Ind. Eng. Chem. Res. 2007, 46, 1657-1660
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APPLIED CHEMISTRY Middle Distillate Fuel Stability: Straightforward Methods for Improving Quality George W. Mushrush,*,†,‡ Heather D. Willauer,† James H. Wynne,§ Matthew Laskoski,§ Christopher T. Lloyd,§ and Teddy M. Keller§ NaVy Technology Center for Safety and SurViVability, NaVal Research Laboratory, 4555 OVerlook AVenue SW, Washington, DC 20375, Chemistry Department, George Mason UniVersity, 4400 UniVersity DriVe, Fairfax, Virginia 22030, and Materials Chemistry Branch, Code 6120, NaVal Research Laboratory, 4555 OVerlook AVenue SW, Washington, DC 20375
Fuel storage instability reactions of middle distillate fuels continue to be a major problem for the Department of Defense. Unlike civilian fuels, military fuels can remain in storage tanks for one or more years. As fuels are drawn from these tanks, the tanks are subsequently topped off with more recently purchased fuels. In many cases, the mixed fuels are not compatible resulting in sediment and sludge formation. This fuel incompatibility results in degradation reactions that form solids which will plug nozzles and filters and render the entire contents of the storage tank unusable. Previous research has shown that certain specific polar organic nitrogen functional groups are involved in fuel instability reactions. These organonitrogen compounds are difficult to completely remove during refining. We report on a simple inexpensive procedure for the removal of these organonitrogen compounds before fuels are mixed in storage tanks. Introduction The military is the largest consumer of middle distillate fuel in the world. Anything that impacts this market has ramifications for the civilian market. Middle distillate fuels such as Number 2 diesel are defined by an average set of chemical and physical characteristics. These include an average carbon number of C13 up to C21 and a distillation range of 150-400 °C (300-750 °F). Military fuels must also meet many other criteria such as API gravity, flash point, pour point, water solubility, cetane number, and total sulfur, just to name a few.1 The typical procedure for military fuels is simply to top off storage tanks with new fuels, as stored fuels are withdrawn.1 Thus, fuels of different ages from different geographical sources and refineries are mixed. These fuels can have different chemical characteristics which upon mixing results in storage oxidative instability problems.2-5 These degraded fuels fuel must be returned to a refiner or recycler to be reprocessed at significant expense. It was found that as little as a few ppm of organic nitrogen compound can induce reactions that cause an entire storage tank of middle distillate fuel to degrade. We define degradation in terms of sediment and insoluble gum formation, mg of solids/ 100 mL of fuel.6,7 The organic compounds that have been implicated in these systems have been indoles, carbazoles, and their alkyl-substituted congeners.8 It is difficult and expensive to remove the last traces of these nitrogen compounds from the finished fuels during the refining process. * To whom correspondence should be addressed. Address: George Mason University, Chemistry Department, 3E2, 4400 University Drive, Fairfax, VA 22030. Tel.: (703) 993-1080. Fax: (703) 993-1055. E-mail:
[email protected]. † Navy Technology Center for Safety and Survivability, Naval Research Laboratory. ‡ George Mason University. § Materials Chemistry Branch, Code 6120, Naval Research Laboratory.
This report demonstrates that the final filtering during the filling and before mixing of these middle distillate fuels in a storage tank would prevent oxidative instability reactions. Suitable filtering material would include activated clay, silica gel, or carbon nanotubes. All of these materials will remove active nitrogen compounds and result in mixed fuels that exhibit excellent storage stability. Carbon nanotubes are much too expensive at present to warrant their use, but they may be readily available in the future. We have treated an unstable fuel with activated clay, silica gel and carbon nanotubes, identified the organic compounds that each removes and then used these extracts to examine fuel stability when mixed with a stable fuel. Experimental Chemicals and Materials. Unless otherwise stated, chemicals were reagent-grade and were obtained from Aldrich Chemical Co. (Milwaukee, WI) and used without additional purification. The middle distillate fuels used in this study were obtained from our extensive collection of both stable and unstable fuels. The unstable fuel was a Spanish refined (No. 2, 83 ppm total organic nitrogen) diesel, and the stable fuel was an American refined (No. 2, 6 ppm total organic nitrogen) diesel. Both fuels have been used as benchmarks for stability studies. The unstable fuel over many years has been found to form 3.0-3.5 mg of sediment/100 mL of fuel, thus ranking it as very unstable. The stable American refined (No. 2) diesel has historically formed 0.3-0.5 mg sediment/100 mL fuel and thus showing that it is a very stable fuel. Both fuels are stored under refrigeration until used. Fuel Blends. The unstable Spanish fuel was treated in three different fashions. The fuel (15 mL) was mixed in separate vials with 10 g each of activated clay, silica gel, and carbon nanotubes. Each mixture was stirred for 30 min and then filtered through a fine glass frit. The filtered fuel was collected. Then 10 mL of each filtered fuel was added to 90 mL of the stable
10.1021/ie061430m CCC: $37.00 © 2007 American Chemical Society Published on Web 02/16/2007
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Scheme 1. Synthesis of Cobalt Complex 1 in the Presence of Excess 2
American diesel to give a total volume of 100 mL. These three samples were then analyzed for storage stability by the ASTM gravimetric procedure D-5304.8 The activated clay, silica gel, and carbon nanotube filter materials were then rinsed with cyclohexane to remove excess fuel and dried by gentle air vacuum. Each filter material was then treated with 50 mL of methylene chloride. The collected methylene chloride filtrates were then reduced in volume to approximately 4 mL and analyzed by combined capillary column GC/MS. The removal of any remaining organic nitrogen residues from the filter media proved straightforward. Each filter media was subjected to a methanol rinse and the GC/MS of this rinse material confirmed the removal of adsorbed fuel components Instrumental Methods. The extracts were submitted for capillary gas chromatography/ mass spectrometry (GC/MS) analysis. The GC/MS system was a Hewlett-Packard 5890 capillary gas chromatograph equipped with a Hewlett-Packard 5971 mass selective detector operating in electron ionization mode. The column utilized was a J&W DB-5ms (5% phenyl) methylpolysiloxane film. The carrier gas was helium and a flow rate of 24 cm/s. The temperature program has an initial temperature of 70 °C, then 20 °C per minute to 290 °C with a 10 min hold. The injection temperature, detector temperature, and source temperatures were 250, 280, and 250 °C, respectively. The solvent delay was set at 4 min. Carbon Nanotubes Synthesis, CNT. The in situ synthesis of CNTs occurs in high yield and in a bulk solid composition from pyrolysis of a mixture of the cobalt hexacarbonyl complex 1 of 1,2,4,5-tetrakis(phenylethynyl)benzene and an excess amount of compound 2 (1,2,4,5-tetrakis(phenylethynyl)benzene). Compound 2 was chosen as the carbon source due to the inherent symmetry within the structure and potentially easy pathway to the formation of CNTs. Moreover, compound 2 is synthesized in one step starting from a commercially available material, 1,2,4,5-tetrakis(bromo)benzene.9 The cobalt complex 1 was formed from the reaction of cobalt octacarbonyl, Co2(CO)8, with 2 at room temperature in a dry, 50:50 mixture of hexane and CH2Cl2 (Scheme 1). The reaction product was concentrated, dried, and used as prepared in our studies. The interaction of alkynes with cobalt octacarbonyl is well documented.10 Shaped CNT-containing compositions can be readily fabricated by our novel method. The composition can be tailored to have mainly CNTs or varying amounts of CNTs and cobalt nanoparticles. For our study, a precursor composition containing a 1:20 molar ratio of 1:2 was melted, heated to 1000 °C, and held for 1 h, producing CNTs in greater than 70% yields. The growth of the CNTs proceeds in the solid phase during the carbonization process above 600 °C. The cobalt species (atoms, clusters, and/or nanoparticles) are responsible for and directly
aid in the formation and size of the CNTs. The formation of CNTs, formed during the carbonization of compositions formulated from 1 and an excess amount of 2, occurs from interaction of the cobalt species with the developing aromatic fused ring structures. During the pyrolysis of a mixture of 1 and 2, a progressive increase in the formation of polycondensed ring structures occurs resulting in the formation of carbon nanoparticles from pairing of free radicals.11,12 The developing large condensed ring structures and/or carbon nanoparticles react with the cobalt species to initiate the CNT growth process.13 Only a small amount of cobalt nanoparticles is necessary to initiate the formation of CNTs. With the proper heat treatment, high concentrations of CNTs are formed in the carbonaceous composition along with some amorphous carbon. The solid carbonaceous CNT composition was pulverized, converted into a powdered form and used in the purification of the fuel. Elemental analysis showed that the CNT composition used in this study contained about 1.2 wt % of cobalt. The multiwalled carbon nanotubes were about 107.5 nm in length and 21.5 nm in diameter. These isolated nanotubes contained less than 15% amorphous carbon. These carbon nanotubes have been characterized by both X-ray and SEM.14 Amorphous carbon has been found to be a less desirable filter media for isolating organic nitrogen compounds than either activated clay or silica gel in our earliest work.3 Storage Stability Test Procedure. The American No. 2 and the Spanish No. 2 diesel as well as the composite fuel blends were tested for storage stability and chemical instability reactions. They were tested by a gravimetric technique described in ASTM D5304-99.8 A brief description of this method is as follows: 100 mL sample of the blends in 125 mL borosilicate brown glass bottles were subjected to a 16 h, 90 °C timetemperature regimen at 100 psig overpressure of pure oxygen. After the reaction period, the samples were cooled to room temperature. The samples were filtered, and the sediment determined by a gravimetric procedure. Results and Discussion Civilian fuels are used within a few days or weeks of production; thus, storage stability and oxidative instability reactions have little practical significance to this market. However, military fuels are stockpiled and stored for one or more years. How these fuels behave in long-term storage is of major importance, and if the fuel degrades, it could then plug filters, injector nozzles, and otherwise could damage engine components. It is not only what happens to an individual fuel, but what can happen when fuels from different refineries of the same company or from different companies are blended in storage facilities. Our laboratory has found that small amounts of an unstable fuel can induce instability reactions in an otherwise stable fuel. The accelerated storage stability results from the two fuels studied in this report are interesting in that 10% of an unstable fuel induced instability in a very stable fuel. Table 1 shows the results of adding the unstable fuel to an otherwise stable fuel and the consequences for storage stability. The ideal would be to stop these instability reactions before they occur. It is quite expensive to reprocess a fuel once these reactions start to occur, but could the fuel be treated before use by some simple filtering method on site? The results in Table 1 clearly show that filtration through media such as activated clay, silica gel, or carbon nanotubes will accomplish this goal. Activated clay would be the cheapest solution, but silica gel and carbon nanotubes are also relatively close in cost. The
Ind. Eng. Chem. Res., Vol. 46, No. 6, 2007 1659 Table 1. Fuel Instability Results for Both the Stable Fuel and Unstable Fuel and the Unstable Fuel Filtrates from the Activated Clay, Silica Gel, and Carbon Nanotubes Separation fuel unstable fuel stable fuel 90 mL stable +10 mL unstable +10 mL activated clay treated +10 mL silica gel treated +10 mL nanotubes treated
sediment, mg/100 mL fuel 3.8 0.3 2.6 0.8 0.9 0.9
nanotubes used in this report cost approximately $30-$50/ pound, depending on scale of manufacture.9 This is about the same cost of the silica gel used, but 3 times that of the price of activated clay. All three methods are quite effective at improving overall fuel quality. While a wide range of organic nitrogen compounds are found in middle distillate fuels, it has been speculated that only specific organic nitrogen species are actively involved in fuel degradation.7,15 The indoles and substituted indoles such as carbazoles are the organic nitrogen compounds that have been found to be the most implicated in fuel instability reactions.16 Table 2 shows the results of the GC/MS analysis of the fuel filtrates from the unstable fuel when it was treated with activated clay, silica gel, and carbon nanotubes. The number of alkylsubstituted derivatives is represented by n, and the percentage refers to the individual nitrogen compound area percentage compared to the total nitrogen area percentage. Pyridine compounds make up the bulk of all organic nitrogen compounds found in the three filtration fractions, of which 56 derivatives comprising 61.7% from the activated clay, 54 derivatives comprising 63.0% from the silica gel, and 61 derivatives comprising 62.0% from the carbon nanotubes. Pyridine and its alkylsubstituted derivatives have been shown to have little effect on fuel stability.3 The ratio of quinolines to tetrahydroquinolines indicated that this fuel was not extensively hydrotreated; quinolines make up about 20%, compared to the 5%-6% for the tetrahydroquinolines. Both classes of these compounds have also been found to be innocuous, as far as fuel instability reactions are concerned.3 The only pyrrole nitrogen found was the relatively long alkyl chain C5-C7 substituted pyrroles. The only pyrroles implicated in fuel instability reactions are the short chain compounds such as 2-methyl or 3-methyl pyrroles. The total percentage of these compounds found in the three extracts was very small, less than 1%. The alkyl-substituted indoles varied with 11 derivatives comprising 2.2% from the activated clay, 15 derivatives comprising 2.9% from the silica gel, and 9 derivatives comprising 3.8% from the carbon nanotubes extract. There were fewer carbazoles derivatives (a substituted indole), in the activated clay extract, a total of 3 derivatives comprised 6.9% of the total organic nitrogen isolated, while 4 derivatives, 7.3%, were found in the silica gel and 3 derivatives made up 6.9% of the total area from the carbon nanotubes extract. Our laboratory has found carbazoles and indoles to be the most deleterious organonitrogen species as far as a predictor of fuel instability.3 Only about 2% of the total organic nitrogen compounds could not be identified by GC/MS. This was primarily due to partial oxidation of some compounds and other reactions in these complicated mixtures. Conclusion Fuel instability is a problem that continues for the military. When fuels are mixed, undesirable reactions can occur. This
Table 2. GC/MS Analysis of the Unstable Fuel Filtrates from the Activated Clay, Silica Gel, and Carbon Nanotubes Separations Activated Clay compound class, organonitrogen carbazoles C0 C1 C2 total indoles C0 C1 C2 C3 C4 C5 C6 total pyridines C1 C2 C3 C4 C5 C6 C7 C8 total pyrroles C5 C6 C7 total quinolines C1 C2 C3 C4 C5 C6 C7 total tetrahydroquinolines C0 C1 C2 C3 C4 C5 total total nitrogen unidentified
n
area %
Silica Gel
Nanotubes
n
n
area %
1 1 1 3
3.2 1.4 2.3 6.9
1 1 2 4
3.7 2.1 1.5 7.3
2 3
2.7 2.1 2.1 6.9
1 4 2 2 1 1
0.4 0.7 0.3 0.2 0.2 0.1
3 5 3 4
11
2.2
15
0.5 1.3 0.7 0.3 0.4 0.1 0.1 2.9
2 3 1 1 1 1 1 9
0.5 1.1 1.7 0.2 01 0.1 0.1 3.8
1 5 8 10 8 10 10 4 56
1.3 8.9 7.5 17.9 14.7 7.2 3.7 0.5 61.7
1 6 7 8 7 8 7 4 54
1.4 7.4 9.3 19.2 14.1 8.4 3.7 0.4 63.9
1 10 14 11 5 8 12
1.7 6.9 13.1 19.0 11.1 7.2 3.0
61
62.0
2 1 1 4
0.3 0.2 0.1 0.6
2 1 0 3
0.3 0.4 0.7
2 1 2 5
0.2 0.1 0.1 0.4
1 1 4 5 4 10 9 34
0.6 5.7 5.1 1.9 2.6 4.3 2.1 20.0
1 3 5 7 7 4 7 34
0.5 4.0 4.0 2.2 2.0 3.5 1.4 17.77
1 4 3 5 5 4 5 27
0.7 5.0 6.0 2.9 3.0 3.1 0.7 19.3
1 10 6 0 7 6 30 140
1.3 2.1 1.7
1 8 7 2 5 6 29 164
1.1 1.7 1.5 0.1 0.7 0.3 5.5 98.0
1 6 6 0 7 5 25 147
1.2 1.6 1.7
0.9 0.3 6.3 97.7 2.3
2.0
1
area %
0.5 0.1 5.1 97.5 2.5
report used 90 mL of a stable fuel and 10 mL of an unstable fuel to induce instability to the stable fuel. The unstable fuel was treated with activated clay, silica gel, and carbon nanotubes. All three methods of separation resulted in an extract that did not induce instability reactions upon addition to a stable fuel. The methylene chloride extracts from the activated clay, silica gel and the carbon nanotubes were subjected to GC/MS analysis. The range and identity of the suite of organic nitrogen compounds was determined for each extract. Of the compounds identified, pyridines comprised the largest percentage (approximately 60%) of organic nitrogen with quinolines and tetrahydroquinolines making up about 26% and the indoles and carbazoles comprising about 10% of the total. Both indoles and carbazoles organonitrogen compounds have been linked to fuel instability reactions. All three filter media were capable of removing about 98% of the organic nitrogen compounds present in the unstable Spanish fuel. The silica gel adsorbent was slightly better at removing the carbazole and indole compounds present.
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It is recommended that all middle distillate fuels be treated before being added to partially filled fuel storage tanks. Literature Cited (1) ASTM, American Society for Testing Materials. Standard Specification for Fuel Oils; Annual Book of ASTM Standards, ASTM D975-96; ASTM: Philadelphia, 1997; Part 05.01, pp 165-168. (2) Malhotra, R.; St. John, G. A. Examination of Fuel Deposits from Various Sources by Pyrolysis/Field Ionization Mass Spectrometry. In Proceedings of the 3rd International Conference on Stability and Handling of Liquid Fuels, London, September 1988; Hiley, R. W., Penfold, R. E., Pedley, J. F. Eds.; Institute of Petroleum: London, U.K., 1998; Vol. 3, pp 525-537. (3) Beal, E. J.; Cooney, J. V.; Hazlett, R. M.; Morris, R. E.; Mushrush, G. W.; Beaver, B. D.; Hardy, D. R. Mechanisms of Syncrude/Synfuel Degradation; Final Report, DOE/BC/87001232; Department of Energy: Washington, DC, 1984. (4) Walls C. L.; Beal, E. J.; Mushrush, G. W. Nitrogen compound distribution in a heating oil. J. EnViron. Sci. Health 1999, 34(1) 31. (5) Hudlicky, M. In Oxidations in Organic Chemistry; ACS Monograph 186, American Chemical Society: Washington, DC, 1990. (6) Wynne, J.; Stalick, W. J.; Mushrush, G. W. Fuel Instability Studies: The Synthesis of Long Chain Alkyl Substituted Indoles. Pet. Sci. Technol. 2000, 18 (1 & 2), 221. (7) Taylor, W. F.; Frankenfeld, J. W. Deposit formation from deoxygenated compounds, effects of trace nitrogen and oxygen compounds. Ind. Eng. Chem. Prod. Res. DeV. 1978, 17, 86. (8) ASTM, American Society for Testing Materials. Standard Test Method for Assessing Distillate Fuel Storage Stability by Oxygen OVerpressure; Annual Book of ASTM Standards, ASTM D5304-99; ASTM: Philadelphia, 1999; Part 05.03, pp 569-572.
(9) Jones, K. M.; Keller, T. M. Synthesis and characterization of multiple phenyl-ethynyl benzenes via cross-coupling with activated palladium catalyst. Polymer 1995, 36, 187. (10) Hsu, M. A.; Yeh, W. Y.; Lee, G. H.; Peng, S. M. Preparation of Mo, Co and iron group heterometallic clusters linked by cyclotetradeca1,8-diyne ligand. J. Organomet. Chem. 1999, 588, 32. (11) Bruck, S. D. Thermal degradation of an aromatic polypyromellitimide in air and vacuum. III. Pyrolytic conversion into a semiconductor. Polymer 1965, 6, 319. (12) Fitzer, E. From polymers to polymeric carbon. A way to synthesize a large variety of new materials. Pure Appl. Chem. 1980, 52, 186. (13) Iyer, V. S.; Vollhardt, P. C.; Wilhelm, R. Near-quantitative solidstate synthesis of carbon nanotubes from homogeneous diphenylethynecobalt and nickel complexes. Angew. Chem. Int. Ed. 2003, 42, 4379. (14) Keller, T. M.; Qadri, S. B.; Little, C. A. Carbon nanotube formation in situ during carbonization in shaped bulk solid cobalt nanoparticle compositions. J. Mater. Chem. 2004, 14, 3063 (15) Mushrush, G. W.; Honeychuck, R. V.; Stalick, W. M.; Beal, E. J.; Hardy, D. R. The reaction of 3-methyl indole with para-ethylbenzene sulfonic acid: Fuel instability reactions. Fuel, Sci., Technol., Int. 1995, 13 (6), 793. (16) Goetzinger, J. W.; Thompson, C. J.; Brinkman, D. W. A ReView of the Storage Stability Characteristics of Hydrocarbon Fuelss1982; Report No. DOE/BETC/IC-83/19523: Department of Energy: Washington, DC, 1983.
ReceiVed for reView November 7, 2006 ReVised manuscript receiVed December 7, 2006 Accepted December 15, 2006 IE061430M
