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Cool flames in butane oxidation
COOL FLAMES iN BUTANE OXIDATION
529
Williams & Wilkins Co., Baltimore (1949); also ref. 7. 11. HARDING, A. J., AND NORRISH, R. G. W Proc. Roy. Soc., A212, 291 (1952). 12. PAQUOT, C.: Bull. Soc. chim., 12, 120 (1945).
9. PEASE, R. N.: J. Am. Chem. Soc., 60:2244 (1938). 10. CHAMBERLAIN, G. H. N., AND WALSH, A. D.:
Third Symposium on Combustion, Flame and Explosion Phenomena, p. 368. The
51
COOL FLAMES IN BUTANE OXIDATION By J. BARDWELL
Introduction A common feature of the combustion of many gaseous fuels is the occurrence of cool flames. Such flames are recognizable by their pale luminescence, their low velocity, and their tendency to recur in the same reaction mixture. Studies of cool flames have included investigations of their range of occurrence l, spectroscopic and photoelectric study of the luminescence ~, 3 and analysis for chemical products and intermediates 4, s. Considerable speculation has arisen about the chemical processes responsible for cool flames but no general agreement has yet been reached 6, 7. In the gas-phase oxidation of n-butane, cool flames appear at convenient pressures, and permit systematic investigation of their kinetics. In this paper the relationship of the cool flame to other types of combustion will be considered and the phenomenon of multiple cool flames discussed.
Experimental APPARATUS
The apparatus consisted of a silica vessel connected to a manometer and to an appropriate vacuum system. The reaction vessel was cylindrical in shape, 6 cm in diameter and 9 cm long, and was supported vertically in a furnace in a region where the temperature was uniform to within ~ I ~ The reactant gases were admitted separately to the vessel and the combustion followed by pressure measurements, by visual observation through a window, and by chemical analysis. The n-butane used was supplied by the Matheson Co. and was 99 per cent pure.
ANALYSIS FOR PEROXIDES
The combustion was interrupted at appropriate stages by the rapid withdrawal of the gases into an evacuated pipette. Peroxides were determined iodimetrically by a method similar to that suggested by Wagner, Smith and Peters 8. The sample was shaken with a mixture of isopropanol (25 cc) and glacial acetic acid (1 cc) and transferred to a 250 cc Erlenmeyer flask. A saturated solution of sodium iodide in isopropanol (5 cc) was then added and the solution refiuxed gently for five minutes. The solution was cooled, 5 cc of water was added and the liberated iodine was titrated with 0.01N sodium thiosulfate. The result was expressed as the partial p~essure of peroxides in the combustion gases at the moment of sampling. ANALYSIS FOR ALDEHYDES
For this analysis the sample of combustion gases was shaken with water. A small aliquot was used for analysis of formaldehyde by the chromotropic acid method of Bricker and Johnson 9. The remainder was analyzed for total aldehyde by a bisulfite method similar to that of Goldman and Yagoda 1~ Analysis of mixtures of formaldehyde and higher aldehydes by this method was successful only when the final pH was adjusted to about 9.7, prior to titration with iodine. When the p H was lower than this figure the rate of dissociation of the bisulfite-formaldehyde complex was too small; when the pH was higher, interference was encountered because of formation of iodoform. The optimum pH was obtained by addition of a carbonate-bicarbonate buffer.
530
KINETICS OF COMBUSTION REACTIONS
Results IGNITION DIAGRAMS
Experiments with butane-oxygen mixtures at different temperatures and pressures revealed at least six different types of combustion. (1) Slow combustion: In this type, no lumi500
I-
TWO* STAGE
GLOW CO.SUST,O. Z0r
~0
Z~O
~0
*~0
~00
TOTAL PRESSURE (mm.)
FIG. 1. Conditions of temperature and pressure for combustion in butane-oxygen mixtures containing 50 per cent butane. The numbers shown indicate the number of cool flames observed.
'-~ozTo co.~v~;"o%. ',os\,~"..~..~ q~'-,7"~'~._r;,_.7G~_;,;__. T,O_,t SL
ED ~
TWO STAGE
leO 200 500 TOTAL PRESSURE (mm.]
400
FIG. 2. Conditions of temperature and pressure for combustion in butane-oxygen mixtures containing 33 per cent butane. The numbers shown indicate the number of cool flames observed. nescence was observed, nor was there any sudden increase of pressure. (2) Single cool flame: After an appreciable induction period, a flicker of bluish light was observed in the reaction vessel. At the same time a moderate pressure-pulse occurred. This was usually followed by a period of slow combustion. (3) Multiple cool flames: Pale flames up to five in number passed in succession through the reaction mixture. The time intervals between flames ranged from about one second up to one
hundred seconds, depending on the temperature and pressure. (4) Simple ignition: A single bright flame passed through the mixture and was accompanied by a large pressure increase. (5) Two-stage ignition: The hot flame was similar in appearance to that found with simple ignition, but was preceded by a cool flame. The interval between flames was usually about one second. (6) Ignition at second cool flame (delayed ignition): Ignition was delayed until the passage of the second cool flame, usually a minute or more after the first cool flame. The inflammation was usually accompanied by an audible click. The conditions of temperature and pressure that gave rise to each type of combustion when an equimolecuiar mixture of butane and oxygen was used are shown in Figure 1. The corresponding ignition diagram for mixtures containing 33 per cent butane is shown in Figure 2. The main features of these diagrams are similar to those found with other hydrocarbon fuels 1, 7, e.g., there is a low-temperature ignition peninsula at about 275~ and a large cool-flame region between 275~ and 400~ The principal difference between the diagrams for 50 per cent butane and 33 per cent butane is the occurrence in the latter of a second peninsula for delayed ignition. The effect of mixture composition on the ignition limits and cool-flame limits at 287~ is shown in Figure 3. Of particular interest here is the manner in which the different types of ignition intervene as the partial pressure of each reactant is changed. FACTORS
INFLUENCING
THE
COOL-FLAME
FRE-
QUENCY
When multiple cool flames occurred there was an appreciable time-lag between successive flames. The symbol 0~ will be used for the time-lag between the first and second cool flames and its reciprocal will be called the cool-flame frequency. The induction period, i.e., the time-lag between the admission of the reactants to the vessel and the occurrence Of the first cool flame, will be called 61. Its reciprocal is a measure of the rate of the chemical reactions in the early stages of oxidation. The effect of temperature on the cool-flame frequency and on the reciprocal induction period in the range where multiple cool flames are found is shown in Figure 4. The dependence of fre-
531
COOL FLAMES IN BUTANE OXIDATION
quency on temperature is large and approximately exponential in form; for each ten-degree rise of temperature the frequency increases by a factor of about 3.2. The plot shown in Figure 4 yields an activation energy of approximately 73,000 calories. I t is interesting to note that the reciprocal induction period shows a similar dependence on temperature in this region. The cool-flame frequency also varies with reactant pressure. The variation with the initial partial pressure of each reactant, the other being held constant is shown in Figure 5. The reciprocal induction period is also shown and its increase with each partial pressure resembles that of the cool-flame frequency.
TEMPERATURE 306 300 295
310
i.2
("C) 290
"b~e,,~
log(~-)
\'-.\
~A
e,---" "oo . ~ o - -
i
|
1,72
,
1.74
L76
I000 / T (~
%
1.78
1.80
FIG. 4. Effect of temperature on the reciprocal induction period and cool-flame frequency. Butane: 100 mm, oxygen: 100 nun. .~2.O0
I\
E E
,0]
DELAYEDe'".e~. TWO- STAGE ~IGNITION ,.~ O--..O.,,,.,. IGNITION
~ o c o o , ~"r-'-" "-
(n w re.
o-I00 z bJ >.-
\
OOLLA.E
g 5o
0"0~0
0
.0~
ear~
.oef
9.0! /
:ol 0
0
O-
s~o* co.s~sT,oN i .01
'
f.,,.,r"
,~o ' 2~ BUTANE PRESSURE (turn.)
g'.0,1
~d: ~ ~
'
FIG. 3. Effect of mixture composition on inflammation limits at 287~ T H E F O R M A T I O N OF P E R O X I D E S
The combustion gases were analyzed for peroxides during a typical oxidation giving rise to two cool flames. The vessel temperature w a s 284~ and the reaction mixture was: butane, 120 ram; oxygen, 120 ram. Under these conditions the induction period was about 90 seconds and the interval between flames 50 sec. The results, including the pressure-time record, are shown in Figure 6. The most striking feature of the variation of peroxide concentration with time is the sharp drop accompanying the second cool flame.
s'o
9
&
I,,'l*t o r e . . . - -
INITIALBUTANEPIIESSlIIE(Nil.)
(-.)
FIG. 5. Effect of reactant pressure on the reciprocal induction period and cool-flame frequency at 287~ (a) Diagram at left: oxygen pressure, 110 mm. (b) Diagram at right: butane pressure 80mm. eo
|
~o
Ap ~ (ram)20
|
/
J
~t~
~EE
T H E F O R M A T I O N OF A L D E H Y D E S
The corresponding variation of aldehyde concentration with time is shown in Figure 7. I t is evident that formaldehyde is the principal aldehyde formed, making up approximately 80 per cent of the total aldehyde. I t is also clear that the
TIME (see.)
FIG. 6. Pressure-time record and variation of peroxide concentration during cool-flame oxidation of butane at 284~ Butane pressure: 120 ram; oxygen pressure: 120 mm.
532
KINETICS OF C O M B U S T I O N REACTIONS
most favorable period for aldehyde formation is immediately prior to and during the first cool flame. At no stage in cool-flame oxidation does aldehyde concentration undergo a decrease comparable to that of peroxides at the second cool flame.
A few measurements were made to assess the effect of a two-stage ignition on the concentration of peroxides and aldehydes. The temperature was 284~ and the pressure of butane and oxygen 140 mm each. Under these conditions the first cool flame gave rise, after an interval of 1.3 sec to a hot flame. The hot flame reduced the peroxide partial pressure from 2.7 mm to less than 0.2 mm. The total aldehyde pressure fell from 14 mm to mm.
II II
I
{ram.)
i~ 3O 0
TO,OI Aldehyde
I~
Without inert gas, two cool flames were observed, but when sufficient inert gas was added the second cool flame was eliminated. The partial pressures of argon, nitrogen and water vapor required to just suppress the second cool f a m e were approximately 250, 150, and 90 mm respectively. The induction period, however, was unaffected by the presence of inert gas. A similar inhibitory effect was observed when nitrogen was added to mixtures that gave rise to delayed ignitions, i.e., oxidation where the second cool flame ignited the fuel-oxygen mixture. A t 284~ with a butane pressure of 100 mm such delayed ignitions occurred when the oxygen pressure was in the range, 130 to 150 mm. In the presence of 180 mm of nitrogen, however, the ignition was suppressed, two cool flames only being observed. Nitrogen was found to have a small effect on the limiting pressure of reactants for the occurrence of a single cool flame and for the onset of two-stage ignitions. In the presence of 180 mm of nitrogen each of these limits at 284~ was increased by about 10 mm.
turn.) FO
~-~Forrnold lhyde
Discussion IGNITION LIMITS
_-r-~-:~.~" " 50
TIME
, I00
, 1,50
, 200
(see.)
FIG. 7. Pressure-time record and variation of concentrations of formaldehyde and total aldehyde during cool-flame oxidation of butane at 284~ Butane pressure: 120 mm.; oxygen pressure; 120 ram. E F F E C T OF ADDED A L D E H Y D E S
When a small amount of acetaldehyde was added along with the butane and oxygen the induction period was considerably reduced. The subsequent course of the reaction was, however, unaffected by the initial addition of acetaldehyde. When formaldehyde was used as an additive, the effect was more complex. The induction period was lengthened but the subsequent reaction was made more vigorous. For example, at 284~ when 10 mm of formaldehyde was added, the pressure limit for a two-stage ignition was lowered from 280 mm to 240 mm. EFFECT OF INERT GAS The influence of inert gas at 284~ was tested by adding various amounts of argon, nitrogen and water vapor along with a reaction mixture composed of 120 mm each of butane and oxygen.
The relation of the cool flame to other types of combustion may conveniently be considered with reference to Figures 1, 2 and 3 which show the range of occurrence of each type of oxidation. The ignition diagram for mixtures containing 50 per cent butane (Fig. 1) shows many points of resemblance to those for propane u and butanone TM but the pressure limits are lower, as might be expected from the greater ease of oxidation of butane 13. The pressure-limit for ignition varies comparatively little between 300~ and 400~ but rises sharply as the temperature falls below 270~ As with propane 14 there is a well defined peninsula in the ignition curve at about 275~ Although this paper is concerned primarily with the characteristics of oxidation in the coolflame zone, attention should be called to the peculiarities of ignition in the low-temperature range, i.e., below about 400~ Unlike the simple inflammation that occurs at higher temperatures, ignition here is a two-stage process, the hot flame occurring a second or two after a cool flame. A still more complicated type of ignition occurs when the proportion of fuel in the mixture is reduced. Figures 2 and 3 show that in certain regions of temperature and pressure where multi-
533
COOL FLAMES IN BUTANE OXIDATION
ple cool flames might be anticipated, there occurs a delayed ignition. Here the first cool flame is normal, in the sense that immediately after it the reaction rate falls to a very low value. There follows a period of autocatalytic reaction (characteristic of multiple cool flames) at the end of which a true ignition occurs. Experimentally such delayed ignitions are easily distinguished from two-stage ignitions by the much longer quiescent period after the first cool flame, this interval amounting to as much as 100 sec in some instances. Figures 2 and 3 show that the conditions required for such ignitions are a temperature between 280~ and 310~ less than 50 per cent butane in the mixture, and a total reactant pressure somewhat below the limit for two-stage ignition. The exact reactions responsible for such delayed ignitions are not yet clear, nor is it obvious why the second cool flame should succeed in igniting the mixture when the first flame has failed to do so. A clue to this anomaly may, however, be contained in the analytical results for peroxides and aldehydes during multiple coolflame oxidation. Figures 6 and 7 show that substantial quantities of both peroxides and aldehydes are present immediately prior to the second cool flame and it is possible that these substances are induced to participate in some highly exothermic reaction by the flame. The favorable effect of excess oxygen and the unfavorable influence of inert gas support the view that the chemical processes in delayed ignitions are rather different from those responsible for two-stage ignitions. COOL-FLAME LIMITS
To the left of the curves for ignition in Figures 1 and 2 is the boundary between the areas of cool flames and slow combustion. On the low-temperature side this curve appears as an extension of the ignition boundary; on the high-temperature side, however, it intersects the ignition boundary at almost a right angle. Malherbe and Walsh 15have examined the form of the cool-flame boundary with several fuels and have concluded that it is of a composite nature, occasionally showing two lobes instead of a single lobe as in Figures 1 and 2. Careful study during the present investigation of the pressuretemperature limits for mixtures containing 33 per cent and 50 per cent butane failed to uncover any such inflections in the cool-flame boundary.
For equimolecular mixtures of butane and oxygen the areas of single and multiple cool flames are also distinguished in Figure 1, the maximum number of flames being four. Figure 2 shows in greater detail the behavior in the low-temperature region with a mixture containing 33 per cent butane. Here the maximum number of flames is five, this number being attained at temperatures in the vicinity of 310~ The number of flames, and their frequency diminish with decrease of either temperature or reactant pressure. As the temperature is increased the frequency increases to such an extent that the individual flames can no longer be distinguished, the multiple flames being replaced by a single glow lasting for several seconds. P E R I O D I C I T Y OF COOL FLAMES
The periodic nature of the cool-flame reaction has aroused the curiosity of many investigators. Salnikov 16 related the waxing and waning of reaction rate to the accumulation and decomposition of a combustion intermediate. If the combustion is represented by: A--* X--* B chemical periodicity may result if both of the following conditions are satisfied: (1) The temperature coefficient of the second step is greater than that of the first. (2) The second step is highly exothermic. In these circumstances, the concentration of X and its rate of decomposition may rise to such values that isothermal conditions can no longer be maintained. In the ensuing self-heating however, the concentration of X may be considerably reduced if the postulated relation of temperature coefficients holds. A repetition of the cycle then becomes possible, provided, of course, that true ignition does not intervene. By an extension of Salnikov's theory, Bardwell and Hinshelwood 1~were able to account for many of the properties of the cool flame in the oxidation of butanone. Application of similar ideas to the foregoing results for butane leads to the following conclusions: (1) The theory accounts satisfactorily for the general form of the pressure-time curve in Figure 6. The pressure-pulse is ascribable to moderate self-heating in the cool flame. The low rate of reaction immediately after each flame is consistent with the partial destruction of a combustion intermediate.
534
KINETICS OF COMBUSTION REACTIONS
(2) The effect of temperature on the coolflame frequency and the parallel effect on the reciprocal induction period (Fig. 4) suggest that the chemical processes occurring between flames are similar to those preceding the first flame. (3) The similarity of the effect of reactant pressure on the cool flame frequency, to its effect on the reciprocal induction period (Fig. 5) supports the conclusion in (2). The chemical structure of the intermediate, X, cannot, however, be conclusively deduced from the results presented in this paper. Several investigators have postulated peroxides as the essential oxidation intermediates at low temperatures17, is; others have suggested aldehydes 19. The Salnikov theory predicts an appreciable reduction in the concentration of intermediate with the passing of the cool flame. Figure 6 shows that with peroxides such a reduction does occur with the passing of the second cool flame in butane oxidation. No similar reduction occurs with the first flame, however. The same conclusion has been drawn from experiments with diethyl ether ~~ From the standpoint of the Salnikov theory, the postulate of intermediate aldehydes is still less satisfactory since their concentration does not decrease significantly at any stage in the reaction (Fig. 7). An alternative interpretation of the periodicity of cool flames has been proposed by FrankKamenetzki ~1 and elaborated by Walsh 22. In this theory two intermediates are postulated, the second being formed autocatalytically from the first, but capable of destroying it. I t can be shown that such interplay of intermediates may result in a sinusoidal variation in the concentration of each. Walsh suggests that the two combustion intermediates are peroxide and formaldehyde. The analytical results in Figures 6 and 7 fail to provide any support for this interpretation.
Summary The ranges of temperature and pressure in which slow combustion, ignition, and cool flames occur in the oxidation of n-butane have been determined. When multiple cool flames occur their frequency is an exponential function of temperature and varies with the pressure of each reactant. Peroxides and aldehydes are formed, the concentration of peroxides increasing sharply prior to and during the first cool flame, but suffering a
sharp reduction in the second flame. Aldehydes accumulate throughout the reaction.
Acknowledgments The author is indebted to the National Research Council of Canada for a grant in aid of this research, and to Muriel Finlayson, Nan Shu, and B. Sokalski for assistance with the experimental work. REFERENCES 1. TOWNEND, D. T. A : Chem. Rev., 21, 259 (1937). 2. UBBELOHDE,A. n . : Proc. Roy. Soc., A152, 354, 378 (1935). 3. OUELLET, L., LEGER, E., AND OUELLET, C.: J. Chem. Phys., I8, 383 (1950). 4. LEGER, E., AND OUELLET, C. : J . Chem. Phys., 21, 1310 (1953). 5. BAILEY, H. C., AND NORRISH, R. G. W.: Proc. Roy. Soc., A212, 311 (1952). 6. LEWIS, B., AND VON ELBE, G.: Combustion, Flames, and Explosions of Gases. New York, Academic Press, 1951. 7. Josw, W.: Explosion and Combustion Processes in Gases. New York, McGraw-Hill Book Co., 1946. 8. WAGNER, C. D., SMITH, R. H., AND PETERS, E. D. : Ind. & Eng. Chem. (Anal. Ed.), 19, 976 (1947). 9. BRICKER, C. E., AND JOHNSON, H. n. : Ind. & Eng. Chem. (Anal. Ed.), 17, 400 (1945). 10. GOLDMAN,F. H., AND YAGODA,H. : Ind. & Eng. Chem. (Anal. Ed.), 15, 377 (1943). 11. NEWlTT, D. M., AND THORNES, L. S. : J. Chem.
Soc., 1656 (1937). 12. BARDWELL, J., AND HINSHELWOOD, C. : Proc.
Roy. Soc., A205, 375 (1951). 13. BARDWELL, J.: Proc. Roy. Soc., A207, 470 (1951). 14. DAY, R. A., AND PEASE, R. N.: ft. Am. Chem.
Soc., 62, 2234 (1940). 15. MALHERBE, F. E., AND WALSH, A. D. : Trans.
Faraday Soc., $6,835 (1950). 16. SALNIKOV,I. E.: C. R. Acad. Sci. (U.R.S.S), 60, 405 (1948). 17. WALSH, A. D.: Trans. Faraday Soc., ~3, 297 (1947). 18. EGERTON, A. C.: Nature, 121, 10 (1928). 19. Noauisn, R. G. W.: Disc. Faraday Soc., 10, 269 (1951). 20. EASTWOOD, T. A., AND HINSHELWOOD, C. : J.
Chem. Soc., 733 (1952). 21. FRANK-KAMENETZKI, D. A.: C. R. Acad. Sci. (U.R.S.S.), 25,671 (1939). 22. WALSH, A. D.: Trans. Faraday Soc., ~3, 305 (1947).
529
Williams & Wilkins Co., Baltimore (1949); also ref. 7. 11. HARDING, A. J., AND NORRISH, R. G. W Proc. Roy. Soc., A212, 291 (1952). 12. PAQUOT, C.: Bull. Soc. chim., 12, 120 (1945).
9. PEASE, R. N.: J. Am. Chem. Soc., 60:2244 (1938). 10. CHAMBERLAIN, G. H. N., AND WALSH, A. D.:
Third Symposium on Combustion, Flame and Explosion Phenomena, p. 368. The
51
COOL FLAMES IN BUTANE OXIDATION By J. BARDWELL
Introduction A common feature of the combustion of many gaseous fuels is the occurrence of cool flames. Such flames are recognizable by their pale luminescence, their low velocity, and their tendency to recur in the same reaction mixture. Studies of cool flames have included investigations of their range of occurrence l, spectroscopic and photoelectric study of the luminescence ~, 3 and analysis for chemical products and intermediates 4, s. Considerable speculation has arisen about the chemical processes responsible for cool flames but no general agreement has yet been reached 6, 7. In the gas-phase oxidation of n-butane, cool flames appear at convenient pressures, and permit systematic investigation of their kinetics. In this paper the relationship of the cool flame to other types of combustion will be considered and the phenomenon of multiple cool flames discussed.
Experimental APPARATUS
The apparatus consisted of a silica vessel connected to a manometer and to an appropriate vacuum system. The reaction vessel was cylindrical in shape, 6 cm in diameter and 9 cm long, and was supported vertically in a furnace in a region where the temperature was uniform to within ~ I ~ The reactant gases were admitted separately to the vessel and the combustion followed by pressure measurements, by visual observation through a window, and by chemical analysis. The n-butane used was supplied by the Matheson Co. and was 99 per cent pure.
ANALYSIS FOR PEROXIDES
The combustion was interrupted at appropriate stages by the rapid withdrawal of the gases into an evacuated pipette. Peroxides were determined iodimetrically by a method similar to that suggested by Wagner, Smith and Peters 8. The sample was shaken with a mixture of isopropanol (25 cc) and glacial acetic acid (1 cc) and transferred to a 250 cc Erlenmeyer flask. A saturated solution of sodium iodide in isopropanol (5 cc) was then added and the solution refiuxed gently for five minutes. The solution was cooled, 5 cc of water was added and the liberated iodine was titrated with 0.01N sodium thiosulfate. The result was expressed as the partial p~essure of peroxides in the combustion gases at the moment of sampling. ANALYSIS FOR ALDEHYDES
For this analysis the sample of combustion gases was shaken with water. A small aliquot was used for analysis of formaldehyde by the chromotropic acid method of Bricker and Johnson 9. The remainder was analyzed for total aldehyde by a bisulfite method similar to that of Goldman and Yagoda 1~ Analysis of mixtures of formaldehyde and higher aldehydes by this method was successful only when the final pH was adjusted to about 9.7, prior to titration with iodine. When the p H was lower than this figure the rate of dissociation of the bisulfite-formaldehyde complex was too small; when the pH was higher, interference was encountered because of formation of iodoform. The optimum pH was obtained by addition of a carbonate-bicarbonate buffer.
530
KINETICS OF COMBUSTION REACTIONS
Results IGNITION DIAGRAMS
Experiments with butane-oxygen mixtures at different temperatures and pressures revealed at least six different types of combustion. (1) Slow combustion: In this type, no lumi500
I-
TWO* STAGE
GLOW CO.SUST,O. Z0r
~0
Z~O
~0
*~0
~00
TOTAL PRESSURE (mm.)
FIG. 1. Conditions of temperature and pressure for combustion in butane-oxygen mixtures containing 50 per cent butane. The numbers shown indicate the number of cool flames observed.
'-~ozTo co.~v~;"o%. ',os\,~"..~..~ q~'-,7"~'~._r;,_.7G~_;,;__. T,O_,t SL
ED ~
TWO STAGE
leO 200 500 TOTAL PRESSURE (mm.]
400
FIG. 2. Conditions of temperature and pressure for combustion in butane-oxygen mixtures containing 33 per cent butane. The numbers shown indicate the number of cool flames observed. nescence was observed, nor was there any sudden increase of pressure. (2) Single cool flame: After an appreciable induction period, a flicker of bluish light was observed in the reaction vessel. At the same time a moderate pressure-pulse occurred. This was usually followed by a period of slow combustion. (3) Multiple cool flames: Pale flames up to five in number passed in succession through the reaction mixture. The time intervals between flames ranged from about one second up to one
hundred seconds, depending on the temperature and pressure. (4) Simple ignition: A single bright flame passed through the mixture and was accompanied by a large pressure increase. (5) Two-stage ignition: The hot flame was similar in appearance to that found with simple ignition, but was preceded by a cool flame. The interval between flames was usually about one second. (6) Ignition at second cool flame (delayed ignition): Ignition was delayed until the passage of the second cool flame, usually a minute or more after the first cool flame. The inflammation was usually accompanied by an audible click. The conditions of temperature and pressure that gave rise to each type of combustion when an equimolecuiar mixture of butane and oxygen was used are shown in Figure 1. The corresponding ignition diagram for mixtures containing 33 per cent butane is shown in Figure 2. The main features of these diagrams are similar to those found with other hydrocarbon fuels 1, 7, e.g., there is a low-temperature ignition peninsula at about 275~ and a large cool-flame region between 275~ and 400~ The principal difference between the diagrams for 50 per cent butane and 33 per cent butane is the occurrence in the latter of a second peninsula for delayed ignition. The effect of mixture composition on the ignition limits and cool-flame limits at 287~ is shown in Figure 3. Of particular interest here is the manner in which the different types of ignition intervene as the partial pressure of each reactant is changed. FACTORS
INFLUENCING
THE
COOL-FLAME
FRE-
QUENCY
When multiple cool flames occurred there was an appreciable time-lag between successive flames. The symbol 0~ will be used for the time-lag between the first and second cool flames and its reciprocal will be called the cool-flame frequency. The induction period, i.e., the time-lag between the admission of the reactants to the vessel and the occurrence Of the first cool flame, will be called 61. Its reciprocal is a measure of the rate of the chemical reactions in the early stages of oxidation. The effect of temperature on the cool-flame frequency and on the reciprocal induction period in the range where multiple cool flames are found is shown in Figure 4. The dependence of fre-
531
COOL FLAMES IN BUTANE OXIDATION
quency on temperature is large and approximately exponential in form; for each ten-degree rise of temperature the frequency increases by a factor of about 3.2. The plot shown in Figure 4 yields an activation energy of approximately 73,000 calories. I t is interesting to note that the reciprocal induction period shows a similar dependence on temperature in this region. The cool-flame frequency also varies with reactant pressure. The variation with the initial partial pressure of each reactant, the other being held constant is shown in Figure 5. The reciprocal induction period is also shown and its increase with each partial pressure resembles that of the cool-flame frequency.
TEMPERATURE 306 300 295
310
i.2
("C) 290
"b~e,,~
log(~-)
\'-.\
~A
e,---" "oo . ~ o - -
i
|
1,72
,
1.74
L76
I000 / T (~
%
1.78
1.80
FIG. 4. Effect of temperature on the reciprocal induction period and cool-flame frequency. Butane: 100 mm, oxygen: 100 nun. .~2.O0
I\
E E
,0]
DELAYEDe'".e~. TWO- STAGE ~IGNITION ,.~ O--..O.,,,.,. IGNITION
~ o c o o , ~"r-'-" "-
(n w re.
o-I00 z bJ >.-
\
OOLLA.E
g 5o
0"0~0
0
.0~
ear~
.oef
9.0! /
:ol 0
0
O-
s~o* co.s~sT,oN i .01
'
f.,,.,r"
,~o ' 2~ BUTANE PRESSURE (turn.)
g'.0,1
~d: ~ ~
'
FIG. 3. Effect of mixture composition on inflammation limits at 287~ T H E F O R M A T I O N OF P E R O X I D E S
The combustion gases were analyzed for peroxides during a typical oxidation giving rise to two cool flames. The vessel temperature w a s 284~ and the reaction mixture was: butane, 120 ram; oxygen, 120 ram. Under these conditions the induction period was about 90 seconds and the interval between flames 50 sec. The results, including the pressure-time record, are shown in Figure 6. The most striking feature of the variation of peroxide concentration with time is the sharp drop accompanying the second cool flame.
s'o
9
&
I,,'l*t o r e . . . - -
INITIALBUTANEPIIESSlIIE(Nil.)
(-.)
FIG. 5. Effect of reactant pressure on the reciprocal induction period and cool-flame frequency at 287~ (a) Diagram at left: oxygen pressure, 110 mm. (b) Diagram at right: butane pressure 80mm. eo
|
~o
Ap ~ (ram)20
|
/
J
~t~
~EE
T H E F O R M A T I O N OF A L D E H Y D E S
The corresponding variation of aldehyde concentration with time is shown in Figure 7. I t is evident that formaldehyde is the principal aldehyde formed, making up approximately 80 per cent of the total aldehyde. I t is also clear that the
TIME (see.)
FIG. 6. Pressure-time record and variation of peroxide concentration during cool-flame oxidation of butane at 284~ Butane pressure: 120 ram; oxygen pressure: 120 mm.
532
KINETICS OF C O M B U S T I O N REACTIONS
most favorable period for aldehyde formation is immediately prior to and during the first cool flame. At no stage in cool-flame oxidation does aldehyde concentration undergo a decrease comparable to that of peroxides at the second cool flame.
A few measurements were made to assess the effect of a two-stage ignition on the concentration of peroxides and aldehydes. The temperature was 284~ and the pressure of butane and oxygen 140 mm each. Under these conditions the first cool flame gave rise, after an interval of 1.3 sec to a hot flame. The hot flame reduced the peroxide partial pressure from 2.7 mm to less than 0.2 mm. The total aldehyde pressure fell from 14 mm to mm.
II II
I
{ram.)
i~ 3O 0
TO,OI Aldehyde
I~
Without inert gas, two cool flames were observed, but when sufficient inert gas was added the second cool flame was eliminated. The partial pressures of argon, nitrogen and water vapor required to just suppress the second cool f a m e were approximately 250, 150, and 90 mm respectively. The induction period, however, was unaffected by the presence of inert gas. A similar inhibitory effect was observed when nitrogen was added to mixtures that gave rise to delayed ignitions, i.e., oxidation where the second cool flame ignited the fuel-oxygen mixture. A t 284~ with a butane pressure of 100 mm such delayed ignitions occurred when the oxygen pressure was in the range, 130 to 150 mm. In the presence of 180 mm of nitrogen, however, the ignition was suppressed, two cool flames only being observed. Nitrogen was found to have a small effect on the limiting pressure of reactants for the occurrence of a single cool flame and for the onset of two-stage ignitions. In the presence of 180 mm of nitrogen each of these limits at 284~ was increased by about 10 mm.
turn.) FO
~-~Forrnold lhyde
Discussion IGNITION LIMITS
_-r-~-:~.~" " 50
TIME
, I00
, 1,50
, 200
(see.)
FIG. 7. Pressure-time record and variation of concentrations of formaldehyde and total aldehyde during cool-flame oxidation of butane at 284~ Butane pressure: 120 mm.; oxygen pressure; 120 ram. E F F E C T OF ADDED A L D E H Y D E S
When a small amount of acetaldehyde was added along with the butane and oxygen the induction period was considerably reduced. The subsequent course of the reaction was, however, unaffected by the initial addition of acetaldehyde. When formaldehyde was used as an additive, the effect was more complex. The induction period was lengthened but the subsequent reaction was made more vigorous. For example, at 284~ when 10 mm of formaldehyde was added, the pressure limit for a two-stage ignition was lowered from 280 mm to 240 mm. EFFECT OF INERT GAS The influence of inert gas at 284~ was tested by adding various amounts of argon, nitrogen and water vapor along with a reaction mixture composed of 120 mm each of butane and oxygen.
The relation of the cool flame to other types of combustion may conveniently be considered with reference to Figures 1, 2 and 3 which show the range of occurrence of each type of oxidation. The ignition diagram for mixtures containing 50 per cent butane (Fig. 1) shows many points of resemblance to those for propane u and butanone TM but the pressure limits are lower, as might be expected from the greater ease of oxidation of butane 13. The pressure-limit for ignition varies comparatively little between 300~ and 400~ but rises sharply as the temperature falls below 270~ As with propane 14 there is a well defined peninsula in the ignition curve at about 275~ Although this paper is concerned primarily with the characteristics of oxidation in the coolflame zone, attention should be called to the peculiarities of ignition in the low-temperature range, i.e., below about 400~ Unlike the simple inflammation that occurs at higher temperatures, ignition here is a two-stage process, the hot flame occurring a second or two after a cool flame. A still more complicated type of ignition occurs when the proportion of fuel in the mixture is reduced. Figures 2 and 3 show that in certain regions of temperature and pressure where multi-
533
COOL FLAMES IN BUTANE OXIDATION
ple cool flames might be anticipated, there occurs a delayed ignition. Here the first cool flame is normal, in the sense that immediately after it the reaction rate falls to a very low value. There follows a period of autocatalytic reaction (characteristic of multiple cool flames) at the end of which a true ignition occurs. Experimentally such delayed ignitions are easily distinguished from two-stage ignitions by the much longer quiescent period after the first cool flame, this interval amounting to as much as 100 sec in some instances. Figures 2 and 3 show that the conditions required for such ignitions are a temperature between 280~ and 310~ less than 50 per cent butane in the mixture, and a total reactant pressure somewhat below the limit for two-stage ignition. The exact reactions responsible for such delayed ignitions are not yet clear, nor is it obvious why the second cool flame should succeed in igniting the mixture when the first flame has failed to do so. A clue to this anomaly may, however, be contained in the analytical results for peroxides and aldehydes during multiple coolflame oxidation. Figures 6 and 7 show that substantial quantities of both peroxides and aldehydes are present immediately prior to the second cool flame and it is possible that these substances are induced to participate in some highly exothermic reaction by the flame. The favorable effect of excess oxygen and the unfavorable influence of inert gas support the view that the chemical processes in delayed ignitions are rather different from those responsible for two-stage ignitions. COOL-FLAME LIMITS
To the left of the curves for ignition in Figures 1 and 2 is the boundary between the areas of cool flames and slow combustion. On the low-temperature side this curve appears as an extension of the ignition boundary; on the high-temperature side, however, it intersects the ignition boundary at almost a right angle. Malherbe and Walsh 15have examined the form of the cool-flame boundary with several fuels and have concluded that it is of a composite nature, occasionally showing two lobes instead of a single lobe as in Figures 1 and 2. Careful study during the present investigation of the pressuretemperature limits for mixtures containing 33 per cent and 50 per cent butane failed to uncover any such inflections in the cool-flame boundary.
For equimolecular mixtures of butane and oxygen the areas of single and multiple cool flames are also distinguished in Figure 1, the maximum number of flames being four. Figure 2 shows in greater detail the behavior in the low-temperature region with a mixture containing 33 per cent butane. Here the maximum number of flames is five, this number being attained at temperatures in the vicinity of 310~ The number of flames, and their frequency diminish with decrease of either temperature or reactant pressure. As the temperature is increased the frequency increases to such an extent that the individual flames can no longer be distinguished, the multiple flames being replaced by a single glow lasting for several seconds. P E R I O D I C I T Y OF COOL FLAMES
The periodic nature of the cool-flame reaction has aroused the curiosity of many investigators. Salnikov 16 related the waxing and waning of reaction rate to the accumulation and decomposition of a combustion intermediate. If the combustion is represented by: A--* X--* B chemical periodicity may result if both of the following conditions are satisfied: (1) The temperature coefficient of the second step is greater than that of the first. (2) The second step is highly exothermic. In these circumstances, the concentration of X and its rate of decomposition may rise to such values that isothermal conditions can no longer be maintained. In the ensuing self-heating however, the concentration of X may be considerably reduced if the postulated relation of temperature coefficients holds. A repetition of the cycle then becomes possible, provided, of course, that true ignition does not intervene. By an extension of Salnikov's theory, Bardwell and Hinshelwood 1~were able to account for many of the properties of the cool flame in the oxidation of butanone. Application of similar ideas to the foregoing results for butane leads to the following conclusions: (1) The theory accounts satisfactorily for the general form of the pressure-time curve in Figure 6. The pressure-pulse is ascribable to moderate self-heating in the cool flame. The low rate of reaction immediately after each flame is consistent with the partial destruction of a combustion intermediate.
534
KINETICS OF COMBUSTION REACTIONS
(2) The effect of temperature on the coolflame frequency and the parallel effect on the reciprocal induction period (Fig. 4) suggest that the chemical processes occurring between flames are similar to those preceding the first flame. (3) The similarity of the effect of reactant pressure on the cool flame frequency, to its effect on the reciprocal induction period (Fig. 5) supports the conclusion in (2). The chemical structure of the intermediate, X, cannot, however, be conclusively deduced from the results presented in this paper. Several investigators have postulated peroxides as the essential oxidation intermediates at low temperatures17, is; others have suggested aldehydes 19. The Salnikov theory predicts an appreciable reduction in the concentration of intermediate with the passing of the cool flame. Figure 6 shows that with peroxides such a reduction does occur with the passing of the second cool flame in butane oxidation. No similar reduction occurs with the first flame, however. The same conclusion has been drawn from experiments with diethyl ether ~~ From the standpoint of the Salnikov theory, the postulate of intermediate aldehydes is still less satisfactory since their concentration does not decrease significantly at any stage in the reaction (Fig. 7). An alternative interpretation of the periodicity of cool flames has been proposed by FrankKamenetzki ~1 and elaborated by Walsh 22. In this theory two intermediates are postulated, the second being formed autocatalytically from the first, but capable of destroying it. I t can be shown that such interplay of intermediates may result in a sinusoidal variation in the concentration of each. Walsh suggests that the two combustion intermediates are peroxide and formaldehyde. The analytical results in Figures 6 and 7 fail to provide any support for this interpretation.
Summary The ranges of temperature and pressure in which slow combustion, ignition, and cool flames occur in the oxidation of n-butane have been determined. When multiple cool flames occur their frequency is an exponential function of temperature and varies with the pressure of each reactant. Peroxides and aldehydes are formed, the concentration of peroxides increasing sharply prior to and during the first cool flame, but suffering a
sharp reduction in the second flame. Aldehydes accumulate throughout the reaction.
Acknowledgments The author is indebted to the National Research Council of Canada for a grant in aid of this research, and to Muriel Finlayson, Nan Shu, and B. Sokalski for assistance with the experimental work. REFERENCES 1. TOWNEND, D. T. A : Chem. Rev., 21, 259 (1937). 2. UBBELOHDE,A. n . : Proc. Roy. Soc., A152, 354, 378 (1935). 3. OUELLET, L., LEGER, E., AND OUELLET, C.: J. Chem. Phys., I8, 383 (1950). 4. LEGER, E., AND OUELLET, C. : J . Chem. Phys., 21, 1310 (1953). 5. BAILEY, H. C., AND NORRISH, R. G. W.: Proc. Roy. Soc., A212, 311 (1952). 6. LEWIS, B., AND VON ELBE, G.: Combustion, Flames, and Explosions of Gases. New York, Academic Press, 1951. 7. Josw, W.: Explosion and Combustion Processes in Gases. New York, McGraw-Hill Book Co., 1946. 8. WAGNER, C. D., SMITH, R. H., AND PETERS, E. D. : Ind. & Eng. Chem. (Anal. Ed.), 19, 976 (1947). 9. BRICKER, C. E., AND JOHNSON, H. n. : Ind. & Eng. Chem. (Anal. Ed.), 17, 400 (1945). 10. GOLDMAN,F. H., AND YAGODA,H. : Ind. & Eng. Chem. (Anal. Ed.), 15, 377 (1943). 11. NEWlTT, D. M., AND THORNES, L. S. : J. Chem.
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Soc., 62, 2234 (1940). 15. MALHERBE, F. E., AND WALSH, A. D. : Trans.
Faraday Soc., $6,835 (1950). 16. SALNIKOV,I. E.: C. R. Acad. Sci. (U.R.S.S), 60, 405 (1948). 17. WALSH, A. D.: Trans. Faraday Soc., ~3, 297 (1947). 18. EGERTON, A. C.: Nature, 121, 10 (1928). 19. Noauisn, R. G. W.: Disc. Faraday Soc., 10, 269 (1951). 20. EASTWOOD, T. A., AND HINSHELWOOD, C. : J.
Chem. Soc., 733 (1952). 21. FRANK-KAMENETZKI, D. A.: C. R. Acad. Sci. (U.R.S.S.), 25,671 (1939). 22. WALSH, A. D.: Trans. Faraday Soc., ~3, 305 (1947).