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High Heat Sink Jet Fuels. 2. Stabilization of a JP-8 with Model Refined Chemical Oil/Light Cycle Oil (RCO/LCO)-Derived Stabilizers Maria Sobkowiak,† Caroline Burgess Clifford,† and Bruce Beaver*,†,‡ The Energy Institute, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802, and Department of Chemistry and Biochemistry, Duquesne UniVersity, Pittsburgh, PennsylVania 15282 ReceiVed August 21, 2006. ReVised Manuscript ReceiVed January 2, 2007
JP-900 is the generic name given to a future jet fuel that will be required to handle an anticipated thermal stress of ∼900 °F (482 °C) for several hours. We report flowing rig scouting results, under approximate JP900 conditions, examining the effect on both oxidative and pyrolytic stability of the addition of a few volume percent of model refined chemical oil/light cycle oil (RCO/LCO)-derived compounds to a petroleum-derived JP-8. Tetralin, tetralone, and tetralol were used as model hydroaromatic compounds, which, in principle, can be obtained via the hydrotreatment of RCO/LCO blends. Our scouting results suggest that a jet fuel with improved heat sink capabilities could likely be formulated by adding 1% v/v of hydroaromatic compounds to JP-8.
Introduction Edwards has noted that the development of future jet aircraft will require solving “the difficult struggle to increase hydrocarbon fuel thermal performance while minimizing fuel system deposits.”1 An added dimension of this problem is that future fuels must be developed in the context of the economics of the jet fuel market: a market that is primarily driven by commercial airlines using predominately legacy aircraft. Thus, market economics limits the ability of producers to develop new jet fuels for military aircraft. Consequently, the economic reality of the jet fuel business is such that, at least for the foreseeable future, military and commercial aircraft must continue using the same fuel. New approaches for providing high heat sink fuels2 for the military might be enabled by the steady development of a better mechanistic understanding of fuel oxidative and pyrolytic degradation pathways;3 such an understanding may enable the development of additives,4 stabilizers,5 and/or intrinsically stable fuels.6-22 * To whom correspondence should be addressed. Email: beaver@ duq.edu. † The Energy Institute, The Pennsylvania State University. ‡ Department of Chemistry and Biochemistry, Duquesne University. (1) Edwards, T. J. Propul. Power 2003, 19 (6), 1089-1107. (2) Increased fuel heat sink capabilities may be possible with the development of new technologies for on board fuel deoxygenation. See, for example: Spadaccini, L. J.; Huang, H. J. Eng. Gas Turbines Power 2003, 125, 686-692. (3) Beaver, B.; Gao, L.; Burgess Clifford, C.; Sobkowiak, M. Energy Fuels 2005, 19, 1574-1579. (4) Beaver, B. D.; Burgess Clifford, C.; Fedak, M. G.; Gao, L.; Iyer, P. S; Sobkowiak, M. Energy Fuels 2006, 20 (4), 1639-1646. (5) We define additives as chemical components that are effective in the parts per million (ppm) concentration range, whereas chemical components that are effective in the 1%-2% v/v range are defined as fuel stabilizers. (6) Coleman, M. M.; Selvaraj, L.; Sobkowiak, M.; Yoon, E. Energy Fuels 1992, 6 (5), 535-539. (7) McKinney, D. E.; Bortiatynski, J. M.; Hatcher, P. G. Energy Fuels 1993, 7, 578-581. (8) Selvaraj, L.; Sobkowiak, M.; Song, C.; Stallman, J. B.; Coleman, M. M. Energy Fuels 1994, 8 (4), 839-845.
With declining global petroleum reserves, and the anticipated strengthening of future jet engine emission standards (i.e., particulates), we are exploring the potential use of coal-derived liquids in the formulating of future fuels. We have recently shown that a jet fuel derived from a 1:1 mixture of petroleum (i.e., light cycle oil (LCO)) and coal feed stocks (i.e., refined chemical oil (RCO)) can be produced from existing refinery processes.16,21,22 After extensive hydrotreatment, the fuel is rich (9) Yoon, E.; Selvaraj, L.; Song, C.; Stallman J. W.; Coleman, M. M. Energy Fuels 1996, 10 (5), 806-811. (10) Coleman, M. M.; Sobkowiak, M.; Fearnley, S. P.; Song, C. Prepr. Pap.sAm. Chem. Soc., Pet. Chem. 1998, 43 (3), 353-356. (11) Fearnley, S. P.; Sobkowiak, M.; Coleman, M. M. Prepr. Pap.s Am. Chem. Soc., Pet. Chem. 1998, 43 (3), 357-359. (12) Sobkowiak, M.; Yang, R.; Song, C.; Coleman, M. M.; Beaver, B. Prepr. Pap.sAm. Chem. Soc., Pet. Chem. 2000, 45 (3), 470-473. (13) Beaver, B.; Gao, L.; Fedak, M. G.; Coleman, M. M.; Sobkowiak, M. Energy Fuels 2002, 16, 1134-1140. (14) Song, C.; Eser, S.; Schobert, H. H.; Hatcher, P. G. Energy Fuels 1993, 7, 234-243. (15) Butnark, S.; Badger, M.; Schobert, H. H. Prepr. Pap.sAm. Chem. Soc., Pet. Chem. 2001, 45 (3), 493-495. (16) (a) Schobert, H. H.; Badger, M. W.; Santoro, R. J. Prepr. Pap.s Am. Chem. Soc., Pet. Chem. 2002, 47 (3), 192-194. (b) See, also: Gu¨l, O.; Rudnick, L. R.; Schobert, H. H. Energy Fuels 2006, 20 (4), 16471655. (17) Strohm, J. J.; Andre´sen, J. M.; Song, C. Prepr. Pap.sAm. Chem. Soc., Pet. Chem. 2000, 45 (3), 449-453. (18) Strohm, J. J.; Andre´sen, J. M.; Song, C. Prepr. Pap.sAm. Chem. Soc., Fuel Chem. 2001, 46 (2), 487-489. (19) Strohm, J. J.; Andre´sen, J. M.; Song, C. Prepr. Pap.sAm. Chem. Soc., Pet. Chem. 2002, 47 (3), 189-191. (20) Strohm, J. J. Novel Hydrogen Donors for Improved Thermal Stability of Advanced Aviation Jet Fuels, Master’s Thesis, The Pennsylvania State University, University Park, PA, 2002. (21) (a) Fickinger, A. E.; Badger, M. W.; Mitchell, G. D.; Schobert, H. H. Energy Fuels 2004, 18 (4), 976-986. (b) Guel, O.; Rudnick, L. R.; Schobert, H. H. Energy Fuels 2006, 20 (4), 1647-1655. (22) 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., 2005. (23) Corporan, E.; DeWitt, M.; Wagner, M. Fuel Process. Technol. 2004, 85, 727-742.
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Figure 1. Temperature and carbon deposition profile for flow reactions. Conditions were as follows: JP-8 neat, silcosteel, flow rate of 2.23 mL/min, for 4 and 5 h.
in various hydrogenated naphthalenes, such as substituted decalin and tetralin (THN) derivatives, and has a low heteroatom content. This fuel may serve directly as JP-900, because of its pyrolytic stability, which is inherent in its hydrocarbon molecular structure, and its excellent thermal oxidative stability, which is due to the low heteroatom content. Alternatively, only modest hydrotreatment16,21b of the LCO/ RCO blend produces a mixture that is rich in hydroaromatic derivatives of THN. In principle, this material can be readily blended into JP-8 as a fuel thermal stabilizer. There are several reports by Strohm and co-workers17-20 of significant oxidative and pyrolytic stabilization when stressing a conventional jet fuel that contains a few volume percent of THN and its derivatives. Similar observations were made by Coleman et al. with various stabilizers;6 most significantly, it was shown that the addition of 5% v/v of benzyl alcohol into a Jet A-1 fuel, followed by stressing for 3 h in a tubing bomb at 425 °C, in the presence of 100 psig of air, results in minimal visible oxidative and pyrolytic deposits, compared to the neat fuel. The goal of this work is to serve as a scouting study to confirm the observations of Strohm and co-workers17-20 that the addition of 1% v/v of each THN and THNone to a conventional jet fuel will significantly improve its thermal stability under high-temperature flow conditions. Experimental Section Materials. The fuel tested was a JP-8 (POSF-3804), which was obtained from the U.S. Air Force. Silcosteel coated tubing (Catalog No. 20595) was purchased from Restek Corporation. A thermocouple probe with a lead wire attached and with a stripped end (Part No. HKMTSS-062E-12) was purchased from Omega Engineering, Inc. 1,2,3,4-Tetrahydronaphthalene (THN), R-tetralol (THNol), and R-tetralone (THNone) were purchased from Aldrich in the highest purity available and were used as received. Flow Reactor Studies. Reactions were conducted in a single tube flow reactor. The length of tubing in the reactor furnace was 36.2 in. (90 cm), with an outer diameter (OD) of 0.0625 in. and an inner diameter (ID) of 0.0400 in. A liquid hourly space velocity
(LHSV) of 182 h-1 was used. The fuel was exposed to air at atmospheric pressure and then pumped with the pressure maintained at ∼550 psig. The reactor temperature calibration profile was obtained using multiple runs, with dodecane as the fuel, with a thermocouple probe placed into the bulk fuel, using a 1/16 in. silcosteel Tee at different tube positions. The standard conditions have a thermocouple placed at the top of the reactor, just before the exit from the furnace, to measure the bulk fuel exit temperature. The reactor was heated in such way to keep the exit temperature constant at 675 °C and held at this temperature for 4 or 5 h. Characterization of Carbon Deposition. Upon completion of the reaction, the reactor tubing was washed three times with pentane to remove any residual fuel, followed by removal from the reactor. Afterward, the reaction tubing was cut into 2-cm pieces, washed again with pentane, and dried in a vacuum oven at 100 °C overnight. Upon completion of the drying, the total carbon deposition on each piece was determined using a LECO model RC 412 multiphase carbon analyzer.
Results and Discussion A JP-8 fuel was stressed in a flow reactor using silcosteelcoated tubing up to 675 °C for 4 h.24 Our experimental conditions approximately correlate to a fuel residence time of 55 cm is the pyrolytic regime. The data has a “spiky” appearance, because not much deposit was generated under our experimental conditions. Since this study was completed, we have subsequently learned that POSF-3804 is a thermally stable jet fuel with a low concentration of polar aromatic compounds that are believed to be the precursors for (24) The use of silcosteel is known to passivate surfaces and reduce oxidative deposits by approximately one-half, and it dramatically reduces pyrolytic deposits. See: Ervin, J. S.; Ward, T. A.; Williams, T. F.; Bento, J. Energy Fuels 2003, 17, 577-586.
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Figure 2. Carbon deposition profile for flow reactions. Conditions were as follows: JP-8 neat, and JP-8 with 1% v/v of each THN and THNone, silcosteel, fuel exit temperature of 675 °C, flow rate of 2.23 mL/min, for 5 h.
thermal oxidative deposits.25 Several runs for longer durations with the data averaged would likely provide larger deposit peaks with the more-typical smoother curves. Nonetheless, the data presented in Figure 1 suggest good reproducibility, in that the oxidative deposit maximums for both runs are slightly less than ∼100 µg/cm2, whereas both pyrolytic deposit maximums are 300-400 µg/cm2. Next, the JP-8 was stressed following the addition of 1% of each THN and R-tetralone (THNone). The results are reported in Figure 2, with the results of the 5-h stress for the neat fuel added for comparison. Interestingly, the observed oxidative deposit maximum for THN/THNone addition is approximately the same as that of the neat fuel at ∼100 µg/cm2, whereas the pyrolytic deposit maximum is reduced from ∼350 to ∼200 µg/ cm2. Most interestingly, at a distance of >65 cm, the pyrolytic deposition is almost zero! However, the addition of stabilizers visually seems to have increased overall oxidative deposits. The addition of 2% v/v of coal-derived hydroaromatic thermal stabilizers (i.e., THN/THNone) into current jet fuels may exacerbate the smoke point of the fuels and possibly particulate emissions. To address this issue, we similarly stressed the JP-8 after the addition of only 1% v/v of tetralin (THN). In Figure 3, a decrease is noted in both the oxidative and pyrolytic deposition maximum, compared to the neat JP-8 fuel. Figure 3 suggests that THN addition decreases both the oxidative and pyrolytic deposit maximums from ∼100 µg/cm2 to ∼25 µg/ cm2 and from ∼350 µg/cm2 to ∼150 µg/cm2, respectively. A similar experiment is shown in Figure 4, which shows that the presence of 1% R-tetralol (THNol) also results in a decrease in both the maximum for thermal oxidative deposit (up to ∼25 µg/cm2) and the pyrolytic deposit maximum (up to ∼150 µg/ cm2). On balance, our results agree with those of Strohm et al.,18 in that stressing a JP-8 fuel in the presence of air while passing through a silcosteel tube, after the addition of various models of coal-derived stabilizers generally results in a reduction of both the oxidative and pyrolytic fuel depositions. In the next (25) Balster, L. M.; Zabarnick, S.; Striebach, R. C.; Shafer, L. M.; West, Z. J. Energy Fuels 2006, 20, 2564-2571.
paper in this series,26 we present data which suggest that the hydroaromatic stabilizers limit the extent of conversion of a fuel’s polar aromatic compounds into deposits. However, there are also some differences in our results, which are most likely due to the different flow conditions and different JP-8 fuels used in the studies.27 Strohm et al. have shown that the addition of 1% v/v of each THN and THNone to their JP-8 fuel results in improved oxidative stabilization.18 Their neat fuel had an oxidative deposit maximum of ∼600 µg/cm2, whereas the presence of the stabilizer combination reduced the oxidative deposit maximum to ∼100 µg/cm2. We have not observed a similar result with our JP-8 fuel; our oxidative deposit maximums are ∼100 µg/cm2 for both the neat fuel and with THN/ THNone addition. In fact, visual inspection of the total oxidative deposit region in Figure 2 (up to a distance of 55 cm) suggests that our JP-8 fuel actually produced more oxidative deposit with THN/THNone addition. However, we have observed that, in the pyrolytic region, the deposit maximum was reduced to ∼200 µg/cm2 from ∼350 µg/cm2 with stabilizer addition while the deposit is almost zero from distances of 65-85 cm. Strohm et al. also noted a reduction in the pyrolytic maximum from ∼4000 µg/cm2 to ∼1250 µg/cm2 with stabilizer addition. A recent mechanistic hypothesis that is used to account for oxidative deposits in stressed middle distillate fuels can be invoked to account for the aforementioned observations.3 Briefly, it is suggested that soluble macromolecular oxidatively reactive species (i.e., SMORS)28 are formed by a repetitive series of oxidative oligomerization steps that involve indigenous polar reactive molecules, such as phenol and indole derivatives. As the SMORS molecular weight increases, the largest molecules become insoluble, agglomerate, and then precipitate as microspherical oxidative deposits. (26) Beaver, B.; Sobkowiak, M.; Burgess Clifford, C.; Wei, Y.; Fedek, M. Energy Fuels 2007, 21, XXX-XXX. (27) Both studies used silicosteel tubing and different fuels, with Strohm et al.18,19 using a liquid hourly space velocity (LHSV) of 450 h-1 with a tube surface exit temperature of 772°C for a 3-h stress. We used a value of LHSV ) 182 h-1 with a tube exit temperature of 675 °C and, typically, a 5-h stress. (28) Hardy, D. R.; Wechter, M. A. Energy Fuels 1994, 8, 782-787.
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Figure 3. Carbon deposition profile for flow reactions. Conditions were as follows: JP-8 neat, and JP-8 with 1% v/v of THN, silcosteel, fuel exit temperature of 675 °C, flow rate of 2.23 mL/min, for 5 h.
Figure 4. Carbon deposition profile for flow reactions. Conditions were as follows: JP-8 neat, and JP-8 with 1% v/v of THNol, silcosteel, fuel exit temperature of 675 °C, flow rate of 2.23 mL/min, for 5 h.
In addition, we suggest that SMORS structurally contain reactive benzylic hydrogen atoms that are pyrolytically labile. Thus, lower-molecular-weight SMORS that have yet to precipitate as oxidative deposits can donate benzylic hydrogen atoms to stabilize fuel paraffins in the pyrolytic regime. This hypothesis is consistent with the observations of Coleman et al.,6 who used infrared spectroscopy to monitor fuel functional group changes while stressing a Jet A-1 fuel in air at 425 °C in stainless steel tubing bombs for several hours. It was determined that the fuel alkene concentration gradually increases and then decreases while the aromatic hydrocarbon concentration increases. We suggest that these results are consistent with a pyrolytically activated benzylic hydrogen loss from SMORS,
followed by molecular coupling at the benzylic position. Subsequent pyrolytically activated benzylic hydrogen losses would yield SMORS-localized alkene functional groups. Additional SMORS modifications via thermally promoted DielsAlder reactions could form larger aromatic structures that ultimately precipitate. If the aforementioned hypothesis is correct, then the addition of the THN/THNone stabilizer may protect the fuel’s indigenous polar oxidatively reactive molecules from oxidation; this limits their conversion to SMORS, ultimately forming both oxidative and pyrolytic deposits. However, oxidation of the THN/THNone stabilizer also produces polar oxidation products that may
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become insoluble.29 We suggest that, with Strohm et al.’s JP-8 fuel, the presence of indigenous polar molecules can solubilize oxidized “polar stabilizer molecules”. This is analogous to the hypothesized stabilization of polar asphaltenes in crude oil by resins.30 The presence of the THN/THNone stabilizer in our JP-8 fuel (POSF-3804) would also limit SMORS production and oxidative deposits from the fuel’s polar oxidatively reactive compounds. However, we suggest that POSF-3804 does not contain enough indigenous polar molecules to solubilize polar material that has been derived from stabilizer oxidation. Hence, we observe no oxidative stabilization with stabilizer addition to POSF-3804, because the nature of the fuel is such that the oxidized THN/THNone precipitates. However, our JP-8 with THN/THNone addition had minimal pyrolytic deposits at the highest temperatures (at distances of >65 cm in Figure 2). This is consistent with the absence of pyrolytic precursors (i.e., SMORS) in the high-temperature regime with POSF-3804. The absence of pyrolytic precursors, coupled with the presence of unoxidized THN/THNone, which can function as a pyrolytic stabilizer, can account for the absence of pyrolytic deposits in the high-temperature region of Figure 2. In summary, we suggest that jet fuel thermal degradation progresses through the following general chemical sequences: (i) SMORS formation; (ii) “heavy” SMORS deposition as oxidative deposits; (iii) pyrolytically activated SMORS modification, followed by deposition; and, finally, (iv) pyrolytically activated paraffin modification and deposition. This suggestion is consistent with the work of Edwards,31 who examined both oxidative and pyrolytic deposits from three different Jet A fuels (29) We have found that stressing solutions (5% v/v) of both THN and THNol in dodecane in tubing bombs at 250 °C under 100 psi of air results in visible deposit formation after 3-4 h. A similar experiment with THNone results in visible deposit formation after 2-3 h. Neat dodecane under the same conditions does not form deposits. See details in the next paper in this series. (30) Leon, O.; Rogel, E.; Espidel, J.; Torres, G. Energy Fuels 2000, 14, 6-10. (31) Edwards, T. Prepr. Pap.sAm. Chem. Soc., Pet. Chem. 1996, 41 (2), 481-487.
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and the paraffin specialty fuels Norpar 13 and Exxsol D80. Stressing these fuels, in the presence of air in a flowing rig with a stainless steel tube to a final fuel temperature of ∼650 °C, produced results that were consistent with our hypothesis. Namely, all the Jet A fuels produced SMORS, as evidenced by the formation of an oxidative deposit in the 1-2 ppm range. In addition, one Jet A fuel produced a pyrolytic deposit of ∼2 ppm, whereas the two others produced only ∼0.2 ppm. By comparison, the paraffin specialty fuels do not form SMORS, as evidenced by their lack of oxidative deposit formation. However, the absence of SMORS in these fuels results in the formation of a large amount of pyrolytic deposits from the paraffins: ∼5 ppm for Norpar 13 and ∼7 ppm for Exxsol D80. Conclusions We have shown that, under high-temperature flow reaction conditions, the addition of 1% v/v of either tetralin (THN) or R-tetralol (THNol) enhances fuel stability. We have suggested that this result is due to the stabilizer limiting thermal oxidative transformations of the fuel’s polar aromatics into insolubles. In addition, we have corroborated earlier reports17-19 that, when a 1:1 mixture of THN and R-tetralone (THNone) is added at a level of 2% v/v as a stabilizer, the total fuel deposits are significantly decreased. Interestingly, our results with THN/ THNone addition to POSF-3804 reveal that, relative to the neat fuel, oxidative deposits are equivalent (or possibly enhanced) while the total pyrolytic deposits are strikingly decreased. We have suggested that this observation is due to the poor solubility characteristics for POSF-3804. Acknowledgment. This project was jointly supported by the U.S. Air Force Research Laboratory/ Propulsion Directorate and by the U.S. Department of Energy. We also wish to thank the reviewers of this manuscript for many helpful suggestions and Jon Strohm, Semih Eser, Chunshan Song, Mike Coleman, and Harold Schobert for their help and encouragement. EF060422F
