August, 1960
SHOCK TUBEEXPERIMENTS O N PYROLYSIS OF ETHAXE
occur is iiidicated by the observed decrease in [G(HcI,]C C I ~ a t lower CCl, concentrations. This quantity is less than the expected value for G(cI, a t approximately lScY, CC14 and becomes negative a t lower conrentrations. Our results were not extended to 1-ery low CC14 concentrations because of an increasing sensitivity of [G(HcI~] CClr to small errors in HCl determination. A conciw discussion of such interactions has been given by Ihirton and Lipskye8 Our experimental (SI AI. B u r t m a n d S. LipsLy, T H i s JOURNAL, 61, 14G1 (1957).
1025
results do not permit speculatioii c o n c ~ n i i i gthe nature of the energy transfer processes occurring in this particular system. They do, however, show a significant “protection” of CHCl, by CCL. A mechanism is suggested in which excited or ionized CHCls molecules, which in the presence only of other CHCL molecules would dissociate or react with adjacent molecules, may lose energy of exeitation to CC14. The energized CC14 molecules undergo energy dissipation without chemical change. This work has been supported by Atomic Energy Commission Grant KO.AT-(40-1)-2055.
SHOCK TUBE EXPERIMENTS O S THE PYROLYSIS OF ETHAN2 B Y GORDON B. SKINXER -4KD WILLIAM E. BALL A/ oraanlo Chcniical Company, Research and Engineering Division, Research Department, L)a?jton, Ohio Receloed February 1 1 , 1860
The kinetics of ethanc pyrolysis have been studied in the range 1057-141S°K. in a single-pulse shock tube. Mixturre. of methane, iethylene and hydrogen with ethane also have been studied, to determine their effect on the kinrtics. Thew results and literature data a t lower temperatures have been correlated by a modified Rice-Herzfeld free-radical mechanism, ahich accounts quite well for the observed effects.
Introduction The kinetics of ethane pyrolysis have been studied previously in either tubular flow or static bulb reactors. The earlier work, in the temperature range 824-973”K., has been summarized by Steacie’ who has also discussed current thinking on the mechanism of the reaction. An important paper by Davis and Williamson2 recently has appeared, describing experiments with a flow reactor a t 03i-1044”K. calculations based on a free-radical pyrolysis mechanism have recently been made by Snow, Peck and Von Fredersdorff.3 The present paper deals with shock tube experiments in the range 1057-1418°K. With data available over this wide temperature range, more can now be said about the reaction mechanism. Experimental Tlir shock tube drscribed in an earlier p a p d was usrd, nith idcnticd techniques and mrthods of calculation. E?perimental temperatures were corrected for variations in temperature due to minor pressure fluctuations during the runs, and for the heat effects due to chemical rraction. Vapor chromatographic analysrs were made for CHa, 1 1 2 , C2H6, C‘JL and CzH2 in all experiments, and for propane, prop) lcne, butane and 1 ,%butadiene in scveral CYpcrimrnts. Experiments W(Y e carrictl out with the gas mi\turrs listed in Tablr I. Total rcwtion prrswres were fivc atmospherrs, and dwcll timrs xtmut t IT-o milliseconds. Phillips reagent grade ethane, 1I:itheson C.P. methanc and etli) lrne, and .lirco hydrogen aiid argon were used without further purification. The gas mixtures were always analyzed bcfore reaction, and none of the products were ever found in the original mixtures. ( 1 ) E. W. R. Steacie, “Atomic a n d Free Radical Reactions,” 2nd ed.. Reinhold Puhl. Carp., New York, N. Y., 1954. (2) H. G. Davis a n d K. D. Williamson. Fifth World Petroleum Congress, Section I\’. Paper 4. 19.59. (3) R . H. Siiow, R . E. Peck and C. G . Von Fredersdorff. A.I.Ch.E. J., 6 , :304 (lS;?). (4) G . B. S l ~ i n n c rand It. A. Ruehril-cin, THIS. J O C R S A L , 63, 1736
(1959).
TABLE I REACTION MIXTURE COMPOSITIOKS Mixture
CPIIS
1 2 3
G 0.5
.5 .5 .5
4
5 G
ti
7 8
5.4 G
--Mole CZII4
.. .. .. .. .. ..
To of componentCHI I
.
.. ..
ir
.. ..
12
2.5 11.2 ti
0.ti G
..
0.6
..
..
-Ar
!I4 99 5 87 5 97 88.3 88 9.7 4 88
Results As is the case a t lower temperaturey the main products of ethane pyrolysis were found t o be ethylene and hydrogen in equal molar amounts. Small amounts of methane were found in most of the runs with Mixture 1, the ethylene/methane ratio averaging 21, with no noticeable temperature dependence. For Mixtures 2 and 3 no methane was found since the original ethane concentration was too lorn, and for Mixtures 4-6 any methane formed was obscured by what was already present. E’or Mixture 7 the ethylene/methane ratio averaged 20, arid for Mixture 8, the ratio was 10. Traces of n-butane ere found in a few runs with Mixt3ureI , the ethylriic/butane ratio averaging about 140. Some acetylene aiid 1,3-butadiene were foiiiid for JIixtures i and 8 at the higher teinperature5, but these seemed to be primarily ethylene pyrolysis products. Conversions were held to less than about 20% to minimize complications such as approach to equilibrium, decomposition of products, decrease of rate due to decreased reactant concentration, and excessive temperature drop due to heat of reaction. From the results for Mixtures 1 and 2 the reaction seemed to be first order, so the results were expressed a5 first-order rate constants in
GORDON B. SKISXERAND WILLIAME. BALL
1026
Ir0L 64
TABLE I1 R A T E CONSTANTS I N ETHAXE PYROLYSIS Each pair of numbers gives first lO4/T, OK., and then k, sec.-1 Mixture 1 : 0.46, 1.45; 9.38, 2.45; 9.33, 2.7; 9.15, 4 . 2 ; 8.99, 10.1; 8.76, 17.2; 8.50, 42; 8.25, 101; 8.20, 88 Mixture 2 : 9.02, 10.1; 8.91, 14.0; 8.79, 20.0; 8.53, 39; 8.45, 64; 8.42, 61; 8.02, 185 Mixture 3: 9 32, 6 3 : 9 31, 3 2; 9 31, 4 5 ; 9 12, 9 4; 8 99, 12 8; 8 95, 26; 8 94, 18; 8 90, 26; 8 68, 53; 8 55, 125; 8 51, 87 Mrt'urc f : 8.31, 9 . 4 ; 8.09, 18; 8.02, 27; 7.72, 5 2 ; 7.57, 74; 7. 46, 124 Mixture E : 8.19, 6 4: 7.97, 11.4; 7 . i 4 , 24; 7.30, 91; 7.27, 122; i . 0 5 , 214 AIixture C : 9.18, 2 . 6 ; 9.02, -4.6; 8.93, 11.1; 8.64, 20.6; 8.55, 20.6; 8.35, 41; 8.20, 54; 8.12, 7 2 K s t u r e 7 : 8.89, 12.6; 8 73, 21.5; 8.58, 38; 8.42, 51; 8.27, 72 IIisture I;: 9 . 1 5 , 3.7; 9.01, 8 . 4 ; S.!)O, 4 . 6 ; 8.66, 14.1; 8.63, 18.6; 8.40, 42; 8.36, 4.3; 8.18, ti5: 8.01. 83; ?.!)I, 100 : 7 86, 134
Table I[. The results are corrected for the decrease in amount of reactant as the reaction proceeded I a small correction qiiice the conversioiis were lov;) but iiot for any reverse reaction. The problem of approach to equilibrium is not nearly as serious 2 t these high temperatures as it is a t lower temperature;. I:or example, a t 1200°K. a sample of ethaile at 0.3 atmosphere initial pressure will be Inorc thaii 9S% decomposed a t equilibrium, under constant pre5sure conditions. Lea\t-quareb equations of the type log k =
u c +7 1
TABLE I11 Ls.wi-s~~-anss CONSTANTS FOR ETHASEI'YIEOLYSIS
c
1 2 3 4 5 6
13.65 12.00 l(i.8.i 11.40 11,x 12.2~1
I
11.10 11.38
-
8
D
14,170 12,850 17,390 12,520 13,400 1 2 , 7-40 11,260 I 1 , no0
D
TABLE IT' FREER A D I ~ . I LREACTIOXS 15 ETHANE PYROLY5IS log d , -1 or
SIC
Reactions
( 1 ) C?He+ 2CH:i
(2) CH,
+ CrH6
+
( 3 ) CZHj -+CrHi
were calculated from the data of Table 11. The values of C' and L> are given in Table 111. Since it is difficult, to visualize how the curves lie wit'h differing values of both C and D, column 4 of Table III s h o w the value of C if the average D value (13:290) is used for all the curves. In this colunin, a change of 1 in C corresponds to a factor of 10 in rate. These curves, of course, apply over the range of temperatures given in Table 11. Consideriiig t>hescatter of the dat,a and the limited temperature range covered, inore weight should be given to the ahsolute values of the rate const'ants than to tmheactivat,ion energies derived from the tempera-Lure dependence.
3 I ist 11 rc
calculated from the amounts of rebidual ethane in column 2 .
C if = 13,290
12.87 12.08 13.17 12.00 11.68 12.71 12.93 12.67
IBRl cslc. C if
D
= 13,290
13.03 12.73 12.84 12.00 11.81 12.84 12.97 12 60
The sxandard deviation of the points from the curves of Table 111, expressed in terms of log k , is 0.074, which corresponds to about 20% in k , or about 10" in temperature. Table T- summarizes results from runs with Mixture 2 at higher temperatures and conversions, times being 2 milliseconds as before. Actual product yields, uncorrected for the decrease in ethane concentration with time, are shown. Firstorder rat e coilstants for ethane disappearance were
+ CrH6 (6) H + CJls-t 17) H + CH:j (4j H
-c
CI-I,
+ GH;
cc. s m - 1
cal.
k"
1k 7 12.5 12 8 11 7 1-L 2 12 3 12 3 I2 11.I ii 14 114 4 13 6
79 300 -1,500 14,500 18,000 42,600 3,900 6,200 12,200 0 97,800 101,000 0
11.9 11.2 13.6
8,000 10,000 0
r
k I'
+H Ha + CrHj
k !'
h. r
CzHe H~ C;H* j + CH, (9) 2CyHfi+ C:Hio I + C2H4 CpHtir (12) H CHh-Hz CH.3
k
+
-+
r I'
k
+ +
+
+ GHj
tion
k r
C,Hy i i: CHI CIH~C k represents t,he forward rate constaiit,
(13) CHj
+
-L
+
r
the reverse
rate constant. 'YliBLE I'ROUUCT
1-
1)ISTRIBUTION IS R C S S \VfT11
11IXTLRE
2
AT
HIGHER TEMPERATERES, 2 NSEC.TIXE 10*(T, Moles produet/100 moles CzIIs originally present H2 O h . CzHs C2H4 CzFh CHa
7.54 7.31 7.13 6.91 6.39 6.10 5.74
37.4 19.1 12.4 6.8 3.4 1.7 1.3
54.5 62.8 60.1 57.9 40.6 31.1 9.2
3.6 9.6 16.4 24.7 44.5 54.0 76.2
4.5 8.3 11.6 li3.2 20.6 21.7 21.3
64.5 84.6 86.7 128.2 129.7 1-10.0 lti5.3
k, sec.''
490 830 1040 13-10 ItiOO 2040 2180
Discussion The relative rates of ethane dccompositioii can he estimated from column 4 of Table 111. The data for Mixtures 1 and 2 (ethane partial prei.*5ures 0.3 and 0.025 atmosphere) suggest a reaction order of one, or slightly less than one. Hydrogen iiicreases t'he rate of ethane decomposition slightly, while met,hane and et,hylene reduce the rate. These data, and the earlier data at loner temperatures, can be interpreted by a modified Rice-Herzfeld
Alugust, 1960
S H O C K T U B E EXPERIhlESTs O X PYROLYSIS O F
inechanisrr,, as discussed by Steacie, Davis and Williamsoii, and SnoJv-. The data of this paper permit mole accurate calculations of the individual free radical reaction rates, partly because of the extended I emperature range, and partly because the variouq compositions studied here emphasized certain rea ctioiis which were not important under previously studied conditions. Qualitative evidence for the free-radical chain inechanisni of ethane decomposition comes from v-hich 5,hoir.s that the apparent activation Table ITq energy falls off a t high conversions. At the highect teinpeiature of Table \-, the rate constants for (>thanepymlyhii are only a few per cent. of those calculated hy extrapolating the low-conversion rates of TJlile I1 or 111. I n a chain reaction the chain length nould be expected to shorten as the ethane di -appeared and was replaced by more stable product molecule
