THE J O U R N A L OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
AUE.. 1922
725
Sulfur Compounds and Oxidation of Petroleum Oils’~z By C. E. Waters3 DEPARTMENT O F COMMERCE, BUREAUOF STANDARDS, WASHINGTON, D . C.
In general, mineral lubricating oils which contain large percentages of sulfur compounds are more easily oxidized than those in which there is less sulfur. This is true whether the oils are heated to 250” C . or exposed to sunlight. The residues left behind on cracking and partly distilling 09 oils, as in the Conradson carbon residue test, tend to retain the surfur in the oil. The bearing of these facts upon the formation of sludge in transformer and turbine.oils, and of carbon in internal combustion engines is pointed out in the following paper.
I
N earlier work on the oxidation of mineral lubricating 0i1~,4it was found that, contrary to expectation, the addition of free sulfur to an oil caused no great difference in the amount of asphaltic matter formed by heating it in the air. I n other words, the presence of sulfur did not appreciably affect the rate of oxidation of the oil. This result was not only unexpected, but it was contrary to what others have found when oil is heated in contact with sulfur and sulfur dioxide.6 If there is any explanation other than experimental errors, it is that the amount of sidfur added, 0.10 per cent, was too small to affect appreciably the rate of oxidation of the oil. Data accumulated during the past two years seem to uphold this explanation in general. CARBONIZATION VALUEOF OILS I n a “carbonization” test for motor oils developed by the author, the oil is heated to 250’ C. in contact with air, for 2.5 hrs., diluted with petroleum ether, and the precipitate is filtered off and weighed. The result, calculated as percentage of the weight of original oil, is called the “carbonization value.’’ From time to time the carbonization values and sulfur content of motor oils are determined, simply to accumulate data on the possible relation between the two. I n nearly every case it has been found that oils which contain much sulfur in combination almost always have comparatively high carbonization values. In general, low sulfur and low carbonization go together, but not always, because other things than sulfur compounds can increase the rate of oxidation of the oiL6 The data on thirty-seven oils, all that have been examined, can be summarized in Table I. TABLEI-REL.4TION NUMBEROF OILS 8
O F SULFUR
Carbonization Value Per cent Trace-0.10
6
0.11-0.30
14
0.31-0.75
4 5
0.76-1.25 1.26-4.55
CONTENT
TO
CARBONIZATION VALUE
Sulfur Per cent 0.04-0.08 (One with 0 21) 0.18-0.29 (One each with 0.05 and 0.90) 0.17-0.63 (Two with 0.82) 0.17-0.61 0.37-0.46
There are irregularities in the table, and the data are too few to be the basis of any well-founded generalization, but Received March 27, 1922. Published by permission of the Director of the Bureau of Standards. a Chemist, Bureau of Standards. 4 Bur. Stds., Technologzc Paper 4 (1911), 9 and 11; THIS JOURNAL,3 (1911),814. 6 Holde and Eickmann, Mitt. kgl. Materialprufungsamt, 1907, 145; 2. angew. Chem., 20 (1907),1263, 1923;Southcombe, J . SOC.Chem. I n d . , 80 (1911), 261. 6 Bur. Stds., Technologic Paper 177 (19201,8; Circ. 99 (1920), 13. 1 2
there seems to be a tendency toward high carbonization with high sulfur. The matter is complicated by the presence in the oils of substances other than sulfur which can cause accelerated oxidation, as in the case of the oil which contained but 0.05 per cent of sulfur but had a carbonization value of 0.29. Four other oils with relatively high sulfur had carbonization values which were unexpectedly low. These results can be accounted for on the supposition that the sulfur compounds resisted oxidation.’ This is not an improbable assumption. It is not the intention of this paper to formulate a general indictment against oils which contain more than a few hundredths of a per cent of sulfur, because there is not enough experimental and practical evidence to justify such a course. However, it does not seem unfair to say that such oils, except those used for cutting, are not above suspicion. No chemist, if forced to choose between two oils identical in every respect except their sulfur content, would select the one with the greatest percentage of that element as being the one least likely to deteriorate when used under oxidizing conditions, If he did make such a selection, how would he justify it?
EFFECT OF HEATON SULFUR COMPOUNDS IN OILS Granting for the sake of discussion that sulfur compounds are contributory causes to rapid oxidation, what happens to these compounds when oil containing them is heated in the air? Part is lost either by direct evaporation or after cracking, and hydrogen sulfide and oxides of sulfur are to be looked for in the escaping gases and vapors. A considerable part of the sulfur is to be found in the asphaltic matter which is thrown down by the addition of petroleum ether to the oxidized oil. The oil filtered off from the precipitate contains less sulfur than it did before heating. A 6-g. sample of California cylinder stock, which contained 0.80 per cent of sulfur, was heated in an all-glass apparatus so constructed that a slow current of air could be passed into the oil and out through a flask partly filled with a solution of sodium hypobromite. The oil was kept a t 240’ to 260” C. for 3 hrs. Only a film of oily dist,illate passed over. When the hypobromite was acidified, the addition of barium chloride immediately produced a dense precipitate of barium sulfate. Drops of moisture on the inside of the tube leading into the flask were strongly acid to congo red paper, which does not change color with weak acids. T.his oil gave off hydrogen sulfide when heated in a test tube to 250°, and perhaps the drops were formed by oxidation of this gas. The following experiments show that only part of the sulfur is lost when oils are heated in air, and that this element takes part in the formation of the asphaltic matter thrown down by the addition of petroleum ether. Three oils were used: C, the California cylinder stock just discussed; 0, a commercial lubricant of unusually high carbonization value; and M, a motor oil t o which topped Mexican crude had been added to increase its sulfur content. Ten-gram samples were heated to 250’ C. for 2.5 hrs. in open flasks. When cold, the flasks were weighed to determine the loss by evaporation. The residual oil was then diluted with 50 cc. of petroleum ether, 7 The definite statement that oils may contain sulfur so well “protected” that it resists oxidation was recently made to the author by a chemist employed at one of the large eastern refineries.
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T H E JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEiMISTRY
Vol. 14, No. 8
and the flasks were corked. Xext day the precipitates were filtered off in Gooch crucibles, washed with petroleum ether, dried at 105’ C., weighed, and analyzed for sulfur. The oily filtrates were freed as completely as possible from the ether by warming first on the steam bath and then in an oven at 105’. It is believed that the error resulting from this procedure was not unduly large.
almost any apparatus from which air was reasonably well excluded would serve the purpose. KO form of apparatus can duplicate the conditions in an engine. I n the second place there was no need of driving off all but the residue of coke, for then no sulfur would be left unless strongly basic metals were present. Besides, no one claims that the deposits in an engine are really carbon. Apart from soot, dust, and similar extraneous matter, they consist largely TABLE11-SPECIALDATAO N THREEOILS of the last portions of the destructive distillation of the oil and C 0 M Oil ..................................... fuel, or oxidation products, or both.8 4.55 1.91 Carbonization value per c e n t . . . . . . . . . . . . . . 0.10 0.46 0.39 Sulfur in unhkatcd dil, per cent. . . . . . . . . . . . 0.80 When only a small part of the oil was left, the heating was 3.37 22.06 15.54 Evaporation loss, per c e n t . . . . . . . . . . . . . . . . . stopped. After cooling, the crucibles were cleaned as well as Sulfur in asphaltic precipitate, per cent. . . . . 1.05 2.03 4.88 Sulfur in oil filtered off, per c e n t . . . . . . . . . . . 0.72 0,35 0.26 possible on the outside, and weighed. The sulfur in the resiTOTAL SULFUR ACCOUNTSD FOR dues was then determined in order to ascertain whether it 0.0112 0.0093 I n asphaltic precipitate, gram.. . . . . . . . . . . . . 0.0001 tended to remain behind or to be driven off with the oil In oil filtered off, g r a m . . . . . . . . . . . . . . . . . . . . 0.0695 0.0253 0.0215 ~ vapors. 0.0365 0.0308 Sulfur found, g r a m . . . . . . . . . . . . . . . 0.0696 The following oils were employed: C,, the California 0.080 0.046 0.039 Sulfur in unheated oil, gram.. . . . . . - - _ _ cylinder stock already discussed; P, a paraffin base motor Sulfur lost, g r a m . . . . . . . . . . . . . . . . . 0.0104 0.0095 0,0082 oil; N, a naphthene base motor oil; S, a heavy red oil con20.65 21.03 Percentage of sulfur lost . . . . . . . . . . 1 3 , O O taining “sulfonic acid^;"^ and W, a white medicinal oil from It is plain from the figures in Table I1 that there is a ten- California mixed with topped Tampico crude. dency for the sulfur to accumulate in the asphaltic matter In the case of every oil except C, the distillation residue which is formed by oxidation and polymerization. Some is contained a somewhat higher percentage of sulfur than the also to be found in the highly oxidized varnish-like coating unheated oil. This is shown most clearly by S and W. The on the walls of the flasks. With these three oils the amount percentage of distillation residue, except for C, was less than of this varnish was less than 3 mg., so that a quantitative the percentage of the original sulfur retained by the residue. determination of sulfur was out of the question, although This shows that the sulfur.is not driven off as fast as the oil qualitative tests were obtained. during the cracking and distillation. It is also significant
..........
O i l , . .......................... Sulfur in oil.. . . . . . . . . . . . . . . . . . . . . . . . . . . .
TABLE I I I S U L F U IN R DISTILGATIONRSSIDUES (All figures are exoressrd as oercentaees) ._ .
c
- 0- . 8 0
CI
1
P
-9.80 0.24
P 0.18 34.2 0.20 37.7
N
0.19
46.0 0.20
48.4
N 0.19 16.2
S 0.44 13.6 . 0.99
22.1
30..5
0.26
S
0.44 22.2 0.77 38.6
s t
W
0.44 10.0 1.24 28.1
0.79 17.4 2.49 54.9 .
W
0.79
23.3 2.12 62.8
W 0.79 3.4 7.24 31.1
Only 4.20 g. of or! were left for this test.
If the data given above are not in conflict with the behavior of oils in actual use, the deposits formed from turbine and transformer oils should contain sulfur. A typical steam turbine sludge analyzed by Mr. Anderson of this Bureau was found to contain 0.33 per cent of sulfur, or 0.65 per cent, calculated on the oil-free basis. Similarly, the oil-free deposit in a transformer contained, according to an analysis by Mr. Berryman, 0.57 per cent of sulfur. CARBON FROM INTERNAL COMBUSTION ENGINES A few samples of “carbon” taken from internal combustion engines have also been analyzed. In twelve analyses, the percentages of sulfur found in the oil-free material were from 0.52 to 3.08. The amounts of ash, 2.87 to 11.87 per cent, varied independently of the sulfur. The benzenesoluble portions of three carbon deposlts contained 0.94, .1.33, and 1.52 per cent of sulfur, respectively. I n order t6 learn something more about the accumulation of sulfur in the f‘~arbon” deposits formed in internal combustion engines, a few oils were subjected to the following; treatment: 5-g. samples were heated in 100-cc. porcelain crucibles which rested on a wire triangle at the bottom of a larger iron crucible. The porcelain crucibles were covered with watch glasses, so that the heating could be stopped when only a small part of the oil was left. This form of apparatus was used in preference to the Conradson carbon residue apparatus because it was easier to stop the distillation at any desired point short of dryness. It might be objected that the simpler apparatus mould not give results comparable with those obtained’ with Conradson’s. There seems to be little ground for such an objection. I n the first place, the object was to learn something of the fate of the sulfur during destructive distillation of the oil, and
that the smaller the amount of the distillation residue for a given oil, the greater the percentage of sulfur it contains. This is most evident with S and W. I n the last experiment with W, the residue weighed only 0.17 g., yet it yielded almost 0.090 g. of barium sulfate. It consisted of loose flakes of carbon and a varnish-like coating on the walls of the crucible. Because the residue of carbon cannot possibly carry more than traces of sulfur, it follows that at the very end of the distillation there must be a rapid change in the sulfur content. It is difficult to check this by analysis, because the distillation cannot be stopped a t the desired point, even though its progress can be watched. Besides, not enough sulfur for a satisfactory determination would be left. A simple distillation test was therefore made. About 5 cc. of oil, containing 0.88 per cent of sulfur, were distilled from a retort made by drawing out the open end of a hard glass test tube. Throughout, the distillation the evolution of moderate amounts of hydrogen sulfide could be detected by means of lead acetate paper. At the end, when it became necessary to heat part of the retort to dull redness, hydrogen sulfide came off more freely than before. The conclusion must be drawn that the presence of even considerable percentages of sulfur in a “carbon” deposit in an internal combustion engine is not evidence in favor of either the oxidation test or the carbon residue test. Finally, in order to ascertain whether the presence of sulfur compounds caused rapid oxidation a t ordinary temperatures, a series of experiments was made by exposing;oils to the action of sunlight and air for 100 hrs. Ten-gram samples were :Bur. Stds., Civc. 99, 39. Published statements bearing on this point are there discussed. 9 Distributed in 1920 by F.R . Baxter for codperative work on sulfur b y Committee D-2, American Society for Testing Materials.
