Ind. Eng. Chem. Res. 1996, 35, 837-843
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Application of JFA-5 as an Antifouling Additive in a Jet-A Fuel E. Grant Jones,*,† Walter J. Balster, and Larry P. Goss† Systems Research Laboratories, An Operation of Calspan SRL Corporation, 2800 Indian Ripple Road, Dayton, Ohio 45440-3696
A commercial Jet-A aviation fuel, POSF-2827, was treated with an antifouling additive package, DuPont Jet Fuel Additive No. 5 (JFA-5), at a concentration of 12 mg/L. Fouling of heated stainless-steel (304) tubes from the neat and treated fuels was studied over the temperature range 155-300 °C using a single-pass heat exchanger and at 185 °C in flask tests. The rate of autoxidation in the treated fuel was slowed over the temperature range 155-225 °C, but total surface fouling evaluated after 100% conversion of dissolved oxygen was not reduced. Significant reduction (factor of 2-3) in surface fouling was observed at the higher temperatures, 255 and 300 °C. This improvement was traced to one component of the package, namely, a metal deactivator present at 2 mg/L. Additive packages such as JFA-5 which contain antioxidant, dispersant, andsin particularsmetal-deactivator components may reduce Jet-A fuel fouling in certain temperature regimes. Introduction The proprietary DuPont Jet Fuel Additive No. 5, known as JFA-5, is used in military aviation fuels to improve thermal oxidation stability. In particular, since 1970 (according to MIL T-25524), it has been required for use in a U.S. Air Force specialty fuel, Thermally Stable Jet Fuel (JPTS), at concentrations of 9-12 mg/L (Martel, 1987). The antifouling additive package JFA-5 is an ashless mixture of polymers, organic amines, and amides in kerosene (Martel, 1987); the package contains dispersant, antioxidant, and metal-deactivator components. A recent Materials Safety Data Sheet lists nonproprietary components and their percentages (DuPont, 1991). Treatment with JFA-5 has been reported to reduce Coker-tube deposit ratings and filter pressure differentials (DuPont, 1960). Recently, reductions in surface deposition on 316-stainless-steel surfaces have been reported (Heneghan et al., 1993) through the use of 12 and also 50 mg/L JFA-5 in a Jet-A aviation fuel (POSF-2827) during studies conducted at 300 °C in a single-pass heat exchanger. The Jet-Fuel ThermalOxidation Tester (JFTOT) breakpoint of a shale-derived JP-4 fuel has been reported to be improved by use of JFA-5 (Boos and Dues, 1986). As part of a broad-range investigation into thermal oxidation stability of aviation fuels, POSF-2827 has served as a reference since it meets all USAF specifications and is amenable to study because it has a propensity to foul heated surfaces (Heneghan et al., 1993; Jones et al., 1993; Jones and Balster, 1993). Quantification of surface and bulk insolubles formed in POSF-2827 fuel during thermal stressing under nearisothermal conditions has provided a very detailed picture of this fuel at 185 °C (Jones et al., 1993; Jones and Balster, 1994) and an outline of its behavior over the temperature range 155-225 °C (Jones and Balster, 1994). Deposition profiles as a function of stress duration have been found to correlate closely with autoxidation. Knowledge of autoxidation and the temperature dependence for the formation of insolubles has established POSF-2827 fuel as a baseline for ranking the performance of individual additives and additive pack† Present address: Innovative Scientific Solutions, Inc., 3845 Woodhurst Court, Beavercreek, OH 45430. FAX: (513) 4299734.
0888-5885/96/2635-0837$12.00/0
ages in reducing surface and bulk fouling (Anderson et al., 1994). From a fundamental standpoint, documented changes resulting from the use of additives may aid in understanding the complex routes leading to insolubles and also in developing new additive combinations to minimize fouling. We now report the behavior of JFA-5 in mitigating surface and bulk insolubles in the Jet-A aviation fuel POSF-2827 over a broad temperature range. The goal of the current study was to make a simple one-to-one empirical comparison between POSF-2827 fuel, neat and treated with 12 mg/L JFA-5. Comparison is predicated on the quantity of surface and filtered insolubles measured after conversion of 100% of the dissolved oxygen, primarily in dynamic tests. Results include those from (1) near-isothermal (low-flow-rate) dynamic deposition experiments at 155-225 °C (10-deg increments), (2) direct measurements of dissolved oxygen as a function of stress time over the temperature range 155-205 °C, (3) non-isothermal (higher-flow-rate) dynamic deposition experiments at 255 and 300 °C, (4) particle-size measurements at 185 °C, and (5) static flask experiments conducted isothermally at 185 °C. Experiments at 255 and 300 °C were conducted to determine whether earlier findings (Heneghan et al., 1993) that JFA-5 reduces deposition on 316-stainlesssteel surfaces also hold for 304 stainless steel and to ascertain which of the three major constituents of JFA-5snamely, antioxidant, dispersant, or metal deactivatorsmakes the most significant contribution. In order to aid in visualizing the effects of JFA-5, most data are presented as graphs that directly compare the results from both neat and treated fuel. Cases where the neat-fuel results have been reported elsewhere are referenced and briefly reviewed along with the impact of fuel treatment. It should be noted that additive performance is specific to each fuel and the current results relate to JFA-5 performance in a single fuel. Experimental Section The baseline is a commercial straight-run distillate Jet-A fuel designated POSF-2827. It meets all USAF specifications and has a breakpoint of 539 K from the JFTOT. Formation of insolubles has been attributed to autoxidation reactions of inherent sulfur compounds (0.079 wt %) which may also tend to inhibit autoxidation © 1996 American Chemical Society
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Ind. Eng. Chem. Res., Vol. 35, No. 3, 1996
Figure 1. Schematic diagram of near-isothermal flowing test rig (NIFTR).
(Jones and Balster, 1993; Jones et al., 1992). Fuel was used without prior filtration. The proprietary additive JFA-5, obtained from DuPont, was used at a concentration of 12 mg/L. The individual components comprising JFA-5snamely, antioxidant (PL-1694), dispersant, and metal deactivator (MDA; N,N′-disalicylidene-1,2-propanediamine)swere received in a carrier solvent and used in selected experiments at concentrations of 3, 4.9, and 2 mg/L, respectively. The concentration selected for each component was based upon its inherent level in JFA-5 (DuPont, 1994). The instrument and techniques used for the dynamic experiments have been described previously (Jones et al., 1993; Jones and Balster, 1993) and will be reviewed here only briefly. All dynamic experiments were conducted using air-saturated fuel flowing through 304stainless-steel tubing clamped in the copper block of the Near-Isothermal Flowing Test Rig (NIFTR) shown in Figure 1. The instrument is a single-pass heat exchanger with a 81.3-cm heated section; when used under conditions of slow fuel-flow rates and low block temperatures, this instrument provides near-isothermal conditions for reaction along the tube. System pressure was maintained above 2.3 MPa in order to ensure a single liquid phase. Fuel was passed through 0.318-cm (0.125in.) -o.d., 0216-cm (0.085-in.)-i.d. commercial tubing and an in-line 0.45-µm filter. In non-isothermal experiments at higher temperature, a second in-line filter (0.2 µm) was used. Surface deposits on 5.1-cm (2-in.) segments of tubing and insolubles on the filter were quantified by carbon burnoff using a LECO RC-412 surface-carbon analyzer. Reaction time or stress duration in isothermal tests was determined from the flow rate, tube dimensions, and location along the tube axis. The calculated times were reduced by 15% to compensate for fuel expansion. Deposition rates were determined from the amount of carbon in each segment, the increment of stress time within that segment, and the total quantity of fuel passed. Each segment provides one data point, representing the deposition rate averaged over the entire test time (i.e., usually 6 h). Deposition rate is expressed in units of micrograms of carbon per unit stress time per unit volume of fuel passed through the system. In the
non-isothermal tests at 300 °C, deposition rate is expressed in units of micrograms per square centimeter per hour of testing, and the abscissa is shown as distance along the tube rather than stress duration. Dissolved oxygen was measured in separate experiments using the entire 81.3-cm tube as a reaction cell; stress duration was changed by varying the flow rate. An in-line gas chromatographic detector, designed and built by Rubey and co-workers (1995), was employed for oxygen measurements. Photon correlation spectroscopy (PCS) was used for submicron particle sizing after the isothermally stressed fuel had cooled to room temperature. The instrument and techniques were based on those outlined by O’Hern et al. (1993). Static tests were conducted isothermally under ambient pressure at 185 °C using a Pyrex flask with continuous stirring and oxygen sparging. Bulk insolubles were collected by aliquot filtration every 30 min, and surface deposits were collected on a series of 302B-stainless-steel disks; each disk was suspended in the fuel for a different period of time. Both surface and bulk insolubles measured in flask tests exhibited a zeroorder growth with time; thus, pseudo-zero-order rates could be determined. These techniques have been described previously (Jones et al., 1992). Results and Discussion Near-Isothermal Dynamic Studies. A comparison of the 6-h-averaged surface-deposition rates at 185 °C (flow 0.25 mL/min) and at 205 °C (0.5 mL/min) is shown in Figure 2. These results are typical of experimental data obtained over the temperature range 155-225 °C. Differences between surface deposition from neat and from treated fuel are evidently minor at 205 °C but are more pronounced at 185 °C. Generally, deposition profiles from the treated fuel extend to longer times at lower temperatures. The integrated area under these profiles is the total surface deposit arising from complete reaction of the dissolved oxygen present in 1 mL of fuel saturated initially with respect to air at room temperature. In other experiments the quantity of deposits was reduced to a negligible level (
