CHEMICAL ACTIOS IN THE GLOW DISCHARGE. XIY
THE IGNITION OF HPDROGES-OXYGEN MIXTURES A. KEITH BREWER
AYD
P. D. KUECK
F e r t i l i z e r I n [ esfigntions. Rn!.cnzr os C h e m i s t r y and S U I I S , C . . 4 9 / i ~ i i l t i i r c ,Tl7nshinqton, D.C.
S.Deptr, t r n c t ~ to j
E c c c i i f t i 3Iag 10, 19S!+
In previous papers (1, 2) tlie factors influencing the ignition of esplo ive mixtures of various gases in the condensed discharge were discussed in detail. The present comniunication describes a similar study of ignition in the positive column of the g l o discharge. ~ A 2H2:02mixture. was cho. '11 for the study. APPARATCS AND METHOD
The apparatus was the same as that described in the study of chain reactions in the positive column (3). I n determining the conditions necessary for ignition, the discharge tube was first, filled to the desired pressure, as read by a dibutyl phthalate manometer. The anode was lonered to exclude any positive column, and the discharge started a t some lo^ current. The anode v a s then raised until tlie desired length of positive column was introduced and the current was increased until igiiitiori occurred, both the current and voltage being read at the instant of ignition. The fall of potential through the positive colunin \vas obtained by subtracting iliat for tlie negative glow from the potential drop betv een the electrodes. RESULTS
The eflccf of pccssiiw
Tlir. effect' of pressure of the purc 2II2: 0 7 niisturv 011 tlic poiver input in tlic positii-e column for ignition is slion-n in table 1. It will be observed that (iV)P is constant t o within tlici limits of esperimenf.nl error. Thus thc relit,ionship bctn-een power input and pressure cnn he espressed in the form (il') = k,'P. This is in distinction to the condciised discliargr, in whicli it was shoivn tliat Q = k , l P , n-here Q is the quantity of electricity ncccssary for ignition. The hyperbolic relation between pon-er and pressure in the positive column is to be expected, from the fact tlint tlie heat' developed as n-c,ll as the number of active centers formed is proportional to the pon.er espended. 10.51
1052
A . K E I T H B R E K E R AND P . D . KUECK
I n the condensed discharge, on the other hand, the spark jumps such a short distance that only part of the energy of the electron is dissipated in the gas; the production of active centers, therefore, is proportional to the number of electrons flowing rather than to the power. TABLE 1 Efect of pressure of the mixture on t h e p o u e r input jor ignition
v
P
i X 103
mm. Hg
amperes
volts
29.2 20.0 14.6
17.0 30.0 55.0
420 350 250
70
209 210 201
I
60
501 / 50
0 X
IO00
900 800 700
600
c" P 400 500
300 200 IO0
0
I
2
3
-4
Electrode Separation cm.
5
FIG.1. THEEFFECTO F ELECTRODE SEPARATIOS
T h e eflect of electrode separations I n figure 1 are given the current, potential drop through the positive column, and the power input a t ignition for various separations. It will be seen that while the ignition current decreases linearly with a n increase in the electrode separation ( d ) , the ignition voltage undergoes a rapid increase. The resultant power consumption increases only slowly with the length of the positive column, changing about 40 per cent for a
1053
CHEMICAL ACTION I N GLOW DISCHARGE. XIV
threefold increase in electrode separation. The propagation wave was set up a t some place in the positive column and not a t either of the electrodes; in so far as could be told the electrodes themselves had no direct effect on the ignition.
T h e e$ect of the H2:O2 ratio The effect of varying the hydrogen: oxygen ratio a t constant pressure on the power input for ignition is shown in figure 2.
I
I
I
I
I
FIG. 2. THE EFFECTOF
THE
I
I
I
I
HYDROGES-OXYGEN RATIO
The ignitability as given by 1/W, and the power consumed, W , are both plotted. The power required for ignition reaches a minimum in a mixture containing 25 per cent hydrogen and 75 per cent oxygen; this power consumption is only 18 per cent of that required for the combining 2Hz:02 mixture. This is in distinct contrast t o the condensed discharge, in which excess oxygen had only a negligible effect on the quantity of electricity required for ignition.
1054
A. KEITH BREWER AND P. D. KCECK
The e$ect of added gases The effect of various amounts of added gases on the power input for ignition is illustrated in figure 3. The partial pressure refers t o the amount of gas added t o 20 mm. of the 2H2:O2mixture. The data show that the addition of hydrogen and helium t o the explosive mixture results in an increase in the poll-er for ignition, Ivhile water vapor, argon, nitrogen, oxygen, aiid nitrous oxide lower the explosive limit in the order named. The curves for oxygen and nitrous oxide were not symnietrical, owing primarily t o changes they produced in the character of the positive column. Since the total pressure in the discharge was increased
P= 2
20 mm (2H2 : 02)
A
Partial Pressure of Added Gas (mm Hg) Frr,. 3. THE EFFECT OF ADDEDGAGE'
when the gas was added, the relative effects of theqe gases must be determined by a comparison viith the 2H2: O2curve aiid not with the power input at 20 mm. It will be observed that the order in n-hich the various gases affect the ignitability is the same as that in n-liicli they affected the rate of reaction in the positiT-e coluniii for currents bclon- that of ignition (3). The order is distinctly different from the effect of foreign gases on the rate in the negative glow (4). Likewise it is not at all the qanie as that found for ignition in the condensed discharge (21, where the order was more like that found in the negative glow, in that the ignition energy n-as 1011-ered by hydrogen
CHEJIICAL ACTION I X GLOW DISCHARGE. XIV
1055
and raised by most other gases in accordance with the complexity of their molecules. The order is slightly different from that found by Hinshelwood and Gibson ( 5 ) for thermal ignition where the gases are placed H z O > A > NZ>He.
The effect of glass icalls The presence of surfaces in close proxiniity t o the positive column increased markedly the power necessary for ignition, whereas such surfaces several centimeters removed from the path of the discharge had no effect. The power input for ignition was substantially the same in 1-, 3-, and 5liter reaction chambers. I n the case of a discharge between two aluniiiium rods 5 mm. long and 2 cm. apart, ignition occurred for the same power input when a glass cylinder 2.7 cin. long and 3.2 mi. in diameter was surrounding the discharge as when it was absent; the axis of the cylinder was placed perpendicular to the discharge. (In these tests the axis of the discharge was horizontal.) On the other hand, !$-hen the cylinder was replaced by a glass rectangle 1 cm. wide, 3.8 cm. long, and 3.2 cni. high, ignition could not be induced with the power available, which was over twice that used for the cylinder. JJ7hen the cylindcr was placed parallel t o the direction of the positive column, the axis of both being vertical, the effect ivas much more pronounced; the gas could not be ignited with the available power input without adding excess oxygen to the explosive mixture. The effect of the surface material was studied by surrounding the positive column with a concentric glass cylinder 1 in. long and 1+in. in diameter, the cylinder being used both clear and heavily plated with silver on the inside. The diameter of this cylinder was sufficiently large to produce no effect on the power required to ignite the gas by a condensed discharge. The gas mixture chosen for the test was composed of one part of hydrogen and t n o parts of oxygen. The excess oxygen and the silver were chosen to test the effect of a possible production of ozone on the ease of ignition. The results showed that ignition occurred with 38 per cent smaller power input for silver than for the glass cylinder. T h e effect of temperature The ignition power input for various temperatures of the furnace surrounding the discharge tube is shon-n in figure 4. These experiments were carried out at constant gas density, the pressure a t room temperature being 20 cni. on the dibutyl phthalate manometer, which is equivalent t o 1.54 cm. of mercury. The data show that while the potential drop through the positive column increased somewhat with temperature, the actual power input for ignition decreased 8.75 tinies in raising the temperature from 300°K. to 600°K.
1056
A . K E I T H B R E W E R AND P. D. KUECK DISCUSSIOX O F RESULTS
The data just presented show that ignition in the positive column is distinctly different from that in the condensed discharge, in respect to the manner in which the ignitability is affected by the hydrogen:oxygen ratio, the presence of foreign gases, and the gas pressure. There is, however, a distinct analogy between the rate of reaction in the negative glow and ignition in the condensed discharge. I n the preceding article of this series a chain mechanism was developed which is adequate to account for the difference in reactivity in the p0siti.c.e column and in the negative glow (3). This mechanism involves two disI
I
d=
I
I
I
I
4.7 cm.
P3000K=
15.4 mm
-
-
-500
-
v
c
)
- 200
I 300
l 350
l
l 400
l
l 450
l
l
l
500
Temperature FIG.4. TI-IEEFFECT OF
l 550
l
l 600
l
. 650
O K
TE3fPERATURE
tinct steps in the ignition procesls: first, the production of active centers; and second, the setting up of reaction chains about these centers. The important consideration in this niechanism is that for propagation to occur the active center must be in a gas of energy density such that it can receive its energy of activation, E , by collision with the energy-rich molecules through which it is moving. This condition can be brought about by the influx of energy from the outside (electrical, thermal, etc.), or by the heat liberated in the primary reaction about the active centers initially formed in the discharge. This mechanism can no17 be used to account for the factors underlying ignition in the positive column, and for the differences found here and in
CHEMICAL ACTIOS I N GLOW DISCHARGE. S I V
1057
the condensed discharge. Obviously ignition is dependent on the conditions which favor chain growth. I n the case of the effect of added gases, as shown in figure 3, it will be seen that the various gases decrease the porn-er input for ignition in the order of their respective abilities to retard the rate of diffusion out of the discharge. This determines the order in which these gases increase the energy density in the path of the discharge, and thus increase the chance of the active centers receiving the activation energy, E , by collision with energy-rich molecules. Flame propagation, a condition in which the heat of reaction is capable of supplying the necessary activation energy, starts somewhere in the space occupied by the positive column, and is encouraged, therefore, by any condition which raises the energy density of this region. The curves illustrated in figure 3 are in general segments of hyperbolas, as is to be expected from the data in table 1. From this it follows that the effect produced by the added gases is proportional to the number of molecules in the path of the discharge. I11 the case of the condensed discharge (2) foreign gases raise the power input for ignition in order of their ability to absorb energy from the discharge, the order being almost directly opposite to that observed in the positive column. I n this case the energy is all introduced at one time and in a very small volume. Since the gas molecules surrounding this volume are in the normal state, active centers diffusing out are lost. For flame propagation t o occur i t is necessary that a quantity of gas burn in the path of the spark sufficient t o produce the energy-rich molecules that supply the energy E requisite for the continuance of reaction chains; the problem, therefore, is one of the production of a necessary number of reaction centers. In consequence, the presence of a foreign gas which absorbs energy from the spark decreases the number of active centers, and hence necessitates a higher power input to burn the ainourit of gas required for propagation. Since the rate of reaction in the negative glo\T is proportional to the rate of production of active centers, it follows from the above considerations that the order in 11-hich various gases affect the rate must be the same as that in n-hich they affect the ignition in the condensed discharge. It should be mentioiled that while added gases also absorb energy in the positive colu m n , this effect is necessarily qiiiall coinpared to the gain from the reaction chaiiis made possible by the retardations in the rate of diffusion. The importance of the energy density of the gas surrounding the region through n-hich the discharge passes can be obtained by comparing the difference in the ignition energy per unit volume for the positive column and for the condensed discharge. An exact estimation cannot be made, but the order of magnitude can be obtained as follows: Referring t o figure 1 it will be seen that about 50 xi-atts is required for an electrode separation of 1 cm.; under tliese conditions the positive column is about 10 mm. in diameter and 5 mm. long, n-ith a volumr of about 400 cu. nim. The power T H E JOL-RXAL O F P H Y S I C A L CFlE%IISTRY, T-OL. X X S Y I I I , KO.
8
1058
A . KEITH BREWER A S D P. D . KUECK
expended per cubic millimeter is 0.125 joule per second. In the condensed discharge a t 1 cin. pressure, explosion occurq 11itli 11 microfarad3 capacity a t about 40 i-olts, the spark occupying not more than 1 cu. mni. of volume; the energy per cubic millimeter is 0.0098 joule. I n the absence of iiiductance in the circuit the period of the discliarge is betiyeen 1 0 3 arid 10-5 seconds; this represents an expenditure of at least 98.0 joules per cubic millimeter per second. Since soiiie energy is necessarily lost to the elcctrodes, a loss of as niucli as 50 per cent n o d d still gi\ e an energy deniity four hundred times greater for the condensed discharge than for the positive coluiiin. The effect of the energy density in the gas surrounding tlie discharge on the formation of reaction chains can be seen also from the mrious other factors iiifluencing the ignition in the positii e column. Thus, a twofold increase in the absolute teiiiperature results in a ninefold decrease in the poiTer input. Again, the p o m r input per unit volume of positive colunin is decreased when tlie electrode separation is increased, as slion n in figure 1. This uould be very difficult to understand, since the ignition starts a t localized points and not throughout the entire discharge at once, n.ere it not for the fact that the energy f l o w from one segment of discharge into another, and thereby raises the energy density over that obtained from the power expended per unit volume alone; especially is this true in the present case where the axis of the discharge was vertical, so that the transfer of energy upward from lower portions of the discharge was aided by convection. The effect of surfaces near the discharge is also associated with the energy distribution. It is generally considered that reaction chains are broken upon contact with a surface. This, doubtless, accounts for the decrease in the reaction rate described in the previous conimunication (3), especially n here propagation is possible. Walls surrounding the discharge break the chains, and, therefore, in limiting the reaction prevent thp energy density from being raised to a point TI here ignition can take place. The fact that a silver surface lowers the ignition point less than does glass may be due to its ability to reflect the heat rays, thereby tending t o raise the temperature of the gas in the path of the discharge. SCRIMART
The factors iiifluenciiig ignition in the positive column are contrasted with thobe obtained previously in the condensed discharge. Ignition does not occur in the negative glow, the S,'H2+ratio remaining alniost constant irrespective of the pressure and power input. The results sholv that the addition of foreign gases lowers the pon er input for ignition in accordance with the ability of these gases to retard the rate of diffusion; in the condensed discharge the order is almost the opposite, tlie n-ork being increased in proportion to the amount of energy ab-
CHEMICAL A C T I O S I N GLOW DISCHARGE. XIY
1059
sorbed by the gas. The most easily ignited mixture of hydrogen and oxygen in the positire colunin is one containing 25 per cent hydrogen and 7 5 per cent oxygen. h hyperbolic relation exists between power input for ignition and pressure. The ignitability is increased ninefold for a twofold increase in teniperature. I t increases n-it,h the length of the positive column. It' is decreased by t,he presence of surfaces near the discharge, hut is unaffected by surfaces more than about 1 cm. removed. The results are interpreted in tlie light of tlie cluster chain hypothesis presented in the preceding paper. The importance of the energy densit'y of the gas surrounding the positive column is est'iniated by a coniparison of the power input for ignition per unit' volume of explosive mixture in the positive column and in the coiideiised discharge. REFERENCES (1) (2) (3) (4) (5)
BREKER, A. K.: Proc. K a t . Acad. Sci. 13, 689 (1927). BREWER ASD DEMIKG: J. Ani. Cheni. SOC.62, 4225 (1930). BREWER A N D KVECK: J. P h p . Chem. 38,889 (1934). BREWER ASD VESTHATER: J. Phys. Chem. 34, 2343 (1930). GIBSON ASD HISSHELWOOD: Proc. Roy. Soc. London 119A.591 (1928).
