Energy & Fuels 2007, 21, 987-991
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High Heat Sink Jet Fuels. 3. On the Mechanisms of Action of Model Refined Chemical Oil/Light Cycle Oil (RCO/LCO)-Derived Stabilizers for JP-8 Bruce Beaver,*,† Maria Sobkowiak,‡ Caroline Burgess Clifford,‡ Yunjing Wei,† and Mitch Fedek† Department of Chemistry and Biochemistry, Duquesne UniVersity, Pittsburgh, PennsylVania 15282, and The Energy Institute, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802 ReceiVed August 21, 2006. ReVised Manuscript ReceiVed January 2, 2007
JP-900 is a generic name representing a future jet fuel that will be required to handle the anticipated thermal stress of ∼900 °F (482 °C) for several hours. In the previous paper in this journal, we showed that the addition, to a JP-8 fuel, of model compounds structurally similar to those potentially derived from the hydrotreatment of refined chemical oil/light cycle oil (RCO/LCO) blends enhances both the thermal oxidative and pyrolytic stability in a flow rig under approximate JP-900 conditions. In this paper, we report model studies designed to clarify the mechanisms by which the model stabilizers (tetralin, R-tetralol, and R-tetralone) function at 250 and 425 °C.
Introduction In the previous paper in this journal,1a we corroborated the oxidative and pyrolytic stability enhancements in a JP-8 fuel under model JP-900 flow conditions via the addition of a few volume percent of various hydroaromatic stabilizers. The model stabilizers examined are structurally similar to those that can be produced from the hydrotreatment of refined chemical oil/ light cycle oil (RCO/LCO) blends.1b-d Hydroaromatic stabilizers, tetralin and R-tetralol, were observed to improve both oxidative and pyrolytic stability. To explain this observation, we suggested that the presence of the stabilizers limits the oxidation of the fuel’s polar aromatic compounds and their subsequent deposit formation. Interestingly, our results with a 1:1 mixture of tetralin (THN) and R-tetralone (THNone) reveal that, relative to the neat fuel (POSF-3804), oxidative deposits are equivalent or possibly enhanced, whereas the total pyrolytic deposits are strikingly decreased. We have suggested that this observation is due to the poor solubility characteristics of the JP-8 fuel. In this paper, we experimentally test aspects of the aforementioned hypotheses. Experimental Section Materials. Decane, dodecane, tetradecane, 1,2,3,4-tetrahydronaphthalene (THN), R-tetralone (THNone), R-tetralol (THNol), * To whom correspondence should be addressed. E-mail: beaver@ duq.edu. † Department of Chemistry and Biochemistry, Duquesne University. ‡ The Energy Institute, The Pennsylvania State University. (1) (a) Sobkowiak, M.; Burgess Clifford, C.; Beaver, B. Energy Fuels 2007, 21, XXX-XXX. (b) Fickinger, A. E.; Badger, M. W.; Mitchell, G. D.; Schobert, H. H. Energy Fuels 2004, 18, 976-986. (c) Gu¨l, O.; Rudnick, L. R.; Schobert, H. H. Energy Fuels 2006, 20 (4), 1647-1655. (d) Balster, L. M.; Corporan, E.; DeWitt, M. J.; Edwards, J. T.; Ervin, J. S.; Graham, J. L.; Lee, S. Y.; Pal, S.; Phelps, D. K.; Rudnick, L. R.; Santoro, R. J.; Schobert, H. H.; Shafer L. M.; Striebich, R. C.; West, Z. J.; Wilson, G. R.; Woodward, R.; Zabarnick, S. Submitted to Fuel Process. Technol. (e) See also: Edwards, T. Prepr. Pap.sAm. Chem. Soc. Pet. Chem. 1996, 41 (2), 481-487.
butylated hydroxy toluene (BHT), and 2,4,6-trimethylphenol (TMP) were purchased from Aldrich Chemical Co. in the highest purity available and were used as received. Instrument and Parameters. Gas chromatography was performed at Duquesne University on a Shimadzu model 14A gas chromatograph with a model CR 501 detector. An Agilent model J&W DB-1301 bonded and cross-linked (6%-Cyanopropyl-phenyl)-methylpolysiloxane (30 m × 0.536 mm × 1.0 µm) column was used. The column was maintained at an initial temperature of 100 °C for 4 min. The temperature then was increased at a program rate of 10 °C/min to a final temperature of 260 °C for 6 min. The attenuation of the detector was adjusted from 3 to 8, to observe small peaks. Flask Oxidation Study. In a typical experiment, into a 150 mL three-neck round-bottom flask is added the reaction mixture composed of 15 mL of decane (solvent), 172 µL of tetradecane (internal standard), 41 µL of 1,2,3,4-tetrahydronaphthalene (0.02 M THN), 40 µL of THNone (0.02 M), and 0.0661 g of BHT (0.02 M). The flask was equipped with a reflux condenser and a gas adapter, which was connected to an oil bubbler. The outside of both the condenser and the flask was wrapped with aluminum foil to prevent light from entering into the container. Both outer necks of the flask were fitted with rubber septa, with a glass pipet inserted through one as a gas inlet. Oxygen was rapidly bubbled into the solution for 5 min prior to sampling for quantitative GC analysis. The reaction was started by immersing the flask into a hot oil bath. Vigorous oxygen bubbling was maintained during the course of the reaction while samples were taken at appropriate time intervals for GC analysis. Control experiments with the addition of a metal deactivating additive (N,N-bis(salicylidene)-1,2-propanediamine) suggests that trace metals are not involved in THN auto-oxidation. Calculation of Reaction Rates. The known molarity of the initial THN solution was assumed to be the mean of the result of the initial GC analysis. After a time t (given in minutes), the peak area ratios (THN/C14) for 4-6 points were determined and assumed to be the fraction of THN still present. The initial
10.1021/ef0604238 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/21/2007
988 Energy & Fuels, Vol. 21, No. 2, 2007
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Table 1. Visible Descriptions of Each Additive (THN, THNol, THNone, each 5% v/v) in Dodecane, under Mild Oxidative Conditions (250 °C) and Severe Oxidative Conditions (425 °C) in Static Tube Bombs under 100 psig of Air Visual Description time (h) 1 2 3 4 6 12 0.25 0.50 1 2 3 4 6
dodecane plus THN
dodecane plus THNol
Temperature ) 250 °C pale yellow liquid, no deposit light yellow liquid, no deposit light yellow liquid, no deposit light yellow liquid, no deposit yellow liquid, no deposit yellow liquid, no deposit yellow liquid, no deposit yellow liquid, no deposit yellow liquid, trace deposit suspended in liquid yellow liquid, trace deposit yellow liquid, small deposit yellow liquid, small deposit clear pale yellow liquid, no deposit clear pale yellow liquid, no deposit clear pale yellow liquid, no deposit clear yellow liquid, no deposit clear yellow liquid, no deposit clear dark yellow liquid, no deposit clear dark yellow liquid, deposit
Temperature ) 425 °C clear pale yellow liquid, no deposit clear pale yellow liquid, no deposit clear yellow liquid, no deposit clear yellow liquid, no deposit clear orange liquid, no deposit clear orange liquid, no deposit orange liquid, deposit
rate of reaction was derived from the slope of a plot of molar THN loss as a function of time. Tubing Bomb Study. Studies were performed at the Energy Institute, using the same tubing bomb methodology as that described previously.2 Mixtures of 10 mL of dodecane and 5% v/v tetralol (for example) were thermally stressed for 1, 2 ,3 ,4, and 6 h at 425 °C under 100 psi of air and up to 12 h at 250 °C. All samples were initially analyzed visually and then by gas chromatography/mass spectroscopy (GC/MS). GC/MS analysis was conducted on the liquid products using a Shimadzu model GC-174 that was coupled with a Shimadzu model QP-5000 MS detector. The column used was a Restek model XT I5 column with a coating phase of 5% diphenyl/ 95% dimethyl polysiloxane and was heated from 40 °C to 290 °C at a heating rate of 12 °C/min. Results and Discussion Hypothesis 1. In the previous paper in this journal,1a we suggest that hydroaromatic stabilizers, such as THN, THNol, and R-tetralone (THNone), can, in principle, form polar oxidation products when a jet fuel is thermal stressed. We also suggested that the fuel’s inherent solvating capacity determines the extent of precipitation of the oxidized model coal stabilizers. Similar to crude oils, where a fuel’s solvating capacity is believed to be related to the ability of the fuel resins to solvate polar asphaltenes,3 we suggest that similar effects are involved in the deposition processes in thermally stressed jet fuels. To test the aforementioned hypothesis, we examined the reactions of the various hydroaromatic stabilizers, under different degrees of oxidative stress, for the approximate time of initial appearance of visible deposits in the nonpolar solvent dodecane. Although this experiment exposes the model compounds to a higher oxygen concentration than the 60-70 ppm of oxygen experienced in a jet fuel system, we believe that our results will provide useful deposit formation trends. In addition, the hydroaromatic stabilizers are added at a level of 5% v/v to ensure the presence of some unreacted polar stabilizer to solvate the more-polar oxidized stabilizer, which likely happens in a fuel. (2) Coleman, M. M.; Selvaraj, L.; Sobkowiak, M.; Yoon, E. Energy Fuels 1992, 6 (5), 535-539. (3) (a) Leon, O.; Rogel, E.; Espidel, J.; Torres, G. Energy Fuels 2000, 14, 6-10. (b) See also: Gonzalez, G.; Sousa, M. A.; Lucas, E. F. Energy Fuels 2006, 20, 2544-2551.
dodecane plus THNone yellow liquid, no deposit orange liquid, no deposit orange liquid, no deposit orange liquid, black deposit dark orange liquid, black deposit dark orange liquid, black deposit clear yellow liquid, no deposit cloudy yellow liquid, small deposit cloudy orange liquid, deposit clear dark orange liquid, deposit cloudy orange liquid, deposit clear dark orange/brown liquid, deposit
Table 1 describes the visible results for hydroaromatic stabilizer oxidation with two stress conditions in tubing bombs: a mild stress, which is defined as heating at 250 °C, and a severe stress, which involves heating at 425 °C. Although heating at 425 °C represents the lower end of the pyrolytic region, previous work with petroleum-derived jet fuels suggests that most fuel deposits in this regime are the result of autooxidation.1d Table 1 reveals that THNone is the most active deposit promoter. In the mild auto-oxidative model studies (i.e., 250 °C), visible deposits are noted with THNone after 4 h of stressing, whereas deposition is noted after 6 h with both THN and THNol. Similar experiments that were performed in the severe stress regime (i.e., 425 °C) suggest that THNone is even a more reactive deposit promoter; visible deposit formation is first noted at 30 min in the presence of THNone verses 6 h with THN or THNol. Note that stressing neat dodecane under all conditions results in no visible deposit formation. The first formed products of THN autoxidation, at temperatures of
