US 3328276 A
Description (OCR text may contain errors)
June 27. 1967 H. SCHMIDT ET AL MFTHOD FOR THE PRODUCTION OF A DIRECT CURRENT ARC PLASMA BEAM SUITABLE FOR CRACKING REACTIONS Filed 'Dec. 12, 1963 United States Patent 3,328,276 METHOD FDR THE PRUDUCTION OF A DIRECT CURRENT ARC PLASMA BEAM SUITABLE FOR CRACKING REACTIONS Herbert Schmidt, Heinz Gladisch, and Walter Jahnentz, Marl, Germany, assignors to Chemische Wei-ire Huls Aktiengesellschaft, Marl, Germany, a corporation of Germany Filed Dec. 12, 1963, Ser. No. 330,179 Claims priority, application Germany, May 28, 1963, C 30,056 11 Claims. (Cl. 204170) By bringing a gas into contact with an electric arc of high energy concentration it is possible to heat the gas to a temperature of several thousand degrees whereby a substantial percentage of the gas molecules will be dissociated and the atoms ionized. Such gas is called plasma gas and can be employed as a so-called plasma beam in various fields of application.
Various methods are known for the production of a plasma beam. On pages 2 and 3 of Chen1.-Ing.-Techn., 35 (1963 there is described a process for the production of a plasma beam in which hydrogen is conducted through a polyphase are which burns between three carbon electrodes. On page 53 of Ind. Engng. Chem., Fundamentals l (1962) a plasma nozzle is described in which a direct current arc burns between a tungsten electrode and a water cooled copper electrode and in which the gas is introduced tangentially. The apparatus described on page 287 of Ind. Engng. Chem 52 (1960), generates a direct current are between two tungsten electrodes and the gas is introduced in the direction of the cathode. The use of a high-frequency alternating current of to 100 megacycles, generating a high-frequency torch discharge at a temperature of approximately 4,000 C. is disclosed on pages 232 to 235 of Elektro-Waerme 19 (1961).
Fields of application for a plasma beam are for example material testing, cutting and melting of metals and other substances, manufacture of materials having a high melting point such as titanium nitride and magnesium nitride, and also the manufacture of highly endothermic compounds such as cyanogen, acetylene and ethylene (Chem.-Ing.-Tech. 35 (1963)), pages 1 to 10; Ind. Engng. Chem. 53 (1961), pages 341 to 342.
When acetylene and ethylene are prepared by the dissociation of hydrocarbons, the plasma beam serves as heat carrier which transmits the energy to the hydrocarbon molecules to be dissociated. Therefore, the gas which is being transformed within the electric arc into a plasma beam is referred to as carrier gas. Normally, it will not take part in the reaction.
The dissociation of hydrocarbons by the plasma beam method is of importance from an engineering as Well as an industrial aspect because it generates a substantially lower amount of soot, resulting in a lower hydrocarbon consumption and greater ease in reconditioning of the dissociated gases than other methods which utilize electrical current as the energy source, for example the electrical arc method of Huels (Angew. Chem, 1948, page 257) or the Ediger method (Chim. et Ind, 79, 1958, pages 432 to 438). Furthermore, the plasma beam method permits a much better control of the course of the reaction by a selection of the thermal conditions. Hydrogen is the most suitable carrier gas because other gases will either take part in the reaction resulting in undesirable reaction products or are too costly, e.g. argon.
However, great difficulties are encountered if the plasma beam method is carried out by use of hydrogen as the carrier gas, impairing the economy and industrial usefulness of the dissociation of hydrocarbons. The great thermal conductivity of the hydrogen causes an extreme contraction of the discharge path of the electrical are, ex-
tremely high focal point temperatures and an excessive consumption of the electrodes.
Tungsten or molybdenum, materials with high melting points, could be used as electrodes in order to reduce consumption thereof but are not feasible for economic reasons because of the high price of these materials. If graphite or carbon electrodes are used the consumption will require complicated devices for compensation of the losses by forward movement of the electrodes or frequent replacements.
The employment of cooled electrodes will reduce the consumption only slightly but will lead to enormous thermal losses. If water is used as cooling agent, the thermal losses will reach 30 to 50% of the electrical are energy with the heat transfer at the cooled electrodes taking place partly by convection from the plasma gas, partly by the radiation of the arc, and partly directly through the starting points of the arc at the electrodes, that is the focal points.
Use of hydrogen will lead to further difi'iculties if standard 50 cycle alternating current is employed because the great thermal conductivity of the hydrogen will prevent at high voltages-and consequently wide spans between electrodes-a continuing arc discharge. For this reason it becomes necessary to utilize alternating current arcs at very high amperages, low voltages and extremely narrow gaps between electrodes. It is possible to attain a continuing burning of the alternating current are at industrially practical voltages by preionization of the hydrogen or by utilization of polyphase current but these measures are complicated and costly; furthermore, in this case it will not be possible to utilize a so-called vortex burner with tangential gas feed.
The above given explanation proves that in case of the known methods for the production of a plasma beam it is not possible to combine and realize simultaneously loW thermal losses and low electrode consumption "with a simple design of an electrical arc furnace.
It has been found that it is possible to overcome the difficulties existing heretofore and to produce a plasma beam which is particularly suitable industrially for cracking reactions by heating hydrogen by means of a direct current, vortexstabilized are burning between two cooled hollow electrodes by piping into the hollow electrode serving as cathode, especially at its initial cathode point, .a stream of gas consisting of a hydrogen-hydrocarbon mixture containing from 6 to 25 hydrogen atoms for each carbon atom, and by heating said mixture together with a hydrogen stream supplied through a vortex chamber within the direct current arc, with the ratio of the operational voltage, expressed in volts, to the operational current expressed in amperes, ranging between 5 and 15.
Any individual feature of the above described process standing alone is insufiicient to accomplish the object of the invention but in combination they will do so.
The modification of the known apparatus used for the production of a vortex-stabilized electric arc makes it possible to reduce the thermal losses by convection of plasma gas to the wall of the hollow electrodes because through the vortex of gas at the wall of the electrodes a narrow zone of low temperature is maintained while the radiant heat of the electric arc, its major portion burning within the vortex chamber, is transferred to the tangentially entering hydrogen and heats the hydrogen without any thermal losses. Therefore, heat is given up substantially only through the focal points of the electrodes.
The operation of the are by direct current makes it possible to increase the arc voltage and to lower the arc current to such an extent that a substantial reduction in the concentration of energy at the focal points and consequently lower temperatures are attained in comparison with known methods of identical capacities. The reduction in the concentration of energy at the focal points has the effect that thermal losses and electrode consumption will be reduced accordingly.
Since the focal point temperatures have a functional relationship to the amperage of the arc equal to a power of more than 1, the focal point temperature will be reduced, for example by a factor of more than the 4.5 if the amperage is lowered by a factor of 4.5 by a change of the voltage-current ratio from .5 to 10 without affecting the power output.
It was found that operations can be carried out in a particularly advantageous manner if the voltage-current ratio ranges from 5 to 15, and preferably between 8 and 12. If the voltage-current ratio is increased above 15, no further improvement is attained as to consumption of the electrodes and the higher voltage will require costly engineering features of the apparatus. Voltage-current ratios of less than 5 will lead to excessive consumption of the electrodes.
Due to the proper selection of the voltage-current ratio it is possible to reduce the consumption of the cooled, hollow electrode which is made of iron and serves as anode to such extent that the life time of such anode becomes acceptable for industrial purpose. However, it is a known fact that the consumption is substantially greater at the cathode side than at the anode side and additional measures are called for in order to protect the former. The invention makes it possibe to reduce also consumption at the cathode focal point to the extent that the life time of the cathode becomes acceptable for indusrial purposes by supplying the cathode area with hydrogen which is mixed with small quantities of hydrocarbons. Any gaseous or liquid hydrocarbons can be employed, and preferably the lower parafflnes such as methane, ethane and propane.
The employment of a carbon concentration greater than 1 C-atom to 6 H-atoms Within the area of the cathode focal point was found to be inappropriate because such concentrations will lead to deposits of graphite within the cathode which will interfere considerably with the operation of the arc. A carbon concentration of less than 1 C-atom to 25 H-atoms is also unsuitable because such low concentration fails to reduce the cathode consumption significantly. It was found that a concentration of 1 C-atom to 10 to H-toms is particularly advantageous. This value corresponds, for example, to a molar concentration of 15 to of methane or 7.5 to 12.5% of ethane.
The hydrocarbons can be introduced into the hollow electrode serving as cathode either in pure form or admixed with hydrogen, depending on the degree of dilution by the hydrogen entering the hollow electrode from the area of the vortex chamber. The introduction can be accomplished through apertures within the wall of the holow electrode, with the apertures being arranged in accordance with flow conditions within the hollow electrode and in such manner that the desired carbon concentration within the area of the cathode focal point is maintained with a minimum quantity of hydrocarbons being employed. A preferred method is the tangential introduction of the hydrocarbon, or hydrocarbon-hydrogen mixture, into the upper end of the electrode. Said tangential introduction must be accomplished in the same direction as the direction of rotation of the flow of hydrogen which arrives from the vortex chamber and stabilizes the elec tric arc.
Due to the low partial presure of the added hydrocarbons and their further dilution by the hydrogen introduced from the vortex chamber the splitting of the hydrocarbons can be accomplished almost without the formation of soot with the result that a hydrogen-plasma beam is produced which is practically free of soot and has a very low content of dissociated products.
If a certain soot content can be tolerated in the plasma gas it becomes possible to add the amount of hydrocarbon required for the C-concentration within the cathode to the carrier-hydrogen. By a suitable design of the hollow cathode-electrode it is possible to create a gas vortex within the electrode which will move hydrocarbons in sufficient quantity from the area of the vortex chamber to the cathode focal point.
The above described measures make is possible to utilize iron as material for the electrodes.
The life of the electrodes does not depend solely on the amount of consumption of the electrode material as such but also on the distribution of the consumption over the entire cooled surface of the electrodes.
It was found that a uniform distribution of the consumption can be attained if the starting points of the are are forced, in addition to the rotation at the inside of the hollow electrodes, into a fluctuating movement in longitudinal direction of the electrodes. This movement can be accomplished either by moving back and forth the entire arc while maintaining its overall length, or by periodic changes in the length of the arc and can be effected either by electric means, for example periodic voltage and/or current variations, or by engineering measures effecting the flow conditions, for example periodic changes in the quantity and/or velocity of rotation of the gas delivered to the are, changes which can be accomplished in the socalled vortex chamber. It is also possible to influence the length of the arc and/or to force the arc to move in longitudinal direction of the electrodes by an externally applied magnetic field which fluctuates periodically as to direction and/or magnitude. In the latter case it will become necessary to construct the apparatus for the production of the plasma beam wholly or in part of non-magnetic material. A measure of the above described kind will increase the life time of the electrodes at least by the factor 5.
In the following examples an electric arc furnace, diagrammatically illustrated in the accompanying drawings, is employed. Its significant components are: a cylindrical vortex chamber 1 made of iron, an insulator 2, a watercooled hollow electrode 3 made of iron and serving as cathode, and a water-cooled hollow electrode 4 made of iron and serving as anode. The drawing depicts the introduction of the hydrogen-hydrocarbon mixture tangentially and in the same direction as the direction of the vortex within the vortex chamber as set forth in the following Example 1(d).
Example 1 (a) Hydrogen at a speed of approximately 30 m./sec. is introduced into the vortex chamber 1 of the electric arc furnace tangentially to its perimeter and a direct current arc is ignited between the electrodes 3 and 4. The are is stabilized by the hydrogen vortex, and the hydrogen, transformed into a plasma gas, emerges through the hollow anode 4 in the form of a plasma beam. The are requires a power of kw., the operational voltage is 225 v. and the operational current 450 amp. Therefore, the voltage-current ratio is .5. After 30 seconds of operations the cathode is burned through and the arc has to be turned off. Due to the short time period of operation the amount of thermal losses can not be determined.
(b) In the same apparatus with hydrogen used as carrier gas a direct current are is ignited with the following electric factors: power 100 kw., operational voltage 1000 v., operational current 100 amp. The voltage-current ratio is now 10 in contrast to Example 1(a). The are can now be maintained for 5 minutes, instead of 30 seconds. After this time the water cooled iron cathode is burned through. Again, thermal losses could not be determined.
(c) In the same apparatus a direct current are, at 225 volts and 450 amps, the voltage-current ratio being .5, is ignited. A mixture of 30% of methane and 70% of hydrogen is introduced into the hollow cathode tangentially and in the same direction as the direction of the hydrogen vortex in the vortex chamber, the volume ratio between the hydrogen introduced into the vortex chamber and the mixture being :1. By the introduction of carbon atoms to the region of the cathode focal point the life of the cathode can be increased from 30 seconds to 2 hours. The cooling water of the electrodes consumes of the electric are energy.
(d) In the same apparatus a direct current are is ignited with an operating voltage of 1000 v. and an operating current of 100 amp., the voltage-current ratio therefor being 10. Hydrogen is utilized as carrier gas and a mixture of 30% of methane and 70% of hydrogen is introduced into the hollow cathode tangentially and in the same direction as the direction of the hydrogen vortex in the vortex chamber (volume-ratio hydrogen to mixture 10:1). This method of operation increases the arc operation from 5 minutes (of Example l(b)) to more than 100 hours. After an operation of 100 hours the hollow cathode is still not burned through. Upon disassembly an annular burned-out area was found inside the electrode, approximately 5 mm. in width and a maximum depth of 1 mm. Thermal losses due to cooling water are now limited to 8%.
Example 2 An electric arc is operated in the apparatus under the same conditions as set forth in Example 1(d). By periodic variations of J -10% in the quantity of hydrogen being introduced into the vortex chamber the arc is forced into a back-and-forth movement within the water-cooled portions of the electrodes. The frequency of the back-and forth movement is 1 per second. After 100 hours of operations the cathode is burned off throughout a length of approximately 50 mm. on the average to a depth of .1 to .15 mm.
Example 3 The electric power of the direct current arc furnace, illustrated in the drawing, is 100 kw. (1000 v., 100 amps) with a voltage-current ratio of 10. 23 m. (N.T.P.) of hydrogen are introduced per hour through line 5 and vortex ring 6 tangentially into the vortex chamber 1 of the plasma furnace to serve as carrier gas. For the purpose of carburizing the cathode focal point 2.3 m. (N.T.P.) of a 30% methane-hydrogen mixture per hour are introduced tangentially through line 7 into the cathode 3. Propane, the hydrocarbon to be cracked, is injected through line 8 into the hot plasma gases. The dissociated gases formed within the reaction chamber 9 are chilled by the water-cooled device 10. Under the above-given operating conditions a tail gas of the following composition is produced:
Volume percentage Acetylene 14.80 Higher acetylenes .87 Ethylene 4.93 Higher olefines .88 Methane 7.36 Higher paraffines 2.64 Hydrogen 68.52
The amount of soot is 1.5 g. of soot/kWh. The energy losses due to the cooling water are approximately 8.4%.
1. Method for the production of a plasma beam suitable for effecting cracking reactions which comprises passing a stream of a hydrogen-hydrocarbon gas mixture containing from 6 to 25 hydrogen atoms for each carbon atom through a hollow cooled cathode and through a direct current vortex-stabilized arc maintained between said cathode and a cooled anode and passing a stream of hydrogen through a vortex chamber surrounding said arc, the ratio of operational voltage, expressed in volts, to the operational current, expressed in amperes, of said arc ranging between 5 and 15.
2. Method according to claim 1 in which the hydrogenhydrocarbon mixture is introduced tangentially into the hollow electrode serving as cathode.
3. Method according to claim 2 in which the hydrogenhydrocarbon mixture is introduced into the cathode in direction of the stream of hydrogen which is introduced into the vortex chamber.
4. Method according to claim 1 in which the hydrocarbon is introduced directly into the cathode, and the hydrogen required for the preparation of the mixture is delivered to the cathode from the vortex chamber.
5. Method according to claim 1 in which the mixture of hydrogen and hydrocarbons contains between 10 and 15 H-atoms for each C-atom.
6. Method according to claim 1 in which the electric arc is moved periodicaly back and forth in longitudinal direction of the electrodes by periodic current changes.
7. Method according to claim 1 in which the electric arc is moved back and forth periodically in longitudinal direction of the electrodes by periodic variation of the quantity of the introduced gases.
8. Method according to claim 1 in which the electric arc is moved back and forth periodically in longitudinal direction of the electrodes by an external magnetic field which changes periodically its magnitude and/or its direction.
9. Method according to claim 1 in which water-cooled hollow electrodes made of iron are used for the production of the electric are.
10. Method according to claim 1 in which the electric arc is moved periodically back and forth in longitudinal direction of the electrodes by periodic voltage changes.
11. Method according to claim 1 in which the electric arc is moved back and forth periodically in longitudinal direction of the electrodes by periodic variation of the velocity of rotation of the introduced gases.
References Cited UNITED STATES PATENTS 3,119,758 1/1964 Orbach 204171 3,217,056 11/1965 Sennewald et al 204171 JOHN H. MACK, Primary Examiner.
R. K. MIHALEK, Assistant Examiner.