Energy & Fuels 1989, 3, 262-267
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hours; and k is the first-order rate constant. The square of the correlation coefficient, r2,for the data is 0.965. The calculated values for Rol and Rmlare 15.45 and 16.50 s-l, respectively. Rmlrepresents the spin-lattice relaxation rate for a system in which the intermolecular interactions are at equilibrium. The first-order rate constant for the molecular association is 0.0139 h-l at a mean temperature of 32 “C. The only interpretation attached to the parameters in eq 5 is that the “driving force for a ~ s o c i a t i o n ”is~the ~ remaining nonassociated molecules and that the association asymptotically approaches the value of Rmlas t m. That is, mechanistically, the association of molecules is a pseudo-first-order rate process in which the amount of associated molecules exceeds the amount of nonassociated molecules.
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(15)Tahiri, M.; Sliepcevich, C. M.; Mallinson, R. G. Energy Fuels 1988,2, 93-100.
To a first approximation, eq 2 may be used to explain the data for the extracted tar sand bitumen. At a constant temperature, the increase in the relaxation rate with time suggests that the molecular weight as well as the kinematic viscosity (TIP) increases after quenching. However, any increase in viscosity is caused by a decrease in the mobility of molecules that results from an increase in the molecular weight of these molecules in the semiliquid phase. It is assumed that the density does not change significantly with reassociation of the molecules. Thus, reassociation of the bitumen molecules begins after quenching and continues for nearly a week before the intermolecular-interaction equilibrium is reestablished. Acknowledgment. We express thanks and appreciation to the U.S. Department of Energy for funding of this work under Cooperative Agreement No. DE-FC2183FE60177 and to Turner for the treatment of the kinetic data.
Methods for Quantifying JFTOT Heater Tube Deposits Produced from Jet Fuels Robert E. Morris*>+and Robert N. Hazlett* Chemistry Division, Navy Technology Center for Safety and Survivability, Code 6180, U S . Naval Research Laboratory, Washington, D.C. 20375-5000, and Hughes Associates, 2730 University Boulevard West, Wheaton, Maryland 20902 Received August 29, 1988. Revised Manuscript Received November 18, 1988
One measure of the thermal stability of aviation fuels is the quantity of deposits formed on heated metal surfaces. In accelerated stability tests conducted in accordance with the JFTOT procedure (ASTM D3241), the rating methods currently employed involve either visual comparisons or measurements of reflected light by the tube deposit rater (TDR), both of which are sensitive to deposit color and surface texture. In this study, deposits formed on stainless-steel JFTOT heater tubes have been examined by the TDR, a gravimetric carbon combustion method, and two new nondestructive techniques for determining deposit volumes based on measruements of dielectric strength and optical interference. Measurements of total carbon content by combustion were used as a reference. It was found that the dielectric and interference methods correlated well with the combustion analyses and each other, while the total TDR often yielded misleading results.
Introduction The thermal oxidation of liquid fuels is often accompanied by the formation of insoluble reaction products, either as suspended particulate or as a gum that adheres to heated surfaces. Modern aircraft engine designs and aerodynamic heating of wing surfaces place severe thermal stress on the fuel, increasing the likelihood of the formation of insoluble deposits. Aircraft fuel system deposits can be responsible for a variety of problems including decreased efficiency of engine heat exchangers, seizing of fuel control valves and injector fouling. It is known that thermally initiated fuel degradation is accelerated by the presence of oxygen through autoxidative processes involving free-radical chain reactions. The jet ‘U.S. Naval Research Laboratory. Hughes Associates. f
0887-0624/89/2503-0262$01.50/0
fuel thermal oxidation tester (JFTOT) is widely used to characterize the thermal oxidation stability of a fuel. In the JFTOT, the fuel is stressed under conditions of high oxygen availability and slowly increasing temperatures. The quantities of insoluble products formed under these conditions constitute a measure of the deposit-forming characteristics of the fuel. In accordance with standard ASTM D3241 test procedures,’ the formation of adherent insolubles on the heated tube is characterized by visual comparison with color standards. The highly subjective nature of the visual method of rating heater tube deposits was revealed in a round-robin effort conducted by the Coordinating Research Council.2 The poor precision of visual ratings from unusual and highly colored deposits (1) ASTM Thermal Oxidation Stability of Aviation Turbine Fuels (JFTOT Procedure). In Annual Book of ASTM Standards; ASTM: Philadelphia, PA, 1976;Part 25, ASTM D3241-74.
0 1989 American Chemical Society
JFTOT Heater Tube Deposits
resulted in random errors that exceeded the differences between the values. To increase the reliability of the measurement, the tube deposit rater was developed. The tube deposit rating (TDR) is based on the measurement of the attenuation of reflected white light by a photocell. The instrument is calibrated so that high TDR values correspond to thick coatings that have low reflectance. From comparisons with deposit thicknesses measured by auger spectroscopy, neither the visual rating method nor the TDR were found3 to be adequate in quantifying tube deposits. While less subject to operator judgement than the visual rating method, the TDR can be influenced by the optical properties of the deposit. Quantification of tube deposits by combustion to carbon dioxide has been investigated, with the assumption that the deposit is composed primarily of carbon. Measurements of total carbon contents per unit area per unit time have been used4 to study the influences of dissolved oxygen on the rates of deposit formation from thermally stressed jet fuels onto 316 stainless-steel tubes. These studies were later extended to examine the effects of trace amounts of sulfur- and nitrogen-bearing compound^^-^ on deposition rates. Combustion analyses of total carbon content have been utilized' to quantify deposition rates from jet fuels stressed under a wide variety of experimental conditions in a special test apparatus. Carbon was then determined from deposits on sintered stainless-steel filters and from the inner walls of heated 316 stainless-steel tubes. A lower detection limit of 200 pg of carbon on the tube sections was reported. The precision of combustion analyses on standard aluminum JFTOT heater tubes has been shown8 to be poor, compared to stainless steel, due to the difficulty with which quantitative removal of carbon from the aluminum surface can be attained. This behavior seems related to the relatively thick porous layer of aluminum oxide that coats aluminum metal surfaces. This may account, in part, for the findings that the amount of carbon formed on stainless-steel tubes thermally stressed in the JFTOT generally exceeded8 that from aluminum tubes by a factor of 2. In addition, migration of magnesium compounds in 6061 T 6 alloy aluminum heater tubes at elevated temperatures has been reported to inhibit deposition. Heater tubes comprised of 304 stainless steel do not form porous oxide coatings, allowing much lower detection limits, nor is magnesium inhibition possible. However, standard JFTOT procedures employ 21/2-in.aluminum heater tubes, and a method for accurately quantifying deposits is needed. Two techniques for determining heater tube deposit volumes have been developed. One techniqueg is based (2) Tube Deposit Rating Techniuqes Panel. 'Investigation of Techniques for Evaluating Oxidative Stability Deposits of Aviation Turbine Fuel"; CRC Project No. CA-43-67; CoordinatingResearch Council, Inc.: New York. New York. Oct 1974. (3) Martel, C. R.; Bradley, R. P. "Comparison of Rating Techniques for JFTOT Heater Tube Deposits"; Interim Report No. AFAPL-TR-7549, Air Force Aero Propulsion Laboratory, Wright-PattersonAir Force Base: Dayton, OH, Oct 1975. (4) Taylor, W. F. Ind. Eng. Chem. Prod. Res. Deu. 1974,13,133-138. (5) Taylor, W. F. Ind. Eng. Chem. Prod. Res. Deu., 1976, 15, 64-68. (6) Taylor, W. F.; Frankenfeld,J. W. Ind. Eng. Chem. R o d . Res. Deu. 1978, 17, 86-90. (7). Giovanetti,A. J.; Szetela, E. J. "Long Term Deposit Formation in Aviation Turbine Fuel at Elevated Temperature"; NASA Report No. CR-179579;United Technologies Research Center: East Hartford, CT, Apr 1985. (8) Kendall, D. R.; Mills, J. S. "The Influence of JFTOT Operating Parameters on The Assessments of Fuel Thermal Stability"; SAE Paper No. 851871; Society of Automotive Engineers: Warrendale, PA, 1985. (9) Stavinoha, L. L.; Barbee, J. G.; Yost, D. M. "Thermal Oxidative Stability of Diesel Fuels"; BFLRF Report No. 205, Defense Technical Information Center No. AD A173850; Southwest Research Institute: Feb 1986.
Energy & Fuels, Vol. 3, No. 2, 1989 263 Table I. Selected Properties of Jet A Test property value total acidity, mg of KOH/g
