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Energy & Fuels 1996, 10, 1176-1180
Thermal Stability of Jet-Fuel/Paraffin Blends Lori M. Balster and Walter J. Balster Systems Research Laboratories, An Operation of Calspan SRL Corporation, 2800 Indian Ripple Road, Dayton, Ohio 45440-3696
E. Grant Jones* Innovative Scientific Solutions, Inc., 3845 Woodhurst Court, Beavercreek, Ohio 45430-1658 Received April 19, 1996. Revised Manuscript Received August 6, 1996X
The thermooxidative stability of blends of a straight-run Jet-A fuel (POSF-2827) and a paraffinic/cycloparaffinic solvent (Exxsol D-80) has been studied in a single-pass tubular heat exchanger operated isothermally at 185 °C. Autoxidation of most blends is found to be significantly slower than that of either fuel or solvent. Surface fouling relative to the solvent is increased by addition of jet fuel; this is attributed to reactions of natural antioxidants present in the fuel. Surface fouling relative to the jet fuel is reduced following paraffin addition under conditions of partial O2 conversion; this is attributed to a lower initiation rate as the concentration of aromatics is reduced. The impact of these findings for mitigation of surface fouling in aircraft fuel lines is discussed.
Introduction Fuels in modern military aircraft serve as the primary thermal sink by dissipating waste heat from aircraft subsystems such as environmental control, hydraulics, power generation, airframe, and engine.1,2 As a consequence, fuel temperature is increased and the fuel degrades, primarily by autoxidation at temperatures below 300 °C and by pyrolysis at temperatures above 400 °C. Within the autoxidative regime, reaction products such as insoluble gums and solids cause local and downstream surface fouling and filter blockage. Surface fouling reduces heat-exchange efficiency and disrupts fuel distribution by causing hysteresis in flow-divider valves and altered spray patterns in nozzles. This problem results in costly downtime and may ultimately lead to system failure. It has been predicted that such problems will be exacerbated in future military aircraft.1 Methods that are currently available to reduce fouling include (1) special refining techniques such as hydrotreatment that improves fuel thermal stability by removing heteroatom impurities, (2) complex fuel management such as recirculation that can enhance component cooling by multiple fuel passes prior to combustion, (3) alteration of heated surfaces by alloy and finish selection and by application of specialized coatings, and (4) introduction of additives such as antioxidants to delay autoxidation, metal deactivators to reduce free radical initiation, and dispersants to reduce agglomeration. Fouling that occurs in the autoxidative temperature regime and its mitigation using approaches 3 and * Author to whom correspondence should be addressed [telephone (513) 252-4264]. X Abstract published in Advance ACS Abstracts, September 15, 1996. (1) Edwards, T.; Anderson, S. D.; Pearce, J. A.; Harrison, W. E. HighTemperature JP FuelssAn Overview. AIAA Paper 92-0683, Presented at the 30th Aerospace Sciences Meeting and Exhibit, Reno, NV, January 6-9, 1992. (2) Edwards, T. USAF Supercritical Hydrocarbon Fuels Interests. AIAA Paper 93-0807, Presented at the 31st Aerospace Sciences Meeting and Exhibit, Reno, NV, January 11-14, 1993.
4 are current topics of study in our laboratory.3,4 Dilution of one fuel with a second fuel may be viewed as a variation on additive introduction. In a companion work5 (hereafter referred to as the fuel/fuel study), blends of two aviation fuelssone hydrotreated and one straight-runswere evaluated by measurements of dissolved O2 and hydroperoxides and by quantification of insoluble products. The conclusion was that only through 10-fold dilution of a lesser quality fuel with a prime hydrotreated fuel could a more thermally stable fuel be achieved at 185 °C. This finding precluded blending as a cost-effective method for utilizing higher molecular weight refinery byproducts and was consistent with reports of reduced stability following blending.6-10 However, this result differed with recent conclusions of Zabarnick and co-workers11 that dilution with a hydrotreated fuel may yield a simple and relatively inexpensive method of improving the thermal stability of a highly depositing fuel. The different conclusions were of additional concern because both experimental studies were conducted isothermally using the same straight-run fuel. Zabarnick’s conclusions were based on lower temperature (140 °C) experiments, diluting not with a hydrotreated fuel but rather with a paraffinic/cycloparaffinic solvent, Exxsol D-80. (3) Jones, E. G.; Balster, W. J.; Rubey, W. A. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1995, 40 (4), 655-659. (4) Jones, E. G.; Balster, L. M.; Balster, W. J. Energy Fuels 1995, 9, 906-912. (5) Jones, E. G.; Balster, L. M.; Balster, W. J. Energy Fuels 1996, 10, 509-515. (6) Gomes, H. O.; Pereira, R. C. L. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1994, 39 (3), 917-922. (7) Hazlett, R. N. Fuel Sci. Technol. Int. 1988, 6, 185-208. (8) Datschefski, G. Fuel Sci. Technol. Int. 1988, 6, 609-631. (9) Speck, G. E. Feasibility of Marine Diesel Fuel as an Emergency Aircraft Fuel. Report NAPC-PE 44; Naval Air Propulsion Center, Trenton, NJ, Feb 1981. (10) Goetzinger, J. W.; Ripley, D. L.; French, C. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1989, 39 (1), 804-808. (11) Zabarnick, S.; Zelesnik, P.; Grinstead, R. R. J. Eng. Gas Turbines Power 1996, 118, 271-277.
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Thermal Stability of Jet Fuel Blends
Energy & Fuels, Vol. 10, No. 6, 1996 1177
Table 1. Fuel/Solvent Properties type treatment total S (%) flash point (°C) sp gravity 15.6/15.6 °C aromatics (vol %) cycloparaffins paraffins distillation 10% (°C) distillation 50% (°C) distillation 90% (°C)
POSF-2827
Exxsol D-80
Jet-A straight run 0.079 50 0.807 19
paraffinic solvent
179 207 245
0.0003 73 0.79 0.8 41 58 206 212 225
It appeared that the discrepancy might be related to different reaction conditions or to fundamental differences between the use of a hydrotreated fuel and a paraffinic solvent as a diluent. Both diluents are subject to rapid oxidation resulting from the absence of natural antioxidants, but they differ in their aromatic content. On the basis of oxidation rates, we expected similar behaviors in the two blending systems. The goals of the current investigation were to resolve the above-stated difference and to gain further insight into the thermal stability of fuels. The approach taken was to study the thermal stability of fuel/solvent blends at 185 °C and to compare the results with those from Zabarnick’s11 lower temperature (140 °C) flask tests and also with those from our analogous fuel/fuel-blending experiments. Experimental Section Table 1 summarizes some physical properties of the fuel and solvent blendstocks. Exxsol D-80, consisting only of paraffins and cycloparaffins, contains neither aromatics nor natural sulfur inhibitors. Blends were made by volume and were designated according to the percentage of POSF-2827. Each was initially saturated with respect to air at room temperature and used after passage through a 15-µm filter. All fuel stressing occurred at 185 °C under 2.3 MPa of pressure during slow flow through 0.318-cm-o.d., 0.216-cm-i.d. tubing that was clamped tightly within a Cu block heat exchanger. The details of the near-isothermal flowing test rigs (NIFTR) and test procedures have been summarized previously5 and are reviewed here only briefly. Dissolved O2 and surface deposition rates were measured in separate experiments. O2 was measured in-line at system pressure using a GC technique12 following varied residence times in a heated tube. Tubing used in these experiments was passivated by the Silcosteel13 process to minimize possible surface catalytic effects from the stainless steel walls. Normal heptane and hexadecane were obtained from Aldrich Chemical Co. and were used as received. Surface deposition was studied following fuel passage through 183-cm heated stainless steel tubes; a fuel flow rate of 0.20 mL min-1 provided up to 28 min of stressing. During the course of 72-h tests, 864 mL of fuel was passed through the heat exchanger. Deposits collected on the inner walls were quantified by surface carbon burnoff of 5.1-cm tube segments. Under isothermal conditions, reaction or stress time is proportional to distance along the tube and can be calculated from the flow rate and tubing dimensions. The total surface carbon (micrograms per milliliter) was obtained as a function of stress time by summing carbon along the tube. Bulk insolubles collected on two sets of in-line Ag membrane filters (0.45 and 0.20 µm) were similarly quantified by surface carbon burnoff. (12) Rubey, W. A.; Striebich, R. C.; Tissandier, M. D.; Tirey, D. A.; Anderson, S. D. J. Chromatogr. Sci. 1995, 33, 433-437. (13) Silcosteel tubing, Restek Corp., Bellefonte, PA.
Results and Discussion Autoxidation in Blends. Reaction of O2 and formation of ROOH involve a series of free radical chain processes. A simplified mechanism for autoxidation includes initiation (reactions 1 and 2), propagation (reactions 3 and 4), and termination (reaction 5).
RH + O2 f R•
(1)
ROOH f RO• + HO•
(2)
R• + O2 f ROO•
(3)
ROO• + RH f ROOH + R•
(4)
ROO• + ROO• f nonradical products
(5)
On the basis of observed inhibited autoxidation and the correlation between deposition and autoxidation of POSF-2827 fuel4 as well as reported14,15 ties between S-containing inhibitors and insolubles, reaction 6 has been introduced5 as a first step leading to insolubles in the neat fuel.
ROOH + R′S f insolubles
(6)
An excess of S-containing natural inhibitors, R′S, is available in POSF-2827 fuel to limit [ROOH]. This mechanism has been used to explain autoxidation and deposition occurring in fuel/fuel blends.5 Figure 1 shows the disappearance of dissolved O2 for each blendstock and 12 blends. The solvent, with no natural inhibitors, undergoes rapid autoxidation that is complete within 2 min. In contrast, the straight-run fuel undergoes slow autoxidation, requiring more than 13 min for completion. Autoxidation of each component is significantly altered by blending. Blends obtained by adding solvent to fuel undergo increasingly slower autoxidation, culminating in an approximately 2-fold reduction for the 30% blend. Additional dilution causes further slowing of the initial autoxidation rate, but reaction for these fuel-lean blends eventually accelerates at higher conversion. As the blends become more fuel-lean, this occurs earlier. The effects of dilution of fuel and solvent are summarized in Figure 2 by considering the reaction time required to achieve the same fraction of O2 conversion. First, the increased times in the fuel-lean blends on the left in Figure 2 are delays that might be expected because the concentration of natural inhibitors increases as small quantities of fuel are added. These natural inhibitors include primary antioxidants such as phenols or aromatic amines that intercept and stabilize free radicals, thereby slowing propagation, and also secondary antioxidants that operate by destroying ROOH and reducing self-initiation. In neat POSF-2827 fuel the concentration of hydroperoxides, [ROOH], does not increase under the current test conditions.5 Thus, inhibition may occur primarily by reaction 6 in a secondary antioxidant effect. Very similar antioxidant behaviors and delays in autocatalysis have been observed by both headspace pressure measurements11 at (14) Kendall, D. R.; Mills, J. S. Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 360-366. (15) Bolshakov, G. F. Sulfur Rep. 1987, 7 (Part 5), 379-392.
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Figure 3. Surface and bulk insolubles evaluated near complete conversion.
Figure 1. Autoxidation of blends (a) >40% and (b)
