Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 156-159
156
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60
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20
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60
z 50 0 l3
& LO w
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I
I
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Figure 5. Variation of cetane number with carbon types.
not contain ignition improvers; and (d) cetane number range is from 20 to 75.
Conclusions An assessment of the relationship between ignition quality and fuel composition is given. Carbon groups that have dominant effect on the ignition quality of the fuels have been specified. A novel technique to characterize the fuel chemistry in terms of these carbon groups of the fuel molecules by proton nuclear magnetic resonance spectrometry has been presented. A group of diesel fuels, whose cetane numbers were determined on several standard cetane rating engines at different locations, have been analyzed by using the proposed technique of fuel characterization, and the resulting
data have been used to obtain a correlation expression for the prediction of the cetane number of diesel fuels of a wide range of ignition ratings. The accuracy of the correlation has been found to be better than the accuracy of the cetane engine measurements on the basis of the spread of the cetane numbers determined by a number of standard cetane rating engines. Literature Cited Aggarwal, S. K.; Slrlgnano, W. A. "Proceedings of the 20th International Symposium on Combustion"; Combustion Institute: Pittsburgh, PA, 1985. Annamaiai, K.; Madan, A.; Mortada, Y. I.ASME Paper 84-WAIHT-18, American Society of Mechanlcai Englneers: New York, 1984. ASTM Test Method D613, ASTM Standards, Pt. 47, 1979. Azev, V. S.;Tugoiukov, V. M.; Kukushkin, A. A,; Livshits, S.M. Khim. Tekhno/. Top/. Masel 1978, 14, 51-53. Backhouse, T.;Ham, A. J. Fuel 1949, 28, 248-252. Chao, B. H.; Matalon, M.; Law, C. K. Combust. Flame 1985, 39, 43-51. Chiu, H. H.; Kim, H. Y.; Croke, E. J. "Proceedings of the 19th International Symposium on Combustion"; Combustion Institute: Pittsburgh, PA, 1983; pp 971-980. Correa, S. M.; Sichel, M. "Proceedings of the 19th International Symposium on Combustion"; Combustion Institute: Pittsburgh, PA, 1983; pp 981-991. Giavincevski, B.; Gulder, 0. L.; Gardner, L. SAE Paper No. 841 341, Society of Automotive Engineers: Warrensdaie, PA, 1984. Guider, 0. L.;Wong, J. K. S. "Proceedings of the 20th International Symposium on Combustion"; Combustion Institute: Pittsburgh, PA, 1985. Kondo, T. et al. J. Jpn. Pet. Inst. 1984, 27, 247-251. Kreulen, D. J. W. J . Inst. Pet. Techno/. 1937, 23, 253-285. Labowsky, M. Combust. Sci. Techno/. 1880, 22, 217-226. Labowsky, M.; Rosner. D. E. I n "Evaporation-Combustion of Fuels"; (Tung, J. T., Ed.); American Chemical Society: Washington, DC, 1976; Vol. 166, pp 63-79. Ohuchi, H.; Ohi, A.; Aoyama, H. J . Jpn. Pet. Inst. 1882, 25, 205-212. Olson, D. R.: Meckei, N. T.;Quillian, R. D., Jr. SAE Paper No. 263A, Society of Automotive Engineers: Warrensdaie, PA, 1980.
Received for review August 27, 1985 Revised manuscript received October 22, 1985 This work was presented a t the Symposium on the Chemistry of Cetane Number Improvement, Division of Petroleum Chemistry, Inc., 189th National Meeting of the American Chemical Society, Miami Beach, FL, April 28-May 3, 1985. Released as NRCC 21730.
Effect of Organic Sulfur Compounds on Cetane Number John N. Bowden' and Edwin A. Frame Belvoir Fuels and Lubricants Research Facility, South west Research Institute, San Antonio, Texas 78284
The evaluation of engine lubricating oils by standardized engine tests often requires the use of a diesel fuel containing 1 wt % sulfur, preferably as naturally occurring sulfur compounds. Since diesel fuels with this level of sulfur are not readily available, the addition of tert-butyl dlsulfMe to the test fuel is permitted. The addition of this compound to diesel fuel produced a noticeable increase in cetane number of the fuel. Addition of other sulfur-containing compounds such as mercaptans, sulfides, thiophene, and dibenzothiophene did not significantly affect cetane number. The effects of the sulfur compounds on other properties such as accelerated stability and carbon residue were found to be insignificant for most of the compounds investigated.
Introduction During the early 1940s, hundreds of materials were investigated as cetane-number improvers (Bogen and Wilson, 1944). These included alkyl nitrates and nitrites; aldehydes, ketones, ethers, and esters; peroxides; aromatic nitro compounds; metal derivatives; oxidation products; polysulfides; aliphatic hydrocarbons; nitration products; and oximes and nitroso compounds. Acetone peroxide and alkyl nitrates were found to be the most effective materials for cetane improvement. Polysulfides and other sulfur compounds were mentioned as having some effect on ce0196-432118611225-0156$01.50/0
tane number, but the extent of this effect was not stated. In the work reported here, some sulfur compounds found in diesel fuels used for lubricating-oil evaluation tests were identified, and effects of a number of organic sulfur compounds on cetane number, carbon residue, and accelerated stability were measured.
Background Extensive investigations have been conducted to identify hundreds of sulfur compounds in petroleum crudes and in distillate fractions (Met al., 1972; Coleman et al., 1970; 0 1986 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986
Nunanov et al., 1957; Obolentsev and Aivazov, 1958). A review of this literature indicates that the predominant species of sulfur compounds found in the diesel-fuel boiling range consist of cyclic aliphatic sulfides and aromatic sulfides including thiophenes, thiophanes, benzothiophenes, dibenzothiophenes, and phenyl sulfides. Thiols (mercaptans), alkyl sulfides, and disulfides are probably in lower concentrations and may be more prevalent in the gasoline boiling range. Many researchers have investigated the effects of diesel-fuel sulfur content on compression-ignition engine wear and deposits. Several investigators added known sulfur compounds to diesel fuel to increase sulfur content. Cloud and Blackwood (1943) used carbon disulfide and diamyl trisulfide to increase fuel sulfur content. Moore and Kent (1947) added thiophene, and Blanc (1947) added thiophene, dodecyl mercaptan, and diamyl sulfide. tert-Butyl disulfide was used by Gergel(l980) and McGeehan (1982) to increase sulfur content of fuel employed in diesel-engine deposit tests. Standard engine tests are used to evaluate and qualify lubricating oils for diesel-engine application. One such test was known as the Caterpillar 1D test, now obsolete, and was formerly used to qualify lubricating oils for the API (American Petroleum Institute) classification for service CD (Method 340.3, Federal Test Method Standard 791B). The diesel-fuel specification for this test required a sulfur concentration of 1.0 f 0.05 w t % sulfur, of which 0.35% was to be derived from naturally occurring sulfur compounds and the balance was to be added in the form of tert-butyl disulfide. Military Specification MIL-F-46162B, Fuel, Diesel, Referee Grade, is intended for use in research, development, and proof-testing of all compression-ignition engines used by the U.S. Army. One of the requirements of this fuel is that it should contain between 0.95 and 1.05 wt % sulfur, preferably naturally occurring sulfur; however, if additional sulfur is required, only tert-butyl disulfide shall be added. tert-Butyl disulfide has become a standard material for addition to test fuels for evaluating effects of fuel sulfur on engine wear, deposits, and lubricant effectiveness. This material is less costly than other sulfur compounds and is more readily available, although others may have less effect on other properties of the fuel. The presence of sulfur compounds in diesel fuel is undesirable because of potential increase in engine wear, corrosion, and sulfur dioxide exhaust emissions. However, it is anticipated that high sulfur fuels will become more common in the near future due to the depletion of lowsulfur crudes and the necessity of refining lower quality, high-sulfur crudes. The chemical reaction that takes place in the combustion chamber of a compression-ignition engine has been explained as a chain reaction initiated by formation of free radicals, which eventually results in heat release (Murphy, 1983). The time required for these processes is related to the molecular structure of the fuel. A long molecule such as hexadecane fractures into free radicals more readily than a compact molecule like isooctane. Molecules containing functional groups such as peroxides, nitrates, nitrites, and disulfides apparently form free radicals easily under the conditions of pressure and temperature encountered in the combustion chamber. These free radicals become ignition sites, which reduce the ignition delay, and result in improved cetane numbers. Experimental Section Two fuels meeting the requirements of MIL-F-46162B
n Y E
157
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BASE FUEL
TBDS
TDDS
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Figure 1. Effect of di- and polysulfides on cetane number.
have been used at the Belvoir Fuels and Lubricants Research Facility (BFLRF) for engine and lubricant research. One fuel had a sulfur content of about 1 w t % due to naturally occurring sulfur compounds, while the other had tert-butyl disulfide added to raise the sulfur content level to 1 wt %. Measured cetane number (ASTM D 613), calculated cetane index (ASTM D 976), gravity, and midpoint boiling temperature were determined for both samples. The fuel with natural sulfur had a cetane number of 43 and a cetane index also of 43, while the fuel with added sulfur had a cetane number of 48 and a calculated index of 41. Both high-sulfur fuels were analyzed for hydrocarbon and sulfur distribution by using fused silica capillary gas chromatographic columns and for identification of sulfur compounds by GC-MS spectroscopy. Sulfur compounds were present throughout their boiling ranges; however, the fuel with added sulfur showed, as expected, a large peak for tert-butyl disulfide. Mass spectrometry analyses of these fuels showed that the compounds present in both fuels are primarily benzothiophene, dibenzothiophene, and their homologues, with the exception of the added tertbutyl disulfide. Sulfides, thiols, and other sulfur compounds may have been present at much lower concentrations, but further analyses for these materials were not pursued. The effects of a variety of sulfur-containing organic compounds on cetane number, carbon residue, and oxidation stability were determined by using a base fuel with a sulfur content of 0.26 wt % and a cetane number of 44.9. The compounds listed in Table I were added to the base fuel in concentrations calculated to raise the sulfur content to 0.75 and 1.00 wt %. The data show that the disulfides and polysulfides increased the cetane number of the fuel significantly, while the sulfides, thiols, and thiophenes did not affect the cetane number. The precision of the cetane-number measurement is stated in ASTM D 613 as follows: "The difference between two test results, obtained by the same operator with the same apparatus, would, in the long run, in the normal and correct operation of the test method, exceed the values in the following table only in 1 case in 20." av cetane no. level 40 44 48 52 56
repeatability limits, cetane no. 0.6 0.7 0.7 0.8 0.9
All the measurements in this work were conducted by the same operator with the same apparatus. Figure 1pictures the effects of di- and polysulfides on cetane number. Butyl sulfide, phenyl sulfide, dibenzothiophene, and thiophene appeared to reduce the cetane number of the fuel ever so slightly, although the differences in values are generally within the repeatability of the ce-
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Table I. Blend Compositions and Results component properties wt% wt% compd empirical sulfur, sulfur, samde added" formula thetl detmd NAb NA~ 0.26 base fuel NAb blend 1 ND' CBHl& 35.95 TBDS ND blend 2 CSH& 35.95 TBDS blend 3 BS CBHlBS 21.92 ND blend 4 21.92 ND Cs"S BS ND blend 5 ClZHlOS 17.21 PS blend 6 17.21 ND ClZHlOS PS 17.40 ND blend I ClZHBS DBT ND blend 8 ClZHBS 17.40 DBT ND blend 9 C4H4S 38.10 T ND blend 10 C4H4S 38.10 T 16.3 blend 11 C12H&3 15.84 DDM 21.8 blend 12 CiIHlBS 21.92 OM 15.6 blend 13 CUHwS2 15.92 TDDS 55.9 blend 14 C8H18S4 52.89 TBPS 26.6 blend 15 C14H14SZ 26.02 BDS
wt%
wt%
compd added 0
sulfur, detmd
2.07 1.37 3.41 2.27 4.36 2.89 4.33 2.95 1.95 1.28 4.61 3.44 4.81 1.33 2.80
0.26 1.02 0.75 0.99 0.75 1.01 0.76 1.01 0.76 1.00 0.75 0.99 1.01 1.06 1.10 1.00
blend properties carbon cetane residue, no. 10% bottoms 44.9 48.7 47.4 44.9 43.8 44.5 43.7 43.1 44.5 43.4 43.6 44.9 45.3 52.0 53.5 41.4
0.11 0.10 0.10 0.10 0.10 0.12 0.11 0.11 0.12 0.11 0.10 0.10 0.09 0.49 1.02 0.12
accel stab., ma/100 mL 0.8 0.4
ND' 0.2
ND 0.4
ND 0.6
ND 0.5
ND 25 0.2 5 5 0.6
"TBDS, tert-butyl disulfide. BS, butyl sulfide. PS, phenyl sulfide. DBT, dibenzothiophene. T, thiophene. DDM, dodecyl mercaptan. OM, n-octyl mercaptan. TDDS, tert-dodecyl disulfide. TBPS, tert-butyl polysulfide. BDS, benzyl disulfide. NA, not applicable. ND, not determined. /* x
n
t
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- 9% JET A IO% HEAVV AROMATIC NAPHTHA
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Figure 2. Effect of tert-butyl disulfide on cetane numbers of lowsulfur fuels: (X) Jet A; (0) 50% Jet A, 50% heavy aromatic naphtha.
tane number test. If this reduction is real, it can probably be attributed to the hydrocarbon portion of these compounds, which are aromatic in three cases and a short (C,) paraffin chain in the other, all of which would tend to reduce the cetane number. A normal paraffin chain would be expected to increase cetane number; however, butane has a high octane number and probably low cetane, and therefore it is not surprising that a small decrease in cetane number was observed with butyl sulfide in the fuel. nOctyl mercaptan showed a slight tendency to increase the cetane number. Next a series of experiments were conducted to determine if tert-butyl disulfide was effective in raising the cetane number of a low-natural-sulfur-content fuel and a low-cetane-number fuel. A Jet A fuel, which approximated an arctic diesel fuel (DF-A), was used as the low-natural-sulfur (
