IKDUSTRIAL ARD ENGIXEERIUG CHEMISTRY
1364
to have some effect on the penetration of these compounds from aqueous emulsions into the insects and apparently reach a most favorable balance with the twelve-carbon product.
VOL. 27,
bo. 11
-4cknow-ledgmeiit The authors are greatly indebted to E. A. Taylor, E. B. Alvord, and other nienibers of the Grasselli Chemical Company who have lent their liberal support and guidance during the course of this invest,igation.
Plant Tolerance Tests Because the twelve-carbon homolog has shown a high relative efficiency, this product was selected for extensive plant tolerance tests. Numerous combinations of both the pure and commercial product in various dispersing agents have been used in laboratory, greenhouse, and field tests. At effective concentrations and in the proper dispersing medium, a sufficiently wide margin of safety to many kinds of plants has been found to warrant the use of such combinations as commercial insecticides.
Literature Cited (1) Peterson, Manual of Entomological Equipment and Methods, P a r t 1, plate 127, No. 2, 4-6, Edwards Bros., 1934. (2) Salsberg and Bousquet, U. S. P a t e n t 1,963,100 ( J u n e 19, 1934). (3) Ibid., 1,993,040 (March 6, 1935). RECEIVED July 11, 1935. Presented before the Dirieion of Agricultural and Food Chemistry at the 8 9 t h Meeting of the American Chemical Society, Y e a r o r k , N. Y , .4pril 22 t o 26, 1935. Contribution No. 158 f r o m the Esperimental Station, E. I. du Pont de Nemours 8: Company, Inc.
Prevention of Gas Exdosions bv I J
Controlling Oxygen Concentration J
W
c
*
E
XPLOSIT’E mixtures of combustible gase.;
G. F. JONES .4ND R. E. KENNEDY and vapors may be rendered nonexplosire by reducing the oxygen concentration beU. S . Bureau of Mines Experiment Station, lorn certain critical values. This i6 of special importance in Pittsburgh, Pa. the repair of equipment, such as gas line.. tank cars, and gas holders which contain combustible gases or vapors. The operations may be carried out with safety, provided the kind The value> obtained by this “reqidual atnioaphere” method and proportions of combustibles present are known and the have been found to vary with the design of the burner, with oxygen is kept below the amount required to propagate the rate a t which the gas is burned, and with the amount of flames of these combustibles. primary air introduced into the gas before burning. The Bureau of Mines has been obtaining data whereby the Table I gives the values obtained by investigators using the explosive or inflammable limits of complex combustible gas residual atmosphere method and, for comparison, values obmixtures, such as mine-fire gases, gases produced during mine tained by the explosion tube method used during the investiexplosions, and natural gases containing high percentages of gation described in this paper. Th_e e $ m t t o which the carbon dioxide and nitrogen, may be determined by calculavalues may J-ary by the residual atmosphere method is aption ( 2 , 4). These investigations hare given information on parent from the data given for natural gas. When natural the oxygen percentages required to prevent explosions of gas heaters were allowed to burn in a 1000-cubic foot sealed many of the commonly occurring combustible gases and chamber, and the atmosphere was sampled and analyzed a t vapors. the time the flames were extinguished, the results showed that Various methods have been used to determine the oxythe critical oxygen ralues depended on the type of heater used gen values below which explosive mixtures are rendered nonexulosive. Rhead 07 and otiers (6, 8) have obtained the oxygen p e r c e n t a g e in An investigation of the values below which the oxygen must be a t m o s p h e r e s a t or below maintained to prevent explosions of combustible gases and vapors is dewhich flames of combustibles scribed. The respective critical oxygen values obtained when carbon diwere extinguished by burning the combustibles in a closed oxide or nitrogen is used as the inert diluent are as follows for the various chamber. As the gas burned gases investigated: methane, 14.6 and 12.1; ethane, 13.4 and 11.0; proin the closed space, the oxypane, 14.3 and 11.4; butane, 14.5 and 12.1; pentane, 14.4 and 12.1; hexane, gen was consumed by the 14.5 and 11.9; ethylene, 11.7 and 10.0; propylene, 14.1 and 11.9; hydrocombustion process. Evengen, 5.9 and 5.0; and carbon monoxide, 5.9 and 5.6 per cent by volume. tually a time w a s r e a c h e d when the oxygen supply beElevated temperatures cause a widening of the inflammable limits of came sufficiently reduced to combustible gases and vapors and therefore a decrease in the oxygen conextinguish the flame. Analycentration necessary to prevent flame propagation. The values given in sis of samples of the atmoethis report hold only for temperatures below PO0 C. and for ordinary phere taken a t the time the variations of atmospheric pressure. flame was extinguished gave the amount of oxvgen present.
I I
I
meters) long and 2 inches (5 cm.) in diameter. Test,s were made by closing the lower end of the explosion tube with a ground-glass plate and evacuating the apparatus with a vacuum pump. The mixtures to be tested were prepared in a 60-liter mercury-sealed gas holder and passed from t'he gas holder through copper tubing into the explosion tube. Water vapor was removed by drying agents before reaching the explosion tube. The mixtures were tested for inflammability by sliding off the ground-glass plate at the bottom of the explosion tube and at the same time passing the flame of an alcohol lamp across the open end. Inflammable mixtures propagated flame to the top of the tube. Flames which propagated only a short distance from the ignition source were considered nonflammable.
TABLE
I.
OXYGES \-ALUES BELOW \\-HIGH FLAMES OF COMBUSTIBLE GASES AND VAPORS ARE EXTINGUISHED Per Cent 0 bv Vol in .ktmospheres below Which Flames .ire Extinguished IIixtures tested in 6-it. 2-in. explosion tube, flames propaFlames burning in gated closed chamber S added to CIO? added to (residual atmos- combustible- combuatiblephere method) air mixture air mixture
Combustible
EXPLOSIOS O F GAS GESERATIYG UXIT
D 4 X h G E CAUSED BY THE AN
ACETYLEXE
in the teat ( t i ) . The flaine of an ordinary yellon--flanie heater burning Pittsburgh natural gas n-itliout primary air supply was extinguished when tlie oxygen concentration x i s reduced to 17.7 per cent; in a radiant type heater in which a large proportion of the primary air supply was introduced into the gas stream before burning, the flaine was not extinguished until the oxygen concentration was reduced to 13.8 per cent. Even this d u e is too high for the prevention of flame propagation in some mixtures, since t'ests made in the G-foot explosion tube showed that t'he oxygen conceritratioii must be reduced to 12.0 per cent when nitrogen is used as a diluent,, to prerent propagation of all mixtures of Pittsburgh natural gas in air. (The Pittsburgh natural gas analyzed: methane, 90.5: ethane, 7.1; propane, 1.G; butane, 0.8 per cent by 1-olunie.) Inspection of the value; obtained by the explosion tube method (Table I) shows that in general they are lower than those obtained by the residual a t m o s p h e r e method. The greatest differences occur for the gases which have a low rate of flame propagation. For example, carbon monoxide gives an oxygen value of 10.2 per cent by the residual atmosphere method and below 6.0 per cent by the explo4on tube method.
H) drogen Carbon monoude Methane Ethane Propane Butane Pentane Hexane Cyanogen Ethylene Propylene Acetylene Pittsburgh natural gas Coal gas
12.1 : I ) 11 0 ( 3 ) 11.4 ( 4 ) 1 2 . 1 (4) 12.1 11.9
....
15 8 16.0 16.4
....
15.1 13.2
..
.
5.9 5.9 14.6 13.4 14.3 14.5 14.4 14.5
10.0
11.9
11.7 14.1
10.1 :S! 1 3 . 8 - 1 7 . 7 :6. 10.9 ':S)
1'7 0
14 4
....
. . I .
(4:' (4'1
....
7) (7J
....
(5:l (6) (fj (3
.... ....
Critical Oxygen Values Tlie oxygen content of an explosire mixture may be reduced by direct absorption of the oxygen by means of special zolut,ione, by dilut'ion with inert gases suc,h as nitrogen or carbon dioxide, or by combinations of these inert gases a:: represented by flue gas or exhaust gax from internal conibustion engines. In this investigation the oxygen content, was reduced by diluting the mixtures x i t h nitrogen and carbon dioxide. The inflammable or esplosiye limits of a large number and wide rariety of mixture. of each combustible in air were determined when both nitrogen and carbon dioxide were used as diluents. Tlie results when plotted with the ratio of added inert to the combustible as the abscissa, and
Explosion Tube JZethod The explosion t,ube method n-as chosen because it closely simulates conditions found in large confined spaces. These conditions require an explosion chamber sufficiently large in diameter to eliminate the cooling effect of the walls of the c h a m b e r a n d l o n g enough to ascertain whether the flames continue to propagate independently of tlie heating effects of the source of ignition: The tests were made under conditions whereby the flames were propagated upward, since flames will propagate upward in atmospheres containing lower percentages of oxygen than when propagated horizontally or downward. The explosion tube described in previous reports ( 2 ) w m 6 feet (1.8
5 . 0 (31 5 . 6 (51
5.6 10.2 15.5
1345
INDUSTRIAL AND ENG NEERING CHEMISTRY
13 6
0
IZ
IO
R A & , : ~ INERT ~ ~ ~GAS
LU
HEXANE
IN FIGURE 1. LIMITSO F INFLAMMABILITY O F HEXANE Am WHEN MIXED WITH VARIOUS PROPORTIONS OF ADDEDNITROGEK AND CARBON DIOXIDE
the percentage of combustible plus added inert that was inflammable as the ordinate, describe the boundary of areas outside of which the mixtures are noninflammable. Graphs of this kind show the oxygen percentage required to prevent explosion of each particular concentration of combustible. However, from a practical viewpoint it is necessary to consider only the minimum oxygen value required t o prevent explosions of any mixtures; accordingly the complete data will be given for only one combustible as an example of the procedure used in obtaining the maximum allowable oxygen percentage for a particular combustible. Figure 1 shows the inflammable areas of mixtures of hexane and air with nitrogen or with carbon dioxide. The percentage of hexane plus inert gas (added nitrogen or carbon dioxide) is plotted on the vertical axis and the ratio of added inert gas to combustible on the horizontal axis. Tests were first made with hexane-air mixtures containing no added inert gas, and the lower and upper inflammable limits were determined; these limits were 1.27 and 6.90 per cent, respectively. Then nitrogen or carbon dioxide was added to the hexane-air mixtures, and the limits were determined. This procedure was continued until the entire inflammable area was defined by the points shown on the curves. The oxygen values below which flames were unable to propagate are shown by a scale on the right vertical axis and are seen to vary with the ratio of added inert gas t o the com-
VOL. 27, NO. 11
bustible present. The mixture of hexane-air and added nitrogen that propagates flame with a minimum concentration of oxygen contains 22 volumes of added nitrogen per volume of hexane. This mixture will propagate flame when the oxygen concentration is 11.9 per cent oxygen or higher, All other mixtures require higher oxygen concentrations as shown. The minimum oxygen concentration which permits flame propagation of hexane-air and added carbon dioxide mixtures is 14.6 per cent. The minimum oxygen values which permit flame propagation should be used to determine the extent to which the oxygen must be reduced to prevent explosions for any particular combustible. Figure 2 gives these minimum oxygen values for hydrocarbons from methane to hexane, and for ethylene, propylene, carbon monoxide, and hydrogen. The results show that, if carbon dioxide is used as a diluent, explosions are prevented a t higher oxygen concentrations than when nitrogen is used as the diluent. The respective critical oxygen values obtained when carbon dioxide or nitrogen is used as the inert diluent are as follows: for the various gases investigated (in per cent by volume): methane, 14.6 and 12.1; ethane, 13.4 and 11.0; propane, 14.3 and 11.4; butane, 14.5 and 12.1; pentane, 14.4 and 12.1; hexane, 14.5 and 11.9; ethylene, 11.7 and 10.0; propylene, 14.1 and 11.9; hydrogen, 5.9 and 5.0; and carbon monoxide, 5.9 and 5.6. The tests indicate that the combustibles which occur in natural gases and the volatile saturated hydrocarbons in petroleum and its products are rendered nonexplosive when sufficient carbon dioxide is added to the combustible mixtures to reduce the oxygen content below 12.5 per cent; if nitrogen is used as the diluent, the oxygen content should be reduced below 10 per cent. Prevention of ethylene explosions requires lower oxygen values than the saturated hydrocarbons, and carbon monoxide and hydrogen require the lowest of any combustibles so far tested. The oxygen content of combustible mixtures containing either carbon monoxide or hydrogen or both should not exceed 5 per cent if explosion hazards are to be eliminated. If automobile exhaust gas or flue gas is used as the diluent, the oxygen content of combustiblegas mixtures should be maintained below 5.0 per cent to eliminate the explosion hazards, since these diluents may contain appreciable amounts of carbon monoxide and hydrogen.
Literature Cited (1) Coward, H. F., a n d Hartwell, F. J., J . Chem. SOC.,129, 1522 (1926). (2) Jones, G. W., Bur. Mines, Tech. Paper 450 (1929). (3) Jones, G. W., and Kennedy, R. E., Bur. Mines, Rept. of Investigat i a s 3172 (1932). (4)Ibid., 3216 (1933). (5) Jones, G. W., and Perrott, G. S t . J., IND.EXQ.CHEM.,19, 985 (1927). (6) Jones, G. W., Y a n t , W. P., a n d Berger, L. B.,Bur. Mines, Tech. Paper 362 (1925). (7) Rhead, T. F. E., J. SOC.Chem. Ind., 37,274T (1918). (8) Vinding, P., and Banner-Voigt, E., Chimie et industrie, 30, 579 (1933). RECEIVED April 2 6 , 1935. Presented before the Division of Gas and Fuel Chemistry at the 89th Meeting of the American Chemical Society, New York, N. Y., April 22 t o 26, 1935. Published by permission of the Director, U. 9. Bureau of Mines. (Not subject to copyright.)
SATURATED HYDROCARBONS FIGURE
\\'HIGH
LLUMINANTS
PERCEYT4GE O X Y G E N I N G ~ MIXTURES s BELOW EXPLOSIOYS A R E P R E V E N T E D IT O R D I U a R Y T F M P E R A TURES IUD PRESSURES
2.
