Synthesis of Acetylene by Pyrolysis of Methane H. H. STORCH AND P. L. GOLDEN,Bureau of NE of the a u t h o r s has
0
summarized e l s e w h e r e (8) previous work on the pyrolysis of methane. Subsequent to that review the work of Kassel (6) and Storch (9) has led to a mechanism of the pyrolysis which may be summarized by the following series of reactions:
+
CHI e CH2 H2 CHz CHs CzHs CZH6 e Ci" Hn CZHI e C2Hz HI C2Hz + 2C Ht
+
++
+
Mines Experiment Station, Pittsburgh,
Pa.
A product has been obtained, containing about 10 per cent of unsaturated hydrocarbons (mainly acetylene), 10 per cent carbon monoxide, 35 per cent hydrogen, and 45per cent methane, bypassing a mixture of 7 5 p e r cent carbon dioxide and 25per cent methane through a tube at 1500" C. (time of contact being 0.03 to 0.04 second) and by subsequently scrubbing out the carbon dioxide. With somewhat longer times of contact the water gas content of the product increases at the expense of the methane, the percentage of unsaturates remaining approximately constant. Steam appears to be less desirable than carbon dioxide as a diluent because the loss of methane as carbon deposited in the reaction tube, which is only a f e w per cent when carbon dioxide is used, mounts to 15 or 20 per cent when steam is used.
tend to yield a s l i g h t l y high figure for the ethylene content. A second sample, after c a r b o n dioxide a n d oxygen r e m o v a l , was led directly into the fuming sulfuric acid, t h u s r e m o v i n g all of the unsaturates, the remainder of the sample b e i n g p a s s e d over c o p p e r oxide a t 300" C. to oxidize hydrogen and carbon monoxide. From these twosets of data were obtained two values for the carbon monoxide content, and single values for higher olefins, a c e t y l e n e , ethylene, and hydrogen. A slow combustion on the residual gas from thesecond sample, followed by measurement of t h e c o n t r a c t i o n a n d carbon dioxide formed, gave the methane content. The acetvlene fieures are likely to be somewhat high a t the expense 'bf the khylene, for the latter is appreciably soluble in cuprous ammonium chloride.
These reactions are all homogeneous except the decomposition of acetylene, which is largely heterogeneous. This m e c h a nism does not take account of the polymerizations which result i n t h e synthesis of b e n z e n e , naphthalene, etc., but it does represent a good approximation of the processes occurring in static systems, and in systems where the contact time is sufficiently short to avoid polymerization of ethylene and acetylene. The above mechanism indicates that, if one can remove the acetylene rapidly enough from the hot zone, a large conversion of methane to ethylene and acetylene might be attained. Storch (9) has shown that this is possible when operating a t low pressures in a carbon filament lamp immersed in liquid oxygen, but, in the static experiments which he made using baths a t 0" and -78" C., the product was largely carbon and hydrogen. It appeared desirable to conduct similar experiments using methane at atmospheric pressure in a flow system.
TABLEI. PYROLYSIS OF METHANE AT ATMOSPHERIC PRESSURE BY GRAPHITE ROD SURROUNDED B Y A WATER-COOLED METAL JACKET
EXPERIMENTS WITH ELECTRICALLY HEATEDGRAPHITEOR CARBONSURFACES
(25 cc. gaa apace and 2.5 sq. cm. heating surface) -COMPOSITION OF PRODUCTSTEMP. Gas VELOCITYCzHn CnH, H.0.a Hn C. Litets/hour -Per cent by oolumc-1.3 0.3 0.5 0.1 1300 212 1.2 4.8 0.9 0.0 1430 212 6.0 1.3 0.5 0.5 1425 212 10.2 212 1.7 0.7 0.2 1490 1.9 18.0 1545 212 2.7 0.5 13.3 2.3 1520 141 0.2 0.2 9.4 141 0.2 2.6 0.3 1600b 13.3 0.1 1.2 141 1.6 15OOc 36.3 3.2 2.0 0.3 160OC 70 Or Higher olefins. b Reactant was 80 per cent Nn 20 per cent CHI instead of pure CHg. C CH, preheated to 1000' C.
A series of experiments was made, using an electrically heated graphite rod surrounded by a water-cooled metal jacket, the methane being passed through the annulus between the graphite rod and the inner surface of the watercooled jacket. Results are given in Table I. The methane was prepared as described by Storch and Golden (IO). The products were analyzed by first absorbing carbon dioxide and oxygen, using potassium hydroxide and alkaline pyrogallol, respectively; the higher olefins were then absorbed in sulfuric acid of 1.6 specific gravity; subsequently the acetylene and carbon monoxide were absorbed by cuprous ammonium chloride, and the ethylene by fuming sulfuric acid containing small amounts of silver and nickel sulfates. Aromatic hydrocarbons were present in only very minute amounts in all of the gases analyzed, and no attempt was made to determine these separately. It is probable that the aromatics present in the gas samples were not removed by 1.6 specific gravity sulfuric acid or the ammoniacal copper chloride solution, but were absorbed in the fuming sulfuric acid and would thus
The acetylene and ethylene concentrations reported in Table I are little, if any, higher than can be obtained by passing methane a t high velocities through narrow-bore tubes. In order to discover the conditions under which the hot-cold tube set-up would €unction, a number of orienting tesbs was made, using an ordinary carbon filament lamp as the reaction vessel. The results obtained with the carbon filament lamp (operating with the bulb cooled by quiescent air a t room temperature in all except the last two experiments of Table 11)are about the same as those obtained in the graphite rod tests of Table I. Apparently not more than 2.5 to 3 per cent of unsaturates caLiexist in the steady state set up in passing pure methane at atmospheric pressure through a capillary at high velocity or over a surface of any kind a t 1500" C. It is only when very drastic cooling conditions are employed that the hot-cold tube idea is successful. Thus in the last experiment listed in Table 11, the walls of the carbon filament lamp were cooled to -78" C., and a total of 8 per cent unsaturates was ob-
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July, 1933
I N D U S T R I A L A h D E N G 1 N E E I1 I N G C H E M I S T R Y
tained in the off-gas, plus 0.6 gram of oil from 24 liters of methane. No carbon or tarry products were formed. The oil was straw colored; about 60 per cent of i t distilled below 130' C . and about 90 per cent helow 200' C. a t atmospheric pressure. This oil was highly unsaturated, as evidenced by rapid absorption of bromine.
769
obtained with the same apparatus, using methane-carbon dioxide, and methane-water mixtures. TABLEHI. QUARTZ TUBEEXYEHIMENT~ U s i ~ oPUKEMETHANE COXTACT Txxa
r ^ - A ~ ~ ~ ~ 8or 1 8Pnoooc-
GH,
CIA'
H.O.
H,
REM**=*
Table I11 shows that, when more than 2 to 3 per cent of uusat.urates is present (compare experiments at 0.03 with 1300 45.0 1.2 Faint log 0.02 second), the amount of hydrogen in the product indicates 1600 13.7 2.8 Deoidediog considerable polymerization. Carbon deposition is evi1500 4.0 3.6 1.1 0.3 20.3 Thieklog 1760 43.0 4.1 1.0 0.5 37.8 Thickfog denced by soot in the off-gas of the 0.03-second experiment. 15w 4.5 0.9 1.1 0.0 Bulb immersed The data of Table IV show the results obtained with "ll 1ram + aleohul methane-carbon dioxide mixtures. The last t a o colunrns of 24 1. CH. Table IV, containing figures on the percentages of metlrane converted to unsaturates and to carbon monoxide, were QUARTZ TUBEEXPERIMENTS calculated upon the assumptions that the amount of carbon The results obtained in Table I1 with the lamp cooled to deposition in the tube was small, that all of the carbon monox-78' C. are unfortunately of little significance so far as tlie ide originated from the reaction CN, + COz = 2C0 + 2H1, chemical industrial utilization of methane is concerned, owing and that the average number of carbon atoms in the higher to the relatively high cost of such cooling operations. The olefins (N.0.) is 4. The first of these assumptions will be practical alternative to such intensive cooling is the use of shown to be substantially valid when Table V is dkcussed. Concerning the second assumption, the only other reactions relatively very low pressures of methane. Previous workers ( 2 , 6) have shown that products con- which could give rise to carbon monoxide are: (1) reaction taining as high as 15 per cent a.eetylene may be obtained by of other hydrocarbons with carbon dioxide, but this would be stoiohiometrically equivalent to the assumed reaction; (2) operation a t about. 50 mm. pressnre and 1500" C. HzO, taking place in the colder portions The advantages inherent in low-pressure operation may COz NI = CO also be obtained by working vith methane diluted with some of the tube; and (3) COS C = 2C0. The second possihilit,y relatively inert gas which can be readily scruhbed out of the was found to be negiigible upon striking an oxygen balance product. Such a process (4)used 50 per cent methane and (Table V), whereas the third presupposes the deposition of 50 per cent hydrochloric acid a t 1400' to 1450' C. and con- carbon which, as has already been stated, is shown by the tact times of ahout 0.00015 second. About 7.5 per cent data of Table V to be a small percentage of the methane. In order to determine the amount of methane lost because acetylene w&s obtained in the product after removal of the hydrochloric acid by scrubbing with water. The use of of carbon deposition, and to check the loss of hydrogen steam or carbon dioxide instead of hydrochloric acid would, through the quartz walls, it w'&s necessary to measure the offlrand, appear undesirable because of nrobable reactions volume of the off-gas. To do this n-ith some degree of acto iorm carLou monoxide and hydrogen. i h e present authors curacy on a small gas sample, watcr must be avoided as the confining fluid because of the solubility f o u n d , however, that these reactions of the hydrocarbons and of the carbon were slow e n o u g h to make it posdioxide. I n obtaining the data of Table sible to obtain 8 to 10 per cent of acetyV, the carbon dioxide plus m e t h a n e lene plus ethylene in the product after was passed through an oil-filled gas r e m o v a l of the c a r b o n dioxide (or meter w h i c h h a d p r e v i o u s l y been water). The use of t h e s e d i l u e n t s equilibrated with the particular mixture has the additional advantage of makto be used, and t h e n c e t h r o u g h the ing it possible to secure a mixture of reaction tube. At the exit of the latt.er water g m and 8 to 10 per cent unsatu. a water-cooled condenser was placed to rates ii desired. The results in Tables cool the product and to condense any IV to VI show that the amonut of carwater or other condensable materials. bon monoxide may be varied from a few From the condenser the product passed per c,ent up to 37 per cent Trith 8 to 10 through a manually o p e r a t e d needle per cent of unsaturates present in most valve into a 4-liter e v a c u a t e d flask. cases. It was found possible to control the The data of Table I11 were obtained flow rate (as indicated by oil flowby passing pure methane through a meters placed just before the entrance 20-cm. hot zone in a 3.2-mm. i. d. to the reaction tube) quite accurately quartz tube, heated by an aliindum without much fluctuation in pressure furuace Found with platinum-rhodium. gradient. The latter was, h o w e v e r , The middle 10 em. of the hot zone were considerably different from that of the a t 1500' to 1550' C. as measured by experiments of Table IV; and, in order a thermocouple placed alongside the to obtain comparable concentrations of quartz tube. T h e g r a d i e n t i n t h e unsaturates, it was necessary to in&em. end p o r t i o n s of the hot zone crease the time of contact by about 25 was fairly steep, dropping from 1500' A WILDGAS WELL to 1300" C. a t the e n d s . The d a t a A conservative estimate of natural ssa wasted in per cent. The product was allowed to feet per day. stream into the flask until a pressure of of Tables IV to VI, i n c l u s i v e , were tiBe unitedstates iB one billion c.
L;iarqhow,
----per
ceni as dUmd0.5 0.0 4.2 0.8 0.0 10.1
+
+
+
I N D U S T R I A L A N D E N G I 3 E E R I h-G C H E M I S T R Y
770
Vol. 25, No.
TABLEIV. QCARTZ TUBEEXPERIMESTS USISQ METHASE-CARBON DIOXIDEMIXTURES CONTACT EXPT. TIME CONSTITUENT Second 1 0.03 Reactants Product COa-free product 2 0.03 Reactants Product Cor-free product 3 0.04 Reactants Product Cor-free product 4 0.03 Reactants Product COz-free product Reactanta 5 0.04 Product COz-free product '
CO? 67.0 59.5
*.
75.0 68.5
..
75.0 57.6
..
90.0 84.1
.H.O.
...
Gas COMPOSITIOS CzHn CpHi CO Per cent by volume
... 13.8 36.5
2.2 7.0
0.3 0.9
2.8 8.9
9.3 29.5
4.0 9.4
0.3 0.7
11.9 25.1
18.8 44.4
...
...
0.1 0.3 0.1 0.2
...
...
33.0 21.1 49.6 25.0 16.6 53.4 25.0
...
2.4 5.9
2.8 6.9
...
...
...
...
...
...
.
I
...
...
..
0.2 1.2
1.2 7.5
0.2 1.2
3.6 22.5
4.2 26.8
... 0.1 0.6
...
...
...
...
1.8 10.8
0.1 0.6
6.2 37.1
5.0 30.0
3:i[
23.3
4.3 4.3
2:6\
22.7
6.0
40.2
26.8
30.0
lj.2
39.0
28.7
7.8
8.2
...1
1;:; 10.0 6.5 40.8 10.0 3.5 20.9
...
.
90.0 83.3
..
TOTALU s - CH4 CONVERTED TO: BATURATES Unsaturates CO % % ..
CH,
0.3 0.7
0.1 0.2
...
...
...
.
Hz
1i::l
i:i/ ...1
9.9
2.0. 12.0)
DIOXIDE MIXTURES TABLE v. QUARTZTUBEEXPERIVESTS USINQ METHANE-CARBON CONTACT
EXPT. TIXE Second 1 0.0375 2 3
4
0.05 0,0375 0.05
TOTAL GAS CONSTITUEXT VOLUMESCOz H . O .
cc.
Reactants Products COz-free products Reactants Products COz-free products Reactants Products COa-free Droducta Reactants Products COS-free products
2835 3080
75.0 68.0
2835 3125
75.0 65.4
2835 2970
90.0 85.7
..
.. ..
2835 2948
..
..
.. ..
90.0 83.3
..
. . . . . . . . . . . .
0.2 0.6
-
Gas COMPOSITION CXHX CaHd CO Ht Per cent by nolume 2.9 9.1
0.1 0.3
3.3 10.3
CHI
li:3 35.3
. . . . . . . . . . . . . . .
0.1 0 3
2.8 8.1
0.3 0.9
6.1 17.5
14.5 42.0
0.0 0.0
1.05 7.3
0.1 0.7
2.6 18.2
4.3 30.1
. . . . . . . . . . . . . . .
. . . . . . . . . . . . . .
0.1 0.6
1.7 10.2
0.1 0.6
5.7
33.1
TOTAL UNSATU-
4.8 28.7
-
RATES
25.0 14.2 45.4 25.0 10.8 31.2 10.0 6.25 43.7 10.0 4.3 25.8
CHI CONVERTED TO: Unsaturates CO
BALANCES O F Gas C Hz Oz cc. cc. cc.
70
70
3:2[
+5
-77
4- 6
29.5
7.2
3:iI 9.3
-55
-172
+ 9
32.4
15.0
+45
-30
+28
23.3
13.2
+25
-104
-12
36.1
24.9
10.0
...
)
1.15
8.0(
...) l L ; {
VI. QUARTZ TUBEEXPERIMES-TS GSISQ METHANE-STEAM MIXTURES TABLE CONTACT
EXPT. TIME Second 1 0.03 2
0.03
3
0.04
4
0.03
5
0.04
CONSTITUENT Reactants HaO-free product Reactants HIO-free product Reactants HXO-free product Reactants HIO-free product Reactante HIO-free product
HIO 68.0
.. ..
77.0 77.0
..
92.0
,.
92.0
,.
H.O.
CzHz
GABCOMPOSITION ClHI CO1 CO Per cent by volume
. . . . . . . . . . . .
0.3
7.4
1.0
0.1
2:4
40:9
0.3
7.0
1.0
0.8
4:l
45:2
. . . . . . . . . . . .
. . . . . . . . . . . . . . 0.8 7.5 0.2 7.4 0.6 . . . . . . . . . . . .
59:4
0.2
4.5
0.6
1.4
l2:4
5-:4
0.2
6.3
0.5
3.2
9:8
60:9
. . . . . . . . . . . .
about 50 cm. was built up; any attempt to fill the flask much beyond 50 cm. resulted in erratic control of the flow rate. From the final pressure and temperature in the receiving flask, and the gas-meter reading combined with an analysis of the product, balances for the carbon, hydrogen, and oxygen were made. All volumes are expressed in cubic centimeters measured a t 28" C. All of the hydrogen balances of Table V are negative, indicating appreciable loss of this material by diffusion through the hot quartz walls. Most of the carbon balances are positive to a small extent, and, taking into account possible errors of measurement, the loss of carbon by deposition on the walls is probably in no case greater than a few per cent of the total methane used. The oxygen balances are all of low positive value (except experiment 4,where it is slightly negative), indicating that little, if any, water was formed by reduction of carbon dioxide. Table VI presents some results using methane-steam mixtures. For introducing the steam, the saturator described by Hawk, Golden, Storch, and Fieldner (S) was used. In all cases (except experiment 4)there is a large excess of hydrogen in the product, indicating considerable polymerization and carbon deposition. If one assumes that the hydrogen leakage was of the same order of magnitude as in the case of methanecarbon dioxide mixtures, the percentage of methane decomposed to carbon and polymers becomes two to three times
TOTAL CH, CONVERTED . UNSATUUnsatu- CO & rates COz C TO:
Ha
CHI
32.0 52.0 23.0 41.6 23.0 24.1 8.0 29.5 8.0 19.3
RATES
...
-
8.7
...
8.3
...
%
%
%
23:5
3:3
5:4
+'S8'.2
2k:4
7;3
5:9
+"7.9
30:5
15:O
5.3
20:3
2i:4
7.0
2817
2i:9
8.2
...
...
HYDROGEN BALANCE
%
10:8
..
+'l'i.9
...
1
-
715
+'7.6
1.7
the figures gii-en in Table VI, and experiment 4 would also show a decided loss of methane to carbon. The figures recorded in Table VI for percentage of methane converted to unsaturates and to carbon oxides would drop appreciably when the loss of hydrogen by leakage is taken into account. It appears therefore that steam is not as desirable a diluent as carbon dioxide.
POSSIBLE ISDUSTRIAL APPLICATION OF PYROLYSIS OF hfETH.4NE-CARBOK
DIOXIDE hIIXTURES
Rherever natural gas is available in large quantities a t a very low cost, such as about 2 cents per 1000 cubic feet, it seems feasible to consider the production of acetylene by the pyrolysis of carbon dioxidemethane mixtures. Temperatures of 1900" to 1600" C. may be obtained by the use of preheated compressed air and preheated natural gas. For refractory materials one is limited in choice to either alundum or silicon carbide. Either of these materials in the shape of a checkerwork heat exchanger could be heated to 1600' C. by a blast of compressed air and natural gas, and then the air shut off and a blast of carbon dioxide and methane substituted in proper quantity so as to obtain a contact time of 0.03 t o 0.04 second. When the temperature had dropped to 1450' C., the air-natural gas blast would be repeated, etc. Such a cyclic process seems preferable to continuous heating of refractory tubes.
July, 1933
I N D U S T R I A L A N D E N G I N E E R I N G C H E M I ST K Y
The carbon dioxide in the product could be scrubbed out with triethanolamine and recovered. Losses of carbon dioxide due to carbon monoxide formation could be readily made up from the flue gases of the heating operation. In this fashion a gas containing about 10 per cent of unsaturates (mainly acetylene), 10 per cent carbon monoxide, 35 per cent hydrogen, and 45 per cent methane could be obtained. The acetylene can be profitably converted into a variety of products for which there are large markets. Thus it is possible to make acetaldehyde, acetic acid, benzene, chloroprene, etc. Additional outlets for acetylene could be provided by hydrogenating it to ethylene. The latter could, for example, be used as the raw material for the production of ethyl alcohol, formaldehyde, ethylene glycol, and/or lubricating oil. By using a longer time of contact of the carbon dioxidemethane mixture, a product can be obtained which contains little or no methane, being mainly a mixture of hydrogen, carbon monoxide, and acetylene. After removal of the acetylene by an appropriate absorption medium, the remainder of the gas could be used for the production of methanol and higher alcohols, or for the production of synthetic motor fuel
771
by passage over the catalyst's developed hy Fixher (1) and by Smith ( 7 ) . The influence of dilution with carbon dioxide on the pyrolysis of hydrocarbons such as ethane, propane, butane, etc., is at present under investigation in this laboratory. LITERATURE CITED (1) E'i,*cher a n d Peters, Brennst0.f-Chem., 12, 286 (1931). (21 Fischer a n d Pichler, Ibid., 13, 381 (1932). ( 3 ) H a v k , Golden, Storch, a n d Fieldner, IBD. ESG. CHEW, 24, 28 (1932). (4) Imperical Chem. I n d . a n d D. Binnie, English P a t e n t 343,881 (SO\,. 1 2 , 1929). (5, Kassel, J . Am. Chem. Soc., 54, 3949 (1932). (6) R u d d e r and Biedermann, BuU. SOC. chim., 47, 710 (1930). EXQ.CHEM.,20, 1341 (1928). (7) S m i t h , H a w k , a n d Reynolds, IND. ( 8 ) Storch, B u r . Mines, Circ. 6549 (1932). (9) Storch, J . d m . Chem. Soc., 54, 4188 (1932). (101 Storch and Golden, I h i d . , 54, 4662 (1932).
RECEIVED January 7, 1933. Presented before the Division of Industrial and Engineering Chemistry a t the 85th Meeting of the American Chemical Society, Washington, D . C . , March 26 t o 31, 1933. Published hy permission of the Director, U . S. Bureau of Mines. (Not subject to copyright.)
Explosive Properties of Propylene Dichloride-Air Mixtures G.
D
w. JOKES,Mr. E. MILLER,AND H. SEARIAX, Bureau of %lines Experiment Station, Pittsburgh, P a .
U R I S G the past few years many new combustible
compounds have been marketed by manufacturers for various uses. Propylene dichloride, heretofore a rather expensive compound, may now be manufactured from cheaper sources of propylene at a cost low enough to compete with other substances as a solvent for oils and greases. This makes it desirable that the fire and explosion hazard attending its use be ascertained. The empiric formula of propylene dichloride is C3H6C12 which is the same as that representing the four known dichloropropanes with which it is isometric. The constitutional formula of propylene dichloride is CH3 ('HCI CH2C1, and this same structural formula represents that one of the dichloropropanes that is also styled dichloroethylmethane. The four dichloropropanes differ from each other in their fundamental characteristics and particularly in their boiling points. The propylene dichloride now being produced commercially has a boiling point of 96.8" C. In order that this material may be used safely, it mas decided to determine its limits of inflammability when mixed with various proportions of air, the pressures which might be developed when these mixtures are ignited in a closed chamber, and how easily the mixtures are ignited.
PURITY OF PROPYLENE DICHLORIDE The propylene dichloride, supplied by the ('arbide and Carbon Chemicals Corporation, conformed to the following specifications : Specific gravity, 20°/200 Weight per gallon. Ib. Acidity, yo Boiling range: Below 93' (199.4' F.) Below 99' C. (210.2° F.i Above 103' C. (217.4' F.) Water
1.159
9.64 Not more than 0.005 Sone Not less than 95 None Substantially anhydrous
Tests were made with the propylene dichloride as received after it had been treated with anhydrous sodium sulfate to remove any traces of water which might be present.
TESTAPPARATUS The lower limit of inflammability in air was determined in an 8-liter closed cylindrical bomb. The bomb was 4 inches in diameter and 38 inches long, and mas mounted in a vertical direction. Flames were initiated near the bottom of the bomb and propagated in an upward direction. This bomb is now used as standard equipment for the determination of the limits of inflammabihy of gases and vapors. The bomb was designed and used in an investigation to determine the explosive properties of acetone-air mixtures ( 2 ) . Reference is made to this report for details of operation and methods used in the preparation and analysis of the vapor-air mixtures. LOWERIKFLAMJIABLE LIhlIT
BKD PRESSURES
DEVELOPED
Table I gives the results of tests made to determine the lower inflammable limit of propylene dichloride vapor in air and the pressures developed in the closed bomb. The lower limit was found to be 3.40 per cent by volume in air. The pressures developed varied from 38 pounds per square inch (above atmospheric) at the lower limit to 67 pounds at the maximum, when 5.02 per cent propylene dichloride was present. The saturation point of propylene dichloride in air at the laboratory temperature was reached when the amounts present equaled 5.63 per cent and the pressure fell to 61 pounds. Calculation shous that the ideal mixture of propylene dichloride in air, which contains just sufficient combustible to consume all the oxygen present and give complete combustion products-carbon dioxide, water vapor, and hydrochloric
