[Regular Paper] Prediction of Physical Properties and Cetane Number of Diesel Fuels and the Effect of Aromatic Hydrocarbons on These Entities (Received March 13, 1995) The gross heat of combustion and cetane number of diesel fuels have been predicted from the experimental values of refractive index; moreover, cetane number has also been predicted from the gross heat of combustion. These predicted values have agreed well with the experimental values. The effects of aromatic hydrocarbons (aromatics, single-ring aromatics, polynuclear aromatics) on such physical properties as density, refractive index, distillation temperature, kinematic viscosity, gross heat of combustion and cetane number of diesel fuels have been studied systematically. Density, refractive index, and distillation temperature (50% and 90%) increased with increasing mole fraction of polynuclear aromatics, but the gross heat of combustion and cetane number of diesel fuels decreased. The results showed that polynuclear aromatics affected the physical properties and cetane number of diesel fuels. 1. Introduction ASTM D4868-90 standard method is used to predict the gross heat of combustion of diesel fuels1), and for such prediction, four experimental data are needed: density, sulfur content, water content, and ash content. We ignored the contribution of the slight contents of water and ash to the gross heat of combustion. Diesel fuels containing low-levels of sulfur, which is related to atmospheric pollution, were produced; so we also ignored the content of sulfur in the fuels. The gross heat of combustion is affected by density. One of the physical properties, which is affected by composition, pressure, and temperature, is the refractive index. Based on the study of T. E. Daubert2), the density of diesel fuels is intimately related to the refractive index. Therefore, the gross heat of combustion predicted by density is predicted also by refractive index. Cetane number is a measure of ignition quality, specifically ignition delay, which is related to the composition and thermodynamic properties of diesel fuels3)-6), and it is determined by the standard engine test method ASTM D6131) which, however, has been widely criticized for rating the ignition quality of diesel fuels because of its poor the repeatability and reproducibility; moreover, it is claimed that the ignition characteristics determined by this method * To whom correspondence should be addressed. do not properly correlate with the ignition delays produced in the diesel engines, particularly for some alternative fuels. In addition to these objections, the cost and operation time involved in the rating of ignition quality led to a search for " nonengine" methods and correlation expressions in terms of the easily measurable physical properties of diesel fuels4)-7). These studies have predicted cetane number, from the carbon type structural composition of the diesel fuel, in other words, cetane number can be induced by certain thermodynamic functions which are also related to the composition of diesel fuels. In this paper, we predicted the cetane number from the gross heat of combustion and refractive index, which are easily measurable thermodynamic entities. We also predicted the gross heat of combustion from the refractive index of diesel fuels, using linear regression analysis. The gross heat of combustion thus resulted gave better results than the values estimated by ASTM D4868-90 method. And the cetane number predicted from refractive index and gross heat of combustion resulted also in better agreement than that evaluated by ASTM D976 using density, 50% distillation temperature. Air-pollution problems caused by such fuels as gasoline and diesel are aggravated with the increasing number of automobiles. It is known that air-pollution results from the sulfur and aromatics found in diesel fuel. Sulfur which can be reduced by desulfurization, and the aromatic
components which affect specifications and cetane number are environmental pollution sources. However, there are restrictive problems with the use of diesel fuels. Recently the relationship between aromatic components and physical properties and cetane number of diesel fuels produced by hydrocracking process of heavy fractions has become a most subject of research. In this research, we obtain various kinds of aromatic components that should be restricted as atmospheric pollutants. Recently, the effect of the aromatics on density and cetane number of pure hydrocarbon compounds6) have been studied. In the mixtures of various kinds of hydrocarbons found, for example in diesel fuels, the effects of such aromatic components on physical properties and cetane numbers have been studied from the compositional data and characteristics of the molecules4),8),9) since 1980. We separated the aromatic hydrocarbons into single-ring and polynuclear aromatic hydrocarbons and systematically studied their effects on cetane number and such physical properties as density, refractive index, distillation temperature, kinematic viscosity, gross heat of combustion of diesel fuels. In this study we hope first, to confirm the kinds of aromatic components causing air-pollution; second, to investigate the effects of various kinds of aromatic hydrocarbons on the physical properties and cetane numbers of diesel fuels; and third, to control these aromatic components to improve the fuel quality. ASTM D4868; sulfur content, ASTM D4294; ash content, ASTM D482; water content, ASTM D1744; cetane number, ASTM D613 standard engine test; and cetane index, ASTM D976. The equation developed by M. R. Riazi10) was used to estimate the molecular weights of the diesel fuels. 2.2. Method of Compositional Analysis of Diesel Fuels High performance liquid chromatographic method was used to analyze the composition of diesel fuels. Compositional details were defined as saturates, single-ring aromatics, and polynuclear aromatics (containing the resins). Detailed conditions of compositional analysis are given in Table 1. Diesel fuel components are eluted in order of saturates, single-ring aromatics, polynuclear aromatics as shown in Fig. 1. 2. Experimental 2. 1. Determination of Physical and Dynamic Properties of Diesel Fuels 32 diesel fuels with no additives, produced from the atmospheric distillation process were used and their physical and dynamic properties were determined using ASTM standard methods1) as follows: density, ASTM D4052; refractive index, ASTM D1218; distillation, ASTM D86; kinematic viscosity, ASTM D445; gross heat of combustion, ASTM D2382; estimation of gross heat of combustion, Fig. 1 High Performance Liquid Chromatogram of Diesel Fuel Table 1 Analytical Conditions of HPLC
Fig. 2 The Relationship between Gross Heat of Combustion (experimental values) and Refractive Index Fig. 3 Predicted Gross Heat of Combustion by Eq. (1) and ASTM D4868 vs. Observed Gross Heat of Combustion 3. Results and Discussion As seen from Fig. 2, which illustrates the change in the gross heat of combustion with the measured refractive index of diesel fuel, the gross heat of combustion decreased in definite proportion with the refractive index. In Fig. 2, it is shown that the relation between the gross heat of combustion and the refractive index could be represented by the following Eq. (1). Where H represents the gross heat of combustion in kpa and n represents the refractive index of unit was considered in the use of Eq. (1). Figure 3 is the plot of gross heat of combustion predicted by Eq. (1) and ASTM D4868-90 against the measured gross heat of combustion. Figure 3 also indicates that gross heat of combustion predicted based on refractive indices are closer to the measured one than that estimated based on ASTM D4868-90. Figure 4 shows the plot of cetane number which represent the kinetic properties of diesel fuel against the measured gross heat of combustion representing thermodynamic properties. The cetane number increase with the gross heat of combustion. This fact indicates that cetane number related with combustion properties of the fuel is directly related to the thermodynamic properties. In Fig. 4, it is shown that the gross heat of combustion and the cetane number both change with the same tendency in proportion with the aromatic components in the saturated portion comprising paraffin and naphthenics, suggesting Fig. 4 The Relationship between Cetane Number and Gross Heat of Combustion (experimental values) that both properties are closely linked with the composition of the fuel. The correlation between cetane number and the gross heat of combustion as determined using the 2nd order polynomial fitting method from Fig. 4 can be represented by Eq. (2): Where CN represents the cetane number and H represents the gross heat of combustion in kpa. (Unit was not considered in calculation of cetane number from this equation.) The combined use of Eqs. (1) and (2) will allow the evaluation of cetane numbers from the measured refractive indices. Figure 5 shows the plot of cetane numbers, which were measured by the standard engine test
Fig. 5 The Relationship between Cetane Number and Refractive Index Fig. 6 Predicted Cetane Numbers by Eqs. (2), (3) and ASTM D976 vs. Observed Values Fig. 7 Effects of Aromatics (total aromatics, single-ring aromatics, polynuclear aromatics, and polynuclear aromatics/total aromatics) on Density of Diesel Fuel
Fig. 8 Effects of Aromatics (total aromatics, single-ring aromatics, polynuclear aromatics, and polynuclear aromatics/total aromatics) on Refractive Index of Diesel Fuel method, versus the measured refractive indices. As shown in Fig. 5, the cetane numbers decrease with refractive indices following a definite tendency. The change in cetane numbers based on refractive indices, determined by the 2nd order polynomial fitting method gives Eq. (3): The results of cetane numbers evaluated from Eqs. (2) and (3) versus those measured by ASTM D613 are graphically represented in Fig. 6 and the results of their comparison with the cetane indices calculated from density and 50% distillation temperatures by ASTM D976 are depicted also in Fig. 6 shows good agreement of the cetane numbers estimated from the gross heat of combustion and the refractive indexes with the cetane numbers actually measured by engine test, and the cetane numbers predicted from the gross heat of combustion and refractive indices of diesel fuels, show better agreement with the measured ones than with the cetane indices calculated by ASTM D976. As seen from the above results and from Fig. 5, the physical and kinetic properties of the diesel fuel, which comprises a multicomponent hydrocarbon system, vary with the composition of the fuel. The effects of aromatic components in the diesel fuel on various physical properties such as density, refractive index, distillation temperature, kinematic viscosity, gross heat of combustion, and the cetane number representing dynamic properties, which define the specification of quality and characteristics of the diesel fuel mixtures of numerous hydrocarbons, have been studied. The effects of total aromatic hydrocarbons, single-ring aromatics, polynuclear aromatics, and the ratio of polynuclear aromatics to the total aromatics on the physical properties (density, refractive indices, kinematic viscosity, gross heat of combustion) and cetane number of diesel fuels, are plotted in Figs. 7 to 11 in which mole fraction was used as the unit of
Fig. 9 Effects of Aromatics (total aromatics, single-ring aromatics, polynuclear aromatics, and polynuclear aromatics/total aromatics) on Viscosity of Diesel Fuel aromatic content. In Figs. 7 to 11, densities, refractive indexes and gross heat of combustion as well as cetane numbers of diesel fuel are shown to vary with the aromatics content and with the ratio of polynuclear aromatics to total aromatics following a specific tendency. Density, refractive index and kinematic viscosity increase generally with increasing content of aromatic components. On closer observation, they decrease with increasing content of single-ring aromatics but increase with increasing content of polynuclear aromatics. The trend of density, refractive index, and kinematic viscosity of pure aromatic hydrocarbons may better be explained by reference to Fig. 12, in which the density of polynuclear aromatics increases with the increase in molecular weight for 1 to 3 carbon atoms in the straight alkyl chain bonded to the benzene ring, but decrease with 4 or more carbon atoms. For the single-ring aromatics, density is substantially constant or slightly decreased over the whole range of molecular weight, with increase in the number of carbon atoms in the bonded alkyl group6). These facts may similarly be extended to explain for the refractive indexes and kinematic viscosities depicted in Figs. 8 and 9. Therefore, the number of carbon atoms in the straight chain alkyl attached to any polynuclear aromatic hydrocarbon in diesel fuels is estimated to be from 1 to 5. Figure 11 relating to cetane number, which is important in evaluating the quality characteristics of diesel fuels, shows a decrease with increasing content of total aromatics or polynuclear aromatics but an increase with increasing content of single-ring aromatics. This observation corresponds substantially to the reverse trend that was seen in density and others mentioned previously. Such a tendency can be well explained by Fig. 13, which illustrates the changing tendency of cetane number with increase in the number of carbon atoms (or molecular weight) included in the straight chain
Fig. 10 Effects of Aromatics (total aromatics, single-ring aromatics, polynuclear aromatics, and polynuclear aromatics/total aromatics) on Gross Heat of Combustion of Diesel Fuel alkyl group of in the saturates pure aromatic hydrocarbons. Furthermore, in Fig. 13, the number in the plot represent the number of carbon atoms in the side chains of these molecules. For example, 5 on the benzene line represents n- pentylbenzene, 5 on the paraffin line represents n- pentane. In the case of polynuclear aromatics, referring again to Fig. 13, cetane number decreases more or less with the molecular weight over the range up to 2 or 3 carbon atoms in the alkyl group bonded to the benzene ring, but slightly increases over the range of 4 or more carbon atoms6). However, such a tendency of increase in cetane number is very dull compared with the marked increasing tendency in the case of single-ring aromatics. Accordingly, the decrease in cetane number due to the total aromatics means that the effect of polynuclear aromatic components is predominant. Figure 10, which concerns with the gross heat of combustion representing direct thermodynamic properties, indicates that the various type of aromatics shown in the figure, have the same tendency as that of the results of cetane number representing the ignition quality of diesel fuels. This fact backs up Fig. 6 which relate to the cetane number derived from the measured value of gross heat of combustion. From Figs. 7 to 11, it is seen that the effects of aromatic components on the physical properties and the cetane number of diesel fuels are dominated by the content of polynuclear aromatic components which is less than that of single-ring aromatics. Figure 14 shows the plots of temperature when 50% and 90% have been distilled off in fractional distillation against the content of total aromatic components, the content ratio of total aromatics to the saturated (paraffin+naphthenics), the content of polynuclear aromatics, and the content ratio of polynuclear aromatics to polynuclear plus singlering aromatics. Both 50% and 90% distillation
Fig. 11 Effects of Aromatics (total aromatics, single-ring aromatics, polynuclear aromatics, and polynuclear aromatics/total aromatics) on Cetane Number of Diesel Fuel Fig. 12 Density of Pure Compounds with Normal Alkyl Side Chains Fig. 13 Cetane Number of Pure Compounds with Normal Alkyl Side Chains
Fig. 14 Effects of Aromatics (total aromatics, aromatics/saturates, polynuclear aromatics, and polynuclear aromatics/single-ring aromatics) on Distillation Temperature (T50%, T90%) of Diesel Fuels temperatures have a tendency to increase particularly with the increase in the amount of polynuclear aromatic components. The fact suggests the need to lower 90% distillation temperature in order to reduce the polynuclear aromatic components. The relationship between physical properties and molecular weights of diesel fuels is given in Fig. 15 which shows that, in the cases of density, refractive index, gross heat of combustion, and cetane number, the long curved line of points in the upper left corner of the plot is the results from the saturates (paraffin+naphthenics), and the other points of the plot are the results from aromatics, especially polynuclear aromatics. 4. Conclusions The gross heat of combustion and cetane number of diesel fuels have been predicted from refractive index. Cetane number has also been predicted from the measured value of gross heat of combustion. The results of gross heat of combustion predicted from the refractive index are better than those estimated by ASTM D4868-90. The cetane numbers predicted respectively from refractive index and gross heat of combustion result in better agreement with the measured ones, and the agreement was better than that of cetane index calculated by ASTM D976. The effects of the aromatic hydrocarbons on the physical and dynamic properties are as follows: 1) Density, refractive index, and 50% and 90% distillation temperatures increase with increasing content of polynuclear aromatics. 2) The gross heat of combustion and cetane number decrease with increasing content of polynuclear aromatics. 3) Polynuclear aromatics are the main components that affect the physical properties and cetane number.
Fig. 15 The Relationship between Physical Properties and Molecular Weight of Diesel Fuels References 1) ASTM, "Annual Book of ASTM Standards," Philadelphia (1990), D-482, D-613, D-976, D-1218, D-1744, D-2382, D-4052, D-4294, D-4868-90. 2) Riazi, M. R., Daubert, T. E., Ind. Eng. Chem., Process Des. Dev., 19, 289 (1980). 3) Backhouse, T., Ham, A. J., Fuel, 28, 246 (1949). 4) Glavincevski, B., Gulder, O. L., Gardner, L., SAE Paper, No. 841341 (1984). 5) Gulder, O. L., Glavincevski, B., Kallio, N. N., SAE Paper, No. 892073 (1989). 6) DeFries, T. H., Indritz, D., Kastrup, R. V., Ind. Eng. Chem. Res., 26, 188 (1987). 7) Choi, J. H., Chun, Y. J., Choi, U. S., Choi, Y. S., Kwon, O. K., J. Korean Ind. & Eng. Chem., 4, 709 (1993). 8) Cookson, D. J., Latten, J. L., Shaw, I. M., Smith, B. E., Fuel, 64, 509 (1985) 9) Gulder, O. L., Glavincevski, B., Ind. Eng. Chem., Prod. Res. Dev., 25, 153 (1986). 10) American Petroleum Institute (API), "API Technical Data Book," 2B2.1 (1986).
Keywords Prediction, Physical properties, Cetane number, Diesel fuel, Aromatics, Refractive index