March, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
Apparently the oxidation characteristics of lubricating oils in the presence of metalsaltsare dependent on the of motor 0% the kind of eng;ne test, and the type Of salt used as an addition agent. This study of the mechanism of oxidation of lubricating oils in the presence of organometallic compounds will be continued.
Literature Cited (1) Dryer, C. G., Brooks, Walter, Morrell, J. C., and Egloff,Guetav, Universal Oil Products Co., Booklet 211, 1937. (2) Evans, E. A., and Kelman, A. L.,Znst. Mech. E ~ L B TLubrica~., tion discussion, Oct., 1937,Group IV,90-9. (3) Gilbert, W. V., Brit. Patent 455,097 (Oct. 14, 1936).
357
(4) Griffith, A. A., and Helmore, W., Ibid., 423,441 (Aug, 1, 1933). Louis, J . Ow. Chenz., 5 , No. 3,253 (1940). (6) Mardles, E. W. J., Tech. Pub. Intern. Tin Research and Dmelopment Council C,No. 2 (1934). (7) Nightingale, D. V., Chem. Rev., 25,329 (1939). (8) Reiff, 0. M. (to Socony-Vacuum Oil C o . ) , U. 9. Patent 2,062,676 (Dec. 1. 1936). (9) Ib&.. 2.191.498'(Feb. 27, 1940). (id, Ibid.; 2,197,832(April 23, 1940). (11)Ibid., 2,197,833(April 23, 1940). (12) Ibid., 2,197,837(April 23, 1940). (13) Reiff, 0.M., and Badertacher, D. E., Ibid., 2,048,466(July 21, 1936). (14) Reiff, 0. M., Giammaria, J. J., and Redman, H. E., Ibid., 2,197,834 (April 23, 1940). (5) Ipatieff, V. N., Pines, Herman, and Schmerling,
Spontaneous Ignition of Hydrocarbons Zones of Nonignition CHARLES W. SORTMAN, HAROLD A. BEATTY, AND S. D. HERON Ethyl Gasoline Corporation, Detroit, Mich.
Spontaneous ignition temperatures in air, and the corresponding time lags, have been determined for a variety of hydrocarbons at atmospheric pressure by the oil-drop or Moore method, using a steel crucible and different air flow rates and liquid drop sizes, Under some conditions of air and liquid feed the readily ignited hydrocarbons, such as cetane and heptane, show two separate temperature zones of nonignition above the minimum ignition temperature, a behavior heretofore unobserved; under other conditions one or both of these zones are eliminated. The conditions of air and liquid feed also have a marked effect on the ignition time lag, especially at low temperatures. Addition of tetraethyllead completely inhibits ignition up to about 850-1000° F. (454-538' C.).
HE so-called spontaneous-ignition temperature of a T combustible liquid is not a definite property but varies according to the method of test. However, the application of one suitable test method to different combustibles will give comparative results which may be of considerable significance. For evaluating the fire hazard arising from the contact of liquids or vapors with a hot surface, the Moore oil-drop or crucible method of test is undoubtedly the simplest to apply in the laboratory, and may be expected to correlate with the actual behavior found in practice on a larger scale. In the Moore test of a given liquid, the principal variables
are: the amount of liquid taken (for a crucible of given size), the nature of the surface material of the crucible, the use of air or of oxygen, and the use of stagnant or of flowing air. It is assumed that the pressure is held a t one atmosphere, and that the liquid is charged in drops and not atomized; these two provisions are in accord with the conditions normally involved in consideration of the fire hazard of a liquid (although a t the same time they prevent the test from showing good correlation with the behavior of the liquid as a fuel in internal-combustion engines). For the same reason the use of air rather than of oxygen is desirable; in the present work a few tests were also made on a 50 per cent oxygen-nitrogen mixture, in order to demonstrate that the concentration rather than the total amount of oxygen is the important variable. The present work is an outgrowth of a series of tests (6) made on different liquids, in connection with their fire hazard when used in aircraft. On this account it was decided to adhere t o the use of a stainless steel crucible and of flowing air, rather than to follow the A. S. T. M. specification (designation D-286-30) of a glass crucible (flask) and of stagnant air. The steel surface gave good reproducibility, without requiring any unusual care in cleaning; while it normally gives somewhat higher ignition temperatures than glass does, the difference is not important. The air flow rate was varied one hundred fold, the lowest rate being practically the equivalent of stagnant air. The existence of one or two well-defined temperature zones of nonignition above the h s t or minimum ignition temperature was noted in the previous report (2) for some of the paraffinic fuels. This type of behavior is familiar in vaporphase oxidations studied by both the flow and static methods, but has not heretofore been reported for the Moore method; and the appearance of two zones of nonignition a t atmospheric pressure is novel in any case. Accordingly, this phenomenon was studied in some detail for cetane as a function of air flow rate and liquid drop size, and for heptane as a function of liquid drop size. For comparison, a number of other hydrocarbons were tested a t one or two liquid drop sizes.
358
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 33, No. 3
tion either began or ceased. These results showed good repr:ducibility, usually better than *2' F. .(*l.ll (3.). Equally good checks were obtained on tests made months apart, and a second complete assembly of apparatus checked the original assembly t o within about 5" F. (2.78" C.) for four differenthydrocarbons.
Results with Various Hydrocarbons
CETANE. The effect of both air flow rate and liquid drop size on the spontaneous ignition of cetane is shown in Figure 3. For air flow rates of 2, 10, 20, and 50 cc. per minute the results are scarcely distinguishable: They show ignition beginning about 450' F. (232' C.) with one zone of nonignition from 550" to 600" F. (288' to 316" C.), and a t low liquid drop sizes, a second zone of nonignition in the neighborhood of 725 ' F. (385" C . ) . At the higher air flow rate of 125 cc. per minute both of these zones are considerably widened, but at the still higher rate of 200 cc. the second zone is entirely wiped out and the first is almost wiped out. This variaFIGURE 1. DIAGRAM OF CRCCIBLE (DIMENSIONB IX INCHES) tion was also demonstrated by a run at a constant temperature of about 725' F., in which the air flow rate was progressively increased from a low value; the result was that ignition Equipment and Procedure occurred a t first, then failed to occur, then reoccurred a t The crucible (Figure 1) was made of stainless steel (18 per cent the higher rates. chromium, 8 per cent nickel, 0.3 per cent selenium) and covered The effect of liquid drop size is not marked, except for with an outer Transite plate. The i nition chamber, of 43-CC. volume, was cleaned after each run by%rushing lightly with steel drops below 10 mg. in size, for which the zones of nonignition wool, although there was usually little or no visible deposit; this are considerably widened as the drop size is reduced, until a t a procedure retained a dull satin finish on the surface and gave of 1.6 mg. the two become one and extend Over a re roducible results. 300" F. (167' C.).range. %sing a variable transformer, the heating current mas reguHEPTANE.As Figure 4 shows, the effect of liquid drop lated to give a temperature rise of 2' F. (1.11' C.) per minute, measured potentiometrically. The air stream was metered size on the ignition of heptane a t an air flow rate of 125 cc. from 2 to 200 cc. per minute and was dried over Drierite. The liquids were delivered from glass droppers, calibrated separately for each liquid by delivery into a weighing tube. Figure 2 illustrates the complete assembly of equi ment. Ofthe hydrocarbons used, the pentane and decane were obtained through the courtesy of Albert L. Henne of the Ohio State University; the xylenes, cyclo entanes, dimethylhexane, and methylethyypentane were furnished by the A. P. I. Hydrocarbon Research Project; the cetane was purified in this laboratory by crystallization; the isododecane was the commercial mixture obtained by hydrogenation of triisobutylene; the others were commercial c. P. or certified materials. An approximate location of the minimum ignition temperature for each given set of conditions was obtained by a preliminary run starting at 250' F. (121.1' C.) nith a fast rate of heating. Then, starting some degrees below this point, the temperature was noted, a drop of liquid was added, and a stop watch was started. If ignition occurred, there was always an evident puff of flame or smoke, and the watch was then stopped; in the absence of this, observation of the interior of the crucible by means of a mirror failed t o show even a glow, except at temperatures above 1000" F. (538' C.). In either event, at the end of 1 minute the crucible was flushed out with an air stream from a hose; then after another 1 minute, another drop of liquid was added. This cycle was repeated up to about 1000° F. or higher. Check determinations \+ere usually made, both on rising and falling temperatures, in the neighborhood of each point where igniFIGURE 2. ASSEMBLED EQUIPMENT
CHEMISTRY
INDUSTRIAL
March, 1941 0
IGNITION
0 IGNITION
AIR FLOW
CC.I MIN.
125 CC./ MIN.
zi*iJ
2,10,20,
AIR FLOW
ticipated, the oxidation-resistant hydrocarbons show no indication of zones of nonignition, whereas the readily ignited decane and decahydronaphthalene evidently have two zones of nonignition similar to those of heptane. Table I lists the minimum ignition temperature and zones of nonignition (if any) for seven other hydrocarbons, together with the eleven compounds in Figure 5 a t an air flow, rate of 125 cc. per minute and a liquid drop size of about 10 mg. (for the compounds in Figure 5 the 10-mg. temperatures were estimated by interpolation). The six aromatic hydrocarbons, which have the highest ignition temperatures, gave a visible glow within the crucible over a temperature range of 15-50' F. (8.3-27.8' C.) below the point of flammation.
STARTS STOPS
AIR FLOW 200
359
Effect of Variables on Ignition
SOCC./MIN.
INCREASED OXYGENCONCEP~TRATION. Tests made on cetane with a 50 per cent oxygennitrogen mixture instead of air are listed in Table 11; they show that the use of this mix10 ture scarcely changed the minimum ignition temperature but did wipe out the zones of 0 nonignition in every case. This effect is 600 700 800 900 400 500 clearly due to the increase in the concenTEMPERATURE,'F. tration of, rather than in the amount, of oxygen. OF AIR FLOW RATEAND IGNITION SIZE ON THE FIGURE3. EFFECT TETRAETHYLLEAD. Addition of 3 cc. tetraIGNITION OF CETANE ethyllead per gallon raised the ignition temperature of cetane from 450" to 932' F. (232.2' to 500' C.) an increase of 482' F. (267.8' C.) a t an air flow of 54 cc. per minute and drop size per minute is pronounced, in contrast to that of cetane. of 8 mg. The previous report (3) showed similar increases: The ignition in the range from 490-525" F. (254-274' C.) occurs only within a limited range of drop sizes; yet even here the distinction between ignition and nonignition was sharp. OTHDR HYDROCARBONS. Figure 5 gives results for nine TABLE 11. SPONTANEOUS IGNITION TEMPERATURES OF CETANE IN 50 PERCENTOXYQEN-NITROGEN other hydrocarbons, together with heptane and cetane, a t Gas 125 cc. per min. air flow and two different liquid drop sizes; Flow Drop the lower sets of points is for a drop in the range 6-8 mg., and Gas Used Ignition Temp. Zones of Nonignition Rate Sine the upper, for a drop about 10 mg. larger in each case. The 2;;. MQ. F. C. F. c. effect of the drop size is nil in some cases but marked in others 125 9.5 Air 457 236.1 558-635 292.2-335 where differences of some 100"F. (56' C.) are found. As an683-780 361.7-415.6 O
1.6
50%Oz-N1
452
233.3
None
Air
485
251.7
540-865
474
245.6
None
450
232.2
451
232.8
548-598 715-738 None
50% 0 2 - N ~
TABLE I.
SPONTANEOUS IGNITION
TEMPERATURES OF VARIOU5
HYDROCARBONS F.
O
C.
Cetane
455
235
Decane Heptane Deoahydronaphthalene Propylcyclopentane
487 498 520 545
252.8 258.9 271.1 285
Methylo yolohexane Pentane 2 3-Dimethylhexane Z:Methyl-3-ethylpentane Methylcyolopentane
740 785 820 862 876
393.3 418.3 437.8 461.1 468.9
Isododecane 2 2 4-Trimethyl entane d-dfethylnaphtfalene o-Xylene Toluene
932 985 1050 1097 1165
500 529.4 565.6 591.7 029.4
Benaene m-Xylene p-Xylene
1205 1272 1275
651.7 688.9 690.6
0
'
F.
560-632 685-775 602-752 613-745 615-698 646-708
..... ..... ..... .....
.....
.....
.....
.....
..... ..... ..... .....
.....
0
9.5
Air 50% 02-NZ
Zones of Nonignition
Ignition Temp. Compound
50
O
282.2-462.8
286.7-314.4 379.4-392.2
c.
293.3-333.3 362.8-412.8 316.7-400 322.8-396.1 323.9-370 341.1-375.6
....... ....... ....... ....... .......
....... ....... .......
....... .......
....... ....... .......
500
600 700 TEMPERATURE, F.
800
OF LIQUIDDROP SIZEON THE IGNITION OF FIGURE 4. EFFECT HEPTANE AT AN AIR FLOW RATEOF 125 Cc. PER MINUTE
360
INDUSTRIAL AND ENGINEERING CHEMISTRY
for heptane, 341' F. (189.4' C.); for isododecane, 63' F. (35" C.); for 2,2,4-trimethylpentane, 92" F. (51.1' C.). I n each case the addition of lead brought the ignition temperature up to about 850-1000' F. (454-538' C.).
Vol. 33, No. 3
ture ignition (at the end of the second zone of nonignition) the lag increased slightly to about 1-2 seconds. The ignition-resistant hydrocarbons, with single ignition temperatures above 700" F. (371' C.), showed time lags of
WTHYLCYCLO I BENZENE EZS
t
!
I
W v)
E
v)
a
0
a
I? U
n
I I
I
400
500
I 800
I I 600 700 TEMPERATURE,. F.
I 900
I 1 1000 1100 TEMPERATURE, 'E
I 1200
I 1300
OF LIQUID DROPSIZEON IGNITION OF HYDROCARBONS AT AN AIR FLOW OF 125 Cc. PER MINUTE FIGURE 5. EFFECT
IGNITION TIMELAGS. For readily igniting hydrocarbons, such as heptane and cetane, the first or low-temperature ignition followed a relatively long time lag, from 5 to 40 seconds. For cetane, with ignition temperatures in the range 450-486' F. (232.2-252.2' C.) this time lag increased from 6.5 to 32 seconds as the drop size was increased from 1.6 to 22.6 mg. at an air flow of 125 cc. per minute; also, i t increased from 9.5 to 40 seconds as the air flow was decreased from 200 to 2 cc. per minute, at a drop size of 9.5 mg.
50
-
CkrANE
4 0
l3
4
HEPTANE
A
30
V
W
AIR FLOW IOCC./MIN. DROP SIZE 2 2 . 6 MG. AIR FLOW 125 c c . 1 M I N . DROP S I Z E 10.4 MG.
IGNITION IGNITION
STARTS STOPS
E 20 z g
t
5
10
0 450
500
550
TEMPERATURE,
600
"F.
FIGURE6. EFFECTOF AIR FLOW AND DROPSIZEON TION TIMELAGS
IQNI-
about 3 seconds a t the ignition point, which decreased slowly with further increase in temperature; in contrast to the foregoing, these lags increased with a decrease in drop size, up to as much as 14 seconds for a 1.7-mg. drop of benzene a t 1205' F. (651.7' C.).
Discussion of Results The foregoing results are in general accord with previous experience in the field of spontaneous ignition temperature measurements, as outlined in recent reviews and discussions of the subject (1). I n their study of ignition of air-vapor mixtures as a function of pressure, Maccormac and Townend (8) showed two small temperature zones of nonignition for heptane and octane a t pressures within a narrow range in the neighborhood of 1.6 atmospheres. These zones were only 5-25' F. (2.8-13.9' C.) wide, but the mixtures used contained 50 per cent excess air over the theoretical for complete combustion; i t appears from their data that richer mixtures would show wider zones of nonignition, and that the pressure a t which these zones occur would be lower. The present results are in agreement with this supposition. Maccormac and Townend (3) also state that they were unable to make measurements on decane, for which the ignition time lags were too short to permit the ignition chamber t o be filled. The present work illustrates the usefulness of the Moore or oil-drop method in this respect; the ability to obtain easily measurable results with decane and even cetane is probably due to the decrease in rate of evaporation of the drop with increasing molecular weight.
Literature Cited In the range of some 100' F. (56' C.) between the first ignition temperature and the beginning of the &st zone of nonignition, the time lags fell off gradually from the initial values to a lag of 1-2 seconds as the temperature was increased. Two typical curves are shown in Figure 6. I n the range between the two zones of nonignition the time lags were uniformly about 0.5-1 second, but a t the third or high-tempera-
(1) Helmore, W., "Science of Petroleum", Vol. 4, p. 2970, Oxford Univ. Press, 1938; Townend, D. T. A. et al., Ibid., p. 2958; Chem. Rev., 21, 259 (1937); J . I n s t . Petroleum, 25, 459 (1939). (2) Heron, 8 . D., and Beatty, H. A., Proc. A m . Petroleum Inst., 9th Mid-Year Meeting, 20, 115-37 (1939). (3) Maccormac, M., and Townend, D. T. A., J . Chem. SOC.,1938, 238.
ENDOF SYMPOSIUM
