INDUSTRIAL
AND
ENGINEERING CHEMISTRY
ANALYTICAL EDITION PUBLISHED
BY
THE
AMERICAN
CHEMICAL
SOCIETY
0
HARRISON
E.
HOWE,MEDITOR
Symporlum on Analytical Methods Used in t h e Petroleum Industry, pages 287 to 321.
Determination of the Heat of Combustion of Gasolines W. H. JONES AND C. E, STARR, JR. Esso Laboratories, Standard Oil Co. of Louisiana, Baton Rouge, La.
D
ETERMINATION of the heat of combustion of gasolines is becoming increasingly important and is ineluded in certain aviation gasoline specifications. The application of this test to gasolines must be accompanied by a more closely controlled procedure than that ordinarily employed by petroleum testing laboratories for heavier fuels, in order to obviate losses of the more volatile components of the gasolines, such as butanes and pentanes, which have the highest heating values. Considerable work has been published on the art of calorimetry since Andrews ( 1 ) first used the bomb calorimeter in 1848. A comprehensive bibliography on this subject is furnished by Kharasch (14). The apparatus, technique, and procedure of modern calorimetry have been described a t considerable length by Dickinson (4,Jessup and Green ( I O ) , Richards and Gucker (do), Rossini (B) and , White (81). The work on the heat of combustion of hydrocarbons has been done mainly on pure compounds in order to study the relation between energy, structure, number of carbon atoms, and the influence of organic groups. For this purpose the apparatus and technique have been developed to such a high degree of precision and accuracy that, according to Rossini (W), measurements of quantities of energy can be made with uncertainties as low as 0.01 to 0.02 per cent. To attain this degree of accuracy considerable time, apparatus, and a precise technique are necessary. However, in the practical application of this test to gasolines, such precision is not warranted. A procedure has been developed whereby six to eight determinations per day can be made on low-boiling (aviation) gasolines, with an accuracy of +0.2 per cent.
shown by the galvanometer and thus a sensitive balance can be maintained. The oxygen used should be at least 99.8 per cent pure and free from any combustible material. Commercial oxygen made by the li uid air process frequently contains as much as 0.3 per cent of hylrogen, possibly from contamination, and should be passed over copper oxide at 600" t o 700" C. before being used for calorimetric work. Keffler (11, l a ) has shown that even electrolytic oxygen may contain considerable impurities. A number of cylinders of commercial oxygen made by the liquid air process were analyzed and found to contain from 0.01 to 0.3 per cent of hydrogen. Oxygen containing no carbonaceous material and an amount of hydrogen not exceeding 0.01 per cent was considered t o be satisfactory without pretreatment. Ordinary pharmaceutical capsules (No. 00) were employed. The ignition wire was pure iron and of No. 34 B. & S. gage. Combustion cups used were made both of platinum and illium. Benzoic acid used in calibrations was obtained from the National Bureau of Standards.
SWITCH
WATER JACKET THERMO.
Apparatus and Materials A calorimeter of the adiabatic type was employed. This consisted of a double-valve oxygen bomb, a calorimeter bucket with stirrer, and a water-jacketed case equipped with a stirrer
and connections to hot- and cold-water lines for maintaining adiabatic conditions. A Beckmann thermometer calibrated by the Xational Bureau of Standards was used t o measure the temperature rise. The jacket temperature was controlled with respect t o the calorimeter bucket temperature by means of two 3-junction thermocouples and a galvanometer as shown in Figure 1. Slight differences in temperature between the bucket and t h e jacket are clearly
(N. 8 . s CALIBRATED) (ZERO OF BECKMANN SET ABOUT 3O X-CONSTAN TAN 0 . 34
BELOW AVERAGE ROOM T E M P € R A W R @
s.
FIGURE1, TEMPERATURE-C]ONTROL ARRANGEMENT
288
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 13, No. 5
Procedure and Discussion The procedure is standard in most respects, and is generally described in the literature. Calibrations were made using benzoic acid, the heating value of which was based on standards of e. m. f. and resistance maintained by the National Bureau of Standards. The heat units employed in the calculations were the same as those used by Rossini (22) which are: 1 calorie = 4.1833 international joules = 4.1850 absolute
joules 1 absolute joule = 9.480 X 10-4 mean B. t. u . 1 calorie per gram 1.7994 B. t. u. per pound
-
20
IO
0
1
SAMPLING.A considerable saving in time is accomplished by basing calculations on a direct weighing of the sample rather than on a weight of sample burned as calculated from an analysis of the products of combustion. Jessup (9) has indicated that calculations on either basis are in close agreement when the FIGCRE3. sample is obviously completely burned. Thin-walled glass bulbs, flattened on two sides to withstand pressure changes, have been used by Richards and Barry (Ill), Jessup (8, 9), and others. The advantages of the glass bulbs are that no corrections are necessary for the heat of combustion of the sample container, and that no absorption of moisture from t h e atmosphere is encountered. For complete combustion of liquid hydrocarbon samples enclosed in a container, such as a glass bulb or gelatin capsule, it is FIGURE 2. IGKITION SYSTEM necessary that the container be completely filled with the liauid. This is done to meve'nt explosions within the oxygen bomb, upon ignition, with consequent scattering of the sample beyond the combustion zone, resulting in incomplete combustion. However, for gasoline samples containing considerable amounts of very volatile hydrocarbons, the use of glass bulbs is not recommended by the authors since, to fill these bulbs completely. it is necessary to heat and cool alternately. Accurate weights of representative samples of gasolines have been obtained by using gelatin capsules and employing a special technique for filling and sealing. When the capsules are first removed from their sealed container, a heat of combustion determination is immediately made on a composite of several capsules selected in a representative manner in order to establish a correction factor for the capsules. At the same time each of the remaining capsules is weighed, the weight being recorded on a slip of paper inserted within the capsule. This procedure eliminates any error in the calculated heat value of the capsule due to change in weight from subsequent moisture absorption. The capsule is sealed by lightly wetting the perimeter of the smaller half with water and inserting it into the other half. This forms a completely sealed capsule which is reweighed before filling with the sample. The gasoline sample, cooled in an ice bath, is drawn into a hypodermic syringe and is then injected into the sealed capsule. The air is allowed to escape from the capsule through a small pinhole near the hole made by the
I ml BE/A P # L I E ~ U I
I CO~J 1
~ D E F ~ L L ~ W I ~ G
3. INITIAL REA2INC l k C . 0 9
FINAL READING DEGREES
EXAMPLE CORRECTION CHART FOR BECKMA" THERMOMETER hypodermic needle. When the capsule is completely filled the two holes are preferably sealed with a small drop of collodion or, when sealing samples of high volatility, by applying a small gelatin atch which has been previously weighed with the c a p sule an8 is wetted with water for sealing. The weight of this water (2 to 3 mg.) is estimated from a number of com arative weighings. If collodion is used, and is properly appyied, its weight is negligible. \There atmospheric conditions of high temperature and humidity are encountered, it is recommended that all sample handling be carried out in a room held a t a constant low temperature and low humidity, so that when cold hydrocarbon samples are injected into the capsules no moisture condensation onto the capsules is encountered. SAMPLEIGNITION. The capsule, completely filled with the sample, is suspended by means of the iron ignition wire near the bottom of an illium, stainless steel, or preferably a platinum cup, which is approximately 3.75 cm. (1.5 inches) deep, as shown in Figure 2. Although good combustion can be obtained in a shallow cup, the deep cup gives complete combustion more consistently for samples sealed in gelatin capsules. Various methods have been described by Richards and collaborators (19, 21) to ensure complete combustion of a hydrocarbon sample. However, it has been shown by some investigators (8) that complete combustion can be obtained without using any foreign materials as combustion aids. In the work on gasolines it has been found that unburned hydrocarbons and carbon monoxide exist in negligible quantities whenever the sample burns with no visual carbon formation. Jessup (8) has recommended using 30 atmospheres of oxygen pressure for complete combustion of hydrocarbons, while Richards and Jesse (21) claimed that with 35 atmospheres an explosion took place and with 20 atmospheres no carbonization occurred. I n the experimental work on gasolines various oxygen pressures ranging from 15 to 40 atmospheres were used. It was found that the use of a pressure of 30 atmospheres resulted in the most consistent complete combustions. MEASUREMENTS. To attain the desired accuracy for gasoline work it is necessary to establish the following tolerances on the measurements involved : Weights of sample and capsule Weight of calorimeter water Thermometer (bucket) reading
10.1 *O. 1 mg gram ~ 0 . 0 0 1c. ~
Further, it is recommended that a sensitive galvanometer be used on the thermocouple circuit shown in Figure 1,so that
May 15, 1941
ANALYTICAL EDITION
the analyst can control the jacket temperature to within 0.1" C. of the bucket temperature during the temperature rise following ignition of the sample. When obtaining the water equivalent of the calorimeter, by burning standardized benzoic acid, the amount of sample should be such that the temperature rise obtained will closely approximate the temperature rise that results subsequently when analyzing the gasoline samples. Banse and Parks (2) report satisfactory results with the use of a Beckmann thermometer, while other investigators (8) employ platinum resistance thermometers for more accurate measurements. In the case of a calibrated Beckmann thermometer much time can be saved by the use of a chart giving the total correction to be added to the nominal temperature difference observed. This total correction, as illustrated in Figure 3, includes the scale, emergent stem, and setting factor corrections as calculated on the basis of a Bureau of Standards certificate received with the thermometer. I n the construction of the chart the total corrections to be applied to the apparent difference between the starting and final temperatures are calculated for final readings of 2", 3", 4",and 5" a t each of several room temperatures covering the range encountered. The points are joined with straight lines, since an appreciable part of the total correction will be due to errors in the scale graduations which are difficult to estimate at points not calibrated. CALCULATIONS.Corrections to the total observed heat of combustion values are made for the fuse wire consumed and for acids formed by the oxidation of nitrogen and sulfur. The latter corrections are relatively small for gasoline work, amounting to only 3 to 5 B. t. u. per pound for samples meeting customary specifications for sulfur. The methods to be applied in precision calorimetry for correcting the data to standard states are described in publications of the National Bureau of standards, particularly by Washburn (29). The net, or lower, heat of combustion values are the ones usually written into gasoline specifications. These are obtained by correcting the gross values, as determined, for the heat of vaporization of water formed. This is most satisfactorily accomplished by actually determining the weight
289
TABLEI. REPRODUCIBILITY OF RESULTS NO.
26 24 23 22 21 16
Values % 100 96 92 88 84 64
Maximum Deviation from Average % 0.26 0.25 0.22 0.19 0.17 0.10
Average Value B . t . u./lb. gross
TABLE11. CHARA~PERISTICS OF CONTROL SAMPLE Boiling point, ' C. Freezing point, C. Refractive index, n%' Specific gravity, dzo
n-Heptane 98.4 -90.68 1.38769 0.6836
Literature Values 98.4 -90.6 1.38777 0.6837
per cent of hydrogen in the gasoline by means of a combustion analysis. RESULTS. Reproducibility of results is exemplified by 25 determinations made on a sample of n-heptane, as shown in Table I. The n-heptane used had the characteristics shown in Table 11. The values obtained by this method may be compared with those of the National Bureau of Standards, which reported values of 20,731 B. t. u. per pound for a different sample of n-heptane from the same source and 20,714 B. t. u. per pound for pure %-heptane (24). These data indicate that the errors of the method used result in low values. The authors recommend, therefore, that the procedure be followed closely with the suggestions offered in order to keep the results within 0.2 to 0.3 per cent of the true values.
Estimation of Heats of Combustion For laboratories that are not equipped to make this test, or for rapid estimations of heats of combustion, a correlation has been derived to yield reasonable values from other d a b which may be more readily obtainable. In Figure 4 a linear
PER GENT HYDROGEN BY WEIGHT
FIGURE 4. CORRELATION OF HEATSOF CODUSTION WITH PERCENTHYDROGEN
INDUSTRIAL AND ENGINEERING CHEMISTRY
290
function is shown to relate the heats of combustion with hydrogen content. This correlation has been derived from the literature values for the heats of combustion of pure hydrocarbons boiling in the gasoline range, as given in Table 111. The paraffin and aromatic hydrocarbons line up very well, but the naphthenes and olefins may diverge considerably. However, for gasolines of usual composition this curve may be applied. I n Table IV there is tabulated a comparison of actual heats of combustion on gasolines with values estimated from the correlation in Figure 4. The hydrogen content of gasolines is not generally determined by routine petroleum testing laboratories. Thus, in order to determine net heating values it is necessary to employ some means for obtaining reasonable approximations of the hydrogen content. Sweeney and Voorhies (27‘) present a correlation of hydrogen content with the average boiling point and gravity. The characterization factor, as given by Watson, Nelson, and Murphy (SO), may also be employed to derive an approximate hydrogen content from readily obtainable inspections.
TABLE
Iv.
COMPARISON
Gasoline Sample
Weight % ’ of Hydrogen
1 2 3 4 5 6 7 8 9 10
15.3 15.2 15.1 14.9 14.3 14.1 14.0 14.0 13.9 13.8 13.6 13.4 13.3 13.2 12.9 12 5 12.5 12.0 10.5
11
12 13 14 15 16 17 18 19
Boiling Point
72.15 72.15 72.15 86.18 86.18 86.18 100.20
36.0 28.0 9.5 68.8 60.2 58.1 98.4
16.77 16.77 16.77 16.37 16.37 16.37 16.10
845.27 843.36 840.61 1002.4 998.54 993.9 1149.7
21,100 21,051 20,983 20.948 20,867 20,770 20,664
19,515 19,466 19,398 19,400 19,319 19,222 19,142
90.0 89.7 80.9 125.6 117.2 109.9 99.2 150.7 142.8 140.6 124.1
16.10 16.10 16.10 15.88 15.88 15.88 15.88 15.72 15.72 15.72 15.72
1148.9 1148.0 1147.9 1316.4 1306.1 1298.4 1303.40 1473.4 1454.1 1458.8 1458.8
20,650 20,633 20,631 20,754 20.591 20,470 20,549 20,688 20,417 20,483b 20,483)
19,128 19,111 19,109 19,252 19,089 18,968 19,047 19,201 18,930 18,996 18,996
122.3
16.72
1458.8
20.483b
18,996
49.5 71.8
14.38 14 38
783.6 937.5
20,121 20,062
18,761 18,702
100.20 2-Methylhexane 100.20 2 3-Dimethylpentane 2:2,3-Trimethylbutane 100.20 114.22 n-Octane 114.22 2-Methylheptane 114.22 2,4-Dimethylhexane 2,2,4-Trimethylpentane 114.22 128.25 n-Nonane 128.25 2-Methvloctane 128 25 2 3-Dimethylhe tane 2:2,5-Trimethyliexane 1 2 8 . 2 5 2,2,4,4 Tetramethyl128 25 pentane Na hthenes 70.13 Zyclopentane 84.16 Methylcyclopentane 1.2 Dimethylcyclo98.18 pentane 1 Methyl 2 ethyl112.21 cyclopentane 112.21 Pro ylcyclopentane 84.16 Cycyohexane 98.18 Methylcyclohexane 1,l Dimethylcyclo112.21 hexane 1,3 Dimethylcyclo112.21 hexane 112.21 Ethylcyclohexane 1,1,3 Trimethylcycln126.23 hexane Aromatics 78.11 Benzene 92.13 Toluene 106.16 o-Xylene 106.16 m-Xylene 106.16 p-Xylene 106.16 Ethylbenzen? 1 3 5-Trimethylbenzene 120.19 1:2:4-Trirnethvlhenzene 120.19 Propylbenzene 120.19 Olefins Pentene-1 70.13 2-Methylbutene-2 70.13 Hexene-1 84.16 2-Methylpentenp-2 84.16 Heptene-1 98.18 98.18 5-hlethylhexene-1 112.21 Octene-1 2,4.4 Trimethylpm112.21 tene-1 126.23 Nonene-1 126.23 3-Methyloctene-1
-
-
-
- -
-
-
-
~~~
HydroHeat of CombustionQ gen Grosa Net Weight % O K . , cal./mole B . t. u./lb. B . t . u./lb.
91.9
14.38
1096.0
20,102
18,742
121.0 131.4 80.8 100.3
14,38 14.38 14,38 14.38
1250.4 1250.4 939.0 1091.4
20,067b 20,067b 20,095 20,018
19,707 19,707 18,735 18,658
119.9
14.38
1242,5
19,939
18,679
122.7 131.6
14,38 14.38
1238.0 1250.4
10,867 20.067)
18,507 18,707
138.7
14.38
1394.7
19,896
18,536
80.1 110.8 144 0 139 3 138.4 136 2 164.6 l69.? 159 o
7.75 8.76 9.5c 9.50 9.50 9.50 10,06 10.06 10.06
782 0 934.2 1090,9 1090.9 1087.1 1089.0 1242.8 1239.8 1245.7
18,030 18,261 18,606 18,506 18,441 18,473 18,622 18,577 18,666
17,298 17.433 17,608 17,608 17,543 17,575 17,670 17,625 17,714
30.2 38 4 63.6 67.3 94.9 84.7 122.5
14.38 14.35 14.38 14.38 14.38 14.38 14.38
8C6.780 795.7 963.90 950.8 1120.9g 1107.1 1277.9q
10,581c 20,431 20,477C 20,346) 20,410C 20,305, 20,378C
19,221 19,071 19,117 18,986 19,050 18,945 19,018
101.2 145.3 136.3
14.38 14.38 14.38
1263.4 1434.90 1419.7
20.252b
20,275b 20,345C
18,915 18,985 18.892
~
(1
b
Heating values of liquid hydrocarbons unless designated b y Calculated values; considered to be accurate within 1%. Values shown are calculated for liquid hydrocarbons.
0.
Heat of Combustion Determined Estimated 20,430 20,460 20,400 20,304 19,988 20,003 19,885 19,866 19,930 19,867 19,840 19,814 19,655 19,640 19,581 19,499 19,420 19,355 18,732
Divergence
- 80 - 120 -- 100 64
20,350 20,340 20,300 20,240 20,020 19,980 19,950 19,950 19,900 19,880 19,830 19,750 19,720 19,690 19,600 19,450 19,450 19,310 18,810
+- 3223 ++ 65 84 +-- 301310 - 64 +++ 65 60 19
+-- 493045 + 7s -
8
Acknowledgments
Molecular Weight
c.
AND I h T E R M I N E D
Av.
(Selected values from literature)
Paraffim n-Pentane Isopentane Neooentane &Hexane 2-Methylpentane 2.3-Dimethylbutane n-Heptane
OF ESTIMATED
HEATSOF COMBUSTION OF GASOLINES
TABLE111. HEATSOF COMBUSTION OF PUREHYDROCARBONS BOILING IN THE GASOLINERANGE
Hydrocarbon
Vol. 13, No 5
The authors desire to express their sincere thanks to A. Voorhies, Jr., and J. A. Hinckley for their helpful suggestions.
Literature Cited (1) Andrews, Pogg. Ann., 75, 27 (1848). (2) Banse and Parka, J . A m . Chem. Soc., 55, 3223 (1933). (3) Brooks, “Chemistry of the Non-
Benzenoid Hydrocarbons”, New York, Chemical Catalog Co., 1922. (4) Dickinson, J. Research Natl. Bur. Standards, 11, 189 (1914). (5) Edgar, J. Am. Chem. SOC.,51, 1483, 1544 (1929). (6) Huffman, I b k , 52, 1547 (1930). (7) Ibid., 53, 3878 (1931). (8) Jessup, J . Research Natl. Bur. Stundards, 18, 115 (1937). (9) Ibid., 20, 589 (1938). (10) Jessup and Green, Ibid., 13,469 (1934). (11) Keffler, J . Am. Chem. SOC., 56, 1259 (1934). (12) Keffler, J . Chem. Phys., 32, 91 (1935). (13) Kharasch, J . Phys. Chem., 29, 625 (1925). (14) Kharasch, J . Research Natl. Bur. Standards, 2, 359 (1929). (15) Knowlton and Rossini, J . Research Natl. Bur. Standards, 22, 415 (1939). (16) Nelson, Oil Gas J., 35, No. 28,46 (1936). (17) Parks, J . Am. Chem. Soc., 52, 1035 (1930). (18) Ibid., 52, 4382 (1930). (19) Richards and Barry, Ibid., 37, 993 (1915). (20) Richards and Gucker, Ibid., 47, 1876 (1925); 51, 712 (1929). (21) Richards and Jesse, Ibid., 32, 268 (1910). (22) Rossini, Chem. Rev., 18, 233 (1936). (23) Rossini, J . Research Natl. Bur. Btandards, 6, 1 (1931). (24) Ibid., 13, 25 (1934). (25) Rossini, Oil Gas J., 33, N o . 52, 64 (1935). (26) Rossini and Knowlton, J . Research Nalt. Bur. Standards, 19, 342 (1937). (27) Sweeney and Voorhies, IND. EXQ. CHEM.,26, 195 (1934). (28) Swietoslowski, J . Am. Chem. SOC.,49, 2478 (1927). (29) Washburn, J . Research Natl. Bur. Standards, 10, 525 (1933). (30) Watson, Nelson, and Murphy, IND. ENO CHEM.,27, 1460 (1935). (31) . . White. “The Modern Calorimeter”. A. C. 13. Monograph No. 42, New Y o r k . Chemical Catalog Co., 1928
