1
1
1
Limits of Flammability of
APPARATUS FOR FLAMMABILITY DETERMINATIONS
E. B. HODGE Commercial Solvents Corporation, Terre Haute, Ind.
0
1. A combustion tube of sufficient diameter and length must be used; 2 inches X 6 feet (5.1 cm. X 1.8 meters) are adequate chemistry is the vapor-phase nitration of aliphatic dimensions. hydrocarbons. I n such nitrations part of the nitric 2. The gases must be thoroughly mixed. acid appears in the exit gas as nitric oxide. This must be re3. T o give comparable results, the directions of flame propagation must be similar; most limits are given for upward propacovered for economic reasons. Also, an excess of hydrocarbon gation. is used, so that hydrocarbon is present along with the nitric 4. The ignition source must be sufficiently strong. A flame oxide. One way to recover the nitric oxide is to oxidize i t to or a very hot spark is commonly used. nitrogen dioxide with air and then remove the nitrogen dioxide with water. I n order to carry out this oxidation safely, it is I n most work of this nature, temperature and pressure need necessary t o have data on the proportions in which nitrogen be controlled only within rather wide limits. Ordinary dioxide, air, and hydrocarbon form combustible or explosive changes in room temperature or barometric pressure usually mixtures, and the present work was carried out for this reahave no significant effect, but since an equilibrium mixture of son. Although this paper concerns only propane, similar NO2 and N204 was involved in the present study, the room work with other hydrocarbons is in progress. temperature was kept between 25" and 27" C. When a combustible gas, such as propane, is added t o air, a I n previous work i t has generally been customary to make concentration is reached where the mixture will just burn. up a gas mixture over a confining liquid. The gas mixture As the proportion of combustible gas is increased, the combuswas analyzed and then drawn into an evacuated combustion tion at first becomes more vigorous, but as more and more tube. Because of the difficulties of handling mixtures concombustible gas is added, the force of the combustion dimintaining nitrogen dioxide, this method could not be used. ishes and finally another point is reached where the mixture Instead, the three gases were passed through carefully caliwill just burn. These two points, where a slight change in brated flowmeters, and the compositions of the mixtures were composition renders the mixture flammable or nonflammable, calculated from the flowmeter calibrations. are known as the limits of flammability of the combustible Flammable limits are usually given in per cent by volume, gas and air. These limits are ordinarily stated in per cent by but the shift of the equilibrium volume of the flammable gas. They have been accurately NiOa 2N02 determined for a number of combustible gases with air and with oxygen (2), and some work has been done on limits with both with change in temperature and change in partial presnitric and nitrous oxides (6); but as far as is known, no data sure, makes the calculation of volume percentages complicated are available on limits with nitrogen dioxide. and not particularly significant; therefore, the limits are The conditions for determining flammable limits were given on a weight basis. The equilibrium mixture of NO2 worked out by Coward and Jones (2). Briefly, they are as and N204 will be called nitrogen dioxide in this article. follows: 1390
NE of the recent developments in the field of organic
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Mixtures of Propane, Air, and Nitrogen Dioxide Materials The propane used was a c . P. grade. It was analyaed by J. A. Riddick of this laboratory in a Rose low-temperature fractionating column and found to be a t least 99.2 per cent propane, with ethane as the only impurity. Compressed air was used. This air was from a system which was in continuous and extensive use. The oxygen content was determined by means of an Orsat apparatus and found to be the same as that of the room air-i. e., 20.8 per cent. The compressed air was dried with calcium chloride before being passed through the flowmeter. Nitrogen dioxide was made in a modification of an apparatus described by J. M. Peterson (6). Approximately 85 per cent nitric acid was fed down a packed column surrounded by a condenser. The oxides of nitrogen, liberated by the action of the acid on copper contained in a flask below the column, passed up the column. I n this way the lower oxides were oxidized to nitrogen dioxide by the acid flowing down the column. A slow stream of oxygen was also passed through the apparatus to aid in oxidizing the lower oxides. The nitrogen dioxide made in this apparatus was about 95 per cent pure. The principal impurity was water. This nitrogen dioxide was distilled, and the first 85 per cent coming over was collected for use.
Apparatus The apparatus used is shown in Figure 1: Propane and air entered at constant pressure, maintained by the blowoff pressure regulators, A and A', which contained mer-
Limits of flammability were determined for the system propane-air-nitrogen dioxide. Mixtures were made by means of flowmeters and checked by analyses for nitrogen dioxide. I n general, the conditions set forth by Coward and Jones (2) were observed. The results are plotted on a triangular diagram in per cent by weight. The upper limits for ternary mixtures lie on a smooth curve between the upper limit for propane and air (13.9 per cent propane) and the upper limit for propane and nitrogen dioxide (33.5 per cent propane). The lower limits for ternary mixtures are on a straight line between the lower limit for propane and air (3.6 per cent propane) and that for propane and nitrogen dioxide (6.4 per cent propane).
cury. The rates of ?ow of propane and air were regulated by stopcocks B and B . With this arrangement constant flow could easily be maintained. Nitrogen dioxide was stored in the stainless steel cylinder, C, which was partially immersed in water. Air was blown through the water to provide agitation, and the temperature was controlled by immersion heater D, which was connected to a Variac transformer. The rate of flow of the nitrogen dioxide was regulated by the water temperature and by the l/a-inch stainless steel valve, F. The nitrogen dioxide passed through '/*-inch stainless steel pipe and then through flowmeter G. Connection was made between the l/s-inch stainless steel pipe and the flowmeter by means of joint H packed tightly with asbestos rope and sealed with paraffin. The flows of propane and air were measured by flowmeters 1 and J . No rubber was used in any part of the apparatus where nitrogen dioxide was present. Ground-glass joints were utilized for all connections exposed to nitrogen dioxide. From the flowmeters the gases passed into mixing device M . (The three-way stopcock, K, and soda lime tube, L, were provided for the pur ose of venting any nitrogen dioxide which might leak from $when the apparatus was not in use.) This mixing device had jets so arranged that the gas streams impinged at the same point on the adjacent wall. The mixture thus obtained was passed through another jet and out the side arm, M shown. It was then conducted into the combustion tube and directed downward. The gases passed up the combustion tube and out to the air. That thorough mixing was accomplished is shown by the fact that consistent results were obtained, and also by the fact that analyses for nitrogen dioxide checked the figure calculated from flowmeter calibrations. The three-way stopcock, N , was used to isolate the combustion tube from the rest of the apparatus before ignition, and also to introduce air for drying the combustion tube. Flask 0 was attached to the inlet tube by a capillary groundglass joint. This flask was used for analyses, which will be discussed subsequently. Combustion tube Q measured 58 X 1.7 inches (147 X 4.3 cm.). The bottom of this tube was closed by a removable caD attached by means of a ground-glass joint. The cap contained two sealed-in tungsten electrodes, P,with a 2-mm. gap between them. A spark coil actuated by four dry cells furnished the spark. The top of the tube was left open but was reduced to about 12 mm., and a T-tube was sealed onto it. The combustion tube was surrounded with magnesia pipe lagging, R, through most of its length. This lagging had a window, S, about 1 inch (2.5 cm.) wide throughout its entire length. The outside of the lagging and the bottom of the combustion tube were painted black to absorb light and render the flame more visible. The combustion tube had two thermocouple wells, T and T',which were to be used when determinations were made at higher temperatures. The combustion tube was in a hood, which was darkened by painting all of the windows black but the front one. A cloth hood was placed over this. This arrangement enabled the flames to be observed in the dark.
Flowmeter Calibration and Analytical Procedure The air and propane flowmeters contained water as the indicating liquid. They were calibrated for slow rates of flow by measuring the water displaced from a bottle during a timed interval. While the water was being displaced, the pressure in the bottle was maintained constant by regulating the rate a t which water was siphoned out of it. Before the propane flowmeter was calibrated, propane was bubbled through the water used for 2 hours. The higher rates of flow (50-100 liters per hour) were measured by using a Sargent wet-test meter, recently repaired and checked. All readings were corrected for water vapor. Three orifices were Cali1391
1392
INDUSTRIAL AND ENGINEERING CHEMISTRY
brated for the propane flowmeter, and three for the air flowmeter. I n the nitrogen dioxide flowmeter 63 per cent nitric acid saturated with nitrogen dioxide was used. This flowmeter was calibrated by running the nitrogen dioxide into a glass tube, cooled in an ice-salt mixture, and weighing the liquid condensed during a timed interval. These calibrations were checked from time to time. Analyses for nitrogen dioxide were made as follows: Flask 0 (volume 211 ml.) was evacuated. Then the flowmeters were set for the mixture desired, and this mixture was passed through the combustion tube for a length of time sufficient to fill the tube with the mixture. Stopcock N was shut, and the capillary stopcock on 0 was opened. Then 0 was removed, and an excess of standard alkali was introduced through the small funnel. The flask was shaken to dissolve the nitrogen dioxide, the alkali was rinsed out, and the excess was titrated with standard acid. I n calculating the volume per cent of nitrogen dioxide, the N20rN02 mixture was assumed to act as a perfect gas. Under room conditions (25O C. and 750 mm. pressure), the molecular volume would be 24.8 liters. The calculations were carried out as follows: Thenumber of equivalents of base used was multiplied by 46, the molecular weight of NO2; this figure was divided by the average molecular weight of nitrogen dioxide a t room temperature and at the estimated partial pressure; the result, which represented the number of moles, was multiplied by 24.8 to convert it to liters and divided by 21.1 [the volume of flask 0 (in liters) X 1001. This gave an approximation of the per cent by volume of nitrogen dioxide in flask 0. Using the figure thus obtained, the partial pressure was again calculated, and a new average molecular weight was found, which in turn was used to obtain a more accurate value for the per cent by volume of nitrogen dioxide in 0. This procedure was continued until no appreciable change occurred in the final result. I n making the calculations, the value for the equilibrium constant for the reaction, N204e 2N02,was taken as 0.139 atmosphere. This value is given by Bodenstein and Boes (1) for 25" C. The figures
VOL. 30, NO. 12
i n Table I are comparisons of the per cent of nitrogen dioxide by volume in the mixtures as calculated from flowmeter calibrations and as found by analysis. The analyses indicate that the flowmeter calibrations were substantially correct and that thorough mixing was obtained.
TABLE I. COMPARISON OF ANALYTICALRESULTS WITH CALCULATIONS FROM FLOWMETER CALIBRATIONS Point No.
Vol. % Nitrogen Dioxide From flowmeter From calibrations analysis 55.0 40.1 33.0 24.9 16.8 13.2 10.0
55.7 40.4 32.7 25.1 16.7 13.5 10.1
Point No. 8 9 10 11 12 13
Vol. % ' Nitrogen Dioxide From flowmeter From calibrations analysis 89.8 67.8 47.0 28.8 20.0 10.9
90.2 67.8 46.7 29.1 20.2 10.8
Experimental Procedure The flowmeters were set t o give the mixture desired, This mixture was passed through the combustion tube until a volume of gas about five times the volume of the tube had been put through it. Stopcock N was then closed, valve F was quickly closed, and stopcock K was turned to vent the nitrogen dioxide. While the tube was being viewed, a spark was passed between electrodes P, which ignited a small tuft of nitrocellulose inserted between the electrodes. If flame traveled to the top of the tube, the mixture was considered t o be flammable. If there was no flame or one that traveled only part way up the tube, the mixture was considered t o be nonflammable. After each ignition, the entire combustion tube was washed with a fine stream of water and dried with hot dry air. Table I1 gives the compositions of mixtures which would just burn and which just refused to bu'rn, and the limit mix-
FIGURE 1. APPARATUSFOR DETERMINING FLAMMABLE LIMITS
DECEMBER, 1938
INDUSTRIAL AhTD ENGINEERING CHEMISTRY
1393
tures which represent the average between those which would burn and those which would not. Points 1to 7, inclusive, are upper limit points; 8 t o 13 are lower limit points. These results are shown graphically in Figure 2. ~~
IJpper and Lower Limits The limits for propane and air mixtures were accurately determined and reported elsewhere ( 2 ) . On a weight basis these limits are 3.6 and 13.9 per cent propane. The limits for propane and nitrogen dioxide were determined and found t o be 6.4 and 33.5 per cent propane by weight. These four limits were plotted on a triangular diagram (Figure 2). Straight lines were drawn between the two upper and the two lower limits, and the area defined by these lines was used as a guide in determining flammable mixtures of the three gases. The upper limit mixtures lie on a smooth curve; the lower limit mixtures are on a NO2 AIR straight line. The flames a t the two limits FIGURE 2. WEIGHT PER CENT COMPOSITIONS OF FLAMMABLE MIXTURES were of a different character. Those a t OF AIR, NITROGEN DIOXIDE,AND PROPANE the upper limit were violet in color and dim, and the flame front took, possibly, a composition between two adjacent mixtures, one of which second to go up the tube; the flames a t the lower limit were yellow or orange, much brighter, and more rapid. The would burn and one of which would not. phenomena accompanying combustions such as those studied These compositions were figured from the flowmeter calibration curves. I n converting volumes of air t o weights, are undoubtedly complicated, and it may be that entirely different types of combustion were encountered at the two density data were taken from the International Critical Tables (4) and corrected to room temperature and pressure limits. Mixtures of propane and nitrogen dioxide lying in the intermediate region between the two limits were violently by means of the perfect gas equations. The density used for explosive. Intermediate mixtures containing the three conair was 1.169 grams per liter a t 25" C. and 750 mm. The density of propane was calculated using the method of Cox stituents were not ignited. The points represent the average from data collected by him (3). The value used was 1.805 grams per liter a t 25" C. and 750 mm. The principal sources of error were flowmeter calibrations, _ _ _ _ ~ maintenance of the proper flowmeter level, and changes in TABLE 11. LIMITCOMPOSITIONS FOR PROPANE, AIR, AND room temperature. These errors are difficult to evaluate NITROGEN DIOXIDE MIXTURES accurately. But judging from the agreement of the analyses % by Weight for Point Shown in Fig. 2 yo by Weight with the values calculated, from the distances on the triangular Point diagram between points which represent flammable and nonAir N0z CsHs Propagation Air NO2 CaHe NO. flammable mixtures, and from the fact that consistent re1 No .. 66.2 33.8 .. 66.5 33.5 .. 66.9 33.1 Yes sults could be obtained (most of the points shown were checked and no inconsistent results were found), the results 11.4 55.1 33.5 2 s:$ 11.6 55.6 32.8 56.1 32.1 11.8 are probably accurate within 2 per cent.
-
9
3
19.8 20.5
48.3 48.3
31.9 31.2
4
32.9 32.9
38.7 40.4
28.4 26.7
5
47.2 47.0
28.2 30.0
24.6 23.0
6
57.8 58.6
23.1 23.1
19.1 18.4
7
64.8 65.3
17.8 18.5
17.4 16.2
8
.. ..
93.8 93.5
6.2 6.5
9
11.5 11.0
82.9 82.4
5.6 6.6
10
66.2 65.6
28.3 28.1
5.5 6.3
11
47.0 46.5
48.4 47.9
4.6 5.6
12
59.6 59.5
35.7 35.1
4.7 5.4
13
75.6 75.0
20.0 20.0
4.4 5.0
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20.1
48.3
31.6
32.9
39.5
27.6
47.1
29.1
23.8
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58.1
23.1
18.8
YZB
65.1
18.1
16.8
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No
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93.6
6.4
$tS
11.2
82.7
6.1
65.9
28.2
5.9
B:$
46.7
48.2
5.1
B:$
59.6
35.4
5.0
75.3
20.0
4.7
&,:
Acknowledgment The author wishes to thank J. Martin and K. H. Hoover of the Commercial Solvents Corporation for helpful advice and criticism. Helpful suggestion8 were also obtained from G. W. Jones of the United States Bureau of Mines.
Literature Cited (1) Bodenstein and Boes, 2. physik. Chem., 100,68-75 (1922). (2) Coward, H. F.,and Jones, G. W., U. S. Bur. Mines, Bull. 279 (1928, rev. in 1931). 13) Cox. E.B.. Oil Gas J . . 33. 16 (1935). ( 4 ) International Critioal'Tables,'Vol. 111, p. 3, New York, McGrawHill Book Co..,~1928. ( 5 ) Lawrence, R. W., private communication. (6) Wal, M. J. van der, Rec. trav. chim., 53, 97-117 (1934). R E C ~ I V EJuly D 4, 1938. Presented before the Division of Physical and Inorganic Chemistry a t the 95th Meeting of the American Chemical Society, Dallas, Texas, April 18 t o 22, 1938.
