INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1951
Table IV.
Oxygenated Compounds i’H
Same Ethylene oxide Propylene oxide Acrolein Methanol Diethyl ether Propionaldehyde Acetone Isopropyl alcohol Ethyl acetate
‘pa
1.09 1.05 1.00 1.04 1.05 1.06 1.00 1.07 1.00
3. 2405 2360 2355 2135 2285 2275 2307 2200 2258
x
lo’, Atmospheres 1.41 0.97 0.75 0.29 0.67 0.61 0.60 0.44 0.47
*
Po
X 103 Atmospheree 0.68 0.53 0.71 0.58 0.25 0.22 0 47 0.89 0.31
PoHa
X 10 , Atmospheres 5.09 4.21 4.10 1.49 2.90 2.54 3.73 1.69 2.95
Maximum Flame Velocity Cm. per Secondb 89 5
67.2 66.1 57.2 49.8 49.9 42.3 41.5 38.2
a ‘p = equivalenoe ratio = fuel a t stoichiometric divided by fuel of mixture. b Flame speed data of Experiment Ino.
the experimental values, 11are within 5 to 1275, and only ethylene deviates by a higher per cent-230jo. I n general, regardless of the structure of the hydrocarbon fuel, the fact that a constant IC value may be used t o predict the fundamental flame velocity from the experimental velocity reflects the deviation of the calculated k value from the average IC. The k value for ethylene is higher than the average. It deviates by more than 6 times the average deviation, which suggests that the over-all rate constant for oxidation of ethylene may be higher than the rate constant for other hydrocarbon oxidations. By analogy acetylene (Figure 2) would have a high /c value also. From this discussion it may be concluded that the hydrocarbonair flame velocity data are consistent with a n active particle diffusion theory of flame propagation. Since the calculated active particle concentrations depend on flame temperature, some thermal mechanism of flame propagation which depends strongly on flame temperature could also give a consistent picture with the hydrocarbon flame velocity data. Figure 4 shows the correlation of flame velocity with the calculated equilibrium flame temperatures. Such a correlation of flame velocity with temperature could be used for the prediction of flame velocities. The data of Experiment Inc. also included nine oxygenated compounds for which sufficient thermochemical data were avail-
t
I
212 1
able to calculate equilibrium product concentrations by the method of Huff and Calvert ( 7 ) . In Figure 5 the relative atom and free radical concentrations are plotted against the fundamental flame velocities for these nonhydrocarbons. The data for this figure are given in Table IV. The flame velocities of five of the nonhydrocarbons-ethyl acetate, acetone, isopropyl alcohol, propionaldehyde, and diethyl ether-appear to be related t o the active particle concentration in the same way as the hydrocarbons. The other four nonhydrocarbons-ethylene oxide, propylene oxide, acrolein, and methanol-deviate from the original hydrocarbon correlation. Furthermore, it appears that the extent of the deviation is about the same as for ethylene. The fact that the flame velocities of the nonhydrocarbons do not correlate with the relative atom and free radical concentratione as well as for the hydrocarbons is not surprising since greater differences in the rate constants might be expected. But the explanation of the apparent uniformity in the deviation of ethylene oxide, methanol, propylene oxide, acrolein, and ethylene from the general hydrocarbon behavior is not obvious. LITERATURE CITED
(1) Badin, Stuart, and Pease, J . Chem. Phys., 17, 314 (1949). (2) Bartholome, Naturwissenschuften, 6, 171 (1949). (3) Calcote, Barnett, and Irby, paper presented at the 116th Meeting of the AMERICAN CHEMICAL SOCIETY, Atlantic City, N. J. (4) Gerstein, Levine, and Wong, IND.ENG.CHEM.,43, 2770 (1951?, (5) Gerstein, Levine, and Wong, J . Am. Chem. SOC.,73, 418 (1951). ( 6 ) Hottel, Williams, and Satterfield, “Thermodynamic Charts for Combustion Processes. Part I,” New York, John Wiley & Sons, 1949. (7) Huff and Calvert, Natl. Advisory Comm. Aeronaut., Tech. Note 1653, 1948. (8) Linnett, and Hoare, “Third Symposium on Combustion, Flame and Explosion Phenomena,” p. 195, Baltimore, Williams & Wilkins, 1949. (9) Sachsse and Bartholome,Z . Elektrochem., 53,183 (1949). (10) Simon, J . Am. Chem. SOC.,73, 422 (1951). (11) Tanford, “Third Symposium on Combustion, Flame and Explosion Phenomena,” p. 140, Baltimore, Williams & Wilkina Co., 1949. (12) Tanford and Pease, J . Chem. Phys., 15, 431,433,861 (1947).
RECEIVED May 16, 1951.
FLAMMABILITY LIMITS OF HYDROCARBON-AIR MIXTURES *
Reduced Pressures JAMES
T. DI PIAZZA AND MELVIN GERSTEIN, National Advisory ROBERT C. WEAST,
-i
Case Institute
A systematic study to determine the effect of molecular structure on the flammability limits of pure hydrocarbonair mixtures at reduced pressures and room temperature is presented. Limit determinations were made in a closed tube with hot-wire ignition at the lower end. The low pressure limit of propagation of 34 mm. of mercury was relatively unaffectedby molecularweight, branching, or unsaturation. The range of 5ammable mixtures, when expressed as per cent stoichiometric fuel, ‘increased with molecular weight. Branched paraffinshad a slightly decreased range of composition limits when compared with straight-chain isomers. The effect of branching was most pronounced when two methyl groups were eubstituted for hydrogen atoms on the same carbon atom.
Committee for Aeronautics, Cleveland, Ohio
o f Technology, Cleveland, Ohio Unsaturation (ethylene excepted) had no significant effect when mono-olefins were compared to analogous saturated hydrocarbons. Flammability‘limit curves of pressure US, fuel concentration were characterized by a two-lobe form. The formation of the fuel-rich lobe was attributed to cool5ame phenomena.
PART of a fundamental combustion program a t the Lewis laboratory of the NACA, a study of the flammability lirmts of pure hydrocarbon-air mixtures a t reduced pressures and room temperature has been made. Previous work on flammability limits dealt with either the determination of limits a t atmospheric pressure for many fuels or the effect of variables such as pressure and temperature for a few
I N D U S T R I A L AND ENGINEERING CHEMISTRY
2722
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selected fuels. The object of this investigation was to determine the effects of molecular structure on flammability a t reduced pressures. Limit curves of pressure versus fuel concentration for a number of pure hydroaarbons were obtained under identical experimental conditions. However, flammability limits, especially at reduced pressure are affected greatly by the apparatus in which they are determined. It was therefore necessary to investigate the effect of experimental variables in order to determine what significance could be attached to the results that were observed.
Vol. 43, No. 12
IGNITION SOURCE o HOT WIRE 0 INDUCTANCE SPARK 0 CAPACITANCE SPARK A GUN COTTON PLATINUM WIRE
360320-
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APPARATUS AND PROCEDURE
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The experimental apparatus and method was essentially the same asZhat described by Coward and Jones (1). The apparatus is shown in Figure 1. The flame tube was a glass cylinder, 4 feet long and 2 inches in diameter, closed a t both ends by rubber s t o p pers. Ignition was accomplished a t the bottom of the vertical tube with a coil of Brown and Sharp No. 26 Piichrome wire. The electrically heated coil was located about 1.5 inches above the rubber stopper. Power input to the coil was kept constant with a
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Figure 1.
Flammability Limits Apparatus
constant voltage transformer. A wattmeter was used to measure power input and an electric timer indicated the time interval over which current was applied. A 60-watt power level was maintained for the results reported. Current to the coil was turned off the instant that flame was visible. If ignition did not occur within 10 seconds after the coil had been turned on, the mixture was called nonflammable. Propagation of flame the entire length of the flame tube was the criterion for flammability. The pressure limits were determined within f 2 mm.-that is, two pressures were found that differed by 4 mm., the higher of which permitted flame propogation. The recorded limit pressure was the average of the two pressures. Determinations vere repeated a t the limit as a check of the reproducibility. The hydrocarbon-air mixtures were prepared and stored in a 47-liter glass carboy. The hydrocarbon was introduced as a vapor and its pressure measured with a metal Bourdon gage, The gage was graduated in 0.2-mm. intervals and was calibrated against a mercury manometer. The total pressure after air had been admitted was measured with a mercury manometer. Mixing was accomplished with a bellows-sealed stirrer. The purity and source of the compounds used are listed in Table I. The air used to prepare combustible mixtures was passed over Ascarite and Anhydrone to remove carbon dioxide and water, respectively. The maximum error in the preparation of hydrocarbon-air mixtures mas not greater than 1 2 % based on the precision of fuel pressure measurement. The total mixture pressure m-as large so that the percentage error in measuring the total pressure was small.
01 40
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PERCENT Figure 9.
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STOICHIOMETRIC
Effect of Ignition Source on Propane-Air Limits
diameter may not be large enough to eliminate or minimize uall effects a t reduced pressures. Huebner and Wolfhard show that the lower pressure limit for propagation varies with the tube diameter; Simon (6) has shown that their data can be correlated with quenching distances determined by burner flash back indicating that wall quenching may be an important effect in establishing the minimum pressure limit. A 2-inch diameter was selected mainly because of the previous amount of n-ork that has been done in a tube of that size. A 4-fOOt tube length was chosen on the assumption that energy supplied to the combustible mixture in excess of the amount required for ignition u-ould be dissipated to the tube walls and in
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EXPERIMENTAL VARIABLES
The experimental variables investigated or considered i-cere the flame tube size, method of ignition, the use of an open or closed flame tube, and the direction of flame travel. Tube Size. The effect of tube diameter on limits has been studied by other investigators who show that there is a negligible increase in flammability limits at atmospheric pressure for tube sizes larger than 2 inches in diameter. However, a 2-inch tube
20 L 0 Figure 3.
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IO0 00 3 00 PERCENT STOICHIOMETRIC FUEL
Effect of Ignition Coil Power Input on Pentane-Air Limits
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1951
2723
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Effect of Upward and Downward Propagation on Propane-Air Limits
Effect of O p e n or Closed Tube Propagation on Propane-Air Limito
the relatively large volume of burned gas before the flame reached the top of the tube. Ignition Source. The ignition source is another factor which is known to affect flammability limits. Three types of ignition were investigated: hot-wire, spark, and guncotton, The hotwire source was described in a previous section. The sparks used were of two types, an inductance or long duration spark and a capacitance or short duration spark. The inductance spark was produced by discharging 25,000-volt alternating current across pointed stainless steel electrodes spaced I / p inch apart. The capacitance spark was produced by discharging a 0.1-microfarad condenser system, having a 10,000-volt potential, across the same electrodes. The guncotton was ignited by electrically fusing a 1/8-inch helix of platinum wire similar to the method used by Jones and Scott (4). The results of the ignition experiments are shown in Figure 2. It is evident that the hot-wire source gave wider composition and lower pressure limits than any of the other sources. The inductance spark, which resembled a glow discharge a t low pressures gave limits that were very close to those obtained with the hot wire. It is significant that the capacitance spark was unable to ignite mixtures that contained more than 200% stoichiometric fuel (propane). The dotted portion of the curve indicates the region where several unsuccessful ignitions were attempted. Although the lean and rich composition limits found with guncotton ignition were about the same as those obtained with the hot wire the low pressure limit of propagation was 40 mm. higher than that obtained with the hot wire. Further experiments were performed with hot-wire ignition to determine the effect of power input, which is roughly proportional to the wire temperature, on flammability limits. The results illustrated in Figure 3 indicate that a t 30 watts the composition limits were narrower and the pressure limits higher than the limits obtained when 45 and 60 watts were impressed. A 60-watt power level was arbitrarily selected to obtain the data reported. Open or closed tube. Another important experimental factor is the use of an open or closed tube for limit determinations. The pressure rises in a closed tube owing to the heating of the gas and an increase in the number of moles of gas during combustion.
This pressure rise and disturbances caused by the movement of the flame may affectthe limit in a closed tube. To investigate the importance of this effect, experiments were conducted in a closed tube and in a tube that was open a t the bottom to a 47-liter .plenum chamber through a a/,-inch bore stopcock. The results of the experiments are shown in Figure 4. The limit curve determined in the closed tube had wider composition limits and a more pronounced rich or second lobe. The low pressure limit was unchanged, but occurred a t richer fuel concentration in the open tube. Because of ease and simplicity of operation, a closed tube was used for the data reported.
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Figure 6.
Character of Flames Near Limits
Direction of Propagation. Figure 5 shows the effect of direction of propagation on limits. Limit curves are plotted for pentane-air mixtures with upward and downward propagation. The lean composition limit is changed very little, but the low pressure limit is about 10 mm. greater for downward propagation. The rich limit, on the other hand, is decreased from 340 to 15001, stoichiometric fuel for downward propagation. It was possible to ignite rich mixtures with the coil located a t the top of the tube, but the flames, once initiated, would not travel downward.
INDUSTRIAL AND ENGINEERING CHEMISTRY
2724
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Effect of Molecular Weigh! on Flammability Limits
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Figure 8.
The remaining data to illustrate the variation of the flamniability limits with fuel type were all obtained in a 2-inch diameter tube closed a t both ends. Ignition was with a hot wire a t the bottom of a vertical tube so that upvard propagation resulted. Limit Flame Characteristics. Figure 6 shows a typical flammability limit curve of pressure versus composition that has
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Flammability Limit Curves for Ethylene and Ethane
characteristic flame shapes superimposed at regions that were comnion to their appearance. The color, speed, and mode of propagation are listed beside each flame. A knowledge of tJhe physical characteristics of the flames may result in a better understanding of the data presented. Perhaps the most important physical characteristics are that the 340flames of the first or fuel-lean lobe are predominantly blue, fill the cross section of the tube, and propagate 300rapidly, whereas the flames of the second or fuel0 I rich lobe are for the most part green, do not fill 260the tube, and propagate slowly. E E Effect of Molecular Weight. Figure 7 has limit 220curves plotted for the normal paraffins to show the ua effect of molecular weight on limits. A large spread 2 180exists in the rich limit in going from methane a t m w 160% stoichiometric fuel to hexane a t 390%. The '140lean limit varies from 40% for methane t o 60% for hexane: the lean limit for each fuel shoas no ap100 preciable change until the pressure is reduced to about 160 mm. T h e n extrapolated to atmospheric 60 pressure, the limits agree well with data available 2 O 0 L in the literature. Below 160 mm. the lean limit 0 ' IO0 ' 2 0 0 ' 300 ' fuel concentration increases for all fuels and apPERCENT STOICHIOMETRIC FUEL proaches a minimum pressure of 34 mm. Since Figure 9. Limit Curves for Propane, Propene, n-Butane, and 1 -Butene lower pressure limits can be obtained in larger tubes (e), the change of the flammability limit beloJT a o PENTANE 0 HEXANE critical pressure of about 160 mm. and the appear340r ; 0 PENTENE 1 0 HEXENE ance of a low pressure limit may be a function of 300 wall quenching or tube diameter. The limit curves I P I [or propane through hexane have two distinct , 260lobes, whereas the limit curves for methane have E E I but one lobe; ethane shows just a tendency toward 220 wthe formation of a second or rich lobe. White (8) a 1803 as well as Spence and Townend ( 7 ) postulated that m m the point of inflection of the limit curve is actually W I40 a the intersection of two curves; one forms the limits CL 100 for normal flames and the other the limits for cool flames. The cool-flame hypothesis for the forma60 tion of the second or rich lobe is upheld by the 20 -I I L L following observations.
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Limit Curves for Pentane, I-Pentene, Hexane, and I-Hexene
1. Methane, which has but one lobe to its limit curve, does not support cool flame combustion (5).
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1951 o 0
2-METHYLPROPANE n-BUTANE
Effect of Branching. The effect of branching on flammability limits is shown in Figures 11, 12, and 13, where branched paraffins are compared with straight-chain isomers. The lean portion of the limit curves for the branched compounds are practically identical with the curves for the straight chain isomer in all six cases. The rich limit, however, decreased appreciably when two methyl groups were substituted for hydrogen atoms on the same carbon atom, and to a lesser degree for the other types of substitution.
o 2-METHYLBUTANE 0
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LITERATURE CITED
(1) Coward, H. F.,and Jones, G. W.. U. S. Bur. Mines, Bull. 279 (1939). (2) Forsythe, J. S., and Townend, D. T. A., Engineering, 149, 127 (1940). (3) Gaydon, A. G., “Spectroscopy and Combustion Theory,” p. 59, London, Chapman and Hall, 1948. (4) Jones, G.W., and Scott, F. E., U. S. Bur. Mines, Rept. Invest. 4473 (May 1949). (5) Jost, Wilhelm, “Explosion and Combustion Processes in Gases.” v . 234, New York, McGrawHill Book Co., 1946. (6) Simon, D. M., J. Applied Phys., 22, No. 1, 103 (1951). (7) Spence, K.,and Townend, D. T. A,, Nature, 155, 330-1 (March 17,1945). (8)White, Albert G., J . Chem. SOC. Trans., CXXI, 1244-70 (1922).
PERCENT STOICHIOMETRIC FUEL Figure 11,
Effect of Branching on Flammability Limits
2. Jost (6) reports that cool flames propagate a t a rate of about 10 to 20 cm. per second. The flames of the rich lobe of the limit curve also propagated a t about 10 to 20 cm. per second. 3. White (8) found that it was impossible to initiate cool flame combustion with a capacitance spark. A capacitance spark was also unable to initiate flame in rich mixtures in this investigation. 4. White (8) was unable to propagate the cool flames of ether and acetaldehyde downward. Flames of the rich lobe of the limit curves determined in this investigation were also unable to 340propagate downward. Effect of Unsaturation. The effect of unsaturation in the form of one double bond is shown in Figures 8,9, and 10 where mono-olefins are plotted with analogous saturated hydrocarbons. Limit curves for ethane and ethylene are shown in Figure 8. The lean composition limit and the low pressure limit are practically unchanged. The rich limit, on the other hand, is more than doubled for ethylene. The dips that appear in the rich portion of the ethylene curve have also been reported by other investigators ( 2 ) . Limit curves for propene, 1-butene, 1-pentene, and 1-hexene appear in Figures 9 and 10, but no significant or regular behavior is shown when these compounds are compared with the normal paraffins.
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Purity, Mole %
Matheson Co Ino Phillip’s Petroieum‘ Co. NACA (prepared jointly b XBS and NACA). d Ohio State Research Founlaation A.P.I. Research Project 45. b
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Source and Purity of Compounds Used
Compound Methane Ethane Propane Butane Pentane Hexane 2-Methylpropane 2-Methylbutane 2 2-Dimethylhutane 2:3-Dimethylbutane 2,2-Dimethylpropane 2-Methylpentane Ethylene Propene 1-Butene 1-Pentene I-Hexene
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Table
RECEIVED May 16, 1951
FUEL
Effect of Branching on Flrmmrbility Limits
1
