US3347774A - Arc furnace for pyrolysis of hydrocarbon to acetylene - Google Patents

Description

Oct, 17, i967 A. F. MYI-:Rs

ARC FURNACE FOR PYROLYSIS OF HYDROCARBON TO ACETYLENE Filed June 24, 1964 V3,073,769, to Doukas, which describes United States Patent O 3,347,774 ARC FURNACE FR PYROLYSIS F HYDRGCARBUN T0 ACETYLENE Arthur F. Myers, Louisville, Ky., assigner to E. I. du

Pont de Nemours and Company, Wilmington, Del., a 5 corporation of Delaware Filed .lune 24, 1964, Ser. No. 377,640

8 Claims. (Cl. 2041-311) ABSTRACT or THE DrscLosUnE 10 This invention is directed to an improved electric arc furnace having significant utility for the pyrolysis of hydrocarbons, said furnace having an outer electrode having a copper or copper alloy base, said electrode having an integrally bonded refractory metal substantially covering the inner surface thereof.

In a furnace of this type, an electric arc is maintained between a central rod-shaped electrode and a surrounding or outer electrode, and this arc is electromagnetically rotatedabout the central electrode, whereby the arc travels in a path along the inner surface of the outer electrode. Such an arc furnace and its use to pyrolyze hydrocarbons to acetylene is described in U.S. Patent No.

operating conditions for minimizing the formation of carbon deposits on the central electrode during pyrolysis. Unfortunately, in the zone and just below where the arc strikes the outer electrode, carbon deposits continue to form. These deposits are dense, hard, and adherent to the electrode wall and their continual buildup have the deleterious effects of decreasing the arc length, arc voltage, power input, and etfectice cross section of the yarc furnace to thereby decrease the productivity of the pyrolysis reaction. Fluffy carbon deposits also form downstream from this zone and have similar adverse effects on furnace operation.

It is known to remove both the hard and the fluffy carbon deposits by intermittently contacting them with scraping devices.

Although the use of such scraping devices eliminates the shut-downs which would otherwise be necessary for removing the carbon by hand, experience with their continuous operation for long periods brings out the fact that the outer electrode, when made of copper, for good electrical conductivity, is eroded at a fairly rapid rate at the zone at which the arc strikes it. Thus. cooling water passages within it eventually become exposed resulting in undesirable leakage of the cooling water into the arc furnace. When this occurs, the furnace must be shut-down and the section of the outer electrode replaced. When more refractory metals than copper, such as molybdenum and tungsten and their alloys are used as the material of construction for the anode, it is found that although these metals are more resistant to arc erosion, the hard carbon deposits formed thereon are harder to remove with scraping devices than from copper and that the refractory metals tend to crack in use, again causing leakage of cooling water into the interior of the furnace. The iluify carbon deposits do not present any problem of removal lfrom the electrode surface by scraping devices.

It is an object of the presentY invention to provide an improved outer electrode, one which both resists arc erosion and enables .the hard carbon deposits to be removed without physicalv impairment of the electrode. Other objects will appear hereinafter.

These and other objects of the present invention will be better understood by reference to the following description and accompanying drawings in which:

FIG. 1 depicts a schematic vertical section of a portion of an arc furnace incorporating an improvedv outer electrode of the present invention;

FIG. 2 is a partial plan view taken along line 2 2 ofFIG.1;and Y y Y FIG. 3 depicts a partial plan view ofranother embodiment of an improved electrode according to the present invention. l e e The arc furnace shown in FIG. 1 is designated as 10. Such a furnace is also depicted in U.S. IPatent No. 3,073,769. Concentrically positioned Within this furnace are the central electrode 12 and outer cylindrical elect-rode 14, which in this instance are the cathode and anode, respectively. A concentric cooling passage 15 is provided between the anode and the outer wall 16 of the arc furnace; the cathode is also water cooled. An arc 17 is struck and maintained between these electrodes and is rotated by an electromagnetic field induced within the anode 14 by an electromagnet 18 which is positioned transverse to the furnace 10 as shown. Hydrocarbon material which is to be pyrolyzed to acetylene is fed into the furnace to flow downward between the electrodes 12 and 14 and through the rotating arc 17.

During such operation, a hard carbon deposit 20 forms along the path travelled by arc 17 over the inner surface of anode 14 in the shape of an annular ring. This hard carbon deposit is periodically dislodged by a scraper 22 which is only representative ofthe many suitable mechanical devices that might be employed. The particular scraper shown has a rectangular cross-section and is mounted upon a post 24 which drives the scraper 22 into the carbon deposit 20 in reciprocating and rotating fashion. Apparatus for driving scraper 22 and additional forms of Scrapers are disclosed in patent application U.S. Ser. No. 245,070, filed Dec. 17, 1962 to Doukas and Reed, now U.S. Patent No. 3,185,754. Removal of the deposit is obtained by the scraping action of a pair of chisel-like edges 26 formed along the uppermost edges of scraper 22. Further scraping can be obtained by the lateral edges 28 of the scraper, which edges lie proximate and parallel for a short distance to the inner surface of the anode. It is under such representative conditions that the difficulties with previously existing anodes Yhave occurred.

In the improved anode 14 shown in FIVG. l, these diflculties are overcome |by having its interior surface consisting of discontinuous plates 30 of an arc-erosion resistant metal mounted in electrically conducting -relationship onto an anode base or body 32 made of copper. Representative examples of arc-erosion resistant metals are such refractory metals as molybdenum, titanium, zirconium, tungsten, alloys thereof, or other metals which are refractory under the conditions of temperature and atmosphere present Within the arc furnace. The plates 30 are integrally bonded, as by brazing, to the copper base 32 of the anode 14 to maintain good electrical and thermal contact therewith. Techniques of obtaining such arbond are explained in the publication, Fansteel Metallurgy, published by the Fansteel Metallurgical Corp., July- August 1963 issue, pages 2 and 3. 1 L

The plates 30 are -to be provided at least beneath the zone of the anode 14 where the hard carbon deposit 20 forms. In the embodiment shown, the plates 30 are rectangular in shape and extend the entire length of the anode 14. The discontinuity of the plates 30 Ais best illustrated in FIG. 2. These plates may be in other shapes so long as they cover a major portion, and preferably more than of the anode surface in the zone struck by the arc. For example, the plates may be square and set in horizontal and vertical' rows or triangular or hexagonal and set in horizontal and diagonal rows. The thickness of the plates 30 is selected to provide sufficient protection 3,.'547,774k Patented Oct.l 17, 1967y to the anode against arc-erosion resistance without cracking as do the anodes made entirely of refractory metal. A thickness of about 1A@ of an inch has been satisfactory.

As shown in FIG. l, the anode 14 is positioned within the arc furnace 10 by threaded engagement at 34 with an upper, ared section 36 thereof. A lower section 38 is provided to abut via a gasket 40 the lower edge of the anode 14. This manner of positioning the anode 14 within the arc furnace is only representative, as obviously many alternative arrangements are possible.

FIG. 3 illustrates another embodiment of the present invention, in which instead of the discontinuous plates 36, the entire interior surface of the anode 14 is composed of a thickness of arc-erosion resistant metal 42 bonded in electrically conducting relationship with the copper base 32. In this embodiment, the arc-erosion resistant metal can be performed into a cylinder which is then integrally bonded to the underlying base 32. A shrink t between these elements is not adequate since, in addition to decreasing the electrical conductivity across the joint, the iit loosens upon repeated heating and cooling during operation of the furnace. The previous discussion on suitable metals for the anode surface, bonding technique, thickness, location, and positioning of the anode also apply for this embodiment.

In operation, the bimetallic anodes herein provided erode on the exposed face about 1/15 as fast as an all-copper anode. In addition, they are much less subject to failure by cracking, and the hard carbon formed on the exposed surface is surprisingly more easily removed than from an anode composed entirely of refractory metal. Hence, the useful life of anodes of the present invention is much greater.

The reason for the relative ease of carbon removal from the improved anodes of the present invention as compared to those of the prior art is not exactly known. It is hypothesized that the refractory metal surfaces of the improved anodes are maintained cooler by the good heat transfer characteristic of the copper base. In this respect and in the respect of anode life, the embodiment in which the refractory metal surface is discontinuous over the copper base has some advantages over the continuous covering. Namely, the discontinuous plates can usually be bonded more securely to the base than the refractory metal sleeve, with the better bond aiding in the transfer of heat between the arc-strike surface and copper base. The discontinuous plates are also better able to travel with the copper base as it expands and contracts to thereby resist cracking despite the high thermal gradients present both radially and longitudinally along the anode surface.

While copper is preferred for use as the base 32 upon which plates 30 or the continuous thickness 4Z of arcerosion resistant metal is integrally bonded, it is contemplated, and included within the term, that copper alloys may also be used for this purpose provided that the desirable properties of thermal and electrical conductivity and ductility are suiciently retained. The polarity of the electrodes can be reversed, and the present invention is still applicable. The present invention is not limited to any particular hydrocarbon material feedstock; some of the suitable feedstocks are methane and numbers 2 and 3 fuel oils.

As many widely different embodiments may be made without departing from the spirit and scope thereof, it

l is to be understood that this invention is not limited to the specific embodiments thereof except as defined in the appended claims.

lVhat is claimed is:

1. In an electric arc furnace for the pyrolysis of hydrocarbons to acetylene by passage of a hydrocarbon containing gas stream through an arc furnace including a reaction chamber, a generally cylindrical anode having an inner surface dening a reaction Zone in said chamber, a rod cathode extending coaxially into said cylinder, a source of electricity connected across said anode and said cathode causing an arc to span said cathode and said anode, means for rotating said arc electromagnetically so the path of said arc defines an axial contact zone at the inner surface of the said anode, means for feeding said gas stream through the reaction zone whereby acetylene is produced, and means for withdrawing said acetylene, the improvement wherein said anode is composed of a material selected from the group consisting of copper and copper alloys, the inner surface thereof exposed at said contact Zone being substantially composed of an integrally bonded refractory metal.

2. In the arc furnace as recited in claim 1 wherein said refractory metal is selected from the group consisting of molybdenum, titanium, zirconium, tungsten and alloys thereof.

3. In the arc f-urnace recited in claim 1 wherein said refractory metal forms a discontinuous cover for said base.

4. In the arc furnace as recited in claim 3 wherein at least about 90% of said exposed surface is composed of said cover.

5. In the arc furnace as recited in claim 1 wherein said refractory metal forms a substantially continuous cover for said base.

y6. In the arc furnace as recited in claim 1 and additionally, scraper means for removing hard carbon deposits which form on said exposed surface of said anode.

'7. In an electric arc furnace for the pyrolysis of hydrocarbons to acetylene by passage of a hydrocarbon containing gas stream through an arc furnace including a reaction chamber, a generally cylindrical anode having an inner surface defining a reaction zone in said chamber, a rod cathode extending coaxially into said cylinder, a source of electricity connected across said anode and said cathode causing an arc to span said cathode and said anode, means for rotating said arc electromagnetically so the path of said arc defines an axial contact zone at 4the inner surface of the said anode, means for feeding said gas stream through the reaction zone whereby acetylene is produced, and means for withdrawing said acetylene, the improvement wherein said anode is composed of copper and the inner surface thereof exposed at said contact zone being substantially composed of discontinuous plates of refractory metal integrally bonded to said anode.

8. In the arc furnace as recited in claim 7, wherein said refractory metal is molybdenum.

References Cited UNITED STATES PATENTS 65 R. K. MIHALEK, Assistant Examiner.

Claims (1)

1. IN AN ELECTRIC ARC FURNACE FOR THE PYROLYSIS OF HYDROCARBONS TO ACETYLENE BY PASSAGE OF A HYDROCARBON CONTAINING GAS STREAM THROUGH AN ARC FURNACE INCLUDING A REACTION CHAMBER, A GENERALLY CYLINDRICAL ANODE HAVING AN INNER SURFACE DEFINING A REACTION ZONE IN SAID CHAMBER, A ROD CATHODE EXTENDING COAXIALLY INTO SAID CYLINDER, A SOURCE OF ELECTRICITY CONNECTED ACROSS SAID ANODE AND SAID CATHODE CAUSING AN ARC TO SPAN SAID CATHODE AND SAID ANODE, MEANS FOR ROTATING SAID ARC ELECTROMAGNETICALLY SO THE PATH OF SAID ARC DEFINES AN AXIAL CONTACT ZONE AT THE INNER SURFACE OF THE SAID ANODE, MEANS FOR FEEDING SAID GAS STREAM THROUGH THE REACTION ZONE WHEREBY ACETYLENE IS PRODUCED, AND MEANS FOR WITHDRAWING SAID ACETYLENE, THE IMPROVEMENT WHEREIN SAID ANODE IS COMPOSED OF A MATERIAL SELECTED FROM THE GROUP CONSISTING OF COPPER AND COPPER ALLOYS, THE INNER SURFACE THEREOF EXPOSED AT SAID CONTACT ZONE BEING SUBSTANTIALLY COMPOSED OF AN INTEGRALLY BONDED REFRACTORY METAL.

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