prediction of physical properties and cetane number of

石油学会誌 Sekiyu Gakkaishi, 38, (6), 421-431 (1995) 421

[Regular Paper]

Prediction of Physical Properties and Cetane Number of Diesel Fuels

and the Effect of Aromatic Hydrocarbons on These Entities

Ju-Hwan CHOI†2)*, Young-Sang CHOI†2), and Oh-Kwan KWON†1)

†1) Tribology Lab., KIST, Seoul 136-791, Korea

†2) Dept. of Chemistry, KOREA Univ., Seoul 136-701, Korea

(Received March 13, 1995)

The gross heat of combustion and cetane number of diesel fuels have been predicted from theexperimental values of refractive index; moreover, cetane number has also been predicted from the grossheat of combustion. These predicted values have agreed well with the experimental values. The effectsof aromatic hydrocarbons (aromatics, single-ring aromatics, polynuclear aromatics) on such physical

properties as density, refractive index, distillation temperature, kinematic viscosity, gross heat ofcombustion and cetane number of diesel fuels have been studied systematically. Density, refractiveindex, and distillation temperature (50% and 90%) increased with increasing mole fraction of polynucleararomatics, but the gross heat of combustion and cetane number of diesel fuels decreased. The resultsshowed 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 dieselfuels1), and for such prediction, four experimental

data are needed: density, sulfur content, water

content, and ash content. We ignored the con-

tribution of the slight contents of water and ash to

the gross heat of combustion. Diesel fuels con-

taining 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 num-ber 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

do not properly correlate with the ignition delays

produced in the diesel engines, particularly forsome alternative fuels. In addition to these objec-tions, the cost and operation time involved in therating of ignition quality led to a search for"nonengine" methods and correlation expressionsin terms of the easily measurable physical pro-

perties of diesel fuels4)-7). These studies havepredicted cetane number, from the carbon typestructural composition of the diesel fuel, in otherwords, cetane number can be induced by certainthermodynamic functions which are also related tothe composition of diesel fuels. In this paper, we

predicted the cetane number from the gross heat ofcombustion and refractive index, which are easilymeasurable thermodynamic entities. We also

predicted the gross heat of combustion from therefractive index of diesel fuels, using linear regres-sion analysis. The gross heat of combustion thusresulted gave better results than the values esti-mated by ASTM D4868-90 method. And thecetane number predicted from refractive index and

gross heat of combustion resulted also in betteragreement than that evaluated by ASTM D976using density, 50% distillation temperature.

Air-pollution problems caused by such fuels as

gasoline and diesel are aggravated with theincreasing number of automobiles. It is knownthat air-pollution results from the sulfur andaromatics found in diesel fuel. Sulfur which canbe reduced by desulfurization, and the aromatic* To whom correspondence should be addressed.

石油学会誌 Sekiyu Gakkaishi, Vol. 38, No. 6, 1995

422

components which affect specifications and cetanenumber are environmental pollution sources.However, there are restrictive problems with theuse of diesel fuels. Recently the relationship be-tween aromatic components and physical prop-erties and cetane number of diesel fuels producedby hydrocracking process of heavy fractions hasbecome a most subject of research. In this re-search, we obtain various kinds of aromatic com-

ponents that should be restricted as atmosphericpollutants. Recently, the effect of the aromaticson density and cetane number of pure hydrocarboncompounds6) have been studied. In the mixturesof various kinds of hydrocarbons found, forexample in diesel fuels, the effects of such aromaticcomponents on physical properties and cetanenumbers have been studied from the composi-tional data and characteristics of the molecules4),8),9)since 1980.

We separated the aromatic hydrocarbons intosingle-ring and polynuclear aromatic hydrocar-bons and systematically studied their effects oncetane number and such physical properties asdensity, refractive index, distillation temperature,kinematic viscosity, gross heat of combustion ofdiesel fuels. In this study we hope first, to con-firm the kinds of aromatic components causingair-pollution; second, to investigate the effects ofvarious kinds of aromatic hydrocarbons on the

physical properties and cetane numbers of dieselfuels; and third, to control these aromatic com-

ponents to improve the fuel quality.

2. Experimental

2. 1. Determination of Physical and DynamicProperties of Diesel Fuels

32 diesel fuels with no additives, produced fromthe atmospheric distillation process were used andtheir physical and dynamic properties were deter-mined using ASTM standard methods1) as follows:density, ASTM D4052; refractive index, ASTMD1218; distillation, ASTM D86; kinematic visco-sity, ASTM D445; gross heat of combustion, ASTMD2382; estimation of gross heat of combustion,

ASTM D4868; sulfur content, ASTM D4294; ashcontent, ASTM D482; water content, ASTMD1744; cetane number, ASTM D613 standardengine test; and cetane index, ASTM D976. The

equation developed by M. R. Riazi10) was used toestimate the molecular weights of the diesel fuels.2.2. Method of Compositional Analysis of Diesel

FuelsHigh performance liquid chromatographic

method was used to analyze the composition of

diesel fuels. Compositional details were definedas saturates, single-ring aromatics, and polynu-clear aromatics (containing the resins). Detailedconditions of compositional analysis are given inTable 1. Diesel fuel components are eluted inorder of saturates, single-ring aromatics, poly-nuclear aromatics as shown in Fig. 1.

Fig. 1 High Performance Liquid Chromatogram of

Diesel Fuel

Table 1 Analytical Conditions of HPLC


423

3. Results and Discussion

As seen from Fig. 2, which illustrates the changein the gross heat of combustion with the measuredrefractive index of diesel fuel, the gross heat ofcombustion decreased in definite proportion withthe refractive index.

In Fig. 2, it is shown that the relation betweenthe gross heat of combustion and the refractiveindex could be represented by the followingEq. (1).

H=93.116-32.239×n (1)

Where H represents the gross heat of combustion

in kPa and n represents the refractive index of

diesel fuel as measured at 20℃. As is clear, no

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 againstthe measured gross heat of combustion. Figure 3also indicates that gross heat of combustion

predicted based on refractive indices are closer tothe measured one than that estimated based onASTM D4868-90. Figure 4 shows the plot ofcetane number which represent the kinetic pro-

perties of diesel fuel against the measured grossheat of combustion representing thermodynamic

properties. The cetane number increase with thegross heat of combustion. This fact indicates thatcetane number related with combustion propertiesof the fuel is directly related to the thermodynamic

properties. In Fig. 4, it is shown that the grossheat of combustion and the cetane number bothchange with the same tendency in proportion withthe aromatic components in the saturated portioncomprising paraffin and naphthenics, suggesting

that both properties are closely linked with thecomposition of the fuel. The correlation betweencetane number and the gross heat of combustion asdetermined using the 2nd order polynomial fittingmethod from Fig. 4 can be represented by Eq. (2):

CN=4.3593×104-1922.2×H+21.214×H2

(2)

Where CN represents the cetane number and Hrepresents the gross heat of combustion in kPa.

(Unit was not considered in calculation of cetanenumber from this equation.) The combined useof Eqs. (1) and (2) will allow the evaluation ofcetane numbers from the measured refractiveindices.

Figure 5 shows the plot of cetane numbers,which were measured by the standard engine test

Fig. 2 The Relationship between Gross Heat of Com-

bustion (experimental values) and Refractive

Index

Fig. 3 Predicted Gross Heat of Combustion by Eq. (1)

and ASTM D4868 vs. Observed Gross Heat of

Combustion

Fig. 4 The Relationship between Cetane Number and

Gross Heat of Combustion (experimental values)


424

Fig. 5 The Relationship between Cetane Number and

Refractive IndexFig. 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


425

method, versus the measured refractive indices.

As shown in Fig. 5, the cetane numbers decreasewith refractive indices following a definite

tendency. The change in cetane numbers basedon refractive indices, determined by the 2nd order

polynomial fitting method gives Eq. (3):

CN=4.2799×104-5.7558×104×n

+1.9374×104×n2 (3)

The results of cetane numbers evaluated fromEqs. (2) and (3) versus those measured by ASTMD613 are graphically represented in Fig. 6 and theresults of their comparison with the cetane indicescalculated from density and 50% distillation tem-

peratures by ASTM D976 are depicted also in Fig. 6shows good agreement of the cetane numbersestimated from the gross heat of combustion andthe refractive indexes with the cetane numbersactually measured by engine test, and the cetanenumbers predicted from the gross heat ofcombustion and refractive indices of diesel fuels,

show better agreement with the measured onesthan with the cetane indices calculated by ASTMD976.

As seen from the above results and from Fig. 5,the physical and kinetic properties of the dieselfuel, which comprises a multicomponent hydro-carbon system, vary with the composition of thefuel. The effects of aromatic components in thediesel fuel on various physical properties such as

density, refractive index, distillation temperature,kinematic viscosity, gross heat of combustion, andthe cetane number representing dynamic prop-erties, which define the specification of qualityand characteristics of the diesel fuel mixtures ofnumerous hydrocarbons, have been studied. Theeffects of total aromatic hydrocarbons, single-ringaromatics, polynuclear aromatics, and the ratio of

polynuclear aromatics to the total aromatics on thephysical properties (density, refractive indices,kinematic viscosity, gross heat of combustion) andcetane number of diesel fuels, are plotted in Figs. 7to 11 in which mole fraction was used as the unit of

Fig. 8 Effects of Aromatics (total aromatics, single-ring aromatics, polynuclear aromatics,

and polynuclear aromatics/total aromatics) on Refractive Index of Diesel Fuel


426

aromatic content.In Figs. 7 to 11, densities, refractive indexes and

gross heat of combustion as well as cetane numbersof 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 con-

tent of aromatic components. On closer observa-tion, 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 in-

creases 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 mole-cular weight, with increase in the number ofcarbon atoms in the bonded alkyl group6). Thesefacts may similarly be extended to explain forthe refractive indexes and kinematic viscositiesdepicted in Figs. 8 and 9. Therefore, the numberof carbon atoms in the straight chain alkyl attachedto any polynuclear aromatic hydrocarbon in dieselfuels is estimated to be from 1 to 5. Figure 11relating to cetane number, which is important inevaluating the quality characteristics of dieselfuels, shows a decrease with increasing content oftotal aromatics or polynuclear aromatics but anincrease with increasing content of single-ringaromatics. This observation corresponds sub-stantially to the reverse trend that was seen indensity and others mentioned previously. Such atendency can be well explained by Fig. 13, whichillustrates the changing tendency of cetane numberwith increase in the number of carbon atoms (ormolecular weight) included in the straight chain

Fig. 9 Effects of Aromatics (total aromatics, single-ring aromatics, polynuclear

aromatics, and polynuclear aromatics/total aromatics) on Viscosity of Diesel

Fuel


427

alkyl group of in the saturates pure aromatic

hydrocarbons. Furthermore, in Fig. 13, the num-

ber 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 withthe gross heat of combustion representing direct

thermodynamic properties, indicates that thevarious type of aromatics shown in the figure, havethe same tendency as that of the results of cetanenumber representing the ignition quality of dieselfuels. This fact backs up Fig. 6 which relate tothe cetane number derived from the measuredvalue of gross heat of combustion. From Figs. 7to 11, it is seen that the effects of aromatic com-

ponents on the physical properties and the cetanenumber of diesel fuels are dominated by thecontent of polynuclear aromatic componentswhich is less than that of single-ring aromatics.

Figure 14 shows the plots of temperature when

50% and 90% have been distilled off in fractionaldistillation against the content of total aromaticcomponents, the content ratio of total aromatics tothe saturated (paraffin+naphthenics), the contentof polynuclear aromatics, and the content ratio of

polynuclear aromatics to polynuclear plus single-ring aromatics. Both 50% and 90% distillation


aromatics, and polynuclear aromatics/total aromatics) on Gross Heat of

Combustion of Diesel Fuel


428


aromatics, and polynuclear aromatics/total aromatics) on Cetane Number

of Diesel Fuel

Fig. 12 Density of Pure Compounds with Normal Alkyl Side

Chains

● Naphthalenes, ● Tetralins, ● Benzenes, ● Cyclohexanes,

● 1-Olefins, ○ Paraffins.

Fig. 13 Cetane Number of Pure Compounds with Normal

Alkyl Side Chains

● Naphthalenes, ● Tetralins, ● Benzenes, ● Cyclohexanes,

● 1-Olefins, ○ Paraffins.


429

temperatures have a tendency to increase

particularly with the increase in the amount ofpolynuclear aromatic components. The fact sug-gests the need to lower 90% distillation temperaturein order to reduce the polynuclear aromatic com-

ponents.The relationship between physical properties

and molecular weights of diesel fuels is given inFig. 15 which shows that, in the cases of density,refractive index, gross heat of combustion, andcetane number, the long curved line of points inthe upper left corner of the plot is the results fromthe saturates (paraffin+naphthenics), and theother points of the plot are the results fromaromatics, especially polynuclear aromatics.

4. Conclusions

The gross heat of combustion and cetanenumber of diesel fuels have been predicted fromrefractive index. Cetane number has also been

predicted from the measured value of gross heat ofcombustion. The results of gross heat of com-bustion predicted from the refractive index arebetter than those estimated by ASTM D4868-90.The cetane numbers predicted respectively fromrefractive index and gross heat of combustionresult in better agreement with the measured ones,and the agreement was better than that of cetaneindex 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 increasingcontent of polynuclear aromatics.2) The gross heat of combustion and cetanenumber decrease with increasing content of

polynuclear aromatics.3) Polynuclear aromatics are the main compo-nents that affect the physical properties and cetanenumber.

Fig. 14 Effects of Aromatics (total aromatics, aromatics/saturates, polynuclear

aromatics, and polynuclear aromatics/single-ring aromatics) on Distilla-

tion Temperature (T50%, T90%) of Diesel Fuels


430

Fig. 15 The Relationship between Physical Properties and Molecular Weight of Diesel Fuels

References

1) ASTM, "Annual Book of ASTM Standards," Philadel-phia (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 TechnicalData Book," 2B2.1 (1986).


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要旨

軽油の物性値とセタン価の推定とそれらの物性値に対する芳香族化合物の影響

Ju-Hwan CHOI†2), Young-Sang CHOI†2), Oh-Kwan KWON†1)

†1) Tribology Lab., KIST, Seoul 136-791, Korea

†2) Dept. of Chemistry, KOREA Univ., Seoul 136-701, Korea

軽油の燃焼熱とセタン価を屈折率より推定し, またセタン価

を燃焼熱より推定した。これらの推定したセタン価と燃焼熱は

それぞれ実験値と一致した。芳香族化合物 (芳香族, 一環芳香

族, 多環芳香族) の物性値 (密度, 屈折率, 蒸留温度, 動粘度,

燃焼熱) や軽油のセタン価への影響を系統的に調べた。密度,

屈折率, 蒸留温度 (50%と90%) は多環芳香族の量が増えれ

ば増加するが, 燃焼熱とセタン価は減少する。軽油の多環芳香

族は物性値, セタン価に影響するという結果が得られた。

Keywords

Prediction, Physical properties, Cetane number, Diesel fuel, Aromatics, Refractive index


prediction of physical properties and cetane number of

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