A N ANALYTICAL APPROACH TO PLASMA T O R C H C H E M I S T R Y J O H N E. A N D E R S O N A N D L A U R A K . C A S E
SpPedu,ayLaboratories, Linde Co., Dioision of L-nion Carbide Corp., Indianapolis 24, Ind.
Available thermodynamic and kinetic data are applied to predict quantitatively the over-all results that are obtained when hot hydrogen from the plasma torch is mixed with methane and the mixture is subsequently quenched. The results of the analytical treatment are compared to experimental data and the agreement is very good. A better understanding of the critical variables in this high temperature process is obtained from the analysis.
WITHIN the last six years, there has been strong interesr in plasma torch chemistry. The plasma torch has stimulated the study of high temperature chemical reactions because of its utility as a research tool and its potential as a production unit. Possible a.pplications for plasma torch chemistry are the deccmposition of compounds to form the elements, the formation of endothermic compounds (either via a decomposition route or by a direct synthesis from the elements): and the formation of free radicals to be used as intermediates in subsequent reactions. An analytical approach to plasma torch chemistry is desirable in order to realize fully the potential of the torch for chemical reactions. .4n analytical study is seriously hampered both because experimental conditions are difficult to define precisely and because the amount of thermodynamic and kinetic data available a t very high temperatures is meager. Hoxvever, in some instances a reasonable and useful analysis can be made. I n this jpaper, available thermodynamic and kineric data are applied to predict quantitatively the over-all results when hot hydrogen from the plasma torch is mixed wirh methane and the mixture is subsequently quenched. This reaction has been chosen both because equilibrium and kinetic data are available for the over-all reactions occurring and because experimental data on this plasma torch chemical reaction have been obtained. The results of the analytical treatment are compared to the experimental data
-PLASMA
T O R C H t -
REACTOR*
QUENCHQUENCH
r WATER
1
I
Figure 1 . equipment
Schematic
INSULATED TUBE
diagram
-4 schematic diagram of the experimental apparatus is shown in Figure 1.
The equipment consisted of three parts: the plasma torch. the reactor. and the quench. An arc was struck between a 5-inch tungsten electrode and a '5-inch nozzle. Hydrogen flowed around the tungsten electrode and through the nozzle, where it was heated by the arc to average temperatures between 5400' and 18,000' R. The hot. hydrogen stream (containing atomic and molecular hydrogen) entered the reaction chamber, where it was mixed quickly with methane. T h e reaction chamber consisted of a 3,/4-inch I.D. insulated carbon tube between 2l/2 and 5 inches long. Holes for the methane feed were provided in the reaction chamber near the plasma torch nozzle. Sek era1 techniques for quickly mixing the plasma torch hydrogen stream and the methane feed were tried. The gas mixture leaving the reaction chamber was quenched by spraying water into the flowing gas mixture or passing the gas mixture through a water-cooled tube. For each test, measurements Lvere made to determine the inlet and outlet rates of all gas streams, the energy in the hydrogen stream leaving the plasma torch, the heat required for quenching, and the chemical constituents of the exit gas stream. Complete over-all material and heat balances were made. Discussion
The following terms are used in the discussion: Acetylene Yield. Per cent methane (on a carbon basis) converted to acetylene. Methane Conversion. Per cent methane that decomposes. Specific Energy Input for Methane. Energy availabk above 540' R (in the plasma torch hydrogen stream and in the form of methane preheat) per pound mole of methane feed.
General Background. In analyzing the experiment, the characteristics of the reaction of methane decomposition to form acetylene and the characteristics of the hydrogen stream leaving the plasma torch should be noted. The important over-al! reactions in the decomposition of methane are: 2CHa + CzH, 3H2 (1 1 CBHB + decomposition products (2) The decomposition of methane to form acetylene (Reaction 1) is a n endothermic reaction ( A H ~ ~ ~=o 162,000 R B.t.u. per pound mole of C B H J and high acetylene concentrations are favored at equilibrium a t high temperatures (Figure 2). Figures 2 to 6 Mere obtained from calculations using available
+
METHANE FEED
THORIATED T U N G S T E N C A T H ODE
WATER COOLED COPPER NOZZLE ANODE
Experimental Equipment
of
experimental
VOL.
1
NO. 3
JULY 1962
161
METHANE
CONCENTRATION
eo
z
E9 600 0
cn
40
E
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n
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A C E T Y LE N E CONCENTRATION
40-
W
(3
0
a
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g
20-
I800 2000 2200 2400 2600 2800 T E M P E R AT U RE
(O
20--
5000 6000 7000 0000 9000 10,000
3000 32 0
R)
TEMPERATURE ( O R )
Figure 2. Effect of temperature on volume per cent concentration of methane, hydrogen, and acetylene and the acetylene yield when thermodynamic equilibrium is established for reaction
Figure 3. Dissociation of molecular to atomic hydrogen a t equilibrium as a function of temperature
+ 3H2
2CH4 + CZHZ
thermodynamic data (2, 4 ) , extrapolated to higher temperatures when necessary. Although the reaction rates for Reactions 1 and 2 both increase with increasing temperature, the relative rate of Reaction 1 increases much faster with increasing temperature than that of Reaction 2. The plasma torch is an efficient and convenient method of heating hydrogen to high temperatures. When the temperature of the hydrogen stream is above 4500' R at atmospheric pressure, the decomposition of the molecular hydrogen to atomic hydrogen is appreciable; above 10,OOOoR it is almost complete (see Figure 3). Because the heat of decomposition is so high (above 180,000 B.t.u. per pound mole of Hz decomposed), the heat content of the hydrogen stream is increased severalfold in the presence of atomic hydrogen (see Figure 4). As a result the predominant portion of the energy in the plasma torch hydrogen stream is available a t high temperatures. This is illustrated in Figure 5, in which the portion of energy available in a hot gas stream above a given temperature level is plotted for a stoichiometric air-methane flame, a stoichiometric oxygen-methane flame, and a plasma torch hydrogen stream at 9900' R. Although the plasma torch stream is heated by an electric arc, the use of the stream as a heat source differs significantly from the direct contact of methane with the tlectric arc. Within the arc the temperature must remain above a given temperature (about 18,000' R) to remain sufficiently electrically conducting to maintain the arc. As a result, the arc acts as an infinite heat source to the methane supplying heat above the 18,000' R temperature level. The plasma torch hydrogen stream, however, is a finite source of heat which will cool as it is mixed with colder gas, and endothermic reactions take place within the stream. Analysis. Unfortunately, the analysis is complicated by complex and not well defined experimental conditions. The temperature of the plasma torch hydrogen stream entering the reaction chamber varies from about 36,000' R a t the center of the stream to as low as 540' R a t the edge. Unknown 162
I & E C PROCESS D E S I G N A N D DEVELOPMENT
temperature fluctuations exist within the reaction chamber. The mixing between the plasma torch stream and the methane feed is a t best not instantaneous or uniform. The contact time within the reactor is known only approximately and is not uniform for all of the gas. The quenching time is not instantaneous. Only the over-all conditions of the inlet and outlet streams are known with accuracy. Thermodynamic Considerations. Some information can be obtained through the use of energy balances and equilibrium considerations. In Figure 6 the energy requirement for making a pound mole of acetylene is plotted against temperature for various yields of acetylene from methane. The yield curves were obtained by calculating the heat required to bring the methane up to the temperature considered plus the heat of
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200-
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D D II S SS SO OC C II A AT T II O ON N
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Figure 4. hydrogen
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Effect of
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TEMPERATURE
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io,oo(
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Measured above 540' R
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