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Branched-chain mechanism of propane-oxygen-fluorineexplosions
BRANCHED-CHAIN
MECHANISM FLUORINE
OF
PROPANE-OXYGEN-
EXPLOSIONS*
G U E N T I t E R VON E L B E AND G E O R G E W H I T E Research and Technology, Atlantic Research Corporation, Alexandria, Virginia
Fluorine diluted with inert gas is injected into a mixture of propane, oxygen, and inert gas at room temperature and the temperature is increased rapidly by adiabatic compression; or, using the same apparatus, a mixture of propane, oxygen, and iner~ gas is adiabatically compressed, and diluted fluorine is injected into the compressed gas within less than a millisecond. The molecular concentrations and the temperature of the mixture at the end of the compression stroke are determined by the compression ratio, the mixture composition, and the initial pressure and temperature. The compression temperature is calculated from the entropy of the mixture, using thermodynamic tables. I t is found t h a t the reaction terminates in explosion between a lower and an upper limit of the concentration ratio [F~]/[O2]. Both limits have been reproduced in numerous runs. The limits coincide at about 350°K and diverge toward higher temperatures, enclosing an explosion peninsula in a diagram of [ F 2 ] / [ O 2 ] versus temperature. The limits are not significantly dependent on the method of experimentation. They are insensitive to changes of the propane concentration and also to changes of ~he pressure, if the latter exceeds a critical limit. The critical limit of ~he concentration ra~io [-F..,]/[()2] corresponds lo the scheme C3H7-~- ()~ - C3H7OO,
(1)
C3H7+F2-~chain breaking,
(2)
CaHTOO +F2---mC3HT,
(a)
CaH;OO q-O2-~chain breaking.
(b)
From reaction-kinetic theory, the explosion peninsuIa is described by the equation
y = (~,- 1)(I 4- {1 - ~-2x/(x- 1)~3} ":) where [with 3 C3H7 in Reaction (a)] x = 2klk~/k2kb = C~ e x p [ - (El --E~.+E~ - - E s ) / R T ] , y = 2k~[F2]/kb[02] = C2([F2]/[()2]) e x p [ - ( E . ~ - E b ) / R T ] .
A close tit between the experimental cm've [ F 2 ] / [ O 2 ] versus T and the theoretical curve of y versus x is obtained using C1 =23.7
E l - E 2 = 3 8 1 0 cal/mole,
C2 = 1.74
E,~--Eb = --2520 cal/mole.
The validiW extends to about 700°K. * Supported by O[tice of Naval Research Contract N00014-69-C-0302. 463
464
OXIDATION AND IGNITION Reaction (a) might be visualized to yield the free radicals COF, CHa, and CH30, which corresponds to n =3 unless the reaction CH30+O2=HCHO+HO2 occurs. The radical HO2 is substantially inert, and similarly, the radical CHeF from Reaction (2) is deactivated by association with O2. The discovery of Reaction (b), which yields aldehydes and HO2, provides a new perspective on the chain mechanism of hydrocarbon oxidation in general. It is proposed that HO2 instead of OH and O is formed, because the reaction energy is decreased by resonance between two structures of the peroxyalkane radical. On association with an aldehyde in an ether-type linkage the resonance is inhibited and the reaction with O2 changes from chain-breaking to chain-branching. This concept explains the high chain-branching propensity of ether/oxygen systems.
Introduction Kinetic experiments with fluorine and hydrocarbons other than methane require rapid preparation of the test mixture, because fluorine reacts with higher hydrocarbons in a few seconds, even at room temperature and low concentrations. Poluektov, Pshezhetskii, and Cherednichenko 1 have studied the effect of small percentages of fluorine on the (p, T) explosion limits of stoichiometric mixtures of oxygen with methane, butane, cyelohexane, and butene, by rapidly injecting fluorine/oxygen mixtures into the hydrocarbon in a quartz tube of 2.5 cm diameter and 15 cm length. The limits were generally found to shift toward lower temperatures and pressures as the fluorine percentage is increased. Above about 3% fluorine, the lowtemperature explosion peninsula of the paraffins was found to have shifted from the range of about 250 ° to 400 ° for the unsensitized reaction, to a range close to room temperature. The authors do not comment on an apparent crossing of their limit curves in several instances; for example, with cyclohexane their 10% F~ curve seems to cross the 7% F2 curve at about 20°C, so that, below this temperature, the limit pressure seems to increase with increasing concentration of F2. In the present experiments, fluorine diluted with nitrogen is injected into a mixture of propane, oxygen, and argon at room temperature and the temperature is increased rapidly by adiabatic compression; or, using the same apparatus, a mixture of propane, oxygen, and argon is adiabatically compressed and diluted fluorine is injected into the compressed gas within less than a millisecond. This approach permits an ageing of the mixture at room temperature over selected time intervals of fractions of a second, but this feature did not prove to be particularly significant in the present work. The principal advantage has been the companion feature of oscilliscopic pressure recording, which permits discrimination between a
rapidly developing and a delayed explosion as well as no ex~olosion. I t has been found that, outside of an upper as well as a lower limit of the concentration ratio EF~2/EOd, there may be no explosion or a substantially delayed explosion. These limits are far more informative than the pressure limits of explosion, which depend on vessel size and surface conditions and do not permit identification of specific elementary reactions in the chain-branching mechanism.
Apparatus and Data Reduction A schematic illustration of the compression apparatus is shown in Fig. 1. A drop weight impinges on a 0.75-in.-diameter piston in the center of a cylinder block, and on two 0.25-in.diameter pistons that are spaced symmetrically from the center. The drop weight is guided by a rod which is firmly clamped to a supporting bar above the block and whose lower end slides in an axial bore hole in the central piston. The 0.75-in. cylinder contains a mixture of hydrocarbon, oxygen, and argon, and one of the 0.25-in. cylinders contains the agent gas, in the present experiments a 15% F2/N2 mixture, that is to be injected into the test mixture. The other 0.25-in. cylinder communicates with the atmospher 9 through a throttled outlet, and serves to prevent any tilting of the drop weight on impact. The three pistons are machined to equal lengths and shoulder heights, and the three cylinders are so dimensioned that, on complete insertion of the pistons to the shoulders, the piston faces approach the cylinder bottoms to within about 0.001 in. At the start of a drop test, with agent injection, the pistons are raised to 63 mm height above the cylinder bottoms, and metal shims of selected thickness are placed on the two smaller pistons. When the weight is dropped, these pistons move correspondingly ahead of the larger piston, so that the agent gas is compressed much more than the test mixture. When
BRANCHED-CHAIN MECHANISM
465
t GUIDE ROD SUPPORT ~1 GUIDE ROD ~ DROPWEIGHT~~--~.,
~
RE MENT SHIMS 3/4 in. D I A PISTON ~ L ~ _ _ _ ~ _ _ ~ 1/4 in D I A D U M M Y ~
PISTON FOR
IA PISTON
.
BALANCE~
PISTON POSITION-k II INDICATING L I N E A R ~td~ TRANSDUCE
LINING
~
CYLINDER ~IE
R~ I / /t ..
LARGE C Y L I N D E R - ~
F I L L LINE AND V A L V E
~Y~-~V/ ~tL ~iL~
,LVE
iE BETWEEN AND AGENT )ER OPENING TOR ABLE LOADED ~VALVE
//c, " " "
, / SHOCK MOUNTED PRESSURE TRANSDUCER
\
I
3/32 in SOFT BRASS TUBE (OIL-FILLED)
FIO. 1. Apparatus fox' adiabatic compression with timed agent injection and recording of pressure and volume change, semi-schematic illustration. the pressure has become sufficiently high to depress the spring-loaded valve at the bottom of the agent cylinder, the passage to the bottom of the larger cylinder is open and rapid iniection occurs. The instant of valve opening can be advanced or retarded by adjusting the spring load. Since the agent piston descends to its full length into the agent cylinder, the transfer of the agent to the larger cylinder is complete except for the small quantity remaining in the connecting passage. When the shoulders of the two shim-loaded pistons touch the cylinder block, the fall of the drop weight is arrested. At this instant, the central piston is at a distance above the cylinder bottom substantially equal to the thickness of the shims. The pressure transducer, used to monitor the
adiabatic compression and subsequent ignition of the gas mixture in the large cylinder, is shockmounted in a separate steel block. This arrangement avoids any blurring of the record or possible damage to the transducer by the largely undamped impact of the drop weight. The pressure is transmitted fl'om the cylinder bottom to the transducer by means of a thin tube of soft brass that does not transmit the impact stresses in the cylinder block. The brass tube is filled with Kei-F oil, in order to reduce the dead space below the cylinder bottom. The piston movement is monitored by a sliding resistor actuated by a rigid rod which is attached to the piston. The pressure and piston movement are recorded by oscilloscope traces, using two channels of an oscilloscope. The instant at which the
466
OXIDATION
AND
agent-inieetion valve opens is recorded by a sudden displacement of the trace of the piston movement, using the motion of the valve stem to break an electrical contact in the slidingresistor circuit. The cylinders are machined from Teflon and inserted into a steel block, as illustrated. They are closed at the bottom by another steel block which is firmly bolted to the upper block and contains the spring-loaded Teflon valve as well as the lines leading to the large and the dummy cylinder. Gas-tight seals are obtained by O-rings in the lower block below the end faces of the Teflon cylinders, by an O-ring around the stem of the spring-loaded valve, and by two O-rings for each piston near the piston face. The pistons slide freely in the Teflon cylinders, but make effective seals for vacuum and pressure in the cylinders. Extensive tests have shown that, with a nonexplosive gas, the recorded pressure change and piston travel closely correspond to theoretical prediction based on the mechanics of free fall and isentropic change of gas pressure and volume. The combination of precision-fitted Teflon and steel restricts the temperature of the block to room temperature, because Teflon has a much higher coefficient of expansion than steel. I t is therefore not possible to vary the adiabaticcompression temperature by varying the initial temperature. However, by using argon as an inert diluent, it has been possible to obtain high-compression temperatures with moderate compression ratios. A lever and timing arrangement, not shown in Fig. 1, permits the agent gas to be injected at room temperature, followed by release of the drop weight and compression after a total time interval of 90 msec or more. This procedure did not significantly change the values of critical concentration ratios EF2]/[-02] that have been obtained by agent injection at the end of the compression stroke. Most of the data reported here have been obtained by the latter method. The gas-inlet valves of the cylinders are connetted to a stainless-steel manifold, which has valves leading to the gas-storage tanks, a vacuum pump, a mercury manometer, and a vessel for preparing the explosive gas mixture to be admitted to the main cylinder. The mixing vessel is a vertically mounted cylindrical glass bulb with stopcocks at both ends. A rubber hose connects the upper stopcock to the manifold valve, and another rubber hose connects the lower stopcock to a water reservoir. With the lower stopcock closed and no water in the vessel, the system is evacuated and propane is admitted to a desired pressure, followed by
IGNITION
oxygen and argon. Then a small quantity of tap water is admitted, and, with both stopcocks closed, the vessel is shaken vigorously within the freedom of movement of the rubber hoses. In this way, uniform mixing of the gases is assured, the water serving as a safe, i.e., nonsparking stirrer. The safety of this procedure derives from the electrolyte in the water, which inhibits the build-up of electrostatic charges. A quantity of the gas mixture at a desired pressure is admitted to the main cylinder at standard distance of 63 mm between piston face and cylinder bottom, while water is flowing into the mixing vessel, in order to maintain the supply pressure and determine the quantity of the remaining gas. Water vapor may be removed by passing the gas over a drying agent, but it has been found that the vapor pressure is too small to have a significant effect on the data. The agent cylinder is charged to a desired pressure with a mixture of 15% ]?2 arid 85% N2 from a storage tank obtained from Mathieson Gas Products. The fluorine percentage has been verified by analysis. The mercury manometer is protected against contact with fluorine by a soda-lime trap. When the cylinders are charged, the drop weight is released from a height of 10 in. above the cylinder block, which yields an adequate piston velocity without damage to the apparatus. Explosion is indicated by a steep pressure rise, which occurs at some time after compression; this time depends on the experimental conditions and represents the ignition lag. The total number of molecules per cm 3, at 760 mm Hg and the initial temperature of 25°C or 298°K, is 2.65X i019. At the end of the compression stroke, the contents of the agent cylinder are added to the main cylinder. The total number of molecules per em ~ is now n (2.65)X I019, where n = E (pJ760)-{- (1/9) (p~/760)] (63/s),
(1)
pi and
Pa are the pressures (mm Hg) in the main and agent cylinders at 63-mm distance between piston face and cylinder bottom, s is the shim thickness which determines the distance between piston face and cylinder bottom in the main cylinder at the end of the compression stroke, and the factor 1/9 is the ratio of the volume of the agent cylinder to the volume of the main cylinder. The number of propane molecules per cm 3 after compression is np (2.65)X i0 i9, where the subscript P denotes propane, and
np = (Pi,p/760) (63/s),
(2)
BRANCHED-CHAIN MECHANISM 0.2
l
]
o
&
&
&
I
I
&
I
467
I
A
-
:
-& 0 . 0 6 u_
o
0.04
a
o
0.03
I
i
500
600
9
300
400
o I
j
700
800
_
m
900
,,
1,000
TEMPERATURE (OK) FIG. 2. Fluorine/oxygen ratio versus temperature for explosion 3x .rod no explosion Q.
Pi,P being the partial pressure of propane before compression. Analogous expressions apply to oxygen and argon, whereas, for fluorine, nF2= [0.15p~/(9) (760)] (63/S).
(3)
The temperature T of adiabatic (isentropic) compression is related to the shim thickness s by the equation T = 298 (63/s)z-~°K.
(4)
The value of y is obtained from the standard entropy of 1 mole of gas mixture at the tentperature T, using
R In (63/s)u= (&,°-- $29a°),~ixt.~
=52fj(&, ° - &~s°)j,
(5)
J
fj being the mole fraction of componeut j. The entropies of the diatomic gases are obtained from
J A N A F tables, and the entropy of propane from the tables published by The American Petroleum Institute. When the agent gas is injected at the end of the compression stroke, it is compressed isentropically much above the pressure p in the main cylinder, before the spring-loaded valve yields and allows the gas to expand to the pressure p. This expansion is substantially isentropie, because the inieetion is extremely rapid, so that the heat loss to the walls of the passage between agent and main cylinders is small. The agent gas thus substantially attains a temperature corresponding to isentropie compression from p~ to p. However, the quantity of injected gas is rather small, the initial pressures p~ and pi are generally not widely different, and the fluorine/nitrogen mixture has a y-value close to the propane/oxygen/argon mixtures. The reported compression temperatures T are therefore based on the y-values of the mixture without agent injection, and the small systematic error inherent in this procedure is believed to be negligible.
TABLE I Test run showing inhibition of explosion by fluorine Test mixture, 5% CaH8+27.4% O2+67.6C/o Ar. Initial pressure p , Agent mixture, 15% F2q-85% N_< Inilial pressure pa. Shim thickness s = 32.6 ram. Run
1
2
3
4
5
p,., mm Hg pa, mm Itg Explosion
360 740 No
360 600 No
360 500 Yes
360 550 No
360 520 Yes
Experimental Data An example of a test run, showing inhibition of explosion above a critical limit of the ttuorine concentration, is presented in Table I. In this instance, no delayed ignition was observed in the negative runs. Reduced data are summarized in Table II. "No explosion" includes sudden large increases of the ignition lag. Figure 2 shows a plot of the ratios of concentrations of fluorine and oxygen EFd/EOd= nr2/no2, versus the temperature T for all data listed in the table. The points marked by tri-
O X I D A T I O N AND I G N I T I O N
468
T A B L E II Summary of data
T~ ° K
3OO 0.36 0.018 0.059 0.0099 0.168
3O0 1.22 0.066 0.217 0.0184 0.085
319 1.3 0.074 0. 243 0.0107 0.044
360 1.8 0.102 0.336 0.0148 0.044
360 1.8 0.102 0.336 0.0148 0.044 +(2) 35,40
360 1.4 0.078 0. 257 0.0144 0.056 + 35
36O 1.2 0.066 0.218 0.0145 0.067
36O 0.98 0.053 0.175 0.0146 0.084
4OO 1.55 0.085 0.282 0.0093 O.033
40O 1.55 0.085 0.282 0.0116 0.041
400 1.55 0.085 0.282 0.0140 0.050 + 16
400 1.55 0.085 0.282 0.0186 0.066 +(2) 7, 7
4O0 1.55 0.085 0.282 0.0186 0.066 + 23*
400 0.98 0.051 0.168 0.0192 0.114
4OO 0.70 0. 034 0.114 0.0192 0.167
470 0.94 0. 046 0.153 0.0256 0.167
475 2.4 0.118 0.390 0.0218 0. 056 + 7
478 2.4 0.118 0. 390 0. 0192 0. 049
475 2.4 0.118 0.390 0. 0163 0. 042
620 2.0 0. 098 0.326 0.0545 0.167 + (3) 2, 2, 2
62O 2.0 0.098 0.326 0.0436 0. 134 +(3) 2, 2, 1
620 2.0 0.098 0.326 0.0340 0. 104
68O 2.5 0. 122 0.410 0.0683 0. 167 +(3) l, 1, 1
680 2.5 0. 122 0.410 0. 0547 0. 134
68O 2.5 0. 122 0.410 0.0342 0,083
8O0 2.9 0.071 0. 492 0.0630 0. 128
810 4.0 0.196 0.650 0. 0548 0.184
-- (3)
760 2.5 0.061 0. 425 0.0544 0.128 --, +(2) 3, 4
800 2.9 0.071 0.492 0.0791 0.161
+(3)
760 2.5 0.061 0.410 0.0681 0. 167 +(3) 4, 1, 1
+ (3)
-- (3)
-
nF~/nO~
835 3.4 0.083 0.576 0.0935 0.161
835 3.4 0. 083 0.576 0.0465 0.081
88O 4.0 0.138 0.675 0.109 0.161 + (2),
1, 0, 1
1 (2)
800 4.5 0. 154 0.760 0. 109 0.161 +(3) 3, 1, 2
890 4.5 0. 154 0. 760 0.098 0. 129 +(2), -2, 3
9O0 4.9 0.168 0.828 0. 134 0. 161
Explosion r, msec
835 3.4 0.083 0.576 0.0740 0. 128 + (2), -2, 3
900 4.9 0.168 0. 828 0.107 0.129 + , -3
910 4.0 O. 098 0. 680 O. 874 O. 128 + (3) 3, 2, 2
910 4.0 0.098 0. 680 0.547 0. 081 +, -2
910 4.0 0. 098 0.680 0.0292 0.043
915 5.2 0. 179 0.878 0. 142 0. 160
915 5.2 0. 179 0.878 0.113 0. 129
915 5.2 O. 179 O. 878 0.0714 0.081
+(3)
+(3)
-- (3)
0, 1, 1
2, 4, 3
n np
nOe nF2 nF2/n02 Explosion T, msec T, °K n np
nO2 nF2
nF~/nO~ Explosion r,
msec
nF~/n02
475 2.4 0.118 0. 390 0. 0274 0. 070
Explosion r, msec
12
T, °K
n np
nO2 nF2
T, °K n np
nO2 nF2
nF2/n02 Explosion r, msec T, ° Z n np
nO2 nF2
+
+(3)
nF2/n02
910 4.0 O. 098 O. 680 O. 109 O. 161
Explosion r, msec
1, 1
T, °K /2p
nO2 nF2
3, 3, 1
+(2),
-
-- (3)
(2)
-- (3)
--
+
+
65
3 620 2.0 0. 098 0.326 0.0272 0.083
+ 2
(3)
1, 0, 0
+(3) 1, 1, 1
920 5.3 O. 183 O. 895 O. 145 O. 162 + (3) 2, O, 2
(2)
BRANCHED-CHAIN MECHANISM
469
TABLE II--Continued T, °K n np nO2 nF~
nF2/nO~ Explosion r, msec
920 5.3 0.183 0. 895 0.115 0.129 + (5), -5(avg.)
920 5.3 0.183 0. 895 0.0725 0.081 -- (4)
1000 6.8 0. 333 1.11 0.093 0.084 -~-(3) 1, 5, 6
* Glass wool inserted. n = (number of molecules per cm~)/2.65 X 10 ~9;Explosion or no explosion indicated by + and - ; P = propane; r =ignition lag in milliseconds; ~-(2) signifies ignition in two successive runs, etc.
angles correspond to ignition, viz., explosion, and the points marked by circles correspond to nonignition. I t is seen that a well-defined explosion region exists, and that the region is bounded by an upper and a lower limit of the ratio [F2]/[O~].
Chain-Branching Mechanism A lower and upper critical limit of the concentration ratio [F2]/[O2] corresponds to the scheme
and (2&/kb) (F2/02) = y,
the solution of Eqs. (6) and (7) is found to be
y= (x--1) (l-i-{1-- [2x/ (x-- l )2-1}112).
X = C l e x p [ - - (E1--E2nLEa--Eb)/RT-1
(11)
(12)
(1)
and
(',3HTd-F2 = chain breaking,
(2)
y = C ~ ( E F d / E O d ) e x p E - (E~--Eb)/RT].
(a)
CaHTOO-t- O2---~chain breaking,
(b)
which is based on chain branching by multiplication of propyl radicals C3H7, and permits chain breaking via Reaction (b) or Reaction (2) to become dom!nant at low or high ratios of the concentration of fluorine and oxygen, respectively. Introducing the rate coefficients /q, etc., and taking n=3, the explosion limit is defined by the equations
ki[O2] [C3HT]+k2EF2]EC3HT] = 3kJF2]EC3HTOO],
(6)
k~[-F2-1EC3HTOO]-~-kb[-O2]EC3H7OO'l =/~I[O2][C3H7].
(7)
Introducing the variables
2klk,/k2kb=x
(8)
(10)
This equation yields the curve of y versus x shown in Fig. 3. Substituting Arrhenius functions for the four rate coefficients, one may write
03H7-~- O2= C3H7OO,
C~H~OO+F2---~nC3HT,
(0)
x is a function of temperature only, whereas y contains the factor [-F2]/[-02-]. The experimental curve of [ F 2 ] / [ 0 2 ] versus T can be fitted to the theoretical curve by choosing C1= 23.7
El-- E2= 3800 cal/mole,
C2= 1.74
Ea-- E b = 2520 cal/mole.
The superposed experimental curve is indicated in Fig. 3 by dots and dashes; it is seen that a nearly perfect fit is obtained. Along the lower branch of the curve, the fit extends to somewhat above 700°K, whereas, at higher temperatures, the experimental curve dips down in agreement with the concept that the free radical generated in Reaction (b) becomes increasingly reactive, and hence Reaction (b) gradually ceases to be a chain-breaking reaction. The detailed course of Reactions (2), (a), and (b) caxx only be inferred from chemical and bond-energy considerations. Reaction (a) might
OXIDATION AND IGNITION
470
TEMPERATURE (°K) 35O
10
8 6
400
500
/
600
700
• CORRESPOND TO E X P E R I M E N T A L EXPLOSION L I M I T USING:
4
>-
X = 23.7.EXP (-1,280/RT)
",,%
k 2
1
3
Y = 1.74 ([F2]/(O21 ) EXP (2,520/RT)
I
I
I
I
I
5
6
7 X---.~
8
9
¥-- (x-11(1±
J
.
2X
1 (x_ 121.
FIG. 3.
be visualized to yield, for example, HF-[-COF-[CH3-I-CHsO, which is exothermic, and would be followed by COF-FC3Hs=HCOF-i-C3HT, or COF-[-F2= COF~Jr-F and F-~-CsHs= HF-[-C3HT; CHz+O2 = HCHO-}-OH and OH-[-C~Hs= H20-~ C3H7; and CH30-[-C~Hs=CH3OH-[-C~HT, unless the reaction CH30-}-O~=HCHO-[-HO~ occurs. This reaction is favored energetically because the energy released in the formation of a carbonyl bond and association of H and O2 is well in excess of the energy required to break a C-H bond. The radical HO2 is substantially inert at low temperatures, because the reactions with C~Hs or F2 to yield H202-}-CsH7 or H O O F + F are endothermic, and is thus captured by the vessel wall. The same consideration applies to other low-molecular peroxidic radicals; one may thus visualfze Reaction (2) to yield, for example, C2H4-FHF+CH2F, followed by deactivation of the radical CH2F due to association with 02 to yield CH2F(OO). For Reaction (b), no deactivating step other than the generation of HO2 can be proposed. There is substantial evidence that C3H~OO may decompose spontaneously to yield CH~CHO-FCH~O,2 which might be followed by CH~O-}-O2=HCHO-[-H02. How-
ever, in the present experiments the decomposition of C3H7OO cannot be significant, because it would eliminate the chain-branching Reaction (a), and an equivalent reaction between CHsO and F2 is not imaginable. Thus, Reaction (b) occurs in collisions between C3H~OO and 02, and since the reaction between a peroxyalkyl radical and oxygen is evidently a very significant step in the chain mechanism of hydrocarbon oxidation, but apparently has thus far not been known to occur, it merits special consideration. Peroxyalkyl Radical Reactions in Hydrocarbon Oxidation The present experiments extend to the temperature range (300°C) of Pease's2 propane/ oxygen data, which show that, toward very low molar ratios O~/C3Hs, 1 mole CH3OH is formed per mole C3Hs reacted, and that CH30H vanishes as the ratio O2/C3Hs is increased. This was thought to represent the sequence C3H7-~-O2--~ CsHTOO--~CH~CHO~-CH~O; CH30-~-C~Hs= CH3OH-~-C~HT; CH~O+O2--~other products. The
BRANCHED-CHAIN MECHANISM
471
change of concept imposed by Reaction (b) may be formulated as follows:
/ 0--0
0--0
I
\1
H3C--C--CH3~H3C--C H
CH3CHO+CH30
/
CH~
H
(b) CH3CHO+HCHO-!-HO2.
Using bond energies, in kc~/mole, C - - C = 82, C - - O = 8 2 , C - - H = 99, C = O = 175, H--O2=78.5, one finds the energy difference between the indicated C3H7OO isomers to be virtually zero; thus, a resonance equilibrium is plausible. The decomposition reaction is exothermic by about 58--RE, the resonance energy, and the 02-Reaction (b) is exothermic by about 131--RE. If OH-t-O were formed instead of HO2, the ~eaction energy would be about 33--RE. This suggests that the reaction would change from chain-br,eaking to chain-branching if the isomer on the right were stabilized, and RE would eorrespondingD drop out. It is generally accepted that the low-temperature chain-branching reaetion of alkane hy~trocarbons involves association of peroxy radicals with aldehydes, and it is now proposed that this association has the effect of stabilizing the structure. One may visualize that an ether-type linkage is formed as illustrated for CaHTOO+CHaCHO: O--O--CHa
I o2 H3C--C" • H . . C t I - * H C H O ~ 2 CH3CHO~OH-~O-~ ~,33 keal/mole. O
CH3
One hydrogen atom is taken to be unassigned and in a bonding role, which would reduce the estimated exothermicity somewhat. With ethers, the structure is preformed. No aldehyde accumulation is required for chain branching and no chain breaking occurs by reactions of peroxy radicals with 02 to yield HO2. This would explain the high propensity of ethers for low-temperature chain branching, including the peculiar violence of ether/oxygen explosions and the propagation of "cool-flame" waves at low temperatures and pressures and in mixtures containing less than 1% oxygen. For diethyl ether, the analogous reaction would be O--O CH3--C
O--O--OH CH~-~H--C.
H
O
CHa
• H ' . CH--~CH3CHO~-2
\ / O
REFERENCES 1. POLUEKTOV, V. A., PSHEZHr~TSKII, S. YA., AND
CHEREDNICHENKO,V. M.: Zhur. Fiz. Khim. 43, 1747 (1969).
\
HCHO-I-OH~O.
CHa
2. PEASE, R. N.: J. Am. Chem. Soc. 57, 2298 (1935); ef. LEwis, B. AND ¥ON ELBE, G.: Combustion, Flames and Explosions of Gases, 2nd ed., p. 127, Academic Press, 1961.
MECHANISM FLUORINE
OF
PROPANE-OXYGEN-
EXPLOSIONS*
G U E N T I t E R VON E L B E AND G E O R G E W H I T E Research and Technology, Atlantic Research Corporation, Alexandria, Virginia
Fluorine diluted with inert gas is injected into a mixture of propane, oxygen, and inert gas at room temperature and the temperature is increased rapidly by adiabatic compression; or, using the same apparatus, a mixture of propane, oxygen, and iner~ gas is adiabatically compressed, and diluted fluorine is injected into the compressed gas within less than a millisecond. The molecular concentrations and the temperature of the mixture at the end of the compression stroke are determined by the compression ratio, the mixture composition, and the initial pressure and temperature. The compression temperature is calculated from the entropy of the mixture, using thermodynamic tables. I t is found t h a t the reaction terminates in explosion between a lower and an upper limit of the concentration ratio [F~]/[O2]. Both limits have been reproduced in numerous runs. The limits coincide at about 350°K and diverge toward higher temperatures, enclosing an explosion peninsula in a diagram of [ F 2 ] / [ O 2 ] versus temperature. The limits are not significantly dependent on the method of experimentation. They are insensitive to changes of the propane concentration and also to changes of ~he pressure, if the latter exceeds a critical limit. The critical limit of ~he concentration ra~io [-F..,]/[()2] corresponds lo the scheme C3H7-~- ()~ - C3H7OO,
(1)
C3H7+F2-~chain breaking,
(2)
CaHTOO +F2---mC3HT,
(a)
CaH;OO q-O2-~chain breaking.
(b)
From reaction-kinetic theory, the explosion peninsuIa is described by the equation
y = (~,- 1)(I 4- {1 - ~-2x/(x- 1)~3} ":) where [with 3 C3H7 in Reaction (a)] x = 2klk~/k2kb = C~ e x p [ - (El --E~.+E~ - - E s ) / R T ] , y = 2k~[F2]/kb[02] = C2([F2]/[()2]) e x p [ - ( E . ~ - E b ) / R T ] .
A close tit between the experimental cm've [ F 2 ] / [ O 2 ] versus T and the theoretical curve of y versus x is obtained using C1 =23.7
E l - E 2 = 3 8 1 0 cal/mole,
C2 = 1.74
E,~--Eb = --2520 cal/mole.
The validiW extends to about 700°K. * Supported by O[tice of Naval Research Contract N00014-69-C-0302. 463
464
OXIDATION AND IGNITION Reaction (a) might be visualized to yield the free radicals COF, CHa, and CH30, which corresponds to n =3 unless the reaction CH30+O2=HCHO+HO2 occurs. The radical HO2 is substantially inert, and similarly, the radical CHeF from Reaction (2) is deactivated by association with O2. The discovery of Reaction (b), which yields aldehydes and HO2, provides a new perspective on the chain mechanism of hydrocarbon oxidation in general. It is proposed that HO2 instead of OH and O is formed, because the reaction energy is decreased by resonance between two structures of the peroxyalkane radical. On association with an aldehyde in an ether-type linkage the resonance is inhibited and the reaction with O2 changes from chain-breaking to chain-branching. This concept explains the high chain-branching propensity of ether/oxygen systems.
Introduction Kinetic experiments with fluorine and hydrocarbons other than methane require rapid preparation of the test mixture, because fluorine reacts with higher hydrocarbons in a few seconds, even at room temperature and low concentrations. Poluektov, Pshezhetskii, and Cherednichenko 1 have studied the effect of small percentages of fluorine on the (p, T) explosion limits of stoichiometric mixtures of oxygen with methane, butane, cyelohexane, and butene, by rapidly injecting fluorine/oxygen mixtures into the hydrocarbon in a quartz tube of 2.5 cm diameter and 15 cm length. The limits were generally found to shift toward lower temperatures and pressures as the fluorine percentage is increased. Above about 3% fluorine, the lowtemperature explosion peninsula of the paraffins was found to have shifted from the range of about 250 ° to 400 ° for the unsensitized reaction, to a range close to room temperature. The authors do not comment on an apparent crossing of their limit curves in several instances; for example, with cyclohexane their 10% F~ curve seems to cross the 7% F2 curve at about 20°C, so that, below this temperature, the limit pressure seems to increase with increasing concentration of F2. In the present experiments, fluorine diluted with nitrogen is injected into a mixture of propane, oxygen, and argon at room temperature and the temperature is increased rapidly by adiabatic compression; or, using the same apparatus, a mixture of propane, oxygen, and argon is adiabatically compressed and diluted fluorine is injected into the compressed gas within less than a millisecond. This approach permits an ageing of the mixture at room temperature over selected time intervals of fractions of a second, but this feature did not prove to be particularly significant in the present work. The principal advantage has been the companion feature of oscilliscopic pressure recording, which permits discrimination between a
rapidly developing and a delayed explosion as well as no ex~olosion. I t has been found that, outside of an upper as well as a lower limit of the concentration ratio EF~2/EOd, there may be no explosion or a substantially delayed explosion. These limits are far more informative than the pressure limits of explosion, which depend on vessel size and surface conditions and do not permit identification of specific elementary reactions in the chain-branching mechanism.
Apparatus and Data Reduction A schematic illustration of the compression apparatus is shown in Fig. 1. A drop weight impinges on a 0.75-in.-diameter piston in the center of a cylinder block, and on two 0.25-in.diameter pistons that are spaced symmetrically from the center. The drop weight is guided by a rod which is firmly clamped to a supporting bar above the block and whose lower end slides in an axial bore hole in the central piston. The 0.75-in. cylinder contains a mixture of hydrocarbon, oxygen, and argon, and one of the 0.25-in. cylinders contains the agent gas, in the present experiments a 15% F2/N2 mixture, that is to be injected into the test mixture. The other 0.25-in. cylinder communicates with the atmospher 9 through a throttled outlet, and serves to prevent any tilting of the drop weight on impact. The three pistons are machined to equal lengths and shoulder heights, and the three cylinders are so dimensioned that, on complete insertion of the pistons to the shoulders, the piston faces approach the cylinder bottoms to within about 0.001 in. At the start of a drop test, with agent injection, the pistons are raised to 63 mm height above the cylinder bottoms, and metal shims of selected thickness are placed on the two smaller pistons. When the weight is dropped, these pistons move correspondingly ahead of the larger piston, so that the agent gas is compressed much more than the test mixture. When
BRANCHED-CHAIN MECHANISM
465
t GUIDE ROD SUPPORT ~1 GUIDE ROD ~ DROPWEIGHT~~--~.,
~
RE MENT SHIMS 3/4 in. D I A PISTON ~ L ~ _ _ _ ~ _ _ ~ 1/4 in D I A D U M M Y ~
PISTON FOR
IA PISTON
.
BALANCE~
PISTON POSITION-k II INDICATING L I N E A R ~td~ TRANSDUCE
LINING
~
CYLINDER ~IE
R~ I / /t ..
LARGE C Y L I N D E R - ~
F I L L LINE AND V A L V E
~Y~-~V/ ~tL ~iL~
,LVE
iE BETWEEN AND AGENT )ER OPENING TOR ABLE LOADED ~VALVE
//c, " " "
, / SHOCK MOUNTED PRESSURE TRANSDUCER
\
I
3/32 in SOFT BRASS TUBE (OIL-FILLED)
FIO. 1. Apparatus fox' adiabatic compression with timed agent injection and recording of pressure and volume change, semi-schematic illustration. the pressure has become sufficiently high to depress the spring-loaded valve at the bottom of the agent cylinder, the passage to the bottom of the larger cylinder is open and rapid iniection occurs. The instant of valve opening can be advanced or retarded by adjusting the spring load. Since the agent piston descends to its full length into the agent cylinder, the transfer of the agent to the larger cylinder is complete except for the small quantity remaining in the connecting passage. When the shoulders of the two shim-loaded pistons touch the cylinder block, the fall of the drop weight is arrested. At this instant, the central piston is at a distance above the cylinder bottom substantially equal to the thickness of the shims. The pressure transducer, used to monitor the
adiabatic compression and subsequent ignition of the gas mixture in the large cylinder, is shockmounted in a separate steel block. This arrangement avoids any blurring of the record or possible damage to the transducer by the largely undamped impact of the drop weight. The pressure is transmitted fl'om the cylinder bottom to the transducer by means of a thin tube of soft brass that does not transmit the impact stresses in the cylinder block. The brass tube is filled with Kei-F oil, in order to reduce the dead space below the cylinder bottom. The piston movement is monitored by a sliding resistor actuated by a rigid rod which is attached to the piston. The pressure and piston movement are recorded by oscilloscope traces, using two channels of an oscilloscope. The instant at which the
466
OXIDATION
AND
agent-inieetion valve opens is recorded by a sudden displacement of the trace of the piston movement, using the motion of the valve stem to break an electrical contact in the slidingresistor circuit. The cylinders are machined from Teflon and inserted into a steel block, as illustrated. They are closed at the bottom by another steel block which is firmly bolted to the upper block and contains the spring-loaded Teflon valve as well as the lines leading to the large and the dummy cylinder. Gas-tight seals are obtained by O-rings in the lower block below the end faces of the Teflon cylinders, by an O-ring around the stem of the spring-loaded valve, and by two O-rings for each piston near the piston face. The pistons slide freely in the Teflon cylinders, but make effective seals for vacuum and pressure in the cylinders. Extensive tests have shown that, with a nonexplosive gas, the recorded pressure change and piston travel closely correspond to theoretical prediction based on the mechanics of free fall and isentropic change of gas pressure and volume. The combination of precision-fitted Teflon and steel restricts the temperature of the block to room temperature, because Teflon has a much higher coefficient of expansion than steel. I t is therefore not possible to vary the adiabaticcompression temperature by varying the initial temperature. However, by using argon as an inert diluent, it has been possible to obtain high-compression temperatures with moderate compression ratios. A lever and timing arrangement, not shown in Fig. 1, permits the agent gas to be injected at room temperature, followed by release of the drop weight and compression after a total time interval of 90 msec or more. This procedure did not significantly change the values of critical concentration ratios EF2]/[-02] that have been obtained by agent injection at the end of the compression stroke. Most of the data reported here have been obtained by the latter method. The gas-inlet valves of the cylinders are connetted to a stainless-steel manifold, which has valves leading to the gas-storage tanks, a vacuum pump, a mercury manometer, and a vessel for preparing the explosive gas mixture to be admitted to the main cylinder. The mixing vessel is a vertically mounted cylindrical glass bulb with stopcocks at both ends. A rubber hose connects the upper stopcock to the manifold valve, and another rubber hose connects the lower stopcock to a water reservoir. With the lower stopcock closed and no water in the vessel, the system is evacuated and propane is admitted to a desired pressure, followed by
IGNITION
oxygen and argon. Then a small quantity of tap water is admitted, and, with both stopcocks closed, the vessel is shaken vigorously within the freedom of movement of the rubber hoses. In this way, uniform mixing of the gases is assured, the water serving as a safe, i.e., nonsparking stirrer. The safety of this procedure derives from the electrolyte in the water, which inhibits the build-up of electrostatic charges. A quantity of the gas mixture at a desired pressure is admitted to the main cylinder at standard distance of 63 mm between piston face and cylinder bottom, while water is flowing into the mixing vessel, in order to maintain the supply pressure and determine the quantity of the remaining gas. Water vapor may be removed by passing the gas over a drying agent, but it has been found that the vapor pressure is too small to have a significant effect on the data. The agent cylinder is charged to a desired pressure with a mixture of 15% ]?2 arid 85% N2 from a storage tank obtained from Mathieson Gas Products. The fluorine percentage has been verified by analysis. The mercury manometer is protected against contact with fluorine by a soda-lime trap. When the cylinders are charged, the drop weight is released from a height of 10 in. above the cylinder block, which yields an adequate piston velocity without damage to the apparatus. Explosion is indicated by a steep pressure rise, which occurs at some time after compression; this time depends on the experimental conditions and represents the ignition lag. The total number of molecules per cm 3, at 760 mm Hg and the initial temperature of 25°C or 298°K, is 2.65X i019. At the end of the compression stroke, the contents of the agent cylinder are added to the main cylinder. The total number of molecules per em ~ is now n (2.65)X I019, where n = E (pJ760)-{- (1/9) (p~/760)] (63/s),
(1)
pi and
Pa are the pressures (mm Hg) in the main and agent cylinders at 63-mm distance between piston face and cylinder bottom, s is the shim thickness which determines the distance between piston face and cylinder bottom in the main cylinder at the end of the compression stroke, and the factor 1/9 is the ratio of the volume of the agent cylinder to the volume of the main cylinder. The number of propane molecules per cm 3 after compression is np (2.65)X i0 i9, where the subscript P denotes propane, and
np = (Pi,p/760) (63/s),
(2)
BRANCHED-CHAIN MECHANISM 0.2
l
]
o
&
&
&
I
I
&
I
467
I
A
-
:
-& 0 . 0 6 u_
o
0.04
a
o
0.03
I
i
500
600
9
300
400
o I
j
700
800
_
m
900
,,
1,000
TEMPERATURE (OK) FIG. 2. Fluorine/oxygen ratio versus temperature for explosion 3x .rod no explosion Q.
Pi,P being the partial pressure of propane before compression. Analogous expressions apply to oxygen and argon, whereas, for fluorine, nF2= [0.15p~/(9) (760)] (63/S).
(3)
The temperature T of adiabatic (isentropic) compression is related to the shim thickness s by the equation T = 298 (63/s)z-~°K.
(4)
The value of y is obtained from the standard entropy of 1 mole of gas mixture at the tentperature T, using
R In (63/s)u= (&,°-- $29a°),~ixt.~
=52fj(&, ° - &~s°)j,
(5)
J
fj being the mole fraction of componeut j. The entropies of the diatomic gases are obtained from
J A N A F tables, and the entropy of propane from the tables published by The American Petroleum Institute. When the agent gas is injected at the end of the compression stroke, it is compressed isentropically much above the pressure p in the main cylinder, before the spring-loaded valve yields and allows the gas to expand to the pressure p. This expansion is substantially isentropie, because the inieetion is extremely rapid, so that the heat loss to the walls of the passage between agent and main cylinders is small. The agent gas thus substantially attains a temperature corresponding to isentropie compression from p~ to p. However, the quantity of injected gas is rather small, the initial pressures p~ and pi are generally not widely different, and the fluorine/nitrogen mixture has a y-value close to the propane/oxygen/argon mixtures. The reported compression temperatures T are therefore based on the y-values of the mixture without agent injection, and the small systematic error inherent in this procedure is believed to be negligible.
TABLE I Test run showing inhibition of explosion by fluorine Test mixture, 5% CaH8+27.4% O2+67.6C/o Ar. Initial pressure p , Agent mixture, 15% F2q-85% N_< Inilial pressure pa. Shim thickness s = 32.6 ram. Run
1
2
3
4
5
p,., mm Hg pa, mm Itg Explosion
360 740 No
360 600 No
360 500 Yes
360 550 No
360 520 Yes
Experimental Data An example of a test run, showing inhibition of explosion above a critical limit of the ttuorine concentration, is presented in Table I. In this instance, no delayed ignition was observed in the negative runs. Reduced data are summarized in Table II. "No explosion" includes sudden large increases of the ignition lag. Figure 2 shows a plot of the ratios of concentrations of fluorine and oxygen EFd/EOd= nr2/no2, versus the temperature T for all data listed in the table. The points marked by tri-
O X I D A T I O N AND I G N I T I O N
468
T A B L E II Summary of data
T~ ° K
3OO 0.36 0.018 0.059 0.0099 0.168
3O0 1.22 0.066 0.217 0.0184 0.085
319 1.3 0.074 0. 243 0.0107 0.044
360 1.8 0.102 0.336 0.0148 0.044
360 1.8 0.102 0.336 0.0148 0.044 +(2) 35,40
360 1.4 0.078 0. 257 0.0144 0.056 + 35
36O 1.2 0.066 0.218 0.0145 0.067
36O 0.98 0.053 0.175 0.0146 0.084
4OO 1.55 0.085 0.282 0.0093 O.033
40O 1.55 0.085 0.282 0.0116 0.041
400 1.55 0.085 0.282 0.0140 0.050 + 16
400 1.55 0.085 0.282 0.0186 0.066 +(2) 7, 7
4O0 1.55 0.085 0.282 0.0186 0.066 + 23*
400 0.98 0.051 0.168 0.0192 0.114
4OO 0.70 0. 034 0.114 0.0192 0.167
470 0.94 0. 046 0.153 0.0256 0.167
475 2.4 0.118 0.390 0.0218 0. 056 + 7
478 2.4 0.118 0. 390 0. 0192 0. 049
475 2.4 0.118 0.390 0. 0163 0. 042
620 2.0 0. 098 0.326 0.0545 0.167 + (3) 2, 2, 2
62O 2.0 0.098 0.326 0.0436 0. 134 +(3) 2, 2, 1
620 2.0 0.098 0.326 0.0340 0. 104
68O 2.5 0. 122 0.410 0.0683 0. 167 +(3) l, 1, 1
680 2.5 0. 122 0.410 0. 0547 0. 134
68O 2.5 0. 122 0.410 0.0342 0,083
8O0 2.9 0.071 0. 492 0.0630 0. 128
810 4.0 0.196 0.650 0. 0548 0.184
-- (3)
760 2.5 0.061 0. 425 0.0544 0.128 --, +(2) 3, 4
800 2.9 0.071 0.492 0.0791 0.161
+(3)
760 2.5 0.061 0.410 0.0681 0. 167 +(3) 4, 1, 1
+ (3)
-- (3)
-
nF~/nO~
835 3.4 0.083 0.576 0.0935 0.161
835 3.4 0. 083 0.576 0.0465 0.081
88O 4.0 0.138 0.675 0.109 0.161 + (2),
1, 0, 1
1 (2)
800 4.5 0. 154 0.760 0. 109 0.161 +(3) 3, 1, 2
890 4.5 0. 154 0. 760 0.098 0. 129 +(2), -2, 3
9O0 4.9 0.168 0.828 0. 134 0. 161
Explosion r, msec
835 3.4 0.083 0.576 0.0740 0. 128 + (2), -2, 3
900 4.9 0.168 0. 828 0.107 0.129 + , -3
910 4.0 O. 098 0. 680 O. 874 O. 128 + (3) 3, 2, 2
910 4.0 0.098 0. 680 0.547 0. 081 +, -2
910 4.0 0. 098 0.680 0.0292 0.043
915 5.2 0. 179 0.878 0. 142 0. 160
915 5.2 0. 179 0.878 0.113 0. 129
915 5.2 O. 179 O. 878 0.0714 0.081
+(3)
+(3)
-- (3)
0, 1, 1
2, 4, 3
n np
nOe nF2 nF2/n02 Explosion T, msec T, °K n np
nO2 nF2
nF~/nO~ Explosion r,
msec
nF~/n02
475 2.4 0.118 0. 390 0. 0274 0. 070
Explosion r, msec
12
T, °K
n np
nO2 nF2
T, °K n np
nO2 nF2
nF2/n02 Explosion r, msec T, ° Z n np
nO2 nF2
+
+(3)
nF2/n02
910 4.0 O. 098 O. 680 O. 109 O. 161
Explosion r, msec
1, 1
T, °K /2p
nO2 nF2
3, 3, 1
+(2),
-
-- (3)
(2)
-- (3)
--
+
+
65
3 620 2.0 0. 098 0.326 0.0272 0.083
+ 2
(3)
1, 0, 0
+(3) 1, 1, 1
920 5.3 O. 183 O. 895 O. 145 O. 162 + (3) 2, O, 2
(2)
BRANCHED-CHAIN MECHANISM
469
TABLE II--Continued T, °K n np nO2 nF~
nF2/nO~ Explosion r, msec
920 5.3 0.183 0. 895 0.115 0.129 + (5), -5(avg.)
920 5.3 0.183 0. 895 0.0725 0.081 -- (4)
1000 6.8 0. 333 1.11 0.093 0.084 -~-(3) 1, 5, 6
* Glass wool inserted. n = (number of molecules per cm~)/2.65 X 10 ~9;Explosion or no explosion indicated by + and - ; P = propane; r =ignition lag in milliseconds; ~-(2) signifies ignition in two successive runs, etc.
angles correspond to ignition, viz., explosion, and the points marked by circles correspond to nonignition. I t is seen that a well-defined explosion region exists, and that the region is bounded by an upper and a lower limit of the ratio [F2]/[O~].
Chain-Branching Mechanism A lower and upper critical limit of the concentration ratio [F2]/[O2] corresponds to the scheme
and (2&/kb) (F2/02) = y,
the solution of Eqs. (6) and (7) is found to be
y= (x--1) (l-i-{1-- [2x/ (x-- l )2-1}112).
X = C l e x p [ - - (E1--E2nLEa--Eb)/RT-1
(11)
(12)
(1)
and
(',3HTd-F2 = chain breaking,
(2)
y = C ~ ( E F d / E O d ) e x p E - (E~--Eb)/RT].
(a)
CaHTOO-t- O2---~chain breaking,
(b)
which is based on chain branching by multiplication of propyl radicals C3H7, and permits chain breaking via Reaction (b) or Reaction (2) to become dom!nant at low or high ratios of the concentration of fluorine and oxygen, respectively. Introducing the rate coefficients /q, etc., and taking n=3, the explosion limit is defined by the equations
ki[O2] [C3HT]+k2EF2]EC3HT] = 3kJF2]EC3HTOO],
(6)
k~[-F2-1EC3HTOO]-~-kb[-O2]EC3H7OO'l =/~I[O2][C3H7].
(7)
Introducing the variables
2klk,/k2kb=x
(8)
(10)
This equation yields the curve of y versus x shown in Fig. 3. Substituting Arrhenius functions for the four rate coefficients, one may write
03H7-~- O2= C3H7OO,
C~H~OO+F2---~nC3HT,
(0)
x is a function of temperature only, whereas y contains the factor [-F2]/[-02-]. The experimental curve of [ F 2 ] / [ 0 2 ] versus T can be fitted to the theoretical curve by choosing C1= 23.7
El-- E2= 3800 cal/mole,
C2= 1.74
Ea-- E b = 2520 cal/mole.
The superposed experimental curve is indicated in Fig. 3 by dots and dashes; it is seen that a nearly perfect fit is obtained. Along the lower branch of the curve, the fit extends to somewhat above 700°K, whereas, at higher temperatures, the experimental curve dips down in agreement with the concept that the free radical generated in Reaction (b) becomes increasingly reactive, and hence Reaction (b) gradually ceases to be a chain-breaking reaction. The detailed course of Reactions (2), (a), and (b) caxx only be inferred from chemical and bond-energy considerations. Reaction (a) might
OXIDATION AND IGNITION
470
TEMPERATURE (°K) 35O
10
8 6
400
500
/
600
700
• CORRESPOND TO E X P E R I M E N T A L EXPLOSION L I M I T USING:
4
>-
X = 23.7.EXP (-1,280/RT)
",,%
k 2
1
3
Y = 1.74 ([F2]/(O21 ) EXP (2,520/RT)
I
I
I
I
I
5
6
7 X---.~
8
9
¥-- (x-11(1±
J
.
2X
1 (x_ 121.
FIG. 3.
be visualized to yield, for example, HF-[-COF-[CH3-I-CHsO, which is exothermic, and would be followed by COF-FC3Hs=HCOF-i-C3HT, or COF-[-F2= COF~Jr-F and F-~-CsHs= HF-[-C3HT; CHz+O2 = HCHO-}-OH and OH-[-C~Hs= H20-~ C3H7; and CH30-[-C~Hs=CH3OH-[-C~HT, unless the reaction CH30-}-O~=HCHO-[-HO~ occurs. This reaction is favored energetically because the energy released in the formation of a carbonyl bond and association of H and O2 is well in excess of the energy required to break a C-H bond. The radical HO2 is substantially inert at low temperatures, because the reactions with C~Hs or F2 to yield H202-}-CsH7 or H O O F + F are endothermic, and is thus captured by the vessel wall. The same consideration applies to other low-molecular peroxidic radicals; one may thus visualfze Reaction (2) to yield, for example, C2H4-FHF+CH2F, followed by deactivation of the radical CH2F due to association with 02 to yield CH2F(OO). For Reaction (b), no deactivating step other than the generation of HO2 can be proposed. There is substantial evidence that C3H~OO may decompose spontaneously to yield CH~CHO-FCH~O,2 which might be followed by CH~O-}-O2=HCHO-[-H02. How-
ever, in the present experiments the decomposition of C3H7OO cannot be significant, because it would eliminate the chain-branching Reaction (a), and an equivalent reaction between CHsO and F2 is not imaginable. Thus, Reaction (b) occurs in collisions between C3H~OO and 02, and since the reaction between a peroxyalkyl radical and oxygen is evidently a very significant step in the chain mechanism of hydrocarbon oxidation, but apparently has thus far not been known to occur, it merits special consideration. Peroxyalkyl Radical Reactions in Hydrocarbon Oxidation The present experiments extend to the temperature range (300°C) of Pease's2 propane/ oxygen data, which show that, toward very low molar ratios O~/C3Hs, 1 mole CH3OH is formed per mole C3Hs reacted, and that CH30H vanishes as the ratio O2/C3Hs is increased. This was thought to represent the sequence C3H7-~-O2--~ CsHTOO--~CH~CHO~-CH~O; CH30-~-C~Hs= CH3OH-~-C~HT; CH~O+O2--~other products. The
BRANCHED-CHAIN MECHANISM
471
change of concept imposed by Reaction (b) may be formulated as follows:
/ 0--0
0--0
I
\1
H3C--C--CH3~H3C--C H
CH3CHO+CH30
/
CH~
H
(b) CH3CHO+HCHO-!-HO2.
Using bond energies, in kc~/mole, C - - C = 82, C - - O = 8 2 , C - - H = 99, C = O = 175, H--O2=78.5, one finds the energy difference between the indicated C3H7OO isomers to be virtually zero; thus, a resonance equilibrium is plausible. The decomposition reaction is exothermic by about 58--RE, the resonance energy, and the 02-Reaction (b) is exothermic by about 131--RE. If OH-t-O were formed instead of HO2, the ~eaction energy would be about 33--RE. This suggests that the reaction would change from chain-br,eaking to chain-branching if the isomer on the right were stabilized, and RE would eorrespondingD drop out. It is generally accepted that the low-temperature chain-branching reaetion of alkane hy~trocarbons involves association of peroxy radicals with aldehydes, and it is now proposed that this association has the effect of stabilizing the structure. One may visualize that an ether-type linkage is formed as illustrated for CaHTOO+CHaCHO: O--O--CHa
I o2 H3C--C" • H . . C t I - * H C H O ~ 2 CH3CHO~OH-~O-~ ~,33 keal/mole. O
CH3
One hydrogen atom is taken to be unassigned and in a bonding role, which would reduce the estimated exothermicity somewhat. With ethers, the structure is preformed. No aldehyde accumulation is required for chain branching and no chain breaking occurs by reactions of peroxy radicals with 02 to yield HO2. This would explain the high propensity of ethers for low-temperature chain branching, including the peculiar violence of ether/oxygen explosions and the propagation of "cool-flame" waves at low temperatures and pressures and in mixtures containing less than 1% oxygen. For diethyl ether, the analogous reaction would be O--O CH3--C
O--O--OH CH~-~H--C.
H
O
CHa
• H ' . CH--~CH3CHO~-2
\ / O
REFERENCES 1. POLUEKTOV, V. A., PSHEZHr~TSKII, S. YA., AND
CHEREDNICHENKO,V. M.: Zhur. Fiz. Khim. 43, 1747 (1969).
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HCHO-I-OH~O.
CHa
2. PEASE, R. N.: J. Am. Chem. Soc. 57, 2298 (1935); ef. LEwis, B. AND ¥ON ELBE, G.: Combustion, Flames and Explosions of Gases, 2nd ed., p. 127, Academic Press, 1961.