I
PAUL E. SAMPLE1 and H. P. SIMONS West Virginia University Engineering Experiment Station, Morgantown, W. Va.
Explosive Reactions of Diborane in BenzeneSaturated Air Benzene has a pronounced effect on controlling the explosive reaction of gas mixtures with various diborane concentrations
THE
vapor phase oxidation of diborane and hydrocarbons in the presence of air largely a t reduced pressure has been studied (4, 7-70) but no data for atmospheric pressure are available. In the work reported here the behavior of diborane was studied when ignited in the presence of air containing 13 mole % benzene vapor. The techniques and the equipment used in this investigation were similar to those used by Poling and Simons (5, 6), with minor alterations made necessary by changes in burning characteristics and flame color. The general handling procedure and toxicological precautions were also similar. The 1 Present address, E. I. du Pont de Nemours & Go., Clinton, Iowa.
Figure 1. Typical pressure-tirhe pattern variation with mole per cent diborane indicates that benzene has a definite effect on burning characteristics VOL. 50, NO. 11
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1699
0
2
Figure 2.
4
6
8
IO
12
14
16
18
20
Effect of diborane concentration is shown on maximum reactor pressure
principal difference was that the air used was saturated by passing through four benzene bubblers, attaining an essentially constant dew point of about
25' C. Reactor product gases were analyzed for carbon dioxide, oxygen. carbon monoxide, hydrogen, and nitrogen (72).
Discussion of Results
Pressure-Time Study. I n the ignition of mixtures of these gases of various diborane concentrations it was apparent that the presence of the benzene had a pronounced affect on the over-all burning characteristics. From a series of photographs in Figure 1 the concentration of diborane increased the slope of the curve from the point of ignition to the first peak. In each case there was an initial rise in pressure inside of the reactor when the combustion started, followed by a drop in pressure and then a second rapid rise. As the mixture became richer in diborane the drop
became more pronounced. This was believed caused by the detonation shock which was then followed by a rapid pressure build-up in the reactor. From the analysis of all of the pressure-time photographs in Figure 1 it was possible to plot the reactor pressure after time intervals of 3, 5, and 10 milliseconds (msec.). This was done by starting at the point of initial reaction as indicated by the first vertical deflection and measuring in the x direction the distance equivalent to the particular time interval under consideration. At this point a measurement of the deflection in the y direction was made and with the aid of pressure calibration information this distance could be converted to the pressure equivalent. In comparing this information with that presented by Poling and Simons (5, 6 ) for ignition of diborane in dry air it was noted that the presence of benzene vapor caused an approximate reduction in the maximum reactor pressure after 3 msec. from 150 p.s.i.g. to
2Y)
a e
45 p.s.i.g. after 5 msec. from 12U to 75 p.s.i.g., and after 10 msec. from 90 to 70 p.s.i.g. I n addition, measurements of the maximum reactor pressure for each run were made (Figure 2). From this the over-all maximum reactor pressure dropped from 200 to 120 p.s.i.g. The presence of the benzene vapor had no effect on the minimum concentration of diborane required to produce a maximum over-all reactor pressure-Le., between 12 and 14 mole yo diborane. Figure 2 shows the effect of the initial diborane concentration on the maximum reactor pressure. This plot is similar to those obtained for pressures after 3, 5, and 10 msec. The curve in Figure 2 is represented by the equation
P, = -31.84
0 c
B2Hb
E"
+ 302
B203
+ 3H10
(1)
+ 3H2
(2)
and
F
C6H6 Initial Diborone Concentrotion, Mole Per Cent
Figure 3. Effect of diborane concentration on time required to reach maximum steady pressure
1700
+ 24.26 Cd - 0.96 Cd2
where P, is the gage pressure and Cd is the initial concentration of diborane in the gas mixture. This equation as well as all others mentioned here were arrived at statistically and are summarized along with the 95% confidence intervals on all regression coefficients in the table. By measuring the time required to reach maximum steady pressure inside the reactor, a measure of the rate of the reactions of a function of the concentration of diborane present in the original gas mixture was distinctly indicated in Figure 3. The time required to reach maximum steady pressure decreased sharply with increasing diborane concentration up to about 7 mole yo, at which concentration the time had reached a minimum of about 4.5 msec. for the entire range from 7 to 20 mole 70. T o ascertain the path of the reactions, many samples of the combustion gas were analyzed. N o appreciable quantities of either oxygen or carbon dioxide were found by these analyses. Sufficient quantities of carbon monoxide, hydrogen, and of course, nitrogen were present to warrant consideration. Figures 4 and 5 show the change in the quantities of carbon monoxide and hydrogen, respectively, with variations in the initial diborane concentration. These curves have been compared with several possible theoretical reaction paths. These are shown as dotted curves and the letter designation indicates the pattern as one of the following : Case (I. Preferential oxidation of diborane followed by oxidation of the benzene to the extent of any remaining oxygen as represented by the reactions
INDUSTRIAL AND ENGINEERING CHEMISTRY
+ 302
6CO
I t should be noted that the reaction which oxidizes benzene completely giving water rather than hydrogen was more favorable according to the standard free
I
a,
DIBORANE EXPLOSIVE REACTIONS
energy calculations but owing to the deficiency of oxygen Reaction 2 was considered more likely. These reactions would then be followed by pyrolysis of any remaining diborane and benzene as follows : B2Hs = 2B
+ 3H2
C ~ H= B 6C
+ 3H2
and (4)
Evidence of the pyrolysis of the benzene was given by the fact that there was a considerable quantity of carbon deposited in the reactor. Case b. The preferential oxidation of diborane was again assumed followed by the oxidation of benzene to the extent of any remaining oxygen. Here, instead of pyrolysis of both the remaining benzene and the diborane only the benzene pyrolyzed. The diborane would undergo complete hydrolysis by the water formed in Reaction 1 together with the water present in the solutions of the Orsat equipment, according to the reaction BzHs
+ 6HzO = 2H8B03 + 6H2
(5)
Case c. This case assumed the same oxidation and pyrolysis patterns for benzene, but this time the excess diborane would hydrolyze only to the extent of the amount of water from Reaction 1 ; any remaining diborane would then pyrolyze. Cases d, e, and f have the same reactions as a, 6, and c, respectively, for diborane but the benzene would undergo a secondary oxidation without pyrolysis of any excess-i.e., omission of Reaction 4. Case g assumes the preferential oxidation of benzene followed by pyrolysis of any excess and since there is never enough oxygen to react completely with the benzene there can be no oxidation of the diborane. In this case the diborane will hydrolyze and pyrolyze according to Reactions 5 and 2. The hydrolysis would occur only to the extent of the amount of water formed in Reaction 1, Case h would be the same as case g, except there would be no pyrolysis of the excess benzene. Figure 4 indicates that the experimentally determined quantity of carbon monoxide in the combustion gas lies somewhere between that required for the assumption of preferential oxidation of the diborane as indicated by curves a through f and that of preferential oxidation of the benzene as indicated by curves g and h. This would be expected since the favoritism of the reactions based on standard free energies are found to cross with increase in temperature, therefore, the reactions are probably competitive rather than preferential.
Mtlai W a n e Concentration, Mole Pbr Cent
Figure 4. Effect of diborane concentration on carbon monoxide in combustion gas is compared with several possible reaction paths
95% Confidence Limits on Coefficent
Statistical Regression Equations and Figure No. .
Regression Equations
2
P,
3
e,
4
~
5
7
-31.84
+ 24.26Cd - 0.96Cd2
= 24.02 - 2 . 9 0 ~ ~ em = 4.49 - 0 . 0 1 7 ~ ~ CC, = 18.25 - 1.41 c d 0.028 cd2
+
Coefficient
B C
B B B C
++
C E ~ = 7.10 2.24Cd -378 499.95cd - 16.2Cd2 Rf
B
B C
However, the use of thermodynamic calculations is limited for this purpose in that such calculations refer to equilibrium conditions. Since there is no real reason to believe that complete . equilibrium is attained within the moving flame front, comparison of the free energies is merely indicative. Figure 5, the corresponding plot for hydrogen concentration, has a wider
95% Confidence Interval f 7.90 & 0.34 f 3.47 f 0.065 f 0.56 & 0.026 0.23 f184.3 & 9.92
*
spread of the experimental points but the statistical curve indicates again a complex pattern of reactions. Rate of Flame Propagation Study. The rate of flame propagation most probably represents a combination of deflagration and detonation. This conclusion was derived from photographs of Figure 6. Photograph 6,A, was typical of oscillograph pictures obtained for
Initial Diborone Concentration, Mole Per Cent
Figure 5. Effect of diborane concentration on hydrogen in combustion gas shows a complex pattern of reactions VQL. 50, NO. 11
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Figure 6. A. 6.
Typical flame propagation patterns
Pattern common for mixtures l o w in diborane concentration which deflagrate Pattern common for mixtures which detonate
in feet per second and Cd is the initial diborane concentration in mole per cent. The presence of the benzene \vapor reduced the maximum rate of flame propagation from 6625 feet per second in dry air as reported by Poling and Simons to 3100 feet per second. The benzene vapor permitted flame propagation velocities as low as 390 feet per second, far below that realized in dry air alone and also apparently lowered the ignition limit for diborane concentration €rom to about 2.4 mole yce about 4.0 mole References Initial Diborane Concentration, Mole Per Cent
Figure 7. The rate of flame propagation increasss rapidly with increase in diborane concentration
mixtures in the range of 4.0 mole y6 diborane which ignited with a sound like that produced by blowing down a long pipe. Photographs in Figure 6,B, were common for mixtures above 4.0 mole 70 which ignited with a loud report. There appeared to be ‘a transition range between 3.5 and 4.0 mole 70 diborane where either deflagration, detonation, or a combination of the two was possible. The sharp break in trace in Figure 6,B (center of photograph) was visible in every instance where the loud report was heard as the apparent result of a detonation. The slope of the trace for a deflagrating mixture was more gradual as in the Figure 6,A, showing that it required more time (measured in the x direction) for a similar quantity of light to reach the photoelectric pickup. Because benzene burns more slowly,
702
the light of this flame front does not reach the first porr in the velocity tube until several milliseconds after the diborane flame front has passed. This explains the second break in the curve as the sweep moves from left to right. However, by the time the flame fronts have reached the second port the expanding gases and the probable increase in rate of advance of the diborane flame front would result in a turbulent flame which would tend to obscure the less intense benzene flame. In Figure 7 the rate of flame propagation is shown to increase rapidly with an increase in the concentration of diborane according to the equation R f = -378
+ 499.95 Cd - 16.2 Cd2
where R, is the rate of flame propagation
INDUSTRIAL AND ENGINEERING CHEMISTRY
(1) Brunswig, H., “Explosives,” Wiley, London, 1912. (2) Kurz, P. F., Fuel 33, 250-1 (1954). (3) Zbid., 35, 318-22 (1956). (4) Kurz, P. F., Tech. Rept. No. 15036-34, March 4, 1955 (TAB, U-Si), 3 (1955).’” (5) Poling, E. L., dissertation, West Virginia University, Morgantown, W. Va., 1955. (6) Poling, E. L., Simons, H. P., U. S. Atomic Energy Comm., Suclear Sci. Abstr. 10, No. 8, Abstr. 59 (1356); IKD,EXG.CHEM.50,1695 (1938). (7) Price, F. P., J . Am. Chem. SOC.72, 5361-5 (1950). (8) Roth, W.,“A Study of the Explosive Oxidation of Diborane,” Fifth Symposium on Combustion, p. 710, Reinhold, VPWYork. 1955. (9j‘Roth,--ilC’., Bauer, W. H., J . Phys. Cizem. 60, 639-41 (1956). (10) Schlesinger, H. I., Burg, A. B., unDublished results of explosive nature bf B ~ in H oxygen. ~ (11) Snedecor, G. W., “Statistical Methods,” Iowa State College Press, 1956. (12) Treadwell, F. P., Hall, T. Williams, “Analytical Chemistry,” Vol. 11, (‘Quantitative,” Wiley, New York, 1942. RECEIVED for review May 16, 1957 ACCEPTEDJuly 28, 1958
