in air-saturated fuel. In isothermal experiments with extended reaction time (to complete O2 conversion), the total quantity of insolubles is proportional to the amount of dissolved O2. Thus, in the absence of other factors, total surface fouling from the recirculated fuel will be only approximately one-half that from neat fuel. Surface fouling for recirculated-fuel blends (represented by open circles in Figure 2) differs significantly (10) Jones, E. G.; Balster, L. M.; Balster, W. J. Energy Fuels 1996, 10, 831-836.
from that for neat fuels, although the quantities are in the expected range, 1.5-3 µg/mL. Deposition from the prestressed fuel blend increases sharply from t ) 0, whereas neat fuels exhibit some delay (induction time) before the highest rate of surface fouling is reached. Because of this difference, total fouling in the recirculation pass exceeds that of the neat fuel over at least the first 5 min of reaction. In the case of POSF-2827, this observation holds for up to 11 min of stressing. In summary, despite reduced O2, surface fouling in the recirculation pass occurs at shorter reaction times, with a rate similar to that eventually achieved with neat fuel. Autoxidation of Neat and Recirculated Fuels at Reduced Initial O2. Since surface deposition and autoxidation are intrinsically related, interpretation of the deposition data requires information on autoxidation of both neat and prestressed fuels at reduced initial O2. Results of these measurements are represented, respectively, by closed triangles and open circles in Figure 3. More uncertainty exists in the latter measurements because of experimental limitations associated with the study of prestressed fuels.
Fuel Recirculation and Surface Fouling
On the basis of Figure 3, we can state that the rate of autoxidation of neat, half-air-saturated fuel is never faster than that of neat, air-saturated fuel. However, because of reduced reactant concentration, the total time needed for depletion of O2 is always less. This finding indicates that the simple reduction of dissolved O2 associated with this test configuration cannot account for the changes in deposition observed in Figure 2. These changes result from prestressing. At reduced O2 content autoxidation of neat and prestressed POSF-2827 fuel is indistinguishable. This result is consistent with the fact that autoxidation of this fuel is not dominated by hydroperoxides. Autoxidation of the other three stressed fuels requires less time (1-4 min) than that of the neat fuels, although the maximum reaction rates achieved at high conversion are similar for both the neat and recirculated fuels, independent of O2 content. Autoxidation rates of prestressed fuels are expected to increase for two reasons. The first is depletion of antioxidants. If antioxidants are totally consumed during prestressing, their overall reduction in a 1/1 blend is 50%; this change may be sufficient to account for the observed differences. The second is the formation of catalytic products. It is doubtful that many hydroperoxides will survive 22 min of stressing at 185 °C, but the presence of even small concentrations of dissolved metals will add to the freeradical pool. In either case, three of the four test fuels are more rapidly oxidized following prestressing, and this factor will contribute to accelerated surface fouling from the recirculated blends. As discussed above, observed differences in fouling from neat and recirculated POSF-2827 fuel cannot be explained by oxidation. The differences in fouling for the other three fuels may be due only in part to oxidation. A more important cause may be the presence of insolubles in prestressed fuel. For example, rapid deposition in neat fuel is delayed almost 4 min, despite the fact that during this time interval, more than 30% of the O2 has been consumed (see Figure 3). We have argued in previous isothermal studies of POSF-2827 fuel that fouling originates as a subset of bulk insolubles that diffuse and adhere to heated surfaces.11 Prestressing may effectively establish conditions for rapid deposition, thereby precluding the need for an induction time. Similar arguments could be made for the other three fuels. This factor may also account for the above observation that deposition from recirculated fuel displays little or no time delay. In summary, two major factors are offered to account for differences in the thermal stability of neat and recirculated fuels, both originating as changes in the prestressed component. The first relates to increases in the rate of autoxidation caused by a combination of reduced antioxidant concentration and the formation of catalytic products. The second relates to reduction or elimination of the time delay in the formation of surface deposits. Mass Balance in Single-Pass and Recirculation Modes. The current experiments provide sufficient reaction time for conversion of all of the dissolved O2. Under such conditions the total surface and bulk insolubles collected in a single pass of neat fuel can be directly compared with similar quantities collected in (11) Jones, E. G.; Balster, W. J. Energy Fuels 1993, 7, 968-977.
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Figure 4. Total insolubles formed in single-pass (a) and recirculation (b) test configurations.
the recirculation configuration. The single-pass total consists of surface and filter carbon from 1.08 L of airsaturated fuel; the recirculation total consists of two surface sums and filter carbon from 2.16 L of airsaturated fuel. The data in Figure 4 confirm that the total amount of insolubles is independent of the test configuration. It should also be noted that three of the test fuels have very similar thermal stability. Implications. Recirculation of prestressed Jet-A fuels has been shown to aggravate existing problems related to surface fouling. Small-scale tests using Jet-A fuels under simplified flow conditions have elucidated potential problems. The current simulation may represent a worst-case scenario. The recirculated blend is only half-air-saturated but consists of 50% stressed fuel. In aircraft fuel lines, large quantities of fuel with limited O2 conversion and limited stressing are recirculated, but the resultant blends are approximately air-saturated. A more realistic test scenario would involve limited O2 conversion during prestressing. We have conducted such experiments on two of the test fuels using 4 min of prestressing during which time
