Method and Device for Hydrogen Sulfide Dissociation in Electric Arc

Abstract

Device for hydrogen sulfide plasma dissociation includes a plasma chemical reactor including an arc plasma generator that has a cathode and an anode; the anode having a working surface for contacting hydrogen sulfide plasma, wherein the working surface is made from a material that includes stainless steel, tungsten or molybdenum; the cathode having a tip for arc attachment where a cathode spot is formed, wherein the cathode tip is made from pure tungsten, pure molybdenum, a tungsten or molybdenum alloy with tungsten as a major component or a composite material in which tungsten or molybdenum is the major component; and a flow path configured to have an inlet for gaseous hydrogen sulfide for dissociation in plasma into hydrogen and sulfur, and an outlet for gaseous products of hydrogen sulfide plasma dissociation. Optionally, the alloy or composite material has up to 10% low work function elements (thorium, cerium, lanthanum, or zirconium).

Claims

1. A device for hydrogen sulfide plasma dissociation, comprising: a plasma chemical reactor including an arc plasma generator that has a cathode and an anode; the anode having a working surface for contacting hydrogen sulfide plasma, wherein the working surface is made from a material that includes stainless steel, tungsten or molybdenum; the cathode having a tip for arc attachment where a cathode spot is formed, wherein the cathode tip is made from pure tungsten, pure molybdenum, a tungsten or molybdenum alloy with tungsten or molybdenum as a major component or a composite material in which tungsten or molybdenum is the major component; and a flow path configured to have an inlet for gaseous hydrogen sulfide for dissociation in plasma into hydrogen and sulfur, and an outlet for gaseous products of hydrogen sulfide plasma dissociation.

2. The device of claim 1, wherein the alloy of the cathode has up to 10% of low work function elements.

3. The device of claim 2, wherein the low work function elements are any of thorium, cerium, lanthanum, and zirconium.

4. The device of claim 1, wherein the composite material of the cathode has up to 10% of low work function elements or their compounds.

5. The device of claim 4, wherein the low work function elements are any of thorium, cerium, lanthanum, and zirconium.

6. The device of claim 1, wherein all surfaces of the arc plasma generator that are in contact with the hydrogen sulfide are kept at temperatures above a temperature of sulfur condensation.

7. The device of claim 1, wherein all conductive surfaces of the arc plasma generator that are in contact with the hydrogen sulfide are made from 316 stainless steel, pure tungsten, pure molybdenum, a tungsten or molybdenum alloy with tungsten or molybdenum as a major component, or a composite material in which tungsten or molybdenum is the major component.

Description

BRIEF DESCRIPTION OF THE ATTACHED FIGURES

[0013] The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.

[0014] In the drawings:

[0015] FIG. 1 shows a hafnium tip of a cathode after 10 minutes of operation in H.sub.2S at 2 A current.

[0016] FIG. 2 shows a cathode with tungsten tip after 30 minutes of operation in hydrogen at 2 A current.

[0017] FIG. 3 shows the same cathode with tungsten tip after 30 minutes of operation in H.sub.2S at 2 A current.

[0018] FIG. 4 shows equilibrium fractions (% mol.) of substances formed from W+3H.sub.2O.

[0019] FIG. 5 shows equilibrium (% mass) of substances formed from W+2H.sub.2S.

[0020] FIG. 6 shows equilibrium fractions (% mass) of substances formed from Mo+2H.sub.2S.

[0021] FIG. 7 shows a schematic of a high-voltage axial plasmatron for dissociation of hydrogen sulfide.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0022] Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings.

[0023] It is known that all metals except gold form sulfides. Also, it is known that during plasma dissociation of H.sub.2S, in addition to sulfur molecules S.sub.n, where n2, in the gas phase, very reactive radicals S and SH form. It is also known that sulfur is the closest element to oxygen regarding their chemical properties as oxidizers. Therefore, it was expected that metals that are not stable in oxygen, are not stable is sulfur atmosphere also.

[0024] In plasma arc devices that are working with oxygen-containing gases (oxygen, air, water vapor), hafnium or zirconium cathodes (so-called thermochemical cathodes) demonstrate very good and stable properties because of formation of very thermally stable electrically conductive films of their oxides or nitrides on the surface of the melted metal crater. There was a small hope that as hafnium electrodes can work with water vapor, they may also work with hydrogen sulfide. The experiment showed rather fast destruction of hafnium insert in the condition of low-current arc in H.sub.2S (FIG. 1). It is possible to see that a protective oxide/nitride film on a crater is destroyed, and surface is irregular, probably because of local bubbling out of dissolved gases (Hafnium melting temperature is 2506 K and boiling temperature is 4876 K).

[0025] Another small hope was that a tungsten cathode can survive in a hydrogen sulfide atmosphere. This hope was supported by the data that tungsten is one of the most stable metals with regard to high-temperature H.sub.2S corrosion (Farber, M. and Ehrenberg, D. M., 1952. High-Temperature Corrosion Rates of Several Metals with Hydrogen Sulfide and Sulfur Dioxide. Journal of The Electrochemical Society, 99(10), pp. 427-434). However, the studied data were obtained at temperatures of about 1200 K while the cathode spot temperature is usually much higher. Thus, it is known that the arc cathode spots have radii of 0.5-2 mm and emit electrons due to the thermionic emission and, therefore, the spot surface temperature is about 3000-4000 K (Jiittner, B., 1997, Properties of arc cathode spots. Le Journal de Physique IV, 7(C4), pp. C4-31).

[0026] Experiments demonstrated a surprising result (FIGS. 2 and 3, showing a roughly 10 mm cathode, with a roughly 4.5 mm tip (at max diameter), in that examplegenerally, the tip can be as small as 1 mm, and as large as 10 mm, for large cathodes). Initially, the tungsten cathode was tested in hydrogen that is known to be safe for tungsten.

[0027] It is possible to see (FIG. 2) that after the sharp cathode tip became dull, there was no further substantial erosion, though the tungsten became darker than its stainless-steel holder.

[0028] When the same electrode was used for 30 minutes in H.sub.2S, no further erosion became visible, but tungsten became shiny metal-white while the stainless-steel holder became dark (FIG. 3).

[0029] To understand why tungsten is stable with H.sub.2S arc and not stable with H.sub.2O arc, thermodynamic equilibrium simulation was made of two mixtures: W+3H.sub.2O (FIG. 4) and W+2H.sub.2S (FIG. 5).

[0030] It is possible to see that there are fundamental differences in these two equilibrium mixtures. At low temperatures, solid tungsten is not a major component and should be converted to WS.sub.2(solid) or WO.sub.2(solid), but reactions are slow at these temperatures and the conversion will take a lot of time. At high temperatures above 2000 K, solid and then liquid tungsten is the only substantial W-containing substance in H.sub.2S atmosphere (FIG. 5). On the other hand, at these high temperatures in H.sub.2O atmosphere (FIG. 4), many gaseous W-containing substances (W.sub.3O.sub.9, W.sub.2O.sub.6, WO.sub.3, WO.sub.2, and WO) are thermodynamically stable in high concentrations and this should result in the fast chemical erosion of a tungsten electrode.

[0031] FIG. 5 shows equilibrium fractions (% mass) of substances formed from W+2H.sub.2S. FIG. 6 shows equilibrium fractions (% mass) of substances formed from Mo+2H.sub.2S.

[0032] Thermodynamic simulation is a good tool to understand the major chemical stability and instability issues, however, it cannot take into account kinetics of the spatially non-uniform electrochemical processes in the cathode vicinity. Because of the thermionic emission, the concentration of electrons near the cathode tip is much higher than that of positive ions. FIG. 5 shows some concentration of W.sup.+ that is equal to that of electrons at very high temperatures. In the vicinity of the cathode spot, the concentration of electrons will be much higher and they will effectively ionize gaseous tungsten because it has the lowest ionization potential among all substances in the mixture. Then positively charged tungsten ions W.sup.+ will be attracted by strong electric field back to the cathode, and thus it will be effective vapor deposition of tungsten on the cathode surface. This is probably the reason why the tungsten cathode became shiny white after the operation in H.sub.2S atmosphere.

[0033] It is known that molybdenum (Mo) is chemically very similar to tungsten. Thermodynamic equilibrium simulation of the mixture Mo+2H.sub.2S (FIG. 6) shows that this similarity will allow probably to use molybdenum cathodes for H.sub.2S plasma generation, however lower melting temperature and higher vapor pressure of molybdenum in comparison with tungsten make tungsten the first choice for manufacturing of arc cathodes. The cathode is typically made of 316 stainless steel, while the cathode tip can be made from pure tungsten, a tungsten alloy (at least 90% tungsten, the rest low work function elements, such as thorium, cerium, lanthanum, or zirconium), pure molybdenum, or an alloy of molybdenum (at least 90% molybdenum, the rest low work function elements), or a composite, such as made from powdered metals, where at least 90% are tungsten grains, the rest low work function elements or their compounds grains, or a composite in which where at least 90% are molybdenum grains, the rest low work function elements or their compounds grains.

[0034] An anode made from stainless steel (SS 316) also demonstrated very good stability at least at low currents, and this is not surprising because it is known that SS316 is stable in H.sub.2S atmosphere, and fast motion of the anode spot that can be arranged by different known ways, e.g., by gas-dynamic or magnetic rotation, can prevent overheating of the metal in the anode spot. Other anode materials can be used also, for example, it is a known practice to make anodes from the tungsten-containing composite materials. Probably, the use of copper as a standard material for arc anodes is not a good choice because of the known high rate of copper corrosion in hydrogen sulfide.

[0035] FIG. 7 shows a schematic of a high-voltage axial plasmatron for dissociation of hydrogen sulfide, according to one embodiment of the invention. In the figure: (1)grounded anode; (2)high-voltage cathode; (3)insulator; (4)plasma-chemical reactor chamber; (5)plasma gas (hydrogen sulfide) inlet; (6)outlet for gaseous products; (7)cathode tip; (8)electric arc discharge.

[0036] Thus, it is possible to make H.sub.2S dissociation in an arc plasma generator (plasmatron).

[0037] Having thus described a preferred embodiment, it should be apparent to those skilled in the art that certain advantages of the described method and apparatus have been achieved.

[0038] It should also be appreciated that various modifications, adaptations and alternative embodiments thereof may be made within the scope and spirit of the present invention. The invention is further defined by the following claims.