Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 153-156
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I I I. Symposium on “The Chemistry of Cetane Number Improvement” Doren Indritz, Chairman 189th National Meeting of the American Chemical Society Miami Beach, Florida, April 28-May 3, 1985
Prediction of Cetane Number of Diesel Fuels from Carbon Type Structural Composition Determined by Proton NMR Spectroscopy Omer L. Giilder’ and Borls Glavlncevskl Division of Mechanical Engineering, M9, National Research Council of Canada, Ottawa, Ontario K1A OR6, Canada
The relationship between ignition quality and carbon type structure of fuels is summarized, and carbon groups that have dominant effect on ignition quality of the diesel fuels are identified. A scheme of characterizing the chemistry of hydrocarbon fuels in terms of these carbon groups using proton nuclear magnetic resonance spectroscopy has been proposed. Through the use of this analysis technique on 67 different diesel fuels, whose cetane numbers were determined on a number of standard cetane rating engines, an empirical expression that relates the cetane number to the carbon type structural composition of the fuels has been developed. The error in the predicted cetane numbers from the developed expression has been found to be smaller than the spread of the multiple standard cetane rating engine determinations.
Introduction The cetane number, which is the present day measure of the ignition quality of diesel fuels, is a lumped quantity that represents the total effects of spray formation, heating, vaporization, turbulent mixing, and chemical induction times under specified conditions of a single-cylinder standard diesel engine. Essentially, the total ignition delay period (physical and chemical induction times) of a given diesel fuel is matched by a blend of two reference fuels, n-hexadecane (n-cetane) and heptamethylnonane, having arbitrarily assigned values of 100 and 15 cetane numbers, respectively. The term ignition quality is generally used to cover the ignition temperature vs. delay characteristics of a fuel when used in an engine. The starting ability and the engine roughness are controlled, to a certain extent, by the ignition delay characteristics of the fuel. Although the effect of hydrocarbon type composition of the fuel on the ignition quality is qualitatively well-known, the inadequacy of analytical methods to identify the amount of each hydrocarbon (their number can be as high as several hundreds) in practical diesel fuels has prevented the establishment of sound models to quantitatively represent the effects of hydrocarbon types on ignition quality. Ignition of a fuel spray is a dynamic phenomenon not determined by a single property of the fuel or fuel-oxidant system, but by the hydrodynamic and thermodynamic conditions of the system as well as the chemical structure of the fuel. For sufficiently dilute sprays, it is possible to employ single droplet vaporization and ignition formulations to predict the physical and chemical induction times; see, e.g., Aggarwd and Sirignano (1985), Giilder and Wong (1985), and Chao et al. (1985). However. nondilute sprays (typical of diesel engines) behave in a manner that is very different from that of single droplets. When the interdroplet spacing is less than 7000 times the droplet diameter (Labowsky and Rosner, 1978; Labowsky, 1980), there is a strong interaction between droplets, and this has a large effect on the rates of heat and mass transport and evaporation. Group combustion models of Chiu et al. (1983) 0196-432 1/86/1225-0153$01.50/0
propose well-defined regions for droplet clouds, where at the nondilute end of the spectrum external sheath evaporation and combustion are observed. In this region, it is proposed that the droplet cloud effectively behaves like a single droplet but with surface conditions determined by the vaporization wave (Correa and Sichel, 1983), and for spherically symmetric quiescent clouds an ignition temperature of the cloud can be estimated (Annamalai et al., 1984). The highly transient nature of fluid flow, pressure, temperature, and spray characteristics in addition to the lack of data on transport and thermophysical properties of common fuels does not permit the estimation of the physical and chemical induction times of the diesel type sprays by the mathematical models mentioned above. The current standard method (ASTM D613, 1979) of rating diesel fuels in respect to ignition quality has been widely criticized because the repeatability and reproducibility of the test method are very poor, and it is claimed that the ignition characteristics determined by this method do not correlate properly with the ignition delays in production diesel engines, particularly for some alternative fuels. In addition to these objections, the cost and operation time involved in rating ignition quality by the standard method led to a search for “nonengine” methods and correlation expressions in terms of the easily measurable physical properties of the fuels. The proposed methods (Kreulen, 1937; Backhouse and Ham, 1949; Azev et al., 1978; Ohuchi et al., 1982) and correlations have been far from satisfactory in estimating the ignition quality of the diesel fuels. In this paper, we present an assessment of the effects of fuel composition on ignition quality, and a simple technique to determine the carbon type structural composition of the diesel fuel. We then develop a correlation in terms of carbon groups to predict the cetane number with good accuracy. Strategy In order to illustrate the effect of fuel composition on Published 1986 American Chemlcai Society
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986 I
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Figure 1. Effect of fuel composition on the ignition quality of hydrocarbon fuels.
ignition quality, the data presented by Olson et al. (1960) are plotted in Figure 1. The base fuel is a blend of 25% n-hexadecane and 75% isooctane, with a cetane number of 38.3. Pure paraffinic (n-, iso-, and cycloalkanes), aromatic, and olefinic hydrocarbons of various carbon numbers were added to the base fuel, up to 20% (v/v). Resulting cetane numbers and corresponding ignition delays measured exhibit a good correlation, Figure 1. Normal alkanes, excluding n-pentane and lower carbon number alkanes, improve the ignition quality. As the chain length gets longer (higher carbon number), the degree of cetane improvement increases. Isoalkanes, depending on the degree of branching, degrade the ignition quality. However, if the branching is concentrated a t one end of the molecule, and the other part of it has a long chain (e.g., 3,4-dimethyldecane and 3,3-diethyloctane), these types of isoalkanes improve the ignition quality. Cycloalkanes and aromatie3 generally reduce the cetane number, unless they have a long normal-alkane chain attached to the ring (e.g., n-hexylbenzene improves the ignition quality). From the foregoing discussion, it is clear that each member of a homologous series of hydrocarbons does not have the same ignition characteristics as the other members of the series. In order to differentiate the degree of effeds we propose a fuel characterization scheme in terms of different carbon groups of the fuel. In the light of the discussion on Figure 1, we classify the carbon types in a hydrocarbon molecule as follows: CA,carbons on mono and condensed aromatic rings; C,, carbons at a-position to aromatic rings; C2, alkane CH,, CH carbons including P-CH,, y-CH, and P-CH, to aromatic rings; and C3, alkane CH, carbons including terminal and branched and y to aromatic rings. In the next section we present the experimental method used to determine these carbon groups from the hydrogen type distribution of diesel fuels. Method of Experimental Analysis In the present study a proton nuclear magnetic resonance spectrometer ('HNMR) has been used to identify the hydrogen type distribution in diesel fuels. The lH NMR spectra have been obtained at 60 MHz on a Varian EM-360 spectrometer. Concentrations of approximately 50/50 v/v of sample to solvent have been used for recording the spectra. Chloroform-dl (99.9%) with 170 Me,Si has been used as solvent. The proton resonance signals of the fuels have been divided into five regions of chemical shift values, and the signals in each region have been assigned to various types of hydrogens relative to
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Figure 2. Regions of five hydrogen types on a typical proton NMR spectrum. Table I. Regions of Hydrogen Types on 'H NMR SDectra hydrogen type chemical shift from Me,Si, ppm 9.0-7.05 7.05-6.5
4.0-1.8 1.8-1.06 1.06-0.5
MelSi (Figure 2). The relative number of hydrogen atoms in each region can be calculated from the area of the signals of the corresponding region. These regions have been specified in Table I. The relative number of carbon atoms is calculated from the area ratio of the hydrogen type to the number of hydrogens per carbon atom. To account for the aromatic ring-junction carbons the hydrogen signal intensity Hdar is multiplied by 1.4. The aromatic ring carbons substituted by alkyl groups are equivalent to a-alkyl hydrogen integral divided by 2.5 (CJ. The sum of 1.4 Hdar,H, and C, gives the number of aromatic carbon atoms, Ck Because of the low concentration of methine and methyl groups in the H, region, it is assumed that two hydrogens correspond to one carbon atom (C2),whereas three hydrogen atoms in the area Hd are equivalent to one carbon atom (C3). The sum of the relative number of carbon atoms of each region is normalized so that CA + c, + c2 + c3 = 100 The calculation procedure is outlined in Figure 2 on a spectrum of a typical fuel. A total of 67 fuels have been analyzed. Cetane numbers of these fuels have been measured on multiple standard cetane rating engines (ranging from 11to 22 engines) located in various institutions in North America. The mean of these determinations for each fuel is taken as the true observed cetane number, These fuels cover a broad range of ignition quality, i.e., from 20 to 75 cetane numbers. The secondary reference fuels, U-11 and T-18, are also included in this study.
m.
Development of a Prediction Equation Through the use of the carbon type composition of 67 fuels in terms of CA,C,, C2,and C3,an expression of the
Ind. Eng.
Table 11. Measures of the Correlation 1.11 standard deviation of residuals, u correlation coefficient, R 0.992 percentage of variation explained by the estimated model 98.323 residual standard deviation expressed as a percentage of 2.41 response mean
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OBSERVED CETANE NUMBER,
Figure 4. Comparison of the spread of cetane numbers determined by multiple engines (11-22 engines) to the error in the predicted cetane numbers. The arrow shows the case where predicted cetane number lies outside the spread of the measured cetane numbers.
20
.30 LO 50 60 OBSERVED CETANE NUMBER,
70
80
Figure 3. Predicted cetane numbers vs. observed cetane numbers.
following functional form has been selected for prediction of the cetane number:
-
GCN = Bo +
-
1: $1 +
+ (B3C, + B4 In C,) +
-
(B5C2 + BGCZ2)+ (B,C3 + B&?) As a result of a multiple regression analysis the following values for Bi have been determined: Bo = 24.3848, B1 = -286.728, B2 = 587.3567 B, 1.5227, B4 -15.882, B, = 0.9778 Be = 0.0047, B7 = -0.2835, B8 = 0.002 Measures of correlation are listed in Table 11.
Discussion Cetane numbers of the 67 fuels predicted by the proposed expression are plotted against the observed cetane in Figure 3. The agreement between the numbers, predicted values and the observed cetane numbers is excellent. A correlation coefficient of 0.992 with a standard deviation of residuals of 1.11 (Table 11) shows that the suggested technique of characterization and the developed correlation expression are very powerful tools in assessing the ignition quality of the diesel fuels. The spread of the measured cetane numbers (i.e., CN-CNi) for each fuel on a number of standard cetane rating engines is shown in Figure 4 for the fuels analyzed. The error in the predicted cetane numbers obtained from the developed expression (i.e., GCN-m) are also shown in the same figure as plotted against observed cetane numbers. The error in the predicted cetane numbers is smaller than the spread of the multiple engine ratings, Figure 4. The predicted cetane number of only 1fuel out of 67 is not within the spread of measured values, although it is still within the general reproducibility band of the standard cetane rating engine. The cetane numbers of the secondary reference fuels U-11and T-18 have been predicted by the proposed ex-
m,
pression as 20 and 75.3, which have cetane ratings of 20.5 and 75, respectively. The success of the present technique of characterization is in its capability to fully describe the fuel chemistry. Previous attempts by Kreulen (1937), Backhouse and Ham (1949),and Kondo et al. (1984) lack this type of description and were not successful. The method proposed by Ohuchi et al. (1982), which tries to correlate the number of hydrogen atoms at 0and farther positions to the aromatic ring (excluding terminal methyl groups) to the ignition quality, produces errors as high as f10 cetane numbers. Of course, it is not possible to attach any immediate physical and/or chemical models to the purely empirical expression presented, due to the complex physicochemical processes occurring in the course of ignition. If it is assumed that the fluid mechanics of the flow in the prechamber of the standard engine does not change appreciably with small changes in compression ratio, the variations in physical induction periods of the fuels would then be governed by the properties such as enthalpy of vaporization, density, specific heat, thermal conductivity, vapor pressure, and molecular diffusivity. These properties are related to molecular structure, and relevant characteristics of the structure are related to atoms, atomic groups, and bond types. Suitable relationships can then be established between identified hydrogen types and fuel properties of concern. So the developed equation lumps the effects of thermophysical properties, which determine the length of the physical induction time with the fluid mechanics of the system, and the chemical induction time into one relationship in terms of carbon groups. This does not shed much light on the physics of the spray ignition, but the developed equation predicts the quantitative changes in cetane number with variations in the fuel's carbon type structural composition. In order to show the quantitative changes in ignition quality with the variations of fuel's carbon type structure, cetane number is plotted as a function of CA,C,, C2, and C3 in Figure 5 for prescribed conditions. In a previous effort (Glavincevski et al., 1984) we obtained a similar expression for the cetane number estimation using 134 fuels. However, 67 of those fuels were tested on one standard engine only for cetane number determination, and the obtained correlation yielded a standard deviation of 1.47, as compared to 1.11 of the present expression. It should be noted that the present expression is valid under the following conditions: (a) fuels are nonolefinic (less than 1%)and heteroatom (0,S, N) content is less than 3%; (b) fraction of cycloalkanes is small; (c) fuels do
Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 156-159
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Literature Cited
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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.
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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
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
