Effects of Extended Duration Testing and Time of Addition of N

Energy & Fuels 1998, 12, 129-138

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Effects of Extended Duration Testing and Time of Addition of N,N′-Disalicylidene-1,2-propanediamine on Jet Fuel Thermal Stability As Determined Using the Gravimetric JFTOT Seetar G. Pande* Geo-Centers, Inc., Lanham, Maryland 20706

Dennis R. Hardy Navy Technology Center for Safety and Survivability, Code 6181, Naval Research Laboratory, Washington, D.C. 20375 Received June 23, 1997. Revised Manuscript Received September 26, 1997X

Two studies were conducted on the effects of the metal deactivator, N,N′-disalicylidene-1,2propanediamine (MDA) on jet fuel thermal stability. Study 1 focused on the effects of MDA on extended duration thermal stability testings ranging from 120 to 152 h. The test fuel was a Jet A, and the effects of MDA were examined with and without the addition of the antioxidant, BHT (2,6-di-tert-butyl-4-methylphenol) as well as with and without added copper. For study 2, the thermal stability test duration was 2.5 h, and the effect of time of addition of MDA, which was also investigated in study 1, was further examined in a test matrix that comprised two JP-5 fuels, two sources of copper, and three concentrations of MDA. Thermal stability was determined using the gravimetric JFTOT. The results of the extended duration testings indicate that even at the relatively high MDA concentrations examined (15 and 35 ppm), MDA exhibited a beneficial effect in the non copper doped commercial (Jet A) and Navy type fuel (Jet A + BHT). The results of the time of MDA addition studies indicate that early addition of MDA is more effective than late addition, for copper-contaminated stored jet fuels.

Introduction The beneficial effects of the metal deactivator additive, N,N′-disalicylidene-1,2-propanediamine, commonly known as MDA, in counteracting the detrimental effects of copper on fuel thermal stability are well recognized.1 However, on extended duration thermal stability tests of a non copper doped Jet A-1 fuel, MDA was interpreted as having no effect after 30 h,2 and, for a non copper doped JP-5 fuel, as having a potentially detrimental effect after 120 h.3 These findings are based on injector feed-arm rig (IFAR) studies2 and on flow reduction studies of two atomizers,3 respectively. Because of the possible impact of these results on the use of MDAswhich is both an optional military specification (MIL-T-5624) thermal stability additive and an optional commercial fuel additive (ASTM D1655)sthe Abstract published in Advance ACS Abstracts, November 15, 1997. (1) Hazlett, R. N. Thermal Oxidation Stability of Aviation Turbine Fuels; ASTM Monograph 1; American Society of Testing and Materials: Philadelphia, PA, 1991; Chapter 1X. (2) Kendall, D. R.; Houlbrook, G.; Clark, R. H.; Bullock, S. P.; Lewis, C. Thermal Degradation of Aviation Fuels in Jet Engine Injector Feed Arms. Part IsResults from a Full-Scale Rig. Paper 87-IGTC-49. Presented at the International Gas Turbine Congress, Tokyo, Japan, October 26-31, 1987. (3) Moses, C. A. “Effect of a Metal Deactivator Fuel Additive on Fuel Deposition in Fuel Atomizers at High Temperature”; Interim Report 281; Belvoir Fuels and Lubricants Research Facility (SwRI) Southwest Research Institute, San Antonio, TX, August 1992. X

effect of MDA on extended duration testing was reinvestigated (study 1). In the reinvestigative study, we revised the protocol used in the initial extended duration testings.2,3 These revisions, which include storage of the test fuels prior to determining their thermal stabilities, the time of addition of MDA, and its concentration are subsequently described. Revision 1 involved storing the test fuels in the presence of the test additive(s) prior to determining their thermal stabilities. The objective of this revision was to decrease the neat fuels’ inherent thermal stabilities such that the effects of the test additives could be better differentiated. This approach is based on the results of an earlier study,4 which indicated that precursors that lead to thermal instability are predominantly formed during long-term storage of the fuel in the presence of copper, and not during the thermal stability test itself. Revision 2 involved the early addition of MDA and hence the use of a freshly refined jet fuel. This adoption is also based on the results obtained from the earlier study4 wherein we found that the early addition of MDA to be especially important for fuels that are stored in the presence of copper. The early addition of MDA is consistent with Pedersen’s recommendation5 for gaso(4) Pande, S. G.; Hardy, D. R. Energy Fuels 1995, 9, 177-182. (5) Pedersen, C. J. Ind. Eng. Chem. 1949, 41, 924.

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130 Energy & Fuels, Vol. 12, No. 1, 1998

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Table 1. Overall Test Matrix for the Two Studies Conducted extended duration testing (study 1)

early vs late MDA addition follow-up study (study 2)

no. and type of fuel: id room temp storage period neat fuel test additivesb antioxidant (AO) MDA, no AO AO + MDA before storage AO + MDA after storage

1 freshly refined Jet A: Jet A 13-16 months a a BHT added in lab at 24 mg/L at 35 ppm MDA at 35 ppm MDA at 15 ppm

2 freshly refined JP-5s: TX, LA 14-15 months non copper doped stored fuels a a AO added at refinery at 17 mg/L na;c JP-5s contain an AO MDA at 20 ppm MDA at 15 ppm

copper doped stored fuels Cu + AO + MDA before storage

Cu as copper wires MDA at 35 ppm

Cu as CuEA and Cu-Ni rods MDA at 20 ppm + 400 ppb Cu (CuEA) MDA at 20 ppm + 6 Cu-Ni rods MDA at 40 ppm + 2000 ppb Cu (CuEA) MDA at 40 ppm + 18 Cu-Ni rods MDA at 15 ppm in all cases

test matrix

Cu + AO + MDA after storage grav JFTOT test duration

ndd min 120 h max 152 h

2.5 h

a Tests conducted as identified. b Unless otherwise stated, all additives were added early, i.e., prior to storage. Antioxidants used at concentrations shown in all tests conducted; and, MDA concentrations are as specified. c na: not applicable. d nd: not determined.

lines, viz., MDA should be added at the earliest possible moment. The late addition of MDA, i.e., after storage, was also examined to determine the effect of time of addition of MDA to non copper doped fuels. Revision 3 was the addition of MDA at high concentrations (∼35 ppm) compared to the military specification6 level of 5.8 mg/L. High concentrations of MDA were used for the following reasons: (a) to compensate for possible depletion on the long-term storage conducted, e.g., losses of MDA may occur due to adherence on the walls of the samples’ containers; and (b) to ensure that the amounts of MDA added will be in excess of the concentration of solubilized copper in the copper dissolution studies. In the extended duration thermal testings (study 1), to facilitate our understanding of the effects of MDA and the antioxidant, BHT, (2,6-di-tert-butyl-4-methylphenol) on jet fuel thermal stability, the individual and combined effects of MDA and BHT were examined using a non copper doped Jet A fuel. In the examination of the combined effects of MDA and BHT, the effects of early versus late addition of MDA (i.e., addition before and after storage) were investigated. And, because of MDA’s well-known effectiveness in improving the thermal stabilities of copper contaminated fuels, the effect of MDA, BHT, and coppersadded at the onset of storage at room temperatureswas also studied. In addition, the effect of early versus late addition of MDA on thermal stability was further examined in a second study (study 2) using neat and copper-doped JP-5 fuels, but at typical test durations of 2.5 h. Also, similar revisions were adopted as for study 1. The results of the time of addition of MDA for both studies 1 and 2 are discussed collectively in study 2 because of their common link. The thermal stabilities of the neat and doped stored test fuels in both studies were determined using the gravimetric JFTOT. This bench test device gives the total weight, in mg/L, of the surface and bulk fuel deposits formed (i.e., deposits formed on a stainless steel strip and the filterables, respectively). (6) Military specification, MIL-T-5624.

Experimental Section Test Matrixes for Studies 1 and 2. Common to the test matrices for studies 1 and 2 are the following: (a) the use of jet fuels that were freshly refined, where the term freshly refined refers to an interim period of 0-4 weeks after refining; (b) the early addition of the test additives, i.e., prior to storage unless otherwise stated; (c) the addition of MDA at relatively high concentrations ranging from 15 to 40 ppm: 15 ppm was used only for those studies involving the late addition of MDA, i.e., after storage, since compensation for possible losses due to depletion on storage was not relevant; and (d) storage of the test fuels at room temperature conditions for a period of approximately 13-16 months prior to determining their thermal stabilities. In study 1, storage of the additized test fuels extended over an interim period of a few months due to the length of time taken to complete the extended testings of the neat and doped fuels in the test matrix. The overall test matrices for both studies 1 and 2 are summarized in Table 1. Some specifics of each study’s test matrix are also briefly described below. Test Matrix for Study 1: Extended Duration Thermal Stability Testing. For this study, the test matrix comprised a field production Jet A fuel, and three additives (BHT, MDA, and copper) which were applied at the concentration levels stated below. The test samples and the order in which the extended duration testings were conducted are listed from first to last as follows: Tests Conducted: Additives and Concentration Levels. 1. None, i.e., the neat fuel for reference. 2. Copper + BHT at 24 mg/L + MDA at 35 ppm. Copper was added in the form of copper wires. The level of copper absorbed during 1 year’s storage, in the presence of BHT and MDA, ranged from approximately 480 ppb after 1.4 months to 4800 ppb after 15 months (see Figure 1). 3. BHT at 24 mg/L + MDA at 35 ppm (early addition of MDA cf., to test 5 below). 4. BHT at 24 mg/L. 5. BHT at 24 mg/L + 15 ppm MDA added after storage (late addition of MDA). 6. MDA at 35 ppm. Test Matrix for Study 2 (see Table 1): A Follow-up Study of Early versus Late Addition of MDA. For this study, the effect of early versus late addition of MDA was examined in both the neat and copper doped fuels. The test matrix comprised the following: two JP-5 fuels; two sources of copper, at two concentration levels/amounts; and three concentration levels of MDA, depending on the copper concentration, and the time of addition. Specifically, two concen-

Effect of MDA on Jet Fuel Stability

Figure 1. Fuel absorption of copper on storage. The Jet A test fuel was stored in the presence of copper as copper wires, BHT at 24 mg/L, and MDA at 30-35 ppm. trations of MDA (20 and 40 ppm) were used in the early MDA addition studies because the test fuels were doped with copper at two levels. For the late addition of MDA investigations (i.e., addition after storage), MDA was added at 15 ppm. Materials for Studies 1 and 2, as Identified. All materials were used as received, unless otherwise specified. Test Fuels (Table 2). For study 1, although no data are available on the specific compositional properties of the Jet A test fuel used in the extended duration studies, the commercial specifications for Jet A fuels, in general, are shown in Table 2. For study 2, the two freshly refined fuels are identified for convenience as TX and LA based on their refinery location, which were Texas and Louisiana, respectively. Note that these are the same two fuels that were used in a previous investigation.7 This current study is an extension of that investigation. On the basis of pertinent compositional and properties data (Table 2), both TX and LA can be categorized as typical JP-5 field production jet fuels.6 The copper contents of all three test fuels were negligibly low, viz., 2-4 ppb. BHT, 2,6-di-tert-butyl-4-methylphenol, was obtained from Pfaltz and Bauer, Inc. (study 1). N,N′-Disalicylidene-1,2-propanediamine commonly known as MDA was obtained as a powder from Pfaltz and Bauer (studies 1 and 2). Copper. For study 1, copper was used in the form of wires from Carlo Erba. For study 2, the following two sources of copper were used: (a) Copper(II) ethyl acetoacetate (CuEA) from Eastman Kodak was added at two concentrations of copper: 400 and 2000 ppb. CuEA was used because it is a convenient source of fuel soluble copper. (b) 90/10 copper/nickel (Cu-Ni) alloy was from Hillman Brass and Copper Co. The alloy was used in the form of rods to facilitate mixing (see Stirring, below). The dimensions of each rod were 2.54 cm × 0.95 cm diameter. In an attempt to obtain different copper concentrations, 6 and 18 rods were used. Cu-Ni was also used because it realistically represents a source of copper contamination in JP-5 fuels. Specifically, 90:10 copper-nickel (Cu-Ni) alloy is used in the fuel piping systems of U.S. aircraft carriers and air capable ships.8,9 Procedures. Pretreatment. The Cu-Ni alloy rods were cleaned by immersing in a mixture of equivolumes of toluene, acetone, and methanol, then rinsed with heptane, and airdried. (7) Pande, S. G.; Hardy, D. R. Energy Fuels 1997, 11, 1019. (8) Shertzer, R. “Investigation of the Reduction of Thermal Stability of Fuel by Copper Contamination on Aircraft Carriers”; Naval Air Propulsion Test Center, NAPTC-PE-14, January 1973. (9) Morgan, R.; Berger, W.; Meehan, R. “Determine the Thermal Stability Properties of Fuel and Copper Concentration”; Pratt and Whitney Aircraft Materials Development Laboratory Work Request, No. 15493, December 1970.

Energy & Fuels, Vol. 12, No. 1, 1998 131 Filtration of Test Fuels. In all cases, the fuels were not filtered prior to the addition of the test additives, but prior to and after conducting the gravimetric JFTOT tests (studies 1 and 2). Two Magna nylon membranes of 0.8 micron pore size were used for filtration in each case. Storage. Because of the stability of the neat test fuels, unless otherwise indicated, the neat and doped test fuels were systematically stored at room temperature (20 °C) and atmospheric pressure for approximately 13-16 months, and hereafter will be referred to simply as storage. Also, unless otherwise indicated, storage was conducted with and without the test additives present, prior to determining their resultant thermal stabilities. Stirring. Copper wires (study 1) and 90:10 Cu-Ni rods (study 2) were used to simplify mixing of the bulk fuel with the added copper on long-term storage. Thus, mixing was effected by either rolling the capped 5-gal cans (study 1) or shaking the 1-gal cans (study 2). Otherwise, during storage the caps were removed and the openings of the respective epoxy-coated cans, containing the test fuels, were covered with aluminum foil that was perforated with small holes for aeration purposes. After one year, all cans were capped. Gravimetric JFTOT. Fuel thermal stability was determined using the gravimetric JFTOT. The method is based on the weight of the total thermal deposits (mg/L) formed when the filtered fuel flowing at 3 mL/min, under a back pressure of 500 psi, is heated at 260 °C for 2.5 h. This was the test duration used in study 2 (see below for the extended duration testing). The filtered fuel flows over a preweighed stainless steel strip (grade 302 and approximately 7 cm long, 0.5 cm wide, and 0.025 mm thick), contained in a heated strip holder. The total thermal deposit is the sum of the deposits formed on the stainless steel strip and the filterables contained in the effluent. The effluent is filtered, and the deposit is washed with heptane, dried at 70 °C for 30 min in an oven, cooled, and weighed. Further details of the method are described elsewhere.4,10 As the test results attest to (Figures 2-5 and Tables 3, 4, and 6), in the gravimetric JFTOT the filterables comprise the bulk of the total thermal deposits. Extended Duration Testing Using the Gravimetric JFTOT (Study 1). For the extended duration testings, at the start of each gravimetric JFTOT test, and after each subsequent startup, we employed a warm-up period of approximately 15 min during which the effluent was not collected. Extended gravimetric JFTOT testing was conducted for a duration period of 120 h for the neat fuel, which was the first fuel examined. Subsequent testings were extended to a maximum of 152 h, in the effort to monitor MDA’s performance on further increasing the test duration (see Figures 2-5). To monitor the trend in deposit formation, with time, for each test fuel after known time intervals, the thermal deposits formed on the strip, and the filterables collected in the effluent were determined. To simulate continuity of the test, the same strip was reused until the test duration was completed. A new/clean strip was used per test fuel. Copper Analyses (Studies 1 and 2). The concentrations of fuel solubilized copper from the copper doped/storage studies, with and without MDA present, were determined at periodic intervals. Copper analyses were performed using graphite furnace atomic absorption spectroscopy at the Naval Air Warfare Center, Aircraft Division, in Trenton, NJ. BHT Analyses (Study 1). BHT analyses were performed using high performance liquid chromatography (HPLC) and an electrochemical detector at the Naval Air Warfare Center, (10) Beal, E. J.; Hardy, D. R.; Burnett, J. C. (a) Proc. 4th Int. Conf. Stability Handling Liq. Fuels, Orlando, FL, Nov. 1991 1992, 245259; (b) In Aviation Fuels Thermal Stability Requirements, ASTM STP 1138; Kirklin, P. W., David, P., Eds; American Society for Testing and Materials, Philadelphia, PA, 1992; pp 138-150.

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Table 2. Available Properties and Compositional Data for the Two JP-5 Fuels Examined and Specification Limits for Jet A Fuels properties/composition (ASTM method, MIL-T- 5624 specification limits for JP-5 fuels) aromatics, vol % (D1319, max 25) olefin, vol % (D1319, max 5) mercaptan sulfur, wt % (D3227, max 0.0020) total sulfur, wt % (D4294, max 0.40) total acid, mg KOH/g (D3242, max 0.015) density, g/mL (D4052, min-max 0.788-0.845) flash point, °C (D93, min 60) freeze point,°C (D2386, max -46 °C) peroxide content, ppm (D3703, max 8) antioxidant corrosion inhibitor fuel system icing inhibitor, vol % (0.15-0.20) copper content, ppb a

Jet Aa ASTM D1655 22 not reqd 0.003 0.3 0.1 0.775-0.840 37.8 -40 not reqd option agreement agreement 3

JP-5 fuels TX

LA

19 0.8 0.0001 0.0069 0.003 0.816b 62.2 -50.5