Producing Acetylene in a Plasma Jet
I
H A N S W. LEUTNER and CHARLES S. STOKES The Research Institute of Temple University Philadelphia 44, Pa.
’
Methane can be converted to acetylene at 8070 yields in an argon plasma
THE
PREPARATION of high-melting nitrides such as titanium nitride (TiN) and magnesium nitride (Mg3N2) from the elements and also endothermic compounds such as cyanogen (CN)z has been accomplished in this laboratory, by using a plasma-jet apparatus (5). In view of the fact that endothermic cyanogen was obtained, a program was initiated to produce another endothermic compound, namely, acetylene, by using the same apparatus. Three different methods were tried to produce acetylene:
Feeding powdered carbon into a hydrogen-plasma jet. 8 Using a methane-plasma jet. Feeding methane into the “flame” of an argon-plasma jet. 0
Of the three methods, the third proved to be most successful. The jet consisted of a I/g-inch diameter, 2% thoriated-tungsten cathode and a water-cooled copper anode (4, 5). Production of Acetylene by Feeding Powdered Carbon into a Hydrogen Jet The operation of a pure hydrogen plasma jet was not possible with the existing equipment. Using a 1 to 3 mixture of hydrogen and argon, it was possible to operate without any major redesign. A graphite insert (l/8 inch thick) in the anode served as thc source of carbon. The graphite slowly sublimed and reacted with the hydrogen. The amount of graphite consumed could be easily determined by loss of weight of the graphite insert.
At a gas flow of 6 to 10 liters per minute of 1 to 3 hydrogen-argon mixture, a very bright stable plasma jet of about 4 cm. in length was obtained. After 2 to 5 minutes, the jet became unstable owing to the loss of graphite from the anode insert, which causes an increase in diameter of the annular jet exit. Table I gives the electrical characteristics of the jet during several runs. The jet was quenched in a watercooled copper chamber; it also filtered out any soot formed. The gases were then passed through four dry-ice traps connected in series and the remaining gases were collected in a gas holder. Besides large quantities of hydrogen and argon, acetylene and a small amount
of methane were found in the reaction products (Table 11). Other hydrocarbons were not found-for example, benzene, which was expected frcm the cyclization of acetylene. Most of the consumed graphite-60 to 807,---was collected in the chamber as extremely fine soot. Probably the quenching of the acetylene was not fast enough, so decomposition of some acetylene into the elements occurred and also some of the soot collected was unreacted carbon. I t was not possible to increase the yield of acetylene by adding to the jet very fine carbon powder in stoichiometric ratio to the plasma mixture. (The carbon powder was National special spectroscopic graphite powder, grade SP-2, National Carbon Co., size: 5-25 microns.) The determination of acetylene was made by gas analysis and by precipitation of copper-I-acetylide, followed by I
Keep your eye on plasma jet processing. With lower power costs, this method could become economical for many reactions, such as: b Manufacturing hydrogen cyanide
b Fixation of nitrogen
b Manufacturing acetylene b Preparing metal nitrides b Manufacturing cyanogen b Reduction of metal oxides
to metal powders
b Preparing thiophosphonitrilic
compounds
VOL. 53,
NO. 5
0
M A Y 1961
341
Table 111. These Are Experimental Conditions and Results of a Typical Experiment Argon arc characteristics Argon flow Reaction time Duration of experiment Total methane input
570 amp. 12 volts 6.84 kw. 8.9 l./min. 0 . 5 msec. 2.67 min.
Total carbon input
2.f925g. (26mg. from graphite insert)
4.4175 1. (2.3665 g. of C)
CARBONBALANCE G. of C % 1.937 = 80.1 as CzHz
METHODOF ANALYSIS Iodometric over CUPCZ
0.1710 =
5 . 7 as Soot 7 . 1 as COP
Gravometric Volumetric
0.1710 =
7.1 as CHa __
Reaction with
0.1395 =
100.0
In this apparatus, methane can b e f e d into an argon plasma through a watercooled annulus
titration with iVa&03 solution ( 6 ) . The 1.5% COZ formed was probably due to a small leak in the apparatus (oxygen in air). Production of Acetylene by Using a Methane-Plasma Jet By replacing hydrogen with methane the following reaction may be expected: 2CH4
+CzHz
+ 3H2 -95.54
kcal.
Methane-argon mixtures of proportions from 1 : 4 to 1 : 30 were tried. However, these mixtures melted the tungsten cathode instantaneously. Further experiments were considered useless with the existing jet design. Production of Acetylene by Feeding Methane into the “Flame” of an Argon-Plasma Jet Because the methane-argon jet was unsuccessful, the introduction of methane into the “flame” of an argon-plasma jet
Table 1.
Ar Flow, Flow, L./
L./Min.
Min.
Amp.
2.0 2.0 2.0 1.5 1.5
3.0 3.0 3.0 4.5 4.5
550 450 300 500 450
342
Table II. These Are the Reaction Conditions and Analytical Results for Two Typical Experiments With Without
Electrical Characteristics of the Hydrogen Jet
Approximate electrode separation, 3.2 mm.
H2
was adopted as an alternate solution. This was accomplished by adding a water-cooled methane feeding annulus (see the figure) to the anode of the plasma jet where the methane was introduced a t an angle of 90” to the argon jet. By using this system, it was possible to convert nearly all the methane in the acetylene and hydrogen (Table 111). The residence time of the methane in the argon plasma was about 0.5 msec. The formation of soot was very small and less than lOy0 of original methane remained unreacted. The freeze-out trap system previously described was used in these experiments. The temperature of the methane-argon mixture, where the best conversion was obtained, was not determined. However, the average calculated temperature of the argon plasma jet used in this case was approximately 12000° K, ( 5 ) .
Volts 21 23 30
14 20
Kw. 11.55 10.35 9.00 7.00 9.00
HP: 1 . 0 l/min. Ar :3.5 l/min.
Carbon Powder 1 . 0 l/min. 3.5 l/min.
134.4 mg. 2 min.
844.0 mg. 2.5 min.
Carbon Powder
Gas flow Carbon consumed Reaction time Carbon
INDUSTRIAL A N D ENGINEERING CHEMISTRY
WOW CUO
Discussion of Results By using a plasma jet it is possible to convert methane nearly completely into acetylene. T h e acetylene produced is easily separated from the argon carrier and the hydrogen, produced as a byproduct of the reaction. The only product thermodynamically possible, in the experiments described here, is acetylene. This can be shown by calculating the heat balance and free energy of the various reactions possible (7, 2). Compared with the high-current arc process for acetylene production (12 to 14YG yield of CzHz), the method described here (plasma process) gives much higher yields (S07G). Compared with the presently used pyrolytic process (3) (15.7yo conversion of methane) the yields are much higher and the separation of acetylene from the products of reaction is much easier. Acknowledgment The authors thank A. V. Grosse for his valuable discussions in carrying out this work and acknowledge the NATOOrganization for the grant of a fellowship. literature Cited (1) Baumann, P., Angew. Chem., B, 10,
257 (1948).
(2) Bergmann, D., “The Chemistry of
Acetylene and Related Compounds,” Interscience, New York, 1948. (3) Grjnenko, B. S., Khim. i Keknol. Toplzva 10, 18 (1956). (4) Stokes, C. S., Knipe, W. W.,IND. END.CHEM.52, 287 (1960). (5) Stokes, C. S., Knipe, W. W., Streng, L. A., J . Electro Chem. 107, 35 (1960). ( 6 ) Treadwell. F. P.. Hall. W. T., “Analytical Chkmistry;” Vol. 2, 9th ed., p. 695, Wiley, New York, 1951. I
,
RECEIVED for review November 4, 1960 ACCEPTED January 30, 1961
