Composition of Fischer-Tropsch Diesel Fuel COBALT CATALYST C. C. WARD, F. G. SCHWARTZ, AND N. G. ADAMS Petroleum Experiment Station, Bureau of Mines, Bartlesuille, Okla.
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Fischer-Tropsch (cobalt catalyst) Diesel fuel was analyzed as part of a program to study the relationships of composition of Diesel fuel and engine performance. This particular fuel was selected because it is a synthetic fuel having unusually high cetane number. The sample of Fischer-Tropsch Diesel fuel was composed primarily of n-paraffins, with minor quantities of polar compounds, alpha-type olefins, and internal-type olefins. The approximate composition of the fuel in volume per cent is: paraffins, 88; polar compounds, 2; alpha-typeolefins, 1.5; internal-type olefins, 8.5. The high cetane number is caused b y the high n-paraffin content of the fuel. Analyses of the composition of many different types of Diesel fuels are necessary fully to evaluate relationships of composition and combustion. This investigation represents one step in such a study, and presents data primarily on n-paraffins, but shows the effect of olefins. It also presents data on a synthetic Diesel fuel of the type that could be made in a n emergency. I t emphasizes the need for a better method of rating high-cetane-number fuels because several fractions rated higher than 100 cetane number. Finally, i t has contributed toward development of analytical methods for components of Diesel fuels.
inch. Under these conditions the product is composed largely of straight-chain hydrocarbons ( I d ) ; consequently, the gasoline from this stock has a low octane number and the Diesel fuel a high cetane number. By contrast, the products from the American modification of the Fischer-Tropsch process ( 4 ) ,which utilizes an iron catalyst, are highly branched hydrocarbons that are predominantly olefinic. The gasoline fraction can be converted to a motor fuel of 80 octane number or more. The Diesel fuel by thie process probably would have a low cetane number. TREATMENT OF SAMPLE
The properties of the Diesel fuel as received by the bureau are given in Table I. TABLEI. PROPERTIES OF FISCHER-TROPSCH DIESELFUEL Cetane No. 80 Density 20' C. 0.7681 Specifia 'gravity, 60/60° F. 0.7721 A.P.I. degrees 51.7 172 Flash point, TCC, O F. Cloud point, a F. 82 30 Pour point " F. Color N . P . ~ . 1.0 Viscosity S.S.U., 100' F. 33.8 0.04 Sulfur, wC. % Corrosion Pass Neutralization No. 0.45 1.4316 Refractive index. nKo 1.4411 ~ ~ ~ ~ ~ ~ N ~ 6 9d e Aniline point, 0 F. 187.7 I
HE high-speed Diesel engine has destroyed a popular fallacy
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T t h a t a Diesel engine can operate on any oil that will burn, The engine manufacturer's recommendations for high-speed Diesel-engine fuels are very restrictive regarding fuel cleanliness, viscosity, carbon residue, ash, and ingition delay. Fuel requirements have been met largely through the Use Of straight-mn gas oils, but the prospect of a continuing adequate S U ~ P ~ofY these oils is not favorable. Contributing factors to this condition are increased demand for Diesel-engine fuels, expanding ments for domestic burner fuel, and the use of straight-run gas oils as charging stocks for catalytic cracking units. Because of the uncertainty of adequate stocks of straight-run gas oils, several organizations have inaugurated programs to learn the chafiacteristios of a gas oil which make it a satisfactory or an unsatisfactory Diesel fuel. The Bureau of Mines is participating in several of these programs, one of which is with the Navy. This cooperative program includes compositionand combustion studies of three types of gas oil: a straight-run stock, a catalytioally cracked stock, and a synthetic stock from Fischer-Tropsch synthesis. This paper presents data obtained on tlie gas oil from Fischer-Tropsch synthesis. The fuel used in this study w m part of 1200 gallons of Diesel fuel in storage a t the Carrihres Kuhlmann plant in Harnes, France, when this plant was captured by the Allied occupation forces near the end of World War 11. The plant had been operated by the Germans to synthesize a crude hydrocarbon stock from carbon monoxide and hydrogen by the FischefiTropsch process. The synthesis process was carried out with a cobalt catalyst at about 390" F. and a t a pressure of about 150 pounds per square
Hydrogen, % Carbon, x ' % Oxygen, %
15 0 84.9 0.14
mately Preliminary 2% of analysis polar compounds. showed that To the simplify fuel contained the analytical approxi-
studies on the hydrocarbon portion of the fpel, these com o m d s were removed by percolating 80 gallons of the fuel througg silica gel. The silica gel columns used were made of stainless steel tubing 3 inches in diameter and 21 feet in length. Each column held about 40 pounds of gel. Properties of the polar material are tabulated in Table 11. TABLE 11. PRQPERTIES OF POLAR MATERIALOF FISCHERTROPSCH DIESELFUEL Density, 20° C. p f i c gravity, 60/8Oo F. iscosity At 68O F., CS. At 100" F., CR. Refractive index, n%? Sulfur, wt. % Bromine No. Cetane No. A.S.T.M. D-158 distillation Initial b.p., F. E.p.
0 8632 0.8650 I
6.608 4.106 1.4572 (approx.) 0.12 7.31 52.2 170 420 604 623 631
LeTourneau and Barusch (9) of the California Research Gorp., working with the eame Fischer-Tropsch fuel, separated 1.98% polar material by silica gel. They analyzed this material and found it contained no basic nor ether-insoluble material. 1117
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100
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z w 80 z
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6 500 w
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Figure 1. Properties of Fischer-Tropsch Diesel Fuel Fractions
Acid extraction separated the polar material into two acid fractions and one neutral fraction. The acid A fraction consisted of relatively strong acids and the acid B fraction apparently consisted of acids of high molecular weight. Some of the data obtained by LeTourneau and Barusch in the analysis of the polar material are shown in Table 111; they estimated the over-all composition t o be as follows: Constituent Neutrals Esters Phenols Alcohols Carbonvls Acids Relatively strong organic acid Weak organic acid
% of Polar Material
from Fischer-Tropsch 87.2 8.4 1.7 24.0 53.0 12.8 10.7 2.1
The removal of this polar material by silica gel treatment increased the cetane number of the fuel from 80 to 88. About 75 gallons of this treated fuel were fractionated at reduced pressure in a column previously used for distillation at atmospheric pressure (16). The fractionating column was 3 inches in diameter and packed to a height of 25 feet with 8/32-inch stainless steel helices. It had a fractionating efficiency of about 80 theoretical plates under total reflux at atmospheric pressure. The still pot had a capacity of 100 gallons and was heated by an immersion electric heater. PROPERTIES OF FRACTIONS
One hundred seventy-seven (0.570,1500 ml.) fractions were obtained at thme operating conditions: overhead pressure, 50 mm. of mercury; pressure drop, 100 mm. of mercury; reflux ratio 8 to 1; and product rate 10 ml. per minute. Density, refractive indexes ( ~ z D , no,and ne),and bromine number were determined on each fraction. A portion of each fraction was sent to the Naval Engineering Experiment Station, Annapolis, Md., for determination of cetane number. The distillation curve, with temperature converted to an atmospheric pressure basis, and the density, bromine number, and cetane number for each fraction are shown in Figure 1. (Tabu-
lated properties on the fractions are available upon request to the authors.) The boiling point and density of an average fraction on each of the well-defined plateaus were compared with corresponding properties of the normal paraffins. The fractions on the plateaus apparently were largely normal paraffins ranging from nonane to nonadecane, as the average difference between the fraction and the corresponding paraffi was only about 3 F. in boiling point and about 0.0007 in density. The density curve changes abruptly at the end of each normal paraffin plateau and shows a maximum value at the beginning of each such plateau and a slight tendency for a second high value just preceding the maximum. These twin peaks are more pronounced in the bromine-number curve. The high values for bromine numbers between the normal paraffins indicated the presence of olefins, and fractions 22 and 25 were selected for study to determine if olefins were present. The olefinic materials in these fractions were concentrated by repeated silica gel percolations and distillations until their bromine number indicated an O
TABLE 111. ANALYSIS OF FRACTIONS FROM POLAR MATERIAL OF FISCHER-TROPSCH FUEL Analysis Wt. % in fraction Carbon % Hydrogkn % Nitrogen, '% Sulfur, % Oxygen, % Ash Bromine No. Molecular weight Acid No. Total acid No. Saponification No. Active hydrogen Peroxide N o . Carbonyl No. Hydroxyl No. R.I. Nao a t 20" C. Density, 20' C.
White Polar Material
Neutrals 87.2 Liebig id.'8l 7 7 . 8 0 12.82 12.85 @%aM 0.02 ... Oxygen bomb 0.20 ... B y difference 10.2 9.4 800' C. 0.12 Electrometric (6) 6 8 Cryoscopic (benzene) 200 214 A.S.T.M. (1) 19.8 0.56 Anhydrous sodium 23.4 5.6 A.S.T.M. ( B ) 42 25 Zerewitinoff (IO) ... 0.30 Iodometric (16) 0 Hydroxylamine (6) . . , 168 Acetyl chloride (18) 46 50 1.4476 ... 0.8573 Method
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Acids Acids A B 10.7 2.1 73.07 . . . 12.15 . . .
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14.8
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247 178 179
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olefin content of about 90%, assuming mono-olefins and 12-carbon-atom molecules aa indicated by their boiling ranges. These bromine-number determinations are thought to be reliable b e cause of the absence of aromatic hydrocarbons in this type of fuel. These two particular fractions were chosen because they coincide with two adjacent peaks in the bromine-number curve between the plateaus for n-undecane and n-dodecane. It was thought that the fractions a t these two distinct peaks, typical of the pairs occurring throughout the distillation, contained .two different olefinic structures of 12-carbon-atom content. However, the infrared spectra of these two concentrates were almost identical, indicating that not only were similar olefins present in each fraction, but they were present in approximately the same ratios to each other. Possibly, the lower boiling point peak is caused by branched olefins with the branch removed from the double bond, while the higher boiling straightchain olefins contribute to the other peak. Both of these types could give essentially the same infrared spectra. A study of the spectra showed that the concentrates are predominantly olefins with internal unsaturation. Based on the correlations of Anderson and Seyfried (S), these olefins were indicated as being of the trans form with no indication of the cis form. These correlations for the cis type, however, are based upon only one cis olefin, and the calibration coefficient obtained is much smaller than the coefficients of other functional groups. Thus, it should be possible for 10 to 15% to be undetected. Using a modified form of the olefin-type analysis outlined by Johnston P t al. ( 8 ) , approximately 85% of the olefins in the concentrate were found to have internal unsaturation while the remaining 15% were of the alpha type. The infrared absorption band a t 11.25 microns showed that a small percentage of these alpha-olefins were of the 2-alkyl-substituted type. These results agree in part with those of other investigators (7) using both infrared and mass spectrometers t o study the isomeric composition of hydrocarbons in the gasoline boiling range of a Fischer-Tropsch fuel produced with a cobalt catalyst. Their results show that 20 to 30% of the olefins are of the alpha type and the remainder are of the internal type, consisting of both cis and trans. They also found in the gasoline boiling range about 10 to 15% of monomethyl substituted, branched-chain hydrocarbons and little or no evidence of dimethyl or ethyl substitution. The cetane numbers as reported by the Naval Engineering Experiment Station are plotted on Figure 1. As expected, the cetane numbers are high for fractions corresponding to the normal paraffin plateaus, and they are lower between plateaus where olefins are present. The cetane numbers tend to increase with boiling point and range from about 60 to slightly over 100. These values above 100 raise a question analogous to one that prevailed several years ago in regard to aviation gasoline, when samples of fuel rated higher than the fuel arbitrarily selected for the high reference standard, Several methods have been standardized for rating gasolines over 100 octane number, but no standard procedure is available for rating Diesel fuels over 100 cetane number. The Naval Engineering Experiment Station laboratory reports the cetane numbers above 100 determined for these samples were obtained by extrapolation using the following procedure: Readings of the handwheel used to adjust the compression ratio of the test engine were taken for the test fuel and for 90 and 100 cetane-number reference fuels. The handwheel settings were then plotted against cetane numbers and the cetane number of the test fuel was read from the straight-line extrapolation of the graph.
It is of interest that the fractions in the boiling range of dodecane (fractions 34 to 39) have cetane numbers of 85 to 90, whereas the literature value for dodecane is 80 (11)to 82 (IS). It was b e lieved that these high values might result from the formation of peroxides, as these compounds are known to be good cetane improvers. No good method is available for determining peroxides; however, the test for hydroperoxides is believed to be fairly reli-
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able. Fractions 34,35,and 36 were found to be free of these conipounds, so it is unlikely that peroxides are present. Another explanation for the high cetane numbers for these fractions is that the cetane number of dodecane is higher than the literature values. This was substantiated by engine tests on a dodecane sample of 96% purity that rated 87’cetane number. The uncertainty of the cetane number of n-dodecane is typical of present knowledge of the cetane numbers of hydrocarbons in the Diesel fuel boiling range. Other than cetane, the only normal paraffins on which cetane numbers are available are decane, dodecane, and tetradecane (11, IS) and data on the ignition characteristics of other hydrocarbons are even less comprehensive. Additional information on the ignition characteristics of hydrocarbons in this boiling range is essential to better understanding of the characteristics of a good Diesel fuel. SUMMARY
This Fischer-Tropsch Diesel fuel from a cobalt catalyst contained approximately 2% of polar material. This material was removed from about 75 gallons of sample by percolating it through silica gel. The hydrocarbon material was fractionated at 50-mm. pressure, and 177 (0.5%1500 ml.) fractions were collected. Selected physical properties, including cetane numbers, were determined on each fraction. The analytical data on the hydrocarbon portion of the fuel showed that it was composed largely of normal para& from Cp to C‘p. It contained about 10% of olefins, of which about 85% had internal unsaturation and the remainder was of the alpha type. The fuel as received had a cetane number of 80, and after the polar material was removed this was increased to 88. The fractions having high percentages of normal paraffins corresponded to the peaka in the cetane number curve and fractions containing olefins had lower cetane numbers. The cetane numbers of the fractions ranged from about 60 to slightly over 100. ACKNOWLEDGMENT
Appreciation is expressed to the Bureau of Ships, Navy Department, for financial assistance and for supplying the FischerTropsch Diesel fuel, to the Naval Engineering Experiment Station for determining the cetane numbers of the fractions, and to the California Research Corp. for permission to include data in Table 111. LITERATURE CITED
(1) Am. SOC.Testing Materials, Designation D 604-46T. (2) Ibid.,D 94-481‘. (3) Anderson, J. A., and Seyfried, W. D., Anal. Chem., 20, 998-1008 (1948). (4) Bruner, F. H., IND.ENO.CEIBM., 41, 2511-15 (1949). (5) Bryant, W. M. D., and Smith, D. M., J. Am. Chem. SOC.,57, 57 (1935). (6) Du Bois, H. D., and Skoog, D. A., Anal. Chem., 20, 624-7 (1948). (7) Friedel, R. A., and Anderson, R. B., J. Am. Chem. Soc., 72, 1212-15 (1950). (8) Johnston, R. W. B., Appleby, W. G., and Baker, M. O., Anal. Chem., 29,805112 (1948). (9) LeTourneau, R. L.,and Barusch, M. R., Calif. Research Corp. report, private communication. (10) Niederl, J. B., and Niederl, “Organic Quantitative Microanalysis,” PP. 263-71, New York, John Wiley & Sons, 1942. (11) Puokett. A. D.. and Caudle, B. H.. Bur. Mines. Inform. Circ. 7474 (1948). (12) Smith, D. M., and Bryant, W. M. D., J . Am. Chem. Soc., 57, 61 (1935). (13) Tiiton, J. A., Smith, W. M., and Hockberger, W. G., IND. ENG. CHEM.,40, 1269-73 (1948). (14) Underwood, A. J. V., Ibid., 32, 449-54 (1940). (15) Ward, C. C., Gooding, R. M.. and EccIeston, B. H.. Ibid., 39, 105-09 (1947). (16) Wheeler, Oil and Soap, 9, 89 (1936). RECEIVED Auguat 10, 1960. Presented before the Division of Petroleum CHEMICAL SOCIETY, Chemistry at the 117th Meeting of the AMERICAN Houston, Tex.
